Electrodeposition of nanostructured chromium conglomerates from Cr(III) dissolved in a deep eutectic solvent: Influence of forced convection

In this work, Cr electrodeposition onto a glassy carbon rotating disk electrode (GCRDE) was studied from Cr(III) ions dissolved in the choline chloride and ethylene glycol eutectic mixture (ethaline) under different GCRDE angular speeds, ω, at 343 K. The ethaline kinematic viscosity, ν = 0.17 cm²s^-1, was estimated, for the first time, from potentiodynamic plots under forced convection conditions, using the Levich equation. Cr electrodeposition was studied from the analysis of potentiostatic current density transients, j-t plots, recorded at different ω. This study was based on the model proposed by Hyde et al. (J. Electroanal. Chem., 534, 13 (2002)) for analysis of j-t plots due to 3D nucleation and diffusion-controlled growth under forced convection, j_dc-fcon(t), complemented by a previous adsorption step, j_ad(t), and the residual water reduction, j_WR(t), taking place on the growing surfaces of the Cr nuclei; thus, j_total(t) = j_ad(t) + j_dc-fcon(t) + j_WR(t). The kinetic parameters, namely: nucleation frequency and number density of active sites for chromium nucleation onto the GCRDE showed small dependence on ω; however, the term j_WR(t) clearly depends on the ω value. Scanning Electron Microscopy, Energy Dispersive Spectroscopy and X-ray Photoelectron Spectroscopy techniques were used to characterize the Cr electrodeposited on the GCRDE surfaces. It was observed that the electrodeposits were formed by quasi-spherical conglomerates (ca. 50 and 200 nm diameter for 0 and 1500 rpm, respectively) of nanoparticles (sized less than 30 nm diameter, in both cases). Moreover, these electrodeposits were formed by chromium and oxygen and its content increases with ω. These nanoparticles exhibit a core–shell structure, where the core was formed by metallic, zero valence chromium, and the shell of Cr(III) (as Cr(OH)₃ and Cr₂O₃), which was consistent with the theoretical mechanism used for the analysis of the j-t plots.