Highly Dispersed Cobalt Oxide on TaON as Efficient Photoanodes for

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Highly dispersed cobalt oxide on TaON as efficient photoanodes for long-term solar water splitting Satnam Singh gujral, Alexandr Nikolaevich Simonov, Masanobu Higashi, Xi-Ya Fang, Ryu Abe, and Leone Spiccia ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.6b00629 • Publication Date (Web): 13 Apr 2016 Downloaded from http://pubs.acs.org on April 21, 2016

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ACS Catalysis

Highly dispersed cobalt oxide on TaON as efficient photoanodes for long-term solar water splitting †

†

‡

§

Satnam Singh Gujral, Alexandr N. Simonov,*, Masanobu Higashi, Xi-Ya Fang, Ryu Abe* and Leone Spiccia*, †

‡

†

School of Chemistry and the ARC Centre of Excellence for Electromaterials Science, Monash University, Victoria, 3800, Australia * E-mail: [email protected]; [email protected]; [email protected];

‡

Department of Energy and Hydrocarbon Chemistry, Graduate School of Engineering, Kyoto University, Kyoto 615-8510, Japan §

Monash Centre for Electron Microscopy, Monash University, Victoria, 3800, Australia

ACS Paragon Plus Environment

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ABSTRACT Photo-electrochemical water splitting into H2 and O2 over a semiconductor-based photocatalyst offers a promising way to achieve the sustainable harvesting and storage of solar energy. However, short diffusion lengths and inefficient separation of the charge carriers in the semiconductors following light absorption result in fast recombination of holes and electrons, and eventually poor performance. Herein, we address this problem by integrating an efficient and robust water oxidation catalyst, cobalt oxide (CoOx), into screen-printed TaON photoanodes premodified with TiO2 coatings for better stability. SEM, TEM, ICP-MS analysis of the Co deposits and electrochemical techniques were used to demonstrate the advantages provided by the photoassisted CoOx electrodeposition method. Specifically, this method allows the selective and facile functionalization of the TiO2-TaON surface with a uniform layer of near-(hemi)spherical CoOx particles having a diameter of 5-15 nm. As compared to the TiO2-TaON photoanodes, the optimized CoOx/TiO2-TaON configuration provides an enhancement in the photocurrent densities of up to two orders of magnitude and a substantial improvement in the long-term stability when tested in borate buffer solutions (pH = 9.2). The highest oxidative photocurrent density of 0.7 mA cm−2 was achieved with CoOx/TiO2-TaON under visible light irradiation (λ > 400 nm; 100 mW cm−2) at 1.2 V vs. reversible hydrogen electrode, and remained stable for at least 24 h. The Co-loading in the best performing photoanode is ca 0.1 wt.% with respect to TaON; higher and lower loadings result in poorer photocatalytic activity and stability. Comparisons of the performance of CoOx/TiO2-TaON with other representative inorganic water photo-electrooxidation systems are provided and discussed. KEYWORDS: Electrodeposition, cobalt oxide, tantalum oxynitride, photocatalytic water splitting, visible light


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INTRODUCTION A comparison of humanity’s energy demands with the amount delivered to our planet by the Sun1,2 leads to the conclusion that solar energy can and should eventually replace non-renewable and/or non-sustainable alternatives as our primary source of energy. An obvious drawback of the solar energy is its intermittent nature, which spurs vigorous efforts to develop technologies for converting photons into storable and transportable forms, viz. chemical energy. From this perspective, solar-powered electrochemical generation of hydrogen from water, within the scope of a global ‘hydrogen economy’,3 is one of the most promising sustainable approaches for harvesting and storing solar energy. This can be accomplished by using very efficient photovoltaics4,5 to power free-standing electrocatalytic cells in which water is decomposed into H2 and O2 (by-product)6. More elegant but much harder to achieve is a photo-electrochemical approach, where a light-harvesting element is integrated with the catalyst(s) for the halfreactions, viz. hydrogen evolution reaction (HER) at the cathode and oxygen evolution reaction (OER) at the anode (Scheme 1).7-11 Simplistically, photo-electrochemical water splitting proceeds in the following elementary sequence: (i) absorption of photons, (ii) conversion of photons into electrical energy (photopotential), (iii) electrocatalytic splitting of water into H2 and O2 powered by the photopotential. Inorganic semiconductors are one class of materials that are capable of efficiently sustaining all three elementary steps and have therefore attracted considerable attention.12 However, the complexity and multi-electron nature of water splitting imposes significant constraints and challenges to the development of suitable semiconductor materials that are efficient, inexpensive and nontoxic.


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Scheme 1. Modus operandi of the CoOx-modified TiO2-TaON photoanode during water splitting under visible light irradiation. Thermodynamically, water splitting is an unfavourable reaction requiring an increase in Gibbs free energy by 237 kJ mol(H2)−1. Theoretically, this can be achieved by applying a potential of 1.23 V, but sustaining efficient water splitting requires additional energy to satisfy the overpotentials of the HER and the OER, and resistance losses.13-16 This excess energy demand can be met by the energy of the photogenerated electrons and holes in a semiconductor having a sufficiently wide band gap, EG > 1.6 eV (Scheme 1).5,16 On the other hand, efficient utilization of solar energy, whose predominant contribution is from photons of the visible and near IR regions (400 nm < λ < 1000 nm), imposes an upper limit for EG of 3 eV.16,17 Finally, the valence and conduction band levels must straddle the redox potentials of the OER and the HER (Scheme 1) to allow unassisted, i.e. with no external applied potential, water splitting for hydrogen production.16,18 If these requirements are met, upon absorption of photons, charge separation and transport occurs followed by the consumption of electrons and holes at the metal cathode|electrolyte (HER) and photoanode|electrolyte interfaces (OER), respectively.19,20 At the same time, detrimental recombination of the charge carriers can occur and, therefore, the whole water splitting process must be achieved within the lifetime of the photogenerated electrons and


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holes.21 Semiconductor photocatalytic materials meeting the indicated criteria are required to alleviate the limitations commonly faced when efficient photo-electrochemical water splitting using sunlight as the only energy source is pursued. Semiconducting metal oxides, and especially TiO2, are the most intensively studied materials for photocatalytic applications owing to their excellent activity, chemical stability and photocorrosion resistance.18,22,23 However, their band gaps are often large (e.g. EG = 3.2 eV for TiO2), restricting light absorption to the UV region of solar radiation (λ < 400 nm) and making them non-responsive in the visible and near IR regions. This limits the achievable photon-tocurrent efficiency under the Sun’s irradiation to 1%. Many attempts have been made to extend the light absorption by tuning the metal oxides band gap via doping,24-28 use of plasmonic nanoparticles29-32 and dyes as sensitizers,12,33-38 creation of oxygen vacancies,39,40 etc. Amongst the TiO2-based photoanode materials, 1D TiO2 nanowires rich in oxygen vacancies showed the highest water splitting photoactivity reported to date. A photocurrent of ca 1.3 mA cm−2 at 0.7 V vs. reversible hydrogen electrode (RHE) was measured in 1 M KOH under simulated AM 1.5G full spectrum irradiation (one sun, light intensity of 100 mW cm−2) which corresponds to an applied-bias-compensated photon-to-current efficiency (AB-PCE) of ca 0.69%.41 Defects created by oxygen vacancies in metal oxides also improve electrical conductivity and charge transfer properties. On the downside, the necessity to apply complex synthetic and fabrication procedures, such as thermal evaporation, chemical vapour deposition, pulsed laser deposition, cold pressing, etc., and high temperatures significantly increases the cost of such materials. Moreover, the long-term stability of defect structures is an issue that needs to be considered. As an alternative, narrow band gap inorganic semiconductors with an inherent capability to absorb visible solar radiation have been intensely studied in recent years. Amongst them,


