Received 19 February 2016; accepted 7 April 2016; published 12 April 2016
With the development of science and technology, a variety of new organics are emerging, many of them are difficult to handle by traditional physical and chemical methods, biochemical methods also have very little effect. Therefore, it requires us to find an effective method to deal with the refractory organics  .
TiO2 is an economic, environmental and inexpensive semiconductor photocatalyst and widely used in the degradation of organic pollutants. TiO2 has special physical and chemical properties and electronic band structure, high photocatalytic activity. It is widely used in the research field of environmental protection and pollution control as photocatalyst  .
On one hand, as a kind of the photocatalyst, TiO2 can only be initiated by the ultraviolet light as it has a relative broad energy band, of which the type of anatase is 3.2 eV and the type of rutile is 3.0 eV. It leads to a lower availability of sunlight  . On the other hand, the photogenerated electrons and holes with a high recombination ratio may cause a decline in the catalytic efficiency. In order to improve the photocatalytic efficiency and effectively inhibit the recombination of photoproduction charge carriers  , the researchers adopt many kinds of methods to improve its performance effectively  -  , and the method of ion doping is reported most  -  .
In this paper, W-doped nano-TiO2 was synthesized through direct feeding by the microwave. Its photocatalytic activity was observed by the catalytic degradation of the methyl orange, and the factors including inventory molar ratio and concentration of W6+-doped, calcined temperature, amount of hydrogen peroxide and acidity of solution were investigated. When the catalyst was 1.0 g/L, pH was 2, C(H2O2) was 3 mL/L, the degradation rate of TiO2 for methyl orange of 20 mg/L reached 79.63% in 40 min.
2.1. Instruments and Reagents
Microwave oven with 650W (Sanle General Electric Corp., Nanjing, China) with refluxing system was used. Powder X-ray diffraction (XRD) was used to characterize the sample. Data were collected on a Shimadzu XRD-6100 X-ray diffractometer (Cu Kα radiation, λ = 0.15418 nm). The morphology and size were determined by SEM. The SEM images were recorded on a Quanta 200 FEG field emission scanning electron microscope. Lambda10 UV-vis spectrometer (Perkin-Elme Corp., USA) was used for monitoring the absorption spectra of photo-degradation of methyl orange.
All the reagents used were of analytical purity. Doubly distilled water was used throughout the experiments.
2.2. Microwave Synthesis of W6+-Doped TiO2 Nanoparticles
The mixture solution including 100 mL pure water, 1.0 g sodium dodecyl sulfate, 2.4 g Ti(SO4)2, a certain quantity of urea and Na2WO4∙2H2O was placed in a microwave refluxing system to react for 20 min with a power microwave radiation of 40% and cool down naturally to the room temperature. Then the precipitate was centrifuged, washed with the distilled water and dried at 60˚C in the vacuum for 4 h, the powder was sintered at 500˚C for 2 h. The final product in white was collected for the characterization.
2.3. Photocatalytic Degradation Experiment of W6+-Doped TiO2
The photocatalytic degradation of the aqueous solution of the methyl orange (20 mg/L) with the as-prepared catalyst dispersed by the ultrasonic vibration, some H2O2 and dilute nitric acid adjusting to a certain pH value was investigated. Then the mixture solutions were placed in a glass beaker, and exposed to the visible light irradiation at the room temperature. Every 10 min, the surface solution in the glass beaker was centrifuged and determined by the UV-visible absorption spectrometer. The photocatalysis reaction lasted until the discoloration of the solution was fulfilled or stopped. The formula of the degradation ratio is as follow
where Dt is the methyl orange degradation rate by the visible light irradiation for the time of t in the presence of the catalyst, A0 shows the absorbance of the methyl orange solution after adding catalysts without the sunlight irradiation, At is the absorbance of the methyl orange solution filled with catalyst under the sunlight irradiation for the time of t.
3. Results and Discussion
3.1. XRD, SEM Analysis of W6+-Doped TiO2
Figure 1 shows the XRD pattern graph of as-prepared sample of 2% W-doped TiO2 sintered at 500˚C for 2 h. The XRD pattern (peak 2θ: 25.2, 37.8, 47.99, 54.9) showed that the product was anatase-type TiO2 (JCPDS File
Figure 1. X-ray diffraction pattern of as-prepared sample (the nanocomposite of inventory molar percentage of 2%).
No. 21-1272). The W doping amount almost has no effect on the TiO2 crystal structure. Adopted Scherrer equation, we can estimate the size of the crystal. The average size of as-prepared sample is 6 nm.
