Tuned MWCNT/CuO/Fe3O4/Polyaniline nanocomposites with exceptional microwave attenuation and a broad frequency band

In this study, novel quaternary MWCNT/CuO/Fe3O4/PANI nanocomposites were synthesized with three different weight ratios of CuO/Fe3O4/PANI to MWCNT (1:3), (1:4), and (1:5), where all of its components were synthesized separately and then combined in specific weight ratios. CuO/Fe3O4/PANI transmission electron microscopy (TEM) images revealed that most nanoparticles were in a CuO/Fe3O4 hybrid form, with a narrow size distribution uniformly dispersed in a polymer background. The TEM and scanning electron microscopy (SEM) images of the MWCNT/CuO/Fe3O4/PANI nanocomposite revealed that the MWCNT was uniformly coated with CuO/Fe3O4/PANI. All three nanocomposites samples demonstrated superior microwave attenuation performance in terms of reflection loss and absorption bandwidth. The minimum reflection losses for MWCNT/CuO/Fe3O4/PANI nanocomposites (1:3), (1:4), and (1:5) were 45.7, 58.7, and 85.4, 87.4 dB, respectively. The absorption bandwidths (RL ≤ −10 dB) of MWCNT/CuO/Fe3O4/PANI nanocomposites (1:3), (1:4), and (1:5) were 6, 7.6, and 6 GHz, respectively.

results in improved impedance matching and inhibits the aggregation, corrosion, and magnetic phase transformation of nanoparticles by producing heat during the microwave absorption process.
The most significant polymer in the category of conductive polymers is polyaniline. It has high electrical conductivity with high polarizability (due to the presence of strong chemical bonds or localized charges), polarization relaxation, and conductive loss and is used to synthesize highly effective microwave absorber composites. Polymers can also cause a better dispersion of NPs and enhance the interfacial area and multiple interfacial reflections between individual nanoparticles. Polyaniline can uniformly conjugate with CNT's surface and other substrates by representing surface tension and reactive chemical sites to bind with functional groups of CNTs, which results in a further increment in the interfacial area 14,15 . Polyaniline can also significantly optimize the impedance matching of EW. Thus, polyaniline can enhance electromagnetic wave absorption via various mechanisms [16][17][18][19][20][21][22] .
The present study synthesized a novel quaternary MWCNT/CuO/Fe 3 O 4 /PANI nanocomposite using a stepby-step approach, ensuring that the CuO/Fe 3 O 4 hybrid nanoparticles were prepared using the optimal protocol to achieve the desired magnetic properties and size distribution. Then, aniline was polymerized on the surface of CuO/Fe 3  Electromagnetic parameters of nanocomposites were calculated in the range of 8-18 GHz through a vector network analyzer. The reflection losses versus frequency curves were plotted based on electromagnetic parameters for all samples with different thicknesses. The results showed that all samples had a high reflection loss ranging from -50 to -87 dB and a broad absorption band exceeding 6 GHz.
In addition to the usual mechanisms of microwave absorption in magnetic and dielectric materials, the unique design of the MWCNT/CuO/Fe 3 O 4 /PANI composite with multiple interfaces and weight ratio optimization of each composite component results in a significant improvement in the composite's microwave absorption property. MWCNT/CuO/Fe 3 O 4 /PANI composites have different dielectric permittivity and magnetic permeability interfaces (conductor/semiconductor or conductor/insulator or semiconductor/insulator and non-magnetic/ magnetic). The accumulation of localized surface charge at the boundary of two materials with different dielectric constants leads to attenuation of the electric component of the incident wave. Moreover, the skin depth decreases due to creating current density at a non-magnetic/magnetic interface, and the incident wave's magnetic field deteriorates 23 .

Experimental
This study prepared a novel MWCNT/CuO/Fe 3 O 4 /PANI quaternary composite using a controllable method involving a three-step process. The composite's constituents were prepared under controlled conditions and then combined to form the final nanocomposite at an optimal weight ratio. First, a hybrid CuO/Fe 3 O 4 nanoparticle was synthesized using the optimal protocol to achieve the best superparamagnetic property. Afterward, nanoparticles were used as an initiator to polymerize aniline and form a CuO/Fe 3 O 4 /PANI nanocomposite. Finally, the MWCNT was used as a substrate to carry and stabilize the CuO/Fe 3 O 4 /PANI nanocomposite.

