Hierarchical porous carbon frameworks derived from Juncus effusus biomass with robust electromagnetic wave absorption properties

Abstract

Biomass-based carbon materials exhibit remarkable potential for electromagnetic wave (EMW) pollution protection due to their extremely low density, inconceivable structure, and exceptional dielectric loss properties. In this study, a biomass-derived electromagnetic absorber (EMA) was developed using Juncus effusus (JE), and the 3D hierarchical porous framework was well maintained during a simple carbonization process, achieving a reflection loss (RL) value of −40.4 dB and a broad effective absorption bandwidth (EAB) of 3.48 GHz for JE-700 at the thickness of merely 1.75 mm. Besides, the maximum radar cross-section (RCS) reduction value of 32.4 dB m² was achieved at an incident angle of 0°. The synergistic effects of multiple loss mechanisms for the JE-derived EMA include conduction loss arising from the carbonized skeleton, multi-reflection of EMWs in regularly arranged micro-triangle units and hollow skeletons, and the numerous interfacial polarizations associated with boundary-type defects present in the amorphous and graphite phase carbon, as well as between the microelements (P, K, and Si) and carbon atoms. Therefore, the inherent 3D hierarchical porous framework offers novel inspirations and insights for designing biomass-derived EMAs.

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1. Introduction

The rapid advancement of electronic and communication technologies greatly enhances human lives in many aspects; however, the related electromagnetic wave (EMW) emissions and interference have been threatening human health and potentially disrupting the normal operation of electronic devices.^1,2 Microwave absorption (MA) materials could absorb the incident waves and subsequently dissipate them, avoiding surface reflection-induced secondary pollution, thus they have generated intensive interest in addressing the EMW pollution.^3,4 Recently, carbon-based porous materials, such as covalent organic frameworks (COFs),⁵ metal–organic frameworks (MOFs),⁶ carbon nanotubes (CNTs),⁷etc., have emerged as promising electromagnetic absorbers (EMAs) due to their lower mass density, stronger corrosion resistance, and better design flexibility advantages over the traditional metal-based EMAs.^8–10 Additionally, the presented pore structure could generate more solid–air interfaces that further facilitate the multi-reflection and dissipation of the incident EMWs.¹¹ However, the synthesis of typical carbon-based porous EMAs demands costly raw materials and involves intricate processes, seriously impeding their practical utilization. Hence, it is urgent to explore sustainable and cost-effective raw materials to produce carbon-based porous EMAs using simple synthesis methods.

Nature has nurtured biomasses with inconceivable delicate structures, such as elaborate periodic porous microstructures and microtubular channels, which inspires us to synthesize EMAs from such renewable resources.^12–14 However, the insulation nature of biomasses greatly restricts their interactions with EMWs. Recent studies have demonstrated that the carbonized biomasses showed excellent MA performance due to the enhanced conductivity and better impedance matching with free air.^15,16 Moreover, the periodic porous architectures were well preserved after pyrolysis, which could promote the incidence of EMWs. For example, Wu et al.¹⁷ fabricated a biomass-based EMA featuring a distinctive hierarchical designed pore structure derived from spinach stems. The dihedral angles shaped micro-sized cavities, elongating the transmission path and creating additional interfaces for the attenuation of the incident EMWs, and a maximum reflection loss (RL) of −62.2 dB was achieved. J. B. Zhou et al.¹⁸ prepared a porous pyrolyzed carbon (PCBC) from natural wood, which preserved the aligned parallel channel structure of the raw material, allowing the incident EMWs to penetrate the channels. The PCBC demonstrated varying EMW attenuation capabilities by tuning the pyrolysis temperature. A RL value of −68.3 dB with a broad effective absorption bandwidth (EAB) of 6.13 GHz at a thickness of 4.28 mm was yielded at a pyrolysis temperature of 680 °C.

Juncus effusus (JE) is a naturally occurring biomass material characterized by its unique cylindrical shape, measuring 2–3 mm in diameter and 80–120 cm in length.^19,20 In recent studies, researchers have demonstrated the potential applications of JE in various fields, such as photocatalysis, adsorption, solar evaporation, thermal management, and strain sensing.^21–23 These applications capitalize on JE inherent benefits, including its light weight, abundant functional groups, and extensive porosity. Moreover, the distinctive 3D reticulated framework within the interior of natural JE, featuring interconnected channels, imparts a hierarchical porous structure. Such a structure shows great advantage in reducing dielectric constants and facilitating impedance matching with air in practical applications.²⁴ In addition, the microelements (N, P, and K) in natural JE sourced from soil may create polarization centers due to their distinct electronegativity compared to carbon atoms when subjected to alternating EMW fields. Hence, employing natural JE biomass as the primary material for the construction of carbon-based EMA is a promising approach.

