Preparation Of Carbon Nitride-based Photocatalytic Materials With Broad Spectrum Response And Research On Degradation Of Organic Pollutants

The energy crisis and environmental pollution are the hotspots that human society pays attention to.As a promising advanced oxidation technology,photocatalytic technology has been widely studied and applied in environmental pollution control.The water pollution problem in environmental pollution is closely related to people’s production and life.Therefore,finding cheap photocatalysts that can absorb sunlight and remove harmful organic pollutants in water has become one of the hot spots that researchers follow.In recent years,graphite-phase carbon nitride（g-C₃N₄）has gained advantages due to its wide source,simple element composition,green environmental protection,low cost and easy availability,good thermal stability and corrosion resistance,suitable band gap width and strong reduction ability.Favored by many scientific researchers.Therefore,a cheap carbon nitride（g-C₃N₄）material is selected as the matrix,and an appropriate strategy is used to improve the light absorption performance of the material and achieve high-efficiency carrier separation efficiency,thereby improving the photocatalytic performance.In this thesis,by adjusting the crystal faces of other semiconductors to construct heterostructures with g-C₃N₄,introducing urea nitrate to construct ultra-thin g-C₃N₄ with defects and porous structures,atomic doping type broad spectrum response g-C₃N₄,atomic doping type three-dimensional g-C₃N₄-based photocatalytic materials and other means designed photocatalytic materials with different light response ranges,and studied the influence of substance composition,physical and chemical properties,energy band structure,photoelectric characteristics and fluorescence absorption on the internal mechanism of photocatalytic reactions.The main contents of this thesis are as follows:（1）The SnS₂（nanoparticles,nanosheets and three-dimensional nanoflowers）/g-C₃N₄ composite materials with different crystal faces exposed were synthesized by a simple hydrothermal method coupled with a calcination method.The XRD results show that the relative intensities of several diffraction peaks（100）,（001）,（101）and（102）indicate that the synthesized SnS₂ samples（nanoparticles,nanosheets and three-dimensional nanoflowers）should be crystallized with different advantages.Surface growth is dominant.Compared with monomer SnS₂ and monomer g-C₃N₄,SnS₂nanoparticle/g-C₃N₄ composite catalytic material exhibits the most excellent activity to degrade Rhodamine B（RhB）under visible light irradiation,compared with g-C₃N₄,its degradation rate is improved by 2.63 times.In addition,under the environment of Na₂S and Na₂SO₃ as sacrificial agents,the SnS₂ nanoparticle/g-C₃N₄ composite exhibits better hydrogen production activity under visible light irradiation,up to 6305.18μmol·h^-1·g^-1（without any precious metal used as a co-catalyst）is about 16.98 times that of SnS₂ nanoparticles.All in all,SnS₂ nanoparticles（SnS₂ nanoparticles/g-C₃N₄composite material）grown on the preferred crystal planes of（001）and（100）exhibited significant photocatalytic activity,which is due to their suitable energy band structure.Photocatalytic redox reaction.The analysis of photocurrent response,linear scanning voltammogram and photoluminescence spectroscopy showed the low recombination rate of photogenerated carriers and effective charge transfer,which may be attributed to the interaction between g-C₃N₄ and SnS₂.（2）A new carbon nitride material（UNU-C₃N₄）is obtained by grinding and calcining urea and urea nitrate,and a new carbon nitride composite material（UNM-C₃N₄）is obtained by grinding and calcining melamine and urea nitrate.UNU-C₃N₄exhibits excellent photocatalytic removal of organic matter and hydrogen production performance.Various spectroscopic characterizations prove that the defects enhance the ability to adsorb oxygen,and the increased specific surface area is beneficial to the generation of more active free radicals,thereby improving the photocatalytic performance of the catalytic material.By introducing additional nitrogen,hydrogen and oxygen atoms into g-C₃N₄,the electron polarization effect is enhanced and the charge transfer rate and charge separation efficiency are improved.At the same time,the results of ESR also show that the prepared new carbon nitride has the ability to extract electrons efficiently,which is conducive to the formation of abundant active oxygen radicals,which also helps to improve the photocatalytic degradation performance.Under simulated sunlight,the synthetic UNU-C₃N₄ and UNM-C₃N₄ materials degrade RhB pollutants at a rate of 2.86 times and 10.66 times that of U-C₃N₄ and M-C₃N₄,respectively.At the same time,the H₂ generation rate of UNU-C₃N₄（or UNM-C₃N₄）is9.93 times（or 13.76 times）that of U-C₃N₄（or M-C₃N₄）.The efficient hydrogen production rates of UNU-C₃N₄ and UNM-C₃N₄ under visible light irradiation are830.94 and 556.79 mol·g^-1·h^-1,respectively.（3）The brown carbon nitride（CY-C₃N₄）modified by oxygen bridges and porous defects was successfully synthesized through cyanoacetic acid and urea.Density functional theory（DFT）calculations show that the introduction of oxygen bridges during calcination polymerization can adjust the electronic structure and energy band position of the new catalyst.In addition,the results of X-ray light emission spectroscopy,13C solid-state nuclear magnetic field and elemental analysis test showed that the oxygen bridge structure was successfully introduced into the carbon nitride frame.The results show that 0.1 CY-C₃N₄ can remove bisphenol A（BPA）and 2-mercaptobenzothiazole（MBT）in 90 min and 20 min,respectively.Its degradation rate is 17.94 times and 3.85 times faster than g-C₃N₄,respectively.In addition,through HPLC-MS analysis,the intermediate products of the reaction process were analyzed in depth,and possible photocatalytic degradation pathways were proposed.Free radical capture tests and ESR spectra show that the formation of hydroxyl radicals（·OH）,superoxide radicals（·O₂^–）,singlet oxygen（¹O₂）and holes（h⁺）all play a key role in photodegradation.