Enhanced microwave absorption capacity of hierarchical structural MnO₂@NiMoO₄ composites

Abstract

Hierarchical hybrid nanostructures are desirable materials for microwave absorption (MA) capacity. However, how to obtain this kind of versatile structural materials still remains a great challenge. In this work, a novel MA composite of MnO₂@NiMoO₄ was synthesized via two-step hydrothermal processes combined with a simple annealing process. As confirmed by X-ray diffraction, scanning electron microscopy, energy-dispersive spectrometry, and transmission electron microscopy analysis, the well-defined NiMoO₄ nanosheets could uniformly cover the surface of the MnO₂ nanorods. Compared with pure MnO₂ nanorods, these hierarchical composite structures could provide a higher superficial area, and more effective components, which will favor the penetration of microwaves into the absorber effectively instead of reflecting it, and then translate it into thermal energy. The minimum reflection loss (RL) value of MnO₂@NiMoO₄ composites was −31.4 dB at 11.2 GHz with a thickness of 3 mm, and the band of reflection loss was below −10 dB when frequency was in the range from 9.6 to 14.1 GHz. However, the minimum RL value of MnO₂ was only −12.5 dB at 10.4 GHz with a thickness of 3 mm. The significantly enhanced microwave absorption of MnO₂@NiMoO₄ composites is mainly attributed to the hierarchical hybrid nanostructures, multi-effective components, good impedance matching, and interfacial polarization between MnO₂ and NiMoO₄. Meanwhile, the surface attached NiMoO₄ is useful to increase the multiple reflection of electromagneticwaves. It is believed that these MnO₂@NiMoO₄ composites could serve as an excellent microwave absorber in practical applications.

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Introduction

Nowadays, considerable investigations have been focused on high efficiency, broad-bandwidth, and lightweight microwave absorption (MA) materials due to the wide use of electronic instruments, such as satellite communication, military and commercial radar systems, personal digital assistants, and other communication devices. Up to now, many kinds of MA materials, including magnetism loss materials (Fe₃O₄,^1–3 Co,⁴ Ni,⁵ etc.), dielectric loss materials (MnO₂,⁶ Co₃O₄,⁷ SnO₂,⁸ etc.) and carbon-based materials (graphene,⁹ carbon nanotube,¹⁰ carbon fibers,¹¹ etc.) have been investigated extensively to cope with the rising electromagnetic interference (EMI) problems. It is well known that the key issue of obtaining excellent MA performance is how to let more microwave into absorber and then rapidly convert into thermal energy. In recent years, the precise design of hetero-nanostructured materials, such as flower-like hollow microspheres,¹² yolk–shell microspheres,¹³ nano-rod array,¹⁴ porous nanostructure,¹⁵ have been quite popular since these hetero-nanostructures could absorb much more microwave. Wang and co-workers¹⁶ demonstrated that graphene@Fe₃O₄@SiO₂@NiO nanosheets hierarchical structures exhibited significantly enhanced MA properties which originated from large surface area and high porosity structure, and the maximum reflection loss (RL) was −51.5 dB at 14.6 GHz with a thickness of only 1.8 mm. Zhao et al.¹⁷ prepared amorphous Ni@TiO₂ and Ni@SiO₂ composite microspheres using a two-step method, and their results showed that interface polarization of the core–shell structure was one of the main factors to generate excellent microwave absorption abilities. Three-dimensional flower-like Co/CoO composites were also prepared.¹⁸ It was found that the optimal reflection loss value reached −50 dB with the frequency bandwidth ranged from 13.8 to 18 GHz.

The oxides of Mn, Ni or Mo have been extensively explored as the MA materials. Guan et al.¹⁹ found that the dielectric loss, which is resulted from the space charge polarization, played the dominant role in the total loss for α-MnO₂ nanowires and β-MnO₂ nanorods. Well crystallized ultra-long MnO₂ nanowires were synthesized through hydrothermal method,²⁰ and the value of maximum RL was −35 dB at 3.13 GHz with a thickness of 3.6 mm. NiO is also an important transition-metal oxide that has been extensively studied in the area of MA because of its dielectric properties.^16,21,22 MoO₃ has also been successfully used to prepare PANI–MoO₃ composites²³ with different weight ratios to improve the MA properties. Binary metal oxides, such as CoFe₂O₄,²⁴ ZnFe₂O₄,²⁵ MnFe₂O₄,²⁶ NiCo₂O₄,²⁷ CoMoO₄,²⁸ ZnMn₂O₄,²⁹ have been found with outstanding MA performance than single component oxides due to their multiple oxidation states, complementary and synergistic effect. In addition, low cost, environment friendliness, and abundant resources enable binary metal oxides to be promising MA materials. Therefore, it can be concluded that the multi-component composites with binary metal oxide and hierarchical structures have a bright prospect for microwave absorption.

