Single-Crystalline Permalloy Nanowires in Carbon Nanotubes

(Invar) were also observed by simultaneously injecting two types of metallocene during the nanotube growth process.14,15. While in situ filling of car...
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J. Phys. Chem. C 2007, 111, 11475-11479

11475

Single-Crystalline Permalloy Nanowires in Carbon Nanotubes: Enhanced Encapsulation and Magnetization Ruitao Lv,† Anyuan Cao,‡ Feiyu Kang,*,† Wenxiang Wang,§ Jinquan Wei,§ Jialin Gu,† Kunlin Wang,§ and Dehai Wu§ Laboratory of AdVanced Materials, Department of Materials Science and Engineering, Tsinghua UniVersity, Beijing, 100084, P. R. China, Department of Mechanical Engineering, UniVersity of Hawaii at Manoa, Honolulu, Hawaii 96822, and Department of Mechanical Engineering and Key Laboratory for AdVanced Manufacturing by Materials Processing Technology of Ministry of Education, Tsinghua UniVersity, Beijing 100084, P. R. China ReceiVed: April 21, 2007; In Final Form: May 29, 2007

We report the use of dichlorobenzene as a carbon source to produce carbon nanotubes with thinner walls and a relatively large cavity, boosting the efficiency of metal encapsulation inside nanotubes. As an example, we show that Permalloy (FeNi) was encapsulated in nanotubes in the form of long nanowires and reached more than 30 wt % of the final product. These Permalloy nanowires are single crystalline, with a typical length of several micrometers and an aspect ratio of >100, and sheathed by several graphene layers. Such Permalloy nanowires show much-enhanced magnetization and microwave absorption properties, compared with Fe or FeNi nanowires produced by the source materials without adding Cl. The improved magnetic properties are due to a more efficient in situ encapsulation of Permalloy and a lesser number of nanotube walls coated on the surface of nanowires.

1. Introduction Permalloys (nickel-iron alloy), discovered by Gustav Elmen in 1913, have a broad range of compositions (20-80 wt % Ni).1 Depending on composition, Permalloys have shown many excellent magnetic properties such as high permeability (>105), high saturation magnetization (e.g., 45 Permalloy), near zero magnetostriction (with minor addition of Cu or Cr), and low thermal expansion (35 Permalloy), with potential applications in recording/writing, communications and electronic equipment, and magnetic shielding.1,2 Synthesis of magnetic nanostructures is crucial for making high-density storage or memory devices.3 In one possible application, Permalloy nanostructures could be used to fabricate nanocomposites that are expected to have high shielding performance as well as improved mechanical strength. There were a few attempts to produce Permalloy nanowires in certain compositions (e.g., Fe26Ni74 and Fe14Ni86) by electrodeposition using porous alumina as the template.4-6 However, such Permalloy nanowires embedded in alumina channels were polycrystalline, and the surface was very rough. Oxidation of nanowires may be a problem after removal of the alumina template, although Co or Ni nanowires could be wrapped by an amorphous carbon layer pre-cast in the channel of the template.7,8 Also, it is unknown whether the distribution of Fe or Ni through the length/diameter of nanowire is uniform or not. A carbon nanotube is an ideal template for housing metal nanowires. Since the catalyst materials used for synthesizing * Corresponding author. Tel.: +86-10-62792618. Fax: +86-1062792911. E-mail: [email protected]. † Department of Materials Science and Engineering, Tsinghua University. ‡ University of Hawaii at Manoa. § Department of Mechanical Engineering, Tsinghua University.

