Mixtures of functionalized aromatic groups generated from diazonium chemistry as templates towards bimetallic species supported on carbon electrode surfaces

Abstract

Mixtures of 4-sulfophenyl and 4-aminophenyl groups were grafted onto carbon electrodes by electrochemical reduction of their corresponding diazonium cations. Two experimental methodologies were tested in order to control primarily the composition of the binary organic films and subsequently the composition of the bimetallic Cu/Pt layers. The composition of the organic layers was controlled either by changing the ratio of the two components in solution and applying a cathodic potential at which both diazonium cations are electrochemically reduced. The organic layers were characterized by cyclic voltammetry, X-ray photoelectron spectroscopy and electrochemical impedance spectroscopy. These binary organic films were subsequently used as templates to load bimetallic species to the carbon surface based on electrostatic interactions of 4-sulfophenyl and 4-aminophenyl groups with Cu2+ and PtCl62− ionic species dissolved in solution, respectively. The metal complexes, electrostatically bounded to the ionic sites of the grafted groups, were reduced by using NaBH4 as reducing agent. The amount of Cu was estimated by stripping voltammetry in a sulfuric acid aqueous solution whereas adsorption/desorption of hydrogen was used to quantify the platinum present on the carbon surface. XPS analysis of the metallic surfaces was also performed to confirm the presence of the metals on the electrode surface. The results indicate that the composition of the bimetallic layers is controlled by the ratio of the 4-sulfophenyl and 4-aminophenyl grafted groups.