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Article
Superhydrophobic and Corrosion-Resistant Electrospun Hybrid
Membrane for High-Efficiency Electromagnetic Interference
Shielding
Mei Yang, Xiaoteng Jia,* Dayong He, Yuying Ma, Ya Cheng, Jing Wang, Yongxin Li,* and Ce Wang*
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ABSTRACT: Superhydrophobic electromagnetic interference (EMI) materials are becoming increasingly important to the longterm service of outdoor all-weather electrical equipment. It is an urgent need to prepare flexible and robust high-performance EMI
shielding materials to work in harsh environments. To this end, we demonstrate a delicate structure design of superhydrophobic EMI
shielding material that possesses desired properties via chemical deposition of silver nanocluster on electrospun polymer nanofibers
followed by stearic acid (SA) modification. The porous electrospun hybrid membrane with a spatially distributed silver coating
enabled excellent electrical conductivity up to 57 319 S cm−1. Notably, superior EMI shielding effectiveness (SE) of 90.14 dB in an
ultrabroadband frequency range is achieved in conjugation with the specific shielding effectiveness (SSE/t) of 14 253 dB cm2 g−1,
owing to the combined effects of favorable porous structure and interfacial polarization. The thin coating of the SA layer endowed
the film with superhydrophobicity (water contact angle up to 156.7°) and superior corrosion resistance with only 6.56% loss in EMI
SE after 40 days incubation in the salt spray tank. The integrated functionalities being achieved in the hybrid membrane, such as high
resistance to mechanical deformation (3.55% loss in EMI SE after 2000 times of bending), self-cleaning property, long-term (12
months) performance stability under high mechanical and chemical tolerance, offer great promise for outdoor all-weather electronic
equipment under harsh environments.
KEYWORDS: electrospinning, silver nanoclusters, electromagnetic interference shielding, corrosion resistant, hydrophobic robust application
1. INTRODUCTION
other harsh service conditions (wet or corrosive conditions).11,12
Generally, the performance of EMI shielding materials is
dominated by material microstructure, thickness, and electrical
conductivity.3,5 Conventional metallic-based material is a good
choice because of its high electrical conductivity. However, the
poor flexibility and ease to be corroded hinder its application in
Due to the rapid development of modern communication
technology and the widespread use of electronic equipment,
electromagnetic wave pollution has become the fourth public
hazard after noise, air, and water pollution.1−5 Artificial
electromagnetic noise not only threatens human health but
also seriously interferes with the operation of precision
instruments.1,6−8 Electromagnetic interference (EMI) shielding
materials possessing lightweight, mechanical flexibility, and
superhydrophobicity are ideal to protect human beings and
outdoor all-weather electronic equipment such as signal stations,
outdoor electromagnetic devices, etc.9,10 One major concern is
to enhance both the efficiency and long-term reliability of EMI
shielding material under external mechanical deformations or
© XXXX American Chemical Society
Received: January 22, 2021
Accepted: April 14, 2021
A
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Article
Scheme 1. Schematic of the Preparation of APAN-Ag-SA-T (0.5−2.5 h) Membranes through a Series of Reactions, Including the
Electrospinning of PAN Nanofiber, Alkali Etching of the Nanofiber, Electroless Deposition of Ag Nanoclusters, and SA
Modification
humid high-salt environments such as coastal areas.11,13−15
Embedding or coating lightweight conductive fillers, such as
carbon-based materials, two-dimensional nanomaterials, and
conducting polymers, with nanostructured polymer matrix have
become popular recently.12,16−19 However, the interfacial
interaction between the polymer matrix and conductive fillers
are vulnerable to be deterred under long-term exposure to
extreme conditions.
Constructing a superhydrophobic surface on EMI shielding
materials cannot only effectively endow the film with multifunctionality, such as being self-cleaning, the electro-photothermal effect,20 waterproof, antibacterial,21 and corrosion
resistant, but also maintain EMI shielding effectiveness.5,19,20,22−24 Currently, considerable efforts have been
devoted to the rational design of superhydrophobic EMI
shielding materials by depositing conductive fillers on various
substrates, such as nylon textiles,23 polymer fabrics,12,21,24,25
carbon aerogels,18 followed by the treatment with low-surface
energy materials. Nevertheless, the performance-tested only
within the X-band frequency range (8.2−12.4 GHz) is still
unsatisfactory owing to the low electrical conductivity (lower
than 1000 S cm−1).26−28 Apart from that, the long-term
reliability test under both the repeated stretching/bending
cycles and corrosive conditions remains rare.
