| Graphitic carbon nitride（g-C3N4）,as a metal-free polymer semiconductor,has attracted widespread attentions owing to its low cost,favorable chemical stability and convenience in synthesis.The suitable band gap（2.7 e V）as well as valance band and conduction band potentials endow g-C3N4 with the ability to harvest visible-light and produce H2 and O2 by photocatalytic water splitting.However,the conventional bulk g-C3N4（BCN）synthesized by thermal polymerization of nitrogen-containing precursors still suffers from inherent deficiencies,such as low specific surface area,poor visible light utilization and severe electron-hole pair recombination.The above drawbacks have led to poor photocatalytic activity of g-C3N4,and seriously limits its application in the photocatalysis field.Therefore,several modification strategies such as structural nanosizing,element doping,heterostructure construction and crystallinity regulation have been developed to improve the photocatalytic performance of bulk g-C3N4.Nevertheless,a single type of modification strategy can only bring limited and partial improvement on visible-light utilization or photo-generated charge separation and transfer efficiency of g-C3N4,which leads to insufficient enhancement of its photocatalytic H2 evolution activity.For instance,structural nanosizing can expose more active sites and shorten the electron transfer distance,while the accompanying quantum confinement effect will narrow the visible-light absorption range.Element doping can optimize the energy band structure and enhance visible-light harvesting ability,but doping or defect sites will serve as the recombination center of photo-generated electrons and holes.Construction of heterojunction is able to promote carriers separation,while g-C3N4 in heterojunctions is mostly melon structure with low crystallinity,which hinders the transfer of photo-induced carriers.Besides,the enhancement of crystallinity can remarkably improve the separation and transfer efficiency of photo-induced electron-hole pairs,but it has little effect on enhancing the utilization efficiency of visible-light.To address these problems,this work proposed novel synthesis approaches and collaboratively employed multi-strategies to modify g-C3N4 by selectively combining multiple modification strategies,and thus achieved synergistic optimization of visible-light utilization as well as photo-generated carriers separation and migration efficiency.The main researches and results are as follows:（1）C,O-codoped nano-structured g-C3N4 samples were prepared by a one-step calcination method,to synchronously realize the nanosizing and element doping.Cooperative modification increases the number of active sites,promotes the separation and transmission of photo-generated carriers,enhances light absorption to a certain extent,and thus improves photocatalytic H2 evolution activity.In specific,C,O-codoped nano-structured g-C3N4 samples（NCN-x）were synthesized in one step by high temperature co-pyrolysis of melamine and ethanol.The synergistic modification of nanosizing and C,O co-doping was verified by microscopic morphology and chemical structure characterizations.The photocatalytic H2 production rate of NCN-30 was 396μmol·g-1·h-1 underλ>420 nm visible-light irradiation,which was 4.25 times that of BCN.The results of optical absorption and photoelectrochemical characterizations show that the synergism of“nanosizing+element doping”increased the number of active sites,enhanced the separation and transfer efficiency of photo-generated carriers,optimized the visible-light absorption,and thus improved the photocatalytic H2 production performance.（2）To further overcome the drawback of low specific surface area of g-C3N4,P-doped g-C3N4 nanosheets with larger specific surface area than that of NCN-30 in the previous chapter were synthesized by one-step calcination method using NH4Cl as gas template and（NH4）2HPO4 as doping agent.The larger specific surface area can provide more active sites,further promote the separation of photo-induced carriers,and finally enhance photocatalytic performance.Specifically,P-doped g-C3N4 nanosheets（PCNS-x）were prepared by calcining dicyandiamide,NH4Cl and（NH4）2HPO4 in one-step calcination.The specific surface area of as-prepared PCNS-3was 36.4 m2·g-1,which is 6.6 times that of BCN.Under visible-light irradiation atλ>420 nm,the H2 production rate of PCNS-3 was 1211μmol·g-1·h-1,which is 8.9 times higher than that of BCN and is superior to that of the single modified g-C3N4 nanosheets（CNS）and P-doped g-C3N4（PCN）.This further demonstrates the positive impacts of the synergistic effect of nanosizing and P-doping on the photocatalytic H2 evolution activity.The promotion mechanism of synergistic modification on the separation and transfer of photo-generated electron-hole pairs has been studied through photoelectrochemical characterizations.