Interlayer Electrostatic Interaction Boosts Microwave Attenuation in Bi₂O₂Se Nanosheets

Abstract

Layered nanosheets play a role in promoting electromagnetic attenuation in the field of microwave absorption (MA). Compared to interlayer van der Waals force, nanosheets with intrinsic electrostatic interaction are promising to optimize the microwave dissipation between layers. Bi₂O₂Se consists of alternating [Bi₂O₂]²⁺ and [Se]^2– layers coupled through intrinsic electrostatic interactions, which may provide additional polarization sites and interlayer charge-transfer pathways. However, the MA performance of these materials has not been reported yet, since their exfoliation behavior and intrinsic polarization mechanisms remain to be explored. Here, Bi₂O₂Se nanosheets were prepared by tip ultrasonication, and their MA behavior was investigated. The exfoliated Bi₂O₂Se nanosheets which show a minimum reflection loss (RL_min) of −36.99 dB at 10.86 GHz with a bandwidth of 1.7 GHz, outperforming their unexfoliated counterparts and traditional absorbers governed by van der Waals stacking. This work reveals the dielectric properties and MA performance of electrostatically stacked nanosheets, but also offers a new insight for designing highly-efficient electromagnetic compatibility regimes.

Introduction

With the rapid development of new-generation communication technology, the internet of things, and artificial intelligence, electromagnetic interference generated by consumer electronic devices has posed potential risks to the operation of precision equipment and raised concerns regarding electromagnetic safety. − Therefore, microwave absorbing materials are important for electromagnetic compatibility because they can convert incident electromagnetic energy into heat through dielectric, magnetic, or coupled loss processes. , Notably, as electromagnetic environments become increasingly complex, precise spectrum management has become essential. In particular, frequency-selective microwave absorbers exhibit unique advantages in scenarios such as autonomous driving, radar conformal design, and anti-interference communications, where specific frequency bands (e.g., X-band) must be absorbed while ensuring the transmission of other useful signals. Layered nanosheets, such as graphene derivatives, MoS₂, MXenes, and g-C₃N₄, have emerged as highly promising components for microwave absorbers due to their large specific surface areas, abundant heterogeneous interfaces, and tunable dielectric properties. − However, achieving strong and targeted microwave attenuation in single-component two-dimensional (2D) materials remains a formidable challenge. To enhance their performance, existing 2D absorbers usually require complex compounding with magnetic metals, which inevitably increases synthesis complexity and absorber density.

The fundamental reason behind the absorption bottleneck in single-component 2D materials lies in their structural nature: currently reported conventional 2D materials primarily rely on weak van der Waals interlayer coupling. This severely restricts interlayer charge transport and intrinsic polarization response. To break this limitation, exploring novel 2D materials with strong interlayer interactions is highly desirable. In striking contrast, Bi₂O₂Se is a structurally unique material composed of alternating [Bi₂O₂]²⁺ and [Se]^2– layers. Unlike typical van der Waals materials, adjacent layers in Bi₂O₂Se are connected by relatively strong electrostatic interactions. This bonding motif can increase local charge separation, enhance interfacial polarization, and facilitate interlayer carrier transport, all of which are closely related to microwave attenuation. , Furthermore, the high carrier mobility of Bi₂O₂Se provides excellent conditions for conduction loss. These characteristics suggest that Bi₂O₂Se could be a promising high-performance single-component dielectric absorber. Nevertheless, its MA properties and the corresponding loss mechanisms have not yet been systematically elucidated.

In this work, we report for the first time the MA performance of layered Bi₂O₂Se nanosheets featuring intrinsic interlayer ionic bonds as a competitive single-component absorber. Unlike previously reported materials, we achieved the scalable preparation and effective cleavage of Bi₂O₂Se nanosheets through tip ultrasonication. Structurally and compositionally, while retaining the majority of the intrinsic ionic bonds, this moderate physical exfoliation process not only reduces the thickness but also cleaves a portion of the inherent interlayer ionic bonds, thereby inducing localized non-stoichiometry (e.g., lattice defects). Mechanistically, this atomic-scale symmetry breaking promotes the exposure of highly active “polarized surfaces” on the nanosheets. The combination of intrinsic structural features and localized defect engineering provides a new perspective for its attenuation efficacy. The synergistic effect between electrostatic interlayer interactions and polarized surfaces endows the Bi₂O₂Se nanosheets with favorable dielectric properties and favorable impedance matching. The results demonstrate that the Bi₂O₂Se nanosheets exhibit a RL_min of −36.99 dB at 10.86 GHz, with an effective absorption bandwidth (EAB) of up to 1.7 GHz, proving their precision and high efficiency in frequency-selective microwave attenuation. This work elucidates the MA properties and loss mechanisms of electrostatically coupled Bi₂O₂Se nanosheets, providing an important theoretical rationale for developing non-van der Waals semiconductors in single-component electromagnetic attenuation applications, while also highlighting the immense potential of Bi₂O₂Se to serve as a high-performance polarization matrix for constructing broadband composite systems in the future.

