Correlation between Surface Area and Photocatalytic Activity for

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Correlation between Surface Area and Photocatalytic Activity for Acetaldehyde Decomposition over Bismuth Tungstate Particles with a Hierarchical Structure Fumiaki Amano,*,†,‡ Kohei Nogami,‡ Masako Tanaka,‡ and Bunsho Ohtani†,‡,§ † Catalysis Research Center, Hokkaido University, Sapporo 001-0021, Japan, ‡Graduate School of Environmental Science, Hokkaido University, Sapporo 060-0810, Japan, and §Research Center for Advanced Science and Technology, The University of Tokyo, Tokyo 153-8904, Japan

Received November 11, 2009. Revised Manuscript Received December 21, 2009 The photocatalytic oxidative decomposition of gaseous acetaldehyde (AcH) in air under visible light irradiation (wavelength >400 nm) was driven by bismuth tungstate (Bi2WO6) polycrystalline particles with a hierarchical structure, flake-ball particles, prepared by hydrothermal reaction at various temperatures. Complete decomposition of AcH into CO2 was proven for all of the flake-ball particle photocatalysts. The rate of CO2 liberation was increased in proportional to the specific surface area for flake-ball particles with similar high degrees of crystallinity. Kinetic analysis assuming Langmuirian adsorption of AcH revealed that the initial rate of photocatalytic decomposition could be reproduced by first-order kinetics with respect to the amount of surface-adsorbed AcH. A linear relationship between the photocatalytic activity and surface area of photocatalysts under conditions in which other physical properties such as the photoabsorption property, crystalline content, exposed crystal facets, and secondary particle size are almost the same was experimentally revealed.

1. Introduction The photocatalytic oxidative decomposition of volatile organic compounds into CO2 is a promising technique for the purification of indoor air under illumination from lighting devices. However, the use of TiO2, a widely investigated semiconductor photocatalyst, is limited by its negligible activity under visible light (vis) irradiation. The development of vis-responsive photocatalysts for air purification has been one of the goals in the research field of photofunctional materials.1-5 Recently, the surface modification of some vis-responsive photocatalysts was found to enhance their photocatalytic activity with respect to the decomposition of organic compounds: for instance, iron(III) oxides for sulfurdoped TiO2 photocatalysts,6,7 copper(I) species,8 platinum particles,9 vanadium(V) species,10,11 iron(III) species,12 and *Corresponding author. Tel: þ81-11-706-9130. Fax: þ81-11-706-9131. E-mail: [email protected]. (1) Asahi, R.; Morikawa, T.; Ohwaki, T.; Aoki, K.; Taga, Y. Science 2001, 293, 269. (2) Balcerski, W.; Ryu, S. Y.; Hoffmann, M. R. J. Phys. Chem. C 2007, 111, 15357. (3) Kisch, H.; Sakthivel, S.; Janczarek, M.; Mitoraj, D. J. Phys. Chem. C 2007, 111, 11445. (4) Nakamura, R.; Okamoto, A.; Osawa, H.; Irie, H.; Hashimoto, K. J. Am. Chem. Soc. 2007, 129, 9596. (5) Wu, G.; Nishikawa, T.; Ohtani, B.; Chen, A. Chem. Mater. 2007, 19, 4530. (6) Ohno, T.; Miyamoto, Z.; Nishijima, K.; Kanemitsu, H.; Feng, X. Y. Appl. Catal., A 2006, 302, 62. (7) Nishijima, K.; Fujisawa, Y.; Murakami, N.; Tsubota, T.; Ohno, T. Appl. Catal., B 2008, 84, 584. (8) Morikawa, T.; Irokawa, Y.; Ohwaki, T. Appl. Catal., A 2006, 314, 123. (9) Morikawa, T.; Ohwaki, T.; Suzuki, K.; Moribe, S.; Tero-Kubota, S. Appl. Catal., B 2008, 83, 56. (10) Higashimoto, S.; Tanihata, W.; Nakagawa, Y.; Azuma, M.; Ohue, H.; Sakata, Y. Appl. Catal., A 2008, 340, 98. (11) Ozaki, H.; Iwamoto, S.; Inoue, M. Catal. Lett. 2007, 113, 95. (12) Ozaki, H.; Iwamoto, S.; Inoue, M. J. Phys. Chem. C 2007, 111, 17061. (13) Gao, B. F.; Ma, Y.; Cao, Y.; Yang, W. S.; Yao, J. N. J. Phys. Chem. B 2006, 110, 14391. (14) Abe, R.; Takami, H.; Murakami, N.; Ohtani, B. J. Am. Chem. Soc. 2008, 130, 7780.

