Energy transfer between rare earths in layered rare-earth hydroxides

Abstract

Energy transfer between rare earths in layered rare-earth hydroxides (LRHs) is worth the intensive study because the hydroxyls that act as the bridge connecting the neighbouring rare earths would generate non-radiative transitions. This study focuses on the energy transfer in the intralayer and the adjacent layers of LRHs. A series of LEu_xTb_1−xHs (x = 0, 0.05, 0.2, 0.5, 0.8, and 0.95) was synthesized, the basal spacing (d_basal) was adjusted from 8.3 to 46 Å through ion-exchange process, and unilamellar nanosheets were prepared through a delamination process. The luminescence behaviours of the samples demonstrated the following: (1) for the delaminated nanosheets, the quenching effect of both Eu³⁺ and Tb³⁺ was hardly observed. This implies that in the intralayer, the efficiency of energy transfer is extremely low, so that highly-concentrated co-doping does not influence the luminescence and by controlling the Eu/Tb molar ratio, white light can be obtained. (2) For small d_basal, e.g., 27 Å, the fluorescence quenching of Tb³⁺ and Eu³⁺ was remarkable, while for large d_basal, e.g., 46 Å, the emission of Tb³⁺ emerged and the self-quenching between Eu³⁺ ions weakened. (3) The energy transfer efficiency deceased with an increase in the distance between adjacent layers. In other words, either the energy transfer between Eu³⁺ and Tb³⁺ or the energy migration between Eu³⁺ ions was more efficient when they were located in adjacent layers than in intralayers even when they were the nearest neighbours.

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Introduction

Rare earths, owing to their attractive optical properties arising from the 4f electronic shells,¹ play an important role in the development of optical materials. Because of the extremely close radii, it is easy to dope several types of rare earth ions into the same host matrix to realize the tuneable emission colour of luminescence materials.² Simultaneously, due to the interaction effects among rare earths,³ the luminescence intensities of a particular rare earth would be enhanced or quenched with the co-doping of other rare earths.⁴ For example, the investigation on energy transfer between Eu³⁺ and Tb³⁺ in Tb₂(WO₄)₃:Eu³⁺ indicated that with an increase of the doped Eu³⁺ concentration, the emission of Tb³⁺ became weaker, accompanying the concentration quenching of Eu³⁺.⁵ Such behaviour was also extensively investigated in many other solid phases, such as TbBO₃,⁶ NaTbF₄:Eu,⁷ SrTiO₃,⁸ and TbPO₄.⁹

Layered rare earth hydroxides (LRHs), with a general formula of Ln₂(OH)_6−mA_m·nH₂O (0.5 ≤ m ≤ 2.0, where Ln stands for trivalent rare earths and A for interlayer anions),¹⁰ are attracting increasing attention since Gándara et al. reported their findings in 2006.¹¹ OH⁻ groups (and H₂O in some cases of m) coordinate with Ln to form polyhedra and act as the bridge connecting the neighbouring rare earths to form the layer; the layers stack to form the layered structure with interlayer gallery containing A and water. Recently, intercalation, ion exchange, and delamination into 2D crystals have been intensively studied due to LRHs' unique physicochemical properties.¹² LRHs can be also used as an ideal precursor to produce oxide phosphors¹³ and the up-conversion host matrix β-NaYF₄,¹⁴ bringing new opportunities for LRHs in practical applications. However, only a few studies on energy transfer in LRHs have been reported. Li et al.¹⁵ found that for L(Y_0.97−xTb_0.03Eu_x)Hs (type m = 1), with an increase of Eu³⁺ concentration, the emission of Tb³⁺ became weaker, suggesting that the energy transferred from Tb³⁺ to Eu³⁺; the same result was observed in PMMA@SA-LGdH:Tb_0.5−xEu_x (x = 0–0.50).¹⁶ Although it is well known that the coordinated OH⁻/H₂O would generate efficient non-radiative emission for the de-excitation of rare earths,¹⁷ this effect of OH⁻/H₂O on energy transfer between the rare earths in LRHs has not been studied extensively. Furthermore, this effect and the structural features of LRHs arouse another issue: the energy transfer between the adjacent layers is different from that in the intralayer due to the layered structure.

To clarify these issues, in this study, we synthesized a series of LEu_xTb_1−xHs (x = 0–0.95) and changed the interlayer distances to a large extent. According to the photoluminescence investigation results, we propose a model for the energy transfer in LRHs.

