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Cite This: ACS Appl. Mater. Interfaces 2018, 10, 11108−11115
Hierarchically Porous Carbons Derived from Biomasses with Excellent Microwave Absorption Performance Zhengchen Wu,† Ke Tian,† Ting Huang,† Wei Hu,† Feifei Xie,† Jingjing Wang,‡,§ Mengxing Su,‡,§ and Lei Li*,† †
College of Materials and Fujian Provincial Key Laboratory of Materials Genome, Xiamen University, Xiamen 361005, China Advanced Materials Academy, Luoyang Ship Material Research Institute, Xiamen 361001, China § State Key Laboratory for Marine Corrosion and Protection, Luoyang Ship Material Research Institute, Qingdao 266101, China Downloaded via TULANE UNIV on January 9, 2019 at 20:06:34 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.
‡
S Supporting Information *
ABSTRACT: A variety of biomass-based carbon materials with two-level porous structure have been successfully prepared by one-step carbonization process. The first level of microscale pores templates from the inherent porous tissues, while the second one of nanopores is produced by the in situ etching by the embedded alkaline metal elements. The superimposed effect of nano and microscale pores endows the hierarchically porous carbons (HPCs) with excellent microwave absorption (MA) performance. Among them, the spinach-derived HPC exhibits a maximum reflection loss of −62.2 dB and a broad effective absorption bandwidth of 7.3 GHz. Particularly, this excellent MA performance can be reproduced using the biomass materials belonging to different families, harvested seasons, and origins, indicating a green and sustainable process. These encouraging findings shed the insights on the preparation of biomass-derived microwave absorbents with promising practical applications. KEYWORDS: biomass, in-suit activation, hierarchically porous structure, carbon, microwave absorption
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INTRODUCTION In the past decades, tremendous efforts dedicated to the fabrication of microwave absorbents have revealed that the superb microwave absorption (MA) performance benefits from the both rationally chosen composition and delicately designed structure.1−5 Conventional metal and metal oxide materials, such as Co, Fe3O4, and NiO demonstrate impressive properties for MA applications.6−11 However, they severely suffer from the inherent drawbacks including high density, poor chemical stability, and high loading content. By contrast, carbonaceous materials are believed to be another promising MA candidates, owing to their lightweight, favorable physicochemistry stability and tunable dielectric loss, which have attracted both industrial and academic interests.12−14 Recently, vast advances in MA theory and fabrication technology have demonstrated that the porous structure brings about considerable enhancement in the MA performance. The nanoscale pores will not induce the scattering of incident microwave and thus act as “effective medium” to satisfy impedance matching.15−17 The presence of microscale pores can act as dihedral angles for the microwave reflection. The microwave reflects many times among the dihedral angles, which increases the propagation path of microwave in the absorber and leads to the loss of electromagnetic energy.18,19 By right of superimposed positive effects of multiscale pores, hierarchically porous structure will significantly contribute to the MA performance. For example, the MA performance of activated carbon with microscale © 2018 American Chemical Society
porous structure was proved to be much better than that of most carbonaceous materials with uniform pores.20 In addition, it was found that carbon hollow microspheres with mesopores could achieve a maximum reflection loss (RL) of −84 dB, almost four times as high as that of carbon hollow microspheres without mesopores and nine times higher than that of compacted carbon microspheres.21 Inspired by the concept, more and more hierarchically porous carbons (HPCs) for MA applications are now designed on the basis of the synergism between composition and structure. Extensively used strategies for the generation of hierarchically porous structure involve templating approaches and etching methods.22,23 However, these processes severely suffer from expensive raw materials and environmentally hazardous preparation technologies. The biomasses possess naturally microscale porous vascular bundles that can work as paths for water and ion transport. Additionally, these biomasses also contain various alkaline metal elements, promoting the photosynthesis, metabolism and translocation of carbohydrates, and synthesis of protein. Therefore, not only can the inherent microscale porous structure be well preserved without templates after carbonization process, but also the embedded alkaline metal elements can activate the biomass-derived carbon Received: November 13, 2017 Accepted: March 7, 2018 Published: March 7, 2018 11108
