Photocatalytic Activity and Filtration Performance of Hybrid TiO2-Cellulose Acetate Nanofibers for Air Filter Applications

A facile method to prepare hybrid cellulose acetate nanofibers containing TiO2 (TiO2-CA nanofibers) by emulsion electrospinning technique was developed for the denitrification and filtration of particulate matters (PMs). This work found that hybrid TiO2-CA nanofibers mainly contain the anatase form of TiO2, contributing to the photodecomposition of NO gas under UV irradiation. The TiO2-CA nanofibers also showed an excellent filtration efficiency of 99.5% for PM0.5 and a photocatalytic efficiency of 78.6% for NO removal. Furthermore, the results implied that the morphology of the TiO2-CA nanofibers, such as micro-wrinkles and protrusions, increased the surface hydrophobicity up to 140°, with the increased addition of TiO2 nanoparticles. The proposed TiO2-CA nanofibers, as a result, would be promising materials for highly efficient and sustainable air filters for industrial and home appliance systems.


Introduction
Air pollution is a serious environmental issue, which is continuously burdening our daily lives. In general, the most concerning air pollutants are sulfur dioxide (SO 2 ), nitrogen oxides (NO and NO 2 ), volatile organic compounds (VOCs), particulate matters (PMs), carbon monoxide (CO), carbon dioxide (CO 2 ), ozone, chlorofluorocarbons, and trace heavy metals [1]. Among air pollutants, PMs are comprised of a complex mixture of sulfate, nitrates, ammonia, sodium chloride, carbon black, mineral dusts, and water. Depending on their size, PMs can be classified as PM 0.5 , PM 2.5 , and PM 10 , which denote particle sizes below 0.5, 2.5, and 10 µm, respectively. These PMs are detrimental to human health, since they can penetrate human bronchi and lungs to cause chronic pulmonary disease and lung cancer [2]. In 2013, the World Health Organization also reported that air pollutants such as PMs are carcinogens to humans and can even induce death [3].
Electrospun nanofibers that remove air pollutants such as PMs and VOCs have been of great interest due to their highly specific surface area, interconnected nanoscale pore structures, nanosized fiber diameters, and porous structure, as well as their versatility for incorporating chemical modifications for air filter applications [4]. Particularly, in many applications, such as biomedical or cosmetic products and air filters [5,6], cellulose nanofibers have received increasing attention due to the advantages of large specific surface area, versatile chemical modifications, and good mechanical properties [7].
For a few years, their photocatalytic activities have been well studied for removing various pollutants, such as PMs, SO 2 , NO, NO 2 , and various VOCs, efficiently in air environments [8]. Particularly, photocatalytic degradation using titanium dioxide (TiO 2 ) has been one of the most generally studied methods, because it powerfully converts rich solar energy into active chemical energy that can decompose harmful pollutants in the air [9]. TiO 2 excites electrons under UV illumination from the valence band to the 2 of 11 conduction band and leaves holes in the valence band. At that moment, the electrons change oxygen molecules to superoxide anions and the holes react with water molecules in the air to produce hydroxyl radicals. These two species, superoxide anions and hydroxyl radicals, are very reactive and capable of decomposing air pollutants such as PMs, SO 2 , NO, NO 2 , and VOCs [10].
Several TiO 2 incorporated nanofibers have been of great interest for the efficient removal of air pollutants, because of their well-defined dimensions, high specific surface areas, and greater photocatalytic activities [11][12][13]. For example, polyacrylonitrile (PAN) nanofibers with embedded commercial photocatalysts, P25 and TiO 2 particles, showed an excellent filtration efficiency of 96.75% for PM 2.5 [14]. Dong et al. studied effective strategies for in situ growth of high adhesion TiO 2 to polyvinylidene fluoride (PVDF) nanofibers via electrospinning, coupled with cold plasma pretreatment and hydrothermal processing [15]. On the other hand, bamboo cellulose acetate fibers grafted with TiO 2 have presented the photocatalytic ability to decompose phenol under UV illumination [16]. Lastly, electrospun TiO 2 entrapped-chitosan hybrid nanofibers were developed for the removal of heavy metal ions [17].
However, there are some major problems that restrict the application of photocatalytic fibers using TiO 2 . First is the brittleness of TiO 2 incorporated nanofibers. Li et al. have reported brittle polycrystalline TiO 2 -incorporated nanofibers produced from a precursor solution, such as titanium alkoxide (Ti(OR) 4 ) with poly(vinyl pyrrolidone), leading to reduced photocatalytic activities until after calcination [18]. Another problem is that the number studies on the application of TiO 2 -incorporated nanofibers and their photocatalytic activities for simultaneous denitrification has not been sufficient thus far. The weak adhesion between TiO 2 and the polymeric nanofibers is also a key obstacle that inhibits their practicability. Moreover, the high photocatalytic activity of TiO 2 -incorporated nanofibers, without the degradation of polymeric substrates, remains a critical challenge.
In order to overcome these drawbacks of TiO 2 -incorporated cellulose nanofiber applications, we designed a fast and facile fabrication method for hybrid cellulose acetate nanofibers containing TiO 2 (TiO 2 -CA nanofibers), by using emulsion electrospinning to reduce weak adhesion and brittleness between TiO 2 and the polymeric nanofibers. There is the need for a detailed investigation of the photocatalytic effects of TiO 2 incorporated into nanofibers for the removal of NO gas. In this study, we present a fast and facile fabrication method for hybrid cellulose acetate nanofibers containing TiO 2 (TiO 2 -CA nanofibers), by using emulsion electrospinning, and evaluate the photodecomposition of nitrogen oxides under UV illumination, as well as the filtration efficiency.

