| With the intensification of environmental pollution, a lot of shortages were emerged in conventional techniques of wastewater treatment, such as mishandling the new pollutants and the formation of secondary pollution after treating. Therefore, extensive attention has been paid to the photocatalytic technology, which is regarded as a new, efficient and clean water treatment technology. However, the commonly used TiO2photocatalyst, due to the wide band gap, only absorb the UV light (λ<387nm). So it is important to develop new photocatalysts to extend the absorb wavelength range into the visible region. In recent years, many researchers have focused on the new visible-light respose photocatalysts to solve these problems.Bi2WO6is the simplest member of the Aurivillius family with layered structures, which are structuraly composed of alternating perovskite-like slab of WO42-and [Bi2O2]2+layers. It has been reported that Bi2WO6has some interesting applications as ferroelectric materials, oxygen ionic conductors, photocatalysts for photocatalytic decomposition of organic contaminants, and so on. Since Kudo and Zou firstly reported that Bi2WO6processed the photocatalytic activity under visible light irradiation, which photodecompose water to generate O2and photodegradation of organic pollutants (CHCl3and CH3CHO). The related research about photocatalytic performance of Bi2WO6and bismuth compounds is continued. In this paper, the Bi2WO6with different morphologies have been synthesized by hydrothermal method. Then, the effets of the surface morphologies on its photocatalytic properties have been systematically studied, and the reasons for this impact are aslo explained. Afterwards, Bi2O2CO3has shown good photocatalytic performance in recent years. However, its band gap is relatively large, which is closed to TiO2. This will influence its practical application. Therefore, Bi2O2CO3coupled with the visible light response Bi2WO6can enlarge light absorb range and improve the photocatalytic activity. Bi2O2CO3/Bi2WO6composites with the different molar ratios were synthesized by hydrothermal process. Then, the detailed photocatalytic properties of the samples were analysized. And the reasons of different photocatalytic performances were studied. All the experimental results were demostrated as follows:Using Bi(NO3)3·5H2O and Na2WO4as raw materials, the various morphologies of Bi2WO6such as coralloid spherical particles, packed nanosheets, fluffy microspheres and plates can be obtained by hydrothermal method. Then, the crystal structure, molecular structure, surface morphology, microstructure, phase composition, BET surface area, light absorption and photocatalytic properties of Bi2WO6were studied by XRD, Raman spectroscopy, SEM, TEM, BET and UV-vis diffuse reflection, respectively. The result shows that the pH value of the precursor has a great influence on the morphology of Bi2WO6sample. But it has only a little influennce on the crystal and molecular structure of the samples. Additionally, the absorption spectra displays that the light absorption properties of the photocatalysts with different morphologies were similar. It means that all the samples have similar light absorption properties, even their morphologies are different. However, the photocatalytic efficiency for the photocatalytic degradation of RhodamineB has a huge difference. Among the samples, the porous fluffy microspheres Bi2WO6presented the best photocatalytic performance. In20min,300mL Rhodamine B solution with the concentration about1×10-5mol/L can be photodegradated completely. However, the photocatalytic efficiencies of other photocatalysts with different morphologies are not good. Therefore, it is believed that the main factor affecting their photocatalytic efficiency is the morphology of Bi2WO6samples, rather than electronic structure. Hence, the in-depth analysis of the porous fluffy microspheres Bi2WO6has been conducted to find the reasons why this sample processed best photocatalytic performance. From the experimental results, it is found that this sample had smaller grain size and more interesting hierarchical architecture morphology than the other samples. Due to this difference of the microstructure, this sample processed larger specific surface area and more holes number than the other photocatalysts. As we know, the high surface area brings more catalytic sites, and the porous structure provides more transport paths for the RhB molecules to get to the catalytic sites, which greatly promote its efficiency of photocatalysis. For the purpose of broadening light absorption range of Bi2O2CO3and improving their photocatalytic efficiency, In this work, Bi(NO3)3·5H2O, Na2WO4, and citric acid were used as the raw materials, and PEG6000as the surfacant, a series of Bi2O2CO3/Bi2WO6composites with different proportions were synthesized by hydrothermal method through controlling the molar content of the Na2WO4. Similarly, the crystal structure, surface morphology, microstructure, element distribution, optical absorption and photocatalytic properties were studied by XRD, SEM, TEM, EDX and UV-vis diffuse reflection, respectively. Meanwhile, the mechanism of crystal growth and photocatalytic mechanism were explored. The experimental results showed that all the synthesized Bi2O2CO3/Bi2WO6composites were composed of clubbed Bi2O2CO3and flake-like Bi2WO6. The growth mechanism of the composites follows the Ostwald hydrothermal aging process. And the intermediate Bi3.84W0.16O6.24is formed duing the synthsis process of Bi2O2CO3/Bi2WO6composite. At the same time, on the composite growing, the trend of the crystal oriented growth was emerged and the heteroj unction was formed at the junction between the rod-like Bi2O2CO3and flake-like Bi2WO6. Seen from the UV-vis absorption spectroscopy, with increasing the molar content of Bi2WO6, the absorption spectrum presents red shift, indicating that the band gap of the composite decreases gradually and the light response scope of the composite oxide is enlarged. In the experiment of the photocatalytic performance, the sample with the Bi2O2CO3content of about40%show the best photocatalytic efficiency. In60min,300mL Rhodamine B solution with the concentration about1×10-5mol/L can be photodegradated completely. However, the other samples showed poor photocatalytic efficiency, even lower than their pure substance. Thus, the photocatalytic mechanism and effects of the heteroj unction existed in the photocatalytic process were discussed. We think the effect of the heteroj unction exists in two sides. When the molar ratio of the Bi2O2CO3and Bi2WO6is suitable, the heteroj unction will be able to exert its capability of separating electrons and holes, and reducing recombination probability of the electrons and holes. Thus, the photocatalytic efficiency will be improved. However, when the ratio is inappropriate, the electrons and holes separated from the heteroj unction may react with the holes or electrons from the monomer catalysts. The electrons and holes will annihilate because of this reaction. So the concentration of electrons and holes brought down, resulting in the lower photocatalytic efficiency of the Bi2O2CO3/Bi2WO6composite. |
PDF Full Text Request