Ion-doped BiVO₄ Prepared By Hydrothermal Method And Its Photocatalytic Properties

Photocatalytic oxidation of organic pollutants has become a pollution control technology in recent years. Bismuth vanadate(Bi VO4) is one of the typical semiconductor materials. Due to its appropriate energy band structure, higher chemical stability, higher sunlight utilization and non-toxicity, it has attracted more and more researchers and has drawn the great attention in photocatalysis field. Monoclinic Bi VO4 shows the higher photocatalytic activity. Yet the photocatalytic activity of monoclinic Bi VO4 is still low due to the rapid recombination rate of electron-hole pairs.The hydrothermal method and microwave-hydrothermal method was used to synthesize Bi VO4 powders successfully. The influence of three factors including the amount of Na OH, reaction temperature, holding time on the preparation of Bi VO4 powders was studied, and the optimal process parameters were obtained; The influence of organic additive PVP, nonmetal ions N, F doping, metal ions Cu doping and rare earth ions co-doping on the Bi VO4 structure, morphology and photocatalytic activity was also studied. The main conclusions are as follows:(1) Bi VO4 photocatalysts are prepared by a hydrothermal method. The influence of adding contents of Na OH and surfactant PVP, reaction temperature and holding time on crystal structure and photocatalytic properties are studied. Under the hydrothermal environment, crystal nucleus growth points are formed on the basis of Bi(OH)3 which then react with vanadate anions to replace OH- to produce Bi VO4 crystals nucleus. Alkaline condition is conducive to the formation of vanadate anions, and yet Bi(OH)3 is of strong stability so that it can easily form Bi-oxide precipitations; Under Acidic conditions, vanadium exist as anionic oligomers, Bi(OH)3 dissolve into Bi3+ ions, which are helpful to the generation of pure monoclinic Bi VO4 nucleus. Wood-like monoclinic Bi VO4 crystals with 4 m L Na OH in precursors and reacting at 160℃ for 12 h reaches the degradation rate of 65% after 6h visible light irradiation. The adding of PVP does not change the crystal structure to maintain monoclinic Bi VO4. Photogenerated elec-tron-hole pairs in porous rod-shaped Bi VO4 with adding 1mmol or 4mmol PVP is low, and the degradation rate reaches 62% after visible light irradiation for 4 h.(2) N-doped Bi VO4 photocatalysts are prepared by a hydrothermal method, and the doping of N does not change the crystal structure. The monoclinic Bi VO4 with N/Bi=1.00 and reacting at 200 reaches the degradation rate of 55% after ℃2h visible light irradiation, with the reaction rate constant of 0.49155 h-1. The N-doped Bi VO4 photocatalysts have low electron-hole recombination rate and good transmission rate; Suitable doping content can help to increase the transmission of carriers and promote the photocatalytic properties. The granular N/Bi VO4 is more favourable than the fibrous one.(3) F-doping Bi VO4 photocatalysts are prepared by a hydrothermal method, getting block and fibrous shapes. The F-doping monoclinic Bi VO4 with F/Bi=3.00 and reacting at 200℃ reaches the degradation rate of 70% after 6h visible light irradiation. The fibrous F/Bi VO4 with F/Bi=1.00 have excellent adsorption of Rh B, but its subsequent poor degradation performance leads to a lower degradation rate. Block granular F/Bi VO4 have more excellent photocatalytic activities than fibrous F/Bi VO4. F-doping Bi VO4 photocatalysts possess obvious increasing transmission of photoinduced electrons, which is one of the important factors to promote the photocatalytic properties.(4) Cu-doping Bi VO4 photocatalysts are prepared by a hydrothermal method. The introduction of Cu changes the phase composition of Bi VO4 powders, Bi2O2.7, Bi and Bi2O3 exist in the product, also leaving great influence on the morphology. The block Cu/Bi VO4 photocatalyst with the doping content of 1at.% and reacting at 160℃ reaches an excellent degradation rate of 60% after 6h visible light irradiation.(5) Sm-Er-Bi VO4 and Eu-Nd-Bi VO4 are synthesized by a microwave hydrothermal method. The co-doped Bi VO4 are transformed from a monoclinic scheelite phase into a stable tetragonal zircon phase. All the Sm-Er-Bi VO4 photocatalysts have obvious degradation properties, reaching 45% after 180 min visible light irradiation, while it is almost no degradation under UV light. Eu-Bi VO4 and Nd-Bi VO4 photocatalysts have visible driven degradation properties, reaching 45% after 180 min, and also have good degradation rate of 90% under UV light for 180 min. The co-doped photocatalysts have lower degradation rate than single rare earth doped photocatalysts.