A STUDY OF THE OXIDATION OF THE SULFUR PASSIVATED NI(100) SURFACE BY STM

The influence of sulphur on the early stages of Ni(100) oxidation has been studied by STM and LEED. It has been observed that sulphur concen trations as low as 16% of a monolayer have a marked effect on oxidatio n behaviour. Oxidation of terraces is completely suppressed and a Lang muir-Hinshelwood process is not observed to occur. Oxide nucleation is observed to occur on step edges, which leads to NiO(111) formation du ring room temperature oxygen adsorption. Strong oxide growth anisotrop y and the occurrence of semi-coherent oxide growth along nickel step e dges results in the formation of oxide laths orientated perpendicular to the step edges. The presence of these laths and the preferential gr owth of an oxide across the lower terrace results in the formation of a characteristic, irregularly edged, triangular oxide morphology. On a nnealing the oxidised sample in a temperature range of 270-290 degrees C, LEED indicates that NiO(111) transforms to NiO(100). Increasing th e annealing temperature to above 500 degrees C causes the oxide spots to become fainter and more diffuse, which is consistent with oxygen di ffusion into the bulk and oxide disproportionation. This was consisten t with the behaviour observed by STM. (C) 1997 Elsevier Science B.V.