Nickel Phosphide-based Composites For Photocatalytic Hydrogen Production

With the development of modern industry and consumption of fossil fuels,a series of problems,such as energy crisis and environmental pollution,are increasingly serious.It is urge to develop clean and sustainable energy.As a technology to produce hydrogen by using solar energy,photocatalytic hydrogen generation is an effective way to solve the above problems.However,the semiconductor materials used in this process often suffer a serious carrier recombination problem,which reduced the hydrogen production efficiency.The introduction of appropriate cocatalysts can effectively solve the problem of carrier recombination in semiconductor materials and improve photocatalytic activity of materials.In this paper,nickel phosphide（Ni_xP_yand Ni₃P-Ni）was used as cocatalyst in order to modify the semiconductor materials.Ni_xP_y/Cd S and Ni₃P-Ni/g-C₃N₄was prepared,which showed excellent performance in addition to the photolysis of aquatic hydrogen,the main work is as follows:（1）Three kinds of Ni_xP_y(Ni₂P,Ni₁₂P₅,Ni₅P₄)crystal phases were prepared by phosphating with Ni₃P as the precursor.The effects of phosphating temperature,phosphating time,Ni/P molar ratio,phosphating gas velocity,phosphorus source on the crystal phase were investigated.The results proved that phosphating temperature and phosphorus source were the main factors that affect the formation of crystal phases.The preparation conditions of the three kinds of Ni_xP_ycrystal phases were as follows:red phosphorus as phosphorus source,mole ratio of Ni/P of 1/4,phosphating gas velocity of 20 m L/min,phosphating time of 2 h,and phosphating temperature of 500^○C（Ni₅P₄）,600^○C（Ni₂P）and 800^○C(Ni₁₂P₅),respectively.Ni₃P,Ni₂P,Ni₁₂P₅,and Ni₅P₄were used as cocatalysts to prepare photocatalytic composites.All the four cocatalysts of Ni_xP_ycan significantly enhance the hydrogen production activity of Cd S.Ni₅P₄/Cd S shows the best hydrogen production capacity,which is5.4 times than that of Cd S(601.5μmol·g^-1·h^-1).Meanwhile,the Ni₅P₄/Cd S composite exhibits good stability for the H₂generation reaction in three cycle experiments.The results of PL characterization showed that the introduction of Ni_xP_ycould effectively inhibit the carrier recombination and enhance the photocatalytic activity of Cd S.（2）Ni₃P-Ni/g-C₃N₄composites were prepared by one-step heating of Ni-P precursor and g-C₃N₄.According to XRD,TEM,and other characterization analysis,it is proved that the Ni₃P-Ni cocatalysts is a porous spherical Ni₃P filled with metallic Ni,which loaded on two-dimensional g-C₃N₄material.Photocatalytic reaction results showed that H₂generation activity of g-C₃N₄was significantly improved due to the introduction of Ni₃P and Ni.When the content of Ni₃P-Ni was 2 wt%in Ni₃P-Ni/g-C₃N₄composites,the optimal rate of hydrogen production is 203.3μmol·g^-1·h^-1.Based on PL and electrochemical characterization analysis,it is confirmed that the metallic Ni plays multifunctional roles in boosting the H₂generation under visible light,which could both accept the photo-generated electrons from g-C₃N₄and enhance the electron transfer to Ni₃P for further H₂generation.Therefore,the synergistic effect between metallic Ni and Ni₃P as a cocatalyst can restrain the recombination of photogenerated electrons of g-C₃N₄due to the suitable cocatalyst structure,leading to an excellent photocatalytic H₂generation activity over Ni₃P-Ni/g-C₃N₄composites.