Hydrogen on Zinc Oxide. Theory of Its Heterolytic Adsorption

An atom superposition and electron delocalization molecular orbital study is made of the adsorption of hydrogen species on the (1010) prism surface of zinc oxide. 4s + 4p surface states on Zn²⁺ play a role in H* adsorption on Zn²⁺ and O^2– surface sites. In the former case they provide the covalent stabilization of the Zn²⁺–H* bond and in the latter case they are reduced, allowing a strong O^2––H⁺ bond to form. Because of this, H₂ adsorbs heterolytically, yielding Zn²⁺–H^– and O^2––H⁺ as the products of dissociative chemisorption. Our findings, which are based on calculations on a Zn₁₄O₁₄ cluster model, including surface relaxation, of the (1010) surface are used to discuss the conductivities of the polar (0001) and (0001) cleavage surfaces, the former consisting of Zn²⁺ and the latter of O^2– It is suggested that the conductivities which are established when H* adsorbs on either surface are the consequences of surface and edge sites associated with step defects and the reduction of step Zn²⁺ surface face and edge states when H* adsorbs to step face and edge O^2– ions, respectively. Heterolytic adsorption of H₂ does not change the insulating property of ZnO.