Electrochemical characterization of Self-assembled monolayers (SAMs) of silanes on indium tin oxide (ITO) electrodes – Tuning electron transfer behaviour across electrode–electrolyte interface

In this work, we have systematically investigated the formation and characterization of Self-assembled Monolayer (SAM) films of several silanes on indium tin oxide (ITO) surfaces. Silane molecules having different domains namely substrate binding domain (siloxanes), electron transport region (aliphatic and aromatic spacer) and terminal functional groups (–SH, –CH3 groups) are employed for the study in order to tune the electron transfer (ET) behaviour across SAM modified electrode–electrolyte interface. Structural characterization of these monolayer films is carried out using X-ray photoelectron spectroscopy (XPS) studies. Wettability (hydrophilic and hydrophobic nature) of such modified electrodes is evaluated using contact angle measurements. ET behaviour of these modified electrodes is investigated by electrochemical techniques namely cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) using K4FeII(CN)6|K3FeIII(CN)6 redox couple as a probe. Disappearance of redox peaks in the CV measurements and formation of semicircle having a higher charge transfer resistance (Rct) values during EIS studies suggest that the resultant monolayer films are compact, highly ordered with very low defects and posses good blocking property with less pinholes. The heterogeneous ET rate constant (k) values are determined from EIS by fitting them to an appropriate equivalent circuit model. Based on our results, we comment on tuning the ET behaviour across the interface by a proper choice of spacer region