The Structure of TiO2−SiO2 Sol−Gel Glasses from Neutron Diffraction with Isotopic Substitution of Titanium and 17O and 49Ti Solid-State NMR with Isotopic Enrichment

Abstract

Neutron diffraction with 46Ti and 48Ti stable isotopes and isotope-enriched 17O and 49Ti MAS NMR have been used to characterize the structure of (TiO2)x(SiO2)1-x sol−gel glass as a function of composition (x = 0.08, 0.18, and 0.41) and calcination temperature (T = 250, 500, and 750 °C). The results reveal the first direct observation of two Ti−O distances in a homogeneous (TiO2)0.18(SiO2)0.82 sol−gel derived glass. In the sample heat treated at 250 °C, the Ti occupies a distorted octahedral environment similar to that found in the mineral ramsayite with four Ti−O bond lengths of around 1.89 Å and two close to 2.11 Å. After heating to 750 °C, two shorter bond distances are observed: a short distance at 1.81 Å due to tetrahedrally coordinated Ti and a longer distance of 1.94 Å due to a minority species of octahedrally coordinated Ti. The (TiO2)0.08(SiO2)0.92 sample exhibits similar behavior. After heating to 250 °C, two Ti−O distances are observed at 1.84 and 2.10 Å consistent with the presence of both tetrahedral and distorted octahedral titanium. Heating to higher temperature (500 or 750 °C) leads to the presence of only a single Ti−O distance at 1.82 Å consistent with all the titanium being substituted in tetrahedral sites within the silica network. 17O NMR on samples at 45 atom % isotopic enrichment is very sensitive to phase separation. The (TiO2)0.18(SiO2)0.82 sample exhibits only a very small amount of phase separation in the form of a weak but nevertheless definite Ti−O−Ti signal. More significant phase separation of TiO2 can be observed in the17O NMR spectrum from the (TiO2)0.41(SiO2)0.59 sample after heating at both 250 and 500 °C. 49Ti NMR spectra are quite broad in all samples but some trends in line width and position are discerned. The results presented here are consistent with, but greatly extend, previous XRD, 17O and 29Si MAS NMR, XANES, and EXAFS studies of these materials.