Architecting functionalized carbon microtube/carrollite nanocomposite demonstrating significant microwave characteristics

Biomass-derived materials have recently received considerable attention as lightweight, low-cost, and green microwave absorbers. On the other hand, sulfide nanostructures due to their narrow band gaps have demonstrated significant microwave characteristics. In this research, carbon microtubes were fabricated using a biowaste and then functionalized by a novel complementary solvothermal and sonochemistry method. The functionalized carbon microtubes (FCMT) were ornamented by CuCo2S4 nanoparticles as a novel spinel sulfide microwave absorber. The prepared structures illustrated narrow energy band gap and deposition of the sulfide structures augmented the polarizability, desirable for dielectric loss and microwave attenuation. Eventually, the architected structures were blended by polyacrylonitrile (PAN) to estimate their microwave absorbing and antibacterial characteristics. The antibacterial properties against Gram-negative Escherichia coli (E. coli) and Gram-positive Staphylococcus aureus (S. aureus) were scrupulously assessed. Noteworthy, the maximum reflection loss (RL) of the CuCo2S4/PAN with a thickness of 1.75 mm was 61.88 dB at 11.60 GHz, while the architected FCMT/PAN composite gained a broadband efficient bandwidth as wide as 7.91 GHz (RL > 10 dB) and 3.25 GHz (RL > 20 dB) with a thickness of 2.00 mm. More significantly, FCMT/CuCo2S4/PAN demonstrated an efficient bandwidth of 2.04 GHz (RL > 20 dB) with only 1.75 mm in thickness. Interestingly, FCMT/CuCo2S4/PAN and CuCo2S4/PAN composites demonstrated an electromagnetic interference shielding efficiency of more than 90 and 97% at the entire x and ku-band frequencies, respectively.


Materials and methods
Materials. Cobalt (II) nitrate hexahydrate, copper (II) acetate monohydrate, N, N-dimethylformamide (DMF), ethanol, and nitric acid (65%) were purchased from Merck. Moreover, sodium sulfide hydrate (60.0-62.0%) was obtained from Samchun Chemicals while PAN was supplied from Sigma-Aldrich. Mueller-Hinton agar was purchased from the IBRESCO meanwhile E. coli ATCC 25922 and S. aureus ATCC 25923 obtained from Darvash Co. were employed to investigate the antibacterial characteristics.

Experimental steps. Preparation of FCMTs by biomass.
CMTs were fabricated by pyrolyzing the harvest of Populus euphratica at 500 °C in an N 2 environment for 3 h. The prepared CMTs were functionalized by a novel modified complementary method. Initially, 0.13 g of CMTs was suspended in 15 ml nitric acid using simultaneously an overhead stirrer and ultrasonic bath for 2 h. Subsequently, the oxygen-containing functional groups fully anchored onto CMTs by a solvothermal process for 2 h at 120 °C. The obtained FCMTs were rinsed by deionized water to natural pH and then were dried at 60 °C. The etching treatments performed by sonochemistry elevate defects at grain boundaries while the functionalizing process was fully done through the solvothermal route. The established defects and functional groups generate the diverse polarization relaxation times, ascending the relaxation loss. More significantly, inserting the oxygen-containing functional groups transfer the sp 2 hybridization of conjugated CMTs to sp 3 hybridization, enhancing polarizability.
