The atomic geometry of the (2x2) phases of O and CO+O on Rh(111)

The local adsorption geometries of the (2 × 2)-1O, (2 × 2)-2O, (2 × 2)-(O + CO) and (2 × 2)-(O + 2CO) phases on the Rh(111) surface have been investigated by analysing low-energy electron diffraction (LEED) intensity data. In all cases, the oxygen atoms were found to occupy the threefold fee site and the topmost layer spacing d12 of Rh(111) was found to be expanded by about 3%. Additional experiments with disordered oxygen overlayers prepared at low adsorption temperatures show a monotonic increase of d12 with oxygen coverage. An O coverage of θO ≈ 0.1 already lifts the contraction of 0.02 Å of the clean surface. This finding is remarkable as for Ru(0001) a substantial expansion of the first layer spacing (≈ 3%) is observed only at very high O coverages, e.g. for the (1 × 1)-O structure. For the high-coverage Rh(111)-(2 × 2)-2O phase a honeycomb arrangement of the O atoms can clearly be ruled out. The (2 × 2)-2O has rather to be regarded as a (2 × 1)-1O phase. In the mixed (2 × 2)-(O + CO) overlayer the CO molecule occupies the on-top position which is also expected from a reduced back-donation of (substrate) electron charge density due to coadsorbed O atoms. The presence of CO molecules weakens the Rh-O bond. Altogether, the structural findings nicely reflect the competition of CO and O for electron charge density at the Rh(111) surface. In the (2 × 2)-(O + 2CO) phase 60% of available hcp sites and 100% of available on-top sites in the (2 × 2) unit cell are occupied.