Effect of Particle Size on the Photocatalytic Activity of WO3 Particles for

Article pubs.acs.org/JPCC

Effect of Particle Size on the Photocatalytic Activity of WO3 Particles for Water Oxidation Fumiaki Amano,*,† Eri Ishinaga,† and Akira Yamakata‡,§ †

Graduate School of Environmental Engineering, The University of Kitakyushu, 1-1 Hibikino, Wakamatsu-ku, Kitakyushu 808-0135, Japan ‡ Graduate School of Engineering, Toyota Technological Institute, 2-12-1 Hisakata, Tempaku-ku, Nagoya 468-8511, Japan § Precursory Research for Embryonic Science and Technology (PRESTO), Japan Science and Technology Agency (JST), 4-1-8 Honcho, Kawaguchi, Saitama 332-0012, Japan S Supporting Information *

ABSTRACT: The particle size of a photocatalyst is one of the factors that determine the photon utilization efficiency. Here, we report the effects of the particle size on the photocatalytic activity of tungsten trioxide (WO3) for water oxidation to evolve molecular oxygen in the presence of sacrificial silver nitrate. For the water oxidation, the photocatalytic activity of large aggregates consisting of particles with a small specific surface area (3 m2 g−1) was greater than that of fine particles with a larger specific surface area (11 m2 g−1). Transient infrared absorption spectra revealed that the recombination of photoexcited electrons in the large particles was slower than that in the fine particles. It was found that the fast recombination in the fine particles mostly occurred on the surface. The photocatalytic activity for water oxidation increased with increasing particle size for spherical diameters smaller than 200 nm. These results showed that large particles with a low surface area-to-volume ratio were suitable for photocatalytic oxygen evolution, which requires long-lived holes, because the surface recombination of photogenerated electrons and holes occurred less frequently.

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INTRODUCTION Photocatalytic solar water splitting is attractive because of its potential for hydrogen production without the use of fossil fuel energy. The effects of the structure and physical properties on the performance of particulate materials must be clarified for solar water splitting; in addition, a feasible design concept for highly efficient photocatalysts has yet to be made clear. Tungsten trioxide (WO3) is known as a photocatalyst for the oxidative decomposition of water to evolve molecular oxygen.1−7 To date, the use of TiO2 photocatalysts has been limited; because of the large band gap (3.2 eV for anatase), these materials cannot absorb much visible light. In contrast, the band gap of WO3 is small enough (approximately 2.6 eV) to allow the absorption of visible light at wavelengths shorter than 470 nm. There have been reports indicating that surfacemodified WO3 photocatalysts are effective for the oxidative decomposition of organic pollutants,8−13 organic synthesis via selective oxidation,14 and photoinduced hydrophilicity15−17 under visible light irradiation. To allow the development of highly efficient WO 3 photocatalysts for water oxidation, the decisive factors determining the photocatalytic activity should be elucidated. Many reports have confirmed the significant effects of the particle size on the photocatalytic activity.18−25 It is generally considered that it is desirable for the grain size of a photocatalyst to be small; i.e., the specific surface area should be large. If the grain size is small, the transport of photogenerated electrons (e−) and holes (h+) from the bulk to the surface becomes easier. Moreover, the surface charge © 2013 American Chemical Society

transfer rate will be improved by an increase in the amount of reactant adsorption.24,26 The photoabsorption properties of semiconductors also depend on the particle size in the nanometer range.27 The crystallinity of the particles is another important factor influencing the rate of e−/h+ recombination, which is believed to occur in crystalline defects in the bulk, and/or at the surface;28,29 the rate of surface charge transfer is affected by the exposed crystalline surface.25,29 Generally speaking, the crystallinity of fine particles is lower than that of the well-grown, large crystalline particles that are frequently prepared using high-temperature calcination. Larger particles with a lower density of crystalline defects and/or with reactive crystalline facets can show superior photocatalytic activity.30,31 However, some reports have indicated that the photocatalytic activity of TiO2 powder does not monotonically increase with decreasing particle size and that there is an optimum particle size of a few dozen nanometers.20−22 Based on the above discussion, it may not be true that photocatalysts with a smaller grain size show higher photocatalytic activity. Kominami et al. reported that the photocatalytic activity of anatase TiO2 nanocrystals for the oxidative decomposition of acetic acid was almost proportional to the specific surface area (the amount of surface-adsorbed acetic acid), but the photocatalytic activity for silver metal deposition by water oxidation depended on the crystallinity.26 The photocatalytic Received: August 23, 2013 Revised: October 10, 2013 Published: October 10, 2013 22584

