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Nano Research 2014, 7(5): 704–716 DOI 10.1007/s12274-014-0432-0

High densities of magnetic nanoparticles supported on graphene fabricated by atomic layer deposition and their use as efficient synergistic microwave absorbers Guizhen Wang1,2,3, Zhe Gao1, Gengping Wan3, Shiwei Lin3, Peng Yang1,2, and Yong Qin1 () 1

State Key Laboratory of Coal Conversion, Institute of Coal Chemistry, Chinese Academy of Sciences, Taiyuan 030001, China University of Chinese Academy of Sciences, Beijing 100039, China 3 Key Laboratory of Chinese Education Ministry for Tropical Biological Resources, Hainan University, Haikou 570228, China 2

Received: 9 December 2013

ABSTRACT

Revised: 15 February 2014

An atomic layer deposition (ALD) method has been employed to synthesize Fe3O4/graphene and Ni/graphene composites. The structure and microwave absorbing properties of the as-prepared composites are investigated. The surfaces of graphene are densely covered by Fe3O4 or Ni nanoparticles with a narrow size distribution, and the magnetic nanoparticles are well distributed on each graphene sheet without significant conglomeration or large vacancies. The coated graphene materials exhibit remarkably improved electromagnetic (EM) absorption properties compared to the pristine graphene. The optimal reflection loss (RL) reaches –46.4 dB at 15.6 GHz with a thickness of only 1.4 mm for the Fe3O4/graphene composites obtained by applying 100 cycles of Fe2O3 deposition followed by a hydrogen reduction. The enhanced absorption ability arises from the effective impedance matching, multiple interfacial polarization and increased magnetic loss from the added magnetic constituents. Moreover, compared with other recently reported materials, the composites have a lower filling ratio and smaller coating thickness resulting in significantly increased EM absorption properties. This demonstrates that nanoscale surface modification of magnetic particles on graphene by ALD is a very promising way to design lightweight and high-efficiency microwave absorbers.

Accepted: 17 February 2014 © Tsinghua University Press and Springer-Verlag Berlin Heidelberg 2014

KEYWORDS atomic layer deposition (ALD), graphene, magnetic nanoparticles, microwave absorption

1

Introduction

A considerable number of theoretical and experimental investigations have been focused on microwave absorbing materials (MAMs) due to the increasing electromagnetic (EM) interference problems arising Address correspondence to [email protected]

from the extensive use of communication devices, such as telecommunications, local area network systems, and radar systems. To date, various MAMs including ferrites [1, 2], metallic magnetic fillers [3, 4], conductive fibers [5, 6], ceramics [7, 8], chiral media [9–11] and metamaterials [12–14] have been extensively researched.

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For practical applications, absorbing materials should have not only a strong absorption and a broad bandwidth, but also light weight and low thickness. Carbon-based materials display outstanding physical properties that make them superior candidates for microwave applications, namely high electrical conductivity, low density, and good stability [15, 16]. One prominent representative of carbon species is graphene, which consists of a single layer of carbon atoms in a closely packed honeycomb two-dimensional (2D) lattice, and exhibits unique physical, chemical, and mechanical properties, which allow it to be used as an EM absorber [17, 18]. However, efficient EM impedance matching between the relative permittivity and permeability of MAMs is an important prerequisite for strong absorption. Pure graphene shows predominantly dielectric loss and very low magnetic loss, due to its weak magnetic properties, which reduces its efficacy in practical applications. Incorporating magnetic constituents in carbon materials is one of the effective ways to improve impedance matching, increase the magnetic loss, broaden the absorption bandwidth, and enhance absorption intensity. Moreover, it has been reported that heteronanostructured materials show great potential as microwave absorbers with lower reflection loss (RL) than single-component absorbers due to effective interfacial polarization and associated relaxation loss induced by multiple interfaces [19–24]. However, when using traditional solvothermal routes [25, 26], thermal decomposition [27, 28] or chemical precipitation methods [29], it is still difficult to realize dielectric– magnetic coupled graphene-based microwave materials with the growth of uniform, high density and highquality magnetic nanoparticles (NPs) firmly anchored on graphene. Atomic layer deposition (ALD) is a vapor-phase thin film growth technique that employs self-limiting chemical reactions allowing atomic-scale thickness and uniformity control [30–32]. In particular, ALD can be employed for the fabrication of complex heterostructures, in which the desired properties of one material acting as a support are combined with complementary and desired properties of a second material. Currently, a vast library of magnetic materials ((Fe3O4 [33], Ni [34–36], Co [35, 36], and NiFe2O4 [37])

