Introducing Strain In Anatase TiO₂?001?Films By Epitaxial Growth And Structural Study

Anatase TiO₂?001??abbreviated as ATO in the following text?surface was widely studies due to its excellent catalytic properties.Some recent theoretical works suggested that the introduction of strain in the ATO surface may lead to the distortion of the surface lattice,and may result in the enhanced reactivity of the surface.Thus,it is necessary to study the effect of strain on the structure of ATO surface.Here we present a systematic study on the epitaxial growth of ATO films on BaTiO₃?001?/SrTiO3?001??abbreviated as BTO and STO,respectively?substrates using pulsed laser deposition?PLD?method,characterized using x-ray diffraction?XRD?,x-ray photoemission spectroscopy?XPS?,scanning transmission electron microscopy?STEM?,and scanning tunneling microscopy?STM?.A thin layer of BTO was first epitaxially grown on STO substrates,with the purpose to introduce the strain in the ATO films by considering the relatively large lattice mismatch between the ATO and BTO.The structure of this thesis is as follow:In chapter ?,the research background of this thesis is introduced.Firstly,literatures concerning the structures,properties and applications of TiO₂ materials are surveyed.Then,the research progress of the structure and defects of ATO surface is given in detail.Lastly,recent studies on tuning the physical or catalytic properties of various materials,including TiO₂,by strain engineering are introduced.In chapter ?,the focus is on the growth of ATO films with the introduction of strain.Firstly,various studies on the preparation of ATO films are surveyed,based on which the requirements for substrate used to introduce strain in ATO films are discussed.Then,the experimental details of thin films preparation methods for BTO films grown epitaxially on Nb-doped STO substrates and ATO films grown on BTO films by PLD method are given.Lastly,the ATO/BTO/STO films are characterized using XRD and STEM.The results indicate that the strain can be partially introduced in the ATO films when the thickness of the BTO layer is about 4-6 nm.In chapter ?,the investigation is on the surface structure of ATO/BTO/STO films.The controversies about the surface structure of ATO are briefly reviewed before the experiments.The XPS results show that the suitable thickness of the ATO films is about or over 15 nm,which can result in a negligible concentration of the outward-diffused Sr and Ba in the surface,and minimize their possible effect on the surface structure.Dominant Ti⁴⁺ oxidation state is observed,showing that the ATO surface is fully oxidized.The surface structure characterized by STM shows that the?1×4?reconstruction is remained as those films directly grown on STO substrate,however,the ridges in the?1×4?reconstructed surface show additional super-periods,which is typically shown as the dim features in the ridges separated by 2-5 lattice distances.On the basis of the high resolution STM images and the fully oxidized characteristic of the surface,we propose that dim features could be caused by the "TiO₂" defects in the ridges,according to the ad-oxygen model?AOM?for the fully-oxidized?1×4?reconstructed ATO surface.