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tungsten(V) oxides13 for nitrogen-doped TiO2 photocatalysts, and platinum particles,14 palladium particles,15 copper(II) oxides,16-18 iron(III) ions,19 and tungsten carbides20 for tungsten(VI) oxide photocatalysts have been reported. The surface modifiers play the roles of electron acceptor and/or cocatalyst promoting the reaction of photoexcited electrons with molecular oxygen, resulting in the retardation of the recombination of photogenerated carriers and a great enhancement in the surface reaction rate between positive holes and organic molecules.14,17,19 However, there are a few metal oxide semiconductor photocatalysts (e.g., Bi2WO6 with a band gap of 2.6-2.8 eV) that exhibited relatively high levels of vis-induced photocatalytic activity for the mineralization of gaseous molecules (apparent quantum efficiency under 420 nm irradiation, ∼5%) without any surface modification.21-24 These pristine photocatalysts would make it easy to understand the factors determining the photocatalytic activity of semiconductor photocatalysts. The purpose of this study was to determine the physicochemical properties affecting the photocatalytic activity of pristine metal oxide photocatalysts in order to acquire information for the development of highly active photocatalysts. Information other than that for TiO2-based photocatalysts is very limited. It is (15) Arai, T.; Horiguchi, M.; Yanagida, M.; Gunji, T.; Sugihara, H.; Sayama, K. Chem. Commun. 2008, 5565. (16) Arai, T.; Yanagida, M.; Konishi, Y.; Iwasaki, Y.; Sugihara, H.; Sayama, K. Catal. Commun. 2008, 9, 1254. (17) Arai, T.; Horiguchi, M.; Yanagida, M.; Gunji, T.; Sugihara, H.; Sayama, K. J. Phys. Chem. C 2009, 113, 6602. (18) Irie, H.; Miura, S.; Kamiya, K.; Hashimoto, K. Chem. Phys. Lett. 2008, 457, 202. (19) Arai, T.; Yanagida, M.; Konishi, Y.; Sugihara, H.; Sayama, K. Electrochemistry 2008, 76, 128. (20) Kim, Y. H.; Irie, H.; Hashimoto, K. Appl. Phys. Lett. 2008, 92, 182107. (21) Amano, F.; Yamakata, A.; Nogami, K.; Osawa, M.; Ohtani, B. J. Am. Chem. Soc. 2008, 130, 17650. (22) Amano, F.; Nogami, K.; Ohtani, B. J. Phys. Chem. C 2009, 113, 1536. (23) Wang, D. F.; Kako, T.; Ye, J. H. J. Am. Chem. Soc. 2008, 130, 2724. (24) Wang, D. F.; Kako, T.; Ye, J. H. J. Phys. Chem. C 2009, 113, 3785.