In this study, a natural biomass-derived EMA with a 3D hierarchical porous framework was developed through a one-step carbonization process. The results showed that the carbonization temperature is related to the electrical conductivity and interfacial polarization of the EMA. Then, the underlying mechanism for interfacial polarization in amorphous and graphite phase carbon was analyzed via Raman image technology to establish the relationship between the boundary-type defects and EMW absorption properties. Benefiting from the hierarchical porous network, abundant polarization centers, and hollow carbon skeleton, the obtained JE-700 EMA exhibited excellent EMW absorption properties (RL ∼ −40.4 dB) with an available commercial thickness of 1.75 mm and a low RCS value of 32.4 dB m² in simulated real environments. Therefore, this study provides an effective strategy for designing and fabricating efficient EMA materials using natural biomasses as precursor materials.

2. Experimental section

2.1. Materials

Natural Juncus effusus (JE) was obtained from Rongshan Co., Ltd (Jiangxi, China). Paraffin (AR, Fusing: 54–56 °C) was purchased from Sinopharm Chemical Reagent Co., Ltd (Shanghai, China). Argon (Ar, 99.99%) was provided by XinXiYi Technology Co., Ltd (Jiangsu, China).

2.2. Preparation of the Juncus effusus-derived EMA

To prepare the JE-derived EMA, the pristine JE was carbonized at 700 °C for 2 h at a heating rate of 2 °C min⁻¹ under an Ar atmosphere, and was denoted as JE-700. Similarly, JE-600 and JE-800 as the control samples were prepared according to the described carbonization process.

2.3. Characterization

The morphologies of the samples were observed using a field emission scanning electron microscope (FE-SEM, SU-8100, Hitachi, Japan), and the configured energy-dispersive X-ray spectrometer (EDS) was used to analyze the elemental composition. Fourier transform infrared spectroscopy (FTIR, Nicolet 10, TFS, USA) was used to analyze the functional groups on samples from 4000 to 400 cm⁻¹, and the resolution was 4 cm⁻¹. The crystal structures of samples were analyzed by X-ray diffraction (XRD, D2, AXS, USA) and the scanning speed and range were 5° min⁻¹ and 5–80°. An X-ray photoelectron spectrometer (XPS, Axis Ultra DLD, Kratos, UK) was utilized to investigate the surface chemical composition of samples using Al Kα X-ray excitation radiation (hν = 1486.6 eV). Thermogravimetric analysis was performed using TGA Instruments (TGA2, Mettler, Switzerland) in the range of 50–800 °C under an N₂ atmosphere. The content and distribution of amorphous and graphite carbon were analyzed using a Raman imaging measurement system (Renishaw, Reflex, UK) in the surface range of 10 μm × 10 μm and a step size of 1 μm. The magnetic properties of the samples were evaluated by using a vibrating sample magnetometer (VSM, Lake Shore, 7404, USA) at 300 K from −20 000 to 20 000 Oe. The electromagnetic parameters of the carbonized-JE were measured using a vector network analyzer (VNA, N5222A, KEYSIGHT) in the frequency range of 1 GHz to 18 GHz with 10 wt% filler loading, and the annular sample had an external diameter of 7.00 mm and an internal diameter of 3.04 mm.

