This journal is c The Royal Society of Chemistry 2013 Catal. Sci. Technol. Cite this: DOI: 10.1039/c3cy00017f Anodic deposition of NiO x water oxidation catalysts from macrocyclic nickel(II) complexes† Archana Singh, a Shery L. Y. Chang, b Rosalie K. Hocking, a Udo Bach cde and Leone Spiccia* a Molecular complexes have been found to be excellent precursors for the deposition of catalytically active metal oxide films. Here, three macrocyclic Ni(II) amine complexes have been used for the electrochemical deposition of NiO x films from either a borate buffer solution (pH 9.2) or more basic conditions (pH = 12.9). The cyclic voltammetry of the complexes in both electrolytes shows very similar features and indicates the deposition of a catalytically active nickel oxide film. The NiO x films have been characterised using infrared (IR) and Raman spectroscopy complemented by scanning electron microscopy. Testing of the water oxidation activity at pH 9.2 and 12.9 showed that the films deposited from macrocyclic Ni(II) complexes exhibit similar catalytic activity to those derived from Ni 2+ salts. The macrocyclic complexes offer the advantage of greater solubility and solution stability over a wider range of deposition conditions. At pH 9.2, the catalytic activity of the NiO x films was significantly higher when using a borate buffer and, in addition, the films were more active at pH 12.9 than at pH 9.2. The NiO x films deposited from molecular complexes were also found to show electrochromic properties. The oxidation of water by these films was enhanced by visible light. Water oxidation currents were observed to increase by B20% under simulated solar radiation. 1. Introduction Hydrogen produced by water splitting has the potential to contribute to efforts to replace carbon based fuels with clean energy sources. 1,2 Ideally, hydrogen should be produced using renewable energy sources, such as the large quantity of sunlight that reaches the earth each year. A number of important developments are being pursued that are focused on technologies that utilize sunlight to produce hydrogen from water and effectively store hydrogen until required for use. 3–10 One approach to produce hydrogen is to convert solar radiation into electrical energy in photovoltaic devices, which is then used to generate hydrogen via the electrolysis of water. Water electrolysis (or water splitting) combines water oxidation (H 2 O " O 2 + 4H + + 4e ) and proton reduction (2H + + 2e " H 2 ). Traditionally, expensive electrode materials, such as Pt, IrO x and RuO x , are most widely used in these electrochemical cells. 8,9,11–13 These materials are very effective catalysts for one or both of the chemical processes associated with water splitting but it is difficult to envisage how they could underpin developments aimed at satisfying the energy require- ments of billions of people. In recent years, there has been a focus on the catalyst derived from earth abundant elements that can efficiently catalyze the thermodynamically demanding and mechanistically complex water oxidation reaction. 3–7,10,14–22 Major advances have been reported on the use of solid-state metal-oxide catalysts, such as CoO x , MnO x and NiO x , and a variety of homogeneous solution catalysts. 7,14–17,21,22 We have been exploring the application of nickel and manganese complexes as precursors in the deposition of respective metal oxides as water oxidation catalysts. 5,6,15,16,23,24 a School of Chemistry and ARC Centre of Excellence for Electromaterials Science, Monash University, Clayton, Victoria 3800, Australia. E-mail: [email protected]b Monash Centre for Electron Microscopy, Monash University, Clayton, Victoria 3800, Australia c Department of Materials Engineering, Monash University, Clayton, Victoria 3800, Australia d Commonwealth Scientific and Industrial Research Organization, Materials Science and Engineering, Flexible Electronics Theme, Clayton South, Victoria 3169, Australia e Melbourne Centre for Nanofabrication, 151 Wellington Road, Clayton, VIC 3168, Australia † Electronic supplementary information (ESI) available: CV measured on a 1.0 mM [Ni(tacn) 2 ] 2+ in NaBi buffer switched before the onset of the E a2 peak and the water oxidation catalytic peak (S1), CV of films in 0.60 M NaBi buffer at different scan rates (S2), concentration dependence of CV of [Ni(tacn) 2 ] 2+ in 0.10 M Na 2 SO 4 (S3). CPE for the deposition of films from NaBi and NaOH electrolytes (S4 and S5), EDX (S6), IR spectra (S7), O 2 evolution measurement (S8), Tafel plot (S9), long term testing (S10) and CV of NiO x -aqua and SWtacn-Bi in 0.10 M NaOH (S11). See DOI: 10.1039/c3cy00017f Received 7th January 2013, Accepted 12th February 2013 DOI: 10.1039/c3cy00017f www.rsc.org/catalysis Catalysis Science & Technology PAPER Downloaded on 13/04/2013 04:58:29. Published on 12 February 2013 on http://pubs.rsc.org | doi:10.1039/C3CY00017F View Article Online View Journal
Transcript
This journal is c The Royal Society of Chemistry 2013 Catal. Sci. Technol.
