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tin dioxide binary nanocomposites in the X-band
Synthetic Metals 257 (2019) 116157
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Synthetic Metals journal homepage: www.elsevier.com/locate/synmet
Facile fabrication and enhanced microwave absorption properties of reduced graphene oxide/tin dioxide binary nanocomposites in the X-band ⁎
Jiabin Zhanga, Ruiwen Shua,b, , Yue Wua, Zongli Wana, Mingdong Zhenga, a b
T
⁎
School of Chemical Engineering, Anhui University of Science and Technology, Huainan, 232001, People’s Republic of China School of Earth and Environment, Anhui University of Science and Technology, Huainan, 232001, People’s Republic of China
A R T I C LE I N FO
A B S T R A C T
Keywords: Reduced graphene oxide Din dioxide Nanocomposites Microwave absorption Filler loading
Herein, reduced graphene oxide/tin dioxide (RGO/SnO2) binary nanocomposites were synthesized through a facile one-pot hydrothermal route by using graphene oxide as a substrate. Results of morphology observations revealed that SnO2 nanoparticles were uniformly loaded on the crumpled surface of thinly flake-like RGO. Moreover, the influence of the contents of RGO and filler loadings on the electromagnetic parameters and microwave absorption properties of RGO/SnO2 binary nanocomposites were elaborately investigated. It was found that the as-prepared binary nanocomposites with the content of RGO of 9.6 wt% exhibited superior microwave absorption properties in the X-band. Remarkably, the minimum reflection loss (RLmin) achieved −53.7 dB at 11.1 GHz and effective absorption bandwidth (EAB, RL ≤ −10 dB) reached 3.7 GHz with a thin matching thickness of merely 2.2 mm. Furthermore, the EAB could reach 14.3 GHz (89.4% of 2–18 GHz) by facilely adjusting the matching thicknesses from 1.5 to 5.0 mm, which spanned the whole Ku, X and C bands. Besides, the possible microwave absorption mechanisms of obtained binary nanocomposites were proposed. This work could be helpful for fabricating graphene-based nanocomposites as high-efficient microwave absorbers.
1. Introduction With the increasingly serious problem of electromagnetic pollution originated from the wide usage of electronic equipment, microwave absorbing materials (MAMs) have gained great attentions in the field of electromagnetic absorption [1–3]. Generally, the characteristics of thin thickness, strong absorption, broad bandwidth and light weight are vitally important for designing of an ideal microwave absorber [4–10]. Therefore, tremendous efforts have been focused on designing and fabricating the high-performance MAMs in the past few decades [1,5–10]. As a novel kind of carbon nanomaterials, reduced graphene oxide (RGO) has been considered as a potential candidate for microwave absorption owing to the advantages such as low density, high specific surface area, residual defects and groups, and notable dielectric loss [11–17]. However, single RGO used as MAMs suffers from inferior impedance matching and poor microwave attenuation loss [11,13,14]. Thus, it is very urgent to enhance the microwave absorption performance of RGO for dealing with the growing problem of electromagnetic pollution. According to the electromagnetic theories, impedance matching and attenuation loss play two key roles for achieving good microwave
⁎
absorption properties [6,18]. Recently, numerous investigations demonstrated that the complexing of dielectric loss materials (ZnO, MnO2 and Co3O4, etc) with RGO to fabricate RGO/semiconductor nanocomposites could be an effective strategy for improving the microwave absorption performance of RGO [19–21]. As one of the most important n-type semiconductor compounds, tin dioxide (SnO2) could be a potential candidate for microwave absorption due to these advantages such as facile synthesis, low cost, good chemical stability and moderate dielectric loss [22–28]. For instance, Zhou et al. synthesized the SnO2 submicron fibers by an electrospinning method [23]. The SnO2 fibers/paraffin wax composite displayed the minimum reflection loss (RLmin) of -19.0 dB. Furthermore, the effective absorption bandwidth (EAB, RL ≤ −10 dB) could achieve 6.0 GHz [23]. Zhao et al. fabricated honeycomb-like SnO2 foams by a template method, which exhibited the RLmin of −37.6 dB at 17.1 GHz and EAB of 5.6 GHz [24]. However, the synthesis procedures of SnO2 fibers or foams are rather complicated, which limits their practical applications in the field of electromagnetic absorption. In our previous work, we fabricated SnO2@MWCNTs (multi-walled carbon nanotubes) nanocomposite by a facile one-pot hydrothermal method and found that the as-prepared nanocomposite showed strong dielectric loss and good microwave absorption performance with a RLmin of −21.6 dB at
Corresponding authors at: School of Chemical Engineering, Anhui University of Science and Technology, Huainan, 232001, People’s Republic of China. E-mail addresses: [email protected] (R. Shu), [email protected] (M. Zheng).
https://doi.org/10.1016/j.synthmet.2019.116157 Received 17 July 2019; Received in revised form 18 August 2019; Accepted 27 August 2019 0379-6779/ © 2019 Elsevier B.V. All rights reserved.
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Fig. 1. Schematic illustration of the synthesis process of RGO/SnO2 binary nanocomposites.
Fig. 2. (a) FT-IR spectra and (b) XRD patterns of the samples of S1, S2 and S3.
Fig. 3. (a) Raman spectra of the samples of S1, S2, S3 and GO and (b) TGA curves of the samples of S1, S2 and S3.
investigations of facile synthesis, influence of the contents of RGO and filler loadings on the electromagnetic parameters and microwave absorption properties of RGO/SnO2 nanocomposites have been rarely reported.
8.6 GHz for a matching thickness of merely 2 mm [25]. Therefore, we believe that superior microwave absorption properties could be achieved through fabricating the reduced graphene oxide/tin dioxide (RGO/SnO2) nanocomposites. To the best of our knowledge, the 2
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Fig. 4. SEM images of the samples of (a) S1, (b) S2 and (c) S3.
2.2. Preparation of RGO/SnO2 binary nanocomposites
Herein, graphene oxide (GO) was used as the substrate to in-situ deposit SnO2 nanoparticles on the surface of RGO for fabricating RGO/ SnO2 binary nanocomposites. Various techniques were adopted to explore the relationships between structure and microwave absorption properties of RGO/SnO2/paraffin wax composites. Moreover, the influence of contents of RGO and filler loadings on the electromagnetic parameters and microwave absorption properties of RGO/SnO2/paraffin wax composites was systematically investigated in the frequency range of 2–18 GHz. Results demonstrated that the as-prepared binary nanocomposites showed superior microwave absorption properties in terms of both RLmin and EAB with a thin matching thickness, which could be used as potential candidates for electromagnetic absorption. Besides, the possible microwave absorption mechanisms were clarified.
