Details on the TiO₂ nanotubes wall structure revealed by HRTEM

Valentin S. Teodorescu¹, Leona C. Nistor¹, Silviu Preda², Maria Zaharescu²,

Marie-Genevieve Blanchin³

1.National Institute of Materials Physics, 077125 Magurele, Ilfov, Romania

2.Institute of Physical Chemistry“Ilie Murgulescu”, Romanian Academy

060021 Bucharest, Romania

3. ILM- Universite Claude-Bernard Lyon 1,  69622 Villeurbanne, France

Titanium oxide nanotubes were previously obtained by hydrothermal treatment using a crystalline precursor [1].  We have prepared titania nanotubes by the hydrothermal method starting with amorphous and crystalline sol-gel TiO₂ precursors [2]. The hydrothermal treatment was realized in the presence of a 10 M NaOH solution at 140°C for various periods of time, from 24 to72 hours. The reaction yield was separated by centrifugation and washed alternately with distilled water and 0.1N HCl solution, down to pH ~6. The sample was dried at 110°C, for 12 h, in air. The HRTEM study was performed on specimens prepared on holey carbon grids.

Figure 1 show a low magnification image of an aggregate of TiO₂ nanotubes obtained from an amorphous sol-gel precursor. The Na content, determined by the EDX, is about 3% for large aggregates, but is less than 1% in the case of small, transparent, nanotubes aggregates.

The high resolution study of the resulted TiO₂ nanotubes, evidenced some interesting details about the nanotube formation. One is the presence of unrolled TiO₂ foils. Figure 2 shows the presence of a TiO₂ unrolled foil in the nanotubes aggregate. It is remarkable that the foil structure is much resistant to the electron beam irradiation compared to nanotubes . The most interesting is the spacing variation between the layers forming the nanotube wall, depending of the number of the layers in the wall.

The HRTEM study reveals the presence of TiO₂ nanotubes with different number of layers in the wall, from 2 to 5, the majority having 3 layers in the wall. A comparative analyze of the wall details in the HRTEM images, evidenced clearly this variation of the spacing between the layers in the wall, depending on the number of layers: the higher the number of layers in the wall the smaller the spacing as revealed in figures 2 and 3a., On the other hand, the inner diameter of the nanotube becomes smaller as the number of layers grows. These measurements are shown in figure 3b.

We can explain these details supposing that the TiO₂ foils formed from the amorphous precursor, are quite defected hindering the foil rolling. This effect explains the massive presence of unrolled TiO₂ foils. On the other hand, the spacings in the rolling foils are also controlled by the foil defects, i.e. they become larger as the foil has more defects. Moreover, the rolling force between the TiO₂ layers is more effective in the case of successive rolling of the foils, reducing the spacing between layers and leading to a smaller inner diameter of the nanotube.

References:

1. G.H. Du, Q. Chen, R.C. Che, Z.Y. Yuan, L.-M. Peng: Preparation and Structure Analysis of

Titanium Oxide Nanotubes. Appl. Phys. Lett. 79, 3702 (2001).

2. S.Preda, V.S.Teodorescu, A.M.Musuc, C.Andronescu, M.Zaharescu,  Influence of the TiO₂ precursors on the thermal and structural stability of titanate-based nanotubes, , J.Mater. Res. Vol 28(3),294-303,2013

Figures:

Figure 1. Low magnification image of a TiO2 nanotubes aggregate (a); higher magnification showing the presence of a TiO2 foils in the nanotube aggregate (b)

Figure 2. Evidence of the spacing variation between the layers in the nanotube wall (a). Comparison of the wall spacings using details from the same image (b).

Figure 3. a-Variation of the spacing between the layers in the nanotube wall. (N- number of the layers in the nanotube wall) ; d(nm) - the spacing between the nanotube wall layers ; b-Variation of the inner diameter D (nm) of the nanotubes.

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