One-step freezing temperature crystallization of layered rare-earth hydroxide (Ln2(OH)5NO3·nH2O) nanosheets for a wide spectrum of Ln (Ln = Pr–Er, and Y), anion exchange with fluorine and sulfate, and microscopic coordination probed via photoluminescence

Abstract

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 SO₄²⁻ (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⁻/SO₄²⁻ 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⁺.