Crystallography of Titanate Nanofibres Nanorods and Nanosheets

During alkaline hydrothermal treatment of titanium dioxide the formation of fibrous, rather than tubular, nanostructures can occur. The XRD patterns of these nanofibrous materials usually show a characteristic reflection at small angles, confirming a layered structure of the nanofibre crystals. In current literature, the crystal structure of these nanofibres corresponds to the structure of layered protonated polytitanates H2TinO2n+7, where n = 5 (ref. 16,17), 6 (ref. 18) and 8 (ref. 16,19), or else channelled TiO2-B (ref. 20).

The nanofibres can be considered as a stack of titanate (100) planes. The length of the fibre usually corresponds to crystallographic axis c, which is considered as the direction of most rapid crystallization in polytitanates.17 The width of the fibre usually corresponds to axes b and a. The interlayer distance in nanofibres is in the range of 0.7-0.8 nm. The calcination of titanate nanofibres results in the consecutive transformation from titanate to TiO2-B (at 400 °C), then to anatase (at 700 °C) and to rutile (at 1000 °C). The nanofibrous morphology disappears at 1000 °C.21

Nanorods obtained by calcination of protonated titanate nanotubes at temperatures above 400 °C, are characterised by a tetragonal, anatase poly-crystalline crystal structure with impurities of amorphous phase.

The crystal structure of nanosheets, which are observed as an intermediate product during nanotube or nanofibre synthesis, is attributed to either the hydrated form of delaminated anatase22 or lepidocrocite-type titanates consisting of shared TiO6 octahedrons.23 In both cases, the simulated XRD patterns are similar to those of titanate nanotubes, making it difficult to discriminate between each particular structure. The use of the instrumental methods which can distinguish between three-coordinated oxygen atoms intrinsic to anatase structure, and four-coordinated oxygen atoms forming the characteristic sequence of edge-sharing TiO6 octahedrons of lepidocrocite-type titanates, may enable an accurate determination of nanosheet crystal structure in the future.

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