Construction Of G-C₃N₄-based Composite Photocatalysts Based On MOFs And Their Derivatives And Study Of Their Photocatalytic Performance For Hydrogen Production

Photocatalytic hydrogen production in water by solar energy is a promising way to alleviate the energy crisis and reduce the environmental pollution.The key challenge for photocatalytic hydrogen production is to design and develop photocatalysts with high efficiency.Graphite phase carbon nitride（g-C₃N₄）has gained extensive attention for its special lamellar structure,visible light response,simple preparation and excellent stability.However,the hydrogen evolution activity of g-C₃N₄is largely limited by some shortcomings,such as low surface areas,weak visible light absorption,high photogenerated electron-hole recombination rate,and so on.Therefore,it is an important way to construct g-C₃N₄-based photocatalysts through enhancing surface areas,increasing the visible light absorption and improving the separation and transfer efficiency of photogenerated charges.Based on this,in this paper,the ultrathin two-dimensional（2D）structure g-C₃N₄is formed by ultrasonic exfoliation.Meanwhile,the ultrathin 2D metal organic framework（MOFs）and their derivatives are used to form heterojunctions with 2D g-C₃N₄,which can accelerate the separation and transfer of photogenerated charges,and effectively enhance the absorption of visible light,thus realizing the construction of highly efficient g-C₃N₄based heterojunctions photocatalysis system.The specific contents are as follows:（1）2D/2D ZnCoMOF/g-C₃N₄p-n heterojunction was fabricated through electrostatic assembly.X-ray diffractometer（XRD）,transmission electron microscope（TEM）,atomic force microscope（AFM）,infrared spectroscopy（FTIR）and X-ray photoelectron spectroscopy（XPS）were used to characterize the phase structure,micro morphology and composition of the obtained samples.The results showed that the 2D/2D ZnCoMOF/g-C₃N₄heterojunction was successfully prepared.The effects of metal centers in MOFs and the addition amount of bimetallic ZnCoMOF on the photocatalytic hydrogen production rate of heterojunction were studied.The results show that the addition of bimetallic ZnCoMOF can improve the photocatalytic hydrogen production performance more effectively than that of single metal MOFs.When the addition of ZnCoMOF was 5 wt%,the photocatalytic hydrogen production rate of ZnCoMOF/g-C₃N₄heterojunction reached 1040.1μmol/g/h,which was 33.2 and 3.5times that of bulk g-C₃N₄and 2D g-C₃N₄,respectively.In addition,UV-vis absorption spectrum,photoluminescence spectrum（PL）,time-resolved fluorescence lifetime spectrum,photocurrent,electrochemical impedance spectroscopy and active species were used to reveal the enhancement and photocatalytic mechanism of 2D/2D ZnCoMOF/g-C₃N₄heterojunction.The results showed that the addition of 2D ZnCoMOF can significantly enhance the visible light absorption,inhibit the recombination of photogenerated electrons and holes,accelerate the separation and transfer of photogenerated charges and prolong the lifetime of carriers.The separation and transfer pathway of photogenerated charges between 2D ZnCoMOF and 2D g-C₃N₄was also clarified.（2）Fe₂O₃/g-C₃N₄/Zn Fe₂O₄composite materials were prepared by electrostatic assembly of 2D bimetallic Zn Fe MOF and 2D g-C₃N₄,and then calcined in air at high temperature.The phase structure,morphology and composition of the composite samples were analyzed by different characterization methods.The photocatalytic activity of different photocatalysts was evaluated by photocatalytic hydrogen production under visible light irradiation.By optimizing the molar ratio of Zn:Fe and the amount of 2D Zn Fe MOF,the optimal composite sample CNZFO-7（i.e.the molar ratio of Zn:Fe was 1:1 and the amount of 2D Zn Fe MOF was 7 wt%of g-C₃N₄）was obtained.The photocatalytic hydrogen production rate of CNZFO-7 was2976.9μmol/g/h,which was 10 times higher than that of 2D g-C₃N₄.Besides,CNZFO-7composite sample showed good photocatalytic cycle stability.UV-vis absorption spectrum,PL,time-resolved fluorescence lifetime spectrum,photocurrent and electrochemical impedance spectroscopy were used to test and analyze the samples.The results indicated that Fe₂O₃/g-C₃N₄/Zn Fe₂O₄composite improved the absorption of visible light,inhibited the recombination of photogenerated electrons and holes,and accelerated the separation and transfer of photogenerated charges.Through the analysis of the valence band,conduction band and energy band structure of Zn Fe₂O₄,Fe₂O₃and 2D g-C₃N₄,as well as the active species experiment and the analysis of the generation position of Pt and Pb O₂nanoparticles,it was concluded that the separation and transfer pathway of photogenerated charges in Fe₂O₃/g-C₃N₄/Zn Fe₂O₄heterojunction can be explained by the double Z-type heterojunction photocatalytic mechanism.（3）2D/2D NiCoMOF/g-C₃N₄composite materials were synthesized by the electrostatic assembly of 2D bimetallic NiCoMOF and 2D g-C₃N₄,and then phosphated at high temperature in inert atmosphere to obtain NiCoP/P-C₃N₄composite photocatalysts.The phase structure,micro morphology and composition of the composites were analyzed by a series of characterization methods,indicating that NiCoP/P-C₃N₄composite photocatalyst was successfully prepared.The photocatalytic hydrogen production performance of NiCoP/P-C₃N₄composite was optimized by adjusting the metal types and the amount of bimetallic NiCoMOF.The optimal sample PCN-NiCoP9（i.e.the amount of bimetallic NiCoMOF was 9wt%of 2D g-C₃N₄）was obtained,and its photocatalytic hydrogen production rate reached1703.2μmol/g/h,which was 2.8 times that of CNMOF9 before phosphating,2.2 and 2.3 times that of PCN-Ni₂P9 and PCN-Co P9 composites,and 2.5 times that of 2D g-C₃N₄with Pt as cocatalyst.Besides,PCN-NiCoP9 composite showed good cycle stability.The photocatalytic mechanism of NiCoP/P-C₃N₄composite was studied by the analysis of optical properties and photoelectrochemical tests.The results showed that the NiCoP/P-C₃N₄composite can effectively improve the absorption of visible light,accelerate the separation and transfer of photogenerated charges,and improve the photocatalytic performance of hydrogen production by the synergistic effect of using NiCoP as the cocatalyst and P doping.