Synthesis of TiO 2 Nanoparticles
10 mL of the titanium isopropoxide (TTIP) as a precursor was mixed with 40 mL isopropyl alcohol and stirred for 30 min. Then 10 mL of a mixture (1:1) of deionized water and isopropyl alcohol was added gradually in drops into the TTIP mixture to form a colloidal solution under vigorous stirring. The pH of the obtained colloidal solution was adjusted using NaOH solution and irradiated by sonication (Power sonic 510, Seoul, Korea) at 40 kHz for 30 min. The colloidal solution was dried in an oven at 110 • C for 3 h and TiO 2 nanoparticles were calcinated at 400 • C for 1 h.

Fabrication of TiO 2 -CA Nanofiber Webs
TiO 2 -CA nanofibers were fabricated using emulsion electrospinning. First, 10 wt% CA pellets were dissolved in a mixture of N,N-dimethylacetamide and acetone at the ratio of 1:2 (v/v). The solution was then stirred at 500 rpm and room temperature for 5 h. Afterwards, 5 or 10 wt% of synthesized TiO 2 nanoparticles of CA pellets were put into the CA solution. To evenly disperse TiO 2 nanoparticles in CA solutions, sonication (Power sonic 510, Seoul, Korea) was employed at 40 kHz for 30 min. The solution was put into a 15 mL syringe and electrospun at a feeding rate of 1 mL h −1 on the roller collector, covered with a melt-blown polyester nonwoven supporter. The distance between the Taylor cone and the collector was 15 cm, and 18 kV was applied for the fabrication of TiO 2 -CA nanofiber webs.

Characteristics of Synthesized TiO 2 Nanoparticles and TiO 2 -CA Nanofibers
An X-ray diffractometer (XRD, D8 Advance, Bruker, Billerica, MA, USA) was used to analyze crystalline phases of the synthesized TiO 2 nanoparticles and TiO 2 -CA nanofibers. The XRD was operated in reflection mode with Cu-K radiation (35 kV, 30 mA) and diffracted beam monochromator, using a step scan mode with a step of 0.075 • and 4 s per step. Diffraction patterns of both anatase and rutile TiO 2 powders were compared with references in the Joint Committee on Powder Diffraction Standards (JCPDS) database.

Contact Angle Measurement and Morphology of TiO 2 -CA Nanofiber Webs
The contact angle of TiO 2 -CA nanofiber webs was measured using a contact angle goniometer (DSA 25, Kruss, Matthews, NC, USA) and the sessile drop technique at room temperature. Then, 10 µL of deionized water (γ LV = 72.8 mN/m) droplet was deposited on CA and TiO 2 -CA nanofiber webs using a syringe, and the measurement was repeated at least five times in order to analyze the hydrophobicity of the TiO 2 -CA nanofiber webs. Additionally, the morphology and average diameter of TiO 2 nanoparticles, untreated CA nanofibers, and TiO 2 -CA nanofibers were analyzed by scanning electron microscopy (SEM, Hitachi, Tokyo, Japan), with an energy dispersive X-ray analyzer (EDX) to evaluate the atom weight percentage on the surface of the TiO 2 -CA nanofibers.