Recent work has demonstrated the foam structure with a high
porosity decorated with metal nanoparticles is crucial to create
effective electrical joints and 3D networked pathways for
electron movement, thereby leading to high EMI SE.29−31 The
freestanding electrospun polymer nanofiber-based membrane
featured with high porosity and conjunction structure is ideal for
the efficient absorption and multireflection of the incident
electromagnetic wave inside the material.1,5 Therein, we
developed a flexible, mechanically durable, superhydrophobic,
and corrosion-resistant EM shielding membrane (Scheme 1) by
the effective exploitation of electrospun nanofiber coated with
electroless plating of Ag-nanoclusters followed by SA
modification. An alkali etching treatment is utilized to engineer
the surface wettability and assist metal deposition, endowing the
effective wrapping of Ag-nanoclusters on high conjugation
electrospun polymer nanofibers.32 Further post-treatment with
low-surface energy material enables high shielding effectiveness
(average value above 90 dB) while ensuring superior long-term
resistance to corrosion environments and mechanical deformation.
2. MATERIALS AND METHODS
Polyacrylonitrile (PAN, Mw = 150 000) was provided by Shanghai
Macklin Biochemical Technology Co., Ltd. Silver nitrate (AgNO3, AR,
99.0%), citric acid (C6H8O7), sodium potassium tartrate (KNaC4H4O6·4H2O), sodium hydroxide (NaOH), and sodium borohydride
(NaBH4) were supplied by Sinopharm Chemical Reagent (China).
Dimethylformamide (DMF) and ethanol (C2H5OH) were used as
received from Tianjin Tiantai Refined Chemicals Co., Ltd. Ethylene
glycol (C2H4O2) and ammonia solution (NH3, 25%) were purchased
from Beijing Chemical Works. All the reagents were directly used
without further purification.
2.1. Preparation of PAN Electrospun Nanofiber. A total of 1.70
g of PAN (Mw = 150 000) was dissolved in 18.3 g of DMF by stirring at
room temperature for 5 h to form a homogeneous and viscous
precursor solution. Then, the spinning precursor solution was
transferred to a 20 mL syringe with a 21g blunt needle and connected
to the positive electrode equipped with a high voltage power supply
(Gamma High Voltage Research, Ormond Beach, FL; dc power
supply). The aluminum foil covered collector was connected to the
negative electrode. In the process of electrostatic spinning, the work
distance between the cathode and anode was fixed at 20 cm, the voltage
was set at 17 kV, the syringe spinning liquid propulsion speed was 0.6
mL/h, and the rotation speed of the roller was 40 m/min. At last, the
obtained PAN electrospun nanofiber membranes were placed into the
vacuum oven at 45 °C for 6 h.
2.2. Preparation of the APAN Nanofiber Membrane via Alkali
Etching. The PAN nanofiber membranes were washed with deionized
water at 85 °C for 15 min and then dried at 60 °C for 30 min. Then, the
clean membranes were immersed in 1 mol/L sodium hydroxide
solution at 125 °C for 60 min, followed by rinsing with a large amount
of distilled water until the surface pH reached 7. Finally, they were dried
in a 60 °C oven for 6 h. Thus, the alkali etched PAN membranes were
obtained and denoted as APAN.
2.3. Synthesis of Ag Seeds on the Surface of APAN. The
obtained APAN nanofiber membranes were cut into a rectangle of 4 × 2
cm2. In the first step, they were soaked in 100 mL of aqueous silver
nitrate solution (0.1 mol/L) for 24 h. In the second step, after washing
with distilled water, the APAN nanofiber membranes were put into a
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beaker containing 2 mol/L glucose ethanol solution for reduction.
Finally, the APAN@Ag seeds membranes were obtained.
2.4. Chemical Deposition of Ag-Nanoclusters and Superhydrophobic Modification. The APAN@Ag seeds membrane was
immersed in the Tollen’s reagent (composed of 2.0 wt % AgNO3
solution and excessive NH3·H2O) under mechanical stirring until the
suspension was clarified. Then, 0.8 g of citric acid as a morphology
control agent was added to the mixture. To this suspension, a freshly
prepared reductant solution of NaBH4 (2.0 mmol, 10 mL) was instilled
dropwise with mechanical stirring for 0.5−2.5 h at room temperature.
The synthetic hybrid membrane was named APAN-Ag-T, where T
stands for deposition times. Subsequently, the superhydrophobic
modification process was conducted by dipping the Ag-nanoclusters
germinated APAN-nanofibers in a stearic acid (SA) ethanol solution
(0.01 mol, 100 mL) for 1 h and then rinsed with deionized water and
finally dried at room temperature. The corresponding superhydrophobic electrospun membrane was denoted as APAN-Ag-SA-T.
2.5. Characterizations. Morphologies and microstructures of the
prepared membranes coated with a thin layer of platinum were
observed by field-emission scanning electron microscopy (FESEM, FEI
Nova NanoSEM) at an acceleration voltage of 15.0 kV. The phase and
the crystallographic structure of the products were detected by the
advanced diffractometer (XRD, Rigaku D/Max) with Cu Kα radiation,
and the 2θ angle of the diffractometer ranged from 20° to 90°. The
optical contact angle (CA) measuring device (OCA20) was used to
measure CAs, and a 4 μL droplet was chosen for each CA measurement.
