Role and Function of Ruthenium Species as Promoters with TaON

ARTICLE pubs.acs.org/JPCC

Role and Function of Ruthenium Species as Promoters with TaON-Based Photocatalysts for Oxygen Evolution in Two-Step Water Splitting under Visible Light Kazuhiko Maeda,†,§ Ryu Abe,‡,§ and Kazunari Domen*,† †

Department of Chemical System Engineering, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan Catalysis Research Center, Hokkaido University, North 21, West 10, Sapporo 001-0021, Japan § Precursory Research for Embryonic Science and Technology (PRESTO), Japan Science and Technology Agency (JST), 4-1-8 Honcho Kawaguchi, Saitama 332-0012, Japan ‡

bS Supporting Information ABSTRACT: The structure of nanoparticulate ruthenium (Ru) species dispersed on TaON as cocatalysts was characterized by X-ray absorption spectroscopy and scanning electron microscopy. TaON specimens loaded with Ru species were prepared by an impregnation method using (NH4)3RuCl6 as a precursor followed by calcination in air and tested as photocatalysts for O2 evolution from an aqueous NaIO3 solution under visible light (λ > 420 nm). While unloaded TaON showed little photocatalytic activity, Ru loading resulted in observable O2 evolution, and impregnation of TaON with 0.5 wt % Ru followed by calcination at 623 K for 1 h provided the highest photocatalytic activity. Structural analyses and (photo)electrochemical measurements revealed that the activity of this photocatalyst is strongly dependent on the generation of optimally dispersed RuO2 nanoparticles, which simultaneously promote both the reduction of IO3- and oxidation of water. Overall water splitting through two-step photoexcitation (Z-scheme) was also achieved using the optimized RuO2/TaON photocatalyst in combination with Pt-loaded ZrO2/TaON as a H2 evolution photocatalyst from an aqueous NaI solution. Experimental results suggested that the efficiency of this system is limited by the competitive oxidation of I- with the valence band holes in RuO2/TaON and the photoreduction of O2 that can occur on the RuO2/TaON surface.

1. INTRODUCTION To address concerns regarding the depletion of fossil fuels and related environmental issues, there is an urgent need for practical methods of producing energy that are clean, renewable, and cheap. Catalytic water splitting using semiconductor particles has attracted attention as a potential means of producing hydrogen from water with solar energy.1 Significant efforts have been made to develop a photocatalyst that is capable of absorbing visible light, the main component of the solar spectrum. However, a satisfactory system has not been established so far. Water splitting through two-step photoexcitation using two different semiconductor photocatalysts has been studied as one potential scheme of such solar water splitting and is called the “Zscheme”.2-16 A schematic energy diagram of Z-scheme water splitting is shown in Scheme 1. In a H2 evolution system, the forward reactions that should occur on the photocatalyst surface are the reduction of protons by conduction band electrons and the oxidation of an electron donor (D) by valence band holes to yield the corresponding electron acceptor (A), as follows 2Hþ þ2e- f H2 ðphotoreduction of Hþ to H2 Þ

ð1Þ

Dþnhþ f A ðphotooxidation of D to AÞ

ð2Þ

r 2011 American Chemical Society

On the other hand, the forward reactions on an O2 evolution photocatalyst are as follows Aþne- f D ðphotoreduction of A to DÞ

ð3Þ

4OH-þ4hþ f O2 þ 2H2 O ðphotooxidation of H2 O to O2 Þ ð4Þ where the electron acceptor generated by the paired H2 evolution photocatalyst is converted to its reduced form (D), and water oxidation occurs with the valence band holes. Thus, a cycle of redox pairs (D and A) occurs, and the water splitting reaction is accomplished. A number of semiconductor materials have been applied to this system, aimed at extending the available wavelength for the water splitting reaction to utilize more visible photons.6-13 In addition to efficient utilization of visible light, it is important to control the selectivity of chemical reactions that occur during Z-scheme water splitting. As shown in Scheme 1, the reduction of Received: October 19, 2010 Revised: January 4, 2011 Published: February 3, 2011 3057

dx.doi.org/10.1021/jp110025x | J. Phys. Chem. C 2011, 115, 3057–3064

The Journal of Physical Chemistry C

ARTICLE

Scheme 1. Schematic Energy Diagrams of Photocatalytic Water Splitting for a Two-Step Photoexcitation Systema

a

C.B., conduction band; V.B., valence band; Eg, band gap; D and A, electron donating and accepting species, respectively.

