Highly Effective Electromagnetic Interference Shielding Materials

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Highly Effective Electromagnetic Interference Shielding Materials based on Silver Nanowire/Cellulose Papers Tae-Won Lee,† Sang-Eui Lee,*,‡ and Young Gyu Jeong*,† †

Department of Advanced Organic Materials and Textile System Engineering, Chungnam National University, Daejeon 34134, Republic of Korea ‡ Samsung Advanced Institute of Technology, Samsung Electronics Company, Ltd., Suwon 16678, Republic of Korea S Supporting Information *

ABSTRACT: We fabricated silver nanowire (AgNW)-coated cellulose papers with a hierarchical structure by an efficient and facile dip-coating process, and investigated their microstructures, electrical conductivity and electromagnetic interference (EMI) shielding effectiveness. SEM images confirm that AgNWs are coated dominantly on the paper surfaces, although they exist partially in the inner parts of the cellulose papers, which demonstrates that the AgNW density gradually decreases in thickness direction of the AgNW/cellulose papers. This result is supported by the anisotropic apparent electrical conductivity of the AgNW/cellulose papers depending on in-plane or thickness direction. Even for a AgNW/cellulose paper obtained by a single dip-coating cycle, the apparent electrical conductivity in the in-plane direction of 0.34 S/cm is achieved, which is far higher than the neat cellulose paper with ∼10−11 S/cm. In addition, the apparent electrical conductivity of the papers in the in-plane direction increases significantly from 0.34 to 67.51 S/cm with increasing the number of dip-coating cycle. Moreover, although the AgNW/cellulose paper with 67.51 S/cm possesses 0.53 vol % AgNW only, it exhibits high EMI shielding performance of ∼48.6 dB at 1 GHz. This indicates that the cellulose paper structure is highly effective to form a conductive AgNW network. Overall, it can be concluded that the AgNW/cellulose papers with high flexibility and low density can be used as electrically conductive components and EMI shielding elements in advanced application areas. KEYWORDS: cellulose paper, silver nanowire, electrical property, EMI shielding, dip-dry process

1. INTRODUCTION As communication instruments, electronic devices, and military equipment are used widely, electromagnetic interference (EMI) shielding of radio frequency radiation continues to be a more important concern in the modern world. Accordingly, conductive polymer composites reinforced with nanoscale fillers are required as new alternative candidates for electrostatic discharge (ESD) and EMI shielding applications based on their own advantages such as lightweight, corrosion resistance, flexibility, and process-efficiency.1−11 The EMI shielding effectiveness (SE) of the polymer composite materials depends on many factors including the nanofillers’ intrinsic conductivity, dielectric constant, aspect ratio, and so on.12 Among the nanoscale fillers such as carbon nanotube (CNT), graphene, and inorganic and metallic materials, silver nanowire (AgNW) with high conductivity, small diameter, and high aspect ratio has been considered as an excellent nanoscale ingredient to create high-performance EMI shielding materials.6 Recently, AgNW has been incorporated in various polymer matrices, including polystyrene,13,14 polyacrylate,15 epoxy,6,16 poly(vinyl alcohol),6 and so on. Although the conductive polymer composites containing conductive nanosized fillers have outstanding advantages compared with conventional metal-based EMI shielding © 2016 American Chemical Society

materials, manufacturing processes for conductive polymer composites are still difficult because of the necessity of processing equipment and the complication of preparation process. To simplify the process for manufacturing highly effective EMI shielding materials, in this study, we have adopted dip-coating process. The dip-coating technique has been proved to be a very simple and efficient manufacturing way in aspects of cost-effectiveness, easy processability with low viscosity solutions, high scalability, and short processing time, compared with other strategies such as spin-coating, barcoating, and so on.17−20 In addition, we have chosen a porous cellulose paper as a substrate for the dip-coating process, because cellulose, as one of the most abundant natural polymers on Earth, is environmentally friendly and easily processable in aqueous solution. For this purpose, we have manufactured a series of AgNW/cellulose papers by controlling the dip-coating cycles and have investigated their microstructural features, electrical conductivity, and EMI SE systematically for their use as electrically conductive and EMI shielding materials in emerging advanced applications. The Received: February 22, 2016 Accepted: May 9, 2016 Published: May 9, 2016 13123

