Investigation of Structural and Luminescent Properties of Sol-Gel-Derived Cr‑Substituted MgAlCr Layered Double Hydroxides.

In the present work, Cr-substituted MgAlCr layered double hydroxides (LDHs) were synthesised through the phase conversion of sol-gel-derived mixed-metal oxides in an aqueous medium. The chromium substitution level in the range of 1 to 25 mol% was investigated. It was demonstrated that all synthesised specimens were single-phase LDHs. The results of elemental analysis confirmed that the suggested synthetic sol-gel chemistry approach is suitable for the preparation of LDHs with a highly controllable chemical composition. The surface microstructure of sol-gel-derived MgAlCr LDHs does not depend on the chromium substitution level. The formation of plate-like agglomerated particles, which consist of hexagonally shaped nanocrystallites varying in size from approximately 200 to 300 nm, was observed. Optical properties of the synthesised MgAlCr LDHs were investigated by means of photoluminescence. All Cr-containing powders exhibited characteristic emission in the red region of the visible spectrum. The strongest emission was observed for the sample doped with 5 mol% Cr ions. However, the emission intensity of samples doped with 1-10 mol% Cr ions was relatively similar. A further increase in the Cr ion concentration to 25 mol% resulted in severe concentration quenching.