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Fe-phthalocyanine decorated graphene oxide on the microwave absorbing performance
Journal of Magnetism and Magnetic Materials 399 (2016) 81–87
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Journal of Magnetism and Magnetic Materials journal homepage: www.elsevier.com/locate/jmmm
Influence of Fe3O4/Fe-phthalocyanine decorated graphene oxide on the microwave absorbing performance Jingwei Li, Junji Wei, Zejun Pu, Mingzhen Xu, Kun Jia n, Xiaobo Liu n Research Branch of Functional Materials, Institute of Microelectronic & Solid State Electronic, High-Temperature Resistant Polymers and Composites Key Laboratory of Sichuan Province, University of Electronic Science and Technology of China, Chengdu 610054, PR China
art ic l e i nf o
a b s t r a c t
Article history: Received 5 May 2015 Received in revised form 16 September 2015 Accepted 21 September 2015 Available online 25 September 2015
Novel graphene oxide@Fe3O4/iron phthalocyanine (GO@Fe3O4/FePc) hybrid materials were prepared through a facile one-step solvothermal method with graphene oxide (GO) sheets as template in ethylene glycol. The morphology and structure of the hybrid materials were characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), Fourier transform infrared spectrophotometer (FTIR) and X-ray diffraction (XRD), respectively. The results indicated that the monodispersed Fe3O4/FePc hybrid microspheres were uniformly self-assembled along the surface of GO sheets through electrostatic attraction and the morphology can be tuned by controlling the amount of 4,4′-bis (3,4-dicyanophenoxy)biphenyl (BPH). As the BPH content increases, magnetization measurement of the GO@Fe3O4/FePc hybrid materials showed that the coercivity increased, while saturation magnetizations decreased. Electromagnetic properties of the hybrid materials were measured in the range of 0.5– 18.0 GHz. The microwave absorbing performance enhanced with the increase of BPH content and a maximum reflection loss of 27.92 dB was obtained at 10.8 GHz when the matching thickness was 2.5 mm. Therefore, the novel electromagnetic hybrid materials can be considered as potential materials in the microwave absorbing field. & 2015 Elsevier B.V. All rights reserved.
Keywords: Graphene oxide Iron phthalocyanine oligomer Magnetite Hybrids Magnetic properties Microwave absorbing properties
1. Introduction According to the recent studies, magnetite (Fe3O4) that is deemed to be a common ferrite material has been widely used in magnetic recording and microwave applications due to their unique magnetic and electric properties [1–3]. Lately, it has been shown that magnetic nano-composites can be of great use to absorb microwave, owing to their advantages over pure Fe3O4 with respect to low cost, light weight, design flexibility and microwave properties [4–9]. As a matter of fact, a diverse class of multifunctional composite materials which is originated from by magnetic have many potential applications such as magnetically guided drug delivery system and ferrofluid technology [10–11]. For example, the heterojunction of Fe3O4 with CNTs has been realized in the solvothermal synthesis, indicating that CNTs/Fe3O4 hybrid materials can expand the applications and reinforce the pristine properties of simplex Fe3O4 and CNTs materials [12–13]. Then CNT–CuPc/Fe3O4 with good electromagnetic properties caused by introducing the CuPc, was fabricated successfully by a facile twostep solvothermal route [14]. In comparison with CNTs, the n
Corresponding authors. Fax: þ 86 28 83207326 E-mail addresses: [email protected] (K. Jia), [email protected] (X. Liu).
http://dx.doi.org/10.1016/j.jmmm.2015.09.072 0304-8853/& 2015 Elsevier B.V. All rights reserved.
physical properties of graphene are similar to them except larger surface areas, so graphene can be regard as unrolled CNTs [15]. In addition, the platelet-shaped materials are more effective to absorb microwave than the rod-shaped and sphere-shaped ones [16– 17]. Szabo et al. have synthesized graphene–Fe3O4 hybrids and the results showed that as-prepared hybrid exhibited excellent electrical property [18]. Recently, the graphene/Fe3O4 hybrids have been prepared via solvothermal reaction [19]. Furthermore, a new kind of magnetic nano-composites has been obtained by introducing the iron phthalocyanine (FePc) to Fe3O4 nanoparticles based on the anisotropic electric conductivity of FePc [20]. In this work, the FePc and Fe3O4 are introduced onto the surfaces of the GO sheets to obtain a new materials with better microwave absorption properties. Graphene exhibits many remarkable advantages in the terms of physical, thermal, mechanical, chemical and electronic performances caused by its large surface area (up to 2600 m2 g 1) and layered structure [21–23]. However, the pure graphene tends to restack and accumulate due to the strong π–π stacking tendency and high cohesive energy, resulting in the inhomogeneous dispersion with organic phase. In contrast, GO sheets can stably disperse in the organic phase and organic solvent to form a colloidal dispersion due to a great deal of hydroxyl, epoxy, carboxyl and carbonyl groups on the surface of GO sheet [24–25].
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In the present study, we report a simple, effective solvothermal approach of preparing GO@Fe3O4/FePc hybrid materials in ethylene glycol by using GO sheets as nanoscale building blocks for new hybrids. The target is to discover the relevance between the physical properties of the hybrid materials and the synthesis parameters so as to control the properties for specific applications, especially in the area of microwave absorption.
