Magnetite Nanocrystals on Multiwalled Carbon Nanotubes as a

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Magnetite Nanocrystals on Multiwalled Carbon Nanotubes as a Synergistic Microwave Absorber Zhijiang Wang,*,†,‡ Lina Wu,§,‡ Jigang Zhou,∥ Wei Cai,⊥ Baozhong Shen,§ and Zhaohua Jiang*,† †

School of Chemical Engineering and Technology and ⊥School of Materials Science and Engineering, Harbin Institute of Technology, Harbin 150001, China § Department of Medical Imaging and Nuclear Medicine, The Fourth Affiliated Hospital, Harbin Medical University, Harbin 150001, China ∥ Canadian Light Source Incorporated, Saskatoon, Saskatchewan S7N 0X4, Canada S Supporting Information *

ABSTRACT: The understanding of the interaction between the building blocks in the hybrids can advance our comprehension of design principles in high-performance microwave absorbing materials. Here, we report a hybrid material consisting of magnetite (Fe3O4) nanocrystals grown on multiwalled carbon nanotube (MWCNT) as a highperformance microwave absorber in the 2−18 GHz band, although Fe3O4 nanocrystals or MWCNTs alone or their physical mixture show little microwave absorption. The hybrid is characterized by transmission electron microscopy, X-ray diffraction, and vector network analysis, X-ray absorption near-edge structures at the C K-edge and Fe L3,2-edge, and electron spin resonance analysis. Microstructural analysis reveals that Fe3O4 nanocrystals are immobilized on the MWCNT surface by a strong interaction. Charges in the MWCNT/Fe3O4 hybrids transfer from the conduction band in Fe3O4 to C 2p-derived states in the MWCNT substrate. Dipole interaction between the magnetic nanocrystals is increased. The synergetic interactions leads to much improved microwave absorption.

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great potential as novel high-performance microwave absorbers. Che et al. found that Fe encapsulated within MWCNT presented an improved ability for microwave absorption.11 The following research confirms that nanohybrids containing both dielectric-absorbing material and a magnetic absorber, such as MWCNT/FeCo,12 MWCNT/CoFe2O4,13 or Fe3O4/SnO2,14 can truly enhance microwave absorption in the range of 2−18 GHz. However, these reported nanocomposites still have not well resolved the shortcomings of low microwave absorption, most of them possessing a limited reflection loss value of higher than −35 dB. It is believed that the interaction in nanohybrids is critical for enhanced microwave absorption. The understanding of such interactions can pave the way for design of higher performance microwave absorbers, but the nature of the interaction remains poorly understood, due to the relatively limited analysis that has been carried out. For microwave-absorption nanohybrids, a one-dimensional structure which can form continuous micronetworks is desired, beneficial for propagation and attenuation of microwaves in the absorbers.15,16 For magnetic absorbers, a lower coercivity is favorable, which increases permeability and enhances absorption in turn.17 Superparamagnetic materials are preferred but

erious electromagnetic interference (EMI) pollution, the fourth and growing source of pollution, is omnipresent and presents extensive threats to biological systems by promoting cancer,1 inducing breakdown of DNA strands,2 and weakening of the immune system.3 In addition, extensive utilization of electronic devices and rapid development of gigahertz (GHz) electronic systems have the potential to cause severe interruption to electronically controlled systems as well as be a source of device malfunctions. To address these issues induced by EMI, efficient microwave absorbers with superior absorption qualities at specific frequency band are becoming highly desirable and necessary. Microwave absorbers are generally classified into two categories: dielectric and magnetic absorbing materials. The dielectric absorbers, including multiwalled carbon nanotube (MWCNT), carbon particles, and SiC, depend on the electronic polarization, ion polarization, and intrinsic electric dipolar polarization in their materials to realize microwave absorption.4−6 Magnetic absorbers, such as Fe, Ni, and Fe3O4, depend on their magnetic properties to attenuate microwaves.7,8 However, in the microwave band of 2−18 GHz, both dielectric absorbers and magnetic absorbing materials have the shortcoming of low attenuation ability when they are utilized alone. Nanohybrids with controlled structure and interface interactions may exhibit synergistic properties9,10 and show © 2013 American Chemical Society

