Reduced graphene oxide aerogel decorated with Mo₂C nanoparticles toward multifunctional properties of hydrophobicity, thermal insulation and microwave absorption

1.   Introduction

With their unique three-dimensional (3D) microstructure, aerogels are evolving into a fascinating architecture for graphene or reduced graphene oxide (rGO) because they cannot only acquire the inherent properties of few carbon atomic layers but also resist the re-stacking of carbon layers induced by π–π interaction. Graphene-based aerogels are materials with extremely high porosity and ultra-low density [1–2]. In the past decade, rGO aerogels have significantly advanced the performance of graphene in the fields of adsorption and energy storage; meanwhile, they further emerged as ideal scaffolds for various active species to be excellent catalysts and electrode materials [3–5]. Recent progress has revealed that in addition to conventional applications, rGO aerogels can be utilized as promising microwave absorption materials to alleviate the increasingly serious electromagnetic (EM) pollution, where the conductive network is greatly helpful in providing sufficient loss capability, and the open skeleton is quite favorable for EM wave attenuation through multiple reflections [6]. What is more exciting is that some subsidiary physical and chemical properties, such as lightweight, thermal insulation, and hydrophobicity, clearly demonstrate rGO aerogels as credible candidates for next-generation microwave absorption materials [7]. However, a single rGO aerogel is incapable of creating well matched characteristic impedance with free space to allow complete transmission of incident EM waves [8–9], meaning there is still plenty of room for its performance reinforcement, and thus, rational design of rGO aerogels would facilitate their practical application to a large extent.

Decorating rGO aerogel with additional components that possess distinguishable EM functions is an effective method for improving microwave absorption performance [10–11]. To date, some EM components, such as magnetic metals, ferrites, metal oxides, and conductive polymers, have demonstrated positive effects on rGO aerogel’s microwave absorption performance [12–15]. However, EM wave absorption materials (EWAMs) require good environmental tolerance in practical applications [16–18]; additional magnetic components may face issues under corrosive and oxidative conditions [19–20]. Comprehensive consideration of multiple performances of rGO aerogel-based composites is becoming a hot topic in the design and fabrication of novel EWAMs [13,17]. Among various effective EM components, increasing attention is being paid to carbides for coupling with rGO aerogel, not only for their desirable EM properties but also for their excellent chemical stability and corrosion resistance [21–24]. SiC and Ti₃C₂, as the most typical carbides, are almost overwhelming in the related rGO aerogels. It is well known that both SiC and Ti₃C₂ are synthesized under highly rigorous conditions (i.e., high-temperature over 1400°C); even Ti₃C₂ requires HF treatment to remove Al species from raw materials [25–27]. In addition, the very large particle size is also unfavorable for their uniform dispersion in rGO aerogel. Du’s group [28–30] has recently found that Mo₂C can also modulate the dielectric properties of the carbon host effectively, achieving a positive improvement in electromagnetic attenuation characteristics. Particularly, Mo₂C can be obtained at moderate temperatures, and thus, its particle size is much smaller than those of SiC and Ti₃C₂, which is quite helpful in creating good dispersion and sufficient interfaces. However, studies on microwave absorption of Mo₂C/rGO aerogel are still inaccessible.

Herein, we demonstrate the successful synthesis of Mo₂C/rGO aerogel through the hydrothermal assembly of graphene oxide (GO), glucose (GL), and ammonium molybdate (AM), followed by freeze-drying and high-temperature pyrolysis. Mo₂C nanoparticles wrapped in crisscross rGO cellular walls effectively regulate complex permittivity and impedance matching of rGO composites and induce the generation of a microconductive network for attenuating the electrical branch of EM waves. It is very interesting that the resulting Mo₂C/rGO not only displays an enhanced EM wave absorption property but also presents significant reinforcements in thermal insulation and hydrophobicity.

2.   Experimental

Briefly, graphene oxide (GO) was synthesized through the modified Hummers’ method [31]. GL (0.54 g) and a certain amount of AM were dissolved in 20 mL of deionized water under magnetic stirring at 25°C. GO powder (60 mg) was then added to the above aqueous solution under vigorous sonication for 5 h. Afterward, the mixture was transferred to a 50 mL Teflon-lined stainless-steel autoclave and heated at 160°C for 12 h. The obtained cylindrical precipitate was washed several times with deionized water and freeze-dried at −45°C for 24 h. Finally, the as-prepared aerogel was transferred into a horizontally tubular furnace and pyrolyzed at 800°C for 3 h with a heating rate of 5°C/min under an Ar atmosphere. The final composites are named Mo₂C/GA-0.5, Mo₂C/GA-1.0, and Mo₂C/GA-2.0 based on the molar ratio of Mo/GL, where the quality of AM are 0.26, 0.52, and 1.04 g, respectively. Control GO was made by pyrolysis at 800°C without AM and GL.

