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The effect of high-temperature annealing on the structure and electrical properties of well-aligned carbon nanotubes
Materials Research Bulletin 42 (2007) 474–481 www.elsevier.com/locate/matresbu
The effect of high-temperature annealing on the structure and electrical properties of well-aligned carbon nanotubes Qian-Ming Gong *, Zhi Li, Ye Wang, Bin Wu, Zhengyi Zhang, Ji Liang Key Laboratory for Advanced Manufacturing by Materials Process, Department of Mechanical Engineering, Tsinghua University, Beijing 100084, People’s Republic of China Received 13 November 2005; received in revised form 28 May 2006; accepted 28 June 2006 Available online 13 November 2006
Abstract Systematic work has been performed on the effect of high-temperature annealing on structural defects and impurities of wellaligned carbon nanotubes (ACNTs) in this paper. ACNTs had been prepared by CVD process with ferrocene as catalyst and then the as-grown samples were experienced heat treatment (HT) from 1800 to 3000 8C. X-ray diffraction, Raman spectroscopy and electron dispersive spectroscopy (EDS), etc., have been used to analyze the effect of annealing. Results indicate that some impurities can be removed once annealing temperature exceeds vaporization point of corresponding metal or non-metal. Desorption of O should be attributed to reduced active sites of dangling covalent bonds after heat treatment. Specious discrepancy about interlayer spacing resulted from XRD and Raman tests show that although high-temperature heat treatment can remove in-plane defects of carbon nanotubes greatly, interlayer spacing between graphene shells could not be reduced effectively because of the special concentric cylindrical structure of nanotubes. Electrical resistivity of ACNTs block is about three orders higher than that of copper even after HT at 3000 8C, and the anisotropy of electrical properties increased once experienced heat treatment at increased temperature. # 2006 Elsevier Ltd. All rights reserved. Keywords: A. Nanostructures; B. Electrical properties; C. Raman spectroscopy; C. X-ray diffraction
1. Introduction The discovery of carbon nanotubes (CNTs) [1] and subsequent tested or theoretically calculated properties, such as exceptional mechanical properties (more than 1 TPa for elastic modulus) [2,3], theoretically super high thermal conductivity (up to 6000 W/m K) and special electronic properties [4], make CNTs the best potential candidate for molecular circuit devices or as new re-inforcement for composites. Compared with initially discovered disordered entangled CNTs (look-like carbon black macroscopically), well-aligned carbon nanotubes (ACNTs) aroused more interest for their excellent field emission properties, which make them suitable candidate for flat panel displays (FPD) [5], higher capacity in hydrogen storage and ultra-higher thermal conductivity in axial direction [4,6]. While actually, few persuasive experiments can shorten the gap between tested results and theoretically calculated results no matter for disordered or well-aligned CNTs. The reasons might be ascribed to the following two sides: on the one hand, to date, there lacks any scaleable production technique that produces high-purity single-walled CNTs, and further, most properties have been just predicted theoretically on the basis of quasi-one-dimensional structure of single-walled
* Corresponding author. Tel.: +86 10 62773641; fax: +86 10 62782413. E-mail addresses: [email protected], [email protected] (Q.-M. Gong). 0025-5408/$ – see front matter # 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.materresbull.2006.06.023
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CNTs. On the other hand, structure defects and impurities inside of CNTs, as a more ubiquitous reason, have made their typical properties discounted, and such results have restricted practical applications of CNTs. As for the latter, high-temperature annealing (or graphitization process) would be a feasible resolution. Extensive work has been performed on probing the effect of high-temperature heat treatment on the properties of carbon fibers and carbon/carbon composites [7]. It has been shown that, as for graphitizable carbon materials, structural reordering happened and some properties improved after annealing, such as an increase in the crystallite size, a decrease in the (0 0 2) interplanar spacing, higher thermal conductivity and better frictional properties, etc. Hamwi et al. reported some improvement in the structure and microstructure of a kind of entangled, sinuous nanotubes after heat treatment at 2800 8C, while no change in the interlayer spacing of the materials appeared [8]. Some research work suggested the removal of metal catalyst particles and surface oxygen by heat treatment [9,10]. Andrews et al. [11] presented detailed study on the effects of graphitization on the structural perfection and residual metal catalyst removal of multi-walled CNTs. Kosaka et al. [12] proved that some kinds of defects could be removed after annealing at 2800 8C based on experimental results that there existed difference in electron spin resonance measurements for annealed and non-annealed samples. In this work, a systematic study of the effects of graphitization from 1800 to 3000 8C on the structure and electrical properties of ACNTs has been performed. X-ray diffraction (XRD), Raman spectroscopy, scanning electron microscopy (SEM), transmission electron microscopy (TEM) and high-resolution TEM (HRTEM) have been used to characterize the changes of tubular structure and anisotropy of ACNTs.