DOI: 10.1021/acsami.6b02218 ACS Appl. Mater. Interfaces 2016, 8, 13123−13132

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ACS Applied Materials & Interfaces

Figure 1. Fabrication process of AgNW/cellulose paper with a hierarchical structure via a dip-coating process: (1) dipping cellulose paper in 0.1 wt % Ag/NW aqueous suspension; (2) repeating the dipping and drying processes for inducing AgNW network on cellulose fibers; and (3) final EMI shielding AgNW/cellulose paper where AgNW density gradually decreases in thickness direction.

Figure 2. SEM images for surfaces and cross sections of (A, B) neat cellulose paper, (C, D) S-10, and (E, F) S-50.

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that of the central parts of the cellulose paper. This result demonstrates that the coating density of nanometer-scale AgNWs, which are deposited on micrometer-scale cellulose fibers, decreases gradually in the thickness direction of the cellulose paper, which leads to a hierarchical structure of the AgNW/cellulose papers. The term “hierarchy” in this study includes two aspects: the combination of a nanofiller (AgNW) and a microfiber framework (microcellulose fiber), and the gradient of AgNW concentration in the cellulose paper along the thickness direction. The microcellulose fibers function as excluded volume facilitating the formation of electrically conductive AgNW networks on their framework. The hierarchical structure has electrical anisotropy and high electrical conductivity, which can be fused into the functional devices, as described below. The structural features of the AgNW/cellulose papers prepared at different dip-coating cycles were characterized by using X-ray diffraction patterns, as shown in Figure 3. For all

experimental EMI SE of the AgNW/cellulose papers was analyzed by comparing with the values calculated from theoretical models.

2. EXPERIMENTAL SECTION 2.1. Materials and Preparation of AgNW/Cellulose Papers. Commercially available cellulose filter paper (541, Whatman) with average pore size of ∼22 μm, 150 mm diameter, and ∼154 μm thickness was adopted as a neat cellulose substrate. An aqueous suspension including 0.5 wt % AgNW was purchased from Aiden Com (South Korea). The diameter and length of AgNWs in the aqueous suspension are evaluated to be 19.5 ± 1.6 nm and 18.2 ± 7.3 μm, respectively, from the SEM and TEM images of AgNWs (Figure S1, Supporting Information). A series of AgNW/cellulose papers with a hierarchical structure were manufactured by dip-coating process, as shown schematically in Figure 1. 0.1 wt % AgNW aqueous suspension was prepared by diluting a commercial AgNW suspension with distilled water. A neat cellulose paper was dipped into the 0.1 wt % AgNW aqueous suspension at room temperature for 3 s and then dried at 60 °C for 15 min. This dip-coating process was repeated by 50 cycles to adjust the amount of AgNW coated on the cellulose paper. To prevent any possible precipitation of AgNWs in the aqueous suspension, the suspension was mechanically stirred for 1 min, before each dip-coating cycle. The AgNW/cellulose papers were named as S-x, where x denotes the number of dip-coating cycle. 2.2. Characterization. The thicknesses of AgNW/cellulose papers manufactured under different dip-coating cycles were measured by using a digital thickness meter. More than 10 measurements for the thickness of a paper sample were carried out, and the results were averaged. The apparent density and porosity of AgNW/cellulose papers were evaluated by measuring the weight and apparent volume of the paper samples with 2 × 2 cm2. The dispersion state and associated morphology of AgNWs coated on the cellulose papers were characterized by observing the surface and cross-section images with aid of a cold-type field emission scanning electron microscope (SEM, S-4800, Hitachi). For the cross-sectional analysis, the neat cellulose and AgNW/cellulose papers were cut with a razor blade. The crystalline structural features of neat cellulose and AgNW/cellulose papers treated with different dip-coating cycles were identified by using X-ray diffraction method (D/MAX-2200 Ultima/PC, Rigaku). The electrical properties of AgNW/cellulose papers were investigated by obtaining current−voltage (I−V) and electric power−voltage (P−V) curves with multiple electrometers (2400, 6517, Keithley Instruments, Inc.). For the electrical experiments, the electrode distance of the papers was set to be 10.0 mm. The EMI SE of AgNW/cellulose papers was measured at room temperature in accordance with ASTM D4935−10. S-parameters were measured in the frequency range of 0.5−1.0 GHz by using a coaxial sample holder complying with the test standard and a network analyzer (Agilent E5071C).