2. Experimental details 2.1. Materials Graphene oxide (GO) was prepared by a modified Hummer's method [26]. FeCl3 6H2O (99%), sodium acetate trihydrate (NaAc), ethylene glycol (EG, 99%), and polyethylene glycol (PEG 2000) was purchased from Kelong reagent Co. Ltd., Chengdu, China. 4,4′-bis (3,4-dicyanophenoxy)biphenyl (BPH) was synthesized in our laboratory [27]. All the materials were used without further purification. 2.2. Synthesis of GO@Fe3O4/FePc Typical synthesis of GO@Fe3O4/FePc hybrid material was carried out in a solvothermal system by reduction reaction between FeCl3 and EG in the presence of GO sheets. 1.20 g FeCl3 6H2O was dissolved in 150 mL EG, followed by the addition of 3.75 g PEG, 0.10 g BPH and 0.10 g GO sheets to form a black solution with the help of an ultrasonic bath. Then, 13.50 g NaAc was slowly added into the solution with vigorous stirring for 30 min and then the mixture was sealed in a teflon-lined stainless-steel autoclave. The autoclave was heated to 200 °C and maintained for 15 h, and then cooled to room temperature. The black product was rinsed with ethanol for several times, and dried at 60 °C overnight. By adjusting the addition content of BPH in the mixture solution at the first step, hybrid microspheres with different contents of phthalocyanine oligomer were obtained. In the work, three samples were prepared and corresponding addition contents of phthalocyanine oligomer in the mixture solution were 0.01 g, 0.10 g and 1.00 g, labeled as GO@Fe3O4/FePc-1, GO@Fe3O4/FePc-2 and GO@Fe3O4/FePc-3, respectively. 2.3. Characterization The synthesized products were characterized by FTIR (Shimadzu, 8000S), UV–vis spectra (Persee, UV1810-PC), X-ray diffraction (XRD) (Rigaku RINT2400 with Cuka radiation), scanning electron microscopy (SEM) (JSM, 6490LV), transmission electron microscopy (TEM) (Hitachi, H-600). Thermogravimetric analysis (TGA) was performed on TA Instruments TGA-Q50 modules at a heating rate of 20 °C/min from room temperature to 800 °C. X-ray photoelectron spectroscopy (XPS) was carried out with an ESCALAB 250 electron spectrometer from ThermoFisher Scientific, USA. Magnetic study was performed by a vibrating sample magnetometer (VSM) (Riken Denshi, BHV-525). Electromagnetic (EM) parameters were measured by a vector network analyzer (Agilent 8720ET), in which the hybrid microspheres were mixed with wax in a mass ratio of 3:1 and compressed to standard ring shapes (outer diameter: 7 mm, inner diameter: 3 mm, and thickness: 2– 4 mm).
3. Results and discussions As seen from Figs. 1 and 2, the morphology of GO@Fe3O4/FePc hybrid materials with different addition amounts of BPH was
Fig. 1. SEM images of GO@Fe3O4/FePc hybrid materials with different additions of BPH: (a) GO@Fe3O4/FePc-1; (b) GO@Fe3O4/FePc-2; and (c) GO@Fe3O4/FePc-3.
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Fig. 2. TEM images of GO@Fe3O4/FePc-2 hybrid materials. Fig. 4. UV–vis spectra of raw GO and GO@Fe3O4/FePc.
Fig. 5. TGA curves of GO@Fe3O4/FePc hybrid materials with different additions of BPH: GO@Fe3O4/FePc-1; GO@Fe3O4/FePc-2; and GO@Fe3O4/FePc-3.
Fig. 3. FTIR spectra of raw GO (a) and GO@Fe3O4/FePc hybrid materials with different additions of BPH: (b) GO@Fe3O4/FePc-1; (c) GO@Fe3O4/FePc-2; and (d) GO@Fe3O4/FePc-3.
studied by SEM and TEM. Fig. 1a–c shows that Fe3O4/FePc hybrid microspheres evenly and densely deposited on both sides of these sheets to form a sandwich-like structure composite. Meanwhile, it can be clearly noticed that the size of Fe3O4/FePc hybrid microspheres is becoming larger and the average spheres size increase from 90 nm to 190 nm with the increasing addition of BPH. As shown in the TEM image (Fig. 2), Fe3O4/FePc magnetic particles formed on GO sheets are nearly monodisperse and the particle size is about 130 nm. All results demonstrate that solvothermal synthesis of GO@Fe3O4/FePc hybrid materials is highly efficient and the sphere size of hybrid materials can be easily controlled by changing the content of organic materials. The chemical groups on the surface of GO@Fe3O4/FePc hybrid materials were investigated by FTIR spectra, as shown in Fig. 3. The FTIR spectrum of raw GO shows a strong and broad absorption band at 3427 cm 1, corresponding to the O–H stretching
Fig. 6. XRD patterns of GO@Fe3O4/FePc hybrid materials with different additions of BPH: (a) GO@Fe3O4/FePc-1; (b) GO@Fe3O4/FePc-2; and (c) GO@Fe3O4/FePc-3.
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Table 1 Magnetization data for GO@Fe3O4/FePc hybrid materials at different additions of BPH: (a) 0.01 g, (b) 0.1 g, and (c) 1 g. Sample
Hc(Oe)
Ms(emu g 1)
FWHM
Crystalline size (nm)
GO@Fe3O4/FePc-1 GO@Fe3O4/FePc-2 GO@Fe3O4/FePc-3
255.1 264.7 290.5
57.4 50.5 42.8
0.624 0.643 0.736
59.81 58.54 51.81
Fig. 8. (a) Magnetization curves at 300 K of GO@Fe3O4/FePc hybrid materials with different additions of BPH: a. GO@Fe3O4/FePc-1; b. GO@Fe3O4/FePc-2; c. GO@Fe3O4/ FePc-3; and (b) photograph of GO@Fe3O4/FePc hybrid material dispersed in ethanol (left) and its response to a magnet (right).
