The Theoretical Study On The Size-dependent Electrochemical Properties Of Metallic Nanomaterials

Compared with bulk materials,nanomaterials with many unique nano properties have the widely application prospects and important values in fields of corrosion protection,biomedicine,catalytic,battery and so on.Many physical and chemical properties of nanomaterials are size-dependent including the chemical and electrochemical responses,lead to the use of the Butler-Volmer equation to be limited.In order to better understand,describe and predict the electrochemical behaviors of metallic nanomaterials,a size-dependent thermodynamic model was derived in this paper.By combining the Young-Laplace equation,the capillary equation with the Gibbs-Duhem equation and the Butler-Volmer equation,this model can be used to simultaneously analyze the geometrical size,stresses,solute segregation and electrochemical properties in nanofilm,nanoparticles and nanograined materials.Then,combining with the Molecular Dynamics simulations,the nanofilm and nanoparticles of pure metals(Au,Pt,Ni,Cu and Fe),as well as the CuZn binary solid solution of nanoparticles and nanograined materials were numerically calculated to illustrate the size-dependent thermodynamic model.The main results of this paper are:(1)According to the thermodynamic equilibrium theory,the size-dependent thermodynamic model of electrochemical properties of pure metallic nanomaterials(nanofilms and nanoparticles)under equilibrium state was obtained by combining the Young-Laplace equation,the capillary equation and the Gibbs-Duhem equation,can be used to simultaneously analyze the relationships of geometrical size(thickness or radius),stresses and electrochemical properties.The numerical calculations of Au,Pt,Ni,Cu and Fe nanomaterials(nanofilms and nanoparticles)showed that:With the decreasing of the nanomaterial size,the effects of the surface significantly enhanced,the magnitude of thermodynamic relaxation and the initial strain increased,the internal stress gradually increased,the surface stress significantly decreased,and the anodic current density decreased while the equilibrium potential increased.(2)According to the thermodynamic equilibrium theory,the size-dependent thermodynamic model of electrochemical properties of binary solid solution nanoparticles under equilibrium state was obtained by combining the capillary equation and the Gibbs-Duhem equation.This model can be used to simultaneously analyze the relationships of geometrical size,stresses,solute segregation and electrochemical properties in nanoparticles.The numerical calculations of CuZn binary solid solution nanoparticles showed that:With the decreasing of the nanoparticle radius,the initial strain magnitude increased,the internal stress gradually increased,the surface stress significantly decreased,the anodic current density decreased while the equilibrium potential increased,the surface segregation magnitude of solute Zn atoms in the CuZn nanoparticles increased.With the increasing of the apparent Zn concentration in the CuZn nanoparticles,the equilibrium potential decreased while the anodic current density and the equilibrium potential difference between the CuZn solid solution components increased,implying that the selective corrosion of solute Zn atoms enhanced.When the apparent Zn concentration of CuZn nanoparticles larger than 15%,the solute Zn atoms in the surface are more active and easier to be oxidized and dissolved,compared with that in the bulk pure Zn material.(3)According to the thermodynamic equilibrium theory,the size-dependent thermodynamic model of electrochemical properties of binary solid solution nanograined materials under equilibrium state was obtained by combining the capillary equation and the Gibbs-Duhem equation.This model can be used to simultaneously analyze the relationships of grain size,stresses,solute segregation and electrochemical properties on nanograined material surface with two phases of boundary and grain.The numerical calculations of CuZn solid solution nanograined materials showed that:With the decreasing of the nanograin size,the internal stress gradually increased,the boundary stress significantly decreased,the boundary segregation magnitude of solute Zn atoms in the CuZn nanograined materials increased,the anodic current density decreased while the equilibrium potential increased.With the increasing of the apparent Zn concentration in the CuZn nanograined materials,the equilibrium potential decreased while the anodic current density and the equilibrium potential difference between the CuZn solid solution components increased,implying that the selective corrosion of Zn atoms enhanced.When the apparent Zn concentration of CuZn nanograined materials larger than 20%,the solute Zn atoms in the surface are more active and easier to be oxidized and dissolved,compared with that in the bulk pure Zn material.