The electrodesorption of 3× 3 R30°thiol and sulfur lattices on Au(111) has been investigated by in situ STM and electrochemical methods. For thiol and sulfur adlayers, the 3× 3 R30°lattice is desorbed in sharp voltammetric current peaks. The electrodesorption kinetics involves nucleation and growth of holes. From the analysis of the dependence of the peak potential (E p ) on the length of the hydrocarbon chain (n), the thiol-thiol interaction results in ≈3 kJ mol -1 /C unit. The value of E p for n ) 0 indicates that the S-Au-( 111) bond energy in 3× 3 R30°thiol lattices is ≈19 kJ mol -1 smaller, and the bond has a lesser ionic character than the S-Au(111) bond in the 3× 3 R30°sulfur lattice. Monte Carlo simulations of a desorption model reproduce well the experimental potentiostatic and potentiodynamic results for thiol desorption.
The electrodeposition of gold on highly oriented pyrolytic graphite (HOPG) from acid aqueous solutions was studied by using electrochemical techniques complemented with ex-situ scanning tunneling microscopy (STM). The kinetics of gold electrodeposition is consistent with a nucleation and three-dimensional growth process under diffusion control from the solution side. As the applied potential moves in the negative direction, the gold crystal density increases, and the crystal shape changes from a Euclidean to a dendritic fractal morphology. This transition can be assigned to the anisotropic surface diffusion of gold adatoms induced by the applied electric potential. A model including a potential-dependent energy barrier at step edges accounts for the morphology transition for gold electrodeposition on HOPG.
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