Synthesis of Copper-Silica Core-Shell Nanostructures with Sharp and Stable Localized Surface Plasmon Resonance

Copper nanoparticles exhibit intense and sharp localized surface plasmon resonance (LSPR) in the visible region; however, the LSPR peaks become weak and broad when exposed to air due to the oxidation of Cu. In this work, the Cu nanoparticles are successfully encapsulated in SiO2 by employing trioctyl-n-phosphine (TOP)-capped Cu nano particles for the sol gel reaction, yielding an aqueous Cu-SiO2 core-shell suspension with stable and well-preserved LSPR properties of the Cu cores. With the TOP capping, the oxidation of the Cu cores in the micro emulsion was significantly reduced, thus allowing the Cu cores to sustain the sol-gel process used for coating the SiO2 protection layer. It was found that the self-assembled TOP-capped Cu nanoparticles were spontaneously disassembled during the sol-gel reaction, thus recovering the LSPR of individual particles. During the disassembling progress, the extinction spectrum of the nanocube agglomerates evolved from a broad extinction profile to a narrow and sharp peak. For a mixture of nanocubes and nanorods, the spectra evolved to two distinct peaks during the dissembling process. The observed spectra match well with the numerical simulations. These Cu-SiO2 core-shell nanoparticles with sharp and stable LSPR may greatly expand the utilization of Cu nanoparticles in aqueous environments.