Composition design and structural characterization of MOFs-derived

Research Article pubs.acs.org/journal/ascecg

Composition Design and Structural Characterization of MOF-Derived Composites with Controllable Electromagnetic Properties Wei Liu,† Lei Liu,† Guangbin Ji,*,† Daoran Li,† Yanan Zhang,† Jianna Ma,† and Youwei Du‡ †

College of Materials Science and Technology, Nanjing University of Aeronautics and Astronautics, No. 29 Yudao Street, Nanjing 210016, P. R. China ‡ Laboratory of Solid State Microstructures, Nanjing University, No. 22 Hankou Road, Nanjing 210093, P. R. China S Supporting Information *

ABSTRACT: Simply and effectively achieving the tunability of the composition and chemical state of each component remains a challenge for modifying the electromagnetic performance of metal− organic-framework-derived (MOF-derived) composites. In this work, quaternary ZnO/Fe/Fe3C/carbon composites have been successfully synthesized by thermal decomposition of FeIII-MOF-5. The composition and chemical state of each component can be effectively controlled by changing the heating temperature. In detail, with increasing temperature, the Fe element would be transformed from Fe3+ to Fe3C and Fe, which also leads to the graphitization and weight loss of carbon. The effects on electromagnetic properties are also investigated, and the ZFC-700 sample possesses optimized reflectionloss (RL) performance with an RL value of −30.4 dB and a broad effective frequency bandwidth of 4.96 GHz at a thin thickness of only 1.5 mm. Conduction loss, interfacial polarization, ferromagnetic resonance, and interference cancelation should be responsible for ideal electromagnetic absorption. The porous quaternary composites not only convert incident electromagnetic energy to heat rather than reflect it back which is in favor of solving electromagnetic pollution, but also reduce the consumption of the metal source and poisonous raw materials for traditional microwave-absorbing materials. KEYWORDS: MOF-derived composites, Metallic carbides, Composition design, Tunable electromagnetic properties, Microwave absorption

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INTRODUCTION Recent years have witnessed expanded growth in electronic devices, accompanied by serious electromagnetic (EM) pollution, which not only threatens human health but also hinders the normal operation of various kinds of commercial and military equipment.1 Electromagnetic-shielding and microwave-absorbing (MA) materials are both commonly used against electromagnetic pollution. Although they can protect organisms and devices, distinct targets and mechanisms lead to varied environmental consequences. When an electromagnetic wave is incident on a material, there would be three forms: the reflectance (R), absorbance (A), and transmittance (T). The relationship can be described as R + A + T = 1.2 Electromagnetic-shielding materials aim to obtain the smallest T while microwave-absorbing materials try to obtain the weakest R. The difference in target results in diverse mechanisms, namely, shielding materials pay more attention to R while absorbing materials emphasize A. From the viewpoint of energy, incident electromagnetic energy can be converted into thermal energy or other forms of energy in absorbing materials while weak transformation occurs in shielding materials. Thus, absorbing materials may be more efficient in completely dealing with electromagnetic pollution. © 2017 American Chemical Society

For practical applications, a microwave absorber must meet the requirements of having thin thickness, being lightweight, and having a broad effective frequency bandwidth (fe) and strong absorption intensity.3 However, traditional MA materials are not able to achieve the balance between reflection-loss (RL) performance and application conditions. Generally, MA materials can be classified as dielectric- and magnetic-loss materials according to the attenuation mechanism.4,5 On one hand, dielectric-loss materials usually possess a strong absorption intensity, while a broad fe is hard to reach and is restricted by the material’s bad impedance matching and limited loss mechanisms including conduction loss and a polarization process.6 On the other hand, a strong absorption intensity and broad fe can be obtained by magnetic-loss materials. Nevertheless, the reported thickness is usually larger than 3 mm, and the filling ratio is mostly larger than 80% which is far from satisfactory. Thus, efforts have been devoted to the synthesis of composites which may integrate the advantageous properties of dielectric and magnetic components.7 Received: May 14, 2017 Revised: July 7, 2017 Published: July 20, 2017 7961

DOI: 10.1021/acssuschemeng.7b01514 ACS Sustainable Chem. Eng. 2017, 5, 7961−7971

Research Article

ACS Sustainable Chemistry & Engineering

Figure 1. PXRD pattern (a) and TEM image (b) of the as-synthesized FeIII-MOF-5.

