INTRODUCTION

As a manifestation of energy, electromagnetic waves have special applications in our everyday life [1]. For instance, telecommunications systems, specialized medical equipment, solar radiation, etc. [2]. Among them, nanomaterials and developed composite have been considered in civilian, industrial, and military fields [3]. The radar systems were expanded during World War II and their development continues to the present day [4]. Therefore, it is necessary to use radar-absorbing material (RAM) to deal with radar systems. RAM plays the main role in controlling the electromagnetic environment [5]. The perfect RAM must have remarkable properties such as low cost [6], low density [7], good thermal stability [8], and strong attenuation capacity over a wide frequency range [9, 10]. 

Nano-scale carbon materials such as carbon black [11], carbon nanotubes [12], carbon fibers [13], biomass carbon [14], graphite [15], and graphite-based materials [16, 17] have provided these features. Graphene quantum dots (GQDs) are graphene fragments that are small enough to cause exaction confinement and a quantum size effect [18]. These carbon nanostructures have low density, high thermal and electrical conductivity, outstanding physicochemical attributes, and optical transmittance [19]. 

Also, many reports have been published about the synergic effects of composites. Reports revealed that metal-oxide composites with various formulations are tunable for microwave absorption peaks [20]. Among these researches, the shape and size of the nanostructure play a key role in improving absorbing performances [21]. The hydrothermal route is a significant protocol for morphology engineering and nanomaterial preparation [22]. In fact, during the hydrothermal process, the conditions of the reaction (e.g. solvent type, temperature, pH adjuster, and molar ratio of precursors) are very important [23]. In other words, the morphology and size of the particles are controlled by the nucleation and grain growth process during the hydrothermal protocol [24]. In this current study, we have designed and fabricated nanoscale GQDs-decorated magnetic composites via a green, low-cost, and easy route (hydrothermal strategy) and the microwave absorbing performances were tested. Several nanocomposites with diverse morphologies were fabricated in various conditions (e.g. green routes, various pH adjusters, temperatures, etc.). Next, GQDs were applied to improve the surface. Due to further active sites on the surface of nanocomposites, morphology engineering can be effective in microwave absorbing. Compared to pure magnetic composites, the GQDs-decorated magnetic composites presented excellent microwave absorbing performances in terms of both maximum reflection loss and the adsorption band width. Experimental data suggested that GQDs can find wide applications in the microwave-absorbing areas.

MATERIALS AND METHODS

Chemicals and typical procedures

The precursors were of analytical grade from Merck without extra purification. The as-prepared nanostructures were proved by Scanning electron microscope (SEM), X-ray diffraction analysis (XRD), Fourier transform infrared (FT-IR), Elemental mapping (EDS), and Value stream mapping (VSM) analysis. 

Fabrication of Fe₃O₄ spherical nanoparticles

Initially, 1 g of FeSO₄.7H₂O and 0.72 g of Fe(NO₃)₃.9H₂O were dissolved completely in distilled water to produce a 50 ml solution at ambient temperature. Under continuous stirring, the previously prepared base solution was added drop by drop to the above solution to bring pH=12. The solution directly got dark after the addition base solution and stirred at 25 °C (15 min). Eventually, the whole mixture was put in a 150 ml Teflon Lined stainless steel autoclave and transferred to an electric oven at 140 °C for 15 hours under hydrothermal conditions. The black solid was collected by an external magnetic field. The prepared Fe₃O₄ nanoparticles were washed with dry ethanol and dried in a vacuum overnight at 65°C [19].

Fabrication of CoFe₂O₄ spherical nanocomposite

FeSO₄.7H₂O (1 g), Fe(NO₃)₃.9H₂O (0.72 g), and CoCl₂.2H₂O (0.59 g) were dissolved in 50 ml of distilled water at ambient temperature. Next, the as-prepared base solution was added droply to the above solution to bring pH=12 under continuous stirring. The mixture was stirred at 25 °C (15 min). Finally, the whole mixture was put in a 150 ml Teflon Lined stainless steel autoclave and transferred to an electric oven at 150 °C for 12 hours under hydrothermal conditions. The black solid was collected by an external magnetic field. The prepared CoFe₂O₄ nanocomposites were washed with dry ethanol and dried in a vacuum overnight at 65°C [19].

