Thermal Stability

A knowledge of the thermal stability of titanate nanotubes is important, since some applications or manipulations (including: catalyst supports or the curing of composite films) require an increased operating temperature. At elevated temperatures there are at least three processes occurring with protonated titanate nanotubes, namely: dehydration, crystal structure transformation and a modification in morphology. All three processes occur simultaneously, and each has a characteristic range of temperatures related to their particular phase transition. Figures 4.11a and b show typical TGA and DSC curves of proto-nated titanate nanotubes. The weight loss and exothermic processes that occur in the temperature range of 25 to 450 °C, are usually associated with the removal of water.

In the temperature range of 25 to 120 °C, desorption of water from nanotube pores results in a loss of approximately 7-8 wt%. A further increase in temperature up to 250 °C, results in the removal of crystallographic water from nanotubular titanates. This process is accompanied by a decrease in interlayer spacing d200 and an increase in density, due to the weight loss and shrinkage of the nanotube volume. According to TGA data, approximately 3-4 wt% of mass is loss during the dehydration of crystallographic water. At temperatures

Figure 4.10 Chemical and structural transformations of titanate nanotubes and nanofibres. (The SEM and TEM images are reproduced from refs. 36,39,45,48,49,51,52).

above 250 °C, topotactic transformation of H2Ti3O7 to the intermediate phases of H2Ti6O13 and H2Ti12O25 may occur,36 resulting in a further decrease in interlayer spacing d200 and a further increase in density. Water loss accounts for an additional mass loss of approximately 1-2 wt%, according to TGA. At temperatures below 400 °C, the removal of the remaining water results in the formation of monoclinic TiO2-(B) nanotubes (see Figure 4.11b). This mono-clinic TiO2-(B) phase can be detected using the characteristic reflection in the

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H2Ti307 xH20 H2Ti307 H2Ti6013 Ti02(B)

Figure 4.11 Thermal transformations of protonated titanate nanotubes. a) Graph of thermogravimetric analysis (TGA), b) differential scanning calorimetric (DSC) curve, c) graph showing density and interlayer distance d200 as a function of calcination temperature, and d) schematic crystal structures of hydrated trititanate and its transformation to hexatitanate, followed by the formation of TiO2-(B). (Density was measured using helium adsorption, d200 structures adapted from ref. 36).

H2Ti307 xH20 H2Ti307 H2Ti6013 Ti02(B)

Figure 4.11 Thermal transformations of protonated titanate nanotubes. a) Graph of thermogravimetric analysis (TGA), b) differential scanning calorimetric (DSC) curve, c) graph showing density and interlayer distance d200 as a function of calcination temperature, and d) schematic crystal structures of hydrated trititanate and its transformation to hexatitanate, followed by the formation of TiO2-(B). (Density was measured using helium adsorption, d200 structures adapted from ref. 36).

XRD pattern at ca. 15 °C (ref. 37,38). A further increase in temperature above 400 °C, results in the transformation of nanotubular TiO2-(B) to anatase solid nanorods, accompanied by a filling of nanotubes hollow cavities and a loss of tubular morphology.

The level of ion-exchanged sodium in nanotubular titanates can determine the thermal stability and the nature of thermal transformations during annealing.39-42 When fully saturated with sodium ions, titanate nanotubes can lose their nanotubular morphology and convert to Na2Ti6O13 nanorods only, at 600 °C.43,44

Similar thermal transformations can occur in nanofibrous titanates (see Figure 4.10). Protonated nanofibres can under calcinations undergo the sequence transformations to nanofibrous TiO2-(B) at 400 °C, followed by transformation to nanofibrous anatase at 700 °C, and then formation to microfibrous rutile at 1000 °C.45,46 The formation of anatase nanofibres is also possible by the hydrothermal treatment of titanate nanofibres in water at 150 °C.47 The sodium substituted nanofibres can be converted to Na2Ti6O13 nanofibres at 500 °C in the presence of hydrogen.48

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