Exciton-like trap states limit electron mobility in TiO2 nanotubes

Nanoparticle films have become a promising low-cost, high-surface-area electrode material for solar cells and solar fuel production1,2. Compared to sintered nanoparticle films, oriented polycrystalline titania nanotubes offer the advantage of directed electron transport, and are expected to have higher electron mobility3,4,5,6,7. However, macroscopic measurements have revealed their electron mobility to be as low as that of nanoparticle films8,9. Here, we show, through time-resolved terahertz spectroscopy10, that low mobility in polycrystalline TiO2 nanotubes is not due to scattering from grain boundaries or disorder-induced localization as in other nanomaterials11,12, but instead results from a single sharp resonance arising from exciton-like trap states. If the number of these states can be lowered, this could lead to improved electron transport in titania nanotubes and significantly better solar cell performance.