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materials based on α-Fe2O3 (EG = 2.2 eV),42 BiVO4 (EG = 2.4 eV),43 WO3 (EG = 2.7 eV)44 and TaON (EG = 2.5 eV)45 are arguably the most promising. However, only tantalum oxynitride is theoretically capable of sustaining the complete water splitting process, i.e. both the OER and HER as shown in Scheme 1, without applying an additional bias and/or introducing a photocatalyst on the cathode. This is because, in most cases, the conduction band position is too positive for the HER. Additionally, recombination losses in narrow band gap inorganic semiconductors usually limit their performance for the OER, and typical onset potentials for the photo-electrocatalytic water oxidation are limited to ca 0.8 V vs. RHE. This problem has been resolved by combining the semiconducting materials with well-known water oxidation catalysts, like IrO2,46-48 CoOx48-51 and NiOx50 to facilitate charge separation/transport and the OER kinetics. Structuring of the photoanodes, e.g. in the form of nanotube or nanorod arrays, has also been demonstrated to improve performance.52-55 Representative examples of notable recent achievements are summarized in Table 1 and further details are available in a recent review on the Ta-based photoanodes for water splitting.56 As mentioned above, the EG and valence and conduction band positions of TaON (ca +2.2 and −0.3 V vs. normal hydrogen electrode (NHE), respectively)45,57 are appropriate for sustaining complete water splitting into H2 and O2 under visible light irradiation. Generally, metal oxynitrides, being coloured and visible light responsive; represent an interesting class of photooxidation catalysts.58-62 Since the first report by Domen and co-workers in 2002,45,58 tantalum oxynitrides have emerged as promising photoanodes for water splitting.46,48,61-66 However, as with most oxynitrides, the presence of the N2p orbitals in the valence band makes TaON prone to oxidative decomposition to molecular nitrogen (2 N3− + 6 h+ → N2) and Ta2O5, which is a poor visible light absorber (EG = 3.9 eV).45 The formation of the tantalum oxide layer


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on the TaON surface hinders hole transport to the electrode|electrolyte interface and degrades the performance of the photoanode. Table 1. Water splitting performance of representative examples of photoanode materials and TaON-based systems. Irradiation conditions

Photoanode Materials

pH

1D TiO2 nanowires

14 d

WO3

0

BiVO4

AM 1.5G λ > 400 nm 6 sun c

AM 1.5G λ > 400 nm 1 sun c

e

7.0-7.3

f

1.3

1.4

1.5

1.2

2.2

2.9

0.2

0.6

1.1

7.3

f

2.4

3.0

3.6

CoOx/BiVO4

7.0

f

1.0

1.2

1.5

NiOx/CoOx/BiVO4

7.0 f

2.7

CoOx/W-doped-BiVO4 AM 1.5G full spectrum 1 sun c

Transient photoactivitya, Long-term J / mA cm−2 photoactivity,b −2 0.7 V 0.9 V 1.23 V J / mA cm

d

3.0

3.5

j

n.a. 0.4 1.6

2.2 3.0 1.9

Fe2O3 IrO2/Fe2O3 TaON nanotube arrays C/Cu2O/TaON nanorod arrays TaON/Ta2O5 nanorods TaON TaON/TaON

14 14 d 14 d

n.a. n.a. 1.4

13.6 g

0.70

2.3

4.3

14 d 6h 6h

3.0 n.a. 0.25

3.0 n.a. 0.55

3.1 < 0.01 1.25

IrO2/TaON

6h

1.2

2.1

3.7

CoOx/TaON

8f

0.80

1.6

3.2

8

f

< 0.1

< 0.1

0.20

RhOx/CoOx/BaTaO2N

8

f

0.50

1.5

3.2

IrOx/CoOx/BaTaO2N

8f

0.30

1.1

2.7

BaTaO2N

d

TaON nanotube arrays TaON/TaON CoOx/TaON CaFe2O4/TaON

14 11.5 f 11.5 f 11.5 f

0.00 0.04 0.12 0.10

0.15 0.08 0.30 0.15

0.50 0.15 0.55 0.55

CoOx/CaFe2O4/TaON

11.5 f

0.20

0.40

0.95

h

TiO2/TaON MnOx/TiO2-TaON

6 6h

0.03 0.23

0.05 0.26

0.14 0.30

CoOx/TiO2-TaON

9.2 i

0.48

0.70

1.02

1.2 (0.83 V, 6 h) n.a. 0.4 (0.80 V, 16 h) n.a. 0.65 (0.80 V, 16 h) 2.3 (0.80 V, 16 h) n.a. n.a. n.a. 2.7 (1.0 V, 1 h) n.a. ----0.35 (1.15 V, 1 h) 2.1 (1.07 V, 1 h) --2.0 (1.07 V, 1 h) 1.0 (1.07 V, 1 h) n.a. --n.a. n.a. 0.50 (1.23 V, 3 h) ----0.45 (0.90 V, 24 h)

Ref. 41 67 50 49 50 50 47 47 52 53 54 68 46,69 46 51 48 48 48 52 70 70 70 70 71 72 Present Work

a

Photocurrent densities at defined potentials vs. RHE derived from the voltametric data. Photocurrent densities derived from the chronoamperometric data at defined potential after defined time (in brackets). Data for the systems exhibiting more than 0.1 mA cm−2 at E < 0.9 V are shown only. c 1 sun = 100 mW cm−2. d1 M KOH or NaOH. e 1 M H2SO4. f 0.1 M phosphate buffer. g 0.5 M NaOH. h 0.1 M Na2SO4. i 0.1 M sodium borate buffer. j No data available. b


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Three major approaches have been developed to improve the stability and performance of the TaON photoanodes.56 The first is a post-necking treatment commonly applied in dye-sensitized solar cells.73-76 The use of TiO2,68,71 Ta2O546,69 or TaON46,69 coatings to interconnect the TaON particles improves charge transport within the electrode and at the electrode|electrolyte interface, and enhances the water photo-oxidation activity (Table 1). The second approach is to couple TaON with efficient metal oxide water oxidation catalysts (e.g., IrO2,46,69 CoOx,51 RhOx,48 MnOx72 or CaFe2O470) similarly to studies on other photoanodes (Table 1). The metal oxide cocatalyst improves charge separation at the oxynitride surface by scavenging the photogenerated holes, which typically have short diffusion lengths. This allows suppression of the oxidative decomposition of TaON and also increases the OER rate due to the higher catalytic activity of the MOx co-catalyst as compared to TaON. Critical parameters for the efficient and robust MOxmodified TaON photoanodes are a high dispersion of the co-catalyst and its intimate connection to the TaON surface. Indeed, a high photo-oxidation activity was reported by Abe and coworkers for the IrO2/TaON system46,69 (Table 1), but the long-term stability of the photoanode was limited by the irregular coverage of TaON with the agglomerated co-catalyst nanoparticles. However, impressive continuous water photo-electrooxidation was possible with the CoOx/TaON materials containing highly-dispersed and homogeneously distributed cobalt oxide nanoparticles prepared by the incipient wetness impregnation method.51 Similar conclusions were drawn in our recent studies where a facile photo-assisted electrodeposition method was used to selectively modify TiO2-covered TaON photoanodes (TiO2-TaON) with MnOx.72 These multi-component photoanodes exhibited AB-PCEs comparable to that of the IrO2/TaON photoanode (Table 1), but their stability was inferior to that of the chemically synthesized CoOxTaON reported earlier.51 The third approach to improve the performance of the TaON-based and


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other photoanodes is the creation of structured electrodes52-55 as already mentioned above. Application of materials with a nano-dimensional, well-ordered morphology effectively has been shown to suppress bulk recombination of the photo-generated holes and electrons, facilitate charge transport and thus allows very impressive photocurrents to be achieved (Table 1). The synthesis of this type of electrode, however, requires use of complex and resource-intensive techniques. In this study, photo-assisted electrodeposition of CoOx is used to pursue improvements in the water photo-oxidation activity of the TaON anodes produced by a simple and cost-effective screen-printing method. Following post-necking with TiO2,71 the fabrication procedures were optimized by applying various electrodeposition protocols at different pHs to selectively functionalize the TiO2-TaON surface with highly-dispersed cobalt oxide nanoparticles. For the first time, titania and cobalt oxides are applied jointly to improve the performance of the TaON photoanodes. This combination together with the utilization of a photo-electrodeposition method to produce CoOx, and thorough optimization of the electrode preparation conditions has allowed the creation of the best performing TaON-based photoanodes for water oxidation reported to date. Comprehensive characterization of the multicomponent electrodes was undertaken using scanning electron (SEM) and transmission electron microscopy (TEM), inductively coupled plasma mass spectroscopy (ICP-MS) and a set of conventional electrochemical techniques, and the products of the photo-electrochemical water splitting were analysed quantitatively by gas chromatography. EXPERIMENTAL SECTION Materials. The mesoporous tantalum oxynitride films were prepared by screen-printing the TaON powder (synthesized as reported elsewhere58) onto fluorine-doped tin oxide (FTO) coated


