Electrospun 3D Curly Electret Nanofiber Air Filters for Particulate Pollutants

By inergency On Mar 28, 2024

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1. Introduction

In light of the escalating environmental challenges, the issue of air quality has garnered significant attention [1]. The airborne particulate pollution resulting from the rapid advancement of industrial civilization, as well as the suspended dust caused by natural meteorological disasters such as sandstorms, have inflicted significant harm upon the ecological environment, public health, and industrial and agricultural productivity. Particularly in terms of health, these harmful particles can also harbor a multitude of viruses or bacteria, leading to the onset of various respiratory diseases and allergic symptoms in humans upon inhalation [2,3]. Tiny particles can penetrate human protective barriers, directly entering the lungs and even the bloodstream, as confirmed by scientific research. This has been closely associated with increased incidences of various health issues such as cardiovascular diseases and lung conditions. Researchers like Pope CA III have pointed out that prolonged exposure to PM pollution significantly increases the risk of mortality from cardiovascular diseases [4]. In addition, gaseous pollutants in the atmosphere also introduce irritants to the human respiratory system, with prolonged exposure to them potentially exacerbating respiratory conditions such as asthma and chronic obstructive pulmonary disease. For instance, in Xinjiang, China, due to harsh natural conditions, occurrences of sandstorms increase, resulting in higher levels of particulate matter in the atmosphere, which significantly elevates the incidence of tuberculosis in Xinjiang compared to other regions [5].

Traditional air filtration materials struggle to effectively filter PM0.3 particles and typically exhibit high air resistance, leading to breathing difficulties when used in masks [6]. Air filtration materials with excessively high pressure drops will also lead to excessive energy consumption when used in industrial air purification systems [7]. In order to protect the public from harmful particles and reduce the energy consumption of air filters, we must persistently develop high-efficiency and low-resistance air filters.

Electrospinning technology is an advanced process for preparing nanofiber membranes. Polymer materials or other material solutions are sprayed into nano- to micron-sized fibers by using electrostatic [8]. The fabrication of nanofibers through electrospinning offers the advantages of being relatively straightforward, efficient, controllable, and facile to produce [9]. In general, the greater the porosity of a nanofiber membrane, the more superior its permeability and adsorption capabilities. Therefore, high specific surface area and elevated porosity are crucial for the preparation of high-performance nanofiber membranes [10,11].

Three-dimensional curly nanofibers possess a substantial specific surface area owing to their distinctive morphology [12]. A curly nanofiber membrane has a fluffy structure, a high porosity that allows molecules and ions to pass through effectively, and a large surface area that provides a large number of reaction and adsorption sites; it also has wide application potential in filtration, sensing, biomedicine, and composite materials [9,13,14,15]. Its curly structure can also increase its mechanical strength and toughness, making it more durable and reliable in its applications [16]. Therefore, the preparation of nanofiber membranes with high porosity and a 3D curly structure is a promising research direction with broad application prospects [15,17,18]. TiO₂ NPs not only have excellent photocatalysis and corrosion resistance but are also a metal oxide with a high dielectric constant [19,20,21]; TiO₂ is often used to increase dielectric constants [22,23,24]. Studies have shown that TiO₂ NPs can be compounded with PVDF-TIFE, which proves that the introduction of TiO₂ NPs can greatly improve the dielectric constant of the material, and the effect of rutile TiO₂ NPs is better than that of anatase TiO₂ NPs. Daehwan Cho et al. found that TiO₂ NPs were added to the polypropylene (PAN) electrospinning membrane by electrospinning and that the PAN electrospinning membrane had larger pore sizes and wider pore size distributions. However, when filtering fine particles, the TiO₂/PAN electrospinning membrane shows better filtration efficiency than the pure PAN electrospinning membrane. This is due to the addition of TiO₂NPs, which improves the charging ability of the PAN electrospinning membrane [25].

In this paper, we successfully prepared 3D curly nanofibers by using the extremely high humidity of the spinning environment to induce fiber formation and realized the highly fluffy accumulation of the electrospinning membrane. Subsequently, the electret-reinforced TiO₂ NPs particles were doped into it, and the prepared electret-reinforced nanofibers still maintained a 3D curly structure, forming a highly fluffy nanofiber membrane. We studied the influence of the mass ratio of TiO₂ NPs in PVDF/TiO₂ on the microstructure and the electret effect of 3D curly nanofibers and tested their air filtration performance, which provided some basic research for preparing high-efficiency and low-resistance air filtration nanofiber membranes with an electret effect. Low pressure drops will ensure the comfort of individuals wearing personal respiratory protective air filters for extended periods. It is expected to reduce energy consumption when applied in industrial air purification systems, demonstrating the potential of material innovation to address air quality challenges within the framework of sustainable development.

