Three-dimensional biotemplate-loaded nickel sulfide vacancies engineered to promote the absorption of electromagnetic waves

Abstract

Vacancy engineering offers an appealing strategy for modifying the electronic structure of transition metals. Transition metals with abundant sulfur vacancies can significantly contribute to the microwave absorption capabilities of absorbers. In this study, an Ni_xS_y@De composite material was synthesized through a straightforward hydrothermal synthesis technique. The effective absorption bandwidth (EAB) of this composite material reached 9.86 GHz at 1.44 mm. A minimum reflection loss (RL_min) of −33.61 dB at 1 mm was achieved, and after mild etching, the RL_min further improved to −93.53 dB at 1.16 mm to achieve a high-attenuation microwave absorption. The exceptional performance of Ni_xS_y@De for the absorption of electromagnetic waves (EMWs) is based on its high dielectric loss, substantial magnetic loss, and excellent impedance matching. This work combines transition metal sulfides with three-dimensional biotemplated diatomite, providing valuable insights into the design of advanced EMW absorbing materials.

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1. Introduction

With the development and progress of wireless communication technology, there has been a rapid and extensive increase in the use of various electronic devices and detection radars.^1–4 These devices are being increasingly used in different frequency bands to address significant issues, such as electromagnetic radiation, pollution, and interference.^5–8 Electromagnetic wave (EMW) absorption materials can effectively absorb EMWs and convert them into thermal energy or alternative forms of energy.^9,10 Additionally, they can dissipate EMWs through interference, thereby reducing unnecessary electromagnetic radiation.^11,12 This property is critical and of great importance in protecting people from radiation hazards and aiding in shielding electronic devices from electromagnetic interference.^13,14 To date, researchers have actively explored and developed various EMW absorption materials, including ceramics,¹⁵ ferrites,¹⁶ carbon-based materials,¹⁷ and conductive polymers,¹⁸ and have made remarkable progress. However, most of the currently investigated EMW absorption materials have some drawbacks, such as a singular loss mechanism, suboptimal impedance matching, and relatively weak absorption capabilities.^19,20 Among them, hierarchically engineered materials are considered to be efficient EMW absorption materials capable of meeting practical applications. In this way, hollow structures,²¹ pomegranate-like structures,²² and microsphere structures²³ have been prepared. Although great progress has been made, the development of EMW absorbers with a small thickness, light weight, wide absorption bandwidth, and strong absorption capacity is still a necessary and urgent task.^24–26

Diatomite (De) is a type of siliceous biogenic sedimentary rock with a layered porous structure and naturally occurring large cavities, which is characterized by low density, lightweight, and a large surface area.²⁷ The unique structure of diatomite grants it a photonic crystal effect, enhancing the surface ion resonance of loaded materials and thereby strengthening the absorption of incident EMWs.^28,29 However, diatomite has poor electrical conductivity. To enhance the microwave absorption capacity of composite materials, materials with a higher electrical conductivity can be loaded onto diatomite. Consequently, diatomite has vast potential for applications in the field of microwave absorption. Li et al.³⁰ prepared a ternary composite material with a MnFe₂O₄/rGO/De coated structure using a one-step hydrothermal method. The coated structure of the ternary composite material resulted in low-frequency microwave absorption capabilities. The minimum reflection loss (RL_min) of the composite material was −76.56 dB at 7.9 GHz and a thickness of 2.485 mm with an effective absorption bandwidth (EAB) of 3.61 GHz, demonstrating increased EMW absorption. Transition metal dichalcogenides (TMDs) are often used as loading materials owing to their high conductivity, high dielectric loss, small bandgap, excellent dielectric properties, and cost-effectiveness.³¹ The introduction of TMDs into microwave absorbers is beneficial for impedance matching, thus improving their absorption efficiency, and making them effective EMW materials.³² For example, Wang et al.³³ prepared Ni_xCo₁S@De composite material using a two-step hydrothermal method, achieving an EAB of 6.16 GHz at 1.6 mm and 7.04 GHz at 4.1 mm. Chang et al.³² synthesized a NiS/MoS₂/Ti₃C₂T_x hybrid material using a two-step method, with an EAB of 5.04 GHz at 2.1 mm and an RL_min of −58.48 dB at 2.4 mm. Through these studies, it is evident that TMDs possess strong microwave absorption capabilities. Besides, many reports show that vacancy engineering is an appealing strategy that can have a great impact on the design and development of high-performance EMW absorption materials, especially for the study of dielectric loss. Sulfur vacancies not only greatly improve the electrical conductivity,³⁴ but also promote the formation of electric dipoles, increase the polarization loss caused by defects in the electromagnetic field environment, and greatly improve the absorption properties of EMW.^35,36 Therefore, we aimed to construct an efficient composite microwave absorption material consisting of TMDs and diatomite by modulating the sulfur vacancy concentration.

In this study, we utilized a one-step hydrothermal approach to grow nickel hydroxide nanoflower precursors on diatomite. Subsequently, sulfurization was performed to improve the absorption characteristics of the nickel-based materials. Finally, we etched the sulfurized material to form the Ni_xS_y@De-etched composite material. As anticipated, Ni_xS_y@De demonstrated an EAB of 9.86 GHz with a thickness of 1.44 mm, the RL_min achieved was −33.61 dB at 1 mm. Mild etching resulted in reduced impurities in the sample, improved stability of the diatomite batch, and an RL_min of −93.53 dB at 1.16 mm.

Compared to previously designed De-based composite materials, the material prepared in this work showcased an extremely thin matching thickness, RL_min, and a broader EAB, indicating superior EMW absorption performance. This work provides a new idea to enhance the EMW of composite materials and demonstrates the feasibility of utilizing diatomite in EMW materials.

2. Experimental section

2.1. Materials and reagents

Nickel(II) nitrate hexahydrate (Ni(NO₃)₂·6H₂O), potassium hydroxide (KOH) purchased from Shanghai Aladdin Co., Ltd, diatomite (white power), ammonium fluoride (NH₄F) purchased from Chengdu Kelong Chemical Reagent Factory, anhydrous ethanol (C₂H₅OH), thioacetamide (C₂H₅NS), and urea (CO(NH₂)₂) were purchased from Chongqing Chuandong Chemical Co. Ltd. For this experiment, we prepared deionized water in the laboratory.

2.2. Synthesis of Ni(OH)₂@De

The synthetic process of the Ni_xS_y@De is depicted in Fig. 1. First, 4.8 mmol of Ni(NO₃)₂·6H₂O and 800 mg of diatomite were completely dissolved in 300 ml of deionized water to form Solution A. Next, 324 mg of NH₄F and 540 mg of CO(NH₂)₂ were added to 300 ml of deionized water to form Solution B. Solution A was then poured and thoroughly mixed with Solution B. The resulting mixture was transferred to a Teflon-lined stainless steel autoclave, sealed, and reacted in a rotary oven at 140 °C for 12 h in a homogeneous reactor. After the reaction was completed, the reaction kettle was cooled to room temperature, and the product was suction filtered. The filtered sample was then dried at 60 °C to obtain the precursor Ni(OH)₂@De.

Fig. 1 A schematic diagram of the synthesis of Ni_xS_y@De-etched.

