BiVO₄/MIL-101 composite having the synergistically enhanced visible light photocatalytic activity

Abstract

In this study, a novel composite containing a Cr based metal–organic framework (MOF), MIL-101 and BiVO₄ was successfully synthesized by hydrothermal method, and was fully characterized by X-ray diffraction, transmission electron microscopy, thermogravimetric analysis, UV-vis diffuse reflectance absorption spectra and photoluminescence emission spectra. Moreover, the photocatalytic activities of BiVO₄/MIL-101 composite and the pure materials were evaluated by measuring the degradation of Rhodamine B under visible light. The results show that the composite exhibits better photocatalytic activities than pure materials, which can be ascribed to the big adsorption capacity of MIL-101 and the enhanced separation of photogenerated charge carriers from assembly of MIL-101 on BiVO₄. The synergistic effect between BiVO₄ and MIL-101 was evaluated by the proposed synergistic factor, which is bigger than 1 for various ratios of BiVO₄ to MIL-101, suggesting there exists positive interactions between pure materials for enhancing photocatalytic activities.

---

1. Introduction

In recent years, metal–organic frameworks (MOFs) have gained widespread attention due to their considerable physical and chemical properties.^1,2 MOFs are an emerging class of highly porous materials, built from organic linkers and inorganic metal nodes, which can exhibit extremely high surface areas, as well as tunable pore size and functionality, and can act as hosts for a variety of guest molecules.³ To date, many studies concerning MOFs research adsorption capacity for gas, such as H₂, N₂, O₂ and CO₂ and organic dyes in aqueous solution.⁴ Although photocatalysis as an energy saving and environment friendly technique has been extensively investigated in removing various organic pollutants in water, few studies cover the photocatalytic activity of MOFs. On the other hand, BiVO₄ has attracted more and more people's interest, due to its relatively narrow band gap energy (<2.5 eV) and high photocatalytic activity in evolution of H₂ (ref. 5 and 6) and O₂ (ref. 7–9) as well as degradation of organic contaminants.^10,11 It is well known that several factors, such as crystal structure, oxygen vacancy density, morphology, surface area, pore structure and band gap energy can influence the performance of a photocatalyst. MIL-101, as one extensively investigated MOF, reveals certain photocatalytic activity, but its big band gap energy and easy recombination of the photogenerated electrons (e⁻) and holes (h⁺) can inhibit the photocatalytic efficiency. Therefore, great efforts have been made to expand the photo-absorption range and facilitate the separation of the photo-induced carriers. Coupling of two semiconducting oxides with variable band gaps has been proven to be a feasible approach to reduce the recombination probability of photogenerated electrons and holes, and thus has been widely employed to improve the photocatalytic activity.¹² Various composite semiconductors, like CuCr₂O₄/TiO₂, BiVO₄/Bi₂O₃, AgBr/WO₃, SnO₂/ZnO, BiOI/TiO₂ and Bi₂S₃/BiVO₄ have been demonstrated to be efficient in separating electron and hole pairs.^13–17

In this work, it is the first time to combine MIL-101 with BiVO₄ to prepare composite via coordination reaction induced self-assembly process, which is a common method to construct MOFs containing composites. The as-obtained BiVO₄/MIL-101 composite has been explored for the degradation of Rhodamine B (RhB) using visible light, and shows superior photocatalytic activity as compared with pure materials and physical mixture. This is due to the synergistic effect of big adsorption capacity of MIL-101 for RhB and the enhanced separation of the photo-induced carriers by compounding.

2. Experimental section

2.1. Synthesis of BiVO₄

All of the chemicals including NaOH, HF, ammonium metavanadate, chromium nitrate nanohydrate, bismuth nitrate pentahydrate, terephthalic acid (TPA) and Rhodamine B (RhB) purchased from Sinopharm Chemical Reagent Company were of analytical grade, and used as received without further purification.

