Photocatalytic and Antimicrobial Properties of Electrospun TiO2–SiO2–Al2O3–ZrO2–CaO–CeO2 Ceramic Membranes

In this study, TiO2-based ceramic nanofiber membranes in the system of TiO2–SiO2–Al2O3–ZrO2–CaO–CeO2 were synthesized by combining sol–gel and electrospinning processes. In order to investigate the thermal treatment temperature effect, the obtained nanofiber membranes were calcined at different temperatures ranging from 550 to 850 °C. Different characterization methods such as X-ray diffraction (XRD), scanning electron microscopy (SEM), Fourier transform infrared (FT-IR), and high-resolution transmission electron microscopy (HR-TEM) were conducted on the obtained membranes to investigate the structural and morphological properties of the nanofibers. The Brunauer–Emmett–Teller surface area of the nanofiber membranes was very high (46.6–149.2 m2/g) and decreased with increasing calcination temperature as expected. Photocatalytic activity investigations were determined using methylene blue (MB) as a model dye under UV and sunlight irradiation. High degradation performances were achieved with the calcination temperatures of 650 and 750 °C because of the high specific surface area and the anatase structure of the nanofiber membranes. Moreover, the ceramic membranes showed antibacterial activity against Escherichia coli as a Gram-negative bacterium and Staphylococcus aureus as a Gram-positive bacterium. The superior properties of the novel TiO2-based multi-oxide nanofiber membranes proved as a promising candidate for various industries, especially the removal of textile dyes from wastewater.


INTRODUCTION
The growth of industrialization and the increase in population bring out environmental pollution. 1,2 Organic pollutant emissions, especially from textile dyes in wastewater, pose a serious threat to certain organisms and human health, leading to toxic effects due to their poisonous nature and nonbiodegradable properties. Removal of these organic dyes from industrial wastewater has become a serious problem, and the selection of the best treatment method plays a vital role in the environment and humanity. 3 Various treatment methods, such as physical methods, biological treatment, chemical oxidation, and electrochemical oxidation, are proposed to overcome the disposal problem of these organic pollutants. 4,5 However, organic dyes have a biopersistent nature and are strongly resistant to most treatment techniques, and some of these methods cause secondary pollution. 2,6,7 In recent years, semiconductor-based photocatalysis has emerged as a promising green technology for the degradation of organic pollutants from textile wastewater due to its unique properties such as being environmentally friendly and cost effective. 2,3,8 Among different semiconductor materials for photocatalytic processes in water treatment applications, TiO 2 , especially in the anatase phase, is the most preferred photocatalyst due to its superior properties such as being inexpensive, nontoxic for the human body and environment, photochemically stable, and relatively easy to fabricate. 4,9−12 TiO 2 -based materials are frequently preferred in many technical and electronic applications apart from this field. 13 However, the relatively large band gap energy (3.2 eV) limits its use to only ultraviolet (UV) irradiation, representing 5% of solar energy. 14 In order to provide an efficient dye degradation process, decreasing the electron−hole recombination rate and enhancing solar energy utilization are critical issues. Recently, it has been proposed to combine TiO 2 with different metals, nonmetals, and lanthanide ions to eliminate disadvantageous properties. Suitable dopant selection plays a vital role in reaching a high degradation rate of dyes under sunlight irradiation. 12,14 Among the different rare-earth metal ions for doping elements, CeO 2 is the most preferred dopant owing to its properties such as being nontoxic, low cost, capability of strong oxidation, nonphotocorrosive, high conductivity, and high chemical stability. 7,15−18 Many studies have been used to improve degradation properties of the photocatalysts for different dyestuffs under UV and solar light by the help of CeO 2 -doped TiO 2 photocatalysts. 16−24 Apart from rare earth elements, the photocatalytic degradation efficiency of TiO 2 could be enhanced by doping an another n-type semiconductor as ZrO 2 . The ionic radius and electronegativity of zirconium (Zr 4+ ) and titanium (Ti 4+ ) are similar. The addition of zirconia induces oxygen defects and the diversity of Ti 4+ to Zr 4+ which leads to the improvement of photocatalytic activity by extending the absorption range from UV to visible light. 14,25 Although zirconia doping brings many good features, single doping has some limitations as restricted improvement for the visible light response of photocatalysts. 26 Therefore, researchers have been concentrated on co-doping instead of single doping of zirconia to the TiO 2 photocatalyst to reach the maximum benefit for the photocatalytic degradation process under UV and sunlight irradiation. 14,25−28 On the other hand, stabilizing the anatase phase of TiO 2 materials is another problem in photocatalytic processes. The doping of Ca 2+ to the TiO 2 ensures the reduction of the rutile, and thus, better photocatalytic activity could be provided. Minchi et al. studied Ca-doped TiO 2 nanomaterials, and they reached higher photocatalytic activity than pure TiO 2 , thanks to the stabilization of the anatase phase by Ca doping. 