Fabrication of NiCo2-Anchored Graphene Nanosheets by Liquid

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Fabrication of NiCo2‑Anchored Graphene Nanosheets by LiquidPhase Exfoliation for Excellent Microwave Absorbers Ruilong Yang,† Bochong Wang,‡ Jianyong Xiang,*,† Congpu Mu,‡ Can Zhang,† Fusheng Wen,*,† Cong Wang,† Can Su,† and Zhongyuan Liu*,† †

State Key Laboratory of Metastable Materials Science and Technology and ‡School of Science, Yanshan University, Qinhuangdao 066004, People’s Republic of China S Supporting Information *

ABSTRACT: Graphene nanosheets (GNSs) were prepared by an efficient liquid-phase exfoliation method, and then the NiCo2/GNS nanohybrids were fabricated using the single-mode microwave-assisted hydrothermal technique. The NiCo2/GNS composites with different GNS proportions were investigated as microwave absorbers. Morphology investigation suggested that NiCo2 nanocrystals were uniformly anchored on the GNS without aggregation. The electromagnetic parameters of NiCo2/GNS nanohybrids could be artificially adjusted by changing the GNS proportion, which led to an exceptional microwave-absorbing performance. A reflection loss (RL) exceeding −20 dB was obtained in the frequency range of 5.3−16.4 GHz for the absorber thicknesses of 1.2−3.2 mm, while an optimal RL of −30 dB was achieved at 11.7 GHz for a thickness of 1.6 mm. The enhanced microwave-absorbing performance indicated that the NiCo2/10 wt % GNS composite has great potential for use as an excellent microwave absorber. KEYWORDS: graphene nanosheets, liquid phase exfoliation, NiCo2, single-mode microwave-assisted hydrothermal, permittivity, microwave absorption, reflection loss, quarter-wavelength matching model

1. INTRODUCTION In the past decade, microwave communication technology and its related electronic devices have been developed rapidly, such as the 4G wireless network system, portable wifi devices, smartphones, and so on.1 People reap benefits from these advanced technologies but are inevitably disturbed by them at the same time. One of the urgent issues that needs to be solved is electromagnetic pollution, which can cause terrible threats to human biological systems.2−4 Therefore, further development of high-efficiency microwave-absorbing materials should be put on the agenda, although microwave absorption techniques have been investigated several decades.5−7 From a physical point of view, the microwave absorption ability of one material is mainly determined by the electromagnetic parameters, both complex permittivity and complex permeability. Hence, microwaveabsorbing materials are normally classified into two categories: the dielectric loss materials corresponding to the complex permittivity, such as ZnO,8 BaTiO3,9 TiO2,10 CuS,11 MnO2,12 and conducting polymer,13 and the magnetic loss materials corresponding to the complex permeability, such as Fe,14 Co,15 Ni,16 FeCo,17 and Fe2O3.18 Due to the imbalance of physical properties, these kinds of microwave absorbers have a fatal drawback. It is difficult to satisfy the impedance matching conditions, which usually lead to a weak absorption.19 Mutual cooperation of the electromagnetic parameters is the core issue in the microwave-absorbing field. For that reason, the exploration of microwave-absorbing materials whose electro© 2017 American Chemical Society

magnetic parameters can be artificially adjusted is imperatively necessary. Fortunately, dielectric/magneto nanohybrids can meet these requirements. Carbon-based nanomaterials are considered as one of the ideal candidates for constructing high-efficiency microwave absorption composites. As an important member, graphene and its oxides have been a research hotspot in the last 10 years.20−25 The reasons are listed as follows: First, graphene has excellent electrical conductivity properties that can lead to a strong dielectric loss.26−29 Second, the building block of graphene nanosheets with large surface area and high aspect ratio is beneficial for constructing a dielectric/magneto nanohybrid and for preventing the agglomeration of magnetic nanoparticles.30−32 These advantages make it easier to control the permittivity of graphene nanohybrids. Conversely, magnetic nanocrystals with proper complex permeability are also very important to tune the microwave absorption performance in this system, such as Fe3O4, Fe, Co3O4, and Ni nanoparticles.33−36 A NiCo alloy that possesses large saturation magnetization and high magnetic permeability has been investigated as a microwave absorber.37 However, most of the studies were focused on the microsized NiCo particles.38−40 The advantages of nanoparticles have not been fully exploited. Received: December 16, 2016 Accepted: March 27, 2017 Published: March 27, 2017 12673

