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A strategy for fabricating nanoporous gold filmsthrough chemical dealloying of electrochemicallydeposited Au-Sn alloysTo cite this article: Yantong Xu et al 2014 Nanotechnology 25 445602

 

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A strategy for fabricating nanoporous goldfilms through chemical dealloying ofelectrochemically deposited Au-Sn alloys

Yantong Xu1, Xi Ke1, Changchun Yu2, Shaofang Liu1, Jie Zhao2,Guofeng Cui1, Drew Higgins3,4, Zhongwei Chen4, Qing Li3 and Gang Wu5

1 Electronic Packaging Electrochemistry Laboratory, School of Chemistry and Chemical Engineering, SunYat-sen University, Guangzhou, 510275, People’s Republic of China2 School of Mechanical and Automotive Engineering, South China University of Technology, Guangzhou,510640, People’s Republic of China3Materials Physics and Applications Division, Los Alamos National Laboratory, Los Alamos, New Mexico87545, USA4Department of Chemical Engineering, Waterloo Institute for Nanotechnology, Waterloo Institute forSustainable Energy, University of Waterloo, Waterloo, Ontario N2L 3G1 Canada5Department of Chemical and Biological Engineering, University at Buffalo, the State University of NewYork, Buffalo, New York 14260, USA

E-mail: xikeciac@gmail.com and cuigf@mail.sysu.edu.cn

Received 21 May 2014, revised 15 September 2014Accepted for publication 16 September 2014Published 17 October 2014

AbstractWe report a novel strategy for the fabrication of nanoporous gold (NPG) films. The fabricationprocess involves the electrodeposition of a gold–tin alloy, followed by subsequent chemicaldealloying of tin. Scanning electron microscopy (SEM) images show a bicontinuous nanoporousstructure formed on the substrates after chemical dealloying. Energy dispersive x-ray (EDX)analysis indicates that there are no impurities in the Au–Sn alloy film with an averagecomposition of 58 at. % Au and 42 at. % Sn. After dealloying, only gold remains in the NPG filmindicating the effectiveness of this technique. X-ray diffraction (XRD) results reveal that the as-prepared Au–Sn alloy film is composed of two phases (Au5Sn and AuSn), while the NPG film iscomposed of a single phase (Au). We demonstrate that this approach enables the fabrication ofNPG films, either freestanding or supported on various conductive substrates such as copper foil,stainless steel sheet and nickel foam. The resulting NPG electrode exhibits enhancedelectrocatalytic activity toward both H2O2 reduction and methanol oxidation compared to thepolished Au disc electrode. Our strategy provides a general method to fabricate high qualityNPG films on conductive substrates, which will broaden the application potential of NPG orNPG-based materials in various fields such as catalysis, optics and sensor technology.

Keywords: nanoporous gold, electrodeposition, chemical dealloying

(Some figures may appear in colour only in the online journal)

1. Introduction

In the past few decades, considerable attention has beendevoted to the production of various gold nanostructuresowing to their unique physicochemical properties, such asnanoparticles [1], nanoshells [2], nanowires [3], nanorods [4]and nanocages [5]. Realizing their potential versatility for

real-world applications requires the assembly of nanoscopicelements into 3D macroscopic structures. There has howeverbeen limited success in obtaining macroscopic arrays of goldnanostructures [6–9]. Recently, nanoporous gold (NPG) hasattracted increasing interest because it can exhibit exemplarycatalytic and optical properties resulting from its monolithicbicontinuous porous structure [10, 11]. NPG is an attractive

