Mesoporous anatase-TiO2 spheres consisting of nanosheets of exposed (001)-facets for [Co(byp)3]2+/3+ based dye-sensitized solar cells

To cope with the mass transport problem of a Co(bpy)(3)](2+/3+) electrolyte as well as to maintain a high dye loading, mono-dispersed microspheres of anatase TiO2 with well-defined (001)-facets (10%, 36%, 58%, and 84%), high surface area (106.5, 102.9, 97.6, and 88.3 m(2)/g), and large mesopores (26.6, 30.6, 33.7, and 38.5 nm) were synthesized by a facile hydrothermal route. We demonstrate that, owing to the large voids among microspheres, an extremely high porous TiO2 film is formed, thus offering a facile freeway for electrolyte diffusion. Besides, with the increase of (001)-facets, the TiO2 film shows an increase in its dye coverage (dye molecules per unit internal surface area). The coverage of porphyrin dye on a highly exposed (001)-facets TiO2 has been discussed. The mirror-like, (001)-facet planes of the film enabled a superior reflectance of >60% at the wavelength region of 400 to 800 nm. It is a crucial property for YD2-o-C8 dye to compensate its weak absorbance in the spectral range of 480 to 620 nm. A superior electron diffusibility through the 2D nanosheets of 3D microspheres was observed. Moreover, a negative shift in the flat-band energy was observed in the case of high content of (001)-facets; this has enabled a higher open-circuit voltage for the dye-sensitized solar cell (DSSC). By optimizing the condition of post-treatment and TiO2 film thickness, a cell efficiency of 11.43% under 1 sun illuminasion was obtained. In addition to their promising use in cobalt based DSSCs, we anticipate that TiO2 nanosheet assemblies will provide long-sought-after material solutions in photocatalysis, water splitting, and lithium ion battery. (C) 2016 Elsevier Ltd. All rights reserved.