3. Results and discussion

3.1. Characterization of the materials

The fabrication process of Juncus effusus-derived hierarchical porous carbons is illustrated in Fig. 1(A). The JE fibers were subjected to a mechanical stripping process prior to the carbonization process. As seen from Fig. 1(B)–(D), the internal microstructure of pristine JE is composed of 3D hierarchical porous frameworks built up by interconnected hollow elementary microfibers with a pore diameter of approximately 3 μm. This framework is mainly composed of cellulose, lignin and hemicellulose. Furthermore, these interconnected microfibers generate numerous regularly distributed triangular-shaped macropores within the interior 3D space, which could facilitate the multi-reflection of the incident EMWs. As cellulose is the main component of natural JE, intensive signals from carbon (C) and oxygen (O) elements were observed from the EDS result depicted in Fig. S1 (ESI†). Additionally, signals belonging to trace elements such as silicon (Si), aluminum (Al), potassium (K), phosphorus (P), and calcium (Ca) were also detected in JE. Fig. 1(E)–(G) illustrate that the microstructure of JE underwent substantial changes during high-temperature carbonization. The primary hierarchical porous framework remaining intact was attributed to the robust mechanical stability from the mutually supportive influence of micro triangular units. Notably, the elemental composition of JE-700 (Fig. S2, ESI†) differs from that of pristine JE. In particular, trace elements like Si, P, and K are believed to be instrumental in establishing the interface polarization by alternating EMW fields to improve the EMW absorption properties of materials due to the electronegativity difference with C atoms.^25,26

Fig. 1 Illustration of the fabrication process of Juncus effusus-derived hierarchical porous carbons (A). The FE-SEM image of pristine JE (B–D) and JE-700 (E–G) at different magnifications.

The chemical structures of JE and JE-700 were examined through FTIR spectroscopic analysis, as depicted in Fig. 2(A). The functional groups of α-cellulose, hemicelluloses, and lignin represent the principal features in the spectrum of JE. The prominent absorption band observed at approximately 3310 cm⁻¹ was associated with the stretching vibrations of –OH groups in cellulose, and the evident peak at 1036 cm⁻¹ was caused by the stretching vibrations of C–O groups. Additionally, the absorption peak at 2921 cm⁻¹ corresponds to C–H stretching vibrations in cellulose and hemicellulose. The noticeable peaks at approximately 1734 cm⁻¹ and 1248 cm⁻¹ were assigned to the stretching vibrations of –C=O and –C–O in hemicelluloses, respectively.^27,28 The peak around 1513 cm⁻¹ corresponds to C=C vibrations within the aromatic ring of lignin. Nevertheless, the chemical structure of JE-700 is markedly different from that of JE due to the transformation of α-cellulose, hemicelluloses, and lignin into derived carbon during the carbonation process. It is evident that the characteristic peaks of –C–O, –C–H, and –C=O disappeared, and the peak attributed to the –C=C stretching vibrations appeared at 1564 cm⁻¹ after carbonization, suggesting the successful generation of carbon.

Fig. 2 FTIR curves of JE and JE-700 (A); XPS spectra of JE and JE-700 (B); C 1s (C) and O 1s (D) spectra of JE and JE-700; XRD pattern of JE and JE-700 (E); TG curves of JE and JE-700 under an N₂ atmosphere (F).

To gain a much deeper understanding of the chemical structure changes of JE before and after the carbonization process, XPS analysis was conducted and the sectra are depicted in Fig. 2(B)–(D). The prominent peaks of pristine JE correspond to C, O, and N elements. In detail, the C 1s spectrum displayed three distinct peaks at 284.6, 286.3, and 288.5 eV, corresponding to C–C or C–H bonds, C–OH or C–O–C bonds, and C–O or C=O bonds,²⁹ respectively. Moreover, the O 1s spectrum was characterized by three distinct peaks at 531.6, 532.9, and 533.5 eV, assigned to O–C, O=C, and –O–H chemical bonds.³⁰ However, the XPS spectrum of JE-700 revealed a notable increase in the intensity of the C 1s peak, accompanied by a decrease of the O 1s peak and the disappearance of the N 1s peak after the carbonization process. Specifically, the emergence of the C=C bond at 285 eV indicated the structural transformation from cellulose to carbon materials, and the noticeably decreased intensity of the C–O bond further confirmed the degradation of hemicelluloses, lignin, and glycosidic linkages. Moreover, the single C=O peak at 532.5 eV verified the presence of oxygen defects in JE-700 during the carbonization process.

