1 Introduction

With the flourishing development of wireless communication, particularly fifth-generation (5G) communication technology, serious concerns about electromagnetic radiation, electromagnetic interference, and electromagnetic pollution have arisen due to rampant electromagnetic wave (EMW) signals [1,2,3]. Microwave absorption materials (MAMs) with exceptional abilities to absorb and dissipate electromagnetic energy have non-negligible potential for addressing electromagnetic hazards [4,5,6]. The EMW attenuation performance characteristics of MAMs are closely associated with the dielectric/magnetic loss parameters (complex permittivity and permeability) and morphologies. Hence, constructing MAMs with elaborately optimized dielectric/magnetic loss properties and absorption-promoted morphology has been considered an effective EMW absorber design criterion [7,8,9].

To obtain MAMs with excellent dielectric/magnetic loss characteristics, researchers have made great efforts to construct multicomponent nanocomposites [4, 10,11,12]. This effort is because the EMW absorption properties of nanocomposites can be significantly optimized by integrating additional dielectric/magnetic components, thereby introducing many new attenuation mechanisms. For instance, Gao et al. [13] reported that Cu-intercalated MoS2 with carbon modification can achieve synergistic multiple polarization. Su et al. [14] demonstrated a ternary-alloy FeCo2Ni/carbon composite, by which EMW absorption can be enhanced through the integration of magnetic–dielectric complementary attenuation capabilities. Li et al. [15] fabricated NiO/Ni particles on N-doped hollow carbon spheres with improved conduction loss and polarization loss. ** and electron transport, which can be effectively modulated through particle size regulation, do** design, and defect engineering to tune the polarization loss characteristics [5i). The poor impedance matching of wc-NbC/CNS may be attributed to the large amount of conductive NbC in the 2D carbon skeleton, which leads to excessively high permittivity parameters. Moreover, compared with NbC conductors, Nb2O5 semiconductors with reduced charge transfer ability slow the electrical neutralization of the dipole under an alternating electromagnetic field and hence reinforce dipole polarization relaxation. As a result, ov-Nb2O5/CNS with plenty of well-crystallized Nb2O5 semiconductors can achieve superior dielectric loss.

Nb2O5 and NbC with different dielectric behaviors can impact the electromagnetic responses of the composites. Off-axis electron holography is conducted to investigate the interaction of ov-Nb2O5/CNS and wc-NbC/CNS with the electromagnetic field (Fig. 5j–m). As shown in Fig. 5k, high-density stray field flux lines robustly penetrate from the inside of the ov-Nb2O5/CN nanosheet and radiate to the outside. This result confirms the significant interaction between the ov-Nb2O5/CNS composite and electromagnetic field. However, no obvious stray field flux lines are triggered to be released from wc-NbC/CNS (Fig. 5m). It can be speculated that the Nb2O5 nanoparticles produce plentiful dielectric polarization and activate a strong induced electromagnetic field to influence the EMW. Moreover, the divergence of stray field flux lines in ov-Nb2O5/CNS builds a multidimensional electromagnetic response network and effectively reinforces the attenuation of electromagnetic energy.

Moreover, the incorporation of Nb2O5 and NbC nanoparticles into the carbon nanosheets generates multiple heterointerfaces. Interfacial charge aggregation occurs due to the disrupted original electric balance at these heterointerfaces, which typically leads to interfacial polarization. Notably, Nb2O5 and NbC with different electronic properties can affect the charge aggregation at Nb2O5–carbon and NbC–carbon interfaces, respectively. Therefore, the charge density distributions of the Nb2O5–carbon and NbC–carbon configurations are investigated to theoretically reveal the interfacial polarization induced by Nb-based nanoparticles (Figs. 6a–f and S11). As depicted in Fig. 6a, b, d, e, charges are unevenly distributed at these heterointerfaces, where irregular yellow and blue regions correspond to the aggregation and dispersion of electrons, respectively. It is apparent that the electrons generally delocalize from the carbon and flow into Nb2O5 at the Nb2O5–carbon heterointerface, while the delocalized electrons flow from the carbon and NbC to the intermediate region at the NbC–carbon interface. These results are supported by the planar average potential of the Nb2O5–carbon and NbC–carbon interfaces (Fig. 6c, f). Moreover, the Nb2O5–carbon heterointerface demonstrates a pronounced charge separation effect, which consequently enhances the interfacial polarization capability. The ov-Nb2O5/CNS with abundant well-crystallized Nb2O5 nanoparticles possesses numerous Nb2O5–carbon heterointerfaces. Consequently, intensified electromagnetic energy dissipation through strengthened interfacial polarization loss can be achieved by ov-Nb2O5/CNS.

