Photoluminescence spectral change in layered titanate oxide intercalated with hydrated Eu³⁺

A number of interesting photoluminescence properties of titanate layered oxide intercalated with hydrated Eu³⁺ have been demonstrated. Photoluminescence intensity of Eu³⁺ decreased rapidly with time during irradiation by UV light having energy higher than the band gap energy of the host TiO (T_1.81.siO₄) layer. This is presumably due to the decrease in energy transfer from the host TiO layer to Eu³⁺ as a result of the change in the hydration state of water molecules surrounding Eu³⁺, which is caused by the hole produced in the TiO valence band. When irradiation was discontinued, the emission intensity gradually recovered. The recovery time increased when the water in the interlayer is removed by heat treatment. This indicates that the state of interlayer water changes during irradiation and returns to its initial state after discontinuation of irradiation. The excitation spectra changed drastically at any given wavelength upon irradiation with UV light. A comparison of the excitation spectra before and after irradiation reveals that only the excitation peak at around the irradiation wavelength decreased upon irradiation, as in the case of spectral hole burning. The hydration state of water molecules surrounding Eu³⁺ presumably changes depending on the irradiation wavelength, leading to the above spectral change because the Eu/TiO film has a superlattice structure producing holes with different energies.