Metal clusters in plasma polymer matrices - Part III. Optical properties and redox behaviour of Cu clusters

Using an inert gas evaporation technique in combination with a simultaneous plasma polymerisation, copper clusters with mean diameters between 2.9 to 5.4 nm have been prepared within plasma polymer matrices from hexamethyldisiloxane (HMDSO) monomer. UV-visible spectroscopy of Cu clusters embedded in a plasma polymer film revealed a red shift and a damping of the surface plasmon frequency and resonance, respectively, with increasing thickness and changing nature of copper oxide layers surrounding the clusters. Experimental transmission UV-visible spectra were compared with simulations using the Maxwell-Garnett theory, which is extended to coated spheres. Owing to larger differences in the dielectric constant CuO layers lead to a stronger red shift than Cu2O coatings. The formation and the reduction of the two copper oxide species during successive treatments of Cu clusters with oxygen, hydrogen and carbon monoxide was observed by in situ diffuse reflectance measurements at a wavelength of lambda=600 nm and a temperature of 250 degrees C. The redox reactions with O-2, CO, H-2 are reversible.