Core/Shell/Shell Nanowire Photocathodes for Neutral pH Water Splitting

Silicon is the most widely used semiconductor for solar cells [ 1–4 ] and has also shown promising performances in photoelectrochemical (PEC) cells for hydrogen production. [ 5–10 ] Silicon photoelectrodes with nano/micro structures such as nano/ micro wire array [ 9,11 ] and black silicon [ 7 ] have exhibited improved PEC performances. Silicon nanowire (NW) arrays, in particular, are of considerable interest due to their distinctive properties, facile and scalable techniques for their fabrication, etc. [ 11 – 19 ] On the other hand, there are two key factors limiting the application of silicon photoelectrodes for practical solar hydrogen generation including their large external biasing potential (high overpotential) required to drive hydrogen or oxygen evolution reaction (HER or OER) and their instability in the electrolyte. Tremendous efforts have been expended to overcome these limiting factors such as using metal catalysts (noble [ 20,21 ] or non-noble [ 22–24 ] ) to reduce the turnon (or onset) potential and by coating the electrodes surface with a corrosion-resistant layer such as TiO 2 [ 25–27 ] for stability improvement. A thin TiO 2 coating layer can signifi cantly enhance the stability, [ 27 ] however due to its poor conductivity, a metal co-catalyst needs to be deposited on its surface to boost the HER or OER. [ 25,26 ] Therefore, a more functional metal oxide coating layer, which can simultaneously enhance the PEC performance and HER/OER kinetics, and electrode stability, is desirable. Recent studies have demonstrated that chemical-vapor and atomiclayer deposited Fe 2 O 3 , [ 17,28 ] atomic layer-deposited MnO, [ 29 ]