Preparation of Well-aligned CNT Arrays Catalyzed with Porous Anodic Aluminum Oxide Template

Transcription

1 CHIESE JOURAL OF CHEMICAL PHSICS VOLUME 19, UMBER 1 FEBRUAR 27, 2006 ARTICLE Preparation of Well-aligned CT Arrays Catalyzed with Porous Anodic Aluminum Oxide Template Xing Chen, Xing-jiu Huang, Jia-rui Huang, Zhong-ying Huang, Wei-hong Xu, Jin-huai Liu Hefei Institute of Intelligent Machines, Chinese Academy of Sciences, Hefei , China (Dated: Received on January 26, 2005; Accepted on June 17, 2005) Well-aligned open-ended multi-walled carbon nanotube (MWCT) arrays were prepared via chemical vapor deposition (CVD) method in porous anodic aluminum oxide (AAO) templates without depositing any transition metals as catalyst. Effects of the CVD temperature and heat treatment were studied in detail. Well-aligned open-ended MWCT arrays were obtained at the CVD temperature above 600 o C; when CVD temperature is reduced to around 550 o C, CTs, CFs and other structures existed at the same time; no CTs or carbon nanofibres (CFs) could be found as the CVD temperature is below 500 o C, and only amorphous carbon in the porous AAO template was found. Experimental results showed that the AAO template is catalytic during the CVD process, and it has the following two effects: to catalyze thermal decomposition of acetylene and to catalyze conversion of carbon decomposed from acetylene into CTs or CFs. Heat treatment could improve the graphitization degree, but it might also introduce new defects. Key words: Well-aligned CT arrays, AAO template, CVD, Catalysis of alumina I. ITRODUCTIO Carbon nanotubes [1] are among the most promising materials for future nanotechnology because of their unique properties, such as unique electrical properties, chemical inertness and mechanical strength [2, 3]. They have been used for fabrication of gas sensors [4] and field emission flat displays [5]. In particular, wellaligned, open-ended, catalyst-free CT arrays will be urgently needed in practical applications [6]. Though well-aligned CT arrays have been prepared via CVD method on different substrates [7 10] or in different porous templates [2, 11 21] with deposited transition metals as catalyst, they usually contain catalyst particles at their tips. Growth of CTs in porous AAO templates via CVD method can be a promising approach [21 31], for that alumina may play a role as catalyst during the preparation processes [21 26]. In addition, the diameter and density of the CTs can be well controlled by changing the characters of the porous AAO templates [27 31]. However, during the process of CTs growth in the porous AAO templates without depositing any transition metals as catalyst, the effects of the deposition temperature and heat treatment on the characters of the as-grown CTs have not been sufficiently studied. Furthermore, the catalytic mechanism of alumina in the depositing reaction also needs to be further studied. In this letter, we report deposition of carbon in the porous AAO templates under different temperatures via CVD method, and the effects of heat treatment on the characters of the CTs are studied. Finally, the catalytic mechanism of alumina in the deposition reaction is discussed. Author to whom correspondence should be addressed. jhliu@mail.iim.ac.cn II. EXPERIMETAL A. Preparation of porous AAO templates A two-step anodization process was adopted for the preparation of the porous AAO templates [22]. Briefly, the aluminum(99.999%) sheet was annealed at 500 o C in 2 for 4 h in order to obtain homogeneous pores in the template. Before anodization, the sheets was cleaned and electropolished to remove the surface layer. The first anodization was carried out under a constant anodization potential of 40 V in oxalic acid at 10 o C for 6h. The as-prepared AAO film was removed by dipping them into a mixture of phosphoric acid (6% wt) and chromic acid (1.8% wt) at 60 o C for 6 h. The second anodization was carried out under the same condition for 12 h. Finally the underlying aluminum layer was dissolved with saturated SnCl 4 solution, and the barrier layer at the bottom of the porous AAO template was removed by aqueous phosphoric acid (5% wt) [17]. Figure 1 is the top view of the surface of the porous AAO template thus prepared. B. Growth of CTs in porous AAO templates The porous AAO templates were placed into a tube furnace. After the temperature of the furnace increased to a scheduled deposition temperature in a 2 flow of 100 sccm, a mixture of 10% acetylene in 2 was introduced at a flow rate of 100 sccm for 40 min, then part of the samples were annealed at the same temperature in 2 for 10 h prior cooling down to room temperature. The samples were firstly milled to remove the amorphous carbon layers on both sides of the templates. CTs or CFs were released from porous AAO templates by dissolving the templates in 2 mol/l aoh ISS /DOI: /cjcp (1) c 2006 Chinese Physical Society

