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Efficient Visible Light Photocatalytic Oxidation of NO on Aerosol Flow-Synthesized Nanocrystalline InVO4 Hollow Microspheres Zhihui Ai,†,‡ Lizhi Zhang,*,† and Shuncheng Lee*,‡ Key Laboratory of Pesticide & Chemical Biology of Ministry of Education, College of Chemistry, Central China Normal UniVersity, Wuhan 430079, People’s Republic of China, and Department of CiVil and Structural Engineering, Research Center for EnVironmental Technology and Management, The Hong Kong Polytechnic UniVersity, Hong Kong, People’s Republic of China ReceiVed: July 24, 2010; ReVised Manuscript ReceiVed: September 12, 2010
In this study, aerosol flow-synthesized (AFS) nanocrystalline InVO4 hollow microspheres (AFS-InVO4) were used to oxidize gaseous NO at indoor air level under visible light and compared with hydrothermally synthesized InVO4 counterpart powder. Results revealed that the AFS-InVO4 hollow spheres exhibited higher photocatalytic activity than the hydrothermally synthesized counterpart. The photocatalytic activity enhancement could be attributed to the large surface area and special hollow structures, which were favorable for the diffusion of intermediates and the deactivation inhibition of photocatalyst during the photocatalytic reaction. Fourier transform infrared spectroscopy results confirmed the generation of nitric acid on the AFS-InVO4 surface during the photocatalysis of NO in the gas phase, suggesting that the oxidation of NO molecules was the major process in this photocatalytic reaction. Multiple runs of the photocatalytic NO removal revealed that the AFS-InVO4 hollow spheres were very stable during photocatalysis. This study presents a promising approach for scaling up industrial production of InVO4 hollow spheres with improved photocatalytic activity for indoor air purification. Introduction Recently, nanostructured materials have emerged as the new building blocks for the construction of light energy harvesting assemblies.1,2 Remarkable progress has been made on the fabrication of nanostructures with well-defined 2D and 3D assemblies, which exhibit improved selectivity and efficiency toward light energy conversion.3-5 Among these nanostructures, inorganic hollow nanostructures with tailored properties have attracted considerable attention because of their potential applications in the fields of catalysis, drug delivery, protection of environmentally sensitive biological molecules, and chemical reactors.2,6 Various hollow spheres, including elements, metal sulfides, and metal oxides, have been prepared via hard templates or soft templates routes.2,5-8 However, the high cost, low yield, and complexity that accompany the introduction of templates have limited the practical application of these approaches. Therefore, it is still a major challenge to develop a facile and template-free route for the mass preparation of inorganic hollow nanostructures. Aerosol flow synthetic (AFS) method is a convenient way for the facile mass production of nanostructured materials because of its simple and inexpensive apparatus and easy control of product composition.8 Recently, carbon,9 TiO2,10 SiO2,11 and AgI12 etc. have been synthesized by this method. Although AFS method has been widely used to produce inorganic powders, it is still a challenge to fabricate hollow structures with AFS method. Nowadays, more and more attention has been paid to indoor air quality with increasing awareness of the public environment * To whom correspondence should be addressed. E-mail: zhanglz@ mail.ccnu.edu.cn;
[email protected]. Phone/Fax: +86-27-67867535. † Central China Normal University. ‡ The Hong Kong Polytechnic University.
and health, especially in urban cities.13,14 It was reported that TiO2 immobilized on different substrates, such as activated carbon and glass fibers, can photocatalytically degrade indoor air pollutants at parts per billion levels in a flow system under UV light irradiation.15-17 However, the relatively wide band gap of TiO2 limits its further application in the visible light region, which accounts for 43% of the incoming solar energy.18,19 In view of the better utilization of solar light and indoor illumination, it is appealing to develop visible light-sensitive photocatalysts active enough for practical application.20 Under such background, non-titania-based photocatalysts, such as BiOX,21 ZnWO4,22 BaBiO3,23 Bi5FeTi3O5,24 NiGa2O4,5 Sr2Sb2O7,4 LiBi4M3O14 (M ) Nb, Ta),25 HNb3O8,26 BiFeO3,27 CdIn2S4,28 Ta2O5,29 and (Ag0.75Sr0.25)(Nb0.75Ti0.25)O3,3 have been recently synthesized and utilized for environmental cleaning under visible light irradiation. InVO4 has been found to be an interesting visible lightresponding photocatalyst.30 It is usually prepared by the solidstate or melting reactions, and thus is inhomogeneous and has large particle size and low surface area. Many other approaches, including sol-gel method and hydrothermal processes, had been developed to prepare nanostructured InVO4.31-33 As expected, these approaches could lead to crystalline products with pronounced photocatalytic performance.31 Despite these advances, the diversity of desired nanostructures for InVO4 still needs to be greatly expanded to meet the ever-increasing nanotechnology and environmental remediation demand. It was demonstrated that InVO4 could produce H2 or degrade organic pollutants in water under the visible light irradiation. However, we still lack the information on its potential application for air purification. In the present study, we report for the first time a facile aerosol flow synthesis of nanocrystalline InVO4 hollow microspheres without the use of any template or organic surfactant.