29 Also, SiO 2 plays a vital role in the production of ceramic materials. SiO 2doped TiO 2 nanomaterials have excellent thermal properties and chemical durability. Doping of SiO 2 to the nanofiber structure causes delay in the anatase to rutile phase transformation for TiO 2 , even at high calcination temperatures. 30−32 In photocatalytic applications, the efficiency of doped TiO 2 materials depends on their shape and surface area. The use of porous nanostructured materials such as nanofibers, nanotubes, nanoparticles, and nanosheets with different properties provides advantages in many different application areas such as gas sensors, supercapacitors, and water treatment applications. 12,33,34 Nanostructured materials, especially TiO 2 in the nanoparticle form or thin films, were used to achieve the best water treatment performance. 35 Although TiO 2 nanoparticles have a large surface area, their fixation and recovery are hardly difficult, and it causes secondary pollution in water. In order to overcome this problem, TiO 2 nanomaterials were prepared by coating on different substrates such as glass plates or ceramic materials. However, it leads peeling problem of coated TiO 2 from the substrate surface. To be able to solve this problem, TiO 2 can be incorporated directly into the glass−ceramic system. Glass ceramics attract much attention due to their lowcost production and can be prepared easily in various sizes and shapes, such as nanorods and nanofibers. 31,36 Electrospinning is a simple, inexpensive, and relatively versatile technique to fabricate glass ceramic nanomaterials which have unique properties such as a high surface area to volume ratio, flexibility, highly porous structure, and easy use, and it is possible to control the nanofiber composition to reach desired results from its properties. 37−39 In recent years, novel doped TiO 2 glass−ceramic nanofiber membranes have been prepared with the aid of sol−gel and electrospinning techniques for the photocatalytic degradation of organic pollutants. 32,40,41 The nanofibrous structure of doped TiO 2 materials not only improves adsorption capacity but also prevents secondary pollution due to its morphology. 42 In this context, many studies have been realized by doping TiO 2 with different materials by combining sol−gel and electrospinning processes. Tobin et al. synthesized TiO 2 −Al 2 O 3 fibers, and doping alumina to TiO 2 structure provided the stability of the anatase phase at even high temperatures. 43 Lotus et al. reported that the electrospun TiO 2 −Al 2 O 3 nanofibers, which calcined at high temperatures, preserved the anatase phase of TiO 2 , and they claimed that the improvement of photocatalytic efficiency could be reached by alumina doping. 40 Also, Ismail et al. reported that TiO 2 −Al 2 O 3 nanocomposites showed better photocatalytic efficiency than pristine titania. 44 Frontera et al. produced nickel, niobium, and tantalum-doped TiO 2 with a high surface area by the sol−gel and electrospinning process to investigate the crystalline structure. 45 49 and TiO 2 /g-C 3 N 50 were produced from sol− gel and polymer mixture using the electrospinning method and calcination processes.
Antimicrobial activity is another important factor in producing efficient nanomaterials for biosafety and sustainable applications. Due to their high qualitative properties such as flexibility, high surface area to volume ratio, and high porosity with continuously interconnected pores, electrospun nanofibers can be good candidates for disinfection applications. 51 TiO 2 nanomaterials, especially in the anatase phase, display antibacterial activity but they have limited inhibition to the growth of antibiotic-resistant bacteria. 52,53 Doping of TiO 2 enhances not only photocatalytic activity but also improves the antibacterial properties of TiO 2 nanomaterials, even in the absence of a light source. Moongraksathum et al. synthesized TiO 2 co-doped with silver and ceria, and they found high antimicrobial activity against E. coli and S. aureus both under UV light and dark conditions. 54 Hassan et al. reached high bactericidal efficiency with electrospun Ce 2 O 3 −TiO 2 nanofibers on Gram-positive and Gram-negative microorganisms, and they commented that the killing of bacteria was originated by cerium oxide atoms. 55 In the present study, it was aimed to improve the antibacterial properties of nanomaterials, including TiO 2 with the help of the synergistic effect of doping materials. To the best of our knowledge, TiO 2 has been doped one or two oxides to overcome its drawbacks for the efficient photocatalytic processes. However, in the present study, it was aimed to synthesize the TiO 2 -based multi-oxide ceramic nanofiber membrane by direct incorporation of the doping materials into the sol−gel solution and using the electrospinning method. It was purposed to benefit the synergistic effect of different oxides, not only utilize the enhancement of photocatalytic activity under both UV and sunlight irradiation but also ensure the antibacterial properties of TiO 2 nanofibers. On the other hand, 10 wt % PAN solution was prepared by using DMF as a solute under continuously stirring for 24 h. As the last step, the sol and polymer solutions were mixed with each other at a ratio of 1/4 (w/w). The obtained solution was stirred for 24 h before the electrospinning process.