Received: January 3, 2013 Revised: February 16, 2013 Published: February 19, 2013 5446

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Figure 1. (a) Schematic illustration for fabrication of MWCNT/Fe3O4 nanohybrid; (b) XRD pattern of the MWCNT/Fe3O4 nanohybrid; (c) TEM images of the MWCNT/Fe3O4 nanohybrid; (d) magnetization curve at 300 K of MWCNT/Fe3O4 nanohybrid, insert showing its response to an external magnet and dispersion state in water after slight shaking.

colored stable solution. The resulting mixture was subsequently heated to 287 °C at a rate of 3 °C min−1 under vigorous stirring and argon protection. After cooling to room temperature, 30 mL of ethyl acetate was added to dilute the solution. Finally, the products were magnetically separated by a magnet and washed with ethanol several times and dried in a vacuum oven. Synthesis of Fe3O4 Nanocrystals. The strategy for fabrication of Fe3O4 nanocrystals is similar with that of MWCNT/Fe3O4 nanohybrids. Initially, iron(III) acetylacetonate was dispersed in 30 mL of triethylene glycol solution. Then the solution was heated to 287 °C at a rate of 3 °C min−1 under argon protection and kept at reflux for 30 min. Lastly, nanocrystals were magnetically separated and dried in a vacuum oven. Figure S1, Supporting Information, shows the TEM morphology and XRD analysis for the prepared Fe3 O4 nanocrystals, which also have a core size of 8 nm, the same as that of Fe3O4 nanocrystals grown on MWCNT. Characterization. X-ray diffraction (XRD) patterns of the products were obtained on a Rigaku D/max-γB diffractometer equipped with a rotating anode and Cu Kα source. Transmission electron microscope (TEM) images were taken on a Tecnai G2 F30 electron microscope under an accelerating voltage of 300 keV. TEM samples were prepared by dropping a diluted dispersion of the as-prepared products onto amorphous carbon-coated copper grids and drying in air. Magnetic measurements were carried out using the physical properties measurement system (PPMS) in a vibrating sample magnetometry (VSM) mode (Quantum Design Inc.) with a magnetic field up to 5 T. ESR spectra were recorded at the X band by a Bruker ER-200DSRC Analytic ESR spectrometer. XANES were

restrict the sizes of the magnetic absorbers to no more than 20 nm or less.18,19 Herein, we fabricate a MWCNT/Fe3O4 nanohybrid in which Fe3O4 nanocrystals with a core size of 8 nm are grown on MWCNT in high density. They exhibit a surprisingly high-performance capability for microwave absorption in the 2−18 GHz band, with minimum reflection loss reaching −41.61 dB at a frequency of 5.5 GHz. This is despite MWCNT and Fe3O4 individually or as a physical mixture having little microwave absorption in this band range. The synergistic interaction between the Fe3O4 and MWCNT substrate plays a key role in this enhanced absorption. On the basis of the X-ray absorption near-edge structures (XANES) at the C K-edge and Fe L3,2-edge and electron spin resonance (ESR) analysis of the materials, the nature of the synergistic interaction was studied.

