Conducting polymer coated metal-organic framework nanoparticles: Facile synthesis and enhanced electromagnetic absorption properties

Conducting polymer coated metal-organic framework nanoparticles: Facile synthesis and enhanced electromagnetic absorption properties

Synthetic Metals 228 (2017) 18–24 Contents lists available at ScienceDirect Synthetic Metals journal homepage: Condu...

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Synthetic Metals 228 (2017) 18–24

Contents lists available at ScienceDirect

Synthetic Metals journal homepage:

Conducting polymer coated metal-organic framework nanoparticles: Facile synthesis and enhanced electromagnetic absorption properties


Yan Wang , Wenzhi Zhang, Xinming Wu, Chunyan Luo, Qiguan Wang, Jinhua Li, Lin Hu School of Materials and Chemical Engineering, Xi’an Technological University, Xi’an, 710021, PR China



Keywords: Polymers Nanocomposites Metal organic framework Microwave absorption property

A hybrid polyaniline coated metal organic framework (MOF/PANI) was designed and fabricated via hydrothermal and in situ chemical polymerization methods The microstructure and morphology were examined by XRD, FTIR, FESEM and TEM. The results showed that MOF (Fe) particles were completely coated by PANI and formed a core-shell composite. The hybrid MOF (Fe)/PANI composite exhibited enhanced EM wave absorption capability compared with MOF (Fe), including the higher absorption intensity (−41.4 dB at 11.6 GHz) and excellent absorption bandwidth (5.5 GHz exceeding −10 dB with an only thickness of 2 mm), which was due to enhanced interfacial effects, attenuation constant and the synergic effect between MOF (Fe) and PANI. Therefore, such a core-shell MOF (Fe)/PANI composite is a promising absorber for application in microwave absorption field.

1. Introduction In recent years, electromagnetic (EM) wave absorption materials have attracted increasing attention because of the rapid development of electromagnetic wave devices [1,2]. The EM wave absorbing materials can convert the incident EM wave into thermal energy to dissipate EM wave. According to the loss mechanism of EM wave, the microwave absorption materials are comprised of dielectric loss and magnetic loss, such as ferrite [3,4], nickel [5], cobalt [6,7], carbon nanotubes [8–10], conducting polymers [11,12] and graphene [13–16]. However, these traditional microwave absorbers can not satisfy the requirements (thin, light, wide and strong) of ideal absorbers at the same time due to a mismatch impedance. In terms of the EM energy principle, a proper impedance matching between permittivity and permeability determines the absorption intensity and wide frequency range characteristics of EM absorbers. Hence, it should be promising and meaningful to study the novel composite materials, which can accord with the demand of high performance EM wave absorbers for improving impedance matching feature. Metal organic framework (MOF) is a kind of hybrid porous materials, which has drawn much attention and shown various potential applications such as gas storage [17], catalysts [18,19], microwave absorption [20], sensors [21] and supercapacitors [22]. Owing to low permittivity and mismatch impedance, pure MOF displays poor microwave absorption properties. In recent years, organicinorganic nanocomposites have turned into a hot issue of research due

Corresponding author. E-mail address: [email protected] (Y. Wang). Received 26 January 2017; Received in revised form 22 March 2017; Accepted 7 April 2017 0379-6779/ © 2017 Elsevier B.V. All rights reserved.

to their synergetic or complementary effects [23]. Among kinds of conducting polymers, polyaniline (PANI) has attracted significant attention for EM wave absorbers because of its controllable conductivity, easy production, low price and good chemical stability [24]. Hosseini et al. [25], prepared MnFe2O4/PANI composites with a coreshell structure with dodecyl benzene sulfonic acid (DBSA) as the surfactant and dopant. The studies showed that there was an interaction between PANI and MnFe2O4, and the maximum reflection loss was −15.3 dB at 10.4 GHz for the 1.4 mm absorption thickness. Zhang et al. [26], synthesized Fe3O4/PANI hybrid microspheres via chemical oxide polymerization process and the composites displayed enhanced microwave absorption, which a maximum absorption of −37.4 dB at 15.4 GHz can be achieved with a PANI thickness of 100 nm. The introduction of PANI can not only decrease the weight of absorbers, but also lead to a proper impedance matching, which can provide an idea for ideal EM wave absorbers. Except for impedance matching characteristic, the microwave absorption properties of materials are closely associated with the special nanostructures. MOF materials possess the special porous structure and result in multiple scattering to increase microwave absorption, which is interesting for improving EM wave absorbing property due to the complicated geometrical morphologies. Herein, we reported the synthesis of novel MOF (Fe)/PANI composite. In the first step, MOF (Fe) particles were prepared by hydrothermal route. Then, PANI was coated on the surface of MOF (Fe) by chemical polymerization method. The microwave absorption results indicate that the core-shell structure exhibits improved microwave absorption capa-

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Fig. 1. Schematic illustration of the fabrication process of MOF (Fe)/PANI composite.

