Synthesis of Plait-Like Carbon Nanocoils in Ultrahigh Yield, and Their

Department and Center for Surface Analysis and Research, Hong Kong Baptist UniVersity,. Hong Kong, P. R. China. ReceiVed: February 28, 2008; ReVised ...
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J. Phys. Chem. C 2008, 112, 10061–10067

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Synthesis of Plait-Like Carbon Nanocoils in Ultrahigh Yield, and Their Microwave Absorption Properties Nujiang Tang,*,† Yi Yang,† Kuanjiuh Lin,*,‡ Wei Zhong,*,† Chaktong Au,§ and Youwei Du† National Laboratory of Solid State Microstructures, and Jiangsu ProVincial Laboratory for NanoTechnology Nanjing UniVersity, Nanjing 210093, P. R. China, Department of Chemistry and Center of Nanoscience and Nanotechnology, National Chung-Hsing UniVersity, Taichung 402, Taiwan, Republic of China, and Chemistry Department and Center for Surface Analysis and Research, Hong Kong Baptist UniVersity, Hong Kong, P. R. China ReceiVed: February 28, 2008; ReVised Manuscript ReceiVed: April 6, 2008

Over Ni nanoparticles generated by means of a combined sol-gel/reduction method, crystalline plait-like carbon nanocoils (CNCs) were synthesized in acetylene pyrolysis at 415 °C. The field-emission scanning electron microscope (FE-SEM) and high-resolution transmission electron microscope (HR-TEM) images reveal that there are often two CNCs in opposite handedness fused in one nanoplait. By optimization of reaction parameters, maximum purities and yields of plait-like CNCs and single CNCs were 99.35 wt % and 99.53 wt %, and ca. 18759.8% and 21269.6%, respectively. The pyrolysis of acetylene was carried out at 415 °C, and no dilute gas such as argon and nitrogen was needed. Thus, we have provided a simple, low-cost, and environmentally friendly approach for the mass production of CNCs with ultrahigh purity. The microwave absorption properties of the as-prepared plait-like CNCs and single CNCs were examined systematically. The results demonstrated that the as-prepared plait-like CNCs exhibit good microwave absorbing ability. The effects of the temperatures for acetylene pyrolysis and for NiO powder reduction in the CNCs synthesis on the morphology, yield, and microwave absorption properties of carbon products were also investigated. Introduction Helical structures are common in proteins, RNAs, and biomolecules. Recently, scientists have put in much effort in the study of the morphologies and physical characteristics of carbon nanohelixes, and suggested that the potential applications of carbon nanohelixes are related to their helical structures and unique physical properties.1–11 For example, the materials have been known to display high superelastic property,12 stereospecific magnetoresistance,13 high hydrogen absorption ability,14 and so forth. Furthermore, because of their special electromagnetic (EM) properties due to the helical structures, carbon nanohelixes can be used in micromagnetic sensors, mechanical microsprings or actuators, high elastic electroconductors, good EM wave absorbers, and so forth.15 Unlike the straight carbon nanofibers (CNFs) or nanotube (CNTs) counterparts, CNCs can be categorized as a kind of chiral material, and if an electrical current passes through a CNC, there would be the generation of an inductive magnetic field. Therefore, CNCs have the potential of being applied in the technology of EM nanotransformers or nanoswitches.16 Moreover, in the current synthesis of CNFs and CNTs, metal powders of transition metals such as Fe, Co, and Ni are often used as catalysts. For improvement of catalytic activity, metal oxides such as Al2O3, SiO2, TiO2, and zeolite are often used as supports. In many potential applications, the catalysts and supports are needed to be removed.17 It goes without saying that the purification processes are burdensome and inevitably increase the cost of carbon production. If the conditions for * Address correspondence to: [email protected]. † Nanjing University. ‡ National Chung-Hsing University. § Hong Kong Baptist University.