Figure 2. PXRD pattern (a) and ICP results (b) of the as-synthesized composites.

properties remain equivocal. Meanwhile, the development of facile yet effective techniques to obtain MOF-derived materials with designed compositions and structures is urgent. Third, energy consumption as well as poisonous substances should be reduced in the synthesis route. Fortunately, FeIII-MOF-5 has been found to be a promising precursor for high-performance MA materials composed of ZnO, Fe, Fe3C, and carbon. Interestingly, the chemical state of Zn, Fe, O, and C elements can be effectively controlled by changing the calcination temperature which involves a transformation from Fe3+ to Fe3C and Fe. Effects on the electromagnetic properties are also discussed which may provide useful hints for the fabrication of novel MA materials against EM pollution.

Recently, metal−organic-framework-derived (MOF-derived) composites have emerged as promising candidates for highperformance MA composites.8−11 Generally, graphitic carbon can be obtained by the catalysis of magnetic metal during the decomposition process of MOFs which ensures a strong dielectric-loss ability. Additionally, a magnetic metal or alloy would bring excellent magnetic loss which would be propitious for impedance matching and a broader fe.12−14 The existence of metal oxides may improve the impedance matching by reducing the electrical conductivity and inducing more interfaces which may strengthen interfacial polarization. Our previous work regarding ZnO/Co/carbon and CuO/carbon may enlighten researchers designing novel microwave absorbers.15,16 We also find that metal carbides can be introduced to ZnO/Co/carbon composites which lead to an excellent MA performance.17 Although improvements have been made, several problems still exist to be solved. First, our previous work pays more attention to the optimization of impedance matching. TiO2, CuO, ZnO, and amorphous carbon can be employed to decrease the high complex permittivity of MOF-derived materials to fulfill excellent impedance matching.15−18 However, the precondition of optimizing impedance matching is that MA materials possess strong attenuation abilities. Hence, more work is requested to enhance the attenuation abilities of MOF-derived materials. Second, previous research about improving the attenuation abilities of MOF-derived microwave-absorbing materials is limited. One way is increasing the conductivity of carbon through promoting pyrolysis temperature.10,13 The other way is inducing components with strong loss abilities, such as graphene, metal, etc.8,12 Unfortunately, this research is in the early stages. The vital roles of composition and structure in deciding the electromagnetic

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EXPERIMENTAL SECTION

Preparation of Porous ZnO/Fe3C/C Composites. FeIII-MOF-5 was synthesized using a refluxing method based on the literature.19 Typically, 2.16 g of ferric acetylacetonate, 1.67 g of Zn(NO3)2·6H2O, 0.34 g of terephthalic acid, and 7.2 g of polyvinylpyrrolidone (PVP, K29−K32) were added into mixtures of 180 mL of ethanol and 300 mL of dimethylformamide (DMF). After being completely dissolved by stirring, the mixture was refluxed at 100 °C for 6 h. The products were collected by centrifugation and washed with DMF and ethanol three times and then vacuum-dried at 60 °C. After being ground, the MOF powders were calcined in N2 with a ramping rate of 5 °C min−1 to 600, 650, 700, and 750 °C and kept for 2 h. The obtained composites are denoted as ZFC-600, ZFC-650, ZFC-700, and ZFC750, respectively. Characterizations. Powder X-ray diffraction (PXRD) patterns were collected by a Bruker D8 Advance diffractometer with Cu Kα irradiation and a scanning range 5−90°. Thermogravimetric (TG) analyses of the MOF precursor were performed by a PerkinElmer Diamond TG/DTA apparatus in N2 at a heating rate of 10 °C min−1. Chemical states of each element were investigated by a PHI 5000 7962


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Figure 3. Raman spectra (a) of the as-prepared composites; XPS survey spectra (b) and Zn 2p (c), C 1s (d), O 1s (e), and Fe 2p (f) spectra for ZFC-600, -650, -700, and -750.

structure of MOF-5.20 On the basis of previous research, thermal treatment temperature plays a crucial role in not only the composition but also the structure.14 Hollow FeIII-MOF-5 octahedra can be seen in Figure 1b which is consistent with previous reports.19 The size of the octahedra is about 200 nm. The composition of the samples is determined by PXRD and ICP−AES. Figure 2a displays the PXRD patterns of ZFC-600, -650, -700, and -750. The characteristic peaks at 31.8°, 34.4°, 36.2°, 47.5°, 56.6°, 62.9°, 68.0°, and 69.0° can be seen for all three samples which should be indexed to the (100), (002), (101), (102), (110), (103), (112), and (201) planes of hexagonal ZnO, respectively.21 The diffraction peaks at 37.8°, 42.9°, 43.8°, 44.6°, 45.0°, 45.9°, 48.6°, and 49.1° matched well with the (210), (211), (102), (220), (031), (112), (131), and (221) planes of orthorhombic Fe3C, respectively.22 Interestingly, ZFC-750 only possesses peaks of cubic Fe. It is generally accepted that ZnO would be formed at less than 500 °C and reduced to Zn metal by carbon around 800 °C during the MOF decomposition process.23 Thus, it is anticipated that elemental