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glass substrates (Dyesol, NSW, Australia; sheet resistance of 10 Ω cm−2) following previously reported procedure.71 All screen-printed TaON films (1.5 µm thick; 0.5 cm × 0.4 cm geometric area; 1 mgTaON cm−2 loading) were subjected to a post-necking treatment with TiO2 (ca 15 wt.% unless otherwise stated) using an ethanolic TiCl4 precursor solution.71 All chemicals were used as obtained from either Merck or Sigma Aldrich. Water purified by reverse osmosis was used for all experimental procedures. Photo-electrochemical methods. Measurements were undertaken at ambient temperature (25 ± 1°C) using a Bio-Logic VSP Modular potentiostat in either a three- or a two-electrode configuration using a Zahner-Elektrik PECC-1 or a custom-made tightly sealed photoelectrochemical cell, respectively. Samples within the photo-electrochemical cell were irradiated using a 150 W xenon arc lamp equipped with a horizontal light beam and an Oriel solar simulator through AM 1.5G and long pass filters (λ > 400 nm). The visible light intensity of 1 sun (AM 1.5G, 100 mW cm−2) was calibrated using a Hamamatsu S1133 photodiode. Band pass filters (average band-width of 10 nm) were used for determination of incident photon-to-current conversion efficiencies (IPCE) and the corresponding light intensities were measured using a Hamamatsu S1133 photodiode. In these IPCE measurements, the photocurrent densities were obtained from either chronoamperograms or voltammograms measured at slow scan rates (v = 0.005 V s−1) under chopped monochromatic light irradiation. Freshly prepared TiO2-TaON films were used as working electrodes for electrodeposition of CoOx and subsequent characterization. To avoid deposition of CoOx on bare FTO surrounding the printed TiO2-TaON films, an inert polyimide tape (Kapton) was used as a mask. In some experiments, CoOx was deposited on unmodified FTO glass (0.2 cm2 area provided by a Kapton tape mask). A high surface area Pt wire and Ag|AgCl|KCl(sat.) (BAS) were employed as


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auxiliary and reference electrodes, respectively. However, all potentials are reported versus reversible hydrogen electrode (RHE), whose potential was calculated as −0.059pH − 0.197 V vs. Ag|AgCl|KCl(sat.), unless otherwise stated. Several experiments were undertaken in a singlecompartment two-electrode configuration using a high surface area Pt wire and CoOx-modified TiO2-TaON films as a counter and working electrode, respectively. Throughout the text, the currents are normalized to the electrochemically active geometric surface area of the electrodes. For the CoOx deposition, aqueous 10 mM CoCl2 with 0.1 M Na2SO4 (pH = 5), or 0.5 mM CoCl2 containing either 0.1 M sodium phosphate (pH = 7) or 0.1 M sodium borate buffer (pH = 9.2) were used, and the resulting materials are denoted as CoOx-S, CoOx-P or CoOx-B, respectively. Use of the deposition solutions immediately after dissolution of CoCl2 in the buffer was imperative to achieve photoanodes with high efficiency. A constant potential, Edep, was applied under 1 sun illumination with λ > 400 nm (hereinafter, visible light) or in the dark for deposition of CoOx on TiO2-TaON, TaON or bare FTO. The as prepared electrodes were carefully and copiously rinsed with water before undertaking characterization and photoelectrochemical measurements. The later included cyclic voltammetric (typically, scan rate, v = 0.025 V s−1), chronoamperometric and electrochemical impedance spectroscopic (EIS; frequency range 1 Hz to 100 KHz, amplitude 0.01 V) measurements in the dark and under continuous or chopped visible light irradiation. Aqueous 0.1 M Na2SO4 (pH = 6) solution, 0.1 M phosphate buffer (pH = 7), or 0.1 M borate buffer (pH = 9.2) were used as supporting electrolyte solutions during photo-electrochemical measurements for the films modified with CoOx-S, CoOx-P or CoOx-B, respectively. Gas chromatography. Quantification of the Faradaic efficiency of the studied photoelectrochemical processes was achieved by gas chromatographic (GC) analysis using an Agilent


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MicroGC 3000A instrument (thermal conductivity detector; MS-5A column; Ar as a carrier gas) securely connected to a tightly sealed custom-made photo-electrochemical cell. In these experiments, a 300 W xenon arc lamp (equipped with a cut-off filter: L-42, HOYA) was used to provide 6 suns simulated visible light irradiation (λ > 400 nm), and the working electrodes had a geometric surface area of the CoOx/TiO2-TaON films of 1 cm2. These experimental conditions were needed to allow measurable amounts of gas to be evolved during measurements. Characterization. The morphology of the photoanode films was examined before and/or after photo-electrochemical studies using field-emission SEM (FEI Nova NanoSEM 450 FEG and FEI Magellan 400 FEG) and TEM (JEOL JEM-2100F). The as prepared films on the FTO glass substrate were mounted over the SEM specimen stubs using a carbon tape and coated with iridium (ca 2-3 nm thick). For TEM studies, the films were scratched from FTO glass and deposited on a TEM grid. The amount of electrodeposited Co was determined using ICP-MS analysis (GBC Optimass 9500). Films were digested in 0.5 ml of concentrated HNO3 (ca 70%) for 3 hours at 60°C and the resulting Co containing solution was diluted 10 times with 2% HNO3 solution to obtain solutions with Co concentrations below 2 µM (ca 100 ppm). A calibration curve was measured using a commercially available cobalt stock solution (ICP-MS reference by AccuStandard) and was used to standardize the raw count rates obtained from the analysis of Co solutions. An indium solution was used as an internal reference (ICP-MS reference by AccuStandard) to apply drift corrections. The measurement precision was about 2-4% in terms of relative standard deviation.

RESULTS AND DISCUSSION The creation of active and robust CoOx-modified TaON water oxidation photoanodes requires stable nanostructures of the co-catalyst to be formed in the vicinity of photoactive sites present


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on the TaON surface. To achieve this, the CoOx electrodeposition conditions were optimized by probing the effects of pH, deposition time and irradiation on the photo-oxidation current densities measured by voltammetry (current-potential dependence) and chronoamperometry (current-time dependence at constant applied potential). The outcomes of these studies are briefly summarized in the following section. The photocatalysts fabricated under the optimized conditions were further studied using a variety of techniques as described below. Optimization of the CoOx Deposition Conditions. Before introducing the cobalt oxides on the photoanodes, the screen-printed TaON films were subjected to a post-necking treatment with a thin TiO2 layer (TiO2-TaON), which is necessary for efficient charge transfer within the photocatalyst and to enhance its stability.71 Direct deposition of CoOx on the TaON layer resulted in very unstable and poorly performing photoanodes. Aqueous CoCl2 solutions containing different supporting electrolytes and varying in pH were used to electrodeposit the cobalt oxides (see Experimental Section). In the discussion to follow, the photoanodes are termed CoOx-S (pH = 5, Na2SO4 (aq.)) CoOx-P (pH = 7, phosphate buffer) and CoOx-B (pH= 9.2, borate buffer). Optimization of the electrodeposition was performed in a constant potential (Edep) mode under visible light irradiation (1 sun, λ > 400 nm) and for various deposition times (tdep = 0.5-5 min). Very positive Edep, in the range 1.75-1.94 V vs. RHE, were needed to deposit sufficient amounts of CoOx. However, deposition at even more positive potentials (>2 V) did not improve the activity and stability under the conditions examined, presumably due to

oxidative deactivation of TaON. Typical

voltammograms

and

chronoamperograms obtained under chopped visible light irradiation for the CoOx/TiO2-TaON films prepared under different conditions are shown in Figs. 1 and S1-2 (Supplementary Information). Note that the photocatalytic activity of CoOx itself is poor under these conditions.