BiVO₄ was synthesized by a hydrothermal method as described previously.¹⁸ In a typical synthesis, 5 mmol (2.425 g) of Bi(NO₃)₃·5H₂O was added into 80 mL of distilled water under mild stirring, and a white suspension was formed immediately due to the hydrolysis of Bi(NO₃)₃. Then, 5 mmol (0.585 g) of NH₄VO₃ was added into the white suspension, and an orange suspension was formed. Then, its pH value was adjusted to 5–6 by adding NaOH, and the suspension was stirred for 1 h, resulting in a light yellow colloid solution. After that, the colloid solution was loaded into a Teflon-lined autoclave, which was kept heating in an oven of 160 °C for 12 h, and then naturally cooled to room temperature. The resulted yellow powder was collected, washed with distilled water and ethanol several times to remove ions and possible remnants, and dried for further characterization.

2.2. Synthesis of BiVO₄/MIL-101

To prepare the composite, firstly 0.100 g BiVO₄, 2.000 g Cr(NO₃)₃·9H₂O and 0.820 g TPA were respectively put into 8 mL deionized water with stirring. Then the suspension of BiVO₄ was ultrasonic for 0.5 h. The three solutions were mixed, to which was added 7 drops of HF. After 1 h stirring, the suspension was placed in a Teflon-lined autoclave bomb and kept in an oven at 220 °C for 24 h. After the reaction, the autoclave was cooled to room temperature, and the light green-colored products were collected by filtration. To remove the unreacted TPA, the as-synthesized composite was further purified by treatment with N,N-dimethylformamide (20 mL) following a method described previously.¹⁹ The purified composite was dried at 80 °C for 3 h, dissolved into 20 mL CHCl₃ for 24 h to remove the adsorbed N,N-dimethylformamide, then separated and dried. Other composites with different ratios of BiVO₄ to MIL-101 can be prepared under the same condition via changing the mass of BiVO₄ from 0.100 g to 0.050 g, 0.300 g and 0.500 g, and the resulted composites are named as 0.1BM, 0.05BM, 0.3BM and 0.5BM, respectively.

For comparison, the physical mixtures were also prepared by simply mixing pure BiVO₄ and MIL-101 with the same ratios as those determined by the following thermogravimetric analysis for the composites.

2.3. Material characterization

The crystallographic structure was characterized by X-ray diffraction (XRD, Bruker advance-D8 power diffractometer with Cu Kα radiation, λ = 0.154178 nm at 40 kV and 100 mA flux). The morphology of materials were examined by transmission electron microscopy (TEM: JEM-2100F, Japan). The valence band X-ray photoelectron spectroscopy (XPS) was conducted using an ESCALAB250 spectrometer. Photoluminescence (PL) emission spectra were measured on a PL measurement system (Fluorolog Tau-3) with the excitation wavelength of 320 nm. The optical properties were investigated using a UV-visible spectrophotometer in a wavelength range of 300 to 900 nm. Thermogravimetric analysis (TGA) was carried out on a Perkin-Elmer Diamond TG thermal analyzer at a rate of 20 K min⁻¹ in atmosphere.

2.4. Photocatalytic experiments

Photocatalytic properties of the as-prepared materials were evaluated by the degradation of RhB in aqueous solution. A 75 W energy-saving lamp (λ < 380 nm was filtered out by a cut off filter, JLT70T830, Philips) was used as light source to provide visible light irradiation. In each experiment, 40 mg of the material was dispersed in 100 mL of RhB solution (initial concentration, C_O = 10 mg L⁻¹), and the mixture was stirred for about 3.5 h in dark to attain equilibrium adsorption on the catalyst surface. Then the solution was exposed to visible light irradiation under magnetic stirring. After each 0.5 h, 5 mL of the mixture was withdrawn, and the catalyst was separated by centrifugation. Photocatalytic activity was evaluated by monitoring the RhB decolorization at the maximum absorption around λ = 553 nm with a UV-vis spectrophotometer. For comparison, we also explored the photocatalytic activity of colorless benzoic acid by the as-prepared materials.