Photocatalytic Activity of TiO 2 -CA Nanofiber Webs for NO Removal
The photocatalytic activities of TiO 2 -CA nanofiber webs were evaluated based on ISO 22197-1:2016 for the removal of NO gas. TiO 2 -CA nanofiber or untreated CA nanofiber (5 cm × 10 cm) samples were placed in the middle of two plain glasses (5 cm × 10 cm) of non-photocatalytic blank samples in the photoreactor. A UV lamp system was placed over the photoreactor, and delivered a UVA irradiance (10 W m −2 ) from 2 × 6 W BLB lamps with a 365 nm emission peak. All three samples were illuminated with UV light. NO x analyzer (T-API, T200, San Diego, CA, USA) was used to measure nitrate concentrations every 1 min. NO gas flowed at a rate of 3 L min −1 containing 1 ppm v of NO in air with 50% of relative humidity at 25 • C under UV light. The concentration of NO in the outlet stream was monitored for 20 min before the light was switched on, and then during the 1 h UV irradiation.

Filtration Efficiency of TiO 2 -CA Nanofiber Webs
For filtration efficiency and penetration (%), TiO 2 -CA nanofiber webs were measured using a TSI-3160 filter tester (TSI Inc., Shoreview, MN, USA). In this system, sodium chloride particles were used as representative PMs. This tester was able to generate sodium chloride nanoparticles with sizes ranging between 100 and 600 nm in the flowing air, and measuring particle penetration versus particle size at 32.28 L min −1 aerosol flow rate and 5.38 cm s −1 face velocity. TiO 2 -CA nanofiber webs were placed at the bottom of a sample holder on a wide-mesh metal net to support the specimen. The percentage penetration of sodium chloride particles passing through the TiO 2 -CA nanofiber webs was determined by the relative particle concentration upstream and downstream of the sample [19]. Figure 1 shows the morphology of the synthesized TiO 2 nanoparticles, which displayed an irregular shape and rough surface without pores. The diameters of the TiO 2 nanoparticles were measured from randomly selected areas of SEM images using Nahwoo imaging software (N ≥ 100) (Iworks 2.0, Suwon, Korea). The average diameter of the TiO 2 nanoparticles was 54 nm (±5.6).

The Morphology of TiO 2 and TiO 2 -CA Nanofibers
Polymers 2021, 13, x FOR PEER REVIEW 4 of 12 sodium chloride particles passing through the TiO2-CA nanofiber webs was determined by the relative particle concentration upstream and downstream of the sample [19]. Figure 1 shows the morphology of the synthesized TiO2 nanoparticles, which displayed an irregular shape and rough surface without pores. The diameters of the TiO2 nanoparticles were measured from randomly selected areas of SEM images using Nahwoo imaging software (N ≥ 100) (Iworks 2.0, Suwon, Korea). The average diameter of the TiO2 nanoparticles was 54 nm (±5.6). As shown in Figure 2, the untreated CA nanofibers had smooth surfaces and no defects. The average diameter size of the CA nanofibers was 278 nm, with a standard deviation of 78 nm; however, the TiO2-CA nanofibers displayed increasing roughness and average diameter size with the addition of TiO2 loadings. CA nanofibers containing 5 or 10 wt% TiO2 induced the formation of abnormal beads on the surface that resulted in an uneven morphology and micro-wrinkles [20]. During electrospinning, a highly volatile solvent can solidify immediately on the surface of the nanofibers while the fluid jet is flying to the collector, whereby it becomes hard for the nanoparticles to be transferred from the core to the shell of the nanofibers [21]. Compared to the diameter range of the 5 wt% TiO2-CA nanofibers, from 200 to 1050 nm, the average diameter of the TiO2-CA nanofibers was 378 nm (σ = 74), as shown in Figure 2c, d. Figure 2e highlights the 454 nm (σ = 126) average diameter of 10 wt% TiO2-CA nanofibers, lying within the 150 to 1300 nm diameter range of 10 wt% TiO2-CA nanofibers. The rough surface and grafted TiO2 nanoparticles are shown in Figure 2g, h. In Figure 2b, f quantitative analyses of each untreated CA nanofiber element and 5 and 10 wt% TiO2-CA nanofibers are compared through an energy dispersive X-ray analyzer (EDX, attached to the SEM). The data show spectra with peaks corresponding to all the different elements. In Table 1, the EDX data show each element concentration percentage for the different samples. For 5 wt% TiO2-CA nanofibers, carbon and oxygen atoms mainly occupied 47.1 wt% and 46.7 wt% of the sample, respectively. Untreated CA nanofibers exhibited no Ti content on the surface. In contrast, 5 and 10 wt% TiO2-CA nanofibers comprised 6.3% and 11.6 wt% Ti on the surface, respectively. From the data, we confirmed that the final TiO2-CA nanofibers maintained a similar weight to the initial addition of TiO2. As shown in Figure 2, the untreated CA nanofibers had smooth surfaces and no defects. The average diameter size of the CA nanofibers was 278 nm, with a standard deviation of 78 nm; however, the TiO 2 -CA nanofibers displayed increasing roughness and average diameter size with the addition of TiO 2 loadings. CA nanofibers containing 5 or 10 wt% TiO 2 induced the formation of abnormal beads on the surface that resulted in an uneven morphology and micro-wrinkles [20]. During electrospinning, a highly volatile solvent can solidify immediately on the surface of the nanofibers while the fluid jet is flying to the collector, whereby it becomes hard for the nanoparticles to be transferred from the core to the shell of the nanofibers [21]. Compared to the diameter range of the 5 wt% TiO 2 -CA nanofibers, from 200 to 1050 nm, the average diameter of the TiO 2 -CA nanofibers was 378 nm (σ = 74), as shown in Figure 2c,d. Figure 2e highlights the 454 nm (σ = 126) average diameter of 10 wt% TiO 2 -CA nanofibers, lying within the 150 to 1300 nm diameter range of 10 wt% TiO 2 -CA nanofibers. The rough surface and grafted TiO2 nanoparticles are shown in Figure 2g,h. In Figure 2b,f quantitative analyses of each untreated CA nanofiber element and 5 and 10 wt% TiO 2 -CA nanofibers are compared through an energy dispersive X-ray analyzer (EDX, attached to the SEM). The data show spectra with peaks corresponding to all the different elements. In Table 1, the EDX data show each element concentration percentage for the different samples. For 5 wt% TiO 2 -CA nanofibers, carbon and oxygen atoms mainly occupied 47.1 wt% and 46.7 wt% of the sample, respectively. Untreated CA nanofibers exhibited no Ti content on the surface. In contrast, 5 and 10 wt% TiO 2 -CA nanofibers comprised 6.3% and 11.6 wt% Ti on the surface, respectively. From the data, we confirmed that the final TiO 2 -CA nanofibers maintained a similar weight to the initial addition of TiO 2 .

