Synergistic Enhancement of Microwave Absorption Using Hybridized

Apr 18, 2017 - In this study, we designed a dual-chirality hierarchical structure to achieve a synergistically enhanced effect in microwave absorption...
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Synergistic Enhancement of Microwave Absorption Using Hybridized Polyaniline@helical CNTs with Dual Chirality Xin Tian,† Fanbin Meng,*,† Fanchen Meng,‡ Xiangnan Chen,§ Yifan Guo,† Ying Wang,† Wenjun Zhu,∥ and Zuowan Zhou*,† †

Key Laboratory of Advanced Technologies of Materials (Ministry of Education), School of Materials Science and Engineering, Southwest Jiaotong University, Chengdu 610031, P R China ‡ Department of Physics and Astronomy, Clemson University, Clemson, South Carolina 29634-0978, United States § Material Science and Engineering, Dalian Maritime University, Dalian 116026, P R China ∥ State Key Laboratory of Meta-RF Electromagnetic Modulation Technology, Kuang-Chi Institute of Advanced Technology, Shenzhen 518057, P R China S Supporting Information *

ABSTRACT: In this study, we designed a dual-chirality hierarchical structure to achieve a synergistically enhanced effect in microwave absorption via the hybridization of nanomaterials. Herein, polyaniline (PANi) nanorods with tunable chirality are grown on helical carbon nanotubes (HCNTs), a typical nanoscale chiral structure, through in situ polymerization. The experimental results show that the hierarchical hybrids (PANi@HCNTs) exhibit distinctly dual chirality and a significant enhancement in electromagnetic (EM) losses compared to those of either pure PANi or HCNTs. The maximum reflection loss of the as-prepared hybrids can reach −32.5 dB at 8.9 GHz. Further analysis demonstrates that combinations of chiral acid-doped PANi and coiled HCNTs with molecular and nanoscale chirality lead to synergistic effects resulting from the dual chirality. The so-called crosspolarization may result in additional interactions with induced EM waves in addition to multiscaled relaxations from functional groups and interfacial polarizations, which can benefit EM absorption. KEYWORDS: helical carbon nanotubes, polyaniline, hybridization, dual chirality, microwave absorption conductivity, and good stability.10−12 Furthermore, the helical carbonaceous nanostructures can induce cross-polarization with continuous microwave irradiation, which leads to resonance losses.13−15 However, the dominant microwave attenuation mechanism of HCNTs is via dielectric losses leading to weak EM impedance matching.16 The mainstream approach to enhance input impedance matching is to incorporate HCNTs with magnetic or dielectric materials.2,12,17 Qin’s group incorporated CNCs with Fe2O3, Fe3O4, and Al2O3 by atomic layer deposition to tune the EM wave absorption.2,12 Jian prepared HCNFs on tetrapod-shaped ZnO whiskers (T-ZnO) through in situ growth on the surface of T-ZnO, and HCNFs with a tetrahollow morphology exhibit remarkable improvements in EM wave losses.18 However, because of the large size, high density, poor resistance to corrosion, high processing costs, and strong reflection of magnetic metals or metal oxides, these methods may have limited applications in microwave

1. INTRODUCTION Since the proposal of Varadan et al.1 that a suspension of chiral microgeometries can endow materials with certain optical activity characteristics, making them attractive as high-efficiency absorbers, many research studies have examined chiral materials as high-efficiency absorbers.2,3 It seems that the chiral structures have the potential to be excellent materials for electromagnetic (EM) wave absorption with high efficiency, light weight, and broad bandwidth. One fact that can explain these characteristics is that the chiral materials have additional parameters of chirality and physically generate cross-polarization from the structural chirality.4−6 In recent years, studies on microwave absorption of chiral materials have focused mainly on carbon micro- or nanocoils (CMCs/CNCs), which are helically structured nanomaterials.7,8 The CMCs/CNCs with specific three-dimensional (3D) structures that are similar to a series of fundamental structures in nature, such as α-helix of proteins, DNA, screw, electric waves, and so on, have been highlighted and have attracted much attention.9 Many investigations have demonstrated that helical carbon nanotubes (HCNTs) and helical carbon nanofibers (HCNFs) can exhibit good EM wave absorption because of their light weight, suitable electrical © 2017 American Chemical Society