3.   Results and discussion

3.1   Microstructure characterization

The synthesis of the Mo₂C/rGO aerogel is illustrated in Fig. 1. AM and GL are first uniformly mixed, and then, an aqueous dispersion of GO is added to the mixed solution. In a mild hydrothermal process, the GO nanosheet will suffer a series of cross-linking reactions induced by π–π interaction to generate an aerogel structure [2,32]. Noteworthily, the structure of pristine rGO aerogel can still be maintained after introducing GL and AM (Fig. S1). The X-ray diffraction (XRD) pattern in Fig. 2(a) shows the characteristic peaks related to MoO₂, suggesting the reduction of molybdate to MoO₂ during the hydrothermal process [33]. In the Fourier transform infrared spectroscopy (FT-IR) spectrum (Fig. S2), the characteristic vibration peak of –COO⁻ in GL-Mo (Fig. S2(a)) and the different characteristic vibration peaks between GO and GO-Mo (Fig. S2(b)) indicate that AM undergoes a redox reaction with GO and GL [34]. However, the active site for growing Mo species will be limited during the self-assembly process of GO, which will be unfavorable for obtaining the final product. The presence of GL provides auxiliary motivation to facilitate the transformation of Mo species. The residual weight of GL-Mo/GO in the thermogravimetric (TG) curves is more than that of GO-Mo (Fig. S3), further revealing that GL will stimulate AM reduction on the surface of self-assembled GO. Finally, molybdate will be moderately reduced, resulting in MoO₂ formation anchored on a self-assembled GO cellular wall. The resulting composite precursor is converted into Mo₂C/rGO aerogel during the high-temperature pyrolysis.

Fig. 1.  Synthesis of the Mo₂C/rGO aerogel.

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Fig. 2.  (a) XRD pattern of GL-Mo. SEM images of (b) Mo₂C/GA-0.5, (c) Mo₂C/GA-1.0, and (d) Mo₂C/GA-2.0. (e) Mercury intrusion–extrusion isotherms. (f) Photographs of Mo₂C/GA-1.0 aerogel resting on S. viridis. TEM images of (g) Mo₂C/GA-0.5, (h) Mo₂C/GA-1.0, and (i) Mo₂C/GA-2.0.

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Fig. 2(b)–(d) shows the scanning electron microscope (SEM) images of Mo₂C/GA-0.5, Mo₂C/GA-1.0, and Mo₂C/GA-2.0, respectively. All the three composites display a hierarchical 3D porous network structure similar to the original profile from the 3D rGO aerogel (Fig. S1(a)), indicating that the formation of Mo₂C nanoparticles does not produce fatal impacts on 3D skeletons. These interconnected nanosheets result in abundant macropores with an average diameter of microns. Different mercury intrusion–extrusion isotherms in Fig. 2(e) indicate their distinguishable microstructure features. Pristine rGO aerogel possesses a high saturated intrusion volume of approximately 28.6 mL/g and low saturated intrusion–extrusion pressure, revealing that mercury easily enters the abundant porous cell cavities. With introduction of Mo₂C nanoparticles, Mo₂C/GA-0.5, Mo₂C/GA-1.0, and Mo₂C/GA-2.0 shows a high decrease in saturated intrusion volume to approximately 11.6, 10.6, and 3.2 mL/g, respectively (Table S1) with increasing saturated intrusion–extrusion pressure. From these results, one can conclude that all these samples possess macroporous structures as identified by SEM [14,35], while the presence of Mo₂C nanoparticles negatively impacts the microstructure to a certain degree. These changes can be further supported by textural parameters (Table S1): porosity and surface area are gradually decreased, while the density is gradually increased. The negative impact of Mo₂C nanoparticles is mainly from their relatively high density, and manipulation of the Mo₂C nanoparticle loading may be helpful in maintaining the lightweight feature of rGO aerogel. When Mo₂C/GA-1.0 with a cylindrical shape (a diameter of 10 mm and a height of 15 mm) is placed on soft Setaria viridis, there is negligible deformation of S. viridis, verifying the ultra-low density of the Mo₂C/GA composites (Fig. 2(f)). The rich heterogeneous interface and trapped air in the aerogel structure contribute to improving the impedance matching, as well as the macropores inducing a microconductive network for prompting multiple reflection and scattering of incident EM waves [36–38]. Transmission electron microscope (TEM) images provide further insight into the microstructures of these as-synthesized composites. The Mo₂C nanoparticles are well dispersed on the surface of rGO nanosheets with an ultra-thin thickness (Fig. 2(g)–(i)). Mo₂C nanoparticles increase significantly with increasing AM content, and there is an even slight aggregation in Mo₂C/GA-2.0. In detail, the lattice fringes of nanoparticles approximately 0.23 nm can be assigned to the (101) plane of hexagonal Mo₂C (inset in Fig. 2(h)) [30].

