[(Y1−xGdx)0.95Eu0.05]2(OH)5NO3·nH2O (0 ≤ x ≤ 0.50) layered rare-earth hydroxides: exfoliation of unilamellar and single-crystalline nanosheets, assembly of highly oriented and transparent oxide films, and greatly enhanced red photoluminescence by Gd3+ doping

Abstract

Well crystallized layered rare-earth hydroxide (LRH) crystals of [(Y_1−xGd_x)_0.95Eu_0.05]₂(OH)₅NO₃·nH₂O (0 ≤ x ≤ 0.50), with a tens of micron-sized lateral, have been hydrothermally synthesized in the presence of the mineralizer NH₄NO₃. Smaller LRH particles, expanding in the ab plane, and a shrinked interlayer distance (c/2) were observed for a higher Gd content. The interaction between the adjacent hydroxide main layers of the LRH has been weakened by inserting water insoluble oleate anions into the interlayer via hydrothermal treatment. Unilamellar and single-crystalline nanosheets with lateral sizes close to those of the parent crystals and a thickness of ∼1.76 nm were delaminated by dispersing the oleate-containing LRH in toluene. Through a quasi-topotactic phase transformation, highly [111] oriented [(Y_1−xGd_x)_0.95Eu_0.05]₂O₃ (0 ≤ x ≤ 0.50) films with a transmittance of up to 81% and a thickness of ∼55 nm have been constructed through spin-coating colloidal suspension of the exfoliated nanosheets on quartz, followed by calcination at 800 °C. Gd³⁺ doping leads to greatly enhanced red emission of Eu³⁺ at 615 nm and obvious red shifting of the charge transfer excitation band (CTB) owing to the smaller electronegativity of Gd³⁺. The asymmetry factor of luminescence, I(⁵D₀ → ⁷F₂)/I(⁵D₀ → ⁷F₁), remains nearly constant at ∼4.9 up to x = 0.05, followed by a steady increase to ∼8.2 at x = 0.50, which has been ascribed to the splitting of the C₁ symmetry from distorted S₆ lattice sites at x > 0.05. Gd³⁺ doping has little influence on the fluorescence lifetime of the 615 nm emission, which was determined to be ∼1.6 ± 0.1 ms.