Design And Synthesis Of All-solid-state Z-scheme Photocatalysts And Their Performances For The Photocatalytic Reduction Of CO₂

In recent years,the rapid industrial development and fast-growing population causes serious energy shortage and environmental crises.Photocatalysis have attracted lots of attentions because of that it has many functions,such as the conversion solar energy into sustainable energy and the organic matter degradation under solar light irradiation.However,the aforementioned applications require photocatalysts with the wide absorption range,long-term stability,high charge separation efficiency and strong redox ability.Unfortunately,it is hard for a single-component photocatalyst to fulfill all aforementioned requirements simultaneously.These requirements can be satisfied by constructing the artificial heterogeneous Z-scheme photocatalytic systems which mimic the natural photosynthesis process and overcome the drawbacks of single-component photocatalysts simultaneously.Such heterogeneous heterojunction systems have a huge potential to solve the current energy and environmental crises faced by the modern industrial development.Herein,this thesis is based on the semiconductor photocatalysts that have been reported earlier.Ti O₂ with a good band position can fulfill the redox potential required for most photocatalytic reactions.Because of the wide band gap,Ti O₂only absorb the UV light.WO₃ has a narrow band gap and positive valence band position,oxidation reaction is easy to occur,but WO₃ can not satisfy the potential of CO₂ reduction.Therefore,it is difficult for a single semiconductor photocatalyst to simultaneously possess wide light-absorption range,strong redox ability and high charge separation efficiency etc.Three-dimensional ordered macroporous materials（3DOM）have been extensively developed as a photonic crystal material.The traditional semiconductor materials prepared into photonic crystals can effectively improve their utilization efficiency of light.Hence,in order to utilize inexpensive and abundant semiconductor materials to achieve efficient photocatalytic reduction CO₂,a Z-scheme heterojunction system was constructed based on 3DOM materials.In this way,the reduction and oxidation reactions were separated through the directional conduction of electrons between two semiconductor materials.The Z-scheme system not only can expand the absorption of visible light and enhance the utilization of sunlight energy,but also can improve the separation efficiency of photogenerated charges and make full use of the photogenerated carriers as much as possible.At the same time,after separating the two half reactions,it is easier to meet the redox potential required for the reaction.Based on the above consideration,there are two catalyst systems in this paper.The specific research content is as follows:We have successfully fabricated all-solid-state Z-scheme ternary photocatalysts,consisting of two isolated photochemical systems of graphitic carbon nitride（g-C₃N₄）and three-dimensional ordered macroporous carbon-coated Ti O₂（3DOM-Ti O₂@C）combined with Pt nanoparticles as electron-transfer system.Photonic crystal structure and carbon-coated nanolayers of 3DOM-Ti O₂@C support enhance visible light-harvesting efficiency.The vectorial photoelectron transferring of Ti O₂@C→Pt→g-C₃N₄ boosts the separation and surface enrichment efficiencies of photogenerated electrons and holes.All-solid-state Z-scheme ternary photocatalyst exhibits the outstanding yields of CH₄(65.6μmol g^-1 h^-1)and high-efficient quantum efficiency（5.67%）during visible-light-driven conversion of CO₂ with H₂O.The surface enrichment of electrons and CO₂ is the rate-determining step of selective CO₂photoreduction.It provids a general insight into the design philosophy of efficient CO₂reduction photocatalysts.The photonic crystal 3DOM-WO₃ was prepared by colloidal cetstal template method and the g-C₃N₄ thin film was in situ grown on the pore wall.Fortunately,the position of the conduction band（CB）of WO₃ is close to the valence band（VB）of g-C₃N₄,photogenerated electrons and holes are easily recombined.Thereby,it was more advantagrous to construct a direct Z-scheme photocatalyst（g-C₃N₄/3DOM-WO₃）.The g-C₃N₄/3DOM-WO₃catalyst overcomes the shortcoming of the mismatch between the potential of VB in traditional semiconductor and the redox potential required for the reaction.By constructing a direct Z-scheme system,the photogenerated electrons and holes can be rapidly separated due to the short distance for charge transfer,and the higher redox potentials could be retained.Photonic crystals were used to expand the absorption of visible light and improve the utilization of sunlight energy.The g-C₃N₄/3DOM-WO₃ exhibits the excellent performances,the formation rates of CO is48.7μmol g^-1 h^-1,respectively.The electron transfer mechanism was verified by characterization and activity testing.However,the selectivity of reaction products cannot be effectively controlled.Therefore,in another chapter,the degree of electron aggregation was improved by combining an electron-transfer intermediate media and the high selectivity of CH₄ was achieved.In summary,g-C₃N₄/Pt/3DOM-Ti O₂@Carbon and g-C₃N₄/3DOM-WO₃photocatalysts have improved the light absorption,separation of electrons and holes,and CO₂ adsorption,which were beneficial to high efficiency visible-light-driven CO₂reduction.This study is expected to throw a new light on the fabrication of high-efficient photocatalyst for CO₂conversion to hydrocarbon.