ARTICLE pubs.acs.org/IECR
Preparation of Visible-Light-Driven Silver Vanadates by a MicrowaveAssisted Hydrothermal Method for the Photodegradation of Volatile Organic Vapors Guan-Ting Pan,† Ming-Hong Lai,‡ Rei-Cheng Juang,§ Tsair-Wang Chung,|| and Thomas C.-K. Yang*,† †
Department of Chemical Engineering and Biotechnology, National Taipei University of Technology, Taipei, Taiwan Department of Disaster Management, Taiwan Police College, Taipei, Taiwan § Green Energy and Environment Research Laboratories, Industrial Technology Research Institute, Hsin-Chu, Taiwan Department of Chemical Engineering/R&D Center for Membrane Technology, Taoyuan, Taiwan
)
‡
ABSTRACT: A variety of visible-light-driven silver vanadates, including R-AgVO3, β-AgVO3, and R-Ag3VO4, were synthesized using a microwave-assisted hydrothermal synthesis method. UV-vis spectroscopy indicated that each of the silver vanadate particles obtained in the study had strong visible-light absorption with associated band gaps in the range of 2.2-2.5 eV. The R-Ag3VO4 crystalline sample with rich hydroxyl functional groups on the surface exhibited the highest degree of photocatalytic activity. The reaction rates of the photodegradation of isopropanol (IPA) and benzene vapors were approximately 8 times higher than those of P25 under visible-light irradiation. Furthermore, the active sites at which catalysts play a role as proton donors (Brønsted acidity) in the photodegradation of VOC were characterized by the temperature-programmed desorption (TPD) method in conjunction with the diffuse reflectance infrared Fourier transform (DRIFT) technique. In addition, the photocatalytic activities of microwaveassisted hydrothermal samples were higher than those of samples produced by conventional hydrothermal techniques. This was due to an increase in the specific surface area and additional hydroxyl functional groups on the surface. These results demonstrate that the microwave-assisted hydrothermal method is an efficient technique for the fabrication of visible-light-responsive silver vanadates with outstanding performance in the photocatalysis of VOCs.
1. INTRODUCTION In recent years, the use of semiconducting compounds as photocatalysts has become increasingly important on providing alternative sources of energy and addressing problems associated with environmental pollution. Among a variety of semiconducting photocatalysts, TiO2 has received a great deal of attention due to its reliable photocatalytic performance and outstanding chemical stability. However, TiO2 is only activated under UV irradiation, because its band gap is in the range of 3.0-3.2 eV. Many methods have been implemented to make TiO2 a visible-lightresponsive photocatalyst, including the doping of metal ions such as Sr, Ag, and Cu.1-4 Nitrogen and hydrogen plasma have been used to enhance the absorption of TiO2 in the range of visiblelight;5,6 however, none of these methods have provided very limited improvements. For this reason, monoclinic and perovskite forms of metal oxides, such as InMO4 (M = V, Nb, Ta),7 BiVO4,8 and Ag3VO49 have been developed for their responsiveness to visible-light. These compounds are commonly prepared through solid-state synthesis under high heat-treatment temperatures (850 °C or higher) with at least 12 h holding time. To reduce energy consumption and preparation costs, an alternate wet chemical process has been widely used to synthesize inorganic materials at lower temperatures. This hydrothermal process is considered a relatively simple and high yield process to manufacture a variety of metal oxides at lower temperatures (less than 200 °C). The as-prepared powders do not normally require high-temperature calcination to improve crystallinity. r 2011 American Chemical Society
In addition, uniform particle distribution can be expected with the hydrothermal method.10-13 A wide range of operating parameters, such as the concentration of precursors, pH, synthesis temperature, and synthesis time, are easily controlled. The use of the hydrothermal method for the preparation of Ag3VO4 was first reported by Hu et al.14 They studied the effects of the ratio of silver to vanadium on the formation of Ag3VO4 and found that Ag3VO4 prepared in excessive vanadium at 160 °C for 48 h exhibited higher visible-light-driven activity than that of samples prepared in a stoichiometric ratio. Huang et al.15 investigated the effects of time on the hydrothermal synthesis of silver vanadates and found that powder synthesized at 140 °C for 4 h exhibited the highest photocatalytic activity. The hydrothermal method produces powders of high purity, good crystalline structure, and uniform particle-size distribution. In addition, other unique properties of powder products can be attained through adjustment of the parameters involved in the hydrothermal process (such as temperature, treatment time, and the composition of solution).16-21 However, the processing time of conventional hydrothermal methods always requires a few hours to a few days.
