Figure S1 exhibits the macroscopic photo of the DB and typical PCF foam, it changes from initial yellowish brown to dark black after the calcination treatment. As shown in Fig. 1a, the PCF foam is light enough to stand easily on a typical dogwood compared to the bulk carbon material. When plugging the DB and PCF foam into water (as shown in Fig. 1b), the former sank to the bottom while the latter floated on the surface. It is found that the density of the samples decreased sharply via an exclusive calcination treatment, and the density of the resulting product decreased gradually with increasing temperature (Fig. 1c); meanwhile, the circumference (perimeter) of the products displays an increasing tendency (Fig. 1d). Obviously, the corresponding density is reduced sharply by two orders of magnitude from 1.157 g/cm3 of DB sample to 0.0254 g/cm3 of the PCF-900 sample. Figure 1e exhibits the TGA-DTG curve of the DB sample. It can be observed that the mass loss is very tiny (only 4.41%) below 75 degrees Celsius (℃), which can be attributed to the decomposition of residual adsorbed water in the raw material [27]. Within the temperature range from 75℃ to 265℃, a sharp mass loss with a ratio of 35.73% occurs, which can be assigned to the decomposition of cellulose and hemicellulose. At this stage, the macromolecular components are broken down into glucose, accompanied by the formation of water, which is released as a gas at this temperature. Another noticeable mass variation (about 17.4%) appears between 265℃ and 375℃, which may be aroused by the further decomposition reaction of the obtained glucose, and different kinds of alcohols and acids can be further obtained. Meanwhile, some gaseous small molecule products, such as CO, CO2, and CH4, are released. When the calcination temperature is above 375℃, the mass variation curve of the DB is moderate, which more likely corresponds to the destruction of the structure of lignin in raw material [28]. At high temperatures (> 450 ℃), the polycondensation reaction occurs quickly, the structure conversion of the polycyclic aromatic hydrocarbons occurs, and the derived carbon is subsequently collected. Figure 1f exhibits the XRD patterns of DB and various PCF samples obtained at different calcination temperatures. For DB, the diffraction peak that appeared at 16° can be assigned to the character of cellulose and hemicellulose. For various PCF foams, the broad diffraction peak around 23° can be assigned to (002) crystal plane of carbon [29], and the overall shape of the diffraction peak reveals the amorphous state and disordered characteristics of the product. It can be speculated that the DB has been converted into derived carbon products above 400℃, which is lower than the calcination temperature of other biomass-derived carbon. It is well known that the stems of herbaceous plants are mainly composed of cellulose, hemicellulose, and lignin. As shown in Fig. 1g, cellulose and hemicellulose have poor thermal stability, which can occur decomposition reactions at low temperatures, while the main component of lignin is a phenylpropane-based polymer, which has significantly higher thermal stability than cellulose and hemicellulose. As the calcination temperature increases, the hydroxyl (-OH) originated from the glucose structure unit of cellulose easily converts to H2O. Meanwhile, CO and CO2 are generated from the glycoside bond (C-O-C) through a dehydroxylation reaction, and hemicellulose's xylose structure unit undergoes a degradation reaction, resulting in small molecule releases. The hydroxyl and methoxy groups on the phenylpropyl structural unit of lignin are easily separated from H2O and CH4. Overall, the organic matter from dried balloonflower pyrolyzes and evaporates completely after calcination treatment.