A and oxidation of D are, respectively, more likely to occur on H2 and O2 evolution photocatalysts, compared to water reduction and oxidation. For this reason, the number of Z-scheme water splitting systems showing a reasonable level of activity under visible light is limited, even though a large number of visible-lightresponsive photocatalysts have been developed.1d Thus, controlling the selectivity for the forward reactions on each photocatalyst is important from the standpoint of establishing a highly efficient Z-scheme system. It has been reported that the intrinsic properties of a photocatalyst allow for selective water reduction and oxidation. Taking O2 evolution photocatalysts, for example, rutile TiO2 and WO3 were presented as such “selective” photocatalysts for water oxidation.8 Owing to their unique surface properties, water oxidation occurs on these photocatalysts even in the presence of electron donors such as Fe2þ and I-, which are thermodynamically more susceptible to oxidation than water. However, one may achieve such selective catalysis by the introduction of a proper nanoparticulate cocatalyst on the photocatalyst surface.17,18 We reported that RuO2-loaded TaON is an effective photocatalyst for O2 evolution to achieve Z-scheme overall water splitting by combining with Pt-loaded TaON as a H2 evolution photocatalyst using an IO3-/I- shuttle redox mediator.11 This is the first example of a two-step water splitting system that is comprised solely of a nonoxide-type compound. TaON alone is not applicable to an O2 evolution system in the presence of an IO3-/I- pair,11 most likely because the competitive oxidation of I- occurs efficiently.19 On the other hand, loading RuO2 nanoparticles on TaON enables water oxidation even in the presence of I-. We tentatively attributed this phenomenon to the functionality of RuO2 as an efficient water oxidation catalyst, which helps to promote O2 evolution while preventing the oxidation of I-.11 However, the role of RuO2 on TaON in water oxidation in Z-scheme water splitting is not altogether clear. It is well-known that the photocatalytic activity for overall water splitting depends strongly on the structure of the loaded cocatalyst.1b It is therefore important to investigate the structural characteristics and functions of the cocatalyst to develop a highly active photocatalytic system. Very recently, we reported an efficient two-step water splitting system using a Pt-loaded ZrO2/TaON composite (H2 evolution photocatalyst) and Pt-loaded WO3 (O2 evolution photocatalyst) in the presence of an IO 3 -/I - redox pair. 13b This system

outperforms a similar system that uses Pt/TaON as a H2 evolution photocatalyst, showing an optimized apparent quantum yield (AQY) as high as ca. 6.3% (at 420.5 nm), the highest among the visible-light-driven nonsacrificial water splitting systems developed so far. Thus, it is interesting to use this ZrO2/TaON photocatalyst for the construction of a Z-scheme water splitting system consisting of only TaON-based photocatalysts. In this study, first, the structural characteristics and function of Ru species on the TaON surface are investigated by X-ray absorption spectroscopy (XAFS), scanning electron microscopy (SEM), and (photo)electrochemical measurements in an attempt to determine the relationship between the catalyst’s structure and its photocatalytic activity for O2 evolution from a NaIO3 solution under visible light (λ > 420 nm). Second, the applicability of the ZrO2/TaON photocatalyst to H2 and O2 evolution half reactions using NaI and NaIO3 as a reversible electron donor and acceptor, respectively, is investigated to construct a more efficient Z-scheme water-splitting system consisting of only TaON-based photocatalysts. The photocatalytic behavior is discussed on the basis of a series of experimental results.

2. EXPERIMENTAL SECTION 2.1. Materials and Reagents. TaON and ZrO2/TaON (Zr/ Ta = 0.1 by mole) photocatalysts were prepared according to a method reported previously.13a The physicochemical properties of TaON and ZrO2/TaON have been reported in our previous papers.13 (NH4)2RuCl6 (Aldrich), RuCl3 3 nH2O (Kanto Chemicals, 99.9%), and H2PtCl6 3 2H2O (Kanto Chemicals, 97% Pt) were used as cocatalyst precursors for TaON and ZrO2/TaON. NaI (Kanto Chemicals, reagent grade) and NaIO3 (Kanto Chemicals, reagent grade) were employed as redox reagents. All chemicals were used without further purification. 2.2. Modification with Cocatalysts. Ru species as cocatalysts were loaded onto TaON and ZrO2/TaON by an impregnation method using (NH4)2RuCl6 or RuCl3 3 nH2O as a precursor to apply them to an O2 evolution system. The impregnated samples were heat-treated in air for 1 h at 523-723 K. The amount of Ru loading ranged from 0 to 3.0 wt %. Pt(1.0 wt %)loaded ZrO2/TaON as a photocatalyst for H2 evolution in twostep water splitting was prepared according to our previous report.13b 3058

dx.doi.org/10.1021/jp110025x |J. Phys. Chem. C 2011, 115, 3057–3064


ARTICLE

Figure 1. Time courses of O2 evolution over (NH4)2RuCl6-impregnated TaON calcined at various temperatures at a fixed amount of Ru loading (0.5 wt %). Reaction conditions: catalyst, 50 mg; aqueous NaIO3 solution, 100 mL (1.0 mM); light source, xenon lamp (300 W) fitted with a cold mirror (CM-1) and a cutoff filter (L42); reaction vessel, Pyrex top-irradiation type; irradiation wavelength, 420 < λ < 800 nm.

