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Functionalization of carbon nanotubes with magnetic nanoparticles: general nonaqueous

synthesis and magnetic properties

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2008 Nanotechnology 19 315604

(http://iopscience.iop.org/0957-4484/19/31/315604)

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Nanotechnology 19 (2008) 315604 (6pp) doi:10.1088/0957-4484/19/31/315604

Functionalization of carbon nanotubeswith magnetic nanoparticles: generalnonaqueous synthesis and magneticpropertiesHui Zhang, Ning Du, Ping Wu, Bingdi Chen and Deren Yang1

State Key Lab of Silicon Materials and Department of Materials Science and Engineering,Zhejiang University, Hangzhou 310027, People’s Republic of China

E-mail: mseyang@zju.edu.cn

Received 15 February 2008, in final form 30 May 2008Published 24 June 2008Online at stacks.iop.org/Nano/19/315604

AbstractA novel approach has been developed to synthesize magnetic nanoparticle and carbon nanotube(CNT) core–shell nanostructures, such as CoO/CNTs and Mn3O4/CNTs, by the nonaqueoussolvothermal treatment of metal carbonyl on CNT templates using hexane as the solvent. Themorphological and structural characterizations indicate that numerous cubic CoO or tetragonalMn3O4 nanoparticles are deposited on the surfaces of the CNTs to form CNT-based core–shellnanostructures. It is revealed that the hydrophobic interaction between nanoparticles and CNTsin hexane plays the critical role for the formation of CNT-based core–shell nanostructures. Aphysical property measurement system (PPMS-9, Quantum Design) analysis indicates that theCoO/CNT core–shell nanostructures show weak ferromagnetic performance at 300 K due to theferromagnetic Co clusters and the uncompensated surface spin states, while the Mn3O4/CNTcore–shell nanostructures display ferromagnetic behavior at low temperature (34.5 K), whichtransforms into paramagnetic behavior with increasing temperature.

1. Introduction

Since the discovery of carbon nanotubes (CNTs) in 1991 [1],a great amount of research work has been focused onthem and on related nanomaterials due to their superiormechanical, chemical, and electronic properties, and potentialapplications in field emission emitters, single moleculartransistors, energy storage, and biomedicine [2]. Recently,many efforts have been devoted to CNT-based core–shellnanostructures synthesized by surface modification withmetal and semiconductor nanoparticles through covalent andnoncovalent bonds for novel applications in a wide varietyof areas [3]. In order to preserve nearly all the propertiesof the CNTs, noncovalent functionalization of CNTs hasattracted more attention [4]. For example, Correa-Duarteet al presented layer-by-layer assembly techniques for thedeposition of CdTe nanocrystals onto CNTs [5]. Liu et alreported the synthesis of Fe2O3/CNT core–shell nanostructures

1 Author to whom any correspondence should be addressed.

by a supercritical-fluid approach [6]. Moreover, SnO2/CNTcore–shell nanostructures have also been fabricated by achemical precipitation method [7]. Despite the successfulexamples so far, numerous efforts also need to be employedto focus on the synthesis, fundamental performance, andapplication of CNT-based core–shell nanostructures.

In recent years, magnetic nanoparticles have attracteda tremendous amount of attention due not only to theirfundamental size-dependent magnetic performance, but alsoto their many technological applications such as magneticstorage media, ferrofluids, contrast agents for magneticresonance imaging, and magnetic carriers for drug delivery [8].Therefore, functionalizing CNTs with magnetic nanoparticlescan combine the features of magnetic nanoparticles andCNTs, which may result in novel chemical and physicalproperties, and thus promising applications. However, onlya few reports about magnetic nanoparticle and CNT core–shell nanostructures have been published [9]. Moreover,several disadvantages reported previously, such as the complex

0957-4484/08/315604+06$30.00 © 2008 IOP Publishing Ltd Printed in the UK1

Nanotechnology 19 (2008) 315604 H Zhang et al

Figure 1. Morphological and structural characterizations of the sample prepared by the nonaqueous solvothermal process in a hexane solutionof Co2(CO)8 and CNTs: (a) XRD pattern; (b), (c) TEM images; (d) HRTEM image. (e), (f) TEM images of the CoO/CNT core–shellnanostructures prepared using 0.5 and 1 g Co2(CO)8, respectively.

synthesis process and the formation of free magneticnanoparticles in the solution, also need to be overcome.Herein, we report a novel approach to synthesize CoO/CNTand Mn3O4/CNT core–shell nanostructures by nonaqueoustreatment of metal carbonyl and CNTs using hexane as thesolvent. Furthermore, the magnetic properties of the CoO/CNTand Mn3O4/CNT core–shell nanostructures were studied witha PPMS-9 (Quantum Design) system.

