Mechanism of Thiol-Induced Color Change of Tungsten Oxide Nanoparticles

Abstract Tungsten oxide (WO3) nanoparticles, with an average size of ca. 50 nm, dramatically change color from light to dark green upon methanethiol (MT) exposure. Proton NMR indicates the presence of physisorbed MT and its oxidation product, dimethyldisulfide. X-ray photoelectron spectroscopy indicates that a maximum of ca. 33% of the W6+ is reduced to W5+ in the process of MT oxidation with creation of oxygen vacancies. Raman spectroscopy shows negligible changes in lattice vibrations, consistent with absence of proton insertion. These results demonstrate that the color change is due to W5+ to W6+ intervalence transitions and support an oxygen vacancy mechanism.