Figure 2. Time courses of O2 evolution over (NH4)2RuCl6-impregnated TaON calcined at 623 K with various Ru loading amounts. Reaction conditions: catalyst, 50 mg; aqueous NaIO3 solution, 100 mL (1.0 mM); light source, xenon lamp (300 W) fitted with a cold mirror (CM-1) and a cutoff filter (L42); reaction vessel, Pyrex topirradiation type; irradiation wavelength, 420 < λ < 800 nm.

2.3. Characterization of Catalysts. The prepared samples were studied by X-ray powder diffraction (XRD, Geiger-flex RAD-B, Rigaku; Cu KR), scanning electron microscopy (SEM; S-4700, Hitachi), and X-ray absorption spectroscopy (XAFS). XAFS measurements were carried out in the NW10A beamline of the Photon Factory (High Energy Accelerator Research Organization, Tsukuba, Japan) using a ring energy of 2.5 GeV and stored current from 60-40 mA (Proposal No. 2008G150) for the measurement of Ru-K edge spectra. The X-ray absorption spectra were measured in transmission or fluorescence mode at room temperature with a Si(111) two-crystal monochromator. Data reduction was performed using the REX2000 program (Rigaku Corporation). The Fourier transforms of k3-weighted EXAFS spectra were typically in the 3.0-12.0 Å region. 2.4. (Photo)electrochemical Measurements. Porous TaON electrodes were prepared by pasting a viscous slurry onto conducting glass according to a previously described method.13b,20 A mixture of 50 mg of as-prepared RuO2/TaON (or TaON calcined in air at 623 K for 1 h without RuO2 loading) powder, 10 μL of acetylacetone (Kanto Chemicals), 10 μL of TritonX (Aldrich, USA), and 250 μL of distilled water was ground in an agate mortar to prepare the viscous slurry. The slurry was then pasted onto fluorine-doped tin-oxide (FTO) glass slides (12 Ω 3 sq-1, transparency 80%, thickness 1 mm; Asahi Glass, Japan) to prepare a 1 3 cm2 electrode, and the sample was calcined in air at 623 K for 1 h. Measurements were performed using a conventional Pyrex electrochemical cell with a platinum wire as a counter electrode and an Ag/AgCl reference electrode under potentiostat control (HSV-100, Hokuto Denko, Japan). Current-voltage curves were measured in an aqueous sodium sulfate solution (Na2SO4, 0.1 M, 100 mL) as a supporting electrolyte. The electrolyte solution was purged with argon prior to the measurements and was maintained at room temperature by a flow of cooling water during the measurements. A 300 W xenon lamp fitted with a cutoff filter was used as a visible light irradiation source. The effective surface area of the electrodes was 1 2.5 cm2. 2.5. Photocatalytic Reactions. Reactions were carried out in a Pyrex top-irradiation reaction vessel connected to a glass closed gas circulation system. Half reactions of H2 and O2 evolution were conducted individually from an aqueous NaI and NaIO3 solution (1.0 mM, 100 mL) suspended with TaON (or ZrO2/ TaON) modified with a cocatalyst (50 mg). For two-step water

splitting, H2- and O2 -evolution photocatalysts (50 mg each) were both suspended in aqueous NaI solutions (0.2 mM, 100 mL). The reactant solutions were evacuated several times to completely remove any air prior to irradiation under a 300 W xenon lamp. The irradiation wavelength was controlled by a combination of a cold mirror (CM-1), a cutoff filter, and a water filter (420 < λ < 800 nm). The reactant solution was maintained at room temperature by a flow of cooling water during the reaction. The evolved gases were analyzed by gas chromatography.

3. RESULTS AND DISCUSSION 3.1. Effect of Calcination Temperature of (NH4)2RuCl6Impregnated TaON and Loading Amount of Ru on Photocatalytic Activity. In general, the photocatalytic activity of a

given material loaded with a cocatalyst is strongly dependent on the physicochemical properties of the cocatalyst because these properties affect the processes of charge injection from the photocatalyst and redox reactions on the cocatalyst. Physicochemical properties are strongly affected by the preparation conditions. Thus, the effects of the calcination temperature for TaON after impregnation of (NH4)2RuCl6 on the activity for O2 evolution from an aqueous NaIO3 solution were investigated. Figure 1 shows the time courses of O2 evolution on Ruimpregnated samples calcined at different temperatures. All tested samples produced a measurable amount of O2 under visible light, regardless of calcination temperature. However, the activity increased with calcination temperature to a maximum at 623 K, beyond which it began to drop significantly. The dependence of the activity of (NH4)2RuCl6-impregnated TaON for O2 evolution on the loading amount of Ru at a calcination temperature of 623 K is shown in Figure 2. No O2 evolution was observed in the absence of Ru loading. The rate of O2 evolution increased markedly with Ru content to a maximum at 0.5 wt % and then decreased upon further loading. It was thus found that the O2 evolution activity of (NH4)2RuCl6-impregnated TaON is dependent on both the calcination temperature and the loading amount. A negligible amount of N2 evolution (ca. 3-4 μmol) was detected in the initial stage of the reaction (first ∼2 h) in some cases. This is most likely due to the oxidation of N3- species near the TaON surface to N2.21 However, the production of N2 was completely suppressed as the reaction proceeded. It is also noted that the rate of O2 3059



