Ultralight and flexible graphene foam coated with Bacillus subtilis as a highly efficient electromagnetic interference shielding film

Ultralight and flexible graphene foam coated with Bacillus subtilis as a highly efficient electromagnetic interference shielding film

Applied Surface Science 491 (2019) 616–623 Contents lists available at ScienceDirect Applied Surface Science journal homepage: www.elsevier.com/loca...

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Applied Surface Science 491 (2019) 616–623

Contents lists available at ScienceDirect

Applied Surface Science journal homepage: www.elsevier.com/locate/apsusc

Full length article

Ultralight and flexible graphene foam coated with Bacillus subtilis as a highly efficient electromagnetic interference shielding film


Chitian Xinga,b, Shoupu Zhua, Zaka Ullahc, Xiaochun Pand, Fan Wue, Xiaobo Zuod, Jianfei Liud, ⁎ ⁎ Mingliang Chena, Weiwei Lia, Qi Lia, , Liwei Liua, a Key Laboratory of Nanodevices and Applications & Collaborative Innovation Center of Suzhou Nano Science and Technology, Suzhou Institute of Nano-Tech and NanoBionics, Chinese Academy of Sciences (CAS), Suzhou 215123, China b Nano Science and Technology Institute, University of Science and Technology of China, Suzhou 215123, China c Centre of Excellence in Solid State Physics, University of the Punjab, Lahore 54590, Pakistan d Institute of Specific Iogistics, Army Academy of Research, Xian 710032, China e School of Mechanical Engineering, Nanjing University of Science and Technology, Nanjing 210094, Jiangsu, China



Keywords: Graphene foam Bacillus subtilis Flexible film Low density EMI shielding

The rapid advancement of electromagnetic appliances in daily life has caused a serious problem of electromagnetic interference (EMI). The demand of efficient materials for the shielding of electromagnetic waves is increasing day by day. Here, we report the assembly of Bacillus subtilis-modified graphene foam (BS-GF) with ultralight weight, significant flexibility and remarkable EMI shielding effectiveness (SE). BS-GF with a thickness of 1 mm and density of 0.18 g cm−3 shows a total EMI-SE of 56 dB in the 8–12 GHz band, and the specific EMI-SE reaches as high as 311 dB cm3 g−1. After folded 5000 times along the central axis, the sample does not show any significant reduction in EMI-SE. These excellent properties come from the light core-shell structure of the bacterial combined with GF, as well as the electrical loss due to GF architecture and the chiral absorbing ability of biomolecules such as DNA in the Bacillus subtilis (BS). The outcomes light the way to large-scale preparation of environmentally friendly and efficient EMI absorbents utilizing the microorganism as precursors.

1. Introduction Extensive efforts are being made to fabricate the electromagnetic interference (EMI) shielding materials which are widely required in a variety of fields ranging from aerospace to consumer electronics, for the elimination of unwanted electromagnetic waves or their undesired scattering to improve the performance or to reduce the electromagnetic pollution [1–3]. The recent research has uncovered the great potential of nanomaterials for EMI shielding owing to their rationally optimized composition and delicately designed structure [4–6]. Suitable modifications in structure and efficient designs are favorable for high electric as well as magnetic losses and can come with fascinating results. It can facilitate the attenuation of incident electromagnetic waves to achieve high EMI performance with low density even at high temperatures [7]. Carbon-based materials having tunable conductivity, low density and excellent mechanical strength are typical dielectric materials and are widely used for EMI shielding [8,9]. Graphene, nanometerthick carbon films and carbon materials with porous structure can absorb electromagnetic waves in the microwave frequency range and

show a significant electromagnetic interference shielding efficiency [10]. The designed structure can adjust the electric and magnetic characteristics of EMI materials which play a major role in determining the EMI performance. The enhancement in electric and magnetic properties of EMI materials has been the focus of current studies [11,12]. The EMI materials without reasonable conductivity and sufficiently high permeability, deliver poor performance. The assembly of composites or hybrid structures of EMI materials is an efficient way to achieve a suitable electric loss and high permeability simultaneously [13,14]. Multi-walled carbon nanotubes (MWCNTs) coupled with diamine cross-linked rGO/FeCo in a soft polymer matrix yield a total shielding effectiveness (SE) of 41.2 dB at 12 GHz [4]. The composite material consisting of MWCNTs and flower-like Fe3O4 nanoclusters, has an EMI shielding coefficient of 64 dB at 18 GHz [12]. Such materials are dense due to filling of metal particles and exhibit good microwave absorption properties. The poly (dimethyl siloxane) (PDMS) cured graphene foam (GF) has a density of only 0.06 g cm−3 and specific SE of 500 dB cm3 g−1 [8]. The higher absorption of electromagnetic waves is