3. Results and discussion

3.1. XRD analysis

Fig. 1 shows the XRD patterns of MIL-101, BiVO₄ and BiVO₄/MIL-101. For BiVO₄, the peaks centered at 2θ = 18.98, 28.94, 30.58 and 35.22° match well with those of the monoclinic BiVO₄ indicating that the BiVO₄ sample has a monoclinic scheelite type crystal structure.^20,21 For MIL-101, all of the diffraction peaks were also in good agreement with those of the standard sample.²² BiVO₄/MIL-101 displays the characteristic peaks of both MIL-101 and BiVO₄, and no new diffraction peaks appear, revealing the coexistence of BiVO₄ and MIL-101. Besides, the intensity of diffraction peaks of the composite has no change as compared to that of BiVO₄ and MIL-101, illustrating the stability of crystal structures for MIL-101 and BiVO₄ in the composite.

Fig. 1 XRD patterns of MIL-101, BiVO₄ and BiVO₄/MIL-101.

3.2. TEM analysis

The morphology of the as-prepared samples was characterized by TEM. Fig. 2 shows low and high resolution TEM images of BiVO₄ and BiVO₄/MIL-101. As shown in Fig. 2(a) and (b), BiVO₄ has schistose shape with the lattice spacing of 0.32 nm as reported previously.²³ Fig. 2(c) and (d) show the images of BiVO₄/MIL-101. After the rotation 90° of Fig. 2(d), we can obtain the mapping as displayed in Fig. 3, from which, it can be seen that V, Bi, as well as Cr element have the same distribution shape, indicating that MIL-101 covers the whole range. In Fig. 3(d), different concentration distribution of O element results from overlapping of O element in BiVO₄ and MIL-101. As indicated in Fig. 2 and 3, MIL-101 was successfully assembled on BiVO₄ with dark and bright parts representing BiVO₄ and MIL-101, respectively.

Fig. 2 TEM images of BiVO₄ and BiVO₄/MIL-101: (a) BiVO₄, inset is a complete plate-like particle of BiVO₄; (b) HRTEM image of BiVO₄; (c) and (d) BiVO₄/MIL-101.

Fig. 3 Distribution of V, Cr, Bi and O elements for Fig. 2(d).

3.3. UV-vis diffuse reflectance absorption spectra and XPS analysis

The energy band structure feature of a semiconductor is considered as a key factor in determining its photocatalytic property.²⁴ Some studies have indicated that MOFs can be used as photocatalyst to decompose water.²⁵ Fig. 4(a) shows the UV-vis diffuse reflectance absorption spectra of BiVO₄ and MIL-101, as indicated, the absorption edge of BiVO₄ is located in the visible light region at about 500 nm, which agrees well with the reported value,²⁶ whereas MIL-101 displays strong absorption in the UV region and weak absorption in the visible light region. The band gap of BiVO₄ and MIL-101 can be calculated by the following equation:

(αhv)² = A(hv − E_g)ⁿ

where α, h, ν, E_g and A each represent the absorption coefficients, Plank constant, light frequency, band gap and a constant, and the calculated results are 2.51 eV and 3.68 eV for BiVO₄ and MIL-101, respectively. Some researches^27,28 reveal that conduction band of BiVO₄ is at 0.3 eV, thus its valence band is at 2.81 eV. Fig. 4(b) shows the valence band of MIL-101 is at 2.09 eV. As a result, the conduction band of MIL-101 is at −1.59 eV which is higher than that of BiVO₄.

Fig. 4 (a) UV-vis diffuse reflectance spectra of BiVO₄ and MIL-101; (b) valence band XPS figure of MIL-101.

3.4. PL analysis

The photoluminescence (PL) emission spectra can be regarded as a direct approach to understand the separation efficiency of photogenerated carriers.^29,30 Fig. 5 shows PL spectra of BiVO₄, MIL-101 and BiVO₄/MIL-101 composite upon excitation at 320 nm. It is obvious that emission intensities of the composite and BiVO₄ are much weaker than that of MIL-101. Possible explanation is that the combination of BiVO₄ and MIL-101 efficaciously separated the photogenerated electrons and holes.

Fig. 5 PL emission spectra for BiVO₄, MIL-101 and BiVO₄/MIL-101.