nanotubes are Fe, Co, or Ni, it is possible to fill the inner cavity of nanotubes with these materials in the form of a long wire during the growth process. Metallocene (e.g., ferrocene) was widely used as a catalyst precursor for aligned nanotubes,9,10 and Fe particles or short rods were frequently seen sitting inside hollow nanotubes.11-13 Alloy rods such as FeCo or Fe65Ni35 (Invar) were also observed by simultaneously injecting two types of metallocene during the nanotube growth process.14,15 While in situ filling of carbon nanotubes is a promising approach to nanostructures of magnetic materials, the efficiency of encapsulation of metal into nanotubes of previous reports seems very low. This can be seen from two facts: (1) most of the encapsulated metal exists as particles or short rods (length < 500 nm),11,12,15 and (2) the metal rods are distributed periodically far away from the next one along the nanotube.11 Such a low encapsulation rate limits the production of a large amount of nanowires with a high aspect ratio for practical applications. Here we used dichlorobenzene (C6H4Cl2) as the carbon source and found that the introduction of Cl radicals facilitates the growth of thin wall multiwalled nanotubes (MWNTs) with a relatively large cavity, making it easier for catalyst material to flow into the nanotube due to capillarity. We have obtained a much higher content of single-crystalline FeNi nanowires (>30 wt % of final product) with the average length increased over an order of magnitude, compared with conventional methods using xylene or benzene as the carbon source (without Cl). These Permalloy nanowires show a soft magnetic property, with enhanced saturation magnetization and lower coercive field, that is attributed to the thinner nanotube walls and a higher efficiency of in situ encapsulation.

10.1021/jp0730803 CCC: $37.00 © 2007 American Chemical Society Published on Web 07/12/2007

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Lv et al.

Figure 1. Characterization of FeNi-MWNTs. (a) SEM image of as-grown MWNT mats. (b) Close view of MWNTs. (c) TEM image of MWNTs containing long metal wires (dark contrast) inside tubes. (d) A 3 µm length metal wire encapsulated in a MWNT. (e) XRD characterization showing existence of both R-FeNi and γ-FeNi structures. Other unmarked peaks are from the XRD holder.

2. Experimental Section MWNTs were grown by the chemical vapor deposition (CVD) process described previously,10 but we used a mixture of ferrocene and nickelocene rather than a single catalyst source. About the same amount of ferrocene and nickelocene powders were dissolved in 10 mL of dichlorobenzene to form solutions with concentrations ranging from 0.05 to 0.2 g/mL and were fed into the CVD furnace by a syringe pump at a constant rate of 0.12 mL/min for 30 min. A mixture of Ar and H2 was flowing through the system at 2000 and 300 sccm, respectively. A quartz slide was placed in the middle of furnace to collect MWNTs, at a reaction temperature 860 °C. 3. Results and Discussion Scanning electron microscopy (SEM) images show that asgrown MWNT mats consist of random nanotubes with an average diameter of ∼30 nm and a length up to hundreds of micrometers (Figure 1a,b). Transmission electron microscopy (TEM) observation reveals metal content encapsulated in MWNTs in the form of long continuous wires (Figure 1c,d). A majority of MWNTs are relatively straight, and most of the encapsulated particles have lengths of a few micrometers although we sometimes observed segmented (versus continuous) particles as well (Figure 1c). X-ray diffraction (XRD) characterization shows the (002) reflections from MWNTs at 2θ ) 27° and that FeNi alloy has been formed, with distinct peaks corresponding to two types of structures as R-FeNi (bccstructured) and γ-FeNi (fcc-structured), respectively (Figure 1e). γ-FeNi is the dominating structure in the products as shown in the XRD pattern. The reason for the formation of these two phases is not fully understood now. Such product (FeNi core with nanotube shell) is denoted as FeNi-MWNTs hereafter. There are no pronounced peaks from other structures such as

pure Fe, pure Ni, or FeC3 (cementite). The reason for these two types of FeNi structures formation is not quite clear now, and the further investigation is needed. Our results are distinct from previous in situ filling methods in the following two aspects. First, the filling of MWNTs by FeNi alloy has been substantially improved. Most of the FeNi nanowires have lengths of about 2 µm, and nanowires of >3 µm were frequently observed (Figure S1). We believe that there are much longer nanowires present in our sample, although it is difficult to find the wire ends for measurement from highly entangled MWNTs under TEM. This length of FeNi nanowires has increased nearly 1 order of magnitude compared with Fe or Invar nanorods with lengths of 130