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suspension remaining after the removal of the precipitate was centrifuged at 9800g for 5 min, to collect the fine particulates, which will be denoted as WO3(KF). The obtained precipitates were dried in an oven at 100 °C. The good separation and clarification of the particles was confirmed using SEM images (using a Hitachi S-5200 instrument) and specific surface area measurements (using a Bel Japan BELSORP-mini instrument). The specific surface area was determined using a BET plot from a nitrogen adsorption isotherm at −196 °C. The sample was evacuated at 200 °C for 2 h before the adsorption isotherm measurements. X-ray diffraction (XRD) patterns were recorded using a Rigaku RINT-2000/PC instrument. Nickel oxide (NiO) was added to a concentration of 30 wt % to act as an internal standard. The crystalline size was estimated from the line broadening at the half-maximum intensity of the diffraction peaks, using the Scherrer equation. Ultraviolet−visible diffuse reflectance spectra were obtained using an ALS SEC2000-UV/ vis spectrometer with a Hamamatsu Photonics L10290 fiber light source, using barium sulfate as a reference. The photocatalytic activity was examined by measuring the oxygen evolution from an aqueous solution of 50 mmol L−1 silver nitrate (AgNO3) under ultraviolet-light irradiation (4Ag+ + 2H2O → 4Ag0 + O2 + 4H+). The silver cation acted as a sacrifice electron acceptor. Fifty milligrams of WO3 powder was dispersed in the 9.0 mL solution in a glass tube with an outside diameter of 18 mm. The suspension was bubbled with argon and photoirradiated using 405 nm violet light-emitting diodes (LEDs) at room temperature under magnetic stirring. The irradiance of the 405 nm wavelength LEDs was measured to be approximately 19 mW cm−2. The amount of evolved oxygen was quantified using a thermal conductivity detector gas chromatograph (TCD-GC) with a Molecular Sieve-5A column. In the case of the photocatalytic oxidation of acetic acid, an aqueous solution of 5 vol % acetic acid was used, and the suspension was bubbled with oxygen. The evolved CO2 was detected using a TCD-GC with a Porpak-Q column. Transient infrared (IR) spectroscopy was performed using a custom setup.33,34 Ethanol suspensions containing WO3 powders (3 mg) were deposited on a calcium fluoride plate with a 16 mm diameter and dried at room temperature. The WO3-coated plate was placed in a stainless-steel cell and evacuated at room temperature. The transient absorbance change was recorded in a vacuum and in the presence of 1.3 kPa of methanol. Third-harmonic light from a Q-switched Nd:YAG laser was used for the photoexcitation (wavelength 355 nm). The power, duration, and repetition rate of the laser pulses were 0.8 mJ cm−2, 6 ns, and 5 Hz, respectively. The IR light emitted from a molybdenum disilicide source was focused on the film, and the transmitted light was dispersed in a grating spectrometer and transformed into an electric signal in a photovoltaic mercury cadmium telluride detector. The signals were averaged over 500 pulses. The signal intensities were constant during the repeated measurements.

activity for water oxidation has also been reported to be inversely proportional to the density of defective sites in TiO2 powders.28 In this reaction, anatase TiO2 particles show very poor activity, and rutile TiO2 with large particle size is efficient.32 The effect of the particle size on the photocatalytic activity has been frequently confirmed for anatase TiO2 nanocrystals for the oxidative decomposition of organic pollutants, but a better understanding is required for visiblelight-sensitive photocatalysts for water splitting. In this study, the effects of the grain size of WO3 powders on photocatalytic water oxidation in the presence of sacrificial silver nitrate were investigated; this was achieved using fine particles and large aggregated particles, which were classified from a commercial, as-prepared WO3 powder. The particle types (fine, and large and aggregated) were expected to exhibit similar physical properties with the exception of their size because the powders were prepared at the same time. For example, because the raw materials in the feed were the same, it was assumed that the purity would be similar as long as there was no segregation of composition. The crystallinity was also assumed to be similar because the heat history during the preparation was the same. We investigated the decay of the photoexcited electrons using transient infrared spectroscopy to determine the correlation between the photocatalytic activity and the e−/h+ recombination rate. The reason for the low (or high) photocatalytic activity of the fine (or large) WO3 particles for water oxidation will be discussed based on the experimental results.

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MATERIALS AND METHODS WO3 powders were sourced commercially, from Aldrich Chemistry and Fluka Chemika in Sigma-Aldrich (St. Louis, MO), Kanto Chemical (Tokyo, Japan), Wako Pure Chemical Industries (Osaka, Japan), and Kojundo Chemical Laboratory (Sakado, Japan). The WO3 powder (Kojundo Chemical Laboratory, purity 99.99%), which will be denoted as WO3(K), consisted of a mixture of fine particulates with sizes of approximately a few hundred nanometers and micrometersized large aggregates, as shown in the scanning electron microscopy (SEM) images in Figure 1. The fine and large

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Figure 1. SEM image of commercial WO3(K) powder.

