Structural Design And Fabrication Of Ultrafine Two-dimensional Nanofiber/Nets For Highperformance Air Filtration

Clean air is not only an important element indispensable for life activities,but also a necessary condition for the sustainable development of human society.However,with the rapid increase in population and development of industry,particulate matter?PM?pollution in air has become more and more serious,and the survival environment that people rely on has gone from bad to worse.Approximately 91%of the world's population lives in places that do not meet the WHO air quality guidelines,and 180 of China's 337 cities have unqualified air quality.Particles with sizes below2.5?m(PM_2.5)are particularly hazardous due to their extremely small size,which could carry large amounts of toxicant,penetrate into bronchi,and deposit in alveoli,causing cancer,cardiovascular diseases and large amount of chronic diseases.Nearly 7 million people worldwide died prematurely due to air pollution in 2016 alone.In addition,the severe haze pollution caused by the high concentration of PMs posed great threaten to environmental climate,ecological balance,and agricultural production.Therefore,there is an urgent need to strengthen the treatment of PM pollution.Existing remediation methods for air pollution mainly include source control and terminal control.However,the source control law is difficult to implement and to achieve results in a short time,due to the current situation of our country with a wide range of pollution and a large degree of pollution.Therefore,how to develop high-efficiency air filtration materials and adopt terminal filtration methods to reduce the harm caused by air pollution is a major issue that needs to be solved urgently.Fiber-based air filtration materials have become the mainstream air filtration materials because of their high cost-effectiveness,easy mass production,and good processability.Currently,the common commercial fibrous air filters mainly include electret melt-blown fiber filters and ultra-fine glass fiber filters.Electret melt-blown fiber filters with large pore sizes can actively capture ultrafine particles while maintaining high air permeability,due to their strong electrostatic adsorption.However,their electret charges are susceptible to rapid decay caused by the water clusters or heat in the environment,resulting in a sharp decrease in filtration efficiency and leading to safety issues.The ultrafine glass fiber filters possess small fiber diameters and pore sizes,which can improve the negative physical interception of PM_2.5 in some extent,but their smooth surfaces usually cause dense stacking,which lead to a higher air resistance.Moreover,the glass fibers are easy to be breaken and fall off due to their intrinsic large modulus,which could lead to secondary pollutions.Compared with traditional micron-fiber air filters,electrospun nanofiber air filters exhibite the characteristics of small diameter,small pore size,and good pore connectivity,resulting in significantly improved PM_2.5filtration efficiency.However,they are almost ineffective for particles with the most penetrating particle size?300nm,due to their sub-micron/micron-scale fiber diameter?usually 0.2-2?m?.Nanofiber/nets are two-dimensional fiber networks with nano-scale fiber diameters?<50 nm?,small pore size?100-500 nm?,and weighted Steiner-tree structures,which can bring an enhanced air slip-effect and effectively intercept particles,thus showing broad application prospects in the field of air filtration.However,the coverage of the nanonets in existing membranes is usually limited and displayed compact stacking structure.It is difficult to further improve its filtering performance only relies on the physical interception effect of the nanonet's passive particle capture.Here,we introduce the structure control and surface property design of the material into the preparation of nanofiber/net air filters,by optimizing the electrospinning/netting technique,and combining with in-situ self-polymerization,self-polarization electret,electrostatic spinning and other multiple technologies.In combination,a nanofiber/net filter material with high nanonet coverage,fluffy packig structures,and both physical interception and surface adsorption is prepared,which expands the types of nanofiber/nets,enhances its ability to capture particulates,and improves the comprehensiveness filtration performance.The main research results obtained are summarized as follows:?1?Based on the"ion-dipole interaction"charging enhancement mechanism,polymer PAN with high dipole moment and high dielectric was used as raw material for nanofiber/net air filters preparation.By introducing cationic TBAC inducer into the low conductivity PAN solution,a novel PAN nanonet air filter with fluffy structure was prepared for the first time,using humidity-induced electrospinning/netting technique.The resultant nanofiber/net filters have the characteristics of small diameter??20 nm?,small pore size??300 nm?,high nanonet coverage,low packing density(0.18 g cm^–3),and strong surface polar adsorption?dipole moment 4.3 D?.The droplet ejection-phase separation forming mechanism of PAN nanonets under the action of TBAC inducer and environmental humidity was analyzed,and a prediction model of the relationship between relative humidity and phase separation of PAN charged droplets was proposed.Based on this model,the morphology,pore structure and stacking structure of these membranes were controlled by tailoring different TBAC content and relative humidity conditions.Moreover,the three-dimensional structure models of fiber filtration membranes with different structures were constructed to simulate their particle trapping process and the air resistance distribution state.The filtration performances indicated that,our fluffy PAN nanonet air filter material with both physical interception and polar adsorption can filter ultrafine particles PM_0.3 with high efficiency?