Nanoporous TiO2 spheres with tailored textural properties: Controllable synthesis, formation mechanism, and photochemical applications

Nanoporous TiO2 spheres have emerged recently as a new class of TiO2 nanomaterials for photochemical applications. Compared with conventional TiO2 nanoparticles, the spherical assemblies consisting of low-dimensional nanocrystallites present significant advantages in terms of structural isotropy, monodisperse nature, structural diversity on nano- and (sub)micro-scales, structural stability, light harvesting property, interconnected nanobuilding blocks with less grain boundary, and easy reclaim. Superior performances have been demonstrated in the fields of photoelectrocatalysis and photovoltaics. Research efforts have been devoted to the rational design of a synthetic strategy for the facile preparation of nanoporous TiO2 spheres. The last decade has witnessed rapid progress in developing a synthesis strategy of nanoporous TiO2 spheres for optimal photochemical applications. Both chemical and physical routes have been extensively developed aiming to ease control over the self-assembly of nanobuilding blocks and, thus, the resultant textural properties and physicochemical performances of the nanoporous TiO2 spheres. In this review, a comprehensive description of different synthetic strategies is first presented, with a special emphasis on the formation mechanism, in particular, the pathway followed by nanocrystallites to self-assemble into a spherical structure. Notable experimental parameters are also discussed for the reproducible and controllable preparation of nanoporous TiO2 spheres with well-defined hierarchical structure, tunable porous microstructure from assembled nanobuilding blocks, and optimal physicochemical properties. Important applications in environmental photocatalysis, solar fuel synthesis, dye-sensitized solar cells (DSCs), and perovskite solar cells (PSCs) are summarized, and the synthesis-component-structure-property relationship in nanoporous TiO2 spheres is highlighted. Finally, perspectives in this rapidly developing field are offered.