3.5. TGA and photocatalytic activities

In order to determine the mass ratio of BiVO₄ to MIL-101 in different composites, TGA was carried out as shown in Fig. 6, from which residual quantity can be obtained for MIL-101, BiVO₄, 0.05BM, 0.1BM, 0.3BM and 0.5BM. Then the BiVO₄ to MIL-101 mass ratio in composites can be calculated by the following formula:

m_BiVO4β + m_MIL-101γ = m_{BiVO4/MIL-101}α (1)

m_BiVO4 + m_MIL-101 = m_{BiVO4/MIL-101} (2)

where is mass ratio of BiVO₄ to MIL-101 in the different composites, and β, α and γ are residue percentages of BiVO₄, different composites and MIL-101, respectively. According to the formula (3), BiVO₄ to MIL-101 mass ratios in 0.05BM, 0.1BM, 0.3BM and 0.5BM are 1 : 37.20, 1 : 11.45, 1 : 2.02 and 1 : 1.28, respectively.

Fig. 6 TGA and residual quantity of MIL-101, BiVO₄, 0.05BM, 0.1BM, 0.3BM and 0.5BM.

Physical mixtures consisting of BiVO₄ and MIL-101 with the same mass ratios as in the composites are named as 0.05PBM, 0.1PBM, 0.3PBM and 0.5PBM, and their photocatalytic properties were also evaluated for degradation of RhB under the same conditions as those for the composites. The photocatalytic degradation process follows the pseudo first-order kinetics, and the value of the rate constant k can be calculated by the following formula:^31,32

ln(C_O/C) = κt (4)

where C_O is the initial concentration of RhB, and C is that at time t. Fig. 7 shows the photocatalytic activities of different catalysts in the degradation of RhB and the fitted rate constant κ by the formula (4). As indicated in Fig. 7(a), most of the composites exhibit better photocatalytic activity than pure materials. This means there exists certain synergistic interaction between MIL-101 and BiVO₄. However, the BiVO₄/MIL-101 ratio should be appropriate. As shown in Fig. 7(a), for very high BiVO₄/MIL-101 ratio, due to imperfect crystallization of MIL-101, the composite has too small surface to adsorb RhB, thus decreasing photocatalytic activity, whereas for very low BiVO₄/MIL-101 ratio, the reduced heterojunction interfaces will result in rapid recombination of photogenerated electron–hole pairs, and the photocatalytic activity will also decrease. Fig. 7(b) shows the photocatalytic kinetics of the physical mixtures which decreases monotonously with the amount of BiVO₄ increasing. In other words, there does not exist obvious synergistic interactions between pure materials in the physical mixtures. Furthermore all of the composites display better photocatalytic activity than the physical mixtures. For the composites, the assembly of MIL-101 on BiVO₄ enhanced the transfer and separation of photogenerated charge carriers through the interface of pure materials as evidenced by above PL analysis.

Fig. 7 Photocatalytic kinetics of (a) MIL-101, BiVO₄ RhB and composites and (b) physical mixtures.

To quantitatively evaluate the synergistic effect, the synergistic factor sκ is proposed and calculated using the following formula:

where κ, κ_BiVO4 and κ_MIL-101 are the fitted reaction rate constants of the composites, BiVO₄ and MIL-101, respectively. The synergistic factors of the composites are 1.730, 1.150, 1.754 and 1.239 for 0.05, 0.1, 0.3 and 0.5BM, respectively, and all are bigger than 1, suggesting there exists positive interactions between pure materials for enhancing photocatalytic activities.