2. Experimental details

All the chemicals were analytical grade. The CNTs werepurchased from Times-nano Nanotech Co., Ltd and were usedwithout further purification. Co2(CO)8 and Mn4(CO)12 werepurchased from Alfa Aesar China (Tianjin) Co., Ltd. In atypical synthesis, 0.25 g Co2(CO)8 and 2 g CNTs were addedto 120 ml hexane under a nitrogen atmosphere; this was thentransferred into Teflon-lined stainless steel autoclaves, sealed,and maintained at 200 ◦C for 20 h. After reaction, the resultingsolid products were centrifuged, washed with distilled waterand ethanol to remove the ions possibly remaining in thefinal products, and finally dried at 60 ◦C for 2 h in air. TheMn3O4/CNT core–shell nanostructures were fabricated by asimilar procedure, except for the replacement of the Co2(CO)8

by Mn4(CO)12.The samples obtained were characterized by x-ray

powder diffraction (XRD) using a Rigaku D/max-ga x-raydiffractometer with graphite monochromated Cu Kα radiation(λ = 1.541 78 A). The morphology and structure of thesamples were examined by transmission electron microscopy

(TEM, JEM 200 CX 160 kV) and high-resolution transmissionelectron microscopy (HRTEM, JEOL JEM-2010). X-rayphotoelectron spectroscopy (XPS) analysis was performedon an AXIS-Ultra instrument from Kratos Analytical, usingmonochromatic Al Kα radiation (225 W, 15 mA, 15 kV)and low-energy electron flooding for charge compensation.Magnetization measurements were carried out using a physicalproperty measurement system (PPMS-9, Quantum Design).

3. Results and discussion

Metal carbonyl is one of the most employed sources forthe synthesis of metal or metal oxide nanoparticles [10].In our previous study, metal atoms were dissociated fromthe metal carbonyl source by a sonolysis technique [11].In this study, a solvothermal technique was employedto induce the decomposition of the metal carbonyl inthe nonaqueous solution. Moreover, the as-synthesizednanoparticles automatically absorbed on the surface of theCNTs by hydrophobic interaction to form CNT-based core–shell nanostructures. Figure 1 shows the morphologicaland structural characterizations of the CoO/CNT core–shellnanostructures prepared by the solvothermal process in ahexane solution of Co2(CO)8 and CNTs. As can be seenfrom the XRD pattern (figure 1(a)), all the diffraction peakscan be indexed as the cubic CoO (JCPDS card No. 78-0431)and CNTs. Figure 1(b) shows a typical TEM image of theCoO/CNT core–shell nanostructures. From this image, it canbe seen that almost all of CNTs are decorated by numerousnanoparticles with diameters of about 10–20 nm, and no

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Figure 2. XPS spectra of the CoO/CNT core–shell nanostructures:(a) survey spectrum; (b) multiplex spectrum.

particles are observed in the sample except at the surfacesof CNTs, which is confirmed by the magnified TEM image(figure 1(c)). Figure 1(d) shows the HRTEM image of anindividual CoO/CNT core–shell nanostructure. The HRTEMimage clearly reveals that there are two kinds of the latticefringe, with lattice spacings of about 0.24 and 0.34 nm,corresponding to the {111} planes of CoO nanoparticles and the{002} planes of CNTs, respectively, which further confirms theformation of CoO/CNT core–shell nanostructures. Moreover,the density of the CoO nanoparticles deposited on the surfaceof the CNTs can be readily tuned by the amount of Co2(CO)8.On increasing the amount of Co2(CO)8 from 0.25 to 0.5 and1 g, an increasing amount of CoO nanoparticles was depositedon the surface of CNTs, as shown in figures 1(e) and (f),respectively.

Figure 2 shows the XPS analysis of the CoO/CNTnanostructures. As can be seen from the survey spectrum(figure 2(a)), the measured elements are cobalt (Co 2s, 2p, 3s,4p), oxygen (O 1s), and carbon (C 1s). The carbon is fromthe CNTs. Cobalt and oxygen are expected from the chemicalcomposition of CoO. Figure 2(b) shows the multiplex spectrumof cobalt. The peak at 778.9 eV is from Co 2p3/2, with ashakeup satellite at 785.0 eV, while the peak at 794.8 eV isinduced by Co 2p1/2, with a satellite peak at 800.8 eV. Thepresence of those two peaks and the highly intense satellitesnear them indicates the presence of Co2+ in the high-spin state,as previously observed [12].

As we know, the decomposition of metal carbonyl canbe induced by several techniques, including thermolysis,sonolysis, photolysis, and chemical assistance [13]. It isbelieved that Co2(CO)8 can also be decomposed into Coatoms under solvothermal conditions in hexane. Moreover,

Figure 3. Morphological and structural characterizations of the sample prepared by the nonaqueous solvothermal process in hexane solutionof Mn4(CO)12 and CNTs: (a) XRD pattern; (b), (c) TEM images; (d) HRTEM image.

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T/K

H/Oe

Figure 4. The M–H curves at 2 and 300 K (a) and ZFC–FC curveswith applied field of 100 Oe (b) of the CoO/CNT core–shellnanostructures.

