Seed Mediated Growth of Oxidation Resistant Copper Nanoparticles with Optical Properties

Metal nanoparticles (NPs) exhibit localized surface plasmonic resonance (LSPR) allowing them to span broad applications including energy, catalysis, biomedicine and sensing, and their optical properties can be tuned by changing the size, shape and surrounding environment of the nanoparticles to achieve a LSPR suitable for the desired application. Copper has a low cost, high conductivity, tunable optical properties, and reported LSPR which make copper nanoparticles (CuNPs) an interesting alternative over other noble metal plasmonics. However, the extinction properties of copper nanostructures can be influenced by both interband and intraband transitions, so a deep understanding of the optical properties is required. Copper also suffers drawbacks such as instability and susceptibility to surface oxidation which can further impact the optical properties of the CuNPs. Copper nanoparticles ranging in size from 20-60 nm were synthesized through a seed mediated growth in tetrahydrofuran using polyvinylpyrrolidone (PVP) as a stabilizing agent. Various average molecular weights of PVP were explored and showed influence on both the optical properties and overall shape and size of the resultant NPs where lower molecular weights of PVP resulted in broader spectra and polydisperse particles while higher molecular weights presented narrower spectra with lower polydispersity. The size of the particles could be tuned by changing the PVP amount and by adding acetic acid to the reaction media. Wide angle x-ray scattering measurements show that the particles consist of Cu metal and that no oxide species were present. The particles remained stable in solution for over a month with no shift to the optical properties over time and were resistant to oxidation after addition of water or hydrogen peroxide, or after heating in air. By examining the extinction and true absorbance measurements we determined that all samples had a true absorption peak due to interband transitions at around 590 nm and peaks observed at longer wavelengths were due to scattering effects. The crystallinity and size of these CuNPs was further studied by transmission electron microscopy (TEM) and small angle x-ray scattering (SAXS). Future work will expand on the synthesis strategy to better tune the shape of the copper NPs and we aim to apply this synthesis strategy to prepare other plasmonic metal nanostructures with controlled sizes and optical properties.