Effect Of Particle Size And Morphology On The Adsorption And Photocatalytic Activity Of Nano-TiO₂

Due to nano-effects,nanomaterials have excellent adsorption and catalytic activity.Both adsorption and catalytic processes take place on the surface of nanomaterials and the surface properties of nanomaterials are related to the particle size and morphology of nanomaterials.The effect of surface properties on adsorption and catalytic performance can be quantified as the effect of the particle size and morphology on the adsorption and catalytic properties.The mechanism and disciplines of this effect need further study.In this paper,the influence of particle size and morphology on the adsorption kinetics and thermodynamic properties of nano-sorbents was analyzed using nano-adsorption theory.In experiments,a variety of morphologies of nano-TiO₂ with different sizes were prepared,and the effects of experimental conditions on the morphology and particle size of nano-TiO₂were studied.The adsorption of basic fuchsine on nano-TiO₂ was used as the research object,and experimental results were compared with the adsorption theory.Furthermore,the effects of particle size and morphology on the photocatalytic degradation of methylene blue by nano-TiO₂ were studied.1)Theoretical analysis of adsorption of nano-TiO₂ by nano-adsorption theoryThe nano-TiO₂ adsorption process was analyzed by the thermodynamic and kinetic theory of nano-adsorption.In thermodynamics theory,the relationship between the particle size and morphology of nano-TiO₂ and the adsorption equilibrium constants and thermodynamic functions were analyzed and the process of derivation was discussed,including the influence mechanism and disciplines of particle size on adsorption equilibrium constants and thermodynamic properties.In the kinetics theory,the influence of particle size and morphology on the kinetic parameters of nano-TiO₂ adsorption was analyzed,including the relationship between the adsorption rate constant,activation energy,pre-exponential factor.2)Preparation of nano-TiO₂In experiments,a variety of morphologies?cubic,spherical,octahedral and decahedral?with different particle sizes of nano-TiO₂ were prepared by two kinds of hydrothermal method.The effects of the amount of titanium source,hydrothermal temperature,morphology control agent and other factors on the morphology and particle size of TiO₂ were studied.3)Experimental study of nano-adsorptionThe adsorption process of basic fuchsin on the nano-TiO₂ was used as research object.The adsorption kinetics and thermodynamic properties of basic fuchsin on the different kinds of nano-TiO₂ at different temperatures were experimentally studied.The effects of particle size and morphology on the adsorption process and equilibrium were studied.4)Photocatalytic experiment of nano-TiO₂The photocatalytic degradation of methylene blue by nano-TiO₂ catalysts with different morphologies and different particle sizes was experimentally performed using prepared nano-TiO₂ particles.The effect of morphology and particle size on photocatalytic performance of nano-Ti O₂ was studied.Through theoretical and experimental research,the following conclusions are drawn:1)Results of theoretical research of nano-TiO₂ adsorptionThe theoretical analysis results show that when the particle size of nano-Ti O₂ is reduced,the adsorption equilibrium constantK^o-increases,and the corresponding standard molar adsorption Gibbs?35?_adsG_m^o-decreases.The standard molar adsorption enthalpy?35?_adsH_m^o-and the standard molar adsorption entropy?35?_adsS_m^o-are influenced by both the surface area of the nanoparticles and the adsorption force.For the kinetic parameters of the adsorption process,as the particle size decreases,the adsorption process will accelerate,that is,the adsorption rate constant k increases and the activation energy Ea for adsorption decreases.2)Preparation of nano-TiO₂The octahedron nano-TiO₂ in the particle size range of 40 nm to 130 nm and the decahedral nano-TiO₂ in the particle size range of 90 nm to 160 nm were obtained by two-steps-hydrothermal method.The experimental results show that the concentration of urotropine is the main factor affecting the particle size of octahedron and decahedral nano-TiO₂,while diethanolamine?DEA?can convert the octahedron nano-TiO₂ into decahedral nano-TiO₂.The cubic nano-TiO₂ in the particle size range of 50 nm to 200 nm and the spherical nano-TiO₂ in the particle size range of 100 nm to 220 nm were prepared by hydrothermal method.The results show that the concentration of DEA is the main factor affecting the morphology,and the amount of tetraisopropyl titanate is the main factor affecting the particle size of nano-TiO₂.3)Effects of morphology and particle size on the thermodynamic properties and kinetic parameters of the adsorption of basic fuchsin on nano-TiO₂The following results were obtained by studying the adsorption of the basic fuchsin on nano-TiO₂:In the aspect of thermodynamics,when the particle size of nano-TiO₂ decreases,the adsorption equilibrium shifts to the right,and the equilibrium adsorption capacity increases.Correspondingly,the adsorption standard equilibrium constantK^o-increases;the standard molar adsorption Gibbs free energy?35?_adsG_m^o-,the standard molar adsorption?35?_adsH_m^o-and the standard molar adsorption entropy?35?^o_{ads m}S^-all decrease;the logarithm of the standard equilibrium constantsln K^o-and?35?_adsG_m^o-,?35?_adsH_m^o-and?35?_adsS_m^o-have linear relationship with the reciprocal of particle size.In terms of dynamics,when the particle size of nano-TiO₂ decreases,adsorption increasing.The adsorption rate constant k increases,while the activation energy Ea and the pre-adsorption factor ln A decrease;and the ln k,Ea,and ln A have a good linear relationship with the reciprocal of the particle size,respectively.That is consistent with adsorption theory analysis.4)Effect of particle size and morphology on the photocatalytic degradation of methylene blue by nano-TiO₂The following conclusions were obtained through experimental research of photocatalytic degradation of methylene blue by TiO₂:The photocatalytic activity of nano-TiO₂ increases with the decreasing of particle size.The photocatalytic activity of nano-TiO₂ with different morphologies has the following order:decahedron>octahedron>cube>sphere.