Electrostatic-spinning construction of HCNTs@Ti₃C₂T _x MXenes hybrid aerogel microspheres for tunable microwave absorption

2.1 Preparation of HCNTs and Ti₃C₂T _x MXenes

The original HCNTs (O-HCNTs) were prepared in our laboratory through chemical vapor deposition [28,29]. Before use, the O-HCNTs were annealed (denoted as A-HCNTs) and washed (denoted as H-HCNTs) to remove amorphous carbon and excess precursors. The F-HCNTs were prepared by immersing the H-HCNTs in a mixed acid (HNO₃/H₂SO₄ = 1/3 by volume) at various set times (0.5, 1, 3, and 6 h). The F-HCNTs modified under different times are denoted as F-HCNTs-X h. Ti₃C₂T _x MXenes were prepared based on the method described in the study of Alhabeb et al. [30].

2.2 Preparation of HCNTs@Ti₃C₂T _x MXenes aerogel microspheres

Totally, 18 mg of HCNTs and 8.4 mL of water were added to 1.6 mL of the Ti₃C₂T _x MXenes dispersion (5 mg·mL⁻¹) and stirred for 5 min. The mixture was then placed in a polypropylene syringe, and the spinning process was started immediately at a voltage of approximately 5 kV under an injection rate of approximately 0.08 mL·min⁻¹. The ice in the frozen droplets was sublimed in a vacuum freeze dryer for 2 days, and hybrid aerogel microspheres were obtained. These aerogels were stored in a sealed vial at −20℃ to avoid oxidization of MXenes.

2.3 Characterization

Scanning electron microscopy (SEM; JEOL JSM-7800F) and transmission electron microscopy (TEM; JEOL JEM-2100) were used to characterize the morphologies. Raman spectra were recorded using a Renishaw InVia Reflex instrument at an excitation wavelength of 532 nm to explore the structure of HCNTs and MXenes. Fourier transform infrared spectroscopy (FT-IR) (Bruker Tensor II) was performed to investigate the structural change of HCNTs after surface treatments. The pore size distribution and specific surface areas were obtained by using conventional nitrogen physisorption measurements (Gold APP V-Sorb 2800P) based on Barrett–Joyner–Halenda (BJH) and Brunauer–Emmett–Teller (BET) methods. The electromagnetic parameters were collected by the coaxial-line method. A vector network analyzer (VNA, Agilent 15071C) was used, and the testing range is 2–18 GHz. The samples were prepared by the same method as the previous work [10].

3.1 Fabrication of HCNTs@MXene

Figure 1a shows the structural evolution of HCNTs and the preparation of HCNTs@MXenes. To form a 3D structure, HCNTs were expected to react with MXenes; thus, a mixed acid was introduced to activate the surfaces [31]. Surface characteristics are crucial not only for constructing 3D structures but also for tuning the MA performance. Of the original HCNTs, the O-HCNTs presented a mirror-symmetrical morphology with unilateral nanotubes in a kink-like helical shape, which has previously been demonstrated to be favorable to the absorption of microwaves (Figure S1a) [32]. When treated with mixed acid, the morphologies, structures, and properties of HCNTs exhibit remarkable changes [33]. This enables better dispersion in water and cooperation with MXenes due to the grafting of oxygen-containing groups. The MXenes used in this study is a type of large-scale 2D nanoflake (Figure S1b) whose surface contains abundant hydroxyl groups, thus offering a large area to hold HCNTs and form 3D architectures. During mixing, HCNTs and MXenes nanoflakes are homogeneously dispersed in water. During electrospinning, the combined force of electric and gravity fields causes the spinning solution to form a Taylor cone at the needle tip. When the voltage is high enough, this Taylor cone breaks away from the needle tip and is immediately frozen in a liquid nitrogen bath [18]. The particles in the spinning solution containing F-HCNTs and MXenes nanoflakes are concentrated on the ice grain boundaries and squeezed to form 3D networks [34]. Because the temperature gradient is from the surface to the center of the droplet, ice grows directionally, and therefore the spatial distribution of MXenes as determined by the grain boundary is radial [35]. In our study, after the ice was sublimated in a vacuum freeze dryer, aerogel microspheres with directional pores were obtained (Figure S2). The specific surface areas were calculated from N₂ adsorption using the BET method (Figure S3). A 3D network structure of aerogel microspheres consisting of macropores was constructed by HCNTs@MXenes junctions. In addition, the MXenes layers are stacked on each other during the dispersion and freeze-drying process. This method hardly forms micropores and mesopores, as confirmed by the BJH pore size distribution. Therefore, the specific surface area measured through N₂ adsorption is relatively low. The specific surface area of pure MXene aerogel microsphere is 137.6 m²·g⁻¹, which got remarkable improvement after adding F-HCNTs, such as the specific surface area of F-HCNTs-0.5 h@MXenes is 252.6 m²·g⁻¹. However, the specific surface area showed a decreasing trend as extending the treating time of F-HCNTs. For F-HCNTs-1.0 h@MXenes, the specific surface area is 248.1 m²·g⁻¹, F-HCNTs-3.0 h@MXenes is 246.9 m²·g^−1, and F-HCNTs-6.0 h@MXenes is 208.3 m²·g⁻¹, indicating that the treatment of mixing acid on HCNTs was favorable to increasing porosity. As mentioned in previous work [33], the morphologies and structures of HCNTs would be destroyed after excessive treatment by mixing acid, resulting in shortened and cut-open nanotubes, which, therefore, causes a decrease in the specific surface area [36].

