Unique Two-Dimensional Multiple Phase Transition of Single-Anchored Aromatic Carboxylic Acids at Electrified Interfaces

We report a unique feature in an adsorption and molecular assembly discovered by classic electrochemical cyclic voltammetry (CV) and electrochemical scanning tunneling microscopy (EC-STM) techniques. With its aromatic carboxylic acid (ACA) composed of one phenyl ring and a single -COOH, benzoic acid (BZA) behaves in a drastically different manner than other ACAs. In this work, we systematically varied the BZA concentration and found that the number of current peaks in cyclic voltammograms (CVs) started to change from one to three at a concentration that we refer to as "critical phase transition concentration (CPC)". Below the CPC, no ordered adlayer can be formed at a negatively charged Au(111) surface. Further, we discovered that the peak position in the CVs shifted as a function of solution concentration, resulting in larger peak separations between either anodic or cathodic peaks as concentration increased. The peak shifting and evolution are attributed to the nature of the BZA structure and the concentration-dependent assembly due to the lack of intermolecular H-bonds formed by the -COOH functional group in the BZAs, evidenced by high-resolution STM images and interpreted by the proposed adsorption models.