Combined STM and TPD Study of Rh(111) Under Conditions of High Oxygen Coverage

Oxygen chemisorption on rhodium surfaces gives rise to several surface structures depending on the total oxygen coverage. In this study, Rh(111) was exposed to O-2 or O + O-2, and the oxygen surface structures formed at coverages greater than or equal to 0.5 ML were imaged using scanning tunneling microscopy (STM). The STM images showed that the (2 x 1)-O adlayer domains are predominant on the Rh(111) surface. Exposure of Rh(111) to O atoms yielded O coverages greater than 0.5 ML; (1 X 1)-O domains were observed to form along terrace step edges, and their areal density increased with exposure. However, (2 x 1)-O adlayers were still present on the surface. The STM images reveal that the surface coverage was appreciably less than the total amount of oxygen, suggesting that O uptake resulted in significant absorption into the selvedge, even at modest surface O coverages and temperatures. We compare these observations to previous surface scattering experiments and calculations and demonstrate that our findings resolve several inconsistencies and clearly demonstrate that despite the apparent simplicity of the O/Rh system subtle details remain important, and multiple O structures were present at any O coverage from 0.25 to >1.0 ML. This indicates the rich complexity of O-transition metal interactions and suggests that accurate models of oxygen on rhodium surfaces must include several coexisting surface structures.