2.3. Synthesis of Ni_xS_y@De

Ni(OH)₂@De composite and thioacetamide powder were dissolved in deionized water at the mass ratio of 1 : 10. The solution was then transferred to a Teflon-lined stainless steel autoclave, sealed, and reacted in a rotary oven at 120 °C for 12 h in a homogeneous reactor. After the reaction was completed, the reaction kettle was cooled to room temperature and suction filtered. The filtered sample was then dried at 60 °C to obtain Ni_xS_y@De composite material.

2.4. Etching of Ni_xS_y@De

An amount of Ni_xS_y@De composite, 56 ml of 3 mol L⁻¹ KOH solution, and 14 ml of anhydrous ethanol were combined in a Teflon-lined stainless steel autoclave and reacted in a rotary oven at 140 °C for 24 h in a homogeneous reactor. After the reaction was completed, the reaction kettle was cooled to room temperature and suction filtered. The filtered sample was then dried at 60 °C to obtain the final sample Ni_xS_y@De-etched.

2.5. Characterization

Morphological and structural characterization of the synthesized materials was conducted using scanning electron microscopy (SEM, Zeiss Auriga) and transmission electron microscopy (TEM, Talos F200S). The crystallographic data and chemical composition of the samples were analyzed by X-ray diffractometry (XRD, Rigaku Ultima IV, Cu/Kα) and X-ray photoelectron spectroscopy (XPS, ESCALAB Xi+). The vacancy levels of the samples were characterized by electron paramagnetic resonance (EPR, Bruker A300). The chemical bonding information of the samples was obtained by Fourier transform infrared spectroscopy (FTIR, Nicolet iS50) to determine their chemical structures. Thermogravimetric analysis (TGA, Mettler 1100LF, N₂ atmosphere, 20 °C min⁻¹ rate of heating). The prepared sample was mixed with paraffin 1 : 1, and its complex permittivity and complex permeability were measured using a vector network analyzer (E5071C) in the frequency range of 1–18 GHz using the coaxial method. The sample powders were measured for resistance (Cryoall CTA-3) by the four-probe method and resistivity and conductivity were calculated.

3. Discussion

Three samples were characterized to observe the surface morphology of the samples. Fig. 2(a–i) presents the SEM images showing the microstructures of three composite materials: Ni(OH)₂@De, Ni_xS_y@De, and Ni_xS_y@De-etched. Firstly, it was observed that the base material, diatomite, remained unchanged in all three samples. Furthermore, we employed a one-step hydrothermal approach to load Ni(OH)₂ nanoflowers onto diatomite and allowed the nanoflowers to grow vertically in situ on diatomite. After sulfurization, a significant change in the sample's morphology was apparent, and the sheet-like structures displayed a reduced length and an uneven loading. This implies that the crystal structure of the sample was changed. Following KOH etching, the flower-like structural morphology of the sample was once again rendered uniform and compact. This suggests that the etching process efficiently eliminated the impurities found in the sample, as well as other compounds created during the sulfurization process. By comparing the three samples, it is evident that the nanosheets grew uniformly and densely, completely encapsulating diatomite without noticeable aggregation. The sheet-like structures are arranged in a manner that forms a porous structure containing an air phase. The introduction of the sulfurization structure enables the effective design of a suitable absorber structure, fulfilling the requirements for EMW scattering, reflection conditions, and impedance-matching mechanisms. Simultaneously, at equivalent magnifications, differences in the interlayer spacing and the size of the sheets among the three samples were observed. The length of the Ni(OH)₂ sheet-like structures was approximately 650 to 750 nm, which decreases to 250 to 350 nm after sulfurization (Fig. S1, ESI†). This reduction in the sheet length leads to increased reflection and scattering of the incident EMWs, facilitating the construction of a conductive network and promoting polarization losses of the incident EMWs,³⁷ as validated by subsequent experimental results.

Fig. 2 SEM images of (a–c) Ni(OH)₂@De, (d–f) Ni_xS_y@De, and (g–i) Ni_xS_y@De-etched at different ratios.

The microstructure and composition of the samples were analyzed in more detail using TEM. Fig. 3(a–d) present the morphology of the entire samples and partially magnified areas. The results align with the findings from the SEM images that the flower-like structure is still retained after sulfurization and etching, and the interface between the diatomite and Ni_xS_y nanoflowers can be observed under high magnification without any obvious voids or gaps at the boundaries, suggesting that the biphasic bond is tightly coupled to provide favorable conditions for scattering and reflection of the EMW and to optimize the impedance matching between the absorber and free space. The inset in Fig. 3d presents the diffraction pattern analysis, which displays recognizable polycrystalline patterns, with the (044), (224), and (113) crystal planes belonging to Ni₃S₄, aligning with the subsequent XRD results. As shown in Fig. 3e, a lattice spacing of 1.69 nm was observed, which can be attributed to the (2̄36) crystal plane of Ni_xS₆. To verify their detailed vacancy features, the EPR images (Fig. S2, ESI†) confirmed that the Ni_xS_y@De samples obtained after sulfurization contained a large number of sulfide vacancies.³⁵ The high-angle annular dark-field scanning transmission electron microscopy-energy-dispersive X-ray spectroscopy (HAADF-EDS) image shown in Fig. 3f reveals that Ni_xS_y@De is composed of Ni, S, O, and Si elements, further confirming the successful sulfurization process.

Fig. 3 TEM images (a–d) of Ni_xS_y@De-etched at different magnifications; the inset in (d) is the diffraction spot analysis. (e) Lattice fringe analysis. (f) HADDF element analysis for Ni, S, O, and Si.

XRD patterns of the three composite materials, Ni(OH)₂@De, Ni_xS_y@De, and Ni_xS_y@De-etched are presented in Fig. 4a. The diffraction peaks at diffraction angles 2θ of 21.64°, 31.34°, 36.11°, 46.86°, and 56.48° can be attributed to the (101), (102), (200), (113), and (104) crystal planes of De (JCPDS 39-1425). The remaining diffraction peaks at 2θ angles peaks of the Ni(OH)₂@De composite were 19.21°, 33.25°, 38.71°, 52.15°, 59.51°, 63.07°, and 73.29°, corresponding to the (001), (100), (011), (012), (110), (111), and (112) crystal planes of Ni(OH)₂ (JCPDS 73-1520), indicating the presence of both diatomite and Ni(OH)₂ phases in the sample Ni(OH)₂@De. In the case of Ni_xS_y@De, besides the characteristic peaks of diatomite, the diffraction peaks at 2θ angles of 31.17°, and 61.51° correspond to the (113), (026) crystal planes of Ni₃S₄ (JCPDS 43-1469), respectively. The diffraction peaks at 2θ angles of 17.91°, 20.25°, 26.23°, 50.07°, and 54.49° are attributed to the (101), (1̄30), (1̄40), (1̄36), and (2̄36) crystal planes of Ni_xS₆ (JCPDS 51-0717), respectively. Compared to Ni(OH)₂@De, the diffraction peaks of Ni(OH)₂ completely disappeared, indicating the complete sulfurization of Ni(OH)₂. The sulfurization product is composed of a mixed phase of Ni₃S₄ and Ni_xS₆, termed Ni_xS_y. In the case of Ni_xS_y@De-etched, the observation of an increased number of diffraction peaks for Ni_xS_y indicates a significant reduction in impurities in the sample after etching, highlighting a more pronounced sulfurization effect, consistent with the SEM results described earlier.

Fig. 4 (a) XRD pattern of Ni(OH)₂@De, Ni_xS_y@De, and Ni_xS_y@De-etched; (b) the total XPS spectrum of Ni_xS_y@De-etched; (c–f) the partial spectra of Ni, S, O, Si.