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EXPERIMENTAL SECTION Materials. MWCNTs, supplied by Shenzhen Nanotech Port Ltd. Co. (China), had a diameter of 20−30 nm and length of 5−15 mm. Iron(III) acetylacetonate and triethylene glycol (TREG) were obtained from Acros and Aldrich, respectively. Deionized water of high resistivity (18.2 MΩ·cm) was collected through a TKA GenPure ultrapure water system. Synthesis of MWCNT/Fe3O4 Nanohybrids. The strategy for fabrication of MWCNT/Fe3O4 nanohybrids is illustrated in Figure 1a. Initially, MWCNT (100 mg) were dispersed in a 30 mL TREG solution in an ultrasonic bath to obtain a dispersed nanotube suspension. The iron precursor (400 mg), iron(III) acetylacetonate, was then added into the suspension, and this mixture was further sonicated for at least 0.5 h to give a black5447

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Figure 2. Microwave absorption characteristics of the samples: (a) MWCNTs, (b) Fe3O4 nanocrystals, (c) physical mixture of MWCNTs and Fe3O4 nanocrystals, (d) MWCNT/Fe3O4 nanohybrid, (e) summary for the best absorption state of each of the samples, (f) reflection loss for the MWCNT/Fe3O4 nanohybrid with different thickness.

spacing of about 0.34 nm in the MWCNT wall are well matched with the XRD analysis. Magnetite nanocrystals on the MWCNT surface are well separated even though the distance is very short. The nanosized Fe3O4 nanocrystals endow the MWCNT/Fe3O4 nanocomposites with a superparamagnetic property and a saturated magnetization up to 28.67 emu g−1 (Figure 1d) at room temperature. Reflection losses of the MWCNT/Fe3O4 nanohybrids were measured in the 2−18 GHz band. Losses were compared to that of pure Fe3O4 nanocrystals with a core size of 8 nm, MWCNTs, as well as a physical mixture of Fe3O4 nanocrystals and MWCNTs with a component mass ratio equivalent to that of the MWCNT/Fe3O4 nanohybrids. Microwave absorption properties of the samples were evaluated by the reflection loss on the basis of the transmission line theory.21−24 The equations are as follows

obtained on the spherical grating monochromator (SGM) beamline (ΔE/E ≈ 10−4) at the Canadian Light Source (CLS) in a surface-sensitive, total electron yield (TEY) mode with the use of specimen current. Microwave-Absorption Measurements. Electromagnetic parameters were measured on a vector network analyzer (Agilent, N5230A) with transmission−reflection mode in the 2−18 GHz band. Samples containing 50 wt % prepared composites were pressed into a shape with an outer diameter of 7.0 mm, inner diameter of 3.0 mm, and thickness of about 3.5 mm for microwave measurement, in which paraffin wax was used as the binder. All reported values were obtained by averaging over the data measured from three samples for each kind of nanomaterials.

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RESULTS AND DISCUSSION The strategy for fabrication of the MWCNT/Fe3O4 nanohybrid by hydrothermal synthesis is illustrated in Figure 1a. The phase composition and morphology of the fabricated nanocomposites were checked with XRD and TEM. There are seven major diffraction peaks observed for the MWCNT/Fe3O4 nanohybrids and shown in Figure 1b. The peaks at 25.98° and 42.78° are assigned to (002) and (100) planes of the MWCNTs, respectively. Diffraction peaks at 30.00°, 35.45°, 43.10°, 53.44°, 57.04°, and 62.58° represent the Bragg reflection from (220), (311), (400), (422), (511), and (440) planes of the cubic spinel crystal structure of magnetite (JCPDS card no. 19-0629). No peaks corresponding to impurities are detected. The diffraction peak broadening of the XRD pattern indicates that the magnetite nanocrystals are significantly small. As calculated by Scherrer’s formula,20 the average crystallite sizes of the magnetite are 7.7 nm. TEM images reveal that the MWCNT has a high-density coating of monodisperse magnetite nanocrystals with a core size of around 8 nm (Figure 1c). The (311) lattice plane of the spinel-structured Fe3O4 with the two adjacent planes at a distance of 0.25 nm and a lattice

Z in = Z0(μr /εr)1/2 tanh[j(2πfd /c)(με )1/2 ] r r

RL(dB) = 20 log

Z in − Z0 Z in + Z0

(1)

