Strategies For Improving Water Oxidation Performance Of Organic G-C₃N₄ Nano-photocatalyst And Application

Solar driven photocatalytic carbon dioxide(CO₂)reduction technology can reduce excess CO₂ emissions into high-value energy fuels,becoming one of the most promising technologies to tackle energy shortage and environmental crisis.It is of great significance to develop efficient and economical semiconductor photocatalytic materials for CO₂-energy conversion to meet the strategic needs of green and sustainable development.Graphitic carbon nitride(g-C₃N₄)is an organic polymer semiconductor with visible light responsive,physically and chemically stable,which has shown great advantages in the field of photocatalytic applications.g-C₃N₄ possess a high conduction band position,which is very conducive to the CO₂ reduction reaction.However,the photocatalytic activity of CO₂ reduction coupled with pure water is still unsatisfactory,which is mainly attributed to the sluggish photogenerated charge separation efficiency of g-C₃N₄,the lack of water oxidation cocatalyst and the insufficient thermodynamic driving force of water oxidation.Therefore,the following work is carried out in this thesis from the perspective of developing water oxidation cocatalyst,ect.to improve the photogenerated charge separation efficiency and water oxidation capacity of g-C₃N₄.Firstly,the modulation of Mn oxygen-bearing nanoclusters cocatalyst on photogenerated holes and water oxidation process were studied.On this basis,Ni oxygen-bearing nanoclusters were further introduced to realize the dual modulation on photogenerated holes and electrons of g-C₃N₄,and the effects on of Mn and Ni oxygen-containing nanoclusters on the photocatalytic performance of g-C₃N₄ was explored.Firstly,ultra-thin g-C₃N₄ nanosheets were prepared by a two-step thermal polymerization with the urea as raw materials followed by acid treatment.Subsequently,the ultra-thin g-C₃N₄nanosheets were premodified by the chitosan oligomers with appropriate molecular weight via a hydrothermal method,and the Ni and Mn oxygen-bearing nanoclusters(diameter about 0.8 nm)were successfully anchored by using the functional groups of chitosan oligomers.The results show that Mn species can capture photogenerated holes and promote water oxidation reaction,while the modified Ni species can capture photogenerated electrons and catalyze CO₂ reduction reaction.The synergistic effect of the Ni and Mn species greatly improves the photocatalytic CO₂ reductiong performance of g-C₃N₄.Secondly,the effects of highly dispersed Ru oxygen-containing nanoclusters modification on the photocatalytic performance of nano-sized g-C₃N₄ were investigated.The decoration of Ru oxygen-containing nanoclusters can largely improve the water oxidation capacity of g-C₃N₄,and thus effectively improve the photocatalytic CO₂reduction activity.The photoactivity of Ru-oxo nanoclusters anchored g-C₃N₄ by premodified chitosan oligomers is much more improved than that of Mn-oxo nanoclusters or Ru O₂ nanoparticles modified one.The obviously improved activity is mainly attributed to the capture of photogenerated holes by the smaller size and highly dispersed Ru-oxo species,which effectively promoted the photogenerated charge separation of g-C₃N₄.Meanwhile,the highly dispersed Ru-oxo nanoclusters could efficiently catalyze the water oxidation process,and thus accelerate the overall photocatalytic CO₂ reduction reaction process.Finally,the effects of B doping and Ru-oxo cluster modification co-modification on the photocatalytic performance of g-C₃N₄ nanosheets were explored.B-doped g-C₃N₄was prepared via calcining the mixture of g-C₃N₄ and sodium borohydride(Na BH₄)in a nitrogen atmosphere firstly,and then Ru oxygen-bearing nanoclusters were successfully anchored by pre-modified chitosan oligomers.The results show that the water oxidation capacity of g-C₃N₄ is greatly improved by the B doping and the further loading of Ru oxygen-containing nanoclusters,and thus it exhibits the superior photocatalytic activity of CO₂ reduction.The improved photoactivities were attributed to the valence band downward to obtain the strong driving force for water oxidation along with extension of visible light response region by B doping and to the capture for photogenerated holes to enhance charge separation and then to accelerate the kinetic process of water oxidation half reaction from the modified Ru-oxo nanoclusters,achieving efficient photocatalytic reduction of CO₂.This thesis provides a new idea for the design and synthesis of highly efficient g-C₃N₄-based photocatalyst for the solar fuel production form the perspective of improving water oxidation performance.In addition,the photocatalytic water oxidation or CO₂reduction is closely related to the photosynthesis process of green plants.This study provides a new direction for promoting the photosynthesis of plants and developing green and sustainable agricultural technologies.