DOI: 10.1021/acsami.6b16144 ACS Appl. Mater. Interfaces 2017, 9, 12673−12679

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Figure 1. Digital photographs and schematic illustration for the fabrication of NiCo2/GNS nanohybrids in this study: (a) preparation of GNS by liquid-phase exfoliation method; (b) GNS scattered in the anhydrous alcohol solution; (c, d) mixed solution of raw materials before and after the microwave-heated reaction, respectively; (e) schematic illustration for the formation of NiCo2/GNS hybrids. helpful in preventing agglomeration. When the reaction was complete, the crude products of nickel−cobalt hydrates were collected by centrifugation and washed with anhydrous ethanol and deionized water several times. Figure 1c,d show the solutions before and after the microwave reaction, respectively. Finally, the NiCo2/GNS nanohybrids were obtained by annealing the crude products at 600 °C for 2 h under H2/Ar atmosphere. In this experiment, graphite (99.99%) was purchased from Shenzhen Xieli Graphite Co., Ltd., China. Analyticalgrade chemical reagents Ni(NO3)2·6H2O (98.0%) and Co(NO3)2· 6H2O (98.0%) were purchased from Alfa Aesar (China) Chemicals Co., Ltd. CO(NH2)2 (99.0%) was purchased from Sinopharm Chemical Reagent Co., Ltd., China. 2.2. Characterization. The crystal structures of NiCo2/GNS nanohybrids were investigated by X-ray diffraction (XRD) with Cu Kα radiation (λ = 1.5406 Å) on a Rigaku SmartLab diffractometer. The structural deformation was measured by Raman spectra at room temperature using a 532 nm laser (Horiba Jobin Yvon LabRAM). The morphology and structure of nanohybrids were characterized by scanning electron microscope (SEM, S-4800 Hitachi) and highresolution transmission electron microscopy (HRTEM, FEI Tecnal G2 F20). The composites for the electromagnetic parameters measurement were prepared by mixing paraffin with 50 wt % NiCo2/GNS nanohybrids. The composites were pressed into a toroidal shape (φout: 7.00 mm, φin: 3.04 mm) and measured by a vector network analyzer (VNA). Complex permittivity and permeability computed from the electromagnetic parameters were used to calculate the microwave reflection loss (RL) of the samples.

Therefore, it is of particular interest to explore the synthesis of NiCo2 nanocrystals combined with graphene nanosheets and investigate the microwave absorption performance of NiCo2/ GNS nanohybrids. In the present work, GNSs were obtained by an efficient liquid-phase exfoliation method, and the NiCo2/GNS nanohybrids with different GNS proportions were fabricated as a microwave absorber for the first time. NiCo2 nanocrystals dispersed uniformly on the GNS network structure, which was beneficial for tuning the electromagnetic parameters of the hybrids. In addition, the microwave absorption performances of the NiCo2/GNS composites were investigated systematically.

2. EXPERIMENTAL SECTION 2.1. Fabrication of Graphene Nanosheets and NiCo2/GNS Nanohybrids. Graphene nanosheets (GNSs) were prepared by an efficient liquid-phase exfoliation (LPE) method, as shown in Figure 1a,b. LPE is one of the most effective and straightforward methods to produce 2D-layered materials, such as graphene. The advantages are not only mass production but also high-quality graphene with fewer defects than produced by the Hummers method. Graphite (0.1 g) was put into 20 mL of anhydrous alcohol and ultrasonically treated for 4 h. After standing and centrifuging, GNSs were collected by the PVDF membrane filter and dehydrated under vacuum drying oven for 24 h. Then NiCo2/GNS nanohybrids were fabricated by a facile synthetic route, seen in Figure 1c−e. First, Ni(NO3)2·6H2O (2 mmol), Co(NO3)2·6H2O (4 mmol), and CO(NH2)2 (8 mmol) were dissolved in 40 mL of mixed solution of anhydrous ethanol and deionized water. Second, GNSs were added into the solution with different mass ratios (5, 10, and 20 wt %). Third, the solution was heated at 95 °C for 24 h under microwave irradiation in a single-mode microwave reactor (NOVA-II, Preekem of Shanghai, China). A single-mode microwaveassisted hydrothermal technique (SMMHT) is an effective and nonpolluting method for the chemical reactions. Moreover, it is very

