Preparation Of Three-dimensional Hierarchical Structure Bi 2 WO 6 , Precious Metal Loading And Its Photocatalytic Properties

As a new photocatalyst, bismuth tungstate （Bi₂WO₆） has potential researchful and practical values for the aspects of raising the utilization ratio of solar energy and environmental pollution control because of its visible light response and special layered structure. In this thesis, three-dimensionally （3D） hierarchical Bi₂WO₆ architectures is fabricated by simply adjusting the pH of the precursor solution through a facile and economical hydrothermal route. The growth mechanism of the Bi₂WO₆ architectures is also suggested. The photocatalytic activities of Bi₂WO₆ structures with a diversity of morphologies have been studied. Furthermore, noble metal （Pd） deposition on the hierarchical Bi₂WO₆ architectures has been performed, showing a remarkably improved photocatalytic activity. Especially, the deposition of noble metal on Bi₂WO₆ structures usually involve Pt, Au, and Ag, and there is no report on the deposition of Pd on Bi₂WO₆ structures up to now. The research results of this thesis are as follows:Bi₂WO₆ 3D flower-like hierarchical architectures, Bi₂WO₆ multilayered disks and Bi₂WO₆ nanoplates have been synthesized through hydrothermal routes at pH=1,6,9 respectively. X-ray diffractometer, scanning electron microscopy （SEM）, transmission electron microscopy （TEM）, physisorption apparatus, X-ray photoelectron spectrometer and UV-Vis spectrophotometer have been used for characterization of Bi₂WO₆ products. The influence of pH values of the precursor solution on the morphology of Bi₂WO₆ products is investigated in detail. The 3D structure of Bi₂WO₆ could transform into 2D structure when pH values increase from 1 to 9. At pH=10, the product would be Bi_3.84W_0.16O_6.24 rather than Bi₂WO₆. We also make a point that Bi₂WO₆ 3D architectures grow through the combination of Ostwald ripening and self-assembly. The photocatalytic results indicate that the 3D Bi₂WO₆ architecture exhibits remarkably improved photocatalytic activity in the photodegradation of rhodamine B than that of other morphological Bi₂WO₆, such as multilayered disks and nanoplates. The enhanced performance should be attributed to two factors:The first one should be coming from structure-induced enhancement of Bi₂WO₆ architectures. The 3D hierarchical Bi₂WO₆ architectures which possess a larger specific surface area than that of the reference samples （Bi₂WO₆ multilayered disks and nanoplates）, is evidently beneficial for the enhanced photocatalytic activity. Meanwhile,3D flowerlike architectures with good stability can effectively prevent aggregation and thus maintain a large active surface area, which result in more opportunities for the efficient diffusion and transportation of RhB molecules and hydroxyl radicals in the photochemical degradation reaction. Secondly, the Bi₂WO₆ architectures have the highest capability of absorbed oxygen species, which can easily capture electrons and accelerate the photocatalytic reactions by creating some high activity materials such as hydroxyl radicals.Reduction method is adopted to deposit Pd nanoparticles on Bi₂WO₆ 3D architectures. Pd@Bi₂WO₆ composite photocatalyst shows a superior performance compared with pure Bi₂WO₆ 3D architectures. The photocatalytic enhanced mechanism of Pd@Bi₂WO₆ composite photocatalyst is explored. We believe that Pd nanoparticles act as an electron acceptor, and photo-induced electrons can be transferred from the conduction band of Bi₂WO₆ to Pd nanoparticles quickly, leading to the increased separation efficiency of electrons and holes. Meanwhile, the absorption in visible light region for Pd@Bi₂WO₆ composite photocatalyst increases due to surface plasmon resonance effect of Pd nanoparticles. In addition, the Pd deposition results in a enhanced capability of absorbed oxygen species, which is also beneficial to the enhancement of photocatalytic performance. The different amount of Pd deposition （Oat%,0.5at%, lat%, 2at%,4at%） is also investigated, and the photocatalytic results show that the 2at% Pd-loaded Bi₂WO₆ sample exhibites the best photocatalytic activity, where the degradation rate can be up to 99% within 50 min.