Photoelectric measurements on nickel and nickel oxide film

Abstract

In order to gain a clearer understanding of how molecular oxygen and hydrogen react with the surfaces of metals and metallic oxides, the electronic work function has been measured during the oxidation of evaporated nickel films and the reduction of nickel oxide with hydrogen. The measurements included changes in the spectral sensitivity of surfaces prepared under varied conditions, and in the photoelectric yield (at constant photon energy) during the admission of gas. Admission of oxygen to nickel films at 20°C resulted in several distinct effects. Lightly oxidized films (< 1 monolayer equivalent total uptake) gave composite spectral sensitivity curves. By using the Fowler equation in the form of a plot of frequency against (photo-electric yield) ½ , these were analyzed in terms of two work functions, which remained unaltered until the films were saturated with respect to the rapid chemisorption of oxygen. As further quantities of oxygen were absorbed, both work functions rose. When the films were stood in vacuo , the lower work function decreased, finally reaching a lower value than that measured for clean nickel; at the same time the films recovered the capacity to adsorb oxygen. The process is regarded as identical with the known ‘thermal regeneration’, namely, the ability of oxidized films to take up oxygen rapidly after they have been heated in vacuo at 200 to 400°C. Although the lower work function fell markedly as thermal regeneration occurred, the larger work function did not fall below about 5∙7 eV. It is considered that the observations point to two kinds of surface structure in the lightly oxidized films. The work function of 5∙7 eV is assigned to areas thinly covered with the nickel oxide crystal lattice, while the lower work function is assigned to areas on which the oxide, as such, does not form. It is suggested that the initial interaction of oxygen with the surface is largely non-polar, and that nickel oxide only forms on certain areas as a result of migration, charge transfer and rearrangement. Those parts of the surface not covered with the nickel oxide crystal lattice undergo modification, as a result of penetration of oxygen into the surface layers of the metallic lattice. Photoemission from nickel oxide films (> 1000Å thick) has been examined, and the interaction of hydrogen with these films was studied in detail. Changes in photoelectric yield show that there is some small degree of chemisorption at 20°C, and it is postulated that this represents formation of H + 2 which either penetrates the nickel oxide lattice or occupies vacant cation sites in or near the surface. At higher temperatures the surface potential becomes increasingly negative as dissociative chemisorption takes place extensively by the mechanism already recognized. With the desorption of water and the continuation of reaction, nuclei of nickel form. The findings are in agreement with, but amplify, the accepted generalized model for oxidation and reduction processes.