Studies on the Adsorption Behavior of 2,5-Dimercapto-1,3,4-thiadiazole and 2-Mercapto-5-methyl-1,3,4-thiadiazole at Gold and Copper Electrode Surfaces

The adsorption behavior of 2,5-dimercapto-1,3,4-thiadiazole (DMcT) and 2-mercapto-5-methyl-1,3,4-thiadiazole (McMT) on Au and Cu electrode surfaces was studied using a 5 MHz quartz crystal microbalance (QCM), cyclic voltammetry, Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), and phase measurement interferometric microscopy (PMIM). Different behaviors were observed for the adsorption of DMcT and McMT on Au and Cu electrodes. Exposing the Au electrode to a McMT solution resulted in the formation of a stable, self-assembled monolayer on the electrode surface. A sharp peak resulting from the reductive desorption (RD) of McMT was observed for McMT chemisorbed on the Au electrode. It was also found that dimer-DMcT (di-DMcT) should be used in order to construct a stable DMcT layer on an Au electrode. Detailed comparisons of charge consumption and mass change during reductive desorption suggest that chemisorbed di-DMcT is monomeric and desorbs completely from the Au electrode in the RD process. However, on a Cu electrode surface, a stable McMT layer could not be constructed. It was also confirmed from PMIM experiments and Raman spectroscopy that DMcT etched copper electrodes, along with concurrent formation of a dimer form of DMcT (di-DMcT). The apparent reason for the different adsorption behaviors between DMcT and McMT is that DMcT is a stronger proton donor and oxidant.