DOI: 10.1021/acsami.7b17264 ACS Appl. Mater. Interfaces 2018, 10, 11108−11115
Research Article
ACS Applied Materials & Interfaces
Figure 1. Digital photos of spinach stem harvested in January, 2017, HPC-S, and the test sample (a); RL curves of the test sample with 30 wt % HPC-S carbonized at 600 °C (b); the comparison of the optimal MA performance of HPC-S with those of the representative carbonaceous materials reported previously (c). where f represents the frequency, c means the velocity of light, d is the thickness, and Zin refers to the normalized input impedance of a metalbacked microwave absorbing layer. Detail preparation process of the test sample is schematically shown in Scheme S1. Characterization. The morphology was observed by scanning electron microscope (SEM, Hitachi SU-70). The X-ray diffraction (XRD) patterns were carried out on a Bruker-Axe X-ray diffractometer with Cu Kα radiation source (40.0 kV, 40.0 mA). The Raman spectra were obtained on a Renishaw inVia spectrometer using laser excitation at 532 nm. Elemental analysis was recorded using CHNOS Elemental Analyzer Vario EL III (Elementar Analysensysteme GmbH, Germany). N2 adsorption/desorption isotherms were measured at 77 K using a Micromeritics TriStar II 3020 static volumetric analyzer. The Brunauer−Emmett−Teller (BET) surface area was calculated within the relative pressure range of 0.05 to 0.20. Pore size distribution was deduced from the absorption isotherms by nonlocal density functional theory.26 The mercury intrusion porosimetry measurement was carried out using a MicroActive Autopore V 9600 porosimeter. Sample was subjected to a pressure cycle starting at approximately 0.49 psi, increasing to 60 000 psi in predefined steps, and the pore size is calculated on the basis of Washborn−Laplace equation: P = −4γ cos θ/d, where P is the pressure, d is the diameter of the pore, and γ and θ are the surface tension and contact angle of the liquid, respectively.27 Conductivity of the samples was determined using a RTS-9 four-point probe instrument. Sample was pressured into sheet before test, and the sheet resistivity was calculated by measuring the potential difference between the two inner points at a given current.
in suit through etching reaction without the need of external activator, producing abundant microporous structure. Thus, HPCs with high specific surface areas are formed.24,25 In comparison with the widely used template and etching strategies, the template-free technique is an environmental friendly and sustainable choice, which is hardly applied for the MA material preparation to our best knowledge. Herein, we reported a variety of biomass-derived HPCs prepared via the one-step carbonization process and the investigation of their MA performance. By adjusting the carbonization temperature and absorbent loading in the paraffin matrix, the obtained HPCs achieve the maximum MA capacity with a RL value of as much as −62.2 dB and a broad effective absorption bandwidth (EAB) of 7.3 GHz. Particularly, the excellent MA performance can be reproduced using the same kind of biomass with different harvested seasons and origins. In addition, the MA performance of HPCs templating from different biomasses is also estimated, which is superior to those of most of reported carbonaceous materials. Detailed investigation reveals that high conduction loss originated from components together with multi-reflection of microwave and strong interfacial polarization promoted by structure results in such remarkable MA performance. These achievements light the way to large-scale preparation of excellent MA absorbents utilizing the biomasses as precursors.
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RESULTS AND DISCUSSION Spinach, a common plant, was selected as the precursor for investigating the MA performance of biomass-derived HPCs (Figure 1a). The additive-free HPC-S was prepared by directly carbonizing spinach stem, followed by washing and drying treatment. Twelve test samples (absorbent/paraffin composite) were fabricated in accordance with the established experimental program, which was combinations of four carbonization temperatures ranging from 500 to 800 °C and three filler loadings of absorbent ranging from 20 to 40 wt % (Figure S1). Among them, the sample with 30 wt % HPC-S that is carbonized at 600 °C exhibits the optimal MA performance. The maximum value of RL (RLmax) attains as much as −62.2 dB at 12.5 GHz with a thickness of 2.71 mm, while the EAB is up to 7.3 GHz, which is almost a half the frequency band of 2.0−18.0 GHz (Figure 1b). To our best knowledge, these values are superior to those of most MA materials reported previously, as summarized in Figure 1c.19−21,28−34 With such outstanding MA performance, the HPC-S can be applied widely in anti-electromagnetic radiation textiles and the radar absorbing coatings. The mechanisms behind the impressive MA performance are described as follows.
