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Article Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX

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Three-Dimensional Architecture Reduced Graphene Oxide−LiFePO4 Composite: Preparation and Excellent Microwave Absorption Performance Jingjing Dong,† Ying Lin,*,† Hanwen Zong,† Haibo Yang,*,† Lei Wang,† and Zhonghua Dai‡ †

School of Materials Science and Engineering, Shaanxi University of Science and Technology, Weiyang District, Xi’an 710021, China Laboratory of Thin Films Technology and Optical Test, Xi’an Technological University, Xi’an, Shaanxi 710032, China

‡

Inorg. Chem. Downloaded from pubs.acs.org by IOWA STATE UNIV on 01/08/19. For personal use only.

S Supporting Information *

ABSTRACT: The present work reports preparation of 3D composites of reduced graphene oxide (RGO) with coral-like LiFePO4 particles in two steps, which involves the fabrication of LiFePO4 particles by the solvothermal method and the subsequent preparation of RGO/coral-like LiFePO4 composites by the etching method. The microwave absorption properties of LiFePO4 particles, coral-like LiFePO4 particles, and the RGO/coral-like LiFePO4 composites were studied. Our results show that the as-prepared RGO/coral-like LiFePO4 composites exhibit significantly improved microwave absorption properties compared with the pure LiFePO4 particles and coral-like LiFePO4 particles. The RGO/corallike LiFePO4 composite (S-60) possesses optimized reflection loss (RL) performance with an RL value of −61.4 dB and a broad effective absorption bandwidth of 4 GHz (from 11.36 to 14.88 GHz and 16.32 to 16.8 GHz) at the matching thickness of only 2.4 mm. This demonstrates that the RGO/coral-like LiFePO4 composites can be superior candidates for lightweight and high-efficiency microwave absorbers.

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INTRODUCTION Currently, high efficiency microwave absorption materials have draw intensive attention owing to the increasing electromagnetic (EM) radiation and interference problems, which have severely threatened human health.1,2 High performance EM absorbers with lightweight, thin thickness, strong attenuation, and broad effective absorption bandwidth are needed.3−5 Traditionally, EM absorbing materials principally include metal oxides. However, the traditional absorbers face many challenges such as poor anticorrosion ability, high density, and low permittivity, which restrict their practical application.6−9 One prominent representative of carbon nanomaterials is reduced graphene oxide (RGO), which has extremely large specific surface area, low density, superior electrical conductivity, and carrier mobilities coupled with residual defects and functional groups.10−17 However, the EM wave absorbing performance of RGO is very weak due to its poor impedance matching and single dielectric loss mechanism.18−20 The RLmax of pure RGO is only −7 dB, which is not an ideal material in microwave absorption applications.21 Hence, great efforts have already been devoted to the incorporation of RGO with oxides to overcome the deficiencies. Feng et al. prepared the ZnFe2O4@SiO2@RGO composites by a “coating−coating” route. The composite exhibited an RLmax of −43.9 dB, and the effective absorption bandwidth was 6 GHz.22 Zhang et al. © XXXX American Chemical Society

synthesized the RGO/α-Fe2O3 nanohybrids and studied their EM wave absorption performances.23 The RLmax reached −33.5 dB and effective absorption bandwidth of 6.4 GHz. Zhang et al. synthesized hybrid CoS2/RGO composite with a RLmax of −56.9 dB and effective absorption bandwidth of 4.1 GHz.24 It has been realized that the outstanding EM absorption properties are attributed to impedance matching and unique microstructure of EM wave absorption materials. To sum up, incorporating diverse ingredients into or on the surface of RGO and giving consideration to both EM attenuation and impedance matching should be done to produce an outstanding absorber.25−29 Lithium iron phosphate (LiFePO4), a classical anisotropic semiconductor material with an olivine-type structure, is composed of a distorted hexagonal close-packed skeleton including Li and Fe at the octahedral sites and P at the tetrahedral sites. LiFePO4 has drawn much attention because of many reasons such as convenient synthesis, thermal stabilities, low cost, and environmental friendliness. However, there have been no reports on the composites of RGO with coral-like LiFePO4 particles being used in the EM wave absorption area. Therefore, we developed a facile etching method to prepare coral-like LiFePO4 particles, anchoring on RGO nanosheets to Received: October 28, 2018