RGO/SnO2 binary nanocomposites were synthesized by a facile hydrothermal strategy. Briefly, graphite oxide (20 mg) was firstly dispersed into 30 mL deionized water by ultrasonic treatment for 1.0 h. Then, SnCl4·5H2O with different additive amounts (1.55, 3.1 and 4.65 g) was completely dissolved into the above aqueous GO dispersions by vigorously stirring. Next, concentrated NH3·H2O was added drop-wise into the reaction mixtures to adjust the pH equals to 10 and vigorously stirred for 15 min. Afterward, the mixture dispersions were poured into a Teflon-lined stainless-steel autoclave (50 mL) and reacted at 160 °C for 18 h. The obtained products were collected by centrifuging, and then purified by repeated washing with deionized water and anhydrous ethanol for several times until pH equals to 7. Finally, the powder-like samples were obtained by grinding dried solid products, which were dried at 55 °C for 24 h in a vacuum oven. For simplicity, the as-prepared RGO/SnO2 nanocomposites with different additive amounts of SnCl4·5H2O were labeled as S1 (1.55 g), S2 (3.1 g) and S3 (4.65 g), respectively. The schematic synthesis procedures of RGO/SnO2 binary nanocomposites were described in Fig. 1. Firstly, aqueous GO dispersions were obtained by ultrasonication of graphite oxide. Then, positively charged Sn4+ could be attached to the negatively charged surface of GO by electrostatic attraction under alkaline conditions in the aqueous GO dispersions [15,28]. Finally, SnO2 particles were in situ deposited on the surface of RGO after hydrothermal reactions and thus RGO/SnO2 nanocomposites were formed.
2. Experimental 2.1. Materials Graphite oxide was provided by Suzhou TANFENG Graphene Tech Co., Ltd (Suzhou, China). Tin tetrachloride (SnCl4·5H2O), ammonium hydroxide (NH3·H2O, 25–28 wt%) and anhydrous ethanol (C2H5OH) were purchased from Sinopharm Chemical Reagent Co., Ltd (Shanghai, China). All the chemical reagents were analytical grade and used without further purification. Deionized water was produced in our laboratory (electrical resistivity ∼ 18.2 MΩ cm).
3
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Fig. 5. TEM images with different magnifications: (a)‒(c), HRTEM image (d) and EDS pattern (e) of the sample of S1.
temperature by using a laser confocal Raman spectrometer (Renishaw2000, UK) in the range of 300–2200 cm-1. Thermal gravimetric analysis (TGA) was performed on a thermogravimetric analyzer (TGA/DSC 3+, Switzerland) from 40 °C to 800 °C in the atmosphere of air with a heating rate of 5 °C min-1. The morphology was observed with a field emission scanning electron microscopy (FESEM, Hitachi-Su8020, Japan) and field emission transmission electron microscopy (FETEM, FEI-TF20, USA) equipped with the energy dispersive X-ray spectrum
2.3. Characterization Fourier transform infrared (FT-IR) spectra were recorded in the wavenumber range of 500–4000 cm−1 using a Nicolet 380 spectrometer (Thermoscientific, USA). The crystalline structure was characterized by X-ray diffraction (XRD, LabX XRD-6000, Japan) with CuKα radiation (λ =0.154 nm) in the scattering range (2θ) of 20–80° with a scanning rate of 2°/min. Raman spectra were acquired at room 4
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Fig. 6. Frequency dependence of reflection loss with different thicknesses for the sample of S1 at different filler loadings: (a) 50 wt%, (b) 60 wt% and (c) 70 wt%.
disorders, and sp2 hybridization, respectively [6,14,28]. Generally, the value of ID/IG is often used to evaluate the degree of disorders [6,14,28]. It can be seen that the values of ID/IG for S1, S2, S3 and GO are 1.09, 1.08, 1.06 and 0.89, respectively. Therefore, the as-prepared binary nanocomposites exhibit obviously enhanced ID/IG compared with GO. This result indicates that the degree of defects become higher originated from the reduction of GO into RGO and loaded SnO2 nanoparticles on the surface of RGO [14,28]. Furthermore, the value of ID/IG decreases as the contents of RGO declining in the binary nanocomposites. Besides, the characteristic Raman scattering peaks of SnO2 in the low wavenumbers range (400–700 cm−1) can be observed in the binary nanocomposites [29]. Thermal gravimetric analysis (TGA) measurements were conducted for determining the contents of RGO in the obtained nanocomposites. As shown in Fig. 3(b), the thermal decomposition process of all the samples can be divided into two stages. Firstly, a small weight loss (∼5.5 wt%) occurs below 250 °C, which is mainly caused by the loss of some oxygen-containing groups such as −COOH and −OH on the surface of RGO. Secondly, an obvious weight loss between 250 and 550 °C, which could be ascribed to the degradation of RGO. Besides, the residual products are believed as the constituent of SnO2 [28]. Thus, the contents of RGO in S1, S2 and S3 can be estimated as 9.6 wt%, 8.7 wt% and 7.6 wt%, respectively. The specific surface areas (SBET) of the samples were calculated by a BET method, as shown in Table S1. The calculated SBET of S1, S2 and S3 were 171.1, 167.0 and 156.7 m2 g−1, respectively. Therefore, the specific surface areas show a slight decline trend with the decreasing of the contents of RGO.
(EDS) device. The Brunauer‒Emmett‒Teller (BET) specific surface area of the samples was measured on a V-Sorb 2800 P analyzer (China) by N2 adsorption at −196 °C. The electric conductivity was measured by a four-point probe method (ST2722-SZ, China). Electromagnetic parameters including the relative complex permittivity (εr = ε'-jε”) and permeability (μr = μ'-jμ”) were measured by a vector network analyzer (VNA, AV3672B-S, China) using the coaxialline method in the frequency range of 2–18 GHz. Before being tested, the as-prepared nanocomposites were homogeneously mixed with paraffin wax (which was transparent to microwaves) in different filler loadings (50 wt%, 60 wt% and 70 wt%) and then pressed into toroidalshaped ring with outer diameter of 7.0 mm, inner diameter of 3.04 mm and thickness of 2.0 mm. 3. Results and discussion 3.1. Structural analysis Fig. 2(a) shows the typical Fourier transform infrared (FT-IR) spectra of S1, S2 and S3. The peaks appearing at 3430, 1623, 1399, 1202 and 1044 cm−1 could be ascribed to the stretching vibrations of −OH, C]C, CeOH, CeOeC and CeO, respectively [29]. Besides, the peak at around 602 cm−1 can be assigned to the characteristic absorption of O–Sn–O [29]. As depicted in Fig. 2(b), the diffraction peaks from X-ray diffraction (XRD) appearing at 2θ = 26.8, 33.7, 37.7 and 51.7° are in good accordance with the (110), (101), (200) and (211) crystal planes of SnO2 (JCPDS 41-1445), respectively [23,27–29]. However, it is difficult to distinguish the diffraction peaks of RGO in all the samples, which could be explained by the relatively low diffraction strength of RGO compared with that of SnO2 in the binary nanocomposites [28,29]. Degree of graphitization of obtained nanocomposites was characterized by Raman spectroscopy. From Fig. 3(a), all the samples show two obvious Raman scattering peaks at 1592 cm−1 (G band) and 1347 cm−1 (D band). The D and G bands signify the sp3 defects or
3.2. Morphological analysis Micromorphology of obtained nanocomposites was observed by scanning electron microscopy (SEM). As shown in Fig. 4(a), the thinly flake-like RGO in the sample of S1 shows a rippled and crumpled morphology. Furthermore, numerously nano-sized SnO2 particles were 5
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Fig. 7. Frequency dependence of reflection loss with different thicknesses: (a) S1, (b) S2 and (c) S3; (d) 3D plots of reflection loss curves: (a’) S1, (b’) S2 and (c’) S3 at a filler loading of 60 wt%.
