Formation of copper(I) oxide nanoparticles at an early stage of copper sol synthesis in the presence of poly(vinylpyrrolidone)

Abstract

The initial stage of the reduction of Cu2+ ions with tert-butylamine borane in aqueous poly(N-vinylpyrrolidone) solutions has been investigated by electronic spectroscopy, X-ray diffraction analysis, transmission electron microscopy, and measurements of pH. It has been shown that a yellow-orange sol of Cu2O with nanoparticle sizes of 2–10 nm is formed at this stage, which is accompanied by a significant decrease in the pH of the reaction system. With a lack of a reducing agent, Cu2O sols remain transparent for 24 h or more, thereby indicating an interaction between oxide nanoparticles and polymer macromolecules. In the presence of excess reducing agent, the formation of a copper metal sol begins after Cu2+ ions have been exhausted. The introduction of an additional portion of Cu2+ ions into the reaction system at this stage leads to the oxidation of copper nanoparticles with the formation of Cu2O particles. Based on the data obtained, it has been concluded that the oxidation of copper metal particles with Cu2+ ions to the univalent state hinders the formation of the copper sol at the initial stage of the reduction. Thus, in the investigated system, Cu2O nanoparticles play the role of a precursor for copper metal particles.