Electrodeposition of Nanostructured Chromium Conglomerates from Cr(III) Dissolved in a Deep Eutectic Solvent: Influence of Forced Convection

Abstract

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, <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mi>ν</mml:mi> </mml:math> = 0.17 <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msup> <mml:mrow> <mml:mi mathvariant="normal">cm</mml:mi> </mml:mrow> <mml:mrow> <mml:mn>2</mml:mn> </mml:mrow> </mml:msup> <mml:msup> <mml:mrow> <mml:mi mathvariant="normal">s</mml:mi> </mml:mrow> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>1</mml:mn> </mml:mrow> </mml:msup> <mml:mo>,</mml:mo> </mml:math> 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 convec t ion, j dc-fcon ( t ), complemented by a previous adsorp t ion step, j ad ( t ), and the residual water reduction, j WR ( t ), taking place on the growing surfaces of the Cr nuclei; thus, j t otal (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 Cr 2 O 3 ), which was consistent with the theoretical mechanism used for the analysis of the j-t plots.

Topics & Concepts