High surface area ordered mesoporous nano-titania by a rapid surfactant-free approach

Abstract

Immersion of solid nanorods of the commercial metal–organic precursor [Ti(OCH₃)₄]₄ into boiling water results in their rapid topotactic transformation into rod-shaped colloid crystals built up of uniform crystalline anatase nanoparticles with average particle diameter 5.0 ± 1 nm forming rather regular wormhole-type mesoporosity with average pore diameter 4.1 nm and a record-high surface area and mesopore volume of 288 m² g⁻¹ and 0.42 cm³ g⁻¹ respectively. This structure emerges through reorganization of the intermediate ordered mesoporous lamellar structure. The distance between lamellae is about 3 nm. They are oriented parallel to the {1 0 0} crystallographic direction in the precursor crystals and originate most probably through contraction and densification of the layers of precursor molecules in the course of the process. Similar mechanisms are observed even for hydrolytic transformation of nanocrystals of other alkoxide precursors, derivatives of volatile alcohols, permitting to achieve surface area up to 350 m² g⁻¹. The obtained material is completely free from organics, but highly hygroscopic. Powders produced within 3 min show very broad X-ray diffraction peaks, indicating low volume coherence domains, but continued refluxing offers a strongly improved XPD pattern after 10 min and a fully crystalline material with coherence domain equivalent to the individual particle size after only 30 min. The latter material exhibits well defined Ti–Ti distances in the structure typical for a crystalline anatase phase according to EXAFS spectroscopy. It displays strong photochemical activity in destruction of methylene blue dye. When an aqueous dispersion of superparamagnetic Fe₃O₄ nanoparticles is applied for hydrolysis the product is a magnetic nanocomposite, easily and rapidly removable from solution by a magnet. The produced fully crystalline porous TiO₂ possesses attractive characteristics as an adsorbent for water remediation tested through adsorption of dichromate anions.