As-prepared solutions were loaded into a plastic syringe equipped with a flat stainless-steel needle. This needle was connected to a high-voltage supply. The electrospinning process was realized by an electrospinning device (Nanospinner 24 Touch, Inovenso Co.) under specific process conditions. The applied voltage was set at 20 kV, and the feeding flow rate was adjusted as 2 mL/h by taking the needle tip to the collector distance as 130 mm. The electrospun nanofibers were deposited as nonwoven mats on a grounded target wrapped with aluminum foil. The electrospinning process was performed at ambient temperature with a relative humidity of 45−55%. The obtained nanofibers were dried in a vacuum oven at 60°C for 24 h. Finally, the dried samples were calcined at different temperatures between 550−850°C for 1 h in a muffle furnace with a heating rate of 2°C/min to remove nitrate residues and to form a ceramic structure.

Characterization.
Different characterization methods were applied to investigate the prepared nanofiber samples. The crystal structures of the nanofiber samples were investigated by using a Panalytical Xpert Pro X-ray diffractometer with Cu-Kα radiation (wavelength = 0.15419 nm). XRD data of each nanofiber were accumulated over the 2θ range from 5 to 80°with a step size of 0.017°. Fourier transform infrared (FT-IR) spectroscopy was used to investigate the structural composition of the obtained nanofibers. Spectra were collected using a Perkin Elmer Spectrum 100 Model spectrometer in transmittance mode between 4000 and 650 cm −1 . Scanning electron microscopy (SEM, JSM-5410, Jeol) was used to determine the surface morphologies and the microstructure of the obtained nanofibers after the electrospinning process. Before SEM measurements, the surface of the samples was coated (SC7620 sputter coater, Quorum Technologies Ltd., United Kingdom) with platinum for 120 s to obtain a conductive surface. The measurement of average fiber diameters was realized by using Image J software (National Institute of Health, USA). For this aim, 50 different points on SEM images were selected randomly for each fiber sample to reach the average fiber diameters, standard deviations, and distribution graphs of nanofiber diameters. High-resolution transmission electron microscopy (HR-TEM, Jeol 2100F) at 200 kV was used for further analysis of the morphology and crystalline structure of the calcined membrane. Prior to analysis, the calcined sample was prepared with a CF200-Cu Carbon film grid. First, the calcined sample was powdered with a pestle. The powder sample suspended in ethyl alcohol was mixed in an ultrasonic cleaner for 45 min, and then a drop was dropped on the grid with a micropipette and left to dry for one night. Electron dispersive spectroscopy (Thermo Scientific Axia ChemiSEM) at 8 kV was used to analyze the distribution of the elements in the calcined nanofiber membrane. The substitution of surface chemical species of the produced nanofiber membranes was investigated by X-ray photoelectron spectroscopy (XPS, Thermo Scientific K-Alpha) analysis. The Brunauer−Emmett−Teller (BET) surface area of each sample was analyzed by using liquid nitrogen with Micromeritics Gemini VII Version 2. Prior to analysis, samples were degassed at 110°C for 8 h. N 2 adsorption−desorption isotherms were measured in the relative pressure range (P/P 0 ) 0.01−1. The Barrett−Joyner− Halenda (BJH) model by an adsorption isotherm was used to reach pore size distribution.

Antibacterial Activity.
