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In Situ Synthesis of Iron/Nickel Sulfide Nanostructures-Filled Carbon Nanotubes and Their Electromagnetic and Microwave-Absorbing Properties Qingmei Su,† Jie Li,† Guo Zhong,† Gaohui Du,*,†,‡ and Bingshe Xu‡ †
Zhejiang Key Laboratory for Reactive Chemistry on Solid Surfaces, Institute of Physical Chemistry, Zhejiang Normal University, Jinhua 321004, China ‡ Key Laboratory of Interface Science and Engineering in Advanced Materials, Ministry of Education, Taiyuan University of Technology, Taiyuan 030024, China ABSTRACT: We present a one-step chemical vapor deposition method to prepare iron/nickel sulfide nanostructures-filled carbon nanotubes (CNTs) by pyrolysis of dimethyl sulfide on stainless steel substrate. Dimethyl sulfide is used as the carbon and sulfur source, and the stainless steel substrate acts as both the catalyst and support. The as-grown products were characterized using scanning electron microscopy, transmission electron microscopy, and X-ray diffraction, and the results demonstrate that the encapsulated compounds in CNTs are Fe7S8 and Ni17S18 with hexagonal crystal structure. The possible growth mechanism is proposed. The complex permittivity and permeability of the CNT composites were measured at a frequency range of 2-18 GHz, and the microwave-absorbing performance was analyzed. With a matching thickness of 2 mm, the maximum reflection loss is about -29.58 dB at 14.80 GHz for the absorber. The bandwidth corresponding to the reflection loss at -10 dB is more than 5.58 GHz. The strong reflection loss is mainly caused by the magnetic loss originating from the encapsulated two-component sulfide nanostructures.
1. INTRODUCTION Carbon nanotubes (CNTs) have stimulated intensive research due to their fundamental and remarkable physical properties and many potential applications, e.g., hydrogen storage,1 sensors,2 probes,3 composites,4 microwave adsorption,5 and nanoelectronic application.6 Due to the quantum confinement effect, the CNTs with the filling or coating materials will exhibit different properties. Thus, CNTs filled with various materials are of great interest in materials science and technology, like crystal transition,7 improvement of catalytic activity,8 acquisition of high coercivity,9 and the enhancement of stability.10 So considerable efforts have been made in recent years to modify CNTs either via inserting molecules or nanostructures into the CNTs7 or decorating the CNT walls through coating nanostructures,11 to achieve new functionality by combining the properties of both the CNTs and the inserted/coated materials. Different methods have been developed to achieve the encapsulation of exotic materials within CNTs. Conventional methods are mainly two-step methods involving the opening of the CNTs and depositing the foreign material by impregnation of a molten precursor or precursor solution followed by a subsequent heat treatment. Typically, CNTs are refluxed in concentrated nitric acids containing precursor salts, by which CNTs can be opened and filled by the precursors.12-14 The technology is complex, and sometimes the tube is destroyed. Alternatively, the one-step method r 2011 American Chemical Society
is based on the arc discharge technique and chemical vapor deposition (CVD) method, and shows certain superiority. CVD method has been widely used because of the mild preparation conditions and the potential for mass production. However, the filled nanostructures obtained from the CVD method are generally the catalytic metals (e.g., Fe, Co, and Ni) and their carbide by pyrolysis of the corresponding organometallic compounds (e.g., ferrocene and cobaltocene) as metal sources.15,16 It is still a challenge to produce more types of substances-filled CNT composites, especially the two-component/multicomponent nanostructures-filled CNTs, by a one-step method. The microwave-absorbing materials have attracted much attention and been widely used in commercial and military applications recently because of the increasing environmental pollution of microwave irradiation. CNTs have many practical and potential applications due to their outstanding electromagnetic and mechanical properties.17,18 The high conductivity makes CNTs capable of dissipating electrostatic charges19 or shielding from electromagnetic radiation.20 Particularly, the CNTs decorated with magnetic materials have great potentials in microwaveabsorbing technologies.21,22 Received: November 28, 2010 Revised: December 12, 2010 Published: January 12, 2011 1838
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Figure 1. SEM images of the as-grown CNTs: (a) the stainless steel substrate was not preheated, and (b) the substrate was preheated at 850 °C.
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Figure 2. XRD pattern of the as-grown product.
