Controllable and Repeatable Synthesis of Thermally Stable Anatase Nanocrystal−Silica Composites with Highly Ordered Hexagonal Mesostructures

Abstract

In this article, we report a controllable and reproducible approach to prepare highly ordered 2-D hexagonal mesoporous crystalline TiO₂−SiO₂ nanocomposites with variable Ti/Si ratios (0 to ∞). XRD, TEM, and N₂ sorption techniques have been used to systematically investigate the pore wall structure, and thermal stability functioned with the synthetic conditions. The resultant materials are ultra highly stable (over 900 °C), have large uniform pore diameters (∼6.8 nm), and have high Brunauer−Emmett−Teller specific surface areas (∼290 m²/g). These mesostructured TiO₂−SiO₂ composites were obtained using titanium isopropoxide (TIPO) and tetraethyl orthosilicate (TEOS) as precursors and triblock copolymer P123 as a template based on the solvent evaporation-induced co-self-assembly process under a large amount of HCl. Our strategy was the synchronous assembly of titanate and silicate oligomers with triblock copolymer P123 by finely tuning the relative humidity of the surrounding atmosphere and evaporation temperature according to the Ti/Si ratio. We added a large amount of acidity to lower condensation and polymerization rates of TIPO and accelerate the rates for TEOS molecules. TEM and XRD measurements clearly show that the titania is made of highly crystalline anatase nanoparticles, which are uniformly embedded in the pore walls to form the “bricked-mortar” frameworks. The amorphous silica acts as a glue linking the TiO₂ nanocrystals and improves the thermal stability. As the silica contents increase, the thermal stability of the resulting mesoporous TiO₂−SiO₂ nanocomposites increases and the size of anatase nanocrystals decreases. Our results show that the unique composite frameworks make the mesostructures overwhelmingly stable; even with high Ti/Si ratios (≥80/20) the stability of the composites is higher than 900 °C. The mesoporous TiO₂−SiO₂ nanocomposites exhibit excellent photocatalytic activities (which are higher than that for commercial catalyst P25) for the degradation of rhodamine B in aqueous suspension. The excellent photocatalytic activities are ascribed to the bifunctional effect of highly crystallized anatase nanoparticles and high porosity.