MWCNT nanocomposites via microwave-assisted polyol process

Journal of Alloys and Compounds 554 (2013) 132–137

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Controlled synthesis and magnetic properties of monodisperse Ni1xZnxFe2O4/MWCNT nanocomposites via microwave-assisted polyol process Huaqiang Wu ⇑, Ning Zhang, Li Mao, Tingting Li, Lingling Xia 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

Article history: Received 6 November 2012 Received in revised form 28 November 2012 Accepted 29 November 2012 Available online 8 December 2012 Keywords: Microwave-assisted polyol process Carbon nanotubes NiZn ferrite Magnetic properties

a b s t r a c t Ni1xZnxFe2O4/MWCNT nanocomposites (x = 0, 0.2, 0.4, 0.5, 0.6, 0.8 and 1) with controllable composition have been successfully synthesized via microwave-assisted polyol process using triethylene glycol solution (TREG) as solvent. Experimental results demonstrated that monodisperse face-centered cubical Ni1xZnxFe2O4 nanoparticles with average size (6 nm) had been attached on the MWCNTs. The composition of ferrite nanoparticles can be controlled through adjusting the atomic ratios of the nickel and zinc salts in the mixed nitrate solution. The magnetic properties of nanocomposites with different Zn contents were measured by vibrating sample magnetometer (VSM). The saturation magnetization (Ms) of Ni1xZnxFe2O4/MWCNT nanocomposites gradually increases when the x is less than 0.5 while decreases when the x is larger than 0.5. Ms reached maximum value when the x is 0.5. The coercivity (Hc) of nanocomposites is low at room temperature, which exhibits characteristic of superparamagnetic. Ó 2012 Published by Elsevier B.V.

1. Introduction Monodisperse ferrite magnetic materials with spinel structure and small dimensions (MFe2O4, M = Fe, Co, Ni, Zn, Mn, Mg, Cd) have attracted a considerable attention because of their outstanding magnetic and electric properties [1–3]. Ni1xZnxFe2O4 (0 < x < 1) is a kind of mixed type spinel ferrites whose general formula is de2þ 3þ 3þ scribed as ½Zn2þ x Fe1x  A ½Ni1x Fe1þx BO4, where ‘A’ represents tetrahedral site, ‘B’ represents octahedral site, and ‘x’ means the degree of inversion. As one of the most versatile magnetic materials that possess properties such as high saturation magnetization, high resistivity, chemical stability and low dielectric loss, NiZn ferrites have extensive applications in fields of magnetic fluids, high-frequency devices and microwave absorber [3–5]. The multi-walled carbon nanotubes (MWCNTs) have been widely functioned as constructive material in various areas of science and technology owing to their fascinating characteristics such as large specific surface area, unique one-dimensional tubular structure, high absorptivity and well stability [6,7]. MWCNT-based nanocomposites have become an interesting area of researches. Inorganic nanoparticles such as metals [8], alloys [9], and semiconducting nanoparticles [10] coated on MWCNTs were widely reported in previous. As we know, magnetic spinel ferrite particles have a strong tendency to agglomerate during their formation. Magnetic ferrite nanoparticles coated MWCNTs could have avoided agglomeration [11,12] and improved their superior performances ⇑ Corresponding author. E-mail address: [email protected] (H. Wu). 0925-8388/$ - see front matter Ó 2012 Published by Elsevier B.V. http://dx.doi.org/10.1016/j.jallcom.2012.11.185

of optical [7], electrical [13], magnetic [14,15], microwave absorption [16] and gas-sensing [17]. Considering the excellent properties of MWCNTs and ferrite nanoparticles, their nanocomposites have promising applications in many aspects, for example, the field of magnetic separation technology [18], target-drug delivery [19], magnetic resonance imaging [6] and clinical diagnosis [20]. Recently, various approaches have been developed to prepare MFe2O4/MWCNT (M = Fe, Co, Ni, Zn, Mn) nanocomposites, such as sol–gel route [14], one-pot method [20], solvothermal method [17,19,21], chemical precipitation [22] and physical mixing [18]. At the same time, polyol process was often used to prepare a variety of non-aggregated nanoparticles [23]. In this process, the polyol triethylene glycol solution (TREG) with high-boiling and great viscosity can easily bind with the obtained nanoparticles, which serves as stabilizer to control the growth of nanoparticles and prevent interparticle aggregating [24]. The polyol method is efficient in achieving nanoparticles of uniform and narrow size distribution without adding surfactant [25]. Nevertheless, all the polyol approaches for preparing the ferrite/MWCNT nanocomposites often require several hours [21,26]. It is well known that microwave method has remarkable advantages of more rapid heating rates, more homogeneous nucleation, shorter crystallization time (even less than several minutes) and less energy consumption, which is attributed to the difference in heating mechanism [27,28]. In this work, Ni1xZnxFe2O4/MWCNT nanocomposites (x = 0, 0.2, 0.4, 0.5, 0.6, 0.8 and 1) with controllable composition and monodispersion were synthesized via microwave-assisted polyol process. The composition of ferrite nanoparticles can be controlled through adjusting the atomic ratios of the nickel and zinc salts in the mixed