Experimental

Synthesis of LEu_xTb_1−xHs (x = 0, 0.05, 0.2, 0.5, 0.8, and 0.95)

Eu₂O₃ and Tb₄O₇ were obtained from Shanghai Aladdin Bio-chem Technology Co., Ltd.; sodium dodecyl sulfate (NaDS), from Tianjin Bodi Chemical Co., Ltd; sodium oleate (NaOA), from J&K Scientific Co., Ltd.; and hexamethylenamine (HMT) and the other reagents from Xi Long Chemical Co., Ltd. Eu/Tb nitrate or hydrochloride was prepared from Eu₂O₃/Tb₄O₇ and the corresponding acid by recrystallization.

Different A anion intercalated LRHs are referred as to A-LRHs below. Cl⁻- and NO₃⁻-LEu_xTb_1−xHs (x = 0, 0.05, 0.2, 0.5, 0.8, and 0.95) were prepared through homogenous precipitation method.¹⁸ Briefly, the rare-earth salts were added into an aqueous solution of sodium salt (NaCl or NaNO₃) and HMT at the molar ratios of (Eu + Tb) salts/sodium salt/HMT = 1/13/1, and then heated in a Teflon-lined stainless steel autoclave at 90 °C for 12 h. After air-cooling to room temperature, the resultant solid was centrifuged, washed with distilled water and then with anhydrous ethanol several times, and finally dried at 40 °C overnight.

DS⁻-LRHs were obtained by treating NO₃⁻-LRHs (0.1 g) in NaDS (0.3708 g, keeping the molar ratio of DS⁻/NO₃⁻ = 3) aqueous solution (80 mL) via the DS⁻/NO₃⁻ ion exchange. After autoclaving at 70 °C for 2 days, the resultant solid was collected after filtering. Then, the solid was washed with deionized water and dried at 40 °C for 24 h. In addition, via the same method, OA⁻-LRHs were prepared by mixing 0.1 g NO₃⁻-LRHs and 1.4982 g NaOA (OA⁻/NO₃⁻ = 12) in 80 mL deionized water and stirring for 24 h.

Synthesis of nanosheets of LRHs

The nanosheets (2D crystals) of the LRHs, noted as NSs-LRHs, were obtained by delamination of the DS⁻-LRHs samples (0.1 g) in formamide (50 mL)¹⁸ after being heated at 40 °C for 3 days. The nanosheets existed in formamide as a colloidal suspension.

Characterization

Powder X-ray diffraction (XRD) measurement was performed with a Phillips X'Pert Pro MPD diffractometer with Cu-K_α radiation (λ = 0.1541 nm) at room temperature. The generator setting was 40 kV and 40 mA, with a step size of 0.017° and a scan time of 10 s per step. For a Rietveld refinement using the RIETAN-FP software,¹⁹ the XRD patterns were collected at room temperature with step size of 0.017°, scan time of 150 s per step, and 2θ ranging from 4.5° to 120°. At the final refinement, the temperature-factors of H₂O were set to be equal to those of OH⁻.

Fourier-transform infrared (FT-IR) spectra were recorded on a Nicolet-380 Fourier-transform infrared spectrometer using the KBr method. Eu and Tb contents were analysed by ICP atomic emission spectroscopy (Jarrell-ASH, ICAP-9000) after the solid samples dissolved in dilute HNO₃ solution, and C, H and N contents were determined by Elementar Vario Elemental analyzer. Atomic force microscope (AFM) observation was carried out in a Veeco NanoScope IIIA microscope after the sample was deposited on a silicon wafer substrate.

Photoluminescence measurements were performed on a Shimadzu RF-5301PC spectrofluorophotometer equipped with a 150 W xenon lamp as the excitation source using monochromator slit widths of 5 nm for NSs-LRHs on both excitation and emission sides and 3 nm for the other samples. The luminescence decay curves were obtained by using Steady State & Time-resolved Fluorescence Spectrometer TemPro-01. All measurements were carried out at room temperature.

