Inner Surface-Functionalized Graphene Aerogel Microgranules with

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Inner surface functionalized graphene aerogel microgranules with static microwave attenuation and dynamic infrared shielding Xiaohan Wu, Jing Lyu, Guo Hong, Xiang-cui Liu, and Xuetong Zhang Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.8b01410 • Publication Date (Web): 29 Jun 2018 Downloaded from http://pubs.acs.org on July 1, 2018

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Inner surface functionalized graphene aerogel microgranules with static microwave attenuation and dynamic infrared shielding ∽

Xiaohan Wu, †,‡ Jing Lyu,† Guo Hong,§ Xiang-cui Liu, Xuetong Zhang,*,† †

Suzhou Institute of Nano-tech and Nano-bionics, Chinese Academy of Sciences, Suzhou

215123, P. R. China. ‡

University of Chinese Academy of Sciences, Beijing, 10000, P. R. China.

§

Institute of Applied Physics and Materials Engineering, University of Macau, Macao.

∽

Research Institute of Chemical Defence, Beijing, 102205, P. R. China

ABSTRACT: Bulk graphene aerogels (GAs) with high electrical conductivity, ultra-low density and high specific surface area have attracted significant attention because of their fascinating performances in energy storage, catalysis, absorption, sensor, electromagnetic shielding, etc. However, graphene aerogel microgranules (i.e. reducing the size of the bulk aerogels into microscale) and their performances in the electromagnetic field have been ignored. Herein, we report a new strategy to make floatable graphene aerogel microgranules with high hydrophobicity (137°), low density (13.5 mg/cm3) and high specific surface area (516 m2/g). These microgranules were synthesized initially from reduced graphene oxide (rGO) hydrogel microparticles and then in situ modified by silica nanoparticles. Further investigations have demonstrated that the resulting silica modified rGO aerogel (SMGA) microgranules possess high-efficient static electromagnetic screening (average 30.3 dB in 8-18 GHz) and dynamic

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infrared shielding (higher than 10 dB during floating in air for 15 min) properties. The work reported here should give much inspiration to make more functional aerogel microgranules used in various emerging fields.

INTRODUCTION Graphene aerogels (GAs), a three-dimensional (3D) porous architecture, can be synthesized from wet chemical assembling or vapor phase growing of the two-dimensional (2D) graphene sheets into a 3D interconnected network. It can be easily envisioned that GAs with low density, large specific surface area, marvelous pore volume and high electrical conductivity can be applied in various fields, such as energy storage

1-3

, catalysis

4, 5,

absorption 6, 7, and sensing devices 8, 9,

which greatly benefitted from the integration of the intriguing properties of the 2D graphene sheets and 3D porous network of the aerogel. GA microspheres have been demonstrated as a novel porous material in drug delivery, absorption, functional composites and hosts for phase change materials

10-12

over recent years. Liao et al

11

prepared GA microspheres by combining

the electrospraying, freeze-casting and thermal reduction techniques in sequence. Wang et al.

12

synthesized GA microspheres by ink jetting-liquid marbling-supercritical fluid drying (ILS) coupling techniques, and then used as the hosts for phase change materials. The resulting GA microspheres have excellent mono-dispersity in diameter and large specific surface area. Park et al.

13

developed a spray-assisted self-assembly process to synthesis graphene microspheres,

which precipitated in a high-temperature organic solvent for energy storage devices. Recently, the 3D porous network of graphene sheets and its composites are thought to be the potential electromagnetic interference (EMI) shielding materials

14-17

due to their low density,

high electrical conductivity and excellent chemical resistance. Bi et al.

14

studied the EMI

shielding mechanisms of GAs made by chemical reduction of graphene oxide (GO) with


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hydrazine vapor. They discussed the influence of the reduction degree of rGO on EMI shielding properties and found that the EMI shielding effectiveness (SE) of GAs increased along with the concentration of hydrazine vapor. Song et al.

