Nanotextured CeO2−SnO2 Composite: Efficient Photocatalytic, Antibacterial, and Energy Storage Fibers

Bacterial infections remain a serious and pervasive threat to human health. Bacterial antibiotic resistance, in particular, lowers treatment efficacy and increases mortality. The development of nanomaterials has made it possible to address issues in the biomedical, energy storage, and environmental fields. This paper reports the successful synthesis of CeO2−SnO2 composite nanofibers via an electrospinning method using polyacrylonitrile polymer. Scanning and transmission electron microscopy assessments showed that the average diameter of CeO2−SnO2 nanofibers was 170 nm. The result of photocatalytic degradation for methylene blue dye displayed enhanced efficiency of the CeO2−SnO2 composite. The addition of SnO2 to CeO2 resulted in the enhancement of the light absorption property and enriched charge transmission of photoinduced electron–hole duos, which conspicuously contributed to momentous photoactivity augmentation. Composite nanofibers exhibited higher specific capacitance which may be accredited to the synergism between CeO2 and SnO2 particles in nanofibers. Furthermore, antibacterial activity was screened against Escherichia coli and CeO2−SnO2 composite nanofibers depicted excellent activity. The findings of this work point to new possibilities as an electrode material in energy storage systems and as a visible-light-active photocatalyst for the purification of chemical and biological contaminants, which would substantially benefit environmental remediation processes.


Introduction
Globally, environmental pollution is a serious problem, inflicting harm to life on the planet. Water pollution, among other types of pollution, has a significant negative impact on living species, including aquatic life. Water pollution is due to the discharge of harmful organic chemicals, such as dyes, acids, and antibiotics, into drinkable water bodies, such as rivers, lakes, and ponds, from textile, chemical, and pharmaceutical facilities. In nature, the majority of organic molecules are carcinogenic. Furthermore, water pollution sequentially causes soil pollution, which has a direct or indirect impact on daily living [1,2]. Synthetic color dyes, particularly those generated during textile washes, combine easily with water in comparison to chemicals and reagents; hence, the combination of effluents has hazardous potential. As a result, industrial effluent must be processed prior to being disposed of in the surroundings [3]. Dyes are colored aromatic organic complexes that capture light or coupling with other semiconductors, such as CeO 2 /ZnFe 2 O 4 [33], TiO 2 -WO 3 -CeO 2 [34], CeO 2 /Ni-Al [35], CeO 2 /MnO 2 [36], CeO 2 /SiO 2 [37], CuO-CeO 2 [38], CaO/CeO 2 [39].
Tin oxide (SnO 2 ) is an n-type semiconductor with a band gap of 3.6 eV. SnO 2 has extensively been utilized as gas sensors, transparent conductive electrodes, solar cells, and photocatalysts [40][41][42]. It is expected that mixing SnO 2 with CeO 2 may enhance its activity for various applications. In the present study, CeO 2 −SnO 2 composite nanofibers (NFs) were manufactured via electrospinning and screened for antibacterial, photocatalytic, and electrochemical applications. For decades, antibiotics have commonly been used to treat bacterial infections. However, the fast growth of antibiotic-resistant bacteria has caused several issues that place a significant burden on the medical community. As a result, the use of nanoparticles as an antibacterial substitute has been investigated. Regarding this situation, metal nanoparticles showed broad-spectrum antibacterial action. Furthermore, the use of nanomaterials in the biomedical area allows researchers to solve the issues of bacterial antibiotic resistance due to their distinct antibacterial mechanisms. Several metal and metal oxide-based nanomaterials were recently fully integrated into antibacterial applications and demonstrated remarkable performances [43]. Among them, Ce-and CeO 2 -based nanomaterials garnered a lot of attention across the world. Numerous investigations demonstrated that CeO 2 nanoparticles have remarkable antibacterial activity [44,45]. Several investigations showed that CeO 2 has an antibacterial impact on Staphylococcus aureus [46,47]. Furthermore, some research studies used agar diffusion and microdilution assays to investigate and confirm P. aeruginosa's sensitivity to CeO 2 [48]. In the same way, SnO 2 also received attention as an antibacterial agent, where it was shown to inhibit the development of several bacterial strains, such as S. aureus and E. coli [49,50]. Furthermore, it was also found that SnO 2 disinfects germs effectively once incapacitated with transition metal ions, for instance, Co@SnO 2 and Ag@SnO 2 nanoparticles exhibit powerful antimicrobial properties [51][52][53].
