The performance of radar absorption of Mn x Fe 3–x O 4 /rGO nanocomposites prepared from iron sand beach and coconut shell waste

: In this work, the Fe 3 O 4 nanoparticles from natural iron sand were doped with Mn and combined with reduced-graphene oxide (rGO) to obtain Mn x Fe 3–x O 4 /rGO nanocomposites with mole fraction variations of the Mn of 0.25, 0.5, and 0.75. The crystalline phase of the synthesized Mn x Fe 3–x O 4 /rGO nanocomposites formed an amorphous phase. The presence of rGO was observed through EDX results. The magnetical properties of Mn x Fe 3–x O 4 /rGO nanocomposites were shown by decreasing the Br, H c J, H max along with increasing of Mn doping. Interestingly, increasing rGO and Mn composition made the absorption bandwidth of the Mn x Fe 3–x O 4 /rGO nanocomposites wider, so that the radar absorption also increased marking by the greater reflection loss that reached −11.95 dB. The increase in the radar absorption performance of Mn x Fe 3–x O 4 /rGO nanocomposites came from the efficient complementarity between dielectric loss and magnetic loss and interfacial polarization between Fe 3 O 4 doped Mn and rGO.

properties, low density, high surface area and good electrical conductivity [40,41], so it is necessary to stabilize the density and dielectric properties of the material from RAM to obtain optimal radar wave absorption capability. Furthermore, the residual and functional group defects cause a polarizing effect that contributes to the increase in the RL value [42]. Thus, in this study, the synthesis of rGO based on coconut shell waste material as a precursor with Mn x Fe 3-x O 4 was carried out. The choice of coconut shell used as a dielectric material because it has a carbon element with a content that reaches 80 wt% [43], so it is able to absorb radar waves well [44]. Meanwhile, rGO has properties in hydroxyl and carboxyl structural defects that support the absorption of radar waves through the principle of surface polarization [36,40]. In addition, rGO has a low density and reduces the aggression of Mn x Fe 3-x O 4 , making it suitable for use as an antiradar dielectric material [45]. The method used for the synthesis of rGO is solid state reaction, because the method is simple. Then, the process of combining Mn x Fe 3-x O 4 (x = 0.25, 0.50, 0.75) with rGO in the form of Mn x Fe 3-x O 4 /rGO nano composites through the coprecipitation method. These results present a discussion of the structure, elemental content, morphology, and RAM potential of the Mn x Fe 3-x O 4 /rGO nanocomposite.

Synthesis of rGO from Coconut Shell Waste
The rGO material was synthesized using the solid-state reaction method. The process began with the cleaned coconut shell which. The coconut shells were washed with distilled water and dried in the sun for 3 d. After drying, the coconut shell was burned to form charcoal. Furthermore, the charcoal was sieved using a 200 mesh sieve to obtain charcoal powder. Then, the sample was carbonated at 400 o C for 5 h. 4 /rGO (x = 0. 25, 0.50, 0.75) The Mn x Fe 3-x O 4 /rGO nanocomposite was synthesized using the co-precipitation method with the following steps. In general, the Mn x Fe 3-x O 4 synthesis procedure has been corresponded in previous studies [21]. At first, the iron sand was washed and dried in the sun to dry. It was then separated using permanent magnets to remove impurities. Next, 20 g of iron sand was dissolved in 58 mL of HCl with a stirring speed of 720 rpm for 30 min at room temperature. The solution was filtered and separated from the residue using filter paper to obtain a solution of FeCl 2 and FeCl 3 . Simultaneously, the rGO powder was dispersed into 50 mL of distilled water and sonicated for 2 h at room temperature. Then, each mole fraction, namely: x = 0.25 was 1.36 g, x = 0.50 was 2.72 g, x = 0.75 was 4.09 g MnCl 2 •4H 2 O and rGO suspension was added to a mixture of 18 mL of FeCl 2 and FeCl 3 solution. The mixing process was carried out with a stirring speed of 720 rpm which was then titrated with 25 mL of NH 4 OH at room temperature. This synthesis process was carried out for 30 min until a black precipitate was produced. The precipitate was washed repeatedly using distilled water until it was odorless with a pH value of 7. Then, each precipitate was filtered using filter paper and dried in an oven at 100 ℃ for 1 h. After drying, 1.5 g of Mn x Fe 3-x O 4 /rGO material was compacted with manual pressure to become pellets with a diameter of 0.5 cm and thickness of 1 cm. It was sintered at 400 ℃ for 1 h. The synthesized material was characterized by instruments. The rGO samples were coded as rGO and the Mn x Fe 3-

