Formation, Characteristics and Electrocatalytic Properties ... · A thesis submitted in conformity...

Formation, Characteristics and Electrocatalytic Properties of Nanoporous

Metals Formed by Dealloying of Ternary-Noble Alloys

by

Adrián Alberto Vega Zúñiga

A thesis submitted in conformity with the requirements for the degree of Doctorate of Philosophy

Department of Chemical Engineering and Applied Chemistry

University of Toronto

© Copyright by Adrián Alberto Vega Zúñiga (2014)

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Formation, Characteristics and Electrocatalytic Properties of Nanoporous

Metals Formed by Dealloying of Ternary-Noble Alloys

Adrián Alberto Vega Zúñiga

Doctor of Philosophy

Department of Chemical Engineering and Applied Chemistry University of Toronto

2014

Abstract

Nanoporous metals formed by electrochemical dealloying of silver from Ag-Au-Pt

alloys, with 77 at.% silver and platinum contents of 1, 2 and 3 at.%, have been studied. The

presence of platinum, which is immobile relative to gold, refine the ligament size and

stabilized the nanostructure against coarsening, even under experimental conditions that

would be expected to promote coarsening (e.g., exposure to high temperature, longer

dealloying times). By adding only 1 at.% Pt to the alloy precursor, the ligament/pore size

was reduced by 50% with respect to that in nanoporous gold (NPG), which was formed on a

Ag-Au alloy with the same silver content as ternary alloys. A further decrease in the

ligament size was observed by increasing the platinum content of the precursor; however,

most of the improvement occurred with 1 at.% Pt.

The adsorbate-induced surface segregation of platinum was also investigated for

these nanoporous metals. By exposing freshly-dealloyed nanostructures to moderate

temperatures in the presence of air, platinum segregated to the ligament surface; in

contrast, in an inert atmosphere (Ar-H2), platinum mostly reverted to the bulk of the

ligaments. This thermally activated process was thermodynamically driven by the interaction

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between platinum and oxygen; however, at the desorption temperature of oxygen, platinum

de-segregated from the surface. Moreover, the co-segregation of platinum and oxygen

hindered the thermal coarsening of the ligaments.

Finally, the electrocatalytic abilities of these nanostructures were studied towards

methanol and ethanol electro-oxidation, in alkaline and acidic media, showing significantly

improved response in comparison to that observed in NPG. The synergistic effect between

gold and platinum atoms and the smaller feature size of the nanostructures were directly

associated with this behaviour. In alkaline electrolyte, the nanostructure formed on the alloy

with 1 at.% Pt showed higher catalytic response than the other two ternary nanostructures,

which could be associated with the platinum/gold ratio on the surface of the structure. In

acidic electrolyte, the nanostructure with the highest platinum content displayed the highest

electrocatalytic response. Furthermore, the presence of platinum changed the selectivity of

both reactions: the concentrations of carbonate produced increased by increasing the

platinum content in the alloy precursor.

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Acknowledgements

My sincere gratitude is due toward my supervisor, Dr. Roger C. Newman for his

continuous guidance, support, encouragement, and valuable feedback throughout this

research work. His support, generosity and guidance gave me the chance to overcome

many of the challenges that I faced during this process.

I also thank Dr. Donald W. Kirk and Dr. Edgar J. Acosta for serving on my Ph.D

committee and providing valuable feedback to strengthen my project. I am also thankful to

Dr. Doug Perovic and Dr. Joey Kish for agreeing to be examiners in the dissertation

defence. Thanks very much to all of them for their valuable comments and suggestions.

I am grateful to Mariusz A. Brik, Jaganathan Ulaganathan, Dorota Artymowicz and

Anatolie Carcea for providing lengthy and valuable discussions generating excellent ideas. I

am especially grateful to Nick Senior for his help, advise, constant support and friendship

during most of this process. Certainly all of them have contributed to achieve my goals

during my Ph.D.

My thanks are also extended to Ilya Gourevich, Peter Brodersen, Neil Coombs,

George Kretchmann, Joel Tang and Sal Boccia for sharing their expertise on specific topics

and for providing their time and minds to solve technical issues.

Special thanks also to the faculty and staff of the Department of Chemical Engineering

and Applied Chemistry for their support and for the friendly environment that I found here. I

would like to acknowledge also the generous funding contributed by NSERC.

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Lastly, I would like to express my deepest appreciation and respect to my family,

especially my parents, Willy and Giselle, my wife Olga and my son Sebastián for giving me

hope, courage and an amazing support throughout this process.

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Dedication

To Olga and Sebastián

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Co-Authorship Statement

All manuscript-based chapters of this thesis were co-authored with my research

supervisor, Dr. Roger C. Newman. My contribution was the planning and execution of the

experiments, the collection and analysis of the experimental data and the writing of the

manuscripts. Dr. Roger. C. Newman largely contributed to the improvement of experimental

plans, the discussion of the results, and the revision of the manuscript drafts.

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Quote

The victory of success is half won when one gains the habit of setting goals and achieving them. Even the most tedious chore will become endurable as you parade through each day convinced that every task, no matter how menial or boring, brings you closer to fulfilling your dreams.

Og Mandino

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Table of Contents

Abstract ....................................................................................................................................... ii

Acknowledgements.................................................................................................................... iv

Dedication .................................................................................................................................. vi

Co-Authorship Statement ......................................................................................................... vii

Quote ........................................................................................................................................ viii

Table of Contents ...................................................................................................................... ix

List of Tables............................................................................................................................ xiv

List of Figures ........................................................................................................................... xv

List of Appendices .................................................................................................................. xxvi

Chapter 1. Introduction ........................................................................................................... 1

1.1 Formation of nanoporous metals by dealloying ................................................ 4

1.2 NPG: formation, characteristics and surface modifications ........................... 11

1.2.1 Overview of proposed dealloying mechanisms of NPG ......................... 13

1.2.2 Coarsening of the nanoporous structure ................................................. 20

1.2.2.1 Overview ........................................................................................ 20

1.2.2.2 Reduction of coarsening in NPG .................................................. 25

1.2.2.2.1 Nanoporous metals formed by dealloying of ternary-

noble alloys (Ag-Au-Pt)…………………………………. 26

1.2.3 Surface modifications of NPG ................................................................. 27

1.3 Beyond NPG: Other nanoporous metals ........................................................ 29

1.4 Applications of nanoporous metals ................................................................. 32

1.4.1 Catalytic abilities of NPMs ....................................................................... 33

1.4.2 Electrocatalytic applications of NPMs ..................................................... 38

1.5 Thesis scope ................................................................................................... 41

1.6 Thesis objectives ............................................................................................. 42

1.7 Thesis layout ................................................................................................... 44

1.8 Reproducibility of experiments ........................................................................ 47

1.9 References ...................................................................................................... 48

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Chapter 2. Nanoporous Metals Fabricated through Electrochemical Dealloying of

Ag-Au-Pt Alloys with Systematic Variation of Au:Pt Ratio1 .......................... 69

2.1 Introduction ...................................................................................................... 69

2.2 Experimental procedures ................................................................................ 71

2.2.1 Materials and dealloying procedures ...................................................... 71

2.2.2 Material characterization ......................................................................... 72

2.3 Results and discussion ................................................................................... 75

2.3.1 Electrochemical measurement of dealloying behavior ........................... 75

2.3.2 Formation and characterization of nanoporous metals .......................... 77

2.3.3 Influence of dealloying parameters on the characteristics of the

layer ......................................................................................................... 86

2.3.3.1 Dealloying temperature ................................................................. 87

2.3.3.2 Dealloying potential ....................................................................... 91

2.3.3.3 Dealloying charge.......................................................................... 94

2.4 Conclusion ....................................................................................................... 97

2.5 Acknowledgements ......................................................................................... 98

2.6 References ...................................................................................................... 98

Chapter 3. Beneficial Effects of the Surface Segregation of Platinum in

Nanoporous Metals Fabricated by Dealloying of Ag-Au-Pt Alloys2 ........... 104

3.1 Introduction .................................................................................................... 104

3.2 Materials and methods .................................................................................. 106

3.3 Results and discussion ................................................................................. 109

3.3.1 Surface chemistry of nanoporous metals after heat treatment ............. 113

3.3.2 XPS and LEIS characterization of the surface of nanoporous

metals .................................................................................................... 118

3.3.3 Surface segregation in the absence of oxygen ..................................... 124

3.4 Conclusions ................................................................................................... 128

3.5 Acknowledgements ....................................................................................... 128

3.6 References .................................................................................................... 129

Chapter 4. Methanol Electro-oxidation on Nanoporous Metals Formed by

Dealloying of Ag-Au-Pt Alloys3 ....................................................................... 134

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4.1 Introduction .................................................................................................... 134

4.2 Experimental procedures .............................................................................. 137

4.2.1 Materials and dealloying procedures .................................................... 137

4.2.2 Materials characterization ..................................................................... 138

4.2.3 Electro-oxidation of methanol ................................................................ 140


4.3.1 Effect of dealloying parameters and tunability of the resulting

nanoporous structures ........................................................................... 152

4.3.2 Methanol electro-oxidation in acidic media ........................................... 160

4.4 Conclusions ................................................................................................... 163

4.5 Acknowledgements ....................................................................................... 165

4.6 References .................................................................................................... 165

Chapter 5. Electro-oxidation of Ethanol on Highly Porous Nanostructures

Obtained by Dealloying of Binary and Ternary Noble-metal Alloys4 ......... 173

5.1 Introduction .................................................................................................... 173

5.2 Experimental ................................................................................................. 175

5.2.1 Materials, dealloying procedures and nanoporous characterization .... 175

5.2.2 Electro-oxidation of ethanol ................................................................... 177


5.4 Conclusions ................................................................................................... 190

5.5 Acknowledgment ........................................................................................... 192

5.6 References .................................................................................................... 192

Chapter 6. Conclusions, Contributions and Recommendations for Future Work....... 200

6.1 Overall conclusions ....................................................................................... 200

6.2 Significance and contribution of the research .............................................. 211

6.3 Potential applications of findings .................................................................. 213

6.4 Limitations of the research and recommendations for future work .............. 214

Appendix A. Supplementary Data ..................................................................................... 218

A.1 Potentiostatic dealloying: comparison between binary and ternary alloys .. 218

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A.2 Formation of surface oxides during dealloying ............................................. 219

A.3 Electrochemical Impedance spectroscopy (EIS) of dealloyed samples ...... 220

A.4 Alternative method to determine the developed surface area in

dealloyed specimens..................................................................................... 222

A.5 Calculation of the mass of the dealloyed layer ............................................. 223

A.6 Calculation of the thickness of the dealloyed layer based on the

retained silver content in the layer ................................................................ 226

A.7 Potentiostatic dealloying at different temperatures: comparison between

all alloys ......................................................................................................... 227

A.8 Nitrogen adsorption/desorption isotherms at 77.35 K – BET Surface

area estimation .............................................................................................. 229

A.9 SEM images of nanoporous structures after two months in dealloying

electrolyte ...................................................................................................... 231

A.10 SEM images of NPG after removing small charge densities ....................... 232

A.11 Estimation of the availability of platinum to segregate to the surface of

the ligaments ................................................................................................. 233

A.12 Angle-resolved X-ray spectroscopy (XPS) analyses for the ternary

alloys before and after segregation of platinum ........................................... 236

A.13 Depth profiling analyses based on angle resolved X-ray Spectroscopy

(XPS) for samples before and after induced-surface segregation of

platinum ......................................................................................................... 238

A.14 Under-potential-deposition (UPD) of hydrogen for samples before and

after surface segregation of platinum was induced ...................................... 241

A.15 Effect of scan rate and agitation speed during methanol electro-

oxidation reaction .......................................................................................... 243

A.16 Methanol electro-oxidation on nanoporous metals obtained at different

dealloying conditions ..................................................................................... 245

A.17 Potentiostatic oxidation of methanol in alkaline solution .............................. 248

A.18 Nuclear magnetic resonance (NMR) spectra: determination of formate

concentrations for the methanol oxidation reaction ...................................... 249

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A.19 Electro-oxidation of ethanol in acidic media ................................................. 251

Appendix B. Simulation of Nanoporous Metals Formed by Electrochemical

Dealloying of Ag-Au-Pt Alloys ..................................................................... 253

B.1 Introduction .................................................................................................... 253

B.2 Modeling conditions and results ................................................................... 253

B.3 References .................................................................................................... 263

Appendix C. List of Publications and Presentations ...................................................... 265

C.1 Publications ................................................................................................... 265

C.2 Presentations ................................................................................................ 266

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List of Tables

Table 2.1 Real compositions of Ag-Au and Ag-Au-Pt alloys. The numbers in brackets represent the 95% confidence interval (CI) calculated from at least seven measurements randomly chosen along the alloys. ............................................ 76

Table 2.2 Physical characteristics and developed surface area of different nanoporous structures. The numbers in brackets represent the 95% CI. ............................. 79

Table 2.3 Physical characteristics, developed surface area and residual Ag of different nanoporous structures formed at different dealloying potentials. The numbers in brackets represent the 95% CI. ....................................................... 93

Table 4.1 Physical characteristics, residual silver and roughness factor of the nanoporous structures developed at 25 °C and 0.55 V vs. MSE in 0.77 M HClO4. Charge density: 5 C cm-2. The number in brackets represents the 95% confidence interval (CI). ........................................................................... 142

Table A.1 Experimental data for the calculation of the mass of the dealloyed layer in NPG grown in 0.77 M HClO4 at 0.55 V vs. MSE and 25 °C. ........................... 225

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List of Figures

Figure 1.1 Nanoporous structure formed by dealloying of silver from Ag77:Au23 alloy in 0.77 M HClO4 solution at 40 °C. ........................................................................... 3

Figure 1.2 Schematic illustration of the electrochemical behavior of binary alloys. Adapted from Pickering [33]. ................................................................................ 9

Figure 1.3 NPG and its various structural variations ranging from microfabrication (adapted with permission from Biener et al. [53]. Copyright 2006 American Chemical Society), tunable pore and ligament size, hierarchical structures based on templating techniques (adapted from Erlebacher and Seshadri [12] with permission of Cambridge University Press) and its chemical variations, ranging from decoration of the surface of the ligaments with metals such as platinum (adapted with permission from Ding et al. [54]. Copyright 2004 American Chemical Society), molecular entities (adapted from Pareek et al. [55] with permission of Royal Society of Chemistry) or even metal oxides, such as TiO2 (adapted with permission from Jia et al.[56]. Copyright 2009 American Chemical Society). ....................................... 13

Figure 1.4 Snapshots taken from Monte Carlo simulations performed on Ag-Au alloys: (a) initial stage of dissolution of silver and formation of terrace island; (b) formation of mounts. Single steps of dealloying mechanism are depicted in (c), where the yellow balls represent gold and the grey balls silver. (a) and (b) are adapted from Erlebacher and Seshadri [12] with permission of the Cambridge University Press. .............................................................................. 19

Figure 1.5 (a) The coordination of atoms depends on their location. The stability of the atoms decreases with the coordination number (CN). (b) Percentage of atoms as a function of the length scale. This is shown for a straight ligament as a simple example (numbers are based on a geometric consideration). Reprinted with permission from Wittstock [157] – Copyright 2010 A. Wittstock. ............................................................................................... 35

Figure 2.1 Results of negative to positive potential scan on Ag-Au and Ag-Au-Pt alloys, recorded at a scan rate of 0.5 mV s-1 at 25 °C. The absolute value of the current density is plotted in this figure: current densities below ~ -0.3 V are cathodic; everything else is anodic. .................................................................... 76

Figure 2.2 Interconnected ligament/pore structure of the Ag77:Au23 after passing 5 C cm-2 at 25 °C and 550 mV vs. MSE. ................................................................... 77

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Figure 2.3 SEM images of freshly-formed nanoporous structures at 550 mV vs. MSE and 25 °C: (a) Ag77:Au23, (b) Ag77:Au22:Pt1, (c) Ag77:Au21:Pt2, (d) Ag77:Au20:Pt3. In all cases 5 C cm-2 were passed. .............................................. 79

Figure 2.4 BET pore size distribution of freshly-formed nanoporous structures. The dealloying conditions for all alloys were the same - temperature: 25 °C, dealloying potential: 550 mV vs. MSE, charge passed: 5 C cm-2. ..................... 80

Figure 2.5 TEM images of freshly-formed nanoporous structures: (a) Ag77:Au23, (b) Ag77:Au22:Pt1. The dealloying conditions for both alloys were the same - temperature: 25 °C, potential: 550 mV vs. MSE, charge passed: 5 C cm-2. ..... 81

Figure 2.6 XRD patterns of (a) Ag77:Au23 and (b) Ag77:Au20:Pt3 before and after dealloying. The as-annealed material in both cases is represented with a solid line, and the as-dealloyed material with a dotted line. The dealloying conditions for both alloys were the same - temperature: 25 °C, potential: 550 mV vs. MSE, charge passed: 5 C cm-2. ...................................................... 82

Figure 2.7 (a) Ultramicrotomed sample; the insert shows a higher magnification of the region close to the dealloying front. (b) Residual Ag profile across the dealloyed layer of freshly-formed nanoporous structures. The Ag content was determined by TEM – EDS of ultramicrotomed samples, and 100% represents the original surface of the sample (SS); 0 % is the dealloying front (DF). Error bars represent 95% CI calculated from at least 3 different measurements across the dealloyed layer. ........................................................ 85

Figure 2.8 Cyclic voltammograms of the Ag-Au-Pt alloys after dealloying at 550 mV vs. MSE, 25 °C and passing a charge density of 5 C cm-2. In all cases, CV profiles were obtained in 1 M H2SO4 solution at 20 mV s-1 and 25 °C. The original Pt content is shown in the figure. The insert shows the fraction of Pt on the surface of the nanostructure, obtained by H-UPD and XPS analyzes, with respect to Pt content of the precursors. The error bars represent 95% CI calculated from triplicate runs. .............................................. 86

Figure 2.9 SEM images of freshly-formed nanoporous structures: (a) Ag77:Au23 dealloyed at 10 °C; (b) Ag77:Au23 dealloyed at 60 °C; (c) Ag77:Au22:Pt1 dealloyed at 10 °C; (d) Ag77:Au22:Pt1 dealloyed at 60 °C; (e) Ag77:Au20:Pt3 dealloyed at 10 °C; (e) Ag77:Au20:Pt3 dealloyed at 60 °C. In all cases, the SEM images were taken approximately in the middle of the DL. ...................... 89

Figure 2.10 Ligament width, normalized surface area, thickness of DL and residual Ag in the DL of freshly-formed nanoporous structures at different temperatures: (□) Ag77:Au23, (●) Ag77:Au23:Pt1, (■) Ag77:Au21:Pt2, (▲) Ag77:Au20:Pt3. Error bars represent 95% CI calculated from triplicate runs in the case of surface area, from at least 30 measurements in the case of the ligament size, from ca. 15 measurements for the thickness of the layer and from 4 different measurements across the dealloyed layer in the case of

xvii

the Ag content. In all cases, the Ag content was measured approximately in the middle of the DL. ....................................................................................... 90

Figure 2.11 Fraction of Pt atoms on the surface of as-dealloyed structures after dealloying at different temperatures. In all cases, dealloying was carried out in 0.77 M HClO4, with a charge passed of 5 C cm-2 and a dealloying potential of 550 mV vs. MSE. The original Pt composition of the precursors is shown in the figure. Error bars represent 95% CI calculated from triplicate runs. ...................................................................................................... 91

Figure 2.12 SEM images of freshly-formed nanoporous structures at different dealloying potentials: (a) Ag77:Au23 at 500 mV, (b) Ag77:Au23 at 550 mV, (c) Ag77:Au22:Pt1 at 500 mV (d) Ag77:Au22:Pt1 at 550 mV, (e) Ag77:Au20:Pt3 at 500 mV, (f) Ag77:Au20:Pt3 at 550 mV. In all cases the charge density passed was 5 C cm-2. All SEM images were taken approximately in the middle of the DL. ................................................................................................. 92

Figure 2.13 SEM images of freshly-formed nanoporous structures after passing different dealloying charges: (a) Ag77:Au23 passing 2.5 C cm-2; (b) Ag77:Au23 passing 40 C cm-2; (c) Ag77:Au22:Pt1 passing 2.5 C cm-2; (d) Ag77:Au22:Pt1 passing 40 C cm-2; (e) Ag77:Au20:Pt3 passing 2.5 C cm-2; (f) Ag77:Au20:Pt3 passing 40 C cm-2. In all cases, the SEM images were taken approximately in the middle of the DL. ....................................................................................... 95

Figure 2.14 Average ligament width, normalized surface area, thickness of DL and average residual Ag in the DL of freshly-formed nanoporous structures at different dealloying charges: (□) Ag77:Au23, (●) Ag77:Au22:Pt1, (■) Ag77:Au21:Pt2, (▲) Ag77:Au20:Pt3. In all cases, the surface area was measured by CV in 1M HClO4 at 25 ºC. Error bars represent 95% CI calculated from triplicate runs in the case of surface area, from at least 30 measurements in the case of the ligament size, from ca. 15 measurements for the thickness of the layer and from 4 different measurements in the case of the average Ag content of the layer. In all cases, the Ag content was measured approximately in the middle of the DL. ...................................... 96

Figure 3.1 (a) SEM image of the freshly-developed nanoporous structure obtained by dealloying of Ag77:Au22:Pt1. (b) Dealloyed layer after ultramicrotoming a sample; in this case, the dealloyed layer rolled as a consequence of the low cutting velocity used during the process. The insert shows a TEM image of the nanoporous structure. The dealloying conditions in all cases were the same – dealloying charge density: 5 C cm-2, temperature: 25 °C and dealloying potential: 550 mV vs. MSE. ...................................................... 111

Figure 3.2 (a) Fraction of Pt atoms on the outermost surface of the ligaments. (b) Pt content across the dealloyed layer obtained by TEM – EDS in ultramicrotomed samples. (c) Ultramicrotomed sample; the insert shows a TEM image of the dealloyed layer showing a highly dense porous structure. For Figures (a) and (b) error bars represent the 95% confidence

xviii

interval (CI): in the case of (a) the CI was obtained by triplicate runs, for (a) the CI was obtained from at least 3 points across the dealloyed layer - 0 % represents the dealloying front (DF) and 100 % represents the original surface of the sample (SS). The nomenclature in the case of (b) is based on the original composition of the alloys. ......................................................... 113

Figure 3.3 (a) Fraction of Pt on the surface of the nanoporous structures after exposure to different temperatures for 2 h; (b) roughness factor (Rf) of the nanostructures; (c) time dependence of the segregation phenomenon at 425 °C; (d) roughness factor (Rf) as a function of exposure time at 425 °C. The different alloys, based on their original composition, are represented as follows: (●) Ag77:Au22:Pt1, (○) Ag77:Au21:Pt2, (♦) Ag77:Au20:Pt3. In all cases, laboratory air was used. Error bars represent 95% CI calculated from triplicate runs. ........................................................................................... 115

Figure 3.4 SEM images of the nanoporous structures formed from Ag77:Au22:Pt1 alloy (a, b, c) and from Ag77:Au20:Pt3 alloy (d, e, f). Images (a) and (d) show the structures right after dealloying; images (b) and (e) show the structures after annealing at 425 °C in the presence of laboratory air; images (c) and (f) show the structures after annealing at 500 °C in the presence of laboratory air. For all the annealed structures, the exposure time was 2 h. .... 117

Figure 3.5 Fraction of Pt on the surface of the nanoporous structure formed on Ag77:Au20:Pt3 after exposure to 300 °C for different times. The dealloying conditions were the following: charge passed 5 C cm-2 at 25 °C and 550 mV vs. MSE. In all cases, laboratory air was used. The error bar represents 95% CI calculated from triplicate runs. .......................................... 118

Figure 3.6 Pt4f, Ag3d and O1s photoelectron spectra of nanoporous structures on Ag77:Au22:Pt1 alloy (a, c, e) and Ag77:Au20:Pt3 alloy (b, d, f) at different conditions: as-dealloyed structures (▬▬); after exposure at 425 °C ( ); after exposure at 500 °C ( ) and after exposure to 600 °C ( ). In the case of high temperature experiments, they were done in laboratory air for 2 h. All spectra were obtained at a take-off angle of 48 °. ... 120

Figure 3.7 A curve-fit of the Pt4f surface level spectra obtained after exposure of the as-dealloyed material to (a) 425 °C and (b) 600 °C. (c) Pt4f spectra and (d) O1s in which the solid line represents the photoelectron spectra of nanoporous structure annealed at 425 °C and the dotted line represents the spectra after electrochemically reducing the surface oxides in 1 M H2SO4. In all cases, samples were annealed for 2 h in laboratory air. All spectra were obtained at a take-off angle of 48 °on Ag77:Au20:Pt3 alloy. ......... 121

Figure 3.8 XPS Pt4f composition with respect to the take-off angle for (a) Ag77:Au22:Pt1 and (b) Ag77:Au20:Pt3 and for different experimental conditions: (◊) right after dealloying, (●) after exposure at 425 °C, (○) after exposure at 500 °C and (▲) after exposure to 600 °C. In the case of all high temperature conditions, laboratory air was used for an exposure time of 2 h. .................... 122

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Figure 3.9 (a) LEIS general survey for the highest platinum content alloy before and after exposure to moderately high temperature; in this case, the surface of the samples was bombarded with 3 keV 4He+ ions. (b) Au-Pt spectra after bombarding the surface of the samples with 5 keV 20Ne+ ions; the insert in this figure shows the Au-Pt spectra after bombarding the surface with 5 keV 40Ar+ ions to improve the sensitivity of the analysis. The signals for Au and Pt standards are also included in the figure. The details about the different experimental conditions are shown in the figure. For all high temperature experiments, samples were exposed for 2 h in the presence of laboratory air unless otherwise state. In all cases, analyses were done without any surface cleaning to avoid any ion-induced mixing of the noble metals. ............................................................................................................... 124

Figure 3.10 Fraction of Pt on the surface of the ligaments and average ligament size of the nanoporous structure formed on Ag77:Au20:Pt3 alloy with respect to temperature. Ar-H2 was used in all cases with an exposure time of 2 h. The insert shows the roughness factor (Rf) with respect to temperature. The error bars represent 95% CI calculated from triplicate runs in the case of the fraction of Pt and from at least 20 measurements in the case of the ligament size. .................................................................................................... 126

Figure 3.11 SEM images of the resulting structures formed after annealing in Ar-H2 atmosphere the nanoporous structure obtained by dealloying Ag77:Au20:Pt3 alloy. (a) Sample annealed at 200 °C; (b) sample annealed at 300 °C; (c) sample annealed at 425 °C, (d) sample annealed at 500 °C. In all cases, the annealing time was 2 h. .............................................................................. 126

Figure 3.12 SEM images of the resulting structures formed after annealing in NPG in (a) Ar-H2 atmosphere and (b) laboratory air. In both cases the exposure temperature was 425 °C and the the annealing time was 2h. ......................... 127

Figure 4.1 Interconnected ligament/pore structure formed on the Ag77:Au23 alloy after passing 5 C cm-2 at 40 °C and 0.55 V vs. MSE. The insert shows a higher magnification SEM image of the nanoporosity. ................................................ 142

Figure 4.2 (a) CV profiles of the different nanoporous structures in 5 M KOH; (b) CV profiles of NPG and of the nanoporous structures formed on the alloy with 1 at.% Pt in 5 M KOH - 1 M CH3OH solution. The insert in (b) shows the CV profile of polycrystalline Pt in 5 M KOH - 1 M CH3OH solution. In (a) and (b) the current density was normalized by the geometrical area of the electrodes. The original platinum content is shown in the figures. All CV profiles were obtained at 10 mV s-1 and 25 °C. ................................................ 145

Figure 4.3 (a) Summary of specific activities of the ternary alloy structures for the CH3OH oxidation reaction at -0.35 V vs. Hg/HgO; the vertical axis is the current density per true area of the electrodes. (b) Concentration of carbonate/formate produced after potentiostatic oxidation for 4000 s at -0.35 V vs. Hg/HgO; during this time, the solution was agitated with a

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magnetic stirrer at ca. 700 rpm. In all cases, a 5 M KOH - 1 M CH3OH solution was used. The error bars represent 95 % CI obtained by triplicate runs. .................................................................................................................. 148

Figure 4.4 CV profiles for the electrooxidation of CH3OH solution in 5 M KOH on the nanoporous structure formed on the ternary structure developed on the alloy with 1 at.% Pt: (a) effect of the scan rates, (b) agitation speeds, (c) scan limit and (d) methanol concentration. For figures (a) to (c), the methanol concentration was 1 M; for figures (b) to (d), the scan rate was 10 mV s-1. Temperature: 25 °C. Details about the scan rates, agitation speeds, scan limit and methanol concentrations are given in the figures. In all cases, the current was normalized by the true area of the electrodes. ...... 151

Figure 4.5 CV profiles for the electrooxidation of CH3OH solution in 5 M KOH on the nanoporous structure formed on the alloy with 1 at.% platinum. In all cases the scan rate was 10 mV s-1 and the temperature 25 °C. Details about the number of scan cycles are indicated in the figures. ......................... 152

Figure 4.6 Effect of the dealloying temperature and dealloying charge density on the main characteristics of the resulting nanoporous structures: (a) effect of dealloying temperature on the size of the ligaments; the insert in this figure shows the average thickness of the DL; (b) fraction of platinum on the surface of the ligaments after dealloying at different temperatures; (c) ligament size of the different structures after passing different charge densities; the insert shows the average thickness of the DL; (d) fraction of platinum on the surface of the ligaments after passing different charges. Details about the dealloying temperature are indicated in the figures (b) and (d). The error bars represent 95 % CI obtained by triplicate runs. ........... 154

Figure 4.7 (a) Summary of specific activities for the CH3OH oxidation reaction of the ternary alloy nanostructures developed under different conditions; the vertical axis is the current density per true area of the electrodes. (b) Concentration of carbonate produced on nanoporous structures dealloyed after passing different charge densities after potentiostatic oxidation for 4000 s; during this time, the solution was agitated with a magnetic stirrer at ca. 700 rpm. The insert in (b) corresponds to concentration of carbonate produced on nanostructures dealloyed at different temperatures. In all cases, the potential was fixed at -0.35 V vs. Hg/HgO. The different dealloying conditions are shown in the figures. The error bars represent 95 % CI obtained by triplicate runs. ....................................................................... 157

Figure 4.8 (a) Ligament size and fraction of platinum on the surface of the ligaments for all ternary structures after exposure to 425 °C for 2 h in the presence of laboratory air; (b) CV profiles in 5 M KOH - 1 M CH3OH solution for the nanoporous structure formed on the alloy with 1 at.% Pt structures before and after segregation of Pt, the CV profiles were obtained at 10 mV s-1 and 25 °C and the current density was normalized by the geometrical area of the electrodes; (c) Summary of specific activities of the ternary alloy nanostructures for the CH3OH oxidation reaction at -0.35 V vs. Hg/HgO;

xxi

the vertical axis is the current density per true area of the electrodes; (d) Concentration of carbonate produced after potentiostatic oxidation for 4000 s at -0.35 V vs. Hg/HgO; during this time, the solution was agitated with a magnetic stirrer at ca. 700 rpm. The error bars represent 95 % CI obtained by triplicate runs. ............................................................................................... 160

Figure 4.9 (a) CV profiles of the different nanoporous structures in 0.5 M HClO4; (b) CV profiles of the nanoporous structures in 0.5 M HClO4 - 1 M CH3OH solution. The insert in (b) corresponds to the CV profiles in 0.5 M HClO4 - 1 M CH3OH solution after segregation of platinum at 425 °C in laboratory air for 2 h. The original platinum composition is shown in (a). All CV profiles were obtained at 10 mV s-1. The temperature in all cases was 25 °C. ............ 162

Figure 5.1 SEM images of the freshly-developed nanoporous structure at 0.55 V vs. MSE and 25 °C in 0.77 M HClO4: (a) Ag77:Au23 (b) Ag77:Au22:Pt1. In all cases, 5 C cm-2 were passed. Inserts in (a) and (b) are TEM image of the same structures. ............................................................................................... 180

Figure 5.2 Average ligament size of the freshly-formed nanoporous structures formed at 0.55 V vs. MSE and 25 °C in 0.77 M HClO4. The insert shows the roughness factor (Rf) as a function of the platinum content of the precursor. Error bars represent 95% confidence interval (CI) calculated from at least 30 measurements in the case of the ligament size, and triplicate runs for Rf. ...................................................................................................................... 181

Figure 5.3 (a) CV profiles of the different nanoporous structures in 4 M KOH; (b) CV profiles of NPG and of the nanoporous structures formed on the alloy with 1 at.% Pt in 4 M KOH - 1 M C2H5OH solution. The insert in (b) shows the CV profile of polycrystalline Pt in 4 M KOH - 1 M C2H5OH solution. In all cases the vertical axis corresponds to the nominal current density. The original platinum composition is shown in (a). All CV profiles were obtained at 10 mV s-1 and 25 °C. .................................................................................... 183

Figure 5.4 CV profiles for the electro-oxidation of ethanol in 4 M KOH – 1 M C2H5OH solution. The insert shows the summary of specific activities (S.a) of the ternary alloy structures for the C2H5OH oxidation reaction at -0.6 V vs. Hg/HgO and of the binary structure taken at -0.1 V vs. Hg/HgO; the vertical axis is the current density per true area of the electrodes. Details about the structures and the number of scan cycles are indicated in the figures. All CV profiles were recorded at 10 mV s-1 and 25 °C. ......................................... 185

Figure 5.5 Current density observed during a prolonged oxidation of 1 M C2H5OH in 4 M KOH solution at -0.6 V vs. Hg/HgO in the case of ternary nanostructures and at -0.1 V vs. Hg/HgO in the case of NPG. In all cases, the solution was de-aerated by high-purity nitrogen and agitated with a magnetic stirrer at ca. 700 rpm. The insert shows the concentration of carbonate produced after 4000 s with respect to the platinum content of the precursor. ................. 186

xxii

Figure 5.6 SEM images of nanoporous structures used to oxidize ethanol in alkaline electrolyte: (a) NPG after 10 potential cycles, (b) ternary nanostructure formed on the alloy with 1 at.% Pt after 10 potential cycles, (c) NPG after 4000 s reaction at -0.1 V, (d) ternary nanostructure formed on the alloy with 1 at.% Pt after 4000 s at -0.6 V. ................................................................ 188

Figure 5.7 CV profiles for all ternary nanoporous structures in 4 M KOH – 1 M C2H5OH solution. In all cases, the structures were exposed to 425 °C for 2 h in the presence of laboratory air. The insert shows the summary of specific activities (S.a) of the ternary alloy structures for the C2H5OH oxidation reaction at -0.6 V vs. Hg/HgO before and after segregation; the vertical axis is the current density per true area of the electrodes. The CV profiles were obtained at 10 mV s-1 and 25 °C. The original platinum composition is shown in the figure and the current density was normalized by the true area of the electrode. ........................................................................................ 190

Figure A.1 Dealloying current of the binary and ternary alloys recorded at 0.55 V vs. MSE and 25 °C in 0.77 M HClO4. The charge density passed was 5 C cm-2 in all cases. Details about the alloys are shown in the figure. ......................... 218

Figure A.2 Negative polarization sweep (1 mV s-1) of freshly dealloyed Ag77:Au23 specimen (0.7 V for approximately 25 s at 25 °C and removing 5 C cm -2) in the dealloyed solution. Insert: linear current density axis plot of the same data ................................................................................................................... 219

Figure A.3 Impedance measurements for all alloys studied in this investigation: (a) Ag77:Au23, (b) Ag77:Au22:Pt1, (c) Ag77:Au21:Pt2, (d) Ag77:Au20:Pt3. In all cases, the charge density passed was 5 C cm-2 and the dealloying potential was 0.55 V vs. MSE. Dealloying temperature: 25 °C. ............................................. 221

Figure A.4 Voltammograms obtained from -0.24 to 0.05 V vs. MSE in 1 M HClO4 solution: (a) NPG, (b) Nanoporous structure formed on the alloy with the lowest platinum content. The insert in both cases shows the dependence of the double layer current at -0.1 V vs. scan rate. The scan rates used are shown in the figures. ......................................................................................... 223

Figure A.5 Representation of the dealloyed specimen, in which the dealloyed layer is identified with the inclined lines. ....................................................................... 224

Figure A.6 (a) Thickness of the dealloyed layer in NPG measured from a cross-sectional metallographic specimen and (b) from a fracture surface using a low magnification SEM image. In both cases, different samples of Ag-Au alloy were dealloyed at 0.55 V vs. MSE at 25 °C and passing a total charge of 5 C cm-2. The metallographic specimen was polished to 0.05 µm

xxiii

using alumina powder; the fracture surface was obtained after manually breaking the dealloyed sample in air. ............................................................... 224

Figure A.7 Dealloying current of all the alloys investigated: (a) Ag77:Au23; (b) Ag77:Au22:Pt1; (c) Ag77:Au21:Pt2; (d) Ag77:Au20:Pt3. In all cases, the current was recorded at 0.55 V vs. MSE in 0.77 M HClO4 at different electrolyte temperatures. The charge density passed was 5 C cm-2 in all cases. Details about the electrolyte temperatures are shown in the figure. ............... 228

Figure A.8 Nitrogen adsorption/desorption isotherms at 77.35 K for all the nanoporous structures. All nanoporous metals were grown in 0.77 M HClO4 at 0.55 V vs. MSE and 25 °C. Dealloying charge: 5 C cm-2. De-gas conditions: 100 °C for 60 min. Details about the alloys are shown in the figure. ...................... 230

Figure A.9 SEM images of nanoporous structures dealloyed at 0.55 V vs. MSE and 25 °C and coarsened for 2 months in 0.77 M HClO4 solution: (a) Ag77:Au23, (b) Ag77:Au22:Pt1, (c) Ag77:Au21:Pt2, (d) Ag77:Au20:Pt3. In all cases 5 C cm-2 were passed. No potential was applied during the coarsening process. ................. 232

Figure A.10 SEM images of NPG dealloyed at 0.55 V vs. MSE and 25 °C: (a) and (b) after removing 0.05 C cm-2, (c) and (d) after removing 0.1 C cm-2. ................. 233

Figure A.11 XPS compositions with respect to the take-off angle for different nanoporous structures: (a) Pt4f for Ag77:Au22:Pt1; (b) Pt4f for Ag77:Au20:Pt3; (c) Ag3d for Ag77:Au22:Pt1; (d) Ag3d for Ag77:Au20:Pt3; (e) Au4f for Ag77:Au22:Pt1; (f) Au4f for Ag77:Au20:Pt3. The experimental conditions for different sets of data are the following: (◊) right after dealloying, (●) after exposure at 425 °C, (○) after exposure at 500 °C and (▲) after exposure to 600 °C. In the case of all high temperature conditions, laboratory air was used for an exposure time of 2 h. ..................................................................... 237

Figure A.12 Depth profile analysis for the lowest platinum content nanostructure (1 at.% Pt) generated from angle-resolved XPS and using the Maximum Entropy algorithm. Details about the elements and the experimental conditions are shown in the figures. ................................................................. 239

Figure A.13 Depth profile analysis for the lowest platinum content nanostructure (3 at.% Pt) generated from angle-resolved XPS and using the Maximum Entropy algorithm. Details about the elements and the experimental conditions are shown in the figures. ................................................................. 241

Figure A.14 (a) CV profiles of the nanostructures formed on Ag-Au and Ag-Au-Pt alloys after dealloying at 0.55 V vs. MSE, 25 °C and passing a charge density of 5 C cm-2; (b) CV profiles for the nanostructure formed on the alloy with 3 at.% Pt after exposure of the as-dealloyed structure to different temperatures in the presence of laboratory air. In all cases, CV profiles

xxiv

were obtained in 1 M H2SO4 at 20 mV s-1 and 25 °C. The original platinum content is shown in (a) and the exposure temperature is shown in (b). .......... 242

Figure A.15 CV profiles for the electro-oxidation of CH3OH solution in 5 M KOH on nanoporous structures formed on ternary alloys: (a) effect of the scan rates on the structure formed on Ag77:Au22:Pt1; (b) effect of the external agitation speed on the structure formed on Ag77:Au22:Pt1; (c) effect of the scan rates on the structure formed on Ag77:Au21:Pt2; (d) effect of the external agitation speed on the structure formed on Ag77:Au21:Pt2; (e) effect of the scan rates on the structure formed on Ag77:Au20:Pt3; (f) effect of the external agitation speed on the structure formed on Ag77:Au20:Pt3. In all cases 5 C cm-2 were passed at 0.55 V vs. MSE and 25°C. ............................................................... 244

Figure A.16 CV profiles of the nanoporous structures formed on (a) the alloy with 1 at.% platinum and (b) the alloy with 3 at.% platinum. In all cases, dealloying was carried out in 0.77 M HClO4 at 0.55 V vs. MSE and at different temperatures. All CV profiles were obtained in 5 M KOH - 1 M CH3OH solution at 10 mV s-1 and 25 °C. Details about the dealloying temperatures are given in the figures. In all cases, the current was normalized by the true area of the electrodes. ................................................. 246

Figure A.17 Effect of the dealloying charge density in the electro-oxidation of methanol on as-dealloyed Ag77:Au22:Pt1. In all cases the nanoporous structure were dealloyed at 25 °C but passing different dealloying charges. All CV profiles were obtained at 10 mV s-1 and 25 °C. The different charge densities are shown in the figure. ........................................................................................... 247

Figure A.18 Current density observed during a prolonged oxidation of 1 M CH3OH in 5 M KOH solution at -0.35 V vs. Hg/HgO in the case of ternary alloys and at -0.05 V vs. Hg/HgO in the case of the binary alloy. In all cases, nanostructures were formed after dealloying the precursors at 0.55 V vs. MSE at 25 °C and with a charge density of 5 C cm-2. The label of the different nanostructures is shown in the figure. ............................................... 248

Figure A.19 NMR spectra for samples prepared/obtained in 5 M KOH – 1 M CH3OH with different formate concentration. For all samples a scan at 90 ° pulse (16.1 µs) and 10 s relaxation delay were used with an acquisition time of 4.5 s and spectral window of 8012 Hz. All samples were diluted in D2O before analysis in NMR..................................................................................... 250

Figure A.20 Calibration curve for the concentration of formate in the 5 M KOH – 1 M CH3OH solution. Every point in this graph corresponds to the area under the peak located at ca. 8.30 ppm. .................................................................... 250

Figure A.21 CV profiles of the highest platinum content alloy (3 at.% Pt) in 0.5 M HClO4 – 1 M C2H5OH solution. The number of cycles during the evaluation is shown in the figure. All CV profiles were obtained at 10 mV s-1 at 25 °C. ....... 252

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Figure B.1 Orientation of the simulation cell with respect to the orthogonal FCC cubic cell. The sides of the simulation cell are of {111} family and the edges are <110>. Red vertices of the simulation cell coincide with the dace centers of the orthogonal FCC cubic cell. ......................................................................... 255

Figure B.2 Comparison between dealloying of a binary (Ag75Au25) and ternary (Ag75Au20Pt5) alloys: (a) anodic current density, (b) charge density passed, (c) thickness of the dealloyed layer and (d) surface increase. These results were obtained by KMC simulation of the dealloying process carried out for a maximum time of 300 s at 1000 mV vs. SHE. Dealloying temperature: 25 °C. The dealloying process in the binary and ternary alloy stopped at 300 s and 200 s respectively. ..................................................................................... 257

Figure B.3 Simulated structures of the nanoporous layer for Ag75:Au25 (a and c) and Ag75:Au20:Pt5 (b and d). In all cases, KMC simulations were used. The color code in these structures is as follows: gray atoms represent silver atoms, yellow atoms represent gold atoms that have not diffused; brown atoms represent gold atoms that diffused; green atoms represent platinum atoms that have not diffused and blue atoms represent platinum atoms that diffused. The green lines in (a) show the outline of the simulation cell and the red arrow is the cell’s side surface. ............................................................ 258

Figure B.4 Cross-sectional view of the dealloyed simulated structures of the nanoporous layer for Ag75:Au25 (a and c) and Ag75:Au20:Pt5 (b and d). In all cases, KMC simulations were used. The color code in these structures is the same as described in Figure B.3. ............................................................... 259

Figure B.5 Content of residual silver trapped in the dealloyed layer formed on the binary and ternary alloy (Ag75:Au25 and Ag75:Au20:Pt5 respectively) after simulating the dealloying process at 1000 mV vs. SHE and 25 °C. The dealloying process in the binary and ternary alloy stopped at 300 s and 200 s respectively. ................................................................................................... 260

Figure B.6 (a) Fraction of the more-noble elements in the binary (Ag75:Au25) and ternary alloy (Ag75:Au20:Pt5); (b) ratio of platinum and gold on the surface of the ligaments. Simulation conditions: dealloying potential - 1000 mV vs. SHE and 25 °C as a dealloying temperature. The dealloying process in the binary and ternary alloy stopped at 300 s and 200 s respectively. .................. 261

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List of Appendices

Appendix A. Supplementary Data ..................................................................................... 218

A.1 Potentiostatic dealloying: comparison between binary and ternary alloys .. 218

A.2 Formation of surface oxides during dealloying ............................................. 219

A.3 Electrochemical Impedance spectroscopy (EIS) of dealloyed samples ...... 220

A.4 Alternative method to determine the developed surface area in

dealloyed specimens..................................................................................... 222

A.5 Calculation of the mass of the dealloyed layer ............................................. 223

A.6 Calculation of the thickness of the dealloyed layer based on the

retained silver content in the layer ................................................................ 226


all alloys ......................................................................................................... 227

A.8 Nitrogen adsorption/desorption isotherms at 77.35 K – BET Surface

area estimation .............................................................................................. 229


electrolyte ...................................................................................................... 231

A.10 SEM images of NPG after removing small charge densities ....................... 232


the ligaments ................................................................................................. 233

A.12 Angle-resolved X-ray spectroscopy (XPS) analyses for the ternary

alloys before and after segregation of platinum ........................................... 236



platinum ......................................................................................................... 238


after surface segregation of platinum was induced ...................................... 241

A.15 Effect of scan rate and agitation speed during methanol electro-

oxidation reaction .......................................................................................... 243

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dealloying conditions ..................................................................................... 245

A.17 Potentiostatic oxidation of methanol in alkaline solution .............................. 248


concentrations for the methanol oxidation reaction ...................................... 249

A.19 Electro-oxidation of ethanol in acidic media ................................................. 251

Appendix B. Simulation of Nanoporous Metals Formed by Electrochemical

Dealloying of Ag-Au-Pt Alloys ..................................................................... 253

B.1 Introduction .................................................................................................... 253

B.2 Modeling conditions and results ................................................................... 253

B.3 References .................................................................................................... 263

Appendix C. List of Publications and Presentations ...................................................... 265

C.1 Publications ................................................................................................... 265

C.2 Presentations ................................................................................................ 266

1

Chapter 1. Introduction

The development of new and more robust materials for applications in chemical

processes, sustainable energy, remediation of the environment and many other fields is one

of the biggest challenges in the 21st century. Porous materials, such as zeolites or aerogels,

offer versatile properties and characteristics (e.g., pore sizes and morphology) that have

been found very useful in a variety of fields [1,2]. Amongst all the different porous materials,

porous metals have gained a lot of interest due to their high electrical conductivity,

mechanical properties and broad application spectrum [3-8]. In recent decades, the

significant growth of nanotechnology, the development of powerful characterization

techniques such as scanning electron microscopes (SEM) and transmission electron

microscopes (TEM), and the remarkable properties displayed by nanomaterials, such as

nanowires, nanotubes and metal clusters [9-11], increase the interest of the scientific

community in the formation and characterization of nanoporous materials. These materials

possess unique characteristics that underline their relevance in fields of great technological

importance such as catalysis, sensors, energy harvesting and optics. According to the

International Union of Pure and Applied Chemistry (IUPAC), materials with a pore size in the

range between 2 and 50 nm should be referred to mesoporous; materials with larger pores

are referred as macroporous and materials with smaller pores are known as microporous.

