Reduced Graphene Oxide-Cu0.5Ni0.5Fe2O4

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Journal of Electronic Materials ISSN 0361-5235

Volume 46 Number 6

Journal of Elec Materi (2017) 46:3707-3713

DOI 10.1007/s11664-017-5386-z

Reduced Graphene Oxide-Cu _0.5 Ni _0.5 Fe ₂ O ₄ - Polyaniline Nanocomposite: Preparation, Characterization and Microwave

Absorption Properties

Tran Quang Dat, Nguyen Tran Ha & Do

Quoc Hung

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Reduced Graphene Oxide-Cu _0.5 Ni _0.5 Fe ₂ O ₄ -Polyaniline

Nanocomposite: Preparation, Characterization and Microwave Absorption Properties

TRAN QUANG DAT,^1,2NGUYEN TRAN HA,¹ and DO QUOC HUNG¹

1.—Physics Department, Le Quy Don Technical University, No. 236, Hoang Quoc Viet Road, Hanoi, Vietnam. 2.—e-mail: dattqmta@gmail.com

Reduced graphene oxide-Cu0.5Ni0.5Fe2O4-polyaniline nanocomposite (RGO- CNF-PANI) was synthesized by a three-step method. The morphology, structure and magnetic properties of composite samples were characterized by scanning electron microscopy, x-ray diffraction, Fourier transform infrared spectroscopy, Raman spectroscopy (RAMAN) and vibrating sample magnetometer. It was found that reduced graphene oxide was exfoliated and deco- rated homogeneously with ferrite nanoparticles having diameters between 11 nm and 21 nm. The polyaniline was coated by anin situchemical oxidation polymerization. The measurement of magnetic properties found the remanence (Mr) and coercive field (Hc) were near zero, indicating that the obtained material was superparamagnetic. The microwave measurements found that the nanocomposite exhibited a good absorption property with the optimum matching thickness of 3 mm in the frequency of 8–12 GHz. The value of the maximum RL was40.7 dB at 9.8 GHz.

Key words: Cu0.5Ni0.5Fe2O4, polyanilin, reduced graphene oxide, absorption, RAM

INTRODUCTION

There has been a growing interest in new appli- cations of an efficient microwave absorbing material in electronic devices and stealth technology.^1,2 Microwave absorption materials can absorb electromagnetic (EM) waves effectively and convert EM energy into thermal energy. For example, the microwave absorbing coatings on the exterior sur- faces of military aircraft and vehicles may help them to avoid detection by radar. Today’s microwave-absorbing materials cannot meet all require- ments for a stealth coating, such as strong absorption in a wide range of absorption frequency, small thickness, and light weight at the same time.³ There are several materials that make excellent candidates for a composite microwave absorption material. Spinel ferrites are currently key materials for advancements in electronics, magnetic storage,

ferro-fluid technology and many bioinspired appli- cations.⁴Ferrite nanoparticles are also widely used for the manufacture of microwave absorbing materials with high performances.⁵ On the other hand, polyaniline (PANI), due to its unique physical and chemical properties including excellent electrical conductivity at ultrahigh radio-frequencies, as well as simplicity and cheapness of its fabrication technology, is one of the most advanced conducting polymers. In the scientific community, it is gener- ally accepted that radar-absorbing properties of PANI are closely related to its structure.⁶ Finally, as the thinnest and most lightweight material in the carbon world, reduced graphene oxide (RGO), which possesses extremely high specific surface area and unique two-dimensional structure, may be the best candidate of electromagnetic wave absorbing materials.⁷

It can be expected that radio-absorbing composite materials containing these three ingredients may combine all their advantages and overcome many of

(Received October 20, 2016; accepted February 15, 2017;

published online February 28, 2017)