Results and discussion

Interlayer spacings and intralayer structure

Fig. 1A depicts the typical XRD patterns of the samples with different interlayer spacings of LEu_0.5Tb_0.5H. The arrangements of the interlayer species are shown in Fig. 1B. The diffraction patterns of the Cl⁻- (Fig. 1A(a)) and NO₃⁻-LEu_0.5Tb_0.5H (Fig. 1A(b)) samples can be well indexed to orthorhombic (a = 12.874(1), b = 7.299(0), c = 8.439(4) Å) and monoclinic (a = 12.849(0), b = 7.119(1), c = 16.355(0) Å, β = 94.88(1)°) symmetries, respectively, as previously reported,²⁰ but the two samples have similar basal spacing (d_basal) values (8.32 and 8.31 Å, respectively). Larger d_basal values are displayed in the samples with different organic anion galleries: ∼25 Å for DS⁻ (Fig. 1A(c)) and ∼46 Å for OA⁻ (Fig. 1A(d)). The latter value was also observed in OA⁻-LEuH and OA⁻-LTbH.²¹ The d_basal values can be construed according to the dimensions and the area per unit charge (S_charge) of the layers and the organic ions.²² S_charge values are 0.51 nm² for OA⁻ as calculated using the ChemOffice software,²³ 0.19 nm² for DS⁻,¹⁸ and 0.23 nm² for the layer calculated according to the lattice parameters of Cl⁻-LEu_0.5Tb_0.5H. The length is ∼22 Å for oleate²¹ and 18.2 Å for DS⁻ and the thickness of the layer is 6.5 Å.¹⁸ Thus, for the OA⁻-LRH samples, the larger S_charge(OA⁻) value compared to S_charge (layer) implies a bilayer arrangement of OA⁻ anions (Fig. 1B), which is in agreement between the predicted d_basal of (2 × 22 + 6.5=) 50.5 Å and the measured value of ∼46 Å. For the same reason, the S_charge(DS⁻) value are smaller than S_charge(layer) value, which allows an alternating antiparallel monolayer arrangement of DS⁻ (Fig. 1B), which predicts a d_basal value of (18.2 + 6.5=) 24.7 Å, agreeing with the observed value of 25.3 Å (Fig. 1A(c)). The thickness of NSs-LEu_0.5Tb_0.5H was measured as 1.57 nm (Fig. 1C), indicating that the unilamellar were obtained.

Fig. 1 (A) XRD patterns of (a) Cl⁻-, (b) NO₃⁻-, (c) DS⁻- and (d) OA⁻-LEu_0.5Tb_0.5H, (B) the schematic representation of the synthesis processes and the d_basal for the samples, and (C) AFM images of NSs-LEu_0.5Tb_0.5H, with the height profiles along the lines in the bottom panel.

Altering x in LEu_xTb_1−xHs invokes barely remarkable d_basal changes for the entire Cl⁻- or NO₃⁻-type samples as shown in the XRD patterns (Fig. S1, S2 and Table S1, ESI†). Moreover, the discrepancy of ∼0.1 Å, changing irregularly with x, could be explained by the very close natures of Eu³⁺ and Tb³⁺ ions. Conversely, the large difference of ∼1.0 Å in d_basal for the organic-type samples (Table S1, and Fig. S3, S4, ESI†) indicates the easy deformation of organic interlayer species. The chemical analysis for the Cl⁻- (Table S2, ESI†) and NO₃⁻-LRHs samples (Table S3, ESI†) indicates good agreements between the formulae of m = 1 type LRHs (i.e. Ln₂(OH)₅(A^x)_1/x·nH₂O (ref. 10)) and the analysis results. For all the samples, the interlayer species except for Cl⁻ are confirmed by the FT-IR spectra (Fig. S5–S7, ESI†). For DS⁻- and OA⁻-LEu_0.5Tb_0.5H, a scanning electron microscopy/energy-dispersive X-ray (SEM/EDX) observation indicated that each element, including Eu, Tb, C, and O (and S for DS⁻-LEu_0.5Tb_0.5H), was distributed uniformly in the samples (Fig. S8, ESI†).

