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.

Mass synthesis, without exfoliation, of Ln₂(OH)₅NO₃·nH₂O layered rare-earth hydroxide (LRH, n ∼ 1.2) nanosheets (down to ∼3 nm thick, in the form of hydrangea-flower like assemblies) via chemical precipitation at the freezing temperature of ∼4 °C was originally developed in this work for a wide spectrum of Ln (Ln = Pr, Nd, Sm, Eu, Gd, Tb, Dy, Y, Ho, and Er). The processing temperature was found to be low enough to effectively suppress the high activation-energy thickness growth of the LRH crystallites along the c-axis while high enough to crystallize the hydroxide main layers (the ab planes). Interlayer chemistry of the LRH nanosheets was explored via room temperature exchange of the interlayer NO₃ ⁻ (trigonal plane) with monovalent F⁻ (sphere) and divalent SO₄ ²⁻ (tetrahedron), and the basal spacing of the product was discussed from the geometric size/spatial orientation of the anions and also from anion–host interactions via electrostatic attraction and hydrogen bonding. Photoluminescence spectroscopies found substantially different behaviors for the anion exchanged products, characterized by the emergence of an additional charge transfer excitation band for LEuH and strong 4f⁸ → 4f⁷5d¹ inter-configurational excitation transition for LTbH, which have been ascribed to the generation of oxide-like coordination environments by the strong hydrogen bonding between the interlayer F⁻/SO₄ ²⁻ and the hydroxyls/water in the hydroxide main layers. Greatly enhanced green emission was observed for the anion exchanged LTbH upon excitation with the 4f⁸ → 4f⁷5d¹ rather than the intra-4f⁸ transitions of Tb³⁺.

Well-crystallized (Y0.97−xTb0.03Eux)2(OH)5NO3·nH2O (x = 0–0.03) layered rare-earth hydroxide (LRH) nanoflakes of a pure high-hydration phase have been produced by autoclaving from the nitrate/NH4OH reaction system under the optimized conditions of 100 °C and pH ∼7.0. The flakes were then converted into (Y0.97−xTb0.03Eux)2O3 phosphor nanoplates with color-tunable photoluminescence. Detailed structural characterizations confirmed that LRH solid solutions contained NO3− anions intercalated between the layers. Characteristic Tb3+ and Eu3+ emissions were detected in the ternary LRHs by selectively exciting the two types of activators, and the energy transfer from Tb3+ to Eu3+ was observed. Annealing the LRHs at 1100 °C produced cubic-lattice (Y0.97−xTb0.03Eux)2O3 solid-solution nanoplates with exposed 222 facets. Multicolor, intensity-adjustable luminescence was attained by varying the excitation wavelength from ∼249 nm (the charge transfer excitation band of Eu3+) to 278 nm (the 4f8–4f75d1 transition ...