Low Temperature Synthesis of Nanocrystallized Titanium Oxides with Layered or Tridimensional Frameworks, from [Ti₈O₁₂(H₂O)₂₄]Cl₈·HCl·7H₂O Hydrolysis

A low-temperature aqueous chemical growth process was developed to produce nanometric titanium oxide with controlled size and structural variety. Gentle hydrolysis of a commercial TiOCl 2 solution, in a controlled relative humidity, leads to the formation of single crystals of [Ti 8 O 12 (H 2 O) 24 ]Cl 8 ·HCl·7H 2 O. Under autogenous pressure at 120 °C, the hydrolysis of the latter by tetramethylammonium hydroxide (noted TMAOH) is mainly governed by the R = Ti/TMAOH molar ratio and thus by the pH value. Two values are particularly important: R = 8/9 and 8/17. The former corresponds to the balance of the Cl − ions of the titanium oxychloride hydrate by TMA + cations and the latter to the stoichiometric ratio for the formation of a layered oxo-hydroxide (TMA) 2 Ti 2 O 4− x (CO 3 ) x (OH) 2 · n H 2 O, x = 0.7, which is obtained under basic conditions. Above R = 8/9, that is, in an acidic medium, the solvothermal treatment at 120 °C directly leads to the crystallization of anatase and then rutile when the pH decreases. For intermediate R values, the amorphous dried solid is required to be rinsed and dried at 110 °C to lead to the crystallization of nanometric titania, either anatase or brookite. TEM studies show that titania nanoparticles exhibit superstructures involving a doubling of the cell parameters that could originate from the adsorption of carbonate, hydrogen carbonate, or hydroxyl groups on the nanocrystallite surface. During evaporation of the solvent, these crystallites can self-assemble to form micrometric platelets.