MWCNT nanocomposites by microwave irradiation

Materials Research Bulletin 47 (2012) 1–5

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Controlled synthesis and magnetic properties of Co1 xNix/MWCNT nanocomposites by microwave irradiation Huaqiang Wu *, Peipei Cao, Ning Zhang, Li Mao, Mingming Li College of Chemistry and Materials Science, Anhui Key Laboratory of Functional Molecular Solids, Anhui Normal University, Wuhu 241000, PR China

A R T I C L E I N F O

A B S T R A C T

Article history: Received 19 May 2011 Received in revised form 1 August 2011 Accepted 19 October 2011 Available online 25 October 2011

Co1 xNix alloy nanoparticles (x = 0.2, 0.5, 0.6, and 0.8) with the diameter 15–28 nm attached on the surface of multi-walled carbon nanotubes (MWCNTs) were prepared to form Co1 xNix/MWCNT nanocomposites by microwave irradiation. Experimental results demonstrated that Co1 xNix alloy nanoparticles with quasi-spherical and face-centered cubic structure had been attached on the MWCNTs, the composition and size of Co1 xNix alloy nanoparticles could be controlled through adjusting the atomic ratios of metal Co to Ni in the mixed acetate solution, the microwave power and microwave irradiation time, respectively. Both the coercivity and the saturation magnetization of Co1 xNix alloy nanoparticles increased with increasing Co concentration from x = 0.8 to 0.5, and decreased when Co concentration was increased from x = 0.5 to 0.2. These confirm that microwave synthesis is promising for fabricating alloy nanoparticles attached on MWCNTs for magnetic storage and ultra high-density magnetic recording applications. Crown Copyright ß 2011 Published by Elsevier Ltd. All rights reserved.

Keywords: A. Alloys A. Nanostructures C. X-ray diffraction D. Magnetic properties

1. Introduction The study of metal-coated carbon nanotubes has attracted many attentions owing to their unique applications such as support media in heterogeneous catalysis [1], fuel cells [2], and magnetic recording [3], and it can avoid the agglomeration of nanoparticles and thus improve their excellent performances [4]. Magnetic particles (Fe, Co, Ni and their alloys) with sizes in the nanometer range exhibit features that are quite different from those of the corresponding bulk magnets [5] due to the fact that the thermal and magnetic energies are comparable in such small particles, and alloy-based nanoparticles are expected to play a crucial role in tailoring new magnetic properties [6]. As important transition metal alloys, CoNi alloys are interesting for their remarkable magnetic properties, while the composition and size of the nanoparticles are considered as the key characteristics affecting their magnetic properties. Recently, there are many reports about the effects of the composition and size on the magnetic properties of CoNi alloy nanoparticles prepared with different methods, for example, double composite template approach [7], heterogenous nucleation [8], polyol process [9], solvothermal method [10], hydrothermal synthetic route [11], polymer assisted synthesis [12] and surfactant-assisted synthesis [13]. Up to now, there have been many reports on depositing

* Corresponding author. E-mail address: [email protected] (H. Wu).

mono-metal nanoparticles onto multi-walled carbon nanotubes (MWCNTs), such as Fe [14], Co [15], Ni [16], Fe2O3 [17], Fe3O4 [18]. Although there are also some reports about bimetallic systems or alloys attached on MWCNTs [19,20], there are few reports about CoNi alloy nanoparticles attached on MWCNTs. As is well-known, microwave irradiation is an attractive method for the synthesis of nanocrystals due to its unique reaction effects, such as rapid volumetric heating and the consequent dramatic increase in reaction rate [21]. Compared with conventional methods, microwave irradiation method has the advantages of short reaction time, small particle size, and rapid formation of nanoparticles with a narrow size distribution, and no serious agglomeration [22]. In our previous work, we have successfully prepared CoNi/MWCNT nanocomposites by microwave irradiation method and studied the effect of particle size on the magnetic properties of CoNi alloy nanoparticles [23]. We found that microwave irradiation in the deposition of nanoparticles onto MWCNTs is a very useful and powerful method to directly control the size, morphology, structure, and loading of nanoparticles. Except for the particle size, the composition is also an important factor which has great effect on the magnetic properties of CoNi alloy nanoparticles. In this work, we attempted to extend microwave irradiation method to prepare composition-controlled Co1 xNix alloy nanoparticles (x = 0.2, 0.5, 0.6, and 0.8) attached on MWCNTs (Co1 xNix/MWCNTs), and studied the effect of the composition of Co1 xNix alloy nanoparticles on the magnetic properties. The Co1 xNix/MWCNT nanocomposites had been characterized by X-ray powder diffraction (XRD), scanning