the composition of obtained nanocomposites. It should be noted that the Cu element comes from the supported copper grid for the measurement of TEM.
uniformly loaded on the surface of RGO. From Fig. 4(b) and (c), it can be found that the SnO2 particles exhibit obvious aggregation in the samples of S2 and S3. Therefore, the dispersion of SnO2 particles become worse with the decreasing of contents of RGO in the binary nanocomposites. The micromorphology and structure of the sample of S1 were further characterized by transmission electron microscopy (TEM). From Fig. 5(a)–(c), the thinly flake-like RGO is almost transparent, which suggests few-layer structure. Besides, SnO2 particles with slight aggregation are loaded on the surface of RGO. As depicted in Fig. 5(d), the high-resolution transmission electron microscopy (HRTEM) image reveals that the inter-plane distance is 0.334 nm, which corresponds to the (110) crystal plane of SnO2 [28]. The EDS pattern demonstrates the existence of Sn, O, C and Cu elements (Fig. 5(e)), which is coincide with
3.3. Microwave absorption properties Our recent investigations demonstrated that the filler loading could significantly influence the microwave absorption properties of absorbers [6,14]. Therefore, we have investigated the influence of filler loadings on the microwave absorption properties of the sample of S1, as shown in Fig. 6. It can be found that the values of RLmin of S1 are −32.1, −43.0 and −34.5 dB for the filler loadings of 50 wt%, 60 wt% and 70 wt%, respectively. Therefore, the S1 displays the optimal microwave absorption performance at a filler loading of 60 wt%. 6
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Fig. 8. Frequency dependence of (a) ε', (b) ε” and (c) tanδe for the samples of S1, S2 and S3.
Fig. 9. Frequency dependence of (a) normalized impedance matching (Z) with a matching thickness of 2.2 mm and (b) attenuation constant (α) for the samples of S1, S2 and S3.
important for determining the microwave absorption properties of absorbers [6,18,28]. Owing to the non-magnetic characteristic of RGO/ SnO2 nanocomposites, the real part (μ') and imaginary part (μ”) of relative complex permeability approximately equal to 1 and 0, respectively. Thus, the magnetic loss of all the samples is negligible. Fig. 8 shows the frequency-dependent real part (ε'), imaginary part (ε”) of relative complex permittivity and dielectric loss tangent (tanδe) of the samples of S1, S2 and S3. From Fig. 8(a), the ε' of S1–S3 shows a declining trend as the frequency increasing with slight fluctuations in the high frequency region, and ε' decreases with the decreasing of contents of RGO. As depicted in Fig. 8(b), the ε” of S1–S3 exhibits similar variation trend as ε' with the increasing of frequency. Moreover, all the samples especially S1 exhibit multiple relaxation peaks (marked by the dashed boxes in Fig. 8(b)) in the high frequency region [30,31]. On the basis of free electron theory, it can be deduced that the ε” enhances with the increasing of electric conductivity [6,32]. We have measured the electric conductivity of RGO/SnO2 nanocomposites (S1, S2 and S3) by a four-point probe method, as displayed in Fig. S1. The values of
For the sake of exploring the influence of the contents of RGO on the electromagnetic parameters and microwave absorption properties of obtained nanocomposites, the filler loading was fixed at 60 wt%. As shown in Fig. 7(a), the sample of S1 exhibits the RLmin of −53.7 dB at 11.1 GHz (X-band) and EAB of 3.7 GHz (9.3–13.0 GHz) with a thin matching thickness of merely 2.2 mm, which suggests superior microwave absorption performance. Furthermore, the EAB could achieve 14.3 GHz (89.4% of 2–18 GHz) by facilely adjusting the matching thicknesses from 1.5 to 5.0 mm, which spans the whole Ku, X and C bands. From Fig. 7(b) and (c), the samples of S2 and S3 present the RLmin of −22.6 dB and −10.2 dB, respectively. Thus, the as-prepared binary nanocomposites exhibit obviously enhanced microwave absorption performance with the increasing of the contents of RGO. Fig. 7(a’)–(c’) display the three-dimensional (3D) plots of reflection loss curves for the samples of S1, S2 and S3. Remarkably, the RLmin corresponding to the maximum microwave absorption could locate at various frequencies by modulating the thicknesses of absorbers [6,18,28]. Generally, the electromagnetic parameters (ε', ε”, μ', μ”) are vitally 7
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electric conductivity for S1, S2 and S3 are 24.7, 6.2 and 4.0 S/m, respectively. Thus, the electric conductivity of obtained nanocomposites enhances with the increasing of contents of RGO, leading to the enhancement of ε”. From Fig. 8(c), the binary nanocomposites exhibit obviously enhanced tanδe in the measured frequency range with the increasing of contents of RGO, suggesting the improved dielectric loss capacity. Notably, the S1 exhibits the strongest dielectric loss among all the samples. As shown in supplementary materials, the Cole‒Cole semicircle theory was deduced. Fig. S2 shows the Cole‒Cole plots of the samples of S1, S2 and S3. It can be observed that all the samples exhibit at least two semicircles, which suggests the existing of dual or multiple Debye dipolar relaxation processes [18,26]. Furthermore, it can be observed that the semicircles are distorted in some degree, which suggests that the Debye relaxation is not the only mechanism for dielectric loss and other mechanisms such as conductive loss and interfacial polarization could be responsible for microwave absorption [18,26]. In general, an ideal microwave absorber should satisfy the two requirements of impedance matching and maximum attenuation [6,18,31]. Normalized impedance matching (Z) is often described as follows [33–36]:
Z=
Zin = Z0
μr 2πfd ⎞ tanh ⎡j ⎛ μ r εr ⎤ ⎢ ⎥ εr ⎣ ⎝ c ⎠ ⎦
(1)
Herein Z0 and Zin are the free space and input impedance, respectively. Electromagnetic attenuation capacity can be evaluated by the attenuation constant (α), which is expressed as the following equation [37–41]:
α=
2 πf × c
(μ′′ε′′ − μ′ε′) +
(μ′′ε′′ − μ′ε′)2 + (ε′μ′′ + ε′′μ′)2
(2)
Fig. 9 displays the frequency-dependent Z and α for the samples of S1, S2 and S3. From Fig. 9(a), both S2 and S3 exhibit the values of Z values notably larger than 1 in the frequency range of 11–17 GHz, which suggests inferior impedance matching characteristic. However, the values of Z of S1 are much closer to the line of optimal impedance matching (Z = 1) than that of S2 and S3, indicating the improved impedance matching. From Fig. 9(b), the values of α are enhanced with the increasing of contents of RGO. Notably, the sample of S1 shows the largest α with a value of 228.9, manifesting the strongest attenuation loss. As the good impedance matching is achieved, most of the incident microwaves can penetrate into the specimen; meanwhile, the strongest microwave attenuation capacity could effectively transform the electromagnetic energies into thermal energies to attenuate the microwaves [18,31]. As a consequence, the S1 demonstrates the best microwave absorption performance among all the samples. The relationship between absorption peak frequency (fm) and matching thickness (tm) can be well clarified by quarter-wavelength (λ/ 4) matching theory, which is often described as follows [6,18,31,37]:
Fig. 10. (a) Frequency-dependent reflection loss, (b) simulations of the tm versus fm under the λ/4 model and (c) normalized impedance matching (Z) as a function of frequency for the sample of S1 with different thicknesses.
tm = Fig. 11. Schematic illustration of the possible microwave absorption mechanisms of RGO/SnO2 binary nanocomposites.
nλ nc = (n=1,3,5,…) 4 4fm |εr μr |
(3)
If tm and fm meet the Eq. (3), a phase cancellation effect could
Table 1 Typical SnO2 related composites as microwave absorbers reported in this work and recent literatures. Samples
Matrix
Thickness (mm)
RLmin (dB)
EAB (GHz)
Ref.