Figure 1

Schematic of preparing HCNTs@Ti₃C₂T _x MXenes hybrid aerogel microspheres.

The obtained aerogel microspheres are presented in Figure 2a–c. Because of the balance force between electric and gravitational fields, the diameters of microdroplets are approximately 800 μm after the droplets exit the needle tip. In addition, the droplets are round, which may be due to comprehensive factors such as uneven liquid nitrogen levels and the vaporization microenvironment [18]. After sublimation, air replaces the ice crystals, and the remaining droplets are porous. The structure of a 3D porous network is significantly affected by the particle properties, which cause entrapment or repulsion by different interaction forces between the particles and solidification fronts [37]. The MXenes flakes are hydrophilic and thus tend to become entrapped by solidification fronts, resulting in curved cell walls inside the HCNTs@MXenes aerogels (Figure 2a′–c′). The oxidation of HCNTs renders them more hydrophilic, which is beneficial in avoiding self-aggregation and binding to MXenes through hydrogen bonding. As the enlarged views show, the HCNTs became more uniformly dispersed on the MXenes flakes as the treatment time increased. However, the aggregation of HCNTs could not be completely avoided. As Figure 2a′–c′ shows, HCNT aggregates dispersed on the MXenes skeleton instead of on single tubes. However, the aggregates were dispersed more evenly when the HCNTs were treated for a longer time, which in turn improved the dispersion of the F-HCNTs in water and their interaction with MXenes. The interactions between the HCNTs and MXenes were confirmed by the Raman spectra, where the characteristic peaks of the graphite structure (D and G bands) at 1,349 and 1,586 cm⁻¹ [38] and peaks of Ti₃C₂T _x MXenes at approximately 200 and 400 cm⁻¹, respectively, could be found [39] (Figure 2d). More importantly, after integration, the widths of the D and G bands broadened, and the I _D/I _G ratio increased from 1.36 to 2.01, which was much higher than the value after modification by acid mixing [33]. The higher I _D/I _G ratio for the F-HCNTs@MXenes was attributed to the introduction of MXenes, which interacted with the F-HCNTs and caused an increase in disordered carbon in these samples [40]. An improved I _D/I _G ratio implied the formation of more defects, which can serve as polarization centers and improve polarization loss. Figure 2e shows the FT-IR spectra of the F-HCNTs@MXenes. The peak at approximately 1,725 cm⁻¹ represents the vibration of C═O in –COOH and demonstrates the successful modification of F-HCNTs. More importantly, the shift of the C–OH vibration bonds from 3,436 to 3,428 cm⁻¹ shows the hydrogen bond interactions among F-HCNTs and MXenes [41].

Figure 2

Morphological and structural analysis of HCNTs@MXenes hybrid aerogels. SEM images of (a) A-HCNTs@MXenes, (b) F-HCNTs-1 h@MXenes, and (c) F-HCNTs-6 h@MXenes. (a′)–(c′) provide details of (a)–(c) under a high magnification, respectively. (d) Raman spectra and (e) FT-IR spectra of HCNTs@MXenes.

