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

Abstract

Abstract 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 CO2, O2 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 O2 and CO2 can undergo chemisorption on ZnO polar surfaces at elevated pressures. On the ZnO ( 0001 ) surface, molecular CO2 (O2) can chemically interact with the top layer Zn atoms, leading to the formation of C O 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 CO2 and O2 exposure. On the ZnO ( 000 1 ¯ ) surface, the molecular chemisorbed CO2 (O2) mainly bond to the surface oxygen vacancies, which also results in an upward band-bending in ZnO ( 000 1 ¯ ). 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 2p3/2 and O 1s in ZnO. Our findings can shed light on the fundamental understandings of CO2 and O2 activation on ZnO surfaces, especially the role of ZnO in heterogeneous catalytic reactions.