Highly Flexible And Porous Structured Silica-Based Nanofibrous Membranes For Multifunctional Applications

Benefiting from the high specific surface area,abundant pore structure,adjustable skeleton structure,easy to function,and favorable acid and alkali resistance,the porous SiO₂ materials have been widely studied and applied in various applications,including industrial catalysis,adsorption separation,ion exchange,etc.The currently prepared porous SiO₂ materials are generally in particle form,such as microporous molecular sieves,mesoporous molecular sieves,and porous silica gel,exhibiting outstanding performance in various applications.However,due to the lack of strong interaction force between the porous SiO₂ particles,there exist serious problems such as easy falling off,brittleness and poor vibration resistance during the application in the air medium.Moreover,when the porous SiO₂ particles were applied in liquid medium,they were easily dispersed and suspended in liquid,leading to their dreadful recoverability and secondary pollution during application.Through spraying or adhering the porous particles to the surface of substrates,or pressing the pellets into a block via vacuum filtration,could improve the mechanical properties of the aggregates to some extent.However,a large amount of adhesive covers the surface of particles,which will seriously block the pore structure on the surface and inside the particles,resulting in a dramatical reduction of the pore channel utilization rate.Additionally,the disadvantage of large rigidity and inflexibility of the matrix makes it greatly limited in practical applications.Alternatively,one-dimensional porous SiO₂ fibrous material simultaneously possessed the advantages of pore structure characteristics of porous particles and large length-to-diameter ratio,favorable continuity and outstanding structural stability of the fibers,which has become the most effective means for solving the bottleneck problem of porous particles.At present,methods for preparing porous SiO₂fibrous materials mainly include hard template synthesis method,soft template synthesis method,mechanical drawing approach,and electrospinning technique.Among them,electrospinning technology has become the most effective approaches for fabricating porous SiO₂ fibers due to its wide range of spinnable raw materials,favorable structural adjustability and strong combination of multiple technologies.However,the currently prepared porous SiO₂ fibers always suffered from the defects of large brittleness and easy fracture,leading to the poor long-term stable serviceability in practical applications.Therefore,the controllable construction of flexible porous SiO₂ nanofibrous membranes and the exploration of pore structure and flexible deformation mechanism possessed important theoretical and practical significance for promoting its practical application.In this paper,we mainly focused on the controllable preparation of electrospun flexible porous SiO₂ nanofiber membranes and their applications in the environmental field.Through the polymer template control method,a variety of flexible porous SiO₂ nanofibrous membranes have been created.We have systematically studied the pore formation mechanism of the SiO₂ nanofibers induced by polymer template.Furthermore,we have investigated the intrinsic relationship between the microstructure and the mechanical properties of the single fibers,and then we initially explored the flexibility mechanism of porous SiO₂ nanofibers.On this basis,the flexible SiO₂-based nanofibrous membrane with multi-stage structure was constructed through in situ loading of active nanoparticles,and its application in environmental fields such as molecular filtration,organic pollutant degradation and catalytic sterilization were explored.The main research results obtained are summarized as follows:?1?Through regulating the species,molecular weight and content of the polymer template in the precursor solution,a variety of highly flexible and porous SiO₂ nanofibrous membranes with different pore structures have been prepared.Firstly,influence of the polymer template type on the pore structure of the nanofiber was investigated.It could be found that the SiO₂ fiber which was prepared using polyvinyl alcohol?PVA?as templates was solid and non-porous.However,the SiO₂fibers with an average pore diameter of 10 nm could be obtained using polyvinyl butyral?PVB?and polyacrylic acid?PAA?as templates,respectively.Secondly,the effects of different molecular weight of PVB on the pore structure of SiO₂ fibers were investigated.It was found that the pore size of the fibers was related to the phase separation degree of PVB molecules.Thirdly,the effects of different PAA contents on the pore structure and flexibility of SiO₂ fibers were investigated.It was found that the higher PAA content would enable the larger specific surface area and pore volume of the fibers,as well as worsen the flexibility of SiO₂ fibers.Furthermore,we applied Focused Ion Beam/Scanning Electron Microscopy to observe the mechanical bending behavior of the porous SiO₂ fibers in