Propriedades multifuncionais dos sistemas cerâmicos de Ce1-(3/4)XPrXO2 (x = 0,04; x = 0,08; x = 0,12) na forma de nanopartículas e filmes
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Data
2020-07-01
Autores
Oliveira, Liliane Lelis
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Universidade Estadual Paulista (Unesp)
Resumo
Neste trabalho obteve-se material semicondutor de óxido de cério dopado com praseodímio utilizando o método de síntese hidrotermal assistida por micro-ondas para em seguida, ser estudado na forma de nanopartículas, por meio da caracterização estrutural, óptica e magnética e na forma de filme, por meio da caracterização elétrica. As nanopartículas de óxido de cério foram dopadas com praseodímio em diferentes concentrações, respeitam a estequiometria Ce1-(3/4)xPrxO2 para os valores de x igual a 0,04; 0,08 e 0,12 e inicialmente foram caracterizadas pelas técnicas de Difratometria de raios – X (DRX), indicando para todas as amostras a fase cúbica tipo fluorita do CeO2 e grupo espacial Fm3m com ordem a longa distância interatômica, e a técnica de Refinamento de Rietveld indicou coerência nos resultados dos parâmetros de rede, além de apontar uma maior concentração de tensões na rede para a amostra dopada em 8% de praseodímio (x = 0,08); A técnica de Infravermelho (IR-FT) indica a síntese completa dos agentes precursores com a ausência do grupo nitrato; A caracterização de espectroscopia Raman além de apresentar ordem a curta distância interatômica dada a presença das unidades CeO8 e PrO8, também mostra o modo F2g para todas as amostras, relacionado com a estrutura tipo fluorita do óxido de cério, além do modo de segunda ordem indicar que o aumento do dopante promove a redução do Ce4+ para Ce3+ e consequentemente, promove a formação de vacâncias de oxigênio; A Espectroscopia de Fotoluminescência (PL) mostra para as amostras dopadas a presença de defeitos rasos, variações de valências dada a redução do Ce4+ para Ce3+ associados às recombinações de espécies e vacâncias de oxigênios; As análises de área de superfície específica B.E.T. mostram que a dopagem do sistema do óxido de cério com praseodímio promove um aumento da área de superfície para o material na forma de nanopartículas; Microscopia Eletrônica de Varredura (MEV) evidencia a presença de aglomerados da ordem de 17-30 nm tanto para o material dopado quanto puro; Com a técnica de Ultravioleta-visível (UV-Vis) foi possível identificar que a dopagem com o praseodímio aumenta o caráter condutor do material, indicando estágios eletrônicos intermediários entre os níveis 2p do oxigênio e o nível vazio 4f do Ce e Pr, indicando a redução do Ce4+ para Ce3+, formando vacâncias de oxigênio e diminuindo o valor do band-gap; A Espectroscopia de Ressonância Paramagnética (EPR) indica a presença de elétrons desemparelhados nos locais de vacâncias de oxigênio para todas as amostras dopadas, com maior concentração para a amostra dopada em 8% de praseodímio; A caracterização Magnética mostra para todas as amostras dopadas um material com múltiplos domínios e que assume dois comportamentos magnéticos distintos: acima da temperatura de bloqueio se comporta como um material Paramagnético, visto que abaixo da temperatura de bloqueio, possui caráter Superparamagnético, apresentando coercitividade e remanescência. Os filmes dopados foram depositados em substrato de alumina pelo método manual screen printing, possuem espessura da ordem de 20 µm. Os filmes apresentam sensibilidade em atmosfera de monóxido de carbono por meio da queda da resistência na faixa de temperatura de 100 °C – 380 °C, apontando maiores sinais de resposta em temperaturas abaixo de 150 °C para as concentrações dopadas em 4% e 8% de praseodímio. Maiores valores da energia de ativação Ea para as amostras submetidas à atmosfera de monóxido de carbono podem indicar que o material tenha atingido um estado de energia mais estável, indicando a oxidação da superfície do filme em elevadas temperaturas e dada a presença do gás redutor (COg). Ainda é indicado o mecanismo de transporte de elétrons por tunelamento, dada a queda brusca da resistência em presença do gás monóxido de carbono.
