Effect of water on Cu electrodeposition from ethaline based deep eutectic solvent

Ionic liquids have been utilized to fabricate metals, alloys and semiconductor materials by the electrochemical scientific community. ILs offer desirable characteristics that make them attractive for electroplating procedures. Examples of such properties are large electrochemical windows, reasonable conductivities, lowvolatility, and environmental compatibility. A novel class of ILs are Deep Eutectic Solvents (DESs) that show similar properties of ILs. In addition, DESs formulated from ammonium salts as choline chloride (ChCl) and hydrogen bond donors (HBD) are one of the most promising electrolytes since they are readily available at a reasonable cost. This work focuses on 'ethaline' which is a DES prepared by mixing choline chloride and ethylene glycol in a 1:2 molar ratio. The main reason for selecting ethaline among other DESs is its availability as bulk reagent, ease of preparation and handling. Numerous metals are produced by the industry of which copper is essential for the manufacturing electronic industry. Although DESs have been used to fabricate Cu deposits, earlier studies have concentrated on low water-containing DESs (< 0.5 wt% H2O). This is in line with previous experiences using ILs because the electrochemical windows of the solvents were reduced drastically (~ 2 V) with the incursion of water. Despite the academic contributions to understand Cu electrodeposition utilizing DESs, these electrolytes are hygroscopic and absorb water from the atmosphere unless special provisions are taken. Information in the literature about the influence of water in Cu electrodeposition from DESs is scarce. Hence, the objective of this study is to understand the influence of water on the morphology of Cu deposits obtained from watercontaining 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 was determined using Karl Fischer titration. The effect of increasing water content on copper coordination in the solvent 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 discusse