Study On The Design And Performance Of Carbon Nitride Based Light Energy Conversion Systems

The efficient conversion and utilization of solar energy is one of the important ways to solve the energy crisis and environmental pollution issues.Among them,photocatalysis can convert solar energy into hydrogen energy or realize the removal of the pollutants,which is considered as one of the promising technologies to solve energy and environmental problems simultaneously.Highly efficient solar energy conversion systems are critical for solar energy utilization.However,the present systems have limited photon conversion efficiency.In this work graphite-like carbon nitride（g-C₃N₄）,a promising solar energy conversion material with good visible light response（band gap about 2.7 e V）and excellent stability,was selected as the matrix to construct solar energy conversion systems.The photogenerated charges behaviors of the systems were tailored by designing composition and optimizing synthesis,for maximizing the utilization of solar energy.（1）Trace copper modified g-C₃N₄（Cu-g-C₃N₄）with excellent photo-Fenton catalytic performance was constructed,and carbon dots（CDs）and Cu co-modified g-C₃N₄（CDs/Cu-g-C₃N₄）with higher photo-Fenton catalytic activity on degradation of different organic pollutants was futher designed,g-C₃N₄ based solar energy conversion systems were synthesized by one-step thermal polymerization.The introduced copper species mainly in the form of Cu⁺,were stabilized in g-C₃N₄ structure through coordination with N.doped into g-C₃N₄ matrix as Cu⁺,and stabilized in g-C₃N₄ structure through coordination with N.It was demonstrated that the introduction of Cu decreased the photodegradation ability of g-C₃N₄ owing to the trapping of the photogenerated charge carriers caused by introduced Cu species,which was ignored in previous reports.But due to the introduced trace-level Cu that avoided producing excessive defects acting as trapping centers of active radicals and simultaneously reduced the consumption of photogenerated charge carriers by Cu species themselves,more active radicals could participate in degrading organic pollutants.Therefore,at an introduction of Cu with theoretical weight percentage of 1 wt%,the obtained Cu-g-C₃N₄ not only exhibited exceptional Fenton-like catalytic activity with low consumption of H₂O₂（4 m M）,but also showed favorable photo-Fenton synergistic catalytic activity.The photo-Fenton degradation rate constant for Cu-g-C₃N₄ reached 0.233 min^-1.Additionally,benefiting from CDs with rich surface states which could further promote the charge transfer and surface interaction,when introducing CDs with theoretical weight percentage of 0.001 wt%,the obtained CDs/Cu-g-C₃N₄showed higher photo-Fenton activity in degrading different organic pollutants.（2）Considering that the rich oxygen-containing groups on the surface of CDs which is beneficial for the adsorption of O₂,the composite of CDs and protonated g-C₃N₄（CDs/p-g-C₃N₄）was designed and the property of photochemical synthesis H₂O₂ was studied.The obtained g-C₃N₄ prepared via the thermal polymerization of urea was first protonated by nitric acid,and then coupled with CDs though theπ-πconjugated interaction between g-C₃N₄ and CDs.The results indicated that CDs/p-g-C₃N₄ exhibited better photochemical synthesis H₂O₂ability than that of p-g-C₃N₄.Moreover,to further tailor the electronic structure of g-C₃N₄ so as to improve solar harvesting and conversion performance,boric acid was used to protonate g-C₃N₄ for simultaneously introducing B into g-C₃N₄.The result demonstrated that the protonated g-C₃N₄ by boric acid（B-p-g-C₃N₄）showed enhanced photochemical synthesis H₂O₂activity.