Encapsulation of Au nanoparticles with well-crystallized anatase TiO ₂ mesoporous hollow spheres for increased thermal stability

Uniform Au_encap/TiO₂ hollow microspheres, in which sub-10 nm Au nanoparticles are coated with a mesoporous anatase TiO₂ shell, are prepared by a protected-calcinating process. The method involves the preparation of SiO₂@Au@TiO₂ colloidal composites, sequential deposition of carbon and then SiO₂ layers through solvothermal and sol-gel processes, crystallization of TiO₂ by calcination and finally etching of the inner and outer silica layers to produce the hollow structure. The protected crystallization process suppresses the excessive growth of TiO₂ and eventually produces mesoporous anatase shells with high surface area. Additionally, by simply controlling the crystallization temperature, it can be convenient to tune the porosity and crystallinity of the TiO₂ shell, meanwhile maintaining the small size of Au nanoparticles. When used as the catalyst for the reduction of 4-nitrophenol, the synthesized Au_encap/TiO₂ catalysts exhibit significantly enhanced catalytic performance. Moreover, the obtained Au_encap/TiO₂ hollow spheres show a superior thermal stability, as it resists sintering during the additional calcination at 500 °C, whereas the sample prepared by deposited Au nanoparticles on commercial P25(Au/P25) was found to sinter severely.