Modification Of Graphite-like Carbon Nitride And The Study On Photocatalytic Performance

Under the pressures of energy crisis and environmental problems,hydrogen energy has become the most promising clean energy source to replace fossil energy because of its pollution-free and high calorific value.Photocatalytic decomposition of water for hydrogen production can make use of abundant solar energy and water to produce hydrogen,effectively avoiding the shortcomings of traditional technology such as high energy consumption and pollution,so it is considered as the most ideal way to develop hydrogen energy.Graphite carbon nitride?g-C3N4?is a kind of polymer semiconductor material with visible-light photocatalytic activity.It has attracted wide attention because of its good stability,low cost and energy band structure suitable for photocatalytic hydrogen evolution.This paper focuses on the problems of low separation efficiency of photogenerated carriers and high activation energy of surface hydrogen generation reaction of g-C₃N₄,and studies as the following:1)In order to improve the separation efficiency of photogenerated carriers in g-C3N4materials,a series of P-doped g-C3N4 materials were prepared.In this part,P-doped g-C3N4 material was prepared by an one-step calcination strategy using urea as raw material and different types of phosphate as dopants.The results show that P element doped into the structure of g-C3N4 homogeneously,the main form of which is high valence P element,and the doping results in a slight increase in the interlayer spacing of g-C3N4.The experimental results of photocatalytic hydrogen evolution show that the photocatalytic activity of g-C3N4 can be significantly improved by appropriate P doping.Steady-state fluorescence measurements show that P-doping effectively improves the separation efficiency of photogenerated carriers.2)The surface chemical bond modification can effectively change the electronic structure of the photocatalytic material.Therefore,we have designed a calcination method which can modify the surface chemical bond of g-C3N4,and successfully prepared g-C3N4with P-N bond on the surface.Compared with the original g-C3N4,its photocatalytic activity for hydrogen evolution increased by four times under visible light irradiation.The existence of surface chemical bonds and the mechanism of their influence on the separation efficiency of photogenerated carriers were determined by means of text characterization and quantum chemical calculation.3)Aiming at the problem of large activation energy of hydrogen evolution on the surface of g-C3N4 material,CoP-modified g-C3N4 photocatalyst was constructed to form a Mote-Schottky junction based on the special metal-like properties of CoP;and the 2D/2D CoP/g-C3N4 composite photocatalyst was prepared by in-situ method to expand the interface,thereby expanding the Schottky effect.Compared with nanoparticle-modified materials,the 2D/2D structure not only enlarges the Schottky junction,but also provides more active sites and shorter electron/carrier transport distances.In addition,in-situ growth control at 2D/2D interfaces will result in a more homogeneous mixture,with stronger interactions between the two components,tighter interfaces,and reduced self-aggregation,which is more conducive to interfacial charge transfer.The results show that the two-dimensional composite has higher photocatalytic activity,and its hydrogen evolution rate is obviously higher than that of g-C₃N₄.