0025-5408/$ – see front matter . Crown Copyright ß 2011 Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.materresbull.2011.10.016

Co0.2Ni0.8

Co0.4Ni0.6 Co0.5Ni0.5

2. Experimental The MWCNTs with a purity of about 95% were provided by Shenzhen Nanoport Company, which were produced via catalytic decomposition of hydrocarbon. For the better anchoring of the nanoparticles, the MWCNTs were refluxed in concentrated nitric acid for 4 h, then purified by distilled water, and finally dried at 100 8C for 24 h. All chemicals were of analytical grade and used as received without further purification. In a typical synthesis, 0.0300 g of the acid-treated MWCNTs was mixed with 0.0187 g of Co(Ac)2
4H2O and 0.0187 g of Ni(Ac)2
4H2O (x = 0.5) in ethylene glycol (25 mL). After stirring the mixture for 24 h, an appropriate sodium hydroxide solution was added to the mixture to adjust the pH to 10. Simultaneously, several drops of hydrazine hydrate were added as a reducing agent. Subsequently, the mixture was added in a 100 mL round-bottom bottle, which was placed in a microwave refluxing system (National N-S570MFS microwave oven, 2450 MHz, 900 W, Sanle General Electric Corp. Nanjing, China) at 600 W for 2 min. The as-obtained sample was washed with distilled water and absolute ethanol several times to remove possible residual impurities. The product was dried at 60 8C in a vacuum oven before being further characterized. By varying the atomic ratio of the cobalt and nickel salts in the mixed acetate solution with x = 0.2, 0.5, 0.6, and 0.8, respectively, the Co1 xNix/ MWCNT nanocomposites were prepared. 2.1. Characterization The as-obtained samples were characterized by X-ray powder diffraction (XRD) on an XRD-6000 (Japan) X-ray diffractometer with Cu-Ka radiation (l = 0.154060 nm) at a scanning rate of 0.05 s 1 in the 2u range from 208 to 808. The morphology and size of as-obtained products were observed by transmission electron microscopy (TEM) carried out on a Hitachi H-800 transmission electron microscope and scanning electron microscopy (SEM) operated on a Hitachi S-4800 scanning electron microscope. Highresolution transmission electron microscopy (HRTEM) and selected area electron diffraction (SAED) were performed using JEM 2010F field emission microscope operated at optimum defocus with accelerating voltages of 200 kV. Energy-dispersive X-ray spectrometry (EDS) was carried out with spectroscope attached to HRTEM, which was used for elemental analysis. The magnetic hysteresis loop of sample was measured by vibrating sample magnetometer (VSM, BHV-55, Japan) with an applied field 10 kOe at room temperature. 3. Results and discussion

fcc (220)

fcc (200)

fcc (111)

electron microscopy (SEM), transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM), selected area electron diffraction (SAED), energydisperse X-ray spectroscopy (EDS) and vibrating sample magnetometer (VSM), respectively. Experimental results demonstrate that Co1 xNix alloy nanoparticles with quasi-spherical and facecentered cubic structure had attached on the MWCNTs. Magnetic measurement confirms that both coercivity (Hc) and saturation magnetization (Ms) of Co1 xNix alloy nanoparticles firstly increase and then decrease with increasing Ni concentration. When x = 0.5, the Ms and Hc reach their maximum.

