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CHINESE JOURNAL OF CATALYSIS Volume 29, Issue 8, August 2008 Online English edition of the Chinese language journal

Cite this article as: Chin J Catal, 2008, 29(8): 680–682.

Received date: 20 June 2008. * Corresponding author. Tel: +86-28-85405213; E-mail: 301qzl@vip.sina.com # Corresponding author. Tel: +86-28-85405322; E-mail: zzhang@scu.edu.cn Foundation item: Supported by the National Natural Science Foundation of China (50774053). Copyright © 2008, Dalian Institute of Chemical Physics, Chinese Academy of Sciences. Published by Elsevier BV. All rights reserved.

Preparation of Titania Nanotube Arrays by Hydrothermal Reaction in Combination with Anodic Aluminum Oxide Template Attached to Aluminum Substrate

LI Gang, LIU Zhongqing*, YAN Xin, ZHANG Zhao#

College of Chemical Engineering, Sichuan University, Chengdu 610065, Sichuan, China

Abstract: TiO2 nanotube arrays were prepared in situ on aluminum substrate via hydrothermal reaction, using ammonium hexafluoroti-tanate as treatment solution and anodic aluminum oxide attached to aluminum substrate as template. Field emission scanning electron mi-croscopy and X-ray diffraction were carried out to characterize the as-synthesized product. The experimental results show that TiO2

nanotube arrays obtained through hydrothermal synthesis exhibit an especial morphology. The surface of the TiO2 nanotube arrays has continuous porous structure, while the cross-section has discrete and unattached tubular structure constituted of densely packed TiO2

nanoparticles with an average particle size of 45 and 25 nm, respectively. Without any further heat treatment, anatase phase crystallinestructure of the as-prepared nanotube arrays using this method has already occurred.

Key words: titania; nanotube array; aluminum substrate; anodic aluminum oxide template; hydrothermal synthesis

Much attention has been paid to TiO2 due to its excellent properties useful in many fields, such as photocatalysis [1], photovoltaic cells [2], and gas sensors [3]. As compared to other forms of TiO2, TiO2 nanotube arrays have much higher specific surface area, unique optical and electronic character-istics, and a superior physical topology that can enhance photocatalytic activity and solar-to-electric energy conversion efficiency remarkably. Up to now, the major approaches to fabricate TiO2 nanotube arrays are the sol-gel template method [4] and electrochemical anodization method [5]. Recently, a novel wet process, a liquid phase deposition template method, has been developed for the preparation of TiO2 nanotube ar-rays. In this method, a metal oxide or hydroxide was deposited into the channels of the anodic aluminum oxide (AAO) tem-plate by the principle that the hydrolysis equilibrium of the metal-fluorine ion complex can be shifted by the ligand ex-change of the metal-fluorine ion complex species and the fluorine ion consumption regent (e.g. boric acid, aluminum oxide). Hsu et al. [6] synthesized TiO2 nanotube arrays of 200 nm diameter in the pores of the AAO template using ammo-nium hexafluorotitanate as deposition solution and boric acid

as fluorine ion consumption regent. Yamanaka et al. [7] de-posited TiO2 nanotube arrays in situ by the reaction of an AAO template with ammonium hexafluorotitanate as the deposition solution. In order to overcome the drawbacks of the fragile texture, low intensity, and inability for successive operations of the AAO template, Jiang et al. [8] successfully prepared TiO2

nanotube arrays using an AAO template undetached from the aluminum substrate as the template. Unfortunately, their ex-perimental results indicated that the TiO2 nanotube arrays fabricated using the liquid phase deposition approaches were amorphous. To obtain an anatase crystalline structure with photocatalytic activity, a crystallization heat treatment at a temperature higher than 400 °C was required.

In this work, TiO2 nanotube arrays with a crystalline struc-ture were synthesized on an aluminum substrate in a solution of ammonium hexafluorotitanate by hydrothermal treatment, using an AAO membrane attached to the aluminum substrate as the template.

A two-step anodization was applied to prepare the AAO template. Aluminum foils of high purity (purity 99.999%, thickness 0.5 mm) were cut into rectangles of 30 mm × 20 mm

LI Gang et al. / Chinese Journal of Catalysis, 2008, 29(8): 680–682

and annealed at 500 °C for 4 h to remove mechanical stress. After degreasing in absolute ethanol, removing of the oxide film in sodium hydroxide, and cleaning in distilled water, the aluminum foils were electrochemically polished in a mixture of absolute ethanol and perchloric acid (V:V, 4:1). Subsequently, the foil was anodized at a constant voltage of 120 V for 1 h using 30 g/L phosphoric acid as electrolyte at atmospheric temperature (23 °C). The previously anodized aluminum foil was soaked in a mixture of chromic acid (1.8%) and phospho-ric acid (6.0%) (V:V, 1:1) for 16 h to remove the formed alu-minum oxide layer. After that, a second anodization was car-ried out under the same conditions as the first anodization process except that the anodization time was prolonged to 2 h. Finally, the AAO template was obtained by enlarging the pore size in 5.0% phosphoric acid for 10 min at 30 °C.

