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Mar 22, 2016 - Graphene as a Shield against Electromagnetic Interference. Faisal Shahzad,. †,‡. Pradip Kumar, ... Electromagnetic interference (EMI) shielding materials have been gaining a rapidly growing ...... samples (PDF). □ AUTHOR ...

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Biomass-Derived Thermally Annealed Interconnected Sulfur-Doped Graphene as a Shield against Electromagnetic Interference Faisal Shahzad,†,‡ Pradip Kumar,† Yoon-Hyun Kim,§ Soon Man Hong,†,‡ and Chong Min Koo*,†,‡ †

Materials Architecturing Research Center, Korea Institute of Science and Technology, Hwarangno 14-gil 5, Seongbuk-gu, Seoul 136-791, Republic of Korea ‡ Nanomaterials Science and Engineering, University of Science and Technology, 176 Gajung-dong, 217 Gajungro, Yuseong-gu, Daejeon 305-350, Republic of Korea § R & D Center, Chang Sung Corporation, 11B-9L, Namdong Industrial Area 320, Seunggicheon-ro, Namdong-gu, Incheon 405-846, Republic of Korea S Supporting Information *

ABSTRACT: Electrically conductive thin carbon materials have attracted remarkable interest as a shielding material to mitigate the electromagnetic interference (EMI) produced by many telecommunication devices. Herein, we developed a sulfur-doped reduced graphene oxide (SrGO) with high electrical conductivity through using a novel biomass, mushroom-based sulfur compound (lenthionine) via a twostep thermal treatment. The resultant SrGO product exhibited excellent electrical conductivity of 311 S cm−1, which is 52% larger than 205 S cm−1 for undoped rGO. SrGO also exhibited an excellent EMI shielding effectiveness of 38.6 dB, which is 61% larger than 24.4 dB measured for undoped rGO. Analytical examinations indicate that a sulfur content of 1.95 atom % acts as n-type dopant, increasing electrical conductivity and, therefore, EMI shielding of doped graphene. KEYWORDS: graphene, sulfur doping, biomass, electrical conductivity, electromagnetic interference shielding



INTRODUCTION Electromagnetic interference (EMI) shielding materials have been gaining a rapidly growing attention because EMI originating from widely used telecommunication devices deteriorates the performance of electronic equipment and also threatens the health of human beings.1−3 Because of graphene’s excellent electrical conductivity, graphene-based materials have recently been extensively investigated as a promising EMI shield.4,5 Several graphene/polymer systems were reported, such as reduced graphene oxide (rGO)/PEI (20 dB),6 rGO/Epoxy (21 dB),7 sulfur-doped reduced graphene oxide (SrGO)/PS (24.5 dB),8 rGO/PS (29.3 dB),9 and rGO/ PS (48 dB).10 The values in the brackets indicate the reported highest EMI SE (shielding effectiveness) of investigated materials. Graphene with magnetic fillers in polymers were also examined to enhance EMI SE, for example, rGO/Fe3O4/ PVC (13 dB),11 rGO/Fe3O4/PANi (26 dB),12 rGO/Ni/PC (29.4 dB),13 and rGO/Fe2O3/PANi (51 dB).14 Similarly, several systems comprising graphene or its blends with magnetic constituents were also investigated, including, rGO/ Fe3O4 (24 dB),1 rGO/Ba Ferrite (32 dB),3 rGO/SnO2 (40 dB),15 and rGO (46 dB).16 Although most of these composite systems show EMI SE values greater than 20 dB, nevertheless, they suffer the disadvantage of relatively large thickness, usually © 2016 American Chemical Society

more than 2 mm, which makes the practical use of these materials insignificant in application in the aerospace sector and communication devices. Recently, ultrathin graphene-based materials with light weight have gained tremendous interest to challenge the thickness issues. Song et al.17 utilized a novel strategy to fabricate a sandwiched wax-graphene paper architecture that showed EMI SE of 19 and 46.3 dB at 0.1 and 0.3 mm in thickness, respectively. Shen et al.18 used high-temperature thermal annealing treatment to develop graphitized GO paper of 8.4 μm, which shows EMI SE of 20 dB. Song et al.19 developed a multilayer graphene architecture with polymer to fabricate EMI shielding material that shows 27 dB at 0.35 mm in thickness. Singh et al.20 developed phenolic-resin-based composite sheets having a thickness of 0.2−0.4 mm, filled with graphene and magnetic nanoparticles to yield EMI values ranging from 16.98 to 42.83 dB. Yuan et al.21 fabricated artificial nacre-like hybrid film of graphene that reaches EMI SE of 20.3 dB having a thickness of only 0.36 mm. Received: January 12, 2016 Accepted: March 22, 2016 Published: March 22, 2016 9361

