Layer-by-Layer Assembled Polyelectrolytes on Ta₃N₅ Pyramid Arrays for Efficient Photoelectrochemical Water Splitting

Ta₃N₅ is a promising material for overall water splitting due to its suitable band positions and high theoretical photocurrent (12.9 mA cm^-2). However, pristine Ta₃N₅ suffers from significant optical, recombination, and resistive losses, resulting in photocurrents and efficiencies far below theoretical expectations. To address these limitations, this work focuses on enhancing photocurrent and overall performance through structural and morphological engineering and surface functionalization. We investigated the potential of polyelectrolytes, poly(sodium-p-styrenesulfonate) (PSS) with anionic groups and poly(diallyl-dimethylammonium chloride) (PDDA) with cationic groups, as a hole transport layer material in Ta₃N₅ pyramid arrays photoelectrodes for enhanced photoelectrochemical water splitting. We synthesized pyramid-shaped Ta₃N₅ using Na₂Ta₂O₆ as a template. The polyelectrolyte combined with catalysts was used to modify Ta₃N₅ to improve charge separation and injection efficiency. After systematically investigating the effect of the polyelectrolyte modification sequence on the photoelectrode, the champion sample Ta₃N₅-PDDA₃PSS₃-Co(OH)_x delivers a photocurrent density of 7.7 mA cm^-2 at 1.23 V vs. RHE under simulated solar illumination, which is over 7-fold higher than that of pristine Ta₃N₅. This work demonstrates that polymer functionalization of nitride semiconductors enables efficient solar-to-fuel conversion.