Oxidation of a Tb2O3(111) Thin Film on Pt(111) by Gas-Phase Oxygen Atoms

We used X-ray photoelectron spectroscopy and temperature-programmed desorption (TPD) to investigate the oxidation of Tb2O3(111) films on Pt(111) by gaseous oxygen atoms. We find that plasma-generated O atom beams are highly effective at completely oxidizing the Tb2O3 films to TbO2 at 300 K, for film thicknesses up to at least seven layers. Heating to similar to 1000 K in ultrahigh vacuum restores the films to the Tb2O3 stoichiometry and produces two distinct O-2 TPD features centered at similar to 385 and 660 K, which we attribute to the release of lattice oxygen from the surface vs bulk trilayers, respectively. We also find that the adsorption of plasma-activated oxygen at 90 K produces a weakly bound state of oxygen on the TbOx films which desorbs between similar to 100 and 270 K during TPD. This oxygen state is consistent with a form of chemisorbed oxygen, possibly an atomic and/or molecular species that bonds on-top of Tb atoms at the surface. TPD experiments of the oxidation of (Tb2O3)-O-18 films by O-16 atom beams demonstrate that oxygen desorption below about 500 K originates almost entirely from the oxygen that is added to the Tb2O3 film and that all isotopic combinations of O-2 desorb from the bulk above 500 K, though the relative amount of O-18 to O-16 which desorbs above 500 K is lower than that determined from the isotopic composition of the oxidized TbOx films. These results support the idea that oxygen desorption below 500 K originates from oxygen species that are localized at the surface and further suggest that the oxide structure only partially accommodates oxygen atoms that incorporate into lattice sites at 300 K.