Figure 3. X-ray diffraction patterns of neat cellulose and AgNW/ cellulose papers manufactured by a dip-coating process: (A) neat cellulose paper, (B) S-1, (C) S-3, (D) S-5, (E) S-10, (F) S-20, (G) S30, (H) S-40, and (I) S-50.

the papers, four dominant diffraction peaks are observed at 2θ = ∼14.7, ∼16.3, ∼22.7, and ∼33.7°, which are associated with the diffractions of 101, 101, 002, and 040 planes, respectively, of cellulose I-form with monoclinic dimensions of a = 0.783 nm, b = 0.819 nm, c = 1.038 nm, α = β = 90°, and γ = 96.55°.21,22 In cases of the AgNW/cellulose papers, typical diffraction peaks associated with AgNW crystals are detected at 2θ = ∼38.1, ∼44.2, ∼64.5, and ∼77.3°, which can be indexed to 111, 200, 220, and 311, respectively.23 In particular, the diffraction peaks related with crystalline AgNW became intense with the increment of the dip-coating cycle, indicating that the amount of AgNW coated on cellulose fibers in the papers increases with the dip-coating cycle. Figure 4A shows a photographic image of a paper crane made of S-30 paper, which supports that the AgNW/cellulose papers are highly flexible and foldable because of the good coating of AgNWs on cellulose fibers of the papers. It is worth noting that the AgNW aqueous suspension used for the dipcoating process was fabricated and stabilized in the presence of

3. RESULTS AND DISCUSSION 3.1. Structural Characterization. To identify the morphological features of AgNW/cellulose papers, we examined SEM images of the surface and cross section of the neat cellulose, S-10, and S-50 papers, as can be seen in Figure 2. The SEM images of the surface and cross-section of the neat cellulose paper exhibit a porous morphology and the smooth surfaces of cellulose fibers (Figure 2A, B). For the AgNW/ cellulose papers, the porous structure is quite similar to that of the neat cellulose paper, as can be seen in the cross-sectional SEM images. On the other hand, it was found that the surfaces of cellulose fibers in the AgNW/cellulose papers were coated uniformly and randomly with interconnected AgNWs and also that AgNWs existed in the inner parts of the cellulose paper, as can be seen in the SEM images of Figure 2C−F, although the density of AgNWs coated on the paper surface was higher than 13125


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primary hydroxyl groups of cellulose.28 It is thus valid to contend that AgNWs can be coated tightly on cellulose papers via the specific interactions meditated by PVP, as represented schematically in Figure 4B. When the apparent thickness of the neat cellulose and AgNW/cellulose papers were measured, the AgNW coating thickness in the papers increased linearly with the dip-coating cycle within the experimental error, as listed in Table 1. The volume fraction (VAgNW) of AgNW of the papers was evaluated by eq 1. In addition, the apparent density (dpaper) and porosity (Ppaper) of the neat cellulose and AgNW/cellulose papers were evaluated by using eqs 2 and 3: ⎡ ⎛ dAgNW ⎞⎛ 1 − WAgNW ⎞⎤ ⎟⎟⎥ ⎟⎟⎜⎜ VAgNW = ⎢1 + ⎜⎜ ⎢⎣ ⎝ d paper ⎠⎝ WAgNW ⎠⎥⎦

d paper =

Ppaper =

(1)

M paper Vpaper dcomposite − d paper dcomposite

(2)

