BaAl2O4:Eu3+ monofásico via métodos Sol-Gel e Pechini modificado: Avaliação das propriedades estruturais, morfológicas e espectroscópicas visando aplicação em dispositivos ópticos
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Data
2018-06-22
Autores
Oliveira, Nagyla Alves de
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Universidade Estadual Paulista (Unesp)
Resumo
Este estudo propõe a produção de BaAl2O4 dopado ou não com Eu3+ a partir dos métodos sol-gel com alcóxido metálico (SG) e Pechini modificado (PM). Uma investigação sistemática inicial estabeleceu as condições de síntese dos precursores, gel e resina polimérica, respectivamente, que resultassem no sistema monofásico, sendo definido pré-calcinação a 300°C/2h e calcinação a 1000°C/2h. Variou-se a concentração do dopante de 0 a 7% em mol, caracterizando-se todas as amostras do ponto de vista estrutural, morfológico, e fotoluminescente. Independentemente do método e porcentagem de dopante, a fase majoritária BaAl2O4 hexagonal, grupo espacial P6322, com Ba2+ em dois sítios distintos de número de coordenação 9 foi obtida. O método SG resultou em pós com maiores tamanhos de cristalito sendo que no caso do método PM houve diminuição com o aumento da dopagem, mantendo a média de 4,85 nm. Por MEV verificou-se que o método PM resulta em partículas também menores e coalescidas na forma de bulk e as via método SG possuem perfil esferoidal. O valores de band gap dos pós foram similares, independentemente do método, mantendo uma média de 4,30 eV. A partir do monitoramento das transições do Eu3+ nos dois sítios de Ba2+, foram avaliados tempo de vida de estado excitado, eficiência quântica, parâmetros de intensidade, além do diagrama de cromaticidade. A matriz apresenta emissão intrínseca. Um mecanismo de transferência de energia envolvendo íon ativador e rede hospedeira foi proposto. A rota SG produziu luminóforos vermelhos com maior intensidade relativa de emissão que aumenta com o aumento da porcentagem de dopante. Os valores de eficiência quântica foram acima de 50% para os luminóforos via ambos os métodos, mais elevados do que os reportados na literatura. Entretanto, BaAl2O4:Eu3+(1%) via método PM sob atmosfera oxidante apresentou emissão anômala de Eu2+, embora instável, a qual é discutida e associada ao efeito dos precursores e à carga de íons da rede. Com base em todos os resultados, ambos métodos resultam na fase desejada com sucesso, e produziram luminóforos vermelhos promissores para serem aplicados em dispositivos ópticos.
The purpose of this study is the production of BaAl2O4 doped or not with Eu3+ from the sol-gel with metal alkoxide (SG) and the modified Pechini (MP) methods. An initial systematic investigation established the synthesis conditions of the precursors, gel and polymer resin, respectively, that resulted in the single-phase system, being defined pre-calcination at 300 °C/2h and calcination at 1000 °C/2h. The concentration of the dopant was varied from 0 to 7 mol%, and all samples were structurally, morphologically and optically characterized. Regardless of the method and percentage of dopant, the BaAl2O4 hexagonal major phase, spatial group P6322, with Ba2+ at two distinct sites of coordination number 9 was obtained. The SG method resulted in powders with higher crystallite sizes and in the case of the MP method there was decrease with increasing doping, maintaining the mean of 4.85 nm. By SEM it was found that the MP method also yields in smaller coalesced particles in the bulk form and the SG method generates particles with spheroidal profile. The band gap values of the powders were similar regardless of the method, keeping an average of 4.30 eV. From the monitoring of the Eu3+ transitions in the two sites of Ba2+, the excited state lifetime, quantum efficiency, intensity parameters, and the chromaticity diagram were evaluated. It was also detected the intrinsic emission of the matrix, and a mechanism of energy transfer involving activator ion and host network was proposed. The SG route produced red phosphors with a higher relative emission intensity that increases with increasing dopant percentage. The quantum efficiency values were above 50% for the phosphors via both methods, much higher than those reported in the literature. However, BaAl2O4:Eu3+ (1%) via MP method under oxidizing atmosphere exhibited anomalous emission of Eu2+, although unstable, which is discussed and associated to the effect of the precursors and the ion charge of the network. Based on all results, both methods result in the desired phase successfully, and produced promising red phosphors to be applied on optical devices.
The purpose of this study is the production of BaAl2O4 doped or not with Eu3+ from the sol-gel with metal alkoxide (SG) and the modified Pechini (MP) methods. An initial systematic investigation established the synthesis conditions of the precursors, gel and polymer resin, respectively, that resulted in the single-phase system, being defined pre-calcination at 300 °C/2h and calcination at 1000 °C/2h. The concentration of the dopant was varied from 0 to 7 mol%, and all samples were structurally, morphologically and optically characterized. Regardless of the method and percentage of dopant, the BaAl2O4 hexagonal major phase, spatial group P6322, with Ba2+ at two distinct sites of coordination number 9 was obtained. The SG method resulted in powders with higher crystallite sizes and in the case of the MP method there was decrease with increasing doping, maintaining the mean of 4.85 nm. By SEM it was found that the MP method also yields in smaller coalesced particles in the bulk form and the SG method generates particles with spheroidal profile. The band gap values of the powders were similar regardless of the method, keeping an average of 4.30 eV. From the monitoring of the Eu3+ transitions in the two sites of Ba2+, the excited state lifetime, quantum efficiency, intensity parameters, and the chromaticity diagram were evaluated. It was also detected the intrinsic emission of the matrix, and a mechanism of energy transfer involving activator ion and host network was proposed. The SG route produced red phosphors with a higher relative emission intensity that increases with increasing dopant percentage. The quantum efficiency values were above 50% for the phosphors via both methods, much higher than those reported in the literature. However, BaAl2O4:Eu3+ (1%) via MP method under oxidizing atmosphere exhibited anomalous emission of Eu2+, although unstable, which is discussed and associated to the effect of the precursors and the ion charge of the network. Based on all results, both methods result in the desired phase successfully, and produced promising red phosphors to be applied on optical devices.
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Palavras-chave
Luminóforo vermelho, Aluminatos de alcalinos terrosos, Terras raras, Estudo sistemático de metodologia, Eu2+ anômalo, Red phosphor, Alkaline earth aluminate phosphors, Rare earth, Systematic study of methodology, anomalous Eu2+