Tandem Core-Shell Si-TaN Photoanodes for Photoelectrochemical Water Splitting.

Nanostructured core-shell Si-TaN photoanodes were designed and synthesized to overcome charge transport limitations of TaN for photoelectrochemical water splitting. The core-shell devices were fabricated by atomic layer deposition of amorphous TaO onto nanostructured Si and subsequent nitridation to crystalline TaN. Nanostructuring with a thin shell of TaN results in a 10-fold improvement in photocurrent compared to a planar device of the same thickness. In examining thickness dependence of the TaN shell from 10 to 70 nm, superior photocurrent and absorbed-photon-to-current efficiencies are obtained from the thinner TaN 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 and NiO water oxidation cocatalysts were deposited onto the Si-TaN to yield active photoanodes that with NiO retained 50-60% of their maximum photocurrent after 24 h chronoamperometry experiments and are thus among the most stable TaN photoanodes reported to date.