Construction And Photocatalytic Activities Of Z-scheme Heterojunctions With Ultrathin Perylene Diimide

Excessively consumption of fossil fuels has made pernicious effects on global ecological environment,thus,it is of great significance to realize CO₂ conversion by using photocatalysis technology.Inspired by natural photosynthesis,the construction of solar-driven artificial Z-scheme heterostructure photocatalytic systems could be seen as a sensible strategy to strengthen the comprehensive utilization of CO₂.It is very promising to develop the low-cost and easily available perylene diimide(PDI)with high carrier migration and wide spectral response,as novel oxidative semiconductor to construct Z-scheme nanoheterojunctions for the photocatalytic CO₂ reduction.This thesis focuses on Z-scheme heterojunctions constructed by ultrathin PDI and the optimized modification of PDI itself,and further reveals the mechanism on the improved photoactivity of CO₂ reduction.In terms of the problems of insufficient light absorption,poor oxygen evolution and high cost in the Z-scheme heterojunction system constructed with oxidative inorganic metal oxides and reductive carbon nitride(g-C₃N₄,CN).Using PDI with deep valence band and full light absorption,as novel oxidative semiconductor to construct Z-scheme system with CN(PDI/CN)can effectively overcome the above shortcomings,thereby effectively improving its photocatalytic activity.In addition,in view of the weak interfacial interactions and high charge transmission resistance between PDI and the reductive counterpart.Graphene modulated two-dimensional PDI/CN Z-scheme heterojunction has been successfully prepared via a two-step?-?induced assembly.The constructed Z-scheme heterojunction shows extended light absorption range,and the dimension-matched PDI/G-CN nanocomposite establishes a high-flux interfacial electron transport channel through the introduced graphene,as a result the driving force for Z-scheme electron transfer can be significantly enhanced.More importantly,graphene modification on CN will enhance the conjugative effect and induce the high dispersion of PDI assemblies.The optimal loading amounts of PDI could be increased from 1 wt.%to 3 wt.%,and the CO₂ reduction efficiency for the optimal ternary photocatalyst with 15.6-time improvement compared with pristine CN nanosheets,which is mainly attributed to the enhanced Z-scheme charge transfer and separation by the interfacial modulation with graphene.In terms of the problems of lacking necessary reduction catalytic active sites and low visible-near infrared light utilization in the constructed PDI/CN Z-scheme heterojunction.A novel ZnPc/PDI Z-scheme heterojunction has been established with wide visible-light response and abundant catalytic sites,by replacing CN with the metal phthalocyanine(MPc)which processes selective light absorption in the 550-800 nm region and central metal with potential catalytic function.Based on the results of Mott-Schottky measurements,detection of free radical reactive species and low-temperature EPR technique,it is confirmed that ZnPc/PDI heterojunction obeys Z-scheme charge transfer mechanism,and the electrons in ligand of ZnPc can further migrate to central metal cation,empowering the catalytic function of Zn for CO₂ reduction.By using 2wt.%loading amounts of ZnPc combined with PDI,the optimized 2ZnPc/PDI nanocomposite exhibits a 6.5-fold enhancement in the photoactivity for visible light-driven CO₂ conversion compared with pristine PDI.In terms of the severe self-agglomeration of ZnPc in novel ZnPc/PDI Z-scheme heterojunction because of its conjugated molecular configuration.Controlling the interface connection to realize high-dispersed ZnPc assembly on PDI can increase ZnPc loading amount,and further effectively enhanced Z-scheme charge transportation.Pre-modification of phosphate groups on PDI can increase the surface sites and strengthen the double-hydrogen-bond induced lateral interfacial connections,which can achieve the suitable interface connection between ZnPc and PDI in modulated ZnPc/P-PDI composites so as to increase the optimal loading amount of ZnPc.It is verified by photophysical and photochemical methods that the introduction of phosphate effectively promoted Z-scheme charge transfer and separation.Moreover,the catalytic function of center coordination metal is more significant due to highly dispersed ZnPc.In addition,the formed negative field on resulting PDI by phosphate groups modification can trap photogenerated holes,efficiently driving water oxidation half-reactions and further improving overall CO₂ reduction reaction.The amount-optimized ternary 3ZnPc/0.6P-PDI heterojunction shows an exceptional photoactivity by?30-fold enhancement compared with PDI.This phosphate modulated strategy is also feasible for the construction of PDI-based Z-scheme heterojunctions with other MPc,such as Fe Pc,Ni Pc and Co Pc.In terms of the poor self-charge separation,limited interface link sites and lack of hydrophilic structures with unmodified PDI.Modifying PDI itself on imide N,N'positions and further constructing novel Z-scheme heterojunction is expected to dramatically promote the photoactivity of CO₂ reduction.Hydrophilic small molecule amino acids contain both amino and carboxyl groups,can be used to modify amide sites of PDI.Furthermore,amino acid modification can provide more connection sites for the interfacial combination between PDI and reductive semiconductor in Z-scheme heterojunction system.Comparing the substitution of amide positions with different amino acids,?alanine-modified small-sized ultrathin PDI nanosheets(PDIC₃aa)exhibits the optimal charge separation and photocatalytic activity.Furthermore,compared with the as-obtained ZnPc/PDI,the PDIC₃aa constructed one exhibits more excellent photogenerated charge separation and photoreduction performance,and the yield of CO₂ conversion to CO is about 38 times than that of pristine PDI.This work provides a research basis and more design possibilities for the subsequent construction of highly competitive PDI based Z-scheme heterojunctions.This thesis develops an efficient organic Z-scheme heterojunction system with PDI supramolecular polymer as novel oxidative semiconductor for CO₂ reduction,and explores the promotion strategies of the Z-scheme charge transfer mechanism.It also provides a new idea for the design and synthesis of low-cost,high-performance organic photocatalytic materials focused on CO₂ conversion.