⇦Chinese Physics Letters, 2020, Vol. 37, No. 4, Article code 045201 Tunable Dielectric Properties of Carbon Nanotube@Polypyrrole Core-Shell Hybrids by the Shell Thickness for Electromagnetic Wave Absorption * De-Ting Wang (汪德汀), Xian-Chao Wang (王显超), Xiao Zhang (张潇), Hao-Ran Yuan (袁浩然), Yu-Jin Chen (陈玉金)** Affiliations Key Laboratory of In-Fiber Integrated Optics (Ministry of Education), and College of Physics and Optoelectronic Engineering, Harbin Engineering University, Harbin 150001 Received 2 January 2020, online 24 March 2020 *Supported by the National Natural Science Foundation of China under Grant No. 51972077, and the Heilongjiang Touyan Innovation Team Program.
**Corresponding author. Email: chenyujin@hrbeu.edu.cn Citation Text: Wang D T, Wang X C, Zhang X, Yuan H R and Chen Y J et al 2020 Chin. Phys. Lett. 37 045201    Abstract Carbon nanotube@polypyrrole (CNT@PPy) hybrids have been successfully fabricated via a simple in situ chemical oxidation polymerization. The thickness of the PPy shell can be finely controlled in the range of 3.0–6.4 nm. The dielectric loss of core-shell hybrids can be tuned by the shell thickness, resulting in a well-matched characteristic impedance that can enhance electromagnetic wave (EMW) absorption performance. Minimum reflection loss of the hybrid with moderate PPy shell thickness can reach $-51.4$ dB at 11.8 GHz with a matching thickness of merely 2 mm. Furthermore, the minimum reflection loss values of the hybrid are below $-30$ dB even at thickness in the range of 1.4–1.9 mm, endowing the possibility of practical application of the hybrids in electromagnetic wave absorption field. DOI:10.1088/0256-307X/37/4/045201 PACS:52.70.Gw, 52.70.Ds, 77.84.Lf, 78.20.Ci © 2020 Chinese Physics Society Article Text With over-exploitation of electromagnetic instruments in the commercial, industrial, and military fields, electromagnetic pollutions have become increasingly prominent. There is an urgent to explore effective ways to solve the electromagnetic pollution.[1–4] Fortunately, electromagnetic wave (EMW) absorbing materials are considered an effective means to eliminate the electromagnetic pollution.[5–7] To date, carbonaceous materials including carbon nanocoils (CNCs),[8] carbon nanosheets (CNSs),[9] and grapheme,[10] have shown promising EM absorption performance. Among these carbonaceous structures, CNTs have attracted great attention in the field of EMW absorption due to their low density, stable chemistry, high specific surface area and low cost.[11–13] However, the poor impedance matching characteristic of pure CNT greatly hinders their practical application. Recently, conducting polymers (CPs) have emerged as promising candidates for EMW absorbers.[14] Polypyrrole (PPy), as a kind of CP, not only possesses the unique properties of low density and effortless large-scale industrial production, but also has tunable electrical conductivity, which is conducive to its practical applications.[15] However, the pure CPs cannot meet the requirements strong absorption with wide absorption bandwidth. Constructing nanocomposites is an effective way to improve the EMW absorption property due to synergistic effect caused by foreign materials in the nanocomposites.[16–18] To date, various CNTs or PPy-based nanocomposites have been reported for high-performance EMW absorbing materials. For example, Wen et al. grew magnetic metal nanoparticles (NPs) on CNTs, showing significantly enhanced EMW properties due to the increased impedance matching performance.[19] Zhao et al. synthesized Fe$_{3}$O$_{4}$-NP-modified CNT with a minimum reflection loss ($R_{\rm L,min}$) of $-35.8$ dB at 8.56 GHz and absorption bandwidth (EAB) of approximately 2.32 GHz.[20] Wang et al. reported Co/PPy nanohybrids through an in situ oxidation polymerization and the $R_{\rm L,min}$ of the hybrid reached $-33$ dB at 13.6 GHz.