Mesoporous TiO2 Mesocrystals: RemarkableDefects-InducedCrystallite-Interface Reactivity and Their in Situ Conversion to SingleCrystals

摘要

Oriented self-assembly between inorganic nanocrystals and surfactants is emerging as a route for obtaining new mesocrystalline semiconductors. However, the actual synthesis of mesoporous semiconductor mesocrystals with abundant surface sites is extremely difficult, and the corresponding new physical and chemical properties arising from such an intrinsic porous mesocrystalline nature, which is of fundamental importance for designing high-efficiency nanostructured devices, have been rarely explored and poorly understood. Herein, we report a simple evaporation-driven oriented assembly method to grow unprecedented olive-shaped mesoporous TiO2 mesocrystals (FDU-19) self-organized by ultrathin flake-like anatase nanocrystals (∼8 nm in thickness). The mesoporous mesocrystals FDU-19 exhibit an ultrahigh surface area (∼189 m²/g), large internal pore volume (0.56 cm³/g), and abundant defects (oxygen vacancies or unsaturated Ti³⁺ sites), inducing remarkable crystallite-interface reactivity. It is found that the mesocrystals FDU-19 can be easily fused in situ into mesoporous anatase single crystals (SC-FDU-19) by annealing inair. More significantly, by annealing in a vacuum (∼4.0 ×10^–5 Pa), the mesocrystals experience an abruptthree-dimensional to two-dimensional structural transformation toform ultrathin anatase single-crystal nanosheets (NS-FDU-19, ∼8nm in thickness) dominated by nearly 90% exposed reactive (001) facets.The balance between attraction and electrostatic repulsion is proposedto determine the resulting geometry and dimensionality. Dye-sensitizedsolar cells based on FDU-19 and SC-FDU-19 samples show ultrahigh photoconversionefficiencies of up to 11.6% and 11.3%, respectively, which are largelyattributed to their intrinsic single-crystal nature as well as highporosity. This work gives new understanding of physical and chemicalproperties of mesoporous semiconductor mesocrystals and opens up anew pathway for designing various single-crystal semiconductors withdesired mesostructures for applications in catalysis, sensors, drugdelivery, optical devices, etc.