The water sliding angle was tested using the tilting plate method. The
final CAs were calculated by averaging the values at five different
locations from the surface. The chemical compositions were analyzed
via X-ray photoelectron spectrometer (XPS, ESCALAB250, Thermo
Scientific) with an Al Kα X-ray source. The mechanical properties of the
hybrid membranes (dimensions: length = 20 mm, width = 5 mm) were
measured by an AGS-H tensile tester (Shimadzu Corporation, Kyoto,
Japan) at a crosshead speed of 30 mm/min. The tensile strength and
elongation at the break were evaluated using stress−strain curves. The
electrical conductivity (σ) of APAN-Ag-T membranes (size of 1.0 cm ×
1.0 cm) was evaluated with a four-point probe instrument (RTS-2,
Guangzhou Four Probe Technology Co., Ltd.). The thermal
gravimetric analysis (TGA) was used to determine the amounts of
Ag in the APAN-Ag-T membranes, which was heated from 25 to 700 °C
at a heating rate of 10 °C/min under an air atmosphere.
2.6. EMI Shielding Performance Measurements. The electromagnetic interference shielding effectiveness (EMI SE) values were
measured via a vector network analyzer (ZV3672B-S, China Electronics
Technology Instruments Co., Ltd.) in the X, Ku, and K bands of 8−26.5
GHz frequency range at room temperature, respectively. Before testing,
each band was checked and debugged using the corresponding
rectangular waveguide adapter to ensure accuracy.
EMI SE was used to calculate the material’s ability to attenuate the
electromagnetic waves, and it is defined as the logarithm of the ratio of
incident power (Pi) and transmitted power (Pt) of an electromagnetic
wave in decibels (dB), as shown in the eq 1.
EMI SE = 10 log
Pi
(dB)
Pt
T = |S12|2 = |S21|2
(3)
A=1−R−T
(4)
According to the above description, electromagnetic shielding mainly
includes reflection loss (SER), absorption loss (SEA), and inside
multiple reflection loss (SEM) of part, so the total shielding efficiency
(SET) can be expressed as
SE T = SE R + SEA + SEM
(5)
Because the electromagnetic waves are reflected many times inside the
material, they are eventually absorbed and dissipated as heat. When SET
≥ 15 dB, the SEM can be negligible. Therefore, the total electromagnetic
shielding efficiency (SET) mainly includes reflection (SER) and
absorption (SEA), namely,
SE T = SE R + SEA
(6)
ij |S21|2 yz
i T zy
zz
zz = − 10jjj
SEA = − 10 logjjj
j 1 − |S |2 zz
k1 − R {
11
{
k
(7)
SE R = − 10 log(1 − R ) = − 10 log(1 − |S11|2 )
(8)
Finally, SEA and SER are attained by using the following equations:
2.7. Corrosion Resistance. First, a Q-fog circulating salt Fog
corrosion tester (CCT, Q-Lab Company) was used to simulate the
corrosion resistance of samples in the marine environment. The test
reagent was dissolved in distilled water with sodium chloride (50 ± 5 g/
L). The pH value was 6.5−7.2, the temperature of the salt spray tank
was controlled at 35 ± 2 °C, and the spraying pressure was controlled
within 8 psi. The test time was 0−40 days for continuous spraying. After
the salt spray test was started, the corrosion of samples was recorded
regularly.
3. RESULTS AND DISCUSSION
3.1. Preparation of Ag-Nanocluster Wrapped PANNanofiber Membrane. Scheme 1 demonstrates the preparation of electrically conductive and superhydrophobic membrane
through a series of reactions, including the electrospinning of
PAN nanofiber, alkali etching of the nanofiber, electroless
deposition of Ag nanoclusters, and SA modification. First, the
PAN nanofiber membrane was obtained via an electrospinning
technique, followed by soaking into NaOH solution to induce
the formation of the superwetting surface via the alkali etching
treatment. The alkali etching herein plays two roles.33 First,
NaOH forms a rough membrane surface and improves the
wettability of the PAN membrane. Besides, NaOH induces the
in situ hydrolysis of −CN groups into −COOH groups and
ultimately realizes the hydrophilicity of the PAN membrane
(Figure S1a). The pure PAN nanofiber (Figure S1b) exhibits
typical −CN peaks at 2243 cm−1. After the alkali etching
treatment, the −CN peak was weakened significantly and
peaks at 1619 and 1388 cm−1 corresponding to the −CO
group appeared, highlighting the formation of the carboxylic
acid group. The etching treatment has also been observed by the
color change of the membranes from white to yellow (Figure
S1c).
