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

Neutron diffraction with Ti-46 and Ti-48 stable isotopes and isotope-enriched O-17 and Ti-49 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 degreesC). 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 degreesC, 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 Angstrom and two close to 2.11 Angstrom. After heating to 750 degreesC, two shorter bond distances are observed: a short distance at 1.81 Angstrom due to tetrahedrally coordinated Ti and a longer distance of 1.94 Angstrom due to a minority species of octahedrally coordinated Ti. The (TiO2)(0.08)(SiO2)(0.92) sample exhibits similar behavior. After heating to 250 degreesC, two Ti-O distances are observed at 1.84 and 2.10 Angstrom consistent with the presence of both tetrahedral and distorted octahedral titanium. Heating to higher temperature (500 or 750 degreesC) leads to the presence of only a single Ti-O distance at 1.82 Angstrom consistent with all the titanium being substituted in tetrahedral sites within the silica network. O-17 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 the O-17 NMR spectrum from the (TiO2)(0.41)(SiO2)(0.59) sample after heating at both 250 and 5 00 degreesC. Ti-49 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, O-17 and Si-29 MAS NMR, XANES, and EXAFS studies of these materials

Atomic-scale structure of gel materials by solid-state NMR

The underlying principles of solid-state NMR spectroscopy are outlined with an emphasis on the physical origins of the interactions that affect NMR spectra so that an understanding of the structural information they convey is clearly understood. The fundamental components of the experimental approach are described. How the experimental data can be analyzed to provide structural characterization of sol-gel materials is illustrated through a series of examples from the literature. The short-range structural sensitivity of NMR means that it is an ideal probe of sol-gel materials since they are structurally disordered. Given the importance of silicates in sol-gel science, 29Si magic-angle spinning (MAS) NMR is a widely used nucleus in solid-state NMR studies of sol-gel materials. However, it is emphasized that to derive maximum benefit fromNMR characterization, a multinuclear approach is used, although each nucleus will have its own particular considerations which are presented. In this second edition, key advances in the experimental methodology (e.g., much higher applied magnetic fields, faster MAS rates, more sophisticated excitation approaches) since 2005 are outlined. The use of first-principles computational approaches to calculate NMR interaction parameters and hence better constrain structure provides an important additional dimension to the NMR approach. Materials where there has been a substantial expansion of sol-gel approaches since 2005 are included, with, for example, novel sol-gel schemes opening up preparation of phosphates where 31P MAS NMR is a sensitive structural probe. Another area where there has been substantial sol-gel activity since 2005 is in the preparation of bioactive calcium silicate-based materials, where multinuclear NMR is an ideal probe, including the use of 43Ca, a quadrupolar nucleus with a small magnetic moment, which has only really become readily accessible in recent years. © Springer International Publishing AG, part of Springer Nature 2018