Fig. 6
figure 6

Charge density distribution of the a–b Nb2O5–carbon configuration and d–e NbC–carbon configuration. Planar average electrostatic potential of c Nb2O5–carbon and f NbC–carbon. g Charge density contour of perfect Nb2O5. h Charge density distribution of ov-Nb2O5 (yellow and blue regions represent aggregation and dispersion of electrons). i-k Micro-CT images of the ov-Nb2O5/CNS-paraffin absorber by 3D reconstruction (dark blue and gray regions refer to the ov-Nb2O5/CNS and paraffin wax phase, respectively). l Schematic illustrations of the multiple scattering and conductive loss mechanism of the 2D lamellar ov-Nb2O5/CNS. (Color figure online)

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Complementary to excellent interfacial polarization loss, the presence of abundant oxygen vacancy defects in Nb2O5 nanoparticles can contribute to the excellent electric dipole polarization in ov-Nb2O5/CNS. The charge distributions of perfect Nb2O5 and oxygen vacancy Nb2O5 configurations are estimated by first-principles calculations to better understand the intensification of electric dipole polarization for ov-Nb2O5/CNS. Figure S12 presents the configurations of perfect Nb2O5 and oxygen vacancy Nb2O5 (ov-Nb2O5). The charge distribution in perfect Nb2O5 is relatively uniformly distributed (Fig. 6g). However, with the presence of two oxygen vacancies, electrons delocalize at the vacancy sites and flow into nearby oxygen atoms, resulting in charge separation (Fig. 6h). Subsequently, electric dipoles are generated in ov-Nb2O5, which can induce the formation of electronic dipole polarization oscillation in an external electromagnetic field. Therefore, Nb2O5 nanoparticles with abundant oxygen vacancies can act as electronic dipole polarization oscillation units, thereby efficiently enhancing the dielectric loss capability of ov-Nb2O5/CNS.

To gain a comprehensive understanding of the dielectric loss mechanisms, Cole–Cole plots based on Debye theory are further investigated. As depicted in Fig. S13, the rich polarization relaxation processes for the ov-Nb2O5/CNS are suggested by a more perfect Cole–Cole semicircle with a large diameter. In contrast, the highly distorted Cole–Cole trajectories observed for other composites suggest a weakened Debye dipolar relaxation.

Furthermore, the 2D lamellar morphology of ov-Nb2O5/CNS is beneficial for establishing abundant nanosheet–paraffin interfaces, which can strengthen the multiple reflection and scattering dissipation characteristics. To enable 3D reconstruction of the phase diagram, the ov-Nb2O5/CNS-paraffin absorber is scanned using a microcomputer tomography (micro-CT) device, as shown in Fig. 6i–j. Image slicing techniques are employed to acquire the nondestructive ov-Nb2O5/CNS phase (blue phase) and paraffin phase (transparent gray phase). The ov-Nb2O5/CNS nanosheets stack in the absorber and form numerous voids, and these voids are filled with paraffin wax. This unique structure can produce multiple macro-heterointerfaces to enhance interfacial polarization loss and prolong the reflection and scattering path of the EMW to enhance attenuation. Additionally, upon removing the paraffin phase, an interconnected 3D conductive ov-Nb2O5/CNS network is observed (Fig. 6k), which can realize intensive conductivity loss. Generally, these results suggest that the ov-Nb2O5/CNS in the absorber can efficiently strengthen the attenuation of EMW by intensifying interfacial polarization loss, multiple reflections, scattering dissipation, and conductivity loss.

