Preparation and Photocatalytic Performance of TiO2 Nanowire-Based Self-Supported Hybrid Membranes

Nowadays, the use of hybrid structures and multi-component materials is gaining ground in the fields of environmental protection, water treatment and removal of organic pollutants. This study describes promising, cheap and photoactive self-supported hybrid membranes as a possible solution for wastewater treatment applications. In the course of this research work, the photocatalytic performance of titania nanowire (TiO2 NW)-based hybrid membranes in the adsorption and degradation of methylene blue (MB) under UV irradiation was investigated. Characterization techniques such as scanning electron microscopy (SEM), transmission electron microscopy (TEM), energy-dispersive X-ray spectroscopy (EDS), X-ray powder diffractometry (XRD) were used to study the morphology and surface of the as-prepared hybrid membranes. We tested the photocatalytic efficiency of the as-prepared membranes in decomposing methylene blue (MB) under UV light irradiation. The hybrid membranes achieved the removal of MB with a degradation efficiency of 90% in 60 min. The high efficiency can be attributed to the presence of binary components in the membrane that enhanced both the adsorption capability and the photocatalytic ability of the membranes. The results obtained suggest that multicomponent hybrid membranes could be promising candidates for future photocatalysis-based water treatment technologies that also take into account the principles of circular economy.


Introduction
Over the past decade, air and water pollution has been on an upward trend, causing concern in everyday life around the world. In general, the industrial sectors accumulate large quantities of wastewater during manufacturing processes, which contains various chemical components such as pharmaceutical residues, dyes or heavy metals and salts [1] that are costly to neutralize and remove. Semiconductors and their composites have been studied in depth to combat the problems associated with waste generation. They include TiO 2 [2], ZnO [3], WO 3 [4], Bi-based [5] and Ag-based [6] materials, etc. [7].
Titanium dioxide (TiO 2 ) is one of the most widely used semiconductor for photocatalytic applications due to its advantageous properties (optical and electronic properties, chemical stability, low cost, lack of toxicity) [8]. However, its practical application is limited, as it can only be effectively excited under UV light due to its wide band gap. There have been numerous attempts to enhance the photocatalytic activity and excitability of titanium dioxide, for example, by synthesizing TiO 2 with various morphologies [9], modifying it with noble metals [10,11], doping it with various elements [12,13] and preparing composites with graphene [14], graphene oxides [15] and other semiconductors [16].
Copper and copper oxide are widely used as dopants on titanium dioxide materials' surfaces to enhance their photocatalytic performance by narrowing the bandgap and improving electron-hole separation with photoexcitation, as reported in [17,18]. Variations of iron oxides are used to modify the properties of titanium dioxide, due to their small band gap [19]. With the combination of iron oxide and titanium dioxide, shallow trap sites appear between the conduction band and the valance band that leads to a reduction in the band gap energy of TiO 2 [20]. Furthermore, the radius of Fe 2+ and Fe 3+ ions is smaller than that of Ti 4+ ; thus, iron ions can diffuse into the TiO 2 lattice to substitute TiO 2 [21]. In addition, iron oxides (magnetite, i.e., Fe 3 O 4 and maghemite, i.e., Fe 2 O 3 ) have special magnetic properties in the nanoscale range [22]. These ferromagnetic properties can be beneficial to improve the recyclability of photocatalysts and prevent particles from clumping together [23].
Photocatalytic membranes (PM) offer a great potential alternative for economical and eco-friendly treatments of wastewater, based on the combination of membrane filtration and photocatalysis [34,35]. Membrane filtration is widely used for drinking water treatment and wastewater treatment [36,37] due to its simple operation and effective removal of various types of pollutants. Using this technology, most organic and small amounts of inorganic substances can be decomposed using solar energy to reduce the harm of pollutants. There are various types of materials that can be used as membranes, such as natural polymers (cellulose [38,39]), green porous nano-membranes [40], graphene [41], carbon nanotube [42], ceramic [43] or synthetic polymer-based (polyvinyl alcohol (PVA), polyacrylonitrile (PAN), polyvinylidene fluoride (PVDF) etc.) [44] membranes [45]. TiO 2 nanowire membranes have been investigated as mechanical microfilters and as photocatalysts to degrade pharmaceutical residues such as trimethoprim and other organic materials including dyes, phenol and humic acid under UV irradiation [46]. A. Hu et al. investigated various types of TiO 2 nanostructured membranes, though the properties of CuO-and Fe 2 O 3 -modified TiO 2 photocatalytic membranes were not explored.
Cellulose-based materials are promising substances that can partly or totally replace synthetic fibers as filters in masks [47] and can be used as membranes or support for other materials and to remove oil and heavy metal ions during water treatment [48]. Furthermore, cellulose has unique features in comparison to the usual supports and thus allows nanoparticles stability, reactivity, recyclability and prevents nanoparticle aggregation. Cellulose also represents a sustainable alternative to known methods with the aforementioned properties. CdS and TiO 2 nanoparticle-nanocellulose hybrid composites have been used as photocatalysts for a model pollutant, i.e., methyl orange degradation. In the CdS case, 82% degradation efficiency was achieved after 90 min irradiation, and the material was reusable up to five times. [49]. A TiO 2 -cellulose hybrid composite was 20% more efficient in degrading methylene orange than pure TiO 2 after 20 min of UV irradiation [50]. In a recent study, it was found that the photocatalytic degradation rate of TiO 2 -cellulose was much higher (99.72%) than that of bare TiO 2 (69.18%) under UV irradiation for 30 min, because cellulose served as a support for TiO 2 nanoparticles distribution and also promoted the adsorption of methyl orange molecules [51]. In this work, the aim was to develop a "green chemistry" solution to industrial wastewater effluents. Cellulose-based membranes were chosen over other materials due to their advantageous properties such as lack of toxicity, low cost, biodegradability and eco-friendliness. Other than this, cellulose membranes have excellent specific surface area, adjustable surface chemistry, hydrophilicity and mechanical strength [52].
In the current paper, we successfully developed titanium dioxide nanowire (TiO 2 NW)based hybrid membranes. The surface of TiO 2 NW was decorated with iron oxide (Fe 2 O 3 ) and copper oxide (CuO) nanoparticles to improve the photocatalytic performance of the as-prepared composites. Furthermore, cellulose fibers were applied as a reinforcement filler material to prepare self-supported hybrid membranes. The photocatalytic properties of the membranes were investigated against methylene blue decomposition under UV irradiation. The results showed that nanofiber-based hybrid membranes can provide an excellent alternative as environmentally friendly solutions for wastewater treatment requiring the degradation of organic pollutants.

