Investigation On Microstructure Modulation And Performance Of Oxide Fibers

Oxide fibers have been extensively applied in environment,energy,thermal insulation fields,owing to large aspect ratio,easy recycling,self-support,and the intrinsic properties of oxide material.It is difficult to employ melt spinning to prepare oxide fibers due to oxide material with high melt point.Precursor fibers spinning is an approach to obtain oxide fibers.In the process from precursor fibers to oxide fibers,organics removal and surface modification are key problems to realize the successful preparation and functionalization of oxide fibers.A lot of research demonstrated that water-assisted treatment is an efficient,simple and convenient treatment method of organic removal and surface modification.Direct water treatment could seriously damage the fiber structure due to the presence of organic compounds in precursor fibers.The study on zirconia fibers by our group has revealed that the organic could be effective removal with water-assisted treatment.Water-assisted treatment in oxide fibers heat treatment is important application prospect but no systematic researching.Based on this point,the influence of heat treatment with the addition of water steam and pressure on the microstructure and catalytic performance of TiO2 nanofibers(NFs)was systematically studied.The effect of pretreatment with atmosphere and pressure on microstructure and near-infrared reflectance of potassium titanate fibers was studied.The influence of changing the temperature range of steam pretreatment on the microstructure and thermal properties of MgO fibers was studied.The main contents and results of the thesis are as follows:I.Effects of heat treatment temperature and atmosphere on the microstructure and catalytic performance of TiO2 nanofibers1.The effect of heat treatment temperature on the microstructure and photocatalytic performance of electrospun TiO2 NFs with as-obtain polyacetictitanium(PET)as titanium source was studied.The crystallinity,crystalline size and phase content were analyzed by X-ray diffraction(XRD)patterns,the Scherrer and phase content formula.The results indicated that the TiO2 NFs possessed good phase structure thermal stability.SEM images exhibited that the TiO2 NFs at 1000 ? had good fibers morphology and structure,indicating the TiO2 NFs also possessed good morphology structure thermal stability.The photocatalytic performance of TiO2 nanofibers was characterized by degrading methyl orange(MO)and rhodanine(RhB),and the results showed that the TiO2 NFs at 900 v had the highest photocatalytic activity.Photoluminescence(PL)spectra indicated the TiO2 NFs obtained at 900 ? had lower electro-hole recombination rate owing to good crystallinity,and with the temperature increasing to 1000 °C,the electro-hole recombination rate increased due to the generation of rutile.The production of ·OH and O2·-in the photocatalytic process was analyzed by preparing the solutions of phthalic acid(TA)and nitrilotetrazolium chloride(NBT).The results showed that the TiO2 NFs at 900 ? had higher production of ·OH and O2·-in the photocatalytic reaction.In summary,the anatase phase TiO2 NFs with higher crystallinity had lower electron-hole recombination rate and higher production of ·OH and 02·-in the process of photocatalysis,which were conducive to higher catalytic activity of the fibers.2.When the precursor fibers were heat treated to 100 ? with the beginning of water steam atmosphere,the effect of atmosphere on the microstructure and photocatalytic activity of TiO2 NFs was studied.Infrared(IR)spectra and XRD patterns indicated that water steam accelerated the removal of organic and promoted the crystalline of TiO2 NFs(150 ?).Crystalline size of the as-obtained TiO2 NFs via water steam treatment was smaller than that in air atmosphere.N2 adsorption and desorption isotherm,SEM and TEM images showed that the template-free TiO2 NFs possessed mesoporous structure via water steam atmosphere treatment.Water steam could obviously improve the surface area of TiO2 NFs.The mechanisms on the crystallinity at a low temperature and the increased surface area via water steam were studied.Water steam as activity energy could accelerate the removal of organics to promote the crystallization at a low temperature.In the process,the structural rearrangement of the TiO62-octahedral units was driven by water steam to form a basic unit of anatase and the constant dissolution-precipitation would contribute to the formation of a great number of mesoscopic pores,yielding large surface area.The photocatalytic degradation of the TiO2 NFs for MO solution was tested under UV irradiation,and the results showed that the TiO2 NFs at 550 ? via water steam possessed the highest photocatalytic activity,which was attributed to the high crystallinity,the synergistic effect of anatase and rutile phase,and high surface area.3.When the precursor fibers were heat treated to 180 ? with the beginning of water steam atmosphere,the effect of atmosphere on