Synthesis and photoluminescence properties of nanostructured mullite/α-Al2O3

Phase transformation of α-Al2O3 to mullite was observed by correlated X-ray diffraction and low-temperature (7 K) time- and energy-resolved photoluminescence methods. The aluminosilicate solids with different Si:Al ratios were prepared after thermal treatment (1400 °C) of ultraporous alumina (UPA) monoliths impregnated with vapour of silica precursors at room temperature. The initial dispersion of Si is homogeneous on a size-scale of 5 nm raw UPA fibres, which grow in size during the thermal treatment stage up to ∼50–100 nm. Three phases were resolved in the considered SiO2/Al2O3 system: silica, α-Al2O3 and their interaction product 2:1 mullite (2Al2O3:SiO2). The 2:1 mullite mass increases and α-Al2O3 mass decreases with an increase of the initial silica content. At the SiO2 content in the initial system above 20 mol.%, the phase transformation to 2:1 mullite is complete and no α-Al2O3 was observed. No mullite phases with either lower or higher Si content were observed. The fundamental absorption onset energy 7.55 eV of 2:1 mullite was measured.