>99.97%?and low resistance?95.5 Pa?,and show robust long-term PM_2.5cycle purification capability.?2?Combining the blend electrospinning/netting technique and the in-situ self-polymerization method,a biomimetic PAN/PDA nanofiber/net air filter with both small pore size and wet-adhesive surface was prepared.By introducing the mussel-like adhesion molecule DA into the precursor solution,the ejection and phase separation of the charged droplets are promoted;and then excellent wet adhesion is endowed with the 2D nanonets with spider web-like structures after the self-polymerization reaction of the DA molecules on the surface.The PAN/PDA nanofiber/net filter has the characteristics of small fiber diameter??27 nm?,small pore size?0.28?m?,high porosity?>92%?,excellent surface adhesion,good mechanical properties,etc.,and shows good performance in particle removal under high humidity conditions.This study focused on the influence of DA content in the solutions on the morphology and structure of nanonets,the distribution of the PDA wet-adhesive functional layer on the fiber surface,and the physical structures,surface properties and mechanical properties of PAN/PDA nanofiber/net materials.The filtration performance of the biomimetic wet-adhesive nanofiber/net material shows that the material can still achieve high-efficiency?99.996%?,low-resistance?108 Pa?filtration for PM_0.3 removal under high-humidity conditions,as well as stable filtration performance and long-term usability under various humidity conditions.?3?Using PVDF as the raw material,a high-efficiency self-polarized electret nanofiber/net air filter was prepared using in-situ electret electrospinning/netting technique.Based on the in-situ ion doping of precursor solutions,the effective control of the charge density at the tip of Taylor cone and the dynamic ejection of the charged droplets are realized.Meanwhile,the polarization/drafting effect of the high-voltage electric field during the electrospinning/netting process is used to promote the orientation arrangement of the dipoles in the PVDF molecules,resulting in the change from non-polar?type to polar?type crystal phase of PVDF molecular chain.Besides the increased probability of polarized charge generation,the high surface potential?6.8 k V?is obtained together with the small pore size?0.26?m?,resulting in the creation of the long-term self-polarized electret PVDF nanonet air filter material.The study focused on comparing the influence of the polymer structure on its self-polarization ability in high-voltage electric fields,on exploring the internal correlation between the nanonet structure and its molecular crystal phase transition,and on analyzing the effect of nanonet coverage on the physical/chemical structures and application performances of the fiber membrane.In addition,the filtration performance and light transmission performance of the filter membranes are systematically studied.The results showed that the electret nanonet filters can achieve a PM_0.3 removal efficiency of 99.998%at only 1/100 of the weight of the conventional microfiber filter material,while maintaining low air resistance of 93 Pa,and they also indicated a long-lasting PM_0.3 removal ability and high light transmittance?84%?.?4?The PVDF-HFP fiber/PVDF nanonet composite filters with gradient pore structures were prepared by combining the electrospinning and electro-netting technology.Utilizing the double diffusion effect between water molecule clusters and solvent molecules during the electrospinning process,the difference in the solidification rate between the inside and outside of the jet is increased,and then the wrinkled PVDF-HFP submicron fibers with rough surfaces are obtained,and the micro-nano multi-level porous structure is realized as well.During the following electro-netting process,based on the initiation of cationic surfactants,the ejection and phase separation of charged droplets were promoted to obtain PVDF electret nanonet core filter layer with both nano-scale diameter??20nm?and Steiner tree structures?pore sizes of?300nm?,resulting in the formation of the composite nanonet filter material with capability of multi-level gradient interception and electrostatic adsorption for PM removal.The effect of the double diffusion between water molecule clusters and solvent molecules on the morphology and performance of the wrinkled fiber was studied,and the forming mechanism of the nanonet layer structure was analyzed,and a nanonet core filter layer with high network coverage was prepared.The filtration performance and radiant cooling performance of the gradient composite filter membrane are systematically studied,and the obtained results show that the composite membrane not only achieves comprehensive filtration performance(quality factor:0.12 Pa^-1)for PM removal,but also indicate a promising radiant cooling function.