Phase‐Controlled Synthesis of Nickel Phosphide Nanocrystals and Their Electrocatalytic Performance for the Hydrogen Evolution Reaction

: The phase of nanocrystals has a key role in the modulation of catalytic properties. Uniform and well-crystallized nickel phosphide nanocrystals with controlled phases (Ni5 P4 , Ni2 P, and Ni12 P5 ) and narrow size distributions are synthesized by a wet chemical method. The phases of the as-synthesized nickel phosphide nanocrystals are controlled by the P/Ni precursor molar ratio, heating process, and time of reaction. Rarely reported nearly monodisperse 5.6 nm Ni5 P4 nanocrystals are successfully synthesized and show superior hydrogen evolution reaction (HER) activity. Only a low overpotential of 103 mV is required to achieve the HER current of 10 mA cm-2 at a low catalyst loading of 0.12 mg cm-2 . The high HER activity is attributed to the high quality of the as-obtained Ni5 P4 nanocrystals, which have the electronic effect from the Ni5 P4 phase and also high surface area owing to the small particle size. A systematic study of the controlled synthesis of nickel phosphide nanocrystals is shown in this paper, and the HER catalytic activity is improved through the phase- and size-controlled synthesis of nanocrystals.