Synthesis of Ti(OH)OF ⋅ 0.66 H₂ O in Imidazolium-based Ionic Liquids

The influence of the cation of imidazolium-derived ionic liquids (ILs) on a low-temperature solution-based synthesis of hexagonal tungsten bronze (HTB) type Ti(OH)OF ⋅ 0.66 H₂ O and bronze-type TiO₂ (B) is investigated. The IL (C_x mim BF₄ ) acts as solvent and also as reaction partner with respect to the decomposition of [BF₄ ]^- , releasing F^- . In the present study, the chain length of the alkyl chain side groups attached to the imidazolium ring was varied (C₂ mim BF₄ to C₁₀ mim BF₄ ), and the obtained solids were analyzed by Powder X-Ray diffraction (PXRD) followed by Rietveld refinement. As a main finding these analyses indicate a transformation of Ti(OH)OF ⋅ 0.66 H₂ O into TiO₂ (B), and upon prolonged reaction time finally also into anatase TiO₂ . Rietveld analysis suggests that when using ILs with longer alkyl chains, the conversion of Ti(OH)OF ⋅ 0.66 H₂ O is slower compared to syntheses performed with smaller alkyl chains. Hence, Ti(OH)OF ⋅ 0.66 H₂ O appears to be metastable and is stabilized by long-chain ILs serving as surfactant attached to the crystallites' surface. In this view, the ILs shield the nanoparticles and thus slow down the conversion into the more stable compounds. This confirms previous findings that ILs act as both, solvent and reaction medium in this reaction, thus enabling the synthesis of peculiar Ti-oxides.