Hybridization-Induced Polarization of Graphene Sheets by

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Surfaces, Interfaces, and Applications

Hybridization-induced Polarization of Graphene Sheets by Intercalationpolymerized Polyaniline towards High Performance of Microwave Absorption Yifan Guo, Jinyang Li, Fanbin Meng, Wei Wei, Qian Yang, Ying Li, Huagao Wang, Fuxi Peng, and Zuowan Zhou ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b04498 • Publication Date (Web): 09 Apr 2019 Downloaded from http://pubs.acs.org on April 9, 2019

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ACS Applied Materials & Interfaces

Hybridization-induced Polarization of Graphene Sheets by Intercalation-polymerized Polyaniline towards High Performance of Microwave Absorption Yifan Guo, Jinyang Li, Fanbin Meng*, Wei Wei, Qian Yang, Ying Li, Huagao Wang, Fuxi Peng, 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. *Corresponding author: [email protected] (Fanbin Meng); [email protected] (Zuowan Zhou)

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Abstract An intercalation polymerization is applied to regulate the hybridizing structures of polyaniline@graphene (PANI@GE). Polarization of graphene sheets is realized, which is attributed to the hybridization by the in situ intercalation-polymerized polyaniline molecules. The polarizing effect on graphene is confirmed by characterizations and DFT calculations, and the results indicate that it exists distinct p-π and π-π interactions between the PANI molecules and the graphene sheets. As a result, this new structural hybrid leads to a high performance of microwave absorption. The minimum reflection loss (RL) of the optimized PANI@GE hybrid can be as low as −64.3 dB at 10.1 GHz with the RL bandwidth of -10 dB being 5.1 GHz (from 8.6 to 13.7 GHz). Further study reveals a special mechanism for the electromagnetic energy consumptions by the structural resonance of the polarized graphene-based hybrids, a complex macromolecule. In addition, the fully separated GE provides a good impedance matching, together with the widelyheld multi-scaled relaxations of the interfacial polarization. Keywords: polarization of graphene, polyaniline, hybridization, structural resonance, microwave absorption


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The structural tuning and functional design of graphene are key issues for its multifunctional applications. By hybridization, doping or grafting, graphene has been expected to be used in various fields, such as microelectronics, supercapacitors, photocatalysis and microwave absorption1-6. Taking the case of microwave absorption (MA), incorporating graphene with lossy materials can introduce more interfaces and polarization centers, together with a good impedance matching, both of which effectively improve the MA performance7-12. Among them, the incorporation with conductive PANI is preferred due to, not only addressing the interface impedance mismatching of graphene13-14, but also bringing obvious Debye relaxation processes to the PANI/graphene composites15-16. In order to introduce more polarization centers and achieving high impedance matching, it is proposed to fabricate ternary or quaternary-composites, such as graphene@polyaniline/NiFe2O4 composites17, magnetic graphene@polyaniline@porous TiO2 ternary composites18, and polyaniline nanorod arrays covalently-grafted graphene@Fe3O4@C19. However, most of the previous reports overemphasize the design of complex synthetic steps for pursuing the enhancement of electromagnetic synergy. Little attention is focused on the structure characteristics of hybrids which might be the key to achieve a high performance MA. In our previous report on the in situ intercalation polymerization, fibrous PANI synthesized in the interlayer of graphite results in the separation of graphene layers20. But the bloated PANI aggregations are not facilitate to the hybridization between PANI molecules and graphene layers. Herein, we further regulate the interfaces of PANI@GE hybrids by fine-tuning the process of in situ intercalation polymerization. Based on the experimental characterizations and the DFT calculations, some new structures have been recognized during this process. The polarization of graphene is achieved because of the special interaction between the PANI molecules and the GE layer. As a result, the macromolecules of the polarized graphene possess high efficient


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consumption of incident microwave energy by structural resonance. The good impedance matching, together with the widely-held multi-scaled relaxations of the interfacial polarization also facilitate the improvement of MA performance. The minimum RL of the optimized PANI@GE hybrid can be as low as −64.3 dB at 10.1 GHz with the RL bandwidth of -10 dB being 5.1 GHz (from 8.6 to 13.7 GHz), indicating a superior microwave absorption performance.