3.2   Phase and composition characterization

Fig. 3(a) illustrates the XRD patterns of rGO aerogel, Mo₂C/GA-0.5, Mo₂C/GA-1.0, and Mo₂C/GA-2.0. The pristine rGO aerogel exhibits two distinguishable peaks located at 26.1° and 44.0°, ascribed to the (002) and (100) planes of graphite carbon (JCPDS 75-1621) [11,39]. This phenomenon suggests that high-temperature treatment induces the re-stacking of rGO nanosheets to some extent. Introducing Mo₂C, the peak at 26.1° becomes weak gradually; meanwhile, some well-resolved peaks appear at 34.3°, 37.9°, 39.4°, 52.1°, 61.6°, 69.5°, and 72.3°, which are precisely matched with the (100), (002), (101), (102), (110), (103), and (200) crystal planes of hexagonal Mo₂C (JCPDS 35-0787), respectively [40]. These results manifest that MoO_x nanoparticles have been converted into Mo₂C nanoparticles during high-temperature pyrolysis and Mo₂C nanoparticles effectively suppress the re-stacking of rGO nanosheets. Raman spectroscopy is further employed to investigate the influence of Mo₂C nanoparticles on the defective degree of rGO aerogel. As shown in Fig. 3(b), rGO aerogel, Mo₂C/GA-0.5, Mo₂C/GA-1.0, and Mo₂C/GA-2.0 all exhibit two distinguishable peaks at 1350 and 1600 cm⁻¹, which can be described as the D and G bands of carbon materials, respectively [41–42]. The D band is always associated with the vibration of sp³ hybridization defects or lattice distortion in the carbon matrix, originating from the disordered arrangement of carbon atoms, while the G band describes the in-plane bond stretching of sp² hybridization carbon atoms [43–44]. The intensity ratio I_D/I_G is an important indicator for the relative graphitization degree of carbon materials [20,45]. Clearly, the rGO aerogel has the smallest I_D/I_G = 0.87 while Mo₂C/GA-0.5, Mo₂C/GA-1.0, and Mo₂C/GA-2.0 give incremental I_D/I_G values from 0.99 to 1.13, indicating numerous defects and the reduced graphitization degree of rGO aerogel with the introduction of Mo₂C.

Fig. 3.  (a) XRD patterns and (b) Raman spectra of rGO aerogel, Mo₂C/GA-0.5, Mo₂C/GA-1.0, and Mo₂C/GA-2.0. (c) TG curves of Mo₂C/GA-0.5, Mo₂C/GA-1.0, and Mo₂C/GA-2.0. Photographs of the WCA test of (d) rGO, (e) Mo₂C/GA-0.5, (f) Mo₂C/GA-1.0, and (g) Mo₂C/GA-2.0. (h) Thermal infrared images of Mo₂C/GA-1.0.

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The relative content of Mo₂C nanoparticles in these composites can be determined exactly through TG because rGO skeletons can be completely combusted to CO₂ in air at high temperatures [46–47]. In Fig. 3(c), the TG curves of Mo₂C/GA-0.5, Mo₂C/GA-1.0, and Mo₂C/GA-2.0 present two obvious weight loss processes, which can be attributed to the removal of physically absorbed water and the combustion of rGO skeletons under air. It is worth noting that besides these two weight loss regions, both Mo₂C/GA-1.0 and Mo₂C/GA-2.0 also display weight increases in the temperature region of 350–400°C due to the oxidation of Mo₂C nanoparticles. The abnormal behaviors of weight variation in Mo₂C/GA-1.0 and Mo₂C/GA-2.0 are attributed to their high Mo₂C nanoparticle loadings, which can offset the decomposition of rGO aerogel. Taking into consideration that Mo₂C totally oxidizes into MoO₃ in high-temperature air, the relative mass content (m, wt%) of Mo₂C nanoparticles can be determined using the following equation [48]:

where R (wt%) is the percentage of the residual mass after the TG test and M represents the molecular weight of the corresponding matters. The specific contents of the Mo₂C nanoparticles in Mo₂C/GA-0.5, Mo₂C/GA-1.0, and Mo₂C/GA-2.0 are 27.6wt%, 40.8wt%, and 56.5wt%, respectively. These results clearly demonstrate that the composition and EM properties of the Mo₂C/GA composites may be easily regulated by the amount of AM in the first hydrothermal process.