The antibacterial activity of the nanofiber membrane calcined at 750°C was investigated against Escherichia coli BC1402 as a Gram-negative bacteria and Staphylococcus aureus ATCC 25923 as a Gram-positive bacteria using the quantitative viable count method modified by Akhtach et al. 56 and Shi et al. 57 E. coli and S. aureus cells were inoculated into tryptic soy broth (TSB) and incubated for 18 h at 37°C. Cells were harvested by centrifugation (5000 rpm, 10 min, 4°C) and resuspended in an equal volume of physiological saline (0.85% NaCl), diluted to 1 × 10 6 colony forming units (CFU)/mL after the incubation period.
All of the nanofiber membranes were sterilized in a UV cabinet for 30 min on each side. Nanofiber membranes (0.8 mg) were dispersed into the 2 mL of a sterile saline (0.85% NaCl) solution containing 10 6 CFU/mL bacterial cultures. The suspension without a nanofiber membrane was used as the positive control. Each of the suspensions was incubated at 37°C for 24 h on a rotary shaker at 300 rpm. After incubation, bacterial suspension was serially diluted in saline solution, plated on TSA, and incubated for 18−24 h at 37°C. After incubation, the number of survival colonies was counted. All of the experiments were repeated twice with three parallels. The antimicrobial activities of the nanofiber membranes were calculated from the following equation. 54 where N 0 and N indicate the average number of colonies in the control group and experimental group (CFU/mL), respectively.

Photocatalytic Degradation Studies.
The photocatalytic activity of the obtained nanofiber membranes was investigated under both UV and simulated sunlight irradiation. Methylene blue (MB) was used as a model dye component for the photocatalytic activity measurements. Ceramic nanofibers (20 mg) were added to 40 mL of 20 mg/L aqueous methylene blue solution in the flask, and the solution was placed in a photoreactor on the mixer. For UV tests, the solution with a catalyst was exposed to UV light in a photoreactor which contains 18 UV-A (each lamp 8 W) lamps. On the other hand, 300 W Osram Ultra Vitalux with a cut-off filter was used as a simulated sunlight source and placed 20 cm away from the photoreactor. The temperature of the photoreactor was controlled with a fan. Prior to irradiation, the suspension of the catalyst in MB was stirred in the dark for 30 min to reach the adsorption−desorption balance. After that, 3 mL aliquots of the samples were taken every 10 min from the stirred solution, and the collected samples were filtered by a syringe filter. The MB absorbance values were measured at 664 nm with a UV−visible spectrophotometer (UV mini-1240 where C 0 is the initial MB concentration and C is the MB concentration after irradiation at a given time.

RESULTS AND DISCUSSION
3.1. Structural Characterization. XRD analysis was conducted to assess the changes in the crystalline structure of the obtained nanofiber membranes. Figure 1 shows the XRD patterns of the membranes before and after the heat treatment processes. Before the calcination process ( Figure  1a), the diffraction peak at 2θ of 16.5°corresponding to the crystal plane (100) represents the PAN linear macromolecules. 58 When the membrane was calcined at 550°C (Figure 1b), the disappearance of this diffraction peak indicates the removal of the PAN polymer with the heat treatment process. However, this temperature was relatively low for the formation of the anatase phase of TiO 2 . In the XRD pattern of the membrane, which was calcined at 650°C, exhibited the reflection at 2θ of 25°corresponding to the (101) plane of the anatase phase (Figure 1c). When the calcination temperature was increased to 750°C, additional peaks at the 2θ value of 48.2°corresponding to the (200) plane of the anatase phase and 2θ of 55.2°related to the (211) plane of the anatase phase ( Figure 1d). Also, the number of peaks was highly increased with the increase of calcination temperature to 850°C, and rutile phase formation started to occur (Figure 1e 64 The lack of any other phase except TiO 2 for the calcined membranes demonstrated that the dopants were well dispersed in the TiO 2 lattice. In this study, it is obviously seen that anatase to rutile phase transformation was delayed. Generally, pure TiO 2 nanofibers have a tendency to form crystalline TiO 2 fully at a calcination temperature of 450°C. However, doping of TiO 2 materials causes the increase of calcination temperature to the formation of crystalline TiO 2 . 30 Researchers have studied different metal oxides and reached anatase to rutile phase transformation at higher temperatures. 14,30,65 Li et al. studied hybrid SiO 2 and TiO 2 nanofiber calcined at 600°C, and they did not reach the crystalline TiO 2 . 