Study On Photocatalytic Performance Of Wood Based Carbon Quantum Dots/Lava Sigold Organic Frame Composites

With the development of industrial society,the problem of water pollution has attracted more and more attention.Under the action of photocatalyst,the use of solar energy photocatalysis to treat pollutants such as organic dyes and heavy metal ions is an important research direction in sewage treatment.In the research of removing pollutants in water,the research of photocatalyst is particularly important.Metal organic frame(MOF)is a new type of three-dimensional hybrid porous material composed of metal-oxygen clusters and organic connectors.Among them,the MIL series metal organic frame is made of iron or titanium and terephthalic acid,which is low-cost and can exhibit good stability in acid and alkali environments.Due to its large specific surface area,diverse pore structure and specific chemical sites point,MIL series metal organic frame has received widespread attention.However,the light response capability of metal-organic frames is limited,and there is room for further optimization of the energy band structure,which limits their application and promotion.In order to improve the photodegradation efficiency of MOF materials under visible light,this paper prepared wood-based carbon quantum dots(CQDs)from poplar wood powder and added them to the MOF precursor solution to prepare CQDs/MIL-125(Ti)/g-C3N4 and CQDs@MIL-88B(Fe).Then the carbon quantum dots and composite photocatalyst were characterized and tested.The performance of the photocatalyst was tested by using methylene blue(MB),rhodamine B(RhB)and hexavalent chromium ion(Cr6+)as the target pollutants.The main research results are summarized as follows:1.Wood-based carbon quantum dots(CQDs)were prepared from poplar wood powder as carbon source,ethylenediamine and citric acid as nitrogen source at 220℃ for 8 h.The average particle size of the prepared carbon quantum dots is about 7.75 nm measured by scanning electron microscope(TEM),and the lattice spacing of 0.31nm is caused by the {002} crystal plane of the graphite structure.Using Fourier transform infrared spectroscopy(FT-IR)and X-ray photoelectron spectroscopy(XRD)to scan the wood-based carbon quantum dots,it is found that the surface of the carbon quantum dots is rich in oxygen-containing groups,which is beneficial for its combination with other materials.Through the characterization of the fluorescence spectrum and CIE chromaticity diagram,it is found that the wood-based carbon quantum dots CQDs exhibit obvious wavelength dependence,that is,the emission wavelength redshifts with the red shift of the excitation wavelength,and the carbon quantum dots appear bright blue under the excitation light.2.Using melamine as the precursor,the graphite phase carbon nitride(g-C3N4)was prepared by thermal polymerization at 550℃.Then,CQDs and g-C3N4 were added to the precursor solution(butyl titanate and terephthalic acid)of the metal organic framework MIL-125(Ti),and the reaction was took place at 150℃ for 24 hours to prepare CQDs/MIL-125(Ti)/g-C3N4.According to X-ray powder diffraction and electron microscopy photos,the addition of CQDs and g-C3N4 did not affect the crystal structure of MIL-125(Ti),but they changed the pore structure of MIL-125(Ti).The particle size of CQDs/MIL-125(Ti)/g-C3N4 ternary composite material is about 500nm,the specific surface area is about 148.16 m2/g,the pore volume is about 0.21 cm3/g,and the average pore diameter is about 5-67 nm.The band structure of MIL-125(Ti)and g-C3N4 enables them to form a heterojunction with each other.The doping of carbon quantum dots CQDs accelerates the transfer of electrons in the heterostructure.Compared to pure MIL-125(Ti)and binary composites MIL-125(Ti)/g-C3N4,CQDs/MIL-125(Ti)/g-C3N4 have stronger light absorption ability and lower electron-hole recombination rate,hence the photocatalytic efficiency improved.3.CQDs were added to the precursor solution(terephthalic acid and anhydrous ferric chloride)of the metal organic framework MIL-88B(Fe),and CQDs@MIL-88B(Fe)was prepared by reacting at 100℃ for 12 hours.Through a series of characterizations,it is found that CQDs are attached to the surface of MIL-88B(Fe),and these two materials combined to form a new photo-induced Fenton-like catalyst.CQDs changed the dominant crystal plane exposed by MIL-88B(Fe),and the absorption peak of oxygen-containing functional groups in FT-IR spectrum became stronger,indicating that CQDs@MIL-88B(Fe)changed the distribution of surface functional groups.CQDs affected the pore structure of the composite material,resulting in an increase in the pore size of CQDs@MIL-88B(Fe)from 3.49 nm to 5.26nm compared with MIL-88B(Fe)and a decrease in specific surface area from 118.25 m2/g to 99.83 m2/g.But from the nitrogen adsorption and desorption curve and the pore size distribution graph,it can be seen that CQDs@MIL-88B(Fe)is still a typical mesoporous material.CQDs enhanced the light absorption ability of CQDs@MIL-88B(Fe)and affected the band gap structure of composite materials.The band gap of CQDs@MIL-88B(Fe)is reduced from 2.1 eV to 1.97 eV.which makes it easier to generate photo-generated loads.In the process of material preparation.CQDs reduces about 6%of Fe3+ in CQDs@MIL-88B(Fe)to Fe2+,which accelerates the conversion between the two kind of iron ions in the photo-induced Fenton-like reaction and also accelerates the generation of hydroxyl radicals;at the same time,the loading of CQDs reduces the carrier recombination rate of CQDs@MIL-88B(Fe)which improves the mobility of electrons.Hence it is easier for H2O2 and Cr6+ to capture exciting electrons,which reults in the acceleration in the process of photo-induced Fenton-like catalysis.