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.