X-ray photoelectron, Cu L3MM Auger and X-ray absorption spectroscopic studies of Cu nanoparticles produced in aqueous solutions: The effect of sample preparation techniques
Stable sols of copper nanoparticles from 10 nm to 50 nm in diameter synthesized in aqueous solutions using several reducing agents and immobilized on pyrographite were characterized by XPS, Cu L3MM Auger and X-ray absorption spectroscopy (Cu L-edge TEY XANES and Cu K-edge XANES and EXAFS in transmission mode) along with UV-vis spectroscopy, AFM, TEM. It was found that the nanoparticles produced in the hydrazine hydrate and sodium borohydride assisted synthesis are composed of surface Cu(II) oxyhydroxides, and the shells formed by cuprite Cu2O and distorted Cu(I) oxide, which showed the Cu L-edge peak shifted to higher energies, above the metallic core. The relative amount of cuprite was usually larger in the hydrazine systems, but it depended on the sample preparation protocol. The nanoparticles prepared using ascorbic acid as a reactant consisted of rather thick Cu(II) overlayer and largely distorted Cu(I) oxide underneath. The post-synthetic sample handling notably affected the results; in particular, sedimentation (instead of drying of the colloidal solution) favored reduction of the oxide layers to metallic copper and agglomeration of the Cu particles in the precipitate, even in the case of ascorbic acid, while the colloidal particles in the supernatant were oxidized.