Low-Temperature STM Study Of Adsorption And Reaction Of Small Organic Molecules On ZnO Surface

Zinc oxide(ZnO)has wide applications and plays active roles in people's life.It can not only be used as sunscreen and bleaches in our daily life,but also applied industrially in synthetic rubber,plastics and coatings.Owing to its wide-gap semiconductive property,ZnO is also used in the semiconductor as well electronics industry.From the chemical point of view,the most important application of ZnO may be serving as the catalyst for industrial synthesis of methanol,the latter being one of the most important energy material,whose synthetic mechanism has long been the center topics of intensive researches.However,even after tremendous studies of over half century,the critical roles of ZnO in the catalytic transformation from syngas to methanol is not completely clear yet,which remains actively as the key questions in recent studies.In particular,little knowledge has been obtained for the interactions between ZnO and the reactant,intermediate species and the product molecules which are inevitably occurring during the methanol synthesis process.Under such circumstance,it become more and more important to perform a systematic surface science research which are carried out on the ZnO single crystal surface and with a variety of surface characterizations at the atomic level.In this thesis,we select ZnO(10(?)0)as the model surface since it is the most stable facet of ZnO and possesses a well-defined atomic structure.We apply mostly the low-temperature scanning tunneling microscopy(LT-STM)as the main technique,but also combine with other surface analytic methods such as x-ray photoelectron spectroscopy(XPS)and temperature-programmed desorption spectroscopy(TPD),to investigate the adsorption/desorption,diffusion,and/or reaction behaviors of small organic molecules on the ZnO(10(?)0)surface which are involved in the synthetic reaction of methanol from syngas.This study is believed of essential importance for deepening our fundamental understandings of the critical roles of ZnO in the catalytic reaction of methanol synthesis.The main results are summarized as following:1.We successfully prepared the clean ZnO(10(?)0)surface with routine ultra-high vacuum(UHV)treatments and characterized the surface structure and electronic properties with low energy electron diffraction(LEED),auger electron spectroscopy(AES),ultraviolet photoelectron spectroscopy(UPS)as well as LT-STM.After that,we exposed the methanol molecules onto the ZnO(10(?)0)at both liquid nitrogen(LN2)and room temperatures and performed detailed STM experiments.We found that at LN2 temperature,methanol adsorbs with both a physisorption and a chemisorption state,the former of which resides on the surface O anion through a hydrogen boding while the latter forms strong bonds with its O atom to a surface Zn ion.Owing to the low adsorption energy,the physisorbed methanol molecules readily diffuse on the surface along the[1-210]direction even at liquid temperature and can be transformed to a chemisorbed state upon tip manipulation as well sa thermal annealing.When dosed at room temperature,we found the methanol molecules dominantly chemisorbed on the surface and form a chain-like structure along the[0001]direction.2.Using the same in situ exposure method,we conducted a series of detailed STM characterization of the adsorption structure of formic acid on the ZnO(10(?)0)surface.Based on the atomically resolved STM data in combination with density functional theory(DFT)calculations,we demonstrate that the formic acid dissociate spontaneously into H and HCOO-upon adsorbing on the ZnO(10(?)0)surface,and can form two types of adsorption configurations depending on their adsorption sites.The type-I configuration has its H and HCOO-binding to the surface O anion and Zn ions belonging to the same Zn-O pair while those of the type-II configuration binding to two adjacent Zn-O pairs,and the former has slightly higher adsorptoin energy hence a slighter higher population on the surface.Owing to the strong binding to the surface,formic acid molecules distribute randomly on the ZnO(10(?)0)surface both at LN2 and room temperatures,but can also gradually assemble into a chain-like structure along the[0001]direction upon heating to around 420 K.When increasing the coverage to saturation,the adsorbed formic acid can finally develop into a superstructure with(2×1)periodicity.These ordered structures,as well as the incorporated defects,were clearly unveiled by our systematic STM measurements in combination with detailed DFT calculations.3.Being a well-acknowledged intermediate in the syngas-to-methanol reaction,we consider that the adsorption structures of formic acid may have great impact over the activation and/or reactions of other reactant molecules.With this idea in mind,we carried out detailed STM experiments of the H2 adsorption on the ZnO(10(?)0)surface preadsorbed with formic acid molecules.As we have proposed,the type-II formic acid has a similar binding configuration as the dissociatively adsorbed H2 which can initiate the chain growth of the H2 molecules through a low-barrier reaction path,which may possibly play a promotive effect over the latter.Indeed,our experiments found that with the presorbed formic acid,the dissociative adsorption of H2 was enhanced significantly and the upper temperature limit was lifted from 40 K to 150 K under UHV conditions.Moreover,the STM data clearly demonstrated that all the H-chains grow from a type-? formic acid.We further considered a few other small molecules on the surface,including H,CO2 and methanol,and found their promotion effects are more or less correlated with the induced charge accumulation at the neighbored surface sites around the pre-adsorbates.These results clearly reinforce the important roles of co-adsorption effects in deepening the understandings of a catalytic reaction mechanism.4.Similar to formic acid,formaldehyde is also well acknowledged as an important intermediate in methanol synthesis.Meanwhile,it is also an important organic pollutant in people's daily life.Through a detailed in-situ exposure experiment in combination with DFT calculations,we demonstrate that the formaldehyde adsorbs on the Zn rows with considerable strength with its H and O atoms binding to the surface O anion and Zn cations,respectively.At room temperature,the adsorbed formaldehyde molecules assemble into straight chain-structures orienting along the[0001]directions,similar to methanol and formic acid as addressed above.We also tried to investigate the reaction of formaldehyde on the ZnO(10(?)0)surface upon thermal or photo excitations.Through a combination of TPD and STM measurements,we found that upon heated to elevated temperature,the formaldehyde can react with the ZnO surface by grabbing a surface oxygen to form formate while leave an oxygen vacancy on the surface.When heated further to higher temperature,these formate species further decompose into CO,CO2 and H2O while the oxygen defects gradually accumulate and develop into larger defects upon the desorption of Zn ions from the surface.As a comparison to thermal reaction,we also shed ultraviolet lights(365 nm)to the sample to trigger photocatalytic reactions.We found upon UV irradiation,small fractions of the formaldehyde desorbs from the surface while the another small fraction reacts even at LN2 temperature When the UV irradiation was performed at room temperature,both the desorption and reaction were greatly enhanced and most of the reacted formaldehyde has transformed into formate.The similarity and the different characteristics of the thermal and photocatalytic reactions of the formaldehyde was analyzed and briefly discussed based on the compositional experimental results as well as DFT calculations.