Synthesis, characterization and photocatalytic application of H3PW12O40/BiVO4 composite photocatalyst
ZHANG Jin1,2, LI Chuang1, WANG Bing1, CUI Hao1, ZHAI JianPing1 & LI Qin1*
1 State Key Laboratory of Pollution Control and Resource Reuse, School of the Environment, Nanjing University, Nanjing 210023, China 2 School of Biochemical and Environmental Engineering, Nanjing Xiaozhuang University, Nanjing 211171, China
Received January 9, 2013; accepted April 12, 2013
A series of H3PW12O40/BiVO4 composite with different H3PW12O40 loadings were prepared using a hydrothermal and impreg-nation method. The prepared composites were characterized by XRD, Raman, SEM, XPS, and DRS techniques. The bandgap of the composite was narrower compared with the as-prepared pure BiVO4. As a novel photocatalytic material, the photocata-lytic performance of the H3PW12O40/BiVO4 composite was investigated by the degradation of methylene blue (MB) dye solu-tion under visible light irradiation and compared with that of pure BiVO4. The results revealed that the introduction of H3PW12O40 could improve the photocatalytic performance and different concentrations of H3PW12O40 resulted in different photocatalytic activities. The highest activity was obtained by the sample with a loading HPW concentration of 10 wt%. The reason for the enhanced photocatalytic activities of H3PW12O40/BiVO4 samples was also discussed in this paper. Moreover, the H3PW12O40/BiVO4 composites retained the catalytic activity after four repeated experiments.
Organic dyes are one of the larger groups of pollutants in wastewaters produced from textile and other industrial pro-cesses. These wastewaters are environmentally harmful and difficult to be degraded by conventional chemical methods. Due to its high mineralization efficiency, short processing cycle and low toxigenicity, heterogeneous photocatalysis has received much attention for the purification of dye-containing wastewater. Among the photocatalysts used, titanium dioxide (TiO2) in the anatase phase is the most widely used one. However, TiO2 can work only under UV light (occupying around 4% of the solar energy), which re-stricts its practical application in wastewater treatment. In order to utilize of solar light as efficiently as possible, look-ing for and preparing different types of visible- light-responsive photocatalysts has become a research
hotspot [1, 2]. In the past few decades, a large number of undoped, sin-
gle-phase semiconductor photocatalysts with particular ab-sorption abilities in the visible light range have been devel-oped, such as BiVO4 [3–5], Bi2Mo3O12 [6], and Bi2TiO4F [7]. Among these photocatalysts, BiVO4 is a promising candi-date for its strong photocatalytic effect on water splitting and organic pollutant decomposing under visible light irra-diation [8]. However, the photocatalytic activity of pure BiVO4 is limited due to the fast recombination rates of pho-togenerated electron-hole pairs. It was reported that loading metals such as Eu [9], Cu [10], Mo and Ag [11] on the sur-face of the photocatalyst could suppress the recombination of photogenerated electrons and holes at the photocatalyst or cocatalyst interfaces. When the metal ions are loaded on the BiVO4 particles, the metals or metallic oxides particles can act as electrontraps, making the photogenerated elec-trons migrate from BiVO4 to the metals. Accordingly, the electron-hole recombination is effectively suppressed. Therefore, more holes would be preserved, reach the surface
doi: 10.1007/s11426-013-4889-6
2 Zhang J, et al. Sci China Chem January (2013) Vol.56 No.1
of the BiVO4, and thus enhance the degradation efficiency of target organic compounds [12, 13]. In addition, Long et al. suggested using composite semiconducting particles by loading Co3O4 onto BiVO4 for phenol degradation. For Co3O4-BiVO4 photocatalyst, a p-n-type heterojunction is formed and the recombination of electron-hole pairs is sup-pressed [14].
In recent years, heteropolyacids (HPAs), particularly Keggin-type heteropoly acids, such as H3PW12O40 (HPW), have received much attention as reagents or catalysts for redox processes involving organic substrates, since they are well-known to be good electron acceptors which can store several electrons per molecule [15–17]. Therefore, they have been considered as mediators to control dynamics of photoinduced electron transfer from conduction bands of semiconductors to other substrates nearby [18]. However, there are two main drawbacks impeding their potential ap-plications in wastewater treatment: (1) the high solubility in solution of HPAs makes them difficult to be reused; (2) HPAs system is inactive under visible light irradiation, which strongly limits the use of solar light. Thus, to develop a heterogeneous photocatalytic system by immobilizing POM in supports such as carbon, silica, titania, zirconia and mixed oxides has attracted considerable attention. In this context, we propose loading HPAs on the surface of BiVO4 to design novel composite photocatalysts.