VersaProbe device. Precise contents of Zn and Fe were provided by inductively coupled plasma−atomic emission spectroscopy (ICP− AES) in which the sample was treated by aqua regia. The bonding state of carbon was further studied by Raman spectra using a Renishaw in Via 2000 Raman microscope. SEM images were obtained from a Hitachi S4800 device, and TEM and HRTEM images were gained from a JEOL, JSM-2010 device. A micromeritics ASAP 2020 system was employed at 77 K for examination of the porous structure of the samples. Static magnetic properties were determined by a vibration magnetometer (Lakeshore 7400). Complex permittivity and permeability were measured by an Agilent PNA N5244A vector network analyzer. The samples were made by mixing 40 wt % wax with samples and then pressing this into a toroidal ring with a ϕout of 7.0 mm and a ϕin of 3.04 mm.

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RESULTS AND DISCUSSION Figure 1a shows the PXRD pattern of the as-prepared FeIIIMOF-5 and the simulated pattern of MOF-5. Prominent peaks at 6.8°, 9.6°, 13.6°, 15.2°, and 22.3° of the sample are found to match well with the simulated pattern. It may be inferred that the inserted Fe3+ ions do not significantly change the crystal 7963


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Figure 4. Deconvolution of ZFC-700 HR-XPS Zn 2p (a), Fe 2p (b), O 1s (c), and C 1s (d) data.

Zn exists in the form of ZnO in this case. It should be noted that elemental Fe exists mainly in the form of Fe and Fe3C. This may result from the shorter thermal treatment time and the unique composition. The contents of each element are given in Figure 2b. The contents of Zn of ZFC-600, -650, -700, and -750 are 38.27%, 36.31%, 34.94%, and 3.35%, respectively. It seems that, at higher temperature, Zn elements may be vaporized or discharged from the composites which also lead to the highest Fe content of 89.89% and the lowest C content of 5.94% for ZFC-750. It should be pointed out that the carbon contents of the samples are all near 17% except for that of ZFC750 (5.94%). Raman spectra are given to characterize the bonding state of carbon. Two noticeable peaks at about 1335 and 1590 cm−1 can be distinguished which are usually defined as the D band and the G band, respectively. It has been well-documented that the D band is related to defects or disorder in carbon materials and that the G band is induced by the sp2-hybridized carbon.10 The relatively high intensity of the G band for ZFC-600, -650, -700, and -750 reveals that carbon is partially graphitized because of the catalytic effect of the Fe species, while the obvious D band may result from the defects and disorder among interfaces with Fe, Zn species, and air. It is widely accepted that the intensity ratio of ID/IG can be a criterion to represent the graphitization degree. As shown in Figure 3a, the values of ID/IG are 0.872, 0820, 0.846, and 0.868 for ZFC-600, -650, -700, and -750, respectively. On the basis of the above discussions, we may infer that the number of graphitic walls developed by thermal treatment at 750 °C is the smallest while ZFC-600, -650, and -700 possess nearly the same graphitic walls. With increasing temperature, the graphitization should be improved according to previous research, while, in this case, the graphitization degrees of ZFC-700 and -750 are lower than that of ZFC-650. A possible reason would be the increased relative intensity of

