Hydrothermal conversion of layered hydroxide nanosheets into (Y_0.95Eu_0.05)PO₄ and (Y_0.96−xTb_0.04Eu_x)PO₄ (x = 0–0.10) nanocrystals for red and color-tailorable emission

Abstract

Tetragonal-structured (Y_0.95Eu_0.05)PO₄ and (Y_0.96−xTb_0.04Eu_x)PO₄ nanocrystals were sacrificially converted from layered rare-earth hydroxide (Ln₂(OH)₅NO₃·nH₂O, LRH) nanosheets via hydrothermal reaction and in the presence of ammonium dihydrogen phosphate (NH₄H₂PO₄). Detailed characterizations of the materials were achieved by the combined techniques of FT-IR, ICP, XRD, FE-SEM, TEM, and optical spectroscopies, and the mechanisms of anion exchange and hydrothermal conversion were discussed. Phase structure and morphology of the product were found to heavily rely on the amount of PO₄³⁻ relative to rare-earth ions. The effects of calcination were studied in detail with the (Y_0.95Eu_0.05)PO₄ red phosphor nanocrystals. The (Y_0.96−xTb_0.04Eu_x)PO₄ ternary phosphors present efficient Tb³⁺ → Eu³⁺ energy transfer through electric dipole–dipole interactions, with which the emission color can be facilely tuned from green to red via yellow by raising the Eu content. The efficiency of energy transfer was analyzed to be ∼40.7% at the optimal Eu³⁺ concentration of 8 at% (x = 0.08).

---

1. Introduction

The orthophosphate of YPO₄ exhibits high thermal stability (melting point ∼2300 °C), extremely low water-solubility (solubility product ∼10⁻²⁵ to 10⁻²⁷),^1,2 and facile substitution of the Y site with various lanthanide ions, and is thus widely employed as a host lattice for luminescence. Previous investigations manifested that the compound may crystallize as one of the following three phases depending on the extent of hydration: monoclinic YPO₄·2H₂O, hexagonal YPO₄·0.8H₂O and tetragonal YPO₄,^3–5 with the latter two being the most frequently encountered. Luminescent materials in nano-dimensions have been drawing keen research interest owing to their widespread applications in physics, chemistry, biology, optoelectronics, displays, and lighting.^6–8 As for YPO₄-based phosphors, C. X. Li et al. prepared nano/microcrystals with both hexagonal and tetragonal structures and well-defined uniform morphologies via hydrothermal reaction. Through a series of control experiments, they concluded that the initial solution pH, citrate chelate (Cit³⁻), and phosphate source (NaPO₄, NH₄H₂PO₄ or Na₅P₃O₁₀) synergistically determine the phase structure and crystal shape of the final product. The luminescence behaviors of Tb³⁺, Eu³⁺, and Dy³⁺ in YPO₄ were also studied, and multicolor emission in a single host lattice was achieved.⁹ Through a CTAB-assisted hydrothermal technique, P. Li et al. synthesized hexagonal structured (Y,Eu)PO₄·0.8H₂O of prismatic particle morphologies and discussed its thermal decomposition, morphology evolution, and luminescence.¹⁰ The growth pattern of nanocrystals is known to be governed by the intrinsic surface energy of the crystallographic face of the nuclei, the type and amount of adsorbed organic capping molecules, the time of crystal growth, and the molecular precursor,¹¹ and morphology selection has always been a widely and intensively discussed topic.^12–14 In this regard, the research group of Y. D. Li made extensive investigation into the crystallization habit of rare-earth orthophosphate and the factors influencing anisotropic growth under hydrothermal conditions.^15,16

Eu³⁺ and Tb³⁺ are well-known efficient activators for various host lattices to attain high quality red and green emissions, respectively. Meanwhile, energy transfer between two types of activators is being widely utilized in the phosphor field to tune the emission color, to produce a specific color that cannot be attained with one single type of activator, and to enhance the desired emission.¹⁷ The Eu³⁺/Tb³⁺ combination is undoubtedly one of the most frequently adopted activator pairs to achieve the aforementioned purposes, since the emission spectrum of Tb³⁺ presents significant overlap with the excitation spectrum of Eu³⁺, which allows efficient Tb³⁺ → Eu³⁺ resonant energy transfer.^18–21 We thus synthesized in this work Eu³⁺ singly-doped and Eu³⁺/Tb³⁺ codoped YPO₄ nanocrystals for red and color tunable emissions, via a hydrothermal nano-conversion technique where nanosheets (∼4 nm) of the Ln₂(OH)₅NO₃·nH₂O layered rare-earth hydroxide (LRH) were used as a new type of sacrificial precursor and ammonium dihydrogen phosphate (NH₄H₂PO₄) as the phosphate source. The LRH compound is a recent type of layered inorganic materials, whose crystal structure is built up via alternative stacking of the [Ln₂(OH)₅(H₂O)_n]⁺ host layers and interlayer NO₃⁻ along the c-axis.²¹ The Ln³⁺ centers have the two types of coordination environments of [Ln(OH)₇(H₂O)] dodecahedron (8-fold coordination) and [Ln(OH)₈(H₂O)] monocapped square antiprism (9-fold coordination). Each LnO₈ unit is linked to two other LnO₈ and four LnO₉ units via edge sharing to form a two-dimensional host layer parallel to the ab plane, with NO₃⁻ sandwiched between two adjacent layers as a free and exchangeable anion for charge balance.²¹ The classic synthesis of LRH via reflux and hydrothermal reactions generally yield micron-sized thick crystals,^21–23 from which nanosheets are obtainable in poor yield and low efficiency via delamination.^24–26 We very recently developed a “freezing-temperature crystallization” technique that can produce LRH nanosheets of only ∼4 nm thick in one step and large quantities.²⁷ The significantly exposed hydroxide main layers by the thinness of the nanosheets would allow fast kinetics of phase conversion reactions. We systematically studied in this work the effects of PO₄³⁻/(Y,Ln)³⁺ (Ln = Eu, Tb/Eu) molar ratio, reaction time, and calcination on phase structure, morphology, and photoluminescent property of the (Y,Ln)PO₄ nanocrystals. Color tunable luminescence was achieved by varying the Tb/Eu molar ratio and the efficiency and mechanism of Tb³⁺ → Eu³⁺ energy transfer were also analyzed in detail. We believe that the nanocrystal preparation can be transferred to technology for practical applications, since the synthesis of LRH is readily achievable via precipitation and the conversion of LRH into phosphate is fast under mild hydrothermal conditions.

