Tandem Core–Shell Si–Ta 3 N 5 Photoanodes for Photoelectrochemical Water Splitting
Here, nanostructured core–shell Si–Ta 3 N 5 photoanodes were designed and synthesized to overcome charge transport limitations of Ta 3 N 5 for photoelectrochemical water splitting. The core–shell devices were fabricated by atomic layer deposition of amorphous Ta 2 O 5 onto nanostructured Si and subsequent nitridation to crystalline Ta 3 N 5 . Nanostructuring with a thin shell of Ta 3 N 5 results in a 10-fold improvement in photocurrent compared to a planar device of the same thickness. In examining thickness dependence of the Ta 3 N 5 shell from 10 to 70 nm, superior photocurrent and absorbed-photon-to-current efficiencies are obtained from the thinner Ta 3 N 5 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–Ta 3 N 5 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 Ta 3 N 5 photoanodes reported to date.