can be produced by ALD, which allows a variety of hybrid nanomaterials to be designed and their complex permittivity and permeability to be tailored to give excellent EM absorbing performance. In this work, we report the ALD-assisted synthesis of Fe3O4/graphene (Fe3O4/G) and Ni/graphene (Ni/G) composites as high-efficiency and lightweight EM absorbing materials. Benefiting from the highly controllable ALD technology, well dispersed Fe3O4 or Ni NPs with narrow size distribution were homogeneously anchored throughout the entire surface of the graphene. The microwave absorption properties of as-prepared samples were investigated in terms of complex permittivity and permeability. The results show that the coated graphene materials exhibit remarkably improved microwave absorption properties compared to the pristine graphene, which can be ascribed to the effective impedance matching, multiple interfacial polarization and increased magnetic loss from the added magnetic constituents.

2 2.1

Experimental Preparation of graphene

Graphite oxide (GO) was prepared according to the modified Hummers method. Graphene was obtained by rapid heating of GO under high vacuum [38]. The as-prepared GO was pre-evacuated to pressure of less than 2.0 Pa, and then a heating schedule with a heating rate of 30 °C/min was executed. An abrupt expansion of GO was observed with a mass of fluffy black powder generated at about 200 °C. The above expanded GO was further annealed at 900 °C for 30 min to afford graphene. 2.2

Synthesis of Fe3O4/G and Ni/G

The ALD process was carried out in a home-made, closed type, hot-wall ALD reactor. Prior to ALD, the graphene was dispersed in ethanol by ultrasonic agitation and then dropped onto a quartz wafer. After being air-dried, the Fe2O3 or NiO coatings were deposited by sequential exposure of the graphene to ferrocene (FeCp2) or nickelocene (NiCp2) and O3. Deposition temperatures for Fe2O3 and NiO were 200 and 150 °C, respectively. FeCp2 and NiCp2 were kept

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at 100 and 80 °C, respectively. Finally, after the ALD process, the samples were transferred to a furnace and reduced at 450 °C in 5% H2/Ar atmosphere for 2 h. 2.3

Sample characterization

X-ray diffraction (XRD) patterns were collected on a Bruker D8 Advance X-ray diffractometer using a Cu Kα source (λ = 0.154056 nm). X-ray photoelectron spectra (XPS) were recorded on an AXIS ULTRA DLD spectrometer (Shimadzu/Kratos) to characterize the surface composition with the Al Kα line as the excitation source. Transmission electron microscopy (TEM) and high-resolution TEM (HRTEM) images were taken on a JEOL-2100 microscope instrument at an acceleration voltage of 200 kV. The composition of the samples was analyzed by energy-dispersive X-ray (EDX) spectroscopy using an EDX attachment to the TEM instrument. Fourier transform infrared (FTIR) spectra were collected on a Bruker TENSOR27 spectrometer with the samples pre-pressed with KBr into pellets. Raman spectroscopy was performed on a Renishaw inVia Reflex Raman microscope using 532 nm green laser excitation. The specimens for measuring the EM properties were prepared by uniformly mixing 10 wt.% graphene or coated graphene with paraffin and pressing the mixture into a cylindrical shape. Then the cylinder was cut into a toroid of 7.00 mm outer diameter and 3.04 mm inner diameter for measurement. The relative permeability and permittivity values of the mixture were determined and obtained by measuring the S11 and S21 parameters between 2 and 18 GHz with an AV3629D network analyzer by using the transmission/reflection coaxial line method.

