The electron injection efficiency and the steady state
absorptance
at different photon energies for a composite system made of Au NPs
embedded in a cerium oxide matrix are reported. Cerium oxide can be
coupled with plasmonic nanoparticles (NPs) to improve its catalytic
properties by visible-light absorption. The present work is a study
of the ultrafast dynamics of excited states induced by ultraviolet
and visible-light excitation in Au NPs combined with cerium oxide,
aimed at understanding the excitation pathways. The data, obtained
by femtosecond transient absorption spectroscopy, show that the excitation
of localized surface plasmon resonances (LSPRs) in the Au NPs leads
to an ultrafast injection of electrons into the empty 4f states of
the surrounding cerium oxide. Within the first few picoseconds, the
injected electrons couple with the lattice distortion forming a polaronic
excited state, with similar properties to that formed after direct
band gap excitation of the oxide. At sub-picosecond delay times, we
observed relevant differences in the energetics and the time dynamics
as compared to the case of band gap excitation of the oxide. Using
different pump energies across the LSPR-related absorption band, the
efficiency of the electron injection from the NPs into the oxide was
found to be rather high, with a maximum above 30%. The injection efficiency
has a different trend in energy as compared to the LSPR-related static
optical absorptance, showing a significant decrease in low energies.
This behavior is explained considering different deexcitation pathways
with variable weight across the LSPR band. The results are important
for the design of materials with high overall solar catalytic efficiency.