Tiomalic acid adsorbed on Au (111) and nanoparticles surfaces

Full text: The self-assembly of thiomalic acid (TMA) on Au(111) and on Au nanoparticles (AuNPs) of different sizes have been studied by X-ray photoelectron spectroscopy (XPS) and electrochemical techniques. Results shows that TMA is adsorbed on the Au(111) surface as thiolate species with a small amount of atomic sulfur (~10%) and a surface coverage lower to that found for alkanethiol on Au(111) due to steric factors. The amount of atomic sulfur markedly increases when the TMA is self-assembly on AuNPs (3-4 nm and 6 nm in size) by the ligand exchange method. We propose that the atomic sulfur is produced as a consequence of C-S bond cleavage, a process that is more favorable at defective sites of the AuNPs surface. The bond scission is also assisted by the presence of an electron withdrawing group, as it is the case of the carboxyl moiety, in alpha position, with the consequent formation of a resonance-stabilized carbanion. The presence of a high local concentration of positive charged molecules increases the stability of negative charged leaving group. Our results demonstrate that the terminal functionalities of thiols are conditioning factors in the final structure and composition of the adlayers. (author)