Propriedades ópticas e estruturais de hidróxidos duplos lamelares luminescentes contendo zinco, alumínio, európio e ligantes orgânicos
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Data
2017-03-10
Autores
Romero, João Henrique Saska [UNESP]
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Universidade Estadual Paulista (Unesp)
Resumo
O desafio da Química no século XXI é a obtenção de materiais promissores em aplicações e perspectivas inovadoras. Um ótimo candidato para estudos específicos é a síntese de materiais luminescentes bidimensionais. Entre as matrizes bidimensionais disponíveis, pode-se destacar os Hidróxidos Duplos Lamelares (HDLs) sintetizados no laboratório por rotas simples e de baixo custo, que têm como fórmula geral [M2+(1-x)M'3+x(OH)2](An-)x/n.zH2O (M, M' = íons metálicos que constituem as lamelas, e An- = ânion interlamelar). Preparou-se as amostras pelo método de coprecipitação e troca iônica. Na primeira etapa do trabalho otimizou-se os parâmetros de síntese. Após, obteve-se as amostras [Zn2Aℓ1-xEux(OH)6]A·zH2O-HDL (A = NO3- ou Cℓ- e x = 0,1 ou 1 ou 5 ou 10% em mol) e a fase formada é de HDL, conforme dados de XRD. O perfil de luminescência indica sítios de baixa simetria, emissão com alta pureza de cor e, que alta concentração de grupos –OH intensifica os processos não-radiativos diminuindo a eficiência quântica. Na segunda etapa do trabalho, as amostras [Zn2Aℓ1-xEux(OH)6]A·zH2O-HDL (A = bca (4-ácido-bifenilcarboxílico) ou acac (acetilacetona) ou tta (2-tenoiltrifluoroacetona) obtidas pelo método de coprecipitação não formam a estrutura lamelar. Assim, optou-se pelo método de troca iônica para as amostras [Zn2Aℓ1-xEux(OH)6]A·zH2O-HDL (A = bca, bpdc (4,4-ácido-bifenildicarboxílico), tta e acac e x = 0,1% em mol). A intercalação das espécies aniônicas na matriz HDL nas condições utilizadas é eficiente no caso do bca e é parcial para aos demais. O processo de luminescência é otimizado porque a intercalação dos ligantes orgânicos diminui os processos não-radiativos, e aumenta as possibilidades de transferência de energia. Também, a emissão com excitação por raios X é de maior intensidade. Finalmente, na última etapa, realizou-se a intercalação do complexo NH4[Eu(bca)4], constituído pelo ligante bca que apresentou as melhores propriedades estruturais e luminescentes. Dessa forma, os três sistemas [Zn2Aℓ1-xEux(OH)6]NO3-HDL x = 0,1% (I), [Zn2Aℓ1-xEux(OH)6]bca-HDL x = 0,1% (II) e [Zn2Aℓ(OH)6][Eu(bca)4]-HDL (III) são comparados. Com relação as propriedades luminescentes, os resultados são instigantes. No sistema (I) observa-se emissão com intensidade consideravelmente alta com a concentração em mol de Eu3+ muito pequena, 0,1%. Os grupos –OH são supressores do processo de luminescência, porém essa interferência é superada, até certo limite, com a intercalação do ligante bca (sistema II). No sistema (III) a luminescência é menos intensa que o sistema (II) e mais intensa que o sistema (I) e a simetria diminui em comparação ao complexo livre. A luminescência excitada por raios X ocorre nos três sistemas com diferentes danos por radiação e os sistemas apresentam alta estabilidade quando excitados com radiação ionizante. Enfim, a matriz HDL atua como um ambiente protetor para as espécies aniônicas orgânicas e de coordenação tornando-se candidatos, em potencial, combinando as contrapartes inorgânica (HDL) e orgânica (ligante) para um sistema de emissão promissor.