Fig. 1. SEM morphology (a) and TEM observation (b) of as-grown ACNTs.
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2. Experimental ACNTs were prepared by chemical vapor deposition (CVD) process in tubular furnace. Xylene/ferrocence solution, which was carried in flowing argon and hydrogen, deposited on flat quartz substrate under catalysis of ferrocene at about 780 8C. Well-aligned CNTs that grew perpendicular to the plate surface up to several millimeters long could be readily obtained after desired reaction time. Graphitization of ACNTs samples were carried out at 1800, 2100, 2300 and 2500 8C for 2 h in a graphite-resistance furnace, while graphitization at 3000 8C only lasted 15 min in case of serious sublimation of carbon materials at that point. Vacuumizing was ahead of heating in a high-purity argon atmosphere. Morphology and microstructure of as-produced and annealed ACNT samples were characterized by field emission scanning electron microscopy (LEO-1350, 10 kV) and HRTEM (JEOL JEM 2010F, 200 kV). The interlayer spacing (d0 0 2) and graphitization degree of the as-grown and heat-treated samples were determined by XRD (Philips, Model APD-10, using Cu Ka radiation (0.15406 nm), 40 kV, 40 mA) and Raman spectroscopy, which was taken at room temperature under ambient conditions (Renishaw RM2000 microscopic confocal Raman spectrometer, an Ar-ion laser excitation (633 nm, 5 mW; resolution 1 cm1)). The position and widths of the peaks were calibrated with respect to silicon internal standard. Interlayer spacing (d0 0 2) and graphitization degree were calculated from the Bragg and Scherrer equations and Maire and Merings equation, respectively [13,14]. Electron dispersive spectroscopy (EDS) (LEO-1350, 20 kV) analysis was used to analyze residual iron content and other impurities in as-produced and graphitized ACNT samples. Electrical resistance of ACNTs was measured in both transverse and longitudinal directions by a standard four-point method. 3. Results and discussion 3.1. Morphology of as-produced ACNTs Typical SEM and TEM observations of produced ACNTs are shown in Fig. 1a and b. Averagely, the produced ACNTs are about 30–50 nm in diameter and up to several millimeters in length. Different from curved vapor-grown carbon fibers, as-produced ACNTs are straight and exhibit clear hollow cores. Inside the tubes, black particles could be discovered. Generally, the particles are distributed uniformly and considered to be agglomerated catalyst particles (Fe) [15]. Consequently, quantitative study about variations of microstructures and contents of ACNTs by heat treatment would be valuable considering their potential properties. 3.1.1. XRD and Raman analyses The XRD and Raman results are shown in Fig. 2 and Table 1, respectively. In principle, for carbon materials, the higher the degree of developed graphitic structure, the smaller the interlayer spacing and the larger the crystalline size. Correspondingly, in XRD patterns (Fig. 2a), it is obviously that the diffraction intensity of (0 0 2) peak for annealed samples is much higher than that non-annealed ones. Furthermore, the intensities of (1 0 0) (1 0 1) and (0 0 4) peaks also increased after high-temperature heat treatment for ACNTs, and all these results indicate improved in-plane (cylindrical layer) and interlayer graphitic structure. However, some discrepancy arises between calculated interlayer spacing, graphitization degree and XRD patterns. In other words, compared with non-annealed ACNTs, higher intensity of (0 0 2) peak for annealed samples does not correspond to smaller interlayer spacing (d0 0 2) or higher graphitization degree (Table 1). The negative value graphitization degree of 3000 8C treated samples indicates that even experienced extreme high-temperature heat treatment, the d0 0 2 is rather larger than that of typical graphite. As a result, another test method, i.e., Raman spectroscopy, has been used to analyze the specious contradiction at the same time. Raman characterization is very sensitive to the breakdown in translational symmetry of carbon materials and can provide some detailed quantitative information about the microstructure [16]. Compared with as-produced ACNTs, a stronger and sharper peak appeared at 1580 cm1 for annealed ones (Fig. 2b), which was the high-frequency E2g firstorder mode (G band). In other words, the intensity of G band is determined by the in-plane microstructure of graphitic structure. Besides, another peaks at 1331 cm1 (D band) and 1615 cm1 (D0 band) exhibit stronger intensities for nonannealed samples (the peak at 1615 cm1 incorporated with the peak at 1580 cm1 for non-annealed ones). As for ordinary carbon materials, the D and D0 bands are ascribed to disorder-induced features, such as the finite particle size effect or lattice distortion [17–19]. Since no carbon nanoparticles existed in ACNTs and CNTs are fibers like, such
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Fig. 2. XRD (a) and Raman (b) patterns of as-produced and annealed ACNTs.
