Preparation of silica and titanium-containing silica hollow spheres at supercritical CO2/H2O interface

We propose a route to fabricate hollow silica and titanium-containing hollow silica spheres at the interface of supercritical (SC) CO2/cetyltrimethyl-ammonium bromide (CTAB) aqueous solution. In this method, tetraethyl orthosilicate (TEOS) and tetrabutyl titanate (TBT) were used as the precursors, SC CO2 was used as both the solvent for the precursors and as the source of Bronsted acid which can catalyze the hydrolysis reactions. The materials prepared were characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), high resolution transmission electron microscopy (HRTEM), X-ray diffractometer (XRD), X-ray photoelectron spectra (XPS), IR and UV–vis techniques. It was demonstrated that the diameter and wall thickness of the hollow silica spheres could be changed within the range of nano-scale by varying pressure of CO2 and the concentration of CTAB, and the diameter and wall thickness decreased with increasing pressure and decreasing CTAB concentration. Increasing concentration of the precursors could increase the size of titanium-containing hollow silica from nano-scale to micron scale. It was also shown that Si atoms were substituted atomically by Ti atoms and there was no bulk TiO2 in the titanium-containing hollow silica spheres.