Nanotechnology

Nanotechnology 25 (2014) 445602 (7pp) doi:10.1088/0957-4484/25/44/445602

0957-4484/14/445602+07$33.00 © 2014 IOP Publishing Ltd Printed in the UK1

candidate as a next generation material for a wide range ofapplications such as heterogeneous catalysis [12, 13] elec-trochemical biosensing [14, 15] optical sensing, [16, 17]energy storage [18, 19] and drug delivery [20]. An effectivemethod for fabricating NPG is by dealloying, in whichnanoporosity is formed through the chemical or electro-chemical dissolution of less noble metals present in goldalloys [21]. Typically, NPG films are fabricated by chemi-cally dealloying commercially available Au–Ag alloy leaves[10, 14, 16–19, 22], leaving behind a freestanding NPG film.However, these ultra-thin films are difficult to handle, arereadily available in only standard compositions (for example,9 carat: 25 at. % Au, or 12 carat: 35 at. % Au), and requireextremely careful transfer methods to be transferred ontovarious substrates for practical applications. All of thesefactors culminate in substantial material integration chal-lenges to be realized for a wide array of potential applications,and alternative synthesis methods for NPG films are highlydesirable for overcoming these issues. There have been somestudies that report the synthesis of NPG by alternative fabri-cation routes such as sputtering [23], electron beam eva-poration [24] and electrodeposition [25]. While methods suchas sputtering and electron beam evaporation require complexand costly high vacuum apparatuses, electrodepositionrepresents the most attractive technique for depositing metalalloy films owing to the low temperature and ambientoperation, low equipment costs and easy scalability. Synthesismethods based on electrodeposition are expected to holdpromise in extending the application potential of NPG invarious fields.

In this paper, we report a unique approach to fabricateNPG films. The process allows for the preparation of NPGfilms that are either: (i) free-standing, (ii) supported on var-ious conductive substrates, or (iii) patterned on flat substrates.This simple but highly effective technique involves electro-chemical co-deposition of Au and Sn to form Au–Sn alloyfilms which are then subjected to chemical dealloying. Au–Snalloy is one of the most promising lead-free solders [26]. TheAu–Sn solder possesses excellent mechanical and thermalproperties making it well suited to optoelectronic packaging[27]. To date, there are few reports on the synthesis of NPGbased on Au–Sn alloy [28]. To the best of our knowledge, thisis the first report on the formation of NPG films from elec-trodeposited Au–Sn alloy films. Our strategy provides ageneral, simplistic and cost-effective method to fabricate highquality NPG films on conductive substrates, which willbroaden the potential applications of NPG or NPG-basedmaterials in various fields.

2. Experimental section

2.1. Materials and substrate preparation

Au–Sn alloy plating solution was obtained from HuizhouLeadao Electronic Material Co., Ltd (Huizhou, China, web-site: www.leadao.cn). HCl (36%), H2O2 (30%), NaOH, NaCl,Na2HPO4·12H2O, NaH2PO4·2H2O, CH3OH, KOH and

HNO3 (70%) were purchased from Xilong Chemical Co., Ltd(Guangzhou, China). All the reagents are of analytical gradeand used as received without further purification. Ultrapurewater with a resistance of 18.2MΩ · cm from a MilliporeMilli-Q purification system was used throughout this work.Copper foil and stainless steel sheets were cleaned ultra-sonically successively in acetone, ethanol and distilled waterfor 15 min each before use. Nickel foam was carefullycleaned with a 5M hydrochloric acid solution in an ultrasonicbath for 30 min in order to remove the nickel oxide layerpotentially existing on the surface, and then thoroughly rinsedwith distilled water. The pre-patterned stainless steel substratewas prepared by a standard photolithography technique. Inbrief, photoresist (Microposit S-1818, Shipley, Marlborough,MA) was spin-cast at 4000 rpm for 50 s onto the stainlesssteel sheet surface and then baked for 1 min at 95 °C. Uti-lizing a contact mask aligner (Q2000, Quintel Corp. San Jose,CA), the photoresist was exposed to UV (345 nm) light for 6 sand was subsequently developed for 40 s (MF-CD-26, Ship-ley, MA).

