Preparation Of Modified Graphite Carbon Nitride And Its Photocatalytic Performance

The emerging energy and environmental problems have become a complex subject on a global scale.Photocatalytic technology,an efficient technical using solar energy,is a powerful solution to the above-mentioned problems.So far,many semiconductors have been synthesized to improve the photocatalytic performance.However,most of the current photocatalytic systems are composed of metal atoms.While improving the photocatalytic performance,it also brings huge cost and highly toxic pollution during treatment.Graphitic carbon nitride,g-C₃N₄,is a kind of metal-free conjugated semiconductors with a similar layer structure like graphite.It is non-toxic and owns good physical and chemical stability,and band gap（EG）of about 2.7 eV and it can absorb visible light.Nevertheless,g-C₃N₄ still inevitably has many shortcomings.In this paper,in view of the high photo-generated carrier recombination rate of g-C₃N₄,we modified g-C₃N₄ from morphology control and crystal structure adjustment,and its photocatalytic performance was significantly improved.The specific content is as follows:（1）By preparing sponge-like 3D hierarchical superstructure g-C₃N₄（SCN）,we introduced a porous structure and C defects into the internal structure to form a N-rich structure,which can affect the excitons state inside g-C₃N₄.The original strong exciton state was converted into a weak exciton state,reducing the number of excitons,and easing the coupling and annihilation between excitons,which can increase the number of carriers.We can generate more electrons and holes of g-C₃N₄,which can migrate to the surface of the g-C₃N₄,participating in the photocatalytic reaction,to improve the rate of the reaction.In addition,the constructed 3D hierarchical superstructure can accelerate the mass transfer in the photocatalytic reaction and own fast migration rate of carriers and higher availability of visible light because of the exists of the internal secondary structure.All these advantages can promote SCN to acquire an excellent photocatalytic H2 production rate of 21.5 mmolh-1g-1 with the assistance of Pt cocatalyst and TEOA sacrificial agent,the apparent quantum yield（AQY）of SCN reached up to 51.3% at λ=380 nm using the TEOA sacrificial agent and Pt cocatalyst.（2）By hydrothermal treatment of bulk g-C₃N₄,part of the triazine ring structure in the internal structure of g-C₃N₄ was rearranged,and part of the structure became long-range order under high temperature and high pressure,resulting in an enhanced crystallinity of g-C₃N₄（CCN）.At the same time,the hydroxyl groups were modified on the surface of CCN.Studies find that the plane of the triazine ring inside the CCN was more complete,the number of broken triazine ring was less,and there were more delocalized electrons between the layers.In addition,because the structure of the CCN was more complete,the number of internal electron-hole recombination centers can be reduced,so that the number of photogenerated electrons and holes were greatly increased.The layer structure makes that electrons and holes can be transferred to the surface of CCN easily,to enhance the photocatalytic performance.What’s more,the CCN can efficiently fix nitrogen in air to generate nitrate,and the rate can reach 34.56 mgh-1g-1,which is much higher than the rate of nitrate generated by g-C₃N₄（PCN）before modification.