Enhanced dielectric properties and electromagnetic wave absorption in ZnO@Ni core–shell hollow spheres for Ku-band broadband applications

Abstract

ZnO@Ni core–shell structures exhibit exceptional electromagnetic wave (EMW) absorption, driven by synergistic dielectric and conductive loss mechanisms. Hydrothermally synthesized ZnO hollow spheres (≈ 500 nm diameter) with hexagonal wurtzite structure were uniformly coated with a 20–60 nm Ni–P shell via electroless plating. Structural characterization (XRD, FE-SEM, TEM) confirmed the core–shell architecture, while Raman spectroscopy revealed compressive lattice stress in ZnO and enhanced defect-related polarization due to nickel deposition. The ZnO@Ni composite achieved a minimum reflection loss of − 17.4 dB at 16.5 GHz (2.3 mm thickness) and a 4.8 GHz effective bandwidth (RL <  − 10 dB), significantly outperforming ZnO. This performance stems from optimized impedance matching (Z ≈ 1, 12–18 GHz), interfacial polarization at Ni–ZnO junctions, and defect-induced dipole polarization (e.g., oxygen vacancies in ZnO). Nickel’s conductivity facilitated conductive networks, boosting ohmic losses, while the hierarchical porous structure prolonged microwave propagation through multiple scattering. The core–shell design balances dielectric and conductive properties, enabling efficient energy dissipation. This work highlights ZnO@Ni’s core–shell potential as a lightweight, broadband absorber for electromagnetic shielding, leveraging interfacial engineering and defect modulation.