Sol-Gel

Sol-gel processes are a method of producing dispersed materials via the growth of metal oxo-polymers in a solvent. The chemistry involved in sol-gel processes is based on the hydrolysis and condensation reactions of metallo-organic compounds such as metal alkoxides (M(OR)n) leading to the formation of metal oxo-polymers. The combination of these metal oxo-polymers can produce bushy structures that invade the whole volume, forming amorphous gels when these oxo-polymers reach macroscopic sizes, or disordered precipitates when reactions produce dense rather than bushy structures. Indeed, amorphous macroscopic networks are not the only possible outcome for such polymerisation reactions. When the polymerised structures do not reach macroscopic sizes, other final states can be obtained, including molecular metal-oxo-organo clusters and sols containing nanocrystalline metal oxide particles .

Titanium oxide based compounds can be obtained through the controlled hydrolysis of Ti(OR)₄ alkoxide, using complexing ligands or protons as inhibitors. The technological importance of titania based materials has been well known for a long time. Indeed, these materials exhibit numerous applications. They can be used as pigments, as powders for catalytic or photocatalytic applications, as colloids and thin films for photovoltaic, electrochromic, photochromic, electroluminescence devices and sensors, as components for antireflecting coatings, as porous membranes for ultrafiltration and even as refractory fibres.
Amongst these applications, many of the corresponding properties depend on the structure of the TiO2 phase (mainly anatase, brookite and rutile), and are driven by the nature and the extension of the surface and thus the size of the titania particles. Consequently, it is of primary importance to have effective characterisation tools that can accurately probe the chemical species constituting the bulk and the surface of these titania based nanomaterials.

Contrary to silica based materials for which ²⁹Si MAS NMR techniques are particularly useful and relevant sol-gel derived titanium oxide based materials cannot be routinely probed by NMR experiments performed on the titanium nuclei. Indeed, titanium isotopes carrying a nuclear spin (⁴⁷Ti^IV, ⁴⁹Ti^IV) possess a strong quadrupolar moment that up to now makes it difficult to monitor high-resolution solid state titanium NMR experiments. Another possibility, which has been more recently investigated, is to characterise transition metal oxo-polymers by probing oxygen atoms that directly observe the connectivity between different metallic atoms using NMR .
Oxygen is a key constituent of many important oxide based materials. Therefore, amongst the quadrupolar nucleus, the ¹⁷O (I = 5/2) has a paramount importance. In addition, oxygen NMR parameters are sensitive to molecular structure and chemical environment, suggesting that direct observation of the ¹⁷O spectra has potential to yield valuable and previously inaccessible information. Oxygen-17 chemical shifts are span over a range of about 1500 ppm and along with quadrupolar coupling parameters provide detailed information on oxygen bonding, solvation, crystallographic symmetry and molecular structure.
The main difficulty of ¹⁷O NMR are its poor sensitivity due to low natural abundance (0.037 % natural abundance) and its quadrupolar momentum (I = 5/2, Q = -2.63.10^-30 m²). However, the quadrupolar interaction broadens the central transition only to the second order and the low natural abundance can be overcome by hydrolysing the precursors with ¹⁷O-enriched water. The relative stability of the C-O bond compared to the Ti-O bond ensures that ¹⁷O is efficiently incorporated in the growing Ti-O-Ti oxide network. This strategy has been widely used in the study of the titanium oxo-polymers formed by sol-gel processing . Indeed, ¹⁷O NMR allows for the differentiation of oxygen sites as a function of the number of titanium atoms bonded to them. Using high resolution solid state ¹⁷O NMR, three titanium-oxo-organo clusters ([Ti₁₂O₁₆(OPrⁱ)₁₆], [Ti₁₆O₁₆(OEt)₃₂] and [Ti₁₈O₂₂(OBuⁿ)₂₆(Acac)₂]) as well as monodisperse nanoparticles of titania anatase having mean oxide network diameters of 20 Å and 30 Å have been fully characterised. The structurally well-defined clusters are used as references to classify the dominant magnetic interactions that contribute to the ¹⁷O NMR resonances in titanium oxide based materials.

For the first time, bulk and surface oxygenated species present in anatase particles are clearly identified and assigned through ¹⁷O NMR.

This study shows that in titanium oxo-based compounds, the µ₂-O sites are dominated by an important chemical shift anisotropy, an interaction that is much weaker for the other µ_n-O sites (n= 3,4,5). For all µ_n-O sites (n = 2,3,4 or 5), the ¹⁷O NMR linewidths are dominated by chemical shift distribution with a minor contribution from second order quadrupolar broadening. However, depending on the degree of distortion from tetrahedral geometry, the µ₃-O sites can also be sensitive to second order quadrupolar effects. Bulk µ₃-O and surface oxo species (µ₃-O, µ₂-O and Acac-Ti) present in titania anatase nanoparticles are identified and clearly assigned. The ratio between bulk and surface species decreases as the particle size is increased. Moreover the surface reconstruction of the nanoparticles in the presence of ¹⁷OH₂ enriched moist air, at room temperature, is demonstrated by ¹⁷O MAS NMR experiments. From these experiments the µ₃-O, µ₂-O, Ti-OH, Acac-Ti and H₂O-->Ti surface species were identified
The methodology presented in this work opens a land of opportunities for improving our knowledge of metal oxide surfaces in nanocomposites materials (for example inorganic charges for polymers, pigments, catalysts and photocatalysts, etc).

Full article
Journal of Materials Chemistry, 1999, 9, 2467-2474

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