Hence, from IUPAC’s standpoint, the term “nanoporous” is meaningless; however, the prefix

nano brings out a intrinsic size scale that goes from 1 to 100 nm [12]. One subgroup of

nanoporous materials is nanoporous metals (NPMs). NPMs can be synthesized by a variety

of techniques such as template synthesis [13-15], surfactant mediated synthesis [16-18],

and dealloying. Some of these techniques provide poor control over size and extent of

porosity; furthermore, while in some methods there is control over these parameters, but

2

only porous metals in the form of thin films can be prepared which have insufficient

mechanical stability. Dealloying or selective dissolution is, among these techniques, an

easy, flexible and economical strategy to produce NPMs [12,19,20]. During dealloying, the

less-noble element is removed from a metallic solid solution (i.e., parent alloy) by electrolytic

dissolution, leaving behind an interconnected structure with remarkable characteristics such

as high surface-to-volume ratio, good mechanical integrity, near-theoretical strength in

compression, and size-scale dependent elastic modulus [20-22].

Although NPMs seem to be rather new, they are not. Gold purification by selective

dissolution can be traced back over 2500 years to the reign of King Croesus (560-547 B.C.)

of Lydia, in present day Turkey [23]. Using a partially refined Ag-Au-Cu alloy (termed

electrum) as a starting material, a process was developed where, under the application of

heat, the silver and copper were converted to metal chlorides, leaving behind gold [24].

Many centuries later, the pre-Columbian civilizations of Central and South America, such as

the Incas, used a superficial dealloying of Au-Cu alloys to generate a shiny gold surface,

giving the work piece the allure of pure gold. They subsequently developed a process that

enabled the selective dissolution of silver from Ag-Au alloys that was remarkably similar to

that used in Croesus’ time, although they were able to modify the process to remove the

requirement of heat [25,26]. Also, in the middle of the Italian Renaissance, even Leonardo

da Vinci discussed in the Codex Atlanticus (ca. 1508) that parting silver from Ag-Au alloys is

possible using potassium nitrate [27].

NPMs, as demonstrated in Figure 1.1, generally display a remarkable consistency in

the ligament and pore widths; moreover, the feature size of the nanostructures could range

over several orders of magnitude, from nanometers, up to microns in diameter under the

right experimental conditions. In addition, it is fair to say that dealloyed materials have an

intrinsic structural, dimensional and compositional complexity of their own: the thickness of

3

the resulting nanoporous sponge may be of several millimeters in depth, while the width of

the ligaments and pores, at the smaller size, is typically only of the order of a few

nanometers; furthermore, while dealloying is progressing, the less-noble element is

removed not only from the dealloying front (i.e., interface between unattached alloy and fully

formed porosity), but also it could be removed from the already formed ligaments inducing a

compositional gradient along the dealloyed layer. This process is closely associated with the

coarsening of the structure (as discussed later).

These, and many other properties, place NPMs in the front line of nanotechnology

research today. In fact, the number of research groups dealing, at least in principle, with

these nanoporous structures has grown drastically in the last few years. Therefore, it is

expected that a lot more will be made of these materials in the near future.

Figure 1.1 Nanoporous structure formed by dealloying of silver from Ag77:Au23 alloy in 0.77 M HClO4 solution at 40 °C.

4

1.1 Formation of nanoporous metals by dealloying

Historically, dealloying has been primarily recognized, within the corrosion community,

for its role in stress-corrosion cracking of different industrial alloys (e.g., dezincification of

brass, nickel-aluminum-bronze alloys) [28,29], although other corrosion phenomena (i.e.,

corrosion pitting) has been associated with dealloying of technologically important alloys

such as the aluminum 2024-T3 alloy [30]. It is only relatively recently that dealloying has

been identified as a route to produce NPMs. Two dealloying techniques have been mostly

used to form NPMs: free-corrosion (or simple-immersion) dealloying and electrochemical

dealloying. The former one refers to the process in which the alloy is submerged in an

oxidizing electrolyte (typically aqueous nitric acid - HNO3) that facilitates the dissolution of

the less-noble component(s). In electrochemical dealloying, on the other hand, the selective

removal of the less-noble element is facilitated by introducing an electric potential to the

system; conventionally, a three-electrode electrochemical cell is used: it consists of an

anode (i.e., the precursor alloy), a counter-electrode, a reference electrode and an

electrolyte (aqueous perchloric acid – HClO4 is commonly used). Even though this

technique requires a more sophisticated set-up, it provides superior process control.

As briefly described before, dealloying consists in the removal of the less-noble

element from a metallic solid solution (i.e., binary or ternary parent alloy) and the

reorganization of the remaining element(s) by surface diffusion. The parent alloy however,

has to meet some basic requirements to be able to facilitate the development of an

interconnected structure such as the one shown in Figure 1.1. Those basic requirements

are the following [31]: (i) the reduction potential of the alloy components must be well-

separated such that one component is soluble in its oxidized state (i.e., so that the element

dissolves); in other words, one component (or possibly more than one) is thus more noble

than the other one; (ii) in the majority of the cases, the composition of the alloy must be

5

neither too rich nor too poor in the more noble components; if the alloy is too rich in the

more-noble species, no dealloying will occur; conversely, if there is too little noble species, it

is very difficult to maintain porosity evolution through the material; (iii) the alloy must be

sufficiently homogeneous with no phase separation prior to dealloying and ideally stress-

free, and (iv) surface diffusion of the more-noble atoms at the alloy/electrolyte interface must

be sufficiently fast to move from their original lattice sites and agglomerate into the

backbone of the nanoporous sponge (see details below).

In ideal solid solutions (i.e., ApB1-p in which B is the more-noble element), each

component has individual thermodynamic properties; thus, it is expected that the anodic

dissolution of A is related to the chemical composition of the solid solution. The partial Gibbs

free energy of A is given by )ln( AA nRTG , where nA is the mole fraction of A. As

AA EyFG , where EA is the difference between the equilibrium electrode potential of

A in the alloy and the corresponding potential of A in the individual phase, y is the number of

electrons transferred and F is the Faraday constant (i.e., 96 458 C mol-1); therefore, the

following relation is obtained:

)ln( AA nyF

RTE (1.1)

By definition nA is always less than 1, so that ∆EA > 0, which means that the partial

potential of the alloy component (i.e., A) is always more positive than the corresponding

individual metal. Consequently, dissolution of metals from alloys should also start at more

positive potentials, as can be seen in Figure 1.2 (explanation below). Considering now the

6

difference in the free energy between the two components (A and B), one can write the

following relations:

)ln(00

A

BABAB

n

nRTGGG

(1.2)

ABAB EyFG (1.3)

where again ni (i = A, B) is the mole fraction of the components of the solid solution, ( ) is

the free energy of the respective component in the standard state, R is the universal gas

constant, T is the absolute temperature and E is the difference in potential.

By substituting the relation given in 1.3 into 1.2 and making the necessary

simplifications, we obtain the following relation:

)ln(0

A

BABABAB

n

n

yF

RTEEEE (2.4)

This relation shows the existence of a difference in the electrochemical potential of

each component of the alloy (i.e., the dealloying potential), which depends on the difference

in their standard reversible potential, and the composition of the solid solution. Moreover, it

7

is also clear that the fraction of less-noble element cannot become zero and that dealloying

cannot proceed to 100%.

A typical example of a solid solution is the Ag-Au alloy. In this case, silver is less-noble

than gold, as observed by comparing their standard potentials ( = +1.68 VNHE;

= +0.80 VNHE) [32]; therefore, when this solid solution is dealloyed, only silver is

selectively removed leaving behind mostly a gold structure with a significant porosity.

Nevertheless, some silver is always trapped in the core of the nanostructure. This

nanoporous metal is typically called nanoporous gold (NPG). More importantly, based on

the difference in the standard potentials, one can easily prepare NPG by dealloying either

Ag-Au or Cu-Au alloys, but not from Au-Pt alloys.

Besides the thermodynamic considerations described before, two additional

parameters are essential for dealloying, the so-called critical potential and the composition

threshold or parting limit. Pickering, in his pioneering work [33,34], was the first one to

introduce the relation between the applied potential and the dissolution rate that

characterizes the electrochemical dealloying process (Figure 1.2). This behavior can be

reproduced by scanning the potential in the positive direction from the standard reversible

potential ( ) of the less-noble metal of the alloy (e.g., A in the case of ApB1-p alloy). At low

overpotentials relative to , the alloy displays passive-like behavior due to the surface

enrichment and blocking influence of the more-noble metal, B. Up to now, it is not

completely clear what is the origin of this passivation; nevertheless, fundamental research in

NPG has shown that at very low overpotentials, there is a formation of monolayer patches of

gold-rich film that after growing to a thickness of two or three monolayers has an

unexpected inverted stacking sequence; by increasing the overpotential, almost pure gold

islands (10-15 monolayers) appears inheriting the inverted stacking sequence. At sufficiently

8

high overpotential, a new substrate oriented gold structure appears and the initial stacking-

inverted layer vanishes, forming the templates for the growth of nanoporous metals. [35].

This passive-like behavior results in a relatively potential-independent current with no

development of bulk porosity. At a certain potential (i.e., empirical critical potential - Ec), the

current quickly rises with the subsequent formation of porosity. This dealloying behaviour

was termed by Pickering as Type I dissolution. At this point, the less-noble element(s) are

removed from the parent alloy and the more-noble element(s) undergo a self-organization

process (i.e., by surface diffusion) producing a three-dimensional bicontinuous porous

network of interconnected ligaments or pillars (i.e., dealloyed layer). In simple terms, Ec

represents a transition between surface smoothening and severe roughening leading to bulk

dealloying and porosity evolution [36,37]. More recently, an intrinsic critical potential was

proposed to exist; this potential does not depend on any adjustable experimental parameter,

such as the scan rate or the electrolyte temperature, and it is a critical point inherent in the

system, corresponding only to the necessity of dissolving the less-noble metal to create a

surface with very high curvature [31,38]. For Ag-Au alloys, it was demonstrated that the

intrinsic critical potential (determined potentiostatically after long periods of time) lies

approximately 100 mV below those determined potentiodynamically (i.e., Ec) [39,40]. This

effect is basically due to the electrode not being at equilibrium during the potentiodynamic

measurement.

9

Figure 1.2 Schematic illustration of the electrochemical behavior of binary alloys. Adapted from Pickering [33].

The applied potential basically determines the rate at which dissolution and diffusion

occur. At high enough potential, the activation barrier required to break bonds of the less-

noble element is reduced, at the same time the surface diffusivity of the more-noble metals

increases; nevertheless, the change in diffusivity is small relative to the exponential increase

in dissolution rate. If the alloy is rich in the less-noble element, the Ec lies closer to ;

however, Ec increases with increasing alloy composition until a certain composition

threshold or parting limit (in some cases also referred as dealloying threshold) is reached.

The parting limit is the percentage of the less-noble element below which dealloying

vanishes, irrespective of how oxidizing is the potential. For an alloy with composition below

the parting limit, the less-noble component will be removed from the first few atomic layers

of the surface, but an enrichment of the more-noble component on the surface will slow

down and eventually shut off the dissolution process. Recently, the connectivity of less-

10

noble metal atoms has been the focus of attention to explain the parting limit [41]; therefore,

in simple terms, below the parting limit there will not be enough interconnected channels of

the less-noble constituent to allow dissolution and to proceed through the thickness of the

material. According with the basic percolation theory, the percolation threshold (i.e.,

composition of silver below which silver atoms do not connect through the thickness of the

alloy) for FCC alloys (e.g., Ag-Au) is 20 at.% silver [42]. However, experimental data

suggest that the parting limit for dealloying Ag-Au is approximately 55 at.% silver [33,43,44].

To solve this discrepancy, ‘high-density percolation’ (HDP) concepts have been used

[41,42]. The idea behind this is that for a cluster of the less-noble element to be dissolvable

in an electrolyte, it must have sufficient width so that anions in the electrolyte can solvate it.

Kinetic Monte Carlo (KMC) simulations confirmed this concept by allowing silver (for the Ag-

Au alloy) to dissolve or not, depending on its local coordination; specifically, it was found

that when 10- and 11- coordinated silver atoms were not allowed to dissolve, the effective

percolation threshold changed to the observed 55 at.% [42]. In some FCC alloys such as

brass, the parting limit for dealloying is actually 20 at.%; in these materials, this threshold

can be accessed if the more noble metal can be exchanged between the electrolyte and the

surface (i.e., it is at or near its equilibrium potential with its aquo-ions) [42]. At the parting

limit, and unless the electrolyte that has been used can also dissolve the more-noble

component of the alloy (e.g., B), a passivating oxide will form (Type II as shown in Figure

1.2).

The component B in the ApB1-p alloy (i.e., the more noble component) will also be

oxidized if the difference between the standard reversible potential between A and B ( -

)

is too small. Under these circumstances, not sufficient overpotential is allowed for the

selective dissolution of the less-noble metal (A in this case); nevertheless, it is worth noting

11

that under these conditions, the significantly differing elemental dissolution rates are still

capable of forming porous material, as in the case of α-brass [36].

1.2 NPG: formation, characteristics and surface modifications

Amongst the different NPMs that can be produced by dealloying, NPG has clearly

dominated in the literature. In recent years the number of publications concerning NPG

steadily increased from ~25 papers in thirteen years (from 1992–2005) [45] to more than

110 publications in 2013 alone (Thompson Reuters - Web of Knowledge, 2014). As

discussed before, NPG can be generated by dealloying of a gold alloy containing silver,

copper or aluminium as less-noble constituents. Besides the remarkable properties and

characteristics of NPG, it is probably accurate to say that most of the development of NPG

and other gold nanomaterials has been driven not only for the fascination that people have

developed for gold (it has been generally viewed as an immutable, non-changing element

and as the ultimate statement of wealth and beauty), but also for the remarkable properties

that gold displays when its feature size decreases down to the nanoscale (e.g.

nanoparticles) [46-48]. Some of the highly positive characteristics of NPG are the following:

a) uniform open porosity; b) very high surface area; c) conductive bicontinuous network with

characteristic length (i.e., width of the ligaments) easily tunable from a few nanometers to

micrometers; d) biocompatible surface to which can be attached all sorts of functional

biomolecules, amongst many others. Some of those characteristics are depicted in Figure

1.3.

In the case of Ag-Au alloys, as precursors for NPG, silver and gold are both metals

with identical face-centered cubic (FCC) lattice structure and almost identical atomic radii,

resulting in alloys with unlimited solid solubility; however, their electrochemical properties

12

are significantly different from each other (i.e., standard reversible potential), which allow the

removal of silver and formation of well-defined structures with open porosity [36,42]. Since

the 1960s, corrosion of gold alloys (i.e., dealloying) has been systematically investigated

[32,34,42,49]. In fact, in the 1980s and 1990s, many researchers studied the corrosion

process of Ag-Au alloys showing the growing interest of the scientific community to

understand the mechanism of dealloying in gold alloys [50,51]. However, it was not until the

development of nanotechnology, in the late 1990s and the early 2000s, that it was revealed

that nano-sized gold can have a remarkable role in technologies such as water purification,

fuel cells, biomedicine, therapeutics and others [52]. Nonetheless, it seems counterintuitive

that an inert material such as gold can be used in applications where the chemical activity of

its surface plays a key role; however, it is the unique combination of gold and its

nanostructure that makes it possible, as will be discussed in Section 1.4.

The mechanical properties of NPG have also been extensively studied [20,57,58].

Amongst many interesting characteristics, it has been determined that the yield stress within

an individual ligament may be as high as 1.45 GPa, which approaches the theoretical shear

strength of the material [59]. Moreover, it was found that by decreasing the ligament size of

NPG enhances the yield strength and stiffness of the material [53,60]. This suggests that

the dislocation activity, which is responsible for the plastic deformation of metals, is strongly

affected by the nanoporous structure. For more details about the mechanical properties of

NPG, refer to the review done by Dou and Derby [58].

13

Figure 1.3 NPG and its various structural variations ranging from microfabrication (adapted

with permission from Biener et al. [53]. Copyright 2006 American Chemical Society), tunable

pore and ligament size, hierarchical structures based on templating techniques (adapted

from Erlebacher and Seshadri [12] with permission of Cambridge University Press) and its

chemical variations, ranging from decoration of the surface of the ligaments with metals

such as platinum (adapted with permission from Ding et al. [54]. Copyright 2004 American

Chemical Society), molecular entities (adapted from Pareek et al. [55] with permission of

Royal Society of Chemistry) or even metal oxides, such as TiO2 (adapted with permission

from Jia et al.[56]. Copyright 2009 American Chemical Society).

1.2.1 Overview of proposed dealloying mechanisms of NPG

A fundamental question regarding the formation of porous metals is the mechanism by

which dealloying is maintained over more than a few atomic layers leading to the three-

dimensional structure shown in Figure 1.1. Historically, different mechanisms have been

proposed to explain how two alloying constituents, intermixed on an atomic scale, can be

separated so effectively by electrolytic action. The majority of the work done towards the

14

development of these mechanisms has been based on NPG; however, the principle behind

this is applicable to other NPMs.

A key aspect of any dealloying mechanism, besides the dissolution of the less-noble

component(s), is the fact that the more-noble metal atoms in the alloy must physically move

from original lattice sites, as occupied in the non-dealloyed material, and reposition

themselves on a highly porous skeleton during the selective dissolution. Typically, however,

the driving force for atomic diffusion follows a concentration gradient or follows a direction

so as to lower the overall surface area (i.e., surface energy), which clearly is not what

happens during the formation of nanoporosity. One might think that knowing the fact that the

more-noble alloy component moves, they might arrange themselves on a surface to form a

dense monolayer, protecting the bulk of the alloy from further dissolution (i.e., passive-like

behavior), which does occur below Ec; however, above Ec the more-noble atoms move in

such a way as to both agglomerate (moving against a concentration gradient as they diffuse

from the base of the pits, where exists a surface with locally bulk alloy composition, to the

noble element-rich pillars of the porous skeleton) and also increase the surface area of the

material as it becomes nanoporous [61]. Clearly, the driving force that moves more-noble

metal atoms during dealloying is counterintuitive. Some of the mechanisms that have been

proposed to explain the whole phenomenon are described below.

One of the first mechanisms to explain the observed enrichment of more-noble atoms

on the porous skeleton was called the Volume Diffusion model and it was postulated by

Pickering and Wagner [34] in the late 60s. In this model, it was suggested that the near-

surface region becomes depleted of the less-noble component, which creates a gradient

perpendicular to the alloy/electrolyte interface position, driving bulk diffusion of the less-

noble component to the surface where it can be dissolved away (this obviously creates an

accumulation of the more-noble adatoms on the surface of the electrode due to the

15

formation of surface vacancies created by preferential removal of the less-noble

component). However, volume diffusion in the unattacked alloy was found to be quite slow;

in fact, at the time this mechanism was proposed, even surface diffusion was considered too

slow (only a few measurements of the surface self-diffusion coefficient had been measured,

and only in vacuum [61]). Therefore, this mechanism also proposed that the injection into

the bulk of vacancies and divacancies (i.e., faster-moving defects) maintains the transport of

the less-noble components to the surface, thereby permitting the propagation of the

dealloying process. This mechanism however did not get enough experimental support.

The Surface diffusion mechanism was, according with Dursun [39], originally proposed

by Gerischer in 1962 and it involves the nucleation and growth of nuclei of the pure, or

almost pure, more-noble component via a surface diffusion process. As the less-noble

element is selectively dissolved from the surface of the alloy, the remaining more-noble

element, which is in a highly disordered state, begins to reorder by surface diffusion

resulting in a nucleation and growth of gold-rich islands. This model, in contrast with the

Volume Diffusion model, establishes that there is no transport of the less-noble atoms to the

electrode surface via volume diffusion. In this model however, it was expected that after the

initial stages of the dissolution of the less-noble element, the alloy becomes completely

passivated when all the surface sites are occupied by more-noble elements. This was

clearly in opposition to practical experience in which formation of nanoporosity was

observed.

Following the work done by Gerischer, Forty et al. [32,42,62] proposed that the

selective dissolution of the less-noble element should lead to the creation of surface

vacancies or adatoms, which migrate across the surface to form pits, steps and other

surface roughening features, or they can assist the migration of the residual more-noble

atoms which leads to the island growth. The coalescence of these islands by migration of

16

the more-noble atoms exposes fresh alloy to the corrosive environment where further

dissolution will occur. The main argument against this new model was also that the

mechanism for sustained three-dimensional porosity development was not clear

(enrichment at the surface of the more-noble element would also shut off the dealloying

process). Nevertheless, this model was later extended by incorporating percolation theory,

using Monte Carlo simulations of dealloying, in order to include the importance of the

placement of atoms, in a randomly packed way, in a solid to account for the appearance of

porous structures [41]. In this way, it was shown that for a randomly packed matrix of gold

and silver atoms (again in the case of NPG), two fundamental processes were necessary to

reproduce the electrochemical behaviour and the morphology of the material: silver

dissolution and gold surface diffusion, in which the driving force was the high curvature of

the initial porosity (Gibbs-Thompson effect) resulting in the coarsening of the structure (see

below). In this model, the transport of the less-noble component to the surface by volume

diffusion was not necessary for dealloying; moreover, the selective dissolution process is

only possible if a continuous path of the more electrochemically active component

(percolation cluster) exists throughout the thickness of the material.

Based on this model, for dealloying systems in which large silver dissolution

overpotentials could be applied without gold oxidation (see Figure 1.2), silver could be

selectively removed from highly coordinated sites (initially silver dissolves from a terrace),

forming neighboring ledges that would continually dissolve until the entire layer was stripped

of suitable silver, clearly resulting in a three dimensional dissolution. The more-noble metal

atoms that are left behind could, at least in principle, stay in place as adatoms, populating

the layer of the material and slowing down further dissolution, but the thermodynamics of

the alloy/electrolyte interface predicts that such a condition would place the surface far out

of equilibrium, thus the adatoms will tend to agglomerate and eventually coarsen. The

17

kinetics of surface diffusion of more-noble metal atoms cannot be accurately described by

simple Fickian diffusion, mainly because the concentration gradient of the more-noble metal

component increases in the growing islands (which again is expected due to the highly non-

equilibrium state of the adatoms on the surface). To adequately describe this phenomenon,

the Cahn-Hilliard diffusion equation, incorporating a concentration dependent mobility, has

been proven very successful [19].

The surface diffusion mechanism was later experimentally verified by Oppenheim et

al. [50], who not only demonstrated, via in situ electrochemical scanning tunneling

microscopy (STM) of alloy dissolution, that surface diffusivities of FCC metal adatoms were

many orders of magnitude faster in the presence of the electrolyte than in air or vacuum,

exceeding the threshold value for nanoporosity formation (i.e., a minimum surface diffusion

coefficient on the order of 10-14 cm2 s-1 was needed for the noble atoms to diffuse on the

order of 1 nm in about 1 s and agree with experimental observations), but also

demonstrated that a terrace atom dissolution of silver from gold-rich alloys require a

significant overpotential, which clearly agrees with the simulations [31]. Moffat et al. [63]

independently confirmed the faster surface diffusivities of gold in an electrolyte after

studying Cu3Au by STM. This model, even though it is based on simplified dissolution and

diffusion rules, successfully predicts many of the aspects associated with the formation of

nanoporous metals formed by dealloying, including the critical potential, parting limit (the 20

% parting limit), formation of unconnected porosity below Ec, etc.

Recently, Erlebacher further developed this Monte Carlo model by introducing kinetic

function to both, silver dissolution/diffusion and gold diffusion [19,36]. By doing that, it was

possible to simulate a “real-time” dealloying that was controlled through alloy composition

and applied potential. The rates of dissolution and diffusion were based on a nearest

neighbour bond-breaking model, where surface atoms were chosen at random with the

18

dissolution/diffusional probabilities calculated for specific circumstances. The rates of

dissolution and diffusion are shown in equations 1.5 and 1.6 respectively:

Tk

Ek

B

bEdiss

exp (1.5)

Tk

Ek

B

bDdiff

exp (1.6)

where α represents the number of nearest neighbouts, Eb the bond energy, which is

considered equivalent for both gold and silver and given initially as 0.15 eV, and was the

applied potential (1.75 eV). kB and T were the Boltzmann constant and absolute temperature

respectively. The E was obtained after fitting experimental results of the dissolution of pure

silver ( E = 1x104 s-1); D is an attempt of frequency of the order of the Debye frequency

and set equal to 1x1013 s-1. Silver dissolution rates were reported to be consistent with the

Butler-Volmer equation in the high overpotential Tafel regime.

Figure 1.4 shows an example of the KMC simulations done for Ag-Au alloys. Figure

1.4a shows the terrace island formed after silver atoms were preferentially removed from

the first atomic layer. Initially, dissolution of silver from a terrace (which is the rate-limiting

step of the process – Figure 1.4c) creates a terrace vacancy. The atoms surrounding the

created vacancy are more susceptible to dissolution, because they have fewer nearest

neighbours, and are removed faster than the initiation step to create a terrace island. The

19

terrace is thus largely stripped of silver, leaving behind the bulk fraction of gold adatoms

(Figure 1.4a, cii). Due to their low co-ordination states and because the sample is immersed

in an electrolyte, the gold adatoms display a very high surface self-diffusion coefficient, and

quickly agglomerate to form islands that also contain residual silver (Figure 1.4ciii). The

selective removal of silver from the underlying layer again creates a high concentration of

gold adatoms that diffuse to the gold islands created from the previous terrace (Figure 1.4b,

civ). This process can thus repeat until the porosity is formed.

Figure 1.4 Snapshots taken from Monte Carlo simulations performed on Ag-Au alloys: (a) initial stage of dissolution of silver and formation of terrace island; (b) formation of mounts. Single steps of dealloying mechanism are depicted in (c), where the yellow balls represent gold and the grey balls silver. (a) and (b) are adapted from Erlebacher and Seshadri [12] with permission of the Cambridge University Press.

20

Even though this improved Monte Carlo model has been remarkably successful to

reproduce/predict the main dealloying characteristics, direct comparison between real

dealloyed structures and those formed by simulation (e.g., ligament size) is not yet possible.

Several simplifications were made to the model to reduce its computational cost. Some of

those simplifications included the removal of the electrolyte by vacuum, which once again

has implications for the relaxation of the metal surfaces [64-67], the assumption that gold

surface self-diffusion was considered to be constant, amongst others. It is expected

however, that these simplifications will be eliminated in the near future as the cost of

computer power falls.

1.2.2 Coarsening of the nanoporous structure

1.2.2.1 Overview

While dealloying of Ag-Au progresses, the size of recently created pores and

ligaments keeps increasing. At the same time that ligaments grow in size, more original

alloy is exposed to the electrolyte allowing removal of silver from the already formed

ligaments, which constitutes the second stage of silver dissolution. Furthermore, even after

most of the silver has been removed from the ligaments, the feature size in the structure

continues to increase as long as NPG is exposed to the electrolyte thanks to the high

surface self-diffusivity of gold in the electrolyte. This process is typically referred as

electrochemical coarsening, post-porosity coarsening or simply coarsening. Even though

coarsening happens quite fast when the material is immersed in the electrolyte, coarsening

happens throughout the entire life of the material, even when it is taken out from solution.

21

NPG is, from the thermodynamic point of view, intrinsically unstable, i.e., its

nanostructure has a very high surface energy. This instability provides the driving force

behind coarsening, which is the Gibbs-Thompson effect or curvature-driven growth [12,36].

During coarsening, the migration of material goes from higher to lower energy sites;

therefore, as smaller ligaments contribute more to the surface energy (on a per volume

basis) than large ligaments, the thermodynamic driving force is larger for their elimination or

incorporation into larger ligaments. As a result, the distance between adjacent ligaments

increases and a significant growth of both ligaments and pore diameters is observed [68].

In order to rationalize the coarsening phenomenon in nanoporous metals, it was

necessary to look at the growth of larger clusters at the expense of smaller clusters

(typically known as Ostwald ripening). This theory was formally developed by Lifshitz and

Slyozov [69] and Mullins [70] through a self-consistent mean-field theory, showing that the

cluster growth asymptotically following the relation:

31

ktRav (1.7)

where Rav is the average cluster radius, k a proportional constant and t is the time. The

number of cluster/islands was likewise predicted to decrease with t-1. After complex

calculations, it was determined that this result was independent of both dimensionality and

surface coverage, which has shown to be correct in two and three dimensions for solid/liquid

[71], and solid/solid systems [72] under mean-field conditions [73]. However, in two

dimensions, the equation that governs the cluster radius was only valid under the adatom

diffusion limited ripening [73] and the time dependence decreases to t0.2-0.25 under conditions

22

where the islands themselves migrate [74]. Moreover, the mean-field approach has been

shown to be inaccurate through shielding effects on islands [75], and the simulated

exponents never reached the asymptotic limit of 1/3.

A different approach considered a two-dimensional growth of static spherical-cap

islands, in mass conservative systems, showing that the average radius increases with t1/4

[76,77]. Simplistically, the ripening process on an atomic level can be considered as the

detachment of atoms from an island or grain boundaries, kinks and ledges; these atoms

then diffuse across the metal surface as adatoms, and eventually coalescence with another

island. Therefore by monitoring the change in surface area (i.e., ripening) of

thermodynamically destabilized metals (i.e., gold and platinum atoms), one can determine

the surface self-diffusion coefficient of metals in contact with an electrolyte. Therefore,

continuing the work done by Chakraverty [76], Alonso et al. [77] and Vazquez et al. [78]

developed a rounded-cap cylindrical column model that enabled the calculation of the

diffusion coefficient, as shown in the following relation:

kT

tDarr s

44

0

4 2 (1.8)

where the average cylinder radius is given by r (r0 at t=0 is given as 1x10-6 cm [78]), is the

surface tension, a is the metal lattice constant and Ds the surface self-diffusion coefficient.

With this equation, a prediction of the growth of the average cylinder radius with time was

possible. In addition, the following relation was derived to make this result even more useful:

23

4

1

0

41 23

)(

r

kT

tDa

Mq

zFtR s

(1.9)

R(t) is the surface roughness at time t and can be experimentally determined as the ratio of

charge passed for the formation of an oxide monolayer between the electrodispersed state

and the initial state, i.e., 0)()( qtqtR . In equation 2.9 the quantity in the first parentheses

represents the roughness and dimensions of the rounded-cap cylindrical columns used to

approximate the surface, M is the molecular weight of the hydrous metal oxide layer (i.e.,

Au2O3), q the electroreduction charge density of the oxide layer at t=0, z the number of

electrons passed per elementary reaction, F is Faraday’s constant, and is the density of

the oxide film. Literature concerning the measurement of Ds clearly demonstrates the validity

of the t1/4 relationship [65-67,77,79].

An obvious question at this point is whether or not this model developed by Alonso,

Vazquez and coworkers would be applicable to nanoporous structures. Graphical

extrapolation from the experimental data recorded by Kelly et al. [80], showed some

variability in the time dependency with respect to the predicted relation (i.e., t0.19-0.31 instead

of the t0.25); Corcoran et al. [81], on the other hand, showed that coarsening of NPG (formed

from Ag70:Au30) under potential control was surface-diffusion dominated and displays t1/4

kinetics at long times. Dursun et al. [82] also confirmed the validity of the Equation 1.8 for

NPG developed under potential control. It is important to say however, that originally this

model allows for random surface diffusion in a surface with negligible curvature, so the

thermodynamic driving force is isotropic. In nanoporous metals, the presence of positive and

negative curvatures generate anisotropic surface diffusion fluxes that, for example, eliminate

smaller ligaments at a faster rate than those with a larger radius [83]. Moreover, this model

24

is based on the surface diffusion-limited scenario, where the electro-dispersed islands are

limited to emit adatoms only from their edges; in the case of nanoporous metals, the entire

surface can be considered ‘edge’ as every surface atom is capable of diffusion. Despite

those arguments, the model developed by Alonso, Vazquez and coworkers seems to

represent the general behaviour followed by the coarsening of nanoporous metals. Later on,

Pugh [84] re-derived Equation 1.8 in its entirely from Fick’s Second Law, and showed that

the surface self-diffusion coefficients obtained from his relation agreed very well with values

reported in literature; nevertheless, he concluded that the most appropriate means of

understanding the coarsening mechanism was through atomistic simulations. Recently,

Kolluri and Demkowicz [85], using atomistic modeling, argued that surface diffusion alone

does not accurately describe the characteristics observed in nanoporous metals after

coarsening (e.g., volume reduction, enclosed voids); in fact, they proposed the existence of

phenomena such as ligaments collapsing and localized plasticity to accurately reproduce

the characteristics observed in coarsened NPG. Nevertheless, and as described since the

beginning of this section, up to now, coarsening is a phenomenon that depends mostly on

surface diffusivity.

Coarsening of the nanoporous structure clearly has a direct impact in the potential

applications of materials such as NPG. The reduction of the surface area in time is not a

desirable property for applications like catalysis and electrocatalysis; nevertheless,

coarsening could be beneficial in applications where the feature size of NPG needs to be

adjusted for particular purposes. Some of the most common strategies that have been

tested/implemented to produce NPG with smaller ligaments (i.e., minimization of coarsening

in solution or ex-situ) are briefly discussed in the next two sections.

25

1.2.2.2 Reduction of coarsening in NPG

Among the different alternatives that have been evaluated, the selection/control of

some of the dealloying parameters, such as temperature and the nature of the electrolyte,

have been found useful. Qian and Chen [86] demonstrated that ultrafine nanoporous gold,

with pore size of about 5 nm (typically the pore sizes could be between 15 – 20 nm), can be

produced by reducing the dealloying temperature of the HNO3 electrolyte from 25 °C to -20

°C in simple immersion procedures. By reducing the temperature, the surface self-diffusivity

of gold was decreased by approximately two orders of magnitude. Snyder et al. [87], on the

other hand, showed that by dealloying Ag-Au in neutral pH silver nitrate solution,

significantly smaller pores and ligaments were obtained if compared with those obtained in

an acidic environment (e.g., HNO3); this observation can be explained by the formation of a

silver oxide layer behind the dissolution front, which reduces the post-porosity coarsening of

the structure. Even though these two approaches are effective to minimize the immediate

coarsening of the structure, they will not prevent coarsening of the structure once the

sample is taken out from the electrolyte.

A different strategy was to deposit small amounts of ternary elements, with lower

surface diffusivity than gold (e.g., platinum), onto the surface of NPG [88-90]. By depositing

platinum on the surface of the ligaments it was observed that no platinum aggregation or

significant coarsening was found after exposure to ambient temperature for two months, or

even after annealing at high temperature conditions. The platinum overlayers typically

adopted a layer-islanding (also called Stranski-Krastanov, SK) growth mode where platinum

atoms grow epitaxially on NPG surfaces. At early stages, platinum forms a ‘wetting layer’

uniformly coating the NPG substrate; however, after a few monolayers platinum starts

forming three-dimensional islands. Moreover, compared with NPG, platinum-plated samples

exhibit a relatively smooth surface morphology, suggesting a preferential growth over

26

ligament sites with high radial curvatures. Even though this technique has proven to be

effective, it requires an extra step in the formation of stable and robust nanoporous

structures. Alternatively, the addition of a ternary element (e.g., platinum) to the Ag-Au

precursor, has emerged as a possible way forward to create stable nanostructures, in one

step process and with extraordinary characteristics.

1.2.2.2.1 Nanoporous metals formed by dealloying of ternary-noble alloys (Ag-Au-Pt)

Bengough and May [91] first reported that adding less than 0.05 wt.% of arsenic to α-

brass helped to protect it from dealloying (i.e., improving its dezincification resistance). Later

on, Newman [92] suggested that pinning of mobile copper step edges by arsenic might

explain this phenomenon. Therefore, it was thought that a similar effect should be seen if

platinum was added to Ag-Au alloys. Surface self-diffusivity of gold, at potentials near the

standard critical potentials (i.e., 0.4 V vs. standard calomel electrode – SCE – for Ag80:Au20

in H2SO4 solution) was on the order of 10-14 cm2 s-1, reaching the benchmark for dealloying;

moreover, it rises further with increasing potential [67]. In contrast, platinum does not have

surface diffusivity more than the order of 10-18 cm2 s-1 at moderate applied potentials in a

variety of electrolytes [93,94]. Therefore, platinum, which does not dissolve, is basically

immobile relative to gold blocking step edges, and slowing down the surface diffusion of

gold, with the subsequent refinement of porosity.

Snyder et al. [95] was the first one to report that adding 6 at.% of platinum to the

‘white-gold’ type of Ag-Au alloy (i.e., Au content of 35 at.%) creates a porous structure with

a smaller length scale, if compared with NPG, and with more stability towards coarsening.

Later, Jin et al. [96] reported on the mechanical properties of nanoporous Au-Pt structures,

formed from (Au1-xPtx)25Ag75 with x ranging from 0.1 to 0.5, and showed that this structures

27

had ligament sizes around 5 nm. Xu et al. [97], also obtained smaller feature sizes after

dealloying Cu-Au-Pt alloys with platinum contents between 4 and 16 at.%.

Clearly, addition of platinum to a binary precursor has practical importance towards the

development of more stable and functional nanoporous metals. This Ph.D project takes

further this idea of adding platinum to the Ag-Au alloy, and focuses on the understanding of

the role of platinum on the formation and most relevant characteristics/properties of the

resulting nanostructures. For that purpose, nanoporous structures formed by

electrochemical dealloying of Ag-Au-Pt alloys, with systematic variation of the platinum

content, and with a maximum platinum concentration of 3 at.%, were studied, as discussed

in more detail in Section 1.5.

