Electron mobility and trapping in TiO₂ nanotubes revealed by time-resolved THz spectroscopy

In the last two decades nanoparticle films have emerged as a low cost high surface-area material forming the basis of promising dye-sensitized solar cells for the production of either electricity or solar fuels. A drawback of using sintered nanoparticle films as photoelectrodes is that these films have low electron mobilities. Oriented polycrystalline titania nanotubes could well be a cost effective alternative. 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. These spectroscopic results also give an unprecedented characterization of 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.