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thought that photocatalytic activity depends on the physical and structural properties of the photocatalyst and that there is a correlation between activity and each property. However, the properties are related to each other. Therefore, in practice, it is difficult to prepare photocatalyst samples that have the same properties except for one. This is understood by considering, for example, that changing the temperature of calcination as a postpreparation process results in different crystalline content as well as specific surface area. Thus, there is no way to determine the relationship between activity and a given property from the results, but the results from trials to solve this problem using statistical analysis have been reported for commercial TiO2 powders.25 We have reported that Bi2WO6 polycrystalline particles exhibited a photocatalytic activity level as high as that of TiO2 for the mineralization of acetic acid in aqueous solution under ultraviolet light (UV) irradiation, probably owing to the high specific surface area and the low density of recombination centers.26,27 The Bi2WO6 particles exhibited a micrometer-sized spherical shape with an assembled structure of a number of flakes, “flake-ball particles”.26,27 The flakes were composed of a number of crystalline platelets. Time-resolved infrared absorption spectroscopy has been recognized as a useful technique for evaluating the photodynamics of TiO2-based photocatalysts.28-30 A recent study on Bi2WO6 photocatalysts has revealed that crystallization by a hydrothermal reaction method enhanced the lifetime of photogenerated carriers and the photocatalytic activity for the decomposition of gaseous acetaldehyde (AcH) whereas amorphous Bi2WO6 exhibited negligible photocatalytic activity because of the fast recombination of photogenerated carriers.21 Crystalline Bi2WO6 induced the complete decomposition of AcH under vis irradiation without any surface modification. The rates of degradation of organic pollutants by heterogeneous photocatalysts have frequently been fitted by a kinetic model assuming that the reaction of surface-adsorbed reactants obeying a Langmuir isotherm is the rate-determining step. Although the adsorption ability of photocatalysts would be related to the surface area and the surface structure, there have been a few works reporting the proportional relation between the specific surface area and the photocatalytic reaction rate for the degradation of organic pollutants.31,32 A systematic work on TiO2 particles prepared by hydrothermal crystallization followed by calcination showed that the photocatalytic activity was almost proportional to the number of surface-adsorbed reactants, which was also proportional to the specific surface area.32 In the study of Bi2WO6 flake-ball particles, we have found that controlling the temperature of the hydrothermal reaction gives flake-ball particles with flakes of different thicknesses with negligible differences in other properties, enabling the extraction of the effect of specific surface area on photocatalytic activity. The characterization and photocatalytic activity of these Bi2WO6 flake-ball particles are (25) Prieto-Mahaney, O. O.; Murakami, N.; Abe, R.; Ohtani, B. Chem. Lett. 2009, 38, 238. (26) Amano, F.; Nogami, K.; Abe, R.; Ohtani, B. Chem. Lett. 2007, 36, 1314. (27) Amano, F.; Nogami, K.; Abe, R.; Ohtani, B. J. Phys. Chem. C 2008, 112, 9320. (28) Yamakata, A.; Ishibashi, T.; Onishi, H. J. Phys. Chem. B 2001, 105, 7258. (29) Yamakata, A.; Ishibashi, T.; Onishi, H. J. Mol. Catal. A: Chem. 2003, 199, 85. (30) Yamakata, A.; Ishibashi, T.; Kato, H.; Kudo, A.; Onishi, H. J. Phys. Chem. B 2003, 107, 14383. (31) Cao, L. X.; Gao, Z.; Suib, S. L.; Obee, T. N.; Hay, S. O.; Freihaut, J. D. J. Catal. 2000, 196, 253. (32) Kominami, H.; Murakami, S.; Kato, J.; Kera, Y.; Ohtani, B. J. Phys. Chem. B 2002, 106, 10501.

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reported here, and the relationship between activity and structural properties is discussed.

2. Experimental Section 2.1. Sample Preparation. The procedure for preparing

Bi2WO6 particles was described elsewhere.26,27 A mixture of bismuth nitrate (5.0 mmol) and an aqueous solution of sodium tungstate (2.75 mmol) was poured into a 100 mL Teflon-lined autoclave to ca. 80% of its capacity, followed by heating in an oven. The atomic ratio of tungsten to bismuth was adjusted to 0.55, which is a 10% excess compared to the stoichiometric amount of Bi2WO6. After 20 h of hydrothermal reaction under autogenerated pressure, the precipitates were collected by centrifugation, washed with water, and dried at 393 K in air. The hydrothermal reaction temperature was changed from 403 to 493 K. Nitrogen-doped TiO2 particles were prepared by holding commercial anatase powders (Ishihara Sangyo Co. ST-01) in a gaseous ammonia stream at 823 K for 3 h.33,34 2.2. Characterization. Scanning electron microscopy (SEM) images were obtained on a JEOL JSM-7400F. Particle size distributions were measured for aqueous suspensions with sodium hexametaphosphate as a dispersant by a laser diffraction/ scattering method using a Shimadzu SALD-7000 analyzer. X-ray diffraction (XRD) patterns were recorded on a Rigaku RINT ULTIMA diffractometer with Cu KR radiation. The fwhm of the XRD peak was estimated after correction of the line broadening by Cu KR2 radiation and emanation due to the optical path in the diffractometer. Raman spectra were recorded on a Perkin-Elmer System 2000R spectrometer with an Nd:YAG laser (1064 nm). The incident laser power was 200-300 mW, and the obtained peak intensities were normalized by the peak area intensity at 988 cm-1 of barium sulfate as an internal standard. Diffuse reflection spectra were recorded using barium sulfate as a standard material on a JASCO V-670 spectrometer equipped with a PIN-757 integrating sphere. The specific surface area was analyzed by the Brunauer-Emmett-Teller method from nitrogen adsorption isotherms at 77 K using a Yuasa Ionics Autosorb-6-AG surface area and pore size analyzer. Samples were evacuated at 393 K before the measurement. AcH adsorption in the dark at room temperature (ca. 298 K) was measured using a glass vacuum line equipped with an MKS Instruments Baratron capacitance manometer. 2.3. Photocatalytic Decomposition of AcH. A photocatalyst powder (50 mg) was spread on a glass plate (15 mm  15 mm) and placed on the bottom of a cylindrical glass vessel with a volume of 330 mL. Gaseous AcH, which is a model reactant of a volatile organic compound with a bad smell, was injected into the vessel filled with ambient air. Unless otherwise mentioned, the AcH concentration was adjusted to ca. 2200 ppm, corresponding to 30 μmol of AcH. After reaching an adsorption equilibrium in the dark at room temperature, photoirradiation was performed using a 300 W xenon arc lamp (ILC Technology CERMAXLX300F) from the top of the glass vessel. A cold mirror was used to decrease the heat from the lamp. In the case of vis irradiation (>400 nm), a cutoff filter (Asahi Techno Glass L42) was used to eliminate UV. The amounts of AcH and CO2 in the gas phase were measured with a gas chromatograph (Agilent Technologies 3000A Micro GC).

