Orthorhombic NiSe Nanocrystals on Si Nanowires for Efficient Photoelectrochemical Water Splitting.

Photocatalytic water splitting is a vital technology for clean renewable energy. Despite enormous progress, the search for earth-abundant photocatalysts with long-term stability and high catalytic activity is still an important issue. We report three possible polymorphs of nickel selenide (orthorhombic phase NiSe, cubic phase NiSe, and hexagonal phase NiSe) as bifunctional catalysts for water-splitting photoelectrochemical (PEC) cells. Photocathodes or photoanodes were fabricated by depositing the nickel selenide nanocrystals (NCs) onto p- or n-type Si nanowire arrays. Detailed structural analysis reveals that compared to the other two types, the orthorhombic NiSe NCs are more metallic and form less surface oxides. As a result, the orthorhombic NiSe NCs significantly enhanced the performance of water-splitting PEC cells by increasing the photocurrents and shifting the onset potentials. The high photocurrent is ascribed to the excellent catalytic activity toward water splitting, resulting in a low charge-transfer resistance. The onset potential shift can be determined by the shift of the flat-band potential. A large band bending occurs at the electrolyte interface, so that photoelectrons or photoholes are efficiently generated to accelerate the photocatalytic reaction at the active sites of orthorhombic NiSe. The remarkable bifunctional photocatalytic activity of orthorhombic NiSe promises efficient PEC water splitting.