A Simple Method of Electrospun Tungsten Trioxide Nanofibers with Enhanced Visible-Light Photocatalytic Activity
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The present study involves the fabrication of tungsten trioxide (WO3) nanofibers by an electrospinning technique using polyvinyl pyrrolidone (PVP)/citric acid/tungstic acid as precursor solution. It was found that the PVP concentration was one of the most crucial processing parameters determining the final properties of WO3 nanofibers. The optimum concentration of PVP was from 75 to 94 g L−1. The average diameter of the nanofibers increases with increasing the PVP concentration, whereas it is decreased after sintering and orthorhombic structure were formed at 500 °C. The photocatalytic properties of the as-synthesized nanofibers were also investigated by degrading methylene blue and twofold efficiency was obtained compared with that of commercial WO3 microparticles.
KeywordsTungsten trioxide Electrospinning Nanofiber Photocatalysis
The textile industry effluent is a major source of water pollution which can destroy the aquatic life and influence world climates change seriously . The main source of water pollution comes from dyeing and finishing process of fabrics in textile industries. It is believed that 10–15 % of the dyes are unintentionally wasted during the dyeing processes in major developing countries . An industrial dye constitutes one of the largest groups of harmful organic compounds which are difficult to degrade naturally. Mostly, the non-fixed dyes especially the azo-dyes and other inorganic salts from textile factories tend to contain considerable amount of toxins and are highly carcinogenic. Therefore, direct contact of these dyes has harmful effects on human [3, 4].
Currently, different strategies have been employed for treating the textile effluents before they are released in water bodies including microfiltration , chemical precipitation , biosorption , membrane separation techniques , nanofiltration , electrochemical treatment  solvent extraction , and coagulation-flocculation . However, the limitations of these strategies include that incomplete ion removal and production of toxic sludge are unavoidable, which require further disposal of the released effluents. The residual dyes can remain for a longer period of time which are non-biodegradable and have no effect due to sunlight . Hence, the aforementioned methods are not fully useful as they can only transform the dyes from one phase to another, creating new kinds of pollutants which further need to be treated with other methods such as advanced oxidation processes by photocatalytic degradation using metal oxides.
Tungsten trioxide (WO3) is one of the most popular metal oxides which have been extensively used as electrochromic devices and gas sensor for water splitting . Recently, it was concluded that WO3 has a broad range of band-gap values which ensures considerable photocatalytic and photoelectrocatalytic activity under visible-light illumination . At this point of view, WO3-based materials as well as their photocatalysis have attracted much attention, for example, nanoparticles for degradation of lidocaine under visible and sunlight irradiation  and porous structure for degradation of rhodamine B dye . Many efforts have also been made to enhance the photocatalytic efficiency of WO3, for example, photo-deposition of platinum in WO3 nanostructures , titania-WO3 nanotubular composite fabricated by electrochemical method , various films based on WO3 , etc. However, WO3 nanofibers with large aspect ratio are rarely reported before [21, 22]. It is believed that increase of surface-to-volume ratio is one of the effective ways to increase the photocatalytic efficiency . In this connection, various nano-structured WO3 such as nanorods , nanofibers , nanowires , and nanoporous films  have been fabricated. Among these nanomaterials, nanofibers with superior porosity, excellent mechanical properties, and desirable chemical properties make them multifunctional capabilities for various applications .
Generally, nanofibers are produced by three techniques of self-assembly , phase separation , and electrospinning , in which the electrospinning is considered to be the most preferred technique due to its facility of quantity production, tunable properties via changing the solution parameters, as well as wide applications. Till now, polymer, ceramic, metal oxide, and lots of composite nanofibers have been successfully prepared via electrospinning technique [31, 32, 33, 34]. Various electrospun WO3-based nanofibers were also investigated widely, for example, WO3 nanofibers for gas-sensing , core–shell WO3/TiO2 nanofibers for photocatalysis , polycrystalline trioxide nanofibers as ammonia sensors , templates for detection of biomarker molecules , and improved photoluminescence . Especially, WO3 nanofibers can be efficiently used for photocatalyst due to its unique physical and chemical properties as above references mentioned. Recently, lots of works have been done to investigate the fabrication and properties of electrospun WO3 nanofibers [25, 26, 37, 39]. However, the existing methods need either expensive precursors or long time process. In this work, we developed a simple electrospun method of fabrication WO3 nanofibers by using inexpensive precursor of polyvinylpyrrolidone (PVP)/citric acid/tungstic acid. The photocatalytic efficiency of the as-synthesized nanofibers was also investigated via degrading methylene blue.
