Plasmon Features of Coinage Metal Nanoparticles Supported on Zeolites

The plasmonic features in the optical response of coinage metal nanoparticles supported on different type of zeolites were studied. The shifts in the plasmon frequency were analyzed for Cu, Ag, and Au nanoparticles in mordenite, β-zeolite, and Y-zeolite. It was shown experimentally that the resonance energy is sensitive both to type of zeolite structure and counter-cation of zeolite, as well as to annealing temperature and chemical composition of zeolite, their SiO₂/Al₂O₃ molar ratio. A theoretical framework was employed to identify physical mechanism for this sensitivity. Within a simple model, the width of the absorption window identified in the imaginary part of the bulk dielectric function of the different metals was seen to play the important role in establishing the range of the plasmon energies available. In terms of an effective dielectric function, the composite medium was fully described by the complex dielectric function of the metal involved, the dissipation-free dielectric function of the zeolite matrix, and the filling fraction which relates the volume of metal inclusions as a fraction of the total sample volume. The sensitivity of the optical spectra is understood in terms of variations in both the dielectric response of the zeolite matrix as well as nanoparticle size.