Performance Of G-C₃N₄ NanoSheets/porous ZnO Composite Photo-catalyst For Photo-catalytic CO₂ Reduction

Photocatalytic CO₂ reduction coupled with the splitting of H₂O to produce fuel such as CH₄,CO and H₂,etc.using solar energy can achieve the conversion and storage of solar energy while simultaneously utilizing CO₂ as carbon resource,which has potential application in solving energy crisis and environmental issues.Therefore,3 series of photo-catalysts were designed,prepared and characterized via SEM,TEM,XRD,FT-IR,PL,UV-vis and so on to investigate the influences of morphology,composition,structure,photo-electric property and band structure of photocatalyst on the performance of photocatalytic reduction of CO₂.ZnS with different morphologies were fabricated successfully through adjusting volume ratio of ethanediamine/water.Research result showed that the concentration of ethylenediamine would have a great influence on the morphology,specific surface area and crystal type of ZnS,which can lead to the formation of defect sites in the ZnS.These defect sites can strengthen the visible light absorption ability,reduce the recombination chance of photo-induced electrons and holes,increase CO₂ adsorption and activation ability and thus improve the photocatalytic reduction capacity of CO₂,which is mainly attributed to the strong reducibility of ethylenediamine and high p H.In addition,the binding of small amount of ethylenediamine on the surface of catalyst can also improve the ability of CO₂adsorption and activation.With the increase of the adding amount of ethylenediamine,the photocatalytic reduction activity of CO₂ over ZnS first increased and then decreased.ZnS with volume ratio of ethanediamine/water=1 performed the highest yield of CO and CH₄ with 4.47μmol/g_cat/h and 11.57μmol/g_cat/h respectively.A ternary composite photocatalysts for ZnS-ZnO/g-CN were prepared by hydrothermal and calcination methods.Studies showed that the addition of melamine can generate thin carbon nitride（g-C₃N₄/g-CN）films in hydrothermal process,improving the specific surface area of catalytic materials,extending performance of visible light absorption,enhancing the separation efficiency of photo-generated carriers,and facilitating the ability of CO₂ adsorption and activation on the surface of the photo-catalyst.With the increase amount of melamine,the photocatalytic activity of composites reduced after rising to the peak.ZnS-ZnO/g-CN with g-CN films mass fraction of 0.5%showed the best photo-activity and product yields of CO and CH₄ were6.47μmol/g_cat/h and 12.32μmol/g_cat/h respectively,which is ascribed to the improvement of adsorption and activation of CO₂ as well as enhancement of visible light absorption and the formation of heterojunction structure between ZnO with ample defects,ZnS and g-CN films after the introduction of g-CN films,effectively improving the separation performance of photo-generated carriers.g-CN films were first thermally deposited on ITO-supported porous ZnO via a two-step process.The loading amount of g-CN films can be controlled by changing amount of g-CN particles and its influence on photo-catalytic CO₂ reduction performance was further discussed.Research results confirmed that the g-CN films can significantly extend the absorption capacity of porous ZnO to visible light,increase the photo-generated carrier life of the photo-catalyst,reduce the electrochemical impedance,and improve the photoelectric current response.With the increase of g-CN films,the photo-catalytic activity of ZnO@g-CN first increased and then decreased.The yields of H₂,CH₄ and CO over PNS-ZnO@NF-g-CN-0.4 with g-CN films mass fraction of～2.6%were the highest,reaching 22.7μmol/g_cat/h,30.5μmol/g_cat/h and 16.8μmol/g_cat/h respectively,which may be attributed to the improvement of adsorption and activation of CO₂ on porous ZnO nano-sheet and g-CN films and the formation of heterojunction structure of the composite catalyst.