Cite this: DOI: 10.1039/c3cy00017f
Anodic deposition of NiOx water oxidation catalystsfrom macrocyclic nickel(II) complexes†
Archana Singh,a Shery L. Y. Chang,b Rosalie K. Hocking,a Udo Bachcde andLeone Spiccia*a
Molecular complexes have been found to be excellent precursors for the deposition of catalytically
active metal oxide films. Here, three macrocyclic Ni(II) amine complexes have been used for the
electrochemical deposition of NiOx films from either a borate buffer solution (pH 9.2) or more basic
conditions (pH = 12.9). The cyclic voltammetry of the complexes in both electrolytes shows very similar
features and indicates the deposition of a catalytically active nickel oxide film. The NiOx films have been
characterised using infrared (IR) and Raman spectroscopy complemented by scanning electron
microscopy. Testing of the water oxidation activity at pH 9.2 and 12.9 showed that the films deposited
from macrocyclic Ni(II) complexes exhibit similar catalytic activity to those derived from Ni2+ salts. The
macrocyclic complexes offer the advantage of greater solubility and solution stability over a wider
range of deposition conditions. At pH 9.2, the catalytic activity of the NiOx films was significantly higher
when using a borate buffer and, in addition, the films were more active at pH 12.9 than at pH 9.2. The
NiOx films deposited from molecular complexes were also found to show electrochromic properties. The
oxidation of water by these films was enhanced by visible light. Water oxidation currents were
observed to increase by B20% under simulated solar radiation.
1. Introduction
Hydrogen produced by water splitting has the potential tocontribute to efforts to replace carbon based fuels with cleanenergy sources.1,2 Ideally, hydrogen should be produced usingrenewable energy sources, such as the large quantity of sunlight
that reaches the earth each year. A number of importantdevelopments are being pursued that are focused on technologiesthat utilize sunlight to produce hydrogen from water and effectivelystore hydrogen until required for use.3–10 One approach to producehydrogen is to convert solar radiation into electrical energy inphotovoltaic devices, which is then used to generate hydrogen viathe electrolysis of water. Water electrolysis (or water splitting)combines water oxidation (H2O " O2 + 4H+ + 4e�) and protonreduction (2H+ + 2e� " H2). Traditionally, expensive electrodematerials, such as Pt, IrOx and RuOx, are most widely used in theseelectrochemical cells.8,9,11–13 These materials are very effectivecatalysts for one or both of the chemical processes associatedwith water splitting but it is difficult to envisage how they couldunderpin developments aimed at satisfying the energy require-ments of billions of people. In recent years, there has been afocus on the catalyst derived from earth abundant elements thatcan efficiently catalyze the thermodynamically demanding andmechanistically complex water oxidation reaction.3–7,10,14–22
Major advances have been reported on the use of solid-statemetal-oxide catalysts, such as CoOx, MnOx and NiOx, and avariety of homogeneous solution catalysts.7,14–17,21,22
We have been exploring the application of nickel andmanganese complexes as precursors in the deposition ofrespective metal oxides as water oxidation catalysts.5,6,15,16,23,24
a School of Chemistry and ARC Centre of Excellence for Electromaterials Science,
Monash University, Clayton, Victoria 3800, Australia.
E-mail: [email protected] Monash Centre for Electron Microscopy, Monash University, Clayton,
Victoria 3800, Australiac Department of Materials Engineering, Monash University, Clayton, Victoria 3800,
Australiad Commonwealth Scientific and Industrial Research Organization, Materials Science
and Engineering, Flexible Electronics Theme, Clayton South, Victoria 3169,
Australiae Melbourne Centre for Nanofabrication, 151 Wellington Road, Clayton, VIC 3168,
Australia
† Electronic supplementary information (ESI) available: CV measured on a 1.0 mM[Ni(tacn)2]2+ in NaBi buffer switched before the onset of the Ea2 peak and thewater oxidation catalytic peak (S1), CV of films in 0.60 M NaBi buffer at differentscan rates (S2), concentration dependence of CV of [Ni(tacn)2]2+ in 0.10 M Na2SO4 (S3).CPE for the deposition of films from NaBi and NaOH electrolytes (S4 and S5), EDX(S6), IR spectra (S7), O2 evolution measurement (S8), Tafel plot (S9), long termtesting (S10) and CV of NiOx-aqua and SWtacn-Bi in 0.10 M NaOH (S11). See DOI:10.1039/c3cy00017f
Received 7th January 2013,Accepted 12th February 2013
Catal. Sci. Technol. This journal is c The Royal Society of Chemistry 2013
In the case of manganese, polynuclear complexes were found todeposit a manganese oxide when doped in Nafion and subjectedto a potential bias. Moreover, the nano-sized MnOx resulting fromthese precursors had a higher surface area and higher catalyticactivity than those formed from [Mn(OH2)6]2+. In the case of nickelcomplexes, NiOx deposited using [Ni(en)3]Cl2 in the presence ofborate buffer produced very homogenous, well adhered NiOx thinfilms that exhibited significantly higher catalytic activity than filmsderived from [Ni(OH2)6]2+ solution under similar conditions.24
Considering that molecular complexes are useful pre-catalysts tothe corresponding metal oxide, we now report the application ofthree macrocyclic nickel(II) amine complexes (Scheme 1) for theelectrochemical deposition of NiOx films in slightly basic boratebuffer and more basic hydroxide media, and then examine theproperties and catalytic activity of the films via IR and Ramanspectroscopy, electron microscopy, linear scan voltammetry andcontrolled potential electrolysis. Since NiOx is a well-knownelectrochromic material,25–28 the changes in the UV-Visiblespectrum on application of a potential bias as well as the effectof light on the catalytic activity of the electro-deposited films areexamined.