To exclude the interference of the difference of composition and carbonization degree, and to investigate the effect of morphology (hole size) on the microwave absorption performance of PCF, we prepared the PCF sample captured from horizontal and vertical cutting directions (as indicated in Fig. 2g & 2h), respectively. Figure 2a-f exhibit the SEM images of the fracture surface of various PCF samples collected from the horizontal direction. It is found that the PCF sample's cross-section is honeycomb-shaped, divided into dense prisms with diverse sizes and irregular shapes along the axial direction, which is different from DB. It can be confirmed that the pore size can be adjusted with increasing calcination temperatures, that is, the higher the temperature, the larger the pore size, which is consistent with the variation tendency of perimeter and density of PCF samples. Figure 2i-n shows the vertical section of the PCF samples, which also presents an irregular prismatic structure. Furthermore, the hole size of each sample is slightly larger than that of the cross-sectional sample. Generally, the porous material can be regarded as a complex composed of solid material and air interspersed with it. Therefore, the pore structure has an important influence on the absorption properties of porous absorbing material. Different sections of the PCF sample may achieve different electromagnetic wave attenuation capacities. Figure S2a displays the typical SEM image of DB captured from the horizontal section. It is found that the fracture surface is smooth with randomly distributed texture patterns appearing in the field of vision, which is composed of numerous primary cellulose. Figure S2b exhibits the SEM image of DB captured from the vertical section. Compared with Fig. S2a, the fracture surface of DB is rough with a more robust and bulky venation microstructure. It is inferred that this phenomenon may be related to the distribution direction of cellulose in the interior. Figure 2o-q reveal the element mapping collected from a typical PCF sample, and C, N, and O atoms can be detected obviously. The abundant C element suggests that the central part of the PCF is carbon, and the N and O elements uniformly distribute in carbon structure to form heterogeneous doping atoms. The doping atoms not only improve the conductivity and conduction loss, but also generate additional dipole polarization loss acting as polarization centers and exerting a positive effect on the dissipation of electromagnetic waves. To intuitively analyze the hole size variation, the statistical results of hole size distribution captured from the horizontal section and vertical sections were exhibited in Fig. S3a-f. Obviously, the hole size in a vertical section is more significant than that of the horizontal section obtained at the same temperature, suggesting that the microstructure of PCF sample can be successfully adjusted by changing the calcination temperature and cutting direction.
FT-IR spectra were employed to investigate the surface functional group information of various PCF samples, and the corresponding characterization results are shown in Fig. 3a. With the increase of calcination temperature, the FT-IR spectra of PCF samples gradually exhibit a flat variation tendency, suggesting that some functional groups of the derived carbon were degraded dramatically, and the absorbance intensity decreased significantly. Figure 3b-d displays the partially enlarged drawing captured from the corresponding region of Fig. 3a, and several character absorption peaks can be recognized. In Fig. 3b, the absorption peak located around 580 cm− 1 can be attributed to the stretching vibration of the C-H bond. It can be found that the relative intensity of the C-H bond does not vary with the calcination temperature of PCF samples. In Fig. 3c, the characteristic peak located at 1045 cm− 1, 1570 cm− 1, and 1630 cm− 1 can be assigned to the stretching vibration of C-O bond in phenolic, alcohols, and ethers components, C = C bond or stretching vibration of the benzene ring skeleton, stretching vibrations of C-N, respectively. Notably, the integral area of the C-O absorption peak gradually decreases, meanwhile, its location shows a slight blue shift, indicating that the changes of microstructure depended on the calcination temperature. The relative intensity of the C-N characteristic peak also gradually decreases, which disappears in PCF-700 ~ PCF-900 samples, suggesting that some nitrogenous-containing functional groups cannot exist stably at high temperatures. Meanwhile, the relative intensity of the C = C characteristic peak increase with the increase of calcination temperature. In Fig. 3d, the characteristic peaks appearing at 2850 cm− 1 and 2920 cm− 1 can be attributed to the stretching vibration of -CH3 and -CH2 [30] from alkanes. Although their relative intensity decreases gradually, the corresponding integral area's ratio remains constant.
Figure 3e presents the Raman spectra of the PCF samples, in which two prominent peaks marked as D band and G band appear around 1300 cm− 1 and 1580 cm− 1, respectively, corresponding to the characteristic peaks of carbon material. It is well known that the D peak represents the vibration of sp3 atoms in disordered graphite [31], and the G peak stands for the in-plane vibration of sp2 atoms in the two-dimensional hexagonal lattice [32].
The ID/IG value (the relative intensity ratio between the D band and the G band) is usually employed to evaluate the graphitization degree or disorder of carbon materials [33]. With the enhancement of calcination temperature, the ID/IG value of the PCF samples shows an increasing trend from 0.72 for PCF-400 to 0.96 for PCF-900. It should be noted that a higher calcination temperature is favorable to enhancing the graphitization degree of carbon-based material. In other words, the PCF-900 sample has a higher graphitization degree than the PCF-700 sample. Therefore, excluding the influence of graphitization degree, the higher ID/IG value of PCF-900 can be attributed to its more disordered lattice structure, which is consistent with the reduced trend of C-N and C-O bonds. These lattice defects can be served as polarization centers and generate abundant dipole polarization to enhance the absorption capacity of incident electromagnetic waves. Figure 3f shows the full-scan XPS spectra of obtained various PCF samples, and three characteristic peaks of C1s, O1s, and N1s can be detected at 285eV, 400eV, and 532eV, respectively. The content variation of each element in PCF samples was exhibited in Fig. 3g.