× 100 (3) 3

where dpaper is the density of cellulose (1.50 g/cm ), dAgNW is the density of AgNW (10.49 g/cm3),29 WAgNW is the weight fraction of AgNW in the papers, Mpaper is the weight of the paper, Vpaper is the volume of the paper, and dcomposite is the density of AgNW/cellulose composite in which all the pores are filled with cellulose. Interestingly, it was found that the apparent density (0.53 ± 0.01 g/cm3) and porosity (64.7 ± 1.5%) of AgNW/cellulose papers are almost identical, irrespective of the dip-coating cycle within the experimental error, although the thickness of AgNW/cellulose papers and the AgNW content (vol %) in the papers increase with the increment of the dip-coating cycle, as listed in Table 1. 3.2. Electrical Property. The current−voltage (I−V) curves in the in-plane direction for the AgNW/cellulose papers manufactured under different dip-coating cycles of 1−50 are shown in Figure 5A. For the neat cellulose and S-1 papers, there is almost no electric current over the applied voltage of 0.1−5.0 V. On the other hand, in cases of S-3, S-5, S-10, S-20, S-30, S-40, and S-50 papers, the electric current increases linearly with the applied voltage and the slopes of the I−V curves increase significantly with the increase of the dip-coating cycle. It demonstrates that AgNWs are interconnected physically on the surfaces of cellulose fibers in the papers and

Figure 4. (A) Photographic image of a paper crane made of S-30 paper and (B) schematic illustration of the specific interactions among AgNW, PVP, and cellulose in the AgNW/cellulose papers.

poly(vinylpyrrolidone) (PVP), as reported in the literatures.24−26 Our FT-IR data of AgNW powder, which was obtained from its aquous suspension, confirm the existence of a stong interfacial interaction between the carbonyl groups of PVP and the AgNW (Figure S2, Supporting Information), which is consistent with X-ray photoelectron spectroscopic investigations in the literature.27 It was also reported that cellulose and PVP blends are miscible at the molecular level via a specific interaction between carbonyl groups of PVP and

Table 1. Structural Parameters and Electrical Properties of Neat Cellulose and AgNW/Cellulose Papers Manufactured by a DipCoating Process

sample code neat paper S-1 S-3 S-5 S-10 S-20 S-30 S-40 S-50

apparent thickness (μm)

AgNW coating thickness (μm)

AgNW content (vol %)

154.5 ± 3.9 156.5 156.6 157.7 159.8 161.3 162.8 164.1 164.2

± ± ± ± ± ± ± ±

3.3 3.6 6.8 2.9 3.5 4.9 3.8 3.2

2.0 2.1 3.2 5.3 6.8 8.3 9.6 9.7

0.01 0.03 0.05 0.10 0.20 0.31 0.42 0.53

apparent density (g/cm3)

porosity (%)

apparent conductivity (inplane direction) (S/cm)

0.54

64.3

1.26 × 10−11

0.55 0.53 0.52 0.52 0.51 0.54 0.54 0.53

63.3 65.0 65.8 65.6 66.2 64.8 65.0 65.6

0.34 3.19 7.66 14.44 39.35 47.03 51.43 67.51 13126

practical conductivity (inplane direction) (S/cm)

apparent conductivity (thickness direction) (S/cm)

practical conductivity (thickness direction) (S/cm)

1.40 × 10−12 24.90 246.70 395.50 440.84 925.01 909.52 878.52 1071.73

8.13 1.27 5.68 8.52 1.50 1.40 1.22 2.78

× × × × × × × ×

10−6 10−3 10−3 10−3 10−2 10−2 10−2 10−2

1.04 1.66 1.16 2.69 6.06 7.34 7.35 1.70

× × × × × × × ×

10−7 10−5 10−4 10−4 10−4 10−4 10−4 10−3


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Figure 5. (A) Current−voltage (I−V) and (B) electric power-voltage (P−V) curves of neat cellulose and AgNW/Cellulose papers in in-plane direction. Photographic images showing an LED lamp under 3 V when AgNW/cellulose papers are used as electrically conductive elements: (C) S-1 and (D) S-30.