2. ExperimentalMaterials
Reagent or analytical grade chemicals were sourced fromcommercial suppliers and used as received unless stated other-wise. Deionized water was used throughout. The nickel(II)complexes, [Ni(tacn)(OH2)3]2+, [Ni(tacn)2]2+, where tacn = 1,4,7triazacyclononane, and [Ni(cyclen)(OH)2]2+, where cyclen =1,4,8,11 tetraazacyclotetradecane, were all prepared and character-ized following the literature procedures.29–31
Electrochemistry
All electrochemical experiments were performed at 22 1C in athree-electrode cell connected to an Epsilon CS3 workstation.The reference electrode was a BAS Ag/AgCl (3 M NaCl) glassbodied electrode, which has a potential of 0.200 V vs. NHE.Pt foil was used as counter electrode. Glassy carbon electrodes(3 mm diameter) or fluorine doped tin oxide (FTO) coated glasswere used as working electrodes. Sodium borate (NaBi) buffer(pH 9.2), 0.10 M NaOH (pH 13) or 0.10 M Na2SO4 (pH 9.2)
solutions were used as electrolytes. Potentials are specified withreference to Ag/AgCl. No corrections for the uncompensatedresistance were made to the potentials reported in this article.
Film deposition and water oxidation testing
The films for water oxidation studies were deposited by con-stant potential electrolysis (CPE) at 1.10 V (vs. Ag/AgCl) using1.0 mM solution of each Ni(II) amine complexes in 0.10 M NaBibuffer. To deposit the films from 0.10 M NaOH solution, anapplied potential of 0.75 V (vs. Ag/AgCl) was used. The filmsdeposited from [Ni(OH2)6]2+, [Ni(tacn)2]2+, [Ni(tacn)(OH2)3]2+
and [Ni(cyclen)(OH2)2]2+ using Bi buffer are abbreviated asNiOx-aqua, SWtacn-Bi, tacn-Bi and cyclen-Bi while the filmdeposited from [Ni(tacn)2]2+ in the 0.10 M NaOH electrolyteis abbreviated as SWtacn-OH. Comparative water oxidationstudies for the films were carried out in the absence of aNi(II) source in 0.60 M NaBi buffer at a constant potential of1.10 V. For this purpose, films with an identical amount of Ni(B0.26 mmol cm�2) were deposited from each Ni(II) precursoron FTO conducting glass by passing similar charge through theelectrochemical circuit. Water oxidation studies in the 0.10 MNaOH electrolyte were carried out at 0.75 V potential vs. Ag/AgCl.Linear scan voltammetry (LSV) studies on the electro-depositedfilm, made at a scan rate of 1.0 mV s�1, were used to establishthe current–potential characteristics and to carry out a Tafelanalysis.25 To investigate the effect of light on water oxidationactivity, a SWtacn-Bi film deposited on FTO glass was exposed tosimulated sun light with an intensity of 200 mW cm�2 equivalentto two suns. A 200 W Oriel xenon lamp was used as light sourceand was calibrated using a Si photodiode. Oxygen detection wasperformed following the procedure outlined in our previouswork using an Ocean Optics fluorescence-probe oxygen sensor.25
Characterization methods
Quantification of the amount of Ni deposited from the differentprecursors by ICPMS was carried out using a GBC Optimass9500 ICP Time of Flight MS. SEM images of the electro-deposited films before and after water oxidation catalysis wererecorded using a JEOL 7001F instrument, operated at 15 kV.Energy dispersive spectroscopy (EDS) has also been employedto measure the composition of the specimen. The films werescraped off from the electrode and then ground with dry,anhydrous KBr to record IR spectra on a Perkin Elmer 1600FTIR spectrometer at a resolution of 1 cm�1. Films depositedon FTO glass were used to record the Raman spectra on aRenishaw Invia Raman microscope at an excitation wavelengthof 514 nm.
Optical measurements
Transmittance spectra of the NiOx films, deposited on trans-parent FTO coated glass, were recorded using a JASCO V-670spectrophotometer. The electrochromic behavior was examinedby chronoamperometry using an EC-lab V10.23 equipment incombination with a three electrode cell filled with the 0.10 MNaOH electrolyte with Ag/AgCl as a reference electrode and Ptwire as a counter electrode. The in situ transmittance of the
Scheme 1 Structure of macrocyclic nickel(II) amine complexes used for thedeposition of NiOx films.
This journal is c The Royal Society of Chemistry 2013 Catal. Sci. Technol.
films was measured at 450 nm and recorded while applyingdouble step chronoamperometry (0.75 V and �0.75 V) measure-ments.29–31
3. Results and discussionCyclic voltammetry of nickel(II) complexes
NaBi buffer (pH = 9.2). CVs were recorded for solutions ofthe complexes shown in Scheme 1 under the same conditionsin 0.10 M NaBi buffer (pH = 9.2).23 For [Ni(tacn)(OH2)3]2+, thefirst CV scan shows an anodic peak at a potential of 0.95 V anda cathodic peak at 0.69 V (Fig. 1A). Upon consecutive scanningthe anodic peak potential shifts from 0.95 V to 0.89 V afterseven cycles. For [Ni(cyclen)(OH2)2]2+, anodic and cathodicpeaks were observed at 0.95 V and 0.67 V, respectively(Fig. 1B). On repeated scanning both complexes showed anincrease in the anodic and cathodic peak currents. For[Ni(tacn)2]2+ (Fig. 1C), the first CV scan shows a broad anodicpeak at Ea1 = 0.67 V and no cathodic peak. Following subsequentscans, however, the anodic peak at 0.67 V decreases in amplitudeand a second anodic peak at Ea2 = 0.93 V appears together with astrong catalytic peak above 1.04 V. A cathodic peak at Ec1 = 0.70 V,which grows in intensity on subsequent scans, accompanies thischange. When the potential was switched either just after Ea1
(ESI,† Fig. S1) or even after Ea2 and before the onset of wateroxidation catalysis (ESI,† Fig. S1) no deposition was observed.