RESULTS AND DISCUSSION Figure 2 shows SEM images of the WO3 particles classified from a commercial WO3(K) powder, in which fine and large particles were mixed in an as-prepared form. The images show the good separation of the fine particles from the large particles. The grain sizes of the large and fine particles were approximately 2−10 μm and 100−300 nm, respectively. The large particles exhibited a rough surface composed of fine particulates, indicating that the micrometer-sized particles were

particles could be separated using centrifugation, using a previously reported method.8,14 The WO3(K) powder (1.0 g) was suspended in deionized water (40 mL) using the application of ultrasonic irradiation for 1 h, and the suspension was then centrifuged at a relative centrifugal force of 160g for 5 min to collect the precipitate, which consisted of large aggregate particles, which will be denoted as WO3(KL). The 22585

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Figure 3. XRD patterns for (a) WO3(KL), (b) WO3(KF), and (c) commercial WO3(K).

aggregates, but slightly different in the range of 32°−35°. This pattern was partly assigned to triclinic WO3 (PDF #321395, a = 0.731 nm, b = 0.752 nm, c = 0.768 nm, α = 88.81°, β = 90.92°, γ = 90.93°) (Figure S1). These results indicated that the large particles mainly consisted of monoclinic WO3, and the fine particles contained triclinic WO3, which had a crystal structure that was more distorted than that of the monoclinic WO3 in the large aggregated particles. The XRD pattern for the commercial WO3(K) was a mixed composite of the patterns for the large and fine particles. The crystalline diameters for the (002), (020), and (200) planes were estimated using the Scherrer equation and are shown in Table 1. The crystalline diameters of the large particles were slightly larger than that of the fine particles. However, there was only a small difference between the crystalline diameters because the large aggregated particles were composed of the fine crystalline particles. Figure 4 shows time courses for the photocatalytic evolution of oxygen over the WO3 particles, under 405 nm wavelength

Figure 2. SEM images of (a) large and (b) fine WO3 particles classified from WO3(K).

secondary particles composed of chemically aggregated fine particles. Table 1 shows the specific surface area values for the Table 1. Physical Properties of Large, Fine, and Commercial WO3(K) Particles sample

SSAa (m2 g−1)

Db (nm)

d(002)c (nm)

d(020)c (nm)

d(200)c (nm)

WO3(KL) WO3(KF) WO3(K)

3.3 11.5 6.0

252 72 137

48 41 43

52 36 43

45 40 45

a

BET specific surface area. bAverage particle diameter estimated from SSA. cThe diameter of crystalline domains, estimated using the Scherrer equation.

classified particles, WO3(KL) and WO3(KF). The finer particles exhibited a larger specific surface area. The specific surface area of the fine particles was approximately 2 times larger than that of the commercial WO3(K), suggesting good separation. Calculations using the specific surface area showed that the commercial particles as supplied were composed of 67% large particles and 33% fine particles. The average particle diameter (D) was estimated from the specific surface area (SSA) and the density (ρ) using the equation D = 6/(SSAρ), assuming spherical-shaped particles with an equivalent diameter. The density ρ is 7.286 g m−3 for monoclinic WO3 (PDF #43-1035). The estimated diameter of the large particles, 252 nm, was much smaller than the micrometer-order diameters determined from the SEM images. This indicated that the large particles were formed via the chemical aggregation of the fine particles. Figure 3 shows XRD patterns for the WO3 particles. The pattern for WO3(KL) was assigned to monoclinic WO3 (PDF #43-1035, a = 0.730 nm, b = 0.754 nm, c = 0.769 nm, α = β = 90.00°, γ = 90.91°) (Figure S1 in Supporting Information). The pattern for WO3(KF) was similar to that for the large

Figure 4. Time courses for the photocatalytic evolution of oxygen over (a) WO3(KL), (b) WO3(KF), and (c) commercial WO3(K) under 405 nm wavelength irradiation.

irradiation. The oxygen evolution was negligible when any of the following experimental variables were absent: photoirradiation, WO3 powder, or silver cations (Figure S2). Figure S3 shows diffuse reflectance ultraviolet−visible spectra for the WO3 particles and the spectrum of the 405 nm wavelength violet LEDs used for the photocatalytic reaction. The photoabsorption edges due to the interband transition from the valence band to the conduction band were located at approximately 460−475 nm. The band gaps of WO3(K), WO3(KL), WO3(KF) were estimated to be 2.63, 2.61, and 2.70 eV. It has been reported that the blue-shift in the absorption spectral edge of TiO2 particles cannot be explained by size 22586

