Novel luminescent yttrium oxide nanosheets doped with Eu³⁺ and Tb³⁺

Abstract

Novel single-layer rare-earth oxide nanosheets were obtained by the exfoliation of Y_(1−X)RE_XOBr (RE = Eu/Tb) layer. First, the Br⁻ ions in Y_(1−X)RE_XOBr were exchanged by benzoate ions under microwave conditions. The interlayer benzoate can chelate RE ions in the oxide layer, and change the structure and luminescence properties of the new layer compound. Then, a colloid of yttrium oxide nanosheets was obtained in n-butanol after the ultrasonic treatment of the benzoate-inserted layer for 30 min. The size and thickness of the yttrium oxide nanosheets were ∼800 nm and 0.8 nm, as measured by transmission electron and atomic force microscopies, respectively. A transparent thin film of the yttrium oxide nanosheets on indium tin oxide glass was prepared by electrophoretic deposition. The thin film showed strong red or green emission upon UV-light excitation based on Eu³⁺ and Tb³⁺ ions, respectively.

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1 Introduction

Layered rare-earth hydroxides (LRHs) were first prepared by Gandara et al. by the hydrothermal treatment of rare-earth (RE) nitrates below pH 6.5.¹ Because of the special electronic, optical, magnetic, catalytic, and medical properties of the RE elements,^2,3 LRHs have attracted considerable attention, and extensive efforts have been made for their synthesis, structural characterization, anion exchange, and exfoliation.⁴ Fogg and Sasaki^2,4,5 successfully synthesized a series of LRHs and studied their structural and photoluminescence (PL) properties. LRHs may be developed as a new type of luminescent materials. The as-synthesized Eu³⁺/Tb³⁺-doped LRHs show typical Eu³⁺ (red) and Tb³⁺ (green) emissions. However, they suffer from an inherent problem: Water molecules and hydroxyl groups are directly coordinated to the RE metal centres, thus drastically quenching the emissions.⁶ This disadvantage can be overcome by a post-treatment of calcination or by the direct synthesis of RE oxide nanosheets.

The structure of RE oxides,⁷ even if slightly modified by anions, is mainly composed of alternating layers of (REO)⁺ complex cations and Xⁿ⁻ anions (Cl⁻, Br⁻, MoO₄²⁻, PO₄³⁻, benzoate, etc.).⁸ The layered structure of RE oxides indicates that these materials exhibit two-dimensional (2D) electronic, magnetic, and luminescence behaviour.⁹

We envisioned that an appropriate anion, Xⁿ⁻, would provide the desired distance between the adjacent (REO)⁺ layers under microwave conditions. Thus, a new family of positively charged RE oxide nanosheets can be produced by the exfoliation of RE oxyhalides. In this study, we report for the first time the synthesis of RE oxide nanosheets using the abovementioned approach. Furthermore, Eu³⁺ and Tb³⁺ ions were doped to the nanosheets to develop novel efficient luminescent materials.

2 Results and discussion

2.1 Synthesis of Y_(1−0.05)RE_0.05OC₆H₅COO

The entire fabrication process is described in detail as follows: First, RE oxides, Y₂O₃ and Eu₂O₃/Tb₄O₇ were dissolved in nitric acid in an appropriate ratio (Y/Eu = 19 : 1). Then, homogenous RE oxalate precursors were obtained using oxalic acid as the precipitant. The calcination of precursors at 800 °C for 2 h in air afforded the corresponding oxides, Y_2(1−0.05)RE_0.1O₃ (RE = Eu/Tb). The Y_0.95RE_0.05OBr layered compounds were prepared by the solid-state reaction of the as-prepared RE oxide, Y_2(1−0.05)RE_0.1O₃, with ammonium bromide (NH₄Br). Four times excess of NH₄Br (M/Br = 1 : 4) was intimately mixed with the oxides Y_2(1−0.05)RE_0.1O₃ (RE = Eu/Tb), and first calcined at 450 °C for 2 h and then at 700 °C for 2 h in air to fabricate the Y_(1−0.05)RE_0.05OC₆H₅COO.

The as-synthesized Y_(1−0.05)RE_0.05OBr was added into a sodium benzoate solution (molar ratio, 1 : 2). After 10 min microwave treatment, the product obtained was separated by filtration, washed with deionized water, and dried to afford Y_(1−0.05)RE_0.05OC₆H₅COO.

2.2 Characterization of Y_(1−0.05)RE_0.05OC₆H₅COO

The anion exchange from Br⁻ to benzoate increased the basal spacing of the (001) plane from 0.825 nm to 1.89 nm, as shown by the X-ray diffraction (XRD) data (Fig. 1).

Fig. 1 X-ray powder diffraction patterns of (a) Y_(1−0.05)Ln_0.05OBr and (b) Y_(1−0.05)RE_0.05OC₆H₅COO.

