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Ultrasonic coating of nanofibrons webs: a feasible approach to photocatalytic water filters.

Abstract In this study, we introduce ultrasound-assisted coating as a simple means of functionalizing nanofibrous membranes for photocatalytic (PC) water filtration. Ti[O.sub.2] nanoparticles, which have an anatase crystalline phase with superior PC activity, were directly synthesized over TPU nanofibers and the coated membranes were characterized using scanning electron microscopy (SEM) and X-ray diffractometry. The SEM images confirmed that the addition of polyethylene glycol reduced the nanoparticle dimensions and yielded greater distribution over the nanofibers. UV-Vis analysis demonstrated that Ag decoration over the coating enhanced the PC performance by up to 69.4% following 2-h UV illumination.

Keywords Nanofibers, Ultrasonic, Coaling, Photocatalytic

Introduction

Due to frequent droughts, population increases, and rapid industrialization, the need for safe water is currently at its greatest. According to UN reports, more than 1.5 billion people are currently unable to obtain safe, clean water (1) and, in developing countries, over 2 million people die annually as a result of water-related diseases. (2) Additionally, the impact of the greenhouse effect on the environment, coupled with climate change, is causing the treatment of waste water to become an increasingly pressing issue. Even in well-developed countries, an exorbitant amount of contaminated water is released into water sources such as rivers, lakes, and underground deposits without treatment, leading to pollution problems. In order to identify the most effective engineering solutions, the contaminants within used water, which can be classified as either particulate matter or biological and chemical components, must be examined. (3-5) For particulate matter and biological material, well-known solutions such as coagulation, flocculation, and pressure-driven membranes are widely used. On the other hand, the treatment of chemically contaminated water remains a challenging issue. To date, various oxidation techniques have been proposed. Of these, photocatalytic oxidation (PCO) is a feasible method for the removal of chemical contaminants from wastewater. (6)

As regards the development of an effective PCO technique, titania (or Ti[O.sub.2]) is by far the most widely investigated semiconductor. This is because of its superior PCO performance, along with its unique properties such as chemical stability, biocompatibility, and low cost. (7) Comprising a filled valence band (VB) and an empty conduction band (CB), illumination by a photon with energy larger than the Ti[O.sub.2] band gap generates an electron in the CB and holes in the VB. (8) Hence, the generated electron and hole produce radicals around the contaminant molecules or directly react with them. (9) Many studies have concentrated on modifying the Ti[O.sub.2] structure so as to enhance its activity in response to visible light, (5,8,10,11) since titania exhibits a low electron transfer rate to oxygen and a high electron-hole recombination rate. This significantly limits the photo-oxidation rate of organic compounds. (8) Photo-generated electron-hole pairs have a recombination time of the order of [10.sup.-9] s; however, the chemical interaction with adsorbed pollutant species has a timescale of [10.sup.-8] to [10.sup.-3] s. (2,9) The decoration of Ti[O.sub.2] with metals is a practical means of enhancing the photocatalytic (PC) efficiency, as the recombination of photo-excited electrons is restricted in metal-doped titania. This is because of the appearance of a semiconductor-metal junction, called the Schottky barrier, in which a space-charge separation region is formed. At the metal-titania interface, electron movement occurs because of the difference in Fermi energy level. Electrons move from the high Fermi energy level (Ti[O.sub.2]) toward the lower Fermi energy level of the metal. Thus, charge separation is achieved through the Schottky barrier that appears at the metal-Ti[O.sub.2] interface. The electron movement leads to excess electrons in the metal, along with excess positive holes in the Ti[O.sub.2] VB. The resultant depletion layer owing to the Schottky barrier at the metal-Ti[O.sub.2] interface has the capability to sustain charge separation. (12) Of such metallic modifications, silver (Ag)-incorporated Ti[O.sub.2] particles have demonstrated enhanced electron-hole separation and interfacial charge transfer ability, as well as increased visible-light excitation. (13) Hence, slower recombination of photo-generated electron-hole pairs and, at the same time, extended excitation wavelengths have been observed. (12)