XRD Analysis of TiO 2 Nanoparticles and TiO 2 -CA Nanofibers
Both anatase and rutile are well known as stable phases of TiO 2 nanoparticles [12]. TiO 2 nanoparticles are most likely to be a mixture of those phases, rather than pure anatase or rutile; therefore, quantitative analysis is highly important. Figure 3 and Table 2 [22]. For 5 or 10 wt% TiO 2 -CA nanofibers, the new diffraction peaks at 22.6 • mainly represented the crystalline region of the cellulose [16,23]. From the XRD analysis, the results show that crystalline TiO 2 nanoparticles consisted of 87.8% anatase and 12.7% rutile forms, and TiO 2 -CA nanofibers contained both anatase and rutile forms of TiO 2 nanoparticles after fabrication.

The Photocatalytic Effect of NO Removal
In order to confirm the inevitability of UV irradiation for photocatalytic reaction, tests of UV irradiation (turning on UV lamp) and darkness (turning off UV lamp) for denitrification were carried out, as exhibited in Figure 4.

The Photocatalytic Effect of NO Removal
In order to confirm the inevitability of UV irradiation for photocatalytic reaction, tests of UV irradiation (turning on UV lamp) and darkness (turning off UV lamp) for denitrification were carried out, as exhibited in Figure 4. The results show that the NO removal efficiency changed greatly with or without UV irradiation. Specifically, the denitrification efficiency in the dark was less than 0.1%, mainly due to the physical adsorption of NO over TiO2-CA nanofibers. After turning on the UV light for 60 min, the NO removal efficiency of 5 or 10 wt% TiO2-CA nanofibers showed a rapid upward trend and increased to 64.5% and 78.6%, respectively. Untreated CA nanofibers, however, showed no effect with or without UV light.
When the UV light was turned off, the removal efficiency of NO decreased rapidly to the initial adsorption equilibrium level in the dark. This implies that there is a significant effect of UV irradiation on NO removal, meaning that UV light is a vital factor in the photocatalytic reaction of NO gas removal.

