Leaflike Structured Multilayer Assembly of Dimercaptothiadiazole on Gold Surface

Adsorption of a heteroaromatic dithiol, 2,5-dimercapto-1,3,4-thiadiazole (DMT), on Au surface from a completely deaerated aqueous solution leads to the formation of a multilayer assembly via hydrogen bonding with water molecules, whereas adsorption from acetonitrile, ethanol, dimethyl sulfoxide, and chloroform solutions leads to the formation of a monolayer. Cyclic voltammetry (CV), attenuated total reflectance (ATR) infrared spectroscopy, Raman spectroscopy, high-resolution X-ray photoelectron spectroscopy (XPS), and scanning electron microscopy (SEM) were used to characterize the monolayer and multilayer assemblies of DMT on Au surface. Since DMT contains two S-H groups, it chemisorbs on Au surface through one of its two S-H groups, while the other S-H group is pointing away from the surface. The presence of free S-H groups on the Au surface was confirmed by CV, ATR-FT-IR, and XPS. It is presumed that the free S-H groups of DMT on the Au surface form a hydrogen bond with the water molecules. Subsequently, DMT molecules in solution form a hydrogen bond with the water molecules attached with DMT on Au surface, and this type of hydrogen bonding network goes on increasing when the soaking time of the Au surface in an aqueous solution of DMT increases. The involvement of water molecules in the multilayer formation was confirmed from the appearance of a broad stretching band around 3300 cm(-1) corresponding to the hydrogen bonded O-H in the ATR-FTIR spectrum, in addition to a binding energy peak at 533.4 eV due to hydrogen-donating water molecules in the O is region of XPS. The absence of a stretching band characteristic for S-S at 537 cm(-1) in the Raman spectrum confirmed that the multilayer assembly was not formed via S-S linkage. However, exposure of DMT multilayer to air shows a stretching band characteristic of S-S, indicating that aerial oxidation leads to the formation of S-S bond. SEM images show that DMT forms a leaflike structured multilayer assembly on Au surface.