the microstructure and photocatalytic activity of TiO2 NFs was studied.Crystallinity and crystalline size of the TiO2 NFs were analyzed by the XRD patterns and Scherrer formula.The crystallinity of TiO2 NFs(180 ?)was higher than that of the TiO2 NFs(100 ?).N2 adsorption and desorption isotherm,SEM and TEM images indicated the TiO2 NFs possessed mesoporous structure.The TiO2 NFs at 550 ? had surface area of 29.10 m2 g-1 with average pore size of 15.73 nm.With the temperature increased to 650 ?,the surface area of TiO2 NFs was 21.57 m2 g-1 and the average pore size was 17.30 nm,which were higher than the surface area(19.53 m2 g-1)and average pore size(15.39 nm)of the as-obtained TiO2 NFs at 650 ? via water steam(from 100 ? beginning).The photocatalytic activity of the TiO2 NFs for MO degradation under UV irradiation was measured.The TiO2 NFs at 650 ? via water steam(from 180 ? beginning)had the highest photocatalytic activity,and was higher than the TiO2 NFs via water steam(from 100 ? beginning)and commercial P25.The photocatalytic perf'ormance of TiO2 NFs could be optimized by the modification of addition temperature of water steam.?.Effects of atmosphere and pressure on the microstructure and catalyticperformance of TiO2 nanofibers1.The effects of atmospheric pressure steam and high-pressure steam atmosphere on the thermal decomposition,crystallization and mesoporous structure of TiO2 NFs were studied.TG curve and XRD patterns showed that the as-obtained TiO2 NFs had good crystallinity via high pressure steam compared to atmospheric steam treatment at 130 ?,and there was about 14.71%residual organics,which was about twice lower than organic of the precursor fibers(39.72%).The addition of pressure further promoted the removal of organics and crystallization of TiO2 NFs.N2 adsorption and desorption isotherms indicated that the TiO2 NFs had mesoporous structure.2.The effects of different atmosphere and pressure on photocatalytic activity of TiO2 NFs under visible light irradiation were studied.The photocatalytic activity of TiO2 NFs heat treated in air under visible light was measured.The results showed that the TiO2 NFs had almost no adsorption on RhB and no photocatalytic activity under visible light.The photocatalytic degradation of the as-obtained TiO2 NFs via water steam for RhB cationic dye under visible light was measured.The results showed that the as-obtained TiO2 NFs via high pressure steam had better adsorption and higher catalytic degradation activity for RhB,and the catalytic degradation of RhB reached 93%after visible light irradiating 80 min.However,both adsorption capacity and catalytic degradation activity of TiO2 nanofibers via atmospheric pressure steam for RhB were low.Dye RhB was degraded 54%after 80 min,and only 17%for commercial P25.Water steam could promote the adsorption and photocatalytic activity of TiO2 NFs under visible light,and the addition of pressure further enhanced the adsorption and photocatalytic activity.The diffuse reflectance spectra(DRS)showed that the TiO2 NFs via water steam atmosphere and P25 had the same band gap.In summary,the photocatalytic degradation of the TiO2 NFs for dye RhB under visible light was due to dye sensitization.The photocatalytic degradation of TiO2 NFs for anionic dye MO and neutral dye phenol red(PR)was measured.The results showed that other dyes also had better sensitization photocatalytic activity to the as-obtained TiO3 NFs via high pressure steam.3.The cycling performance and thermal stability of TiO2 NFs were studied.After five cycles,the photocatalytic degradation of dye RhB over the Tio2 NFs decreased slightly,revealing good stability.Moreover,the morphology and internal porous structure of the cycled TiO2 NFs did not exhibit any significant changes,presenting good structural robustness.Additionally.,the thermal stability of the TiO2 NFs was studied by heat treating at higher temperatures in air.N2 adsorption-desorption isotherms indicated all samples still held the mesoporous structure.It indicated the mesoporous structure of TiO2 NFs exhibited good stability at higher temperatures.With the temperature increasing to 350 ?,the TiO2 NFs had surface area of 135.76 m2 g-1 with an average pore size of 6.07 nm,and the photocatalytic activity slightly lowered,revealing the good thermal stability.The result of reusability and thermal stability further verified the TiO2 NFs could act as a good photocatalyst.4.The effects of water steam and pressure co-modification on surface properties were studied.XPS spectra and pyridine adsorption IR spectra indicated the surface of TiO2 NFs via high pressure steam possessed the higher content of C-O and C=O groups,which could faclitate dye adsorption and fast electron transfer efficiency,and Lewis(L)and Bronsted(B)acid sites,which was easy to adsorb sensitizer.FT-IR absorption spectra of RhB/TiO2 NFs results manifested the modified surface structure of mesoporous TiO2 NFs attacked the COOH groups and the =N+Et2 groups of RhB molecules.RhB molecules adsorbed on the surface