Preparation of CuO/Fe 3 O 4 hybrid nanoparticles.
To obtain a homogeneous solution, 1.51 g copper nitrate and 2.31 g iron oxide were mixed with 40 mL of deionized water and stirred. Then, 0.1 mol NaOH was added dropwise until the pH of the solution reached 13. Finally, the product was collected with a magnet, washed, and dried at 40 °C. were dissolved in 20 mL aqueous HCl solution. Then, the mixture was exposed to ultrasonic waves for 30 min to disperse the NPs (separate nanoparticles in the clusters). The polymerization reaction must be performed at a temperature less than 4 °C. Afterward, 0.8 g of ammonium persulfate (oxidizing agent) was dissolved in 30 mL of deionized water and added dropwise to the mixture. Adding the first oxidizing droplets leads to the appearance of a green-blue color, indicating the beginning of polymerization. When the polymerization was complete, the products were collected by a magnet, and the solution was washed to remove the residual aniline and reach neutral pH 16 ). Subsequently, two suspensions were added together and sonicated for 30 min. The sediment was then separated from the liquid by centrifugation. Finally, the black precipitate was dried in an oven at 50° C. Similar nanocomposites were prepared using weight ratios of MWCNT to CuO/Fe 3 O 4 /PANI of 1:4 and 1:3 16 . The successful synthesis of MWCNT/CuO/Fe 3 O 4 /PANI nanocomposite was verified by determining its chemical bonding and crystalline planes using FT-IR (Nicolet IS10) and X-ray diffraction (XRD) (PANalytical), respectively. The magnetic behavior of products was investigated at room temperature in the presence of a magnetic field using a vibrating sample magnetometer (VSM JDM-13). The TEM technique was used to study the morphology of CuO/Fe 3 O 4 /PANI and MWCNT/CuO/Fe 3 O 4 /PANI nanocomposites using MIRA3 TESCAN and Tecnai. The composite's electromagnetic properties and microwave absorption capability were determined using the transmission/reflection waveguide method on an Agilent vector network analyzer (VNA) E8364B over 8.2-18 GHz. The absorption band at 1562 cm −1 also corresponds to the tensile vibration of C = N corresponding to the quinoid ring in the polyaniline structure. The characteristic peak of carbon nanotubes at 1638 cm −1 is related to the symmetrical expansion vibration of the C = C bonds. The absorption band at 1479 cm −1 is attributed to the C = C functional groups in the benzenoid rings in the polyaniline structure 24 . The absorption band at 1296 cm −1 demonstrates the tensile vibration of the C-H bonds, while the absorption band at 1131 cm -1 exhibits the bending vibration of the C-H bonds, which is a characteristic of polyaniline bonds 25 (Fig. 4b,c) demonstrate that the entire surface of the nanotubes was coated with a uniform thickness of about 40 nm of CuO/ Fe 3 O 4 /PANI nanocomposite, indicating that the nanotubes were buried beneath the polymeric shell. Moreover, the polymer increases the specific surface area of the nanotubes through proper adhesion and reduces surface tension at the interface, in addition to chemical bonding to the functional groups of the CNT. These features contribute to the uniform adhesion of CuO/Fe 3 O 4 /PANI to MWCNT. No apparent aggregation of nanoparticles was observed in the TEM images. Interconnected polymer chains increase the solubility of nanotubes in the reaction medium and prevent them from merging and clustering without using the surfactant. As a result, coating polymer on the surface of CuO/Fe 3 O 4 NPs causes uniform deposition of CuO/Fe 3 O 4 NPs on the outer surface of CNTs, as shown in the TEM images (Fig. 4b,c).

