Theoretical Study On The Structure-Activity Relationship Of Silver-Gold Nanoalloys

Due to its unique atomic and electronic structure,high specific surface area,and quantum confinement effects,the silver-gold nanoalloys exhibit the unique surface activity,chemical stability and biocompatibility.They are widely applied in the fields of electricity,chemical sensing,optics,catalysis and so on.Morphology,size,composition and chemical order play important roles in tuning electronic,sensing,catalytic,optical properties of metal nanoalloys,which makes atomic and electronic structure more complicated.Besides,thiolate-protected Ag-Au nanoalloys exhibit unique molecular properties,making the optical properties of this system particularly unique,and therefore attract more and more researchers' attentions.In this thesis,the electronic and atomic structures of Ag-Au nanoalloys are taken as the starting point.Density functional theory(DFT)method is utilized to investigate the effects of various substances on the electronic,sensing,optical and catalytic properties of Ag-Au nanoalloys.The main research contents and conclusions are as follows:1.Tuning the electronic performance of Ag-Au nanoalloys through ligands.In the experimental and biological environments,various ligands play role in tuning the electronic properties of the Ag-Au nanoalloys.Specifically,the morphology,composition,size,and chemical order of the silver-gold nanoalloys are affected by small molecules and sulfur atom.(1)The interactions between 13-atom Ag-Au nanoalloys(Au13,Au12Ag,Ag12Au,Ag13)and ten ligands(-CN,-COOH,-CH3,-OH,-SH,-NH3,-NO,-NO2,-N(CH3)2,-PH3)are investigated here using DFT method.The adsorption strengths of Ag13 cluster toward these ligands follow the order:Ag13-CN>Ag13-SH>Ag13-OH>Ag13-COOH>Ag13-CH3>Ag13-NO2>Ag13-N(CH3)2>Ag13-NO>Ag13-PH3>Ag13-NH3.As for the same ligand,the adsorption strengths of 13-atom Ag-Au nanoalloys follow the order:Ag 12Au-X>Ag13-X>Au13-X>Au 12Ag-X.Our results also show that the morphology of Au-rich Ag-Au nanoalloy would transfer from icosahedron to truncated octahedron while adsorbing-NO,-N(CH3)2 and-PH3 ligands.The conclusions from our theoretical results are promising to provide guidelines for preparing Ag-Au nanoalloys in the experiments.(2)The adsorption strengths of Ag-Au nanoalloys toward sulfur atom are inverstigated via DFT method,leading to better knowledge of the size,morphology,composition and chemical order effects.The adsorption strengths of Agn and Agn-1 Au(n=13,55,147)clusters toward sulfur atom decrease with increasing the size of clusters.Comparison of morphology also shows that the truncated octahedral Ag42Au13 cluster possesses stronger adsorption strength toward sulfur atom compared to icosahedral and dechedral Ag42Au13 cluster.With increasing the composition of Au,the adsorption strengths of Ag-Au nanoalloys toward sulfur atom increase following the order:Ag55,Ag54Au,Ag42Au13,Au55 cluster.The chemical order is proved to play an important role in electronic performance of Ag-Au nanoalloys via investigating six Ag42Au13 clusters with various chemical order.Our results are promising for guiding preparation and design of Ag-Au nanoalloys utilized in the field of sensing.2.Tuning the sensing performance of Ag-Au nanoalloys through sulphide.We investigated the effects of sulphide on the sensing performance of Ag-Au nanoalloys by DFT and TDDFT methods.As a specific sensor or selective sensor,Ag-Au nanoalloys have been widely used for detecting sulphide in biological environments.The key factors of sensors are sensing mechanism and sensitivity;therefore,investigations are carried out as follows.(1)Taking sulphide atom as an example,we studied the sensing mechanism and tunable sensitivity of Ag-Au nanoalloys as a specific sensor.The interaction between[Ag13]+ cluster and sulphide atom leads to significant changes in the optical spectra,which is the sensing mechanism of Ag-Au nanoalloys.Moreover,the[Ag12Au]+ cluster is more sensitive than the[Ag13]+and[Au13]+ clusters toward sulphide atom in the optical spectra.Besides,the coverage of sulphide atom in the biological environment also affects the optical spectra of[Ag12Au]+ cluster.Compared to the icosahedral[Ag12Au]+cluster,the truncated octahedral[Ag12Au]+ cluster is more sensitive toward sulphide atom in the optical spectra.