2.2. Electrodeposition of Au–Sn alloy films

The electrodeposition was performed in a two-electrodeelectrochemical cell using the freshly pretreated substrates asthe working electrodes and a platinum sheet as the counterelectrode. A KR-3001 30 V/1 A programmable DC source-meter (Kingrang Electronic Technology Co., Ltd, Shenzhen,China) was used as a power supply. The deposition wasperformed at a cathodic current density of 5 mA cm−2 for5 min in an aqueous electrolyte containing 500 mL L−1 Au-Snalloy plating solution. After deposition, the substrates werethoroughly rinsed with distilled water, and then dried in air.

2.3. Fabrication of NPG films

For Au-Sn alloy films deposited on stainless steel sheets, thesubstrates were immersed into 35 wt.% HNO3 at room tem-perature for 3 days. For Au-Sn alloy films deposited on nickelfoam, the substrates were immersed into 5M NaOH+1MH2O2 solution at room temperature for 3 days. For Au-Snalloy films deposited on copper foil, the samples were treatedusing the above-described two procedures, respectively. Afterfree corrosion, the substrates were thoroughly rinsed withdistilled water and dried in air. In the case of the coppersubstrate dealloyed in HNO3 solution, the substrate was dis-solved in the solution, leading to the formation of a free-standing NPG film floating on the solution surface, which wascarefully picked up by a glass slide.

2.4. Characterization

The microstructure and chemical composition of the Au–Snalloy films and NPG films were investigated using a field-emission scanning electron microscope (SEM, JEOL, JSM-6700F, 15 keV) equipped with an energy-dispersive x-rayspectrometer (EDX). X-ray diffraction (XRD) measurementswere carried out on a Rigaku D/max-2200/PC diffractometerusing Cu Kα radiation. All electrochemical characterizations

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were carried out using a Gamry REF 600 potentiostat and aconventional three-electrode electrochemical cell, with Ptplate as the auxiliary electrode, Ag/AgCl electrode or satu-rated calomel electrode (SCE) as the reference electrodes,NPG or Au disc electrodes as the working electrodes,respectively. The electrocatalytic activity of NPG electrodestoward H2O2 reduction was evaluated in 0.1 M phosphatebuffer (pH 7.2) and methanol oxidation in 0.1M KOHsolution.

3. Results and discussion

The fabrication procedure of NPG films through electro-deposition and chemical dealloying is schematically illu-strated in figure 1. First, Au–Sn alloy films wereelectrodeposited onto conductive substrates (e.g. copper foil,stainless steel sheet, Ni foam, etc). Second, the as-depositedAu–Sn alloy thin films were subjected to a chemically deal-loying process to selectively remove the Sn component,which then resulted in formation of supported NPG filmunder the condition that the substrate can survive after thedealloying process, or freestanding NPG film in the casewhen the substrate cannot withstand the etching. Such anelectrodeposition–dealloying procedure for fabricating NPGfilm is simple to conduct, and can be used to obtain free-standing NPG films as well as NPG films supported on a widevariety of conductive substrates.

Figure 2(a) shows a representative optical image of theAu–Sn alloy film galvanostatically deposited on a copper foilsubstrate. It is clearly shown that the deposited area uniformly

turned to a silvery white color after alloy deposition. The as-prepared samples were then dealloyed in two different solu-tions. In the first case, a square of side length 1.5 cm was cutfrom the deposited area and immersed into a 35 wt. % nitricacid bath. After a long enough time period (e.g. 3 days) of freecorrosion, both the tin component and the copper foil substratewere etched away, thereby leaving a freestanding film floatingon the water surface (figure 2(b)). In comparison to the Au–Snalloy film, the freestanding thin film is gold colored, high-lighting the fact that the non-noble elements have been leachedout. This thin film is furthermore mechanically robust enoughto be picked up by various substrates such as silicon and glass.In the second case, the Au–Sn alloy-deposited copper foil wasimmersed into 5M NaOH+1M H2O2 solution. After deal-loying for 3 days, the color of the deposited film changed fromsilver white to gold yellow (figure 2(c)), suggesting the for-mation of gold-based materials.