133

(440)

(422) (511)

(311)

(220)

(400)

x=1

Intensity (a.u.)

nitrate solution. Characterization results indicate that small size nanoparticles with spinel face-centered cubic structure have been monodisperse attached on the surface of MWCNTs. Magnetic measurement reveals that the composition of NiZn ferrite nanoparticles is a key factor of the magnetic properties. The saturation magnetization (Ms) of Ni1xZnxFe2O4/MWCNT nanocomposites gradually increases when x is less than 0.5 while decreases when the x is larger than 0.5. Ms reaches a maximum value when the x is 0.5. The coercivity (Hc) of nanocomposites is low at room temperature, which exhibits characteristic of superparamagnetic.

C(002)

H. Wu et al. / Journal of Alloys and Compounds 554 (2013) 132–137

x=0.8 x=0.6 x=0.5

2. Experimental MWCNTs (purity  95%) were purchased from Shenzhen Nanoport Company, which were produced via catalytic decomposition of hydrocarbon. For better anchoring of the nanoparticles, the MWCNTs were refluxed in boiling concentrated nitric acid for 4 h, then purified by distilled water until pH value reaching neutrality, and finally dried at 100 °C. All other chemicals were of analytical grade and used without further purification. In a typical procedure (x = 0.5), 0.05 g of the acid-treatment MWCNTs was mixed with Ni(NO3)26H2O (0.0303 g, 0.1 mmol), Zn(NO3)26H20 (0.0310 g, 0.1 mmol), Fe(N03)39H20 (0.1680 g, 0.4 mmol) and 1.8 g of sodium acetate (NaAc) in triethylene glycol (TREG) solution by sonication. After stirring for 24 h, the resulting mixture was transferred into a 100 mL round-bottom bottle, which was in a microwave refluxing device (National NN-S570MFS microwave oven, 2450 MHz, Sanle General Electric Corp. Nanjing, China) at 365 W for 8 min. After cooling down to room temperature, the as-obtained sample was washed with distilled water and acetone several times to remove the residual impurities. Finally, the products were dried in a vacuum oven at 60 °C and protected by nitrogen. By varying the atomic ratio of the nickel and zinc salts in the mixed nitrate solution with x = 0, 0.2, 0.4, 0.5, 0.6, 0.8 and 1, respectively, a series of Ni1xZnxFe2O4/MWCNT nanocomposites were synthesized in the same conditions. The crystalline phase of the samples was confirmed by X-ray powder diffraction (XRD) on an XRD-6000 (Japan) with Cu-Ka radiation (k = 0.154056 nm) at a scanning rate of 0.05 s1 in the 2h range from 20° to 80°. The morphology and size of the as-prepared products were observed by scanning electron microscopy (SEM) operated on a Hitachi S-4800 scanning electron microscope, transmission electron microscopy (TEM) carried out on a Hitachi H-800 transmission electron microscope. High-resolution transmission electron microscope (HRTEM) images and selected area electron diffraction (SAED) were taken on the JEM 2010F field emission microscope operated at optimum defocus with accelerating voltages of 200 kV. Energydispersive X-ray spectrometry (EDX) was carried out with spectroscope attached to TEM, which was used for elemental analysis. The magnetic hysteresis loops of samples were measured by vibrating sample magnetometer (VSM, SQUID, America) with an applied field ±5 kOe at room temperature.

3. Result and discussion 3.1. Characterization of the samples Fig. 1 shows the typical XRD patterns of Ni1xZnxFe2O4/MWCNT nanocomposites (x = 0, 0.2, 0.4, 0.5, 0.6, 0.8 and 1) at room temperature. The XRD peak at 2h value of 26.28° according to the carbon nanotubes corresponds to its (0 0 2) crystal plane. The major XRD peaks at 2h value of 30.14°, 35.53°, 43.08°, 53.71°, 57.11°, 62.80° correspond to (2 2 0), (3 1 1), (4 0 0), (4 2 2), (5 1 1) and (4 4 0) planes of face-centered cubic (fcc) NiZn ferrite, respectively, which accord with the previous reports [5,14,27]. All the samples exhibit characteristic of the spinel structure despite different compositions. However, the diffraction peaks are gradually shifted to small angles with the increasing of the content of Zn, which results in the lattice constant increases gradually. The lattice constants were dependent on the composition which calculated by the XRD pattern (See Table 1.). The calculated lattice constant of NiFe2O4 based on the (3 1 1) is a = 8.335 Å, which is consistent with standard value (a = 8.337 Å, JCPDS 74-2081) in the error range. The lattice constant increases from 8.335 Å to 8.435 Å with increasing of the content of Zn, which can be attributed to replacement of smaller Ni2+ (ionic radius r = 0.069 nm) by larger Zn2+ (r = 0.074 nm). The broadness of the peaks indicates that the magnetite crystallites are significantly small [5]. The average size of the nanoparticles as estimated by

