The effect of point defects on the dissolution of zinc oxide in sodium hydroxide solutions

The dissolution behavior of sintered zinc oxide in aqueous solutions of NaOH has been studied as a function of varying concentrations of lithia, alumina, and chromia as solid state dopants. Both the pure and doped zinc oxide samples were found to obey the shrinking particle model of dissolution, and there was a lack of dependence of the dissolution rate on agitation speed. An activation energy of 48.1 kJ/mol (11.5 kcal/mol) was obtained for the undoped ZnO material. It is proposed that the desorption of surface hydroxyl complexes constitutes the rate controlling step. Additions of up to 1.0 mol percent Cr₂,O₃ and Al₂O₃ decreased the dissolution rate. An opposite effect was observed for the addition of Li₂O. Dopant-induced changes in zinc oxide surface chemistry have been used to interpret the observed difference in the effects of the higher-valent and lower-valent dopants. Zinc oxide is an n-type semiconductor with an equivalent amount of zinc interstitials and quasi-free electrons. Doping with alumina and chromia results in an increase in negatively charged point defects (quasi-free electrons), reducing adsorption of OH^- species on the zinc oxide surface and thereby decreasing dissolution. Doping with lithia increases the concentration of positively charged point defects (zinc interstitials), thus facilitating surface hydroxylation and dissolution.