Electrospinning (ES) is one of the most useful techniques to form nanofibers in a diameter of several hundred nanometers (Doshi & Reneker, 1995, Buchko et al., 1999, Huang et al., 2003). The diameter of the nanofibers produced by ES is at least one or two orders of magnitude smaller than those of conventional fiber production methods like melt or solution spinning. As a result, the electrospun nanofibers have high specific surface area (Yamashita, 2007). These nanofibers are well-suited to be used as chemical reaction fields (Nakane et al., 2005, 2007).
Much attention has been paid to the formation of both organic polymeric nanofibers and inorganic nanofibers using ES (Ramakrishna et al., 2005). Many kinds of inorganic nanofibers (SiO2, Al2O3, ZrO2, NiCo2O4, and so on) have been obtained by calcination of organic-inorganic hybrid precursor nanofibers formed by ES (Guan et al., 2004, Shao et al., 2004, Chronakis, 2005, Panda & Ramakrishna, 2007, Krissanasaeranee et al., 2008). The formation of TiO2 nanofibers have been also reported by several research groups. Li and Xia formed anatase-type titanium oxide (TiO2) nanofibers by the calcination of poly(vinyl pyrrolidone) (PVP)-Ti tetraisopropoxide (TTIP) hybrid nanofibers at 500 C in air (Li & Xia, 2003). The TiO2 nanofibers obtained would be a useful material for a photocatalytic reaction, but their usage has not been investigated. Ethanol has been used as the solvent of the spinning solution to form the hybrid precursor nanofibers. Therefore, a spinneret could be stopped up by a solid material because ethanol will evaporate from the tip of the spinneret during the spinning. Furthermore, TTIP is very easily hydrolyzed, and thus a water-free condition is required for the use of TTIP on ES. Another groups also formed TiO2 nanofibers by calcination of TiO2-PVP and TiO2-poly(vinyl acetate) precursors which were formed by ES using organic solvents such as ethanol and dimethylformamide (Kim et al., 2006, Nuansing et al., 2006, Kumar et al., 2007, Ding et al., 2008).
Li and Xia reported the formation of TiO2 hollow-nanofibers (nanotubes) by ES of two immiscible liquids (TTIP-PVP ethanol solution and heavy mineral oil) through a coaxial, two-capillary spinneret, followed by selective removal of the cores and calcination in air (Li & Xia, 2004). The TiO2 nanotubes with uniform and circular cross-sections were obtained by the method. Kobayashi et al. reported the preparation of TiO2 nanotubes using the gelation (self-assembly with a rodlike fibrous structure) of an organogelator (It is not ES.) (Kobayashi et al., 2000, 2002). The organogelator is a cyclohexane derivative that was specially synthesized by this research group. The outer and inner diameters of the TiO2 nanotubes obtained were 150-600 nm and 50-300 nm, respectively. We considered that the TiO2 nanotubes would be easily formed by using the ES process without organogelators.
In this study, we formed TiO2 nanofibers (Nakane et al., 2007) and TiO2 nanotubes (Nakane et al., 2007) by calcination of new precursor nanofibers of poly(vinyl alcohol) (PVA)-titanium compound hybrids. The precursor nanofibers were formed by using ES with water as a solvent. It is most safety method to use water as a solvent on ES. The photocatalytic reaction using the TiO2 nanofibers obtained was also investigated.
2. Materals and methods
2.1. Formation of PVA-Ti compound hybrid precursor nanofibers and TiO2 nanofibers
Two types of precursor nanofibers were formed using ES.
Precursor-1: PVA (degree of polymerization: 1500) 10 wt% aqueous solution was prepared. Titanium lactate (TL) [(OH)2Ti(C3H5O2)2] (5 g) was added to the PVA solution (10 g) to produce transparent PVA-TL mixed solution (spinning solution). TL was kindly gifted from Matsumoto Chemical Industry Co., Ltd., Japan (TC-310, content: TL 35-45%, 2-propanol 40-50%, water 10-20%). The mixed solution was loaded into a plastic syringe (2 ml) equipped with a needle. The solution extrusion rate was 0.015 ml/min. A voltage of 25 kV was applied to the needle, and the PVA-TL hybrid nanofibers were then deposited on a collector. The collector (copper plate) was grounded, and the distance between the tip of the needle and the collector was 10 cm. The PVA-TL hybrid nanofibers obtained were used as a precursor of TiO2 nanofibers.