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quantization effect but direct transition in indirect semiconductor.27 There was only a small difference between the photoabsorption values for the WO3 particles at approximately 405 nm. Under 405 nm wavelength irradiation, WO3(KL) exhibited the highest activity for oxygen evolution from an aqueous solution of silver nitrate. The amount of evolved oxygen was found to be low over the fine particles. The photocatalytic activity of the commercial WO3(K) (which was composed of both types of particles) was between those of the fine and large particles. This order of photocatalytic activity for oxygen evolutionWO3(KL) > WO3(K) > WO3(KF)was also observed under photoirradiation using 380 nm ultraviolet LEDs. Since the large particles were formed via the chemical aggregation of the fine particles, the surface properties of the primary particles would be similar. Therefore, the difference in photocatalytic activity should depend on the rate of e−/h+ recombination rather than the surface reactivity. We examined the behavior of photoexcited electrons using transient IR spectroscopy to probe the dynamics of the photogenerated charge carriers. Figure 5 shows transient IR

Figure 6. Transient IR absorption probed at 2000 cm−1 and triggered by a 6 ns and a 355 nm laser pulse (A) in a vacuum and (B) in the presence of methanol: (a) WO3(KL) and (b) WO3(KF).

was found to be slow in the well-crystallized large particles. This was the reason for the high photocatalytic activity of WO3(KL) for oxygen evolution; it has been reported that long-lived photogenerated holes are necessary to induce four-electron water oxidation.37,38 In the presence of methanol, the fast recombination of WO3(KF) was suppressed by the adsorption of methanol on the surface.39 The lifetime of the photoexcited electrons was elongated because of the competing reaction of the photogenerated holes with the methanol rather than with the photoexcited electrons. The oxidation of methanol is simpler and occurs more easily than the oxidation of water. From a thermodynamic viewpoint, the redox potentials for HCHO/ CH3OH (+0.232 V vs SHE) are more negative than that of O2/ H2O (+1.229 V vs SHE).40 In addition, the oxidation of CH3OH that produces HCHO is a two-electron reaction, which occurs more easily than the four-electron reaction of water oxidation that is required to evolve an oxygen molecule. The methanol adsorbed on the surface could therefore react with the photogenerated holes and suppress the surface recombination with the photoexcited electrons. This indicated that surface recombination was the dominant process in the fine particles. The photoexcited electron decay in WO3(KL) hardly changed in the presence of methanol, as shown in Figure 6a. This was because of the longer lifetime of the photoexcited electrons, even in a vacuum. Bulk recombinationrather than surface recombinationwas the dominant process in the large particles, which had a low surface area-to-volume ratio. For oxygen evolution, long-lived electrons are necessary to induce the four-electron oxidation of water. Fast recombination in the fine particles therefore resulted in a low activity for oxygen evolution. The large, low surface area-to-volume ratio particles exhibited high activity for water oxidation, probably because of the lower levels of recombination on the surface. In contrast with water oxidation, long-lived electrons might not be necessary for the oxidation of organic compounds such as methanol and acetic acid. We therefore tested the photocatalytic activity of WO3 particles for the oxidative decomposition and mineralization of organic compounds in an

Figure 5. Transient IR absorption spectra for WO3(KF). The spectra were recorded at delay times of 0, 1, 2, 3, 5, 10, 20, 50, and 100 μs after photoexcitation under a 6 ns laser pulse at a wavelength of 355 nm in a vacuum.

absorption spectra for the fine WO3 particles. The spectra were recorded a certain time after irradiation under a 355 nm laser pulse in vacuum. Broad, transient IR absorption was observed after a 6 ns pulse of photoexcitation, and the signal decayed within a few hundred microseconds. The broad absorption was assigned to photoexcited electrons in the conduction band and trapped electrons in the defect state. The decay of the signal in vacuum was assigned to the e−/h+ recombination. Figure 6 shows the decay of transient IR absorption (probed at 2000 cm−1) that was triggered by a 355 nm wavelength laser pulse in vacuum in the presence of methanol (which acted as an electron donor). Methanol immediately reacts with photogenerated holes to suppress recombination and generate longlived electrons, as follows:33,34 CH3OH ⎯⎯⎯⎯⎯⎯⎯⎯→ e− + (CH 2OH)• /CH3O• + H+ WO3 + hv

The generated radical species can induce one-electron injection to the conduction band of WO3, generating formaldehyde via the current doubling effect.35,36 (CH 2OH)• /CH3O• → e− + HCHO + H+

In a vacuum, the decay of the photoexcited electrons in WO3(KF) was much faster than that observed for WO3(KL), indicating that the e−/h+ recombination occurred more easily in the fine particles. In contrast, the photoexcited electron decay 22587

