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

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 Cu2-xS NCs, the controlled diffusion of Cu2+ ions into the CuS NCs allows the formation of a unique CuS@Cu2-xS core-shell-like nanostructure as the intermediate and Cu2S NCs in the presence of excess Cu2+ ions. The CuS@Cu2-xS core-shell nanostructure can evolve into homogenous Cu2-xS NCs by further cation diffusion from the Cu2-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.