Study On The Synthesis And Properties Of LaVO₄ Complex Oxides

Rare Earth shows unique physical and chemical properties, especially excellent optical, electrical, magnetic and catalytic properties. It has been applied widely in national economy, science and technology. Rare earth compounds are considered to be new materials of the 21st century in the international area.In this paper, a series of complex oxide LaVO₄ were synthesized by sol-gel and hydrothermal method under different conditions. The as-prepared samples were characterized by XRD, FT-IR, SEM, BET, UV-Vis DRS. Meanwhile, their photocatalytic properties were investigated by the degradation of Rhodamine B under visible light irradiation. The results showed that m- and t-LaVO₄ could be selectively synthesized by both methods under controlled conditions. m-LaVO₄ showed higher photocatalytic activities than t-LaVO₄ under visible light irradiation. The porous m-LaVO₄ prepared by sol-gel method exhibited higher photocatalytic activities compared with the catalysts synthesized by hydrothermal method. The catalyst calcined at 300℃showed the highest photocatalytic activity among all m-LaVO₄. The degradation of Rhodamine B in the presence of m-LaVO₄ crystal calcined at 300℃reached 65.45 % after 6 hours, which was 48.07 % higher in the comparison of conventional photocatalyst TiO₂(Degussa P25) Both adding electron acceptor and adjusting the initial pH value of Rhodamine B solution to acid (pH=3) could enhance photocatalytic ability of m-LaVO₄ remarkably, Rhodamine B could be degraded completely under visible light irradiation after 2 hours in such cases. The photocatalytic activity of m-LaVO₄ under visible light irradiation could make up for the deficiency about useless of sunlight in the presence of TiO₂ (Degussa P25) photocatalyst. It established the foundation of application of complex oxide m-LaVO₄ in the photocatalytic field.m- and t-LaVO₄:Eu³⁺ crystals have been selectively synthesized by hydrothermal and precipitation method without any additives under different pH values. The Photoluminescence properties of the as-prepared products were investigated. It was found that the pH value was crucial to determine the crystal structure. t-LaVO₄:Eu³⁺ could be obtained in the pH range of 6 to 9 and m-LaVO₄:Eu³⁺ in the range of 4 to 5. The samples excited by 280 nm UV lights showed characteristic narrow red emissions with the peak at about 600-620 nm, the photoluminescent property of t-LaVO₄:Eu³⁺ was superior to m-LaVO₄:Eu³⁺. The morphology played an important role in its photoluminescent property, the submicronmeter t-LaVO₄:Eu³⁺ samples prepared by precipitation exhibited regular morphologies. The photoluminescent intensity can be improved by prolonging the aging time. The photoluminescent intensity of the precipitation sample aging for 12 h at room temperature was comparable with the hydrothermal sample prepared at 180℃for 2 h. The dosage of Eu³⁺ was also an important factor that influenced the photoluminescent property greatly, the appropriate dosage of Eu³⁺ was 5 % molar ratio. t-LaVO₄:Eu³⁺ showed good thermodynamic stability below 600℃, the photoluminescent intensity didn't decrease after calcined.