Design And Synthesis Of Heterostructures Based On Two-dimensional Materials For Photocatalytic Nitrogen Fixation

The global industrialization process and increasing world population are accompanied with the unceasing decline of fossil energy,which has also exacerbated the energy crisis,climate change,and environmental deterioration.The development of green eternal alternatives is the main challenges and opportunities human society in the future faces.As the cornerstone of modern civilization,ammonia is an incontrovertible raw material for modern industry and agriculture,which has a crucial effect on human survival and economic growth.Nowadays ammonia is predominantly fabricated via the energy-intensive Haber-Bosch process,however,which gives rise to the excessive consumption of feedstocks and consequent high CO₂ emissions.Exploring and developing renewable,environment-friendly,and green routes to yield NH₃ is very promising and appealing.It’s an optimal option to realize the capture and activation of nitrogen in the atmosphere over semiconductor materials with sunlight as an inexhaustible resource and water as hydrogen sources to synthesize ammonia through photocatalytic nitrogen fixation under mild conditions.Nevertheless,strengthening the adsorption and activation of dinitrogen gas,hampering the recombination of the photogenerated carriers,and improving the stability of catalysts are the keys to the deep applications of the photocatalytic nitrogen fixation directions.Burgeoning layered materials have demonstrated considerable photocatalytic potentials owing to highly exposed active centers,high specific surface area,fast charge transfer,and tunable bandgaps.Especially,two-dimensional metal oxides have grabbed more and more attention thanks to their peculiar properties,such as planar morphology,catalytic edge effect,and tunable bandgap energy,of which these characteristics have been extensively researched in the field of electro-and photo-catalysis.Heterostructures based on two-dimensional materials and wide bandgap semiconductors can broaden the light absorption range,promote carrier transport at the interface,and hasten catalytic reactions through surface modification.Based on the above analysis,our research is grounded on 2D material-based heterojunction materials,focusing on their design,construction and photocatalytic performance.The basic content of this thesis is outlined below:（1）Novel well-designed heterostructure comprising 2D Sb₂O₃nanosheets and one-dimensional（1D）W₁₈O₄₉nanowires were synthesized by solvothermal method coupled liquid phase exfoliation,which efficiently catalyze the photoreduction of N₂ to NH₃ through interfacial synergistic effects.Oxygen defects are introduced to the synthesis process to facilitate the binding and activation of N₂,and the heterostructure interface is used to ameliorate the delivery and separating efficiency of charge carriers,which can effectively realize the multi-electron/-proton transfer process of nitrogen fixation reaction.（2）The effects of different loading,pressure,methanol-water volume ratio,and sacrificial agent types on the photocatalytic nitrogen fixation performance of catalysts were investigated.It was found that under visible light,the heterojunction catalyst with Sb₂O₃ mass fraction of 18%produced a maximum rate of NH₃ yield up to 731μg h^-1 g^-1_cat.in 20 vol.%methanol solution.Ultimately,the long-time stability and repeatability of the catalyst were investigated,indicating the superiority of the heterojunction catalyst.（3）Further,the energy band structure and photocatalytic enhancement mechanism of heterojunction catalysts are analyzed in detail by solid-state UV diffuse reflectance spectroscopy,valence band XPS spectroscopy,and Mott Schottky spectroscopy.It is found that Sb₂O₃/W₁₈O₄₉ photocatalyst has wide light absorption and Z-type heterostructure,which can effectively inhibit photogenerated hole recombination and provide strong photogenerated electrons.At the same time,the photoluminescence spectrum,time-resolved transient photoluminescence spectrum,photocurrent response,and impedance spectrum verify that the catalyst has higher photogenerated electron and hole separation efficiency.（4）The mechanism study implies that exposed Sb₂O₃ at interfaces in the heterojunction are catalytic active centers and the hydrogenation reaction proceeds via the distal pathway.The interfacial electron transfer mechanism dramatically enhances the migration and separation of charge carriers.