In this paper, we discuss the hydrothermal and impregna-tion method of BiVO4-HPA photocatalytic system. The main purposes for designing these materials are: (1) to im-prove the photocatalytic performance of pure BiVO4 under visible light irradiation using the HPW as electron capture agents; (2) to make separation and recovery of the HPAs from the reaction environments easy; (3) Because HPAs share very similar photochemical characteristics with semi-conductor photocatalysts, the combination of these two ma-terials has much higher photocatalytic activity than their pure forms due to the synergistic effect [19–21].
The synthesis, characterization, and catalytic activity of HPW/BiVO4 composites were studied. Then, methylene blue (MB) dye was chosen to test the photocatalytic activity of the HPW/BiVO4 composites under visible light irradia-tion. The reason for the enhanced photocatalytic activities of HPW/BiVO4 composites was also discussed.
2 Experimental
2.1 Synthesis of HPW loaded BiVO4
All chemicals were of analytical grade and used without further treatment. In a typical preparation process, 0.01 mol Bi(NO3)3∙5H2O was dissolved in 20 mL of 32.5% (w/w) HNO3 and stirred for 30 min at room temperature, a trans-parent solution A was formed. Meanwhile, 0.01 mol NH4VO3 was dispersed in 20 mL of 5 mol/L NaOH solution under ultrasonic waves for 30 min to obtain the solution B.
By mixing solution A and solution B, a stable mixture was obtained (stirred for 60 min). The pH of the mixture was adjusted to 7.0 by concentrated ammonia, and the mixture was put under ultrasonic waves for another 10 min. After that, the mixture was sealed in a 50 mL Teflon-lined stain-less autoclave and heated to 195 °C for 6 h. Then, the sys-tem was cooled to ambient temperature. The final product was filtered and washed with distilled water for three times, and dried in vacuum at room temperature for 10 h [22].
The loaded catalysts were prepared using an impregna-tion method. Typically, a specified amount of HPW (0~20 wt%) was dissolved in 20 mL ethanol aqueous solution with ethanol and water (v:v = 1:1), then 1 g of the as-prepared BiVO4 powder was suspended in the above solution. The suspension was stirred using a glass rod during evaporation of the solvent on a water bath at 80 °C. Then the dried powder was collected and heat-treated at 200 °C for 3 h. Finally, the powder was washed three times with distilled water, and dried in an oven at room temperature for 10 h. The corresponding samples were designated as x% HPW/BiVO4 (where x% represents the weight percentage of HPW corresponding to neat BiVO4).
2.2 Characterization
The crystal structure of the composite was investigated by XRD (ARL, Switzerland) in the region of 2 = 10°–65° using Cu Kα radiation ( = 0.15418 nm). Raman spectra were recorded using a HR800 microprobe (JY, France) with an excitation wavelength of 514.5 nm from an Ar ion laser. The surface morphology was investigated by an S-3400NII (Hitachi, Japan) scanning electronspectroscope (SEM). The transmission electron micrographs (TEM) of samples were obtained using a JEM-200CX microscope (JEOL, Japan). X-ray photoelectron spectra (XPS) were measured on a PHI5000 VersaProbe spectrometer (ULVAC-PHI, Japan) with an Al-Kα X-ray source (1486.6 eV). UV-vis diffuse reflectance spectra were recorded on a UV-2450PC spec-trometer (Shimadzu, Japan) with barium sulfate as the ref-erence sample.
