Reactivity of 1,8-octanedithiol monolayers on Au(111). Experimental and theoretical investigation

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

JOURNAL OF ELECTROANALYTICAL CHEMISTRY

ELSEVIER

Lugar: Amsterdam; Año: 2005 vol. 579 p. 13 - 23

0022-0728

The reactivity of 1,8-octanedithiol/Au(1 1 1) self-assembled monolayers (SAMs) formed by the immersion method was investi-gated 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 negativethan for 1-octanethiol indicating a higher stability of dithiol monolayers. A reduction charge of -109 µC/cm2 was obtained for thedithiol which is to be compared to the reductive desorption charge of -83 µC/cm2 for the monothiol. The extra reduction charge ofthe dithiol was attributed to the presence of intralayer disulfide bonds at the SAM/electrolyte interface. EIS measurements con-firmed 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 isattributed to flat lying molecules. In alkaline electrolyte, the complete reduction of the dithiol monolayer occurs at the most negativepotentials at which the herringbone reconstruction of the gold substrate is clearly imaged together with gold islands. Density func-tional calculations confirmed that intralayer disulfide bond formation is energetically favorable when the adjacent alkyl chains arearranged in an antiparallel configuration. We observed that the concentration of surface disulfides for 1,8-octanedithiol monolayersis 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.