Fig. 2. XRD patterns of samples (a. MOF (Fe), b. MOF (Fe)/PANI).

cities in terms of the maximum microwave absorption and effective bandwidth, which is attributed to multi-interfaces, geometric effect and impedance match of MOF (Fe) cores and PANI shells. The maximum reflection loss can achieve −41.4 dB at 11.6 GHz and the absorption bandwidth below −10 dB is 5.5 GHz with a thickness of 2 mm. The microstructure and morphology of the as-prepared composites were examined by XRD, FESEM, TEM, BET and FTIR. 2. Experimental 2.1. Preparation The core-shell MOF (Fe)/PANI composite were prepared as illustrated in Fig. 1. The MOF (Fe) particles were prepared by hydrothermal route. 0.81 g FeCl3·6H2O and 0.5 g terephthalic acid were dissolved in 64.7 ml DMF and stirred for 30 min. The obtained solution was transferred into a 100 ml Teflon-lined autoclave and maintained at 150 °C for 12 h. After letting it cool down to room temperature naturally, the products were washed three times with ethanol and water, then dried at 150 °C for 8 h. 0.4 g of the resulting MOF (Fe) was dispersed in 150 ml acidic aqueous solution and the solution was cooled

Fig. 3. FTIR spectra of MOF (Fe), PANI and MOF (Fe)/PANI composite.

down to 0 °C under stirring, then 0.2 ml aniline was added and stirred for 20 min. The (NH4)2S2O8 (0.5 g) dissolved in 20 ml distilled water was added slowly to the mixture with constant stirring for 24 h at 0 °C. Lastly, the reactants were washed and dried to get MOF (Fe)/PANI composite.


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Fig. 4. FESEM images of MOF (Fe) (a-c) and MOF (Fe)/PANI (d-f).

Fig. 5. TEM images of MOF (Fe) (a, b) and MOF (Fe)/PANI (c, d).

emission scanning electron microscope (FESEM, Quanta 600FEG) and transmission electron microscopy (TEM, JTM-2100). The N2 adsorption-desorption curves were recorded on a Quad-rasorb-SI instrument, and the specific surface area was examined by the Brunauer-EmmettTeller (BET) process. The relative complex permittivity and permeability were measured in the 2–18GHz by a vector network analyzer

2.2. Characterization The crystal phase was characterized by X-ray diffraction (XRD) methods (German Bruker D8 with Cu-Kα radiation, λ = 0.15406 nm) and Fourier transform infrared spectroscopy (Thermo SCIENTIFIC Co.USA). The morphology characteristics were characterized by field