Received: February 22, 2017 Accepted: April 18, 2017 Published: April 18, 2017 15711

DOI: 10.1021/acsami.7b02607 ACS Appl. Mater. Interfaces 2017, 9, 15711−15718

Research Article

ACS Applied Materials & Interfaces

Figure 1. FE-SEM and TEM images of HCNTs (a, d) and PANi@HCNT hybrids doped with HCl (b, e) and D-CSA (c, f), respectively (the molar ratios of ANi to dopant were both 1:1). PANi with nanocone structures are coated on the HCNTs almost vertically. The dashed lines in (e) and (f) represent the interface between the HCNTs and PANi.

absorption.19,20 Thus, it is necessary to incorporate them with materials with light weight, good chemical stability, and microwave absorption properties, such as conducting polymers. Among various conducting polymers, chiral polyaniline (PANi), which is doped with a chiral organic acid, was found to have adjustable EM parameters, making it one of the most promising microwave absorbents.21 Many reports show that by introducing chirality into the PANi chain, asymmetric polarization is produced, which induces different EM responses of the chain to microwave impedance.22,23 Therefore, we proposed a new path to improve the microwave absorption characteristics by introducing chiral conducting PANi into HCNTs, but not magnetic metals or metal oxides, therefore avoiding their disadvantages. We demonstrated the desirable and tunable microwave-absorption performance of the dualchirality architecture synthesized through in situ polymerization of PANi nanorods grown on HCNTs. The nanohybrids exhibit good microwave-absorption characteristics in the measured frequency range of 2−18 GHz, and the results show that there exists a synergistic effect of the dual-chirality nanohybrids.

solution containing a certain amount of ANi and 0.102 g of HCNTs were stirred under ultrasonication for 3 h in an ice bath (0−5 °C). Then, 20 mL of a precooled 1 M D-CSA solution of APS was slowly added to the above mixture with mechanical stirring. The suspension was then transferred to an ice bath (0−5 °C) and stirred for 8 h. The resulting products were washed alternately with methanol and deionized water three times and vacuum-dried in an oven at 60 °C for 24 h. For comparison, different molar ratios of ANi to D-CSA ([ANi]/[D-CSA]: 2:1, 1:1, and 1:2) were used to synthesize PANiCSA@HCNTs with different chiralities. In addition, achiral nanohybrids doped with HCl (denoted PANi-HCl@HCNTs) and a ground mixture of PANi-CSA and HCNTs (denoted PANi-CSA+HCNTs) were synthesized; the molar ratios of ANi to dopant were both 1:1. 2.3. Characterization. The morphologies of the products were observed on a field-emission scanning electron microscope (FE-SEM; JSM-7001F) and a transmission electron microscope (TEM, JEM2100F). Powder X-ray diffraction (XRD) analyses were carried out on a Philips X’Pert PRO X-ray diffractometer with Cu Kα radiation. The chirality of all samples was tested by a diffuse reflectance circular dichroism (DRCD) spectrometer (JASCO J-815). The electrical conductivities of the pressed pellets at room temperature were measured by a standard four-probe method on an RTS-9 digital instrument. The complex permeability and permittivity of the materials were measured using a vector network analyzer (AV3618; CETC) in a frequency range of 2−18 GHz. The samples were mixed with wax and prepared in the shape of a toroid with an outer diameter of 7.0 mm, an inner diameter of 3.04 mm, and a thickness of 4.0 mm. The mass ratio of the samples to wax was 3:7. The sample preparation and measurements were repeated two to three times for each sample to ensure reproducibility of the analytical results (Figure S4).