For application purposes, Ti[O.sub.2] in powder form is not appropriate, since it requires subsequent cleaning from the water following the initial treatment. (8) Thus, in this study, an ultrasonic coating procedure is proposed that immobilizes the Ti[O.sub.2] on nanofibrous webs. This system allows for the simultaneous coating of pristine/Ag-doped Ti[O.sub.2] on previously prepared electrospun webs. Nanofibrous webs provide the largest surface area for coating with an active layer. Theoretically, very high surface roughness for illumination is achievable. Further, the high-energy acoustic cavitation that occurs during ultrasonic treatment provides durable and uniform coating. An extremely high temperature (approximately 5000[degrees]C) and pressure (approximately 500 atm) are contained in the cavities, which is a unique feature of the ultrasonic process. These cavities have lifetimes with a timescale of a few microseconds and the shock waves that emerge following cavity collapse in a liquid suspension cause particle collisions, which have sufficient energy to catalyze many reactions. (14-16) Finally, Ti[O.sub.2] that is synthesized in an ultrasonic bath has more advantageous properties than that synthesized via a conventional sol-gel system, including an extremely high surface area, decreased particle size, and superior PCO activity. (17)

In this study, ultrasonic irradiation was used to decorate silver ions onto Ti[O.sub.2]-coated nanofibrous mats. To the best of our knowledge, this is the first time this approach has been used. Following decoration, the Ag nanoparticles created the possibility for a Schottky barrier to develop, which ensured charge separation. A thermoplastic urethane (TPU) nanofiber mat was used as a template, with the silver nanoparticles as decorative agents on the titania layer. The efficiency of the coated nanofibrous filters was then investigated and evaluated based on the degradation rate of methylene blue (MB) in an aqueous solution. Also, the effect of added polyethylene glycol (PEG) on the Ti[O.sub.2] distribution and PCO performance was evaluated. In order to evaluate the efficacy of the proposed approach, the performance of ultrasonicassisted synthesized pristine/silver-decorated Ti[O.sub.2] and that of a similar commercial product, Degussa, was compared.

Materials and methods

A TPU polymer was purchased from BASF (C95). Dimethyl formamid (DMF %98) and Ethanol (%99) purchased from Merck without any further treatment were used as a solvent for the polymer solution preparation, while AgN[O.sub.3] and titanium tetra isopropoxide (TTIP) were used as the Ag and Ti[O.sub.2] sources for the PC activation, respectively. Titanium(IV) oxide (Degussa 21 nm [greater than or equal to] 99.5%) was purchased from Sigma Aldrich. All chemicals were used without further purification, as obtained from Sigma Aldrich.

TPU nanofiber electrospinning

A neat polyurethane (13 wt%) solution was prepared by dissolving TPU pellets in DMF at 80[degrees]C. Electrospinning was conducted at 24 kV, with a tip-to-collector distance of 150 mm and a solution feed rate of 0.6 mL/h. TPU was selected as the base polymer because of its elasticity.

Ti[O.sub.2] NP synthesis

Electrospun TPU nanofibrous mats (35 mm x 35 mm) were immersed in preformed solutions (Table 1) under ultrasonic irradiation at a frequency of 40 kHz. Nanocrystalline Ti[O.sub.2] was synthesized and coated onto the TPU nanofibers simultaneously by hydrolysis of the TTIP in the presence of ultrasonic irradiation. Then, 3 g of silver nitrate (AgN[O.sub.3]) was added to the ultrasonic bath in order to decorate the Ti[O.sub.2] coating with silver particles. After 5 min of ultrasonic irradiation in solution, the samples were exposed to UV irradiation for 30 min. This was to transform the silver ions into silver atoms, following the method explained in reference (18).

Morphological analysis

The nanoparticle distribution on the nanofibers was examined using a Zeiss EVO MA10 SEM at 15.00-20.00 kV with magnifications ranging from 500 to 20 kX. A tungsten filament was used to create the beam and the reported resolution was 35 [Angstrom] with a 20-kV beam. To watch the elemental composition after Ag decoration, the energy dispersive X-ray spectroscopy (EDS) was recorded using SEM (model Zeiss EVO 60) installed in MEMTEK Laboratories, ITU. The crystal structure of the Ti[O.sub.2] coating was investigated using a Rigaku SmartLab X-ray diffractometer equipped with a rotating anode Cu source with in-line focus geometry, which produces Cu-Ka radiation with a wavelength of 1.54 [Angstrom] generated at 40 k[alpha] and 44 mA. The samples were equatorially scanned within a 2[theta] range (=20[degrees]-80[degrees]) at an increment of 0.1[degrees].