1.2.3 Surface modifications of NPG

One of the most remarkable aspects of NPG (as shown in Figure 1.3) is the flexibility

and tunability of the structure. It has been reported, for example, that by changing the initial

composition of the Ag-Au alloy, nanoporous structures with different morphologies can be

produced; i.e., the higher the initial silver content of the precursor, the higher the porosity

that is formed [98]. By allowing NPG to coarsen in acid environment, the size of the

ligaments/pores changes [68]; moreover, post-dealloying treatment with concentrated acids

(e.g., HCl) leads to the formation of peculiar prismlike structures with nanoporosity

coarsened to 300-500 nm [99]. Thermally-induced coarsening has been also studied

showing that it can give an additional degree of freedom in terms of the tunability of NPG

[45,99-105]. As a result of the exposure of NPG to high temperature, structures with pores

and ligaments in the size regimen between a few nanometers and several micrometers can

be formed without changing the connectivity, ligament and/or pore shape; in fact, by

28

alternating annealing and dealloying, structures with a well-controlled multimodal pore size

distribution can be formed [106]. In addition, the use of casting procedures, analogous to

slip-casting of ceramics, have been used to form nanoporous structures on hollow Ag-Au

particles [107]. Moreover, the use of metals, as mentioned in the case of platinum

deposition, organic compounds, metal oxides or even enzymes, can be used to modify the

surface of NPG, expanding its use in fields like electrochemistry, catalysis and many others

[55,56,89,108-111].

As shown in the case of the ultrafine NPG formed by reducing the dealloying

temperature and/or by using a different electrolyte, the characteristics of the NPG strongly

depend on the dealloying conditions. For example, it has been reported that in free

corrosion dealloying, the size of the ligaments and the amount of residual silver highly

depend in the immersion time and on the concentration of the etchant [68,112]. The

presence of halides (i.e., KCl, KBr, KI) in the electrolyte has also been proven to have a

significant impact in the dealloying characteristics (e.g., critical potential) and nanoporous

morphology [82]. It was observed that the size scale of pores increased with the addition of

halides, with almost an order of magnitude increase thanks to the increase in surface

diffusivity of gold.

The tunability and flexibility of NPG is one of its greatest advantages, and definitely

has contributed to the fascination that the scientific community has developed for NPG.

More examples of modification of NPG can be found in the literature.

29

1.3 Beyond NPG: Other nanoporous metals

The potential applications of NPMs in various fields of science and technology, and the

interest in studying and understanding the dealloying mechanism, has provide great deal of

motivation for studying the formation and characteristics of different systems, some of which

are summarized here. The purpose of this section is just to put in perspective the

importance and flexibility that dealloying has, as a tool to form nanomaterials; therefore, and

because the focus of this work is in NPG and in the nanoporous metals formed from Ag-Au-

Pt alloys, this is not an exhaustive review of all the other alloys that have been dealloyed.

For more details about a particular system, please refer to the references herein.

Nanoporous platinum: Dealloying of platinum-based systems has been virtually

unexplored. Only a few works (if compared with the Ag-Au system) have been focused

on these alloys, mainly because platinum-based alloys are difficult to produce and, in

some cases, intermetallics may complicate the formation of nanoporous metals. For

instance, the Al-Pt system shows multiple intermetallic phases that could complicate

fundamental studies on the dealloying process; however, dissolution of α-Al from the

Al88Pt12 has been reported to result in the formation of nanoporous platinum ribbons

with bimodal channel size distribution [113-115]. Pickering and Kim [116], investigated

the dealloying of Co-Pt and Fe-Pt alloys exposed to HCl/H2 at elevated temperatures

(900–1300 K) and confirmed the presence of porosity; however, due to the elevated

temperatures required for the dealloying in this system, only macroporous platinum was

observed (likely bulk diffusion played also a role here). Dealloying Cu-Pt and Ag-Pt

alloys has been reported, showing pore sizes of between 3 and 4 nm, thanks to the low

surface diffusivity of platinum [117-119]. Nickel has also been electrochemically

dealloyed from the Ni-Pt system in NiSO4 solution showing pore sizes of about 2 nm

30

and a typical core-shell structure with approximately 30 at.% Ni retained in the core of

the structure [120]. Moreover, ternary alloys (i.e., Pt-Ru-Al), with different ratios

between platinum and ruthenium and 80 at.% aluminum, have been dealloyed in 2 M

NaOH showing ligament sizes between 5 and 7 nm and with a pore size distributed

around 4 nm [121].

Nanoporous silver: Nanoporous silver has been formed by dealloying of Al75:Ag25 and

Ag22:Zn78 alloys [113,122]. In the case of the Ag22:Zn78 alloy, dealloying in H2SO4 was

analyzed showing that the resulting nanostructure undergoes a significant coarsening,

with pore sizes that range between 38 and 112 nm, depending on the dealloying

temperature; in addition, it was determined that the surface diffusivity of silver is faster

than that in gold. Dealloying of Al75:Ag25 in HCl at room temperature also gives ligament

size between 100 and 120 nm, which is in some agreement with the results obtained

from Ag22:Zn78 alloy.

Nanoporous palladium: Dealloying of palladium-containing alloys, to fabricate

nanoporous palladium, has been recently investigated using alloys with iron, cobalt and

nickel as sacrificial element [123-125]. As implied from the dealloying theory, the reason

for using those alloying elements is because the standard electrode potentials of these

elements are much lower than that of palladium, and these elements forms single-

phase solid solutions when alloyed with palladium. In the specific case of the Co-Pd

system, which is a FCC alloy, the porosity can be easily tuned by altering the

composition of the precursor, which is one of the advantages of this system.

Nanoporous palladium has been also formed by dealloying Al-Pd alloys in alkaline

solution [113,126], and by dealloying multi-component systems (i.e., Pd30Ni50P20

ribbons), although these alloys have intermetallic and glass phases respectively [127].

31

In these multi-component systems, pore sizes between 30 – 60 nm were observed;

nevertheless, the dealloying process in these systems has not been fully understood;

however, it has been suggested that the mechanism of formation of nanoporosity is

analogous to that in the crystalline systems, such as Au-Ag alloys (i.e., as the less

noble atoms continuously dissolve into the solution, the noble atoms will be driven to

agglomerate into clusters and gradually evolve into a 3D network structure). Dealloying

of Cu-Pd has been studied using single crystals to try to understand the initial steps

during the dealloying process, as well as Cu-Au single crystals [128,129].

Nanoporous Nickel: Nanoporous nickel and nickel-copper have been formed by

dealloying manganese from the Ni-Mn and Ni-Cu-Mn alloys [130]. Nickel-manganese

and nickel-copper-manganese form single-phase solid solutions under the proper

conditions; therefore, these two systems were easily dealloyed. Ligament sizes of 10-20

nm were observed after dealloying under potential control in (NH4)2SO4 solution.

Thermal treatment of these nanoporous structures (preferably under reducing

atmosphere) increases the ligament size to approximately 300 nm.

Nanoporous copper: Nanoporous copper has been formed by dealloying of Mn-Cu with

different copper contents (Mn50:Cu50, Mn25:Cu75, Mn70:Cu30) [131,132], Incrumet (Cu,

43-51 at.% Mn and 3-5 at.% Al) [133] and Cu-Al alloys [113,134,135]. Under the

appropriate dealloying conditions and alloy preparation, Mn-Cu can be effectively

dealloyed either by free-corrosion or potentiostatically control dealloying. Ligament

sizes between 15 nm to 120 nm can be easily formed. Nanoporous copper can also be

fabricated by dealloying Cu-Al alloys in HCl, which results in the formation of ribbons

with ligament size between 100-300 nm.

32

As mentioned before, these are just few common examples of dealloying systems that

have been studied recently; more examples can be found in the open literature.

Furthermore, variations of the typical dealloying process are also reported. For example, it

has been shown that in some cases the more-noble metal is the component that is

dissolved while the less-noble one remains in the structure. A standard example is the Ni-

Cu system, in which nickel films were formed by a process involving electrodeposition of Ni-

Cu alloy followed by dealloying of copper from the alloy in the same solution (1.6 M

Ni(H2NSO3)24H2O and 0.1 M CuSO45H2O). The formation of a passive oxide film on nickel

allows the selective electrochemical etching of copper, which is thermodynamically more

stable than nickel. This approach suggests that is possible to passivate the more-active

component on an alloy to make it kinetically favorable rather than thermodynamically stable

[136].

1.4 Applications of nanoporous metals

As suggested in this review, the flexibility, cleanliness, simplicity and versatility that

dealloying, as a preparation method, offers and the large inventory of NPMs, each of them

with unique mechanical, physical, and chemical properties, suggest that these materials

could be used in a variety of important technological applications such as catalysis, fuel

cells, sensors, actuators, supercapacitors, amongst many more. NPG, for example, has

been used as a heat exchanger [137], as an actuator [138,139], in sensors [102,140,141],

as a catalyst and electrocatalyst [56,88,90,118,142-145] of different reactions and even in

fuel cell applications [54].

33

As discussed in Section 1.5, one of the main goals of this research project was to

evaluate the catalytic/electrocatalytic properties of the nanoporous metals formed from Ag-

Au-Pt alloys. Therefore, to keep this review as relevant as possible, an overview of the most

relevant discoveries of the catalytic/electrocatalytic abilities of NPMs is presented.

Applications such as sensors, actuators, etc., are not included here but can be easily found

in the open literature.

1.4.1 Catalytic abilities of NPMs

Somorjai and McCrea [146] have accurately pointed out that one of the biggest

challenges of the 21st century is to produce catalysts that provide very high selectivity for

catalyst-based chemical processes. However, the knowledge of selectivity is much poorer

than the understanding of what controls the activity or the turnover rate of the catalyst;

nonetheless, there are four main aspects of selectivity that are considered very important: 1)

the surface structure of the metal surface, 2) the selective site blocking, 3) the bifunctional

catalyst, and 4) the oxide metal interface sites. In that regard, nanoporous metal catalysts

could be indeed a step forward towards a 21st century catalyst thanks to their structure,

properties and cleanliness. They offer unique characteristics if compared with other

catalysts (e.g., supported catalysts): (i) they are bulk in nature yet nanoscale in

microstructure, which means they can be easily employed and recovered; (ii) their

preparation methodology is very simple and thus reproducible; (iii) their structural unit is

tuneable in a wide range from a few nanometers to many microns, which allows the study of

the size dependence very easily; (iv) particle agglomeration is not a real concern and (v)

they have extremely clean nanostructured surfaces, because in most cases they are

produced in concentrated acidic media (e.g. nitric acid) [143,147].

34

Catalysis by nanoscale gold has been attracting rapidly growing interest, not only

because of its relatively high activity, but also because of its high selectivity towards many

oxidation reactions. Furthermore, it has been reported that the number of papers has grown

from more or less five per year in the 1980s to approximately 700 in 2005 [148] and to more

than 1200 publications in 2013-2014 (Thompson Reuters - Web of Knowledge, 2014). This

interest in gold has increased because platinum group metals often suffer from poisoning by

strongly chemisorbed carbonaceous intermediates such as carbon monoxide, and gold has

been proven to be highly effective for CO oxidation, which of course opens promising

alternatives for many applications [90]. In fact, nowadays, the three major streams in terms

of gold catalysis are the expansion of new applications, especially to liquid-phase organic

reactions, discussion on the active states of gold, and exploration of new forms of gold

catalysts [148]. Among all the new forms of gold catalyst, NPG is definitively an interesting

candidate.

Despite the extensive research in gold as a catalyst, the catalytic nature of gold

catalysts is not fully understood yet. Zeis et al. [149] suggested that the primary reason that

bulk planar gold is not particularly active is the absence of active sites. It has been found

that the catalytic sites in gold are often crystal surface defect sites, such as step edges. At

these sites, gold atoms with lower coordination number dominate (see Figure 1.5);

therefore, dangling bonds may help with the adsorption of reactants through two main ways:

the electronic (adsorption enthalpy) and the geometric interaction (activation barrier) [150].

In reality, these two effects are hardly separated. Nørskov and co-workers were the first to

introduce the concept of d-band centre to explain the electronic effects on the interaction of

gases with metallic surfaces [151]. The d-band centre is defined as the first moment of the

density of d-states. The position of the d-band centre relative to the Fermi Level is the main

factor controlling adsorption. For example, transition metals tend to have higher d-states in

35

case of low coordination numbers (kinks, steps – Figure 1.5); therefore, these atoms interact

more strongly with adsorbates than atoms in a closed packed surfaces [152]. Additionally,

the geometry of the substrate provides specific adsorption sites, playing a crucial role for

activation of adsorbed molecules. In fact, in the case of gold, the presence of low

coordination atoms even determines whether the surface shows any catalytic activity [153].

This observation leads to the hypothesis that gold surfaces roughed artificially (e.g.,

induced by ion beams) should be more active than planar gold. If crystalline step edges are

helpful for enhancing the catalytic activity of gold, then it seems likely that forms of gold with

a significant number of step sites (e.g. nanoparticle-based gold, NPG) should be good

catalysts. Fujita and co-workers [154] recently provided a detailed study of the surface

atomic structure of NPG, showing that in the curve regions of the ligaments, the surface has

a very high density of atomic steps and kinks. Additional, other authors suggested that some

of the reasons nanosized gold is catalytically active is related to a quantum size effect, the

strain effect, and even a charge transfer effect in the case of supported nanoparticles

[155,156].

Figure 1.5 (a) The coordination of atoms depends on their location. The stability of the

atoms decreases with the coordination number (CN). (b) Percentage of atoms as a function

of the length scale. This is shown for a straight ligament as a simple example (numbers are

based on a geometric consideration). Reprinted with permission from Wittstock [157] –

Copyright 2010 A. Wittstock.

36

Gold has been used as a catalyst for a variety of reactions: low temperature CO

oxidation [46], selective oxidation of benzyl alcohol [158,159], selective oxidation of glycerol

to sodium glycerate [160], the water gas shift reaction [161], among many other reactions,

some of which were summarized elsewhere [162,163]. In most of these cases however,

gold has been used as a supported catalyst. In fact, it was believed that catalytic activity of

this noble metal only occurs when it is in the form of nanometer sized particles supported on

a suitable metal oxide support. In supported gold catalysts, many factors have been

correlated with the catalytic activity, including preparation methods of the catalyst, nature of

the support, pre-treatment conditions, the presence of water in the feed and many others

[142,147]. Among these factors, the catalyst support is usually considered crucial to

determining the catalytic activity. Therefore, “active” and “inert” supports have been

classified in accordance with the reducibility [164] or semiconducting properties of transition

metal oxides [165]. In that regard, using -Fe2O3 and TiO2, as well as CeO2, results in highly

active catalysts, whereas catalysts prepared on other oxides, such as Al2O3 and SiO2,

showed usually low or no activity [166].

NPG, on the other hand, seems to be best described as an inversely supported gold

catalyst or a bimetallic catalyst where the high catalytic activity can be accounted for by the

presence of silver residues left over from the dealloying process [148]. Iizuka et al. [167],

reported in detail the effect of silver contamination and of pre-treatments on the catalytic

activity of gold particles with diameters of 20 to 250 nm and with surface silver

concentrations of 0.2 to 40 wt%. In fact, it was observed that the catalytic activity was more

enhanced by silver contamination (more than three orders of magnitude) than by a decrease

in particle size of gold. In addition, recent experiments on CO oxidation demonstrate that

silver, even if present only in traces (< 1 at %), plays an active role [142,157]. Later on,

Moskaleva et al. [168], modeled different gold surfaces showing that the presence of silver

37

atoms can facilitate the adsorption and dissociation of molecular oxygen and with that

increase the catalytic activity towards carbon monoxide oxidation. NPG always contains

traces of silver, and although one can reduce the overall silver content of NPG to well below

1 at %, there is always a significant amount of Ag at the ligaments surfaces [169]. Fujita et

al. [154], demonstrated that surface reconstruction (i.e., {111} dynamic faceting) in NPG

samples with high concentrations of silver was significantly suppressed; whereas the

reconstruction in similar NPG samples, but with lower concentration of silver was more

pronounced. More importantly, samples with high concentrations of silver exhibits higher

catalytic activity towards CO oxidation than low silver content NPG. Therefore, it was

concluded that the presence of silver not only suppressed the surface reconstruction

dynamics in NPG, but also provided more active silver sites for dissociative O2 adsorption.

NPG has been used to catalyze a significant variety of reactions: Zielasek et al. [144]

and Xu et al. [155] reported on the abilities of NPG to oxidize carbon monoxide at

remarkable low temperatures. Xu et al. [147], showed strong evidence that the metallic gold

atoms on NPG are the intrinsic active sites at which carbon monoxide and oxygen reacts;

moreover, a kinetic study found that the reaction rate of carbon monoxide oxidation on NPG

depends significantly on the CO concentration but only slightly on the oxygen concentration,

suggesting that carbon monoxide adsorption plays a decisive role as the rate-limiting step.

Wittstock et al. [170], recently reported that NPG can catalyze selective oxidative coupling of

methanol to methyl formate with a selectivity above 97% and high turnover frequencies.

Gas-phase selective oxidation of benzyl alcohol to benzaldehyde, with 95% selectivity under

ambient reaction conditions, has also been reported by Han et al. [171]. In addition, NPG

has been used to catalyze aerobic oxidation of D-Glucose to D-gluconic acid with a 99%

selectivity [143] and the oxidation of organosilane compounds with water [172].

38

1.4.2 Electrocatalytic applications of NPMs

Electrocatalysis has been another key area of research in NPG. In view of the global

energy demands, fuel cells have gained a significant relevance. Currently, platinum remains

the more investigated noble metal catalyst and also has the best performance in fuel cells;

however, platinum suffers from poisoning by carbonaceous compounds. Therefore,

exploring the use of NPG as a potential electrocatalyst for applications like fuel cells has

become very important, especially from the point of view that NPG could play an unique role

in the preparation of novel highly efficient electrocatalysts.

Besides all the different materials used to form NPG, it was recently reported that by

dealloying white gold leaves could have remarkable advantages for the use of NPG in

applications such as fuel cells [68,106]. Electroless plating of platinum onto the NPG leaf

substrate has shown remarkable catalytic activities, showing that using platinum-plated

NPG as an electrode material for hydrogen fuel cells is feasible [173]. These Pt - NPG

leaves typically contain less than 50 g cmleaf of platinum and it can be easily integrated into

a membrane electrode assembly in H2/O2 fuel cells. It was reported that upon optimization,

the Pt - NPG nanocatalyst could generate a specific power as high as 4.5 kW g-1 of Pt,

which is close to the US Department of Energy target of 2015 for platinum utilization.

Although it is widely accepted that the highest platinum utilization can be achieved only on

monolayer (or submonolayer) type of structures, to which these NPG-based nanostructures

are close, the most important characteristic of decorated NPG is the easy accessibility to

almost all surface precious-metal atoms by target molecules. Considering the structure

flexibility of NPG and the strong synergistic effect between gold and platinum, decorated

NPG may represent an alternative ultralow precious-metal loading catalyst to traditional

ones.

39

Electro-oxidation of methanol has been also studied using NPG as a catalyst. Zhang

et al. [90] has reported that NPG has a very good catalytic activity towards methanol electro-

oxidation, particularly in alkaline solution; however, it was shown that after long-time

potential cycling the structure significantly coarsens. It is well-known that methanol oxidation

proceeds independently in two potential regions; at lower potentials, methanol is oxidized to

formate/formic acid, whereas at higher potentials, methanol is oxidized to carbonates/CO2

[174]. It has been suggested by some researchers that pre-oxidation species such as Au-

OHads(1-)- ( is the charge transfer coefficient , 0 < <1) play a governing role in the

electrochemistry of gold in alkaline media [175]. More importantly, NPG could react with the

reaction intermediates to further oxidize them, eliminating any possible catalyst poisoning.

Decorating NPG with Pt, on the other hand, offers a real possibility not only to minimize

coarsening, but also to enhance the oxidation of methanol and increase the poisoning

resistance if compared with pure platinum [90,176]. DFT (Density Function Theory)

calculations on Au-Pt catalyst have shown the possible mechanism for the reaction [177]:

the first step in this reaction is the adsorption of methanol on platinum active sites, followed

by dehydrogenation; intermediates like formaldehyde and formates are then present in the

system; COads species can be also identified as an intermediate of the process on platinum

atoms with the subsequent transfer of CO from platinum sites to neighbouring gold sites,

which is possible in view of the favourable adsorption of CO on gold; finally, reactions of Pt-

COads and Au-COads with Au-OHads are expected to happen towards the final product (i.e.

CO32-). Of course, the presence of silver might alter the reaction pathway.

Electro-oxidation of formic acid has also been studied using NPG decorated with

platinum [178]. It was observed that electrooxidation of formic acid was highly sensitive to

the catalyst surface structure; in fact, it was noticed that the least plated structure (i.e., less

platinum on the surface of NPG) exhibited unusually high activity towards FA, which could

40

be related to the nearly ideal platinum utilization. Oxidation of formaldehyde and ethanol in

acidic media was also reported by the authors, showing again that the activity of the

decorated NPG, normalized by the mass of platinum on the nanostructure, was much higher

than that of the commercial Pt/C catalyst. Wang et al. [179] successfully fabricated a

sandwich-type nanostructures electrocatalyst (i.e., NPG-Pt-Au) by depositing a monolayer

of platinum onto the ligament surfaces of NPG by in situ redox replacement of PtCl42- with a

monolayer of Cu deposited by UPD, then, an adequate amount of gold was deposited onto

the platinum overlayer using the same method. It was observed that a greatly enhanced

catalytic activity was achieved by changing the reaction pathway by using gold surface

clusters, which at the same time contributed to the stabilization of the catalyst.

Besides NPG, other nanoporous metals have been used as catalysts for different

electrochemical reactions. Nanoporous Pt-Ru and Au-Pt structures were tested for methanol

and formic acid oxidation, showing remarkable activities (per mass of platinum or per true

area of the electrocatalyst - determined by CO-stripping charge) even higher than the

commercial PtRu/C catalyst (same as before) [97,121]. It was believed that this

enhancement could be due to the nanoscale bicontinuous porous structure that allows good

mass transport and electron conductivity, besides the fact that alloying ruthenium/gold with

platinum effectively modifies the electronic structure of platinum, enhancing the catalytic

activities under appropriate conditions facilitating the removal of species such as carbon

monoxide. Shao et al. [180] reported on the synthesis of a core-shell catalyst consisting of a

platinum monolayer as the shell and porous/hollow Pd-Cu alloy nanoparticles as the core

(these porous nanoparticles were fabricated by electrochemical dealloying of copper via

cyclic voltammetry in 0.1 M HClO4). The catalytic ability of these platinum decorated-

nanoparticles was evaluated towards the oxygen reduction reaction (ORR), showing a

significant enhancement in the activity, which can be explained by the lower oxygen binding

41

energy to the surface. Snyder et al. [120] also showed that a catalytic enhancement towards

ORR can be achieved by using a composite nanoporous Ni-Pt alloy (obtained by

electrochemically dealloying nickel from the Ni-Pt structure in NiSO4 solution) impregnated

with a hydrophobic, high-oxygen-solubility, protic ion liquid. Liu and coworkers [181-183]

reported on the fabrication of nanoporous Pt-Co, Pt-Ni and Pt-based multimetallic alloy

nanowires through the combination of electrodeposition into AAO templates with dealloying.

It was shown that these nanowires exhibit distinctly enhanced electrocatalytic activities

(normalized by the effective area of platinum) towards methanol oxidation as compared with

current state-of-the-art Pt/C and PtCo/C catalyst (also normalized by the effective area of

platinum in the catalyst).

1.5 Thesis scope

The overall objective of this research was to investigate the formation, and principal

characteristics of nanoporous metals formed by electrochemical dealloying of ternary-noble

alloys (i.e., Ag-Au-Pt). More importantly, we aimed to unveil the effect that the ternary

element (i.e., platinum) has on the resulting nanoporous structures, which is the reason why

alloys with a systematic variation in the platinum content (i.e., 1, 2 and 3 at.%) were

evaluated. As shown in section 1.3.2.2.1, previous studies have proven that having platinum

in the dealloying precursor is beneficial to: a) reduce the feature size of the structure, b)

increase the resistance to coarsening and c) increase the catalytic performance of the

resulting structure; however, the field lacked a fundamental study regarding the real effect of

platinum on the main characteristics and properties of the resulting nanostructures. In this

project, all alloys had 77 at.% silver and 23 at.% of more-noble metals, and the nanoporous

structure formed from the binary alloy (Ag-Au) was used as a benchmark nanostructure,

which is a nanostructure that has a more open and compliant structure than other alloys

42

with higher gold content. This study included an in-depth characterization of the resulting

nanostructures, using electrochemical and surface sensitive techniques; moreover, the

effect of different dealloying conditions and experimental protocols were carefully evaluated,

allowing us to create a general understanding of the effect of having a more-noble ternary

element in the precursor material. In addition, an assessment of the catalytic abilities of the

resulting nanostructures towards technologically important electrochemical reactions (e.g.,

methanol and ethanol oxidation) was included.

In summary, our general motivation to pursue this research was to give the field a

better understanding of the role of ternary elements, platinum in this case, in the formation

and characteristics (e.g., ligament size, composition, morphology) of nanoporous metals,

which undoubtedly will have a tremendous impact in science and technology.

1.6 Thesis objectives

Within the thesis scope, the specific objectives of this research were to:

1. Determine the most appropriate experimental conditions to electrochemically dealloying

Ag-Au-Pt alloys in perchloric acid (HClO4) to form clean and well-defined nanoporous

metals.

2. Investigate the effect of platinum, as a ternary element in the Ag-Au-Pt alloys, in the

formation, characteristics and resistance to electrochemical coarsening of the resulting

nanoporous metals. This evaluation was done on the following metrics:

morphology of the nanoporous metals,

ligament and pore sizes of the nanoporous structure,

43

developed surface area as a function of the platinum content on the precursors,

thickness and shrinkage of the dealloyed layer,

silver content that has been retained in the dealloyed layer,

platinum coverage of the surface of the ligaments.

3. Examine the effect of the dealloying temperature, charge density passed and dealloying

potential on the following metrics for each alloy:

resistance to electrochemical coarsening (i.e., ligament size),

true surface area of the resulting structures,

thickness of the dealloyed layer,

amount of retained silver in the dealloyed layer,

concentration of platinum on the surface of the ligaments.

4. Investigate the stability (i.e., resistance to coarsening) of the different nanoporous

structures under more aggressive conditions. Specifically, exposure of the freshly

dealloyed samples to high temperatures and after immersion for extended periods of time

in solution (e.g., dealloying electrolyte). This evaluation was done on the following

metrics:

ligament/pore size of the different nanostructures,

composition of the dealloyed layer,

concentration of platinum on the surface of the ligaments.

44

5. Evaluate the electrocatalytic abilities of all the nanoporous metals, with and without

platinum, towards methanol and ethanol electro-oxidation, in acidic and alkaline

electrolytes, considering the following aspects:

evaluation of the catalytic response of the different nanostructures after dealloying

under different conditions (e.g., temperature, charge density),

investigate the effect of parameters such as the supporting electrolyte

concentration, scan rate, etc., in the catalytic response of these structures,

identify the reaction products for at least the experiments run in alkaline electrolyte

(i.e., amount of carbonate formed).

1.7 Thesis layout

This thesis presents the results, analyses, and conclusions obtained throughout the

course of this research in the form of four manuscripts. Each manuscript constitutes one of

the following four chapters and contributes to meeting one (or more) of the thesis objectives.

The following is a detailed description of how each of the abovementioned objectives has

been met with respect to their presentation in the chapters and/or appendices.

Chapter 2 contributes to meeting Objectives 1, 2 and 3. This manuscript reported a

detailed study on the formation of nanoporous metals from Ag-Au and Ag-Au-Pt alloys in

acidic media (i.e., perchloric acid). The presence of platinum limited the ability of gold to

diffuse on the surface, reducing the ligament size of the structure and increasing its stability

and resistance to coarsening. Aspects such as the ligament and pore size, pore size

distribution, thickness of the dealloyed material, resulting surface area and composition

across the dealloyed layer were reported for all the different nanostructures after

45

characterization by SEM, TEM, X-ray photoelectron spectroscopy (XPS), Brunauer-Emmett-

Teller (BET) surface area, and electrochemical methods such as cyclic voltammetry.

Furthermore, the effect of adjusting dealloying parameters such as the temperature, charge

density and dealloying potential on the characteristics of the resulting structures was also

reported.

Chapter 3 contributes to meeting Objective 4. As a result of the experimental work

done to evaluate the stability of the different structures at high temperatures, it was

observed that under specific conditions (i.e., moderate temperature in the presence of

laboratory air), platinum was induced to segregate to the surface of the ligaments due to its

preferential interaction with oxygen. Moreover, it was found that the co-segregation of

platinum and oxygen hinders the thermal coarsening of the ligaments. A detailed evaluation

of the ligament size, composition of the surface of the ligaments, and roughness factor was

done by SEM, TEM, XPS, low energy ion scattering (LEIS), in addition to electrochemical

methods such as underpotential deposition (UPD) of hydrogen and cyclic voltammetry. This

adsorbate-induce surface segregation phenomenon, to the best of our knowledge, was not

reported in prior studies for nanoporous metals.

Chapter 4 contributes to meeting Objective 5. In this chapter, the electrocatalytic

abilities of these novel nanoporous metals were assessed towards methanol electro-

oxidation in acidic and alkaline electrolytes. In the case of the electrocatalytic experiments

ran in alkaline electrolyte, it was found that the selectivity of the reaction changes with the

platinum content of the precursor – the concentration of carbonate increases with the

platinum content of the precursor, while formate is favored by the nanostructure with the

lowest platinum content. Moreover, it was determined that the most active nanostructure in

alkaline electrolyte was the nanostructure formed on the alloy with 1 at.% Pt; whereas in

acidic media the one formed on the alloy with 3 at.% Pt was the most active one. Aspects

46

such as the influence of the dealloying temperature and charge density passed in the

electrocatalytic response of these materials, as well as the effect of inducing segregation of

platinum to the surface of the ligaments were assessed. The influence of methanol

concentration, scan rate, supporting electrolyte concentration was also examined.

Chapter 5 also contributes to meeting Objective 5. This chapter reports on the

catalytic abilities of ternary-noble nanostructures towards ethanol electro-oxidation in

alkaline electrolyte. Similar to the case of methanol electro-oxidation, evaluation of the

amount of carbonate produced during the reaction was carried out, showing that ternary

nanostructures had a spectacular activity to cleave the C-C bond and produce carbonate as

the dominant product of the reaction. In NPG, no significant amount of carbonate was

detected. Evaluation of the stability of the nanostructure after running the reaction for long

periods of time was performed by SEM of the nanoporous layer.

In Appendix A, supplementary data that, in most cases, was implicit within the

manuscripts is presented. This collection of data includes dealloying/electrochemical data

(e.g., impedance measurements, anodic current vs. time for specific dealloying conditions),

characterization data like the voltammograms in the double layer region of the nanoporous

structure used to determine the resulting area after dealloying, BET adsorption/desorption

isotherms, under-potential-deposition (UPD) of hydrogen, various SEM images, etc.

Additionally, data related to the surface characterization techniques such as the depth

profile analysis obtained by applying the Maximum Entropy algorithm, XPS data, etc. are

also included.

Appendix B provides preliminary results in terms of the simulation of these

nanoporous metals by a modified simulation code called MESOSIM, which is based on a

Kinetic Monte Carlo (KMC) algorithm. This effort was initiated from the beginning of this

47

research; however, discrepancies between the experimental data and the model were not

possible to solve; nevertheless, we believe this work can be carried on by future

researchers.

Finally, Appendix C includes the complete list of contributions (publications and

presentations) done during the course of this research.

1.8 Reproducibility of experiments

Most of the experimental evaluations carried out in this research were done in

triplicate. For most of the dealloying conditions explored in this research, three different

samples were dealloyed under identical conditions to assess the variability of the anodic

current density, impedance measurements, developed surface area, etc. In terms of the

ligament/pore size, at least fifteen measurements of the feature size from different SEM

images of the same sample were done. This strategy was chosen as the best way forward

after confirming that the variability within a sample was similar to the variability between

samples. The same was true for the compositional analysis (by EDS or EPMA) of the as-

dealloyed material and of the as-received material; in some specific cases, three different

samples were evaluated, but in other cases analyses at different points along the material

surface were carried out. For the determination of platinum on the surface of the ligaments,

under-potential-deposition of hydrogen was run in three different samples prepared under

exactly the same conditions. With respect to the catalytic evaluation of the nanoporous

structures, triplicate runs were done for most of the cyclic voltammetry and potentiostatic

evaluations. The amount of carbonate produced during catalytic oxidations was at least run

in duplicate; however, in the case of methanol oxidation, triplicate samples were tested and

evaluated.

48

In the case of potentiodynamic curves, nuclear magnetic resonance tests, BET

surface area and pore size distribution, LEIS and XPS evaluations, duplicate runs were

done in most of the cases to confirm the trend/result without calculating any standard

deviation or confidence interval.

Throughout this work, all error bars represent the confidence interval (95% CI), as

defined by Cummings et al. [184].

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1 A version of this chapter has been published.

Vega, A. A., and Newman, R. C. Nanoporous metals fabricated through electrochemical dealloying of Ag-Au-Pt with systematic variation of Au:Pt ratio, Journal of the Electrochemical Society 2014, 161(1), C1.

69

Chapter 2. Nanoporous Metals Fabricated through Electrochemical

Dealloying of Ag-Au-Pt Alloys with Systematic Variation of

Au:Pt Ratio1

2.1 Introduction

The development of new and more robust materials for applications in chemical

processes, sustainable energy, remediation of the environment and many other fields is one

of the biggest challenges in the 21st century. Porous materials, such as zeolites or aerogels,

offer versatile properties and characteristics (e.g., pore sizes and morphology) that have

been found very useful in a variety of fields [1-2]. Porous metals, another important

subgroup of porous materials, have gained a lot of interest due to their high electrical

conductivity, mechanical properties and broad application spectrum [3-8]. Particularly,

porous precious metals, with pore size distribution below 10 nm, are useful in

electrocatalysis, catalysis, sensing and other applications.

A common strategy to make nanoporous metals is using template growth methods, in

which the metals are plated into the interstices of a porous phase that is later removed [9].

However, these techniques are generally difficult to implement and are time consuming. An

alternative method – dealloying – has proven to be a very effective technique to produce

nanoporous metals. In the dealloying process, one starts with a monolithic alloy in any form

(e.g., bulk or thin film) and selectively dissolves one or more less-noble elements from the

alloy, forming high surface area materials with an interconnected ligament/pore structure

[10-13].

70

Amongst the different nanoporous metals, nanoporous Au (NPG), typically formed by

dealloying of Ag-Au alloys, has been extensively investigated because the starting alloys

display single-phase solid solubility across the entire range of compositions, and produce a

beautiful structure with open porosity [11,12,14,15]. Furthermore, research in NPG has been

motivated by the prospect of applying this material as a heat exchanger [16], actuator

[17,18], sensor [19,20], catalyst [21-25] and electrocatalyst [26-28] that can be used in

applications such as fuel cells [29]. Unfortunately, NPG suffers from a major limitation:

coarsening of the porosity by surface self-diffusion of Au. This coarsening can be a limiting

factor for long term functionality in some applications such as catalysis [28]. Recently,

however it has been reported that adding a third species (i.e., Pt) to the Ag-Au precursor is

beneficial in keeping the structural integrity of the nanoporous layer (i.e., reducing the

coarsening effect) [30]. Pt, which does not dissolve, is immobile relative to Au and blocks

step edges, creating a porous structure with a smaller length scale. Furthermore, it has

been reported that the presence of Pt induces distinctive catalytic properties in the NPG

[31].

The main objective of the present study is to understand and characterize the effect of

Pt on the nanoporous structure. For that purpose, a comparison between the nanoporous

structure formed from binary (Ag-Au) and ternary (Ag-Au-Pt) precursors, with a systematic

variation in Pt content, is made. The main aspects examined are the quantitative

morphology of the nanoporous structure, its composition, and the amount of exposed Pt on

the ligament surfaces, as revealed by underpotential deposition (UPD) of hydrogen. The

effect of dealloying potential, temperature, and charge density on the characteristics of the

resulting nanostructure are examined.

71

2.2 Experimental procedures

2.2.1 Materials and dealloying procedures

Ag-Au alloy with 23 at.% Au was obtained as cold-rolled 200 µm sheet from

Goodfellow Metals, Cambridge, UK. Ag-Au-Pt alloys with nominal Pt contents of 1, 2 and 4

at.% and 77 at.% Ag were obtained as cold-rolled 200 µm sheet from Ames National

Laboratory - US Department of Energy, Iowa, USA. For all experiments, specimens of

approximately 4 mm by 10 mm were cut before annealing at 975 °C for 15 h in the case of

the ternary alloys and at 900 °C for 5 h in the case of the binary alloy. In all cases, the

annealing was performed in H2-Ar atmosphere (2.5 % H2; balance Ar). All the specimens

were used in the as-annealed condition without any further surface preparation. Each strip

of alloy was attached to a copper wire for electrical connection, using lacquer (SPI

Miccroshield) to mask the junction. In all cases, dealloying was done from both sides of the

sample.

Anodic current-potential (polarization) curves and the dealloying itself were

performed in 0.77 M HClO4 solution prepared from Analar grade HClO4 (Alfa-Aesar, 62%).

All solutions were prepared with 18 MΩ∙cm de-ionized water and de-aerated by high-purity

nitrogen purging (min. purity: 99.998%). The electrochemistry was performed using a Gamry

Reference 600TM potentiostat, using an electrochemical cell with a volume of approximately

500 mL; a Pt wire was used as a counter electrode (CE) and mercury/mercury sulfate (MSE,

640 mV vs. SHE) was used as a reference electrode. The reference electrode was housed

in a separate compartment and connected to the electrochemical cell via a Luggin probe.

Most of the specimens were dealloyed at 25 °C, passing an anodic charge density of 5 C

cm-2 at 550 mV vs. MSE.

72

The effects of the dealloying temperature, charge density, and dealloying potential

on the characteristics of the dealloyed layer were systematically studied for all alloys. For

the temperature-controlled experiments, the electrochemical cell was built with a water

jacket to help stabilize the temperature of the electrolyte; a temperature range of 10 – 60 °C

was evaluated. The effect of charge density was analyzed by varying charges from 2.5 to 40

C cm-2 during dealloying at room temperature. Two different dealloying potentials (500 and

550 mV vs. MSE) were selected to study the effect of the applied overpotential on the main

characteristics of the nanoporous structure.

2.2.2 Material characterization

The compositions of the binary and ternary alloys were verified by an electron probe

X-ray microanalyzer (EPMA – CAMECA SX-50/51) equipped with 3 wavelength dispersive

spectrometers (WDS) and operated at 20 kV and 30 nA. Calibration with proper standards

(i.e., Ag80Au20 alloy and elemental Pt) was done prior to any measurement. Probe for EPMA

software (Probe Software Inc.) was used for data analysis. For the microstructural

characterization (e.g., ligament size), dealloyed samples from all alloys were manually

broken under tension in air and the fracture surface was photographed using a scanning

electron microscope (SEM – Hitachi S-5200) with an accelerating voltage of 20 kV. The

SEM pictures were later analyzed with the Image Tool software (provided by The University

of Texas Health Science Centre in San Antonio, USA) to accurately determine the ligament

size (here equivalent to the average width perpendicular to the ligament edge). Cross-

sectional metallographic specimens were prepared for the determination of the dealloyed

layer (DL) thickness and its shrinkage. In all cases, specimens were polished to 0.05 µm

using alumina powder. The cross sectional view of those specimens was photographed

73

using an Olympus PME3 Optical Microscope and the DL thickness was measured using

Clemex Vision Professional software. For the shrinkage of the layer, a section of selected

samples was masked with lacquer (MiccroshieldTM) prior to dealloying; a comparison

between the thickness of the dealloyed and non-dealloyed sections was carried out to

determine the shrinkage of the DL. The relative shrinkage was calculated with respect to the

theoretical thickness of the DL.

X-ray diffraction (XRD) was done on selected samples before and after dealloying.

All the samples were run on a Bruker AXS D8 Discovery Microdiffraction system with a Cu

K point-focus X-ray source operating at 40 kV/40 mA. The system is equipped with a

curved primary graphite monochromator and 2D proportional detector (GADDS). The

experimental data were collected in three frames, each of 400 – 480 s exposure, that cover

the range of 25 ° - 92° (2-theta). The 2D diffraction images were then integrated with the

step size of 0.005° 2-theta and converted to standard I vs. 2-theta diffraction patterns. The

phase identification was done by Diffrac PlusTM data processing software EvaTM v. 8.0 and

Search/MatchTM routine. The profile fitting applications (e.g., Rietveld refinement) were

performed using TopasTM v. 3.0.

The composition of the DL was determined by several methods. Using the

metallographic specimens, the average chemical composition was characterized on the

SEM (JEOL JSM6610-Lv) complemented by an Oxford INCA X-sight energy dispersive X-

ray spectrometer (EDS). An accelerating voltage of 25 kV was used in all cases. The

composition profile across the layer was determined on a transmission electron microscope

(TEM – Hitachi HD-2000) complemented by an Oxford INCA X-sight EDS. For that purpose,

selected samples were embedded in low viscosity resin (SPI-PON 812) and

ultramicrotomed to 30 – 50 nm thickness using the Leica UltraCut R instrument equipped

with a diamond knife. An accelerating voltage of 200 kV was used. Additionally, the surface

74

composition of the dealloyed layer was analyzed using an X-ray photoelectron spectrometer

(XPS – Thermo Scientific K-Alpha) with monochromatized Al K X-ray as excitation source.

AdvantageTM software was used for the elemental quantification.

The true surface area of the electrodes was determined by means of voltammetric

profiles in the double layer region of potentials at different scan rates [32,33]. After the

samples were dealloyed, they were taken from the solution and rinsed with de-ionized

water; CV was then performed in the potential range from -240 to 50 mV vs. MSE in 1 M

HClO4 solution. Scan rates of 20, 50, 100, 200, 300 and 500 mV s-1 were used. In all cases,

28 µF cm-2 was used as the baseline double-layer capacitance for polycrystalline flat metal

surfaces [32]. The surface area was normalized by dividing the true area of the electrode by

the mass of the DL, estimated from the amount of charge passed – Ag removed – and the

thickness of the layer, to account for the real increase in surface area after dealloying.

Brunauer-Emmet-Teller (BET) surface area and pore size distribution was carried out using

the Autosorb-1 Analyzer (Quantachrome Instruments). Nitrogen adsorption/desorption

isotherms were obtained at 77 K after degassing at 100 °C for at least 3 h. For the pore size

distribution calculation, the non-local density functional theory (NLDFT) method/kernel was

used considering the adsorption branch model [34,35]. The BET surface area was also

normalized by the mass of DL.

In order to determine the fraction of Pt on the surface of the nanoporous structure,

UPD of hydrogen was used [36-40]. Immediately after the specimen was dealloyed and

rinsed with de-ionized water, it was immersed in 1 M H2SO4 de-aerated solution (EMD, 95-

98%) in a three-electrode cell with an MSE reference electrode, a Pt wire as a CE and the

dealloyed specimen as a working electrode. CV was also run using a Gamry Reference

600TM potentiostat. The CV curves were obtained at 25 °C between -630 and 0 mV vs. MSE,

at a scan rate of 20 mV s-1. The fraction of the surface with Pt atoms was calculated by

75

integrating the hydrogen adsorption/desorption regions to obtain the charge related with the

formation of a hydrogen monolayer, assuming that the charge associated with the

monolayer formation in polycrystalline Pt was 210 µC cm-2 [39,41].