C (002)

H. Wu et al. / Materials Research Bulletin 47 (2012) 1–5

Intensity(a.u.)

2

Co0.8Ni0.2 20

30

40

50

60

70

80

2 Theta (degree) Fig. 1. The XRD patterns of the Co1 xNix/MWCNT nanocomposites (x = 0.2, 0.5, 0.6, and 0.8).

44.278, 51.488, and 76.458 correspond to the crystal planes of the (1 1 1), (2 0 0), and (2 2 0) of face-centered cubic (fcc) CoNi nanoparticle, respectively, which match well with that of the reported CoNi nanoalloys [11–13]. And there are no observable peaks in the XRD spectra corresponding to those of pure cobalt and nickel, which confirms that Co and Ni formed an alloy, rather than separate grains. All the samples display the same face-centered cubic phase despite different compositions, and the diffraction peaks are gradually shifted to small angles with the increase of Ni concentration. The average size of the Co1 xNix nanoparticles, calculated using the Debye–Scherrer formula based on the full width at half-maximum of the (1 1 1) diffraction peak, is 24.9 nm, 14.5 nm, 13.2 nm, and 11.6 nm, which corresponds to x = 0.2, 0.5, 0.6, and 0.8, respectively. The results indicate that the size of the Co1 xNix alloy nanoparticle decreases with increasing Ni concentration. Fig. 2 shows the morphology, microstructure of the Co1 xNix/ MWCNT nanocomposites examined by TEM. The TEM images show that quasi-spherical Co1 xNix alloy nanoparticles are uniformly attached on the surface of MWCNTs. The average diameter of the Co1 xNix alloy nanoparticle calculated from TEM is 28 nm, 20 nm, 18 nm, and 15 nm (which were listed in Table 1), corresponding to x = 0.2, 0.5, 0.6 and 0.8, respectively, which is consistent with the trend of the change of the size calculated from the Debye–Scherrer formula. HRTEM (Fig. 3a) is employed to investigate the inner structure of CoNi/MWCNT nanocomposites as a typical example, showing well-defined lattice fringes of CoNi nanoparticles. The measured spacing of the crystallographic planes is 0.204 nm and 0.342 nm from the HRTEM images, corresponding to the (1 1 1) and (0 0 2) plane separations of CoNi alloy and MWCNTs, respectively. The SAED (Fig. 3b) can reveal details of the local structure of CoNi alloy, which can be indexed to polycrystalline CoNi. The concentric rings could be assigned as diffractions from the (1 1 1), (2 0 0), and (2 2 0) planes of fcc CoNi, and the centermost ring is assigned as diffractions from the (0 0 2) plane of MWCNT, which is consistent with the XRD results.

3.1. Structure and morphology 3.2. Influence of experimental conditions on size and composition Fig. 1 shows the XRD patterns of Co1 xNix/MWCNT nanocomposites (x = 0.2, 0.5, 0.6, and 0.8) at room temperature. In Fig. 1, the XRD peak at 2u value of 26.108 arising from the carbon nanotubes corresponds to its (0 0 2) crystal plane. The peaks at 2u values of

In our experiments, the size and composition of Co1 xNix/ MWCNT nanocomposites were found to be strongly depended on the experimental conditions such as the microwave irradiation

H. Wu et al. / Materials Research Bulletin 47 (2012) 1–5

3

Fig. 2. SEM and TEM images of Co1 xNix/MWCNT nanocomposites. (a,b) x = 0.2; (c,d) x = 0.5; (e,f) x = 0.6; (g,h) x = 0.8.

