Facile strategy for controllable synthesis of stable mesoporous black TiO₂ hollow spheres with efficient solar-driven photocatalytic hydrogen evolution

Hydrogenated black TiO₂ has been proven to tune the bandgap and utilize solar energy effectively. Herein, we report a facile strategy for controllably synthesizing stable mesoporous black TiO₂ hollow spheres (MBTHSs) with a narrow bandgap via a template-free solvothermal approach combined with a small amine molecule reflux-encircling process and subsequent high-temperature hydrogenation, which are composed of highly crystalline pore-walls, Ti³⁺ in frameworks and surface disorders. The encircled protectors especially ethylenediamine result in high thermostability of the TiO₂ hollow structures, which not only facilitate hydrogenation (600 °C), but also inhibit grain growth and anatase-to-rutile phase transformation as well as retain a high structural integrity. The MBTHSs with a diameter of ∼700 nm possess a relatively high surface area of ∼80 m² g^-1, large pore size and pore volume of ∼12 nm and ∼0.20 cm³ g^-1, respectively. The diameters and wall thicknesses are controllable from ∼500 nm to 1 μm and ∼35 to 115 nm, respectively. The high crystallinity, integrated hollow structure, Ti³⁺ in frameworks and surface disorders of the MBTHSs give rise to an extending photoresponse from the ultraviolet to the visible light region and significant improvement in the solar-driven photocatalytic hydrogen evolution rate (241 μmol h^-1 0.1 g^-1), which is two times as high as that of black TiO₂ nanoparticles (118 μmol h^-1 0.1 g^-1) and almost three times that of pristine mesoporous TiO₂ hollow spheres (81 μmol h^-1 0.1 g^-1), respectively.