Synthesis of zinc oxide nanorods using carbon nanotubes as templates

Synthesis of zinc oxide nanorods using carbon nanotubes as templates

ARTICLE IN PRESS Journal of Crystal Growth 265 (2004) 184–189 Synthesis of zinc oxide nanorods using carbon nanotubes as templates Hua-Qiang Wu, Xia...

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ARTICLE IN PRESS

Journal of Crystal Growth 265 (2004) 184–189

Synthesis of zinc oxide nanorods using carbon nanotubes as templates Hua-Qiang Wu, Xian-Wen Wei*, Ming-Wang Shao, Jia-Shan Gu School of Chemistry and Materials Science, Anhui Normal University, Bejing East Road 1, Wuhu 241000, People’s Republic of China Received 26 March 2003; accepted 28 January 2004 Communicated by M.S. Goorsky

Abstract The filling of carbon nanotubes with zinc oxide via wet chemical techniques is presented. Crystalline zinc oxide was introduced into carbon nanotubes by immersing empty and dried opened tubes in zinc nitrate solution followed by filteration and calcination, which was characterized by means of energy-dispersive X-ray spectroscopy and transmission electron microscopy (TEM). After burning off the carbon nanotubes at 750 C in air for 2 h, crystalline zinc oxide nanorods, with diameters in the range of ca. 20–40 nm and length of up to 1 mm, were formed, and characterized by powder X-ray diffraction (XRD), TEM and HRTEM. XRD indicated that the nanorods were hexagonal ZnO. r 2004 Elsevier B.V. All rights reserved. Keywords: B1.Fullerenes; B1.Nanomaterials; B1.Zinc compounds

1. Introduction One-dimensional nanoscale semiconductor materials are currently the focus of considerable interest because of their great potential for fundamental studies of the roles of dimensionality and size in their physical properties as well as for application in optoelectronic nanodevices and functional materials [1,2]. The ZnO nanostructure is one of the most promising materials

*Corresponding author. Tel.: +00865533869303; fax: +00865533869303. E-mail address: [email protected], [email protected] (X.-W. Wei).

for optoelectronic application due to its wide band gap of 3.37 eV and large exciton binding energy of 60 meV [3–5]. ZnO has also great potential in applications in solar cell [6], sensors [7,8], photocatalysis [9], nanolaser [10,11], etc. There have been various methods, such as MOVPE [12], infrared irradiation [13], thermal evaporation [14], thermal decomposition [15], electrochemical deposition [16], gas reaction [17], catalytic growth [18], catalyst-free CVD [19], CVTC [20–23], solution deposition [24–26] to produce ZnO low-dimensional nanostructures, e.g. nanorods, nanowires and nanobelts [27]. However, there has been limited work on ZnO nanorods produced by templates with nanochannels.

0022-0248/$ - see front matter r 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2004.01.052

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One successful route leading to one-dimensional nanostructures is the template-mediated growth using zeolites, membranes, or nanotubes [28–30]. Carbon nanotubes have been used as templates for preparing zirconia nanotubes [31], metal nanowires [32–36] and carbide- [37], nitride- [28,38], phosphide- [39], and d- and fblock transition metal oxides nanorods [32, 40–44]. In this paper we describe the generation of crystalline ZnO nanorods, with diameters in the range of 20–40 nm and length in the 250–1000 nm range, by templating with carbon nanotubes.

2. Experimental procedure Multi-walled carbon nanotubes (MWNTs) were prepared by the thermal catalytic decomposition of hydrocarbon [45]. The procedure employed by us for preparing ZnO nanorods was as follows. In a typical synthesis, the multiwalled carbon nanotubes (0.5 g) were treated with boiling HNO3 (68%, 100 ml) for 24 h, then washed with water and dried in an oven at 60 C for 24 h. The acid-treated carbon nanotubes (250 mg) were stirred with 50 ml of saturated zinc nitrate solution for 48 h, filtered and washed with water, then dried at 80 C for 11 h, followed by calcination at 500 C for 6 h under argon. The calcined samples were then heated at 750 C in air for 2 h to burn off the carbon nanotubes. X-ray powder diffraction (XRD) was carried out on a Rigaku (Japan) D/max-gA X-ray diffractometer with Cu-Ka radiation (l ¼ 0:154178 nm) at a scanning rate of 0.02 s1 in the 2y range from 10 to 70 . Transmission electron microscopy (TEM) micrographs were taken using a Hitachi Model H-800 transmission electron microscope, with an accelerating voltage of 200 kV. High-resolution transmission electron microscopy (HRTEM) was performed using JEOL 2010 microscopes operated at 200 kV. Energydispersive X-ray (EDX) spectrometry was carried out with spectroscope (Oxford, Link ISIS) attached to HRTEM.