2. EXPERIMENTAL SECTION 2.1. Materials. HCNTs with diameters of ca. 190 nm were prepared by chemical vapor deposition in our laboratory.24 Aniline (ANi) was distilled under reduced pressure (35 mmHg, 85 °C) and stored below 0 °C. Analytical-grade (1S)-(+)-10-camphorsulfonic acid (D-CSA), ammonium peroxydisulfate (APS; (NH4)2S2O8), and 36% hydrochloric acid (HCl) were used as received, without further purification. All experiments were carried out using deionized water. 2.2. Sample Preparation. D-CSA-doped PANi and HCNT hybrids were prepared by growing an array of PANi nanorods on the surface of HCNTs through in situ polymerization (denoted PANiCSA@HCNTs). In a typical experiment, 90 mL of 1 M D-CSA

3. RESULTS AND DISCUSSION The morphological observations were carried out by FE-SEM and TEM, and the results are displayed in Figure 1. As demonstrated in Figure 1a,d, HCNTs with a diameter of ca. 15712

DOI: 10.1021/acsami.7b02607 ACS Appl. Mater. Interfaces 2017, 9, 15711−15718

Research Article

ACS Applied Materials & Interfaces

Figure 2. (a) FTIR spectra and (b) XRD patterns of HCNTs, D-CSA-doped PANi, and PANi@HCNT hybrids doped with HCl and D-CSA.

Figure 3. (a) DRCD spectra of HCNTs, D-CSA-doped PANi, and PANi@HCNT hybrids doped with HCl and D-CSA. (b) The nanoscale and molecular helix of PANi-CSA@HCNTs.

CSA; meanwhile, achiral PANi-HCl@HCNTs do not exhibit these changes.26,27 Figure 2b displays the typical XRD patterns of PANi, HCNTs, and PANi/HCNTs. The diffraction peaks at 26.0, 42.8, and 45.0° correspond to the graphite-like HCNT structure.28 The diffraction peak of PANi centered at approximately 9.0° is attributed to the periodic structure between the dopant and the N atom on the adjacent main chains,29 and the diffractions near 14.8 and 20.5° are assigned to the periodic parallel and perpendicular arrangements of the polymer chains, respectively.30 The peak at approximately 24.5° is attributed to the arrangement regularity of the overall PANi molecular chains. The typical XRD pattern of the PANi-CSA@ HCNT hybrid presents almost the same profile as that of pure PANi-CSA, and the major typical diffraction peaks of the HCNTs (26.0°, 42.8°) can also be found in the diffraction patterns. No new peaks can be found in the patterns of PANi@ HCNTs compared with pure PANi or HCNTs, demonstrating that no additional ordered structure is formed in the hybrids. However, there exists a significant difference between chiral and nonchiral products. The intensity of 2θ = 9.0° in PANi-CSA@ HCNTs is much lower than that in PANi-HCl@HCNTs, which can be attributed to the doping effect on PANi molecular chains by the chiral D-CSA molecules. The flexibility of the PANi molecular chains is greatly decreased due to the larger volume of the D-CSA molecules. In addition, the presence of the D-CSA molecules may induce the formation of a helical configuration in the PANi chains.31 It is well known that the chirality can exhibit optically active properties and deflect polarized light;32 thus, it is beneficial to microwave absorption. To characterize the dual chirality of the

200 nm showed a symmetrical growth mode and were mirror images of each other. After introducing PANi through in situ polymerization and using the chiral D-CSA or achiral HCl as the dopant, the array of PANi nanorods grew on the surface of the HCNTs, thus forming 3D hierarchical structures (Figure 1b,c). Furthermore, as demonstrated in the TEM images (Figure 1e,f), the morphology of the PANi-CSA@HCNTs was similar to that of the PANi-HCl@HCNTs, except for the D-CSAdoped PANi nanorods, which were longer than the HCl-doped samples. Additionally, the morphology of PANi can change from nanoparticle to regular nanorod by changing the ratio of [ANi] to [D-CSA] from 2:1 to 1:2 (as shown in Figure S1). The Fourier transform infrared (FTIR) spectra of the assynthesized PANi@HCNT hybrids and PANi-CSA are shown in Figure 2a. The peaks at 1558, 1471, 1300, and 1125 cm−1 of PANi-CSA@HCNTs can be assigned to the CC stretching vibrations of the quinoid (Q) and benzenoid (B) rings and the stretching vibrations of Ar-N (Ar is the aromatic group) and protonated −NH+, respectively.17,18 The specific peak at a frequency of 1115 cm−1 is associated with the vibrational mode of NQN in the doped PANi chains.11,19 The FTIR spectra of PANi-CSA@HCNTs and PANi-HCl@HCNTs are similar to the spectrum of PANi-CSA, indicating that both the hybrids are in the acid-doped state.20 Compared to that in PANi-CSA, redshifts are observed at the NQN vibrational mode and the CC stretching vibrational mode of the Q-ring and B-ring, indicating obvious interactions between the PANi molecular chains and HCNTs, which can be mainly ascribed to π−π interactions.25 D-CSA-doped PANi is confirmed by the presence of peaks at 1717 and 1025 cm−1, corresponding to the N−H···OC H-bond and SO stretching vibrations in D15713

