Construction Of G-C₃N₄-based Composite Photocatalysts And Regulation Of The Photogenerated Charges Separation

Photocatalytic technology has a wide application prospect in remediating environmental issues,and the construction of highly efficient photocatalysts is the most important content in photocatalysis.g-C₃N₄,as a type of polymeric semiconductor,has attracted wide attention within the photocatalytic field due t o its abundant resources,tunable energy band structure and liable surface modification,including splitting water,photoreduction of CO₂,photodegradation of pollution,organic synthesis and others.However,g-C₃N₄photocatalytic performance is limited since the high recombination rate of photogenerated electron-hole pairs.To improve the separation and transfer efficiency of photogenerated charges,in there,the g-C₃N₄-based composite materials with the enhanced photocatalytic performance were constructed by different modification methods.Then the influence of different regulations on the separation and transfer behavior of phtogenerated charges was studied and analyzed in detail.A nanosheets-assembled Bi₂O₂CO₃-Bi₂O₂（OH）NO₃/g-C₃N₄（BOC-BON/CN）ternary composite photocatalyst was successfully synthesized by ion exchange reaction under hydrothermal conditions.The analysis of formation mechanism stated that Bi（NO₃）₃·5H₂O was hydrolyszed to form Bi₂O₂（OH）NO₃,and part of Bi₂O₂（OH）NO₃ was converted into Bi₂O₂CO₃ through ion exchange reaction with CO₃^2-generated by the hydrolysis of g-C₃N₄.The constructed ternary composite BOC-BON/CN exhibited excellent photocatalytic degradation performance under visible light.The photodegradation rate of Rhodamine B（Rh B）was 3.55 times of the bulk g-C₃N₄.The analysis of band structure and separation and transfer behavior of photogenerated charge stated that the significant increase in photodegradation rate was attributable to the formed type II heterojunction structure be tween Bi₂O₂CO₃,Bi₂O₂（OH）NO₃ and g-C₃N₄,which could promote the separation of photogenerated charges.Besides,Bi₂O₂（OH）NO₃ played an essential role as electron accelerator in the charge transfer process.The internal electric field of Bi₂O₂（OH）NO₃ could accelerate the directed transfer of electron from g-C₃N₄ to Bi₂O₂CO₃,which further improved the separation and transfer efficiency of photogenerated charges.Using glucose as the precursor of carbon,WS₂@C was fabricated by depositing it on the surface of WS₂ with a narrow bandgap under hydrothermal conditions.A Z-scheme heterojunction WS₂@C/g-C₃N₄ composite photocatalyst with lamellar structure was further prepared through combining WS₂@C and g-C₃N₄ by a deposition method.The results of composition and micromorphology studies demonstrated that the introduced carbon was deposited on the surface of WS₂ with a carbon film form and further formed the ternary composite with g-C₃N₄.The analysis of separation and transfer behavior of photogenerated charge revealed that the carbon film played as an electron-mediator in the ternary composites and could improve the separation efficiency of photogenerated charge.Meanwhile,it could change the pathway of photogenerated electrons between WS ₂ and g-C₃N₄,thereby constructing a Z-scheme heterojunction for maintaining the oxidative ability of photogenerated hole.The analysis of light absorption ability found that the introduction of WS₂ effectively broadened the photoreponse range of g-C₃N₄.The ternary 2%-WS₂@C/g-C₃N₄ composite exhibited an excellent photodegradation rate towards Rh B and 2,4-dichlorophenol（2,4-DCP）under visible light.The photodegradation rate of 2,4-DCP by 2%-WS₂@C/g-C₃N₄(0.0104 min^-1)was 3.15and 3.06 times of the pure g-C₃N₄ and binary WS₂/g-C₃N₄ composite,respectively.Besides,the photocatalytic degradation mechanisms and degradation pathway of2,4-DCP were investigated and discussed in detail.The generated superoxide radicals（·O₂^-）,hydroxyl radicals（·OH）and holes（h⁺）by ternary composites could promote the dechlorination reaction of 2,4-DCP effectively and decompose it into smaller organic molecules.Using melamine-cyanuric acid（MCA）supramolecular,urea（U）and oxalic acid as the precursors,a novel O-doped g-C₃N₄ nanosheets（coiled/stacked）homojunction（O-g-C₃N₄-MCAU）photocatalyst was constructed.Under visible light,O-g-C₃N₄-MCAU showed significanatly enhanced photocatalytic efficiency towards 2,4-DCP.And the photodegradation rate of O-g-C₃N₄-MCAU was 0.0230min^-1,which was 15.33,5.48,3.48 and 1.92 times higher than those of g-C₃N₄-M,g-C₃N₄-MCA,g-C₃N₄-U and g-C₃N₄-MCAU,respectively.The optical analysis,photo-electrochemical studies and density functional theory（DFT）calculations were carried out and revealed that the product can promote the photogenerated charge separation and transfer efficiency and adjust energy band structure in different ways.The homojunction enhanced charge separation efficiency on the surface.O doping changed the charge arrangement in the homojuncti on and formed an internal electric field,which enhanced the charge separation efficiency in the bulk phase.Meanwhile,O doping also regulated the band structure by substituting the N atoms,thereby expanding the photo-response range.Furthermore,the nanosheets structure shortened the transfer distance of photogenerated charges from bulk phase to surface.Combined the multi-regulation behavior with active species analysis,the mechanism of the enhanced photocatalytic performance was further proposed.