Photochemical Generation of Nanometer-Sized Cu Particles in Octane

Irradiation with 350 nm photons of anhydrous, air-free octane or toluene solutions of copper(II) oleate containing benzophenone as a photosensitizer and oleoylsarcosine as a stabilizer resulted in metallic Cu particles with nanometer dimensions. Evidence is presented that implicates the hydrocarbon as the predominant H-atom donor in the generation of reductive benzophenone ketyl radicals and a kinetic model is constructed to rationalize the rate dependencies with respect to the Cu2+/Cu+ step. Rates of both Cu2+ consumption and Cu formation vary linearly with light intensity and exhibit a first-order dependence on benzophenone concentration but the latter step shows little dependence otherwise. The initial rate of reactant consumption decreases with increasing concentration of cupric ions or sarcosine. Quenching of the excited state of benzophenone by the stabilizer occurs with a rate constant of k 4 = 1.6 × 105 M–1 s–1 and is explained by the formation of a contact ion pair between the reduced chromophore and oxidized amine which ultimately decays by back electron transfer. The rate decrease induced by cupric ions results from the quenching of the excited state of benzophenone by the copper(II) complex with a rate constant of k 5 = 6.1 × 105 M–1 s–1. The resulting colloids proved to be stable in an anaerobic environment for at least a month and require more than 12 h to oxidize upon exposure to air. Upon removal of the octane solvent, the particles can be redispersed in a variety of low-dielectric media such as chloroform, carbon tetrachloride, hexane, or toluene.