Received: June 16, 2010 Accepted: December 23, 2010 Revised: December 3, 2010 Published: January 26, 2011 2807
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Recently, the use of microwave irradiation in hydrothermal treatment has been seen as an efficient and practical method to fabricate various metal oxides, such as TiO2,22 CoMoO4,23 Fe2O3,24 and ZrO2.25 As compared to conventional hydrothermal synthesis, the use of microwave power can significantly reduce hydrothermal temperature and time.26,27 In this study, the technique of microwave-assisted hydrothermal method was applied to synthesize various silver vanadate crystals. Isopropanol (IPA) and benzene vapors were selected as two model indoor pollutants. The photocatalytic reactivity of silver vanadates obtained from the microwave-assisted and conventional hydrothermal synthesis was compared to that of commercial TiO2 (P-25). In addition, photodegradation rates were also correlated with morphology, surface hydroxyl groups, and the surface acidity of the samples.
2. EXPERIMENTAL SECTION 2.1. Preparation of Catalysts. Silver nitrate (AgNO3) and ammonium vanadate (NH4VO3) were selected as the silver and vanadium precursors for the fabrication of silver vanadium oxides (SVO). Each well-mixed solution was adjusted to predetermined pH values through the addition of ammonia solution. The samples prepared from at pH values of 6, 6.5, 7, 8, 9, 10, and 11 were denoted as samples a, b, c, d, e, f, and g. The suspended solution was placed in a microwave oven equipped with a 3 GHz microwave generator (CEM Discover). Microwave power was set at 50 W, and the reaction was held at 140 °C, for 60 min. Following the microwave-assisted hydrothermal treatment, an orange slurry was obtained. This slurry was rinsed several times using distilled water to remove NO3- and NH4þ residue. Finally, the samples were dehydrated at 110 °C, for 6 h, prior to further testing. In the conventional hydrothermal method, the suspended solution is held in an autoclave at 140 °C, for 240 min.. For further details regarding preparation procedures, please refer to our previous work.28 The sample prepared by the conventional method with hydrothermal solution of pH = 7 was denoted as sample h, and commercially available TiO2 (Degussa P25) was denoted as sample i. 2.2. Sample Characterization. The crystalline phase of the powders was identified by an X-ray diffraction (PANalytical X’Pert PRO) with Cu radiation (λ = 0.15418 nm) from 25° to 60° (2θ). Surface morphology was examined using a field emission scanning electron microscope (Hitachi S-3000N). UV-vis spectra (300-700 nm) were collected using a spectrophotometer (JASCO V-500) equipped with an integrated sphere assembly. The surface reactivity of the samples was monitored using an FTIR spectrophotometer (PerkinElmer spectrum GX) with a diffuse reflectance infrared Fourier transform accessory (DRIFT) attached. The sample holder inside the DRIFT was equipped with a heating cartridge controlled by a PID controller. The spectra of the sample were recorded between room temperature and 250 °C. The IR spectra were scanned in the region of 4000-1000 cm-1 at the resolution of 4 cm-1. To achieve a fast response and high sensitivity, we used an MCT detector.29 In addition, the spectra were smoothed and presented in the Kubelka-Munk units for quantitative analysis. Prior to each measurement, the sample was degassed inside an airsealed holder at 250 °C for 30 min with a purge of dried N2 (10 mL/min). 2.3. Evaluation of Photocatalytic Activity. Photodegradation of VOCs (isopropanol and benzene) was carried out in a
Figure 1. XRD patterns of as-prepared silver vanadates.
