Ultrathin Exfoliated TiO2 Nanosheets Modified with ZrO2 for Dye-Sensitized Solar Cells

In the present work, we use chemical exfoliation to fabricate ultrathin two-dimensional anatase TiO 2 nanosheets (NSs) for application as photoanode materials in dye-sensitized solar cells. For the first time, colloidal Ti 0.91 O 2 NSs are synthesized via chemical exfoliation of a layered precursor (H x Ti 2– x /4 □ x /4 O 4 ·H 2 O; □: vacancy, x = 0.7) through ion exchange with tetrabutylammonium (TBA + ) cations. The as-prepared Ti 0.91 O 2 NSs are well-dispersed and ultrathin with a lateral size of up to a few micrometers. Subsequent acid treatment induces colloidal Ti 0.91 O 2 to reassemble and precipitate into a gelation form, followed by thermal annealing to convert the Ti 0.91 O 2 gelation into anatase TiO 2 nanosheets for applications as photoanode materials in DSSCs. Because of the enhanced light absorption and dye absorption resulting from the high surface area of ultrathin TiO 2 nanosheets, the DSSC consisting of 8.8 μm thick TiO 2 nanosheet film delivers the highest energy conversion efficiency of 4.76% and the largest short-circuit current of 9.30 mA cm –2, among DSSCs based on TiO 2 nanosheet films of various thicknesses. It is noted that an overly thick TiO 2 NS film will not further increase DSSC efficiency because the thicker layer results in a longer pathway for electron transport and more electron–hole recombination. Moreover, a ZrO 2 ALD coating combined with TiCl 4 treatment on TiO 2 NS film can effectively enhance the efficiency of DSSCs to 7.33% by significantly creating more surface area for more dye loading and preventing electron–hole recombination between TiO 2 and the dye/electrolyte, respectively.