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aqueous solution under oxygen. CO2 evolution was not observed for the photocatalytic oxidation of methanol, but it was observed for acetic acid. This was probably because the evolution of CO2 should occur more easily from acetic acid than from methanol. Considering the electrons needed for the reaction, six-electron oxidation is necessary to evolve CO2 via the oxidation of methanol, but CO2 can be evolved via the oneelectron oxidation of acetic acid, as follows:

Table 2. Specific Surface Area of Samples after Calcination at 800 °C and the Effect of Calcination on the Photocatalytic Activity for Oxygen Evolution SSAa (m2 g−1)

CH3OH + 6h+ + H 2O → CO2 + 6H+

r(O2)b (μmol h−1)

sample

after 800 °C calcination

before calcination

after 800 °C calcination

WO3(K) WO3(KL) WO3(KF)

1.5 1.0 1.5

14.7 23.1 6.6

24.0 23.9 23.9

a BET specific surface area. bRate of oxygen evolution in the initial period from 0 to 40 min.

CH3COOH + h+ → CH3• + CO2 + H+

Figure 7 shows time courses for the photocatalytic evolution of CO2 from an aqueous solution of 5 vol % acetic acid over

recombination sites, resulting in an increase in the photocatalytic activity. In contrast, the photocatalytic efficiency of WO3(KL) was not changed significantly after they were subjected to calcination at 800 °C, although the specific surface area was reduced from 3.3 to 1.0 m2 g−1 by the heating. This change in the specific surface area corresponded to a growth in the average spherical diameter from 252 to 861 nm (see the SEM image shown in Figure S5). It should be noted that the WO3 crystal structures in the fine and large particles were not the same. Figure S6 shows XRD patterns for the samples calcined at 800 °C. The patterns for the calcined samples were not the same; the calcined samples prepared from WO3(KF) contained both triclinic and monoclinic phases, even after calcination. However, the photocatalytic activities of the calcined samples were very similar. This indicated that the difference in the crystal structure of the WO3 particles (monoclinic or triclinic) was not crucial for the photocatalytic activity for the oxidation of water. Figure 8 shows a summary of the photocatalytic activities determined in this study; specifically, the effects of the specific

Figure 7. Time courses for CO2 evolution during the photocatalytic decomposition of acetic acid over (a) WO3(KL), (b) WO3(KF), and (c) commercial WO3(K) under 405 nm wavelength irradiation.

WO3(K) particles in the presence of oxygen. The rate of CO2 evolution was enhanced by the presence of oxygen (Figure S4). The rate of photocatalytic CO2 evolution over WO3(KF) was higher than that measured over WO3(KL) in this reaction; this was in contrast with the results for the oxidative splitting of water. This indicated that the adsorption of acetic acid and oxygen on the surface of the fine particles resulted in the suppression of surface recombination, in a fashion similar to that observed for methanol. Therefore, higher photocatalytic activity was observed for the fine particles. The order of photocatalytic activity for the oxidative decomposition of acetic acidWO3(KF) > WO3(K) > WO3(KL)was also observed under photoirradiation using 380 nm wavelength ultraviolet LEDs. It was concluded that fine particles with a large surface area are suitable for the photocatalytic mineralization of acetic acid because long-lived photogenerated holes are not necessary to induce this reaction. The holes can react with the surfaceadsorbed acetic acid rather than the photoexcited electrons. In the case of water oxidation, which requires long-lived holes, large particles with a low surface area-to-volume ratio are suitable for photocatalytic oxygen evolution. To confirm the effect of particle size on the photocatalytic activity for oxygen evolution, we tested the activity of WO3 particles after calcination performed at 800 °C. Table 2 shows the effects of calcination at 800 °C on the specific surface area and the photocatalytic activity. Calcination at 800 °C decreased the specific surface area for WO3(K) and WO3(KF) and increased their photocatalytic activities for oxygen evolution. The decrease in the specific surface area was due to crystal growth (see the SEM images shown in Figure S5). These results indicated that the crystal growth that occurred at high temperatures reduced the density of surface defects acting as

Figure 8. Effect of (A) specific surface area and (B) average spherical diameter on the rate of photocatalytic oxygen evolution in the initial period from 0 to 40 min: (closed circle) commercial WO3 powders, (open circles) powders calcined at 800 °C, (a) WO3(KL), (b) WO3(KF), and (c) commercial WO3(K).

surface area and the average particle diameter on the photocatalytic activity for oxygen evolution are illustrated. The specific surface area and photocatalytic activities of six commercial samples (Figure S7) have been added to Figure 8. The XRD patterns indicated that some commercial samples exhibited both monoclinic and triclinic WO3, and there were differences in the crystalline composition (Figure S8). Despite these differences, the photocatalytic activity decreased monotonically with increases in the specific surface area, as shown in Figure 8A. These results confirmed that the difference in crystal structure was not crucial for the photocatalytic activity of WO3 particles for water oxidation. For spherical diameters smaller 22588

