REACTIVITY AND ELECTRONIC-PROPERTIES OF 2 MICROCRYSTALLINE ZNO SURFACES - INTERACTION WITH CO AND CO/O-2

A comparative spectroscopic study has been made of two ZnO powders coming from zinc carbonate decomposition (ex-carbonate) and from zinc metal combustion (Kadox). The morphological differences between the samples have been found to be at the origin of their different reactivity. In particular, on the more perfect, lower surface area Kadox microcrystals, CO reacts at RT with adsorbed oxygen ions O-2 producing CO2 (g) and releasing electrons to the conduction band, without formation of surface species and defective levels in the gap. For the smaller, less defined, higher surface area ex-carbonate microcrystals the reduction with CO occurs by reaction with surface lattice oxygen, more uncoordinated and accessible than in the Kadox, giving rise immediately to carboxylate surface species and donor defective levels localized in the gap. Also the nature of the fundamental transition has been found to be different: direct-like in the Kadox, involving band tailing in the excarbonate. © 1990 Pergamon Press plc.