Hole Removal Rate Limits Photodriven H2 Generation Efficiency in CdS-Pt and CdSe/CdS-Pt Semiconductor Nanorod-Metal Tip Heterostructures

Semiconductor-metal nanoheterostructures, such as CdSe/CdS dot-in-rod nanorods with a Pt tip at one end (or CdSe/CdS-Pt), are promising materials for solar-to-fuel conversion because they allow rational integration of a light absorber, hole acceptor, and electron acceptor or catalyst in an all-inorganic triadic heterostructure as well as systematic control of relative energetics and spatial arrangement of the functional components. To provide design principles of such triadic nanorods, we examined the photocatalytic H-2 generation quantum efficiency and the rates of elementary charge separation and recombination steps of CdSe/CdS-Pt and CdS-Pt nanorods. We showed that the steady-state H-2 generation quantum efficiencies (QEs) depended sensitively on the electron donors and the nanorods. Using ultrafast transient absorption spectroscopy, we determined that the electron transfer efficiencies to the Pt tip were near unity for both CdS and CdSe/CdS nanorods. Hole transfer rates to the electron donor, measured by time-resolved fluorescence decay, were positively correlated with the steady-state H-2 generation QEs. These results suggest that hole transfer is a key efficiency-limiting step. These insights provide possible ways for optimizing the hole transfer step to achieve efficient solar-to-fuel conversion in semiconductor-metal nanostructures.