Reaction of atomic oxygen with a Pt(1 1 1) surface: chemical and structural determination using XPS, CAICISS and LEED

The adsorption of thermally cracked atomic oxygen on the Pt(1 1 1) surface has been investigated using X-ray photoelectron spectroscopy (XPS), co-axial impact collision ion scattering spectroscopy (CAICISS), and low energy electron diffraction (LEED). High concentrations of atomic oxygen (∼2 × 1015 atoms cm−2) can be cleanly adsorbed on the Pt(1 1 1) surface at room temperature by dosing the surface with externally generated atomic oxygen at relatively low exposures (∼50 L). Two states of oxygen are readily resolved by XPS with O 1s binding energies at 530.8 and 530.2 eV. These states are assigned to chemisorbed oxygen and to an oxidic oxygen state, respectively. XPS spectra of the Pt 4f region confirm the existence of an oxide species with the appearance of a peak at 2.4 eV higher binding energy than the bulk Pt 4f5/2 peak. Using a combination of angle-resolved XPS, CAICISS, and LEED, evidence is provided to suggest that penetration of oxygen into the surface occurs most likely by an exchange mechanism resulting in the reconstruction of the Pt atoms in the first two atomic layers. On annealing the oxygen covered surface at 500 °C the oxidic layer is observed to decompose and a p(2 × 2)-O reconstruction is observed. A CAICISS study of this reconstructed surface presents evidence in favour of O atoms occupying h.c.p. sites over f.c.c. sites. Low exposures (∼5 L) of atomic oxygen result in a sharp p(2 × 2) reconstruction of the Pt(1 1 1) surface and a single species in the O 1s spectrum at a binding energy of 530.8 eV. Both the p(2 × 2)-O overlayer and the oxide species are shown to be extremely sensitive to the electron and ion beam.