However, to the best of our knowledge, no research work has been published on the antibacterial behavior of CeO 2 −SnO 2 composite NFs. This paper describes the facile manufacturing of CeO 2 −SnO 2 composite NFs via electrospinning and their incredible photocatalytic, electrochemical applications, and antibacterial activity against E. coli. The composite was categorized using various physicochemical techniques, such as XRD, SEM, TEM, FT-IR, PL, and UV-vis. The photocatalytic activity was measured using MB disintegration under visible light radiation. The mechanism of improved photocatalytic activity of composite CeO 2 −SnO 2 NFs was interpreted. The electrochemical efficiency of synthesized materials was scrutinized in terms of the cyclic voltammetry performance, which displayed that the CeO 2 −SnO 2 composite possessed outstanding electrochemical efficacy as a supercapacitor material. Overall, the outcomes of this research highlight the innovative possibilities of utilizing these 1D high-aspect-ratio composite NFs as an electrode in energy storage systems and as a visible light active photocatalyst for the purification of chemical and biological contaminants, which would greatly assist in environmental remediation procedures. Henceforward, composites of these two materials (CeO 2 −SnO 2 ) can serve as an excellent preliminary point for augmenting electrodes and photocatalytic efficiency. Additionally, this combination will be an imminent antimicrobial material with multiple functionalities. To recapitulate, the novelty of this work is that we used a facile electrospinning technique to fabricate composite NFs using cost-effective precursors. The CeO 2 −SnO 2 composite NFs with a high aspect ratio are an auspicious future material and can be applied for antibacterial, photocatalyst, and electrode purposes, all in one. In the general process, for the synthesis of CeO 2 and CeO 2 -SnO 2 composite NFs, PVP (15 wt%) solution was primed by liquefying PVP in DMF on a magnetic stirrer as abovementioned. Tin(II) ethylhexanoate (0.15 mL) was supplemented with 2 mL of ethanol, then transferred into 10 mL of PVP solution. Ce(NO 3 ) 3 ·6H 2 O (1.0 g) was added to produce the final solution. The acquired sol-gel was transported into a 10 mL needle with a stainless steel spike. A copper pin coupled to a high voltage generator was implanted in solution as a positive terminal, while a ground iron barrel roofed with a polyethylene leaf assisted as the counter electrode. The mixture was retained as a capillary by regulating the inclination angle. A voltage of 17.5 kV was maintained to produce the final resultant assortment. The distance between the tip of the needle and the collector was held at 15 cm. Primarily, as-spun composite NFs were desiccated at 80 • C for 24 h in a vacuum. Additionally, to eliminate the polymer, composites were calcined in air at 500 • C at a rate of 2 • C/min for 2 h.

Classification of Pure CeO 2 and CeO 2 −SnO 2 NFs
The XRD outlines of pure and composite NFs were measured using a Rigaku/Max-3A (Tokyo, Japan) with Cu Kα radiation (λ = 1.540 Å) over Bragg angles between 20 • to 80 • . To scan surface features, pristine and composite NFs were examined using field emission scanning electron microscopy (FE-SEM, JSM6700, JEOL, Tokyo, Japan) and highresolution transmission electron microscopy (HRTEM-H-7650 Hitachi, Co., Tokyo, Japan). The elemental conformation of the samples was scrutinized using energy-dispersive X-ray spectroscopy (EDS) coupled to an SEM instrument. The EDS of the sample was taken on carbon tape with a Pt coating. The photoluminescence (PL) spectra of the NFs were measured at room temperature using the 325 nm line of a He-Cd laser at a power of 25 mW for excitation (Kimmon Koha, JP/IK 3302 R). The light absorbance of NFs was measured by means of a UV-vis diffused reflectance spectrum (UV-DRS, 525 Shimadzu).

