Density
of Deep Trap States in Oriented TiO<sub>2</sub> Nanotube Arrays
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Abstract
Correlations between the population
of deep trap states in an array
of TiO<sub>2</sub> nanotubes (NT) and the dynamic photocurrent responses
under supra-band-gap illumination are investigated. Ordered arrays
of TiO<sub>2</sub> NT of 10 μm length, 125 nm inner diameter,
and 12 nm wall thickness featuring strong anatase character were obtained
by anodization of Ti in ethylene glycol solution containing NH<sub>4</sub>F. Cyclic voltammograms at pH 10 show the characteristic responses
for nanostructured TiO<sub>2</sub> electrodes, in particular a sharp
cathodic peak as the electron density in the film increases. These
responses are associated with the population of deep trap states with
an average value of 5 × 10<sup>4</sup> electrons per NT. Dynamic
photocurrent measurements clearly show that the characteristic rise
time of the photocurrent increases as the potential is increased above
the onset region for charging deep trap states. At potentials in which
deep trap states are fully depopulated in the dark, photocurrent rise
time approaches values just below 1 s, which is more than 3 orders
of magnitude slower than the estimated <i>RC</i> time constant.
The occupancy of the deep trap states under photostationary conditions
is a fraction of the density of states estimated from voltammetric
responses. These findings are discussed in the context of current
views about trap states in high surface area TiO<sub>2</sub> electrodes