Corresponding authors. E-mail addresses: [email protected] (Q. Li), [email protected] (L. Liu).

https://doi.org/10.1016/j.apsusc.2019.06.107 Received 4 January 2019; Received in revised form 31 May 2019; Accepted 11 June 2019 Available online 12 June 2019 0169-4332/ © 2019 Elsevier B.V. All rights reserved.

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Fig. 1. Schematic diagram showing the procedure for the assembly of 3D GF coated with BS.

for 3 min. After that, the argon flow was set at 30 sccm and hydrogen was also introduced into the furnace at 30 sccm. The temperature was increased to 950 °C at a rate of 25 °C min−1 and maintained for 10 min and the flow of hydrogen and argon was set at 40 sccm and 20 sccm, respectively. Afterward, the flow of hydrogen, argon and methane was set at 20 sccm, 30 sccm and 10 sccm for 15 min at 950 °C, respectively. Finally, the furnace was quenched to room temperature under the argon flow rate of 60 sccm. The resultant product was wetted with 75% ethanol and etched with 1 M FeCl3 aqueous solution for 1 d. The product was washed with deionized water till it became neutral. After adding with 6 M nitric acid, the sample was heated at 80 °C for 2–3 h and then washed with deionized water. Ultimately, 3D GF was successfully obtained.

ascribed to three-dimensional conductive network of lightweight GF. The graphene foam/poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) materials have also been prepared with superior EMI shielding performance due to the unique design and structure of the materials for higher degree of reflection and scatter of electromagnetic waves [11]. Bacillus subtilis (BS) as light core-shell structure particles can be thought as structural optimization additives for EMI shielding. The potential advantages of BS are as follows. Firstly, these common bacteria are harmless and easy to breed [15], and thus they are suitable for multiple experiments to get the best loading conditions [16]. Secondly, these bacteria with uniform structure have the size of a single cell is ~0.7 × 2 μm2 [17,18], uniform coloration, no capsule and peripheral flagella. Such bacteria consisting of the outermost cell wall (the main component is peptidoglycan), the inner cell membrane, the cytoplasmic matrix and the innermost nuclei (containing a large amount of DNA) possess an ideally formed core-shell structure. This structure contributes to multiple reflection losses on the surface and inside of the bacteria stacked on the GF [19]. Thirdly, the bacteria contain a large number of biochiral molecules such as DNA with chiral absorbing ability [20]. Fourthly, the porous network structure of the three-dimensional (3D) GF is not easily destroyed when BS is loaded. In comparison with the wide use of GF loaded with metal or metal oxide structures, GF loaded with BS is a facile and sustainable choice. Herein, we report a Bacillus subtilis-modified GF materials (BS-GF) for the investigations of their EMI shielding performance. After studying the living habits of BS, a strategy for attaching the BS to GF and then coating on PDMS (BS-GF-PDMS), has been proposed. The prepared composites show great potential for EMI shielding. The outcomes can light the way to large-scale preparation of efficient EMI absorbents utilizing the microorganism as precursors.

