Design,Synthesis And Catalytic Performance Of Ni(Fe,Cr)-Based Layered Double Hydroxide Heterojunction Photocatalysts

Due to excessive discharge of organic contaminants,exhaustion of fossil fuel resources and accompanied release of harmful gases in modern society,it is crutial to develop new technologies for environmental purification and renewable energy production.Photocatalysis capable to degrade harmful pollutants,generate H₂ and O₂ by water splitting,and convert CO₂ to hydrocarbon fuel by utilizing solar energy.One of the pre-requirements to commertial application of photocatalysis is the development of environmental friendly,efficient and inexpensive photocatalysts.In this thesis,we focus on design and synthesis of highly efficient heterojuction composite photocatalystic systems based on noble-metal-free and visible light-responsive semiconductors with staggered band alignment.The photocatalytic performance was evaluated by degradation of organic pollutants,water splitting and reduction carbon dioxide.A systmetic study was carried out to explore the structural and morphological,electronic optical and photoelectrochemical properties,and the charge transfer mechanism.Layered double hydroxides（LDHs）are a family of two-dimensional layered materials,with the advantages of low-cost,high chemical stability,large specific surface area,adjustable chemical composition and band structure,and excellent adsorption properties.Thus,LDHs provide an ideal platform for designing heterojunction composite photocatalysts.This paper explored three LDH-based heterojunction composite photocataly tic systems,including:（1）NiFe-LDH/g-C₃N₄ heterojunction photocatalysts:Samples with different molar ratios of Ni/Fe were prepared by a simple hydrothermal method,and the photocatalytic activity was evaluated by degradating methylene blue（MB）and water splitting under visible light.It is found that the removal efficiency of MB over NiFe-LDH/g-C₃N₄（94%）was 6 times that of pure NiFe-LDH and hydrogen production（212.9μmol/g/h）of NiFe-LDH/g-C₃N₄was 53 times that of pure NiFe-LDH.Based on the results of trapping experiments and ESR measurements,the enhanced photocatalytic performance of NiFe-LDH/g-C₃N₄ was attributed to the Z-scheme charge transfer mechanism,i.e.the photoexcited electrons in lower conduction band（CB）of NiFe-LDH recombine with photoexcited holes in the higher valuence band（VB）of g-C₃N₄ while chage carriers with stronger redox ability（electrons in g-C₃N₄and holes in NiFe-LDH）diffuse to surface to participate photocatalytic reactions.（2）NiFe-LDH/Cu₂O heterojunction photocatalyst:Samples with different reaction time were prepared by co-precipitation method and the photocatalytic performance was evaluated by MB degradation and CO₂reduction under visible light.It is found that the removal efficiency of MB over NiFe-LDH/Cu₂O from 20%for Cu₂O and 45%for NiFe-LDH increased to 93%.In addition,CH₄ production of NiFe-LDH/Cu₂O was approximately 5.6 and 6.9 times that of NiFe-LDH and Cu₂O,respectively.Based on the results of trapping experiments and ESR measurements,the enhanced photocatalytic performance of NiFe-LDH/Cu₂O was attributed to the Z-scheme charge transfer mechanism,i.e.the photoexcited electrons in the CB of NiFe-LDH recombine with photoexcited holes in the VB of Cu₂O while chage carriers with stronger redox ability（electrons in Cu₂O and holes in NiFe-LDH）diffuse to surface to participate reactions.（3）NiCr-LDH/Cu₂O heterojunction photocatalyst:Samples with different reaction temperatures were prepared by co-precipitation method and the photocatalytic activity was measured by degradating MB and water splitting.It is found that the removal efficiency of MB over NiCr-LDH/Cu₂O（82%）was about 4 times that of Cu₂O and 2.8 times that of NiCr-LDH.And hydrogen production after adding sacrificial agents（Na₂SO₃and Na₂S）was 11 times that of in pure water.Based on a detailed study of structural,optical and electrochemical properties,Z-scheme photocatalytic mechanism was proposed to ex plain the enhanced photocatalytic performance of NiCr-LDH/Cu₂O.