Scanning Tunneling Microscopy Studies of Single Atoms and Molecules on TiO2 Surface
Author:Dong Shi Hui
The energy shortage and environmental pollution have become increasingly serious problems with the rapid economic and social developments.Solar energy is an inexhaustible source of energy on which human beings depend.How to efficiently convert solar energy through various methods has become the essential question of energy science.Since 1972,Fujishima and Honda first discovered the photocatalytic water splitting into hydrogen and oxygen on titanium dioxide(TiO2),which made photocatalysis become the research focus of the solar energy conversion into chemical energy in the past 40 years.Except the photocatalytic water splitting,TiO2 provides amazing activity in catalytic oxidation of carbon monoxide,methanol photodecomposition and other organic degragation,which exhibit potential ability to solve both energy and environmental problems.Therefore,the TiO2-related researches have scientific significance and strategic importance,and the TiO2 has become an important model system in the field of surface science based on transition metal oxides.TiO2 has three kinds of crystal phases:rutile,anatase and brookiete,and each phase envolves different stable facets.In this thesis,we present our studiesincluding:(1)the adsorption configurations and the electronic structures of gold(Au)single atom(SA)on rutile TiO2(110)surface.(2)the interaction of carbon monoxide and oxygen with platinum(Pt)single atoms on the surface of rutile TiO2.(3)the adsorption of methanol(CH3OH)molecules on anatase TiO2(001)with surface reconstruction.In chapter 1,First,I briefly introduce the sructures,electronic properties and the applications of different polymorphs of TiO2 and single atom catalysis.I also introduce the principles of scanning tunneling microscope(STM)and the related spectroscopic methods used in our experimental studies.I briefly review some previously reported results on the STM studies of TiO2 and properties of metal contacting with semiconductor.In chapter 2,I will introduce the photo-response of Au SAs on the(110)surface of rutile TiO2.By using the in situ electron beam(E-beam)evaporation method,the controllable growth of Au SAs on TiO2 are realized.It is found that Au SA could stably adsorb on the surface of rutile TiO2(110)not only at the oxygen vacancies(Ov),but also at the Ti5c sites.The dI/dV spectra measurements indicate a gap state,locating at 1.2 eV below Fermi level,can be only detected for the Au SA at Tisc but not for those at Ov’s.Such a localized state for Au SA at Ti5c can be assigned to the metal-induced gap state(MIGS)at the Au/TiO2 interface.It is observed that the Au SAs adsorbed at Ti5c can diffuse under ultraviolet light illumination,which is very similar to the results when the negative pulses(hole injection)are applied on Au SAs by the STM tip.Combining with the density functional theory,we reveal the photo-induced diffusion of Au SAs is a result of the transfer of photo-excited hole to Au SAs through the MIGS,where the hole transfer can weaken the bonding orbital of Au-Ti and also reduce the diffusion barrier.Such a MIGS-assisted hole transfer provides a potential correlation at the Au-TiO2 co-catalytic surface,which is promising to promote the reactions at low temperatures.In chapter 3,I describe our study on the interaction of CO and O2 with the Pt SAs on rutile TiO2(110)surface.As a widely used noble metal catalyst,Pt has shown high catalytic activity in the CO oxidation.Molecular-level understanding of the interaction and chanrge transfer between CO,O2 and Pt is of great significance to get insights into the catalytic mechanisms.Using E-beam evaporation in situ,method,I prepare Pt SAs on rutile TiO2(110).It is observed that the Pt SAs locate almost at the Ov’s,which is consistent with our calculation results that the adsorption energy of Pt SA at the Ov is the most favorite.After dosing CO and O2 to the Pt pre-adsobed surface,we find that the structure of Pt-CO shows brighter contrast at positive bias in the STM images,while the Pt-O2 shows darker contrast,indicating different charge transfer between Pt and molecules.Both the Pt-CO and Pt-O2 indicate asymmetric configurations with the O bonding to an adjencent Ti5c sites.Combining with the density functional theory,we discuss the charge transfer states between Pt atoms and molecules,which lays the foundation for us to further study the catalytic oxidation of CO on the surface by Pt SAs.In chapter 4,I study the adsorption behavior of CH3OH on anatase TiO2(001)surface with surface reconstruction.First,the adsorption behaviors of CH3OH moleculse on TiO2 surface at different temperatures are studied.At 80 K,with the increase of coverage,CH3OH can adsorb on the surface not only on the ridge sites,but also on the terrace sites.At room temperature,a small amount of CH3OH exposure only leads to dissociated adsorption at the surface defects.When a large amount of CH3OH melecules are exposed to the surface,the original 1×4 reconstructed surface will be transformed into 1 ×3 reconstruction.With the appearance of such 1×3 reconstruction,the lattice constant of the surface will change slightly.This change lead to CH3OH molecules can adsorb on the ridge sites of the surface at room temperature.In combination with the results of temperature programmed desorption(TPD),the adsorption at room temperature caused by this reconstruction is likely to be the active source of surface C-C coupling reaction.