Characterization of Hydrothermally Prepared Titanate Nanotube Powders by Ambient and In Situ Raman Spectroscopy

This letter reports on the successful synthesis of hydrogen titanate nanotubes (H-Ti-NT) and TiO2 (anatase) nanotubes and their thermal solid-state transformational chemistry and clarifies some of the confusion surrounding their literature Raman vibrational assignments. Hydrothermally prepared titanate nanotube powders with negligible (<0.1 wt % Na, H-Ti-NT) and high (similar to 7.0 wt % Na, Na/H-Ti-NT) Na content, that underwent freeze-drying and thermal treatments, were prepared and characterized with ambient and in situ Raman spectroscopy. The H-Ti-NT phase gives rise to Raman bands at similar to 195, 285, 458, similar to 700, 830, and 926 cm(-1). The Raman bands above 650 cm(-1) were found to be sensitive to the presence of moisture, which indicates that they are related to surface vibrational modes. The titanate nanotube Raman band at similar to 926 cm(-1) was shown riot be related to a Na-O-Ti vibration, which was previously assigned in the literature, since its intensity does not vary with Na content, which varied by a factor of > 70. The nanotubular H-Ti-NT phase was found to be thermally stabilized, <700 degrees C, by Na that had been entrapped during synthesis, The Na-free H-Ti-NT phase, however, transformed to TiO2 (anatase) nanotubes upon heating above 200 degrees C and was stable up to 700 degrees C.