Ion Doping Modification Of Bi₂WO₆ And Its Photocatalytic Properties

Photocatalytic oxidation of organic pollutants has become a pollution control technology in recent years. Bismuth tungstate(Bi2WO6) is one of the simplest Aurivillius oxides, possessing a layered structure with the perovskite-like slab of WO6 and Bi2O2. Due to its appropriate energy band structure, Bi2WO6 has good response to visible light, using solar energy more efficiently. So it has attracted more and more researchers. Yet the photocatalytic activity of orthorhombic Bi2WO6 is still low due to the rapid recombination rate of electron-hole pairs.In this study, the pure orthorhombic Bi2WO6 nanoparticles were synthesized by a microwave-hydrothermal method. The non-metal ions and rare earth ions doping modification was carried on. The effects of ions doping on phase structure, microstructure and photocatalytic properties of Bi2WO6 nanoparticles were studied. The samples were characterized by XRD、SEM、TEM、XPS and other analytical tools. The photocatalytic activity of these catalysts was evaluated using rhodamine B and total organic carbon(TOC) content. The photocatalytic degradation mechanism of ion-doped Bi2WO6 was deeply analyzed. Main results are as follows:(1)High purity and crystallinity Bi2WO6 was prepared using bismuth nitrate(Bi(NO3)3·5H2O) and sodium tungstate(Na2WO4·5H2O) as raw materials by microwave-hydrothermal method through controlling reaction temperature. Flower-like Bi2WO6 was a multistage layer assembly process. With the sample prepared at180℃, the decolourization rate of rhodamine B was more than 95% after irradiated under the visible light for 90 min and the photocatalytic reaction rate constant(k) got up to 0.04167min-1.(2)Na N3、NH4Cl and H4N2·H2O were added in the precursor solution and N/Bi2WO6(NBWO) photocatalyst was prepared. H4N2·H2O had an important influence on the structure and properties of the product. When the content of H4N2·H2O was low, N entered into the crystal lattice of the Bi2WO6, obtaining porous loose layer structure including orthorhombic Bi2WO6 and cubic Bi2O3. The content of surface adsorption oxygen was larger and the photocatalyst with high adsorption properties was prepared. N/Bi2WO6 accorded with Langmuir isothermal adsorption model, belonging to the monolayer chemical adsorption, and the maximum adsorption quantity was 12.92mg/g. After adsorption in dark environment, the decolourization rate of rhodamine B was above 95% with NBWO-0.75 and the TOC removal rate reached 30%. It increased to 40% after irradiated under the visible light for 120 min, showing that part of rhodamine B had been degraded after adsorption; When the content of H4N2·H2O was high, Bi2WO6、Bi2O3、Bi2W2O6 were generated. The redshift happened in the Bi2WO6 and light energy utilization extended to the near infrared area for NBWO-2. The decolourization rate of rhodamine B was above 98% after irradiated under the visible light for 60 min.(3)In order to improve the photocatalytic efficiency of Bi2WO6 further, N-F/Bi2WO6 was synthesized. The results showed that doping content and microwave reaction temperature both had an effect on the structure and properties of N-F/Bi2WO6. F ions entered into the crystal lattice partly replacing O and Bi2O3/Bi2WO6-xF2 x heterojunction was prepared under higher temperature(220℃ and 240℃). The morphology was uniform and regular nano square. Its efficiency of photocatalytic decolourization rate of rhodamine B was more than 95% under 90 min visible light irradiation. Electrochemical test fully proved the better charge separation efficiency and electron transport properties as well as high current density of Bi2O3/Bi2WO6-xF2 x. The improvement of the photocatalytic activity of Bi2O3/Bi2WO6-xF2 x was because p-n junction increased the transfer efficiency of photogenerated carrier inside the semiconductor, effectively inhibited the recombination probability of electronic-hole. F-ion doping effectively reduced the band gap of the product, improving the utilization rate of light energy, thus improved the photocatalytic activity of Bi2O3/Bi2WO6-xF2 x.(4)In order to widely explore the effect of ion-doping on the photocatalytic properties of Bi2WO6, Ce(NO3)3·6H2O and Er(NO3)3·6H2O were used respectively as rare earth element additive to prepared Ce/Bi2WO6 and Er/Bi2WO6. A lot of Ce entered into the Bi2WO6 lattice changing the structure of Bi2WO6, generating Ce O2 crystallites. The oxygen vacancy caused by Ce doping made red-shift, improving the utilization rate of light energy, which had a great impact on the photocatalytic activity. The decolourization rate of rhodamine B was 93% after irradiated under visible light for 80 min with 10% Ce doping. Ion conversion function of Ce3+ and Ce4+ ions and oxygen vacancies within the semiconductor would act as electronic capture agent, effectively restraining the recombination of photogenerated electronic-hole inside the sample and improving the photocatalytic activity while large Ce O2 deposited on the surface of Bi2WO6 would reduce the photocatalytic activity. Er3+ doped into the Bi2WO6 lattice, partly replacing Bi ions in Bi2WO6. The photocatalytic activity of Bi2WO6 was improved by the increase of oxygen vacancy, finally achieving the goal of doping modification.