Preparation And Photocatalytic Performance Of Carbon Quantum Dots/TiO₂ Modified Porous G-C₃N₄

g-C₃N₄ polymer semiconductor plays an important role in the field of photocatalysis because of its wide source of raw materials,facile preparation method,acid and alkali corrosion resistance,high temperature resistance and suitable band gap.However,g-C₃N₄ also has some disadvantages,such as low specific surface area,limited absorption range of visible light and easily recombination of photogenerated carriers.This paper constructs a porous structured g-C₃N₄ by a template method.The porous structure contributes increased specific surface area of the g-C₃N₄,resuling in the increase of active sites for the photocatalytic reaction.Carbon quantum dots（CQDs）have excellent visible light absorption,up-conversion photoluminescence and carrier transport efficiency.The modification of porous g-C₃N₄ with CQDs may enhance its light absorption capacity and carrier transport efficiency.TiO₂nanoparticles are also considered to incorporate to form a heterostructure to delay carrier recombination.The characterization of the phase,structure,morphology,pore structure and photocatalytic degradation properties of porous g-C₃N₄ and TiO₂/CQDs co-decorated porous g-C₃N₄ hybrids were investigated.The relationship between properties and structure was discussed.The main research contents and results are as follows:（1）The SiO₂ nanospheres with controlled particle size and uniform size distribution have been prepared by a hydrolysis method.Porous g-C₃N₄ with uniform pore size was synthesized by using SiO₂ nanospheres as the hard template and monocyanamide as the precursor,through thermal polymerization and chemical etching process to obtain the porous g-C₃N₄ with tunable pore sizes and high specific surface area.The results showed that the photocatalytic degradation ability of porous g-C₃N₄ to was significantly improved,compared with bulk g-C₃N₄.Compared with the bulk g-C₃N₄,the degradation rate of the PCN-50-580 sample,which has a specific surface area of 92 m²/g,to the methylene blue solution was up to 71.27%,while the degradation rate of PCN-50-580 sample to gaseous benzene was up to 66.74%.It is proposed that the improvement of photocatalytic performance resulted from the high specific surface area,which could adsorb more pollutants and provide more reactive sites for the photocatalytic reactions.（2）On the basis of porous g-C₃N₄,the CQDs and TiO₂ nanoparticles were introduced by the impregnation method and followed by the hydrothermal method to form the hybrids.The results showed that the hybrids maintained the porous structure of porous g-C₃N₄ with high specific surface area up to 131 m²/g.The light absorption capacity and photo-generated carrier generation efficiency of the hybrids were significantly improved.The photocatalytic degradation efficiency of TCPCN-2 to gaseous benzene reached 96.08%within 120 min,which was 1.4 times that of porous g-C₃N₄ and 1.5 times of pure TiO₂.The first-order chemical reaction kinetics simulation of the photocatalytic degradation of gaseous benzene was carried out.The photocatalysis process of hybrids was consistent with the basic law of first-order chemical reaction kinetics.The rate constant of degradation of gaseous benzene of TCPCN-2 reached 0.2726 min^-1,which was 2.9 times that of porous g-C₃N₄ and 3.7times of pure TiO₂.（3）In this work,g-C₃N₄,CQDs and TiO₂ have formed heterostructures.When excited by light,g-C₃N₄ and TiO₂ generate both photogenerated electrons and holes at their own conduction band and valence band positions,respectively.At this time,the electrons on the g-C₃N₄ conduction band will migrate to the conduction band position of TiO₂ through the transmission of CQDs and then combine withO₂ to generate·O₂^-.The h⁺on the valence band of TiO₂ migrates to the valence band of g-C₃N₄through the transport of CQDs.Both·O₂^-and h⁺participate in the oxidative degradation of benzene in the photocatalytic reaction.In this process,CQDs exhibit the excellent visible light absorption capacity to broaden the absorption of visible light range of the hybrids.Their charge transport characteristics can also promote the separation of carriers in hybrids.CQDs also have up-conversion photoluminescence,which can convert long wavelength light into short wavelength light,leading to the increase of the light absorption capacity of the hybrids.Hence the heterogeneous structure fabricated in the hybrids can prolong the transmission path of photogenerated carriers,increase the diffusion and migration probability of carriers,delay the recombination of carries,and then enhance the photocatalytic degradation ability of hybrids.