The reactions of oxygen at dark and irradiated zinc oxide surfaces

Abstract

Studies of the kinetics of adsorption of oxygen on zinc oxide over the temperature range from 25 to 390 $^\circ$C have provided evidence for two distinct types of chemisorption, one prevalent below 200 and the other above 300 $^\circ$C. This pattern of activity has been confirmed by measurements of desorption rates over the same temperature range. The mode of preparation and pretreatment of the oxido oxort a strong influence on the adsorption behaviour, and these differences are accentuated when the processes of adsorption and desorption are studied in the presence of irradiation in the ultra-violet and visible. Photodesorption of oxygen is confirmed to be the normal behaviour for zine oxide, but photo-adsorption has been observed under conditions of high excess zinc concentration. The photo-effects are especially marked below 300 $^\circ$C. The adsorption studies have been followed up by experiments on the rate of the intermolecular oxygen reaction $^{18}$O$_2$+$^{16}$O$_2 \leftrightharpoons$ 2$^{18}$O$^{16}$O, and on the influence of irradiation on this catalysis. It is evident that irradiation stimulates both adsorption and desorption, but the balance between them depends on the previous history of the specimen. The experiments with heavy oxygen have also included a brief study of oxygen exchange with zinc oxide at 400 to 500 $^\circ$C. The results as a whole are discussed in terms of the model of zinc oxide as an n-type semiconductor with interstitial zinc, and oxygen chemisorbed as O$^-$ and O$^{2^-}$, respectively, are held to be mainly responsible for the phenomena observed. The relationship with conductivity studies is emphasized and the depletive chemisorption of oxygen, forming a boundary layer, is discussed in some detail. The depletion of electrons is not exhaustive for normal specimens of zinc oxide, and the treatment of this case leads to an expression consistent with the observed kinetics. Several possible mechanisms for photo-adsorption are put forward, and the association with high donor concentrations is discussed. Interstitial zinc diffusing under the influence of the electric field of chemisorbed oxygen is considered to play an important role in specimens heated above 300 $^\circ$C.