Preparation Of Doped And Well-dispersed Perovskite Nanocrystals

The properties of multi-component perovskites are good in optics, catalysis, magnetism, electricity, sensor for gas detection. They also have been entitled for almighty materials. Perovskite-structure BiFeO₃ is typical ferroelectric materials and antiferromagnetic materials. Curie temperature is 1110K and Nier temperature is 650K. At present, the reports on the BiFeO₃ focus on the study of ferroelectric film applying in semiconductor memorizer.The reports on low temperature synthetic methods of BiFeO₃ mainly include the pyrogenation of oxalate and tartaric acid complex. The carrying temperature in two methods is all about 650°C. The photocatalytic-activites of samples obtained from the above methods is almost consistent. In order to enhance the photocatalytic-activities of samples,BiFeO₃,the precursor solution was prepared by mixing appropriate amounts of Bi(NO₃)3·5H₂O and Fe(NO₃)3·9H₂O dissolved in nitrate, and tartrate was added to compensate. The obtained solution was dried in an oven at 90-100°C, then calcinations at 650°C. The calcined powders were milled with lithium nitrate. Then the mixture is heated at the melting point of the LiNO₃ for some time, following by washing the mixture to remove the LiNO₃. The result indicates that the photo-catalytic activities of BiFeO₃ samples enhance greatly after crystallization in melting salt. Experiment shows that the photo-catalytic activities of BiFeO₃ samples are affected by ratios of LiNO₃ to BiFeO₃ and crystallization time in melting salt. There is not any report on the photo-catalytic activities of BiFeO₃ which is developed in melting salt up to now.The sol-gel method possess of many advantages in preparation catalysts of high activities. In its initial stage, precursors are controlled by adding chelating agent to slow the hydrolytic rate of ion and mix well even in the molecule level. The temperature is low for form the final product. The size is little and the sample obtained by sol-gel method is pure and well in dispersivity. So the sol-gel process is favorable for synthesize the perovskite materials with catalytic property.LaCoO₃ is synthesized with a new modified method. LaCoO₃ solid with perovskite structure is a good material in catalysis. LaCoO₃ is a semiconductor and its band gap is about 0.6 eV. So its absorbance corresponds well with visible-light. We prepared well-dispersed Mg-doped LaCoO₃ nanocrystals by a modified sol-gel method.Controllable particle size can be obtained by introduction excess magnesium nitrate into the reactant mixture. During the calcination process, MgO phase serves as pinning particle to inhibit the growth and agglomeration of Mg-doped LaCoO₃ nanocrystals. After remove MgO by dissolving in diluted acetic acid, well dispersed Mg-doped LaCoO₃ nanocrystals with large surface is obtained and with the increasing of MgO content. With the increasing of MgO content from 0 to 4, the size of LaCo0.9Mg0.1O₃ nanocrystals gradually decreases from 31.6 to 13.2 nm.Moreover, synthesis the Mg-doped LaCrO₃ with this modified method. When we calcinations the powders have not got pure phase, because Cr is volatile in this process. CaO is also used in preparation of LaCoO₃ nanocrystals with this sol-gel method. With the increasing of CaO content from 0 to 6, the size of LaCoO₃ nanocrystals gradually decreases from 26.7 to 18.2 nm.