Adsorption, structure, and phase transition of organic molecules at electrode-electrolyte interfaces without and with copper UPD

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Chen, Kuo-Hao
Li, Zhihai
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Scanning probe microscopy (SPM) is a powerful technology in nanoscience and nanotechnology. As a type of SPM technique, Scanning Tunneling Microscopy (STM) not only can image surface and molecules with anultrahigh resolution, but also can be employed in various environments. In our research, Electrochemical Scanning Tunneling Microscopy (EC-STM) was used to study molecular adsorption, metal deposition, and structural phase transition at electrode electrolyte interfaces. The adsorption and self-assembly of organic molecules at electrode electrolyte interfaces is related to many chemical reactions and processes such as electron transfer, organic phase transition, and heterogeneous catalysis. Studying the interactions between adsorbates (molecules) and metal substrates can provide valuable information to molecular devices, sensors, and the design of catalysts and liquid crystals. In this work, we have investigated molecular adsorption of aromatic trimesic acid (TMA), terephthalic acid (TPA), and benzoic acid (BZA) on a Au (111) electrode in the present of 0.1M HClO4 and/or 0.05M H2SO4 electrolyte using cyclic voltammetry (CV) and EC-STM techniques. Our experiments reveal that TPA with two carboxyl groups at the para position of phenyl ring forms H-bonded flat-orientated linear structures and TMA with 3 carboxyl groups at the 1,3,5-position of phenyl ring forms H-bonded flat-orientated hexagonal structures at negative potentials. These experimental results indicate that intermolecular interactions (H-bonds) play a dominant role compared with molecule-substrate interactions. Further, we found that BZA, which has only one carboxyl group, cannot form long range ordered H-bonded adlayers at low concentration (below 6 mM). We discovered how the concentration of molecules affect the formation of adlayer structures and CV current response, and we detected the transition concentration we called the Critical Phase-Transition Concentration (CPC). In addition, we explored the “diffusion-controlled” assembly of alkanedithiols on Au(111) in 0.05M H2SO4 electrolyte. This is a novel method that has never been employed before. We observed the self-assembled monolayer (SAM) of 1,8-octanedithiols (ODT) where ODT molecules form flat-oriented ordered adlayer. We did not successfully monitored the process of the formation of SAM yet and one has to find the optimal experimental conditions and parameters to discover the molecular level details of the SAM formation to reveal the assembly mechanisms. Finally, copper underpotential deposition (Cu UPD) has been investigated by EC-STM in combination with electrochemical CV. The CV of Au(111) in 1 mM Cu2+ and 0.05 M H2SO4 shows that there are two set of current peaks where potential regions were separated into Phase I, Phase II, and Phase III, respectively. Cu UPD (√3 × √3) R30° structure was discovered in Phase II using EC-STM. The high-resolution EC-STM images allow the determination if the surface coverage of co-adsorbed SO4 2- r. Cu UPD (√3 × √3) R30° adlayer was further used as a template to detect molecular adsorption of the trace amount of ODT from air to explore the formation mechanism of SAM at Au(111)/electrolyte interfaces.