RGO/SnO2 (S1) RGO/SnO2 (S1) Ni-doped SnO2@MWCNTs SnO2-graphene Fe-doped SnO2@MWCNTs Fe-doped SnO2/RGO
Paraffin Paraffin Paraffin Paraffin Paraffin Paraffin
2.2 1.5 2.5 2.0 1.5 5.3
−53.7 −43.0 −39.2 −15.28 −44.5 −29.0
3.7 3.6 3.6 1.3 4.5 2.7
This work This work [25] [26] [27] [28]
Notes: MWCNTs denoted multi-walled carbon nanotubes. 8
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References
effectively attenuate the incident microwaves [6,18,31,37]. As depicted in Fig. 10(a), it can be seen that the reflection loss peaks of S1 shift to lower frequency with the increasing of tm. Fig. 10(b) describes the simulations of tm versus fm under the λ/4 model. The pentagram signifies the experimental tm (denoted as tmexp). Significantly, all the tmexp are exactly located at the fitted λ/4 curve. This finding indicates that the λ/4 rule essentially determines the relationship between tm and fm. Therefore, it is valuable to design the thickness of absorbers according to the quarter-wavelength matching theory. Besides, the strongest RL peak (-53.7 dB at 11.1 GHz and 2.2 mm) corresponds well with the optimal impedance matching (Fig. 10(c)). Fig. 11 describes the possible microwave absorption mechanisms of RGO/SnO2 binary nanocomposites. Firstly, the residual oxygen-containing groups such as −COOH and −OH, and structure defects on the surface of RGO could induce the dipole polarization and defect polarization under the alternating electromagnetic fields, respectively [28,40]. Secondly, numerously heterogeneous interfaces among paraffin matrix, RGO and SnO2 nanoparticles significantly enhance the interfacial polarization relaxation [28]. Lastly, the synergistic effects of dielectric loss derived from interfacial polarization, defect polarization and dipole polarization, and conduction loss originated from RGO [28,40], which notably improve the microwave absorption performance. As shown in Table 1, we have summarized the reported SnO2 related composites as the microwave absorbers in this work and recent literatures. It is obvious that the as-prepared RGO/SnO2 nanocomposite (S1) exhibited superior microwave absorption properties with strong absorption, broad bandwidth and thin thickness among the reported SnO2 related composites.
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Sci. 66 (2014) 112–255. [23] M. Zhou, F. Lu, B. Liu, J. Yang, X. Zeng, Electrospun SnO2 submicron fibers for broadband microwave absorption, J. Phys. D-Appl. Phys. 48 (2015) 495303. [24] B. Zhao, B. Fan, Y. Xu, G. Shao, X. Wang, W. Zhao, R. Zhang, Preparation of honeycomb SnO2 foams and configuration-dependent microwave absorption features, ACS Appl. Mater. Interfaces 7 (2015) 26217–26225. [25] L. Lin, H. Xing, R. Shu, L. Wang, X. Ji, D. Tan, Y. Gan, Preparation and microwave absorption properties of multi-walled carbon nanotubes decorated with Ni-doped SnO2 nanocrystals, RSC Adv. 5 (2015) 94539–94550. [26] X. Wang, J. Yu, H. Dong, M. Yu, B. Zhang, W. Wang, L. Dong, Synthesis of nanostructured MnO2, SnO2, and Co3O4: graphene composites with enhanced microwave absorption properties, Appl. Phys. A 119 (2015) 1483–1490. [27] H. Xing, Z. Liu, L. Lin, L. Wang, D. Tan, Y. Gan, X. Ji, G. Xu, Excellent microwave absorption properties of Fe ion-doped SnO2/multi-walled carbon nanotube composites, RSC Adv. 6 (2016) 41656–41664.
4. Conclusions In summary, RGO/SnO2 binary nanocomposites were successfully prepared by a facile one-pot hydrothermal strategy. Results demonstrated that numerous SnO2 nanoparticles were uniformly loaded on the crumpled surface of thinly flake-like RGO. Furthermore, it was found that both the contents of RGO and filler loadings had remarkable influence on the microwave absorption properties of as-prepared nanocomposites. Significantly, the obtained nanocomposites showed excellent microwave absorption properties with the RLmin of -53.7 dB and EAB of 3.7 GHz for a thin matching thickness of 2.2 mm. Besides, the possible microwave absorption mechanisms were clarified and mainly attributed to the enhanced dielectric loss from multiply interfacial polarization, dipole polarization and defect polarization, conduction loss from RGO and optimized impedance matching. Therefore, it was believed that the obtained nanocomposites could be used as high-efficiency microwave absorbers in the field of electromagnetic absorption.
Acknowledgments This work was financially supported by the National Natural Science Foundation of China (Grant No. 51507003), China Postdoctoral Science Foundation (Grant No. 2019M652160), Foundation of Provincial Natural Science Research Project of Anhui Colleges (Grant No. KJ2019A0119), Lift Engineering of Young Talents and Doctor’s Start-up Research Foundation of Anhui University of Science and Technology (Grant No. ZY537).
Appendix A. Supplementary data Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.synthmet.2019. 116157. 9
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[35] Y. Wang, X. Wu, W. Zhang, C. Luo, J. Li, Y. Wang, Fabrication of flower-like Ni0.5Co0.5(OH)2@PANI and its enhanced microwave absorption performances, Mater. Res. Bull. 98 (2018) 59–63. [36] D. Li, B. Zhang, W. Liu, X. Liang, G. Ji, Tailoring the input impedance of FeCo/C composites with efficient broadband absorption, Dalton Trans. 46 (2017) 14926–14933. [37] R. Shu, W. Li, X. Zhou, D. Tian, G. Zhang, Y. Gan, J. Shi, J. He, Facile preparation and microwave absorption properties of RGO/MWCNTs/ZnFe2O4 hybrid nanocomposites, J. Alloys Compd. 743 (2018) 163–174. [38] J. Feng, F. Pu, Z. Li, X. Li, X. Hu, J. Bai, Interfacial interactions and synergistic effect of CoNi nanocrystals and nitrogen-doped graphene in a composite microwave absorber, Carbon 104 (2016) 214–225. [39] B. Quan, G. Xu, D. Li, W. Liu, G. Ji, Y. Du, Incorporation of dielectric constituents to construct ternary heterojunction structures for high-efficiency electromagnetic response, J. Colloid Interface Sci. 498 (2017) 161–169. [40] R. Shu, G. Zhang, J. Zhang, X. Wang, M. Wang, Y. Gan, J. Shi, J. He, Fabrication of reduced graphene oxide/multi-walled carbon nanotubes/zinc ferrite hybrid composites as high-performance microwave absorbers, J. Alloys. Compd. 736 (2018) 1–11. [41] L. Wang, X. Bai, B. Wen, Z. Du, Y. Lin, Honeycomb-like Co/C composites derived from hierarchically nanoporous ZIF-67 as a lightweight and highly efficient microwave absorber, Compos. Part B-Eng. 166 (2019) 464–471.