The main body of the skeleton is composed of carbon, which increases gradually with the increase of carbonization temperature. At the same time, the content of O and N elements decreases, suggesting that the chemical composition of the PCF sample obtained can be perfectly adjusted by changing the pyrolysis temperature. Figure 3h-j shows the high-resolution N1s, C1s, and O 1s spectra of various PCF samples[34]. As exhibited in Fig. 3h, three peaks at 395.2eV, 397.6eV, and 403eV can be stripped from N1s spectra, representing pyridine-N, pyrrolic-N, and graphite-N, respectively [35]. With the increase of the carbonization temperature, both the relative intensity of the pyridine-N peak and the integrated area ratio of pyridinic-N to graphitic N gradually decreases, which is consistent with the change of C-N, C-O, and C = C bonds. The high-resolution C1s spectra of PCF samples obtained at different temperatures were shown in Fig. 3i, which can be divided into the following two peaks, C-C/C = C at 284.8 eV and C-N/C-O at 286.5 eV [36]. The integrated area ratio of the former to the latter gradually increases, indicating the enhanced order of graphite lattice. In addition, the peaks located at 527.8 eV and 530.5 eV (Fig. 3j) represent defective oxygen vacancy and surface-adsorbed oxygen, respectively. With the increase of carbonization temperature, the percentage of oxygen vacancy gradually increases.
The relative complex permittivity (εr = ε′-jε") and the relative complex permeability (µr = µ′-jµ") are the two key parameters that determine the absorption performances of microwaves [37]. The real part of the complex permittivity and complex permeability represents the ability of the absorbing material to store electromagnetic energy. In contrast, the imaginary part represents the ability of the absorbing material to lose electromagnetic energy. Since PCF samples are non-magnetic, their wave absorption properties are mainly determined by the permittivity, that is, the real and imaginary parts of permeability are 1 and 0, respectively. Figure 4a & b show the real part and imaginary part curves of the permittivity taken from each sample in the horizontal cutting directions, respectively. It can be seen from the plots that the values of both the real part and imaginary parts of the samples increase significantly with the increase of the calcination temperature, suggesting that the PCF samples obtained at high temperature possess the strongest energy storage capacity for electric filed. In the frequency range of 2 ~ 18 GHz, the real part values of the permeability for PCF-400H, PCF-500H, PCF-600H, and PCF-700H samples remain largely unchanged. The subtle difference of ε' values among them is closely related to their similar microstructure and surface-adsorbed functional groups. In Fig. 4b, the ε'' values of PCF-400H, PCF-500H, PCF-600H, and PCF-700H samples basically remained around 0 without fluctuation, which is an order of magnitude lower than corresponding ε' values, indicating that these samples can be regarded as wave-transparent materials with poor attenuation capacity for electromagnetic waves [38]. The real and imaginary parts of the PCF-800H sample show small fluctuations in the range of 12.1GHz to 18GHz, and 9.6GHz to 189.6GHz, respectively, which can be assigned to the resonance peak related to the better carbon network structure and good electrical conductivity. For the PCF-900H sample, the real part values gradually decrease from 13 at 2 GHz to 6.6 at 18 GHz, and the imaginary part values of 4.3 at 4 GHz increase gradually to 4.8 at 18 GHz, and both of them display multiple fluctuations at higher frequencies.
Figure 4c & d show the real and imaginary curves of the permeability for the vertical samples, respectively. Similar to the curves for each horizontal cutting sample, the values of the real and imaginary parts for each vertical cutting PCF sample also increase significantly with increasing reaction temperature. In the frequency range from 2 to 18 GHz, the real part values of the PCF-400V, PCF-500V, PCF-600V, and PCF-700V samples remain constant around 4.5, and their imaginary part values locate at 1.2 respectively. The real part values of the PCF-800V sample gradually decrease from 9.7 at 2GHz to 5.2 at 18GHz. The imaginary part value is 2.8 near 2GHz and gradually increases to 4.2 at 18GHz. For the PCF-900V sample, the real part values gradually decrease from 16.7 at 2GHz to 6.6 at 18GHz, meanwhile, its imaginary part value also decreases from 2.9 at 2GHz to 4 at 18GHz. Noteworthily, the imaginary part values of the PCF-900V are more significant than that of the PCF-900H, indicating its more robust attenuation capability for the electromagnetic wave. Furthermore, both the PCF-900H and PCF-900V display the synchronous change in S and C bands and achieve maximum value in Ku band and X band respectively, which is attributed to the dielectric polarization relaxation behavior, and suggesting their good dissipation capacity for electrical field energy. Figure 4e and Fig. 4f exhibit the dielectric loss tangent (tanδ) curves of PCF-900H and PCF-900V, in which the samples achieve higher tanδ values than other samples (especially in Ku band), which is consistent with the variation of ε'' curves, revealing their good dielectric loss.
