Positron Annihilation Study Of The Pore Structure Of Ordered Mesoporous Materials And Its Effect On Various Properties

In the last three decades,mesoporous materials have wide applications in gas adsorption and separation,catalytic reaction and low dielectric materials due to their controllable pore size,large specific surface area and uniform pore structure.A series of mesoporous materials with different pore structure and pore size can be synthesized by using different templates and synthetic methods.In terms of characterization,the main methods employed to characterize the pore structure of porous materials are small angle x-ray scattering?SAXS?,gas adsorption/desorption and high resolution transmission electron microscopy?HRTEM?.These methods provide complementary information about pore structures,but they still have disadvantages,and cannot provide perfect pore structure information.Therefore,exploring other characterization methods to obtain more comprehensive information of pore structure and study the relationship between pore structure and performance is still an important subject.Positron annihilation spectroscopy is a very sensitive method to study pore structure.In the field of positron annihilation,a large number of semi-empirical formulas have been proposed for the relationship between the lifetime of positronium and the radius of the pore.In this thesis,we prepared several typical mesoporous materials,and studied the pore structures by means of positron annihilation analysis,small angle x-ray scattering?SAXS?,high resolution transmission electron microscopy?HRTEM?and N₂ adsorption/desorption.The effects of pore structure on dielectric properties,photocatalytic degradation and thermoelectric properties were also studied.Combined with different characterization methods,we further discussed the way to characterize the pore structure by positron annihilation technique.Porous silica SBA-15 was synthesized using triblock copolymer P123 as structure template and tetraethyl orthosilicate(TEOS,C₈H₂₀O₄Si)as silicon source.The pore structure was adjusted by using swelling agent 1,3,5-trimethylbenzene?TMB?.Small angle X-ray scattering?SAXS?and high resolution transmission electron microscopy?HRTEM?indicate that the pore morphology changes in three stages with increasing TMB/P123 ratio.In the first stage?TMB/P123<0.2?,the pore type is a typical cylindrical hole.When TMB/P123 is 0.2–0.5,the pore morphology is a curved cylinder.In the final stage when TMB/P123 ratio is higher than 0.5,ordered mesoporous silica is converted into mesoporous silica foam.The average pore size estimated from N₂ adsorption measurements increases from 6.5 nm to 17.2 nm with increasing TMB/P123 ratio from 0 to 2.Positron lifetime measurement indicate two long lifetime components?₃ and?₄,which are due to o-Ps annihilation in micropores and mesopores,respectively.It was noted that the mesopores are open pores and the micropores are closed pores.With the increase of TMB ratio,the number and size of micropores show nearly no change,but the mesopore size increases gradually with increasing TMB/P123 ratio up to 0.5,which are in agreement with results of the N₂ adsorption/desorption measurements.However,after the pores evolves into mesoporous foam,the pore size obtained from o-Ps lifetime is less than that of N₂ adsorption/desorption measurements.Ordered mesoporous SiO2?KIT-6?was synthesized using triblock copolymer P123 as the surfactant and tetraethyl orthosilicate as silicon source.Small-angle X-ray scattering and High resolution electron microscope measurements indicate the 3d cubic Ia3d symmetry of the pore structure of KIT-6 synthesized at 30?-120?.When the synthesis temperature increases to 180?,the order of pores was deteriorated.The pore size was estimated by nitrogen adsorption/desorption measurements,which increases from 3.8 nm to 18.5 nm as the synthesis temperature increases from 30?to 180?.With the increase of mesopore size,the pore wall thickness shows continuous decrease.Positron annihilation lifetime was measured for the synthesized KIT-6.The lifetime spectrum can be resolved to two short and two long lifetime components.The two long lifetimes?₃ and?₄ correspond to o-Ps lifetime in micropores and mesopores,respectively.The size of mesopores was estimated from the o-Ps lifetime by using Goworeck's model for pores with shape of cylindrical tube,which shows continuous increase with synthesis temperature,and is consistent with the N₂ adsorption/desorption measurements.Due to the decrease in wall thickness,the number of micropores in the wall shows subsequent decrease.In addition,the second lifetime component?₂ is also found to be a sensitive parameter for the pore size,which shows the same trend as lifetime?₄ with increasing synthesis temperature.The dielectric properties of the synthesized KIT-6 was found to decrease systematically with increasing synthesis temperature.This reveals that the porous structure is an effective way to prepare low dielectric constant materials.Highly ordered mesoporous titanium dioxide was prepared by nano-casting method using mesoporous silica SBA-15 as hard template and tetrabutyl titanate as titanium source.The results of SAXS,WAXRD,HRTEM and N₂adsorption/desorption measurements show that mesoporous titanium dioxide has an inverse pore structure with SBA-15.The pore size and specific surface area of mesoporous titanium dioxideare are 4.5 nm and 116 m²/g,respectively.The pore wall is highly crystallized,which is a mixed phase of anatase and rutile.The mesoporous titanium dioxide was characterized by positron annihilation lifetime measurements.The long lifetime component?₄ was 57 ns,but the corresponding intensity was only0.9%.This is caused by the inhibition and quenching effect of titanium dioxide on the formation and annihilation of o-Ps,which makes the pore size and relative number of pores obtained from the lifetime of o-Ps to be smaller than the actual value.Finally,the photocatalytic properties of TiO₂ were characterized.The adsorption and photocatalytic properties of mesoporous TiO₂ were better than those of commercial P25 and TiO₂ nanocrystal.This may be the result of the interaction of ordered pore structure,high specific surface area and highly crystallized pore wall.Finally,the relationship between the pore structure and thermoelectric properties of ordered mesoporous carbon?CMK-3?was studied.Ordered mesoporous carbon CMK-3 was prepared by nano-casting method using SBA-15 as hard template and sucrose as carbon source.The prepared CMK-3 were sintered with different SPS temperature.The results of SAXS,HRTEM,N₂ adsorption/desorption and PALS show that unsintered CMK-3 has a highly ordered pore structure with a specific surface area of 1225 m²/g and the pore size of 4 nm.The porosity of CMK-3decreases with the increase of sintering temperature.The increase of sintering temperature leads to densification of CMK-3.The carrier concentration increases with the increase of sintering temperature,leading to an increase in conductivity and a decrease in Seebeck coefficient.The thermal conductivity of CMK-3 sintered at low temperature is lower than that sintered at high temperature.This can be explained by the retention of more pores in CMK-3 by low temperature sintering,resulting in more phonons being scattered and lowering the lattice thermal conductivity of the samples.It is shown that the pore structure in materials is an effective way to reduce the lattice thermal conductivity.The maximum ZT value of CMK-3 is 4.56×10^-4,which is better than that of ordinary organic conductive polymers.Its thermal stability is higher than that of organic materials,and is believed to be a new type of thermoelectric materials.