In this work, MnO₂ nanorods were selected as templates to synthesize hetero-nanostructured MnO₂@NiMoO₄ composites due to its high dielectric property and good MA capacity.^19,20 The surface distributed binary metal oxide, NiMoO₄ nanosheets on the MnO₂ nanorods could offer high surface area, light weight and good dielectric loss. To the best of our knowledge, fabrication of hierarchical nanostructures composed of NiMoO₄ nanosheet and MnO₂ nanorod for MA properties has not been reported yet. Herein, the MnO₂@NiMoO₄ composites were successfully prepared by three-steps. MnO₂ rods were firstly synthesized by hydrothermal method, followed by the surface precipitation of NiMoO₄ precursor nanosheets. Finally, the MnO₂@NiMoO₄ nanostructures were obtained by annealing. This study aims to develop a facile method to prepare high performance absorber, as well as to investigate the synergistic interaction between MnO₂ and NiMoO₄, and figure out the relationship between structure and MA properties.

Experimental

Materials

Manganese sulfate monohydrate (MnSO₄·H₂O), polyethylene glycol (PEG-400), urea (CH₄N₂O) were purchased from Aladdin Chemical Reagent Co., Ltd. Potassium permanganate (KMnO₄) were purchased from Yantai Sanhe Chemical Reagent Co., Ltd. Nickel acetate tetrahydrate (C₄H₆NiO₄·4H₂O), hexaammonium heptamolybdate tetrahydrate ((NH₄)₆Mo₇O₂₄) were purchased from Sinopharm Chemical Reagent Co., Ltd. Ethanol was purchased from Tianjin Fuyu Fine Chemical Co., Ltd. All reagents were of analytical grade and used without further purification.

Preparation of MnO₂ nano-rods

The MnO₂ nano-rods were prepared by a hydrothermal method as described below. Briefly, MnSO₄·H₂O (4 mmol, 0.676 g) and KMnO₄ (20 mmol, 3.161 g) were first dissolved in 10 mL and 20 mL deionized water, respectively. 2 mL PEG-400 and 10 mL deionized water were added in a 50 mL beaker, stirred it for 10 minutes, then MnSO₄ aqueous solution was added and the resulting mixture was stirred it for 30 minutes. After that, KMnO₄ aqueous solution was added and stirred for 10 minutes. The mixture was transferred into a 50 mL Teflon-lined stainless steel autoclave and heated at 160 °C for 12 h, then cooled to room temperature naturally. Finally, the precipitates were centrifuged and washed with deionized water and ethanol several times and dried at 60 °C in an oven.

Synthesis of MnO₂@NiMoO₄ core–shell nanostructures

The MnO₂@NiMoO₄ core–shell nanostructures were synthesized through co-precipitation method using MnO₂ as template. The as-prepared MnO₂ nano-rods (0.12 g) were dispersed in 60 mL deionized water, followed by the addition of C₄H₆NiO₄·4H₂O (2 mmol, 0.498 g), (NH₄)₆Mo₇O₂₄ (0.32 mmol, 0.4 g), and urea (8 mmol, 0.48 g). The mixture was stirred for 10 minutes and transferred into a 100 mL Teflon-lined stainless steel autoclave, then heated at 160 °C for 1 h. After that, MnO₂@NiMoO₄ precursors were centrifuged and washed with deionized water and ethanol several times and dried at 60 °C in a oven, followed by annealing at 400 °C for 2 h in air atmosphere to obtain MnO₂@NiMoO₄ composites. For comparison, NiMoO₄ nanosheets were prepared through the same procedures, but without the MnO₂ template.