A

DOI: 10.1021/acs.inorgchem.8b03043 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry Scheme 1. Schematic Illustration of the Fabrication Process of RGO/Coral-like LiFePO4 Composites

Figure 1. (a) XRD patterns of RGO, LiFePO4 particles, coral-like LiFePO4 particles, and RGO/coral-like LiFePO4 composite (S-60) and (b) Raman spectra of GO, LiFePO4 particles, coral-like LiFePO4 particles, and RGO/coral-like LiFePO4 composite (S-60). composites were fabricated by varying the RGO content (RGO = 40, 60, and 80 mg), and the products are denoted as S-40, S-60, and S-80, respectively. Materials Characterization. The synthesized pure LiFePO4 particles, coral-like LiFePO4 particles, and RGO/coral-like LiFePO4 composites were characterized by X-ray diffraction (XRD), Raman spectra, X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), transmission electron microscopy (TEM), energy dispersive X-ray analysis (EDX), and Brunner−Emmet−Teller (BET) method. Electromagnetic Measurements. Electromagnetic parameters were tested on a vector network analyzer (VNA, HP8720ES, Agilent, United States) in the range of 2−18 GHz. Coaxial specimens were prepared by uniformly mixing 30 wt % of RGO/coral-like LiFePO4 composites with wax and were compressed to standard rings (Φout of 7.0 mm and Φin of 3.04 mm). The RL was calculated according to the following equations:32 ÅÄÅ 2π ÑÉÑ μ ÑÑ με Zin = Z0 r tan hÅÅÅÅj Ñ r fd Ñ r ÅÅÇ c ÑÑÖ εr (1)

form RGO/coral-like LiFePO4 composites, and we used the etching properties of a DMF solution of methyl mercaptoacetate (complexing agent) and hydrazine (reducing agent) to obtain a nanohybrids of plenty of coral-like LiFePO4 particles and RGO nanosheets. The manufacturing process of RGO/ coral-like LiFePO4 composites is illustrated in Scheme 1. The superior EM absorbing performances could be obtained for the RGO/coral-like LiFePO4 composite (S-60) with RLmax of −61.4 dB and a broad effective absorption bandwidth of 4 GHz at 2.4 mm. The enhanced EM wave absorption properties are attributed to the optimal impedance matching and strong interfacial polarizations of the RGO/coral-like LiFePO4 composites. Thus, the RGO/coral-like LiFePO4 composite is a promising candidate as an EM wave absorption material.

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EXPERIMENTAL PROCEDURE

Materials. Graphene was obtained according to the Hummers method.30 Lithium hydroxide (LiOH·H2O), ferrous sulfate (FeSO4· 7H2O), phosphoric acid (H3PO4, 85 wt %), methyl mercaptoacetate, DMF, and hydrazine monohydrate (N2H4·H2O) were supplied by Sinopharm Chemical Reagent Co., Ltd. All reagents were used as received without further purification. Synthesis of RGO/Coral-like LiFePO4 Composites. LiFePO4 particles were prepared through a solvothermal method.31 The RGO/ coral-like LiFePO4 composites were fabricated by etching method. Briefly, 0.2 g of LiFePO4 particles and graphite were ultrasonically dispersed with 200 mL of DMF. The solution was heated to 80 °C under the water bath. Then, hydrazine (6 mL) and methyl mercaptoacetate (1.5 mL) were added. After N2 protection for 30 min, the reaction was ended by cold ethanol after etching for 60 min. The products were washed with deionized water and ethanol three times and then freeze-dried for 12 h. The RGO/coral-like LiFePO4

RL(dB) = 20 lg

Zin − Z0 Zin + Z0

(2)

where Z0 is the impedance of free space (377 Ω), Zin stands for the input impedance of RGO/coral-like LiFePO4 composites, μr and εr are the complex permeability and permittivity, f is the microwave frequency, d is the thickness, and c is the velocity of light.

