Electrocrystallization of rhodium clusters on thiolate-covered polycrystalline gold.

This study reports on the electrochemical deposition of rhodium metal clusters on a polycrystalline gold electrode, modified with a monolayer of dodecanethiol through self-assembly from solution. The deposition process was investigated using cyclic voltammetry, chronoamperometry, and electrochemical quartz crystal microbalance. It is shown that the presence of the thiol monolayer drastically alters the nucleation and growth mechanism compared with the mechanism on the bare gold electrode. The small uncovered gold domains, located at the imperfections in the thiolate monolayer which are induced by the gold nanoroughness, act as nucleation sites for small rhodium clusters. At longer times, these clusters can outgrow the organic monolayer. The resulting surface morphology was characterized by scanning electron microscopy. Rhodium electrocrystallization on the bare gold substrate resulted in an ensemble of a very large amount of very small clusters that are difficult to distinguish from the gold roughness. In contrast, in the presence of a self-assembled monolayer (SAM) of dodecanethiol covalently attached to the gold electrode, the resulting deposit consisted of an ensemble of hemispherical particles. The size distribution of the rhodium particles obtained by using double step chronoamperometry was compared to the ones obtained with cyclic voltammetry and "classical" chronoamperometry. It is shown by X-ray photoelectron spectroscopy that the SAM is still present after rhodium deposition on the thiolate-covered gold substrate. Because the rhodium clusters are directly attached to the gold substrate and can thus easily be electrified, the resulting interface could be used as a composite electrode consisting of a random array of gold supported rhodium nano/microparticles separated from each other by an organic phase. On the other hand, it is shown that the SAM is easily removed by electrochemical oxidation without dissolving the rhodium clusters and, thus, leaving a different array of rhodium clusters on the gold surface compared with the topography obtained in the absence of the SAM. From this point of view, substrate modification with such "removable" organic monolayers was found to be an interesting tool to tune the nano- or microtopography of electrochemically deposited rhodium.