MB decomposition test

A custom-made UV irradiation system was used to conduct the PCO performance test. The MB degradation rate under UV illumination was used as a means of evaluating the PCO performance of the nanofibrous samples. (12,19) An explanation of the evaluation technique based on MB concentration loss is available in the literature. (19-22) In order to evaluate the PC activity of the Ti[O.sub.2], a [10.sup.-5] M MB aqueous solution was prepared. The as-prepared functionalized nanofibrous mats were floated in the MB aqueous solution for periods of 10 min, 1 and 2 h under UV illumination. The UV lamp used in the study had a wavelength of 365 nm and power of 9 W, and the decomposition rate was evaluated using UV-Visible spectroscopy (Philips UVA PLS/PLA-9 W, irradiation 1.66 W).

Results and discussion

Morphological analysis

Scanning electron microscopy (SEM) was used to analyze the nanofiber diameter and the distribution of the nanoparticles on the nanofibrous webs. As can be seen in Fig. 1a, the average diameter of the TPU nanofibers was found to be 164 [+ or -] 33 nm. A smaller fiber diameter would yield a larger surface area, which is important for catalytic reactions. On the other hand, the ultrasonic process did not alter the fibrous structure, as can be seen in Fig. 1b. However, agglomeration occurred in the pristine Ti[O.sub.2] sample, since TTIP is highly sensitive to water. In the case of the TTIP-based coating sample, Ti[O.sub.2] particles on the microscopic scale were obtained with no proper distribution. However, the addition of PEG to the solution resulted in Ti[O.sub.2] particles of nanoscopic dimensions with a more suitable distribution on the nanofibrous web. The homogenous distribution of the nano-scale Ti[O.sub.2] particles in Figs, 1c and 1d can be attributed to the hydroxyl groups of the PEG which acted as a surfactant resulted in inhibition of agglomeration of synthesized Ti[O.sub.2] particles. (23,24) Note that dimensional control is extremely important as regards PCO performance, because decreased dimensions enhance the interaction between the Ti[O.sub.2] particles and dye (MB) molecules, as well as the interaction between the UV light and the Ti[O.sub.2] particle surfaces. This justifies the strategy of introducing a chemical agent to enhance the PCO activity of the Ti[O.sub.2].

Additionally, EDS analysis was also conducted to prove Ag decoration on Ti[O.sub.2]. As shown in Fig. 2b, Ag decoration is apparent over Ti[O.sub.2] particles, whereas without the presence of AgN[O.sub.3] in precursor, coating was solely composed of C, Ti, and O.

The well-known crystal structures of Ti[O.sub.2] are anatase and rutile, and it is important to note that the physical properties of a material are dependent on its crystal structure as to highly PC activity. The Ti[O.sub.2] particles synthesized in the presence of ultrasonic irradiation exhibited an anatase crystalline phase, as seen in Fig. 3. The crystal structure of the sonochemically synthesized anatase Ti[O.sub.2] may be due to the localized high temperature and high pressure induced by the ultrasonic irradiation. Clearly, the primary advantage of ultrasound-induced anatase Ti[O.sub.2] synthesis is that the resultant material does not require any additional heat treatment. Thus, this novel method can be evaluated as being both cost effective and rapid, in comparison with other methods such as sol-gel synthesis.

