Control of TiO2 Structures from Robust Hollow Microspheres to Highly Dispersible Nanoparticles in a Tetrabutylammonium Hydroxide Solution

A new process for controlling the structure of TiO2 from hollow microspheres to highly dispersible nanoparticles has been developed by altering the concentration of tetrabutylammonium hydroxide (TBAH) in the solvothermal reaction of titanium isopropoxide. Robust and size-controllable hollow TiO2 microspheres, constructed by the assembly of 18 nm TiO2 nanoparticles, were synthesized at relatively high TBAH concentration. The diameters of hollow spheres, with a shell thickness of ∼250 nm, were controlled to 1.5−4 μm by varying the concentration of TBAH in the range of 0.1−0.5 M. After calcination at 450 °C, the hollow microspheres were not appreciably deformed and were still floating on the surface of the water. However, highly dispersible TiO2 nanoparticles with an average diameter of 13 nm were obtained at a low TBAH concentration such as 9.2 mM. The colloidal particle size of TiO2 in an aqueous suspension at pH 2 was 12.5−13.5 nm, which indicates that the each nanoparticle is completely separated. The overall procedure is simple and highly reproducible, and large-scale synthesis is available at low cost.