Remarkable Decrease in the Oxidation Rate of Cu Nanocrystals Controlled by Alkylamine Ligands

Colloidal solutions of copper nanoparticles (7.2 +/- 1.1 nm 1.6 diameter), stabilized by alkylamine ligands, show a remarkably long persistence (several months) of the localized surface plasmon resonance (LSPR) when exposed to ambient air at room temperature. The oxidation kinetics of these nanoparticles have been investigated by optical spectroscopy and modeled using numerical simulations. Three distinct oxidation regimes are evidenced: (i) A fast regime in which oxygen is adsorbed and dissociated on the nanoparticle to form preoxide islands; (ii) a slower regime where the coalescence of the oxide islands takes place up to the formation of a complete Cu(2)0 shell; (iii) and finally an extremely slow oxidation of the residual copper core and eventually the formation of hollow Cu20 nanoparticles. The adsorption rate of oxygen on copper nanoparticles is controlled by the amount of alkylamine ligands in solution (from 0.1 to 2 mol equiv).