Co atoms are oxidized into CoO because of the oxygendissolved in the solution from the surrounding ambient air.Due to the hydrophobic surface of the CoO nanoparticles,the as-synthesized CoO nanoparticles automatically absorbon the surface of CNTs by hydrophobic interaction to formCoO/CNT core–shell nanostructures [14]. Although the exactmechanism for the formation of the CoO/CNT core–shellnanostructures should be further investigated, the approachpresented here can be extended to fabricate other oxide/CNTcore–shell nanostructures, such as Mn3O4/CNT core–shellnanostructures. Figure 3 shows the morphological andstructural characterizations of the Mn3O4/CNT core–shellnanostructures prepared by solvothermal decomposition ofMn4(CO)12 using CNTs as the matrix in hexane. The XRDpattern (figure 3(a)), TEM image (figures 3(b) and (c)), andHRTEM image (figure 3(d)) reveal that tetragonal Mn3O4

nanoparticles with size of about 10–20 nm were depositedon the surface of CNTs and that Mn3O4/CNT core–shellnanostructures were formed. These results definitely illustratethe versatility of our approach for the synthesis of CNT-basedcore–shell nanostructures.

The magnetic measurements for the CoO/CNT core–shell nanostructures were performed on a Quantum DesignPPMS magnetometer. Figure 4(a) shows the magnetizationversus applied magnetic field (M–H ) curves at 2 and 300 K.As can be seen, the M–H plots at 2 and 300 K bothdisplay a typical hysteresis loop with different saturation

T/K

H/Oe

Figure 5. The M–H curves at 5 and 300 K (a) and ZFC–FC curveswith applied field of 100 Oe (b) of the Mn3O4/CNT core–shellnanostructures.

magnetization, remanent magnetization, and coercivity. Thetemperature dependence of the magnetization of the sampleis further characterized by the zero-field-cooling (ZFC) andfield-cooling (FC) procedures in an applied magnetic fieldof 100 Oe between 2 and 300 K, as shown in figure 4(b).It is found that the ZFC curve gradually deviates fromthe FC curve with decrease of the temperature at about300 K. Upon further cooling, the ZFC plot exhibits acusp centered at about 6 K, and the FC data sequentiallyincrease. The variable temperature magnetic data clearlyindicate that the CoO/CNT core–shell nanostructures exhibitweak ferromagnetic behavior at room temperature, whichconflicts with the antiferromagnetic performance of bulkCoO [15]. Recently, the weak ferromagnetic behavior ofCoO nanomaterials with the size of several nanometers atlow temperature or room temperature has also been observed,which is attributed to the uncompensated surface spin states orferromagnetic Co clusters [16]. As a result, it is believed thatthe weak ferromagnetic performance of the CoO/CNT core–shell nanostructures comes from the ferromagnetic Co clustersand the uncompensated surface spin states.

Figure 5(a) shows the magnetization versus appliedmagnetic field curves of the Mn3O4/CNT core–shellnanostructures at 5 and 300 K. It is found that theM–H curve exhibits a typical hysteresis loop at 5 Kwith saturation magnetization, remanent magnetization, and

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coercivity values of about 21.35 emu g−1, 10.72 emu g−1,and 5535 Oe, respectively, which are lower than the valuesfor pure Mn3O4 nanoparticles due to the existence ofCNTs [17]. However, the M–H curve shows the linearrelationship at 300 K. Therefore, the magnetization analysisindicates that Mn3O4/CNT core–shell nanostructures exhibitferromagnetic behavior at low temperature, which transformsinto paramagnetic behavior with increasing temperature.The temperature-dependent magnetization of the sample wasfurther characterized by the ZFC and FC measurements in anapplied magnetic field of 100 Oe between 5 and 120 K asshown in figure 5(b). As can be seen, the magnetization inthe ZFC and FC plots first slowly increases, and then sharplyincreases at a certain temperature. However, upon furthercooling, the FC data begin to deviate from the ZFC data. Themagnetization in the ZFC plot reaches the maximum point at34.5 K, which is defined as the blocking temperature (TB).It is lower than that of the bulk Mn3O4 due to the smallsize [18]. Therefore, the ZFC and FC analysis further confirmsthe apparent transition from paramagnetic to ferromagneticbehavior of the Mn3O4/CNT core–shell nanostructures withdecreasing temperature, which is consistent with the previousreport [19].

4. Conclusion

CoO/CNT and Mn3O4/CNT core–shell nanostructures havebeen fabricated by the nonaqueous solution route in a hexanesolution of metal carbonyl and CNTs at 200 ◦C. Thehydrophobic interaction between nanoparticles and CNTs inhexane plays the critical role for the formation of CNT-basedcore–shell nanostructures. Moreover, the CoO/CNT core–shell nanostructures show weak ferromagnetic performanceat 300 K due to the ferromagnetic Co clusters and theuncompensated surface spin states, while the Mn3O4/CNTcore–shell nanostructures display an apparent transition fromparamagnetic to ferromagnetic behavior with decreasingtemperature. It is believed that the magnetic CNT-basedcore–shell nanostructures may show promising applications inbiomedicine, as catalysts, and in batteries.

Acknowledgments

The authors appreciate the financial support from the Programfor Changjiang Scholar and Innovative Team in University,Program 973 (No. 2007CB613403), ZiJin Project, ZhejiangProvincial Natural Science Foundation of China (Y407138),and the Doctoral Program of the Ministry of Education ofChina (No. 20070335014). They thank Professors YouwenWang and Yuewu Zeng for the TEM measurements.

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