Architecting FCMT/CuCo 2 S 4 nanocomposite. Firstly, the copper and cobalt salts in stoichiometric amounts were dissolved in a mixture of deionized water/ethanol = 50% (v/v) and then FCMTs by 10 Wt. % were dispersed in the solution by an ultrasonic bath and overhead stirrer, simultaneously. Afterward, the sodium sulfide in a molar ratio of S 2− /Cu 2+ = 6 was separately dissolved in the solvent and added to the aforementioned solution, following that the suspension was treated for 1 h. Next, it was transferred into a p-polyphenylene lined stainless steel autoclave and annealed for 8 h at 200 °C. The architected nanocomposite was rinsed several times and      Optical characteristics. Figure 5 exhibits the light absorptions (λ = 200-800 nm) and energy band gaps of FCMT, CuCo 2 S 4 , and FCMT/CuCo 2 S 4 structures. The more polarizability is in the clear trade-off with the narrower energy band gap. Particularly, the augmenting polarizability promotes the dielectric loss in microwave absorbers. The energy band gap was defined as the distance between the valence and conduction band. As indicated, the anchoring spinel nanoparticles onto FCMTs led to the red shift of absorption edge. The following equations were used to reveal the energy band gaps: (αhν) 2 = hν − Eg, α = -1/t lnT, and T = 10 −A , where T, A, α, ν, t, h, and E g are obtained by the transmittance, absorbance, absorption coefficient, frequency, thickness, Planck constant, and energy gap, given by Kubelka-Munk theory 73 . It is found that the energy band gap was diminished in the nanocomposite, realized by the produced interactions at the interfaces between FCMTs and nanoparticles as well as augmented average size of the nanoparticles reducing the distance between HOMO and LUMO, desirable for Maxwell-wagner effect and microwave attenuating 50 . The achieved results introduce the prepared nanocomposite as a promising photocatalyst.   Table 1. As revealed, H c of the nanocomposite was amplified owing to the spin pinning at heterogeneous interfaces. More interestingly, intermediate structures produced by the oxygencontaining functional groups, anchored onto FCMTs, enhance the size of nanoparticles in the nanocomposite, influencing the magnetic features, defined by Snoek's law [74][75][76] . The observed ferromagnetic property of FCMTs is ascribed to their unique morphology as well as the produced crystal defects and distortions along the complementary oxidative treatments, hence, the presented factors induce delocalize electronic structures and develop localize dipole moments [77][78][79][80] . Natural resonance equation states that the isotropic magnetic exchange interactions and magnetization are the vital keys tuning the microwave absorbing bandwidth [81][82][83][84] .
Antibacterial properties. The agar diffusion method was applied to investigate the antibacterial characteristics of samples. Figure 7 and Table 2  The quarter wavelength mechanism denotes that there is a clear trade-off between the matching frequency and thickness, tuned by the relative complex permeability and permittivity 95,96 . Accordingly, CuCo 2 S 4 /PAN and FCMT/CuCo 2 S 4 /PAN composites demonstrated the thinner matching thicknesses, compared to FCMT/PAN composite. Figure 9 depicts the matching  www.nature.com/scientificreports/ thickness versus maximum RL and efficient bandwidth (RL > 10 dB) of the absorbers, as well as Fig. 10 represents a comparative diagram related to the maximum RL and efficient bandwidth (RL > 10 dB) of the reported results and this research [51][52][53][54][55][56][97][98][99][100][101] . Frequency dependence of complex permittivity and permeability of the samples has been depicted in Fig. 11. The real part of permittivity is originated from dipole and interfacial polarizations 53,54,102 . It can be seen that FCMTs, ornamented by CuCo 2 S 4 nanoparticles, indicated the augmented relaxation loss mechanism due to the intrinsic characteristics of FCMTs, nanoparticles, and PAN, as well as, the emerged exclusive interactions at heterogeneous interfaces. Conductive loss is the key factor, boosting the imaginary part of permittivity 53,103 . As indicated, anchoring the nanoparticles onto FCMTs amplifies the conductive loss mechanism, compared to the FCMT/PAN composite. The observed notches at permeability curves are generated from the natural and exchange resonances 82,103 . It is found that CuCo 2 S 4 nanoparticles and FCMT/CuCo 2 S 4 nanocomposite showed the considerable imaginary part of permeability, derived from the intrinsic features of