Electroless Plating of Graphene Aerogel Fibers for Electrothermal and

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Interface-Rich Materials and Assemblies

Electroless plating of graphene aerogel fibers for electrothermal and electromagnetic applications Xiaohan Wu, Guo Hong, and Xuetong Zhang Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.8b04007 • Publication Date (Web): 15 Feb 2019 Downloaded from http://pubs.acs.org on February 19, 2019

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Electroless plating of graphene aerogel fibers for electrothermal and electromagnetic applications Xiaohan Wu, †,‡ Guo Hong,§ Xuetong Zhang,*,†,║

†Suzhou

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

Suzhou 215123, China. ‡University

§Institute

of Chinese Academy of Sciences, Beijing, 10000, China.

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

║Department

of Surgical Biotechnology, Division of Surgery & Interventional Science,

University College London, London, NW3 2PF, UK.

KEYWORDS: graphene aerogel, electromagnetic interference shielding, composite fiber, electrothermal heating.

ABSTRACT: Graphene aerogel fibers (GAFs) with low density, high specific surface area and high porosity can be used as the host material to incorporate another

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component and thus form multifunctional fibers, which have potential applications in wearable devices, thermoregulating apparatus and sensors, etc. However, the intrinsically low electric conductivity of GAFs hampers themselves in the fields of electrothermal heating and electromagnetic interference (EMI) shielding. Herein, we report a new aerogel fiber composed by graphene sheets and nickel nanoparticles with low density (55-192 mg/cm3), high electric conductivity (0.8×103-4.5×104 S/m), and high specific surface area (49-105 m2/g). The graphene/Ni aerogel fibers (GNAFs) were synthesized initially from reduced graphene oxide (rGO) hydrogel fiber followed by an electroless plating process. Further investigations have demonstrated that the resulting GNAFs possess excellent electrothermal property, faster electrothermal response, high mechanical and electrical stability as the electric wire, and excellent EMI shielding performance as the composite filler. The saturated temperature of GNAFs can reach 174 oC with an applied voltage of only 5 V, and the heating rate surpasses those of commercial Kanthal and Nichrome wires about 2.1 times and 2.6 times, respectively. The EMI shielding effectiveness of GNAFs is higher than 30 dB at the long bandwidth of


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12.5-20 GHz. Specifically, it can shield more than 99.99% of incident wave at the bandwidth of 15-20 GHz.

INTRODUCTION In recent years, there is a growing interest for functional fibers with low density, high electrical conductivity and excellent mechanical strength. Nanostructured carbon building blocks, such as carbon nanotubes and graphene sheets, can be candidate materials for functional fibers due to their unique structure and excellent properties1-5. Motivated by their remarkable flexibility, mechanical and electrical conductivity, carbon nanotube (CNT) fibers as well as graphene ones have been fabricated for the applications in the fields of conductive wires1,6-7, energy storage8-10, heaters11-12 and wearable electronics2,

13.

However, both carbon nanotube fibers and graphene fibers

are normally single-component and difficult to incorporate another component due to their solid constriction, which limits their functionalities for many potential applications, such as catalysis, thermoregulating garment, self-cleaning fabrics and electromagnetic interference shielding (EMI) components. To solve above problems, researchers have introduced metal particles on the surface of nanostructured carbon fibers for obtaining 3 ACS Paragon Plus Environment

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composite fibers with higher electrical conductivity, strong mechanics14-15 and expanded applications. Randeniya et al.16 electrodeposited Au or Cu particles onto CNT fiber surface, which increased the conductivity of CNT fibers from 5 × 104 S/m to 2−3 × 107 S/m. Zhang and Li et al.17 electrodeposited Cu on CNT fibers, and the composite fibers exhibited a metal-like conductivity (from 4.08 × 106 to 1.84 × 107 S/m). However the mass density of CNT-Cu fibers can reach 3.08 g/cm3, which is twice the mass density of CNT fibers and much higher than that of graphene fibers, due to their solid constriction and the high mass density of metal. Kim et al.18 prepared graphene fiber (GF)-Cu wires with significantly enhanced electrical, mechanical and microwave heating properties. However, the structure of GF-Cu wires was heterogeneous, the inside of GFs didn’t contain Cu at all. Graphene aerogel fibers (GAFs), a one-dimensional (1D) porous architecture, can be synthesize via wet spinning process with subsequent special drying process. It is much easy for graphene aerogel fibers to combine with guest materials to form homogenous multifunctional composite fibers. Li et al.19 in our group has fabricated flexible, strong, and self-cleaning graphene aerogel composite fibers with tuneable 4 ACS Paragon Plus Environment

