The Preparation And Photocatalytic Water Splitting Properties Of Bi_0.5Y(0.5)VO₄

Nowadays, it is very urgent for human beings to develop clean, no-polluting hydrogen energy, since the energy and environmental problem caused by the large-scale exploitation and use of fossil energy. Among many methods of hydrogen production, the heterogeneous photocatalytic water splitting technique which was developed from"Fujishima-Honda"is increasingly under the spotlight. It is meaningful to design and develop solid photocatalysts which can stoichiometrically split water into hydrogen and oxygen. BiVO₄ is being one of the highlights because it can respond to visible light. But it is regretted that BiVO₄ can only produce O2 from AgNO3 aqueous solution, can not reduce H₂O to H₂ because of its property of band structure.In this thesis, in order to adjust the band structure and photocatalytic property, a solid solution Bi_1-xY_xVO₄ was synthesized in 6 hours with hydrothermal pretreatment. Their physical and chemical properties were characterized by XRD, SEM, DRS, BET and so on. The results showed that Bi_1-xY_xVO₄ was solid solution of BiVO₄ and YVO4, its crystal structure was I41/amd. Bi_1-xY_xVO₄ was found to be capable of overall water splitting when loaded with co-catalysts. Bi_0.5Y_0.5VO₄ loaded with wt 1%Pt-1%Cr₂O₃ showed the best photocatalytic activity, Rate of H₂ and O2 was 80.5μmol/h and 43.8μmol/h, respectively.The effect of pH value was studied in detail. The results showed that the morphology of hydrothermal product under different pH value varied from each other. pH=1 was favorable for the dispersion and combination of Bi3+, Y3+ and V3+, whereas Bi_0.5Y_0.5VO₄ produced under pH=1 had little crystal defects and showed the best photocatalytic activity.The effect of the temperature of hydrothermal reaction was studied in detail. The results showed that Bi_0.5Y_0.5VO₄ synthesized under lower hydrothermal temperature was not stable when lighted for long time. When temperature exceeded 120oC, the products can keep stable and the Bi_0.5Y_0.5VO₄ prepared under 140oC showed the best photocatalytic activity.The effect of calcination temperature was studied in details. The results showed that calcination temperature had great effect on crystal structure. The hydrothermal product was tetragonal, when calcination temperature was 600oC, it turned to monoclinic, when calcination temperature was 700oC, 800oC, or 900oC, it kept tetragonal. It was proved that the middle product didn't form stable structure. The Bi_0.5Y_0.5VO₄ prepared under 800oC showed the best photocatalytic activity.Bi_1-xY_xVO₄ was prepared and characterized to study the effect of ratio of component on the photocatalytic activity. With increasing of the amount of Y, the absorption edge blue shifted, on one hand, the band gap became wider, on the other hand, the range of wavelength that can be absorbed became narrower. Combined influence, when x=0.5, Bi_0.5Y_0.5VO₄ showed the highest photocatalytic activity.When loaded with wt 1% Pt or wt 1% Cr₂O₃ or wt 1% RuO2, Bi_0.5Y_0.5VO₄ began to have photocatalytic activity. Bi_0.5Y_0.5VO₄ loaded Pt or Cr₂O₃ had higher activity than loaded with RuO2.It was found that the loading sequence of wt 1% Pt and wt 1% Cr₂O₃ had a significant effect on photocatalytic activity, when Cr₂O₃ was loaded after Pt, the rate of O2 exceeded H₂, the stoichiometric of H₂ and O2 was not 2:1. This is because Cr₂O₃ acts as O2 site and can decrease the over potential of oxygen evolution. The first co-catalyst was covered with the second co-catalyst, the uneven area of exposure is disadvantaged to activity. So Bi_0.5Y_0.5VO₄ loaded with Pt and Cr₂O₃ at the same time showed the highest photocatalytic activity.Bi_0.5Y_0.5VO₄ loaded with wt 1% Pt-1% Cr₂O₃ showed highest photocatalytic activity, comparing with Bi_0.5Y_0.5VO₄ loaded with wt 1% Pt-0.5% Cr₂O₃, 1% Pt-1.5% Cr₂O₃, and 1% Pt-2% Cr₂O₃. the lower content of Cr₂O₃ cause less O2 site, the higher content of Cr₂O₃ cause concentration.