The effects of Gd3+ substitution on the crystal structure, site symmetry, and photoluminescence of Y/Eu layered rare-earth hydroxide (LRH) nanoplates

Abstract

Well crystallized nanoplates of the (Y_0.95−xGd_xEu_0.05)₂(OH)₅NO₃·nH₂O ternary layered rare-earth hydroxides (LRHs), synthesized hydrothermally, have been investigated with emphasis on the effects of Gd³⁺ substitution for Y³⁺ on the structural features and optical properties. Characterizations of the materials were achieved by the combined techniques of XRD, FT-IR, TEM, DTA/TG, and optical spectroscopies. The results showed that Gd³⁺ substitution leads to linearly expanded ab plane, shortened interlayer distance (c/2), and reduced hydration (smaller n value) of the crystal structure. As a consequence, the Ln³⁺ partially shifts from the C_4v to C₁ site symmetries and thus leads to systematically altered photoluminescence behaviors. Under the ⁷F₀→⁵L₆ transition excitation of Eu³⁺ at 394 nm, both the ⁵D₀→⁷F₂ to ⁵D₀→⁷F₄ and the 595 nm ⁵D₀→⁷F₁ to 590 nm ⁵D₀→⁷F₁ intensity ratios linearly increase towards a higher Gd³⁺ content. The incorporated Gd³⁺ cations selectively sensitize emission from the C₁-site Eu³⁺ and produce a new charge transfer (CT) excitation band at ∼254 nm. With this, the desired 615-nm red emission is obtainable either under intra-4f⁶ transition excitation of Eu³⁺ or by exciting the CT band. The materials have similar fluorescence lifetimes of 0.85 ± 0.05 ms for the 615-nm emission, irrespective of the Gd³⁺ content and excitation wavelength.