Fabrication of GQDs-decorated Fe₃O₄ and CoFe₂O₄ spherical nanocomposite

The hydrothermal technique is the foremost common route for the massive production of GQDs. The mixture of 1g of citric acid, 0.4 ml of ethylenediamine, and distilled water (50 cc) was mixed for 2 minutes (25 °C). Then, the as-fabricated Fe₃O₄ or CoFe₂O₄ magnetic nanostructures (1 g) were poured into the above mixture and sonicated for 2 min. Next, the mixture was put in a 150 ml Teflon Lined stainless steel autoclave and placed in the electric oven at 180 °C for 9 hours under hydrothermal conditions. At completion, the resulting sediment was collected under an external magnetic field and washed with dry ethanol. The separated precipitate, finally, dried at 55 °C for 24 hours under vacuum conditions [25].

RESULTS AND DISCUSSION

In general, there are various approaches to the fabrication of nanostructures which are sol-gel, microwave, precipitation, etc. [26]. Among them, the hydrothermal technique is an effective route fabrication and growth of advanced nanomaterials. In addition, engineering the structure morphology (shape and size) plays a main role in structural properties. Besides, solvent, pH adjuster, reaction time, temperature, etc. are fundamental parameters in nanostructure size and morphology engineering [26, 27]. Moreover, many research papers have been reported about magnetic nanoparticles/composites. These nanostructures have been remarkably expanded because of their facile preparation, good performance, high stability, and high absorption [28]. In this study, the surface morphology of the Fe₃O₄, CoFe₂O₄, Fe₃O₄/GQDs, and CoFe₂O₄/GQDs were investigated by the FE-SEM route. In the case of Fe₃O₄ NPs, KOH and NH_3(aq) were used as pH adjusters. FE-SEM images of Fe₃O₄ NPs in the presence of two different pH-adjusters are shown in Fig. 1. From Fig. 1a-b, the Fe₃O₄ NPs could be recognized as having spherical and bulk shapes in the presence of KOH and NH_3(aq) at 170 °C, respectively. As shown in Fig. 1c-d for appropriate morphology, the temperature was changed. Fig. 1c-d showed the formation of non-unique shapes at 120 and 160 °C, respectively. 

FE-SEM images of the CoFe₂O₄ composite are shown in Fig. 2. It is clear that the spherical and heterojunction shapes of the CoFe₂O₄ composite were formed using KOH for 12 h at 150 °C and 180 °C, respectively (Fig. 2a-b). When the pH adjuster was changed to an ammonia solution, the spherical morphology was converted to a hexagonal shape at the same conditions (Fig. 2c).

Since GQDs can accommodate –COOH and –OH functional groups, their binding to magnetic nanostructures includes stability [29]. Fig. 3 exhibits the FE-SEM images of GQDs-decorated Fe₃O₄ and CoFe₂O₄ magnetic composites. As shown in Fig. 3 the Fe₃O₄ and CoFe₂O₄ spherical morphologies are changed after modification with GQDs. Also, for more investigation of Fe₃O₄/GQDs and CoFe₂O₄/GQDs NCs were applied. As can be seen in Fig. 4, the spherical structure of Fe₃O₄/GQDs and CoFe₂O₄/GQDs NCs were generated from dense nanosphere building blocks through the self-assembly process.

The XRD patterns of Fe₃O₄, CoFe₂O₄, Fe₃O₄/GQDs, and CoFe₂O₄/GQDs structures are illustrated in Fig. 5. As shown in Fig. 5a, the nano-sized Fe₃O₄ Miller’s index is seen; the intensity ratio and position of these peaks have admissible corresponding to the standard pattern (JCPDS No. 03-0863) [30]. According to the Debye formula (D= kλ/βcosθ), the crystallite size was measured about 42 nm. To date, different papers have been reported about the XRD pattern of the CoFe₂O₄ composite. The obtained XRD pattern in this study is totally in agreement with previous reports. The characteristic peaks can be observed in Fig. 5b that data match with reference code (JCPDS No. 80-1540) [31]. The CoFe₂O₄ composite crystallite size, which is calculated by the Debye formula, was determined 56 nm. Investigation of the detailed XRD graph of the CoFe₂O₄ composite has confirmed this description. In the case of Fig. 5c-d, a broad peak at 2θ=24° is also related to the (002) plane corresponding to the presence of GQDs in the magnetic nanostructure [32].