Photocatalytic Degradation
Photocatalytic activity of CeO 2 −SnO 2 composite has been performed by disintegration of MB dye in visible light treatment using 450 W Xenon lamp. In 250 mL of a solution, there was a dye concentration of 10 mgL −1 and 250 mg of photocatalyst; this was stirred for 1 h in the dark (at a neutral pH). Afterward, around 3 mL of aliquots was taken at regular time intervals from the solution and centrifuged. The absorbance of MB dye mixture was analyzed by a UV-Vis spectrophotometer (Shimadzu UV-3101, UV Probe) at a 664 nm absorbance peak.

Electrochemical Characterization
Cyclic voltammetry was undertaken in a three-electrode configuration by means of a potentiostat (Digi-Ivy, USA). A glassy carbon electrode was used, while Ag/AgCl and Pt were used as the working standard and the counter electrode, respectively. The specific capacitance (Cs) was measured from CV curves calculated graphically by integrating the area under the CV curve with help of the following equation [54]: where w is the mass of the electrode and υ is the sweep rate (V/s).

Antimicrobial Activity
The antimicrobial actions of CeO 2 and CeO 2 −SnO 2 composite NFs were screened against E. coli, which is ever-present throughout the atmosphere. The bacterial standard cultures were held on nutrient agar (NA) plates. Bacterial culture from the agar plates was Nanomaterials 2023, 13, 1001 5 of 17 seeded in 5 mL NS solution and adjusted to 1 × 10 6 CFU/mL. To evaluate the antibacterial effect, different concentrations of CeO 2 NFs and CeO 2 −SnO 2 composite NFs were prepared and screened. Preliminary screening was conducted using an agar diffusion technique, as described previously [55]. In short, 25 mL of agar holding 1 mL of microbial culture was placed in Petri dishes. Approximately 50 µL from each dosage was put into a 4 mm diameter well. Dishes were pre-incubated for 3 h at room temperature to enable the prediffusion of samples and then incubated for 24 h at 37 • C. As a negative control, DMSO was utilized, and the standard drug ciprofloxacin was used as the positive control. The inhibition potency was described as the absence of bacterial growth in the vicinity of holes and a caliper was used to determine the inhibition zone.

Results and Discussion
Electrospinning is a simple and versatile technology that uses electrostatic repulsion between surface charges to extract nanofibers from a viscoelastic fluid and can be used for a variety of materials [56]. In the present study, it was utilized successfully to make composite NFs with diameters as small as hundreds of nanometers. The XRD configuration of CeO 2 and CeO 2 −SnO 2 NFs after calcination at 500 • C are displayed in Figure 1. Specifically, all of the reflection crests in Figure 1a [26]. A small peak was observed at 52.5 • , which was probably the peak of Ce(OH) 3 [57]. In the XRD configuration of CeO 2 −SnO 2 (Figure 1b), major diffracted crests identical to those of CeO 2 NFs were visualized. Additionally, diffraction peaks from the tetragonal SnO 2 (JCPDS card no. 41-1445) were also observed [58], which demonstrates the effective materialization of CeO 2 −SnO 2 composite NFs. No other mixes of Ce and Sn were detected using XRD.
where w is the mass of the electrode and υ is the sweep rate (V/s).

Antimicrobial Activity
The antimicrobial actions of CeO2 and CeO2−SnO2 composite NFs were screened against E. coli, which is ever-present throughout the atmosphere. The bacterial standard cultures were held on nutrient agar (NA) plates. Bacterial culture from the agar plates was seeded in 5 mL NS solution and adjusted to 1 × 10 6 CFU/mL. To evaluate the antibacterial effect, different concentrations of CeO2 NFs and CeO2−SnO2 composite NFs were prepared and screened. Preliminary screening was conducted using an agar diffusion technique, as described previously [55]. In short, 25 mL of agar holding 1 mL of microbial culture was placed in Petri dishes. Approximately 50 µL from each dosage was put into a 4 mm diameter well. Dishes were pre-incubated for 3 h at room temperature to enable the pre-diffusion of samples and then incubated for 24 h at 37 °C. As a negative control, DMSO was utilized, and the standard drug ciprofloxacin was used as the positive control. The inhibition potency was described as the absence of bacterial growth in the vicinity of holes and a caliper was used to determine the inhibition zone.