ARTICLE

Figure 3. Ru-K edge XANES spectra for (NH4)2RuCl6-impregnated TaON calcined at various temperatures for a given level of Ru loading (0.5 wt %).

Figure 4. Fourier transforms of k3-weighted Ru-K edge EXAFS spectra for (NH4)2RuCl6-impregnated TaON calcined at various temperatures for a fixed amount of Ru loading (0.5 wt %).

evolution decreased with reaction time in all cases. This is attributed to the accumulation of I- in the reactant solution, causing the following reverse reaction, which can compete with water oxidation (eq 4)

RuO2-modified Ti electrodes prepared from RuCl3, followed by calcination in air.22 The dependence of the size of RuO2 nanoparticles on the calcination temperature that we observed in this study is essentially identical to that reported by Takasu et al. The morphology of the samples prepared with different Ru loadings was observed by SEM. Figure 5c shows an SEM image of the sample calcined at 623 K with 3.0 wt % Ru content. It was observed that the 10-30 nm RuO2 particles in this sample were more densely deposited on the TaON surface than in the case of the 0.5 wt % sample (Figure 5a). 3.3. Relationship between the Structure and O2 Evolution Activity. As shown in Figure 1, the photocatalytic activity of (NH4)2RuCl6-impregnated TaON was found to depend on the calcination temperature. XAFS measurements indicate that part of the loaded Ru species remains in the precursor form after calcination below 573 K (Figure 3). Calcination at 623 K results in almost complete decomposition of the precursor and the formation of RuO2 nanoparticles with an average diameter of less than 30 nm (Figure 5a). However, upon calcination at higher temperatures, the nanoparticles undergo some aggregation (Figure 5b). Thus, the abrupt increase in activity from 573 to 623 K is thought to be due primarily to the formation of active RuO2 nanoparticles, while the decrease in activity upon calcination above 623 K is attributable to the aggregation of nanoparticles. The partial oxidation of the TaON surface upon calcination at higher temperatures is also considered to be the secondary reason for the drop in activity.20 These ideas are wellconsistent with the result of XRD measurement, which revealed that calcination at elevated temperatures contributed to the generation of the RuO2 and Ta2O5 phase (Figure S1, Supporting Information). The RuO2 loading amount was also found to be an important factor contributing to the enhancement of photocatalytic activity. In the range between 0 and 0.5 wt % Ru content, the activity increased significantly, most likely due to the increased density of active sites. However, excess Ru loading resulted in a decrease in activity, presumably due to the excess coverage of TaON with RuO2, as observed in SEM images (Figure 5c), which can hinder light absorption by TaON. It can thus be concluded that the formation of RuO2 nanoparticles with a high dispersion on the TaON surface is the most important factor in the enhancement of photocatalytic activity for O2 evolution from an aqueous NaIO3 solution under visible light. According to the study by Takasu et al., RuCl3 transforms into RuO2 at 573 K, and calcination at higher temperatures leads to aggregation of RuO2 particles, which is evident from the marked

I-þ 3H2 O þ 6hþ f IO3-þ 6Hþ

ð5Þ

This is also supported by the fact that an intentional addition of NaI into the reactant solution results in a marked decrease in the O2 evolution activity.11 3.2. XAFS Measurement and SEM Observation. The valence state of Ru species in samples calcined at different temperatures was investigated by XAFS measurements. Figure 3 shows the Ru-K edge XANES spectra for samples with 0.5 wt % Ru after calcination at varying temperatures. Data for reference samples of (NH4)2RuCl6 and RuO2 are shown for comparison. The Ru-K edge spectra for samples calcined below 573 K clearly differ from those of (NH4)2RuCl6 and RuO2. The spectra exhibit a change at 623 K and start resembling the spectra for the RuO2 reference, indicating that the impregnated Ru species on the TaON surface decomposes and converts to ruthenium(IV) oxide at elevated calcination temperatures. Figure 4 shows the Fourier transforms (FT) of the k3weighted Ru-K edge EXAFS spectra for the same samples. The FT of EXAFS peaks could be assigned according to a previous report by Takasu et al.22 The samples calcined below 573 K exhibit a peak assigned to the Ru-Cl shell at ca. 2 Å, in addition to a peak appearing at ca. 1.5 Å that is assignable to the first Ru-O shell configuration. This indicates that the structure of (NH4)2RuCl6 is partially preserved upon calcination below 573 K. However, the characteristic peak weakens with increasing calcination temperature and is almost undetectable at 623 K, in good agreement with the change in the Ru-K edge XANES spectra (Figure 3). In addition, the peaks due to higher shell configuration gradually become stronger with increasing calcination temperature, suggesting that the particle size of RuO2 increased upon calcination. This is supported by SEM observations. As shown in Figure 5a, the sample calcined at 623 K with 0.5 wt % Ru contains featureless RuO2 nanoparticles with a size of 10-30 nm deposited on larger TaON particles. At the highest calcination temperature of 723 K (Figure 5b), however, nanoparticles larger than 50 nm formed, with some aggregation, in addition to smaller nanoparticles (10-30 nm). It was thus found that calcination at higher temperatures leads to aggregation of RuO2 nanoparticles on TaON. Takasu et al. conducted XAFS measurements on