3.3   Hydrophobicity and thermal insulation

The interconnected macropore structure and variable chemical components also bring tailorable hydrophobicity and thermal insulation. The hydrophobic degrees of rGO aerogel, Mo₂C/GA-0.5, Mo₂C/GA-1.0, and Mo₂C/GA-2.0 are measured by the water contact angle (WCA) test. A lamina of aerogels is first adhered to a glass slide, and then, a drop of water is released on their surface. The WCA of the glass slide is measured to be 31.5° under similar conditions (Fig. S4), showing apparent hydrophilicity. Mo₂C/GA-0.5, Mo₂C/GA-1.0, and Mo₂C/GA-2.0 all exhibit favorable hydrophobicity with WCA values of 132.3°, 133.2°, and 134.9°, respectively, which are superior to that of the rGO aerogel (89.3°, Fig. 3(d)–(g)). The improved hydrophobicity of the Mo₂C/GA composites can be mainly ascribed to the following two aspects. One is that the introduction of AM and GL in the precursor and high-temperature pyrolytic reaction will both induce the reduction of the hydrophilic group on the surface of pristine GO. The other is that the increased nanoscale surface roughness from introducing Mo₂C nanoparticles and sufficient micron-scale roughness of the aerogel structure work together to reduce the contact area between water and solid. The hydrophobicity of aerogel not only will endow it with the characteristics of waterproof and corrosion resistance but can also achieve self-cleaning by removing contaminants and dirt when water droplets slide on its surface.

Thermal stability and thermal insulation are also necessary for the application of microwave absorption materials in harsh environments such as aerospace. Thermal stability below 300°C is validated by TG (Fig. 3(c)). The superior thermal insulation property will maintain the coating layer at a desirable temperature, which can protect targets from high-temperature damage and detection of infrared apparatus [49]. The thermal insulation performance of rGO aerogel, Mo₂C/GA-0.5, Mo₂C/GA-1.0, and Mo₂C/GA-2.0 was verified using an infrared imaging camera, where cylindrical samples were arranged at the center of a heat plate at 130°C, and thermal infrared images were captured at 1, 10, and 30 min (Fig. S5(a)–(c) and Fig. 3(h)). When the actual temperature (Sp1 point) of heating plate reached ~130°C, the detected temperatures (Sp2 point) of Mo₂C/GA-0.5 are 46.8, 47.2, and 47.6°C (Fig. S5(b)), which are inferior to those of pristine rGO under the same conditions (Fig. S5(a)). The similar color between the Sp2 point and the surrounding environment also indicated good infrared invisibility. With increasing Mo₂C nanoparticles, the detected Sp2 temperatures of Mo₂C/GA-1.0 (Fig. 3(h)) and Mo₂C/GA-2.0 (Fig. S5(c)) gradually decrease to ~42 and ~36°C, revealing that the incremental Mo₂C nanoparticles contribute to thermal insulating properties. The relative thermal conductivities of these composites are in the order rGO > Mo₂C/GA-0.5 > Mo₂C/GA-1.0 > Mo₂C/GA-2.0 (Table S2). We further increased the temperature of the heat plate to 200°C, and the detected temperatures (Sp1 point) of Mo₂C/GA-1.0 were 49.9, 64.9, and 66.3°C at 1, 10, and 30 min intervals, respectively (Fig. S6). Although the temperature of the Sp1 point increases slightly at a higher heating temperature, the obtained Mo₂C/GA-1.0 composite still has remarkable thermal insulation performance. The thermal conductivity of Mo₂C/rGO aerogel relies on the conduction of the solid phase, gas phase, and radiative heat transfer, associated with thermal conduction, thermal convection, and thermal radiation, respectively [37,50]. Mo₂C nanoparticle loading with relatively low thermal conductivity can enormously decrease the intensity of thermal conduction in a solid phase, and the randomly distributed pores in aerogel also impede heat transfer by radiation [51–52]. The meritorious thermal insulation and hydrophobicity stimulate the resistance and durability of Mo₂C/rGO aerogel, guaranteeing its multifunction and promising application prospects in harsh environments.