Benzoates are well-known ligands for RE ions. The bonding between the interlayer benzoate anions and RE ions in the oxide layers was studied by Fourier transform infrared (FT-IR) spectroscopy. The FT-IR spectra of the novel layered compounds are shown in Fig. 2. A peak observed at ∼1594 cm⁻¹ can be attributed to the C–C stretching vibration in benzene. A peak observed at 704 cm⁻¹ can be attributed to the C–H absorption in benzene, and the strong absorptions at 1535 and 1407 cm⁻¹ can be attributed to the carboxylate stretching modes.¹⁰ All the absorptions can be attributed to the benzoate group. Furthermore, the FT-IR results indicate the interactions between the benzoate and RE ions. There are three common types of carboxylate coordination modes for metal ions: monodentate, chelating bidentate, and bridging bidentate. These coordination modes can be distinguished in IR spectra by the different separations between the carboxylate antisymmetric and symmetric stretching absorption bands (Δν). In general, the band separations are 350–500 cm⁻¹ for monodentate binding, 150–180 cm⁻¹ for bridging, and less than those for chelating.¹¹ After the ion exchange procedure, the separation value between the two bands was 128 cm⁻¹, indicating the “chelating binding” of carboxylate groups to RE ions in Y_(1−0.05)Eu_0.05OC₆H₅COO powders.

Fig. 2 Fourier transform Infrared spectra of Y_(1−0.05)Eu_0.05OC₆H₅COO.

The XRD results of indicate that this swollen structure exhibits long-range order; however, the interlayer interaction should have been significantly weakened. The ultrasonic treatment in a large amount of n-butanol might have imposed a transverse sliding force on the swollen phase, leading to the exfoliation of the layered material. Thus, the intercalated sample (0.1 g) was dispersed in 100 mL n-butanol. The turbid mixture gradually became a translucent colloidal suspension after 30 min ultrasonic treatment. Meanwhile, nitrogen gas was used to protect the sample from oxidation. After the unexfoliated residue was removed by centrifugation at 5000 rpm for 15 min, the resulting colloidal suspension was stable, and no sediment was observed upon standing for a long time. A clear Tyndall light scattering of the colloidal suspension was observed, as shown in Fig. 3a, indicating the occurrence of delamination.

Fig. 3 (a) Tyndall effect of translucent colloidal suspension, (b) transmission electron microscopy (TEM) of yttrium oxide nanosheets doped by Tb³⁺, and (c) TEM of yttrium oxide nanosheets doped by Eu³⁺.

The ultra-thin and homogeneous nature of the nanosheets was confirmed by the transmission electron microscopy (TEM) image shown in Fig. 3b and c. The sheets show a very faint contrast, demonstrating their unique thickness, and the insets of Fig. 3b and c show the selected area electron diffraction (SAED) patterns of individual nanosheets obtained during the TEM study. They display a set of well-arranged spots, which can be indexed to tetragonal Y_(1−0.05)RE_0.05OBr with a single-crystalline structure along the (001) zone axis. The inhibited growth direction of the nanosheets was established as (001) because the nanosheets with a large area lie along the Cu grid. This proved that the intralayer structure of the Y_(1−0.05)RE_0.05OBr maintained the original crystallinity during the exfoliation.

The TEM image cannot display the thickness of nanosheets. Therefore, the morphology of the as-obtained nanosheets was examined by atomic force microscopy (AFM). The AFM image in Fig. 4 shows 2D ultrathin sheets with lateral dimensions of up to several hundred nanometres, even though small amounts of fragments were also observed. The height profile scan indicates that the nanosheets are fairly flat with an average thickness of ∼0.8 nm, confirming a single layer of nanosheet. The thickness was measured at steps between a nanosheet and substrate surface; the upper part of the height profile indicated that a small nanosheet overlapped on another considering the basal spacing of REOBr. Therefore, it can be proposed that the nanosheets in the suspension consist of one to two monolayers, which are quite different from the normal nanosheets prepared by hydrothermal process.¹²

Fig. 4 Typical atomic force microscopy (AFM) image of the {Y_(1−0.05)RE_0.05O⁺}_n nanosheets deposited on Si substrate. The height profile along the line from X to Y is shown in the right panel.