10.1021/jp106906s 2010 American Chemical Society Published on Web 10/11/2010
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Furthermore, we test the photocatalytic activity and stability of these aerosol flow-synthesized InVO4 (AFS-InVO4) hollow spheres on degradation of NO at indoor air level (400 ppb) under visible light irradiation and compared them with hydrothermally synthesized InVO4 (HTS-InVO4) counterpart. Experimental Section Synthesis. Indium chloride tetrahydrate (InCl3 · 4H2O) and ammonium metavanadate (NH4VO3) obtained from China National Medicines Corporation Ltd. were used as the precursors. All the chemicals were of analytical grade and used as received without further purification. Deionized water was used throughout the experiments. InVO4 hollow microspheres were directly aerosol flow synthesized from a mixed aqueous solution containing InCl3 · 4H2O and NH4VO3. In a typical synthesis, 2 mmol of NH4VO3 was dissolved in 50 mL of acidic aqueous solution at 80 °C to form a transparent solution (the pH of the solution was adjusted to about 5 with 1 M nitrate acid solution) and then cooled to room temperature. After the addition of 50 mL of a solution containing 2 mmol of InCl3 · 4H2O into the above NH4VO3 solution under stirring, the resulting stable and yellow solution was nebulized using an ultrasonic nebulizer at 1.7 MHz ( 10% (Yuyue402AI, Shanghai, China) and carried by air flow with a suction pump through a quartz tube surrounded by a furnace thermostatted at different temperatures (500-800 °C). The reaction proceeded quickly as droplets passed through the high-temperature quartz tube with the diameter of 3.5 cm and the length of 0.8 m. The products were collected in a percolator with distilled water, then filtered by a fritted glass funnel, washed thoroughly with distilled water and ethanol, and finally dried in an oven at 50 °C. The yield of the AFS-InVO4 powders is about 65-70%. The representation of the typical AFS apparatus was showed in Figure S1 (Supporting Information). For comparison to our AFS samples, InVO4 counterpart powder was prepared using a hydrothermal method (Supporting Information). Characterizations. X-ray diffraction (XRD) patterns were obtained on a Philips Xpert System X-ray diffractometer with Cu Ka radiation (λ ) 1.54178 Å). Scanning electron microscopy (SEM) images were performed on a JEOL 6490 scanning electron microscope. Transmission electron microscopy (TEM) and selected area electron diffraction (SAED) patterns were performed on a JEOL JEM-2010 electron microscope operating at 200 kV. The samples for TEM were prepared by dispersing the final powders in ethanol; the dispersion was then dropped on carbon-copper grids. Furthermore, the obtained powders deposited on a copper grid were observed by a high-resolution transmission electron microscope. Fourier transform infrared (FTIR) spectra were recorded with a Perkin-Elmer System 2000 FTIR spectrometer with the standard KBr pellet method at room temperature. UV-vis diffuse reflectance spectra were recorded at room temperature with the Cary 300 UV-visible spectrophotometer equipped with an integrated sphere. X-ray photoemission spectroscopy (XPS) was recorded on a PHI 5600 multitechnique system with a monochromatic Al KR source (Physical Electronics) operated at 150 W (15 kV, 10 mA). The nitrogen adsorption and desorption isotherms at 77 K were measured using a Micrometritics ASAP2010 system after samples were vacuum-dried at 473 K overnight. Photocatalytic Experiments. The photocatalytic activity experiments on the resulting samples for oxidation of NO in air were performed at ambient temperature in a continuous flow reactor. The volume of the rectangular reactor, which was made of stainless steel and covered with Saint-Glass, was 4.5 L (10
Figure 1. XRD patterns of the InVO4 samples aerosol flow synthesized at different temperatures: (a) 500 °C; (b) 600 °C; (c) 700 °C; and (d) 800 °C.