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Unexpectedly, photo-assisted deposition of CoOx-S on TiO2-TaON resulted in a negligible enhancement of the oxidative photocurrent densities at potentials below ca 1.0 V and even deteriorated the performance of the photoanode at more positive potentials (Fig. S1A). The maximum activity of CoOx-S/TiO2-TaON prepared under irradiation was achieved with tdep = 3 min. In contrast, both CoOx-P/TiO2-TaON and CoOx-B/TiO2-TaON outperformed the parent Co-free films at all tdep examined and the optimal deposition time was 1 min (Figs. 1 and S1B). Chronoamperometric measurements at the very positive potential of 1.5 V were undertaken to assess the stability of the films towards corrosion (Fig. S2). In accordance with our expectations, CoOx significantly enhanced the stability of the TaON-based photoanodes, especially when electrodeposition and testing were performed in borate buffer solutions. Importantly, even the CoOx-S/TiO2-TaON system showed reasonable short-term stability and higher activity than TiO2-TaON under these strongly oxidising conditions (Fig. S2A). 1.4 Lightmin 0.5 min 0.5 1Light min1 min 2Light min2 min CoO -B/TiO -TaON x 2 3Light min3 min 4Light min4 min 5Light min5 min 4x5 uL TiO 2-TaON TaON plain

1.2 1

J / mA cm−²

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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0.8 0.6 0.4 0.2 0 0.3

0.5

0.7

0.9

1.1

1.3

1.5

E vs. RHE / V

Figure 1. Voltammograms (v = 0.025 V s−1) obtained under chopped visible light irradiation (1 sun, λ > 400 nm) for CoOx-B/TiO2-TaON compared to TiO2-TaON (black dotted) and TaON (black solid) films in contact with aqueous 0.1 M borate buffer (pH 9.2). CoOx was electrodeposited from a 0.5 mM CoCl2 aqueous solution buffered with 0.1 M borate (pH 9.2) at a constant potential (Edep = 1.94 V vs. RHE) and for varying illumination times (1 sun, λ > 400 nm). Voltammograms are corrected for the corresponding dark current at 0.55 V to facilitate photocurrent comparisons.


14

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Previously, we demonstrated that post-necking the screen-printed TaON photoanodes with titania improved their stability and activity.71 We undertook extensive optimization of the TiO2deposition procedure and adopted here the conditions providing TiO2-TaON samples with the best performance to prepare the CoOx-modified anodes. To further demonstrate the importance of the TiO2 coating for stable and efficient operation of the catalysts under study, CoOx-B/TiO2TaON electrodes with 50% lower and 50% higher than the optimal TiO2 loadings and also titania-free CoOx-B/TaON electrodes were prepared and tested for water oxidation. As is shown in Fig. S3, within the potential range examined, the photocurrent densities were highest for the films containing optimized amounts of TiO2 (e.g., 0.93 mA cm−2 at 1.25 V) as compared to CoOx-B/TiO2-TaON films with lower (0.63 mA cm−2) and higher (0.38 mA cm−2) loadings and especially to the titania-free CoOx-B/TaON films (ca 0.05 mA cm−2). The optimization studies emphasized the effect of the CoOx and TiO2 loadings on the photooxidation activity of the TaON photoanodes. In particular, sufficient CoOx co-catalyst amount is needed to allow notable synergistic effects, but should be below the limit when it starts interfering with light absorption by tantalum oxynitride. These findings are consistent with the reports on the photo-oxidation activity of TiO2-TaON,71 and TiO2-TaON modified with (photo)electrodeposited MnOx.72 The influence of the preparation conditions on the performance of the CoOx/TiO2-TaON photoanodes is discussed in the following section. Effect of Irradiation on CoOx Loading and Microstructure. The amount of CoOx electrodeposited onto the TiO2-TaON photoanodes was determined by ICP-MS (Table 2). At very positive deposition potentials, water (photo)oxidation contributed substantially to the deposition current transients (Fig. S4). Indeed, at the least positive Edep examined, the Faradaic efficiency for deposition of CoOx-S was 17-18%, and only a few percent for deposition of


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CoOx-P and CoOx-B (Table 2). This difference in Edep used for experiments with sulphate and buffered solutions was also reflected in the effect of irradiation on the amount of CoOx deposited on the electrode surface. For CoOx-S, twice the amount of cobalt was deposited under illumination as compared to in the dark (Table 2). This observation confirms that oxidation of dissolved Co2+(aq.) species is promoted by TaON under visible light irradiation. In contrast, irradiation reduced the efficiency of CoOx deposition by ca 3-fold when more positive Edep were applied in buffered solutions at higher pH (see ICP-MS data, Table 2). At the same time, very similar currents, dominated by water rather than Co2+(aq.) oxidation, were measured during dark and photo-assisted deposition of both CoOx-P and CoOx-B (see deposition charge data, Table 2 and Fig. S4). Therefore, the specific (Co-amount weighted) water electrooxidation activity is ca 3 times higher for the CoOx co-catalyst produced on the TiO2-TaON surface under irradiation. Table 2. Properties of the CoOx/TiO2-TaON films produced under different conditions. Sample

Edep / V vs. RHE

CoOx-S/TiO2-TaON

1.75

CoOx-P/TiO2-TaON

1.81

CoOx-B/TiO2-TaON

1.94

CoOx-B/FTO CoOx-B/TiO2-FTO

1.94

Deposition solution 10 mM Co2+ (pH = 5, Na2SO4) 0.5 mM Co2+ (pH = 7, phosphate) 0.5 mM Co2+ (pH = 9.2, borate) 0.5 mM Co2+ (pH = 9.2, borate)

Faradaic Transient ΓCo a / μg cm−2 Deposition efficiency for activity at conditions ICP-MS b Deposition deposition d, 1.05 V e, charge c % J / mA cm−2 dark, 3 min

1.3

6.9

18

0.55

light, 3 min

2.6

15

17

0.10

dark, 1 min

0.83

49

1.7

0.76

light, 1 min

0.31

56

0.56

0.32

dark, 1 min

2.5

87

2.9

0.87

light, 1 min

0.93

92

1.0

0.75

2.0

62

3.1

---

1.7

60

2.8

---

dark, 1 min

a

Amount of Co normalized to the geometrical area of the screen-printed TaON film. b ICP-MS analysis was undertaken on solutions obtained by digesting the electrodes in HNO3 (see Experimental). c Estimated by using the charge measured during Co2+(aq) electrodeposition (Q / C) and Faraday’s law, NCo = , where NCo / mol is the theoretical maximum of CoII oxidized to CoOx, F = 96485 C mol−1 is the Faraday constant, n is the number of electrons transferred, which is assumed to be 1.8 on the basis of the XANES studies on electrodeposited CoOx.77 d Ratio of the Co content derived from the ICP-MS analysis to that estimated from the deposition charge. e Photocurrent density derived from voltametric measurements (Fig. 5).


16

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Differences in the properties of the various CoOx/TiO2-TaON photoanodes were confirmed by cyclic voltammetry (Fig. 2). The CoOx-S/TiO2-TaON and CoOx-B/TiO2-TaON films as well as the TaON-free CoOx-B/FTO control sample exhibited well-defined voltammetric responses characteristic of electrodeposited cobalt oxides78 (Fig. 2A and 2C). Previous detailed ex situ and in situ XAS studies have concluded that these electrodeposited materials are based on Cooxo/hydroxo structural motifs similar to those found in the CoO(OH) heterogenite phase.77-80 Less pronounced CoOx signals were found for CoOx-P/TiO2-TaON (Fig. 2B) due to the lower amount of deposited cobalt oxide (Table 2). Generally, the charge associated with the CoOx redox transformations measured by cyclic voltammetry agreed well with the differences in the cobalt concentrations determined by ICP-MS (cf. data in Fig. 2 and Table 2). 120

A

light Series4

CoOx-B/TiO2-TaON

dark Series1

dark, CoOx-B/FTO Series7

80

TiO2-TaON Series3

40 0 -40 -80

J / µA cm−2

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Catalysis

120

B

light Series2 dark Series7

80

CoOx-P/TiO2-TaON

E vs. RHE / V

TiO2-TaON Series3

40 0 -40 -80 120

C

light Series4 dark Series1

CoOx-S/TiO2-TaON

E vs. RHE / V

80

TiO2-TaON Series3

40 0 -40 -80 0.75

0.95

1.15

1.35

1.55

E vs. RHE / V

Figure 2. Cyclic voltammograms (scan rate, v = 0.025 V s−1) in the dark for the TiO2-TaON (black), CoOx-B/TiO2-TaON (red), CoOx-P/TiO2-TaON (green), CoOx-S/TiO2-TaON (blue) and CoOx-B/FTO (purple) films in contact with either 0.1 M borate (pH 9.2) (red, purple) or 0.1 M phosphate (pH = 7) buffer (green), or 0.1 M Na2SO4 (pH = 6) (blue). CoOx was electrodeposited in the dark (dashed curves) or under illumination (solid curves). Other conditions were as defined in Table 2.