3.2 MA performance

The MA performance of F-HCNTs@MXenes was studied based on the transmission line theory by evaluating the experimental data, which includes the complex permittivity (ε = ε′ − jε″) and permeability (μ = μ′ − jμ″). Electromagnetic parameters (2–18 GHz) were collected by the coaxial-line method on VNA. The real and image parts of ε(ε′) and μ(μ′) and their loss tangents (calculated by Eqs. (1) and (2)) of F-HCNTs@MXenes wax composites with different treatment times are shown in Figure 3. Here, ε′ represents the dielectric and polarization properties of materials. In addition, the increase of ε′ indicates that the material is more susceptible to polarization in the magnetic/electric fields. ε′′ represents the dielectric loss ability of materials. In Figure 3a, ε′ tends to decrease with increasing frequency, which can be explained by the hysteresis increase of polarization relative to electric-field changes at higher frequencies [42,43]. It is noteworthy that strong resonance peaks of F-HCNTs-0.5 h@MXenes exist over the entire testing frequency range, particularly in the low-frequency band. However, the curves of the other F-HCNTs@MXenes are relatively flat. In addition, the variation law of the ε″ shows a similar rising trend to ε′ in the testing frequency range, as shown in Figure 3b. Obviously, multiple resonance peaks exist at 2–18 GHz in the ε″ curves. This phenomenon can be explained in two respects. First, interfacial polarization relaxation occurred at the heterointerfaces and junction regions between the HCNTs and Ti₃C₂T _x MXenes. On the other hand, the microwave dissipation ability of absorbers was unstable, resulting in severe fluctuation of the ε″ curves [44,45]. In addition, ε′ of F-HCNTs@MXenes increased with an increase in treatment time of the F-HCNTs from 1 to 6 h, which was contrary to the results of the F-HCNTs. The enhanced dielectric and polarization properties of F-HCNTs@MXenes may have benefitted from the improved dispersion of F-HCNTs on MXenes, which also contributed to ε″, indicating that an increase in the treatment time was beneficial to electromagnetic wave loss. The contribution of conductive and polarization loss to dielectric loss can be analyzed and calculated through Debye's theory. As shown in Eq. (3), ε″ can be classified as ε p ″ which is induced by relaxation and ε c ″ which is generated from conductivity, where τ is the relaxation time, σ is the conductivity, and ω is the angular frequency. By fitting with a nonlinear square fitting mode, the contribution of polarization loss and conductive loss can be obtained (see details in the Appendix) [18]. As Figure S4 shows, ε c ″ was generally higher than ε p ″ , which can be ascribed to the formation of conductive networks [46]. Thus, ε c ″ plays a key role in dielectric loss. Lightly modified F-HCNTs-0.5 h@MXenes and well-dispersed F-HCNTs-6 h@MXenes showed higher ε c ″ than the others, which was consistent with the previous results.

Figure 3

Measured frequency dependence of (a) ε′, (b) ε″, (c) tan(ε′/ε″), (d) μ′, (e) μ″, and (f) tan(μ′/μ″) of F-HCNTs@MXenes.

The tan δ of hybrids is displayed in Figure 3c, demonstrating an obvious dependence on treatment time and implying a remarkable improvement in surface modification and dispersion of F-HCNTs on MXenes to transform electromagnetic energy to other forms [47]. Figure 3d shows a decreasing trend of the μ, and the F-HCNTs-0.5 h@MXenes exhibited the highest capacity in terms of storing and converting electromagnetic energy and had many response peaks in the whole testing range (Figure 3e–f). The magnetic loss capability originated from the catalysts inside the HCNTs. Note that previous studies have shown that ferrite used for catalysis can attenuate electromagnetic energy [5,28]. However, after prolonged treatment with mixed acid, a portion of the catalysis was etched, weakening magnetic loss capability.

Reflection loss (RL) values that directly reflect the MA performance were calculated using Eqs. (4) and (5), where Z ₀ is the impedance of free space, Z _in is the impedance of input-characteristic, μ _r and ε _r are the measured complex permeability and permittivity, respectively, d is the thickness of the tested sample, f is the microwave frequency, and c is the speed of light, respectively

As Figure 4a–b shows, the RL of pristine HCNTs and MXenes is less than −10.0 dB that is the basic requirement of practical use. After integrating them into aerogels, the MA performance can be significantly enhanced. It reached −31.8 dB at 9.7 GHz for the minimal RL (RL_min) of F-HCNTs-0.5 h@MXenes with a thickness of 0.7 mm (Figure 4c), and the high MA performance was maintained with increasing thickness. Interestingly, the effective RL shifted to a low-frequency band when the thickness was greater than 3.5 mm, which is desirable for modern MA materials [48]. When the surface treatment time increased to 1 h (Figure 4d), the MA performance decreased slightly. However, as the treatment time increased, the MA capacity shifted to a high-frequency band. For F-HCNTs-3.0 h@MXenes, an RL_min of −35.5 dB at 10.6 GHz was reached with a thickness of 2.2 mm, and the effective bandwidth was 2.2 GHz (9.7–11.9 GHz). For F-HCNTs-6.0 h@MXenes, the RL_min reached −26.4 dB covering 13.0–16.0 GHz with a thickness of 1.2 mm.