DOI: 10.1021/acsami.7b02607 ACS Appl. Mater. Interfaces 2017, 9, 15711−15718

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resulting in poor EM wave absorption. However, the PANiCSA@HCNT hybrids showed a new dielectric and magnetic response peak in the low- and high-frequency ranges, respectively, which can be ascribed to the new chiral asymmetric center introduced by the CSA-doped PANi molecular chain. Moreover, both the helical nanotube structure and helical PANi molecular chains can produce crosspolarization; thus, both the polarization relaxed loss and interface polarization loss are enhanced. According to the transmission line theory, the microwave reflection loss (RL) could be calculated from the measured complex permeability and permittivity data using the following equations36

as-prepared PANi-CSA@HCNTs, the DRCD spectra were recorded (Figure 3). It is obvious that all samples show the Cotton effect, indicating they all have chirality; the deflection of polarized light occurring in the HCNTs resulted from the helical nanotube morphology; and a stronger Cotton effect can be seen for PANi-CSA@HCNTs compared to that for the original HCNTs, revealing that PANi-CSA@HCNTs have a highly asymmetric structure in the polymer chains. However, the D-CSA molecules do not show any Cotton effect at wavelengths longer than 300 nm,20 inferring that the deflection of polarized light occurring in PANi-CSA is ascribed to the helical structure of the PANi chains (Figure 3b). Moreover, it is observed that the ellipticity of the PANi-CSA@HCNT hybrids increases significantly with increasing D-CSA concentrations, whereas the HCl-doped product exhibits a relatively weak circular dichroism response. This result further confirms the existence of dual chirality in PANi-CSA@HCNTs, a contribution from the nanoscale and molecular helical structures, which can also be proved by the FTIR and XRD characteristics simultaneously (Figure S2). The FTIR spectra of three PANiCSA@HCNT samples synthesized with different [ANi]/[DCSA] are displayed in Figure S2a. The relative intensity of the peaks at 1717 and 1025 cm−1 increases with increasing D-CSA concentrations, indicating that the interaction between the PANi and D-CSA molecules also increased. Meanwhile, the intensity of the characteristic XRD diffraction peak ascribed to the periodic parallel arrangement of the PANi chains increased, and its position changed from 15.3 to 14.6° due to the heightened steric hindrance effect of the D-CSA molecules (as shown in Figure S2b), thereby confirming that both the degree of doping of the PANi chains and the chirality of the material are enhanced with increasing D-CSA concentrations. It can be further confirmed by the electrical conductivities of these samples, as shown in Table 1. For the as-prepared nanohybrids, the electrical conductivities increased with increasing concentration of D-CSA due to the enhanced doping effect on PANi molecular chains.

HCNTs PANi-CSA@HCNTs

[ANi]/[D-CSA] 2:1 1:1 1:2

electrical conductivity (S/cm) 0.34 0.19 0.46 0.60

± ± ± ±

(1)

Z in = Z0 μr /εr tan h[j(2πfd)/c εrμr ]

