Thermal stability of titanate nanotubes

D. Králová¹, R. Kužel², J. Kovářová¹, J. Dybal¹, M. Šlouf¹

¹ Institute of Macromolecular Chemistry, Academy of Sciences of the Czech Republic, Heyrovskeho nam. 2, 162 06 Praha 6, Czech Republic

² Department of Condensed Matter Physics, Faculty of Mathematics and Physics, Charles University, 121 16 Praha 2, Ke Karlovu 5, Czech Republic

kralova@imc.cas.cz

Keywords: titanium dioxide, titanate nanotubes, electron diffraction.

Abstract

Titanate nanotubes (Ti-NT) were prepared by hydrothermal synthesis from four different TiO₂ powders: anatase micropowder (mA), rutile micropowder (mR), anatase nanopowder (nA), and rutile nanopowder (nR). As we use the nanotubes as filler in molten polymers, we investigated their structural changes at elevated temperatures (up to 800 °C) by a number of methods: transmission electron microscopy (TEM), selected-area electron diffraction (SAED), powder X-ray diffraction (PXRD), thermogravimetric analysis (TGA) and Raman scattering (RS). The shapes of nanotubes were not changed as proved by TEM. The structure of single sheets, from which the nanotubes were formed, was also stable as confirmed by SAED at high scattering vectors q. Packing of sheets and chemical bonding between the sheets was, however, strongly dependent on the temperature, as indicated by PXRD at low q, TGA and mRS.

Introduction

In the past decade, titanate nanotubes (Ti-NT) have attracted much attention because of their interesting structure, morphology and potential applications. A few studies have reported also their thermal stability at high temperatures. Basically, all were in agreement that structure of Ti-NT was stable until about 300 °C. Above this temperature, the crystalline structure of Ti-NT changed, usually into anatase or rutile [1-5]. The aim of this work was to confirm morphological stability of Ti-NT at elevated temperatures that we used during polymer composites preparation. Moreover, we wanted to compare the differences among Ti-NT synthesized from various TiO₂ crystal sizes and modifications.

Experimental

Titanate nanotubes (Ti-NT) were synthesized by hydrothermal synthesis as reported in our previous work [6]. Briefly, Ti-NT were synthesized from four different TiO₂ powders (anatase micropowder (mA), rutile micropowder (mR), rutile nanopowder (nR), and anatase nanopowder (nA)). Initial concentration of TiO₂ was 0.1 g; reaction time was 48 hours.

The morphology of the nanotubes was investigated by TEM. A droplet of the Ti-NT aqueous suspension was deposited on a carbon-coated copper grid, left to evaporate and then inspected in a transmission electron microscope (TEM; Tecnai G² Spirit 120, FEI, Czech Republic). For investigation of thermal stability, specimens prepared on TEM grids were heated at 300 °C for 1 hour and then observed in TEM. The crystalline structure at high scattering angles q > 1.4 Å^-1 was obtained from SAED on the same microscope.

The crystalline structure and its thermal stability was also characterized by powder x-ray diffraction (PXRD, diffractograms at low scattering angles q < 1.4 Å^-1, temperatures up to 800 °C with step of 50 °C, each temperature was hold for 1 h before measurement), thermogravimetric analysis (TGA, analyses up to 800 °C, heating rate 5 °C/min) and Raman microscopy (RS, temperature up to 350 °C, step 50 °C heating rate 10 °C/min, temperature held 10 min before each measurement).

Results

TEM micrographs (Fig.1a, c) proved that Ti-NT morphology was not affected by heating up to 300 °C, which was the maximum temperature used during melt mixing of polymer composites with Ti-NT. It has been demonstrated [7] that Ti-NT are formed by rolled sheets, which are composed of titania octahedra. The periodic distances within the sheets are low and so the corresponding diffractions are observed at high scattering vectors (q > 1.4 Å^-1 » 2q(CuKa) > 20°), whereas interplanar distances within the rolled sheet are in the range of nanometers, which corresponds to lower scattering vectors (q < 1.4 Å^-1 » 2q(CuKa) < 20°). The periodic interatomic distances within single sheets, observed at high q by SAED, did not change (Fig. 1b, d). On the other hand, the interplanar distances due to rolling of the sheets, observed at low q by PXRD, exhibited significant shifts (Fig. 2). Moreover, changes at elevated temperatures, indicated by PXRD, were confirmed by TGA (decrease of sample mass with the temperature, most likely release of water) and also by RS (change of Raman spectra starting at 150 °C).

Conclusion

The morphology of our laboratory-synthesized titanate nanotubes was stable at elevated temperatures (up to 300 °C) as proved by TEM (Fig.1a, c). The crystal structure of single sheets was also stable, as confirmed by SAED (Fig.1b, d). Packing of sheets and chemical bonding between the sheets was, however, strongly dependent on the temperature, as indicated by PXRD at low q (Fig. 2). Structural changes at elevated temperatures were proved also by TGA and mRS.

References

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Acknowledgements.

Financial support through grants KAN200520704 a GACR 203/07/0717 is gratefully acknowledged.

Figure 1. TEM micrographs and ED patterns of Ti-NT heated up to 300°C (a,b) and non-heated Ti-NT (c,d); no significant difference was observed.

Figure 2. PXRD of Ti-NT synthesized from micro-rutile (A), nano-rutile (B), micro-anatase (C) and nano-anatase showed significant changes.