High temperature Raman spectroscopy of titanate nanotubes Andreja Gajovic ´ a, * , Ivica Frišc ˇic ´ a,1 , Milivoj Plodinec a,1 , Damir Ivekovic ´ b a Ruder Boškovic ´ Institute, Bijenic ˇka 54, HR-1002 Zagreb, Croatia b Faculty of Food Technology and Biotechnology, University of Zagreb, Zagreb, Croatia article info Article history: Received 19 August 2008 Received in revised form 19 December 2008 Accepted 24 December 2008 Available online 12 January 2009 Keywords: Titanate nanotubes Thermal stability Phase transition High temperature Raman spectroscopy Transmission electron microscopy abstract Titanate nanotubes were synthesized by hydrothermally treating the TiO 2 powder with NaOH solution. Their formation and thermal stability was discussed in the view of different TiO 2 precursors and Na + con- tent. The starting precursors for nanotube synthesis were either anatase or the mixture of anatase, rutile and high-pressure TiO 2 II phase (TiO 2 II) as a major component. The samples with various Na/Ti ratios were prepared by ion exchange of interlayer Na + cations with H + ions under controlled pH conditions. The thermal stability and the structural changes of nanotubes were studied in situ at high temperatures using Raman spectroscopy (RS) and transmission electron microscopy (TEM). We found that hydrothermal treatment of mixture of TiO 2 phases (anatase, rutile and TiO 2 II) leads to the formation of titanate nanotubes with structure and morphology similar to these obtained from pure anatase. However, their temperature stability was reduced, so transform to anatase at temperatures around 80 °C. In the case of nanotubes obtained from anatase precursor, Raman bands characteristic for titanate nanotubes were visible up to 300 °C. At 500 °C titanate nanotubes in H-form, H 2 Ti 3 O 7 , were completely transformed to anatase, while Na-form, Na 2 Ti 3 O 7 , showed phase transition to hexatitanate, Na 2 Ti 6 O 13 , nanowires. Ó 2009 Elsevier B.V. All rights reserved. 1. Introduction Titanate nanotubes are especially interesting because of their high aspect ratio and high specific surface area. In last few years, properties of TiO 2 nanostructures were extensively studied due to their potential applications in solar cells [1–3], electronics [4], chemical sensors, hydrogen sensing and storage [5,6], (photo)catal- ysis [7,8], as mesoporous material for ion exchange [9,10], and in lithium batteries [11]. TiO 2 -based one-dimensional nanostructures can be made by several different methods. For TiO 2 -nanofiber, electro-spinning synthesis [12] and hydrogen treatment [13] have been proposed. Titanate nanotubes can be synthesized by anodic porous alumina templating [14,15], anodic oxidation of a titanium sheet [16], car- bon nanotube inner templating [17], supramolecular assemblies templating [18], but due its simplicity and cheapness, the most used method to produce titanate nanotubes is hydrothermal meth- od proposed first by Kasuga et al. [19]. However, the mechanism of nanotube formation, growth and the influence of synthesis param- eters on finale crystal structure and chemical composition are still the subject of the numerous studies. Kasuga et al. [20] and some other authors [21,22] stated that the acid washing of products after hydrothermal treatment is crucial for sodium exchange and forma- tion of nanotubes. But, several other authors found strong evi- dences that nanotubes are formed during the hydrothermal reaction [23–28]. Today, synthesis of titanate nanotubes became a routine and it is generally accepted that the nanotubes have a layered titanate structure. Nevertheless, there is big discrepancy in proposed crystal structure and chemical composition of titanate nanotubes. Different authors proposed the crystal structure as TiO 2 anatase with tetragonal structure [20,27,29–31], trititanates (H 2 Ti 3 O 7 ) with monoclinic structure [23,24,30–33],H 2 Ti 3 O 7 ÁnH 2 O [34], Na x H 2Àx Ti 3 O 7 [9,35–37], tetratitanate H 2 Ti 4 O 9 ÁH 2 O [26] and lepidocrocite titanate H x Ti 2Àx/4 h x/4 O 4 (h is vacancy) with ortho- rhombic structure [21,38]. There is also discrepancy in proposed models of formation of titanate nanotubes in hydrothermal environment. A number of authors agree with model of rolling up of nanosheets into the nanotubes, proposing formation of two-dimensional layered struc- tures after braking of bonds in three-dimensional TiO 2 structure, stripping off and rolling in alkaline solution [21,25,27, 29,32,33,38]. However, Kukovecz et al. [39] proposed that nano- tubes are formed by orientated crystal grow starting from nanolo- ops formed due to local fluctuations of ion concentration on surface of TiO 2 . In hydrothermal method TiO 2 powder, used as a starting mate- rial, can be in form of anatase [10,19,29,35,37–45], rutile [20,33], mixture of anatase and rutile [57], natural rutile [56], brookite 0022-2860/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.molstruc.2008.12.072 * Corresponding author. Tel.: +385 14561106; fax: +385 14680112. E-mail address: gajovic@irb.hr (A. Gajovic ´). 1 Undergraduate student of Department of Physics, Faculty of Science, University of Zagreb, Bijenic ˇka 32, HR-10000 Zagreb, Croatia. Journal of Molecular Structure 924–926 (2009) 183–191 Contents lists available at ScienceDirect Journal of Molecular Structure journal homepage: www.elsevier.com/locate/molstruc