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thermal function under multi-stimuli. Like bulk graphene aerogels, the graphene aerogel fibers is an excellent ultralight material with low density, large surface area and marvelous pore volume. The capillary force generated in the pores can be beneficial to incorporate other component uniformly, and the large surface area can provide abundant attaching points for many active nanoparticles to form homogenous multifunctional composite fibers. However, this kind of composite fibers possess quite low electrical conductivity, which is extremely abominable for applications in conducting wires, heaters, and EMI. Moreover, only the porous structure was kept in the synthetic process, the obtained composite fibers can acquire low density. Therefore, partial keeping the porous structure of the GAFs while introducing active components to form homogenous multifunctional aerogel fibers is still quite challenging. In this work, to obtain multifunctional aerogel fibers with low density, we chosen Ni to prepare composite graphene aerogel fibers with GAFs through simple electroless plating process. Ni can build stronger interfacial strength with nanostructured carbon fibers than other metals, such as Cu20. Besides high electrical conductivity, Ni has excellent magnetic and catalytic property21-22, which empowers the composite fibers to 5 ACS Paragon Plus Environment

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be used in the field of heating, electrical conducting and electromagnetic shielding. Therefore, Ni is an excellent guest material for GAFs to form multifunctional composite fibers. The graphene/Ni aerogel fibers (GNAFs) were fabricated via wet spinning of graphene hydrogel fibers, pre-treat graphene hydrogel fibers, Ni2+ in-situ reduction in the hydrogel fibers, and followed by supercritical drying with CO2. The porous architecture of the graphene hydrogel fibers provided abundant channels for Ni2+ to cross to absorb on the inner graphene sheets, and thus the resulting nickel nanoparticles obtained on both the surface and internal of the GNAFs uniformly. The resulting GNAFs possess excellent electrothermal property as heater, high mechanical and electrical stability as wire, and excellent EMI shielding performance. To the best of our knowledge, this is the first report on the electroless plating of metal nanoparticles in the graphene gel fibers, and the electroless plating of Ni nanoparticles into graphene aerogel fibers might provide a new and simple method to synthesis homogenous multifunctional composite aerogel fibers with low density, controlled porosity and conductivity for electrothermal and EMI shielding applications.


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EXPERIMENTAL SECTION Materials. Graphite powder (400 mesh) was purchased from Qingdao Tianheda Graphite Co., Qingdao, China. P2O5, K2S2O7, 30% H2O2, HCl, H2SO4, HI, NiSO4.6H2O, NaOH, KNaC4H4O6.4H2O, N2H4.H2O were purchased from Sinopharm Chemical Reagent Company. These chemicals were used without further purification. Synthesis of graphene aerogel fibers (GAFs). Graphene oxide (GO) were first prepared from graphite powder by a modified Hummers method reported in our previous work23, and the GO dispersion was centrifuged at high speed (15000 rpm) to obtain liquid crystalline GO (≈ 50 mg/mL). According to our previous work19, graphene aerogel fibers were fabricated by spinning of 4 mL liquid crystalline GO into HCl aqueous solution, and followed by adding of HI to reduce GO at 80 oC for 3 h to obtain the graphene hydrogel fibers. Then, washing with ethanol and supercritical drying with CO2 (Sc-CO2 drying, 40 oC,

10 MPa) for 12 hours in sequence to obtain the corresponding graphene aerogel

fibers. Electroless Ni plating. Before the electroless Ni plating process, the graphene hydrogel fibers needed to be pre-treated. The pre-treatment including absorption of


