| The photocatalysis of titanium dioxide(TiO2)has provided an effecient strategy for solar energy conversion that can be used for water spliting and pollutant degradation.Such promising applications have made TiO2 photocatalysis a hot research topic in energy and environment science in the past four decades.As an important prototypic system for the surface science study of transition metal oxides,it is of great significance to study the adsorption of small molecules,photochemical reaction,interfacial electronic coupling,electron-boson couplings of metallic state,which may provide significant implications to understand the mechnisms of photocatalysis and to improve photocatalytic efficiency.In this dissertation,the mechanism of the chemical reaction between H2O and CH3OH on anatase TiO2(001)film surface,the interfacial electron structure of H2O molecule after adsorption,and the electron-boson coupling of metal state were systematically studied by using ultraviolet photoelectron spectroscopy(UPS),X-ray photoelectron spectroscopy(XPS)and angular resolution photoelectron spectroscopy(ARPES).In chapter 1,as a basis of the research work of this thesis,I give a systematical introduction on the research system and photoelectron spectroscopy.First,the crystal structure,surface reconstruction,defect and electronic structure of TiO2 are introduced.Second,the principle,classification,theory,experimental system and analyzer of photoelectron spectroscopy(PES),especially the angle-resolved photoelectron spectroscopy(ARPES),are introduced.In chapter 2,I describe our study on the hydrogen bond(H-bond)networks promoted photo-excited H2O dissociation on A-TiO2(001)surface.On A-TiO2(001)surface,the dissociation of H2O is almost negligible when the H2O coverage is less than sub-monolayer coverage.However,when the coverage is higher than 1 ML,we find that the H2O dissociation can be obviously observed.Combining with the density functional theory,it is found that the H-bond networks at H2O/TiO2 interafce can form when the H2O coverage is above 1ML,which can not only lower the H2O dissociation barrier,but also can assist the proton and hole transfer at the interface.These findings reveal the catalytic reaction at the interface of A-TiO2(001)at higher water coverages,which stress the important role of hydrogen bond network in the dissociation process of H2O,providing an in-depth understanding of the microscopic process of H2O dissociation.In chapter 3,I investigate the microscopic mechanism of photocatalytic dissociation of CH3OH at different sites on A-TiO2(001)surface.Methanol(CH3OH)is a good hole scavenger,which has higher oxidation efficiency than H2O and has been used as model system to study photooxidation and organic reaction.I have studied the effects of different factors such as the coverage and UV light illumination time on the photo-excited dissociation of CH3OH at A-TiO2(001)surface by in-situ STM and PES.In situ STM images allow us to determine the coverage of 1ML CH3OH adsorption,which corresponds to two CH3OH molecules adsorbed at ridge and terrace sites,respectively.That is.four methanol molecules in the 1×4 reconstructed unit.XPS spectra shows at low temperature the dissociation of CH3OH is photo-excited,while at near-room temperature the dissociation of CH3OH is a spontaneous process at the defect sites.Furthermore,the UPS spectra shows the dissociation of CH3OH at ridge and terrace sites can result in the gap states(GSs)with different peak positions.These results reveal the active characteristics of ridge and terrace sites in the photo-excited reaction of CH3OH.In chapter 4,I mainly describe study on the electronic coupling between the molecular orbitals and valence bands(VBs)at the H2O/A-TiO2(001)interface.Interfacial charge transfer is the premise of catalytic reaction,which depends greatly on the coupling of interfacial electronic states.Photocatalytic oxidation of H2O,photo-hole from the valence band of TiO2(valence band,VB)transferred to H2O highest occupied level(the highest occupied molecular orbit,HOMO)is an important step of reaction.Therefore,direct detection of the coupling of HOMO and VB at the H2O/TiO2 interface is an important aspect of understanding the catalytic micro-mechanism.By using ARPES,I first study the dispertions and distribution of the VBs and O 2s core level of pristine A-TiO2(001).The the core-level and valence bands with contributions from the surface and bulk of A-TiO2(001)are clearly distinguished and an inerting surface model of AOM-ADM mixture is proposed.Further,the VBs from surface O atom are found to interact with the molecular orbitals of H2O directly,that is,the dispersion of the surface VBs is observed to change by strong coupling with the HOMO of water when the H2O is gradually dosed onto A-TiO2(001)surface.Combining with DFT calculation,I find that the H2O adsorption can cause charge redistibition at different surface O atoms,which lead to the dispersion change of the surface VBs.These results provide direct evidence that the HOMO of H2O can hybridize with the VBs of TiO2,which provides insights for the charge transfer processes at the H2O/TiO2 interface.In chapter 5,the coupling mechanism between the conductive metallic state formed by electron doping and the boson is studied.We used angular resolved photoelectron spectroscopy(ARPES)to irradiate A-TiO2(001)surface with different intensies,and observed the Frohlich polaron formed by long-range electron-phonon coupling under low electron doping concentration.At high electron concentration,the long-range electron-phonon coupling is suppressed by the dynamical screening,which evolves into the short-range electron-phonon coupling with a kink feature.At the same time,the plasma frequency is much larger than phonon frequency,and long-range electron-plasmon coupling became dominant to form plasmonic polaron.The coexisting plasmonic peaks and kink features enable us to quantitatively distinguish the two kinds of coupling and help us to understand the complex many body interaction. |
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