Results and Discussion

As shown in Figure a, Bi₂O₂Se nanosheets were obtained by tip ultrasonication of high-quality bulk crystals in deionized water (details are described in Experimental Section). The liquid-phase exfoliation process is expected to not only effectively reduce the lateral sizes and thickness of the crystals, but also cleave the interlayer ionic bonds, leading to the exposure of abundant polarized sites on the nanosheet surfaces. Because Bi₂O₂Se is essentially nonmagnetic, its microwave attenuation is expected to originate mainly from dielectric loss. In addition to the multiple reflections commonly observed in layered absorbers, the alternating [Bi₂O₂]²⁺ and [Se]^2– layers can generate local charge separation under an alternating electromagnetic field, and the exfoliation-induced asymmetric interfacial charge distribution, thereby contributing to polarization loss (Figure b).

1.

Schematics of (a) the preparation of Bi₂O₂Se nanosheets and (b) the proposed MA mechanism.

The as-exfoliated Bi₂O₂Se nanosheets were characterized by X-ray photoelectron spectroscopy (XPS) confirming the presence of Bi, O, and Se, as shown in Figure a–c. − (Detailed parameters, including binding energy values, charge calibration methods, fitting constraints, and background functions, are provided in Supporting Information Note S1). It is worth noting that, compared to the unexfoliated sample (Figure S1, Supporting Information), the characteristic peaks of Bi 4f, O 1s, and Se 3d in the exfoliated samples shift toward higher binding energies. In Figure b, the O 1s spectrum can be chemically deconvoluted into two components: the low binding energy peak (530.95 eV) is assigned to the intrinsic lattice oxygen within the [Bi₂O₂]²⁺ layers, while the high binding energy peak (532.70 eV) is attributed to surface-chemisorbed oxygen or hydroxyl species. This prominent adsorbed oxygen component strongly indicates that the physical cleavage process exposes an abundance of highly active coordinatively unsaturated sites on the flake surfaces, leading to facile surface oxidation and chemical adsorption in air. Meanwhile, the oxygen-containing groups attached to the defect sites can induce local charge imbalance, which is generally favorable for the formation of additional dipole polarization under an alternating electromagnetic field. Furthermore, the overall shift of the characteristic peaks toward higher binding energies in the XPS spectra implies a change in local charge states induced by exfoliation.

2.

Characterization of Bi₂O₂Se nanosheets. XPS spectra of Bi 4f (a), O 1s (b), and Se 3d (c) in Bi₂O₂Se nanosheets. (d) The SEM image of Bi₂O₂Se nanosheets. (e) Raman spectra of Bi₂O₂Se crystal (blue line) and nanosheets (red line). (f) The measured (black line) and standard XRD pattern (gray line) of Bi₂O₂Se nanosheets.

To investigate the morphological and compositional evolution of the exfoliated Bi₂O₂Se, the SEM image (Figure d) illustrates its micromorphology. Regarding the elemental composition, the EDS results of the bulk sample (Figure S3 and Table S1) demonstrate that the atomic ratio of Bi to Se elements is close to the stoichiometric value (2:1). This value significantly reduced after the exfoliation process, which simultaneously results in a high oxygen content. These results are consistent with the surface-sensitive XPS data, which can be attributed to the oxidation and chemical adsorption occurring on the highly active surfaces upon exposure to air.

To exclude the ambient surface interference and reveal the intrinsic structure, high-angle annular dark-field scanning transmission electron microscopy and corresponding energy-dispersive X-ray spectroscopy maps of Bi, O, and Se elements (Figure S5) were used to reveal the spatial distribution of Bi, O, and Se in the nanosheets, confirming the local loss of Bi elements and the generation of polarized sites (Table S3). Furthermore, the particle size distribution histogram derived from SEM images (Figure S6) indicates that the lateral dimensions of the exfoliated Bi₂O₂Se sample are predominantly distributed in the range of 0.5–1 μm. These statistical and characterization results collectively verify the effectiveness of the physical exfoliation process.

