Sol-Gel

Redispersible TiO2 anatase nanoparticles have been obtained by hydrolysis of Ti alkoxides complexed with acetyl acetone in acid medium. The "xerosols" are readily redipersible in both organic and aqueous solvents with no signs of aggregation. Surface and bulk characterization was carefully conducted using various experimental techniques (QELS, SAXS, XRD, NMR 1H, ¹³C, ¹⁷O, IR-Raman spectroscopy, UV-Visible absorption and Chemical analysis as well as thermal behaviour (DTA,TGA)).
For the first time, titanium oxo based nano-objects have been probed using ¹⁷O MAS and 3Q-MAS NMR. Three Titanium oxo-organo clusters ([Ti₁₂O₁₆(OPrⁱ)₁₆], [Ti₁₆O₁₆(OEt)₃₂] and [Ti₁₈O₂₂(OBuⁿ)₂₆(Acac)₂]), and monodisperse nanoparticles of titania anatase having 20 Å and 30 Å in oxide core diameter, have been characterised by ¹⁷O NMR.

By varying the ratio of Acac/Ti(OR)4 the bulk nanoparticles size can be selectively fixed between 1.5 and 4 nm.

Their stability is tailored by a double surface protection. A first complex layer formed by the acetyl acetone ligands linked to surface Ti atoms and a diffused solvation layer containing acetyl acetone molecules , water and PTS (Par-toluene sulfonic acid) (as shown below)

Schematic representation of ¹⁷O species, identified through ¹⁷O NMR, present in the bulk and at the surface of the titania nanoparticles. Ti atoms are represented by small black spheres and O atoms by large red spheres. The presence of p- toluenesulfonate molecules close to the surface is in agreement with ¹³C NMR data reported in the literature.

Courtesy by E. Scolan

Precursor chemistry and in depth knowledge of reactions mechanism allows the Sanchez's lab and more specifically the Jean Pierre Jolivet's team to obtain TiO2 nanoparticles in all crystalline polymorphic forms; Anatase, Rutile and Brookite

EXAMPLE OF BROOKITE SYNTHESIS (Courtesy by Jolivet's research group)

The starting molecular precursor is Ti(OH)2Cl2(OH2)2. The coexistence of Hydroxo and aqueous ligands in the coordination sphere of Titanium allows the condensation of the complex through olation reactions. Cl^- ligands more difficult to remove than the aquo ligands are present in the oligomers. Those after the complete removal of the water molecules the Cl^- ligands are eliminated by condensation which proceeds by oxolation reaction and simultaneously removal of HCl and the formation of m ₃- oxo bridges between octahedra. The sharing corners between these blocks allows the elimination of the entire Cl^- content present initially at the precursor molecule and leads to the construction of the Brookite structure.

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