(2)

where Zin, Z0, μr, εr, d, c, and f denote the input characteristic impedance at the absorbing material/free-space interface, the impedance of free space, the measured relative complex permittivity, permeability, thickness of the microwave-absorption material, velocity of light, and the microwave frequency in free space, respectively. Occasionally, the RL value of −10 dB is comparable to the attenuation of a 90% incident microwave. Accordingly, materials with an RL value of less than −10 dB are regarded as ideal microwave absorbers.37 On the basis of the measured EM parameters (the complex values of permeability and permittivity), the RL could be calculated at the given frequencies and thicknesses using eqs 1 and 2, respectively. To simulate the microwave losses, the three samples of PANi-CSA@HCNT hybrids with different thicknesses were doped with different [ANi]/[D-CSA] ratios (2:1, 1:1, and 1:2) in Figure 5. Figure 5a−c displays visual 3D RL curves to help compare the microwave-absorption properties of the PANi-CSA@HCNT hybrids with different degrees of chirality; meanwhile, the samples with [ANi]/[D-CSA] = 2:1 and 1:2 had relatively low minimum RLs at −12.3 and −17.0 dB, respectively. However, the sample with [ANi]/[D-CSA] = 1:1 exhibited a distinctly higher RL than the minimum RL reached, −32.5 dB. Moreover, different RL values can be tuned by choosing an appropriate thickness of the absorbent layer between 2 and 5 mm in the frequency range of 2−18 GHz for the PANi-CSA@HCNT sample with [ANi]/[D-CSA] = 1:1, as demonstrated in Figure 5d. It could be observed that the absorption bandwidth with an RL below −10 dB was obtained in the frequency range of 4−18 GHz with a thickness of 2−5 mm. These results revealed that the microwave-absorption intensity and the frequency range of the PANi-CSA@HCNT hybrids could be conveniently controlled by adjusting the degree of chirality. For comparison, we also prepared pristine HCNTs, PANi-CSA, and PANi-HCl@HCNTs and simulated their 3D RL curves (Figure S3). Compared with samples with single chirality, the PANi-CSA@HCNTs with 1:1 [ANi]/[DCSA] exhibited superior microwave-absorption performances. In this study, the specific microwave-absorption performance of PANi-CSA@HCNTs can be explained as follows. Generally, the microwave-absorption performance improves when the dielectric contribution matches the magnetic contribution on the basis of the requirement of the input impedance.38 A Δfunction method is used to effectively evaluate the EM impedance matching degree according to eq 3.39

Table 1. Electrical Conductivities of HCNTs and PANi@ HCNT Hybrids with Different Molar Ratios of ANi to D-CSA ([ANi]/[D-CSA]) sample

(Z in − Z0) (Z in + Z0)

RL = 20 log

0.02 0.07 0.04 0.03

Microwave absorption is highly related to the EM parameters, including complex permittivity (εr = ε′ + jε″) and complex permeability (μr = μ′ + jμ″), where the real parts of complex permittivity (ε′) and complex permeability (μ′) represent the storage capability of electric and magnetic energy and the imaginary parts (ε″ and μ″) represent the loss capability of the electric and magnetic energies.33 The power of dielectric and magnetic losses is characterized by the dielectric loss angle tangent (tan δε = ε″/ε′) and magnetic loss angle tangent (tan δμ = μ″/μ′), respectively.34,35 Figure 4 demonstrates the measured EM parameters of the HCNTs and the PANi-CSA@HCNT hybrids in a frequency range of 2−18 GHz. As shown in Figure 4c,f, tan δε of the PANi-CSA@ HCNTs is much higher than tan δμ, whereas the pristine HCNTs show relatively low tan δμ and tan δε values, thus 15714

DOI: 10.1021/acsami.7b02607 ACS Appl. Mater. Interfaces 2017, 9, 15711−15718

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Figure 4. Frequency dependence of the (a) real (ε′) and (b) imaginary parts (ε″) of complex permittivity, the (d) real (μ′) and (e) imaginary parts (μ″) of complex permeability, and the corresponding (c) dielectric loss tangent (tan δε) and (f) magnetic loss tangent (tan δμ) of HCNTs and DCSA-doped PANi@HCNT hybrids (as shown in Figure 1), with different molar ratios of ANi to D-CSA ([ANi]/[D-CSA]: 2:1, 1:1, and 1:2; displayed in Figure S1).

Figure 5. Three-dimensional representations of the RL of D-CSA-doped PANi@HCNT hybrids with different molar ratios of ANi to D-CSA: (a) 2:1, (b) 1:1, and (c) 1:2. (d) Calculated RL of the PANi-CSA@HCNTs with different thicknesses (2−5 mm) in a frequency range of 2−18 GHz.