DOI: 10.1021/acsami.6b00418 ACS Appl. Mater. Interfaces 2016, 8, 9361−9369

Research Article

ACS Applied Materials & Interfaces Scheme 1. Synthesis of Sulfur-Doped Graphenea

(a) GO, (b) prereduction of GO to obtain rGO at 200 °C, (c) doping reaction between rGO and lenthionine at 400 °C, (d) laminate formation of SrGO-400 at room temperature, (e) high-temperature annealing treatment to obtain SrGO-1100. a



In spite of significant progress made in recent years for development of graphene, the intrinsic defects originating from the synthesis process limits the full potential of this wonderful material in terms of electrical properties. Doping of graphene with heteroatoms has been widely accepted as a mean to overcome the intrinsic defects and tailor the electronic properties.22−24 A particularly exciting and new case among these dopants is sulfur that provides excellent electronic and chemical properties to rGO when the sulfur atom is bonded to aromatic rings of carbon or shares a conjugated planar system with a delocalized π-electron. However, the sulfur doping method is not straightforward and carries technological challenge due to the large atomic size of sulfur compared with carbon that requires much larger formation energy (3.806 eV) as compared with commonly doped nitrogen atom (0.973 eV), which has a similar atomic size to carbon.23,24 So far, several approaches for S doping has been proposed, including, but not limited to, using sulfur powder,22 benzyl disulfide,25 hydrogen disulfide,26 carbon disulfide,27 magnesium sulfate,28 sodium sulfate,29 and others.30,31 Among the reported precursors, most of them suffered from toxic nature, complicated synthesis protocols, long synthesis time, and cost, which limit the practical use of such materials.26,27,32,33 Herein, we proposed a novel environmentally friendly mushroom-derived biomass compound, “lenthionine”, through which a C−S−C type bond structure with a substantial doping content of 1.95 atom % was obtained with similar or better performance than many of the reported literature values.29,34 Similarly, the EMI SE values of corresponding doped laminates reached an outstanding value of 38.6 dB at a relatively very small thickness (150 μm), one of the best achievements among studied graphene materials.1,3,35 This approach of sulfur doping not only provides an excellent alternative material for EMI shielding but also opens new opportunities to use in several other applications that require doped graphene and can be a green alternative to toxic precursors used for sulfur doping.

EXPERIMENTAL SECTION

Material Preparation. Graphene oxide (GO) was prepared using a modified Hummer’s method from natural graphite flakes (SigmaAldrich; size of flakes greater than 80 μm).36,37 Lenthionine was purchased from Haihang Industry, China. Other chemicals, including hydrochloric acid (37%), hydrogen peroxide (30%), sulfuric acid (95− 97%), and potassium permanganate (99%), were purchased from Sigma-Aldrich. Quartz tubes were used for thermal reduction and doping reactions. Doping Conditions. To determine the doping conditions, the reaction temperature for doping was first selected from the TGA thermogram of lenthionine dopant (Supporting Information, Figure S1). As the thermogram reveals, the sulfur-containing compound started evaporation at above 200 °C, similar to the pristine sulfur.8 The doping reaction was performed at 400 °C, as, at that temperature, some lenthionine reacts with graphene, excess lenthionine is completely evaporated, and no lenthionine compound residues are left. Prereduction of Graphene Oxide. Thermal reduction of GO to rGO results in exceptional volume change with a strong exothermic reaction around 180−200 °C, which results in the spread and outflow of rGO powder from quartz tubes. To overcome this issue, we first performed a prereduction treatment of GO (as shown in Scheme 1) by heating the GO from room temperature to 200 °C at a heating rate of 10 °C/min and keeping for 5 min, followed by furnace cooling. Without the prereduction treatment, the GO and lenthionine mixed product was spread throughout the tubes and results in wastage of materials as well as inefficient doping. Synthesis of SrGO at 400 °C. The prereduced rGO powder and lenthionine (1:2.5 and 1:5 w/w) were first mixed in a mortar and pestle and then agitated on a vibrator for 30 min until no separate particle of lenthionine remains. The mixed product was heated to 400 °C at a heating rate of 10 °C min−1 under an argon atmosphere and kept for 1 h. SrGO-400 (1:2.5) represents the sulfur doping at 400 °C with an rGO-to-lenthionine ratio of 1:2.5. Similarly, the SrGO-400 (1:5) corresponds to samples synthesized at 400 °C with an rGO-tolenthionine ratio of 1:5. Upon completion of the reaction, the SrGO powder was cooled to room temperature under argon. SrGO-400 (1:2.5) and SrGO-400 (1:5) laminates were made through compression of the powder in a 10 mm die at high loading (15 Ton) for 30 min (Scheme 1). Subsequently, the 10 mm pellet was cut into a toroidal shape with specially designed cutters (outer diameter: 7 mm, inner diameter: 3 mm) as shown in the Supporting Information 9362

