Photocatalytic Property of TiO2-Vermiculite Composite Nanofibers via Electrospinning
© Tang et al. 2015
Received: 21 March 2015
Accepted: 12 June 2015
Published: 1 July 2015
Titanium dioxide (TiO2) is one of the most common photocatalysts. In this study, TiO2-vermiculite composite nanofibers with a mesh structure and a diameter of approximately 300 nm were prepared via sol–gel approach combined with electrospinning technique. The samples were characterized by X-ray diffraction, scanning electron microscopy, ultraviolet–visible spectroscopy, etc. The photocatalytic property was also evaluated. The TiO2-vermiculite composite nanofibers annealed at 550 °C for 3 h exhibited the best absorption and photo-degradation ability for the treatment of methylene blue. The results implied that the combination of mineral vermiculite powders with TiO2 enhanced the absorption-degradation performance of the as-prepared photocatalytic materials, consequently promoting the materials’ ability to degrade methylene blue.
KeywordsTiO2 Vermiculite Photocatalysis Electrospinning
Titanium dioxide (TiO2) is one of the most common photocatalytic materials due to its unique energy band structure and non-toxicity. Several methods have been reported to synthesize TiO2 nanoparticles, nanowires, nanotubes, nanofibers, etc. However, most of the photocatalytic materials currently used consist of nanoparticles . But the photodegradation rate of photocatalytic materials is rather low because the powders easily agglomerate. Moreover, the band gap of TiO2 is 3.2 eV, which is an intrinsic property of the material. Decreasing the energy band would allow for the activity of TiO2 by UV light, which accounts for 5 % of the whole spectrum of sunlight. Therefore, broadening the responding spectral region of TiO2 is an effective method to increase the photocatalytic property of such materials [2, 3].
Electrospinning is a novel way to obtain one-dimensional inorganic nanofibers, such as TiO2, ZnO, Al2O3, SnO2, and BaTiO3. The electrospinning equipment consists of a high-voltage power supply, a spinneret, a syringe pump, and a collector. The precursor solution is jetted by the high-voltage power supply via the spinneret and the syringe pump, and nanofibers can then be obtained on the collector. By controlling the viscosity of the precursor, the high-voltage strength, and the distance between the spinneret and the collector, which could all affect the diameter and morphology of the nanofibers, we were able to obtain TiO2 polymers and amorphous TiO2 by annealing the nanofibers prepared via the electrospinning process [4, 5]. Vermiculite is a low cost and abundant layered silicate mineral raw material. It features a large surface area and specific absorption property. The nanofiber photocatalytic material used in this study was obtained by combing vermiculite and TiO2 nanofibers, which increases the volume surface area of the composites fibers and promotes the fibers’ ability to decompose chemical wastes materials [6, 7].
In this study, vermiculite and TiO2 composite nanofibers were synthesized by combining the sol–gel process with the electrospinning technique. The photocatalytic ability of the nanofiber composites was assessed by studying the degradation of methylene blue (MB) under irradiation with UV light. The results showed that the utilization of TiO2 composite nanofibers containing 2 wt.% of vermiculite resulted in a remarkable absorption and an enhanced degradation of MB.
Synthesis of the TiO2-Vermiculite Composite Nanofibers
The initial step in the synthesis of the TiO2-vermiculite composite nanofibers was the preparation of a precursor via the sol–gel process. To fabricate the precursor, 1.5 g of polyvinyl pyrrolidone (PVP, Mc = 1,300,000, Alfa Aesar), 20 ml of ethanol, 5 ml of acetic acid, and 5 ml of tetrabutyltitanate were mixed with different amounts of vermiculite powders and stirred for 2 h to ensure a homogeneous mixture. The obtained precursors were then transferred into the electrospinning instrument to fabricate the vermiculite/PVP/Ti(OCH(CH3)2)4 nanofibers. An electrical potential of 15 kV was applied between the nozzle and the collector in order to eject the sol–gel precursor to the collection board which was installed a distance of 15 cm to the nozzle. In order to obtain nanofibers with a homogenous size distribution, the propulsion speed was set to 2 ml/h. After a thermal treatment for 3 h at 550 °C, the vermiculite/PVP/Ti(OCH(CH3)2)4 fibers were transformed into TiO2-vermiculite composite nanofibers.
