Opto-electronic properties of anodized TiO2 nanotube arrays investigated using electron energy loss spectroscopy

A study of the nanoscale crystallinity of anodized TiO2 nanotubes is reported with the aim of demonstrating its influence on the localized optical and electronic properties of the structure. By employing scanning transmission electron microscopy, coupled with electron energy loss spectroscopy, x-ray diffraction and electron energy filtered jump-ratio imaging to probe changes in the electron near edge fine structure of the Ti L-3,L-2 ionization edge, it is found that nanotubes annealed at 450 degrees C in air for 3 h crystallize in the anatase polymorph along their walls, with the underlying thick oxide barrier layer being predominantly ruffle. An increase in annealing temperature to 600 degrees C results in the co-existence of anatase and ruffle along the nanotube length with the nanotube top and bottom showing crystallization in anatase and rutile, respectively. Valence EELS shows that the localized energy band-gap is inconsistent along the nanotube length, deviating between that typically measured for anatase (3.2 eV) and ruffle (3.0 eV). The obtained band-gap of 3.21 eV at the nanotube top and 3.44 eV at nanotube bottom is attributed to the co-existence of anatase and ruffle phases even at a low annealing temperature of 450 degrees C.