Kinetics at the nanoscale: formation and aqueous oxidation of copper nanoparticles

A simple method to prepare copper nanoparticles under the ambient atmosphere, in an aqueous environment, is developed utilizing solid sodium borohydride as the reducing agent and sodium citrate as a stabilizer and complexing agent. This constitutes a model system having a stability of several hours, sufficient to allow kinetic measurements. The localized surface plasmon resonance band of copper nanoparticles in the UV-Vis spectrum is used to determine the rate of formation of copper nanoparticles and assess the beginning of the oxidation process. The effect of temperature, copper sulfate and sodium borohydride concentrations on the copper nanoparticle formation rate is investigated. It is found that the kinetic data obey a first order rate law with respect to both sodium borohydride and copper sulfate. Based on the kinetic data, a novel mechanism of the reduction reaction is envisaged, involving three possible pathways. As solid sodium borohydride is an important hydrogen storage material, the results of this work are relevant to the field of portable fuel cells. The optical properties of copper nanoparticles have been simulated by using the Discrete Dipole Approximation method and the Mie theory and a good agreement was found between the theoretical and experimental characteristics of the copper plasmon band. The data obtained in this work provide valuable information on the kinetics of reactions at the nanoscale.