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 mesocrystaUine 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 doubleqayered TiO2 nanosheet arrays -35 μm long, which were prepared by conversion from the precursor nanoarrays produced via secondary hydrothermal growth.