Study On Structure/Valence Transition And Their Mechanisms Of Small-Size Gold-Based Nanoclusters

Metal nanoclusters,as a new kind of functional nanomaterial,have attracted more and more attention from researchers because of their promising applications in detection,sensing,catalysis,bio-imaging and biomedicine.Nanoclusters exhibit a metal core protected by surface ligands,and their sizes lie between metal-organic complexes and metal nanoparticles,which is a key link in the evolution of metal condensed matter.One of the most interesting aspects of nanoclusters is their atomically precise structure,which provides a suitable platform for studying the properties of nanoclusters.On the one hand,the structures of nanoclusters provide an accurate theoretical model for studying their size evolution,fluorescence origin and catalytic mechanism,so that the relevant mechanism research can be carried out at the atomic level.On the other hand,the structure-property relationship established based on the precise structure of nanoclusters makes it possible to regulate the properties of nanoclusters by adjusting and changing their structures.In this context,the transformation of nanoclusters is considered to be the most suitable research object for understanding the size evolution of nanoclusters and exploring their structure-property relationship because it can establish the structural correlation between nanoclusters.However,the implementation of inter-nanocluster transformation is often random,which greatly restricts the research on mechanism and property of this process.It is still very challenging to realize the transformation between nanoclusters in a controllable and directional way,and the relevant methods of studying the evolution mechanism and properties of nanocluster based on the transformation process between nanoclusters are still immature.Therefore,this work aims to control the transformation between nanoclusters and to develop methods and systems suitable for the research of transformation between nanoclusters.The main contents and achievements are as follows:1.A controlled diphosphine-mediated method has been developed to achieve directional tailoring of thiolate-protected gold nanoclusters.On the one hand,in order to obtain the minimum size of nanoclusters and explore the nucleation mechanism of nanoclusters,using small-size nanoclusters as precursors,three transitional size gold nanoclusters were synthesized via this method and their crystal structures were characterized,they are Au₁₃（SR）₈L₂（BPh₄）(Au₁₃ for short),Au₁₄（SR’）₁₀L and Au₁₆（SR’）₁₁L’（R=adamantyl,R’=cyclohexyl,L=diphenylphosphonbutane,L’=1-diphenylphosphone-2-phenylphosphone-benzene）.By combining the crystal structure of Au₁₃ and DFT calculation,the evolution pathway from Au₁₂（SR）₁₂ complex to Au₁₃nanocluster was revealed,and the nucleation mechanism was revealed to be the transformation from Au₄S₄ ring to Au₅ kernel during this process.In addition,the atom-by-atom evolution pathway from Au₁₂（SR）₁₂ to Au₁₈（SR）₁₄ was completely revealed in this work.On the other hand,the role of different diphosphine ligands in the regulation of nanocluster size was systematically studied using Au₂₃（SR）₁₆^- as a template via this controlled diphosphine-mediated method.Crystal structure analysis shows that when the length of diphosphine ligand is short,the tailoring product of Au₂₃（SR）₁₆^- would be the Au₂₁（SR）₁₂L₂⁺（L=diphosphine）,while when the length of diphosphine prolong to be close to the length of Au₁（SR）₂ unit on the surface of Au₂₃（SR）₁₆^-,the tailoring product would be the Au₂₂（SR）₁₄L.Further,the introduction of rigid groups into diphosphines can give Au₂₂（SR）₁₄L with a distorted structure or Au₂₅（SR）₁₆L₂⁺with a new surface configuration.2.The spontaneous transformation of gold nanoclusters and its mechanism were studied systematically.On the one hand,adamantanethiol-protected gold nanocluster Au₂₂（SR）₁₆ was synthesized and its crystal structure was characterized.It was found experimentally that Au₂₂（SR）₁₆ could spontaneously transform into Au₂₁（SR）₁₅.Kinetics study showed that the spontaneous transformation process obeys first-order kinetics and the reaction rate has obvious solvent effect.Compared with Au₂₁（SR）₁₅,Au₂₂（SR）₁₆ inserted an additional Au-SR into the peripheral structural unit.The structural difference between them causes the obvious difference of absorption spectrum and emission spectrum between them.On the other hand,Au₂₂（SR）₁₅H nanoclusters containing hydride ligands were synthesized by using the steric thiolate ligand dimethyl-adamantanethiol.Au₂₂（SR）₁₅H in solution can spontaneously undergo isomerism transformation to form Au₂₂（SR）₁₅H in another configuration,and this process is reversible.Combining crystal data and theoretical calculations,the structure of the nanoclusters and the positions of the hydride ligands were determined.DFT calculation showed that the transformation process is caused by the migration of[Au-H]unit in the nanocluster.More importantly,by introducing phosphine ligands into nanoclusters,the intermediate of the transformation process was captured,of which the structure is highly similar to that of the theoretically calculated intermediate.It is the first time that reversible isomerization has been found in hydrido nanoclusters,and the transformation mechanism has been revealed by combining theoretical calculation with the capture of intermediates by phosphine ligands.3.The reversible transformation of charge state in Au-Cu alloying nanoclusters was studied and realized.An Au-Cu alloying nanocluster[Au₁₈Cu₃₂（SPh Cl）₃₆]^2-was synthesized.Its conversion to the–3 charged nanocluster[Au₁₈Cu₃₂（SPh Cl）₃₆]^3-and its reverse process were achieved by electrochemical redox methods,addition of H₂O₂/Na BH₄ and oxidation by silica in air atmosphere.Crystal structure analysis shows that the structure of nanoclusters,especially the surface structure,is significantly affected by the change of charge state.This is the first time that the reversible charge state transformation of Au-Cu alloy nanoclusters has been achieved experimentally.This study provides a new perspective for understanding the charge state transformation process of Au-Cu alloying nanoclusters.