(2)

where f is the frequency of the electromagnetic waves, d is the thickness of the absorber, c is the velocity of electromagnetic waves in free space, μr and εr are, respectively, the relative complex permeability and permittivity, Z0 is the impedance of air, and Zin is the input impedance of the absorber. Figure 2 shows the microwave absorption characteristics of the four samples. The lowest reflection loss for Fe3O4 nanocrystals was −3.31 dB (∼53.3% absorption) for all frequencies between 2 and 18 GHz with the thickness of the absorber in the range of 0.5−5 mm. The MWCNTs exhibited little microwave absorption, similar to a previous report.11 When mixing the Fe3O4 nanocrystals and MWCNTs together, there was no obvious enhancement to the ability for the microwave absorption, with the best reflection loss value achieved being −3.50 dB (∼55.3% absorption). A substantial 5448

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Figure 3. Permittivity and permeability characterizations of the four samples in the 2−18 GHz range: (a) real part of complex permittivity spectra, (b) imaginary part of complex permittivity spectra, (c) real part of complex permeability spectra, and (d) imaginary part of complex permeability spectra.

similar values and frequency dependence with those of the MWCNTs. On the contrary, for the MWCNT/Fe3O4 hybrid absorber, the values of complex relative permittivity and permeability are balanced, possessing a far more complex relative permittivity than Fe3O4 nanocrystals as well as a higher complex relative permeability as compared to the MWCNTs. In particular, the μ″ curve of MWCNT/Fe3O4 hybrid shows a new broad peak at 3.5−7.6 GHz. The notable difference of the complex relative permittivity and permeability between MWCNT/Fe3O4 nanohybrids and the physical mixture of MWCNTs and Fe3O4 nanocrystals indicates that strong interactions occur between the Fe3O4 and the MWCNT in the MWCNT/Fe3O4 hybrid. Trying to reveal the nature of these interactions, XANES at the C K-edge and Fe L-edge as well as ESR analysis of the MWCNT/Fe3O4 nanohybrid were examined. Figure 4 shows the C K-edge XANES for MWCNTs and MWCNT/Fe3O4 nanohybrids. C K-edge XANES investigates the unoccupied molecular orbital of MWCNTs with carbon p characters. The area under resonance, pending no countervailing symmetry arguments, is proportional to the unoccupied densities of state (DOS).25 Two main peaks at 285.0 and 291.3 eV for both samples are attributed to the transitions from C 1s to the unoccupied states of CC π* and C−C σ* character, respectively. The transitions at ∼288 eV are attributed to the CO π* groups.26 The 286.6 and 289.5 eV features should be attributed to Fe3O4 nanocrystals−MWCNT interaction, which elongates the C−C bond of MWCNT at the interface, resulting in a shape resonance closer to the threshold. On the basis of the C K-edge XANES analysis, several features are worthy of note. First, the π* transition intensity, which reflects the unoccupied

increase to the microwave absorption abilities was noted after the MWCNT was decorated by high-density Fe3O4 nanocrystals. Minimum reflection losses reached as low as −41.61 dB at 5.5 GHz, which was comparable to the almost 100% of EM wave absorption. It is worth noting that this minimum loss is significantly lower than those reported in the literature, i.e., MWCNT/CoFe2O4 (−18.0 dB),13 Fe3O4/SnO2 (−27.40 dB),14 and MWCNT/FeCo (−28.20 dB).12 The absorption bandwidth with a reflection loss below −10 dB is up to 8.4 GHz (from 3.0 to 11.4 GHz) for the absorber with the thickness in 2.0−5.0 mm. The hybrid shows an excellent electromagnetic wave attenuation performance, including high absorption efficiency, strong absorption, and a wide operation frequency band width. Microwave propagation in electromagnetic media is largely determined by the complex relative permittivity (εr = ε′ − jε″) and permeability (μr = μ′ − jμ″) of the materials. Figure 3a and 3b presents the real part (ε′) and imaginary part (ε″) of the complex relative permittivity of the measured samples in the 2− 18 GHz range, respectively. MWCNT, a typical dielectric loss material for microwaves, has the highest ε′ and ε″ values compared to others, while Fe3O4 nanocrystals possess the lowest ε′ and ε″ values. A broad peak around 5.0−9.0 GHz for the MWCNTs is shifted to the range of 3.8−8.0 GHz when they are decorated with Fe3O4 nanocrystals in ε′. In the case of real and imaginary parts of the complex permeability (Figure 3c and 3d), Fe3O4 nanocrystals have higher μ′ and μ″ values than other samples. For MWCNTs both the real and the imaginary parts of the complex permeability are negligible. The most interesting phenomenon is that both the complex relative permittivity and the permeability of the mixture show very 5449

