First-principles Studies Of The Single-molecule Chemical Reactions And The Photocatalytic Mechanism On TiO₂Surfaces

With the rapid advances in theoretical methods and computational techniques, first-principles calculation has become an essential scientific tool. Using theoretical calcula-tion, we can simulate and interpret the experimentally observed phenomena and predict properties which are difficult to observe in experiments. Theoretical calculation plays an important role in the development of information, energy, environment and many other aspects. Titanium dioxide is one of the most promising photocatalysts for solar energy conversion and environmental cleanup, and also its great potential in hydrogen production from photocatalytic or photoelectrochemical water splitting has attracted a lot of research interests. In this thesis, we explore the single-molecule chemical reac-tions and the photocatalytic mechanism on titanium dioxide surfaces by first-principles calculation. We have studied the structure and the reactivity of anatase TiO2(001)1×4reconstructed surface, level alignment for H2O on anatase TiO2(101) suface, and the photoemission excitation pathways in rutile TiO2(110). These studies are beneficial for the understanding of the photocatalytic process and mechanism.In chapter1, we give an introduction to the theoretical approaches for the electronic structure calculation, from Hartree-Fock method to density functional theory. Then the functionals for exchange and correlation, basis, and pseudo-potentials are presented. In the end of this chapter, we briefly describe some frequently used simulation packages.In chapter2, we conduct a systematic investigation of the local structure and chem-ical activity of anatase TiO2(001)1×4reconstructed surface by first-principles calcu-lations in combination with experiments. We first briefly review the crystal structure, electronic structure, and photocatalytic properties of titanium dioxides. Then some ex-perimental technologies are introduced:the scanning tunneling microscope (STM), X-ray photoelectron spectroscopy (XPS) and ultraviolet photoelectron spectroscopy (UP-S). Based on the experimental results and first-principles calculations, we provide a new structural model, in which the surface is oxidized. Two types of intrinsic point defects are identified, and the bright defect is the most active cite for H2O and O2adsorption.In chapter3, we apply many-body quasipartical (QP) GW techniques to determine the level alignment of H2O on anatase TiO2(101) surface, and compare the results to H2O on rutile TiO2(110). We begin by presenting the Green function techniques and GW approximation. Then QP GW calculations on H2O/TiO2surfaces are performed. A variety of coverages of intact and dissociated H2O on stoichiometric and defective anatase TiO2(101) are considered, and comparison to H2O on rutile TiO2(110) are made thoroughly. We find the highest H2O PDOS peak is higher for dissociated H2O than intact H20. We also find that the highest H2O PDOS peak for anatase TiO2(101) is closer to VBM than rutile TiO2(110). This suggests hole trapping is more favorable on anatase TiO2(101) than rutile TiO2(110) and may explain why anatase is more photocatalytically active than rutile for H2O photooxidation.In chapter4, we investigate the photoinduced electron dynamics within the con-duction band of reduced rutile TiO2(110) surface in combination with multiphoton photoemission (mPP) spectroscopy. In the beginning the multiphoton photoemission (mPP) experiments and experimental results are demonstrated. Then we calculate the transition densities for d-d transtions in rutile. Our calculation verifies the t2g-eg transi-tion observed in experiments, and gets the similar results with experiments. This work expands our understanding of the charge-carrier dynamics within the conduction band of TiO2.