Raman spectra of ZnO powders and thin films obtained during exposure to H at different concentrations, temperatures, and electrochemical conditions indicated two distinct effects of H on the defect structure of the oxide. At lower concentrations of H in a gaseous environment at approximately 400C, the H diffused into the ZnO and occupied O vacancies contributing to a reduction in intensity of Raman bands associated with the defect. At higher concentrations of hydrogen, generated during electrochemical polarization in aqueous solution at 25C, the H diffused into the ZnO, both filling O vacancies and populating interstitial sites in line with recent observations reported in the literature. In this latter role, H imparted sufficient electron density to the Zn and/or structural disorder to enhance vibration modes that were forbidden or typically weak in native ZnO. The results argue for the pervasive and varied chemistry of H in this oxide and may help explain the dependence of physical properties, especially electrical conductivity, on fabrication methods and environmental conditions.

Raman Study of the Influence of Hydrogen on Defects in ZnO. Windisch, C.F., Exarhos, G.J., Yao, C., Wang, L.Q.: Journal of Applied Physics, 2007, 101[12], 123711