3. Results and Discussion 3.1. Morphology and Crystalline Content of Flake-Ball Particles. The morphology of Bi2WO6 particles prepared by the hydrothermal reaction was observed by SEM. It was confirmed that flake-ball particles composed of a number of crystalline flakes were obtained (Figure 1). The particle size of each flake-ball (33) Amano, F.; Abe, R.; Ohtani, B. Trans. Mater. Res. Soc. Jpn. 2008, 33, 173. (34) Irie, H.; Watanabe, Y.; Hashimoto, K. J. Phys. Chem. B 2003, 107, 5483.

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Figure 3. Raman spectra of samples prepared by hydrothermal reaction at (a) 403, (b) 433, (c) 463, and (d) 493 K. “S” symbols indicate peaks of barium sulfate as an internal standard material.

Figure 1. FE-SEM images of flake-ball particles prepared by hydrothermal reaction at (a, b) 403, (c, d) 433, (e, f) 463, and (g, h) 493 K.

Figure 2. Area intensity and fwhm of the Bi2WO6 131 XRD peak of flake-ball particles prepared at several hydrothermal reaction temperatures.

particle was in the range of 3-5 μm (Figure S1 in Supporting Information). These results were consistent with the particle size distributions measured by a laser diffraction/scattering method (Figure S2 in Supporting Information). SEM images at higher magnification indicated that the thickness of the rectangular platelets composing the flakes increased with an elevation of the preparation temperature. Figure S3 in Supporting Information shows XRD patterns of flake-ball particles prepared at 403-493 K. All of the flake-ball particles contained “russellite” Bi2WO6 crystals (JCPDS card no. 39-0256). The peak-height intensity was found to increase along with the preparation temperature possibly because of the increase in the content and size of the crystallites. Figure 2 shows 7176 DOI: 10.1021/la904274c