To further confirm the formation of the crystalline structure in JE-700, XRD spectra of JE and JE-700 were collected and are shown in Fig. 2(E). The JE displayed two diffraction peaks at 16.7° (110) and 22.8° (200),³¹ which correspond to the cellulose Iβ crystal pattern and the crystalline structure of cellulose, respectively. In contrast, the new strong peak at 14.1° in JE-700 indicates the incomplete graphitization degree and the presence of abundant deficiency in the carbonization process. The peak at 43.2° is related to the crystal surfaces of graphite-3R (003) and graphite-2H (002) along with amorphous carbon, both coexisting at lower carbonizing temperatures.³² Thermogravimetric (TG) analysis was performed to assess the thermal stability of both JE and JE-700. As illustrated in Fig. 2(F), JE-700 experienced a slight weight reduction in the temperature range of 50–200 °C due to the loss of adsorbed water, and the weight remained relatively constant as the temperature increased to 800 °C, yielding a residue weight of 89%. In contrast, JE displayed a significant degradation in the examined temperature range, where a significant degradation of hemicellulose and lignin was observed within the temperature range of 150–300 °C, and a substantial mass reduction ascribed to the degradation of α-cellulose was seen between 300 and 400 °C, ultimately leading to a residue weight of approximately 26%.

To investigate the effect of carbonization temperature on the distribution and proportion of amorphous and graphite phase carbon within the JE-derived hierarchical porous carbons, the Raman imaging technique was adopted to visually and precisely analyze the corresponding I_D/I_G ratio. The D peak at 1350 cm⁻¹ was associated with the symmetric mode related to the K-zone boundary sonar A_1g, signifying the presence of defects and microcrystalline graphite, and the G peak at 1580 cm⁻¹ matched the E_2g symmetry mode, attributing to the vibrational motion of sp² hybrid bonds.³³ Previous studies have shown a positive correlation between the I_D/I_G ratio and the extent of defects.³⁴ As shown in Fig. 3(A), the Raman image of JE is filled with pink, indicating that the range of I_D/I_G is between 0.8 and 0.9. As the carbonization temperature increases, noticeable changes are observed in the Raman images. The yellow regions (0.9–1.0) denote higher enhancement in I_D/I_G ratios, and even some green regions emerged in Fig. 3(C) and (D). This suggested that the graphite nanocrystals or graphite-rich regions increased in the JE-derived hierarchical porous carbons as the carbonization temperature rose. Consequently, many boundary-type defects appeared between the amorphous and graphite phase carbon due to the different dielectric properties in alternating EMW fields. The inhomogeneous distribution of positive and negative charges at the interfaces of these defects generates spatial electric dipole moments and polarization relaxation, thereby increasing the dielectric loss for the EMW absorption material. However, a lower I_D/I_G ratio (0.7–0.8) occurred as the carbonization temperature increased to 800 °C (Fig. 3(D)). It suggested that boundary-type defects were eliminated due to the transformation of amorphous carbon into regular graphite carbon structures.

Fig. 3 Raman images of JE (A), JE-600 (B), JE-700 (C), and JE-800 (D).

3.2. Electromagnetic wave absorption properties

The electromagnetic properties of the JE-derived hierarchical porous carbons were investigated using their complex permittivity (ε_r = ε′ − jε′′) and complex permeability (μ_r = μ′ − jμ′′) at various carbonization temperatures. The RL was calculated based on the transmission line theory and the metal backplane model to evaluate the electromagnetic wave absorption performance.^35,36

Herein, Z_in refers to the input impedance of the materials, Z₀ denotes the intrinsic impedance in free space, f stands for the frequency of EMWs, and d and c indicate the thickness of materials and the light speed in a vacuum, respectively. Commonly, EMA materials should exhibit an RL value lower than −10 dB, implying an absorption efficiency of approximately 90%, and the associated frequency range is defined as the EAB. Fig. 4 presents the calculated RL values and the corresponding 3D curves of the JE-derived hierarchical porous carbon absorbers at various carbonization temperatures with thicknesses ranging from 1 to 4 mm across the 1 to 18 GHz frequency range. JE-700 demonstrated excellent EMW absorption performance, showing a minimum RL value of −40.4 dB at 14.3 GHz, along with a relatively wide EAB of 3.48 GHz and an available commercial thickness of 1.75 mm. In addition, JE-700 has varying thicknesses of 2.05 mm and 2.35 mm that demonstrated remarkable EMW absorption features, with the corresponding RL values of −16.2 dB and −21.3 dB. The EAB encompassed the entire C band (4–8 GHz) when the thickness ranged from 3.85 mm to 3.25 mm, and the X-band (8–12 GHz) was integrally covered from 2.65 mm to 2.05 mm. Additionally, the EAB encompassed the entire Ku band (12–18 GHz) with the thickness from 2.05 mm to 1.15 mm. Furthermore, we comprehensively and systematically evaluated the performance of this and previously reported absorbers based on three dimensions (RL_min, thickness, and EAB_max) as shown in Fig. S3 (ESI†). The results established JE-700 as a promising EMA compared to biomass derived EMA materials ever reported.^37–40

Fig. 4 3D reflection loss curves and 2D contour mappings of JE-600 (A) and (D), JE-700 (B) and (E), and JE-800 (C) and (F) in the frequency range of 1–18 GHz.

