Growth mechanisms and kinetics of photo-induced silver nanoparticles in mesostructured hybrid silica films under UV or visible illumination
Résumé
The growth of silver nanoparticles in mesostructured hybrid silica films under laser illumination is investigated by optical absorption spectroscopy and transmission electron microscopy. At low laser doses in the UV range, a blue-shift of the plasmon resonance is observed for increasing particle size. This size dependence is interpreted on the basis of the Mie theory assuming a two-layer core-shell model resulting from the localization of the nanoparticles in the block copolymer part of the film. At higher laser doses, the observed red-shift of the resonance wavelength and decrease of the absorbance level at resonance are attributed to the formation of a high refractive index shell around the nanoparticles that thickens slowly with the dose. The growth kinetics of silver nanoparticles is also studied for different illumination wavelengths in the UV and visible ranges. Surprisingly, the nanoparticles also grow under visible illumination. This is partly attributed to the release of electrons by the degrading copolymer under illumination. The decrease of the photo-reduction process rate with the incident wavelength increase has been evaluated quantitatively. We propose an autocatalytic model that fits well with the experimental data and suggest that the absorption of some incident wavelengths by the generated small silver clusters boosts their own growth.