the D band. Two aspects should be taken into consideration. On one hand, increased temperature would favor the formation of tiny crystalline domains.10 On the other hand, some defects would be generated during the high-temperature carbothermal reduction.10 In this case, some lattice carbon may capture oxygen in ZnO which may lead to enhancement of the D band. For further probing of the chemical state of as-prepared composites, XPS is employed. The purity of the composites and existence of Zn, Fe, O, and C elements are confirmed by the XPS survey spectrum. Figure 3c shows the Zn 2p spectra of three samples with two similar peaks at 1021.8 and 1044.8 eV, which correspond to Zn 2p3/2 and Zn 2p1/2. C 1s spectra are shown in Figure 3d with a single peak at around 284.8 eV which may be ascribed to CO2, C, COH, OCO, and Fe3C. A remarkable difference can be seen in the O 1s spectra. The peak at 530.6 eV of ZFC-600 should be the O2− ions of ZnO or Fe2O3. Other peaks at around 532.3 eV can be distinguished which might be attributed to CO or CO.24 Remarkable differences also exist in the Fe 2p spectra. Peaks at 711.1 and 725.1 eV for ZFC-600 belong to Fe3+ 2p3/2 and Fe3+ 2p1/2. Similarly, peaks at 710.9 and 724.9 eV for ZFC-650 as well as peaks at 710.2 and 724.4 eV should be ascribed to Fe3+ ions which may come from the oxidation of Fe after exposure in air with less carbon coating. Meanwhile, peaks at 707.3 and 719.8 eV for ZFC-600, peaks at 707.0 and 719.4 eV for ZFC650, peaks at 706.7 and 720.0 eV for ZFC-700, and peaks at 707.0 and 720.0 eV for ZFC-750 all reflect the binding energies of Fe 2p3/2 and Fe 2p1/2, respectively. Noticeably, Fe3C, namely, Fe+, should be observed clearly according to XRD results. This is because XPS is a surface analysis technique which focuses on the chemical state of elements less than several tens of nanometers. For a further understanding of the chemical state of each element in ZFC-700, the HR-XPS data are analyzed in Figure 4. 7964


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Figure 5. TEM image (a) and HRTEM images (b, c) of ZFC-600. TEM image (d) and HRTEM images (e, f) of ZFC-650. TEM image (g), HRTEM image (h), and electron diffraction pattern (i) of ZFC-700. TEM image (j), HRTEM image (k), and EDX result (l) of ZFC-750.

As for Zn, two peaks belonging to Zn2+ can be well-fitted with HR-XPS spectra, indicating that ZnO is stable after thermal treatment at 700 °C. The Fe 2p spectrum of ZFC-700 can be deconvoluted into five peaks. Peaks at 711 and 724 eV belong to Fe3+ ions, and peaks at 706.8 and 719.9 eV pertain to Fe, while the characteristic peak at 708.1 eV of Fe+ in Fe3C can be observed, too. We may infer that the Fe3+ in FeIII-MOF-5 may be reduced gradually to Fe by carbon at a moderate temperature (more than 600 °C) and further react with carbon to form Fe3C. It is necessary to point out that our results seem to contradict the previous report on Fe/C nanocubes derived from prussian blue.10 This can be explained by the fact that the existence of ZnO would affect the carbon content and chemical environment of Fe to some extent; the relatively higher carbon content in precursors probably favors the formation of Fe3C. The O element in ZFC-700 shows a remarkable difference with other samples, and the HR-XPS spectra can be deconvoluted into three peaks including 530.6 eV for metal oxides, 532.3 eV for CO, and 533.1 eV for CO. The O species may be ZnO, Fe2O3, adsorbed CO2, H2O, surface OH, COOH, etc. Additionally, the chemical state of carbon of all samples is similar, and the C 1s HR-XPS spectrum of ZFC-700 can be deconvoluted into four peaks: in detail, the peak at 284.3 eV corresponds to graphitic carbon, the peak at 284.8 eV is a signal of CO2, the peak at 285.4 eV should be assigned to COH, and the peak at 289.1 eV may originate from OCO. This

is consistent with Raman data and proves the existence of graphitic carbon in the composites.25 In conclusion, the as-prepared materials are composed of ZnO, Fe, Fe3C, and partially graphitc carbon. In detail, ZnO is quite stable under this circumstance with the slight chemical state change of O which is caused by the reduction to Zn at high temperature. As for Fe species, reduction from Fe3+ to Fe and reaction with carbon to form Fe3C step by step should be responsible for the varied chemical state. As for carbon, the catalytic effect of Fe species substantially improves the graphitization degree, and high temperature and the carbothermal reduction may induce more defects which may become polarization centers. For identification of the structure of the composites, TEM and HRTEM images are offered in Figure 5. After calcination in N2 at 600 °C, the octahedral structure is basically collapsed, and ∼15 nm nanoparticles uniformly embedded in the carbon matrix are obtained. In Figure 5b, the lattice fringes of 0.238 nm can be clearly observed which can be assigned to the (121) interplane spacing of Fe 3 C. Because of the catalytic graphitization effect of Fe and Fe3C, few graphene layers can be seen coating on nanoparticles. The lattice fringes belonging to ZnO can also be found, and it is noteworthy that the coated carbon is amorphous. When the temperature rises up to 650 °C, the nanoparticles would grow and partially aggregate with each other. Fe nanoparticles can grow up to ∼100 nm, and a part of ZnO would form a nanorod-like structure which is 7965


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Figure 6. Nitrogen adsorption−desorption isotherms (a) and pore size distribution (b) of the as-prepared composites.

