Fluorinated TiO2 Hollow Photocatalysts for Photocatalytic Applications

Recently, extensive studies have been carried out to synthesize spherical microassemblies with hollow interiors and specific surface functionalizations, which usually exhibit fascinating enhanced or emerging properties and have promising applications in catalysis, photocatalysis, energy conversion and storage, biomedical applications, etc. With particular emphasis on the results obtained mainly by the authors' research group, this review provides a brief summary of the recent progress on the fabrication and potential photocatalytic applications of fluorinated TiO2 porous hollow microspheres (F-TiO2 PHMs). The synthesis strategies for F-TiO2 PHMs include a simplified two-step templating method and template-free method based on the fluoride-mediated self-transformation (FMST) mechanism. Compared to the two-step templating method, the template formation, coating, and removal steps for the FMST method are programmatically proceeded in black-box-like one-pot reactions without additional manual steps. The four underlying steps involved in the fabrication of F-TiO2 PHMs through the FMST pathway, nucleation, self-assembly, surface recrystallization, and self-transformation, are presented. By controlling these four steps in the FMST pathway, F-TiO2 PHMs can be successfully fabricated with a high yield by a simple one-pot hydrothermal treatment. The multi-level microstructural characteristics (including the interior cavity and hierarchical porosity) and compositions of hollow TiO2 microspheres as well as the primary building blocks can be well tailored. The unique superstructures of the F-TiO2 PHM photocatalysts provide advantages for photocatalytic applications by improving the light harvesting, mass transfer, and membrane antifouling. In addition, the in situ-introduced surface fluorine species during the formation of F-TiO2 PHMs provide significant surface fluorination effects, which are not only favorable for the adsorption and activation of reactant molecules, but also beneficial for surface trapping and interfacial transfer of photo-excited electrons and holes. Moreover, the porous hollow superstructures exhibit considerably better compatibility and tolerance to guest modifications, and thus the photocatalytic performances of F-TiO2 PHMs can be increased by synergetic host and guest modifications, such as ion doping, group functionalization, and nanoparticle loading. The light-harvesting range and intensity can be increased, the charge recombination can be reduced, mass transfer and adsorption can be promoted, and the surface reactivity can be tuned by introducing specific surface functionalities or nanoparticular cocatalysts. Consequently, the entire photocatalytic process can be systematically modulated to optimize the overall photocatalytic performance. The as-prepared F-TiO2 PHMs typically integrate the merits of interior cavity, hierarchical porosity, and surface fluorination and are open to synergetic host-guest modifications, which provides abundant compositional/structural parameters and specific physicochemical properties for systematically modulating the interconnected photocatalytic processes and promising potential photocatalytic applications.

Automated summary

Extensive studies have focused on synthesizing spherical microassemblies with specific surface functionalizations and hollow interiors for various applications. Focusing on the fabrication of fluorinated TiO2 porous hollow microspheres (F-TiO2 PHMs), this review highlights the simplified two-step templating method and template-free method based on the fluoride-mediated self-transformation (FMST) mechanism. The unique superstructures of F-TiO2 PHMs provide advantages for photocatalytic applications by improving light harvesting, mass transfer, and membrane antifouling, while in situ-introduced surface fluorine species enhance adsorption, activation of reactants, and surface trapping of photo-excited electrons and holes.