2.3 Photocatalytic degradation of MB
Photocatalytic activities of the samples were determined by the decolorization of MB under visible light irradiation. A 500 W Xe-illuminator served as a light source through UV cut-off filters to completely remove any radiation below 420 nm. Experiments were carried out at ambient temperature as follows: the same amount (0.03 g) of photocatalyst was added into 50 mL of a 10 mg/L MB solution. Before illu-mination, the solution was stirred for 30 min in darkness in order to reach the adsorption-desorption equilibrium for MB. At appropriate intervals, about 4 mL suspension was col-lected, and then centrifugalized to remove the photocatalyst particles. The concentrations of the remnant MB were mon-
Zhang J, et al. Sci China Chem January (2013) Vol.56 No.1 3
itored by checking the absorbance of solutions at 664 nm using UV-2550 spectroscopy (Shimadzu, Japan).
Photolysis experiments were performed in the absence of the photocatalyst, using the same experimental setup previ-ously described for the photocatalytic system [23].
3 Results and discussion
3.1 XRD patterns
The XRD patterns of the HPW, pure BiVO4, and HPW-loaded BiVO4 are presented in Figure 1. Characteris-tic peaks at 15.2°, 18.7°, 28.6°, 30.5°, 35.0°, 46.1°, and so on are observed for pure BiVO4 diffraction patterns, and indexed to monoclinic BiVO4 (JCPDS No. 14-0688). After HPW loading, the HPW/BiVO4 composites maintained a monoclinic structure regardless of HPW content, indicating that HPW loading does not have an effect on BiVO4 struc-ture [24]. From the XRD patterns it is also found that the diffractions originating from the Keggin unit are hardly ob-served for the as-prepared composites, which may be due to the well dispersion of the HPW [19].
3.2 Raman spectra
Raman spectroscopy may provide structural information and is also a sensitive method for the investigation of the crystallization, local structure, and electronic properties of materials. Figure 2 shows the Raman spectra of pure BiVO4 and HPW/BiVO4 samples. The spectra are dominated by an intense band near 820 cm1, assigned to the symmetric V–O stretching mode. The peaks at 366, 326 cm1 are assigned to the symmetric VO4
3 deformation and the asymmetric VO4
3 deformation, respectively. The peaks at 152, 220 cm1 are the external vibrations mode [25]. The intensity of the peaks decreased after loading with HPW. This is be-cause Raman spectroscopy can only survey the surface structure. When a small amount of HPW was added to the precursor, hetero particles were dispersed at the surface of
Figure 1 XRD patterns of pure HPW (a), pure BiVO4 (b), 5% HPW/BiVO4 (c) and 10% HPW/BiVO4 (d) samples.
Figure 2 Raman spectra of as-prepared pure BiVO4, 5% HPW/BiVO4 and 10% HPW/BiVO4 samples.
larger BiVO4 particles thereby decreasing the intensity of Raman light emission at the BiVO4 substrate [26]. On the contrary, the values of the full width at half maximum (FWHM) increase with increasing HPW content. It is well known that Raman widths are more sensitive to the degree of crystallinity, defects and structural disorders. Generally speaking, the more disordered the structure is, the higher catalytic activity it would have [27, 28].
3.3 SEM and TEM micrographs
The representative microstructures of the as-prepared sam-ples are shown in Figure 3. Figures 3(a) and 3(b) are the SEM images of pure BiVO4 and 10 wt% HPW/BiVO4 sam-ples. It can be seen that the pure and HPW-loaded BiVO4 samples are slightly agglomerated into irregular shape, and the particle size of the HPW/BiVO4 is a litter smaller than that of pure BiVO4 prepared under the same condition. Fig-ures 3(c) and 3(d) give the TEM images of pure BiVO4 and 10% HPW/BiVO4 samples, respectively. Small hetero parti-cles are deposited as isolated islands or form a thin layer on the surface of large BiVO4 particles, and the dispersion of the composite is better than that of pure BiVO4. Figure 3(e) further reveals that HPW was successfully loaded on the BiVO4 during the impregnation process, and Bi, V, O, P and W elements can be identified from the EDS image of 10% HPW/BiVO4 composite.