（3）The above modified g-C3N4 is still a melon structure,of which the in-plane hydrogen bond will hinder the electron transmission.Enhancing the crystallinity of g-C3N4 can significantly improve the carrier separation and transfer efficiency.Therefore,nano-structured crystalline g-C3N4,namely poly（heptzine imide）（PHI）,was synthesized by one-step calcining method to simultaneously achieve structural nanosizing and crystalline optimization.Collaborative optimization significantly promotes the migration of photo-generated electron-hole pairs,and thus remarkably enhances the H2 production performance.To be more specific,nano-structured PHI was prepared by calcining melamine with KOH and NH4Cl,and it achieved the controllable regulation of in-plane or interlayer crystallinity by adjusting the mass ratio of KOH and NH4Cl.Specifically,a high KOH/NH4Cl ratio results in PHI-IPO0.5 with a higher in-plane integrity while a low ratio leads to PHI2.5 with a more orderly interlayer stacking.Under visible-light irradiation atλ>420 nm,the photocatalytic H2 production rate of PHI was much higher than BCN,of which PHI-IPO0.5 and PHI2.5 were 1943 and 1601μmol·g-1·h-1,13.6 and 11.2 times higher than that of BCN,respectively.The enhanced photocatalytic activity was mainly attributed to the positive contribution of nanosizing and crystallinity modulation to the optical response ability and photo-generated carrier separation and transfer efficiency,which was confirmed by photoelectrochemical characterizations.In addition,PHI-IPO0.5 surpassed PHI2.5 in photocatalytic H2 production activity,illustrating that in-plane highly ordered structure is more conducive to visible-light absorption and carrier separation and migration than the interlayer stacking order structure.（4）Homojunction structure has been verified to be an efficient strategy to promote the separation and transfer of photo-induced electron-hole pairs.Therefore,on the basis of enhancing crystallinity,nanoscale crystalline g-C3N4 homojunction（poly（heptazine imide）/poly（triazine imide）,abbreviated as PHI/PTI）was synthesized by a molten salt-assisted method.It realized the synergistic modification of g-C3N4 by"structural nanosizing+crystalline regulation+homojunction structure",further improving the separation efficiency of photo-generated carriers,and significantly enhancing the photocatalytic H2 production performance.In concrete,nanoscale PHI/PTI homojunction（PHI/PTI-550）was synthesized by calcining melon-based g-C3N4（BCN）with Li Cl,and it was found for the first time that a single Li Cl could induce deep deamination of BCN to synthesize PHI.The synergistic modification of“nanosizing+crystallinity regulation+homojunction structure”was validated by morphological,crystal and chemical structure characterizations.The synchronous implementation of the three on the visible-light absorption and the efficiency of photo-generated carrier separation resulted in an improved photocatalytic H2production rate of 2017μmol·g-1·h-1 for PHI/PTI-550 under visible light irradiation atλ>420 nm,which was 18.3 times that of BCN.（5）Although PHI-based crystalline g-C3N4 has significantly improved photo-generated carriers separation and transfer properties owing to its high crystallinity,its visible-light utilization efficiency is still insufficient.Therefore,a novel type of amino-rich red PHI-based crystalline g-C3N4 nanoparticles were synthesized by one-step molten salt-assisted calcination method,which synchronously realized the structural nanosizing,crystallinity optimization and molecular doping.The synergistic modification simultaneously enhanced the visible-light absorption as well as the separation and transfer of photo-generated carriers,and thus greatly improved the photocatalytic H2 production activity of PHI under low energy visible-light irradiation.Concretely,red PHI nanoparticles（RPHINP-x）were obtained by heating melamine with KCl/KSCN molten salt.The smaller nanoparticle size（about 50nm in diameter）of RPHINP-x promoted photo-generated carrier separation by shortening the electrons and holes transfer distance.Besides,the amino-rich-induced n→π*electronic transition significantly broadened the visible-light response range to 800 nm,and thus the visible-light utilization was greatly enhanced.Meanwhile,the acceptor energy level can capture the excited electrons to promote the separation of electron-hole pairs.The synergistic effect of the nanoparticle structure and n→π*electronic transition leaded to a favorable H2 production rate of RPHINP-1(297μmol·g-1·h-1),which was 25times higher than that of PHI under visible-light irradiation atλ>510 nm. |
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