Research On The Performance And Mechanism Of All-Organic Photocatalytic Materials In The Solar Energy-Hydrogen Energy Conversion

With the rapid development of industry and the continuous growth of population,the contradiction between energy consumption and human development is becoming increasingly prominent.Clean energy has a direct impact on social and economic development and the survival of human society.Exploring and developing green and efficient renewable clean energy has become a research focus in the field of energy and chemical research.Among numerous renewable energy sources,hydrogen（H₂）,as a high-density energy carrier and clean energy,has advantages such as good thermal conductivity,high calorific value,renewability,zero-pollution,and abundant sources,which can meet the requirements of sustainable energy utilization.From the perspective of green chemistry and clean energy,the catalytic water splitting hydrogen production technology driven by sunlight is an ideal strategy to produce hydrogen energy and reduce energy consumption in the chemical industry at the same time.The key to the development of photocatalysis technology is to develop efficient,stable photocatalysts with a wide absorption range.Among many catalytic materials with a wide spectral response range,organic supermolecule perylene diimide（PDI）has been widely used in recent years,such as photocatalytic degradation of pollutants,photocatalytic dehalogenation,photocatalytic oxygen evolution from water and other photocatalytic fields.However,a single component of PDI does not have sufficient reduction catalytic ability and cannot be used as a catalyst main body alone in the photocatalytic complete decomposition of water reaction process,seriously restricting the development and application of PDI as a broad spectrum absorption photocatalyst material.In view of the shortcomings of PDI in the photocatalytic overall water splitting to produce hydrogen,this paper reduced PDI to a double-electron PDI radical(:PDI^2-/PDI^2-)with a free radical-quinone hybrid structure through chemical reduction.At the same time,in the process of:PDI^2-/PDI^2-self-assembly,the direct transition of S₀→T₁Excited state under the condition of non heavy metals was found for the first time.The effective absorption from visible light to near-infrared light induced by upconversion of triplet-triplet annihilation（TTA）was detected.This paper delves into the photocatalytic reaction mechanism of all organic semiconductors based on:PDI^2-/PDI^2-,opening up a new way to construct a all-organic semiconductor photocatalytic overall water splitting system,and providing meaningful exploration for further research on all-organic semiconductor photocatalysts.Main results are as follows:1.Molecular design and physicochemical properties of stable PDI radicals（1）N,N’-bis（propionic acid）-perylene-3,4,9,10-tetracarboxylic acid diimide,with high hydrophilicity and symmetric structure,was prepared by using propionic acid groups to replace the imide sites at both ends of perylene-3,4,9,10-tetracarboxylic acid diimide through calcination.The physical and chemical properties of the prepared hydrophilic PDI were characterized in detail by water contact angle test,Fourier transform infrared spectroscopy（FTIR）,X-ray diffraction energy spectrum（XRD）,Electronic scanning electron microscopy（SEM）and other technical means.The results showed that the hydrophilicity of PDI based photocatalytic materials could be effectively improved by grafting hydrophilic groups on the imide site.（2）By chemical reduction,the PDI is reduced to stable PDI anionic radicals.Due to the presence of free radical ion dipoles,the as-prepared PDI anionic radicals exhibit higher hydrophilicity than PDI,which is beneficial for promoting the adsorption and dissociation of water molecules during the photocatalytic process.At the same time,the steady-state fluorescence test proved that the radical quinone type tautomerism conversion process would occur during the reduction of PDI,thus forming a radical quinone type hybrid structure(:PDI^2-/PDI^2-).This unique hybrid structure has less recombination energy,thus accelerating the rate of electron transfer.2.Synthesis of PDI/Zn_0.8Cd_0.2S composites for efficient visible light-driven photocatalytic overall water splittingA PDIs/Zn_0.8Cd_0.2S composite photocatalytic system with organic-inorganic heterostructure was constructed by hydrothermal method.Through in-situ UV-visible absorption spectroscopy analysis,it was determined that Zn_0.8Cd_0.2S with strong reducibility will reduce PDIs to:PDI^2-under visible light excitation.Although:PDI^2-formed by reduction of Zn_0.8Cd_0.2S does not have high chemical stability,the photocatalytic activity of the composite photocatalyst was significantly improved when it was applied to overall water splitting under the reaction condition of isolating oxygen(the highest hydrogen evolution rate was 71.98μmol·g^-1·h^-1,with a maximum oxygen evolution rate of 32.44μmol·g^-1·h^-1)and exhibits excellent stability within tens of hours,indicating that the presence of:PDI^2-has a promoting effect on the photocatalytic hydrogen production performance.3.Preparation of:PDI^2-/PDI^2-all-organic semiconductors and research on the performance and mechanism in the solar energy-hydrogen（1）Inspired by the self-assembly strategy of PDI,a controllable self-assembly based on:PDI^2-/PDI^2-was achieved for the first time using solvent phase transfer,and:PDI^2-/PDI^2-all-organic semiconductor photocatalysts with nanobelt structures were successfully prepared.With the increase of self-assembly time,Triplet state spin coupling occurs between:PDI^2-/PDI^2-molecules,which improves the photocatalytic performance of:PDI^2-/PDI^2-nanoblets by an order of magnitude,with the maximum hydrogen evolution rate reaching 409.7μmol·g^-1·h^-1 and maximum oxygen evolution rate of 171.2μmol·g^-1·h^-1.It exhibits an apparent quantum efficiency（AQY）of 1.96%underλ=550 nm illumination,making it the first all-organic semiconductor photocatalyst that can independently achieve photocatalytic overall water splitting.（2）Through a series of photochemical characterizations,the photocatalytic reaction mechanism based on:PDI^2-/PDI^2-all-organic semiconductor photocatalysts was elucidated.Different from the traditional triplet state fusion upconversion excitation path,the triplet state spin-coupling of:PDI^2-/PDI^2-all-organic semiconductors during the self-assembly process can enable them to achieve direct excitation of S₀→T₁,and then through the triplet state fusion upconversion process,the overall water splitting driven by near-infrared light is finally generated into H₂ and O₂.