Journal of ELECTRONIC MATERIALS, Vol. 46, No. 6, 2017

DOI: 10.1007/s11664-017-5386-z

2017 The Minerals, Metals & Materials Society

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their disadvantages. Thus, Huang et al.⁸ have fabricated and investigated a nanocomposite con- sisting of RGO, NiFe₂O₄ nanoparticles and PANI (RGO-PANI-NF). They found that the material possessed high radar absorbing performance. How- ever, in our experimental results, Cu0.5Ni0.5Fe2O4

nanoparticles far exceed NiFe2O4 nanoparticles in their magnetic properties; namely, that its satura- tion magnetization is almost two times more than that of NiFe₂O₄ nanoparticles. Therefore, the replacement of NiFe2O4 by Cu0.5Ni0.5Fe2O4 in the composite may likely lead to an improvement in microwave absorption properties of the composite material. With such a hope, in this study, we aimed to synthesize a reduced graphene oxide-Cu0.5Ni0.5

Fe2O4ferrite-polyanilin (RGO-CNF-PANI) nanocomposite and explore its properties, including its microwave absorption.

EXPERIMENTAL

Synthesis of Reduced Graphene Oxide (RGO) RGO was synthesized from graphite powder by a modified Hummers method.⁹ The detailed process- ing is described as below: In the first step, 1 g of graphite (99%) and 0.5 g of NaNO3were mixed with 50 mL of H2SO4 (98%) in a three-necked flask at 0C. The mixture was stirred for 1 h. Then 3 g of KMnO4 was added to the suspension, and the mixture was stirred at 10C for 2 h. The obtained suspension was stirred at room temperature for 25 min, then underwent sonication for 5 min in an ultrasonic bath. After repeating the stirring-sonication cycles for 15 times, 200 mL of distilled water was added to the treated suspension to quench the reaction. After the reaction is quenched, additional ultrasonic treatment was performed for 2 h. Then, after adjusting the pH at6 by the addition of 1 mol/L NaOH solution, the suspension was further sonicated for 1 h. After that, a solution containing 20 g of L-ascorbic acid dissolved in 200 mL of distilled water was slowly added to the exfoliated graphite oxide suspension at room temperature.

The process of reduction continued for 1 h at a temperature of 95C. The resultant black precipi- tates were collected by using filter paper and then were washed with a 1 mol/L HCl solution and distilled water to neutral pH. Finally, the collected and precooled precipitate was then dried in a vacuum chamber, without any heating, to obtain RGO powder.

Synthesis of RGO-CNF

RGO-CNF composite was prepared by the hydrothermal method. Firstly, an initial mixture for hydrothermal synthesis was prepared. It con- tained 4 mmol of FeCl3. 6H2O, 1 mmol of NiCl2. 6H2O and 1 mmol of Cu (NO3)2. 6H2O, 60 mL of ethylene glycol, 40 mL of distilled water, an appropriate amount of aqueous RGO (100 mg RGO/10 mL

distilled water) and 4 mL of an ammonia solution (25%). The mixture was accurately stirred for 4 h in order to achieve good uniformity. The mixture was then subjected to hydrothermal treatment at constant temperature of 190C for 24 h using an 150 ml Teflon-lined autoclave - reactor. After the hydrothermal treatment, the reactor with the mixture was given to cool freely to room temperature, and the precipitate was then separated from the mixture by a permanent magnet. The collected precipitate was repeatedly washed with distilled water and ethanol, then dried at 60C for 12 h. The resulting product is the dry synthesized RGO-CNF nanocomposite.

Synthesis of RGO-CNF-PANI

In our experiments, the RGO-CNF-PANI composites were synthesized by polymerization of a mixture containing aniline and RGO-CNF composite powder prepared by the method described above.