3.4 XPS analysis
Figure 4 shows Bi4f, V2p, P2p and W4f high-resolution XPS spectra of the as-fabricated 10% HPW/BiVO4 sample. As shown in Figure 4(a), the sample exhibits the spin-orbit split-ting signals of Bi4f7/2 and Bi4f5/2 at BE = 158.4 and 163.7 eV, respectively, which are characteristic of Bi3+ [29]. The V2p region is displayed in Figure 4(b) with the characteristic peaks at 516.1 and 529.9 eV ascribed to the V2p3/2 and V2p1/2,
4 Zhang J, et al. Sci China Chem January (2013) Vol.56 No.1
Figure 3 SEM images of pure BiVO4 (a) and 10 wt% HPW/BiVO4 (b); TEM images of pure BiVO4 (c) and 10 wt% HPW/BiVO4 (d); EDS spec-trum of HPW/BiVO4 (e) samples.
respectively. Furthermore, P2p spectra (Figure 4(c)) exhibits a peak at 133.2 eV, which is shifted to lower binding energy by 2.8 eV in comparison with bulk HPW (136.0 eV), im-
plying the formation of bonding between HPW and BiVO4 [30]. Two peaks at 35.0 and 37.1 eV are assigned to W4f7/2 and W4f5/2, respectively. The energy position of this doublet corresponds to the W6+ oxidation state [2].
3.5 DRS analysis
The optical absorption of the as-prepared samples was measured by a UV-vis spectrometer. Figure 5 presents the UV-vis diffuse reflectance spectra of pure HPW, pure Bi-VO4 and 10% HPW/BiVO4 samples. In comparison to pure HPW, the optical absorption is enhanced significantly in the region of 400–530 nm (in visible light region) for pure BiVO4
and 10% HPW/BiVO4 samples. The band gap absorption edge of pure BiVO4 is determined to be 535 nm, corre-sponding to the band gap energy (Eg) of 2.32 eV by the equation Eg = 1239.6/g. After HPW was loaded, the ability of light absorption was enhanced for BiVO4 samples. Moreover, the onset of the spectrum for 10% HPW/BiVO4 appears about 10 nm red shift compared with that of pure BiVO4, and the value of Eg is 2.27 eV for 10% HPW/BiVO4 sample. The observed weak red shift over HPW/BiVO4 is caused by the charge-transfer transition between the HPW and the BiVO4 conduction or valance band [24, 31–32].
In addition, the conduction band (CB) edge of a semi-conductor at the point of zero charge can be calculated by Equation (1):
Zhang J, et al. Sci China Chem January (2013) Vol.56 No.1 5
Figure 5 UV-vis diffuse reflectance spectra of pure HPW (a), pure Bi-VO4 (b) and 10% HPW/BiVO4 (c) samples.
where X is the absolute electronegativity of the semicon-ductor, expressed as the geometric mean of the absolute electronegativity of the constituent atoms, which is defined as the arithmetic mean of the atomic electron affinity and the first ionization energy, and for BiVO4, the value of X is 6.035; Ec is the energy of free electrons on the hydrogen scale (ca. 4.5 eV); Eg is the band gap of the semiconductor [14]. According to the values of Eg estimated and Equation (1), the calculated conduction band edge (ECB
0) and valence band (EVB
0) at the point of zero charge for the as-prepared pure BiVO4 sample are 0.375 and 2.695 eV, respectively. For 10% HPW/BiVO4 sample, ECB
0 and EVB0 are 0.40 and
2.670 eV, respectively. These data demonstrate that the electronic structures of BiVO4 were slightly changed due to the introduction of Keggin-type HPW [33].
3.6 Photocatalytic activity
The photocatalytic performance of HPW/BiVO4 composites was evaluated by measuring the degradation of MB from an aqueous solution under visible light irradiation. Temporal changes in the concentration of MB were monitored by examining the variations in maximal absorption in UV-vis spectra at 664 nm, and the results of MB degradation in the presence of different samples are shown in Figure 6(a). It can be seen that all the HPW/BiVO4 composites show better photocatalytic activities than the pure BiVO4. The photo-degradation rates of MB reached 93% after irradiation for 360 min in the presence of the 10% HPW/BiVO4 samples, while the photodegradation rate of MB over pure BiVO4 was 59% after irradiation for 360 min under the same con-ditions. The suitable content of HPW on BiVO4 was about 10 wt% from the experimental results, and it shows less photocatalytic activities with higher HPW content, which is because the excess HPW may cover the active sites on the BiVO4 surface or may act as a recombination center and thus reduce the efficiency of charge separation [9, 34]. The neat photodegradation rate of MB in the absence of any
Figure 6 Photocatalytic degradation of MB dye solution using pure Bi-VO4 and HPW/BiVO4 samples as photocatalyst: (a) conversion in terms of the ratio of remaining MB concentration (C) to initial concentration (C0); (b) linear relationship of ln(C0/C) versus time for degradation of MB.
photocatalyst was about 10% after 6 h, which is far below that of the photocatalytic process.
