Preparation,characterization Of ZnO And Metal-doped ZnS Nanosheets And Their Photocatalytic Performance

In the progress of photocatalytic reduction of CO₂,the adsorption and activation of CO₂ on the surface of the photocatalyst is the first and key step to continue the reaction.This paper aims to explore a new way to improve the CO₂absorption ability on the surface of photocatalysts in order to improve the efficiency of the CO₂ photoreduction reaction.Thus,we designed two kinds of photocatalysts:one is about ZnO nanomaterials,we controlled the morphologies,surface areas and surface defects of ZnO nanomaterials in order to in-situ form basic zinc carbonate which was active intermediates among H₂O and CO₂ atmosphere and improve the efficiency of this reaction;the other is using metal doped ZnS-ethylenediamine?En?inorganic-organic hybrid nanosheets as photocatalysts,we used organic alkali to enhance CO₂ chemisorb and activation ability and doped sulfide-based semiconductors with narrowing band into the hybrid nanosheets to improve the vision light absorption ability of the photocatalysts.Three ZnO samples with different morphologies,surface areas and defect contents were fabricated by calcinating nitrate,solvothermal route and calcinating ZnS?En?_0.5nanosheets in the air,characterized by SEM,TEM,XRD,FT-IR,room temperature PL spectra,UV-vis,TPD and discussed the influence of morphologies,structure and band gaps of the photocatalysts to the photoreduction of CO₂.The results showed that the morphologies of as-prepared ZnO nanomaterials were nanoparticles,nanorods and nanosheets,but they are all wurtzite.The ZnO nanosheets exhibited more porous structure,bigger surface areas,more zinc vacancies and oxygen vacancies defect sites,better light absorption ability and separation ability of photogenerated electrons and holes compared with ZnO nanoparticles and nanorods.Bigger surface areas and more defects of ZnO nanosheets can promote the absorption of H₂O and CO₂ and formation of basic zinc carbonate,which was active intermediate species for photoreduction of CO₂ to CO and CH₄.The production rate for H₂,CO and CH₄ were 112.69?mol/g_cat/h,406.77?mol/g_cat/h and 20.16?mol/g_cat/h under UV light when the reaction temperature was 200 ^oC using this as-prepared ZnO nanosheets as photocatalyst,which was much higher than using ZnO nanoparticles and nanorods as photocatalysts.On the contrary,ZnO nanoparticles and ZnO nanorods had few defect sites,leading to weak adsorption for CO₂,thus they showed inferior photocatalytic activities.Cu²⁺,Co²⁺and Ni²⁺doped ZnS-En nanosheets were fabricated by ethylenediamine solvothermal route,characterized by many equipments,such as SEM,TEM,UV-vis,PL and so on,and tested in photoreduction of CO₂.The results showed that although nanosheets were damaged with increasing of the content of doping metals,the metal doped hybrid photocatalysts remained the morphologies of nanosheets basically.When Cu was doped into the ZnS nanosheet,CuS nanoparticles became the primary form of Cu ion.A lot of worm-like holes were formed on the ZnS nanosheet and surface areas of the samples were increased;when Co and Ni were doped into the ZnS nanosheets,metel ions were doped into the crystal lattice,which had little influence on the morphologies and structures of ZnS nanosheet.After Cu,Co and Ni ion were doped into the ZnS nanosheets,the bad gap of the hybrid photocatalysts were narrowed down,which expanded the light absorption spectrum of the photocatalysts to the visible light region.Doping metal ions could improve the efficiency of photoreduction CO₂.In particular,1%Cu-ZnS-En sample showed the best photocatalytic performance,which production rate for CO,H₂ and CH₄ were 93.62?mol/g_cat/h,60.79?mol/g_cat/h and24.27?mol/g_cat/h under visible light when the reaction temperature was 200 ^oC.The enhanced photocatalytic ability might be due to appropriate band gaps,better light absorption ability and strengthened CO₂ chemisorb and activation ability which was caused by the component of ethylenediamine of the metal doped ZnS-En hybrid photocatalysts.