× 30 × 15 cm (H × L × W)). One dish containing the 0.2 g of photocatalyst powders was placed in the middle of the reactor. A 300 W commercial tungsten halogen lamp (General Electric) was used as the simulated solar light source. The lamp was vertically placed outside the reactor above the sample dish and a glass filter was placed to remove light below 420 nm. Four minifans were fixed around the lamp to prevent a rise in temperature of the flow system. The photocatalyst samples were prepared by coating an aqueous suspension of our sample onto to a dish with a diameter of 12 cm. The dishes containing the photocatalyst were pretreated at 60 °C until complete removal of water in the suspension and then cooled to room temperature. NO gas was selected as the target pollutant for the photocatalytic degradation at ambient temperature. The NO gas was acquired from a compressed gas cylinder at a concentration of 48 ppm NO (N2 balance, BOC gas) with traceable National Institute of Standards and Technology (NIST) standard. The initial concentration of NO was diluted to about 400 ppb by the air stream supplied by a zero air generator (Thermo Environmental Inc. model 111). The desired humidity level of the NO flow was controlled at 70% (2100 ppmv) by passing the zero air streams through a humidification chamber. The gas streams were premixed completely by a gas blender, and the flow rate was controlled at 4 L min-1 by a mass flow controller. After the adsorption-desorption equilibrium among water vapor, gases, and photocatalysts was achieved, the lamp was turned on. The concentration of NO was continuously measured by a chemiluminescence NO analyzer (Thermo Environmental Instruments Inc. model 42c), which monitors NO and NO2 with a sampling rate of 0.7 L/min. The removal rate (%) of NO was one minus the ratio of the NO concentration in the outlet to the initial concentration of NO in the feeding stream. The reaction of NO with air was ignorable when performing a control experiment with or without light in the absence of photocatalyst. Results and Discussion XRD Patterns. The powder XRD pattern provides crystallinity and phase structures information for the samples obtained by AFS method at different temperatures (Figure 1). The peaks of the sample prepared at 500 °C could be indexed to a pure monoclinic phase (JCPDF 38-1135) of InVO4, while the sample obtained at 600 °C consisted of mixed monoclinic (JCPDF 38-
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Figure 2. SEM images of the as-obtained InVO4 obtained at 500 °C (a), 600 °C (b), 700 °C (c), and 800 °C (d), and EDX pattern of the samples obtained at 700 °C (e).
1135) and orthorhombic phases (JCPDF 48-898) of InVO4. A pure orthorhombic phase (JCPDS 48-0898) was achieved when the synthesis temperature was equivalent to or higher than 700 °C. The intensity of the XRD peaks was found to increase with increasing temperature. This implied that the high temperature led to better crystallization. No crystalline peaks from impurities such as In2O3, V2O5, and In(OH)3 were observed. The hydrothermally synthesized counterpart could be indexed to a pure orthorhombic phase (JCPDS 48-0898) of InVO4 (Figure S2, Supporting Information). SEM Images and EDX Analysis. The morphology of the resulting InVO4 samples obtained at different temperatures was investigated by SEM (Figure 2a-d). The SEM images show that all the AFS-InVO4 samples consist of polydispersive microspheres with diameters ranging from 0.5 to 3 µm. As presented in Figure S3 (Supporting Information), the HTS-
InVO4 counterpart consists of particles roughly ranging from 0.5 to 1.5 µm. Furthermore, energy-dispersive X-ray (EDX) spectroscopy analysis was utilized to determine the chemical composition of the AFS samples. EDX analysis (Figure 2e) reveals that the AFS samples are composed of indium, vanadium, and oxygen elements. The average atomic ratio of In/V/O is about 1.07:1:4.12 within the instrumental accuracy, suggesting the AFS samples are pure InVO4. XPS Spectra. The surface element compositions of the AFSInVO4 spheres synthesized at 700 °C were studied by XPS (Figure 3). The binding energies obtained in the XPS analysis were corrected for specimen charging by referencing the C 1s line to 284.5 eV. The survey XPS spectrum (Figure 3a) reveals that the AFS hollow spheres are composed of In, V, O, and adventitious C elements, which is consistent with the EDX result. The carbon peak is attributed to adventitious hydrocarbon
Photocatalytic Oxidation of NO on AFS-InVO4
Figure 3. XPS spectra of the InVO4 hollow spheres obtained at 700 °C, (a) survey of the sample, (b) In 3d, and (c) O 1s.
