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Cite this: Chem. Commun., 2013, 49, 258 Received 18th September 2012, Accepted 9th November 2012
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Magnetic carbon nanostructures: microwave energy-assisted pyrolysis vs. conventional pyrolysis† Jiahua Zhu,a Sameer Pallavkar,a Minjiao Chen,a Narendranath Yerra,ab Zhiping Luo,c Henry A. Colorado,d Hongfei Lin,e Neel Haldolaarachchige,f Airat Khasanov,g Thomas C. Ho,a David P. Young,f Suying Wei*b and Zhanhu Guo*a
DOI: 10.1039/c2cc36810b www.rsc.org/chemcomm
Magnetic carbon nanostructures from microwave assisted- and conventional-pyrolysis processes are compared. Unlike graphitized carbon shells from conventional heating, different carbon shell morphologies including nanotubes, nanoflakes and amorphous carbon were observed. Crystalline iron and cementite were observed in the magnetic core, different from a single cementite phase from the conventional process.
Coating nanomaterials to form a core–shell structure has greatly widened their use in fields such as catalysis, optics, electronics and biomedical drug delivery due to their unique physiochemical properties.1 The major merits of the core–shell design are to (1) protect core material from oxidation or dissolution in harsh environments, typically with a carbon shell;2 (2) engineer the surface to obtain unique physiochemical properties such as catalytic activity,1d,e fluorescence and anti-corrosion,3 and enhanced microwave shielding;4 (3) use surface atoms more efficiently in catalytic reactions and help to retain high catalytic activity on recycling.5 Recently, Wei et al. reviewed the state-ofart of synthesis, property characterizations and applications of multifunctional composite core–shell nanoparticles.1c Among various nanomaterials, 3d transition metals including Fe, Co
a
Integrated Composites Laboratory (ICL), Dan F. Smith Department of Chemical Engineering, Lamar University, Beaumont, TX 77710, USA. E-mail:
[email protected]; Tel: +1-409 880-7654 b Department of Chemistry and Biochemistry, Lamar University, Beaumont, TX 77710, USA. E-mail:
[email protected]; Tel: +1-409 880-7976 c Department of Chemistry and Physics and Southeastern North Carolina Regional Microanalytical and Imaging Consortium, Fayetteville State University, Fayetteville, NC 28301, USA d Department of Materials Science and Engineering, University of California Los Angeles, Los Angeles, CA 90095, USA e Department of Chemical and Materials Engineering, University of Nevada Reno, Reno, NV 89557, USA f Department of Physics and Astronomy, Louisiana State University, Baton Rouge, LA 70803, USA g University of North Carolina at Asheville, Asheville, NC 28804, USA † Electronic supplementary information (ESI) available: Experimental details, GC-MS, SEM, TEM and TGA. See DOI: 10.1039/c2cc36810b
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and Ni nanoparticles are of great interest due to their unique magnetic properties and catalytic activity.6 However, these bare nanoparticles are readily oxidized or even ignite spontaneously upon exposure to air. An inert carbon shell is often introduced to protect the nanoparticles against oxidation. The reported methods to obtain carbon shells include magnetron and ion-beam co-sputtering,7 high temperature annealing,8 catalytic chemical vapour deposition,9 and pyrolysis of organometallic compounds or polymers.2,10 All these methods are either depositing a carbon shell on the particle surface or using heat energy to carbonize carbon precursors. The complex manufacturing process and high cost limit their large-scale yield. Microwaves are electromagnetic waves with frequencies ranging from 300 MHz to 300 GHz. The standard frequency used in various applications is 2450 MHz. Dipolar polarization and conduction losses are the two main heating mechanisms to heat the irradiated material.11 The heating rate for microwave heating is very high based on the fact that the microwaves are absorbed by the semiconductors rather than by the surroundings to generate a large amount of heat. The cooling rate could also be high because the ambient surrounding is not heated during the process. The fast cooling rate during annealing can cause a unique phase segregation yielding novel structures such as Pd and Si clusters from amorphous Pd82Si18 alloys.12 Thus, microwave could be a promising technology to synthesize core–shell nanoparticles with unique morphology and crystalline structure. Here a facile microwave-assisted approach to convert epoxy nanocomposites into core–shell structural magnetic nanoparticles is reported. The experimental setup, microwave analysis and nanoparticle synthetic procedures are detailed in the ESI.† Different carbon shell morphologies including carbon nanotubes, carbon nanoflakes and amorphous carbon shells were observed. Meanwhile, phase segregation and crystallization of the magnetic core were revealed in the products from the microwave pyrolysis process. The carbon shell morphology, and the component and crystalline structure of magnetic core obtained from conventional pyrolysis were also studied for comparison. This journal is
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Fig. 2 EFTEM of M60 (a) zero-loss map, (b) Fe map, (c) C map.
