First-principles Study Of Electronic Properties Of Several Metal Oxides

Recently, the rapid development of synthesis methods and applications of low-dimensional materials, various nanoplates, nanorods, nanowires, nanotubes and semi-conductor composites with given crystal facet have become the extensive investigated systems in metal oxides. With the booming of experiments, theoretical work also has made great development. For example, density functional theory (DFT) calculations and simulations can not only be used to explain and understand experimental observa-tions, but also give some interested theoretical predictions. In this dissertation for Ph. D, we perform extensive DFT-based first-principles calculations, and explore the facet-dependent electrochemical property toward heavy metal ions (HMIs) of nanocrystals, electronic structures and optical absorption properties of TiO2nanotubes and arrays, photo-response behavior of semiconductor composites, and impact of oxygen vacancy on band structure engineering of codoped anatase TiO2. This dissertation contains the following chapters.In Chapter1, the basic theory and framework of the first-principles method are briefly reviewed, including Hartree-Fock method, the fundamental theorem of DFT, exchange-correlation functional, plane wave basis set as well as pseduopotential. At last, we introduce two computational packages adopted in this paper.In Chapter2, we focus on the facet-dependent electrochemical properties of metal oxide nanocrystals toward heavy metal ions. Recently, nanoscale metal oxides as nov-el modifiers have been reported in electrochemical detection of HMIs, which is one of the central issues of water environment pollution. Combining the electroanalytical ex-perimental measurements and DFT simulations, we find that the (111) facet of CO3O4nanoplates has better electrochemical sensing performance than that of the (001) facet of CO3O4nanocubes. Adsorption of HMIs (i.e. Pb) is responsible for the difference of electrochemical properties. Similarly, we find that the electrochemical performances of the α-Fe2O3nanostructures also depend on their exposed facets for the first time. These theoretical findings clearly reveal that the selectivity electrochemistry response mechanism is strongly related to the adsorption behavior of HMIs on nanoscale metal oxides, which not only suggests a promising new strategy for designing high perfor-mance electrochemical sensing interface through the selective synthesis of nanoscale materials exposed with different well-defined facets, but also provides a deep under-standing for a more sensitive and selective electroanalysis at nanomaterials modified electrodes.In Chapter3, we investigate electronic and optical properties of TiO2nanotubes and arrays. In the past years, the synthesis, properties, modifications, and applications of TiO2nanomaterials have attracted much research attention. Here, we investigate the stability, electronic structures and optical adsorption properties of the single-walled TiO2nanotubes and arrays based on extensive first-principles calculations. We find that the SWTONTs energetically prefer the S2n structure, which is constructed by anatase TiO2(101) monolayer. Compared with S2n (-n, n) SWTONTs, the calculated Young’s modulus of the Dnd (-n, n) SWTONTs are more stiff due to their relative large strain energies. The band gaps of hexagonal TONTAs are not sensitive to their apertures and less than that of TiO2bilayer. The narrowing band gaps of TONTAs originate from the edge states mainly contributed by the Ti and O atoms at the core region. The calculated optical adsorptions of both SWTONTs and TONTAs indicate that they display anisotropic feature. These results clearly reveal that the electronic and optical properties of TiO2nanostructures are strongly related to their symmetry, dimension and morphology, which provide useful insights into the understanding of the related experimental observations.In Chapter4, we study the photo-response behavior of the hybrid a-MoO3and TiO2composites. Coupling between semiconductors has been shown an effective method for tuning carrier separation and light absorption efficiency. Here, we explored the electronic and optical properties of monolayer (ML) and bilayer (BL) of α-MoO3sheets covered on TiO2(001) surface. Theoretical results show that the stable Mo-O-Ti bonds form at the interfaces between the α-MoO3and TiO2(001) surface. The charge transfer from TiO2(001) surface to α-MoO3sheet results in the offset of the valence bands, which enhances the optical absorption in the visible region, indicating that this proposed composite is a good light-harvesting semiconductor. For the BL a-MoO3/TiO2composite, the predicted band alignment benefits the separation of pho-togenerated carriers. Interestingly, electrons move to the outer layer α-MoO3. Clearly, these theoretical findings show insight into the future design and development of novel hybrid metal oxides.In Chapter5, we explore the impact of oxygen vacancy on band structure engi-neering of co-doped anatase TiO2. TiO2is one of the most important photocatalyt-ic semiconductors, doping with various impurities is an effective method to enhance their photoactivities. Here, we investigate the effect arising from oxygen vacancy on geometric and electronic properties of compensated and non-compensated co-doped anatase TiO2systems. Theoretical calculation results show that oxygen vacancy prefers to occupy the neighboring site of n dopants. The presence of oxygen vacancy can ef-fectively reduce the unoccupied impurity bands in co-doped anatase TiO2, which are the recombination centers for the photogenerated electron-hole pairs. Moreover, the alignment of conduction bands in co-doped systems are not sensitive to oxygen vacan-cy, which benefits for hydrogen production via water splitting. Finally, we examine the optimal growth conditions, and find that oxygen vacancy is easily introduced in (V+N) and (Cr+N) co-doped systems under O-poor condition, while for (V+C) and (Cr+C) co-doped systems, O-rich condition is needed to produce oxygen vacancies. Clearly, these theoretical findings are helpful for designing co-doped TiO2systems.