Synthesis And Photocatalytic Performance Study Of Metal Nanoclusters And Their Derivatives

Metal nanoclusters and their derived materials have become an ideal photocatalyst material system by virtue of their unique advantages such as definite composition structures and strong designabilities,and have shown a broad application prospect in the field of photocatalysis.However,their applications in photocatalytic field are still in their infancy and problems remian,such as low visible light photocatalytic activity,poor stability,as well as limited types of materials and reactions.So how to achieve a perfect balance between photocatalytic activity and photostability through reasonable design,and further expand the catalyst material system and the type of catalytic reaction are of great significance for the practical application of metal nanoclusters in the field of photocatalysis.In this thesis,a series of new type metal nanoclusters-based photocatalysts with high efficiency and stability have been synthesized through the reasonable structural and functional design,employing metal-organic frameworks construction and ligand engineering method.Furthermore,the photocatalytic performance,structure-activity relationship and photocatalytic mechanism have also been explored.Specific research contents are as follows:A series of new Ce-oxo cluster-based metal-organic framework materials,Ce-UiOs have been synthesized by solvothermal method using the coordination of Ce-oxo cluster with different organic ligands.The photocatalytic performance,photostability and photocatalytic reaction mechanism of Ce-UiOs for the selective catalytic oxidation of benzyl alcohol under visible light have been studied further.The results show that Ce-UiOs have better visible light photocatalytic activities and photostabilities than pure Ce-oxo clusters.Compared with Zr-UiOs,Ce-UiOs have stronger visible light absorption capacities and photocatalytic activities.This can be attributed to the more negative ligand-metal charge transfer energy in Ce-UiOs,which is more favorable to the separation of photocarriers.Moreover,the structures of organic ligands will significantly affect the carrier separation efficiency of the photocatalysts,thus influencing their photocatalytic activities.In addition,the introduction of the unique 4f vacant orbital of Ce is beneficial to the expansion of the light absorption threshold.The corresponding mechanism investigation discloses that the Ce-oxo cluster undergoes valence state changes and activates O₂ to generate superoxide radicals which further oxidize the substrates.Four different organic ligands,chiral NHCs,2-phenylethylmercaptan,Captopril and organophosphorus are selected,and four kinds of Au nanoclusters Au₁₀（CNHC）₆Cl₃,Au₂₅（PET）₁₈,Au₂₅（Capt）₁₈and Au₁₃（PR）₅Cl₂ based on these ligands are prepared through direct reduction methods.Their composition and optical properties are studied by UV-Vis spectroscopy,electrospray ionization mass spectrometry and circular dichroism spectroscopy.Compared with Ce-oxo cluster and Ce-UiOs,the bandgaps of these Au nanoclusters are all less than 2.1 e V,which exhibit better ability to absorb visible-light.Moreover,their bandgaps are all greater than the activation energy of the singlet oxygen（～0.97 e V）,which means they all have the abilities to activate oxygen to produce singlet oxygen.And the circular dichroism results show that the Au₂₅（Capt）₁₈ and Au₁₀（CNHC）₆Cl₃ nanoclusters possess good optical activities.Meanwhile,the chiral Au₁₀（CNHC）₆Cl₃ nanocluster is prepared here for the first time and possesses a series of absorption characteristics of circular dichroism in 290～470 nm region.The absorption anisotropy factor reaches 2.2×10^-3 at 294 nm.Taking selective oxidation of sulfide as a model reaction,the photocatalytic activities of Au₁₀（CNHC）₆Cl₃,Au₂₅（PET）₁₈,Au₂₅（Capt）₁₈and Au₁₃（PR）₅Cl₂nanoclusters are studied.And the results reveal that the prepared Au₁₀（CNHC）₆Cl₃nanocluster has good photocatalytic activity,the conversion and selectivity are close to 100%.Moreover,compared with the thiol and organophosphorous ligand protected Au nanoclusters,such as Au₂₅（PET）₁₈,Au₂₅（Capt）₁₈ and Au₁₃（PR）₅Cl₂,the Au₁₀（CNHC）₆Cl₃ nanocluster shows the best photostability and retains 99.1%conversion and 91.7%selectivity after three photocatalytic cycles.Further photocatalytic mechanism studies through electron spin resonance analysis confirm its ability to generate ¹O₂,and further verify its strong photostability.This thesis takes metal nanoclusters as the core material and constructs a series of metal nanoclusters-based photocatalysts with promoted photocatalytic activities and stabilities through rational design of structure and function.