3.6. Photocatalytic mechanism of BiVO₄/MIL-101 composites

It is widely known that photocatalytic degradation of pollutants under UV or visible light illumination takes place via the oxidization or reduction of toxic organic molecules with active species (e.g., ˙OH, ˙OOH, and O₂⁻) induced by the photogenerated electrons (e⁻) and holes (h⁺) on photocatalyst surfaces. As shown in Fig. 8(a), there exists synergistic photocatalytic effect for the composites. BiVO₄ has a narrowing band gap^33–35 which is smaller than that of MIL-101.²⁵ Therefore, under visible light irradiation, due to optical sensibilization, the photogenerated electrons produced by RhB rapidly transfer to the conduction band (CB) of BiVO₄. At the same time, h⁺ produced by BiVO₄ transfer to the valence band of MIL-101, resulting in effective separation of the photogenerated electrons and holes. Meanwhile, the electrons located on the CB of BiVO₄ can react with O₂ adsorbed on the surface of the composite to produce ˙O₂⁻ which creates ˙OOH after protonation. The ˙OH radicals can be produced from the trapped electron after formation of the HOO˙ radical³⁶ and further decompose RhB. h⁺ on the surface of MIL-101 can directly decompose RhB adsorbed by MIL-101 or react with H₂O to yield ˙OH to decompose RhB in the aqueous solution. Besides, the as-obtained BiVO₄/MIL-101 composite and pure BiVO₄ have been explored for the degradation of colorless benzoic acid using visible light. From Fig. 8(b), it can be seen that both the composite and pure BiVO₄ can degrade colorless benzoic acid, and the composite has higher photocatalytic activity. Moreover, the reason why benzoic acid can be degraded by BiVO₄ is that BiVO₄ itself can produce photogenerated electrons and holes under visible light. The photocatalytic mechanism of degradation of benzoic acid by BiVO₄/MIL-101 composite is similar to that of RhB. But the main difference is that photogenerated electrons for degradation of benzoic acid is only from BiVO₄, but not at the same time from optical sensibilization of dyes like degradation of RhB. As a result, the amount of photogenerated electrons is less for degradation of benzoic acid, and the photocatalytic activity of benzoic acid is lower than that of RhB. Due to big adsorption capacity of MIL-101 and fast separation of photogenerated charge carrier between interface of BiVO₄ and MIL-101, the degradation of RhB is enhanced, and the adsorption equilibrium of BiVO₄/MIL-101 composite for RhB is broken, leading to rapid migration of RhB from aqueous solution to the surface of the composite. In this way, RhB is photodegraded more efficiently on the composites than on the pure materials.

Fig. 8 (a) Schematic diagram of separation of electron–hole pairs over BiVO₄/MIL-101 composite under visible light; (b) degradation of benzoic acid by BiVO₄ and BiVO₄/MIL-101 composite.

4. Conclusions

In summary, BiVO₄/MIL-101 composite photocatalysts have been successfully synthesized by hydrothermal method. The results of photocatalytic experiments indicate that BiVO₄/MIL-101 composites exhibit enhanced photocatalytic activities for the degradation of RhB under visible light irradiation as compared with pure materials. The synergistic effect is due to the big adsorption of MIL-101 and the enhanced separation of photogenerated charge carrier between the interface of BiVO₄ and MIL-101.

Acknowledgements

This work was supported by the Anhui Provincial Natural Science Foundation (no. 1508085MB28), and the National Natural Science Foundation of China (Grant 51372062).