The adsorption kinetics for the MB dye onto pure BiVO4/ HPW and BiVO4 composites are presented in Figure S1 (Supporting Information). This figure shows that the amount of MB absorbed onto the photocatalyst from aque-ous solution increases quickly with time, and equilibrium is established within 30 min for all different photocatalysts. Alt-hough the amount of MB adsorbed onto HPW/BiVO4 com-posites is higher than that of pure BiVO4 sample at the ad-sorption equilibrium, the amount of MB dye adsorbed on the surface of the HPW/BiVO4 is not more than 1.5 mg/g, which is very low. Therefore, the adsorption of H3PW12O40 (HPW) is not the main reason for the enhanced photocatalytic activity of HPW/BiVO4 composites.
In addition, the 20 wt% HPW/BiVO4 sample has a high degree of aggregation (Figure S2, Supporting Information), which could be the cause of decrease of its adsorption prop-erties.
It was demonstrated that the photocatalytic degradation of MB follows Langmuir-Hinshelwood first-order reaction kinetics behavior [35], and can be expressed as Equation (2):
6 Zhang J, et al. Sci China Chem January (2013) Vol.56 No.1
0
1ln
Ck
t C (2)
where k is the apparent reaction rate constant (min1), C0 and C are the MB concentration when reaction time is 0 and t (min), respectively. The linear relationship of ln(C0/C) versus time for degradation of MB using different samples is shown in Figure 6(b). Also, the rate constants and linear regression coefficients from Figure 6(b) are listed in Table 1. The apparent reaction rate constant (k) was calculated to be 0.0023, 0.0054, 0.0066, 0.0061 and 0.0045 min1 corre-sponding to pure BiVO4, 5% HPW/BiVO4, 10% HPW/ BiVO4, 15% HPW/BiVO4 and 20% HPW/BiVO4, respec-tively. The k of HPW loaded BiVO4 samples is enhanced compared with that of pure BiVO4, which means MB deg-radation rate will be higher. Thus the catalysts’ photocata-lytic activity has been improved by loading HPW [36]. A possible mechanism of photocatalytic activity enhancement is proposed as illustrated in Figure 7.
The electron(e)-hole(h+) pairs are generated on the Bi-VO4 surface under visible-light excitation (Equation (3)), then photogenerated electrons are instantly trapped into unoccupied W5d states of the Keggin Unit by HPW, and HPW is reduced producing the heteropoly blue (HPWred) (Equation (4)). The reduced HPW (HPWred) is known to be moderate reducing agent, so that it can sensitize the photo-chemical reduction of dioxygen (Equation (5)) to yield su-peroxides (∙O2
) [16, 19]. The suggested sequence of reac-
Figure 7 A proposed visible light photodegradation mechanism of or-ganic compounds on HPW/BiVO4 photocatalyst.