Fig. 1 HRTEM of (a) M20, (b) M40, (c) M60 and (d) C60.
After microwave pyrolysis at H2(5%)/Ar gas flow rates of 20, 40 and 60 cc min 1, the core–shell particle samples are denoted M20, M40 and M60, respectively. Particles after conventional pyrolysis at a gas rate of 60 cc min 1 are denoted C60. Fig. 1 shows the TEM microstructures of the nanoparticle surface after microwave and conventional pyrolysis. M20 is coated by a uniform carbon layer with some carbon residues attached on the surface, Fig. S2(b) (ESI†). The enlarged interfacial area, Fig. 1(a), depicts the single-walled carbon nanotubes (CNTs) grown on the nanoparticle surface, marked by an arrow. Normally, CNT growth requires feed gases, such as hexane and ethanol, in the presence of catalysts.13 Recently, Liu et al. developed a microwave irradiation method to synthesize CNTs from conductive polypyrrole without introducing any feed stock gases.14 The observed CNT growth on the nanoparticle surface is attributed to two continuous steps. First, the oxide layer of the nanoparticles was reduced to pure metal during microwave pyrolysis to serve as highly active catalytic sites. Second, CNT growth was observed on the active sites of nanoparticles in the presence of the microwave-degraded small molecular species, such as dodecane and hexadecane as demonstrated by the gas chromatography/mass spectrometry analysis, refer to ESI† (S2.3). Even though the measured bulk temperature (B400 1C) is relatively low, the nanoparticle surface temperature could reach above 600 1C due to the direct heating on individual nanoparticles for CNT growth.15 With an increase in the gas rate to 40 cc min 1, the nanoparticles are more likely to be wrapped by carbon nanoflakes and assembled to a ‘‘flower’’ like structure, Fig. 1(b). Close to the interface, an amorphous carbon shell could be observed, Fig. S3(b) (ESI†). The M60 comes together with large pieces of carbon residues in the background, Fig. S4 (ESI†). The amorphous carbon shell is about 5 nm thick, Fig. 1(c). With respect to the conventional pyrolysis, the carbon shell shows a graphitized structure with a d-spacing of 3.50 Å, Fig. 1(d).16 The significant difference in carbon shell morphology is due to the unique heating mechanism and rapid cooling rate in microwave pyrolysis. Microwave absorbed by nanoparticles generates a heat flux towards the surroundings that follow This journal is
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a radiation pattern, which allows an unusual phase transition of polymers at the metal/polymer interface. Conventional heating generates heat from outside and transfers it inside the composites, and the heat transfer is limited by the thermal conductivity of the composite itself. Therefore, both heating and cooling rates are relatively slow as compared to the microwave heating, which is beneficial to crystalline structure growth of a carbon shell.17 Energy-filtered TEM (EFTEM) is conducted on M60 to further clarify the elemental distribution of the core–shell structure. The zero loss image (a), Fe map (b), and C map (c) are shown in Fig. 2. The EFTEM mapping provides a 2-dimensional elemental distribution. A brighter area in the elemental map indicates a higher concentration of the corresponding element in that area. Fig. 2(b) depicts the iron map showing the bright iron core with the same particle shape as that in Fig. 2(a). Carbon mapping, Fig. 2(c), shows the dark nanoparticles surrounded by a bright carbon substrate. To precisely identify the specific component and fraction of ¨ssthe magnetic core in each sample, room-temperature Mo bauer spectra are characterized and the results are shown in Fig. 3. M20, M40 and M60 show a combination of two magnetically split sextet patterns, Fig. 3(a–c). In Fig. 3(a), the fitting results show a main component at isomer shift (IS) = 0 mm s 1 and corresponding HI = 330 kOe, which represents a spectral contribution of 20% metallic iron in the magnetically ordered state.18 And the other component at IS = 0.19 mm s 1 and HI = 206 kOe depicts the cementite (Fe3C).19 M40 and M60 show ¨ssbauer spectrum patterns to that of M20, indicating similar Mo the same specific components (metallic iron and cementite) as M20. Moreover, M20, M40 and M60 are analyzed to have an increased metallic iron fraction of 20, 26 and 29%, respectively.
