Research Article Cite This: ACS Sustainable Chem. Eng. 2019, 7, 11795−11805
pubs.acs.org/journal/ascecg
Achieving Excellent Electromagnetic Wave Absorption Capabilities by Construction of MnO Nanorods on Porous Carbon Composites Derived from Natural Wood via a Simple Route Shun Dong, Weikang Tang, Peitao Hu, Xiaoguang Zhao, Xinghong Zhang,* Jiecai Han, and Ping Hu* National Key Laboratory of Science and Technology for National Defence on Advanced Composites in Special Environments, Harbin Institute of Technology, No. 2 Yikuang Street, Nan Gang District, Harbin 150080, People’s Republic of China Downloaded via NOTTINGHAM TRENT UNIV on July 19, 2019 at 02:36:37 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.
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ABSTRACT: Absorbers with light weight, low filler loading, high absorption capacity, and broad absorption bandwidth are highly desirable for electromagnetic (EM) wave absorption application, and extensive efforts in designing excellent performance biomass-derived microwave absorbents using sustainable and renewable materials have been made. Here, for the first time we constructed flexible and high-performance EM-absorbing materials of porous biomass-derived carbon (PBDC) decorated with in-situ grown MnO nanorods (MnOnrs) by a simple process. The chemical composition and microstructural feature of these MnOnrs/PBDC composites are highly dependent on the content of MnOnrs controlled through the concentration of potassium permanganate, and thus their EM properties could be also manipulated. Compared with the pure PBDC, the MnOnrs/PBDC composites exhibited excellent EM wave absorption performance with the minimum reflection loss (RLmin) of −51.6 dB at 10.4 GHz with a thickness of 2.47 mm and a qualified bandwidth of 14.2 GHz with an integrated thickness from 1.00 to 5.00 mm. Notably, the microwave absorption capacity of this new kind of composite is not so susceptible to the content of MnOnrs as those common carbon-based absorbers, which could be attributed to the synergistic effect between PBDC and MnOnrs as well as the hierarchical structure. This work may provide a new guideline for development of biomass as a low-cost, green, and renewable high-performance, carbon-based absorber. KEYWORDS: Wood, MnO, Biomass, Electromagnetic wave absorption
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INTRODUCTION With the rapid development of wireless communication, networks, and electronic devices, serious electromagnetic (EM) pollution has been caused by EM interference and radiation,1−3 resulting in severely threatened information safety and human health. Thus, high-performance EM wave absorption or shielding materials have been systematically explored due to their tremendous contribution to solving EM pollution.4−6 To date, compared with EM-shielding material, EM wave absorption material has aroused more and more interest owing to its low reflection and high consumption toward incident EM waves;7−9 an ideal EM wave absorber should be lightweight, lower filler loading, strong absorption capacity, and broad frequency bandwidth.10−12 Although lots of single-component EM wave absorbers, including carbon materials, metal oxides, magnetic metals, and conductive polymers, have been prepared so far,13,14 constructing composites with multicomponents to improve EM wave absorption performance is still the mainstream study direction.15−17 In addition, it is worth pointing out that the morphology configurations play a crucial role in the EM wave absorbing property, and great efforts have been made to © 2019 American Chemical Society