Structural characterizations and microwave absorption measurements

X-ray diffraction (XRD) measurements were carried out on a Rigaku D-MAX 2500/PC using Cu K_α radiation (λ = 1.54056 Å), operating at 40 kV and 100 mA. The morphology and internal structures of samples were examined by a transmission electron microscope (TEM, JEOL JEM-2100) and field emission scanning electron microscopy (FESEM, JEOL JSM-6700F). Energy-dispersive X-ray spectrometer (EDS) was attached to the SEM. The chemical state of the surface of MnO₂@NiMoO₄ was investigated using X-ray photoelectron spectroscopy (XPS) (ESCALAB 250XI, Thermo Fisher Scientific). The thermogravimetry (TG) of MnO₂@NiMoO₄ precursor was carried out in nitrogen atmosphere with heating rate of 5 °C min⁻¹ (STA449C, Netasch Corp.). The UV-vis spectra of MnO₂ and MnO₂@NiMoO₄ were recorded on a UV-vis-NIR spectrophotometer (Lambda 950, Perkin Elmer Instruments, USA). The Raman spectrum of MnO₂@NiMoO₄ was measured using a LabRAM HR confocal Raman system with 532 nm diode laser excitation at room temperature. The surface area was calculated using the Brunauer–Emmett–Teller (BET) method based on the adsorption data.

For electromagnetic parameter measurements, specimens were prepared by uniformly mixing 50 wt% MnO₂, MnO₂@NiMoO₄ and NiMoO₄ samples with paraffin, and the resulting mixtures were pressed into a cylindrical shaped compact with an inner and outer diameter of 3.04 mm and 7.00 mm, respectively. The complex relative permittivity and permeability of the composites were measured on an Agilent N5244A vector network analyzer using the transmission/reflection coaxial line method from 2.0 GHz to 18.0 GHz.

Results and discussion

The experimental procedure for the synthesis of MnO₂@NiMoO₄ composite is illustrated in Fig. 1a. First, brown MnO₂ rods were prepared by a hydrothermal process, green NiMoO₄ precursor nanosheets were then deposited continuously onto the surface of MnO₂ rods. Subsequently, brown MnO₂@NiMoO₄ composites were obtained by an annealing process, which was performed at 400 °C for 2 h in air atmosphere. SEM images of the as-prepared MnO₂, MnO₂@NiMoO₄ precursors, and MnO₂@NiMoO₄ composites are shown in Fig. 1b–d, respectively. It can be seen that MnO₂ has the smooth rod shape, and the diameter is about 0.2–1.5 μm. Well defined NiMoO₄ precursor can uniformly grown on MnO₂ rods, which can be easily transferred to NiMoO₄ by annealing in the air and the nanosheets features still remained unchanged. According to the literature of Mazzocchia et al.,³⁰ the green MnO₂@NiMoO₄ precursor of Ni_1+δxMoO₄·nH₂O·mNH₃ (δx, n, and m values may be slightly changed depending on the experimental conditions, such as pH, precipitation temperature, and duration of aging) was obtained in alkaline condition, and the reaction equation³¹ can be described as follows.

Mo₇O₂₄⁶⁻ + 4H₂O → 7MoO₄²⁻ + 8H⁺

Fig. 1 (a) The representative synthetic procedure of the MnO₂@NiMoO₄ composites. SEM images of MnO₂ nanorods (b), MnO₂@NiMoO₄ precursors (c) and MnO₂@NiMoO₄ composites (d). XRD pattern of MnO₂@NiMoO₄ composites (e). EDS analysis of MnO₂ and MnO₂@NiMoO₄ composites (f).

