Formation of Cu and Cu₂O nanoparticles by variation of the surface ligand: preparation, structure, and insulating-to-metallic transition

Abstract

Copper and copper (I) oxide nanoparticles protected by self-assembled monolayers of thiol, carboxyl, and amine functionalities [X(CH₂)_n-CH₃, where X can be -COOH, -NH₂, or -SH] have been prepared by the controlled reduction of aqueous copper salts using Brust synthesis. The optical absorption spectrum (λ_max=289 nm) is found to be invariant with the nature of the capping molecule while the particle shape and distribution are found to depend strongly on it. A comparison of the protection efficiency for different capping agents such as dodecanethiol (DDT), tridecylamine (TDA), and lauric acid (LA) suggests that although zerovalent Cu is initially formed for dodecanethiol, all other cases allow oxidation to Cu₂O nanoparticles. Despite the variation in particle size and relative stability, nanoparticles have been found to form oxides after a few days, especially for the case of LA and TDA surface capping. For all the samples studied, the size has been found to be 4-8 nm by high-resolution transmission electron microscopy. The protective ability is found to be better for dodecanethiol SAM (similar to the case of Au and Ag nanoparticles), while the order of capping effeciency varies as Cu-DDT>Cu-TDA>Cu-LA. In the present study we also demonstrate a reversible metal-insulator transition (MIT) in capped nanoparticles of Cu using temperature-dependent electrical resistivity measurement. However, the LA-capped sample does not show any such transition, possibly due to the oxide formation.