Reactivity of 1,8-octanedithiol monolayers on Au(1Â 1Â 1): Experimental and theoretical investigation

The reactivity of 1,8-octanedithiol/Au(1 1 1) self-assembled monolayers (SAMs) formed by the immersion method was investigated by means of cyclic voltammetry (CV), scanning tunneling microscopy (STM), electrochemical impedance spectroscopy (EIS) and quantum mechanical calculations. The potential of the reductive desorption process for the dithiol is 99 mV more negative than for 1-octanethiol indicating a higher stability of dithiol monolayers. A reduction charge of −109 μC/cm2 was obtained for the dithiol which is to be compared to the reductive desorption charge of −83 μC/cm2 for the monothiol. The extra reduction charge of the dithiol was attributed to the presence of intralayer disulfide bonds at the SAM/electrolyte interface. EIS measurements confirmed that these intralayer linkages increase the stability of the monolayer with respect to the homologue 1-octanethiol/Au(1 1 1). The onset of the reductive desorption of the dithiol is characterized by the appearance of a striped structure which is attributed to flat lying molecules. In alkaline electrolyte, the complete reduction of the dithiol monolayer occurs at the most negative potentials at which the herringbone reconstruction of the gold substrate is clearly imaged together with gold islands. Density functional calculations confirmed that intralayer disulfide bond formation is energetically favorable when the adjacent alkyl chains are arranged in an antiparallel configuration. We observed that the concentration of surface disulfides for 1,8-octanedithiol monolayers is lower than for 1,6-hexaneditiol monolayers. A model was proposed to explain the effect of chain length on the reactivity of α,ω-alkanedithiols.