Theoretical insight into surface structures of pentlandite toward hydrogen evolution.

Pentlandite (Fe,Ni)S is a promising transition-metal catalyst for the hydrogen evolution reaction. However, little is explained about the long activation process that has been observed in experiments, and its facet-dependent hydrogen evolution activity is still theoretically unrevealed. To explain some experimental phenomena and to guide subsequent studies, density functional theory calculations are used to study the main synthetic surfaces: (111) and (311) in this work. The results show that the small metal cube plays an important role in the surface stability, and it is suggested that such cubes remain intact during catalysis. The linking sites serve as a bridge across the metal cubes and are the main catalytic active sites for hydrogen evolution. This is because the metal cubes can tune the electronic structures of the linking sites, and then the free energy of the linking sites is optimized. The (311) surface is a composite surface that consists of (100) and (111) facets and has the profile of a step. A surface conversion between the (311) and (111) facets may occur when the cube layer length increases. Therefore, the active sites can be feasibly engineered by the surface structures, and this could be helpful in further applications of (Fe,Ni)S.