Slow Relaxation of Surface Plasmon Excitations in Au<sub>55</sub>: The Key to Efficient Plasmonic Heating in Au/TiO<sub>2</sub>

Gold nanoparticles distinguish themselves from other nanoparticles due to their unique surface plasmon resonance properties that can be exploited for a multiplicity of applications. The promise of plasmonic heating in systems of Au nanoparticles on transition metal oxide supports, for example, Au/TiO<sub>2</sub>, rests with the ability of the surface plasmon in Au nanoparticles to effectively transfer energy into the transition metal oxide. Here, we report a critical observation regarding Au nanoparticle (Au<sub>55</sub>) surface plasmon excitations, that is, the relaxation of the surface plasmon excitation is very slow, on the order of several picoseconds. Starting from five plasmon states in Au<sub>55</sub> nanoparticles using nonadiabatic molecular dynamics simulations, we find that the relaxation time constant resulting from these simulations is ∼6.8 ps, mainly resulting from a long-lived intermediate state found at around −0.8 eV. This long-lived intermediate state aligns with the conduction band edge of TiO<sub>2</sub>, thereby facilitating energy transfer injection from the Au<sub>55</sub> nanoparticle into the TiO<sub>2</sub>. The current results rule out the previously reported molecular-like relaxation dynamics for Au<sub>55</sub>.