In situ scanning tunneling microscopy (STM) is proposed as an option to investigate the electrochemical reactivity of nontrivial systems. For this purpose the controversial electro-oxidation and electro-reduction of thiourea at Au(111) surfaces under potential control and constant temperature are considered. Sequential STM imaging show thiourea adsorption in striped arrays that evolve to a hexagonal close-packed (hcp) structure when the electron surface charge density is decreased. The transient hcp structure is electro-oxidized to formamidine disulfide (FDS) that slowly yields adsorbed sulfur. These results show that STM is a powerful tool to understand the reactivity of adsorbed molecules at conducting surfaces, by sequential imaging at the molecular level.

Electrodesorption potentials for alkanethiolate self-assembled monolayers (SAMs) on polycrystalline Cu and Cu(111) surfaces were determined by using electrochemical techniques combined with Auger electron spectroscopy. For a given alkanethiolate SAM, the electrodesorption potentials from Cu are shifted 0.6 V in the negative direction with respect to those found on Au. Calculations based on density funtional theory for methanethiolate desorption from Cu(111) show that these potential differences reflect differences in the energy for introducing an electron into the alkanethiolate−metal system and also in the energy to break the alkanethiolate S-head−metal bond. On the other hand, the alkanethiolate−alkanethiolate interaction energies at SAMs remain practically independent of the substrate.