To gain deeper insights into the elemental composition and chemical valence states of the material, XPS was utilized to analyze the Ni_xS_y@De-etched composite. The comprehensive XPS spectrum depicted in Fig. 4b unequivocally verifies the existence of Ni, O, S, and Si elements within the Ni_xS_y@De-etched sample. However, the presence of carbon peaks in the spectrum is ascribed to the adsorption of C-containing substances, such as CO₂, from the surrounding environment. It was observed that the Si element content after etching was very low, indicating that the etching effect was very significant. As shown in Fig. 4c, the Ni 2p spectrum consists of two pairs of satellite peaks at 860.0 and 877.0 eV and 862.8 and 879.9 eV and two pairs of spin–orbit doublet peaks corresponding to Ni 2p_3/2 and Ni 2p_1/2 of Ni²⁺ (852.1 and 869.4 eV) and Ni³⁺ (855.6 and 873.4 eV) Ni²⁺ and Ni³⁺, respectively, distributed in Ni₃S₄ and Ni_xS₆.^38,39 In Fig. 4d, the S 2p spectrum was analyzed, revealing four distinct peaks, with the peaks located at 160.0 and 161.1 eV corresponding to S 2p_3/2 and S 2p_1/2 of S²⁻, respectively, and the other two peaks at 162.2 and 167.7 eV are associated with the presence of S–C bonds and the oxidation of Ni₃S₄ and Ni_xS₆,⁴⁰ respectively. The presence of S–O indicates the oxidation of sulfur in the sample, while the appearance of the S–C bond is probably due to the sulfurization of the trace amount of carbon present in the diatomite. In the O 1s high-resolution spectrum shown in Fig. 4e, the peak associated with the O element originates from the O–Si bond (530.8 eV).⁴¹

Furthermore, the chemical bond types within these three samples were investigated in greater detail through FTIR spectroscopy. In Fig. 5a, the absorption peak at 3412.96 cm⁻¹ is attributed to the stretching vibration of O–H.⁴² The appearance of the absorption peaks at 2926.01, 2847.43, 1623.80, and 1110.82 cm⁻¹ are attributed to asymmetric C–H stretching, symmetric C–H stretching vibration,⁴³ C=O stretching vibration, and C–O stretching vibration,^42,44 respectively. A small peak is observed for Ni_xS_y@De and Ni_xS_y@De-etched samples around 1025.48 cm⁻¹. This peak can be linked to the S=O stretching mode, indicating the presence of sulfate ions resulting from the surface-adsorbed moisture.⁴² The absorption peak at 620.98 cm⁻¹ is caused by the stretching vibration of the Ni–O bond. In both Ni(OH)₂@De and Ni_xS_y@De samples, an absorption peak appeared at 470.55 cm⁻¹, which was identified as the stretching vibration of the Si–O bond.⁴⁵ This peak disappeared after etching, indicating the effective removal of diatomite from the samples. Therefore, a combination of XRD, XPS, and FTIR spectroscopy, it can be confirmed that Ni(OH)₂ was successfully loaded onto diatomite and partially sulphurized, resulting in the formation of a Ni_xS_y composite consisting of two phases, Ni_xS₆ and Ni₃S₄. Thus, the Ni_xS_y@De composite was obtained.

Fig. 5 (a) FTIR and (b) TGA spectra of Ni(OH)₂@De, Ni_xS_y@De, and Ni_xS_y@De-etched.

To investigate the heat loss characteristics of the composite materials, TGA analyses were carried out on the three samples as shown in Fig. 5b. Specifically, for the Ni(OH)₂@De sample, there was a 1.06% mass loss from room temperature to 213 °C, followed by a subsequent 6.49% mass loss from 213 °C to 800 °C. For the Ni_xS_y@De sample, there was a 10% mass loss from room temperature to 131 °C, followed by an additional 8.84% mass loss from 131 °C to 267 °C, and finally, a 21.04% mass loss from 267 °C to 800 °C. The thermal stability of the samples before and after etching remained essentially unchanged. The results show that all three samples exhibit good thermal stability up to 400 °C and the weight loss remains below 20%. The weight loss of the three samples below 120 °C is mainly due to the evaporation of the adsorbed water. The weight loss of Ni(OH)₂@De at 230–800 °C is related to the decomposition of Ni(OH)₂ and the dehydration of diatomite, and the weight loss of the sulphurized samples at 120–400 °C is related to the removal of the oxygen-containing functional groups and the dehydration of diatomite, and the weight loss above 400 °C is related to the oxidation of Ni_xS_y and the dehydration of diatomite.^46,47 Considering the properties of Ni(OH)₂, we speculate that the occurring reactions can be expressed by eqn (1) and (2):

Ni(NO)₃·6H₂O → Ni(OH)₂ + H₂O + NO₂↑ (1)

Ni(OH)₂ + CH₃CSNH₃ → Ni_xS₆ + Ni₃S₄ + CH₃CONH₂ + H₂O (2)

The electromagnetic parameters are fundamental factors that dictate the electromagnetic properties of the wave-absorbing materials, specifically the relative complex permittivity (ε_r = ε′ − jε′′) of Ni(OH)₂@De, N_ixS_y@De, and Ni_xS_y@De-etched are shown in Fig. 6a and b. In general, ε′ and ε′′ reflect the ability of the wave-absorbing material to store and dissipate electrical energy, respectively.⁴⁸ The reduction in complex permittivity optimizes impedance matching, causing EMWs to enter the absorbers to the greatest extent possible for dissipation.⁴⁹ For Ni(OH)₂@De, the curves of ε′ and ε′′ hardly fluctuate and remain flat at 2.6 and 0.2, respectively, however, in general, the materials with stable real and imaginary parts are suitable to be used as the substrate materials. The ε′ of Ni_xS_y@De and Ni_xS_y@De-etched show a decreasing trend with increasing frequency, which can be attributed to the characteristic dispersion response due to the enhanced hysteresis of the dipole polarization response to the electric field, which also improves the impedance matching of the incident microwave.^50,51 The ε′ and ε′′ values of the last two samples undergo a huge enhancement with respect to Ni(OH)₂@De, suggesting that sulfide increases the dielectric properties of the samples in the form of phase components, vacancies, and defects.⁵² The presence of abundant defects and a large number of interfaces after sulfurization was demonstrated by XRD, EPR, and SEM, leading to significant polarization,⁵³ thus synergistically contributing to the dielectric attenuation performance.⁵⁴ According to the electronic free-electron theory, high conductivity leads to high ε values, which in turn increases the dielectric loss and leads to improved performance, which is in agreement with the later conclusions. For the magnetic permeability (μ_r = μ′ − jμ′′) of the three samples, the trend of the curves is almost the same, with a small enhancement occurring in the vulcanized samples, μ′ and μ′′, but not much increase in the contribution of the magnetic losses, which we will analyze in detail later. As far as tan δ_ε and tan δ_μ are concerned, the dielectric loss tangent and the magnetic permeability tangent have the same values at low frequencies, but as the frequency increases, tan δ_ε remains stable and tan δ_μ undergoes a sharp decrease, indicating that tan δ_ε dominates the losses.

Fig. 6 (a) The real part and (b) the imaginary part of the dielectric constants, (d) the real part and (e) the imaginary part of the permeability, (c) the dielectric loss tangent, and (f) the magnetic loss tangent.

