Construction Of Transition Metal Phosphide/g-C₃N₄ Photocatalytic System And Study On Their Photocatalytic Performance

Hydrogen is not only clean and pollution-free,but also has a high energy density(140MJ?kg^-1),which is considered to be one of the most promising clean energy sources.At present,hydrogen is mainly obtained through high energy-consuming processes such as steam reforming of methane and electrolysis of water.Therefore,it is urgent to seek clean and green hydrogen production technology.Photocatalytic water splitting to produce hydrogen is a green sustainable technology that can directly convert solar energy into hydrogen energy.However,the high recombination rate of photogenerated electrons and holes leads to low hydrogen production quantum efficiency of photocatalysts,which prevents the large-scale application of this technology.The development of excellent cocatalysts to promote the separation of photogenerated electrons and holes is the core of research in the field of photocatalytic hydrogen production.Transition metal phosphides?TMPs?are an efficient hydrogen evolution co-catalyst with abundant element reserves.In this paper,Ni₂P,Co₂P and Ni_xCo_2-xP?0<x?1?are loaded as hydrogen evolution co-catalysts on g-C₃N₄nanosheets?CNNs?,which effectively suppress the recombination of photo-generated electrons and holes,reduce the over-potential of hydrogen production,and increase the activity of photocatalytic hydrogen production.The main research contents and results are as follows:?1?Preparation of Ni₂P/CNNs and Co₂P/CNNs by mechanical grinding method and study of hydrogen production performance.Using nanoscale red phosphorus?RP?as the phosphorus source,pure Ni₂P and Co₂P nanoparticles were synthesized by solvothermal method,and then mechanically milled with CNNs?urea as precursor?to prepare a series of Ni₂P/CNNs and Co₂P/CNNs with different amounts.Experiments results show that compared with pure CNNs,the composite photocatalysts have strong visible light absorption capacity,fast photo-generated carrier separation characteristics and low hydrogen evolution overpotential.The highest photocatalytic hydrogen production efficiency of Ni₂P/CNNs and Co₂P/CNNs are 2941?mol?g^-1?h^-1and 3156?mol?g^-1?h^-1,which are 22 times and 24 times that of CNNs(134?mol?g^-1?h^-1),respectively.Under 400nm monochromatic light,their apparent quantum efficiency?AQE?reach 17.2%and17.4%,respectively.?2?Preparation of Ni_xCo_2-xP/CNNs?0<x?1?by mechanical grinding method and study of hydrogen production performance.A series of Ni_xCo_2-xP nanoparticles were prepared by solvothermal method using nanoscale RP as a phosphorus source,adjusting the input ratios of different Ni and Co sources.Ni_xCo_2-xP/CNNs photocatalyst was obtained after mechanical grinding with CNNs?urea as precursor?.It is found that when Ni:Co was 1:3,the obtained Ni_0.5Co_1.5P has better hydrogen evolution activity.The highest hydrogen production rate of Ni_0.5Co_1.5P/CNNs is 5162?mol?g^-1?h^-1?AQE is 18.5%under 400 nm light?,which is 39 times that of CNNs(134?mol?g^-1?h^-1).It is higher than single metal phosphide?Ni₂P or Co₂P?and Pt-modified CNNs,which are 1.8 times,1.6 times and 1.5times,respectively.The main reason for the enhanced hydrogen production performance is the Schottky Junction formed by Ni_xCo_2-xP and CNNs,which can inhibit the recombination of photo-generated carriers,accelerate the charge transfer,reduce the hydrogen-evolution overpotential and promote hydrogen-production kinetics.?3?Preparation of Ni₂P/CNNs by in-situ solvothermal method and study of hydrogen production performance.With nanoscale RP as the phosphorus source,Ni₂P was closely grown on CNNs?melamine as precursor?in one step by solvothermal method.The experiment found that the hydrogen production rate of 17.5%Ni₂P/CNNs can reach 3344?mol?g^-1?h^-1,which is 68 times that of CNNs(49?mol?g^-1?h^-1),and higher than the Ni₂P/CNNs(2253?mol?g^-1?h^-1)obtained by mechanical grinding.After 15 h of cyclic hydrogen production experiments,the hydrogen production rate does not decrease significantly,showing excellent hydrogen production stability.In-situ introduction of Ni₂P not only has good interface contact,but also can accelerate the transfer of interface charge,thereby promoting the kinetics of hydrogen evolution reduction reaction.