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the peaks in the L-edge of MWCNT/Fe3O4 nanohybrids as compared to these of Fe3O4 nanocrystals suggests a charge relocation from the conduction band with Fe 3d feature in Fe3O4 to C, which is consistent with the above C K-edge results. This observation further confirms formation of covalent bonding in the hybrid which is via Fe−O−C bonding. XANES at the C K-edge and Fe L-edge strongly indicates that Fe3O4 nanocrystals interact with MWCNTs by covalent bonding in the nanohybrid, which results in charges transferred from the conduction band in Fe3O4 nanocrystals to the C 2p-derived states in MWCNT substrate. Figure 6 presents the ESR spectra of the MWCNT/Fe3O4 nanohybrids, pure Fe3O4 nanocrystals, and MWCNTs. The Figure 4. XANES C K-edge of MWCNT and MWCNT/Fe3O4 nanohybrid.

DOS of π* character, is reduced in MWCNT/Fe 3 O 4 nanohybrids as compared to MWCNTs. This provides direct evidence that charge transfer from Fe3O4 to C 2p-derived π* states in MWCNT has occurred at the interface. Second, the intensity increase of the 288 eV transitions in the MWCNT/ Fe3O4 hybrid suggests that there is an oxidized C environment at the interface of the nanohybrids, which withdraws a significant amount of charge from C, resulting in a localized, high density of the unoccupied state of C p character. These features indicate that there is strong interaction between Fe3O4 and MWCNT in the MWCNT/Fe3O4 nanohybrids through Fe−O−C bonding. The Fe L3,2-edge XANES of the MWCNT/Fe3O4 nanohybrids and Fe3O4 nanocrystals are shown in Figure 5, which

Figure 6. ESR spectra of MWCNT, Fe3O4 nanocrystal, and MWCNT/Fe3O4 nanohybrid.

principle of ESR relies on the transition between Zeeman components of the electronic levels. Splitting at a given external magnetic field yields information on the magnetic moment μ of the atoms or ions involved in the resonance transitions.30−32 ESR spectra of both MWCNT/Fe3O4 nanohybrids and Fe3O4 nanocrystals show a nearly symmetric signal, which is typical for superparamagnetic iron oxide nanocrystals. When Fe3O4 nanocrystals are bonded with MWCNT, the ESR spectrum shows an increased line width from 1950 G of Fe3O4 nanocrystals alone to 2643 G of MWCNT/Fe3O4 hybrid and shifts to lower fields. The shift depends on the local chemical (i.e., electronic) environment of the unpaired electron. Line width broadening is attributed to spin disorder (frustrations) coming from mainly antiferromagnetic interactions between the neighboring spins in magnetic nanocrystals.33,34 This clearly indicates that the Fe3O4 nanocrystals bonding with MWCNT enhances the exchange interactions between magnetic nanocrystals, which results in the change of magnetic moment μ. The charge redistribution at the dielectric matter significantly affects the discharge parameters, electronic polarization, ion polarization, intrinsic electric dipolar polarization, and polarized centers.35 Meanwhile, the dipolar interactions of magnetite nanoparticles and the charge redistribution vary the magnetic moment of the materials. All of these lead to the complex relative permittivity εr and permeability μr of the MWCNT/ Fe3O4 hybrid well balanced when compared with their physical mixtures, as presented in Figure 3. An important parameter relating to the reflection loss is the concept of matched characteristic impedance, where the characteristic impedance of the absorbing materials should nearly approach that of the free space so as to achieve zero reflection at the front surface of the

