Research Article Received: 5 August 2009 Accepted: 23 September 2009 Published online in Wiley Interscience: (www.interscience.wiley.com) DOI 10.1002/jrs.2561 A Raman spectroscopic and TEM study on the structural evolution of Na 2 Ti 3 O 7 during the transition to Na 2 Ti 6 O 13 Hongwei Liu, Dongjiang Yang, Zhanfeng Zheng, Xuebin Ke, Eric Waclawik, Huaiyong Zhu and Ray L. Frost * The synthesis of sodium hexatitanate from sodium trititanate was characterized by Raman spectroscopy, X-ray diffraction (XRD) and high-resolution transmission electron microscopy (HRTEM). The structural evolution from trititanate to hexatitanate was studied using Raman spectra, XRD and HRTEM techniques. It was found that the Raman bands at 279 cm -1 corresponding to very long Ti – O bonds and at 883 cm -1 corresponding to the very short Ti – O bonds decrease in intensity and finally disappear during the transition from sodium trititanate to sodium hexatitanate. The band at 922 cm -1 corresponding to an intermediate-length Ti–O bond was observed to become stronger with the increase in temperature, indicating that there is no terminal oxygen atom in the crystal structure of Na 2 Ti 6 O 13 and that all the oxygen atoms become linearly coordinated by two titanium atoms. Furthermore, the TiO 6 octahedron in Na 2 Ti 6 O 13 are more regular because the very long (2.2 Å) or very short (1.7 Å) Ti – O bonds disappear. It is revealed that the phase transition from trititanate to hexatitanate is a step-by-step slipping process of the TiO 6 octahedral slabs with the loss of sodium cations, and a new phase with formula Na 1.5 H 0.5 Ti 3 O 7 has been discovered as an intermediate phase to interlink Na 2 Ti 3 O 7 and Na 2 Ti 6 O 13 . Copyright c  2010 John Wiley & Sons, Ltd. Keywords: titanate; sodium trititanate; sodium hexatitanate; crystal structure; Raman spectroscopy; transmission electron microscopy Introduction Nanostructured materials have received much attention be- cause of their novel properties, which differ from those of bulk materials. [1] There is also great interest in the development of ti- tanates and TiO 2 -based solids with nanoscale dimensions and high morphological specificity, [2,3] such as nanofibers, [4] nanosheets [5] and nanotubes [6] because of their demonstrated potential in solar energy conversion, [7] photocatalysis [8,9] photovoltaic devices [10,11] and as carriers for metallic nanoparticles. [12] It is known that the alkali-metal titanates have a typical A 2 Ti n O 2n+1 crystallographic structure. All of these structures have a monoclinic structure with almost the same b value. [13,14] Generally, alkali-metal titanates with a high alkali metal content (n = 2, 3, 4) possess an open-layered structure. They can be used as cation exchangers and catalysts because of their distinctive intercalation ability and catalytic activity. [15 – 18] On the other hand, alkali-metal titanates with a low alkali-metal content (n = 6, 7, 8) possess a tunnel structure and exhibit high insulating, mechanical and chemical stability. [19,20] A powerful way to prepare titanate nanotubes and nanowires is by hydrothermal treatment of titania in the presence of concentrated alkaline medium. Unfortunately, despite intensive investigations, a consensus regarding fundamental knowledge about the details of the structure of these hydrothermally synthesized nanostructures remains unclear. Taking just one example, in contradiction with previous investigations, Sun and Li [21] report that as-synthesized nanotubes are titanates and can be described by the formula Na x H 2−x Ti 3 O 7 . Ther- mal treatment of these materials leads to the formation of different titanates with the general formula Na 2 Ti n O 2n+1 . Re- cently, it was also mentioned that as-synthesized nanotubes or nanorods have an even more complex structure, namely, Na x H 2−x Ti n O 2n+1 ·yH 2 O. [22] Treatment of these nanoparticles with an HCl solution produced nanotubes and nanorods of H 2 Ti 3 O 7 . The complete assignment of the Raman spectra of sodium titanate is still not available. The first Raman measurement for Na 2 Ti 3 O 7 was made by Bamberger and Begun. [23] Several published data on the Raman spectra of titanate and titania exist in the literature, [24 – 27] but the evolution of Raman spectra during the phase transition of trititanate to hexatitanate has not been reported. It is necessary to discover the details of these above phase transitions by a comprehensive method involving Raman spectroscopy, X-ray diffraction (XRD) and transmission electron microscopy (TEM). In the present work, we report the Raman spectroscopic analysis of the structural evolution of the phase transition from sodium trititanate to sodium hexatitanate fibers. A mechanism is proposed to interpret the crystallographic structures and changes in the structures and associated parameters. ∗ Correspondence to: Ray L. Frost, Inorganic Materials Research Program, School of Physical and Chemical Sciences, Queensland University of Technology, Brisbane, QLD 4001, Australia. E-mail: r.frost@qut.edu.au Inorganic Materials Research Program, School of Physical and Chemical Sciences, Queensland University of Technology, Brisbane, QLD 4001, Australia J. Raman Spectrosc. (2010) Copyright c  2010 John Wiley & Sons, Ltd.