EXPERIMENTAL SECTION
Preparation of Biomass-Derived HPCs. Spinach stem was cleaned, dried, and finally heated at 600 °C for 2 h with a heating rate of 5 °C/min. The resulting sample was washed with distilled water until the filtrate appeared neutral. After being dried under vacuum, the final product was obtained and named HPC-S. For comparison, the samples were carbonized at 500, 700, and 800 °C, respectively. In addition, two kinds of spinach stem bought in different seasons were also used to prepare the HPCs. The other HPCs starting from different biomass precursors were prepared with the same procedures. Microwave Absorption Measurements. The test sample was prepared by pressing the homogeneous HPCs/paraffin mixture into a toroidal-shaped pipe with an outer diameter of 7.0 mm and an inner diameter of 3.0 mm. The loadings of HPC-S in test samples were 20, 30, and 40 wt %, respectively. The electromagnetic parameters were measured by an Agilent N5222A vector network analyzer using the coaxial-line method. Subsequently, the relative complex permeability (εr = ε′ − jε″) and permeability (μr = μ′ − jμ″) in the frequency range of 2−18 GHz could be obtained. The RL values were calculated based on the transmission line theory:
Z in =
μr /εr tanh| − j(2πfd /c) εrμr |
RL(dB) = 20 log|Z in − 1/Z in + 1| 11109
DOI: 10.1021/acsami.7b17264 ACS Appl. Mater. Interfaces 2018, 10, 11108−11115
Research Article
ACS Applied Materials & Interfaces
porosimetry (Figure 2c). The microstructure of HPC-S is further explored by N2 adsorption/desorption experiment performed at 77 K. The isotherm curve exhibits a rapid N2 uptake at a very low pressure region (P/P0 < 0.01), followed by a continuous increase in the rest of P/P0 range and a mesoporecaused adsorption−desorption hysteresis loop (Figure 2d), indicative of the presence of micro-/meso-/macropores,35 as plotted in the Figure S3. The large micropore volume of 0.16 cm3 g−1 and high specific surface area of 345.3 m2 g−1 are believed to be produced by alkaline metal element activation process. In comparison with KOH activated biomass materials, the HPCs in our case are obtained without external activator, suggesting a green carbonization process. Thanks to the superimposed effects of both nano and microscale pores, hierarchically porous structure contributes to the MA performance of the biomass-based HPCs. The microsized cavities will act as dihedral angles to promote the reflection of microwave inside the absorber and lead to the loss of electromagnetic energy by increasing the propagation path of microwave (Figure S4).18,19 It is well-known that an incident wave is not sensitive to the particles and structure that are smaller than a sensing wavelength, thus the nanosized pores can work as an “effective medium”.15 The effective permittivity of HPC-S can be calculated using Maxwell Garnet model:
As shown in Figure 2a, pristine spinach stem is mainly made of the epidermis protecting inside tissues and the vascular
Figure 2. SEM figures of dried (a) and carbonized (b) spinach stem; the pore size distribution obtained by mercury intrusion porosimetry (c), and the N2 absorption−desorption isotherm (d) of HPC-S carbonized at 600 °C.
bundles conducting water and inorganic salts. After carbonization, the obtained carbon material principally inherits the porous structure with pore size ranging from 2 to 30 μm (Figure 2b and Figure S2). The statistical microscale pore size distribution and porosity are characterized by mercury intrusion
⎡ (2ε + ε2) + 2v(ε2 − ε1) ⎤ εeffMG = ⎢ 1 ⎥ε1 ⎣ (2ε1 + ε2) − v(ε2 − ε1) ⎦
Figure 3. Dependence of carbonization temperature on XRD patterns (a), Raman spectra (b), elemental components (c), real (d) and imaginary (e) parts of the complex permittivity, dielectric loss tangent and characteristic impedance (f), and RL curves (g to i) of HPC-S when the absorbent loading is 30 wt %. The RL curves of HPC-S carbonized at 600 °C are shown in Figure 1b. 11110
DOI: 10.1021/acsami.7b17264 ACS Appl. Mater. Interfaces 2018, 10, 11108−11115
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Figure 4. Dependence of filler loading on the complex permittivity (a and b), dielectric loss tangent and characteristic impedance (c), and RL (d and e) of HPC-S/paraffin composites when the carbonization temperature of HPC-S is 600 °C. The RL curves of composite with 30 wt % HPC-S are shown in Figure 1b.
