Preparation,Characterization And Photocatalytic Performance Of Ag-Doped BiVo₄ Composites

In this paper, BiVO₄, Bi₂WO₆, Bi₂MoO₆ and metal-doped BiVO₄ were prepared by hydrothermal method, characterized by using SEM, TEM, XRD, XPS, Raman, PL and UV-Vis spectra and tested in photocatalytic hydrogen production from an ethanol-triethylamine aqueous solution. The significant influences of morphologies, composition and crystalline structure and energy band structure of composites on their photo-absorption and photocatalytic performance were also discussed.The results show that the as-prepared composite oxide of BiVO₄, Bi₂WO₆ and Bi₂MoO₆ exist in the from of m/t-BiVO₄, monoclinic phase Bi₂WO₆ and ?-Bi₂MoO₆, among which the BiVO₄ has the highest hydrogen production rate due to superior light absorption properties and appropriate band structure. For BiVO₄ semiconductor materials,the pH values, the types of surfactant and molar ratio of V/Bi during the preparation effect greatly the morphologies, crystalline structure, photo-absorption and photocatalytic performance of BiVO₄. BiVO₄ preapared with the preparation conditions of pH=9.0, CTAB as surfactant and V/Bi molar ratio of 1 has the highest photocatalytic performance, which is related with structure, morphologies and photo absorption properties.After the doping of Co, Ni, Cu, Zn and Ag metal ion, monoclinic phase BiVO₄ was gradually transformed into the mixture of monoclinic and tetragonal phase,and heterojunction has been formed between m-BiVO₄ and t-BiVO₄, the amout of heterostructure is associated with the content of tetragonal phase and the kinds of metal. The BiVO₄ doped by Co, Ni and Cu ion mainly contain m-BiVO₄ and a small amount of t-BiVO₄, so the number of heterojunction structure is small and which has little effect on photocatalytic activity; The BiVO₄ doped by Zn ion mainly contains tetragonal phase BiVO₄, which reduces the photocatalytic activity of BiVO₄; When Ag is incorporated,the square-sheet like morphology of m-BiVO₄ is gradually changed to dispersive sphere, and monoclinic structure is progressively converted to monoclinic/tetragonal heterostructures. For Ag-BiVO₄ semiconductor materials, a tight interfacejunction has been formed between m-BiVO₄ and t-BiVO₄ in a nanosize level and the amount of this heterojunction is related to the ratio of m-BiVO₄/t-BiVO₄ and Ag content. The as-prepared Ag-doped BiVO₄ composites have much higher photocatalytic activities for H2 evolution from the ethanol-triethylamineaqueous solution when compared to pure BiVO₄. The 4Ag-BiVO₄ exhibitsthe highest photocatalytic activity, and hydrogen production rate of 205?mol·h-1·g-1 can be obtained, which is more than 2.5 times that of undoped monoclinic BiVO₄. The highly improved performance of the composites can be attributed to the efficient charge transfer and separation,resulting from the coupling of Ag₄V₂O₇ and heterostructured BiVO₄ system with a proper ratio of m-BiVO₄ and t-BiVO₄.Calculation of energy band and photocatalytic mechanism of Ag-BiVO₄ showed that under the radiation of light, the electrons in CB edge of Ag₄V₂O₇ with potential more negative than that of t-BiVO₄ and m-BiVO₄ are produced, so photo-generated electrons on the Ag₄V₂O₇ surface would easily transfer in sequence to t-BiVO₄ and m-BiVO₄. The electrons located on the m-BiVO₄ can reduce H2 O to H2. At the same time, the photoinduced holes at the VB top of Ag₄V₂O₇ will directly oxidize ethanol to the final product CO2. In this process, heterojunction between m-BiVO₄ and t-BiVO₄ and the coupling of Ag₄V₂O₇ have a very important effect on improving the light absorption performance of composite materials and the transmission speed and separation efficiency of photo-generated charge.