Figure 6. Apparent and practical electrical conductivity of neat cellulose and AgNW/cellulose papers (A) in the in-plane direction and (B) thickness direction. Schematic diagrams for the electrical conductivity measurement direction of neat cellulose and AgNW/cellulose papers in the (C) in-plane direction and (D) thickness direction.

the electric power of S-3, S-5, S-10, S-20, S-30, S-40, and S-50 papers is quadratically proportional to the applied voltage, which is consistent with the expression P = IV = V2/R.

the networking degree is strengthened with the increment of the dip-coating cycle. Figure 5B represents the electric power− voltage (P−V) curves of AgNW coated papers. It is found that 13127


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percolation, and α the critical exponent. For the AgNW/ cellulose papers in the in-plane direction, the straight lines of logσ versus log(p − pc) plots with α = 0.923 and pc = 0.000082 (0.0574 wt %) for the apparent electrical conductivity and with α = 0.563 and pc = 0.000090 (0.0629 wt %) for the practical electrical conductivity give good fits to the experimental data (Figure 7A). In case of the thickness direction, the straight lines

Accordingly, when an external voltage of 3 V is applied to AgNW/cellulose papers, a light emitting diode (LED) lamp is lit, except for the neat cellulose and S-1 papers, and also the LED light is brighter for the papers with higher electrical conductivity, as can be seen in Figure 5C,D. From the slopes of the I−V curves in Figure 5A, the apparent electrical conductivity of the AgNW/cellulose papers in the inplane direction was calculated and also the resulting electrical conductivity of only AgNW coating layers was evaluated by considering the AgNW coating thickness (2.0−9.7 μm) of the papers by using the following eq (Figure 6A): σ=

1 1 L = × ρ R A

(4)

where σ is the electrical conductivity, ρ is the electrical resistivity, R is the resistance, A is the cross-sectional area, and L is the length or thickness of a sample between two electrodes. Similarly, the apparent electrical conductivity in the thickness direction was measured for the AgNW/cellulose papers with 154.5−164.2 μm thickness and the practical electrical conductivity was calculated for only AgNW coating layers with 2.0−9.7 μm thickness, as shown in Figure 6B. In addition, the apparent and practical conductivity of the AgNW/cellulose papers in the plane or thickness direction was summarized in Table 1. For the neat cellulose paper, the apparent electrical conductivity in the in-plane direction was measured to be ∼10−11 S/cm, indicating that the neat cellulose paper is electrically insulating. On the other hand, in cases of the AgNW/cellulose papers, the apparent conductivity in the inplane direction is found to increase significantly from 0.34 S/ cm for S-1 to 67.51 S/cm for S-50 with the increase of the dipcoating cycle. The practical electrical conductivity in the inplane direction for only AgNW coating layers also increases from 24.90 S/cm for S-1 to 1071.73 S/cm for S-50, which are far higher than those of the apparent conductivity of the papers (Figure 6A). It means that the electric current can flow effectively through interconnected AgNWs on the cellulose papers. Similarly, the apparent and practical electrical conductivity in the thickness direction for the papers also increases from 8.13 × 10−6 and 1.04 × 10−7 S/cm for S-1 to 2.78 × 10−2 and 1.70 × 10−3 S/cm for S-50 with the increment of the dip-coating cycle (Figure 6B), although they are far lower than those in the in-plane direction (Table 1). For example, the apparent electrical conductivity of S-50 paper in the thickness direction (2.78 × 10−2 S/cm) is ∼105 times lower than the conductivity in the in-plane direction (1071.73 S/cm). The noticeable difference in the electrical conductivity according to the in-plane or thickness direction is associated with the different coating degree of AgNWs on the surface and inner parts of the papers as well as the anisotropic orientation of cellulose fibers in the papers, as represented in Figure 6C,D. As confirmed from the above SEM images, AgNWs are more densely coated on the cellulose paper surface, compared to the inner part of the papers, and AgNW-coated cellulose fibers are located dominantly along the in-plane direction of the papers. It is generally known that, when the conductive nanofiller content reaches the electrical percolation threshold, a conductive path is formed in the composite matrix due to the network formation of the conductive filler. To determine the electrical percolation threshold for the AgNW/cellulose papers, a power law relation σ ∝ (p − pc)α was adopted,30 where σ is the electrical conductivity of the papers, p the AgNW volume fraction, pc the critical volume fraction at the electrical

Figure 7. log σ vs. log (p − pc) plots based on a power law relation and experimentally measured apparent and practical electrical conductivity data for AgNW/cellulose papers in the in-plane direction (A) and in the thickness direction (B). The solid lines are drawn by using the adjustable parameters (α and pc) of the power law relation.