purification were harsh, there could be damage to carbon production, resulting in structural defects and/or surface modification. As for CNTs synthesis, another problem is the presence of other carbon entities as impurities that are commonly generated at high temperatures (often above 650 °C). Therefore, a high yield but low temperature route involving a support-free catalyst is desirable for low-cost generation of high-purity CNTs. Herein, we report the low-temperature synthesis of crystalline CNCs in ultrahigh yield over a support-free catalyst, and the CNCs display a novel plait-like structure. The pyrolysis of acetylene was conducted over Ni nanoparticles derived from a sol-gel method combined with hydrogen reduction. The purity of the plait-like CNCs was ca. 99.35%. Moreover, the pyrolysis reaction was carried out at a relatively low temperature of 415 °C, and there was no need of using dilute gas such as argon or nitrogen or those of sulfur compounds in the synthetic process. The CNC material has been demonstrated to be a good lightweight absorber for microwave radiation. Experimental Section For the generation of Ni nanoparticles, 0.01 mol NiCl2 · 6H2O and 0.015 mol citric acid monohydrate were mixed well in 100 mL absolute ethanol and stirred for 4 h at 60 °C, followed by a procedure similar to that reported elsewhere.18,19 In brief, with the evaporation of ethanol at 80 °C and heating of the residual at 350 °C in air for 4 h, the xerogel was turned into NiO; then, 0.025 g of the NiO powder was spread on a ceramic plate which was placed inside a quartz reaction tube (4.39 cm in diameter and 45 cm in length, equipped with temperature and gas-flow controls), and the NiO was reduced in H2 at 375 °C for 1 h. The pyrolysis of acetylene was conducted at 415 °C for 0.5 h at atmospheric pressure over the reduced nickel nanoparticles.

10.1021/jp8017293 CCC: $40.75  2008 American Chemical Society Published on Web 06/14/2008

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Figure 1. XRD pattern of as-prepared catalyst precursor powder.

With cooling to room temperature, approximately 3.032 g of “as-prepared” plait-like CNCs could be collected. Thermoanalysis was carried out on a Perkin Elmer TGA7 series Thermal Analysis System under the conditions of 3.9 mg sample heated in air at a rate of 5 °C /min. The phase of the samples was determined by XRD with Cu KR radiation (model D/Max-RA, Rigaku, Japan) at room temperature. Raman spectroscopic investigation was performed using a Jobin-Yvon LABRAM HR800 instrument with 514.5 nm Ar laser excitation. The morphologies of the samples were examined by HR-TEM (model JEOL-2010, Japan) operated at an accelerating voltage of 200 kV, and by FE-SEM (model JSM-6700F, JEOL, Japan) operated at an accelerating voltage of 5 kV. The complex permeability µ (µ ) µ′ - jµ′′) and permittivity  ( ) ′ - j′′) of samples were measured using an Agilent PNA E8363B (USA) network analyzer in the frequency range of 2 to 18 GHz. For microwave measurement, a composite sample containing 35 wt % of CNCs (with paraffin being binder matrix) was pressed into a ring with outer diameter of 7 mm and inner diameter of 3 mm. Results and Discussion Microstructures and Microwave Absorption Properties of As-Prepared Plait-Like CNCs. The XRD pattern of the asprepared nickel oxide powder shows lines corresponding to NiO, and since no signals of other phases are detected, the as-prepared catalyst precursor is single-phase NiO powder (Figure 1). The results also show that the samples are polycrystalline with no preferred grain orientation. By using 0.025 g NiO nanoparticles as catalyst precursor, 3.032 g of CNC material was generated by CVD of acetylene at 415 °C. The yield (weight ratio of carbon to nickel) was ca. 15334%, and the measured Ni content of the sample was ca. 0.65 wt %. The results suggested that the carbon purity of the as-prepared CNCs was high, up to ca. 99.35 wt % without any need of purification. The quantitative data were confirmed by thermogravimetric analysis (TG). As shown in Figure 2a, the 99.03% weight loss of as-prepared CNCs in the temperature range 450-700 °C confirmed the aforementioned percentage of plait-like CNCs (shown in Figure 3). The leftover ca. 0.97 wt % can be attributed to NiO powder. The results indicated that the Ni nanoparticles prepared through the sol-gel method followed by hydrogen reduction are catalytically active. This low-temperature CVD approach is viable for large-scale production of plait-like CNCs. Moreover, the peaks of the X-ray diffraction pattern of as-prepared plait-like CNCs are typical (d(002) ) 0.34 nm; d(101) ) 0.204 nm) of graphite structure, confirming the formation of graphitic carbons in the pyrolysis of acetylene at 415 °C (Figure 2b). Moreover, there is no indication of XRD signals assignable to Ni particles, implying that the particle size of Ni is small and the Ni content in the

