Solution-Phase Activation and Functionalization of Colloidal WS2 Nanosheets with Ni Single Atoms

Single-atom functionalization of transition-metal dichalcogenide (TMD) nanosheets is a powerful strategy to tune the optical, magnetic, and catalytic properties of two-dimensional materials. In this work, we demonstrate a simple solution-phase method to generate nucleophilic sulfide sites on colloidal WS2 nanosheets that subsequently serve as ligands for Ni single atoms. These materials can be controllably functionalized with varying amounts of Ni on the surface ranging from 9% to 47% coverage with respect to W. High-resolution scanning transmission electron microscopy coupled to electron energy loss spectroscopy and X-ray absorption spectroscopy indicate that adsorbed Ni species bind as single atoms at low coverage and a mixture of single atoms and multimetallic clusters at high coverage. The Ni single atoms adsorbed on WS2 show altered electronic properties, and both the electronic perturbation and isolated atom geometry play a role in enhancing the intrinsic catalytic activity of Ni-WS2 samples for the electrochemical oxygen evolution reaction.