Fabrication Of D-A Type Photocatalysts Based On G-C₃N₄ And The Study Of Their Photocatalytic Performance

Excessive CO₂ is emitted into the air when a large amount of fossil fuel is consumed,which greatly challenges the earth’s ecosystem.In the context,the adoption of CO₂recycling technologies turns out to be the optimal way to solve this problem.Among the technologies,photocatalytic reduction of CO₂ is one of the best solutions,as it catalyzes the conversion of CO₂ into renewable fuels by using solar energy.Therefore,the development of new-type efficient photocatalysts has become the focus of the research field of photocatalytic reduction of CO₂.Graphite-phase carbon nitride（g-C₃N₄）is an organic polymer semiconductor with two-dimensional layered structure made by the sp² hybridization of atom C and N.It has been widely studied because of its excellent catalytic activity in photocatalytic CO₂ reduction caused by the following advantages:its unique electronic band structure,easy modification of surface functional groups,cheap price and low toxicity,simple synthesis process,as well as its excellent optical and chemical stability.In the meanwhile,there are some disadvantages such as a small range of visible light absorption,low specific surface area and easy recombination of photogenerated electrons and holes,and hence it is imminent for researchers to optimize the structure of g-C₃N₄ to improve its catalytic performance for CO₂ reduction.Catalysts with D-A（donor-acceptor）structure formed by alternating connection of electron donor units and electron acceptor units are widely used in the preparation of solar cells and photocatalytic reactions.g-C₃N₄ produces a large amount of photogenerated electrons-holes under the irradiation of visible light;different electron deficient units（A）are introduced to have a D-A structure so as to make internal photogenerated carriers realize directional separation and transfer,and the specific surface area and energy band structure of g-C₃N₄ are regulated to improve the photocatalytic performance of g-C₃N₄.Transition metal ions are easy to coordinate with CO₂ molecules to reduce the activation energy of the reduction reaction due to the unique electronic structure of transition metal ions.Thus,they are widely used as the catalytic active center of CO₂ reduction.However,there are limited studies of the selectivity of reduction products of different transition metal ions in photocatalytic CO₂ reduction.Based on the above ideas,the following parts were focused on in the paper:Five D-A photocatalysts-g-CN-PTCDA,g-CN-NTCDA,g-CN-PMDA,g-CN-C₆₀ and g-CN-FN-were made by embedding several classical electron-deficient（A）groups,such as perylene tetracarboxylic anhydride（PTCDA）,naphthalene tetracarboxylic anhydride（NTCDA）,benzene tetracarboxylic anhydride（PMDA）,footballene(C₆₀)and fluorenone（FN）,into g-C₃N₄（D）framework through co-thermal polymerization.The experimental results showed that the doping of a small number of electron-deficient groups did not change the structure and morphology of g-C₃N₄itself;however,the introduction of electron-deficient groups narrowed the band gap of g-C₃N₄,reduced the recombination rate of photogenerated electron-holes and increased the carrier concentration,which effectively improved the catalytic CO₂reduction of g-C₃N₄.The introduction of electron-deficient molecule PTCDA’s covalent bond showed the highest performance of visible light photocatalytic CO₂ reduction.The doping amount of PTCDA was controlled to regulate the morphology,electronic and band gap structure and optical properties of g-CN-PTCDA photocatalyst.The experimental results showed that the photocatalyst g-CN-1 mg PTCDA had the best photocatalytic CO₂ reduction when the doping amount was 1mg.g-CN-PTCDA exhibited significantly enhanced visible light absorption due to the excellent visible light capture capability of PTCDA.At the same time,the introduction of PTCDA inhibited the polymerization process of urea to a certain extent.The specific surface area of the prepared g-CN-PTCDA nanosheets increased,the photogenerated electron-holes were easier to migrate to the catalyst surface,and the D-A structure in the catalyst molecule also contributed to the directional separation and transfer of photogenerated carriers internally,thereby ensuring that the photocatalytic CO₂reduction performance of g-CN-PTCDA was23s times higher than that of unmodified g-C₃N₄.Transition metals easily coordinated with CO₂ molecules as the catalytic active center due to their empty d orbital,which reduced the activation energy of CO₂ reduction process and improved the catalytic performance;different transition metal ions affected the selectivity of CO₂ reduction products because of their different electronic structures and different coordination modes with CO₂.In Chapter 4 of the paper,a series of M-g-CN-PTCDA photocatalysts were prepared by hydrothermal coordination of different transition metal（CO,Cu,Fe）precursors.The experimental results showed that after the transition metals were introduced as the catalytic active site,the band gap of the composite catalyst was narrowed,the visible light absorption range was widened,and the separation and transmission efficiency of photogenerated electron-holes were significantly improved.The photocatalytic CO₂reduction test indicated that the catalytic performance and the selectivity of photocatalytic CO₂ reduction products varied with different transition metals as catalytic active centers,when transition metals Cu and Fe were used as catalytic active centers,the photocatalytic CO₂ reduction was mainly a side reaction hydrogen evolution reaction.When transition metal Co was used as the catalytic active center,the photocatalytic CO₂reduction performance was greatly improved,378 times that of unmodified g-C₃N₄and 17 times that of g-CN-PTCDA.Furthermore,more studies of the catalytic mechanism of Co-as the main reduction product-is being carried out.