These results are consistent with our previous observation for[Ni(en)3]2+ where no film deposition was observed on reversingthe CV scan just before the onset peak for water oxidationcatalysis. Thus, as for [Ni(en)3]2+, the macrocyclic Ni(II) complexesexhibit features characteristic of a surface deposition process butin contrast to [Ni(OH2)6]2+, the deposition of nickel oxide from themacrocyclic complexes coincides with water oxidation.24 Afterseveral cycles, the CVs of the macrocyclic complexes show anodicand cathodic peaks at similar potentials to those of solutioncontaining [Ni(H2O)6]2+ salt suggesting that a similar electroactivematerial is being deposited in each case. The anodic and cathodicpeaks persist when the electrode is rinsed, placed in fresh bufferand CVs are recorded again in the absence of Ni(II) precursors.The linear dependence of the peak amplitude on the scan rate(ESI,† Fig. S2A) is also indicative of the deposition of an electro-active species. In the absence of NaBi buffer, no increase in peakcurrent density was observed even at a higher concentration of thecomplex (1–10 mM [Ni(tacn)2]2+) and no catalytic wave wasobserved (ESI† Fig. S3). In a recent publication,24 we postulatedthat the buffer may be aiding the deligation of the macrocyclicligands via protonation and binding of borate to higher valentnickel centres.
NaOH (0.10 M, pH = 12.9). The macrocyclic Ni(II) complexeswere found to be soluble and stable in the 0.10 M NaOH electrolyte.The CV scan for a 1.0 mM solution of Ni(tacn)(OH2)3]2+ showed ananodic peak at 0.525 V and a cathodic peak at 0.350 V [Fig. 2A]. Thefirst CV scan on a 1.0 mM [Ni(tacn)2]2+ solution in 0.10 M NaOHshowed two anodic peaks at 0.45 V and 0.70 V and a cathodic peakat 0.39 V, see Fig. 2B. On repeated scanning the complex showed asingle anodic peak at 0.535 V and a cathodic peak at 0.39 V. As forthe experiments in NaBi buffer, in both cases the peak current ofthe anodic peak increases simultaneously with the catalytic peakcurrent as the number of CV scans increases. Thus, we canconclude that an electroactive material can be deposited from themacrocyclic metal complex in alkaline solution. The difference inE1/2 values at pH 12.9 (0.10 M NaOH) and pH 9.2 (0.10 M NaBibuffer) is higher than expected for a 1e�/1H+ coupled electrontransfer reaction and is in agreement with a 2e�/3H+ coupledprocess as also confirmed in our previous studies.24 For example,[Ni(tacn)2]2+ showed E1/2 values of 0.460 V and 0.810 V in 0.10 MNaOH (pH = 13) and NaBi buffer (pH = 9.2), respectively. Thedifference in the E1/2 value (0.35 V) is close to what would beexpected for a 2e�/3H+ couple process (0.88*pH = 0.88*3.7 =0.325 V).
As for the films deposited from NaBi buffer, the electrodeswere rinsed with water and then transferred into fresh 0.60 MNaBi buffer with no Ni(II) precursor present in the electrolyte.The CV scans run at different scan rates (ESI,† Fig. S2B)revealed that the anodic and cathodic peaks together with aprominent catalytic current peak can still be observed indicatingthe presence of catalytically active species.
Film deposition
Nickel oxide (NiOx) films were electrodeposited by controlledpotential electrolysis (CPE) from solutions of each macrocyclicnickel(II) complex and water oxidation catalysis by these films
Fig. 1 Continuous cyclic voltammetry scans of 1.0 mM solutions of:[Ni(tacn)(OH2)3]2+ (A), [Ni(cyclen)(OH2)2]2+ (B) and [Ni(tacn)2]2+ (C) in 0.10 MNaBi buffer recorded at a scan rate of 100 mV s�1. Arrows indicate the increase incurrent density over a consecutive number of scans.
Catal. Sci. Technol. This journal is c The Royal Society of Chemistry 2013
was investigated, as outlined previously.24 For the NaBi buffer,the films were deposited using 1.0 mM solutions of [Ni(tacn)2]2+,[Ni(tacn)(OH2)3]2+and [Ni(cyclen)(OH2)2]2+ by applying a constantpotential of 1.10 V (vs. Ag/AgCl). During thirty minutes of CPE at1.10 V similar currents of 1.2 to 1.3 mA cm�2 were measured forall precursors (ESI,† Fig. S4). For comparison, NiOx films werealso deposited from solution of [Ni(tacn)2]2+ in 0.10 M NaOH atan applied potential of 0.75 V (ESI,† Fig. S5). A current density of1.5 mA cm�2 was achieved over a similar period of time and wasaccompanied by the formation of green coloured films.