construct precise morphology to achieve better absorbing capability other than simple composition stacking.18−20 Up to now, carbon-based materials have become one of the most remarkable candidate EM wave absorbers due to their low density and tunable electrical conductivity, while individual carbon materials exhibit poor absorbing properties owing to the high electrical conductivity, resulting in strong skin effects during EM wave incidences.9 Therefore, introducing other components into the carbon materials should be an effective approach to enhance EM wave absorption properties.21−31 For example, Peng et al. reported that carbon nanotubes/crystalline Fe nanocomposites have excellent microwave absorption properties, in which reflection loss (RL) could be as low as −25 dB at 1.2 mm and almost lower than −10 dB from 2 to 18 GHz.32 Qin et al. adopted atomic layer deposition to coat carbon nanocoils (CNCs) with magnetic Fe3O4, and the shell thickness of Fe3O4 had an important influence on the microwave absorption performReceived: April 15, 2019 Revised: May 26, 2019 Published: May 30, 2019 11795
DOI: 10.1021/acssuschemeng.9b02100 ACS Sustainable Chem. Eng. 2019, 7, 11795−11805
Research Article
ACS Sustainable Chemistry & Engineering ance. The minimum RL (RLmin) reaches −40.3 dB for a 24 nm Fe3O4 shell, −21.5 dB for a 12 nm Fe3O4 shell, and −11.8 dB for a 6 nm Fe3O4 shell, respectively.33 Cao et al. fabricated a three-dimensional (3D) hierarchical Co3O4-reduced graphene oxide (rGO) hybrid architecture by a facile strategy, and the RL value reached −61 dB at Co3O4/rGO ratio of 2:1.34 It was worth pointing out that the carbon composition as a dielectric loss material played a vital role in these composites, while the current carbon resources were high-cost and non-renewable, and it is urgent to develop green, low-cost, friendly, and renewable, carbon-based materials with excellent EM wave absorption performance. Recently, numerous of natural biomaterials such as wood, cotton, and others have acted as templates or carbon precursors to manufacture advanced biomorphic functional carbon materials applied in different application fields, which could inherit the unique and hierarchical porous structure of raw biomaterials, and porous biomass-derived carbon (PBDC) materials should be an effective strategy to synthesize green, low-cost, and renewable, carbon-based materials.13,35−37 Moreover, diverse manganese oxides have been incorporated into carbon materials to enhance EM wave attenuation due to their effective permittivity, low density, and environment friendliness.38−40 For instance, Singh et al. introduced MnO2 into graphene nanoribbons (GNRs) to improve EM interference shielding effectiveness, which presented absorption dominated high shielding effectiveness of −57 dB at 3 mm.38 More recently, Li et al. found that core-shell MnO@ carbon nanowires (MnO@C NWs) achieve substantially enhanced microwave absorption, suggesting the suitable impedance matching induced by the synergetic effect between MnO and carbon.40 Although the microwave absorption properties of MnO/carbon composites have been reported, incorporating MnO nanostructures into PBDC is rarely investigated, and it is urgent to study the EM wave absorption capacity of MnO nanostructures/PBDC for the application of EM absorbers. Herein, for the first time, we constructed a 3D hybrid structure by introducing MnOnrs into PBDC via a simple route and the effect of MnOnrs content on the EM wave absorption performance of the composites was investigated. The natural wood and KMnO4 were used as the raw precursors to prepare PBDC and MnOnrs, respectively, and a simple hydrothermal activation of KOH followed by carbonization process was adopted to convert the dried natural wood into the porous carbon material, and then the MnOnrs were incorporated into the PBDC by a facile hydrothermal process followed by annealing. The content of MnOnrs could be controlled by adjusting the concentration of KMnO4 and the synergetic function between carbon and MnO upon adsorption performance at a detail level. Exhilaratingly, the 3D MnOnrs/ PBDC composites possess suitable impedance matching and enhanced EM wave absorption performance, with the RLmin of −51.6 dB at 10.4 GHz when the thickness is 2.47 mm, and the maximum effective absorption bandwidth (EAB) can cover 4.7 GHz. The rationally designed 3D MnOnrs/PBDC composites with high-performance EM absorption may promote the development of MnO combined with other carbon materials as efficient EM wave absorbers.
