Enhanced electromagnetic wave absorption property of binary ZnO/NiCo2O4 composites

Abstract: Nowadays, metal oxide-based electromagnetic wave absorbing materials have aroused widely attentions in the application of telecommunication and electronics due to their selectable mechanical and outstanding dielectric properties. Herein, the binary ZnO/NiCo₂O₄ nanoparticles were successfully synthesized via hydrothermal reaction and the electromagnetic wave absorption properties of the composites were investigated in detail. As a result, benefiting from the dielectric loss, the as-obtained ZnO/NiCo₂O₄-7 samples possessed a minimum reflection loss value of −33.49 dB at 18.0 GHz with the thickness of 4.99 mm. This work indicates that ZnO/NiCo₂O₄ composites have the promising candidate applications in electromagnetic wave absorption materials in the future.

Keywords: ZnO particles; dielectric loss; magnetic loss; electromagnetic wave absorption

1 Introduction

In recent years, due to the rapid applications of electronic devices, electromagnetic wave pollution, considered as the fourth pollution source, has aroused widely attentions in the scientific community ^[1–6]. This is because electromagnetic wave not only threats information security but also the health of human beings ^[7–9]. In the past few years, numerous effects have been conducted to weaken or eliminate this pollution ^[10–15]. To date, electromagnetic wave absorbing materials are considered as the most popular strategies due to the various candidate materials with their low reflection ^[16–18]. Generally, the efficiency of electromagnetic wave absorbing performance is highly dependent on the electromagnetic wave absorption materials, including structures and components ^[19,20]. Until now, numerous electromagnetic wave absorbing materials, including carbon ^[21–24], ceramics ^[25–28], conductive polymers ^[29], metal oxides ^[30–33], and magnetic metals ^[34–39], have been investigated and applied in real. Although those single-component materials have been widely studied in the past few years, constructing the composites with unique structures to improve the electromagnetic wave absorption properties via synergistic effects is still a mainstream trend. 

Among various composites, metal oxide semiconductor based materials are considered as the most promising candidates due to their unique electronic property, diverse forms, and tailored dielectric and magnetic loss. Liu et al. ^[40] synthesized a CoNi@SiO₂@TiO₂ microsphere. The results demonstrated that as-obtained samples exhibited remarkable microwave absorption properties with the reflection loss of –58.2 dB at 10.4 GHz with the thickness of 2.1 mm. However, the fabrication process is rather complex. Wang et al. ^[41] also reported a flexible broadband CC@ZnO electromagnetic wave absorbing material. Benefiting from high concentration of polarized charge, multiple reflection, and orientation polarization, the CC@ZnO composites exhibited excellent electromagnetic wave absorption performance with reflection loss reaching –43.6 dB with the thickness of 2.0 mm. Zhou et al. ^[34] synthesized a hierarchical CoNi@SiO₂@C composite through adjusting the dose of phenolic resin and achieved remarkable and broad bandwidth absorbing capability at the thickness only 2.2 mm. This advanced work also involved complication process and it is difficult to achieve amount of the samples. Recent advances in the preparation of hybrid electromagnetic wave absorbing materials clearly demonstrate that rational design microstructure is also critical in determining absorption properties. NiCo₂O₄ is considered as a good electromagnetic wave absorbing material for its high dielectric loss ^[42–45]. For example, Zhou et al. ^[44] revealed that the NiCo₂O₄ nano-flakes possess a minimum reflection loss of –25.5 dB at 4.5 GHz. However, such nano-flakes possessed weak electromagnetic wave absorbing performance. After synthesizing hierarchical core–shell C@NiCo₂O₄@Fe₃O₄ composites ^[46], the samples exhibited significant enhancement electromagnetic wave absorbing properties and the reflection loss increased to –43.0 dB at 13.4 GHz. Recently, Chang et al. ^[47] reported the different morphologies of NiCo₂O₄ particles as well as the electromagnetic wave absorbing performances. The results suggested that as-fabricated samples showed the large effective absorption bandwidth of 5.81 GHz. In addition, this work fabricated different morphologies of NiCo₂O₄ via adjusting the agent for the first time. Although numerous works have been conducted to fabricate NiCo₂O₄-based electromagnetic wave absorbing materials, most of them still restrict by its complex process. Thus, it remains a challenge to develop NiCo₂O₄- based electromagnetic wave absorbing materials with hybrid structure in a simple way. 