microscopic scale.Based on the characterization of single fiber mechanical properties and bending behavior observation,we preliminarily analyzed the flexibility mechanism of the porous SiO₂ nanofibers.?2?The highly flexible SiO₂/SnO₂ composite nanofibrous membrane was fabricated by means of the blending electrospinning approach through embedding the SnO₂ crystalline nanoclusters?average particle size of 5-10 nm?into the amorphous SiO₂ matrix.The specific surface area of the fabricated SiO₂/SnO₂ composite membrane was 219.5 m² g^-1 and pore volume was 0.14 cm³ g^-1.Firstly,we investigated the effect of SnO₂ crystalline phase on the mechanical properties of SiO₂fibrous membranes.It was found that tensile strength of the composite membranes increased from0.89 to 4.15 MPa after the introduction of SnO₂ crystalline nanoclusters.Subsequently,we investigated the selective adsorption properties of porous SiO₂/SnO₂ composite nanofibrous membranes.The results showed that the prepared fiber membranes exhibited outstanding selective adsorption properties towards various organic molecules,which possessed different molecular sizes or electrical properties.Based on the high adsorption selectivity,abundant adsorption sites on the fiber,and highly open pore structures between fibers,the obtained SiO₂/SnO₂ nanofibrous membrane could separate the organic molecule mixtures with a high efficiency of more than 99.2%.In addition,the membrane also displays excellent stability and can be reused for ten consecutive filtration-regeneration cycles,highlighting the superior recyclability.?3?Our approach,for the first time,fabricated the porous copper doped carbon/silica nanofibrous membranes?Cu@C/SiO₂ NFMs?with superb flexibility via facile electrospinning technique and in-situ carbonization reduction approach.The specific surface area of the fabricated was 133.6 m² g^-1 and the pore volume was 0.16 cm³ g^-1.Firstly,we investigated the effects of contents of Cu nanoparticles in the fiber on the pore structure and flexibility performance.The results showed that with the increase of Cu particle contents,the specific surface area increased and the mesopore volume fraction decreased,thus leading to the decrease of the flexibility of the fibrous membrane.Moreover,according to the thermal analysis and the crystal structure characterization of the fibrous membrane during the carbonization process,it could be clarified that the structural evolution of the Cu nanoparticles followed the sequence of CuCl·2H₂O?CuCl?Cu.In addition,the catalytic performance of Cu@C/SiO₂ NFMs was systematically assessed through activating persulfate for the elimination of organic pollutants in water.The fabricated Cu@C/SiO₂ NFMs provided outstanding catalytic performance towards organic pollutants with a high removal efficiency of 95%in 40 min and a rapid removal speed of 0.054 min^-1.Moreover,the membranes exhibited outstanding recyclability,which could be facilely regenerated through being directly separated from water without any post-processing.Through examining the changes in the surface chemical structure and crystal structure of the fiber before and after the reaction,we clarified the degradation mechanism of this system.And the degradation path of the organic pollutant was further obtained through examining the intermediates in different degradation processes using high performance liquid chromatography mass spectrometer.?4?Our methodology,for the first time,fabricated the flexible metallic Co-embedded inorganic nanofibrous membranes?CINMs?with tunable interconnected mesoporous structures and monodispersed Co nanoparticles,via double-template electrospinning technique and in situ carbonization reduction method.The pore structure of CINMs could be effectively adjusted through regulating the mass ratio of PAA to PVB.The specific surface area of the obtained CINMs could reach 251.4 m² g^-1 and the pore volume could reach 0.35 cm³ g^-1.Afterwards,we investigated the effect of the mass ratio of PAA to PVB on the pore structure and mechanical properties of the obtained fibers.It was shown that the specific surface area and pore volume of the fiber increased with the increase of PVB contents,and the flexibility of the fiber membrane gradually decreased.According to the thermal analysis and the crystal structure characterization of the fibrous membrane during the carbonization process,it could be clarified that the structural evolution of the Co nanoparticles followed the sequence of Co?NO₃?₂·6H₂O?Co₃O₄?CoO?Co?hexagonal phase??Co?cubic phase?.The monodispersed Co particles throughout fibers could activate peroxymonosulfate to effectively produce reactive oxygen species?ROS?,enabling the complete bacterial inactivation with a 7 log reduction within only 3 min.Benefitting from the integrated features of high porosity,robust mechanical property,and rapid ROS production,the obtained CINMs exhibit an ultra-high bactericidal efficiency of 99.99999%and a high permeate flux of 3.4×10⁴ L m^-2 h^-1 merely driven by gravity?¹.2 kPa?,which were superior to the currently reported membranous antibacterial agent.