Modification Of BiVO₄Composite Photocatalyst And Its Photocatalytic Properties

As a semiconductor material, bismuth vanadate (BiVO4) has been used asyellow pigment for many years. In recent years, due to the narrow band gap,higher stability, higher sunlight utilization and non-toxicity, BiVO4has drawn thegreat attention in photocatalysis field. There are three crystal structures of BiVO4:monoclinic scheelite structure, tetragonal scheelite structure and tetragonalzircon structure. Among these three crystal forms, monoclinic scheelite BiVO4shows the higher photocatalytic activity than the other forms under visible lightirradiation. Yet the photocatalytic activity of monoclinic BiVO4is still low due tothe rapid recombination rate of electron-hole pairs.In this paper, BiVO4powders were prepared by the microwavehydrothermal method using NH4VO3and Bi(NO3)35H2O as raw materials. Thecrystal structure, the morphology, and the photophysical properties of thesamples were all characterized by XRD, FESEM, TEM, XPS, UV-vis and BETsurface area techniques. The photocatalytic performances of the samples wereevaluated by the decolorization of rhodamine B under UV and simulatedsun-light irradiation. The selective preparation of monoclinic BiVO4and dopingmodification of BiVO4is well studied in this study.(1)In order to study the selective synthesis of monoclinic BiVO4, BiVO4powders were prepared at different pHs, using NaOH as the pH adjuster. Thecorresponding relationship among pH values of the precursor, crystalline phase,morphology and photocatalytic performance of the powders was also discussed.The pH synthesis region of monoclinic BiVO4was obtained. The possiblemechanism of phase transformation at different pH conditions was alsodiscussed. The results showed that rod-like and dendritic BiVO4with a pure monoclinic structure could be obtained within a very wide pH range(pH=4.26-9.76). At pH=7.81, the monoclinic BiVO4with irregular rod-likemorphology exhibited the best visible light photocatalytic activity and thedegradation rate of RhB after5h simulated sun-light irradiation was50%.(2)In order to enhance the photocatalytic efficiency of BiVO4, C/BiVO4powders were synthesized using C6H12O6as a carbon source. The effects ofdoping content of carbon, reaction time and temperature on the crystalline phase,morphology and photocatalytic performance of the powders were also discussed.The results showed that the appearance of tetragonal BiVO4was induced by thedoping carbon, low temperature made the amount of tetragonal BiVO4more. Thephotocatalytic efficiency of15at.%C/BiVO4powders prepared at200℃for40min was36%higher than that of pure BiVO4under UV-light irradiation and29%under simulated sun-light irradiation, respectively.(3)In order to improve the photocatalytic efficiency of BiVO4further,N/BiVO4powders with exposed (040) facets were synthesized using NaN3as anitrogen source. The effects of doping content of nitrogen, reaction time andtemperature on the crystalline phase, morphology and photocatalyticperformance of the powders were also discussed. The results showed that theN/BiVO4nanoplates preferentially grew along the (040) facet and their band gapwere smaller than that of pure BiVO4. They exhibited the higher activity thanpure BiVO4. The photocatalytic efficiency of110at.%N/BiVO4powdersprepared at200℃for40min showed the highest photocatalytic efficiency. Itsphotocatalytic efficiency was96%higher than that of pure BiVO4underUV-light irradiation and34%under simulated sun-light irradiation, respectively.(4)In order to improve the visible photocatalytic activity of BiVO4,Er/BiVO4and Yb/BiVO4powders were synthesized using Er(NO3)36H2O andYb(NO3)36H2O as the europium and ytterbium source, respectively. The effectsof doping content of Er3+and Yb3+on the crystalline phase, morphology andvisible photocatalytic performance of the powders were also discussed. Theresults showed that both Er/BiVO4and Yb/BiVO4had the tetragonal structureand they were nano-scaled particles. Er/BiVO4powders showed weak absorptionin the visible light regions (490nm,525nm and605-620nm). The visiblephotocatalytic activity of Er/BiVO4and Yb/BiVO4with tetragonal structure was better than that of monoclinic BiVO4. By doping Er3+and Yb3+, thephotocatalytic efficiencies of BiVO4were enhanced by90%.