Preparation, Characterization And Photocatalytic Property Of Metal Doped TiO₂ And Bi₂WO₆

Since Japanese scientists Fujishima and Honda discovered the photocatalytic splitting of water on TiO2 single crystal electrodes in 1972, semiconductor photocatalysis has attracted considerable research interest. Among the various oxide semiconductor photocatalysts, TiO2 is extensively investigated because of its biological and chemical inertness, strong oxidizing power, nontoxicity and long-term stability against photo and chemical corrosion. However, there are two major shortcomings on the utilization of solar light. Due to its wide band-gap of 3.2 eV, TiO2 can only be excited by ultraviolet or near-ultraviolet radiation. On the other hand, the high recombination rate of photogenerated electron/hole pairs results in low photon efficiency. For solving these problems, two foreland domains about semiconductor photocatalysts, modification of TiO2 and exploitation of new photocatalysts, have been developed.On this base, metal ion doped TiO2 and Bi2WO6 were prepared by a chemical solution decomposition method and the photocatalytic activities were evaluated by the degradation of organic compounds. This dissertation mainly includes:Al doped TiO2 was prepared by a chemical solution decomposition method and treated with concentrated nitric acid. The samples were characterized by XRD, TEM and UV-Vis. XRD patterns indicated that no doped sample obtained by heat treatment at 550℃for 2 h consisted of rutile and anatase, while Al doped sample under the same condition was pure anatase. The TEM photos exhibited that the particle size of Al doped TiO2 was 25 nm smaller than the no doped one. The photocatalytic activities of the samples were evaluated by degradation of methyl orange under UV light. The results revealed that Al doped TiO2 had better photocatalytic activity, which could be further enhanced by acid treatment. The reason for the superior photocatalytic activity was that the acid treatment removed part of Al2O3 nanoparticles in the TiO2 samples and made them possess a higher specific surface area.Fe doped TiO2 was prepared by a chemical solution decomposition method and treated with concentrated nitric acid. XRD patterns indicated that the sample obtained by heat treatment at 500℃was a mixture of rutile and Fe2TiO5. A mixture of rutile, Fe2Ti05 and Fe2O3 was obtained when it was treated at 600℃. Moreover, while increasing the heat treatment temperature, the samples had a better crystallinity. The UV-Vis diffuse reflectance spectra exhibited that the samples after being pretreated at 500 and 600℃both had a response to the visible light. Comparing with the samples treated at 500℃, there was an obvious red shift for the ones obtained by being treated at 600℃. The degradation of Rhodamine B under visible light was employed to evaluate the photocatalytic activity of the samples. The results revealed that all the Fe doped TiO2 had photocatalytic activity for the degradation of Rhodamine B, and the acid treatment had no effect on the photocatalytic activity.Bi2WO6 photocatalysts were fabricated by a chemical solution decomposition method and treated with concentrated nitric acid. XRD patterns indicated that the sample obtained by heat treatment at 700℃was Bi2WO6 as the main product with byproduct (Bi14W2O27). The diffraction peaks of Bi14W2O27 disappeared by acid treatment, leaving pure phase Bi2WO6 as the only product. In order to understand why pure phase Bi2WO6 could be obtained after the acid treatment, the composition of the acid-treated supernatant liquids was analyzed by ICP. The results suggested that the most possible process to achieve the pure phase Bi2WO6 was that Bi14W2O27 was converted into Bi2WO6 by selective leaching bismuth oxide in Bi14W2O27. The SEM images revealed that catalysts prepared by acid treatment at 70℃possessed a nanosheet-like structure. The UV-Vis diffuse reflectance spectra showed that the samples with and without the acid treatment both had a response to the visible light. Photocatalytic activity of the samples was evaluated by the degradation of Rhodamine B under visible light irradiation. The results showed that the acid treatment could enhance photocatalytic activity of the catalysts, and the ones obtained by the acid treatment at 70℃exhibited the highest photocatalytic activity.