The photoluminescence (PL) spectrum of nanoparticulate zinc oxide (ZnO) excited by 320-340 nm light consists of a relatively sharp ultraviolet peak at 380 nm and a broad visible peak at 550 nm. In general, the former originates from excitonic recombination (electron/hole pair), and the latter is due to surface defects, including oxygen vacancies. Since visible emission of ZnO nanoparticles is sensitively affected by surface states and morphology, chemisorption and physisorption of gases on ZnO may be monitored by measuring changes in the PL spectrum. Recent experimental measurements of PL changes of ZnO nanoparticles upon exposure to various gases and vapors are discussed. Possible adsorption mechanisms are suggested, aided by thermal gravimetric analysis (TGA) and X-ray photoelectron spectroscopy studies. Density functional theory (DFT) calculations on ZnO nanoclusters with co-adsorbed surface hydroxyls also have been performed to understand the bonding of gases to the ZnO nanoparticles. In order to determine the role of hydroxyl groups in the PL spectrum of nanoparticulate ZnO, emission spectra of the particles have been measured as a function of temperature and correlated with TGA results. In the case of reversible adsorption of hydrogen and oxygen molecules, the PL changes may be ascribed to transfer of electrons to or from the conduction band which affects the probability of excitonic recombination, resulting in UV emission intensity changes. For irreversible chemisorption of molecules such as methanethiol and sulfur dioxide, it is found that surface hydroxyls or lattice atoms play a key role in the surface reactions. Additionally, comparing the ratio of the intensities of the UV and visible emission peaks may be a reliable indication of adsorption. Towards the goal of developing a field-deployable hazardous chemical detection system, a customized portable UV LED-based fluorometer has been constructed and tested.

Gas and vapor dependent photoluminescence changes and surface chemistry of zinc oxide nanoparticles

254th National Meeting (2017)

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