Electrochemical preparation and characterization of cadmium deposits on carbonaceous substrates for nitrate reduction reaction

Abstract

The electrodeposition of Cd on two different carbonaceous substrates, highly ordered pyrolytic graphite (HOPG) and glassy carbon (GC), was studied in a mildly acidic sulfate solution, using electrochemical techniques and scanning electron microscopy (SEM). Cyclic voltammetric results showed that the metal reduction process occurs in a single stage involving two-electron transfer. Potentiostatic current transients performed at different potentials, exhibited typical features of a three-dimensional nucleation process and diffusion-controlled growth. Similar values for the diffusion coefficient for Cd+2 ions were derived from the Cottrell and the Randles-Sevcik equations, which are in agreement with those reported in the literature. Using the theoretical model of Sharifker and Hills, a progressive nucleation and growth mechanism of Cd crystals was determined on both substrates. Cd deposits were obtained on carbon supports by applying appropriated potentiostatic pulses. SEM images showed a more homogeneous distribution of Cd crystals deposited on the HOPG substrate surface. A higher number of particles was found for the systems at more negative potential values, which was attributed to greater activation of growth nuclei. The analysis of the particle size distribution indicated a predominance of larger sizes for the systems supported on GC and, conversely, a greater number of particles smaller than 100 nm were found for the HOPG. The predominant morphologies were hexagonal structures with hollow centers. Preliminary linear sweep voltammetry analysis in nitrate solutions indicated the presence of two well defined nitrate reduction peaks for the Cd particles/HOPG modified electrode, which can be used for nitrate detection.