Hydrothermal-assisted exfoliation of Y/Tb/Eu ternary layered rare-earth hydroxides into tens of micron-sized unilamellar nanosheets for highly oriented and color-tunable nano-phosphor films

Abstract

Efficient exfoliation of well-crystallized (Y0.96TbxEu0.04-x)2(OH)5NO3 · nH2O (0 ≤ x ≤ 0.04) layered rare-earth hydroxide (LRH) crystals into tens of micron-sized unilamellar nanosheets has been successfully achieved by inserting water insoluble oleate anions (C17H33COO−) into the interlayer of the LRH via hydrothermal anion exchange at 120°C, followed by delaminating in toluene. The intercalation of oleate anions led to extremely expanded interlayer distances (up to approximately 5.2 nm) of the LRH crystals and accordingly disordered stacking of the ab planes along the c-axis and also weakened interlayer interactions, without significantly damaging the ab plane. As a consequence, the thickness of the LRH crystals increased from approximately 1 to 10 μm, exhibiting a behavior similar to that observed from the smectite clay in water. Highly [111]-oriented and approximately 100-nm thick oxide films of (Y0.96TbxEu0.04-x)2O3 (0 ≤ x ≤ 0.04) have been obtained through spin-coating of the exfoliated colloidal nanosheets on quartz substrate, followed by annealing at 800°C. Upon UV excitation at 266 nm, the oxide transparent films exhibit bright luminescence, with the color-tunable emission from red to orange, yellow, and then green by increasing the Tb3+ content from x = 0 to 0.04.Electronic supplementary materialThe online version of this article (doi:10.1186/s11671-015-0828-0) contains supplementary material, which is available to authorized users.

General Synthesis of Layered Rare-Earth Hydroxides (RE = Sm, Eu, Gd, Tb, Dy, Ho, Er, Y) and Direct Exfoliation into Monolayer Nanosheets with High Color Purity

“…A clear Tyndall scattering effect was observed for the nanosheet suspension in the inset of Figure 5c, indicating the exfoliated nanosheets are in a well-dispersed state. Compared with the previous reports on LREHs nanosheets with sizes generally below 100 nm 37,39,46 or lacking a clearly identified sheet-like morphology, 36,38 the nanosheets obtained herein exhibit a larger size as well as a well-defined shape. More importantly, the thicknesses of more than 200 nanosheets of various rare-earth elements (Sm to Er and Y) were examined statistically, all featured with a thickness of ∼1.0 nm and ranges from 0.91 to 1.13 nm.…”

“…This is in stark contrast with the previously reported nanosheets of two or more layers and a corresponding thickness larger than 1.5 nm. 18,19,[35][36][37][38][39]46 The characterization results clearly confirmed that DS − -intercalated LREHs were exfoliated into monolayer nanosheets through a sonication in formamide for a short time (within 1 h). This is regarded as the first universal method developed for an exfoliation of layered hydroxides containing a large series of rare-earth ions ranging from Sm to Er and Y.…”

“…Though several rare-earth hydroxide nanosheets have been prepared in this way, the thickness was generally above 1.5 nm, which indicated that bi-or few-layer nanosheets were obtained other than a single-layer product. 18,36,38 Moreover, the exfoliated nanosheets were of a tiny size, generally smaller than 200 nm. 19,37,39 A systematic study to develop a universal method for attaining high-quality monolayer nanosheets with a direct exfoliation of LREHs is still of great importance.…”

“…Soft chemical procedures have been established for the successful exfoliation of various layered compounds including metal oxides, − layered double hydroxides (LDHs), − metal dichalcogenides, , and MAX phase compounds. − Similarly, rare-earth hydroxide nanosheets can be prepared by exfoliating their layered hosts in suitable solvents. In most cases, as typical inorganic counterions such as Cl – and NO 3 – are not desirable for a direct exfoliation of LREHs, ion-exchange or interlayer modification is required before exfoliation. ,,− As schematically illustrated in Figure a, interlayer ions (Cl – ) in synthesized LREHs can be substituted by larger organic moieties such as dodecyl sulfate anions (C 12 H 25 SO 4 – , DS – ), leading to a substantial interlayer gallery expansion from ∼0.8 to ∼2.6 nm . LREH nanosheets can be prepared by a subsequent swelling/delamination in a suitable solvent.…”

“…LREH nanosheets can be prepared by a subsequent swelling/delamination in a suitable solvent. Though several rare-earth hydroxide nanosheets have been prepared in this way, the thickness was generally above 1.5 nm, which indicated that bi- or few-layer nanosheets were obtained other than a single-layer product. ,, Moreover, the exfoliated nanosheets were of a tiny size, generally smaller than 200 nm. ,, A systematic study to develop a universal method for attaining high-quality monolayer nanosheets with a direct exfoliation of LREHs is still of great importance. In this aspect, formamide is well-known as an effective exfoliating solvent for layered hydroxides including LDHs and LREHs.…”