Fabrication Of Novel Porphyrin/Graphene-Based Assemblies And Their Photocatalytic Performance For Hydrogen Production

The increasingly serious energy crisis and the environmental pollution caused by fossil fuel combustion made the demand for renewable clean energy essential.The combustion value for hydrogen energy is high,and the combustion product is water.Therefore,hydrogen energy is a kind of sustainable and easily stored green energy.Among the various preparation methods for hydrogen energy,conversing solar energy into hydrogen energy by photocatalysis has attracted extensive attentions of researchers at home and abroad in recent years.Among them,photocatalysts have been the main concern because it is closely related to the efficiency of hydrogen production.In recent years,graphene-based photocatalysts have been studied,and it is found that the microstructures and preparation methods have close relationship with their properties.The electronic structures of porphyrins are complement with that of graphene,and the optical and electronic properties of porphyrins are easily regulated by adjusting the metal centers in porphyrin macrocycles.Therefore,using porphyrin molecules as functional units to assemble with graphene,the reaction mode is more flexible,and the microstructure and properties of graphene can be carefully regulated.In this paper,a series of novel graphene/porphyrin composite systems were assembled rapidly by non-covalent interaction,and the assembly mechanisms of porphyrin and graphene were studied.The photocatalytic properties for the porphyrin/graphene composite systems and the mechanisms of photoelectron transfer and transformation were also studied in detail.The main research contents are as follows:A new type of nanohybrid?GO/THPP/PSA?was noncovalently constructed by anchoring5,10,15,20-tetrakis-?4-hydroxylphenyl?porphyrin?THPP?and 1-pyrenesulfonic acid hydrate?PSA?in graphene oxide?GO?.The assembly mechanism of the nanohybrid was explored in detail.The results showed that THPP and PSA were attached in the GO by?-?stacking interaction and hydrogen bond.Compared with pure GO,GO/THPP or GO/PSA,the GO/THPP/PSA nanohybrid showed better photocatalytic activity for hydrogen evolution.The mechanism of electron transfer in the GO/THPP/PSA nanohybrid was investigated.It was shown that light absorption and separation of electron/hole pairs were improved dramatically dueto wider light response and multi-channel electrons transfer in the hybrid.The results could initiate new ideas for constructing other graphene-based functionalized materials with high photocatalytic activity.By facilely pre-implanting Co²⁺ions in the graphene oxide?GO?,a novel5,15-diphenyl-10,20-di?4-pyridyl?porphyrin?DPyP?pillared GO(GO-Co²⁺-DPyP)was fabricated by means of electrostatic interaction and coordination interaction.It was shown that the morphology and the structure of the GO-DPyP nanocomposite were modified by introducing Co²⁺ions on the interface between GO and DPyP.Furthermore,it was found that the photocatalytic activity for hydrogen evolution over the GO-Co²⁺-DPyP was evidently higher than that without Co²⁺?GO-DPyP?.It confirmed that strong interaction and efficient electron transfer between DPyP and GO were the important reasons for the enhanced photocatalytic activityfor hydrogen evolution.Subsequently,it will be a simple and efficient approach to optimize the transfer pathway of photogenerated electrons and to improve photocatalytic performance by implanting metal ions in the interface of nanocomposites.In order to study the effect of valence state of the cobalt on the structure and properties of GO-Co-DPyP composite,a novel GO-Co-DPyP composite with metal Co as interfacial bridging agent was prepared under hydrogen atmosphere.The results of XRD and Raman spectra showed that the pillared angle between DPyP molecule and GO was increased due to stronger coordination between the cobalt and the N on pyridine group around the DPyP.It was beneficial to the light absorption and the transfer of photogenerated electrons.In addition,it was shown that the amount of hydrogen production over the GO-Co-DPyP composite was about 1.6 times higher than that of GO-Co²⁺-DPyP composite.Combined with fluorescence spectra and photocurrent-response,the essential reasons for the enhanced photocatalytic activity over the GO-Co-DPyP complexes were elucidated.This study provides a theoretical and experimental basis for exploring the effects of valence state of metal ions on the assembly and photocatalytic properties of GO based functional composites.Furthermore,a novel mixed-valence cobalt bridged graphene oxide/porphyrin complex GO-Co²⁺/Co-DPyP was achieved.Transmission electron microscopy?TEM?results showed that there existed a few of cobalt particles with larger size at the edge of the GO-Co²⁺/Co-DPyP composite.Further studies were carried out by means of XPS and Raman spectroscopy.It was found that the valence state of Co²⁺was only changed at the edge of the lamellae of the composite,while many Co²⁺ions still existed on the surface and inside of the composite It was shown that the efficiency of photocatalytic hydrogen production over the GO-Co²⁺/Co-DPyP composite was about 2.5 times higher than that of GO-Co²⁺-DPyP composite.This may be related to the partial conversion of Co²⁺to elemental Co,which resulted in more catalytic active sites.In addition,it was found that the interfacial resistance of the GO-Co²⁺/Co-DPyP composite was lower,and the efficiency of photoelectron transfer was higher,which were of also the main reasons for the high photocatalytic activity over the GO-Co²⁺/Co-DPyP composite.In this paper,by means of non-covalent interactions,a series of novel graphene/porphyrin composite systems were constructed by using porphyrin molecules as functional units.The mechanisms of assembly and photocatalytic hydrogen production for the composites were studied in detail.It provided some new ideas for the design and preparation of porphyrin/grapheme-based composites with high photoelectricity and photocatalytic activity.