Interaction of oxygen and Ni on W(110)

Abstract

The adsorption of Ni on O-covered W(110) and of oxygen on Ni-covered W(110) and subsequent behavior on heating have been studied via thermal desorption, XPS, Auger, work function, and LEED measurements. Adsorption of Ni on O/W(110) does not lead to NiO formation, but on heating to T > 400 K to dewetting and at even higher temperatures to segregation which seems complete near 1000 K. After segregation Ni forms (111)-oriented islands, while O is forced into a p(1 × 8) structure, which corresponds to p(1 × 1) with regularly spaced defect lines every eighth (non-primitive) unit cell, running along 〈100〉 directions. There is evidence for a dead zone between Ni and O islands, as also observed in other systems. Ni desorption temperature is reduced appreciably, relative to Ni on clean W(110). Adsorption of oxygen on Ni W (110) occurs on a saturated Ni monolayer (corresponding to Ni W = 1.29 , i.e. virtually to the density of a Ni(111) plane) with constant sticking coefficient s = 0.74 at 90 K up to O W = 0.9 . Maximum O uptake corresponds to O Ni ≈ 1 or O W ≈ 1.3 . For lower Ni coverage, Ni W = 0.94 ( Ni 0.73 W(110) relative to saturation Ni coverage), s is initially higher but decreases more rapidly than on Ni 1 W (110) . In both cases O adsorption leads to disorder of the composite surface. For Ni W (110) chemisorption occurs up to O W = 0.3 at 90 K, accompanied by a linear increase in work function. Above this coverage oxidation sets in, as indicated by Ni core level shifts; φ remains constant to nearly saturation coverage, where a steep increase, attributable to O 2 species occurs. On heating these desorb at 150–200 K. NiO decomposes between 400 and 600 K with O becoming chemisorbed on the W surface. Additional heating leads to desorption of WO 2 between 1000 and 1250 K, with remaining O coverage O/W = 0.7. Ni-O segregation occurs above 600 K. Oxygen adsorption also occurs on Ni 1O 0.6/W(110) to the extent of O W = 0.7. Similarities and differences between these results and those obtained for O-Cu, O-Pd and O-Ag interactions on W(110) are discussed.