Mark A. Holland

Changes in the Zr environment in zirconia–silica xerogels with composition and heat treatment as revealed by Zr K-edge XANES and EXAFS

X-ray absorption spectroscopy at the Zr K-edge is an important technique for probing the environment of Zr. Here it is applied to zirconia–silica xerogels with composition 0.07⩽x⩽0.40, where x is the molar ratio Zr:(Zr+Si). Reference samples of crystalline ZrO2, ZrSiO4, BaZrO3 and liquid Zr n-propoxide were also examined. New XANES (X-ray adsorption near edge structure) results are presented for zirconia–silica xerogels, and compared with previous EXAFS (extended X-ray absorption fine structure) results. For high Zr contents (x=0.4) there is a separate, amorphous ZrO2 phase, which before heat treatment is similar to Zr hydroxide, and after heat treatment at 750°C is similar to an amorphous precursor of tetragonal ZrO2. For low Zr contents (x=0.1) there is atomic mixing of Zr in the SiO2 network, and the environment of Zr is more similar to that in Zr n-propoxide compared to other reference samples. New in situ XANES and EXAFS results are presented for x=0.1 xerogels heated at 250°C. These clearly show that the Zr environment depends on ambient moisture in addition to heat treatment.

The effects of different heat treatment and atmospheres on the NMR signal and structure of TiO2-ZrO2-SiO2 sol-gel materials.

The effects of different heat treatment schemes (i.e. successively or directly heated to particular temperatures) and atmospheres (air or nitrogen) on the solid-state NMR spectra obtained from (TiO(2))(0.15)(ZrO(2))(0.05)(SiO(2))(0.80) sol-gel materials are investigated. A combination of 1H, 13C, 17O and 29Si NMR is used. 29Si MAS NMR indicates that the extent of condensation of the silica-based network strongly depends on the maximum temperature the sample has experienced, but the condensation is largely independent of the details of the heat treatment scheme and atmosphere used. For sol-gel produced silicate-based materials the results show that the equilibrium structure at each temperature is reached rapidly compared to the time (2h) spent at that temperature. The 17O NMR results confirm that a nitrogen atmosphere does significantly reduce loss of 17O from the structure but care must be taken since there could be differential loss of 17O from the regions having different local structural characteristics.

Comparing the atomic structures of binary MO2-SiO2 (M = Ti, Zr or Hf) xerogels

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.

Structure of (Ta2O5)x(SiO2)1-x xerogels (x = 0.05, 0.11, 0.18, 0.25 and 1.0) from FTIR, 29Si and 17O MAS NMR and EXAFS

A combination of 29Si and 17O MAS NMR, EXAFS and FT-IR spectroscopy has been used to study the atomic structure of (Ta2O5)x(SiO2)1 − x (x = 0.05, 0.11, 0.18 and 0.25) xerogels prepared by reacting partially-hydrolysed tetraethyl orthosilicate with tantalum(V) ethoxide. Amorphous tantala, a-Ta2O5, xerogels have also been prepared and their structures studied in detail for the first time. Results have shown that in all these materials, Ta adopts predominantly 5-fold coordination with respect to oxygen. For the mixed oxide xerogels, partial phase separation of the two component oxides occurs for x > 0.11.

Characterisation of sol–gel prepared (HfO2)x(SiO2)1−x (x=0.1, 0.2 and 0.4) by 1H, 13C, 17O and 29Si MAS NMR, FTIR and TGA

The HfO2-SiO2 system is attracting interest as a possible new dielectric material in semiconductor devices. Knowledge of the location of hafnium within the silica network and the effect hafnium has on the structure will be central to the successful use of this material system in this application. Here, sol-gel techniques have been used to manufacture (HfO2)x(SiO2)1-x samples (x=0.1, 0.2 and 0.4, each heat treated at 250, 500 and 750 degrees C) and these have been characterised by magic angle spinning (MAS) NMR (1H, 13C, 17 O, 29Si), Fourier transform infrared spectroscopy (FTIR) and thermogravimetric analysis. 29Si MAS NMR showed that increasing the hafnia content decreases the connectivity of the silicate network, i.e. increases the range of differently connected SiO4 (Qn) units with more having increased numbers of non-bridging oxygens (i.e. lower n). FTIR and 17 O MAS NMR showed unequivocally that the x=0.4 sample phase-separated at higher temperatures, while in the x=0.1 sample the hafnium was homogeneously mixed into the SiO2 phase without any phase separation.

Transition metal atom sites in ternary ZrO2-TiO2-SiO2 xerogels

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.

Synchrotron-based studies of transition metal incorporation into silica-based sol-gel materials

Previous structural studies on titania- and zirconia-silica xerogels have shown the occurrence of homogeneous mixing at low metal content, and phase separation at high metal content. The use of additional, complementary, synchrotron-based methods can contribute to a fuller structural description of these materials. We present new x-ray absorption near edge structure (XANES) and SAXS results for (ZrO 2 ) x (SiO 2 ) 1-x , xerogels and compare them with previous results for (TiO 2 ) x (SiO 2 ) 1-x xerogels. Significant differences between (TiO 2 ) x (SiO 2 ) 1 . and (ZrO 2 ) X (SiO 2 ) 1-x xerogels are observed in the affects of heat treatment on the coordination of homogeneously mixed metal atoms, and in the development of phase separated metal oxide regions.