Gas and vapor dependent photoluminescence changes of zinc oxide nanoparticles

Nanoparticulate zinc oxide (ZnO) excited by 320-340 nm light consists of a sharp ultraviolet and a broad visible emission peak. The former originates from excitonic recombination, and the latter appears due to surface defects, including oxygen vacancies. Since visible emission of ZnO nanoparticles is affected by surface states and morphology, adsorption of gases on ZnO may be monitored by measuring changes in photoluminescence (PL). In this presentation, we report recent experimental measurements in PL changes of ZnO nanoparticles upon exposure to various gases and vapors. Possible adsorption mechanisms are suggested aided by thermal gravimetric analysis and X-ray photoelectron spectroscopy. Density functional theory (DFT) calculations on zinc oxide clusters with co-adsorbed surface hydroxyls also have been studied to understand the bonding of adsorption on ZnO nanoparticles. In the case of reversible adsorption, the PL changes may be explained by the transfer of electrons to or from the conduction band, and these may directly affect excitonic recombination. For irreversible chemisorption, it is found that surface hydroxyls or lattice oxygen atoms play a key role in the surface reaction.