The reflection loss (RL) is usually used to evaluate the microwave absorption properties of the absorbing material. According to transmission line theory [39], the RL curve can be described as
$$\text{R}\text{L}\left(\text{d}\text{B}\right)=20\text{l}\text{o}\text{g}\left|\frac{{\text{Z}}_{\text{i}\text{n}}-{\text{Z}}_{0}}{{\text{Z}}_{\text{i}\text{n}}+{\text{Z}}_{0}}\right|$$
\({\text{Z}}_{\text{i}\text{n}}={\text{Z}}_{0}\sqrt{\frac{{{\mu }}_{\text{r}}}{{{\epsilon }}_{\text{r}}}}\text{tan}\text{ℎ}\left(\text{j}\frac{2{\pi }\text{f}\text{d}}{\text{c}}\sqrt{{{\mu }}_{\text{r}}{{\epsilon }}_{\text{r}}}\right)\)
in above equations, Z0 is the air impedance, Zin stands for the input impedance, f represents the frequency, d is the sample thickness, c is the speed of light, εr and µr are the relative complex permittivity and magnetic permeability, respectively. In addition, effective absorption bandwidth (EAB) is generally employed to measure the frequency coverage range of the absorber for incident electromagnetic wave. Figure S4 (a-f) and Fig. S5 (a-f) show the RLmin plots versus thickness and frequency of the horizontal cutting PCF samples prepared at different calcination temperatures, respectively. The RLmin values of the PCF samples increase gradually with the increasing of the calciantion temperature. However, the RLmin values of PCF-400H, PCF-500H and PCF-600H samples are less than − 10 dB, which does not meet the requirements of EAB. It is almost certain that their poor microwave absorption performances is aroused by their weak imaginary part values of permittivity and poor dielectric loss capacity. For the PCF-700H, the EAB value is also small, and the optimal RLmin of -15.74 dB can be achieved at 5.5 mm matching thickness. When the calcination temperature reaches to 800℃, the PCF-800H displays the minimal RL value of -12.18 dB at 2.2 mm thickness, meanwhile, its EAB of 2.4 GHz is also measured at 2.36 mm thickness. Figure S4f, Fig. S5f and Fig. 5b exhibit the RLmin values of PCF-900H. As seen from these figures, the location appearing RLmin values gradually shift to low frequencies with the increase of the thickness, the RLmin of -24.65 dB is obtained when the matching thickness is 1.57 mm, while the widest EAB reaches to 6.24 GHz (11.76 GHz ~ 18 GHz) at 1.86 mm, covering the whole Ku band. Figure S5 and Fig. 5a show the RL relationship of various PCF samples versus frequency and matching thickness captured from the vertical cutting direction respectively. It is obvious that the RLmin values of the vertical cutting PCF samples gradually increase, when the calcination temperature is higher than 600°C, the EAB values (<-10 dB) can be recognized. As shown in Fig. S4f, Fig. S5f and Fig. 5c, the PCF-900V exhibits outstanding electromagnetic wave absorption performances with the the minimum RL value of -46.95 dB at a thin thickness of 1.46 mm, meanwhile, and its optimal EAB value of 5.52GHz is achieved at 1.72mm thickness, indicating its good microwave absorption performance. The measurement results of different PCF samples were summarized, as exhibited in Fig. 5d. It can be clearly found that the RLmin and EAB values PCF-800 and PCF-900 samples is larger than that of PCF-400 ~ PCF-700, suggesting that higher calcination temperature is beneficial to obtain better absorption performances, which may be related to the structure integrity of carbon skeleton and conductivity of the as-synthesized product. Generally, the RLmin values of vertical cutting PCF samples with large hole size is better than that of horizontal cutting PCF samples, suggesting that the larger the hole size, the greater the reflection loss capacity, which is consistent with the results of Fig. 4b & c.