Fig. 3. (a) HRTEM images; (b) SAED pattern of the obtained CoNi/MWCNT nanocomposites.

time, the microwave power and the atomic ratios of metal Co to Ni in the mixed acetate solution. By adjusting the microwave irradiation time and the microwave power suitably, the size of CoNi alloy nanoparticles can be controlled. CoNi/MWCNT nanocomposites have been prepared as shown in Figs. 4 and 5 at different microwave irradiation time and microwave power, respectively. In Fig. 4a, it can be seen that when the microwave irradiation time is 1 min, the MWCNTs are attached with fewer CoNi nanoparticles unevenly and the diameter is about 22 nm, when the microwave irradiation time is 3 min, there are some aggregated CoNi nanoparticles attached on the MWCNTs in Fig. 4c. While in Fig. 4b, CoNi nanoparticle disperses uniformly and the diameter is about 25 nm. So we chose 2 min as microwave

irradiation time to prepared CoNi/MWCNT nanocomposites. In Fig. 5, it can be seen that the size of CoNi alloy nanoparticles increase with increasing the microwave power. When the microwave power is 400 W, the loading of CoNi alloy nanoparticle is a little in Fig. 5a, when the microwave power is 800 W in Fig. 5c, the particle aggregated severely. While when the microwave power is 600 W, the CoNi alloy nanoparticles disperse uniformly in Fig. 5b. Therefore, we chose that the microwave power is 600 W and the microwave irradiation time is 2 min as the optimal experimental condition. The composition of Co1 xNix alloy nanoparticles can be controlled by adjusting the atomic ratios of metal Co to Ni in the mixed acetate solution. Different composition of Co1 xNix/

Table 1 Magnetization data for Co1 xNix/MWCNT nanocomposites measured at room temperature. 1

x:x (Nominal value)

0.8:0.2 0.5:0.5 0.4:0.6 0.2:0.8

EDS measured value (at.%) Co Ni

Crystallite size (nm)

Hc (Oe)

Ms (emu/g)

77.2 50.8 39.3 20.1

28 20 18 15

221.1 260.5 216.1 157.0

4.0 14.1 10.5 10.0

22.8 49.2 60.7 79.9

4

H. Wu et al. / Materials Research Bulletin 47 (2012) 1–5

Fig. 4. SEM images of CoNi alloy nanoparticles attached on carbon nanotubes at 600 W. (a) 1 min; (b) 2 min; (c) 3 min.

Fig. 5. SEM images of CoNi alloy nanoparticles attached on carbon nanotubes reduced for 2 min. (a) 400 W; (b) 600 W; (c) 800 W.

Fig. 6. EDS spectrums of Co1 xNix/MWCNT nanocomposites. (a) x = 0.2; (b) x = 0.5; (c) x = 0.6; (d) x = 0.8.

H. Wu et al. / Materials Research Bulletin 47 (2012) 1–5

MWCNT nanocomposites (x = 0.2, 0.5, 0.6, and 0.8) have been synthesized under microwave irradiation which the power is 600 W at 2 min (See Fig. 2). The EDS spectrums shown in Fig. 6 correspond to Fig. 2 of different composition of Co1 xNix/MWCNT nanocomposites, which indicate the presence of Co, Ni, and C in all samples. It is obvious that the silicon peak is caused by the silicon chip used to clamp the nanocomposites. The carbon peak comes from the carbon nanotubes. The oxygen peak in the spectrum probably originates from the unavoidable surface-adsorption of oxygen onto the samples from exposure to air during sample processing. The atomic ratio of Co to Ni for the majority of Co1 xNix alloy nanoparticle according to EDS quantitative microanalysis has been listed in Table 1, which is close to the stoichiometry of Co1 xNix (x = 0.2, 0.5, 0.6, and 0.8). 3.3. Magnetic measurement Magnetic properties of the Co1 xNix/MWCNT nanocomposites were investigated at room temperature using a VSM with an applied field 10 kOe  H  10 kOe. Fig. 7 shows the hysteresis loops of Co1 xNix/MWCNTs. The coercivity (Hc) and the saturation magnetization (Ms) were listed in Table 1. It can be seen that the Hc of the Co1 xNix/MWCNTs increases with the increase of the size from 15 nm to 20 nm, and then decreases from 20 nm to 28 nm. This change trend can be related to the theory of the magnetic domain [24], there is a critical size ds of single domain particles. When the particle size (D) is smaller than ds, the magnetic particles are single domain particles. When D is larger than ds, the magnetic particles are multi-domain particles. The Hc of single domain particles increases with the increase of D, while the Hc of the multidomain particles decreases with the increase of D. In our experiments, the size of the Co1 xNix alloy nanoparticles increases with increasing Co concentration form 15 nm to 28 nm. According to the literature [25], the critical size ds of CoNi alloy nanoparticle is about 25 nm. So when the particle size of CoNi alloy nanoparticle increased to 25 nm, the single domain turned to multi-domain, the coercivity decreased. On the other hand, this reduction in coercivity may be due to the microstructure, the K1 anisotropy constant of the sample, and the treatment process [26]. The Ms increases with the increase of Co concentration from x = 0.8 to 0.5, and decreases when Co concentration was increased from x = 0.5 to 0.2. This trend is consistent with the reported literatures [12,27]. It can be explained that as for CoNi alloy, due to different magnetic moment of Co and Ni, that is 2.07 and 0.98 mB/atom [28], respectively, the Ms increases with Co concentration from x = 0.8 to 0.5, when Co concentration increased from x = 0.5 to 0.2, the Ms decreases on the contrary, this may be related to the change of