These results demonstrate that the mechanical forces during the ultrasonication process are not only expected to effectively increase the geometric surface area of the material, but also cleave the interlayer ionic interactions in Bi₂O₂Se and induces a local loss of Bi elements, which is hypothesized to facilitate the abundant exposure of [Se]^2– polarized surfaces on the surface of the nanosheets. This atomic-level surface reconstruction can disrupt the inherent charge symmetry of the crystal, which is supported by. the overall shift of the characteristic peaks toward higher binding energies in the XPS spectra. Under an alternating microwave electric field, these cleavage-induced asymmetric polarized sites on the surface act as ultra-high-density dipole polarization centers, , thereby significantly enhancing the dielectric polarization capacity of the material. Figure e shows a typical Raman spectrum of exfoliated Bi₂O₂Se samples (red line) under 532 nm laser excitation. The characteristic A_1g mode is observed at ∼152.5 cm^–1, which is red shifted compared with the unexfoliated samples (blue line). Such a phonon softening phenomenon can be rationally attributed to the weakened interlayer ionic coupling, which results from the reduced nanosheet thickness. Furthermore, it is impacted by the local strain and structural disorder introduced by ionic bond cleavage and surface reconstruction during the exfoliation process. ,

The X-ray diffraction (XRD) pattern of the Bi₂O₂Se nanosheets (Figure f) agrees well with the standard Bi₂O₂Se phase (PDF#73-1316), indicating that the main crystal structure is retained after exfoliation. In comparison to the bulk (Figure S2, Supporting Information), the relative intensities of the (004) and (006) reflections decrease and most diffraction peaks broaden after ultrasonication. According to the Scherrer equation, the broadening of diffraction peaks qualitatively reflects the reduction in the coherent scattering domain size during the ultrasonic exfoliation process. Furthermore, this peak broadening can also be attributed to the local lattice strains and microscopic defects introduced during the intense ultrasonic exfoliation.

The MA performance of an absorber is governed by its complex electromagnetic parameters. The relative complex permittivity is expressed as ε_r = ε′ – jε″, and the relative complex permeability is expressed as μ _r = μ′ – jμ″. The real components (ε′ and μ′) describe the capacity for electric and magnetic energy storage, whereas the imaginary components (ε″ and μ″) describe dielectric and magnetic energy dissipation, respectively. Because Bi₂O₂Se is nonmagnetic, its absorption performance is mainly determined by the dielectric response, while μ′ and μ″ are expected to remain close to 1 and 0, respectively.

Figure a shows that ε′ first increases and then decreases within the X-band. Because the strong ionic bonding in Bi₂O₂Se mediates interlayer charge transport more effectively than van der Waals coupling, contributing to a relatively stable real permittivity across the X-band. As shown in Figure b, two distinct loss peaks at around 10 (in the X-band) and 16 GHz (in the Ku-band), which correspond to enhanced dielectric dissipation. These features are also reflected in the dielectric loss tangent (tan δ = ε″/ε′, Figure c), consistent with both dielectric relaxation and conductive loss mechanisms in the Bi₂O₂Se nanosheets.

3.

Dielectric properties of Bi₂O₂Se nanosheets. (a) real part (ε′), (b) imaginary part (ε″), and (c) loss tangent (tan δ = ε″/ε′) of the complex permittivity. Cole–Cole plots of Bi₂O₂Se nanosheets with the mass fractions of (d) 90%, (e) 70%, and (f) 50%.

To further uncover the mechanisms of dielectric loss, we plotted the Cole–Cole curves for absorbers at various mass fractions of 90%, 70%, and 50%, respectively (Figure d–f). Based on the Debye relaxation theory, each single semicircle corresponds to a typical polarization relaxation process. The curves for all three samples are highly distorted, displaying a composite configuration consisting of a prominent major semicircle, multiple overlapping minor arcs, and a conspicuous linear tail on the right side. This multi-semicircle characteristic, which deviates from the ideal single Debye semicircle, provides definitive evidence for the synergistic interplay between multiple polarization relaxations and conduction loss within the material.

