Tandem Core-Shell Si-Ta₃N₅ Photoanodes for Photoelectrochemical Water Splitting

Nanostructured core-shell Si-Ta₃N₅ photoanodes were designed and synthesized to overcome charge transport limitations of Ta₃N₅ for photoelectrochemical water splitting. The core-shell devices were fabricated by atomic layer deposition of amorphous Ta₂O₅ onto nanostructured Si and subsequent nitridation to crystalline Ta₃N₅. Nanostructuring with a thin shell of Ta₃N₅ results in a 10-fold improvement in photocurrent compared to a planar device of the same thickness. In examining thickness dependence of the Ta₃N₅ shell from 10 to 70 nm, superior photocurrent and absorbed-photon-to-current efficiencies are obtained from the thinner Ta₃N₅ shells, indicating minority carrier diffusion lengths on the order of tens of nanometers. The fabrication of a heterostructure based on a semiconducting, n-type Si core produced a tandem photoanode with a photocurrent onset shifted to lower potentials by 200 mV. CoTiO_x and NiO_x water oxidation cocatalysts were deposited onto the Si-Ta₃N₅ to yield active photoanodes that with NiO_x retained 50-60% of their maximum photocurrent after 24 h chronoamperometry experiments and are thus among the most stable Ta₃N₅ photoanodes reported to date.