2. Experimental section

2.1 Synthesis of LRH nanosheets and conversion into (Y,Ln)PO₄ (Ln = Eu, Eu/Tb) nanophosphors

The starting lanthanide sources are Ln₂O₃ (Ln = Y, Tb, and Eu; 99.99% pure; Huizhou Ruier Rare-Chem. Hi Tech. Co. Ltd., Huizhou, China), and the nitrate solution of Ln was prepared by dissolving the oxide with a proper amount of hot nitric acid.

In a typical synthesis, 150 ml of an aqueous solution containing 15 mmol of (Y_0.95Eu_0.05)³⁺ or (Y_0.96−xTb_0.04Eu_x)³⁺ was cooled to 2–5 °C under magnetic stirring, to which ammonium hydroxide solution (NH₄OH, 1 mol L⁻¹; analytical grade, Shenyang Chemical Reagent Factory, Shenyang, China) was dropwise added until pH ∼ 8.5 for direct precipitation of layered hydroxide nanosheets.²⁷ The Eu content in the (Y_0.96−xTb_0.04Eu_x)³⁺ combination was varied in the range of x = 0–0.10 to reveal its effects on optical properties. After aging for 1 h, the precipitate was collected via centrifugation and was washed with distilled water three times to remove by-products, rinsed with absolute ethanol, followed by drying in an air oven at 50 °C for 24 h. For each run of anion exchange and hydrothermal reaction, 0.5 mmol of the LRH nanosheets was first dispersed in ∼70 ml of deionized water, to which a certain amount of ammonium dihydrogen phosphate (NH₄H₂PO₄; 1 mol L⁻¹, analytical grade, Shenyang Chemical Reagent Factory) was then dropwise added. The resultant suspension was constantly stirred for 30 min before being transferred into a Teflon-lined stainless steel autoclave of 100 ml capacity. The autoclave was tightly sealed and put into an electric oven preheated to 150 °C. After 24 h of reaction, the autoclave was left to cool naturally to room temperature and the hydrothermal product was collected via centrifugation. The wet precipitate, after being washed as aforementioned, was dried in the air at 70 °C for 24 h to yield a white powder for characterization and further processing. The effects of reaction parameters (Table 1) were studied with (Y_0.95Eu_0.05)PO₄ for example.

Table 1 Hydrothermal condition for sample synthesis

Sample ID	R = PO4^3−/(Y + Eu)^3+	Reaction time (h)
S1	1.5	0.5 (room temp.)
S2	1.5	3
S3	1.5	6
S4	1.5	9
S5	1.5	15
S6	1.5	24
S7	1.1	24
S8	1.2	24
S9	2	24

---

2.2 Characterization techniques

Phase identification was performed via X-ray diffractometry (XRD, Model PW3040/60, Philips, Eindhoven, The Netherlands) operated at 40 kV/40 mA using nickel filtered Cu-Kα radiation (λ = 0.15406 nm) and a scanning speed of 4.0° 2θ per minute. Morphology and microstructure of the products were analyzed by field emission scanning electron microscopy (FE-SEM, Model JSM-7011F, JEOL, Tokyo, Japan) operated under 15 kV and transmission electron microscopy (TEM, FEM-3000F, JEOL) under 300 kV. Fourier transform infrared spectroscopy (FT-IR, Nicolet iS5, Thermal Fisher Scientific, USA) was performed by the standard KBr method. Element contents of the product was analyzed via inductively coupled plasma (ICP) spectroscopy (Model IRIS Advantage, Jarrell-Ash Japan, Kyoto, Japan) with a detection limit of 0.01 wt%. Photoluminescence and fluorescence decay of the YPO₄:Ln³⁺ phosphors were analyzed at room temperature with an FP-8600 fluorospectrophotometer (Jasco, Tokyo).

3. Results and discussion

3.1 Formation process of (Y_0.95Eu_0.05)PO₄ nanocrystals

FT-IR analysis was performed on the three typical samples of S1, S4 and S6, and the results are presented in Fig. 1. The LRH precursor exhibits a sharp and strong absorption peak at 1385 cm⁻¹, which is characteristic of uncoordinated nitrate anions widely observed for NO₃⁻-LRHs.²⁸ The absorption at ∼3448 cm⁻¹ and the shallow shoulder near 1639 cm⁻¹ prove the existence of hydration water and are assignable to the O–H stretching vibrations (ν₁ and ν₃) and the H–O–H bending mode (ν₂), respectively.²⁹ The band observed in the 3500–3700 cm⁻¹ region (centered at 3587 cm⁻¹) arises from stretching vibrations of hydroxyls (OH⁻).^29,30 These findings conform well to the chemical formulae of NO₃⁻-LRHs. For the anion-exchange product S1, it is clearly seen that the NO₃⁻ absorption at 1385 cm⁻¹ and the hydroxyl vibration at ∼3587 cm⁻¹ disappeared while new bands that correspond to the bending (ν₄, at ∼531/638 cm⁻¹) and stretching (ν₃, at ∼1078 cm⁻¹) vibrations of PO₄³⁻ emerged in the IR spectrum.³¹ As no other species are clearly identifiable, it can thus be said that a hydrated phosphate has been formed even after very short (30 min) reaction at room temperature. Elemental analysis found 32.3 ± 0.1 wt% of Y, 2.92 ± 0.1 wt% of Eu and 12.3 ± 0.1 wt% of P for S1, leading to a Y : Eu : PO₄ molar ratio of 0.950 : 0.050 : 1.030. The sample can thus be approximately expressed as (Y_0.95Eu_0.05)PO₄·nH₂O, where the n value was calculated from the Y content to be ∼4. This indicates that the phosphate anions released from NH₄H₂PO₄ readily replace not only the interlayer NO₃⁻ but also the hydroxyls in the [Ln(OH)₇H₂O] and [Ln(OH)₈H₂O] polyhedrons that comprise the host layers of LRH owing to its significantly higher coordination capability.³² The IR spectra of samples S4 and S6 exhibit features similar to S1, but the weaker water absorptions at ∼3448 and 1639 cm⁻¹ imply dehydration of the product under hydrothermal conditions. Dehydration would also lessen the intramolecular hydrogen bonding between H₂O and PO₄³⁻, thus shifting the stretching mode of PO₄³⁻ (around 1070 cm⁻¹) towards lower energy (smaller wavenumber).³³ TG analysis (Fig. S1†) found that all the three samples decompose via two major stages up to 500 °C, with the first one (up to ∼270 °C) largely due to the evaporation of surface absorbed water while the second one (∼270–500 °C) to the removal of hydration water. From the total weight losses of ∼23.01%, 14.06% and 9.6%, samples S1, S4 and S6 were calculated to have successively smaller n values of ∼3.11, 1.70 and 0.96, respectively.

