【作者】 张莹；

【导师】 徐安武；

【作者基本信息】 中国科学技术大学 ， 纳米化学， 2010， 硕士

【摘要】 近几年,由于在可见光下表现出的良好的光催化活性, Cu 2O和Bi 2 WO 6广泛引起研究者们的注意。另外,许多研究证明了多面体在催化方面具有潜在的应用价值。本论文以Cu 2O和Bi 2 WO 6为研究对象,发挥水热合成技术在形貌控制合成方面的有利优势,制备了具有26面体和18面体形貌的Cu 2O和八面体形貌的Bi 2 WO 6,我们研究了形貌对光催化性能的影响,发现由于{110}, {100}, {111}晶面表面结构不同导致Cu 2O26面体和18面体的光催化活性优于立方体和八面体。但是, Bi 2 WO 6八面体可见光下降解罗丹明B表现的光催化性极差。本论文中,我们详细研究了多面体的生长机理和在光催化性能方面的应用可能性,主要包括以下几个方面的研究工作:1、利用硬脂酸作为添加剂调控液相反应过程,我们用简单的水热方法成功制备了氧化亚铜26面体和18面体;通过改变实验条件还合成了Cu 2O八面体和立方体。对多面体的结构运用XRD,SEM等详细进行了表征,提出26(18)面体的合成是晶面吸附-稳定进而长大的过程。此外,采用对比研究的方法,通过对甲基橙溶液进行光降解,发现26面体和18面体比八面体和立方体有更好的光催化活性,第一次提出了氧化亚铜{110}晶面的活性高于{111}晶面和{100}晶面。我们从晶面表面原子排布详细分析了这一结论的合理性,在本工作中{110}晶面表面具有最多的悬键数目,极性强,活性位多,导致光生电子和空穴能够有效分离,进而提高了光催化活性。这一工作对形貌控制晶体的合成及提高其光催化性能起到了积极的促进作用,同时对多面体在光催化剂方面的应用也提供了良好的正面实例。2、我们通过调节前驱物溶液pH值,用水热法合成了纳米级γ-Bi 2 WO 6假正八面体,经研究发现前驱物溶液的pH值和水热温度是产物影响的关键因素,并从热力学和动力学两方面分析了八面体形成的机理。γ-Bi2WO6晶体的生长过程综合了原位结晶过程和Gibbs-Thomson的晶体生长熟化过程,大晶核沿[131]晶面长大成为八面体。通过降解罗丹明B溶液,发现γ-Bi2WO6较差的光催化活性,我们从八面体带隙宽(2.94ev),和八面体表面无缺陷中心或活性中心而导致光生电子和空穴在表面和体相复合两方面对其进行了合理的解释。这个工作进一步证明了只有当多面体表面晶面的活性中心数目较多时,才会有好的光催化性能;为合成pH条件影响层状化合物光催化性能方面提供了很好的解释途径。更多还原

【Abstract】 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.更多还原