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A combination of extended X-ray absorption fine structure (EXAFS), X-ray absorption near-edge structure (XANES), Fourier transform infrared (FTIR), and 17O nuclear magnetic resonance (NMR) spectroscopies, small-angle X-ray scattering (SAXS), and X-ray diffraction (XRD) has been used to study the atomic structure of (TiO2)x(ZrO2)y(SiO2)1-x-y xerogels (0.05 ≤ x, y ≤ 0.15). The samples were prepared by reacting Ti butoxide and Zr n-propoxide with acetylacetone, to reduce reactivity of the metal alkoxides, and then reacting with partially hydrolyzed tetraethyl orthosilicate. EXAFS and XANES results imply that simultaneous insertion of Zr and Ti oxides into a silica host network causes the metals to adopt environments similar to those observed for the corresponding binary Zr oxide− and Ti oxide−silica systems. XANES, SAXS, and XRD indicate that the samples with up to 20 mol % total metal loading remain homogeneous with heat treatment up to 500 °C. 17O NMR indicates that both Ti- and Zr-rich samples with 20 mol...

The incorporation of transition-metal oxides into silica can give materials with useful optical, electronic or catalytic properties. For example, ZrO2-SiO2 and HfO2-SiO2 materials are of interest due to their high dielectric constants. Here we present a comparison of extended X-ray absorption fine structure and small-angle X-ray scattering results for acid-catalysed binary (MO2) x (SiO2)1 − x (M = Ti, Zr or Hf) xerogels, with x up to 0.4 and heat treatments up to 750°C. Detailed observations for TiO2-SiO2 and ZrO2-SiO2 xerogels provide a basis for interpretation of new results for HfO2-SiO2 xerogels. At low concentrations metal atoms are homogeneously incorporated into the silica network. Ti adopts coordinations of 4 or 6, and Zr and Hf both adopt higher coordination of 6 or 7 (the larger coordinations being due to ambient moisture). At higher concentrations, phase separation of metal oxide occurs. Such regions become clearly separated from the silica network for TiO2, but remain very finely mixed with silica network for ZrO2 and HfO2.

There has been much work on the binary TiO2-SiO2 and ZrO2-SiO2 materials prepared by sol-gel because of the beneficial properties resulting from incorporation of Ti and Zr. In contrast the ternary TiO2-ZrO2-SiO2 xerogels have been relatively little studied. We report the results of a study of those xerogels having Zr:Ti:Si ratios of 5:15:80, 10:10:80 and 15:5:80 heated to 750°C and to 1000°C. The study includes X-ray diffraction, small angle X-ray scattering, X-ray absorption spectroscopy at Ti and Zr K-edges, and 17O MAS-NMR. The study has benefited from close comparison with similar previous studies of the binary systems. The metal atoms in the ternary systems are shown to be predominantly homogeneously mixed in the silica network, as observed for the respective binary systems. The clear exception is for the sample with a minority of Zr, which after heat treatment at 750°C shows the presence of phase separation attributed to the formation of an amorphous precursor of ZrTiO4; at 1000°C this phase crystallises. In samples with higher Zr content the crystallisation of a ZrO2 tetragonal phase was observed. The data obtained illustrate well the strength of a research methodology in which a common batch of samples is studied using a coherent suite of modern structural probes.

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 A and two close to 2.11 A. After heating to 750 °C, two shorter bond distances are observed: a short distance at 1.81 A due to tetrahedrally coordinated Ti and a longer distance of 1.94 A 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 A consistent with the presence of both tet...

Amorphous (TiO2) x (SiO2)1−x (x = 0.08, 0.18, and 0.41) sol-gel formed materials have been characterised by a combination of X-ray and neutron diffraction, infra-red and 29Si and 17O magic angle spinning NMR spectroscopy. This combination allows new insight into the fundamental structural role titanium additions play in silica, entering the network at x = 0.08 but largely phase separating at x = 0.41. At an intermediate value of x = 0.18 there is complex coordination behaviour with initially some Ti—O—Ti linkages forming and both TiO4 and TiO6 coordinations present. It appears that at 500°C for the x = 0.18 sample all titanium is present in Ti—O—Si linkages but that this situation is unstable on further calcination. The new information presented here is amalgamated with that from our previous EXAFS, XANES and SAXS on the same samples to produce the most complete view to date of the local and mesoscopic structural behaviour of the (TiO2) x (SiO2)1−x system produced by the sol-gel method.