Δ = |sin h2(Kfd) − M |

corresponds to Δ-value smaller than 0.2, in PANi-CSA@ HCNTs is larger than that in PANi-HCl@HCNTs, indicating that the impedance matching can be increased by introducing dual chirality. The schematics of the microwave attenuation mechanism of the PANi-CSA@HCNTs are demonstrated in Figure 6. It has been reported that HCNTs are typically chiral materials with helical nanotube morphology and are expected

(3)

The smaller Δ value implies better EM impedance matching. Figure S5 shows the calculated Δ-value maps of PANi-HCl@ HCNTs and PANi-CSA@HCNTs with thicknesses of 2−5 mm. Δ-Values smaller than 0.2 correspond to RL below −8 dB. It can be seen in the figure that the blue region, which 15715

DOI: 10.1021/acsami.7b02607 ACS Appl. Mater. Interfaces 2017, 9, 15711−15718

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ACS Applied Materials & Interfaces

Figure 6. Schematics of microwave attenuation mechanism of PANi-CSA@HCNTs.

Figure 7. Comparison of calculated RL of PANi-CSA@HCNTs with (a) HCNTs, PANi-CSA, and PANi-CSA+HCNTs and (b) PANi-HCl@ HCNTs with an absorbent layer thickness of 3.7 mm at a frequency range of 2−18 GHz. The molar ratio of [ANi]/[D-CSA] was 1:1.

−7.5 and −5.4 dB, respectively. Interestingly, the obtained PANi-CSA@HCNTs showed a breakthrough in the improvement of microwave absorption. With a thickness of 3.7 mm, the peaks reached an optimal absorption of −32.5 dB at 8.9 GHz and a bandwidth below −10 dB at 5.1 GHz (7.1−11.2 GHz). Furthermore, the ground mixture presented as PANi-CSA +HCNTs shows a much lower RL value than the PANi-CSA@ HCNTs, indicating that the combination of HCNTs and chiral PANi could synergistically enhance the microwave-absorption properties. As one can see in Figure 7b, after the hybridization of PANi and HCNTs, the PANi@HCNTs doped by both DCSA and HCl exhibited superior microwave-absorption performances than the HCNTs or PANi-CSA; however, the dual-chirality PANi-CSA@HCNT hybrids exhibited a much more enhanced effect than the single-chiral PANi-HCl@ HCNTs. The minimum value of the former was −32.5 dB, and its microwave-absorption value of less than −10 dB was found in the range of 7.1−11.2 GHz, whereas that of the latter was only −13.6 dB (8.3−11.8 GHz). Furthermore, RLs below −15 dB can be detected in PANi-CSA@HCNTs, with a bandwidth of 3.6 GHz, which was not possible in PANi-HCl@ HCNTs. This may help explain why the capability of microwave attenuation in PANi-CSA@HCNTs is superior to that in PANi-HCl@HCNTs. Moreover, the calculated RL in the PANi-CSA@HCNTs was much higher than that in both PANi-HCl@HCNTs and PANi-CSA. In addition, the results of electrical conductivity can also be used to support the dual-chirality effect in PANi-CSA@ HCNTs. We mentioned before that the imaginary parts (ε″) of permittivity represent the loss capability of the electric energy, which is given as

to interact with microwaves in a mechanism different from that of nonchiral materials; the special interaction likely occurs when microwaves transmit along the helix. As a result, the cross-polarization leads to an induced current in the helix.12 The structural characterization confirmed that asymmetric polarization occurs in the PANi backbone because of polaron/ bipolaron polarization due to the chiral molecules that act as new centers of polarization and cause additional relaxation.10 Hence, the dual chirality of PANi-CSA@HCNT hybrids can form cross-polarization and improve the EM-absorption performance. In addition, the special core−shell and interfacial structures of the PANi@HCNT hybrids could also expand the microwave propagation path through multiple reflections than via single-component absorbers, due to effective interfaces and their associated relaxation losses.40 Moreover, we believe that there exists some synergistic effect produced by dual chirality resulting from the helical nanotube structure and PANi helical molecular chains. The structural symmetry is decreased by the introduction of chiral D-CSA molecules, thus increasing the polarization of the dual-chirality nanohybrids. Moreover, the helix-shaped nanostructure of HCNTs also lacks structural symmetry because of the inherent pentagons and heptagons formed in their growth stage.41 The interfacial polarization, molecular polarization, and group polarization will be synergistically generated simultaneously during the hybridization of D-CSA-doped PANi and HCNTs. This can be confirmed by comparing the microwave-absorption behaviors of pristine materials, such as HCNTs and PANi-CSA, and dual-chirality hybrid PANi-CSA@HCNTs. The calculated RL results of the materials in the frequency range of 2−18 GHz with a simulated absorbent layer thickness of 3.7 mm are shown in Figure 7. As demonstrated in Figure 7a, the RLs of individual chiral HCNTs and PANi-CSA, which are relatively lower than those of the PANi-CSA@HCNT hybrids, exhibit poor microwave-absorption characteristics with minimum values of