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RESULTS AND DISCUSSION Characterization of the Materials. The crystal structure and phase purity of products were investigated by XRD (Figure 1a). For LiFePO4 particles and coral-like LiFePO4 B


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Inorganic Chemistry

Figure 4. (a) N2 adsorption−desorption isotherms and (b) pore size distribution of the prepared LiFePO4 particles, coral-like LiFePO4 particles, and RGO/coral-like LiFePO4 composite (S-60); C 1s spectra of (c) coral-like LiFePO4 particles and (d) RGO/coral-like LiFePO4 composite (S-60).

detected, revealing that the high purities of all the samples were successfully synthesized. Figure 1b shows Raman spectroscopy of GO, LiFePO4 particles, coral-like LiFePO4 particles, and RGO/coral-like LiFePO4 composite. Three prominent peaks at 150−500 cm−1 (Fe−O) and four prominent peaks at 500−1100 cm−1 (PO43−) were observed in the LiFePO4 particles.33 The peaks at ∼1350 cm−1 and ∼1587 cm−1 are correspond to the D and G-band of carbon, respectively. The D band is due to the vibration of the k-point photons of A1g symmetry, while the G band arises from the in-plane vibration of sp2 carbon atoms.34,35 The intensity ratio of D band to G band (ID/IG) for RGO/coral-like LiFePO4 composite (1.01) is higher than that of GO (0.91), indicating the increased degree of defects in the RGO/corallike LiFePO4 composite compared to GO. SEM images of LiFePO4 particles, coral-like LiFePO4 particles, RGO/coral-like LiFePO4 composite (S-60) at different magnifications, and elemental maps of each element of RGO/coral-like LiFePO4 composite (S-60) are shown in Figure 2. As found for LiFePO4 particles (Figure 2a), the LiFePO4 particles are uniformly formed into a rugby-like structure, and the average grain size is about 0.4−0.7 μm. After etching, the surface of LiFePO4 particles changed, and the morphology of LiFePO4 particles transformed into a coral-like structure (Figure 2b). Figures 2c and d show the detailed morphology of the RGO/coral-like LiFePO4 composite, from which can be seen that the coral-like LiFePO4 particles composed of numerous nanorods are successfully attached onto the RGO surface. The size of coral-like LiFePO4 particles becomes smaller when graphene is added. During the etching process, lithium and iron ions are dissolved in the mixed solution; subsequently, a great amount LiFePO4 fragments were recrystallized and aggregated to form coral-like particles on the surface of the RGO sheets. Moreover, Figure 2e show the EDS spectra of the RGO/coral-like LiFePO4 composite, which reveal that the content (weight ratio) of Fe, P, O, and C are 20.07, 9.95, 37.96, and 32.01%, respectively. Moreover, the elemental mapping by EDS of the RGO/coral-like LiFePO4 composite was detected and furnished evidence that four elements Fe, P, O, and C are in the composites (Figure 2f).

Figure 2. Typical FESEM images of (a) LiFePO4 particles, (b) corallike LiFePO4 particles, and (c and d) RGO/coral-like LiFePO4 composite (S-60) at different magnifications. (e) EDX of RGO/ coral-like LiFePO4 composite (S-60) and (f) elemental maps of Fe, P, O, and C in RGO/coral-like LiFePO4 composite (S-60).

Figure 3. TEM images of (a) LiFePO4 particles, (b) coral-like LiFePO4 particles, and (c) RGO/coral-like LiFePO4 composite (S60) and (d) HRTEM image of the S-60.

particles, all the diffraction peaks match well with space group Pbam (JCPDS card no. 40-1499). The decrease in the peak intensities of the coral-like LiFePO4 particles compared to the LiFePO4 particles indicated that the porous structure contains abundant nanosized fragments. For RGO/coral-like LiFePO4 composite (S-60), the peaks of RGO cannot be observed in the sample, suggesting the poor crystallinity and low content of RGO in the composites. Furthermore, no impurities were C


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Inorganic Chemistry

Figure 5. Complex permittivity (a, b), the complex permeability (c, d), dielectric loss tangent (e), and magnetic loss tangent (f) of the RGO/corallike LiFePO4 composites.

