Electronic structure investigation of the room temperature coadsorption of oxygen 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 Ni(100) by means of O Is and K 2p X-ray photoemission spectroscopy (XPS), relative work function and O Is X-ray absorption spectroscopy (XAS) measurements. 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 formation 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 uptake 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 oxidation 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 thus 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-insulator type of transition. The O Is XPS data rule out the possibility that this 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 bilayered structure in which oxygen is adsorbed underneath the alkali metal layer at K coverages above the work function minimum. However, we find that the alkali-induced minimum of the work function is not related to the occurrence of structural properties, in particular the formation of the (3 x 3) structure, or electronic properties such as the metal-insulator transition of the K layer. Therefore a coplanar alkali-oxygen coadsorption model cannot be ruled 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.