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nm–1 μm, which suggests good crystalline nature. As can be seen in Fig. 4(d–f), the exposed surfaces of MOF (Fe) polyhedrons are not smooth, which confirms the formation of PANI layer outside the surfaces of polyhedrons. From Fig. 5(a and b), it can be seen that the MOF (Fe) particles mainly present leaf-like structure and the size of particles is consistent with the result of FESEM images. A great amount of PANI was coated on MOF (Fe) to form a core-shell structure, which had a thickness of about 10–30 nm and a regular leaf-like structure in Fig. 5(c and d). The rough edge suggests that a thin cladding layer of PANI coated the surface of MOF (Fe). The porosity and specific surface area of as-prepared products can be determined by the nitrogen adsorption-desorption measurements. As seen in Fig. 6a, the hysteresis loop of MOF (Fe) is developed between 0.8P/P0 and 1.0 P/P0, revealing the presence of meso-pore (2–50 nm) and macro-pore (> 50 nm). According to the N2 adsorption-desorption curves, BET surface area, pore diameter and pore volume of MOF (Fe) are about 22.36 m2 g−1, 134.22 nm and 0.11 cm3 g−1, respectively. In Fig. 6b, the area of hysteresis loop (MOF (Fe)/PANI) is much greater than MOF (Fe), demonstrating that the introduction of PANI is beneficial to increasing the porosity and BET surface area of the MOF (Fe). As calculated by the BET method, the BET surface area of MOF (Fe)/PANI is measured to be 90.23 m2 g−1, and the pore diameter and pore volume achieve 55.27 nm and 0.124 cm3 g−1, respectively. The larger BET surface area and higher pore volume makes contribution to high performance microwave absorption materials. Fig. 7 illustrates the complex relative permittivity (ε = ε′–jε″), permeability (μ = μ′–jμ″), dielectric loss tangent (tan δε = ε″/ε′), magnetic loss tangent (tan δm = μ″/με′), attenuation constant (α) and impedance matching characteristics (Z). From Fig. 7a and b, we can observe that the ε′ and ε″ values of MOF (Fe)/PANI have a slightly decrease in the range of 4.9–11.2 and 0.5–5.8 in 2–18 GHz, respectively. It is clearly observed that both ε΄ and ε̋ values are higher than those of MOF (Fe), which is attributed to the high conductivity of PANI coated on the MOF (Fe), the results indicates that the existence of PANI enlarges the dielectric constant. Meanwhile, the higher dielectric constant of MOF (Fe)/PANI may be attributed to the improved interfacial polarization and appearance of localized defects as bipolaron/polaron. In Fig. 7c and d, it is clearly that the μ′ values of all the samples present complex fluctuations in the 2–18 GHz, whereas, the μ″ values are large at low frequency, and decrease gradually with the increasing frequency, exhibit broad resonance peaks around 14–14.9 GHz. Based on the above data, we can calculate the dielectric loss and magnetic loss and the results are illustrated in Fig. 7e and f. It can be observed that the tan δε values of MOF (Fe)/PANI vary between 0.06 and 0.54 over 2–18 GHz range, which is higher than those of MOF (Fe). Meanwhile, the tan δm values of two samples are similar and play an important role in low-frequency region. The complementarities between permittivity and permeability are also very crucial on the excellent microwave absorbing properties. The tan δε values are equal to the tan δm, the absorber will show the best EM absorption properties [31]. From Fig. 7e and f, we can see that the tan δε and tan δm values of MOF (Fe)/PANI vary from 0.06–0.54 and 0.06–1.4, respectively, thus the relatively complementarities between dielectric loss and magnetic loss can enhance the EM wave absorption property of MOF (Fe)/PANI. In addition, the impedance matching characteristics (Z = |Zin/Z0|) in the equation, is another decisive factor to EM wave absorption property, which is an important parameter for improving absorption of electromagnetic wave at the air-absorber interface [32].

Fig. 6. N2 adsorption-desorption isotherms and pore diameter distribution of MOF (Fe) (a) and MOF (Fe)/PANI (b).

(HP8720ES), and the measured samples were prepared by a cylinder shape (OD 7 mm and ID 3.04 mm), uniformly blended with a paraffin matrix (70 wt%). 3. Results and discussion The crystal structures were examined by XRD and the results were illustrated in Fig. 2. For MOF, the well-crystallization diffraction peaks demonstrated the high crystallinity of the samples, which are accorded with the MOF–88B (Fe) structure [27]. For MOF (Fe)/PANI, these diffraction peaks had remained unchanged and were much weaker than those of MOF (Fe), which indicated that the crystal structure of MOF (Fe) was well preserved and there was no decomposition of the framework structure. Meanwhile, no other peaks can be detected, revealing that the high purities of MOF (Fe)/PANI composite were successfully synthesized. Fig. 3 shows FTIR spectra of MOF (Fe)/PANI composite. The samples are obtained by KBr tabletting and the weight ratio of KBr to sample is 100:1. The characteristic peaks at 1546 and 1405 cm−1 are ascribed to the C]N and C]C stretching vibrations of quinonoid and benzene rings, respectively [28]. The peak at 1308 cm−1 is associated with CeN stretching of the benzenoid ring [29]. The bands at 1161 and 1103 cm−1 are related to vibrational modes of N]Q]N (Q is the quinonic type rings). The peaks at 1019, 879, 817 and 757 cm−1 are ascribed to the aromatic ring CeH out-of-plane bending [30]. Furthermore, the strong absorption band at 547 cm−1 could be corresponded to the characteristic band of MOF (Fe), which suggests successful formation of MOF (Fe)/PANI composite. The morphology and microstructure of the synthesized-products were examined by FESEM and TEM. From Fig. 4(a–c), MOF (Fe) particles composed of polyhedrons, with an average size of about 500

Z = Z in Z 0 =



⎡ ⎛ ⎤ 2πfd ⎞⎟ μ ε ⎥ εr tanh ⎢j ⎜⎝ c⎠ r r ⎣ ⎦


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Fig. 7. The complex permittivity (a, b), the complex permeability (c, d), dielectric loss tangent (e), magnetic loss tangent (f) and attenuation constant (h) of samples, the relationship between Zin/Z0 and electromagnetic wave frequency of MOF (Fe)/PANI composite (g) with different coating thicknesses.


