Rare earth modification of smectites and their use in luminescent up-conversion emission

Abstract

The spectroscopic response of synthetic smectites after cation exchange with selected rare earth elements (REE) in the UV-VIS region was investigated for luminescence activity. Hectorite (Optigel SH) and Sumecton (Sumecton-SA), both swellable smectites, underwent modification through exchange reactions using Er³⁺ and equimolar mixture of Er³⁺/Yb³⁺ cations. REEs were chosen for their optical activity, stemming from multiple 4f-4f electron orbital levels and excitation/deexcitation transitions induced in the UV-VIS region. The optical responses of raw and modified smectites were verified after heating of exchanged samples up to temperatures of 400 °C, 800 °C, and 1200 °C for 1 h, to synthesize new optically active materials. Efficient up-conversion was observed in samples heated at 1200 °C, exhibiting characteristic green ∼525 and 550 nm, ²H_11/2/⁴S_3/2 → ⁴I_15/2 and red ∼660 nm, ⁴F_9/2 → ⁴I_15/12 emissions, due to new phases formed from smectite-REE matrices. The X-ray diffraction and scanning electron microscopy analysis confirmed the presence of the Er₂Si₂O₇ and/or Yb₂Si₂O₇ phase. The presence of REE cations induced homogeneous growth of ∼100 to 300 nm large crystals found by scanning electron microscopy in Sumecton, while larger crystal particle size distribution up to 5 μm particles were found for the REE modified Hectorite.