Preparation Of Novel Graphitic Carbon Nitiride (g-C₃N₄) Photocatalysts And Its Mechanism For Organic Pollutants Degradation And H₂ Production In Water

Considering the rapid growth of population and industrialization in the past few decades,the energy supply challenges have led the world to utilize fossil fuels more than ever and subsequently release a massive amount of industrial and urban toxic waste into water resources.Accordingly,consuming fossil fuel energy sources is inevitably detrimental to the environment and has destructive effects like water pollution,causing some human health problems.Thus,to deal with these pollutants'destructive effects and,subsequently,their environmental remedies,many different novel procedures in materials science and engineering,including physical,biological,and semiconductor-based photocatalysis technology,have been utilized to address energy conversion and environmental issues.Among these different green methods,semiconductor-based photocatalysis has gained remarkable interdisciplinary attention for its potential in solar energy and environmental applications like water splitting,degradation of organic pollutants,carbon dioxide reduction,and water purification.Among different semiconductors,polymeric graphitic carbon nitride(g-C₃N₄),as a metal-free photocatalyst,is an attractive option considering several advantages,such as visible light response,nontoxicity,cheap,and cheap high chemical stability.However,it is far from being used for practical applications due to its downsides,including a high recombination rate of photogenerated electron-hole charges,poor charge transfer properties,fewer active sites,insufficient visible light absorption,and low specific surface area.In this respect,this thesis was an attempt to surmount these obstacles by using non-metal doping,heterojunction construction,and morphology optimization to speed up the charge transfer and separation and enhance the exposure of solar light absorption of the g-C₃N₄.Contribution1:This contribution was an attempt to develop a novel two-dimensional g-C₃N₄/WO₃/Mo S₂(CWM)ternary nanocomposite through a facile co-calcination and a hydrothermal process to reach a highly-efficient photocatalyst for organic pollutant elimination under visible light.The Tungsten trioxide(WO₃)and Molybdenum disulfide(Mo S₂)nanoparticles were dispersed on the ultra-thin g-C₃N₄ nanosheets in which the electronegative g-C₃N₄ facilitates the formation of oxygen vacancies in WO₃.The crystal structures,chemical state,elemental composition,morphologies,and optical properties were investigated.The EDX mappings further indicated that Mo S₂ was highly dispersed in the composite.Furthermore,the CWM presents the most significant redshift among all samples and,subsequently,higher visible light absorption,which indicates its remarkable photocatalytic performance,resulting from the more photogenerated electron-hole pairs.Contribution2:This contribution tackles the low specific surface area of g-C₃N₄ by modifying g-C₃N₄ structure through a thermal-chemical exfoliation in which the bulk g-C₃N₄ was obtained first under thermal exfoliation.It then was exposed to an acidic etching using hydrofluoric acid by hydrothermal process.The crystal structures,chemical state and elemental composition,morphologies,and their optical properties investigation showed acidic etching not only exfoliated g-C₃N₄ nanosheets by disrupting weak van der Waals forces between layers,which led to the formation of a monolayer or a few layers of g-C₃N₄ nanosheets but also made disordered defects on its surface and created plicated g-C₃N₄ nanosheets.Contribution3:The results in this thesis clearly confirm that the photocatalytic activity of the g-C₃N₄ based photocatalyst can efficiently be enhanced.The novel two-dimensional g-C₃N₄/WO₃/Mo S₂(CWM)ternary nanocomposite Compared to pure g-C₃N₄,WO_3,and binary composites,exhibited higher photocatalytic activities for various organic pollutants removal under visible light irradiation.For instance,the CWM showed a removal ratio of?99%for Rh B after only10 min irradiation of visible light(?>420 nm)and nearly 100%for ciprofloxacin after 2 h of operation.The results showed that^?OH radicals are the main active species for organic degradation,suggesting a direct Z-scheme heterojunction in CWM that improved spatial separation of charge carriers.Furthermore,the collection of electrons is significantly enhanced by Mo S₂ for oxygen reduction reaction,and the increased oxygen vacancies of WO₃ further enhanced the separation of electron-hole pairs;therefore,it led to effective suppression of charge carriers'recombination.The obtained exfoliated and plicated 2-D g-C3N4 nanosheets showed that under visible-light illumination,a hydrogen evolution rate of 54.13?mol h^-1g^-1(without co-catalyst)and a specific surface area of 121.4 m²g^-1,which were about 4.7 and 2.5 times,respectively,higher than CNs F-0%.It also showed remarkably enhanced photocatalytic performance in removing various organic pollutants.This remarkable improvement probably arises from porous nanosheets and an increased number of active sites resulting from the exfoliated g-C₃N₄ nanosheets,which subsequently enhanced charge separation efficiency.To recapitulate,the third contribution offers a Z-scheme heterojunction,which improved spatial separation and transfer of photo-induced charge carriers.Furthermore,the decorated Mo S₂ speeds up the usage of electrons for oxygen reduction reaction(ORR),which further hinders the combination of charge carriers.This work provides a useful example for designing novel visible-light-driven photocatalysts without noble metals'assistance,which might facilitate their practical applications as semiconductor photocatalysts with high efficiencies and robust stabilities.Furthermore,we addressed the importance of acidic etching,which not only exfoliated g-C₃N₄nanosheets by disrupting weak van der Waals forces between layers,leading to the formation of a monolayer or a few layers of g-C₃N₄ nanosheets but also made disordered defects on its surface and created plicated g-C₃N₄ nanosheets.Furthermore,it showed the highest degradation rate for photocatalytic degradation of various organic pollutants.This significant improvement should be attributed to the structural distortion of g-C₃N₄nanosheets through acid treatment due to the formation of nitrogen vacancies and distorted C-N planes,which could significantly increase the specific surface area and the number of active sites,therefore promoting the separation of photogenerated electron/hole pairs.