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nickel ions by immersing graphene hydrogel fibers into 100 mL 0.5 M nickel sulfate solution at room temperature for 24 h, and in-situ reduction of nickel ions by immersing graphene hydrogel fibers with nickel ions into 100 mL 0.2 M NaBH4/ethanol solution under ambient temperature for 5 h. After the pre-treatment, the graphene hydrogel fibers were immersed into 150 mL plating solutions composed of 0.1 M NiSO4.6H2O, 0.7 M C4H4KNaO6.4H2O, 0.085 M N2H4.H2O, 0.75 M NaOH and reacted at 90 oC for a certain time to obtain graphene/Ni hydrogel fibers. Finally, the graphene/Ni hydrogel fibers were washed with ethanol at least four times, and supercritically dried with CO2 (40 °C, 10 MPa) for 12 hours to obtain graphene/Ni aerogel fibers (GNAFs). To investigate the effect of plating time, the products with different plating time of 10 min, 15 min, 20 min, 30 min were synthesized, which were named as GNAFs-1, GNAFs-2, GNAFs-3 and GNAFs-4 respectively. Characterizations. The morphology of the samples was examined by scanning electron microscope (SEM, Hitachi S-4800) with the acceleration voltage of 10-20 kV. Transmission electron microscope (TEM) measurement was carried out on Tecnai G2F20 S-Twin with the acceleration voltage of 200 kV. Raman spectra were recorded using a LabRAM HR Raman Spectrometer (LabRAM HR, Horiba-JY) fitted with a 50 mW laser at an excitation wavelength


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of 532 nm. The crystal structure was characterized by the X-ray diffraction (XRD, D8 Advance, Bruker AXS) with a scanning rate of 0.02 s over an angular range of 10-80o (2θ). The pore size distribution and average pore diameter of the samples analyzed by the BJH nitrogen adsorption and desorption method (ASAP 2020, Micromeritics, USA). The surface area of the aerogels was determined by the Brunauer - Emmett - Teller (BET) method, based on the amount of N2 adsorbed at pressures 0.05 < P/Po < 0.3. Thermogravimetric analysis (TGA) was performed on a TG 209 F1 thermogravimetric analyser (NETZSCH, TG 209 F1 Iris). The heating rate was set as 10 oC/min under N2 and the analytical temperature region was set from ambient temperature to 900

oC.

X-ray photoelectron spectroscopy (XPS) was performed using an AXIS Ultra

spectrometer with a high-performance Al monochromatic source operated at 15 kV. The electrical resistances of GAFs and GNAFs were measured by using a CHIChief 600D electrochemical workstation, and the electrical conductivity can be calculated by the equation: κ = IL/UA, where κ is the electrical conductivity, I is the current that cross the sample, U is the voltage applied in the sample, L is the length of sample current goes through, and A is the cross area of current. The cross section of GNAFs was assumed circular and the external diameters of GNAFs were measured under an optical microscope to calculate the cross-section area. The external diameter of each sample was measured three times for getting an average value and the diameter was in the range of 160.2~ 193.8 µm. The ends of single fiber were bonded to two pieces of copper foil by conductive silver adhesive, respectively, and the electrodes were applied to the copper foils. A vector network analyzer (N5227A) was used to measure the S-parameters and complex permittivity in 10 MHz-26.5


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GHz. For electromagnetic parameter measurements, the GNAFs and GAFs were cut into shortfibers about 10 mm, followed by mixing with wax at certain filling ratios and compressed to standard ring shape with an outer diameter of 3.5 mm, inner diameter of 1.52 mm. The samples were designated at different proportions of GNAFs with 5 wt.%, 10 wt.%, 15 wt.% and 20 wt.%.

RESULTS AND DISCUSSION Synthesis of the GNAFs. The synthetic pathway of GNAFs is shown in Scheme 1. It involves the spinning and pretreatment of graphene hydrogel fibers, electroless plating of graphene hydrogel fibers, and Sc-CO2 drying of graphene/Ni hydrogel fibers. Graphene hydrogel fibers were prepared by spinning of a uniform GO liquid crystal into HCl aqueous solution, and subsequently reduced by hydroiodic acid (HI)

19.