RGO/coral-like LiFePO4 composite, respectively. Here, the composite RGO/LiFePO4 has a specific surface area larger than those of the LiFePO4 particles and coral-like LiFePO4 particles. The pore size distribution of the RGO/coral-like LiFePO4 composite reflects a broad distribution around 50 nm, which is much greater than LiFePO4 particles and corallike LiFePO4 particles. The elemental components of LiFePO4 particles and corallike LiFePO4 particles were further investigated by XPS measurements (Figure S1). From Figure S1a, the four elements of Li, Fe, P, and O are characterized in the survey spectrum. In Figure S1b, the Li 1s peak of coral-like LiFePO4 particles slightly moved to a higher binding energy due to the existence of surface defects compared to LiFePO4 particles.38 For Figure S1c, the surface of LiFePO4 particles seem to be mainly composed of Fe3+ and Fe2+ phase, and the peak positions corresponds to 711.4 and 724.6 eV, respectively. The relatively high intensity of Fe3+ signal indicates that the surface of LiFePO4 particles is enriched with Fe3+. Whereas after

Figure 3 shows the TEM images of LiFePO4 particles, corallike LiFePO4 particles, and RGO/coral-like LiFePO4 composite (S-60). Figure 3a shows that the LiFePO4 particles show a rugby-like shape, which is consistent with the SEM image of the LiFePO4 particles. Compared with the LiFePO4 particles, the coral-like LiFePO4 particles are composed of numerous nanorods (Figure 3b). Figure 3c shows that the coral-like LiFePO4 particles are supported by RGO sheets, indicating an excellent adhesion between RGO and coral-like LiFePO4 particles is formed. In Figure 3d, the interplanar spacing of 2.5 Å can be observed, which corresponds to the (131) plane of orthorhombic LiFePO4. The surface area and pore size were studied by using N2 adsorption/desorption isotherms. As seen in Figures 4a and b, all samples have a small hysteresis loop from P/P0 = 0.65 to 1, indicating the presence of porous structure.36,37 The BET surface areas, pore volumes, and pore sizes are summarized in Table S1. The surface areas were 10.98, 51.89, and 59.66 m2 g−1 for LiFePO4 particles, coral-like LiFePO4 particles, and D


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Inorganic Chemistry

Microwave Absorbing Properties. Figure S2 showed the probable EM parameters and reflection loss curves of the LiFePO4 particles and coral-like LiFePO4 particles. In Figure S2a and b, both ε′ and ε″ of LiFePO4 particles and coral-like LiFePO4 particles present a similar fluctuation tendency and display a high peak at ∼16 GHz. Moreover, the ε′ and ε″ values of coral-like LiFePO4 particles are higher than those of LiFePO4 particles, demonstrating that the coral-like LiFePO4 particles have higher dielectric loss than LiFePO4 particles. This may be due to the formation of many vacancies in corallike LiFePO4 particles, which results in multiple scattering, dipole polarization, and interfaces polarization generated.43,44 The values of μ′ and μ″ for LiFePO4 particles and coral-like LiFePO4 particles have similar fluctuation tendency with increasing the frequency. For LiFePO4 particles shown in Figure S2e, the RLmax value can be up to 17.8 dB at 16.2 GHz with a thickness of 3.2 mm. As can be seen from Figure S2f, the EM absorbing performance of coral-like LiFePO4 particles was enhanced obviously compared to the LiFePO4 particles in terms of RLmax. The RLmax of coral-like LiFePO4 particles with a thickness of 3.2 mm is 30.2 dB at 16.3 GHz. Figure 5 shows the complex permittivity (ε′, ε″), complex permeability (μ′, μ′′), loss tangent (tan δε = ε″/ε′, tan δμ = μ″/μ′) of RGO/coral-like LiFePO4 composite with different RGO contents. ε′ and μ′ are on behalf of the storage capability of EM energy, while ε″ and μ′′ represent dissipation ability of EM energy.45 The values of ε′ of the RGO/coral-like LiFePO4 composites show a slight decrease with increasing frequency (Figure 5a), and the ε″ values tend to have the same trend in the testing frequency region (Figure 5b). To characterize energy conversion, the contributions of relaxation and charge transport to ε″ (εp″ and εc″, respectively) are separated. Three peaks appear near f = 6.8, 9.5, and 15.6 GHz, demonstrating multiple relaxation in RGO/coral-like LiFePO4 composites.46,47 In general, ε′ and ε″ are expressed by the Debye theory as follows:48