Results and Discussion According to the DFT calculation, the aniline cation made of aniline and H+ tends to be drawn towards the electron-enriched zone and then intercalates into the interlayer of graphite20. Subsequently, an in situ polymerization leads to the separation of graphite into graphene sheet. It is necessary to recognize the detailed onsite-forming structures and related functional regulations. A series of PANI@GE hybrids were formed with polymerization time of 1, 2, 4, 6, 8 and 12 h, respectively. PANI@GE−x h refers to hybrids with different polymerization duration, where x refers to the reaction time. The morphologies of the associated hybrids were characterized by SEM and TEM as depicted in Figure 1 and Figure S1. In the first 4 hours of in situ intercalation polymerization, the stacked graphite can be still recognized in PANI@GE−1 h (Figure 1a, 1a') and PANI@GE−2 h (Figure S1a), indicating limited exfoliation of graphite. When the reaction goes to 4 hours, the highly-stacked layers of natural graphite (NG) are exfoliated into few-layered graphene as proved by TEM image in Figure 1b'. Meanwhile, the SEM image (Figure 1b) depicts homogeneous distribution of PANI, which results in a sufficient contact between the PANI and the graphene molecules. With a longer reaction time, superfluous PANIs coat on the graphene (Figure 1c, S1b and S1c), and massive aggregations of PANIs are distinguished. Thus, the PANI@GE hybrids synthesized with 1, 4 and 12 h are selected for further structural analysis due to their representative morphological characteristics.


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Figure 1. Morphology of PANI@GE hybrids. a-c. SEM and a'-c'. TEM images of PANI@GE hybrid obtained by polymerizing time of 1 h, 4 h and 12 h, respectively. Detailed characterizations were carried out to recognize the structures of the PANI@GE hybrids. The XRD patterns for the PANI@GE hybrids are illustrated in Figure 2a. For the sample of PANI@GE−1 h, the sharp characteristic peak for the (002) plane of NG is observed at 2θ=26.5°, indicating the existence of graphite layers. However, this peak disappears in PANI@GE−4 h and PANI@GE−12 h samples. That means NG has been effectively exfoliated in 4 h or a longer reaction duration, which agrees with the morphology characterization in Figure 1. Raman spectra were used to investigate the interactions in PANI@GE. As shown in Figure 2b, the D-band and G-band of graphene tend to overlap with the peaks of PANI to form a diffuse peak, indicating the formation of hybrid structure in PANI@GE−4 h. Although depicting the same tendency, the Dband and G-band of graphene can still be identified in PANI@GE−1 h, which is due to the unexfoliated graphite as confirmed by SEM/TEM images and XRD pattern. However, the Raman spectrum for the PANI@GE−12 h is similar to bare PANI (Figure S2b). That is attributed to the existence of massive PANI aggregations, thus the signal for hybrids is submersed by that of the PANI.


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As the interactions may influence the valence state of the elements in hybrids, XPS analysis were carried out and the spectra are exhibited in Figure 2c-e. The N 1s spectrum of PANI@GE−1 h (Figure 2c) is de-convoluted into four peaks. These peaks refer to four type of N atoms in –NH+(402.35 eV), –NH+·- (400.73 eV), -NH- (399.36 eV) and =N- (397.97 eV), respectively21. But for PANI@GE−4 h, the peaks of –NH+- and –NH+·- shift to lower binding energy while the peaks of –NH- and =N- shift to higher binding energy. This phenomenon confirms that the interfacial hybridization acts on the N atoms in hybrids, and the PANI@GE−4 h has a higher degree of interfacial hybridization than the PANI@GE−1 h. However, these peaks for PANI@GE−12 h shift back, which are near to bare PANI (Figure S2c). Obviously, the aggregations of PANI in PANI@GE−12 h can’t donate to the interactions between graphene and PANI. Moreover, the ponderous PANI coating on graphene make it difficult to obtain the XPS signal from the hybrid interfaces. Therefore, the spectrum of the PANI@GE−12 h is similar to that of the bare PANI.

Figure 2. Structural characterization of PANI@GE hybrids. a. XRD patterns and b. Raman spectra (color coded). XPS (N 1s) of PANI@GE synthesized with different polymerization duration. c. 1 h, d. 4 h and e. 12 h.