Results and Discussion
Electrospinning is a simple and versatile technology that uses electrostatic repulsion between surface charges to extract nanofibers from a viscoelastic fluid and can be used for a variety of materials [56]. In the present study, it was utilized successfully to make composite NFs with diameters as small as hundreds of nanometers. The XRD configuration of CeO2 and CeO2−SnO2 NFs after calcination at 500 °C are displayed in Figure 1. Specifically, all of the reflection crests in Figure 1a were assigned to the cubic fluorite structure of CeO2 (JCPDS no. 65-2975) at angles of 28.5°, 33.2°, 47.5°, 56.4°, and 59.1°, which relate to the (111), (200), (220), (311), and (222) crystal planes, respectively [26]. A small peak was observed at 52.5°, which was probably the peak of Ce(OH)3 [57]. In the XRD configuration of CeO2−SnO2 (Figure 1b), major diffracted crests identical to those of CeO2 NFs were visualized. Additionally, diffraction peaks from the tetragonal SnO2 (JCPDS card no.  were also observed [58], which demonstrates the effective materialization of CeO2−SnO2 composite NFs. No other mixes of Ce and Sn were detected using XRD.  The morphologies of pure and composite NFs obtained after heating at 500 • C were demonstrated using SEM ( Figure 2). From Figure 2a, it can be perceived that these arbitrarily oriented NFs had a uniform and continuous structure. Their lengths were measured as being several micrometers. The average diameter of the pure CeO 2 NFs was 400 nm, whereas the average diameter of CeO 2 −SnO 2 composite NFs was lower at 170 nm, which might have been due to the interaction between Ce and Sn ( Figure 2b). sequently, it was observed that a different salt addition had a considerable impact on the spinnability of the polymer solution. Additionally, because of the increased conductivity, the electric field accelerated the ions and the interactions between the polymer and the salt resulted in a less compact polymer structure. Our findings show that when salt (Ce and S) ions were propelled by an electric field, the polymer chain followed the ions' motion, thereby decreasing the fiber diameter size. Analogous studies were reported for polymeric fibers by electrospinning in the presence of salts [59,60].   (111) and (211) of CeO2 and SnO2, respectively. It further verified the formation of a heterojunction between CeO2 and SnO2. The inset in Figure 3 demonstrates the low-resolution image of NFs. The TEM morphology of the CeO2−SnO2 NFs again showed that the average diameter of the composite NFs was 170 nm (inset of Figure 3). The corresponding selected area electron diffractions (SAED) arrangement of these NFs displayed a good ring pattern with no dislocations or imperfections, which can be attributed to the high crystalline phase of the composite sample, indicating the polycrystalline nature of the composite NFs (inset of Figure 3). Figure 4 displays the FT-IR bands of the virgin CeO2 and composite CeO2−SnO2 NFs. The existence of a strong band prior to 600 cm −1 could be assigned to the Ce-O-Ce stretching vibration [61] (Figure 4a). Both spectra revealed bands around 3450 cm −1 and 1637 cm −1 conforming to O-H widening vibration of the remaining water molecules and hydroxyl clusters. In the nanocomposite NFs spectra (Figure 4b), the occurrence of a band at 610 cm −1 corresponded to the stretching mode of SnO2 [62]. The presence of both CeO2 and SnO2 peaks suggested the integration of these materials in the final composite NFs. The arrows clearly show the characteristic sheath on the composite NFs, which was due to the presence of SnO 2 (Figure 2c). The different salt content significantly increased the solution conductivity, resulting in thinner electrospun fibers. Due to increased spinnability in the presence of Ce and Sn precursors, uniform nanofibers were generated. Consequently, it was observed that a different salt addition had a considerable impact on the spinnability of the polymer solution. Additionally, because of the increased conductivity, the electric field accelerated the ions and the interactions between the polymer and the salt resulted in a less compact polymer structure. Our findings show that when salt (Ce and S) ions were propelled by an electric field, the polymer chain followed the ions' motion, thereby decreasing the fiber diameter size. Analogous studies were reported for polymeric fibers by electrospinning in the presence of salts [59,60]. Figure 3a demonstrates the EDS results, which confirmed the presence of Ce, Sn, and O in the CeO 2 −SnO 2 composite NFs. EDS analysis for the composite yielded an average atomic ratio of 1:0.15 for Ce/Sn. The detailed microstructure and morphology of composite NFs were further scanned using HR-TEM, the results