15

prepared an ultra-light flexible graphene/texture

composites by modifying a 3D carbon texture with in situ generation of graphene aerogels. 27 dB was achieved by the GA-carbon texture hybrid with a thickness of 2 mm in the X band region. Chen et al. 16 reported a one-step process for graphene-based foam composite by growing graphene sheets on a nickel foam in chemical vapor deposition (CVD), coating a thin layer of polydimethylsiloxane (PDMS), and etching the nickel foam with hydrochloric acid (HCl). The resulting graphene/PDMS foam shows excellent flexibility and high EMI SE of 30 dB at a low graphene loading < 0.8 wt.%. However, all the above works only shields electromagnetic waves at static, and no study have been reported about the microwave or infrared shielding at dynamic state using GAs. On the other hand, all the above bulk GAs can’t be processed into various shapes and it is difficult to incorporate them with other materials. Compared to bulk GAs, the advantages of using microgranules for EMI and infrared shielding were: 1) the microgranules not only can shield electromagnetic wave at static, but also can shield electromagnetic wave at dynamic (floating in the air); 2) the microgranules can compound with other materials easily and can be processed into various shapes compared with bulk GAs, which can be conveniently applied to a variety of electromagnetic protection appliance. So far, the above GA microspheres10-13 can’t float in air due to their large size and easy water-absorption to form aggregation in air without further hydrophobic modification. Ultra-low density, small size, good dispersity and high-efficient EMI shielding are four keys to obtain GA microgranules that can shield electromagnetic waves at dynamic state.


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Therefore, we proposed a novel method to synthesize silica nanoparticle modified rGO aerogel (SMGA) microgranules. The rGO hydrogel was firstly synthesized by reducing GO with vitamin C, then in situ modified with silica nanoparticles and hydrophobized reagent in sequence, and finally dried with supercritical CO2 fluid. The resulting SMGA microgranules showed ultra-low density (13.5 mg/cm3), high specific surface area (516 m2/g), small size (D50 22.7 µm) with narrow size distribution, and good dispersity when floating in air. More importantly, the EMI shielding performance of the SMGA microgranules was not only better than that of the GA microgranules in 2-18 GHz at static state, but also excellent in infrared range at dynamic state (when floating in air).

EXPERIMENTAL SECTION Materials. Graphite (crystalline powder, 400 mesh), P2O5, K2S2O7, 30% H2O2, HCl, H2SO4, tetraethoxysilane (TEOS), Vitamin C (VC) and dimethylformamide (DMF) were purchased from Sinopharm Chemical Reagent Company. Trimethylchlorosilane (TMCS) was purchased from Aladdin Industrial Corporation. These chemicals were used without further purification. Graphene oxide (GO) was prepared from graphite powder by a modified Hummers method reported in our previous study 18. Silica sol was synthesized according to literatures 19, 20, and the molar ratio of TEOS: water: ethanol is 1:1.3:2.5. Synthesis of rGO hydrogel fractal microgranules. VC with a mass ratio of VC to GO of 5:1 was added into a 250-mL-beaker containing 150 mL 3 mg/mL GO aqueous solution with vigorous stirring for several minutes until complete dissolving. The corresponding rGO hydrogel was obtained by keeping the mixture without stirring for 12 hours at 60 °C, which was defined as the aging process. Then the hydrogel was washed with ethanol to remove the remaining VC in


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the hydrogel network. After solvent-exchange, the hydrogel was grinded by the colloid mill to obtain rGO hydrogel fractal microgranules and then dispersed in ethanol. Synthesis of silica nanoparticles modified inner surface of rGO aerogel (SMGA) fractal microgranules. Typically, a fixed amount of condensed silica was added into 100 mL ethanol suspension containing rGO hydrogel microgranules with concentration of 4.5 mg/mL, then 10 mL of ammonium hydroxide were added into above mixture with stirring for 30 s. During this process silica nanoparticles sol-gelled on the rGO sheets. After aging at 45 °C for 12 h, rGO/SiO2 hybrid hydrogel microgranules was experienced solvent-exchange process to replace ethanol and impurities with DMF. Inner surface modification process was preceded by refluxing with addition of 15 mL TMCS for 3 h at 50 °C. Finally, the silica nanoparticles modified rGO hydrogel microgranules were washed with ethanol, and supercritically dried with CO2 (40 °C, 10 MPa) for 12 hours to obtain SMGA fractal microgranules. To investigate the effect of synthetic conditions, the products with the initial mass ratios of GO to SiO2 of 8 : 1, 4 : 1, 2 : 1, 1 : 1 were synthesized, which were named as SMGA 8, SMGA 4, SMGA 2, and SMGA 1 respectively. For comparison, rGO aerogel (GA) microgranules and silica aerogel (SA) microgranules were also synthesized according to the method reported elsewhere21. Characterizations. Raman spectra were recorded using a LabRAM HR Raman Spectrometer (LabRAM HR, Horiba-JY) fitted with a laser at an excitation wavelength of 532 nm. The crystal structure was characterized by the X-ray diffraction (XRD, D8 Advance, Bruker AXS). A field emission scanning electron microscope (FESEM, Hitachi S-4800) and transmission electron microscope (TEM, Tecnai G2 F20 S-Twin) were used to investigate the morphology and microstructure of the samples. The pore structure of the samples was analyzed using a Surface Area Analyzer (Micrometrics, ASAP 2020 HD88). The Brunauer–Emmett–Teller (BET) method


