*2.2.2 EM properties of core-shell Ni@CuO composites*

*Electromagnetic Materials and Devices*

shell composites are obtained under this procedure.

clearly observed from **Figure 5a, b**. The inset SAED pattern of the CuO particles indicated that CuO particles are polycrystalline (**Figure 5a**). The HRTEM image (inset of **Figure 5b**) displays that the lattice spacing is 0.276 nm, which is in good agreement with the (110) lattice spacing of CuO. Based on the SEM and TEM results, it can be concluded that the CuO is deposited on the surface of Ni, the core-

**Figure 6** exhibits the morphologies of the obtained products with different molar ratio of the CuCl2·2H2O to Ni microspheres. Noticeably, the surfaces of all samples turns coarser in comparison with the pure Ni microspheres, which indicates the successful coating of the CuO nanoparticles on the pristine Ni surfaces. Furthermore, the shape and coverage density of CuO materials could be controlled by tuning the content of precursor (Cu2+). When the molar ratio of the CuCl2·2H2O to Ni microspheres in the precursor solution is 1: 0.85 (S-1), one can find (**Figure 6a, b**) that the Ni microspheres are coated by a large number of CuO nanorices. But, due to the low content of precursor (Cu2+), we just could obtain thin CuO shell. If the

*FESEM images of hierarchical Ni/CuO core-shell heterostructures with different molar ratio: (a, b) S-1; (c, d)* 

**158**

**Figure 6.**

*S-2; and (e, f) S-3 [37] (permission from RSC).*

To compare and assess the EM wave absorption properties of Ni, Ni/CuO coreshell composites, and CuO nanoflakes, the paraffin (30 wt%, which is transparent to microwave) are mixed with as-obtained products, and pressed into a ring shape with an outer diameter of 7.00 mm and an inner diameter of 3.04 mm. The microwave absorption abilities of these as-fabricated products could be evaluated by the RL values, which could be simulated on the basis of the complex permeability and permittivity with the measured frequency and given layer thickness [38, 39]. As presented in **Figure 7a**, the three Ni/CuO composites show the superior microwave-absorption properties to those of the pure Ni microspheres and the CuO nanoflakes. Taking an example, when the thickness is 2 mm, the S-1 sample exhibits the enhanced EM-wave absorption with the minimal RL value of −15.6 dB at 11.9 GHz among the five samples. From Eqs. (1) and (2), one can find that the thickness of the absorber is one important factor, which would affect the position of minimal RL value and the absorption bandwidth. Therefore, the RL values of Ni/CuO samples with different thicknesses are also calculated. Compared with S-2 (**Figure 7c**) and S-3 (**Figure 7d**) samples, the S-1 (**Figure 7b**) displays the outstanding microwave absorption performances. The lowest RL of the S-1 sample is −62.2 dB at 13.8 GHz with the only thickness of 1.7 mm. The effective absorption (below −10 dB) bandwidth can be tuned between 6.4 GHz and 18.0 GHz by adjusting thickness in 1.3–3.0 mm.

#### **Figure 7.**

*(a) Comparison of RL of the five as-obtained samples with a thickness of 2.0 mm. The RL values of (b) S-1, (c) S-2, and (d) S-3 samples with various thicknesses [37] (permission from RSC).*

Notably, the optimal RL peaks gradually shift toward lower frequencies with an increased absorber thickness, which can be described by quarter-wavelength cancelation model that the incident and reflected waves in the absorber are out of phase 180° and causing the reflected waves in the air-absorber interface are totally cancelled [40]. The enhanced microwave absorption property of core-shell Ni/CuO composites can be obtained by tuning the content of CuO. The rice-like CuO shell is expected to be helpful for the dissipation proprieties of the core/shell composites. The CuO shells are covered on the surface of Ni microspheres to produce the special core-shell structure, which brings metal-dielectric hetero-interfaces to cause interfacial polarization. It is supposed that the interfacial polarization taken place in heterostructures consisting of at least two constituents [17, 41, 42]. This type of polarization occurring at the interfaces results from the movement of charge carriers between different compositions, which accumulate the moving charge at these interfaces. When irradiated by alternating EM fields, the accumulated charge would redistribute periodically between Ni cores and CuO shells, which are favorable for the microwave dissipation. However, for the S-2 and S-3 samples, thanks to the high content of CuO, we cannot observe the synergistic effect between Ni cores and CuO shells, which gives rise to inferior microwave absorption.