Metal-based materials with exceptional intrinsic conductivity
show excellent EMI shielding performance.34 The high bonded
coating on the polymer nanofiber is achieved via electroless
plating of Ag seeds, which can provide nucleation active sites on
the nanofiber surface and aid in the subsequent growth of metal
particles.35 Ag seeds are adsorbed on the surface of APAN with
the carboxylic acid group, followed by treatment with reducing
agent glucose (named as APAN@Ag seeds). The Ag seeds have
been confirmed with the XRD pattern (Figure S2). Afterward,
we used citric acid as the shape control agent to regulate the
(1)
When the electromagnetic wave propagates to the shielding material,
the following three processes will occur: First, part of the electromagnetic wave will reflect on the surface of the material, which largely
depends on the impedance matching between the shielding material
and the external space medium. The remaining electromagnetic waves
will enter the material, some of which will be absorbed by the material
and dissipated in the form of heat, and some of which will generate
multiple reflections and absorption at the defects or interfaces inside the
shielding material and finally dissipate in the same way. Finally, the
remaining electromagnetic waves pass through the material.1
The reflection coefficient (R), transmission coefficient (T), and
absorption coefficient (A) were calculated by the S parameters (S11 and
S21 or S12 and S22) according to the following equations:
R = |S11|2 = |S22|2
Article
(2)
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XRD analysis is presented in Figure 2 to ascertain the crystal
structure of APAN-Ag-T and APAN-Ag-SA-T membranes with
different chemical deposition times. Five diffraction peaks at
38.42°, 44.53°, 64.68°, 77.69°, and 81.79° were ascribed to the
(111), (200), (220), (311), and (222) crystal planes of the Ag
diffraction peak (JCPDS No. 04-0783), respectively (Figure
2A). The results indicate that the prepared Ag cluster has a facecentered cubic structure with a high crystallinity-centered cubic
of Ag.36 Meanwhile, the intensity of the diffraction peak
increases with the extension of chemical deposition times,
revealing that the deposition amount of Ag is directly
proportional to time. After 2 h of deposition, the intensity of
the diffraction peak does not change anymore, indicating the Ag
particles growth on the fiber reaches saturation. After the SA
modification (Figure 2B), the diffraction peaks of (111) are
slightly lower compared with APAN-Ag-T. It can be inferred
that SA can protect the surface of Ag nanoclusters and prevent
their corrosion.
XPS is utilized to further verify the chemical composition and
oxidation states of APAN-Ag-2.0 and APAN-Ag-SA-2.0
membranes. As Figure 2C−H emerged, two main peaks located
at 368.45 and 374.45 eV are observed, assigned to Ag 3d5/2 and
Ag 3d3/2, respectively, indicating the formation of metallic Ag.32
The curve-fitted C 1s core energy level spectrum contains a
strong peak at a binding energy of 284.8 eV. Meanwhile, the C 1s
spectral peak intensity of APAN-Ag-SA-2.0 is enhanced,
indicating the influence of the introduction of long-chain alkyl
groups on the membrane. To explore the thickness of the stearic
acid layer, TEM is utilized to further verify the APAN-Ag-SA-2.0
membrane (Figure S5). The Ag nanoparticles were uniformly
and tightly distributed on the membrane. The thickness of the
stearic acid layer is in the range of 3−8 nm.
3.3. EMI Shielding Performance and Stability under
Different Environments. Excellent electrical conductivity is
the key factor to obtain high EMI SE.37 Figure 3A presents that
the surface electrical conductivities can be controlled by
adjusting the deposition time. When the deposition time is 0.5
h, Ag nanoparticles grow into small isolated crystals, yielding a
conductivity of only 58.53 S cm−1. When the deposition time is
prolonged to 1 h, the conductivity was 5602.95 S cm−1 and the
Ag content jumped from 13.58% to 61.16% from 0.5 to 1 h. This
trend matches with the results obtained from TGA and SEM. In
contrast, the conductivity increased by over 1000-fold to
60856.34 S cm−1 after 2 h deposition with the coalescence of
Ag crystallites into a continuous layer. It should be pointed out
that the conductivity of SA-modified samples (APAN-Ag-SA2.0) is 57319.2 S cm−1, with only a 5.81% decrease compared
with APAN-Ag-2.0. The high conductivity is among the best
values for EMI shielding materials even at a relatively low density
(2.04 g cm−3), attributed to the continuous 3D Ag network
formed by the efficient exploitation of the electrospun nanofiber
structure. The alkaline etching enables strong conjugation of Ag
nanoclusters rather than loose contact, significantly reducing the
contact resistance among the metal layers in the nanofiber.
Moreover, the inset of Figure 3A shows an optical image of an
APAN-Ag-SA-2.0 membrane as a conductor to light an LED
bulb with a total power of 3 W under an external voltage of 9 V.
To verify that the nanofiber films retain a stabilized electrical
conductivity under harsh environments, we separately immersed
APAN-Ag-SA-2.0 films in simulated acid rain (pH = 5.5),
simulated seawater (3.5% NaCl solution), alkaline (1 M
NaOH), acid (1 M HCl), and DI water solutions for 6 h and
then picked them up for electrical conductivity testing. We also
particle diameter. Ag nanoparticles have uniformly adhered to
APAN via the chemical deposition process. The reaction times
(T) were set as 0.5, 1.0, 1.5, 2.0, and 2.5 h, respectively (APANAg-T). Moreover, a green modification with SA is utilized to
engineer the surface hydrophobicity and decrease surface
energy, yielding the final hydrophobic membrane APAN-AgSA-T.
