In situ observation of H2 dissociation on the ZnO (0001) surface under high pressure of hydrogen using ambient-pressure XPS

The interaction of H-2 molecules with a ZnO (0001) single crystal surface has been studied over a wide pressure (10(-6)-0.25 Torr) and temperature (300-600 K) range using ambient pressure X-ray photoelectron spectroscopy (AP-XPS). ZnO is well-known for interstitial hydrogen and hydrogen atoms in ZnO are believed to be incorporated by the dissociative adsorption of H-2 molecules in the atmosphere and their subsequent diffusion into the bulk. The dissociative adsorption of H-2 has been investigated at elevated pressures because H-2 molecules are not dissociated on the ZnO single crystal surface under ultrahigh vacuum (UHV) conditions. When the pressure is increased to several mTorr, the dissociative adsorption of H-2 takes place to form OH bonds on the surface. At 0.25 Torr, the ZnO surface is saturated with H atoms and the coverage is estimated to be 1.1 x 10(15) atoms/cm(2) at 300 K. At higher surface temperatures, the equilibrium between the dissociative adsorption of gas-phase H-2 molecules and the associative desorption of surface H atoms is established. While maintaining the equilibrium, the surface has been monitored successfully in situ by utilizing AP-XPS. (C) 2018 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.