To better compare the catalytic activity, films were depositedby passing a similar amount of charge (50 mC cm�2) throughNaBi solutions of each macrocyclic complex. Identical amountsof Ni (0.25 to 0.27 mmol cm�2) were deposited, as was confirmed byICPMS. A Faradaic efficiency of 50–55% was calculated based on theamount of deposited nickel oxide, which is close to the value ofB60% reported by Nocera and co-workers.14 In our case, a lowervalue was expected as film deposition was coincident with significantoxygen evolution. The films deposited from [Ni(tacn)2]2+ in 0.10 MNaOH contained approximately 1.5 times more Ni per cm2, corre-sponding to a Faradaic efficiency of 70–75%. A possible reason forthis higher Faradaic efficiency may be that at the applied potential of0.75 V in the more basic solution less charge is consumed by theoxygen evolution process. To facilitate comparisons of catalyticactivity, a similar amount of Ni (B0.26 mmol cm�2) was alsodeposited from 0.10 M NaOH (vide infra).
Film characterization
SEM images of the different films show that different complexeslead to the deposition of very homogenous films with well definedgrain boundaries (Fig. 3). The SWtacn-Bi and SWtacn-OH filmsshow very similar morphology (Fig. 3C and D). Energy dispersiveX-ray (EDS) analysis was performed directly on the films depositedon FTO (ESI,† Fig. S6 shows the EDS of SWtacn-Bi (top) andSWtacn-OH films (bottom)). The elemental composition wasfound to be similar for the two films except that the SWtacn-OHfilms contain some Na+ ions whereas the NaBi derived films donot. However, the presence of Na+ does not affect the activity ofthe films. SEM images of the SWtacn-Bi and SWtacn-OH filmswere also taken after 30 minutes of water oxidation catalysis inNaBi performed using CPE (Fig. 3E and F). As shown in Fig. 3,relatively little change in the morphology of the films hadoccurred during testing as described below; the films remainquite homogenous with well defined grain boundaries. The filmthickness was measured using a Veeco Dektak 6M stylus profilo-meter. On passing a charge of 50 mC cm�2 SWtacn-Bi, tacn-Bi andcyclen-Bi films showed an average thickness of 0.12, 0.11 and0.13 mm, which compare well to the thickness of films derivedfrom [Ni(en)3]2+ and Ni2+.24 For the SWtacn-OH films, however, onpassing a similar amount of charge films of higher thickness(0.20 � 0.02 mm) were obtained. The higher thickness is inkeeping with the higher amount of Ni deposited from the NaOHelectrolyte as confirmed by ICP-MS. Since all the films exhibitedsimilar performance (vide infra), further characterization focusedmainly on the SWtacn-Bi films.
The IR spectra of [Ni(tacn)2]2+ and SWtacn-Bi film, recordedin KBr disks (ESI,† Fig. S7), clearly support the deposition of the
Fig. 2 Continuous CV scans of 1.0 mM solutions of [Ni(tacn)(OH2)3]2+ (A) and[Ni(tacn)2]2+ (B) in 0.10 M NaOH. Arrows indicate the increase in current densityon repeated scanning.
Fig. 3 SEM images of the freshly prepared tacn-Bi (A), cyclen-Bi (B), SWtacn-Bi (C),SWtacn-OH (D), films; E and F show the SWtacn-Bi and SWtacn-OH films after30 minutes of CPE testing in 0.60 M NaBi buffer.
This journal is c The Royal Society of Chemistry 2013 Catal. Sci. Technol.
NiOOH type films and at the same time show significantdifferences in their residual composition.23,26 The N–H vibra-tions found in the 3300–3500 cm�1 region for the complex arereplaced by a broad peak at 3450 cm�1, due to O–H vibrations,and H2O bending vibrations at 1625 cm�1 attributable tothe presence of lattice water in the films. A (C–H) vibration at2930 cm�1 indicates the presence of residual carbon from themacrocyclic ligands in the film.26 However, no bands werefound in the IR spectrum suggesting the incorporation ofborate into the films. The Raman spectrum of the SWtacn-Bifilm shows two strong peaks at 481 and 558 cm�1 (Fig. 4), as hasbeen reported for g-NiOOH phase of NiOx.23,26,31
Water oxidation
Linear scan voltammetric (LSV) studies were used to comparethe water oxidation activity of the nickel oxide films. The LSVsof the different films (with the same amount of Ni depositedB0.26 mmol Ni cm�2) in NaBi buffer, shown in Fig. 5 (top),indicate that they exhibit similar catalytic activity. Controlledpotential electrolysis at an overpotential (Z) of 0.60 V showed anincrease in current over the first 30 minutes which stabilised at1.2�1.3 mA cm�2 for all films, in agreement with LSV results(Fig. 5, bottom). This current density was similar to thatobserved for the films deposited from [Ni(OH2)6]2+ but lowerthan those made from [Ni(en)3]2+.11 The films deposited from[Ni(tacn)2]2+ in the 0.10 M NaOH electrolyte (Ni content =0.26 mmol Ni cm�2) were found to have a similar catalytic activityto that of the NaBi derived NiOx films when tested at pH 9.2.
The Faradic efficiency for water oxidation was determined bymeasuring oxygen evolution using SWtacn-Bi films in NaBibuffer in the absence of a Ni(II) ion at an applied potential of1.1 V. As shown in Fig. S8 (ESI†), after two hours of operationsufficient charge was passed through the electrolysis cell toproduce 12 mmol of dioxygen (i.e., 48 mmol of electrons),compared to 10.8 mmol of dioxygen collected in the headspace.