3.2. Characterization of Ag-Nanocluster Wrapped
PAN-Nanofiber Membrane. Figure 1 shows the morphology
Figure 1. SEM images of (A) the electrospun PAN nanofibers, (B)
alkali etching treatment of PAN nanofibers, (C) Ag-seeds wrapped
APAN nanofibers, and (D) APAN-Ag-0.5, (E) APAN-Ag-1.0, (F)
APAN-Ag-1.5, (G) APAN-Ag-2.0, (H) APAN-Ag-2.5, and (I) APANAg-SA-2.0 hybrid membranes.
of the PAN nanofibers and the membranes in the subsequent
processing, including APAN nanofibers, APAN@Ag seeds
nanofibers, APAN-Ag-T (T = 0.5, 1.0, 1.5, 2.0, and 2.5 h)
nanofibers, and APAN-Ag-SA-2.0 nanofibers. PAN nanofibers
with a smooth and round surface are chaotically distributed with
diameters ranging from 120 to 220 nm (Figure 1A). The APAN
nanofibers show similar diameter distribution with twisted
structure after the alkali etching, resulting in the decrease of pore
size and the formation of a staggered network structure. As
shown in Figure 1C−H, only small-sized Ag nanoparticles are
immobilized on the nanofiber surfaces in the first 0.5 h of
chemical deposition. With the extension of deposition time, the
particles become more compact and assembled into the
nanoclusters (Figure 1G). The fiber maintains the porous
structure with an increased diameter when the accumulation of
Ag nanoparticles is continuous (the specific diameter distribution is shown in Figure S3). Finally, after the hydrophobic
treatment with SA, an ultrathin molecular film can be formed on
the surface of the Ag nanocluster membrane which maintained
the rough structure (Figure 1I). The SEM images showed that
Ag nanoparticles were uniformly and tightly distributed on each
fiber. A high conductive three-dimensional network was formed
for efficient electron transport, thus providing the basis for EMI
shielding application. The TGA (Figure S4) result demonstrates
that the weight percentage of Ag in the corresponding fiber
membrane was 13.58%, 61.16%, 71.26%, 79.19%, and 83.42%
for 0.5, 1.0, 1.5, 2.0, and 2.5 h deposition, respectively.
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Figure 2. Characterization and surface composition of the electrospun hybrid membranes. XRD patterns of APAN-Ag-T (A) and APN-Ag-SA-T (B)
membranes (T = 0.5−2.5 h). (C) XPS wide-scan spectra of APAN-Ag-2.0 membrane and the corresponding Ag 3d (D) and C 1s (E) core-level spectra.
(F) XPS wide-scan spectra of APAN-Ag-SA-2.0 membrane and the corresponding Ag 3d (G) and C 1s (H) core-level spectra.
of the APAN-Ag-2.0 membrane reaches a value up to 92.04 dB,
indicating more than 99.999999% incident radiation is
blocked.39 After the SA modification, the EMI SE value remains
at 90.14 dB. When the time is extended to 2.5 h, the conductivity
increases slightly to 61671.8 S cm−1 with the EMI SE value of
93.57 dB.
Generally, the total shielding effectiveness (SET) includes the
absorption loss (SEA) and reflection loss (SER), related to the
multireflection at the interface, electric dipoles, and mobile
charge carriers.5 To explore the EMI shielding mechanisms, the
average values of SET, SEA, and SER at three bands (X-band, Kuband, and K-band) were intuitively plotted in Figure 3D. It can
be seen that the absorption loss contributes the most to the
shielding performance for APAN-Ag-T (0.5−2.5 h) membranes.
Regardless of the testing frequency range and deposition times,
SEA is the dominant factor, which contributes to the highest
proportion of 78.98% for the total SET. The proportion is the
conducted a 12 month long-term placement experiment to
further explore the stability of its electrical conductivity. The
conductivity stability (Figure 3B) shows APAN-Ag-SA-2.0
placed under different conditions exhibits no significant
decrease compared to the original sample after 12 months.
EMI SE refers to the reduction of electromagnetic
interference and is defined as the logarithmic ratio of incident
to the transmitted power.1,38 The EMI shielding ability of
APAN-Ag-T (0.5−2.5 h) hybrid membranes (average thicknesses of 38 μm) are conducted in an ultrabroadband frequency
range of 8 to 26.5 GHz, including the X-band, Ku-band, and Kband, respectively. Figure 3C illustrates the total shielding
effectiveness (SET) increased with longer chemical deposition
time. Besides, there is a strong correlation between the EMI SE
and conductivity. When the reaction time is 1 h, the EMI SE
value reaches about 20.1 dB, meeting the commercial demand of
consumer electronic products (99% attenuation). The EMI SE
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Figure 3. (A) Electrical conductivity and density of APAN-Ag-T (0.5−2.5 h) and APAN-Ag-2.0-SA hybrid membranes (n = 5, bars represent the
standard deviation). (B) Comparison of the electrical conductivity under different treatment conditions (n = 5, bars represent the standard deviation).