The Filtration Efficiency of TiO2-CA Nanofiber Webs
Various sizes of PMs were tested for filtration efficiencies of 10 wt% TiO2-CA nanofiber webs. Figure 5a shows the filtration efficiency (%) and penetration (%) of 10 wt% TiO2-CA nanofibers, depending on NaCl particle size. Clearly, the filtration efficiencies against all sizes of PM were higher than 97%. TiO2-CA nanofiber webs showed increased efficiencies from 97.1 to 99.6% in particle sizes 0.1 (PM0.1) to 0.6 μm (PM0.6). However, penetration decreased from 2.93 to 0.42%. The pressure drop was consistently near 3.13 mm H2O for most particle sizes. The results show that the NO removal efficiency changed greatly with or without UV irradiation. Specifically, the denitrification efficiency in the dark was less than 0.1%, mainly due to the physical adsorption of NO over TiO 2 -CA nanofibers. After turning on the UV light for 60 min, the NO removal efficiency of 5 or 10 wt% TiO 2 -CA nanofibers showed a rapid upward trend and increased to 64.5% and 78.6%, respectively. Untreated CA nanofibers, however, showed no effect with or without UV light.
When the UV light was turned off, the removal efficiency of NO decreased rapidly to the initial adsorption equilibrium level in the dark. This implies that there is a significant effect of UV irradiation on NO removal, meaning that UV light is a vital factor in the photocatalytic reaction of NO gas removal.

The Filtration Efficiency of TiO 2 -CA Nanofiber Webs
Various sizes of PMs were tested for filtration efficiencies of 10 wt% TiO 2 -CA nanofiber webs. Figure 5a shows the filtration efficiency (%) and penetration (%) of 10 wt% TiO 2 -CA nanofibers, depending on NaCl particle size. Clearly, the filtration efficiencies against all sizes of PM were higher than 97%. TiO 2 -CA nanofiber webs showed increased efficiencies from 97.1 to 99.6% in particle sizes 0.1 (PM 0.1 ) to 0.6 µm (PM 0.6 ). However, penetration decreased from 2.93 to 0.42%. The pressure drop was consistently near 3.13 mm H 2 O for most particle sizes. SEM images of 10 wt% TiO2-CA nanofibers after capturing NaCl nanoparticles (represented as PMs) are shown in Figure 5b. Some 0.1-0.6 μm sized PMs were captured at the surface and around nanofibers, revealing a strong electrostatic force was present at the surface of 10 wt% TiO2-CA nanofibers, which attracted PMs that are much smaller than the thru-holes because of the electrostatic force created by air flow friction. A study on the electrosurface properties and interaction of cellulose nanofibers and TiO2 nanoparticles stated that cellulose nanofibers and titanium dioxide nanoparticles were attracted to one another due to electrostatic forces on the surface [24]. SEM images of 10 wt% TiO 2 -CA nanofibers after capturing NaCl nanoparticles (represented as PMs) are shown in Figure 5b. Some 0.1-0.6 µm sized PMs were captured at the surface and around nanofibers, revealing a strong electrostatic force was present at the surface of 10 wt% TiO 2 -CA nanofibers, which attracted PMs that are much smaller than the thru-holes because of the electrostatic force created by air flow friction. A study on the electrosurface properties and interaction of cellulose nanofibers and TiO 2 nanoparticles stated that cellulose nanofibers and titanium dioxide nanoparticles were attracted to one another due to electrostatic forces on the surface [24].

