In situ and dynamic monitoring of the self-assembling and redox processes of a ferrocenylundecanethiol monolayer by electrochemical quartz crystal microbalance

Abstract

The self-assembling process of ferrocenylundecanethiol on a gold surface has been measured in situ using the electrochemical quartz crystal microbalance (EQCM). Initial adsorption was fast and was followed by the slow adsorption whose rate was 2 orders of magnitude smaller. The number of adsorbed thiol molecules, estimated from the frequency change, suggests the formation of the monomolecular layer of the thiol. This number agrees well with that calculated from the electrochemical charge associated with oxidationtreduction of the monolayer in 1 M HClOd. The EQCM was also used to follow the mass change during the oxidationtreduction of the ferrocenylundecanethiol monolayer. It was confirmed that anions are incorporated into the self-assembled layer upon oxidation of the ferrocene group. IntroductionControlled modification of electrode surfaces with monolayer film of organic molecules has attracted much attention because of the relevance to the construction of molecular devices. The modified electrode is also usefui for the understanding of electron transfer mechanism at electrode/electrolyte interfaces at a molecular level. Oriented monolayer assembly can be created by both the Langmuir-Blodgett and the self-assembling methods using various alkane derivatives and substrates.' The selfassembled monolayers of alkane derivatives with sulfurcontaining head groups on gold substrates have been widely examined recently, since the binding between S atoms and Au surface is strong and the S-anchored monolayers thus formed are in well-oriented ~tructure.~-~ Structure and orientation of these monolayer assemblies have been examined by infrared reflectance spectroscopy? ellipsometry? X-ray photoelectron spectroscopy,lO etc. Although these techniques give useful information, these studies have been conducted only under ex situ conditions. To understand the structure of the assembled monolayer and electron-transfer mechanism, it is essential to conduct quantitative in situ monitoring of mass transport both at an assembling process and at an electron transfer process so that we can quantify the number of adsorbed molecules and the movement of ion and solvent accompanying an electron transfer process. A recently developed electrochemical quartz crystal microbalance (EQCM)ll is one of the best techniques to obtain this quantitative information. So far, the EQCM has been used in only a few works as far as self-assembled monolayers are concerned. Buttry and his co-workers have examined mass transport during the redox of self-assembled layers of viologen derivatives12

Electrochemical Oxidative Formation and Reductive Desorption of a Self-Assembled Monolayer of Decanethiol on a Au(111) Surface in KOH Ethanol Solution

“…Similar tendencies have been seen in SAM formation without potential control in a solution containing only alkanethiol. [10][11][12][13][14]21 Sweep Rate Dependence of Desorption Peak Potential. Figure 2 shows that the desorption potential shifted negatively as the sweep rate became faster.…”

“…5,6 The self-assembly of thiols on metals such as gold and silver is achieved by chemical bond formation between the substrate atoms and sulfur atoms of alkanethiols and hydrophobic interaction among the alkyl chains. 7 The formation process and the structure of alkanethiol SAMs on gold single crystal surfaces have been studied in detail by using various techniques, including IR spectroscopy, [8][9][10] quartz crystal microbalance (QCM), 11,12 scanning tunneling microscopy (STM), [13][14][15] atomic force microscopy (AFM), 16,17 X-ray diffraction, 18 X-ray photoemission spectroscopy (XPS), 19,20 second harmonic generation spectroscopy (SHG), 21 sum frequency generation spectroscopy (SFG), 22,23 and impedance spectroscopy. 24,25 It is now well-known that the alkanethiols are arranged in striped structures when the coverage is low and in a ( 3 × 3)R30°and/or c(4 × 2) structure with a tilt angle of ca.…”

Electrochemical oxidative adsorption and reductive desorption of a self-assembled monolayer of decanethiol on the Au(111) surface in KOH+ethanol solution

“…The self-assembly is achieved by a chemical bond formation between the substrate atoms and sulfur atoms of the thiols and the hydrophobic interaction between the alkyl chains. The formation process and the structure of the thiol SAMs on Au(111) have been studied in detail by various techniques including IR [4,5], quartz crystal balance (QCM) [6,7] and scanning tunneling microscopy (STM) [8][9][10]. It is now well known that the alkanethiols are arranged in striped structures when the coverage is low [8][9][10] and in a (√3 x √3)R30° or c(4√3 x 2√3) structure with a tilt angle of 30° from the surface normal when the coverage becomes high [11].…”

Construction of multilayers of bare and Pd modified gold nanoclusters and their electrocatalytic properties for oxygen reduction

“…The oxidation/reduction wave of the ferrocence group attached to the GNCs was observed at around 380 mV. The redox process of the surface attached ferrocene group can be expressed as [26,32,33],…”