For this purpose, first of all, a mixture comprised of aniline and RGO-CNF powder was prepared by dissolving 2 mmol of alinine in 70 ml of solution containing 20 mmol HCl, then in the resulting and precooled at 0C solution, was added appropriate RGO-CNF powder. For the polymerization to take place, 30 mL of aqueous solution containing 2 mmol of ammonium persulfate was slowly poured into the prepared mixture. The reaction of synthesis was continued at constant temperature of 0C for 16 h with vigorous mechanical stirring at about 400–

500 rpm. After the synthesis process is ended, the solution was subjected to filtration to separate the precipitation containing RGO-CNF-PANI composite from the reaction mixture. The collected precipitation was then repeatedly washed with deionized water and ethanol. Washing stopped only when the filtrate became colorless. Finally, the wet collected composite was dried in a vacuum at a temperature of 60C for 24 h using a vacuum oven. Then the material was ready for use in further experiments.

Materials Characterization and Microwave Absorption Experiments

The morphologies and crystal structures of the obtained composites were characterized using scanning electron microscopy (SEM—S4800), x-ray diffraction (XRD, Bruker D5 with Cu Ka1

k= 1.54056 A˚ ), Fourier transform infrared spectroscopy (FTIR, Perkin Spectrum Two) and Raman spectroscopy (LabRAM HR 800). Magnetic measurements were performed with a vibrating sample magnetometer (VSM, DMS 880 in magnetic fields up to 13.5 kOe).

To measure the microwave absorption properties, samples were prepared by homogeneously mixing paraffin wax with 30 wt.% of the products. All samples with a thickness of 2 mm, 3 mm, and 4 mm were cut into rectangular shapes with dimensions as required for waveguide measurements at 8–

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12 GHz bands. The sample is then inserted in the waveguide so that it fills the cross-section of the waveguide tightly in order to prevent any leakage of electromagnetic energy. The complex permittivity and permeability of the materials were measured by an Agilent PNA E8362C vector network analyzer.

The reflection loss (RL) is calculated according to transmission line theory, which is summarized by the following equations¹⁰:

RLðdBÞ ¼20 logjZ_in1j Zinþ1

j j ð1Þ

Zin¼ ffiffiffiffiffi l_r er

r

tanh j2pfd c

ffiffiffiffiffiffiffiffi erl_r

p

; ð2Þ

whereZin is the input impedance of the absorber,c is the velocity of electromagnetic waves in free space, f is the frequency and d is the sample’s thickness.

RESULTS AND DISCUSSION

Morphology of the obtained RGO is shown on Fig.1. According to this figure, the thin RGO leaves are separated from graphite, but they are strongly deformed and wrinkled. Figure2 shows the SEM image of the RGO-CNF composite. It can be seen that the CNF particles cover almost completely the entire surface of the RGO, so that the RGO leaves cannot be seen except only at the boundaries between the blocks of RGO-CNF.

In order to analyze the CNF particle size distribution, only the most clear-cut particles on the SEM-image were used to carry out measurement of the particle size. The results are shown in Fig.3. In this figure, one can see that the particle sizes range from 11 nm to 21 nm in diameter. The distribution curve has a typical bell-shape with the maximum in the region of 15–16 nm. Figure4shows the surface morphology of the composite RGO-CNF-PANI. After the aniline to RGO-CNF polymerized and

transformed into PANI, the polymer would cover almost the entire surface of the RGO-CNF. Now, in the SEM image we do not observe RGO-CNF blocks, but only RGO-CNF-PANI layers that can be seen quite clearly.

Raman scattering spectra of the materials, measured by using 610 nm wavelength laser radiation, are shown in Fig.5. In the case of RGO-material, peaks of D-band (at 1330 cm¹) and G-band (at 1586 cm¹) appeared in the spectrum. It is known that the G-band peak corresponds to the first order scattering of the mode E2g related to the sp² bond, and that the D-band peak corresponds to K-point phonon of the A1g mode related to the sp³-bond.¹¹ The intensity ratioID/IGwas greater than one. This indicates that the recovery process of oxide graphene to produce graphene occured.¹²In the case of the RGO-CNF-PANI sample, along with the afore- named peaks, the emergence of other peaks are observed. Dixit et al.¹³ believed the peaks at 489 cm¹ and 569 cm¹ correspond to the T2g - mode oscillation and the peaks at 665 cm¹ and

Fig. 1. SEM image of RGO.

