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

Electrochemical scanning tunneling microscopy (EC-STM) and electrochemical quartz crystal microbalance (E-QCM) techniques in combination with DFT calculations have been applied to reveal the static phase and the phase transition of copper underpotential deposition (UPD) on a gold electrode surface. EC-STM demonstrated, for the first time, the direct visualization of the disintegration of (√3 × âˆš3)R30° copper UPD adlayer with coadsorbed SO42- while changing sample potential (ES) toward the redox Pa2/Pc2 peaks, which are associated with the phase transition between the Cu UPD (√3 × âˆš3)R30° phase II and disordered randomly adsorbed phase III. DFT calculations show that SO42- binds via three oxygens to the bridge sites of the copper with sulfate being located directly above the copper vacancy in the (√3 × âˆš3)R30° adlayer, whereas the remaining oxygen of the sulfate points away from the surface. E-QCM measurement of the change of the electric charge due to Cu UPD Faradaic processes, the change of the interfacial mass due to the adsorption and desorption of Cu(II) and SO42-, and the formation and stripping of UPD copper on the gold surface provide complementary information that validates the EC-STM and DFT results. This work demonstrated the advantage of using complementary in situ experimental techniques (E-QCM and EC-STM) combined with simulations to obtain an accurate and complete picture of the dynamic interfacial adsorption and UPD processes at the electrode/electrolyte interface.

Molecular Imaging of Viologen Adlayers and In Situ Monitoring Structural Transformations at Electrode-Electrolyte Interfaces.

The effects of temperature and molecular concentration on the ordering of two-dimensional (2D) nanostructures have been investigated at the well-defined Au(111)-electrolyte interface. In comparison to the assembly of thiolated alkanes or hydrogen-bonded nonthiolated molecules, fabricating large aromatic thiolated molecules into a highly ordered adlayer on a surface remained a challenge. In this study, we demonstrated the importance of controlling the assembly conditions and procedures for the formation of ordered adlayers of redox-active viologen derivatives. The assembly conditions that were explored include the variation of molar concentration of assembly solutions, assembly time, and thermal annealing. We report that the optimal assembly conditions for creating highly ordered thiolated viologen derivatives on a Au(111)-(1 × 1) electrode surface are to limit the time in which the electrode is immersed in a deoxygenated 0.05 mM ethanolic viologen solution (preheated to 70 °C) to 45 s, followed by thermal annealing in absolute ethanol for 12 h. Highly ordered molecular adlayers were imaged by electrochemical scanning tunneling microscopy (STM), revealing the molecular packing of low-coverage adlayers. Furthermore, in situ STM combined with cyclic voltammetry (CV) allowed for the exploration of the structural transformation and potential limit of reductive and "oxidative" desorption of adlayers within the electrochemical potential range of the sample potential (ES) from -0.95 V to -0.10 V vs SCE.

Probing Molecular Nanostructures of Aromatic Terephthalic Acids Triggered by Intermolecular Hydrogen Bonds and Electrochemical Potential.

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.