3

precursors, which is a useful strategy to eliminate this predicament. Ozone molecules can be physisorbed on the surface of graphene, and act as nucleation sites for effective deposition [39]. Figures 1(b) and 1(c) show low-magnification TEM images of Fe2O3/G obtained by applying 100 ALD cycles of Fe2O3 deposition. It can be seen that ALD is an efficient method to deposit highly homogeneous Fe2O3 films on the surfaces of graphene. The Fe2O3 layers are about 2 nm thick, as observed from the edge of graphene. The HRTEM image (Fig. 1(d)) shows no obvious lattice fringes, which indicates the poor crystallinity of as-deposited Fe2O3. The deposited Fe2O3 can be readily converted to magnetic Fe3O4 by a reduction process. Fe3O4/G composites were obtained by annealing Fe2O3/G at 450 °C for 2 h under a mixture of H2/Ar flow in a tube furnace. The structure and morphology of as-prepared Fe3O4/G were investigated by means of XRD, XPS and TEM. Figure 2(a) presents the XRD profiles of graphene, Fe2O3/G and Fe3O4/G (prepared by 100 cycles of Fe2O3 deposition, and then a hydrogen reduction, denoted as 100-Fe3O4/G) composites. For the graphene, the diffraction peak at 24.7° can be attributed to the (002) planes of a graphitic structure with short-range order in stacked graphene sheets. No clear diffraction peaks of Fe2O3 were found for products after 100 ALD cycles of Fe2O3 deposition (no diffraction peak was seen even for 200 ALD cycles of Fe2O3 deposition, see Fig. S1 in the Electronic Supplementary Material (ESM)),

Results and discussion

Figure 1(a) displays representative TEM images of the starting graphene used in this study. Typical curved 2D structures with many stripelike wrinkles can be observed. In fact, using ALD technology it is difficult to precisely control thickness and conformally deposit materials onto graphene, because ALD is a surface-reaction-limited process, and graphene, being sp2-bonded, has no out-of-plane covalent functional groups to initiate the ALD reaction. In this work, ozone was selected as a reactant to react with metal

Figure 1 (a) TEM image of pristine graphene. (b) and (c) TEM and (d) HRTEM images of Fe2O3/G obtained by applying 100 ALD cycles of Fe2O3 deposition.

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which is probably due to the poor crystallinity and low content of Fe2O3 in the composites. After 100-Fe3O4/G composites were formed by hydrogen reduction, the relevant peaks can be indexed to the face-centered cubic crystal structure of Fe3O4 according to Joint Committee on Powder Diffraction Standards (JCPDS), powder diffraction file No. 01-1111. To further prove the structural conversion from Fe2O3 to Fe3O4, XPS analyses were performed. As shown in Fig. 2(b), the peaks generally shift to high binding energy and broaden for the reduced sample due to the appearance

of Fe2+ 2p3/2 and Fe2+ 2p1/2. In our case, the Fe 2p line shapes for the Fe2O3 differ significantly from that of Fe3O4. The binding energies of Fe 2p3/2 and Fe 2p1/2 are 711.1 and 724.7 eV for Fe2O3, and 711.8 and 725.2 eV for Fe3O4, respectively. The 2p peak of Fe2+ indicated by the arrow is observed only in Fe3O4, which is in good agreement with the literature [40, 41]. The XPS analysis clearly reveals that the Fe2O3 is effectively converted to Fe3O4 by the reduction process. Figures 3(a)–3(c) show low-resolution TEM images of the assynthesized 100-Fe3O4/G nanohybrids. It can be clearly

Figure 2 (a) XRD patterns of graphene, 100-Fe2O3/G and 100-Fe3O4/G composites. (b) XPS spectra of Fe 2p core-level lines for 100-Fe2O3/G and 100-Fe3O4/G composites.