2 80 Chin. J. Chem. Phys., Vol. 19, o. 1 Xing Chen et al. TABLE I The phenomena at different CVD temperatures T / o C Colors of exhaust gas Colors of templates after reaction 400 Achromatism Brown(a little transparence) 450 Light white fog-like Dark brown 550 White fog-like Jet-black 600 Light milky-yellow Black 700 Milky-yellow Black(no surface reflection of light) solution, and then by washing with deionized water several times. Some of the templates were partially dissolved by aqueous phosphoric acid (5% wt) to get well-aligned CTs arrays. C. Sample characterization Morphology of the CTs was characterized by field emission scanning electron microscope(fei Sirion 200 FEG, American). We used transmission electron microscope (TEM, JEM-200CX, Japanese) to observe their structures. The degree of graphitization of the CTs was characterized by Raman spectra apparatus (RAMALOG-6, American). III. RESULTS AD DISCUSSIO A. CTs or CFs grown in porous AAO templates at different CVD temperatures Table I lists the color of the exhaust gas and templates after reaction at different temperatures. The color of the exhaust gas proceeded from achromatism to milky-yellow and that of the templates after reaction get darker as the CVD temperature increased from 400 to 700 o C. From the experiments, it can also be found that the color of the aluminum around the templates will not change much after reaction if the condition is well controlled, especially when the CVD temperature is below 600 o C. This suggests catalytic effect of alumina to acetylene decomposition. In addition, the rates of acetylene decomposition increased with the CVD temperature. o CTs or CFs could FIG. 1 SEM image of the surface of porous AAO be found even though the templates turned black after the templates were thoroughly dissolved in the 2 mol/l aoh solution when the CVD temperature was below 500 o C. This indicates that even though acetylene decomposed at the temperature below 500 o C and carbon was deposited in the porous AAO templates, no CTs or CFs were formed. Figure 2 shows the SEM images of the CFs or CTs grown in the porous AAO at different CVD temperature with heat treatment. Figure 2(a) is the SEM image of the sample as the CVD temperature is 550 o C, in which CFs grew out of the templates, and they have bamboo-like structures as shown in Figure 3(a). The mechanism of the bamboo-like carbon structures need to be studied in the future. Figure 2(b) and (c) are the top view of the templates and the dispersed sample at the CVD temperature of 600 o C, respectively. In order to get clear SEM image, the top of the template was coated with a thin gold film. It was found that the CTs are open-ended, well-aligned and uniform. The diameter of the CTs grown in the porous AAO templates is about 50 nm on average. Comparing to the porous AAO templates as shown in Figure 1, it is a little larger than that of the porous AAO templates. This indicates that the holes of the porous AAO templates enlarge during the deposition process [20]. As shown in Figure 3(d), CTs have similar structures at the CVD temperature of 700 o C as those at 600 o C. At the same time, it can be found that the surfaces of the CTs deposited at the CVD temperature of 700 o C are a little coarser than those at 600 o C. This might be related to the rate of decomposition of acetylene. As the temperature increases, the rate would be increasing. However, the rate of the formation of CTs didnot increase very much. As shown in Fig.3, the structures of the carbon deposited at different CVD temperatures are quite different from each other. At 550 o C CVD temperature, the deposited carbon showed different structures, such as curved bamboo-like structure which grew out of the templates, hollow CTs and solid CFs. As the CVD temperature increased to 600 o C or even higher, the deposited carbon formed hollow CTs structure. The thickness of the CTs at 600 o C is a little larger than that of the CTs at 700 o C. This indicates that the rate of the decomposition of acetylene and formation of CTs increased with the CVD temperature. The -junction structure in Figure 3(d) is related to the structures of the templates.

