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

Abstract

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.

Probe the Dynamic Adsorption and Phase Transition of Underpotential Deposition Processes at Electrode–Electrolyte Interfaces

“…The working electrode (WE) used in the EC-STM study was a single-crystal Au(111) disc from Princeton Scientific (10 mm in diameter and 2 mm in thickness, miscut <0.4°). The electrode preparation details can be found from previous publications. , A solution of 1 mM CuSO 4 in 0.05 M H 2 SO 4 was used for the Cu UPD experiments with a copper wire (0.5 mm in diameter) in Cu 2+ (1 mM) as a quasi-reference electrode and a platinum wire (0.5 mm in diameter) as a counter electrode (CE), respectively. Electrochemical potentials for Cu UPD experiments reported in this work refer to the Cu 2+ /Cu quasi-reference electrode.…”

“…In situ STM techniques can image the potential-dependent surface adsorption processes and discern the copresence of the intermediate surface species with high spatial resolution. 17 E-QCM techniques provide complementary real-time information on the surface adsorbates and their composition and coverage during the dynamic adsorption and electrode reactions. 18 It provides high mass accuracy and sensitivity for a thin and rigid film.…”

“…, electrochemical STM (EC-STM) and electrochemical quartz crystal microbalance (E-QCM), that allow the observation of the potential-dependent dynamic phase transition of order vs disorder surface adsorption processes in the presence of parallel Faradaic processes at atomic dimensions and in real-time. In situ STM techniques can image the potential-dependent surface adsorption processes and discern the copresence of the intermediate surface species with high spatial resolution . E-QCM techniques provide complementary real-time information on the surface adsorbates and their composition and coverage during the dynamic adsorption and electrode reactions .…”