SURFACE-CHEMISTRY AND ELECTRONIC EFFECTS OF H-2 (D(2)) ON 2 DIFFERENT MICROCRYSTALLINE ZNO POWDERS

A FT-IR spectroscopic study has been made on two ZnO powders differently prepared: one obtained by thermal decomposition of ZnCO3 (ex-C), the other by zinc metal combustion (Kadox). The morphological differences between the samples have been proposed to be at the origin of the different reactivity and electronic phenomena. In particular, on the more perfect, lower surface area Kadox microcrystals, H-2 reacts at RT to give hydroxyls and hydrides mainly reversibly adsorbed (type I); the hydrogen irreversibly adsorbed (type II) is mainly due to an IR inactive species formed by a reductive mechanism releasing electrons to the conduction band. On smaller. less defined, higher surface area ex-carbonate microcrystals type I adsorption occurs in smaller amounts (113 of the amount on Kadox), type II adsorption is mainly due to the formation of hydroxyls and hydrides, paralleled by release of electrons trapped into defective shallow levels localized in the gap.