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and the Barrett–Joyner–Halenda (BJH) model were utilized to calculate the BET specific surface area (SSA) and the pore size distribution, respectively. Thermogravimetric analysis (TGA) was performed on a TG 209 F1 thermogravimetric analyser (NETZSCH, TG 209 F1 Iris). The heating rate was set as 10 °C/min under N2 and the analytical temperature region was set from ambient temperature to 900 °C. XPS was performed using an AXIS Ultra spectrometer with a high-performance Al monochromatic source operated at 15 kV. The XPS spectra were taken and all binding energies were referenced to the C1s neutral carbon peak at 284.8 eV, and elemental compositions were determined from peak area ratios after correction with a sensitivity factor for each element. Size distribution of silica modified rGO microgranules, rGO hydrogel microgranules and silica hydrogel microgranules were performed on a Bettersize 3000 Laser particles size analyzer. A vector network analyzer (AV3672C) was used to measure the S-parameters and complex permittivity in 2-18 GHz. For electromagnetic parameter measurements, the aerogel microgranules were mixed with paraffin at certain filling ratios and compressed to standard ring shapes with an outer diameter of 7 mm, inner diameter of 3 mm, and thickness of 2 mm. The SMGA samples were designated at different proportions of SMGA microgranules (5 wt%, 10 wt%, 15 wt% and 20 wt%) while GA and SA samples were designated at 10 wt%. The extinction spectra of aerogel microgranules were acquired using a Nicolet 6700 Fourier Transform Infrared Spectrometer (FT-IR, Nicolet 6700, Thermo Fisher Scientific). The specimens for FT-IR measurements were prepared by grinding 5 mg aerogel microgranules and equvilent KBr together, and then compressed into thin pellets under 10 MPa. The electrical conductivity was obtained by the four-probe method. The infrared shielding performance to mid-infrared (3-5 µm and 8-14 µm) of aerogel microgranules was tested in a smoke chamber with dimension of 3.0 m


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× 1.6 m ×1.6 m and effective volume of 6 m3. 6 g of aerogel microgranules were sprayed into the smoke chamber by the spray nozzle with high-pressure air at a flow rate of 40 L/min. After the infrared signal passed the aerosol, its power data was collected by the thermal infrared imager INFRATEC8300. Size distribution of SMGA microgranules were performed on HELOS (Hi200) & RODOS, R5 particle size analysis.

RESULTS AND DISCUSSION Synthesis of the inner surface modified graphene aerogel (SMGA) microgranules. The SMGA fractal microgranules were prepared via a synthetic process illustrated in Figure 1. GO aqueous dispersion was reduced by VC to form the bulk rGO hydrogel. Then the bulk rGO hydrogel (Figure S1a) was grinded by the particle colloid grinder to obtain rGO hydrogel fractal microgranules, and we found the characteristic of viscoelastic solid was lost (Figure S1b). The 3D porous network of the rGO hydrogel contains abundantly water, which can tolerate the breaking force of grinding and protect the 3D porous network from collapse. If we directly grind the bulk GAs, the 3D porous network will be damaged. The optimal grinding time was in the range of 1-5 min, grinding time lower than 1 min caused the wide size range of the microgranules, while grinding time higher than 5 min formed cotton-like product due to lasting friction. In order to prevent GA microgranules from aggregation in air, the inner surface modification was realized by in situ growth of the silica nanoparticles on the graphene sheets, following by the TMCS treatment of the rGO/SiO2 fractal microgranules. The condensed silica was chosen as the precursor of silica nanoparticles. The appropriate mixing time