Table 1 Rate constants of MB photodecomposition and linear regression coefficients from a plot of ln(C0/C) = kt with different samples
Photocatalysts Regression equation R2 k (min1)
pure BiVO4 y = 0.0895 + 0.0023x 0.9602 0.0023
5% HPW/BiVO4 y = 0.09118 + 0.00544x 0.9923 0.0054
10% HPW/BiVO4 y = 0.08354 + 0.0066x 0.9899 0.0066
15% HPW/BiVO4 y = 0.1180 + 0.0061x 0.9918 0.0061
20% HPW/BiVO4 y = 0.12504 + 0.00453x 0.9685 0.0045
tions portrays the HPW as an electron carrier between the photocatalyst particle and dioxygen, which results in the retardation of the recombination of the photoexcited h+-e pairs on BiVO4. Accordingly, the enhanced quantum effi-ciency can be obtained compared with pure BiVO4. Simul-taneously, the resulting holes can react with the adsorbed water and OH derived from H2O and form hydroxyl radical (∙OH) (Equation (6) and Equation (7)). The generated ∙O2
would further interact with H2O adsorbed to produce more ∙OH radicals (Equation (8)). Finally, these active species oxidize the organic compounds (MB dye) (Equation (9–11)) [37, 38]. BiVO BiVO e4 4 ( )h h (3)
ox redBiVO e HPW BiVO HPW4 4( ) (4)
red oxHPW O HPW O2 2 (5)
2BiVO H O BiVO OH H4 4( )h (6)
BiVO OH BiVO OH4 4( )h (7)
O H O OH OH2 2 (8)
oxBiVO Org BiVO Org4 4( )h (9)
oxO Org Org2 (10)
OH Org MB photogradation products( ) (11)
3.7 Stability of catalyst
The reproducibility of the catalytic activity of the photo-catalyst was also studied, and 10% HPW/BiVO4 was chosen to evaluate the reproducibility of the composite catalyst. After the first catalytic test, the catalyst was removed from aqueous solution, and then it was washed with distilled wa-ter and dried at room temperature. Figure 8 shows results from the four successive runs for the photodegradation of MB under the same experimental conditions. No significant loss of activity was found in four successive runs and the removal of MB remains about 90% in each cycle, testifying
Figure 8 Effect of number of recycling cycles on the degradation rate and conditions: photocatalyst of 0.6 g/L; MB of 10 mg/L and irradiation time of 360 min.
Zhang J, et al. Sci China Chem January (2013) Vol.56 No.1 7
that the photocorrosion of BiVO4/FACs was negligible [39]. HPW loading is 9.6% after four times’ catalytic cycles through ICP-AES analysis, which shows the as-prepared catalyst was stable during the photodegradation of MB.
4 Conclusions
Novel HPW/BiVO4 photocatalysts have been prepared by a hydrothermal and impregnation method. After loading HPW, the BiVO4 particles retain monoclinic scheelite structures and the ability of visible light absorption is en-hanced. The photocatalytic activities of the HPW-loaded samples are higher than that of pure BiVO4 and the highest photodegradation efficiency of MB is obtained at 10 wt% HPW content. The enhanced photocatalytic performance under visible light irradiation is attributable to the efficient separation of photogenerated electron-hole pairs in the HPW and BiVO4 coupling system.
The authors gratefully acknowledge financial supports from the Founda-tion of State Key Laboratory of Pollution Control and Resource Reuse of China, the China Postdoctoral Science Foundation funded project (2012M511254), the National Natural Science Foundation of China (51008154), the Natural Science Research Project of Jiangsu Province’s Education Department (12KJD610004), the Scientific Innovation Research Foundation of Graduate Student of Jiangsu Province (CXZZ12-0063), as well as the Scientific Research Foundation of Graduate Student of Nanjing University (2012CL10).
1 Hu C, Tang YC, Yu JC, Wong PK. Photocatalytic degradation of cationic blue X-GRL adsorbed on TiO2/SiO2 photocatalyst. Appl Catal B: Environ, 2003, 40: 131–140
