Modification Of Graphitic Carbon Nitride For Enhanced Photocatalytic Activity In Hydrogen Evolution

With the rapid development of photocatalytic technology,metal-free two-dimensional polymer semiconductor graphitic carbon nitride（g-C₃N₄）,benefiting from its suitable band structure can absorb visible light and excellent stability compared with those of traditional semiconductor photocatalysts,as well as natural abundance and simple preparation,has been widely used for photocatalytic water splitting,pollutants degradation,selective organic synthesis and biomedical fields as a new type of photocatalyst with excellent performance.However,the photocatalytic activity of the original g-C₃N₄is unsatisfactory owing to its shortcomings such as small specific surface area,wide bandgap and serious photogenerated electron-hole recombination.Modification strategies for its inherent shortcomings have been continuously researched to improve its photocatalytic activity.In this paper,the bandgap of g-C₃N₄was modified by metal ion doping primarily.Then the amorphous carbon nitride nanosheet（a-CNN）were prepared by metal vapor thermal etching method to narrowing the bandgap while increase the specific surface area.Lastly,loading bismuth（Bi）nanospheres on a-CNN to accelerate the separation of photo-generated carriers,thereby enhance the photocatalytic activity of g-C₃N₄.（1）Fe-doped g-C₃N₄（x%Fe-CN）was synthesized by thermal polycondensation with melamine and iron nitrate nonahydrate as precursors.The morphology,structure,optical and photocatalytic properties of the materials were proposed from SEM,XRD,UV-vis and PL,and the structure model of a perfect Fe ion intercalated g-C₃N₄was proposed.The effects of Fe doping on the photocatalytic activity were evaluated by water splitting for hydrogen evolution under visible light irradiation with Pt cocatalyst.The results show that Fe doping narrow the band gap and broaden the light absorption range compared with pure g-C₃N₄.Especially,the sample with Fe content of 0.5 wt%exhibit highest activity in photocatalytic hydrogen generation rate of 338.68μmol·g^-1·h^-1,indicating that Fe doping could promote the hydrogen generation rate of g-C₃N₄,while excessive Fe might break the sheet structure and inhibit activity.（2）A method of metal vapor thermal etching has been designed to prepare amorphous carbon nitride nanosheets（a-CNN）.The morphology,specific surface area,structure,optical and photocatalytic properties of the materials were proposed from SEM,N₂adsorption and desorption tests,XRD,UV-vis and PL,and the mechanism of metal vapor thermal etching was elucidated.The formation mechanism of a-CNN was elucidated.The photocatalytic activity of a-CNN were evaluated by water splitting for hydrogen evolution under visible light irradiation with Pt cocatalyst.The results show that,the thickness of a-CNN after exfoliation is about 4-5 nm,and the bandgap of a-CNN is obviously decreased compared with bulk g-C₃N₄（b-CN）.a-CNN presents remarkable enhanced photocatalytic hydrogen production rate of 3077.10μmol·g^-1·h^-1,which is ascribed to the increased specific surface area and the broadened visible light absorption range,bringing more reactive sites and higher visible light utilization.Simultaneously,the surface defects of C₃N₄were increased by amorphous process,which facilitate the adsorption and dissociation of H₂O and thereby conducive to the photocatalytic hydrogen evolution.（3）Bi nanospheres were loaded on a-CNN to prepare nanocomposites（x%Bi/CNN）by hydrothermal method.The morphology,structure,optical and photocatalytic properties of the materials were proposed from SEM,XRD,UV-vis and PL.The effects of Bi loading on the photocatalytic activity were evaluated by water splitting for hydrogen evolution under visible light irradiation without Pt cocatalyst.The results show that the Bi nanospheres loading on the surface of a-CNN with the existence form of a simple substance,and forms a Schottky barrier with a-CNN.Bi metal can trap photoelectron and efficiently enhance the separation of photoinduced charge.Moreover,Bi/CNN composite reveal enhanced light absorption in the visible light region owing to the SPR effect of the semimetal Bi.Bi-loaded a-CNN present further improved the photocatalytic hydrogen generation activity especially in the sample with Bi content of 1.0 mol%,which is 7.77μmol·g^-1·h^-1with no cocatalyst case.Excessive Bi greatly reduces the utilization of visible light by a-CNN owing to the absorption of most of the incident light,resulting in a decrease of photocatalytic activity.