Hybrids of Reduced Graphene Oxide and Hexagonal Boron Nitride

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Hybrids of Reduced Graphene Oxide and Hexagonal Boron Nitride: Lightweight Absorbers with Tunable and Highly Efficient Microwave Attenuation Properties Yue Kang,†,‡ Zhenhua Jiang,† Tian Ma,‡ Zengyong Chu,*,† and Gongyi Li† †

College of Science, National University of Defense Technology, Changsha 410073, PR China The Quartermaster Research Institute of General Logistics Department, Beijing 100010, PR China

‡

S Supporting Information *

ABSTRACT: Sandwichlike hybrids of reduced graphene oxide (rGO) and hexagonal boron nitride (h-BN) were prepared via heat treatment of the self-assemblies of graphene oxide (GO) and ammonia borane (AB). TG-DSC-QMS analysis indicate a mutually promoted redox reaction between GO and AB; 900 °C is a proper temperature to transfer the hybrids into inorganic sandwiches. XRD, XPS, and Raman spectra reveal the existence of h-BN embedded into the rGO frameworks. High-resolution SEM and TEM indicate the layer-bylayer structure of the hybrids. The content of h-BN can be increased with increase of the mass ratio of AB and the highest heat treatment temperature. The complex permittivity and the microwave absorption are tunable with the variation of the content of h-BN. When the mass ratio of GO/AB is 1:1, the microwave absorption of the hybrid treated at 900 °C is preferable in the range of 6−18 GHz. A minimum reflection loss, −40.5 dB, was observed at 15.3 GHz for the wax composite filled with 25 wt % hybrids at the thickness of 1.6 mm. The qualified frequency bandwidth reaches 5 GHz at this thickness with a low surface density close to 1.68 kg/m2. The layer-bylayer structure of the hybrid makes great contributions to the increased approaches and possibilities of electron migrating and hopping, which has both highly efficient dielectric loss and excellent impedance matching for microwave consumption. KEYWORDS: graphene, reduced graphene oxide, hexagonal boron nitride, 2D stacks, self-assembly, layer-by-layer, microwave-absorbing property conventional RAMs.14−21 These materials include carbonyl irons,14 core−shelled nanoparticles,15,16 controlled carbon fibers,17,18 decorated carbon nanotubes,19,20 and newly arising graphene.7,8,22−28 As a monolayer of sp2 carbon atoms in a honeycomb structure, graphene draws extensive scientific and industrial attention recently, mostly due to its light weight,7 high electrical conductivity,22 and high thermal conductivity.22−24 As the thinnest and zero-band gap material in the carbon family, many of the unique physical properties of graphene stem from its unusual electronic structure near the Fermi level.24 Graphene is highly desirable as an electromagnetic wave absorber because of its large interface and high dielectric loss. However, the electromagnetic parameters of pristine graphene are too high to meet the requirement of impedance match, which results in strong reflection and weak absorption. In this aspect, graphene or fully reduced graphene oxide (frGO) have demonstrated

1. INTRODUCTION Nowadays, stealth is becoming more and more important for modern weapons. Structures are required to absorb the emitted electromagnetic wave energy and to minimize the reflection of the electromagnetic wave in the direction of the enemy radar receiver.1−4 Studies have been focused on effective radar absorbing materials (RAMs) that have a wide absorption frequency,5 high absorption capability,6 lightweight,7 good thermal stability,8 and antioxidation capability.5−8 According to the electromagnetic energy conversion principle, the relative complex permittivity (εr = εr′ − jεr″), the relative complex permeability (μr = μr′ − jμr″), and the proper matching of the complex permittivity and permeability determine the reflection and attenuation characteristics. The magnetic loss materials, such as ferrites,9−11 are relatively heavy and mainly effective in MHz range, which is hard to meet the demands of lightweight. The dielectric loss materials, such as SiC fibers,12,13 have attracted increasing interests due to their good radar-absorbing performance at high temperatures, but their dielectric properties often do not meet the practical requirements. In recent years, various strategies have been demonstrated. It is generally regarded that nanometer-sized materials are much better than © 2016 American Chemical Society

Received: September 19, 2016 Accepted: November 3, 2016 Published: November 3, 2016 32468

