Synthesis of hollow TiO2@SiO2 spheres via a recycling template method for solar heat protection coating

The development of a simple and environmentally friendly method to synthesize hollow TiO2 structures is urgent for realizing high NIR reflectance and low thermal conductivity. Hollow TiO2@SiO2 structures have been successfully synthesized by a recycling template method. The synthesis process involves four steps as follows: 1) preparation of SiO2 spheres, 2) synthesis of SiO2@TiO2 core-shell structure by coating mesoporous TiO2 shell on SiO2 and crystallization by calcination, 3) etching the SiO2@TiO2 to form the hollow TiO2 spheres, and 4) coating the hollow TiO2 spheres with a SiO2 protection layer to produce hollow TiO2@SiO2 structures. The SiO2 serves as a template to support and synthesize hollow TiO2 shell, and then after being etched, to form a Na2SiO3 solution as a coating agent to protect TiO2 shell from photocatalytic activity. The as-synthesized hollow TiO2@SiO2 structures not only have high NIR reflectance, but also have a high thermal barrier performance due to the hollow structure. Moreover, the NIR reflectance of the coating made from hollow TiO2@SiO2 reached 94.9%, and the temperature of the inner surface of the coating board is reduced about 16 °C, compared with the surface without coating. The weather resistance studies reveal that the hollow TiO2@SiO2 sample is very stable under the UV irradiation. In particular, the hollow TiO2@SiO2 structures synthesized by using this method shows multifunctional responses towards solar heat.