with α = 1.297 and pc = 0.000095 (0.0664 wt %) for the apparent electrical conductivity data and α = 1.001 and pc = 0.000095 (0.0664 wt %) for the practical electrical conductivity are matched well with the experimental results (Figure 7B). Theoretically, the α value as an index of the system dimensionality has been predicted to be 1.3 and 1.94 for ideal 2- and 3-dimensional percolation systems, respectively.30 Accordingly, the α values of 0.563−1.297 as well as the low pc values of 0.0574−0.0664 wt % for the AgNW/cellulose paper system demonstrate that AgNWs are coated uniformly by forming a quasi-2-dimensional network structure in the cellulose papers with high AgNW contents above ∼0.0664 wt %. This result is supported by the fact that both α and pc values 13128


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Figure 8. EMI shielding characteristics of AgNW/cellulose papers: (A) Coefficients of reflection, absorption, and transmittance at 1.0 GHz and their dependence on electrical conductivity; (B) SER, SEA, and SET in decibels at 1.0 GHz; (C) SET in the frequency range of 0.5−1.0 GHz.

Table 2. Comparison between Structural Parameters, Electrical Properties, and SET Values (at 1.0 GHz) of EMI Shielding Materials. SET was Calculated at the Thickness of 160 μm filler loading material AgNW/cellulose (this study) 35

MWNT/cellulose SWNT/rubber36 SWNT/epoxy37 MWNT/PDMS38 Aerogel-like carbon39 Pyrolyzed-BC aerogel40 MWNT/cellulose aerogel41 AgNW/epoxy6 AgNW/PVA42 CuNW/PS42 AgNW/PI foam43 AgNW/PI foam (with both surfaces spraycoated with AgNW)43 a

dfiller (g/cm3)

dmatrix (g/cm3)

dcomposite (g/cm3)

wt %

vol %

electrical conductivity (S/cm)

skin depth (μm)

10.49

0.54

0.53

9.57

0.53

67.51

194

1.85 1.35 1.35 1.85

1.0 1.0 1.0

6.71 100.00 0.14 3.00 1.33 0.40 1.80 5.26 76.92 20.00

614 159 4250 1590 1380 2520 1190 694 181 356

10.49 10.49 8.96

1.0 1.25

0.76 1.04 1.04 1.05 0.12 0.003 0.095 1.43 1.77 1.13 0.017 0.022

16.5 15.8 15.0 10.0 2.5

11.56 12.20 5.67

33.3 33.3 13.0

4.54 5.62 1.68

SET (dB) 46.4 (Cal.) 48.6 (Mea.) 26.5 49.6 3.1 20.0 14.0 6.9 16.2 24.5 47.4 35.8 5.4a 17.0a

Measured at 5 mm thickness.

⎛ T ⎞ ⎟ SEA = −10log⎜ ⎝1 − R ⎠

for the electrical conductivity data in the in-plane direction are even lower than those in the thickness direction. 3.3. EMI Shielding Property. In general, EMI shielding effectiveness (SE) of a material is expressed in decibel (dB). A SE value of 30 dB, corresponding to 99.9% attenuation of a EM radiation, is considered an adequate level of shielding for many applications.31−33 SE of a material can be described by coefficients of reflectance (R), absorbance (A), and transmittance (T). The coefficients T and R can be drawn from magnitudes of S-parameters S11 and S21 as shown in eq 5, and A can be drawn from the law of energy conservation in eq 6: |S11|2 =

PR = R, PI

A=1−R−T

|S21|2 =

PT =T PI

(PR + PA + PT = PI)

⎛ T ⎞ ⎟ SE T = −10log T = −10log(1 − R ) − 10log⎜ ⎝1 − R ⎠ = SE R + SEA

(9)

SET is strongly related to the physical parameter, skin depth δ (=1/ πfμσ ), where f is the frequency of microwave, and μ and σ are the magnetic permeability and the electrical conductivity of the material, respectively. The skin depths of S-1 and S-30 are ∼7.0 mm and ∼230 μm, respectively, but the thicknesses of the materials are ∼160 μm, which means all the AgNW/ cellulose papers can be treated electrically thin (t/δ ≤ 1.3). For the electrically thin materials, SET can be theoretically predicted by using eq 10:34

