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Understanding the Role of Nanoscale Hetero-Interfaces in Core/Shell Structures for Water Splitting: Covalent Bonding Interaction Boosts the Activity of Binary Transition Metal Sulfides.

The appropriate catalyst model with precisely designed interface is highly desirable for revealing the real active site at atomic level. Herein, we report a proof-of-concept strategy for creating exposed and embedding interface model by constructing unique Co9S8 core with full (Co9S8/FWS2) and half WS2 shell (Co9S8/HWS2) to uncover the synergistic effect of hetero-interfaces on the catalytic performances. Tailoring the heteroepitaxial growth of WS2 shell, the Co9S8/HWS2 with exposed Co-S-W interfaces leads to the exceptional electron density changes on edged-S atoms with large amounts of lone-pair electrons. Meanwhile, the unique Co9S8/HWS2 could accelerate the kinetic adsorption of hydrogen and oxygen-containing intermediates. Such Co9S8/HWS2 electrocatalysts show extremely low overpotential of 78 mV and 290 mV at current density of 10 mA cm-2 for hydrogen evolution reaction (HER) and oxygen evolution reaction (OER). Using Co9S8/HWS2 as both cathode and anode, an alkali electrolyzer delivers at a quite low cell voltage of 1.60 V (10 mA cm-2). The operando Raman spectroscopy and electron spin resonance (ESR) results both indicate the presence of S-S terminal and S-S bridging with unsaturated S atoms during the HER process. The present work reveals the synergistic effects of nanoscale interfaces on overall electrocatalytic water splitting.