| Abstract
| - In situ scanning tunneling microscopy images of self-assembled monolayers (SAMs) of 4-mercaptopyridine(4-MPy) on Au(111) recorded in neat 0.1 M H2SO4 solutions provided evidence for a potential-inducedphase transition over the range 0.40−0.15 V versus saturated calomel electrode. Analysis of the data wasconsistent with the presence of a (5 × √3) and (10 × √3) superstructure (phase A) at the positive end,that is, 0.40 V, for which the local coverage, θloc, is about 0.2 (two 4-MPy molecules per unit cell), whichcompresses at the negative end, that is, 0.15 V, to yield a much denser superstructure (phase B, θloc ca.0.5). This behavior is unlike that reported for the 4-MPy-Au(111) SAM prepared by identical means, in0.1 M HClO4 (or in sulfate solutions of a much higher pH) for which only the (5 × √3) superstructure wasobserved over the same potential range. The compression associated with the phase A to phase B transitionis attributed to the formation of a hydrogen-bonded network of bisulfate coordinated in turn to the 4-MPylayer via the acidic hydrogens of the pyridinium moieties. Such conditions promote better packing ofadsorbed 4-MPy species, which are aided by intermolecular π−π ring interactions, resulting in higher localcoverages.
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