Preparation And Study Of Carbon Nitride-Based Photocatalysts For Water Splitting By Visible Light

With the rapid increase of global population,energy crisis and environmental issues become more and more increasingly serious.Thus,it is very essential to explore and develop clean,economic and sustainable energy.At present,solar-driven water splitting into hydrogen（H2）is a promising method in various H2 production technologies on account of its clean and abundant.It is of great importance for this technology to develop photocatalysts.Among the reported semiconductors,carbon nitride（g-C3N4）has attracted much interest in photocatalysis because of its low-cost,easily tailoring energy bandgap and chemical stability.However,bulk g-C3N4（BCN）as an organic polymeric semiconductor represents fast recombination of photoinduced charge carriers resulting in poor photocatalytic H2 production performance to a large extent.Therefore,it is essential for g-C3N4 to explore excellent strategies for boosting its photoactivity.In this paper,three methods for improving photoactivity of g-C3N4based on its drawbacks about poor separation efficiency of photoinduced charge carrier are proposed,including to the design of ultrathin two-dimensional（2D）g-C3N4nanosheets,the improvement of hydrophilicity about g-C3N4 and the construction of NiCo-LDH/g-C3N4 composites.The research contents of this paper include the following three parts:（1）Rapid recombination of photoinduced charge carriers for g-C3N4 results in poor photocatalytic activity.Herein,we report a“bottom-up”approach to prepare relatively high-yield g-C3N4 nanosheets（CNSx:1）by hydrothermal method of ammonium bicarbonate（NH4HCO3）and dicyandiamide.The synthesis of CNS4:1 with thickness of4 nm and specific surface area of 70.03 m2/g was successfully confirmed by SEM,TEM,AFM and BET,etc.The separation ability of photocarriers was systematically analyzed by the steady-state PL,EIS and transient photocurrent reponse measurements.The optimal sample showed an outstanding photocatalytic H2 production performance(879.79μmol·g^?1·h^?1)under the visible light exposure（λ>420 nm）,which was about2.8 times higher with respect to BCN.Finally,optimized sample after 16 h cyclic stability test showed stable and SEM,TEM and XRD before and after the cyclic reaction also illustrated its stability.The more efficient performance of as-prepared 2D CNSx:1 could be attributed to rapid separation of photogenerated exciton and large specific surface area.（2）The polymer carbon nitride（PCN）obtianed by directly traditional thermal-polymerization always shows hydrophobic and insoluble character,dramatically limiting its photocatalytic activity.To this end,we report a facile in-situ surface-amination strategy for synthesis of high-hydrophilic PCN by copolymerizing urea with indole.The introduction of surface amino-groups about PCN was successfully confirmed by XPS,FT-IR and static droplet contact angle test.The UV-vis DRS spectra exhibited that capability of optical absorption for g-C3N4 was extended by this method.The EIS and transient photocurrent reponse measurements showed that surface-ammoniated PCN represented more rapid separation efficiency of photoinduced charge carriers.The results showed that optimal surface-ammoniated sample exhibited a remarkable enhancement for the H₂ production performance(2792.55μmol·g^?1·h^?1)under the visible light illumination,which outperformed most reported hydrophilic PCN-based materials thus far.Finally,optimized sample after 16 h cyclic stability test showed stable and XRD before and after the cyclic reaction also illustrated its stability.The more efficient photocatalytic performance of as-prepared hydrophilic g-C3N4 can be contributed to:（i）the improved separation efficiency of photoinduced charge carriers;（ii）the expansion of visible light absorption;（iii）amino-group（-NH2）enriching proton（H⁺）from water to promote the rapid production of H2.（iiii）the high hydrophilicity resulting in the acceleration of the interface reaction rate.（3）The g-C3N4 shows poor the separation efficiency of photoinduced charge carriers.Herein,NiCo layered double hydroxide（NiCo-LDH）coupled with g-C3N4 was constructed by simple mechanical composite method.The fabrication of NiCo-LDH/g-C3N4 composites was successfully confirmed by the XRD,FT-IR,XPS,SEM and TEM.The transient photocurrent response and EIS showed that the separation ability of photoinduced excitons was improved.The effect of different loading amount of NiCo-LDH on the H₂ production performance under visible light irradiation（λ>400 nm）was systematically studied.Results reflected that the photoactivity of 5 wt%NiCo-LDH was the best,which showed 6.4-fold in comparison with that of pristine g-C3N4.Finally,optimized sample after 12 h cyclic stability test showed stable.The reason for the improvement of the photocatalytic performance of the NiCo-LDH/g-C3N4 composite system can be attributed to the higher efficiency of photogenerated exciton separation caused by the loading of NiCo-LDH cocatalyst.