2.2.2. Loading BS on GF The 3D GF was infiltrated with 75% ethanol, and then dipped in a glucose solution of 0.1 g cm−3 for 10 min, dried in an oven at 65 °C for 10 min, and mixed with a solution of BS containing a culture solution at a constant temperature of 35 °C. After 3 days incubation, the samples were vacuum dried overnight at 65 °C. Finally, the sample was cured using PDMS to form a thin film of BS-GF-PDMS (Fig. 1). 2.3. Characterization The morphology of BS-GF was observed by scanning electron microscopy (SEM, Quanta FEG 250, FEI Co., Hillsboro, OR, USA, operating voltage = 20 kV) and high resolution transmission electron microscopy (HRTEM, JEM-1011, JEOL, Japan). Element distribution of BS-GF was investigated by EDS mapping of SEM (Quanta FEG 250, FEI Co., Hillsboro, OR, USA). The X-ray diffraction (XRD) tests were carried out on X-ray diffractometer (XRD, D8 Advance with lynxEye and SolX (Bruker AXS, WI, USA), Cu-Kα radiation, λ = 1.5418 Å) from 10°to 90°with a scanning rate of 10°/min−1. The Raman spectra were recorded by a Raman spectrometer (LabRam HR800-UV-NIR, HORIBA JobinYvon, Paris, France, excitation laser wavelength = 532 nm). The Fourier transform infrared (FT-IR) spectroscopy of samples was recorded by a FT-IR spectrophotometer (Spotlight 400/400 N, Perkin Elmer, USA). X-ray photoelectron spectroscopy (XPS) was obtained by a PHI instrument (PHI 5000 VersaProbe, Ulvac-Phi, Japan). The EMI SE in 8–12 GHz of samples were tested by network analyzer (Keysight, PNAN5227A) with the waveguide transmission method.

2. Experimental section 2.1. Materials and chemicals Nickel Foam (1.6 mm, 320 g m−2) was purchased from Cangde Liyuan, China. BS (100 billion spores g−1) was purchased from the Pesticide Factory of IPP of CAAS. Polydimethylsiloxane (PDMS, 184 SILICONE ELASTOMER) was purchased from SYLGARD. 2.2. Materials synthesis

3. Results and discussion 2.2.1. Synthesis of 3D GF by CVD method Nickel Foam with a thickness of 1.6 mm and an area density of 320 g m−2 was cut into 2.5 × 1.5 cm2 pieces and washed in acetone and ethanol successively. Finally, the foam was rinsed in deionized water and dried at 60 °C for 24 h. The treated nickel foam was placed in CVD furnace and argon gas was introduced into the furnace at 60 sccm

The BS-GF-PDMS shows excellent flexibility and can be randomly bent without any damage, as shown in Fig. 2a. The SEM images of the BS-GF exhibit the 3D interconnected porous graphene network architecture (Fig. 2b). Compared with the smooth surface of the 3D GF, the surface of BS-GF become rough due to the coating of a layer of bacteria 617

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Fig. 2. (a) Digital photograph of flexible BS-GF-PDMS sheet with thickness ~1 mm. (b, c, d) SEM images of BS-GF, and (e) EDS mapping of BS-GF.

Fig. 3. (a) and (b) TEM images of BS-GF.

spectrum of GF given in Fig. 4b, a sharp G-peak at ~1582 cm−1 and a weak 2D-band at ~2706 cm−1 are observed. The intensity ratio of G and 2D peaks (IG/I2D) is 2.63 for GF and 2.80 for BS-GF which confirms the deposition of multilayer graphene [21]. The G and 2D peaks of GF are similar to those of BS-GF, indicating that the graphene structure is not destroyed after loading of BS, which is the guarantee of the good electromagnetic SE. Interestingly, a small characteristic peak of at 1250 cm−1 can be seen in the Raman spectrum of BS-GF, which is attributed to the existence of BS [22]. FTIR data of GF does not show obvious characteristic peaks in the range of 1000–2500 cm−1, while FTIR data of BS-GF has a C]O stretching vibration peak near 1640 cm−1, and a CeN stretching vibration peak near 1100 cm−1, respectively. The two characteristic peaks are mainly derived from the main components of BS (Fig. 5).

