Growth Processes and Electrochemical Behaviors of Aromatic Thiol Self-Assembled Monolayers on Au(111)

Abstract

Structures and electrochemical behaviors of self-assembled monolayers (SAMs) formed aromatic thiols on Au(111) were investigated by scanning tunneling microscopy and cyclic voltammetry measurements. It is found that electrochemical behaviors of aromatic thiol SAMs are strongly influenced by two-dimensional SAM structures, which can be controlled by introducing a methylene group between aromatic ring and thiol group as well as by changing the immersion time in a dilute thiol solution.

I. Introduction Fundamental aspects such as film formation, structures, and properties of alkanethiol self-assembled monolayers (SAM) on metals are relatively well-understand [1]. But recently, aromatic thiols have also moved into the focus of interest since they represent an interesting alternative to alkane-based thiol SAMs. Because of their higher rigidity and stronger intermolecular interactions compared with those of their aliphatic counterparts, aromatic thiols offer the chance of better control over the structure of SAMs. In this work, in order to understand a relationship between SAM structures and electrochemical properties for aromatic thiol SAMs, the growth processes and electrochemical behaviors of aromatic thiol SAMs on Au(111) were charterized by scanning tunneling microscope (STM) and cyclic voltammetry (CV).

II. Experiment p-Toluenethiol (TT) and 4-methylbenzyl mercaptan (MBM) were used in this study. Aromatic thiol SAMs were prepared by immersing the gold substrates in 0.5 M ethanol solutions of TT and MBM for various dipping time. STM imaging was performed using NanoScope E (Veeco, Santa Barbara, CA) with a Pt/Ir tip (80:20). All STM images were obtained in constant current mode in air at room temperature. Electrochemical experiments were carried out with the BAS-100, employing a three-electrode cell: the SAMs of aromatic thiols on gold electrode acted as the working electrode, the platinum wire as the counter electrode, and the Ag/AgCl wire as the reference electrode.

III. Results and Discussion STM images of Fig. 1 show growth processes of MBM SAMs on Au(111), which were prepared as a function of immersion time in 0.5 M ethanol solution. As the immersion time increases, the phase transitions of MBM SAMs occur from the gas phase to well-ordered phase. Contrary to alkanethiol SAMs, we could not find any intermediate phase between liquid-like phase and well-ordered phase. On the other hand, the total area of ordered phases was gradually increased as a function of immersion time. However, in the case of TT SAMs, we observed no significant phase transitions and appeared only as a disordered phase. From the CV results in Fig. 2, we found that the electrochemical behaviors of aromatic thiol SAMs have strong relation with two-dimensional SAM structures

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