Fig. 7. (a) XPS wide scan and (b) Fe 2p peaks of GO@Fe3O4/FePc peaks of GO@Fe3O4/FePc hybrid materials.
vibration, and the strong peak around 1631 cm 1 is attributed to aromatic C ¼C [28]. In the FTIR spectrum of final product (Fig. 3b– d), the absorption band at 577 cm 1 is assigned to the characteristic peak of Fe3O4 [29]. Meanwhile, the absorption bands at 1075 and 1275 cm 1 correspond to the stretching vibration of phthalocyanine ring skeleton [30–31]. Due to the content of iron phthalocyanine in GO@Fe3O4/FePc-1 and GO@Fe3O4/FePc-2 is small, the characteristic peaks of phthalocyanine macrocycles are weak [29]. FePc is formed from the coordination between the nitrogen atom in phthalocyanine ring and iron atoms [30,32]. Furthermore, iron phthalocyanine can be further confirmed by UV–vis spectra (Fig. 4). The bands at 328 and 661 nm are characteristic absorption bands of FePc. The thermal properties of the GO@Fe3O4/FePc were determined by TGA in N2 atmosphere. TGA curves of the GO@Fe3O4/FePc
hybrid materials are provided in Fig. 5. The T5% (initial decomposition temperatures) of hybrid materials decreases from 324 °C to 304 °C with increasing BPH content, which is attributed to the increase of organic compound with lower degradation temperature in the hybrid microspheres [33]. Fig. 6 shows the XRD patterns of GO@Fe3O4/FePc hybrid materials with different additions of BPH. According to the literature [34], the XRD curve of the GO sheets displays a diffraction peak at a 2θ value around 10°, owing to interlamellar water trapped between the hydrophilic GO sheets. After surface decoration, no obvious diffraction peaks of GO are observed in the GO@Fe3O4/ FePc composite, because the regular stack of GO is destroyed by the intercalation of Fe3O4/FePc hybrid microspheres, which is consistent with other reported works about GO-based composites [34–37]. Meanwhile, the main diffraction peaks of hybrid microspheres at (111), (220), (311), (400), (422), (511) and (440) are all consistent with those of pure Fe3O4 [38]. The full width at half maximum (FWHM) corresponding to (311) peak from XRD patterns along with the calculated crystallize size via Scherrer equation are summarized in Table 1. It was clear that the increased FWHM and decreased crystallize size were obtained as the increasing of BPH loading content, which can be ascribed to the following two reasons: one was the lower crystallinity; the other
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Fig. 9. Permittivity and permeability of the GO@Fe3O4/FePc hybrid materials with different additions of BPH: (a) real part of permittivity, (b) imaginary part of permittivity, (c) real part of permeability, and (d) imaginary part of permeability.
was the hybrid spheres were the mixture of noncrystalline and nanocrystalline. The XRD results confirmed that the Fe3O4/FePc hybrid microspheres have been successfully deposited onto the GO sheets. To further demonstrate the chemical composition, XPS analysis in Fig. 7a reveals the elements C, N, O and Fe present on the GO@Fe3O4@FePc. As shown in Fig. 7b, the peaks located at 711 and 724.5 eV correspond to Fe 2p3/2 and Fe 2p1/2, respectively, which further confirms that Fe3O4 exists in the sample. The magnetic hysteresis curves of GO@Fe3O4/FePc are obtained by using a vibrating sample magnetometer as shown in Fig. 8a. The hysteresis loops of all samples illustrate the strong magnetic response to a varying magnetic field. The value of coercive force (Hc) and saturated magnetization (Ms) are summarized in Table 1. The coercivity of GO@Fe3O4/FePc hybrid materials increased with the increasing content of BPH. Since the coercivity is mainly dependent on the crystalline size of Fe3O4 nanoparticles and the coercivity of magnetic nanoparticles is obviously enhanced via the promotion of crystal growth with single domain region, the highest coercivity obtained from GO@Fe3O4/FePc-3 sample could be attributed to its crystalline size that is very close to the single domain size ( 50 nm) of magnetite nanoparticles (see Table 1). Table 1 presents the dependence of the saturation magnetization of the GO@Fe3O4/FePc hybrid materials at 300 K. Their saturation magnetizations are 57.4, 50.5 and 42.8 emu g 1. The saturation magnetization values of pure Fe3O4 (80 emu g 1) prepared by the
similar method are bigger than that of obtained hybrid materials [4]. This difference in saturation magnetization is attributed to various contents of GO sheets existing in the hybrid materials and different relative Fe contents. The black samples could be quickly separated from their dispersion by holding the samples close to a magnet (Fig. 8b). This result demonstrates that the GO@Fe3O4/FePc hybrid microspheres have high magnetic sensitivity. As shown in Fig. 9a and b, the complex permittivity of the waxGO@Fe3O4/FePc hybrid material with different additions of BPH in the range of 0.5–18.0 GHz is illustrated. The real part (ε′) and the imaginary part (ε′′) of the permittivity of the hybrid materials changed largely as the addition of BPH increased (Fig. 9a and b). It can be seen that the tendency variation of GO@Fe3O4/FePc-1 and GO@Fe3O4/FePc-2 is almost similar, which is attributed to the fewer BPH content. The ε′ is critically dependent on the frequency for the hybrid with the low content of BPH and the ε′ decreased with increasing frequency. It can be found that the values of ε′ of GO@Fe3O4/FePc-1 and GO@Fe3O4/FePc-2 declined from 15.2 to 6.6, over the 0.5–18.0 GHz frequency range. Accordingly, it can be noticed from Fig. 9b that the imaginary part ε′′ of GO@Fe3O4/FePc3 exhibits a slightly increase then declining linearly. According to the free electron theory [39], ε′′ E1/2πε0ρf, where ρ is the resistivity. In these hybrid materials, GO@Fe3O4/FePc-3 possesses a lower ε′′ value, indicating a higher electric resistivity. In the general case, the high electric resistivity and appropriate dielectric
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Fig. 10. Reflection loss of (a) GO@Fe3O4/FePc hybrid materials with different additions of BPH for a layer thickness of 3 mm and (b) GO@Fe3O4/FePc-2 for different layer thicknesses.