The major polarization semicircle in the Cole–Cole plot fundamentally originates from the intrinsic crystal architectures of Bi₂O₂Se. In contrast to other two-dimensional materials with interlayer van der Waals force, Bi₂O₂Se with alternative stack of [Bi₂O₂]²⁺ and [Se]^2– layers possess a natural spatial separation of the positive and negative charge centers. The interaction between the [Bi₂O₂]²⁺ and [Se]^2– layers stimulates a massive number of electrons to transfer and accumulate between the layers. Simultaneously, the linear tail on the right side of the Cole–Cole plot indicates the presence of conduction loss.

Although strong dielectric loss is necessary for microwave attenuation, excessive permittivity can cause impedance mismatch, leading to strong reflection at the absorber surface and preventing incident waves from entering the material. Therefore, high absorption requires a balance between attenuation capability and impedance matching. As shown in Figure c, the moderate tanδ values of the Bi₂O₂Se indicate that the nanosheets provide dielectric loss without causing excessively severe mismatch. This balance is important for achieving deep RL at practical thicknesses.

The RL values of Bi₂O₂Se/paraffin composites with different filler loadings and matching thicknesses were calculated from the measured electromagnetic parameters according to transmission line theory ,

$$\mathrm{R}\mathrm{L}=20\log \left|\frac{Z_{\mathrm{i}\mathrm{n}}-Z_0}{Z_{\mathrm{i}\mathrm{n}}+Z_0}\right|$$ 1

$$Z_{\mathrm{i}\mathrm{n}}=Z_0\sqrt{{\mu }_{\mathrm{r}}/{\epsilon }_{\mathrm{r}}}\tan \mathrm{h}\left[j\left(\frac{2\pi \mathit{fd}}{c}\right)(\sqrt{{\mu }_{\mathrm{r}}{\epsilon }_{\mathrm{r}}})\right]$$ 2

where Z _in represents the input impedance, Z ₀ is the impedance of free space, f denotes the frequency of the incident microwave, d indicates the sample thickness, and c represents the speed of light in vacuum. The EAB is defined as the frequency range over which RL is lower than −10 dB, corresponding to more than 90% attenuation of the incident microwave power. As shown in Figure , samples were prepared by ultrasonic exfoliation for 30 min at different filler loading levels. Among the tested samples, the 90 wt % Bi₂O₂Se/paraffin composite exhibits optimal MA performance, with RL_min of −36.99 dB at 10.86 GHz and a matching thickness of 2.6 mm. The 70 and 50 wt % composites show weaker absorption (Figure b,c), indicating that the filler content strongly affects both the attenuation capability and impedance matching. At low filler loading, insufficient conductive pathways and fewer polarization interfaces limit dielectric loss. At high filler loading, the increased number of [Bi₂O₂]²⁺/[Se]^2– interfaces, and nanosheet contacts enhances dielectric attenuation. However, the filler loading must still be controlled to avoid excessive permittivity and surface reflection.

4.

MA performance of Bi₂O₂Se/paraffin composites. RL curves of the composites with 90% (a), 70% (b), and 50% (c) filler loadings, respectively, measured at the thicknesses ranging from 2.0 to 5.0 mm.

As shown in Table S4, we compare the wave absorption performance of the exfoliated Bi₂O₂Se nanosheets with that of the bulk Bi₂O₂Se, pure Bi₂Se₃, and several representative dielectric absorbers. − As shown in Figure S7, the unexfoliated bulk Bi₂O₂Se with a high filler loading of 90 wt % suffers from severe interfacial impedance mismatch, causing the vast majority of incident electromagnetic waves to be reflected at the material surface rather than penetrating into the interior. Nevertheless, it still achieves a minimum reflection loss (RL_min) of −31.3 dB, demonstrating a distinct intrinsic advantage among single-component material systems. This performance is inferior to that of the exfoliated nanosheets, the RL_min of which is −36.99 dB. The comparative results demonstrate that Bi₂O₂Se possesses superior microwave attenuation efficacy relative to analogous bismuth-based materials. These results further substantiate that the exfoliation process significantly optimizes the interfacial characteristics and polarization response by exposing abundant active sites and polarization centers, thereby effectively enhancing the ultimate microwave absorption performance.