(5) (6)

where PI, PR, PA, and PT are the incident, reflected, absorbed, and transmitted powers of an electromagnetic wave, respectively. SET is defined as the transmitted power quantity in decibels (dB). Therefore, SET can be described by the sum of SER and SEA defined as eqs 7 and 8: SE R = −10log(1 − R )

(8)

⎛ Z tσ ⎞ SE T_Cal = 20log⎜1 + 0 ⎟ ⎝ 2 ⎠

(10)

Figure 8 represents EMI shielding characteristics of AgNW/ cellulose papers. Figure 8A shows the dependence of the coefficients, R, A, T, on the cycle number of dip-dry process

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Figure 9. SET values (at 1.0 GHz) of AgNW/cellulose papers as functions of (A) electrical conductivity and (B) density in comparison with those of other materials in literatures. (C) Schematic EMI shielding mechanism for AgNW/cellulose papers with thin or thick AgNW coating layers.

and AgNW/polyimide foams.37,43 Therefore, the SET values of all the materials in the references are calculated at 160 μm thickness and at 1.0 GHz in eq 10, and the results are illustrated in Figure 9. Figure 9A shows that all the calculated SET values of the AgNW/cellulose papers as well as the other materials are proportional to the electrical conductivity, which just obeys eq 10. On the other hand, Figure 9B reveals that the AgNW/ cellulose papers can be lighter but more effective in EMI shielding, compared with other materials. Although the SET value of 26.7 dB for S-5 paper is comparable to that (26.5 dB) of MWCNT/cellulose, the density (0.52 g/cm3) of S-5 is even lower than that (0.76 g/cm3) of the material. In addition, a high SET value of 43.4 dB is achieved for S-30 paper with a relatively low density of 0.54 g/cm3. Overall, high SET values for the AgNW/cellulose paper can be easily attained with just a little sacrifice of the density even at many dip-coating cycles. CNTbased composites covered a wide range of electrical conductivity.36−38 Among them, the electrical conductivity of 100 S/cm as well as the calculated SET value of 49.6 dB was obtained for a rubber composite including 15.8 wt % supergrown SWCNT with a few millimeters in length and a few nanometers in diameter. Our calculation shows that S-50 paper can have EMI shielding of 52.2 dB on the basis of the same mass (same area and higher thickness of 322.2 μm = (tAgNW/cellulose × dSWCNT/rubber)/dAgNW/cellulose = (164.2 μm × 1.04 g/cm3)/0.53 g/cm3, which exceeds that of the supergrown

and electrical conductivity. Figure 8B displays the SER, SEA, and SET calculated in eqs 5−9. An increase in the cycle number of dip-coating led to an increase in electrical conductivity, which induced an increase in SE Figure 8A exhibits there was a rapid increase in the reflectance, R, at three dip-coating cycles, which means the dominant shielding mechanism changed from absorbance to reflectance around the number of cycles. The apparent electrical conductivity and SET at three dip-coating cycles are ∼3.19 S/cm and ∼22.8 dB, respectively. This indicates that the dip-coating process is very efficient in forming a conductive network of AgNWs on cellulose fibers. A SET of 48.6 dB is achieved at 50 dip-coating cycles where the electrical conductivity is 67.51 S/cm. Figure 8B includes the SET values calculated by eq 9, which exhibits a good agreement with the measured data. Figure 8C indicates the SET of the conductive cellulose papers is independent of frequency, as anticipated in eq 10. The SET values of the AgNW/cellulose papers can be quantitatively compared with referred materials in terms of electrical conductivity and density, as shown in Table 2 and Figure 9.6,35−43 All the materials in the comparison can be considered to be electrically thin, as the skin depths are satisfied by the criteria, t (160 μm)/δ ≤ 1.3. All the references include electrical conductivity data, but some of them do not possess SET values. Also, measured frequency ranges are different from that of this study,6,35,38,39,42 except SWCNT/epoxy composites 13130


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67.51 S/cm exhibits high EMI SE of ∼48.6 dB at 1 GHz. Overall, it was revealed that highly flexible and low density AgNW/cellulose papers with a hierarchical structure generated by a dip-coating process could be utilized as electrically conductive elements and EMI shielding materials in advanced application fields.