In general, the EMW absorption features are correlated with the complex permittivity, complex permeability, and suitable matching between them.⁴¹Fig. 5 illustrates the complex permittivity of JE-derived hierarchical porous carbons, as well as the dielectric loss tangent angle (tan δε) subjected to varying carbonization temperatures. The real parts (ε′) signify the energy storage properties, while the imaginary parts (ε′′) indicate the energy loss properties of materials.⁴²Fig. 5(A) and (B) show that the ε′ and ε′′ gradually reduce as the frequency increases corresponding to the theoretical dielectric behavior. Specifically, the ε′ values decreased from 2.5 to 2.3, 13.4 to 9.3, and 22.1 to 8.9, and their ε′′ values correspondingly changed from 0.3 to 0.1, 4.1 to 3.2, and 18.3 to 5.0 for JE-600, JE-700, and JE-800 across the entire frequency range. Additionally, as the carbonization temperature increased from 600 °C to 800 °C, the ε′ and ε′′ of JE-derived hierarchical porous carbons had a significant increase. Fig. 5(A) and (B) make it evident that the JE-800 exhibited superior dielectric storage capacity and energy loss properties due to the prevalence of extensive graphite regions in the hierarchical porous carbons. The dielectric loss is represented by tan δε = ε′′/ε′, which assesses the dielectric loss performance.⁴³ As depicted in Fig. 5(C), the tan δε curves for JE-derived hierarchical porous carbons exhibited several resonance peaks throughout the frequency range of 1–18 GHz, indicating multiple polarization relaxations due to the dielectric losses within EMA materials.⁴⁴ The dielectric loss of porous carbons experienced a gradual rise when the carbonization temperature was raised from 600 °C to 800 °C. This was attributed to the presence of heterogeneous interfaces resulting from amorphous carbon and graphite nanocrystals, and the interfacial polarization centers caused by the abundant boundary-type defects. Additionally, the growth of graphite nanocrystals formed extensive conductive areas with robust conduction loss capability. The polarization loss induced by vacancy-type defects and conduction loss primarily contributed to the dielectric loss. The relationship between ε′ and ε′′ can be defined according to the following Debye theory.⁴⁵ If the ε′′–ε′ plot forms some Cole–Cole semicircles, this corresponds to a single Debye relaxation process, and the arc shape indicates the relaxation process with several relaxation times.⁴⁶

Fig. 5 The real parts ε′ (A) and imaginary parts ε′′ (B) of the complex permittivity, dielectric loss tangent (tan δε = ε′′/ε′) curves (C), and Cole–Cole semicircles of JE-600 (D), JE-700 (E), and JE-800 (F).

Herein, ε_s and ε_∞ represent the static and relative dielectric permittivity at a high-frequency limit, respectively. The Cole–Cole semicircle curves for JE-derived hierarchical porous carbons at various carbonization temperatures are depicted in Fig. 7D–F. The results revealed that JE-700 exhibited more distinct semicircles when compared with JE-800 and JE-600, suggesting the occurrence of dipole relaxation. The dielectric loss in JE-700 was attributed to the synergistic effect of the interfacial polarization caused by the boundary-type defects and the conductive network due to extensive graphite regions. It is noteworthy that the conduction loss dominated in JE-800 as the tan δε was characterized by a pronounced straight-tail at low frequencies because of the highly graphitized carbon structures.