on the surface of samples. The high-resolution XPS spectra of In 3d regions were also investigated (Figure 3b). The two strongest peaks in the high-resolution XPS spectra at 452.1 and 445.2 eV are assigned to In 3d3/2 and In 3d5/2, respectively. They are characteristic of In3+ in InVO4.34 The binding energy of In 3d5/2 in the obtained InVO4 is higher than that of elemental In, and similar to that of In(OH)3, suggesting that the indium atoms in the AFS samples are positively charged by the formation of direct bonds with oxygen. Meanwhile, the high-resolution of XPS O 1s spectra are also recorded (Figure 3c). The broad peak
J. Phys. Chem. C, Vol. 114, No. 43, 2010 18597 of O 1s can be fitted by two peaks at binding energies of 530.0 and 531.5 eV. The dominant peak at 530.0 eV is characteristic of oxygen in InVO4, and the other peak at around 531.5 eV suggests the presence of other components such as hydroxide groups, H2O, and carbonate species adsorbed on the surface. TEM and High-Resolution TEM Images. TEM images provide further insight into the microstructure and morphology of the AFS products (Figure 4). As shown in the lowmagnification bright-field TEM images of the AFS samples obtained at different temperatures (Figure 4a,c,e,g, the AFSInVO4 spheres possess diameters of about 0.5-3 µm and wall thicknesses of several hundred nanometers. The strong contrast between the dark edge and the bright center is a powerful evidence for the hollow nature. The high-magnification TEM images show that the AFS-InVO4 hollow microspheres obtained at different temperatures are composed of nanoparticles of about several nanometers in size (Figure 4b,d,f,h). The ED patterns on different spheres or different positions of an individual hollow sphere were essentially identical, indicative of polycrystalline nature (Figure 4i). Detailed structural analysis of the AFS-InVO4 hollow microspheres obtained at 700 °C was performed by highresolution TEM (HRTEM). Figure 4j shows HRTEM image of the edge part in an individual InVO4 hollow sphere. As shown in Figure 4j, the shell layer is compact and the diameters of the nanoparticles in the shell are in the range of 5-10 nm. Although these nanoparticles are tightly agglomerated, their grain boundaries are clearly distinguishable, confirming the polycrystalline nature of the InVO4 hollow spheres. According to the TEM observations, we think the formation of nanocrystalline InVO4 hollow microspheres is different from that of TiO2 during the AFS process.35 In general, when the solute concentration at the center of the droplet is higher or equal to the equilibrium saturation of the solute at the temperature of the droplet, precipitation would occur throughout the droplet; i.e., volume precipitation occurs. If the solute concentration at the center of the droplet is lower than the equilibrium saturation of the solute at the temperature of the droplet, precipitation may take place only at the droplet surface; i.e., surface precipitation occurs.36 Owing to the relatively low solubility of InVO4, surface precipitation theory may govern the formation of AFS-InVO4 hollow spheres. When the liquid droplets are sprayed into a tubular reactor under pyrolysis conditions, each precursor droplet would serve as a microreactor whose temperature varies as it travels along the high-temperature furnace tube. As the solvent in the precursor solution evaporates from the droplet surface, the concentration of InVO4 would increase at the surface of the droplet, resulting in nucleation and growth of InVO4 nanocrystals on the liquid droplet surface exposed to the air and then inward the entire droplet. During the nucleation and growth of InVO4 nanocrystals, water would rapidly evaporate and/or other gases would evolve at the same time, eventually producing nanostructured InVO4 hollow spheres. UV-Vis Spectra. We studied the optical absorption of the AFS-InVO4 to estimate their energy band gaps. As presented in Figure 5, the UV-vis spectra of all the AFS-InVO4 hollow spheres show a broad absorbance in the visible region (Figure 5a). This permits the InVO4 hollow spheres to respond to a wide range of solar spectrum and utilize visible light for photocatalysis. In addition, the absorbance increases with the increase of temperatures, suggesting that the temperatures could affect the crystal and electronic structures of final products. The energy band gaps of AFS-InVO4 samples could be estimated from the tangent line in the plot of the square root of Kubelka-Munk functions against photon energy (Figure 5b). The tangent line,
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Figure 5. (a) UV-vis diffuse reflectance absorption spectra of the as-prepared nanocrystalline InVO4 hollow spheres obtained at different temperatures and (b) the plots of (Rhυ)1/2 vs photo energy (hυ).