where ε1 and ε2 are the permittivity of matrix material and air in the pores, respectively, and v represents the volume fraction of the pore.16,17,36 The larger the volume of air is, the lower the value of effective permittivity is, because the value of permittivity of dielectric matrix is much higher than that of air. Therefore, the presence of nanosized pores in HPC-S can decrease the permittivity and optimize the Zim matching, as explained in the following. It is well-known that the closer the values of characteristic impedance (Z im = μ0 /ε0 μr /εr ) of absorbent are to 1 (the values of air’s Zim), the more the microwave can propagate into the absorbent, which also means more matched Zim.28 For the carbonaceous material, the values of μr are much lower than those of εr in the microwave frequency. Therefore, the decrease in the values of εr leads to the increase in the values of μr /εr , which makes the values of Zim closer to 1. In addition, the chemical composition (degree of graphitization) of HPC-S can also be adjusted by carbonization temperature, to obtain the optimal MA performance collocating hierarchically porous structure. As shown in Figure 3a, two broad diffraction peaks centered at 24.3° and 43.3° can be observed in all patterns, indicating the formation of graphitic carbon, albeit with disorder.37 The Raman spectra of HPC-Ses obtained at different carbonization temperatures are shown in Figure 3b. All the samples display two peaks at 1351 and 1590 cm−1, corresponding to the D and G bands, respectively.38 The integrated intensity ratio of D and G bands (ID/IG) increases from 0.91 to 0.99 with raising the annealing temperature from 500 to 700 °C. According to the phenomenological three-stage model proposed by Ferrari and Roberston, the gradually increased ID/IG value of HPC-S is exactly in the transitional stage from amorphous carbon to nanocrystalline graphite.39 When increasing carbonization temperature to 800 °C, the ID/ IG value reduces to 0.96 because of the enlarged sp2 carbon domain, demonstrating the transformation from nanocrystalline graphite to graphite. The dependence of carbonization temperature on elemental components of HPC-Ses is shown in Figure 3c. The carbon content grows continuously with the
increase of carbonization temperature. While the tendency is opposite for the oxygen and nitrogen contents, suggesting the decrease of defects.40 Moreover, free conduction electrons will appear in great numbers at the time when the carbonization temperature is high enough to drive most of the heteroatoms out of the carbon, which in conjunction with a gradual replacement of the high opaque intermolecular barriers by low resistance C−C contacts (enlarged nanocrystalline graphite domain) leads to the enhancement in conductivity.41 The complex permittivity plots of HPC-Ses carbonized at different temperatures are collected in Figure 3d and e for comparison. It is evident that both the values of ε′ and ε″ increase with the increasing of carbonization temperature, which may be explained as following. Four kinds of polarization including electronic, atomic, Debye (dipolar) and interfacial polarization can exist in a heterogeneous system.42 Usually, the former two kinds occur at higher frequencies,43 which can be excluded from the following discussion. On the basis of Debye polarization theory,44 the complex permittivity can be deduced as εs + ε∞ ⎞2 ⎛ ⎛ εs − ε∞ ⎞2 2 ⎜ε′ − ⎟ + (ε ″ ) = ⎜ ⎟ ⎝ ⎝ 2 ⎠ 2 ⎠
where εs is the static permittivity and ε∞ is relative dielectric permittivity at the high-frequency limit. When the plot of ε″ versus ε′ (Cole−Cole plot) is a semicircle, it represents one Debye relaxation process.45 In our case, the Cole−Cole plots of these four samples only demonstrate slight fluctuations and have no evident Debye peak (Figure S5), suggesting that dipolar polarization is negligible while interfacial polarization may play a major role during polarization process. This result is consistence with the previous report that denied the existence of dipolar polarization in biomass-derived carbons carbonized above 500 °C.46 Usually, the interfacial polarization is caused by the aggregation of carriers at the interfaces.47 The increment of carbonization temperature facilitates not only the enlargement of specific surface area of HPC-S (Figure S6), but also the generation of free electrons. Eventually, the increase of values 11111
DOI: 10.1021/acsami.7b17264 ACS Appl. Mater. Interfaces 2018, 10, 11108−11115
Research Article
ACS Applied Materials & Interfaces
Figure 5. Dependence of the frequency on the complex permittivity (a) and RL of HPCs prepared using spinach stems harvested in July 2016 (b) and in July 2017 (c), respectively.