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Figure 1. Schematic illustration for the synthesis of SMGA microgranules.

of rGO hydrogel fractal microgranules with the condensed silica was longer than 8 h. If the mixing time was shorter than 8 h, the condensed silica couldn’t enter into the pores of rGO hydrogel fractal microgranules completely, which further aroused heterogeneity of silica nanoparticles on the graphene sheets (Figure S2). The same experimental time was also suitable for the mixing of TMCS and the rGO/SiO2 fractal microgranules. After the inner surface modification, supercritical CO2 drying was introduced to acquire the corresponding SMGA microgranules. When the SMGA was dried at ambient pressure, the volume of SMGA microgranules was severely shrunk, indicating that the SMGA can’t be dried by the ambient


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pressure drying technique. SMGA2 was chosen as a typical SMGA for the comparisons with GA and SA, except for special instruction. Morphological structure of SMGA, GA and SA microgranules. As can be seen in the Figure 2a, SA microgranules were built up by uniform nanoparticle network. Compared with the GA microgranules (Figure 2b), SMGA microgranules (Figure 2c, Figure S3) also exhibited a welldefined and interconnected 3D network with abundant pores with a wide size distribution, a typical structure of aerogels. The difference was that SMGA microgranules with a lot of nanoparticles distributed onto the graphene sheets had thicker lamella than that of the GA microgranules. It can be seen in Figure 2d (scanning transmission electron microscope, STEM) that a lot of sphere-like silica nanoparticles in Figure 2a with a size about 5 nm were anchored homogenously onto both sides of the graphene sheets, and no apparent aggregates appeared. The STEM image of SMGA8, SMGA4, SMGA2 and SMGA1 (Figure S3e-h) showed that the silica nanoparticles became more intensive along with the weight proportion of condensed silica. Furthermore, the TEM image of Figure 2e also showed that all silica nanoparticles were anchored onto the graphene sheets and no free nanoparticles were observed. The selected area electron diffraction (SEAD) pattern (the inset of Figure 2f) of SMGA microgranules showed only diffraction rings of rGO aerogel after silica modification, indicating that the silica nanoparticles increased the disorder of graphene sheets (Figure S4a and b showed the TEM and electron diffraction of GA microgranules). A statistical size analysis indicated that the median diameter (D50) of silica modified rGO gel microgranules with further TMCS modification was 17.90 µm (Figure 2f), which was larger than that of rGO hydrogel microgranules (12.06 µm, Figure S5a). The D50 of rGO/SiO2 gel microgranules without further TMCS modification


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(Figure S5b) were also larger than that of rGO hydrogel microgranules, indicating that after compositing the rGO and silica, the size of resulting gel microgranules became larger.

Figure 2. Structure characterization of GA and SMGA microgranules. (a) SEM images of SA, (b-c) SEM images of GA (b) and SMGA (c). Insets show the low magnification SEM images of GA and SMGA microgranules, respectively. (d) STEM images of SMGA. (e) TEM images of SMGA. Insets show the electron diffraction. (f) Size distribution with a Gauss fit of SMGA gel microgranules.

Characterization of the aerogel microgranules. Raman spectroscopy was applied for the analysis of GA, SMGA and SA microgranules, as shown in Figure 3a. Two peaks at 1340 cm-1 and 1578 cm-1 can be prominently observed from both GA and SMGA microgranules corresponding to D and G band. Furthermore, the intensity ratios of D to G band (ID/IG) of GA and SMGA microgranules were both 1.4, and the D, G band of SMGA showed no shift in comparison with those of GA, indicating that the structure of GA is maintained after the compound procedure of rGO with silica. The SMGA8, SMGA4, SMGA2 and SMGA1 have comparable Raman shift (Figure S9b). The XPS characterization showed the Si-O-Si (104.2 eV) arising from the silica nanoparticles anchored on the graphene sheets while the Si-C (108.4 eV)