Precursor-2: Pure PVA nanofibers formed by ES were immersed in a titanium alkoxide [titanium tetraisopropoxide (TTIP)] (10wt%)-ethanol solution for 10 minutes. The treated nanofibers were washed in fresh ethanol and then PVA-TTIP hybrid precursor nanofibers were obtained.
These precursor nanofibers obtained were calcined up to a given temperature in an electric furnace (in air), and TiO2 nanofibers were formed.
2.2. Apparatus and procedure
The structure of the nanofibers was observed by scanning electron microscope (SEM) (Hitachi S-2400, Japan). Thermogravimetric (TG) analysis was performed in air and a heating rate of 10 C/min (Shimadzu DTG-60, Japan). X-ray diffraction (XRD) measurement was taken using a CuK with a Ni filter (40 kV, 30 mA) (Shimadzu XRD-6100, Japan). The nitrogen adsorption isotherms (-196 C) of the TiO2 nanofibers were measured by Micromeritics TriStar 3000, USA.
The photocatalysis of the TiO2 nanofibers was evaluated using the photocatalytic decomposition of methylene blue (3,7-bis(dimethylamino) phenothiazin-5-ium chloride; C16H18ClN3S) ( Nakane et al., 2007 ). The TiO2 nanofibers formed (5 mg) were dispersed in methylene blue (1×10-5 mol/l) aqueous solution (50 ml). Three ml of test liquid was taken from this solution and fed in a quartz cell. The test solution was irradiated with white light using an extra-high pressure mercury vapor lamp (Ushio Inc., Japan), and the absorbance at 665 nm, which is the maximum absorption wavelength of methylene blue, was measured by an absorptiometer (JASCO, CT-109, Japan). The decomposition rate of methylene blue was calculated from the absorbance.
3. Results and discussion
3.1. TiO2 nanofibers obtained from PVA-TL hybrid nanofibers
Fig. 1 shows the SEM image of PVA-TL hybrid nanofibers (precursor-1). The fiber diameters of the PVA-TL nanofibers are 200-350 nm, and the fibers have a smooth surface without macropores. The specific surface area and the pore volume of the hybrid nanofiber are 1.80 m2/g and 0.00764 cm3/g, respectively. Thus, the hybrid nanofiber is considered a nonporous material.
Fig. 2 shows TG curves of the pure PVA and the PVA-TL hybrid nanofibers. The weight residue of pure PVA becomes zero at 550 C, and that of the PVA-TL hybrid is 25% at 600 C. White residues (TiO2) were obtained after measurement for PVA-TL hybrid.
Fig. 3 shows the SEM image of the TiO2 nanofibers obtained by the calcination of PVA-TL hybrid nanofibers at 400 C for 5 hours. Compared to the image shown in Fig.1, the fiber diameter of the TiO2 nanofibers is 70-80% of that of the PVA-TL hybrid nanofibers, with the space between the fibers made denser by calcination. The residues were brittle, but maintained the shape of the PVA-TL hybrid non-woven mat, although shrinkage occurred due to the calcination.
Fig. 4 shows XRD curves of the TiO2 obtained by calcination of the PVA-TL nanofibers at 400-700 C for 5 hours. Anatase-type TiO2 is formed at 400-600 C, and the peak intensities increase with calcination temperature. Rutile-type (rutile-anatase mixed) TiO2 is formed at 700 C. It is well-known that anatase is superior to rutile for photocatalysis. Thus, a calcination temperature of 600-700 C would be an effective condition when using the TiO2 nanofibers as a photocatalyst.