(C) EMI SE of APAN-Ag-T (0.5−2.5 h) hybrid membranes with an average thickness of 38 μm in a broadband frequency range of 8−26.5 GHz. (D)
Total EMI SE (SET) and its absorption (SEA) and reflection (SER) of the APAN-Ag-T (0.5−2.5 h) hybrid membranes in the X-band, Ku-band, and Kband.
quotient of the sum of the mean SEA values obtained at all bands
and different processing times and the sum of the corresponding
SET values (SEA/SET).
The high microwave absorption and low reflection indicate an
absorption-dominated shielding mechanism (Figure 4A). This
can be attributed to the efficient microstructure design, taking
advantage of the interlaced Ag layer, interconnected pores, and
interfacial polarization derived from the interface between the
Ag layer and insulating polymer nanofiber. When the membrane
is exposed to an incident EM wave, part of it is reflected by free
electrons due to the formation of surface plasmon resonance on
the surface of the metal cluster.29 Moreover, the remaining
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Figure 4. (A) Schematic description of the EMI shielding mechanism. (B) Specific EMI SE (SSE) and absolute EMI SE (SSE/t) of APAN-Ag-T (T =
0.5−2.5h) and APAN-Ag-SA-2.0 membranes. (C) Comparison of the shielding performance with other materials in terms of EMI SE and absolute
EMI SE (SSE/t).
Table 1. EMI Shielding Performance of Various Fabrics and Film-Based Shielding Materials
samples
EMI SE (dB)
density (g cm−3)
thickness (mm)
SSE (dB cm3 g−1)
MXene
MXene foam5
MXene/PDMS/BN41
RGO/CNT@epoxy/AgNW42
PPCB30 foam43
HTPAHs-based C−Cu44
GO/PPy coated wool45
PVDF/CNT/graphene46
AgNW/WPU foam47
PPy/Ag/PET fabric48
CF/PC/Ni film49
copper50
silver foil51
carbon fabric51
CPAN NF/Ag NPs-9051
APAN-Ag-2.0 (this work)
APAN-Ag-SA-2.0 (this work)
57
70
40
40
41
58.7
22.2
27.58
64
13.5
72.7
90.2
58.49
47.11
87.57
92.04
90.14
2.31
0.22
0.008
0.006
0.001
0.01
60
2.0
2.236
0.1
2.3
0.45
0.31
3.1
10
109
0.032
0.028
0.031
24.6
318
1
0.12
1.65
0.098
0.57
0.045
0.29
1.7
8.96
10.49
1.10
2.69
2.08
2.04
341.7
35.58
35.58
226.53
1422
46.55
42.76
10
5.58
42.83
32.93
44.25
44.18
SSE/t (dB cm2 g−1)
30844
53030
56.9
178
178
1557
10970
1034
1379
32
5575
3929
10289
15803
14253
shielding performance. We have compared the SE, SSE, and
SSE/t values of the prepared electrospun hybrid membranes in
Figure 4B. APAN-Ag-2.0 membrane with a density of 2.08 g
cm−3 and thickness of 28 μm reaches the maximum SSE and
SSE/t values of 44.25 dB cm3 g−1 and 15803 dB cm2 g−1,
respectively. Therefore, the chemical deposition time of 2 h was
waves that pass through the membrane will hit the high electron
density of Ag nanoclusters and produce ohmic losses, which will
cause the EM wave energy to drop.
Considering that the density and thickness are both key
factors affecting the EMI shielding performance, the practical
parameters, SSE and SSE/t, are used to evaluate the EMI
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Figure 5. (A) Stress−strain curves and (B) the corresponding Young’s modulus of PAN, APAN, APAN-Ag-2.0, and APAN-Ag-SA-2.0 membranes.
(C) Flexibility test and EMI SE changes of the APAN-Ag-SA-2.0 membrane during 2000 bending cycles.
the three hybrid membranes at three bands. After 12 months of
placement, APAN-Ag-SA-2.0 had an EMI SET loss of 7.9%,
1.9%, and 3.73% in the three bands, respectively. In the
calculation of SEA, SER, and SET of APAN-Ag-SA-2.0 sample,
the absorption part accounts for 78.2% of EMI, which is
consistent with the previous conclusion.
3.4. EMI Shielding Stability under Mechanical Bending. The membrane-based EMI shielding materials should have
enough mechanical strength for practical applications. As shown
in Figure 5A, the tensile strength of the APAN-Ag-T membrane
is continuously enlarged after chemical deposition; however, the
elongation at the break markedly decreases from 20.73% to
14.98% caused by surface defects of Ag nanoparticles on the
surface. After SA modification, the tensile strength of APAN-AgSA-2.0 slightly increases to 25.23 MPa. The Young’s modulus
corresponding to the four membranes is shown in Figure 5B.