Further characterization of the films was conducted usingscanning electron microscopy (SEM) and energy dispersivex-ray (EDX) spectroscopy. Figure 3(a) shows SEM images ofthe Au–Sn alloy film deposited on a copper foil substrate. It isrevealed that the alloy film consists of numerous diamond-like grains with an average size of ca. 300 nm densely packedon the substrate surface. Figure 3(b) presents a typical EDXspectrum of the Au–Sn alloy film with the determined com-position shown in the inset. It clearly shows that the as-deposited film contains only Au and Sn (the Cu signal comesfrom the copper foil substrate). The atom ratio of Sn to Au inthe film is 42:58, as derived from the EDX analysis. Theseresults verify that gold and tin were successfully co-depositedon the copper substrate, with gold present in a higher atomicconcentration in the alloy film. Figures 4(a) and (b) show low-and high-magnification SEM images of the Au–Sn alloy filmsafter chemical dealloying in 35 wt. % nitric acid solution. It isshown that a very uniform porous structure, with an averageligament diameter of about 150 nm was formed as a result ofthe dealloying process. The cross-sectional SEM image(figure 4(c)) reveals that the continuous porous network wasformed across the entire thickness of the film. RepresentativeEDX spectroscopy (figure 4(d)) of the NPG film reveals thatthe porous structure consists of almost pure gold and the tincontent is below the EDX detection limit (∼0.5 wt. %). Thisreveals that tin can be nearly completely etched away fromthe Au–Sn alloy in nitric acid, leading to the formation of anopen bicontinuous nanoporous structure comprised entirely ofgold. It is worth mentioning that, in the majority of previousstudies [16, 29–31], there are always some residual less-noblemetal component (e.g. Ag) trapped in the NPG films after thedealloying process. The possible reason for this discrepancyin results is that the difference of the standard electrodepotentials between Au and Sn (1.83 V) is much greater thanthat between Au and Ag (0.9 V).

X-ray diffraction (XRD) measurements were conductedto further examine the crystal phase of the Au–Sn alloy filmsbefore and after chemical dealloying. Samples prepared withlonger electrodeposition time (30 min) were employed inorder to overcome the issue of weak diffractive patternsobtained when using an electrodeposition time of only 5 min.

Figure 1. Schematic illustration of the fabrication of NPG films byelectrodeposition of Au–Sn alloy film on conductive substratesfollowed by chemical dealloying.

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Figure 2. Representative optical images of the Au–Sn alloy film deposited on a copper foil substrate (a) before and (b), (c) after chemicaldealloying in (b) 35 wt. % HNO3 solution for 3 days or in (c) 5 M NaOH+ 1 M H2O2 solution for 3 days.

Figure 3. (a) SEM image of Au–Sn alloy film deposited on copper foil substrate before chemical dealloying and (inset) high-magnificationimage with a scale bar of 300 nm. (b) EDX spectrum of the Au–Sn alloy film.

Figure 4. (a) Low-magnification SEM, (b) high-magnification SEM and (c) cross-sectional SEM images of NPG film after chemicaldealloying. (d) EDX spectrum of the NPG film.

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Figure 5 shows XRD patterns of the Au–Sn alloy film and theNPG film. The XRD results reveal that the as-prepared Au–Sn alloy film is composed of two phases: Au5Sn (PDF No.31-0568) and AuSn (PDF no. 08-0463). The peaks located at2θ values of 35.2°, 37.8°, 39.9°, 52.4°, 62.9°, 69.7°, 74.1°and 77.2° can be indexed to the diffractions from (110),(006), (113), (116), (300), (119), (220) and (223) crystalplanes of Rhombohedral Au5Sn, while the peaks located at 2θvalues of 41.7°, 48.7°, 51.5°, 59.5° and 75.5° can be indexedto the diffractions from (110), (200), (201), (202), and (223)crystal planes of hexagonal AuSn. The diffraction pattern ofthe NPG film matches well with that of Au (PDF no. 04-0784). Such results indicate that only Au is present in theNPG films, in agreement with the EDX analysis (figure 4(d)).These results confirm that high purity, bicontinuous NPGnanostructures can be effectively obtained through chemicaldealloying of Au–Sn alloy film.