2 Cao MN, Lin JX, Lu J, You YL, Liu TF, Cao R. Development of a polyoxometallate-based photocatalyst assembled with cucurbit[6]uril via hydrogen bonds for azo dyes degradation. J Hazard Mater, 2011, 186: 948–951
3 García-Pérez UM, Sepúlveda-Guzmán S, Martínez-delaCruz A, Ortiz-Méndez U. Photocatalytic activity of BiVO4 nanospheres ob-tained by solution combustion synthesis using sodium carbox-ymethylcellulose. J Mol Catal A: Chem, 2011, 335: 169–175
4 Zhang XF, Quan X, Chen S, Zhang Y. Effect of Si doping on photo-electrocatalytic decomposition of phenol of BiVO4 film under visible light. J Hazard Mate, 2010, 177: 914–917
5 Lu Y, Luo YS, Kong DZ, Zhang DY, Jia YL, Zhang XW. Large- scale controllable synthesis of dumbbell-like BiVO4 photocatalysts with enhanced visible-light photocatalytic activity. J Solid State Chem, 2012, 186: 255–260
6 Martínez-dela CA, Obregón AS. Synthesis and characterization of nanoparticles of a-Bi2Mo3O12 prepared by co-precipitation method: Langmuir adsorption parameters and photocatalytic properties with rhodamine B. Solid State Sci, 2009, 11: 829–835
7 Fan J, Hu XY, Xie ZG, Zhang KL, Wang JJ. Photocatalytic degrada-tion of azo dye by novel Bi-based photocatalyst Bi4TaO8I under visi-ble-light irradiation. Chem Eng J, 2012, 179: 44–51
8 Kohtani S, Koshiko M, Kudo A, Tokumura K, Ishigaki Y, Toriba A, Hayakawa K, Nakagaki R. Photodegradation of 4-alkylphenols using BiVO4 photocatalyst under irradiation with visible light from a solar simulator. Appl Catal B:Environ, 2003, 46: 573–586
9 Zhang AP, Zhang JZ. Effects of europium doping on the photocatalytic behavior of BiVO4. J Hazard Mater, 2010, 173: 265–272
10 Xu H, Li HM, Wu CD, Chu JY, Yan YS, Shu HM, Gu Z. Preparation,
characterization and photocatalytic properties of Cu-loaded BiVO4. J Hazard Mater, 2008, 153: 877–884
11 Liu KJ , Chang ZD, Li WJ, Che P, Zhou HL. Preparation, characteri-zation of Mo, Ag-loaded BiVO4 and comparison of their degradation of methylene blue. Sci China Chem, 2012, 55: 1770–1775
12 Lee DK, Cho IS, Lee SW, Bae ST, Noh JH, Kim DW, Hong KS. Ef-fects of carbon content on the photocatalytic activity of C/BiVO4 composites under visible light irradiation. Mater Chem Phys, 2010, 119: 106–111
13 Ge L. Novel Pd/BiVO4 composite photocatalysts for efficient degra-dation of methyl orange under visible light irradiation. Mater Chem Phys, 2008, 107: 465–470
14 Long MC, Cai W, Cai J, Zhou B, Chai X, Wu Y. Efficient photo-catalytic degradation of phenol over Co3O4/BiVO4 composite under visible light irradiation. J Phys Chem B, 2006, 110: 20211–20216
15 Kozhevnikov IV. Catalysis by heteropoly acids and multicomponent polyoxometalates in liquid-phase reactions. Chem Rev, 1998, 98: 171–198
16 Yoon MJ, Chang JA, Kim YG, Choi JR. Heteropoly acid-incorporated TiO2 colloids as novel photocatalytic systems resembling the photo-synthetic reaction center. J Phys Chem B, 2001, 105: 2539–2545
17 Akid R, Darwent JR. Heteropolytungstates as catalysts for the pho-tochemical reduction of oxygen and water. J Chem Soc Dalton Trans, 1985, 1: 395–399
18 Sivakumar R, Thomas J, Yoon MJ. Polyoxometalate-based molecu-lar/nano composites: Advances in environmental remediation by photocatalysis and biomimetic approaches to solar energy conversion. J Photochem Photobiol C: Photochem Rev, 2012, 13: 277–298
19 Lu N, Zhao YH, Liu HB, Guo YH, Yuan X, Xu H, Peng HF, Qin HW. Design of polyoxometallate-titania composite film (H3PW12O40/ TiO2) for the degradation of an aqueous dye Rhodamine B under the simulated sunlight irradiation. J Hazard Mater, 2012, 199–200: 1–8
20 Li D, Guo Y, Hu C, Jiang C, Wang E. Preparation, characterization and photocatalytic property of the PW11O39
7/TiO2 composite film towards azo-dye degradation. J Mol Catal A: Chem, 2004, 207: 183–193