HRTEM and EDS Analysis of TiO 2 NW@Fe 2 O 3 and TiO 2 NW@CuO Nanocomposites
Heat-treated nanocomposite samples were investigated by the HRTEM technique. Figure 1 shows HRTEM micrographs of the prepared TiO 2 NW@Fe 2 O 3 and TiO 2 NW@CuO nanocomposites. These images revealed that the fabrication of both nanocomposites was successful, although different nanocomposite structures were observed during TEM. The HRTEM images showed that inorganic nanoparticles (Fe 2 O 3 and CuO) were attached to the surface of TiO 2 NW. Figure 1a,b show that Fe 2 O 3 and CuO nanoparticles adhered on TiO 2 NW, respectively, and segregated particles could not be observed.
In a recent study, it was found that the photocatalytic degradation rate of TiO2-cellulo was much higher (99.72%) than that of bare TiO2 (69.18%) under UV irradiation for min, because cellulose served as a support for TiO2 nanoparticles distribution and a promoted the adsorption of methyl orange molecules [51]. In this work, the aim was develop a "green chemistry" solution to industrial wastewater effluents. Cellulose-bas membranes were chosen over other materials due to their advantageous properties su as lack of toxicity, low cost, biodegradability and eco-friendliness. Other than this, cel lose membranes have excellent specific surface area, adjustable surface chemistry, hyd philicity and mechanical strength [52].
In the current paper, we successfully developed titanium dioxide nanowire (T NW)-based hybrid membranes. The surface of TiO2 NW was decorated with iron ox (Fe2O3) and copper oxide (CuO) nanoparticles to improve the photocatalytic performan of the as-prepared composites. Furthermore, cellulose fibers were applied as a reinfor ment filler material to prepare self-supported hybrid membranes. The photocataly properties of the membranes were investigated against methylene blue decomposit under UV irradiation. The results showed that nanofiber-based hybrid membranes c provide an excellent alternative as environmentally friendly solutions for wastewa treatment requiring the degradation of organic pollutants.

