Comprehensive Study of the Electrodeposition of Nickel Nanostructures from Deep Eutectic Solvents: Self-Limiting Growth by Electrolysis of Residual Water

The electrodeposition of nickel nanostructures on glassy carbon was investigated in 1:2 choline chloride-urea (1:2 ChCl-U) deep eutectic solvent (DES). By combining electrochemical techniques with ex situ FE-SEM, XPS, HAADF-STEM, and EDX, the electrochemical processes occurring during nickel deposition were better understood. Special attention was given to the interaction between the solvent and the growing nickel nanoparticles. The application of sufficiently negative potential results in the electrocatlytic hydrolysis of residual water in the DES, which leads to the formation of a mixed layer of Ni/Ni(OH)2(ads). In addition, hydrogen bonds between hydroxide species and the DES components could be formed, quenching the growth of the nickel clusters favoring their aggregation. Due to these processes, a highly dense distribution of nickel nanostructures can be obtained within a wide potential range. Understanding the role of residual water and the interactions at the interface during metal electrodeposition from DESs is essential to produce supported nanostructures in a controllable way for a broad range of applications and technologies.