Synthesis of Electrospun PAN/TiO2/Ag Nanofibers Membrane As Potential Air Filtration Media with Photocatalytic Activity.

The PAN/TiO2/Ag nanofibers membrane for air filtration media was successfully synthesized with electrospinning method. The morphology, size, and element percentage of the nanofiber were characterized by a scanning electron microscopy-energy dispersive spectroscopy, while X-ray fluorescence and FTIR were used to observe the chemical composition. The water contact angle and UV-vis absorption were measured for physical properties. Performance for air filtration media was measured by pressure drop, efficiency, and quality factor test. TiO2 and Ag have been successfully deposited in nonuniform 570 nm PAN/TiO2/Ag nanofibers. The nanofiber membrane had hydrophilic surface after TiO2 and Ag addition with a water contact angle of 34.58°. UV-vis data showed the shifting of absorbance and band gap energy of nanofibers membrane to visible light from 3.8 to 1.8 eV. The 60 min spun PAN/TiO2/Ag nanofibers membrane had a 96.9% efficiency of PM2.5, comparable to results reported in previous studies. These properties were suitable to be applied on air filtration media with photocatalytic activity for self-cleaning performance.

Introduction

Air pollution, which consists of hydrocarbon, nitrogen oxide, and particulate matter 2.5 (PM2.5) is harmful to the respiratory system and may even increase premature deaths.[1] Several studies report that high PM2.5 concentration could cause various respiratory infections and cognitive impairment such as Alzheimer’s disease and dementia.[2,3] In addition, many bacteria, such as Micrococcus, Staphylococcus, andAerococcus, are found in urban areas and also harm human health.[4] Wearing masks is one of the common solutions to minimize personal exposure to pollution and to prevent infection. Nowadays, the N95 facemask is one of the most effective masks used in the commercial sector with 95% efficiency. However, it is only recommended for 8 h of use and must be replaced with a new mask.[5] Unfortunately, the use of disposable masks increases the number of facemask waste that leads to environmental pollution.[6] Furthermore, it might lead to a financial burden for some people. Therefore, facemasks with self-cleaning ability and good air filtration performance are required.

Wang et al.[7] have recently reported that they have successfully developed PAN/TiO2/Ag nanofiber membrane for application in wastewater purification. It has excellent photocatalytic activity in degrading dyes under visible light and good antibacterial activity againstEscherichia coliandStaphylococcus aureus. The study demonstrates the possibility of developing nanofiber membrane with self-cleaning performance. In addition, the pure polyacrylonitrile (PAN) nanofiber membrane and PAN nanofibers with active substance are known as good air filtration media with high removal of PM2.5.[8−10]

Nanofibers are one-dimensional structures having diameters of less than 1 μm.[11] Nanofibers have been used as high-performance air filtration because of their thin fiber diameter, wide surface area, good surface adhesion, low density, and high porosity.[12] Their pore structures are small and tortuous and are linked like a membrane, allowing collection of PM2.5 particles from the air while minimizing pressure drop.[13]

Electrospinning is one of the most common methods to synthesize nanofibers.[14] The principal components in the electrospinning process are a high-voltage source, a syringe pump, a nozzle, and a collector.[15] The continuous strand of a polymer solution is ejected through a nozzle by the syringe pump while the solution is drawn to the collector by the high electrostatic force. Nanofibers morphology can be controlled by adjusting process parameters including high voltage, the distance between nozzle and collector, flow rate, and the concentration of polymer solution.[16]

In this paper, we will discuss the synthesis of the PAN/TiO2/Ag nanofiber membrane as an air filtration media with self-cleaning properties. In our view, it is a novel development and has not been reported previously. The PAN/TiO2/Ag nanofiber membrane will be produced through electrospinning method. Its performance will be compared to the PAN nanofiber membrane, other air filter membranes in previous studies, and commercial Semi-High Efficiency Particulate Air Filter (HEPA).