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The microstructure of the CoOx deposits was analysed using SEM for all samples synthesized under optimized conditions (Fig. 3 and Figs. S5-S6) and with TEM for the CoOx-B/TiO2-TaON films prepared using the photo-assisted method (Fig. 4). Inspection of the SEM micrographs obtained for TiO2-TaON (Fig. 3A), CoOx-B/FTO (Fig. 3B), CoOx-B/TiO2-FTO (Fig. 3C and S6) and CoOx/TiO2-TaON (Fig. 3D-F and S5) reveals that cobalt oxide is deposited in the form of small particles. This is corroborated by TEM analysis that allowed fine visualization of the CoOx-B/TiO2-TaON interface (Fig. 4). The micrographs in Fig. 4 clearly show that (semi)spherical cobalt oxide nanoparticles with diameters of 5-15 nm are embedded into the thin disordered TiO2 layer (2-3 nm) that uniformly covers the TaON surface. The higher magnification TEM images of the CoOx particles suggest that these very small crystallites exhibit an interplanar spacing of ca 0.22 ± 0.01 nm under conditions relevant to Fig. 4. This is consistent with the (111) plane for CoOOH (PDF 26-480) suggested as a major structural motif of electrodeposited CoOx,80 but presence of other phases cannot be ruled out on the basis of this TEM data. Most importantly, the selectivity of deposition and degree of agglomeration of CoOx particles depended strongly on the preparation conditions. As demonstrated below, this has significant implications for the catalytic properties of the CoOx/TiO2-TaON photoanodes.


18

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Figure 3. SEM images obtained for (A) TiO2-TaON, (B) CoOx-B/FTO, (C) CoOx-B/TiO2-FTO, (D) CoOx-S/TiO2-TaON, (E) CoOx-P/TiO2-TaON and (F) CoOx-B/TiO2-TaON. Electrodeposition of CoOx was performed in the dark (i) or under illumination (ii) for 1 min (E, F) or 3 min (D) (see also Table 2).


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Figure 4. TEM images obtained for CoOx-B/TiO2-TaON. Photo-assisted electrodeposition of CoOx was undertaken for 1 min and as indicated in Table 2. The importance of irradiation for achieving selective modification of the TiO2-TaON surface with CoOx is best demonstrated by the SEM data for the CoOx-S/TiO2-TaON and CoOx-B/TiO2TaON films (Figs. 3D, F and S5B). In the dark, CoOx-S and CoOx-B were electrodeposited randomly and non-uniformly on both the TiO2-TaON aggregates and the TiO2-coated FTO surface that remained uncovered after screen-printing of TaON, the latter being more preferable for cobalt oxide deposition from the sulphate solutions (see Figs. 3D, F). In contrast, photoassisted electrodeposition resulted in the selective and homogeneous deposition of CoOx-S and CoOx-B on TiO2-TaON (Figs. 3D, F and S5B). Surprisingly, irradiation had no obvious influence on the selectivity of electrodeposition for the CoOx-P/TiO2-TaON system under the conditions employed (see Figs. 3E and S5A). In this case, small isolated islands of cobalt oxide were found on both the TaON-covered and TaON-free areas of the electrode. The lowest degree of agglomeration of the CoOx particles was achieved when using the photoassisted method, buffered Co2+ solutions and tdep = 1 min (Fig. 3F, ii). Increasing the amount of


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ACS Catalysis

electrodeposited cobalt oxide, as provided by higher tdep, improved the coverage of the TiO2TaON surface, but also resulted in its agglomeration (Fig. S5). A higher degree of agglomeration was typical for the CoOx-S/TiO2-TaON films (tdep = 3 min) irrespective of the illumination conditions (Fig. 3D). Taken together, the electrochemical and microscopic data provide consistent evidence for the selective immobilization of highly dispersed CoOx particles on the TiO2-TaON surface when using the photo-assisted electrodeposition method and Co2+ precursor solutions containing sodium sulphate (pH = 6.0) or borate buffer (pH = 9.2). The very positive deposition potentials used in this work also allowed CoOx to be formed on the TiO2-TaON surface in the dark, but with notable amount of cobalt oxide also deposited on the TaON-free TiO2-FTO surface. Photo-oxidation Activity of CoOx/TiO2-TaON. Differences in the cobalt oxide loadings, location and microstructure were reflected in the water oxidation activities of CoOx/TiO2-TaON photocatalysts, which was initially assessed using voltammetry under chopped visible light irradiation (1 sun, λ > 400 nm) (Fig. 5). In the dark, the water oxidation current densities were well below 0.1 mA cm−2 at potentials less positive than ca 1.5 V as expected for the CoOxcatalyzed OER.78,80 On the contrary, a well-defined photo-oxidative response was observed for all CoOx/TiO2-TaON materials at potentials as low as 0.35-0.40 V (Fig. 5). The only exception was CoOx-S/TiO2-TaON synthesized via photo-electrodeposition. Thus, the synergistic effect of wiring an efficient electrooxidation catalyst, CoOx, to the TaON-based photoanode has been confirmed, consistent with the previous reports on IrO2-,46 CoOx-51 and MnOx-72 TaON materials.


21

ACS Catalysis

1.2

A

1

dark, 3 min light, 3 min CoOx-S/TiO2-TaON TiO2-TaON TaON

0.8 0.6

Light on/off

0.4 0.2 0 -0.2 1.2

J / mA cm−²

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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B

dark,1 1min min Dark light,11min min Light 4x5 TiOuL 2-TaON TaON plain

C

dark,1 1min min Dark light,11min min Light 4x5 TiOuL 2-TaON plain TaON

1 0.8

CoOx-P/TiO2-TaON

0.6 0.4 0.2 0 -0.2 1.2 1 0.8

CoOx-B/TiO2-TaON

0.6 0.4 0.2 0 -0.2 0.3

0.5

0.7

0.9

1.1

1.3

1.5

E vs. RHE / V

Figure 5. Voltammograms (v = 0.025 V s−1) obtained under chopped visible light irradiation (1 sun, λ > 400 nm) for the CoOx-S/TiO2-TaON (blue), CoOx-P/TiO2-TaON (green), CoOx-B/TiO2-TaON (red), TiO2-TaON (black-dotted) and TaON (black-solid) films in contact with (A) 0.1 M Na2SO4 (pH = 6), (B) 0.1 M phosphate (pH = 7) (B) or (C) 0.1 M borate buffer (pH = 9.2). CoOx was electrodeposited in the dark (dashed curves) or under illumination (solid curves) for a set period of time (see also Table 2). As initially found in the optimization studies, photo-assisted modification of the TiO2-TaON films with CoOx-S deteriorated the water oxidation activity (Fig. 5A). This is rationalized in terms of suppression of light absorption by excessive amounts of cobalt oxide agglomerates created on the photoactive tantalum oxynitride surface, as demonstrated by SEM and ICP-MS analysis (see Fig. 3D and Table 2). Longer tdep might also lead to enhanced TaON degradation during deposition. However, it is noted again that a decrease in tdep and the amount of the photo-


22

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ACS Catalysis

electrodeposited CoOx-S species resulted in even poorer performance (Fig. S1). In contrast, a reduction in the amount of deposited CoOx, achieved through the use of more dilute buffered Co2+ precursor solutions (Table 2), maintained efficient light absorption and improved photooxidation performance as compared to the parent TiO2-TaON photoanode (Fig. 5B-C). For example, photocurrent densities of ca 0.82 and 0.38 mA cm−2 at 1.2 V (corresponds to the OER overpotential of ηOER = −0.03 V) were produced by the CoOx-P/TiO2-TaON films synthesized by dark and photo-assisted electrodeposition, respectively. In comparison, ca 0.13 mA cm−2 was measured for TiO2-TaON. In borate buffer (pH = 9.2), even higher improvements were achieved with the best performing CoOx-B/TiO2-TaON films prepared in the dark (0.97 mA cm−2 at 1.2 V) or under illumination (0.85 mA cm−2 at 1.2 V) (Fig. 5C). At less positive potentials, i.e. very negative ηOER, enhancements in the photocurrent densities of up to 20-fold (in the 0.35-0.70 V range) were possible by depositing CoOx onto the TiO2-TaON photoanodes (Fig. 5B-C). EIS analysis, performed at 1.15 V under visible light irradiation, confirmed the outcomes of the voltammetric measurements shown in Fig. 5. Indeed, deposition of CoOx onto the TiO2TaON films decreased the impedance, i.e. facilitated charge transfer, under photocatalytic conditions for all samples examined, with the exception of only CoOx-S/TiO2-TaON prepared via the photo-assisted approach (Fig. S7). More pronounced improvements in the charge transfer properties under irradiation were always found for samples modified with CoOx using the dark electrodeposition method. The major contribution to the changes in the EI spectra induced by the CoOx is a substantial decrease in the Faraday impedance, which reflects the rate of charge transfer associated with water oxidation at the electrode|electrolyte interface. This conclusion rests upon semi-quantitative simulations undertaken using the equivalent circuit employed previously to analyse the TiO2-TaON photoanodes.71 EIS experiment-simulation comparisons


