Copper Electrodeposition from a Water-Containing Choline Chloride-Based Deep Eutectic Solvent

Abstract

Ionic liquids are increasingly being used to electrodeposit metals, alloys and semiconductor materials. The popularity of these solvents arises from their desirable characteristics such as large electrochemical windows, reasonable conductivities, and good environmental compatibility. An important class of ionic liquids are deep eutectic solvents (DESs). DESs have been shown to be stable under atmospheric conditions, are non-flammable and have low vapour pressures. DESs formulated from choline chloride (ChCl) hold special promise since they are readily available at a reasonable cost. The focus of this study is on ‘ethaline’ prepared by mixing choline chloride with ethylene glycol in a 1:2 molar ratio. Although DESs have been used to electrodeposition, earlier studies have concentrated on systems with a low water content (< 0.5 wt%). This is in line with earlier studies using other ionic liquids, since water can reduce the electrochemical window. However, these DES systems are hygroscopic, and absorb water from the atmosphere unless special precautions are taken. Not only can water absorption have an adverse effect on the electrochemical window, it can also affect deposit composition and morphology. Conversely, other researchers have shown that water can protect against the anodic decomposition of the solvent itself and prevent the formation of chlorinated by-products. Information about the impact of water content on metal electrodeposition from DESs is, however, relatively scarce. The objective of this study is to understand the influence of water on the morphology of copper electrodeposited from water-containing DESs. We first examined the effect of adding various concentrations of water (3 – 15 wt%) on the stability of the solution; in particular this was obtained from the polarisation behaviour of metal deposition. It was noted that water addition affected both the limiting current for copper deposition and the deposition potential. A second series of experiments were carried out to assess how much water is absorbed by the DES depending on the storage conditions. The water content in these experiments were determined using Karl Fischer titration and NIR spectroscopy. The effect of increasing water content on copper speciation was explored using UV-Vis spectroscopy. Subsequently, Cu films were electroplated on a steel rotating disc substrate from electrolytes containing different weight percentages of water. The deposits were characterised using Scanning Electron Microscopy (SEM) and Energy Dispersive X-ray Spectroscopy (EDS). SEM images showed that the morphology of Cu deposits was also dependent on the water content of the DES. The relationship between the observed changes in electrochemical characteristics and deposit properties with increasing water content will be discussed.