title: Self-assembled monolayers of 4-mercaptopyridine on Au(111): a potential-induced phase transition in sulfuric acid solutions.
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-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 x square root(3)) and (10 x square root(3)) superstructure (phase A) at the positive end, that is, 0.40 V, for which the local coverage, theta(loc), 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, theta(loc) 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 x square root(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 pi-pi ring interactions, resulting in higher local coverages.
authors.affiliation: Abteilung Elektrochemie, Universität Ulm, 89069 Ulm, Germany.
authors.forename: Thorsten
authors.initials: T
authors.lastname: Baunach
authors.forename: Valentina
authors.initials: V
authors.lastname: Ivanova
authors.forename: Daniel A
authors.initials: DA
authors.lastname: Scherson
authors.forename: Dieter M
authors.initials: DM
authors.lastname: Kolb
mesh_terms: D006046:Gold / Q000737:chemistry*; D010084:Oxidation-Reduction; D044367:Phase Transition; D011522:Protons; D011725:Pyridines / Q000737:chemistry*; D012996:Solutions / Q000737:chemistry; D013464:Sulfuric Acids / Q000737:chemistry*; D013499:Surface Properties
chemical_list: D011522:Protons; D011725:Pyridines; D012996:Solutions; D013464:Sulfuric Acids; C042861:4-thiopyridine; D006046:Gold; C033158:sulfuric acid