the peak-area intensity and fwhm of the most intense diffraction peak at 28.8° (131 diffraction peak). The peak area intensity reached saturation at a reaction temperature of 433 K. However, the fwhm of the peak gradually decreased with an elevation of the preparation temperature. It is generally recognized that the area intensity of an XRD peak is in proportion to the mass of the crystalline part, though the intensity depends on the size of the crystalline particle when the particle is larger than several micrometers. If an appropriate standard sample can be used, then the mass ratio of the crystalline part to the sum of crystalline and amorphous parts, that is, the crystalline content, could be estimated from the peak area intensity. The peak area intensities of flake-ball particles prepared at temperatures g433 K were negligibly changed by calcination at 873 K for 2 h in air. Because the sample calcined at a high temperature could be regarded as an appropriate standard sample for high-purity single-phase crystalline Bi2WO6, it is concluded that the crystalline content in the sample prepared at g433 K is almost 100%. This was also supported by Raman spectra (Figure 3). The intensity of the peak at 796 cm-1 in the Raman spectra, assigned to one of the W-O stretches in the Bi2WO6 lattice, increased with an elevation of the preparation temperature to less than 433 K and was negligibly changed for samples prepared at 433-493 K, indicating that the crystalline contents of samples prepared at temperatures g433 K are almost constant. Thus, the samples prepared at hydrothermal reaction temperatures of 433-493 K have different fwhm values of a 131 XRD peak. Figure S4 in Supporting Information shows the crystallite size (i.e., the thickness measured in the direction vertical to the (131) lattice plane in the strict sense, calculated from the fwhm using the Scherrer equation for flake-ball particles). Also plotted is the thickness of platelets determined from SEM images (Figure 1), and these two plots showed similar trends with respect to preparation temperature. Considering that the platelets are parallel to the (010) lattice plane, intersecting the (131) plane at an angle of 54.9°, the ratio of the thickness of the platelets to crystalline size estimated by the Scherrer equation is calculated to be ca. 0.82. The difference between the obtained thicknesses might be related to an underestimation of the crystallite size calculated using the Scherrer equation. From the above-described structural analyses, it is concluded that the flake-ball particles prepared by hydrothermal reaction Langmuir 2010, 26(10), 7174–7180

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Figure 4. Effects of the hydrothermal reaction temperature on (b) the specific surface area of flake-ball particles and (O) the initial rate of CO2 liberation in photocatalytic AcH decomposition.

Figure 5. Time courses of (A) AcH degradation and (B) CO2 liberation in the photocatalytic decomposition of gaseous AcH (ca. 30 μmol) in air under vis irradiation (>400 nm) over flake-ball particles prepared by hydrothermal reaction at (a) 403, (b) 433, (c) 463, and (d) 493 K and (e) nitrogen-doped TiO2 particles. The AcH injected at 0.0 h was immediately decreased by adsorption. In the case of nitrogen-doped TiO2, it took a long time to reach the adsorption equilibrium, probably owing to the large surface area.

at 433-493 K are assemblies of crystalline Bi2WO6 platelets with different thicknesses. The surface area of one platelet with a difference in thickness would not be very different because the difference in the surface area of the sides of platelets is small. The weight of each platelet, however, depends on the thickness; the number of platelets included in the unit weight of a sample is decreased with the elevation of the preparation temperature. As a result, the specific surface area was decreased with the elevation of the temperature as shown in Figure 4. Figure S5 in Supporting Information shows the pore diameter distributions and total pore volumes of flake-ball particles. The pore diameter was drastically increased with an increase in the thickness of platelets. However, the change in the total pore volume was found to be relatively small. 3.2. Photocatalytic Activity of Flake-Ball Particles. The photocatalytic decomposition of gaseous AcH in air was performed to evaluate the activities of flake-ball particles prepared at several hydrothermal reaction temperatures. Figure 5 shows the time courses of AcH degradation and CO2 liberation in the photocatalytic oxidative decomposition of AcH under vis Langmuir 2010, 26(10), 7174–7180

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Figure 6. Relationship between the specific surface area and the initial rate of CO2 liberation in the photocatalytic decomposition of AcH over flake-ball particles prepared by hydrothermal reaction at (a) 403, (b) 413, (c) 423, (d) 433, (e) 463, and (f) 493 K.

