Self-Assembled Monolayers of 4-Mercaptopyridine on Au(111):  A Potential-Induced Phase Transition in Sulfuric Acid Solutions

• Published in: Langmuir 2004-03, Vol.20 (7), p.2797-2802
• Main Authors: Baunach, Thorsten, Ivanova, Valentina, Scherson, Daniel A, Kolb, Dieter M
• Format: Article
• Language: English
• Subjects: Gold - chemistry; Oxidation-Reduction; Phase Transition; Protons; Pyridines - chemistry; Solutions - chemistry; Sulfuric Acids - chemistry; Surface Properties
• ISSN: 0743-7463; 1520-5827
• DOI: 10.1021/la035389t

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-induced phase transition over the range 0.40−0.15 V versus saturated calomel electrode. Analysis of the data was consistent 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), which compresses 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, in 0.1 M HClO4 (or in sulfate solutions of a much higher pH) for which only the (5 × √3) superstructure was observed over the same potential range. The compression associated with the phase A to phase B transition is attributed to the formation of a hydrogen-bonded network of bisulfate coordinated in turn to the 4-MPy layer via the acidic hydrogens of the pyridinium moieties. Such conditions promote better packing of adsorbed 4-MPy species, which are aided by intermolecular π−π ring interactions, resulting in higher local coverages.