DOI: 10.1021/acsami.6b11843 ACS Appl. Mater. Interfaces 2016, 8, 32468−32476

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ACS Applied Materials & Interfaces excellent performance for microwave shielding rather than absorbing.7,8,25 The ability to tailor the conductive properties of graphene, especially the opening of a band gap, is critical for its potential use as RAMs. Graphene oxide (GO) is thus a graphene derivative with wide band gaps.22,23 They have many chemically reactive oxygen functionalities, including carboxylic acid groups at the edges and epoxy and hydroxyl groups on the basal planes. A controlled oxidation/reduction process provides the modulation of its electronic, optical, and mechanical properties, so partially reduced graphene oxide (prGO) have found much enhanced microwave absorbing properties.26−28 But GO or prGO are not stable at high temperatures and/or in reductive atmospheres, which limits their applications at high temperatures.22−24,29 Doping, including layer stacking, is regarded as one of the most feasible methods to control the semiconducting properties of graphene.30−33 The dopant can modulate the band structure of graphene, resulting in a metal-to-semiconductor transition.34,35 Theoretical and experimental studies have shown the possibility of making p- and n-type graphene by substituting C atoms with B and N atoms.31 Previous studies have found that B−N and C−C bonds tend to segregate in the B−C−N hybrids.35 It is also possible to form layer-by-layer hybrid structures using graphene and graphene-like hexagonal boron nitride (h-BN).36h-BN is an isomorph of graphene and has the lattice parameters similar to those of graphene. The heterostructures have been constructed by atomic layer stacking of graphene with h-BN through chemical vapor deposition (CVD).34,35 The layered structures provide a new approach to modify the electronic structure of graphene, and provide new possibilities in the stealthy field. Recently, Tang and co-workers37 reported a GO-based hybrid nanostructure with ammonia borane (NH3BH3, AB). AB is a precursor to h-BN, but higher heat treatment was not carried out in the literature. Here we demonstrate a modified assembling and annealing route to make the sandwichlike rGOh-BN hybrids in large scale. GO was used as the starting template; h-BN was formed and attached to rGO by annealing the self-assembled hybrids of GO and AB. The content of h-BN in the hybrids can be easily controlled, and the microwave absorbing properties of the hybrids can be tuned and optimized easily. The heat-treated rGO-h-BN sandwiches are lightweight and thermal stable.

Figure 1. Illustration of the formation of rGO-h-BN hybrids. ultrasonic treatment the mixture was warmed to 40 °C to form a thick slippery liquid, which was then dried at the same temperature for 24 h in vacuum with a dynamic pump. The dried mixture was then heated to different temperatures with a heating rate of 100 °C/h in N2. The heated samples were cooled to the room temperature and black or gray powders, rGO-h-BN, were obtained. A series of rGO-h-BN hybrid samples could be obtained by varying the mass ratios of the raw materials and changing the highest heat treatment temperature (HHTT) in N2. The preparation conditions of the typical samples are listed in Table 1. The sample BCN50−900 is

Table 1. Typical Hybrids and Their Processing Parameters sample BCN20−900 BCN33−900 BCN50−900 BCN66−900 BCN33−1000 BCN33−1100 BCN33−1200

GO (wt %) AB (wt %) 80 67 50 34 67 67 67

20 33 50 66 33 33 33

highest temperature in N2 (°C) 900 900 900 900 1000 1100 1200

thus named because the mass fraction of AB was 50 wt % in the raw materials and the HHTT is 900 °C. Other products are named as BCN20−900, BCN33−900, BCN66−900, BCN33−1000, BCN33− 1100, and BCN33−1200 accordingly. 2.3. Characterization. Thermochemical evolution of the GO−AB assemblies was performed up to 1200 °C in 2 h through a TG-DSCQMS combination analysis using the Netzsch equipment (STAA449C16/G+ QMS403) in Ar. The surface microstructures of the rGO-h-BN hybrid were characterized by scanning electron microscope (SEM) using the JSM-6700F microscope. The crystalline structure was investigated by X-ray diffraction (XRD) on a D8ADVANCE type, using Cu Kα radiation with 2θ from 10 to 90°. Raman spectroscopy (514 nm, Ar+ ion laser) was used to study the defect structures. The Raman spectra were obtained using a laser confocal Raman spectrometer (LABRAM-010) in the range from 400 to 2000 cm−1. UV−visible (UV/vis) spectra were recorded with a UV1800 spectrophotometer and quartz cells with 1 cm path length. Transmission electron microscopy (TEM) was conducted using a JEM-2100F electron microscope at an acceleration voltage of 200 kV with a CCD camera. X-ray photoelectron spectroscopy (XPS) was investigated using K-Alpha 1063 type with focused monochromatized Al Kα radiation (1486.6 eV), to determine changes in the atomic ratios. 2.4. Dielectric Measurement. In a typical experiment, the hybrids (25 wt %) and paraffin wax (75 wt %) were added into preheated ether solution at 70 °C under vigorous stirring. After the