In previous works, ZnO nanoparticles had been successfully fabricated by a simple method and the as-synthesized samples showed good electromagnetic wave absorbing performance ^[48]. In this work, a hybrid structure with binary ZnO/NiCo₂O₄ nanoparticles synthesized through hydrothermal reaction using nitrate and ammonium hydroxide and then the as-obtained powders were heated at 300℃ . This simple method provides a new strategy to obtain the binary ZnO/NiCo₂O₄ composites with excellent electromagnetic wave absorption performance.

2 Experimental 

2. 1 Materials 

Zinc nitrate (Zn(NO₃)₂·6H₂O), cobalt nitrate (Co(NO₃)₂·6H₂O), nickel nitrate (Ni(NO₃)₂·6H₂O), and ammonia aqueous solution (NH₃·H₂O, 25–30 wt%) were of analytical grade without further purification and purchased from Shanghai Aladdin Biochemical Technology Co., Ltd., China. 

2. 2 Synthesis of ZnO/NiCo₂O₄ nanoparticles

The ZnO/NiCo₂O₄ nanoparticles were fabricated by a facile hydrothermal reaction strategy. Typically, 0.30 g of Zn(NO₃)₂·6H₂O, 0.29 g of Ni(NO₃)₂·6H₂O, and 1.16 g of Co(NO₃)₂·6H₂O were gradually dissolved in deionized water (40 mL) using magnetic stirring. Then, different amounts of NH₃·H₂O (0.5, 0.7, and 1.0 mL) were added into the mixture solution and the color of mixture solution changed from milk white to baby blue. The corresponded specimens were labeled as ZnO/NiCo₂O₄-5, ZnO/NiCo₂O₄-7, and ZnO/NiCo₂O₄-10, respectively. Subsequently, the solution was transformed into a Teflon reactor and carried out at 200℃ for 18 h. The resulting black particles were washed by ethanol and distilled water for several times. Finally, the obtained brown powders were treated at 300℃ at air atmosphere for 2 h to obtain ZnO/NiCo₂O₄ nanoparticles. 

2. 3 Characterization 

The phase structure was characterized by X-ray diffraction (XRD) using a Rigaku D/max-3C equipped with Cu Kα radiation source (λ = 0.1541 nm) operating at 40 kV. The morphologies and microstructures were analyzed by a field-emission scanning electron microscope (FE-SEM, Helions Nanolab600i, USA) and a transmission electron microscope (TEM, CM300, Philips, the Netherlands), respectively. The magnetic characters of the as-prepared samples were observed via a vibrating sample magnetometer (VSM, Lakeshore 7407, USA) at room temperature. The specific surface and pore size distribution were analyzed by nitrogen adsorption–desorption isotherms (ASAP2020, USA). To evaluate the electromagnetic parameters, a vector network analyzer (Agilent Technologyies N5230A, USA) was utilized in 2.00–18.00 GHz. The powders were homogeneous mixed with paraffin at a mass ratio of 1:1 and then pressed into a circle shape (Φ_outer = 7.00 mm, Φ_inter = 3.04 mm) with the thickness of 2.5 mm. The reflection loss values were calculated by the electromagnetic parameters according to the transmission line theory.

