Study On Hydrothermal Synthesis,characterization And Properties Of Bi-Fe-O Composite Oxide Crystallites

The oxide semiconductor materials with visible-light photocatalytic effect and gas sensitive characteristics can effectively deal with the problems of energy crisis,environmental pollution and safety monitoring.The special crystal structure,visible-light response and gas sensitivity of Bi-Fe-O composite oxide crystallites have attracted wide attention in the field of photocatalysis and gas sensing.In this work,a hydrothermal process,in combination with solvothermal and coprecipitation process were used to synthesize three different structures and morphologies of Bi-Fe-O composite oxide crystallites.The formation technology and growth mechanism of three structures of oxide crystallites was studied.The photocatalytic process and the mechanism of photo-Fenton-like reaction under visible light irradiation were studied.The photocatalytic and gas-sensing properties of different composite oxides were compared.The applied electric field polarization treatment and the magnetic field magnetization treatment on the photocatalytic and gas-sensing properties of the composite oxides were discussed.Pure phase Bi₂₅FeO₄₀,BiFeO₃ and Bi₂Fe₄O₉ crystallites were synthesized by PVA assisted hydrothermal process at Bi/Fe=1:1,hydrothermal temperature of 200 ^oC,reaction time of 24 h,NaOH concentration of 2 M,5 M and 10 M,respectively.The controllable synthesis of Bi-Fe-O composite oxide crystallites was achieved.In the process of nucleation,PVA changes the dissolution rate of Bi³⁺and Fe³⁺,limits the growth rate of the nucleus,and thus is beneficial to the controllable synthesis of three phase structures.Various morphologies of Bi-Fe-O composite oxide crystallites were obtained by adjusting the process parameters:BiFeO₃ particles synthesized at Bi/Fe=1:1 are mostly in cubic morphology with particle size of 10²0?m,600 nm thick disc and octahedral BiFeO₃ crystallites can be obtained at Bi/Fe=1:2;Bi₂Fe₄O₉ crystals are rod shape with the length of 2?m in shorter hydrothermal time,sheet shape with the thickness of 30²00 nm in high concentration of NaOH and comb teeth shape with the addition of surfactant CA;Bi₂₅FeO₄₀ particles prepared by hydrothermal method are 10?m larger cubes,which change into 50 nm nanoparticles in a solvothermal method.BiFeO₃ nanoparticles of 60 nm were synthesized by the coprecipitation process with NH₃H₂O as precipitator.BiFeO₃,Bi₂Fe₄O₉ and Bi₂₅FeO₄₀ samples with different particles sizes and crystal morphologies have an Eg around 2.0 eV,with good absorption in the visible band.In the photocatalytic reaction solution,the addition of trace H₂O₂ forms the Fenton-like effect,which greatly improves the photocatalytic properties of the Bi-Fe-O materials.BiFeO₃ and Bi₂Fe₄O₉ have a better photocatalytic performance of MO degradation than Bi₂₅FeO₄₀.Among the samples,the MO degradation rate of BiFeO₃ nanoparticles and Bi₂Fe₄O₉ fragment after 3 h of visible light irradiation is91%and 85%respectively.The MO degradation rate of the disc BiFeO₃ reaches 98%when the concentration of H₂O₂ is 80mm and the addition of catalyst is 2.0 g/L.BiFeO₃,Bi₂Fe₄O₉ and Bi₂₅FeO₄₀ crystallites show certain gas sensitivity.The best working temperature is 260 ^oC.Among the samples,BiFeO₃ and Bi₂Fe₄O₉ have good selectivity for ethanol and acetone gas,while Bi₂₅FeO₄₀ has good selectivity for alcohol gases.The sensitivity of BiFeO₃ nanoparticles gas sensor is high,the sensitivity to 400 PPM ethanol is 8.3.The response-recovery rate of Bi₂Fe₄O₉ gas sensor is fast,and response recovery time is within 5 s.The photocatalytic properties and gas sensitive properties of the wafer BiFeO₃crystallites were significantly enhanced by polarization and magnetization treatment.The photocatalytic degradation rate of BiFeO₃ samples was increased by 40.6%and46.9%,respectively,and the sensitivity to 400 PPM ethanol gas was increased by18.0%and 36.1%,respectively.