Figure 6. Plots of μ″(μ′)−2f−1 for the samples produced at various frequency.

etching, the peaks at 710.6, 723.2, and 725.1 eV of Fe 2p corresponding to ferric(II) can be found for coral-like LiFePO4 particles, indicating that ferric(III) is reduced to ferric(II) by hydrazine.39,40 Besides, it can be found from the XPS P 2p (Figure S1d) that the peak at 133.2 eV could be indexed to PO4 group, indicating that phosphate groups are preserved. For Figure S1e, the O 1s spectrum with the peak positions at 530.4, 531.5, and 533.2 eV are attributed to O2−, ordered lattice oxygen ions, and chemically/physically adsorbed water.41 After etching, the O 1s peak located at 531.5 eV is strengthened, indicating an increase in the number of lattice oxygen ions in the coral-like LiFePO4 particles. Further, from the high-resolution of C 1s XPS spectra provided in Figure 4, it can be observed that the C 1s spectrum is deconvoluted into three peaks, which correspond to the O−CO (288.4 eV), C−O (∼285.6 eV), and C−C/CC (284.6 eV) respectively, and the C−C/CC band of the composite is weaker than that of coral-like LiFePO4 particles, indicating that the GO is reduced.42

ε′ = ε∞ +

εs − ε∞ 1 + ω 2τ 2

(3)

Figure 7. (a−c) Frequency dependence of reflection loss (RL) of RGO/coral-like LiFePO4 composites with different RGO contents (S-40, S-60, and S-80); (d−f) simulations of the thickness (tm) of absorber versus frequency (f m) of RGO/coral-like LiFePO4 composites under the λ/4, 3λ/4, and 5λ/4 model. E


Article

Inorganic Chemistry ε″ =

εs − ε∞ 1 + ω 2τ 2

ωτ +

σ ωε0

(4)

where εs is the static permittivity, ε∞ represents the relative permittivity, ω is the angular frequency, τ means the polarization relaxation time, σ is the alternative conductivity, and ε0 is the dielectric constant in vacuum. Based on eqs 3 and 4, the decline in ε′ and ε′′ benefit from an increase in ω. The ε″ exhibits a gradual enhancement with increasing the content of RGO, which can be explained rationally by the effective medium theory. At the same time, two peaks were observed in the plots of ε′′, indicating the effects of interface and defect polarization on the dielectric properties. In Figure 5c, the variation tendency of μ′ for RGO/coral-like LiFePO 4 composites is basically the same, implying their similar storage ability of magnetic energy. In Figure 5d, the imaginary parts μ″ curves of RGO/coral-like LiFePO4 composites display the same variation trends in the 2−18 GHz frequency range with slight fluctuations. For RGO/coral-like LiFePO4 composites, there are five relaxation peaks in the spectrum of μ″, which correspond to multiple magnetic resonance of effective interface polarization.49 Generally, the magnetic loss mostly stems from eddy current loss, natural resonance, and exchange resonance.50,51 The eddy current loss could be explained by the equation of C0 = μ″(μ′)−2f−1 (Figure 6). If magnetic loss only stems from the eddy current loss, the value of C0 is constant when frequency increases. The values of C0 of the samples were similar at 9.3−18 GHz. This suggests that the eddy current loss greatly contributes to magnetic loss in the high frequency region. Three peaks at 9.3−13.4, 13.4−16.4, and 16.4−18 GHz were observed in the C0 plot of RGO/corallike LiFePO4 composites. The peaks at 4.0 and 8.5 GHz for S60 were caused by natural resonance and the exchange resonance, respectively. The values of dielectric and magnetic loss of RGO/coral-like LiFePO4 composites are shown in Figures 5e and f, indicating the main contribution of the magnetic loss in low-frequency region and dielectric loss in the high-frequency area to the EM wave absorption property of RGO/coral-like LiFePO4 composites. Figure 7 shows the RL curves of RGO/coral-like LiFePO4 composites with various thicknesses over 2−18 GHz. It is obvious that, without the introduction of RGO, the RL values of the coral-like LiFePO4 particles on EM are poor. However, the RL values of the specimens (S-40, S-60, and S-80) have obviously enhanced after the introduction of RGO. Obviously, S-60 has EM absorbing performances superior to those of S-40 and S-80; in the investigated region, the RLmax value reaches −61.4 dB at 12.6 GHz, and the effective absorption bandwidth is 4 GHz (from 11.36 to 14.88 GHz and 16.32 to 16.8 GHz). The RLmax of S-40 is −33.9 dB with a thickness of 2.9 mm, and