defects as dislocations in graphene sheets, tube ends and finite crystalline size should be the origin of the two peaks. Accordingly, the ratio of the intensity of D peak to the intensity of G peak (R value, i.e., the ratio of integral area of corresponding peaks) would be a reasonable index to evaluate the crystal planar domain size of graphite, the higher the 1/R value, the better the developed graphitic structure. Even though there is no pronounced variation in d0 0 2 values between non-annealed and annealed samples, notable increase in 1/R value exhibits, which indicates that with the increase in heat treatment temperature, more defects would be removed and the size of in-plane graphitic structure of CNTs would become larger. Another distinct change about CNTs can be figured out in Fig. 3. A layer of amorphous carbon coating around CNTs disappeared after annealing. Further more, the more clear 0 0 2 layers confirmed better in-plane graphitic structure, which are corresponding to the Raman test results. As a whole, high-temperature treatment could remove inplane defects in some degree while could not decrease d0 0 2 substantially. However, such trend is quite different from heat treatment on ordinary PAN-derived carbon fiber [7]. The contradiction might be explained by the following Table 1 XRD and Ramana data analysis results Samples
2u (degree)
d0 0 2 (nm)
gb (%)
R
1/R
As-produced HTT 1800 8C HTT 2100 8C HTT 2300 8C HTT 2500 8C HTT 3000 8C
25.73 25.74 25.62 25.69 25.64 25.78
3.460 3.458 3.474 3.465 3.472 3.453
23.26 20.93 39.53 29.07 37.21 15.12
1.22 0.56 0.48 0.46 0.23 0.17
0.82 1.79 2.08 2.17 4.35 5.88
a b
XRD and Raman tests were carried out in the direction that is normal to the axes of CNTs. Graphitization degree (%) = (0.344 d0 0 2)/(0.3440 0.3354) 100 [14], dgraphite(0 0 2) = 0.3354 nm and dturbostratic = 0.3440 nm.
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Fig. 3. HRTEM photos of non-annealed (a) and annealed (b) CNTs.
possible reasons, on the one hand, the heat treatment time is too limited for d0 0 2 to contract; on the other hand, the concentric cylindrical structure of CNTs limited the contraction of d0 0 2. Actually, for CNTs, d0 0 2 is determined by the variation of diameters of neighboring layers, whereas any change of diameter would bring about adding or removing a band of carbon atoms, that is, breaking down of the firm hexagonal network of curved sheet should be fulfilled first, which needs rather more activation energy and active sites than the energy needed for the contraction of d0 0 2 for ordinary planer carbon materials. Thus, compared with traditional carbon materials, interlayer spacing of tubular CNTs changes less even experienced high-temperature heat treatment. Table 2 Iron and other impurities in non-annealed and annealed ACNT samples (wt.%) Samples
O
Si
Fe
As-produced HTT 1800 8C HTT 2100 8C HTT 2500 8C HTT 3000 8C
4.20 4.35 4.03 3.64 3.35
0.13 0.14 0.11 0.05 0.00
3.05 0.24 0.07 0.02 0.00
Note. All the samples were tested in direction that normal to the axis of CNTs; HTT, heat treatment temperature.