x=0.4 x=0.2 x=0 20

30

40

50

60

70

80

2θ Degree Fig. 1. The XRD patterns of the Ni1xZnxFe2O4/MWCNT nanocomposites (x = 0, 0.2, 0.4, 0.5, 0.6, 0.8 and 1).

Table 1 Magnetization data for Ni1xZnxFe2O MWCNT nanocomposites measured at room temperature. Ni1xZnxFe2O4

NiFe2O4 Ni0.8Zn0.2Fe2O4 Ni0.6Zn0.4Fe2O4 Ni0.5Zn0.5Fe2O4 Ni0.4Zn0.6Fe2O4 Ni0.2Zn0.8Fe2O4 ZnFe2O4

EDX measured value (at.%) Ni

Zn

Fe

1.26 2.58 1.42 0.49 0.67

0.34 1.61 1.41 0.78 2.77

3.07 8.07 5.45 2.10 6.76

Crystal constant a (Å)

Hc (Oe)

Ms (emu/g)

8.335 8.398 8.405 8.408 8.418 8.421 8.435

6.6 4.3 3.8 1.6 5.7 12.2 7.9

11.57 14.42 15.55 19.33 6.58 4.83 4.74

the Debye–Scherrer formula based on the full-width half-maximum of the (3 1 1) diffraction peak was about 6 nm. Fig. 2 shows the SEM images of the samples with different composition (Fig. 2(a)–(g), x = 0, 0.2, 0.4, 0.5, 0.6, 0.8, 1). According to the SEM images, the overall tubular structure of MWCNTs remains intact after activating treatment. It can be seen that quasi-spherical Ni1xZnxFe2O4 nanoparticles with small dimension have been uniformly attached on the surface of MWCNTs. Fig. 3 shows the morphology of the ferrites for which Ni0.5Zn0.5Fe2O4 as a typical example. From the TEM images of Ni0.5Zn0.5Fe2O4/MWCNT nanocomposites (Fig. 3a), the MWCNTs have been coated with high dispersions and high loadings of ferrite nanoparticles without detectable aggregation, which indicates the monodisperse magnetite nanoparticles on MWCNTs are formed. TEM image in Fig. 3b reveals the nanoparticles coated on MWCNTs are quasi-spherical and monodisperse distributed with the average particle size is 6 nm, which is consistent with the above size calculated by the XRD analysis. Fig. 4 shows the HRTEM images, SAED pattern and EDX spectrum of Ni0.5Zn0.5Fe2O4/MWCNT nanocomposites. HRTEM is generally employed to investigate the internal structure of nanocomposites. After a detailed analysis on the lattice fringes, the interplanar spacing is 0.34 nm and 0.25 nm (Fig. 4a), which corresponding to the (0 0 2) plane of MWCNTs and (3 1 1) plane of NiZn ferrite, respectively. As shown in Fig. 4b, the SAED pattern exposed the prepared Ni0.5Zn0.5Fe2O4 is spinel polycrystalline structure. The concentric

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H. Wu et al. / Journal of Alloys and Compounds 554 (2013) 132–137

Fig. 2. SEM images of Ni1xZnxFe2O4/MWCNT nanocomposites ((a)–(g), x = 0, 0.2, 0.4, 0.5, 0.6, 0.8 and 1).

rings would be assigned as diffractions from different crystal planes of MWCNTs and spinel NiZn ferrite. The centermost ring is assigned as diffractions from the (0 0 2) plane of MWCNTs, other rings could be indexed to the (2 2 0), (3 1 1), (4 0 0), (5 1 1), and (4 4 0) planes of Ni0.5Zn0.5Fe2O4, which is consistent with the XRD results. The chemical composition of Ni1xZnxFe2O4/MWCNTs was qualitatively detected by EDX spectrum. The EDX spectrum of Ni0.5Zn0.5Fe2O4/ MWCNT nanocomposites shown the presence of Ni, Zn, Fe, O, Cu and C in the sample (See Fig. 4c). It is obvious that the copper peak is caused by the copper grid used to clamp the nanoparticles and the carbon peaks come from the carbon nanotubes. The atomic ratio between Fe, Ni and Zn for the majority of NiZn ferrite nanoparticle according to EDX quantitative microanalysis has been listed in Table 1, which is close to the stoichiometry of Ni1xZnxFe2O4 (x = 0.2, 0.4, 0.5, 0.6 and 0.8).

