Infra-red spectroscopic studies of zinc oxide surfaces

Abstract

After storage under ambient conditions zinc oxide, prepared by the combustion of metallic zinc, carries both carbonate and hydroxylic species. The former are removed by heating the oxide in oxygen and water vapour at 673 K. Subsequent cooling to ambient temperatures in a moist oxygen atmosphere yields a surface carrying hydroxyls. The corresponding infra-red absorption spectrum exhibits four major absorption bands. Two narrow bands are present at 3670 and 3620 cm^–1 and two broad absorptions are centred about 3555 and 3440 cm^–1. Outgassing the sample at 625 K removes the surface species corresponding to the 3555 cm^–1 band and reduces the concentration of that associated with the absorption at 3440 cm^–1. The intensities of the 3670 and 3620 cm^–1 absorptions are little affected by this outgassing procedure. Rehydroxylation of such outgassed samples by exposure to water vapour at ambient temperatures indicates that the thermal dehydroxylation process involves a “mobile” surface state. The absorption bands observed in the spectra are tentatively assigned to isolated and hydrogen-bonded surface hydroxyls sited on different surface crystal planes. Studies of the adsorption of CO₂ show that this gas chemisorbs rapidly at ambient temperatures on a partially dehydroxylated sample to form a carbonate species but the chemisorption is much reduced if the surface is in the “fully” hydroxylated state. HCl gas also reacts with the surface layers of zinc oxide but in such a way as to promote the formation of the bulk hydroxide on exposure of the chlorine-containing sample to water vapour at ambient temperatures.