[21] Yu et al. synthesized Ag/PPy sponge via a solvothermal, in situ polymerization and freeze-drying processes. The $R_{\rm L,min}$ of the hybrids reached $-33$ dB at 8.8 GHz.[22] However, most of the nanocomposites mentioned here have mainly been focused on coupling magnetic nanomaterials with CNTs or PPy. The agglomeration among magnetic nanomaterials limits their practical applications. Meanwhile, to achieve strong absorption toward EMW, large filler mass loading of these nanomaterials is usually required. Therefore, it is highly desirable to develop a lightweight CNT-based EMW absorbing material with excellent EMW absorption characteristics. Herein, we fabricate lightweight CNT@PPy core-shell hybrids to boost EMW absorption property by a facile method. Thickness of the PPy shell is well controlled by modulating the usage volume of pyrrole during the chemical oxidation polymerization process. The CNT@PPy core-shell hybrid with $R_{\rm L,min}$ of $-51.4$ dB at 11.8 GHz as the thickness of the absorber film is merely 2 mm. In addition, when the thickness of the absorbing film is only 1.6 mm, EAB value can reach 4.9 GHz. Furthermore, the filler mass loading of the CNT@PPy core-shell hybrid is only 20 wt%, suggesting its feasibility of practical applications in EMW absorption field. The CNT@PPy core-shell hybrids were synthesized via in situ chemical oxidation polymerization. In our experiment, 0.075 g of CNTs was dispersed in 200 mL of 0.5 mol/L H$_{2}$SO$_{4}$ solution and ultrasonicated over 1 h to form a homogeneous suspension. Pyrrole with a given volume was then added into the CNT suspension slowly under magnetic stirring for 20 min. Then, 0.10 g of ammonium peroxydisulfate (APS) dissolved in 100 mL of 0.5 mol/L H$_{2}$SO$_{4}$ solution was slowly added into the above suspension. The polymerization was carried out at $-5^{\circ}\!$C for 1 h with constant magnetic stirring. The core-shell CNT@PPy hybrids were filtered and rinsed with the anhydrous ethanol and distilled water for several times. The hybrid powders were dried at 40$^{\circ}\!$C under vacuum for 24 hours.[23] In order to study the role of PPy shell thickness in the EMW absorption properties, the volume of pyrrole was adjusted. The obtained hybrids were named as CNT@PPy-1, CNT@PPy-2 and CNT@PPy-3 as the volume of pyrrole was 0.05, 0.1 and 0.15 mL, respectively (Fig. 1). The electromagnetic parameters of the absorbing materials were measured using a vector network analyzer (Anritsu MS4644AVectorstar). The cylindrical sample (with 3.00 mm inner diameter, 7.00 mm outer diameter and 3.00 mm thickness) was prepared by mixing CNT@PPy core-shell hybrids with a paraffin matrix. The filler mass loading of CNT@PPy core-shell hybrids into the paraffin matrix was controlled to be 20 wt%.
cpl-37-4-045201-fig1.png
Fig. 1. Schematic diagram of the synthesis of CNT@PPy core-shell hybrids.
cpl-37-4-045201-fig2.png
Fig. 2. SEM images of CNT and the CNT@PPy hybrids.
To control the PPy shell thickness, different volumes of pyrrole were adopted. The SEM images indicate that the CNT@PPy core-shell hybrids have morphologies and lengths slightly different from those of the pristine CNTs (Fig. 2). The TEM image shows that the pristine CNTs have no coating layer on their outmost surfaces (Fig. 3(a)). However, obvious PPy layers are observed at outmost surfaces of CNT@PPy core-shell hybrids (Figs. 3(b)–3(d)). The thicknesses of PPy shells in CNT@PPy-1, CNT@PPy-2 and CNT@PPy-3 are $3.0\pm0.7$ to $4.9\pm0.2$ and $6.4\pm0.9$ nm, respectively, suggesting that the shell thickness can be controlled by tuning the usage amount of pyrrole.
cpl-37-4-045201-fig3.png
Fig. 3. TEM images of CNT and the CNT@PPy hybrids.
cpl-37-4-045201-fig4.png
Fig. 4. (a) XRD patterns of CNT and CNT@PPy hybrids, (b) survey scans for XPS spectra of the CNT and CNT@PPy-2, (c) C $1s$, and (d) N $1s$ peaks of CNT@PPy-2.
