Tungsten(II) chloride hydrates with high solubility in chloroaluminate ionic liquids for the electrodeposition of Al–W alloy films

Abstract

• Tungsten(II) chloride hydrates showed high solubility in AlCl 3 -based ionic liquids. • Al–W alloy films with high W contents were obtained at high current densities. • Current efficiencies close to 100% were achieved even in the baths with hydrates. • Spectroscopic techniques were used to identify the chemical species in the bath. The electrodeposition of Al alloys using chloroaluminate ionic liquids (ILs), such as 1-ethyl-3-methylimidazolium chloride (EMIC)–AlCl 3 , has been of significant interest as a surface coating technology. To minimize the contamination of the ILs with water, which can inhibit Al deposition, anhydrous metal chlorides were generally used as metal precursors in the previous studies on Al alloy electrodeposition. However, many anhydrous transition metal chlorides have limited solubility in ILs, making it difficult to obtain Al-based alloy films with a high alloy content at a high current density. This article describes the electrodeposition of Al–W alloy films using W(II) chloride hydrates, W 6 Cl 12 ·2H 2 O and (H 3 O) 2 [W 6 Cl 14 ]·7H 2 O, as W precursors. These hydrated salts were found to have much higher solubilities in EMIC–AlCl 3 ILs than anhydrous W 6 Cl 12 . Therefore, owing to the high solubility of the hydrated salts, Al–W alloy films with a high W content could be electrodeposited even at high current densities. Furthermore, the use of hydrated salts had no negative effect on the quality of the electrodeposited films or the current efficiency. A chemical speciation of the Al and W ions in the ILs was carried out using Raman spectroscopy, Fourier-transform infrared spectroscopy, extended X-ray absorption fine structure spectroscopy, and UV-visible spectroscopy. The spectral data conclusively indicated that the dissolved species derived from the hydrated salts were different compared to those derived from the anhydrous salt. The high solubility of the hydrated W salts and the success of Al–W alloy electrodeposition encourage the use of hydrated salts in chloroaluminate ILs.