Luminescent properties of amorphous and crystalline materials for artificial lighting obtained via sol-gel type Pechini method

Full text: There is a growing interest in phosphorescent materials for generating artificial light as active medium for LEDs. However, these devices have yet certain limitations due to low emission in the red spectral region. Therefore, many efforts have been paid to materials doped with europium because the dual valence state that this atom can take, i.e., Eu²⁺ and Eu³⁺. In this work, we investigated Eu₂O₃ doped CaO-Al₂O₃ -SiO₂ -MgO nanocrystalline powders prepared by sol-gel Pechini method. The resultant xerogels were powdered and afterwards they were calcined in 10 g amounts in air at 700, 800, 900 and 1000 °C for 12 hours. The TGA curve indicated that for elimination of water and organic compounds the as prepared sample need to be calcined up to 540 °C. The XRD pattern as prepared sample showed the typical amorphous halo, whereas the sample calcined at 700 °C showed crystalline peaks being the most intense at 2θ = 29,33° due to SiO₂ phase, also additional crystalline phases were identified: AlMg, Si, MgO and CaSi₂ . The samples calcined at 1000 °C had the following phases Ca₂Al(AlSi)₂O₇ , 3CaOAl₂ O₃ and SiO₂ . The FTIR spectrum of unannealed powders showed absorption bands of C-H, H₂O and SiCH₃ . After calcination at higher temperatures these bands were eliminated and bands attributed to AlO₄ around 800 cm^-1, Si-O-Si, Si-O-Al and SiO₄ between 1500 – 500 cm^-1 were identified. The AFM measurements performed on the sample surfaces showed grains of about 100 nm. The luminescence spectrum presented Eu³⁺ typical emission peaks, being the most intense centered at 615 nm (red region) due transition ⁵D₀ → ⁷F₂ . Further peaks were also observed at 580, 595, 657 and 706 nm which are due transition ⁵D₀ → ⁷F₀ , ⁵D₀ → ⁷F₁ , ⁵D₀ → ⁷F₃ and ⁵D₀ → ⁷F₄ . These results suggests that such materials are candidates as red phosphors. References: [1] LIMA, S. M. et. al., Optical Society of America, 2012. [2] OLIVEIRA, L. H. et. al., J. of Alloys and Compd., 2013. (author)