Effect of Adsorption on the Photoluminescence of Zinc Oxide Nanoparticles

Photoluminescence (PL) changes of ZnO nanospheres at room temperature have been measured during exposure to gases and vapors using a traditional fluorometer and a portable, ultraviolet (UV) light-emitting diode-based instrument. Thermal gravimetric analysis indicates that the nanospheres are essentially fully hydroxylated, with OH groups and H atoms attached to surface Zn and O sites, respectively. The PL spectrum has both a UV excitonic emission peak and a visible, defect-related one. Exposure to the gases and vapors studied, whether they physisorb or chemisorb, causes a decrease in the intensity of the visible emission peak relative to pure nitrogen, although to different degrees. Electron-donating molecules, such as hydrogen and methanol, cause a reversible increase in the UV emission peak intensity due to formation of an electron-rich accumulation layer around the nanoparticles. Electron-withdrawing molecules, such as oxygen and water, cause a corresponding decrease due to a depletion layer. For reactive adsorption, such as by sulfur dioxide and methanethiol (MT), surface hydroxyl groups play an important role in reactivity and PL changes. X-ray photoelectron spectroscopy, coupled with density functional theory calculations, confirms that MT adsorption occurs by replacement of hydroxyl groups adsorbed on Zn sites, while SO 2 adsorption leads to sulfite formation and removal of H atoms attached to O sites. The latter process causes a more dramatic decrease in visible emission, and it is postulated that hydroxyl groups formed by adsorbed H on O sites act as efficient charge traps that enhance visible PL. Their removal decreases visible PL, partially shutting down this energy pathway and causing an increase in UV emission. While SO 2 and MT adsorption occur mainly by replacement of surface hydroxyls, benzene adsorbs at defect sites, such as oxygen vacancies. HCl and Cl 2 decrease the visible and UV emission peaks by transforming the surface of the ZnO to ZnCl 2, while H 2 S causes the emergence of a PL peak at 422 nm, which is postulated to arise from the formation of Zn interstitial defects.