By analyzing the electromagnetic parameters, we need to specifically discuss the dielectric loss of the samples, to further prove our inference, we tested the conductivity of the three samples, and the results are shown in Table 1.

Table 1 Comparison of the electrical resistivity and electrical conductivity of Ni(OH)₂@De, Ni_xS_y@De, and Ni_xS_y@De-etched at 30 °C

Sample	Electrical resistivity (μΩ m)	Average electrical resistivity (μΩ m)	Average electrical conductivity (S m^−1)
Ni(OH)2@De	976934051.962	932068294.342	0.00107288
	914925151.340
	904345679.724
NixSy@De	5616.026	5580.162	179.206267
	5562.144
	5562.316
NixSy@De-etched	5760.246	5689.778	178.897981
	5695.880
	5613.207

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The electrical conductivities of Ni(OH)₂@De, Ni_xS_y@De, and Ni_xS_y@De-etched were 0.0011 S m⁻¹, 179.21 S m⁻¹, and 178.90 S m⁻¹, respectively, and their conductivity trends are consistent with ε′′, which is in line with the free-electron equation. Sulfurization has a great role in enhancing conductivity, and in the case of our samples, an order of magnitude increase in conductivity occurs after sulfurization, which is further demonstrated in the DFT calculations later on. The results show that Ni_xS_y@De-etched has good electrical conductivity, indicating excellent EMW absorption properties. In addition to analyzing the conductivity, the Debye relaxation is also an important way to analyze the dielectric loss of absorbing materials.⁵⁵ Usually, eqn (5) can be derived from eqn (3), (4), and (5),^54,56 and can represent the relationship between ε′ and ε′′, and the semicircular curve of ε′ − ε′′ proves the existence of the Debye relaxation process, also called Cole–Cole semicircles. Each semicircle represents a Debye relaxation process; therefore, the Cole–Cole semicircle can be used to identify the frequency bands in which polarization occurs. The conductivities of Ni_xS_y@De and Ni_xS_y@De-etched are close, but ε′′ is smaller because Ni_xS_y@De-etched has more semicircular arcs and irregularly dense regions (Fig. 7), which suggests that it has more polarization relaxation processes and stronger conductivity losses.

where τ is the polarization relaxation time, ε_s is the static dielectric constant, and ε_∞ is the high-frequency limiting dielectric constant. As shown in Fig. 7(a–c), it can be observed in the Cole–Cole plot that Ni(OH)₂@De, Ni_xS_y@De-etched has more semicircles, which implies that more relaxation processes occur, suggesting the presence of multiple Debye relaxation and other dielectric loss mechanisms in the absorber, related to the interface between De and the loaded Ni-based compound. Also, smooth tails can be observed at high frequencies for both samples, this suggests that the conduction losses also affect the dielectric losses.⁵⁷ For the Ni_xS_y@De sample, on the other hand, we can see a very long straight line, which is a characteristic of strong conductive loss, proving that the Ni_xS_y nanosheets form a three-dimensional conductive network in the diatomite matrix. The high conductivity allows it to have few semicircles, but after etching, more polarization occurs at the interface between diatomite and Ni_xS_y due to the etching of a portion of SiO₂, resulting in more semicircles being created.

Fig. 7 (a–c) Cole–Cole curves of three samples, (d) eddy current loss diagram, (e) attention constant diagram, and (f) impedance matching diagram.

Therefore, we further analyze the magnetic loss, which in the microwave band is mainly manifested as hysteresis, domain wall resonance, eddy current loss, natural resonance, and exchange resonance.⁵⁸ However, in a weak magnetic field, hysteresis arises from irreversible magnetization and can be ignored. Domain wall resonance occurs only in the 1–100 MHz range of multi-domain magnetic materials Eddy current loss is an important source of magnetic loss, and the effect of eddy current loss on the absorption effect can be initially analyzed based on calculations, and eqn (6) (ref. 59) is usually used to determine this effect:

c₀ = μ′′(μ′)⁻²μ′′(f)⁻¹ = 2πμ₀d²δ (6)

where d is the thickness of the absorber and μ₀ is the vacuum permeability. The variation of the c₀ value with frequency is a useful indicator for analyzing the mechanism of the magnetic loss. In cases where eddy current loss occurs within the absorber material, the c₀ value remains relatively stable and does not exhibit significant changes with frequency. Fig. 7d illustrates the correlation curve between c₀ and the frequency of each sample. It is evident that within the 8–12 GHz range, the c₀ value remains relatively stable, suggesting that magnetic loss primarily arises from eddy current effects. Conversely, in the 1–8 GHz range, the c₀ value varies across frequencies, indicating that eddy current loss alone does not account for the magnetic loss. Other significant factors such as hysteresis loss, natural resonance, and exchange resonance also contribute to the magnetic loss of the material.⁵⁸

A high dielectric loss is essential for achieving exceptional performance in EMW absorbing materials. However, for effective loss mechanisms in the material, a high attenuation constant and favorable impedance matching are crucial prerequisites. The attenuation constant (α) serves as a key metric to assess the absorption capacity (eqn (7) (ref. 60)).

As depicted in Fig. 7e, the attenuation coefficient of Ni_xS_y@De exhibits a more prominent increase in frequency compared to the other samples. This observation suggests a superior attenuation of the incident EMW and better absorption performance for Ni_xS_y@De. The enhanced polarization relaxation and conductive loss contribute to this improved absorption capacity. It is important to emphasize that while a larger α is necessary, it is not solely sufficient for the absorber to achieve excellent absorption performance—it is also intricately linked to impedance matching. To mitigate EMW reflection, it is imperative to tailor the impedance-matching properties of the material. The introduction of Ni_xS_y on diatomite significantly enhanced both the impedance matching capability and the EMW attenuation capability. Usually, |Z_in − Z_o| should be as close to 0 as possible, i.e., |Z_in/Z_o| = 1, to facilitate better penetration of EMW into the material. Fig. 7f depicts |Z_in/Z_o| versus frequency. Ni_xS_y@De's |Z_in/Z_o| can be close to 1 in most of the region, which matches perfectly with our EAB results.

We analyzed the 1D, 2D, and 3D plots of the relationship between frequency, thickness, and reflection loss in the 1–18 GHz band for three samples by electromagnetic parameter calculations (Fig. 8). In general, RL_min values less than −10 dB are considered to have an absorption efficiency of 90% for EMW, and wave-absorbing materials on this basis are consistent with practical applications. As can be seen in Fig. 8, the RL values of Ni(OH)₂@De do not reach −10 dB at any thickness and frequency; therefore, there is no effective absorption of EMWs. When Ni(OH)₂ introduces Ni_xS_y on the surface of diatomite by reacting with thioacetamide, the RL_min increases dramatically, and for Ni_xS_y@De, the RL_min is −33.61 dB at 18.0 GHz, with a matching thickness of 1.0 mm. It is worth mentioning that, due to the dual effect of dielectric and magnetic losses, the impedance matching of Ni_xS_y@De also greatly improved due to both dielectric loss and magnetic loss, and the EAB is very good. With a thickness of 1.44 mm, the corresponding EAB is 9.86 GHz (8.14–18 GHz), which almost completely covers the X and Ku bands. Interestingly, when we adjust the thickness to 4.6 mm, the EAB expands to 10.55 GHz (1.85–4.39 GHz, 9.35–13.96 GHz, and 14.6–18 GHz), covering the complete C-band and most of the X, and Ku-bands. By adjusting the thickness, the purpose of realizing the absorption in different bands can be achieved, which has a strong practical application, and further illustrates that the performance of the samples is greatly improved after sulfurization of the samples due to the incorporation of vacancy engineering.⁵² To further improve the performance of the sample, we etched the sample and surprisingly, the RL_min is −93.53 dB and the EAB is 7.79 GHz at Ni_xS_y@De-etched with a thickness of only 1.16 mm. Its excellent broadband absorption can reach 7.79 GHz at an ultra-thin thickness of 1.16 mm (10.21–18.0 GHz). We infer that the etching process reduces the broken diatomite and some magazine elements produced during the sizing process, resulting in a more homogeneous loading of Ni_xS_y and better batch stability of diatomite, which not only increases the RL_min and reduces the matching thickness for optimal performance but also has an effect on the interaction between Ni_xS_y and diatomite at the material interface, which affects the impedance matching and slightly reduces the EAB, but the performance is still surprising and noteworthy.

