Effect Of Particle Size And Morphology On The Reaction Kinetic Parameters Of Nano-ZnO

Compared with the corresponding bulk materials,nanomaterials have a lot of special properties,and these properties are mostly involved in the reaction of nanomaterials,which are mainly depended on the particle size and morphology of the nanoparticles.But,the influences of the particle size and morphology of nanoparticles on the reaction kinetic parameters are not completely clear,which severely restricts the development and application of the nano-reaction kinetic theory.In this paper,the effects of particle size and morphology on the reaction kinetic parameters and the surface thermodynamic properties of nanoparticles are investigated by combining experiments and theories.Theoretically,the relations between the apparent activation energy and the pre-exponential factor of the nano-reaction and the particle size and morphology are derived based on the transition state theory in first;then the Arrhenius equation is used to derive the relation between the reaction rate constant and the particle size and morphology,and the influence regularities and influence mechanism of particle size and morphology of nanoparticles on the reaction kinetic parameters were studied.In the experiment,ZnO nanospheres or nanowires with different particle sizes were prepared by hydrothermal method,and the morphology and crystal structure of the prepared products were characterized by SEM and XRD,respectively.Take the reaction of nano-ZnO with acrylic acid dilute solution or the solution of sodium bisulfate as the research system.Then the influence regularities of particle size and morphology of nanoparticles on the kinetic parameters were summarized,and compared with the corresponding relations.In addition,the surface thermodynamics of nanoparticles are able to obtained by the reaction kinetic parameters,and the influence regularities of particle size and morphology on the surface thermodynamics of nanoparticles were discussed.Studies indicate that:(1)ZnO nanospheres and ZnO nanowires were synthesized by hydrothermal method through controlling experiments conditions.For the preparation of spherical ZnO,the amount of Zn(CH₃COO)₂,the hydrothermal time,and hydrothermal temperature all have effect on the particle size of ZnO nanoparticles:the particle size of ZnO will increase by increasing the amount of Zn(CH₃COO)₂,and prolonging the hydrothermal time or increasing the hydrothermal temperature,the particle size of the nano ZnO increase;for the synthesis of ZnO nanowires,the particle diameter of ZnO is influenced by the concentration of ZnCl₂,the hydrothermal time and the hydrothermal temperature:the particle diameter of ZnO will increase through prolonging the reaction time,and the diameter of nanowires will increases by increasing the reaction temperature,what's more,with increasing the concentration of ZnCl₂,the diameter of ZnO nanowires increases.(2)Both morphology and particle size have remarkable influence on the reaction kinetic parameters;for the reaction of ZnO nanospheres and ZnO nanowires,as the diameter decreases,the reaction rate constant k will increases,while the apparent activation energy E_a and the pre-exponential factor A will decrease.Moreover,there are linear relations between the lnk,lnA,E_a and the reciprocal of diameter.For the reaction of ZnO nanoparticles with the same particle diameter,compare with the nanowire,lnk of spherical nanoparticles is larger,while lnA and E_a are smaller,and the influence of morphology on the reaction kinetic parameters increases with the decrease of particle diameter.These regularities coincide with the related the theory of reaction kinetic.In addition,k is influenced by Gibbs energy of partial molar surface,lnA by surface entropy of partial molar,E_a by surface enthalpy of partial molar.(3)The surface thermodynamic properties of ZnO nanospheres and ZnO nanowires are able to obtained by the reaction kinetic parameters.Both morphology and particle size have remarkable influence on the surface thermodynamic properties of nanoparticles.As particle diameter decreases,the surface thermodynamic properties of nanoparticles(Gibbs energy of partial molar surface G^s,surface entropy of partial molar S^s and surface enthalpy of partial molar H^s)will increase,and for the particle diameter exceeds 20 nm,surface thermodynamic properties are all linearly related to the reciprocal of particle diameter.In addition,the surface thermodynamic properties of nanoparticles with different morphologies(sphere and wire)at the same particle diameter are also different,and the surface thermodynamic properties as follows:sphere>wire.These regularities are also consistent with the theoretical formula of surface thermodynamic.In this paper,the reaction kinetic theory of nano system is improved,which can accurately explain and predict the dependence of reaction kinetic parameters of nanoparticles on particle size and morphology,provide theoretical guidance for the application of nanomaterials reaction and its related fields.