Theoretical Study On The Nucleation Growth Mechanism,Stability And Catalytic Properties Of Ligand Protected Gold Nanoclusters

The synthetic chemistry of precious metal nanoclusters,particularly gold nanoclusters,has developed rapidly recent years,and a large number of ligand-protected gold nanoclusters with size less than 2 nm and atomically precise composition and structure have been successfully synthesized and structurally resolved by experimental single crystal X-ray diffraction.These clusters have attracted widespread interest due to their unique size-dependent physicochemical properties.Though experimental work on the synthesis and structure determination of ligand-protected gold nanoclusters has made great progress,and theoretical studies have carried out extensive analytical and structural prediction work on the unified structural expressions of such nanoclusters and their electronic structures,the understanding of the nucleation mechanisms,structural evolution patterns,thermodynamic stability and ligand effects of the clusters is not yet advanced enough.In this research,the elementary reaction steps of nucleation and growth,descriptors of thermodynamic stability,structure prediction of large-sized nanoclusters,catalytic properties and optical properties et al.of thiolate ligand protected gold nanoclusters were investigated in detail based on density functional theory（DFT）.The nucleation and growth of Au^（I）-SR oligomers with the reducing agent Na BH₄into thiolate-protected gold nanoclusters（SR-Au NCs,Au_n（SR）_m）was investigated at the atomic level,basing on the 2e^–reduction mechanism discovered in previous experiments.AIMD simulations and DFT calculations results showed that the initial reactant was determined to be[BH₄-Au_n（SR）_n]^–,following three active intermediates containing Au-H or Au-H-Au,[H-Au_n（SR）_n]^–,[H-Au_n（SR）_nBH₃]^–and H-Au_n（SR）_n-1were obtained by hydrolysis or desorption of small molecules.Then,these three types of reactive intermediates may react with reaction precursors or other reactive intermediates,and finally generate 2e^–gold nanoclusters with Au₃ or Au₄ cores.The generation of reactive intermediates containing Au-H or Au-H-Au and the nucleation reaction are the key steps of the 2e^–reduction mechanism.A new method to describe the thermodynamic stability of Au_n（SR）_m were investigated by proposing two thermodynamic descriptors,the average formation energy(E_ave)and the average ligand stabilization energy（ALSE）,which relate the thermodynamic stability of Au_n（SR）_m to their atomic structure and ligand stability effects.Taking 48 gold nanoclusters with definite atomic composition reported,the E_aveand ALSE of them were calculated.The results showed a good linear relationship between E_ave and N_Au/N_SR and between ALSE and N_Au/N_SR.Therefore,E_ave and ALSE can be used as descriptors to study the thermodynamic stability and ligand effect of Au_n（SR）_m.The structure and properties of Au₃₁₇（SR）₁₁₀ nanocluster with gold atomic number greater than 300 were predicted for the first time by theoretical calculations.A large-scale gold nanocluster Au₃₁₇（SR）₁₁₀ with an icosahedral core was constructed based on the structural laws of experimentally synthesized large-scale nanoclusters such as Au₂₄₆（SR）₈₀ and Au₂₇₉（SR）₈₄ et al.The E_ave,radial distribution functions,ultraviolet-visible（UV-vis）absorption spectra of Au₂₄₆（SR）₈₀,Au₂₇₉（SR）₈₄ and Au₃₁₇（SR）₁₁₀nanoclusters were compared.And the compatibility of ligand of Au₃₁₇（SR）₁₁₀nanocluster under the stabilization of two real ligands S-PhCOOH（p-MBA）and S-CH₂Ph was investigated.The calculation results showed that the Au₃₁₇（SR）₁₁₀nanocluster may be in metallic state,and the nanocluster has good structural stability,its possible ligand is p-MBA.The catalytic performance of gold nanoclusters protected by thiolate ligands in hexagonal close-packed configuration was investigated.A series of hexagonal close packing（hcp）and double hcp（4H）gold nanoclusters were formed by structural evolution in one-dimensional direction with face-centered cubic（fcc）Au₄₀（SR）₂₄nanocluster as seed nanocluster,and the differences between the structures and properties of gold nanoclusters with different packing styles were explored.The E_ave,UV-vis and absorption spectrum et al.of Au₇₁（SR）₃₆-4H and Au₇₁（SR）₃₆-hcp nanoclusters showed that they have good structural stability and little difference in optical and electronic properties.Taking Au₇₁（SR）₃₆ nanocluster as an example,the differences in the catalytic CO oxidation performance between 4H and hcp configurations were investigated by removing a monomeric motif from the surface of the clusters.It was found that gold nanoclusters with different inner core stacking modes have different rate-determining step of catalytic reactions due to differences in the environment near the surface reaction sites,which in turn affects its catalytic performance.The performance of Au₂₂（L⁸）₆ nanocluster and nanoclusters with phosphine ligands substituted by N-heterocyclic carbene（NHC）for the electrocatalytic CO₂reduction reaction were studied.The stable structures of Au₂₂（L⁸）_6-x（NHC）_2xnanoclusters（x=0-6）,which NHC-doped Au₂₂（L⁸）₆ nanoclusters with different contents were determined by calculation,and their different reaction pathways for the electrocatalytic CO₂ reduction reaction were calculated.The results showed that by adjusting the doping amount of NHC at Au₂₂（L⁸）_6-x（NHC）_2x nanoclusters（x=0-6）,the main products of electrocatalytic reduction of CO₂ are converted among formic acid,formaldehyde and methanol.