HRTEM and EDS Analysis of TiO2 NW@Fe2O3 and TiO2 NW@CuO Nanocomposites
Heat-treated nanocomposite samples were investigated by the HRTEM techniq Figure 1 shows HRTEM micrographs of the prepared TiO2 NW@Fe2O3 and TiO2 NW@C nanocomposites. These images revealed that the fabrication of both nanocomposites w successful, although different nanocomposite structures were observed during TEM. T HRTEM images showed that inorganic nanoparticles (Fe2O3 and CuO) were attached the surface of TiO2 NW. Figure 1a and b show that Fe2O3 and CuO nanoparticles adher on TiO2 NW, respectively, and segregated particles could not be observed. Furthermore, from the analysis of the HRTEM images, the average particle sizes Fe2O3 and CuO nanoparticles were calculated using the iTEM software (Olympus S Furthermore, from the analysis of the HRTEM images, the average particle sizes of Fe 2 O 3 and CuO nanoparticles were calculated using the iTEM software (Olympus Soft Imaging Solutions). The average particle size of these components was determined by measuring 100 individual particles in both samples. Based on these calculations, it was found that Fe 2 O 3 nanoparticles had a diameter in the range of 20-30 nm, as can be seen in Figure 1a, while the average diameter of CuO nanoparticles was 2-3 nm, as shown in Figure 1b.
EDS analysis was performed to determine the elements in the as-prepared nanocomposites. Figure 2a,b show the EDS spectra and confirmed that the most significant signals originated from carbon (C), oxygen (O), titanium (Ti), potassium (K), copper (Cu) and iron (Fe). The presence of TiO 2 NW, Fe 2 O 3 and CuO in the samples was confirmed by the Ti, Fe, Cu and O peaks, as shown in the spectra (Figure 2), while in the case of the TiO 2 NW@Fe 2 O 3 nanocomposite sample, the Cu peak originated from the sample holder (a lacey Cu grid), and the peak of K was due to the residual KOH that was used during the preparation of the TiO 2 NW.
Imaging Solutions). The average particle size of these components was determined measuring 100 individual particles in both samples. Based on these calculations, it found that Fe2O3 nanoparticles had a diameter in the range of 20-30 nm, as can be see Figure 1a, while the average diameter of CuO nanoparticles was 2-3 nm, as show Figure 1b.
EDS analysis was performed to determine the elements in the as-prepared nanoc posites. Figure 2a and b show the EDS spectra and confirmed that the most signifi signals originated from carbon (C), oxygen (O), titanium (Ti), potassium (K), copper and iron (Fe). The presence of TiO2 NW, Fe2O3 and CuO in the samples was confirme the Ti, Fe, Cu and O peaks, as shown in the spectra (Figure 2), while in the case of the NW@Fe2O3 nanocomposite sample, the Cu peak originated from the sample holde lacey Cu grid), and the peak of K was due to the residual KOH that was used during preparation of the TiO2 NW.

SEM and EDS Analysis of the Hybrid Membranes
In order to gain information about the surface morphology of the as-prepared hy membranes, SEM analysis was performed. The surface nature and morphology of a cellulose membrane (Figure 3a

SEM and EDS Analysis of the Hybrid Membranes
In order to gain information about the surface morphology of the as-prepared hybrid membranes, SEM analysis was performed. The surface nature and morphology of a neat cellulose membrane (Figure 3a To determine the elemental composition and confirm the presence of Fe 2 O 3 and CuO nanoparticles in the as-prepared hybrid membranes, EDS analysis was performed for each sample. The data of the EDS analysis in Table 1 revealed the atomic percentages (at%) of the detected elements in the samples. The most significant signals originated from carbon (C), oxygen (O) and titanium (Ti), confirming the presence of cellulose and TiO 2 NW in the hybrid membranes. Furthermore, iron (Fe) and copper (Cu) signals were detected, which related to the Fe 2 O 3 and CuO nanoparticles. Other elements were also observed, such as sodium (Na), potassium (K) and fluorine (F), which originatied from the preparation procedure of TiO 2 NW. The results of EDS analysis from HRTEM and SEM showed good correlations. To determine the elemental composition and confirm the presence of Fe2O3 and CuO nanoparticles in the as-prepared hybrid membranes, EDS analysis was performed for each sample. The data of the EDS analysis in Table 1 revealed the atomic percentages (at%) of

