The effects of atmosphere and calcined temperature on photocatalytic activity of TiO2 nanofibers prepared by electrospinning
© Hu et al.; licensee Springer. 2013
Received: 5 September 2013
Accepted: 11 December 2013
Published: 30 December 2013
TiO2-based nanofibers were synthesized using a sol–gel method and electrospinning technique. The as-spun composite fibers were heat-treated at different temperatures (500°C, 550°C, 600°C, and 650°C) and atmospheres (ammonia and nitrogen) for 4 h. The fibers had diameters of 50 to 200 nm and mainly featured anatase and rutile phases. The anatase phase decreased and the rutile phase increased with increasing temperature. Different nitrogen conditions exerted minimal effects on the TiO2 crystalline phase. Different nitriding atmospheres during preservation heating yielded various effects on fibers. The effect of nitrogen in ammonia atmosphere is better than that in nitrogen atmosphere. The fibers heat-treated at 600°C and subjected to preservation heating in NH3 showed high photocatalytic activity.
KeywordsTiO2 nanofibers Electrospinning Sol–gel Photocatalytic activity
Heterogeneous photocatalysis has been extensively investigated by researchers for the degradation of organic pollutants [1, 2]. As a very promising photocatalyst, TiO2 shows high chemical stability, high photocatalytic activity, low cost, and non-toxicity. However, the materials exhibit photocatalytic activities only under UV light at wavelengths of less than 387.5 nm. UV light accounts for only 4% of the solar light. Therefore, synthesizing a TiO2 photocatalyst with visible-light responses for environmental protection is important [3–7]. The catalytic activity of TiO2 is easily influenced by the agglomeration of the TiO2 particles. TiO2 thin films are considered excellent photocatalytic materials because of the large specific surface area of their particles, which improves catalytic efficiency through increased contact with pollutants .
To improve the catalytic performance of TiO2 photocatalyst, researchers have investigated many methods to modify Ti. Doping with metal ions, such as the rare earth metal ions (Er, Yb, Y, and Eu) or the noble metal crystals, for example, has been performed to enhance catalytic efficiency of Ti [9–12]. However, rare metal dopant photocatalysts have low thermostability and short life spans. Furthermore, rare metals and noble metals are expensive. Several studies report that the doping of TiO2 with non-metals, such as carbon, nitrogen, sulfur, boron, and fluorine, shifts the optical absorption edge of TiO2 toward lower energies, which increases its photocatalytic activity in the visible-light region . The nitrogen process is a low-cost and efficient way of modifying TiO2 to develop TiO2 fiber catalysts.
The catalytic activity of TiO2 is easily affected by the agglomeration of TiO2 particles. Thus, TiO2 thin films are considered as favorable photocatalytic materials. In recent years, the preparation of nanofibers by electrospinning has attracted significant research attention [14, 15]. In this paper, we prepare TiO2 fibers by electrospinning and modify them using nitrogen at high temperatures.
The precursor for electrospinning was prepared by the sol–gel method. In a typical synthesis, 1.5 g of polyvinylpyrrolidone (PVP, molecular weight = 1,300,000) was dissolved in 20 mL of ethanol, after which 5 mL of acetic acid and 5 mL of tetrabutyl titanate were added to the above solution under magnetic stirring. After 1 h of stirring at 70°C in a water bath, the resultant orange solution was used as the electrospinning precursor.
Methylene blue (MB; concentration 20 mg/L in distilled water) was used as a model pollutant to measure photocatalytic activity of the TiO2 catalysts.
P25 TiO2 (Degussa; anatase phase, 20%; rutile phase, 80%) was used as standard photocatalytic material.
In the electrospinning procedure, the precursor solution was loaded into a 5-mL syringe with a stainless steel needle. An electric voltage of 15 kV was supplied between the needle and the collection target covered with aluminum foil. The distance between the needle and the collection target was 15 cm. A flow rate of 0.15 mm/min was supplied by a syringe pump. A white nanofiber mat was prepared by electrospinning.
PVP-Ti composite fibers were prepared by electrospinning. The as-obtained fibers were calcined at a temperature range of 500°C to 650°C at a heating rate of 1°C/min. Preservation heating was performed for 4 h under flowing N2 and NH3 surroundings.