Young’s modulus of APAN-Ag-SA-2.0 has been increased from
the original 106.3 to 196.23 MPa. The result manifests that Ag
nanoparticles act as a reinforcing filler for PAN membranes. The
dense cross-linked network decorated with Ag nanoparticles not
selected as the optimal parameters. The EMI performance
parameters under different sedimentary times were displayed in
Table S1. As listed in Figure 4C (details in Table 1), the
mechanical flexibility and easy coating capability offered by
MXenes and their composites provide high EMI shielding
efficiency.40 Compared with other electromagnetic shielding
materials, the electrospun hybrid membranes demonstrate
much higher shielding performance in an ultrabroadband
frequency range than most other shielding architectures with
similar or even larger thickness, including metal-based, carbonbased, and hybrid shielding materials.42−51 In the realization of
its superhydrophobic modification, stearic acid is nontoxic and
biocompatible as a modification. The sample has excellent
electromagnetic shielding performance and can achieve fullband shielding at the same time. Accordingly, the SSE and SSE/t
of APAN-Ag-SA-2.0 were slightly reduced to 44.18 dB cm3 g−1
and 14253 dB cm2 g−1, mainly due to the increase in thickness
(31 μm) after SA modification. The durability of the hybrid
membranes is also an important parameter in practical
applications. Figure S6 shows the EMI shielding stability of
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Figure 6. Surface wetting measurements under various conditions. (A) Water contact angle and (B) the corresponding images of the pure PAN and
superhydrophobic APAN-Ag-SA-T (0.5−2.5 h) hybrid membranes. (C) Contact angles of APAN-Ag-SA-2.0 with different droplets (n = 5, bars
represent standard deviation); inset shows the contact angle images. (D) SEM and optical images of different droplets on APAN-Ag-2.0 (hydrophilic
membrane) and APAN-Ag-SA-2.0 (superhydrophobic membrane).
is 87.6°. With the extension of reaction time and SA
modification, Ag nanoparticles gradually form hierarchical
nanoclusters, and the surface roughness increases, yielding an
increased WCA of 156.7° for 2 h deposition, realizing a
superhydrophobic surface. When the deposition time is 2.5 h,
the core−shell structure of Ag clusters formed between the fibers
and metal gradually become smooth, and the roughness
decreased obviously with a WCA of 153.2°. This phenomenon
can be observed by the SEM micrograph shown in Figure 1H.
Here, we mainly simulated the ocean and acid rain
environment and explored the droplet states under strong
acid/alkali conditions (Figure 6C). The contact angle value is
still greater than 150°, indicating their stability against corrosive
environments. We also tested the droplet states of APAN-AgSA-2.0 over a wide range of pH (1−14) (Figure S7) and found
that the WCAs of the droplet remained above 150° for all
samples. Figure 6D demonstrates the morphologies of SA
modified (left) and unmodified (right) samples. The modified
sample surface is covered with a white thin layer with reduced
roughness. The optical image (middle) shows the state
only improves the electrical conductivity but also greatly
increases the mechanical properties. The excellent flexibility
and stability of APAN-Ag-SA-2.0 are proven by bending and
folding tests (Figure 5C). The optical images show the
electrospun membranes can endure repeated bending without
any fragmentation. Especially, EMI SE still maintained 87.68 dB
even after 2000 cycles of bending, which is 3.55% lower than the
initial value. All in all, the membrane displays outstanding
shielding stability and durability against repeat folding.
3.5. Water Contact Angle Measurements and SelfCleaning Behavior. Generally, the construction of a superhydrophobic surface is inseparable from the abundant roughness
and low surface free energy.22,52 The moderate growth of Ag
nanoclusters could increase roughness. Meanwhile, SA has a
long alkyl chain, low surface energy, and capillary effect, which
can significantly reduce the free energy of the membrane surface,
thus ensuring superhydrophobicity.
As shown in Figure 6A, the wettability of APAN-Ag-T-SA
(0.5−2.5 h) membranes was detected by water contact angle
(WCA) measurements. The WCA for the pure PAN membrane
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Figure 7. Corrosion resistance measurements of the superhydrophobic APAN-Ag-SA-2.0 membrane. (A) Contact angle change (n = 5, bars represent
standard deviation) and (B) EMI SE change in the salt spray tank environment for different days. (C) Comparison of APAN-Ag-2.0 (hydrophilic
membrane) and APAN-Ag-SA-2.0 (superhydrophobic membrane) placed in the salt spray tank for 40 days.
days, the conductivity of the sample was 40218.24 S cm−1, which
was reduced by 29.71% compared with the initial sample. The
performance loss of the SA modified sample was much lower
than that of the unmodified sample. The above experiments
prove that the SA modification can prevent the oxidation and
corrosion of the Ag nanoclusters and endow the hybrid
membrane with reliability in the harsh outdoor environment.
differences of different droplets on the two surfaces. Superhydrophobic materials manifest the characteristics of waterproof
and self-cleaning. The water droplets can be rolled off
immediately on the declining surface with a lower inclined
angle (α) of 10.0°, along with taking away the dust or pollutants
from the surface. Hence, the self-cleaning properties hold great
promise in effectively reducing material pollution.
