Sol-gel Synthesis And Study On The Doping Modification Of BiFeO₃ Nanoparticles

The multiferroic materials possess both ferroelectric?or antiferroelectric?and ferromagnetic?or antiferromagnetic?ordering within a single phase.In recent years,significant attention has been paid to magnetoelectric?ME?multiferroics which display the potential applications in advanced spintronic and ME devices.Among all the multiferroic materials studied so far,Bi FeO₃ exhibits high ferroelectric Curie temperature?TC¹103 K?and high G-type antiferromagnetic Néel temperature?TN⁶43 K?,which make it one of the prime candidates for room-temperature ME applications.In addition to the above-mentioned applications,BiFeO₃ is also considered as a kind of visible-light-responsive photocatalyst due to its suitable band gap?2.1-3.0 eV?and excellent chemical stability.In this thesis work,we prepared Bi_0.97Na_0.03Fe_1-xNi_xO₃ and Bi0.9La0.1Fe1-xZnxO₃ nanoparticles via a facile sol-gel method,and detailed studies of structural,magnetic,electrical and optical properties were investigated.We selected alkali metal?AM?Na1+ ions and transition metal?TM?Ni2+ ions as dopants to substitute Bi³⁺ ions and Fe³⁺ ions of BiFe O₃,respectively,and prepared Bi_0.97Na_0.03Fe_1-xNi_xO₃?x=0,0.005,0.01,0.015?nanoparticles via a facile sol-gel method.Weak ferromagnetism and exchange bias phenomenon without field cooling were observed in the samples.The oxygen vacancy concentration and leakage current density are increased with increasing the Ni content.However,with the increase of Ni content,the band gap of Bi_0.97Na_0.03Fe_1-xNi_xO₃ nanoparticles first decreases and then increases.To explain the abnormal phenomenon,the interplay of oxygen vacancy donor and hole acceptor was analyzed and a phenomenological qualitative model based on the electronic energy band was proposed.Additionally,the threshold switching?TS?behavior appears in Bi_0.97Na_0.03Fe_1-xNi_xO₃ samples with x=0.01,0.015 and the effect is qualitatively explained by introducing a conducting channel model based on the high-density mobile charges.We selected rare earth?RE?La³⁺ ions and transition metal?TM?Zn2+ ions as dopants to substitute Bi³⁺ ions and Fe³⁺ ions of BiFe O₃,respectively,and prepared Bi0.9La0.1FeO₃?BLFO?and Bi0.9La0.1Fe0.99Zn0.01O₃?BLFZO?nanoparticles via a sol-gel method carried out in our early work.The oxygen vacancies and holes increase with Zn doping analyzed through X-ray photoelectron spectroscopy?XPS?,which could contribute to the increase of leakage current density.However,with the increase of the defects?oxygen vacancies and holes?,the band gap of BLFZO also is increased.To explain the abnormal phenomenon,the bandwidth of occupied and unoccupied bands was analyzed based on the structural symmetry driven by the Fe-O-Fe bond angle and Fe-O bond anisotropy.