[28] J. Zhang, R. Shu, Y. Ma, X. Tang, G. Zhang, Iron ions doping enhanced electromagnetic wave absorption properties of tin dioxide/reduced graphene oxide nanocomposites, J. Alloys Compd. 777 (2019) 1115–1123. [29] M. Mishra, A. Singh, B. Singh, S. Dhawan, Performance of a nanoarchitectured tin oxide@reduced graphene oxide composite as a shield against electromagnetic polluting radiation, RSC Adv. 4 (2014) 25904–25911. [30] J. He, X. Wang, Y. Zhang, M. Cao, Small magnetic nanoparticles decorating reduced graphene oxides to tune the electromagnetic attenuation capacity, J. Mater. Chem. C 4 (2016) 7130–7140. [31] Y. Wu, R. Shu, J. Zhang, R. Sun, Y. Chen, J. Yuan, Oxygen vacancy defects enhanced electromagnetic wave absorption properties of 3D net-like multi-walled carbon nanotubes/cerium oxide nanocomposites, J. Alloys Compd. 785 (2019) 616–626. [32] L. Yin, T. Chen, S. Liu, Y. Gao, B. Wu, Y. Wei, G. Li, X. Jian, X. Zhang, Preparation and microwave-absorbing property of BaFe12O19 nanoparticles and BaFe12O19/ Fe3C/CNTs composites, RSC Adv. 5 (2015) 91665–91669. [33] Y. Wang, X. Gao, C. Lin, L. Shi, X. Li, G. Wu, Metal organic frameworks-derived FeCo nanoporous carbon/graphene composite as a high-performance electromagnetic wave absorber, J. Alloys Compd. 785 (2019) 765–773. [34] W. Liu, Q. Shao, G. Ji, X. Liang, Y. Cheng, B. Quan, Y. Du, Metal–organic-frameworks derived porous carbon-wrapped Ni composites with optimized impedance matching as excellent lightweight electromagnetic wave absorber, Chem. Eng. J. 313 (2017) 734–744.
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Synthetic Metals journal homepage: www.elsevier.com/locate/synmet
Facile fabrication and enhanced microwave absorption properties of reduced graphene oxide/tin dioxide binary nanocomposites in the X-band ⁎
Jiabin Zhanga, Ruiwen Shua,b, , Yue Wua, Zongli Wana, Mingdong Zhenga, a b
T
⁎
School of Chemical Engineering, Anhui University of Science and Technology, Huainan, 232001, People’s Republic of China School of Earth and Environment, Anhui University of Science and Technology, Huainan, 232001, People’s Republic of China
A R T I C LE I N FO
A B S T R A C T
Keywords: Reduced graphene oxide Din dioxide Nanocomposites Microwave absorption Filler loading
Herein, reduced graphene oxide/tin dioxide (RGO/SnO2) binary nanocomposites were synthesized through a facile one-pot hydrothermal route by using graphene oxide as a substrate. Results of morphology observations revealed that SnO2 nanoparticles were uniformly loaded on the crumpled surface of thinly flake-like RGO. Moreover, the influence of the contents of RGO and filler loadings on the electromagnetic parameters and microwave absorption properties of RGO/SnO2 binary nanocomposites were elaborately investigated. It was found that the as-prepared binary nanocomposites with the content of RGO of 9.6 wt% exhibited superior microwave absorption properties in the X-band. Remarkably, the minimum reflection loss (RLmin) achieved −53.7 dB at 11.1 GHz and effective absorption bandwidth (EAB, RL ≤ −10 dB) reached 3.7 GHz with a thin matching thickness of merely 2.2 mm. Furthermore, the EAB could reach 14.3 GHz (89.4% of 2–18 GHz) by facilely adjusting the matching thicknesses from 1.5 to 5.0 mm, which spanned the whole Ku, X and C bands. Besides, the possible microwave absorption mechanisms of obtained binary nanocomposites were proposed. This work could be helpful for fabricating graphene-based nanocomposites as high-efficient microwave absorbers.
1. Introduction With the increasingly serious problem of electromagnetic pollution originated from the wide usage of electronic equipment, microwave absorbing materials (MAMs) have gained great attentions in the field of electromagnetic absorption [1–3]. Generally, the characteristics of thin thickness, strong absorption, broad bandwidth and light weight are vitally important for designing of an ideal microwave absorber [4–10]. Therefore, tremendous efforts have been focused on designing and fabricating the high-performance MAMs in the past few decades [1,5–10]. As a novel kind of carbon nanomaterials, reduced graphene oxide (RGO) has been considered as a potential candidate for microwave absorption owing to the advantages such as low density, high specific surface area, residual defects and groups, and notable dielectric loss [11–17]. However, single RGO used as MAMs suffers from inferior impedance matching and poor microwave attenuation loss [11,13,14]. Thus, it is very urgent to enhance the microwave absorption performance of RGO for dealing with the growing problem of electromagnetic pollution. According to the electromagnetic theories, impedance matching and attenuation loss play two key roles for achieving good microwave
⁎
absorption properties [6,18]. Recently, numerous investigations demonstrated that the complexing of dielectric loss materials (ZnO, MnO2 and Co3O4, etc) with RGO to fabricate RGO/semiconductor nanocomposites could be an effective strategy for improving the microwave absorption performance of RGO [19–21]. As one of the most important n-type semiconductor compounds, tin dioxide (SnO2) could be a potential candidate for microwave absorption due to these advantages such as facile synthesis, low cost, good chemical stability and moderate dielectric loss [22–28]. For instance, Zhou et al. synthesized the SnO2 submicron fibers by an electrospinning method [23]. The SnO2 fibers/paraffin wax composite displayed the minimum reflection loss (RLmin) of -19.0 dB. Furthermore, the effective absorption bandwidth (EAB, RL ≤ −10 dB) could achieve 6.0 GHz [23]. Zhao et al. fabricated honeycomb-like SnO2 foams by a template method, which exhibited the RLmin of −37.6 dB at 17.1 GHz and EAB of 5.6 GHz [24]. However, the synthesis procedures of SnO2 fibers or foams are rather complicated, which limits their practical applications in the field of electromagnetic absorption. In our previous work, we fabricated SnO2@MWCNTs (multi-walled carbon nanotubes) nanocomposite by a facile one-pot hydrothermal method and found that the as-prepared nanocomposite showed strong dielectric loss and good microwave absorption performance with a RLmin of −21.6 dB at
Corresponding authors at: School of Chemical Engineering, Anhui University of Science and Technology, Huainan, 232001, People’s Republic of China. E-mail addresses: [email protected] (R. Shu), [email protected] (M. Zheng).
https://doi.org/10.1016/j.synthmet.2019.116157 Received 17 July 2019; Received in revised form 18 August 2019; Accepted 27 August 2019 0379-6779/ © 2019 Elsevier B.V. All rights reserved.
