Structure and Mechanism of Photooxidation of Self-assembled Monolayers of Alkylthiols on Silver Studied by XPS and Static SIMS

Abstract

XPS and static SIMS have been employed to characterize the structures and the photooxidation of self-assembled monolayers (SAMs) of alkylthiols on silver. Samples were prepared by immersing silver substrates in thiol solutions for a range of times from 30 min to 40 h, but no changes were observed in either the compositions of the monolayers determined by XPS or the static SIMS spectra. Static SIMS spectra exhibited a range of characteristic molecular species that facilitated quantification of changes in the SAM structure following exposure to UV light. Photooxidation led to the formation of alkylsulfonate species at a rate that depended on the length of the adsorbate alkyl chain. However, evaluation of XPS compositional data suggested that significant amounts of unoxidized sulfur remain even when SIMS data indicate the absence of intact, unoxidized thiolate species in the monolayer. In agreement with the results of other workers, we propose that photoirradiation of SAMs on silver leads to two processes: photooxidation of thiol adsorbates to alkylsulfonates and sulfur−carbon bond scission. The latter process, not observed in an earlier study of SAMs on Au, leads to the formation of significant quantities of inorganic sulfur that are subsequently slowly oxidized to yield inorganic oxides of sulfur.