Mesocrystalline TiO₂ nanosheet arrays with exposed {001} facets: Synthesis via topotactic transformation and applications in dye-sensitized solar cells

A facile, fluorine-free approach for synthesizing vertically aligned arrays of mesocrystalline anatase TiO₂ nanosheets with highly exposed {001} facets was developed through topotactic transformation. Unique mesocrystalline {001}-faceted TiO₂ nanosheet arrays vertically aligned on conductive fluorine-doped tin oxide glass were realized through topotactic conversion from single-crystalline precursor nanosheet arrays based on lattice matching between the precursor and the anatase crystals. The morphology and microstructure of the {001}-faceted TiO₂ nanosheets could be readily modulated by changing the reactant concentration and annealing temperature. Owing to enhanced dye adsorption, reduced charge recombination, and enhanced light scattering arising from the exposed {001} facets, in addition to the advantageous features of low-dimensional structure arrays (e.g., fast electron transport and efficient charge collection), the obtained TiO₂ nanosheet arrays exhibited superior performance when they were used as anodes for dye-sensitized solar cells (DSSCs). Particularly, {001}-faceted TiO₂ nanosheet arrays ~15 μm long annealed at 500 ℃ showed a power conversion efficiency of 7.51%. Furthermore, a remarkable efficiency of 8.85% was achieved for a DSSC based on double-layered TiO₂ nanosheet arrays ~35 μm long, which were prepared by conversion from the precursor nanoarrays produced via secondary hydrothermal growth.