In the pre-treatment process, the graphene

hydrogel fibers with negative charges due to the incomplete reduction intended to absorb sufficient Ni2+ ions through electrostatic adsorption from the nickel sulfate solution, and the absorbed Ni2+ ions were in-situ reduced by NaBH4 on the graphene sheets to form nickel nuclei. In the electroless plating process, the above nickel nuclei could act as autocatalytic active centers to induce the growth of nickel nanoparticles24. The N2H4.H2O was the dominant reducing agent in the plating solution, and the reaction mechanism was shown in the following equation. Ni2 + + N2H4 +4OH ― = 2Ni(s) + N2(g) + 4H2O (1) In the reaction, NaOH played an important role on the growth rate of nickel nanoparticles. Generally, the higher the concentration of NaOH, the faster the growth of nickel nanoparticles according to the previous research23-25. It is found that the optimum concentration of NaOH was 0.75 M. Besides, temperature was another important factor of the electroless plating reaction.


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The reaction rate was very slow when the temperature was lower than 90 oC. In this work, we controlled the content of nickel nanoparticles on graphene sheets through time controlling of plating while the reaction temperature was maintained at 90 oC.

Scheme. 1 Schematic illustration of the electroless plating GNAF.

Morphological structure of the GNAFs. As can be seen in the Figure1a-b, the SEM images of GNAFs showed that the framework of the composite fibers was built by close stacking of graphene sheets in parallel, which was in accordance with the structure of GAFs (Figure S1), and abundant nickel nanoparticles were anchored homogenously onto the surface of GNAFs even in the gullies of the fibers. As shown in Figure 1c-d, the SEM images of GNAF cross section showed that the nickel nanoparticles were wrapped by graphene sheets, and no bare graphene sheets were observed, which indicated that the nickel nanoparticles can be uniformly grown on the surface and cross section of 11 ACS Paragon Plus Environment

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GNAFs in the process of electroless plating with enough reaction time. According to Figure S1 and Figure S2, the nickel nanoparticles became denser on the framework and the cross section of GNAFs with the longer reaction time, and a compact Ni layer was observed on the surface of GNAFs (Figure S2c, f) in 30 min. The 3D porous network structure of the graphene hydrogel fibers provided abundant channels for Ni2+ to penetrate through the surface into the inner and adsorb on graphene sheets in the processes of pre-treatment and electroless plating. Moreover, the micro-size of the diameter of graphene hydrogel fibers was beneficial to the adsorption process because Ni2+ is difficult to enter the central position of bulk graphene hydrogels. As shown in Figure 1e and Figure S3, GNAFs can be tied into a knot and twist into a rope without fracture trace, which exhibited excellent elasticity. The TEM (Figure1f, Figure S4) and STEM (Figure1g) images of GNAFs both showed that all of nickel nanoparticles with the diameter about 100 nm were anchored on graphene sheets evenly and no free nickel nanoparticles were observed. The selected electron area diffraction (SEAD) pattern (Figure S5) of GNAFs showed diffraction rings of rGO and the polycrystalline nature of Ni. Figure1 h-j showed the elemental mappings of C (red), Ni (yellow), and O (orange) 12 ACS Paragon Plus Environment

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of the area. As observed, C and O elemental signals were uniformly distributed over the graphene sheet while Ni elemental signals consistent with the nickel nanoparticles on the graphene sheet.

Figure 1. a - e) SEM images of GNAFs. f) TEM images of GNAFs. g) STEM images of GNAFs. h - j) EDS mapping images of GNAFs.

Characteristics of the GNAFs. The phase structure and purity of as-synthesized samples (GAFs, GNAF-1, GNAF-2, GNAF-3, GNAF-4) were examined by XRD spectroscopy. As shown in Figure 2a, the peak at 2θ = 23o corresponding to the facet (002) of graphene, and the other three characteristic diffraction peaks of GNAFs can be indexed to the (111), (200) and (220) planes of the face centred cubic (fcc) phase of nickel (JCPDS file card No. 04-0850). No impurity peaks are detected, indicating that the nickel obtained in this work is consisted of a pure phase. The characteristic peaks of GNAFs increase sharpness from GNAF-1 to GNAF-4, demonstrating that the degree of crystalline of nickel nanoparticles increasingly higher with the extended plating time. As shown in the Raman spectra (Figure 2b), two peaks at 1340 cm-1 and