Figure 8. Three-dimensional representation of the reflection loss values for RGO/coral-like LiFePO4 composites with different RGO contents (S-40, S-60, and S-80).

Figure 9. (a−c) ε′−ε″curves of the RGO/coral-like LiFePO4 composites (S-40, S-60, and S-80). F


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Inorganic Chemistry

Figure 10. Complex impedance of the composites: (a) imaginary part and (b) real part.

to other two samples, S-60 displays the best EM wave absorption property, demonstrating that the RGO/coral-like LiFePO4 composite (S-60) has superior EM absorbing ability. The thickness of absorber (tm) at peak frequency (f m) satisfies the formula: tm = nc /(4fm |μr ||εr| ) (n = 1, 3, 5, ...).52 The relationship between tm and f m is simulated based on the λ/4 model, as shown in Figures 7d−f. Clearly, all the t m corresponding to the peaks are in good agreement with the λ/4 model of samples. Figure 8 shows the 3D RL date for RGO/coral-like LiFePO4 composites with various thicknesses (2.1−3.1 mm) in the frequency range of 2−18 GHz. It is clearly seen that the EM absorbing performances of the RGO/ coral-like LiFePO4 composites can be easily tuned by adjusting the thickness of absorber. According to eqs 3 and 4, the relationship between ε′ and ε″ can be described as

Figure 11. Possible mechanism for the microwave absorption of RGO/coral-like LiFePO4 composites.

2 ε + ε∞ yz2 ij i ε − ε∞ yz jjε′ − s zz + (ε″)2 = jjj s zz 2 { k k 2 {

the effective absorption bandwidth is 6 GHz (from 11.44 to 15.52 GHz and 16.08 to 18 GHz). The RLmax of S-80 is −43.8 dB with a thickness of 2.9 mm, and the effective absorption bandwidth is 4.72 GHz covering 8.96−13.68 GHz. Compared

(5)

Thus, the plot of ε′ vs ε″ represents several semicircles, which is called the Cole−Cole semicircles while each

Figure 12. Reflection loss values versus effective bandwidth of typical rGO-based materials in this work and recent literature. G


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Inorganic Chemistry

frequency bandwidth, and thin thickness, which are promising for EM interference shielding applications.

