Electron Deep Trap States in Oriented TiO2 Nanotubes Arrays
Energy states in the band gap of nanostructured semiconductors (trap states) play an important part in the electron transport through such semiconductors. The presence of 'deep trap states', states far below the conduction band, especially has been identified as being detrimental to the efficiency of photovoltaic and photocatalytic devices using such semiconductors, since they slow electron transport and may act as recombination centres. Here, localised deep trap states located at around -0.2V vs. Ag/AgCl in anatase TiO2 nanotubes (NT) have been investigated in the dark and under supra band gap illumination using electrochemical methods. The ordered arrays of anatase NT of up to 23 μm in length were synthesised by the anodization of Ti foils. Cyclic voltammograms of such NT arrays were found to have the characteristic capacitative peak that is associated with deep trap states in nanocrystalline TiO2. Chopped frequency illumination responses confirmed that there are kinetic limitations for electrons escaping from these traps. In addition, using the charge extracted from photocurrent transient measurements it has been shown that the number of electrons in deep trap states under illumination is directly proportional to the NT length. The density of states of these monoenergetic deep traps was explicitly calculated using the zero temperature approximation of the Fermi Dirac function. Finally, the number of electrons trapped in a single NT under illumination has been calculated to be 7000 ± 1000 per micron of length. From this information we suggest that these deep trap states arise due to oxygen vacancies at grain boundaries.