The AAO template undetached from the aluminum substrate was transferred into a Teflon-lined stainless steel autoclave filled with 0.1 mol/L ammonium hexafluorotitanate (capacity 500 ml, tamping degree of 80%). After it was sealed, the autoclave was heated at 140 °C for 90 min. Then, the sample was taken out, cooled rapidly, washed with distilled water and absolute ethanol repeatedly, and dried in ambient air.

The morphology of the AAO template and the resulting samples synthesized by the hydrothermal method were ob-served using an Inspect F field emission scanning electron

microscope (FE-SEM, FEI Corporation). The phase purity and crystal structure of the obtained samples were examined by X-ray diffraction (XRD) using a Philips X Pert PRO diffrac-tometer equipped with Cu K radiation ( = 0.154056 nm) in the 2 range 23º to 75º, employing a scanning rate of 0.3º. The accelerating voltage was set at 40 kV with 40 mA flux.

Fig. 1 shows the FE-SEM images of the AAO template fab-ricated by the two-step anodization. Fig. 1(a) illustrates that the surface of the AAO template was continuous with nearly oval pores. The pore size was distributed in the range of 135–370 nm with a mean size of 250 nm, and the thickness of the wall was approximately 55 nm. The pores were distributed uni-formly on the entire surface and no etching or merging of pores was observed. As shown in Fig. 1(b), the pores were tubular with a length of 9 m. The channels were parallel to each other and adjacent pores shared a wall. It can also be seen from Fig. 1(b) that intersections between channels existed in some local areas, which was probably due to the high temperature of the electrolyte (23 °C). It is well known that a low temperature is beneficial to prepare an AAO template with uniform pore size and parallel channels [9].

The FE-SEM images of the product obtained by the hydro-thermal reaction using the AAO template are shown in Fig. 2.

Fig. 2. FE-SEM images of the resulting nanotube arrays from AAO template by the hydrothermal method. (a) Top view; (b) Cross-sectional view.

Fig. 1. FE-SEM images of the as-prepared AAO template by two-step anodization. (a) Top view; (b) Cross-sectional view.

LI Gang et al. / Chinese Journal of Catalysis, 2008, 29(8): 680–682

The top view image (Fig. 2(a)) shows that the surface of the resulting product was continuous and porous and was the same as the pristine AAO template. The difference between them was just the roughness of their surfaces. The surface of the AAO template was very smooth, while the product fabricated by the hydrothermal method had a very rough surface consti-tuted of deposited nearly spherical particles with a mean size of 45 nm. The average pore size of the pores enclosed by these nearly spherical particles was 180 nm, and the thickness of the wall was around 170 nm.

As can be seen from the cross-sectional image (Fig. 2(b)), there was a discrete and unattached tubular structure in the direction vertical to the aluminum substrate, which was dif-ferent from the shared tube wall between the tubes of the AAO template. The pore wall was also constituted by many depos-ited particles with an average particle size of 25 nm. It can be seen that the product obtained by the hydrothermal reaction with the AAO template had three structural configurations: many fine nanoparticles packed to form tubes and many par-allel and discrete tubes constituted nanotube arrays vertical to the aluminum substrate.

The XRD pattern of the surface of the nanotube arrays pre-pared by the hydrothermal treatment of the AAO template in ammonium hexafluorotitanate solution is presented in Fig. 3. The result showed that all of the diffraction peaks can be in-dexed to standard TiO2 (JCPDS Files No. 21-1272), which indicated that the product was TiO2 nanotube arrays with the anatase phase structure. Another notable characteristic was that the peaks were broad, indicating poor crystallinity and non-perfect crystal plane growth of the product. The average crystalline size of the TiO2, which comprised the TiO2 nano-tube arrays, was 14.9 nm calculated by the Scherrer equation. It is reasonable to conclude that the as-prepared nanotube arrays were constituted from agglomerated particles derived from fine TiO2 crystallites, which is consistent with the results of the FE-SEM images.

In summary, TiO2 nanotube arrays were deposited in situ by the reaction of [TiF6]2– and AAO at 140 °C by hydrothermal synthesis using ammonium hexafluorotitanate as deposition solution and AAO attached to aluminum substrate obtained from a two-step anodization as the template. The TiO2 nano-tube arrays exhibited a special morphology, that is, it had a continuous and porous surface and a discrete and unattached

tubular interior. There was no need of a subsequent heat treatment to fabricate TiO2 nanotube arrays with the anatase phase structure, and the use of this method can decrease the crystallization temperature of the amorphous nanotubes, which provides a new way for the preparation of TiO2 nanotube arrays with a crystalline phase via the template method.

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Fig. 3. XRD pattern of the as-synthesized nanotube arrays.