3060



ARTICLE

Figure 6. Current-voltage curves for porous RuO2/TaON and TaON electrodes under intermittent visible irradiation (420 < λ < 800 nm) in a 0.1 M Na2SO4 solution. Scan rate: 20 mV 3 s-1.

Figure 5. SEM image of (NH4)2RuCl6-impregnated TaON calcined at (a) 623 and (b) 723 K for the same Ru loading (0.5 wt %). Image (c) shows an analogue calcined at 623 K with 3.0 wt % Ru loading.

growth of the second shell peak in the FT of EXAFS spectra.22 We also prepared RuO2-loaded TaON samples from RuCl3 by calcination at 573 K, following the method given in this study, and tested their performance. The sample prepared using RuCl3 yielded an activity (ca. 12 μmol h-1) comparable to the optimized activity obtained using (NH4)2RuCl6 with calcination at 623 K. Thus, both (NH4)2RuCl6 and RuCl3 are useful for preparing RuO2-loaded TaON since the calcination temperature

is high enough to convert the precursors into RuO2 and low enough to suppress aggregation of RuO2. 3.4. Role of RuO2 Cocatalyst on TaON in O2 Evolution. It was found that RuO2 on TaON has a significant impact on O2 evolution from an aqueous NaIO3 solution under visible light. Although it has been suggested that RuO2 on a photocatalyst can work as both a water reduction and oxidation site during the water-splitting reaction,17,23,24 the role of the RuO2 cocatalyst in Z-scheme water splitting has yet to be investigated. Because TaON functions as an n-type semiconductor, the photooxidation behavior can be monitored by constructing a photoelectrochemical cell with the material.19,20,25 Figure 6 shows current-voltage curves for TaON-based electrodes under intermittent irradiation with visible light (λ > 420 nm). The TaON electrode generated an anodic photocurrent associated with water oxidation upon irradiation with visible light, consistent with previous reports.19,20,25 The introduction of RuO2 cocatalysts onto the TaON surface resulted in a marked increase in photocurrent, indicating that water oxidation on the loaded RuO2 nanoparticles occurs more efficiently than on the TaON surface. The same trend has been reported for TaON electrodes modified with colloidal IrO2,25 which is wellknown as a water oxidation catalyst.26 The promotional effect of RuO2 on water oxidation is also supported by the fact that RuO2/ TaON is able to produce O2 from an aqueous NaIO3 solution even in the presence of NaI, which is thermodynamically more susceptible to oxidation than water.11 The behavior of the RuO2/TaON electrode with respect to the reduction of IO3- was investigated by monitoring the cathodic response in an aqueous NaIO3 solution under dark conditions. As shown in Figure 7, an appreciable cathodic current was observed when the RuO2/TaON electrode was employed and increased with a negative potential. However, the electrode did not give any cathodic current in the absence of IO3- ions in the potential range examined here. It was also confirmed that there was no cathodic current for the unmodified TaON electrode in an aqueous NaIO3 solution even at a relatively negative potential (-0.3 V vs Ag/AgCl). These results suggest that RuO2 nanoparticles on TaON are capable of catalyzing the reduction of IO3- as well as promoting water oxidation. As reported in the previous paper,13a Pt/ZrO2/TaON also catalyzes the IO3- reduction, which is a backward reaction of two-step water splitting using an IO3-/I- redox pair occurring over an H2 evolution photocatalyst. 3.5. Two-Step Water Splitting Using Pt/ZrO2/TaON and RuO2/TaON with an IO3-/I- Shuttle Redox Mediator. We 3061



