Excited-state dynamics of size-dependent colloidal TiO₂-Au nanocomposites

The ultrafast excited-state dynamics of size-dependent TiO₂-Au nanocomposites synthesized by reducing gold nanoclusters to the surface of colloidal TiO₂ nanoparticles are studied using pump-probe transient absorption spectroscopy with 400 nm excitation pulses. The results show that the relaxation processes of the plasmon depletion band, which are described by electron-phonon and phonon-phonon scattering lifetimes, are independent of the gold nanocluster shell size surrounding the TiO₂ nanoparticle core. The dynamics corresponding to interfacial electron transfer between the gold nanoclusters and the TiO₂ bandgap are observed to spectrally overlap with the gold interband transition signal, and the electron transfer lifetimes are shown to significantly decrease as the nanocluster shell size increases. Additionally, size-dependent periodic oscillations are observed and are attributed to acoustic phonons of a porous shell composed of aggregated gold nanoclusters around the TiO₂ core, with frequencies that decrease and damping times that remain constant as the nanocluster shell size increases. These results are important for the development of improved catalytic nanomaterial applications.