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Page 24 of 44

are not reported for the multicomponent CoOx/TiO2-TaON photoanodes studied herein due to ambiguities in the parameterization of the relevant model. Analysis of the voltammetric and EIS data confirms that introduction of CoOx on the TiO2TaON films improves transfer of the photo-generated holes to the catalytically active sites on the photoanode surface and thus facilitates water oxidation. This is also expected to supress the oxidative self-deactivation of tantalum oxynitride. Overall, these results highlight the intricate dependence of the catalytic properties of the multicomponent CoOx/TiO2-TaON photoanodes on their structure and composition. A seemingly unexpected outcome of these experiments is the finding that all samples produced using the photo-assisted deposition method were outperformed by the CoOx/TiO2-TaON photocatalysts prepared in the dark (Fig. 5 and Fig. S7). As discussed above, a substantial and sometimes dominating portion of CoOx electrodeposited in the dark was immobilized on the TaON-free areas of the photoanodes, as best seen with the CoOx-S/TiO2TaON samples (Fig. 3). These coarse structures may be connected to the light-harvesting TaON particles in some ill-defined way and can hardly contribute to the efficient water photo-oxidation exemplified in Fig. 5. More realistically, the high activity of the CoOx/TiO2-TaON photoanodes prepared by dark electrodeposition may originate from the comparatively small loadings of CoOx on the TiO2-TaON surface (see micrographs (i) in Figs. 3D-F) whose formation was possible owing to very positive Edep used. Lower amounts of CoOx should improve light absorption by the underlying TaON, while good dispersion of the catalyst should enhance charge separation and the rate of water oxidation. To confirm this hypothesis, a CoOx-B/TiO2-TaON photoanode was prepared using a very short photo-electrodeposition of 1 s and its performance was only ca 1015% worse than that of an optimized CoOx-B/TiO2-TaON sample prepared with tdep = 1 min. This observation suggests that very low loadings of the CoOx co-catalyst on the TaON-based


24

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ACS Catalysis

photoanodes are sufficient to sustain a high water photo-oxidation activity. However, the above conclusion is derived from comparatively fast and non-steady state voltammetric and EIS measurements. Therefore, in the next section we investigate the long-term stability of the highlydispersed cobalt oxide and/or the underlying TiO2-TaON under catalytic conditions. Stability of the Photoanodes. The stability of the semiconductor-based photocatalytic materials is one of the most critical problems to tackle in the development of the solar water splitting and other technologies. As explained above, one well-established reason for the instability of the TaON-based photocatalysts is oxidative self-deactivation attributable to the short diffusion lengths of the photo-generated holes, and their reaction with N3− to produce N2 and Ta2O5.19,46,69 The post-necking treatment resolves this problem to some extent,71 but introduction of a co-catalyst capable of efficiently scavenging holes from the TaON surface should improve the stability further.46,51,72 However, as noted above, optimization of the amount and structure of the co-catalyst, CoOx in our case, is important. In what follows, we focus mainly on stability of the CoOx-P/TiO2-TaON and CoOx-B/TiO2-TaON films as they produced higher photo-oxidation activities than samples prepared and tested in sodium sulphate solutions. Chronoamperograms under chopped irradiation conditions at a range of applied potentials confirmed a significant enhancement in the photo-oxidation activity of TiO2-TaON upon modification with CoOx-P and CoOx-B (Fig. 6). The comparatively fast deterioration in the performance of the cobalt-free TiO2-TaON further emphasized the advantages of coupling the TaON-based photoanodes to efficient water oxidation catalysts such as CoOx. However, even during these short-term measurements the CoOx-modified samples deteriorated in performance. In particular, the photocurrent density measured for CoOx-P/TiO2-TaON prepared by dark electrodeposition decreased continuously, even at 0.5 V. The CoOx-P/TiO2-TaON film produced


25

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Page 26 of 44

using photo-assisted deposition was more stable, but still prone to degradation at potentials ≥1.2 V. The CoOx-B/TiO2-TaON photoanodes exhibited much better short-term stability, although some degradation of the samples produced by dark electrodeposition was observed at 1.5 V. The data in Fig. 6 were obtained by changing the potential from more to less positive values. Therefore, degradation suffered by each type of CoOx/TiO2-TaON under more oxidising conditions contributed to the photocatalytic performance achieved during subsequent measurements with smaller applied biases. Analysis of Fig. 6 from this angle also emphasises the advantage of the CoOx-B/TiO2-TaON synthesised under irradiation over the other photoanodes examined.

Figure 6. Chronoamperograms obtained for water photo-oxidation at defined potentials (vs. RHE) by CoOx-B/TiO2-TaON (red, blue), CoOx-P/TiO2-TaON (green, magenta) and TiO2-TaON (black) films in contact with 0.1 M borate (pH 9.2) (red, blue, black) or 0.1 M phosphate (pH = 7) (green, magenta) buffers under chopped visible light irradiation (1 sun, λ > 400 nm). CoOx was electrodeposited in the dark (blue, magenta) or under illumination (red, green) for 1 min (see also Table 2). To compare the longer-term performance of the CoOx/TiO2-TaON films, chronoamperometric stability tests, accelerated by using a harsh potential of 1.5 V, were undertaken. Inspection of the data in Fig. 7A highlights the poor stability of all CoOx/TiO2-TaON materials prepared in the dark, as is highlighted by decrease in photocurrent densities of 80-90% during initial period of test (10-15 min, Fig. 7A) for the CoOx-S/TiO2-TaON and CoOx-P/TiO2-TaON films. Similarly


26

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ACS Catalysis

prepared anodes, but under illumination, were more stable with only 20% degradation during the initial 15 min and much slower degradation thereafter. The stability of the CoOx-P/TiO2-TaON anodes was improved to an extent by increasing the amount of photo-electrodeposited CoOx to facilitate scavenging of the photo-generated holes, but there is a concomitant loss in initial activity (Fig. S8A). Of the photoanodes prepared by dark electrodeposition, the CoOx-B/TiO2TaON films showed the best stability. A 30% decrease in the photocurrent density was found during the 2 h test, and extension of these measurements to 12 h showed that the performance of these CoOx-B/TiO2-TaON photoanodes decreases by more than 60% (Fig. 7). In contrast, the CoOx-B/TiO2-TaON film prepared by using photo-assisted deposition exhibited much improved stability. These films suffered a deterioration of only 20% during the initial 2 h, but then maintained a quasi-steady state for at least 10 h (Fig. 7B). Increasing the CoOx-B loading did not provide further significant improvements in stability (Fig. S8B).


