Revealing the Structural Complex of Adsorption and Assembly of Benzoic Acids at Electrode−Electrolyte Interfaces Using Electrochemical Scanning Tunneling Microscopy

Self-assembly of benzenecarboxylic acids on well-defined noble metals has been intensively investigated using surface-sensitive techniques. However, most studies were focused on the formation of nanostructures via benzenetricarboxylic and benzenedicarboxylic acids, which are composed of multiple carboxylic acid functional groups in either the meta or para positions of the benzene ring, allowing the formation of long-range ordered molecular arrays through −COOH-mediated intermolecular hydrogen bonds. Two-dimensional nanostructures of benzoic acid molecules that are composed of a single carboxylic acid functional group on the phenyl ring at metal–electrolyte interfaces were rarely reported using scanning tunneling microscopy (STM) because there is only one carboxylic acid functional group for each benzoic acid available to form intermolecular hydrogen bonds, making it difficult to construct long-range ordered nanoarchitectures. In this work, we employed electrochemical scanning tunneling microscopy (EC-STM) in combination with electrochemical cyclic voltammetry (CV) techniques to explore the adsorption and phase formation of benzoic acids (BZAs) at Au(111)/electrolyte interfaces. Our experiments show how the electrolyte, molecular concentration, electrochemical potential, and co-adsorption of aqueous ions affect the adsorption and self-assembly of BZA molecules. It is found that the BZA molecules are not assembled into long-range ordered structures in the presence of a sulfuric acid electrolyte due to the strong competing co-adsorption of sulfate ions on a gold electrode. BZA molecules can form flat-oriented ordered adlayers in a perchloric acid electrolyte (containing weakly adsorbed ClO4– ions) at a negatively charged surface only when the concentration of the molecular solution reaches above 6 mM. Below 6 mM, the CVs of BZA on Au(111) in 0.1 M HClO4 show only one pair of adsorption/desorption peaks. When the BZA concentration increases to 6 mM, the voltammogram exhibits three pairs of peaks, corresponding to the structural transformation of disordered phase [phase I, Esample (ES): −0.600 to −0.190 V], linear stripe pattern (phase II, ES: −0.190 to 0.108 V), zigzag pattern (phase III, ES: −0.108 to −0.066 V), and upright packing pattern (phase IV, ES: −0.066 to 0.300 V). These phases and molecular adlayers were revealed by STM in the four electrochemical potential regions. Effect of parameters (electrolyte ions, concentration, and electrochemical potential) explored in this study will provide valuable information for the formation of molecular adlayers, adsorption and self-assembly, materials, corrosion inhibition, and molecular devices.