3.4   EM parameters

The EM parameters, relative complex permittivity (ε_r = ${\varepsilon }_{\mathrm{r}}'$ − j${\varepsilon }_{\mathrm{r}}''$) and relative complex permeability (μ_r = ${\mu }_{\mathrm{r}}'$ − j${\mu }_{\mathrm{r}}''$), are extremely important indicators for the EM properties and performance of EWAMs [53–55]. Fig. 4(a) and (b) shows the ${\varepsilon }_{\mathrm{r}}'$ and ${\varepsilon }_{\mathrm{r}}''$ values of rGO aerogel, Mo₂C/GA-0.5, Mo₂C/GA-1.0, and Mo₂C/GA-2.0 in the frequency range of 2.0–18.0 GHz. The pristine rGO aerogel has the highest ${\varepsilon }_{\mathrm{r}}'$ and ${\varepsilon }_{\mathrm{r}}''$ values, as well as typical frequency dispersion in the studied frequency interval, where ${\varepsilon }_{\mathrm{r}}'$ decreases from 35.1 at 2.0 GHz to 17.6 at 15.0 GHz and remains constant in the rest frequency range, and ${\varepsilon }_{\mathrm{r}}''$ sharply declines from 35.0 to 15.3 in the frequency range of 2.0–10.0 GHz and then slowly decreases to 11.2 in the frequency range of 10.0–18.0 GHz. Both Mo₂C/GA-0.5 and Mo₂C/GA-1.0 display a similar downtrend of ${\varepsilon }_{\mathrm{r}}'$ and ${\varepsilon }_{\mathrm{r}}''$, whose ${\varepsilon }_{\mathrm{r}}'$ values decrease from 18.9 and 14.8 at 2.0 GHz to 13.5 and 10.1 at 18.0 GHz, respectively. The ${\varepsilon }_{\mathrm{r}}''$ values of Mo₂C/GA-0.5 and Mo₂C/GA-1.0 exhibit an approximate tendency and confirm a faint variation in the range of 2.0–18.0 GHz, indicating that they may exhibit similar EM attenuation properties. When more Mo₂C nanoparticles are introduced, Mo₂C/GA-2.0 gives inferior ${\varepsilon }_{\mathrm{r}}'$ and ${\varepsilon }_{\mathrm{r}}''$ values and they almost remain unchanged in the range of 2.0–18.0 GHz, revealing unsatisfactory dielectric loss capability. On the whole, the ${\varepsilon }_{\mathrm{r}}'$ and ${\varepsilon }_{\mathrm{r}}''$ values of rGO aerogel, Mo₂C/GA-0.5, Mo₂C/GA-1.0, and Mo₂C/GA-2.0 display coincident frequency dispersion behaviors, which may contribute from the hysteretic reorientation of dipoles in the applied EM field [56–57]. Dielectric loss tangents ($\mathrm{tan}{\delta }_{\mathrm{e}}={\varepsilon }_{\mathrm{r}}''/{\varepsilon }_{\mathrm{r}}'$) are employed to intuitively uncover the dielectric loss capability of these composites [58]. In Fig. 4(c), the $\mathrm{tan}{\delta }_{\mathrm{e}}$ values of rGO aerogel, Mo₂C/GA-0.5, Mo₂C/GA-1.0, and Mo₂C/GA-2.0 show a similar tendency to the ${\varepsilon }_{\mathrm{r}}''$ values, that is, rGO > Mo₂C/GA-0.5 > Mo₂C/GA-1.0 > Mo₂C/GA-2.0 in the studied frequency range. It is well known that dielectric loss derives from conductivity loss and polarization loss [59]. From the Raman spectra, relative graphitization gradually decreases from rGO to Mo₂C/GA-2.0, implying the degradation of conductivity with the introduction of Mo₂C nanoparticles. Based on free-electron theory (${\varepsilon }_{\mathrm{r}}'' \approx \dfrac{\sigma }{\omega {\varepsilon }_{0}}$, where σ is conductivity, ω is angular frequency, and ${\varepsilon }_{0}$ is the dielectric constant of free space), the contribution from conductivity loss will be weakened correspondingly. In terms of polarization loss, interfacial polarization and dipole polarization loss are favored for EM energy attenuation because the flexibility and quickness (10⁻¹² to 10⁻¹⁶ s) of electronic polarization and ionic polarization are unable to effectively convert EM energy [60]. Dipole orientation polarization comes from the hysteretic reorientation of dipoles in an applied EM field, and interfacial polarization mainly depends on the asymmetrical distribution of space charges at heterogeneous interfaces, which can generate electric dipole moment and dissipate EM energy [61]. In our case, introducing Mo₂C nanoparticles decreases the relative graphitization degree of rGO aerogel, and more functional groups and defect sites are produced; thus, dipole orientation polarization may be increased moderately. On the other hand, Mo₂C nanoparticles also enhance interfacial polarization because they create considerable interfaces. The existence of the polarization process can be demonstrated with the semicircle of the ${\varepsilon }_{\mathrm{r}}'$ vs. ${\varepsilon }_{\mathrm{r}}''$ curves shown in the following formula [62]:

Fig. 4.  (a) $\boldsymbol {\varepsilon }_{\mathbf{r}}'$, (b) ${\boldsymbol{\varepsilon }}_{\mathbf{r}}''$, and (c) $\mathbf{tan}{\boldsymbol{\delta }}_{\mathbf{e}}$ values of rGO aerogel, Mo₂C/GA-0.5, Mo₂C/GA-1.0, and Mo₂C/GA-2.0 in the range of 2.0–18.0 GHz.

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where ε_s and ε_∞ are static dielectric constant and dielectric constant at infinite frequency, respectively. Debye’s theory indicates that each polarization process will correspond to a semicircle in the ${\varepsilon }_{\mathrm{r}}'$ vs. ${\varepsilon }_{\mathrm{r}}''$ plot. From Fig. 5, all samples exhibit obvious semicircles, and the number of semicircles gradually increases from rGO to Mo₂C/GA-2.0, demonstrating that more polarization processes are induced with the loading of more Mo₂C nanoparticles. In addition to these semicircles, there is also an apparent straight line in the curves of rGO, Mo₂C/GA-0.5, and Mo₂C/GA-1.0 (Fig. 5(a)–(c)), which is attributed to the contribution from conductivity loss [62–63]. The disappearance of this straight line in Mo₂C/GA-2.0 again verifies the reduction in conductivity loss (Fig. 5(d)). However, it is worth noting that the tanδ_e values do not increase with increasing polarization loss process, suggesting that the accumulative polarization loss in Mo₂C/rGO cannot offset the reduction in conductivity loss. That is to say, conductivity loss plays a dominant role in determining the dielectric losses of these Mo₂C/rGO composites. By the way, all these composites show ${\mu }_{\mathrm{r}}'$ and ${\mu }_{\mathrm{r}}''$ values at approximately 1 and 0 (Fig. S7), respectively, because both Mo₂C and rGO aerogel are non-ferromagnetic and cannot produce the magnetic loss mechanism. Thus, in this case, dielectric loss is mainly responsible for EM energy attenuation.

Fig. 5.  Cole–Cole semicircles (${\boldsymbol{\varepsilon }}_{\mathbf{r}}'$ vs. ${\boldsymbol{\varepsilon }}_{\mathbf{r}}''$) of (a) rGO aerogel, (b) Mo₂C/GA-0.5, (c) Mo₂C/GA-1.0, and (d) Mo₂C/GA-2.0.

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3.5   EM absorption performance

Based on transmission line theory, the reflection loss (RL) values of rGO aerogel, Mo₂C/GA-0.5, Mo₂C/GA-1.0, and Mo₂C/GA-2.0 can be concluded from EM parameters using the following equations [64]:

where Z_in and Z₀ are the input impedance of absorbent and impedance of free space; c, f, and d are the velocity, frequency of EM waves in free space, and the absorber thickness. Fig. 6(a)–(d) reveals the 3D RL maps of rGO aerogel, Mo₂C/GA-0.5, Mo₂C/GA-1.0, and Mo₂C/GA-2.0, where x, y, and z axes represent the frequency (f), thickness (d), and RL values, respectively. In Fig. 6(a), the pristine rGO aerogel with minimum RL (mRL) of not exceeding −10 dB cannot offer effective microwave absorption performance, and the qualified response bandwidth (frequency range when the RL value is less than −10.0 dB) only covers 0.5 GHz (17.5–18.0 GHz) with 3.0 mm thickness. After introducing the Mo₂C nanoparticles, the mRL characteristic of Mo₂C/GA-0.5 has been effectively promoted, where its mRL value is −21.5 dB at 18.0 GHz with 1.3 mm thickness, and the corresponding qualified response bandwidth can reach up to 4.8 GHz (13.2–18.0 GHz) at 1.5 mm thickness (Fig. 6(b)). When Mo₂C nanoparticles continue to increase, Mo₂C/GA-1.0 exhibits enhanced EM attenuation characteristics with an mRL value of −63.3 dB at 7.3 GHz, and the qualified response bandwidth can exceed 5.1 GHz at 1.7 mm thickness (Fig. 6(c)). In addition, the mRL of Mo₂C/GA-1.0 can reach or even be less than −20 dB in the prescribed thickness range of 1.5–5.0 mm. It is regrettable that more Mo₂C nanoparticles in Mo₂C/GA-2.0 induce obvious retrogression of microwave absorption performance, which not only is manifested in a prompt recession in mRL value but also has caused a substantial reduction in the qualified response bandwidth (Fig. 6(d)). In addition to the qualified response bandwidth in specific thickness, the integrated qualified bandwidth in the thickness range between 1.0 and 5.0 mm has been proposed as an important index to highlight the customizable mRL characteristics and qualified bandwidth in the studied frequency range. It can be deduced that the integrated qualified bandwidths of rGO aerogel, Mo₂C/GA-0.5, Mo₂C/GA-1.0, and Mo₂C/GA-2.0 are 1.0 GHz (17.0–18.0 GHz), 14.5 GHz (3.5–18.0 GHz), 14.2 GHz (3.8–18.0 GHz), and 12.8 GHz (5.2–18.0 GHz), respectively. Some recent references demonstrate that qualified response bandwidth and mRL value are crucial parameters for evaluating microwave absorption performance [65–71]. Although the thicknesses corresponding to the maximum qualified response bandwidth of Mo₂C/GA-0.5 are thinner than those of Mo₂C/GA-1.0, Mo₂C/GA-1.0 achieves great progress in terms of broad qualified bandwidth and mRL value, making it a highly attractive candidate for the composite. In Table 1, the property parameters of Mo₂C/GA-1.0 with those of literature-reported rGO-based composites are further compared, and it is exciting that Mo₂C/GA-1.0 displays distinguishable superiorities in broad qualified response bandwidth, low filler loading, and small thickness.

Fig. 6.  3D RL maps of (a) rGO aerogel, (b) Mo₂C/GA-0.5, (c) Mo₂C/GA-1.0, and (d) Mo₂C/GA-2.0 with variable thicknesses in the frequency range of 2.0–18.0 GHz. (e) Attenuation constants of rGO aerogel, Mo₂C/GA-0.5, Mo₂C/GA-1.0, and Mo₂C/GA-2.0.

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Table  1.  Microwave absorption properties of rGO-based composites in the literature and this work

Material	Filler loading / wt%	Thickness / mm	Bandwidth (range) / GHz	Ref.
FeNi3/N-GN	50	1.84	5.1 (10.7–15.8)	[65]
rGO/MnFe2O4	70	1.7	5.2 (12.8–18.0)	[66]
Fe3O4@LAS/rGO	50	2.1	4.0 (10.7–14.7)	[67]
rGO/MWCNTs/NiFe2O4	50	1.6	5.0 (13.0–18.0)	[68]
Fe3O4@SiO2@MnO2/rGO	30	3.2	4.3 (7.1–11.4)	[69]
rGO/ZnxFe3−xO4@MnO2	50	3.1	4.5 (7.0–11.5)	[70]
rGO–Mo–WO3	8	3.5	5.2 (7.8–13.0)	[71]
Mo2C/GA-1.0	9	1.7	5.1 (12.9–18.0)	This work

 | Show Table

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As stated above, the presence of Mo₂C nanoparticles has a negative effect on dielectric loss but a positive effect on mRL values and qualified bandwidth; thus, investigating the relationship between dielectric loss and microwave absorption performance is necessary. It is widely accepted that loss capability and impedance matching comprehensively determine the RL characteristic of EWAMs [72–73]. The attenuation constant (α) is always employed to feature the overall attenuation characteristic, as shown in the following equation [74–75]:

The α values of rGO aerogel, Mo₂C/GA-0.5, Mo₂C/GA-1.0, and Mo₂C/GA-2.0 all give a monotonous increase from 2.0 to 18.0 GHz (Fig. 6(e)), implying their strong attenuation in the high-frequency range with in the order of rGO > Mo₂C/GA-0.5 > Mo₂C/GA-1.0 > Mo₂C/GA-2.0 at any specific frequency point, which is highly consistent with that of $\mathrm{tan}{\delta }_{\mathrm{e}}$ values (Fig. 4(c)). This similar order again indicates the dominant role and the main pathway of conductivity loss in dielectric loss and EM energy attenuation, respectively. Nevertheless, the highest α value cannot have the best microwave absorption performance. This phenomenon can be described with impedance matching, which represents the gap of characteristic impedance between free space and microwave absorption medium [18,76]. A satisfactory impedance matching requires a small gap so that most incident EM waves can penetrate the surface of microwave absorption materials rather than being reflected on their boundary [61]. The delta function ($\left|\varDelta \right|$) has been widely employed to identify the matching degree of characteristic impedance and can be estimated using the following equations [28]:

where K and M are related to ${\varepsilon }_{\mathrm{r}}'$ and ${\mu }_{\mathrm{r}}'$, and their relationships are revealed by the following equations:

where ${\delta }_{\mathrm{m}}$ is the magnetic loss factor. Fig. 7 illustrates the calculated impedance matching maps of rGO aerogel, Mo₂C/GA-0.5, Mo₂C/GA-1.0, and Mo₂C/GA-2.0. According to Eqs. (6)–(8), $\left|\varDelta \right|$ close to zero means complete impedance matching, while $\left|\varDelta \right|$ < 0.4 means competent impedance matching [29,45]. The area coverage of $\left|\varDelta \right|$ < 0.4 for rGO, Mo₂C/GA-0.5, Mo₂C/GA-1.0, and Mo₂C/GA-2.0 is 5.2%, 42.1%, 63.6%, and 26.1%, respectively. Hence, the well-pleasing microwave absorption property of Mo₂C/GA-1.0 is contributed by its decent impedance matching and dielectric loss.

Fig. 7.  Calculated $\left|\boldsymbol{\varDelta }\right|$ values of (a) rGO aerogel, (b) Mo₂C/GA-0.5, (c) Mo₂C/GA-1.0, and (d) Mo₂C/GA-2.0 in the frequency range of 2.0–18.0 GHz with various coating thicknesses of 1.0–5.0 mm.

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Fig. 8 illustrates the EM attenuation mechanism in the Mo₂C/GA aerogel. When incident EM waves encounter the Mo₂C/GA aerogel coating attached to a metal plate, most of them can penetrate the surface of the Mo₂C/GA aerogel due to its good impedance matching and undergo a series of attenuation and loss processes inside the Mo₂C/GA aerogel. On the one hand, the hierarchical 3D porous network structure in the aerogel effectively extends the propagation route of incident EM waves and results in diffuse scattering and repeated energy dissipation, which is quite beneficial to the attenuation of EM energy. On the other hand, the cellular structure also provides sufficient surfaces to adhere to Mo₂C nanoparticles, which will generate abundant interfacial polarization. At the same time, residual groups and defects in the Mo₂C/rGO aerogel can induce satisfying dipole polarization loss for auxiliary energy loss.

Fig. 8.  Schematic illustration of the microwave absorption mechanisms in Mo₂C/GA with a plate metal backing.

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4.   Conclusion

In this work, a multifunctional Mo₂C/rGO aerogel has been rationally fabricated by a hydrothermal process followed by freeze-drying and high-temperature pyrolysis. The presence of Mo₂C nanoparticles brings significant improvements in the hydrophobicity and thermal insulation of the rGO aerogel. More importantly, Mo₂C nanoparticles have considerable effects on the EM properties of the rGO aerogel, where both the real and imaginary parts of relative complex permittivity gradually decrease with increasing Mo₂C nanoparticle loading. Although the decrease in relative complex permittivity results in the recession of dielectric loss, it generates better impedance matching at the incident interface of EM waves. Tailoring the Mo₂C nanoparticle loading, a good balance between attenuation ability and impedance matching may produce considerable microwave absorption performance, which is actually superior to those of rGO aerogel-based composites in the literature. The multifunctional characteristics of the Mo₂C/rGO aerogel herein inspire possible application in harsh conditions, and this work hopes to provide insight into fabricating multifunctional materials.

Acknowledgements

This work was financially supported by the China Postdoctoral Science Foundation (No. 2021MD703944), the Fund of Science and Technology on Near-Surface Detection Laboratory (No. 6142414211808), and the Director Fund of State Key Laboratory of Pulsed Power Laser Technology (No. SKL2021ZR06), and the National Natural Science Foundation of China (No. 21776053).

Conflict of Interest

The authors declare no conflict of interest.

Supplementary Information

The online version contains supplementary material available at https://doi.org/10.1007/s12613-022-2570-9.