Silica-directed growth of Anatase TiO2 hierarchical hollow microspheres

Recently, hierarchical hollow nanostructures (HHNs) have received a great attention for their unique or enhanced physicochemical properties1. Numerous strategies have been developed to synthesized TiO2 hollow nano/micro spheres, although obtaining HHNs with tunable properties remains a great scientific challenge. Here, we use ultrasonic spray pyrolysis (USP) for the generation of TiO2 hierarchical hollow spheres (HHSs). A suspension containing colloidal silica, H2O and a TiIV complex, was nebulized using a home-made ultrasound generator (1.65 MHz). The resulting mist was carried in a hot furnace (1000 °C) by a gas (air) stream. After exiting the hot zone, spherical particles a few hundred nanometers in size (microspheres) were collected in H2O-filled bubbler2. The microspheres were then isolated from this solution by centrifugation and etched with HF 10 wt.% solution for 75 min. The resulting hollow nanostructure was confirmed by TEM analysis. We control the morphology of the microspheres by varying the Ti precursor/SiO2 molar ratio. In addition, by using SiO2 nanoparticles with different size (12, 35-50, 70-100 nm), we obtained TiO2 HHSs characterized by meso- or macroporosity. The XRD patterns of TiO2 solid spheres (obtained without template, T_USP) and HHSs showed samples with dramatically different phase composition. T_USP was composed by 36% in anatase and 64% in rutile; otherwise, by adding increasing amounts of SiO2 to the precursor solution, we obtained samples composed by an increasing content of anatase (up to 100%). We propose that the SiO2 surface plays a key role in this template-directed process (figure) suggesting a possible nucleation mechanism. The photocatalytic activities of the USP microspheres have been evaluated using the NOx (gas phase) degradation as a probe reaction.