Figure 2 shows the SEM pattern graph of as-prepared sample of 2% W-doped TiO2 sintered at 500˚C for 2 h. It shows that the majority of the catalyst is a relatively flake structure, and some particles attached to it. The size of most of the flakes is 50 to 500 nm.
3.2. Photocatalytic Activity of W6+-Doped TiO2 Nanocomposite
Figure 3 shows the typical visible absorption spectra of the photo-degradation for the methyl orange. It can be seen that with the photocatalytic degradation, the absorbance of the solution decreases gradually. After 10, 20, 30, 40 min of the irradiation, the degradation rate of TiO2 for methyl orange of 20 mg/L reached 35%, 65%, 75%, 79.63%, respectively. After 40 min of the irradiation, the solution in the range of the visible light has almost no absorption, indicating that the methyl orange has been degraded thoroughly. When the methyl orange solution is directly put under the illumination without the catalyst, the results show that the solution does not fade by continuous illumination for 2 h, and the detection by the UV-visible absorption spectrometer finds that the absorbance does not change.
3.3. The Mechanism of Reaction
The mechanism of the microwave synthesis may be that at one side of the surfactant sodium dodecyl sulfate is a long chain of carbon-based water-repellent, and at the other side is the hydrophilic group with the charge. It forms micelles in the aqueous solution. W6+-doped nano-TiO2 will grow around the micelle surface precipitation. Washed products and some parts of sodium dodecyl sulfate were wiped clean and further heat-treated, sodium dodecyl sulfate ash residue decomposes and forms a spherical porous structure which is aggregated by many small particles. It can be seen clearly from the SEM images, the majority of the catalyst is arelatively regular structure. It can increase the specific surface area in the photocatalyst. Larger surface area is advantageous for the adsorption of oxygen on the particle-surface, and enhances the acceptable probability of the electrons. Thereby it reduces the electron and hole composite probability, and prolongs the life of the electron and hole. So it improves the catalytic properties  .
Figure 2. Scanning electron micrograph of as-prepared sample.
Figure 3. Visible absorption spectra of photocatalytic degradation of methyl orange. (1.0 g/L of the nanocomposite of inventory molar percentage of 2%, pH was 2, C(H2O2) was 3 mL/L, 20 mg/L of methyl orange).
At the same time, sodium tungstate may distribute evenly in the titanium dioxide crystal after W6+ and Ti4+ mixed evenly. W belongs to the d areas element. But the ion radius of W6+ is 68 pm, ion radius of Ti4+ is 68 pm. W6+ is easier to destroy the original crystal lattice of TiO2, which brings the distortion to the crystal lattice and enhances the catalyzed efficiency. Expecting W6+-doped TiO2 nanocomposites can extend the photo-carrier lifetime to raise catalytic properties effectively.
When W doping content is too large or too small, the catalytic properties are not so good. When the doping amount is greater than the optimum value, as the decrease of the distance of the trapped carrier, doping ions form the electron-hole recombination center, and the excess doping concentration may reach saturation, produce kainotype and reduce TiO2 effective surface area, thus reduce the photocatalytic efficiency.
Calcined temperature can stabilize the crystalline form of the synthesized product, but too large or too low will affect the catalytic properties. The product is irregular polygon block when the temperature is too low. And when the temperature is too high, it can cause further growth of nano particles by sintering or crystal, increased particle size, decreased photocatalyst specific surface area, and eventually it can lead to the decrease in the photocatalytic efficiency.
In this paper, the W6+-doped nano-TiO2 was synthesized through direct feeding microwave radiation. The method of direct feeding microwave radiation is very simple. The sample of 2% W-doped TiO2 is sintered at 500˚C for 2 h. XRD patterns show that the crystal is anatase, and SEM pictures show that the majority of the catalyst is a relatively flake structure, and some fine particles attach to it.
A certain amount of W doping and calcined temperature can improve the photocatalytic efficiency of TiO2, the proper amount of H2O2 can improve the visible light photocatalytic efficiency of the W-doped nanosized composite, the acidity of the solution in the acidic range affects little on the degradation efficiency, but also has effects on the degradation time. It turns out that the best efficiency is obtained on the degradation when the catalyst of 2% W-doped TiO2 sintered at 500˚C for 2 h is 1.0 g/L, pH is 2, and H2O2 with the volume fraction of 0.003 under the visible light is added. The as-prepared photocatalyst is an excellent photocatalyst.
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