ε″ = 15716

εs − ε∞ 2 2

1+ωτ

ωτ +

δ ωε0

(4) DOI: 10.1021/acsami.7b02607 ACS Appl. Mater. Interfaces 2017, 9, 15711−15718

ACS Applied Materials & Interfaces

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where εs is the static permittivity, ε∞ is the high-frequency relative dielectric permittivity, ω is the angular frequency, τ is the polarization relaxation time, and δ is the electrical conductivity. Given the insignificant variation of electrical conductivity of all PANi-CSA@HCNTs (Table 1), the broad dielectric response peak at 8−12 GHz could be caused by the hybridization and dual chirality in PANi-CSA@HCNTs rather than the electrical polarization loss. Even though there was no obvious difference in the morphology of the chiral and achiral hybrids, we believe that these results are borne from a synergistic effect between the dual chirality resulting from the combination of the chiral PANi molecular chains and the chiral structure of the HCNTs. These phenomena facilitated the efficient synergy between the permittivity and permeability and were beneficial to improve the input impedance matching of the PANi-CSA@ HCNT hybrids. Furthermore, the dual-chirality synergistic effect of the helical nanotube structure and chiral doping effect endowed the PANi-CSA@HCNT hybrids with specific microwave-absorption characteristics.

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (M.F.). *E-mail: [email protected]. Tel/Fax: +86-28-87600454 (Z.Z.). ORCID

Zuowan Zhou: 0000-0002-5732-4650 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This study was financially supported by National Natural Science Foundation of China (No. 51573149), the Science and Technology Planning Project of Sichuan Province (Nos. 2016RZ0050 and 2016GZ0224), and Shenzhen Key Laboratory of Ultrahigh Refractive Structural Material.