Characterization
X-ray diffraction characterization (XRD; PANalytical, X'Pert Pro, Cu-Kα 1.5406 Å) was used to determine the phase, lattice parameters, and crystal size of the samples. Investigation of the lattice parameters, crystal size, and density of Mn x Fe 3-x O 4 /rGO was carried out based on the data XRD with refinement rietveld method using Rietica program. Next, identification of microscopic morphology and elemental content was used for Scanning Electron Microscopy (SEM, FEINSPECT-S50) characterization. Particle Size Analyzer (PSA, Mastersizer 3000) was used to identify the particle size of the samples. Then, to study the magnetic properties and absorption ability of the radar waves of the Mn x Fe 3-x O 4 /rGO nanocomposite, Vector Network Analyzer (VNA, Advantest R3770, 8-12 GHz (X-Band)) and Permagraph (Magnet Physik Streigroever GmbH) were used to study the magnetic properties and absorption capabilities of radar waves.

Structure of Mn x Fe 3-x O 4 /r-GO nanocomposite
XRD patterns of all the samples are displayed in Figure 1. First, the data confirm the presence of only magnetite (PDF No. 19-0629) without any impurities for all samples from other study. In that study, the data of XRD supported by lattice parameter and crystal volume ( Figure 2). The diffraction peaks shift to a lower angle as x increases, implying that the lattice parameter increases with Mn 2+ incorporation [21]. Besides, the ionic radius of Mn 2+ (0.89 Å) is larger than that of Fe 2+ (0.77 Å) and Fe 3+ (0.64 Å) [46]. Exactly, Mn 2+ ions were inserted into Fe 3 Figure 1, the synthesized rGO sample qualitatively shows two amorphous peaks at 2θ = 24 o (002) and 43 o (001) according to Acharya et al. [47] by modified Hummers method and Lavin-Lopez et al. [48] by chemical reduction, heat reduction and multiphase reduction methods. This study confirmed that coconut shell was successfully synthesized into rGO material through the solid-state reaction method.
The nanocomposite compound showed a two-phase structure consisting of Mn x Fe 3-x O 4 and rGO compounds. The diffraction pattern of the nanocomposite sample is different compared to the single-phase rGO (a) and Mn x Fe 3-x O 4 (b). Therefore, the two main phases which are Mn x Fe 3-x O 4 and rGO respectively can be identified in the diffraction pattern of the nanocomposite without the appearance of additional diffraction peaks. The Mn x Fe 3-x O 4 (a) pattern appears more than the rGO pattern (b), this is because the amount of Mn x Fe 3-x O 4 content is more than the rGO content.    511) and (440). The X-ray diffraction pattern of the Mn x Fe 3-x O 4 /rGO nanocomposite shows a characteristic spinel ferrite structure and no other phases are formed [28,49,50]. The structural study confirms spinel structure formation with Fd3m space group for the 0 ≤ x ≤ 0.5 samples. The combination with rGO did not significantly affect the shift in the X-ray diffraction pattern of Mn x Fe 3-x O 4 , which has been clarified by previous studies [21,51,52]. The RAM1 sample also shows no rGO peak was found which could be caused by the XRD characterization scan (2θ angle increment) which was too fast. In another report, the disappearance of the X-ray diffraction pattern of rGO caused by the interfacial bonding of Mn x Fe 3-x O 4 with rGO resulted in a separate structure of rGO thus preventing the repetition of rGO [18]. This was also clarified by a previous study [21]. The hierarchical size of the nanocomposites was explained in detail the particle size with Particle Size Analyzer (PSA).