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permanganate (KMnO4), alcohol, and deionized (DI) water were bought from Harbin Jierui Chem. Co. All reagents were employed without further purification. Materials Synthesis. The first step is the fabrication of PBDC as follows. The cube teak wood was first immersed in 1 mol/L KOH solution for 10 min with ultrasonic assistance, and the mixture was poured into a Teflon-lined stainless-steel autoclave and kept at 100 °C for 6 h. The as-obtained wood was dried in a vacuum oven at 50 °C and then placed in the center of a tube furnace with a flowing N2 atmosphere at 50 mL/min and then carbonized at 1050 °C for 3 h. The product, namely PBDC, was obtained when the tube furnace cooled down to room temperature naturally. The second stage is the preparation of MnOnrs/PBDC composites, and the detailed process is as follows. The PBDC samples were first soaked in the KMnO4 solution with different concentrations (2, 4, and 6 mmol/L) and then transferred into the Teflon-lined stainlesssteel autoclave and maintained at 160 °C for 12 h. The as-prepared samples were washed with alcohol and DI water three times and then dried in a vacuum oven at 50 °C. Finally, the samples were put into the tube furnace and then annealed at 800 °C for 3 h under the flow of N2 atmosphere at 50 mL/min. The as-prepared MnOnrs/PBDC samples were denoted as S-n (where n refers to the concentration of KMnO4), that is S-2, S-4, and S-6, respectively. Materials Characterizations. The crystallite structures of the samples were analyzed by X-ray diffraction (XRD), and a Cu Kα radiation at a generator voltage of 40 kV was used for the XRD analysis. Raman spectroscopy of the samples was recorded using a Renishaw inVia Raman microscope with a 633 nm incident laser to analyze the composition and state of carbon in the samples. The morphologies and microstructure of the samples were characterized by field-emission scanning electron microcopy (SEM, HELIOS NanoLab 600i, USA) and high-resolution transmission electron microscopy (TEM and HRTEM, Tecnai G2-F30, USA). Measurements of Microwave Absorption Properties. The microwave absorption properties of the as-obtained PBDC and MnOnrs/PBDC composites were evaluated by two important parameters, namely, complex permittivity (εr, εr = ε′ − jε″) and complex permeability (μr, μr = μ′ − jμ″) within 2−18 GHz through a vector network analyzer (Agilent N5230A, USA). The as-prepared samples were mixed with paraffin wax in a mass ratio of 30 wt % at about 85 °C, and then the mixture powder was pressed into a toroidal shape with an outer diameter of 7 mm and an inner diameter of 3.04 mm.
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RESULTS AND DISCUSSION The MnOnrs/PBDC composites were fabricated via a simple hydrothermal activation of KOH followed by carbonization, hydrothermal, and subsequent annealing method. Figure 1 shows that the macroscopic morphology of the product at different stages during the synthetic procedure. The dried wood after hydrothermal activation of KOH shown in Figure 1a exhibits a yellowish color. The colors of as-prepared samples after carbonization are dark black (Figure 1b) and then turn greyish green after annealing at 800 °C, namely, the color of
EXPERIMENTAL SECTION
Materials. Cube teak wood (10 × 10 × 10 mm3) was purchased from a local market, potassium hydroxide (KOH), potassium
Figure 1. Macroscopic morphology of the product during the different synthetic procedures. 11796
DOI: 10.1021/acssuschemeng.9b02100 ACS Sustainable Chem. Eng. 2019, 7, 11795−11805
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Figure 2. (a) XRD and (b) Raman spectra of all samples.
Figure 3. SEM images of the obtained MnOnrs/PBDC composites: (a−b) S-2, (c−d) S-4, and (e−f) S-6.
The crystal structures of all synthesized samples were examined by XRD, and the typical XRD patterns are shown in Figure 2a. From the results, all the samples present two broad peaks centered at about 25.0° and 44.0° due to the (002) and (101) planes of an amorphous carbon phase owing to the low carbonization temperature.35 After incorporation of MnOnrs, diffraction peaks observed from the MnOnrs/PBDC composites beyond the amorphous carbon phase could be assigned to MnO phase, and the peaks near 35.0, 40.5, 58.8, and 70.3° could be well indexed to the (111), (200), (220), and (311) crystal planes of cubic MnO (JCPDS card No. 07-0230),40,43 suggesting that the MnOnrs were successfully introduced into the PBDC by a facile hydrothermal and subsequent annealing approach, which is also confirmed by the following Raman spectra (Figure 2b and Figure