present work present work present work ref 18 refs 13, 14 ref 7 ref 8

can be attributed to the following two aspects: on one hand, the diamagnetic property of MWNTs might screen/weaken the magnetic field produced by the embedded FeNi alloy,16 causing reduced magnetization of FeNi nanowires inside thick nanotube walls compared with those inside thin walls; on the other hand, the larger metal content in the thin-walled nanotubes also can lead to a larger magnetization. The Ms value of our Permalloy nanowires is comparable to Fe1-xNix (x ) 0.25∼0.75) nanoparticles uniformly attached to MWNTs by wet chemistry (40∼120 emu/g),17 and a value of 240 Oe) or Ni nanowires (>130 Oe) inside MWNTs prepared in alumina template.7,8 Note that FeNi nanowires encapsulated in thin wall MWNTs have a much-reduced coercive field compared with those in thick walls (>200 Oe) or FeNi nanoparticles attached to MWNTs (260 Oe).17 As is known, the bulk materials of FeNi possess a lower coercive field than FeCo and pure Fe because FeNi alloys have the weakest magnetic anisotropy.1 So the

Single-Crystalline FeNi Nanowires in C Nanotubes

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composition might be one reason for the decrease of the FeNiCNT coercive field. In addition to the composition, the crystalline texture is also one of the important factors which decide the coercive field of magnetic materials.1 The grain boundaries will restrain the movement of magnetic domains. Therefore, the more grain boundaries, the higher the coercive field will be needed. So, the single-crystalline nature of as-grown FeNi nanowires might be another important reason for their lower coercive field compared with those of the polycrystalline alloy structures.4-6,17 For the applications of microwave absorption, the initial permeability (µi) of absorber should be as high as possible, while the permeability of ferromagnetic materials can be expressed as follows:1

M2s µi ) aK + bλξ

(1)

where Ms is the saturation magnetization, a and b are two constants determined by the material composition, λ is the magnetostriction constant, ξ is an elastic strain parameter of crystal, and K is a constant to decide the direction of easy axis, which can be expressed as

K ) kHcMs

(2)

Here k is a proportion coefficient, and Hc is the coercive field. Substituting eq 1 with eq 2, we thus can get the following equation:

µi )

M2s akHcMs + bλξ

(3)

It can be seen from eq 3 that both higher Ms and lower Hc are favorable to the improvement of the µi value, which will in turn enhance the microwave absorption. Microwave reflection loss (in dB) of Fe- and FeNi-MWNTs in the range of 2∼18 GHz is shown in Figure 4d. The composite samples for testing microwave absorption properties were made by uniformly mixing the FeNi-MWNTs (or Fe-MWNTs) products in a paraffin matrix (transparent to electromagnetic waves) and pressing the mixture into a cylindrical compact. Then the compact was cut into toroidal shaped samples of 7.0 mm outer diameter and 3.0 mm inner diameter with 30 wt % FeNi- or Fe-MWNTs (sample thickness ) 2 mm). The electromagnetic parameters were measured in the range of 2∼18 GHz using an HP 8722ES vector network analyzer. Preliminary results of the reflection loss R (dB) of FeNi-MWNTs (or Fe-MWNTs) composites, according to the transmit-line theory,18 are plotted in Figure 4d. The occurrence of absorption, or minimal reflection of the microwave power, shifted from about 3.2 GHz for FeMWNTs to 7.5 GHz for FeNi-MWNTs, while the latter shows doubled reflection loss at the same loading of MWNTs in the composites. 4. Conclusions In summary, we described a simple way (introducing Cl in the carbon source) to produce Permalloy (FeNi) nanowires in nanotubes with enhanced encapsulation. The Permalloy is single crystalline with a composition of Fe50Ni50, and in principle Permalloy at a range of compositions (e.g., 25-75 wt % Ni) could be synthesized by adjusting the Fe/Ni concentration in the source material. We believe the current encapsulation efficiency (>30 wt %) could be further improved by optimizing

