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Physica E 37 (2007) 241–244 www.elsevier.com/locate/physe
ZnO nanotubes by template wetting process B.I. Seo, U.A Shaislamov, M.H. Ha, S.-W. Kim, H.-K. Kim, Beelyong Yang Department of Advanced Nano Materials for Information Technology, Kumoh National Institute of Technology, Yangho-dong, Gumi, Gyeongbuk 730-701, Korea Received 8 May 2006; accepted 26 July 2006 Available online 12 September 2006
Abstract ZnO nanotubes have been fabricated within the nanochannels of porous anodic alumina templates by template wetting process. In this method, pore walls of the alumina template were wetted by polymeric ZnO source. After heat treatment procedures, the template was selectively etched off to release the nanotubes. Field-emission scanning electron microscopy (FE-SEM) investigations showed that nanotubes have smooth wall morphologies and well-defined diameters corresponding to the diameter of the applied template. In addition, X-ray diffraction measurements showed that our ZnO nanotubes realized in the template-wetting process have polycrystallinestructural properties. r 2006 Elsevier B.V. All rights reserved. PACS: 81.20.Fw; 78.67.Ch; 77.84. s; 07.85.Jy Keywords: ZnO; Nanotube; Template; Wetting
1. Introduction Recently, considerable research efforts have been focused on fabrication and characterization of one-dimensional (1D) nanomaterials because of their fundamental importance in physics and potential applications for realizing nanodevices. ZnO is recognized as a promising material for short-wavelength optoelectronic applications because of its superior optical properties of a wide direct band gap of 3.37 eV and a large exciton binding energy of 60 meV [1,2]. Various kinds of methods were introduced to prepare 1D ZnO nanostructures such as nanowires, nanobelts, and nanotubes [3–5]. Single-crystal semiconductor nanotubes would be advantageous in potential nanoscale electronics, optolelectronics, and biochemical-sensing applications [6,7]. There are some reports demonstrating preparation of ZnO nanotubes by vapor phase growth method [5]. However, this method generally requires hightemperature processes. Very recently, Zhang et al. [8] have reported preparation of self-assembled ZnO tubes on sapphire substrates by a Corresponding author. Tel.: +8254 478 7741; fax: +8254 478 7769.
E-mail address:
[email protected] (B. Yang). 1386-9477/$ - see front matter r 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.physe.2006.07.025
metalorganic chemical vapor deposition method. However, regretfully, grown tubes have quite large diameters of around 800 nm and precise control of tube thickness should be overcome. In this regard, we describe preparation of ZnO nanotubes via a template-wetting process introducing nanoporous alumina templates in order to realize precise control of nanotube size and dimension.
2. Experiment and discussion High-purity Al sheets (99.99%) were used to fabricate the porous anodic alumina membranes. The Al sheet was firstly electropolished in a mixed electrolyte solution containing phosphoric acid, sulfuric acid, and DI water with a small amount of chromic acid under a constant DC voltage. Anodization with a platinum wire as a cathode and the Al sheet as an anode was conducted in 10 wt% phosphoric acid under 160 V DC voltages (Fig. 1). The electrolyte was maintained at 3 1C by using circulator. To make the porous structures more regular and uniform, the so-called ‘‘two-step’’ method was used [9–11]. After anodization of 30 min, the sample was immersed in a mixture of chromic acid and phosphoric acid at 60 1C to
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remove the porous alumina layer formed in the first anodization. The surface of the remaining aluminum film had a highly ordered indented hole array upon it, due to the barrier layer structure formed at the bottom of the alumina pores in the first anodization. Anodization of the remaining aluminum layer, under the same conditions, resulted in AAO nanoporous arrays of better uniformity and straighter pore channels. ZnO nanotubes were fabricated by the template-wetting process. The wetting phenomena described above are the key to template wetting for the preparation of 1D nanoand micro-objects. Organic polymers are among the materials with low surface energy [12]. A polymeric melt or solution can be loaded with considerable proportions of low molar mass compounds. When a polymer-containing liquid is brought into contact with or a polymer melt is molten onto the surface of a porous membrane, wetting of the pore walls occurs while individual chains diffuse onto and along the pore walls. As a prerequisite, the pore walls must have a high surface energy. The processes discussed here are microscopic in nature, in contrast to phenomena such as the Lotus effect [13], which are associated with the macroscopic wettability of structured surfaces. Then, a mesoscopic precursor film will wet the walls of the pores in
OUTLET INLET
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DC POWER SUPPLY 220 V AC
Fig. 1. Experimental setup for anodization process.