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3. Results and discussion 3.1. Filling of MWNTS with ZnO The carbon nanotubes used had an inner diameter in the range 3–15 nm and an outer diameter in the range of 20–50 nm (with lengths of up to a few microns), which were checked by HRTEM. The nanotubes were almost open after being treated with nitric acid for 24 h. Fig. 1a is a typical TEM image of the calcined samples of zinc nitrate-carbon nanotubes, it indicated that carbon nanotubes were partially filled with the zinc material. About 40% of the open nanotubes (B20% of the volume, on average) contained zinc material inside. Some zinc-containing material was observed on the exterior of the nanotubes. Close examination of the zinc material inside the tubes (Fig. 1b) showed lattice fringes with an observed fringe separation ( consistent with the interlayer separation of 2.48 A, of the (1 0 1) crystal plane of zinc oxide (ZnO). It is interesting to note that the (1 0 1) ZnO plane is always aligned at 40–50 to the graphite layer (0 0 2) of the tube. Most of the ZnO crystallites have diameters less than or nearly equal to the internal cross-section of the tubes (B15 nm) and have lengths of 10–30 nm. These crystallites are also found at a considerable distance from the opened ends of the tubes, due possibly to the zinc nitrate solution being sucked into the tubes as they were opened, with crystalline ZnO being formed during calcination. The growth of the ZnO crystallites may be influenced by the surface structure of the inner tube, and the elongated shape of the ZnO crystallites may reflect their surface-wetting properties. The chemical composition of the zinc nanoparticles inside carbon nanotubes was analyzed using the energy-dispersive X-ray spectrometer attached to the high-resolution electron microscope. The EDX spectrum (Fig. 1c) of an individual nanoparticle shows zinc, oxygen, carbon and copper. It is obvious that the copper peak is caused by the copper grid used to clamp the nanoparticles. The carbon comes from carbon nanotube. EDX quantitative microanalysis indicates a 1:1 Zn:O composition within

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Fig. 1. (a) TEM image, (b)HRTEM image and (c) EDX spectrum of carbon nanotubes filled with zinc oxides.

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experimental error and is consistent with stoichiometric ZnO. 3.2. ZnO nanorods

10

20

30

40

50

(103) (200) (112) (201)

(110)

(102)

Intensity

(002)

(100)

(101)

On the removal of the nanotube template, the resulting oxidic species showed the presence of interesting nanostructures. Fig. 2 shows the XRD pattern of a sample of the zinc oxide powders so obtained. The positions of the XRD peaks show good agreement with those of the JCPDS (36– 1451) data of the zinc oxide with hexagonal phase ( c ¼ 5:2066 A). ( The peaks at 2y [46] (a ¼ 3:2498 A,    values of 31.9 , 34.5 , 36.4 , 47.6 , 56.7 , 62.9 , 66.4 , 68.0 and 69.2 correspond to the crystal planes of (1 0 0), (0 0 2), (1 0 1), (1 0 2), (1 1 0), (1 0 3), (2 0 0), (1 1 2), (2 0 1), respectively, of the crystalline zinc oxide. The average size of the assynthesized crystalline ZnO, calculated from the half-width of the (1 0 0) diffraction peak using the Scherrer formula [47], is 27.9 nm. The TEM image of the zinc oxide sample of Fig. 2 is shown in Fig. 3a, which reveals that the zinc oxide powders consist of mainly nanorods. It can be seen that the nanorods have diameters of ca. 20–40 nm and lengths of 250–1000 nm. This result accords with the XRD results for the particle diameter.

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2 (degree) Fig. 2. XRD pattern of a zinc oxide sample produced by removal of carbon nanotubes on calcination at 700 C in air for 2 h.

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An HRTEM image (Fig. 3b) of a part of a nanorod displayed that the distance between parallel lattice fringes is equal to the spacing of ( of ZnO. The selectedthe (1 0 1) planes (2.48 A) area electron diffraction pattern of the nanorod (Fig. 3c) can also be indexed to the reflection of hexagonal ZnO structure. It has been reported that ZnO was produced on the outermost shells of MWNTs in the forms of ultrathin films, quantum dots, or nanowires/ nanorods by heating zinc with MWNTs at various temperatures. Zinc oxide nanowires were grown on the surface of MWNTs without the presence of any catalyst [44]. In our case, the formation of ZnO inside carbon nanotubes was achieved by wet chemistry that is a convenient method, and ZnO nanorods were obtained by burning off the carbon nanotubes. The formation of crystalline nanorods of ZnO in the present study is noteworthy. A possible mechanism [42] of formation of the nanoparticles is that the decomposition of the oxide precursor [Zn(NO3)2] in the hot combustion zone of the nanotubes gives rise to the ZnO crystals in situ. The crystals could become elongated as a result of the evolution of gases (NO2, H2O) during the transformation. Furthermore, the rod-like ZnO nanoparticles are generally larger than the starting nanotube template attributable to merging at high temperature of smaller particles of ZnO into larger ones in order to reduce the surface energy. Another possibility is that the zinc oxides in neighboring carbon nanotubes or in the same nanotube coalesce to form the rod-like nanostructures during the course of the template removal.

4. Conclusions The present study establishes that rod-like nanoparticles of ZnO, having diameters in the range of ca. 20–40 nm and lengths of up to 1 mm, can be prepared readily via wet chemistry by using carbon nanotubes as a template. The method offers certain advantages, such as providing rodlike crystalline nanoparticles in fairly large dimensions and good yields.

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Fig. 3. (a) TEM, (b) HRTEM images and (c) selected area electron diffraction pattern of the zinc oxide sample shown in Fig. 2.

Acknowledgements We thank Prof. Shuyuan Zhang for his assistance with HRTEM, Anhui Provincial Excellent Young Scholars Foundation (No. 04046065) and Natural Science Foundation (No. 00045122), the Education Department (Nos. 2003KJ140, 2001KJ115ZD) of Anhui Province, the State Education Ministry (EYTP, SRF for ROCS) and National Natural Science Foundation (No.

20271002, key project No. 20391001) of PR China for financial support. Especially thanks to Mr. M.Z. Qu, Chengdu Institute of Organic Chemistry of Chinese Academy of Sciences, for generous providing MWNTs samples.

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