Fig. 1 FT-IR spectra for the as-precipitated LRH and the products of S1 (anion exchanged at room temperature for 0.5 h), S4 (hydrothermally reacted for 9 h) and S6 (hydrothermally reacted for 24 h). The PO₄³⁻/(Y + Eu)³⁺ molar ratio (R) is 1.5 for all the three samples.

Fig. 2 shows XRD patterns of the pristine LRH and samples S1–S6. The LRH exhibits a series of 00l and non-00l diffractions that correspond to the layered rare-earth hydroxide of Ln₂(OH)₅NO₃·nH₂O.²⁷ The characteristic LRH diffractions disappeared for S1, indicating collapse of the layered structure and disintegration of the hydroxide main layers, as also seen from the results of morphology analysis shown later. The seriously broadened diffraction peaks cannot be indexed to any crystalline form of rare-earth orthophosphate, possibly owing to its high water content. Though sample S2 is almost identical to S1, tetragonal (Y,Eu)PO₄ (JCPDS No. 83-0658, space group: I4₁/amd) crystallizes in sample S3 via consumption of the unidentifiable phase. Pure tetragonal (Y,Eu)PO₄ is formed for samples S4–S6, and gradually sharper XRD diffractions (better crystallinity) were observed along with increasing hydrothermal time.

Fig. 2 XRD patterns for the LRH, the room-temperature product S1, and the products obtained via hydrothermal reaction for 3 h (S2), 6 h (S3), 9 h (S4), 15 h (S5), and 24 h (S6).

FE-SEM observation (Fig. 3a) revealed micron-sized and flower-like assemblies of LRH nanoflakes, while TEM analysis (Fig. 3b) found nanosheets of up to 3–6 nm in thickness. The very thin nature of the nanosheets is also perceivable from their flexibility. Selected area electron diffraction (SAED, the inset in Fig. 3b) yielded ring-like patterns that correspond to the (220) and (620) planes, further confirming that the hydroxide host layers of the LRH are well ordered as revealed by XRD. Sample S1 generally retains the flower-like skeletons of the LRH precursor (Fig. 3c), but the individual nanosheets were eroded and disintegrated by PO₄³⁻ interaction at room temperature (Fig. 3d). HR-TEM (the inset 1 in Fig. 3d) revealed crystalline objects with interplanar spacing of ∼0.37 nm, and the very tiny crystallite size (∼5 nm) may explain the significantly broadened XRD peaks (Fig. 2, sample S1) and the rather vague diffraction rings (the inset 2 in Fig. 3d). The above observations may thus suggest that the hydrated product of (Y_0.95Eu_0.05)PO₄·4H₂O (S1) was formed via a dissolution-reprecipitation pathway. Though the 9 h product (S4, Fig. 3e) contains both dispersed and aggregated spindle-like particles, the 24 h product (S6, Fig. 3f) exclusively consists of monodispersed spindles. Longer reaction time favors Ostwald ripening and thus a more uniform particle morphology.

Fig. 3 FE-SEM (a, c, e and f) and TEM (b and d) micrographs showing morphologies of the LRH (a and b) and the three typical samples of S1 (c and d), S4 (e), and S6 (f). The insets in (b and d) are the corresponding SAED pattern and HR-TEM lattice image.

3.2 Phase and morphology evolution

Fig. 4 compares XRD patterns of the products synthesized with different PO₄³⁻/(Y + Eu)³⁺ molar ratio R, from which it is seen that those of up to R = 1.5 (S7, S8 and S6) are all well indexable with tetragonal YPO₄ (JCPDS No. 83-0658, space group: I4₁/amd). The samples were calculated to have crystallite sizes of ∼32 nm for S7, 39 nm for S8 and 42 nm for S6 via broadening analysis of the (200) diffraction with the Scherrer equation. SEM observation found that increasing R from 1 to 1.5 led to gradually more elongated particles, and the aspect ratio was assayed to be ∼2.1 for S7 (Fig. 5a), ∼3.2 for S8 (Fig. 5b) and ∼3.6 for S6 (Fig. 3f). This is consistent with the report of Peng et al.,^34,35 who suggested that a higher monomer concentration will encourage 1D crystal growth of phosphate. Further raising the R value to 2 (S9) led to partial crystallization of hexagonal phosphate (Fig. 4; JCPDS No. 42-0082, space group: P6₂22). Vanetsev et al. recently reported that hexagonal YPO₄·0.8H₂O tends to be lower in crystallinity than tetragonal YPO₄ under the same synthetic conditions.³⁶ The high PO₄³⁻ concentration at R = 2 would fasten chemical reactions, preventing sufficient crystallization of the product and thus the formation of hexagonal phosphate.

Fig. 4 XRD patterns for the products synthesized via hydrothermal reaction under the different PO₄³⁻/(Y + Eu)³⁺ molar ratio (R) of 1 : 1 (S7), 1.2 : 1 (S8), 1.5 : 1 (S6) and 2 : 1 (S9). The black dots in the XRD pattern of S9 denote hexagonal phosphate.

