Study On Manipulating Structures And Performances Of Oxide-based Catalysts For Water Splitting

The supply of secure, clean, sustainable energy is arguably the most important scientific and technical challenge facing humanity in the21st century. The utilization of solar energy to replace fossil fuels is a promising method for meeting the growing global energy demands and solving the current environmental problem, which is significant for the sustainable development of China’s future economic strategy. The most attractive approach to solar energy conversion and storge is using photoelectrochemical cells (PEC) to split water into hydrogen and oxygen. TiO2was a proper candidate for photoelectrodes due to inexpensive, stable and nontoxic. However, the large intrinsic band gap of TiO2allows only the absorption of ultraviolet light but prohibits the effective absorption of visible light. Meanwhile, the electrochemical activities of conventional semiconductor photoelectrode materials are generally low, which seriously affect the energy conversion efficiency of the photo induced water splitting. This dissertation presents a comprehensive investigation of the relationship between electronic structure and photochemical/electrochemical properties of TiO2anode and Co3O4-based catalysts by using the density functional theory (DFT), linear sweep voltammetry (LSV), cyclic voltammetry (CV), electrochemical impedanc spectroscopy (EIS), electron microscope, X-ray diffraction (XRD) and X-ray absorption fine structure (XAFS) analysis methods.The main content in this dissertation is as follows:1. Structural and Optical Absorption investigation on P-doped TiO2The efficient absorption of visible-light is crucial for improving the photocatalytic activity of foreign species doped TiO2With first-principles calculations, we explore the effects of phosphorous doping on mediating the photocatalytic activity of anatase. It is found that the P impurity tends to occupy the cation site in TiO2; more importantly, there exists a critical phosphorous concentration of about0.7%for maximizing the absorption of solar light. The optical energy gap is narrowed by～0.3eV at the low doping concentration of0.7%, while it increases with P concentration at the higher P concentrations region of0.7-3.1at.%. These results suggest that the dopant concentration dependence might be responsible for numerous seemingly controversies of optical absorption observed in experiments. This finding points to a possibility of tailoring the optical absorption of TiO2by varying the dopant content.2. Improvement of activity by manipulating surface structures of NixCo3-x04Controllable synthesis and modification is important for the construction of highly efficient and stable material. Here, we report a possible approach to manipulate ions distribution and local structure of spinel Co3-xNixO4in a long range by the moderate electric field driven. The continued increasing of oxidation and reduction peaks of CV along with XPS results exhibit surface enrichment process of Ni and XAFS measurement strongly suggests an occupation tendency from octahedral to tetrahedron is along with the migration. The transport of Ni is believed to carry out along the channels parallel with the tetrahedron ion sites and is based on the position exchange of Ni and Co ions whose balance is change when close to the surface. This discovery provides a guide for the future design and optimization of functional material and also reveals some new information in fundamental physics.3. Electracatalytic activity of Co3O4controlled by solution ambientThe investigation of the relationship between the electrocatalytic performance and the electrolyte environment is significant for photoelectrochemical water splitting applications. A quantitative research of Co3O4nanowire arrays is performed to provide a function that describes the influences of solution pH values and reactant diffusion processes on the catalytic activity of water splitting. According to the linear sweep voltammetry and electrochemical impedance spectroscopy, the diffusion process is the major factor that hinders the catalytic current of Co3O4electrode when the solution pH is decreasing. Through improving convection such as stirring, the diffusion layer thickness can be markedly decreased, yielding four to five times enhancement of catalytic current density which indicated a possible way to extend the efficiency of Co3O4to near neutral environment. These results provide the quantitative theoretical supports for maintaining the activities of water splitting catalysts in the design of PEC.4. Roles of synchrotron radiation of XAFS on study of ancient ceramicsThe Fe K-edge X-ray absorption near-edge structure (XANES) spectra of a series of ceramic shards were measured by fluorescence mode to reveal the color-generating techniques of Chinese porcelain. The analysis disclosed the intrinsic relationships among the chemical form of the iron, the firing conditions, and the colors of the ceramics. The results indicated that the coloration for different ceramics depends on the valence states of iron as the main color element in glaze and the proportion of Fe2+and Fe3+is attributed to the baking technology. The findings provide important information for archaeologist on the coloration researches.