Colloidal Ni2P Nanocrystals Encapsulated in Heteroatom-Doped Graphene Nanosheets: A Synergy of 0D@2D Heterostructure Toward Overall Water Splitting

Transition-metal phosphide (TMP) nanostructures have been extensively studied for hydrogen evolution reaction (HER) and oxygen evolution reaction (OER). However, phase-controlled synthesis of colloidal Ni2P nanocrystals (NCs) or related heterostructures remains challenging and their use as bifunctional electrocatalysts in overall water splitting (OWS) is not systematically studied. Herein, zero-dimensional (0D) colloidal Ni2P NCs are synthesized using a robust solution-phase method and encapsulated in two-dimensional (2D) N- and S-doped graphene (NSG) nanosheets via facile ex situ sonication to form a 0D@2D Ni2P@NSG heterostructure. The interaction between surface functionalities of Ni2P NCs and defective NSG via strong van der Waals force provides a robust sheath to Ni2P NCs when encapsulated in NSG nanosheets, further enhancing the specific surface area and active site exposure. Density functional theory calculations indicate that the dual interaction of N and S dopants with Ni2P benefits the synergistic effect of optimized water and hydrogen free energy adsorption. As a result, Ni2P@NSG electrocatalysts manifest high catalytic activity toward HER and OER, and a two-electrode alkaline electrolyzer assembled by Ni2P@NSG as both an anode and a cathode requires only 1.572 V to reach a current density of 10 mA/cm2.