Molecular Adsorption And Reaction On Single Crystal Metal Oxide Surfaces Studied By UHV-FTIRS

Since Fujishima and Honda discovered the photocatalytic water splitting on TiO₂ electrode in 1972,it has aroused a research upsurge of photocatalysis in the past few decades.Photocatalysis shows great potential in water splitting,photodegrade organic and inorganic pollution,solar cells and CO₂ photoreduction fields.The understanding of photocatalytic processes at molecular level is important for designing and optimizing photocatalysts to get higher catalytic efficiency and selectivity.In heterogeneous catalysis,the surface of a catalyst is the place where the reactions happen.The studies of interaction between surfaces and molecules are critical to understanding the catalytic mechanism.As two kinds of the most widely used photocatalysts,TiO₂ and ZnO are usually regarded as model catalysts to study the molecular adsorption and reaction processes.In recent years,by using multitechnique surface analysis methods,many progresses of photocatalytic mechanisms have been achieved on single crystal TiO₂ and ZnO surface.But still many fundamental surface chemistry processes of TiO₂ and ZnO are not well understood so far.More surface analysis methods to characterize surface processes from different sights are needed.Infrared reflection absorption spectroscopy?IRRAS?can distinguish different adsorbed molecules on surfaces and is very sensitive to the chemical surrounding changes of adsorbed molecules,thus it is usually regarded as an ideal method to study the molecular adsorption sites,adsorption configurations,reaction pathways and intermolecular interaction.IRRAS has been successfully applied to single crystal metal surfaces and achieved a wealth of information on metal surfaces on the atomic scale.However,IRRAS studies on single crystal oxide surfaces are challenging due to the inherent experimental difficulties arising from the insulating and semiconducting properties of many oxide materials.We built a new-designed state-of-the-art ultrahigh vacuum-Flourier transform infrared spectrometer?UHV-FTIRS?.The innovative design permits to record high-quality IR spectra on single crystal oxide surfaces with high sensitivity and long-term stability.In this thesis,by using UHV-FTIRS combined with density functional theory?DFT?calculations,we systematically investigated the molecular adsorption sites,adsorption configurations,intermolecular interaction and adsorbate-surface charge transfer on TiO₂?110?and ZnO?10???0?surfaces.The main results are as follows:1.We used CO and NO as probe molecules to study the polaron-involved molecular adsorption on reduced TiO₂?110?surfaces.Two typical scenarios were proposed:for CO adsorption,the subsurface polaron only transfers to the surface five-coordinated Ti?Ti5c?cation underneath CO,the surrounding change results in a weak shift of the molecular vibrational frequency;for NO adsorption,the subsurface polaron electron transfers directly to the Ti:3d-NO:2p hybridization orbital,the strong intramolecular modification causes a large shift of the molecular vibrational frequency.These scenarios are determined by the energy-level matching between the polaron state and the lowest unoccupied molecular orbital?LUMO?of adsorbed molecules.2.By using polarization-and azimuth-resolved IRRAS,we studied the NO adsorption and reaction pathways on perfect and reduced TiO₂?1010?surfaces.On perfect TiO₂?110?surfaces,we identified the cis-?NO?₂/Ti&Ti configuration of?NO?₂ dimer and gave the NO reaction pathway:NO ?cis-?NO?2/Ti&Ti ?N₂O + Oa.On reduced TiO₂?110?surfaces,the oxygen vacancy?Vo?alters the NO adsorption and pathway.We found three types of Vo-related NO configurations,and they all have strong interaction with polarons introduced by Vo.We also clarified the NO reaction pathways on reduced TiO₂?110?and found that all reaction intermediates convert to N₂O finally.3.We studied the growth of Au clusters on TiO₂?110?surface,then characterized and manipulated of charge state of these Au clusters.Using CO as a probe,we found that the as-grown Au clusters on oxidized rutile TiO₂?110?surfaces are electrically neutral based on the stretching vibrational frequency of CO.Through the reaction of?NO?₂?N₂O + Oa on the small fraction of bare TiO₂?110?surface,we succeeded in driving the electrically neutral Au0 to positively charged Au? state at the interface perimeter of Au cluster,reflected by the large blue-shift?20?26 cm-1?of CO stretching frequency on Au clusters with NO exposure.The magnitude of blue shift depends on both the Oa number and the Au cluster size.4.We studied the CO₂ adsorption and interaction on TiO₂?110?surfaces.With increasing CO₂ exposure,CO₂ adsorbs in succession at the Vo sites,on the Ti5c sites nearest to Vo,Ti5c sites away from Vo,and the bridging oxygen?Obr?sites at the low temperature of 90 K.The coupling occurred between neighboring CO₂ adsorbed on Ti5c sites,leading the v3?OCO?asymmetric stretching vibrations to splitting into two absorption bands in IR spectra.Two kinds of coupled geometries of adjacent CO₂ on Ti5.sites are determined by DFT calculations.For the higher CO₂ coverage??1.5 ML?,the horizontal adsorption configuration along[1???0]azimuth of CO₂ adsorbed on Obr sites is identified for the first time using polarization-and azimuth-resolved IRRAS in experiments.5.We studied the formation and evolution of orientation-specific CO₂ chains on ZnO?10???0?surfaces.We observed the fine structures of CO₂ vibrational levels on ZnO?10???0?surfaces,which are attributed to the formation of CO₂ chains with different lengths along[0001]direction according to DFT calculations.Such novel chain adsorption mode results from the relatively large attractive interaction between CO₂ and Zn3c atoms in[0001]direction.Further experiments indicate that the short chains at low coverage evolve into long chains by annealing.At higher CO₂ coverage,the as-grown?2×1?phase first evolve into an unstable local?1×1?phase below 150 K,and then into a stable well-defined?2×1?phase above 150 K.