| Graphitic carbon nitride(g-C3N4)as a polymeric photocatalyst has attracted numerous attention due to the advantages of high structure stability,easy availability and visible light response.Nevertheless,g-C3N4 still suffers from low charge separation efficiency and sluggish surface reaction kinetic,mainly caused by the poor charge mobility,strong Coulomb interactions between electron and hole,lacking of necessary active sites.To overcome these two drawbacks,we have developed a series of strategies,such as the construction of crystal carbon nitride phase junction,modifying carbon nitride with electron acceptor,constructing crystal carbon nitride nanoneedle array to improve the charge separation,loading Pt nanoparticles to boost charge separation and facilitate the surface reaction rate.Furthermore,the relationships between the charge transfer and the photocatalytic performance of carbon nitride were systematically investigated.The main content and results are shown as follows:(1)Cystalline phase junction between triazine and tri-s-triazine carbon nitride was constructed by using the ionothermal treatment strategy.The influence of junction stability and crystallinity of carbon nitride to the charge separation were systematically investigated.Compared with the traditional heterojunction,the construction of crystalline carbon nitride phase junction could improve the junction stability and enlarge the junction area,leading to the giant enhancement of charge separation efficiency.The photocurrent density of the phase junction was tested to be 1.5 ?A·cm-2,which was 2.0 times higher than that of g-C3N4.Results showed that the kinetic constant of the crystalline phase junction for p-chlorophenol degradation was 1.2 h-1,which was 7.5 and 1.6 times higher than those of tri-C3N4 and tri-s-tri-C3N4,respectively.Moreover,the crystalline phase junction photocatalyst also displayed high photocatalytic H2 evolution activity with a rate of 144?mol·h-1,which was 28.2 times higher than g-C3N4.Furthermore,no obvious catalytic activity decay was found after four consecutive H2 evolution experiments,demonstrating the high structure stability of the crystalline carbon nitride phase junction.(2)Mimicking the electron storage and release process of green plant's photosynthesis,cyano group and potassium atom were installed on g-C3N4 to endow the material with electron storage property,so as to conquer the strong Coulomb interactions between the electron and hole.The effects of electron storage and electron storage capacity of carbon nitride to the charge separation were investigated.The PL results revealed that the storage and release of photogenerated electron at the electron storage site could significantly enhance the charge separation efficiency.When the introduced amount of cyano group and potassium were 2.3 atom%and 4.0 atom%,the cyano group and potassium co-modified g-C3N4(CCN)exhibited the highest photocatalytic activity.Under visible light irradiation,CCN displayed efficient H2 evolution activity with AQE reached 55%,which was the highest value reported so far.Moreover,CCN exhibited efficient H2O2 production activities with a rate of 210?mol·h-1,which was 35.0 times higher than that of g-C3N4.(3)3D branched crystalline g-C3N4 nanoneedle photocatalyst(3DBC-C3N4-N)was synthesized by using the combined methods of ionothermal and quenching treatment.The effect of electric field distribution on the photocatalyst to the charge separation was investigated.It was found that the high curvature tip of the nanoneedle could intensify the local electric intensity by concentrating the photogenerated electrons around the tip area,therefore significantly enhancing electron-hole separation.As a result,the nanoneedle shaped catalyst displayed higher charge separation efficiency than that of nanorod shaped catalyst.The photocurrent density on 3DBC-C3N4-N was tested to be 0.16 ?A·cm-2,which was 2.3 times higher than that of 3DBC-C3N4-R.The AQE for H2 evolution at 420 nm on 3DBC-C3N4-N reached 47.5%,higher than most reported carbon nitride based materials.Moreover,3DBC-C3N4-N also displayed enhanced photocatalytic activity toward p-chlorophenol degradation,where the TOC removal reached 70%after 4 h photocatalytic reaction.(4)Modifying the photocatalyst with Pt nanoparticles was an effective way to enhance surface reaction rate by reducing the H2 evolution energy barrier.Thus,Pt nanoparticles were deposited on g-C3N4 by using an impregnation method.Results showed that coating the Pt nanoparticle with an alkali metal shell(MOx,M=Li,Na,K)could facilitate electron transfer from Pt to the absorbed proton,therefore improving the H2 evolution rate.As a result of the improved surface reaction kinetic,the Pt@MOx modified g-C3N4 showed excellent photocatalytic performance toward H2 evolution coupled with p-chlorophenol degradation,where more than 6.4 ?mol(30 ?mol·g-1·h-1)H2 was generated with the TOC removal reached 30%within four hours photocatalysis.(5)Single atom Pt modified carbon nitride photocatalyst(SA-Pt/g-C3N4)was prepared via the cationic ion exchange process,and the effect of Pt coordination environment to the photocatalytic activity was rationally investigated.Scanning transmission electron microscopy(STEM)and X-ray absorption spectroscopy(XAS)characterizations revealed that Pt was coordinated with four N atoms within the six-cavity of g-C3N4,and confined by the adjacent layers of carbon nitride.The photocurrent density on SA-Pt/g-C3N4 was tested to be 0.5 ?A·cm-2,which was 6.3 times higher than that of g-C3N4.DFT calculations revealed that the polarization effect from the interlayer interactions could reduce the H2 evolution barrier on Pt,improving the photocatalytic H2 evolution activity.With the Pt loading content of 8.7 wt%,SA-Pt/g-C3N4 showed high photocatalytic performance toward H2 evolution coupled with p-chlorophenol degradation.After 4 h photocatalytic reaction,the H2 generation rate reached 64 ?mol·g-1·h-1 with TOC removal of 70%.The results demonstrating here indicated that adjusting the Pt coordinating environment could successfully enhance the surface reaction rate,thus improving the photocatalytic activity of g-C3N4.To sum up,the manipulations of charge transfer process could successfully improve the charge transfer efficiency and surface reaction rate of g-C3N4,significantly improving the photocatalytic performance toward organic pollutant degradation and H2 evolution.The strategies we developed for the manipulations of charge separation and surface reaction rate may provide new insights for designing high-efficient photocatalyst. |
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