Synthesis of Luminescent La2O3:Eu3+ Nanosheet By Graphite Oxide Nanosheet Templating
Authors: Saito Ryota , Ayato Yusuke , Mochizuki Dai , Takenaka Sakae , Sugimoto Wataru 
Published: February 2020

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→7Fj (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 O2- 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