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 (Cx 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.
Keywords: Rietveld refinement; ionic liquids; nanoparticles; titanium oxide; titanium oxyhydroxy fluoride.
© 2020 The Authors. Published by Wiley-VCH GmbH.