Electrochemical study and physicochemical characterization of iron nanoparticles electrodeposited onto HOPG from Fe(III) ions dissolved in the choline chloride-urea deep eutectic solvent

Iron nanoparticles, FeNPs, were electrodeposited onto the HOPG electrode surface from Fe(III) ions dissolved in the choline chloride-urea eutectic mixture using potentiostatic current density transients. The morphology of the FeNPs, supported onto HOPG, was characterized by means of AFM and SEM. From these techniques it was found that most of the FeNPs were formed by nanostructured hemispheric particles, monodisperse in size (displaying diameters of (60 ± 8) nm with 30 nm height), that were homogeneously distributed on the HOPG surface. Furthermore, from EDX and XPS it was determined that the iron electrodeposit was constituted by core-shell type particles with zero-valence iron as the core and a shell composed by a mixture of FeO, Fe₂O₃, and Fe(OH)₃. From analysis of experimental current density transients, it was found that the electrodeposition mechanism of FeNPs involves multiple 3D nucleation with diffusion controlled growth and that residual water reduction occurs on the growing surface of the FeNPs as the applied potential becomes more negative. These models involve contributions to the overall current due to: an adsorption process, iron 3D nucleation with diffusion-controlled growth and residual water reaction over the growing surfaces of the Fe nuclei. The proposed models help determining the charge percentage due to each individual contribution to the total process.