DOI: 10.1021/acsami.6b00418 ACS Appl. Mater. Interfaces 2016, 8, 9361−9369

Research Article

ACS Applied Materials & Interfaces

Figure 1. (a) Low-magnification TEM image of SrGO (1:5)-1100. (b) High resolution TEM image taken from SrGO (1:5)-1100 edge as marked by the red square in panel (a). (c) Intensity line profile showing the number of graphene layers and interlayer distance of a selected area in panel (b). (d) and (e) show the SAED patterns of the circled areas of (1) and (2) in panel (a), respectively. (f) SEM images of SrGO (1:5)-400; inset shows the red circle area at larger magnification, and (g) SrGO (1:5)-1100; inset shows the red circle area at larger magnification. (h) SEM image of a SrGO (1:5)-1100 particle; (i, j) the corresponding elemental mapping of sulfur and EDS spectra. (Figure S2) to fit in the coaxial sample holder. Powder processing after 400 °C annealing treatment for formation of laminates is easy and controllable, which were subsequently used to make high-temperature annealed laminates. High-Temperature Annealing Treatment of SrGO Laminates at 1100 °C. To get high-temperature annealed SrGO-1100 (1:2.5) and SrGO-1100 (1:5) laminates, SrGO-400 (1:2.5) and SrGO-400 (1:5) laminates were further heated up to 1100 °C at a heating rate of 10 °C min−1 and kept for 1 h, followed by cooling in an inert atmosphere. Synthesis of rGO at 400 and 1100 °C. A powder of rGO-400 was synthesized according to the same procedure as SrGO-400 (1:2.5) and SrGO-400 (1:5), except that the lenthionine was not added. One set of samples was kept for examination, while the other was heated to 1100 °C, designated as rGO-1100. Materials Characterization. The morphologies of graphene samples were characterized with transmission electron microscopy (TEM, Tecnai F20 G2, FEI, USA) and field-emission scanning electron microscopy (FE-SEM, Inspect F50, FEI, USA) techniques equipped with an energy-dispersive X-ray spectrometer (EDS). Samples for TEM measurement were prepared by dipping the TEM grid in a dilute suspension of graphene/ethanol and drying in ambient conditions before measurement. Samples for SEM measurements were prepared by fracturing the graphene laminates, which were subsequently mounted on carbon tape attached with the aluminum sample holder. The samples were coated with platinum for 40 s at 15 mA in a coating chamber. The chemical states and elemental compositions of graphene samples were investigated using the X-ray photoelectron spectroscopy technique (XPS, K-Alpha, Thermo Scientific, USA) with Al Kα as X-ray source at a power of 72 W. Raman spectroscopy was conducted using a Raman spectrometer with a 532 nm Ar-ion laser (LabRam HR, Jobin-Yvon, France). Thermogravimetric analysis (TGA) was carried out using a thermal analyzer (Q50, TA Instruments, USA) under a nitrogen gas flow of 30 mL min−1 and at a heating rate of 10 °C min−1. The DC electrical conductivity of the samples was examined using a four-pin probe (MCP-TP06P PSP) method with a Loresta GP meter (MCP-T610 model, Mitsubishi Chemical, Japan). EMI SE of each sample was measured using a network analyzer (ENA5071, Agilent Technologies, USA) with a coaxial wire method consisting of a toroidal-shaped specimen (ϕout = 7 mm, ϕin = 3.04 mm, and thickness of 150 μm). Thickness of the specimen was controlled by mounting the toroidal sample on a stainless steel holder grooved to 0.150 mm depth (see the