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3.1.2. Effect of heat treatment on impurities in ACNTs The results of EDS analyses, which measured the residual iron contents and other impurities, are shown in Table 2. It is natural for as-grown ACNTs to contain Fe and O elements, while it is not clear about the reason of the existence of silicon. Presumably, it might be ascribed to the tough corrosive solution of ferrocene and xylene for it is not possible for carbon to react with quartz at 780 8C. Generally, the element of O could be ascribed to the strong physical adsorption ability of ACNTs, which led to almost constant adsorbed O element. While with increase of heat treatment temperature, the contents of O decreased somewhat especially at 3000 8C. It has been considered unanimously that adsorption properties of graphite are mainly brought about by its dangling covalent bonds at defective sites, thus, the higher the heat treatment temperature, the better the graphitic crystal structure and the less the active sites, consequently, the less the adsorbed O content. As for Si contents, as expected, almost no loss of Si was found below its vaporization temperature at about 2350 8C, whereas once heat treatment temperature exceeded that point (2350 8C), essentially all silicon was removed from ACNTs. At the same time, Fe content decreased dramatically once ACNTs experienced heat treatment at 1800 8C and was almost thoroughly removed after heat treatment at 2500 8C (Fig. 4). Different from previous reports that residual catalyst encapsulated within nanotubes could not be removed by annealing treatment [10], almost all impurities were completely removed by heat treatment in this study. This may be due to open end of the nanotubes prior to annealing or opening closed tips by heat treatment that allows unhindered escape of Fe and Si vapors from ACNTs. 3.1.3. Effect of heat treatment on electrical properties of ACNTs Generally, electrical properties are determined by their structures, especially for single-walled CNTs, semiconductivity or metal character is determined only by their chiralities, while for multi-walled CNTs, most properties
Fig. 4. EDS analyses of as-produced (a) and annealed (b) ACNTs (at 2500 8C).
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Table 3 Electrical resistivitya of as-grown and heat-treated ACNTs Sample ACNTs
a b
HTT (8C) R? Rjj AI(=R?/Rjj) b
1800 8C
As-grown 3
1.10 10 5.86 103 5.33
2500 8C 4
3.40 10 5.03 103 14.79
3000 8C 4
1.21 10 4.68 103 38.68
6.5 105 4.3 103 66.15
Electrical resistivity r = RWH/L (V m); while W, H and L denote length, height and width of ACNTs samples, respectively. Anisotropic index of electrical properties.
are similar to polycrystalline graphite. But for ACNTs, their anisotropic character brought about by their directional growth deserves further attention. Table 3 lists electrical resistivity of as-grown and heat-treated samples of ACNTs. It is obviously that electrical resistivity of ACNTs is much higher than that of metal copper (1.7 108 V m at room temperature), even after heat treatment at 3000 8C, the reason deserves further study. Although CNTs in ACNTs grew independently with each other, they contacted with each other for local curvature of the long tubes. Hence the contacts resulted in channels of electrons and exhibited definite resistivity instead of open circuit or insulation. Different from that of parallel direction, the resistivity in vertical direction almost kept constant after heat treatment, this should be ascribed to the limited and almost constant contacting points in the whole block. Better graphitic structure in axial-parallel direction brought about by heat treatment aforementioned led to better electric conductivity naturally. As a result, with the increase of heat treatment temperature, the ratio of R?/Rjj increased greatly, which means greater anisotropy for ACNTs. As for improvement of the anisotropy or electric conducting mechanism of ACNTs, more work leaves to be carried out and will be explored in the following research, such as densifying the ACNTs block with other matrices or modifying the CNTs themselves, etc. 4. Conclusions Detailed work has been performed on the effect of heat treatment on as-produced ACNTs in this study. CVD process might be a promising method to produce ACNTs in large scale, while heat treatment would be a necessary process to remove impurities and structural defects in ACNTs. Summarily, On the one hand, it has been proved that high-temperature annealing is an effective way to remove residual catalyst (Fe) and other impurities (such as Si and O contents) once heat treatment temperature exceeds vaporization temperature of corresponding contents, besides, the decrease of O contents with increased annealing temperature might be ascribed to the disappearing of active-dangling covalent bonds in defective graphitic materials. On the other hand, specious discrepancy about the effect of annealing has been observed by XRD and Raman tests, respectively, i.e., different from 1/R value by Raman, d0 0 2 almost keep constant with increased annealing temperature, which has been ascribed to the reason that high-temperature treatment can remove in-plane defects greatly while interlayer spacing can not be reduced effectively because of concentric cylindrical structure. Additionally, electrical resistivity of ACNTs block is about three orders higher than that of copper or silver even after HT at 3000 8C. The anisotropic index, i.e., the ratio of resistivity between vertical and parallel directions, increased once the heat treatment temperature increased, which should be ascribed to the improved in-plane structure of CNTs along axial direction after heat treatment and also should be ascribed to limited and almost constant contact points in the vertical direction. Acknowledgements This research was financially supported by the National Science Council of the People’s Republic of China and Postdoctoral Fund of China. The author would like to thank Prof. Qizhong Huang, Prof. Xiang Xiong, Prof. Hongbo Zhang and Mr. Jinlv Zuo of Central South University, and Prof. Zhongyu Xu of Hunan University for their help and instructions in heat treatment of the samples. References [1] S. Iijima, Nature 354 (1991) 56–58. [2] C. Goze, P. Bernier, L. Henrard, L. Vaccarini, E. Hernandez, A. Rubio, Synth. Met. 103 (1999) 2500–2501.
