Formation of Layer‐by‐Layer Assembled Cocatalyst Films of S²⁻‐Stabilized Ni₃S₄ Nanoparticles for Hydrogen Evolution Reaction

Abstract It remains challenging to develop alternatives, based on widely abundant elements, to highly active Pt catalysts for the hydrogen evolution reaction. Metal sulfide nanoparticles (NPs) are potential candidates for such catalysts because of their metallic features and intrinsic high surface area; however, their poor electrocatalytic activity, owing to the presence of surface ligands, is an obstacle to practical use. Here we report the selective synthesis of oleylamine (OAm)‐stabilized Ni₃S₄ (Ni₃S₄/OAm) NPs. The insulating ligands were exchanged with S²⁻ and loaded on electrodes by a layer‐by‐layer (LbL) assembly method with metal cation linkers. The enhanced catalytic properties were confirmed by electrochemical measurements. A LbL assembled film with Ni²⁺ as a linker (11 cycles) showed an overpotential of 555 mV@‐4 mA cm⁻² at pH 6.8 and a stable current density for 12 h. In addition, when this LbL assembly method was applied to a CdS/Cu(In,Ga)Se₂ photocatalyst electrode, the resulting photoelectrode showed an enhanced photocurrent compared with that of the Ni₃S₄/OAm NPs‐spin coated and bare photoelectrodes. These results indicate the versatility of LbL assembly for fabrication of ligand‐free NP electrocatalyst films.