3. Results and Discussion

3.1. FESEM Analysis

The humidity of the electrospinning environment impacts the evaporation of the solvent, thus influencing the solidification process of the spinning jet into fibers. It stands as one of the pivotal factors in the electrospinning process. The electron microscope images depicting the morphology of PVDF nanofiber membranes prepared through electrospinning in environments with low humidity (40RH%) and high humidity (95RH%) are illustrated in Figure 1.

According to Figure 1a, it is evident that PVDF nanofibers fabricated in a low-humidity environment exhibit a uniform distribution in a typical linear morphology, with the fiber surface appearing remarkably smooth at the microscopic level, as depicted in Figure 1c. However, as the humidity exceeds 95RH%, the structure of the fabricated PVDF nanofibers undergoes significant changes. As per Figure 1b, the nanofibers exhibit a 3D curly morphology. Upon comparing Figure 1b with Figure 1d, it becomes apparent that the fiber surface prepared in a high-humidity environment is rougher. Figure 1e,f represent side views of the two samples, respectively. It is observable that the fiber membrane prepared in a low-humidity environment forms a densely packed membrane, while the fiber membrane prepared in an ultra-high-humidity environment assumes a highly voluminous 3D curly structure.

According to the FESEM analysis, relative humidity is a crucial factor that influences the 3D structure of electrospinning, benefiting from the favorable interaction between water and solvent. A high concentration of water molecules will enhance solvent evaporation and modulate the exchange rate between water molecules and DMF, thereby directly impacting the trajectory of the jet and, consequently, the morphology of the nanofibers deposited on the receiving substrate. At a humidity level of 95%, the weak electric field force during stretching will induce residual stress in the polymer network structure, leading to the retraction of highly intertwined molecular chains. This phenomenon results in the observation of curly fibers when spinning in a high-humidity environment [26,27]. According to Figure 1b,d, it is evident that the fiber surface prepared in a low-humidity environment appears smooth under microscopic observation, while the surface of the PVDF nanofiber prepared in a high-humidity environment exhibits irregular gullies and a rougher texture. This phenomenon may be attributed to the accelerated evaporation of the DMF solvent in the spinning jet in a high-humidity environment. The DMF solvent rapidly volatilizes and absorbs heat during the drawing process, causing a decrease in the jet temperature, which leads to the condensation of water vapor on the fiber surface, forming water droplets [28].

The process comprises a large amount of water vapor contained in the air being uniformly dispersed in the space in the form of atomized nano-scale droplets, and the droplets formed by condensation and the nano-scale droplets in the space in a high-humidity environment will occupy the space on the fiber surface; after the fiber is completely dried, the droplets evaporate from the liquid to the air, vacating the original space, forming pits, and finally the PVDF fiber with rough surface appears.

When the fiber membrane is utilized for filtration, the 3D curly fiber structure can result in reduced air resistance. However, the space support effect brought about by this 3D curly structure will also marginally decrease the filtration efficiency of the fiber membrane. It is generally considered an effective strategy to enhance filtration efficiency by imparting an electrostatic electret effect to the nanofiber filter without compromising the filter pressure drop. In line with this perspective, TiO₂ NPs were introduced into the spinning solution as an electret enhancer, and PVDF nanofibers with an electret effect were prepared in a single step through electrostatic spinning. Figure 2 shows the SEM images of PVDF/TiO₂ composite nanofibers with TiO₂ NP contents of 0.25 wt%, 0.5 wt%, 0.75 wt%, and 1.0 wt%. In comparison with pure PVDF in Figure 1b, the introduction of 0.25 wt% and 0.5 wt% TiO₂ NPs appears to have no effect on the crimp morphology, and with the addition of TiO₂ NPs, the fiber diameter decreases slightly in Figure 3. This may be attributed to the incorporation of TiO₂ NPs, causing the solution to carry more charges, and the fibers are drawn thinner under the action of the electric field stretching. However, if the content of nano-titanium dioxide is further increased, the curling phenomenon of the nanofibers will weaken or even disappear, and the nano-titanium dioxide particles will also agglomerate. The weakening and disappearance of the curly structure may reduce the porosity of the fiber, and the increase in agglomeration may lead to uneven charge distribution, and it even has the reunion fall off.

3.2. EDS Analysis of 3D Curly Fiber Membranes

Figure 4 shows the EDS element distribution diagram of a 3D curly nanofiber composite membrane with a mass fraction of 0.5 wt% added TiO₂ electret particles. The C, O, F, and Ti elements are marked in red, yellow, green, and purple, respectively. By combining Figure 4 and Table 2, it can be seen that the proportion of the C and F elements is relatively high. The fiber outline can be slightly observed through the C and F element distribution diagram. This is because the fiber-based material PVDF is dominated by C elements and F elements. The O element occupies a low proportion, and the Ti element occupies the lowest proportion. It is almost impossible to see the fiber outline from the element distribution of O and Ti, and it shows that O and Ti elements are evenly distributed on the nanofiber membrane. The electret particle TiO₂ NP was successfully compounded on 3D curly nanofibers.