Figure 2. (a) Thermogravimetric (TG) curve; (b) powder X-ray diffraction pattern; and (c) Raman spectrum of as-prepared CNCs obtained in acetylene pyrolysis at 415 °C.

as-prepared sample low. In addition, the results of Raman investigation offered further information on the characteristics of the graphite structure. The Raman spectrum (Figure 2c) of as-prepared CNCs exhibits two peaks, one at ca. 1331.6 cm-1 and the other at ca. 1600 cm-1. It is known that crystalline graphites (such as highly oriented pyrolitic graphite) show a characteristic Raman peak at 1580 cm-1 (called G-band), and carbon materials with disordered structures give an intense “defect-induced” band at 1350 cm-1 (called D-band). Over the as-prepared plait-like CNCs, the appearance of the D-band as well as the broad half-width (100.4 cm-1) are evidence for the presence of disorder and/or distortion in the CNCs sample.20 In other words, the existence of defects in the plait-like CNCs is highly likely; it is understandable because the temperature of acetylene pyrolysis was relatively low. Accordingly, we deduced that there is the formation of graphitic and a certain amount of amorphous carbon during the complete decomposition of acetylene. Figure 3a,b shows the FE-SEM images of as-prepared plaitlike CNCs. The FE-SEM images reveal that there are plait-like CNCs, and nozzles of the CNCs are indicated by the white arrows (Figure 3a). One can see that the plait-like structure exhibits two single CNCs of opposite handedness growing alongside each other in a fused manner with almost identical diameter, length, pitch, and coil number. Shown in Figure 3c is a typical HR-TEM image. As shown in Figure 3b,c, the plait is composed of two single CNCs; the left side is left-handed,

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Figure 3. FE-SEM images of plait-like CNCs obtained in acetylene pyrolysis at 415 °C: (a) Low magnification with white arrows indicating nozzles of plait-like CNCs; (b) plait-like CNC with two CNCs fused in opposite handedness; the left side is left-handed, and the right side is right-handed. HR-TEM images of plait-like CNC: (c) low magnification and (d) high magnification.

and the right side is right-handed, giving a discrete plait-like structure. The two single CNCs are coiled in a regular and tight fashion, showing very short coil pitch and short pitch along the fused CNCs. The diameter of a thread of CNC is ca. 20 nm. For the plait-like structure, the diameters of the single coil and plait are ca. 35 and 75 nm, respectively. According to the FESEM images, the length of the plaits is ca. 10 µm. To the best of our knowledge, such a nanoscaled CNC species of plait-like structure has never been reported before. Over microscaled Ni catalysts reported elsewhere, pyrolysis of acetylene was conducted at a relatively high temperature of 750 °C, and the products were amorphous spring-like carbon micro/nanocoils.13 As shown in Figure 3d, despite a pyrolysis temperature of 415 °C (much lower than 750 °C), the CNCs obtained are not amorphous but crystalline. The two fused pieces