The crystal structure and surface composition of CNT and CNT@PPy core-shell hybrids were further analyzed by XRD and XPS data. Figure 4(a) shows the x-ray diffraction patterns of CNT and CNT@PPy core-shell hybrids. The diffraction peaks at 25.5$^{\circ}\!$ and 43.5$^{\circ}\!$ correspond to (002) and (100) lattice planes of CNTs.[24] In addition, for CNT@PPy core-shell hybrids, not only the two diffraction peaks of CNTs are obviously observed, but also the diffraction peak of CNT@PPy core-shell hybrids at 2$\theta =12.5^{\circ}$ can be clearly found.[25] Furthermore, the intensity of the diffraction peak from PPy is increased with the increasing shell thickness, confirming successful coating of PPy on CNTs. Figure 4(b) compares the XPS survey spectra of pristine CNT and CNT@PPy-2. The C, N and O signals are observed in the XPS survey spectra of CNT@PPy-2, while only C and O signals appear in the XPS survey spectra of the pristine CNT, further confirming the successful coating of the PPy shell on the CNT.[26] In the high-resolution C $1s$ spectra of the CNT@PPy-2, the peaks with binding energies of 284.5, 285.8, 286.6, 287.8 and 289.5 eV can be indexed to C–C, C–N, C–O, C=O and O–C=O groups, respectively.[27] The N $1s$ spectrum of CNT@PPy-2 can be fitted by three deconvoluted peaks at 399.3, 399.8 and 401.4 eV, corresponding to –N=, –NH– and N$^{+}$, respectively (Fig. 4(d)).[26,28] It is clear that the –NH– is the dominant n-doping type in the hybrids. In addition, numerous functional groups (C–N and C–O) of carbon-heteroatoms in the hybrids can be considered as dipole active sites (Figs. 4(b) and 4(c)). Their symmetry centers are different from the original balance points, resulting in intensive dipole polarization, which further promotes the enhancement of EMW absorption performance.[29]
Table 1. Detailed comparison of the EMW absorption property of CNT@PPy hybrids to other absorbers.
Sample $R_{\rm L,min}$ (dB) EAB (GHz) Thickness (mm) References
Nanotube@TiO$_{2}$ sponge $-31.8$ 3.43 2.0 [2]
RGO-Fe$_{3}$O$_{4}$ composites $-15.8$ 2.80 2.0 [7]
RGO/MnFe$_{2}$O$_{4}$ composites $-29.0$ 4.88 3.0 [8]
Al$_{2}$O$_{3}$-coated FeCo nanocapsules $-30.8$ 3.2 2.0 [10]
24-Fe$_{3}$O$_{4}$/Al$_{2}$O$_{3}$/CNCs $-28.3$ 3.5 2.0 [13]
SiC-Fe composites $-31.6$ 4.4 2.0 [17]
Fe$_{3}$O$_{4}$/MWNTs hybrids $-35.8$ 2.32 3.0 [20]
Co/PPy nanocomposites $-33.0$ 4.77 2.0 [21]
Ag@PPy-26.2 sponge $-33.0$ 2.5 3.0 [22]
CNT@PPy-2 $-51.4$ 4.9 2.0 This work
CNT@PPy-3 $-43.1$ 4.3 1.5 This work
To elucidate the effect of the shell thickness on EMW absorption property, the reflection loss of the CNT@PPy core-shell hybrids is calculated,[30] $$\begin{align} &Z_{\rm in} =Z_{0} \sqrt {\frac{\mu_{\rm r} }{\varepsilon_{\rm r} }} \tanh\Big({\frac{j2\pi fd}{c}\times \sqrt {\mu_{\rm r} \varepsilon_{\rm r} } } \Big),~~ \tag {1} \end{align} $$ $$\begin{align} &R_{{\rm L}} (dB)=20\log \Big| {\frac{Z_{\rm in} -Z_{0} }{Z_{\rm in} +Z_{0} }} \Big|,~~ \tag {2} \end{align} $$ where $Z_{\rm in}$ represents the input impedance of the absorber, $Z_{0}$ stands for the impedance of free space, $f$ is the frequency of EMW, $c$ is the velocity of EMW in free space, $d$ refers the absorbers' thickness, and $\mu_{\rm r}$ and $\varepsilon_{\rm r}$ represent the complex permittivity and complex permeability, respectively. Figure 5 compares the EMW absorption properties of CNT@PPy core-shell hybrids with that of the pristine CNT. The $R_{\rm L,min}$ of the pristine CNT are only $-18.2$ dB at 17.5 GHz (Fig. 5(a)). In contrast, after being coated with the PPy shells, the EMW absorption properties are significantly improved (Figs. 5(b)–5(d)). Typically, the $R_{\rm L,min}$ values of CNT@PPy-1, CNT@PPy-2 and CNT@PPy-3 can reach $-26.5$ dB at 4.9 GHz, $-51.4$ dB at 11.8 GHz and $-43.1$ dB at 8.7 GHz, respectively. Furthermore, the EAB of the pristine CNT is only 1.2 GHz at $d$ of 2.0 mm. In contrast, CNT@PPy-1, CNT@PPy-2 and CNT@PPy-3 have the EAB values of 2.4 GHz at $d$ of 3.0 mm, 4.9 GHz at $d$ of 1.6 mm and 