Table 1. MA Performance of HPCs Derived from Different Biomass Precursors precursor (Family)
organ
Tc (°C)a
FL (wt %)b
RLmax (dB)
EAB (GHz)
thickness (mm)
pak choi (Brassicaceae)
stem leaf stem leaf stem
600 600 700 600 600 600 600 700 600
50 30 50 30 30 30 30 30 50
−51.5 −53.6 −52.8 −41.4 −51.7 −25.6 −62.2 −41.2 −22.6
5.1 4.8 5.1 5.1 1.5 6.6 7.3 4.6 7.8
2.15 2.10 2.52 2.40 4.64 2.20 2.71 1.50 2.50
cabbage (Brassicaceae) celery (Apiaceae) kelp (Laminariaceae) spinach (Chenopodiaceae)
a
stem stem leaf
Carbonization temperature. bFiller loading of absorbent.
of ε′ may be the result of the enhanced interfacial polarization caused by more massive accumulation of electrons at larger interfaces; the increase of values of ε″ may arise from enhanced polarization loss and conduction loss induced by increased conductivity (Figure S7). The dependence of frequency on the complex permeability of HPC-S is shown in Figure S8. Despite with slight fluctuations, it can be found that the values of μ′ and μ″ are close to 1 and 0, respectively, suggesting the negligible magnetic loss. The fluctuations are thought to be induced by the microsized pores, as explained in the following. The microporous structure can cause the scattering of local electric field. As the electric and magnetic fields are mutually concomitant EM wave propagation, the direction of magnetic field will change when an electric field changes its direction. The varying magnitude and direction of magnetic field will induce electric currents on conductive circular paths according to Lenz’s law. The flowing currents decay in the microscale porous structure by converting into heat, resulting in partial energy attenuation of the scattered magnetic fields.48 Attenuation ability (dielectric loss for carbonaceous material, tan δ = ε″/ε′) and characteristic impedance (Zim) are two main factors that determine the MA capacity. Majority of microwave can enter into the absorbent when the Zim is well matched with that of the air, and the incident microwave can be dissipated as much as possible when the absorbent has strong attenuation ability.49 The values of tan δ and Zim of HPC-Ses carbonized at different carbonization temperatures were measured and plotted in Figure 3f. With the increasing of carbonization temperature, the values of tan δ evidently increase while the values of Zim decrease, suggesting the enhanced dielectric loss and mismatched Zim. On the basis of the balance between tan δ and Zim, HPC-S carbonized at 600 °C demonstrates the optimum MA performance. Moreover, it can be found that dried spinach stem exhibits poor MA performance because of the negligible dielectric loss (Figure S9), highlighting the importance of carbonization process.
