Sulfur dioxide and nitrogen dioxide adsorption on zinc oxide and zirconium hydroxide nanoparticles and the effect on photoluminescence

摘要

Nanoparticulate zinc oxide and micron-size zirconium hydroxide powders have been exposed to sulfur dioxide and nitrogen dioxide by flowing the gases, diluted with nitrogen, over powder samples. X-ray photoelectron spectroscopy (XPS), Raman spectroscopy, and thermogravimetric analysis (TGA) indicate strongly bound, chemisorbed SO_3 and NO_3 surface species. Two pre-treatments of the nanoparticulate ZnO samples prior to gas exposure have been investigated: (1) drying overnight in a vacuum oven and (2) hydrating the samples by placing them overnight in water-saturated air. A dramatic difference in reactivity of ZnO is observed, with approximately two-fold and ten-fold greater uptake of NO_2 and SO2, respectively, measured by XPS for the hydrated samples relative to the dried ones. Transmission electron microscopy (TEM) demonstrates that the greater uptake arises from a morphology change in the case of the hydrated samples. For zirconium hydroxide, no morphology change is observed for hydrated samples, and SO_4 (ads), in addition to SO_3 (ads), is indicated by XPS. ZnO and Zr(OH)_4 both exhibit photoluminescence (PL) spectra, with peak intensities that change dramatically due to hydration and subsequent exposure to SO_2 and NO_2 gases. Dosing of the powders with these gases effectively reverts the PL spectra to those corresponding to less hydration.