Figure 5. XANES Fe L3,2-edge of Fe3O4 and MWCNT/Fe3O4 nanohybrid.

provides the electronic and chemical structure information on the orbital symmetry and spin state of the materials. Spectral features arise from dipole transitions (Δl = ± 1, Δj = ± 1,0). The Fe L3-edge and L2-edge are dominated by the electronic transitions from 2p3/2 to 3d5/2,3/2 and from 2p1/2 to 3d3/2, respectively. The local ground state of Fe ions is a mixture of configurations where 3dn, 3dn+1 L (denoting ligand hole), and 3dn+2 L2 with n = 5 and 6 for Fe3+ and Fe2+ ions, respectively.27−29 The final state in the L-edge absorption is predominantly a mixture of configurations 2p3dn+1 and 2p3dn+2 L. Spectra from the measured samples in Figure 5 show a constancy of edge shapes for given oxidation states. The intensity of the peaks reflects the unoccupied 3d state in the Fe atoms available for mixing with the O 2p states. The decrease of 5450

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materials.7,23,24 Large ε″ and negligibly small μ″, such as MWCNTs, or vice versa like Fe3O4 nanocrystals, usually endow them with a poor matched state. The interactions between Fe3O4 nanocrystals and MWCNT support in the MWCNT/ Fe3O4 hybrids are of great benefit in balancing the permeability and permittivity. This results in an improvement of matching impedance for the microwave absorber with the free space. Furthermore, the interfacial polarization and associated relaxation also contribute to the microwave attenuation. Finally, significant microwave absorption is achieved.

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CONCLUSIONS Although Fe3O4 and MWCNT alone as well as their physical mixture showed little ability for the microwave absorption, their hybrid material exhibited considerable electromagnetic absorbing ability at a frequency of 2−18 GHz, with minimum reflection loss reaching −41.61 dB at a frequency of 5.5 GHz and reflection loss below −10 dB up to 8.4 GHz with a thickness of 2.0−5.0 mm. Microstructural analysis revealed that Fe3O4 nanocrystals were immobilized on the MWCNT surface by a strong interaction. Charges transferred from the conduction band in Fe3O4 to the C 2p-derived states in MWCNT substrate and increased dipolar interactions between the magnetic nanocrystals took place in the MWCNT/Fe3O4 hybrid. As a result, the hybrids achieved significant microwave absorption.

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ASSOCIATED CONTENT

S Supporting Information *

TEM, XRD, and PPMS characterization on Fe3O4 nanocrystals. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Email: [email protected]; [email protected]. Author Contributions ‡

These authors contributed equally.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors appreciate the financial support of the National Natural Science Foundation of China (Nos. 51102067, 81101087, 81130028, and 31210103913), Science and Technology Research Project of Heilongjiang Education Department (No. 12511325), China Postdoctoral Science Foundation (20110491052, 2012T50321, 2012M510992), Heilongjiang Postdoctoral Foundation (LBH-Z10139, LBHZ11054), Medical Scientific Research Foundation of Heilongjiang Province Health Department (2011-165), and Fundamental Research Funds for the Central Universities (Grant No. HIT NSRIF. 2010067). C.L.S. is supported by the NSERC, NRC, CIHR of Canada, and University of Saskatchewan.

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REFERENCES

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The Journal of Physical Chemistry C

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dx.doi.org/10.1021/jp4000544 | J. Phys. Chem. C 2013, 117, 5446−5452