On the electrodeposition of titanium in ionic liquids

Abstract

The ability to electrodeposit titanium at low temperatures would be an important breakthrough for making corrosion resistant layers on a variety of technically important materials. Ionic liquids have often been considered as suitable solvents for the electrodeposition of titanium. In the present paper we have extensively investigated whether titanium can be electrodeposited from its halides (TiCl₄, TiF₄, TiI₄) in different ionic liquids, namely1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)amide ([EMIm]Tf₂N), 1-butyl-1-methylpyrrolidinium bis(trifluoromethyl-sulfonyl)amide ([BMP]Tf₂N), and trihexyltetradecyl-phosphonium bis(trifluoromethylsulfonyl)amide ([P_14,6,6,6]Tf₂N). Cyclic voltammetry and EQCM measurements show that, instead of elemental Ti, only non-stoichiometric halides are formed, for example with average stoichiometries of TiCl_0.2, TiCl_0.5 and TiCl_1.1. In situSTM measurements show that—in the best case—an ultrathin layer of Ti or TiCl_x with thickness below 1 nm can be obtained. In addition, results from both electrochemical and chemical reduction experiments of TiCl₄ in a number of these ionic liquids support the formation of insoluble titanium cation–chloride complex species often involving the solvent. Solubility studies suggest that TiCl₃ and, particularly, TiCl₂ have very limited solubility in these Tf₂N based ionic liquids. Therefore it does not appear possible to reduce Ti⁴⁺ completely to the metal in the presence of chloride. Successful deposition processing for titanium in ionic liquids will require different maybe tailor-made titanium precursors that avoid these problems.