continuous-flow annular photoreactor.30 Photocatalysts in quantities of 0.05 g were evenly distributed within the annular reactor. Various concentrations of the inlet gas stream (20 mL/min) were prepared by bubbling oxygen through IPA and benzene solvents at various temperatures. The photocatalyst was initially purged by dry nitrogen (100 mL/min) for 30 min before introducing the VOCs. After the adsorption was saturated, the catalyst was irradiated with a white fluorescent lamp (TFC, FL10W-EX). The main emission band of the white fluorescent lamp was at 545 nm. In addition, an ultraviolet lamp (PHILIPS, TLD 10 W/08) with the main emission band at 360 nm irradiated the P25 for further comparison. The light intensity was 1.3 mW/cm2 measured by a detector 1 cm from the lamp. The chemical compositions of the outlet and inlet streams were analyzed by a quadrapole mass spectrometer (SRS QMS300). In addition, the DRIFT technique in conjunction with a mass spectrometer was used to monitor the change of adsorbed VOCs as well as gas-phase streams during the photodegradation on the catalyst. Time-dependent IR spectra were recorded both in the dark and under light-irradiation conditions. A fixed amount of gas was periodically withdrawn from the sampling port and analyzed by a mass spectrometer.
3. RESULTS AND DISCUSSION 3.1. XRD and Analysis of Morphology. The XRD patterns of silver vanadate powders prepared by the microwave-assisted hydrothermal method (50W) are shown in Figure 1. The varieties of crystalline phases including R-AgVO3, β-AgVO3, R-Ag3VO4, and Ag2O were observed. For the solution at pH= 6.0, R-AgVO3 was the main product (sample a). As the pH value increased to 6.5, β-AgVO3 became the dominant phase (sample b). For pH values ranging between 7-9 (samples c-e), 2808
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Figure 3. Diffuse reflectance spectra of as-prepared silver vanadates.
Figure 2. SEM micrographs of as-prepared silver vanadates: (a) sample a, (b) sample b, (c) sample c, (d) sample d, (e) sample e, (f) sample f, (g) sample g, and (h) sample h.
the main product was R-Ag3VO4. When the pH value exceeded 10, Ag2O became the main crystalline phase for samples f and g. As a result, the pH value of the hydrothermal solution was found to be a significant factor in the determination of the crystalline properties of the final product. Sample h had mixed phases of Ag4V2O7 and a-Ag3VO4. Additionally, the morphology of the samples was also influenced by pH value of the hydrothermal solution, as summarized in Figure 2a-e. It appears that most of the samples were rodshaped, with diameters ranging between 0.22 and 0.49 μm and lengths between 30 and 180 μm. Only sample f, Ag2O crystallite, showed aggregated particles with a width of a few micrometers, as shown in Figure 2f. Irregular ragged particles with sharp edges and clear grain boundaries were observed in sample h, as shown in Figure 2h. 3.2. UV-Vis Spectra. The UV-vis spectra of the sample powders were obtained with a JASCO V-500 spectrophotometer employing an integrating sphere. The absorption spectra were converted to Kubelka-Munk units for quantitative analysis. Figure 3 shows the spectra of the samples prepared at five different pH values. Each of these samples showed intensive absorption in the region of visible-light. Furthermore, the spectral patterns of the R-Ag3VO4 samples prepared at pH = 7.0, 8.0, 9.0 were similar to samples produced by conventional methods.
Figure 4. Time-dependent concentrations of reactants (IPA and benzene), main intermediates, and final products of the photocatalytic process with sample c.