Noted that the optimal value RLmin is achieved by constructing a Bi₂O₂Se/wax composite with a high filler loading of 90 wt % and a thickness of 2.6 mm, which shows a relatively narrow EAB of 1.7 GHz. This indicates that the exfoliated Bi₂O₂Se exhibits pronounced frequency-selective absorption characteristics in the X-band. Although the 90 wt % loading inevitably increases the overall density of the composite, its robust attenuation performance at specific frequencies validates its targeted application potential as a dielectric component in coaxial composite absorbers. In future research, we mainly focus on overcoming the current limitation of high-loading composites. These exfoliated nanosheets as the dielectric polarization ingredients are favorable to introduce magnetic loss or construct multidimensional architectures in the related composites, which optimizes their impedance matching in the case of low loading. These strategies will facilitate overcoming the bandwidth limitations of single-component materials, paving the way for customized, frequency-selective electromagnetic protection.

The thickness-dependent RL behavior is consistent with the quarter-wavelength matching model, in which the absorption peak shifts with absorber thickness because destructive interference occurs when the phase difference between waves reflected at the air/absorber and absorber/metal interfaces approaches π. The optimum thickness of 2.6 mm therefore reflects the combined effects of intrinsic dielectric loss and geometrical phase matching. The deep RL_min at 10.86 GHz suggests that, at this thickness and loading, Bi₂O₂Se provides sufficient attenuation after electromagnetic waves enter the absorber while maintaining acceptable impedance matching. As shown in Figure , to systematically quantify and evaluate how the electrostatically coupled Bi₂O₂Se structure achieves this ideal balance, we further calculated the relationship between the impedance matching ratio (|Z _in/Z ₀|) and the RL for samples with different filler loadings at their optimal matching thicknesses, as well as the variation of the attenuation constant (α) with frequency. The calculation formula for the impedance matching ratio has been provided above; according to transmission line theory, the attenuation constant is defined as follows

$$\alpha =\frac{\sqrt{2}\pi f}{c}\sqrt{{\mu }^{″}{\epsilon }^{″}-{\mu }^{\prime }{\epsilon }^{\prime }+\sqrt{{({\mu }^{″}{\epsilon }^{″}-{\mu }^{\prime }{\epsilon }^{\prime })}^2+{({\mu }^{\prime }{\epsilon }^{″}+{\mu }^{″}{\epsilon }^{\prime })}^2}}$$ 3

From the perspective of electromagnetic wave absorption, optimized impedance matching serves as the primary prerequisite for achieving favorable MA. Essentially, it ensures that incident electromagnetic waves can maximally penetrate the air-absorber interface and propagate into the material’s interior, rather than undergoing severe reflection at the surface. As shown in Figure a, the sample with a mass fraction of 90% achieves a locally ideal matching state (|Z _in/Z ₀| ≈ 1) around 10.9 GHz, thereby ensuring the maximum incidence of microwaves. In contrast, the other samples suffer from severe impedance mismatch within the same frequency band, causing a substantial portion of the incident waves to be directly reflected at the surface, which severely limits their RL performance.

5.

Synergistic effect of impedance matching and attenuation characteristics. (a–c) Correspondence between the impedance matching (|Z _in/Z ₀|) and RL for Bi₂O₂Se samples with different mass fractions at their optimal matching thicknesses; (d–f) variation curves of the attenuation constant (α) with frequency at various mass fractions.

Furthermore, the attenuation constant (α) determines the intrinsic capability of the material to dissipate the microwaves that enter its interior. As illustrated in Figure d, the optimal sample exhibits a prominent attenuation peak (α ≈ 200) at 10.5 GHz. This dissipation window precisely coincides with its ideal impedance matching characteristics in the frequency domain, realizing an excellent intrinsic physical synergistic effect.

In summary, the excellent microwave attenuation performance of the moderately exfoliated Bi₂O₂Se nanosheets originates from the synergistic effect of multiple mechanisms. First, the natural asymmetric charge distribution, arising from the intrinsic interlayer ionic interactions between [Bi₂O₂]²⁺ and [Se]^2–, induces intrinsic dipole polarization under an alternating electromagnetic field, providing the fundamental dielectric loss. Secondly, the moderate exfoliation process partially disrupts the interlayer confinement, not only effectively mitigating the geometric impedance mismatch of the bulk material, but also allowing the anticipated exposed surface or edge defects (such as [Se]^2– active sites) to act as additional polarization centers, thereby significantly enhancing the polarization loss. Third, the anticipated increased contact area resulting from the exfoliation process further intensifies the polarization effects at the heterogeneous interfaces. Fourth, the semiconducting nature and high carrier mobility of Bi₂O₂Se can contribute to the conductive loss, especially in the case of formation of partial charge-transport pathways between the nanosheets at high filler loading. Finally, the layered morphology increases multiple reflections and scattering, extending the propagation path of microwaves within the absorber and increasing the probability of energy dissipation.