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The effect of high-temperature annealing on the structure and electrical properties of well-aligned carbon nanotubes Qian-Ming Gong *, Zhi Li, Ye Wang, Bin Wu, Zhengyi Zhang, Ji Liang Key Laboratory for Advanced Manufacturing by Materials Process, Department of Mechanical Engineering, Tsinghua University, Beijing 100084, People’s Republic of China Received 13 November 2005; received in revised form 28 May 2006; accepted 28 June 2006 Available online 13 November 2006
Abstract Systematic work has been performed on the effect of high-temperature annealing on structural defects and impurities of wellaligned carbon nanotubes (ACNTs) in this paper. ACNTs had been prepared by CVD process with ferrocene as catalyst and then the as-grown samples were experienced heat treatment (HT) from 1800 to 3000 8C. X-ray diffraction, Raman spectroscopy and electron dispersive spectroscopy (EDS), etc., have been used to analyze the effect of annealing. Results indicate that some impurities can be removed once annealing temperature exceeds vaporization point of corresponding metal or non-metal. Desorption of O should be attributed to reduced active sites of dangling covalent bonds after heat treatment. Specious discrepancy about interlayer spacing resulted from XRD and Raman tests show that although high-temperature heat treatment can remove in-plane defects of carbon nanotubes greatly, interlayer spacing between graphene shells could not be reduced effectively because of the special concentric cylindrical structure of nanotubes. Electrical resistivity of ACNTs block is about three orders higher than that of copper even after HT at 3000 8C, and the anisotropy of electrical properties increased once experienced heat treatment at increased temperature. # 2006 Elsevier Ltd. All rights reserved. Keywords: A. Nanostructures; B. Electrical properties; C. Raman spectroscopy; C. X-ray diffraction
1. Introduction The discovery of carbon nanotubes (CNTs) [1] and subsequent tested or theoretically calculated properties, such as exceptional mechanical properties (more than 1 TPa for elastic modulus) [2,3], theoretically super high thermal conductivity (up to 6000 W/m K) and special electronic properties [4], make CNTs the best potential candidate for molecular circuit devices or as new re-inforcement for composites. Compared with initially discovered disordered entangled CNTs (look-like carbon black macroscopically), well-aligned carbon nanotubes (ACNTs) aroused more interest for their excellent field emission properties, which make them suitable candidate for flat panel displays (FPD) [5], higher capacity in hydrogen storage and ultra-higher thermal conductivity in axial direction [4,6]. While actually, few persuasive experiments can shorten the gap between tested results and theoretically calculated results no matter for disordered or well-aligned CNTs. The reasons might be ascribed to the following two sides: on the one hand, to date, there lacks any scaleable production technique that produces high-purity single-walled CNTs, and further, most properties have been just predicted theoretically on the basis of quasi-one-dimensional structure of single-walled
* Corresponding author. Tel.: +86 10 62773641; fax: +86 10 62782413. E-mail addresses: [email protected], [email protected] (Q.-M. Gong). 0025-5408/$ – see front matter # 2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.materresbull.2006.06.023
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CNTs. On the other hand, structure defects and impurities inside of CNTs, as a more ubiquitous reason, have made their typical properties discounted, and such results have restricted practical applications of CNTs. As for the latter, high-temperature annealing (or graphitization process) would be a feasible resolution. Extensive work has been performed on probing the effect of high-temperature heat treatment on the properties of carbon fibers and carbon/carbon composites [7]. It has been shown that, as for graphitizable carbon materials, structural reordering happened and some properties improved after annealing, such as an increase in the crystallite size, a decrease in the (0 0 2) interplanar spacing, higher thermal conductivity and better frictional properties, etc. Hamwi et al. reported some improvement in the structure and microstructure of a kind of entangled, sinuous nanotubes after heat treatment at 2800 8C, while no change in the interlayer spacing of the materials appeared [8]. Some research work suggested the removal of metal catalyst particles and surface oxygen by heat treatment [9,10]. Andrews et al. [11] presented detailed study on the effects of graphitization on the structural perfection and residual metal catalyst removal of multi-walled CNTs. Kosaka et al. [12] proved that some kinds of defects could be removed after annealing at 2800 8C based on experimental results that there existed difference in electron spin resonance measurements for annealed and non-annealed samples. In this work, a systematic study of the effects of graphitization from 1800 to 3000 8C on the structure and electrical properties of ACNTs has been performed. X-ray diffraction (XRD), Raman spectroscopy, scanning electron microscopy (SEM), transmission electron microscopy (TEM) and high-resolution TEM (HRTEM) have been used to characterize the changes of tubular structure and anisotropy of ACNTs.