ARTICLE

Table 1. Rates of H2 and O2 Evolution Half Reactions for Z-Scheme Water Splittinga activityb/μmol h-1 reactant entry

catalyst

solution

H2

O2 0

1

1.0 wt % Pt/ZrO2/TaON

NaI

2.9

2

1.0 wt % Pt/TaON

NaI

1.0

0

3

0.5 wt % RuO2/ZrO2/TaON

NaIO3

0

8.2

4

0.5 wt % RuO2/TaON

NaIO3

0

13.0

a

Figure 7. Current-voltage curves for porous RuO2/TaON electrodes under dark conditions in an aqueous solution containing 0.1 M Na2SO4 and 10 mM NaIO3. Scan rate: 20 mV 3 s-1.

have previously reported that ZrO2/TaON exhibits higher photocatalytic performance for H2 evolution from aqueous methanol solutions than TaON, while the activity for O2 evolution from aqueous AgNO3 solution is lower.13a Similarly, photocatalytic activities of ZrO2/TaON and TaON with some modifications for individual H2 and O2 evolution were examined using NaI and NaIO3 as an electron donor and acceptor, respectively. As tabulated in Table 1, the rate of H2 evolution from the NaI solution using Pt/ZrO2/TaON (entry 1) was higher than that achieved by Pt/TaON (entry 2). On the other hand, the O2 evolution activity of RuO2/ZrO2/TaON from the NaIO3 solution (entry 3) was lower than that of RuO2/TaON (entry 4). These results strongly suggest that the most effective combination for Z-scheme water splitting using TaON-based photocatalysts is Pt/ZrO2/TaON for a H2 evolution system and RuO2/ TaON for an O2 evolution system. By applying the best combination (ZrO2/TaON for H2 evolution and TaON for O2 evolution), as revealed by the results of half reactions (Table 1), two-step water splitting was attempted under visible light. Figure 8 shows the time course of H2 and O2 evolution over a mixture of Pt/ZrO2/TaON and RuO2/TaON from an aqueous NaI solution under visible light (420 < λ < 800 nm). Both H2 and O2 were evolved almost stoichiometrically, but the rates of H2 and O2 evolution gradually decreased. Nevertheless, the total production of H2 and O2 after 5 h was 55 μmol, which is substantially larger than the total amount of NaI employed as a shuttle redox mediator (20 μmol). No simultaneous evolution of H2 and O2 was observed when one component of the system (Pt/ZrO2/TaON, RuO2/TaON, NaI) was absent. In addition, no gas evolution was observed without irradiation. These results clearly demonstrate that overall water splitting proceeds photocatalytically according to the Z-scheme principle. Compared to an analogous system using Pt/ZrO2/TaON and Pt/WO3 with an IO3-/I- redox mediator,13b however, the rates of H2 and O2 evolution achieved in the present system are about 3-5 times lower. This is at least in part attributable to the competitive oxidation of I- by holes photogenerated in RuO2/ TaON, which hinders the water oxidation process, resulting in lower overall efficiency. It has been revealed that the activity of O2 evolution from an aqueous NaIO3 solution over RuO2/ TaON drops by 80% in the presence of a small amount of NaI (1.0 mM).11 On the other hand, nearly 70% of the original activity of Pt/WO3 for O2 evolution is kept even when a relatively large amount of NaI (10 mM) is added.8 These results indicate

Reaction conditions: aqueous solution, 100 mL (1.0 mM); light source, xenon lamp (300 W) fitted with a cold mirror (CM-1); reaction vessel, Pyrex top-irradiation type; irradiation wavelength, 420 < λ < 800 nm. b Initial rate of gas evolution.

Figure 8. Time courses of H2 and O2 evolution over a mixture of Pt/ ZrO2/TaON and RuO2/TaON. Reaction conditions: catalyst, 50 mg each; aqueous NaI solution, 100 mL (0.2 mM); light source, xenon lamp (300 W) fitted with a cold mirror (CM-1) and a cutoff filter (L42); reaction vessel, Pyrex top-irradiation type; irradiation wavelength, 420 < λ < 800 nm.

that competitive oxidation of I- occurs more efficiently on RuO2/TaON than on Pt/WO3. This is also supported by the fact that the optimal NaI concentration for Z-scheme water splitting using RuO2/TaON was lower (0.2 mM) than that using Pt/WO3 (1.0 mM).13b The O2 evolution rate in a Z-scheme system consisting of Pt/ZrO2/TaON and RuO2/TaON actually decreased as the NaI concentration increased from 0.2 to 1.0 mM, signaling that the competitive oxidation of I- was more pronounced. Another important aspect of the present RuO2/TaON photocatalyst is that the reduction of O2, a backward reaction of water splitting,17a can occur on both RuO2 and TaON. As shown in Figure 9, RuO2/TaON and TaON electrodes both generated a cathodic photocurrent assignable to O2 reduction, the former giving a lower overvoltage for the reaction. Ota et al. have also reported that TaOxNy is an active electrocatalyst for the reduction of O2.27 This suggests that, during Z-scheme water splitting using RuO2/TaON as an O2 evolution photocatalyst, photoreduction of O2 occurs on RuO2/TaON, resulting in a decrease in the water-splitting rate. As demonstrated, the RuO2 cocatalyst is essential for activating TaON as an O2 evolution photocatalyst in a two-step water splitting system with Pt/ZrO2/TaON and an IO3-/I- redox mediator because it serves the functions of hosting reduction sites for IO3- and promoting water oxidation. However, the intrinsic activity of RuO2/TaON for the reverse reaction, i.e., the reduction of O2, and the competitive oxidation 3062