The XRD pattern (Fig. 1e) revealed that the MnO₂@NiMoO₄ composites exhibited the diffraction peaks of MnO₂ (JCPDS no. 24-0735) and NiMoO₄ (JCPDS no. 45-0142),^32,33 which confirmed the existence of two kinds of substances. The low intensity of diffraction peaks of NiMoO₄ indicated the poor crystallization of NiMoO₄ sheets in the MnO₂@NiMoO₄ composites. In addition, the energy dispersive spectroscopy (EDS) measurement showed the presentation of Mn, Ni, Mo, and O elements in MnO₂@NiMoO₄ composite, which further demonstrated that this composite was composed of MnO₂ and NiMoO₄. Further information about the morphology and structure of MnO₂@NiMoO₄ composite were obtained from SEM and TEM images of Fig. 2. As shown in Fig. 2a and b, the MnO₂ rods interrelated and crossed to form uniform snowflakes-like morphology. The low magnification image demonstrates that NiMoO₄ nanosheets uniformly surround MnO₂ rod and the morphology of NiMoO₄ precursor can be well retained after annealing (Fig. 2c and d). The high-resolution image shows that the thickness of NiMoO₄ nanosheets is about 50 nm (Fig. 2e). The selected area electron diffraction (SAED) image (Fig. 2f) reveals the polycrystalline nature of NiMoO₄ sheets and can be readily indexed to (22-2), (132), and (44-3) crystal planes of the NiMoO₄ phase, which is consistent with the XRD characterization.

Fig. 2 SEM images of MnO₂@NiMoO₄ composites (a and b). Low magnification TEM images of MnO₂@NiMoO₄ precursor (c), MnO₂@NiMoO₄ composites (d), high magnification of the NiMoO₄ nanosheets (e), and SAED pattern of the NiMoO₄ nanosheets (f).

The surface chemical state and elemental character of the MnO₂@NiMoO₄ composites were investigated through XPS measurements and the corresponding results were presented in Fig. 3. Three typical peaks: Ni 2p, O 1s, and Mo 3d were observed clearly in the survey spectrum (Fig. 3a). In the Ni 2p spectra (Fig. 3b), the fitting peaks at 856.0 eV and 873.7 eV should be attributed to Ni 2p_3/2 level, while the fitting peaks at 861.2 eV and 879.8 eV could be indexed to Ni 2p_1/2 level.^34,35 In the O 1s spectra, three fitting peaks were observed (Fig. 3c). The fitting peaks at 529.6 eV and 530.7 eV corresponded to the typical metal–oxygen bond. The fitting peak at 531.2 eV should be assigned to the oxygen in defects, contaminants, and surface species.³⁶ In the Mo 3d spectra (Fig. 3d), two peaks with binding energies of 232.2 eV and 235.3 eV could be assigned to Mo 3d_5/2 and Mo 3d_3/2, respectively.^34,37 The binding energy peaks of Mo 3d were separated by 3.1 eV, which signified the existence of different oxidation states of Mo(VI) (Mo 3d_5/2 and Mo 3d_3/2).³⁸ These results confirmed the successful preparation of MnO₂@NiMoO₄ composites.

Fig. 3 XPS spectra of (a) survey spectrum, (b) Ni 2p, (c) O 1s, and (d) Mo 3d for MnO₂@NiMoO₄ composites.

The TG curve (Fig. 4a) indicated the mass reduction of MnO₂@NiMoO₄ precursor in the N₂ atmosphere. The first mass loss occurred from room temperature to 180 °C due to the evaporation of adsorbed water. The second mass loss happened from 180 °C to 450 °C which should be assigned to the removal of H₂O and NH₃ in Ni_1+δxMoO₄·nH₂O·mNH₃ precursor.³⁹ The UV-vis spectrum of MnO₂@NiMoO₄ composites (Fig. 4b) showed a strong absorption at 211 nm and a shoulder at 230 nm, which was mainly attributed to the charge-transfer transition (from 2p orbitals of oxygen to 4d orbitals of molybdenum) inside the MoO₄²⁻ anion.^40,41 The Raman spectrum exhibited the typical peaks of NiMoO₄ and MnO₂ (Fig. 4c). The main peaks at 935, 889, and 811 cm⁻¹ could be assigned to the β-NiMoO₄ phase.⁴² The symmetric stretching mode of Mo–O was observed at 935 cm⁻¹. The asymmetric stretching and bending modes of oxygen in O–Mo–O located at 889 and 356 cm⁻¹, respectively.⁴³ The peak centred at 553 cm⁻¹ should be assigned to the characteristic feature of MnO₂.⁴⁴ The nitrogen adsorption and desorption measurements were employed to investigate the Brunauer–Emmett–Teller (BET) surface area of MnO₂@NiMoO₄ composite. As shown in Fig. 4d, a hysteresis loop in the range of 0.5–1.0 p/p₀ was observed for MnO₂@NiMoO₄ composites, indicating the presence of mesoporous structure,^45,46 and the BET surface area value was measured to be 109.7 m² g⁻¹. However, the MnO₂ nanorods showed a character of non-porous materials and the BET surface area value was only 0.06 m² g⁻¹, which was much lower than that of MnO₂@NiMoO₄ composite. The pore size distribution of the MnO₂@NiMoO₄ composite was calculated by Barrett–Joyner–Halenda (BJH) method. As shown in Fig. 4d, the distributions of pores centered at 4.7 nm and the pore volume was 0.29 cm³ g⁻¹. Therefore, NiMoO₄ nanosheets provided a large surface area coating for MnO₂ rod, which could greatly improve the chance of penetration of microwave into the absorber.