Fig. 5 FE-SEM and TEM micrographs showing morphologies of the products obtained with PO₄³⁻/(Y + Eu)³⁺ molar ratio (R) of (a) 1 : 1 (S7), (b) 1.2 : 1 (S8) and (c) 1.5 : 1 (S6). Part (d) is the HR-TEM lattice analysis of S6, and the insets in (c) and (d) are the corresponding SAED and Fourier transformation patterns, respectively.

Low magnification TEM observation of sample S6 (Fig. 5c) found that the spindle-like particles have a length of about 200 nm and a width of about 35 nm. The aspect ratio calculated here (∼5.7) is larger than that (3.6) from SEM micrograph (Fig. 3f), since the SEM sample was sputtered with gold for electrical conductivity. Selected area electron diffraction (SAED, the inset in Fig. 5c) yielded sharp spots that correspond to the (200), (112) and (312) planes of tetragonal (Y_0.95Eu_0.05)PO₄, indicating that the product is well crystallized. The excellent crystallinity is also evidenced by the well resolved lattice fringes of the individual nanocrystals (Fig. 5d), where the spacing of ∼0.259 nm corresponds well to the (112) plane of tetragonal YPO₄ (d₁₁₂ = 0.256 nm). Fourier transformation of the lattice fringe yielded well defined diffraction spots (the inset in Fig. 5d), indicating that each individual nanocrystal is of single crystalline.

3.3 The effects of calcination on structure and properties of (Y_0.95Eu_0.05)PO₄

The effects of annealing on characteristics of the phosphate crystallites were investigated with sample S6 for example. It was found that, in the range of this study, calcination did not induce any change to the tetragonal phase structure (Fig. S2†) but led to gradually sharper XRD diffractions for the higher-temperature product owing to crystal perfection and growth. SEM observation revealed that the spindle-like morphologies were essentially retained at 800 (Fig. 6a) and 900 °C (Fig. 6b) but slightly fragmented at 1000 °C (Fig. 6c and d). HR-TEM analysis of the 1000 °C product (the inset (1) in Fig. 6d) clearly reveals the (200) plane with an inter-planar spacing of ∼0.346 nm, which is quite close to the 0.345 nm of YPO₄ in the standard data file. SAED analysis (the inset (2) in Fig. 6d) yielded well defined spots, indicating not only high crystallinity but also single crystalline nature of each particle.

Fig. 6 FE-SEM (a–c) and TEM (d) micrographs showing morphologies of the products calcined from S6 at (a) 800, (b) 900, and (c and d) 1000 °C. The insets in (d) are the corresponding HR-TEM image (1) and SAED pattern (2).

The effects of calcination on photoluminescence of the (Y_0.95Eu_0.05)PO₄ red phosphors are studied in Fig. 7 for sample S6. The PLE spectra (Fig. 7a) all exhibit excitation bands at ∼217 nm, which are assignable to the excitation of electrons from the 2p orbital of O²⁻ in the PO₄³⁻ group to the 4f orbital of Eu³⁺. The other excitations in the longer wavelength region are arising from the intra-4f⁶ electronic transitions of Eu³⁺. The phosphors exhibit sharp emissions upon UV excitation at 217 nm, which are associated with the transitions from the excited ⁵D₀ state to the ⁷F_J (J = 0–4) ground states of Eu³⁺ as labeled in Fig. 7b.^37,38 Intensities of the PLE and PL bands were greatly enhanced by calcination at 800 °C, mainly due to crystal perfection/growth and the elimination of fluorescence quenching water molecules, hydroxyls, and surface dangling bonds.³⁹ Morphology of the nanocrystals does not appreciably affect emission peak position of the doped lanthanide ions since the luminescence only involves 4f electrons, which are well shielded by 5s²5p⁶ electronic shells. Intensity of the emission, however, is clearly dependent on crystal morphology (size/shape). The asymmetry factor of luminescence (intensity ratio of ⁵D₀ → ⁷F₂ to ⁵D₀ → ⁷F₁ transitions) was calculated to be around 1.1, though a stronger ⁵D₀ → ⁷F₁ magnetic dipole transition was expected since the Eu³⁺ ions would replace the Y³⁺ in YPO₄ to inherit a centrosymmetric D_2d point symmetry.^40,41 The abnormally higher intensities of the ⁵D₀ → ⁷F_2,4 emissions may mainly be due to the elongated crystallite morphology, which give rise to distortion of the Eu³⁺ local symmetry as well demonstrated for Sr₃Ga₂O₅Cl₂:Eu³⁺, Bi³⁺ phosphors.⁴²

Fig. 7 PLE (a) and PL (b) spectra of the (Y_0.95Eu_0.05)PO₄ phosphors (S6) calcined at various temperatures.

Fluorescence decay kinetics of the (Y_0.95Eu_0.05)PO₄ phosphors were studied for the ⁵D₀ → ⁷F_J (J = 1, 2) emissions at 593 and 618 nm. The decay curves (Fig. S3 and S4†) can be fitted with the single-exponential function of I = A exp(−t/τ) + B, where τ is the fluorescence lifetime, t the delay time, I the relative intensity of emission, and A and B are constants. The derived lifetime values are summarized in Table 2, where it is seen that the lifetime increased dramatically by calcination at 800 °C and then slightly decreases at higher annealing temperatures. The abrupt increase is primarily owing to the removal of luminescence quenching defects and surface species, while the subsequent decrease is mainly due to crystallite/particle growth.⁴³ It is also observed that the 593 nm emission (⁵D₀ → ⁷F₁) has a longer lifetime than the 618 nm one (⁵D₀ → ⁷F₂) in each case, since the former arises from Eu³⁺ taking higher point symmetries and is parity-allowed while the latter from Eu³⁺ residing at less symmetric sites and is parity-forbidden. The Commission International de L'Eclairage (CIE) chromaticity coordinates were calculated to be about (0.62, 0.38) for the uncalcined powder and about (0.64, 0.36) for all the three calcined ones (Fig. S5†).