S2). Raman spectra within 250− 1000 and 1000−2000 cm−1 wavelength range of all samples ware revealed in Figure 2b and Figure S2, which could be used to verify the formation of MnOnrs in the PBDC. As shown in Figure 2b, the peak located at 651.0 cm−1 could be ascribed to the Mn−O vibration similar to the results reported by previous literature reports,40,43 demonstrating the effective growth of MnOnrs via the current method. Further, from Figure S2, the two evident peaks at about 1340 and 1590 cm−1 could be attributed to typical D and G bands of carbon, representing amorphous and graphitized carbon, respectively.9,35 It is worth pointing out that the intensity ratio of the D band to G band (ID/IG) could be employed to evaluate the contents of defects
MnOnrs/PBDC composite (Figure 1c), indicating that the MnO has been successfully introduced into the PBDC. As for the growth process of MnOnrs, it can be described as follows: the MnO2 phase would be formed after the hydrothermal process within the KMnO4 solution, and the MnO could be obtained through a phase transition in the N2 atmosphere during the subsequent annealing and carbonization process.41,42 Figure S1 displays the microstructure of the products viewed from the cross-sectional at different stages during the synthetic procedure, corresponding to the macroscopic morphology of the product shown in Figure 1. It can be seen that there are lots of pores with a few to a dozen microns after the hydrothermal activation of KOH and high-temperature carbonization and activation, which could be attributed to the complex KOH activation.35 The KOH activation process could be summarized by following sentences: first, KOH could react with the carbon component and convert into K2CO3 under a suitable temperature; next, the K2CO3 would decompose into K2O and CO2, and the above intermediate species could further react with carbon to form CO; further, these generated gases, including H2, CO2, and CO, would be released from the system, leading to the formation of a plentiful porous structure.35 From Figure S1c, there is a thin film with several islands grown on the carbon wall, and the detailed morphology is characterized by following SEM and TEM. 11797
DOI: 10.1021/acssuschemeng.9b02100 ACS Sustainable Chem. Eng. 2019, 7, 11795−11805
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ACS Sustainable Chemistry & Engineering and the graphitization degree of carbon,9,35 and the values of ID/IG are 0.94 for PBDC, 0.91 for S-2, 0.89 for S-4, and 0.88 for S-6. Compared with that of the fully graphitized carbon, the values of ID/IG for all the samples obtained by current work are much higher, indicating the amorphous features of carbon in the samples,44 which is consistent with the analysis of XRD spectra. In addition, the high values of ID/IG demonstrate that the samples exhibit a low graphitization degree and plenty of defects, and the samples with abundant defects would act as effective polarization centers to improve the EM wave absorption capacity of MnOnrs/PBDC composites.45 The morphology and structure of the prepared MnOnrs/ PBDC samples were characterized by the SEM and TEM analysis. Figure 3 reveals the microstructure of MnOnrs/ PBDC composites viewed from the cross-sectional with different concentrations of KMnO4, namely, the S-2, S-4, and S-6 samples, respectively. Compared with the pristine PBDC, there are lots of nanoparticles and nanorods coated on the carbon walls, and the content of nanorods increases with the concentration of KMnO4, suggesting that the content of MnOnrs could be effectively adjusted by the concentration of KMnO4 in the hydrothermal process. When the concentration is up to 6 mmol/L, both inner and outer walls of PBDC are fully covered by highly dense MnOnrs, and the MnOnrs are 100−200 nm in diameter and ∼2 μm in length, as shown in Figure 3e−f. From the macroscopic and microscopic morphology of the MnOnrs/PBDC composites (Figure 1 and Figure 3), it can be concluded that introducing the MnOnrs does not destroy the porous structure of PBDC, and this conductive network would restrict the incident EM wave within the composite, leading to the enhanced EM wave attenuation. The TEM and HRTEM images of the composites, MnOnrs and wood-derived carbon obtained in the S-6 sample are presented in Figure S3 and Figure 4. Figure S3 reveals the microstructure of the MnOnrs/PBDC composite, and it can be clearly seen that the MnOnrs and PBDC both exist, and the length of the MnOnrs is about several micrometers. Figure 4a displays that MnOnrs are about 100 nm in diameter with a