of the TiO2 NFs via high pressure steam with =N+(Et)2 group could be up to 46%,which was higher than surface electrostatic adsorption model of TiO2 NFs via atmospheric pressure steam.In summary,high-pressure water steam treatment facilitated dyes adsorbed on surface of TiO2 NFs,and the electrostatic adsorption model could promote the fast electron transfer from chromophore of dyes to conduction band of TiO2 under visible light.RhB sensitization of TiO2 NFs was applied to the photocatalytic degradation of MO and MB solutions.After RhB sensitization of mesoporous TiO2 NFs,MB and MO degradation were also enhanced and the degradation efficiency of RhB wassuppressed.?.Effects of air and high-pressure steam pretreatment atmosphere on microstructure and near-infrared(NIR)reflectivity of potassium titanates1.Potassium titanate precursor fibers were prepared by electrospinning with tetrabutyl titanate as titanium source and potassium acetate as potassium source.The effects of changing the mole ratio of K/Ti on the thermal decomposition,crystallization,crystal phase,morphology and infrared reflection of potassium titanate fibers were studied.XRD patterns and Raman spectra indicated K2Ti6O13 fibers with the mole ratio of 1:2.5 at 1000 ? possessed good thermal stability at higher temperature.SEM and TEM images indicated morphologies evolved from fibers with acicular gains to fibers with nanorods with the temperature increasing.With the temperature increasing,the NIR reflectivity of K?Ti6O13 fibers enhanced.The K2Ti6O13 fibers with mole ratio of 1:2.5 at 1100 ? possessed the highest NIR reflectivity of 98%,which was attributed to good crystallinity and fine nanorods morphology in length and diameter2.The effects of high-pressure steam pretreatment on thermal decomposition,crystal phase and NIR reflectivity of potassium titanate precursor fibers were studied.TG/DSC curves showed that potassium titanate precursor fibers with different mole ratio of K/Ti all had high solid content,and about 20%of organic in precursor fibers could be removed via high-pressure steam pretreatment(130 ? holding for 40 min).XRD patterns indicated that the potassium titanate fibers at 500 ? were still amorphous.Compared to no high-pressure steam pretreatment,crystallization temperature increased 100 ?,while K2TisO17 could appeared at lower temperature(about below 100 ?).When K/Ti molar ratio was 1:3,the fibers were crystallized with K2T16O13 at 600 ?.High-pressure steam pretreatment could promote the orgaincs removal of fibers and crystallined in K2Ti6O13 phase at lower temperatures.When molar ratio of K/Ti was 1:2.5,K2Ti6O13 fibers at 1100 ? had a small amount rutile present.DRS of K-2Ti6O13 fibers with K/Ti molar ratio of 1:2.5 showed that K2T16O13 fibers at 1000 ? had highest NIR reflectivity of 96%,and with the temperature increasing to 1100 ?,NIR reflectivity of K2Ti6O13 fibers had a slight decrease owing to the presence of a small amount of rutileIV.Effect of water steam pretreatment on microstructure and thermal performance of MgO fibers1.The MgO precursor fibers were prepared by centrifugal spinning with magnesium oxide as the magnesium source and citric acid as the ligand.IR spectra and TG/DSC curves indicated that three carboxyl groups and one hydroxyl group in citric acid molecules were coordinated with Mg atoms.In addition,the effects of air and water steam atmosphere on the thermal decomposition and crystallization of MgO fibers were studied.The results of IR spectra and XRD patterns indicated that the water steam could accelerate the removal of organics and promote crystallization at a low temperature.2.The effects of different temperature ranges(from 200 ? to 350 ?,450 ?,600 ?)via water steam atmosphere pretreatment on the crystalline size,morphology,pore structure and thermal conductivity of MgO fibers were studied.The calculation results by Scherrer formula showed the crystalline size of MgO fibers via air atmosphere was small,and the crystalline size of as-obtained fibers via water steam pretreatment increased.MgO fibers via water steam pretreatment(200-450 ?)had largest crystalline size.SEM images showed that the surface of MgO fibers had obvious cracks and the growth of grains was slow.MgO fibers via water steam pretreatment was layered structure in inner.With increase of pretreatment temperature via water steam atmosphere,particles of MgO fibers at 1000 ? presented successively spherical,polyhedral and lamellar morphologies.The mechanisms of atmosphere and pretreatment temperature on the grain evolution of MgO fibers were discussed.It was mainly attributed to the etching dissolution of MgO crystal surface in the water steam atmosphere and the formation and decomposition of intermediate phase Mg(OH)2.The thermal conductivity of the fiber sheets prepared by MgO fibers heat treated at 1000 ? was measured.MgO fiber sheets with MgO fibers via water steam pretreatnment(from 200 ? to 450 ?)had the lowest thermal conductivitymainly attributed to the multilayer structure inside the fibers..