Electron Transport, Trapping and Recombination in Anodic TiO2 Nanotube Arrays
Benjamin D. Wiltshire,
Abdelrahman M. Askar,
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.
Keywords: Cyclic voltammetry, electrochemical impedance spectroscopy, IMPS/IMVS studies, open-circuit voltage decay,
photoconductivity, photoluminescence, resistivity, TiO2 nanotubes arrays.
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