2.3 Results and discussion

Prior to any electrochemical test, the composition of all the alloys was verified by

EPMA. As shown in Table 2.1, the composition of the binary alloy was in excellent

agreement with the nominal composition; the ternary alloys, on the other hand, showed

smaller Pt contents. All the alloys hereafter are identified by the experimentally determined

composition.

2.3.1 Electrochemical measurement of dealloying behavior

Figure 2.1 shows anodic current-potential curves (polarization curves) for the ternary

alloys and for the binary alloy in 0.77 M HClO4 at 25 °C. As observed in the figure, the

presence of Pt did not influence the ‘empirical’ critical potential (Ecrit) of the alloys; i.e., at the

specific scan rate used for this work, the Ecrit was the same for all the alloys, lying in the

range of 280 – 340 mV. Conventionally, it has been predicted that the Ecrit results from a

balance between the rate of dissolution of the less-noble element and surface diffusion of

the more-noble element [42]; therefore, it was hypothesized that adding Pt to the precursor

would lower the Ecrit. However, it was recently proposed that Ecrit should depend mostly on

the rate limiting step in the dissolution process, which has been identified as the nucleation

of terrace vacancies [30]. Following this, it was predicted that alloys with the same Ag

concentration should have virtually the same Ecrit, which is what was observed. In Figure 2.1

76

the high anodic current densities on the right-hand side of the figure correspond to the

formation of interconnected nanoporosity by dealloying of Ag (Figure 2.2).

Table 2.1 Real compositions of Ag-Au and Ag-Au-Pt alloys. The numbers in brackets represent the 95% confidence interval (CI) calculated from at least seven measurements randomly chosen along the alloys.

Nominal composition Bulk Composition (by EPMA, at.%)

Ag Au Pt

Ag77:Au23 77.0 (0.2) 23.0 (0.1) NA

Ag77:Au22:Pt1 77.1 (0.3) 22.0 (0.2) 0.9 (0.1)

Ag77:Au21:Pt2 77.2 (0.2) 21.3 (0.1) 1.6 (0.2)

Ag77:Au19:Pt4 77.1 (0.2) 19.8 (0.3) 3.1 (0.1)

Figure 2.1 Results of negative to positive potential scan on Ag-Au and Ag-Au-Pt alloys, recorded at a scan rate of 0.5 mV s-1 at 25 °C. The absolute value of the current density is plotted in this figure: current densities below ~ -0.3 V are cathodic; everything else is anodic.

77

Figure 2.2 Interconnected ligament/pore structure of the Ag77:Au23 after passing 5 C cm-2 at 25 °C and 550 mV vs. MSE.

2.3.2 Formation and characterization of nanoporous metals

The porosity evolution in nanoporous metals, as originally described by Forty and

others [15,43,44], is the result of the selective dissolution of the less-noble element (i.e., Ag

in the case of the Ag-Au alloys) and the surface diffusion of the more-noble element (e.g.,

Au atoms) at the alloy/electrolyte interface. Consequently, by controlling these two

processes, a tunable nanoporosity can be obtained. Moreover, the presence of Pt in ternary

alloys, slows down the surface diffusion of Au and with that reduces the ligament/pore size

of the resulting structure. Figure 2.3 shows the nanoporous structures formed on the binary

and ternary alloys. In all cases, the dealloying potential was kept constant at 550 mV vs.

MSE. At this potential, rapid dissolution of Ag was permitted while Au oxide formation was

avoided. The oxidation of Au during dealloying would limit its surface diffusion kinetics,

hindering the initial ligament coarsening (i.e., postporosity coarsening) that establishes the

morphology and allows the second stage of Ag elimination. Senior and Newman [45]

reported that a large cathodic peak at ~520 mV vs. MSE was found when potential

backscans were carried out from the dealloying potential (i.e., ~700 mV vs. MSE) on

78

Ag77:Au23, associating the location of this peak with that reported for reversible monolayer

oxidation of pure polycrystalline Au to AuOH. At higher applied potentials (~800 mV vs.

MSE) a monolayer of Au2O3 started to form [46,47]. Therefore, to confirm that at 550 mV

neither hydroxide nor oxide were formed, backscans were also carried out from the

dealloying potential on the ternary alloys and on Ag77:Au23 (figures not shown here). As

expected, no cathodic peak was found using this sequence.

As demonstrated in Figure 2.3, the ligament size decreased with increasing Pt

content in the precursor material (see Table 2.2). In the case of NPG, the average ligament

size was ca. 14 nm whereas for the alloys with 1, 2 and 3 at.% Pt the ligament size was ca.

6.8, 6 and 4.3 nm respectively. The pore size distribution, on the other hand, showed

additional features, as shown in Figure 2.4. NPG had a much wider pore size distribution

than the ternary alloys, with a mean pore size of 17 nm. With the presence of Pt, the pore

size was significantly reduced (e.g., the alloy with 1 at. % Pt had a mean pore size of ca. 11

nm). As Pt has a much lower surface diffusion rate than Au, during dealloying the Pt

embedded in exposed terraces should segregate to the edges of the growing Au-rich

islands and stabilize them, ultimately reducing the length scale of porosity. [30] All

nanoporous structures were also observed by TEM (Figure 2.5 for NPG and the ternary

alloy with 1 at.% Pt) showing that the pore sizes (light regions) were ca. 18 and 8 nm for the

binary and ternary alloys respectively, in agreement with the BET analysis.

79

Figure 2.3 SEM images of freshly-formed nanoporous structures at 550 mV vs. MSE and 25 °C: (a) Ag77:Au23, (b) Ag77:Au22:Pt1, (c) Ag77:Au21:Pt2, (d) Ag77:Au20:Pt3. In all cases 5 C cm-2 were passed.

Table 2.2 Physical characteristics and developed surface area of different nanoporous structures. The numbers in brackets represent the 95% CI.

Precursor Mean ligament

width (nm)a

Mean

thickness of

the DL (um)b

Relative

shrinkage of

the DL (%)c

Surface area (by

CV, m2 g

-1DL)

d

BET surface

area (m2 g

-1DL)

e

Ag77:Au23 13.8 (0.7) 7.8 (0.2) 9.2 17.3 (0.8) 16.5

Ag77:Au22:Pt1 6.8 (0.4) 9.1 (0.9) 9.4 30.7 (1.3) 29.3

Ag77:Au21:Pt2 6.0 (0.2) 9.4 (0.2) 10.0 32.8 (0.8) 33.8

Ag77:Au20:Pt3 4.3 (0.2) 9.7 (0.5) 10.2 35.2 (1.4) 36.1

a,b The 95% CI was calculated from at least 30 measurements for each quantity. c The relative shrinkage of the layer was determined with respect to the theoretical thickness of the DL. d

Triplicate runs were done to calculate the 95% CI. e The BET surface area was obtained at P/Po = 0.05-0.1.

80

Figure 2.4 BET pore size distribution of freshly-formed nanoporous structures. The dealloying conditions for all alloys were the same - temperature: 25 °C, dealloying potential: 550 mV vs. MSE, charge passed: 5 C cm-2.

XRD patterns on the as-annealed and as-dealloyed samples are shown in Figure

2.6. As observed, the binary and the ternary alloy with the highest Pt content are both

polycrystalline materials with a face-centered-cubic (fcc) structure. In the case of the binary

alloy (Figure 2.6a), the as-annealed material was mainly oriented to the (220) plane,

whereas in the ternary alloy (Figure 2.6b), the precursor was mainly oriented to the (200)

plane. More importantly, for both alloys, the initial crystal face orientation was preserved

during dealloying. Additionally, there were some texture effects evident in the patterns, but

no foreign phases. The peak shifts that were observed in both alloys, but especially in the

case of the ternary alloy, are not fully understood at this point.

81

Figure 2.5 TEM images of freshly-formed nanoporous structures: (a) Ag77:Au23, (b) Ag77:Au22:Pt1. The dealloying conditions for both alloys were the same - temperature: 25 °C, potential: 550 mV vs. MSE, charge passed: 5 C cm-2.

82

Figure 2.6 XRD patterns of (a) Ag77:Au23 and (b) Ag77:Au20:Pt3 before and after dealloying. The as-annealed material in both cases is represented with a solid line, and the as-dealloyed material with a dotted line. The dealloying conditions for both alloys were the same - temperature: 25 °C, potential: 550 mV vs. MSE, charge passed: 5 C cm-2.

As observed in Table 2.2, there was not a significant difference in the thickness of

the DL in the ternary alloys; the binary alloy, on the other hand, showed a thinner layer that

could be explained based on the smaller amount of residual Ag that remained in the

ligaments. The elemental composition profiles across the DL were determined by TEM –

EDS on ultramicrotomed samples. Figure 2.7a shows an image of an ultramicrotomed

sample, in which the dealloyed layer and non-dealloyed material are clearly visible; the

insert in this figure shows more clearly the porosity of the DL. Figure 2.7b presents the Ag

compositional gradient for all alloys, showing that the ternary alloys always have a higher Ag

content than the binary alloy. Additionally, the Ag profile in the ternary alloys remained

rather constant with the different Pt concentrations. In NPG the Ag content increased from

ca. 28 at.% at the edge of the sample to around 57 at.% close to the dealloying front. For

83

the ternary alloys, on the other hand, the profile was less pronounced, increasing from ca.

47 at.% at the edge of the sample to ca. 56 at.% in the region close to the dealloying front.

This retained Ag must exist buried in the core of the ligaments, with a shell comprised of Au

and Pt. While dealloying was progressing and Ag was removed from the dealloying front,

the Ag removal from the already formed ligaments continued; however, because Pt reduces

Au mobility, the post-porosity coarsening was hindered, resulting in higher Ag contents. In

other words, the smaller Ag content in the NPG was due to exposure of buried Ag to the

electrolyte during coarsening behind the dissolution front. Consequently, the dealloyed layer

was thicker in the ternary alloys. Coincidently perhaps, the Ag content in the vicinity of the

dealloying front was close to the classical parting limit of ca. 55 at. % in NPG [48].

Associated with the thickness of the DL, the relative shrinkage of the layer is also shown in

Table 2.2. It was determined that there was no measurable difference between the

shrinkage in the different alloys; in other words, even though Pt did help in stabilizing the

structure (i.e., reducing the coarsening of the ligaments), it did not have a clear effect on the

shrinkage of the DL.

The changes in ligament size and thickness of the layer have an impact on the

developed surface area. As shown in Table 2.2, the surface area increased with Pt

concentration, with the largest increment occurring at the lowest Pt addition. The

nanoporous structure with 1 at. % Pt in the precursor had a surface area of 30.7 m2 g-1DL,

whereas NPG had a surface area of 17.3 m2 g-1DL. The structure formed from the alloy with

3 at. % Pt showed an increase of 15 % in surface area with respect to the lowest Pt content

alloy. The surface area measured by electrochemical means (i.e., CV in acidic media)

showed very good agreement with the BET surface area. Moreover, equally good

agreement was obtained with the capacitance ratio calculated from impedance

measurements before and after dealloying (results not shown here).

84

In the formation of nanoporous metals from Ag-Au-Pt alloys, the surface coverage of

Pt is another significant factor to evaluate. Hydrogen UPD, measured from CV using

standard procedures from the literature, was used to estimate the concentration of Pt on

ligament surfaces in 1 M H2SO4 (see Figure 2.8). Different scan rates were tested to confirm

that there were no significant mass transport (diffusional) limitations during the process. As

shown in Figure 2.8, the CV profiles showed obvious adsorption and desorption regions in

the potential region between -630 mV and -400 mV vs. MSE. As expected, the current

density in these regions increased with the Pt content of the precursor. By integrating the

charge associated with the hydrogen adsorption minus the contribution from the double

layer charging, Pt surface areas of approximately 1.4, 2.7 and 6.9 % of the real surface area

of the nanoporous structure (e.g., BET surface area in Table 2.2) were obtained for the

alloys with 1, 2 and 3 at. % of Pt respectively (see insert in Figure 2.8). XPS analyses in the

near-the-surface region of the ligaments yielded Pt contents similar to the ones obtained by

H-UPD analysis (see also insert in Figure 2.8).

The presence of Pt not only stabilized the structure against electrochemical

coarsening, but also conferred stability even in high-temperature environments. Treatment

of NPG at 400 °C in the presence of laboratory air for 2 h, increased the ligament size to

292 nm (immediately after dealloying the ligament size was ca. 14 nm). In similar conditions,

structures with Pt displayed much better resistance to coarsening: the alloy with 1 at. % Pt

coarsened to 33 nm whereas the ligament size of the alloy with 3 at. % Pt increased to 9

nm. Nevertheless, it was recently found that the presence of oxygen is a determinant

parameter in the thermal stability and main characteristics of nanoporous metals formed

from Ag-Au-Pt [49]. Coarsening after long periods in solution was also evaluated, showing

that the average ligament size for NPG was 30 nm after 2 months in HClO4 solution (no

85

potential applied). For ternary alloys, the ligament size increased to ca. 9, 7 and 5 nm for the

alloys with 1, 2 and 3 at. % Pt respectively.

Figure 2.7 (a) Ultramicrotomed sample; the insert shows a higher magnification of the region close to the dealloying front. (b) Residual Ag profile across the dealloyed layer of freshly-formed nanoporous structures. The Ag content was determined by TEM – EDS of ultramicrotomed samples, and 100% represents the original surface of the sample (SS); 0 % is the dealloying front (DF). Error bars represent 95% CI calculated from at least 3 different measurements across the dealloyed layer.

86

Figure 2.8 Cyclic voltammograms of the Ag-Au-Pt alloys after dealloying at 550 mV vs. MSE, 25 °C and passing a charge density of 5 C cm-2. In all cases, CV profiles were obtained in 1 M H2SO4 solution at 20 mV s-1 and 25 °C. The original Pt content is shown in the figure. The insert shows the fraction of Pt on the surface of the nanostructure, obtained by H-UPD and XPS analyzes, with respect to Pt content of the precursors. The error bars represent 95% CI calculated from triplicate runs.

2.3.3 Influence of dealloying parameters on the characteristics of the layer

In electrochemical dealloying, there are multiple parameters that affect the

nanoporosity formation and the characteristics of the resulting nanoporous structure. These

parameters include the dealloying potential, temperature and the dealloying charge density,

which includes an effect of time as well as depth. By adjusting these parameters, the

nanoporous structure is tunable in terms of the residual Ag in the nanoporous layer, the size

of the ligaments/pores, the depth of the dealloyed layer, and, consequently, the developed

surface area. Therefore, these parameters were examined through a series of controlled

experiments (see Section 2.2.1), in order to understand their influence on the characteristics

of the dealloyed layer. A detailed discussion is presented below.

87

2.3.3.1 Dealloying temperature

Simultaneously with the dissolution of the less-noble element in the alloy and the

nanoporosity formation, there is a coarsening process of the microstructure. This coarsening

is significantly obvious in NPG; in fact, Dursun et al. [50] estimated, based on theoretical

percolation considerations, that the initial pore size for Ag70Au30 dealloyed in HClO4 is ca.

1.6 nm, even though its typical final pore size is higher than 10 nm. This coarsening has

been described as a surface-diffusion dominated process at the alloy/electrolyte interface

that depends, among other parameters, in the surface diffusivity of Au and the temperature

of the electrolyte [50,51]. All this is in agreement with the literature for the relaxation of

roughened Au surfaces [52-54]. Recently, Qian and Chen [55] demonstrated that ultrafine

nanoporous Au, with a pore size of about 5 nm, can be produced by reducing the

temperature of HNO3 electrolyte from 25 to -20 °C in simple immersion exposures; it was

estimated that by this reduction in the dealloying temperature, the surface diffusivity of Au

was decreased by approximately two orders of magnitude. Conversely, by increasing the

dealloying temperature, more significant coarsening is expected.

Figure 2.9 shows SEM images for the binary alloy and the ternary alloys with ca. 1

and 3 at. % Pt, dealloyed at 10 and 60 °C. As expected, NPG exhibited significant

temperature dependence: after dealloying at 10 °C, the ligament size was ca. 12 nm,

whereas after dealloying at 60 °C, it increased to 28 nm (see also Figure 2.10a). The

presence of Pt in the ternary alloys minimized the effect that the electrolyte temperature has

on the coarsening of the nanostructure. The alloy with 1 at.% Pt in the precursor showed

only a 25 % increase in the ligament size for the same temperature range (from ca. 5.8 nm

at 10 °C to ca. 7.4 nm at 60 °C). A similar tendency was observed for the alloys with 2 and 3

at.% Pt. The thickness of the DL also changed with the dealloying temperature. In the case

of ternary alloys, it was observed that the thickness increased with temperature, whereas for

88

the binary alloy it decreased (Figure 2.10b). The changes in the depth of the layer were

associated with the residual Ag content of the layer (Figure 2.10c) in order to account for the

total charge passed. The average residual Ag content in the layer was obtained by SEM –

EDS assuming that the incident electron beam did not penetrate through the dealloyed

layer, exciting the substrate. Unfortunately, the beam penetration profile in porous materials

has not been well documented; however, the mean free path is likely to be larger than that

corresponding to a void-free metal. As shown in Figure 2.10c, the average residual Ag in the

dealloyed layer tended to increase with the dealloying temperature for the ternary alloys,

showing values between 52 and 55 at.%. It was hypothesized that this behaviour has to do

with the mobility of Pt at different temperatures: at 10 °C Pt is so immobile that it just sits

there and has to be dragged along by the step edges, whereas at 60 °C it starts to get some

limited mobility, so it more easily forms an efficient decoration of the step edges. NPG, on

the other hand, showed a decrease in the Ag content (from approximately 42 at.% at 10 °C

to 26 at.% at 60 °C), which was consistent with the observed tendency of the DL

morphology (i.e., reduction of the thickness with higher temperatures). These changes in

ligament sizes and thickness of the layer had a direct impact in the developed surface area

of the structure (Figure 2.10d). By increasing the temperature from 10 to 60 °C the surface

area of NPG decreased 60 % (from approximately 24 m2 g-1DL to 10 m2 g-1

DL). On the ternary

alloys, the surface area was always higher than in the binary alloys, although they also

showed a reduction with temperature.

By changing the dealloying conditions, not only can the ligament size be modified,

but moreover, by increasing the dealloying temperature the fraction of Pt atoms on the

surface of the ligaments increased. Figure 2.11 shows the fraction of Pt atoms on the

surface of the ligaments after dealloying at different temperatures. It is clear that by

increasing the dealloying temperature from 10 °C to 60 °C, the fractional coverage of Pt

89

atoms steadily increased for all alloys, doubling the Pt coverage at the upper temperature.

This increase in the Pt coverage on the surface could be associated with the dealloying

mechanism itself and/or other processes that were favored under these experimental

conditions (e.g., preferential adsorption of OH groups on Pt atoms).

Figure 2.9 SEM images of freshly-formed nanoporous structures: (a) Ag77:Au23 dealloyed at 10 °C; (b) Ag77:Au23 dealloyed at 60 °C; (c) Ag77:Au22:Pt1 dealloyed at 10 °C; (d) Ag77:Au22:Pt1 dealloyed at 60 °C; (e) Ag77:Au20:Pt3 dealloyed at 10 °C; (e) Ag77:Au20:Pt3 dealloyed at 60 °C. In all cases, the SEM images were taken approximately in the middle of the DL.

90

Figure 2.10 Ligament width, normalized surface area, thickness of DL and residual Ag in the

DL of freshly-formed nanoporous structures at different temperatures: (□) Ag77:Au23, (●)

Ag77:Au23:Pt1, (■) Ag77:Au21:Pt2, (▲) Ag77:Au20:Pt3. Error bars represent 95% CI calculated

from triplicate runs in the case of surface area, from at least 30 measurements in the case of the ligament size, from ca. 15 measurements for the thickness of the layer and from 4 different measurements across the dealloyed layer in the case of the Ag content. In all cases, the Ag content was measured approximately in the middle of the DL.

91

Figure 2.11 Fraction of Pt atoms on the surface of as-dealloyed structures after dealloying at different temperatures. In all cases, dealloying was carried out in 0.77 M HClO4, with a charge passed of 5 C cm-2 and a dealloying potential of 550 mV vs. MSE. The original Pt composition of the precursors is shown in the figure. Error bars represent 95% CI calculated from triplicate runs.

2.3.3.2 Dealloying potential

In electrochemical dealloying, the applied potential primarily reduces the activation

barrier required for the dissolution process of the less-noble element (i.e., energy required to

break bonds). If this potential is relatively high with respect to the ‘empirical’ critical potential,

and no oxidation of the more-noble element occurs, the rate of dissolution is significantly

faster than the diffusion rate [56], which favors the formation of porosity. If, on the contrary,

the applied potential is relatively close to the ‘empirical’ critical potential, the diffusion rate of

Au is sufficient to significantly block Ag dissolution creating an imbalance between these two

processes and making more difficult the porosity evolution. The effect of potential on Au

diffusivity in acidic electrolytes has been reported elsewhere [57].

The effect of the dealloying potential on the characteristics of the different

nanoporous structures was studied by reducing the applied potential by 50 mV with respect

92

to the benchmark potential (i.e., 550 mV). As observed in Figure 2.12, the ligament size in

NPG increased by reducing the overpotential, which was due to the longer time available for

coarsening at a lower current density and fixed charge density. The ligament width in NPG

increased from 14 nm at 550 mV to approximately 24 nm at 500 mV (see also Table 2.3).

For the ternary alloys, the SEM images (Figure 2.12c-f) showed that the the ripening

process is less significant. For instance, the ligament size for the alloy with 1 at.% Pt

increased from approximately 7 nm to 9 nm. Table 2.3 shows a comparison between all the

alloys.

Figure 2.12 SEM images of freshly-formed nanoporous structures at different dealloying potentials: (a) Ag77:Au23 at 500 mV, (b) Ag77:Au23 at 550 mV, (c) Ag77:Au22:Pt1 at 500 mV (d) Ag77:Au22:Pt1 at 550 mV, (e) Ag77:Au20:Pt3 at 500 mV, (f) Ag77:Au20:Pt3 at 550 mV. In all cases the charge density passed was 5 C cm-2. All SEM images were taken approximately in the middle of the DL.

93

With the coarsening of the structure, the resulting surface area also changed. As

shown in Table 2.3, the surface area decreased by approximately 54% in the binary alloy by

reducing the applied potential by 50 mV. By adding Pt to the precursor, the effect was

reduced (e.g., for the alloy with 3 at.% the surface area decreased only 36% by reducing the

applied potential). In the case of the ternary alloys, it was demonstrated in Table 2.3 that for

the conditions tested, the residual Ag concentration had a significant dependence upon

dealloying potential, showing higher Ag content at lower overpotentials. For the binary alloy,

on the other hand, the Ag content decreased upon reduction of the dealloying potential. In

NPG, at the same time that the structure was coarsening, Ag trapped in the core of the

ligaments was also exposed to the electrolyte, reducing the concentration of Ag in the DL.

This agrees with the fact than the DL gets thinner at lower overpotential. On the ternary

alloys, thanks to the hindering of coarsening, more Ag was trapped in the layer and,

consequently, thicker layers were observed (results not shown here).

Table 2.3 Physical characteristics, developed surface area and residual Ag of different nanoporous structures formed at different dealloying potentials. The numbers in brackets represent the 95% CI.

Precursor

500 mV vs. MSE 550 mV vs. MSE

Mean

ligament

width (nm)a

Surface area

(by CV, m2 g

-

1DL)

Average Ag in

the dealloyed

layer (by EDS,

at.%)c

Mean

ligament

width (nm)a

Surface area

(by CV, m2 g

-

1DL)

b

Average Ag in

the dealloyed

layer (by EDS,

at.%)c

Ag77:Au23 24.3 (1.1) 8.5 36.9 (0.4) 13.8 (0.7) 18.6 (1.5) 40.9 (1.6)

Ag77:Au22:Pt1 8.7 (0.3) 13.5 55.4 (0.9) 6.8 (0.4) 30.7 (1.3) 50.1 (0.7)

Ag77:Au21:Pt2 7.4 (0.2) 17.0 58.8 (0.7) 6.0 (0.2) 32.5 (1.4) 53.8 (1.2)

Ag77:Au20:Pt3 5.6 (0.2) 22.9 60.0 (0.7) 4.3 (0.2) 36.2 (1.4) 52.0 (1.0)

a The 95% CI was calculated from at least 30 measurements for each quantity. b Triplicate runs were done to calculate the 95% CI. c

The 95% CI was calculated from 4 different analyses in the dealloyed layer.

94

2.3.3.3 Dealloying charge

Even though it is recognized that the diffusion of the more-noble element(s) and the

dissolution of the less-noble ones are altered by changes in electrolyte temperature and

applied potential, the charge passed has an indirect impact in the ligament width and on the

resulting surface area. By increasing the charge density, which is directly proportional to the

amount of Ag removed from the alloy, there is more time for coarsening of the ligaments

behind the dealloying front. This was especially true in the case of NPG in which the

average ligament size after removing 2.5 C cm-2 was 12 nm, but when removing 40 C cm-2

the average ligament size increased to 21 nm (see SEM pictures in Figure 2.13a,b and

Figure 2.14a). In this particular case, the time ratio between the higher and lower charge

was approximately 25. The presence of Pt, on the other hand, suppressed the relative

coarsening of the ligaments (Figure 2.13). The average ligament size for all ternary alloys

was below 10 nm.

Figure 2.14b shows the relation between the amount of charge passed during

dealloying and the thickness of the DL. It was observed that the thickness of the layer in

ternary alloys was always thicker than in the NPG, which could be explained due to the

higher retained Ag content of the layer (Figure 2.14c). In all cases, the Ag content tended to

decrease when the dealloying charge increased, but the binary alloy had much lower Ag left

in the layer. For instance, when removing 20 C cm-2, the average retained Ag in the binary

alloys was ca. 28 at.%, while the alloy with 1 at.% Pt has approximately 45 at.% of Ag left.

SEM – EDS line scans across the dealloyed layers (results not shown here) confirmed that

the concentration gradient in the ternary alloys is not as pronounced as it is in the Ag-Au

alloy.

95

Figure 2.13 SEM images of freshly-formed nanoporous structures after passing different dealloying charges: (a) Ag77:Au23 passing 2.5 C cm-2; (b) Ag77:Au23 passing 40 C cm-2; (c) Ag77:Au22:Pt1 passing 2.5 C cm-2; (d) Ag77:Au22:Pt1 passing 40 C cm-2; (e) Ag77:Au20:Pt3 passing 2.5 C cm-2; (f) Ag77:Au20:Pt3 passing 40 C cm-2. In all cases, the SEM images were taken approximately in the middle of the DL.

The developed surface areas for all alloys, at different dealloying charges, are

shown in Figure 2.14d. Even though the thickness of the layer increased by increasing the

dealloying charge, the coarsening effect gained importance, reducing the surface area of

the nanoporous structure. The normalized surface area of NPG decreased by approximately

40% when the charge density increased from 2.5 C cm-2 to 40 C cm-2. For the alloy with 1

at.% Pt, the surface area only decreased by 19 % (from 32 m2 g-1DL when 2.5 C cm-2 were

passed to 26 m2 g-1DL when 40 C cm-2 were passed). The other two ternary alloys had

similar trends, but they displayed slightly higher surface area. Approximately 70 C cm -2 were

needed to dealloy the sample all the way through.

96

Figure 2.14 Average ligament width, normalized surface area, thickness of DL and average residual Ag in the DL of freshly-formed nanoporous structures at different dealloying

charges: (□) Ag77:Au23, (●) Ag77:Au22:Pt1, (■) Ag77:Au21:Pt2, (▲) Ag77:Au20:Pt3. In all cases,

the surface area was measured by CV in 1M HClO4 at 25 ºC. Error bars represent 95% CI calculated from triplicate runs in the case of surface area, from at least 30 measurements in the case of the ligament size, from ca. 15 measurements for the thickness of the layer and from 4 different measurements in the case of the average Ag content of the layer. In all cases, the Ag content was measured approximately in the middle of the DL.

In summary, by changing the dealloying charge, especially in the case of the Ag-Au

alloy, characteristics such as the retained Ag content and the thickness of the dealloyed

layer can be modified. For the ternary alloys however, it was found that even when large

97

charges are removed, the main characteristics of the dealloyed material (e.g., Ag content,

ligament size) remain relatively constant.

2.4 Conclusion

(a) The ‘empirical’ critical potential does not show any significant dependence on the Pt

content of the alloy.

(b) The addition of Pt to the Ag-Au precursor, thanks to the lower surface diffusivity of Pt

with respect to Au, refines the ligament size and stabilizes the nanoporous structure

against coarsening, even under experimental conditions that promote coarsening of the

structure (e.g., exposure to air at elevated temperature). By adding less than 1 at. % Pt

to the precursor, the ligament width reduces in 51% with respect to the binary

counterpart.

(c) Dealloyed layers formed from ternary alloys have higher Ag contents than those formed

from NPG. This is associated with the hindered post-porosity coarsening of the

structure thanks to the presence of low-mobility Pt. Moreover, the Ag content across the

dealloyed layer in ternary alloys shows a less pronounced gradient than in the binary

alloy.

(d) By controlling parameters like the dealloying temperature, charge density and

dealloying potential, the morphology and characteristics of the nanoporous structure

can be modified either by changing the ligament size, surface area and/or the

composition of the dealloyed layer.

98

2.5 Acknowledgements

The authors would like to acknowledge the Natural Sciences and Engineering

Research Council (NSERC) of Canada for financial support, the Centre of Nanostructure

Imaging at the University of Toronto for their help imaging the nanoporous structures, and

Ames National Laboratory - US Department of Energy for preparing most of the alloys used

in this research.

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2 A version of this chapter has been published.

Vega, A. A., and Newman, R. C. Beneficial effects of the surface segregation of Pt in nanoporous metals fabricated by dealloying of Ag-Au-Pt alloys, Journal of the Electrochemical Society 2014, 161(1), C11.

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Chapter 3. Beneficial Effects of the Surface Segregation of Platinum in

Nanoporous Metals Fabricated by Dealloying of Ag-Au-Pt

Alloys2

3.1 Introduction

For our present purposes, nanoporous metals may be defined as bulk nanostructured

materials that are formed by electrochemical selective dissolution or dealloying of

homogeneous alloys. During dealloying, the less-noble element is removed from the alloy,

at the same time that the remaining element (or elements) undergoes a self-organization

process (i.e., driven by surface diffusion), forming a three dimensional bicontinuous porous

metallic network with fully interconnected channels. Among the different nanoporous metals

that can be made, nanoporous gold (NPG), typically formed by dealloying of Ag-Au alloys,

has a very distinctive structure and remarkable properties that have caught the attention of

the scientific community since experimental work on the precursor alloy, and on the

corrosion-derived nanoporous Au, was done to study the mechanism of alloy corrosion [1-

8]; however, coarsening of the porosity by surface self-diffusion of Au becomes a major

problem and a potential limiting factor for long term functionality [9-10]. In the past, it has

been shown that the presence of small amounts of ternary elements (e.g., arsenic in α-

brass) slows down the surface diffusion of copper improving the dezincification resistance of

the material [11,12]; therefore, the idea of adding a ternary element to the NPG to improve

the coarsening resistance seems reasonable. Snyder et al. [13] first reported that by adding

Pt (~6 at.%) to the ‘white-gold’ type of Ag-Au alloy (i.e., Au content of 35 at.%), the

coarsening of the nanoporous layer is significantly reduced. Pt, which does not dissolve, is

105

immobile relative to Au and blocks step edges, creating a porous structure with a smaller

length scale and higher stability. Moreover, the presence of Au and Pt on the surface of the

nanostructure may have synergistic effects that can lead to superior properties if compared

with the pure metals [14]. Recently, we have characterized Ag-Au-Pt alloys with 77 at. % Ag

and Pt contents of ca. 1, 2 and 3 at.%, with the idea of understanding better the real role of

Pt on the characteristics of the resulting nanoporous structures [15].

In nanoporous metals, as well in many other nanometre scale materials, in which the

distinction between surface and bulk regions becomes indistinct, changes in the

reaction/environment conditions can induce changes in the structure and/or composition of

the surface. Under specific conditions, it is possible that one or more of the alloy

components enriches in the surface region, and not necessary in accordance with the

behaviour of a flat alloy crystal. This (possibly modified) form of surface segregation has a

significant impact in a number of technologically important applications such as

heterogeneous catalysis. Among different metallic nanomaterials, surface segregation

studies in Au-based bimetallic/multimetallic systems have been done recently; for instance,

in Au-Pt nanoparticles it has been found that Au is thermodynamically favored to segregate

to the surface of the alloy thanks to the slightly larger size of Au, and lower surface energy

relative to Pt [16-18]. The same trend has been also observed in Au-Pt flat alloy surfaces

[19-21]. The presence of an adsorbed layer however, can induce significant changes with

respect to the underlying trends. Recently, Dhouib and Guesmi [22] reported, based on

Density Functional Theory (DFT) calculations, that Ni, Pt and Pd should segregate to the

surface of M-Au alloys in the presence of oxygen. Experimental data on such phenomena

are scarce; however, Wang et al. [23] reported that Pt can even segregate to the surface of

multimetallic Au/FePt3 nanoparticles thanks to the adsorption of oxygen and/or OH- groups.

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In this article, we report on the thermally-activated Pt surface segregation

phenomenon seen in nanoporous structures formed from Ag-Au-Pt alloys. It is shown that

by exposing nanoporous metals to surprisingly low temperatures in the presence of

laboratory air, there is a clear tendency of Pt to segregate to the surface of the ligaments.

Some of the results included here are obtained from underpotential deposition - UPD - of

hydrogen, X-ray photoelectron spectroscopy (XPS) and low energy ions scattering (LEIS),

besides the comparison with the equivalent thermally driven process in absence of oxygen.

Our focus is the interplay of surface composition and nanoscale morphology: surface

segregation in nanoporous metals modifies the thermally induced coarsening of the

ligaments as well as their surface chemical composition.

3.2 Materials and methods

Ag-Au-Pt alloys with Pt contents of ca. 1, 2 and 3 at.% Pt and 77 at.% Ag were

obtained as cold-rolled 200 µm sheet from Ames National Laboratory - US Department of

Energy, Iowa, USA. The composition of all alloys was determined by an electron probe X-

ray microanalyzer (EPMA – CAMECA SX-50/51) equipped with 3 wavelength dispersive

spectrometers (WDS) and operated at 20 kV and 30 nA. Calibration with proper standards

(i.e., Ag80Au20 alloy, and elemental Pt) was done prior to any measurement. Probe for EPMA

software (Probe Software Inc.) was used for data analysis. Before any electrochemical

experiment, all specimens were annealed in Ar-H2 (2.5 % H2; balance Ar) at 975 °C for

approximately 15 h. All specimens were then used in the as-annealed condition without any

further surface preparation. Each sample (approximately 4 mm by 10 mm strips) was

attached to a copper wire for electrical connection, using lacquer (SPI Miccroshield) to mask

the junction. In all cases, dealloying was done from both sides of the sample.

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The dealloying process was carried out in 0.77 M HClO4 solution prepared from

Analar grade HClO4 (Alfa-Aesar, 62%). All solutions were prepared with 18 MΩ∙cm de-

ionized water and de-aerated by high-purity nitrogen purging (min. purity: 99.998%). All

electrochemistry was performed using a Gamry Reference 600TM potentiostat, using an

electrochemical cell with a volume of approximately 500 mL; a Pt wire was used as a

counter electrode (CE) and mercury/mercury sulfate (MSE, 640 mV vs. SHE) was used as a

reference electrode. The reference electrode was housed in a separate compartment and

connected to the electrochemical cell via a Luggin probe. All of the specimens were

dealloyed at 25 °C, passing an anodic charge density of 5 C cm-2 at 550 mV vs. MSE.

Immediately after dealloying, the connecting wire of selected samples was removed

and the specimens were thoroughly rinsed with de-ionized water and dried with air before

exposed them to moderately elevated temperature (200 – 600 °C). In the majority of the

cases, samples were heated in laboratory air; however, analysis of some samples heated in

Ar-H2 mixture (2.5 % H2; balance Ar) was done to study the effect of oxygen on the surface

composition evolution of the nanoporous material.

After the exposure to temperature, each sample was reconnected to a copper wire (as

described before) to measure the true area of the specimen and the fraction of Pt atoms on

the surface by classical electrochemical methods. The true area of the electrodes was

estimated by means of voltammetric profiles in the double layer region of potentials (i.e., -

240 to 50 mV vs. MSE) at different scan rates in 1 M HClO4 solution [24,25]. In all cases, 28

µF cm-2 was considered the baseline double-layer capacitance for polycrystalline Au and Pt

[24]. This assumption proved to be valid after comparing the results with BET surface area

and impedance measurements [15]. The equivalent area of Pt on the surface of the

ligaments was estimated by UPD of hydrogen [26,27]. All specimens were immersed in 1 M

H2SO4 solution (EMD, 95-98%) in a three-electrode cell with a MSE reference electrode, a

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Pt wire as CE and the dealloyed specimen as a working electrode. The solution was de-

aerated for approximately 15 min before the experiment. Cyclic Voltammetry (CV) curves

were obtained at 25 °C between -630 and 0 mV vs. MSE, at a scan rate of 20 mV s-1. The

equivalent area of Pt on the surface of the structure was calculated by integrating the

hydrogen adsorption/desorption regions to obtain the charge related with the formation of a

hydrogen monolayer, assuming that the charge associated with the monolayer formation in

polycrystalline Pt was 210 µC cm-2 [28,29]. The fraction of Pt on the surface of the resulting

structure was estimated by dividing the equivalent area of exposed Pt by the estimated true

area of the electrode. The roughness factor (Rf) was determined by dividing the true area of

the electrode by its geometrical area. For some specific samples, an electrochemical

reduction of any oxidized species on the surface was carried out at ~-300 mV vs. MSE in 1

M H2SO4 solution right after the heat treatment. A qualitative determination of Ag on the

surface of the ligaments was done by means of CV in 1 M NaOH solution (Alfa-Aesar, 97%)

between -200 mV and 600 mV vs. mercury/mercury oxide (Hg/HgO – 20% KOH, 100 mV

vs. SHE) at a scan rate of 50 mV s-1 [30].

For the microstructural characterization of the nanoporous layer (e.g., ligament size),

dealloyed samples from all alloys were manually broken under tension in air and the fracture

surface was photographed using a scanning electron microscope (SEM – Hitachi S-5200)

with an accelerating voltage of 20 kV. The SEM pictures were later analyzed with the Image

Tool software (provided by The University of Texas Health Science Centre in San Antonio,

USA) to accurately determine the ligament size (here equivalent to the average width

perpendicular to the ligament edge). The composition profile across the layer was

determined on a transmission electron microscope (TEM - Hitachi HD-2000) complemented

with an Oxford INCA X-sight energy dispersive X-ray spectrometer (EDS). For that purpose,

selected samples were embedded in low viscosity resin (SPI-PON 812) and microtomed to

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30 – 50 nm thickness using the Leica UltraCut R instrument equipped with a diamond knife.

An accelerating voltage of 200 kV was used.

For some selected samples, X-ray photoelectron spectrometer (XPS – Thermo

Scientific Theta Probe) and high sensitivity low energy ion scattering (HS-LEIS – Qtac100)

were used for surface characterization. For both techniques, dealloyed/heat-treated

samples were placed in the XPS/LEIS chamber without any additional surface treatment.

For the XPS, a monochromatized Al K X-ray (h = 1486.68 eV) was used as an excitation

source and the analyzer pass energy was fixed at 30 eV. The calibration of the equipment

was done with elemental Au, Cu and Ag to cover the full energy spectrum. Thermo

AdvantageTM software (version 4.78) was used for the elemental quantification and for peak

fitting estimation. Angle-resolved XPS analysis was done to generate a non-destructive

depth profile of the composition of the near-surface region in the nanoporous structure. AR

Process software (version 5.59) was used to process the angle-dependent XPS data. LEIS

experiments were performed using three different ions depending on the required

information: for the general survey of the surface, 4He+ was used with an energy of 3 keV

and with a target current of 5 nA; for the better resolution of the Au-Pt spectra, 20Ne+ was

used with an energy of 5 keV and a target current of 1 nA, and for the separation of the Au

and Pt signal, 40Ar+ was used to improve the sensitivity of the analysis with an energy of 5

keV and with a target current of 1 nA. In all cases, the analyzed area was approximately 1

mm2 and the measurement time was 45 s.


Figure 3.1a shows a SEM image of the nanoporous metal formed by dealloying of Ag-

Au-Pt alloy with ca. 1 at.% Pt. In our previous work [15], we reported that the average

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ligament size of these nanostructures was ca. 6.8 nm, whereas the ligament size formed on

the Ag-Au alloy with the same Ag content was ca. 14 nm. As Pt has a much lower surface

diffusion rate than Au, during dealloying the Pt embedded in exposed terraces should

segregate to the edges of the growing Au-rich islands and stabilize them, ultimately

reducing the length scale of the resulting structure. As a result, the ligament size decreased

with increasing Pt content in the precursor (e.g., the ligament size obtained from the

precursor with 3 at.% Pt was ca. 4.3 nm). In Figure 3.1b, a lower magnification image of the

nanoporous layer is shown, in which the interconnected pore/ligament structure is evident.

In the insert of this figure, a TEM image suggests that the mean pore size of this structure

was about 8 nm. The pore size also decreased with increasing the Pt content in the

precursor.

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Figure 3.1 (a) SEM image of the freshly-developed nanoporous structure obtained by dealloying of Ag77:Au22:Pt1. (b) Dealloyed layer after ultramicrotoming a sample; in this case, the dealloyed layer rolled as a consequence of the low cutting velocity used during the process. The insert shows a TEM image of the nanoporous structure. The dealloying conditions in all cases were the same – dealloying charge density: 5 C cm-2, temperature: 25 °C and dealloying potential: 550 mV vs. MSE.

