Abstract: The possibility of using light to drive chemical reactions has highlighted the role of photocatalysis as a key tool to address the environmental and energy issues faced by today?s society. Plasmonic photocatalysis, proposed to circumvent some of the problems of conventional semiconductor catalysis, uses hetero-nanostructures composed by plasmonic metals and semiconductors as catalysts. Metal-semiconductor core-shell nanoparticles present advantages (i.e., protecting the metal and enlarging the active sites) with respect to other hetero-nanostructures proposed for plasmonic photocatalysis applications. In order to maximize light absorption in the catalyst, it is critical to accurately model the reflectance/absorptance/transmittance of composites and colloids with metal-semiconductor core-shell nanoparticle inclusions. Here, we present a new method for calculating the effective dielectric function of metal-semiconductor core-shell nanoparticles and its comparison with existing theories showing clear advantages. Particularly, this new method has shown the best performance in the prediction of the spectral position of the localized plasmonic resonances, a key parameter in the design of efficient photocatalysts. This new approach can be considered as a useful tool for designing coated particles with desired plasmonic properties and engineering the effective permittivity of composites with core-shell type inclusions which are used in photocatalysis and solar energy harvesting applications.

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