STM investigation of the (2 × 2)O and (2 × 1)O structures on Rh(1 1 1)

The adsorption of oxygen on Rh(1 1 1) was investigated by STM. Dosing of O2 on the Rh(1 1 1) surface at 300 K until saturation leads to a (2 × 1)O structure with an O coverage of 0.5. The (2 × 1)O structure forms large, well-ordered domains. After lower O2 exposures a (2 × 2)O structure with an O coverage of 0.25 could be resolved at low temperatures, whereas at 300 K disordering and the high mobility prevented resolution of oxygen. The O atoms quickly react with H2 at 300 K, but no similar low temperature reaction channel as on Pt(1 1 1) was observed. Under slightly worse vacuum conditions a different (2 × 2) structure was observed that does no longer transform into the (2 × 1)O structure at higher O2 exposures. This structure, which is not reactive with H2 and displays CO and CO2 desorption in TDS, is interpreted as the (2 × 2)O + CO structure. Its formation by unintended coadsorption of CO can explain controversial observations in previous work.