Formation of size-tunable dandelion-like hierarchical rutile titania nanospheres for dye-sensitized solar cells

A sol–gel method with a modified solvent comprising of three simple steps under low-temperature conditions was used to synthesize mono-disperse rutile TiO2 nanospheres with dandelion-like hierarchical morphology (DHRS) as a light-scattering layer for dye-sensitized solar cells (DSSC). 1,2-Ethanediol (ED) served as a key retardation agent in the second step at 25 °C to slow the hydrolysis and condensation of the TiCl4 aqueous solution prepared in the first step at 0 °C; the subsequent nucleation of a rutile type phase occurred homogeneously in the third step at 70 °C to generate the rutile TiO2 nanospheres with highly uniform sizes. The spherical sizes of DHRS were well controlled with the volume ratios of the ED-water solvent mixture in the second step, for which the DHRS of diameter 650–200 nm were produced from the solvent system containing 5–25% ED. A detailed mechanism is presented to rationalize the formation of nanospheres of uniform size in six steps: 1) hydration and hydrolysis, 2) hydrolysis and retardation, 3) hydrolysis and condensation, 4) homogeneous nucleation, 5) aggregation and growth of rod-like crystals and 6) DHRS formation. The DSSC device with a scattering layer made of DHRS (size ~300 nm) performed comparably to that with a scattering layer made of a commercially available TiO2 paste, making this nano-material a cost-effective alternative for future DSSC commercialization.