3 Results and discussion

The phase and crystal structure of the as-obtained powders are analyzed by XRD and the results are shown in Fig. 1(a). The peaks located at 32°, 34°, 36°, 52°, 47°, 57°, 59°, and 62° can be confirmed to hexagonal ZnO phase (PDF#99-0111). The remain peaks, including 31°, 33°, 37°, 38°, 39°, 44°, and 65° can be indexed to the NiCo₂O₄ phase (PDF#20-0781). The detailed formation of ZnO and NiCo₂O₄ phases can be described as followings: Firstly, the cationic of Ni²⁺ and Co²⁺ diffused around the Zn(OH)₂ particles, and formed the nucleus of NiCo₂(OH)₆ for the minimization of its surface energy according to Eqs. (1)–(3). Then, during the heat-treatment process, the Zn(OH)₂ and NiCo₂(OH)₆ hybrid particles converted into ZnO and NiCo₂O₄ hybird nanoparticles simultaneously according to Eqs. (4) and (5) ^[43–45,47], respectively. Meanwhile, no other peaks of the impurity are observed in XRD pattern, indicating that the high purity of the as-obtained powders. Figures 1(b)–1(d) demonstrate the morphologies of the as-obtained powders observed by SEM. As shown in Fig. 1, both the ZnO/NiCo₂O₄-5 and ZnO/NiCo₂O₄-7 samples are irregular shape particles ranging from several nanometers to hundred nanometers (Figs. 1(b) and 1(c)). The SEM image of ZnO/NiCo₂O₄-10 apparently represents a well-regulated structure (Fig. 1(d)). Amounts of well-regulated particles arrange together. The different morphologies of ZnO/NiCo₂O₄ nanoparticles are attributed to the concentration of the alkalinity. As reported in Ref. ^[45], with increasing the concentration of alkalinity, i.e., increasing the amount of NH₃·H₂O, the ZnO nanoparticles are easy to form the large-dimensional growth units and therefore, form the well-regulated particles. 

NH₃ + H₂O → NH4⁺ + OH^– (1) 
 Zn²⁺ + 2OH– → Zn(OH)₂ (2) 
 2Co²⁺ + Ni²⁺ + 6OH^– → NiCo₂(OH)₆ (3) 
 Zn(OH)₂ → ZnO + H₂O (4) 
 NiCo₂(OH)₆ + 1/2O₂ → NiCo₂O₄ + 3H₂O (5) 

Fig. 1 (a) XRD patterns and (b–d) SEM images of the as-obtained samples: (b) ZnO/NiCo₂O₄-5, (c) ZnO/NiCo₂O₄-7, and (d) ZnO/NiCo₂O₄-10.

To further confirm the microstructure of the ZnO/NiCo₂O₄ nanoparticles, TEM was utilized and the results are shown in Fig. 2. From Fig. 2(a), it can be found that as-synthesized nanoparticles form an aggregate particle. As seen from Fig. 2(b), the lattice plane of the main phase is about 0.26 nm corresponding to (002) lattice type plane in hexagonal structure of ZnO (P63mc). In addition, the nano-dots can be identified as NiCo₂O₄ phase. The lattice space of the nano-dot is about 0.23 nm (Fig. 2(c)), consistent with the distance of (222) plane of the NiCo₂O₄. The surface-scanning element mapping (Fig. 2(d)) clearly demonstrates the distribution of elements. Obviously, the Zn elements distribute uniformly in the nanoparticle; whereas the Ni and Co elements show discontinuous distribution in the nanoparticle. 

Fig. 2 TEM and high resolution TEM (HRTEM) images of the ZnO/NiCo₂O₄-7 nanoparticles: (a) TEM, (b, c) HRTEM, and (d) EDS mapping.

To confirm the porous attributes of the ZnO/NiCo₂O₄ nanoparticles, the nitrogen adsorption test was performed. The nitrogen adsorption–desorption isotherms and pore size distribution curves of all samples are shown in Fig. 3. It can be seen that as-obtained ZnO/NiCo₂O₄ samples represent type IV curves with a hysteresis loop (Fig. 3(a)), suggesting the existence of mesopores in the sample. Meanwhile, the Brunner−Emmet−Teller (BET) surface area of the ZnO/NiCo₂O₄-5, ZnO/NiCo₂O₄-7, and ZnO/NiCo₂O₄-10 samples are calculated to 86.85, 22.57, and 80.13 m²/g, respectively. In addition, the pore size of the ZnO/NiCo₂O₄ sample mainly ranges in 0–40 nm calculated by the Barrett–Joyner–Halenda (BJH) method. Therefore, we can conclude that the as-obtained ZnO/NiCo₂O₄ sample contains mesopores and micropores, simultaneously. 