of which are presented in Figure 3b. The surface consisted of two sets of lattices with spacings of 0.312 nm and 0.17 nm, corresponding to the interspace area between (111) and (211) of CeO 2 and SnO 2 , respectively. It further verified the formation of a heterojunction between CeO 2 and SnO 2 . The inset in Figure 3 demonstrates the low-resolution image of NFs. The TEM morphology of the CeO 2 −SnO 2 NFs again showed that the average diameter of the composite NFs was 170 nm (inset of Figure 3). The corresponding selected area electron diffractions (SAED) arrangement of these NFs displayed a good ring pattern with no dislocations or imperfections, which can be attributed to the high crystalline phase of the composite sample, indicating the polycrystalline nature of the composite NFs (inset of Figure 3). Figure 4 displays the FT-IR bands of the virgin CeO 2 and composite CeO 2 −SnO 2 NFs. The existence of a strong band prior to 600 cm −1 could be assigned to the Ce-O-Ce stretching vibration [61] (Figure 4a). Both spectra revealed bands around 3450 cm −1 and 1637 cm −1 conforming to O-H widening vibration of the remaining water molecules and hydroxyl clusters. In the nanocomposite NFs spectra (Figure 4b), the occurrence of a band at 610 cm −1 corresponded to the stretching mode of SnO 2 [62]. The presence of both CeO 2 and SnO 2 peaks suggested the integration of these materials in the final composite NFs.   The optical characteristics of bare and composite NFs were assessed using UV-DRS ( Figure 5). Figure 5A demonstrates the UV-DRS results of the CeO2 and CeO2−SnO2 NFs, which show the absorption edge within the visible region. The spectrum of the CeO2-SnO2 composite presented a slight red shift, which coordinated well with pure CeO2 and could be ascribed to exciton partial leakage into the CeO2 matrix. To estimate the band gaps of   The optical characteristics of bare and composite NFs were assessed using UV-DRS ( Figure 5). Figure 5A demonstrates the UV-DRS results of the CeO2 and CeO2−SnO2 NFs, which show the absorption edge within the visible region. The spectrum of the CeO2-SnO2 composite presented a slight red shift, which coordinated well with pure CeO2 and could be ascribed to exciton partial leakage into the CeO2 matrix. To estimate the band gaps of be ascribed to exciton partial leakage into the CeO 2 matrix. To estimate the band gaps of the prepared samples, the modified Kubelka-Munk function was plotted for (Ahν) 2 versus the energy of the exciting light (hν) using the following formula: sus the energy of the exciting light (hν) using the following formula: From the inset in Figure 5A, the calculated band gaps of the CeO2 NFs and CeO2−SnO2 composite were found to be 2.87 eV and 2.67 eV, respectively. This means that the smaller band gap of CeO2−SnO2 composite NFs will harness more visible light than CeO2 NFs. These results are consistent with an earlier report [63]. The PL emission spectra of the CeO2 and CeO2−SnO2 composite samples were documented using an excitation wavelength ( Figure 5B). The PL spectrum of CeO2 had prominent peaks at 364, 421, 483, and 530 nm [64][65][66]. Moreover, the CeO2−SnO2 composite showed weaker emission peaks than CeO2, indicating a lower recombination frequency of the photogenerated electronhole pair.  From the inset in Figure 5A, the calculated band gaps of the CeO 2 NFs and CeO 2 −SnO 2 composite were found to be 2.87 eV and 2.67 eV, respectively. This means that the smaller band gap of CeO 2 −SnO 2 composite NFs will harness more visible light than CeO 2 NFs. These results are consistent with an earlier report [63]. The PL emission spectra of the CeO 2 and CeO 2 −SnO 2 composite samples were documented using an excitation wavelength ( Figure 5B). The PL spectrum of CeO 2 had prominent peaks at 364, 421, 483, and 530 nm [64][65][66]. Moreover, the CeO 2 −SnO 2 composite showed weaker emission peaks than CeO 2 , indicating a lower recombination frequency of the photogenerated electron-hole pair. The photoactivity of pure and composite nanofibers was assessed for the breakdown of MB dye under visible light ( Figure 6). Figure 6A shows the MB concentration change calculated from its main absorption peak at 664 nm. From a comparative experiment, it can clearly be seen that CeO 2 −SnO 2 composite photocatalyst displayed better photoactivity than the pure CeO 2 NFs ( Figure 6B). After 125 min of visible light irradiation, the photodegradation effectivenesses of the CeO 2 and the CeO 2 −SnO 2 composite photocatalysts for MB were 42% and 85%, respectively. The better photodegradation efficiency of the CeO 2 -SnO 2 composite can be ascribed to the development of heterojunction between CeO 2 and SnO 2 and the parting of photogenerated electron-hole pairs.