Fig. 8 From left to right: Ni(OH)₂@De, Ni_xS_y@De, and Ni_xS_y@De-etched. (a–c) the 3D loss diagram, (d–f) contour plot of RL with thickness and frequency, and (g–i) frequency-loss 1D diagram.

For a more comprehensive understanding of how the absorption properties of EMW vary with thickness, (Fig. S3, ESI†) shows the quarter-wavelength model for both Ni_xS_y@De and Ni_xS_y@De-etched samples, where the frequency corresponding to RL_min shifts to lower frequencies with increasing thickness,⁶¹ which roughly agrees with the fitting curves given by eqn (8),⁶² further demonstrating the reliability of the data.

t_m = nc/[(4f)(|ε_r||μ_r|^1/2)] (n = 1,3,5,…) (8)

The density of states of theoretical models is calculated to investigate the excellent EMW absorption performance of Ni_xS_y@De-etched. The structural model of Ni₃S₄ is constructed based on the results of XRD. The model of Ni_xS_y@De-etched consists of the hexagonal Ni₃S₄, which is determined from the XRD data. From the analysis of the total and partial density of states (TDOS and PDOS) of Ni₃S₄ (Fig. 9a), the energy band of Ni₃S₄ near the Fermi level is primarily influenced by the Ni 3d state. To verify the promising application of Ni_xS_y@De-etched in practical applications, the simulation of the Radar Cross Section (RCS) was conducted utilizing the CST software. RCS is a physical metric quantifying the intensity of a target's echo when exposed to radar wave irradiation.⁶³ The RCS area of a target can be likened to the projected area of a metal ball that produces a similar echo signal.⁶⁴Fig. 9(b–e) show the 3D RCS simulation results for a perfect electric conductor (PEC), Ni(OH)₂@De, Ni_xS_y@De, and Ni_xS_y@De-etched at different intensities, respectively. Fig. 9f shows the RCS values of the absorber at different incidence angles. Ni_xS_y@De-etched has the lowest RCS value in the −90° to 90° incidence angle range.

Fig. 9 (a) TDOS and PDOS of Ni₃S₄. RCS simulation results of the samples: (b) PEC and the PEC substrates covered with (c) Ni(OH)₂@De, (d) Ni_xS_y@De, (e) Ni_xS_y@De-etched. (f) RCS values of samples at different angles. (g) Comparative summary of EMW absorption performances of some representative sulfide-based absorbers.

To visualize the impact of the coating on reducing the Radar Cross Section (RCS) value more intuitively, the absorber value is subtracted from the PEC value to obtain the reduction. The analysis shows that all absorbers contribute positively to reducing RCS values, with Ni_xS_y@De-etched exhibiting a more pronounced effect compared to other EMW absorbers (Fig. S4, ESI†). Ni_xS_y@De-etched has the highest noise reduction value at 10°, which reaches 44.49 dB m². The simulation results of RCS align well with the EMW attenuation results, providing strong evidence for the significant potential of Ni_xS_y@De-etched in the fabrication of efficient EMW absorbers. A comparison of the EAB and thickness of some reported dielectric microwave absorbing composites is given in Fig. 9g,^32,64–71 where it can be seen that the Ni_xS_y@De-etched and Ni_xS_y@De composites prepared in this study have a larger EAB and thinner thickness.

By analyzing the electromagnetic parameters and EMW absorption performance, we can elucidate the EMW attenuation mechanism of Ni_xS_y@De-etched composites that can be summarized as follows (Fig. 10). First, the combination of Ni₃S₄, Ni_xS₆, and diatomite optimizes the impedance matching of the absorber, reducing the surface reflection of the EMW, enhancing the attenuation capability, and ultimately obtaining excellent absorption performance. Second, the three-dimensional porous structure of diatomite as well as the two-dimensional small lamellar layer structure and the abundant specific surface area of Ni_xS_y cause multiple reflections and attenuation of the EMW entering the material, which enhances the EMW loss. Third, the abundant interfacial polarization formation between Ni_xS_y and diatomite, in conjunction with the paraffin wax, further contributes to the enhancement of EMW absorption, and the defective polarization caused by sulfurization, this flow of charge from the Ni–O bond to the Ni–S bond induces a variance in local electron density and disrupts the symmetry of the substituent sites. Consequently, spatial charge separation occurs. Under the influence of an applied electromagnetic field, this heterogeneous structure oscillates, generating dielectric polarization and significantly boosting macroscopic dielectric loss capability. The attenuation ability of the composite is improved. Finally, mild etching reduces the number of ineffective absorbers and increases the batch stability, which produces conduction losses while decreasing the dielectric properties to increase impedance matching, and the three-dimensional conductive network formed by diatomite's holes and Ni_xS_y lamellae facilitates the promotion of electron transfer. Under the combined effect of the above mechanisms, Ni_xS_y@De-etched composites obtained satisfactory EMW absorption properties.

Fig. 10 Schematic illustrations for the EMW absorption mechanism of Ni_xS_y@De-etched absorbers.

4. Conclusion

In this study, uniform and dense Ni(OH)₂ nanoflowers were grown on diatomite using a hydrothermal method. Ni_xS_y@De was prepared through sulfurization and the samples were mildly etched. The results indicate that sulfurization treatment significantly improved the dielectric loss, impedance matching characteristics, and absorption properties of the Ni_xS_y@De composites. At a thickness of 1.44 mm, the EAB was 9.86 GHz, and the RL_min at 1 mm was −33.61 dB. After mild etching, the RL_min at 1.16 mm further increases to −93.53 dB, resulting in high attenuation of microwave absorption. This composite material surpasses previous findings in terms of bandwidth and matched thickness, signifying its excellence. The exceptional ability to absorb EMW is mainly attributed to the high content of sulfur vacancy defect polarization and the synergistic effect of several mechanisms, such as multiple reflection and scattering incident waves, interfacial polarization, and generation of conduction loss. This study demonstrates the feasibility of diatomite as an EMW absorption material and provides new ideas for the development of new high-broadband EMW absorption materials.

Conflicts of interest

The authors affirm that they have no known financial or interpersonal conflicts that would have appeared to have an impact on the research presented in this study.

