Relationship between interlayer anions and photoluminescence of layered rare earth hydroxides

Abstract

The effect of interlayer anions on the photoluminescence of layered rare earth hydroxides was investigated with the rare earth (RE)-doped layered gadolinium hydroxynitrate as a representative base matrix for efficient and stable anion-exchange reactions. Eu³⁺, Tb³⁺, and Ce³⁺ were employed as RE activator ions for red, green, and blue emissions, respectively. The excitation and emission behaviors of Gd_1.80RE_0.20(OH)₅X·nH₂O (LGdH:RE) were systematically compared for various interlayer inorganic and organic anions, where X = F⁻, Cl⁻, I⁻, OH⁻, ClO₃⁻, S²⁻, CO₃²⁻, SO₄²⁻, terephthalate, 2-naphthoate, and dodecylsulfate members were obtained by the exchange reaction of corresponding X = NO₃⁻ members. Interestingly, a close relationship was found between the UV-Vis absorption spectra of aqueous solutions containing X anions and the excitation behavior of LGdH:RE. Thus, NO₃⁻, I⁻, and S²⁻ anions showing high absorbance in aqueous solution consistently shielded the excitation light for the ⁸S_7/2 → ⁶I_J transition of Gd³⁺ and the 4f → 5d interconfigurational transitions of Tb³⁺ and Ce³⁺ to turn off the corresponding emissions from LGdH:Eu, LGdH:Tb, and LGdH:Ce. In contrast, the effect of terephthalate and 2-naphthoate, despite high absorbance in aqueous solutions, was significantly different depending on the RE activator ion. It is proposed that an identical interlayer anion in the gallery of LRHs can filter or sensitize the UV energy for excitation of RE³⁺-doped LRHs and its role (whether as a filter, a sensitizer, or just a spacer) is determined by the nature of activator ions.