In accordance with the dealloying mechanism [2,31,32], the surface of nanoporous

dealloyed metals (i.e., the ligament surfaces) should be almost entirely covered with the

more-noble metal (s). In the case of nanoporous metals formed from Ag-Au-Pt alloys, the

surface of the ligaments should be mostly covered with Au and Pt; therefore, one

particularly interesting aspect of these nanostructures is the concentration of Pt atoms on

the surface, as well as the Pt content across the dealloyed layer. It was expected that the

112

higher the Pt content of the precursors, the higher the coverage of Pt on the surface of the

ligaments. In agreement with this expectation, for simple dealloying without any post-

treatment, the fraction of Pt on the surface of the ligaments increased from 1.6 % to 3.0 %

when the Pt content of the precursor increased from 1 at.% to 2 at.% (Figure 3.2a). The

highest Pt content alloy had 6.6 % of Pt on the surface. The Pt composition profiles across

the dealloyed layer (Figure 3.2b) were determined by TEM – EDS on ultramicrotomed

samples (see Figure 3.2c). In Figure 3.2c, the substrate (i.e., non-dealloyed region) and the

dealloyed layer are clearly visible; the insert in this figure shows evidence of the porosity of

the layer. Figure 3.2b shows that the Pt compositional concentration remained relatively

constant across the dealloyed layer for all alloys. In the case of the lowest Pt content alloy,

the Pt content in the region close to the surface of the sample was ca. 3 at.%, whereas in

the vicinity of the dealloying front it was ca. 2 at.%. In the case of the highest Pt content

material, the average composition across the layer was ca. 8.5 at. %. In all cases, the ratio

Au to Pt was nominally the same as the original alloy, confirming that only Ag was removed

during the corrosion process. The average amount of retained Ag in the layer was ca. 50 at.

% for the alloy with originally 1 at. % Pt; whereas for the alloy with the highest Pt content it

was ca. 52 at. % [15].

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Figure 3.2 (a) Fraction of Pt atoms on the outermost surface of the ligaments. (b) Pt content across the dealloyed layer obtained by TEM – EDS in ultramicrotomed samples. (c) Ultramicrotomed sample; the insert shows a TEM image of the dealloyed layer showing a highly dense porous structure. For Figures (a) and (b) error bars represent the 95% confidence interval (CI): in the case of (a) the CI was obtained by triplicate runs, for (a) the CI was obtained from at least 3 points across the dealloyed layer - 0 % represents the dealloying front (DF) and 100 % represents the original surface of the sample (SS). The nomenclature in the case of (b) is based on the original composition of the alloys.

3.3.1 Surface chemistry of nanoporous metals after heat treatment

The presence of Pt on the surface of the ligaments impacts not only the resistance to

coarsening of the structure, but it might also have an effect on its reactivity. These two

114

factors would have a major role in the potential applications of these materials in areas as

demanding as catalysis and sensing.

It was observed that the content of Pt on the surface of the ligaments can be tuned by

exposing freshly-prepared nanoporous metals to moderately elevated temperature (< 500

°C) in the presence of air (see Figure 3.3a). When nanoporous structures were exposed to

elevated temperature, there was a mixing between the top surface of the ligaments and the

layers underneath; during that process, Pt was pinned on the surface of the resulting

structure due to its strong interaction with oxygen. It was surprising that this segregation

occurred at such relatively low temperatures, where the lattice diffusion is quenched;

however, there may be analogous phenomena in nanoparticles systems. As suggested in

literature [22], and shown below in section 3.3.3, if oxygen was not present, Pt did not

segregate to the surface. This adsorbate-induced surface segregation phenomenon, to the

best of our knowledge, has not been reported before for nanoporous metals. Besides the

clear tendency of Pt to segregate to the surface at temperatures lower than 500 °C, CV in 1

M NaOH solution suggested that Ag was also exposed to the surface of the ligaments,

which is a phenomenon that has been previously reported in NPG [33]. This was concluded

after comparing a distinctive cathodic peak (located at ~150 mV vs. Hg/HgO) between the

different nanoporous structures and a pure Ag electrode (these voltammograms were

consistent with studies done specifically on Ag electrodes in NaOH solutions [30]).

Moreover, it was observed that the tendency of Ag to segregate to the surface of the

ligaments was minimized in the alloys with higher Pt content in the precursor.

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Figure 3.3 (a) Fraction of Pt on the surface of the nanoporous structures after exposure to different temperatures for 2 h; (b) roughness factor (Rf) of the nanostructures; (c) time dependence of the segregation phenomenon at 425 °C; (d) roughness factor (Rf) as a function of exposure time at 425 °C. The different alloys, based on their original composition, are represented as follows: (●) Ag77:Au22:Pt1, (○) Ag77:Au21:Pt2, (♦) Ag77:Au20:Pt3. In all cases, laboratory air was used. Error bars represent 95% CI calculated from triplicate runs.

As shown in Figure 3.3a, the maximum coverage of Pt was obtained in the

temperature range between 425 °C and 475 °C showing values of ca. 10%, 19% and 32%

for the alloys with ca. 1, 2 and 3 at.% Pt in the precursors, respectively. These maximum

concentrations of Pt atoms on the surface of the ligaments were supported by our

calculations regarding the maximum coverage of Pt that was possible in accord with the

concentration of Pt on the bulk of the ligaments (Figure 3.3b). At slightly higher

temperatures (i.e., 500 °C and 600 °C), the fraction of Pt significantly decreased. At 600 °C,

for instance, it was determined that the maximum fraction of Pt was ca. 4.0 % for the alloy

116

with 3 at.% of Pt in the precursor. In this range of temperatures, the desorption of oxygen

from surface Pt atoms was responsible for the reduction of Pt coverage [34-36], reverting to

the usual Au-enrichment behaviour of binary Au-Pt in vacuum [16-18]. The roughness factor

(Rf), indicative of the amount of thermal coarsening of the nanoporous structure, decreased

for all alloys with increasing temperature, as shown in Figure 3.3b; nevertheless, it was

observed that the resistance to coarsening increased with the Pt content of the precursor.

SEM pictures of some selected samples are shown in Figure 3.4. The ligament width

in the alloy with 1 at.% Pt (Figure 3.4a, b and c) increased from ca. 7 nm to 38 nm when

exposed to 425 °C; at 500 °C however, the ligament width increased to ca. 380 nm. In the

case of the alloy with 3 at.% Pt in the precursor (Figure 3.4d, e and f), the ligament size only

increased from 4 nm to 9 nm at 425 °C. After exposure to 500 °C, the average ligament

width increased to ca. 14 nm. As a comparison, and in order to see the ability of NPG to

coarsen, the ligament size in the binary alloy after annealing for 2 h at 425 °C in air was ca.

370 nm (see later in the discussion). This is in agreement with coarsening trends for NPG

under similar experimental conditions [37].

As demonstrated in Figure 3.3c, the segregation of Pt to the surface of the ligaments

happened very quickly at 425 °C. In the first 30 minutes of exposure the Pt concentration on

the surface increased by a factor of four; at longer times, small changes were obtained with

no further significant changes after 2 h of exposure. The same was true for the change in

the Rf (Figure 3.3d). It was also determined that nanoporous structures exposed for almost

70 h at 300 °C (see Figure 3.5 for the higher Pt content alloy) did not show any significant

difference in the Pt composition with respect to the fraction obtained after 2 h (i.e., ca. 17

%). This result suggested that the structure was in equilibrium at even lower temperatures

than the temperature that showed the maximum Pt coverage (i.e., 425 °C). At this moment,

117

however, the transport mechanism that these Pt atoms follow is still unknown for us,

especially because at these temperatures the lattice diffusion coefficients are very small.

Figure 3.4 SEM images of the nanoporous structures formed from Ag77:Au22:Pt1 alloy (a, b, c) and from Ag77:Au20:Pt3 alloy (d, e, f). Images (a) and (d) show the structures right after dealloying; images (b) and (e) show the structures after annealing at 425 °C in the presence of laboratory air; images (c) and (f) show the structures after annealing at 500 °C in the presence of laboratory air. For all the annealed structures, the exposure time was 2 h.

118

Figure 3.5 Fraction of Pt on the surface of the nanoporous structure formed on Ag77:Au20:Pt3 after exposure to 300 °C for different times. The dealloying conditions were the following: charge passed 5 C cm-2 at 25 °C and 550 mV vs. MSE. In all cases, laboratory air was used. The error bar represents 95% CI calculated from triplicate runs.

3.3.2 XPS and LEIS characterization of the surface of nanoporous metals

XPS analysis was done on samples with 1 and 3 at.% Pt in the precursor before and

after different heat treatments. XPS spectra were taken at a number of different take-off

angles to increase the relative surface sensitivity; nevertheless, Figure 3.6 only shows the

spectra taken at approximately 48 ° as an example. It was estimated that with this take-off

angle we were probing skin depths of < 5 nm. A more detailed surface composition of

ligaments with micro-XPS will be beneficial for the characterization of this phenomenon; this

constitutes part of our future work with these materials. As demonstrated in Figure 3.6a and

Figure 3.6b, after heat treatment at 425 °C for 2 h in the presence of laboratory air, there

was a stronger signal for Pt in the lower and higher Pt content structure than that

immediately after dealloying. In both structures, it was observed that the Pt4f spectral region

has at least five strong peaks; these peaks disappeared after annealing the lower Pt content

119

sample at 500 °C, but remained in the case of the other alloy (Figure 3.6b). These peaks

were due to the formation of different adsorption states of Pt-O on the surface. Figure 3.7a

shows the curve fitting for those Pt peaks showing slightly different binding energies than

previously reported in literature [38-42]. These differences could be due to the presence of

Au and/or Ag and the high concentration of step Pt atoms on the surface [43-45]. At high

enough temperature, oxygen desorbed (presumably between 500 and 600 °C) allowing the

surface to partially regain its metallic form, which was particularly evident after annealing the

higher Pt content structure at 600 °C (see Figure 3.7b). At this temperature, a peak at ca.

70.6 eV and another one at ca. 74.0 eV were in close agreement with the binding energies

for metallic Pt; however, there was still some evidence that after exposure to this

temperature some other species still remained on the surface of the nanostructure (peak at

77.5 eV). A similar effect was observed after electrochemically reducing the surface oxides

(at ~-300 mV vs. MSE in H2SO4 solution) from the as-annealed sample (at 425 °C). Right

after the sample was annealed at the pre-selected temperature, it was carefully reconnected

to a copper wire and immersed in 1 M H2SO4 solution; immediately after the electrochemical

reduction, the sample was analyzed in XPS showing only two strong peaks with binding

energies ca. 70.6 eV and ca. 74.0 eV (Figure 3.7c).

Figure 3.6c and Figure 3.6d show the XPS spectra for Ag3d in both alloys. As the

temperature increased, the intensity of the signal increased suggesting that Ag also tended

to segregate towards the surface of the structures, but without a specific interaction with

oxygen. In the lower Pt content alloy there was a stronger signal for Ag than in the other

alloy, in agreement with the tendency determined by our qualitative determination of Ag on

the surface (CV in NaOH solution). No chemical shift was observed in the Ag3d, or Au4f

spectra (not shown here), under any of the experimental conditions used here. The O1s

spectra are shown in Figure 3.6e (for the alloy with 1 at.% Pt) and Figure 3.6f (for the alloy

120

with 3 at.% Pt). After annealing these two structures in air at 425 °C, there was a shift in the

binding energy of oxygen with respect to the signal obtained in the sample right after

dealloying (from ca. 532 eV in the sample after dealloying to less than 530 eV for the

annealed sample). This peak has been associated with the adsorbed oxygen on Pt surfaces

[39]. By annealing the alloy with originally 1 at.% Pt at 500 °C, the signal for O1s decreased

and slightly shifted again to higher binding energies. The other alloy did not show a

significant change in the O1s at 500 °C, but at 600 °C there was a decrease in the peak

signal with its corresponding shift. The electrochemical reduction of surface oxide species

also induced a shift in the O1s peak to higher binding energies (Figure 3.7d).

Figure 3.6 Pt4f, Ag3d and O1s photoelectron spectra of nanoporous structures on Ag77:Au22:Pt1 alloy (a, c, e) and Ag77:Au20:Pt3 alloy (b, d, f) at different conditions: as-dealloyed structures (▬▬); after exposure at 425 °C ( ); after exposure at 500 °C ( ) and after exposure to 600 °C ( ). In the case of high temperature experiments, they were done in laboratory air for 2 h. All spectra were obtained at a take-off angle of 48 °.

121

Figure 3.7 A curve-fit of the Pt4f surface level spectra obtained after exposure of the as-dealloyed material to (a) 425 °C and (b) 600 °C. (c) Pt4f spectra and (d) O1s in which the solid line represents the photoelectron spectra of nanoporous structure annealed at 425 °C and the dotted line represents the spectra after electrochemically reducing the surface oxides in 1 M H2SO4. In all cases, samples were annealed for 2 h in laboratory air. All spectra were obtained at a take-off angle of 48 °on Ag77:Au20:Pt3 alloy.

Figure 3.8 shows the Pt composition obtained by XPS at different take-off angles.

Evidently, the Pt content in the near-surface of the ligaments depended on the exposure

temperature; however, it was not obvious to what extent the Pt is enriched in only the

outermost atom layer. Using the angle-resolved XPS data, we generated a depth profile

analysis (not shown here) for the lower and higher Pt content alloys. The maximum entropy

algorithm was used to simulate the angle-dependent photoemission intensity [46]. As

pointed out before, it has been shown, both by theoretical and experimental approaches,

that roughness could have a significant effect on the outcome of the analysis [47-49];

122

nonetheless, no correction for the surface roughness was done at this stage. It was found

that indeed, the predicted Pt compositions on the top surface of the structure agreed very

well with our hydrogen UPD values; however, at this moment we cannot make any definitive

conclusion about the Pt enrichment in the outermost atom layer.

Figure 3.8 XPS Pt4f composition with respect to the take-off angle for (a) Ag77:Au22:Pt1 and (b) Ag77:Au20:Pt3 and for different experimental conditions: (◊) right after dealloying, (●) after exposure at 425 °C, (○) after exposure at 500 °C and (▲) after exposure to 600 °C. In the case of all high temperature conditions, laboratory air was used for an exposure time of 2 h.

In addition to the XPS analysis, a LEIS analysis performed on the higher Pt content

alloy is shown in Figure 3.9. This analysis was done in specimens before (right after

dealloying) and after segregation of Pt (at temperatures > 200 °C). Figure 3.9a shows a

general survey of the surface, indicating the positions of the peaks for O (ca. 1100 eV), Ag

(ca. 2550 eV) and Au-Pt (ca. 2700 eV). Evidently, the signal for oxygen got stronger after

exposure of the as-dealloyed samples to the heat treatment; moreover, the Ag peak

increased with increasing temperature. In the absence of oxygen (sample exposed to 425

123

°C in Ar-H2), the signal for Au-Pt was very weak (see Section 3.3.3 below), whereas the

sample exposed to 425 °C in air had the strongest Au-Pt signal. It is important to mention

however, that this analysis was done without any surface cleaning to avoid any damage to

the surface; therefore, the presence of hydrocarbons on the surface masked some of the

signal for the analysis (especially for Pt and Au). Figure 3.9b shows the signal of Au-Pt for a

different set of sample after targeting the surface with 20Ne+ ions to obtain better resolution;

as expected, the as-dealloyed sample and the sample exposed to 425 °C had the stronger

signal, whereas a sample exposed at 500 °C showed a very weak signal (this sample had a

very disruptive layer of hydrocarbons on the surface). The insert in this figure shows a better

sensitivity analysis of the Au-Pt spectra, which allowed us to separate the two signals: in the

as-dealloyed sample, the signal consisted mainly of Au; the surface of the sample after

exposure to 425 °C, on the other hand, contained mainly Pt. In both cases, a comparison

with appropriate standards was made (see figure). In general, all these results are in

agreement with our main findings; nevertheless, a gentle cleaning of the surface will be

beneficial to have a better signal to noise ratio.

124

Figure 3.9 (a) LEIS general survey for the highest platinum content alloy before and after exposure to moderately high temperature; in this case, the surface of the samples was bombarded with 3 keV 4He+ ions. (b) Au-Pt spectra after bombarding the surface of the samples with 5 keV 20Ne+ ions; the insert in this figure shows the Au-Pt spectra after bombarding the surface with 5 keV 40Ar+ ions to improve the sensitivity of the analysis. The signals for Au and Pt standards are also included in the figure. The details about the different experimental conditions are shown in the figure. For all high temperature experiments, samples were exposed for 2 h in the presence of laboratory air unless otherwise state. In all cases, analyses were done without any surface cleaning to avoid any ion-induced mixing of the noble metals.

3.3.3 Surface segregation in the absence of oxygen

The absence of oxygen severely reversed the segregation of Pt to the surface of the

ligaments. Figure 3.10 shows the fraction of Pt on the surface of the ligaments after

125

exposure of Ag77:Au20:Pt3 samples to an Ar-H2 atmosphere at different temperatures.

Clearly, the concentration of Pt on the surface was much smaller than the one observed in

the presence of air (i.e., there was de-segregation of Pt in Ar-H2 atmosphere, as expected

from literature studies on binary Au-Pt alloys [17,20]). For example, after 2 h at 425 °C, the

fraction of Pt was ca. 2 at. % whereas at the same temperature but in the presence of air

the fraction of platinum was ca. 32 % (see Figure 3.3a). This tendency was also confirmed

after doing the LEIS analysis on one sample exposed to 425 °C in the absence of oxygen,

showing practically no indication of Pt being on the surface of the nanostructure (Figure

3.9a). In addition, the ligament size in these samples was massively bigger than the

ligament size of samples coarsened in air (see Figure 3.11): the ligament size of samples

exposed at 425 °C in Ar-H2 was almost 90 nm whereas samples annealed at the same

temperature, in air, showed an average ligament size of ca. 10 nm. At higher temperature

(i.e., 500 °C), the ligament size in the Ar-H2 atmosphere was ca. 170 nm (Figure 3.11d); in

air the ligament width was 14 nm (Figure 3.4f). These ligament widths had the expected

effect on the roughness factors of these structures (insert Figure 3.10). Again, using NPG as

a reference, it was observed that the ligament size in the binary alloy was approximately

360 nm when exposed to 425 °C for 2 h in Ar-H2 (Figure 3.12a). As discussed before, the

ligament size of NPG after annealing in air (Figure 3.12b) was basically the same as in the

case of Ar-H2 annealing. Therefore, it was evident that the co-segregation of Pt and O

hindered the coarsening process in these nanostructures produced from ternary Ag-Au-Pt

alloys.

126

Figure 3.10 Fraction of Pt on the surface of the ligaments and average ligament size of the nanoporous structure formed on Ag77:Au20:Pt3 alloy with respect to temperature. Ar-H2 was used in all cases with an exposure time of 2 h. The insert shows the roughness factor (R f) with respect to temperature. The error bars represent 95% CI calculated from triplicate runs in the case of the fraction of Pt and from at least 20 measurements in the case of the ligament size.

Figure 3.11 SEM images of the resulting structures formed after annealing in Ar-H2 atmosphere the nanoporous structure obtained by dealloying Ag77:Au20:Pt3 alloy. (a) Sample annealed at 200 °C; (b) sample annealed at 300 °C; (c) sample annealed at 425 °C, (d) sample annealed at 500 °C. In all cases, the annealing time was 2 h.

127

Figure 3.12 SEM images of the resulting structures formed after annealing in NPG in (a) Ar-H2 atmosphere and (b) laboratory air. In both cases the exposure temperature was 425 °C and the the annealing time was 2h.

In summary, the co-segregation of Pt and O on these nanoporous metals offers an

interesting way of engineering these nanostructures. Through being able to modify the

composition of the ligament surfaces, and thanks to the stability of the structures, it is

evident that these materials have great flexibility, and have potential applications in fields

like catalysis, in which the surface composition and structure are crucial.

128

3.4 Conclusions

(a) The Pt coverage on the surface of the nanoporous dealloyed structure formed from

ternary Ag-Au-Pt alloys can be tuned by exposing nanoporous metals to surprisingly

modest temperature (< 500 °C) in the presence of air, with a maximum coverage

obtained in the temperature range between 425 °C and 475 °C. At higher temperatures,

there is a decrease in the fraction of Pt on the surface due to desorption of oxygen from

Pt atoms and consequent de-segregation of Pt.

(b) In the absence of oxygen, a de-segregation of Pt is evident in the range of temperatures

studied (200 – 500 °C). The presence of oxygen is the driving force to bring Pt to the

surface of the ligaments thanks to its strong interaction with Pt. Moreover, it was found

that the co-segregation of Pt and O hinders the coarsening process in nanoporous

metals.

(c) The co-segregation of O and Pt is a very quick process. It was observed that in

approximately 30 min, almost maximum coverage is achieved with no changes after 2

hours of exposure at 425 °C. Even prolonged experiments at lower temperatures than

the optimum segregation temperature suggests that the structure is in equilibrium.

(d) We still do not understand the transport mechanism that these Pt atoms follow for the

lower temperatures in the range studied. In the range of temperatures that were

studied, lattice diffusion coefficients are very small and should not make a significant

contribution to this process.


The authors wish to thank P. Brodersen, from the Surface Interface Ontario at the

University of Toronto, for his help in the performance and analysis of the XPS/LEIS

129

experiments. The authors wish also to acknowledge R. ter Veen, from TASCON GmbH –

Münster, Germany, and T. Grehl, from ION-TOF GmbH – Munster, Germany, for their help

running the LEIS experiments. The financial support from Natural Science and Engineering

Research Council (NSERC) of Canada is also gratefully acknowledged.

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3 A version of this chapter has been submitted for publication. Vega, A. A., and Newman, R. C. Methanol electro-oxidation on nanoporous metals formed by dealloying of Ag-Au-Pt alloys.

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Chapter 4. Methanol Electro-oxidation on Nanoporous Metals Formed

by Dealloying of Ag-Au-Pt Alloys3

4.1 Introduction

For centuries, gold has been recognized for its beauty and rarity, but also for its

nobleness and inability to corrode. Historically, gold is an element that has been considered

as a poor catalyst towards many technologically important reactions like methanol electro-

oxidation or CO oxidation; however, relatively recently it was observed that when this

element is subdivided down to the nanoscale, it becomes a very reactive material [1-3].

Thus, the rapid development of nanotechnology and nanoscience has stimulated the

application of gold in disciplines like sensing and catalysis. Some of the reactions that have

been catalyzed by nano-sized gold are low temperature CO oxidation [1], selective oxidation

of benzyl alcohol [4,5], the water gas shift reaction [2,6] and many other reactions, some of

which have been summarized elsewhere [7-9]. Among the different alternatives to study

gold as a catalyst, gold nanoparticles have been commonly used. These nanoparticles,

typically on a support, show a superior chemical stability, which enable them to act as

superior material for catalysis; however, small particles may suffer from shape stability

issues and sintering over time, which reduces their active surface area and limits their long-

term functionality [10-12]. Additionally, their processing usually involves relatively

complicated preparation and assembly procedures [13-15]. Moreover, in supported gold

catalysts, many factors have been correlated with the catalytic activity, including preparation

methods of the catalyst, nature of the support, pre-treatment conditions, presence of water

and others [15-17].

135

Another alternative to produce nano-sized gold is to dealloy Ag-Au alloys to form

nanoporous gold (NPG). By selectively leaching the silver from the Ag-Au alloy, highly

porous nanostructures are formed, in which the interconnected interstices and channels

extend in three dimensions. NPG has a desirable structure for catalysis, not only because of

its high surface area, but also because of it does not require a support. Moreover, it has

been reported that the remarkable catalytic properties of NPG, which are also applicable to

gold nanoparticles, are related to the high density of atomic steps and kinks on the surface

of the ligaments, which act as active sites for catalysis [18]. In addition, it has been

suggested that the residual silver in NPG played a significant role in its catalytic abilities, not

only helping for adsorption/dissociation of molecules during chemical/electrochemical

reactions, but also stabilizing the surface steps and kinks [18-20]. Furthermore, NPG is

relatively easy to produce and its feature size is tunable from a few to hundreds of

nanometers by post-heat treatment. Unfortunately, NPG is prone to coarsening, reducing its

functionality due to loss of surface area [21]. It has been shown that the microscopic surface

diffusion kinetics of gold along the alloy/electrolyte interface are not only responsible for

porosity evolution during dealloying but also for the post-fabrication coarsening of the

structure [22]. To minimize this coarsening, it has been reported that small amounts of

platinum can be deposited onto the NPG film, with the resulting material having better

stability even at high temperature conditions [21,23-25]; however, this technique is time

consuming and represents an extra step during the catalyst preparation process. Another

alternative is to add platinum to the precursor material, as shown by Snyder et. al. [26] after

adding approximately 6 at. % of platinum to the ‘white-gold’ type of Ag-Au alloy (i.e., gold

content of 35 at.%). Moreover, we recently probed that by adding as little as 1 at. % of

platinum to the alloy precursor, the coarsening of the structure is significantly minimized

[27]. In addition to the beneficial effect of platinum towards the stabilization of the

nanostructure, it is expected that the presence of platinum and gold on the surface of the

136

nano-structure will offer unique catalytic properties thanks to the synergy between these two

elements [28-35]. It is believed that the strong chemisorption of oxygen species onto the

porous gold will provide many reactive oxygenated species which may promote further

oxidation of carbonaceous intermediates typically absorbed on the platinum surface, which

may also reduce the typical poisoning effect [36]. Thus, exploring the electrocatalytic

properties of nanoporous structures with small amounts of platinum has a significant

importance, not only to have a better understanding of the effect of platinum in the catalytic

response of these novel nanostructures, but also in the steps towards the optimization and

characterization of potentially robust nanocatalysts.

In this article, the electrocatalytic abilities of dealloyed multi-component alloy systems

are reported. Specifically, alloys of Ag-Au-Pt have been selected with a maximum platinum

content of 3 at.%. By selectively removing some of the silver from the ternary alloys, porous

nanostructures were formed with well-controlled length scales and morphologies. The effect

of some dealloying conditions (i.e., dealloying temperature and charge density passed) are

evaluated in terms of the catalytic response of the resulting nanostructures. In addition,

nanoporous structures with a platinum-enriched surface, obtained by an adsorbate-induced

surface segregation mechanism, are also investigated. Methanol electro-oxidation in basic

medium was selected as our target system; however, some preliminary data of methanol

oxidation in acidic media are also shown. NPG formed by dealloying of Ag-Au alloys (with

the same silver content as the ternary systems) is used as a control material.

137

4.2 Experimental procedures

4.2.1 Materials and dealloying procedures

Ag-Au alloy with 77 at.% silver was obtained from as cold-rolled 200 m sheet from

Goodfellow Metals, Cambridge, UK. Ag-Au-Pt alloys with platinum content of 1, 2 and 3

at.% and 77 at.% silver were obtained as cold-rolled sheet from Ames National Laboratory –

US Department of Energy, Iowa, USA. For all the experiments, specimens were cut into

strips of approximately 4 mm by 10 mm before annealing at 900 °C for 5 h in the case of the

binary alloy and at 975 ºC for 15 h in the case of the ternary alloys. In all cases, the




Miccroshield) to mask the junction and the isolated copper wire. In all cases, dealloying was

done from both sides of the sample.

Dealloying was carried out potentiostatically in a three-electrode electrochemical cell,

with platinum coil as the counter electrode (CE) and mercury/mercurous sulphate (MSE,

0.64 V vs. SHE) as the reference electrode (RE). The reference electrode was housed in a

separate compartment and connected to the electrochemical cell via a Luggin probe. In all

cases 0.77 M HClO4 solution, prepared from Analar grade HClO4 (Alfa-Aesar, 62%), was

used. All solutions were prepared with 18 MΩ∙cm de-ionized water and de-aerated by high-

purity nitrogen purging (min. purity: 99.998%). The electrochemistry was performed using a

Gamry Reference 600TM potentiostat. Most of the specimens were dealloyed at 25 °C,

passing an anodic charge density of 5 C cm-2 at 0.55 V vs. MSE; however, to study the

effect of dealloying temperature and charge density in the catalytic abilities of the

nanoporous metals, different conditions were used. A dealloying temperature range of 25 –

138

60 °C was evaluated (for the same charge density passed), whereas the effect of charge

density was analyzed by varying charges from 2.5 to 20 C cm-2 during dealloying at 25 °C.

For some selected samples, the fraction of platinum exposed on the surface of the

ligaments was tuned by the adsorbate-induce surface segregation phenomenon at 425 °C in

the presence of laboratory air. For that purpose, the connecting wire was removed

immediately after dealloying and the specimens were thoroughly rinsed with de-ionized

water and dried with air before exposed them to the target temperature. This phenomenon

has been recently reported by the authors [37].

4.2.2 Materials characterization

A detailed description of characterization of these nanoporous structures has been

given previously [27]. In summary, for the microstructural characterization (e.g., ligament

size), dealloyed samples were manually broken under tension in air and the fracture surface

was photographed using a scanning electron microscope (SEM – Hitachi S-5200) with an

accelerating voltage of 20 kV. The SEM pictures were later analyzed with the Image Tool

software (provided by The University of Texas Health Science Centre in San Antonio, USA)

to accurately determine the ligament size (here equivalent to the average width

perpendicular to the ligament edge). The thickness of the dealloyed layer (DL) was

determined after analyzing cross-sectional metallographic specimens. In all cases,

specimens were polished to 0.05 µm using alumina powder. The cross sectional view of

those specimens was photographed using an Olympus PME3 Optical Microscope and the

DL thickness was measured using Clemex Vision Professional software. The composition of

the DL was determined by SEM (JEOL JSM6610-Lv) complemented by an Oxford INCA X-

139

sight energy dispersive X-ray spectrometer (EDS). An accelerating voltage of 25 kV was

used in all cases.

To evaluate the true surface area of the dealloyed specimens and the fraction of

surface platinum atoms on the surface of the structure, classical electrochemical methods

were used. The true area of the electrodes was estimated by means of voltammetric profiles

in the double layer region of potentials (i.e., -0.24 to 0.05 V vs. MSE) at different scan rates

in 1 M HClO4 solution [38,39]. In all cases, 28 µF cm-2 was considered the baseline double-

layer capacitance for polycrystalline gold and platinum [38]. This assumption proved to be

valid after comparing the results with Brunauer-Emmett-Teller (BET) surface area and

impedance measurements [27]. The equivalent area of platinum on the surface of the

ligaments was estimated by underpotential deposition (UPD) of hydrogen [40,41]. All

specimens were immersed in 1 M H2SO4 solution (EMD, 95-98%) in a three-electrode cell

with a MSE as a RE, a platinum wire as CE and the dealloyed specimen as a working

electrode. The solution was de-aerated for approximately 15 min before the experiment.

Cyclic voltammetry (CV) curves were obtained at 25 °C between -0.63 and 0 V vs. MSE, at

a scan rate of 20 mV s-1. The equivalent area of platinum on the surface of the structure was

calculated by integrating the hydrogen adsorption/desorption regions to obtain the charge

related with the formation of a hydrogen monolayer, assuming that the charge associated

with the monolayer formation in polycrystalline platinum was 210 µC cm-2 Pt [42,43]. The

fraction of platinum on the surface of the resulting structure was estimated by dividing the

equivalent area of exposed platinum by the estimated true area of the electrode. The

roughness factor (Rf), as an indication of the developed surface area during the dealloying

process, was determined by dividing the true area of the electrode by its geometrical area.

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4.2.3 Electro-oxidation of methanol

In the case of the electrocatalytic studies in basic media, dealloyed specimens were

immersed in a solution containing methanol (Caledon, 99.8%) in 5 M KOH (Aldrich, 90%).

Methanol concentrations of 0.3, 1 and 3 M were used for the analysis. The KOH/CH3OH

solution was prepared by diluting/dissolving the reactants with de-ionized water with a

resistivity of 18 MΩ∙cm. Prior to any measurement, the solution was de-aerated by high-

purity nitrogen purging (min. purity: 99.998%). A Gamry Reference 600TM potentiostat was

used to perform CV between -0.9 and 0.2 V vs. mercury/mercury oxide RE (Hg/HgO – 20%

KOH, 0.1 V vs. SHE). The effect of the positive scan limit was evaluated by changing it from

0 to 0.4 V vs. Hg/HgO. Scan rates of 3, 10, 30 and 100 mV s-1 were used. To assess

qualitatively the presence of any mass transfer effects during reaction, a magnetic stirrer

was used. The approximate rotation velocity of the stirrer was measured with a digital laser

tachometer (Fieldpiece, HP2234C). All current densities were normalized by the true area of

the electrodes unless otherwise stated. Polycrystalline platinum was used as a control

material.

Additionally, potentiostatic experiments at -0.35 V vs. Hg/HgO (in the case of the

ternary alloys) and at 0.05 V vs. Hg/HgO (for the binary alloy) were carried out for 4000 s in

the 5 M KOH – 1 M CH3OH solution. During these experiments, the solution was agitated

with a magnetic stirrer with an approximated rotation velocity of 700 rpm. An evaluation of

the products obtained as a result of the oxidation was done. The formation of formate was

evaluated by nuclear magnetic resonance spectroscopy (H-NMR – Agilent DD2 500 MHz).

For this analysis, 50 µL of solution was diluted in 5 mL of deuterium oxide (D2O - Cambridge

Isotope Laboratories, 99.9%). Samples were then run using a scan at 90 ° pulse (16.1 µs)

and 10 s relaxation delay, with an acquisition time of 4.5 s and spectral window of 8012 Hz.

Standards with different formate contents were prepared for calibration purposes. The

141

quantification of carbonate was done by titrating 5 mL of resulting solution with 1.2 M BaCl2

solution (ACP, 99%). To avoid the precipitation of Ba(OH)2, the pH was brought down from

ca. 15.0 to ca. 10.0 by adding 2 mL of concentrated HCl (BioShop, 36.5-38%) and a few

drops of 2 M HCl solution. The HCl was added as quickly as possible to minimize the

unnecessary exposure of the solution to air. Right after that, a solution of BaCl2 was added

drop wise while the pH was kept between 10.0 and 10.5 to avoid the dissolution of BaCO3,

which occurs at pH values around 6 [44-46]. Filtration of the obtained precipitate was done

using a 0.45 µm membrane filter (PALL) followed by a gravimetric determination. The

contribution to the carbonate content from original slight carbonation of the KOH was

measured by titrating the solution prior to the reaction.

For the electrocatalytic experiments in acidic media, selected specimens were

immersed in a solution of 0.5 M HClO4 (Alfa-Aesar, 60-62%) and 1 M methanol (Caledon,

99.8%). The HClO4/CH3OH solution was prepared by diluting the reactants with de-ionized

water with a resistivity of 18 MΩ∙cm. Prior to any measurement, the solution was de-aerated

by high-purity nitrogen purging (min. purity: 99.998%). A Gamry Reference 600TM

potentiostat was again used to perform CV between -0.45 and 0.35 V vs. MSE.


The catalytic and electrocatalytic properties of nanoporous metals have received a lot

of attention in recent years, not only because of the high surface area and interconnected

nanoporosity of these structures (see Figure 4.1 as an example), but also because of the

potential synergistic effect between the alloying elements, which is particularly true for the

nanoporous structures formed on Ag-Au-Pt precursor alloys. By dealloying these ternary

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alloys, nanoporous metal structures with distinctive characteristics and properties are

formed, as summarized in Table 4.1, where NPG was used as a benchmark material.

Figure 4.1 Interconnected ligament/pore structure formed on the Ag77:Au23 alloy after passing 5 C cm-2 at 40 °C and 0.55 V vs. MSE. The insert shows a higher magnification SEM image of the nanoporosity.

Table 4.1 Physical characteristics, residual silver and roughness factor of the nanoporous structures developed at 25 °C and 0.55 V vs. MSE in 0.77 M HClO4. Charge density: 5 C cm-2. The number in brackets represents the 95% confidence interval (CI).

Precursor alloys

Thickness of the DL (µm)a

Ligament width (nm)a

Pore size (nm)b

Roughness factor (Rf)

c

Ag77:Au23 7.8 (0.2) 13.8 (0.7) 17.2 894 (17) Ag77:Au22:Pt1 9.1 (0.9) 6.8 (0.4) 11.6 1941 (238) Ag77:Au21:Pt2 9.4 (0.2) 6.0 (0.2) 8.1 2486 (88) Ag77:Au20:Pt3 9.7 (0.5) 4.3 (0.2) 5.8 2679 (81)

a The 95% CI was calculated from at least 30 measurements for each quantity. b Obtained by BET pore size distribution analysis using the NLDFT method and considering the adsorption branch model. c The 95% CI was calculated from triplicate runs.

The thickness of the DL, for the same charge density passed, did not vary significantly

in ternary alloys; however, the layer measured in the binary alloy was thinner than in the

other alloys. This effect has to do with the amount of residual silver that remains buried in

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the core of the ligaments, which was smaller in the binary alloy due to exposure of buried

silver to the electrolyte during coarsening in solution, driven by surface diffusion of gold [27].

The ligament size, on the other hand, decreased with the platinum content of the alloy

because the presence of platinum reduced gold surface mobility, hindering the coarsening

of the structure. In the case of the binary alloy, the average ligament size was ca. 14 nm

whereas for the alloy with 1 at.% Pt the ligament size drops to ca. 7 nm. By increasing the

amount of platinum in the precursor, the size of the ligaments kept decreasing to a value of

ca. 4 nm. The median pore size of NPG was ca. 17 nm; in the ternary alloys, by increasing

the platinum content, the median pore size decreased from ca. 12 to ca. 6 nm. The fraction

of platinum on the surface of the ligaments was 1.4, 3.0 and 6.6 % for the alloys with 1, 2

and 3 at.% Pt in the precursor respectively [27]. According with the platinum content of the

precursor, and the estimated distance between surface platinum atoms on the ligaments, it

is believed that this fraction of platinum represents the minimum platinum coverage of the

ligaments [47,48]. Therefore, we recognize that this is an important area for further

development, which could be critical for better understanding of the catalytic properties of

these nanomaterials. It is important to mention that at this temperature, the surface platinum

content is in accord with the fraction of silver removed, which is not the case for higher

dealloying temperatures. The roughness factor (see also Table 1) showed that by reducing

the ligament size of the structure, the increase in surface area was substantial.

Figure 4.2a shows the CV profiles of all the nanoporous structures in the supporting

electrolyte (5 M KOH). This concentration of the KOH was experimentally determined for the

methanol oxidation reaction after observing that at lower concentrations, the reaction was

limited by depletion of OH- owing to the extremely high nominal current density (see Figure

4.2b). As shown in this figure, NPG had an extended double-layer region between -0.9 V

and 0.25 V vs. Hg/HgO. At 0.25 V an oxidation peak started that was ascribed to the

144

formation of a monolayer of gold surface oxides [49]. A reduction peak at 0.1 V was

associated with the electrochemical reduction of those oxides. For the ternary alloys, the

oxidation of gold started at a potential 0.1 V less than that of NPG. The reduction peak was

also slightly shifted to less positive potential if compared with that for NPG. The magnitude

of this reduction peak decreased with increasing platinum content of the alloy. At ca. -0.5 V

there are also reduction peaks that were attributed to the presence of oxidized platinum in

the structure (no influence of silver was detected at this potential).

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Figure 4.2 (a) CV profiles of the different nanoporous structures in 5 M KOH; (b) CV profiles of NPG and of the nanoporous structures formed on the alloy with 1 at.% Pt in 5 M KOH - 1 M CH3OH solution. The insert in (b) shows the CV profile of polycrystalline Pt in 5 M KOH - 1 M CH3OH solution. In (a) and (b) the current density was normalized by the geometrical area of the electrodes. The original platinum content is shown in the figures. All CV profiles were obtained at 10 mV s-1 and 25 °C.

Figure 4.2b shows the CV profiles for the methanol oxidation reactions of NPG and of

the nanostructure formed on the ternary alloy with originally 1 at.% Pt. In both cases, the

vertical axis corresponds to the nominal current density (i.e., normalized by the geometrical

area of the electrode), and the positive scan limit was fixed at 0.2 V to avoid the region in

which gold and silver oxides started forming. Both nanostructures, but particularly the

ternary one, had very high current density (the peak current in the binary structure was at

146

ca. 8 times smaller than that in the ternary nanostructure). In NPG, the rising current in the

forward scan (approximately at -0.2 V) can be ascribed to the characteristic methanol

oxidation on the surface (see Figure 4.2a for NPG). The pre-oxidation species such as Au-

OHads(1-)- ( is the charge transfer coefficient , 0 < <1) are considered by some authors to

play a governing role in the electrochemistry of Au in alkaline media [50]. Eventually, the

gradual exhaustion of Au-OHads(1-)- species slows down and eventually stops the methanol

oxidation reaction at about 0.15 V vs. Hg/HgO. This potential was very close to the potential

at which the gold oxide monolayer started forming. In the case of the ternary alloy, there

was a sharp increase in current at more negative potentials than in the binary alloy

(approximately at -0.75 V). This increase in the current density was again related to the

characteristic methanol oxidation on the surface (see Figure 4.2a for ternary alloys). The

peak potential in the forward scan was observed at -0.2 V; whereas another oxidation peak

was observed at ca. -0.3 V during the backward sweep, which represents the reaction

resuming after whatever was blocking it at higher potentials dissolved off or was reduced.

The insert in Figure 4.2b shows that a polycrystalline platinum electrode had oxidation

peaks, in the forward and backward scans, that agreed very well with those observed in the

ternary nanostructures, and with those reported elsewhere for porous platinum electrodes

[51]. Moreover, it was observed that the peak current density in the ternary nanostructure

was ca. 40 times higher than that with the platinum electrode. The catalytic response of

these ternary nanoporous metals was also significantly higher than that reported for

platinum decorated NPG under relatively similar experimental conditions [21,52].

Dealloying, by definition, dramatically increases the surface area of the material, which

depends, among other things, on the size of the ligaments. As shown in Table 4.1, the Rf

increased by increasing the platinum content of the precursor. Therefore, to account for that

effect, the current density was normalized by the true area of the electrode. Following this

147

approach, it was observed that the CV profiles of all ternary nanostructures, under the same

experimental conditions, showed the same trend (see Figure 4.2b). However, it was found

that the nanostructure formed on the alloy with 1 at.% Pt had slightly higher true methanol

oxidation current density than the other two, as observed after comparing the specific

activities (taken at -0.35 V vs. Hg/HgO) of the three ternary alloys (Figure 4.3a). We

rationalized this result based on the relative ratio of platinum and gold atoms on the surface

of the ligaments. As pointed out before, gold and platinum have been reported to display a

synergistic effect that enhances their catalytic activity towards many reactions, including

methanol electrooxidation [52-54]; however, as suggested elsewhere [28,52,55,56], in

alkaline electrolytes, where gold becomes a very active element, there is an optimum

fraction of platinum atoms surrounded by gold atoms that maximizes the yield of the

reaction. Gold atoms are believed to play an important role providing oxygenated species

(e.g., adsorption sites for OH groups and/or chemisorption of HO2- intermediates) in the

methanol oxidation process; therefore, this relative ratio between elements represents a

compromise balance of activation of platinum sites and promotion of gold sites by

adsorption of OH groups. We are not suggesting, of course, that the nanoporous structure

with originally 1 at.% Pt, represents that optimum composition; however, having most of the

platinum atoms surrounded by gold atoms, in a highly porous structure, could have a

significant role in the observed behaviour. Mott and coworkers [28] predicted, by DFT

calculations, that a bifunctional electrocatalytic property may be operative in the case of Au-

Pt nanoparticles in alkaline electrolyte, where adsorption of CO and OH- on gold and

dehydrogenation of methanol on platinum sites are involved. In acidic electrolytes, on the

other hand, the most active structure was the one with higher platinum content (see Section

4.3.2 below).

