Ultrafast Carrier Dynamics in Water-Splitting Photocatalysts

Semiconductors that photocatalytically split water into hydrogen and oxygen using solar energy represent promising renewable energy technologies. Charge carrier dynamics on sub-nanosecond timescales dictate the overall efficiency of charge separation, and consequently, photocatalytic activity of these materials. Using transient absorption spectroscopy, we investigated photoexcited carrier trapping, recombination, and interfacial charge transfer dynamics in water-splitting niobate nanosheets. Ca$_{2}$Nb$_{3}$O$_{10}$ generated H$_{2}$ from water with 0.22{\%} quantum yield, but no O$_{2}$ was formed. Carriers relaxed with second-order kinetics on a sub-nanosecond time scale that depended on the nanosheet size. Methanol was used as a sacrificial donor to separate electron and hole dynamics and identify absorption spectra for the trapped carriers. In aqueous methanol, hole scavenging was observed within 100 ps in direct competition with electron-hole recombination. The nanosheets were also functionalized with metal and semiconductor nanoparticles to form novel photocatalyst nanostructures. Colloidal IrO$_{2}$ was attached to Ca$_{2}$Nb$_{3}$O$_{10}$ to sensitize the catalyst for visible light absorption. Interfacial electron transfer stabilized the charge separation in IrO$_{2}$, and O$_{2}$ was generated.