Study On The Adsorption Behavior And Mechanism Of Small Hydroxyl-containing Molecules On Non-polar ZnO(11(?)0) Surface

Zinc oxide(ZnO)is a rather common metal oxide which is widely used in daily life and industry.In the field of heterogeneous catalysis,ZnO is an important component of catalysts for the synthesis of methanol from syngas on an industrial scale.As a result,great efforts have been put in the investigation of the various properties of ZnO.However,understanding of the surface of ZnO is still limited.In the last decades,due to the rapid development of surface-sensitive techniques,numerous endeavors had been focused on this system and yielded plenty of information.In this work,we had investigated the atomic structure of the non-polar ZnO(11(?)0)surface and the adsorption of small molecules(water and methanol)by using scanning tunneling microscopy,X-ray photoelectron spectroscopy,complemented with density functional theory calculations.The experimental and theoretical information of ZnO surface which has become available here allows to understand the process of heterogeneous reactions on an atomic-level.The main results and conclusions of this dissertation are summarized as follows:1.After sputtering and annealing at?700? in ultra-high vacuum,the single crystals of ZnO(11(?)0)exhibit a atomic-smooth un-reconstructive(1×1)surface.The surface morphology,composition and structure were systematically evaluated using surface analysis methods.The STM images of(11(?)0)surface display well-defined rectangular terraces with the steps running along either[0001]or[1100]directions.The atomic-resolved STM images show that a sublattice with only one kind of atoms in the unit cell.The separation between the "sawtooth"atomic chains extended along the[0001]direction is equal to the length of bulk unit cell.No indication of surface vacancy on the surface is observed in such atomic-resolved images.Missing surface atoms in the atomic rows are just observed occasionally.Combined with DFT theoretical calculations,Surface atoms miss easily than bulk atoms.And for different type of surface defect such as Zn vacancy,O vacancy and Zn-O dimer vacancy,the missing O atoms are predicted to be the favored defect type on the non-polar ZnO(11(?)0)surface.2.Based on the acquisition of clean ZnO(11(?)0)surface,we had investigated the water adsorption on the ZnO(11(?)0)surface by monitoring the adsorption of water molecules form single molecule to full-layer,and focused on the mechanism of the switch of different adsorption modes during increasing of water coverages.The results show that initially,the intact water molecule adsorb on the ZnO(11(?)0)surface stably by forming OH2O-Zn3c bonding and two HH2O-O3c hydrogen bondings with surface Zn3c and O3c atoms,respectively.The adsorbed water molecules induce surface relaxation around the adsorption site,which hamper the adsorption of subsequent molecules,resulting in the favorable formation of monomer at low coverage.At critical coverage around 0.12 ML,water molecules begin to dissociate on surface forming pairs of hydroxyl ions at Zn3c site beside adsorbed dimer on the adjacent surface row.Finally,half-dissociated water monolayer can form through a mixed and stable hydrogen bondings network.Furthermore,during the adsorption process,the water molecules act as electron donors,inducing downward band bending near the ZnO surface.When the water dissociation sets in,the OH accept net charges from the ZnO surface,which gradually compensates the adsorbed molecular water donated charges to the surface and thus levels off the band bending.3.In order to study the mechanism of adsorption on ZnO(11(?)0)in depth,we had investigated the adsorption of methanol on the ZnO(11(?)0)surface by monitoring the adsorption from single molecule to full-layer.The results indicate that methanol adsorb on the perfect ZnO(11(?)0)surface molecularly,and the dissociation only occurs at the step along the[1100]direction on the surface.The intact methanol molecule forms a OcH3OH-Zn3c bonding and single HCH3OH-O3c hydrogen bonding with surface Zn3c and O3c atom,respectively.There is no strong competition for O3c atom when methanol molecules adsorbed at the adjacent Zn3c site on the surface.This may be the reason for the molecular adsorption of methanol on the prefect ZnO(11(?)0)surface even at high coverage.The adsorbed methanol molecules also induce surface relaxation around the adsorption site,which promote the subsequent molecules to form island-like cluster.During the adsorption,the methanol molecules act as electron donors,inducing downward band bending near the ZnO(11(?)0)surface(0.68 eV).In this thesis,we show a detailed investigation of the adsorption behavior of different small molecules on the non-polar,defect-free ZnO(11(?)0)surface,and revealed whether the adjacent adsorbed molecules have a competition for the surface O3c atoms would be the key factor of the adsorption mode.And the charge transfer at the interface between the adsorbate and ZnO induce the band bending of the(11(?)0)surface.It has laid a foundation to understand the mechanism of the reactions on the ZnO surface in the synthesis of methanol and photocatalytic hydrogen production at the molecular level.