Structure and electrochemistry of 4,4'-dithiodipyridine self-assembled monolayers in comparison with 4-mercaptopyridine self-assembled monolayers on Au(111).

4,4'-Dithiodipyridine (PySSPy) monolayers on Au(111) were investigated by cyclic voltammetry, X-ray photoelectron spectroscopy (XPS) and in situ scanning tunneling microscopy (STM). The studies were performed in solutions of different anions and pHs (0.1 M H2SO4, 0.1 M HClO4, 0.1 and 0.01 M Na2SO4, 0.1 and 0.01 M NaOH). The cyclic current-potential curves in H2SO4 show current peaks at about 0.4 V, which are absent for all other electrolytes at this potential. The XPS data suggest that PySSPy adsorbs via the S endgroup on the gold surface and the S-S bond breaks during adsorption. From the chemical shift of the N(ls) peak, it is concluded that in acidic media the self-assembled monolayer (SAM) is fully protonated, whereas in basic solution it is not. The pKa is estimated to be 5.3. STM studies reveal the existence of highly ordered superstructures for the SAM. In Na2SO4 and H2SO4, a (7 x mean square root of 3) structure is proposed. However, whereas in Na2SO4 solutions the superstructure does not change with potential, in 0.1 M H2SO4 the superstructure is observed only negative of the current peak at +0.4 V. At more positive potentials, the film becomes disordered. The results are compared to those for 4-mercaptopyridine (PyS) SAMs. XPS experiments and current-potential curves indicate that both molecules adsorb in the same manner on Au(111), that is, even in the case of PySSPy the adspecies is PyS. The STM results, however, call for a more subtle interpretation. While in Na2SO4 solutions the observed superstructures are the same for both SAMs, markedly different structures are found for PySSPy and PyS SAMs in 0.1 M H2SO4.