Tuning hydrogen adsorption on pure and doped ZnO (0001¯) surfaces by a simple electron counting model

Abstract

Hydrogen (H) adsorption strengths on oxygen-terminated (0001¯) surfaces of pure and doped wurtzite ZnO are investigated under varying H surface coverage conditions. Consistent with the prediction of the classical electron counting rules, a 12 monolayer (ML) of adsorbed H changes the electronic structure of pure ZnO (0001¯) surface from metallic to semiconductor state by saturating unpaired electrons of surface oxygen atoms. This closed-shell electron configuration of the ZnO (0001¯) surface significantly reduces the adsorption strengths of subsequent H atoms, making the dissociative adsorption of a H2 molecule endothermic. We apply a simple electron counting model to predict and tune the coverage-dependent H adsorption strengths on general polar semiconductor surfaces. This model is confirmed by our investigations of H adsorption on (0001¯) surfaces of ZnO with a series of dopant elements (Na, Mg, Al, Ti, Fe, Sn, etc.). It can also be applied to H adsorption on other similar polar semiconductors, such as ZnO (0001¯) containing O vacancies, wurtzite GaN (0001¯), and zincblende ZnS (1¯1¯1¯) surfaces.