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To gain a profound understanding of the interactions, the ATR-FTIR spectra were applied to track the structure of PANI@GE during in situ intercalation polymerization. Figure 3a-3c shows the second-order derivative derived from the ATR-FTIR spectra. The bands at ~1303 (Figure 3a) and 1559 cm-1 (Figure 3b) are attributed to the C-N and C=N stretching vibrations in Q=N-B (where Q represents the quinone ring and B the benzene ring) structure in polyaniline, respectively22. In bare PANI, the C-N and C=N stretching vibration exhibits red and blue shift (Figure S3), respectively. It is the electron cloud trade-off in C-N and C=N that leads to the different shifts on their FTIR spectra. However, both the tertiary amine group attached to quinone ring display blue shift (Figure 3a and 3b) in the PANI@GE hybrids, which is quite different from that of the bare PANI. In the hybrids, graphene could provide PANI with extra electrons so that the electron density near both tertiary amine groups are increased, which results in the coinstantaneous blue shift of C-N and C=N stretching vibrations in the PANI@GE hybrids. On the other hand, it is also noticeable that the secondary amine group in B-N-B structure displays a red shift during the in situ intercalation polymerization (Figure 3c), indicating a decreased electron density around. It is noteworthy that the shift of peaks stops after 4 hours of in situ intercalation polymerization (green lines in Figure 3). That is due to the saturation of contact area between PANI and graphene as shown in Figure 1b, which has resulted in sufficient interface hybridization. A longer reaction time contribute little to the hybridization because of the aggregations of PANI. Thus, the PANI@GE fabricated via in situ intercalation polymerization displays a distinct structure. Not only effective exfoliation of graphite is achieved, but also homogeneous intercalation and hybridizing structure can be obtained, which forms strong interfacial interactions between the PANIs and the graphenes.


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Figure 3. Second derivative ATR-FTIR spectra for a. C-N group and b. C=N group stretching vibration in Q=N-B, where both of them display blue shift during the in situ intercalation polymerization. The black numbers (1303 and 1559 cm-1) and blue ones (1306 and 1565 cm-1) in the graphs indicate the peak positions before and after polymerization, respectively. c. The second derivative ATR-FTIR spectra of C-N stretching vibration in B-N-B, which displays red shift from 1244 to 1241 cm-1. The structure of PANI@GE was studied by first principle calculations to further confirm the experimental results. The calculated electron density difference map is shown in Figure 4a. Here, N1 and N2, N3 and N4 correspond to the secondary amine in B-N-B structure and the tertiary amine in Q=N-B structure. The red and yellow areas correspond to the regions of electron accumulation and electron depletion near the tertiary and secondary amine groups in PANI, respectively, depicting the same trend as shown in the FTIR spectra (Figure 3). The electronic density of state (DOS) and partial density of state (PDOS) are calculated for revealing the nature of the interactions in the hybrids, which has been proved by structural characterization. The displacement of outer electrons of PANI in hybrids is observed in Figure 4c. That can intrinsically be the result of the interactions between C 2p orbits (in graphene) and N 2p orbits (in PANI) as confirmed in Figure S4. It is considered to be a convincing proof for the p-π interactions between secondary/tertiary amine in PANI and graphene. Moreover, π-π stacking between the hexagonal


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plane of graphene and the benzene/quinone ring also contributes to the PANI and graphene interactions. The electron accumulation on benzene/quinone ring of PANI (Figure 4a) leads to the blue shifts of C=C (Figure S5a) and C=N (Figure S5b) stretching vibration in Q=N-B, which is attributed to the π-π interactions formed during in situ intercalation polymerization.

Figure 4. Simulation results of PANI@GE hybrids. a. The electron density difference map of PANI@GE with an isovalue = 0.002. The electronic density of state (DOS) for b. PANI@GE hybrids, c. the partial density of state (PDOS) for PANI in hybrids and d. the DOS of PANI. To investigate the MA performance of the PANI@GE hybrids, the frequency-dependent electromagnetic (EM) parameters were measured, and the reflection loss RL of the microwave were calculated according to the transmission line theory23. Figure 5a demonstrates the RL values of the PANI@GE hybrids for the assumed thickness of 2.9 mm. It is found that the RL peak gradually shifts to a lower frequency in the initial stage of in situ intercalation polymerization. The RL value is down to -64.3 dB (at 10.1 GHz) for the PANI@GE−4 h sample with the RL bandwidth of -10 dB being 5.1 GHz (from 8.6 to 13.7 GHz). The thickness dependence of the RL value for the PANI@GE−4 h is shown in Figure 5b, indicating a wide frequency bandwidth. Thus, the


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synthesized hybrids exhibit a superior microwave absorption ability, better than that of most PANI@GE composites as summarized in Figure S8.