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than 200 nm, the photocatalytic activity increased significantly with increasing particle size, suggesting that the surface area-tovolume ratio of a 200 nm particle was low enough to suppress fast recombination on the surface. The photocatalytic activity did not decrease when the particle size increased beyond 200 nm, although the adsorption of reactant molecules such as silver cations should decrease with increases in the particle size. The plateau in the photocatalytic activity observed for particles larger than 200 nm indicated that the rate-determining step for photocatalytic oxygen evolution in the presence of silver cations was not the adsorption process. For water oxidation, the most important factor deciding the photocatalytic efficiency was likely the surviving fraction of long-lived photogenerated holes under photoirradiation.

CONCLUSIONS The effects of particle size on the photocatalytic activity of WO3 particles were investigated using fine particles and large aggregated particles, which were obtained via the separation of as-prepared commercial WO3(K) particles. Large particles with a low surface area-to-volume ratio exhibited a photocatalytic activity for water oxidation that was higher than that of fine particles. In the case of the oxidative decomposition of organic compoundswhich can become adsorbed on the surface and can block surface recombinationfine particles with a large surface area exhibited a photocatalytic activity that was comparable to that of the large particles. When the fine particles were calcined at 800 °C, the grain size was increased, which resulted in the improvement of the photocatalytic activity because of the decrease in the surface area-to-volume ratio. The results showed that for water oxidation the optimum WO3 particle size is greater than 200 nm. The reason for the low photocatalytic activity of the fine particulate WO3 for water oxidation was found to be the fast recombination that occurred on the surface. It was concluded that large particles with a low surface area-to-volume ratio are suitable for photocatalytic oxygen evolution requiring long-lived photogenerated holes. Bulk recombination, rather than fast surface recombination, was the dominant process in the large particles. This was the reason for the high photocatalytic activity of the large particles for the four-electron oxidation of water. ASSOCIATED CONTENT

S Supporting Information *

Diffuse reflectance ultraviolet−visible spectra; spectra of the light sources used for the photocatalytic reaction; SEM images and XRD patterns for the calcined particles; photocatalytic activities of commercial WO3 powders. This material is available free of charge via the Internet at http://pubs.acs.org.

