Effect of Adsorption on the Photoluminescence of Zinc Oxide Nanoparticles

Photoluminescence (PL) changes of ZnO nano spheres 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 SO2 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 SO2 and MT adsorption occur mainly by replacement of surface hydroxyls, benzene adsorbs at defect sites, such as oxygen vacancies. HCI and Cl-2 decrease the visible and UV emission peaks by transforming the surface of the ZnO to ZnCl2, while H2S causes the emergence of a PL peak at 422 nm, which is postulated to arise from the formation of Zn interstitial defects.