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Morphological and Structural Changes in Titanate Nanotubes Induced by Thermal Annealing

During the last few years, titanate-based nanotubes have attracted a great deal of attention due to their great potential for application in a wide range of areas such as (photo)catalysis, solar energy conversion, hydrogen sensing, and energy storage. However, there is yet a considerable ambiguity regarding the exact structure and chemical composition of titanate nanotubes, and the mechanism of nanotube formation and growth. Currently it is accepted that the nanotubes have a layered titanate structure, with an interlayer spacing of about 0.8– 0.9 nm, and the interlayer space occupied by water and alkali metal cations that can be easily exchanged with H+ ions. In this work the influence of the Na+ content on the thermal stability of titanate nanotubes have been studied in detail. Samples of titanate nanotubes with various Na/Ti atomic ratios were prepared and annealed in the temperature range between 100 and 600 °C. Morphological and structural changes in the nanotubes induced by the thermal treatment were studied by transmission electron microscopy (TEM), selected area electron diffraction (SAED), Raman spectroscopy, and Fourier transform infrared (FTIR) spectroscopy. Titanate nanotubes in the Na-form were synthesized hydrothermally by treating the anatase TiO2 powder with NaOH solution. The samples with various Na/Ti ratios were prepared by ion-exchange of interlayered Na+ cations with H+ ions under controlled pH conditions. A typical low-magnification TEM image of synthesized nanotubes is shown in Fig 1a. At higher magnification, numerous bundle-like structures were observed (Fig 1b), consisting of individual nanotubes sticked together (Fig 1c). SAED pattern (insert in Fig 1a) indicates that nanotubes have layered trititanate (Na2Ti3O7/H2Ti3O7) structure [1]. In Raman spectrum of nanotubes (Fig 2) several broad bands appear in the crystal lattice vibration region. Broad absorption band in the range 2500 – 3650 cm-1 and the band at 1625 cm-1 in FTIR spectrum (Fig 3), as well as the diversity in bands intensities and shapes in the OH stretching region of Raman spectrum (insert in Fig 2) indicate presence of structurally bound OH groups and hydrogen bound water molecules in the nanotube structure. IR absorption bands attributed to water and OH groups decreased in intensity during the thermal treatment of all samples, indicating the loss of both interlayered water molecules and structurally bound OH groups. Based on the TEM, SAED, FTIR and Raman observations of the thermally annealed samples it was concluded that the replacement of the interlayered Na+ cations with H+ ions has a great influence on the thermal stability of titanate nanotubes. In the case of nanotubes with low Na/Ti ratio dehydration leads to phase change and loss of tubular structure at lower temperatures than in the case of samples with higher Na/Ti ratios.

nanotubes; titanates; TEM; SAED; Raman spectroscopy

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