Fig. 6 illustrates the real part (μ′) and imaginary part (μ′′) of the complex permeability, and the magnetic loss tangent (tan δμ) of JE-derived hierarchical porous carbons at various carbonization temperatures. The μ′ reflects the magnetic storage ability, while the μ′′ describes the magnetic loss properties.⁴⁷ In Fig. 6(A) and (B), the μ′ value and μ′′ value of all samples exhibited a similar variation trend of initially increasing and then decreasing with the rise in carbonization temperature. This behavior indicated a strong connection between the magnetic energy storage and magnetic energy loss characteristics of JE-derived hierarchical porous carbons and the boundary-type defect number caused by amorphous carbon and graphite nanocrystals.⁴⁸ Previous studies also showed that the lattice structure of graphene is distorted or missing carbon atoms, resulting in the formation of spin-hybridized states that make it exhibit weak magnetic performance.^49–51 The loss curve (tan δμ = μ′′/μ′) depicted in Fig. 6C reveals that the tan δμ value of the porous carbons experienced an initial increase followed by a decrease as the carbonization temperature raised. This trend is consistent with the alterations in μ′ and μ′′ and further suggested that JE-700 possessed a slightly enhanced magnetic loss capacity compared to other samples. Magnetic losses are typically attributed to exchange resonances, natural resonances, eddy current effects, domain wall resonances, and hysteresis losses.⁵² The hysteresis loops of JE-derived hierarchical porous carbons (JE-700) are depicted in Fig. 6(D), which exhibited a coercivity of ±65 Oe and a saturation magnetization strength of ±0.0035 emu g⁻¹. The results verified that the porous carbons of JE-700 are practically nonmagnetic, and the low lever magnetism originated from spin induced by defects and not itself. The effects of domain wall resonances and hysteresis losses on magnetic losses should be ruled out when the frequency exceeds 1 GHz. Therefore, eddy current losses and exchange resonances may be the main reasons contributing to the magnetic permeability response of JE-derived hierarchical porous carbons.⁵³ To gain deeper insights into the magnetic loss mechanism of JE-derived hierarchical porous carbons, the C₀ is obtained using the following equation:⁵⁴

C₀ = μ′′(μ′)⁻²f⁻¹ = 2πμ₀d²σ (7)

where μ₀ is the permeability in a vacuum, and σ represents the electrical conductivity. If C₀ remains nearly constant or has minimal variation as the frequency increases, the principal contributor is the eddy current loss.⁵⁵ As shown in Fig. 6(E), the significant fluctuation peaks at 8.3, 12.6, and 16.7 GHz are attributed to the exchange resonance of magnetic graphite nanocrystals within the JE-700. Furthermore, the C₀ values remain relatively consistent within the frequency ranges of 5.6–7.5, 9.7–10.7, and 13.9–14.7 GHz. This consistency suggests the prevalence of eddy current losses within these frequency bands. According to the electromagnetic theory, the incident EMWs generate eddy currents, and the distribution becomes increasingly concentrated at the air–carbon interface as the EMW frequency increases. This phenomenon is known as the skin effect, which fundamentally leads to the attenuation of internal propagation of EMWs within the EMA materials, and a more pronounced skin effect corresponds to higher eddy current loss. Moreover, the local electrical network established by the hierarchical porous carbon framework could generate internal polarization in response to EMWs. The intensity and vector of the polarization electric field lag from the original by a certain angle and produced the corresponding current, establishing eddy currents and transforming electrical energy into heat. The graphite nanocrystal phase exhibits higher conductivity than amorphous carbon in hierarchical porous carbons, resulting in a heightened level of eddy current loss as the carbonization temperature increases. Subsequently, the attenuation constant (α) of JE-derived hierarchical porous carbons was analyzed according to eqn (8) and are presented in Fig. 6(F).⁵⁶

Fig. 6 The real parts (μ′) (A) and imaginary parts (μ′′) (B) of the complex permeability, magnetic loss tangent (tan δμ = μ′′/μ′) (C), M–H curves of JE-700 (D), C₀ curve (E) and attenuation constant (F) of JE-derived hierarchical porous carbons.

The α value of JE-derived hierarchical porous carbons increased as the carbonization temperature increased. Although the JE-800 exhibited the highest α value across the entire frequency range, the optimal EMW absorption characteristic among the three samples was exhibited by JE-700, as shown in Fig. 7A1–C1. As a result, the impedance matching values (Z) of all three samples in different frequencies and thicknesses were further investigated according to eqn (9) and (10) to validate the compatibility of the incident EMWs.⁵⁷

Fig. 7 Reflection loss dependent thickness and frequency, simulations for the t_mversus frequency subject to the λ/4 model, and impedance matching of JE-600 (A), JE-700 (B), and JE-800 (C).

