Reduction of oxygen pre-treated Ni(1 1 1) by H2 exposure: UPS and MIES studies compared with Monte Carlo simulations

The reduction of oxygen pre-treated Ni(1 1 1) under hydrogen has been followed by metastable induced electron spectroscopy (MIES) and ultraviolet photoelectron spectroscopy (UPS) for a substrate temperature of 600 K. Both techniques have allowed us to report on the evolution of the electronic structure of the valence band over hydrogen exposure. These investigations showed the fast formation and desorption of water molecules, but no hydroxyl groups. The oxygen coverage of the surface depends on the Ni 3d band intensity during hydrogen exposure. By using the MIES technique, we observed that the reduction reaction is preceded with a induction period. These periods are not seen with UPS data. This difference can be attributed to the fact that MIES gives information about the outermost layer of the surface, whereas UPS probes the first innerlayers. So it indicates that oxygen atoms are desorbed from the surface, and then subsurface oxygen atoms diffuse to the oxygen-depleted surface. To interpret these experimental results, we carried out simple Monte Carlo simulations based on the fact that the reduction takes place on the surface along the NiO island perimeters. This simulation fits well the experimental data when a lateral mobility of subsurface oxygen atoms is taken into account, i.e. the reaction leads to the reduction of the three-dimension NiO before the outermost layer oxygen desorption.