Publicação
Synthesis and characterization of new layered and microporous photoluminescent silicates
| Resumo: | This thesis reports the synthesis, structural characterization and photoluminescence properties of new layered and microporous rare-earth silicates. These materials have been characterized by powder and single crystal Xray diffraction, scanning electron microscopy, thermal analysis, elemental analysis, nuclear magnetic resonance, infrared, Raman and photoluminescence spectroscopies. The crystalline rare-earth silicates were obtained by hydrothermal synthesis at moderate temperatures and pressures. Layered rare-earth silicates AV-22, K3[(RE)Si3O8(OH)2], RE3+=Y3+, Eu3+, Er3+ ,Tb3+, Gd3+ and Ce3+ are host-guest systems suitable for engineering multifunctional materials with tuneable properties. The structure of AV-22 was solved by single crystal X-ray diffraction. The Tb- and Eu-AV-22 samples are visible emitters (green and red, respectively) with output efficiency comparable to that of standards used in commercial lamps, while Er-AV-22 is a roomtemperature infrared phosphor. The incorporation of Ce3+ and Tb3+ in the same layered silicate induces and effective Ce3+ to Tb3+ energy transfer channel. Upon calcination at 650 ºC, AV-22 materials undergo a phase transformation to small-pore framework AV-23, K3[RESi3O9], RE3+=Y3+, Eu3+, Er3+ and Tb3+. The radiance values of Tb-AV-23 and standard Tb3+ green phosphors (Gd2O2S:Tb) are similar. The calcination process increases the intensity of the Er3+ emission (essentially due to the removing of OH groups) and the importance of the Er3+-Er3+ interactions as a quenching emission channel. Microporous lanthanide silicates AV-24, K7Ln3Si12O32⋅3H2O, Ln3+=Eu3+, Sm3+ ,Tb3+ and Gd3+ are the first reported to contain Ln-O-Ln dimmers isolated in a siliceous matrix and exhibiting a unique emission feature: the lifetime of the 5D0 excited state is remarkably long, ca. 10 ms at 10 K. The structure of all these silicates allows the inclusion of a second (or even third) type of Ln3+ ion in the framework and, therefore, the fine-tuning of their photoluminescence properties. |
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| Autores principais: | Kostova, Mariya Hristova |
| Assunto: | Química Fotoluminescência Silicatos microporosos Terras raras |
| Ano: | 2007 |
| País: | Portugal |
| Tipo de documento: | tese de doutoramento |
| Tipo de acesso: | acesso aberto |
| Instituição associada: | Universidade de Aveiro |
| Idioma: | inglês |
| Origem: | RIA - Repositório Institucional da Universidade de Aveiro |
| Resumo: | This thesis reports the synthesis, structural characterization and photoluminescence properties of new layered and microporous rare-earth silicates. These materials have been characterized by powder and single crystal Xray diffraction, scanning electron microscopy, thermal analysis, elemental analysis, nuclear magnetic resonance, infrared, Raman and photoluminescence spectroscopies. The crystalline rare-earth silicates were obtained by hydrothermal synthesis at moderate temperatures and pressures. Layered rare-earth silicates AV-22, K3[(RE)Si3O8(OH)2], RE3+=Y3+, Eu3+, Er3+ ,Tb3+, Gd3+ and Ce3+ are host-guest systems suitable for engineering multifunctional materials with tuneable properties. The structure of AV-22 was solved by single crystal X-ray diffraction. The Tb- and Eu-AV-22 samples are visible emitters (green and red, respectively) with output efficiency comparable to that of standards used in commercial lamps, while Er-AV-22 is a roomtemperature infrared phosphor. The incorporation of Ce3+ and Tb3+ in the same layered silicate induces and effective Ce3+ to Tb3+ energy transfer channel. Upon calcination at 650 ºC, AV-22 materials undergo a phase transformation to small-pore framework AV-23, K3[RESi3O9], RE3+=Y3+, Eu3+, Er3+ and Tb3+. The radiance values of Tb-AV-23 and standard Tb3+ green phosphors (Gd2O2S:Tb) are similar. The calcination process increases the intensity of the Er3+ emission (essentially due to the removing of OH groups) and the importance of the Er3+-Er3+ interactions as a quenching emission channel. Microporous lanthanide silicates AV-24, K7Ln3Si12O32⋅3H2O, Ln3+=Eu3+, Sm3+ ,Tb3+ and Gd3+ are the first reported to contain Ln-O-Ln dimmers isolated in a siliceous matrix and exhibiting a unique emission feature: the lifetime of the 5D0 excited state is remarkably long, ca. 10 ms at 10 K. The structure of all these silicates allows the inclusion of a second (or even third) type of Ln3+ ion in the framework and, therefore, the fine-tuning of their photoluminescence properties. |
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