All of the above results are obtained with samplesdeposited on a copper foil substrate. However, potentialconductive substrate materials are not only limited to copper.In order to demonstrate the applicability and versatility of oursynthetic strategy, Au–Sn alloy thin films were electro-deposited onto various conductive substrates, such as non-patterned or pre-patterned stainless steel sheets and nickelfoam. As in the case of the copper foil substrate, the obtainedAu–Sn alloy films on these substrates initially appear silverywhite (figures 6(a), (e) and (g)), and turn a gold color afterchemical dealloying (figures 6(b), (f), and (h)) suggesting thesuccessful formation of the NPG film. As in the case of thecopper foil substrate, the NPG film on stainless steel sheetsexhibits the bicontinuous nanoporous structure (figures 6(c)and (d)). Similar results were obtained with a pre-patternedstainless steel substrate which was prepared by means ofstandard photolithography techniques. It is shown that theSun Yat-sen University logo was electrodeposited on thesubstrate, which appears silvery white (figure 6(e)) and turns agold color (figure 6(f)) after chemical dealloying. In addition

to two-dimensional NPG films, this approach can also be usedto fabricate three-dimensional NPG films. For example, theAu–Sn alloy film can be electrodeposited on a nickel foam(figure 6(g)) substrate. Once again, after chemical dealloyingthe film turned a gold color (figure 6(h)). SEM images(figures 6(i) and (j)) reveal that the nanoporous structure wasuniformly wrapped around the ligaments of nickel foam afterchemical dealloying. In this case, it should be noted that analkaline etching solution was used in the dealloying processto avoid the dissolution of the nickel foam substrate. On thebasis of the above results, it can be said in principle that theNPG structure can be generated on any type of conductivesubstrate by the method proposed in the present work.

Such NPG films are expected to have a large electro-chemical active surface area (ECSA) and exhibit superiorelectrocatalytic activity owing to the presence of highly por-ous and rough nanostructures. We take NPG films supportedon stainless steel substrate as an example with which toevaluate the electrochemical properties of the NPG electrode.The ECSA of NPG electrode was determined in comparisonwith Au disc electrode. The cyclic voltammogram (CV)curves of these electrodes were recorded in N2-saturated0.5 M H2SO4 and are presented in figure 7. These voltam-mograms display the current peaks associated with the for-mation of surface gold oxide during the anodic scan and thesubsequent gold oxide reduction during the cathodic scan.The ECSA of these electrodes was calculated by the chargeintegration of the gold oxide reduction peak by assuming avalue of 390 μC cm−2 as the conversion factor [32]. TheECSA of the NPG electrode was 35.8 cm2 in comparison to6.7 cm2 of the Au disc electrode. The increased ECSA valueof NPG electrode can be attributed to its special porousstructure with more active sites exposed to the solution. Theelectrocatalytic performance of the NPG electrode wasinvestigated through the electrochemical reduction of H2O2

and oxidation of methanol. Figure 8(a) displays CV curves ofthe H2O2 reduction activities of the NPG and Au disc elec-trodes in a 0.1 M phosphate buffer (pH 7.2) with 5 mM H2O2