The onset wavelength was 550 nm, and the band gap was determined to be 2.2 eV.31 The spectra of sample a (R-AgVO3) 2809
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Table 1. Summary of Surface Area, Photocatalytic Activities, and Photon Efficiency of the Samples apparent rate constant (min-1) surface area (m2/g)
sample ID
IPA
benzene
photon efficiency (%) IPA
benzene
sample a (pH = 6.0, R-AgVO3)
3
0.32
0.24
0.74
0.43
sample b (pH = 6.5, β-AgVO3)
13
0.49
0.30
1.13
0.54 1.17
sample c (pH = 7.0, R-Ag3VO4)
20
0.76
0.65
1.75
sample d (pH = 8.0, R-Ag3VO4)
19
0.47
0.45
1.08
0.81
sample e (pH = 9.0, R-Ag3VO4)
17
0.47
0.42
1.08
0.76
sample h (conventional hydrothermal)
2
0.138
0.123
0.07
0.05
sample i (P-25, excited by visible-light)
49
0.09
0.08
0.21
0.14
sample i (P-25, excited by UV-light)
49
0.35
0.16
0.81
0.29
and sample b (β-AgVO3) had a steeper absorption curve in the visible-light region. On the basis of the onset wavelength, the band gaps of samples a and b were found to be 2.5 and 2.3 eV, which was consistent with the values reported by Konta et al.32 3.3. Photocatalytic Reactivity. The time-dependent concentration of the gaseous composition was monitored by a quadrapole mass spectrometer (SRS QMS300) at the inlet and outlet of the photoreactor. The results for sample c are shown in Figure 4a and b, where acetone was the main intermediate compound for the photodegradation of IPA and phenol was the main intermediate for the benzene system. In both cases, noticeable quantities of CO2 were observed after 25 min of the photodegradation reaction. In addition, the mass balance of carbon atoms was examined for both cases. As irradiation time increased, IPA (=mass of C/3) decreased rapidly, and acetone (=mass of C/3) began to increase until the formation of CO2 (=mass of C/1). The time-dependent mass balance ratio of carbon atoms between inlet and outlet was close to 1, during irradiation. In the case of the benzene photodegradation, the concentration of benzene (=mass of C/6) began to drop, and phenol (=mass of C/6) was noticed as the main intermediate. A large quantity of CO2 (=mass of C/1) formed at 25 min. Similarly, the mass balance ratio of carbon atoms between the inlet and outlet was close to 1 during irradiation. In the analysis of photodegradation kinetics, the LangmuirHinshelwood model assumes that the rate of surface reaction (R) is proportional to the surface coverage (θ) and to the intrinsic rate constant (k). R¼
-dc kKC ¼ kθ ¼ dt 1þKC
ð1Þ
where C is the concentration of the reactant, and K (μM-1) is the equilibrium adsorption constant. For the case of 1 . KC, the Langmuir-Hinshelwood kinetic model was simplified to follow the pseudo-first-order rate equation: R ¼ kapp C
ð2Þ
where kapp (μM min -1) is the apparent rate constant. The values of kapp obtained from the analysis of reactantdegradation data points are summarized in Table 1. The apparent rate constant (kapp) of the R-AgVO3 and β-AgVO3 crystallites was lower than that of the R-Ag3VO4 compounds. This tendency indicates that the R-Ag3VO4 crystalline was the most favorable phase to photodegrade VOCs. One possible reason for this is the fact that the photogenerated holes from R-Ag3VO4 crystallite have a faster migration rate toward the reactive sites than those of
Figure 5. DRIFT spectra of (a) IPA and (b) benzene degraded on 50 W 140 °C 1 h samples c during the 60 min adsorption under dark condition.
R-AgVO3 or β-AgVO3 under visible-light illumination.32 In comparison with the degradation rates of two pollutants, the values of the apparent rate constant (kapp) on IPA were higher than those of benzene for each of the samples. Because IPA is a hydrophilic molecule with a dipole moment of 1.7 D, and benzene is considered a nonpolar molecule, the photocatalysts appeared to have a stronger affinity toward the hydrophilic molecules. According to the Langmuir-Hinshelwood kinetic model, photocatalysts undergoing faster adsorption with reactants have the higher photocatalytic efficiency. 2810
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Table 2. Intensity of OH Peaks of the IR Spectrum (Region from 3000 to 3700 cm-1) for the Samples Preheated at 250 °C isolated-OH H-bonded O-H OH group chemically adsorbed at 3200 cm-1
no. sample a
stretching vibration at 3425 cm-1
H2Oad ν(OH) triply coordinated at 3634 cm-1
ν(OH)/ 3550 cm-1
total area
67.90
612.18
63.83
84.15
828.06
103.51
765.46
110.28
285.27
1264.52
29 194.43
408.77
471.25
1682.94
31 757.40
9644.76
5149.44
1109.80
3229.44
19 133.44
6520.31
2794.38
611.13
3406.18
13 332.00
7.74
84.98
1.12
5.89
99.73
(pH = 6.0, R-AgVO3) sample b (pH = 6.5, β-AgVO3) sample c (pH = 7.0, R-Ag3VO4) sample d (pH = 8.0, R-Ag3VO4) sample e (pH = 9.0, R-Ag3VO4) sample h (conventional hydrothermal)