Figure 3 (a)–(c) TEM and (d) HRTEM images of 100-Fe3O4/G composites. The insets in (b) and (c) are the SAED pattern and size distribution analysis, respectively. www.theNanoResearch.com∣www.Springer.com/journal/12274 | Nano

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seen that the surfaces of graphene are densely covered by Fe3O4 NPs with a narrow particle size distribution and an average size of 5.6 nm (the inset in Fig. 3(c)). The distribution of Fe3O4 NPs on each graphene sheet is uniform, and no significant conglomeration of Fe3O4 NPs or large vacancies on graphene are observed. A very few larger particles appear in the wrinkles, since migration and aggregation of particles occur more easily in these locations. The inset in Fig. 3(b) exhibits corresponding selected area electron diffraction (SAED) pattern recorded on an individual 100-Fe3O4/G sheet. Several rings can be assigned to diffraction planes of the cubic phase of Fe3O4, in agreement with the XRD data. The HRTEM image highlights the well-defined crystalline lattice spacings of 0.25 and 0.49 nm, which can be indexed as (311) and (111) crystal planes of Fe3O4 (Fig. 3(d)), respectively. It is also worth mentioning that the Fe3O4/G nanohybrids exhibit good stability under ultrasonic vibration—uniform Fe3O4 NPs are still firmly anchored on graphene after ultrasonic treatment (40 kHz) for 1 h (Fig. S2 in the ESM). The content and particle size of Fe3O4 NPs in the nanohybrids may have an effect on the EM absorption properties. Thus, the relationship between Fe3O4 particle size and the number of ALD cycles was also

investigated. The TEM images of Fe2O3/G structures obtained by employing 50 and 200 ALD cycles of Fe2O3 deposition show that the thickness of the Fe2O3 layers increases with increasing cycle number (Fig. S3 in the ESM). The thickness is about 4 nm after 200 ALD cycles, which corresponds to a growth rate of about 0.2 Å per cycle. As shown in Figs. 4(a) and 4(b) and Figs. 4(d) and 4(e), after the same reduction treatment, the Fe2O3 layers can also be converted to uniform Fe3O4 NP films. The average diameters of Fe3O4 NPs measured by TEM images are 4.5 and 9.4 nm after 50 and 200 cycles of Fe2O3 deposition, respectively (denoted hereafter as 50-Fe3O4/G and 200-Fe3O4/G). XRD profiles (Fig. S4 in the ESM) and the lattice fringes from HRTEM images (Figs. 4(c) and 4(f)) also reveal the features of cubic phase Fe3O4. EDX analysis (Fig. S5 in the ESM) of the as-synthesized Fe3O4/G products using different numbers of ALD cycles of Fe2O3 indicates the presence of Fe, C, and O elements and the Fe atom ratio increases with increasing number of ALD cycles. FTIR and Raman spectroscopy were also employed to further characterize the surface functionalities of graphene and Fe3O4/G hybrids qualitatively. As shown in Fig. 5(a), the broad absorption peaks at about

Figure 4 TEM and HRTEM images of (a)–(c) 50-Fe3O4/G and (d)–(f) 200-Fe3O4/G. | www.editorialmanager.com/nare/default.asp

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Figure 5 (a) FTIR and (b) Raman spectra of graphene and Fe3O4/G hybrids.

3,400 cm–1 and the band at 1,635 cm–1 correspond to the surface-adsorbed water and the hydroxyl groups. The absorption peak at 1,580 cm–1 is assigned to the components from the skeletal vibrations of un-oxidized graphitic domains (δ C=C). The band at 571 cm–1 is related to the Fe–O stretching vibration modes and the intensity of the Fe–O band clearly increases with increasing number of ALD cycles, consistent with the formation of Fe3O4 [42]. Figure 5(b) shows Raman spectra of the as-synthesized graphene and Fe3O4/G composites. The Raman-active E2g mode at 1,585 cm–1 (G-band) is the characteristic for graphitic sheets, whereas a broad D-band centered at 1,345 cm–1 can be attributed to the presence of sp3 defects within the graphene sheets. The fundamental Raman scattering peaks for Fe3O4 are also observed at 330, 510, and 673 cm–1 corresponding to the Eg, T2g, and A1g vibration modes, respectively [43]. Microwave absorption properties were investigated by mixing 10 wt.% of the samples with paraffin. The RL curves of graphene–paraffin and Fe3O4/G–paraffin were derived from the relative complex permittivity and permeability at a given frequency and layer thickness according to the transmit line theory, which can be expressed as the following Eqs. (1) and (2) [44, 45]: Zin  Z0 ( r /  r )1/ 2 tanh [j (2 fd / c )( r  r )1/ 2 ]