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Fig. 1. Schematic illustration of the synthesis process of RGO/SnO2 binary nanocomposites.
Fig. 2. (a) FT-IR spectra and (b) XRD patterns of the samples of S1, S2 and S3.
Fig. 3. (a) Raman spectra of the samples of S1, S2, S3 and GO and (b) TGA curves of the samples of S1, S2 and S3.
investigations of facile synthesis, influence of the contents of RGO and filler loadings on the electromagnetic parameters and microwave absorption properties of RGO/SnO2 nanocomposites have been rarely reported.
8.6 GHz for a matching thickness of merely 2 mm [25]. Therefore, we believe that superior microwave absorption properties could be achieved through fabricating the reduced graphene oxide/tin dioxide (RGO/SnO2) nanocomposites. To the best of our knowledge, the 2
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Fig. 4. SEM images of the samples of (a) S1, (b) S2 and (c) S3.
2.2. Preparation of RGO/SnO2 binary nanocomposites
Herein, graphene oxide (GO) was used as the substrate to in-situ deposit SnO2 nanoparticles on the surface of RGO for fabricating RGO/ SnO2 binary nanocomposites. Various techniques were adopted to explore the relationships between structure and microwave absorption properties of RGO/SnO2/paraffin wax composites. Moreover, the influence of contents of RGO and filler loadings on the electromagnetic parameters and microwave absorption properties of RGO/SnO2/paraffin wax composites was systematically investigated in the frequency range of 2–18 GHz. Results demonstrated that the as-prepared binary nanocomposites showed superior microwave absorption properties in terms of both RLmin and EAB with a thin matching thickness, which could be used as potential candidates for electromagnetic absorption. Besides, the possible microwave absorption mechanisms were clarified.
RGO/SnO2 binary nanocomposites were synthesized by a facile hydrothermal strategy. Briefly, graphite oxide (20 mg) was firstly dispersed into 30 mL deionized water by ultrasonic treatment for 1.0 h. Then, SnCl4·5H2O with different additive amounts (1.55, 3.1 and 4.65 g) was completely dissolved into the above aqueous GO dispersions by vigorously stirring. Next, concentrated NH3·H2O was added drop-wise into the reaction mixtures to adjust the pH equals to 10 and vigorously stirred for 15 min. Afterward, the mixture dispersions were poured into a Teflon-lined stainless-steel autoclave (50 mL) and reacted at 160 °C for 18 h. The obtained products were collected by centrifuging, and then purified by repeated washing with deionized water and anhydrous ethanol for several times until pH equals to 7. Finally, the powder-like samples were obtained by grinding dried solid products, which were dried at 55 °C for 24 h in a vacuum oven. For simplicity, the as-prepared RGO/SnO2 nanocomposites with different additive amounts of SnCl4·5H2O were labeled as S1 (1.55 g), S2 (3.1 g) and S3 (4.65 g), respectively. The schematic synthesis procedures of RGO/SnO2 binary nanocomposites were described in Fig. 1. Firstly, aqueous GO dispersions were obtained by ultrasonication of graphite oxide. Then, positively charged Sn4+ could be attached to the negatively charged surface of GO by electrostatic attraction under alkaline conditions in the aqueous GO dispersions [15,28]. Finally, SnO2 particles were in situ deposited on the surface of RGO after hydrothermal reactions and thus RGO/SnO2 nanocomposites were formed.
2. Experimental 2.1. Materials Graphite oxide was provided by Suzhou TANFENG Graphene Tech Co., Ltd (Suzhou, China). Tin tetrachloride (SnCl4·5H2O), ammonium hydroxide (NH3·H2O, 25–28 wt%) and anhydrous ethanol (C2H5OH) were purchased from Sinopharm Chemical Reagent Co., Ltd (Shanghai, China). All the chemical reagents were analytical grade and used without further purification. Deionized water was produced in our laboratory (electrical resistivity ∼ 18.2 MΩ cm).
3
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Fig. 5. TEM images with different magnifications: (a)‒(c), HRTEM image (d) and EDS pattern (e) of the sample of S1.
temperature by using a laser confocal Raman spectrometer (Renishaw2000, UK) in the range of 300–2200 cm-1. Thermal gravimetric analysis (TGA) was performed on a thermogravimetric analyzer (TGA/DSC 3+, Switzerland) from 40 °C to 800 °C in the atmosphere of air with a heating rate of 5 °C min-1. The morphology was observed with a field emission scanning electron microscopy (FESEM, Hitachi-Su8020, Japan) and field emission transmission electron microscopy (FETEM, FEI-TF20, USA) equipped with the energy dispersive X-ray spectrum
2.3. Characterization Fourier transform infrared (FT-IR) spectra were recorded in the wavenumber range of 500–4000 cm−1 using a Nicolet 380 spectrometer (Thermoscientific, USA). The crystalline structure was characterized by X-ray diffraction (XRD, LabX XRD-6000, Japan) with CuKα radiation (λ =0.154 nm) in the scattering range (2θ) of 20–80° with a scanning rate of 2°/min. Raman spectra were acquired at room 4
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Fig. 6. Frequency dependence of reflection loss with different thicknesses for the sample of S1 at different filler loadings: (a) 50 wt%, (b) 60 wt% and (c) 70 wt%.
disorders, and sp2 hybridization, respectively [6,14,28]. Generally, the value of ID/IG is often used to evaluate the degree of disorders [6,14,28]. It can be seen that the values of ID/IG for S1, S2, S3 and GO are 1.09, 1.08, 1.06 and 0.89, respectively. Therefore, the as-prepared binary nanocomposites exhibit obviously enhanced ID/IG compared with GO. This result indicates that the degree of defects become higher originated from the reduction of GO into RGO and loaded SnO2 nanoparticles on the surface of RGO [14,28]. Furthermore, the value of ID/IG decreases as the contents of RGO declining in the binary nanocomposites. Besides, the characteristic Raman scattering peaks of SnO2 in the low wavenumbers range (400–700 cm−1) can be observed in the binary nanocomposites [29]. Thermal gravimetric analysis (TGA) measurements were conducted for determining the contents of RGO in the obtained nanocomposites. As shown in Fig. 3(b), the thermal decomposition process of all the samples can be divided into two stages. Firstly, a small weight loss (∼5.5 wt%) occurs below 250 °C, which is mainly caused by the loss of some oxygen-containing groups such as −COOH and −OH on the surface of RGO. Secondly, an obvious weight loss between 250 and 550 °C, which could be ascribed to the degradation of RGO. Besides, the residual products are believed as the constituent of SnO2 [28]. Thus, the contents of RGO in S1, S2 and S3 can be estimated as 9.6 wt%, 8.7 wt% and 7.6 wt%, respectively. The specific surface areas (SBET) of the samples were calculated by a BET method, as shown in Table S1. The calculated SBET of S1, S2 and S3 were 171.1, 167.0 and 156.7 m2 g−1, respectively. Therefore, the specific surface areas show a slight decline trend with the decreasing of the contents of RGO.