Fig. 4 (a) TG curve of MnO₂@NiMoO₄ precursor. (b) UV-vis spectra of MnO₂@NiMoO₄ composites. (c) Raman spectrum of MnO₂@NiMoO₄ composites. (d) Nitrogen adsorption–desorption isotherm curves of the MnO₂ nanorods and MnO₂@NiMoO₄ composites, and the inset is the pore size distribution of MnO₂@NiMoO₄ composites.

The microwave absorption properties of MnO₂@NiMoO₄ materials were evaluated by the values of reflection loss (RL), which depend on permittivity, permeability, and thickness of composite layers. According to transmission line theory, the value of RL can be evaluated by the following equation:⁴⁷

where Z₀ is the impedance of free space and Z_in is the input characteristic impedance, which can be expressed as below:⁴⁸
where d is the absorber thickness, c is the velocity of light, and f is the microwave frequency. The real (ε′, μ′) and imaginary (ε′′, μ′′) parts of the complex relative permittivity and permeability can be measured by a vector network analyzer. Using the following equations: ε_r = ε′ − jε′′, μ_r = μ′ − jμ′′, the values of ε_r and μ_r can be obtained. In general, RL value less than −10 dB corresponds to more than 90% microwave absorption, a threshold value which is usually required for materials as suitable absorbers.

Fig. 5 shows the comparison of calculated RL curves in the frequency range of 2–18 GHz for MnO₂, MnO₂@NiMoO₄ and NiMoO₄/paraffin composites with a thickness of 3 mm. It was found that MnO₂@NiMoO₄ composites exhibited enhanced MA performance compared with MnO₂ or NiMoO₄ alone. The minimum RL value was found to be −31.4 dB at 11.2 GHz, and the band of RL below −10 dB was from 9.6 to 14.1 GHz. However, the minimum RL values of MnO₂ and NiMoO₄ were −12.5 dB at 10.4 GHz and −11.0 dB at 11.1 GHz, respectively. In order to determine the influences of thickness and its corresponding frequency on the MA properties in detail, three-dimensional RL values of MnO₂@NiMoO₄ are shown in Fig. 6. It was found that the attenuation peaks shifted to lower frequency according to quarter-wavelength cancellation model.^47,49

Fig. 5 Microwave RL curves of MnO₂, MnO₂@NiMoO₄ and NiMoO₄/paraffin composites with a thickness of 3 mm in the frequency range of 2–18 GHz.

Fig. 6 Three-dimensional representation of RL of MnO₂@NiMoO₄/paraffin composites in the frequency range of 2–18 GHz.

In order to investigate possible mechanism for the MA reinforcement of the MnO₂@NiMoO₄ composites, the real (ε′) and imaginary parts of relative permittivity (ε′′) and real (μ′) and imaginary (μ′′) permeability of three samples were discussed. As shown in Fig. 7, both the values of ε′ and ε′′ for MnO₂ decreased with the increasing frequency. For MnO₂@NiMoO₄ composites, the trend of ε′ and ε′′ was more gentle. A minimum value (ε′) and a maximum value (ε′′) at about 12.5 GHz were observed, which should be associated with the resonant behavior.³ Similarly, resonant behavior at about 10.6 GHz was also observed with NiMoO₄. It is well known, if we want to obtain high MA performance, ε′′ value should not be too high or too low due to the requirement of impedance matching.⁵⁰ As shown in Fig. 7a the MnO₂@NiMoO₄ composites just have the proper ε′′ value, which makes impedance matching occurred. In addition, there are two other factors contributing to dielectric loss of MnO₂@NiMoO₄ composites,⁵¹ one is the synergistic effect, which comes from the effective components of MnO₂ and NiMoO₄; the other is the dipole relaxation loss and interface polarization. The former origins from heterogeneity and defects in the system and the latter from the different dielectric constants of the two media.¹⁶ Owning to the absence of magnetic component in the composites, the μ′ and μ′′ values were not high (Fig. 7b). For MnO₂ and MnO₂@NiMoO₄, the change of μ′ and μ′′ curves was gentle, however, for NiMoO₄, obvious peaks at about 11.0 GHz were observed. It is noteworthy that some μ′′ values are negative, which means that the magnetic energy is radiated out and transferred into the electric energy.¹⁷

