Shape-dependent plasmonic response of rod-like Pd–TiO ₂ core–shell nanoparticles for enhanced photocatalysis and sensing

Abstract In this study, the spectral response of Pd–TiO 2 core–shell systems is theoretically investigated as a function of core size, shape (cylindrical and ellipsoidal), shell thickness, and host-medium refractive index using the finite-difference time-domain method. The extinction spectra reveal the excitation of two dipolar plasmonic resonances: a low-energy symmetric mode and a high-energy antisymmetric mode. At a fixed shell thickness, the resonance wavelengths of both modes exhibit size-dependent tunability across the UV and visible regions. In contrast, increasing the shell thickness at constant core size progressively suppresses the symmetric mode while enhancing the antisymmetric one, owing to the stronger influence of the shell polarization. The cylindrical Pd–TiO 2 nanostructure demonstrates higher refractive-index sensitivity (276 nm RIU −1 ) than the prolate-ellipsoidal counterpart (254 nm RIU −1 ). The calculated spectral overlap integral (SOI) and absorbed photon flux (APF) are also higher for cylindrical geometries, indicating more efficient light concentration and photon absorption, which directly support their enhanced sensing and photocatalytic performance. Compared with spherical Pd–TiO 2 core–shells, the reduced symmetry of rod-like geometries enables broader resonance tunability and stronger sensing performance. These findings provide fundamental insights for guiding synthesis strategies to optimize Pd–TiO 2 nanostructures for targeted applications, including enhanced light harvesting in solar cells and improved photocatalytic activity.