Table 2 Fluorescence lifetime for the ⁵D₀ → ⁷F₁ (593 nm) and ⁵D₀ → ⁷F₂ (618 nm) transitions of (Y_0.95Eu_0.05)PO₄

Conditions	Uncalcined	800 °C	900 °C	1000 °C
593 nm (ms)	2.71 ± 0.04	4.74 ± 0.06	4.59 ± 0.03	4.46 ± 0.03
618 nm (ms)	2.53 ± 0.04	4.45 ± 0.05	4.36 ± 0.04	4.31 ± 0.05

---

3.4 Characterization and optical properties of doubly doped (Y_0.96−xTb_0.04Eu_x)PO₄

Since only a slight increment in emission intensity was observed for the (Y_0.95Eu_0.05)PO₄ phosphor upon raising calcination from 900 to 1000 °C, the doubly doped (Y_0.96−xTb_0.04Eu_x)PO₄ phosphors were thus synthesized with the hydrothermal conditions of sample S6, followed by calcination in flowing H₂ (200 ml min⁻¹) at 900 °C for 2 h. XRD analysis (Fig. S6†) shows that the products (x = 0–0.10) all exhibit diffractions corresponding to tetragonal YPO₄. The lattice constants a and c, calculated from the (200) and (101) diffractions, become steadily larger with increasing Eu³⁺ addition (Table S1†), confirming the formation of solid solution.

We firstly studied the excitation and emission behaviors of Tb³⁺ in the Eu³⁺-free sample of (Y_0.96Tb_0.04)PO₄ (Fig. S7†). The excitation spectrum recorded by monitoring the intra-4f^{8 5}D₄ → ⁷F₅ transition at 546 nm (Fig. S7a†) consists of two bands in the short UV region for the spin-allowed (low-spin, LS; at ∼222 nm) and spin-forbidden (high-spin, HS; at ∼266 nm) inter-configurational 4f⁸ → 4f⁷5d¹ transitions, respectively.^18,44 The other bands in the longer UV region of 275–400 nm represent intra-4f⁸ transitions of Tb³⁺ (Fig. S7a,† the inset). The whole excitation spectrum is dominated by the LS transition at 222 nm. The PL spectrum (Fig. S7b†) obtained under 222 nm excitation consists of sharp lines ranging from 470 to 650 nm that are associated with transitions from the excited ⁵D₄ state to the ⁷F_J (J = 6–2) ground states, with the green emission at 546 nm (⁵D₄ → ⁷F₅) being the most prominent.

The excitation spectra of (Y_0.96−xTb_0.04Eu_x)PO₄ are shown in Fig. S8a and b† for the red emission of Eu³⁺ at 618 nm and the green emission of Tb³⁺ at 546 nm, respectively. Monitoring the Eu³⁺ emission at 618 nm produced an overlapped excitation band in the ∼200–250 nm region due to superimposition of the LS transition of Tb³⁺ (∼222 nm) and the charge transfer (CT) excitation of Eu³⁺ (∼215 nm). Intensity of the 215 nm excitation improves monotonically with increasing Eu³⁺ content up to x = 0.08 and then decreases, indicating concentration quenching at x > 0.08. It is clearly seen from Fig. S8b† that the excitation spectrum of Tb³⁺ consists of both 4f⁸ → 4f⁷5d¹ and 4f–4f transitions, with the LS at ∼222 nm being the strongest in each case. The LS intensity remarkably and steadily decreases towards a higher Eu³⁺ content and finally becomes negligible at x = 0.10, suggesting the occurrence of efficient Tb³⁺ → Eu³⁺ energy transfer.

Since the LS band of Tb³⁺ and the CT band of Eu³⁺ significantly overlaps, we excited the (Y_0.96−xTb_0.04Eu_x)PO₄ phosphors at 266 nm (the HS transition of Tb³⁺) to better analyze the mechanism and efficiency of Tb³⁺ → Eu³⁺ energy transfer. The obtained PL spectra are shown in Fig. 8, from which it is seen that the characteristic emissions of Tb³⁺ and Eu³⁺ emerged simultaneously for the codoped samples, but the Tb³⁺ emission was drastically weakened at a higher Eu³⁺ content (larger x, Fig. 8 and 9). This further confirms an efficient energy transfer (ET) from Tb³⁺ to Eu³⁺.²⁰ The observed 620 nm luminescence of samples x = 0.02–0.10 is superimposed from the 618 nm emission of Eu³⁺ (⁵D₀ → ⁷F₂) and the 622 nm emission of Tb³⁺ (⁵D₄ → ⁷F₃), and the results of deconvolution are shown in Fig. 8 as inset. Luminescence properties of the phosphors are further analyzed in Fig. 9, where it is seen that the 618 nm (⁵D₀ → ⁷F₂) and 697 nm (⁵D₀ → ⁷F₄) emissions of Eu³⁺ similarly improve up to x = 0.08 (C_Tb+Eu = 12 at%) and then decrease, following the tendency observed from the excitation spectra (Fig. S8a†). The optimal total activator concentration of 12 at% found in this work is almost identical to the value determined for the (Y,Eu)PO₄ binary phosphor.^45,46 The red to green intensity ratios of I₆₁₈/I₅₄₆ and I₆₉₇/I₅₄₆ steadily increase towards a higher Eu³⁺ content, implying that the emission color can be tailored. The two intensity ratios are expected to increase further at x > 0.10 since the emission intensity of Tb³⁺ is approaching zero.

Fig. 8 Photoluminescence (PL) spectra of the (Y_0.96−xTb_0.04Eu_x)PO₄ phosphors calcined at 900 °C. The inset shows deconvolution of the ∼620 nm emission by Gaussian fitting for samples x = 0.02–0.10.

Fig. 9 Relative emission intensities of Tb³⁺ (546 nm) and Eu³⁺ (618 and 697 nm) and the I₆₁₈/I₅₄₆ and I₆₉₇/I₅₄₆ intensity ratios, as a function of the Eu³⁺ content (the x value).