Fig. 3 (a) Nitrogen sorption isotherms of the samples and (b) BJH for the samples. 

Figure 4 shows the hysteresis loop of ZnO/NiCo₂O₄-7 nanoparticles at room temperature. The saturation magnetization (Ms) and the coercivity (Hc) of nanoparticle are 0.0007 emu/g and 60 Oe, respectively (Fig. 4(b)). The results clearly indicate that synthesized nanoparticles are nonmagnetic. This is mainly due to formation of nonmagnetic ZnO and NiCo₂O₄ phases. It is well known that ZnO is a multifunctional semiconductor and NiCo₂O₄ is a nonmagnetic material at room temperature. Therefore, the as-synthesized ZnO/NiCo₂O₄-7 nanoparticles are nonmagnetic. 

Fig. 4 (a) Hysteresis loop and (b) partly hysteresis loop for ZnO/NiCo₂O₄-7 nanoparticles. 

To evaluate the electromagnetic wave absorption performance of the binary ZnO/NiCo₂O₄ nanoparticles, complex permeability and permittivity of the as-synthesized samples were analyzed. Figure 5 demonstrates the dates for the as-obtained samples in the range of 2.0–18.0 GHz. From Figs. 5(a) and 5(b), it can be seen that both the real (ε') and imaginary (ε'') parts of the permittivity increase with increasing the NH₃·H₂O content. The maximum values of ε' reach 5.78, 6.85, and 6.96 for the samples of ZnO/NiCo₂O₄-5, ZnO/NiCo₂O₄-7, and ZnO/NiCo₂O₄-10, respectively. Meanwhile, the maximum values of ε'' increase to 0.39, 0.50, and 0.67 for the samples of ZnO/NiCo₂O₄-5, ZnO/NiCo₂O₄-7, and ZnO/NiCo₂O₄-10, respectively. It is generally accepted that the ε' represents the storage energy ability via polarization; whereas ε'' shows the loss of energy by relaxation. Obviously, increasing the NH3·H2O content is good for increasing both real (ε') and imaginary (ε'') parts of the permittivity, which means both the storage energy ability and transformation electromagnetic energy to heat energy are improved. Figure 5(c) represents the dielectric loss tangent (ε''/ε') of the synthesized samples. It is clear that the values of ε''/ε' also increase with increasing the NH₃·H₂O content and reach 0.068, 0.074, and 0.097 for the samples of ZnO/NiCo₂O₄-5, ZnO/NiCo₂O₄-7, and ZnO/NiCo₂O₄-10, respectively. In addition, the values of ε''/ε' also increase with increasing frequency, indicating that dielectric loss mechanism enhanced with increasing frequency. 