Figure 4

Calculated RL: (a) H-HCNTs, (b) Ti₃C₂T _x MXenes, (c) F-HCNTs-0.5 h@MXenes, (d) F-HCNTs-1.0 h@MXenes, (e) F-HCNTs-3.0 h@MXenes, and (f) F-HCNTs-6.0 h@MXenes.

To explain the difference in MA of the F-HCNTs@MXenes, the corresponding parameters were analyzed. The dielectric loss mainly comes from dipole, ion, and electron polarization, where electron and ion polarization were relatively weaker in the microwave range. As mentioned before, the main role of dielectric loss is electric dipole polarization, whose relaxation process can be expressed by Debye dipole relaxation, which is the Cole semicircle:

where ε _∞ and ε _s are the dielectric constant at the infinite frequency and static dielectric constant, respectively. Thus, ε′ and ε″ can be described by

Then, the relationship between ε′ and ε″ can be derived from [49]:

As shown in Eq. (9), the relationship between ε′ and ε″ is similar to the equation of circles in the first quadrant, in which each semicircle represents a Debye relaxation process. Figure 5a depicts the semicircles for F-HCNTs@MXenes, indicating the existence of multiple dielectric relaxations. In addition, the radius of semicircles increased as the treatment time increased, which may have derived from an increase in the number of contact interfaces, defects, surface groups, and the amount of conductive loss. The relatively high value of F-HCNTs-0.5 h@MXenes may have derived from the residual ferrite. Conductivity is also a critical factor affecting the dielectric loss of hybrids, and the existence of a tail in the Cole–Cole curves confirmed the increase in conductive loss derived from the highly conductive Ti₃C₂T _x MXenes. The semicircles indicate the relaxation processes derived from the interfaces between the HCNTs and Ti₃C₂T _x MXenes flakes. Approximately six types of relaxation processes occurred in the hybrids, where a detailed explanation of each semicircle is provided as follows. Only when the material exhibits impedance matching and loss characteristics excellent MA performance can be achieved. To explain the excellent MA performance of F-HCNTs-0.5 h@MXenes, the loss characteristics (attenuation constant α) using Eq. (10) (Figure 5b) were first evaluated, where α showed the same trend as the electric constant, implying that the balance between the residual catalysts and surface groups of HCNTs when adjusted by the treatment time was vital to the microwave loss capability. A more efficient loss characterization may have resulted from the conductive loss. Second, the impedance matching degree (delta Δ) using Eq. (11) was calculated, where M and K can be obtained using Eqs. (12) and (13). An excellent impedance matching degree of MA absorbers should be equal to or close to the impedance matching degree of the free-space interfaces. An excessive impedance matching degree causes reflection, and low impedance matching degree causes transmission of most electromagnetic waves, where a condition of Δ < 0.2 is acceptable [50]. As Figure 5c–f shows, F-HCNTs-1.0 h had the largest area, indicating a superior impedance matching degree, whereas the MA performance was poor due to its low attenuation capability (Figure 5b). The impedance matching degree agreed with the aforementioned RL results, and more importantly, the high impedance matching degree in the S band was the critical factor that endowed the F-HCNTs@MXenes with excellent MA performance in the low-frequency band

Figure 5

(a) Cole–Cole curves and (b) attenuation factors of F-HCNTs@MXenes, (c)–(f) impedance matching degree of F-HCNTs-0.5 h@MXenes, F-HCNTs-1.0 h@MXenes, F-HCNTs-3.0 h@MXenes and F-HCNTs-6.0 h@MXenes, respectively.

In summary, the controllable structural design promotes the controllable MA performance of F-HCNTs@MXenes, which is derived from the impedance matching and attenuation constant that are optimized in the process of structural regulation. The surface groups, defects, and conductive loss of both Ti₃C₂T _x MXenes and HCNTs and interfacial polarization between the two components and themselves attribute to the dielectric loss. In addition, the surface modification of HCNTs affected the dielectric loss capability of F-HCNTs@MXenes, which was mainly caused by competition between the residual catalysts, conductive loss, and polarization loss. According to the aforementioned results, a possible MA mechanism for the F-HCNTs@MXenes hybrids is shown in Figure 6: (1) the polarization at interfaces derived from the moving velocities of different induced charges on Ti₃C₂T _x MXenes and HCNTs; (2) the polarization loss resulting from the dipoles of heterointerfaces, defects, amorphous carbon, TiO₂, and surface groups; (3) the conductive loss derived from the HCNTs and Ti₃C₂T _x MXenes flakes; and (4) the pores that optimize the impedance matching and elongate the multiple reflection and scattering.

Figure 6

Schematic of the potential MA mechanisms of HCNTs@MXenes.