(EDS) device. The Brunauer‒Emmett‒Teller (BET) specific surface area of the samples was measured on a V-Sorb 2800 P analyzer (China) by N2 adsorption at −196 °C. The electric conductivity was measured by a four-point probe method (ST2722-SZ, China). Electromagnetic parameters including the relative complex permittivity (εr = ε'-jε”) and permeability (μr = μ'-jμ”) were measured by a vector network analyzer (VNA, AV3672B-S, China) using the coaxialline method in the frequency range of 2–18 GHz. Before being tested, the as-prepared nanocomposites were homogeneously mixed with paraffin wax (which was transparent to microwaves) in different filler loadings (50 wt%, 60 wt% and 70 wt%) and then pressed into toroidalshaped ring with outer diameter of 7.0 mm, inner diameter of 3.04 mm and thickness of 2.0 mm. 3. Results and discussion 3.1. Structural analysis Fig. 2(a) shows the typical Fourier transform infrared (FT-IR) spectra of S1, S2 and S3. The peaks appearing at 3430, 1623, 1399, 1202 and 1044 cm−1 could be ascribed to the stretching vibrations of −OH, C]C, CeOH, CeOeC and CeO, respectively [29]. Besides, the peak at around 602 cm−1 can be assigned to the characteristic absorption of O–Sn–O [29]. As depicted in Fig. 2(b), the diffraction peaks from X-ray diffraction (XRD) appearing at 2θ = 26.8, 33.7, 37.7 and 51.7° are in good accordance with the (110), (101), (200) and (211) crystal planes of SnO2 (JCPDS 41-1445), respectively [23,27–29]. However, it is difficult to distinguish the diffraction peaks of RGO in all the samples, which could be explained by the relatively low diffraction strength of RGO compared with that of SnO2 in the binary nanocomposites [28,29]. Degree of graphitization of obtained nanocomposites was characterized by Raman spectroscopy. From Fig. 3(a), all the samples show two obvious Raman scattering peaks at 1592 cm−1 (G band) and 1347 cm−1 (D band). The D and G bands signify the sp3 defects or
3.2. Morphological analysis Micromorphology of obtained nanocomposites was observed by scanning electron microscopy (SEM). As shown in Fig. 4(a), the thinly flake-like RGO in the sample of S1 shows a rippled and crumpled morphology. Furthermore, numerously nano-sized SnO2 particles were 5
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Fig. 7. Frequency dependence of reflection loss with different thicknesses: (a) S1, (b) S2 and (c) S3; (d) 3D plots of reflection loss curves: (a’) S1, (b’) S2 and (c’) S3 at a filler loading of 60 wt%.
the composition of obtained nanocomposites. It should be noted that the Cu element comes from the supported copper grid for the measurement of TEM.
uniformly loaded on the surface of RGO. From Fig. 4(b) and (c), it can be found that the SnO2 particles exhibit obvious aggregation in the samples of S2 and S3. Therefore, the dispersion of SnO2 particles become worse with the decreasing of contents of RGO in the binary nanocomposites. The micromorphology and structure of the sample of S1 were further characterized by transmission electron microscopy (TEM). From Fig. 5(a)–(c), the thinly flake-like RGO is almost transparent, which suggests few-layer structure. Besides, SnO2 particles with slight aggregation are loaded on the surface of RGO. As depicted in Fig. 5(d), the high-resolution transmission electron microscopy (HRTEM) image reveals that the inter-plane distance is 0.334 nm, which corresponds to the (110) crystal plane of SnO2 [28]. The EDS pattern demonstrates the existence of Sn, O, C and Cu elements (Fig. 5(e)), which is coincide with
3.3. Microwave absorption properties Our recent investigations demonstrated that the filler loading could significantly influence the microwave absorption properties of absorbers [6,14]. Therefore, we have investigated the influence of filler loadings on the microwave absorption properties of the sample of S1, as shown in Fig. 6. It can be found that the values of RLmin of S1 are −32.1, −43.0 and −34.5 dB for the filler loadings of 50 wt%, 60 wt% and 70 wt%, respectively. Therefore, the S1 displays the optimal microwave absorption performance at a filler loading of 60 wt%. 6
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Fig. 8. Frequency dependence of (a) ε', (b) ε” and (c) tanδe for the samples of S1, S2 and S3.
Fig. 9. Frequency dependence of (a) normalized impedance matching (Z) with a matching thickness of 2.2 mm and (b) attenuation constant (α) for the samples of S1, S2 and S3.
important for determining the microwave absorption properties of absorbers [6,18,28]. Owing to the non-magnetic characteristic of RGO/ SnO2 nanocomposites, the real part (μ') and imaginary part (μ”) of relative complex permeability approximately equal to 1 and 0, respectively. Thus, the magnetic loss of all the samples is negligible. Fig. 8 shows the frequency-dependent real part (ε'), imaginary part (ε”) of relative complex permittivity and dielectric loss tangent (tanδe) of the samples of S1, S2 and S3. From Fig. 8(a), the ε' of S1–S3 shows a declining trend as the frequency increasing with slight fluctuations in the high frequency region, and ε' decreases with the decreasing of contents of RGO. As depicted in Fig. 8(b), the ε” of S1–S3 exhibits similar variation trend as ε' with the increasing of frequency. Moreover, all the samples especially S1 exhibit multiple relaxation peaks (marked by the dashed boxes in Fig. 8(b)) in the high frequency region [30,31]. On the basis of free electron theory, it can be deduced that the ε” enhances with the increasing of electric conductivity [6,32]. We have measured the electric conductivity of RGO/SnO2 nanocomposites (S1, S2 and S3) by a four-point probe method, as displayed in Fig. S1. The values of
For the sake of exploring the influence of the contents of RGO on the electromagnetic parameters and microwave absorption properties of obtained nanocomposites, the filler loading was fixed at 60 wt%. As shown in Fig. 7(a), the sample of S1 exhibits the RLmin of −53.7 dB at 11.1 GHz (X-band) and EAB of 3.7 GHz (9.3–13.0 GHz) with a thin matching thickness of merely 2.2 mm, which suggests superior microwave absorption performance. Furthermore, the EAB could achieve 14.3 GHz (89.4% of 2–18 GHz) by facilely adjusting the matching thicknesses from 1.5 to 5.0 mm, which spans the whole Ku, X and C bands. From Fig. 7(b) and (c), the samples of S2 and S3 present the RLmin of −22.6 dB and −10.2 dB, respectively. Thus, the as-prepared binary nanocomposites exhibit obviously enhanced microwave absorption performance with the increasing of the contents of RGO. Fig. 7(a’)–(c’) display the three-dimensional (3D) plots of reflection loss curves for the samples of S1, S2 and S3. Remarkably, the RLmin corresponding to the maximum microwave absorption could locate at various frequencies by modulating the thicknesses of absorbers [6,18,28]. Generally, the electromagnetic parameters (ε', ε”, μ', μ”) are vitally 7