3. RESULTS AND DISCUSSION As illustrated in Figure 1, the graphene nanosheets and NiCo2/ GNS nanohybrids were fabricated through a facile synthetic route. The homogeneous color in Figure 1b reveals that the GNS are dispersed uniformly and stably in the solution. In the microwave reactor, NiCo2 nanocrystals are in-suit nucleated and deposited on the surface of GNS as illustrated in Figure 1e. 12674

DOI: 10.1021/acsami.6b16144 ACS Appl. Mater. Interfaces 2017, 9, 12673−12679

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ACS Applied Materials & Interfaces

in the NiCo2/GNS nanohybrids. Raman spectra of GNS and NiCo2/GNS nanohybrids with different proportions are shown in Figure 2b. There are two typical peaks of graphene: the D band locates at 1345 cm−1 associating to the vibration of sp3 defect and the disorder sites, and the G band centers at 1576 cm−1 relating to the in-plane sp2 hybridization vibration. The intensity ratio between the D band and G band (ID/IG) is usually used to represent the defects and disorders in carbonbased materials.41−43 On the basis of the Raman results (NiCo2/5 wt % GNS ID/IG = 0.25, NiCo2/10 wt % GNS ID/IG = 0.35, NiCo2/20 wt % GNS ID/IG = 0.40, GNS ID/IG = 0.81), the ratios increase with the GNS proportion in NiCo2/GNS nanohybrids, which suggest that more disorders and defects are introduced in GNS. Moreover, the peak centered at 2710 cm−1 corresponds to the 2D band in graphene. Obviously, the intensity of the 2D band is lower than that of the G band, which is consistent with the multilayer property of graphene foam.44,45 The morphologies of GNS, NiCo2 nanocrystals, and NiCo2/ GNS nanohybrids were analyzed by SEM and HRTEM. Parts a−c of Figure 3 show the SEM and HRTEM images of GNS. The GNS powder has a two-dimensional flake nanostructure with a smooth surface, and it is nearly transparent from the TEM observation. The interplanner spacing is measured as 0.34 nm, which is consistent with the XRD result. In Figure 3d,e, the NiCo2 nanocrystals show a little agglomeration. Parts g and h of Figure 3 present the SEM and TEM images of NiCo2/GNS nanohybrids. Clearly, NiCo2 nanocrystals with sizes of 10−100

Figure 2a shows the crystallographic structures of GNS, NiCo2 nanocrystals, and NiCo2/GNS nanohybrids with different GNS

Figure 2. (a) XRD patterns of pure GNS, NiCo2 nanocrystals, and NiCo2/GNS nanohybrids with different GNS proportions; (b) Raman spectra of GNS and NiCo2/GNS nanohybrids with different GNS proportions.

proportions, respectively. The pure GNS shows a diffraction peak around 26.5° that is related to the (002) crystal plane with an interplanar distance of 0.34 nm. All NiCo2 nanocrystals and NiCo2/GNS nanohybrids have the diffraction peaks located at 44.3°, 51.6°, and 76.0°, corresponding to the (111), (200), and (220) crystal planes of face center cubic structure Ni and Co, respectively. The diffraction peaks of Ni oxides and Co oxides are not detected, indicating a pure phase of NiCo alloy. The intensity of the GNS (002) peak increases with its proportion

Figure 3. SEM images of (a) GNS, (d) NiCo2 nanocrystals, and (g) NiCo2/GNS nanohybrids. Representative TEM images of (b) GNS, (e) NiCo2 nanocrystals, and (h) NiCo2/GNS nanohybrids. HRTEM images of (c) GNS, (f) NiCo2 nanocrystals, and (i) NiCo2/GNS nanohybrids. 12675

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Figure 4. Frequency dependence of (a) real part and (b) imaginary part of permittivity, (c) dielectric loss tangent, and (d) reflection loss under a constant thickness for NiCo2/GNS composites with different GNS proportion.