XRD and Specific Surface Area Analysis of the Hybrid Membranes
In order to determine the degree of crystallization of the as-prepared nanocomposites and membranes and to identify and describe the crystal structure of these materials, XRD analysis was performed. As can be seen in Figure 4a Cellulose  45  55  ------TiO2 NW  -64  36  -----TiO2 NW@Fe2O3/cellulose  20  50  23  3  -3  1  -TiO2 NW@CuO/cellulose  25  58  11  -3 2 -1

XRD and Specific Surface Area Analysis of the Hybrid Membranes
In order to determine the degree of crystallization of the as-prepared nanocomposites and membranes and to identify and describe the crystal structure of these materials, XRD analysis was performed. As can be seen in Figure 4a [54].
The pure materials, the membranes and the hybrid membranes were also characterized by the N 2 adsorption technique to determine their specific surface areas and pore diameters, as can be seen in Table 2. It was found that there was no significant difference between the surface areas of the as-prepared hybrid membranes. Both hybrid membranes had a specific surface area of approx. 120 m 2 /g.

Photocatalytic Efficiency of the Hybrid Membranes
The photodegradation efficiency of the synthesized membranes was tested using methylene blue dye as a model pollutant, under UV light irradiation. The results revealed the almost complete removal of MB. Approx. 90% of the initial MB dye was decomposed by the hybrid membrane containing a photocatalyst, i.e., the TiO 2 NW@Fe 2 O 3 and TiO 2 NW@CuO nanocomposites. Figure 5 presents the removal efficiency of MB of the TiO 2 NW@Fe 2 O 3 /cellulose and TiO 2 NW@CuO/cellulose hybrid membranes under UV light.
between the surface areas of the as-prepared hybrid membranes. Both hybrid membranes had a specific surface area of approx. 120 m 2 /g.

Photocatalytic Efficiency of the Hybrid Membranes
The photodegradation efficiency of the synthesized membranes was tested using methylene blue dye as a model pollutant, under UV light irradiation. The results revealed the almost complete removal of MB. Approx. 90% of the initial MB dye was decomposed by the hybrid membrane containing a photocatalyst, i.e., the TiO2 NW@Fe2O3 and TiO2 NW@CuO nanocomposites. Figure 5 presents the removal efficiency of MB of the TiO2 NW@Fe2O3/cellulose and TiO2 NW@CuO/cellulose hybrid membranes under UV light.  It is important to note here that only the hybrid membranes are shown because it was reported that structure deformations appear in pure cellulose exposed to UV light. This is the reason why we applied a lower amount of cellulose in the hybrid membranes. The high removal of MB by the membranes reflects their potential to degrade organic dye molecules. The excellent degradation efficiency could be attributed to the synergistic effects of the TiO 2 NW with nanoparticles (Fe 2 O 3 , CuO) on their surface and the adsorption of MB on cellulose. As the present study was carried out under UV light, significant differences could be observed under visible light; therefore, we will explore this possibility in a further study. In addition, since pure cellulose does not contain any photocatalyst, a pure cellulose membrane is not expected to possess photocatalytic activity. However, its adsorption capacity can be non-negligible; therefore, we performed adsorption studies for 2 h of this and other membranes, as shown in Figure 6. Since a small amount of cellulose was used in the hybrid membranes for the reason mentioned above and the adsorption capacities appeared to be similar for all materials, it can be concluded that titanate alone is capable of adsorbing MB molecules. In the literature, several works have reported enhanced adsorption of different organic pollutants onto Fe 2 O 3 and CuO, making these compounds promising components of membranes for wastewater treatment [19,55].
fects of the TiO2 NW with nanoparticles (Fe2O3, CuO) on their surface and the adsorption of MB on cellulose. As the present study was carried out under UV light, significant differences could be observed under visible light; therefore, we will explore this possibility in a further study. In addition, since pure cellulose does not contain any photocatalyst, a pure cellulose membrane is not expected to possess photocatalytic activity. However, its adsorption capacity can be non-negligible; therefore, we performed adsorption studies for 2 h of this and other membranes, as shown in Figure 6. Since a small amount of cellulose was used in the hybrid membranes for the reason mentioned above and the adsorption capacities appeared to be similar for all materials, it can be concluded that titanate alone is capable of adsorbing MB molecules. In the literature, several works have reported enhanced adsorption of different organic pollutants onto Fe2O3 and CuO, making these compounds promising components of membranes for wastewater treatment [19,55]. Figure 6. Adsorption capacity of pure cellulose (green marks) as well as TiO2 NW@Fe2O3/cellulose (blue marks) and TiO2 NW@CuO/cellulose (red marks) hybrid membranes in MB adsorption tests.