The sketch of the doping of Mn into Fe 3 O 4 can be seen in Figure 3. The composition and cationic distributions of the Mn x Fe 3-x O 4 particles for x composition between 0 and 1 can be analyzed using the following formula in Eq 1 [53]: The Mn x Fe 3-x O 4 /rGO composite fabrication process was carried out using the co-precipitation method, by first doping Mn on Fe 3 O 4 which was reacted with HCl to produce FeCl 2 and FeCl 3 . This was carried out to get the same phase, so that the reaction can take place [54]. Mn x Fe 3-x O 4 /rGO composite preparation through stoichiometric calculations has resulted in composite compounds that have been mixed with the ultrasonic method. Variations in the composition were carried out as many as 3, namely x = 0.25, x = 0.5, and x = 0.75, resulting in 3 (three) samples named consecutively as RAM1, RAM2, and RAM3. The purpose of adding Mn to Fe 3 O 4 is to improve the performance of Fe 3 O 4 as a radar absorbing material (RAM). In line with the research of Taufiq et al. [54]. stated that Fe 3 O 4 still has a low absorption ability of radar waves. The reaction that occurs is described in Eq 2.
The iron metal ions (Fe 2+ and Fe 3+ ) enclosed in the square bracket represent those in the octahedral positions, while the other metal ions (Mn 2+ and Fe 3+ ) in the round bracket represent those in the tetrahedral positions depending on the x value. Six oxygen atoms surround the metal ions in the octahedral sites, while four oxygen atoms surround the metal spinel structure can be classified into three types: inverse, ions in the tetrahedral sites. Generally, Fe 3 O 4 has an inverted spinel structure [54]. In the Mn x Fe 3-x O 4 system, the spinel, which has eight Fe 3+ ions located in the tetrahedral mixed, and normal spinel. The systems for x = 0.25, 0.5, and 0.75 form mixed spinel structure because Mn 2+ ions partially shift the Fe 3+ ions in the tetrahedral positions. Physically, the Mn x Fe 3-x O 4 crystal structure modeled using the Vesta program can be seen in Figure 4 below.  Figure 5 relates the rGO structure modeled using the ChemDraw program. Basically, graphene is composed of thin atoms of 2D sp 2 carbon layers that form a honeycomb-like structure. Meanwhile GO, which is a graphene derivative, contains sp 2 and sp 3 carbons and binds to a fairly high hydroxyl group (-OH) [55,56]. This is very different from rGO which has few oxygen groups and sp 3 carbon. Consequently, rGO is more similar to graphene and also has structural defects [57]. The curve of the measurement results with PSA can be seen in Figure 6. The particle size characterization of the iron sand sample that has been separated from the non-magnetic material that has been mashed and filtered shows that the raw material has a homogeneous particle size with a single peak with an average value of 531.8 nm (Table 1). This powder was detected to be still in micro size because it still has a size of more than 100 nm. This is because there is still a coagulation process or clumping at the time of measurement, resulting in a precipitate [58]. The particle size characterization for the detected rGO samples had an average particle size of 1359 nm. This size is larger than the iron sand sample. The deposition process that occurs in rGO powder is faster than iron sand powder, so the clumping process is faster and is detected as having a larger particle size.  In general, the addition of the composition of Mn and rGO to Fe 3 O 4 nanoparticles causes the crystallinity level of Fe 3 O 4 to increase teorically [12]. This is proven in Figure 6 which shows that the distribution peaks of Mn x Fe 3 O 4 /rGO in nanocomposites (x = 0.25 to x = 0.75) appear sharper when compared to pure Fe 3 O 4 . The increase in Mn composition in Mn x Fe 3-x O 4 /rGO nanocomposites results in an increase in the intensity of the Mn x Fe 3-x O 4 /rGO peak except for x = 0.75 which is lower when compared to x = 0.5. This is because the x = 0.75 particle distribution is lower than other nanocomposites of particle sizes (440.4 nm), but the average of particle size from x = 0.25 to x = 0.75 increase. The increase in particle size is caused by the large number of Mn particles that push Fe. Theoretically, the size of Fe 2+ and Fe 3+ ions is larger than that of Mn 2+ ions. However, when the number of Mn 2+ ions is more pressing the Fe 2+ ions will result in clumping, resulting in a larger particle size than before being pushed by Mn 2+ . This phenomenon is theoretically reinforced that in the voltaic series, Mn is to the left of Fe, so it has more reactive properties and is stronger against Fe. The more Mn metal ions push Fe from Fe 3 O 4 , the larger the particle size of the Mn x Fe 3-x O 4 nanocomposite. This is also confirmed in Table 1 Figure 7. As shown in Figure 7a, the morphology of the nanocomposites consists of 3 particle shapes namely spheres, chunks, and worm-like structures. Sequentially, these shapes describe the Fe 3 O 4 nanoparticles, Mn [60,61], and rGO [62]. Qualitatively, it is observed that the sphere of Fe 3 O 4 tends to agglomerate. The size of Fe 3 O 4 nanoparticles is smaller than that of Mn which has relatively irregular chunks. In addition, the Mn surface also shows some agglomeration of Fe 3 O 4 , while rGO fills the gap between Fe 3 O 4 and Mn [63]. Illustrated in Figure 5, rGO is a carbon material that is activated with acids to form OH-functional groups, so that the rGO will bind to the positive charge of Fe 3 O 4 which causes the Fe 3 O 4 to fill the surface of the carbon material. Based on Figure 5a, few rGO surfaces are covered by agglomeration of Fe 3 O 4 . This is because the density of Fe 3 O 4 is higher than that of rGO [64].

The magnetic properties of Mn x Fe 3-x O 4 /rGO
Magnetic properties in the form of hysteresis loops for Mn x Fe 3-x O 4 /rGO nanocomposite samples can be seen in Figure 8. All curves show the characteristics of soft and tall soft magnetic hysteresis loops. A summary of the magnetic properties that obtained in this research, can be seen in Table 2. Theoretically, Fe has higher magnetic properties than Mn. Thus, when more Mn replaces Fe, the magnetic properties will decrease. However, it must be analyzed further through the results of the reflection loss of the synthesized sample.

Reflection loss of Mn x Fe 3-x O 4 /rGO
Reflection loss is a characteristic that shows the ability of radar wave absorption from Mn x Fe 3-x O 4 /rGO. Reflection loss of each sample has been characterized using VNA with the results of the reflection loss and frequency curves as shown in Figure 9. The results show that there has been absorption of electromagnetic waves in the frequency range between 7-13 GHz with the absorption frequency range at 2-14 GHz. At this frequency, it shows that the Mn x Fe 3-x O 4 /r-GO nanocomposite can be applied as a RAM material in the X-Band 8-12 GHz frequency range. Figure 9 shows RL of the Mn x Fe 3-x O 4 /rGO nanocomposites at a frequency of 8-12 GHz which were calculated by using Eqs 3 and 4.

RL = 20log
(3) Z in value was calculated by using Eq 3 below.
where Z in and Z in are input and output impedance values. While µ r and Ɛ r are the permeability and complex permittivity of the material, c represents the velocity of the radar in a vacuum, f is the frequency of radar, and d is the thickness of the material when tested [65]. In Figure 9 for the sample of Mn 0.25 Fe 2.75 O 4 /rGO nanocomposite, absorption of electromagnetic waves has occurred in the frequency range between 8.9-11.5 GHz with the absorption peak frequency at 11.5 GHz of −11.8 dB with a thickness of 1.5 mm adsorption field. Likewise for the Mn 0.5 Fe 2.5 O 4 /rGO nanocomposite sample, absorption occurs in the frequency range between 9-11.3 GHz with the absorption peak frequency at 11.3 GHz of −11.5 dB. Meanwhile, for the Mn 0.75 Fe 2.25 O 4 /r-GO nanocomposite sample, absorption has occurred in the frequency range between 8.8-11.5 GHz with the absorption peak frequency at 11.5 GHz of −  [17]. This is due to the presence of Mn 2+ ions which can increase the polarization of the space charge [69]. The ability of magnetic loss in Mn x Fe 3-x O 4 material occurs when the direction of the magnetic moment which is relatively antiparallel in the tetrahedral and octahedral regions will be rectified [19]. Generally, Mn x Fe 3-x O 4 materials can produce magnetic loss consisting of eddy current loss and natural resonance loss [30]. Furthermore, the magnetic loss capability is more dominant due to the eddy current loss phenomenon, as expressed in Eq 5 [28].