Figures 5(d) and 5(e) show the real (μ') and imaginary parts (μ'') of complex permeability of the ZnO/NiCo₂O₄ samples. Compared with complex permittivity of the synthesized samples, both μ' and μ'' show a sharp decrease with increasing frequency. Generally, μ' and μ'' stand for the storage magnetic energy ability and magnetic loss energy, respectively. Herein, compared with other magnetic materials ^[14,15,20], ZnO/NiCo₂O₄ samples have the low values of μ'. Thus, the stored magnetic energy ability of the ZnO/NiCo₂O₄ samples can be neglected. As shown in Figs. 5(e) and 5(f), some resonance peaks are found for the three samples. It is generally accepted that such resonance is attributed to the natural resonance, exchange resonance, and eddy current effect. According to Ref. ^[38], the exchange resonance usually was produced at high frequencies. Thus, we can infer that the fluctuates at low frequencies (4.0–8.0 GHz) and at high frequencies (12.0–18.0 GHz) should be associated with natural resonance and exchange resonance, respectively. In addition, the NiCo₂O₄ particles are in single domain range and therefore domain-wall displacement has a little effect on the magnetic energy loss ^[42]. Since the magnetic hysteresis, ferromagnetic resonance, and domain-wall displacement has a little effect on the magnetic energy loss, the eddy current also needs to be investigated. Usually, the C₀ curve C₀ = μ''(μ')^–2f^-1, where f stands for the frequency of electromagnetic wave, is used to evaluate the eddy current effect and the results are shown in Fig. 6. Generally, if the eddy current effect had contribution to the magnetic loss, the values of C₀ are constant when increasing the frequency. As it can be seen from Fig. 6(a), the C₀ values of the ZnO/NiCo₂O₄ samples gradually decrease within the frequency range of 2.0–18.0 GHz, suggesting that eddy current effect has no contribution for the magnetic energy loss in this range. Based on the above analysis, it can conclude that magnetic loss has a little effect on the electromagnetic wave absorbing performance. 

Fig. 5 Electromagnetic parameters of the ZnO/NiCo₂O₄ range of 2.0–18.0 GHz: (a) real parts of permittivity (ε'); (b) imaginary parts of permittivity (ε''); (c) ε''/ε'; (d) real parts of permeability (μ'); (e) imaginary parts of permeability (μ''); and (f) μ''/μ'.

According to the Debye dipolar relaxation theory, the complex form of real parts and imaginary parts of permittivity can be described as followings ^[49–52]: 

 [ε' – (ε_s + ε_∞)/2]² + (ε'')² = [(ε_s – ε∞)/2]² (6)

where ε_s represents the stationary frequency dielectric constant and ε_∞ stands for the infinite frequency dielectric constant. Theoretically considering that the plot of ε'' versus ε' is a single semicircle according to Eq. (6). In fact, the plots show three parts of semicircles for all ZnO/NiCo₂O₄ samples (Fig. 6(b)), indicating the coexistence of multiple polarization processes, suggesting the enhanced Debye polarization process produced by multiple structures. When an alternation field applies on the samples, the charges will redistribute alternatively between ZnO particles and NiCo₂O₄ nano-dots. Thus, in addition to the dielectric loss of ZnO particles, the interfacial relaxation between ZnO and NiCo₂O₄ nano-dots also produces. 

Fig. 6 (a) C₀ values and (b) Cole–Cole plots for the ZnO/NiCo₂O₄ samples. 

In recent years, the impedance matching degree (Δ), which is the characteristic impedance of the sample between the free spaces, is used to elucidate the microwave absorbing capabilities in depth. The Δ can be described as following ^[53–55]: 

where d stands for the thickness of the sample; c is the velocity of light in vacuum; δ_ε and δ_μ are the dielectric loss angle and magnetic loss angle, respectively. Generally, the ideal value of Δ defined at 0.4 (| Δ |  < 0.4) ^[56,57]. It can be seen from Figs. 7(a)–7(c) that all the samples have the terrible values of Δ in the frequency range of S, C, and X bands. When increasing the thicknesses of ZnO/NiCo₂O₄-7 and ZnO/NiCo₂O₄-10 samples, the values of Δ gradually possess a little matching region at Ku band, suggesting that the as-synthesized samples may possess excellent electromagnetic wave absorption properties only in Ku band and a thickness more than 3.00 mm. However, the ZnO/NiCo₂O₄-5 samples still suffer a terrible matching region. Apart from the Δ, the attenuation constant (α) is another essential factor in determining the electromagnetic absorbing properties. The α usually represents the ability of dissipating the incident electromagnetic wave via dielectric and magnetic loss and can be described as following ^[58,59]；