In m = 1 type LRHs, two types of polyhedra, [Ln(OH)₇(H₂O)] and [Ln(OH)₈(H₂O)] (noted as LnO₈ and LnO₉, respectively), should exist. A LnO₈ polyhedron links to two other LnO₈ polyhedra and four LnO₉ polyhedra via edge-sharing, forming the host layers.^20a Eu³⁺ or Tb³⁺ ions in the space group P2₁2₁2 have three Wyckoff sites, which are 2a, 2b and 4c, corresponding to LnO₈, LnO₈ and LnO₉, respectively. In the co-doping case, the smaller ion Tb³⁺ (0.92 Å), rather than Eu³⁺ (0.95 Å),² tends to be coordinated in LnO₈,²⁴ but the radial difference is too small to determine the distribution. As shown in Fig. 2A, three distribution models are proposed to conduct Rietveld refinements for Cl⁻-LEu_0.5Tb_0.5H using the analysed composition of Eu_0.51Tb_0.49(OH)_2.45Cl_0.42(CO₃)_0.06·0.87H₂O (Table S2, ESI†). The refinement result based on the model of a random distribution (Fig. 2A(a)) is shown in Fig. 3 and Table 1. The calculated pattern is in good agreement with the measured pattern (Fig. 3); the R-factors (Table 1), the refined crystal model, and 3D electron density distribution image (Fig. 2B) are satisfactory. However, employing the other two models (Fig. 2A(b) and A(c)) leads to slight differences, including those in R-factors (Fig. S9, S10 and Table S4, S5, ESI†). It could be concluded that it is difficult to use the XRD technique to determine which model is preferred because the two rare earths have slight differences in radius and atomic scattering factor.²⁵ It should be noted that we also combined the fluorescent theoretical calculation to determine the possible distribution of Eu³⁺ and Tb³⁺ in the layers, but in the system, it is difficult for LRHs to be calculated using Gaussian 09 programs.²⁶ Fig. 4 presents the lattice parameters, calculated by the Rietveld refinements (Fig. S11–S14 and Table S6–S9, ESI†) as a function of Eu³⁺ contents. Both a and b increase linearly with the increase in Eu³⁺ content, thus following Vegard's law.²⁷ This implies that continuous solid solutions were formed; the same has been reported in LRHs of Y/Eu,^13b,28 Y/Tb²⁹ and Eu/Gd,³⁰ which can be explained by the slight distortion energy of the crystal lattice.²⁹

Fig. 2 For Cl⁻-LEu_0.5Tb_0.5H with the space group P2₁2₁2, (A) three Eu³⁺/Tb³⁺ distribution models of the layer projected along [001] directions, (a) random distribution, (b) Tb³⁺ in LnO₈ and Eu³⁺ in LnO₉, (c) Tb³⁺ in LnO₉ and Eu³⁺ in LnO₈; and (B) refined structural models based on the random distribution, (a) crystal structure, (b) 3D electron-density distribution image (isosurface density level of 0.5 Å⁻³). T stands for the rare earth Eu³⁺ or Tb³⁺.

Fig. 3 Fitting patterns of Cl⁻-LEu_0.5Tb_0.5H. The experimental and simulated intensities are plotted as dotted and solid lines, respectively; the line at the bottom is their intensity difference, the tick marks indicate the positions of all possible Bragg reflections from the structure model.

Table 1 Structure parameters of Cl⁻-LEu_0.5Tb_0.5H^a

Species	Wyckoff index	x	y	z	g	B/Å^2
a Space group: P21212 (no. 18); a = 12.874(0) Å; b = 7.299(2) Å; c = 8.439(3) Å; V = 793.0(4) Å^3; Rwp = 7.329%, Rp = 5.726%, RR = 19.458%, Re = 2.834%, S = 2.5864%. T stands for the rare earth Eu^3+ or Tb^3+, g stands for the occupancy, B stands for the temperature-factor.
T1	4c	0.271(5)	0.248(8)	0.929(9)	1.0	0.841(4)
T2	2b	0	0.5	0.949(3)	1.0	0.841(4)
T3	2a	0	0	0.930(5)	1.0	0.841(4)
OH(1)	4c	0.485(9)	0.270(2)	0.906(6)	1.0	0.303(0)
OH(2)	4c	0.083(0)	0.229(2)	0.838(1)	1.0	0.303(0)
OH(3)	4c	0.160(8)	0.411(9)	0.085(9)	1.0	0.303(0)
OH(4)	4c	0.646(3)	0.449(8)	0.925(7)	1.0	0.303(0)
OH(5)	4c	0.143(7)	0.775(5)	0.828(5)	1.0	0.303(0)
H2O(1)	4c	0.198(1)	0.827(2)	0.378(3)	1.0	2.922(5)
H2O(2)	2b	0	0.5	0.608(5)	0.36	2.922(5)
H2O(3)	2a	0	0	0.629(0)	0.54	2.922(5)
Cl	4c	0.125(6)	0.216(9)	0.430(9)	1.0	2.723(7)

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Fig. 4 The correlation of lattice parameters of (a) a and (b) b with the Eu³⁺ content in Cl⁻-LEu_xTb_1−xHs.