-1

Ms (emu.g ) of Co1-xNix/MWCNTs

15

Co0.5Ni0.5 Co0.4Ni0.6

10

Co0.2Ni0.8

5 Co0.8Ni0.2

0 -5 -10 -15

-5000

0

5000

H(Oe) Fig. 7. the hysteresis loops of Co1 xNix/MWCNT nanocomposites.

5

anisotropy constant [29]. On the other hand, Ms decreases with decreasing particle size at single domain particles which arises from spin rotation on the surface of the crystals, while Ms decreases with increasing particle sizes can be attributed to both domain wall and spin rotation contribute at multi-domain particles [30]. 4. Conclusions Co1 xNix alloy nanoparticles with controlled composition (x = 0.2, 0.5, 0.6, and 0.8) and size were successfully attached on the surface of MWCNTs by microwave irradiation method. The composition and size of Co1 xNix alloy nanoparticles can be controlled through adjusting the atomic ratios of metal Co to Ni in the mixed acetate solution, the microwave power and microwave irradiation time, respectively. Co1 xNix alloy nanoparticles with quasi-spherical and face-centered cubic structure dispersed uniformly on the surface of MWCNTs to form Co1 xNix/MWCNT nanocomposites. Magnetic measurement shows that both Hc and Ms of Co1 xNix alloy nanoparticles firstly increase with increasing Co concentration from x = 0.8 to 0.5, and then decrease from x = 0.5 to 0.2. These results demonstrate that the microwave irradiation method is promising for preparation of magnetic alloy nanoparticles attached on MWCNTs for magnetic storage and the ultra high-density magnetic recording applications. Acknowledgments We thank Anhui Provincial Natural Science Foundation (No. 070414179) and National Natural Science Foundation (No. 20901003) of the People’s Republic of China for financial support. References [1] Z.J. Liu, Z. Xu, Z.Y. Yuan, D. Lu, W. Chen, W. Zhou, Catal. Lett. 72 (2001) 203. [2] E.S. Steigerwalt, G.A. Deluga, C.M. Lukehart, J. Phys. Chem. B 106 (2002) 760. [3] Z.J. Liu, Z.D. Xu, Z.Y. Yuan, W.X. Chen, W.Z. Zhou, L.M. Peng, Mater. Lett. 57 (2003) 1339. [4] Y. Liu, W. Jiang, L. Xu, X.W. Yang, F.S. Li, Mater. Lett. 63 (2009) 2526. [5] X.C. Sun, X.L. Dong, Mater. Res. Bull. 37 (2002) 991. [6] C. Chen, O. Kitakami, S. Okatomo, Y.J. Shimada, J. Appl. Phys. 86 (1999) 2161. [7] M. Wen, Y.F. Wang, F. Zhang, Q.S. Wu, J. Phys. Chem. C 113 (2009) 5960. [8] D. Ung, G. Viau, C. Ricolleau, F. Warmont, P. Gredin, F. Fievet, Adv. Mater. 