rough surface, and the HRTEM image in Figure 4b taken from the edge of the sample demonstrates that the lattice space is about 0.26 nm, in accordance with the distance of the (111) plane of the cubic MnO, agreeing with the XRD result.40 From the TEM and HRTEM images in Figure 4c−d, the woodderived carbon displays a disordered graphite-like structure, indicating an amorphous carbon structure, which is also in keeping with the analysis of the XRD pattern. Combined with the above results, it is reasonable to believe that MnOnrs were successfully incorporated into the PBDC, and this unique structure might exhibit excellent microwave absorption performance along with multicomponents. The EM wave absorption performance of all as-prepared samples has been tested by the vector network analyzer, which could be reflected by the RL value. According to the transmission line theory, the RL value depends on the relative complex permittivity (ε′, ε″) and permeability (μ′, μ″) and is calculated by the following equations:40,46 RL (dB) = 20lg
Zin =
Zin − 1 Zin + 1
ÄÅ ÉÑ ÅÅ 2πfd ÑÑ ÑÑ με tanhÅÅÅj r ÑÑ r ÅÅÇ c εr ÑÖ
(1)
μr
(2)
where Zin represents the normalized input impedance, f, d, and c refer to the applied frequency of microwave, the thickness of the sample, and the velocity of EM wave in free space, respectively. The value of εr and μr lies in the real part permittivity (ε′) and the imaginary part permittivity (ε″), and the real part permeability (μ′) and the imaginary part permeability (μ″), respectively. In addition, the ε′ and ε″ are related to the dielectric properties, and the μ′ and μ″ are associated with the magnetic properties, and then the ε′ and μ′ represent the storage capacity of the microwave, while ε″ and μ″ stand for the loss of microwave capacity.35 Generally, an excellent EM wave absorption property originates from efficient complementarities between the εr and μr of samples, and the corresponding tangents, tan δe = ε″/ε′ and tan δμ = μ″/μ′, might be responsible for the dielectric loss and magnetic loss, respectively. To investigate the EM wave absorption properties, the obtained curves for ε′, ε″, μ′, and μ″ in a frequency range 2− 18 GHz are displayed in Figure 5 and Figure 6, and the corresponding tangents, tan δe and tan δμ, were also calculated. Owing to its low conductivity compared with that of PBDC, the value of ε′ is much lower than that of PBDC similar to the results of ε″ shown in Figure 5a−b, demonstrating that dielectric properties are dependent on the conductivity, while the average values of ε′ for MnOnrs/ PBDC composites are pretty close due to the large gap of conductivity between the carbon and MnO. It can be seen that ε′ values for all samples decrease with the frequency increasing, accompanied by some fluctuation, presenting the typical dispersion characteristics, which could be attributed to the orientation polarization of electric dipoles lagging behind the period change of the electric field.9,47 Note, there four peaks at ∼7.0, 9.0, 12.0, and 16.5 GHz in the ε″ curves, suggesting the multi-relaxations formed in all samples. Moreover, the ε″ values for PBDC, S-2, and S-4 exhibit a decreasing trend with the rise of frequency, while the ε″ value for S-6 increase with an increase in the frequency due to a larger space charge polarization. Compared with those of PBDC, S-2, and S-4, lots
Figure 4. TEM and HRTEM images of (a−b) the MnOnrs and (c− d) the wood-derived carbon. 11798
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Figure 5. (a) Real part of complex permittivity of all samples, (b) imaginary part of complex permittivity of all samples, and (c) tangent dielectric loss values of all samples.
Figure 6. (a) Real part of complex permeability of all samples, (b) imaginary part of complex permeability of all samples, and (c) tangent magnetic loss values of all samples.
PBDC, S-2, S-4, and S-6 samples are about 0.2−(−0.1), 0.9− 0.1, 0.2−0.05, and 0.1−(−0.3) in the measured frequency, respectively. The peaks in μ′ and μ″ indicate the magnetic resonance, and the strong magnetic resonance in low frequency could be attributed to the natural resonance, and the multi-
of MnOnrs result in a larger surface area in the interfaces, leading to the enhanced interfacial polarization and dipole moment.48 The μ′ values (Figure 6a) of PBDC, S-2, S-4, and S-6 samples are about 1.05−1.20, 0.90−1.20, 0.85−1.20, and 1.00−1.35, respectively. From Figure 6b, the μ″ values of 11799
DOI: 10.1021/acssuschemeng.9b02100 ACS Sustainable Chem. Eng. 2019, 7, 11795−11805
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Figure 7. Simulated curves for EM wave loss of (a) PBDC, (b) S-2, (c) S-4, and (d) S-6.
