Multiband optical response of Cu@TiO₂ core–shell nanoparticles: toward next-generation plasmonic photocatalysts

Abstract This work presents a comprehensive finite-difference time-domain (FDTD) study of the multiband optical response and light-harvesting characteristics of Cu@TiO 2 core–shell nanoparticles. By systematically varying the copper core radius and TiO 2 shell thickness, the interplay between plasmonic and dielectric resonances is quantitatively elucidated. The hybrid nanostructure supports three distinct optical modes: an ultraviolet interband transition and two visible-range hybridized plasmon resonances arising from bonding and antibonding coupling between the Cu core plasmon and TiO 2 shell polarization. Increasing the shell thickness or core radius enhances the spectral overlap with the solar spectrum, yielding up to an order-of-magnitude improvement in absorbed photon flux (APF) and hot-carrier generation compared with a dielectric reference. The optimized configuration achieves a high refractive index sensitivity of ∼120 nm RIU −1 , indicating strong field confinement and efficient plasmon–dielectric interaction. The consistent enhancement in the spectral overlap integral (Γ), APF, and hot-electron injection rate demonstrates that shell-thickness and core-size engineering offer powerful routes to tailor optical absorption, carrier generation, and sensing performance. These findings establish Cu@TiO 2 as a cost-effective, broadband-active alternative to noble-metal hybrids for plasmon-enhanced photocatalysis and optical sensing applications.