Figure 5. Microwave absorption capacity of PANI@GE. a. The RL of PANI@GE with a thickness of 2.9 mm. b. The thickness dependence of reflection loss for PANI@GE-4 h. The superior MA performance for the PANI@GE−4 h sample should be attributed to its hybridizing structures. As proved above, the strong p-π and π-π interactions exist between the confined PANI molecules and the graphene sheets, which induce the electron cloud transfer at the interfaces. This leads to some changing of the symmetrically patterned π electron cloud on the graphene sheets, and the graphene is transformed to be a polarized structure due to the asymmetric electron cloud distribution as shown in Figure 4a. It is known that a polar molecule is able to move under the electromagnetic field because of its non-coincident center of positive and negative charge. Similarly, the polarized graphene acts with the incident microwave, a representative alternating electromagnetic field, resulting in the structural resonance to consume the microwave energy. Thus, the strong and sharp RL peak in Figure 5 is demonstrated by the structural resonanceinduced microwave absorption. For other PANI@GE hybrids, either unexfoliated graphite (Figure 1a, a') or ponderous PANI coating/aggregations (Figure 1c, c') hinder the structural resonance, only lead to a weak MA performance.


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The complex permittivity of PANI@GE hybrids is given in Figure 6 to further understand their MA performance. The real part (ε′) and imaginary part (ε″) of the complex permittivity as displayed in Figure 6a and 6b. In general, ε′ and ε″ of each samples decrease with an increase of frequency, indicating a frequency dispersion behavior induced by the enhanced polarization lagging in the high frequency24. The ε′ for PANI−1 h is in the range of 5.0-7.7, and that value increase to 5.4-9.7, 6.1-12.0 and 6.8-12.2 with the intercalation polymerization time of 4, 6 and 12 h, respectively, suggesting the improvement of energy storage. It is known that the introduction of dielectric PANI can improve the dielectric property of graphene-based materials18. Therefore, the higher ε′ values are mainly arise from the growth of PANI during the polymerizing process. The imaginary part of the complex permittivity symbolizes the dissipation capability of the PANI@GE hybrids. From Figure 6b, it can be observed that the ε′′ of hybrids also have an upward tendency. That is attributed to the exfoliation of NG and the formation of interfaces between the PANI and the graphene, which enhances the interfacial polarization loss18, 25. The dielectrical dissipation factors (tan δε) derived from complex permittivity are presented in Figure 6c. It can be seen that PANI@GE−6 h and PANI@GE−12 h, which contain massive free PANIs, display higher values of tan δε, indicating strong dielectric loss capability. However, a high dielectric loss ability can cause the impedance mismatching, which is detrimental to MA performance26.


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Figure 6. Electromagnetic parameters of PANI@GE. a. real part (ε′) and b. imaginary part (ε″) of the complex permittivity, and c. the dielectrical dissipation factor (tan δε), the arrow points out the resonance peak for PANI@GE-4 h. A resonance peak (as marked by the arrow in Figure 6c) is observed only for PANI@GE−4 h in the frequency range 11-15 GHz, implying the dielectric relaxation behavior of the hybrids27-29. The relaxation process can be described by a Cole−Cole semicircle, which is deduced based on the Debye theory30. The Cole−Cole plots for PANI@GE hybrids are given in Figure 7 together with their electrical conductivities. The optimized hybridizing structure (PANI@GE−4 h) possesses the lowest electrical conductive and the largest radius of Cole-Cole semicircle while the tail became shorter than other samples as illustrated in Figure 7b. These phenomena indicate that the enhancement of polarization loss and the weakening of conductive loss in PANI@GE−4 h when compared with other samples, which is due to the formation of intercalation and hybridizing structure24. As mentioned above, PANI@GE−4 h has realized sufficient contact and interfacial hybridization between PANI and graphene, thus the p-π and π-π interactions can introduce more polarization center at interface. Moreover, the polarized graphene has an asymmetric electron cloud, which causes further polarization relaxation on the graphene plane. Therefore, the multiscaled polarization relaxations are realized to improve the microwave loss in the intercalation hybrids. Although the enhancement of polarization loss in PANI@GE−4 h, conductivity loss also contributes to its microwave loss, which is recognized by the conspicuous tail in Cole-Cole plot24, 31.

When it comes to PANI@GE− 6h and PANI@GE−12 h, the Cole-Cole semicircle fade away

in Figure 7c and 7d, combining with an increment of electrical conductivity. This change originates from the increasing content of PANI aggregations. Therefore, the dielectric loss is dominated by the conductivity loss of the massive PANI, which overshadows the interfaces polarization32.


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Figure 7. Cole-Cole plots of PANI@GE synthesized with different polymerization duration. a. 1 h, b. 4 h, c. 6 h and d. 12 h. Apart from the loss ability, impedance matching is another factor for improving MA performance. A delta-function method was proposed to evaluate the degree of EM impedance matching as depicted in ref. 33. The calculated delta value maps for PANI@GE hybrids are given in Figure 8. In general, the small delta value (|Δ|

Hybridization-Induced Polarization of Graphene Sheets by

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