258 Chem. Commun., 2013, 49, 258--260 This journal is c The Royal Society of Chemistry 2013

Cite this: Chem. Commun.,2013, 49, 258

Magnetic carbon nanostructures: microwaveenergy-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

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 greatlywidened their use in fields such as catalysis, optics, electronics andbiomedical drug delivery due to their unique physiochemicalproperties.1 The major merits of the core–shell design are to(1) protect core material from oxidation or dissolution in harshenvironments, typically with a carbon shell;2 (2) engineer thesurface to obtain unique physiochemical properties such ascatalytic activity,1d,e fluorescence and anti-corrosion,3 andenhanced microwave shielding;4 (3) use surface atoms moreefficiently in catalytic reactions and help to retain high catalyticactivity on recycling.5 Recently, Wei et al. reviewed the state-of-art of synthesis, property characterizations and applications ofmultifunctional composite core–shell nanoparticles.1c Amongvarious nanomaterials, 3d transition metals including Fe, Co

and Ni nanoparticles are of great interest due to their uniquemagnetic properties and catalytic activity.6 However, these barenanoparticles are readily oxidized or even ignite spontaneouslyupon exposure to air.

An inert carbon shell is often introduced to protect thenanoparticles against oxidation. The reported methods to obtaincarbon shells include magnetron and ion-beam co-sputtering,7

high temperature annealing,8 catalytic chemical vapour deposi-tion,9 and pyrolysis of organometallic compounds or polymers.2,10

All these methods are either depositing a carbon shell on theparticle surface or using heat energy to carbonize carbon precur-sors. The complex manufacturing process and high cost limit theirlarge-scale yield. Microwaves are electromagnetic waves with fre-quencies ranging from 300 MHz to 300 GHz. The standardfrequency used in various applications is 2450 MHz. Dipolarpolarization and conduction losses are the two main heatingmechanisms to heat the irradiated material.11 The heating ratefor microwave heating is very high based on the fact that themicrowaves are absorbed by the semiconductors rather than by thesurroundings to generate a large amount of heat. The cooling ratecould also be high because the ambient surrounding is not heatedduring the process. The fast cooling rate during annealing cancause a unique phase segregation yielding novel structures such asPd and Si clusters from amorphous Pd82Si18 alloys.12 Thus, micro-wave could be a promising technology to synthesize core–shellnanoparticles with unique morphology and crystalline structure.

Here a facile microwave-assisted approach to convert epoxynanocomposites into core–shell structural magnetic nano-particles is reported. The experimental setup, microwave ana-lysis and nanoparticle synthetic procedures are detailed in theESI.† Different carbon shell morphologies including carbonnanotubes, carbon nanoflakes and amorphous carbon shellswere observed. Meanwhile, phase segregation and crystal-lization of the magnetic core were revealed in the productsfrom the microwave pyrolysis process. The carbon shell morpho-logy, and the component and crystalline structure of magneticcore obtained from conventional pyrolysis were also studied forcomparison.

a Integrated Composites Laboratory (ICL), Dan F. Smith Department of Chemical

Engineering, Lamar University, Beaumont, TX 77710, USA.

E-mail: zhanhu.guo@lamar.edu; Tel: +1-409 880-7654b Department of Chemistry and Biochemistry, Lamar University, Beaumont,

TX 77710, USA. E-mail: suying.wei@lamar.edu; Tel: +1-409 880-7976c Department of Chemistry and Physics and Southeastern North Carolina Regional

Microanalytical and Imaging Consortium, Fayetteville State University,

Fayetteville, NC 28301, USAd Department of Materials Science and Engineering, University of California Los

Angeles, Los Angeles, CA 90095, USAe Department of Chemical and Materials Engineering, University of Nevada Reno,

Reno, NV 89557, USAf Department of Physics and Astronomy, Louisiana State University, Baton Rouge,