Fig.5 shows the relationship of the calcination temperature of the hybrid nanofibers and the pore characteristics (specific surface area and pore volume) obtained from the nitrogen adsorption isothermes (-196 C) of the TiO2 nanofibers. The specific surface area and pore volume of the TiO2 nanofibers decreases with increasing the calcination temperature. This is due to the sintering of the TiO2 by the calcination. The average pore diameters (dp), which were assumed to be cylindrical in shape, were based on the specific surface area (S) and pore volume (V) for each TiO2 nanofiber: dp=4V/S. The dp of the TiO2 nanotubes were 3.8 nm (calcination temperature: 400 C), 7.4 nm (500 C), 17.5 nm (600 C), and 44.0 nm (700 C). Consequently, the TiO2 nanofibers obtained in this study are classified as mesoporous materials.
3.2. TiO2 nanotubes obtained from PVA-TTIP hybrid nanofibers
Fig.6 shows the SEM images of (a) pure PVA nanofibers formed by ES, and (b) PVA-TTIP hybrid precursor nanofibers (precursor-2). The fiber diameter of a PVA nanofiber is ca. 200 nm, and the fiber diameter of the precursor nanofiber becomes 1.3 times larger that that of the PVA nanofiber. Both fibers have a smooth surface. White residues (TiO2) were obtained by calcination of the hybrid precursor nanofibers at 500 C for 5 hours. The sample weight after calcination was 12.1% of the one before calcination.
Fig.7 shows the SEM image of the residue after calcination of the precursor nanofiber at 500 C for 5 hours. As can be seen from this image, hollow TiO2 nanofibers (TiO2 nanotubes: outer diameter, ca.440 nm, inner diameter, ca.270 nm) were formed. The nanotubes obtained are seemed to have a uniform diameter as compared to the diameter range reported by Kobayashi et al. (Kobayashi et al., 2000). We determined that a precursor nanofiber with a
skin-core structure would be obtained by our method; the skin layer is a PVA-TTIP hybrid, and the core is pure PVA. TTIP will penetrate into the PVA matrix when the PVA nanofibers are immersed in TTIP-ethanol solution, and a PVA-TTIP hybrid layer will be formed. The interaction between the PVA and TTIP could not be identified, but the hybrid would be formed by a coordination bond between the titanium and the oxygen of the hydroxyl group on the PVA molecules (Nakane et al., 2003). The structure of the TiO2 nanotubes obtained would be reflected in the skin-core structure of the precursor nanofibers. Schematic illustration of the formation of TiO2 nanotubes by our method is shown in Fig.8.
The diameter of the nanotube can be controlled by changing the diameter of the pure PVA nanofiber. However, nanotubes were not obtained when the outer diameter was ca.200 nm (Fig.9 (a), formed of non-hollow nanofibers) or ca.700 nm (Fig.9 (b), breaking of TiO2 tube wall). Thus, the range of the outer diameter which can be formed in this study is 300–600 nm.
Fig.10 shows the XRD curves of the residues (TiO2) obtained by calcination of the precursor nanofibers at 400-800 C for 5 hours. Anatase type TiO2 is mainly formed at 400-600 C, and the peak intensities increase with an increase in the calcination temperature. Rutile type (rutile-anatase mixed) TiO2 is formed above 600 C.
Fig.11 shows the nitrogen adsorption isothermes (-196 C) of the TiO2 nanotubes (calcination temperature range: 400-700 C). The adsorption amount of the TiO2 nanotubes decreases with an increase of the calcination temperature. This is due to the sintering of TiO2. The specific surface areas of the TiO2 nanotubes were obtained from Fig.10 using the B.E.T. equation. The areas are 75.9 m2/g (calcination at 400 C), 38.8 m2/g (500 C), 17.4 m2/g (600 C) and 6.4 m2/g (700 C) (the area of pure PVA nanofiber was 3.8 m2/g). The average pore diameters of the TiO2 nanotubes were 8.3 nm (400 C), 14.8 nm (500 C) and 21.4 nm (600 C). These pore sizes are not reflected the hollow size of the TiO2 nanotubes, because the hollow size is several hundred nanometers. Therefore, it is likely that the TiO2 nanotubes have mesopores on their nanotube wall (Fig.12). The mesopores would be through-holes formed by thermal decomposition of PVA in the precursor hybrid nanofibers. By the presence of the mesopores, the specific surface area of the TiO2 nanotubes becomes larger, and the photocatalytic reaction using the TiO2 nanotubes would occur effectively. The TiO2 nanotube calcined at 700 C had a non-porous wall due to the sintering.