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Figure 4.3 (a) Summary of specific activities of the ternary alloy structures for the CH3OH oxidation reaction at -0.35 V vs. Hg/HgO; the vertical axis is the current density per true area of the electrodes. (b) Concentration of carbonate/formate produced after potentiostatic oxidation for 4000 s at -0.35 V vs. Hg/HgO; during this time, the solution was agitated with a magnetic stirrer at ca. 700 rpm. In all cases, a 5 M KOH - 1 M CH3OH solution was used. The error bars represent 95 % CI obtained by triplicate runs.

It has been suggested that among the three likely product of methanol oxidation by

gold in alkaline media (i.e., formaldehyde, formate and carbonate), formate is the most

probable [50,57]; whereas carbonate tends to be the dominant product when platinum-

based catalysts are used [51,58]. To identify the oxidation products formed on these

nanoporous metals, potentiostatic experiments at -0.35 V vs. Hg/HgO were carried out for

4000 s. The quantification of formate by H-NMR is shown in Figure 4.3b. The concentration

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of formate decreased with increasing platinum content of the precursor. No indication of

formaldehyde has been found; nevertheless, disproportionation of formaldehyde in highly

basic media, according with Cannizzaro’s reaction, would occur readily under the current

experimental conditions [59]. Carbonate, on the other hand, could easily be determined by

titration with barium solution. As shown also in Figure 4.3b, the concentration of carbonate

produced during the oxidation increased with increasing platinum content of the alloys,

which could be a consequence of the higher platinum coverage on the surface of the

ligaments. These results suggest that the ratio between the oxidation products could be

modified by changing the platinum content of the structure. The low amount of carbonation

that occurred during the titration procedure was demonstrated by the nearly zero carbonate

analysis obtained for the binary alloy (not shown in Figure 4.3b). The overall current

efficiency for the nanostructures with originally 1, 2 and 3 at.% was 85, 94 and 98 %

respectively, indicating that no significant losses occurred during the electrocatalytic

process.

The effects of different potential scan rates, different agitation speed of the solution

(by a magnetic stirrer), different potential scan limits and methanol concentrations were

assessed for all ternary structures. NPG was not considered further as ternary alloys were

always more catalytic. Figure 4.4 shows the effect of those parameters for the alloy with 1

at. % Pt; similar trends were observed for the other ternary alloys. As shown in Figure 4.4a,

the oxidation current was dependent on the potential scan rate (); in fact, the peak current

was linear with 0.5 for all nanostructures. This clearly indicated a diffusion controlled

process for methanol oxidation in the bulk solution boundary layer. The oxidation current

density associated with the half peak potential (i.e., -0.4 V vs. Hg/HgO) is nearly linear with

0.5 for the alloys with 2 and 3 at.% Pt, but independent of the scan rate in the case of the

lower platinum content alloy (see Figure 4.4a). In the case of the different agitation speeds

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(i.e., between ca. 200 rpm and 970 rpm) it was observed that the peak current increased

with the agitation speed. For example, in the case of the ternary alloy with 1 at.% Pt (Figure

4.4b), the peak current increased from ca. 200 µA cmtrue (no agitation) to 550 µA cmtrue

at

800 rpm (at 970 rpm there were no further changes with respect to 800 rpm). These

changes in the peak current were entirely related to the diffusion limitation at the top surface

of the catalyst. No changes were found in the oxidation current below ca. -0.3 V. Similar

trends were observed for the other two ternary alloys (results not shown here). In terms of

the effect of the scan limit (Figure 4.4c), it was clear that the oxidation peak in the reverse

scan was first observed when the scan limit was 0.2 V; at less positive limits, no oxidation

peak was shown, but instead a hysteresis was found. This hysteresis could be associated to

the blocking of active sites during that stage of the oxidation process. At more positive scan

limit (i.e., 0.4 V), two main features were observed in the reverse scan: a smaller oxidation

peak at ca. -0.4 V and a cathodic peak at ca. 0.1 V. This cathodic peak was related to the

reduction of gold oxide formed on the anodic scan, which also confirmed that gold surface

oxide formation occurs at potentials higher than 0.2 V. In terms of the effect of the methanol

concentration (Figure 4.4d), it was observed that the oxidation current increased linearly

with the increasing concentration of methanol; moreover, it was observed that the peak

potential moved to less negative potentials while increasing the methanol concentration.

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Figure 4.4 CV profiles for the electrooxidation of CH3OH solution in 5 M KOH on the nanoporous structure formed on the ternary structure developed on the alloy with 1 at.% Pt: (a) effect of the scan rates, (b) agitation speeds, (c) scan limit and (d) methanol concentration. For figures (a) to (c), the methanol concentration was 1 M; for figures (b) to (d), the scan rate was 10 mV s-1. Temperature: 25 °C. Details about the scan rates, agitation speeds, scan limit and methanol concentrations are given in the figures. In all cases, the current was normalized by the true area of the electrodes.

One method to test the electrochemical performance of these nanoporous metals was

by repeated potential cycling. Figure 4.5 shows the effect of different cycles in the ternary

structure with originally 1 at.% platinum showing a very stable activity towards methanol

oxidation, i.e., the specific activity at -0.35 V decreased from 192 µA cmtrue to 130 µA cmtrue

after 40 continuous cycles. During the backward sweep the oxidation peak height was

reduced with subsequent scans, which could be associated with small changes in the

nanoporous structure. Similar behaviours were observed in the other ternary structures. The

binary alloy (not shown here) showed an 80% reduction in the peak current after 40 cycles.

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Figure 4.5 CV profiles for the electrooxidation of CH3OH solution in 5 M KOH on the nanoporous structure formed on the alloy with 1 at.% platinum. In all cases the scan rate was 10 mV s-1 and the temperature 25 °C. Details about the number of scan cycles are indicated in the figures.

4.3.1 Effect of dealloying parameters and tunability of the resulting nanoporous

structures

One of the main advantages of nanoporous metals is that they are highly tunable. By

changing the composition of the precursor and/or dealloying conditions, changes in the

ligament size, the surface composition (i.e., fraction of platinum exposed on the surface of

the ligaments), and other characteristics were observed [27]. The effect of the dealloying

temperature and charge density are some of the characteristics of the dealloyed layer that

are summarized in Figure 4.6. By increasing the dealloying temperature, the rate of gold

surface diffusion increased, accelerating the coarsening of the ligaments (Figure 4.6a). This

agreed well with the literature associated with the relaxation of roughened gold surfaces [60-

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62]. The average ligament width in NPG changed from ca. 14 nm at 25 °C to almost 28 nm

at 60 °C. In ternary alloys, a much smaller increase of approximately 10% was found for the

nanostructure with originally 1 at. % Pt (from ca. 6.8 nm at 25 °C to 7.4 nm at 60 °C).

Variations in the dealloying temperature also induced changes in the depth of the layer

(insert in Figure 4.6a); in general, the thickness of the layer, for a constant charge density,

increased with temperature on the ternary alloys, whereas in the binary alloy it tended to

decrease. These tendencies were related to the fraction of silver retained in the layer (not

shown here but discussed in [27]). Moreover, by increasing the dealloying temperature, the

fraction of platinum atoms on the surface of the ligaments increased, as it is shown in Figure

4.6b. By changing the dealloying temperature from 10 °C to 60 °C, the fractional coverage

of platinum atoms increased for all ternary structures, doubling the platinum coverage at the

upper temperature. This increase in the platinum coverage on the surface could be

associated with the dealloying mechanism itself and/or other processes that were favored

under the experimental conditions used here (e.g., preferential adsorption of OH groups on

platinum atoms as suggested before for multimetallic nanoparticles [63]).

154

Figure 4.6 Effect of the dealloying temperature and dealloying charge density on the main characteristics of the resulting nanoporous structures: (a) effect of dealloying temperature on the size of the ligaments; the insert in this figure shows the average thickness of the DL; (b) fraction of platinum on the surface of the ligaments after dealloying at different temperatures; (c) ligament size of the different structures after passing different charge densities; the insert shows the average thickness of the DL; (d) fraction of platinum on the surface of the ligaments after passing different charges. Details about the dealloying temperature are indicated in the figures (b) and (d). The error bars represent 95 % CI obtained by triplicate runs.

Changing the dealloying time (for a given potential) changed the charge density, which

is directly proportional to the amount of silver removed from the alloy. Moreover, the

dealloying time affects the porosity of the structure, because through extended exposure,

the ligaments eventually coarsen. Figure 4.6c shows the changes in ligament size for all the

alloys with respect to charge density. The average ligament width in the case of the binary

alloy changed from ca. 12 nm (while passing 2.5 C cm-2) to almost 17 nm when the charge

density increased to 20 C cm-2. The presence of platinum clearly reduced the coarsening

effect; for all ternary alloys, the ligament widths were below 10 nm for all the charge

155

densities studied. The thickness of the layer increased with the charge density and the

platinum content (insert in Figure 4.6c). The average retained silver tended to decrease

when the dealloying charge increased, with the binary alloy showing the lowest silver

content in the layer; however, this effect saturated and the residual silver never decreased

beyond a certain point (results not shown here). The platinum coverage of the surface of the

ligaments tended also to increase with the charge density (Figure 4.6d); however no

obvious difference was observed between samples in which the dealloying charges were

2.5 and 5 C cm-2; at higher charge densities (i.e., 12.5 and 20 C cm-2) higher coverage of

platinum was detected. Once again, this effect could be related to the dealloying mechanism

itself, and/or with the extended dealloying time, i.e., longer exposure of the dealloyed layer

to the electrolyte, where preferential adsorption of OH groups on platinum might induce

platinum to segregate to the surface, either by surface diffusion mechanism or simple place

exchange between atoms.

All these characteristics had an impact in the electrocatalytic response of the

nanoporous structures. Figure 4.7a shows the specific activity (taken from cyclic

voltammograms at -0.35 V vs. Hg/HgO – not shown here) for the three ternary

nanostructures developed under different experimental conditions (i.e., temperature, charge

density passed). As observed, the current density significantly increased by decreasing the

dealloying charge density. For the experimental conditions reported here, the highest

specific activity was obtained for structures where 2.5 C cm-2 were passed; conversely, the

sample in which 20 C cm-2 were removed showed the lowest specific current density.

Initially, it was assumed that thicker layer (i.e., higher charge densities) could give higher

current densities; however, the abovementioned trend suggested otherwise. In fact, these

results also suggested that the oxidation reaction might happen mostly in the vicinity of the

top surface of the structure. It is important to mention that the effects of mass transport

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limitation within the layer are not negligible, although they are smaller that might be

assumed since the effective diffusivity within the layer is lower than that for the bulk, but not

by orders of magnitude. The concentration of carbonate produced by the ternary structures

with respect to the charge density passed is shown in Figure 4.7b. After removing 5 C cm-2,

the carbonate concentration was smaller than that after passing 2.5 C cm-2; however, when

passing higher charge densities (i.e., 12.5 and 20 C cm-2); a relatively constant

concentration of carbonate was obtained, with an average value of 1.5 mg L-1 cmtrue . This

effect was particularly clear in the structures with originally 2 and 4 at.% Pt. These results

suggested that on nanostructures with thick DLs, the concentration of carbonate in solution

was small if compared with that in thinner layers, which could indicate that the carbonate

formation was counteracted by the thickness of the DL. At this point however, the existence

of any mass transfer limitations or partially blockage of the pores in thick DLs (20 µm and

thicker) cannot be ruled out. The average current efficiency for all ternary nanostructures

when 2.5 C cm-2 were removed was ca. 65%; whereas for the nanostructures when 20 C

cm-2 were removed was ca. 80%. In all cases, carbonate was considered as the only

product of the oxidation.

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Figure 4.7 (a) Summary of specific activities for the CH3OH oxidation reaction of the ternary alloy nanostructures developed under different conditions; the vertical axis is the current density per true area of the electrodes. (b) Concentration of carbonate produced on nanoporous structures dealloyed after passing different charge densities after potentiostatic oxidation for 4000 s; during this time, the solution was agitated with a magnetic stirrer at ca. 700 rpm. The insert in (b) corresponds to concentration of carbonate produced on nanostructures dealloyed at different temperatures. In all cases, the potential was fixed at -0.35 V vs. Hg/HgO. The different dealloying conditions are shown in the figures. The error bars represent 95 % CI obtained by triplicate runs.

The effect of the dealloying temperature was studied by comparing the electrocatalytic

response of nanostructures developed at different temperatures but at constant dealloying

charge density (i.e., 5 C cm-2 as shown also in Figure 4.7a). By increasing the temperature

of the electrolyte, the specific activity of the nanostructures tended to increase. The

158

concentration of carbonate in solution (see insert in Figure 4.7b) increased by increasing the

dealloying temperature, which could be associated with the higher platinum coverage of the

structures (Figure 4.6b). The current efficiency for the ternary nanostructures developed at

60°C was 85, 93 and 98% for the alloys with 1, 2 and 3 at.% Pt respectively. Once again, for

this analysis carbonate considered the only product of the oxidation.

Besides the effect of the abovementioned dealloying parameters, the effect of

platinum enrichment on the surface of the ligaments (i.e., induced surface segregation of

platinum) had a role in the catalytic response of the resulting structures. As shown in Figure

4.8a, after exposure of the as-dealloyed structures to 425 °C in the presence of laboratory

air, the ligament size increased in all alloys (in the case of the structure formed on the alloy

with 1 at. % Pt, the ligament size increased from ca. 7 nm right after dealloying to 37 nm

after annealing); nevertheless, the nanostructure developed on the alloy with 4 at.% Pt had

the lowest increment in ligament size. More importantly, by exposing the as-dealloyed

structures to air at moderately elevated temperatures, the fraction of platinum on the surface

of the structures increased (see also Figure 4.8a) if compared with the results of as-

dealloyed specimens (minimum coverage) [37]. Figure 4.8b shows a comparison between

the CV profiles of the nanostructures formed on the 1 at.% alloy before and after exposure

to 425 °C. In both cases, the vertical axis corresponds to the nominal current density. Even

after exposure to this moderate temperature, the electrode showed a very high activity, e.g.,

at -0.3 V the difference in the nominal current density was only 20% with respect to the

nominal current in as-dealloyed samples. However, to account for the effect of surface area,

the specific activity (normalized by the true area of the electrodes) was plotted in Figure

4.8c, showing that inducing segregation of platinum induced a very significant increase in

the activity of the electrocatalyst. The nanostructure developed on the alloy with 1 at.% Pt

displayed the highest increment (ca. 400%) with respect to the as-dealloyed nanostructure.

159

The other nanostructures also showed higher catalytic response after segregation than the

as-dealloyed samples, although the activity decreased by increasing the platinum content of

the precursor. As before, we could rationalize this finding based on the relative ratio

between platinum and gold on the surface of the ligaments. Moreover, it is important to

mention that the structure formed on the alloy with 1 at.% Pt was the one that showed more

traces of silver on the surface than the other two ternary structures [37]. This silver comes

from the core of the ligaments after coarsening occurs. Silver could change the catalytic

behaviour of the nanostructure by providing additional oxygenated species that interact with

methanol species adsorbed on the surface [64,65]. For instance, it has been reported that

silver significantly enhances the catalytic activity shown by NPG towards gas-phase

methanol oxidation, promoting the total oxidation all the way to CO2 [65].

The concentration of carbonate produced by the nanostructures after platinum

segregated to the surface is shown in Figure 4.8d. The concentration of carbonate was

basically the same for all structures (ca. 7.5 mg L-1 cmtrue ), which means that for the

structure formed on the alloy with 1 at.% Pt (i.e., ca. 10 % of the surface atoms were

platinum) there was an increase of ca. 300 % with respect to the as-dealloyed structure;

whereas for the structure formed on the alloy with 4 at.% Pt, an increase of ca. 60 % was

observed with respect to the as-dealloyed structure. However, the current efficiency

(considering carbonate as the only product of the oxidation) was 20, 62 and 80% for the

nanostructures with originally 1, 2 and 3 at.%, which suggests that a significant amount of

formate was produced under this experimental conditions, especially in the case of the

lowest platinum content alloy. It is believed that the interaction between gold and platinum

surface atoms with the silver atoms exposed to the surface might be responsible for this

trend.

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Figure 4.8 (a) Ligament size and fraction of platinum on the surface of the ligaments for all ternary structures after exposure to 425 °C for 2 h in the presence of laboratory air; (b) CV profiles in 5 M KOH - 1 M CH3OH solution for the nanoporous structure formed on the alloy with 1 at.% Pt structures before and after segregation of Pt, the CV profiles were obtained at 10 mV s-1 and 25 °C and the current density was normalized by the geometrical area of the electrodes; (c) Summary of specific activities of the ternary alloy nanostructures for the CH3OH oxidation reaction at -0.35 V vs. Hg/HgO; the vertical axis is the current density per true area of the electrodes; (d) Concentration of carbonate produced after potentiostatic oxidation for 4000 s at -0.35 V vs. Hg/HgO; during this time, the solution was agitated with a magnetic stirrer at ca. 700 rpm. The error bars represent 95 % CI obtained by triplicate runs.

4.3.2 Methanol electro-oxidation in acidic media

To better understand the electrocatalytic properties of these novel nanoporous metals,

a preliminary assessment of their catalytic response in an acidic electrolyte has been

performed. Figure 4.9a shows the CV profiles of all the nanoporous structures in 0.5 M

HClO4. As for the basic solutions, the concentration of the supporting electrolyte was also

experimentally determined choosing the concentration at which the highest current density

161

was observed (results not shown here). As can be seen in this figure, the double-layer

region in NPG extended from -0.45 V to 0.10 V vs. MSE. At 0.10 V an increase in the

current density was observed, which could be associated with the beginning of the oxidation

of gold. No reduction peaks were observed in the range of potentials under investigation. At

higher scan limits, an oxidation peak, with the subsequent reduction peak, were associated

with gold. For the ternary alloys, reversible waves caused by hydrogen

adsorption/desorption at potentials lower than 0.3 V were observed. An increase in the

oxidation current was detected for all ternary alloys at potentials higher than 0.10 V, with the

alloy with 3 at.% Pt showing the increase in current density at slightly more negative

potentials than the other two ternary alloys. A characteristic reduction peak around 0 V was

associated with the reduction of platinum oxides that were formed during the anodic scan

[51,66].

Figure 4.9b shows the CV profiles for the methanol oxidation reactions of all

nanostructures in acid. The positive scan limit was fixed at 0.35 V to minimize the impact of

surface oxides, and evaluate the catalytic response of these nanostructures mostly in a

region below the empirical critical potential of these alloys (i.e., ca. 0.35 V in 0.77 M HClO4)

[27]. In NPG, there was basically no catalytic response in this region of potentials; at higher

potentials however, it has been reported that gold becomes a relatively active catalyst for

methanol oxidation [57]. In the case of the ternary alloys, there was an obvious increase in

current at -0.10 V; this increase in the current density was related to the characteristic

methanol oxidation on the surface (see Figure 4.9a). The peak potential in the case of the

nanostructure formed on the alloy with 1 at.% Pt was located at ca. 0.23 V; whereas in the

case of the nanostructure formed on the 4 at.% Pt alloy, the peak potential shifted to more

positive potential (ca. 0.32 V). The peak current was ca. 2.5 times higher in the 3 at.% Pt

alloy than in the 1 at.% Pt. In contrast with the results obtained in alkaline electrolyte, the

162

highest platinum content nanostructure was the most active catalyst; nevertheless, it is clear

that these nanostructures displayed higher electrocatalytic response in alkaline media. This

result agreed well with the observations made on supported gold-platinum nanoparticles

when evaluated as electrocatalysts for methanol oxidation in acidic electrolyte, where high

contents of platinum were required to maximize the activity of the catalyst [28,55,56].

Figure 4.9 (a) CV profiles of the different nanoporous structures in 0.5 M HClO4; (b) CV profiles of the nanoporous structures in 0.5 M HClO4 - 1 M CH3OH solution. The insert in (b) corresponds to the CV profiles in 0.5 M HClO4 - 1 M CH3OH solution after segregation of platinum at 425 °C in laboratory air for 2 h. The original platinum composition is shown in (a). All CV profiles were obtained at 10 mV s-1. The temperature in all cases was 25 °C.

163

As was reported at the end of Section 4.3.1, by exposing the as-dealloyed structures

to 425 °C in the presence of air, there was a clear tendency of platinum to segregate to the

surface of the ligaments; therefore, it was an obvious step to analyze the catalytic response

of these tuned structures towards methanol oxidation in HClO4 supporting electrolyte. The

insert in Figure 4.9b shows the CV profiles of all ternary structures in a solution containing

methanol. As can be seen, the structure formed on Ag77:Au20:Pt3 displayed higher specific

activity than that in the as-dealloyed sample (e.g., at 0.2 V, the current density in the as-

annealed sample was 40% higher than that in the as-dealloyed electrode). The peak

current, on the other hand, slightly decreased with the heat treatment. In the backward scan,

the sample after segregation had a well-defined oxidation peak, which was not clearly

observed in the sample before the heat treatment. Increasing the fraction of platinum on the

surface from ca. 6 to ca. 30 % did improve the electrocatalytic response of the

nanostructure, even though the surface area slightly decreases (i.e., ligament size

increased). In the other two nanostructures, a decrease in the electrocatalytic activity was

observed, with the bigger decay in the one formed on the alloy with 1 at.% Pt.

4.4 Conclusions

(a) Novel nanoporous structures have been formed by electrochemically removing silver

(i.e., by dealloying) from Ag-Au-Pt alloys with platinum contents of 1, 2 and 3 at. %. In

these structures, the surface is mostly covered by the more-noble elements (i.e., gold

and platinum) through the whole thickness of the DL.

(b) The characteristics of the resulting nanoporous metals depend on the platinum content

of the precursor. By increasing the platinum concentration, the ligament size decreases

and the platinum coverage on the surface of the ligaments increases. Moreover, by

having gold and platinum on the surface of the nanostructure, the catalytic response

164

towards methanol oxidation is significantly enhanced showing an onset of the reaction

at more negative potentials, higher current densities and longer stability and

electrocatalytic activity.

(c) Based on the original platinum content of the ternary alloy precursors and the estimated

distance between platinum atoms in the nanoporous material, it is believed that the

fraction of platinum obtained by UPD of hydrogen represents the minimum platinum

coverage of the ligaments. This clearly becomes an important area for further

development to better understand the catalytic properties of these and other

nanomaterials. Low energy ion scattering (LEIS) and X-ray photoelectron spectroscopy

(XPS) could be critical tools to understand this phenomenon.

(d) Among all the three ternary nanostructures, the one developed on the alloy with 1 at.%

Pt shows the highest activity towards methanol electrooxidation in alkaline electrolyte.

Gold and platinum clearly display a remarkable synergistic effect, however, the ratio

gold to platinum seems to play an important role in alkaline electrolytes.

(e) In alkaline media, formate and carbonate were identified as the main products of the

oxidation reaction. In the case of NPG, mostly formate was detected (the amount of

carbonate formed during the reaction was minimal), whereas a combination of the two

products was detected when the ternary nanostructures were used. More importantly,

by increasing the platinum content of the ternary precursor, the ratio of the reaction

products can be modified.

(f) By changing dealloying parameters like temperature and charge density passed, the

characteristics of the resulting nanoporous structures change (e.g., platinum coverage

on the surface and ligament size). Moreover, the catalytic performance towards

methanol oxidation also changes. By increasing the dealloying temperature, the specific

activity of all nanostructures increases; whereas by increasing the charge density, the

specific activity decreases for all structures. The concentration of carbonate produced

165

increases also with the dealloying temperature and decreases with an increase in the

charge density.

(g) By inducing segregation of platinum to the surface of the ligaments (after exposure to

425 °C in the presence of air), there is a significant increase in the current density in all

ternary structures in alkaline media, especially in the one formed on the alloy with 1

at.% Pt.

(h) In the presence of an acidic electrolyte (HClO4), the highest platinum content alloy is the

one showing the highest specific activity, in contrast with the tendency observed in

alkaline electrolyte; nonetheless, these nanostructures displayed higher electrocatalytic

abilities in alkaline electrolyte.


The authors wish to thank D. Burns and J. A. Tang, from the Nuclear Magnetic

Resonance Facility at the Department of Chemistry - University of Toronto, for their help in

the performance of the NMR experiments. The authors wish also to acknowledge the

financial support from Natural Sciences and Engineering Research Council (NSERC) of

Canada.

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4 A version of this chapter has been submitted for publication. Vega, A. A., and Newman, R. C. Electro-oxidation of ethanol on highly porous nanostructures obtained by dealloying of binary and ternary noble-metal alloys.

173

Chapter 5. Electro-oxidation of Ethanol on Highly Porous

Nanostructures Obtained by Dealloying of Binary and

Ternary Noble-metal Alloys4

5.1 Introduction

Numerous studies have dealt with the electro-oxidation of low molecular weight

alcohols, like methanol and ethanol, with the ultimate goal of using them in fuel cells [1-7].

Amongst the possible electrocatalysts, platinum seems to be the most active metal for

dissociative adsorption of most low molecular weight alcohols (e.g., methanol), although it is

well-known that platinum is readily poisoned by strongly adsorbed species; this

disadvantage of platinum has forced the scientific community to shift to binary and/or ternary

platinum-based electrocatalysts (e.g., Pt-Ru, Pt-Ru-W) to minimize the poisoning effect [8-

12]. Gold, on the other hand, has been generally recognized as a poor catalyst; however, it

was recently reported that when this element is subdivided down to the nanoscale, it

becomes a very reactive material [13-15]. In addition, gold has proven to have higher

resistance to poisoning than platinum [16]. Gold electrocatalysis has been mainly studied

using nanoparticles, typically on a support [17-19]; however, nanoparticles may suffer from

shape stability issues and sintering over time, which reduces their active surface area and

with that their functionality [20,21]. Another alternative is to use nanoporous gold (NPG),

which is typically formed by dealloying Ag-Au alloys. During dealloying, one starts with a

monolithic alloy in any form (e.g., bulk or thin film) and selectively dissolves the less-noble

element from the alloy (silver in the case of Ag-Au alloys) leaving an interconnected

ligament/pore structure with a very good control of porosity and size [22-24]. Clearly, NPG

174

has a desirable structure for catalysis, not only because of its high surface area, but also

because it does not require a support.

The catalytic/electrocatalytic properties of NPG, and of gold nanoparticles, are related

to the high density of atomic steps and kinks on the surface, which act as active sites for

catalysis [25]. Moreover, the residual silver in NPG (mostly retained in the core of the

ligaments) could play a significant role in its catalytic abilities, not only helping during the

adsorption/dissociation of molecules during chemical/electrochemical reactions, but also

stabilizing the surface steps and kinks [25-27]. Unfortunately, NPG is prone to coarsening of

the structure by the rapid surface diffusion kinetics of gold along the alloy/electrolyte

interface, which reduces its functionality due to loss of surface area [28,29]. Adding small

amounts of platinum to the Ag-Au master alloy efficiently suppresses the coarsening of the

structure, thanks to the lesser surface diffusivity of platinum with respect to gold [30,31]. The

presence of platinum and gold on the surface of the nanostructure may offer unique catalytic

properties thanks to the synergy between these two elements [32-39]. Furthermore, the

presence of gold, and its strong interaction with oxygen species, may also reduce the

poisoning effect of platinum [32,40].

The main objective of the present study is to evaluate the electrocatalytic abilities of

nanoporous metals towards ethanol electro-oxidation in alkaline electrolyte. Specifically,

nanoporous structures formed on Ag-Au and on Ag-Au-Pt alloys, with a systematic variation

in the platinum content and the same silver content as in the binary alloy, were studied. By

selectively removing some of the silver from the alloy precursors, porous nanostructures

were formed with well-controlled length scale and morphologies. The effect of the platinum-

enriched nanoporous structures, obtained by adsorbate-induced surface segregation

mechanism, towards the electro-oxidation of ethanol was also evaluated.

175

5.2 Experimental

5.2.1 Materials, dealloying procedures and nanoporous characterization

Ag-Au alloy with 77 at.% silver was obtained from as cold-rolled 200 m sheet from

Goodfellow Metals, Cambridge, UK. Ag-Au-Pt alloys with platinum content of 1, 2 and 3

at.% and 77 at.% silver were obtained as cold-rolled sheet from Ames National Laboratory –

US Department of Energy, Iowa, USA. For all the experiments, specimens were cut into

strips of approximately 4 mm by 10 mm before annealing at 900 °C for 5 h in the case of the

binary alloy and at 975 ºC for 15 h in the case of the ternary alloys. In all cases, the




Miccroshield) to mask the junction and the isolated copper wire. In all cases, dealloying was

done from both sides of the sample.

Dealloying was carried out potentiostatically in a three-electrode electrochemical cell,

with platinum coil as the counter electrode and mercury/mercurous sulphate (MSE, 0.64 V

vs. SHE) as the reference electrode. The reference electrode was housed in a separate

compartment and connected to the electrochemical cell via a Luggin probe. In all cases

0.77 M HClO4 solution, prepared from Analar grade HClO4 (Alfa-Aesar, 62%), was used. All

solutions were prepared with 18 MΩ∙cm de-ionized water and de-aerated by high-purity

nitrogen purging (min. purity: 99.998%). The electrochemistry was performed using a Gamry

Reference 600TM potentiostat. All specimens were dealloyed at 25 °C, passing an anodic

charge density of 5 C cm-2 at 0.55 V vs. MSE. For some selected samples, the fraction of

platinum exposed on the surface of the ligaments was tuned by the adsorbate-induce

surface segregation phenomenon at 425 °C in the presence of laboratory air. For that

176

purpose, the connecting wire was removed immediately after dealloying and the specimens

were thoroughly rinsed with de-ionized water and dried with air before exposed them to the

target temperature. This phenomenon has been previously reported by the authors [41].

A detailed description of characterization of these nanoporous structures has been

given elsewhere [30]. For the microstructural characterization (e.g., ligament size),

dealloyed samples were manually broken under tension in air and the fracture surface was

photographed using a scanning electron microscope (SEM – Hitachi S-5200) with an

accelerating voltage of 20 kV. The SEM pictures were later analyzed with the Image Tool

software (provided by The University of Texas Health Science Centre in San Antonio, USA)

to accurately determine the ligament size (here equivalent to the average width

perpendicular to the ligament edge). Transmission electron microscope (TEM – Hitachi HD-

2000) was also used for the nanoporous characterization; for this purpose, selected

samples were embedded in low viscosity resin (SPI-PON 812) and untramicrotomed to 30 –

50 nm thickness using Leica UltraCut R instrument equipped with a diamond knife. An

accelerating voltage of 200 kV was used.

The true area of the electrodes was estimated by means of voltammetric profiles in the

double layer region of potentials (i.e., -0.24 to 0.05 V vs. MSE) at different scan rates in 1 M

HClO4 solution [42-43]. In all cases, 28 µF cm-2 was considered the baseline double-layer

capacitance for polycrystalline gold and platinum [42]. This assumption proved to be valid

after comparing the results with Brunauer-Emmett-Teller (BET) surface area and impedance

measurements [30].

The equivalent area of platinum on the surface of the ligaments was estimated by

underpotential deposition (UPD) of hydrogen [44-45]. All specimens were immersed in 1 M

H2SO4 solution (EMD, 95-98%) in a three-electrode cell with a MSE as a reference

177

electrode, a platinum wire as counter electrode and the dealloyed specimen as a working

electrode. The solution was de-aerated for approximately 15 min before the experiment.

Cyclic voltammetry (CV) curves were obtained at 25 °C between -0.63 and 0 V vs. MSE, at

a scan rate of 20 mV s-1. The equivalent area of platinum on the surface of the structure was

calculated by integrating the hydrogen adsorption/desorption regions to obtain the charge

related with the formation of a hydrogen monolayer, assuming that the charge associated

with the monolayer formation in polycrystalline platinum was 210 µC cm-2 Pt [46-47]. The

fraction of platinum on the surface of the resulting structure was estimated by dividing the

equivalent area of exposed platinum by the estimated true area of the electrode. The

roughness factor (Rf), as an indication of the developed surface area during the dealloying

process, was determined by dividing the true area of the electrode by its geometrical area.

5.2.2 Electro-oxidation of ethanol

In all cases dealloyed specimens were immersed in 4 M KOH (Aldrich, 90%) - 1 M

ethanol (Aldrich, ≥ 99.5%) solution. The KOH/C2H5OH solution was prepared by

diluting/dissolving the reactants with de-ionized water with a resistivity of 18 MΩ∙cm. Prior to

any measurement, the solution was de-aerated by high-purity nitrogen purging (min. purity:

99.998%). A Gamry Reference 600TM potentiostat was used to perform CV between -0.9

and 0.2 V vs. mercury/mercury oxide (Hg/HgO – 20% KOH, 0.1 V vs. SHE) reference

electrode. All current densities were normalized by the true area of the electrodes unless

otherwise stated. Polycrystalline platinum was used as a control material.

Additionally, potentiostatic experiments at -0.6 V vs. Hg/HgO (in the case of the

ternary alloys) and at -0.1 V vs. Hg/HgO (for the binary alloy) were carried out for 4000 s in

the KOH/C2H5OH solution. During these experiments, the solution was agitated with a

178

magnetic stirrer with an approximated rotation velocity of 700 rpm. The carbonate

concentration in the reaction medium after the oxidation was determined by titrating 5 mL of

resulting solution with 1.2 M BaCl2 solution (ACP, 99%). To avoid the precipitation of

Ba(OH)2, the pH was brought down from ca. 15.0 to ca. 10.0 by adding ca. 2 mL of

concentrated HCl (BioShop, 36.5-38%) and a few drops of 2 M HCl solution. The HCl was

added as quickly as possible to minimize the unnecessary exposure of the solution to air.

Right after that, a solution of BaCl2 was added drop wise while the pH was kept between

10.0 and 10.5 to avoid the dissolution of BaCO3, which occurs at pH values around 6 [48-

50]. Filtration of the obtained precipitate was done using a 0.45 µm membrane filter (PALL)

followed by a gravimetric determination. The contribution to the carbonate content from

original slight carbonation of the KOH was measured by titrating the solution prior to the

reaction.


The ligament size (here defined as the average width perpendicular to the ligament

edge) for some selected samples is shown in Figure 5.1. As can be seen, the ligament size

decreased by increasing the platinum content of the alloy. The presence of platinum in the

alloy reduced gold mobility, hindering the electrochemical coarsening of the structure – in

the binary alloy (Figure 5.1a) the average ligament size was ca. 14 nm, whereas for the

nanostructure formed on the alloy with only 1 at.% Pt (Figure 5.1b) the ligament size

dropped to ca. 7 nm (see also Figure 5.2). The inserts in Figure 5.1 showed TEM images of

the two structures suggesting that the pore sizes (light regions) were ca. 17 nm and 8 nm for

the binary and ternary alloy respectively, which agrees well with the pore size distribution

analysis done by the Brunauer-Emmett-Teller (BET) method (not shown here but reported

179

elsewhere [30]). With higher concentrations of platinum in the precursor, the size of the

ligaments kept decreasing to a value of ca. 4 nm when the precursor had 3 at.% Pt (see

Figure 5.2). The fraction of platinum on the surface of the ligaments was 1.4, 3.0 and 6.6 %

for the alloys with 1, 2 and 3 at.% Pt. Maroun et al., [51] and Bergbreiter et al. [52] found

that individual Pt atoms did not readily participate in the UPD of hydrogen, but that small

domains rich in platinum were required; therefore, after estimating the minimum distance

between platinum atoms on the surface of the ligaments, it is believed that the fraction of

platinum reported here only represents the minimum coverage of the ligament surface.

Further research has to be done to understand/resolve this issue. The roughness factor (Rf,

insert in Figure 5.2) increased with the platinum content of the structure, showing that by

reducing the ligament size, the increase in surface area was substantial.

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Figure 5.1 SEM images of the freshly-developed nanoporous structure at 0.55 V vs. MSE and 25 °C in 0.77 M HClO4: (a) Ag77:Au23 (b) Ag77:Au22:Pt1. In all cases, 5 C cm-2 were passed. Inserts in (a) and (b) are TEM image of the same structures.

181

Figure 5.2 Average ligament size of the freshly-formed nanoporous structures formed at 0.55 V vs. MSE and 25 °C in 0.77 M HClO4. The insert shows the roughness factor (Rf) as a function of the platinum content of the precursor. Error bars represent 95% confidence interval (CI) calculated from at least 30 measurements in the case of the ligament size, and triplicate runs for Rf.

As mentioned before, ethanol electro-oxidation was evaluated in alkaline electrolyte (4

M KOH). The concentration of the supporting electrolyte was experimentally selected after

observing that at lower concentrations, the reaction was limited by depletion of OH- owing to

the extremely high nominal current density of the nanostructures (see below). Figure 5.3a

shows the CV profiles of all the nanoporous structures in the supporting electrolyte, showing

that NPG had an extended double-layer region between -0.9 V and 0.25 V vs. Hg/HgO. At

0.25 V an oxidation peak started that was ascribed to the formation of a monolayer of gold

surface oxides in highly basic solutions; a reduction peak at 0.1 V was associated with the

electrochemical reduction of those oxides. For ternary alloys, the oxidation of gold started at

~0.15 V and the reduction peaks were slightly shifted to less positive potentials, if compared

with that for NPG. The magnitude of this reduction peak decreased with increasing platinum

182

content of the alloy. The reduction peaks at ca. -0.5 V, which also change with the platinum

content of the alloy, were attributed to the presence of oxidized platinum in the structure (no

influence of silver was detected at this potential). Figure 5.3b shows the CV profiles for the

ethanol oxidation reaction of NPG and of the nanostructure formed on the ternary alloy with

1 at.% Pt. In both cases, the vertical axis corresponds to the nominal current density (i.e.,

normalized by the geometrical area of the electrode), and the positive scan limit was fixed at

0.2 V to avoid the region in which gold and silver oxides might form. Clearly, both

nanostructures, but particularly the ternary one, had very high anodic current densities. In

NPG, the rising current in the forward scan (at ca. -0.4 V) can be ascribed to the

characteristic ethanol oxidation on the surface of the structure; a peak potential at 0 V

corresponded to a current density of ca. 2.5 x 105 µA cm . In the case of the

nanostructure formed on the ternary alloy, there was a sharp increase in current at much

more negative potential than in the binary nanostructure (ca. -0.82 V), which agrees well

with the fact that small organic compounds (e.g., formaldehyde, methanol, ethanol) had

been oxidized on Au-Pt electrodes at lower potentials than on gold [53,54]. The broad peak

potential in the forward scan started at -0.5 V and finished at 0.05 V, and corresponded to a

current density of ca. 3.0 x 105 µA cm . During the backward scan at least two not-well

defined oxidation peaks were observed at ca. 0 and -0.35 V (better resolved peaks were

identified when the positive scan limits were set at higher values - e.g., 0.4 V vs. Hg/HgO,

results not shown here). These two peaks represent the reaction resuming after whatever

was blocking the surface of the nanostructure at higher potentials dissolves off or was

reduced. The same behaviour was followed by the other two ternary nanostructures. The

insert in Figure 5.3b shows the CV of polycrystalline platinum electrode in the same solution

containing ethanol. The oxidation peaks in forward and backward scans were located at ca.

-0.3 V; which is in agreement with the general trend reported for platinum electrodes in the

183

literature [55,56]. The nominal current density in the ternary nanostructure was more than

50 times higher than that on the platinum electrode.

Figure 5.3 (a) CV profiles of the different nanoporous structures in 4 M KOH; (b) CV profiles of NPG and of the nanoporous structures formed on the alloy with 1 at.% Pt in 4 M KOH - 1 M C2H5OH solution. The insert in (b) shows the CV profile of polycrystalline Pt in 4 M KOH - 1 M C2H5OH solution. In all cases the vertical axis corresponds to the nominal current density. The original platinum composition is shown in (a). All CV profiles were obtained at 10 mV s-1 and 25 °C.

184

Figure 5.4 shows the CV profiles of all ternary nanostructures in KOH - C2H5OH

solution. The vertical axis in this figure corresponds to the true current density (i.e.,

normalized by the true area of the electrode) of the reaction. As can be seen, there was no

significant difference in the oxidation current density among ternary nanostructures at

potentials lower than 0.6 V; however, at higher potentials the nanostructure formed on the

alloy with 1 at.% Pt had slightly higher ethanol oxidation current density than the other two

nanoporous metals. A similar trend was observed by the authors when oxidizing methanol in

alkaline electrolyte [57]. It is hypothesized that the relative surface concentration (ratio

between platinum and gold) and the fact that the reaction was run in alkaline media are

mostly responsible for this trend [58]. The insert in Figure 5.4 shows the specific activities

(taken at -0.6 V vs. Hg/HgO) of all ternary nanostructures after multiple cycles. For all the

ternary nanostructures, there was a decrease in the specific activity after the first cycle;

however, no significant differences were observed after the following cycles. On the binary

nanostructure, the specific activity significantly decreased with every cycle.

In agreement with the trend observed by Beltowska-Brzezinska and Luczak [59], the

electrocatalytic activity of ternary nanostructures towards ethanol oxidation was higher than

that towards methanol oxidation under similar experimental conditions (reported previously

by the authors [57]). In the case of the ethanol oxidation, the reaction started at ca. -0.82 V,

which was approximately 0.12 V more negative than in the case of methanol, and

significantly higher true current densities were recorded at potentials lower than -0.5 V. As

discussed in detailed by Beltowska-Brzezinska and Luczak, by increasing the length of the

aliphatic carbon chain, the free-enthalpy of adsorption of alcohol molecules increased,

resulting in larger oxidation currents; in addition, the decrease in the α C–H bond energy in

the secondary alcohol (e.g., ethanol), lead to lower values of the apparent activation energy

than in primary alcohols. In simple words, secondary alcohols are more easily oxidized than

185

primary ones on gold-rich catalyst. A similar trend was observed for NPG: the ethanol

oxidation reaction started at ca. -0.4 V, whereas methanol oxidation started at ca. -0.15 V.

Additionally, it was observed that NPG showed higher maximum true oxidation current

density than ternary nanostructures; nevertheless, the onset of the reaction on NPG started

at ca. 0.4 V more positive than in the ternary nanostructures.

Figure 5.4 CV profiles for the electro-oxidation of ethanol in 4 M KOH – 1 M C2H5OH solution. The insert shows the summary of specific activities (S.a) of the ternary alloy structures for the C2H5OH oxidation reaction at -0.6 V vs. Hg/HgO and of the binary structure taken at -0.1 V vs. Hg/HgO; the vertical axis is the current density per true area of the electrodes. Details about the structures and the number of scan cycles are indicated in the figures. All CV profiles were recorded at 10 mV s-1 and 25 °C.