PVP of M w = 1,300,000 was purchased from Sigma Aldrich in USA. Commercial WO3 powders (mean particle size 30 μm) were obtained from Kanto Chemical Company and methylene blue (99 % purity) from Showa Chemical Company in Japan. Ammonia solution was from Duskan Pure Chemical Company Ltd, and citric acid monohydrate (99.5 % purity) was from Samchun Pure Chemicals in Republic of Korea.
2.2 Preparation of Solution for Electrospinning
The WO3 nanofibers were synthesized using an electrospinning method. Commercially available WO3 powders were dissolved in hot ammonia solution with stirring firstly. After the mixture was cooled down to room temperature, the insoluble materials were removed by filtration after which an amount of citric acid was added as chelating agent for binding WO3 . Then, PVP (75 g L−1) were added into the above solution followed by magnetic stirring for about 1 day. The weight ratio of PVP/citric acid/WO3 was kept constant at a ratio of (1.1:2.7:1.0). The mixed solution was aged on hot plate for different time to form different concentration of PVP/citric acid/tungstic acid precursor gel for electrospinning. In this work, three kinds of precursor solution were prepared in which the concentration of PVP was 75, 94, and 140 g L−1, respectively.
2.3 Electrospinning Solution
A high voltage power supply (CPS-60 K02V1, Chungpa EMT Co.in Republic of Korea) capable of generating voltages up to 30 kV was used as the electric field source for spinning of nanofibers. Solutions to be electrospun were supplied through a 20-mL plastic syringe attached with stainless steel having 22 gages, (0.7 mm OD × 0.4 mm ID). The feeding rate of solution to be electrospun was adjusted to be 0.2 mL min−1 by using a syringe pump (KDS-100, KD Scientific, United States). The syringe was aligned perpendicular to flat bed collector of electrospinning apparatus. The wire originated from the positive electrode (anode) was connected with the needle tip of plastic syringe with an alligator clip. Finally, the solutions were electrospun at +18 kV. The distance between the needle tip and the flat bed metallic collector covered with aluminum foil was kept at 15 cm. The as-spun nanofibers were vacuously dried at 60 °C for 24 h in the presence of P2O5 to remove the residual moisture from used solvents. Further on, the samples were subjected to calcination at 500 °C for 2 h with a heating rate of 1 °C min−1.
The morphology of the samples was examined using a scanning electron microscope (SEM, Hitachi, Japan S-3500N). The samples were coated with a thin layer of platinum–palladium for 50 s with the discharge current of 10–15 mA for three reparative cycles using (Hitachi, Japan E1010) ion sputter. After coating, the micrographs were taken at an accelerating voltage of 25 kV, and images were captured with magnifications of 1 and 20 K. The thermal properties were tested by thermogravimetry/differential scanning calorimetry instrument (SETARAM TG/DSC 92). The samples were heated from 30 to 600 °C under a continuous nitrogen purge of 20 mL min−1 with a heating rate of 20 °C min−1. Information about the phases and crystallinity of nanofibers was obtained using X-ray diffractometer (XRD, X’pert-pro MPD) with Cu, Cr (λ = 1.540 A) radiation over Bragg angle ranging from 20° to 60°. For photocatalytic property, the samples were suspended (1 g L−1) in methylene blue solution (20 ppm) in a batch reactor and illumination was applied using a commercial haloline visible-light illumination (500 W, Osram). The evolution of the methylene blue concentration was measured by UV–Vis spectrometry (Cary 100, Varian) following its 660 nm characteristic band.
3 Results and Discussion
WO3 nanofibers were fabricated by an electrospinning technique using PVP/citric acid/tungstic acid as precursor solutions. The average diameter of nanofibers is 80–100 nm and increases with PVP concentration. After sintering at 500 °C for 2 h, the diameter was reduced and the structure was transferred from amorphous to orthorhombic. The photocatalytic activity of WO3 nanofibers was found much higher than that of commercial WO3 powders in degradation of methylene blue under visible-light illumination.
This study was supported by the National Research Foundation of Korea (NRF)—Grants funded by the Ministry of Science, ICT and Future Planning (2014R1A2A2A01004352) and the Ministry of Education (2009-0093816), Republic of Korea.
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