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bringing by TMCS treatment (Figure S6), and the FT-IR characterization also showed the Si-OSi bond (1094 cm-1) and Si-C bond (840 cm-1) (Figure S7, S9a). There was no evidence that chemical bonds were formed between graphene sheets and SiO2 nanoparticles. Previous study has shown that there is ultra-strong adhesion between graphene and amorphous SiO2, which including van der Waals and interactions originating from graphene clinging to the undercoordinated Si and the nonbridging O defects on amorphous SiO2 22. The XRD patterns of SMGA microgranules, GA microgranules and SA microgranules were shown in Figure S8, indicating that the diffraction peak at 23o (002) of rGO sheets widened with the modifying process by silica nanoparticles. In order to further confirm the porous 3D network structure of the hybrid aerogel microgranules, the N2 adsorption-desorption isotherms of GA, SMGA and SA microgranules were tested. As shown in Figure3b and FigureS9c, all the adsorption-desorption curves exhibit type-IV isotherm with a H3 hysteresis loop, indicating a characteristic of open wedge-shaped mesoporous structure

23

. The Brunauer-Emmett-Teller (BET) surface areas of SMGA

microgranules was 516 m2/g, slightly lower than that of silica microgranules, but much higher than that of the GA microgranules, as shown in table S1. This indicates that silica nanoparticles attached onto the graphene sheets would increase the specific surface area of the resulting hybrid aerogel microgranules.


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Figure 3. Property comparison of SMGA with GA and SA. (a) Polarized Raman spectra of GA, SMGA and SA microgranules. (b) Nitrogen adsorption and desorption isotherms of GA, SMGA and SA microgranules. (c) TGA curves of GA, SMGA and SA microgranules. (d) Contact angles of GA, SMGA and SA microgranules.

Thermo gravimetric analysis (TG) was carried out in nitrogen to investigate thermal stability of GA, SMGA and SA microgranules, as shown in Figure 3c. It can be seen from this figure that all the microgranules show different mass losses below 100 oC due to de-intercalation of water absorbed in the microgranules. Because of excellent hydrophobic property, SA microgranules adsorbed less water than GA and SMGA microgranules, and thus showed the least mass losses. Moreover, SMGA microgranules show less mass losses than GA microgranules, indicating that SMGA microgranules possessed better hydrophobic property than GA microgranules. The above phenomenon is consistent with the description in Figure 3c. The SA microgranules shows a weight loss of about 13.24 % at 100 oC-740 oC due to the degradation of –OC2H5 and –CH3


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groups

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, exhibiting an excellent thermal stability up to 900 oC. GA and SMGA microgranules

both show a weight loss at 100 oC-250 oC corresponding to the removal of oxygen-containing groups of the rGO sheets. The remaining small fraction of stable oxygen-containing groups on the graphene nanosheets is removed during the TG heating scan. 78.4 % initial mass of SMGA microgranules is reserved at 900 oC, much more than that of GA microgranules, indicating that silica nanoparticles attached onto the graphene sheets increased the thermal stability of SMGA microgranules. Figure 3d showed the static contact angles of GA, SMGA and SA microgranules. Compared with GA microgranules, SMGA microgranules modified with silica nanoparticles by sol-gel process have larger contact angel (137°), indicating the excellent hydrophobicity. The SMGA8, SMGA4, SMGA2 and SMGA1 have comparable hydrophobicity (Figure S9d). SMGA microgranules possessed the ability of formation of a liquid marble from a rolling water droplet (Figure S10) due to the excellent hydrophobicity. The water droplet was absolutely encapsulated in the SMGA microgranules as shown in the photograph of the Figure S10b. EMI shielding property. The permittivity behavior of microwave attenuation materials has important effect on the shielding performance25. As shown in Figure 4a, the real permittivity ε’ of complex permittivity (εr) of SMGA are in the range of 8.06-33.59, which are much higher than that of GA in the long range of 2-18 GHz. The ε’ of EMI shielding materials is the energy stored by polarization, which consists of interface and orientation polarization. The SMGA microgranules possess higher specific surface area than GA microgranules (as shown in table S1) due to the presence of SiO2 nanoparticles. The interface between graphene and SiO2 nanoparticles give rise to the interfacial polarization (called as the Maxwell–Wagner effect) 25, 26,