The challenge of chemistry in the 21st century has being the study of promising materials for applications and innovative perspectives. A great example is the synthesis of two-dimensional luminescent materials. Among the available structures, it can be highlighted Layered Double Hydroxides (LDH) synthetized in the laboratory by simple route and low cost. LDH have the general formula [M2+(1-x)M'3+x(OH)2](An-)x/n.zH2O (M, M' = layer metal ions, and An- = interlayer anion). The samples have been prepared by co-precipitation or ion exchange method. In the first stage, several experimental steps have performed to acquire the better synthesis parameters. Then, [Zn2Aℓ1-xEux(OH)6]A·zH2O-LDH (A = NO3- or Cℓ- and x = 0.1 or 1 or 5 or 10 at.%) samples were achieved according to XRD data. Luminescent properties indicate low-symmetry sites, emission with high color purity, and low quantum yield due to the high -OH groups concentration that enhances the non-radiative processes. In the second step, attempts to prepare [Zn2Aℓ1-xEux(OH)6]A·zH2O-LDH (A = bca (biphenyl-4-carboxylic acid) or acac (acetylacetone) or tta (2-thenoyltrifluoroacetone) by co-precipitation method did not lead to a LDH structure. Thus, it has been chosen the ion exchange method to prepare [Zn2Aℓ1-xEux(OH)6]A·zH2O-LDH (A = bca, bpdc (biphenyl-4,4′-dicarboxylic acid), tta e acac e x = 0.1 at.%) samples. Anionic species intercalation into LDH under the used conditions is efficient to the bca and is limited to the others. The luminescence processes were optimized because the organic ligands intercalation has decreased non-radiative processes, increasing the possibilities of energy transfer. Also, X-ray excited optical luminescence with higher than intensity are observed. Finally, in the last step, the NH4[Eu(bca)4] complex has been intercalated into LDH because bca has presented the better structural and luminescence properties. In this way, the three systems, [Zn2Aℓ1-xEux(OH)6]NO3-LDH X = 0.1 at.% (I), [Zn2Aℓ1-xEux(OH)6]bca-LDH x = 0.1 at.% (II) and [Zn2Aℓ(OH)6][Eu(bca)4]-LDH (III) are compared. The luminescent properties are instigating. System (I) show high emission with very small (0.1 at. %) Eu3+ concentration is observed. The bca intercalation/coordination increases de luminescence intensity of the doped LDH because decreases the -OH groups, luminescence suppressors by multi-phonon process, and increases the -* energy transfer processes (system II). The luminescence in the system (III) is less intense than the system (II) but more intense than system (I) and the symmetry is different from the free complex. X-ray excited optical luminescence occurs for all systems with different radiation damage. All rhe systems have high stability. Finally, the LDH structure acts as a protective environment for the anionic and coordination species. Thus, LDH systems become potential candidates combining the inorganic and organic properties for promising emission materials.
The challenge of chemistry in the 21st century has being the study of promising materials for applications and innovative perspectives. A great example is the synthesis of two-dimensional luminescent materials. Among the available structures, it can be highlighted Layered Double Hydroxides (LDH) synthetized in the laboratory by simple route and low cost. LDH have the general formula [M2+(1-x)M'3+x(OH)2](An-)x/n.zH2O (M, M' = layer metal ions, and An- = interlayer anion). The samples have been prepared by co-precipitation or ion exchange method. In the first stage, several experimental steps have performed to acquire the better synthesis parameters. Then, [Zn2Aℓ1-xEux(OH)6]A·zH2O-LDH (A = NO3- or Cℓ- and x = 0.1 or 1 or 5 or 10 at.%) samples were achieved according to XRD data. Luminescent properties indicate low-symmetry sites, emission with high color purity, and low quantum yield due to the high -OH groups concentration that enhances the non-radiative processes. In the second step, attempts to prepare [Zn2Aℓ1-xEux(OH)6]A·zH2O-LDH (A = bca (biphenyl-4-carboxylic acid) or acac (acetylacetone) or tta (2-thenoyltrifluoroacetone) by co-precipitation method did not lead to a LDH structure. Thus, it has been chosen the ion exchange method to prepare [Zn2Aℓ1-xEux(OH)6]A·zH2O-LDH (A = bca, bpdc (biphenyl-4,4′-dicarboxylic acid), tta e acac e x = 0.1 at.%) samples. Anionic species intercalation into LDH under the used conditions is efficient to the bca and is limited to the others. The luminescence processes were optimized because the organic ligands intercalation has decreased non-radiative processes, increasing the possibilities of energy transfer. Also, X-ray excited optical luminescence with higher than intensity are observed. Finally, in the last step, the NH4[Eu(bca)4] complex has been intercalated into LDH because bca has presented the better structural and luminescence properties. In this way, the three systems, [Zn2Aℓ1-xEux(OH)6]NO3-LDH X = 0.1 at.% (I), [Zn2Aℓ1-xEux(OH)6]bca-LDH x = 0.1 at.% (II) and [Zn2Aℓ(OH)6][Eu(bca)4]-LDH (III) are compared. The luminescent properties are instigating. System (I) show high emission with very small (0.1 at. %) Eu3+ concentration is observed. The bca intercalation/coordination increases de luminescence intensity of the doped LDH because decreases the -OH groups, luminescence suppressors by multi-phonon process, and increases the -* energy transfer processes (system II). The luminescence in the system (III) is less intense than the system (II) but more intense than system (I) and the symmetry is different from the free complex. X-ray excited optical luminescence occurs for all systems with different radiation damage. All rhe systems have high stability. Finally, the LDH structure acts as a protective environment for the anionic and coordination species. Thus, LDH systems become potential candidates combining the inorganic and organic properties for promising emission materials.
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Palavras-chave
Hidróxidos duplos lamelares, Európio, Luminescência, Troca iônica, Espectroscopia de luminescência com excitação por raios X