irradiation (>400 nm). The complete decomposition of AcH is proven by the molar yield of CO2 (ca. 60 μmol), being twice as large as the molar amount of AcH in the feed (ca. 30 μmol). It was found that all of the Bi2WO6 flake-ball particles prepared at 403-493 K induced the complete decomposition of AcH into CO2, although the irradiation time for complete decomposition was significantly different among samples. If the crystalline content is the sole factor affecting the photocatalytic activity, then the flake-ball particles prepared at 433-493 K should exhibit higher rates of AcH degradation and CO2 liberation than those of a sample prepared at 403 K. However, the highest photocatalytic activity was observed for flake-ball particles prepared at 403 K, which exhibited a CO2 liberation rate much higher than that of nitrogen-doped TiO2 photocatalysts. It was confirmed that nitrogendoped TiO2 could not induce the complete decomposition of AcH under vis irradiation. The yield of CO2 is less than 30 μmol even after 30 h of irradiation probably because of the generation of persistent intermediates such as formic or acetic acid. Indeed, the present nitrogen-doped TiO2 was found to be less active for acetic acid decomposition when UV was eliminated from the light beam.33 Because pure TiO2 (P25, Nippon Aerosil) exhibits no vis absorption as shown in Figure S6 in Supporting Information, the photocatalytic activity was very low under vis irradiation.21 Figure S7 in Supporting Information shows the results under full arc light irradiation (>290 nm). The rates of AcH degradation and CO2 liberation over flake-ball particles were lower than those over P25 under the condition of illumination with UV light. The initial rate of CO2 liberation on a time scale of 5-15 min after the commencement of photoirradiation was determined from the time courses (Figure S8 in Supporting Information). Figure 4 shows the effect of the preparation temperature on the initial rate of CO2 liberation over flake-ball particles. The rate decreased with the elevation of temperature in the range of 403-493 K. Figure 4 also shows specific surface areas of the flake-ball particles. The elevation of the preparation temperature was found to decrease the specific surface area monotonically, as has been observed for the CO2 liberation rate. In general, the photocatalytic efficiency of photocatalysts under the same irradiation conditions depends on the photoabsorption property, the rate of surface reaction of photogenerated carriers with surfaceadsorbed reactants, and the rate of electron-hole recombination.35 Figure 6 shows the relationship between the specific surface area and the photocatalytic activity. It was found that the initial rate of CO2 liberation over flake-ball particles was proportional to their specific surface area, the line passed through (35) Ohtani, B. Chem. Lett. 2008, 37, 217.

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Figure 7. Relationship between the concentration of gaseous AcH after reaching equilibrium adsorption before photoirradiation and the initial rate of photocatalytic AcH degradation over flake-ball particles prepared at 433 K. The curve shows a kinetic equation fitting the experimental data.

the origin of the coordinate axes, and the rates deviated from the proportional relation at a higher specific surface area. Flake-ball particles with a specific surface area of less than 20 m2 g-1 were prepared at 433-493 K. Their crystalline contents were found to be similarly high as shown by XRD measurements. However, flake-ball particles with a high specific surface area were likely to possess an amorphous fraction. These results indicate that the photocatalytic activity was proportional to the specific surface area only in the case of the crystalline content being similar. A similar trend was also confirmed under full arc light irradiation with a xenon lamp (Figure S9 in Supporting Information). It has been proposed that the photocatalytic activity increasing in proportion to the specific surface area of semiconductor photocatalysts indicates that the electron-hole recombination rate is much higher than the surface reaction rate and the recombination rate is presumed to be almost the same for each photocatalyst.22 In the case of the amorphous fraction being small, the electron-hole recombination would be slow, as was suggested by the results of time-resolved infrared absorption spectroscopy showing that the lifetime of photogenerated carriers of amorphous Bi2WO6 could be ignored in the microsecond region and was greatly enhanced after hydrothermal reaction at 403 K, resulting in crystallization.21 Therefore, we can assume that the lifetimes of photogenerated carriers in crystallized flakeball particles prepared at 433-493 K are on a similar level and are sufficient long to promote a surface reaction. Because the light absorption and scattering properties of flake-ball particles were negligibly influenced by the preparation temperature (Figure S6 in Supporting Information), the photocatalytic activity could be assumed to depend on the rate of surface reaction of photogenerated carriers with reactant molecules. It was suggested that the specific surface area is related to the surface reaction rate, resulting in the proportional relation between specific surface area and photocatalytic activity. It should be noted that the photocatalytic activity was evaluated for static powder samples with the same weights under vis irradiation. Particles located near the surface could absorb a greater number of photons than could particles located in the inner part. However, the photoabsorption of flake-ball particles is not as high for wavelengths >400 nm. Therefore, we assume that some photons could penetrate into the inner part. 3.3. Kinetics of Photocatalytic Reaction. The effect of the concentration of AcH on the photocatalyst activity was investigated using flake-ball particles prepared at 433 K. The concentration of AcH in the gas phase of the reaction mixture after the establishment of the adsorption equilibrium was measured in the dark. Figure 7 shows the relation between the equilibrium 7178 DOI: 10.1021/la904274c

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Figure 8. Adsorption isotherms of AcH at room temperature on flake-ball particles prepared by hydrothermal reaction at (a) 403, (b) 433, (c) 463, and (d) 493 K. The curves show Langmuirian equations fitting the experimental data.

