Alkaline Hydrothermal Synthesis of Titanate Nanotubes and Nanofibres

In 1998, Kasuga et al?2 first reported a simple method for the preparation of TiO2 nanotubes without the use of sacrificial templates, involving the treatment of amorphous TiO2 with a concentrated solution of NaOH (10moldm~3) in a PTFA-lined batch reactor at elevated temperatures. In a typical process, several grams of TiO2 raw material can be converted (<100%) to nanotubes at temperatures in the range of 110-150 °C, followed by washing with water and HCl (0.1 moldm~3). It has since been demonstrated that all polymorphs of TiO2 (anatase, rutile,23,24 brookite25 or amorphous forms) can be transformed to nanotubular or nanofibrous TiO2 under alkaline hydrothermal conditions.

Figure 2.4 shows typical TEM images of titanate nanotubes and nanofibres obtained by such treatments. Generally, the nanotubes have a multi-wall morphology (with typically four walls) and a distance between successive layers of approximately 0.72 nm (in the protonated form). The inner diameter of nanotubes is in range from 2 to 20 nm and the majority of tubes are open at both ends. Each tube tends to have a constant diameter along its length. The distribution of internal diameters in the sample of nanotubes is relatively wide

Figure 2.4 TEM images of a) and c) titanate nanotubes, b) and d) nanofibres and e) multilayer nanosheets, produced by the alkaline hydrothermal treatment of anatase with NaOH (lOmoldm"3) at 140 0C (for nanotubes and nanosheets) and at 190 OC (for nanofibres). A SEM image of f) an agglomerate of titanate nanotubes. (Images for a), c), d), e) and f) are reproduced with kind permission from ref. 29).

Figure 2.4 TEM images of a) and c) titanate nanotubes, b) and d) nanofibres and e) multilayer nanosheets, produced by the alkaline hydrothermal treatment of anatase with NaOH (lOmoldm"3) at 140 0C (for nanotubes and nanosheets) and at 190 OC (for nanofibres). A SEM image of f) an agglomerate of titanate nanotubes. (Images for a), c), d), e) and f) are reproduced with kind permission from ref. 29).

when compared with those of the nanotubes produced using templating methods. Figure 2.4a shows the radial cross-section of a selected nanotube (left hand side) having a pronounced seam, and the axial cross-section of another nanotube (right hand side) having an asymmetrical number of walls. The nanotubes can be long - up to several microns, resulting in aspect ratios of several order of magnitude. Nanotubes are randomly assembled into agglomerate particles as seen in Figure 2.4f. Their size and shape depends on the synthetic conditions. The typical size of agglomerates is several microns. Nanotubes can be isolated from the bundles into an aqueous colloidal solution using surfactants and mild physical treatment (e.g., by stirring or ultrasound).

Nanofibres, produced by hydrothermal treatment of TiO2 with NaOH (10moldm~3) at temperatures higher than 170 °C, are characterised by a solid elongated morphology with a typical width being in the range of 20 to 200 nm and a length exceeding several microns, as shown in Figures 2.4b and 2.4d. These samples of nanofibres are also characterised by a wide distribution of nanofibre widths and lengths. The nanofibres also have a layered structure with a characteristic interlayer distance of approximately 0.72 nm (in the protonated form).

Sometimes, it is possible to observe partially wrapped multi-layer nanosh-eets, however, each sample of nanotubes contains a small amount of non-wrapped multilayered nanosheets, as shown in Figure 2.4e. It is believed that these nanosheets play an important role in the mechanism of nanotube formation.

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