First principles study of oxide semiconductors for solar energy applications

The objectives of this thesis are to understand the electronic structures of oxides and oxynitrides for photocatalytic water splitting, examine the Casimir interaction between oxides, and explore possible approach to bridge the Casimir force and material properties for advanced material research. The studies were performed in the framework of the density functional theory, many-body perturbation theory, i.e, the GW approximation and Bethe-Salpeter equation, as well as the Casimir-Lifshitz approach. The thesis consists of two sets of results. In the first part (papers I-VI), the electronic structures of oxynitrides, i.e., ZnO-GaN and ZnO-InN, with different compositions and local structures have been studied. The oxynitrides reduce the band-gap energies significantly compared to the binary counterparts, enabling the oxynitrides to act as visible light active photocatalysts. Formation of cluster--like structures further reduces the band-gap and delocalizes the valence bands, benefiting higher optical absorption. Furthermore, the energy levels between oxynitride and water were aligned using a surface model adapted from semiconductor heterostructure. In the second part (papers V-IX), the electronic structures of oxides as well as the Casimir interactions have been examined. In particular, we investigated the differences of optical and electronic properties between SnO2 and TiO2 polymorphs in terms of band-edge characters and electron-phonon coupling. In addition, we synthesized a mesoporous material possessing two types of pore structures (one is hexagonal ordered with pore diameter of 2.60 nm and the other is disordered with pore diameter of 3.85 nm). The pore framework contains four-coordinated titanium and oxygen vacancies, verified by both experimental measurements and density-functional theory calculations. Utilizing the predicted properties of the materials, we studied the Casimir interactions. A stable equilibrium of Casimir force is achieved in planar geometry containing a thin film and porous substrates. Both the force and equilibrium distance are tuned through modification of the material properties, for instance, optical properties and porosity. Furthermore, we adapted this concept to study the interactions between gas bubbles and porous SiO2 in water. A transition from repulsion to attraction is predicted, which highlights that the bubbles may interact differently at different surface regions.QC 20150423