Supporting Information, Figure S2) and polishing on fine emery paper to balance the thickness. Thickness measurement was performed on locations around the periphery of toroidal laminate to obtain the average value. The incident EM waves had a power of 0 dBm, which corresponds to 1 mW.



RESULTS AND DISCUSSION The TEM micrographs of the SrGO (1:5)-1100 sample and their corresponding selected-area electron diffraction (SAED) patterns are presented in Figure 1a−e. A flake-like transparent and crumpled structure analogous to few-layer graphene sheets can be observed from the micrograph in Figure 1a. The number of graphene layers was identified by taking an HR-TEM image (Figure 1b) at the edge of graphene sheets bounded by the red square region in Figure 1a. Using the intensity line profile (Figure 1c), 3−5 layers were observed with a d spacing of 0.37 nm, slightly larger than that of graphite (0.33 nm). This may be attributed to the residual oxygenated group and sulfur atoms, which protrude out of the graphene plane and slightly increase the bond length when substitutionally incorporated into the carbon matrixes. The SAED pattern of a transparent region marked as circle (1) in Figure 1a is shown in Figure 1d. The well-defined hexagonal structure with six distinctive spots maintaining the sp2 structure of carbon was observed, confirming that the sulfur atoms did not disturb the hexagonal graphitic order of graphene sheets.34,38 In contrast, Figure 1e is an SAED pattern taken from the less transparent area marked as circle (2) in Figure 1a that shows typical dispersive ring-like patterns. Such a pattern depicts the polycrystalline nature of graphene sheets and arises from the merging of diffraction spots having a greater number of graphene layers in the selected area.39,40 Figure 1f shows the SEM micrograph of the top surface for the SrGO (1:5)-400 laminate, indicating that the graphene sheets are nicely packed (inset of Figure 1f). Figure 1g presents the SEM micrograph of the surface of the SrGO (1:5)-1100 laminate in which a closer look (inset of Figure 1g) reveals microvoids generated as a result of out-diffusion of gaseous species during the high-temperature annealing process. The SEM micrograph of a particle broken from the SrGO 9363

DOI: 10.1021/acsami.6b00418 ACS Appl. Mater. Interfaces 2016, 8, 9361−9369

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Figure 2. XPS spectra of graphene samples. (a) Full-scan XPS spectra of rGO-400, SrGO (1:2.5)-400, SrGO (1:5)-400, rGO-1100, SrGO (1:2.5)1100, and SrGO (1:5)-1100; inset shows the enlarged portion with comparison of C 1s and S 2p spectra of rGO-1100 and SrGO (1:5)-1100. (b−e) S 2p spectra of doped graphene.

