Photoluminescent properties of ion-exchangeable layered oxides

Photoluminescent properties of host-guest compounds based on ion-exchangeable layered oxides were studied. Cs₂Ti₂O₅, K₄Nb₆O₁₇, and KNb₃O₈ showed blue luminescence around 420-450 nm, whereas the luminescence of KTiNbO₅, CsTi₂NbO₇, K₂Ti₄O₉, Cs₂Ti₅O₁₁, and Cs₂Ti₆O₁₃ was observed around 500 nm. These results indicate that the two-dimensional sheets of titanate and niobate hosts were photoactive. The coordination state in layered structures is an important factor for the luminescent properties. The emission spectrum of KTiNbO₅ was red-shifted by replacing K⁺ between the TiNbO^-₅ layers by H⁺, showing that the energy structure of the host materials was changed by the guest intercalated between the layers. On the other hand, for layered perovskite oxides (K₂La₂Ti₃O₁₀ and KLaNb₂O₇) doped with rare earth ions in the perovskite layers, the luminescence of the guest rare earth ions doped was observed by photoexciting the host layered oxides. The host luminescence of the Pr³⁺-, Eu³⁺-, and Tb³⁺-doped layered perovskite oxides was completely quenched, suggesting the importance of redox properties. Tb³⁺ intercalated in the interlayers of Cs₂Ti₅O₁₁ also showed the luminescence with the host excitation. These results indicate that the excitation energy formed by exciting the host layered oxides migrates in the two-dimensional oxide layers and is trapped by the guest rare earth ions doped in the oxide layer and intercalated in the interlayer, resulting in the luminescence.