Electronic structure of TiO(2) nanotube arrays from X-ray absorption near edge structure studies

We report an X-ray absorption near edge structure (XANES) investigation of several TiO(2) 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 TiL(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 TiO(2) was also observed when Li ions were intercalated into annealed ATNTA albeit no noticeable reduction of Ti(4+) 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 TiO(2) 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.