(1:5)-1100 laminate, which was tested for EDS analysis and elemental mapping, is shown in Figure 1h. Figure 1i exhibits that the sulfur atoms were uniformly distributed in the graphene layers, and the representative EDS spectrum (Figure 1j) confirms the sulfur atoms along with carbon and oxygen. The full-scan XPS spectra of doped- and undoped-graphene samples were obtained (Figure 2a). Intrinsic doping of sulfur atoms in graphene was characterized from the presence of S 2p and S 2s peaks at binding energies of 164 and 227 eV.41 The sulfur-doped samples at low temperature (at 400 °C) as well as after the high-temperature annealing process (at 1100 °C) show the S 2p and S 2s signatures. The inset of Figure 2a shows the well-resolved C 1s peak of SrGO (1:5)-1100 with sulfur signals which were absent in rGO-1100. In all doped cases,22,25,26 sulfur is present either in the thiophenic state (−C−S−C−) or in the form of sulfoxide groups (−C−SOx−C, x = 2, 3, 4). A close look at fine-scanned S 2p spectra with high resolution (Figure 2b−e) discloses the nature of S-doping. At low temperature (400 °C), sulfur atoms are present in thiophenic form (a lower energy doublets 2p3/2 and 2p1/2 arising from spin−orbit coupling at 163.8 and 165 eV) as well as in the sulfoxide state (a higher energy doublets at 167.2 and 168.5 eV). After the high-temperature annealing process, the S 2p spectra (Figure 2c,e) reveal only thiophenic groups. It indicates the instability of sulfoxide groups at higher temperatures that were observed in the final product in pure thiophenic form suitable to impart the n-type doping contribution.42 As presented in the elemental composition from XPS analyses in Table 1, the sulfur content was improved in doped samples when the lenthionine loading was increased. The sample SrGO (1:2.5)-400 showed a sulfur content of 1.36 atom %, which increased to 2.65 atom % for SrGO (1:5)-400. After the high-temperature annealing process of doped laminates, the sulfur content decreased to 1.25 atom % for SrGO (1:2.5)-1100 and 1.95 atom % for SrGO (1:5)-1100 owing to the removal of sulfoxide species that were present at the lower-temperature processed product. The sulfur content was found to saturate at a level of 2 atom %, as we assume that sulfur doping in graphene depends more on the available vacant

Table 1. Elemental Compositions of Graphene Samples Obtained from XPS Analysis sample

C

O

S

C/O

rGO-400 SrGO-400 (1:2.5) SrGO-400 (1:5) rGO-1100 SrGO-1100 (1:2.5) SrGO-1100 (1:5)

88.62 87.89 87.16 98.14 96.69 96.33

11.38 10.75 10.20 1.86 1.73 1.71

0 1.36 2.65 0 1.25 1.95

7.7 8.2 8.5 52.7 55.6 56.1

sites. Once the doping sites are occupied, a further increase in temperature or initial precursor loading has minimal effect on raising the resultant sulfur content. The C/O ratio of sulfurdoped graphene also improved compared with undoped samples, suggesting a positive role of sulfur on the reduction reaction.43 In the Raman spectra of all the graphene samples (Figure 3), the G bands, originating from in-plane vibration of sp2 carbon atoms, were the most prominent feature of the majority of graphitic materials. However, the D band corresponds to a

Figure 3. Raman spectra of rGO-400, SrGO (1:2.5)-400, SrGO (1:5)400, rGO-1100, SrGO (1:2.5)-1100, and SrGO (1:5)-1100 with ID/IG ratios. 9364

DOI: 10.1021/acsami.6b00418 ACS Appl. Mater. Interfaces 2016, 8, 9361−9369

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SrGO, which is much larger than the value of 4.5 S cm−1 measured for rGO. The increase in electrical conductivity after sulfur doping is ascribed to n-type doping effect as sulfur atoms increase the concentration and charge of electrons when doped in the graphene layers. The equation σ = enμ (σ is conductivity, μ is mobility, n and e are concentration and charge of electrons, respectively) indicates an increase in electrical conductivity owing to n-type doping contribution. The extra valence electrons contributing from sulfur atoms become free carriers and increase the electron density in the graphene layers. The strong electron-donating ability of the thiophene−S bond raises the Fermi level higher than the Dirac point of pristine graphene toward the conduction band to increase the overall conductivity of S-doped graphene.22,42,48 The EMI SE is a measure of the material’s ability to block the EM waves and can be expressed as the logarithm of the ratio of incident power (PI) to transmitted power (PT) in decibels as

