Recently, more sophisticated crystallographic measurements, using total X-ray diffraction and the atomic pair distribution function obtained from synchrotron radiation,28 have demonstrated that the three-dimensional structure of "TiO2-type'' nanotubular materials produced by the alkaline hydrothermal method can be interpreted as an arrangement of TiO6 octahedra in corrugated layers. The particular arrangement of octahedrons may depend on the morphology of the nanostructures and the encapsulation of water or sodium ions. The results agree with the local structure of nanotubes obtained using have revealed that the assembly of TiO6 octahedra is different from that in anatase, but some anatase-like structures may be presented in the nanotubes.29 It is likely that as-synthesised nanotubular materials correspond more closely to sodium titanates, than to anatase or TiO2-(B). This conclusion is also supported by (i) the frequent observation of a characteristic reflection at small angles in the XRD pattern;18 (ii) the low isoelectric point (ca. 3) and the negative value of zeta potential in aqueous solutions, due to the acid-base dissociation of titanates;30 (iii) the prefered adsorption of positively charged ions from aqueous solutions on the surface of nanotubes;31 (iv) the very pronounced ion-exchange properties of nanotubes, allowing almost stoichiometric amounts of alkaline ions to be exchanged;32 and (n) a dependence of the interlayer distance between the layers in the nanotube walls on the amount of alkaline ions contained within.33,34
The precise crystal structure of titanate nanotubes is still the subject of systematic investigations, including neutron diffraction studies. It is possible, however, to conclude that the structures have several common features. Firstly, it is a well defined, layered structure with a relatively large interlayer distance ca. 0.7-0.8 nm, resulting from the observation of a characteristic reflection (200) in the XRD patterns at small 20 values of ca. 10°. Secondly, an atom of hydrogen situated in these interlayer cavities can be exchanged with alkali metal ions in the aqueous suspension. Thirdly, the layers of the (100) plane consist of edge- and corner-sharing TiO6 octahedra, building up to zigzag structures. When protonated titanate nanotube samples are calcinated at moderate temperatures (less than 450 °C), the appearance of the TiO2-B phase is likely. The phase transformation of titanate nanotubes and their stability will be discussed in Chapter 4.
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