17 (2005) 338. [9] P. Elumalai, H.N. Vasan, M. Verelst, P. Lecante, V. Carles, P. Tailhades, Mater. Res. Bull. 37 (2002) 353. [10] M.J. Hu, Y. Lu, S. Zhang, S.R. Guo, B. Lin, M. Zhang, S.H. Yu, J. Am. Chem. Soc. 130 (35) (2008) 11606. [11] S.L. Pan, Z.G. An, J.J. Zhang, G.Z. Song, Mater. Lett. 64 (2010) 453. [12] Md.H. Rashid, M. Raula, T.K. Mandal, J. Mater. Chem. 21 (2011) 4904. [13] L.P. Zhu, H.M. Xiao, S.Y. Fu, Eur. J. Inorg. Chem. (2007) 3947. [14] Q.F. Liu, W.C. Ren, Z.G. Chen, B.L. Liu, B. Yu, F. Li, H.T. Cong, H.M. Cheng, Carbon 46 (2008) 1417. [15] Z.P. Dong, K. Ma, J.G. He, J.J. Wang, R. Li, J.T. Ma, Mater. Lett. 62 (2008) 4059. [16] X.J. Zhang, W. Jiang, D. Song, J.X. Liu, F.S. Li, Mater. Lett. 62 (2008) 343. [17] H.Q. Cao, M.F. Zhu, Y.G. Li, J. Solid State Chem. 179 (2006) 1208. [18] D. Shi, J.P. Cheng, F. Liu, X.B. Zhang, J. Alloys Compd. 502 (2010) 365. [19] W.X. Chen, J.Y. Lee, Z.L. Liu, Mater. Lett. 58 (2004) 3166. [20] H.Q. Wu, P.S. Yuan, H.Y. Xu, D.M. Xu, B.Y. Geng, X.W. Wei, J. Mater. Sci. 41 (2006) 6889. [21] N.N. Xia, D.S. Yuan, T.X. Zhou, J.X. Chen, S.S. Mo, Y.L. Liu, Mater. Res. Bull. 46 (2011) 687. [22] H.Q. Wu, Q.Y. Wang, Y.Z. Yao, C. Qian, X.J. Zhang, X.W. Wei, J. Phys. Chem. C 112 (2008) 16779. [23] H.Q. Wu, P.P. Cao, W.T. Li, N. Ni, L.L. Zhu, X.J. Zhang, J. Alloys Compd. 509 (2011) 1261. [24] G.D. Tang, D.L. Hou, M. Zhang, L.H. Liu, L.X. Yang, C.F. Pan, X.F. Nie, H.L. Luo, J. Magn. Magn. Mater. 251 (2002) 42. [25] D. Mercier, J.C.S. Levy, G. Viau, F. Fievet-Vincent, F. Fievetet, P. Toneguzzo, O. Acher, Phys. Rev. B 62 (2000) 532. [26] M.H. Xu, W. Zhong, X.S. Qi, C.T. Au, Y. Deng, Y.W. Du, J. Alloys Compd. 495 (2010) 200. [27] R. Brayner, M.J. Vaulay, F. Fievet, T. Coradin, Chem. Mater. 19 (2007) 1190. [28] H.T. Jeng, D.S. Wang, J. Magn. Magn. Mater. 317 (2007) 46. [29] Z.J. hou, D.F. Shen, Z.Q. Zou, L.Y. Chen, J.L. Wang, Phys. Lett. A 271 (2000) 115. [30] K. Kawano, M. Hachiya, Y. Iijima, N. Sato, Y. Mizuno, J. Magn. Magn. Mater. 321 (2009) 2488.