30 In another study, Vieira et al. synthesized CeO 2 -doped TiO 2 photocatalysts and reported that the increase in calcination temperature did not cause the change of the crystallinity of the material. 65 The transformation of the anatase to rutile phase was inhibited in the Ce-doped TiO 2 materials after heat treatment even at 600°C. Kapusuz et al. reported that Zr doping to the TiO 2 suppressed considerably the formation of the rutile phase. 14 FT-IR analysis was performed to investigate the chemical composition of the obtained nanofiber membranes. Fourier transform infrared (FT-IR) graphs of the membranes, which were obtained before and after the calcination processes, are given in Figure 2. For all samples, the broad absorption band in the range of 3650−3100 cm −1 corresponded to H−OH stretching vibrations. The gradual decrease of this peak after the calcination process indicates the evaporation of hydroxyl groups. 66 The absorption peak at 1630 cm −1 corresponded to hydrated species for the membrane before calcination, and this peak shifted to 1648−1654 cm −1 after the heat treatment process. 67 When the FT-IR graph of the sample before the calcination process was examined (Figure 2a), the peak at 2925 cm −1 corresponds to the C−H stretching vibration of CH 2 strains. The typical peak at 2244 cm −1 was assigned to the stretching vibrations of nitrile groups (−CN−) in PAN chains. 68 Also, the peak at 1737 cm −1 belongs to the bending modes of adsorbed water. The peaks at 1452 and 1332 cm −1 indicated NO 3 −1 stretching. The peak at 1236 cm −1 corresponded to the −C�O stretching vibrations. All these peaks before the calcination process put forward the presence of the PAN polymer. The disappearance of all these peaks after the calcination process showed that the polymer was removed at a high temperature successfully (Figure 2b−e). 69 On the other hand, the peak at 1027 cm −1 that was assigned to asymmetric ν(Si−O−Si) vibrations for the membrane before calcination shifted to the 1040−1073 cm −1 after the calcination process. However, the characteristic peak that belongs to the Si−O−Al vibration can be observed at the same wavelength, and thus, it was thought that this bond coincided with the asymmetric ν(Si−O−Si) and cannot be detected exactly. 67

SEM−EDS Analysis.
The surface morphologies of the as-fabricated nanofiber membranes were observed by SEM analysis. Figure 3 shows the SEM images with low and high magnifications and the distribution of diameters for each nanofiber membrane before and after the calcination processes. Before the calcination process, nanofibers distributed homogeneously without any bead structure, and the surface of the fibers was smooth with normal distribution (Figure 3a). The mean diameter was measured as 122.9 ± 24.0 nm. After heat treatment that was applied at different temperatures, the measured diameters of the membranes decreased and varied in the range of 51.9−65.5 nm owing to the destruction of the polymeric structure. Moreover, the obtained ceramic nanofiber membranes maintained integrity and homogenous distribution, although high calcination temperatures were applied to the nanofibers (Figure 3b−e). Since all electrospun ceramic nanofiber membranes had randomly oriented and uniformly  , and (e) 850°C after calcination.

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http://pubs.acs.org/journal/acsodf Article distributed nanofibers with relatively low diameters, they can have high adsorption and photocatalytic degradation properties due to the high surface area to volume ratio. EDS mapping results and spectra can be seen from Figure 4. All elements were uniformly distributed throughout the whole ceramic nanofiber. The well-mixed heterojunction, which was composed of Ti, Si, Zr, Al, Ca, and Ce, was verified ( Figure  4a−f). The co-existence of these elements was further confirmed with the EDS spectrum taken as a representative area from nanofibers (Figure 4g). The resulting elemental ratios were found to be similar to theoretical synthesis ratios, and it was demonstrated that the structure mainly contains high TiO 2 , which is in accordance with XPS analysis.  the rutile phase of the TiO 2 (110) crystal plane. 70,71 This result confirmed the only presence of the TiO 2 crystal lattice fringe in the ceramic nanofiber structure. As a result of this, ingredients other than TiO 2 were embedded in anatase crystal planes with little disturbance to the orientation. The difference between gray and white zones showed the existence of overlapping direction patterns. The highlighted locations in Figure 5e,j indicated the overlapping of more than two crystal patterns. Therefore, the thickness of nanofiber walls increased in similar locations.