In general, anatase has significantly greater photoactivity than rutile, the other well-known Ti[O.sub.2] crystalline phase. (25) This behavior is attributed to the band-gap size; anatase and rutile crystalline structures have 3.2 and 3.0 eV band gaps, respectively. The higher band gap is the source of the greater redox potential of the anatase structure. Additionally, anatase crystals have a higher surface hydroxyl area density. Hydroxyls play an important role in slowing the recombination of the photo-generated electron--hole pairs. (12) In addition, Perez et al. (26) have also demonstrated that decreased particle size causes an increase in the band-gap energy of Ti[O.sub.2]. It can then be inferred that the exciton energy levels of Ti[O.sub.2] with lower particle size are higher than those of Ti[O.sub.2] samples with greater particle size. It should also be noted that excitons with higher energies may be the source of the superior PC activity observed for this material. As seen in Fig. 2, the as-synthesized Ti[O.sub.2] nanoparticles with anatase crystalline structures match the results of the study conducted by Perez et al. exactly. (26) It can also be inferred that the PEG addition resulted in the synthesis of Ti[O.sub.2] with lower dimensions, as indicated by the SEM images. Hence, Ti[O.sub.2] with reduced dimensions exhibits superior PCO activity because of the higher energy levels of its excitons.

[FIGURE 1 OMITTED]

MB decomposition performance of PC filters

UV-Vis spectroscopy measurements were performed after illumination of the MB/water solutions, in which the samples were embedded for periods of 10 min, 1 and 2 h. The as-prepared MB solution was taken as the base line. It is clear from Fig. 4 that the MB solutions treated with functionalized nanofibrous mats were dramatically clarified as a result of the PC activity of the Ti[O.sub.2] in the solution. As can be seen in Fig. 4a, the presence of the PEG enhanced the PCO efficiency of the filters compared to the performance of the TTIP-based filters. It can then be concluded that PEG addition is an effective strategy for controlling the dimensions of the as-synthesized nanoparticles. Therefore, the PEG-induced highly specific surface area increased the interaction between the Ti[O.sub.2] and UV light, as well as the interaction between the MB and Ti[O.sub.2] particles. These increased interactions may be the source of the PCO performance enhancement. Additionally, it can be hypothesized that other reactions may exist which resulted in intermediate products which did not decompose fully. Note that the TTIP-based filter may require a longer period of time for effective performance to be demonstrated, since the UV-Vis spectrum curves indicate absorption in the range of 450-550 and 700-800 nm. On the other hand, the synthesized Ti[O.sub.2] with added PEG achieved complete MB decomposition.

[FIGURE 3 OMITTED]

Considering Fig. 4b, it can be clearly stated that the Ag decoration contributed substantially to the performance of the PC filters, as expected. As explained previously, both charge separation and retarded recombination induced a PCO performance enhancement. Further, additional enhancement of the PCO efficacy was also expected following the reduction of the silver ions to atoms by the UV irradiation. However, the Ag-based filters with no UV irradiation exhibited superior performance to the filter that was exposed to the UV radiation, which was unexpected. Hence, further investigation and improvement of the method used to obtain silver atoms is required in forthcoming studies, in order to explain this result.

In addition, a comparison of the PC effectiveness of the Ti[O.sub.2] particles synthesized via the sonochemical reaction with that of Degussa (commercial Ti[O.sub.2] nanoparticles) was conducted. Although Degussa is a well-known and widely used PC, the sonochemically synthesized Ti[O.sub.2] with silver decoration demonstrated far superior PC activity, as can be seen in Fig. 4c. The excellent performance of the synthesized Ti[O.sub.2] can be attributed to the reduced dimension and significantly more uniform distribution of the Ti[O.sub.2] nanoparticles. This uniform distribution was due to the presence of the PEG, and can be considered to be another source of the superior performance of PEG-based PC filters. Following Fig. 4, it is also important to emphasize that PC filters have the capability to degrade organic compounds with a decomposition rate that is directly proportional to the UV irradiation time.