Results and Discussion

Characterization of the Nanofiber Membrane

Figure shows the morphology and the diameter of PAN and PAN/TiO2/Ag nanofiber membranes at 2000 times and 10000 times magnifications. On the basis of Figure a, the PAN nanofibers’ diameter shows normal distribution with the average of 358.9 nm, standard deviation of 0.59, and coefficient of variation (CV) 0.16. This PAN nanofiber membrane has uniform fibers since the CV value is below 0.3. On the basis of the histogram of Figureb, the addition of TiO2/Ag results in an increase in the diameter of nanofibers to 570 nm followed by a wider size distribution with the CV of 0.37. This result corresponds to the results of Ren et al. and Ji et al., who reported increasing diameter of PAN after TiO2 addition.[17,18]

Figure 1

SEM images of (a) PAN and (b) PAN/TiO2/Ag nanofiber membranes. (c) EDS spectrum of the PAN/TiO2/Ag nanofiber membrane.

Figure b also shows the agglomeration of TiO2/Ag particles on the nanofiber membrane. The presence of TiO2 and Ag is shown on the SEM image as grain-like structures in the fiber. The agglomeration is due to high particle bonding among TiO2 and poor dispersion of Ag, similar to the findings of Wang et al.[19] and Ren et al.[17]

Figure shows X-ray fluorescence (XRF) patterns of PAN and PAN/TiO2/Ag, and XRF has been used to determine the presence of Ti and Ag. The blue line with multiple low peaks represents the PAN nanofiber membrane with no Ti and Ag in the fiber. Meanwhile, two high-intensity peaks have appeared after the addition of TiO2 and Ag. Those peaks correspond to Lβ1 Ag at 3.011 keV and Kα TiO2 at 4.533 keV.[3,20−24]

Figure 2

XRF patterns of PAN and PAN/TiO2/Ag nanofiber membranes.

Furthermore, to support the XRF data above, we have measured the elemental composition of the PAN/TiO2/Ag nanofiber membrane with the energy dispersive spectroscopy (EDS) spectrum. Figure c shows the EDS spectrum of the PAN/TiO2/Ag nanofiber membrane, which consists of Ti, Ag, and elements of PAN. The percentage of each element is given in Table . The C and N elements originate from PAN with the chemical formula (C3H3N).[25] The O element is from the oxide of Ti. The Al element is from the aluminum foil used as the substrate of the sample.

Table 1

Percentage of Elemental Composition in the PAN/TiO2/Ag Nanofiber Membrane

element	wt %
C	49.11
N	12.95
O	10.25
Al	2.18
Ti	16.05
Ag	9.46

Figure shows FTIR spectra (absorbance) of PAN and PAN/TiO2/Ag nanofiber membranes. The peaks between 500 and 1000 cm–1 on the PAN/TiO2/Ag nanofiber membrane are from TiO2. The PAN nanofiber membrane shows characteristic peaks for the stretching vibration of the nitrile group (C≡N) at 2242 cm–1 and the weak ether peak (C—O—C) at 1078 and 1260 cm–1.[26,27] The peaks at 1360, 1450, 1624, 2870, and 2930 cm–1 correspond to the vibration of aliphatic CH groups, which also originate from PAN molecules.[27−31] After TiO2 and Ag are added, there are significant changes including two new peaks at 663 cm–1 related to the lattice vibration of TiO2 (Ti—O stretching)[32] and at ∼1300 cm–1 for TiO2—Ag bonding.[32−34] Accordingly, the broad peak at 1335 cm–1 is attributed to TiO2—Ag bonding, which indicates that the Ag and TiO2 have been successfully deposited in the PAN nanofiber membrane. A new broad peak at 3427 cm–1 on PAN/TiO2/Ag is attributed to the stretching vibration of the −OH due to the absorption of water molecules and CO2 from the air.[33] The peaks from DMF[35] (1677, 1388, 1092, and 659 cm–1) do not appear on the PAN/TiO2/Ag nanofiber membrane. On the basis of the result above, it can be concluded that the electrospun nanofibers do not contain residue solvent.

Figure 3

FTIR spectra of PAN and PAN/TiO2/Ag nanofiber membranes.

