Direct Evidence of Ozone as the Active Oxidant in “Photooxidation” of Alkanethiols on SERS-Active Silver

Abstract

Surface-enhanced Raman spectroscopy (SERS) has been used to study the mechanism of “photooxidation” of decanethiol self-assembled on roughened Ag. Direct exposure of self-assembled decanethiol to light from a low-pressure Hg lamp under ambient conditions results in adsorbed oxidized sulfur species. Absorbance spectroscopy was used to quantify ozone levels (∼ 20 ppm) generated by the Hg lamp. When decanethiol samples were exposed to the same amount of 254 nm light from the lamp but were environmentally isolated, so that oxygen was present but ozone generated by the lamp could not diffuse to the surface, no oxidation was observed. In addition, when decanethiol samples were exposed to an equivalent amount of electrically generated ozone in the absence of light, the SERS oxidation product spectra were identical to those generated by the lamp under ambient conditions. Quantitative investigation of the kinetics of oxidation yielded second-order rate constants of 0.0029 ± 0.0005 ppm⁻¹ s⁻¹ and 0.0024 ± 0.0006 ppm⁻¹ s⁻¹, for electrically generated ozone and the lamp, respectively. The two rate constants are the same within error, indicating that under the conditions of our experiment the mechanism of oxidation is dominated by ozone and not light.