Synthesis And Photocatalytic Properties Under Visible Light Of Bi₂WO₆

In this paper, the mechanism of semiconductor photocatalysis, the main factors affecting photocatalytic properties of semiconductor photocatalyst and modification technologies of semiconductor photocatalyst are briefly summarized and reviewed. The current domestic and foreign research progresses on multi-oxide semiconductor and application of semiconductor photocatalysis in purifying environment are given a detailed overview. On this basis, Bi₂WO₆ was selected as research subject. New methods and materials were used to control the morphology of the Bi₂WO₆, and the photocatalytic degradation mechanism was discussed, which will provide guidance to theory research and practical application of visible-light responsive Bi₂WO₆ photocatalyst.The as-prepared samples were characterized by X-ray Diffraction (XRD), Field Emitted Scanning Electron Microscopy (FESEM), UV-Vis diffuse reflectance spectroscopy (UV-vis DRS), Fourier Transform Infrared Spectroscopy (FT-IR) and Brunauer-Emmett-Teller (BET).Bi₂WO₆ powders have been prepared by Hydrothermal Synthesis (HS) and Molten Salt Synthesis (MMS). In addition, Bi₂WO₆ powders have been prepared by Solid-state Reaction Method (SSR) for comparison. The photocatalytic activity of Bi₂WO₆ powders was evaluated by the photocatalytic decomposition of methyl orange (MO) under visible light (λ>400 nm). The experimental results show that samples synthesized by HS had the highest visible light activity. The reasons for different photocatalytic activities were the samples prepared by HS have the smallest grain size and the largest surface area. The smaller the particle size, the smaller the probability of electron and hole gets, the higher the photocatalytic activity gets; the greater the surface area, the greater the reaction area is, the stronger the absorption capacity gets, the higher the photocatalytic activity gets. The required temperature of MMS (350℃) was far below than that of SSR (900℃). This is due to the formation of melted salt, which enhanced mobility of reaction ingredients in liquids. So the activity of reactants and the contact area of reaction improved, too. Reaction progress and the subsequent cleaning process of MSS were conducive to the elimination of impurities. Products with high purity were formed. Therefore, MMS may be the simplest way to synthesize multi-oxide powder with high purity.The morphology of Bi₂WO₆ synthesized by HS was controlled by adjusting pH value of hydrothermal precipitation precursor. It has gradually changed from three-dimensional flower spherical structure to two-dimensional sheet structure when pH values increased from 1 to 7, and photocatalytic activities decreased. Adding different surfactants (poly vinyl pyrrolidone (PVP), sodium dodecyl benzene sulfonate (SDBS), ethylene diamine tetraacetic acid (EDTA)) has realized the controlled self-assembly of Bi₂WO₆ nano-structure. Formation mechanisms and their photocatalytic activities of Bi₂WO₆ in the presence of organic agents were discussed. The three major formation steps of round flake Bi₂WO₆ are as follows: first, with the reaction conditions dramatically changed, a large number of small Bi₂WO₆ nanocrystals formed. Then these nanocrystals crystallized into Bi₂WO₆ nanoplates with single crystal structure. Bi₂WO₆ nanoplates with small sizes self-assembled again, forming a circular sheet unit. To further understand the progress of photocatalytic degradation methyl orange, mechanism of methyl orange degradated by hydrothermal products was initially explored. In the end, the best conditions for photocatalytic degradation and stability of Bi₂WO₆ were inspected.