Tunable d-Band Centers of Ni₅P₄ Ultra-Thin Nanosheets for Highly-Efficient Hydrogen Evolution Reaction

Although the recent advance of ultra-thin 2D nanosheets for hydrogen evolution reaction (HER) is remarkable, there are still substantial challenges to reliably control their physioelectric and electrochemical properties to employ as highly-efficient electrocatalysts. Herein, based on complementary theoretical and experimental studies, the d-band center position of ultra-thin 2D Ni₅P₄ nanosheets can be manipulated by simple heteroatom doping. Interestingly, the Fe-doped nanosheets yield the lowest d-band center position, but they do not display the optimal Gibbs free energy of adsorbed H atoms due to the imbalance of adsorption and desorption of adsorbed H atoms. With the proper Co doping (i.e., 20%), the nanosheets exhibit the best electrocatalytic performance along with an excellent stability. The overpotential is only 100.5 mV at 10 mA cm⁻² with a Tafel slope of 65.8 mV dec⁻¹, which is superior than those of Fe-doped, Cu-doped, and pristine Ni₅P₄ nanosheets. Ultraviolet photoelectron and X-ray photoelectron spectroscopy further verify the downshift of d-band centers of nanosheets by optimal doping, illustrating that Ni with the lower binding energy mainly dominates the active sites. All these results provide a valuable design scheme of dopants to control the d-band center position of nanosheets for next-generation highly-efficient HER electrocatalysts.