(Fig. 2c). The bacteria stick fast on 3D GF as cylindrical particles with size around 1–2 μm (Fig. 2d). Oxygen and sulfur are also present at the chemical mapping spectra of BS-GF (Fig. 2e). These elements mainly contribute from biologically active substances such as polypeptides of BS. The chemical mapping further confirmed that the BS bacteria were uniformly distributed on the 3D GF (Fig. 2e). The BS loading in BS-GF could reach up to 43.2 mg cm−3. TEM images further show the structure of BS-GF. It can be seen from Fig. 3 that BS with different sizes and similar shapes are attached to the surface of GF. XRD analysis is conducted to investigate the crystallographic phases of the BS-GF. The XRD patterns given in Fig. 4a show the diffraction peak at ~26° which corresponds to graphene/graphene foam. There is no prominent characteristic peak for bacteria in the patterns, demonstrating that the bacteria exist in an amorphous state. In Raman 618

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Fig. 4. (a) XRD patterns of GF and BS-GF. (b) Raman spectra of GF and BS-GF. Table 1 Atomic contents of C,N, and O elements in GF and BS-GF. Name

C1s N1s O1s

Atomic% GF


96.45 0.4 2.85

86.59 1.93 10.9

also can be seen from the Table 1 that the content of N1s and O1s of BSGF is much higher than that of GF, which is due to the abundant biomacromolecules in BS biomolecules. The EMI shielding factor reflects the electromagnetic shielding ability of the material. The total shield effectiveness (SET) is expressed in terms of shielding by absorption (SEA), by reflection (SER) and multiple reflections (SEMR). If the sample is thicker than the skin depth, the multiple reflections can be ignored because the multiple reflected waves will be absorbed by the sample. Therefor the total shielding effectiveness can be expressed as [26]:


Fig. 5. FTIR data of the GF and BS-GF.

The electromagnetic SE of the absorber is determined by using a vector network analyzer to test S11 (or S22) and S12 (or S21) in the 8–12 GHz band using a waveguide cavity. The two sets of data reflect the reflection power ratio R and the transmission power ratio T:

Both BS-GF and GF have C1s peak at 285 eV and O1s peak at 532 eV in the Fig. 6. In addition, the XPS of BS-GF exhibits other characteristic peaks, such as N1 s peak (at 400 eV) and Cl2p3 peak (at 1072 eV). It

Fig. 6. (a) XPS data of GF. (b) XPS data of the BS-GF. 619

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Fig. 7. (a) Schematic diagram of electromagnetic waves shielding process (A: incidence and reflection. B: absorption. C: multiple internal reflections. D: transmission). (b) EMI-SE of GF-PDMS. (c) EMI-SE of BS-GF-PDMS. (d) Comparison of EMI-SE of lightweight carbon-based materials with different thicknesses: 1. MWCNT-Fe3O4 [12], 2. GF [8], 3. CNT-MLGEP [10], 4. Graphene/CNF/PDMS foam [23], 5. CNTs/rGO/PDMS foam [23], 6. CNT sponge/epoxy composite [24], 7. MWCNT/WPU foam [25] and 8. rGO-MDA-FeCo/MWCNT [4].

S11 = 10 log(R)

as DNA) in BS, have chiral absorption characteristics. However, its electromagnetic SE is significantly weaker than that of BS-GF. The core-shell structure of BS plays an important role in electromagnetic waves shielding. After carbonizing BS-GF, the total EM SE decreased to 26–28 dB, as shown in Fig. 8c. The conductivity of the bacteria was increased but their electromagnetic SE was decreased significantly after the carbonization. The reason is that the BS was squashed after carbonization, the load density was reduced and the columnar microcavity structure was also destroyed. After carbonization, the organic molecules with chiral absorbing ability were transformed into amorphous carbon, so the chiral absorbing ability was lost. The SET of BS is 9–15 dB, and the shielding effect is mostly due to the absorption effect (Fig. 8d). For GF, the electromagnetic shielding mechanism mainly depends on the electrical loss derived from both direct absorption and the absorption of multiply reflected waves in the 3D conductive network (Fig. 9a). For BS-GF, the high EMI SE is attributed to the adsorption of BS on the graphene surface and the synergistic effect of BS and GF. BS on the graphene surface increases the specific surface area which results in increase in absorption of multiply reflected waves (Fig. 9b). The BS has a microcavity structure and 3D GF has a hollow tube structure; these both enhance the reflection and absorption of electromagnetic waves in the microcavity. The BS also contains a large amount of chiral organic molecules (especially DNA) inside the nucleus. These organic molecules cause chiral absorption on the way of electromagnetic waves travelling through the cell again and again (Fig. 9c). Thus, the synergy effect of BS with GF is much greater than its own separate effect. When electromagnetic waves are incident on GF structure, the thin layer of uniform bacteria on the surface provides the countless scattering sites. The structure of the bacteria and the internal chiral absorption also increase their EMI efficiency in multiple