The process of Tb³⁺ → Eu³⁺ energy transfer is well studied and documented in the literature,^{17,20,21,45,47,48} and is schematically shown in Fig. S9.† Exciting the (Y_0.96−xTb_0.04Eu_x)PO₄ phosphor at 266 nm raises electrons from the 4f⁸ ground state to the 4f⁷5 d¹ energy level of Tb³⁺, followed by relaxation to the ⁵D₃ and then ⁵D₄ state to yield the observed ⁵D₄ → ⁷F_J (J = 3–6) emissions. Since the ⁵D₄ state of Tb³⁺ lies above the ⁵D_0,1 states of Eu³⁺ in the energy diagram for the excited states of rare-earth ions^47,49 and the ⁵D₄ → ⁷F_J Tb³⁺ emissions and the ⁷F₀ → ⁵D_0,1 Eu³⁺ excitations present significant spectral overlap, the resonant energy transfer from Tb³⁺ to Eu³⁺ may excite Eu³⁺ electrons to the ⁵D_0,1 energy levels, and back-jumping of the excited electrons from the lowest lying ⁵D₀ state to the ⁷F_J ground states (J = 1–4) then produces the observed Eu³⁺ red emissions.⁴⁷ Emissions of the (Y_0.96−xTb_0.04Eu_x)PO₄ phosphors under 266 nm excitation were calculated to have CIE chromaticity coordinates of around (0.36, 0.55) for x = 0, (0.44, 0.49) for x = 0.02, (0.51, 0.44) for x = 0.04, (0.56, 0.40) for x = 0.06, (0.59, 0.38) for x = 0.08 and (0.60, 0.37) for x = 0.10 (Fig. 10A). The emission color can thus be finely tuned from green to red via yellow with increasing x from 0 to 0.10, as also seen from the vivid multicolor luminescence of the phosphors under 254 nm irradiation from a hand-held UV lamp (Fig. 10B).

Fig. 10 CIE chromaticity diagram (A) and multi-color emission under 254 nm radiation from a hand-held UV lamp (B) of the (Y_0.96−xTb_0.04Eu_x)PO₄ phosphors calcined at 900 °C, with (a) x = 0, (b) x = 0.02, (c) x = 0.04, (d) x = 0.06, (e) x = 0.08 and (f) x = 0.10.

The mode of energy transfer (ET) is governed by the average separation distance (R) between the Tb³⁺ donor and Eu³⁺ acceptor, and ET may occur via either exchange interaction or electric multipole interaction. Exchange interaction generally requires an overlap of the donor and acceptor orbitals and an R value of less than 0.3–0.4 nm, or, otherwise, electric multipole interaction may dominate.⁵⁰ The R value can be estimated from the following equation proposed by Blasse and Bril.^51,52

where C_Tb+Eu is the total concentration of Tb³⁺ and Eu³⁺ ions, N is the number of sites that the rare-earth ions can occupy per unit cell and V is the cell volume. The unit cell of tetragonal YPO₄ has 4 cation sites and therefore N = 4.⁵³ The cell volume can be calculated with V = a²c, where a and c are the lattice constants shown in Table S1.† The results of calculation yielded R = 1.318, 1.198, 1.114, 1.049 and 0.997 nm for C_Tb+Eu = 0.06, 0.08, 0.10, 0.12 and 0.14, respectively. The R value is significantly larger than 0.3–0.4 nm in each case, indicating that Tb³⁺ → Eu³⁺ ET may largely take place via electric multipole interactions.

The energy transfer mechanism for exchange/multipolar interactions has been discussed by several authors,^54–56 and can be analyzed by:

where C is the total concentration of Tb³⁺ and Eu³⁺ and it can be replaced with the Eu³⁺ content in this work since the Tb³⁺ content is fixed, and η₀ and η are the luminescence quantum efficiencies of the Tb³⁺ sensitizer in the absence and presence of Eu³⁺, respectively. The η₀/η ratio can be approximated as the luminescence intensity ratio I_S0/I_S. The relation of ln(I_S0/I_S) ∝ C corresponds to exchange interaction, while those of (I_S0/I_S) ∝ C^n/3 with n = 6, 8 and 10 correspond to dipole–dipole, dipole–quadrupole and quadrupole–quadrupole interactions, respectively. The results of analysis are presented in Fig. 11, where the best linear relationship was found for n = 6 by comparing the fitting factor value of R². Therefore, the energy absorbed by Tb³⁺ is dominantly transferred to Eu³⁺ via a non-radiative electric dipole–dipole mechanism, conforming to the conclusion drawn from the earlier analysis of critical separation distance (R_c).

Fig. 11 Dependence of ln(I_S0/I_S) on C (a) and (I_S0/I_S) on C^6/3 (b), C^8/3 (c), and C^10/3 (d) for the Tb³⁺ emission.

Fluorescence decay kinetics of the 546 nm Tb³⁺ and 618 nm Eu³⁺ emissions were analyzed against the Eu³⁺ content (0 ≤ x ≤ 0.10) via single exponential fitting (Fig. S10 and S11†), and the derived lifetimes are summarized in Table 3. Apparently, the lifetime of Tb³⁺ dramatically decreased from ∼4.72 ms for x = 0 to 3.62 ms for x = 0.02, followed by slow yet steady shortening. The sudden decrease is mainly owing to Tb³⁺ → Eu³⁺ energy transfer and is frequently observed for donor/acceptor co-doped systems.^57,58 The lifetime of Eu³⁺ emission continuously decreases with increasing x, which can be explained as follows. Raising the Eu³⁺ content shortens the separation distance among luminescent centers, which increases the probability of not only non-radiative Tb³⁺ → Eu³⁺ energy transfer but also radiative (resonant) energy transfer among them followed by non-radiative transfer to surface sites. This may also account for the continuous lifetime shortening of Tb³⁺ emission for x = 0.02–0.10.