loss contribute to the improvement of the microwave absorption properties. Fig. 9c and d shows the complex permeability of the prepared hybrid material at different additions of BPH in the frequency of 0.5–18 GHz. Obviously, the real parts μ′ demonstrates slight dependence on the change of frequency which is supposed to occur at lower frequency. As in Fig. 9c, the real parts μ′ of complex permeability for all samples decrease with increasing frequency, which is attributed to both ferromagnetic resonance and eddy current loss [40]. It showed that μ′ of complex permeability appear a resonance peak at around 6 GHz which shifts to low frequency with the increasing of the BPH content. It can be mainly due to addition the interface effects between the Fe3O4 and phthalocyanine oligomer [41]. Fig. 9d depicts the imaginary part of permeability μ′′ of the three kinds of hybrid materials depends on frequency. In the range of 0.5–6 GHz, the value of μ′′ decreased with the increase of BPH content and the resonance peaks were observed at around 1.17 GHz. The Kittel equation has been widely used to calculate the natural resonance frequency of magnetite materials; the natural resonance of the sphere-shaped magnet is [42]: fr ¼ γHa where γ ¼ 28 GHz T 1, is the gyromagnetic ratio and Ha¼ 4|K1|/
3 μ0Ms is the effective anisotropy field. The saturation magnetization μ0Ms of GO@Fe3O4/FePc-1 is 0.542 T and the anisotropic coefficient K1 for the fcc-type bulk magnetite is about 9 103 J m 3, so the theoretical calculation of the natural resonance frequency of GO@Fe3O4/FePc-1 should be γHa E1.2 GHz, which matches well with the experimental data of 1.26 GHz. Meanwhile, the theoretical calculation of fr for GO@Fe3O4/FePc-2 and GO@Fe3O4/FePc-3 should be about 1.08, 1.02 GHz, respectively, which is close to the experimental data of 1.12 GHz. This demonstrates that the as-synthesized product shows mainly sphere-like morphology and the magnetic loss is caused mainly by the natural resonance in the microwave region. For the three samples, it also can be demonstrated that the multi-resonance peaks, which are shown in the imaginary part of permeability in the range of 6.0– 18.0 GHz, are the result of dimensional resonance, spin wave excitations, surface effect and the small size effect [43]. Thus, the permeability of the as-obtained samples comes mainly from stronger natural resonance and enhanced eddy current loss, rather than domain wall resonance and magnetic hysteresis, due to the microspheres size increasing after the addition of BPH and the change of microstructure [44]. Generally, the reflection loss (RL) value could be used to predict the microwave absorbance of the products. Therefore, the RL is calculated by using the relative complex permeability and permittivity for further revealing the superior microwave-absorbing properties of GO@Fe3O4/FePc. Fig. 10a presents the relationship between the calculated frequency and RL of the hybrid materials with the different additions of BPH ranging from 0.01 g to 1 g at a layer thickness of 3 mm. It can be noticed that the reflection loss peak move to a high frequency by increasing BPH. Fig. 10b shows the reflection loss curves of hybrid materials with 0.1 g BPH for the layer thickness ranging from 2.0 to 5.0 mm. It can be seen that the RL is dependent sensitively on the thicknesses of GO@Fe3O4/FePc. According to literatures [45,46], the microwave-absorbing properties of materials are better when the negative reflection loss value is larger. In 4.6 to 8.7 GHz, the reflection loss value r 17 dB by turning thickness between 3.0 to 5.0 mm suggests that the composites can absorb 90% of the electromagnetic wave [47]. In contrast, with the increase of the thickness of the samples, the reflection loss peaks towards to low-frequency and become narrow. The maximum reflection loss value at 27.92 dB suggests the optimal impedance compatibility at a critical point when the thickness is 2.50 mm. It demonstrates that the novel hybrid materials have remarkable microwave-absorbing properties with thin thicknesses. Thus, the strong microwave absorption suggests the potential applications of GO@Fe3O4/FePc in the field of microwave absorbing [48].
4. Conclusions In summary, GO@Fe3O4/FePc hybrid materials were synthesized through a simple and effective solvothermal route. Their morphology and structure of the hybrid materials can be tuned by the addition amount of BPH. As the increase of BPH content, the magnetization measurement of the hybrid materials indicated that coercivity increased while saturation magnetization decreased. Moreover, the GO@Fe3O4/FePc hybrid materials possess high resistivity. The microwave absorbing properties were enhanced with the increasing addition of BPH and a maximum reflection loss of 27.92 dB was obtained at 10.8 GHz for sample with 1.00 g BPH when the matching thickness was 2.5 mm. Thus, the novel composites are believed to have potential applications in microwave absorbing filed.
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Acknowledgments The authors wish to thank for financial support of this work from the National Natural Science Foundation of China (Nos. 51173021, 51373028, and 51403029), “863” National Major Program of High Technology (2012AA03A212), Ningbo Major (key) Science and Technology Research Plan (2013B06011) and South Wisdom Valley Innovative Research Team Program.