27

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A

1.4

light CoO -B/TiO -TaON Series1 x 2 dark Series4 light Series2 CoOx-P/TiO2-TaON dark Series7 light Series3 CoOx-S/TiO2-TaON dark Series6 TiO2-TaON Series5

1.5 V

J / mA cm−²

1.2 1.0 0.8 0.6 0.4 0.2 0.0 0

0.2

0.4

0.6

0.8

1

t/h

B

1.4

CoOx-B/TiO2-TaON dark, 1.5 V light, 1.5 V light, 1.2 V light, 0.9 V

J / mA cm−²

1.2 1.0 0.8 0.6 0.4 0.2 0.0 0

2

4

6

8

10

12

t/h CoOx-B/TiO2-TaON

C

1.4

light, 1.2 V light, 0.9 V light, 0.5 V

1.2

J / mA cm−²

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 28 of 44

1.0 0.8 0.6 0.4 0.2 0.0 0

2

4

6

8

10

12

14

t / min

Figure 7. Chronoamperograms obtained for water photo-oxidation under visible light irradiation (1 sun, λ > 400 nm) by (A) CoOx-B/TiO2-TaON in 0.1 M borate buffer (pH = 9.2) (red), CoOx-P/TiO2-TaON in 0.1 M phosphate buffer (pH = 7) (green), CoOx-S/TiO2-TaON in 0.1 M Na2SO4 (pH = 6) (blue), TiO2-TaON in 0.1 M borate buffer (pH 9.2) (black), and by (B) CoOx-B/TiO2-TaON 0.1 M borate buffer (pH = 9.2). (C) Water splitting in a two-electrode configuration using the CoOx-B/TiO2-TaON photoanode and Pt cathode in 0.1 M borate buffer (pH 9.2). CoOx was electrodeposited in the dark (dashed curves) or under illumination (solid curves) for 1 min (red, green, purple, brown, magenta) or 3 min (blue) (see also Table 2). Each


28

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chronoamperogram was obtained using a freshly prepared electrode not used previously for longterm catalytic testing. The initial drop in performance of the CoOx/TiO2-TaON films may originate from ineffective hole scavenging by the randomly deposited CoOx and/or changes to the CoOx co-catalyst layer during long-term measurements. Both factors can promote oxidative self-deactivation of TaON. For CoOx-P/TiO2-TaON, comparison of the SEM micrographs of the as-prepared films and those after water photo-oxidation (1 h, 1.5 V) reveals that a substantial portion of the CoOx-P deposits has been scavenged from the TiO2-TaON surface during the catalytic measurements (cf. Figs. 3E and 8A). This observation explains the continuous degradation in performance of these particular photoanodes (Fig. 7). Dissolution/reprecipitation of CoOx is a well-established phenomenon in water oxidation at very positive potentials even in alkaline solutions.78,81 Presumably, very low co-catalyst loadings in CoOx-P/TiO2-TaON lead to poor stability at positive potentials. Comparison of the SEM images for the as-synthesized and tested CoOx-B/TiO2-TaON suggests that these samples did not suffer significant irreversible cobalt oxide depletion even after 15 h at 1.5 V, but structural changes in the CoOx layer were found (cf. Figs. 3F and 8B). For the catalyst prepared by dark CoOx-B electrodeposition, the co-catalyst was almost completely removed from the TaON-free surface, and presumably redeposited on the TiO2-TaON particles (Fig. 8Bi). For these photoanodes, dissolution/redeposition of CoOx resulted in agglomeration and formation of dense layers of CoOx aggregates on the TaON surface, which was reflected in a continuous deterioration in performance. Thus, the CoOx electrodeposited in the dark is not stable at positive potentials. It suffers from irreversible dissolution at lower pH (CoOx-S and CoOx-P) or forms coarser deposits at higher pH via dissolution/redeposition (CoOx-B). In contrast, minor morphological changes were observed for the corresponding catalysts prepared


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by photo-assisted electrodeposition. These changes explain the initial drop in activity and subsequent quasi-stable operation of the photoanode (Fig. 7B).

Figure 8. SEM images obtained for (A) CoOx-P/TiO2-TaON after 1 h and (B) CoOx-B/TiO2TaON after 15 h of testing at 1.5 V (vs. RHE) under continuous visible light irradiation (1 sun, λ > 400 nm) in contact with (A) 0.1 M phosphate (pH = 7) and (B) 0.1 M borate buffer (pH 9.2). CoOx was electrodeposited over TiO2-TaON films in the dark (i) or under illumination (ii) for 1 min (see also Table 2). Photo-electrodeposition of very small amounts of CoOx-B onto TiO2-TaON (e.g., using tdep = 1 s) also resulted in unsatisfactory stability (data not shown). In contrast, CoOx/TiO2-TaON photoanodes prepared via photo-assisted electrodeposition (tdep = 1-3 min) exhibit good stability, especially when prepared and tested at higher pH (9.2). Notably, the CoOx-B/TiO2-TaON anodes


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always outperformed the best CoOx-S- and CoOx-P-modified samples prepared in sulphate and phosphate solutions but tested in the borate buffer at pH 9.2. Enhancements in the photooxidation activity of CoOx-S/TiO2-TaON and CoOx-P/TiO2-TaON achieved by using buffered solutions with pH 9.2 for water oxidation were not substantial (below 10%), similarly to our previous findings on TiO2-TaON71 and MnOx/TiO2-TaON72 photoanodes. This observation suggests that the performance of the studied multi-component photoanodes is mainly determined by the capacity of the CoOx co-catalyst to withdraw photogenerated holes from TaON, rather than by the intrinsic electrocatalytic properties of cobalt oxides, which are pH- and supporting anion-dependent.78,82 At potentials of 1.2 and 0.9 V, which correspond to water oxidation overpotentials of ca −0.03 and −0.33 V, a very stable operation was found for the CoOx-B/TiO2-TaON film prepared under illumination (Fig. 7B and S9). Very similar performance was achieved by this photoanode when it was used for water splitting under visible light irradiation in a two-electrode configuration, using a high-surface area Pt cathode. The products of water splitting in three- (1.2 V vs. RHE) and two-electrode (∆E = 1.2 V) configurations were analyzed by gas chromatography using larger electrodes and higher intensity visible light irradiation (6 suns) to achieve significant product yields. In both cases, H2 and O2 gases were evolved in a 2:1 stoichiometry with a close to 100% Faradaic efficiency during the time course of 2 h measurements (Fig. 9 and Table S1). Water splitting was still possible with the application of a 0.5 V potential between the CoOx-B/TiO2-TaON photoanode and Pt cathode in a two-electrode cell (J = 0.10 mA cm−2), but the efficiency of the process was negligible at 0.3 V (J < 0.010 mA cm−2). We also examined the effect of concentration of the supporting borate buffer solution on the performance of the CoOx-B/TiO2-TaON films prepared by photo-electrodeposition. Increasing


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the borate buffer (pH = 9.2) concentration from 0.1 to 0.5 M allowed slight (10-15%) improvement in the activity and more importantly provided better stability of the photoanode at the harsh potential of 1.5 V and under 3 sun visible light irradiation (Fig. S10). Apart from a decrease in the Ohmic losses provided by higher concentration of the electrolyte, this improvement can be attributed to facilitated proton transfer.51,83-86 100 Amount of Evolved Gas / µmol

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H2 H2 O2 O2 e−/2 e-/2 −/4 e e-/4

80

60

40

20

0 0

0.5

1

1.5

2

t/h

Figure 9. Evolution of H2 and O2 during water splitting by the CoOx-B/TiO2-TaON photoanode under continuous visible light irradiation (6 suns, λ > 400 nm) at 1.2 V vs. RHE and by a high surface area Pt cathode in contact with 0.1 M borate buffer (pH 9.2). Measurements were done in a three-electrode configuration. CoOx was photo-electrodeposited for 1 min (see also Table 2). Efficiency of the Photoanodes. Quantification of the capacity of a photo-electrocatalyst to convert irradiation into current can be done by calculating the incident photon-to-current conversion efficiency (IPCE)17,87 as a function of the wavelength of the light using the formula: IPCE(λ) = 1239.8

,

where 1239.8 / J C−1 nm is the product of the Planck’s constant and the speed of light divided by electron charge, J / A cm−2 is the photocurrent density, λ / nm is wavelength of the monochromatic light, and Pmono / W cm−2 is intensity of the monochromatic light. The IPCE spectra (Fig. 10) for the best performing CoOx-B/TiO2-TaON films, prepared by photo-assisted electrodeposition, were calculated using the water oxidation photocurrent


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densities derived from the chronoamperograms and comparatively slow voltammograms for the parent TiO2-TaON films. The sums of the photocurrent densities measured under monochromatic light irradiation (within the 390-530 nm range) were in agreement with those obtained under non-filtered 1 sun visible light irradiation within ca 10% error.