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which enhance the ε’ of SMGA microgranules. Moreover, according to the XPS spectra in Figure S6, there are residual oxygen functional groups such as hydroxyl, epoxy and carbonyl

Figure 4. EMI shielding performance of SMGA, GA microgranules (10 wt.% in wax) in 2-18 GHz. (a) real permittivity; (b) imaginary permittivity; (c) tanδ; (d) Typical Cole–Cole plot of SMGA microgranules (the semicircle dotted lines represent the dielectric relaxation processes). (e) EMI SE; (f) SEA; (g) SER; (h) SEA/SE and SER/SE.

groups on graphene generated in the process of chemical reduction. The functional groups with the defects generated in the process of preparing graphene oxide act as polarized centers, accounting for the high ε’ values of SMGA microgranules27, 28. The ε’ decreases with frequency in the 2–18 GHz range, which similar to earlier reports25, 27, 29.


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The imaginary permittivity ε” of complex permittivity is the dielectric dissipation or loss, which depend on dipolar polarization, associated relaxation phenomena, natural resonance and dominant electronic, and interfacial polarization4, 30-32. As shown in Figure 4b, the ε” is also much higher than that of GA in the range of 2-18 GHz. The dielectric tangent loss (tan δ = ε”/ε’), a dissipation factor implies the energy loss in the shield materials, has also been evaluated. As shown in Figure 4c, the average value of SMGA is 0.9, while the average value of GA is only 0.6, indicating that the SMGA have much microwave loss than GA in the range of 2-18 GHz. The dielectric loss of the SMGA microgranules could be described by the Debye theory and the Cole-Cole semicircles can describe the relaxation process, which has an important influence on the permittivity behavior of microwave shielding materials. According to the Debye relaxation process33, the relationship between ε’ and ε” can be written as

− ∞ + " = − ∞

(1)

where εs is the static permittivity, ε∞ is the relative dielectric permittivity at the high-frequency limit. The ε’- ε” curve is shown in Figure 4d. There are three semicircles in the Cole–Cole plot, which demonstrates that there are three dielectric relaxation processes in the SMGA microgranules. Each semicircle corresponding to one Debye relaxation process25, which reinforce the dielectric loss of SMGA microgranules. The Maxwell–Wagner effect generated at the interface between graphene can give rise to the associated relaxation26, 34-36, which enhance the dielectric loss. Moreover, the interfaces of heterogeneous media can catch charge carriers, which give rise to the interfacial and space charge relaxation process25, 30. The distortion of the Cole–Cole semicircles can reveal the presence of the interfacial polarization, conductance loss and the dielectric loss due to the functional groups and defects except for the dielectric relaxation.


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The dielectric performance determines the EMI shielding characteristics. EMI shielding effectiveness (SE) is the ability to shield devices from the EM wave source and is defined as the logarithmic ratio of transmitted power (Pt) to incoming power (Pi) of an EM wave. The total EMI SE including absorption effectiveness (SEA), reflection effectiveness (SER), and multiple internal reflection effectiveness (SEMR), which can be calculated based on the measured Sparameters as follows: Pt EMI SE = −10 log = −10log|S | = −10 log|S | 2 Pi

EMI SE = SE + SE + SE!

(3)

|S | |S | SE = −10 log " # = −10log " # 4 1 − |S | 1 − |S | SE = −10 log1 − |S | = −10log 1 − |S |

(5)

where %&'( % represents the power transmitted from port ' to port (. Absorption requires dielectric losses and/or magnetic polarization

37

while reflection is related to charge carriers. As shown in

Figure 4f, the SEA of SMGA microgranules are much higher than that of GA microgranules in the range of 2-18 GHz, and the SEA of SMGA microgranules increases with frequency, which is consistent with the trend of the dielectric tangent loss (tan δ). Combining Figure 4e and Figure 4g, the SEA of SMGA microgranules is the major contributor for the total EMI SE. The values of SEA/SE are much higher than SER/SE in the most measuring frequency range, as shown in Figure 4e. The absorption effectiveness mainly depends on the electrical conductivity and skin depth. Therefore, the skin depth (the depth when the field drops to 1/e of the incident value) of the wax/SMGA microgranules must be evaluated38. When the σ » 2πfεo, the skin depth can be calculated using the relation39