2. EXPERIMENTAL PROCEDURES 2.1. Raw Materials. Ammonia borane (NH3BH3, AB, 97 wt %) was purchased from Sigma-Aldrich. Hydrogen peroxide (30 wt %, H2O 2) and anhydrous tetrahydrofuran (THF) was obtained commercially from Guo Yao Co. Ltd. Graphite particles (10−15 μm) were purchased from Xinghe Graphite Co. Ltd. (Qingdao, China). Potassium permanganate (KMnO4) and concentrated sulfuric acid (98 wt %, H2SO4) were obtained from Beijing Chemical Work (Beijing, China). High-purity nitrogen (99.999%) was obtained from Ri Zhen Co. Ltd. (Changsha, China). All these chemicals were used without further purification. Deionized water used in all the experiments was produced from a Millipore-ELIX water purification system. 2.2. Synthesis. The typical process for the fabrication of rGO-hBN hybrids is illustrated in Figure 1. The interaction between AB cation and negatively charged oxygen of GO promotes the selfassembly of the hybrids.37 Large and single-layer GO nanosheets were prepared by a modified Hummers method as reported.38,39 The obtained GO was thoroughly mixed with a solution of AB in anhydrous THF for 3 h under constant ultrasonication. After 32469


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Figure 2. TG-DSC-QMS analysis of the original assembled hybrids. (a) TG and (b) DSC curves; mass spectra of (c) AB, (d) GO, and (e) GO−AB with a mass ratio of 1:1.

peak, 1205 J/g, located at 229.3 °C. It is related to the large amount formation of carbon dioxide and water. As a combination of AB and GO, the DSC curve of AB-GO assembly is relatively smooth. But a lower temperature, 124.5 °C, and a higher temperature, 238.0 °C (419.5 J/g), were found for the corresponding endothermic peaks, indicating a mutual promotion of the redox reactions between AB and GO. Figure 2c shows that hydrogen is the main evolution gas from AB, associated with some ammonia and borane (eq 1). hBN will be obtained as a high-yield residue experiencing borazine or PB. Carbon dioxide and water are the main evolution gases from GO (Figure 2d and eq 2), coming from the oxygen containing groups. The mass spectra of GO−AB are shown in Figure 2e, and five main species are observed as expected. However, an extended release of hydrogen and carbon dioxide was observed, which is somewhat different from the literature.37 We think that GO here acts as a catalyst for the hydrolysis reaction of AB and then the nascent hydrogen is used to reduce GO, as shown in eqs 3 and 4.41,42

ether was completely evaporated, the mixture was cooled to room temperature. A portion of the resulting mixture was pressed into toroidal shape (Φout, 7.0 mm; Φin, 3.0 mm). The transmission/ reflection (T/R) coaxial line was used to determine the electromagnetic parameters.40 The measurement setup consists of an Agilent 8720ET vector network analyzer with a synthesized sweep oscillator source and an S-parameter test set. A gold-plated coaxial air line with a precision 7 mm connector interface was used to hold the samples. The relative complex permittivity ε= ε′ − jε″ (ε′ and ε″ are the real and imaginary parts of the complex permittivity, respectively) of the composite samples was calculated from the measured T/R coefficients over the frequency range of 2−18 GHz. The reflection loss, RL (dB), was calculated based on the electromagnetic parameters using a RAMCAD software.40

