Surface-enhanced Raman spectroscopy of aromatic thiols and disulfides on gold electrodes

Surface-enhanced Raman spectroscopy (SERS) has been used to characterize monolayers formed from benzenethiol (BT), benzenemethanethiol (BMT), p-cyanobenzenemethanethiol (CBMT), diphenyl disulfide (DPDS), and dibenzyl disulfide (DBDS) on roughened gold electrodes. All five species adsorb dissociatively as the corresponding thiolates. The charge transfer and electrostatic interactions between the adsorbates and the surface depend strongly on the applied potential. The aromatic ring in BT is tilted relative to the mean surface plane at all applied potentials. The rings in BMT and CBMT also are tilted, but stepping the potential to positive or negative extremes causes a reduction in surface coverage and permits the BMT and CBMT molecules to lie more flat on the surface. Full coverage can be restored by reimmersing the electrode in thiol solution. Monolayers formed from symmetric disulfides are exactly like those formed from the corresponding thiols. if the gold electrode is immersed for prolonged periods in disulfide solution and not rinsed with fresh ethanol, multilayers form, which can be electrochemically reduced (S-S bond cleavage). The monolayers formed from all five compounds are robust over the potential window for the gold electrode; complete desorption occurs between +800 and +1000 mV vs SCE, coincident with oxidation of the surface, and between -1000 and -1200 mV due to hydrogen generation (reduction of water) at the interface.