Electron Transport, Trapping and Recombination in Anodic TiO 2 Nanotube Arrays

Anodically synthesized TiO₂ nanotube arrays (TNTAs) constitute an exciting ordered large bandgap semiconductor nanoarchitecture for use as scaffolds and active layers for solutionprocessable devices including but not limited to, optoelectronic sensors, photovoltaics, photodetectors, photocatalysts and photoelectrochemical cells. Charge transport, trapping and recombination are key attributes of the material architecture that significantly influence the properties and performance of the resulting optoelectronic devices, thus motivating this review article. Since nanocrystalline mesoporous TiO2 films (np-TiO₂) are actively researched for the same applications, in many cases, TNTAs and np-TiO₂ are direct competitors and it is therefore meaningful to compare the optoelectronic properties of the two architectures head-to-head. In addition, there exists a whole host of TNTA-specific applications such as bottom-up fabricated photonic crystals, bulk heterojunction organic solar cells and metallodielectric metamaterials that leverage the ordered channel architecture. Recent studies have established the order of magnitude superior recombination lifetimes in sensitized TNTAs as compared to sensitized np-TiO₂ as well as the salutary effect of lower structural disorder in TNTAs resulting in trap-free electron diffusion coefficients approaching those of single crystals and two orders of magnitude larger than np-TiO₂. Photoconductivity measurements using bandgap illumination in both single nanotubes and nanotube ensembles have resulted in similar values of the mobility-lifetime product (10^-5-10^-4 cm²V^-1), which are four to six orders of magnitude higher than in nanoparticle electrodes. At the same time, TiO₂ nanotubes have a larger trap density and a greater average trap-depth than nanoparticulate Ti₂ films, pointing to the importance of synthesis modification to improve material quality and post-synthesis techniques for trap passivation.