Enhanced Semicondutor Photocatalysis Performance By Plasmonic Nanostructures

Plasmonic nanomaterials,due to their strong light harvesting ability from visible to near-infrared light,hot charge carrier generation and electrical field enhancement,have been widely used in semiconductor photocatalysis.The photocatalytic performance of semiconductors can be enhanced through plasmon resonance energy transfer,plasmonic sensitization and photothermal effect of plasmonic nanostructures.By optimizing the morphology of the hybrid nanostructure,the photocatalytic activity can be improved by taking full advantage of the synergistic effect between the plasmonic component and the semiconductor.Moreover,the economic cost of noble metals can be reduced by exploring alternative plasmonic nanostructures,such as heavily doped transition metal oxides.In this thesis,we studied the photocatalytic performance（including photocatalytic nitrogen reduction to ammonia and water splitting to produce hydrogen）of the nanohybrids of Au nanoparticles and graphitic carbon nitride（g-C₃N₄）.Moreover,the photocatalytic activity of plasmonic MoO_3-x/g-C₃N₄ heterostructure toward hydrogen evolution was investigated.The main results are as follows:（1）The Au nanoparticles embedded nitrogen-deficient hollow mesoporous carbon nitride nanospheres,synthesized by hydrogen reduction,was used for photocatalytic nitrogen fixation to produce ammonia.The nitrogen vacancies in the carbon nitride spheres,serving as the sites for N₂ chemisorption and activation,can capture photoexcited electrons generated from the carbon nitride nanospheres and plasmon resonance excitation of the embedded Au nanoparticles for N₂ reduction.The concentration of the nitrogen vacancies can be reduced by calcinating the nanostructures in air.The embedded nanostructure can maximize the utilization efficiency of the surface plasmonic effect of Au nanoparticles,as well as the nitrogen activation effect of nitrogen vacancies in carbon nitride spheres,thus promoting the efficiency of nitrogen photoreduction.The optimal Au-embedded carbon nitride nanohybrid with a Au loading amount of 7.4 wt%exhibits an ammonia production rate of 783.4μmol h^–1 g^–1 under visible light irradiation（λ>420 nm）based on the synergistic interaction between the nitrogen vacancies and the localized surface plasmon resonance.Combining with the interfacial plasmon-induced charge separation effect,the hybrid nanostructure achieves a solar-to-ammonia conversion efficiency of0.032%under simulated sunlight irradiation in pure water.（2）The Au nanocrystals（including Au nanospheres and Au nanorods）were assembled on the surface of g-C₃N₄ nanosheets by electrostatic attraction.The optimized 18 nm-sized Au nanospheres/g-C₃N₄ with a Au loading amount of 2.0 wt% shows the highest hydrogen generation rate,when the nanohybrids were subjected for photocatalytic water splitting.The photocatalytic hydrogen evolution activity decreases with increase in the size of the Au nanocrystals.Moreover,the hydrogen generation rate decreases with the decrease in the match extent between the plasmon wavelength of the Au nanocrystals and the emission spectrum of the light source.Hydrogen generation rates of Au/g-C₃N₄ under monochromatic light demonstrate that the photogenerated electrons transfer from g-C₃N₄ nanosheets to Au nanocrystals to participate the hydrogen evolution in short-wavelength spectral region,while hot electrons stemmed from the plasmonic excitation of Au nanocrystals can overcome the Schottky barrier between Au and g-C₃N₄,and inject into g-C₃N₄ for water reduction.（3）A unique nanohybrid was prepared by hydrothermally in-situ growing two-dimensional oxygen-deficient molybdenum oxide nanoplates onto acidified g-C₃N₄nanosheets for photocatalytic hydrogen evolution.The resultant MoO_3-x/g-C₃N₄nanohybrids can construct a Z-scheme heterojunction to improve the photogenerated charge carrier separation efficiency.Moreover,the abundant oxygen vacancies on the surface of MoO_3-x,can excite its localized surface plasmon resonance to enhance the visible light harvesting capacity of the photocatalyst.Benefiting from the synergetic effect between interfacial Z-scheme heterojunction and the plasmon sensitization of MoO_3-x,the nanohybrid achieves a remarkably improved photocatalytic H₂ evolution efficiency under visible light（λ>420 nm）and plasmon resonance region（λ>590 nm）.Electron spin resonance and fluorescence spectra further confirmed that the photoexcited charge carrier recombination has been significantly inhibited by the interface Z-scheme heterojunction,thus boosting the photocatalytic activity.In summary,the employment of plasmonic metal nanoparticles or heavily doped metal oxide is an effective strategy to enhance the photocatalytic performance of semiconductor and improve the efficiency of solar-to-chemical energy conversion.