Fig. 1. SEM morphology (a) and TEM observation (b) of as-grown ACNTs.
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2. Experimental ACNTs were prepared by chemical vapor deposition (CVD) process in tubular furnace. Xylene/ferrocence solution, which was carried in flowing argon and hydrogen, deposited on flat quartz substrate under catalysis of ferrocene at about 780 8C. Well-aligned CNTs that grew perpendicular to the plate surface up to several millimeters long could be readily obtained after desired reaction time. Graphitization of ACNTs samples were carried out at 1800, 2100, 2300 and 2500 8C for 2 h in a graphite-resistance furnace, while graphitization at 3000 8C only lasted 15 min in case of serious sublimation of carbon materials at that point. Vacuumizing was ahead of heating in a high-purity argon atmosphere. Morphology and microstructure of as-produced and annealed ACNT samples were characterized by field emission scanning electron microscopy (LEO-1350, 10 kV) and HRTEM (JEOL JEM 2010F, 200 kV). The interlayer spacing (d0 0 2) and graphitization degree of the as-grown and heat-treated samples were determined by XRD (Philips, Model APD-10, using Cu Ka radiation (0.15406 nm), 40 kV, 40 mA) and Raman spectroscopy, which was taken at room temperature under ambient conditions (Renishaw RM2000 microscopic confocal Raman spectrometer, an Ar-ion laser excitation (633 nm, 5 mW; resolution 1 cm1)). The position and widths of the peaks were calibrated with respect to silicon internal standard. Interlayer spacing (d0 0 2) and graphitization degree were calculated from the Bragg and Scherrer equations and Maire and Merings equation, respectively [13,14]. Electron dispersive spectroscopy (EDS) (LEO-1350, 20 kV) analysis was used to analyze residual iron content and other impurities in as-produced and graphitized ACNT samples. Electrical resistance of ACNTs was measured in both transverse and longitudinal directions by a standard four-point method. 3. Results and discussion 3.1. Morphology of as-produced ACNTs Typical SEM and TEM observations of produced ACNTs are shown in Fig. 1a and b. Averagely, the produced ACNTs are about 30–50 nm in diameter and up to several millimeters in length. Different from curved vapor-grown carbon fibers, as-produced ACNTs are straight and exhibit clear hollow cores. Inside the tubes, black particles could be discovered. Generally, the particles are distributed uniformly and considered to be agglomerated catalyst particles (Fe) [15]. Consequently, quantitative study about variations of microstructures and contents of ACNTs by heat treatment would be valuable considering their potential properties. 3.1.1. XRD and Raman analyses The XRD and Raman results are shown in Fig. 2 and Table 1, respectively. In principle, for carbon materials, the higher the degree of developed graphitic structure, the smaller the interlayer spacing and the larger the crystalline size. Correspondingly, in XRD patterns (Fig. 2a), it is obviously that the diffraction intensity of (0 0 2) peak for annealed samples is much higher than that non-annealed ones. Furthermore, the intensities of (1 0 0) (1 0 1) and (0 0 4) peaks also increased after high-temperature heat treatment for ACNTs, and all these results indicate improved in-plane (cylindrical layer) and interlayer graphitic structure. However, some discrepancy arises between calculated interlayer spacing, graphitization degree and XRD patterns. In other words, compared with non-annealed ACNTs, higher intensity of (0 0 2) peak for annealed samples does not correspond to smaller interlayer spacing (d0 0 2) or higher graphitization degree (Table 1). The negative value graphitization degree of 3000 8C treated samples indicates that even experienced extreme high-temperature heat treatment, the d0 0 2 is rather larger than that of typical graphite. As a result, another test method, i.e., Raman spectroscopy, has been used to analyze the specious contradiction at the same time. Raman characterization is very sensitive to the breakdown in translational symmetry of carbon materials and can provide some detailed quantitative information about the microstructure [16]. Compared with as-produced ACNTs, a stronger and sharper peak appeared at 1580 cm1 for annealed ones (Fig. 2b), which was the high-frequency E2g firstorder mode (G band). In other words, the intensity of G band is determined by the in-plane microstructure of graphitic structure. Besides, another peaks at 1331 cm1 (D band) and 1615 cm1 (D0 band) exhibit stronger intensities for nonannealed samples (the peak at 1615 cm1 incorporated with the peak at 1580 cm1 for non-annealed ones). As for ordinary carbon materials, the D and D0 bands are ascribed to disorder-induced features, such as the finite particle size effect or lattice distortion [17–19]. Since no carbon nanoparticles existed in ACNTs and CNTs are fibers like, such
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Fig. 2. XRD (a) and Raman (b) patterns of as-produced and annealed ACNTs.