HR-TEM
The selected area electron diffraction (SAED) pattern of the sample (Figure 5k,l) indicated the high crystallinity of anatase TiO 2 in the obtained ceramic nanofiber structure. The ringtype diffraction pattern implied the well-crystallized structure of the nanofiber without the formation of any other phases. 71,72 The radius of apparent circles was measured and matched with Miller indices and diffraction angles (2θ). According to the measurements (Table 1), SAED results were similar to XRD results in Figure 1d showing the anatase phase of TiO 2 . 73

BET Surface Area.
Specific surface area investigations of the obtained samples were performed by BET analysis. Table 2 shows the BET surface area, Barrett−Joyner−Halenda (BJH) pore size, and pore volume results of the membranes that calcined at different temperatures. Since the shrinkage of the ceramic matrix induced the enlargement of pores, the specific surface area decreased with increasing temperature. This result is coincident with the other related studies on TiO 2 -based materials for photocatalysis. 43,74 The high surface area to volume ratio induces to increase in adsorption capacity, and thus, more organic pollutants could be adsorbed on TiO 2based nanofiber materials. BET surface areas of the obtained nanofibers (82.9−149.2 m 2 /g) except the calcined membrane at 850°C were found to be higher than that of commercial TiO 2 (50 m 2 /g), which indicated the improvement of adsorption and photocatalytic properties. 75 Nitrogen adsorption−desorption isotherms of the calcined membranes can be seen from Figure 6. According to the IUPAC classification, both calcined membranes present a direct hysteresis loop that belongs to the type IV isotherm. These results indicated the mesoporous structures of the obtained samples. 74,76 3.5. Photocatalytic Activity. Methylene blue (MB) is a cationic dye that is used in different industries such as coloring, textile, and petrochemical industries, which leads to water pollution. The removal of this pollutant dye from wastewater is a big issue for human health and the environment. 77 In this study, the photocatalytic degradation of 20 ppm MB with prepared nanofiber membranes was investigated under both UV and simulated sunlight irradiation. First, photocatalysts were stirred in the dark for 30 min to reach adsorption− desorption equilibria. The concentrations of MB decreased remarkably for all samples, as can be seen in Figure 7, due to the high adsorption capacity of the obtained nanofiber membranes. According to calcination temperature, the calcined nanofibers were coded as NC-550, NC-650, NC-750, and NC-850. The adsorption step is the essential process for heterogeneous photocatalysis that stimulates the interaction between dye molecules and photocatalysts, and thus, the degradation of the adsorbed dye molecules on the photocatalyst surface could be realized more easily. 78,79 The high adsorption capacity of the fabricated nanofiber membranes indicated that a large amount of MB dye molecules were exhibited on the surface. Therefore, the superoxide anion radicals and hydroxyl radicals, which were generated from electron (e−)-hole (h+) coupled reaction with oxygen and water, ensured the possibility of reaching a higher degradation ratio for MB molecules. 21,61 After the adsorption−desorption process was completed, the photocatalytic performances of the nanofiber membranes were investigated by opening UV and sunlight resources for 90 min of exposure time on the samples. As can be seen in Figure 7, the maximum degradation efficiency was achieved with the ceramic nanofiber material calcined at 650°C under UV light (90.7%), while it was achieved with the sample calcined at 750°C under simulated sunlight irradiation (88.5%). Although adsorption capacity was found as very high at the calcination temperature of 550°C, the photocatalytic degradation efficiency was lower than that of other calcined samples at 650 and 750°C. The reason for this   lower efficiency was that there was no anatase phase formation at this temperature, which was confirmed with XRD analysis. The surface area of nanomaterials plays an important role in adsorption capacity and photocatalytic degradation efficiency. Generally, the high surface area provides to increase the photocatalytic efficiency of nanomaterials by ensuring the adsorption of more pollutants onto the nanomaterial surface.
On the other hand, photocatalytic activity depends on not only the surface area but also the structural, morphological, and optical properties of nanomaterials. Since the amorphous TiO 2 enables electron−hole recombination on the surface easily, TiO 2 in the anatase phase is considered to show higher photocatalytic activity than TiO 2 in an amorphous structure. 61 On the other hand, the nanofiber membrane calcined at 850°C has the lowest adsorption capacity and photocatalytic degradation efficiency compared to other calcined membranes due to having the lowest surface area and the rutile phase structure. These results revealed that both the crystalline structure and surface area played a significant role in determining the photocatalytic efficiency of the fabricated nanofiber materials.