As depicted in Fig. 7(A3), (B3) and (C3), the JE-700 exhibited superior impedance-matching properties compared to other samples across the entire frequency range. The moderate electrical conductivity and rich interface polarization centers are beneficial for reducing the reflection characteristics of EMWs at the air–carbon interfaces and improving the loss in the EMA interiors.⁵⁸ Therefore, the synergistic effect of well-balanced impedance matching and excellent attenuation capability is crucial for the EMW absorption performance for JE-derived hierarchical porous carbons. In addition, the EMW absorption mechanisms of hierarchical porous carbons were investigated using the 1/4 wavelength model in Fig. 7(A2), (B2) and (C2), which established a relationship between the matching thickness (t_m) and frequency (f_m), corresponding to the minimum RL value. The incident and reflected EMWs are 180° out of phase at the air interface, resulting in a complete cancellation of the reflected EMWs. Fig. 7(A1), (B1) and (C1) illustrate that the minimum value of the RL curve for JE-derived hierarchical porous carbons shifted towards a lower frequency with increased thickness. The thickness with a blue triangle, denoting the practical thickness, closely aligned with the calculated results. The RL curves aligned well with the 1/4 wavelength model, effectively elucidating the EMW absorption characteristics of JE-derived hierarchical porous carbons.

The RCS simulation distribution was performed to validate the application prospects of JE-derived hierarchical porous carbons in practical applications. RCS distribution is a measurement of the echo intensity of the target when exposed to EMWs. With the same echo signal, the RCS area of the target can be equivalent to the projection area of a metal sphere.⁵⁹ The model used to simulate this is shown in Fig. 8(A), which includes a composite absorber layer (200 × 200 × 1.75 mm³) and a perfect electrical conductor (PEC, 200 × 200 × 1 mm³). The simulated frequency was set to 14.3 GHz. Fig. 8(B)–(E) show the 3D RCS distribution simulation results of PEC, JE-600, JE-700, and JE-800, respectively. The perfect electrical conductor of PEC showed the greatest scattering signal, but JE-700 demonstrated the highest EMW absorption performance with a minimum RCS projection area. The RCS values of the EMA at the angle of incidence of EMWs from −90° to 90° are shown in Fig. 8(F), where JE-700 possesses the lowest RCS value. In detail, the effect of the EMA in reducing the RCS distribution area could be measured by subtracting the value of PEC. When the angle of incidence was 0°, the RCS reduction value of the JE-700 could reach a maximum of 32.4 dB m², efficiently preventing the reflection and scattering of EMW from the electrical conductor (PEC) and showing great potential for practical applications.

Fig. 8 Scheme of the CST simulation model (A). RCS simulation results at 14.4 GHz of PEC (B), JE-600 (C), JE-700 (D), JE-800 (E) and RCS simulation curves of different samples and degrees (F).

3.3. Electromagnetic wave absorption mechanisms

The EMW attenuation mechanisms of the JE-derived hierarchical porous carbons are summarized based on the analysis of the electromagnetic parameters and the EMW absorption performance shown in Fig. 9. First, the hierarchical porous framework and hollow tubes of JE-700 could reflect and attenuate the incident EMW several times, enhancing the EMW reflection loss. Second, the rich interfacial polarizations associated with boundary-type defects are presented between amorphous and graphite phases, as well as between the micro elements (P, K, and Si) and carbon atoms, which improved the attenuation ability of the EMA. Finally, the conductive carbonized skeleton could promote the electron transport and contribute to enhancing the conduction loss properties. With the combined effect of the above mechanisms, JE-derived hierarchical porous carbons exhibited excellent EMW absorption performance.

Fig. 9 Schematic illustration of EMW attenuation mechanisms of Juncus effusus biomasses hierarchical porous carbons.

4. Conclusions

In this study, a novel biomass-based EMA with an inherent 3D hierarchical porous network structure was successfully prepared through a simple one-step carbonization process without any external activators. The combined impact of conduction loss, multi-reflection, and interfacial polarization in the JE-derived hierarchical porous carbons imparted the biomass-based EMA outstanding EMW absorption characteristics, achieving a low RL value (−40.4 dB), a wide EAB (3.48 GHz) and a high RCS reduction value (32.4 dB m²). Hence, the natural JE with a distinctive 3D hierarchical porous framework structure offers a novel insight and approach for developing high-performance EMA materials.

Conflicts of interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

This work was financially supported by the National Natural Science Foundation of China (Grant no. 52203117) and the Fundamental Research Funds for the Central Universities (JUSRP122005).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d4tc00223g

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