δe can contribute to the defects and disorders of GNS and also the dipole and interfacial polarization effects of the nanohybrids. Theoretically, high ε″ and tan δe values indicate large microwave energy attenuation in the NiCo2/GNS composites. However, it does not mean that a good reflection loss can be obtained. As one example, reflection losses for NiCo2/GNS composites under 1.6 mm thickness are shown in Figure 4d. NiCo2 and NiCo2/5 wt % GNS composites have the comparable RL results because of their similar electromagnetic parameters. The RL reaches −30 dB at 11.7 GHz for NiCo2/10 wt % GNS composites, indicating that 99.9% of microwave power is consumed. Further increasing the GNS proportion, the complex permittivity values increase a lot, but the RL becomes even worse than the pure NiCo2 nanocrystals. The reason is quite simple: impedance mismatch owing to the unilateral excessive increase of the complex permittivity. That is why we emphasize this fact in the Introduction: the exploration of materials whose electromagnetic parameters can be artificially adjusted is imperatively necessary to improve the microwave absorption performance. To evaluate one microwave material, there are four criteria: strong absorption (RL), broad frequency ( f), thin thickness (d), and lightweight (ρ). Except for the last one, the criteria are plotted in Figure 5a−d using the 3D color plot for the NiCo2/ GNS composites with different GNS proportions. Figure 5a shows the RL values of pure NiCo2 nanocrystals. The RL has a minimum value around −25 dB, and it can achieve −10 dB in a wide range of thickness and frequency. Frankly speaking, the microwave absorption ability of NiCo2 nanocrystals is not bad. By introducing the GNS, the absorption performances of NiCo2/GNS composites are tremendously enhanced. Clearly, the NiCo2/10 wt % GNS composite has the best performance, and the RL values under different thickness are shown in Figure 5e. The RLs exceeding −20 dB in the frequency range of 5.3− 16.4 GHz are obtained for the absorber thicknesses of 1.2−3.2 mm, while an optimal RL of −30 dB is achieved at 11.7 GHz for a thickness of 1.6 mm. Generally, a typical RL value of −10 dB corresponding to 90% absorption is suitable for practical

nm are uniformly anchored on the GNS without any agglomeration and also make the surface of GNS coarse. The interplanner spacings of NiCo2 nanocrystals and nanohybrids have the same values of 0.23 nm, corresponding to the (111) crystal planes of NiCo2. The selected area elemental mapping of NiCo2/GNS nanohybrids was also performed (Figure S1). The results confirm that the Ni and Co elements are uniformly distributed in the carbon matrix. To investigate the microwave absorbing performance of a material, one common way is to analyze the electromagnetic parameters (complex permittivity and permeability) obtained from the vector network analyzer. The frequency dependence of reflection loss (RL) can be estimated by the following equations46 Z in = Z0(μr /εr)1/2 tanh[j(2πfd /c)(με )1/2 ] r r RL = 20 log|(Zin − Z0)/(Z in + Z0)|

(1)

where Z0 and Zin are the impedance of air and sample, respectively, f is the microwave frequency, d is the sample thickness, and c is the velocity of light. Parts a and b of Figure 4 show the frequency dependence of the real part (ε′) and imaginary part (ε″) of permittivity for NiCo2/GNS composites with different GNS proportions, respectively. For pure NiCo2 nanocrystals, the values of ε′ and ε″ almost remain constant through the entire frequency range (ε′ = 11 and ε″ = 1). The low values are mainly due to the weak conductivity of metallic nanoparticles. When the GNS proportion is increased from 5 to 20 wt %, ε′ increases from 10.6 to 28.3 and ε″ changes from 1 to 15.9 at 10 GHz. The enhancement of permittivity is quite reasonable because of the excellent electrical conductivity property of graphene. Dielectric loss tangent (tan δe = ε″/ε′) is always used to represent the dielectric loss capacity of the microwave absorber, shown in Figure 4c. It can be seen that the dielectric loss tangent is enhanced with the GNS proportion, and the nanohybrids with 20 wt % GNS have the largest value. The enhancement of tan 12676