4.3 GHz at $d$ of 1.5 mm, respectively. In addition, in comprehensive view of the $R_{\rm L,min}$, EAB and the absorber film thickness, the CNT@PPy-2 and CNT@PPy-3 exhibit superior EMW absorption property. Furthermore, even at very small $d$ values (1.4–1.9 mm), the $R_{\rm L,min}$ values of CNT@PPy-2 and CNT@PPy-3 still exceed $-40$ dB, confirming their excellent EMW absorption properties (Fig. 6). In addition, the filler mass loading of the CNT@PPy into paraffin matrix is only 20 wt%. The lower filler mass loading, outstanding EMW absorption performance and wide absorption bandwidth of CNT@PPy-2 and CNT@PPy-3 suggest that they are promising candidates for lightweight absorbers. Compared to the CNT-based absorbers reported previously, CNT@PPy-2 and CNT@PPy-3 have favorably comparable or superior EMW absorption property (Table 1). To understand the superior EMW absorption properties of CNT@PPy core-shell hybrids, the impedance matching characteristics were evaluated. According to Eqs. (1) and (2), if input impedance of an absorber matches with the impedance of free space, the following formulas should be met, $$\begin{align} &{\rm Re } {Z_{\rm in} } \to Z_{0},~~ \tag {3} \end{align} $$ $$\begin{align} &{\rm Im } {Z_{\rm in} } \to 0,~~ \tag {4} \end{align} $$ where ${\rm Re }{{Z}_{\rm in} } $ and ${\rm Im }{{Z}_{\rm in} }$ represent the real and imaginary parts of ${Z}_{\rm in}$, respectively. For convince, normalized impedance ${Z}={Z}_{\rm in} /{Z}_{0}$ is used to estimate the impedance matching characteristic. When the absorbers have better impedance matching characteristics, the modulation of $Z$ is close to 1. As shown in Fig. 7, $|Z|$ values of the CNT@PPy-2 and CNT@PPy-3 are closer to 1 than those of the pristine CNT and CNT@PPy-1. Therefore, the enhanced EMW absorption performance of CNT@PPy-2 and CNT@PPy-3 is attributed to their better impedance matching characteristic. In addition, EMW absorption of an absorber is closely dependent on its EM parameters.
cpl-37-4-045201-fig5.png
Fig. 5. (a) Three-dimensional $R_{\rm L}$ of the CNT and CNT@PPy hybrids with different thicknesses (1.0–5.0 mm) from 2.0 to 18.0 GHz: (a) CNT, (b) CNT@PPy-1, (c) CNT@PPy-2, and (d) CNT@PPy-3.
cpl-37-4-045201-fig6.png
Fig. 6. The $R_{\rm L,min}$ values of (a) CNT@PPy-2 and (b) CNT@PPy-3 at very small $d$ values.
cpl-37-4-045201-fig7.png
Fig. 7. The $|Z|$–$f$ curves of CNT and CNT@PPy hybrids with different thicknesses (1.0–5.0 mm) from 2.0 to 18.0 GHz: (a) CNT, (b) CNT@PPy-1, (c) CNT@PPy-2, and (d) CNT@PPy-3.
Figure 8 shows the complex permittivity of the pristine CNT and CNT@PPy core-shell hybrids. According to the EM-wave energy conversion theory, $\varepsilon '$ stands for the storage ability of electric energy and $\varepsilon ''$ refers the dissipation ability of electric energy.[27] Figure 8(a) displays that the $\varepsilon '$ of the pristine CNT and CNT@PPy core-shell hybrids. All the $\varepsilon '$ values have a decreasing trend with increasing frequency, which is associated with the frequency dispersion behavior.[31] The $\varepsilon '$ values of CNT@PPy-1, CNT@PPy-2 and CNT@PPy-3 are about 12.7–8.1, 15.7–9.6 and 18.2–11.3, respectively, larger than those of CNT (8.2–5.7), suggesting that the PPy shells can improve the storage ability of electric energy. Obviously, $\varepsilon '$ values decrease in this order CNT@PPy-3 $>$ CNT@PPy-2 $>$ CNT@PPy-1, which is caused by the different amounts of pyrrole added. As we know, PPy has a strong charge storage ability. CNT@PPy-3 with the thickest PPy shell possesses the strongest charge storage ability among the three samples, and thereby it has the highest $\varepsilon '$ value. Thus, the dielectric properties of hybrids can be controlled by tuning the thickness of the PPy shell.[32]
cpl-37-4-045201-fig8.png
Fig. 8. (a) Real and (b) imaginary part of the permittivity for the CNT and CNT@PPy hybrids, (c) the dielectric loss of the CNT and CNT@PPy hybrids, and (d) attenuation constant of the CNT and CNT@PPy hybrids.