Figure 7. M−H loops of ZFC-600 (a), ZFC-650 (b), ZFC-700 (c), and ZFC-750 (d) measured at room temperature.

temperature: Nanoparticles would grow and aggregate with each other. A change of chemical state of Fe, Zn, O, and C may also lead to disordered structure. In addition, the porous structures should not be neglected. N2 sorption isotherms and pore distributions of all samples are given. Typical type-IV isotherms can be found which indicate a mesoporous character (see Figure 6a). The BET surface areas are 69.71, 64.55, 48.28, and 8.41 m2 g−1 for ZFC-600, -650, -700, and -750, respectively. This is consistent with SEM and TEM studies in which ZFC-750 is mainly composed of large nanoparticles while a porous structure can be observed in other samples. Meanwhile, the surface area is less than that from a previous report which should originate from a higher calcination temperature.19 The average pore size is about 12 nm which further proves the mesoporous structures of the sample. It is interesting that pores of nearly 2.0 and 2.5 nm are obvious in ZFC-600, -650, and -700, which may be inherited from the porous structures of MOFs. When the MOF structure is destroyed at about 750 °C, only broad peaks of about 2.2 nm

confirmed by the HRTEM image in Figure 5e. Meanwhile, ZnO nanoparticles can also be seen in Figure 5f. With increased temperature to 700 °C, larger nanoparticles are obvious, as well as more graphene layers. On the basis of Figure 5h, we may speculate that large Fe nanoparticles catalyze the graphitization of carbon and also supply more opportunities for the formation of Fe3C. In addition, rings of Fe3C, ZnO, and Fe can all be seen in Figure 5i, which further confirmed the composition of the asmade samples. EDS mapping results of ZFC-700 are displayed in Figure S3, indicating that ZnO disperses uniformly in the carbon matrix and that Fe mainly exists in large particles. After thermal treatment at 750 °C, Fe nanoparticles have grown to more than 1 μm, and few ZnO and carbon species can be found. The morphology evolution can also be described by SEM images in Figure S2. ZFC-600 possesses a similar shape with FeIII-MOF-5, while massive nanorods can be observed for ZFC-650 which is consistent with the TEM study. As for ZFC700, nanoparticles grow to large particles and become larger for ZFC-750. In a word, ordered structure is unstable at a higher 7966


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Figure 8. Real parts (a) and imaginary parts (b) of complex permittivity of all samples. Real parts (c) and imaginary parts (d) of complex permeability of all samples.

previous research, the relation between ε″ and electrical conductivity can be expressed as

can be seen which may be the pores in unreacted carbon in ZFC-750 (see Figure 6b). Moreover, for an evaluation of the magnetic properties of the samples, hysteresis loops are exhibited in Figure 7. Typical ferromagnetic behavior can be seen for all the samples with Ms values of 33.0, 50.3, 58.9, and 103.5 emu g−1 for ZFC-600, -650, -700, and -750, respectively. They are all smaller than the literature value of 140 emu g−1 for Fe3C, which should be the reason for the existence of nonmagnetic ZnO and carbon,26 while, for the Hc value, a “V”-shaped trend can be found with increasing temperature in which 231, 88, and 136 Oe belong to ZFC-600, -650, and -700, respectively. Meanwhile, ZFC-750 possesses the smallest Hc value of 54.9 Oe. This may be related to the varied size and distinct chemical state of Fe species in each sample.12 Caused by a diverse structure and distinct composition, the electromagnetic properties of the as-prepared materials are unique. In particular, complex permittivities (εr) and complex permeabilities (μr) are supplied. From Figure 8a and 8b, one can see that the real permittivity (ε′) values of ZFC-600 decrease from 9.06 at 2 GHz to 7.56 at 18 GHz while the imaginary permittivity (ε″) values fluctuate around 1.7. The dielectric properties of ZFC-650 are very similar to those of ZFC-600. The ε′ values decline from 10.16 to 8.38 in the measuring range, and the ε″ values hover around 1.5. An obvious change can be noticed for ZFC-700 in which the ε′ values drop from 14.94 to 10.58, and the ε″ values rise and drop between 4.12 and 3.30. Generally, ε′ represents the storage capability, and ε″ stands for the loss ability of dielectric energy.27 However, ZFC-750 possesses a complex permittivity that is too high and can be ignored in later discussions. It can be deduced that the loss abilities enhance with increasing temperature as well as the storage capabilities. On the basis of