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1578 cm-1 corresponding to D and G band of graphene can be prominently observed from both GAFs and GNAFs’ cross sections. The D, G band of GNAFs showed no shift in comparison with those of GAFs, indicating that the structure of GAFs was maintained after the electroless plating process. In general, the intensity ratios of D to G band (ID/IG) is a useful indicator to evaluate the density of defects in graphene - based materials26. It could be seen that the values of ID/IG of GNAFs were higher than that of GAFs, suggesting that there were more sp2 domains formed in the GNAFs due to the secondary reduction of oxygen – containing functional groups on grapheme sheets in the process of electroless plating. The percentage of nickel nanoparticles on surface of GNAFs was higher than those in cross sections due to the surface can absorb endless nickel ions, which resulted in decay of the Raman signal intensity on the surface (Figure S6). The XPS spectra of GNAFs showed the signals belong to Ni2p, C1s, and O1s, respectively. As shown in Figure 2c, the peaks of C1s, and O1s located at 285 eV and 532 eV respectively. Ni2p spectrum contained two peaks, which located at 852.3 eV and 854.2 eV belonged to Ni0 and Ni2+, respectively. The existence of Ni2+ was due to the Ni2+ absorbed on graphene sheets which didn’t react completely in the process of pre-treatment.


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Figure 2. a) XRD spectra of GNAFs and GAFs. b) Raman spectra of GNAFs’ and GAFs’ cross section. c) XPS spectra of GNAFs. d) Tensile stress-strain curves of GNAFs and GAFs. e) TGA curves of GNAFs and GAFs. f) Nitrogen adsorption and desorption isotherms of GNAFs and GAFs.

As can be seen in Figure 2d, mechanical measurements demonstrated that GNAFs exhibited typical stress versus strain profiles, similar to that of GAFs under tensile loading. The average values of fracture strength (5.69 MPa) and fracture strain (9.59 %) of GNAFs are lower than those of GAFs (7.79 MPa, 20.67 %), respectively. The fracture strength of GNAF-1 was higher than GAFs and strain was lower than GAFs, which may be due to that the nickel nanoparticles on graphene sheets could act as welding spots for structure defects to enhance the strength of GNAFs. The stress and strain of GNAF-2, GNAF-3, GNAF-4 were lower than GAFs, which may be due to that the rapid growing number of nickel nanoparticles with reduction time increase the distance between graphene sheets and reduce the tensile strength of GNAFs. The thermal stability of GAFs and GNAFs were investigated in nitrogen. As can be seen in Figure 2e, the weight loss of GNAFs was lower than that of GAFs, suggesting the existence of the Ni nanoparticles in the GAFs for the plated samples. In order to confirm the 3D porous network 15 ACS Paragon Plus Environment

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structure of GNAFs, the N2 adsorption-desorption isotherms of GAFs and GNAFs were tested. As shown in Figure 2f, all the adsorption-desorption curves of GNAFs exhibit type-IV isotherm, indicative of a characteristic open wedge-shaped mesoporous structure27. The BET surface areas of GNAFs were lower than that of GAFs (as shown in Table.S1), and the surface areas of GNAFs decreased from GNAF-1 to GNAF-4, which can be attributed to the mass of nickel nanoparticles which was much heavier than graphene while the larger-size nickel nanoparticles (>100 nm) didn’t contribute the surface area of GNAFs. Electrothermal property of GNAFs. The electric conductivity of GNAFs was investigated by I-E curves, and the GNAFs exhibited excellent electric conductivity. As shown in Figure 3a, the electric conductivity of GNAF-1 (822.5 S/m) was far higher than that of GAFs (126.2 S/m) and the electric conductivity of GNAFs increased rapidly with the plating time. When the plating time reached 30 min, the electric conductivity of GNAF-4 increased to 4.5 × 104 S/m, and a compact nickel layer on the surface of GNAF-4 can be observed in Figure S1a, which can explain why the electric conductivity of GNAF-4 was much higher than the other cases. Electrothermal performance of GNAFs was tested through immobilizing both ends of a bunch of GNAFs (length of 2 cm) by copper tapes, and connecting to a direct current (DC) power supply. When thermal energy generated at a constant applied voltage, i.e., the GNAFs reached equilibrium with heat dissipation, the temperature of GNAFs would saturate, which can be called saturated temperature (TS). As shown in Figure 3b, the TS of GNAFs were much higher than that of GAFs and the TS of GNAFs increase rapidly from GNAF-1 to GNAF-4 at 5 V, which was consistent with the trend of electric conductivity of GNAFs. According to previous studies, Joule heating was generated from inelastic collision between electrons and photons when electric current pass through the GNAFs in an electric field23. The higher electric conductivity of


