Structure and reactivity of iridium oxide layers grown on Ir(100) by oxidation at sub-ambient O 2 pressures

We used low energy electron diffraction (LEED) and temperature programmed desorption (TPD) to investigate the structure and reactivity of iridium oxide layers prepared by oxidizing Ir(1 0 0) at 765 K and O2 pressures ranging from 0.05 to 5 Torr. Our LEED results provide evidence that Ir(1 0 0) oxidation at O2 pressures up to 0.20 Torr produces a mixture of Ir oxide structures present as small domains, including a commensurate IrO2(1 0 1) structure that coexists with other structures. Oxidizing from 0.50 to 1 Torr causes formation of a commensurate IrO2(1 1 0)R27° structure and a sharp rise in the oxygen uptake from ~8 to 20 ML (monolayer) as the films exhibit signs of roughening. Further increasing the O2 pressure from 1 to 5 Torr causes the IrO2(1 1 0)R27° structure to be replaced with a so-called IrO2(1 1 0)-aligned structure, for which the IrO2(1 1 0) lattice vectors align with those of the Ir(1 0 0) substrate. We find that the oxidized Ir(1 0 0) surfaces become increasingly reactive toward the dissociation and oxidation of CH4 as IrO2(1 1 0) develops on the surface, and observe that the IrO2(1 1 0)-aligned structure is more reactive than the IrO2(1 1 0)R27° phase. Our findings demonstrate that the oxide phase evolution on Ir(1 0 0) is sensitive to the O2 pressure in the range from 0.05 to 5 Torr, and that the development of reactive IrO2(1 1 0) structures requires elevated O2 pressures (>0.5 Torr) and temperature.