Professor, Department of Chemistry, University of Massachusetts Lowell; Editor, Applied Surface Science, USA

Speech Title: Metal Oxide Surface Chemistry and Optochemical Sensing
Abstract: The adsorption of gases on metal oxide nanoparticles and surfaces has been investigated by a variety of surface science techniques, including thermal desorption spectroscopy, X-ray photoelectron spectroscopy (XPS), ultraviolet photoelectron spectroscopy (UPS) and thermal gravimetric analysis (TGA). Examples of gases investigated include sulfur dioxide, methanethiol, methanol, and benzene. The effects of co-adsorbed water and the presence of hydroxyl groups have also been studied. The surface science experiments have been accompanied by density functional theory (DFT) calculations on metal oxide clusters toward the goal of understanding the energetics of adsorption. Of particular interest are photoluminescent metal oxide nanoparticles, such as zinc oxide, cerium oxide and zirconium oxide. The ultimate goal of this research is to lay the foundation for an understanding of the processes involved in "optochemical sensing", in which the photoluminescence of the metal oxides changes due to adsorption.
Not surprisingly, adsorbed hydroxyl groups have been found to dramatically affect both photoluminescence and the adsorption mechanism. For example, in the case of methanethiol on single crystal ZnO(0001), room temperature adsorption on the atomically clean surface occurs by thiolate bonding to zinc sites, with the liberated hydrogen atom attaching to oxygen sites. In contrast, on a hydroxylated surface, adsorption occurs by a condensation reaction, accompanied by water desorption. For ZnO nanoparticles, the replacement of adsorbed hydroxyl groups by methanethiolate groups causes a decrease in the visible photoluminescence of the particles. Methanol does not adsorb at room temperature on hydroxylated ZnO.
Toward the goal of demonstrating the practicality of optochemical sensing, a portable UV LED-based fluorometer has been constructed and tested. Initial results demonstrate the potential use of the instrument for various applications, including hazardous chemical detection.