Pressure-dependent band-bending in ZnO:A near-ambient-pressure X-ray photoelectron spectroscopy study

摘要

ZnO-based catalysts have been intensively studied because of their extraordinary performance in lower olefin synthesis,methanol synthesis and water-gas shift reactions.However,how ZnO catalyzes these reactions are still not well understood.Herein,we investigate the activations of CO_(2),O_(2)and CO on single crystalline ZnO polar surfaces at room temperature,through in-situ near-ambient-pressure X-ray photoelectron spectroscopy(NAP-XPS).It is revealed that O_(2)and CO_(2)can undergo chemisorption on ZnO polar surfaces at elevated pressures.On the ZnO(0001)surface,molecular CO_(2)(O_(2))can chemically interact with the top layer Zn atoms,leading to the formation of CO_(2)^(δ-)(O_(2)^(δ-))or partially dissociative atomic oxygen(O-)and hence the electron depletion layer in ZnO.Therefore,an apparent upward band-bending in ZnO(0001)is observed under the CO_(2)and O_(2)exposure.On the ZnO(0001)surface,the molecular chemisorbed CO_(2)(O_(2))mainly bond to the surface oxygen vacancies,which also results in an upward bandbending in ZnO(0001).In contrast,no band-bending is observed for both ZnO polar surfaces upon CO exposure.The electron-acceptor nature of the surface bounded molecules/atoms is responsible for the reversible binding energy shift of Zn 2 p_(3/2)and O 1 s in ZnO.Our findings can shed light on the fundamental understandings of CO_(2)and O_(2)activation on ZnO surfaces,especially the role of ZnO in heterogeneous catalytic reactions.