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GNAFs, the more electrons in GNAFs, and the more Joule heating. As shown in Figure 3c, the Ts of GNAF-4 were 42, 134, and 174 oC when input voltages were only 1, 3, and 5 V respectively, while the Ts of aerogel-directed fibers (ASFs) was only about 40 oC under electric field of 30 V18, indicating the extremely high electrothermal efficiency of GNAF heaters. The neat graphene fibers (GF) showed higher Ts (425 oC), but it doesn’t have porous structure and high surface area (Table S3). As can be seen in Figure 3d, comparing with GAFs, the GNAFs exhibit faster electrothermal response, especially the GNAF-4 (a heating rate of 61.6 oC/s and a cooling rate of 85.7 oC/s), which was much faster than those of Kanthal (a heating rate of 29.1 oC/s

and a cooling rate of 53.46 oC/s) and Nichrome wires (a heating rate of 23.69 oC/s and a

cooling rate of 21.2 oC/s). The order of current density of a bundle GNAFs used for electrothermal experiments can reach 104 A/m2, which was comparable to the power cables. According to the excellent electrothermal property of GNAFs, only low power consumption was needed to drive the GNAFs to work for general heating requirements of human beings. Temperature distribution of the GNAFs was monitored by a real time infrared thermal camera. Figure 3e showed infrared image of GNAF-3 at operating voltage of 5 V, indicating that the thermal energy generated by electric field distribute uniformly in a bunch of GNAFs. As shown in Figure 3f and Figure S7, a light-emitting diode (LED) connected using GNAF-4 was turned on at 3 V, and can last at least 60 min (Figure S8). Moreover, taking GNAF-3 as an example, the morphology of GNAF-3 (as can be seen in Figure S9) had no obvious change after electrothermal test in comparison with the morphology before electrothermal test. The electrical conductivity and density of GNAF-3 after electrothermal test were 2.6 × 103 S/m and 123.3 mg/cm3, respectively, which differ with the values before electrothermal test very few. This demonstrated that the GNAFs had excellent mechanical and electrical stability. 17 ACS Paragon Plus Environment

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Figure 3. a) Conductivity of GNAF with different electroless plating times. b - c) Electrothermal performance of GNAF and GAF. b) Temperature profiles as a function of time at 5V. c) Electrothermal performance of GNAF-4 at different DC voltages. d) Heating rate and cooling rate of GNAF and GAF. e) Infrared image of GNAF-3 at operating voltage of 5V. f) Demonstration of LEDs connected via GNAFs (operating voltage 3 V).

EMI shielding property of GNAFs. The EMI shielding effectiveness (SE) is defined as the logarithmic ratio of transmitted power (Pt) to incoming power (Pi) of an EM wave and is measured in decibels (dB). For most applications, an SE value of 20 dB corresponding to 90% shielding of EM radiation is considered as a satisfactory level of shielding28. The total EMI SE consists of reflection (SER), absorption (SEA), and multiple reflection (SEM) components.