Potentiostatic oxidation of ethanol on all nanostructures is shown in Figure 5.5. As

can be seen in this figure, for the ternary structures the current density dropped from ca. 90

µA cmtrue to ca. 40 µA cmtrue

in approximately 200 s and then remained constant; while for

NPG the current dropped from 170 µA cmtrue and steadily decreased up to the end of the

186

experiment. These trends agreed well with the predictions made from their respective

voltammograms (e.g., Figure 5.4 for ternary nanostructures). Although NPG initially

displayed higher oxidation current density, after 1500 s the electrocatalytic activity of ternary

nanostructures increased in relation to NPG, i.e., there was a pronounced deactivation of

NPG with respect to the other alloys. This deactivation could be related to the formation of a

“passive” layer that blocks the active sites of the catalyst, as has been reported before for

gold nanoparticles and other catalytic materials [6,60]. No significant difference was

observed in the catalytic activity between ternary nanostructures.

Figure 5.5 Current density observed during a prolonged oxidation of 1 M C2H5OH in 4 M KOH solution at -0.6 V vs. Hg/HgO in the case of ternary nanostructures and at -0.1 V vs. Hg/HgO in the case of NPG. In all cases, the solution was de-aerated by high-purity nitrogen and agitated with a magnetic stirrer at ca. 700 rpm. The insert shows the concentration of carbonate produced after 4000 s with respect to the platinum content of the precursor.

187

In alkaline solution, different mechanisms have been proposed to explain how the

ethanol oxidation reaction proceeds on gold, identifying acetate (that could eventually being

converted to a ketone) as the main product of the oxidation [60-62]. The same product have

been identified when platinum is used as a catalyst [63]; however, it has been reported that

by using platinum-based electrocatalysts (e.g., Pt-Rh), the selectivity for carbonate over the

other products increases [64]. According with the work done by Beltowska-Brzezinska in Au-

Pt electrodes, the oxidation of secondary alcohols such as ethanol in alkaline electrolyte

would proceed as follows: initially, there is an interaction between pre-adsorbed OH- groups

and the alcohol molecules to initiate the cleavage of the C-H bond, then the intermediate

that is formed would react further with additional adsorbed OH- groups to end up with two

OH- groups directly linked to the α-carbon; finally, this second intermediate would react with

hydroxyl groups in solution (similar to a Eley-Rideal step) until the acetate ion is formed [54].

In our case however, analyses done in the solution after the potentiostatic oxidation

experiments showed that ethanol was oxidized to carbonate when the reaction was

catalyzed by the ternary nanostructures, with a carbonate concentration that increased by

increasing the platinum content of the alloy precursor (insert in Figure 5.5). The low amount

of carbonation that occurred during the titration procedure was demonstrated by the nearly

zero carbonate analysis obtained for NPG. More importantly, the current efficiency analysis

(considering carbonate as the only product) suggested that carbonate was the only product

of the oxidation. This result clearly implies that ternary nanoporous structures (with a small

amount of platinum) are sufficiently active to cleave the C-C bond in ethanol, which

undoubtedly is the most difficult step in the reaction. It is important to mention that in the

case of the nanostructures formed on the alloy with initially 2 and 3 at.% Pt, the current

efficiency was higher than 100%, which we cannot rationalize at the moment. All

experiments were conducted in a well de-aerated system, and the sample used to

188

determine the concentration of carbonate in solution had minimal contact with air before any

analysis.

After these prolonged experiments and after multiple potential cycling, no significant

change in the ligament size of the structures was observed, as can be seen in Figure 5.6 for

NPG and for the nanostructure formed on the alloy with 1 at.% Pt. In the case of NPG, the

ligament size after cycling the potential ten times (Figure 5.6a), and after holding the

reaction for 4000 s at -0.1 V (Figure 5.6c) was ca. 14 nm, with no change with respect to the

ligament size of freshly prepared structures (see Figure 5.1a and Figure 5.2). The same is

true for the ternary structure formed on the alloy with originally 1 at.% Pt, in which no

difference in ligament size was observed under any of the experimental condition used here

(Figure 5.6b and d). Neither was any significant difference seen in the other two ternary

structures (not shown here).

Figure 5.6 SEM images of nanoporous structures used to oxidize ethanol in alkaline electrolyte: (a) NPG after 10 potential cycles, (b) ternary nanostructure formed on the alloy with 1 at.% Pt after 10 potential cycles, (c) NPG after 4000 s reaction at -0.1 V, (d) ternary nanostructure formed on the alloy with 1 at.% Pt after 4000 s at -0.6 V.

189

The fraction of platinum atoms on the surface of the ligaments can be modified not

only by adjusting the dealloying conditions (e.g., temperature, charge density passed)

[30,57], but also by inducing surface segregation of platinum at moderate temperature in the

presence of oxygen [41]. Changing the surface composition of the structure will have an

impact on the catalytic response of these nanoporous metals. Figure 5.7 shows the CV

profiles of all ternary nanostructures after inducing segregation of platinum at 425 °C. As

observed in the figure, the modified structure formed on the alloy 1 at.% Pt displayed the

highest current density with a broad peak current of ca. 1300 µA cmtrue , which represents an

increase of almost 900% with respect to the current density of the equivalent freshly

dealloyed structures. The insert in Figure 5.7 shows a comparison between the specific

activities (taken also at -0.6 V) of as-dealloyed structures and structures after segregation.

The ternary nanostructure formed on the alloy with 1 at.% Pt showed a specific activity that

was more than 6 times higher after segregation of platinum than the as-dealloyed

nanostructures. The other two ternary nanostructures also showed higher specific activity

towards ethanol oxidation; however, the increase with respect to the as-dealloyed structures

was much lower. In all cases, broad and well-defined oxidation peaks at -0.3 V were

observed during the backward sweep. It is believed that the relative ratio between gold and

platinum on the surface of the ligaments, and the traces of silver identified on the surface of

the structures right after segregation had a role in the observed trend [41]. It is well-known

that traces of silver could significantly change the catalytic behaviour of nanoporous metals

[25,65,66].

190

Figure 5.7 CV profiles for all ternary nanoporous structures in 4 M KOH – 1 M C2H5OH solution. In all cases, the structures were exposed to 425 °C for 2 h in the presence of laboratory air. The insert shows the summary of specific activities (S.a) of the ternary alloy structures for the C2H5OH oxidation reaction at -0.6 V vs. Hg/HgO before and after segregation; the vertical axis is the current density per true area of the electrodes. The CV profiles were obtained at 10 mV s-1 and 25 °C. The original platinum composition is shown in the figure and the current density was normalized by the true area of the electrode.

5.4 Conclusions

a) Novel nanoporous structures have been formed by electrochemical dealloying of Ag-Au

and Ag-Au-Pt alloys, with 1, 2 and 3 at.% Pt, in HClO4 solution. In the ternary

nanostructure, the characteristics of the resulting nanoporous metals depend on the

platinum content of the precursor. By increasing the platinum concentration, the

ligament size decreases and the platinum coverage on the surface of the ligaments

increases, although it is considered that this coverage represents the minimum fraction

of platinum on the ligament surface.

191

b) Nanoporous metals formed on ternary alloys displayed a very high electrocatalytic

response towards ethanol oxidation in alkaline electrolyte. In all ternary nanostructures,

the onset of the oxidation reaction started at approximately 0.4 V more negative than

that in NPG, with the nanostructure formed on the alloy with 1 at.% Pt displaying the

highest current density at potentials higher than 0.6 V (no significant difference was

observed at lower potentials).

c) Compared with the methanol electro-oxidation reaction, NPG displayed significantly

higher true activity towards ethanol electro-oxidation (i.e., in alkaline electrolyte

secondary alcohols are more easily oxidized than primary ones on gold). A similar effect

was also observed in ternary nanostructures where the onset of ethanol electro-

oxidation was 0.12 V more negative than that in methanol oxidation.

d) The concentration of carbonate detected after potentiostatic oxidation of ethanol

increased with the platinum content of the alloy precursor. More importantly, based on

the current efficiency of the process (considering carbonate as the only product), it was

determined that carbonate was the only product of the oxidation, suggesting that the

activity of ternary nanostructures was so spectacular that the C-C bond was easily

cleaved. No detectable concentration of carbonate was found when the reaction was

catalyzed by NPG.

e) The ligament size in all nanostructures did not significantly change after running the

ethanol electro-oxidation reaction through multiple cycles or after relatively long

potentiostatic experiments; nevertheless, NPG displayed a constant decrease in

specific activity in time; whereas ternary nanostructures showed much higher stability.

The blockage of the active catalytic sites, especially on NPG, due to the formation of a

“passive” layer during the reaction has been considered as a possible reason for the

electrocatalytic activity to decay.

192

f) The effect of enriching the surface of the ligaments with platinum (after inducing

segregation at moderate temperatures in air) showed that, once again, the structure

formed on the alloy with 1 at.% Pt displayed the highest true current density amongst

the other ternary nanostructures, with a 900% increase with respect to the current

density of freshly-dealloyed structures. The other two ternary nanostructures also

showed an increase in the specific activity towards ethanol oxidation; however, the

increase with respect to the as-dealloyed structures was much lower.

5.5 Acknowledgment

The authors thank I. Gourevich, from the Center for Nanostructure Imaging at the

University of Toronto for his help imaging the nanoporous structures. The financial support

of NSERC is also gratefully acknowledged.

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200

Chapter 6. Conclusions, Contributions and Recommendations for

Future Work

6.1 Overall conclusions

Nanoporous metals formed by electrochemical dealloying of Ag-Au-Pt alloys, with a

systematic variation of the platinum content, were investigated. Nanoporous gold (NPG),

formed on Ag-Au alloys with the same content of silver than ternary alloys, was included in

the investigation as a control material. The presence of small amounts of platinum limited

the mobility of gold on the surface of the material, reducing the ligament size of the

nanoporous structure and improving its stability towards post-porosity coarsening. By

adding only 1 at.% Pt to the Ag-Au, the ligament size decreased by 50% if compared with

that in NPG. By increasing the Pt content of the precursor, further reduction in the ligament

size was observed; however, the major improvement was achieved with only 1 at.% Pt. The

platinum coverage of the surface of the ligaments increased with the platinum content of the

precursor; however, changes in the dealloying conditions (i.e., dealloying temperature and

charge density passed) changed the fraction of platinum on the surface of the

nanostructure. Under the proper experimental conditions (i.e., moderate temperature in the

presence of air) the fraction of platinum on the surface of the ligaments can be also modified

by an adsorbate-induced surface segregation mechanism, which have not been reported

before for this kind of nanomaterials. Finally, the electrocatalytic abilities of the ternary

nanoporous metals (formed under different experimental conditions and protocols) were

significantly higher than those of NPG. The presence of platinum not only increased the

stability of the structures but also modified the catalytic response and selectivity of these

nanoporous metals.

201

The overall conclusions derived from this research are as follows:

Formation of nanoporous metals from Ag-Au-Pt noble alloys with a systematic variation of

platinum content and the same concentration of the less-noble metal (Ag)

The presence of platinum did not influence the ‘empirical’ critical potential of the

alloys. Conventionally, it was predicted that the presence of platinum would lower

the ‘empirical’ critical potential by slowing down the surface diffusion of gold and,

therefore, facilitating the dissolution of silver (i.e., passivation on the surface by gold

would be less favored); however, the lack of influence of platinum on the critical

potential suggested that the critical potential depends mostly on the rate limiting step

in the dissolution process (i.e., nucleation of terrace vacancies) and not on the

balance between dissolution and surface diffusion. In simple words, alloys with the

same concentration of the less-noble metal (i.e., silver) will have virtually the same

critical potential.

A significant reduction in the ligament and pore sizes was observed in the ternary

structures dealloyed under the same experimental conditions as their binary

counterpart. Moreover, the biggest reduction in the length scale of porosity was

observed in the lowest platinum content alloy; however, further increase in the

platinum content showed additional reduction in the feature size of the structure.

This reduction of the ligament and pore sizes in ternary alloys induced an increase in

the developed surface area, with the largest increment occurring at the lowest

platinum addition. These effects have been associated with the limited

electrochemical coarsening of the ternary nanostructures thanks to the platinum

decoration of the step edges of the growing gold-rich structures.

For the binary and ternary alloys the initial crystal face orientation (FCC in both

cases) was preserved during dealloying, which agrees with the behaviour reported in

202

literature. More importantly, neither foreign or secondary phases were identified in

ternary alloys, which is rather a common phenomenon in alloys with platinum.

The amount of silver retained in the dealloyed layer (for the same amount of charge

density passed) was always higher in ternary alloys than in the binary alloy.

Moreover, a very shallow concentration gradient across the dealloyed layer was

measured for the ternary alloys, which is close to the classical parting limit of ca. 55

at.%; whereas for NPG the concentration gradient was significantly higher - while

dealloying was progressing and silver was removed from the dealloying front, the

silver removal from the already formed ligaments continued, inducing a more

pronounced silver gradient across the dealloyed layer. The presence of platinum on

the structure reduced this gradient (i.e., the post-porosity – or electrochemical –

coarsening of the structure was hindered) keeping higher silver contents retained in

the dealloyed material. The difference in the silver concentration across the

dealloyed layer induced thicker layers in ternary alloys (in average 20% thicker

layers than in the binary alloy) to compensate for the amount of silver that has to be

removed.

The platinum composition across the dealloyed layer remained relatively constant

across the dealloyed layer, whereas the fraction of platinum on the surface of the

ligaments increased with the platinum content of the precursor. Previous studies

suggested that individual platinum atoms do not readily participate in the UPD of

hydrogen, and that small platinum domains are required for hydrogen adsorption.

Therefore, based on the original platinum content of the ternary alloy precursors, and

the estimated distance between platinum atoms in these nanoporous material, it is

believed that this fraction of platinum, obtained by UPD of hydrogen, represents the

minimum platinum coverage of the ligaments. In addition, the ratio gold-platinum in

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the dealloyed layer was nominally the same as the original alloy, confirming that only

silver was removed during the corrosion process.

The relative shrinkage (or contraction) of the dealloyed layer did not show any

measurable difference between alloys. Even though platinum did increase the

stability of the nanoporous materials by increasing the post-porosity coarsening

resistance, it did not have a distinctive effect on the shrinkage of the layer.

The stability of the nanoporous structure formed on ternary alloys was confirmed

even in more aggressive environments, (i.e., high temperature exposure or

immersion in an electrolyte for at least two months). This observation indicated that

even under conditions that facilitate coarsening of the structure the presence of

platinum reduced gold mobility and, consequently, reduced the ligament size. Higher

stability was observed by increasing the platinum content of the precursor.

Direct comparison between the characteristics of real nanoporous metals, and

simulated nanostructures obtained by Kinetic Monte Carlo algorithm (i.e., MESOSIM)

was not possible. After modifying the original MESOSIM code to include a ternary-

noble element in the structure, it was concluded that the characteristics of simulated

nanostructures (e.g., ligament size, composition of the less-noble element retained

inside the layer) did not agree with the experimental results; moreover, it was

observed that in all simulated structures, but particularly in NPG, the lack of realistic

coarsening in the simulations could be partyially responsible for this disagreement.

Even though this KMC algorithm has been remarkably successful in reproducing the

electrochemical and morphological behaviours of dealloyed systems, the

simplifications that were made to this algorithm to reduce its computational cost

significantly limited its use.

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Effect of dealloying parameters on the most relevant characteristics of the dealloyed

material

Increasing the temperature of the electrolyte (from 25 °C to 60 °C) increased the

tendency of the nanoporous structures to coarsen; however, the presence of

platinum significantly minimized that effect, with the highest resistance to coarsening

observed in the structure with the highest platinum content in the precursor. The

thickness of the dealloyed layer (for the same charge density passed) increased with

the dealloying temperature in all the nanostructures formed on ternary alloys;

whereas for the binary nanostructures it steadily decreased. This is again related to

the amount of silver retained in the dealloyed layer. In NPG, due to the accelerated

coarsening of the structure, the amount of silver in the layer was significantly

reduced; whereas in ternary nanostructures the amount of silver was much greater

and close to the classical parting limit of 55 at.%. This tendency could be related with

the mobility of platinum at different temperatures: an increase in the electrolyte

temperature induced an easier and more efficient decoration of the step edges,

retaining more silver on the structure and, therefore, increasing the thickness of the

layer.

The platinum coverage on the surface of the ligaments increased with the dealloying

temperature. Although the actual mechanism through which that change occurred is

unknown, it was hypothesized that it could be associated with the dealloying

mechanism itself (where more-noble metals cover almost entirely the surface of the

ligaments) and/or parallel processes (e.g., segregation of platinum atoms to the

surface of the ligaments due to preferential adsorption of OH groups) that were

favored under such experimental conditions. The same tendency was observed

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when higher charge densities were removed: by increasing the charge density, the

platinum coverage on the ligaments increased.

All nanoporous metals formed on ternary alloys, but particularly the nanostructure

with the highest platinum content, displayed more stability and higher coarsening

resistance at lower dealloying potentials. Reducing the dealloying potential

significantly increased the dealloying time of all alloys, and with that the possibility of

significantly coarsening the ligaments, as happened in the case of NPG.

Increasing the dealloying time increased the charge density during dealloying, and

with that the tendency of the ligaments to coarsen. However, the average ligament

size in the nanostructures formed on ternary alloys did not drastically change with

the charge density; moreover, higher coarsening resistance was obtained by

increasing the platinum content of the alloy precursor. In the binary alloy, on the

other hand, the higher the charge density, the bigger the ligament size. The

thickness of the dealloyed layer increased with the charge density in all

nanostructures, but more so in the case of ternary alloys; the average concentration

of silver retained in the layer evidently decreased in the binary structure (due to

coarsening), whereas in ternary alloys the reduction in silver content is less

pronounced.

Adsorbate-induced surface segregation of platinum to the surface of the ligaments

Exposure of as-dealloyed ternary nanostructures to moderately elevated

temperature (< 500 °C), in the presence of air, induced an increase in the platinum

coverage on the surface of the ligaments, with a maximum (approximately 20% with

respect to the as-dealloyed coverage) obtained at 425 – 475 °C. It is believed that

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the strong interaction between oxygen and platinum atoms promoted that

enrichment, in contrast with the reported gold-enrichment behavior of binary Au-Pt

system in vacuum. At higher temperatures (> 500 °C), the fraction of platinum on the

surface decreased, which agreed well with the temperature at which oxygen

desorbed from surface platinum atoms.

Even thought the actual transport mechanism of platinum during segregation is not

clear, it is recognized that lattice diffusion is virtually quenched in the studied range

of temperatures (< 500 °C). We hypothesized that there may be analogous

phenomena in nanoparticle systems to explain this phenomenon.

By exposing nanoporous metals to air, different adsorption states of Pt-O were

formed on the surface of the ligaments. Small differences in the binding energies of

those Pt-O compounds with respect to the values reported in literature were

associated with the presence of gold and/or silver and the high concentration of

steps on the surface. More importantly, after exceeding the experimentally

determined temperature threshold (ca. 500 °C), platinum tended to regain its metallic

state. The same result was obtained after electrochemically reducing the surface

oxides in H2SO4 solution right after the heat treatment.

The roughness factor (Rf) of the nanoporous structures, here used as an indication

of the thermal coarsening of the nanostructure, decreased for all alloys with

increasing exposure temperature; nevertheless, the resistance to coarsening

increased by increasing the platinum content of the precursor. A significant drop of

the Rf was observed at ca. 500 °C exposure in air, which agreed well with the

reduction of platinum coverage on the surface of the ligaments.

The platinum enrichment on the surface of the nanostructure in air was a process

that occurred in a very short time. An almost maximum coverage was observed in

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only thirty minutes of exposure to the appropriate temperature, with small changes at

longer times: after more than seventy hours under the effect of temperature in the

presence of air, no changes in the platinum content of the sample or the R f (i.e.,

ligament size) was detected. An apparent equilibrium condition was observed even

at lower temperatures than that at which the maximum coverage occurred.

In the absence of oxygen (i.e., heat treatment in an inert atmosphere, Ar-H2), no

segregation of platinum was detected at any temperature; in fact it desegregated as

already known for binary Au-Pt systems . More importantly, in the absence of

oxygen, a massive coarsening of the structure was observed (i.e., the ligament size

was significantly bigger than that observed in the presence of air). NPG, on the other

hand, did not show any significant difference if annealed at moderate temperature in

air or in an inert atmosphere. Clearly, the co-segregation of platinum and oxygen

hindered the coarsening process in these nanoporous metals.

Besides the clear tendency of platinum to segregate to the surface of the ligaments,

it was determined that silver was also exposed to the surface of the ligaments after

the exposure of as-dealloyed samples to moderate temperatures. Considering the

fact that significantly higher silver concentrations were retained inside the ligaments

in ternary nanostructures, and that the relatively higher mobility that gold and

platinum had at the studied temperatures (i.e., coarsening), it was expected that

silver was exposed on the surface of the ligaments; nevertheless, it was observed

that this tendency was minimized by increasing the platinum content of the

precursor, which decreased the coarsening of the structure.

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Electro-oxidation of methanol in alkaline and acidic media on nanoporous metals

Much higher activity towards methanol electro-oxidation in alkaline electrolyte was

observed on ternary nanostructures than in NPG: the onset of the reaction started at

more negative potentials in all ternary structures, with a peak potential that at. 300

mV more negative than the binary nanostructure and with a peak current that was at

least four times higher than that in NPG. The strong synergistic effect between gold

and platinum, and the smaller feature sizes on ternary nanoporous metals could be

associated with this increase in activity.

Among the three ternary nanostructures, the one formed on the alloy with 1 at.% Pt

showed the highest electrocatalytic activity in alkaline electrolyte. The relative ratio of

platinum and gold atoms on the surface of the ligaments was most likely associated

with this trend. On the contrary, in acidic electrolyte, ternary nanostructures

displayed increasing activity with increasing platinum content of the precursor (the

most active nanostructure under this conditions was the one formed on the alloy with

3 at.% Pt), which is in agreement with the literature concerning Au-Pt bimetallic

catalysts in acidic media. NPG only showed limited electrocatalytic activity in alkaline

electrolyte, but virtually none in acidic electrolyte in the range of potentials under

investigation.

Increasing the platinum content of the nanostructures changed the selectivity of the

reaction in alkaline electrolyte: the higher the platinum content on the nanoporous

metal, the higher the concentration of carbonate and the lower the concentration of

formate in solution. For NPG, only formate was detected as a major product of the

oxidation. In none of the cases, was formaldehyde detected, possibly due to the

disproportionation of formaldehyde in highly basic media (i.e., Cannizzaro’s

reaction).

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After characterizing the methanol electro-oxidation reaction by cyclic voltammetry at

different scan rates, it was concluded that this reaction was diffusion controlled in the

bulk solution boundary layer near the peak current density. The same conclusion

was obtained after studying the effect of the external agitation on the system.

The electrocatalytic response of all ternary nanostructures changed by changing

their dealloying conditions: samples dealloyed at higher temperatures showed higher

specific activity than those dealloyed at room temperature, moreover, samples in

which small charge densities were passed showed higher specific activity than

samples in which large charge densities were removed. Increasing the temperature

of the dealloying electrolyte increased the relative platinum coverage of the

ligaments, without changing much the ligament size, which correlate with the higher

specific activities and a higher amount of carbonate produced during the oxidation.

Higher charge densities increased the thickness of the layer, but decreased the

specific activity of the catalyst. The effect of mass transport limitations within the

layer are not negligible and could have a role in these trends.

By inducing segregation of platinum to the surface of the ligaments by heating in air,

it was found that the electrocatalytic response in alkaline electrolyte was enhanced,

particularly in the nanostructure developed on the alloy with only 1 at.% Pt. The

relative surface composition of the nanostructure (silver was detected in the surface

of the nanostructures) could contribute to the observed enhancement. The

concentration of carbonate produced for these nanostructures remained constant

between them, but significantly higher than the amounts produced by as-dealloyed

structures. The current efficiency analysis suggest that a significant amount of

formate was produced on the nanostructure formed on the alloy with originally 1 at.%

Pt after segregation; whereas carbonate was the dominant product on the

nanostructure with the highest platinum content. In acidic electrolyte, the most active

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nanostructure after segregation was again the one developed on the alloy with 3

at.% Pt.

Ethanol electro-oxidation on nanoporous metals in alkaline electrolyte

Nanoporous metals formed on ternary alloys displayed a very high electrocatalytic

response towards ethanol oxidation in alkaline electrolyte. In all ternary

nanostructures, the onset of the oxidation reaction started at approximately 0.4 V

more negative than that in NPG, with the nanostructure formed on the alloy with 1

at.% Pt displaying the highest current density at potentials higher than 0.6 V (no

significant difference was observed at lower potentials).

Compared with the methanol electro-oxidation reaction, NPG displayed significantly

higher true activity towards ethanol electro-oxidation. As has been well documented

in literature, in alkaline electrolyte secondary alcohols are more easily oxidized than

primary ones on gold. The increase in the free-enthalpy of adsorption and the

decrease in the C-H bond energy in secondary alcohols lead to higher oxidation

currents than in the case of primary alcohols. A similar effect was also observed in

ternary nanostructures where the onset of ethanol electro-oxidation was 0.12 V

more negative than that in methanol oxidation.

The concentration of carbonate detected after potentiostatic oxidation of ethanol

increased with the platinum content of the alloy precursor. More importantly, based

on the current efficiency of the process (considering carbonate as the only product),

it was determined that carbonate was the only product of the oxidation, suggesting

that the activity of ternary nanostructures was so spectacular that the C-C bond was

easily cleaved. No detectable concentration of carbonate was found when the

reaction was catalyzed by NPG.

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The ligament size in all nanostructures did not significantly change after running the

ethanol electro-oxidation reaction through multiple cycles or after relatively long

potentiostatic experiments; nevertheless, NPG displayed a constant decrease in

specific activity in time; whereas ternary nanostructures showed much higher

stability. The blockage of the active catalytic sites, especially on NPG, due to the

formation of a “passive” layer during the reaction has been considered as a possible

reason for the electrocatalytic activity to decay.

The effect of enriching the surface of the ligaments with platinum (after inducing

segregation at moderate temperatures in air) showed that, once again, the structure

formed on the alloy with 1 at.% Pt displayed the highest true current density

amongst the other ternary nanostructures, with a 900% increase with respect to the

current density of freshly-dealloyed structures. The other two ternary nanostructures

also showed an increase in the specific activity towards ethanol oxidation; however,

the increase with respect to the as-dealloyed structures was much lower.

6.2 Significance and contribution of the research

Although in the last few decades, the development and characterization of all kinds of

nanomaterials has gained a lot of attention from the scientific community, the search for

new, more efficient and more robust nano-sized materials is a very active field. The

formation, characterization and tunability of nanoporous metals from Ag-Au-Pt alloys, with

small platinum content, and with proven stability and flexibility, contribute to this field by

offering a fundamental understanding of the role of ternary elements in nanoporous metals;

moreover, it confirms the fact that dealloying is an pliable, clean and relatively easy way of

producing a new class of nanomaterials with remarkable properties.

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To the best of our knowledge, this research is the first comprehensive study to look in

depth in the effect that ternary elements have on the characteristics of the nanoporous

metals formed by electrochemical dealloying. The selection of the alloy precursors, with a

systematic variation of the platinum content and with the same silver content as the control

material (Ag-Au alloy with 77 at.% silver, which is an structure that has been characterized

for its more open and compliant structure) allowed us to identify not only small changes in

the nanoporous structures as a result of the presence of platinum, but also general trends

regarding the development and characteristics of the resulting dealloyed layers. In addition,

the detailed characterization carried out during this investigation, covering aspects such as

the ligament size, composition across the dealloyed layer, pore size distribution, changes in

the thickness of the dealloyed layer, etc., provide the field with a well-supported data that

can establish the base on which related research regarding the effect of ternary components

in nanoporous metals can be done. Moreover, the evaluation of some of the dealloying

parameters (temperature, potential and charge density) in the characteristics of these novel

nanoporous metals significantly enhanced the contribution of this research. In the past,

similar studies have been done for binary alloys (especially NPG) but, in the best of our

knowledge, this has never been done for ternary nanostructures. Through this

characterization, it was possible not only to confirm the higher stability that platinum induced

in the nanoporous metals, but also to identify alternative ways to change the platinum

coverage on the surface of the ligaments. All this clearly confirms the extraordinary flexibility

of these nanostructures.

In addition to the contributions made to the field of dealloying and formation of

nanoporous metals, the findings obtained in this research can also applied to fields like

surface science (i.e., adsorbate-induced surface segregation of platinum in nanoporous

metals) and catalysis (i.e., methanol and ethanol electro-oxidation). The preference of

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platinum to segregate to the surface of Au-Pt structures (and nanostructures) in the

presence of air has been predicted and reported before; however, this has never been

reported for nanoporous metals, and its kinetic effects are extraordinary. Through this

research we unveiled the experimental conditions in which the platinum content on the

surface of nanoporous metals could be modified and up to what extent. However, even

though there are still aspects of this phenomenon that are not completely clear for us (in

particular, transport mechanism of platinum to the surface of the ligaments), the importance

of the observed trends could be vast, and potentially open new research clusters within the

surface science and corrosion community. In terms of the catalytic abilities of these

nanomaterials, the fact that very high activities towards methanol and ethanol oxidation

reactions were obtained with very small platinum contents significantly helps to support the

search for new nanocatalysts with applications in clean energy and other fields. The

synergistic effect between the elements in these nanostructures was evident; more

importantly, it was determined that the selection of parameters such as the supporting

electrolyte, depending on the composition of the nanoporous structure, is essential to

maximize the yield of the reaction.

Undoubtedly, the observations made in this research will significantly contribute not

only to general understanding of dealloying, but also to the research and development

activities being done towards formation of robust nanomaterials and towards the use of

nanoporous metals in applications like electrocatalysis.

6.3 Potential applications of findings

As indirectly pointed out before, the observations and findings obtained during this

investigation could be relevant for on-going research in the field of metal nanostructures for

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applications in batteries, fuel-cells, sensors, membranes, among many others. Nowadays,

there is a constant effort to find not only a catalyst that is relatively cheap but, more

importantly, efficient and stable. Nanoporous metals, especially those fabricated on Pt-

based alloys, have been proven as potential good candidates for those applications;

however, platinum is an expensive metal and it suffers from drawbacks like deactivation due

to poisoning. The development of robust bimetallic-nanoporous metals, with minimal

platinum content, is indeed an attractive proposition for applications like catalysis and

electrocatalysis where, besides exploiting the remarkable activity displayed by nano-sized

gold, higher resistance and stability of platinum are expected.

6.4 Limitations of the research and recommendations for future work

The limitations encountered during this project, and specific recommendations for

future investigations, either in the same or in collateral areas, are presented below:

Compare the characteristics of the ternary nanostructures developed in this research

with those obtained in nanoporous metals developed on Ag-Pt alloys. The original

goal of our investigation was to compare the characteristics of the ternary

nanostructures with the nanostructures formed on binary alloys with the same silver

content (Ag-Au and Ag-Pt); however, severe inhomogeneities experimentally found

in the Ag-Pt alloy forced us not to include the material in our research. Therefore, we

strongly recommend to continue the same line of research using Ag-Pt alloy, not only

to characterize the nanoporous structure that can be developed on this alloy and

compare it with those obtained on ternary nanostructures, but also to expand our

knowledge in an material that has not been extensively investigated. Moreover, it

would be also important to expand that characterization to nanoporous structures

215

formed on the Ag-Pt alloy but with different platinum contents (e.g., 5, 15 at. %). This

will also benefit parallel projects that been carried out in this research group.

As shown in this research, the major reduction in the ligament size occurred in the

alloy with 1 at.% Pt. Therefore, it would be important to evaluate noble-ternary alloys

with even smaller platinum contents to determine up to what extent the platinum

stabilizes the nanoporous structure and induces higher resistance towards

electrochemical coarsening. Moreover, this evaluation could be extended to assess

the catalytic response of nanoporous structures with platinum content lower than 1

at. %, in alkaline electrolyte, to further assess the synergistic effect between gold

and platinum and the electrocatalytic response of such structures.

It is highly recommended to evaluate the chemical composition profile across

individual ligaments in ternary alloys to: i) confirm the core-shell structure of the

ligaments, ii) determine the platinum content in the core and in the shell of the

ligaments. Several attempts were tried to do this analysis as part of this project;

however, the preparation of the sample has been the major limitation; nevertheless,

information like this will be really beneficial for the understanding of the distribution of

atoms within ligaments.

Further research is needed in terms of the role of individual platinum atoms in the

UPD of hydrogen. This clearly becomes an important area for further development,

especially to better understand the catalytic properties of these and other

nanomaterials. Low energy ion scattering (LEIS) and X-ray photoelectron

spectroscopy (XPS) could be critical tools to understand this phenomenon.

Set the necessary conditions to dealloy material under inert-atmosphere conditions

(e.g., using a glove box) and, more importantly, with the capability of being able to

transfer the resulting nanoporous structures to vacuum chambers to do surface

216

sensitive analysis such as LEIS or XPS. In our experimental work, hydrocarbon

contamination on the surface of the structures was observed, and even though we

were able to obtain robust results, producing cleaner surfaces will be highly

beneficial.

The catalytic/electrocatalytic response of these nanoporous metals should be further

investigated. For electrochemical reactions, the use of nanoporous rotating disc

electrodes is strongly recommended to do a more precise evaluation. Moreover, it is

also recommended to evaluate the catalytic abilities of these structures in reactions

such as CO oxidation (gas-phase reaction) or glucose oxidation (liquid-phase

reaction).

Improve the method to determine the amount of carbonate in solution during/after

the electrochemical reactions. Specifically, a more robust method (i.e., anaerobic) is

desirable to reduce, as much as possible, any potential carbonation that might affect

the results.

As an additional step to evaluate the role of ternary elements during dealloying, it is

recommended that their effect has to be assessed in a different system/alloy. It is

well known that nanoporous copper, from Cu-Mn alloys, suffers some practical

limitations such as disintegration of the layer, severe coarsening oxidation, etc.

Therefore, it would be interesting to assess the role of small contents of platinum in

Cu-Mn alloys and determine how much stability is gained in the resulting ternary

structure.

Continue the effort of modeling these ternary nanoporous structures. Even though

KMC simulations seem to underestimate the coarsening of the structure and with

that, the real characteristics of the nanoporous structures, alternative approaches

could be considered. For example, combining KMC simulation with molecular

217

dynamics (MD) and/or density function theory (DFT) could provide additional

information and an extra degree of accuracy by including important effects such as

the presence of the electrolyte. Undoubtedly, further research in this area would be

highly beneficial.

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Appendix A. Supplementary Data

A.1 Potentiostatic dealloying: comparison between binary and ternary alloys

Most of the dealloying procedures during this research were carried out

potentiostatically at 0.55 V vs. MSE, and for a fixed charged density passed. Figure A.1

shows the dealloying current for the different alloys. As can be seen, the lower the platinum

content of the precursor, the longer the time that is required to remove a specific charge (5

C cm-2 in the case of the figure).

Figure A.1 Dealloying current of the binary and ternary alloys recorded at 0.55 V vs. MSE and 25 °C in 0.77 M HClO4. The charge density passed was 5 C cm-2 in all cases. Details about the alloys are shown in the figure.

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A.2 Formation of surface oxides during dealloying

Depending on the dealloying potential, oxidation of gold could occur during the

process. Gold oxides would limit its surface diffusion kinetics, hindering the initial ligament

coarsening. After dealloying the Ag-Au alloy at 0.7 V vs. MSE, a negative potential sweep

was carried out as shown in Figure A.2.

Figure A.2 Negative polarization sweep (1 mV s-1) of freshly dealloyed Ag77:Au23 specimen (0.7 V for approximately 25 s at 25 °C and removing 5 C cm-2) in the dealloyed solution. Insert: linear current density axis plot of the same data

In the region from 0.7 V to 0.57 V, the current was anodic, undergoing a switch to

cathodic between 0.57 V to 0.49 V before changing sign again. This behaviour can be

interpreted as the oxidation of gold above 0.57 V with its reduction below this value. The

reduction of gold resulted in a large cathodic charge than the anodic current corresponding

to silver dissolution, as the gold oxide/hydroxide covered a significantly large surface area.

When all the gold has been converted, the specimen reverted to the normal dealloying

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process until 0.3 V was reached. At this point, the dealloying current stopped, and the

cathodic current recorded was the reduction of the charge stored on the dealloyed layer

surface (similar to a capacitor). No indication of gold oxidation was observed at 0.55 V vs.

MSE.

A.3 Electrochemical Impedance spectroscopy (EIS) of dealloyed samples

Impedance spectroscopy has been used recurrently to probe the structure of the

dealloyed layer and the developed surface area. Bode plots of the impedance for a wide

range of frequencies are given in Figure A.3. The linear gradient of impedance versus

frequency below 10 Hz represents purely capacitance behaviour. At low frequencies, the

values of the phase angle are more negative than -87 °, which implies the absence of

Faradaic processes (pure capacitance being -90°). The tale observed at high frequencies is

characteristic of porous materials. Throughout the project, the surface area of the porous

layer was probed using frequencies between 1 and 0.1 Hz.

In the absence of Faradaic reactions, the capacitance of an electrode can be given by:

imZfC

2

1 (A.1)

where f is the frequency at which the impedance is measured (Hz), Zimis the imaginary

component of the impedance of the specimen ( cm2) and C is the specific capacitance (F

cm-2). The capacitance ratio between the value after and before dealloying can be

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considered as proportional to the developed surface area of the sample (also known as the

roughness factor – Rf).

Figure A.3 Impedance measurements for all alloys studied in this investigation: (a) Ag77:Au23, (b) Ag77:Au22:Pt1, (c) Ag77:Au21:Pt2, (d) Ag77:Au20:Pt3. In all cases, the charge density passed was 5 C cm-2 and the dealloying potential was 0.55 V vs. MSE. Dealloying temperature: 25 °C.

222

A.4 Alternative method to determine the developed surface area in dealloyed

specimens

Besides the impedance spectroscopy measurements showed before, the developed

surface area of dealloyed specimens was obtained by recording voltammograms in the

double layer region of potentials at different scan rates, as shown in Figure A.4 for two of

the ternary structures studied in this project. Current measured at the selected potential (-

0.1 V), located in the middle of the polarizability range, was then plotted against the scan

rate (insert in Figure A.4). The double layer capacitance was then calculated from the slope

of the current density versus scan rate, assuming again a negligible faradaic contribution to

the overall current. The developed surface area was calculated by dividing the calculated

capacitance by the gold double layer capacitance, which was assumed to be 28 µF cm -2

(assumption that seems to be correct after comparing the obtained results with impedance

results and BET values).

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Figure A.4 Voltammograms obtained from -0.24 to 0.05 V vs. MSE in 1 M HClO4 solution: (a) NPG, (b) Nanoporous structure formed on the alloy with the lowest platinum content. The insert in both cases shows the dependence of the double layer current at -0.1 V vs. scan rate. The scan rates used are shown in the figures.

A.5 Calculation of the mass of the dealloyed layer

The real surface area of dealloyed specimens was normalized by the mass of the

dealloyed layer to account for the real increase in surface area. The following sample of

calculations is just to describe the method and assumptions considered for that purpose.

To visualize the approach, we considered that the dealloying specimen as a rectangle,

in which the layer on the surface represents the dealloyed layer, as it is shown in Figure A.5.

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The thickness of the dealloyed layer was determined by measuring the layer in

metallographic specimens, and confirmed by SEM images taken from all dealloyed

materials. As an example, Figure A.6 shows the thickness of the dealloyed layer on NPG

measured from a cross-sectional metallographic specimen (Figure A.6a) and from the

fracture surface of the as-dealloyed material (low magnification SEM image). In both cases,

the thickness of the layer for NPG was ca. 7.8 µm (see Table A.1)

Figure A.5 Representation of the dealloyed specimen, in which the dealloyed layer is identified with the inclined lines.

Figure A.6 (a) Thickness of the dealloyed layer in NPG measured from a cross-sectional metallographic specimen and (b) from a fracture surface using a low magnification SEM image. In both cases, different samples of Ag-Au alloy were dealloyed at 0.55 V vs. MSE at 25 °C and passing a total charge of 5 C cm-2. The metallographic specimen was polished to 0.05 µm using alumina powder; the fracture surface was obtained after manually breaking the dealloyed sample in air.

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The experimental parameters/data for the specific case of NPG are shown in Table A.1:

Table A.1 Experimental data for the calculation of the mass of the dealloyed layer in NPG grown in 0.77 M HClO4 at 0.55 V vs. MSE and 25 °C.

Alloy Ag77:Au23 Faraday constant (F) 96 458 C mol-

2

Charge density passed (q) 5 C cm-2 Molecular weight of silver

(MWAg) 107 g mol-1

Thickness of the layer (T1) 7.8 µm Density of silver (dAg) 10.5 g cm-3

Approximate Shrinkage (Sh)

1.1 µm Fraction of silver ([Ag]) 0.77

Geometrical area of the sample (A)

0.38 cm2 Density of gold (dAu) 19.3 g cm-3

True area of the sample 342 cm2 Fraction of gold ([Au]) 0.23

The mass of the ‘dealloyed layer’ before dealloying is given by the following relation:

AuAgL dAudAgShTAMass 1

(A.2)

Substituting the values listed in Table A.1 and doing the necessary unit conversions,

the mass of the layer before dealloying was estimated to be 4.2 mg. The mass of silver

removed during the dealloying process can be calculated by the relation A.3:

AgAg MWAF

qMass (A.3)

The amount of silver removed by passing 5 C cm-2 was approximately 2.09 mg.

Therefore, the mass of the dealloyed layer (DL) was calculated by removing the mass of

silver from the mass of ‘layer’ before dealloying, as shown in A.4:

226

AgLDL MassMassMass (A.4)

For this particular case, the mass of the dealloyed layer was ca. 2.1 mg.

A.6 Calculation of the thickness of the dealloyed layer based on the retained

silver content in the layer

The thickness of the dealloyed layer can be also estimated by the composition of the

dealloyed layer (i.e., amount of silver retained in the layer). Considering the same

experimental data/parameters listed in Table A.1, the average composition of silver in NPG

(α = 0.45 at.% based on EDX results) and the molecular volume of silver (MVAg = 10.2 cm3

mol-1), the thickness of the layer can be estimated by:

AgF

MVqThickness

Ag

DL )( (A.5)

where represents the fraction of silver removed during the dealloying process and can be

estimated by:

1

1Ag

Au (A.6)

Substituting the respective values in Equation A.6, the fraction of silver removed was

0.76; therefore, the thickness of the layer (Equation A.5) was estimated to be 9.1 µm. As

227

shown in Table A.1, the experimentally determined thickness was 7.8 µm. Thus, the

difference in thickness is considered to be the shrinkage of the dealloyed layer (Sh), which

in this case was approximately 1.3 µm. This value of Sh agrees very well with the

experimentally estimated value listed in Table A.1.


all alloys

As described along this thesis, some samples of each alloy were dealloyed

potentiostatically at 0.55 V vs. MSE at different electrolyte temperatures. Figure A.7 shows

the dealloying current for the different alloys and for the four different dealloying

temperatures used along this research.

228

Figure A.7 Dealloying current of all the alloys investigated: (a) Ag77:Au23; (b) Ag77:Au22:Pt1; (c) Ag77:Au21:Pt2; (d) Ag77:Au20:Pt3. In all cases, the current was recorded at 0.55 V vs. MSE in 0.77 M HClO4 at different electrolyte temperatures. The charge density passed was 5 C cm-2 in all cases. Details about the electrolyte temperatures are shown in the figure.