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δ = 1/√πσμf

(6)

where σ is the electrical conductivity, f is the frequency, εo = 8.854 ×10 shielding material without magnetic substance, µo= 4π ×10

-7

-12

F/m, µ =µoµr (µr=1 in

H/m). The σ of SMGA

microgranules was 126.8 S/m while the GA microgranules’ was 482.9 S/m. The σ of SMGA microgranules satisfy the condition of Eq. (5). The SEA could be given as15 01 = 8.6865/6

(7)

where d is the shielding thickness. According to Eq. (6), the skin depth decreases with the frequency, which result in the increase of the value of 7/8. According to Eq. (7), the high value of 7/8 lead to high SEA, indicating that the values of SEA increase with the frequency increase, which is in good agreement with the observed results in Figure 4f and 4h. Moreover, the average skin depth of SMGA microgranules is found to be 0.5 mm, which is smaller than the thickness of the SMGA sample itself, indicating that the multiple reflection can be neglected.

Figure 5. Illustrations demonstrate the EMI shielding mechanisms of SMGA microgranules.


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We propose a schematic illustration of the microwave shielding mechanism based on the microstructure characteristics of the SMGA microgranules, as shown in Figure 5. When the incoming EM waves were incident on the surface of SMGA microgranules, electron at the graphene sheets generated an internal electric field is opposite to the external electric field, which results in an immediately reflection of part of the incoming EM waves due to the impedance mismatch 14, 40. The residual of the incoming EM waves passed through the surface of aerogel microgranules and interacted with the aerogel microgranules’ electric dipoles, resulting in energy loss. According to the Maxwell-Wagner-Sillars (MWS) principle, in the 3D porous network structure of SMGA microgranules, the grapheme sheets separated by air can be treated as capacitors14, 41, and those in close but separated by SiO2 nanoparticles also can be represented by nanocapacitors. Furthermore, the physically touched rGO sheets without SiO2 nanoparticles can be seen as resistors (see Figure 5). The permittivity of SiO2 is higher than that of air, indicating that the nanocapacitors composed by graphene sheets and SiO2 nanoparticles possess the capacity to store more energy. The 3D network structure of SMGA microgranules own abundant such capacitors contributing to absorbing the EM waves, while the GA microgranules only has the capacitors composed by graphene sheets and air gaps. According to the previous studies14, the porous network structure of GA can improve the impendence matching, and the synergistic effects of the graphene sheets with SiO2 nanoparticles also can reduced the impedance mismatch at the interface between the air and the SMGA, which resulted in more microwave to penetrate into the SMGA. Meanwhile, the network structure can increase the propagation path of microwaves, which also enhance the microwave shielding performance of SMGA42-43.


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When the mass ratio of GO to SiO2 was 2:1, the EMI SE of SMGA microgranules was highest, indicating that the synergistic effect of graphene sheets and SiO2 nanoparticles is the most beneficial for EMI shielding performance, as shown in Figure S11a. Typically, the EMI SE level as satisfactory for most practical applications is 20 dB (99 % shielding)

25, 44

. The present

SMGA microgranules showed EMI SE of over 20 dB in the long range of 8.8-18 GHz when the weight percentage was equal or greater than 10% (see Figure S11c), and showed EMI SE of over 30 dB (99.9 % shielding) when the frequency in the 12.4-18 GHz, which is high enough for practical application as a qualified shielding material. Table 1 shows the comparison of our results with those of recently reported GAs-based materials. Our ultra-light SMGA microgranules exhibits a highly competitive EMI performance in comparison with others. Especially, the SMGA microgranules possesses relatively higher EMI SE in the long range of 218 GHz. Table 1. EMI shielding performance of typical GAs-based composites. Samplea GA Epoxy/p-GA-900 oC (0.33 wt%) CF/RGO-800 oC GA(Chemically reduced) GAT(thermally reduced) GA Epoxy/anisotropic GAs (0.33 wt%) GA/cellose S-RGOA GA-carbon texture Wax/SMGA(20wt%) a

