Preparation And Characterization Of Fe-based Oxides Narrow Bandgap Semiconductors

In recent years, the environment pollution is becoming more and more seriousall over the world with the development of the global economics and industry.Semiconductor photocatalysts with high photocatalytic activity have received moreand more attention because of their important role on environmental applicationssuch as air purication, water disinfection, hazardous waste remediation and waterpurification. In genereal, there have two main categories for semiconductorphotocatalysts: one is the titania dioxide and its modification by doping method, theother is the novel narrow bandgap semiconductor materials. The latter, narrowbandgap semiconductor semiconductor materials with characters of absorbing thevisible light, can degrade the organic pollutants and split the water under visible light,thus became the focus in the research in recent years.In this thesis, the related research progress and preparation method on narrowbandgap semiconductor were summarized. The focus of present study is about thepreparation and characterization of iron related compounds such as BiFeO₃ with theperovskite structure, SiO₂/YFeO₃ comporiste, and the aurivillius phase layeredBi₅Ti₃FeO₁₅. Hydrothermal method and solid-state reaction method were adopted toprepare the BiFeO₃ and Bi₅Ti₃FeO₁₅, respectively. And the SiO₂/YFeO₃ compositephotocatalyst was synthesized via sol-gel method. Among various preparationparameters, the ratio of raw material, synthesis temperature, the reaction time and thenumber of the reaction times are taken into account to obtain the targer products,together with the measurement of phase rype, morphology, photocatalytic properties.In addition, beside its semiconductor characters, the magnetic property ofBi₅Ti₃FeO₁₅ product was studied in present study.BiFeO₃, a semiconductor with visible-light response and photocatalyticdegradation ability, was synthesized via hydrothermal and solid-state reactionmethod using Bi₂O₃, Fe₂O₃ and nitrate salts as starting materials. Th effect of rawmateriald ratio, reaction temperature and different kinds of Fe₂O₃ raw materials onvisible-light degradation efficiency of MO was investigated. Furthermore, thesignificant difference of visible-light degradation efficiency between products prepared via solid-state method and those prepared by hydrothermal method wasrevealed and clarified from the point view of particle morphology and lattice defects.It is revealed that the impurities of Bi₂₅FeO₄₀ co-exists with increase of Bi₂O₃/Fe₂O₃ ratio, also the impurities of Bi₂Fe₄O₉ is readily to exist with increase ofreaction temperature with fixed Bi₂O₃/Fe₂O₃ ratio of 1. Also, using nano sized Fe₂O₃raw material provides high surface energy and promotes the solid-reaction rate,therefore benefits the BiFeO₃ product. It was found that the optimal solid-reactionconditions for relatively pure BiFeO₃ product are: the Bi₂O₃/Fe₂O₃ ratio of 1,sintered temperature of 830℃, and sintering time of 3 hours.As for the hydrothermal method route, the target phase BiFeO₃ is easily toobtain with increase of the temperature under fixed reaction time, or with increase ofthe reaction time under the fixed reaction temperature. Photocatalytic degradationtests showed that the BiFeO₃ with small amount of impurities prepared in thesolid-state method exhibits better photocatalytic degradation efficiency on MO thanthat of pure BiFeO₃ prepared by hydrothermal under visible light irradiation. It isbelieved that, besides the structure defects and the particle morphology, theimpurities played an assist-role on degradation process.Second, SiO₂/YFeO₃ composite was synthesized via Pechini sol-gel method.The relationship amony the morphology, the number of the preparation cycles, andthe photocatalysis efficiency of SiO₂/YFeO₃ was investigated. It was demonstratedthat increase the prepation cycles could improve the photocatalytic degradationefficiency of products.Finally, the aurivillius phase layered Bi₅Ti₃FeO₁₅ were prepared byhydrothermal and solid-state reaction method respectively. The growth of theBi₅Ti₃FeO₁₅ prepared by hydrothermal method under the different reaction time wasinvestigated. The results revelaed that pure phase of the Bi₅Ti₃FeO₁₅ could beobtained through 48h reaction under hydrothermal condition or sintered for 5 hoursat 900℃via solid-state method. Besides its semiconductor characters, weakferromagnetism at room temperature was also detected for the pure phase ofBi₅Ti₃FeO₁₅ product.