The photon efficiency (ζ) for each photocatalyst was calculated using the following equation:33-35 ζ¼
-dc=dt 100 Po
where Po was denoted as the photon flux of the lamp, and dC/dt was the reaction rate. In this study, the photon flux was measured as 3.98 10-10 einstein/s 3 m3, and the reaction rate was calculated from the slope of the conversion curves after the first 5 min of the reaction. Table 1 shows that sample i (P25) had a higher photocatalytic activity under UV irradiation as compared to P25 illuminated by visible-light. With regard to the photocatalysis of IPA, the quantum efficiency of P25 under UV-light irradiation (ξ = 0.81) was 4 times higher than that of P25 under visible-light irradiation (ξ = 0.21), because its band gap was 3.0-3.2 eV. Similar results were also observed in the case of benzene photocatalysis, where the quantum efficiency of P25 under UV-light irradiation (ξ = 0.29) was 2 times higher than that of P25 under visible-light irradiation (ξ = 0.14). However, sample c still exhibited the highest reactivity as well as the highest quantum efficiency under visible-light irradiation. These results confirm that the silver vanadate samples were more active under visible-light irradiation, and their photocatalytic rate was even higher than that of P25 under illumination of UV-light. Furthermore, R-Ag3VO4 photocatalyst prepared via microwave-assisted hydrothermal treatment inherited higher quantum efficiency over photodegradation of VOCs than those prepared by conventional hydrothermal methods.28 To investigate the adsorption process and intrinsic surface reaction of the photocatalyst, the DRIFT technique was adopted to monitor changes in adsorbed VOCs on the photocatalysts, in dark conditions and under visible-light illumination. Figure 5a shows three peaks in the low wavenumber region (1500-1200 cm-1) assigned to be the δ(CH) mode of IPA. In addition, a particularly intensive band at 2978 and a weak band at 2888 cm-1 were observed, corresponding to the stretching ν(CH) mode of methyl groups of IPA. The band at 3660 cm-1 was assigned as the hydrogen-bonded hydroxyl group for IPA or hydroxyl groups from adsorbed water. The intensity of C-H stretching and C-H
bending bands reached a steady level at 30 min, indicating adsorption equilibrium of IPA with sample c. In the case of benzene adsorption, a number of significant bands were visible, including the ν4 parallel band at 684 cm-1 and three perpendicular bands ν12 at 3048 cm-1, ν13 at 1484 cm-1, ν14 at 1038 cm-1, as shown in Figure 5b.36 In the case of the photocatalytic reaction, the intensity of the ν(CH) mode of methyl groups from IPA (peak at 2978 cm-1) began decreasing after 3 min of irradiation, as shown in Figure 5a. Similarly, the intensity of the δ(CH) mode of benzene (peak at 1484 cm-1) decreased, as illustrated in Figure 5b. Prior to the following set of DRIFT measurements, the samples were all preheated to 250 °C to remove adsorbed water. The broad peaks in the range of 3700-3000 cm-1 may have been due to the overlapping of various OH groups. The deconvolution of these peaks was conducted according to peak assignments with Gaussian curves as the basis of the peak. The peaks located at 3200 and 3694 cm-1 corresponded to the OH groups chemically adsorbed on the surface of the catalyst. The peaks at 3425, 3634, 3663, and 3687 cm-1 were, respectively, ascribed to hydrogenbonded O-H stretching vibration, triple coordination, ν(OH) of bridged hydroxyls with Brønsted acidity, and linear ν(OH). The peak at 3550 cm-1 was assigned to the OH stretching vibration for molecularly adsorbed water, because it coexisted with the OH bending vibration at 1636 cm-1. The intensity of individual OH peaks and their total areas are summarized in Table 2. Figure 6a and b illustrates the correlation between the intensity of OH groups on each sample and the photodegradation rates of IPA and benzene, respectively. The values of the apparent rate constant for IPA were consistent with the intensity of OH groups in the order of samples c > sample d > sample e > sample b > sample a. The same tendency was also observed in the case of benzene, indicating that surface hydroxyl groups of the as-prepared samples are a significant factor influencing photocatalysis. 3.4. Verification of Brønsted Acid Sites. To further identify the acid-base properties occurring on the surface of the catalyst, a temperature-programmed desorption method, in conjunction with the purge of the probe molecule (ammonia gas), was implemented using DRIFT technique. The spectroscopic results 2811
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Figure 7. DRIFT spectra of NH3 species adsorbed on silver vanadates at 250 °C.