3.3. Photocatalysis of TiO2 nanofibers and TiO2 nanotubes
The photocatalysis of the TiO2 nanofibers and nanotubes was investigated. Fig.13 (a) (b) show the relationship between the decomposition rate of methylene blue and the irradiation
time of white light for each TiO2. Each figure includes the result of commercially available anatase type TiO2 nanoparticles (ST-21, particle size: 20 nm, specific surface area: ca.50 m2/g, Ishihara Sangyo Kaisha, LTD. Japan).
In Fig.13 (a), the photocatalysis of the TiO2 nanofibers calcined at 600 C and 700 C is higher than that of the TiO2 nanofibers calcined at 400 C and 500 C, but the major differences between each TiO2 nanofiber are not observed. The crystallinity of anatase-type TiO2 increases with calcination temperature, though the specific surface area becomes lower. The photocatalysis of the TiO2 nanofibers would be affected by both the crystallinity and pore characteristics of the TiO2 nanofibers. The TiO2 nanofibers have good photocatalysis, but the properties of these TiO2 nanofibers are inferior to that of ST-21. The specific surface area of the TiO2 nanofiber calcined at 400 C (56.4 m2/g) is higher than that of ST-21, but the ST-21 excels in photocatalysis. This would be due to the difference of the crystallinity of anatase.
In Fig.13 (b), the photocatalysis of the anatase type TiO2 nanotubes (calcination at 500 C and 600 C) is clearly higher than that of ST-21, and the nanotube (calcination at 600 C) shows the highest photocatalysis in this experiment. Also, the photocatalysis of the rutile-anatase mixed TiO2 nanotube (calcination at 700 C) is equivalent to that of ST-21. The specific surface area of ST-21 is higher than that of TiO2 nanotubes, but the TiO2 nanotubes excel in photocatalysis. At the present stage the reason why is uncertain, but the structure of the nanotube might contribute to the efficient photocatalysis of TiO2. In other words, electron holes will be formed at the surface of the hollow when the TiO2 nanotubes are irradiated. And the excited electron moves to inside the hollow because it might be the non-irradiation area (the electron density might be low). Thus the oxidation site and the reduction site would be separated, and the photocatalysis would proceed efficiently.
Titanium oxide (TiO2) nanofibers were formed by calcination of poly(vinyl alcohol) (PVA)- Ti lactate hybrid precursor nanofibers in air. The fiber diameters of the PVA-Ti lactate hybrid nanofibers were 200-350 nm, and the fiber diameters of the TiO2 nanofibers were 70-80% of those of the PVA-Ti lactate hybrid nanofibers. The specific surface area and average pore diameter of the TiO2 nanofibers calcined at 500 C for 5 hours were 21.0 m2/g and 7.4 nm, respectively.
TiO2 hollow-nanofibers (nanotubes) were formed by calcination of PVA-Ti alkoxide hybrid nanofibers. The outer and inner diameters of the TiO2 nanotubes calcined at 500 C for 5 hours were ca.440 nm and ca.270 nm, respectively. The specific surface area of the nanotubes was 38.8 m2/g, and the existence of mesopores (average pore diameter, 14.8 nm) on the nanotube wall was indicated by the nitrogen adsorption isotherm (-196 C). The photocatalysis of the TiO2 nanotubes was superior to that of commercially available anatase type TiO2 nanoparticles.
The TiO2 nanofibers and nanotubes have the advantage of being easily fixed on other materials, such as a refractory fabric, without a binder by the fiber length (TiO2 powders such as ST-21 requires a binder in order to be fixed on other materials.). Fig.14 shows the SEM images TiO2 nanofibers formed on carbon microfibers without a binder. These fibers are expected to be used as an air filter.
The authors express their gratitude to Dr. Shinji Yamaguchi, Dr. Naoki Shimada and Ms. Kaori Yasuda for their helpful cooperation. This work was supported by the Ministry of Education, Culture, Sports, Science and Technology-Japan.