In this work, we present a one-step CVD method to achieve the filling of iron/nickel sulfide nanostructures within the CNTs. To the best of our knowledge, it is the first time that Ni17S18, Fe7S8 two-component nanostructures are encapsulated in the CNTs. The microstructures of the products have been examined, and their formation mechanism has been proposed. Moreover, the CNT composites exhibit excellent microwave-absorbing performance due to the presence of the filled sulfide nanostructures.
2. EXPERIMENTAL SECTION 2.1. Preparation of Substrate. The stainless steel foil (type 304) was cleaned with ethanol in an ultrasonic bath for 30 min and was etched in hydrochloride acid (35%) solution at room temperature for 10 min. Subsequently, it was preheated at 850 °C in Ar for 30 min to modify its surface structures to be suitable for uniform CNT growth. By means of our experiments we demonstrated the pretreatment plays a key role in growing uniform CNTs. 2.2. Preparation of CNTs. CNTs were synthesized in a conventional thermal CVD system consisting of a horizontal furnace fitted with a quartz tube. The prepared stainless steel substrate was placed directly in the quartz tube at the center of the furnace, parallel to the gas flow. Argon was flowed into the quartz tube with a flow rate of 200 sccm for 20 min to purge it before the CVD furnace was heated up to 1000 °C; 100 sccm of H2 was allowed to bubble through dimethyl sulfide (C2H6S, liquid) and flowed into the quartz tube to initiate the CNT growth, and meanwhile the Ar flow rate was set at 500 sccm. After the reaction for 30 min, H2 was closed and the chamber was cooled down naturally to room temperature with the purge of Ar (200 sccm). 2.3. Characterization. The morphology, density, composition, and microstructures of the products were analyzed using a field emission scanning electron microscope (SEM, Hitachi S-4800), transmission electron microscope (TEM, JEOL 2010F), and powder X-ray diffraction (XRD, Cu KR radiation, Philips PW3040/60). The electromagnetic parameters of the nanostructures-filled CNT composites were measured by the coaxial line method at a microwave frequency range of 2-18 GHz with an N5230A vector network analyzer, and the samples were prepared with a 20 wt % mixing ratio of CNTs/paraffin in a toroidal shape with an external diameter of 7 mm, an internal diameter of 3 mm, and a thickness of 1 mm.
3. RESULTS AND DISCUSSION 3.1. SEM Analysis. Two experiments have been carried out to investigate the effect of the thermal pretreatment of substrate on the CNT growth. Shown in Figure 1a is the SEM image of the
Figure 3. TEM image of the CNTs filled with nanoparticles (a), nanorods (b), and nanowires (c and d).
CNTs obtained from the stainless steel substrate without pretreatment, whereas in Figure 1b, the substrate was preheated at 850 °C for 30 min. It is observed that the CNTs are not uniform in Figure 1a, and there are some impurities mixed in the product. In contrast, the CNTs in Figure 1b seem to be more uniform. These CNTs have smooth walls with a uniform diameter of about 80-100 nm and a length of a few micrometers. Obviously, the quality of the CNTs depends on the pretreatment, which can facilitate the recrystallization process, remove residual carbonaceous materials and impurities, and generate nanoscale active catalytic sites for the growth of the uniform CNTs. 3.2. XRD Analysis. The XRD pattern of the sample is shown in Figure 2. The main diffraction peaks can be assigned to Ni17S18 (JCPDS card no. 76-2306), Fe7S8 (JCPDS card no. 76-2308), and graphite. Some weak diffraction peaks in Figure 2 might derive from a few compounds of metal carbide and cannot be identified owing to the very low content. In our experiments, the synthesis of the filled CNT composites is achieved by the decomposition of C2H6S, which is used in our experiments as the carbon and sulfur source for the in situ growth of sulfide nanostructures-filled CNTs. The results confirm that the sulfidation reaction occurred as expected during the CNT growth. 3.3. TEM Analysis. Figure 3 shows some typical TEM images of the as-grown CNTs and reveals that a few nanostructures are sitting inside the hollow CNTs. The filling is in the form of nanoparticles in Figure 3a and nanorods in Figure 3b. The filled nanorod is 35 nm in diameter and 450 nm in length. In Figure 3c, we can clearly see that the filling segments are discrete with short separation (as indicated with arrowheads), while some CNTs were filled with long nanowires as shown in Figure 3d. We thought that the close segments are possible to form a long nanowire through the fusion at high temperature. The nanowires are several micrometers in length and about 60 nm in diameter, 1839
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Figure 4. HRTEM image (a) of an Fe7S8-filled CNT and its corresponding SAED pattern (b) and EDX spectrum (c); HRTEM image (d) of an Fe7S8/ Ni17S18-filled CNT and its corresponding FFT pattern (e) and EDX spectrum (f).
