XPS study on thermally and electrochemically prepared oxidic adlayers on iridium

An Ir(111) surface has been oxidized in 0.1 MPa of flowing oxygen gas at 900 K for 0.5 h. X-ray photoemission spectra showed that a thick surface film was formed with Ir 4f {7}/{2} and Ir 4 f{5}/{2} levels at 61.9 and 64.8 eV binding energy, respectively. The O Is signal had a binding energy of 530.0 eV. An atomic O-to-Ir ratio of 2.7 ± 0.4 was calculated from the peak areas. The surface oxide could be decomposed to the elements at 850 to 900 K in ultra-high vacuum. This temperature was much higher than the decomposition temperature of 500 K for bulk IrO ₂, which gave the same XPS binding energies, but an O-to-Ir ratio of only 2.4 ± 0.2. Therefore, the surface oxide seems to be deficient in Ir cations relative to the bulk oxide phase. Anodic polarization of the same Ir surface for 2 h at 298 K in IN H ₂SO ₄ at 3 V versus standard hydrogen electrode resulted in Ir 4 f{7}/{2} and Ir 4 f{5}/{2} signals at 61.2 and 64.1 eV, respectively, that were shifted by only 0.3 eV relative to the clean metal. The O Is signal at E_B = 531.1 eV was rather broad and composed of several oxygen species. Because of the determined O-to-Ir ratio of 1.2, the adlayer at 3 V is interpreted as a very thin film of a hydrated indium oxyhydroxide. This surface film decomposed in UHV already at 400 K.