Search

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.

Adsorption of thiols on ZnO(0001) and ZnO nanorods has been investigated using X-ray and ultraviolet photoelectron spectroscopies (XPS and UPS). Ultrahigh vacuum (UHV) dosing of sputter-cleaned ZnO(0001) with methanethiol (MT), 1-dodecanethiol (DDT), and 3-mercaptopropyltrimethoxysilane (MPTMS) leads to S2p peaks with a binding energy of 163.3 eV. Similar results for MPTMS are obtained for sputter-cleaned ZnO(0001) that is pre-dosed with water to form hydroxyl groups. In all cases, the absence of a free thiol S2p peak at 164.2 eV indicates that bonding to the surface occurs via the thiol end of the molecule. A DDT-dosed ZnO(0001) sample stored for 10 days in UHV and heated to temperatures as high as 150 °C exhibits minimal changes in its S/Zn atomic ratio, confirming chemisorption and the presence of a strong bond to the surface. UPS shows that MT adsorption on sputtered ZnO(0001) leads to a 0.7 eV increase in work function and perturbation of the MT molecular orbitals, again consistent with chemisorption. Dry ZnO nanorods have been exposed to MT while monitoring their photoluminescence. XPS and Raman spectroscopy confirm thiol adsorption. Relative to dry ZnO, adsorption causes a decrease in intensity of the visible emission peak, but the UV peak remains unchanged. These results indicate that Zn S bond formation quenches radiative decay to the valence band from defect states, possibly by methanethiolate adsorption filling oxygen vacancies.

ZnO is an alternative to TiO 2 -based dye-sensitized solar cells (DSSCs). Adsorption of cis -ruthenium-bis[2,2′-bipyridine]-bis[4-thiopyridine] onto ZnO nanorods has been studied using X-ray and ultraviolet photoelectron spectroscopies (XPS and UPS). XPS indicates chemisorption with a surface density of ca. 1 × 10 15 molecules/cm 2 , confirming the possibility of using thiol-terminated dyes for ZnO-based DSSC devices. The energy level diagram, based on UPS and absorbance spectroscopy, indicates that the LUMO of this dye is lower in energy than the ZnO conduction band edge, providing minimal enthalpic driving force for photovoltaic electron injection. However, optimization of thiol-functionalized Ru dyes could result in competitive ZnO-based DSSCs.