3.6. Corrosion Resistance of the Electrospun Hybrid
Membrane. The corrosion resistance is of great significance for
the protection of equipment under the high salt environment
along the coast. We compare the sample change under different
storage days through the salt spray chamber experiment (spray
pressure is 8 psi, the temperature is 35 °C, the electrolyte is 5%
NaCl solution). Figure 7A shows the contact angle changes of
the sample in the salt spray tank for different days. During this
process, the contact angle of the sample decreases continuously.
When the sample is placed for 40 days, the contact angle is
150.6°, which still maintains a good superhydrophobic property.
Similarly, we investigated the EMI shielding performance in this
environment. As demonstrated in Figure 7B, the average value of
EMI SE was reduced by 5.94 dB with a 6.56% loss after 40 days.
Figure 7C illustrates optical images of the modified and
unmodified membranes after 40 days of placement. The
APAN-Ag-SA-2.0 membrane surface is as clean as new, while
the surface of APAN-Ag-2.0 becomes dull and heavy, indicating
that it has been subjected to corrosion and oxidation. To further
explore the microstructure changes of the membrane, SEM was
used to characterize the morphology changes (Figure S8). The
Ag particles still adhered firmly to the nanofiber surface, and the
cluster structure of the fiber was not corroded after a highconcentration salt fog treatment. After 40 days of salt spray
treatment, the performance changes of APAN-Ag-2.0 are shown
in Figure S9. The average value of EMI SE was reduced by 13.38
dB with a 14.74% loss after 40 days. After being placed for 40
4. CONCLUSIONS
Superhydrophobic and high-efficiency electromagnetic shielding membranes have been successfully developed through the
combination of electrospun nanofibers and layered roughness.
The superhydrophobic electrospun hybrid membranes exhibit
excellent electrical conductivity, mechanical flexibility, selfcleaning, and superior corrosion resistance. A high WCA of
156.7° and a sliding angle lower than 8.0° are achieved. The
superhydrophobic APAN-Ag-SA-2.0 membrane has an EMI SE
above 90 dB, and the SSE/t reaches the maximum value of
14253 dB cm2 g−1, which is better than pure metal foils and most
synthetic EMI shielding materials. It also exhibits excellent
shielding stability under external mechanical deformations or
simulated harsh environments. Therefore, the superhydrophobic electrospun hybrid membranes have potential applications in
a large variety of fields in aerospace, defense, and portable and
wearable smart electronics.
■
ASSOCIATED CONTENT
sı Supporting Information
*
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acsaelm.1c00076.
FTIR, optical images, and XRD patterns of PAN and
APAN membranes; diameter distributions; TGA and
EMI parameters of APAN-Ag-T membranes; TEM, SEM,
total EMI SE (SET), absorption (SEA), and reflection
J
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Performance Electromagnetic-Interference Shielding. Adv. Mater.
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AUTHOR INFORMATION
Corresponding Authors
Xiaoteng Jia − State Key Laboratory of Integrated
Optoelectronics, College of Electronic Science and Engineering,
Jilin University, Changchun 130012, China; orcid.org/
0000-0001-5630-8838; Email: xtjia@jlu.edu.cn
Yongxin Li − Key Lab of Groundwater Resources and
Environment of Ministry of Education, College of New Energy
and Environment, Jilin University, Changchun 130021,
China; Email: liyongxin@jlu.edu.cn
Ce Wang − Alan G. MacDiarmid Institute, College of
Chemistry, Jilin University, Changchun 130012, China;
orcid.org/0000-0003-3204-5564; Email: cwang@
jlu.edu.cn
Authors
Mei Yang − Alan G. MacDiarmid Institute, College of
Chemistry, Jilin University, Changchun 130012, China
Dayong He − Alan G. MacDiarmid Institute, College of
Chemistry, Jilin University, Changchun 130012, China
Yuying Ma − Alan G. MacDiarmid Institute, College of
Chemistry, Jilin University, Changchun 130012, China
Ya Cheng − Alan G. MacDiarmid Institute, College of
Chemistry, Jilin University, Changchun 130012, China
Jing Wang − Alan G. MacDiarmid Institute, College of
Chemistry, Jilin University, Changchun 130012, China
Complete contact information is available at:
https://pubs.acs.org/10.1021/acsaelm.1c00076
Notes
The authors declare no competing financial interest.
■
ACKNOWLEDGMENTS
This work was supported by the research grants from the
National Natural Science Foundation of China (Grant
21875084), the Project of the Education Department of Jilin
Province (Grant JJKH20211039KJ), the Project of the Department of Science and Technology of Jilin Province (Grant
20190101013JH), and Jilin Province Development and Reform
Commission (Grant 2020C023-5).
■
Article
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