Initial stages of hydrogen reduction of NiO(100)

The reduction of single crystal NiO(100) under hydrogen has been followed by AES, XPS and LEED for the pressure range of 1.0 × 10^?7 to 1.3 × 10^?6 Torr and for substrate temperatures of 150–350°C. The kinetics of reduction are controlled both by the rate of removal of lattice oxide at the surface and by the diffusion of subsurface oxygen to the oxygen-depleted surface. The rate of oxygen removal is first-order in surface oxide concentration and in hydrogen pressure. An induction period precedes the reduction reaction and its length is postulated to be controlled by surface defect concentration. The stoichiometric and reduced lattice oxygen species appear to be chemically identical and give a single symmetric XPS peak at 529.4 eV. Nickel spectra indicate a shift in XPS binding energies from those expected of the oxide to those of nickel metal early in the reduction process, although LEED indicates the NiO(100) surface lattice to remain the stable structure for surface reduced to approximately 20% of the stoichiometric oxygen concentration. Ni(100) island formation is observed, with Ni 〈010〉 and 〈001〉 directions along the NiO 〈010〉 and 〈001〉, respectively, but only after the NiO surface is severely depleted in oxygen.