With C 0 as the value representing the eddy current loss. If depicted in a graph C 0 with f, then the eddy current loss is indicated by a constant value of C 0 . In the study of Tajik et al., the Mn x Fe 3-x O 4 material showed a constant C 0 value at frequencies above 8 GHz [70]. In another report, Modaresi et al. reported the same [28]. Conversely, if the value of C 0 is not constant, it can be indicated that there is a natural resonance loss phenomenon from the magnetic material. The relatively small crystal size has the potential to increase the RL value, which is caused by the exchange length resonance of the magnetic energy of a material [55]. Thus, the Mn x Fe 3-x O 4 material can be used as a potential magnetic material in RAM applications.   Furthermore, rGO is a unique material, which refers to its special characteristic which is it can produce dielectric loss. In the report of Liu et al., dielectric materials depend on the phenomenon of dipole and electron polarization [37]. Inside an atom, the influence of the electric field from electromagnetic waves will cause electron polarization. If viewed at the molecular level, each molecule has a dipole moment, which when applied by an external electric field will form a charge separation or dipole polarization. In the rGO system, dipole polarization occurs in the structural defects and the presence of various functional groups [71]. For example, Qiao et al. representing the interaction of electromagnetic waves with the structural defects of rGO, will result in an asymmetric charge distribution to form a dipole formation [72]. This causes the dipole to continue to rotate in electromagnetic waves, which in turn converts electromagnetic wave energy into heat energy [73]. In fact, electrons on the surface of rGO can interact with electromagnetic wave energy [74]. Consequently, the electrons will migrate and collide with the rGO lattice, where the energy of the electromagnetic waves is converted into heat energy [73]. Debye's dipolar relaxation theory is usually used to confirm the ability of dielectric loss, as expressed in Eq 6 [28].
Where ε s and ε ∞ are static permittivity and relative dielectric permittivity. If and are expressed as x and y axes, a semi-circular Cole-Cole graph is formed. According to Chai and colleagues, the presence of a semi-circle pattern indicates a relaxation process, which is caused by the absorption of the electric field from electromagnetic waves [67]. Furthermore, Shu et al. reported that the distortion of the semi-circular Cole-Cole graph in the rGO material indicates the presence of polarization [74]. Furthermore, based on the previous SEM image of the composite sample in Figure 7, rGO forms cavities and is filled with Mn x Fe 3-x O 4 particles. The presence of cavities in rGO has the potential to increase the value of reflection loss, which is caused by electromagnetic waves undergoing multiple reflections [75]. Thus, the rGO material has potential in RAM applications. Although, based on Table 3, the reflection loss value of rGO is still low at −5 dB, so it is necessary to combine it with magnetic materials. Mn x Fe 3-x O 4 deposition strategy on the surface of rGO is one of the appropriate ways to produce microcurrent networks [76]. The mechanism, Mn x Fe 3-x O 4 acts as an electron-hopping bridge between adjacent rGO layers [55]. More deeply, electron-hopping is expressed as contact conductivity (σ contact ) as a function of temperature (T) as in Eq 7 [76].
Where K h is the prefactor, U is the barrier potential and k B is the Boltzmann constant. As the equation above, conductivity is caused by electron-hopping followed by an increase in temperature.
The micro current network that is formed will produce a conduction loss [55]. The phenomenon of conduction loss is characterized by a decrease in the permittivity value as the frequency increases, which is useful for weakening the energy of electromagnetic waves. However, Cao and colleagues reported that the phenomenon of relaxation or an increase in temperature that is too high causes a conductivity network not to form [71]. Consequently, at high frequencies, the dielectric loss capability will decrease. One solution is to adjust the mass composition of rGO with Mn x Fe 3-x O 4 . In addition, the presence of interfacial interactions between Mn x Fe 3-x O 4 and rGO has the potential to produce interfacial polarization [13]. Physically, the interface interaction accumulates the migration and diffusion of the carrier so as to form a spatial separation consisting of positive and negative charges [77]. Consequently, between the boundaries of the Mn x Fe 3-x O 4 and rGO materials, a balance formation is formed in the form of a region of spatial charges and an electric field. However, changing the direction of the electromagnetic wave field can change the charge pattern so that the electromagnetic wave energy will be dissipated [78]. Interfacial polarization can also be identified through Debye's dipolar relaxation theory as studied above. According to Table 3, the Mn x Fe 3-x O 4 /rGO composite of the three variations showed fantastic results, namely higher reflection loss values than the single material. The combination of magnetic and dielectric materials provides a synergy effect with good impedance match results (Z ~ 1) [79]. Furthermore, this composite system contains a magnetic material, this is in accordance with the results of the reflection loss value obtained from the tests that have been carried out.
