Crystallographic dependence of 4-dimethylaminopyridine electrosorption on gold

Abstract

Using differential capacity we have compared the electrosorption of dimethylaminopyridine (DMAP) on Au(poly), Au(100), and Au(111) electrode surfaces. Thermodynamic analyses of charge density data have provided quantitative information concerning the Gibbs excess of the films and the bonding motif between the adsorbate and the Au(111) surface. The results indicate that the orientation of DMAP is highly dependent on electrode potential, electrolyte pH, and surface crystallography. At pH equal to the pKa of the conjugate acid, the dominant adsorbed species is the free base, DMAP, but its adsorption state depends on the electrical state of the Au(111) surface. Negative polarizations favor a low coverage state with individual DMAP molecules coordinated through both the aromatic ring and the exocyclic tertiary amine. Positive polarizations induce a phase transition that provides a high density monolayer of DMAP coordinated to the Au(111) surface through the lone pair of the endocyclic nitrogen. The phase transition at high pH is unique to Au(111) as Au(100) and Au(poly) has been shown to only exhibit the high density state of adsorption. In slightly acidic electrolytes the adsorbed species on Au(111) and Au(100) is the protonated ion, DMAPH+, and no phase transitions were observed. The data provide the basis from which a rational strategy for synthesizing anisotropic gold nanocrystals can be developed.