Plasmonic Au/p-GaN photocathodes for artificial photosynthesis: Ultrafast hot-carrier dynamics and photoelectrochemical CO_2 reduction

Abstract

Plasmonic-metal nanostructures offer unique opportunities for solar photocatalysis via photo-excitation of highly energetic "hot" carriers at both metal-semiconductor and metal-electrolyte interfaces that can drive photochem. reactions. While examples of hot-electron-driven processes have been widely reported, little is known about the nature of plasmon-derived hot holes and their role in hot-carrier photocatalysis. Here, we report the demonstration of plasmonic Au/p-type GaN photocathodes for photoelectrochem. CO_2 redn. Despite an interfacial Schottky barrier to hot hole transport of more than 1 eV across the Au/p-GaN heterojunction, plasmonic Au/p-GaN photocathodes exhibit photoelectrochem. properties consistent with the injection of hot holes into GaN upon plasmon excitation of Au nanoparticles. We further monitored the carrier dynamics of hot-hole injection via ultrafast transient absorption spectroscopy; we obsd. plasmon-induced hot-hole transfer from Au to p-GaN occurs within the 200 fs instrument response of our exptl. setup, placing hothole transfer from a plasmonic metal to a p-type semiconductor on similar timescales as hot-electron transfer to an n-type semiconductor. Moreover, the ultrafast (t < 200 fs) injection of hot-holes into p-GaN exerts a profound influence on the dynamics of hot electrons left behind on the Au nanoparticles, lowering the electronic temp. and reducing the electron-phonon coupling time. The plasmonic Au/p-GaN photocathodes were further employed for plasmon-driven CO_2 redn. in aq. electrolytes, demonstrating improved selectivity for CO prodn. over H_2 evolution upon plasmon excitation. Taken together, our results offer new insight into the ultrafast dynamics of plasmon-induced hot holes and demonstrate a photoelectrochem. device for harvesting them to selectively drive solar-to-fuel energy conversion with metal nanostructures.