Effect of Halide Impregnation on the Structure and Surface Characteristics of Rutile

Abstract

Titanium dioxide (TiO₂, rutile) subjected to fluoride (1 M and 2 M NH₄F) and iodide (1 M KI) treatment has been investigated by combining several approaches: structural and textural characterization as well as surface acidity and basicity. Fluorination reduces the crystallinity at both concentrations studied, the higher concentration being more effective. Iodination has the same effect.

Fluoride ions replace all basic OH groups whereas iodination does not lead to full replacement due to competition with the Ti⁴⁺ substrate. A distinct exotherm is observed at 228°C and 235°C, respectively, for the F⁻ and I⁻treated samples, resulting from energetic changes leading to the activated dissociation of water and restoration of the basic OH groups.

A surface compound is apparently formed on fluorination and iodination, and this decomposes endothermally at 270–275°C. In addition, impregnation with I⁻ appears to increase the photosensitivity of the TiO₂ leading to the formation of a carbonate with atmospheric CO₂ which generates an endotherm at 285°C and whose existence has been confirmed by IR spectral analysis.

Specific surface areas and pore structure analyses demonstrate differences not only upon increasing the F⁻ concentration but also between the F⁻ and I⁻ treated samples, where the addition of the latter markedly reduced the surface parameters. Two groups of pore sizes could be recognized with most samples. Increasing the F⁻ concentration increased the sintering temperature to 550°C, the value for the low F⁻ sample being 400°C.