Evidência de ferromagnetismo diluído assistido por vacâncias de oxigênio em nanopartículas de CeO2 dopado com cromo trivalente

• Autor(a) principal: Ferreira, Nilson dos Santos
• Data de Publicação: 2014
• Tipo de documento: Tese
• Idioma: por
• Título da fonte: Repositório Institucional da UFS
• Texto Completo: https://ri.ufs.br/handle/riufs/5246
• topic: Física; Magnetismo; Ferromagnetismo; Oxigênio; Cromo; Nanopartículas; Óxido de cério; Propriedades ópticas do óxido de cério; CNPQ::CIENCIAS EXATAS E DA TERRA::FISICA

In this thesis, we studied the structural, chemical, optical and magnetic properties of the undoped and chromium doped cerium oxide (CeO2) nanoparticles. The nanoparticles were synthesized by the sol-gel method and were all calcined at 400 oC. The 1, 3, and 5% nominal percentages of chromium in doped cerium oxide were in agreement with the energy dispersive X-ray spectroscopy (EDX) results. Structural characterization of the nanoparticles using X-ray diffraction (XRD) and Raman spectroscopy confirmed that all samples exhibited a single-phase fluorite structure. In addition, high-resolution transmission electron microscopy (HRTEM), and scanning transmission electron microscopy (STEM) images showed the nanoparticles were to be nearly spherical in shape in both undoped and doped samples. The particle sizes in both undoped and Cr-doped samples were found to be about 5-9 nm, which are comparable with the XRD and HRTEM findings. Lattice constant and the strain in Cr-doped CeO2 nanoparticles were increasing with increase in doping concentration levels. Systematic X-ray photoelectron spectroscopy (XPS), UV-Vis absorption spectroscopy and electron paramagnetic resonance spectroscopy (EPR) studies reveled that the substitution of Ce4+ by Cr3+ ions leaded to formation of Ce3+ ions and oxygen vacancies into the CeO2 host lattice, naturally formed to ensure charge neutrality. Magnetic properties of these nanoparticles were measured using a superconducting quantum interface device (SQUID) magnetometer. The results clearly showed that all Cr3+-doped samples presented room-temperature ferromagnetism. We observed that increasing dopant content from 1 to 5% increased the remanent magnetization from 0.001 emu/g to a maximum of 0.01 emu/g. All these experimental analyses suggested that the observed room-temperature ferromagnetism in Cr-doped CeO2 nanoparticles is closely suggested to result from the concomitant formation of localized Cr3+-VO- Ce3+ and Cr3+-VO- Cr3+ complexes.