Synthesis of Luminescent La₂O₃:Eu³⁺ Nanosheet By Graphite Oxide Nanosheet Templating

Rare earth oxides, when doped with another rare earth element, shows unique optical properties. For example, Eu³⁺-doped La₂O₃ (La₂O₃:Eu³⁺) is known as a red phosphor and has been used in various luminescent devices such as lasers, displays, etc. Processing of high quality thin films under ambient condition is desired for large-scale applications. In particular, exfoliated nanosheet colloids can be used to form high quality films by wet-chemical processing. The advantages of using nanosheet colloids includes, high crystallinity which is reflected from the starting layered crystal, ease of thickness control by layer-by-layer deposition, and room temperature fabrication. To obtain La₂O₃ nanosheets, we focus on a templating method using graphite oxide (GO) nanosheet.^1,2 This method can synthesize transition metal oxide nanosheets, for example, TiO₂, ZrO₂, NbO₂, SnO₂, and Ta₂O₅. In this study, we synthesized La₂O₃ nanosheets doped with a Eu³⁺by the GO templating method.

GO was synthesized from graphite by Hummer's method. GO was exfoliated in water by ultrasonication. This suspension was centrifuged and the precipitate was vacuum dried after removal of the supernatant, resulting in restacked GO nanosheet powder. The GO nanosheet powder was re-exfoliated in 2-methoxyethanol, and then La(OC₃H₇)₃ or a mixture of La(OC₃H₇)₃ and Eu(OC₃H₇)₃ was added. After reaction of the rare earth species with the GO nanosheets for 2 days, the product was purified by centrifugally collected by repeated washing with 2-methoxyethanol. The product was re-dispersed in 2-methoxyethanol, transferred to an autoclave, and solvo-thermally treated at 180˚C for 6 hours. The precipitated product was dried under vacuum and then heat-treated at 650˚C in air to give the final products La₂O₃ and La₂O₃:Eu³⁺ nanosheets.

TEM images of La₂O₃ and La₂O₃:Eu³⁺ nanosheet (Figure A and B) shows sheet like morphologies with several hundred nanometers in width. The SAED patterns of the samples (Figure A and B insets) show sharp diffraction spots with a six-fold symmetry, indicating that these nanosheets are single crystalline. All of the XRD peaks of the samples can be indexed based on the hexagonal La₂O₃ structure. The La₂O₃:Eu³⁺ nanosheet shows red luminescence attributed to ⁵D₀→⁷F_j (j=0,1,2,3,4) transition (Figure C). The excitation spectrum of the red luminescence shows two peaks centered at 280 and 227 nm. The strong peak at 280 nm is attributed to the charge transfer transition between O^2- and Eu³⁺. The weak peak at 227 nm is attributed to the host absorption. These results indicate that La₂O₃ nanosheet was doped with Eu³⁺. The Eu content in La₂O₃:Eu³⁺ nanosheet was 3.4 mol% measured by ICP. Two phosphorescent lifetimes of 1.05 and 0.25 ms were observed. The lifetime of 1.05 ms has been observed in previous research with similar of Eu³⁺ concentration³. The lifetime of 0.25 ms is very short, suggesting aggregation of Eu³⁺ species in La₂O₃nanosheet.

The authors thank Prof. S. Tsubaki and Prof. Y. Wada at Tokyo Inst. Tech. for fluorescence lifetime measurements. This work was partially supported by an Advanced Low Carbon Technology Research Development Program, JST-ALCA.

1) S. Takenaka, S. Uwai, S. Ida, H. Matsune and M. Kishida, Chem. Lett., 42, 1188 (2013).

2) S. Takenaka, S. Miyake, S. Uwai, H. Matsune and M. Kishida, J. Phys. Chem. C, 119, 12445 (2015).

3) S. Lee, S. Jang, J. Kang and Y. Sohn, J. Mater. Sci. Eng. B, 201, 35 (2015).

Figure 1