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.

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.

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 D_t 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, A_t is the absorbance of the methyl orange solution filled with catalyst under the sunlight irradiation for the time of t.

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

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.

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.

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 [12] .

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.