Density of deep trap states in oriented TiO₂ nanotube arrays

Correlations between the population of deep trap states in an array of TiO₂ nanotubes (NT) and the dynamic photocurrent responses under supra-band-gap illumination are investigated. Ordered arrays of TiO₂ 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 NH₄F. Cyclic voltammograms at pH 10 show the characteristic responses for nanostructured TiO₂ 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 × 10⁴ 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 RC 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 TiO₂ electrodes.