The photodegradation rate constants of MB by utilizing the prepared samples calcined at different temperatures conformed with the pseudo-first order kinetics: 77 where C and C 0 are the concentration of the sample after a given time t and 0, respectively, t is light irradiation time, and k is the reaction rate constant. The photodegradation behavior of the samples was identified with the Langmuir−Hinshelwood mechanism, which could be obtained from the relationship of ln (C/C 0 ) versus irradiation time (t) for MB (Figure 8). Rate  constants were reached from the slope of linear fitting the data, as shown in Figure 8, for each sample calcined at different temperatures and are listed in Table 3. The highest removal rate for MB was accomplished with the nanomaterial calcined at 650°C under UV light, while it was reached with the sample calcined at 750°C under sunlight irradiation. The reason for this was discussed above, and the results were found to be coherent with the photodegradation efficiencies. The rate of photocatalytic reactions is influenced by various factors, such as calcination temperature. The increasing calcination temperature may lead the conflicting impacts on the surface area and crystallinity of the photocatalysts, which result in an unpredictable effect on photocatalytic degradation rates. 80 In this study, the rate of reaction was accelerated with increasing calcination temperature up to a range of anatase to rutile phase transformation of TiO 2 , which is one of the most important factors in reaching the high degradation rates for containing TiO 2 materials. This result was also confirmed by the faster lightening of the blue color of MB during the experiment. The clear improvement of the photocatalytic degradation efficiency for MB removal was achieved by using the synergistic effect of different dopants on the TiO 2 nanofibers. The photocatalytic properties of bare TiO 2 and doped TiO 2 materials have been reported in different studies. The degradation efficiencies and the related rate constants differ from each other owing to the changing operational conditions. Sanguino et al. studied MB photocatalytic degradation under visible light, and they resulted in the photocatalytic degradation rate constant as 100 times higher than the bare TiO 2 (5 × 10 −5 min −1 ). 81 Moongraksathum et al. synthesized TiO 2 −CeO 2 films to investigate the MB degradation and measured the first-order rate constants as 0.552 and 0.283 h −1 for CeO 2 -doped TiO 2 films under UV and visible light irradiation, respectively, while they reached the half of the rate constants for undoped TiO 2 film. 82 Abdi et al. added ZrO 2 to the TiO 2 structure for the photodegradation of Rhodamine B, and they reached the firstorder rate constants as 0.0036 min −1 for pure TiO 2 and 0.013 min −1 for the TiO 2 /ZrO 2 photocatalyst at 180 min. 83 Hussain et al. investigated the MB dye degradation performance of the metal organic framework-derived Co 3 O 4 @ZnO nanomaterial, and they reached 55% MB degradation at 80 min reaction time under visible light irradiation. 84 When compared with the literature studies, it is obviously seen that promising results were obtained with the doped nanofibers containing anatase TiO 2 calcined at 650 and 750°C under both UV and sunlight irradiation.
3.6. XPS Analysis. The XPS analysis was performed to obtain the surface chemical species of the as-prepared nanofiber membrane that calcined at 750°C. Since the maximum photocatalytic efficiency for the degradation of 20 ppm methylene blue under simulated sunlight irradiation and the excellent nanofiber morphology (bead-free structure) was reached by using the membrane calcined at 750°C, XPS analysis was applied by using this membrane. As seen in Figure  9a (Figure 9b). 85 Since no peak of Ti 3+ was observed, it was concluded that TiO 2 was quite stable in the system. 54 Figure 9c displays the O 1s spectra with the peaks at 530.08 and 531.18 eV, which corresponded to the lattice oxygens and surface hydroxyl groups. 86 In Figure  9d, the single observed peak at 103.58 eV can be attributed to the Si 2p. 76,79 As shown in Figure 9e, the peaks exhibited at 182.18 and 184.58 eV can be assigned to the Zr 3d5/2 and Zr 3d3/2, respectively. 80 Figure 9f shows the Al 2p spectra, which is related to the peak at 74.87 eV. 87 As displayed in Figure 9g, the two spin-orbital components exhibited at 347.38 and 351.18 eV corresponded to the Ca 2p spectrum. 53 In Figure  9h, the two diffraction peaks at 885.78 and 900.98 eV can be attributed to the Ce 3d5/2 and 3d3/2 with the Ce 3+ oxidation state, respectively. On the other hand, the observed peaks at 899.38 and 916.88 eV related to CeO 2 , which indicated the presence of the Ce 4+ oxidation state. 87,88 The coexistence of Ce 3+ and Ce 4+ oxidation states is beneficial for the surface reactions by supporting reactive oxygen species (ROS) formation in photocatalytic processes. 54,87 3.7. Antibacterial Activity. The antibacterial ability of the nanofiber membrane calcined at 750°C was tested against two different bacteria: Escherichia coli BC 1402 as a Gram-negative bacterium and Staphylococcus aureus ATCC 25923 as a Grampositive bacterium. The viable cell count method was used, and

