Formation of Trititanate Nanotubes by Non-Hydrothermal Methods - Optical Properties and Surface-Exciton Dynamics Studied by Photoluminescence Spectroscopy

Formation of Trititanate Nanotubes by Non-Hydrothermal Methods

  Trititanate (H₂Ti₃O？) nanotubes were prepared by a non-hydrothermal digestion of TiO(OH)₂ solution with 5 M NaOH, and their optical properties and exciton dynamics were studied by using steady-state and time-resolved photoluminescence (PL) spectroscopy. Based on TEM, AFM and XRD measurements, the tubular structure was identified to be the same as that of trititanate crystalline multi-walled scroll nanotubes prepared hydrothermally, which has intershell d-spacing of 0.79 ㎚ with inner diameters of 5 ㎚ and the lengths of several tens of nanometers. The UV-absorption spectrum of trititanate nanotubes showed weak surface-state absorption at 425 ㎚ as well as the absorption maximum band around 350 ㎚ with onset energy 3.03 eV corresponding to the indirect band gap energy. The steady-state PL spectra of the trititanate nanotubes showed a band-edge emission band around 360 ㎚ as well as a broad surface emission band originating from widely dispersed surface-state energy levels (2.84, 2.65, 2.41 2.21 2.03 eV), exhibiting higher relative intensity ratio of the band-edge emission to the surface emission as compared to that of TiO₂ nanoparticles. These results with PL excitation spectra indicate that the density of the surface states was found to be higher in the nanotubes than in TiO₂ nanoparticles. The PL decay profiles were measured by using the time correlated single photon counting system (TCSPC) adopting fs-Ti-Sappire laser at 350 ㎚, and they were analyzed to fit a tri-exponential equation. The decay times depend on monitoring emission wavelength, and at least four different decay time components (~28 ㎰, ~70 ㎰, ~700 ㎰ and ~4 ㎱) were resolved, implying the existence of charge-carriers trapping surface states with different energy levels. The two longer decay times are attributed to the deep-trap surface states of trititanate nanotubes, which are much longer than those of TiO₂ nanoparticles. These results suggest that the nonradiative route to the surface excitons recombination is more dominant in trititanate nanotubes than in titania nanoparticles because of strong coupling of excitons wave functions with lattice phonons.