exhibiting a high aspect ratio. The different forms of nanostructures are resulted from the different growth rates between the CNT and the filled segment. To form long and continuous nanowires, the growth rate of CNTs should be consistent with that of the filled nanowires. If the CNT growth rate is higher, the overgrown CNT part will not be filled; namely, the as-grown CNT will be filled with nanoparticles or nanorods. The growth rates of the CNT and the filled nanostructures are determined by the local vapor concentration; the vapor concentration fluctuation leads to the filling of materials in different forms. It is necessary to note that the vapor concentration variation is inevitable at the moment in experiment when C2H6S vapor is just opened or closed because the concentration in the reaction chamber suffers a variation between none and the designed value. High-resolution TEM (HRTEM) has been used to reveal the microstructures of the CNT composites. A typical HRTEM image of a nanostructure-filled CNT is shown in Figure 4a. The interface between the CNT and nanostructures is clearly shown, and the filled material is crystalline with clear lattice fringes. The selected-area electron diffraction (SAED) pattern obtained from this CNT is shown in Figure 4b, revealing the filled material is Fe7S8 with hexagonal crystal structure. The energy-dispersive X-ray (EDX) spectrum in Figure 4c was recorded from the center of the CNT in Figure 4a, confirming the filled CNT is composed of C, S, and Fe. The Cu signal in the EDX spectrum originated from the copper TEM grid. The above results demonstrate that the CNT in Figure 4a is filled with Fe7S8 nanowire. Another HRTEM image of a CNT is shown in Figure 4d. It can be seen clearly that the filled material in CNT contains two parts, and the corresponding fast Fourier transform (FFT) pattern shown in Figure 4e gives the lattice parameters well-indexed as the hexagonal crystal structure of Ni17S18 and Fe7S8. In other words, this CNT is filled with two substances. The EDX spectrum in Figure 4f recorded from the center of the CNT reveals that Ni and Fe are both present together with C and S. These TEM results demonstrated that the two-component sulfide nanostructures can be encapsulated within the CNTs by a simple, one-step CVD growth process. 3.4. Growth Mechanism. According to the above experimental results and analysis, the growth mechanism of the sulfide nanostructures-filled CNTs could be reasonably proposed as
follows. C2H6S was used as the carbon and sulfur source in our experiments. As C2H6S was introduced into the reactor, it thermally decomposed and released CxHy and H2S, resulting in a coexistence of the sulfidation of metal catalysts and the growth of CNTs. The presence of sulfur might cancel the strong driving force against filling by drastically reducing the surface tension; in other words, the metal sulfide could be easier to enter into the CNTs than the metal by capillary action when the CNTs grew.23 There were plenty of metal nanoparticles in the stainless steel substrate, so a continuous filling could proceed to form long nanowires in the CNTs. Another possible formation mechanism is based on a volume increase induced by the phase transition.7 In the reaction process, metal nanoparticles (e.g., Ni and Fe) were possibly sucked into the CNTs through capillary action at high temperature; subsequently, these nanoparticles reacted with H2S to form metal sulfide. Sun et al. reported that multiwalled CNTs can cause large pressure on their cores, and the internal pressure can reach values higher than 40 GPa.24The large pressure can plastically deform and extrude solid materials that are encapsulated inside the cores. During the CNT growth in the experiments, a high pressure could form between the encapsulated particle and the carbon shell due to the volume expansion of the metal nanoparticles when it underwent the phase transition from metal to metal sulfide. The CNT would act as a robust nanoscale mold to extrude and deform the expanding compound into a nanowire. 3.5. Microwave-Absorbing Performance. To investigate the intrinsic reasons for the microwave-absorbing performance, the complex permittivity and permeability of the as-grown CNTs have been measured and analyzed by a coaxial line method. The reflectivity (R) of the materials with different thicknesses can be calculated from the equations shown below from the electromagnetic parameters:25 Z - 1 in RðdBÞ ¼ 20 lg Zin þ 1 ð1Þ
Zin 1840
1=2 εr 2πfd 1=2 ¼ tanh j ðμr εr Þ c μr
ð2Þ
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Figure 5. Frequency dependence of the permittivity and permeability spectra (a) and the dielectric/magnetic loss tangent (b) of the filled CNTs/ paraffin.