Electromagnetic parameters. The electromagnetic (EM) parameters of MWCNT/CuO/Fe 3 O 4 /PANI
nanocomposites were calculated to determine their absorption capacity. To this end, each nanocomposite was homogeneously dispersed in paraffin with a 25 wt% filler content. Complex permittivity (ε) and complex permeability (µ) are electromagnetic parameters that are intrinsic to each absorbent material and determine a matter's ability to absorb waves were achieved by a network vector analyzer (Anritsu.37269D) in the 8.2-18 GHz frequency range, presented in Fig. 6.
The real part of permittivity and permeability εʹ and µʹ, which show the degree of the polarizability of absorbent material, is a measure of the ability to store electrical and magnetic energy. εʺ and µ″, also known as dielectric and magnetic loss, refer to the attenuation of the incident wave's electric and magnetic energy within   www.nature.com/scientificreports/ the material, respectively 30,31 . εʺ and μ″, the imaginary parts, are related to polarization relaxation and spin rotation relaxation (aligning dielectric and magnetic dipole with alternative electric and magnetic fields of incident waves), respectively. εʺ originated from different polarization such as the electron, nuclei, dipoles, and interfacial polarization relaxation (space accumulated charge polarization). Here, space-charge polarization is the primary factor in the dielectric loss. This is due to additional polarizations at higher frequency ranges 32 . Interfacial polarization is caused by space-charge accumulation at the interface of materials with different dielectric constants. For conductive materials, εʺ typically originates from conduction loss as well as polarization relaxation 33 . From  34,35 . Dispersion of nanoparticles within the polymer results in a significant increase in the nanoparticles' joint surface with the polymer. Additionally, the nanotube's surface is coated with a polymer layer. The nanotubes' high surface-tovolume ratio creates a large interface between the nanotubes and the polymer. It is well established that the larger the interface area, the more space charge, and polarization relaxation occurs 36 . The presence of a conductive loss in polyaniline and CNT is another phenomenon that significantly contributes to the εʺ of nanocomposites. The polarization relaxation of the space charge at the NPs/PANI interface, which occurs at high frequencies (> 14 GHz), and the conductive loss of PANI appear to be the dominant mechanisms. In addition, multiple reflections occurred between separated CuO/Fe 3 O 4 NPs, causing more incident wave dissipation 37,38 . As a result, the nanocomposite (1:5) with the highest amount of CuO/Fe 3 O 4 /PANI nanocomposite represents the highest εʺ and dielectric tangent loss (tanδ ε ) at a higher frequency range.
As illustrated in Fig. 6, the µʹ curves of all samples are similar. Except at the frequency extremes, the µʹ values of nanocomposites (1:4) and (1:5) are nearly identical, while the µʹ values of nanocomposites (1:3) are slightly lower. This observation is associated with the higher amount of Fe 3 O 4 NPs in nanocomposites (1:4) and (1:5) than (1:3). µʹ demonstrates the possibility of magnetic dipoles in this regard. Since magnetic dipoles can only occur in magnetic materials with unpaired spin, by increasing the amount of magnetite, µʹ values increased. μ″ is related to spin rotation relaxation that initially corresponds to natural resonance in the ferromagnetic domain wall resonance, occurring at lower frequencies (< 100 MHz), exchange resonance, and eddy current 39 . our nanocomposites contain CuO/Fe 3 O 4 with single domain superparamagnetic nanoparticles. Hence, exchange resonance and eddy current loss are involved in magnetic dissipation, occurring at high frequencies (> 13 GHz) 33 . Since ferrites have a relatively high electrical resistance 40 , they have a low dielectric loss and low eddy current loss. However, by incorporating CuO, a semiconductor with a narrow bandgap, to form a CuO/Fe 3 O 4 hybrid, conductivity and eddy current loss may be increased. An increment in μ″ values and magnetic tangent loss (tanδ μ ) www.nature.com/scientificreports/ of nanocomposite (1:5) over 13 GHz was expected. Generally, tanδ ε is higher than tanδ μ for all samples, indicating dielectric loss makes a larger contribution to microwave dissipation which has often been observed 41 . The relaxation (especially polarization relaxation) is an essential dielectric loss mechanism in the absorption of EM waves.
The Cole-Cole model can be used to describe the relaxation process (Fig. 7). The Debye relaxation model is used to interpret ε scattering mechanisms. In this model, the Cole-Cole curve is presented in the shape of semicircles. Each semicircle represents a separate Debye relaxation. The radius of the semicircle exhibits relaxation time. The larger the radius, the longer the rest time. Long relaxation time corresponds to relaxation with higher periodicity and lower frequency 42 . Thus, resonance occurs at lower frequencies for these relaxations; vice versa, resonance occurs at higher frequencies for the smaller semicircle radius (short relaxation time). In all three composites, one or two semicircles were observed, indicating multiple relaxation processes occur for the composites, proving the contribution of the Debye relaxation in increasing the dielectric dispersion of the composites 43,44 . The Cole-Cole curve of nanocomposite (1:3) is composed of large and small semicircles and indicates the presence of three relaxation resonances, two of which have long relaxation times and one of which has a short relaxation time. Nanocomposite (1:4) exhibits two large semicircles (with a higher radius than that of nanocomposite (1:3)) and a minute one which indicates two long relaxation times and a short one. Nanocomposite (1:5) shows two medium semicircles and two equal relaxation times. As per the statements above, polarization relaxation is a significant factor in the incident wave's attenuation. The resonance peaks in the RL diagram depend on these relaxation frequencies. Thus, in nanocomposite (1:4) with the highest relaxation frequency, the minimum reflection loss is located at the lowest frequency, whereas in nanocomposite (1:3) to (1:5) with increasing relaxation frequency, the minimum reflection loss is located at higher frequencies.
Reflection losses and absorption bandwidths. The reflection loss values for various sample thicknesses were calculated using the EM parameters: where Z in is the input impedance of the microwave absorbing material, Z 0 denotes the impedance of free space, j denotes the imaginary unit, d represents the thickness of the absorbent, f is the microwave frequency, c denotes the velocity of light, and RL min is the reflection loss.  Figure 9 shows the loss constant (α) as the main effective factor in the range of microwave absorption. The loss constant (α) is used to evaluate the ability of integral absorption loss, calculated through the following formula:  www.nature.com/scientificreports/ The higher values (α) indicate the more powerful the magnetic and dielectric loss is if the microwave can reach the inner space of the absorber. The magnetic and dielectric loss tangent are two key factors influential in the absorption efficiency of an absorber. The higher values of loss tangent indicate the high ability to convert electromagnetic waves to other forms of energy. The attenuation constant was significantly increased in the 10-18 GHz frequency range. Meanwhile, the (1:5) exhibited a larger attenuation constant, which was also well consistent with the results of the RL curves.
Several absorption mechanisms discussed in Sect. 4.1 contributed to the remarkable absorption capacity of the MWCNT/CuO/Fe 3 O 4 /PANI nanocomposites, significantly greater than comparable composites reported in Table 1.