In order to evaluate the microwave absorption performance of PCF-900H and PCF-900V more intuitively, the two samples were compared systematically from the perspective of reflection loss, effective absorption bandwidth, matching thickness and location frequency, and the corresponding radar graph was shown in Fig. 5e. Obviously, the area enclosed by the irregular closed loop corresponding to PCF-900V sample is obviously larger than PCF-900H, suggesting that the comprehensive absorption properties of the former is better than later. Figure 5f and Tab. S1 exhibit the comparison of electromagnetic wave absorption performance of the PCF-900V with other materials, the result shows that the PCF-900V has a wider EAB with excellent RLmin value, which is in line with the characteristics of "thin, wide, light and strong" absorbing materials.
According to Debye theory [40], ε' and ε'' can be expressed as
in which,\({{\epsilon }}_{\text{s}}\), \({{\epsilon }}_{{\infty }}\), \(f\), \({{\epsilon }}_{0 }\)and \({\tau }\) stand for static dielectric constant, optical dielectric constant, frequency, vacuum dielectric constant and polarization relaxation time, respectively. The dielectric relaxation can be evaluated by the following Cole-Cole equation:
$${(\epsilon ’-\frac{{\epsilon }_{s}+{\epsilon }_{\infty }}{2})}^{2}+{\left(\epsilon ‘’\right)}^{2}=({\frac{{\epsilon }_{s}-{\epsilon }_{\infty }}{2})}^{2}$$
A polarization relaxation process, can be reflected by a semicircle in Cole-Cole curve [41]. It is well known that the adhered adsorption functional groups on the porous carbon framework and the abundant O and N doping atoms on the carbon framework act as polarization centers, generating intense polarization relaxation and dielectric loss. As shown in Fig. S6 and Fig. 6a & b, the semicircles can be marked out, and the PCF-900H and PCF-900V samples achieve more semicircles quantity than other samples, which mainly occurs in X band and Ku band, suggesting their more polarization relaxation processes in these frequency range, which plays a significant role in enhancing the attenuation capacity of absorbing material [42, 43]. Moreover, the PCF-900H has the same number of semicircles as PCF-900V, and their relaxation phenomena are mainly concentrated in the Ku band. Attenuation coefficient and impedance matching and are two crucial indexes to investigate the microwave absorption performance [44]. Figure 6c & d shows the attenuation constant of the PCF samples from horizontal and vertical cutting directions, respectively. Obviously, the attenuation constant values display the similar variation tendency with the curve of complex permittivity, indicating that the dielectric loss occupies a dominate position in determining the electromagnetic waves absorption properties. Compared with other samples, PCF-900H and PCF-900V achieve approximate maximum values in Ku band and strong microwave attenuation capacity in this frequency range. More importantly, it also implies that the different microwave absorption performances of horizontal and vertical cutting samples are almost negligibly affected by the attenuation coefficient. The impedance matching represents the capacity of the incident electromagnetic wave introduced into the absorber [45], which can be calculated by the Zin value. Ideal impedance matching condition can be obtained while the value of |Zin-1| infinitely approaches 0 [46], indicating that all of the incident electromagnetic wave enters the sample ultimately. Figure 6e & f and Fig. S7 show the impedance matching plots of various PCF samples captured from horizontal cutting and vertical cutting, respectively. It can be seen that the impedance matching values of the PCF-900V and PCF-900V are closer to 1 than other samples in the frequency range of 2 ~ 18 GHz at the preferred thickness of themselves, which is more favorable to the absorption of electromagnetic waves. It should be noted that the plots of impedance matching versus thickness versus frequency for the above two samples are consistent with the plots of reflection loss variation (Fig. 5a & b), implying that the reflection loss performances of PCF-900H and PCF-900V are highly influenced by impedance matching.
On the basis of the above measured results, the attenuation process and working mechanism of the PCF sample on the incident electromagnetic waves are demonstrated, the corresponding schematic diagram was shown in Fig. 7. Firstly, as a typical derived carbon material, the PCF sample maintains the good electrical conductivity common to other carbon family members, which can produce some conductive loss under the action of the applied electromagnetic field. Secondly, the peculiar honeycomb structure of the PCF contains a large number of three-dimensional cavities, which arouses multiple reflection and scattering effects on the incident electromagnetic wave, and improves the impedance matching at the same time. Thirdly, the rich nitrogen, oxygen and other exotic atoms from the DB raw material are inherited by the PCF sample, who act as polarization centers together with a large number of dangling bonds or functional groups adsorbed on the surface, which produce strong dipole polarization loss to the incident electromagnetic waves and further weaken the energy of electromagnetic waves.