Studies on Preparation and Property of Graphitic Carbon Nitride Based Photocatalytic Materials

Author:Lu Zhao

Supervisor:song wu lin


Degree Year:2017





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.