21 Ozer R, Ferry J. Investigation of the photocatalytic activity of TiO2- polyoxometalate systems. Environ Sci Technol, 2001, 35: 3242–3246
22 Zhang AP, Zhang JZ. Hydrothermal processing for obtaining of Bi-VO4 nanoparticles. Mater Lett, 2009, 63: 1939–1942
23 Wang B, Li Q, Wang W, Li Y, Zhai JP. Preparation and characteriza-tion of Fe3+-doped TiO2 on fly ash cenospheres for photocatalytic ap-plication. Appl Surf Sci, 2011, 257: 3473–3479
24 Yu CL, Yang K, Yu JC, Cao FF, Li X, Zhou XC. Fast fabrication of Co3O4 and CuO/BiVO4 composite photocatalysts with high crystallinity and enhanced photocatalytic activity via ultrasound irradiation. J Alloy Compd, 2011, 509: 4547–4552
25 Galembeck A, Alves OL. Bismuth vanadate synthesis by metallo- organic decomposition: Thermal decomposition study and particle size control. J Mater Sci, 2002, 37: 1923–1927
26 Zhou B, Zhao X, Liu HJ, Qu JH, Huang CP. Synthesis of visible- light sensitive M-BiVO4 (M = Ag, Co, and Ni) for the photocatalytic degradation of organic pollutants. Sep Purif Technol, 2011, 77: 275– 282
27 Hardcastlet FD, Wachs IE. Determlnation of vanadium-oxygen bond distances and bond orders by Raman spectroscopy. J phys Chem, 1991, 95: 5031–5041
28 Liu FD, He H, Ding Y, Zhang CB. Effect of manganese substitution on the structure and activity of iron titanate catalyst for the selective catalytic reduction of NO with NH3. Appl Catal B: Environ, 2009, 93: 194–204
29 Jiang HY, Dai HX, Meng X, Ji KM, Zhang L, Deng JG. Porous olive- like BiVO4: Alcoho-hydrothermal preparation and excellent visible- light-driven photocatalytic performance for the degradation of phenol. Appl Catal B: Environ, 2011, 105: 326–334
30 Zhang LX, Jin QZ, Huang JH, Liu YF, Shan L, Wang XG. Modification of palygorskite surface by organofunctionalization for application in immobilization of H3PW12O40. Appl Surf Sci, 2010, 256: 5911–5917
8 Zhang J, et al. Sci China Chem January (2013) Vol.56 No.1
31 Li FB, Li XZ, Hou MF, Cheah KW, Choy WC. A enhanced photo-catalytic activity of Ce3+-TiO2 for 2-mercaptobenzothiazole degrada-tion in aqueous suspension for odour control. Appl Catal A: Gen, 2005, 285: 181–189
32 Choi WY, Termin A, Hoffmann MR. The role of metal ion dopants in quantum-sized TiO2: Correlation between photoreactivity and charge carrier recombination dynamics. J Phys Chem, 1994, 98: 13669–13679
33 Cheng B, Wang WG, Shi L, Zhang J, Ran JR, Yu HG. One-pot tem-plate-free hydrothermal synthesis of monoclinic BiVO4 hollow mi-crospheres and their enhanced visible-light photocatalytic activity. Int J Photoenergy, 2012, 2012, doi: 10.1155/2012/797968
34 Zhang X, Zhang Y, Quan X, Chen S. Preparation of Ag doped BiVO4 film and its enhanced photoelectrocatalytic (PEC) ability of phenol degradation under visible light. J Hazard Mater, 2009, 167: 911–914
35 Wang DS, Wang YH, Li XY, Luo QZ, An J, Yue HX. Sunlight pho-
tocatalytic activity of polypyrrole-TiO2 nanocomposites prepared by ‘in situ’ method. Catal Commun, 2008, 9: 1162–1166
36 Bi JH, Li J, Wu L, Zheng HR, Su WY. Effects of aluminum substitu-tion on photocatalytic property of BiVO4 under visible light irradia-tion. Mater Res Bull, 2012, 47: 850–855
37 Fu YS, Sun X, Wang X. BiVO4-graphene catalyst and its high pho-tocatalytic performance under visible light irradiation. Mater Chem Phys, 2011, 131: 325–330
38 Nikola C. Castillo, laura ding, andre heel, thomas graule, cesar pulgarin, on the photocatalytic degradation of phenol and dichloroacetate by BiVO4: The need of a sacrificial electron acceptor. J Photochem Photobiol A: Chem, 2010, 216: 221–227
39 Zhao WR, Wang Y, Yang Y, Tang J, Yang Y. Carbon spheres sup-ported visible-light-driven CuO-BiVO4 heterojunction: Preparation, characterization, and photocatalytic properties. Appl Catal B: Environ, 2012, 115: 90–99