LA 70803, USAg 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

Received 18th September 2012,Accepted 9th November 2012

DOI: 10.1039/c2cc36810b

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This journal is c The Royal Society of Chemistry 2013 Chem. Commun., 2013, 49, 258--260 259

After microwave pyrolysis at H2(5%)/Ar gas flow rates of 20, 40and 60 cc min�1, the core–shell particle samples are denoted M20,M40 and M60, respectively. Particles after conventional pyrolysis at agas rate of 60 cc min�1 are denoted C60. Fig. 1 shows the TEMmicrostructures of the nanoparticle surface after microwave andconventional pyrolysis. M20 is coated by a uniform carbon layerwith some carbon residues attached on the surface, Fig. S2(b) (ESI†).The enlarged interfacial area, Fig. 1(a), depicts the single-walledcarbon 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 synthesizeCNTs from conductive polypyrrole without introducing any feedstock gases.14 The observed CNT growth on the nanoparticle surfaceis attributed to two continuous steps. First, the oxide layer of thenanoparticles was reduced to pure metal during microwave pyro-lysis to serve as highly active catalytic sites. Second, CNT growth wasobserved on the active sites of nanoparticles in the presence of themicrowave-degraded small molecular species, such as dodecaneand hexadecane as demonstrated by the gas chromatography/massspectrometry analysis, refer to ESI† (S2.3). Even though the mea-sured bulk temperature (B400 1C) is relatively low, the nanoparticlesurface temperature could reach above 600 1C due to the directheating on individual nanoparticles for CNT growth.15 With anincrease in the gas rate to 40 cc min�1, the nanoparticles are morelikely to be wrapped by carbon nanoflakes and assembled to a‘‘flower’’ like structure, Fig. 1(b). Close to the interface, an amor-phous carbon shell could be observed, Fig. S3(b) (ESI†). The M60comes together with large pieces of carbon residues in the back-ground, Fig. S4 (ESI†). The amorphous carbon shell is about 5 nmthick, Fig. 1(c). With respect to the conventional pyrolysis, thecarbon shell shows a graphitized structure with a d-spacing of3.50 Å, Fig. 1(d).16 The significant difference in carbon shellmorphology is due to the unique heating mechanism and rapidcooling rate in microwave pyrolysis. Microwave absorbed by nano-particles generates a heat flux towards the surroundings that follow

a radiation pattern, which allows an unusual phase transition ofpolymers at the metal/polymer interface. Conventional heatinggenerates heat from outside and transfers it inside the composites,and the heat transfer is limited by the thermal conductivity of thecomposite itself. Therefore, both heating and cooling rates arerelatively slow as compared to the microwave heating, which isbeneficial to crystalline structure growth of a carbon shell.17

Energy-filtered TEM (EFTEM) is conducted on M60 tofurther clarify the elemental distribution of the core–shellstructure. The zero loss image (a), Fe map (b), and C map (c) areshown in Fig. 2. The EFTEM mapping provides a 2-dimensionalelemental distribution. A brighter area in the elemental mapindicates a higher concentration of the corresponding elementin that area. Fig. 2(b) depicts the iron map showing the brightiron core with the same particle shape as that in Fig. 2(a).Carbon mapping, Fig. 2(c), shows the dark nanoparticlessurrounded by a bright carbon substrate.

To precisely identify the specific component and fraction ofthe magnetic core in each sample, room-temperature Moss-bauer spectra are characterized and the results are shown inFig. 3. M20, M40 and M60 show a combination of two magne-tically split sextet patterns, Fig. 3(a–c). In Fig. 3(a), the fittingresults show a main component at isomer shift (IS) = 0 mm s�1

and corresponding HI = 330 kOe, which represents a spectralcontribution of 20% metallic iron in the magnetically orderedstate.18 And the other component at IS = 0.19 mm s�1 and HI =206 kOe depicts the cementite (Fe3C).19 M40 and M60 showsimilar Mossbauer spectrum patterns to that of M20, indicatingthe same specific components (metallic iron and cementite) asM20. Moreover, M20, M40 and M60 are analyzed to have anincreased metallic iron fraction of 20, 26 and 29%, respectively.