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REFERENCES

(1) Maeda, K.; Higashi, M.; Lu, D.; Abe, R.; Domen, K. Efficient Nonsacrificial Water Splitting through Two-step Photoexcitation by Visible Light Using a Modified Oxynitride as a Hydrogen Evolution Photocatalyst. J. Am. Chem. Soc. 2010, 132, 5858−5868. (2) Abe, R.; Higashi, M.; Domen, K. Overall Water Splitting under Visible Light through a Two-Step Photoexcitation between TaON and WO3 in the Presence of an Iodate-Iodide Shuttle Redox Mediator. ChemSusChem 2011, 4, 228−237. (3) Ma, S. S. K.; Maeda, K.; Abe, R.; Domen, K. Visible-Light-Driven Nonsacrificial Water Oxidation over Tungsten Trioxide Powder Modified with Two Different Cocatalysts. Energy Environ. Sci. 2012, 5, 8390−8397. (4) Miseki, Y.; Kusama, H.; Sugihara, H.; Sayama, K. Cs-Modified WO3 Photocatalyst Showing Efficient Solar Energy Conversion for O2 Production and Fe(III) Ion Reduction under Visible Light. J. Phys. Chem. Lett. 2010, 1, 1196−1200. (5) Amano, F.; Li, D.; Ohtani, B. Fabrication and Photoelectrochemical Property of Tungsten(VI) Oxide Films with a FlakeWall Structure. Chem. Commun. 2010, 46, 2769−2771. (6) Amano, F.; Tian, M.; Wu, G.; Ohtani, B.; Chen, A. Facile Preparation of Platelike Tungsten Oxide Thin Film Electrodes with High Photoelectrode Activity. ACS Appl. Mater. Interfaces 2011, 3, 4047−4052. (7) Amano, F.; Tian, M.; Ohtani, B.; Chen, A. Photoelectrochemical Properties of Tungsten Trioxide Thin Film Electrodes Prepared from Facet-Controlled Rectangular Platelets. J. Solid State Electrochem. 2012, 16, 1965−1973. (8) Abe, R.; Takami, H.; Murakami, N.; Ohtani, B. Pristine Simple Oxides as Visible Light Driven Photocatalysts: Highly Efficient Decomposition of Organic Compounds over Platinum-Loaded Tungsten Oxide. J. Am. Chem. Soc. 2008, 130, 7780−7781. (9) Sadakane, M.; Sasaki, K.; Kunioku, H.; Ohtani, B.; Ueda, W.; Abe, R. Preparation of Nano-Structured Crystalline Tungsten(VI) Oxide and Enhanced Photocatalytic Activity for Decomposition of Organic Compounds under Visible Light Irradiation. Chem. Commun. 2008, 6552−6554. (10) Arai, T.; Horiguchi, M.; Yanagida, M.; Gunji, T.; Sugihara, H.; Sayama, K. Complete Oxidation of Acetaldehyde and Toluene over a Pd/WO3 Photocatalyst under Fluorescent- or Visible-Light Irradiation. Chem. Commun. 2008, 5565−5567. (11) Arai, T.; Yanagida, M.; Konishi, Y.; Iwasaki, Y.; Sugihara, H.; Sayama, K. Promotion Effect of CuO Co-Catalyst on WO3-Catalyzed Photodegradation of Organic Substances. Catal. Commun. 2008, 9, 1254−1258. (12) Arai, T.; Yanagida, M.; Konishi, Y.; Sugihara, H.; Sayama, K. Utilization of Fe3+/Fe2+ Redox for the Photodegradation of Organic Substances over WO3 Photocatalyst and for H2 Production from the Electrolysis of Water. Electrochemistry 2008, 76, 128−131. (13) Arai, T.; Horiguchi, M.; Yanagida, M.; Gunji, T.; Sugihara, H.; Sayama, K. Reaction Mechanism and Activity of WO3-Catalyzed Photodegradation of Organic Substances Promoted by a CuO Cocatalyst. J. Phys. Chem. C 2009, 113, 6602−6609. (14) Tomita, O.; Abe, R.; Ohtani, B. Direct Synthesis of Phenol from Benzene over Platinum-Loaded Tungsten(VI) Oxide Photocatalysts with Water and Molecular Oxygen. Chem. Lett. 2011, 40, 1405−1407. (15) Miyauchi, M. Photocatalysis and Photoinduced Hydrophilicity of WO3 Thin Films with Underlying Pt Nanoparticles. Phys. Chem. Chem. Phys. 2008, 10, 6258−6265. (16) Liu, Z.; Miyauchi, M. Visible-Light Induced Superhydrophilicity on a WO3/ITO/CaFe2O4 Heterojunction Thin Film. Chem. Commun. 2009, 2002−2004. (17) Shibuya, M.; Miyauchi, M. Site-Selective Deposition of Metal Nanoparticles on Aligned WO3 Nanotrees for Super-Hydrophilic Thin Films. Adv. Mater. 2009, 21, 1373−1376. (18) Serpone, N.; Lawless, D.; Khairutdinov, R.; Pelizzetti, E. Subnanosecond Relaxation Dynamics in TiO2 Colloidal Sols (Particle sizes Rp = 1.0−13.4 nm). Relevance to Heterogeneous Photocatalysis. J. Phys. Chem. 1995, 99, 16655−16661.

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AUTHOR INFORMATION

Corresponding Author

*Phone +81-93-695-3372; e-mail [email protected] (F.A.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Japan Securities Scholarship Foundation and JSPS KAKENHI Grant Numbers 23655187 and 23686114. 22589

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The Journal of Physical Chemistry C

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structured alpha-Fe2O3 and TiO2. J. Am. Chem. Soc. 2011, 133, 10134−10140. (39) Colbeau-Justin, C.; Kunst, M.; Huguenin, D. Structural Influence on Charge-Carrier Lifetimes in TiO2 Powders Studied by Microwave Absorption. J. Mater. Sci. 2003, 38, 2429−2437. (40) Bird, A. J.; Parson, R.; Jordan, J. Standard Potentials in Aqueous Solution; CRC Press: Boca Raton, FL, 1985.