3.3. FTIR Analysis Nanofiber Membranes

Figure 5 shows FTIR spectra of different nanofiber membranes, respectively. At 1401 cm⁻¹, there is the stretching vibration absorption peak of CH2 [29]. At 1274 cm⁻¹ and 1177 cm⁻¹ there are peaks due to the symmetrical and asymmetrical stretching of CF2, which are the main peaks in PVDF and represents the fluoride part in PVDF. At 1071 cm⁻¹, the vibration of CF2 indicates a special bending vibration of fluoride groups [30]. At 876 cm⁻¹, there is one of the characteristic peaks of PVDF; the peak at 836cm⁻¹ is the bending vibration absorption peak of the CH bond [31]. For the Ti-O stretching vibration, a new broad characteristic absorption peak appears in the range of 500–800 cm⁻¹, which may appear as one or more peaks. The tensile vibration of the O-Ti-O bond in doped TiO₂ is the main reason for the appearance of additional peaks and peak shifts. It shows that TiO₂ NP was successfully compounded into PVDF nanofibers.

3.4. XRD Analysis Nanofiber Membranes

The morphology of the membranes was studied using X-ray diffraction patterns as shown in Figure 6. PVDF has been found so far to crystallize in five polymorphs and forms [32], but mainly in α, β, and γ forms [33]. The diffraction peak at the diffraction angle of 18.5° is the α phase (020) crystal plane, and the diffraction peak at the diffraction angle of 20.6° corresponds to the β phase [34]. The XRD patterns of H-P and L-P fiber membranes are almost the same, indicating that ultra-high environmental humidity has little effect on the crystal form of the prepared 3D curly PVDF nanofibers.

H-P/T1, H-P/T2, H-P/T3, and H-P/4 fiber membranes produced new rutile TiO₂ diffraction peaks at 27.4°, 36.1°, and 54.4°, confirming that TiO₂ NP was introduced into the fiber membranes [35]. As the TiO₂ NP content increases, the α phase diffraction peak of PVDF gradually becomes weaker, indicating that the addition of TiO₂ NPs may inhibit the generation of the α phase of PVDF.

3.5. Nanofiber Membrane Surface Electret Effect

Based on the dominant dipole charge in PVDF fiber, the surface potential of the fiber membrane is negative [36]. Each sample was tested at 20 points. Figure 7 shows that the surface potential of densely packed PVDF nanofiber membrane (P-L) prepared in a low-humidity environment is about −6.42 kV.

The surface potential of the 3D curly PVDF nanofiber membrane (H-L) prepared in a high-humidity environment is significantly reduced. This may be due to the fact that when the nanofibers are prepared in a high-humidity environment, water molecules in the air take away part of the charge, causing the resulting nanofibers to have a reduced charge.

After adding electret particle TiO₂ NPs, the surface potential of the 3D curly nanofiber membrane increased significantly. This is because the addition of TiO₂ NPs increased the area between the crystalline and amorphous regions and introduced polar groups. The Maxwell–Wagner effect is caused by the interface between PVDF and TiO₂ NPs [37,38]. The charges in PVDF and TiO₂ NPs will move in the same direction under the action of an external electric field. Due to the different charge transport capabilities of PVDF and TiO₂ NPs, a charge barrier is generated at the interface, making it difficult for the charges to transfer at the interface. Accumulate to form interface charges [39], it can be seen from the XRD pattern that the addition of TiO₂ NPs also affects the crystal phase formation of PVDF fiber, which may also be another reason for its better performance in terms of electret effect. With the increase of TiO₂ content, the electret effect decreases, which may be due to the formation of charge channels due to the aggregation of TiO₂, resulting in uneven charge distribution and decreased charge storage capacity, but it is still significantly higher than that of H-P a fibrous membrane without adding TiO₂.

3.6. Porosity of Nanofiber Membranes

Compared with ordinary woven materials, electrospinning membranes have higher porosity, but most of them are dense fiber membranes like L-P fiber membranes.

The porosity of the fiber membrane was evaluated by the following formula:
where V₀ represents the volume of the membrane in its natural state, and V refers to the absolute dense volume of the material. Because V₀ and V are not easy to obtain directly, it is deduced from the following formula:
where m represents the quality of the membrane, which is measured by a high-precision electronic scale; ρ denotes the density of raw materials; S represents the area of the sample membrane; and h represents the thickness of the membrane that was obtained based on the SEM images of side views.