of CNCs display graphitic layers that are peripheral and circulate around the tube axes. Apparently, the CNCs crystallize along the direction of growth. At this point of presentation, we would like to emphasize that the synthesis of crystalline plait-like CNCs with matched growth in opposite coiled mode over the Ni nanoparticles prepared by us is unique. The EM properties and EM wave-absorbing characteristic of the as-prepared CNCs were investigated in a frequency range 2-18 GHz. The complex permeability of the composite sample (shown in Figure 4a) shows that the values of the real and imaginary parts of complex permeability are close to 1 and 0, respectively, which can be related to the low content of magnetic Ni nanoparticles in the sample. The complex permittivity of the sample vs frequency (Figure 4b) indicated that ′ decreases with a rise in frequency, and is low in the adopted frequency

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Figure 4. (a-d) Permeability spectra, permittivity spectra, loss tangent spectra, and reflection loss of the composite sample containing 35 wt % of as-prepared plait-like CNCs obtained at the decomposing temperature of 415 °C and paraffin, respectively; all of them were measured in the frequency range of 2 to 18 GHz.

range. The tangent of dielectric and magnetic loss can be expressed as tan δE ) ′′/′ and tan δM ) µ′′/µ′, respectively. Figure 4c shows the frequency dependence of tan δE and tan δM. It is observed that tan δE is much higher than tan δM, suggesting that the magnetic parameters of the composite are very low, and the reflection loss is mainly attributed to electrical loss, especially to the low ′. In other words, the low ′ of the composite plays a key role in the absorption of EM. According to transmission line theory, the reflection coefficient R (dB) of electromagnetic wave (perpendicular incidence) at the surface of a single-layer material backed by a perfect conductor can be defined as21

| |

R ) 20 log

Zin - Z0 Zin + Z0

(1)

where Z0 is the characteristic impedance of free space

Z0 )



µ0 0

(2)

and Zin is the input impedance at free space and material interface

Zin )



µ0µ tanh(j2πft√µ0µε0ε) ε0ε

(3)

(f and t are the frequency of the electromagnetic wave and the thickness of the material, respectively.) The matching frequency (fm) and matching thickness (tm) were calculated using the theory of absorbing wall. Figure 4d shows the reflection loss of EM wave (at different composite thickness) calculated by using the EM parameters of as-prepared plait-like CNCs.

It can be seen that the sample exhibits good EM wave absorbing ability in the high-frequency range. With tm of 1.8 mm, the maximum reflection loss Rmax is ca. -20 dB. The bandwidth corresponding to the reflection loss below -10 dB is more than 4.31 GHz. With increasing thickness, the peak value of reflection loss shifts to lower frequency. Liu et al. 22 studied the EM wave absorbance of SWNTs/SCPU composites with the increase of SWNTs loading from 1 to 25 wt % in the range 2-18 GHz. They found that the EM wave absorbance of the composite with a loading of 5 wt % can reach up to -21.9 dB. But with further increase of the SWNT loading to 25 wt %, the EM wave-absorbing ability decreases. For example, they found that the absorption peak of the composite with a loading of 25 wt % decreased to ca. -4 dB. This result differs significantly from that observed in our sample; our experiments indicate that the plait-like CNCs have good EM wave absorbing ability since the loading is high, up to 35 wt %. The bandwidth of our plait-like CNCs is wider than that of the SWNTs/SCPU composites.22 We envisage that the EM wave absorbance of the as-prepared plait-like CNCs is low at the low-frequency range. Usually, it is unlikely for a single material to afford thin, light, wideband absorbing properties. Nevertheless, it is possible to fabricate composite materials with such characteristics. Indeed, the absorbance of the as-prepared plait-like CNCs can be enhanced by filling or coating them with magnetic nanoparticles. In such a way, a magnetic nanoparticles/plait-like CNCs composite can be excellent in EM absorption. In other words, being strong in absorbing ability and broad in bandwidth, the plait-like CNCs can potentially be a lightweight material for EM absorption, especially when doped with other magnetic absorbing materials.