Figure 10. IPCE spectra obtained at defined potentials for CoOx-B/TiO2-TaON (solid curves) and TiO2-TaON (dashed curves) in contact with 0.1 M borate buffer (pH 9.2). The photocurrent densities were derived from the chronoamperograms (CoOx-B/TiO2-TaON) or voltammograms (v = 0.005 V s−1) (TiO2-TaON) obtained under chopped monochromatic light irradiation. Conditions for CoOx photo-electrodeposition (1 sun, λ > 400 nm; tdep = 1 min) are given in Table 2. Lines are visual guides. Comparison of the IPCE spectra for the CoOx-modified and cobalt-free TiO2-TaON films does not reveal significant differences in their shape, which confirms that the light-harvesting material is still TaON (Fig. 10). As expected, IPCE values increased with the potential applied and reached 23-27% for the CoOx-B/TiO2-TaON films within the potential range 1.1-1.3 V with λ = 410 nm, cf. 0.6-1.1% for the parent TiO2-TaON films. One fundamental property of a semiconductor photocatalyst, TaON in the particular case, that defines the IPCE is the efficiency of separation of the photo-generated charge carriers. As anticipated (see Scheme 1 and relevant discussions in Introduction), the IPCE data clearly show that this is substantially improved upon modification of TaON with TiO2 and CoOx. The most significant enhancement in IPCE (by ca


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45-fold) was achieved at potentials more negative than 0.9 V. These IPCE values can be compared to those reported for the photoanodes based on other semiconductors, e.g., CoOx/BiVO4 (25% at 1.2 V),50 NiOx/CoOx/BiVO4 (58% at 1.2 V),50 IrO2/Fe2O3 (38% at 1.23 V),47 CaFe2O4/TaON (30% at 1.23 V),70 and WO3 (80% at 1.2 V).67 A widely employed route to enhancing the current generated by a photo-electrocatalyst is to use concentrated irradiation with much higher light intensities (P). Previously, we have established that the photon-to-current efficiency of the TaON-based photoanodes decreases when the intensity of visible is higher than 1 sun.71 The CoOx/TiO2-TaON system exhibited a similar behaviour (Fig. 11). Thus, although the presence of CoOx enhances substantially the capacity of TiO2-TaON to utilize high intensity visible light irradiation, the performance at P > 1 sun still seems to be limited by the intrinsic properties of the TaON films. As noted above, unassisted water splitting could not be achieved using the CoOx/TiO2-TaON photoanodes and a platinum cathode (Fig. 7B), but we have demonstrated near 100% Faradaic efficiency of this HER/OER system when an additional driving force was applied. The photocatalytic efficiency compensated for this supplementary bias can be calculated as:21 AB-SHE =

.

,

where AB-SHE is applied-bias-compensated solar-to-hydrogen efficiency, J / A cm−2 is the photocurrent density derived from chronoamperometric measurements, εF is the Faradaic efficiency for the HER/OER, 1.23 V is the theoretical potential for water splitting, Eapp is the applied potential vs. RHE and P / W cm−2 is the light intensity. The highest AB-SHEs were achieved at 0.9 V for all photoanodes examined (Fig. S11). The CoOx-B/TiO2-TaON catalyst prepared using photo-assisted electrodeposition delivered an efficiency of 0.15% at 0.9 V and 0.05 % at very small potential of 0.5 V. This is stable for at


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least 24 hours and is at least two orders of magnitude higher than for the parent TiO2-TaON anode. The AB-SHEs for the other samples studied were all lower and also degraded over time. 2.0

A

J / mA cm−²

1.6

1.5 V 1.3 V 1.1 V 0.9 V 0.7 V 0.5 V

1.2

0.8

0.4

0.0 0

0.5

1

1.5

2

2.5

3

2.5

3

Light Intensity / sun 0.5

B

0.4

J / mA cm−2

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1.5 V 1.3 V 1.1 V 0.9 V 0.7 V 0.5 V

0.3

0.2

0.1

0 0

0.5

1

1.5

2

Light Intensity / sun

Figure 11. Effect of visible light irradiation (λ > 400 nm) intensity on the oxidative photocurrent densities, derived from voltammetric measurements (v = 0.025 V s−1) at the defined potentials (vs. RHE), for (A) CoOx-B/TiO2-TaON and (B) TiO2-TaON in contact with 0.1 M borate buffer (pH 9.2). CoOx was photo-electrodeposited (tdep = 1 min) (see Table 2). Lines are visual guides. Comparisons of the photocatalytic efficiency (IPCE, AB-SHE or AB-PCE) of CoOx/TiO2TaON to those achieved with other water photo-electrooxidation systems, especially the best ones using BiVO4 as a light-harvester (Table 1), suggest that despite the efforts made, the TaONbased photoanodes seem to be still lagging behind.11,19,20,46,48,51,57,58,68-72 Nevertheless, the


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performance of the CoOx-B/TiO2-TaON photoanode under visible light (IPCE 23-27%) is comparable to that of the CoOx-modified BiVO4 tested under full spectrum of the simulated 1 sun irradiation (IPCE 25%) (Table 1). Moreover, direct comparisons of the TaON-based and other photoanodes studied under either visible light or full AM 1.5G spectrum can be misleading. Indeed, the photocurrent density generated by TaON water oxidation anodes tested under full simulated sun spectrum can exceed that under light with λ > 400 nm by at least an order of magnitude at moderately positive applied potentials (below ca 0.6 V vs. RHE).52 The most impressive performances of the TaON-based photo-electrocatalysts achieved so far were obtained when using full solar spectrum,53,54 rather than its visible part only. However, extensive optimization studies would be needed to create a CoOx-modified TaON photoanode that is highly efficient under full AM1.5G spectrum due to partial absorption of UV-light by the CoOx cocatalyst.80 Further improvements in the dispersion of the CoOx co-catalyst and especially fine-tuning of the TaON micro/nanostructure52-55 could lead to enhancements in internal charge transport and concomitant increases in the photon-to-current efficiencies of these multicomponent photoanodes. We note that the CoOx-B/TiO2-TaON system reported herein outperforms all TaON-based photocatalysts for water oxidation tested under visible light irradiation reported to date, notwithstanding the fact that TaON films deposited via a simple screen-printing process, rather than more advanced nanostructured arrays, were used in our investigations (Table 1). An important advantage of CoOx-B/TiO2-TaON is its exceptional stability even under very harsh oxidising conditions. These achievements were made possible through the use of a photoassisted electrodeposition strategy to functionalize the TiO2-TaON photoanodes with CoOx.


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What is equally important, however, is the comprehensive optimization of both the titania postnecking and co-catalyst deposition protocols.

CONCLUSIONS The strategy employed herein to enhance the water photo-oxidation capacity of the TaONbased photoanodes via post-necking with TiO2 coatings followed by introduction of a layer of highly-dispersed CoOx particles has allowed creation of an efficient and very robust catalyst. A high degree of dispersion of the CoOx particles and their homogeneous coverage of the TiO2TaON surface was achieved by photo-assisted electrodeposition from slightly alkaline solutions (borate buffer with pH = 9.2), where the holes generated by TaON oxidize the Co2+ aqua ions. Among the TaON-based photoanodes, the CoOx/TiO2-TaON system reported herein exhibits the highest oxidative photocurrent densities at potentials below 1.23 V (vs. RHE) under visible light irradiation and stability that allows steady operation for at least 24 h. The comprehensive optimization study reported herein can also serve as a guide for selective and fine functionalization of semiconductor catalysts for water splitting and other processes. Further advances in the efficiency of the TaON-based photocatalysts for water splitting may become possible via considered engineering of the microstructure and/or doping of the tantalum oxynitride nanostructures to enhance the intrinsic efficiency of the material to convert photons into electricity. Subsequent procedures for modification of TaON with protective coatings and efficient water oxidation co-catalysts seem to be well-established now. Hopefully, this strategy will eventually allow unassisted water splitting to be accomplished without the need to apply an additional bias or couple the photo-electrochemical cell to a photovoltaic device.


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ASSOCIATED CONTENT Supporting Information Optimization data for CoOx electrodeposition under different conditions and their effect on photoactivity measured voltammetrically and chronoamperometrically, transients for the electrodeposition of CoOx, SEM images (before and/or after stability test), EIS data, chronoamperommetric data under various conditions, comparison of Faradaic efficiency and amount of H2 and O2 gases evolved from water splitting in three- and two-electrode configuration, and AB-SHE data. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Authors *E-mail for L.S.: [email protected] *E-mail for R.A.: [email protected] *E-mail for A.N.S.: [email protected]

ACKNOWLEDGEMENTS The authors acknowledge the Australian Research Council for providing financial support for this study through ARC Centre of Excellence for Electromaterials Science (Grant No. CE140100012), the School of Chemistry (Monash University) for providing research facilities and scholarships to carry out this work, the Monash Centre for Electron Microscopy (Monash University) for providing microscopy facilities, and Dr. Massimo Raveggi (Monash University) for assisting with the ICP-MS experiments.


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Highly Dispersed Cobalt Oxide on TaON as Efficient Photoanodes for

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