First Principles Studies On Surfaces

The studies on surfaces are very attractive. Surfaces and interfaces have wide applications, such as in catalysis, molecular device, electron emission device, etc. In experiments, X-Ray techniques are efficient tools to determine the structure of contact region and the species of adsorbate. Theoretical investigations can not only explain experimental phenomena but also give some clues for new experiments. With the progress in computational methods and enhancement of computational ability, density functional theory(DFT) based first-principles calculation has been widely used to study the reaction mechanisms, electronic structures of surfaces and with the development of theoretical simulations, the X-Ray adsorption spectra, photoelectron spectra etc are able to be calculated.In the first part of Chapter 1, we introduced the basic idea of density functional theory, from original Thomas-Fermi model, Hohenberg-Kohn theorem, to Kohn-Sham equation. We also briefly introduced the widely used exchange-correlation function-als.In the second part, we gave a brief introduction of some DFT based simulation packages used in the following work, such as VASP, DMol³,SIESTA etc. At the end of this chapter, we introduced some experimental methods which are used to study the properties of surfaces.In Chapter 2, the catalysis of gold is investigated. Though gold is very inert, oxygen pre-covered gold surface has shown its activity in some oxidation process. In the first part of this chapter, we carefully studied the adsorption of NO and oxygen atom on perfect gold surface and the reaction mechanisms of NO oxidation on oxygen precovered gold surface. Our calculations indicate that the NO oxidation process is that NO adsorbs easily to oxygen pre-covered surface to form adsorbed NO₂ or NO₃, thenthe products desorb from the surface to form gas phase NO₂.The rate-limit step is the desorption step. The active energy of the reaction is closely related to the coverage of pre-covered oxygen. After this work, we investigated the CO oxidation by activated water on oxygen pre-covered Au surface. Because of the strong hydrogen bonds be- tween water and adsorbed oxygen, waters are easily dissociated to OH groups which can oxide CO to COOH. With the help of adsorbed oxygen, hydroxyl, or water, COOH can dissociate to CO₂.On the other hand, after water extract hydrogen atom from COOH, water becomes adsorbed OH₃.And if two OH₃ are generated, we can get one extra oxygen atom on the surface which can oxide another CO to form more CO₂ or activate another water molecule. The activation energies of the elementary steps are low enough to ensure the proceeding of the reaction at low temperature. At the third part, we investigate the adsorption of oxygen molecules on Au/Ni(111) alloy surface and the following CO oxidation. Different adsorption structures of oxygen molecule are discovered by our calculations. The evolution of HREELS spectra is attributed to the elementary steps proceeding at different temperatures. Except for the reaction mechanism of surface reaction, we also calculated the vibrational frequencies of reactants,products and intermediates. We expect the calculations can give some clue for the experiments.In chapter 3, we focus on the electronic structures of surfaces. The first one is about FeO ultrathin film on Pt(l11) surface. A more realistic geometry structure with different magnetic structures are studied. A special structure is found with which we can explain the experimental phenomena such as STM images, surface potential and zero-bias abnormal etc. In the second work, the electron photoemission mechanism of diamondoid monolayers on gold and silver surfaces are intensively studied with periodic combined with cluster calculations. We find the LUMO level lies under vacuum level at ground state, while the excited electrons can push the LUMO of molecules above the vacuum level. Electrons accumulate on these orbital and then emit into the vacuum. The process is typically a final state effect, and can not be simply explained by the ground state's picture. This source of monochromatic electrons may find application in technologies, such as electron microscopy, electron beam lithography, and field-emission flat panel displays.In Chapter 4, we turn our attention to the simulation of X-Ray spectra with periodic boundary condition. In the first work, we studied the adsorption of 1,4-dithiol benzene molecules on gold surface, nanowire and dot. We find that the molecule prefers to adsorb on the most coordinated gold atoms, while the structure with higher adsorp- tion energy may not correspond to higher conductance. By calculating the S 1s XAS spectra and UPS spectra, we can get some relationship between the spectra and the adsorption structures. In the second work,we studied the adsorption and dissociation of waters on rutile TiO₂(110) surface with first principles molecular dynamics simulation. We then calculated the XPS spectra with (Z+1) approximation and attribute the origin of the peaks in experimental spectra. After the excitation of core electron, the system have some interesting phenomena which provides an interesting field for theoretical simulation.