Photoluminescence of LRHs with different interlayer spacings

The emission and excitation spectra of NO₃⁻- and DS⁻-LEu_xTb_1−xHs are shown in Fig. 5 and S15, ESI.† When x = 0, the excitation spectra (Fig. 5a, and S15a, ESI†) monitored with 544 nm emission of Tb³⁺ (⁵D₄ → ⁷F₅) consist of the characteristic f → f transition lines of Tb³⁺ from the ⁷F₆ ground state to the different excited states. Moreover, upon excitation into the ⁷F₆ → ⁵D₃ transition of Tb³⁺ at 376 nm, the peaks in the emission spectra (Fig. 5b and S15b, ESI†) can be assigned to the characteristic emission of Tb³⁺, relating to ⁵D₄ → ⁷F_j (j = 6, 5, 4, 3) levels at 489, 544, 584 and 620 nm.³¹ However, with Eu³⁺ doping, both NO₃⁻- and DS⁻-LRHs samples primarily exhibit the characteristic emission of Eu³⁺, corresponding to ⁵D₀ → ⁷F_0,1,2,3,4 transitions, while the emission of Tb³⁺ is hardly observed (Fig. 5d and S15d, ESI†). The presence of the excitation bands of Tb³⁺ in the excitation spectra (Fig. 5c and S15c, ESI†) when monitored at 613 nm with ⁵D₀ → ⁷F₂ of Eu³⁺ indicates that an efficient energy transfer occurred from Tb³⁺ to Eu³⁺ in these samples,³² also suggesting that Eu³⁺ had been successfully doped into the LTbH lattice. When x < 0.5, the luminescence intensity of Eu³⁺ increases with the increasing Eu³⁺ concentration; however, when x ≥ 0.5, it becomes weaker and weaker. This result suggests that the luminescence of Eu³⁺ is a competitive result of a dual effect: (1) the increase of Eu³⁺ concentration enhances the probability of energy transfer from Tb³⁺ to Eu³⁺ and (2) it also increases the probability of nonradioactive energy migration between Eu³⁺ ions to quenching centres, where the excitation energy is lost nonradiatively.³³

Fig. 5 (a, c) Excitation and (b, d) emission spectra of NO₃⁻-LEu_xTb_1−xHs. The peaks at 483 and 492 nm (dashed line) are due to spectral responses of the equipment by the blank.

For OA⁻-LEu_xTb_1−xHs with larger d_basal, the luminescence behaviour changes greatly as compared to that of NO₃⁻ and DS⁻-LEu_xTb_1−xHs. In the excitation spectra monitored at 544 nm (Fig. 6A(a)), no matter whether Eu³⁺ is doped, the peaks attributed to the intra-4f⁸ transitions from ⁷F₆ to ⁵L_10–7, ⁵G_6–2, and ⁵D_2,3 states of Tb³⁺ are observable; as monitored with Eu³⁺ emission (Fig. 6A(b)), the excitation bands of Tb³⁺ still exist. Under 376 nm excitation, the characteristic emission of Tb³⁺ emerges noticeably. The luminescence intensity of Tb³⁺ decreases with increasing x from 0 to 0.95 and the intensity for Tb³⁺ at x = 0.05 is slightly lower than that at x = 0. When x < 0.8, the luminescence intensity of Eu³⁺ increases gradually with the increasing Eu³⁺ concentration; when x ≥ 0.8, it wears off (Fig. 6A(c)).

Fig. 6 Excitation spectra monitored at (a) 544 nm, (b) 613 nm and emission spectra excited at (c) 376 nm of (A) OA⁻- LEu_xTb_1−xHs (the peaks at 483 and 492 nm of the dashed line are due to spectral responses of the equipment by the blank) and (B) NSs-LEu_xTb_1−xHs.

Overall, under the same measurement condition, the quenching of Tb³⁺ weakens with the increase in d_basal, and the quenching concentration of Eu³⁺ observable at x = 0.5 for NO₃⁻- and DS⁻-LEu_xTb_1−xHs and at x = 0.8 for OA⁻-LEu_xTb_1−xHs demonstrates that the expansion extent of the interlayer distances plays a decisive role in the energy transfer. Such energy transfers are realized primarily between the adjacent layers rather than in the intralayer, where the rare earths are even closer.

Photoluminescence of 2D crystals

In the case of exfoliated 2D crystals, an infinite interlayer distance or only the energy transfer in the intralayer could be reasonably considered. Under excitation at 376 nm, the emission spectra (Fig. 6B(c)) of NSs-LRHs show both luminescence intensities of Eu³⁺ and Tb³⁺ in proportion to their own concentrations. Furthermore, as monitored at λ_em = 613 nm, the very weak bands in the excitation spectra (Fig. 6B(b)) imply that only a small amount of energy was transferred from Tb³⁺ to Eu³⁺. This behaviour reveals that the energy transfer between the neighbouring rare earths in the intralayer could be negligible. This result is in agreement with the observations in the layered compounds discussed above.