SWCNT-reinforced rubber composite, even though AgNWs with ∼18.2 μm in average length and ∼19.5 nm in average diameter used in this study have less surface area than the supergrown SWCNTs. Conductive aerogels had a relatively low shielding performance due to the low density (< 0.2 g/ cm3).39−41 The shielding performance of metal-wire/polymer composites was also evaluated to have high SET values.6,42 The high densities of the metal wires (dAg = 10.49 g/cm3, dCu = 8.96 g/cm3) and polymer matrix (∼1.0 g/cm3) turned out to be a disadvantage over porous AgNW/cellulose composite. A porous AgNW structure was also compared with our conductive cellulose papers.43 AgNW/PI foam had 0.017 g/ cm3 and 5.4 dB in measured density and SE, but its thickness was 5 mm. An additional AgNW spray-coating on both outer surfaces enhanced SE to 17.0 dB with 0.022 g/cm3 in density, which is illustrated in Figure 9B. Therefore, the cellulose papers with a quasi-2-dimensional conductive network of AgNWs can be good candidates for EMI shielding applications in case that space is not a critical factor for installation. It is highly conjectured that the high SET values of the AgNW/cellulose papers are attributed to the high intrinsic electrical conductivity of AgNW, as well as the conductive network of AgNWs formed in cellulose papers by the dipcoating process that also causes the gradient of AgNW density in the thickness direction, as illustrated in Figure 9C. The schematic distribution of AgNWs in the cellulose papers is drawn from the SEM images in Figure 2 and the anisotropic electrical conductivity data in Table 1. Therefore, the AgNW/ cellulose paper may be simplified as a three-layered structure consisting of two highly conductive AgNW surface layers with high practical electrical conductivity values (Table 1) and a cellulose layer as a spacer between two outer surfaces. Typically cellulose has a dielectric constant of 3−4.44 The wavelength in the cellulose changes to λ = λ 0 / εr ≈ 30/ 4 = 15 cm at 1 GHz. The order difference in the wavelength and the paper thickness is ∼103. Therefore, the surface localization of AgNWs with the practical electrical conductivity at the thickness increment of 9.7 μm is expected to create a same order of EMI shielding performance, compared to AgNW/cellulose with the apparent electrical conductivity.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.6b02218. SEM and TEM images of AgNWs and FT-IR spectra of AgNWs and neat PVP. (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*Tel.: +82-42-821-6617. E-mail: [email protected]. *Tel.: +82-31-8061-1105. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by a grant from the Fundamental R&D Program for Technology of World Premier Materials funded by Ministry of Trade, Industry & Energy, Republic of Korea.

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REFERENCES

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4. CONCLUSIONS In summary, a series of AgNW-coated cellulose papers with high flexibility, a low apparent density of ∼0.53 g/cm3, and a high porosity of ∼64.7% were fabricated via an efficient and simple dip-coating process, and their microstructural features, electrical property, and EMI shielding performance were investigated systematically by taking into account the dipcoating cycle. SEM images exhibited that AgNWs are coated dominantly on the cellulose fibers of paper surfaces, but they existed partially in the inner parts of cellulose papers, which indicates that the AgNW density decreases gradually in the thickness direction of the AgNW/cellulose papers. It was confirmed by the anisotropic apparent electrical conductivity of the AgNW/cellulose papers in the in-plane or thickness direction. Accordingly, relatively high apparent electrical conductivity in the in-plane direction of 0.34 S/cm was attained even for S-1 paper with only 0.01 vol % AgNW. In addition, the apparent electrical conductivity of the AgNW/ cellulose papers in the in-plane direction increases significantly from 0.34 S/cm for S-1 to 67.51 S/cm for S-50. As a result, the AgNW/cellulose paper with apparent electrical conductivity of 13131


Research Article


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Highly Effective Electromagnetic Interference Shielding Materials

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