(19) Xu, N.; Shi, Z.; Fan, Y.; Dong, J.; Shi, J.; Hu, M. Z. C. Effects of Particle Size of TiO2 on Photocatalytic Degradation of Methylene Blue in Aqueous Suspensions. Ind. Eng. Chem. Res. 1999, 38, 373−379. (20) Wang, C. C.; Zhang, Z.; Ying, J. Y. Photocatalytic Decomposition of Halogenated Organics over Nanocrystalline Titania. Nanostruct. Mater. 1997, 9, 583−586. (21) Zhang, Z.; Wang, C. C.; Zakaria, R.; Ying, J. Y. Role of Particle Size in Nanocrystalline TiO2-Based Photocatalysts. J. Phys. Chem. B 1998, 102, 10871−10878. (22) Almquist, C. B.; Biswas, P. Role of Synthesis Method and Particle Size of Nanostructured TiO2 on Its Photoactivity. J. Catal. 2002, 212, 145−156. (23) Jang, H. D.; Kim, S. K.; Kim, S. J. Effect of Particle Size and Phase Composition of Titanium Dioxide Nanoparticles on the Photocatalytic Properties. J. Nanopart. Res. 2001, 3, 141−147. (24) Amano, F.; Nogami, K.; Tanaka, M.; Ohtani, B. Correlation Between Surface Area and Photocatalytic Activity for Acetaldehyde Decomposition over Bismuth Tungstate Particles with a Hierarchical Structure. Langmuir 2010, 26, 7174−7180. (25) Wu, Q.; Liu, M.; Wu, Z.; Li, Y.; Piao, L. Is Photooxidation Activity of {001} Facets Truly Lower than That of {101} Facets for Anatase TiO2 Crystals? J. Phys. Chem. C 2012, 116, 26800−26804. (26) Kominami, H.; Murakami, S. Y.; Kato, J. I.; Kera, Y.; Ohtani, B. Correlation between Some Physical Properties of Titanium Dioxide Particles and Their Photocatalytic Activity for Some Probe Reactions in Aqueous Systems. J. Phys. Chem. B 2002, 106, 10501−10507. (27) Serpone, N.; Lawless, D.; Khairutdinov, R. Size Effects on the Photophysical Properties of Colloidal Anatase TiO2 Particles: Size Quantization or Direct Transitions in This Indirect Semiconductor? J. Phys. Chem. 1995, 99, 16646−16654. (28) Ikeda, S.; Sugiyama, N.; Murakami, S. Y.; Kominami, H.; Kera, Y.; Noguchi, H.; Uosaki, K.; Torimoto, T.; Ohtani, B. Quantitative Analysis of Defective Sites in Titanium(IV) Oxide Photocatalyst Powders. Phys. Chem. Chem. Phys. 2003, 5, 778−783. (29) Amano, F.; Yamakata, A.; Nogami, K.; Osawa, M.; Ohtani, B. Effect of Photoexcited Electron Dynamics on Photocatalytic Efficiency of Bismuth Tungstate. J. Phys. Chem. C 2011, 115, 16598−16605. (30) Amano, F.; Prieto-Mahaney, O. O.; Terada, Y.; Yasumoto, T.; Shibayama, T.; Ohtani, B. Decahedral Single-Crystalline Particles of Anatase Titanium(IV) Oxide with High Photocatalytic Activity. Chem. Mater. 2009, 21, 2601−2603. (31) Amano, F.; Yasumoto, T.; Mahaney, O. O. P.; Uchida, S.; Shibayama, T.; Terada, Y.; Ohtani, B. Highly Active Titania Photocatalyst Particles of Controlled Crystal Phase, Size, and Polyhedral Shapes. Top. Catal. 2010, 53, 455−461. (32) Ohno, T.; Sarukawa, K.; Matsumura, M. Photocatalytic Activities of Pure Rutile Particles Isolated from TiO2 Powder by Dissolving the Anatase Component in HF Solution. J. Phys. Chem. B 2001, 105, 2417−2420. (33) Yamakata, A.; Ishibashi, T. A.; Onishi, H. Electron- and HoleCapture Reactions on Pt/TiO2 Photocatalyst Exposed to Methanol Vapor Studied with Time-Resolved Infrared Absorption Spectroscopy. J. Phys. Chem. B 2002, 106, 9122−9125. (34) Yamakata, A.; Ishibashi, T. A.; Kato, H.; Kudo, A.; Onishi, H. Photodynamics of NaTaO3 Catalysts for Efficient Water Splitting. J. Phys. Chem. B 2003, 107, 14383−14387. (35) Peter, L. M. Dynamic Aspects of Semiconductor Photoelectrochemistry. Chem. Rev. 1990, 90, 753−769. (36) Santato, C.; Ulmann, M.; Augustynski, J. Photoelectrochemical Properties of Nanostructured Tungsten Trioxide Films. J. Phys. Chem. B 2001, 105, 936−940. (37) Tang, J.; Durrant, J. R.; Klug, D. R. Mechanism of Photocatalytic Water Splitting in TiO2. Reaction of Water with Photoholes, Importance of Charge Carrier Dynamics, and Evidence for FourHole Chemistry. J. Am. Chem. Soc. 2008, 130, 13885−13891. (38) Cowan, A. J.; Barnett, C. J.; Pendlebury, S. R.; Barroso, M.; Sivula, K.; Graetzel, M.; Durrant, J. R.; Klug, D. R. Activation Energies for the Rate-Limiting Step in Water Photooxidation by Nano22590

dx.doi.org/10.1021/jp408446u | J. Phys. Chem. C 2013, 117, 22584−22590