As can be seen in this figure, the current density increased with temperature; for

example, Figure A.7a shows the dealloying current for the Ag-Au alloy in which at 60 °C the

current density was higher than at other temperatures. The same is true for the other alloys;

however, it was noticed that higher values of current densities were obtained by increasing

the platinum content of the precursor. For example, Figure A.7d shows that the current

densities for the alloy with 3 at.% were the highest among all the samples (except for the

current density at 10 °C which was approximately the same for all alloys). Increasing the

229

electrolyte temperature increased the surface diffusivity of the alloy constituents (e.g., gold

and platinum). It is believed that by increasing temperature, platinum decorated more

efficiently step edges, which induced changes in the amount of silver retained, thickness of

the layer and even on the dealloying current.

A.8 Nitrogen adsorption/desorption isotherms at 77.35 K – BET Surface area

estimation

Samples of all nanoporous metals were used to measure the surface area by means

of the BET method. As can be seen in Figure A.8, the shape of these isotherms is somehow

atypical: some of them present behaviours similar to a Type IV adsorption, while some

others have similarities to the H1/H2 hysteresis loops. The main characteristics of a Type IV

adsorption is the non-uniform desorption isotherm relative to the adsorption isotherm and

the capillarity condensation during the adsorption mechanism. On the other hand, H2

hysteresis occurs because desorption in the sample does not happen under equilibrium

conditions; rather, desorption occurs via either a pore blocking or cavitation mechanism,

which means that the larger mesopores are connected to the micropores or narrow

mesopores and can only empty after these "neck" have emptied. Based on these isotherms,

it is possible that cavitation is the predominant desorption mechanism because of the tale

steep step down in the desorption branch around 0.4-0.6 P/P0 (see Figure A.8a and b). For

this kind of behaviour during the desorption mechanism, the pores must be more cylindrical

in shape, according with the physical adsorption theory of nanoporous materials. Therefore,

based on these behaviors, it can reasonably conclude that these samples have micropores

as well as mesopores.

230

Figure A.8 Nitrogen adsorption/desorption isotherms at 77.35 K for all the nanoporous structures. All nanoporous metals were grown in 0.77 M HClO4 at 0.55 V vs. MSE and 25 °C. Dealloying charge: 5 C cm-2. De-gas conditions: 100 °C for 60 min. Details about the alloys are shown in the figure.

Based on these characteristics, the NLDFT (non-local density functional theory) kernel

that was used for all the samples was the N2 at 77K on silica (slit/cylindrical pores). This

kernel is designed to model the micropores as "slit-shaped" and the mesopores as

"cylindrical shaped" which seems to be case in these samples. Moreover, the adsorption

branch kernel has to be considered because of the H2-like hysteresis loops. This NLDFT

kernel would give just an approximation of the pore size distribution since no model for

231

metal alloys was available; moreover, in principle it correctly takes into account that there is

a delay in condensation in the adsorption branch of the isotherm due to metastable fluids in

the pore. This is also the reason that, in all cases, NLDFT provides a higher degree of

accuracy than the BJH (Barrett, Joycer and Halenda) method, which is based on the Kelvin

equation and assumes equilibrium desorption conditions, which does not seem to be

present in these samples; in addition, BJH also does not take into account the delay in

condensation.


electrolyte

As described in the Chapter 2, samples of all alloys were kept in 0.77 M HClO4

solution for two months (no potential applied) to determine the ligament size after that time,

and to estimate the resistance to coarsening of the different nanoporous structures. Figure

A.9 shows the SEM images of the different nanostructures after two months in the

dealloying electrolyte. As can be seen, the average ligament size in NPG was 30 nm;

whereas for the alloy with 1 at. % Pt the ligament size was approximately 9 nm. The

structure with the highest platinum content had an average ligament size of ca. 5 nm.

232

Figure A.9 SEM images of nanoporous structures dealloyed at 0.55 V vs. MSE and 25 °C and coarsened for 2 months in 0.77 M HClO4 solution: (a) Ag77:Au23, (b) Ag77:Au22:Pt1, (c) Ag77:Au21:Pt2, (d) Ag77:Au20:Pt3. In all cases 5 C cm-2 were passed. No potential was applied during the coarsening process.

A.10 SEM images of NPG after removing small charge densities

In our attempt to estimate how quickly the ligaments coarsen during the dealloying

process, Ag-Au alloy was dealloyed at the same potential (0.55 V vs. MSE) but removing

only 0.05 and 0.1 C cm-2. Normally, we removed charge densities higher than 2.5 C cm-2

(i.e., 5, 20, 40 C cm-2). As can be seen in Figure A.10a, the ligament size of NPG after

removing only 0.05 C cm-2 was ca. 7 nm, which is half of the size observed after removing 5

C cm-2. After removing 0.1 C cm-2 the ligament size was approximately 9 nm. Indeed, the

coarsening process in NPG is very fast and the ligaments grow very quickly in short periods

of time.

233

Figure A.10 SEM images of NPG dealloyed at 0.55 V vs. MSE and 25 °C: (a) and (b) after removing 0.05 C cm-2, (c) and (d) after removing 0.1 C cm-2.


the ligaments

To estimate if the amount of platinum available within the ligament was enough to

support the platinum coverage after surface segregation, the following theoretical calculation

was carried out:

Assuming that ligaments were cylinder with h height and with a radius of r, the volume

of a ligament can be calculated by:

hrVlig 2 (A.7)

234

The face-centered unit cell was considered for our calculations, with a total volume

given by:

3aV cellunit (A.8)

where a is the crystal lattice constant. Now, the total number of atoms in a ligament and the

number of only platinum atoms in the same ligament are given by the following relations:

3

2

4#a

hratomslig

(A.9)

DLlig Pta

hratomsPt

3

2

4#

(A.10)

where [PtDL] is the average platinum content in the dealloyed layer.

The surface of the cylinder (i.e., the surface of the ligament) is given by the relation

A.11:

hrSurflig 2 (A.11)

For the calculations, only the (111) plane was considered. The area of the (111) plane

is given by the following relation:

2

32

)111(

aA plane

(A.12)

235

The number of atoms on the surface of the ligaments is given by dividing Equation

A.11 by Equation A.12 and multiplying it by the number of atoms that are in the (111) plane

(2 atoms). Therefore, the following expression is obtained:

38#

2

a

hratoms ligSurf

(A.13)

The number of platinum atoms on the surface of the ligaments is given by multiplying

Equation A.13 by the concentration of platinum on the surface of the ligaments [Ptsurf]:

3

8#2

a

hrPtatomsPt surfligSurf

(A.14)

Comparing Equations A.10 and A.14, the following relation can be obtained:

a

rPtPt

ligDL

surf

2

3 (A.15)

For the observed surface segregation results to be theoretically possible, the relation

A.15 has to be true. Indeed, this relation is satisfied for all three ternary structures. As an

example, for the highest platinum content alloys, the term on the right was ca. 12% higher

than the term on the left, using the following values: [Ptsurf] = 30.36%; [PtDL] = 8 at.%; rlig =

2.15x10-9 m; a = 4.08x10-10 m.

236

A.12 Angle-resolved X-ray spectroscopy (XPS) analyses for the ternary alloys

before and after segregation of platinum

Figure A.11 shows the near-the-surface silver, gold and platinum composition,

obtained by XPS at different take-off angles, for the nanostructures developed on the alloys

with 1 and 3 at.% Pt. Evidently, the platinum content in the near-surface of the ligaments

(Figure A.11a, b) depended on the exposure temperature; for example, at 425 °C the

platinum composition was the highest in both nanostructures (ca. 12 and 24 at.% for the

nanostructure formed on the alloy with 1 and 3 at.% respectively for a take-off angle of 10 °);

however, at higher temperatures, the platinum composition decreased (specially in the case

of the nanostructure formed on the alloy with 1 at.%). The silver content in the near-surface

of the ligaments (Figure A.11c, d) was similar in both nanostructures, with the highest silver

contents at the highest annealing temperature, which is also an indication of the coarsening

of the structure. The gold concentration (Figure A.11e, f) consequently decreased by

increasing the annealing temperature to values around 30 at.%.

237

Figure A.11 XPS compositions with respect to the take-off angle for different nanoporous structures: (a) Pt4f for Ag77:Au22:Pt1; (b) Pt4f for Ag77:Au20:Pt3; (c) Ag3d for Ag77:Au22:Pt1; (d) Ag3d for Ag77:Au20:Pt3; (e) Au4f for Ag77:Au22:Pt1; (f) Au4f for Ag77:Au20:Pt3. The experimental conditions for different sets of data are the following: (◊) right after dealloying, (●) after exposure at 425 °C, (○) after exposure at 500 °C and (▲) after exposure to 600 °C. In the case of all high temperature conditions, laboratory air was used for an exposure time of 2 h.

All this data was used to generate the depth profiling across the first few nanometers

of the dealloyed layer, using the Maximum Entropy algorithm, to try to determine if the

segregation of platinum to the surface of the ligaments happened only on the outermost

surface or if there was any trend across the top part of the layer.

238



platinum

To obtain the depth profiling analyses of samples before and after segregation of

platinum, AR Process software (version 5.59) was used to process the angle-dependent

XPS data (Appendix A.12). Some of the properties of the nanoporous metals, such as

relative density and approximate composition of the dealloyed layer, were estimated to be

able to obtain the depth profile for the different samples. It is important to mention that it is

well-documented that the roughness of these nanostructures could have a significant effect

on the outcome of the analysis; nevertheless, no correction for the surface roughness was

done at this stage.

Figure A.12 shows the depth profiling results obtained for the ternary alloy developed

on the alloy with 1 at.% Pt. As can be seen, the modeling was done for the first 3 nm of the

sample and the predicted platinum concentration on the surface of the structure changed

from ca. 3 at.% (in the as-dealloyed sample) to ca. 10 at.% Pt (after exposure to air at 425

°C). At higher temperatures, the concentration of platinum decreased significantly, to values

close to zero after exposure to 600 °C. These results are in good agreement with the results

obtained by under-potential-deposition (UPD) of hydrogen (see Figure 2.3, Chapter 2).

In some of the graphs shown in Figure A.12, there are trends that were not expected

at all. According with the prediction of the model, there was a significantly higher platinum

concentration underneath the surface than that on the surface of the ligaments (in the

sample annealed at 425 °C, a platinum concentration of approximately 20 at.% was

predicted at ca. 0.25 nm depth). In addition, in the as-dealloyed sample and in the sample

annealed at 425 °C, there was a sudden increase in the platinum concentration near the 3

239

nm depth. Similar observations were made for the predicted results in the structure formed

on the alloy with 3 at.% Pt. Based on all these facts, we were not able to conclude if the

enrichment of platinum was only in the outermost atom layer.

Figure A.12 Depth profile analysis for the lowest platinum content nanostructure (1 at.% Pt) generated from angle-resolved XPS and using the Maximum Entropy algorithm. Details about the elements and the experimental conditions are shown in the figures.

Figure A.13 shows the depth profiling results obtained for the ternary alloy developed

on the alloy with 3 at.% Pt. As can be seen, the modeling was also done for the first 3 nm of

240

the sample; the predicted platinum concentration on the surface of the structure changed

from ca. 10 at.% (in the as-dealloyed sample) to ca. 25 at.% Pt (after exposure to air at 425

°C). At 500 °C, the concentration of platinum dropped to 20 at.% and at 600 °C, it further

decreased to ca. 5 at.%. Once again, these results are in good agreement with the results

obtained by under-potential-deposition (UPD) of hydrogen (see Figure 2.3, Chapter 2).

Once again, based on the trends observed in these predicted results, we were able to

conclude if the enrichment of platinum was solely on the outermost surface of the layer. The

assessment of the effect of the huge roughness of these structures has to be further

investigated to determine its effect on the current results.

241

Figure A.13 Depth profile analysis for the lowest platinum content nanostructure (3 at.% Pt) generated from angle-resolved XPS and using the Maximum Entropy algorithm. Details about the elements and the experimental conditions are shown in the figures.


after surface segregation of platinum was induced

As discussed in detail in Chapter 3, exposure of as-dealloyed specimens to moderate

temperature in the presence of laboratory air will induce changes in the surface

composition, i.e., the fraction of platinum atoms will increase compare with the as-dealloyed

samples. Under-potential-deposition (UPD) of hydrogen was used primarily to assess the

242

platinum coverage on the surface of the ligaments. Figure A.14 shows the UPD of hydrogen

for samples before and after segregation.

Figure A.14 (a) CV profiles of the nanostructures formed on Ag-Au and Ag-Au-Pt alloys after dealloying at 0.55 V vs. MSE, 25 °C and passing a charge density of 5 C cm-2; (b) CV profiles for the nanostructure formed on the alloy with 3 at.% Pt after exposure of the as-dealloyed structure to different temperatures in the presence of laboratory air. In all cases, CV profiles were obtained in 1 M H2SO4 at 20 mV s-1 and 25 °C. The original platinum content is shown in (a) and the exposure temperature is shown in (b).

As can be seen in Figure A.14a, the CV profiles showed obvious adsorption and

desorption regions in the potential region between -0.63 V and -0.4 V vs. MSE for all the

243

nanostructures formed on ternary alloys. As expected, the current density in this region

increased by increasing the platinum content of the precursor; the binary alloy did not show

any sign of adsorption/desorption of hydrogen. In the region of potentials between -0.4 V

and 0 V, the double-layer region is observed. Figure A.14b shows two important

characteristics of as-annealed samples: i) adsorption and desorption regions increased with

the annealing temperature, with a maximum observed at 425 °C; ii) the double-layer region

of potentials decreased by increasing the annealing temperature, which is a sign of the

decrease in surface area by exposure to high temperatures.

A.15 Effect of scan rate and agitation speed during methanol electro-oxidation

reaction

The effect of different potential scan rates and agitation speed (by an external source,

i.e. magnetic stirrer) during the electrochemical oxidation of methanol in 5M KOH – 1 M

CH3OH solution is shown in Figure A.15.

As can be seen in Figure A.15a, b and c, the oxidation current was dependent on the

potential scan rate (); in fact, the peak current was linear with 0.5 for all the nanostructures,

which clearly indicated a diffusion controlled process for methanol oxidation in the bulk

solution boundary layer. The oxidation current associated with the half peak potential (i.e., -

0.4 V vs. Hg/HgO) is nearly linear with 0.5 for the alloys with 2 and 3 at.% Pt (Figure A.15c

and e), but independent of the scan rate in the case of the lower platinum content alloy at

rates higher than 3 mV s-1 (see Figure A.15a). In the case of the different agitation speeds

(i.e., between ca. 200 rpm and 970 rpm) it was observed that the peak current increased

with the agitation speed. For example, in the case of the ternary alloy with 1 at.% Pt (Figure

244

A.15b), the peak current increased from ca. 200 µA cmtrue (no agitation) to 550 µA cmtrue

at

800 rpm (at 970 rpm there were no further changes with respect to 800 rpm). These

changes in the peak current were entirely related to the diffusion limitation at the top surface

of the catalyst. No changes were found in the oxidation current below ca. -0.3 V. Similar

trends were observed for the other two ternary alloys (Figure A.15d and f).

Figure A.15 CV profiles for the electro-oxidation of CH3OH solution in 5 M KOH on nanoporous structures formed on ternary alloys: (a) effect of the scan rates on the structure formed on Ag77:Au22:Pt1; (b) effect of the external agitation speed on the structure formed on Ag77:Au22:Pt1; (c) effect of the scan rates on the structure formed on Ag77:Au21:Pt2; (d) effect of the external agitation speed on the structure formed on Ag77:Au21:Pt2; (e) effect of the scan rates on the structure formed on Ag77:Au20:Pt3; (f) effect of the external agitation speed on the structure formed on Ag77:Au20:Pt3. In all cases 5 C cm-2 were passed at 0.55 V vs. MSE and 25°C.

245


dealloying conditions

It was expected that samples dealloyed under different conditions (e.g., temperature

and charge density passed) would have different catalytic response towards methanol

electro-oxidation in alkaline media. Figure A.16 shows the CV profiles for the dealloyed

Ag77:Au22:Pt1 (Figure A.16a) and Ag77:Au20:Pt3 (Figure A.16b) at different temperatures. As

can be seen in Figure A.16a, the voltammograms did not significantly change with respect

to the different dealloying temperatures: in all cases, there was a sharp increase in current

density at approximately -0.7 V that was associated with the characteristic methanol

oxidation on the surface. It was observed however, that for the structures dealloyed at 40°C

and 60°C, the peak potential shifted to ca. 0.33 V (structures dealloyed at 25 °C showed a

peak potential at ca. 0.22 V), which could be associated with the change in the platinum

content on the surface of the ligaments. The peak current during the forward scan did not

show any significant change between nanostructures; during the backward sweep, an

oxidation peak at ca. -0.30 V was observed.

In Figure A.16b, the CV profiles for the nanostructure formed on the alloy with 3 at.%

Pt are shown. As can be seen, by increasing the dealloying temperature, the peak current

significantly decreased and the peak potential shifted to more negative values (ca. -0.34 V).

CV profiles on the nanostructure formed on the alloy with 2 at.% Pt (not shown here)

showed also a decrease in the peak current with increasing dealloying temperature and an

intermediate value for the current density if compared with the other two ternary structures.

It is hypothesized that the relative ration of gold to platinum has an effect on these trends.

246

Figure A.16 CV profiles of the nanoporous structures formed on (a) the alloy with 1 at.% platinum and (b) the alloy with 3 at.% platinum. In all cases, dealloying was carried out in 0.77 M HClO4 at 0.55 V vs. MSE and at different temperatures. All CV profiles were obtained in 5 M KOH - 1 M CH3OH solution at 10 mV s-1 and 25 °C. Details about the dealloying temperatures are given in the figures. In all cases, the current was normalized by the true area of the electrodes.

The effect of the charge density on the CV profiles of these nanoporous metals is

exemplified here by the nanostructure formed on the alloy with 1 at.% Pt (Figure A.17) after

dealloying passing different charges. As observed, all voltammograms showed a sharp

increase in current around -0.6 or -0.7 V with the corresponding oxidation peaks during the

backward sweep; however, the highest current density was observed when the lowest

charge density was passed (i.e., 2.5 C cm-2); conversely, the sample in which 20 C cm-2

247

were removed showed the lowest current density. Initially, it was assumed that thicker layer

(i.e., higher charge densities) could give higher current densities; however, the

abovementioned trend suggested otherwise. In fact, these results also suggested that the

oxidation reaction might happen mostly in the vicinity of the top surface of the structure.

Moreover, it is recognized that the effects of mass transport limitation within the layer are

not negligible, even though they are smaller that might be assumed since the effective

diffusivity within the layer is lower than that for the bulk. In simple words, having very thick

DLs (e.g., 35 µm), did not seem to be beneficial to obtain higher oxidation currents. The

same behaviour was observed for the other ternary alloys (not shown here).

Figure A.17 Effect of the dealloying charge density in the electro-oxidation of methanol on as-dealloyed Ag77:Au22:Pt1. In all cases the nanoporous structure were dealloyed at 25 °C but passing different dealloying charges. All CV profiles were obtained at 10 mV s-1 and 25 °C. The different charge densities are shown in the figure.

248

A.17 Potentiostatic oxidation of methanol in alkaline solution

Figure A.18 shows the potentiostatic oxidation of methanol in alkaline electrolyte (5 M

KOH – 1 M CH3OH). All samples were dealloyed at 0.55 V vs. MSE at 25 °C and passing a

dealloying charge of 5 C cm-2. The potentiostatic experiment was run for 4000 s in a solution

that was de-aereated for at least 30 min with high-purity nitrogen gas (min. purity: 99.998%).

In all these experiments, the counter electrode (platinum wire) was kept in a separate

compartment to minimize the effect of secondary reaction that might affect our results.

Figure A.18 Current density observed during a prolonged oxidation of 1 M CH3OH in 5 M KOH solution at -0.35 V vs. Hg/HgO in the case of ternary alloys and at -0.05 V vs. Hg/HgO in the case of the binary alloy. In all cases, nanostructures were formed after dealloying the precursors at 0.55 V vs. MSE at 25 °C and with a charge density of 5 C cm-2. The label of the different nanostructures is shown in the figure.

As can be seen in Figure A.18, the alloy with the lowest platinum content has the

higher current density, which agrees with cyclic voltammograms done in the same solution.

The other two ternary alloys did not show any significant difference. It is important to notice

249

that the current in ternary alloys did not significantly decrease in time; whereas the binary

alloy did have an obvious decrease in current density.


concentrations for the methanol oxidation reaction

After electro-oxidation of methanol in alkaline electrolyte, selected samples were

prepared to determine the concentration of formate produced during the reaction. No

formaldehyde was detected in any of the samples. All samples were analyzed in the nuclear

magnetic resonance (NMR) spectrometer. Figure A.19 shows some of the NMR spectra for

two samples with different concentration of formate and for the stock solution (5 M KOH – 1

M CH3OH). As can be observed in this figure, the formate peak was located at ca. 8.30 ppm

and the area under the peak changed with the amount of formate in solution. No significant

peak was observed in the case of the stock solution. The peak for water and methanol were

located at ca. 4.5 and 3.2 ppm respectively (not shown in Figure A.19)

In order to estimate the concentration of formate that was formed, aliquots were

prepare with different concentrations of formate. The area under the peak was correlated

with the formate concentrations to generate a calibration curve (Figure A.20). This

calibration curve was used to determine the concentration of formate produced during

methanol electro-oxidation on the different nanoporous structures.

250

Figure A.19 NMR spectra for samples prepared/obtained in 5 M KOH – 1 M CH3OH with different formate concentration. For all samples a scan at 90 ° pulse (16.1 µs) and 10 s relaxation delay were used with an acquisition time of 4.5 s and spectral window of 8012 Hz. All samples were diluted in D2O before analysis in NMR.

Figure A.20 Calibration curve for the concentration of formate in the 5 M KOH – 1 M CH3OH solution. Every point in this graph corresponds to the area under the peak located at ca. 8.30 ppm.

251

A.19 Electro-oxidation of ethanol in acidic media

Electro-oxidation of ethanol was analyzed mostly in alkaline electrolyte (i.e., 4 M

KOH); however, some preliminary runs in acidic media (i.e., 0.5 M HClO4) were performed

with the ternary alloy with the highest platinum content. As can be seen in Figure A.21, there

is an increase in current after -0.3 V vs. MSE and it continued increasing up to the positive

scan limit (0.3 V vs. MSE). According with the CV profiles in the supporting electrolyte (refer

to section 4.2 in Chapter 4), the double-layer region in this ternary alloy extended from -0.3

V to 0.1 V vs. MSE; at higher potentials, the current increased dramatically, which could be

a sign for the beginning of gold oxidation. Therefore, the increase in current observed at -

0.3 V could be ascribed to the oxidation of ethanol. The positive scan limit was fixed at 0.35

V to minimize the impact of surface oxides and evaluate the catalytic response of this

nanostructure in the region below the empirical critical potential (i.e., ca. 0.35 V in 0.77 M

HClO4). It was observed that in the forward scan, there was an oxidation peak at ca. 0 V; no

well-developed oxidation peaks were found in the backward sweep. It was also observed

that after completing multiple cycles, the current density decreased dramatically; for

instance, in the first cycle the peak current was 12 µA cmtrue but during the 10th cycle, the

peak current dropped to 7 µA cmtrue . It is possible that the formation of a “passive” layer

during the process is responsible for blocking the active surface and consequently the

decrease in current was observed.

252

Figure A.21 CV profiles of the highest platinum content alloy (3 at.% Pt) in 0.5 M HClO4 – 1 M C2H5OH solution. The number of cycles during the evaluation is shown in the figure. All CV profiles were obtained at 10 mV s-1 at 25 °C.

253

Appendix B. Simulation of Nanoporous Metals Formed by

Electrochemical Dealloying of Ag-Au-Pt Alloys

B.1 Introduction

The formation of porosity has been explored extensively in binary alloys by atomistic

computer simulation. Erlebacher [1,2], following the work done by Sieradzki, Forty and

others [3-8], developed a simulation code called MESOSIM to explore the kinetics and

thermodynamics of nanoporosity evolution in binary FCC alloys using the Kinetic Monte

Carlo (KMC) method. In KMC, the time evolution of an ensemble of atoms is tracked as

atoms diffuse and dissolve according to simple rate laws meant to mimic realistic surface

diffusion and/or electrochemical dissolution events. This model has proven to be very

success to predict the characteristics/properties of nanoporous metals; therefore, initial

steps were taken towards the implementation of a model (based on the original MESOSIM

code) to describe the dealloying of Ag-Au-Pt alloys and reproduce the principal

characteristics of the resulting nanostructures. This effort was carried out in collaboration

with Dr. Dorota Artymowicz, who had worked simulating the formation of nanoporous metals

by dealloying of Ag-Au alloys [9].

B.2 Modeling conditions and results

Some of the necessary assumptions to implement this model have been reported

elsewhere [1,9]; however, a summary is presented here:

1) Two types of site-coordination dependent transitions – dissolution and surface

diffusion – are possible in the system.

254

2) All atoms having at least one un-coordinated bond, which corresponds to three

vacancies among their 12 nearest neighboring sites, are capable of a transition.

3) Atoms of the less noble element, i.e. ‘silver’, can diffuse on the surface or dissolve,

while atoms of the more noble, i.e. ‘gold’ or ‘platinum’, can only diffuse.

4) Rate of dissolution and diffusion events of an individual atom are calculated as

follows:

(B.1)

and

(B.2)

where n is the number of nearest neighbors of the atom, Eb is the interatomic bond energy,

ɸ is the applied electrochemical overpotential, k is the Boltzmann constant and T is the

absolute temperature. Eb is adjusted for the different atom pairs (e.g., 0.15 eV for Ag-Au

pair) and its value was estimated based on the sublimation energy for the different metals

and on the interaction between the different atoms. The pre-exponential factor Cdiffusion is an

attempt frequency of the order of the Debye frequency (for silver, platinum and gold, Cdiffusion

is 1x1013, 3x1013 and 1x1014 s-1 respectively). The pre-exponential factor Cdissolution is set to

1x104 s-1. This value is obtained after fitting experimental results of the dissolution of pure

silver (i.e., ~1 monolayer of silver atoms dissolve per second) [1]. As an initial

approximation, binary and ternary hypothetical alloys (i.e., Ag75:Au25 and Ag75:Au20:Pt5

respectively) were defined and the dealloying parameters were selected similarly to the

255

actual experimental conditions: 1100 mV vs. standard hydrogen electrode (SHE) as a

dealloying potential, and 300 s as a maximum dealloying time.

The dealloying cell has been defined to have 128 x 128 x 128 atoms. These values

represent the x, y and z positions along the cubic cell (Figure B.1). Additionally, it was set

that the dealloying front cannot pass a value of z = 8; in other words, atoms with z > 8 do

not take part in the process (i.e., they cannot be dissolved or diffused).

Figure B.1 Orientation of the simulation cell with respect to the orthogonal FCC cubic cell. The sides of the simulation cell are of {111} family and the edges are <110>. Red vertices of the simulation cell coincide with the dace centers of the orthogonal FCC cubic cell.

The main results obtained after simulating the formation of nanoporous metals from

binary and ternary alloys are shown in Figure B.2. Figure B.2a shows a comparison

between the dissolution current for the binary and for the ternary alloy. As shown, there was

no significant difference between the two alloys, which agrees with our experimental data,

even though experimentally the current steadily dropped; while in the simulations the current

dropped and then increased to reach a stable value that remained the same until the

sample has been dealloyed all the way through, i.e., after 100 s, the current densities start

256

decreasing in both alloys, which is associated to the event in which the dealloying front has

reached the z = 8 level. In terms of the dealloying charge (Figure B.2b), it was found that for

both alloys there was a steady increase in the charge in the first 100 s, but then it leveled off

in a value of ca. 18 mC cm-2 (after 100 s of dealloying). This behaviour was because at 100

s the dealloying front was reached; after that, the only available silver was that trapped

inside the ligaments. As discuss later on in this appendix, coarsening of these simulated

structures was minimum (which was not the case in real samples); therefore, the availability

of the retained silver was very limited and its contribution to the total charge was almost

negligible. For the thickness of the dealloyed layer (Figure B.2c), it was found that there was

not significant different between the binary and ternary alloy (ca. 28 nm for both alloys

according with the simulation). Based on our experimental results, ternary nanostructures

always had thicker layers than the binary, which was related to residual silver trapped in the

layers (see Chapter 2 and discussion below).

In terms of the increase in surface area, it was observed that the surface area was

higher in the ternary alloy than in the binary alloy (Figure B.2d), which is in agreement with

the general tendency found in our experiments. However, our experiments suggested that

the surface area in alloys with platinum was significantly higher than in alloys without

platinum; e.g., the surface area in the ternary alloy with the highest platinum content (i.e.,

Ag77:Au:20:Pt3) was almost three times higher than in the binary alloy, which is directly

associated with the significant difference in ligament size between binary and ternary alloys

(see Chapter 2). Figure B.3 shows that indeed, the structure simulated on the binary alloy

had slightly bigger ligaments than that in the ternary alloy; nevertheless, the difference is

minimal if compared with the experimentally determined values. Figure B.4 shows a cross-

sectional view of the nanoporous structures from which a clearest view of the small

difference between ligament sizes amongst structures can be seen.

257

Figure B.2 Comparison between dealloying of a binary (Ag75Au25) and ternary (Ag75Au20Pt5) alloys: (a) anodic current density, (b) charge density passed, (c) thickness of the dealloyed layer and (d) surface increase. These results were obtained by KMC simulation of the dealloying process carried out for a maximum time of 300 s at 1000 mV vs. SHE. Dealloying temperature: 25 °C. The dealloying process in the binary and ternary alloy stopped at 300 s and 200 s respectively.

0 100 200 3000

5

10

15

20

time [s]

passed c

harg

e [

mC

/cm

2]

0 100 200 3000

5

10

15

20

25

30

time [s]

deallo

yin

g d

epth

[nm

]

0 100 200 3000

5

10

15

20

25

time [s]

surf

ace incre

ase

0 100 200 30010

-3

10-2

10-1

100

time [s]

dis

solu

tion c

urr

ent

density [

mA

/cm

2]

binary

ternary

(a) (b)

(c) (d)

258

Figure B.3 Simulated structures of the nanoporous layer for Ag75:Au25 (a and c) and Ag75:Au20:Pt5 (b and d). In all cases, KMC simulations were used. The color code in these structures is as follows: gray atoms represent silver atoms, yellow atoms represent gold atoms that have not diffused; brown atoms represent gold atoms that diffused; green atoms represent platinum atoms that have not diffused and blue atoms represent platinum atoms that diffused. The green lines in (a) show the outline of the simulation cell and the red arrow is the cell’s side surface.

It is important to mention that, according with the simulations (Figure B.3 and Figure

B.4), the dealloying time did not have a significant effect in the coarsening of the structure,

especially in the binary alloy. However, based on our experiment observations, the

dealloying time has a significant effect in the binary alloy, whereas in ternary alloys that

effect was reduced.

259

Figure B.4 Cross-sectional view of the dealloyed simulated structures of the nanoporous layer for Ag75:Au25 (a and c) and Ag75:Au20:Pt5 (b and d). In all cases, KMC simulations were used. The color code in these structures is the same as described in Figure B.3.

In terms of the composition of the dealloyed layer, it was found that the binary alloy

should have more residual silver than the ternary alloy (Figure B.5). This result is opposed

to our experimental findings. As discussed in Chapter 2, ternary alloys trapped more silver

inside the ligaments than binary alloys. This was also confirmed by doing TEM – EDS

analysis on ultramicrotomed samples for the three ternary alloys. In all cases, it was

determined that more silver was left behind in ternary alloys due to the lower coarsening

kinetics (i.e., platinum slows down gold mobility and with that the coarsening of the

structure). The disagreement between simulated and experimental results was probably due

to the lack of coarsening in the simulated structures. One of the biggest disadvantages of

KMC is the impossibility of simulating the dealloying process in an electrolyte-like

environment; in fact, from the code point of view, the electrolyte had been removed and

replaced by the equivalent of vacuum. This of course has implications for the relaxation of

260

the metal surface (i.e., surface diffusivities of metals) [10-12], which will directly impact the

coarsening of the simulated structure. This lack of coarsening in the structures could explain

the small difference in ligament size between binary and ternary structures.

Figure B.5. Content of residual silver trapped in the dealloyed layer formed on the binary and ternary alloy (Ag75:Au25 and Ag75:Au20:Pt5 respectively) after simulating the dealloying process at 1000 mV vs. SHE and 25 °C. The dealloying process in the binary and ternary alloy stopped at 300 s and 200 s respectively.

In addition of the amount of retained silver in the nanoporous structures, information

about the surface atoms was obtained for the binary and ternary alloys. Figure B.6a shows

that in the ternary alloy gold represents the 75% on the surface atoms and platinum the

remaining 25%; whereas in the binary alloy most of the surface was covered with only gold.

Approximately 5% of the surface atoms were silver atoms that did not dissolved during the

electrochemical process. These maximum relative compositions were obtained after

approximately 100 s, before that a steady increase in the fraction of more-noble elements

was observed (i.e., silver was steadily removed in that time-frame). Figure B.6b shows that

the platinum-to-gold ratio increases to a value slightly higher than 30% after 200 s of

dealloying, which is not possible since the original ratio of platinum-to-gold was 0.25 and

0 100 200 3000

0.2

0.4

0.6

0.8

time [s]

[Ag]

in d

eallo

yed layer

ternary

binary

261

should remain the same during dealloying (more-noble elements do not dissolve during the

process). Moreover, the platinum-to-gold ratio also gave us an estimate of the fraction of

platinum atoms on the surface of the ligaments, which in our experimental results was ca. 7

% for the alloy with 3 at.% Pt. Even though in this case the “ternary alloy” had higher content

of platinum (5 at.%), the simulated values we obtained were very high and did not represent

the trend we observed experimentally.

Figure B.6 (a) Fraction of the more-noble elements in the binary (Ag75:Au25) and ternary alloy (Ag75:Au20:Pt5); (b) ratio of platinum and gold on the surface of the ligaments. Simulation conditions: dealloying potential - 1000 mV vs. SHE and 25 °C as a dealloying temperature. The dealloying process in the binary and ternary alloy stopped at 300 s and 200 s respectively.

0 100 200 3000.5

0.6

0.7

0.8

0.9

1

time [s]

fraction

[MN] ternary

[Au] bianry[Au] ternary

0 50 100 150 2000.1

0.15

0.2

0.25

time [s]

[Pt]

to [

Au]

ratio

262

After analyzing these preliminary results, it was obvious to us that we were not able to

properly model the coarsening of the structure, especially in the case of NPG, in which

coarsening has rather aggressive consequences in the ligament size. As described before,

while dealloying progresses, the size of pores and ligaments keeps increasing; at the same

time that ligaments grow in size, more original alloy is exposed to the electrolyte allowing

removal of silver from the already formed ligaments. That is the reason why the amount of

silver retained in NPG was always lower than that in the ternary alloys, in which coarsening

is significantly hindered (i.e., significantly smaller ligaments than the binary alloy). As a

consequence, the thickness of the layer was smaller in NPG and the developed surface

area was much higher in ternary alloys. In our attempt to improve the model, some of the

parameters in the model (e.g., bonding energy between atoms) were adjusted; however, no

significant improvement in the final outcome of the model was achieved.

This Monte Carlo model has been remarkably successful to reproduce/predict the

main dealloying characteristics; however, it is clear that the direct comparison between real

dealloyed structures and those formed by simulation (e.g., ligament size, coarsening) is not

yet possible. The simplifications that have been implemented to reduce its computational

cost limit its use to correlate specific characteristics of the nanoporous material. In addition,

it was recently reported that other phenomena could play a role during the coarsening

process. Kolluri and Demkowicz [13] argued that localized plasticity of ligaments is a

process that not only could explains coarsening, but also could explain the formation of

enclosed bubbles and reduction of the of the volume in NPG, which are not intuitive if

coarsening is only associated with surface gold diffusion. At this point however, we are not

suggesting that the outcome of the current model is solely because the abovementioned

processes are not being considered; but we believe that at this stage is important to

263

consider possible parallel processes that could participate in the coarsening of these

structures. Unfortunately, the MESOSIM code is limited to the surface diffusion process.

Although the comparison between simulated and real structures would be very

valuable for the fundamental understanding of the role of ternary noble-elements during the

dealloying process, the main goal of this research project was the experimental

characterization of ternary alloys, and working in this model required a significant amount of

time, which became a limiting factor for us. Therefore, we suggested to continue this effort

in the near future, but not necessarily as part of this project.

B.3 References





3. H. W. Pickering. (1983). Characteristic Features of alloy polarization curves, Corros.

Sci., 23, 1107.

4. H. W. Pickering and C. Wagner. (1967). Electrolytic dissolution of binary alloys

containing a noble metal, J. Electrochem. Soc., 114, 698.


silver-gold alloys in nitric acid, Philos. Mag. A, 42, 295.





264


simulations of corrosion - selective dissolution of binary-alloys, Philos. Mag. A, 59,

713.









12. C. Alonso, R. C. Salvarezza, J. M. Vara, A. J. Arvia, L. Vazquez, A. Bartolome and A.

M. Baro. (1990). The evaluation of surface diffusion coefficient of gold and platinum

atoms at electrochemical interfaces from combined STM-SEM imaging and

electrochemical techniques, J. Electrochem. Soc., 137, 2161.

13. K. Kolluri and M. J. Demkowicz. (2011). Coarsening by network restructuring in model

nanoporous gold, Acta Mater., 59, 7645.

265

Appendix C. List of Publications and Presentations

C.1 Publications

Vega, A., and R. C. Newman. Electro-oxidation of ethanol on highly porous

nanostructures obtained by dealloying of binary and ternary noble-metal alloys, submitted

to Electrochimica Acta (May, 2014).

Vega, A., and R. C. Newman. Methanol electro-oxidation on nanoporous metals formed

by dealloying of Ag-Au-Pt alloys, submitted to the Journal of the Electrochemical Society

(May, 2014).

Vega, A., and R. C. Newman. Nanoporous metals: formation, characteristics, tunability

and prospects for catalytic activity, in Dekker Encyclopedia of Nanoscience and

Nanotechnology, 3rd Edition, S. E. Lyshevski, Editor, CRC Press: New York, 2014, pp.

3217-3229.

Vega, A., and R. C. Newman. Beneficial effects of adsorbate-induced surface

segregation of Pt in nanoporous metals fabricated by dealloying of Ag-Au-Pt alloys.

Journal of the Electrochemical Society, 161, pp. C11-C19, 2014.

Vega, A. and R. C. Newman. Nanoporous metals fabricated through electrochemical

dealloying of Ag-Au-Pt with systematic variation of Au:Pt ratio. Journal of the

Electrochemical Society, 161, pp. C1-C10, 2014.

Vega, A. and R. C. Newman. Fabrication of nanoporous metals by dealloying of ternary

alloys: evaluation of their electrocatalytic properties, Technical proceedings of the 2012

NSTI Nanotechnology Conference and Expo, NSTI-Nanotech, pp. 510-513, 2012.

266

C.2 Presentations

Vega, A., R.C. Newman*. (2014). Formation, characteristics and tunability of highly

porous nanomateriales obtained by dealloying of ternary noble-metal alloys. Paper

accepted for oral presentation at the 5th International Conference of Porous Media and its

Applications in Science, Engineering and Industry (ICPMV), Kona, HI, United States of

America, June 22-27.

Vega, A.*, R.C. Newman. (2014). Formation of nanoporous metals by dealloying of

ternary noble-metal alloys: characteristics, tunability and prospects for catalytic activity.

Paper accepted for oral presentation at the 15th Topical Meeting of the International

Society of Electrochemistry, Niagara Falls, ON, Canada, April 27-30.

Vega, A., R.C. Newman*. (2013). Surface chemistry and morphology of nanoporous

metals synthesis from Ag-Au-Pt precursors. Oral presentation delivered at the 14th

Topical Meeting of the International Society of Electrochemistry, Santiago de Queretaro,

QUE, Mexico, September 9-13.

Vega, A.*, R.C. Newman. (2013). Nanoporous metals from dealloying of ternary noble-

metal alloys: composition, morphology, stability and prospects for catalytic activity. Paper

presented as oral presentation at the 223rd ECS meeting, Toronto, ON, Canada, May 12-

16.

Vega, A., R. C. Newman*. (2013). Manipulating the morphology and surface chemistry of

nanoporous metals for potential (electro)catalytic applications. Oral presentation

delivered as an invited speaker at the 25th Canadian Materials Science Conference,

London, ON, Canada, May 7-10.

Vega, A.*, R. C. Newman. (2012). Fabrication and characterization of nanoporous metals

by electrochemical dealloying of AgAu(Pt) alloys. Paper presented as oral presentation at

267

the Student Corrosion Research Symposium held at University of Toronto, Toronto, ON,

Canada, June 25.

Vega, A.*, R. C. Newman. (2012). Fabrication of nanoporous metals by dealloying of

ternary alloys: evaluation of their electrocatalytic properties. Paper accepted for poster

presentation at the TechConnect World Conference and Expo 2012, Santa Clara, CA,

United States of America, June 18-21.

Vega, A.*, R. C. Newman. (2012). Fabrication of nanoporous metals by dealloying of

AgAu(Pt) alloys. Paper accepted for oral presentation at the 24th Canadian Materials

Science Conference, London, ON, Canada, June 5-8.

Vega, A.*, R.C. Newman. (2012). Assessment of the electrocatalytic activity of

nanoporous metals formed by dealloying of AgAu(Pt) alloys with systematic variation of

Au to Pt ratio. Paper accepted for oral presentation at the ECS Spring meeting, Seattle,

WA, United States of America, May 6-10.

Vega, A.*, R. C. Newman. (2012). Formation and electrocatalytic activity of nanoporous

metals formed by dealloying of AgAu(Pt) alloys with systematic variation of Au to Pt ratio.

Poster presented at the Spring Symposium of the ECS Canadian section, Montreal, QC,

Canada, April, 27.

Artymowicz, D., A. Vega, R. C. Newman*. (2012). Progress in modeling alloy corrosion –

Ternary alloys. Paper accepted for oral presentation at the NACE Conference, Utah,

United States of America, March 12-14.

Vega, A.*, R. C. Newman. (2011). Dealloying of AgAu(Pt) alloys and formation of

nanoporous metals. Paper presented as oral presentation at the Student Corrosion

Research Symposium held at McMaster University, Hamilton, ON, Canada, July 4.

Vega, A. A*, D. Artymowicz, R. C. Newman. (2010). Electro-oxidation of methanol by

nanoporous gold (NPG) formed by dealloying of binary and ternary alloys. Poster

268

presented at the Gordon Graduate Research Seminar on Aqueous Corrosion, London,

New Hampshire, United States of America, July 7-8.

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