Thermodynamics And Electrochemical Properties Of (Au,Ag)-Co Alloy Nanoparticles

The mixture of two or more metallic elements in metallic materials to produce intermetallic compounds and alloys can greatly expand the range of applications of alloys due to synergistic effects that enhance their specific properties.The desire for nanocomposites with well-defined and controlled properties and structures,together with the flexibility offered by intermetallic compounds,has led to a keen interest in nanoalloys.In this thesis,the synthesized Au-Co alloy nanoparticles（NPs）were firstly coated with SiO₂ and characterized using ultraviolet-visible spectrophotometer（UV-Vis）,transmission electron microscopy（TEM）,and High-resolution transmission electron microscopy（HR-TEM）,and differential scanning calorimetry（DSC）was used to measure the melting temperature of Au-Co NPs;then the thermodynamic data of the nanoalloys of Au-Co,Ag-Co were optimized according to the Nano-CALPHAD thermodynamic model.The nanophase diagrams of Au-Co,Ag-Co of different sizes were evaluated;finally,the catalytic hydrogen precipitation properties of Au-Co with different cathode electrode substrates under alkaline conditions was investigated in combination with the catalytic properties of Co and Au stability;the main research of this thesis is as follows.（1）Au₆Co₄,Au₇Co₃,Au₈Co₂ NPs were prepared by chemical co-reduction using HAu Cl₄ and Co Cl₂ as precursors,and Au-Co@SiO₂ NPs with core-shell structured silica encapsulation were obtained in combination with the St（?）ber method,and the formation of the alloys was demonstrated using UV-Vis with TEM results.Melting information for Au₆Co₄@SiO₂,Au₇Co₃@SiO₂ and Au₈Co₂@SiO₂ was obtained using DSC at 1193.6 K,1243.9 K and 1211.5 K.The melting heat absorption signal of the Au-Co bulk alloy was also observed in the DSC curves,and the results indicate that the melting temperature of Au-Co NPs decreases with size The results indicate that the melting temperature of Au-Co NPs decreases with decreasing size.（2）The variation of surface tension,surface segregation and surface energy of Au-Co and Ag-Co alloys as a function of size,alloy composition and temperature was calculated and fitted according to Butler’s equation and the Liquid-drop model.The calculations show that Au and Ag surface tensions are more likely to be segregation on the nanoparticle surface due to their larger difference with Co.In the CALPHAD thermodynamic model,the surface Gibbs energy of Au-Co and Ag-Co nanoalloys of different sizes was introduced and combined with the DSC experimental data to optimize the thermodynamic parameters obtained for Au-Co and Ag-Co.The nanophase diagrams were evaluated for radii of 8.83nm,10.20 nm and 14.30 nm and 7.5 nm,13.0 nm,21.0 nm and Ag-Co phase diagrams at7.5 nm,13.0 nm and 21.0 nm,and zero-variance reaction data were summarised.The thermodynamic properties such as activity and enthalpy of Au-Co and Ag-Co nanoalloys at the corresponding sizes were calculated based on the nanophase diagrams.（3）The Au-Co/GO nanomaterials were obtained by loading the prepared Au-Co NPs onto graphene oxide（GO）in combination with hydrothermal method,and the materials were characterised using TEM and XRD.The electrochemical analysis performance of Au-Co/GO materials was designed and prepared using glassy carbon electrode（GCE）,carbon cloth（CC）,and nickel gauze（NG）as cathode substrates,and linear voltammetry scanning（LSV）,cyclic voltammetry（CV）,and alternating current impedance（EIS）methods under alkaline conditions to test the electrochemical hydrogen precipitation performance of Au-Co/GO materials,respectively.The comparison shows that Au₅Co₅/GO and Au₃Co₇/GO have higher catalytic hydrogen precipitation ability than GCE and CC as cathode electrode substrates and NG as electrode substrate,but Au₅Co₅/GO performs more stably in durability tests.