Low Onset-Potential Z-Scheme Ta₃N₅-based Photoanode with Enhanced Light Harvesting and Charge Transport

Simultaneous enhancement of light harvesting and charge transport in photoelectrochemical (PEC) systems is a major challenge to achieving high solar-to-hydrogen efficiency. Here, a Ta3N5-Si Z-scheme system is constructed to facilitate charge transport pathways from generation to catalysis, taking advantage of the exquisite bandgap and band position of Ta3N5. The tailored Ta3N5-Si junction with an NbNx electron mediator effectively establishes a Z-scheme charge transport and enhances the driving force for water oxidation, reducing the onset potential by an increment in photovoltage. Moreover, the nitrogen-doped CoFeOx co-catalyst boosts hole dynamics and kinetics at the surface level, resulting in improved hole extraction for water oxidation catalysis. The synergy between the above strategies cooperatively expedites the charge separation and transport in a Ta3N5 photoanode, which decreases the photocurrent onset potential from 0.69 to 0.27 V versus the reversible hydrogen electrode, a reduction of 420 mV. This result represents one of the lowest onset potentials observed for Ta3N5-based photoanodes. A systematic approach to enhancing photovoltage and photocurrent expands the design concept of metal nitride-based PEC devices.