Preparation And Photocatalytic Activity Of Lanthanum Ferrite,Strontium Titanate And Its Hybrid Composite

ABO₃ perovskite composite oxides are widely used in photocatalytic degradation of organic pollutants and hydrogen production of water photolysis due to the good structure stability,optical/electrical properties,non-toxic and low price.In this thesis,the influences of preparation method and process conditions on the properties and the visible light excited photocatalytic activity of the typical perovskite composite oxides,i.e.lanthanum ferrite?i.e.,LaFeO3?,strontium titanite?i.e.,SrTiO3?and its silver/silver chloride hybrid composite?i.e.,SrTiO3-Ag/AgCl?were investigated.In addition,the photocatalytic mechanisms of the materials above were also discussed.The main research contents and results are given as follows:?1?LaFeO₃ composite oxide was prepared by an ultrasound-assisted sol-gel method.The influence of ultrasound treatment time on the thermal behavior of LaFeO3 xerogel was analyzed.The result shows that the suitable ultrasound treatment time is 40 min.After 40-min ultrasound assisted treatment,the formation temperature of LaFeO₃ crystalline phase can be decreased to500oC with an average crystalline size of 18.65 nm.The primary reason why ultrasonic assistance can decrease the calcination temperature of LaFeO₃ efficiently is that the instantaneous high temperature and high pressure generated from the burst of ultrasonic cavitation bubble can contribute to the reduction of the critical nucleation work and critical radius of LaFeO₃ colloidal nucleus during sol-gel process.The formation of more compact and dense LaFeO₃ nuclei can make NO₃^-existing in LaFeO₃ colloidal nucleus decomposition rapidly and completely at lower temperature,further promoting citric acid oxidized,combustion and decomposition.Thus,the weight loss rate of LaFeO3 xerogel in the first stage?i.e.,room temperature to 172oC?is improved effectively,resulting in the quick formation of LaFeO₃crystalline phase at a lower temperature.Compared to LaFeO₃ derived without ultrasonic assistance,LaFeO₃ derived via ultrasonic assistance exhibits a better photocatalytic activity and a photodegradation efficiency of methyl orange?MO?of 89%after 180-min visible light illumination.The photodegradation process of MO on the as-synthesized LaFeO3 follows the pseudo-first-order kinetic model.The reaction rate constant?Kapp?of MO on LaFeO3 with ultrasound assistance is almost twice greater than that on LaFeO₃ without ultrasonic treatment.?2?Perovskite LaFeO₃ nanoparticles with an orthorhombic structure were synthesized by a sol-gel route and subsequent vacuum microwave calcination.The effects of vacuum microwave calcination temperature and duration on the formation of LaFeO3 single phase were investigated.The use of vacuum microwave calcination can effectively reduce the crystallization time of LaFeO₃ nanoparticles.The as-synthesized LaFeO₃ nanoparticles have a perfect crystal structure,a single-phase composition without any impurity,uniform particle size,and a suitable energy band of¹.86 eV.Furthermore,LaFeO3 nanoparticles calcined via vacuum microwave calcination have a superior visible light photocatalytic efficiency on organic dye methyl orange?MO?and methyl blue?MB?,and a higher photodegradation efficiency rate,compared to those calcined via the conventional calcination.LaFeO₃nanoparticles obtained by vacuum microwave calcination at 700°C for 30 min exhibit a superior photocatalytic efficiency,and the MB and MO dye solutions in the presence of LaFeO3nanoparticle become colorless during 240 min treatment under visible light irradiation.The photodegradation process of MB on the as-synthesized LaFeO₃ follows the pseudo-first-order kinetic model.The reason that the formation temperature and duration of La FeO₃ decrease in vacuum microwave calcination can be since a)the microwave oscillation of molecular dipole results in a homogeneous distribution of heat inside the material.Heat transfers from the center of the material to the outside without temperature gradient.Thus,LaFeO₃ xerogels can be heated uniformly and transformed into the crystalline phase at lower temperature and less time;and b)the explosive effect of massive gas produced by rapid microwave heating can contribute to the instant formation of monodisperse LaFeO₃ nanoparticles with the massive crystallines in a vacuum state.?3?SrTiO₃ nanoparticles were synthesized by a low-temperature hydrothermal method without alkali as a mineralizer.The reaction temperature and holding time both have dominant effects on the photocatalytic activity of the as-synthesized SrTiO₃.The results show that the suitable hydrothermal temperature and holding time are 120°C and?36 h,respectively.SrTiO3nanoparticles consist of homogeneous polyhedral shape like poly-crystallites with the size of approximately 45 nm and a porous structure and a great specific surface area of 28.08 m²·g^-1.The MO aqueous solution can be photodegraded absolutely after 240 min light illumination in the presence of the as-synthesized nanoparticles.The photodegradation process of MO on the as-synthesized SrTi O3 follows the first-order kinetic model,and exhibits a good photocatalytic stability after 5-cycling runs.According to the calculated results of the first principle,SrTiO₃ is a sort of indirect semiconductor.?4?An efficient visible light responsive photocatalyst hybrid composite of SrTiO₃-Ag/AgCl was synthesized via a hydrothermal method and subsequent chemical precipitation/in-situ photoreduction at room temperature.The transient photocurrent response and electrochemical impedance under visible light illumination indicate that SrTiO₃-Ag/AgCl composite possesses a more intense photocurrent response and a smaller surface resistance than SrTiO₃ and Ag/AgCl due to the lower electrons-holes recombination.SrTiO₃-Ag/AgCl composite exhibits a promoted visible light excited photocatalytic activity in photodecomposition of methyl orange,rhodamine B and phenol,compared to SrTiO₃ and Ag/AgCl.A possible photocatalytic mechanism was proposed,indicating that the synergistic effect of surface plasmonic resonance of Ag⁰ photoreduced from AgCl and decreased the recombination rate of photogenerated carriers through transferring electrons from the surface of Ag⁰ to SrTiO₃ are contributed to the excellent photocatalytic activity of SrTiO₃-Ag/AgCl.Moreover,the photodegradation reaction process of methyl orange,rhodamine B and phenol on SrTiO₃-Ag/AgCl follows the pseudo-first-order kinetic model,and the reaction rate constants are approximately 10 times greater than those on Ag/AgCl.The four-recycling photocatalytic process of methyl orange on SrTiO3-Ag/AgCl also indicates superior stability and durability.Superoxide radical?·O₂^-?is one of the most important reactive species followed by h⁺in visible light excited photodegradation of RhB in aqueous solution in the presence of the as-prepared STO-Ag/AgCl?12:1?catalyst.