Fig. 2. SEM image of RGO-CNF

Fig. 3. Particle size distribution of CNF.

Reduced Graphene Oxide-Cu0.5Ni0.5Fe2O4-Polyaniline Nanocomposite: Preparation, Characterization and Microwave Absorption Properties

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699 cm¹ and correspond to the Eg and A1g mode vibrations in CNF material, respectively. Wang et al.¹⁴ found that the peak at 1156 cm¹ was associated with compression-expansion oscillations related to the C-H bonds, the peak at 1210 cm¹ corresponded to oscillations of the compression- expansion type in the benzenoid ring, and the peak at 1478 cm¹corresponded to the oscillation related to the N=C=N bonds, which took place in the quinoid di-imine. Thus, we see that the observed above peaks are evidence of the presence of CNF and PANI components in the prepared composite material.

FTIR spectra of the RGO material are shown in Fig.6a. We note that the spectra contains absorption lines that are characteristic for GO and RGO.

The lines appearing in the spectra are attributed to typical oscillations in functional groups that are available in GO and RGO, namely: C=O stretching vibration (1720 cm¹), C=C stretching vibration (1640 cm¹), O-H deformation (1410 cm¹), C-O (epoxy) stretching vibration (1190 cm¹), and the C-O (alkoxy) stretching (1070 cm¹).¹² It is inter- esting to note that these lines are significantly weaker in comparison with similar lines in the FTIR spectra of GO.¹⁵In our opinion, this is probably due to the fact that the GO has been reduced to RGO.

Figure 6b shows the FTIR spectra of the RGO- CNF-PANI material. The spectra shows a peak at 1562 cm¹corresponding to the (N=Q=N) vibrations of quinonoid-rings, a peak at 1482 cm¹, which corresponds to the (C=C) in the vibrations of benzenoid ring, and a peak at 1293 cm¹corresponding to the C-N stretching vibration. The peaks at 1244 cm¹ and 1053 cm¹ correspond to the vibrations of C-O bonds. The peak at 790 cm¹ corresponds to vibrations of C-H bonds.¹⁵In addition, the absorption peak at 563 cm¹ corresponds to the vibrations of the type Fe-O. These peaks indicate the interactions that occur within the material.

In the XRD pattern of RGO (Fig.7a), there is a broad peak corresponding to RGO at about 24.44, with an interlayer spacing of 0.54 nm. A few characteristic peaks for planes such as (220),

Fig. 4. SEM image of RGO-CNF-PANI.

Fig. 5. Raman spectra of RGO (a), RGO-CNF-PANI (b).

Fig. 6. FTIR spectra of RGO (a), RGO-CNF-PANI (b).

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(311), (400), (422), (511) and (440) were observed in the XRD patterns of the RGO-CNF, RGO-CNF- PANI (Fig.7b and c). X-ray diffraction data identi- fied that the CNF particles have face-centered cubic trevorite structure. The value of crystallite size of the CNF nanoparticles was evaluated using the Scherrer formulad=kk/bcosh, wherek is equal to 0.94,k is the x-ray wavelength,b is the peak full width half maxima (FWHM) and h is the diffraction peak position. Results obtained by calculation with (311) peak indicate that the crystallite sizes of CNF in RGO-CNF, RGO-CNF-PANI composites are 15 nm, 17 nm, respectively. This result is in good agreement with the previous analysis of the SEM image.

Room temperature magnetization for RGO-CNF- PANI composite was investigated and is shown in Fig.8. The VSM measurement shows that the obtained material is typically superparamagnetic with remanence and coercive field being near to zero. The saturated magnetization value is esti- mated to be 40 emu/g.