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REFERENCES

(1) Varadan, V. K.; Varadan, V. V.; Lakhtakia, A. On the Possibility of Designing Anti-Reflection Coatings Using Chiral Composites. J. WaveMater. Interact. 1987, 2, 71−81. (2) Zhao, S.; Gao, Z.; Chen, C.; Wang, G.; Zhang, B.; Chen, Y.; Zhang, J.; Li, X.; Qin, Y. Alternate Nonmagnetic and Magnetic Multilayer Nanofilms Deposited on Carbon Nanocoils by Atomic Layer Deposition to Tune Microwave Absorption Property. Carbon 2016, 98, 196−203. (3) Xu, Y.; Yuan, L.; Zhang, D. A Chiral Microwave Absorbing Absorbent of Fe−CoNiP Coated on Spirulina. Mater. Chem. Phys. 2015, 168, 101−107. (4) Sun, G. C.; Yao, K. L.; Liu, Z. L. Influence of Helix Parameters on the Reflectance of Microwave by Chiral Composites. IEEE Trans. Electromagn. Compat. 1999, 41, 350−353. (5) Sabah, C.; Uckun, S. Reflection and Transmission Coefficients of Multiple Chiral Layers. Sci. China, Ser. E: Technol. Sci. 2006, 49, 457− 467. (6) Sun, G.; Yao, K.; Liu, Z.; Huang, Q. A Study on Measuring the Electromagnetic Parameters of Chiral Materials. J. Phys. D: Appl. Phys. 1998, 31, 2109−2111. (7) Zhao, D. L.; Shen, Z. M. Preparation and Microwave Absorption Properties of Carbon Nanocoils. Mater. Lett. 2008, 62, 3704−3706. (8) Liu, L.; He, P.; Zhou, K.; Chen, T. Microwave Absorption Properties of Carbon Fibers with Carbon Coils of Different Morphologies (Double Microcoils and Single Nanocoils) Grown on Them. J. Mater. Sci. 2014, 49, 4379−4386. (9) Chen, X.; Motojima, S. Morphologies of Carbon Micro-Coils Grown by Chemical Vapor Deposition. J. Mater. Sci. 1999, 34, 5519− 5524. (10) Lau, K. T.; Lu, M.; Hui, D. Coiled Carbon Nanotubes: Synthesis and Their Potential Applications in Advanced Composite Structures. Composites, Part B 2006, 37, 437−448. (11) Tang, N. J.; Zhong, W.; Au, C. T.; Gedanken, A.; Yang, Y.; Du, Y. Large-Scale Synthesis, Annealing, Purification, and Magnetic Properties of Crystalline Helical Carbon Nanotubes with Symmetrical Structures. Adv. Funct. Mater. 2007, 17, 1542−1550. (12) Wang, G.; Gao, Z.; Tang, S.; Chen, C.; Duan, F.; Zhao, S.; Lin, S. W.; Feng, Y. H.; Zhou, L.; Qin, Y. Microwave Absorption Properties of Carbon Nanocoils Coated with Highly Controlled Magnetic Materials by Atomic Layer Deposition. ACS Nano 2012, 6, 11009− 11017. (13) Bi, H.; Kou, K. C.; Ostrikov, K. K.; Yan, L. K.; Wang, Z. C. Microstructure and Electromagnetic Characteristics of Ni Nanoparticle Film Coated Carbon Microcoils. J. Alloys Compd. 2009, 478, 796−800. (14) Bajpai, V.; Dai, L.; Ohashi, T. Large-Scale Synthesis of Perpendicularly Aligned Helical Carbon Nanotubes. J. Am. Chem. Soc. 2004, 126, 5070−5071.

4. CONCLUSIONS We have synthesized a dual-chirality hybrid of PANi-CSA@ HCNTs via an in situ synthesis of PANi nanorods on the surface of HCNTs and achieved an enhanced synergistic microwave-absorption effect. The PANi-CSA@HCNT hybrids showed a tunable chirality and a significant enhancement in EM losses compared to those of both pure PANi and HCNTs. The maximum RL value of the as-prepared hybrids reached −32.5 dB at 8.9 GHz. Structural characterization demonstrated that there are π−π and H-bond interactions between PANi molecules and HCNTs, and the combination of chiral aciddoped PANi and coiled HCNTs with molecular and nanoscale chirality, respectively, leads to synergistic effects from the dual chirality. Enhanced microwave-absorption performance resulted from the cross-polarization in both the HCNTs and the chiral PANi backbone, which produced additional interactions with the induced EM wave. The cooperation of the multiscaled relaxations from functional groups and the interfacial polarizations can synergistically benefit the EM-absorption characteristics. In addition, the EM-absorption performance could simply be controlled by tuning the degree of doping of the chiral structure in PANi. We believe that this article will provide a new pathway toward the controllable design of highperformance or multifunctional devices based on helical carbon nanostructures and their hybrids.

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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.7b02607. SEM images, FTIR spectra, and XRD patterns of PANiCSA@HCNT hybrids with different molar ratios of ANi to D-CSA; 3D representations of RL of HCNTs, PANiHCl@HCNTs, and PANi-CSA to ANi; SEM images, XRD patterns, FTIR spectra, and 3D representations of the RL of D-CSA-doped PANi@HCNT hybrids in different batches ([ANi]/[D-CSA] = 1:1); and the calculated Δ-value maps of PANi-HCl@HCNTs and PANi-CSA@HCNTs with thickness of 2−5 mm (PDF) 15717

DOI: 10.1021/acsami.7b02607 ACS Appl. Mater. Interfaces 2017, 9, 15711−15718

Research Article

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DOI: 10.1021/acsami.7b02607 ACS Appl. Mater. Interfaces 2017, 9, 15711−15718