Table 3 Fluorescence lifetime for the 546 nm Tb³⁺ emission (τ_R(Tb³⁺)) and the 618 nm Eu³⁺ emission (τ_R(Eu³⁺)) of the (Y_0.96−xTb_0.04Eu_x)PO₄ phosphors calcined at 900 °C

x	0	0.02	0.04	0.06	0.08	0.10
τR(Tb^3+)/ms	4.72 ± 0.02	3.62 ± 0.02	3.24 ± 0.03	3.11 ± 0.03	2.80 ± 0.06	2.70 ± 0.08
τR(Eu^3+)/ms	—	4.21 ± 0.04	3.94 ± 0.05	3.71 ± 0.03	3.49 ± 0.02	3.18 ± 0.02

---

In the absence of concentration quenching, the efficiency (η_ET) of Tb³⁺ → Eu³⁺ energy transfer can be calculated from the fluorescence lifetime with the equation

η_ET = 1 − τ/τ₀ (4)

where τ and τ₀ are the fluorescence lifetime of Tb³⁺ emission in the presence and absence of Eu³⁺ acceptor, respectively. The η_ET value calculated from Table 3 increased steadily from ∼23.3% for x = 0.02 to 40.7% for x = 0.08. The high efficiency of energy transfer is primarily owing to spectral overlapping between the ⁵D₄ → ⁷F_J emission of Tb³⁺ and the ⁷F_0,1 → ⁵D_0,1,2 excitation absorptions of Eu³⁺.⁴⁵

4. Conclusion

Red emitting (Y_0.95Eu_0.05)PO₄ and multi-color emitting (Y_0.96−xTb_0.04Eu_x)PO₄ nanophosphors have been successfully synthesized via sacrificial conversion of layered rare-earth hydroxide (LRH) nanosheets in the presence of ammonium dihydrogen phosphate (NH₄H₂PO₄), followed by hydrothermal treatment and calcination. Under hydrothermal reaction at 150 °C and PO₄³⁻/RE³⁺ (RE: rare-earth) molar ratios of up to 1.5, tetragonal structured orthophosphate was suggested to crystallize via dehydration of a REPO₄·4H₂O intermediate formed at room temperature via dissolution of LRH. Longer hydrothermal time up to 24 h was found beneficial to the formation of monodispersed phosphate spindles. Partial crystallization of hexagonal orthophosphate was observed at the higher PO₄³⁻/RE³⁺ molar ratio of 2.0. Calcination up to 1000 °C did not change phase purity of the product but leads to greatly improved luminescence and altered fluorescence lifetime. With efficient Tb³⁺ → Eu³⁺ energy transfer, the emission color of (Y_0.96−xTb_0.04Eu_x)PO₄ can be finely tuned from green to red by raising the content of Eu³⁺. The efficiency of energy transfer was assayed to be ∼40.7% at the optimal Eu³⁺ content of 8 at%, and the transfer was attributed to electric dipole–dipole interactions. Successively shortened fluorescence lifetime was observed for both the 546 nm green emission of Tb³⁺ and the 618 nm red emission of Eu³⁺ along with increasing Eu³⁺ content.

Acknowledgements

This work was supported in part by the National Natural Science Foundation of China (Grants No. 51172038, U1302272, and 51302032), the Fund of the State Key Laboratory of Advanced Technologies for Comprehensive Utilization of Platinum Metals (SKL-SPM-201505), the Special Fund for Fundamental Research in Central Universities (Grant No. N140204002), and Grant-in-Aid for Scientific Research (JSPS KAKENHI, No. 26420686).