The FT-IR spectroscopy study of pure Fe₃O₄ and CoFe₂O₄ nanostructures have been exploited to achieve more exact data about the fabrication of GQDs decorated magnetic nanoparticles (Fig. 6). The absorption bands in the approximate region of 509 cm^-1 belong to Fe-O stretching vibration [33]. Two peaks at 3423 cm^-1 and 1622 cm^-1 also correspond to -OH stretching and bending vibration, respectively, due to absorbed H₂O by the surface Fe₃O₄ nanoparticles (Fig. 6a). Based on the FT-IR spectrum of CoFe₂O₄ composite represented in Fig. 6b, the two bands at 509 cm^-1 and 1022 cm^-1 are related to Fe-O and Co-Fe stretching vibration [34]. Compared to the Fig. 6a graph, new bands approximately at 1072, 2858, and 2989 cm^-1 in the Fig. 6c spectrum are attributed to C=O, -CH _sp2, and -CH _sp3 stretching vibration of GQDs [35]. Besides, the Fig. 6d spectrum shows that various peaks at 600, 615, 780, 1388, 1680, 2920, and 3385 cm^-1 are related to Fe-O, Fe-Co²⁺, Fe-Co³⁺, C-O, C=O, -CH _sp2, and -CH _sp3, respectively.

Fig. 7 reveals the EDS analysis and elemental mapping of Fe₃O₄, CoFe₂O₄, Fe₃O₄/GQDs, and CoFe₂O₄/GQDs composites. In each case, the EDS patterns prove the presence of main elements in nanostructures. Also, the percentage content of elements was tabulated in the EDS pattern. Moreover, the elemental mapping route depicted that all elements were well distributed in each nanostructure.

The magnetic properties of the as-fabricated Fe₃O₄, CoFe₂O₄, Fe₃O₄/GQDs, and CoFe₂O₄/GQDs composites were measured by a VSM analysis at room temperature. The saturation magnetization (Ms) values of the Fe₃O₄ and CoFe₂O₄ sphere composite are approximately 82 and 79 emu/g, respectively, which is suggestive of powerful magnetic attributes (Fig. 8a-b). The Fe₃O₄ and CoFe₂O₄ composites were decorated with GQDs. The magnetic attributes were reduced sharply (Fig. 8c-d). This change reveals that this decline could be a result of the existence of a non-magnetic layer. Therefore, the GQDs decorated magnetic composites were formed. 

RL calculation for GQDs-decorated magnetic nanostructures with various concentrations of GQDs is shown in Fig. 9. Based on data, maximum absorption of microwave radiations for 5% and 10% of Fe₃O₄/GQDs NCs (2 mm thickness) were reported -19 and -26 dB, respectively, at a frequency of 0-20 GHz (Fig. 9a-b). Similarly, at a frequency of 0-20 GHz, maximum absorption of microwave radiations was registered at -17 and – 27.5 dB for 5% and 10% of CoFe₂O₄/GQDs NCs with a thickness of 2 mm (Fig. 9c-d). The RL was increased with the increase of GQDs concentration. Moreover, the results showed that RL position changes in Fe₃O₄/GQDs NCs (from 5 to 9.5 GHz) and CoFe₂O₄/GQDs NCs (from 12 to 16 GHz), respectively. 

Also, to compare the microwave absorbing of N-GQDs/CoFe₂O₄ and N-GQDs-Fe₃O₄ nanocomposites with reported other materials, we have summarized the results in Table 1.

CONCLUSION

This study describes the design of microwave absorption magnetic nanostructures using GQDs based on surface morphology engineering. GQDs-decorated magnetic nanostructures were prepared via a hydrothermal route. The spherical shape was selected as the best morphology. The influence of spherical-shaped Fe₃O₄/GQDs and CoFe₂O₄/GQDs nanostructures on the reflection loss were investigated. The experimental results show that GQDs can increase the absorption of microwave radiation. Also, RL curves reveal that increasing GQDs concentration has a direct effect on increasing the absorption of microwave radiations.

CONFLICT OF INTEREST

The authors declare that there is no conflict of interests regarding the publication of this manuscript.