Acknowledgements

The authors gratefully acknowledge the financial support provided by Graduate Scientific Research and Innovation Foundation of Chongqing, China (CYB22007, CYS23071), Projects (No. 2020CDJXZ001) supported by the Fundamental Research Funds for the Central Universities, the Technology Innovation and Application Development Special Project of Chongqing (Z20211350 and Z20211351), Scientific Research Project of Chongqing Ecological Environment Bureau (No. CQEE2022-STHBZZ118). The authors also thank the Electron Microscopy Center of Chongqing University for its material characterizations.

References

1. Z. Wu, H.-W. Cheng, C. Jin, B. Yang, C. Xu, K. Pei, H. Zhang, Z. Yang and R. Che, Dimensional Design and Core–Shell Engineering of Nanomaterials for Electromagnetic Wave Absorption, Adv. Mater., 2022, 34(11), 2107538.
2. L. Liang, W. Gu, Y. Wu, B. Zhang, G. Wang, Y. Yang and G. Ji, Heterointerface Engineering in Electromagnetic Absorbers: New Insights and Opportunities, Adv. Mater., 2022, 34(4), 2106195.
3. A. Iqbal, F. Shahzad, K. Hantanasirisakul, M.-K. Kim, J. Kwon, J. Hong, H. Kim, D. Kim, Y. Gogotsi and C. M. Koo, Anomalous absorption of electromagnetic waves by 2D transition metal carbonitride Ti₃CNT_x (MXene), Science, 2020, 369(6502), 446–450.
4. P. Yi, Z. Yao, J. Zhou, B. Wei, L. Lei, R. Tan and H. Fan, Facile synthesis of 3D Ni@C nanocomposites derived from two kinds of petal-like Ni-based MOFs towards lightweight and efficient microwave absorbers, Nanoscale, 2021, 13(5), 3119–3135.
5. L. Yang, Y. Wang, Z. Lu, R. Cheng, N. Wang and Y. Li, Construction of multi-dimensional NiCo/C/CNT/rGO aerogel by MOF derivative for efficient microwave absorption, Carbon, 2023, 205, 411–421.
6. Z. Xiang, Y. Shi, X. Zhu, L. Cai and W. Lu, Flexible and Waterproof 2D/1D/0D Construction of MXene-Based Nanocomposites for Electromagnetic Wave Absorption, EMI Shielding, and Photothermal Conversion, Nano-Micro Lett., 2021, 13(1), 150.
7. L. Sun, Q. Zhu, Z. Jia, Z. Guo, W. Zhao and G. Wu, CrN attached multi-component carbon nanotube composites with superior electromagnetic wave absorption performance, Carbon, 2023, 208, 1–9.
8. L. Wang, X. Li, X. Shi, M. Huang, X. Li, Q. Zeng and R. Che, Recent progress of microwave absorption microspheres by magnetic-dielectric synergy, Nanoscale, 2021, 13(4), 2136–2156.
9. H. Lv, Z. Yang, B. Liu, G. Wu, Z. Lou, B. Fei and R. Wu, A flexible electromagnetic wave-electricity harvester, Nat. Commun., 2021, 12(1), 834.
10. M. Ling, F. Wu, P. Liu, Q. Zhang and B. Zhang, Fabrication of Graphdiyne/Graphene Composite Microsphere with Wrinkled Surface via Ultrasonic Spray Compounding and its Microwave Absorption Properties, Small, 2023, 19(7), 2205925.
11. Y. Cheng, J. Z. Y. Seow, H. Zhao, Z. J. Xu and G. Ji, A Flexible and Lightweight Biomass-Reinforced Microwave Absorber, Nano-Micro Lett., 2020, 12(1), 125.
12. X. Qiu, L. Wang, H. Zhu, Y. Guan and Q. Zhang, Lightweight and efficient microwave absorbing materials based on walnut shell-derived nano-porous carbon, Nanoscale, 2017, 9(22), 7408–7418.
13. H. K. Choi, A. Lee, M. Park, D. S. Lee, S. Bae, S.-K. Lee, S. H. Lee, T. Lee and T.-W. Kim, Hierarchical Porous Film with Layer-by-Layer Assembly of 2D Copper Nanosheets for Ultimate Electromagnetic Interference Shielding, ACS Nano, 2021, 15(1), 829–839.
14. Q. Liu, X. Xu, W. Xia, R. Che, C. Chen, Q. Cao and J. He, Dependency of magnetic microwave absorption on surface architecture of Co₂₀Ni₈₀ hierarchical structures studied by electron holography, Nanoscale, 2015, 7(5), 1736–1743.
15. M. Zhang, Z. Li, T. Wang, S. Ding, G. Song, J. Zhao, A. Meng, H. Yu and Q. Li, Preparation and electromagnetic wave absorption performance of Fe₃Si/SiC@SiO₂ nanocomposites, Chem. Eng. J., 2019, 362, 619–627.
16. D. Lan, M. Qin, J. Liu, G. Wu, Y. Zhang and H. Wu, Novel binary cobalt nickel oxide hollowed-out spheres for electromagnetic absorption applications, Chem. Eng. J., 2020, 382, 122797.
17. G. Liu, C. Wu, L. Hu, X. Hu, X. Zhang, J. Tang, H. Du, X. Wang and M. Yan, Anisotropy engineering of metal organic framework derivatives for effective electromagnetic wave absorption, Carbon, 2021, 181, 48–57.
18. Y. Jiao, F. Wu, A. Xie, L. Wu, W. Zhao, X. Zhu and X. Qi, Electrically conductive conjugate microporous polymers (CMPs) via confined polymerization of pyrrole for electromagnetic wave absorption, Chem. Eng. J., 2020, 398, 125591.
19. B. Wang, H. Chen, S. Wang, Y. Shi, X. Liu, Y. Fu and T. Liu, Construction of core-shell structured Co₇Fe₃@C nanocapsules with strong wideband microwave absorption at ultra-thin thickness, Carbon, 2021, 184, 223–231.
20. Q. Wu, B. Wang, Y. Fu, Z. Zhang, P. Yan and T. Liu, MOF-derived Co/CoO particles prepared by low temperature reduction for microwave absorption, Chem. Eng. J., 2021, 410, 128378.
21. C. Wei, M. He, M. Li, X. Ma, W. Dang, P. Liu and J. Gu, Hollow Co/NC@MnO₂ polyhedrons with enhanced synergistic effect for high-efficiency microwave absorption, Mater. Today Phys., 2023, 36, 101142.
22. Z. Jiang, Y. Gao, Z. Pan, M. Zhang, J. Guo, J. Zhang and C. Gong, Pomegranate-like ATO/SiO₂ microspheres for efficient microwave absorption in wide temperature spectrum, J. Mater. Sci. Technol., 2024, 174, 195–203.
23. Z. Jiang, H. Si, Y. Li, D. Li, H. Chen, C. Gong and J. Zhang, Reduced graphene oxide@carbon sphere based metacomposites for temperature-insensitive and efficient microwave absorption, Nano Res., 2022, 15(9), 8546–8554.
24. Y. Zhang, Y. Huang, T. Zhang, H. Chang, P. Xiao, H. Chen, Z. Huang and Y. Chen, Broadband and Tunable High-Performance Microwave Absorption of an Ultralight and Highly Compressible Graphene Foam, Adv. Mater., 2015, 27(12), 2049–2053.
