Electrodeposition of zero-valent selenium nanoparticles onto glassy carbon electrode from ethaline deep eutectic solvent

Zero-valent trigonal selenium nanoparticles (SeNPs) were potentiostatically formed at room temperature (298 K) on the surface of a glassy carbon electrode (GCE) from SeO₂ dissolved in ethaline, a deep eutectic solvent composed of choline chloride and ethylene glycol. The mechanisms and kinetics of SeNP electrochemical nucleation and growth on GCE were investigated using both potentiodynamic and potentiostatic techniques. From cyclic voltammetry, the equilibrium potential (Eeq) of the Se(IV)/Se(0) redox couple, the exchange current density (j0), and the charge transfer coefficient (α) for the faradaic reaction Se(IV)ethaline + 4e−GCE/Se(0) $$\rightleftarrows$$ Se(s) were determined to be Eeq = 96 mV vs. Ag QRE, j0 = (1.2 ± 0.1) µA·cm⁻², and α = 0.4 ± 0.1. Analysis of the experimental potentiostatic current density transients (j–t plots) recorded at various overpotentials revealed that the SeNP electrodeposition mechanism involves three consecutive processes: (i) Adsorption and double-layer charging, (ii) Multiple 3D nucleation and diffusion-controlled growth of SeNPs, and (iii) Residual water reduction on the growing SeNP surfaces (2H2ODES + 2e− (Se) $$\rightleftarrows$$ H2(g) + 2OH− (DES)). The applied theoretical model enabled deconvolution of the total j–t response into these individual contributions, allowing for extraction of key kinetic parameters, including the SeNP nucleation frequency (A), the number density of active nucleation sites (N0), and the diffusion coefficient of Se(IV) in ethaline, DSe(IV) (298 K) = (4.91 ± 0.04) x 10− 9 cm2s− 1. Furthermore, from the dependence of A on overpotential, beside j0 and α, additional thermodynamic parameters were determined: the surface energy (σ), the critical nucleus size (nc), and the Gibbs free energy for the formation of the critical nucleus (ΔG(nc)) across the range of applied overpotentials. SEM analysis revealed the formation of smooth, quasi-spherical SeNPs with an average diameter of (118 ± 26) nm, uniformly distributed across the GCE surface. High-resolution XPS spectra recorded in the Se 3d region confirmed the presence of selenium in its zero-valent state. The Raman spectrum displays characteristic signals at 236 and 141 cm⁻¹, indicating that the predominant allotropic form of selenium in the nanoparticles is trigonal Se. Additionally, the optical band gap of the electrodeposited film was estimated using UV–Vis spectroscopy and the Kubelka–Munk method. A band gap of 1.7 eV was obtained, which is consistent with previously reported values for trigonal Se nanoparticles.