Figure 8(b) shows the $\varepsilon ''$ values of these samples. The CNT@PPy-2 and CNT@PPy-3 have similar $\varepsilon ''$ values over 2.0–18.0 GHz, which are greatly larger than those of CNT@PPy-1 and pure CNT. This result indicates that the PPy shell with thickness less than $\sim $7 nm can enhance the $\varepsilon ''$ values greatly. However, as the shell thickness exceeds $\sim $7 nm, the increase of the $\varepsilon ''$ values becomes inapparent. The reason may be related to the sources of the $\varepsilon ''$. In general, the $\varepsilon ''$ of carbon-based materials comes from conduction loss and polarization loss. In this work, CNT and PPy mainly contribute to conduction and polarization losses, respectively. Although the polarization loss will be increased with the increase of the thickness of the PPy shell, the conduction loss will also be decreased due to the PPy coating. Therefore, the balance between conduction loss and polarization loss results in the similar $\varepsilon ''$ values of CNT@PPy-2 to that of CNT@PPy-3. It is also worth noting that in the high-frequency range exceeding 10 GHz, the $\varepsilon ''$ values of all the samples remain almost constant without significant attenuation, indicating that the dissipation capacity of the samples are quite stable.[32] Figure 8(c) displays the changes of the dielectric loss tangent ($\tan\delta_{\rm e} =\varepsilon ''/\varepsilon '$) of these samples with change of the frequency. It is clearly observed that $\tan\delta_{\rm e}$ of CNT@PPy-2 at 2 GHz is about 0.60, then decreases with the increasing frequency, down to approximately 0.42 at 18 GHz, while the $\tan\delta_{\rm e}$ of CNT@PPy-3 declines from 0.62 to 0.36 in the region of 2.0–18.0 GHz. Thus, the $\tan\delta_{\rm e}$ values of CNT@PPy-2 are comparable to those of CNT@PPy-3 at a low frequency, and significantly higher than those of CNT@PPy-3 as the frequency is larger than 7 GHz. Meanwhile, the $\tan\delta_{\rm e}$ values increase in the order CNT@PPy-2 $\sim$ CNT@PPy-3 $>$ CNT@PPy-1 $>$ CNT, suggesting that the dielectric loss property is increased by the PPy coating. Therefore, the improved EMW absorption property of the hybrids is attributed to the increasing dielectric loss. In addition, the attenuation constant $\alpha$ can be used to assess attenuation capability of an absorber toward the incident EMW,[33] $$\begin{alignat}{1} \!\!\!\!\alpha =\,&\frac{\sqrt 2 {\rm \pi }f}{c}\big\{(\mu ''\varepsilon ''-\mu '\varepsilon ')^{2}\\ &+\big[(\mu ''\varepsilon ''-\mu '\varepsilon ')^{2}+(\mu '\varepsilon ''+\mu ''\varepsilon ')^{2}\big]^{1/2} \big\}^{1/2},~~ \tag {5} \end{alignat} $$ as shown in Fig. 8(d), the CNT@PPy-2 and CNT@PPy-3 possess larger $\alpha$ values than those of other samples, suggesting their outstanding attenuation abilities toward the incident EMW. Considering the $R_{\rm L}$, $\alpha$ and small usage of pyrrole monomer for final product, the CNT@PPy-2 is more suitable for high-performance EMW absorber. In summary, we have successfully fabricated CNT@PPy core-shell hybrids. The shell thickness could be controlled in the range of 2.0–7.0 nm. After PPy coating, the EMW absorption properties of CNTs are improved greatly. The optimized hybrids exhibit excellent EMW absorption property with $R_{\rm L,min}$ of $-51.4$ dB with a matching film thickness of 2.0 mm and an EAB of up to 3.5 GHz. Furthermore, the $R_{\rm L,min}$ values of the optimized hybrid are below $-30$ dB when the thickness is in the range of 1.4–1.9 mm, which can benefit the practical application. The improved EMW absorption properties of the hybrids are attributed to the increasing dielectric losses and better impedance matching characteristics. 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