The photoactivity of pure and composite nanofibers was assessed for the breakdown of MB dye under visible light ( Figure 6). Figure 6A shows the MB concentration change calculated from its main absorption peak at 664 nm. From a comparative experiment, it can clearly be seen that CeO2−SnO2 composite photocatalyst displayed better photoactivity than the pure CeO2 NFs ( Figure 6B). After 125 min of visible light irradiation, the photodegradation effectivenesses of the CeO2 and the CeO2−SnO2 composite photocatalysts for MB were 42% and 85%, respectively. The better photodegradation efficiency of the CeO2-SnO2 composite can be ascribed to the development of heterojunction between CeO2 and SnO2 and the parting of photogenerated electron-hole pairs. Figure 7 shows the reusability capacity of CeO2−SnO2 composite NFs for the degradation of MB dye. It was demonstrated that even after five cycles, the CeO2−SnO2 composite still exhibited good photocatalytic efficiency. It can be perceived that as the number of cycles increased, the composite photocatalytic activity marginally declined. The CeO2−SnO2 catalyst degraded 85% of the MB dye solution on its initial application, but after repeated applications (5th cycle), it had achieved a decolorization efficiency of 81%.   Figure 7 shows the reusability capacity of CeO 2 −SnO 2 composite NFs for the degradation of MB dye. It was demonstrated that even after five cycles, the CeO 2 −SnO 2 composite still exhibited good photocatalytic efficiency. It can be perceived that as the number of cycles increased, the composite photocatalytic activity marginally declined. The CeO 2 −SnO 2 catalyst degraded 85% of the MB dye solution on its initial application, but after repeated applications (5th cycle), it had achieved a decolorization efficiency of 81%. This shows that the CeO2−SnO2 catalyst was a reliable and efficient composite for dye degradation. Erstwhile, many similar studies also reported on composite nanomaterials for MB dye degradation (Table 1). A potential mechanism for photocatalytic MB degradation using a CeO2−SnO2 composite is hypothesized and illustrated in Figure 8. Under visible light irradiation, photons are absorbed by the CeO2 catalyst when energy is equivalent to or greater than the band gap. CeO2 gets excited and generates electrons (e − ) and holes (h + ).
CeO2 + hν → (eCB − )CeO2 + (eVB + )CeO2 (3) The photoexcited electrons are transported to the SnO2, as the conduction band (CB) position of CeO2 is more negative than SnO2. Meanwhile, the transported electrons in the CB of SnO2 will interact with dissolved oxygen molecules to generate superoxide radical anions (O2 − ). The superoxide radicals react with water and produce hydroperoxide radicals (HO2) and hydroxyl radicals (OH). Subsequently, the holes on the valence band (VB) This shows that the CeO 2 −SnO 2 catalyst was a reliable and efficient composite for dye degradation. Erstwhile, many similar studies also reported on composite nanomaterials for MB dye degradation (Table 1). A potential mechanism for photocatalytic MB degradation using a CeO 2 −SnO 2 composite is hypothesized and illustrated in Figure 8. Under visible light irradiation, photons are absorbed by the CeO 2 catalyst when energy is equivalent to or greater than the band gap. CeO 2 gets excited and generates electrons (e − ) and holes (h + ). CeO 2 + hν → (e CB − ) CeO2 + (e VB + ) CeO2 (3) to produce OH. These radicals have a strong ability to degrade dye molecules (Figure 8). The MB dye is photo-oxidized into carbon dioxide and water molecules or inorganic ions as degradation products [63].