ARTICLE

’ ASSOCIATED CONTENT

bS

Supporting Information. Results of XRD measurement for (NH4)2RuCl6-impregnated TaON calcined at different temperatures (Figure S1). This material is available free of charge via the Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*Tel.: þ81-3-5841-1148. Fax: þ81-3-5841-8838. E-mail: domen@ chemsys.t.u-tokyo.ac.jp. Figure 9. Current-voltage curves for porous RuO2/TaON and TaON electrodes under dark conditions in an aqueous solution containing 0.1 M Na2SO4 with Ar or O2 bubbling. Scan rate: 5 mV 3 s-1.

of I- contribute to lowering the performance of RuO2/TaON as an O2 evolution photocatalyst using an IO3-/I- redox mediator. It is also noted that water formation from H2 and O2 on Pt/ ZrO2/TaON and the reduction of O2 on Pt, as elucidated in our previous studies,13b,28 contribute to the activity decrease during Z-scheme water splitting. If the occurrence of the reverse reactions of O2 photoreduction and water formation is the primary cause of the deactivation observed in the reaction, the activity should be recoverable by evacuation of the reaction system. However, as presented in the preliminary communication, the activity did not recover in this way,11 indicating that irreversible deactivation of the catalyst occurs during the water-splitting reaction. A trace amount of N2 evolution was detected during Z-scheme water splitting using Pt/ ZrO2/TaON and RuO2/TaON, as described in the section on O2 evolution over RuO2/TaON from an aqueous NaIO3 solution. On the other hand, such N2 evolution has not been observed in a similar system consisting of Pt/ZrO2/TaON and Pt/WO3.13 It is therefore considered that part of the photogenerated holes in RuO2/TaON is consumed by self-decomposition of the oxynitride (eq 6 below),1b which would lead to irreversible deactivation, even though the loaded RuO2 nanoparticles help to catalyze water oxidation. 2N3-þ6hþ f N2

ð6Þ

Replacement of RuO2 with other catalytic materials that can effectively suppress the self-decomposition of the oxynitride (in other words, promote water oxidation) and do not exhibit activity for the reverse reactions is thus essential for improving the water-splitting efficiency but still remains a challenge.

4. CONCLUSION Ru species on TaON were investigated as cocatalysts to promote O2 evolution in a two-step water-splitting system with Pt/ZrO2/TaON as a H2 evolution photocatalyst and an IO3-/Iredox mediator. Using (NH4)2RuCl6 as the precursor, RuO2 nanoparticles with an optimal distribution, as determined by the preparation conditions, were demonstrated to act as a bifunctional catalyst for TaON to promote the reduction of IO3- and the oxidation of water. In a two-step water splitting system consisting of Pt/ZrO2/TaON (H2 evolution photocatalyst) and RuO2/TaON (O2 evolution photocatalyst), several kinds of reverse reactions seemed to occur, thereby lowering the overall efficiency of the system.

