Drastic enhancement of photoluminescence properties from anion-pillared layered Y/Eu hydroxides with oxometallate species

Abstract

Layered rare earth hydroxides (LRHs) capable of anion exchange without altering the primary layer structure have garnered significant interest in various chemical domains. Specifically, by manipulating the type and concentration of anions, precise control can be exercised over the morphology, interlayer spacing, and photoluminescent behavior of LRHs. In this study, we successfully synthesized three distinct interlayer compounds. Anions containing various transition metal oxyanions exchanged with NO₃⁻ anions in the interlayer region, resulting in reduced or unaltered interlayer spacing. Photoluminescence spectra exhibited additional charge transfer bands (CTB). Under the CTB excitation, a symmetry transition of Eu³⁺ coordination from 9-fold (C₄ᵥ) to 8-fold (C₁) and anion exchange with MoO₄²⁻ and WO₄²⁻ reduced interlayer spacing (from 0.90 nm to 0.858 nm and 0.794 nm). The symmetry reduction led to the formation of Eu³⁺ activators with enhanced asymmetry, resulting in an 8.5-fold and 2.5-fold improvement in photoluminescence intensity. Furthermore, enhancement in fluorescence performance after the exchange of vanadate anions with LRH's NO₃⁻ can be attributed to the efficient transfer of absorbed energy by the vanadate anions to Eu³⁺ ions in LRHs, achieved through an antenna effect, ultimately increasing a 30.2-fold photoluminescent efficiency, despite maintaining the unaltered interlayer spacing and the original coordination symmetry. This study showcases the synthesis of high-quality layered compound materials using a hydrothermal approach, opening up new prospects for inserting a broader range of anion species into layered rare earth hydroxides and fabricating high-quality nanosheets.