Fig. 7 The real (ε′) and imaginary (ε′′) parts of complex permittivity (a), and the real (μ′) and imaginary (μ′′) parts of permeability (b) of MnO₂, MnO₂@NiMoO₄ and NiMoO₄/paraffin composites.

The eddy current loss, domain-wall resonance, and magnetic hysteresis are the main contributors to magnetic loss.⁵² For composites lack of magnetic particles, domain-wall resonance and magnetic hysteresis would have less influence. To determine whether the eddy current loss plays a significant role in magnetic loss or not, the equation was used as follows: μ′′ ≈ 2πμ₀(μ′)²σd²f/3, where d is the diameter of the nanoparticle, σ is the electric conductivity, μ₀ is the permeability of vacuum. According to this equation, if the magnetic loss mainly originates from the eddy current loss, the values of μ′′(μ′)⁻²f⁻¹ ≈ 2πμ₀σd²/3 should be constant at the varying frequency. As shown in Fig. 8, when f is higher than 5 GHz the values of μ′′(μ′)⁻²f⁻¹ remains almost constant for MnO₂ and MnO₂@NiMoO₄ composites, confirming that the eddy current loss is the main contribution to magnetic loss. For NiMoO₄, except the region of 10.4–12.4 GHz, the eddy current loss is the main contributor to magnetic loss.

Fig. 8 The values of μ′′(μ′)⁻²f⁻¹ versus the frequency for MnO₂, NiMoO₄ and MnO₂@NiMoO₄ composites, respectively.

It is well known that, when the incident wave impinges surface of materials, most of electromagnetic energy enters into the materials and interacts with electrons and charged particles, and then energy is dissipated in the form of heat. However, a portion of the wave will not be absorbed by the materials and reflected back, leading to the reduced MA performance. In current research, dihedral angles could be formed among the NiMoO₄ sheets,⁵³ which would increase the multiple reflection, leading to the higher losses of electromagnetic energy.⁵⁴ A possible electromagnetic wave absorbing mechanism is presented in Fig. 9. The enhanced attenuation should be attributed to the complementary properties of MnO₂ and NiMoO₄, the heterogeneous core/shell structures, the high surface area NiMoO₄ sheets morphology, and the occurrence of the multiple reflections.

Fig. 9 A possible mechanism for the microwave absorption on the surface of MnO₂@NiMoO₄ composites.

Conclusions

The MnO₂@NiMoO₄ composites have been successfully synthesized using hydrothermal process and a simple annealing process. The structure and morphology of products were characterized by XRD, SEM, and TEM, and the results showed that NiMoO₄ sheets were uniformly coated on the surface of MnO₂ rods. MA measurement indicated that the minimum RL value of MnO₂@NiMoO₄ composites was −31.4 dB at 11.2 GHz with a thickness of 3 mm, and the frequency for band of RL below −10 dB was observed from 9.6 to 14.1 GHz, which outperformed the pure MnO₂ rods or NiMoO₄ sheets. The high MA capacity of MnO₂@NiMoO₄ composites should mainly arise from the complementary properties of MnO₂ and NiMoO₄, the heterogeneous core/shell structures, the high surface area NiMoO₄ sheets morphology, and the occurrence of multiple reflections. It is envisioned that these MnO₂@NiMoO₄ composite will be a new type of MA materials for varied applications.

Acknowledgements

This work was supported by the Natural Science Foundation of China (51173087) and Qingdao Innovation Leading Expert Program.

Notes and references

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