σAC = ε0ε″2πf

(1)

where the terms are as follows: σAC is the electrical conductivity, ε0 is the dielectric constant of free space, ε″ is the imaginary permittivity, and f is the microwave frequency.12 As we can see, the ε″ values of ZFC-600, -650, and -700 show fluctuation within a narrow range. Thus, we may speculate that the improvement of electrical conductivity should be responsible for the rise of ε″ values. On one hand, the varied content of each component should be stressed. In detail, the decreased content of semiconductive ZnO and the increased content of more conductive Fe species are all conducive to electrical conductivity. When it comes to complex permeability, a similar trend can be observed (see Figure 8c and 8d). The real permeability (μ′) values of ZFC-600 decreased from 1.20 to 1.02, and the imaginary permeability (μ″) values go down from 0.17 to 0.03 in the measuring frequency range. It should be noted that the permeabilities of ZFC-650 and -700 are quite different from that of ZFC-600. The μ′ values of ZFC-650 and -700 fall from 1.30 to 1.07 and from 1.33 to 1.09, respectively. The μ″ values of ZFC-650 reach the highest point at 10.5 GHz (0.25) and the lowest point at 18 GHz (0.10). With regard to ZFC-700, μ″ values of 0.23 at 10 GHz and 0.10 at 10 GHz are the highest and lowest, respectively. This may indicate that ZFC-650 and ZFC-700 possess stronger storage and loss abilities of magnetic energy. The Globus equation is as follows: μ ∝ (Ms 2D/K1)1/2

(2)

where, for a higher complex permeability, a higher Ms, a larger grain size (D), and a smaller magnetocrystalline anisotropy 7967


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Figure 9. 3D representation of RL values of ZFC-600 (a), ZFC-650 (b), and ZFC-700 (c). RL curves of as-made composites at 1.5 mm (d).

constant (K1) are required.12 In this case, larger Ms values for ZFC-650 and -700 and a larger grain size may contribute to the larger permeability. For comparison of the dielectric-loss abilities, dielectric-loss tangents (tan δε = ε″/ε′) are calculated in Figure S4a. The shape of the curves resembles each other, and ZFC-700 possesses the highest value which means it possesses the strongest dielectric-loss abilities. For comparison of the magnetic-loss abilities, magnetic-loss tangents (tan δμ = μ″/ μ′) are shown in Figure S4b. The values of ZFC-600 directly decrease from 0.15 to 0.03 within several peaks, while ZFC-650 and ZFC-700 possess almost the same curves with values going up from about 0.13 at 2 GHz to 0.20 at 10 GHz and dropping to 0.09 at 18 GHz. We should point out that ZFC-700 possesses the strongest magnetic-loss capability. Considering the strong attenuation ability of the as-prepared materials, the reflection-loss performance can be expected. According to the transmission line theory, the reflection-loss (RL) values can be gained by experimentally obtaining electromagnetic parameters based on the following formulas:28 ⎛ μ ⎞1/2 Z in = Z0⎜ r ⎟ tanh[j(2πfd /c)(με )1/2 ] r r ⎝ εr ⎠

(3)

RL = 20 log|(Z in − Z0)/(Z in + Z0)|

(4)

performance can be seen. The minimum RL value can reach −15.54 dB at 4.65 mm, and the maximum fe is about 2.88 GHz at 4.8 mm. Obviously, ZFC-600 cannot meet the requirement for practical applications even at a high frequency which is related to its relatively low complex permittivity and permeability. With increasing temperature, better RL performance can be gained because of strengthened attenuation abilities. The minimum RL value is up to −18.69 dB at 2.35 mm with fe from 9 to 11.96 GHz. Moreover, an fe value of 3.52 GHz can be obtained at both 2.10 and 2.15 mm with a minimum RL value of approximately −17 dB. However, it is still far from practical use. When the attenuation ability is strongest (ZFC-700), excellent microwave absorption behavior can be attained. It is worth pointing out that the −50.5 dB can be reached at 7.44 GHz with a thickness of 2.6 mm, and a broad fe of 4.96 GHz could be obtained at only 1.5 mm. In summary, ZFC-700 shows obvious superiority in RL performance which is able to fulfill the requirements of having thin thickness, being lightweight, and having a broad effective frequency bandwidth and strong absorption intensity. Specifically, fe values of 4.48, 4.96, 4.76, 4.28, and 3.72 GHz could be realized at 1.4, 1.5, 1.6, 1.8, and 2.0 mm for ZFC-700, respectively. In addition, we can find that the RL peaks shift to a lower frequency with increasing thickness which can be explained by the quarter-wavelength matching model: tm = nc /(4fm (|εr||μr |)1/2 )