concentration of AcH and the initial rate of AcH consumption on a timescale of 0-10 min after the commencement of photoirradiation. As shown in Figure S10 in Supporting Information, two CO2 molecules were not directly liberated from the degradation of one AcH molecule, at least in the initial stage, though a 2-fold larger amount of CO2 was confirmed after prolonged irradiation, suggesting the presence of intermediate species during the photocatalytic reaction. Assuming that the rate of surface reaction is slower than the rates of AcH adsorption and CO2 desorption and that the possible intermediates do not influence AcH adsorption, the rate is first order with respect to the amount of adsorbed AcH. Because the adsorption of AcH on the photocatalyst surface would obey the Langmuirian adsorption isotherm, the rate of AcH degradation could be expressed as follows -

d½AcH K½AcH ¼ kxθ ¼ kx dt 1 þ K½AcH

ð1Þ

where k is the apparent rate constant, x is the maximum amount of AcH adsorption, θ is the surface coverage, and K is the equilibrium constant of Langmuirian adsorption.36 Figure S11 in Supporting Information shows the ratio of the equilibrium concentration of AcH to the rate of AcH decomposition versus the equilibrium concentration. The plots could be fit to a straight line, indicating that the rate obeys eq 1. The value of K was calculated to be 0.035 Pa-1 from the slope and the y intercept. Figure 8 shows AcH adsorption isotherms measured in the dark at room temperature. All of the data for flake-ball particles prepared at different temperatures obey Langmuirian isotherms. The K and x values were calculated from those isotherms. Figure 9 shows the effect of the specific surface area on K and x values for flake-ball particles. The K values were similar among the flake-ball particles. This indicates that the surface property for AcH adsorption is similar among flake-ball particles probably because they exhibited regular platelet shape with largely exposed crystal facets corresponding to the (010) lattice plane of orthorhombic Bi2WO6 crystallites.37,38 The K value of amorphous Bi2WO6, ca. 0.02 Pa-1, is significantly different from those of crystalline flake-ball particles. The K value of flake-ball particles, ca. 0.04 Pa-1, is similar to the value obtained in the photocatalytic decomposition of AcH (0.035 Pa-1). These results support the results of the kinetic analysis of the photocatalytic reaction; the kinetically obtained K value was similar to that estimated from an adsorption isotherm measured in the dark.35,36 The x value was (36) Sopyan, I.; Watanabe, M.; Murasawa, S.; Hashimoto, K.; Fujishima, A. J. Photochem. Photobiol., A 1996, 98, 79. (37) Zhang, C.; Zhu, Y. F. Chem. Mater. 2005, 17, 3537. (38) Zhou, L.; Wang, W. Z.; Zhang, L. S. J. Mol. Catal. A: Chem. 2007, 268, 195.

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Figure 9. Relationship of the specific surface area to (solid symbols) K and (open symbols) x values estimated from Langmuirian adsorption isotherms: (circles) flake-ball particles and (squares) amorphous Bi2WO6 particles.

found to be proportional to the specific surface area of flake-ball particles. This indicates that the amount of AcH adsorbed on flake-ball particles is proportional to the specific surface area regardless of the partial pressure of AcH because their K values were also similar. The slope of the fitting line was found to be 4.65 (μmol m-2), indicating that the area occupied by one molecule of adsorbed AcH is 0.36 nm2. On the basis of the assumption that the average cross-sectional area of adsorbed molecules (a) is the same as that of the corresponding plane of cubic closest packing in the liquefied gas, the following equation is obtained39 pffiffiffi a ¼2 3

