Photophysics and Electron Transfer Dynamics of Type-Ii and Quasi Type-Ii Heterostructure Nanocrystals

Type-II and quasi type-II heterostructure nanocrystals are known to exhibit extended excited-state lifetimes compared to their single material counterparts because of reduced wavefunction overlap between the electron and hole. Thus, type-II heterostructures are promising materials for solar-to-fuel conversion, as extended excited-state lifetimes make transfer of charges to a catalyst more competitive with intrinsic nanocrystal decay processes. However, due to fast and efficient hole trapping and non-uniform morphologies, the photophysics of dot-in-rod heterostructures are more rich and complex than this simple picture. Using transient absorption spectroscopy, we observe that the behavior of electrons in the CdS “rod” or “bulb” regions of non-uniform ZnSe/CdS and CdSe/CdS dot-in-rods is similar regardless of the “dot” material, which supports previous work demonstrating that hole trapping and particle morphology drive electron dynamics. Furthermore, we show that the longest lived state in these dot-in-rods is not generated by the type-II or quasi type-II band alignment between the dot and the rod, but rather by electron-hole dissociation that occurs due to fast hole trapping in the CdS rod and electron localization to the bulb. We propose that specific variations in particle morphology and surface chemistry determine the mechanism and efficiency of charge separation and recombination in these nanostructures, and therefore impact their excited-state dynamics to a greater extent than the heterostructure energy level alignment alone. When coupled to a [Fe-Fe] hydrogenase, which catalyzes reduction of protons to H2, we observe faster rates of electron transfer and higher quantum efficiency of electron transfer with CdSe/CdS and ZnSe/CdS dot-in-rods, but only from the bulb and interface states. Furthermore, the total efficiency of electron transfer of the ensemble is highest when the bulb/interface state is directly populated. This indicates that the bulb morphology is essential to efficient electron transfer in a dot-in-rod hydrogenase system