Engineering the CoSe2 phase transition in a Br-induced confined space for high-performance electromagnetic wave absorption

Abstract

Manipulating the crystal phase structure of nanomaterials in confined spaces to create unique heterogeneous interfaces holds great potential for enhancing the microwave absorption (MA) capabilities. In this work, we successfully confined CoSe₂ nanoparticles in a multi-chambered hollow structure using a two-step controlled annealing process. It was noteworthy that employing different levels of Br doping strategies induced a controlled transition from the o-CoSe₂ phase to c-CoSe₂ phase, thereby regulating the dielectric parameters. Density functional theory revealed that Br-doped o-CoSe₂ introduces defect sites that influence the electrons in the Co 3d orbitals, leading to a reconstruction of the original electronic structure and driving the rotation of the Se₂²⁻ units to form an o-CoSe₂/c-CoSe₂ heterointerface. The difference in work functions of the two phases stimulated electron migration, which caused the separation of dense charges and accumulation at the interface to form a heterojunction capacitor, enhancing interfacial polarization relaxation. Simultaneously, the substitution of Br atoms for Se atoms disrupted the intrinsic charge structure, generating dipole polarization centers. The multi-chambered hollow structure with nano- and micro-pores achieved graded absorption of electromagnetic waves, optimizing the impedance matching characteristics. Consequently, the minimum reflection loss of mix-CoSe₂-Br₁@HC reached −60.6 dB, with an effective absorption bandwidth of 5.9 GHz. Moreover, RCS simulations showed that the RCS values of mix-CoSe₂-Br₁@HC remained below −15 dBm² across the −90° to 90° detection range, demonstrating its excellent radar wave attenuation. Therefore, this study provides a novel strategy for the regulation of heterogeneous components within a confined space and expands the application of phase transition engineering in the field of MA.