The PVP-Ti composite fibers and calcined Ti fibers were characterized by various techniques such as thermogravimetry-differential scanning calorimetry (TG-DSC), x-ray diffraction (XRD), x-ray photoelectron spectroscopy (XPS), fluorescence microscopy-scanning electron microscopy (FM-SEM), transmission electron microscopy (TEM), and UV-Visible (UV–vis) spectrophotometry diffuse reflectance spectroscopy. The TG-DSC instrument was operated at a heating rate of 10°C/min in air and used to determine the thermal decomposition behavior of PVP-Ti composite fibers. Phase analysis of calcined fibers was performed using a Rigaku D/max-rA (Rigaku Corporation, Tokyo, Japan) 12 kW x-ray powder diffractometer using CuKα radiation (2θ = 10° to 80°). XPS spectra were recorded by a Thermo Fisher ESCALAB 250 Xi XPS instrument (Thermo Fisher Scientific, Hudson, NH, USA). The morphology and size of the calcined Ti fibers were observed by FM-SEM and TEM. UV–vis diffuse reflection spectra were used to determine the absorption spectra of the heat-treated fibers. Finally, the catalytic activity of the calcined fibers was detected by UV–vis.
The photocatalytic activity of the calcined fibers was investigated by the degradation of a standard solution of MB in a photochemical reactor. The photocatalytic reactor contained a lamp with a 500-W UV tube manufactured by Shanghai Bilon Instruments Co., Ltd. (Minhang District, Shanghai, China). About 20 mg of photocatalytic materials, including the heat-treated fibers at different temperatures and P25 TiO2 powders, was added into quartz tubes. About 50 mL of 20 mg/L MB solution was then added to the tubes. The mixed solutions were placed in the photocatalytic reactor, stirred in the dark for 60 min, and then exposed to UV light irradiation. UV–vis spectroscopy was used to detect the solution absorption.
Results and discussion
Thermoanalysis of composite fibers
Phase analysis of calcined fibers
Morphological analysis of calcined fibers
Photocatalytic activity of calcined fibers
Wide-band gap analysis of calcined TiO2 fibers
In the XPS spectrum of N1s (Figure 8C), the dominant peak at about 400.08 eV is attributed to the adsorption of N2 due to surface nitriding. The surface nitriding has weakly nitrogen effects. This N element exerts no effects on the chemical status of Ti and O in the crystal lattice. Thus, the peak positions of Ti2p and O1s either did not change or changed only slightly. The chemistry status of N2p did not form leading to the weak visible-light photocatalytic activity . The O1s spectra of the samples are shown in Figure 8D. The O1s peaks of the samples were observed at 529.96 and 531.64 eV. The first peak had a binding energy of 529.96 eV, which is characteristic of metallic oxides; this result is in agreement with the O1s electron binding energy arising from the Ti lattice [21, 22]. In the other peak at 531.64 eV, there were several opinions to interpret the status of O1s. Emeline et al.  reported that the second peak is closely related to hydroxyl groups (−OH), which result mainly from chemisorbed water. The nitriding TiO2 may have more hydroxyl groups on its surface than pure TiO2. With increased surface hydroxyl content, catalysis can trap more photogenerated holes and prevent electron–hole recombination. Some studies have reported that this shift occurs mainly because of the anionic N in O-Ti-N linkages. Babu et al.  reported that the peak at 531.6 eV may be caused by the nitriding process changing the Ti-O crystal lattice due to the N or C doping.
In summary, TiO2 fibers doped with non-metals (C and N) and with diameters of 100 nm were successfully produced by the electrospinning technique. The photocatalytic activity of the fibers during MB degradation was investigated after heat treatment under different atmospheres (NH3 and N2).
TG-DSC results showed that the organic groups of the composite decomposed completely at 479°C. XRD analysis showed different crystalline structures of the fibers under various heat-treatment conditions. Ti fibers containing both anatase and rutile phases showed better photocatalytic performance. SEM images showed that the diameter of the fibers ranged from 50 to 200 nm. As the temperature increased, the crystalline phases of TiO2 changed and exerted significant effects the nitriding process and diameter of fibers. At higher temperatures, the surface of the TiO2 fibers was rough, which can increase their specific surface area and improve photocatalysis. However, when the temperature was too high, TiO2 is given priority to trend to transform to rutile phase from anatase phase, which is detrimental for photocatalysis.
The different nitriding atmospheres of preservation heating had different effects on the fibers. The effects of nitrogen in ammonia were better than those of nitrogen because ammonia activity is higher than nitrogen activity. However, nitrogen is more economical and environment-friendly than ammonia. Heat-treated fibers at 600°C are efficient catalysts for the photocatalytic degradation of MB.
The authors greatly appreciate the Fundamental Research Funds for the Central Universities for financial support (grant nos. 2652013126 and 2652013051).
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