Fig. 3 Room temperature Mo ¨ssbauer spectra of (a) M20, (b) M40, (c) M60 and (d) C60.
Chem. Commun., 2013, 49, 258--260
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Fig. 4 Room temperature magnetic hysteresis loops of C60 and M60.
The corresponding cementite fraction in M20, M40 and M60 decreases gradually from 80 to 74 and 71%. In Fig. 3(d), C60 shows a single cementite phase without any pure metallic iron even at a gas rate of 60 cc min 1. Selected area electron diffraction (SAED) pattern analysis indicates the crystalline phases of Fe3C (115 and 020) and Fe (106) of M20, M40 and M60 (PDF#00-003-0400, PDF#50-1275), while only the Fe3C pattern is observed for C60, Fig. S6 (ESI†). These results demonstrate that both purging gas rate and pyrolysis method play significant roles in phase segregation and crystallization of the magnetic core. The crystalline phase difference from microwave and conventional pyrolysis is primarily due to the tremendous difference in the cooling rate. The microwave process acquires an extremely rapid cooling rate, Fig. S1 (ESI†), which allows partial phase segregation of cementite species into individual carbon and iron. Therefore, both metallic Fe and Fe3C are observed in M20, M40 and M60. In the conventional process, the cooling takes hours, Fe and C tend to form a homogeneous Fe3C phase. The major difference of conventional and microwave pyrolysis processes based on the magnetic epoxy nanocomposites has been illustrated in Scheme S2 (ESI†). In conventional heating, the only applicable external heat flux towards the nanocomposites is a thermal diffusion limited process. Microwave generates a similar heat flux towards the nanocomposites by heating the microwave absorbing SiC foam. At the same time, the metal nanoparticles absorb microwave energy and generate a heat flux towards the composite surface following a radiation pattern, which allows an unusual transformation of both metal core and carbon shell especially at the metal–epoxy interface. Furthermore, M60 exhibits a much higher saturation magnetization of 117.2 emu g 1 than that of C60 (47.6 emu g 1), Fig. 4. The weight fraction of the carbon shell and the magnetic core in each sample can be calculated based on ¨ssbauer spectra results, refer to ESI.† Thus the the magnetic and Mo microwave pyrolysis possesses obviously greater advantages to obtain nanostructures with high magnetization. In conclusion, core–shell structural magnetic nanoparticles with different carbon morphologies have been synthesized using a microwave pyrolysis process. Various carbon morphologies including carbon nanotubes, carbon nanoflakes and amorphous carbon shells were grown on the nanoparticle surface upon gradually increasing the purging gas rate. 260
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Communication The graphitized carbon shell on the nanoparticle surface was observed from a conventional pyrolysis process. And only a pure Fe3C magnetic core was obtained from a conventional pyrolysis process. In addition, as a result of the extremely fast cooling rate of the microwave pyrolysis, phase segregation occurred within the magnetic core to form a mixture of pure iron and cementite. The higher purging gas rate and the larger fraction of iron could be obtained and thus larger magnetization of the core–shell nanoparticles. This process is very general and can be used to produce other carbon coated magnetic nanostructures as well as dielectric semiconductors, which are ongoing and will be detailed later. This work is supported by the Seeded Research Enhancement Grant (REG) of Lamar University and NSF CBET 11-37441 managed by Dr Rosemarie D. Wesson. The support from National Science Foundation with an account number of CMMI 10-30755 managed by Dr Mary Toney to obtain TGA and DSC is appreciated. DPY acknowledges support from the NSF under grant #DMR-1005764.
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