the Cl concentration in the source material and the concentration of catalyst precursor. Such FeNi nanowires, encapsulated in MWNTs with very thin walls, show soft magnetic behavior with enhanced saturation magnetization and microwave absorption properties. Acknowledgment. The authors are grateful for the financial support from the National Natural Science Foundation of China (Grant No. 50572047). This project was also partly supported by the National Center for Nanoscience and Technology, China. We also appreciate the kindly help of Dr. Gan Lin with the HRTEM observation and TGA measurement of Fe-MWNTs. Supporting Information Available: TEM images and EDX pattern of FeNi-MWNTs samples produced with dichlorobenzene can be seen in Figures S1, S2c, and S4. The TEM image of FeNi-MWNTs samples produced with benzene and xylene can be seen in parts a and b of Figure S2, respectively. TGA curves of Fe-MWNTs and FeNi-MWNTs produced with different precursors can be seen in Figure S3. The HRTEM image of a MWNT filled with Fe nanowire produced by xylene can be seen in Figure S5. The volume fraction calculation of FeNi nanowires in CNTs is also included. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Tian, M. Magnetic Materials; Tsinghua University Press: Beijing, 2001, p 68. (2) http://www.aacg.bham.ac.uk/magnetic_materials/soft_magnets.htm. (3) Chou, S. Y.; Wei, M. S.; Krauss, P. R.; Fischer, P. B. J. Appl. Phys. 1994, 76, 6673-6675. (4) Zhu, H.; Yang, S.; Ni, G.; Tang, S.; Du, Y.; J. Phys.: Condens. Matter 2001, 13, 1727-1731. (5) Khan, H. R.; Petrikowski, K. J. Magn. Magn. Mater. 2000, 215216, 526-528. (6) Liu, Q.; Wang, J.; Yan, Z.; Xue, D. Phys. ReV. B. 2005, 72, 144412. (7) Bao, J.; Tie, C.; Xu, Z.; Suo, Z.; Zhou, Q.; Hong, J. AdV. Mater. 2002, 14, 1483-1486. (8) Bao, J.; Zhou, Q.; Hong, J.; Xu, Z. Appl. Phys. Lett. 2002, 81, 4592-4594. (9) Rao, C. N. R.; Sen, R.; Satishkumar, B. C.; Govindaraj, A. Chem. Commun. 1998, 1525-1526. (10) Andrews, R.; Jaeques, D.; Rao, A. M.; Derbyshire, F.; Qian, D.; Fan, X.; Dickey, E. C.; Chen, J. Chem. Phys. Lett. 1999, 303, 467-474. (11) Cao, A.; Zhang, X.; Wei, J.; Li, Y.; Xu, C.; Liang, J.; Wu, D.; Wei, B. J. Phys. Chem. B. 2001, 105, 11937-11940. (12) Zhang, X.; Cao, A.; Wei, B.; Li, Y.; Wei, J.; Xu, C.; Wu, D. Chem. Phys. Lett. 2002, 362, 285-290. (13) Grobert, N.; Hsu, W. K.; Zhu, Y. Q.; Hare, J. P.; Kroto, H. W.; Walton, D. R. M.; Terrones, M.; Terrones, H.; Redlich, Ph.; Ru¨hle, M.; Escudero, R.; Morales, F. Appl. Phys. Lett. 1999, 75, 3363-3365. (14) Elı´as, A. L.; Rodrı´guez, J. A.; McCartney, M. R.; Golberg, D.; Zamudio, A.; Baltazar, S. E.; Lo´pez-Urı´as, F.; Munoz-Sandoval, E.; Gu, L.; Tang, C. C.; Smith, D. J.; Bando, Y.; Terrones, H.; Terrones, M. Nano Lett. 2005, 5, 467-472. (15) Grobert, N.; Mayne, M.; Terrones, M.; Sloan, J.; Dunin-Borkowski, R. E.; Kamalakaran, R.; Seeger, T.; Terrones, H.; Ru¨hle, M.; Walton, D. R. M.; Kroto, H. W.; Hutchison, J. L. Chem. Commun. 2001, 471-472. (16) Heremans, J.; Olk, C. H.; Morelli, D. T. Phys. ReV. B. 1994, 49, 15122-15125. (17) Wu, H. Q.; Cao, Y. J.; Yuan, P. S.; Xu, H. Y.; Wei, X. W. Chem. Phys. Lett. 2005, 406, 148-153. (18) Yusoff, A. N.; Abdullah, M. H.; Mansor, A. A.; Hamid, S. A. A. J. Appl. Phys. 2002, 92, 876-882.