Photocatalytic Performance Of Graphitic Carbon Nitride And Its Optimization

Graphitic carbon nitride(g-CN)has attracted worldwide attention as a multipurpose phtotocatalyst due to its low cost,facile synthesis,appealing electronic band structures and extraordinary thermal/chemical stability.However,the practical application of g-CN is still hindered by its high recombination rate of the photogenerated charge carries as well as the poor surface accessibility.In this thesis,we first prepared g-CN powders via thermal condensation under different atmospheres,revealing the relationship between their photocatalytic activities and the related properties such as structure,visible-light absorption,specific surface area,electron transfer capability and so on;then,a novel approach based on plasma surface modification was developed to further optimize the photocatalytic activity of g-CN.Melamine was used as starting material to prepare g-CN by a direct thermal condensation process under different atmospheres,including air,nitrogen,argon,and vacuum.The obtained samples were systematically characterized to investigate their structures,bonding states,light absorption performances,electron transfer capabilities;by which the difference on their photocatalytic performances were carefully discussed.The sample synthesized under air atmosphere not only had a higher specific surface area of20.15 cm~3/g but also contained oxygen functional groups;as a consequence,it showed the superior photocatalytic activity.While the samples synthesized under both nitrogen and argon atmospheres exhibited worse photocatalytic activities due to their lower specific surface areas and the higher recombination rate of the photogenerated charge carriers.The sample synthesized in vacuum possessed outstanding light absorption performance and low recombination rate of the photogenerated charge carries,but had a relatively lower specific surface area of 14.30 cm~3/g;therefore,it showed lower photocatalytic activity than that synthesized under air atmosphere.Plasma surface modification was employed to tune the photocatalytic activity of g-CN.The bulk properties of g-CN,including the structure,bonding feature,light absorption and electronic band structures,had no changes after oxygen or nitrogen plasma treatments,while the specific surface area exhibited a minor adjustment;however,its surface composition and surface bonding state were remarkably modified,and the surface micromorphology also showed some differences.Oxygen-plasma treatment showed stronger etching effect and pronounced surface codification ability.The specific surface area of the treated sample increased to 17.6 cm~3/g,and the introduced oxygen functional groups played a key role in reducing the recombination rate of the photoexcited charge carries.As a consequence,the oxygen-plasma-treated sample showed a much superior photocatalytic activity,which is about 4.2 times higher than that of the pristine g-CN for the degradation of rhodamine B under visible light irradiation.The photocatalytic activity of nitrogen-plasma-treated sample exhibited a slight decrease due to its relatively lower specific surface area.Furthermore,both of the plasma-treated samples showed an impressive stability on their photocatalytic performances thanks to the unique high thermal/chemical stability of g-CN.