Research On The Photocatalytic Preparation Of H₂ And H₂O₂ Of The Sulfur-Containing Vacancy Zn₃In₂S₆ Nanocomposites

The increasing global demand for energy and the aggravation of environmental deterioration are two pressing challenges facing the sustainable development of human society in the 21st century.At present,green and renewable energy sources have received wide attention as alternatives to non-renewable fossil fuels.As an inexhaustible source of energy,solar energy plays an important role in alleviating the energy crisis and environmental problems in the modern world,reducing dependence on fossil energy,and eventually replacing the energy generated by such fuels.Photocatalytic decomposition of water to produce hydrogen only needs sufficient solar energy as power,and the process conditions are mild,green,pollution-free.The photocatalytic production of H₂O₂on semiconductor catalysts is a sustainable strategy for converting solar energy into valuable chemicals.The core of photocatalytic technology is the development of photocatalysts.The ternary metal sulfide Zn₃In₂S₆has been proved to be a good candidate material due to its suitable band structure,excellent visible light response,satisfactory electronic properties,low cost and non-toxicity.However,the pure Zn₃In₂S₆has low quantum efficiency,slow charge transfer dynamics,and relatively fast electron-hole pair recombination,which cannot achieve satisfactory performance.Hence,it is of practical significance to adopt effective strategies to modify Zn₃In₂S₆and thus enhance its photocatalytic activity.In this thesis,sulfur vacancy was first introduced into pure Zn₃In₂S₆（Vs-Zn₃In₂S₆）,and then Vs-Zn₃In₂S₆was combined with nested g-C₃N₄,CdS quantum dots and UiO-66 nanoparticles to construct heterojunction structures.The synergistic effect of defect engineering and heterojunction engineering could effectively improve the visible light response capacity of Zn₃In₂S₆-based materials,and promote the separation and transfer of charge carriers.The morphology,structure and elemental composition of the prepared materials were studied through a series of characterization means.The photocatalytic capacity and practical application potential of the materials were evaluated by photocatalytic decomposition of water for hydrogen evolution and preparation of H₂O₂.Moreover,the reaction mechanisms of the composites were studied and the electron transport processes were elaborated.The following were the specific research contents of this paper:（1）The Vs-Zn₃In₂S₆was first prepared by solvothermal method,and then g-C₃N₄/Vs-Zn₃In₂S₆（CN/Vs-ZIS）heterostructures were prepared by in-situ method.The photocatalytic activity of the prepared materials was tested by photocatalytic decomposition of water for hydrogen evolution and preparation of H₂O₂.As a result,all the CN/Vs-ZIS heterostructures possessed much enhanced photocatalytic activities.When the weight ratio of g-C₃N₄was 2 wt%（2CN/Vs-ZIS）,CN/Vs-ZIS exhibited the highest visible-light photocatalytic performances.The H₂generation rate of2CN/Vs-ZIS under visible light（λ>420 nm）was 6.55 mmol g^-1h^-1,which was 1.76and 6.06 times higher than those of Vs-Zn₃In₂S₆and g-C₃N₄,and the apparent quantum yield（AQY）was 18.6%at 420 nm.Meanwhile,the two-hour yield of H₂O₂of 2CN/Vs-ZIS was 792.02μM,about 4.72 and 6.04 times that of pure Vs-Zn₃In₂S₆and g-C₃N₄.The enhanced reaction mechanisms for the photocatalytic H₂and H₂O₂productions were also investigated.This work undoubtedly demonstrates that the synergistic effects of defect and heterojunction engineering will be the great promise for improving photocatalytic efficiency of Zn₃In₂S₆based materials.（2）The CdS quantum dot/Vs-Zn₃In₂S₆nanoflower composites（CdS/Vs-ZIS）were prepared by an in situ hydrothermal method.The composite had the best photocatalytic activity when the weight ratio of the CdS quantum dot was 5 wt%（5CdS/Vs-ZIS）.The H₂preparation rate of 5CdS/Vs-ZIS was 2.60 mmol g^-1h^-1,which was 1.37 and 4.73 times that of pure Vs-Zn₃In₂S₆and CdS.The preparation rate of H₂O₂was 152.55μmol g^-1h^-1,which was 2.33 and 1.54 times that of Vs-Zn₃In₂S₆and CdS.Meanwhile,after five cycles of experiments,the performance of the composite had not decreased significantly,indicating that it had good stability.The crystal structure,morphology,chemical composition and carrier migration of the prepared materials were studied by various characterization techniques.In addition,the active species of the reaction were determined through trapping experiments to further study the transport mechanism of the reaction electrons.（3）UiO-66/Vs-Zn₃In₂S₆composites（UiO/Vs-ZIS）were prepared by a simple in situ hydrothermal method.The photocatalytic capacity of the prepared materials was evaluated by the experiments of hydrogen evolution and H₂O₂preparation under visible light.Compared with pure Vs-Zn₃In₂S₆and UiO-66,the photocatalytic activity of UiO/Vs-ZIS was significantly improved.When the weight ratio of UiO-66 was 5wt%,the photocatalytic hydrogen development rate of the composite was 5.36 mmol g^-1h^-1,which was 2.8 times higher than that of the pure Vs-Zn₃In₂S₆.The apparent quantum yield（AQY）was 21.9%at 420 nm.The H₂O₂two-hour yield was 161.62μM,about 2.06 times that of Vs-Zn₃In₂S₆.In addition,5UiO/Vs-ZIS had good H₂and H₂O₂preparation performance in 5 cycles and in Yangtze River water and seawater.Therefore,it showed that UiO/Vs-ZIS composites had excellent stability and practical application potential.In addition,the mechanism of producing H₂and H₂O₂from UiO/Vs-ZIS composites was studied in detail.