Synthesis of TiO2 Nanowires (TiO2 NW)
Recently, we showed the preparation of TiO2 NW using the so-called solvothermal process [56]. In brief, a homogeneous TiO2 suspension was transferred into a Teflon ® -lined autoclave. The autoclave was kept in a dryer at 160 °C for 24 h. The as-prepared TiO2 NWs Figure 6. Adsorption capacity of pure cellulose (green marks) as well as TiO 2 NW@Fe 2 O 3 /cellulose (blue marks) and TiO 2 NW@CuO/cellulose (red marks) hybrid membranes in MB adsorption tests.

Synthesis of TiO 2 Nanowires (TiO 2 NW)
Recently, we showed the preparation of TiO 2 NW using the so-called solvothermal process [56]. In brief, a homogeneous TiO 2 suspension was transferred into a Teflon ® -lined autoclave. The autoclave was kept in a dryer at 160 • C for 24 h. The as-prepared TiO 2 NWs were washed with 0.1 M HCl and deionized water until a neutral pH was reached. The products were dried and calcined at 500 • C for 1 h.

Synthesis of TiO 2 NW@Fe 2 O 3 /Cellulose Membranes
For the synthesis of the hybrid membrane, firstly, a TiO 2 NW@Fe 2 O 3 nanocomposite was prepared. The calculated amount of FeCl 3 × 6H 2 O precursor was dissolved in 100 mL of distilled water to obtain a homogeneous solution. Then, 0.95 g of previously prepared TiO 2 NWs was added to the solution and stirred for 1 h, then transferred to the autoclave for 9 h at 90 • C. The product obtained was washed with 0.1 M NaOH to adjust the pH to 7, dried for 12 h at 50 • C and then calcinated for 2 h at 500 • C using a static furnace. The load of the Fe 2 O 3 nanoparticles in the final composition was 5 w/w %. In the next step, 0.2 g of the as-prepared TiO 2 NW@Fe 2 O 3 nanocomposite powder was dispersed in 100 mL of distilled water for 1 h, then 5 g of cellulose solution (1 w/w %) was added to the solution and stirred for 1 h. Finally, the preparation of the cellulose-based hybrid membranes was accomplished by vacuum filtration through a PVDF membrane (total mass of 250 mg/membrane), followed by drying in a furnace for 30 min at 40 • C.

Synthesis of TiO 2 NW@CuO/Cellulose Membranes
In the same way, a calculated amount of (Cu(CH 3 COO) 2 × H 2 O) was dissolved in 100 mL of EtOH and left under vigorous stirring for 30 min to ensure complete dissolution. Then, 0.95 g of TiO 2 NW was added directly to the solution, which was kept under vigorous stirring for 1 h. The mixture was poured into an autoclave and placed in a static furnace at 150 • C for 12 h. The final product was collected and washed using vacuum filtration and calcinated for 2 h at 500 • C. The load of the CuO nanoparticles in the final composition was 5 w/w %. To prepare the TiO 2 NW@CuO/cellulose membranes, 0.2 g of the above prepared composite was dipped into 100 mL of EtOH for 1 h, then 5 g of cellulose (1%) was added to the solution for another 1 h, and finally, the membrane was obtained by vacuum filtration using a PVDF membrane (total mass of 250 mg/membrane), followed by drying in a furnace for 30 min at 40 • C.

