Paper Title
Adsorption and Electropolymerization of 3,4-Ethylenedioxythiophene Molecules on Au(111) Surfaces in Neutral and Acid Solutions

Abstract
Electrochemical scanning tunneling microscopy (STM) and cyclic voltammetry (CV) are utilized to study the adsorption behavior of 3,4-ethylenedioxythiophene (EDOT) on Au(111) surfaces, as well as the electropolymerization of the EDOT monolayer into poly(ethylenedioxythiophene) (PEDOT). The CV results show that in both neutral phosphate buffer solution (PBS, pH = 6.8) and perchloric acid solution, the EDOT adsorption is a surface oxidative reaction which initiates at a potential of 0.0 V (vs. Ag/AgCl reference), and the corresponding desorption of the EDOT adlayer occurs at a more negative potential of 0.15 V. Furthermore, to perform the polymerization of EDOT molecules, a more positive potential of 0.45 V should be applied. The STM images reveal that in the PBS solution, a highly uniform EDOT monolayer prepared by gradually increasing the electrode potential to control a slow adsorption (Figure 1a). However, it is difficult to obtain a EDOT monolayer with uniform orientation in the acid solution, possibly ascribed to the strong interaction between the EDOT molecules and Au(111) surfaces. After the preparation and observation of the EDOT adlayer, cyclic potentials between 0.0 to 0.5 V are applied to perform the electropolymerization. In the PBS solution, the polymerization occurs mainly on the adsorbed EDOT molecules, and a PEDOT monolayer with well-ordered arrangements of polymer chains is obtained (Figure 1b). On the other hand, in the acid system, the EDOT molecules in the bulk solution involve in the polymerization reaction, resulting in a disordered PEDOT layers on Au(111) surfaces. Impedance spectroscopy analysis indicates that compared to the PEDOT layer prepared using the acid system, the PEDOT monolayer obtained using the PBS system displays a lower charge transfer resistance. Keywords - PEDOT, Monolayer, Scanning Tunneling Microscopy, Cyclic Voltammetry