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electric conductivity for S1, S2 and S3 are 24.7, 6.2 and 4.0 S/m, respectively. Thus, the electric conductivity of obtained nanocomposites enhances with the increasing of contents of RGO, leading to the enhancement of ε”. From Fig. 8(c), the binary nanocomposites exhibit obviously enhanced tanδe in the measured frequency range with the increasing of contents of RGO, suggesting the improved dielectric loss capacity. Notably, the S1 exhibits the strongest dielectric loss among all the samples. As shown in supplementary materials, the Cole‒Cole semicircle theory was deduced. Fig. S2 shows the Cole‒Cole plots of the samples of S1, S2 and S3. It can be observed that all the samples exhibit at least two semicircles, which suggests the existing of dual or multiple Debye dipolar relaxation processes [18,26]. Furthermore, it can be observed that the semicircles are distorted in some degree, which suggests that the Debye relaxation is not the only mechanism for dielectric loss and other mechanisms such as conductive loss and interfacial polarization could be responsible for microwave absorption [18,26]. In general, an ideal microwave absorber should satisfy the two requirements of impedance matching and maximum attenuation [6,18,31]. Normalized impedance matching (Z) is often described as follows [33–36]:
Z=
Zin = Z0
μr 2πfd ⎞ tanh ⎡j ⎛ μ r εr ⎤ ⎢ ⎥ εr ⎣ ⎝ c ⎠ ⎦
(1)
Herein Z0 and Zin are the free space and input impedance, respectively. Electromagnetic attenuation capacity can be evaluated by the attenuation constant (α), which is expressed as the following equation [37–41]:
α=
2 πf × c
(μ′′ε′′ − μ′ε′) +
(μ′′ε′′ − μ′ε′)2 + (ε′μ′′ + ε′′μ′)2
(2)
Fig. 9 displays the frequency-dependent Z and α for the samples of S1, S2 and S3. From Fig. 9(a), both S2 and S3 exhibit the values of Z values notably larger than 1 in the frequency range of 11–17 GHz, which suggests inferior impedance matching characteristic. However, the values of Z of S1 are much closer to the line of optimal impedance matching (Z = 1) than that of S2 and S3, indicating the improved impedance matching. From Fig. 9(b), the values of α are enhanced with the increasing of contents of RGO. Notably, the sample of S1 shows the largest α with a value of 228.9, manifesting the strongest attenuation loss. As the good impedance matching is achieved, most of the incident microwaves can penetrate into the specimen; meanwhile, the strongest microwave attenuation capacity could effectively transform the electromagnetic energies into thermal energies to attenuate the microwaves [18,31]. As a consequence, the S1 demonstrates the best microwave absorption performance among all the samples. The relationship between absorption peak frequency (fm) and matching thickness (tm) can be well clarified by quarter-wavelength (λ/ 4) matching theory, which is often described as follows [6,18,31,37]:
Fig. 10. (a) Frequency-dependent reflection loss, (b) simulations of the tm versus fm under the λ/4 model and (c) normalized impedance matching (Z) as a function of frequency for the sample of S1 with different thicknesses.
tm = Fig. 11. Schematic illustration of the possible microwave absorption mechanisms of RGO/SnO2 binary nanocomposites.
nλ nc = (n=1,3,5,…) 4 4fm |εr μr |
(3)
If tm and fm meet the Eq. (3), a phase cancellation effect could
Table 1 Typical SnO2 related composites as microwave absorbers reported in this work and recent literatures. Samples
Matrix
Thickness (mm)
RLmin (dB)
EAB (GHz)
Ref.
RGO/SnO2 (S1) RGO/SnO2 (S1) Ni-doped SnO2@MWCNTs SnO2-graphene Fe-doped SnO2@MWCNTs Fe-doped SnO2/RGO
Paraffin Paraffin Paraffin Paraffin Paraffin Paraffin
2.2 1.5 2.5 2.0 1.5 5.3
−53.7 −43.0 −39.2 −15.28 −44.5 −29.0
3.7 3.6 3.6 1.3 4.5 2.7
This work This work [25] [26] [27] [28]
Notes: MWCNTs denoted multi-walled carbon nanotubes. 8
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References
effectively attenuate the incident microwaves [6,18,31,37]. As depicted in Fig. 10(a), it can be seen that the reflection loss peaks of S1 shift to lower frequency with the increasing of tm. Fig. 10(b) describes the simulations of tm versus fm under the λ/4 model. The pentagram signifies the experimental tm (denoted as tmexp). Significantly, all the tmexp are exactly located at the fitted λ/4 curve. This finding indicates that the λ/4 rule essentially determines the relationship between tm and fm. Therefore, it is valuable to design the thickness of absorbers according to the quarter-wavelength matching theory. Besides, the strongest RL peak (-53.7 dB at 11.1 GHz and 2.2 mm) corresponds well with the optimal impedance matching (Fig. 10(c)). Fig. 11 describes the possible microwave absorption mechanisms of RGO/SnO2 binary nanocomposites. Firstly, the residual oxygen-containing groups such as −COOH and −OH, and structure defects on the surface of RGO could induce the dipole polarization and defect polarization under the alternating electromagnetic fields, respectively [28,40]. Secondly, numerously heterogeneous interfaces among paraffin matrix, RGO and SnO2 nanoparticles significantly enhance the interfacial polarization relaxation [28]. Lastly, the synergistic effects of dielectric loss derived from interfacial polarization, defect polarization and dipole polarization, and conduction loss originated from RGO [28,40], which notably improve the microwave absorption performance. As shown in Table 1, we have summarized the reported SnO2 related composites as the microwave absorbers in this work and recent literatures. It is obvious that the as-prepared RGO/SnO2 nanocomposite (S1) exhibited superior microwave absorption properties with strong absorption, broad bandwidth and thin thickness among the reported SnO2 related composites.
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4. Conclusions In summary, RGO/SnO2 binary nanocomposites were successfully prepared by a facile one-pot hydrothermal strategy. Results demonstrated that numerous SnO2 nanoparticles were uniformly loaded on the crumpled surface of thinly flake-like RGO. Furthermore, it was found that both the contents of RGO and filler loadings had remarkable influence on the microwave absorption properties of as-prepared nanocomposites. Significantly, the obtained nanocomposites showed excellent microwave absorption properties with the RLmin of -53.7 dB and EAB of 3.7 GHz for a thin matching thickness of 2.2 mm. Besides, the possible microwave absorption mechanisms were clarified and mainly attributed to the enhanced dielectric loss from multiply interfacial polarization, dipole polarization and defect polarization, conduction loss from RGO and optimized impedance matching. Therefore, it was believed that the obtained nanocomposites could be used as high-efficiency microwave absorbers in the field of electromagnetic absorption.
Acknowledgments This work was financially supported by the National Natural Science Foundation of China (Grant No. 51507003), China Postdoctoral Science Foundation (Grant No. 2019M652160), Foundation of Provincial Natural Science Research Project of Anhui Colleges (Grant No. KJ2019A0119), Lift Engineering of Young Talents and Doctor’s Start-up Research Foundation of Anhui University of Science and Technology (Grant No. ZY537).
Appendix A. Supplementary data Supplementary material related to this article can be found, in the online version, at doi:https://doi.org/10.1016/j.synthmet.2019. 116157. 9
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