Ultrafast charge-carrier localization and separation dynamics in CdS nanowires wrapped in C3N5 nanosheets

Nanowires are broadly used in photocatalytic and photoelectrochemical applications due to advanced encapsulation techniques and the large surface area of nanowire networks. However, future sustainable nanotechnologies depend on our ability to harness ultrafast charge-carrier dynamics in emerging nanowires. In this work, we combine time-resolved terahertz spectroscopy (TRTS) and time-resolved photoluminescence (TRPL) to explore ultrafast charge-carrier dynamics in CdS nanowires wrapped in C3N5 nanosheets. Our fluence-dependent TRTS results reveal that the photoexcited charge-carrier dynamics are consistent with the Drude-Smith model and not the plasmon model or the Bruggeman effective medium theory. We find that CdS nanowires possess bulk-like short-range (<14 nm) charge-carrier mobilities that are unaltered by post-growth encapsulation with C3N5 nanosheets. In situ encapsulation is observed to reduce the short-range mobility in the CdS nanowire core. The Drude-Smith localization parameters indicate that the nanowires are fully depleted, even at our highest photoexcitation densities. In turn, photoexcited holes are rapidly transported to the nanowire surface, which affects our TRTS and TRPL lifetimes. We simultaneously model the TRTS and TRPL lifetimes with a 1D diffusion model that contains hot-carrier cooling, intrananowire charge-carrier diffusion, surface recombination, bimolecular recombination, and charge-separation. Our model reveals that sub-nanosecond charge-separation can prolong carrier lifetimes into nanosecond timescales and that post-growth encapsulation increases surface recombination at the CdS-C3N5 interface. Since this has been linked to increased photocatalytic efficiency, our model provides a direct link between the ultrafast carrier dynamics and solar-driven photocatalytic efficiency in CdS nanowires and explains recently observed control over photocatalytic pathways.