The Photocatalytic Water-splitting Performance Of G-C₃N₄ Based Photocatalyst Enhanced By Transition Metal Phosphide

It is an ideal way to overcome the problem of fossil fuel crisis and environmental pollution by converting solar energy into clean and environmental-friendly hydrogen fuel through photocatalytic decomposition of water by semiconductors.Graphite phase carbon nitride（g-C₃N₄）,as a nonmetallic semiconductor,has attracted extensive attention because of its good responsive to visible light,wide species of precursors,simple preparation,unique photochemical properties and suitable band structure.However,due to suffering from narrow small specific surface area,light absorbtion range and serious photocarrier recombination,the g-C₃N₄ shows limited conversion efficiency of solar energy to hydrogen energy.Therefore,the photocatalytic performance of g-C₃N₄ would be improved by morphology regulation,heterojunction construction and cocatalyst modification technology in this study.The porous g-C₃N₄（PCN）was successfully synthesized by hard template method,and the the ratio of hard template to precursor was optimized.Next,the Type II PCN-Cd S heterojunction was constructed by the solvothermal method.By adjusting the temperature and precursor feed amount during solvothermal reaction,the influence of the loading amount of Cd S combined on PCN on the photocatalytic performance was studied.In order to accelerate the photoelectron separation and transfer efficiency of PCN-Cd S,Ni₂P was loaded on the surface of PCN-Cd S,and the effect of the loading amount of Ni₂P cocatalyst on the photocatalytic performance was investigated,and its photocatalytic mechanism was explored.The results show that when triethanolamine（TEOA）was acted as the sacrificial agent,the hydrogen evolution rate of the PCN-Cd S-Ni₂P composite is up to 2905.86μmol g^-1 h^-1,which is as high as about 14,18 and 279 times of PCN-Cd S,PCN-Ni₂P and PCN,respectively.The enhanced photocatalytic activity is mainly ascribed to the light scattering and slow photon effect of PCN,which increases the photon utilization and generates substantial photogenerated electrons.The Type II PCN-Cd S heterojunction not only broadens the light absorbtion range,but also achieves the directional separation of photogenerated electron-hole.Besides,as an electron trap,Ni₂P can rapidly capture photogenerated electrons and provide active sites for H⁺/H₂ reduction reaction.They form a“production-transport-utilization”mode of photogenerated electrons,realizing the efficient utilization of electrons.In addition,PCN-Cd S-Ni₂P photocatalyst exhibits good stability and reusability,there is no obvious decline in activity and no change in morphology and composition during 12 h cycle test.In order to further improve the photocatalytic hydrogen evolution activity of PCN,the bimetallic phosphide（CoNiP）was used to modify the PCN-Cd S heterojunction.By adjusting the loading content of CoNiP and the ratio of Co,Ni and P,the structure of photocatalyst was optimized.The effect of CoNiP on the photocatalytic performance of PCN-Cd S and the mechanism were investigated.Under the visible light（λ≥400 nm）,CoNiP can more significantly improve the photocatalytic water-splitting activity and durability of PCN-Cd S heterojunction,compared with Co₂P and Ni₂P.When TEOA was acted as the sacrificial agent,the photocatalytic water splitting rate of PCN-Cd S-CoNiP reached up to 3462.71μmol g^-1h^-1.Moreover,PCN-Cd S-CoNiP can also have photocatalytic overall water splitting capacity,the photocatalytic rate is about 122.08μmol g^-1h^-1.By TPC,EIS,PL and TRPL characterization,it is proved that the doping of the second metal in the metal phosphide can reduce the impedance of charge transfer,accelerate the transfer of electron,and prolong the life of photogenerated electrons.And the light absorbtion range of PCN-Cd S was widened to 680 nm,the light scattering and the photon utilization rate were increased by CoNiP modification.TEM shows that the doping of the second metal can reduce the nanometer size of metal phosphide and expose a great deal of active sites.Mott-Schottky test results depict that the modification of CoNiP on the surface of PCN-Cd S leads to more negative Fermi energy levels,and so increases the driving force of photocatalytic hydrogen production.What’s more,the PCN-Cd S-CoNiP composite showed better stability than PCN-Cd S-Co₂P and PCN-Cd S-Ni₂P during the 12 h cycle,which is due to the C atoms in PCN were replaced by P atoms to form a positive charge center（P⁺）,and therefore it can be closely combined with CoNiP in the form of P⁺-P^δ--Co^δ+/Ni^δ+chemical bridge,thus forming stable structure.