Potential Dependent Adsorption Geometry of 2,5-Dihydroxybenzoic Acid on a Au(111) Surface: An in Situ Electrochemical Scanning Tunneling Microscopy Study

Adsorption and electrochemical behaviors of 2,5-dihydroxybenzoic acid (DHB) at the electrochemical interface of Au(111) electrode and 0.1 M aqueous perchloric acid solution have been studied by cyclic voltammetry and in situ electrochemical scanning tunneling microscopy. Under modulation of the substrate potential, three distinct phases labeled as I, II, and III are observed. DHB molecules take the flat-lying configuration and form the herringbone structure in phase I, which exists at lower potential range. DHB molecules take upstanding configuration and form short-range ordered phase II at higher potential range. A transitional phase consisted of alternately arranged flat-lying and upstanding DHB molecules is disclosed at intermediate potential. The potential induced structural transition can be ascribed to the deprotonation process of carboxylic group and redox reaction of hydroquinone moiety. The present study provides important experiment evidence for understanding the adsorption, redox, and structural transition of hydroquinone molecules on solid electrodes.