Based on the results of the previous research, it was shown that, if the RL value obtained <−15 dB, it indicates that if 96.9% of the radar waves are absorbed by the nanocomposite. Meanwhile, if the RL value obtained <−20 dB, then the absorbed wave is almost 99.0% [80]. In this research, the RL value of nanocomposites has a range between −2 to −14 dB or <−20 dB. Thus, it can be concluded that the waves absorbed by nanocomposites in this study are around 96.9-99.0%. Two main aspects affecting the increase of radar absorption by Mn x Fe 3-x O 4 /rGO nanocomposites in this study are first, the efficient complementarity between dielectric loss and magnetic loss which is indicated by the relative permittivity and permeability values must be fulfilled [68,81]. If Fe 3 O 4 nanoparticles stand alone as a compiler of RAM, it can be construed that the resulting RL value is still low. This is due to the large disparity between permeability and permittivity which interferes with impedance matching. Thus, in the Mn x Fe 3-x O 4 /rGO system, the Fe 3 O 4 nanoparticles act as absorbents of the magnetic parts, and rGO, is as dielectric absorbers of incoming radar. Besides, the rGO also acts as a nucleation site for Fe 3 O 4 which prevents or reduces the aggregation of these nanoparticles. Second, there is interfacial polarization between Fe 3 O 4 -rGO, where multi-interfaces on nanocomposites will produce significant polarization interfaces that will increase the value of dielectric loss at high frequencies [65]. This is indicated when the addition of rGO absorption material occurs at a frequency of around 11 GHz. Thus, the development of Mn x Fe 3-x O 4 /rGO nanocomposites in this research provides new opportunities for large-scale development for the application of high-performance microwave absorbing materials based on local natural materials through environmentally friendly synthesis.

Practical implication and outlook
Uniquely, in this study, reduced-graphene oxide (rGO) synthesized from coconut shell waste has the potential to be developed as a dielectric material from RAM. This is relevant to the data presented in Table 3 that rGO can absorb electromagnetic waves of −5 dB [52] added with the findings from this study which combines natural materials of natural iron sand and coconut shell waste, it produces an RL value of −11.95 dB. In general, coconut shell waste is very easy to obtain from the community, especially in Indonesia. Theorically, coconut shell has a high cellulose content, so it contains a high carbon content as well [82,83]. Carbon has the ability to absorb electromagnetic waves, so it has the potential to be developed as a dielectric material. Further research on the use of natural materials can be developed towards extracting the potential of the material itself. Natural iron sand can be extracted as a source of Fe 2 O 3 which can be synthesized into Fe 3 O 4 with benefits as an environmentally friendly magnetic material [84], antimicrobial [69], and anti-cancer agent [79]. Meanwhile, coconut shell waste can be synthesized into rGO with the benefit of being a dielectric material for Radar Absorbing Material (RAM) [56], corrosion protection [85], and electrochemical sensor [70].

Conclusions
The Mn x Fe 3-x O 4 /rGO nanocomposites were successfully synthesized by using an environmental precipitation method. Meanwhile, with the addition of rGO in the Mn x Fe 3-x O 4 /rGO nanocomposites, their RL increases significantly because the nanocomposites consist of magnetic loss and dielectric loss which increases the radar absorption. Interestingly, the RL value of the Mn x Fe 3-x O 4 /rGO nanocomposites ranges from −2.0 to −14.0 dB which shows their radar absorption capability is in the range of 96.9%-99.0%.