References

1. J. Y. Lee, O. K. Farha, J. Roberts, K. A. Scheidt, S. T. Nguyen and J. T. Hupp, Metal–organic framework materials as catalysts, Chem. Soc. Rev., 2009, 38, 1418–1429.
2. H. C. Zhou, J. R. Long and O. M. Yaghi, Introduction to metal–organic frameworks, Chem. Rev., 2012, 112, 673–674.
3. S. T. Meek, J. A. Greathouse and M. D. Allendorf, Metal–organic frameworks: A rapidly growing class of versatile nanoporous materials, Adv. Mater., 2011, 23, 249–267.
4. J. L. Wang, C. Wang and W. B. Lin, Metal–Organic Frameworks for light harvesting and photocatalysis, ACS Catal., 2012, 2, 2630–2640.
5. U. M. García-Pérez1, S. Sepúlveda-Guzmán, A. Martínez-de la Cruz and J. Peral, Selective synthesis of monoclinic bismuth vanadate powders by surfactant-assisted co-precipitation method: study of their electrochemical and photocatalytic properties, Int. J. Electrochem. Sci., 2012, 7, 9622–9632.
6. G. P. Nagabhushana, G. Nagaraju and G. T. Chandrappa, Synthesis of bismuth vanadate: its application in H₂ evolution and sunlight-driven photodegradation, J. Mater. Chem. A, 2013, 1, 388–394.
7. A. Kudo, K. Omori and H. Kato, A novel aqueous process for preparation of crystal form-controlled and highly crystalline BiVO₄ powder from layered vanadates at room temperature and its photocatalytic and photophysical properties, J. Am. Chem. Soc., 1999, 121, 11459–11467.
8. D. N. Ke, T. Y. Peng, L. Ma, P. Cai and K. Dai, Effects of hydrothermal temperature on the microstructures of BiVO₄ and its photocatalytic O₂ evolution activity under visible light, Inorg. Chem., 2009, 48, 4685–4691.
9. A. Kudo, K. Ueda, H. Kato and I. Mikami, Photocatalytic O₂ evolution under visible light irradiation on BiVO₄ in aqueous AgNO₃ solution, Catal. Lett., 1998, 53, 229–230.
10. G. S. Li, D. Q. Zhang and J. C. Yu, Ordered mesoporous BiVO₄ through nanocasting: a superior visible light-driven photocatalyst, Chem. Mater., 2008, 20, 3983–3992.
11. L. Hou, W. Z. Wang, L. S. Zhang, H. L. Xu and W. Zhu, Single-crystalline BiVO₄ microtubes with square cross-sections: microstructure, growth mechanism, and photocatalytic property, J. Phys. Chem. C, 2007, 111, 13659–13664.
12. J. Yan, L. Zhang, H. Yang, Y. Tang, Z. Lu, S. Guo, Y. Dai, Y. Han and M. Yao, CuCr₂O₄/TiO₂ heterojunction for photocatalytic H₂ evolution under simulated sunlight irradiation, Sol. Energy, 2009, 83, 1534–1539.
13. M. L. Guan, D. K. Ma, S. W. Hu, Y. J. Chen and S. M. Huang, From hollow olive-shaped BiVO₄ to n–p core–shell BiVO₄@Bi₂O₃ microspheres: controlled synthesis and enhanced visible-light-responsive photocatalytic properties, Inorg. Chem., 2011, 50, 800–805.
14. H. Xu, Y. Xu, H. Li, J. Xia, J. Xiong, S. Yin, C. Huang and H. Wan, Synthesis, characterization and photocatalytic property of AgBr/BiPO₄ heterojunction photocatalyst, Dalton Trans., 2012, 3387–3394.
15. L. Zheng, Y. Zheng, C. Chen, Y. Zhan, X. Lin, Q. Zheng, K. Wei and J. Zhu, Network structured SnO₂/ZnO heterojunction nanocatalyst with high photocatalytic activity, Inorg. Chem., 2009, 48, 1819–1825.
16. X. Zhang, L. Zhang, T. Xie and D. Wang, Low-temperature synthesis and high visible-light-induced photocatalytic activity of BiOI/TiO₂ heterostructures, J. Phys. Chem. C, 2009, 113, 7371–7378.
17. D. K. Ma, M. L. Guan, S. S. Liu, Y. Q. Zhang, C. W. Zhang, Y. X. He and S. M. Huang, Controlled synthesis of olive-shaped Bi₂S₃/BiVO₄ microspheres through a limited chemical conversion route and enhanced visible-light-responding photocatalytic activity, Dalton Trans., 2012, 5581–5586.
18. G. C. Xi and J. H. Ye, Synthesis of bismuth vanadate nanoplates with exposed {001} facets and enhanced visible-light photocatalytic properties, Chem. Commun., 2010, 46, 1893–1895.
19. E. Haque, N. A. Khan, J. E. Lee and S. H. Jhung, Facile purification of porous metal terephthalates with ultrasonic treatment in the presence of amides, Chem.–Eur. J., 2009, 15, 11730–11736.
