Mild yet phase-selective preparation of TiO2nanoparticles from ionic liquids – a critical study

Abstract

The phase selective synthesis of nanocrystalline TiO₂, titania, in ionic liquids (ILs) is explored. The influence not only of the IL but also of the Ti-precursor, pH, and temperature is investigated. Sonochemical synthesis, microwave synthesis and conventional heating are compared. In the case of Ti(OⁱPr)₄ (OⁱPr = isopropyl) as the Ti-source the ILs [C₄mim][Tf₂N] (1-butyl-3-methylimidazolium bis(trifluoromethanesulfonyl)amide), [C₃mimOH][Tf₂N] (1-(3-hydroxypropyl)-3-methylimidazolium bis(trifluoromethanesulfonyl)amide), [C₄Py][Tf₂N] (butylpyridinium bis(trifluoromethanesulfonyl)amide), [N₁₈₈₈][Tf₂N] (methyltrioctylammonium bis(trifluoromethanesulfonyl)amide), and [P₆₆₆₁₄][Tf₂N] (tetradecyltrihexyl phosphonium bis(trifluoromethanesulfonyl)amide) led at ambient temperature to TiO₂ in the form of anatase. The morphology of nano-anatase is controlled by the IL cation. Anatase nanospheres with a crystal size below 10 nm are obtained in [C₃mimOH][Tf₂N], [P₆₆₆₁₄][Tf₂N] and [C₄Py][Tf₂N], whilst nanorods with a length and diameter of ∼10 to 20 and 5 nm are formed in [N₁₈₈₈][Tf₂N] and spindle-shaped particles with an average length of 10–25 nm are formed in [C₄mim][Tf₂N]. Calcination at temperatures above 730 °C leads to rutile. When using TiCl₄ as the Ti-precursor an anatase–rutile mixture forms under ambient conditions. Pure rutile can be obtained under ambient conditions in the presence of an appropriate volume of aqueous HCl. At moderate to high pH values pure anatase can be obtained even from TiCl₄. The photocatalytic activity of the obtained TiO₂ materials has been assessed by the photodegradation of an aqueous methyl orange solution under UV light. The results indicate that the photocatalytic activity of anatase–brookite mixtures obtained in [C₄mim][Tf₂N], [N₁₈₈₈][Tf₂N] and [P₆₆₆₁₄][Tf₂N] is higher than that of pure anatase which is formed in [C₃mimOH][Tf₂N] and [C₄Py][Tf₂N] and competitive with commercially available catalysts.