Reactivity and Structural Aspects of Cesium and Oxygen States at Cu(110) Surfaces: An XPS and STM Investigation†

Structural and reactivity aspects of Cu(110)−Cs and Cu(110)−Cs/O overlayers have been investigated by XPS and STM. The development of cesium-induced structures at a Cu(110)−Cs overlayer has been followed as a function of cesium coverage at 295 K and the reactivity of the overlayer first to oxygen and subsequently to ammonia and carbon dioxide studied. For cesium concentrations up to 1.3 × 1014 cm-2 an incommensurate pseudo square structure is observed which is in registry with the copper substrate in the 100 direction. This coexists, at 1.3 × 1014 Cs adatoms cm-2, with a structure consisting of 110-orientated rows which are not atomically resolved but have a spacing in the 100 direction of 1.1 nm. At a cesium concentration of 1.5 × 1014 cm-2, patches of structure with an inter-row spacing of 0.7 nm are present, and at a cesium concentration of 1.9 × 1014 cm-2, only the latter spacing is observed. An increase in cesium concentration to 2.1 × 1014 cm-2 results in an increase in the inter-row spacing to 1.1 nm. Exposure of cesium-modified surfaces to oxygen results in the development of new terraces superimposed upon the cesium-modified surface with the latter structurally unchanged by the formation of the oxygen adlayer, although there is a change in the XP binding energy of the Cs(3d) peaks. The new terraces consist of 100-orientated chains similar to those observed at unmodified Cu(110) surfaces; however, at low cesium concentrations these chains are arranged in (3 × 1) as well as (2 × 1) domains and at high cesium concentrations a c(6 × 2)O structure develops with an exposure of oxygen as low as 10 L. The adsorption of cesium at partially preoxidized surfaces results in the formation of a c(2 × 4) structure and also structures which have no simple relationship with the substrate lattice and which we assign to strained Cu−O (2 × 1) structures. At a surface with a complete monolayer of oxygen present the cesium forms 110 chains with a minimum interchain spacing of 0.5 nm and alternate chains showing well-resolved maxima with a spacing of 0.5 nm. The adatoms in between the chains are less well-defined suggesting a degree of mobility in the 110 direction. In comparison with oxygen at unmodified Cu(110) surfaces, oxygen states at the cesium-modified surface are unreactive to ammonia at 295 K, reaction only occurring after exposures of 300 L at 490 K. However, the cesium surface is reactive to carbon dioxide chemisorption at 295 K resulting in a surface carbonate with extensive migration of cesium and the emergence of areas of the underlying copper surface.