Nanostructured materials usually possess special properties. The luminescent property of nanostructured RE materials is an interesting topic. The photoluminescent properties of two types of layered compounds and the as-prepared nanosheets were studied. Fig. 5A shows the excitation spectra of all the samples doped by Eu³⁺ ions, monitored within the ⁵D₀–⁷F₂ line at 616 or 620 nm. The high and broad bands near 285 nm are generated from the charge transfer between O²⁻ and Eu³⁺ ions in the oxide layer. The weak sharp peaks in the range 350–420 nm can be attributed to the f–f transitions of Eu³⁺ ion. Because the benzoate anion slightly affects these peaks, the excitation spectra of all the samples doped by Eu³⁺ ions are similar. Fig. 5B shows the excitation spectra of all the samples doped by Tb³⁺ ions monitored at 545 or 547 nm. Strong spin-allowed (low-spin, LS) and spin-forbidden (high-spin, HS) inter-configurational Tb³⁺ 4f–5d transition bands were observed consistent with the previous reports.¹³ Other weak peak lines can be attributed to the f–f transition of Tb³⁺ ions. The LS transition bands were much stronger than the HS bands in the spectra of the novel benzoate-inserted layered compounds and the corresponding yttrium oxide nanosheets, significantly different from that of YOBr doped by Tb³⁺ ions. This is probably because the HS and LS bands from the 4f–5d transitions are significantly affected by the coordination environment of Tb³⁺ ions. Therefore, the chelation of benzoate ions with the RE ions in the novel benzoate-inserted layered compounds and the corresponding yttrium oxide nanosheets causes the changes in the HS and LS bands.

Fig. 5 Excitation spectra of the samples doped by Eu³⁺ (A) or Tb³⁺ (B), different host: (a) YOBr; (b) YOC₆H₅COO; (c) yttrium oxide nanosheet suspension.

Because the PL spectra of RE ions are sensitive to their surroundings, they always act as structured probes. Herein, the room-temperature PL spectra of the nanosheets doped by Eu³⁺ and Tb³⁺ ions were studied and compared.

Fig. 6 shows the emission spectra of the samples doped by Eu³⁺ (A) and Tb³⁺ (B) upon the Eu–O charge transfer (286 or 285 nm) or f–d transition of Tb³⁺ (LS, 211 nm or HS, 249 nm) excitation, respectively. The emission spectra consist of the typical ⁵D₀–⁷F_J (J = 0–4) transitions of Eu³⁺ or the ⁵D₄–⁷F_J (J = 3–6) transitions of Tb³⁺.¹⁴ In the spectra of the novel benzoate-inserted layered compounds and the corresponding yttrium oxide nanosheets, a clear broadening and slight blue-shift of these peak lines were observed. These changes in the spectra also indicate the effects of the interlayer benzoate on the luminescent properties. The chelation of benzoate ions with RE ions changes the surrounding of the luminescence centres and causes the broadening of the blue-shifted peak lines slightly.

Fig. 6 Emission spectra of the samples doped by Eu³⁺ ions (A) or Tb³⁺ ions (B), different host: (a) YOBr; (b) YOC₆H₅COO; (c) oxide nanosheets suspension.

Both the results from excitation and emission spectra indicate the important effects of the coordinated benzoate ions on the PL properties. Herein, the strong ⁵D₀–⁷F₂ emission of the Eu³⁺-doped nanosheets and the ⁵D₄–⁷F₅ emission of the Tb³⁺-doped nanosheets indicate that they have great potential as red and green luminescent materials.

Thin films of RE compounds possess important applications in display devices. Because of the positive charge on the nanosheets, it is important to prepare luminescent thin films using yttrium oxide nanosheets doped with Eu³⁺ and Tb³⁺. Uniform thin films were fabricated by the electrophoretic deposition of the positively charged {Y_(1−0.05)RE_0.05O⁺}_n on an ITO substrate. The films show good fluorescence property (Fig. 7A) and good light transmission property in the visible region (Fig. 7B).

Fig. 7 (A) Red and green light emissions of the films obtained by the electrophoretic deposition under UV irradiation. (B) Transparency comparison among the indium tin oxide (ITO) glasses attached to the films of the nanosheets doped by (a) Eu³⁺, (b) Tb³⁺, and (c) blank.

Similarly, the UV-visible absorption spectra of the thin films were recorded. The curves in Fig. 8 indicate good transmittance for both the nanosheet thin films in the visible electromagnetic spectral region.

Fig. 8 Optical absorption spectra of the nanosheet thin films doped with (a) Eu³⁺ and (b) Tb³⁺.

3 Conclusions

In summary, we successfully exfoliated layered yttrium oxybromides Y_(1−0.05)RE_0.05OBr doped with Eu/Tb in n-butanol after the anion exchange between Br⁻ and benzoate ions under microwave conditions. To the best of our knowledge, this is the first synthesis of single-layer yttrium oxide nanosheets by the exfoliation method. Furthermore the effects of coordinated benzoate ions lead to the broadening and blue-shift of the peak lines in the emission spectra of the novel benzoate-inserted layered compounds and the corresponding yttrium oxide nanosheets. These nanosheets are promising building blocks for transparent thin films that exhibit strong red or green emission. This study provides a method to afford RE oxide nanosheets as a new member of the functional nanomaterial. In the future, we will explore other RE elements to develop efficient luminescent materials using this methodology.

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