| As an important photocatalytic material, titanium dioxide (TiO2) has been widely studied. However, its large intrinsic band-gap (>3.0 eV) limits the photocatalytic response of TiO2-based catalysts to ultraviolet portion, which only takes up -5% of the solar energy. In order to extend its ability to absorb and utilize the abundant visible light,doping is proved to be an efficient way to narrow its bandgap experimentally and theoretically. But so far, the doping geometries and the corresponding electronic properties have not been well characterized. Although the electronic band structure of TiO2 is important for understanding the photocatalytic mechanism and for improving its photocatalytic activity, the description about the electronic band structure, even for the widely studied valence band, is not sufficient. The occurrence of the electronic states near the Fermi energy under or after the UV light irradiation is not well understood. In this dissertation, the main topic is to address the doping effect on the bandgap reduction, the better description of the valence band, and the occurred states under/after UV light irradiation of rutile and anatase TiO2 surfaces.In chapter 1, I give a brief introduction on the research system and the main experimental techniques involved in this dissertation , including the applications,structure and electronic properties of TiO2, the introduction of pulsed laser deposition(PLD) technique for sample preparation, scanning tunneling microscope (STM) for surface structure characterization, scanning tunneling spectroscopy (STS), and photoelectron spectroscopy (PES) technique used for element composition analysis and band structure characterization. Following the introduction of the various experimental methods, their application in the research of TiO2 are referred.In chapter 2, I introduce the self-designed ultra-high vacuum PLD-STM-ARPES combined system, including the system components and its performance test. In this combined system, the preparation, surface morphology and structure characterization in real space and electronic band structure characterization in reciprocal space can be realized in-situ, which is quite helpful for studying the intrinsic properties of the material and developing novel materials.In chapter 3, we prepared the Cr-N codoped rutile TiO2(110) single crystal thin film by PLD method, and the measured band gap is reduced to 1.9 eV, which extended to the visible light region. In the codoped sample surface, two types of new discernable defects (peanut-like and butterfly-like) were resolved by STM,the STS spectra.acquired at different sites indicate that the tunning of the surface electronic property should be delocalized. Combing with theoretical calculations and simulations, we gave the reasonable doping geometries and the corresponding origin of electronic states.In chapter 4, the electronic band structure of anatase TiO2(001) single crystal thin film and rutile TiO2(110) were characterized by ARPES. For both samples, we obtained the clear valence band structures, which are consistent with the theoretical calculations.It is quite helpful for understanding the photocatalytic reaction mechanium of TiO2 and offering important information for how to improve the photo catalysis activity of TiO2 through its electronic properties. In addition, we used the helium light source and synchrotron radiation light source to adequately study the electronic states of anatase TiO2(001). The three-dimensional (kx-ky-kz) band structure around Fermi level were acquired by tuning the energy of excited light with different polarization. |
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