Figure 4. TEM images of the AFS-InVO4 hollow spheres obtained at different temperatures, respectively: (a) and (b) 500 °C, (c) and (d) 600 °C, (e) and (f) 700 °C, (g) and (h) 800 °C. (i) ED pattern of AFSInVO4 microspheres obtained at 700 °C; (j) high-resolution TEM image of AFS-InVO4 microspheres obtained at 700 °C.
which is extrapolated to (Rhυ)1/2 ) 0, indicates the band gaps of the AFS-InVO4 hollow spheres obtained at 500, 600, 700, and 800 °C are about 1.43, 1.39, 1.69, and 1.59 eV, respectively.
Visible Light-Induced Photocatalytic Removal of NO. The AFS-InVO4 hollow spheres were used to photocatalytically oxidize NO to demonstrate their potential indoor air purification application (Figure 6). Figure 6a shows the variation of NO concentration (C/C0) with irradiation time over the AFS-InVO4 hollow spheres. Here, C0 is the initial concentration of NO, and C is the concentration of NOx after photocatalytic degradation for t. As a comparison, direct photolysis of NO, and photocatalytic oxidation of NO on the HTS-InVO4 counterpart, were also performed under identical conditions. As shown in Figure 6a, NO could not be photolyzed under visible light irradiation (λ > 420 nm), whereas the photodegradation of NO on the HTSInVO4 powders was 13%. Interestingly, all the AFS-InVO4 hollow spheres showed higher photocatalytic activity under visible light and their NO removal efficiencies were 17%, 15.5%, 25%, and 19.3% for the AFS-InVO4 hollow spheres synthesized at 500, 600, 700, and 800 °C, respectively. Obviously, the photocatalytic activity of monoclinic InVO4 prepared at 500 °C and mixed phase of monoclinic and orthorhombic InVO4 prepared at 600 °C were lower than those of orthorhombic InVO4 powder prepared at 700 or 800 °C. This may be attributed to crystalline phase-dependent photocatalytic activity of InVO4 and their different band gap values. It is well-
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Figure 7. Stability of nanocryatalline InVO4 hollow microspheres aerosol flow synthesized at 700 °C after multiple runs of photodegradation of NO.
Figure 6. (a) Plots of the visible light-induced decrease in NOx concentration vs irradiation time (λ > 420 nm) in the presence of different InVO4 and the photolysis of NO; (b) dependence of ln(C/C0) on irradiation time under visible light in the presence of different InVO4.
known that the crystalline phase of the photocatalysts is one of the most important factors influencing their photocatalytic activity. For instance, anatase TiO2 with band gap of 3.2 eV exhibits much better photocatalytic activity than rutile TiO2 with band gap of 3.0 eV for the degradation of organic pollutants under UV irradiation in most cases. The higher photocatalytic activity of orthorhombic InVO4 hollow spheres aerosol flow synthesized at 700 °C compared to that of the sample with the same crystal phase synthesized at 800 °C could be attributed to its larger band gap (1.69 eV) because it is commonly accepted that a larger band gap corresponds to a more powerful redox ability. For a clearly quantitative comparison, we used the Langmuir-Hinshelwood model (L-H) to describe the rates of photocatalytic oxidation of NO on different InVO4. The initial photocatalytic degradation of NO was found to follow mass transfer-controlled first-order kinetics approximately as a result of low-concentration target pollutants, as evidenced by the linear plot of ln(C/C0) versus photocatalytic reaction time t. The initial rate constants of the NO degradation over the AFS-InVO4 hollow spheres obtained at 500, 600, 700, and 800 °C under visible light irradiation were estimated to be 0.026, 0.029, 0.038, and 0.032 min-1, respectively. These constants are significantly higher than that (0.011 min-1) over the HTS-InVO4 counterpart