a similar manner to the formation of a precursor film on a flat substrate. ZnO source was mixed with dissolved poly (D,L-lactide) in proper quantities to prepare the wetting solution. The prepared solution then dropped onto porous template, after waiting several minutes letting the solution to cover pore walls, samples baked in an oven at 200 1C. During baking, the applied solution evaporates and the ZnO source solidifies within the pore walls. Then, the alumina template embedded with ZnO, annealed in a tube furnace at 700 1C for 3 h. When heat treatments were completed, the template were selectively etched avay in 30 wt% KOH solution at room temperature to release nanotubes. The released nanotubes were thoroughly washed with DI water and the nanotube-containing solution was dropped on the Si wafer for field-emission scanning electron microscopy (FE-SEM) examination. The resulting FE-SEM images of porous alumina with diameters of about 230 nm using bulk aluminum after twostep anodization in 10 wt% phosphoric acid solution are shown in Fig. 2(a). Surface image (Fig. 2(a)) of porous alumina shows well-ordered close paced pores. In Fig. 2(b), the surface image of commercial nanoporous alumina template is shown, which was actually used in nanotube process pore diameter enlarged by chemical etching in 5 wt% phosphoric acid. The thickness of the porous layer is estimated to be about 6 mm/h. High regularity of pores can be reached by anodization of imprinted Al [19,20]. Organic materials and most polymers are considered as low-energy materials with respect to their surface energies, whereas inorganic materials are referred to high-energy materials. Low-energy liquids spread rapidly on high-energy surfaces. Therefore, the pore walls will be covered with a mesoscopic film if they exhibit a high surface energy. The underlying driving forces are due to short-range as well as long-range Van der Waals interactions between the wetting liquid and the pore walls. After the solvent had been evaporated, the solidified ZnO source formed a thin film covering both the pore walls and the surface of the template. Fig. 3(a) depicts
Fig. 2. FE-SEM images of porous anodic alumina: (a) surface image of commercial nanoporous alumina; (b) surface of nanoporous alumina prepared using bulk Al in 10 wt% phosphoric acid at 160 DC V.
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Fig. 3. FE-SEM images of ZnO nanotubes prepared by wetting of nanoporous template: (a) top view of ZnO nanotubes partially embedded to the template, (b) side wall view of ZnO nanotubes, (c) high-magnification FE-SEM image showing open ends of ZnO nanotubes.
500 (101)
ZnO, 700°C
(100) 400 (002) Intensity(a.u)
FE-SEM top view image of ZnO nanotubes in low magnification. It is clearly seen that vertically aligned tubes partially embedded to the template. Fig. 2(b) shows side walls of the ZnO nanotubes, which are smooth, straight, and have the same diameter along the length. From the open ends of ZnO tubes (Fig. 3c), it is easy to estimate the diameter of the tubes. They correspond to the diameter of applied template of about 200 nm X-ray diffraction pattern of the ZnO nanotubes arrays embedded in alumina template as shown in Fig. 4. XRD investigations were performed after the annealing at 700 1C. The result indicates strong reflections corresponding to (1 0 0), (0 0 2), and (1 0 1) planes are observed along with weaker reflections of (1 0 2), (1 1 0), (1 0 3), and (1 1 2) planes, indicating ZnO nanotubes are [16].
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3. Summary ZnO nanotubes have been successfully prepared within nanopores of porous alumina membrane templates by the template-wetting method. FE-SEM images show that the grown ZnO nanotubes are vertically well-aligned in the templates. The diameters of as-prepared nanotubes are around 200 nm, which coincide with the employed template pore diameters. Our ZnO nanotubes fabricated at low
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Angle(2θ)
Fig. 4. XRD pattern of ZnO nanotubes.
temperature are very promising for applications such as chemical and biological sensors with high sensitivity, optoelectronic devices, and solar cells. In addition, the template-wetting method introduced in this study may be
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greatly helpful for realizing core-shell nanotube–nanowire heterojunctions. Acknowledgment This work was supported by the Korean Research Foundation Grant (KRF-2004-002-D00241). References [1] K. Hummer, Phys. Stat. Sol. (b) 56 (1973) 249. [2] A. Mang, K. Reimann, St. Ruhenacke, Solid State Commun. 94 (1995) 251. [3] S.-W. Kim, Sz. Fujita, Sg. Fujitha, Appl. Phys. Lett. 86 (2005) 153119. [4] Z.W. Pan, Z.R. Dai, Z.L. Wang, Science 291 (2001) 1947.
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