RL  20 log (Zin  Z0 ) / (Zin  Z0 )

(1) (2)

where Zin is the input impedance of the absorber, Z0

the impedance of free space, μr the relative complex permeability, εr the complex permittivity, f the frequency of microwaves, d the thickness of the absorber, and c the velocity of light. A RL value of –10 dB is comparable to 90% microwave absorption. In general, materials with RL values of less than –10 dB absorption are considered as suitable EM wave absorbers. Figure 6(a) shows a comparison of calculated RL curves in the frequency range of 2–18 GHz for the product/paraffin composites with a thickness of 1.5 mm. The RL properties of pristine graphene are expected to arise mainly from dielectric loss, with a minimum RL of –3.3 dB. Compared to graphene, the RL performances toward EM waves of coated graphene are enhanced substantially. The minimum RL of 50-Fe3O4/G, 100-Fe3O4/G, and 200-Fe3O4/G are –7.9, –36.4, and –13.6 dB at 9.5, 14.1, and 17.0 GHz, respectively. The microwave absorption values less than –10 dB for 100-Fe3O4/G and 200-Fe3O4/G are in the ranges of 12.4–16.9 GHz and 15.2–18.0 GHz corresponding to a bandwidth of 4.5 and 3.8 GHz, respectively. To reveal in detail the influence of thickness on the absorption properties, three-dimensional RL values of 100-Fe3O4/G and 200-Fe3O4/G are shown in Figs. 6(b) and 6(c). The minimum RL reaches –46.4 dB at 15.6 GHz for 100-Fe3O4/G with a thickness of only 1.4 mm, and –29.5 dB at 9.7 GHz with a thickness of 2.4 mm for 200-Fe3O4/G. Moreover, RL values below –20 dB can be obtained by choosing an appropriate thickness of the absorbent layer between 1.2 and 5 mm in the frequency range of 3.4–18.0 GHz


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for 100-Fe3O4/G. All of these values are evidently improved relative to the pristine graphene, demonstrating that Fe3O4/G with its uniform and controllable deposit of Fe3O4 NPs can greatly ameliorate the microwave absorbing performance, with high absorption efficiency, strong absorption, and a wide operation frequency bandwidth. Furthermore, compared with other recently reported materials [21, 22, 46–49], the composites have a lower filling ratio and smaller coating thickness with significantly increased electromagnetic absorption properties, as shown in Table 1. The contents of other materials in the paraffin matrix reported in the literature are usually more than 50 wt.% with a thickness exceeding 2 mm, whereas for our 100-Fe3O4/G nanohybrids the content is 10 wt.% and the thickness is 1.4 mm. These results demonstrate that Fe3O4/G nanohybrids have not only excellent absorption performance but also light weight. In general, the reflection and attenuation properties of EM wave absorbers depend on the matching frequency, layer thickness of absorbers, and the relative complex permeability and permittivity, which are determined by their nature, shape, size, and microstructure [50, 51]. In the present work, the enhanced absorption properties can be explained in the following terms. First, the enhanced EM absorption properties are related to the increased impedance matching. According to transmission line theory, the RL is improved when the dielectric contribution matches the magnetic contribution based on the requirement of the input impedance [44]. The complex permittivity Table 1 EM absorption properties of some recently reported additives EM absorbing material

Figure 6 Microwave absorption properties of the products calculated by using the measured relative complex permittivity and permeability values according to transmission line theory. (a) Reflection loss curves of the product/paraffin composites with a thickness of 1.5 mm in the frequency range of 2–18 GHz. Three-dimensional representations of the reflection losses of (b) 100-Fe3O4/G and (c) 200-Fe3O4/G.

Absorbent RLmin Thickness (mm) content (wt.%) (dB)

Ref.