resonance peaks in high frequency (>10 GHz) could be indexed to the exchange resonance due to the presence of MnOnrs.9,48 Meanwhile, the natural resonance would occur in the sample when the μ″ value is close to zero, and the negative μ″ values may be ascribed to the radiation of magnetic energy, which stems from an induced magnetic field caused by the charge movement under the alternating electric field according to the Maxwell equation.9,49 Normally, the magnetic loss consists of natural resonance, domain wall resonance, hysteresis loss, and eddy current, whereas the domain wall resonance and hysteresis loss make little contribution in the gigahertz range since they mainly are active in weak field or low frequency.8,50 In order to confirm the role in current study, the values of C0 are calculated, which could be expressed as C0 = μ″/{(μ′)2f}, and the C0 values would be almost constant if the eddy current loss plays a major role in the magnetic loss.8,9 After C0 curves were plotted in Figure S4, it is obvious to see that C0 values for all samples fluctuated with the increase of frequency within 2.0−18.0 GHz, signifying that the eddy current loss could be neglected in the present work, and the natural resonance is responsible for the process of magnetic loss.8,9 Furthermore, the values of tan δe and tan δμ are calculated to evaluate the dielectric loss and magnetic loss abilities of the microwave, respectively, and the relatively higher value means a higher loss ability. As illustrated in Figure 5c and Figure 6c, there is no tan δe value for MnOnrs/PBDC samples except S-6 is larger than that of PBDC, while, apart from S-6, the tan δμ value for MnOnrs/PBDC samples is always bigger than that of PBDC. Specifically, the dielectric loss should dominate in EM wave absorption performance when the tan δe value is much larger than the tan δμ value, and then the dielectric losses of PBDC and S-6 samples might play
a major role in EM wave absorption capacity. The reason for this phenomenon in S-6 sample should be attributed to the radiation of magnetic energy, and the intrinsic magnetic loss could not counteract the radiation of magnetic energy, resulting in the negative μ″ and magnetic tangent values.8 For S-2 and S-4 samples, the dielectric loss and magnetic loss are together responsible for the EM wave absorption capacity. Notably, although MnO is an antiferromagnetic phase, the nano-size effect along with high-temperature treatment under N2 might be responsible for the magnetic performance. The RL values have been calculated based on transmission line theory, which should be lower than −10 dB for practical application, indicating that over 90% of the incident microwave could be absorbed. Figure 7 gives 3D RL maps of all samples in the frequency range of 2.0−18.0 GHz with integrating absorber thicknesses from 1.00 to 5.00 mm, and some typical RL curves at some certain thicknesses are provided in Figure S5. Obviously, the RL values of the PBDC sample (Figure 7a) are no lower than −10 dB within 5.0 mm thickness in the measured frequency range, showing the poor EM wave absorption performance. Exhilaratingly, all MnOnrs/PBDC composites exhibit superior EM wave absorption properties, reaching a RLmin value of −51.6 dB at 10.4 GHz with 2.47 thickness for the S-2 sample, −51.2 dB at 13.4 GHz with 2.09 thickness for the S-4 sample, and −51.0 dB at 5.5 GHz with 3.52 thickness for the S-6 sample, respectively. In addition, all MnOnrs/PBDC composites perform wide EAB values, such as 3 GHz (11.1−14.1 GHz, 2.03 mm) for the S-2 sample, 4.7 GHz (8.6−13.3 GHz, 2.51 mm) for the S-4 sample, and 3.4 GHz (9.3−12.7 GHz, 2.0 mm) for the S-6 sample, respectively. Noteworthily, the qualified bandwidth over −10 dB is a more valuable evaluation indicator in microwave absorption for 11800
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calculated delta value with the thickness ranging from 0.5 to 5.0 mm against the evaluated frequency range from 2 to 18 GHz. Clearly, the terrible impedance matching performance (Figure 8a) for the PBDC sample accounts for its weak EM wave absorption performance, which should be resulting from the redundantly large ε″ values. Reversely, all MnOnrs/PBDC composites possess a large area with a delta value close to zero compared with the PBDC sample, suggesting an excellent impedance matching, and the sample S-6 possesses the biggest area with a delta value close to zero compared with other specimens, which is confirmed by the results of the qualified bandwidth over −10 dB with a very close RL value. Furthermore, attenuation constant (α), as a concept features the amplitude attenuation of the EM wave, is calculated by the following equation to investigate the loss ability8,9
practical application compared with the RL value (