The impedance matching of the absorber should be close to that of the free space, resulting in zero reflection at interface. When Z is equal or close to 1, most of the microwaves can enter into the absorber and will be converted to heat to dissipate, resulting in zero reflection. The Z values for MOF (Fe)/PANI with different thicknesses are illustrated in Fig. 7g. For a layer of 2 mm, the maximum RL (−41.4 dB) can be gained at 11.6 GHz and the Z is equal 1, revealing that the well-matched characterized impedance also play an important role on microwave absorption properties. With respect to the attenuation constant (α) of sample, it can be expressed by the following equation [33]:


2 πf × c

(μ"ε" − μ′ε′) +

(μ"ε" − μ′ε′)2 + (μ′ε" + μ"ε′)2

RL = 20 log (Z in − Z 0 ) (Z in + Z 0 )

Z in = Z 0

⎛ 2πfd μr εr ⎞ μr tanh ⎜j ⎟ c εr ⎝ ⎠



In the above equations, Zin is the input characteristic impedance of the absorber, Z0 is the impedance of free space, c and f are the velocity and frequency of electromagnetic wave in free space, d is the thickness of the absorber, and εr and μr are the complex relative permittivity (εr = ε'–jε″) and complex permeability (μr = μ'–jμ″) of the absorber medium, respectively. According to the Eqs. (3) and (4), the calculated RL values of samples with different thicknesses are shown in Fig. 8. As illustrated in Fig. 8a, the MOF (Fe) exhibits a maximum RL value of −13.9 dB at 14.4 GHz for a thickness of 3.5 mm and the bandwidth exceeding −10 dB is very narrow, revealing weak EM wave absorption abilities. After coating PANI, it can be found that the core-shell MOF (Fe)/PANI structure reflects significantly improving EM wave absorp-


It is clearly seen from Fig. 7h that the core-shell MOF (Fe)/PANI structure always possess larger attenuation constant than MOF (Fe), suggesting its superior attenuation ability for the incident EM waves. As we know, according to the transmission line theory, microwave absorption performance of materials can be estimated by the reflection loss values, which could be calculated based on the following equations


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strated by the following facts. Firstly, the enhanced interfacial polarization, dipole polarization and charge transfer between MOF (Fe) and PANI are beneficial for EM wave absorption [35]. Secondly, multiple reflection and scattering of the incident EM wave happen in the pores or cavities, which is advantageous for extension of EM wave energy attenuation [36]. Thirdly, the dissipation of microwave energy stems from the dielectric loss, magnetic loss, good impedance matching (Z), improved attenuation constant (α), which are vital for the microwave absorption performance. In addition, the enhanced microwave absorption performance of the core-shell MOF (Fe)/PANI composite may be due to the polyhedron geometrical morphology of MOF (Fe) [28]. In short, the core-shell MOF (Fe)/PANI composite can meet the advantages of intensive absorption, broad bandwidth, light weight feature and is supposed to be a potential in the area of microwave absorption materials.

4. Conclusion In summary, a novel MOF (Fe)/PANI composite was fabricated by hydrothermal and chemical oxidative route. The polyhedron MOF (Fe) particles were first obtained, and then PANI grew around the MOF (Fe) particles via oxidative polymerization method. Investigations of microwave absorbing properties indicate that the core-shell composite exhibits significantly improved EM wave absorption properties in terms of the maximum absorption intensity and absorption bandwidth. The maximum RL values of the composite can reach −41.4 dB at 11.6 GHz and the absorption bandwidth below −10 dB is up to 5.5 GHz with a thickness of only 2 mm. It is believed that MOF (Fe)/PANI core-shell composite can be developed as a promising EM wave absorber with the merits of thin thickness, lightweight, broad bandwidth and strong absorption.

Fig. 8. Reflection loss curves of MOF (Fe) (a) and MOF (Fe)/PANI composite (b).

tion performance, in terms of both the maximum RL value and the absorbing bandwidths (Fig. 8b). The maximum RL value of the MOF (Fe)/PANI composite can achieve −41.4 dB at 11.6 GHz and the absorption bandwidth of RL below −10 dB is 5.5 GHz (from 9.8 to 15.3 GHz) for an only thickness of 2 mm. The results suggest that the adding of PANI can not only enhance the EM wave absorption capacities, but also decrease the thickness Fig. 9 shows the schematic illustration of the EM wave absorption mechanism for MOF (Fe)/PANI composite. The superior EM wave absorption capacities of MOF (Fe)/PANI composite can be demon-

Acknowledgements The authors acknowledge the financial support from the National Natural Science Youth Foundation of China (Grant No 51303147, No 51502233 and No 21506167) and President’s Fund of Xi’an Technological University (project No. XAGDXJJ16002 and XAGDXJJ5011).

Fig. 9. Schematic illustration of the absorption mechanism for MOF (Fe)/PANI composite.


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