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Figure 5. FE-SEM image of plait-like CNCs obtained in acetylene pyrolysis at 425 °C over Ni catalyst pre-reduced in H2 at 375 °C.

Effects of Pyrolysis Temperature for Acetylene Decomposition. In this study, the effects of pyrolysis temperature for acetylene decomposition on the morphology, yield, and microwave absorbing characteristics of the carbons were investigated. We found that the pyrolysis temperature has little influence on the morphology but has a profound effect on the yield of carbon products. In this part of investigation, the NiO powder was first heated at 375 °C in H2 for 1 h and then to 425 °C, and the other conditions were kept as before. At a pyrolysis temperature of 425 °C, CNCs production was ca. 3.705 g in each run, a yield of up to ca. 18759.8%, and a purity of up to ca. 99.35 wt%. Compared to the FE-SEM image (Figure 3a) of plait-like CNCs obtained at 415 °C, the carbons obtained at 425 °C (see Figure 5) are also plait-like CNCs of high purity, indicating that a change of pyrolysis temperature from 415 to 425 °C only increases the yield of plait-like CNCs but has little effect on the morphology of the CNCs sample. The complex permeability (Figure 6a) and the complex permittivity (Figure 6b) are similar to those of the sample obtained at pyrolysis temperature of 415 °C. Figure 6c is the reflection loss of EM wave (at different composite thickness) calculated by using the EM parameters of plait-like CNCs obtained at 425 °C, and the sample also shows the good absorbing ability of the EM wave in the high-frequency range. Both the Rmax and the bandwidth are lower than those of the sample obtained at 415 °C, indicating a slight decrease in microwave absorbing ability. Compared to the case of pyrolysis at 415 °C, there is significant increase in yield but a slight decrease in microwave absorbing ability (see Figure 6c). Effects of the Temperature for Ni Catalyst Reduction. We also found that the temperature for NiO powder reduction has a profound influence on the morphology and yield as well as the microwave absorption properties of carbon products. If the temperature for acetylene pyrolysis was 425 °C and the temperature for NiO powder reduction was 400 °C rather than 375 °C (see Experimental Section), and with the other conditions kept as before, the decomposition of acetylene became very fast and was close to completion in 12 min; the carbon yield was ca. 4.198 g in each run. It is clear that the decomposition of acetylene at 425 °C was drastic, and the yield was extremely high, up to ca. 21269.6%. It is worth pointing out that, despite lower pyrolysis temperature, the CNC yield over the Ni nanoparticles at 425 °C is much higher than that over Fe nanoparticles at 450 °C.19 An extremely high yield of carbon nanotubes (ca. 3050%) was reported over FeMoMgO at 900 °C by Jeong et al.23 To the best of our knowledge, a CNC yield as high as 21269.6% is unprecedented. The amount of Ni in

Figure 6. (a) Permeability spectra, (b) permittivity spectra, and (c) reflection loss of the composite containing 35 wt % of as-prepared plaitlike CNCs obtained in acetylene pyrolysis at 425 °C; all of them were measured in the frequency range of 2 to 18 GHz.

the yield was ca. 0.47 wt %, suggesting that the carbon purity of the as-prepared CNCs was up to ca. 99.53 wt %, much higher than that of CNCs collected over Fe reported before.19 As shown in the FE-SEM images (Figure 7a,b), there is a dense population of single helicals among the yield and the amount of plait-like CNCs is low. One can see typical unplaited CNCs (the nozzle of one has been indicated with a white arrow in Figure 7b) similar to those previously reported.19 Based on the results, one can deduce that the rate of acetylene pyrolysis has a determining effect on CNC selectivity and yield. With a rise in reduction temperature of Ni catalyst (from 375 to 400 °C) and the enhanced rate of acetylene pyrolysis, selectivity to single CNCs is high; the consequence is a poor yield of plait-like CNCs.