（4）A new type of carbon and oxygen co-doped porous g-C₃N₄（PACN）was successfully prepared by one-step calcination of urea and malic acid for photodegradation of bisphenol A（BPA）and selective oxidation of toluene to benzaldehyde.The degradation rate of F-0.5-PACN to BPA is 23.58 times that of g-C₃N₄.Without any organic solvent,the formation rate of benzaldehyde is 5.43 times that of the original g-C₃N₄.At the same time,the density of states and electronic structure of the theoretical simulation structure are calculated by DFT.The results show that the introduction of oxygen and carbon can adjust the electronic structure and obtain a narrower band gap.In addition,under long-wavelength（λ≥550nm）and near-infrared light（λ≥760nm）,F-0.5-PACN has a certain photocatalytic degradation ability for BPA,which indicates that the synthesized material has broad-spectrum photocatalytic activity.The results of solid-state nuclear magnetic carbon spectroscopy(¹³C NMR)and secondary ion mass spectrometry（SIMS）showed that C and O were introduced into the g-C₃N₄ framework.In addition,the intermediate products of the degradation process were detected by liquid chromatography-mass spectrometry（HPLC-MS）,a possible BPA degradation path was proposed,and a possible photocatalytic reaction mechanism was given.（5）Orange carbon nitride（AF-C₃N₄）co-modified with oxygen junction bands and pore defects was successfully synthesized using urea and ammonium formate.In addition,the energy band structure of its simulated structure was calculated by DFT,which showed that the introduction of oxygen connecting bands can adjust its energy band structure.The photocatalytic degradation rate of 0.3AF-C₃N₄ for bisphenol A and2-mercaptobenzothiazole is 8 times and 2.73 times that of g-C₃N₄,respectively.In addition,0.3AF-C₃N₄ also shows certain photocatalytic activity under different wavelengths of light（blue,green and red）,which indicates that the synthesized material has broad-spectrum photocatalytic activity.In addition,we proposed a possible photocatalytic degradation path through HPLC-MS analysis.Free radical capture experiments and ESR spectra show that the generated hydroxyl radicals（·OH）,superoxide radicals（·O₂^-）and holes（h⁺）caused the improvement of photodegradability,and the increased the concentration of singlet oxygen（¹O₂）and reduced hydrogen peroxide content further prove that the active oxygen groups play an important role in the photocatalytic degradation process.（6）Through the simple hydrothermal method coupled thermal polymerization of dicyandiamide and polyethylene glycol-polyglycerol-polyethylene glycol triblock copolymer,a broad spectrum response carbon and oxygen co-doped three-dimensional carbon nitride structure was synthesized（PEPE-C₃N₄）.Through elemental analysis,X-ray photoelectron spectroscopy,solid-state nuclear magnetism and electron loss spectroscopy,the internal electronic structure of PEPE-C₃N₄ was proved.The experimental results show that the PEPE-C₃N₄/persulfate（PS）system exhibits high efficiency in the photocatalytic degradation process of MBT and BPA.Compared with the original carbon nitride,the synergistic effect of photocatalysis and persulfate activation resulted in 12.17 times and 30.78 times increase in the degradation rate of 2-mercaptobenzothiazole（MBT）and bisphenol A（BPA）.At the same time,the synthesized PEPE-C₃N₄ also has photocatalytic activity under blue light（450-462 nm）,indicating that PEPE-C₃N₄ has a broad spectrum response photocatalytic degradation ability.Based on liquid chromatography-mass spectrometry（LC-MS）analysis,the intermediate product was analyzed and its possible degradation pathway was inferred.（7）Through the thermal polymerization of urea,hydroxyacetic acid and phytic acid,a broad-spectrum oxygen and phosphorus-doped porous g-C₃N₄（HAPA-CN）was synthesized.The internal electronic structure was proved by solid-state nuclear magnetism（NMR）and X-ray spectroscopy（XPS）.The surface electron density on HAPA-CN is enhanced by the induction of oxygen and phosphorus.0.05 HAPA-CN shows excellent photocatalytic degradation performance for bisphenol A（BPA）and 2-mercaptobenzothiazole（MBT）.The enhanced photocatalytic activity can be attributed to the broad-spectrum utilization of sunlight and improved charge separation efficiency.The degradation experiment results show that the 0.05 HAPA-CN/PS system shows a higher efficiency in the photodegradation process of BPA and MBT.Compared with g-C₃N₄,the coupling of photocatalysis and persulfate oxidation resulted in an increase in the degradation activity of BPA and MBT,which increased by 68.11 times and 11.18times,respectively.0.05 HAPA-CN exhibits high efficiency photocatalytic degradation ability under long-wave irradiation of blue light（450-462 nm）,green light（510-520nm）and red light（610-625 nm）,indicating that the synthesis catalyst has a broad spectrum response performance.On the basis of liquid chromatography-mass spectrometry（LC-MS）analysis,the possible intermediate products of degradation of MBT and BPA were discussed.In addition,through a series of characterization methods,such as ESR,free radical capture experiment and H₂O₂ determination,the active groups that may exist in the system were proved,and the possible photocatalytic mechanism was inferred.