Ultrafast Dynamics of Electron Injection from Cu Embedded Nanoparticles in CeO₂ Aerogels

Wide bandgap semiconductors, such as TiO₂ and CeO₂, have photocatalytic properties activated in the UV that can be used to drive electrical currents and enhance electrochemical reactions. The activity range of wide bandgap semiconductors can be extended into the visible by coupling the semiconductor to a metal nanostructure in close contact. A localized surface plasmon resonance is excited in the metal nanostructure with visible light, and excited electrons with enough momentum can be injected into the semiconductor before relaxation and recombination with the metal nanostructure. The injection time and subsequent relaxation of free electrons from Cu nanoparticles photodeposited onto a CeO₂ aerogel is investigated using ultrafast transient absorption spectroscopy. The Cu nanoparticles are excited with pulsed visible light and the appearance of free electrons in the CeO₂ aerogel are detected with mid-infrared light. The transient signals appear within 300 fs, providing a timeframe for electron transfer from Cu to CeO₂. Transient signals are measured over a range of mid-infrared probe wavelengths, attributed to free or trapped electrons in CeO₂. Unlike other transition metal oxides, CeO₂ has an additional empty 4f state between its conduction and valence bands that can store electrons and form small polarons. The role of the 4f state in electron-phonon decay is explored by probing the 4f state directly with 266 nm light and 350 nm probe. The results presented here examine the utility of using Cu nanoparticles to enhance the properties of CeO₂ as a photocatalyst.