Polarity- and Pressure-Dependent Hydrogen Dynamics on ZnO Polar Surfaces Revealed by Near-Ambient-Pressure X-ray Photoelectron Spectroscopy

ZnO-based catalysts have been widely used in industrial reactions involving syngas conversions. Hydrogen dynamics on the surface of the catalyst is an essential process for understanding the mechanisms of such reactions. As a polar material, however, the role of ZnO surface polarity on hydrogen dynamics has not been studied under operando conditions. Here, we investigate the behavior of polar (0001) and (000 (1) over bar) surfaces of single-crystal ZnO under different H-2 pressures using near-ambient-pressure X-ray photoelectron spectroscopy. We found that the (000 (1) over bar) surface shows a monotonic and irreversible band bending with increasing H-2 pressure. In contrast, the (0001) surface shows two opposite responses depending on H-2 pressure, which are reversible in pressure cycles. This polarity-and pressure-dependent hydrogen dynamics is clearly understood with the assistance from first-principles calculations. In particular, the amphoteric behavior of atomic H is identified to play a key role.