Here, the terms are as follows: Zin is the input impedance of the absorber, Z0 is the impedance of free space, d is the thickness of the absorber, and c is the velocity of an electromagnetic wave in free space. In general, strong absorption intensity (RL ≤ −10 dB), broad effective frequency bandwidth ( fe), and thin thickness are demanded for high-performance microwave absorbers. Figure 9a−c presents the calculated results at different thicknesses from 1 to 5 mm. As for ZFC-600, poor

(5)

Here, the terms are as follows: tm is the thickness of the absorber, f m is the peak frequency, and n is constant (n = 1, 3, 5, ...). Thus, it is easy to understand that when tm is larger, f m is lower. The fact also proves that interference cancelation contributes to the attenuation of incident microwaves.29 Figure 9d further shows the RL curves of ZFC-600, -650, and -700 at 7968


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Figure 10. Attenuation constants (a) and comparison of impedance matching at 1.5 mm (b) of ZFC-600, -650, and -700.

Figure 11. Cole−Cole plot (a) and C0 values (b) of ZFC-700.

the RL values of ZFC-700 are more negative than −10 dB, indicating that better impedance matching is in favor of more satisfying microwave absorption performance. However, for ZFC-600 and ZFC-650, the values are not close to 1, and impedance matching is even worse at a high-frequency range above 15 GHz. Thus, promising RL performance requirements cannot be fulfilled by ZFC-600 or ZFC-650. Comparisons of attenuation constants and impedance matching are shown in Figure 10a and Figure 10b, respectively. It is interesting that ZFC-650 possesses weak absorption at a high-frequency range which should be ascribed to the strong attenuation at a high frequency. The fact also illustrates that attenuation and impedance matching are both critical points for designing a microwave absorber while attenuation ability should receive more attention. Hence, the attenuation mechanisms are further discussed. It is generally accepted that conduction loss and interfacial polarization loss contributed most in the microwave frequency range for dielectric absorbers. On the basis of Debye theory, the relations between ε′ and ε″ can be roughly written as

1.5 mm. We can clearly see that, with increasing temperature, RL peaks shift to a lower frequency, and RL values become more negative. This is decided by the increasing complex permittivity and permeability which are benefits for better RL performance at thin thicknesses. For a comprehensive understanding of the attenuation properties of the samples, attenuation constants are provided on the basis of the following equation: α=

2 πf c

(μ″ε″ − μ′ε′) +

(μ″ε″ − μ′ε′)2 + (μ′ε″ + μ″ε′)2

(6)

Obviously, with increasing temperature, the attenuation constants improve. ZFC-700 possesses the largest α values from 39.19 to 267.11 in the entire microwave frequency range. Another point is that α values gradually increase with rising frequency, which illustrates that the as-prepared composites show a stronger attenuation ability in the high-frequency range.16 Although attenuation ability is important for a microwave absorber, the impedance matching cannot be ignored, either. If the characteristic impedance of the microwave absorber is not close to the impedance of air, the incident wave would mostly reflect at the interface.9 Thus, excellent impedance matching is necessary for developing high-performance microwave-absorbing materials. |Zin/Z0| values can be employed for determination of the condition of impedance matching, and when it is 1, which means that the input impedance equals the impedance of air, perfect impedance matching is achieved. When the thickness is 1.5 mm, |Zin/Z0| values of 0.8−1.2 cover the frequency range 11.8−15.72 GHz, and in this frequency range,

εs + ε∞ ⎞2 ⎛ ⎛ εs − ε∞ ⎞2 2 ⎜ε′ − ⎟ + (ε ″ ) = ⎜ ⎟ ⎝ ⎝ 2 ⎠ 2 ⎠

(7)

Herein, εs and ε∞ are the static permittivity and the relative permittivity at infinite frequency. Thus, the curve of ε′ to ε″ corresponds to a single semicircle which indicates 1 Debye relaxation process.30 The Cole−Cole plot of ZFC-700 is given in Figure 11a, and we can easily divide it into two segments. One is a straight line in the ε′ range 14−15 which is related to conduction loss. In this case, it mainly comes from graphitic carbon and Fe species. 7969