solution. The onset potential of H2O2 reduction appears at 0 Vat the NPG electrode while an onset potential of −0.1 V isobtained at the Au disc electrode. The lower onset potential atthe NPG electrode reveals its higher electrocatalytic activity,which is consistent with a previous report [33]. Figure 8(b)shows the CV curves of the methanol oxidation activities ofthe NPG and Au disc electrodes in a 0.1 M KOH with 1.5 Mmethanol solution. The catalytic activity of the NPG electrodeis superior to that of the Au disc electrode, which agrees wellwith a previous study [34]. The peak current density ofmethanol oxidation on the NPG electrode is 1.81 mA cm−2,which is about 3 times higher than that on the Au disc elec-trode (0.65 mA cm−2). Obviously, the increase in peak currentdensity of methanol oxidation at the NPG electrode was notas much as that in ECSA. Similar results were obtained inprevious studies [34–36]. This phenomenon may be attributedto the fast kinetics of the reaction species which fully reacts atthe outer porous layer before these species can diffuse into theinner porous surface. The above results confirm that the NPGnanostructure exhibits a higher catalytic activity which may

Figure 5. XRD patterns of Au–Sn alloy films (black) before and(red) after chemical dealloying.

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Nanotechnology 25 (2014) 445602 Y Xu et al

be attributed to the presence of more active sites associatedwith its bicontinuous porous structure.

4. Conclusions

In conclusion, we have reported the fabrication of high purityNPG structures by chemical dealloying of electrochemicallydeposited Au–Sn alloy films. This novel synthetic strategy isnew, simplistic, rapid and readily reproducible. Moreover,this technique is efficient for the preparation of not onlyfreestanding NPG films, but also NPG structures supported onvarious conductive substrates such as non-patterned or pre-patterned stainless steel sheet, copper foil and nickel foam.The resulting NPG electrodes displayed enhanced electro-catalytic activity toward the H2O2 reduction and methanoloxidation than the Au disc electrode. Further investigationmay pave the way to wider applications of these NPGnanostructures in biosensing, surface-enhanced Raman scat-tering (SERS) and energy storage.

Figure 6. Electrodeposition of Au–Sn alloy films on various conductive substrates. (a)–(d) Optical image of Au–Sn alloy film deposited onstainless steel sheet (a) before and (b) after chemical dealloying in 35 wt. % nitric acid solution, and typical (c) low-magnification and (d)high-magnification SEM images of the NPG film. (e), (f) Optical image of the Sun Yat-sen University logo deposited on pre-patternedstainless steel sheet (e) before and (f) after chemical dealloying in 35 wt. % nitric acid solution. (g)–(j) Optical images of Au–Sn alloy filmdeposited on nickel foam (g) before and (h) after chemical dealloying in 5 M NaOH+1 M H2O2 solution. (i) Low-magnification SEM imageof the nickel foam and (j) high-magnification SEM image of the NPG ligaments.

Figure 7. Cyclic voltammograms of (a) Au disc electrode and (b)NPG (supported on stainless steel substrate) electrode in 0.5 MH2SO4 at a scan rate of 50 mV s−1. The inset displays the magnifiedimage of curve a.

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Acknowledgments

G F Cui gratefully acknowledges the financial support byNational Natural Science Foundation of China (51271205,50801070), Guangzhou Pearl Technology the Nova SpecialProject (2012J2200058), Excellent Young College TeachersDevelopment Program in Guangdong Province (Yq2013006),Research and Application of Key Technologies Oriented theIndustrial Development (90035-3283309, 90035-3283321),Science and Technology Plan Projects of Guangzhou city(2013Y2-00102), Huizhou city (2012B050013012) and DaYaGulf district of Huizhou city (20110108, 20120212). Dr X Keacknowledges the support from China Postdoctoral ScienceFoundation (2014M562234).

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Figure 8. The electrocatalytic activity of the NPG electrode (red) and Au disc electrode (black): (a) H2O2 reduction in 0.1 M phosphate buffer(pH 7.2) and (b) methanol oxidation in 0.1 M KOH solution at a scan rate of 50 mV s−1. Current densities are normalized to the geometricelectrode area.

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