Electronic structure of TiO₂ nanotube arrays from X-ray absorption near edge structure studies

We report an X-ray absorption near edge structure (XANES) investigation of several TiO₂ nanotube arrays, including the as-prepared nanotube arrays from electrochemical anodic oxidation of Ti foil (as-prepared ATNTA), as-prepared nanotube arrays after annealing at 580 °C (annealed ATNTA) and annealed ATNTA after electrochemical intercalation with Li (Li-intercalated ATNTA). XANES at the O K-edge and Ti L_3,2 and K edges shows distinctly different spectral features for the as-prepared and the annealed ATNTA, characteristic of amorphous and anatase structures, respectively. Intercalation of Li into annealed ATNTA induces a surprising, yet spectroscopically unmistakable, anatase to rutile transition. XANES at the Li K-edge clearly shows ionic features of Li in ATNTA. The charge relocation from Ti 3d to O 2p at the conduction band in TiO₂ was also observed when Li ions were intercalated into annealed ATNTA albeit no noticeable reduction of Ti⁴⁺ to Ti ³⁺ was observed. The O K-edge shows a distinctly enhanced feature in the multiple scattering regime, indicating a close to linear O-Li-O arrangement in Li-intercalated ATNTA. These results show bonding changes between Ti and O resulting from the interaction of Li ions in the TiO₂ lattices. Such bonding variation has also been supported by X-ray excited optical luminescence (XEOL), which suggests Li⁺-defect interactions. The implications of these results are discussed.