Figure 6. Reflectivity of the as-grown metal sulfide nanostructures-filled CNT composites with the matching thickness of 1 (a), 2 (b), 3 (c), and 4 mm (d), respectively.
where Zin is the normalized input impedance at free space and the material interface, εr = ε0 - jε00 and μr = μ0 - jμ00 are the complex relative permittivity and permeability of the material, d is the thickness of the absorber, and c and f are the velocity of light and the frequency of the microwave in free space, respectively. The impedance matching condition is determined by the combinations of six parameters: ε0 , ε00 , μ0 , μ00 , f, and d, and the R value versus frequency can be evaluated at a specified thickness from the εr and μr. Figure 5a shows the variations of the relative permittivity and permeability of the CNT composites with the frequency. The values of the real part and the imaginary part of the relative permittivity (ε0 and ε00 ) depend on the frequency exhibiting the same trends, which indicates that the CNT composites have a stable dielectric loss. The real and the imaginary parts of the relative permeability (μ0 and μ00 ) of the sample as shown in Figure 5 are almost constant. It is evident that the values of ε0 are much larger than ε00 . The dielectric/magnetic loss tangent can be expressed as tan δE = ε00 /ε0 and tan δM = μ00 /μ0 , respectively. Figure 5b shows frequency dependence of the dielectric/magnetic loss tangent of the CNT composites. It is observed that the value of the dielectric loss is much smaller than that of the magnetic loss in frequencies ranging from 14 to 18 GHz. It suggests that the microwave absorption at the high-frequency region originates mainly from the magnetic loss. The absorbing ability of the materials can be calculated according to the foregoing eqs 1 and 2. Figure 6 shows the reflection loss of the CNT composites with different thicknesses. It can be observed that the absorbing peaks shift toward lower frequency with increasing thickness, and the bandwidths of the reflection loss at -5 dB are 7.70 GHz (curve b), 6.82 GHz (curve c), and 2.92 GHz (curve d), respectively. Double absorbing peaks appear on curves b, c, and d; the CNT composites with
the thickness of 2 mm achieve the maximum absorbing value of -29.58 dB at 14.80 GHz (curve b), and the bandwidth of the reflection loss at -10 dB in curve b is 5.58 GHz. In previous literatures, the microwave absorption of the Fe-filled CNTs was first reported by Che et al., and the maximum reflection loss was found to be 25 dB;21 Zou et al. studied the Ni nanowires-filled CNTs and obtained the maximum reflection loss of -23.1 dB;26 Zhu et al. investigated the microwave absorption of Sn-filled CNTs and found the maximum reflection loss was about -25 dB.27 The maximum reflection loss of our CNT composites is superior to these previous results, and the enhancement can be attributed to the magnetic loss from the two-component sulfide nanostructures within the CNTs.
4. CONCLUSIONS In summary, the iron/nickel sulfide nanostructures-filled CNTs have been synthesized via a one-step CVD method by decomposing C2H6S on stainless steel substrate for the first time. C2H6S is used as the carbon and sulfur source to realize the simultaneous growth of the sulfide nanostructures and the CNTs. TEM results clearly show that the encapsulated Fe7S8 and Ni17S18 nanostructures are single-crystalline, and they are in the forms of nanoparticles, nanorods, or nanowires up to several micrometers in length. The filling of nanostructures within the CNTs is possibly ascribed to the capillary action or the volume expansion due to phase transition. The microwave-absorbing performances of the obtained CNT composites have been investigated in the frequency range of 2-18 GHz. The maximum reflection loss is -29.58 dB for the matching thickness of 2.0 mm, and the bandwidth corresponding to the reflection loss at -10 dB is about 5.58 GHz. The strong reflection loss is caused by the magnetic loss originating from the encapsulated iron/ nickel sulfide nanostructures. With the increasing thickness, the maximum reflection peak shifts to lower frequency. These results indicate that the as-synthesized CNT composites have potential applications in the microwave-absorbing field. ’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected].
’ ACKNOWLEDGMENT This work was supported by the National Science Foundation of China (no. 10904129), the Science and Technology Project of 1841
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Zhejiang Province (no. 2009C31068), and the Natural Science Foundation of Zhejiang Province, China (no. Y4090594).
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