defects as dislocations in graphene sheets, tube ends and finite crystalline size should be the origin of the two peaks. Accordingly, the ratio of the intensity of D peak to the intensity of G peak (R value, i.e., the ratio of integral area of corresponding peaks) would be a reasonable index to evaluate the crystal planar domain size of graphite, the higher the 1/R value, the better the developed graphitic structure. Even though there is no pronounced variation in d0 0 2 values between non-annealed and annealed samples, notable increase in 1/R value exhibits, which indicates that with the increase in heat treatment temperature, more defects would be removed and the size of in-plane graphitic structure of CNTs would become larger. Another distinct change about CNTs can be figured out in Fig. 3. A layer of amorphous carbon coating around CNTs disappeared after annealing. Further more, the more clear 0 0 2 layers confirmed better in-plane graphitic structure, which are corresponding to the Raman test results. As a whole, high-temperature treatment could remove inplane defects in some degree while could not decrease d0 0 2 substantially. However, such trend is quite different from heat treatment on ordinary PAN-derived carbon fiber [7]. The contradiction might be explained by the following Table 1 XRD and Ramana data analysis results Samples
2u (degree)
d0 0 2 (nm)
gb (%)
R
1/R
As-produced HTT 1800 8C HTT 2100 8C HTT 2300 8C HTT 2500 8C HTT 3000 8C
25.73 25.74 25.62 25.69 25.64 25.78
3.460 3.458 3.474 3.465 3.472 3.453
23.26 20.93 39.53 29.07 37.21 15.12
1.22 0.56 0.48 0.46 0.23 0.17
0.82 1.79 2.08 2.17 4.35 5.88
a b
XRD and Raman tests were carried out in the direction that is normal to the axes of CNTs. Graphitization degree (%) = (0.344 d0 0 2)/(0.3440 0.3354) 100 [14], dgraphite(0 0 2) = 0.3354 nm and dturbostratic = 0.3440 nm.
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Fig. 3. HRTEM photos of non-annealed (a) and annealed (b) CNTs.
possible reasons, on the one hand, the heat treatment time is too limited for d0 0 2 to contract; on the other hand, the concentric cylindrical structure of CNTs limited the contraction of d0 0 2. Actually, for CNTs, d0 0 2 is determined by the variation of diameters of neighboring layers, whereas any change of diameter would bring about adding or removing a band of carbon atoms, that is, breaking down of the firm hexagonal network of curved sheet should be fulfilled first, which needs rather more activation energy and active sites than the energy needed for the contraction of d0 0 2 for ordinary planer carbon materials. Thus, compared with traditional carbon materials, interlayer spacing of tubular CNTs changes less even experienced high-temperature heat treatment. Table 2 Iron and other impurities in non-annealed and annealed ACNT samples (wt.%) Samples
O
Si
Fe
As-produced HTT 1800 8C HTT 2100 8C HTT 2500 8C HTT 3000 8C
4.20 4.35 4.03 3.64 3.35
0.13 0.14 0.11 0.05 0.00
3.05 0.24 0.07 0.02 0.00
Note. All the samples were tested in direction that normal to the axis of CNTs; HTT, heat treatment temperature.
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3.1.2. Effect of heat treatment on impurities in ACNTs The results of EDS analyses, which measured the residual iron contents and other impurities, are shown in Table 2. It is natural for as-grown ACNTs to contain Fe and O elements, while it is not clear about the reason of the existence of silicon. Presumably, it might be ascribed to the tough corrosive solution of ferrocene and xylene for it is not possible for carbon to react with quartz at 780 8C. Generally, the element of O could be ascribed to the strong physical adsorption ability of ACNTs, which led to almost constant adsorbed O element. While with increase of heat treatment temperature, the contents of O decreased somewhat especially at 3000 8C. It has been considered unanimously that adsorption properties of graphite are mainly brought about by its dangling covalent bonds at defective sites, thus, the higher the heat treatment temperature, the better the graphitic crystal structure and the less the active sites, consequently, the less the adsorbed O content. As for Si contents, as expected, almost no loss of Si was found below its vaporization temperature at about 2350 8C, whereas once heat treatment temperature exceeded that point (2350 8C), essentially all silicon was removed from ACNTs. At the same time, Fe content decreased dramatically once ACNTs experienced heat treatment at 1800 8C and was almost thoroughly removed after heat treatment at 2500 8C (Fig. 4). Different from previous reports that residual catalyst encapsulated within nanotubes could not be removed by annealing treatment [10], almost all impurities were completely removed by heat treatment in this study. This may be due to open end of the nanotubes prior to annealing or opening closed tips by heat treatment that allows unhindered escape of Fe and Si vapors from ACNTs. 3.1.3. Effect of heat treatment on electrical properties of ACNTs Generally, electrical properties are determined by their structures, especially for single-walled CNTs, semiconductivity or metal character is determined only by their chiralities, while for multi-walled CNTs, most properties
Fig. 4. EDS analyses of as-produced (a) and annealed (b) ACNTs (at 2500 8C).