It is clear that MB degradation was observed via the absorbance spectra of the dye molecule throughout the irradiation as shown in Fig. 4. MB degradation rate as a function of irradiation time was also studied. Typically, disappearance of MB is directly correlated with time and can be predicted with Langmuir-Hinshelwood reaction kinetics. (27,28) The examination of MB concentration (C/[C.sub.0]) was conducted with the reaction time via 1n (C/[C.sub.0] = k x t, where k is the reaction rate constant; t, time; [C.sub.0] is the initial MB concentration; and C is the MB concentration at a specific irradiation time. Figure 5 displays the time dependency of C/[C.sub.0] under UV irradiation of the filters. It is important to note that direct photolysis of MB was in negligible scale. On the other hand, it is clear that MB degradation varies as Ti[O.sub.2] (TTIP + PEG + Ag) > Ti[O.sub.2](TTIP + PEG) > Ti[O.sub.2] (TTIP). The variation in MB degradation rate can be attributed to charge separation phenomena, higher exciton energy in anatase crystal structure, decreased particle dimension, and better distribution of nanoparticle on nanofibrous web. Besides that, MB degradation amount can be evaluated by Fig. 5 throughout 120 min. It is evident that 69.4% MB degradation takes place by Ti[O.sub.2](TTIP + PEG + Ag) filter. Silver decoration resulted in higher performance of the PC filters probably as a result of retarded charge recombination.

[FIGURE 4 OMITTED]

[FIGURE 5 OMITTED]

Conclusions

An ultrasonically assisted Ti[O.sub.2] coating has been proposed as an approach to producing PC water filters. This novel technique is promising as it is based on the ceramic coating of polymeric membranes, which enhances both chemical and thermal durability. Further, this system facilitates metal doping/decoration, which enhances PC performance. The dimensions of the as-synthesized Ti[O.sub.2] particles are controlled by the addition of PEG, and it has been shown that PEG has beneficial effects on the PCO performance of the Ti[O.sub.2], leading to smaller particle size and significantly more uniform particle distribution. Additionally, UV-Vis measurements have demonstrated that Ag decoration enhances the performance by approximately 70%, as it causes charge separation at the Ti[O.sub.2]-Ag interface. Further, the PC water filter developed here exhibited superior performance to a similar commercially available product. The findings of this paper will lead to the development of more effective water filtration systems, which have considerable potential applications worldwide.

DOI: 10.1007/s11998-015-9718-8

R. Simsek, Y. Polat, E. S. Pampal, O. Agma, A. Kilic ([mail])

TEMAG Labs, Istanbul Technical University, Gumussuyu, 34437 Istanbul, Turkey

e-mail: alikilic@itu.eclu.tr

R. Simsek

Department of Textile Engineering. Faculty of Technology, Marmara University, Istanbul, Turkey

E. S. Pampal

Department of Textile Engineering, Faculty of Engineering, Bartin University, Bartin, Turkey

Acknowledgments The authors gratefully acknowledge the ITU Scientific Research Fund (ITU-BAP) for financial support of this work. Thanks are also due to Prof. Ahmet Gul and Dr. Barbaros Akkurt for their analytical assistance and fruitful discussions.

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Table 1: Process parameters and solution compositions

Filter/sample name      Solution composition         Process

TTIP                    100 mL Dl water + 2 mL       45-min ultrasonic
                          TTIP
PEG-TTIP                50 mL ethanol + 0.4 mL PEG   45-min ultrasonic
                          + 2 mL TTIP + 50 mL
                          distilled water
PEG-TTIP-Ag             50 mL ethanol + 0.4 mL       45-min ultrasonic
                          PEG + 2 mL TTIP + 50 mL
                          distilled water + 3 g
                          AgN[O.sub.3]
PEG-Ti[O.sub.2]-Ag-UV   50 mL ethanol + 0.4 mL PEG   45-min ultrasonic,
                          + 2 mL TTIP + 50 mL          30-min UV
                          distilled water + 3 g
                          AgN[O.sub.3]

Fig. 2: Elemental composition of coatings via ultrasonic-assisted
synthesis (a) Ti[O.sup.2] (from TTIP, ultrasonicated for
45 min) (b) Ti[O.sup.2]-Ag (from PEG-TTIP-Ag ultrasonicated for
45 min) Silver peaks around 3.05 eV are apparent for
decorated samples

Element    wt%      at%

C K        44.70    59.29
O K        33.70    33.55
TiK        21.47    07.14

Element    wt%      at%

C K        22.55    44.30
O K        28.72    42.36
AgL        38.96    08.52
TiK        09.78    04.82
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Author:Simsek, Ramazan; Polat, Yusuf; Pampal, Esra Serife; Agma, Onur; Kilic, Ali
Publication:Journal of Coatings Technology and Research
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Geographic Code:1USA
Date:Jan 1, 2016
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