A structural study of (TiO2)x(SiO2)1–x (x=0.18, 0.30 and 0.41) xerogels prepared using acetylacetone

Abstract

A combination of ²⁹ Si and ¹⁷ O MAS NMR, EXAFS and FTIR spectroscopy have been used to study the atomic structure of (TiO ₂ ) _x (SiO ₂ ) _1–x (x=0.18, 0.30 and 0.41) xerogels prepared using an acetylacetone stabilised Ti(OPr ⁱ ) ₄ precursor. In the as-prepared materials, Ti is present in a distorted octahedral coordination with a significant proportion still complexed by acetylacetone. For the first time in such an amorphous solid, a ¹⁷ O NMR resonance has been observed at 110 ppm which is attributed to (O ₅ )Ti-O-Si groups. Heat treatment of these xerogels tends to convert TiO ₆ into TiO ₄ where the Ti is substituted into the silica network. Our results are in accord with previous work which suggests that the upper limit for solubility of TiO ₂ in SiO ₂ is ca. 15 mol%. Clear evidence of some phase separation in the (TiO ₂ ) _0.41 (SiO ₂ ) _0.59 sample after heat treatment to 750 °C is presented, although the ¹⁷ O MAS NMR results show that the use of acetylacetone significantly increases the amount of Ti-O-Si bonding at this composition compared to samples prepared without it.