Inter-diffusion of Cu2+ ions into CuS nanocrystals confines the microwave absorption properties

Abstract

We report inter-diffusion of as-synthesized covellite CuS nanocrystals (NCs) with Cu²⁺ cations at room temperature in the presence of a mild reducing agent (i.e., ascorbic acid). Unlike the previously-reported incorporation of Cu⁺ or Cu²⁺ ions with CuS that generates homogenous Cu_2−xS NCs, the controlled diffusion of Cu²⁺ ions into the CuS NCs allows the formation of a unique CuS@Cu_2−xS core–shell-like nanostructure as the intermediate and Cu₂S NCs in the presence of excess Cu²⁺ ions. The CuS@Cu_2−xS core–shell nanostructure can evolve into homogenous Cu_2−xS NCs by further cation diffusion from the Cu_2−xS shell toward the center (i.e. the CuS core) under E-beam irradiation during TEM test or when the core–shell NCs are stored in toluene for some time (e.g. four weeks). Notably, the CuS/paraffin composite with filler loading of 50 wt% and thickness of 1.95 mm displays a minimum reflection loss (RL) of −54.84 dB at 12.6 GHz. By adjusting the thickness in the range of 1–5 mm, the effective absorption bandwidth (below −10 dB, 90% microwave absorption) could reach 15 GHz (3–18 GHz). In particular, the diffusion of Cu²⁺ into CuS NCs leads to dampening of the NIR plasmon absorbance and a decrease in free carrier (i.e., hole) density and conductivity, which accounts for the confinement effect on the microwave absorption properties of the resulting samples.