’ ACKNOWLEDGMENT This work was supported by the Research and Development in a New Interdisciplinary Field Based on Nanotechnology and Materials Science program of the Ministry of Education, Culture, Sports, Science and Technology (MEXT) of Japan, and The KAITEKI Institute, Inc. Acknowledgement is also extended to the Global Center of Excellence (GCOE) Program for Chemistry Innovation through Cooperation of Science and Engineering and Nippon Sheet Glass Foundation for Materials Science and Engineering. ’ REFERENCES (1) (a) Lee, J. S. Catal. Surv. Asia 2005, 9, 217. (b) Maeda, K.; Domen, K. J. Phys. Chem. C 2007, 111, 7851. (c) Osterloh, F. E. Chem. Mater. 2008, 20, 35. (d) Kudo, A.; Miseki, Y. Chem. Soc. Rev. 2009, 38, 253. (e) Youngblood, W. J.; Lee, S.-H. A.; Maeda, K.; Mallouk, T. E. Acc. Chem. Res. 2009, 42, 1966. (f) Maeda, K.; Domen, K. J. Phys. Chem. Lett. 2010, 1, 2655. (2) Bard, A. J. J. Photochem. 1979, 10, 59. (3) Tennakone, K.; Wickramanayake, S. J. Chem. Soc., Faraday Trans. 2 1986, 82, 1475. (4) Sayama, K.; Yoshida, R.; Kusama, H.; Okabe, K.; Abe, Y.; Arakawa, H. Chem. Phys. Lett. 1997, 277, 387. (5) Fujihara, K.; Ohno, T.; Matsumura, M. J. Chem. Soc., Faraday Trans. 1998, 94, 3705. (6) Sayama, K.; Mukasa, K.; Abe, R.; Abe, Y.; Arakawa, H. Chem. Commun. 2001, 2416. (7) Kato, H.; Hori, M.; Konta, R.; Shimodaira, Y.; Kudo, A. Chem. Lett. 2004, 33, 1348. (8) Abe, R.; Sayama, K.; Sugihara, H. J. Phys. Chem. B 2005, 109, 16052. (9) Abe, R.; Takata, T.; Sugihara, H.; Domen, K. Chem. Commun. 2005, 3829. (10) Higashi, M.; Abe, R.; Teramura, K.; Takata, T.; Ohtani, B.; Domen, K. Chem. Phys. Lett. 2008, 452, 120. (11) Higashi, M.; Abe, R.; Ishikawa, A.; Takata, T.; Ohtani, B.; Domen, K. Chem. Lett. 2008, 37, 138. (12) Sasaki, Y.; Iwase, A.; Kato, H.; Kudo, A. J. Catal. 2008, 259, 133. (13) (a) Maeda, K.; Terashima, H.; Kase, K.; Higashi, M.; Tabata, M.; Domen, K. Bull. Chem. Soc. Jpn. 2008, 81, 927. (b) Maeda, K.; Higashi, M.; Lu, D.; Abe, R.; Domen, K. J. Am. Chem. Soc. 2010, 132, 5858. (14) Sasaki, Y.; Nemoto, H.; Saito, K.; Kudo, A. J. Phys. Chem. C 2009, 113, 17536. (15) Wang, X.; Liu, G.; Chen, Z.-G.; Li, F.; Wang, L.; Lu, G. Q.; Cheng, H.-M. Chem. Commun. 2009, 3452. (16) Abe, R.; Shinmei, K.; Hara, K.; Ohtani, B. Chem. Commun. 2009, 3577. (17) (a) Maeda, K.; Teramura, K.; Masuda, H.; Takata, T.; Saito, N.; Inoue, Y.; Domen, K. J. Phys. Chem. B 2006, 110, 13107. (b) Maeda, K.; Masuda, H.; Domen, K. Catal. Today 2009, 147, 173. 3063



ARTICLE

(18) (a) Maeda, K.; Teramura, K.; Lu, D.; Saito, N.; Inoue, Y.; Domen, K. Angew. Chem., Int. Ed. 2006, 45, 7806. (b) Maeda, K.; Teramura, K.; Lu, D.; Saito, N.; Inoue, Y.; Domen, K. J. Phys. Chem. C 2007, 111, 7554. (c) Yoshida, M.; Takanabe, K.; Maeda, K.; Ishikawa, A.; Kubota, J.; Sakata, Y.; Ikezawa, Y.; Domen, K. J. Phys. Chem. C 2009, 113, 10151. (19) Nakamura, R.; Tanaka, T.; Nakato, Y. J. Phys. Chem. B 2005, 109, 8920. (20) Abe, R.; Takata, T.; Sugihara, H.; Domen, K. Chem. Lett. 2005, 34, 1162. (21) Hitoki, G.; Takata, T.; Kondo, J. N.; Hara, M.; Kobayashi, H.; Domen, K. Chem. Commun. 2002, 1698. (22) Arikawa, T.; Takasu, Y.; Murakami, Y.; Asakura, K.; Iwasawa, Y. J. Phys. Chem. B 1998, 102, 3736. (23) Lee, Y.; Teramura, K.; Hara, M.; Domen, K. Chem. Mater. 2007, 19, 2120. (24) Maeda, K.; Sakamoto, N.; Ikeda, T.; Ohtsuka, H.; Xiong, A.; Lu, D.; Kanehara, M.; Teranishi, T.; Domen, K. Chem.—Eur. J. 2010, 16, 7750. (25) Abe, R.; Higashi, M.; Domen, K. J. Am. Chem. Soc. 2010, 132, 11828. (26) Harriman, A.; Pickering, I. J.; Thomas, J. M.; Christensen, P. A. J. Chem. Soc., Faraday Trans. 1 1988, 84, 2795. (27) Ishihara, A.; Doi, S.; Mitsushima, S.; Ota, K. Electrochim. Acta 2008, 53, 5442. (28) Yoshida, M.; Takanabe, K.; Maeda, K.; Ishikawa, A.; Kubota, J.; Sakata, Y.; Ikezawa, Y.; Domen, K. J. Phys. Chem. C 2009, 113, 10151.

3064


Role and Function of Ruthenium Species as Promoters with TaON

Recommend Documents