Studies On Preparation And Property Of Graphitic Carbon Nitride Based Photocatalytic Materials

Water contamination issues have received great attention in recent years.However,emission of sewage water poses great threat to the environment and damages people's health,including waste water from indutries like papermaking,dyeing,tanning,metallurgy and electroplating,emissions of daily life,improper use of pesticide and fertilizer,arbitrary emissions of medical supplies and personal care products.Photocatalytic oxidation technology has become a promising technology in the wastewater treatment because it can use sunlight to decompose products sufficiently.Graphitic carbon nitride?g-C₃N₄?photocatalyst has the advantages including low cost,simple preparation process,high yield,visible light response,excellent thermal stability,chemical stability and so on.So it has been regarded as one of the most promising photocatalysts.However,the photocatalytic activity of the pure g-C₃N₄ is not ideal,herein,several methods have been used to improve the photocatalytic activity including screening optimal precursor,doping metal cation,constructing semiconductor composites,modifing noble metal,and building Z-scheme composite catalyst.And the relationship between material structure and photocatalytic performance is discussed in depth,which provides a new perspective for the subsequent design of other photocatalytic materials.Firstly,several different precursors were chosen to prepare the pure g-C₃N₄,including urea,melamine,thiourea,dicyandiamide and guanidine hydrochloride.Moreover,the visible-light degradation performance of methyl orange and tetracycline hydrochloride were discussed respectively.The results showed that the g-C₃N₄semiconductor material prepared with urea had better visible-light photocatalytic activity than the others.The g-C₃N₄ prepared with urea had the highest specific surface area,up to93.5 m²/g,which is far greater than g-C₃N₄ from other precursor.This was related to the higher content of heteroatoms in the urea,which would release a large amount of ammonia,water vapor and carbon dioxide during the thermal decomposition process,and thus,occupy the locus between structural units.Therefore,the g-C₃N₄ from urea had the best photocatalytic activity.However,the poor light absorption capacity and the poor separation efficiency of photogenerated electron hole pairs still restricted its unsatisfactory photocatalytic performance.Then,the g-C₃N₄ derived from urea was selected as the modification object.In order to improve its visible light absorption ability and the separation efficiency of photogenerated electron-hole pairs,Fe³⁺ion was doped into the lattice of g-C₃N₄ by impregnation method assissted with sonication.And the XRD and XPS results showed that Fe³⁺ion was successfully incorporated into the framework of g-C₃N₄.Fe³⁺doping modified the optical properties as well as the electronic structure of the material.Such that visible-light absorption ability of Fe³⁺-g-C₃N₄ was improved,and Fe³⁺could directly be reduced to form Fe²⁺through reacting with the photogenerated electrons,whereafter O₂was reduced by Fe²⁺to superoxide radical·O₂^-.So Fe³⁺doping promoted the photogenerated separation of electron-hole pairs effectively.The results showed that the Fe³⁺-g-C₃N₄ had improved photocatalytic activity of methyl orange and tetracycline hydrochloride,and the reaction rate constants were 2.06 and 2.65 times of the pure g-C₃N₄,respectively.Later,a simple hydrothermal strategy was adopted to prepare C-TiO₂/g-C₃N₄nanocomposite.In the process of hydrothermal synthesis,ethylene glycol was added as carbon source so that carbon atoms could be doped into the lattice of TiO₂,simultaneously the C-TiO₂ nanoparticles could be in situ growth on the surface of g-C₃N₄.The visible-light absorption ability of C-TiO₂/g-C₃N₄ was enhanced,which was verified by UV-Vis absorption spectroscopy.Interfacial charge transfer through C-Ti bond and N-Ti bond also played a crucial role in inhibiting the recombination of electron-hole pairs,which was confirmed by XPS analysis.Therefore,the photocatalytic degradation performance of the C-Ti O₂/g-C₃N₄ was greatly improved,and the reaction rate constants of methyl orange and tetracycline hydrochloride were 3.77 times and 3.20 times of the pure g-C₃N₄,respectively.The most important reactive oxygen species was further tested to be superoxide radicals?·O₂^-?.Next,the Pt quantum dots were deposited onto the surface of g-C₃N₄ by heating with an oil bath.After the modification with Pt quantum dots,the visible-light photocatalytic degradation performance was greatly improved,and the reaction rate constant of methyl orange and tetracycline hydrochloride was 7.82 times and 4.30 times of the pure g-C₃N₄.Pt quantum dots could improve the specific surface area of this catalyst,and resulted in forming a schottky barrier between g-C₃N₄ to achieve the fast separation of electron-hole pairs,so that the photocatalytic performance of the material was improved.Finally,a Z-scheme composite catalyst was constructed.This structure was beneficial to induce the effective separation of electrons with strong reduction ability and holes with strong oxidation ability,so the redox reaction rate was improved.C-TiO₂/Pt/g-C₃N₄Z-scheme composite catalyst showed a strong visible light absorption ability.The ESR results indicated the presence of large amounts of superoxide radicals?·O₂^-?and hydroxyl radicals?·OH?in the system,implying the electron from g-C₃N₄ could directly reduce O₂to·O₂^-,the holes from C-TiO₂ could directly oxidize H₂O to·OH.Therefore,the photocatalytic performance of C-TiO₂/Pt/g-C₃N₄ was further improved,and the degradation rate of methyl orange and tetracycline hydrochloride is 9.87 times and 5.24 times of g-C₃N₄,respectively.