Figure 6. Relationship between the intensity of OH groups and the photodegradation rate constants of (a) IPA and (b) benzene.
helped to determine the interaction of ammonia with metal ions. According to the literature, the bands at 1425 and 1670 cm-1 assigned to the asymmetric and symmetric deformation modes of ammonium ions (NH4þ) were correlated with ammonia protonation on Brønsted acid sites.37,38 In addition, the band at 1605 cm-1 was assigned to the asymmetric deformation mode of ammonia species adsorbed on Lewis sites.38 Therefore, significant band vibrations at 3230 and 1425 cm-1 were observable when ammonia gas was adsorbed on the Brønsted sites of silver vanadates. If the ammonia gas is adsorbed on the Lewis sites, the unpaired electron of nitrogen atom will make the band adsorbed at 1650 cm-1. In the TPD experiments, the dehydrated sample was purged by a gas mixture (5% NH3 in N2) at a flow rate of 30 mL/min for 30 min, and DRIFT spectra were recorded every 10 min. At the end of the saturation process, the samples were flushed with N2 flow (30 mL/min) and heated to 250 °C. Each preset temperature was held for 30 min for the collection of DRIFT spectra. Figure 7 shows in situ DRIFTS spectra of NH3 species adsorbed on as-prepared silver vanadates at 250 °C. Intensive bands at 1425 and 1625 cm-1 were observed on the surface of sample c, indicating that the largest amount of NH3 adsorption occurred on sample c. On the other hand, sample a showed the least NH3 adsorption on its Brønsted acid sites. As a result, sample c inherited the richest Brønsted acid sites and Lewis sites, verifying that the intensity of Brønsted acidity and Lewis sites of the catalysts was responsible for the catalytic activity. Apparently, Bronsted acid sites played a dominant role in the photocatalytic activities in the sample studied. Besides, Niwa et al.39 and Wang et al.40 reported that the Brønsted acid
sites take the form of surface hydroxyl groups. These hydroxyl groups act as the traps of the holes and generate reactive hydroxyl radicals, which are able to oxidize the adsorbed substrates. Furthermore, the highly polarized state of strong Brønsted acid sites restricts undesirable electron-hole recombination due to more effective hole-trapping procedure.40 Additionally, a series of papers41-43 also reported that both the Br€onsted and the Lewis acidity sites of the catalysts can work as catalytic centers for the adsorption on the metal oxide surface. In this work, RAg3VO4 crystals have the strongest intensity in hydroxyl functional groups, which were beneficial to the adsorption kinetics of adsorbates on the catalyst surface. Together with its higher quantum efficiency, the R-Ag3VO4 compound was verified to inherit the highest photocatalytic activity among all the silver vanadates compounds.
4. CONCLUSIONS Visible-light active silver vanadate photocatalysts were synthesized using a direct and simple microwave hydrothermal process. The crystalline phase and surface hydroxyl groups of as-prepared samples contributed to the photocatalytic activities of IPA and benzene degradation. The crystalline structure of silver vanadates could be fine-tuned by adjusting hydrothermal time, temperature, and pH values of the bath solution. The microwave-assisted hydrothermal method provided a quick, efficient process for synthesizing silver vanadate compounds, which inherit higher photocatalytic activities than P25. In our results, R-Ag3VO4 crystals with an abundance of surface hydroxyl groups proved to be a significant factor in the enhancement of photocatalytic activity. In addition, the Brønsted acid sites of the as-prepared photocatalysts were responsible for the degradation of VOC. In comparison with conventional hydrothermal methods, the preparation of Ag3VO4 by means of microwave-assisted hydrothermal treatment reduced the time required for synthesis and lowered energy costs. It also provided samples with increased surface area and enhanced quantum efficiency. ’ AUTHOR INFORMATION Corresponding Author