In this research, praseodymium-doped cerium oxide nanoparticles were obtained using the microwave-assisted hydrothermal synthesis to then be studied by structural, optical and magnetic characterization and in a solid film shape, through electrical characterization. The cerium oxide nanoparticles were doped with praseodymium in different concentrations, respecting the stoichiometry Ce1-(3/4)xPrxO2 for x values equal 0.04; 0.08 and 0.12. The material were initially characterized by X-ray diffraction techniques (XRD), indicating for all samples the CeO2 fluorite-like cubic phase and Fm3m spatial group, with long-distance interatomic order. The Rietveld's refinement indicated consistency in the results of the lattice parameters, in addition to pointing out a higher concentration of stresses in the lattice for the sample doped in 8% praseodymium (x = 0.08); The Infrared technique (IR-FT) indicates the complete synthesis of precursor agents with the absence of the nitrate group; The Raman characterization presents a short interatomic distance order given the presence of CeO8 and PrO8 units, it also shows the F2g mode for all samples, related to the fluorite-like structure of cerium oxide. In addition, the second order mode indicates that the increase in doping promotes the reduction of Ce4+ to Ce3+ and, consequently, promotes the formation of oxygen vacancies; Photoluminescence Spectroscopy (PL) shows for the doped samples the presence of shallow defects, variations in valences given the reduction from the Ce4+ to Ce3+ associated with recombination of species and oxygen vacancies; The Specific Surface Area Analyzes B.E.T. shows that the doping of the cerium oxide system with praseodymium promoted an increase in the surface area for the material in the form of nanoparticles; Scanning Electron Microscopy (SEM) shows the presence of agglomerates of the order of 17-30 nm for both doped and pure material; Using the Ultraviolet-visible technique (UV-Vis) it was possible to identify that doping with praseodymium increases the conductive character of the material, indicating intermediate electronic stages between the 2p levels of oxygen and the empty level 4f of Ce and Pr, indicating the reduction of Ce4+ to Ce3+, forming oxygen vacancies and decreasing the bandgap value; Paramagnetic Resonance Spectroscopy (EPR) indicates the presence of unpaired electrons in the oxygen vacancy sites for all doped samples, with a higher concentration for the doped sample in 8% of praseodymium; The Magnetic characterization shows for all doped samples a material with multiple domains and that assumes two distinct magnetic behaviors: above the blocking temperature it behaves such a Paramagnetic material, and below the blocking temperature, it has Superparamagnetic character, presenting coercivity and remnant. The doped films were deposited on an alumina substrate using the manual screen printing technique, and showed ~20 µm thickness. The films present sensitivity in a carbon monoxide atmosphere dropping the resistance in the temperature range of 100 ° C - 380 ° C, indicating greater signs of response at temperatures below 150 ° C for the samples doped in 4% and 8 % of praseodymium. The highest values of activation energy Ea for samples submitted to the carbon monoxide atmosphere may indicate that the material has reached a more stable energy state, indicating the oxidation of the film surface at high temperatures and given the presence of the reducing gas (COg ). The electron transport mechanism by tunneling is also indicated, given the sharp drop of the resistance values in presence of carbon monoxide gas
In this research, praseodymium-doped cerium oxide nanoparticles were obtained using the microwave-assisted hydrothermal synthesis to then be studied by structural, optical and magnetic characterization and in a solid film shape, through electrical characterization. The cerium oxide nanoparticles were doped with praseodymium in different concentrations, respecting the stoichiometry Ce1-(3/4)xPrxO2 for x values equal 0.04; 0.08 and 0.12. The material were initially characterized by X-ray diffraction techniques (XRD), indicating for all samples the CeO2 fluorite-like cubic phase and Fm3m spatial group, with long-distance interatomic order. The Rietveld's refinement indicated consistency in the results of the lattice parameters, in addition to pointing out a higher concentration of stresses in the lattice for the sample doped in 8% praseodymium (x = 0.08); The Infrared technique (IR-FT) indicates the complete synthesis of precursor agents with the absence of the nitrate group; The Raman characterization presents a short interatomic distance order given the presence of CeO8 and PrO8 units, it also shows the F2g mode for all samples, related to the fluorite-like structure of cerium oxide. In addition, the second order mode indicates that the increase in doping promotes the reduction of Ce4+ to Ce3+ and, consequently, promotes the formation of oxygen vacancies; Photoluminescence Spectroscopy (PL) shows for the doped samples the presence of shallow defects, variations in valences given the reduction from the Ce4+ to Ce3+ associated with recombination of species and oxygen vacancies; The Specific Surface Area Analyzes B.E.T. shows that the doping of the cerium oxide system with praseodymium promoted an increase in the surface area for the material in the form of nanoparticles; Scanning Electron Microscopy (SEM) shows the presence of agglomerates of the order of 17-30 nm for both doped and pure material; Using the Ultraviolet-visible technique (UV-Vis) it was possible to identify that doping with praseodymium increases the conductive character of the material, indicating intermediate electronic stages between the 2p levels of oxygen and the empty level 4f of Ce and Pr, indicating the reduction of Ce4+ to Ce3+, forming oxygen vacancies and decreasing the bandgap value; Paramagnetic Resonance Spectroscopy (EPR) indicates the presence of unpaired electrons in the oxygen vacancy sites for all doped samples, with a higher concentration for the doped sample in 8% of praseodymium; The Magnetic characterization shows for all doped samples a material with multiple domains and that assumes two distinct magnetic behaviors: above the blocking temperature it behaves such a Paramagnetic material, and below the blocking temperature, it has Superparamagnetic character, presenting coercivity and remnant. The doped films were deposited on an alumina substrate using the manual screen printing technique, and showed ~20 µm thickness. The films present sensitivity in a carbon monoxide atmosphere dropping the resistance in the temperature range of 100 ° C - 380 ° C, indicating greater signs of response at temperatures below 150 ° C for the samples doped in 4% and 8 % of praseodymium. The highest values of activation energy Ea for samples submitted to the carbon monoxide atmosphere may indicate that the material has reached a more stable energy state, indicating the oxidation of the film surface at high temperatures and given the presence of the reducing gas (COg ). The electron transport mechanism by tunneling is also indicated, given the sharp drop of the resistance values in presence of carbon monoxide gas
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Palavras-chave
Óxido de cério, Óxido de Praseodímio, Síntese hidrotermal assistida por micro-ondas, Cerium oxide, Praseodymium oxide, Carbon monoxide, Microwave-assisted hydrothermal synthesis, Nanopartículas, Monóxido de carbono, Material cerâmico