Theoretical And Experimental Research Into The Phase Transition Thermodynamics And Electrochemical Thermodynamics Of Nanoparticles

The phase transition and electrochemistry problems of nanoparticles are always involved in the preparations,researches and applications of nanomaterials.The phase transition thermodynamics and electrochemical thermodynamics present great differences when compared with their bulk counterparts if the size of particles is reduced to nanometer scale.Currently,the rationality and application scope of different melting models and the influence regularity and the influence mechanism of shape on the phase transition temperature remain unclear,and the research into the electrochemical thermodynamics of the core-shell structural nanometer-sized electrode(double dispersed phases)has not been reported.Therefore,the aim of this subject is to investigate the above problems.(1)Theoretical and experimental research into the phase transition of nanoparticlesTheoretically,based on different phase transition models,the corresponding accurate relations were obtained to describe relations between the thermodynamic properties of phase transition,respectively,and particle size of nanoparticles with different shapes.Then by the comparison of the results of pure theoretical calculation and literature results,the rationality and respective application scope of different phase transition models and the influence regularity of shape on the phase transition temperature of nanoparticles were discussed.The results show that Reiss and Rie models are rational,and the Reiss model is applicable to the initial melting stage and the Rie model to the later stage.Moreover,when the particle size of nanoparticles is smaller(r < 10 nm),the thermodynamic properties of phase transition are influenced by the combined action of surface tension and specific surface area,and the smaller the particle size,the dramatic depression of the thermodynamic properties of phase transition is observed;when the particle size is larger,due to the almost unchanged surface tension,the thermodynamic properties of phase transition are mainly influenced by the specific surface area and present linear relationships with the reciprocal of particle size.For nanoparticles with different shapes but in indentical volume,the specific surface area becomes the major influence factor on the thermodynamic properties of phase transition,and the the more the shape deviates from sphere,the lower the thermodynamic properties of phase transition.Experimentally,the melting of nano-Cu and the phase transition of nano-Pb Ti O3 are chosen as the research systems.Nano-Cu and nano-Pb Ti O3 with different particle sizes were prepared by the methods of microwave and sol-gel,respectively,and the particle size and shape of the nanoparticles are charactered by X-ray diffractometer(XRD)and scanning electron microscope (SEM).The influence factors and their influence regularities on the particle size of nano-Cu and nano-Pb Ti O3 nanoparticles were analysed and summarized.Then the thermodynamic properties of phase transition of nanoparticles with different sizes were determined by the differential scanning calorimeter(DSC),and the influence regularities of particle size on the thermodynamic properties of phase transition were obtained.The results indicate that the thermodynamic properties of phase transition of nanoparticles decrease with the particle size reducing and present linear relationships with the reciprocal of particle size,respectively.The experimental results coincide with the aforementioned theoretical relations of phase transition.(2)Theoretical and experimental research into the electrochemical thermodynamics of nanoparticlesTheoretically,the nanometer electrode model with core-shell structure was first proposed,that is,both the reactant and the product of the electrode are nanoscaled and the composite nanoparticles are formed by reactant coating product.By introducing surface chemical potential,the relation between the molar reaction Gibbs energy and the particle size of dispersed phase of core-shell structural nanometer electrode was derived for the first time.Then combined with the relation between the molar reaction Gibbs energy and the electrode potential and the basic equations of thermodynamics,the relations between the electrode potential,the temperature coefficient of the electrode potential,the molar reaction enthalpy,the molar reaction entropy and the reversible reaction heat,respectively,and the particle size of dispersed phase were derived.Theoretical analysis reveals that the size of composite nanoparticles and the shell thickness have effect on the electrochemical thermodynamics of core-shell structural nanometer electrode;under certain conditions,when the particle size of inner core remains unchanged and the shell thickness decreases,the electrode potential and the equilibrium constant decrease,while the temperature coefficient,the molar reaction Gibbs energy,the molar reaction entropy,the molar reaction enthalpy and the reversible reaction heat increase;on the contrary,the influence regularities are also opposite.Experimentally,the core-shell structural nano-Ag2 O electrode was chosen as the research system,and the influence regularities of size and shell thickness on the electrochemical thermodynamics of core-shell structural nanometer electrode.Firstly,the Ag nanoparticles with different sizes were prepared by the method of liquid chemical reduction and the particle size and shape of the nanoparticles are charactered by XRD and SEM.The influence factors and their influence regularities on the particle size of nanoparticles were analysed and summarized.Then,by the method of electrochemical oxidation on the surface of Ag nanoparticles,the core-shell structural nanometer electrode with nano-Ag as core and nano-Ag2 O as shell was prepared for the first time.By determining the electrode potentials of the nonometer electrode with different particle sizes at different temperatures,the influence regularities of particle size and shell thickness on the electrochemical properties were obtained.The results demonstrate that the electrochemical properties of core-shell structural nano-Ag2 O electrode are related to the particle size and the shell thickness of composite nanoparticles;when the particle size decreases,the electrode potential and the equilibrium constant increase,while the temperature coefficient,the molar reaction Gibbs energy,the molar reaction entropy,the molar reaction enthalpy and the reversible reaction heat decrease(more negative);furthermore,the electrode potential,the temperature coefficient of the electrode potential,the molar reaction Gibbs energy,the logarithm of equilibrium constant,the molar reaction entropy,the molar reaction enthalpy and the reversible reaction heat linearly vary with the reciprocal of particle size.Additionally,the electrode potential increases with the increasing shell thickness(decreasing size of inner core)if the size of composite nanoparticle is not changed basically.The relations derived herein between the thermodynamic properties of phase transition,respectively and particle size and shape are able to quantitatively predict and explain the thermodynamic properties of phase transition of nanoparticles with different sizes and different shapes and to describe the phase transition behaviors of nanoparticles,and further to provide theoretical reference and guidance for the problems of thermodynamics of phase transition involved in the designs,preparations and applications of nanomaterials.The derived thermodynamic relations of core-shell structural nanometer electrode to particle size and shell thickness are capable of describing the particle size and shell thickness on the electrochemical thermodynamics of core-shell structural nanometer electrode,explaining the related electrochemical phenomenon and further providing significant guidance for preparing highly sensitive electrochemical sensors.