Fe3O4/TiO2

50

–20.6

5.0

[21]

Fe3O4

68

–42.7

6.9

[42]

Fe2Ni2N

50

–25.5

3.0

[40]

(Fe, Ni)/C nanocapsules

40

–26.9

2.0

[22]

Fe3O4/SnO2

80

–27.4

4.0

[43]

MWCNT/Fe3O4

50

–41.6

3.4

[41]

100-Fe3O4/G

10

–46.4

1.4

This work


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real part (ε'), permittivity imaginary part (ε"), permeability real part (μ'), and permeability imaginary part (μ") of the graphene–paraffin and Fe3O4/G– paraffin were investigated in the frequency range of 2–18 GHz (Fig. 7). The pristine graphene shows negligible permeability due to the lack of magnetic composition and very high ε' and ε" values from 66.4 to 9.1 and from 126.9 to 24.9, respectively. This means that the magnetic loss and the dielectric loss are out of balance in this case, inducing poor EM wave absorption. By decorating with Fe3O4 NPs, the coated graphene shows reduced imaginary permittivity and increased imaginary permeability compared to pristine graphene, which helps improve the level of impedance matching. However, since excessively low ε" caused by superfluous thick Fe3O4 coatings also negatively affects performance, the 200-Fe3O4/G exhibits poorer absorption abilities than G/100-Fe3O4. Second, the improvement in microwave absorption partly originates from the multiple associated relaxation of effective interface polarization. Benefiting from the highly controllable ALD technology, the Fe3O4

NPs obtained after the reduction process were integrated firmly with the surface of graphene without agglomeration. The charge transfer between graphene and Fe3O4 also occurs with little hindrance. Therefore, the interfacial polarization and associated relaxation should contribute to the enhanced EM absorption properties. Conventionally the relaxation process, which can be described by a Cole–Cole semicircle, has an important influence on the permittivity behavior of microwave absorption materials. According to the Debye dipolar relaxation [52], the relative complex permittivity (εr) can be expressed by the following equation: εr  ε'  iε"  ε 

εs  ε 1  iωτ0

(3)

where τ0, εs, and ε are the relaxation time, the static dielectric constant, and the dielectric constant at infinite frequency, respectively. From Eq. (3), it can be deduced that ε'  ε 

εs  ε 1  (ωτ0 ) 2

(4)

Figure 7 Measured frequency dependence of (a) real and (b) imaginary parts of complex permittivity and (c) real and (d) imaginary parts of permeability.


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ωτ0 εs  ε )

ε" 

1  (ωτ0 )

(5)

2

According to Eqs. (4) and (5), the relationship between ε' and ε" can be further deduced, 2

εs  ε    εs  ε  2  ε'    (ε")    2    2 

2

(6)

Thus the plot of ε' versus ε" is a single semicircle, which is usually defined as a Cole–Cole semicircle, and each semicircle corresponds to one Debye relaxation process. Plots of ε" versus ε' for graphene and Fe3O4/G composites are shown in Fig. 8, where three superimposed Cole–Cole semicircles are found for all the Fe3O4/G samples, which may suggest that there are ternary dielectric relaxation processes. Under a sufficient electromagnetic field, electrons may gain enough energy to surmount the interface between the graphene and the Fe3O4 NPs and move onto the Fe3O4 particles, and thus an interfacial behavior of accumulating space charges is expected for Fe3O4/G composites [53]. In addition, the graphene/paraffin or NPs/paraffin interfacial polarization may contribute to other two weaker relaxation processes. Third, the magnetic loss is also an important factor that contributes to electromagnetic wave attenuation

in Fe3O4/G because an obvious enhancement in μ" with increasing amount of Fe3O4 can be seen in Fig. 7(d). In the microwave frequency band, magnetic loss mainly comes from eddy current effects, natural resonance and exchange resonance. The eddy current loss contribution to the imaginary part of permeability is related to the thickness (d) and the electrical conductivity (σ) of the composite, which can be expressed by the equation    2 0 (  )2  d 2 f / 3 , where 0 is the permeability of vacuum. According to this equation, if the magnetic loss only originates from the eddy current loss, the values of  (  )2 f 1 should be constant when the frequency is changed. As shown in Fig. 9, the value of  (  )2 f 1  20 d 2 / 3 remains approximately constant when f >6.5 GHz for 100-Fe3O4/G and 200-Fe3O4/G, which confirms that magnetic loss is caused mainly by the eddy current loss in this frequency band. The natural resonance occurs usually at a lower frequency than the exchange resonance. The peaks at 4.0 and 6.1 GHz for 100-Fe3O4/G can be attributed to the natural resonance and the exchange resonance, respectively. Besides Fe3O4, ALD technology can be also used to synthesize large amounts of other magnetic materials, such as Ni, Co and NiCo2O4. In the present work, the