Thus, the Faradic efficiency of the film was ca. 90%. The activityof the films was also examined by a Tafel analysis via a plot ofthe overpotential vs. log of current density (ESI,† Fig. S9). Theslopes of the Tafel plot in the NaBi electrolyte were similar forall films (100–110 mV per decade) and close to that obtained forNiOx-aqua films, derived from [Ni(OH2)6]2+, indicating that thefilms deposited by using either electrolytes are equally efficientcatalysts for water oxidation. The lack of evidence for theincorporation of borate into the layered oxide lattice suggeststhat the role of the buffer is to promote the movement ofprotons and anionic species in response to changes inthe nickel oxidation state and proton release through wateroxidation as recently proposed for layered cobalt oxide wateroxidation catalysts.32
Testing of catalytic activity of the SWtacn-Bi film for a longerperiod of time (B4.5 h Fig. S10 in ESI†) showed little change incurrent density after the initial stabilisation of 30 minutes.Since nickel oxide films have long been known as efficient wateroxidation catalysts in alkaline medium,33–35 films depositedfrom [Ni(tacn)2]2+ in NaBi buffer and NaOH electrolyte were alsotested in the 0.10 M NaOH electrolyte. The LSV traces measured
Fig. 4 Raman spectrum of the SWtacn-Bi film in the coloured and bleachedstate.
Fig. 5 Linear scan voltammogram for nickel oxide films derived from eachmacrocyclic nickel(II) complex and [Ni(OH2)6]2+ recorded in NaBi buffer at a scanrate of 1 mV s�1 (top) and controlled potential electrolysis (bottom) performed at1.10 V in 0.60 M NaBi buffer for the films derived from the complexes.
Catal. Sci. Technol. This journal is c The Royal Society of Chemistry 2013
for the SWtacn-Bi and SWtacn-OH films in 0.10 M NaOH andCPE for SWtacn-Bi performed at 0.75 V vs. Ag/AgCl are shown inFig. 6. The onset potential for water oxidation for both films was0.63 V vs. Ag/AgCl (Z B 0.37 V) and the dependence of thecatalytic activity on the applied potential is very similar for thetwo films. Again, this indicates that a common material has beendeposited. CPE performed on the SWtacn-Bi film at 0.75 V (Z =0.49 V) produced a current density of 1.5–1.6 mA cm�2 over twohours of testing (cf., 1.2–1.3 mA cm�2 at pH 9.2). Thus, in morebasic medium the films show a slightly higher activity and anonset of catalysis at a lower overpotential than at pH 9.2.
Optical properties and water oxidation under illumination
The well-established fact that nickel oxide films exhibit electro-chromic properties25–27,36 led us to investigate the opticalproperties of the SWtacn-Bi and NiOx-aqua films in 0.10 Maqueous NaOH solution. In situ UV-Visible spectrophotometrywas used to study the NiOx-aqua (thickness 0.14 mm) andSWtacn-Bi (thickness 0.11 mm) films. The SWtacn-Bi film showsan anodic peak at 0.58 V and a cathodic peak at 0.40 V (ESI,†Fig. S11A) while the corresponding values for [Ni(OH2)6]2+ are0.55 V and 0.41 V (ESI,† Fig. S11B), respectively. On passingthrough the anodic peak, the films transition from transparentto coloured, attributed to the conversion from Ni(OH)2 toNiOOH.25,28
NiðOHÞ2 , NiOOHþHþ þ e�
Bleached Coloured
It has been proposed that during the cathodic scan, H+ ionsintercalate into NiO(OH) with simultaneous reduction of nickelfrom a higher oxidation state to NiII resulting in bleachingof the films.25,28 Anodic oxidation of the films results inde-intercalation of H+ and oxidation of Ni(II) to produce thecoloured films.25 This was further confirmed by the Ramanspectrum of the bleached and coloured films (Fig. 4).The coloured SWtacn-Bi film showed two intense peaks at481 and 558 cm�1, characteristic of the g-NiOOH phase, upon
bleaching the two peaks are replaced by a broad peak centeredat 589 cm�1 characteristic of a-Ni(OH)2.31
The ex situ optical transmittance spectra of the coloured andbleached states of the films are shown in Fig. 7. In the colouredstate, there is a substantial increase in light absorption over the350–700 nm region. The formula DOD = log(Tb/Tc), where Tb
and Tc are the transmittance of the films in the bleached andcoloured states, was used to determine the optical density oftwo films. The DOD value calculated at 450 nm for the NiOx
aqua film (0.49) is significantly higher than that for the SWtacn-Bi film (0.37), and is a consequence of the greater transparencyof the former. The colouring efficiency (CE) of the films wasdetermined by double step chronoamperometry with in situtransmittance measurement at 450 nm, as shown in Fig. 8. TheCE was calculated using the formula CE = DOD/Q, where Q isthe charge intercalated during the colouration process.Although the two films show different DOD values, the CEvalues were the same (18.5 cm2 C�1 for the SWtacn-Bi film and17.5 cm2 C�1 for the NiOx-aqua film) and in agreement withthose reported in the literature (11–17.5 cm2 C�1).26 The reasonfor the higher DOD value for the NiOx-aqua film relative to theSWtacn-Bi film but similar CE values is not clear at present,although a similar observation has been reported by Sunget al.27 We note that, in addition to differences in CE arisingfrom variations in the transparency of the films in the restingstate, the hydroxyl content of the films has been suggested toinfluence this property.25
Given the effect of the applied bias on the optical propertiesof the NiOx films, the water oxidation activity of SWtacn-Bifilms was examined by LSV in NaBi buffer with and withoutvisible light illumination (Fig. 9). As can be seen from Fig. 9,when irradiated with a light intensity of 2 suns (200 mW cm�2)a significant increase in current density was observed over the
Fig. 6 LSV (scan rate = 1 mV s�1) for the SWtacn-Bi and SWtacn-OH films carriedout in 0.10 M NaOH. Inset shows the CPE of the SWtacn-Bi film at 0.75 V vs. Ag/AgCl in 0.10 M NaOH.