20. L. L. Zhang, G. Q. Tann, S. S. Wei, H. J. Ren, A. Xia and Y. Y. Luo, Microwave hydrothermal synthesis and photocatalytic properties of TiO₂/BiVO₄ composite photocatalysts, Ceram. Int., 2013, 39, 8597–8604.
21. S. Usai, S. Obregón, A. I. Becerro and G. Colón, Monoclinic–tetragonal heterostructured BiVO₄ by yttrium doping with improved photocatalytic activity, J. Phys. Chem. C, 2013, 117, 24479–24484.
22. G. Ferey, C. Mellot-Draznieks, C. Serre, F. Millange, J. Dutour, S. Surble and I. Margiolaki, A chromium terephthalate-based solid with unusually large pore volumes and surface area,, Science, 2005, 309, 2040–2042.
23. S. W. Cao, Z. Yin, J. Barber, F. Y. C. Boey, S. C. J. Loo and C. Xue, Preparation of Au–BiVO₄ heterogeneous nanostructures as highly efficient visible-light photocatalysts, ACS Appl. Mater. Interfaces, 2012, 4, 418–425.
24. M. Shang, W. W. Wang, L. Zhou, S. M. Sun and W. Z. Yin, Nanosized BiVO₄ with high visible-light-induced photocatalytic activity: ultrasonic-assisted synthesis and protective effect of surfactant, J. Hazard. Mater., 2009, 172, 338–344.
25. A. Corma, H. García and F. X. Llabrés i Xamena, Engineering metal–organic frameworks for heterogeneous catalysis, Chem. Rev., 2010, 110, 4606–4655.
26. J. Cao, C. C. Zhou, H. L. Lin, B. Y. Xu and S. F. Chen, Surface modification of m-BiVO₄ with wide band-gap semiconductor BiOCl to largely improve the visible light induced photocatalytic activity, Appl. Surf. Sci., 2013, 284, 263–269.
27. J. Su, X. X. Zou, G. D. Li, X. Wei, C. Yan, Y. N. Wang, J. Zhao, L. J. Zhou and J. S. Chen, Macroporous V₂O₅–BiVO₄ composites: effect of heterojunction on the behavior of photogenerated charges, J. Phys. Chem. C, 2011, 115, 8064–8071.
28. P. Madhusudana, J. R. Ran, J. Zhang, J. G. Yu and G. Liu, Novel urea assisted hydrothermal synthesis of hierarchical BiVO₄/Bi₂O₂CO₃ nanocomposites with enhanced visible-light photocatalytic activity, Appl. Catal., B, 2011, 110, 286–295.
29. J. G. Yu, H. G. Yu, B. Cheng, X. J. Zhao, J. C. Yu and W. K. Ho, The effect of calcination temperature on the surface microstructure and photocatalytic activity of TiO₂ thin films prepared by liquid phase deposition,, J. Phys. Chem. B, 2003, 107, 13871–13879.
30. C. Yu, K. Yang, J. C. Yu, F. Cao, X. Li and X. Zhou, Fast fabrication of Co₃O₄ and CuO/BiVO₄ composite photocatalysts with high crystallinity and enhanced photocatalytic activity via ultrasound irradiation, J. Alloys Compd., 2011, 509, 4547–4552.
31. C. H. Wu, H. W. Chang and J. M. Chern, Basic dye decomposition kinetics in a photocatalytic slurry reactor, J. Hazard. Mater., 2006, 137, 336–343.
32. R. G. Chen, J. H. Bi, L. Wu, Z. H. Li and X. Z. Fu, Orthorhombic Bi₂GeO₅ nanobelts: synthesis, characterization, and photocatalytic properties, Cryst. Growth Des., 2009, 9, 1775–1779.
33. Z. X. Zhao, H. X. Dai, J. G. Deng, Y. X. Liu, Y. Wang, X. W. Li, G. G. Bai, B. Z. Gao and C. T. Au, Porous FeO_x/BiVO_4−δS_0.08: highly efficient photocatalysts for the degradation of Methylene Blue under visible-light illumination, J. Environ. Sci., 2013, 25, 2138–2149.
34. Z. Q. He, Y. Q. Shi, C. Gao, L. N. Wen, J. M. Chen and S. Song, BiOCl/BiVO₄ p–n heterojunction with enhanced photocatalytic activity under visible-light irradiation, J. Phys. Chem. C, 2014, 118, 389–398.
35. L. Chen, Q. Zhang, R. Huang, S. F. Yin, S. L. Luo and C. T. Au, Porous peanut-like Bi₂O₃–BiVO₄ composites with heterojunctions: one-step synthesis and their photocatalytic properties, Dalton Trans., 2012, 9513–9518.
36. T. X. Wu, G. M. Liu and J. C. Zhao, Photoassisted degradation of dye pollutants. v. self-photosensitized oxidative transformation of Rhodamine B under visible light irradiation in aqueous TiO₂ dispersions, J. Phys. Chem. B, 1998, 102, 5845–5851.

---