O2 − + H2O → HO2 + OH − h + + H2O → OH − + H + (6) MB dye + Radical species (O2 − ,OH − ) → Degradation products (CO2 + H2O) (7) Figure 8. Schematic representation of the energy band configuration and electron−hole pair separation in CeO2−SnO2 NFs. Figure 9A demonstrates the cyclic voltammograms (CVs) of the pure CeO2 NFs and CeO2−SnO2 NFs. The CeO2−SnO2 composite NFs indicated higher current values and better voltammetric shapes compared with pure CeO2. The CeO2−SnO2 composite exhibited improved specific capacitance (413.9 Fg −1 ) compared with the CeO2 NFs (263.9 Fg −1 ) at a scan rate of 10 mV/s. No redox peaks were observed in the CV of the bare CeO2 NFs, which might have been due to the reason that there was little or no oxidation-reduction (corrosion or degradation) process going parallel to the double-layer charging. The presence of redox peaks in the CeO2−SnO2 composite NFs indicates that reversible and incessant faradaic oxidation-reduction responses were involved during the charging and discharging procedure [75,76]. It can be assumed that the electrochemical performances of composites were enhanced due to the synergistic effect between CeO2 and SnO2. Figure 9B shows the CV curves for different scan rates for the CeO2−SnO2 composite. The CeO2−SnO2 composite NFs confirmed elevated specific capacitance values of 413.9, 294.94, 147.6, and 16.1 F/g at scan rates of 5, 10, 50, and 100 mVs −1 , correspondingly.
In the antibacterial bioassay, CeO2 NFs and CeO2−SnO2 composite NFs were tested against specified bacteria using an agar diffusion experiment at various doses. The CeO2 and CeO2−SnO2 composite NFs were found to be the most promising, with MICs of 50 µg/mL for E. coli ( Figure 10). The augmented activity of the CeO2−SnO2 composite NFs might be attributed to their form and wide surface area [77], as well as the synergistic The photoexcited electrons are transported to the SnO 2 , as the conduction band (CB) position of CeO 2 is more negative than SnO 2 . Meanwhile, the transported electrons in the CB of SnO 2 will interact with dissolved oxygen molecules to generate superoxide radical anions (O 2 − ). The superoxide radicals react with water and produce hydroperoxide radicals (HO 2 ) and hydroxyl radicals (OH). Subsequently, the holes on the valence band (VB) of SnO 2 migrate to the VB of the CeO 2 since the valence band potential of CeO 2 is more negative than that of SnO 2 . Simultaneously, the generated h + can oxidize water molecules to produce OH. These radicals have a strong ability to degrade dye molecules ( Figure 8). The MB dye is photo-oxidized into carbon dioxide and water molecules or inorganic ions as degradation products [63].
MB dye + Radical species (O 2 − , OH − ) → Degradation products (CO 2 + H 2 O) (7) Figure 9A demonstrates the cyclic voltammograms (CVs) of the pure CeO 2 NFs and CeO 2 −SnO 2 NFs. The CeO 2 −SnO 2 composite NFs indicated higher current values and better voltammetric shapes compared with pure CeO 2 . The CeO 2 −SnO 2 composite exhibited improved specific capacitance (413.9 Fg −1 ) compared with the CeO 2 NFs (263.9 Fg −1 ) at a scan rate of 10 mV/s. No redox peaks were observed in the CV of the bare CeO 2 NFs, which might have been due to the reason that there was little or no oxidation-reduction (corrosion or degradation) process going parallel to the double-layer charging. The presence of redox peaks in the CeO 2 −SnO 2 composite NFs indicates that reversible and incessant faradaic oxidation-reduction responses were involved during the charging and discharging procedure [75,76]. It can be assumed that the electrochemical performances of composites were enhanced due to the synergistic effect between CeO 2 and SnO 2 . Figure 9B shows the CV curves for different scan rates for the CeO 2 −SnO 2 composite. The CeO 2 −SnO 2 composite NFs confirmed elevated specific capacitance values of 413.9, 294.94, 147.6, and 16.1 F/g at scan rates of 5, 10, 50, and 100 mVs −1 , correspondingly.
