Preparation Of Anion-regulated Porphyrin Nanoassemblies And Their Photocatalytic Hydrogen Production Performance

With the continuous acceleration of industrialization and urbanization,our country’s total energy consumption has leaped to the forefront of the world,and the economic growth rate is accelerating day by day.The problems of energy shortage and irrationality and the resulting environmental pollution have become increasingly prominent.Hydrogen has received extensive attention due to its high calorific value and its excellent properties such as producing non-toxic and non-polluting products during combustion.Photocatalytic water splitting is an ideal way to prepare hydrogen.How to produce an efficient and stable catalyst for photocatalytic splitting of water to finish hydrogen production has been the goal pursued by scholars at all the world.Porphyrin is the main light-absorbing substance for photosynthesis in nature.Its unique large π-conjugated system makes it have strong absorption in the ultraviolet and visible light regions,and has excellent chemical stability and electron transport property.It is very suitable for photocatalyst making water splitting for hydrogen production.The photocatalytic activity and stability of porphyrins can be further improved by assembling porphyrin molecules to form nano-self-assemblies,and controlling the morphology and size of the assemblies,and realizing efficient photocatalytic water splitting for hydrogen production.In the natural photosynthesis system,porphyrin molecules are mainly arranged in the form of Jtype aggregation to construct an efficient light energy capture and transmission system.It has lower excitation energy and higher utilization of sunlight.How to controllably adjust the degree of overlap between molecules in porphyrin assembly J-type aggregates,so as to obtain higher photocatalytic activity and clarify their structure-activity relationship,is a difficulty in the current research in the field of porphyrin self-assembly.In this work,we constructed a series of porphyrin self-assemblies with different anions using the core protonated 5,10,15,20-tetrakis(4-hydroxyphenyl)porphyrin(THPP)as building block.Due to the strong electrostatic interaction between anions and positively charged porphyrin core,they can form ion pairs in the axial position of porphyrins and the stacking degree of porphyrin molecules can be regulated by anions with different sizes as steric hindrance.As a result,the J-type aggregates with controllable porphyrin molecular overlap in the assembly were successfully prepared to realize efficient photocatalytic hydrogen production.Based on the above background,we mainly carry out the following two parts of work:(1)Preparation of THPP-sodium alkylcarboxylate and its photocatalytic hydrogen production application.In this chapter,we choose alkylcarboxylate anions with different alkyl chain length as steric hindrance to regulate the internal structure of porphyrin self-assemblies.Based on this,we successfully prepared a series of porphyrin self-assemblies through acid-base neutralization assisted micelle confined controllable self-assembly method with central core protonated THPP and alkylcarboxylate anions with different length as building units.A series of tests such as SEM,TEM,XPS,Raman,FTIR and XRD proved that the introduction of alkylcarboxylate anions regulated the morphology and internal structure of the self-assemblies,and there was charge transfer between the protonated THPP and the alkylcarboxylate anions in the self-assembly,indicating that they self-assembled via strong ionic bonds between them rather than simple doping.UV-Vis test showed showed that the absorption peak of the self-assembly red shifted with the addition of anions,indicating that the degree of overlap between porphyrin molecules in J-type aggregates reduced with the introduction of alkylcarboxylate anions.These tests demonstrated the effectiveness of the design of regulating the porphyrin stacking structures through the introduction of alkylcarboxylate anions.The visible light photocatalytic hydrogen production test showed that the photocatalytic performance of HBr/THPP-alkylcarboxylate slef-assemblies had been improved significantly.The hydrogen production rate was increased by 2 to 4.8 times,depending on the size of the alkylcarboxylate anion added,which proves that the introduction of alkylcarboxylate anions to tune the stacking structure of porphyrins self-assemblies and improve the photocatalytic hydrogen production performance of porphyrins was effective.(2)Preparation of HBr/THPP-sodium halide salt and its photocatalytic hydrogen production application.In the first part of the work,we found that the introduced alkylcarboxylate anion has limited ability to regulate the degree of overlap between porphyrin molecules in the aggregate with the size of the anion,which may be due to the fact that the size of the anion is too large to participate in porphyrin selfassembly efficiently.In this section,we introduce smaller sodium halides to regulate the self-assembly process of porphyrin core protonated THPP porphyrins.A series of tests such as SEM,TEM,XPS,Raman,FTIR and XRD proved that the introduction of halogen anions also regulated the morphology and internal structure of the self-assemblies,and there was also charge transfer between the protonated THPP and the halogen anions in the self-assemblies,indicating that they self-assembled via strong ionic bonds between them rather than simple doping.UV-Vis test showed showed that the absorption peak of the self-assembly further red shifted with the addition of halogen anions in comparision with the introduction of alkylcarboxylate anions,indicating that the degree of overlap between porphyrin molecules further reduced in J-type aggregates.These tests demonstrated the effectiveness of the design of regulating the porphyrin stacking structures through the introduction of halogen anions.The visible light photocatalytic hydrogen production test showed that the photocatalytic performance of HBr/THPP-NaCl self-assemblies was 185.1mmol/g/5h,which was further improved compared with the hydrogen production performance of the alkylcarboxylate anions involved self-assembly,and also proves that the halogen anions were more effective to tune the stacking structure of porphyrins self-assemblies and improve the photocatalytic hydrogen production performance of porphyrins in comparision with the alkylcarboxylate anions.