Electronic structure investigation of the room temperature coadsorption ofoxygen and potassium on Ni(100): from oxygen submonolayer coverage to saturated NiO/Ni(100) via an Ni(100)-(3 x 3)-(K+O) structure.

We have investigated the room temperature potassium-promoted oxidation of N i(100) by means of O Is and K 2p X-ray photoemission spectroscopy (XPS), re lative work function and O Is X-ray absorption spectroscopy (XAS) measureme nts. Within the potassium coverage range we examine in this study (theta(K) =0.10-0.25 ML), coadsorption of about 0.5 ML of oxygen results in the forma tion of a (3 x 3) pattern which is peculiar to the coadsorption of alkalis and oxygen on Ni(100). This structure is stable against further oxygen upta ke or against adsorption of other species such as CO. However, by means of O Is XPS and XAS observations we determine an overall potassium-enhanced ox idation rate of about two orders of magnitude. The O Is XPS data show that oxygen coadsorbed with K on Ni(100) is still bonded primarily to Ni and thu s rule out the formation of potassium oxide or other KxOy compounds such as peroxide or superoxide. The line-shape analysis of the K 2p XPS data show that upon oxygen coadsorption the alkali metal layer undergoes a metal-insu lator type of transition. The O Is XPS data rule out the possibility that t his transition is due to a large charge transfer between coadsorbed species so other mechanisms are discussed. The work function changes induced by O on the K-precovered surface are consistent with the formation of a bilayere d structure in which oxygen is adsorbed underneath the alkali metal layer a t K coverages above the work function minimum. However, we find that the al kali-induced minimum of the work function is not related to the occurrence of structural properties, in particular the formation of the (3 x 3) struct ure, or electronic properties such as the metal-insulator transition of the K layer. Therefore a coplanar alkali-oxygen coadsorption model cannot be r uled out. The O Is XAS data show that the presence of potassium on Ni(100) lowers the O coverage for the onset of the formation of cubic NiO. (C) 2000 Elsevier Science B.V. All rights reserved.