3. RESULTS 3.1. Thermochemical Evolution. AB (NH3BH3) is a stable chemical hydride that has been extensively investigated as a potential hydrogen storage material due to its high hydrogen content (19.6 wt %). It has been recognized as highly efficient reduction agent for GO at lower temperatures.41,42 GO was also regarded as the promoter for dehydrogenation of AB,37 but insights into the redox reaction within the hybrid nanostrucures of GO−AB at higher temperatures are absent. Figure 2 shows the TG-DSC-QMS analysis of GO−AB with a mass ratio of 1:1, compared with those of pure AB and GO. Original TGDSC profiles are provided in Figure S1. The TG curve of GO−AB in Figure 2a is very similar to that of GO. More than half of the mass loss occurred before 400 °C. The residue yield of GO−AB and GO at 900 °C are 37.5 and 21.8 wt %, respectively, but their residue yields are both close to 17.0 wt % at 1200 °C. However, AB itself is very stable beyond 400 °C, with a yield up to 74.3 wt % at 1200 °C. This is because the dehydrogenation of AB mainly occurred below 400 °C, during which AB transforms to polyborazylene (PB).37FigFigure 2b shows DSC curves of the three samples. AB shows exothermic behavior. There is a small endothermic peak at 117.3 °C and a sharp exothermic peak at 126.1 °C (Figure S1), which is related to the first stage of dehydrogenation, associated with the formation of polyaminoborane.37 Another exothermic peak at 152.1 °C stands for the second stage of dehydrogenation associated with further polymerizing into borazine and PB. GO shows endothermic behavior and there is a exothermic

a NH3BH3 → Borazine or PB or h‐BN + b NH3 + c BH3 + d H 2 Δ

(1)

GO → rGO + e H 2O + f CO2 Δ

(2)

GO

NH3BH3 + 2H 2O ⎯→ ⎯ NH3 + HBO2 + 6H

(3)

GO + g H → rGO + hH 2O + i H 2

(4)

Figure 2 also indicates that 900 °C is an important transition temperature because hydrogen release is greatly decreased beyond 900 °C, indicating a nearly complete conversion. 3.2. Microstructure and Composition. Typical SEM images of the hybrids are shown in Figure 3. Among these samples, BCN50−900 (Figure 3c) has a sandwichlike structure up to micrometer-scale. The ultrathin rGO nanosheets are wellstacked and interconnected. When the mass ratio of AB is lower than 50 wt %, however, the layer-by-layer structure is very loose, as shown in the images of BCN20−900 (Figure 3a) and BCN33−900 (Figure 3b). This is due to the deficiency of AB molecular within the GO sheets, which acts as adhesive to assemble them.37 Thus, the higher the mass ratio of AB, the 32470


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Figure 3. Typical SEM images of the hybrids. (a) BCN20−900, (b) BCN33−900, (c) BCN50−900, and (d) BCN66−900.

Figure 5. XPS spectra of the hybrids. (a) Full spectrum of BCN33− 900 and BCN33−1200, (b−d) deconvolved curves of BCN33−900 and BCN33−1200 shown in solid lines and dashed lines, respectively. The curves are deconvolved by Gaussian fitting, thereby indicating possible bonding structures.

better the stacking that could be obtained. As shown in Figure 3d, the layer-by-layer structure is much more denser for BCN66−900. Figure 4 shows the XRD patterns of the hybrids. The samples derived from different mass ratios of AB are shown in

corresponds to B−O bonding. The C 1s peak of BCN33− 900 shown in Figure 5c can be deconvolved into five peaks, centered at 283.88, 284.78, 285.9, 286.98, and 288.38 eV, respectively. The main peak located at 284.78 eV is close to the C−C bonding observed in graphene. The peak located at 286.98 eV is assigned to C−O bonds originally present in the GO nanosheets. The peaks located at 285.9 and 283.88 eV are corresponding to C−N and C−B bonds. The N 1s peak in Figure 5d can also be deconvolved into three peaks corresponding to N−B (398.08 eV), B−N−C (398.78 eV), and pyrrolic N (400.18 eV), respectively. The middle peak corresponds to N bonded to both C and B, and the last corresponds to N bonding in a pyrrolic-like configuration. According to the intensity of the major peaks in B 1s and N 1s spectra, B−N bonding is the main configuration for B and N atoms, which strongly implies the existence of h-BN domains in the hybrid nanosheets.43−45 Compared to that of BCN33−900, the decreased intensity of the B−O peak of BCN33−1200 suggests that thermal annealing at a higher temperature removes oxygen-containing functionalities as well as sp3 carbons. At the same time, the C 1s peak of the C−B bonding is broadened. That is to say, thermal annealing at a higher temperature leads to the better reduction of GO, as well as the simultaneous incorporation of N and B. The atomic ratios of B, C, N, and O in the different hybrids are listed in Table 2. The chemical formulas for BCN66−900, BCN50−900, BCN33−900, BCN20−900, and BCN33−1200 are B 1.0 C 0.4 N 0.6 O 1.0 , B 1.0 C 1.5 N 0.7 O 0.4 , B 1.0 C 4.9 N 0.7 O 0.7 , B1.0C9.5N1.0O1.0, and B1.0C3.7N0.9O0.4, respectively. It indicates that the content of B and N can be increased with the increase of mass ratio of AB as well as the HHTT. In order to investigate the distribution of h-BN, XPS depthscan of the elemental content was further carried out on the hybrid of BCN20−900, which has the lowest AB loadings. As shown in Figures 6a and S7, there is only a slight C and O richness on the surface of the hybrid. B and N remains almost constant in depth upon etching for 200 s. It indicates that h-BN has been successfully embedded into the graphene frameworks, existing between the rGO layers.