On a similar note, FTIR spectra indicates that there is bonding between Ag and TiO2, indicating that there are reactions between TiO2 and Ag. A previous study by Zhang et al.[36] has reported that the incorporation of Ag into TiO2 results in partial reaction between the two substances. This reaction occurs when Ag is in the form of Ag+. In this study, AgNO3 is used as the Ag source, which would yield Ag+ ions and eventually react with TiO2. Furthermore, the existence of Ag+ ions would trap some electrons from TiO2, causing charge separation. The electrons would then be transferred to oxygen to form highly oxidative species, which would be beneficial to photocatalytic as well as antibacterial activity of the nanofiber. A similar result has been demonstrated by Rupa,[37] who have analyzed the photodegradation performance of TiO2/Ag nanoparticles. The key of this photocatalytic process is the photogenerated electron–hole pairs. To generate the electron, absorbed light must have sufficient energy that is equal to or larger than the band gap energy. The smaller the band gap energy is, the easier the photogenerated process will be. Absorbance of the PAN nanofiber in Figure a mostly occurs with a wavelength of less than ∼300 nm, which is in the UV light region. Meanwhile, the PAN/TiO2/Ag nanofiber membrane is able to absorb light with a higher wavelength up to 400 nm, which is in the visible light region.

Figure 4

(a) UV–vis spectra of PAN and PAN/TiO2/Ag. (b) Band gap energy plots of PAN and PAN/TiO2/Ag.

Furthermore, Figure b displays the band gap energy plots of the PAN and PAN/TiO2/Ag nanofiber membranes, which show the shifting band gap energy of the PAN nanofiber membrane from 3.8 to 1.8 eV. The mechanism of band gap shifting in TiO2/Ag has also been reported by several studies.[24,32,37] Under visible light, Ag acts as a photosensitizer, collecting the visible light to generate the electron to the conduction band of TiO2. The charge separation generated by the existence of Ag would create sites near the conduction band of TiO2. The shifting of the Fermi energy nearer to the conduction band would result in the narrowing of the band gap energy, thus improving the photocatalytic activity. The improvement in photocatalytic activity is also supported by various studies that demonstrate TiO2/Ag to have good photocatalytic activity under visible light.[7,23,33,38−40]

Photocatalytic properties of PAN/TiO2/Ag are related to the membrane’s ability in dye degradation. Irradiating each sample with UV and visible light enables further observations. Table shows the results of exposing the PAN nanofiber membrane and the PAN/TiO2/Ag nanofiber membrane samples to UV light for 1 h after adding methylene orange and methylene blue solutions. The PAN nanofiber membrane shows excellent dye degradation on 60 ppm methylene orange drops after 1 h of UV irradiation. However, the degradation with 80 ppm methylene orange drop does not reach completion. The degradation is because of PAN’s chemical properties as a polymer that can serve as a photocatalyst.[41−43] Meanwhile, the methylene blue droplets on the PAN nanofiber membrane only slightly degrade after 1 h of irradiation. On the other hand, the PAN/TiO2/Ag nanofiber membrane loses all of its dyes and turns brown as if it has been burned. It can be explained that the browning of nanofiber membrane is due to the existence of Ag which is oxidized and browned after UV irradiation.

Table 2

Degradation of Dyes under UV Light for 60 min on a Nanofiber Membranea

PAN on the left side of each the pictures and PAN/TiO2/Ag on the right side of each the pictures (photos were taken by Sri Hartati).

Table shows the color degradation of methylene blue under visible light on the PAN nanofiber membrane and the PAN/TiO2/Ag nanofiber membrane after adding 60 and 80 ppm methylene blue, respectively. The 60 ppm methylene blue dyes on the PAN/TiO2/Ag nanofiber membrane fade and disappear within 20 min. Meanwhile, the 80 ppm methylene blue vanishes within 25 min under visible light. In addition, the color of methylene orange at the concentration of 60 ppm completely vanishes after 25 min under visible light. Meanwhile, at the 80 ppm concentration, it disappears within 30 min.