25. J. Yan, Y. Huang, Y. Yan, L. Ding and P. Liu, High-Performance Electromagnetic Wave Absorbers Based on Two Kinds of Nickel-Based MOF-Derived Ni@C Microspheres, ACS Appl. Mater. Interfaces, 2019, 11(43), 40781–40792.
26. X. Li, L. Wang, W. You, L. Xing, X. Yu, Y. Li and R. Che, Morphology-controlled synthesis and excellent microwave absorption performance of ZnCo₂O₄ nanostructures via a self-assembly process of flake units, Nanoscale, 2019, 11(6), 2694–2702.
27. C. Z. Zhang, D. S. Wang, L. C. Dong, K. L. Li, Y. F. Zhang, P. A. Yang, S. Yi, X. J. Dai, C. Q. Yin, Z. L. Du, X. F. Zhang, Q. Zhou, Z. Y. Yi, J. S. Rao and Y. X. Zhang, Microwave Absorption of alpha-Fe₂O₃@diatomite Composites, Int. J. Mol. Sci., 2022, 23(16), 9362.
28. Y. F. Zhang, R. Cai, D. S. Wang, K. L. Li, Q. Sun, Y. T. Xiao, H. Teng, X. H. Huang, T. Sun, Z. H. Liu, K. X. Yao, Y. X. Zhang and P. A. Yang, Lightweight, Low-Cost Co₂SiO₄@diatomite Core-Shell Composite Material for High-Efficiency Microwave Absorption, Molecules, 2022, 27(3), 1055.
29. Z. Q. Yan, J. Cai, Y. G. Xu and D. Y. Zhang, Microwave absorption property of the diatomite coated by Fe-CoNiP films, Appl. Surf. Sci., 2015, 346, 77–83.
30. Q. Li, W. Guo, X. Kong, J. Xu, C. Xu, Y. e. Chen, J. Chen, X. Jia and Y. Ding, MnFe₂O₄/rGO/Diatomite composites with excellent wideband electromagnetic microwave absorption, J. Alloys Compd., 2023, 941, 168851.
31. Z. H. Xu, L. Tang, S. W. Zhang, J. Z. Li, B. L. Liu, S. C. Zhao, C. J. Yu and G. D. Wei, 2D MoS₂/CuPc heterojunction based highly sensitive photodetectors through ultrafast charge transfer, Mater. Today Phys., 2020, 15, 100273.
32. M. Chang, Z. Jia, S. He, J. Zhou, S. Zhang, M. Tian, B. Wang and G. Wu, Two-dimensional interface engineering of NiS/MoS_2/Ti₃C₂T_x heterostructures for promoting electromagnetic wave absorption capability, Composites, Part B, 2021, 225, 109306.
33. D. S. Wang, Y. Z. Hu, Z. Y. Cui, P. X. Yang, Z. L. Du, Y. Hou, P. A. Yang, J. S. Rao, C. Wang and Y. X. Zhang, Sulfur vacancy regulation and multipolarization of Ni_xCo₁S nanowires-decorated biotemplated structures to promote microwave absorption, J. Colloid Interface Sci., 2023, 646, 991–1001.
34. J. Liu, L. Zhang and H. Wu, Anion-Doping-Induced Vacancy Engineering of Cobalt Sulfoselenide for Boosting Electromagnetic Wave Absorption, Adv. Funct. Mater., 2022, 32(26), 2200544.
35. S. Hui, L. Zhang and H. Wu, Cationic doping induced sulfur vacancy formation in polyionic sulfide for enhanced electromagnetic wave absorption, J. Colloid Interface Sci., 2023, 629, 147–155.
36. C. Li, D. Li, L. Zhang, Y. Zhang, L. Zhang, C. Gong and J. Zhang, Boosted microwave absorption performance of transition metal doped TiN fibers at elevated temperature, Nano Res., 2023, 16(2), 3570–3579.
37. Z. Du, D. Wang, X. Zhang, Z. Yi, J. Tang, P. Yang, R. Cai, S. Yi, J. Rao and Y. Zhang, Core-Shell Structured SiO₂@NiFe LDH Composite for Broadband Electromagnetic Wave Absorption, Int. J. Mol. Sci., 2023, 24(1), 504.
38. W. Wei, Z. Guo, J. Xu, Z. Fang, J. Zhang, Y. Jia and L. Mi, Novel Ni₃S₄/NiS/NC composite with multiple heterojunctions synthesized through space-confined effect for high-performance supercapacitors, Int. J. Extreme Manuf., 2023, 5(1), 015504.
39. D. Jiang, H. Liang, W. Yang, Y. Liu, X. Cao, J. Zhang, C. Li, J. Liu and J. J. Gooding, Screen-printable films of graphene/CoS₂/Ni₃S₄ composites for the fabrication of flexible and arbitrary-shaped all-solid-state hybrid supercapacitors, Carbon, 2019, 146, 557–567.
40. D. Wu, X. Xie, J. Zhang, Y. Ma, C. Hou, X. Sun, X. Yang, Y. Zhang, H. Kimura and W. Du, Embedding NiS nanoflakes in electrospun carbon fibers containing NiS nanoparticles for hybrid supercapacitors, Chem. Eng. J., 2022, 446, 137262.
41. B. Zhou, Y. Li, Y. Zou, W. Chen, W. Zhou, M. Song, Y. Wu, Y. Lu, J. Liu, Y. Wang and S. Wang, Platinum Modulates Redox Properties and 5-Hydroxymethylfurfural Adsorption Kinetics of Ni(OH)₂ for Biomass Upgrading, Angew. Chem., Int. Ed., 2021, 60(42), 22908–22914.
42. E. C. Linganiso, B. W. Mwakikunga, N. J. Coville and S. D. Mhlanga, Observation of the structural, optical and magnetic properties during the transformation from hexagonal NiS nano-compounds to cubic NiO nanostructures due to thermal oxidation, J. Alloys Compd., 2015, 629, 131–139.
43. G. I. Titelman, V. Gelman, S. Bron, R. L. Khalfin, Y. Cohen and H. Bianco-Peled, Characteristics and microstructure of aqueous colloidal dispersions of graphite oxide, Carbon, 2005, 43(3), 641–649.
44. H. Gu, Y. Huang, L. Zuo, W. Fan and T. Liu, Electrospun carbon nanofiber@CoS₂ core/sheath hybrid as an efficient all-pH hydrogen evolution electrocatalyst, Inorg. Chem. Front., 2016, 3(10), 1280–1288.
45. W. C. Geng, L. B. Duan and Q. Y. Zhang, Humidity Sensing Property of Al-Doped Mesoporous Silica, Appl. Mech. Mater., 2012, 249–250, 992–997.
46. L. Luo, S.-H. Chung and A. Manthiram, Rational Design of a Dual-Function Hybrid Cathode Substrate for Lithium–Sulfur Batteries, Adv. Energy Mater., 2018, 8(24), 1801014.
47. Q. Chen, W. Chen, J. Ye, Z. Wang and J. Y. Lee, l-Cysteine-assisted hydrothermal synthesis of nickel disulfide/graphene composite with enhanced electrochemical performance for reversible lithium storage, J. Power Sources, 2015, 294, 51–58.
48. B. Li, Z. Ma, X. Zhang, J. Xu, Y. Chen, X. Zhang and C. Zhu, NiO/Ni Heterojunction on N-Doped Hollow Carbon Sphere with Balanced Dielectric Loss for Efficient Microwave Absorption, Small, 2023, 19(12), 2207197.