Fig. 7 Ex situ optical transmittance spectra of the SWtacn-Bi and NiOx-aquafilms in the coloured (biased at +0.75 V) and bleached (biased at �0.75 V) statesmeasured in 0.10 M NaOH. The films were deposited from NaBi buffer at 1.1 V vs.Ag/AgCl.
This journal is c The Royal Society of Chemistry 2013 Catal. Sci. Technol.
tested potential range. This was further confirmed by a lighton–off CPE experiment carried out at 1.10 V vs. Ag/AgCl in 0.60 MNaBi (inset of Fig. 9), which revealed an increase in current from1.22 mA cm�2 to 1.43 mA cm�2, corresponding to a photocurrentof 0.21 mA cm�2. At 1.25 V, the photocurrent measured by LSVwas much higher reaching about 1 mA cm�2, or approximately25% of the total electrolysis current. Sun et al.37 recently coupledsol–gel derived NiOx with a silicon photoanode thereby showingthat water could be oxidised at a potential below the thermo-dynamic limit.37 Recognizing the potential of this approach, theoptimization and improvement of the photon driven wateroxidation activity of NiOx films is worth pursuing.
4. Conclusion
Nickel oxide films have been deposited from three macrocyclicnickel(II) complexes dissolved in borate buffer (pH 9.2) and
shown to exhibit similar catalytic activity to that of filmsdeposited from [Ni(OH2)6]2+. Moreover, by using a macrocyclicNi(II) complex to deposit films from more alkaline solution(pH 12.9), we have demonstrated that metal complexes offer theopportunity to broaden the range of conditions over whichactive catalysts can be deposited, such as those under which themetal ions precipitate metal hydroxides and/or oxides. Theoverpotential for the onset of catalysis, Tafel slopes, IR, Ramanand SEM images obtained for the films prepared from thedifferent precursors suggests that the same material has beenformed and that a common reaction mechanism is in opera-tion. Besides being efficient water oxidation catalysts, the filmsare electrochromic showing substantial increases in absorptionin the 350–700 nm region on oxidation. A notable feature ofthese electrochromic films is the significant improvement incatalytic activity observed upon irradiation with visible light.The present results illustrate that the facile deposition of NiOx
films, which are promising water oxidation catalysts, can inprinciple be achieved by using a plethora of already availablemetal complexes.
Acknowledgements
This work was supported by the Australian Research Council(ARC) through the Australian Centre of Excellence for Electro-materials Science (ACES). The authors acknowledge the facilitysupport from the Monash University Center for Electron Micro-scopy. We would like to thank Assoc. Prof. Bjorn Winter-Jensenand Dr Orawan Winter-Jensen for their assistance with thein situ UV-Visible absorption studies. AS is grateful to MonashGraduate Research Scholarship and International PostgraduateResearch Scholarship from Monash University.
Notes and references
1 N. S. Lewis and D. G. Nocera, Proc. Natl. Acad. Sci. U. S. A.,2007, 104, 20142.
2 D. G. Nocera, Daedalus-Us, 2006, 135, 112–115.3 R. Brimblecombe, J. Chen, P. Wagner, T. Buchhorn,
G. C. Dismukes, L. Spiccia and G. F. Swiegers, J. Mol. Catal.A: Chem., 2011, 338, 1–6.
4 R. Brimblecombe, G. C. Dismukes, G. F. Swiegers andL. Spiccia, Dalton Trans., 2009, 9374–9384.
5 R. Brimblecombe, D. R. J. Kolling, A. M. Bond,G. C. Dismukes, G. F. Swiegers and L. Spiccia, Inorg. Chem.,2009, 48, 7269–7279.
6 R. Brimblecombe, A. Koo, G. C. Dismukes, G. F. Swiegersand L. Spiccia, ChemSusChem, 2010, 3, 1146–1150.
7 M. Dinca, Y. Surendranath and D. G. Nocera, Proc. Natl.Acad. Sci. U. S. A., 2010, 107, 10337–10341.
8 L. Duan, F. Bozoglian, S. Mandal, B. Stewart, T. Privalov,A. Llobet and L. Sun, Nat. Chem., 2012, 4, 418–423.
9 N. D. McDaniel, F. J. Coughlin, L. L. Tinker and S. Bernhard,J. Am. Chem. Soc., 2008, 130, 210–217.
10 R. Shinar and J. H. Kennedy, Sol. Energy Mater., 1982, 6,323–335.
Fig. 8 In situ transmittance vs. time curves for the SWtacn-Bi (blue) and NiOx-aqua (black) films for applied potentials of 0.75 V and �0.75 V. tb and tc are thebleaching and colouring times.