(Figure 6b). Such an activity enhancement of hollow spheres should first be attributed to their large specific surface area. The specific surface area of hollow spheres obtained at 700 °C calculated by the BET method was 25.1 m2 · g-1, whereas the specific surface area of the hydrothermally synthesized counterpart was merely 5.5 m2 · g-1 (Figure S4, Supporting Information). It is reasonable that a larger surface area corresponds to more active surface sites for photocatalytic oxidation reaction. Meanwhile, the hollow structure could also significantly promote the photocatalytic activity by allowing multiple reflection of visible light within the internal cavity and the facile diffusion of reactants and intermediates as well as products.37 The stability of a photocatalyst is important for its practical application. The N-doped TiO2 and sulfide photocatalysts often suffer from instability under repeated use for air purification applications.20 This is because the intermediates generated by photocatalysis can accumulate on the surface of the photocatalyst, which possibly deactivates the photocatalyst during the gasphase photocatalytic process.38 In contrast, these reaction intermediates could be removed from the photocatalyst surface during aqueous-phase photocatalytic reaction, which can alleviate the deactivation of photocatalyst to some degree. In the case of AFS-InVO4 hollow spheres, as demonstrated by XRD spectra after the reaction with NO (Figure S5, Supporting Information), the crystal structure of the photocatalyst was unchanged even after the prolonged irradiation (24 h). To test the stability of AFS-synthesized InVO4 hollow spheres for NO photocatalytic oxidation, we repeated the photocatalytic oxidation experiments with the same photocatalyst eight times. As shown in Figure 7, after eight recycles of photocatalytic oxidation of NO, the AFS-InVO4 hollow spheres catalyst did not exhibit any significant loss of activity, confirming that AFSsynthesized InVO4 hollow spheres are promising for air purification application. FT-IR Spectra. Figure 8 represents the FT-IR spectra of the 700 °C aerosol flow-synthesized and used InVO4 hollow spheres. The broadband at 3430 cm-1 is believed to be associated with the stretching vibrations of hydrogen-bonded surface water molecules and hydroxyl groups. Additionally, the bands at 1630 cm-1 correspond to the existence of large numbers of residual hydroxyl groups, which imply the O-H vibrating mode of traces of adsorbed water. The band located at 750 and 490 cm-1 can
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Ai et al. of the HTS-InVO4 counterpart materials; N2 adsorption-desorption isotherms of AFS-InVO4 hollow spheres and HTS-InVO4 counterpart powders; XRD of the used AFS-InVO4 hollow spheres. These materials are available free of charge via the Internet at http://pubs.acs.org. References and Notes
Figure 8. FT-IR spectra of the 700 °C aerosol flow-synthesized and used InVO4 hollow microspheres.
be ascribed to the V-O and In-O vibrations of InVO4 hollow spheres, respectively. The bands at 1380, 970, 892, and 750 cm-1 are ascribed to the stretching vibrations of NO3-. Comparing the two spectra, we found that the stretching vibrations of NO3- appear only in the IR spectra of used InVO4 catalysts, confirming the photocatalytic oxidation of NO.38 Furthermore, the bands at 3430 and 1630 cm-1 of the used InVO4 becomes stronger, suggesting more surface hydroxyl groups exist after the accumulation of NO3-. These results agree well with other reports.39 No organic groups were found to be adsorbed on the surface on the basis of IR spectra. Conclusions In summary, we have demonstrated a facile and rapid AFS method to synthesize nanocrystalline InVO4 hollow microspheres with visible light-driven photocatalytic activity for the first time. In comparison with the hydrothermally synthesized InVO4 counterpart powders, the AFS-synthesized nanocrystalline InVO4 hollow microspheres showed improved performance in photocatalytic degradation of NO. Furthermore, its activity and crystal structure did not change after long-term photocatalytic reactions. This work provides a new route for the design of nanostructured multicomponent single-phase photocatalysts with improved photocatalytic activity. Acknowledgment. This work was supported by the National Science Foundation of China (Grants 20777026, 20977039, 21073069, and 91023010), Program for Distinguished Young Scientist of Hubei Province (Grant 2009CDA014), Program for Innovation Team of Hubei Province (Grant 2009CDA048), SelfDetermine Research Funds of CCNU from the Colleges’ Basic Research and Operation of MOE (Grants CCNU09A02014 and CCNU09C01009), and National Basic Research Program of China (973 Program) (Grant 2007CB613301). This work was also partially supported by the Hong Kong Polytechnic University (GYX0L). Supporting Information Available: The representation of the typical AFS apparatus; the hydrothermal preparation of InVO4 counterpart materials; XRD patterns and SEM images
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