Engineering Heterogeneous NiS 2 /NiS Cocatalysts with Progressive Electron Transfer from Planar p-Si Photocathodes for Solar Hydrogen Evolution.

The sluggish transfer of electrons from a planar p-type Si (p-Si) semiconductor to a cocatalyst restricts the activity of photoelectrochemical (PEC) hydrogen evolution. To overcome such inefficiency, an elegant interphase of the semiconductor/cocatalyst is generally necessary. Hence, in this work, a NiS 2 /NiS heterojunction (NNH) is prepared in situ and applied to a planar p-Si substrate as a cocatalyst to achieve progressive electron transfer. The NNH/Si photocathode exhibits an onset potential of +0.28 V versus reversible hydrogen electrode (V RHE ) and a photocurrent density of 18.9 mA·cm -2 at 0 V RHE , as well as a 0.9% half-cell solar-to-hydrogen efficiency, which is much superior compared with those of NiS 2 /Si and NiS/Si photocathodes. The enhanced performance for NNH/Si is attributed to the contact between the sectional n-type semiconducting NNH and the planar p-Si semiconductor through a p-Si/n-NiS/n-NiS 2 manner that functions as a local pn-junction to promote electron transfer. Thus, the photogenerated electron is transferred from p-Si to n-NiS within NNH as the progressive medium, followed by to Ni 2+ and/or S 2 2- of the defect-rich n-NiS 2 phase as the key active sites. This systematic work may pave the way for planar Si-based PEC applications of heterogeneous metal sulfide cocatalysts through the progressive transfer of electrons.