Fig. 9 LSV of the SWtacn-Bi film in NaBi buffer with (green) and without (black)illumination (200 mW cm�2), recorded at a scan rate of 1 mV s�1. Inset: CPE performedat 1.10 V vs. Ag/AgCl with chopped light illumination (200 mW cm�2) for 30 s.
Catal. Sci. Technol. This journal is c The Royal Society of Chemistry 2013
11 D. J. Blakemore, N. D. Shley, G. W. Olack, C. D. Incarvito,G. W. Brudvig and R. H. Crabtree, Chem. Sci., 2011, 2, 94–97.
12 J. W. Jurss, J. J. Concepcion, J. M. Butler, K. M. Omberg,L. M. Baraldo, D. G. Thompson, E. L. Lebeau, B. Hornstein,J. R. Schoonover, H. Jude, J. D. Thompson, D. M. Dattelbaum,R. C. Rocha, J. L. Templeton and T. J. Meyer, Inorg. Chem.,2012, 51, 1345–1348.
13 D. Moonshiram, J. W. Jurss, J. J. Concepcion, T. Zakharova,I. Alperovich, T. J. Meyer and Y. Pushkar, J. Am. Chem. Soc.,2012, 134, 4625–4636.
14 D. K. Bediako, B. Lasalle-Kaiser, Y. Surendranath, J. Yano,V. K. Yachandra and D. G. Nocera, J. Am. Chem. Soc., 2012,134, 6801–6809.
15 R. Brimblecombe, A. Koo, G. C. Dismukes, G. F. Swiegersand L. Spiccia, J. Am. Chem. Soc., 2010, 132, 2892–2894.
16 R. Brimblecombe, G. F. Swiegers, G. C. Dismukes andL. Spiccia, Angew. Chem., Int. Ed., 2008, 47, 7335–7338.
17 R. K. Hocking, R. Brimblecombe, S. Chang, A. Singh,M. Cheah, C. Glover, W. H. Casey and L. Spiccia, Nat. Chem.,2011, 3, 461–466.
18 B. A. Pinaud, Z. Chen, D. N. Abram and T. F. Jaramillo,J. Phys. Chem. C, 2011, 115, 11830–11838.
19 D. D. Qin, C. L. Tao, S. Friesen, T. H. Wang, O. K. Varghese,N. Z. Bao, Z. Y. Yang, T. E. Mallouk and C. A. Grimes, Chem.Commun., 2012, 48, 729–731.
20 J. J. Stracke and R. G. Finke, J. Am. Chem. Soc., 2011, 133,14872–14875.
21 Y. Surendranath, M. W. Kanan and D. G. Nocera, J. Am.Chem. Soc., 2010, 132, 16501–16509.
22 Y. Surendranath, D. A. Lutterman, Y. Liu and D. G. Nocera,J. Am. Chem. Soc., 2012, 134, 6326–6336.
23 B. Sakintuna, F. Lamari-Darkrim and M. Hirscher, Int. J.Hydrogen Energy, 2007, 32, 1121–1140.
24 A. Singh, S. Chang, R. K. Hocking, U. Bach and L. Spiccia,Energy Environ. Sci., 2013, 6, 579–586.
25 H. Liu, W. Zheng, X. Yan and B. Feng, J. Alloys Compd., 2008,468, 356.
26 M. A. Vidales-Hurtado and A. Mendoza-Galvan, Solid StateIonics, 2008, 179, 2065.
27 K. Ahn, Y. Nah and Y. Sung, J. Vac. Sci. Technol., A, 2002, 20,1468–1474.
28 S. I. Cordoba-Torresi, A. Hugot-Le Goff and S. Joiret,J. Electrochem. Soc., 1991, 138, 1554–1559.
29 Y. S. Nechaev and O. K. Alexeeva, Int. J. Hydrogen Energy,2003, 28, 1433–1443.
30 G. K. R. Srinivasan, J. Chem. Sci., 2009, 121, 145–147.31 B. S. Yeo and A. T. Bell, J. Phys. Chem. C, 2012, 116, 8394–8400.32 J. B. Gerken, J. G. McAlpin, J. Y. C. Chen, M. L. Rigsby,
W. H. Casey, R. D. Britt and S. S. Stahl, J. Am. Chem. Soc.,2011, 133, 14431–14442.
33 M. R. Gennero de Chialvo and A. Chialvo, Electrochim. Acta,1988, 33, 825–830.
34 M. Lyons and M. Brandon, Int. J. Electrochem. Sci., 2008, 3,1386–1424.
35 X. Y. Wang, H. Luo, H. P. Yang, P. J. Sebastian andS. A. Gamboa, Int. J. Hydrogen Energy, 2004, 29, 967–972.
36 A. I. Inamdar, A. C. Sonamane, S. M. Pawar, Y. Kim,J. H. Kim, P. S. Patil, W. Jung, H. Im, D. Kim and H. Kim,Appl. Surf. Sci., 2011, 257, 9606–9611.
37 K. Sun, N. Park, Z. Sun, J. Zhou, J. Wang, X. Pang, S. Shen,S. Y. Noh, Y. Jing, S. Jin, P. K. L. Yua and D. Wang, EnergyEnviron. Sci., 2012, 5, 7872–7877.