Figure 8 Typical Cole–Cole semicircles (ε" versus ε') for graphene and Fe3O4/G composites in the frequency range of 2–18 GHz. | www.editorialmanager.com/nare/default.asp

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size and loading density of Ni NPs increase with increasing number of ALD deposition cycles of NiO. XRD patterns (Fig. S8 in the ESM) and highmagnification TEM and HRTEM analyses (Fig. S9 in the ESM) further reveal the good crystallinity and dispersion of the as-synthesized Ni NPs. The Ni/G composites also exhibit obviously improved ability for microwave absorption. As shown in Fig. 10(d), the minimum RL of 200-Ni/G is –21.1 dB at 15.1 GHz and the bandwidth corresponding to RL below –10 dB is higher than 3.8 GHz at a thickness of 2.0 mm.

Figure 9 Plots of μ"( )–2f –1 vs. frequency for 50-Fe3O4/G, 100-Fe3O4/G and 200-Fe3O4/G.

same procedure was also applied to the synthesis of Ni/G composites. TEM images (Fig. S6 in the ESM) show that NiO/G with a controllable thickness of NiO can be obtained by applying 50, 100, and 200 ALD cycles. Similarly, NiO/G can be converted to uniform Ni NPs after a hydrogen reduction process (Fig. S7 in the ESM). Figures 10(a)–10(c) show the TEM images of Ni/G composites and it is evident that the particle

4

Conclusions

In summary, Fe3O4/G nanocomposites have been synthesized through an ALD technique, in which densely packed and Fe3O4 NPs with a narrow particle size distribution are uniformly deposited on the graphene nanosheets. The as-prepared Fe3O4/G composites show superior microwave absorption performances compared with those of pristine graphene. The minimum RL reaches –46.4 dB at 15.6 GHz for 100-Fe3O4/G with a thickness of only 1.4 mm and RL below –20 dB can be obtained by choosing an

Figure 10 TEM images of Ni/G obtained by applying (a) 50, (b) 100, and (c) 200 cycles of NiO deposition and then a hydrogen reduction process. (d) Microwave reflection loss curves of the product/paraffin composites with a thickness of 2.0 mm. www.theNanoResearch.com∣www.Springer.com/journal/12274 | Nano

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appropriate thickness of the absorbent layer between 1.2 and 5 mm in the frequency range of 3.4–18.0 GHz. Moreover, the content of the composites in the matrix is only 10 wt.%. Such enhanced microwave absorption properties are attributed to the efficient complementarity between complex permittivity and permeability, interfacial polarization between the magnetic NPs and graphene, and magnetic losses associated with efficient separation of magnetic NPs. In addition, Ni/G composites were also prepared and exhibit obviously improved ability for microwave absorption. This study shows that depositing magnetic materials on the surface of graphene by ALD technology is an efficient way to fabricate lightweight materials with strong EM absorption characteristics.

Acknowledgements This work was financially supported by the National Natural Science Foundation of China (Nos. 21376256, 21173248, 21203229, 51362010), the Hundred Talents Program of the Chinese Academy of Sciences, the Hundred Talents Program of Shanxi Province, and in-house projects of the State Key Laboratory of Coal Conversion of China (Nos. Y2BWLD1931, Y3BWLE1931). Electronic Supplementary Material: Supplementary material (XRD patterns and TEM images of Fe2O3/G, TEM images of 100-Fe3O4/G after the ultrasonic process, XRD patterns and EDX spectra of Fe3O4/G, TEM images and XRD patterns of NiO/G, TEM images of Ni/G, XPS spectra of 200-NiO/G and 200-Ni/G) is available in the online version of this article at http://dx.doi.org/10.1007/s12274-014-0432-0.

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