Figure 4. XRD patterns of the hybrids. (a) Samples treated at 900 °C and (b) samples derived from 33 wt % of AB. The inset is the enlarged region taken from BCN33−1200.

Figure 4a. The characteristic peaks of h-BN centered at about 26 and 42° (2θ) could be observed, belonging to the (002) and (100) planes, respectively.35,36 The broadening of the peaks indicates the mixed (002) planes of h-BN with those of rGO. The samples obtained at different HHTTs are shown in Figure 4b. It can be inferred that the crystalline size of the product increases with increasing of the HHTT. The characteristic (100) peak centered at about 42° (2θ) is clearly observed for the sample of BCN33−1200. Another much stronger peak centered at 26.22° (2θ) corresponds to the (002) planes of hBN, which mixes with those of the few-layered graphene.30,31 It confirms that highly crystalline graphene sheets can be well recovered at sufficiently high temperatures. XPS spectra of the hybrids are shown in Figures 5 and S2− S6. The full spectrum of BCN33−900 in Figure 5a indicates the existence of B, C, N, and O elements. The B 1s signal of BCN33−900 is shown in the solid line in Figure 5b. It can be deconvolved into two bands at around 191.18 and 192.98 eV, assigned to three different kinds of B atoms, bonded to C, N, and O, respectively. The main peak at 191.18 eV contains both B−C and B−N bonds. In general, the binding energy of B−C is lower than that of B−N because the electronegativity of C is lower than N.43,44 The deconvolved peak at 192.5 eV 32471


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ACS Applied Materials & Interfaces Table 2. Atomic Content and Atomic Ratios of the Hybrids BCN66−900

BCN50−900

BCN33−900

BCN20−900

BCN33−1200

element

atom %

ratio

atom %

ratio

atom %

ratio

atom %

ratio

atom %

ratio

B C N O

35.64 15.84 22.38 36.15

1.0 0.4 0.6 1.0

27.19 41.34 20.10 11.37

1.0 1.5 0.7 0.4

13.67 66.89 9.33 10.11

1.0 4.9 0.7 0.7

8.03 76.55 7.72 7.70

1.0 9.5 1.0 1.0

16.68 62.25 14.44 6.64

1.0 3.7 0.9 0.4

Figure 6. (a) XPS depth-scan of the elemental content of BCN20− 900; (b) Raman spectra of the hybrids.

Typical Raman spectra of the hybrids are shown in Figure 6b. The peaks centered at around 1331 and 1560 cm−1 correspond to D and G bands, respectively.26−28 G band is generally observed in single crystalline graphite and attributed to the inplane bond stretching of sp2 C pairs.46 D band is associated with the defects or lattice distortion. 2D band centered at around 2606.5 cm−1 is typically used to indicate the quality of graphene films.36,47 Specifically for BCN50−900, the dominant peaks of D, G and 2D bands are located at 1331, 1560, and 2643 cm−1 respectively. The intensity ratio of D band and G band, ID/IG, is directly related to the average defect distance, LD, and defect density.48,49 Experimental study has shown that ID/IG increases as LD decreases (stage 1, ID/IG ∝ 1/LD2), reaches a maximum at LD ≈ 3 nm and decreases toward zero for LD < 3 nm (stage 2, ID/IG ∝ LD2).48 In this study, ID/IG of BCN50−900 is lower for than those of BCN20−900 and BCN33−900 but higher than that of BCN66−900. BCN50− 900 might be located at the transition state from stage 1 to stage 2. It indicates that higher h-BN content leads to the decreased defect distance, i.e., the increased defect density. The incorporation of h-BN clusters could decrease the graphene domains and increase the grain boundaries between rGO and hBN. This will increase the microwave consumptions remarkably. Typical TEM images of BCN50−900 are shown in Figure 7. The low-magnification images in Figure 7a,b indicate the presence of hybrid layers similar to few-layered graphene.7,8 The h-BN particles spread randomly throughout the rGO sheets with the diameter ranging from 3 to 12 nm. The HRTEM image in Figure 7c indicates a flakelike nanodomain assigned to h-BN. The flakes are transparent and stable under the electron beam. To further evaluate the nanodomain, SEAD was carried out and is shown as an insert image. It reveals the distinctive hexagonal structure of h-BN and graphene.37,50 In addition, the layer-to-layer distance (d-spacing) observed in Figure 7c is 0.36 nm. This is larger than the d-spacing (0.335 nm) of pristine graphite,36,47,49 partially due to the increased defect densities in the hybrid. EDS analysis shown in Figure 7d further confirmed the presence of B, C, N, and O. 3.3. Optical Band Gap. It is possible to build graphene-hBN hybrid layers based on the structural similarity between h-

