Controllable Synthesis And Modification Of Nickel Phosphide For Dye Sensitized Photocatalytic Hydrogen Production

The zero carbonization of fuels is an attractive option for solving energy and environmental problems.With the advantages of high energy density and zero product pollution,hydrogen energy is an ideal alternative to fossil fuels.The most efficient scheme to zero carbonization of fuels is to harvest and exploit hydrogen energy.Photocatalytic hydrogen production is an excellent approach to obtain"green hydrogen"and convert solar energy to chemical energy.However,the photocatalytic performance of the most promising photocatalysts are still not quite satisfied due to the easy recombination between charge carriers and few active reaction sites.It is an outstanding strategy to overcome these limitations by introducing co-catalytic materials.The most commonly used co-catalysts are represented by the precious metal Pt,but their scarcity and high price prevent them from realizing large-scale practical applications.Therefore,it is a hot research spot in photocatalysis to search for inexpensive,non-precious metal co-catalysts with comparable performance to Pt.It is reported that various transition metal phosphides can be applied as non-precious metal co-catalysts for photocatalytic hydrogen production.Nickel phosphide draws widespread attention owing to its low hydrogen production overpotential,excellent electrical conductivity,and abundant source and inexpensive preparation cost.It is demonstrated in many studies that the crystalline phase and morphology of catalysts are playing a critical role in the catalytic performance.It is an attractive measure to improve its intrinsic activity by adjusting the phase structure of nickel phosphide.Individual nickel phosphide is an excellent HER electrocatalyst but exhibits poor photocatalytic performance.Dyes are excited in the presence of visible light to generate electrons and holes.An additional co-catalyst is usually required to enhance the hydrogen production,and if the dye is applied to photocatalytic dissociation of water.Dye-sensitized nickel phosphide systems can synergize the effects of both,thus providing excellent activity and stability.In this paper,the synthesis,modification of different phases of nickel phosphide and their photosensitized hydrogen evolution were studied.This work is divided into the following two parts.In the first section,we prepared nickel phosphide by low-temperature calcination using NiCl₂·6H₂O as the nickel source and Na H₂PO₂·H₂O as the phosphorus source,respectively.By modulating the Ni/P ratio of the raw materials,three different phases of Ni₁₂P₅,Ni₂P and Ni₅P₄were synthesized successfully.The synthesized Ni_xP_y was characterized by XRD,XPS,SEM,TEM,EDX,Raman,FT-IR,EIS and FL.We examined the dye-sensitized photocatalytic hydrogen production activity of Ni_xP_y in which EY was employed as the sensitizer,Ni_xP_y as the co-catalyst,and trimethylamine as the sacrificial agent.It was revealed that Ni₅P₄ presented the highest dye-sensitized hydrogen production activity and excellent stability compared to Ni₁₂P₅ and Ni₂P,which were 6.7 times and 3.2 times higher than Ni₁₂P₅ and Ni₂P,respectively,with the best apparent quantum efficiency（AQY）of 37.22%at 470 nm.The high activity is attributed to the highest P content in Ni₅P₄ and the rapid electron transfer between the excited state dye and Ni₅P₄.In this paper,the possible mechanism of hydrogen production is explored.In the second part,the NCDs/Ni₅P₄ composites were obtained by loading carbon dots（NCDs）on Ni₅P₄ by a simple mechanical grinding method.The as-prepared samples were also characterized by means of XRD,XPS,SEM,TEM,Raman,EIS,etc.We have investigated the photosensitized performance of NCDs modified Ni₅P₄ for hydrogen production with EY as the catalyst（sensitizer）and NCDs/Ni₅P₄ as the co-catalyst.It is shown that NCDs can not only inhibit Ni₅P₄ aggregation,but also serve as electron transport mediators for EY and Ni₅P₄ and accelerate carrier separation.When the loading of NCDs was 7.5 wt%,the photosensitized hydrogen production rate of NCDs/Ni₅P₄ was 28737μmol h^-1 g^-1,which was 2.3 times higher than that of pure Ni₅P₄,and the apparent quantum efficiency（AQY）at 450 nm was 45.33%.In addition,the mechanism of dye-sensitized photocatalytic hydrogen evolution has also been analyzed and discussed.