It can be seen from Fig. 7(d) that α values of the ZnO/NiCo2O4 samples increase in the frequency range of 2.0–18.0 GHz. The detailed α values for ZnO/NiCo₂O₄-5, ZnO/NiCo₂O₄-7, and ZnO/NiCo₂O₄-10 are 55.65, 73.64, and 84.85, respectively. It is worth noting that α values are in good consistence with the complex permittivity and permeability parameters (Fig. 5). Therefore, it concluded that the synthesized ZnO/NiCo₂O₄ samples only possess excellent electromagnetic wave absorbing capabilities in high frequencies (> 16 GHz) when both the α and Δ are taken into consideration. 

Fig. 7 (a–c) Color maps of the Δ: (a) ZnO/NiCo₂O₄-5; (b) ZnO/NiCo₂O₄-7; (c) ZnO/NiCo₂O₄-10; and (d) α values of ZnO/NiCo₂O₄ samples. 

The electromagnetic wave absorbing properties of the synthesized samples are evaluated by their reflection loss (RL). According to transmission line theory, RL values can be calculated by the following equations ^[60–64]: 

Figure 8 displays the color maps for the RL of ZnO/NiCo₂O₄ in the frequency range of 2.0–18.0 GHz as well as the typical RL curves at a certain thickness. Clearly, all the samples possess poor electromagnetic wave absorption performance at the frequency below ~16.0 GHz. The results are in good consistence with the electromagnetic parameters, α and Δ. Among those samples, ZnO/NiCo₂O₄-7 exhibits an excellent electromagnetic wave absorption performance, and its minimum RL value even increases to –33.49 dB at 18.0 GHz with the thickness of 4.99 mm (Fig. 8(b)). ZnO/NiCo₂O₄-10 shows inferior reflection loss intensities, and the minimum RL value is –27.46 dB at 17.92 GHz with the thickness of 4.99 mm (Fig. 8(d)). However, the ZnO/NiCo₂O₄-5 shows poor electromagnetic wave absorption properties and the minimum RL values only reaches –6.52 dB at 18.0 GHz with the thickness of 5.0 mm (Fig. 8(f)). Usually, incident electromagnetic wave is considered as –10.0 dB (means 90.0% of absorption efficiency). In this regard, the ZnO/NiCo₂O₄-5 is not an electromagnetic wave absorption material due to the gap between relative complex permittivity and permeability resulting in mismatched impedance. However, when increasing the NH₃·H₂O content, the numbers of ZnO and NiCo₂O₄ nanoparticles are enhanced, leading to increase the dielectric loss and interfacial relaxation. Table 1 represents the electromagnetic wave absorbing performance of the related materials. As observed from Table 1, ZnO/NiCo₂O₄-7 and ZnO/NiCo₂O₄-10 indeed display their promising candidates, the electromagnetic wave absorption materials at high frequencies.

Fig. 8 (a–f) Color maps of the reflection loss: (a, b) ZnO/NiCo₂O₄-5; (c, d) ZnO/NiCo₂O₄-7; and (e, f) ZnO/NiCo₂O₄-10. 

Table 1 Electromagnetic wave absorbing performance of the related materials reported in recent years

4 Conclusions

In summary, the ZnO/NiCo₂O₄ hybrid nanoparticles were successfully synthesized by the hydrothermal reaction method. The increase of NH3·H2O content could increase the relative complex permittivity, visibly enhancing the dielectric loss of the samples. In addition, the unique structure and interfacial relaxation played a critical role on the dissipation of electromagnetic waves. The results also demonstrated that impedance matching degree and attenuation constant improved with increasing the content of NH₃·H₂O at high frequencies. Benefiting from the hybrid structure and components, the as-synthesized ZnO/NiCo₂O₄-7 sample possessed an excellent RL value of –33.49 dB at 18.0 GHz with the thickness of 4.99 mm. This result suggested that ZnO/NiCo₂O₄ was a promising candidate in microwave absorption applications in the future. 

References: Omitted