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http://pubs.acs.org/journal/acsodf Article aureus, while the positive control sample had 1.3 × 10 7 CFU/ mL for E. coli and 1.59 × 10 7 CFU/mL for S. aureus. The photographs of agar plates spread with the control cell suspension and those subjected to nanofiber membrane are given in Figure 10. The obtained nanofibrous membranes exhibited a very high antimicrobial rate of 99.94 and 99.91% against E. coli and S. aureus, respectively. Also, the results of these bacterial inhibitions are stated as the logarithmic decrease (log reduction), as shown in Table 4. However, the obtained nanofibrous membrane exhibited highly promising inhibition of both E. coli and S. aureus. Gram-positive (S. aureus) microorganisms were found to be more resistant to the nanofiber membrane. The reason for the higher inhibition rate of Gram-negative (E. coli) microorganisms when compared with Gram-positive (S. aureus) microorganisms was the thicker peptidoglycan layer of S. aureus which coincided with literature studies. The difference between the cell wall structures of the two microorganisms changed the diffusion properties. 89,90 In literature studies, the antibacterial activities of TiO 2 and doped TiO 2 materials were investigated under UV and sunlight irradiation in general. 86,91,92 In a study, TiO 2 nanomaterials did not show visible inhibitory effect against tested microorganisms; this result was explained with low diffusion and solubility in the solid medium. 93 However, Zr-doped TiO 2 nanoparticles showed higher antibacterial activity by the formation of reactive oxygen species (ROS). 52 The antibacterial mechanism of metal oxide nanofiber membranes occurs through the formation of ROS under both UV and dark conditions. However, as can be seen on nanofiber materials containing TiO 2 , there is a positive relationship between light irradiation and antibacterial activity due to the enhancement of ROS production. 94 In addition, metal doping decreases the crystalline size, which enables higher antibacterial activity. 52 In contrast, the main reason for the high inhibition rate of microorganisms by the obtained nanofiber membrane was the reaction with the cell wall, which led to the killing of bacteria.
In this study, the synergistic effect of doping materials provided to reach efficient antibacterial activity despite the absence of light irradiation. 86,91

CONCLUSIONS
Highly efficient TiO 2 -based multi-oxide nanofiber membranes were successfully fabricated via sol−gel and electrospinning methods in the system of TiO 2 −SiO 2 −Al 2 O 3 −ZrO 2 −CaO− CeO 2 and calcined at different temperatures from 550 to 850°C . XRD analysis revealed that the low calcination temperature of 550°C was not enough for anatase phase formation for TiO 2 , and anatase to rutile phase transformation was observed at a high temperature of 850°C. Moreover, the anatase cyrstalline formation was determined by HR-TEM. Due to the synergistic effect of doping materials to TiO 2 , remarkably high total degradation rates of MB were achieved. The photocatalyst calcined at 650°C showed the highest MB degradation rate under UV, whereas a similar result was obtained with the membrane calcined at 750°C under sunlight irradiation. Although the specific surface area decreased with increasing calcination temperature, the photodegradation rate of MB was enhanced up to a certain level of crystallinity of nanofiber membranes. It could be inferred that both the degree of crystallinity and surface area impressed the adsorption capability and photocatalytic performance of nanofiber membranes. In addition to these superior properties, the membrane calcined at 750°C exhibited very high inhibition rates against E. coli and S. aureus. The synergistic effect of doping materials not only improved photodegradation properties but also ensured the inhibition of the growth of microorganisms such as E. coli and S. aureus. Furthermore, compared with TiO 2 -coated photocatalytic materials, the direct incorporation of TiO 2 in glass ceramic systems prevented the peeling problem. To sum up, an excellent 3D multi-oxide doped TiO 2 ceramic nanofiber, which can be used in various applications requiring both antibacterial and photocatalytic properties, was obtained in this study. Future studies will be conducted to the obtained sample in a continuous process to be able to see the long-term stability and the efficiency of the sample for the waste-water treatment systems.