Study On The Modification Of G-C₃N₄ Semiconductor And The Performance In Treatment Of Organic Matter In Water

In recent years,g-C₃N₄ photocatalyst is widely used in photocatalytic degradation of pollutants,photolysis of water to produce hydrogen,and production of sensors due to its low cost and easy preparation,non-toxic and harmless,stable physicochemical properties,abundant raw materials and unique electronic structure.Furthermore,its forbidden band width is only about 2.7 eV,which can absorb visible light;the inherent graphite-like phase layer structure is conducive to the transmission of electrons.However,some natural defects still exist in g-C₃N₄ and limit its large-scale application.Such as low visible light utilization,high carrier recombination rate and low quantum yield.Therefore,this paper aims at improving the photocatalytic performance of g-C₃N₄ from structural heterojunction,introduction of precious metals,construction of Z-type mechanism,transition metal ion doping and so on,achieve efficient degradation of organic pollutants.The physicochemical properties and photocatalytic properties of the prepared samples were investigated and analyzed,and the possible mechanisms were proposed.This proves the rationality and feasibility of these schemes,and provides new ideas and methods for g-C₃N₄ application in water pollution control.The specific research contents are as follows:?1?Firstly,g-C₃N₄ was prepared by calcination from urea,and then it was combined with Bi₂WO₆ under hydrothermal conditions.Finally,Pt nanoparticles were deposited on the surface of g-C₃N₄/Bi₂WO₆ by photoreduction to form Pt-g-C₃N₄/Bi₂WO₆ Z-type heterojunction.Bi₂WO₆ was used to construct the heterojunction because its band gap edge position(E_CB=+0.51 eV,E_VB=3.21eV)matched very well with g-C₃N₄(E_CB=-1.06 eV,E_VB=+1.52 eV).For g-C₃N₄ and Bi₂WO₆,The noble metal Pt?0.66 eV?was introduced because it has a more suitable Fermi level relative to Ag,Cu,Au,and thus it is easier to construct a Pt-g-C₃N₄/Bi₂WO₆ Z-type heterojunction.It was shown by XPS and HRTEM that Pt was dispersed in the form of a simple substance on the surface of g-C₃N₄/Bi₂WO₆.Subsequently,Pt-g-C₃N₄/Bi₂WO₆ was applied to degrade rhodamine B under visible light,and it was found that it exhibited higher photocatalytic activity than the single component or two-component catalyst.because:First,metal platinum has a plasmon resonance effect that can significantly enhance the absorption of visible light;Second,metal platinum can serve as a bridge for electron transport and a recombination center for carriers;Third,the formed Z-type heterojunction can be guaranteed the system has strong redox ability and can also ensure the effective separation of photogenerated electron hole pairs.Based on a series of characterizations,capture agent experiments and ESR,we propose a possible photocatalytic mechanism.?2?The use of transition metal?TM?ions doped g-C₃N₄ to activate PMS for organic degradation is a highly efficient advanced oxidation technique.First,we prepared g-C₃N₄ according to the previous work,and then prepared a series of Mn ion doped g-C₃N₄ photocatalysts by a mild chemical water bath method.The main reasons for doping with manganese ions are:first,the d orbital of Mn can be hybridized with the p orbital of g-C₃N₄ while reducing the band gap of g-C₃N₄;secondly,in the previously studied TM-doped g-C₃N₄ system,Mn has the strongest binding energy;Third,in these TMs,the Mn doping system has the highest carrier density;Fourth,Mn has diamagnetic properties,its doping system is easily recycled in a strong magnetic source environment.The photocatalytic performance of the prepared samples was evaluated by the degradation efficiency of phenol?20 mg/L?in 30 min under full-light conditions.It was found that the photocatalytic activity was highest when the mass ratio of Mn ions was 10%.The effects of pH,PMS dosage and different ions on the activity of the photocatalytic system were investigated and various effects were explained in detail.The reason for the high activity of this series of catalysts is that there is a synergistic effect of photocatalysis and chemical reaction in the system,which can produce more SO₄^·-;electrons in the g-C₃N₄ conduction band react with PMS to produce SO₄^·-,at the same time,the separation efficiency of carriers is also improved;XPS results show that Mn²⁺and Mn³⁺exist in the M?10?CN photocatalyst,they can also directly activate PMS.Finally,combined with the relevant characterization results,energy band structure analysis and the capture agent experiment proposed a reasonable mechanism.