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Contents lists available at ScienceDirect
Journal of Magnetism and Magnetic Materials journal homepage: www.elsevier.com/locate/jmmm
Influence of Fe3O4/Fe-phthalocyanine decorated graphene oxide on the microwave absorbing performance Jingwei Li, Junji Wei, Zejun Pu, Mingzhen Xu, Kun Jia n, Xiaobo Liu n Research Branch of Functional Materials, Institute of Microelectronic & Solid State Electronic, High-Temperature Resistant Polymers and Composites Key Laboratory of Sichuan Province, University of Electronic Science and Technology of China, Chengdu 610054, PR China
art ic l e i nf o
a b s t r a c t
Article history: Received 5 May 2015 Received in revised form 16 September 2015 Accepted 21 September 2015 Available online 25 September 2015
Novel graphene oxide@Fe3O4/iron phthalocyanine (GO@Fe3O4/FePc) hybrid materials were prepared through a facile one-step solvothermal method with graphene oxide (GO) sheets as template in ethylene glycol. The morphology and structure of the hybrid materials were characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM), Fourier transform infrared spectrophotometer (FTIR) and X-ray diffraction (XRD), respectively. The results indicated that the monodispersed Fe3O4/FePc hybrid microspheres were uniformly self-assembled along the surface of GO sheets through electrostatic attraction and the morphology can be tuned by controlling the amount of 4,4′-bis (3,4-dicyanophenoxy)biphenyl (BPH). As the BPH content increases, magnetization measurement of the GO@Fe3O4/FePc hybrid materials showed that the coercivity increased, while saturation magnetizations decreased. Electromagnetic properties of the hybrid materials were measured in the range of 0.5– 18.0 GHz. The microwave absorbing performance enhanced with the increase of BPH content and a maximum reflection loss of 27.92 dB was obtained at 10.8 GHz when the matching thickness was 2.5 mm. Therefore, the novel electromagnetic hybrid materials can be considered as potential materials in the microwave absorbing field. & 2015 Elsevier B.V. All rights reserved.
Keywords: Graphene oxide Iron phthalocyanine oligomer Magnetite Hybrids Magnetic properties Microwave absorbing properties
1. Introduction According to the recent studies, magnetite (Fe3O4) that is deemed to be a common ferrite material has been widely used in magnetic recording and microwave applications due to their unique magnetic and electric properties [1–3]. Lately, it has been shown that magnetic nano-composites can be of great use to absorb microwave, owing to their advantages over pure Fe3O4 with respect to low cost, light weight, design flexibility and microwave properties [4–9]. As a matter of fact, a diverse class of multifunctional composite materials which is originated from by magnetic have many potential applications such as magnetically guided drug delivery system and ferrofluid technology [10–11]. For example, the heterojunction of Fe3O4 with CNTs has been realized in the solvothermal synthesis, indicating that CNTs/Fe3O4 hybrid materials can expand the applications and reinforce the pristine properties of simplex Fe3O4 and CNTs materials [12–13]. Then CNT–CuPc/Fe3O4 with good electromagnetic properties caused by introducing the CuPc, was fabricated successfully by a facile twostep solvothermal route [14]. In comparison with CNTs, the n
Corresponding authors. Fax: þ 86 28 83207326 E-mail addresses: [email protected] (K. Jia), [email protected] (X. Liu).
http://dx.doi.org/10.1016/j.jmmm.2015.09.072 0304-8853/& 2015 Elsevier B.V. All rights reserved.
physical properties of graphene are similar to them except larger surface areas, so graphene can be regard as unrolled CNTs [15]. In addition, the platelet-shaped materials are more effective to absorb microwave than the rod-shaped and sphere-shaped ones [16– 17]. Szabo et al. have synthesized graphene–Fe3O4 hybrids and the results showed that as-prepared hybrid exhibited excellent electrical property [18]. Recently, the graphene/Fe3O4 hybrids have been prepared via solvothermal reaction [19]. Furthermore, a new kind of magnetic nano-composites has been obtained by introducing the iron phthalocyanine (FePc) to Fe3O4 nanoparticles based on the anisotropic electric conductivity of FePc [20]. In this work, the FePc and Fe3O4 are introduced onto the surfaces of the GO sheets to obtain a new materials with better microwave absorption properties. Graphene exhibits many remarkable advantages in the terms of physical, thermal, mechanical, chemical and electronic performances caused by its large surface area (up to 2600 m2 g 1) and layered structure [21–23]. However, the pure graphene tends to restack and accumulate due to the strong π–π stacking tendency and high cohesive energy, resulting in the inhomogeneous dispersion with organic phase. In contrast, GO sheets can stably disperse in the organic phase and organic solvent to form a colloidal dispersion due to a great deal of hydroxyl, epoxy, carboxyl and carbonyl groups on the surface of GO sheet [24–25].
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In the present study, we report a simple, effective solvothermal approach of preparing GO@Fe3O4/FePc hybrid materials in ethylene glycol by using GO sheets as nanoscale building blocks for new hybrids. The target is to discover the relevance between the physical properties of the hybrid materials and the synthesis parameters so as to control the properties for specific applications, especially in the area of microwave absorption.