In addition to the carbonization temperature, another parameter, the absorbent loading in paraffin, also has significant influence on the MA performance.50 The correlation between the complex permittivity of as-prepared HPC-S/paraffin composite and different absorbent loading is shown in Figure 4a and b when the carbonization temperature of HPC-S is 600 °C. Since the values of ε′ of paraffin are close to those of the air (Figure S10), the values of ε′ of the test samples increase along with the HPC-S content, according to the effective medium theory.44 The similar tendency of the values of ε″ versus absorbent loading can be ascribed that the aggregation of HPCS particles enhances the conduction loss. According to the aggregation-induced charge transport mechanism and conducting network model proposed by Cao et al.,44,51,52 migrating electrons move directionally or hopping electrons jump across the interfaces between absorbent particles respectively, when the microwave propagates into the composites. Higher absorbent content leads to a decrease in the energy barrier for hopping electrons and causes a dense microcurrent interconnected network (Figure S11), which promotes the conduction loss of the test samples. The dependence of HPC-S loading on tan δ and Zim of test sample is shown in Figure 4c, respectively. Similarly, the values of tan δ increase with the increasing absorbent loading while the values of Zim decrease, indicating the enhancement of dielectric loss and mismatching of Zim. The optimum MA capacity can be obtained by tuning the absorbent loading to achieve the best match between tan δ and Zim. Eventually, the best MA performance is obtained when using a 30 wt % absorbent loading (Figure 1b). With the guidance of above results, the other biomassderived carbonaceous materials with excellent MA performance can be obtained according to the following criterions. First, biomass precursors with abundant microsized porous structure and the embedded alkaline metal elements are essential. Second, the optimal match between tan δ and Zim can be 11112
DOI: 10.1021/acsami.7b17264 ACS Appl. Mater. Interfaces 2018, 10, 11108−11115
Research Article
ACS Applied Materials & Interfaces achieved by adjusting carbonization temperature and filler loading of absorbent. The reproducibility of the MA performance of biomass-based HPCs is confirmed using spinach stems harvested in different seasons (July 2016 and July 2017, respectively) and at different origins. As shown in Figure S12, the harvest season and origin hardly have affection to the hierarchical characteristic of the HPC. By adjusting carbonization temperature and absorbent loading, HPCs templating from these spinach stems achieve the similar complex permittivity and MA performance (Figure 5). The other four kinds of biomasses, including pak choi (Brassicaceae), cabbage (Brassicaceae), celery (Apiaceae), and kelp (Laminariaceae) were chosen to investigate the influence of family difference on the biomass-derived MA materials. After the same washing, drying and carbonization process, all the biomass-derived HCPs demonstrate the hierarchically porous structure and high porosity (Figure S13). Their MA performance is measured and summarized in Table 1. It is found that the MA performance of these absorbents are superior to those of most carbonaceous absorbents reported previously though they are classified as different families. For instance, the HPCs derived from the stems of pak choi and cabbage have the RLmax over −50 dB and the EAB wider than 5.0 GHz; the HPC derived from the spinach leaf exhibits an ultra-wide EAB of 7.8 GHz with the thickness of 2.50 mm. These facts reveal that the MA performance of biomass-based HPCs is predominantly by their porous structure and chemical composition.
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AUTHOR INFORMATION
Corresponding Author
*Tel: +86-592-2186296. Fax: +86-592-2183937. E-mail: lilei@ xmu.edu.cn. ORCID
Lei Li: 0000-0003-2732-9116 Author Contributions
All authors contributed to the experimental design and data analyses. The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (Nos. 51373143 and 21674087) and the Natural Science Foundation of Fujian Province (No. 2014J07002).
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REFERENCES
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CONCLUSION In conclusion, a new kind of biomass-based MA materials has been successfully prepared using biomasses belonging to different families. Templating from the inherently their microscale porous tissue structure and activated by the embedded alkaline metal elements, carbon materials with hierarchically porous structure and high surface areas are formed after one-step carbonization process without the need of external activator. The obtained HPCs demonstrate excellent MA performance superior to those of most carbonaceous absorbents reported previously. Such outstanding MA performance results from the strong conduction loss, interfacial polarization and multiple reflections of microwave induced by highly conductivity components and hierarchically porous structure. The detailed investigation also indicates that the harvesting seasons and origins have no influence on the MA performance of the biomass-based HPCs. These accomplishments indicate the promising MA application values of the green and sustainable biomass-based HPCs. Furthermore, it is believed that the investigation on the hierarchically porous structure in this work provides new inspirations and insights for the preparation of novel porous microwave absorbents with the stronger reflection loss in the wider frequency band.
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curves of HPCs; schematic models of conductive networks and multiple reflections of microwave; the schematic illustration of experimental conditions (PDF)
ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.7b17264. SEM images and Cole−Cole plots of HPC-S; SEM images, N2 absorption−desorption isotherms, and RL curves of other biomass-derived HPCs; complex permittivity and RL curves of paraffin and dried spinach; the dependence of carbonization temperature on the conductivity and complex permeability of HPC-S, RL 11113
DOI: 10.1021/acsami.7b17264 ACS Appl. Mater. Interfaces 2018, 10, 11108−11115
Research Article
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