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Figure 7. (a). FE-SEM images of the CNC sample obtained at the decomposition temperature of 425 °C, and the catalyst precursor of NiO powder was reduced in H2 at 400 °C. (b). FE-SEM image of a typical single CNC; the arrow indicates the nozzle of the single CNC.

Figure 8. (a-d) Permeability spectra, permittivity spectra, loss tangent spectra, and reflection loss of the composite sample containing 35 wt % single CNCs obtained at the decomposing temperature of 425 °C, respectively; all of them were measured in the frequency range of 2 to 18 GHz.

One can appreciate that the growth of carbon species is complex and is subject to subtle changes of Ni nanoparticles. It is apparent that one can optimize the yield of single or plait-like CNCs by regulating the temperature for Ni catalyst reduction and acetylene pyrolysis. The results of this study also reveal that the Ni nanoparticles prepared by the sol-gel method combined with hydrogen reduction are effective for the synthesis of single as well as plait-like CNCs. With the adoption of an appropriate temperature for catalyst reduction or acetylene pyrolysis, the rate of CNC formation and selectivity to plaitlike or single CNCs can be manipulated. Since single CNCs can be synthesized in high yield at 425 °C, the material was also investigated for EM wave absorption. The complex permeability (in the 2-18 GHz frequency range) of the composite with 35 wt % single CNCs is shown in Figure

8a. Because the content of magnetic Ni nanoparticles is low (ca. 0.47 wt %), the values of real and imaginary parts of complex permeability are also close to 1 and 0, respectively. The complex permittivity vs frequency of the composite is shown in Figure 8b. It is interesting to find that the ′′ value decreases with a rise in frequency, and is higher than that of the plait-like composite as shown in Figure 4b; it is apparent that the single CNCs obtained are lower in electric resistivity and better conductivity24,25 than the plait-like CNCs obtained at a pyrolysis temperature of 415 °C. Figure 8c shows that the value of tan δE is much higher than that of Figure 4c, indicating that the dielectric loss of the single CNCs composite is much higher than that of the plait-like composite. From Figure 8d, one can see that the EM wave absorbing ability of the single

Synthesis of Plait-Like CNCs in Ultrahigh Yield CNTs composite is much less than that of the plait-like composite, especially in the high-frequency range (Figure 4d). In general, low conductivity and proper dielectric loss are advantageous for microwave absorption. The high conductivity of single CNCs can be due to two factors. First, the single CNCs synthesized at 425 °C (10 °C higher than that adopted (415 °C) for plait-like CNCs synthesis) are relatively higher in degree of graphitization and hence higher in conductivity. Second, based on the results of FE-SEM studies, we found that the aspect ratio of the single CNCs is bigger than that of the plait-like CNCs (plausibly a result of better yield of the former), and a higher aspect ratio means higher conductivity. In other words, the results herein confirm that the low-temperature route can fabricate ultrahigh yield of plait-like CNCs as well as produce carbon materials of good microwave absorbing ability. Conclusions In summary, we reported a novel structure of plait-like CNCs synthesized with ultrahigh yield via a simple, low-temperature, and environmentally friendly method. By regulating the temperature for Ni catalyst reduction and acetylene pyrolysis, one can control the CNC yield as well as selectivity to plait-like and/or single CNCs. The maximum purities and yields of plaitlike CNCs and single CNCs were 99.35 wt % and 99.53 wt % and ca. 18759.8% and 21269.6%, respectively. Thus, we have reported a simple, low-cost approach for the fabrication of plaitlike and single CNCs. The plait-like CNC material obtained at 415 °C displays good microwave absorbing ability, especially in the high-frequency range. The material can be considered a good lightweight candidate for microwave absorption. Acknowledgment. This work was financially supported by the Jiangsu Natural Science Foundation for Young Innovator (BK2007522),theNationalNaturalScienceFoundation(50602023), the National Major Project of Fundamental Research: Nanomaterials and Nanostructures (2005CB623605), P. R. China, and the National Science Council of Taiwan (NSC96-2627-M005-001).

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