Electronic Structures Manipulation And Surface Modification For Photoanode Materials

Sunlight is the only renewable energy source to replace fossil fuels and produce clean energy for the rising global energy demand. To continuously supply energy when sunlight is not available, a cost-effective storage of solar energy is demanded. Capturing solar energy in a chemical form exhibits a much larger energy density than foregoing artificial storage methods. Recently, a central thrust of the energy-related researches has focused on artificial photosynthesis of water splitting to convert solar energy into chemical energy, since hydrogen is ultimately clean and is to be used in fuel cells to construct clean energy systems. This dissertation presents a comprehensive investigation of the atomic structure, electronic structure and photochemical/electrochemical properties of doped TiO2materials and Co-based surface modification by using x-ray absorption fine structure (XAFS), density functional theory, X-ray diffraction (XRD), electron microscope and electrochemical/photochemical analysis methods.The main content in this dissertation is as follows:1. Electronic structure investigation on I-doped rutile TiO2Using density functional theory, the structural and electronic properties of iodine cation-doped rutile TiO2are studied. The total energy calculations show that iodine substituting for titanium sites in TiO2matrix is energetically favorable. The electronic stmcture calculations reveal that iodine doping induces a delocalized band consisting of I5s states and O2p states at the top of the valence band of TiO2-Due to this delocalized state, the band gap is markedly narrowed by about0.4eV, the optical absorption is extended to the visible light region, and the excited electron-hole pairs are expected to have better mobility. Moreover, the conduction band edge is raised above the reduction level of H2/H2O by I-doping, which enables the achievement of high photocatalytic efficiency of I-doped rutile TiO2.2. Photochemical activity investigation on codoping TiO2Using UV-vis absorption spectroscopy, Photocatalysis study, UV-photoelectron and density functional theory calculations, we studied the atomic structure, eletronic structure and photoactivity of N-S and Fe-P codoping TiO2. Photocatalysis study demonstrates that the photoactivity under visible irradiation or UV light for both samples are enhanced, due to a red-shift of absorption edge to700nm. Importantly, the UV-light photocatalytic results show a three-fold expanded photoactivity for the (N+S)-TiO2in comparison with the pristine/monodoped TiO2NCs, suggesting a highly improved quantum efficiency of photoconversion. UV-photoelectron spectroscopy experimental results show that the (N+X)-codoping causes a delocalization on the band gap states and band-to-band optical absorption. First-principles calculations reveal that the strong donor-acceptor interactions between the anionic-N and cationic-X dopants give rise to the formation of delocalized continuum bands, leading to an effective increase of the quantum efficiency. For Fe-P codoping system, it introduces continuum states above the valence band maximum and therefore significantly narrows the band gap by1.2eV. In addition, the separation probability of photocarriers is greatly improved due to the delocalized nature of the Fe majority and minority spin states caused by the P-weakened crystal field.3. The design of composite TiO2/FeTiOx@Co-Pi nanotube arraysXAFS spectroscopy, photoelectrochemical analysis were used to study the the very efficient nanostructured oxygen-evolving photoanode of Ti02/FeTiOx@Co-Pi nanotube arrays. The Fe k edge XAFS spectroscopy of this sample exhibit the same features with the Ti k edge XAFS spectoscopy of TiO2, indicating the substitution site of Fe. When coupled to TiO2FeTiOx that harvests visible photons, the cocatalyst of Co-Pi efficiently catalyzes the evolution of oxygen generation with an IPCE in excess of40%in UV region and remained as high as>30%at465nm. The saturation photocurrent density of Ti02/FeTiOx@Co-Pi at a lower potential of-0.55V vs Ag/AgCl corresponds to a photo-conversion efficiency of2.2%, the best value for an anatase TiO2photoanode. The surficial composite treatment increases the donor density of TiO2nanotubes by2orders of magnitudes, passivates the surface states on TiO2/FeTiOx surface, and facilitates the charge separation and transportation to improve the quantum efficiency.4. Electrochemical activity investigation on PO43-related Co3O4catalystXAFS spectroscopy, electrochemical analysis and density functional theory were used to study the phenomenon that the electrochemical performance of C03O4has been strengthened by phosphate. The Co k edge XAFS spectroscopy of the electrochemically prepared Co3O4and P-Co3O4and thermochemistry prepared Co3O4exhibited that their all have the Co4O4cube structure. The V-I curve current density traces of three samples confirmed their properties are strongly related with the electrolyte. It is worth noticing that P-Co3O4shows better performance under non-phosphoric electrolyte. The first-principles calculations showed that H2O prefers to dissociate on P-Co3O4or Pi-Co3O4surface which ensures the easy getting of OH, making Co3O4with catalytic activity under neutral aqueous solution.