The real and imaginary parts of complex permittivity and complex permeability of samples with thickness of 3 mm as a function of the microwave frequency are shown in Fig.9. It can be seen that the real (e¢) and imaginary part of the permittivity (e¢¢) and real (l¢) and imaginary part (l¢¢) of the permeability of the material are almost constant in the whole range of measured frequency. This inter- esting frequency dependence of dielectric and magnetic properties seems to be very convenient for designing a microwave absorbing coating made from RGO-CNF-PANI composite.

Figure10 shows the dependence of e¢¢ from e¢for prepared composite, measured in the 8–12 GHz frequency interval. It is easy to see in Fig.4 the so-called ‘‘Cole – Cole semicircles’’. As each Cole–

Cole semicircle corresponds to one Debye relaxation process,¹⁶ we can conclude that a multi-dielectric relaxation process occurs in the material, which

contributes to the improvement of the dielectric performance of the samples.

It is known that the magnetic loss due to eddy currents at a given frequency can be calculated by the formula C₀ =l¢¢/(l¢²9f),⁸ where f is the frequency. The results of calculations for the composite material for the frequency range 8–12 GHz are presented in Fig.11. It can be seen that C0 is changing slightly with frequency, with its value fluctuating around a certain mean value. This means that in the RGO-CNF-PANI composite there is little dependence of eddy currents on the frequency.

Figure12 shows the microwave reflection loss (RL) as function of frequency for the three RGO- CNF-PANI samples with thicknesses of 2 mm, 3 mm and 4 mm, respectively. The most important aspect of Fig.12is that each curve is almost entirely at a level lower than 10 dB for all the frequency range from 8 GHz to 12 GHz. It means that the

Fig. 7. XRD patterns of RGO (a), RGO-CNF (b), RGO-CNF-PANI (c).

Fig. 8. Room temperature magnetic hysteresis loops of RGO-CNF- PANI.

Fig. 9. The real and imaginary part of complex permittivity and complex permeability of RGO-CNF-PANI.

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prepared RGO-CNF-PANI composite is very effec- tive absorbing materials for the X band microwaves.

Secondly, with increasing sample thickness, the RL minima shifts to lower frequencies. The best RL results were attained for the sample with 3 mm thickness. In this case, RL was lower than 10 dB over the whole frequency, and the maximum of RL attained a very impressive value of 40.7 dB at a frequency of 9.8 GHz. In our opinion, this shifting is related to the interference of microwaves on the absorbing structures.

In our opinion, the excellent microwave absorption performance of the RGO-CNF-PANI composite is related to their original composition and structure. In fact, they contain RGO and PANI, which are very well-known dielectric loss materials.^17,18 On the other hand, the addition of CNF nanoparticles to the composite, which have outstanding magnetic properties, can considerably improve loss

characteristics of the absorbers.¹⁹In particular, the presence of RGO with its unique 2D-crystal structure in the PANI matrix can lead to new mecha- nisms of energy dissipation for the oscillations of magnetic CNF nanoparticles in the electromagnetic field of the microwave. So, one can expect that using a RGO-CNF-PANI nanocomposite will give new degrees of freedom for variation and enhancement of microwave absorbing materials.

CONCLUSION

In conclusion, a RGO-CNF-PANI nanocomposite was prepared by a three-step method. SEM, RAMAN, FTIR, XRD and VSM have been used to study the morphology, structure and magnetic properties of the composites. The ternary nanocomposite has shown excellent microwave absorption properties; the RL values were lower than -10 dB for all the X-band frequencies, and a maximum RL up to 40.7 dB was attained for the samples with 3 mm thickness at 9.8 GHz. Therefore, RGO-CNF- PANI composite could be a suitable and promising microwave absorbing material.

ACKNOWLEDGEMENT

This research was completed with financial sup- port from Le Quy Don Technical University, Viet- Nam.

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Fig. 10. The typical Cole-semicircle curve of RGO-CNF-PANI.

Fig. 11. TheC0- f values of RGO-CNF-PANI.

Fig. 12. Reflection loss of RGO-CNF-PANI samples.

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