Kazuyoshi Izawa

Dynamic Control of Photoluminescence for Self‐assembled Nanosheet Films Intercalated with Lanthanide Ions by Using a Photoelectrochemical Reaction

Semiconductor oxide nanosheets synthesized by exfoliation of layered oxides are two-dimensional crystals with a thickness of about 1 nm. New layered materials and their films can be reassembled by electrostatic self-assembly deposition (ESD) and by layer-by-layer (LBL) techniques, respectively. Since the nanosheets have a negative charge in aqueous solution they can be used with various cationic species as the starting materials. Layered materials prepared from nanosheets and lanthanide (Ln) ions are promising as new functional materials because Ln ions have unique properties, such as luminescence and magnetic properties, that are attributable to the 4f electron orbital. For example, the titanate layered oxide intercalated with Eu ions prepared from titanate nanosheets and Eu ions has unique luminescence properties. The layered oxide gives a red emission from the Eu ions which is induced by energy transfer through excitation of the bandgap of the titanate nanosheet, and the emission from the Eu ions is promoted by intercalated water molecules. Furthermore, spectral hole burning caused by the intercalated water molecules was observed in the excitation spectra at room temperature. Nanosheets of TiOx, NbOx, and TaOx give a high photocurrent during the photoelectrochemical reaction under UV illumination with an energy higher than that of the bandgap. This finding indicates that a large charge separation is produced between the holes in the valence band and the electrons in the conduction band during excitation of the bandgap. Consequently, layered oxide materials intercalated with Ln ions simultaneously exhibit both photoluminescence and a photoelectrochemical reaction during excitation of the bandgap on illumination with UV light. The study reported herein demonstrates a new form of dynamic control over the photoluminescence of Ln ions intercalated in self-assembled nanosheet films of TiOx and NbOx. The photoluminescence properties of Ln ions are changed by factors such as a change in the pH value and the addition of anionic species. However, it is difficult to dynamically control the photoluminescence properties of Ln ions. In the present system, the emission intensities of the intercalated Eu and Tb ions can be readily controlled by varying the applied potential. The nanosheet/Ln (Ti1.81O4 nanosheet/Eu 3+ (TiO/Eu) and Nb6O17 nanosheet/Tb 3+ (NbO/Tb)) films were prepared and fixed on boron-doped diamond electrodes by the LBL technique. The chemical compositions of the TiO/Eu and NbO/Tb films were EuxTi1.81O4 (x = 0.20–0.30) and TbyNb6O17 (y= 1.30–1.50), respectively. These are close to the theoretical neutral compositions (Eu0.25Ti1.81O4 and Tb1.33Nb6O17). The Ln ions were sandwiched between nanosheets (see Figure S-1 in the Supporting Information). Figure 1 shows a sche-

Synthesis and Photoluminescence Properties of Niobate Layered Oxides Intercalated with Rare Earth Ions by Electrostatic Self-Assembly Methods

The niobate layered oxides intercalated with Tb3+ or Eu3+ ions were prepared by the electrostatic self-assembly deposition method. The Tb3+ ions in the interlayer exhibited green luminescence by energy transfer from the host NbO6 layer to the guest Tb3+ ions, which was based on the niobate nanosheet band gap excitation. The Tb3+ emission intensity decreased with decreasing interlayer water molecules, indicating that the presence of interlayer water molecules is inevitable for the Tb3+ emission assigned to the energy transfer. On the other hand, the emission intensity of Eu3+ in the interlayer was weaker than that of Tb3+.

A New Approach for the Preparation, Exfoliation, and Intercalation Properties of Niobium Layered Sulfide Material

We have developed the new process that can synthesize a layered niobium sulfide under heating a layered oxide with H2S/N2 gas mixture. It was confirmed that K0.34(H2O)xNbS2 was obtained as a single phase and high crystallinity under heating K4Nb6O17 · 3H2O with the gas at 750°C for 10 h and washing with distilled water. The layered sulfide had the large plate‐like shapes that have 50 µm wide length on average. Potassium ions in the interlayer of K0.34(H2O)yNbS2 could be exchanged to hydronium ions by stirring 2M H2SO4. It was also confirmed that tetrabutylammonium could intercalate in interlayer of proton exchanged form. In addition, colloidal solution was obtained after ultrasonication of proton exchanged form. According to the AFM image, the thickness of sulfide nanosheets had 4.2 Å, which roughly corresponded with the thickness of NbS2 single layer.