Fig. 7 shows the Commission International de L'Eclairage (CIE) chromaticity coordinates for NSs-LRHs excited at 284 and 318 nm. The related emission spectra are shown in Fig. S16, ESI.† The (x, y) coordinates vary systematically from green to red with the doped concentration of Eu³⁺, manifesting that the as-obtained NSs-LEu_xTb_1−xHs can show multicolour emissions in the visible region when excited by the selective excitation bands. It should be noted that the colour coordinate (0.3552, 0.3562) of x = 0.8 samples excited under 284 nm results in white light emission. It is inevitable to obtain the white light emission in the Eu³⁺ and Tb³⁺ co-doping 2D crystals because the energy transfer in the intralayer is negligible.

Fig. 7 CIE chromaticity coordinates for NSs-LEu_xTb_1−xHs excited at 284 and 318 nm.

Luminescence lifetimes

Two possible routes of energy transfer between Eu³⁺ and Tb³⁺ in LRHs are proposed in Scheme 1. Route (I), as shown in Scheme 1, represents that energy is transferred through the OH group, which bridges adjacent Eu³⁺ and Tb³⁺ located in the intralayer. The OH groups de-excite the energy transferred from Tb³⁺, making the transfer inefficient. In route (II), the energy is directly transferred from Tb³⁺ to Eu³⁺ as the two ions reside in the adjacent layers. The results discussed above indicate that the transfer would be more efficient through route (II) than route (I). In other words, route (II) might be the main procedure of energy transfer in LRHs. To further clarify this, we investigated the luminescence lifetimes for LTbH (x = 0) and LEu_0.05Tb_0.95H (x = 0.05). The luminescence decay curves of Tb³⁺ are depicted in Fig. 8.

Scheme 1 Illustration of energy transfer between Eu³⁺ and Tb³⁺ in LRHs.

Fig. 8 Luminescence decay curves of LTbH and LEu_0.05Tb_0.95H monitored at 544 nm, corresponding to ⁵D₄–⁷F₅ level of Tb³⁺.

As shown in Fig. 8, the curves can be fitted into a double-exponential function according to eqn (1),

I(t) = I₀ + A₁ exp(−t/τ₁) + A₂ exp(−t/τ₂) (1)

where I and I₀ are the phosphorescence intensities; A₁ and A₂, constants; t, decay time (ms); and τ₁ and τ₂, the decay times (ms) for the exponential components, respectively.³⁴ The efficiency of energy transfer (η_ET) from the donor Tb³⁺ to the acceptor Eu³⁺ could be calculated using eqn (2),

η_ET = 1 − τ/τ₀ (2)

where τ and τ₀ are the luminescence lifetimes of Tb³⁺ for the doped and undoped samples at the same donor concentration.³⁵ The calculated τ and η_ET are listed in Table 2. The η_ET reaches 98.1% for NO₃⁻-LEu_0.05Tb_0.95H and 95.0% for DS⁻-LEu_0.05Tb_0.95H; we ignored the minor change in the Tb³⁺ concentration. The luminescence lifetimes of Tb³⁺ (⁵D₄) are generally observed in the range of milliseconds because of the forbidden nature of the f–f transition,³⁶ but in the present case, the lifetimes of NO₃⁻-, OA⁻- and NSs-LEu_0.05Tb_0.95H change from microsecond to millisecond, corresponding to the increase of d_basal. This indicates that the small interlayer distance has high η_ET. The lifetime of OA⁻-LEu_0.05Tb_0.95H is very close to that of OA⁻LTbHs, denoting that its η_ET becomes extremely insignificant. Such a result is observable in the case of the 2D crystals of NSs-LRHs (Table 2). Since the energy transfer probability from Tb³⁺ to Eu³⁺ is proportional to R⁻⁶, where R is the average distance between the ions,³⁷ the expansion of the interlayer distance and even delamination, leading to the increase of R, lower the energy transfer efficiency.