2. Experimental details 2.1. Materials Graphene oxide (GO) was prepared by a modified Hummer's method [26]. FeCl3 6H2O (99%), sodium acetate trihydrate (NaAc), ethylene glycol (EG, 99%), and polyethylene glycol (PEG 2000) was purchased from Kelong reagent Co. Ltd., Chengdu, China. 4,4′-bis (3,4-dicyanophenoxy)biphenyl (BPH) was synthesized in our laboratory [27]. All the materials were used without further purification. 2.2. Synthesis of GO@Fe3O4/FePc Typical synthesis of GO@Fe3O4/FePc hybrid material was carried out in a solvothermal system by reduction reaction between FeCl3 and EG in the presence of GO sheets. 1.20 g FeCl3 6H2O was dissolved in 150 mL EG, followed by the addition of 3.75 g PEG, 0.10 g BPH and 0.10 g GO sheets to form a black solution with the help of an ultrasonic bath. Then, 13.50 g NaAc was slowly added into the solution with vigorous stirring for 30 min and then the mixture was sealed in a teflon-lined stainless-steel autoclave. The autoclave was heated to 200 °C and maintained for 15 h, and then cooled to room temperature. The black product was rinsed with ethanol for several times, and dried at 60 °C overnight. By adjusting the addition content of BPH in the mixture solution at the first step, hybrid microspheres with different contents of phthalocyanine oligomer were obtained. In the work, three samples were prepared and corresponding addition contents of phthalocyanine oligomer in the mixture solution were 0.01 g, 0.10 g and 1.00 g, labeled as GO@Fe3O4/FePc-1, GO@Fe3O4/FePc-2 and GO@Fe3O4/FePc-3, respectively. 2.3. Characterization The synthesized products were characterized by FTIR (Shimadzu, 8000S), UV–vis spectra (Persee, UV1810-PC), X-ray diffraction (XRD) (Rigaku RINT2400 with Cuka radiation), scanning electron microscopy (SEM) (JSM, 6490LV), transmission electron microscopy (TEM) (Hitachi, H-600). Thermogravimetric analysis (TGA) was performed on TA Instruments TGA-Q50 modules at a heating rate of 20 °C/min from room temperature to 800 °C. X-ray photoelectron spectroscopy (XPS) was carried out with an ESCALAB 250 electron spectrometer from ThermoFisher Scientific, USA. Magnetic study was performed by a vibrating sample magnetometer (VSM) (Riken Denshi, BHV-525). Electromagnetic (EM) parameters were measured by a vector network analyzer (Agilent 8720ET), in which the hybrid microspheres were mixed with wax in a mass ratio of 3:1 and compressed to standard ring shapes (outer diameter: 7 mm, inner diameter: 3 mm, and thickness: 2– 4 mm).
3. Results and discussions As seen from Figs. 1 and 2, the morphology of GO@Fe3O4/FePc hybrid materials with different addition amounts of BPH was
Fig. 1. SEM images of GO@Fe3O4/FePc hybrid materials with different additions of BPH: (a) GO@Fe3O4/FePc-1; (b) GO@Fe3O4/FePc-2; and (c) GO@Fe3O4/FePc-3.
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Fig. 2. TEM images of GO@Fe3O4/FePc-2 hybrid materials. Fig. 4. UV–vis spectra of raw GO and GO@Fe3O4/FePc.
Fig. 5. TGA curves of GO@Fe3O4/FePc hybrid materials with different additions of BPH: GO@Fe3O4/FePc-1; GO@Fe3O4/FePc-2; and GO@Fe3O4/FePc-3.
Fig. 3. FTIR spectra of raw GO (a) and GO@Fe3O4/FePc hybrid materials with different additions of BPH: (b) GO@Fe3O4/FePc-1; (c) GO@Fe3O4/FePc-2; and (d) GO@Fe3O4/FePc-3.
studied by SEM and TEM. Fig. 1a–c shows that Fe3O4/FePc hybrid microspheres evenly and densely deposited on both sides of these sheets to form a sandwich-like structure composite. Meanwhile, it can be clearly noticed that the size of Fe3O4/FePc hybrid microspheres is becoming larger and the average spheres size increase from 90 nm to 190 nm with the increasing addition of BPH. As shown in the TEM image (Fig. 2), Fe3O4/FePc magnetic particles formed on GO sheets are nearly monodisperse and the particle size is about 130 nm. All results demonstrate that solvothermal synthesis of GO@Fe3O4/FePc hybrid materials is highly efficient and the sphere size of hybrid materials can be easily controlled by changing the content of organic materials. The chemical groups on the surface of GO@Fe3O4/FePc hybrid materials were investigated by FTIR spectra, as shown in Fig. 3. The FTIR spectrum of raw GO shows a strong and broad absorption band at 3427 cm 1, corresponding to the O–H stretching
Fig. 6. XRD patterns of GO@Fe3O4/FePc hybrid materials with different additions of BPH: (a) GO@Fe3O4/FePc-1; (b) GO@Fe3O4/FePc-2; and (c) GO@Fe3O4/FePc-3.
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Table 1 Magnetization data for GO@Fe3O4/FePc hybrid materials at different additions of BPH: (a) 0.01 g, (b) 0.1 g, and (c) 1 g. Sample
Hc(Oe)
Ms(emu g 1)
FWHM
Crystalline size (nm)
GO@Fe3O4/FePc-1 GO@Fe3O4/FePc-2 GO@Fe3O4/FePc-3
255.1 264.7 290.5
57.4 50.5 42.8
0.624 0.643 0.736
59.81 58.54 51.81
Fig. 8. (a) Magnetization curves at 300 K of GO@Fe3O4/FePc hybrid materials with different additions of BPH: a. GO@Fe3O4/FePc-1; b. GO@Fe3O4/FePc-2; c. GO@Fe3O4/ FePc-3; and (b) photograph of GO@Fe3O4/FePc hybrid material dispersed in ethanol (left) and its response to a magnet (right).