(2)Taking S,SH,Cys,H2S as examples of reactive sulphide species,we studied the selectively sensing mechanism and tunable selectivity of Ag42Au13 and Ag55 nanoalloys as a selective sensor.Compared to the Ag55 cluster,the Ag42Au13 cluster possesses stronger adsorption strengths toward these reactive sulphide species,which is due to the fact of increased hybrid coupling between silver and gold atoms caused by reactive sulphide species.The adsorption strengths of Ag42Au13 and Ag55 clusters toward reactive sulphide species follow the order:S,SH,Cys,H2S.Moreover,the coverage of sulphide atoms in biological environments tend to be inversely proportional to the adsorption strengths of Ag42Au13 cluster.Our results are promising to provide theoretical guidelines for designing efficient and low-cost Ag-Au nanoalloys utilized in the field of sensing.3.Tuning the optical performance of Ag-Au nanoalloys through thiolate ligands.We investigated the effects of thiolate ligands on the optical performances of Ag-Au nanoalloys by TDDFT method.Thiolate-protected Ag-Au nanoalloys exhibit unique molecular properties,making the optical properties of this system particularly unique.Based on the theoretical results of optical spectra,combining with independent component map of oscillator strength(ICM-OS)analytical method,the origin reason of thiolate tuning the optical performance of Ag-Au nanoalloys is determined.(1)Investigation of optical spectra of(Au-Ag)36(SR)24(R=CH3,Ph)with doping Ag atoms was carried out.Theoretical results of various doping sites and numbers of doping Ag atoms show that,Ag atoms prefer energetically to dope on the Ex-Td sites in the(Au-Ag)36(SCH3)24 nanomolecules while doping only a few Ag atoms.However,Ag atoms prefer energetically to dope firstly on the 4 Ex-St-Near sites and then on the 4 Ex-Td sites for the high-doping cases,termed as Ag8Au28(SCH3)24[4Ex-St-Near&4Ex-Td].Compared to experiments,theoretical optical spectra of low-doping case,Ag4Au32(SPh)24[4Ex-St-Near]nanomolecule,agrees well with the experimental optical spectra for the incoming gold-to-silver metal ratio of 1:0.1 and 1:0.15 cases.However,for the theoretical optical spectra of high-doping case,AggAu28(SPh)24[4Ex-St-Near&4Ex-Td]nanomolecule,agrees well with the experimental optical spectra for the incoming gold-to-silver metal ratio of 1:0.2 cases.ICM-OS plots provide evidence that the increase of total oscillator strength is determined by the combing contribution of dipole and oscillator strength of independent component,[<?i|Z|??>Pi?(Z)].(2)Investigation of optical spectra of Au36(SePh)24 nanomolecule was carried out by TDDFT method.Compared to Au36(SPh)24 nanomolecule,the Au36(SePh)24 nanomolecule possesses stronger oscillator strength in the LV-visible region of optical spectra,which is consist with experimental conclusions.ICM-OS plots provide evidence that long-range off-diagonal single-particle positive contributions of the Au36(SePh)24 nanomolecule are the origin reasons of stronger oscillator strengths in the optical spectra.Our work verifies that the model obtained by theoretical calculation could efficiently predict experimental optical spectra,and this also provide guiding directions for theoretically predicting optical spectra in the future.4.Tuning the hydrogen evolution reaction catalytic mechanism of Ag-Au nanoalloys considering solvent effect.By analyzing each step of electrochemical hydrogen evolution reaction via DFT method,investigation on catalytic mechanism of Ag12Au cluster as electrochemical HER catalyst is carried out.The Ag12AuHm nanocomposite firstly reaches the "resting state"during the reaction,that is,the saturated Ag12AuH11 is reached.Then the Volmer-Heyrovsky path is followed to complete with a biased environment of 0.9 V.The first step(Ag12AuH11?Ag12AuH12)is the rate-determine step of the entire reaction with an energy barrier of lower than 0.742 eV;the second step(Ag12AuH12?Ag12AuH13)is performed spontaneously by thermodynamics,and the reaction quickly produces H2 molecule.Considering solvent effect,our research shows that the increase of the chemical affinity in the Ag12AuH11 nanocluster(specifically the charge process and the protonation process)leads to a decrease of the energy barrier of HER,so that the first and second steps of the Volmer-Heyrovsky mechanism can reach better balance.Therefore,this finding gives us an idea for future catalyst design:when a third metal is introduced,the catalyst may be more easily attached to the electrode and the process of electron exchange may be quickly achieved.The catalytic mechanism we obtained can be used in the design of catalyst,especially for improving the catalytic efficiency.Another significance of this research is that,Ag-Au nanoalloys are also expected to become a substitute for Pt catalyst as one of the low-cost,high-efficiency catalyst options.