Table 3

Degradation of Methylene Blue under Visible Light for Several Minutes on a Nanofiber Membranea

PAN on the left side of each the pictures and PAN/TiO2/Ag on the right side of each the pictures (photos were taken by Sri Hartati).

Wang et al. reported that dye degradation of the PAN/TiO2/Ag nanofiber membrane depends on the chemical structure and properties of the pollutants.[7] Therefore, the difference in degradation time between methylene blue and methylene orange is affected by their properties. Thus, the PAN/TiO2/Ag nanofiber membrane photocatalytic is proven to occur under visible light irradiation.

The wettability of samples can be characterized by observing the water contact angle on the membrane’s surface. It represents the hydrophobicity and hydrophilicity of the surface. A hydrophobic surface can be observed when the contact angle is higher than or equal to 90°. Meanwhile, a hydrophilic surface has a contact angle lower than 90°.[44]Figure shows the water contact angle on the PAN and PAN/TiO2/Ag nanofiber membranes. As shown in Figure a, the surface of the PAN nanofiber membrane yields the nearly spherical shape of a sessile drop with a 123.56° contact angle. This indicates that the PAN nanofiber membrane can be categorized as a hydrophobic membrane. On the other hand, the PAN/TiO2/Ag nanofiber membrane has a 34.58° contact angle, as shown in Figure b, indicating that the PAN/TiO2/Ag nanofiber membrane is hydrophilic. It has been confirmed that the hydrophobic PAN nanofiber membrane became hydrophilic after the addition of TiO2/Ag. This is due to the existence of TiO2, in accordance with several studies about TiO2 as nanoparticles or nanotube or nanofibers that show a water contact angle lower than 90° and consistently provide hydrophilic surface.[19,45,46]

Figure 5

Image of a water droplet on the surface of (a) PAN and (b) PAN/TiO2/Ag nanofiber membranes.

Moreover, the addition of Ag to TiO2 decreases the optical band gap of TiO2, thus, photocatalytic TiO2 and Ag are easier to react with water under visible light, which potentially forms the hydrophilic surface.[7,39] Self-cleaning based on the hydrophilic surface is achieved by the photocatalytic role, which is usually generated by TiO2.[18,40,47−49] Several studies have reported that the photogenerated hole of TiO2 reacts with water in 1 μs and the electron capture reaction enhances the water absorption.[50,51] These reports indicate that the PAN/TiO2/Ag nanofiber membrane can react with water. Thus, the PAN/TiO2/Ag nanofiber membrane becomes hydrophilic.

In the air filter application, the hydrophobic membrane is excellent as an air filter because its self-cleaning surface allows the water droplets to roll up particles on the surface easily.[52] But this process only cleanses the surface of the membrane, while the trapped particle on the fiber sidelines is not removed in this self-cleaning process. The hydrophilic nanofiber membrane can clean up the particle on the sideline with the photocatalytic process. It yields reactive oxygen species that can oxidize the organic molecules and kill the bacteria. In addition, TiO2 and Ag have been reported to have high antibacterial activity.[42,53]

Air Filtration Performance

Particulate air filters are classified into two categories based on the position of the captured particles including surface filters and depth filters. A surface filter is the common filtration hindering the particles larger than the pores of the membrane. The collector of the particle can be made from metal wire mesh, a perforated plate, or a chemical porous membrane (cellulose acetate). Meanwhile, on the depth filters, particle capturing is on the inside medium layer. A depth filter with a high solid fraction is formed by granular filling layers, porous filter media, and thick filter paper. On the other hand, a depth filter with a low solid fraction generally forms a fibrous filter, a thin paper air filter with high efficiency, a foam media filter, etc.[54] A membrane filter in nanofiber form has many advantages to filter ultrafine particles. Woven or nonwoven nanofiber membrane filtration with porosity is highly capable of filtering PM2.5 in the air. The performance of a nanofiber membrane as filtration media can be evaluated by several parameters, such as quality factor, efficiency, and pressure drop.[55]