3.4 Possible Application Prospects

In practical applications, EMW absorbing materials are mainly used as coatings or plates. To verify the application potential of ov-Nb2O5/CNS, the composite is mixed with EMW-transmitting cyanate ester to form a mixed resin. The functional resin can successfully cure into a rectangular plate (80 mm × 40 mm × 2 mm), as shown in Fig. 7a. Moreover, the ov-Nb2O5/CNS-cyanate plate demonstrates excellent machinability, allowing it to be precisely cut to a size of 22.9 mm × 10.2 mm × 2 mm to investigate the microwave absorption performance using a waveguide method (Fig. S14). As shown in Figs. 7b–c and S15, the ov-Nb2O5/CNS-cyanate plate achieves a satisfying RLmin value of − 19.8 dB at 12.38 GHz with a thickness of 1.88 mm, and the EAB reaches 2.41 GHz with a thickness of 2.10 mm. Since the absorption mechanism of EMW absorbers is dominated by transforming electromagnetic energy into thermal energy, the heat dissipation capacity is an imperative factor in practical applications. When the ov-Nb2O5/CNS-cyanate plate is placed on a heating platform (160 °C), the temperature of the plate increases rapidly to 90.2 °C (56.4% of 160 °C) within 10 s and reaches a stable temperature of approximately 150 °C (93.8% of 160 °C) after 180 s (Figs. 7d–f and S16). Overall, ov-Nb2O5/CNS-800 demonstrates excellent application potential by curing into an excellent microwave-absorbing, machinable, and heat-dissipating ov-Nb2O5/CNS-cyanate plate.

Fig. 7
figure 7

a Digital photographs of the ov-Nb2O5/CNS-cyanate plate. b Three-dimensional reflection loss diagrams and c reflection loss contour maps of the ov-Nb2O5/CNS-cyanate plate. d–f Thermal infrared images of the ov-Nb2O5/CNS-cyanate plate on a heating platform (160 °C)

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4 Conclusions

In conclusion, we demonstrate the successful synthesis of ultrafine (~ 10 nm) Nb2O5 nano-semiconductors with rich oxygen vacancies in carbon nanosheets (ov-Nb2O5/CNS) for achieving high-attenuation electromagnetic wave absorption. Semiconductive Nb2O5 nanoparticles endow ov-Nb2O5/CNS with more strengthened charge polarization at the Nb2O5–carbon hetero-interface than NbC conductors, which facilitates interfacial polarization loss. Additionally, Nb2O5 nanocrystals with abundant oxygen vacancies reinforce electric dipole polarization inside the semiconductor. The 2D lamellar morphology of ov-Nb2O5/CNS strengthens the multiple reflection and scattering dissipation characteristics in the absorbers. Therefore, when integrated with outstanding polarization relaxation, intensified electromagnetic response, and excellent impedance matching, ov-Nb2O5/CNS achieves a superior EMW absorption capability with an unparalleled RLmin of − 80.8 dB (> 99.999999% wave absorption) at 7.11 GHz (2.76 mm); additionally, it exhibits a wide effective absorption bandwidth of 3.37 GHz at 1.30 mm. Moreover, the composite shows excellent application potential by curing into a microwave-absorbing, machinable, and heat-dissipating ov-Nb2O5/CNS-cyanate plate. The ov-Nb2O5/CNS-cyanate plate achieves a satisfactory RLmin value of − 19.8 dB at 12.38 GHz with a thickness of 1.88 mm, and the EAB reaches 2.41 GHz with a thickness of 2.10 mm. Our findings provide novel insights into the design and development of dielectric semiconductor-based carbon composites for electromagnetic wave absorption and related applications.