Low-Temperature STM Study Of Small Molecules Adsorbed On ZnO(10(?)0)Surface

Zinc oxide(ZnO)is a technically important material which has found wide applications in many fields.Particular in heterogeneous catalysis,ZnO has been used as an industrial catalyst for water-gas-shift and methanol-synthesis reactions since half-century ago.Despite the tremendous efforts in previous studies,lucubrating the catalytic mechanism over a ZnO surface has remained an unsolved issue.Recently a large number of surface chemistry researches have been conducted around the ZnO-based model systems,which has provided valuable information for understanding the active role of ZnO in related catalytic reactions.In this dissertation,we had performed detailed investigations of small gaseous molecules including CO,CO2,H2 and H2O which are widely involved in industrial synthesis of methanol and water-gas shift reactions,on the surface of a ZnO(10(?)0)single crystal.We mainly used low-temperature scanning tunneling microscopy(LT-STM)in combination with other surface characterization techniques and density functional theory calculations(DFT,together with collaborators),to investigate the surface adsorption and activation of these molecules directly from the atomic level.We emphasized the application of in-situ exposure technique,which can ultimately exclude the interference of contaminations and provide the most direct evidence of the adsorption details of the molecules.The achieve experimental results not only help to reveal the key roles of ZnO in transforming the small molecules into higher grade chemicals,but also pave a solid ground for extended STM researches on ZnO surfaces.The main findings of this thesis are summarized in the following:1.We realized the routine preparation of an atomically flat ZnO(10(?)0)surface under UHV conditions.Low energy electron diffraction(LEED)and auger electron spectroscopy(AES)were applied to confirm the surface structure and cleanness.Atomically resolved images were obtained at liquid nitrogen temperature(77 K).Detailed scanning tunneling spectroscopy(STS)in combination with angle-resolved ultraviolet photoelectron spectroscopy(ARUPS)measurements revealed the n-type semiconductivity of the ZnO crystal as well as the band-bending effect of the exposed surface.We also found that the as-prepared ZnO(10(?)0)surface is free of any point defect at the top surface but populated with low concentration of subsurface defects.Real surface defects can be formed upon reducing the ZnO surface with CO at elevated temperatures.2.On the basis of a routine preparation of clean ZnO(10(?)0)surface,we further carried out a detailed investigation of the adsorption behavior of CO and CO2 molecules.For the first time in the world,to the best of our knowledge,we obtained the high resolution STM images of C02 adsorbed on ZnO(10(?)0),as well as their characteristic topographic changes under different tip states and imaging biases.The STM results in combination with the theoretical calculations clearly demonstrated the tridentate configuration of CO2 binding concomitantly with one surface O and two surface Zn ions.Upon precise controlling the exposure and repeated scanning at the same surface area,we clearly observed the growth of one-dimensional chains of C02 on the ZnO(10(?)0)surface,whose asymmetric kinetic preference along the[0001]direction was well explained by the CI-NEB calculations.Along with the increasing exposure,the density of CO2 chains grows gradually but no condensate islands were observed.However,a double layer CO2 adsorption can be observed at sufficiently large exposure,which presents a complete film with(2×1)superstructure.3.In-situ exposure of CO also resulted in the direct observation of CO molecules binding to the Zn chains on the ZnO(10(?)0)surface.Detailed STM imaging with metallic tip as well as CO-terminated tips,in combination with DFT simulations,demonstrated that the CO molecule adsorbs in a linear fashion and tilts away from the surface normal.Moreover,due to the relatively low binding strength,CO was found diffusing readily along the[1210]direction of the surface even at 77 K.And the migration barrier was experimentally determined as(-0.085 ± 0.02)eV based a series of variable temperature STM measurements.Upon dosing CO2 together with CO,we found that the co-adsorbed CO2 molecules and their chain structures can significantly increase the surface binding of CO,hence largely suppress its diffusion on the surface.In this case,the reaction opportunity of CO with other surface species can be substantially raised.4.Molecular hydrogen(H2)is the important component in syngas.It is also one of the most important energy sources on earth.Therefore,we also investigated the interaction of H2 with the ZnO(10(?)0)surface under cryogenic conditions.Repeated in-situ and ex-situ exposing experiments demonstrated that H2 can efficiently adsorb and form an interesting one-dimensional chain structure on the ZnO(10(?)0)surface at a temperature below 50 K.While no adsorption can be observed when sample was warmed above 50 K.More interestingly,annealing experiments clearly evidenced that the formed chain structure of H2 can be well stabilized until?200 K.High resolution STM images in combination with DFT calculations revealed that upon adsorption H2 splits into atomic hydrogen which subsequently bind to the Zn and O ions belonging to two neighbored Zn-O pairs.And the heterolytic dissociation is mediated by precursor state of physisorption.5.H2O is the last molecule we study in this thesis,owing to its importance in nature as well as its reagent role in water-gas shift reaction.Our in-situ exposing experiments demonstrate that at 77 K H2O adsorbs randomly on the ZnO(10(?)0)surface and keeps the molecular form.These H2O molecules can be dissociated via directly injecting holes into the antibonding orbitals through tip manipulations.The yielded H and OH radicals subsequently bind to surface O and Zn ions,respectively,and form equivalent surface hydroxyls.Exposing at room temperature leads to directly dissociative adsorption of H2O which readily grows into one-dimensional chains along the[0001]direction of ZnO.No isolated H2O molecules can be observed.Further increasing the coverage finally leads to the formation of a(2×1)superlattice,which is identified as half-dissociated water monolayer by referring to the literature reports.