Pressure drop is one important parameter that must be considered in air filtration application. The pressure drops, or pressure gradient along the airflow direction across the nanofiber membrane, have a direct impact on the air filtration performance. An ideal air filtration system must have both high efficiency and low pressure drop. However, the high efficiency of air filtration is often accompanied by a high pressure drop.[56−58] Good air filtration must have a linear plot between the pressure drop and face velocity as well as obtain a straight line on data plotting (ΔP) against face velocity which follows Darcy’s law.[59]

Figure shows the linear fitting pressure drop of PAN and PAN/TiO2/Ag nanofiber membranes. According to Figure , the PAN nanofiber membrane has a higher pressure drop than the PAN/TiO2/Ag nanofiber membrane for the same spinning time. This is due to the small pores possessed by the PAN nanofiber membrane with small average size nanofibers. The small pores hinder the air flow through the membrane, which leads to an increase in the pressure drop.[58,60]

Figure 6

Pressure drops of the PAN/TiO2/Ag membrane.

Meanwhile, the PAN/TiO2/Ag nanofiber membrane shows a lower pressure drop than the PAN nanofiber membrane. It is due to the wide diameter distribution and morphology of the nanofibers. On the basis of the SEM image (Figure b), the fiber diameter varies widely from 100 to 1200 nm, with an average diameter of 570 nm. The existence of large nanofibers among small PAN/TiO2/Ag nanofiber membrane creates space between them. It provides the pathways for air to flow through the membrane.[13,56] Therefore, the air particles easily pass through the membrane, which leads to the lowering of the pressure drop. On the other hand, the PAN/TiO2/Ag nanofiber membrane (spun for 60 min) shows the highest pressure drop. It is acceptable since additional time increases the thickness of the membrane. In general, when the morphology of nanofibers is uniform, the thick membrane yields a high pressure drop since the membrane passes less air. Furthermore, increasing the membrane thickness will improve its ability to capture particles, leading to higher efficiency of the PAN/TiO2/Ag nanofiber membrane.

Efficiency is another parameter to determine good air filtration media. The efficiency of PM2.5 has been measured to test the air filter performance of the PAN and PAN/TiO2/Ag nanofiber membranes. Incense smoke used in this study as PM2.5, as it represents a real pollutant.[56]Table shows that the PAN/TiO2/Ag nanofiber membrane has a lower efficiency on filtering the PM2.5 than the PAN nanofiber membrane, both spun for 30 min. This is due to the PAN nanofiber membrane composition, which consists of the uniform small nanofiber. As mentioned above, the small size fiber of the PAN nanofiber membrane produces small pores. Furthermore, the small pores will effectively capture the particles, indicating the enhanced efficiency of the membrane. Meanwhile, the PAN/TiO2/Ag nanofiber membrane has nonuniform diameter distribution, causing the existence of large pores. These large pores will pass more air particles, which lowers the efficiency of the membrane.[56,58] Indeed, increasing the spinning process increases the efficiency of the membrane. A 96.9% efficiency has been obtained for the PAN/TiO2/Ag membrane for 60 min of spinning, as shown in Table .

Table 4

Filtration Efficiency of PM2.5 of PAN and PAN/TiO2/Ag Nanofiber Membranes

	efficiency (%)
test	PANa	PAN/TiO2/Aga	PAN/TiO2/Agb
1	81.7	58.8	94.9
2	82.7	58.7	96.9
3	82.4	59.5	97.6
4	82.7	61.7	97.7
5	83.2	62.9	97.4
mean	82.6 ± 1.3	60.3 ± 0.4	96.9 ± 2.7

Spin for 30 min.

Spin for 60 min.

The filtering efficiency of NaCl particles has also been measured in this study. Figure shows the efficiency of the 30 min spinning of PAN and PAN/TiO2/Ag nanofiber membranes for NaCl particles with a diameter range of 24–362 nm. For fine particle air filtration, there are three particle capturing mechanisms including diffusion, interception, and impaction. The diffusion mechanism is based on the Brownian motion caused by the collision of gas molecules and particles.[54] Particles that move by Brownian diffusion in their direction can deviate from the air flow through the filter and then hit and stick to the nanofiber.[61,62]

Figure 7

Efficiency vs the NaCl particles diameter on the PAN nanofiber membrane and the PAN/TiO2/Ag nanofiber membrane for 30 min of spinning.

