Dissertation
Dissertation > Mathematical sciences and chemical > Chemistry > Physical Chemistry ( theoretical chemistry ),chemical physics > Chemical kinetics,catalysis > Catalytic > Catalyst

Hydrothermal Synthesis, and Photocatalytic Study of Cu2O and Bi2WO6 Polyhedra

Author ZhangYing
Tutor XuAnWu
School University of Science and Technology of China
Course Nanochemistry
Keywords shape control polyhedra photocatalytic Cu2O γ-Bi2WO6
CLC O643.36
Type Master's thesis
Year 2010
Downloads 141
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With good photocatalytic activities under visible light irradiation, Cu2O and Bi2WO6 have attracted a lot of attentions from researchers nowadays. Besides, polyhedra are believed to have great potential applications in catalytic field. Using the advantages of hydrothermal method in liquid media reactions at low temperatures, we successfully synthesized 26 faces and 18 faces Cu2O polyhedra with distinctive photocatalytic properties and Bi2WO6 octahedra with poor ones. We investigated the shape effects of Cu2O polyhedral microcrystals on photocatalytic activity. It is found that due to the differences on structures of {110}, {100}, {111} crystal planes, Cu2O 26(18) faces polyhedra have better absorption and photodegrading ability than Cu2O octahedral and cubes in micro scale. However, Bi2WO6 octahedra exhibited much poor activity on degrading RhB. We analyzed in detail the mechanism of synthesizing these polyhedra and their differences in photocatalytic abilities in this dissertation. The main results can be summarized following:1 With use of stearic acid as a structure-directing agent, we successfully produced perfect mixed 26-facet and 18-facet polyhedra of Cu2O micro crystals via a hydrothermal process. Cu2O octahedra and cubes were also prepared under hydrothermal method by changing conditions. After samples characterized by XRD, SEM and so on, we proposed synthesis of Cu2O 26(18) faces polyhedra is due to an absorption-stabilization of {110} crystal planes. We investigated the three polyhedra photodegrading methyl orange, and 28(16) polyhedra turned out to be the best under visible light irradiation. We analyzed the reasons at the respect of atomic arrangements on the outside lay of basic crystal planes. The better photocatalytic activity of Cu2O 26(18) faces polyhedra than octahedra and cubes is a result of more dangling bonds or more active sites on {110} crystal planes that can trap the electrons or holes and make the carriers separated, which will enhance the sample’s photodegrading efficient. This finding implies that the performance of catalysts can be enhanced by shape control as shape determines the number of atoms located at the edges, corners, or surfaces. It also could promote the synthesis of other materials and the applications of polyhedra with active surfaces in the catalytic field.2 By changing the pH value of precursor solution to 10 or more,γ-Bi2WO6 pseudo-regular octahedra with different ranges of diameters can be prepared hydrothermally without any additives. It is demonstrated that the effect of pH value and reaction temperature are the key factors on the morphology of the final products. We proposed the formation ofγ-Bi2WO6 pseudo-regular octahedra is an integration of in situ-crystallizing process and nucleus ripping process when the bigger nucleus grows at the cost of small ones in the [131] direction. We proposed that the poor photodegrading efficient is due to the wide band gap ofγ-Bi2WO6 pseudo-regular octahedra (2.94ev) and the atomic structure of {131} planes where few dangling bonds exist at the surface which result in {131} planes few active sites to trap the electrons and holes and the carriers recombining at the surface or in the bulk. This work further implies that only when the crystal planes are active surfaces, the photocatalytic ability of the polyhedra can be improved and it is also important in explaining the pH effects on the photocatalytic activity of Bi2WO6.

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