Growth of oriented single-crystalline rutile TiO ₂ nanorods on transparent conducting substrates for dye-sensitized solar cells

Dye-sensitized solar cells (DSSCs) made from oriented, one-dimensional emiconductor nanostructures such as nanorods, nanowires, and nanotubes are receiving attention because direct connection of the point of photogeneration with the collection electrode using such structures may improve the cell performance. Specifically, oriented single-crystalline TiO ₂ nanorods or nanowires on a transparent conductive substrate would be most desirable, but achieving these structures has been limited by the availability of synthetic techniques. In this study, a facile, hydrothermal method was developed for the first time to grow oriented, single-crystalline rutile TiO ₂ nanorod films on transparent conductive fluorine-doped tin oxide (FTO) substrates. The diameter, length, and density of the nanorods could be varied by changing the growth parameters, such as growth time, growth temperature, initial reactant concentration, acidity, and additives. The epitaxial relation between the FTO substrate and rutile TiO ₂ with a small lattice mismatch plays a key role in driving the nucleation and growth of the rutile TiO ₂ nanorods on FTO. With TiCl ₄-treatment, a light-to-electricity conversion efficiency of 3% could be achieved by using 4 μm-long TiO ₂ nanorod films as the photoanode in a DSSC.