The interception mechanism occurs when the particle with a certain size is not captured by Brownian diffusion.[62] The particle will follow the streamline of air flow perfectly and deposit on the fiber due to interception effect by the fiber.[61] The particle will touch the fiber directly when the distance of particle is half of the diameter of the fiber.[63] Meanwhile, the impaction mechanism occurs when the particle could not capture via the Brownian diffusion and interception mechanism. The particle will deviate from the streamline air flow due to the inertial effect of the particle or the impact of the external forces, electric or gravitational.[61,63]

Mostly, the particles below 100 nm are collected by the diffusion mechanism, while interception and impaction are effectively involved with particles sizes of 500 nm or above.[59,64,65] The combination of three mechanisms will yield the efficiency graph, as shown in Figure . From this graph, the most penetrating particle size (MPPS) can be determined along with the particle size at the lowest efficiency. In general, the MPPS range is between 50 and 500 nm.[58,59,66−69] The MPPS of both the PAN/TiO2/Ag nanofiber membrane and the PAN nanofiber membrane are 98.84 nm. However, the small pores of the PAN nanofiber membrane effectively capture the particles, resulting in a higher efficiency PAN nanofiber membrane in comparison to the PAN/TiO2/Ag nanofiber membrane.

Comparison with a Previous Study

In this study, we also compare the PAN/TiO2/Ag nanofiber membrane with previous study about filter membrane. It has explored the possibility of membrane application as respirator media. The comparison takes into account the quality factor of each membrane: the higher the quality factor, the better the membrane quality. For this, we have used the PAN/TiO2/Ag nanofiber membrane for 60 min of spinning due to the higher pressure drop and efficiency.

Figure shows the comparison of the quality factor of the PAN/TiO2/Ag nanofiber membrane followed by membranes reported in other studies with PM2.5 as flow particles. The gray bar represents our sample, whereas the red bars are from the literature. Bowin95 (commercial mask), Bowin99 (commercial mask), and ABS are the air filter membranes, which have been reported by Zulfi et al.[56] Meanwhile, the PVA, PVDF, PAN (nanofiber based filter), and commercial Semi-HEPA filter has been reported by Kim.[65] The quality factor can be calculated with the following formula:where η is the efficiency and P is the pressure drop of the membrane.

Figure 8

Quality factor of PAN and PAN/TiO2/Ag compared to results of other studies and commercial membranes.

According to Figure , the PAN/TiO2/Ag nanofiber membrane has a 2.45 × 10–2 Pa–1 quality factor. It is almost at the same level with the ABS air filter membrane. This quality factor is higher than the quality factor of a commercial mask and other air filtration media. On the basis of this result, we are optimistic that the PAN/TiO2/Ag nanofiber membrane can be applied as air filtration media with self-cleaning ability.

Experimental Section

Material and Synthesis

PAN and N,N-dimethylformamide (DMF) were purchased from Sigma-Aldrich, Singapore. TiO2 and AgNO3 from Merck were purchased from a local supplier. The fabrication of nanofiber membrane was using electrospinning, which had been used by Zulfi et al.[56] The precursor solution was dissolved with DMF by a magnetic stirrer at 50 °C for 12 h with specific concentrations based on Table .

Table 5

Precursor Solution for PAN and PAN/TiO2/Ag Nanofiber Membranes

precursor	PAN	TiO2	Ag
PAN	10 wt %
PAN/TiO2/Ag	10 wt %	0.5 wt %	2 wt %

The precursor solution was poured into a 10 mL syringe with an 0.8 mm inner diameter needle. It was placed on the electrospinning syringe pump with a 15 cm long distance to the rotary drum collector. The drum collector had a 5.5 cm diameter and a 12 cm length. It was wrapped with aluminum foil to facilitate nanofiber collection. A high voltage of 15 kV and flow rate of 0.5 mL/h were applied in this process.