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(2)

EMI SE = SEA + SER + SEMR

SER is related to the impedance mismatching between the free space and shielding material while SEA is associated with dielectric loss and/or magnetic polarization29-30. SEM occurs at internal of materials and can be attributed to absorption for porous materials31. All above components can be calculated based on the measured Sparameters as follows: EMI SE = -10log

SEA = -10log

(

(PP ) = -10log|S21|2 = -10log |S12|2 t i

|S21|2 1 - |S11|

2

) = -10log(

SER = -10log (1 - |S11|

2

|S12|2 1 - |S22|

2

)

) = -10log(1 - |S22|2)

(3)

(4)

(5)

Where |Sij| represents the power transmitted from port i to port j. As can be seen in Figure 4a, the EMI SE of all GNAF samples were higher than 20 dB at the bandwidth of 9 ~ 20 GHz, and the highest EMI SE of GNAFs reaches 60 dB, demonstrating that GNAFs can satisfy shielding requirement at relatively wide frequency bandwidth. As can be seen in Figure 4b and c, the SEA of GNAFs were much close to the EMI SE, and the SER of GNAFs were lower than 2 dB, indicating that the SEA of GNAFs is the major


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contributor for the total EMI SE. The EMI SE of GNAFs are much higher than that of GAFs in the range of 1 ~ 20 GHz, which can attribute to the presence of Ni nanoparticles. The parameters of EMI shielding materials, including complex permittivity (εr =ε’ jε”) and complex permeability (μr=μ’-μ”), were measured to help explaining the EMI shielding mechanism of GNAFs. The real parts (ε’, μ’) are associated with the electrical and magnetic energy stored, while the imaginary parts (ε”, μ”) are related to the energy dissipation or loss, which depend on relaxation phenomena, natural resonance, dominant electronic, and interfacial polarization in EMI shielding materals32. The permittivity (ε’, ε”) are shown in Figure S10a, b. The value of ε’ and ε” of GNAFs were higher than that of GAFs in the most range of 10 ~ 20 GHz, which attributed to the high electrical conductivity of GNAFs. As can be seen in Figure S10c, d, the permeability (μ’) of GNAF-2 and GNAF-3 were higher than that of others, and the permeability (μ”) of GNAF-3 and GNAF-4 were higher than that of others, indicating that GNAF-3 had better ability of magnetic energy storage and loss. The loss tangents, including dielectric constant tangent (tan δε= ε”/ε’) and magnetic constant tangent (tan δm=μ”/μ’), which can 20 ACS Paragon Plus Environment

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Langmuir

be used to verify the electromagnetic loss capacity of EMI shielding materials, were also evaluated33. As can be seen in Figure 4d, the tan δε of GNAF-1 was higher than that of GAFs and other GNAF samples, indicating that the GNAF-1 had most microwave dissipation generated by dominant electronic property in the range of 1 ~ 20 GHz. However, the tan δm of GNAF-1 was lower than other GNAF samples due to the low magnetic property with the fewer Ni nanoparticles, as shown in Figure 4e. The tan δm of GNAF-1 was lower than 0.06 in the range of 9 ~ 20 GHz. That is why the GNAF-1 presented better EMI shielding performance in the range of 1 ~ 12 GHz. The tan δm of GNAF-3 (>0.07) and GNAF-4 (>0.1) were higher than that of other samples in the range of 12 ~ 20 GHz, and the tan δs of GNAF-3 was higher than GNAF-4 in the range of 9 ~ 20 GHz, which resulted in the highest EMI SE of GNAF-3 in the range of 12 ~ 20 GHz. Importing nickel nanoparticles in GNAFs enhance the EMI shielding effect when frequency was higher than 1GHz, but when frequency was lower than 1GHz, the reinforcing effect was weak. As shown in Figure 4e, the tan δm of GNAFs was not higher than that of GAFs, which can explain why the shielding effect were little at low frequency. The microwave penetrated the porous structure of GNAFs, and generated 21 ACS Paragon Plus Environment

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multiple reflection in pores until completely dissipated. According to Maxwell – Wagner effect, the interfacial relaxation generated at the interface between graphene and Ni nanoparticles can enhance the microwave dissipation loss34. The sample thickness was another factor for the EMI shielding of materials, and taking GNAF-3 at 10wt% as an example (Figure 4f), the thicker the sample, the higher the EMI SE. The EMI SE of GNAF-3 at 10 mm can reach 60 dB at 12 GHz, which was much higher than many pure carbon-based materials, such as aerogel-like carbon35 (

Electroless Plating of Graphene Aerogel Fibers for Electrothermal and

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