S12 = 10 log(T)


T = PT /PIN According to the definition of EMI SE:

SER = −10 log(1 − R) SE T = −10 log(T) SEA = SE T − SER The electromagnetic SE of the GF is plotted for frequency range of 8–12 GHz as shown in Fig. 7b. The total SE of GF is 20–23 dB with absorption 10–16 dB and reflection 7–9 dB. The total SE of BS-GF reached 50–55 dB, as shown in Fig. 7c. Fig. 7d shows that the BS-GFPDMS has the highest EMI shielding performance among the lightweight nanocarbon-based materials. The density of BS-GF-PDMS is ~0.18 g cm−3, with thickness of ~1 mm, and its EMI SE reaches to ~55 dB. The higher EMI SE and lower density make BS-GF-PDMS an ideal EMI shielding material especially for applications where lightweight is an important requirement. Fig. 8a shows that the SET of culture fluid was 20–25 dB which is approximately equal to the SET of pure GF. Consequently, the culture medium did not have any contribution in higher electromagnetic SE of BS-GF. Fig. 8b shows the electromagnetic shielding performance of the GF coated with protoplast which was obtained by ultrasonic disruption, separation and purification. The performance of this sample is significantly better than GF, evidencing that the biomacromolecules (such 620

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Fig. 8. (a) EMI SE of culture fluid @ GF, (b) protoplast @ GF, (c) carbonized BS @ GF and (d) BS-PDMS.

The strategy and experimental method for the loading of core-shell biomaterials on GF have also been tested for Yeast. The total EMI SE of GF coated with Yeast reached up to 43 dB for frequency range 8–12 GHz, as shown in Fig. 11. This shows that the current study can open new ways for the assembly of efficient electromagnetic shielding materials.

reflection losses, thanks to the special three-dimensional structure of GF and the internal conductive loop. In order to estimate the flexibility and lifespan of BS-GF-PDMS, it was folded up to 5000 times along the central axis of the long side. The value of SET of a sample was measured after 500, 1000, 1500, 2000 folds, and the maximum shielding effectiveness (Max SET) was plotted with number of folds, as shown in Fig. 10b. After the sample was bent, the electromagnetic SE did not decrease significantly. The Max SET was 53.06064–0.00134 N (dB) and the slope was only −0.00134. The results evidence that the large number of bending or folding of the material did not affect its EMI SE and it can be used for long period.

4. Conclusions In summary, a new strategy for the fabrication of lightweight GF with exclusive core-shell structure coated with microorganism has been

Fig. 9. Schematic illustration of EMI shielding mechanisms. 621

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Fig. 10. (a) EMI SET of BS-GF during bends up to 2000. (b) Maximum SET of BS-GF for frequency range 8–12 GHz during bends up to 5000.

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Fig. 11. EMI SE of GF coated with Yeast.

developed. The BS bacteria are uniformly loaded on the surface of GF. The suitable core-shell structure makes the multiple reflection loss of electromagnetic waves in GF and their absorption is increased significantly. The chiral molecules in BS also enhance the absorption of electromagnetic waves. BS-GF-PDMS with a low density of 0.18 g cm−3, has a high EMI SE of 56 dB, a remarkable Specific EMI-SE of 311 dB cm3 g−1 and superior durability proved by 5000 bending tests. The proposed material can be used as a promising environmentally friendly EMI shielding material.

Acknowledgments This work was supported by the National Natural Science Foundation of China [Grant Nos. 61605237 and 11474310], the Military Commission Logistics Department [Grant Nos. BY117J013], the State Key Program of National Natural Science Foundation of China [Grant No. 61734008], Jiangsu Province Postdoctoral Research Funding Scheme [Grant No. 2018K158C], and the Projects of Jiangsu Province and Suzhou City [Grant No., BE2016006-3, BK20150366, BK20150367, 1501131B, and SYG201629]. The authors are also grateful for the technical support of Nano-X, the Platforms of Characterization & Test, and Nanofabrication Facility from Suzhou Institute of Nano-Tech and Nano-Bionics.


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