Thiol adsorption on metal oxide nanoparticles

The adsorption of 2-naphthalenethiol (2-NPT) and methanethiol (MT) on 13 different metal oxide nanoparticles, of approximately 30 nm average primary particle size, has been investigated. In the case of 2-NPT, which is fluorescent, a screening method to assess adsorption was developed that consists of mixing the nanoparticles with a dilute ethanolic solution of 2-NPT and performing several cycles of centrifuging and rinsing with ethanol. Fluorescence measurements on the re-dispersed particle suspensions were then used to diagnose whether or not adsorption had occurred. Complementary experiments were performed by mounting powder samples of each of the metal oxide nanoparticles onto sample stubs and performing X-ray photoelectron spectroscopy (XPS) before and after in situ dosing with MT. In both cases, adsorption was observed only on ZnO, TiO₂, and In₂O₃. Adsorption did not occur on Al₂O₃, CeO₂, Fe₂O₃, Gd₂O₃, Ho₂O₃, NiO, SiO_x, WO₃, Y₂O₃, and ZrO₂. Density functional theory (DFT) calculations were performed using small metal oxide clusters, assuming that dissociative adsorption occurs by replacement of a hydroxyl group attached to a metal site and the formation of water. The theoretical and experimental results generally agree, suggesting that this is indeed the adsorption mechanism for most of the nanoparticles. The agreement also suggests that the size and geometry of the nanoclusters play a minor role and that the relative strengths of the metal-sulfur and metal-hydroxyl bonds dictate thiol adsorption. This work has important implications related to the functionalization of metal oxide nanoparticles and surfaces.