49. B. Zhao, Y. Li, Q. Zeng, L. Wang, J. Ding, R. Zhang and R. Che, Galvanic Replacement Reaction Involving Core–Shell Magnetic Chains and Orientation-Tunable Microwave Absorption Properties, Small, 2020, 16(40), 2003502.
50. B. Zhao, Y. Du, H. Lv, Z. Yan, H. Jian, G. Chen, Y. Wu, B. Fan, J. Zhang, L. Wu, D. W. Zhang and R. Che, Liquid-Metal-Assisted Programmed Galvanic Engineering of Core–shell Nanohybrids for Microwave Absorption, Adv. Funct. Mater., 2023, 33(34), 2302172.
51. M. Li, X. Song, J. Xue, F. Ye, L. Yin, L. Cheng and X. Fan, Construction of Hollow Carbon Nanofibers with Graphene Nanorods as Nano-Antennas for Lower-Frequency Microwave Absorption, ACS Appl. Mater. Interfaces, 2023, 15(26), 31720–31728.
52. W. Zhao, S. Yuan, S. Lei, Z. Zeng, J. Dong, F. Jiang, Y. Yang, W. Sun, X. Ji and P. Ge, Tailoring Rational Crystal Orientation and Tunable Sulfur Vacancy on Metal-Sulfides toward Advanced Ultrafast Ion-Storage Capability, Adv. Funct. Mater., 2023, 33(5), 2211542.
53. B. Zhao, Y. Du, Z. Yan, L. Rao, G. Chen, M. Yuan, L. Yang, J. Zhang and R. Che, Structural Defects in Phase-Regulated High-Entropy Oxides toward Superior Microwave Absorption Properties, Adv. Funct. Mater., 2023, 33(1), 2209924.
54. B. Zhao, Z. Yan, Y. Du, L. Rao, G. Chen, Y. Wu, L. Yang, J. Zhang, L. Wu, D. W. Zhang and R. Che, High-Entropy Enhanced Microwave Attenuation in Titanate Perovskites, Adv. Mater., 2023, 35(11), 2210243.
55. J. Wang, Z. Wu, Y. Xing, B. Li, P. Huang and L. Liu, Multi-Scale Design of Ultra-Broadband Microwave Metamaterial Absorber Based on Hollow Carbon/MXene/Mo₂C Microtube, Small, 2023, 19(14), 2207051.
56. F. Pan, Z. Liu, B. Deng, Y. Dong, X. Zhu, C. Huang and W. Lu, Lotus Leaf-Derived Gradient Hierarchical Porous C/MoS₂ Morphology Genetic Composites with Wideband and Tunable Electromagnetic Absorption Performance, Nano-Micro Lett., 2021, 13(43), 1–17.
57. Z. X. Wang, X. F. Li, L. Y. Wang, Y. P. Li, J. Y. Qin, P. T. Xie, Y. P. Qu, K. Sun and R. H. Fan, Flexible multi-walled carbon nanotubes/polydimethylsiloxane membranous composites toward high-permittivity performance, Adv. Compos. Hybrid Mater., 2020, 3(1), 1–7.
58. L. Gai, H. Zhao, F. Wang, P. Wang, Y. Liu, X. Han and Y. Du, Advances in core-shell engineering of carbon-based composites for electromagnetic wave absorption, Nano Res., 2022, 15(10), 9410–9439.
59. M. Qin, H. Liang, X. Zhao and H. Wu, Filter paper templated one-dimensional NiO/NiCo₂O₄ microrod with wideband electromagnetic wave absorption capacity - ScienceDirect, J. Colloid Interface Sci., 2020, 566, 347–356.
60. H. Lv, G. Ji, X. H. Liang, H. Zhang and Y. Du, A novel rod-like MnO₂@Fe loading on graphene giving excellent electromagnetic absorption properties, J. Mater. Chem. C, 2015, 3(19), 5056–5064.
61. M. Li, W. Zhu, X. Li, H. Xu, X. Fan, H. Wu, F. Ye, J. Xue, X. Li, L. Cheng and L. Zhang, Ti₃C₂T_x/MoS₂ Self-Rolling Rod-Based Foam Boosts Interfacial Polarization for Electromagnetic Wave Absorption, Adv. Sci., 2022, 9(16), 2201118.
62. X. Cui, X. Liang, W. Liu, W. Gu and Y. Du, Stable microwave absorber derived from 1D customized heterogeneous structures of Fe₃N@C, Chem. Eng. J., 2019, 381(7), 122589.
63. Y. Wu, Y. Zhao, M. Zhou, S. J. Tan, R. Peymanfar, B. Aslibeiki and G. B. Ji, Ultrabroad Microwave Absorption Ability and Infrared Stealth Property of Nano-Micro CuS@rGO Lightweight Aerogels, Nano-Micro Lett., 2022, 14(1), 171.
64. H. T. Jiang, C. J. Wang, B. W. Cui, X. D. Xu, M. F. Li, Z. H. Xu, H. X. Tan, C. G. Wang and Y. X. Wang, Lotus Leaf Derived NiS/Carbon Nanofibers/Porous Carbon Heterogeneous Structures for Strong and Broadband Microwave Absorption, Small, 2023, 2304918.
65. T. Zhu, W. Shen, X. Y. Wang, Y. F. Song and W. Wang, Paramagnetic CoS₂@MoS₂ core-shell composites coated by reduced graphene oxide as broadband and tunable high-performance microwave absorbers, Chem. Eng. J., 2019, 378, 122159.
66. B. Zhao, G. Shao, B. B. Fan, W. Y. Zhao, Y. J. Xie and R. Zhang, Synthesis of flower-like CuS hollow microspheres based on nanoflakes self-assembly and their microwave absorption properties, J. Mater. Chem. A, 2015, 3(19), 10345–10352.
67. H. J. Wu, J. L. Liu, H. S. Liang and D. Y. Zang, Sandwich-like Fe₃O₄/Fe₃S₄ composites for electromagnetic wave absorption, Chem. Eng. J., 2020, 393, 124743.
68. P. B. Liu, S. Gao, X. D. Liu, Y. Huang, W. J. He and Y. T. Li, Rational construction of hierarchical hollow CuS@CoS₂ nanoboxes with heterogeneous interfaces for high-efficiency microwave absorption materials, Composites, Part B, 2020, 192, 107992.
69. J. L. Liu, L. M. Zhang, D. Y. Zang and H. J. Wu, A Competitive Reaction Strategy toward Binary Metal Sulfides for Tailoring Electromagnetic Wave Absorption, Adv. Funct. Mater., 2021, 31(45), 2105018.
70. W. B. Huang, Z. Y. Tong, Y. X. Bi, M. L. Ma, Z. J. Liao, G. L. Wu, Y. Ma, S. Y. Guo, X. Y. Jiang and X. P. Liu, Synthesis and microwave absorption properties of coralloid core-shell structure NiS/Ni₃S₄@PPy@MoS₂ nanowires, J. Colloid Interface Sci., 2021, 599, 262–270.
71. Y. N. Dong, X. J. Zhu, F. Pan, Z. Xiang, X. Zhang, L. Cai and W. Lu, Fire-retardant and thermal insulating honeycomb-like NiS₂/SnS₂ nanosheets @ 3D porous carbon hybrids for high-efficiency electromagnetic wave absorption, Chem. Eng. J., 2021, 426, 131272.

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Footnotes

† Electronic supplementary information (ESI) available. See DOI: https://doi.org/10.1039/d3nr05275c

‡ These authors contributed equally to this work.

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