Figure 7. Typical TEM images of BCN50−900. (a and b) Lowmagnification images. (c) HRTEM image, indicating h-BN nanodomain combined with the rGO lattice. The insert in panel c is the corresponding SEAD of the h-BN and graphene region. (d) EDS patterns.

BN and graphene. Theoretical calculations indicate that the electronic properties of the hybrid layers are located between those of graphene and the monolayer of BN due to their great electronic differences.51 The UV/vis absorption curves can be used to investigate the band gap of the hybrid samples based on the optically induced transition, as shown in Figure 8. The

Figure 8. Optical properties of the hybrids. (a) UV/vis absorption spectra and (b) plot of ε1/2/λ versus 1/λ. Parameters in b are derived from the UV/vis absorption spectra.

optical band gap could be explained by the following Tauc’s equation.50 ω 2ε = (hω − Eg )2

(5)

where ε is the optical absorbance and ω = 2π/λ is the angular frequency of the incident radiation. On the basis of Tauc’s formulation, it is speculated that the plot of ε1/2/λ versus 1/λ is a straight line at the absorption range. Therefore, the 32472


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ACS Applied Materials & Interfaces intersection point with the x-axis is 1/λg. The optical band gap can be calculated based on Eg = hc/λg. As shown in Figure 8b, the first absorption edge of BCN50− 900 corresponds to a band gap of 4.4 eV, belonging to h-BN domains in the hybrid. The second absorption edge suggests a band gap of 1.5 eV, related to the combined domains of B−C− N. For BCN66−900, the calculated gap wavelengths are about 245 and 450 nm, corresponding to the band gaps of 5.0 and 2.7 eV, respectively; while for BCN33−900, they are 4.1 and 1.5 eV, respectively. It can be reasonably inferred that higher mass ratios of AB leads to higher band gaps of the hybrid. In addition, higher heat treatment temperature also leads to a relative higher band gap, if we compare the band gaps of BCN33−1200 (5.3 and 4.1 eV) to those of BCN33−900 (4.1 and 1.5 eV). We should say that there is no direct relationship between optical band gaps and microwave frequencies, but by using band gaps, we can tell whether or not h-BN or combined phases of B−C−N are formed. It is generally an indicator of the whole conductivity of the hybrid, influencing the complex permittivity of the hybrid. 3.4. Complex Permittivity. The real part (ε′) and the imaginary part (ε″) of the complex permittivity are shown in Figure 9a,b, respectively. Complex permeability (μ = μ′ − jμ″) is not shown because the samples have no magnetic loss, namely, μ′ ≈ 1 and μ″ ≈ 0.

magnetic wave under perpendicular wave incidence at the surface of a single-layer material backed by a perfect conductor can be defined by eq 6.54 RL (dB) = 20 lg

μ ε

tanh

μ ε

tanh

2πjfd εμ c 2πjfd εμ c

−1 +1

(6)

where RL is the reflection loss, f is the frequency, d is the thickness of the material, c is the light velocity in vacuum, and j is the imaginary unit. The complex permittivity is ε = ε′ − jε″ and the complex permeability is μ = μ′ − jμ″. When a material is evaluated as being qualified for microwave absorbing applications, RL is generally below −10 dB. It means that 90% of the microwave energy will be absorbed. Accordingly, when RL is below −20 dB, 99% of the microwave energy will be absorbed.54 In this work, we measured the electromagnetic parameters of the wax composites incorporated with 25 wt % hybrids and used a RAMCAD software to obtain the RL curves versus frequency. Generally, the absorption peaks move to lower frequencies with higher thickness. Figure 10a gives the detailed reflection

Figure 10. Calculated RL curves of the wax composites. (a) BCN50− 900 at different thicknesses and (b) different samples at 1.6 mm.

