Equilibration of Photogenerated Charge Carriers in Plasmonic Core@ Shell Nanoparticles

Noble metal nanoparticles support localized surface plasmon resonances (LSPRs), which are light-driven oscillations of free electrons. Thanks to their strong dependence on the metal’s electron density, these resonances can be used to optically probe the equilibration of photogenerated charge carriers at metal/semiconductor interfaces, a process of paramount importance in energy conversion and sensing applications. In practice, however, it is often difficult to obtain quantitative insight from the observed plasmonic effects, as the spectral position, intensity, and line width of LSPRs can be influenced by different competing contributions, such as particle size distribution, surface oxidation, and changes in the dielectric environment. Here, we develop a two-step synthesis of Ag@TiO2 and Au@TiO2 core@shell nanoparticles and measure their plasmon resonance shifts during UV irradiation and charge equilibration in the dark. We show that the observed optical response can be fully accounted for by an accumulation of photogenerated electrons in the TiO2 shells and that charge transfer to the metal cores is negligible. Our results challenge the established understanding of charge equilibration in hybrid metal@semiconductor nanostructures.