Contact Angle Analysis
The hydrophobicity of untreated CA or TiO 2 -CA nanofiber webs was subsequently measured, with the results of water contact angle measurements from different samples illustrated in Figure 6. In all membranes incorporated with TiO 2 nanoparticles, the value of the contact angles increased with the addition of TiO 2 loadings. To be specific, the contact angle of untreated CA nanofibers was 112.5 • , while that of TiO 2 -CA nanofibers increased from 127.8 • to 140 • with the addition of TiO 2 nanoparticles. When TiO 2 nanoparticles were incorporated with CA nanofibers, the hydrophobicity of TiO 2 -CA nanofibers improved due to the surface roughness, as shown in Figure 2g,h. This supports previous research indicating an improved surface hydrophobicity that stems from morphology traits such as micro-wrinkles and protrusions of nanofibers through the mitigation of wetting and decreasing water-membrane contact area [25]. Guan also studied the relationship between hydrophilicity and photocatalytic activity: a surface with more hydrophilicity resulted in less photocatalytic activity [26]. When photocatalysts included a hydrophobic surface with a nano-or microstructure, it enhanced photocatalytic activities [27]. Since the charges on the hydrophobic surface are transferred quickly to radicals by acceptors (oxygen molecules), oxygen molecules can effectively capture electrons and change oxygen molecules to superoxide anions, which can be attributed to photocatalytic activities [28,29]. Due to these fast transitions to radicals on a more hydrophobic surface, 10 wt% TiO 2 -CA nanofibers showed a higher NO removal, up to 78.6%, than 5 wt% TiO 2 -CA nanofibers. A hydrophobic surface also presents a strong electrostatic force by air flow friction that attracts more PMs to the surface. In this work, the greater hydrophobic surface of 10 wt% TiO 2 -CA nanofibers could cause a stronger interaction between filters and PMs than 5 wt% TiO 2 -CA nanofibers, leading to the removal of air pollutants. The hydrophobicity of untreated CA or TiO2-CA nanofiber webs was subsequently measured, with the results of water contact angle measurements from different samples illustrated in Figure 6. In all membranes incorporated with TiO2 nanoparticles, the value of the contact angles increased with the addition of TiO2 loadings. To be specific, the contact angle of untreated CA nanofibers was 112.5°, while that of TiO2-CA nanofibers increased from 127.8° to 140° with the addition of TiO2 nanoparticles. When TiO2 nanoparticles were incorporated with CA nanofibers, the hydrophobicity of TiO2-CA nanofibers improved due to the surface roughness, as shown in Figure 2g, h. This supports previous research indicating an improved surface hydrophobicity that stems from morphology traits such as micro-wrinkles and protrusions of nanofibers through the mitigation of wetting and decreasing water-membrane contact area [25]. Guan also studied the relationship between hydrophilicity and photocatalytic activity: a surface with more hydrophilicity resulted in less photocatalytic activity [26]. When photocatalysts included a hydrophobic surface with a nano-or microstructure, it enhanced photocatalytic activities [27]. Since the charges on the hydrophobic surface are transferred quickly to radicals by acceptors (oxygen molecules), oxygen molecules can effectively capture electrons and change oxygen molecules to superoxide anions, which can be attributed to photocatalytic activities [28,29]. Due to these fast transitions to radicals on a more hydrophobic surface, 10 wt% TiO2-CA nanofibers showed a higher NO removal, up to 78.6%, than 5 wt% TiO2-CA nanofibers. A hydrophobic surface also presents a strong electrostatic force by air flow friction that attracts more PMs to the surface. In this work, the greater hydrophobic surface of 10 wt% TiO2-CA nanofibers could cause a stronger interaction between filters and PMs than 5 wt% TiO2-CA nanofibers, leading to the removal of air pollutants.

Conclusions
In this study, we have shown a facile and efficient fabrication method to prepare hybrid TiO2-CA nanofibers for photocatalytic denitrification activity and the improved filtration of PMs, and evaluated the photodecomposition of nitrogen oxides under UV illumination, as well as the filtration efficiency for the removal of air pollutants.
The photocatalytic effects of TiO2-CA nanofiber denitrification decomposed NO gas up to 78.6%. A single-layer TiO2-CA nanofiber web effectively filtered and captured PMs (0.1-0.6 μm) up to 99.6%. In addition, further investigations on the possible mechanism between the filter performance and the roughness of the nanofiber filter, as well as the photocatalytic ability to decompose other VOCs, such as toluene, SO2, and formaldehydes under ultraviolet irradiation are required. As a result, this investigation demonstrates a novel method of fabricating TiO2-CA nanofibers and utilizing their high photocatalytic

Conclusions
In this study, we have shown a facile and efficient fabrication method to prepare hybrid TiO 2 -CA nanofibers for photocatalytic denitrification activity and the improved filtration of PMs, and evaluated the photodecomposition of nitrogen oxides under UV illumination, as well as the filtration efficiency for the removal of air pollutants.
The photocatalytic effects of TiO 2 -CA nanofiber denitrification decomposed NO gas up to 78.6%. A single-layer TiO 2 -CA nanofiber web effectively filtered and captured PMs (0.1-0.6 µm) up to 99.6%. In addition, further investigations on the possible mechanism between the filter performance and the roughness of the nanofiber filter, as well as the photocatalytic ability to decompose other VOCs, such as toluene, SO 2 , and formaldehydes under ultraviolet irradiation are required. As a result, this investigation demonstrates a novel method of fabricating TiO 2 -CA nanofibers and utilizing their high photocatalytic efficiency for a wide range of potential applications, including protective clothing systems, sensors, industrial and home appliance filtration systems, and much more. TiO 2 -CA nanofibers are a promising natural alternative to replace synthetic polymers in air filter applications.