Fig. 7. (a) XPS wide scan and (b) Fe 2p peaks of GO@Fe3O4/FePc peaks of GO@Fe3O4/FePc hybrid materials.
vibration, and the strong peak around 1631 cm 1 is attributed to aromatic C ¼C [28]. In the FTIR spectrum of final product (Fig. 3b– d), the absorption band at 577 cm 1 is assigned to the characteristic peak of Fe3O4 [29]. Meanwhile, the absorption bands at 1075 and 1275 cm 1 correspond to the stretching vibration of phthalocyanine ring skeleton [30–31]. Due to the content of iron phthalocyanine in GO@Fe3O4/FePc-1 and GO@Fe3O4/FePc-2 is small, the characteristic peaks of phthalocyanine macrocycles are weak [29]. FePc is formed from the coordination between the nitrogen atom in phthalocyanine ring and iron atoms [30,32]. Furthermore, iron phthalocyanine can be further confirmed by UV–vis spectra (Fig. 4). The bands at 328 and 661 nm are characteristic absorption bands of FePc. The thermal properties of the GO@Fe3O4/FePc were determined by TGA in N2 atmosphere. TGA curves of the GO@Fe3O4/FePc
hybrid materials are provided in Fig. 5. The T5% (initial decomposition temperatures) of hybrid materials decreases from 324 °C to 304 °C with increasing BPH content, which is attributed to the increase of organic compound with lower degradation temperature in the hybrid microspheres [33]. Fig. 6 shows the XRD patterns of GO@Fe3O4/FePc hybrid materials with different additions of BPH. According to the literature [34], the XRD curve of the GO sheets displays a diffraction peak at a 2θ value around 10°, owing to interlamellar water trapped between the hydrophilic GO sheets. After surface decoration, no obvious diffraction peaks of GO are observed in the GO@Fe3O4/ FePc composite, because the regular stack of GO is destroyed by the intercalation of Fe3O4/FePc hybrid microspheres, which is consistent with other reported works about GO-based composites [34–37]. Meanwhile, the main diffraction peaks of hybrid microspheres at (111), (220), (311), (400), (422), (511) and (440) are all consistent with those of pure Fe3O4 [38]. The full width at half maximum (FWHM) corresponding to (311) peak from XRD patterns along with the calculated crystallize size via Scherrer equation are summarized in Table 1. It was clear that the increased FWHM and decreased crystallize size were obtained as the increasing of BPH loading content, which can be ascribed to the following two reasons: one was the lower crystallinity; the other
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Fig. 9. Permittivity and permeability of the GO@Fe3O4/FePc hybrid materials with different additions of BPH: (a) real part of permittivity, (b) imaginary part of permittivity, (c) real part of permeability, and (d) imaginary part of permeability.
was the hybrid spheres were the mixture of noncrystalline and nanocrystalline. The XRD results confirmed that the Fe3O4/FePc hybrid microspheres have been successfully deposited onto the GO sheets. To further demonstrate the chemical composition, XPS analysis in Fig. 7a reveals the elements C, N, O and Fe present on the GO@Fe3O4@FePc. As shown in Fig. 7b, the peaks located at 711 and 724.5 eV correspond to Fe 2p3/2 and Fe 2p1/2, respectively, which further confirms that Fe3O4 exists in the sample. The magnetic hysteresis curves of GO@Fe3O4/FePc are obtained by using a vibrating sample magnetometer as shown in Fig. 8a. The hysteresis loops of all samples illustrate the strong magnetic response to a varying magnetic field. The value of coercive force (Hc) and saturated magnetization (Ms) are summarized in Table 1. The coercivity of GO@Fe3O4/FePc hybrid materials increased with the increasing content of BPH. Since the coercivity is mainly dependent on the crystalline size of Fe3O4 nanoparticles and the coercivity of magnetic nanoparticles is obviously enhanced via the promotion of crystal growth with single domain region, the highest coercivity obtained from GO@Fe3O4/FePc-3 sample could be attributed to its crystalline size that is very close to the single domain size ( 50 nm) of magnetite nanoparticles (see Table 1). Table 1 presents the dependence of the saturation magnetization of the GO@Fe3O4/FePc hybrid materials at 300 K. Their saturation magnetizations are 57.4, 50.5 and 42.8 emu g 1. The saturation magnetization values of pure Fe3O4 (80 emu g 1) prepared by the
similar method are bigger than that of obtained hybrid materials [4]. This difference in saturation magnetization is attributed to various contents of GO sheets existing in the hybrid materials and different relative Fe contents. The black samples could be quickly separated from their dispersion by holding the samples close to a magnet (Fig. 8b). This result demonstrates that the GO@Fe3O4/FePc hybrid microspheres have high magnetic sensitivity. As shown in Fig. 9a and b, the complex permittivity of the waxGO@Fe3O4/FePc hybrid material with different additions of BPH in the range of 0.5–18.0 GHz is illustrated. The real part (ε′) and the imaginary part (ε′′) of the permittivity of the hybrid materials changed largely as the addition of BPH increased (Fig. 9a and b). It can be seen that the tendency variation of GO@Fe3O4/FePc-1 and GO@Fe3O4/FePc-2 is almost similar, which is attributed to the fewer BPH content. The ε′ is critically dependent on the frequency for the hybrid with the low content of BPH and the ε′ decreased with increasing frequency. It can be found that the values of ε′ of GO@Fe3O4/FePc-1 and GO@Fe3O4/FePc-2 declined from 15.2 to 6.6, over the 0.5–18.0 GHz frequency range. Accordingly, it can be noticed from Fig. 9b that the imaginary part ε′′ of GO@Fe3O4/FePc3 exhibits a slightly increase then declining linearly. According to the free electron theory [39], ε′′ E1/2πε0ρf, where ρ is the resistivity. In these hybrid materials, GO@Fe3O4/FePc-3 possesses a lower ε′′ value, indicating a higher electric resistivity. In the general case, the high electric resistivity and appropriate dielectric
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Fig. 10. Reflection loss of (a) GO@Fe3O4/FePc hybrid materials with different additions of BPH for a layer thickness of 3 mm and (b) GO@Fe3O4/FePc-2 for different layer thicknesses.