Table 2 The lifetimes τ for Tb³⁺ and energy transfer efficiency (η_ET) when x = 0 and 0.05 upon excitation with 374 nm

x	NO3^− (μs)	DS^− (μs)	OA^− (ms)	NSs (ms)
0	454	229	0.853	1.30
0.05	8.51	11.4	0.734	1.25
ηET (%)	98.1	95.0	14.0	3.85

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The above results support strongly that the energy transfer between Eu³⁺ and Tb³⁺ in route (II), as illustrated in Scheme 1, is predominant in LRHs.

Conclusions

Due to the unique structure of LRHs, the de-excitation of OH groups on the energy transfer between rare earths is effective in the intralayer. This leads to the fact that the efficiency of the energy transfer from Tb³⁺/Eu³⁺ to Eu³⁺ in the intralayer is very weak, and efficient energy transfer is primarily achieved when the rare earths locate in adjacent layers. White light can be obtained in the 2D crystals of LEu_0.8Tb_0.2H under 284 nm. This might be of potential use as a natural white light source under ultraviolet light excitation.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This study was supported by the National Science Foundation of China (Grants. 51572031 and 51272030) and Program for Changjiang Scholars and Innovative Research Team in University.

Notes and references

1. A. Gupta, N. Brahme and D. P. Bisen, J. Lumin., 2014, 155, 112.
2. H. Shen, S. Feng, Y. Wang, Y. Gu, J. Zhou, H. Yang, G. Feng, L. Li, W. Wang, X. Liu and D. Xu, J. Alloys Compd., 2013, 550, 531.
3. L. G. V. Uitert and S. Iida, J. Chem. Phys., 1962, 37, 986.
4. E. Nakazawa and S. Shionoya, J. Chem. Phys., 1967, 47, 3211.
5. Z. Hou, Z. Cheng, G. Li, W. Wang, C. Peng, C. Li, P. a. Ma, D. Yang, X. Kang and J. Lin, Nanoscale, 2011, 3, 1568.
6. J. Yang, C. Zhang, C. Li, Y. Yu and J. Lin, Inorg. Chem., 2008, 47, 7262.
7. Z. Chen, Z. Geng, D. Shao, Z. Zhou and Z. Wang, CrystEngComm, 2012, 14, 2251.
8. G. García-Rosales, F. Mercier-Bion, R. Drot, G. Lagarde, J. Roques and E. Simoni, J. Lumin., 2012, 132, 1299.
9. W. Di, X. Wang, P. Zhu and B. Chen, J. Solid State Chem., 2007, 180, 467.
10. K.-H. Lee and S.-H. Byeon, Eur. J. Inorg. Chem., 2009, 2009, 4727.
11. F. Gándara, J. Perles, N. Snejko, M. Iglesias, B. Gómez-Lor, E. Gutiérrez-Puebla and M. Á. Monge, Angew. Chem., Int. Ed., 2006, 45, 7998.
12. X. Wu, J.-G. Li, Q. Zhu, W. Liu, J. Li, X. Li, X. Sun and Y. Sakka, J. Mater. Chem. C, 2015, 3, 3428.
13. (a) K.-H. Lee, B.-I. Lee, J.-H. You and S.-H. Byeon, Chem. Commun., 2010, 46, 1461; (b) Q. Zhu, J.-G. Li, C. Zhi, X. Li, X. Sun, Y. Sakka, D. Golberg and Y. Bando, Chem. Mater., 2010, 22, 4204.
14. (a) B. Shao, Q. Zhao, Y. Jia, W. Lv, M. Jiao, W. Lu and H. You, Chem. Commun., 2014, 50, 12706; (b) B. Shao, Q. Zhao, W. Lv, M. Jiao, W. Lu and H. You, J. Mater. Chem. C, 2015, 3, 1091.
15. X. Wu, J.-G. Li, J. Li, Q. Zhu, X. Li, X. Sun and Y. Sakka, Sci. Technol. Adv. Mater., 2013, 14, 015006.
16. L. Liu, M. Yu, J. Zhang, B. Wang, W. Liu and Y. Tang, J. Mater. Chem. C, 2015, 3, 2326.
17. R. M. Supkowski and W. D. Horrocks Jr, Inorg. Chim. Acta, 2002, 340, 44.
18. Y. Zhao, J.-G. Li, M. Guo and X. Yang, J. Mater. Chem. C, 2013, 1, 3584.