References

1. F. H. Firsching and S. N. Brune, J. Chem. Eng. Data, 1991, 36, 93.
2. A. Rouanel, J. J. Serra, K. Allaf and V. P. Orlovskii, Inorg. Mater., 1981, 17, 76.
3. R. Kijkowska, E. Cholewka and B. Duszak, J. Mater. Sci., 2003, 38, 223.
4. S. Lucas, E. Champion, D. Bregiroux, D. Bernache-Assollant and F. Audubert, J. Solid State Chem., 2004, 177, 1302.
5. H. J. Song, L. Q. Zhou, L. Li, T. Wang, F. Hong and X. R. Luo, Mater. Sci. Eng., B, 2013, 178, 1012.
6. G. Hebbink, J. Stouwdam, D. Reinhoudt and E. Beggel, Adv. Mater., 2002, 14, 1147.
7. P. Schuetz and F. Caruso, Chem. Mater., 2002, 14, 4509.
8. M. Q. Tan, Z. W. Ye, G. L. Wang and J. L. Yuan, Chem. Mater., 2004, 16, 2494.
9. C. X. Li, Z. Y. Hou, C. M. Zhang, P. P. Yang, G. G. Li, Z. H. Xu, Y. Fan and J. Lin, Chem. Mater., 2009, 21, 4598.
10. P. Li, Y. Liu, Y. X. Guo, X. L. Shi, G. Q. Zhu and H. Q. Zuo, Ceram. Int., 2015, 41, 6621.
11. S. M. Lee, S. N. Cho and J. Cheon, Adv. Mater., 2003, 15, 441.
12. X. G. Peng, Adv. Mater., 2003, 15, 459.
13. Z. A. Peng and X. G. Peng, J. Am. Chem. Soc., 2001, 123, 1389.
14. Y. N. Xia, P. D. Yang, Y. G. Sun, Y. Y. Wu, B. Mayers, B. Gates, Y. D. Yin, F. Kim and H. Q. Yan, Adv. Mater., 2003, 15, 353.
15. R. X. Yan, X. M. Sun, X. Wang, Q. Peng and Y. D. Li, Chem.–Eur. J., 2005, 11, 2183.
16. Z. Y. Huo, C. Chen, D. R. Chu, H. H. Li and Y. D. Li, Chem.–Eur. J., 2007, 13, 7708.
17. J.-G. Li and Y. Sakka, Sci. Technol. Adv. Mater., 2015, 16, 014902.
18. D. L. Geng, M. M. Shang, D. M. Yang, Y. Zhang, Z. Y. Cheng and J. Lin, J. Mater. Chem., 2012, 22, 23789.
19. Q. Zhu, M. Xiong, J.-G. Li, W. G. Liu, Z. H. Wang, X. D. Li and X. D. Sun, RSC Adv., 2015, 5, 36122.
20. X. L. Wu, J.-G. Li, J. K. Li, Q. Zhu, X. D. Li, X. D. Sun and Y. Sakka, Sci. Technol. Adv. Mater., 2013, 14, 015006.
21. B. Lu, J.-G. Li, X. D. Sun and Y. Sakka, J. Am. Ceram. Soc., 2015, 98, 2480.
22. F. X. Geng, R. Z. Ma and T. Sasaki, Acc. Chem. Res., 2010, 43, 1177.
23. Q. Zhu, J.-G. Li, C. Y. Zhi, X. D. Li, X. D. Sun, Y. Sakka, D. Golberg and Y. Bando, Chem. Mater., 2010, 22, 4204.
24. L. F. Hu, R. Z. Ma, T. C. Ozawa and T. Sasaki, Chem.–Asian J., 2010, 5, 248.
25. K.-H. Lee, B. I. Lee, J. H. You and S.-H. Byeon, Chem. Commun., 2010, 46, 1461.
26. Q. Zhu, J.-G. Li, X. D. Li, X. D. Sun, Y. Qi, M. Y. Zhu and Y. Sakka, Sci. Technol. Adv. Mater., 2014, 15, 014203.
27. X. L. Wu, J.-G. Li, Q. Zhu, W. G. Liu, J. Li, X. D. Li, X. D. Sun and Y. Sakka, J. Mater. Chem. C, 2015, 3, 3428.
28. F. X. Geng, Y. Matsushita, R. Z. Ma, H. Xin, M. Tanaka, N. Iyi and T. Sasaki, Inorg. Chem., 2009, 48, 6724.
29. J. A. Gadsden, Infrared Spectra of Minerals and Related Inorganic Compounds, Butterworth, Newton, 1975.
30. Z. L. Xiu, Z. S. Yang, M. K. Lu, S. W. Liu, H. P. Zhang and G. J. Zhou, Opt. Mater., 2006, 29, 431.
31. N. K. Sahu, R. S. Ningthoujam and D. Bahadur, J. Appl. Phys., 2012, 112, 014306.
32. Z. H. Wang, J.-G. Li, Q. Zhu, X. D. Li and X. D. Sun, Dalton Trans., 2016 10.1039/c5dt01983d.
33. S. A. Brandán, S. B. Díaz, R. C. Picot, E. A. Disalvo and A. B. Altabef, Spectrochim. Acta, Part A, 2007, 66, 1152.
34. Z. A. Peng and X. G. Peng, J. Am. Chem. Soc., 2001, 123, 1389.
35. Z. A. Peng and X. G. Peng, J. Am. Chem. Soc., 2002, 124, 3343.
36. A. S. Vanetsev, E. V. Samsonova, O. M. Gaitko, K. Keevend, A. V. Popov, U. Mäeorg, H. Mändar, I. Sildos and Y. V. Orlovskii, J. Alloys Compd., 2015, 639, 415.
37. J.-G. Li, Q. Zhu, X. D. Li, X. D. Sun and Y. Sakka, Acta Mater., 2011, 59, 3688.
38. Q. Zhu, J.-G. Li, X. D. Li and X. D. Sun, Acta Mater., 2009, 57, 5975.
39. Q. Zhu, J.-G. Li, X. D. Li and X. D. Sun, Curr. Nanosci., 2010, 6, 496.
40. G. Blasse and B. C. Grabmaier, Lumin Mater, Springer Verlag, Berlin, Germany, 1994, p. 65.
41. P. C. de Sousa Filho and O. A. Serra, J. Phys. Chem. C, 2011, 115, 636.
42. Z. P. Ci, R. N. Guana, K. H. Niea, L. M. Liua, L. L. Hana, J. C. Zhanga and Y. H. Wang, Mater. Res. Bull., 2015, 70, 822.
43. Q. Zhu, J.-G. Li, R. Z. Ma, T. Sasaki, X. J. Yang, X. D. Li, X. D. Sun and Y. Sakka, J. Solid State Chem., 2012, 192, 229.
44. F. X. Geng, Y. Matsushita, R. Z. Ma, H. Xin, M. Tanaka, F. Izumi, N. Iyi and T. Sasaki, J. Am. Chem. Soc., 2008, 130, 16344.
45. W. H. Di, X. X. Zhao, S. Z. Lu, X. J. Wang and H. F. Zhao, J. Solid State Chem., 2007, 180, 2478.
46. J. J. Xiao, Y. Y. Gao, J. Zhang, Y. X. Liu and Q. B. Yang, J. Rare Earths, 2012, 30, 515.
47. G. H. Dieke and H. M. Crosswhite, Appl. Opt., 1963, 2, 675.
48. R. Q. Li, Y. Liu, N. N. Zhang, L. L. Lin, L. Liu, Y. M. Liang and S. C. Gan, J. Mater. Chem. C, 2015, 3, 3928.
49. R. T. Wegh, A. Meijerink, R. J. Lamminmaki and H. Jorma, J. Lumin., 2000, 87–89, 1002.
50. D. L. Dexter and J. H. Schulman, Chem. Phys., 1954, 22, 1063.
51. G. Blasse and A. Bril, J. Chem. Phys., 1969, 51, 3252.
52. G. Blasse, J. Solid State Chem., 1986, 62, 207.
53. G. K. Liu, V. V. Zhorin, S. T. Li and J. V. Beitz, J. Chem. Phys., 2000, 112, 373.
54. C. H. Huang, W. R. Liu and T. M. Chen, J. Phys. Chem. C, 2010, 114, 18698.
55. C. K. Chang and T. M. Chen, Appl. Phys. Lett., 2007, 90, 161901.
56. W. J. Yang, L. Luo, T. M. Chen and N. S. Wang, Chem. Mater., 2005, 17, 3883.
57. W. Di, X. Wang, P. Zhu and B. Chen, J. Solid State Chem., 2007, 180, 467.
58. S. Mukherjee, V. Sudarsan, R. K. Vatsa, S. V. Godbole, R. M. Kadam, U. M. Bhatta and A. K. Tyagi, Nanotechnology, 2008, 19, 325704.

---

Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra00434b

---