Self-assembly provides unique routes to create supramolecular nanostructures at well-defined surfaces. In the present work, we employed scanning tunneling microscopy (STM) in combination with electrochemical techniques to explore the adsorption and phase formation of a series of aromatic carboxylic acids (ACAs) at Au(111)/0.1 M HClO4. Specific goals are to elucidate the roles of electrochemical potential and directional hydrogen-bonding on the structures and orientation of individual ACAs that form nanoarchitectures. ACAs are prototype materials for supramolecular self-assemblies via stereospecific hydrogen bonds between neighboring molecules. In this study, we mainly focus on a special ACA, terephthalic acid (TPA), which is almost insoluble in water, making the assembly of this molecule from aqueous solution challenging. Depending on the applied electric field, TPA molecules form distinctly different, highly ordered adlayers on Au(111) triggered by directional intermolecular hydrogen bonds. At low electrochemical potentials, TPA molecules are planar oriented, forming a potentially infinite hydrogen-bonded adlayer without any observed domain boundaries. The increase of the electrode potential triggers the deprotonation of one carboxylic acid functional group of TPA; additionally, this is accompanied by an orientation change of molecules from planar to perpendicular. In contrast, structural “defects” and multiple domain boundaries were found at this positively charged surface. The assembled nanostructures of TPA are compared with other ACAs (trimesic acid, benzoic acid, and isophthalic acid), and corresponding adsorption models were built for all molecular adlayers, showing that intermolecular hydrogen-bonding plays a determining role in the formation of two-dimensional ACA nanostructures.

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 phasetransition 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.