leaking through the disturbance in the inner cell content and distortion. A few previous investigations also showed that CeO2 nanoparticles have exceptional antibacterial activity [44,45]. In particular, some studies found that CeO2 has an antibacterial effect on S. aureus [46,47]. Furthermore, agar diffusion and microdilution experiments were utilized in certain studies to examine and validate P. aeruginosa's sensitivity to CeO2 [48]. Similarly, SnO2 also attracted interest as an effective antibacterial, where it was shown to inhibit the development of several bacterial strains, such as S. aureus and E. coli [49,50]. Furthermore, it was observed that SnO2 supplemented with transition metal ions effectively disinfects microorganisms and possesses potent antibacterial activities [51][52][53].  In the antibacterial bioassay, CeO 2 NFs and CeO 2 −SnO 2 composite NFs were tested against specified bacteria using an agar diffusion experiment at various doses. The CeO 2 and CeO 2 −SnO 2 composite NFs were found to be the most promising, with MICs of 50 µg/mL for E. coli ( Figure 10). The augmented activity of the CeO 2 −SnO 2 composite NFs might be attributed to their form and wide surface area [77], as well as the synergistic influence of CeO 2 and SnO 2 in the composite NFs. However, the antibacterial activities of the virgin CeO 2 NFs and CeO 2 −SnO 2 composite NFs were identical at a low concentration, the antibacterial impact varied with the increase in concentration and incubation time. The chemical components of the CeO 2 −SnO 2 composite NFs primarily reacted with the bacterial outer cell wall before diffusing inside the inner wall, producing disarray and leaking through the disturbance in the inner cell content and distortion. A few previous investigations also showed that CeO 2 nanoparticles have exceptional antibacterial activity [44,45]. In particular, some studies found that CeO 2 has an antibacterial effect on S. aureus [46,47]. Furthermore, agar diffusion and microdilution experiments were utilized in certain studies to examine and validate P. aeruginosa's sensitivity to CeO 2 [48]. Similarly, SnO 2 also attracted interest as an effective antibacterial, where it was shown to inhibit the development of several bacterial strains, such as S. aureus and E. coli [49,50]. Furthermore, it was observed that SnO 2 supplemented with transition metal ions effectively disinfects microorganisms and possesses potent antibacterial activities [51][52][53].

Conclusions
In summary, CeO2−SnO2 composite NFs were prepared via electrospinning. As aforementioned, many strategies for removing MB and other textile dyes from industrial effluent were reported in the literature. However, in the present study, the photocatalysis approach was used. The addition of SnO2 to CeO2 significantly improved the photocatalytic MB degradation property and photoelectrochemical performance. After 125 min of irradiation, a photodegradation efficiency of 85% was achieved using the CeO2−SnO2 composite photocatalyst. Moreover, the CeO2−SnO2 composite NFs also showed higher specific capacitance. It is believed that these CeO2−SnO2 composite NFs with excellent photoactivity and electrochemical performance can be very promising for high-performance electrode materials in supercapacitors and as an efficient alternative photocatalyst. Rationally, due to their distinctive functional mechanism towards pathogens via reversible oxidation state transition between Ce(III) and Ce(IV), (IV) CeO2 nanoparticles with lower toxicity work as powerful antibacterial agents as well. The outcome of this study indicates the prospects of CeO2−SnO2 composite NFs to be exploited as an antimicrobial material, an electrode, and as a visible light active photocatalyst, which would greatly aid in the environmental cleanup process.

Conclusions
In summary, CeO 2 −SnO 2 composite NFs were prepared via electrospinning. As aforementioned, many strategies for removing MB and other textile dyes from industrial effluent were reported in the literature. However, in the present study, the photocatalysis approach was used. The addition of SnO 2 to CeO 2 significantly improved the photocatalytic MB degradation property and photoelectrochemical performance. After 125 min of irradiation, a photodegradation efficiency of 85% was achieved using the CeO 2 −SnO 2 composite photocatalyst. Moreover, the CeO 2 −SnO 2 composite NFs also showed higher specific capacitance. It is believed that these CeO 2 −SnO 2 composite NFs with excellent photoactivity and electrochemical performance can be very promising for high-performance electrode materials in supercapacitors and as an efficient alternative photocatalyst. Rationally, due to their distinctive functional mechanism towards pathogens via reversible oxidation state transition between Ce(III) and Ce(IV), (IV) CeO 2 nanoparticles with lower toxicity work as powerful antibacterial agents as well. The outcome of this study indicates the prospects of CeO 2 −SnO 2 composite NFs to be exploited as an antimicrobial material, an electrode, and as a visible light active photocatalyst, which would greatly aid in the environmental cleanup process.