Modification,Performance And Mechanism Of Graphitic Carbon Nitride For Photocatalytic Hydrogen Production

Photocatalytic hydrogen production is a sustainable technology to convert solar energy into green hydrogen energy,which is of great significance to meet the Chinese large-scale hydrogen energy demand in the future to alleviate the energy shortage,demonstrating a potential application prospect.Graphite carbon nitride（g-C₃N₄）,as a fascinating metal-free conjugated polymer photocatalytic hydrogen production material,has been widely studied and developed due to its suitable energy band structure,good chemical/thermal stability,non-toxic,abundant sources,facile synthesis amd so on.However,the few surface active sites and the easy recombination of carriers are still key problems of g-C₃N₄to be solved.Therefore,it is a still current key research topic to improve the photocatalytic hydrogen production performance of g-C₃N₄by developing novel modification methods.Based on this,g-C₃N₄was modified through three strategies containing heterostructure construction,molecular edge-grafting and single atom modification in the paper.The morphology,microstructure,photoelectrochemical properties,physicochemical properties,hydrogen production performance and mechanism of the modified photocatalysts were systematically studied,as follows:（1）0D/2D SnO₂/g-C₃N₄heterostructure photocatalyst was constructed using urea and SnCl₄·5H₂O as raw materials by a one-step calcination process with SnO₂nanodots anchoring on the surface of g-C₃N₄nanosheets.The surface modification effect of SnO₂nanodots not only improves the light absorption capacity and reduces the band gap,but also effectively promotes the migration and separation of photoelectric charge between interfaces as an electron transport bridge.The photocatalytic H₂production rate of the best SnO₂/g-C₃N₄-1 is 1398.2μmol h^-1g^-1,which is about 6.06 times comparing with g-C₃N₄(230.8μmol h^-1g^-1).The AQE reaches up to 4.5%at 420 nm.At the same time,after 16 h continuous cycle hydrogen production experiment,the hydrogen production rate has barely reduced,indicating it has excellent stability and reusability.（2）Pyridine ring boundary modified g-C₃N₄photocatalyst（2AP-CN）was prepared by the pyridine ring grafted on the edge of g-C₃N₄nanosheets using urea and2-aminopyridine as raw materials through a one-step thermal polymerization process.The edge modification effect of the pyridine ring can induce electron directional transfer from the center to the edge of the nanosheet,which can effectively promote charge migration and separation.Meanwhile,the CB of g-C₃N₄was directly moved down to Fermi level and the band gap was reduced,which could effectively absorb visible light and promote the electron transition from VB to CB,thus improving the photocatalytic activity.The photocatalytic hydrogen production rate of the best2AP-CN-15 is 6317.5 umol g^-1h^-1,is about 3.9 times compared with g-C₃N₄(1632.5umol g^-1h^-1)and the AQE reaches up to 20.1%at 420 nm.（3）Ru single atom modified g-C₃N₄photocatalyst（RuCN）was prepared by Ru monatom uniformly anchored on the surface of g-C₃N₄nanosheets using urea and ruthenium acetylacetone as raw materials through a one-step calcination process.Ru single atom can not only provide abundant active sites and reduce overpotential for hydrogen evolution reaction,but also induce electron transfer from g-C₃N₄to Ru single atom,thus promoting efficient charge separation.Meanwhile,the modification effect of Ru single atom directly causes the Fermi energy level to move significantly upward and pass through CB,which makes the photogenerated electrons more easily excited from VB to CB,thus improving the photocatalytic activity.The photocatalytic hydrogen production rate of the optimal Ru CN-0.5%sample reached 629.9 umol g^-1h^-1,which was 6 times of than that of the optimal Pt/g-C₃N₄-3%sample(105.0 umol g^-1h^-1),and the AQE at 420 nm reached 9.7%.At the same time,after 100 h cumulative cycling experiments,the hydrogen production rate has barely declined,indicating its superior stability and reusability.