In situ formation of nickel sulfide quantum dots embedded into a two-dimensional metal-organic framework for water splitting

The synergistic interaction between metal-organic frameworks (MOFs) and transition metal sulfides (TMS) has been a research hotspot in the field of electrocatalytic water splitting. Herein, nickel sulfide quantum dots@NiFe-terephthalic acid nanosheet (NSQDs@NiFe-TPA) composites were prepared by a two-step hydrothermal method. The size and crystal structure of nickel sulfides were easily modulated by adjusting the concentration of the sulfurizing agent. The in situ formed NiS/Ni3S2 quantum dots with grain sizes of around 5 nm were evenly dispersed on the surface of NiFe-TPA nanosheets, and the overpotential was only 219 mV at 10 mA cm(-2), and 90% current density could be maintained at 1.5 V (vs. RHE) for 60 h. When the concentration of the sulfurizing agent was increased up to 0.15 mmol L-1, the as-prepared Ni3S2@NiFe-TPA exhibited an excellent HER performance. The overpotential is only 109 mV to reach a current density of 10 mA cm(-2), and it was attenuated by 20 mV after a 60 h stability test at a current density of 20 mA cm(-2). Furthermore, the overall water-splitting electrolyzer assembled with NSQDs@NiFe-TPA and Ni3S2@NiFe-TPA as anodic and cathodic electrodes exhibited a low cell voltage of 1.66 V at a current density 10 mA cm(-2), and almost no attenuation was observed after a 60 h stability test. The outstanding electrocatalytic properties of the as-prepared catalyst are due to the synergy of nickel sulfides and 2D MOFs that offers abundant accessible active sites, rapid ion/electron transportation, and convenient O-2/H-2 release channels.