semicircle is assigned to a Debye relaxation process. Figure 9 shows the ε′−ε″ curves of the RGO/coral-like LiFePO4 composites. It is demonstrated that several semicircles for RGO/coral-like LiFePO4 composites were obtained. For S-40, three different semicircles were found, and for S-60 and S-80, four semicircles were discovered (Figures 9b and c). However, for S-60, its semicircles are bigger than S-80, which means that S-60 has an excellent dielectric relaxation processes due to the hierarchical and porous structures of S-60.53 Additionally, the defects and groups in RGO cause self-doping, resulting in an additional relaxation processes.54−57 The improved microwave absorbing performance of RGO/ coral-like LiFePO4 composites primarily depends on two crucial factors: better impedance matching characteristics and enhanced EM wave attenuation. On the basis of eq 2, the RLmax will occur when Zin″ approaches 0 Ω while the corresponding Zin′ approaches 377 Ω.58 Figure 10 displays the complex impedance of RGO/coral-like LiFePO4 composites with the layer thickness of 3 mm. At 14.7 GHz, for S-60 composite, Zin″ is close to −10.8 Ω, and the corresponding Zin′ is 373.3 Ω, which are very close to the required values of 0 and 377 Ω. For S-80, the Zin″ is −17.9 Ω, and the corresponding Zin′ is 487.2 Ω. The Zin″ is very close to 0 Ω, but Zin′ is far from 377 Ω, resulting in a poor EM wave absorbing performance compared to S-60. However, for S-40, Zin″ and Zin′ are −108.8 and 452.4 Ω, respectively, at 15.8 GHz, which is the highest real impedance in the samples. Thus, the EM absorbing performance of S-40 is not ideal. The microwave absorption mechanism is presented in Figure 11. First, there are abundant defects and functional groups in coral-like LiFePO4 particles and RGO providing amount of dipoles. Second, the good electrical conductivity of RGO sheets provide better EM absorbing performances. Third, the multiple interfaces between LiFePO4−LiFePO4, LiFePO 4-RGO, and RGO-RGO can be considered as capacitor-like structure, and electromagnetic waves can be efficiently adsorbed. Furthermore, the magnetic loss of the RGO/coral-like LiFePO4 composites stems from eddy current loss, natural resonance, and exchange resonance. From what has been discussed above, the RGO/coral-like LiFePO4 is supposed to be a potential in the area of microwave absorption materials. The EM wave absorption performances of RGO/coral-like LiFePO4 composite absorbed together with other RGO-based composites reported recently are displayed in Table S2.59−66 It can be concluded that RGO/coral-like LiFePO4 composites exhibit outstanding performance at a rather broadband effective frequency and thin thickness. It can be observed more intuitively from Figure 12.67−76 Therefore, the RGO/ coral-like LiFePO4 composite can be used as a high-efficiency microwave absorber.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.8b03043.

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XPS spectra of LiFePO4 particles and coral-like LiFePO4 particles (Figure S1); electromagnetic parameters and reflection loss curves of LiFePO4 particles and coral-like LiFePO4 particles (Figure S2); comparison of porous characteristics of LiFePO4 particles, coral-like LiFePO4 particles, and RGO/coral-like LiFePO4composite (Table S1); and EM wave absorption performance for rGObased composites (Table S2) (PDF)

AUTHOR INFORMATION

Corresponding Authors

*Tel: +86-29-86168688; Fax: +86-29-86168688; E-mail: [email protected]. *Tel: +86-29-86168688; Fax: +86-29-86168688; E-mail: [email protected]. ORCID

Haibo Yang: 0000-0003-1828-3750 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is supported by the National Natural Science Foundation of China (Grant 51772177), the Science Fund for Distinguished Young Scholars of Shaanxi Province (Grant 2018JC-029), the Shaanxi Science & Technology Coordination & Innovation Project of China (Grant 2017TSCXL-GY-08-05), and the Industrialization Foundation of Education Department of Shaanxi Provincial Government (Grant 16JF002).

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REFERENCES

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CONCLUSIONS In this study, RGO/coral-like LiFePO4 composites with excellent EM wave absorbing properties were successfully prepared via a facile etching approach. The EM wave absorbing properties of the composites containing different RGO amounts were studied. The results show that the RLmax of RGO/coral-like LiFePO4 composite (S-60) achieves −61.4 dB with the matching thickness of only 2.4 mm and effective absorption bandwidth of 4.0 GHz. Therefore, the acquired RGO/coral-like LiFePO4 composites were ideal candidates for EM wave absorber with highly efficient performance, broad H


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Inorganic Chemistry

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