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

Abstract

The role of ZnO polar surfaces in the activation of small gas molecules were studied by in-situ NAP-XPS in realistic pressure conditions. 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 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 CO 2 and O 2 exposure. On the ZnO ( 000 1 ¯ ) surface, the molecular chemisorbed CO 2 (O 2 ) 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 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.