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ACS Sustainable Chemistry & Engineering Table 1. Microwave Absorption Performance of Similar Materials sample

filling ratio (wt %)

optimal thickness (mm)

RLmin (dB)

fe (GHz)

ref

ZnO/SiC/wax C/ZnO/wax Fe−Fe3C/C/wax RGO/ZnO/wax ZnO/Fe/wax C/Fe3C/wax ZnO/C@/Co@C/wax ZnO/Co3ZnC/Co/C/wax ZFC-700/wax

30 40 25 15 60 10 50 50 60

2.5 1.75 1.5 2.4 1.5 3.6 1.9 1.9 1.5

−31.31 −52 −17.9 −54.2 −40 −62.6 −28.8 −32.4 −30.4

6.60 2.5 4.0 6.7 4.0 6.5 4.2 5.24 4.96

32 33 34 35 36 37 15 17 this work

The other curve is multiple semicircles in the ε′ range 10− 14. It reveals that multiple relaxation processes exist in the composites upon irradiation by microwaves. As we know, interfacial polarization usually occurs between components with a distinct electrical conductivity.31 Therefore, interfacial polarization may occur in interfaces among ZnO/C, Fe3C/C, ZnO/ Fe3C, C/air, Fe/C, Fe/Fe3C, and so on. Moreover, it should be noted that the porous structure of the composites might facilitate the scattering of incident waves and also be conducive to the increase of dipole amount which further strengthens the interfacial polarization. Meanwhile, we could not neglect magnetic loss. In most cases, the ferromagnetic resonance and eddy current effect are the main magnetic-loss mechanisms in the microwave frequency range. C0 is always used to confirm that eddy current loss dominates the magnetic loss on the basis of the following equation.

C0 = μ″(μ′)−2 f −1

dB and a broad fe of 4.96 GHz can be reached at a thin thickness of only 1.5 mm which makes ZFC-700 an excellent MA material. Conduction loss, interfacial polarization, ferromagnetic resonance, and interference cancelation should be responsible for the satisfying RL performance.

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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/acssuschemeng.7b01514. SEM images of as-prepared products, and PXRD pattern, electromagnetic parameters, and RL curves of ZFC-750 (PDF)

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AUTHOR INFORMATION

Corresponding Author

(8)

*E-mail: [email protected]. Phone: +86-25-52112902.

As we can see in Figure 11b, C0 values are not constant in the entire frequency range which means that eddy current is not the main loss mechanism and that ferromagnetic resonance of Fe3C should be the dominating magnetic-loss mechanism. In summary, the influence of carbon focuses on the following: supplying a highly conductive matrix which ensures a high ε″ value and strong conduction loss, and isolating Zn and Fe species which guarantees many more interfaces and enhanced interfacial polarization. Fe and Fe3C offer mainly conduction loss and ferromagnetic resonance. ZnO is in favor of strong interfacial polarization. In addition, recent progress in similar microwave materials is summarized in Table 1. Compared with samples from similar work, ZFC-700 shows outstanding performance at a thin thickness and may be more appealing. Nevertheless, a breakthrough is demanded in decreasing the filling ratio and further broadening the effective frequency bandwidth.

ORCID

Lei Liu: 0000-0001-5816-8699 Guangbin Ji: 0000-0002-5150-3949 Author Contributions

The paper was written through contributions of all authors. All authors approved the final version of the paper. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support from the National Nature Science Foundation of China (11575085), Qing Lan Project, Six Talent Peaks project in Jiangsu Province (XCL-035), and the Priority Academic Program Development of Jiangsu Higher Education Institutions is gratefully acknowledged.

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CONCLUSIONS Novel microwave-absorbing composites of ZnO/Fe/Fe3C/ carbon have been successfully synthesized by thermal decomposition of FeIII-MOF-5. The composition and chemical state of each component can be effectively controlled by changing the calcination temperature. ZnO is relatively stable while physical and chemical properties of Fe and C elements are tunable. With increasing temperature, the Fe element would be transformed from Fe3+ to Fe3C and Fe during the carbonization process. This process also leads to the graphitization and weight loss of carbon. The effects on electromagnetic properties are also investigated, and ZFC-700 possesses the strongest attenuation ability with better impedance matching at a thin thickness. RL values of −30.4

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Composition design and structural characterization of MOFs-derived

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