Syntheses of TiO2(B) nanowires and TiO2 anatase nanowires by hydrothermal and post-heat treatments

https://doi.org/10.1016/j.jssc.2005.04.025Get rights and content

Abstract

TiO2(B) nanowires and TiO2 anatase nanowires were synthesized by the hydrothermal processing in 10 M NaOH aq. at 150 °C followed by the post-heat treatment at 300–800 °C. As-synthesized Na-free titanate nanowires (prepared by the hydrothermal treatment and repeated ion exchanging by HCl (aq.) were transformed into TiO2(B) structure with maintaining 1-D morphology at 300–500 °C, and further transformed into anatase structure at 600–800 °C with keeping 1-D shape. At 900 °C, they transformed into rod-shaped rutile grains. Microstructure of these 1-D TiO2 nanomaterials is reported.

Graphical abstract

Systhesis of titanate nanowires (precursor for TiO2(B) and anatase nanowires).

  1. Download : Download full-size image

Introduction

One-dimensional TiO2-related materials, such as nanotubes, nanowires, and nanofibers have attracted particular interest because of their unique microstructure and promising functions. After the pioneer work on TiO2-related nanotubes preparation by Kasuga et al. [1], [2] the hydrothermal method in alkali solution has become one of the most powerful techniques to prepare a wide range of TiO2-related 1-D nanomaterials. In their original work [1], [2] single crystal nanotubes (firstly reported as TiO2-anatase) with small diameter of ca.10 nm were obtained by the hydrothermal treatment of TiO2 powder in 10 M NaOH aqueous solution, without using any templates.

Many groups have tried to modify the processing or to analyze the structure of the nanotubes, and have reported that the synthetic mechanism should be the sheet folding [3], [4], [5]; the nanotubes are composed of a layered titanate rather than TiO2 anatase, that is, reported as H2Ti3O7·xH2O [6], [7], [8], NaxH2−xTi3O7 [9], H2Ti4O9·H2O [10], H2Ti2O4(OH)2 [11], and so on.

The hydrothermal method has been expanded to prepare other TiO2-related 1-D nanomaterials, such as K2Ti6O13 nanowires [12], H2Ti3O7–H2Ti6O13 nanofibers [13], and TiO2(B) nanowires [14]. In general, hydrothermal treatment at a slightly higher temperature (∼150 °C or higher) or in stronger alkali solution (conc. NaOH(aq.) or KOH(aq.)) results in the formation of solid nanowires (or even long nanofibers) rather than scrolled nanotubes, because the normal unidirectional crystal growth becomes preferential at these conditions. Although the nanotube structure is attractive due to its high surface area, titanate nanotubes with free-alkali ions are usually unstable at high temperatures (at ∼500 °C) and convert into anatase particles [8], [15], [16]. To maintain the 1-D nanostructure at higher temperature (typically at 500–800 °C), the solid nanowire form should be more favorable.

As mentioned above, Armstrong et al. have recently synthesized TiO2(B) nanowires via hydrothermal treatment and post-heat treatment [14]. TiO2(B) is a metastable polymorph formed by the dehydration of layered or tunnel-structured hydrogen titanate first synthesized in 1980 [17], [18], [19], [20], and also called as monoclinic TiO2 [21]. Owing to its low density and tunnel structure, TiO2(B) can be a promising Li intercalation host material [14], [22]. Although some properties of hydrothermally synthesized TiO2(B) nanowires have been reported [14], [23], further studies are required to put them into actual applications.

In this paper, synthesis of TiO2(B) nanowires by hydrothermal and post-heat treatments will be reported in detail. Furthermore, synthesis of TiO2 anatase nanowires by the similar processing (obtained by post-heat treatment at higher temperature) will be also reported. As is reported earlier by Brohan et al., TiO2(B) transforms into anatase above ∼550 °C [24]. Thus, by optimizing the post-heat treatment temperature, TiO2 anatase nanowires are successfully obtained.

Section snippets

Synthesis of titanate nanowires by hydrothermal synthesis

A commercial, fine TiO2 (anatase) powder (Ishihara Sangyo Ltd., ST-01, ∼300 m2/g) was used as a starting material. A total of 2 g of TiO2 powder and 25 mL of 10 M NaOH aqueous solution were put into a Teflon-lined stainless autoclave (the rate of TiO2 powder and NaOH aq. is 0.08 g/mL). The autoclave was heated and stirred at 150 °C for 72 h. After it was cooled down to room temperature, it was washed by H2O and filtered in the vacuum. The obtained precipitation was put into 500 mL of HCl aqueous

As-synthesized nanowires

Fig. 2 shows the SEM images, TEM images and nitrogen adsorption isotherms of the samples prepared by hydrothermal method for 72 h at (a–c) 120 °C and (d–f) 150 °C, respectively. Both samples were H2O washed, acid treated at pH2 for 24 h, 3 times, and then freeze-dried. As described in introduction part, the 120 °C-treated sample was composed of titanate nanotubes and the 150 °C-treated one was composed of titanate nanowires. From the SEM images, both diameter and length of nanowires were larger than

Conclusions

Na-free titanate nanowires were prepared by the hydrothermal synthesis of 150 °C for 72 h and repeated HCl treatment. The apparent 1-D morphology of TiO2-related nanowires was thermally stable at any post-heat treatment temperature in this study. At about 300 °C, they began to change into TiO2(B) nanowires, and at about 600 °C, transformed into anatase-type TiO2 nanowires. At higher temperature than 900 °C, they begin to change into rutile-type TiO2 rod-like grains.

Acknowledgments

A part of this work has been supported by 21COE program “Establishment of COE on Sustainable Energy System” and “Nanotechnology Support Project” of the Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan.

References (29)

  • Y.Q. Wang et al.

    Chem. Phys. Lett.

    (2002)
  • Y.F. Chen et al.

    Mater. Chem. Phys.

    (2003)
  • M. Zhang et al.

    J. Molec. Catal. A: Chem.

    (2004)
  • R. Yoshida et al.

    Mater. Chem. Phys.

    (2005)
  • R. Marchand et al.

    Mater. Res. Bull.

    (1980)
  • M. Tournoux et al.

    Prog. Solid State Chem.

    (1986)
  • T.P. Feist et al.

    Solid State Ionics

    (1988)
  • T.P. Feist et al.

    J. Solid State Chem.

    (1992)
  • S. Yin et al.

    J. Mater. Proc. Tech.

    (2003)
  • L. Brohan et al.

    Mater. Res. Bull.

    (1982)
  • Z.-Y. Yuan et al.

    Colloids Surfaces A

    (2004)
  • T. Kasuga et al.

    Langmuir

    (1998)
  • T. Kasuga et al.

    Adv. Mater.

    (1999)
  • Q. Chen et al.

    Acta Crystallogr. B

    (2002)
  • Cited by (269)

    View all citing articles on Scopus
    View full text