Fig. 1 HRTEM of (a) M20, (b) M40, (c) M60 and (d) C60.

Fig. 2 EFTEM of M60 (a) zero-loss map, (b) Fe map, (c) C map.

Fig. 3 Room temperature Mossbauer spectra of (a) M20, (b) M40, (c) M60 and (d) C60.

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260 Chem. Commun., 2013, 49, 258--260 This journal is c The Royal Society of Chemistry 2013

The corresponding cementite fraction in M20, M40 and M60decreases gradually from 80 to 74 and 71%. In Fig. 3(d), C60 showsa single cementite phase without any pure metallic iron even at a gasrate of 60 cc min�1. Selected area electron diffraction (SAED) patternanalysis indicates the crystalline phases of Fe3C (115 and 020) and Fe(106) of M20, M40 and M60 (PDF#00-003-0400, PDF#50-1275), whileonly the Fe3C pattern is observed for C60, Fig. S6 (ESI†). Theseresults demonstrate that both purging gas rate and pyrolysis methodplay significant roles in phase segregation and crystallization of themagnetic core. The crystalline phase difference from microwave andconventional pyrolysis is primarily due to the tremendous differencein the cooling rate. The microwave process acquires an extremelyrapid cooling rate, Fig. S1 (ESI†), which allows partial phasesegregation of cementite species into individual carbon and iron.Therefore, both metallic Fe and Fe3C are observed in M20, M40 andM60. In the conventional process, the cooling takes hours, Fe and Ctend to form a homogeneous Fe3C phase.

The major difference of conventional and microwave pyrolysisprocesses based on the magnetic epoxy nanocomposites has beenillustrated in Scheme S2 (ESI†). In conventional heating, the onlyapplicable external heat flux towards the nanocomposites is athermal diffusion limited process. Microwave generates a similarheat flux towards the nanocomposites by heating the microwaveabsorbing SiC foam. At the same time, the metal nanoparticlesabsorb microwave energy and generate a heat flux towards thecomposite surface following a radiation pattern, which allows anunusual transformation of both metal core and carbon shellespecially at the metal–epoxy interface. Furthermore, M60 exhibitsa much higher saturation magnetization of 117.2 emu g�1 than thatof C60 (47.6 emu g�1), Fig. 4. The weight fraction of the carbon shelland the magnetic core in each sample can be calculated based onthe magnetic and Mossbauer spectra results, refer to ESI.† Thus themicrowave pyrolysis possesses obviously greater advantages toobtain nanostructures with high magnetization.

In conclusion, core–shell structural magnetic nanoparticleswith different carbon morphologies have been synthesizedusing a microwave pyrolysis process. Various carbon morphol-ogies including carbon nanotubes, carbon nanoflakes andamorphous carbon shells were grown on the nanoparticlesurface upon gradually increasing the purging gas rate.

The graphitized carbon shell on the nanoparticle surface wasobserved from a conventional pyrolysis process. And only a pureFe3C magnetic core was obtained from a conventional pyrolysisprocess. In addition, as a result of the extremely fast cooling rate ofthe microwave pyrolysis, phase segregation occurred within themagnetic core to form a mixture of pure iron and cementite. Thehigher purging gas rate and the larger fraction of iron could beobtained and thus larger magnetization of the core–shell nano-particles. This process is very general and can be used to produceother carbon coated magnetic nanostructures as well as dielectricsemiconductors, which are ongoing and will be detailed later.

This work is supported by the Seeded Research Enhance-ment Grant (REG) of Lamar University and NSF CBET 11-37441managed by Dr Rosemarie D. Wesson. The support fromNational Science Foundation with an account number of CMMI10-30755 managed by Dr Mary Toney to obtain TGA and DSC isappreciated. DPY acknowledges support from the NSF undergrant #DMR-1005764.

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Fig. 4 Room temperature magnetic hysteresis loops of C60 and M60.

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