Giant two-photon absorption of anatase TiO2 in Au/TiO2 core-shell nanoparticles

Abstract

We report on deep-to-near-UV transient absorption spectra of core-shell Au/SiO2 and Au/TiO2 nanoparticles (NPs) excited at the surface plasmon resonance of the Au core, and of UV-excited bare anatase TiO2 NPs. The bleaching of the first excitonic transition of anatase TiO2 at ∼3.8  eV is a signature of the presence of electrons/holes in the conduction band (CB)/valence band (VB) of the material. We find that while in bare anatase TiO2 NPs, two-photon excitation does not occur up to the highest used fluences (1.34  mJ/cm2), it takes place in the TiO2 shell at moderate fluences (0.18  mJ/cm2) in Au/TiO2 core-shell NPs, as a result of an enhancement due to the plasmon resonance. We estimate the enhancement factor to be of the order of ∼108–109. Remarkably, we observe that the bleach of the 3.8 eV band of TiO2 lives significantly longer than in bare TiO2, suggesting that the excess electrons/holes in the conduction/valence band are stored longer in this material.