Formation mechanism of H2Ti3O7 nanotubes by single-step reaction of crystalline TiO2 and NaOH has been investigated via transmission electron microscopy examinations of series specimens with different reaction times and extensive ab initio calculations. It was found that the growth mechanism includes several steps. Crystalline TiO2 reacts with NaOH, forming a highly disordered phase, which recrystallized into some H2Ti3O7 thin plates. H-deficiency on the top surface leads to an asymmetrical environment for the surface Ti3O2-7 layer. The calculations of the surface tension, elastic strain energy, interlayer coupling energy, and Coulomb force indicated that the asymmetrical environment is the principal driving force of the cleavage of the single sheets of H2Ti3O7 from the plates and the formation of the multiwall spiral nanotubes.

A simple one step hydrothermal reaction among TiO 2 nanoparticles and KOH solution was found to result in potassium titanate nanowires. The diameters of these nanowires are about 10 nm and the lengths range from 500 nm to 2 m. The nanowires were analyzed by a range of methods including powder x-ray diffraction ͑XRD͒, high resolution electron microscopy ͑HREM͒, selected area electron diffraction, electron energy loss spectroscopy, XRD and HREM image simulations. The structure of the nanowires is determined to be of the type of K 2 Ti 6 O 13 . Based on HREM observations of the growth process of the nanowires, we propose that the growth of the namowire was initiated by the formation of the K 2 Ti 6 O 13 nuclei inside the anatase matrix following the crystallographic relation (200) nanowire //(101) anatase . These nuclei subsequently grew to form one-dimensional nanowires via preferential growth along the ͓010͔ direction. Absorption experiments show that the potassium titanate nanowires are wide-band semiconductors with a band width E g ϳ3.45 eV.

A simple solid-phase method was developed for synthesis of high-quality boehmite nanorods by steam-assisted wet-gel conversion process. γ-Al2O3 nanorods with clear-cut edge have been successfully obtained by thermal treatment of boehmite nanorods at 600 °C for 5 h. The nanorods of boehmite and γ-Al2O3 were characterized by X-ray diffraction (XRD), Raman spectroscopy, and field emission scanning electron microscope (FESEM) with energy-dispersive spectroscopy (EDS), transmission electron microscopy (TEM), and high-resolution tranmission electron microscopy (HRTEM) with selected area electron diffraction (SAED) spectroscopy, X-ray photoelectron spectroscopy (XPS), Fourier transform infrared (FTIR), magic-angle spinning (MAS) 27Al nuclear magnetic resonance (NMR), and thermogravimetric and differential thermal analysis (TGA-DTA). The growth of boehmite was found to be greatly affected by the conditions used for precipitation of Al(NO3)3 with NH4OH aqueous solution. When precipitation was conducted under pH 5.0 or pH 7.0, nanorods of boehmite with lengths in the range of 100−400 nm and uniform diameters (20−30 nm) were obtained by steaming the solid wet gel at 200 °C for 48 h, whereas irregular boehmite particles were obtained from the wet gel precipitated under alkaline condition (pH 10.0). The directed crystallization of boehmite nanorods was attributed to the preferential growth along the longitudinal axis under supersaturated hydrothermal condition with the assistance of steam. The 1D nanostructure of boehmite was well preserved after being converted to γ-Al2O3 without sintering among the highly dispersed nanorods. This approach allows us to fabricate high-quality nanorods of boehmite and γ-Al2O3 with controlled size and morphology using cost-effective solid-phase chemical methods at a large scale.

No abstract

Potassium hexaniobate nanotubes have been synthesized at room temperature and characterized by high-resolution transmission electron microscopy (HRTEM) and energy-dispersive x-ray analysis. It is shown that HRTEM images may be used effectively to determine the helicity of the nanotubes, and, in particular, it is found that almost all nanotubes have their axes pointing within a few degrees from the [100] direction of the K4Nb6O17 structure.