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Table 3 Electrical resistivitya of as-grown and heat-treated ACNTs Sample ACNTs
a b
HTT (8C) R? Rjj AI(=R?/Rjj) b
1800 8C
As-grown 3
1.10 10 5.86 103 5.33
2500 8C 4
3.40 10 5.03 103 14.79
3000 8C 4
1.21 10 4.68 103 38.68
6.5 105 4.3 103 66.15
Electrical resistivity r = RWH/L (V m); while W, H and L denote length, height and width of ACNTs samples, respectively. Anisotropic index of electrical properties.
are similar to polycrystalline graphite. But for ACNTs, their anisotropic character brought about by their directional growth deserves further attention. Table 3 lists electrical resistivity of as-grown and heat-treated samples of ACNTs. It is obviously that electrical resistivity of ACNTs is much higher than that of metal copper (1.7 108 V m at room temperature), even after heat treatment at 3000 8C, the reason deserves further study. Although CNTs in ACNTs grew independently with each other, they contacted with each other for local curvature of the long tubes. Hence the contacts resulted in channels of electrons and exhibited definite resistivity instead of open circuit or insulation. Different from that of parallel direction, the resistivity in vertical direction almost kept constant after heat treatment, this should be ascribed to the limited and almost constant contacting points in the whole block. Better graphitic structure in axial-parallel direction brought about by heat treatment aforementioned led to better electric conductivity naturally. As a result, with the increase of heat treatment temperature, the ratio of R?/Rjj increased greatly, which means greater anisotropy for ACNTs. As for improvement of the anisotropy or electric conducting mechanism of ACNTs, more work leaves to be carried out and will be explored in the following research, such as densifying the ACNTs block with other matrices or modifying the CNTs themselves, etc. 4. Conclusions Detailed work has been performed on the effect of heat treatment on as-produced ACNTs in this study. CVD process might be a promising method to produce ACNTs in large scale, while heat treatment would be a necessary process to remove impurities and structural defects in ACNTs. Summarily, On the one hand, it has been proved that high-temperature annealing is an effective way to remove residual catalyst (Fe) and other impurities (such as Si and O contents) once heat treatment temperature exceeds vaporization temperature of corresponding contents, besides, the decrease of O contents with increased annealing temperature might be ascribed to the disappearing of active-dangling covalent bonds in defective graphitic materials. On the other hand, specious discrepancy about the effect of annealing has been observed by XRD and Raman tests, respectively, i.e., different from 1/R value by Raman, d0 0 2 almost keep constant with increased annealing temperature, which has been ascribed to the reason that high-temperature treatment can remove in-plane defects greatly while interlayer spacing can not be reduced effectively because of concentric cylindrical structure. Additionally, electrical resistivity of ACNTs block is about three orders higher than that of copper or silver even after HT at 3000 8C. The anisotropic index, i.e., the ratio of resistivity between vertical and parallel directions, increased once the heat treatment temperature increased, which should be ascribed to the improved in-plane structure of CNTs along axial direction after heat treatment and also should be ascribed to limited and almost constant contact points in the vertical direction. Acknowledgements This research was financially supported by the National Science Council of the People’s Republic of China and Postdoctoral Fund of China. The author would like to thank Prof. Qizhong Huang, Prof. Xiang Xiong, Prof. Hongbo Zhang and Mr. Jinlv Zuo of Central South University, and Prof. Zhongyu Xu of Hunan University for their help and instructions in heat treatment of the samples. References [1] S. Iijima, Nature 354 (1991) 56–58. [2] C. Goze, P. Bernier, L. Henrard, L. Vaccarini, E. Hernandez, A. Rubio, Synth. Met. 103 (1999) 2500–2501.
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