Morphologic and Structural Modulation of Novel Porous ZnCds Derived from ZIF-8 for Photocatalytic Hydrogen Evolution by Water Splitting

Author:Chen Jian Min

Supervisor:li ying wei


Degree Year:2019





Hydrogen has attracted much attention as the ideal clean energy with high energy density and efficiency.Hydrogen evolution from photocatalytic water splitting is one of the effective ways to alleviate the dependence on fossil resources and reduce environmental pollution.Up to now,all of the photocatalytic materials can not show satisfactory catalytic activity and recycling stability in the absence of cocatalyst that would greatly restrict their practical application.Metal-organic frameworks(MOFs)are a class of porous materials constructed by coordination of metal ions/clusters and oxygen-and/or nitrogen-containing organic ligands.Compared with traditional porous materials,MOFs with variable structure,large specific surface area and high porosity have attracted much research attention,particularly in heterogeneous catalysis.Based on the aforementioned backgrounds,the development of low-cost,highly efficient and stable catalysts is the key issue for the catalytic process.In this thesis,some new strategies for preparation of novel ZnCdS-based catalysts have been developed by using ZIF-8 material as both the precursor and self-template.The relationship between the structure and photocatalytic water splitting activity of the catalyst has been investigated in detail.As reported previously,semiconductor particles with smaller sizes will benefit for separation and transfer of the photogenerated electron-hole pairs,and thus achieving higher photocatalytic efficiency.Visible-light responsive ZnCdS QDs are designed and fabricated rationally by simple calcination and ion-exchange using zeolitic-imidazolate-framework-8(ZIF-8)as both the precursor and template.The ZnCdS QDs exhibit a hydrogen production rate of 3.70 mmol h-1 g-1 under visible-light irradiation,which is much higher than that of bulk ZnCdS,and even outperforms most of the ZnCdS solid-solutions reported previously under visible-light irradiation(λ>420 nm)and noble-metal cocatalyst-free conditions.The outstanding catalytic performance of the ZnCdS QDs could be ascribed to multiple vital merits,including the suitable band matching,strong electron coupling,uniform and abundant active sites,which would facilitate separation and migration of photo-induced charge carriers,and also shorten the charge diffusion distances.In order to further improve the catalytic activity,hollow dodecahedral ZnCdS is prepared via simple sulfurization and cation-exchange using ZIF-8 as the template.The hollow cages and mesoporous structures can endow the Zn1-xCdxS solid solutions with significantly improved visible-light utilization and charge carrier separation and transfer.In addition,BET surface areas of hollow ZnCdS cages are also significantly enhanced with the introduction of Cd as compared to that of ZnS cages,which may provide abundant exposed active sites and decrease the charge transport distance.Moreover,suitable band matching and strong electron coupling in these solid solutions may be simultaneously achieved via ion-exchange,featuring the balance between light absorption ability and the potential of conduction band of the Zn1-xCdxS photocatalysts.Consequently,the hollow Zn0.6Cd0.4S cage material exhibits the highest hydrogen production rate of 5.68 mmol h-1 g-1 under cocatalyst-free and visible-light irradiation(λ>420 nm)conditions.Furthermore,these hollow ZnCdS cages show excellent long-term stability,maintaining high photocatalytic activity for hydrogen evolution over a number of cycles.Catalysts with Z-scheme heterojunction structure usually exhibit higher catalytic activity as compared with single-component materials.Novel sandwich-shelled ZnCdS/ZnO/ZnCdS cages are fabricated with“one stone”-ZIF-8.The ZnCdS,derived from cation-exchanged ZnS,and ZnO with well-defined interfaces also have staggered band structure configurations by virtue of the fine adjustment of the composition.The Zn0.5Cd0.5S/ZnO/Zn0.5Cd0.5S cages exhibit a H2 production rate of 28.6 mmol g–1 h–1 and long-term durability,achieving the highest activities among ZnCdS and ZnO families under noble-metal-free conditions.The remarkable performance could be ascribed to the unique morphology of the sandwich-shell and hollow interior with integrate multiple vital merits for photocatalysis,such as the enhanced light-harvesting ability,abundant active sites,shortened charge diffusion distances,and Z-scheme mechanism featuring preserved strong redox ability and improved charge separation and migration.