Understanding Surface Activity of Oxides from Oxide-Metal Interface Formation

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We propose a technique to estimate the concentration of reactive oxidation sites on an otherwise nonreacting oxide surface; it involves evaporating submonolayer amounts of a nonreacting metal on the oxide and carefully analyzing the fraction that undergoes chemical bonding, using x-ray photoelectron spectroscopy. In this report, Ni has been evaporated on alumina, magnesia, and silica surfaces, and the growing Ni 2p photoelectron peak carefully monitored. It was observed that during the initial stages of bonding, a fraction of the first monolayer of Ni undergoes oxidation to form NiO. This oxidized component grows initially and then saturates to a certain level. This saturation level, expressed as a fraction ( Theta ) of the first monolayer, can be used as a measure of oxygen-active sites such as unattached oxygen bonds. We found that on epitaxially polished sapphire, Theta was as high as 20% of a monolayer. It was considerably reduced when the surface was sputtered with Ar+ ions in vacuum, a process which is known to create an oxygen-deficient Al2O3 surface. In MgO crystals (which do not tend to lose oxygen on ion sputtering), Theta remained practically unchanged. Such observations confirm the feasibility of using this technique as a probe for surface oxygen activity. It was found that increased level of dissolved magnesia in the sapphire lattice reduced the fraction of oxidized component. On the other hand, excessive doping with MgO (which leads to second phase spinel precipitates) seems to increase it considerably. The former observation is attributed to defect chemistry of the corrundum lattice and the latter has been discussed in light of earlier mass transport measurements. Silica surfaces were found to offer negligible oxidation sites, irrespective of sample history. Using this technique, one can systematically study how the concentration of reactive sites (which determines the amount of interfacial bonding) will be influenced by surface modifications.