Reductive Desorption of octanethiol and octanedithiol on Au(111). A combined electrochemical and STM study

M. L. CAROT; M. J. ESPLANDIU; E. M. PATRITO; V. A. MACAGNO

Düsseldorf, Alemania

Congreso; 53rd Annual Meeting of the International Society of Electrochemistry; 2002

We have investigated the reductive desorption of 1-octanethiol and 1,8-octanedithiol layers on Au(111) by Cyclic Voltammetry (CV), Electrochemical Impedance Spectroscopy (EIS) and Scanning Tunneling Microscopy (STM) in both acidic and alkaline electrolytes.             With our CV measurements we first characterized the package and density of thiolates on the surface in the potential region in which no faradaic processes occur. After that, the reductive desorption and the oxidative readsorption processes were monitored. The area of the reduction peaks of the thiol and dithiol layers corresponded to those when the SAMs are in the highest monolayer density. For the dithiols, such results are in agreement with those found for other dithiols (i.e. 1,6-hexanedithiol). In the oxidative thiol readsorption process, the area and the shape of the peaks depend on the pH and the nature of the thiol in correspondence with their different solubilities in the electrolyte.             STM measurements in the double layer potential region showed highly ordered octanedithiol layers and a disordered structure for the dithiol. The onset of the desorption process was characterized by areas with a high adsorbate mobility starting from defects, such as steps, vacancy island rims and domain boundaries. During and after the reductive thiol desorption, step edges become very rough, indicating that the gold substrate is quite involved in the process. Moreover, very stable gold islands with monoatomic height were formed from material extracted mainly from the steps. The dithiol desorption was characterized by the formation of aggregates with a tube-like shape following the substrate symmetry directions. These structures seem to result from some kind of micellization process which resembles the (half-)cylindrical hemimicelles found by assembling surfactants on metal surfaces.