series of defects, such as bond angle disorder, bond length disorder, structural disorder, and heteroatom dopants in the graphene lattice.26,44 The D-to-G band intensity ratio is often used to characterize the defect density in graphene materials. The spectrum of the sample rGO-400 showed an ID/IG ratio of 0.87 that slightly increased to 0.91 and 0.92 after doping to SrGO (1:2.5)-400 and SrGO (1:5)-400 products, respectively. This observation indicates that sulfur moieties bring some distortion in the graphene lattice due to differences in atomic size and bond lengths. However, interestingly, after hightemperature annealing, the ID/IG ratio of doped graphene was slightly decreased to 1.13 (for SrGO (1:2.5)-1100) and 1.10 (for SrGO (1:5)-1100) from 1.18 (for rGO-1100). During the high-temperature thermal annealing process, as the oxygenated species are being removed, more in-plane defects are created, leading to an increase in the number of graphitic domains that are smaller in size and raise the D bands of Raman spectra.45 In the case of sulfur-doped samples at high temperatures, the oxygenated groups as well as out-of-plane and edge located sulfoxides groups (−C−SOx−C, x = 2, 3, 4) are also eliminated. Moreover, because there is no new sulfur atoms injected in the systems, the already present thiophenic sulfur (−C−S−C−) promotes formation of new domains of conjugated C atoms bonded in sp2 hybridization, which makes the D band slightly better.40,46 A downshift in the G band of spectra is associated with n-type doping effect of heteroatoms.44,47 Similar to previous reports,26,40 we observed a down-shift of 7−8 cm−1 in doped samples after the high-temperature annealing process. In the examination of electrical conductivity for doped- and undoped-graphene samples, an average value of 66 S cm−1 was found for the SrGO (1:5)-400 sample, 52.7% larger than 43.2 S cm−1 observed for rGO-400 (Figure 4). The high-temperature

⎛P ⎞ SE T (dB) = 10 log⎜ I ⎟ ⎝ PT ⎠

(1)

Generally, EM waves are attenuated by three mechanisms: reflection (SER), absorption (SEA), and multiple reflections (SEM). The contribution from multiple reflections (SEM) is overlooked when EMI SE due to absorption is larger than 10 dB, as, in that case, most of the re-reflected waves will be absorbed within the shielding material.49,50 Therefore, SET can be expressed in terms of reflection and absorption contributions as SE T = SE R + SEA

(2)

Theoretically, SER and SEA can be expressed as

51

⎛ σ ⎞ SE R = 39.5 + 10 log⎜ ⎟ ⎝ 2πfμ ⎠

(3)

SEA = 8.7d πfμσ

(4)

where σ is the electrical conductivity, μ is magnetic permeability, f is frequency, and d is the thickness of the shield. Equations 3 and 4 show the strong dependence of electrical conductivity and thickness of shielding materials on the total EMI shielding. In actual, when an EM wave hits a shielding surface, the incident wave may be reflected, absorbed, or transmitted through the shield. A two port network analyzer directly measured the scattering parameters (S11, S12, S21, S22) that are correlated with reflection (R) and transmission coefficients (T) as49,50 T = S12 2 = S21 2 , R = S11 2 = S22 2

Figure 4. Electrical conductivity values of doped- and undopedgraphene samples.

SEA and SER can be written in terms of scattering parameters as

annealing treatment restored the graphitic structure and reduced the oxygenated content to reach an outstanding conductivity value of 311 S cm−1 for SrGO (1:5)-1100, which is a significant improvement over rGO-1100 with a conductivity value of 205 S cm−1. The increase in electrical conductivity after doping with sulfur atoms is interesting, which was recently reported by a few other researchers.22,32,42 Yun et al.22 found electrical conductivity of 17.4 S cm−1 for SrGO, which is 2 orders of magnitude larger than that of rGO (0.32 S cm−1). An electrical conductivity 2.2 times larger than that of the undoped counterpart was reported for SrGO (22.8 vs 10.4 S cm−1).32 Wang et al.42 observed a conductivity value of 20 S cm−1 for

⎞ ⎛ ⎛ 1 ⎞ 1 ⎟ = 10 log⎜ ⎟ SE R = 10 log⎜ 2 ⎝1 − R ⎠ ⎝ 1 − S11 ⎠

(5)

⎛1 − S 2 ⎞ ⎛1 − R ⎞ 11 ⎟ = 10 log⎜ ⎟ SEA = 10 log⎜ 2 ⎝ T ⎠ ⎠ ⎝ S21

(6)

Then, SET can be deduced from eqs 5 and 6 as eq 7 SE T = 20 log(S21)

(7)

The variations of EMI SE of doped- and undoped-graphene samples versus frequency are exhibited in Figure 5a,b. The 9365

