On the electrochemical formation of nickel nanoparticles onto glassy carbon from a deep eutectic solvent

Abstract

This work deals with the electrochemical synthesis and characterization of nickel nanoparticles, NiNPs, onto a glassy carbon electrode, GCE, from a deep eutectic solvent prepared by mixing choline chloride and urea in 1:2 M ratio at 60 °C. From the potentiodynamic study, up to 3 energetically different reduction stages can be identified during the Ni(II) reduction process. Two of them (R1 and R2) are mass-transfer controlled process. A crossover between the cathodic and anodic voltammetric scans, characteristic of nucleation processes was clearly observed. From this singular point, the equilibrium potential, Eeq ((-505 ± 5) mV vs. Ag QRE) of the system Ni(II)/Ni(0) was measured. From Tafel plots, the exchange current density of the nickel deposition (j0 = (1.7 ± 0.3) μAcm−2) and its corresponding energy transfer coefficient (α = 0.79 ± 0.2) were obtained. From analysis of potentiostatic current density transients, it was possible to deconvolve the different individual contributions to the total current, namely: i) multiple 3D nucleation and mass transfer controlled-growth of NiNPs and ii) residual water reduction on the growing surfaces of the NiNPs. Furthermore, from integration of the respective j-t plots of each of these contributions it was possible to note that the amount of charge of each process depends of the deposition time and the applied potential and can be controlled by a suitable selection of these experimental variables. Due to water presence, the reduction to hydrogen and hydroxyl ions, precipitation of Ni(OH)2 (s) on the surfaces of the NiNPs, is likely to occur. SEM and HRTE images, taken on the GCE surfaces after potentiostatic deposition of nickel on the R1 potential zone, show the formation of NiNPs all over the surface. Moreover, XPS analysis of the electrode surface indicates the presence of metallic nickel, Ni(0), and NiO/Ni(OH)2 compounds. However, XPS spectra recorded from the same surfaces after a sputtering process, revealed the presence of pure Ni(0), which confirms that the NiNPs obtained after electrodeposition are core-shell type with a core formed by metallic nickel and a thin shell of NiO/Ni(OH)2 (ad).