Figure 9. Complex permittivity of the wax composites. (a) the real part (ε′) and (b) the imaginary part (ε″).

coefficients for BCN50−900. The thickness ranges from 0.2 to 3.0 mm. The reflection loss below −20 dB was obtained in the 8.6−18.0 GHz range within the thickness of 1.4−3.0 mm, and a minimum value of −40.5 dB was observed at 15.3 GHz and 1.6 mm. Using the same method, we obtained RL of different hybrids at a fixed thickness, 1.6 mm, shown in Figure 10b. The absorption peaks of the hybrids move to lower frequencies with higher content of graphene. Their characteristic attenuation properties were extracted and are shown in Table 3. Table 3 also illustrates the qualified frequency bandwidth ( f E) corresponding to RL ≤ −10 dB. The broader the qualified frequency bandwidth, the better the performance of the material. A qualified frequency bandwidth larger than 3.5 GHz could be obtained for BCN50−900 and BCN33−900. For BCN50−900, the qualified bandwidths for RL ≤ −10 and −20 dB reach 5.0 GHz (13.0−18.0 GHz) and 1.9 GHz (14.3−16.2 GHz), respectively.

A general phenomenon could be observed for the rGO-h-BN hybrids, that is, the higher the content of graphene, the higher the value of the permittivity. In addition, both the real parts and the imaginary parts decrease with increasing frequency, attributed to the relaxation effect of the composites, but the rGO-h-BN hybrids show some unique properties, for example, some fluctuations could be observed from BCN20−900. This is because graphene constructs the conductivity network, leading to accelerated electron hopping. It can be observed that ε′ and ε″ of BCN66−900 are both lower than those of all the others. Among all these samples, the content of h-BN is the highest in BCN66−900. This indicates that the insertion of h-BN lowers the complex permittivity, which can be interpreted rationally according to the effective medium theory.53 The effect of HHTT could be clearly observed when comparing the complex permittivity of BCN33−900 and BCN33−1200. Figure 9 illustrates that ε′ and ε″ of BCN33− 900 are both lower than those of BCN33−1200. This is because graphene is better recovered with increased HHTT, leading to a higher increase of complex permittivity, but it has a higher impact on the increase of ε″ than the increase of ε′. 3.5. Microwave Reflection Loss. According to transmission line theory, the reflection loss (RL) of the electro-

4. DISCUSSIONS The efficiency of dielectric loss can be evaluated using the dielectric loss tangent according to eq 7:52 ε″ tan δ = (7) ε′ where δ is the dielectric loss angle. 32473


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ACS Applied Materials & Interfaces Table 3. Characteristic Attenuation Properties of the Hybrids (d = 1.6 mm) itemsa

BCN66−900

BCN50−900

BCN33−900

BCN20−900

BCN33−1200

RLmin (dB) f m (GHz) f E (GHz)

−5.5 17.8 0

−40.5 15.3 5.0

−23.4 13.4 3.7

−9.0 17.7 0

−5.7 11.4 0

a RLmin is the minimum RL in the studied range; f m is the optimal frequency associated with RLmin; f E is the qualified frequency bandwidth corresponding to RL ≤ −10 dB.

mass ratio of AB and the HHTT can adjust the location of the reflection peaks and influence the RL intensities. Table S1 lists the performance comparison of the present study with some related reports. The qualified bandwidth reaches 5 GHz at 1.6 mm. The density of the wax composite of rGO-h-BN is about 1.05 g/cm3, so the surface density is close to 1.68 kg/m2 when it is covered on the metal foil as radar absorbing materials. Compared to other graphene fillers or hybrids with some magnetic fillers, such as rGO/MnFe2O4,55 rGO/CoFe2O4,56 and Fe3O4/GCs,57 the samples prepared in this study have comparable or even much better microwave absorbing performance at the low weight loadings or at the low thickness. The mechanism of the excellent microwave absorbing performance is proposed and illustrated in Figure 12.

Accordingly, Figure 11a shows the variation of the dielectric loss tangent (tan δ) as a function of frequency. The dielectric

Figure 12. Illustration of the electronic transportation and microwave consumption.