DOI: 10.1021/acsami.6b00418 ACS Appl. Mater. Interfaces 2016, 8, 9361−9369

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than the reflection part in all of the samples. Furthermore, EMI SE was found to slightly decrease with the increase in frequency. The possible reason could be understood from the fact that, when the input signals first strike the graphene laminate, the highly electrical conductive surface reflects back some of the EM waves while some EM waves are absorbed in the material. However, as the shield is too thin (150 μm), only a limited part of incoming signals can be blocked at high frequencies (having more energy) while the rest of the signals transmit through the material.52 Favorable impedance matching conditions of air with the graphene laminates at lower frequencies to absorb more EM waves, and the inverse relationship of frequency with the reflection contribution could be another reason for better EMI at lower frequencies. The power relationship (eq 1) can be used to calculate the shielding efficiency. When there is no shielding, all of the incident waves are transmitted and the shielding efficiency value tends to be zero. For example, the values of 20, 30, and 40 dB for SE correspond to 99, 99.9, and 99.99% blockage of incident radiation, respectively. SrGO (1:5)-1100 showed an SE value of 38.6 dB, which means it can block 99.98% of the incident radiation, well above the 30 dB (99.9% shielding) generally recommended for commercial devices.49,50 Review of literature reports for graphene-based materials, biomass-derived sulfur-doped graphene presents a highly competitive EMI SE performance compared with other graphene-based materials with consideration of the thickness factor (Table 2). Chen et al.53 reported a significant increase in EMI SE of GNS/poly(dimethylsiloxane) (PDMS) foam composite from 24 to 33 dB when the material thickness was increased from 1.0 to 3.0 mm, highlighting the significance of thickness in EMI shielding materials. Among the pristine graphene materials of significance, B,N-doped graphene64 showed a promising value of 42 dB for 1.2 mm thickness. The SrGO in this work can deliver almost similar EMI shielding performance with 10 times less thickness (150 μm), thereby, clearly outperforming the other doped-graphene materials. Another imperative feature of SrGO is the decent bandwidth (almost 4 GHz) above 35 dB, which is significant as most of the modern communication devices work in the MHz−GHz transition range, such as FM radio (88−108 MHz), TV (54− 220 MHz), mobile phones (824−849 MHz), global positioning

Figure 5. Variations of EMI SE of doped- and undoped-graphene samples. (a) rGO-400, SrGO (1:2.5)-400, SrGO (1:5)-400; (b) rGO1100, SrGO (1:2.5)-1100, SrGO (1:5)-1100; (c) shielding due to reflection; (d) shielding due to absorption.

three samples of rGO-400, SrGO (1:2.5)-400, and SrGO (1:5)400 showed EMI SE values of 17.6, 23.6, and 26.5 dB at 25 MHz and values of 12.0, 14.3, and 18.6 dB at 4 GHz, respectively (Figure 5a). After the high-temperature annealing process, similar to the increase in electrical conductivity, a significant increase in EMI SE was also observed. Figure 5b shows the EMI SE values of 24.4, 31.0, and 38.6 dB at 25 MHz for rGO-1100, SrGO (1:2.5)-1100, and SrGO (1:5)-1100 graphene laminates. The heavily doped graphene laminate SrGO (1:5)-1100 revealed an increase of 58% in SE value compared with that of rGO-1100. The dotted region in the lower frequency region of Figure 5b is shown in detail in the Supporting Information (Figure S2). To further elucidate the shielding contribution, the reflection and absorption parts were calculated for high-temperature annealed samples, presented in Figure 5c,d. Obviously, both of the SEA and SER for SrGO (1:5)-1100 showed better results than the rest of the samples and shielding contribution due to absorption was found more

Table 2. EMI Shielding Performance of Typical Graphene-Based Shielding Materials filler

matrix

t (mm)a

rGO rGO rGO/CuS rGO rGO rGO rGO rGO rGO/Ag rGO foam rGO few-layer graphene N-doped graphene biomass-derived SrGO

PDMSe PANif PVDFg PUh PVDF wax PEOi epoxy PANi

1 2.4 2.5 2 2 3 10 1 2

σ (S m−1)b 180 20 1 10−4