Bismuth Based Semiconductor Heterojunctions: Design, Preparation And Photocatalytic Performances

Semiconductor-mediated photocatalysis has become one of the most “green” and environmentally friendly technologies for solving energy shortages and environmental pollution. A prerequisite for the development of photocatalytic technology is to obtain efficient photocatalysts. Recently, bismuth(III)-based semiconductor photocatalysts have been demonstrated to exhibit superior photocatalytic activities under visible-light irradiation(λ > 400 nm), since Bi 6s and O 2p levels can form a preferable hybridized conduction band(VB) to show strong oxidative ability for degrading organic pollutants. However, there are still some drawbacks hindering their practical application, such as the unsatisfactory photo-response range and short photogenerated electron-hole pair lifetime. It is well known that the construction of semiconductor heterostructure is an efficient method for the improvement of photocatalytic activity, and several heterostructures(semiconductor-semiconductor heterostructure, semiconductor-metal heterostructure, semiconductor-carbon group heterostructure, and multicomponent heterostructure) have been developed. To improve the photocatalytic activity of Bi2MoO6, we have prepared some Bi2MoO6-based heterostructure photocatalysts, such as Bi2MoO6-Pt and Bi2S3/Bi2MoO6, and their photocatalytic performances have been investigated respectively. In addition, as an important n-type semiconductor, WO3 has been given considerable attention to due to its narrow band gap(2.4-2.8 eV), nontoxicity, stable physicochemical properties, resilience to photocorrosion and high oxidation power of valence band(VB) holes. However, the rapid recombination of photogenerated charge carriers leads to poor photocatalytic activity, inhibiting its photocatalytic applications. To improve the photocatalytic activity of WO3, we also have prepared BiOBr/WO3 p-n heterojunction photocatalysts, and its photocatalytic performance has been investigated. The major research contents and findings are listed as follows:1. The Bi2MoO6-Pt heterojunctions have been prepared via a solvothermal-photoreduction method. It consists of three-dimensional Bi2MoO6 microspheres with diameters ranging from 1 to 4 μm and Pt nanoparticles with the mean size of 2.5 nm. The photocatalytic activity and stability of Bi2MoO6-Pt heterojunctions have been evaluated by the photocatalytic degradation of rhodamine B(RhB) and parachlorophenol(4-CP) under visible-light irradiation(λ > 400 nm). As a result, Bi2MoO6-Pt heterojunctions exhibit higher photocatalytic activity than that of pure Bi2MoO6, and Bi2MoO6-Pt-2(the loading content of Pt is 0.3 wt%) obtains the highest degradation efficiency. The recycling experiment confirms that it is essentially stable during the photocatalytic reaction process.2. The Bi2S3/Bi2MoO6 heterojunction has been prepared by a solvothermal method. It consists of flower-like superstructures with diameters ranging from 1 to 3 μm, which are built from Bi2MoO6 nanosheets with a thickness of about 15 nm decorated with Bi2S3 nanoparticles with diameter of ~3.5 nm. Furthermore, the photocatalytic activity of the Bi2S3/Bi2MoO6 heterojunction has been evaluated through the degradation of rhodamine B(RhB) dye and colorless parachlorophenol(4-CP) under visible-light irradiation(λ > 400 nm). The results demonstrate that the Bi2S3/Bi2MoO6 heterojunction exhibits higher photocatalytic activity in degrading RhB and 4-CP than single Bi2S3 or Bi2MoO6. More importantly, the photocatalytic activity of the Bi2S3/Bi2MoO6 heterojunction is superior to the sum of the activities of two individual photocatalysts(Bi2MoO6 and Bi2S3). These results strongly reveal that there is a synergic effect in Bi2S3/Bi2MoO6 heterojunction.3. BiOBr/WO3 p-n heterojunctions with different molar ratios(the theoretical molar ratios of BiOBr and WO3 are 1/0.5, 1/1 and 1/2, respectively.) have been prepared through an electrospinning-calcination-solvothermal method. The photocatalytic potential of these p-n heterojunctions has been investigated through the degradation of rhodamine B(RhB). The results demonstrate that BiOBr/WO3 p-n heterojunctions exhibit higher photocatalytic activity in degrading RhB than single WO3 or BiOBr. Especially, when the theoretical molar ratios of BiOBr and WO3 is 1/1, BiOBr/WO3 p-n heterojunction gains access to highest photocatalytic activity, and even higher than the sum of the activities of two individual photocatalysts with the same weight of components(WO3 and BiOBr). Further recycling experiment confirms that BiOBr/WO3-1/1 is essentially stable during the photocatalytic process.