Characterization

The morphology and size of the PAN/TiO2/Ag nanofiber membranes were investigated using a scanning electron microscope (SEM Thermoscientific Quanta 650) with 2000 times and 10000 times magnifications. SEM images of the nanofibers were analyzed using the Image MIF v3.0 software to obtain the size distribution of nanofibers on 100 fibers randomly. The average distribution diameter was analyzed statistically. The fiber uniformity was determined from the coefficient of variation (CV) given as follows.where σf is the standard deviation and μf is the average fiber diameter. The value of CV below 0.3 indicated that the nanofibers were uniform while above 0.3 indicated nonuniform nanofibers.[69]

The presence of the Ti and Ag was observed using X-ray fluorescence (XRF Rigaku NEX OC+ EZ series number QC1520) with an operating range between 2000 and 15000 keV. The EDS spectrum (SEM Thermoscientific Quanta 650) was used to confirm the percentage of the PAN/TiO2/Ag nanofiber membrane elemental composition.

The functional groups in the PAN/TiO2/Ag nanofiber membrane were analyzed using Fourier transform infrared spectrometry (Thermo-fisher Scientific NICOLET IS10 FTIR spectrometer) with a spectral range of 500–4000 cm–1. The absorbance of PAN and PAN/TiO2/Ag nanofiber membranes was characterized by a double beam UV–vis spectrometer (Labtron LUS-B13 series number M18P21090201)

The wettability of PAN and PAN/TiO2/Ag nanofiber membranes was determined by examining the surface characteristics through the measurement of the water contact angle by using a contact angle meter (CAAI 2320). The 5 μL water droplet was dropped from the needle, which was controlled by a syringe pump after the sample was placed in the holder. Then the droplet on the surface was captured by a camera. The examination was done on five repetitions.

Air Filter Performance

Pressure drops, the efficiency of NaCl particles, the efficiency of PM2.5 particles, and the quality factor of the fabricated nanofiber membrane were measured in this study. Pressure drop and PM2.5 efficiency were measured according to the procedure by Zulfi et al.[56]Figure S1 shows the scheme of the air filtration test system for the efficiency of NaCl particles. Before the NaCl particles with various size were flowed to the membrane, the charge of aerosols (NaCl particles) was dried and neutralized with a concentration below 10 000 particles per cm3. The measurement of the particles with and without samples was averaged for each condition. The efficiency of filtration was measured by comparing the average deviation of the particles without a membrane according to the standard particle filtration method, which was explained in ASTM F2299. It was done with 5.3 cm/s of face velocity, 1.6 LPM of QIN, 5 LPM of QDIL, and 1 LPM of QAE. The entire test was using 5.3 cm/s face velocity, which was commonly accepted as an air filtration test standard.

Conclusions

PAN/TiO2/Ag nanofiber membrane has been successfully synthesized through the electrospinning method. The resulting samples are the PAN nanofiber membrane before and after addition TiO2/Ag. Morphology of the PAN nanofiber membrane shows the uniform distribution, while the PAN/TiO2/Ag nanofiber membrane is nonuniform. The EDS and XRF graphics show the peaks of Ti and Ag, which indicate that Ti and Ag are successfully deposited onto the PAN nanofibers. The FTIR spectra show Ti and Ag bonding is obtained. It also confirms that the PAN/TiO2/Ag nanofiber membrane has a bonding with water molecules. Furthermore, addition of TiO2 and Ag causes conversion of the hydrophobic PAN to be hydrophilic. It also shifts the absorbance and band gap energy to visible light. According to an air filtration performance test, the PAN/TiO2/Ag nanofiber membrane after 60 min spinning has the highest efficiency of 96.9% for PM2.5. Its quality factor is better than those of commercial masks and several air filters from other research. We conclude that the PAN/TiO2/Ag nanofiber membrane has the potential to be used as an air filtration medium with self-cleaning properties.