3.2. Magnetic measurement Fig. 5 shows the hysteresis loops of Ni1xZnxFe2O4/MWCNTs. Magnetic properties of the Ni1xZnxFe2O4/MWCNT nanocompos-

ites were investigated at room temperature using a VSM with an applied field 5 kOe 6 H 6 5 kOe. The saturation magnetization value (Ms) of the Ni1xZnxFe2O4/MWCNT magnetic nanocomposites (x = 0, 0.2, 0.4, 0.5, 0.6, 0.8 and 1) were listed in Table 1. It is observed that Ms of the Ni1xZnxFe2O4/MWCNT nanocomposites gradually increased with the increase of Zn2+ content until x = 0.5, and rapidly decreased for x larger than 0.5 (as shown in Fig. 5). Ms reached a maximum value of 19.33 emu g1 when the x is 0.5. In terms of spinel ferrite, the composition-dependence on magnetic behavior may have relation with the distribution of cations in A and B sites and superexchange interaction and the non-collinear nature of moments in the B-site. On the basis of Neel’s two sublattice model of ferrimagnetism [5,15], the magnetic moment per formula unit is expressed as:

MðXÞ ¼ jMBðxÞ  M AðxÞ j

ð1Þ

where, MB and MA are the B- and A-sublattice magnetic moment in lB respectively. Actually, the net magnetic moment determined the saturation magnetization value. For zinc ferrite, non-magnetic ion Zn2+ and magnetic ion Fe3+ are distributed in A and B sites,

H. Wu et al. / Journal of Alloys and Compounds 554 (2013) 132–137

Fig. 3. (a and b) TEM images of Ni0.5Zn0.5Fe2O4/MWCNT nanocomposites.

Fig. 4. HRTEM images (a), SAED pattern (b), and EDX spectrum (c) of Ni0.5Zn0.5Fe2O4/MWCNT nanocomposites.

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Fig. 5. The hysteresis loops of Ni1xZnxFe2O4/MWCNT nanocomposites (x = 0, 0.2, 0.4, 0.5, 0.6, 0.8 and 1).

respectively. It is common believed that ZnFe2O4 should not exhibit magnetic performance as B–B superexchange interaction dominate and the B-sites magnetic moment is antiparallel to each other. Nevertheless, the distribution of cation can be changed obviously when the grain size is decreased to the nano size. A part of Zn2+ may enter into B-site and Fe3+ enter into A-site simultaneity. As a result, the net moment is departed from zero which attributed to the enhancement of A–B interaction. For NiZn ferrite, Zn2+ and Ni2+ ions prefer to inhabit the A- and B-sites, respectively, while Fe3+ prefers to inhabit both A-sites and B-sites. Ms increases with the increasing of the content of Zn2+ until x = 0.5, which is due to the substitution of nonmagnetic ion Zn2+ may push Fe3+ to the B-site. However this migration towards B-sites would lead to the increase of Fe3+ concentration in B-sites, which gives rise to antiparallel spin coupling and spin canting, resulting in the weakening of A–B exchange coupling, and thereby decreases the net magnetic moment [24,27,29].

When the content of Zn larger than 0.5, Ms would be decreased due to the A–B exchange interaction gets weaker than B–B interaction. Ms reached maximum when the x is 0.5, which agreed with the former literature [30]. The coercivity (Hc) of the Ni1xZnxFe2O4/ MWCNT nanocomposites was also displayed in Table. 1. The values are so small at room temperature even can be neglected, which exhibits characteristic of superparamagnetic [12,20,21].

4. Conclusions In summary, we have provided a novel microwave-assisted polyol process to synthesize monodisperse Ni1xZnx ferrite nanoparticles attached on the MWCNTs with different composition (x = 0, 0.2, 0.4, 0.5, 0.6, 0.8 and 1). The composition of ferrite nanoparticles can be controlled through adjusting the atomic ratios of the nickel

H. Wu et al. / Journal of Alloys and Compounds 554 (2013) 132–137

and zinc salts in the mixed nitrate solution. The results indicated that face-centered cubic structure Ni1xZnxFe2O4 nanoparticles with average size (6 nm) have monodisperse attached on the surface of MWCNTs. The magnetic property investigation showed that Ms increases firstly and then decreases with increasing the content of Zn. It would be reached a maximum when the x is 0.5. Hc is extremely small at room temperature, which exhibits characteristics of superparamagnetic. These results demonstrated our synthetic strategy is a facile, effective and promising route for fabricating composition-controllable magnetic ferrite-MWCNT nanocomposites of uniform and monodispersion. Acknowledgements 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.

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