The vermiculite/PVP/Ti(OCH(CH3)2)4 fibers were characterized by the simultaneous application of thermogravimetry and differential scanning calorimetry (TG-DSC). The phase composition of the TiO2-vermiculite composite nanofibers was studied by D/max-rA X-ray diffraction (XRD, Rigaku Corporation, Japan, Cu Kα radiation, λ = 1.5406 Å). The microstructures and nanostructures of the nanofibers were characterized by scanning electron microscopy (SEM, JEOL JSM6700F, Japan) and transmission electron microscopy (TEM, 300 kV, FEI-tecnai-G3-F20, Philips, Netherlands). The nanofibers were ultrasonically dispersed in ethanol and then dropped onto carbon-coated copper grids prior to the TEM investigations. Diffuse reflectance ultraviolet–visible (UV–vis) absorption spectra were recorded using a Carry 5000 UV–vis-NIR spectrophotometer with an integrating sphere attachment.
The photocatalytic property of the nanofibers was evaluated as follow: the nanofiber samples were suspended into the MB solution (10 mg/L) and the solution was stirred for 30 min under exclusion of light. The adsorption equilibrium of the solution was reached when the methylene blue concentration remained in balance. Then, the solution was stirred by magnetic stirring and irradiated with UV light using a 500 W high-pressure mercury lamp performance in order to evaluate the photocatalytic property. A 3 ml of the pollutant solution was sampled every 20 min and centrifuged for the separation of the upper clear solution. The concentration of MB was analyzed using a UV–vis spectrophotometer (L5, INESA) at the characteristic wavelength of 664 nm.
Results and Discussion
Both the anatase and the rutile phase were found in the composite nanofibers exposed to the thermal treatment at 550 °C (Fig. 3c). In addition, the electron diffraction pattern obtained for the TiO2-vermiculite composite nanofibers also revealed that the nanofibers are polycrystalline. As demonstrated in literature, the coexistence of the anatase and the rutile phase may enhance the photocatalytic property [12, 13]. Therefore, for a suitable TiO2 nanofibers to vermiculite particle ratio (here, 2 wt.% of vermiculite particles), the TiO2-vermiculite composite nanofibers show an optimized photocatalytic property.
The photodegradation rate is an important factor for the evaluation of photocatalytic materials. Figure 4b compares the degradation rate of the samples. In Fig. 4b, the slope of the line corresponds to the reaction rate of the photodegradation process. The degradation rate of the TiO2 nanofibers with 2 wt.% vermiculite nanoparticles is the highest among all samples. This can be explained as follows: The degradation process is caused by the TiO2. With an increasing amount of vermiculite and a decreasing amount of the catalyst (TiO2), the absorbability is enhanced, whereas the catalytic ability is reduced at the same time. Therefore, the TiO2 nanofibers with 2 wt.% vermiculite nanoparticles show stronger capabilities (including adsorption and photocatalysis) to treat chemical pollutions.
α - optical absorption coefficient;
υ - photon energy;
E g - direct band gap;
h and E D - a constant.
The tangent to the curves (Fig. 5b) at the point of intersection with the x-axis corresponds to the direct band gap energy (E g ) of the samples, which are 3.05, 3.14, 3.08, 3.17, and 3.18 eV for a vermiculite mass fraction of 0, 2, 4, 10, and 50 wt.%, respectively [Additional files 1, 2, 3, 4 and 5]. The band gap energy increases with the amount of vermiculite in the nanofibers (Fig. 5b). The band gap energy of pure TiO2 nanofibers is 3.05 eV due to the band alignment of the rutile and anatase TiO2 phase. Moreover, the addition of a small amount of vermiculite to the TiO2 nanofibers significantly changed the band gap energy. However, adding a larger amount of vermiculite to the nanofibers had negative effect on the band gap. In this paper, the composite nanofibers with 2 wt.% vermiculite showed the most suitable direct band gap and the highest photocatalytic activity.
TiO2-vermiculite nanofiber composites were synthesized by combining a sol–gel process with the electrospinning technique. The TiO2-vermiculite nanofiber composites were obtained after thermal treatment at 550 °C for 3 h. The results of the phase composition analysis indicate that the main phase of the prepared samples was the anatase phase, and a small amount of the rutile phase could also be detected. The analysis of the structure of the composite nanofibers revealed a smooth surface and diameter of approximately 300 nm. The photodegradation of methylene blue by the as-prepared TiO2-vermiculite composite nanofibers was performed under irradiation lights. In this study, the addition of the mineral powder to the TiO2 nanofibers was demonstrated to enhance the adsorption-photocatalytic performance of the photocatalytic material.
The authors greatly appreciate the Fundamental Research Funds for the Central Universities for financial support (grant nos. 2652015021 and 2652013051).
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