As shown in Figure 8, the porosity of a L-P fiber membrane is close to 70%. As can be seen intuitively from Figure 1 and Figure 2, a 3D curly fiber membrane has a highly fluffy structure, and this special spatial structure brings higher porosity. Compared with the tightly packed L-P fiber membrane, the porosity of a fluffy 3D curly fiber membrane is greatly improved. Due to the good supporting effect of 3D crimped fibers and electrostatic repulsion between fibers, the porosity of an H-P fiber membrane reaches about 96.93%. This result is consistent with the side-view results of L-P (densely packed membrane) and H-P (fluffy membrane) shown in Section 3.1. The gap between fibers of the 3D curled membrane is larger, which will be beneficial to the electret effect. After the introduction of titanium dioxide nanoparticles, the porosity decreased slightly, but it was still significantly higher than that of L-P fiber membrane.

3.7. Filtration Performance of Nanofiber Membranes

In order to further study the filtration performance of the prepared nanofiber membrane, PM_0.3 is selected as the filtered particulate matter. The results are shown in Figure 9. The filtration efficiency of PVDF nanofiber membrane (L-P) prepared in a low-humidity environment is about 92.63%, and the pressure drop is about 80.5 Pa. Due to the fluffy structure of the three-dimensional curled fiber membrane, the fiber scaffold has a very high porosity, and a space scaffold structure is formed between the fibers. The airflow can more easily pass through the fiber membrane, and the filtration pressure drop of the H-P fiber membrane is significantly reduced. But, its filtration efficiency is seriously reduced, because the electrostatic adsorption effect of the H-P fiber membrane prepared in a high-humidity environment is low, the fiber is fluffy and the pore size is too large, and the mechanical filtration in PVDF fiber membrane is dominant in this case. After the introduction of electret-enhanced particle TiO₂ NPs, the filtration efficiency is significantly improved.

With the increase in TiO₂ NP content, the filtration efficiency increased from 70.1% to 99.197%. On the one hand, this is attributed to the decreases in the diameter and pore sizes of nanofibers after adding TiO₂, and the slight increase in particles intercepted on the fiber surface and adsorbed inside the fiber through gravity sedimentation and diffusion effects. On the other hand, with the addition of TiO₂, the initial surface potential of the fiber membrane improves the ability of the fiber membrane to store space charges and polarized charges and then improves the electrostatic adsorption ability of the fiber membrane. In this case, the electrostatic effect is dominant.

As particles pass through the nanofiber membrane, they are intercepted and adsorbed by the fiber membrane under the influence of the sum of the interception effect, Brownian diffusion, gravity effect, inertial impaction, and electrostatic deposition [40,41], and, thus, greatly improving the filtration performance of the fiber membrane. When the content of TiO₂ increased from 0 wt% to 0.5 wt%, the filtration performance of the fiber membrane showed an upward trend, and the quality factor reached about 0.096 at 0.5 wt%.

When the TiO₂ NP content is further increased, a decrease in filtration efficiency can be observed, which may be because the aggregation of TiO₂ NPs reduces the electrostatic adsorption effect of the nanofiber membrane. And when the TiO₂ NP mass fraction increases to 1 wt%, it significantly affects the morphology of the fiber, reduces the uniformity of the fiber membrane, and, thus, affects the filtration performance of the electrospinning membrane.

4. Conclusions

In this article, 3D curly nanofibers were prepared by electrostatic spinning in a high-humidity environment above 95RH%. Compared with the densely packed nanofiber membrane prepared in a normal-humidity environment, the 3D curly nanofibers formed a highly fluffy nanofiber membrane, and its filter pressure drop decreased significantly. A suitable quantity of TiO₂ NPs was incorporated as an electret reinforcement to fabricate a highly voluminous 3D curly PVDF/TiO₂ nanofiber membrane. The element distribution, functional groups, and element content on the surface of the fiber membrane were analyzed by EDS, FTIR, and XRD, which showed that TiO₂ NPs had been successfully compounded on PVDF fiber. It provides some basic research for PVDF/TiO₂ composite fiber membranes. The doping of a small amount of TiO₂ NPs has no obvious influence on the 3D curly fluffy structure. The addition of electret particle TiO₂ NPs obviously enhances the electrostatic adsorption of the fiber membrane, greatly improves the electrostatic effect in the process of fiber membrane filtration, and improves the filtration efficiency of 3D curly nanofiber membranes. The filtration efficiency of the H-P/T2 fiber membrane with a TiO₂ NP addition amount of 0.5 wt% is about 99.197%, and the pressure drop is about 49.83 Pa.

The 3D curled nanofiber membrane prepared in this study can not only significantly improve the comfort and efficiency of personal protective equipment but it also reduces energy consumption in the field of industrial air purification, showing the great potential of material innovation in meeting the global air quality challenges and promoting sustainable development.

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