Raman study of the influence of hydrogen on defects in ZnO

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