Thickness(mm) 2 2 5 2.5 2.5 2 2 2 12 2 2

Density(mg/cm3) 75.0 4.8(p-GA) 2.94 5.5 4.5 5.56 -

Bandwith(GHz) 12.4-18.0 8.0-12.0 8.0-12.0 8.0-12.0 8.0-12.0 8.0-12.0 8.0-12.0

59.6 19.0 70 13.5(SMGA)

8.0-12.0 8.0-12.0 8.0-12.0 2.0-18.0

EMI SE(dB) ~20 ~30 DE − ρ B=? g/9η

(8)

Where r was radius of microgranules, ρmicrogranules was the density of microgranules, ρair was the density of air, η was viscosity (dynamic viscosity) of air. Through the above calculating, the maximum settlement velocity of SMGA microgranules was 10-3 m/min. SMGA microgranules can float stably in a stratosphere due to the very low settlement velocity. The actual settlement velocity of the aerogel microgranules was 10-2 m/min, which was faster than the theoretical value due to the measurement cannot completely avoid the influence of wind and the irregular shape of aerogel microgranules. The change of mass concentration with time was also measurement. As shown in Figure 6e, In the first minute the lager aerogel microgranules made up of small microgranules by electrostatic interaction was settled fast, resulting in the mass concentration less than 1 g/m3. After the first 1 minute, the remaining mass of SMGA microgranules was more than that of GA and SA microgranules, and sixth minutes later, the mass concentration decreased smoothly. Through the above analysis, SMGA microgranules can stably float in air and possess the best anti-infrared property. The Tyndall effect of SMGA microgranules floating in air can be observed clearly under a red glow, as shown in Figure 6f.

CONCLUSION 3D porous hydrophobic SMGA microgranules were prepared using silica nanoparticles to modify rGO hydrogel microgranules, followed by TMCS treatment and supercritical CO2 drying in sequence. The resulting SMGA microgranules showed ultra-low density (13.5 mg/cm3), high


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contact angle (137o), large specific surface area (516 m2/g), high thermal stability, and excellent microwave and infrared shielding performances. The average EMI SE of SMGA microgranules was higher than 30 dB (8-18 GHz) and the extinction ratio in the whole mid-infrared range was 100 % (2.5-25 µm). Moreover, we have found that the silica modified rGO aerogel microgranules can float in air and shield more than 65 % infrared in 15 min. Our work offers more possibility for graphene aerogels to be applied in aerosol technology and other frontier fields.

ASSOCIATED CONTENT Supporting Information Picture of rGO hydrogel; Rheology characterization of rGO hydrogel, rGO hydrogel microparticles and rGO/SiO2 gel microparticles; more SEM and STEM images of SMGA microgranules; Size distribution of rGO hydrogel microparticles and rGO/SiO2 gel microparticles; XPS survey spectrum and high-resolution spectra of SMGA microgranules; FTIR and XRD spectra of SMGA, GA and SA microgranule; FT-IR spectra, Polarized Raman spectra, nitrogen adsorption and desorption isotherms and contact angles of SMGA8, SMGA 4, SMGA 2 and SMGA 1; Photographs of liquid marbles made by SMGA microgranules; EMI shielding performance and conductivity of SMGA microgranules at different weight proportion.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] ORCID: Author Contributions


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The manuscript was written by X. W. and was modified by other authors. All authors have given approval to the final version of the manuscript. Notes The authors declare no competing financial interest. ACKNOWLEDGEMENTS This work was financially supported by the Royal Society Newton Advanced Fellowship (NA170184), the National Natural Science Foundation of China (51572285), the Natural Science Foundation of Jiangsu Province (BK20170428) and the National Key Research and Development Program of China (2016YFA0203301). G. Hong Thanks to the Start-up Research Grant (SRG2016-00092-IAPME), University of Macau and Science and Technology Development Fund (081/2017/A2), Macao S.A.R (FDCT).

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Inner Surface-Functionalized Graphene Aerogel Microgranules with

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