Characterization Techniques
For the qualitative characterization, high-resolution transmission electron microscopy (FEI Tecnai G 2 F20 HRTEM, Hillsboro, OR, USA) was used to analyze the morphology of the synthetized TiO 2 NW@Fe 2 O 3 and TiO 2 NW@CuO nanocomposites. To prepare the samples for HRTEM, the nanocomposites were dispersed in ethanol and sonicated for 5 min. On a Cu TEM-grid (300-mesh copper grids, lacey carbon, Ted Pella Inc., Redding, CA, USA), we placed a droplet of each suspension. The diameter of the materials was determined using the ImageJ software, utilizing the HRTEM images and the original scale bar. To determine the elemental composition of the TiO 2 NW@Fe 2 O 3 and the TiO 2 NW@CuO nanocomposites, energy-dispersive X-ray spectroscopy (EDS; AMETEK Inc., Berwyn, PA, USA; active area 30 mm 2 ) coupled to HRTEM was applied.
The surface morphology of the as-prepared TiO 2 NW@Fe 2 O 3 /cellulose and TiO 2 NW@CuO/cellulose membranes was investigated by scanning electron microscopy. SEM and EDS spectroscopy was carried out in a Nova 600i Nanolab (Thermofisher, Eindhoven, The Netherlands) equipped with an EDS system for elemental analysis (EDAX Inc., Mahwah, NJ, USA). The EDS system mounted an Octane Elect Plus X-rays detector. Typical EDS maps and spectra were acquired using acceleration voltage values between 10 kV and 25 kV, with take-off angle of 35 • and Dwell Time of 200 ms. For SEM analysis, the powders were deposited on a sticky carbon tape. Both powders and membranes were imaged directly without any conductive coating. The typical SEM working distance was 5 mm, and the acceleration voltage ranged from 2 kV up to 25 kV, depending on image quality and charging conditions.
To determine the surface area of the raw materials, the nanocomposites and the hybrid membranes, nitrogen adsorption-desorption experiments were carried out at 77 K to determine the Brunauer-Emmett-Teller [24] (BET) specific surface area using an ASAP 2020 instrument (Micromeritics Instrument Corp., Norcross, GA, USA).

Photocatalytic Experiments
The photocatalytic activity of the membranes was evaluated by studying the degradation of methylene blue as a model pollutant in an aqueous solution under UV light irradiation. The membranes were dipped into 100 mL of 0.03 mM MB solution and kept in the dark for two hours to attain the adsorption-desorption equilibrium. After this, the solution containing the pollutant and the photocatalyst (hybrid membranes) was exposed to UV-A lamps at a power between 300 and 500 W (Cosmedico N 400 R7S) for 60 min. The samples were withdrawn at regular time intervals and analyzed using a UV-Vis spectrophotometer (BEL UV-M51). The removal efficiency of the membranes was measured by recording the absorbance at 664 nm, and the degradation efficiency (% deg.) was calculated using the following formula: where, c 0 is the initial concentration at time t = 0, c t is the concentration at time "t", and % deg. is the photodegradation efficiency of the materials in relation to MB removal.

Conclusions
Herein, we presented the synthesis of two types of novel TiO 2 nanowire-based hybrid membranes and studied their adsorption properties and photocatalytic performance in the decomposition of methylene blue under UV light. The as-prepared TiO 2 NW@Fe 2 O 3 /cellulose and TiO 2 NW@CuO/cellulose membranes were characterized by TEM, SEM, EDS and XRD. The results clearly demonstrated that not only the preparation of raw TiO 2 NW, Fe 2 O 3 and CuO decorated TiO 2 NW, but also the production of hybrid membranes was successful.
Comparing the results of photocatalysis, it was found that both types of hybrid membranes showed outstanding performance in removing MB in only 60 min of UV irradiation. The photocatalytic degradation efficiency in MB removal of TiO 2 @Fe 2 O 3 /cellulose was 88%, while that of TiO 2 @CuO/cellulose membrane was up to 90%.
Since we applied UV light in this study, a significant degradation could presumably be observed under visible light irradiation; therefore, we intend to explore this possibility in a future study. Furthermore, we are planning to submit a further study in the near future regarding the microbiological and toxicological properties of the hybrid membranes here presented. It is believed that by exploiting the advantageous properties of the hybrid membrane-based water treatment technologies and solutions presented here, new and sustainable strategies could be implemented for photocatalyst-based water treatment technologies.