3 Chin. J. Chem. Phys., Vol. 19, o. 1 Preparation of CT Arrays with AAO 81 FIG. 2 SEM images of CFs or CTs grown in the porous AAO at different CVD temperatures with heat treatment. (a) 550 o C ; (b) 600 o C, top view; (c) 600 o C, dispersed sample; (d) 700 o C, dispersed sample. B. Effects of heat treatment The degree of graphitization of the CTs is affected by heat treatment at high temperature [30]. In this letter, we adopted heat treatment at the according CVD temperature and at the same time for 10 h. TEM images of CTs grown in the porous AAO at the CVD temperature of 700 o C without heat treatment and with heat treatment are in Figure 4. Figure 4(a) shows the structure of CTS without treatment, which is noncrystalline from the SAED photo at the top right corner. However, CTs grown in the porous AAO templates with heat treatment showed crystallization to some extend as shown in the SAED photo at the top right corner in Figure 4(b). In addition, the thickness of CTs reduces a little after heat treatment. C. Raman spectra of samples obtained under different conditions Hiura et al. had compared the first order Raman spectra of MWCT, of highly-oriented pyrolytic graphite (HOPG) and of glassy carbon etc [32].For MWCTs, the strong peak which occurs in the region of 1580 cm 1 (G-band) can be assigned to one of the two Raman active E2g vibrations of graphite, while the band at around 1350 cm 1 can be attributed to disorder. Raman intensity ratios of the two peaks(id /IG ) and peak bandwidths strongly depend on the crystallinity of the carbon materials. For CTs of high graphitization de- TABLE II D-band, G-band and ID /IG of different samples(unit cm 1 ) T /o C Label D-band G-band ID IG ID /IG gree, the D-band is weak, and sometimes even disappears. The G-band is strong, and the bandwidth of the G-band is narrow. However, the D-band of the CTs prepared via CVD method is often stronger and boarder than their G-band [33]. D-band, G-band and ID /IG of different samples are listed in Table II. When the CVD temperature was above 550 o C, D-band and G-band moved closer to 1350 and 1580 cm 1, respectively, as the CVD temperature increased. In addition, the G-band blue-shifted, and this indicates that the deposited samples have amorphous carbon. As the CVD temperature increased, the ID /IG ratio increased, and this indicates the increase of amorphous carbon and disorder of the samples. The ID /IG ratios of the samples with heat treatment were a little higher than those of the samples without

4 82 Chin. J. Chem. Phys., Vol. 19, o. 1 Xing Chen et al. FIG. 3 TEM images of CFs or CTs grown in the porous AAO of different CVD temperature with heat treatment. (a) 550 o C, (b) 550 o C, (c) 600 o C, (d) 700 o C. FIG. 4 TEM images of CTs grown in the porous AAO at the CVD temperature of 700 o C without or with heat treatment. (a) Without heat treatment, (b) With heat treatment. heat treatment when the CVD temperature was below 550o C. However, the results of the samples at CVD temperatures above 550 o C showed an opposite trend. This indicates that heat treatment could improve the degree of graphitization, and at the same time, heat treatment might introduce new defects, such as nitrogen doping and adhesions of some amorphous carbon to the CTs. In the above discussion, it can be assumed that the alumina may have two roles in CTs or CFs growth in porous AAO templates without deposition of any transition metals as catalyst. The first role is the catalytic effect to the decomposition of acetylene; the second role is as catalyst to the formation of CTs or CFs in the templates. At the CVD temperatures below 550 o C, the first role manipulates the process and the second role almost doesn t exist. However, at CVD temperatures above 550 o C, the two roles co-exist at the same time. In addition, heat treatment can improve the degree of graphitization, but it can also introduce new defects. IV. COCLUSIO We have deposited carbon in the porous AAO templates under different temperatures via CVD method, and the effects of the heat treatment were studied. Wellaligned open-ended MWCT arrays were obtained at CVD temperatures above 600 o C; when the CVD temperature was reduced to around 550 o C, CTs, CFs and other structures existed; no CTs or CFs could be found as the CVD temperatures was below 500 o C, there was only amorphous carbon in the porous AAO

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