nanoparticles. These phenomena are realized by the produced crystal defects, distortions, and dislocations, as well as the induced magnetic dipole moments, established by the unique interactions at grain boundaries 104,105 . Eddy current loss plays a vital role in microwave absorption. The more constant eddy current curve imply to the more eddy current loss mechanism 27 . Evidently, the mechanism of eddy current loss commands in the absorbing media of FCMT/ CuCo 2 S 4 /PAN and FCMT/PAN composites over 14.50 GHz ( Figure S2). Figure 12 exposes Cole-Cole plot, impedance matching (Z), and attenuation constant (α) of the samples. Cole-Cole plot is produced by drawing ε′ versus ε″. Each emerged semicircle denotes one relaxation loss procedure, deduced by Debye relaxation theory 106 . As given by the plot, the semicircles exhibited that the relaxation mechanism in the composites are ordered as FCMT/PAN < CuCo 2 S 4 /PAN < FCMT/CuCo 2 S 4 /PAN. Noteworthy, PAN as a novel absorbing matrix develops the dielectric characteristics of samples due to its functional groups. Z mechanism (Z = 1) is in compromise with the propagation of incident waves in the absorbing matrix [107][108][109] . The achieved results attest that this mechanism is the crucial factor leading to the outstanding microwave attenuation of FCMT/PAN composite. Accordingly, incident waves more influence into the absorbing medium, then absorbing mechanisms such as multiple reflections and scattering as well as the quarter wavelength and canceled waves can be elevated. α and dissipation factor (tan δ- Figure S2) estimate the ability of an absorber for energy conversion 51,100,110 . The achieved results manifest that the more α and |tan δ| are realized by the more imaginary part of permeability and permittivity. Schematic illustration of the microwave absorbing mechanisms has been shown in Fig. 13.  www.nature.com/scientificreports/   www.nature.com/scientificreports/ The electromagnetic interference SEs of the samples with 2.00 mm in thickness were assessed. As known, SE T is the sum of SEs attributed to the reflectance (SE R ) and absorbance (SE A ) 111 . Figures 14 and S3 exhibit the electromagnetic interference SEs of the samples. Interestingly, FCMT/CuCo 2 S 4 /PAN and CuCo 2 S 4 /PAN nanocomposites demonstrated more than 90 and 97% SE T at entire x and ku-band frequencies. It should be noted that the eye-catching SE T of the samples are derived from the absorbance, generated from the dominant microwave absorbing mechanisms existing in their absorbing medium. The achieved results testified that FCMTs as a novel carbon-based material, derived from biomass material, demonstrated outstanding microwave characteristics meanwhile anchoring the nanoparticles onto their structure promoted microwave absorbing features. More significantly, the tailored composites based on PAN, as a practical absorbing matrix, demonstrated the salient microwave absorbing properties as well as considerable SE T . The obtained results clarified that the remarkable microwave features of the samples are essentially generated from the dipole, interfacial, and defect polarization, conductive loss, natural and exchange resonance, eddy current loss, multiple reflections and scattering, impedance matching, as well as quarter wavelength mechanism. Applied equations to investigate microwave absorbing and shielding properties of the samples were arranged in Supplementary materials. www.nature.com/scientificreports/

Conclusion
In this research, a novel morphology of conjugated carbonaceous structures was fabricated using a novel precursor. The prepared CMTs were functionalized based on the solvothermal and sonochemical routes. Moreover, CuCo 2 S 4 nanoparticles were tailored using a solvothermal method and anchored onto FCMTs by an innovative process, as novel microwave absorbing and antibacterial material. All of the analyses revealed that all of the samples were fabricated in good order. Noticeably, PAN was applied as a novel absorbing medium to evaluate  www.nature.com/scientificreports/ the microwave absorbing properties of samples, demonstrating the outstanding microwave features. More significantly, microwave absorbing features and electromagnetic interference SEs of the architected samples were scrupulously dissected, illustrating that the relaxation and conductive loss, natural and exchange resonance, as well as quarter wavelength and eddy current loss are the pioneer mechanisms paving the way for the obtained salient microwave characteristics. www.nature.com/scientificreports/