19. F. Izumi and K. Momma, Solid State Phenom., 2007, 130, 15.
20. (a) F. Geng, Y. Matsushita, R. Ma, H. Xin, M. Tanaka, F. Izumi, N. Iyi and T. Sasaki, J. Am. Chem. Soc., 2008, 130, 16344; (b) F. Geng, Y. Matsushita, R. Ma, H. Xin, M. Tanaka, N. Iyi and T. Sasaki, Inorg. Chem., 2009, 48, 6724.
21. B.-I. Lee, J.-S. Bae, E.-S. Lee and S.-H. Byeon, Bull. Korean Chem. Soc., 2012, 33, 601.
22. G. Huang, S. Ma, X. Zhao, X. Yang and K. Ooi, Chem. Mater., 2010, 22, 1870.
23. http://www.cambridgesoft.com/.
24. L. Hu, R. Ma, T. C. Ozawa and T. Sasaki, Inorg. Chem., 2010, 49, 2960.
25. D. T. Cromer, A. C. Larson and J. T. Waber, Acta Crystallogr., 1964, 17, 1044.
26. M. J. Frisch, G. W. Trucks, H. B. Schlegel, G. E. Scuseria, M. A. Robb, J. R. Cheeseman, G. Scalmani, V. Barone, B. Mennucci, G. A. Petersson, H. Nakatsuji, M. Caricato, X. Li, H. P. Hratchian, A. F. Izmaylov, J. Bloino, G. Zheng, J. L. Sonnenberg, M. Hada, M. Ehara, K. Toyota, R. Fukuda, J. Hasegawa, M. Ishida, T. Nakajima, Y. Honda, O. Kitao, H. Nakai, T. Vreven, J. A. Montgomery Jr, J. E. Peralta, F. Ogliaro, M. Bearpark, J. J. Heyd, E. Brothers, K. N. Kudin, V. N. Staroverov, R. Kobayashi, J. Normand, K. Raghavachari, A. Rendell, J. C. Burant, S. S. Iyengar, J. Tomasi, M. Cossi, N. Rega, J. M. Millam, M. Klene, J. E. Knox, J. B. Cross, V. Bakken, C. Adamo, J. Jaramillo, R. Gomperts, R. E. Stratmann, O. Yazyev, A. J. Austin, R. Cammi, C. Pomelli, J. W. Ochterski, R. L. Martin, K. Morokuma, V. G. Zakrzewski, G. A. Voth, P. Salvador, J. J. Dannenberg, S. Dapprich, A. D. Daniels, O. Farkas, J. B. Foresman, J. V. Ortiz, J. Cioslowski and D. J. Fox, GAUSSIAN 09 (Revision A.02), Gaussian, Inc., Wallingford, CT, 2009.
27. Q. Yuan, Q. Liu, W.-G. Song, W. Feng, W.-L. Pu, L.-D. Sun, Y.-W. Zhang and C.-H. Yan, J. Am. Chem. Soc., 2007, 129, 6698.
28. L. Wang, D. Yan, S. Qin, S. Li, J. Lu, D. G. Evans and X. Duan, Dalton Trans., 2011, 40, 11781.
29. J. Liang, R. Ma and T. Sasaki, Dalton Trans., 2014, 43, 10355.
30. Z. Xu, C. Li, G. Li, R. Chai, C. Peng, D. Yang and J. Lin, J. Phys. Chem. C, 2010, 114, 2573.
31. A. Huignard, T. Gacoin and J.-P. Boilot, Chem. Mater., 2000, 12, 1090.
32. J. Yang, G. Li, C. Peng, C. Li, C. Zhang, Y. Fan, Z. Xu, Z. Cheng and J. Lin, J. Solid State Chem., 2010, 183, 451.
33. L. Zhou, J. Yang, S. Hu, Y. Luo and J. Yang, J. Mater. Sci., 2015, 50, 4503.
34. M. Back, M. Boffelli, A. Massari, R. Marin, F. Enrichi and P. Riello, J. Nanopart. Res., 2013, 15, 1–14.
35. Y.-C. Li, Y.-H. Chang, Y.-S. Chang, Y.-J. Lin and C.-H. Laing, J. Phys. Chem. C, 2007, 111, 10682.
36. R. Chai, H. Lian, C. Li, Z. Cheng, Z. Hou, S. Huang and J. Lin, J. Phys. Chem. C, 2009, 113, 8070.
37. K. Riwotzki, H. Meyssamy, A. Kornowski and M. Haase, J. Phys. Chem. B, 2000, 104, 2824.

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Footnote

† Electronic supplementary information (ESI) available: Additional XRD patterns, FT-IR spectra, chemical analysis results, fitting patterns, structure parameters and fluorescence spectra. See DOI: 10.1039/c7ra12206c

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