DOI: 10.1021/acsami.6b16144 ACS Appl. Mater. Interfaces 2017, 9, 12673−12679

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Figure 5. Color map of the reflection loss for the NiCo2/GNS composites with different GNS proportions: (a) 0 wt %, (b) 5 wt %, (c) 10 wt %, and (d) 20 wt %. (e) Reflection loss for NiCo2/10 wt % GNS composite with different thickness.

where tm is the thickness of absorber, f m is the peak frequency of RL, λm is the quarter-wavelength at f m, c is the velocity of light, and εr and μr are the complex permittivity and permeability at f m, respectively. The frequency dependence of RL at various thicknesses and the frequency dependence of calculated λ/4 thickness for NiCo2/GNS composites with different GNS proportion are shown in Figure S2. Without a doubt, the minimum RL positions are in good agreement with the quarter-wavelength matching model.

application. The excellent microwave absorption performances make the NiCo2/10 wt % GNS composite have great potential for use as a microwave absorber. One more interesting thing in the color map is the “crescent curve” of RL projection in the d− f plane, which represents the minimum RL position under a certain thickness. The crescent curve can be explained by a well-known microwave absorption mechanism, a so-called quarter-wavelength (λ/4) matching model.47 This mechanism has been explained in our previous work16,48 and proved by different research groups using various absorbing materials.49−53 The physical meaning of quarter-wavelength matching model is when a microwave is incident normally on an absorber backed by a metal plate; it is partly reflected from the airabsorber interface and partly reflected from the absorber−metal interface. At a certain frequency point, two reflected waves can be out of phase by 180° and cancel each other for the absorber thickness, satisfying the quarter-wavelength criteria as shown in the following equation48 tm =

n nc λm = (n = 1, 3, 5...) 4 4fm |εrμr |

4. CONCLUSIONS In this work, we prepared graphene nanosheets (GNS) by an efficient liquid-phase exfoliation method. Then the NiCo2/ GNS nanohybrids were fabricated using the single-mode microwave-assisted hydrothermal technique. Morphology and microstructre investigation suggested that NiCo2 nanocrystals were uniformly anchored on the GNS without aggregation. NiCo2/GNS composites with different GNS proportions were fabricated as a microwave absorber for the first time. The complex permittivity of NiCo2/GNS composites could be adjusted artificially due to the outstanding electrical property of graphene. For the NiCo2/10 wt % GNS composite, RL values

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DOI: 10.1021/acsami.6b16144 ACS Appl. Mater. Interfaces 2017, 9, 12673−12679

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ACS Applied Materials & Interfaces exceeding −20 dB in the frequency range of 5.3−16.4 GHz were obtained for the absorber thicknesses of 1.2−3.2 mm, while an optimal RL of −30 dB was achieved at 11.7 GHz for the thickness of 1.6 mm. The exceptional microwave absorbing performance revealed that NiCo2/10 wt % GNS composite has great potential for using as an excellent microwave absorber.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.6b16144. Selected area elemental mapping of Ni, Co, and C elements in NiCo2/GNS nanohybrids; dependence of RL on frequency at various thicknesses for the NiCo2/ GNS composites; dependence of λ/4 thickness on frequency for the NiCo2/GNS composites (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. *E-mail: [email protected]. Tel: +86-335-8074631. Fax: +86-335-8074545. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We gratefully acknowledge financial support from the National Natural Science Foundation of China (Grant Nos. 51271214, 51102206, 51421091, 51571172, and 11404280), National Science Fund for Distinguished Young Scholars (Grant No. 51025103), and Program for New Century Excellent Talents in University (NCET-13-0993).

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REFERENCES

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DOI: 10.1021/acsami.6b16144 ACS Appl. Mater. Interfaces 2017, 9, 12673−12679

Research Article

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DOI: 10.1021/acsami.6b16144 ACS Appl. Mater. Interfaces 2017, 9, 12673−12679