loss contribute to the improvement of the microwave absorption properties. Fig. 9c and d shows the complex permeability of the prepared hybrid material at different additions of BPH in the frequency of 0.5–18 GHz. Obviously, the real parts μ′ demonstrates slight dependence on the change of frequency which is supposed to occur at lower frequency. As in Fig. 9c, the real parts μ′ of complex permeability for all samples decrease with increasing frequency, which is attributed to both ferromagnetic resonance and eddy current loss [40]. It showed that μ′ of complex permeability appear a resonance peak at around 6 GHz which shifts to low frequency with the increasing of the BPH content. It can be mainly due to addition the interface effects between the Fe3O4 and phthalocyanine oligomer [41]. Fig. 9d depicts the imaginary part of permeability μ′′ of the three kinds of hybrid materials depends on frequency. In the range of 0.5–6 GHz, the value of μ′′ decreased with the increase of BPH content and the resonance peaks were observed at around 1.17 GHz. The Kittel equation has been widely used to calculate the natural resonance frequency of magnetite materials; the natural resonance of the sphere-shaped magnet is [42]: fr ¼ γHa where γ ¼ 28 GHz T 1, is the gyromagnetic ratio and Ha¼ 4|K1|/
3 μ0Ms is the effective anisotropy field. The saturation magnetization μ0Ms of GO@Fe3O4/FePc-1 is 0.542 T and the anisotropic coefficient K1 for the fcc-type bulk magnetite is about 9 103 J m 3, so the theoretical calculation of the natural resonance frequency of GO@Fe3O4/FePc-1 should be γHa E1.2 GHz, which matches well with the experimental data of 1.26 GHz. Meanwhile, the theoretical calculation of fr for GO@Fe3O4/FePc-2 and GO@Fe3O4/FePc-3 should be about 1.08, 1.02 GHz, respectively, which is close to the experimental data of 1.12 GHz. This demonstrates that the as-synthesized product shows mainly sphere-like morphology and the magnetic loss is caused mainly by the natural resonance in the microwave region. For the three samples, it also can be demonstrated that the multi-resonance peaks, which are shown in the imaginary part of permeability in the range of 6.0– 18.0 GHz, are the result of dimensional resonance, spin wave excitations, surface effect and the small size effect [43]. Thus, the permeability of the as-obtained samples comes mainly from stronger natural resonance and enhanced eddy current loss, rather than domain wall resonance and magnetic hysteresis, due to the microspheres size increasing after the addition of BPH and the change of microstructure [44]. Generally, the reflection loss (RL) value could be used to predict the microwave absorbance of the products. Therefore, the RL is calculated by using the relative complex permeability and permittivity for further revealing the superior microwave-absorbing properties of GO@Fe3O4/FePc. Fig. 10a presents the relationship between the calculated frequency and RL of the hybrid materials with the different additions of BPH ranging from 0.01 g to 1 g at a layer thickness of 3 mm. It can be noticed that the reflection loss peak move to a high frequency by increasing BPH. Fig. 10b shows the reflection loss curves of hybrid materials with 0.1 g BPH for the layer thickness ranging from 2.0 to 5.0 mm. It can be seen that the RL is dependent sensitively on the thicknesses of GO@Fe3O4/FePc. According to literatures [45,46], the microwave-absorbing properties of materials are better when the negative reflection loss value is larger. In 4.6 to 8.7 GHz, the reflection loss value r 17 dB by turning thickness between 3.0 to 5.0 mm suggests that the composites can absorb 90% of the electromagnetic wave [47]. In contrast, with the increase of the thickness of the samples, the reflection loss peaks towards to low-frequency and become narrow. The maximum reflection loss value at 27.92 dB suggests the optimal impedance compatibility at a critical point when the thickness is 2.50 mm. It demonstrates that the novel hybrid materials have remarkable microwave-absorbing properties with thin thicknesses. Thus, the strong microwave absorption suggests the potential applications of GO@Fe3O4/FePc in the field of microwave absorbing [48].
4. Conclusions In summary, GO@Fe3O4/FePc hybrid materials were synthesized through a simple and effective solvothermal route. Their morphology and structure of the hybrid materials can be tuned by the addition amount of BPH. As the increase of BPH content, the magnetization measurement of the hybrid materials indicated that coercivity increased while saturation magnetization decreased. Moreover, the GO@Fe3O4/FePc hybrid materials possess high resistivity. The microwave absorbing properties were enhanced with the increasing addition of BPH and a maximum reflection loss of 27.92 dB was obtained at 10.8 GHz for sample with 1.00 g BPH when the matching thickness was 2.5 mm. Thus, the novel composites are believed to have potential applications in microwave absorbing filed.
J. Li et al. / Journal of Magnetism and Magnetic Materials 399 (2016) 81–87
Acknowledgments The authors wish to thank for financial support of this work from the National Natural Science Foundation of China (Nos. 51173021, 51373028, and 51403029), “863” National Major Program of High Technology (2012AA03A212), Ningbo Major (key) Science and Technology Research Plan (2013B06011) and South Wisdom Valley Innovative Research Team Program.
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