Enhanced electromagnetic wave absorption property of binary ZnO/NiCo2O4 composites

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.

Among various composites, metal oxide semiconductorbased 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@SiO 2 @TiO 2 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@SiO 2 @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. NiCo 2 O 4 is considered as a good electromagnetic wave absorbing material for its high dielectric loss [42][43][44][45]. For example, Zhou et al. [44] revealed that the NiCo 2 O 4 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@NiCo 2 O 4 @Fe 3 O 4 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 NiCo 2 O 4 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 NiCo 2 O 4 via adjusting the agent for the first time. Although numerous works have been conducted to fabricate NiCo 2 O 4 -based electromagnetic wave absorbing materials, most of them still restrict by its complex process. Thus, it remains a challenge to develop NiCo 2 O 4based 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/NiCo 2 O 4 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/NiCo 2 O 4 composites with excellent electromagnetic wave absorption performance.

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 adsorptiondesorption 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.

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 NiCo 2 O 4 phase (PDF#20-0781). The detailed formation of ZnO and NiCo 2 O 4 phases can be described as followings: Firstly, the cationic of Ni 2+ and Co 2+ diffused around the Zn(OH) 2 particles, and formed the nucleus of NiCo 2 (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 NiCo 2 (OH) 6 hybrid particles converted into ZnO and NiCo 2 O 4 hybird nanoparticles simultaneously according to Eqs. (4) and (5) [43][44][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 demonstrate the morphologies of the as-obtained powders observed by SEM. As shown in Fig. 1, both the ZnO/NiCo 2 O 4 -5 and ZnO/NiCo 2 O 4 -7 samples are irregular shape particles ranging from several nanometers to hundred nanometers (Figs. 1(b) and 1(c)). The SEM image of ZnO/NiCo 2 O 4 -10 apparently represents a well-regulated structure ( Fig. 1(d)). Amounts of well-regulated particles arrange together. The different morphologies of ZnO/NiCo 2 O 4 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 NH 3 ·H 2 O, the ZnO nanoparticles are easy to form the large-dimensional growth units and therefore, form the well-regulated particles.
To further confirm the microstructure of the ZnO/NiCo 2 O 4 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 NiCo 2 O 4 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 NiCo 2 O 4 . 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.  To confirm the porous attributes of the ZnO/NiCo 2 O 4 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/NiCo 2 O 4 samples represent type IV curves with a hysteresis loop ( Fig. 3(a) Therefore, we can conclude that the as-obtained ZnO/ NiCo 2 O 4 sample contains mesopores and micropores, simultaneously. Figure 4 shows the hysteresis loop of ZnO/NiCo 2 O 4 -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 NiCo 2 O 4 phases. It is well known that ZnO is a multifunctional semiconductor and NiCo 2 O 4  www.springer.com/journal/40145 is a nonmagnetic material at room temperature. Therefore, the as-synthesized ZnO/NiCo 2 O 4 -7 nanoparticles are nonmagnetic.
To evaluate the electromagnetic wave absorption performance of the binary ZnO/NiCo 2 O 4 nanoparticles, complex permeability and permittivity of the as-synthesized samples were analyzed. Figure 5  Obviously, increasing the NH 3 ·H 2 O 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 NH 3 ·H 2 O content and reach 0.068, 0.074, and 0.097 for the samples of ZnO/NiCo 2 O 4 -5, ZnO/NiCo 2 O 4 -7, and ZnO/NiCo 2 O 4 -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/NiCo 2 O 4 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/NiCo 2 O 4 samples have the low values of μ'. Thus, the stored magnetic energy ability of the ZnO/NiCo 2 O 4 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 NiCo 2 O 4 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 C 0 curve C 0 = μ''(μ') -2 f --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 C 0 are constant when increasing the frequency. As it can be seen from Fig. 6(a), the C 0 values of the ZnO/NiCo 2 O 4 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.
According to the Debye dipolar relaxation theory, the complex form of real parts and imaginary parts of permittivity can be described as followings [49][50][51][52]: (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/ NiCo 2 O 4 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 NiCo 2 O 4 nano-dots. Thus, in addition to the dielectric loss of ZnO particles, the interfacial relaxation between ZnO and NiCo 2 O 4 nano-dots also produces.
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][54][55]  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  1  (Fig. 8(b)). ZnO/NiCo 2 O 4 -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/ NiCo 2 O 4 -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/NiCo 2 O 4 -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 NH 3 ·H 2 O content, the numbers of ZnO and NiCo 2 O 4 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/NiCo 2 O 4 -7 and ZnO/NiCo 2 O 4 -10 indeed display their promising candidates, the electromagnetic wave absorption materials at high frequencies.
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