*Tel.: 886-2-2771-2171#2533. Fax: þ886-2-27760985. E-mail:
[email protected]. 2812
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’ ACKNOWLEDGMENT Partial financial support from the National Science Council of ROC (NSC 99-2221-E-027-078-MY2) is gratefully acknowledged. We would like to thank Mu-Jung Chen for his assistance in the sample preparation. ’ REFERENCES (1) Kato, H.; Kudo, A. Visible-light-response and photocatalytic activities of TiO2 and SrTiO3 photocatalysts codoped with antimony and chromium. J. Phys. Chem. B 2002, 106, 5029. (2) Ishii, T.; Kato, H.; Kudo, A. H2 evolution from an aqueous methanol solution on SrTiO3 photocatalysts codoped with chromium and tantalum ions under visible light irradiation. J. Photochem. Photobiol., A 2004, 163, 181. (3) Koh, J. K.; Seo, J. A.; Koh, J. H.; Kim, J. H. Templated synthesis of Ag loaded TiO2 nanostructures using amphiphilic polyelectrolyte. Mater. Lett. 2009, 63, 1360. (4) Colon, C.; Maicu, M.; Hidalgo, M. C.; Navio, J. A. Cu-doped TiO2 systems with improved photocatalytic activity. Appl. Catal., B 2006, 67, 41. (5) Irie, H.; Watanable, Y.; Hashimoto, K. Nitrogen-concentration dependence on photocatalytic activity of TiO2-xNx powders. J. Phys. Chem. B 2003, 107, 5483. (6) Liu, H.; Ma, H. T.; Li, X. Z.; Li, W. Z.; Wu, M.; Bao, X. H. The enhancement of TiO2 photocatalytic activity by hydrogen thermal treatment. Chemosphere 2003, 50, 39. (7) Ye, J.; Zou, Z.; Arakawa, H.; Oshikiri, M.; Shimoda, M.; Matsushita, A.; Shishido, T. Correlation of crystal and electronic structures with photophysical properties of water splitting photocatalysts InMO4 (M=V5þ,Nb5þ,Ta5þ). J. Photochem. Photobiol., A 2002, 148, 79. (8) Zhang, X.; Ai, Z.; Jia, F.; Zhang, L.; Fan, X.; Zou, Z. Selective synthesis and visible-light photocatalytic activities of BiVO4 with different crystalline phases. Mater. Chem. Phys. 2008, 103, 162. (9) Konta, R.; Kato, H.; Kobayashi, H.; Kudo, A. Photophysical properties and photocatalytic activities under visible light irradiation of silver vanadates. Phys. Chem. Chem. Phys. 2003, 5, 3061. (10) Wang, J.; Yin, S.; Sato, T. Characterization and evaluation of fibrous SrTiO3 prepared by hydrothermal process for the destruction of NO. J. Photochem. Photobiol., A 2007, 187, 72. (11) Ge, L. Novel Pd/BiVO4 composite photocatalysts for efficient degradation of methyl orange under visible light irradiation. Mater. Chem. Phys. 2008, 107, 465. (12) Witte, K. D.; Busuioc, A. M.; Meynen, V.; Mertens, M.; Bilba, N.; Tendeloo, G. V.; Cool, P.; Vansant, E. F. Influence of the synthesis parameters of TiO2-SBA-15 materials on the adsorption and photodegradation of rhodamine-6G. Microporous Mesoporous Mater. 2008, 110, 100. (13) Li, Y. C. M.; Huang, C. M.; Wang, S. C.; Wang, J. X. Hydrothermal synthesis and thermal decomposing method for synthesizing alumina nanorods. J. Inorg. Mater. 2008, 23, 121. (14) Hu, X.; Hu, C.; Qu, J. Preparation and visible-light activity of silver vanadate for the degradation of pollutants. J. Hazard. Mater. 2008, 151, 17. (15) Huang, C. M.; Pan, G. T.; Li, Y. C. M.; Li, M. H.; Yang, T. C. K. Crystalline phases and photocatalytic activities of hydrothermal-synthesis Ag3VO4 and Ag4V2O7 under visible light irradiation. Appl. Catal., A 2009, 358, 164. (16) Wei, F.; Ni, L.; Cui, P. Preparation and characterization of N-S-codoped TiO2 photocatalyst and its photocatalytic activity. J. Hazard. Mater. 2008, 156, 135. (17) Rajamathi, M.; Seshadri, R. Oxide and chalcogenide nanoparticles from hydrothermal/solvothermal reactions. Curr. Opin. Solid State Mater. 2002, 6, 337. (18) Yao, W. T.; Yu, S. H. Recent advances in hydrothermal syntheses of low dimensional nanoarchitectures. Int. J. Nanotechnol. 2007, 4, 129.
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