On the one hand, when the microwave propagates into the wax composites containing the rGO-h-BN hybrids, free electrons in the graphene and the newly generated electrons in the graphene can migrate within the hybrids, hop across the interfaces between the graphene layers, or hop across between the hybrids. It is widely believed that graphene contains numerous free electrons, which are extremely sensitively to the external microwave field. They acts as the medium to allow the microwave energy convert to electric energy. All these graphene-based conductivities contribute to the high dielectric loss tangent. It can be increased by the increase of the mass ratio of graphene and the HHTT (Figure 11a). Generally, it has a higher impact on the increase of ε″ than the increase of ε′ (Figure 9b), so the mass ratio of GO and the HHTT should be controlled. On the other hand, the incorporation of immobilized h-BN on the graphene provides grain boundaries, leading to the formation of polarization and capacitor-like structure at the interfaces. The defect dipoles generated by the oxygen vacancies or the carbon substitutions could also act as the formation of polarization. All these h-BN-related defects contribute to the excellent impedance matching for microwave propagation and consumption. h-BN defects here can be regarded as resistors and capacitors. Generally, this has a

Figure 11. (a) Loss tangents of the wax composites and (b−f) calculated RL curves versus thickness and frequency.

loss tangent keeps almost constant with increasing frequency for BCN66−900. The dielectric loss tangent is the highest for BCN20−900, which have the lowest AB loading. An obvious relaxation peak could be observed for BCN20−900, which is probably due to the high conductivity and the synchronous polarization effect in the hybrid. The dielectric loss tangent of BCN33−1200 is higher than that of BCN33−900 due to the better recovered graphene sheet. In a word, the dielectric loss of these hybrids varies with the dielectric loss tangent, which are influenced by the mass ratio of AB and the HHTT, i.e., the content of h-BN. Figure 11b−f presents the detailed 3D reflection surfaces of five hybrid samples with the thickness ranging from 0.2 to 3.0 mm. The microwave absorbing performances of BCN33−900 and BCN50−900 are much better than those of the others. In these two samples, the content of h-BN is in a proper range, 23−47 atom %, as summarized in Table 3. Even though the content of h-BN in BCN33−1200 is in this range, ∼31 atom %, its microwave absorbing performance is worsened due to the higher heat treatment temperature. That is to say, both the 32474


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ACS Applied Materials & Interfaces relatively higher impact on the increase of ε′ than the increase of ε″ (Figure 9a). It is reasonable that higher mass ratios of AB leads to higher band gaps and lower conductivities of the hybrid, which is advantageous for microwave propagation into the hybrids. More importantly, when we focus on the microscale sandwichlike rGO-h-BN hybrids, the layer-by-layer structure makes great contributions to the increased approaches and possibilities of electron migrating and hopping, which has both highly efficient dielectric loss and excellent impedance matching. Table S1 confirms that its excellent microwave absorbing property cannot be achieved solely by discrete pristine or doped graphene nanosheets or solely by h-BN, which is an insulator transparency to microwaves. It is the layerby-layer sandwichlike structures that make the point.

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5. CONCLUSIONS GO and AB stack into GO−AB assemblies and transform to rGO-h-BN sandwichlike hybrid material under heat treatment. The content of h-BN can be increased by increasing the mass ratio of AB and the HHTT. When the mass ratio of GO/AB is 1:1, the absorbing capacity of the BCN50−900 is preferable in the range of 6−18 GHz. A minimum RL, −40.5 dB, was observed at 15.3 GHz for the composite filled with 25 wt % BCN50−900 at the thickness of 1.6 mm. The qualified frequency bandwidth (RL ≤ −10 dB) reaches 5 GHz at this low thickness. Graphene-based conductivity contributes to the high dielectric loss tangent, and h-BN-related defects contribute to the good impedance matching. The layer-by-layer structure makes great contributions to the increased possibilities of electron migrating and hopping for microwave consumption. The applications of the lightweight hybrids in the military and civil fields are both highly expected.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.6b11843. TG-DSC profiles of the original assembled hybrids, XPS spectra of the as-prepared hybrids, XPS spectra of the etching hybrids, and comparison of the microwaveabsorbing performance (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Author Contributions

Y.K. and Z.J. contributed equally to this work. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The project was financially supported by National Natural Science Foundation of China (Nos. 11404397 and 61574172), Hunan Provincial Natural Science Foundation for Distinguished Young Scholars (No. 14JJ1001), and Advanced Research Project of NUDT (No. JC11-01-01).

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REFERENCES

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Hybrids of Reduced Graphene Oxide and Hexagonal Boron Nitride

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