Elsevier

Thin Solid Films

Volume 526, 30 December 2012, Pages 116-119
Thin Solid Films

Phase transformation and thermal structure stability of titania nanotube films with different morphologies

https://doi.org/10.1016/j.tsf.2012.11.027Get rights and content

Abstract

Titania nanotube films composed of close-packed and loose-packed titania nanotube arrays were produced by electrochemical anodization of titanium. Phase transformation and thermal structure stability of the titania nanotube films with different morphologies were studied after they were annealed at different temperatures. Results show that the close-packed titania nanotube films started phase transformations from anatase to rutile at a relatively higher temperature, and exhibited higher thermal structure stability than the loose-packed titania nanotube films. The difference in the phase transformation and thermal structure stability of the close-packed and loose-packed titania nanotube films was analyzed from the viewpoints of specific surface area. The results in this paper provided understanding of the morphology effects on phase transformation and thermal structure stability of titania nanotube films.

Highlights

► Close-packed and loose-packed TiO2 nanotube films were prepared. ► The TiO2 nanotube films exhibited different specific surface areas. ► The TiO2 nanotube films exhibited different phase transformation behavior. ► Close-packed TiO2 nanotube films exhibited better thermal structure stability. ► Effects of morphology were discussed from the view point of specific surface area.

Introduction

Nanostructured titania (TiO2) has been extensively studied for applications such as photocatalysts, gas sensors and solar cells [1], [2], [3]. Most recently, one-dimensional TiO2 nanotubes have attracted extraordinary attention because TiO2 nanotubes have been found possessing advanced properties for photocatalysis, gas-sensing and photovoltaic applications [4], [5], [6].

Electrochemical anodization is currently the most often used method to prepare TiO2 nanotubes [7], [8], [9], [10], [11], [12]. The as-anodized TiO2 nanotubes are normally amorphous and annealing treatment is often performed to convert the amorphous phase to anatase or rutile crystalline phase [13], [14], [15], [16]. The crystalline phases of TiO2 formed in the process of annealing strongly affect its physical and chemical properties. Besides the phase transformation, the morphology of TiO2 nanotubes also changes during annealing. As indicated by a number of studies [17], [18], morphological structures of nanomaterials also have great influences on their physical and chemical properties. Therefore, it is necessary to study the phase transformation and thermal structure stability of TiO2 nanotubes before they are used for different applications.

Several previous studies have investigated the effects of the annealing environments, TiO2 nanotube dimensions, and TiO2 nanotubes' substrates on the phase transformation and thermal structure stability of TiO2 nanotube films [19], [20], [21], [22]. However, comparative studies on the phase transformation and thermal structure stability of TiO2 nanotube films with different morphologies have been rarely conducted. In the present work, TiO2 nanotube films with different morphologies were produced by anodization of titanium (Ti). The phase transformation and thermal structure stability of the different TiO2 nanotube films were investigated. Effects of the morphology on the phase transformation and thermal structure stability of TiO2 nanotube films were discussed.

Section snippets

Experimental procedure

Titanium samples with dimensions of 10 mm × 10 mm × 2 mm and purity of 99.7% were polished to a mirror-like finish, ultrasonically cleaned in ethanol, acetone and distilled water successively and then dried in air at room temperature. Anodization was carried out at room temperature using a two-electrode electrochemical cell. The electrolyte was a mixture of ethylene glycol, ammonia fluoride and water. The concentration of ammonia fluoride was 0.25 wt.%. To prepare TiO2 nanotube films with different

Results

Fig. 1 shows the SEM images of the as-anodized TiO2 nanotube films prepared by using electrolytes containing 1 wt.% and 5 wt.% water respectively. As can be seen from the images, the TiO2 nanotube films displayed different morphologies. With the water concentration of 1 wt.%, the as-anodized TiO2 nanotube film was composed of close-packed nanotube arrays and the nanotube arrays were covered by a nanoporous layer. On the other hand, using the electrolyte containing 5 wt.% water, TiO2 film was

Discussion

During annealing process of TiO2, anatase phase appears first, and then anatase transforms to rutile phase at higher temperatures. In the present study, anatase phase appeared at 400 °C in both the close-packed TiO2 nanotube film and the loose-packed film. However, phase transformation from anatase to rutile started at 800 °C in the close-packed film while it started in the loose-packed film at 600 °C. Therefore, the anatase phase was more stable in the close-packed TiO2 nanotube film.

In the

Summary

Close-packed and loose-packed titania nanotube films were produced by anodization of titanium. With the different morphologies, the titania nanotube films exhibited different specific surface areas. The loose-packed titania nanotube films exhibited higher specific surface than the close-packed titania nanotube films. Compared with the loose-packed titania nanotube film, the close-packed nanotube film started phase transformation from anatase to rutile at relatively higher temperature and

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