Using time-resolved THz spectroscopy to study carrier dynamics in TiO₂ nanotubes

Oriented polycrystalline titania nanotubes could well be a cost effective alternative to TiO₂ nanoparticle films which are the basis of dye-sensitized solar cells for the production of either electricity or solar fuels. A technological hurdle when using sintered nanoparticle films as photoelectrodes is that these films have low electron mobilities. It had been hoped that nanotubes would overcome this because they can be many tens of microns long. However, recent macroscopic measurements found electron transport through nanotube and nanoparticle films to be comparable. Here we show that low electron mobility in polycrystalline TiO₂ nanotubes is not due to scattering from grain boundaries but instead due to traps that manifest themselves in a single sharp resonance in the THz spectrum. In addition, we have characterized the time-dependent conductivity after photoexcitation. These spectroscopic results also give an unprecedented window into the shallow electronic defect structure of anatase TiO₂. The TiO2 nanotube spectra are fundamentally and qualitatively different than that for nanoparticles or the bulk material.