!2=3 m pffiffiffi 4 2NA F

ð2Þ

where m is the molecular weight of the gas, NA is Avogadro’s number, and F is the density of the liquefied gas. The value of a for AcH was calculated to be 0.225 nm2 (F at 293 K, 0.783 g mL-1). This indicates that AcH is not adsorbed on the Bi2WO6 surface under closest packing. Kinetic analyses of the photocatalytic reaction revealed that the rate of AcH decomposition over a well-crystallized Bi2WO6 photocatalyst was a surface-reaction-limited process depending on the number of reactants adsorbed on the surface with the Langmuirian isotherm. The reason for the proportional increase in photocatalytic activity along with specific surface area, which was observed for Bi2WO6 flake-ball particles prepared at 433-493 K, was thought to be the proportional increase in the amount of AcH adsorption to the specific surface area. Figure 10 shows the relation between the amount of AcH adsorption and the photocatalytic reaction rate over flake-ball particles. The initial rates for photocatalytic AcH degradation and CO2 liberation were plotted against the adsorption amount calculated from the Langmuirian isotherms assuming a pressure of 220 Pa, which is consistent with the partial pressure of gaseous AcH in air for the photocatalytic reaction. Except for flake-ball particles prepared at 403 K, the crystalline content of which was relatively low, the initial rates were almost proportional to the amount of adsorbed AcH before photoirradiation. This indicates that the specific surface area is a decisive factor in the photocatalytic reaction rate under the present conditions when well-crystallized flake-ball particles are used as photocatalysts. It should be emphasized that the proportional relation between the amount of adsorbed AcH and the activity was not observed for flake-ball particles with a low degree of crystallinity. This deviation from linearity would be dependent on the difference in charge carrier dynamics (the rate of recombination or surface reaction) because the light absorption (39) Emmett, P. H.; Brunauer, S. J. Am. Chem. Soc. 1937, 59, 1937.

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Figure 10. Relationship between amounts of AcH adsorbed at 220 Pa, which was calculated from Langmuirian adsorption isotherms, and photocatalytic reaction rates of flake-ball particles prepared by hydrothermal reaction at (a) 403, (b) 433, (c) 463, and (d) 493 K for the decomposition of gaseous acetaldehyde (2200 ppm) in air: (b) initial rate of CO2 liberation on a timescale of 5-15 min and (O) initial rate of AcH degradation on a timescale of 0-10 min under photoirradiation.

and scattering properties were similar. In the present case, the crystalline content rather than the crystallite size, which is considered to affect the ability of charge carriers to reach the surface, seems to be an important factor because the monotonic increase in crystallite size along with the preparation temperature could not explain the deviation from linearity. These considerations indicate that the photocatalytic activity for the decomposition of gaseous AcH was determined by the crystalline content of bismuth tungstate as well as the specific surface area (e.g., the AcH adsorption ability). Further studies focusing on the effect of crystalline content on the photoexcited carrier dynamics are in progress in our group using time-resolved infrared spectroscopy. The preliminary results suggested that the rate of electron-hole recombination of these samples with high crystalline content was similar and the significant effect of crystallite size on the carrier dynamics was not confirmed.

4. Conclusions Bi2WO6 particles prepared under similar conditions except for reaction temperature in the range of 433-493 K exhibited similar hierarchical architecture, secondary particle size, crystalline shape, exposed crystalline lattice planes, and crystalline content. However, their specific surface areas were different owing to the increase in the thickness of crystalline rectangular platelets with increasing preparation temperature. It was found that the photocatalytic activity for the decomposition of AcH was proportional to the specific surface area when Bi2WO6 flake-ball particles exhibited similar high levels of crystalline content. This proportional relation could be explained by the results showing that the initial rate of AcH decomposition was expressed by first-order kinetics with respect to the amount of surface-adsorbed AcH. The capacity for reactant adsorption is proportional to the specific surface area because the flake-ball particles exhibit exposed Bi2WO6 (010) surfaces, which guarantee similar equilibrium adsorption constants, K, and a cross-sectional adsorption area of an AcH molecule adsorbed on the surface (0.36 nm2). The systematic preparation of hierarchical structure-controlled metal oxide particles enables the elucidation of the effect of only one physical property, which was the specific surface area in this study, on the photocatalytic efficiency of the decomposition of a small organic pollutant in air. Acknowledgment. This work was supported in part by the Global COE Program (project no. B01) from the MEXT and the Project to Create Photocatalyst Industry for Recycling-Oriented DOI: 10.1021/la904274c

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Society supported by NEDO, Japan. F.A. is grateful to the Association for the Progress of New Chemistry, Japan, for financial support. Supporting Information Available: FE-SEM images. Particle size distributions measured by a laser diffraction/ scattering method. XRD patterns. Crystallite size and the

7180 DOI: 10.1021/la904274c

Amano et al.

average thickness of platelets. Pore diameter distributions and total pore volume. Diffuse reflectance spectra. Photocatalytic decomposition of gaseous AcH under full arc irradiation. Relationship between the concentration of AcH and the ratio of the concentration to the initial rate of photocatalytic AcH degradation. This material is available free of charge via the Internet at http://pubs.acs.org.

Langmuir 2010, 26(10), 7174–7180