Conclusion

Bi₂O₂Se nanosheets were prepared by tip ultrasonication and investigated as dielectric microwave absorbers. Structural characterizations confirm that exfoliation reduces the crystal size while largely preserving the Bi₂O₂Se phase, and spectroscopic analysis reveals the cleavage-induced generation of dipole polarization centers. The Bi₂O₂Se/paraffin composite with 90 wt % filler loading exhibits an RL_min of −36.99 dB at 10.86 GHz and an EAB of 1.7 GHz at a thickness of 2.6 mm. The absorption enhancement is attributed to the synergistic effects of impedance matching, interfacial polarization, cleavage-induced polar termination polarization, conductive loss, and multiple reflections/scattering within the nanosheet network. This work demonstrates the potential of electrostatically coupled Bi₂O₂Se nanosheets as dielectric components for MA and provides a design concept for exploring non-van der Waals layered semiconductors in electromagnetic attenuation applications.

Experimental Section

Synthesis of Bi₂O₂Se

Bismuth oxide (Bi₂O₃, 99.99%) and bismuth selenide (Bi₂Se₃, 99.99%) were purchased from Shanghai McLean Biochemical Technology Co., Ltd. All reagents were used as received without further purification. Bi₂O₃ and Bi₂Se₃ powders were weighed at a molar ratio of 2:1, ground, and thoroughly mixed. To suppress reaction between Bi₂O₃ and SiO₂, an additional 10 wt % Bi₂O₃ powder was added. The mixed powder was sealed in an evacuated quartz tube at ∼10^–4 mbar. The sealed quartz tube was heated in a muffle furnace to 1005 K at a rate of 2 K min^–1 and maintained at this temperature for 1 to 2 months to ensure complete reaction and crystal growth. After cooling down to room temperature, the quartz tube was then broken to collect Bi₂O₂Se single crystal. The crystal size distribution is shown in Figure S8, Supporting Information. For exfoliation, 1 g of Bi₂O₂Se crystal powder was dispersed in 10 mL of deionized water. The liquid-phase exfoliation was performed using a Welling Ultrasonic WH001 ultrasonic processor (fixed frequency of 20 kHz, maximum power of 600 W, equipped with a standard 0.6 mm titanium alloy probe). The operating power of the ultrasonic processor was set as 300 W, while a pulse mode of 4 s on and 4 s off was employed to prevent excessive heat accumulation. The total processing time for each sample was maintained at 30 min at room temperature. After sonication, the dispersion was directly freeze-dried to obtain the final exfoliated Bi₂O₂Se powder. The powders were mixed uniformly with a paraffin matrix at selected mass fractions (e.g., 50, 70, 90 wt %) and pressed into standard coaxial ring samples with an inner diameter of 3.04 mm and an outer diameter of 7.00 mm.

Characterization

The chemical composition and valence states were analyzed using X-ray photoelectron spectroscopy (XPS, ESCALAB 250Xi). Crystal structure and composition were analyzed with XRD (Empyrean 3). Raman spectra were collected using a 532 nm excitation laser and a spot size of approximately 1 μm on a Raman spectrometer (HR800, Horiba JY, Japan). Morphology and microstructure were observed using a field-emission SEM (S-4800, Hitachi, Japan). The electromagnetic parameters were measured over 2–18 GHz using a vector network analyzer (E5063A, Keysight) based on the coaxial transmission/reflection method. RL values were calculated from the measured complex permittivity and permeability according to transmission line theory.

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsomega.6c06163.

• Additional material characterizations (XRD, SEM, HAADF-STEM, and bulk crystal images); detailed parameters for XPS measurements and spectral fitting (Note S1); electromagnetic evaluation plots (reflection loss, impedance matching, and attenuation constant); and a performance comparison table with other microwave absorbers (PDF)

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J.Z. and Z.Z. contributed equally to this work.

The authors declare no competing financial interest.