Facile hydrothermal synthesis and multicolor-tunable luminescence of YPO₄:Ln³⁺ (Ln = Eu, Tb)

Abstract

Well-crystallized tetragonal YPO₄ and YPO₄:Ln³⁺ (Ln = Eu, Tb) nanocrystals with cuboid morphology and narrow size distribution have been successfully synthesized through a facile two-step hydrothermal synthesis route using Y₄O(OH)₉NO₃ as a sacrificial precursor for the first time. The structure, morphology and phase evolution process were significantly affected by the reaction conditions (hydrothermal treatment time and concentration of (NH₄)₂HPO₄). The luminescence colors of YPO₄:Tb³⁺, Eu³⁺ phosphors can be easily tuned from green to primrose yellow and then to white by adjusting the concentration of Eu³⁺ ions under 373 nm irradiation. Furthermore, the energy transfer efficiency of YPO₄:0.08Tb³⁺, xEu³⁺ increased gradually and reached about 94.30% when the concentration of Eu³⁺ ions was 0.10. The energy transfer mechanism from Tb³⁺ to Eu³⁺ has been proved to be a quadrupole–quadrupole interaction mechanism. These merits manifest that the yttrium phosphate phosphors with tunable white light emission may have potential applications in white light-emitting diodes.

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1 Introduction

In recent years, hydrothermal synthesis has drawn comprehensive attention, due to its superior characteristics, such as controllable reaction conditions, simple facilities, relatively high yield and low reaction cost, as well as frequent use of water as the reaction medium.¹ As one of the most effective solution-based approaches, hydrothermal synthesis has been of great importance for preparing novel lanthanide (Ln³⁺)-doped nano/micro luminescent materials.^2–4 Especially hydrothermal conversion method has been widely employed. For example, Jia et al.⁵ prepared octahedral LuVO₄ microcrystals through a designed two-step hydrothermal method using Lu₄O(OH)₉NO₃ as a sacrificial precursor. Furthermore, it has been universally acknowledged that the properties of inorganic nano/micro luminescent materials, to some extent, are not only related to their chemical composition and crystal structure, but also their size, shape, and dimensionality.^6,7 Up to now, considerable efforts for hydrothermal synthesis have been dedicated to designing various crystal structures, such as churchite, monazite, rhabdophane and zircon,^8–10 and morphologies (e.g., nanotubes,¹¹ nanorods,¹² nanofibers,¹³ nanosheets¹⁴). In particular, organic additives have been extensively used to control the dynamics of crystal growth and manipulate the shape of materials. However, the additives may result in unavoidable surface contamination and incur impurities in the obtained samples, and the kinetics of the crystal growth with additives is a complicated process. Therefore, it still remains a challenge to develop a mild and environmentally friendly reaction system for fabrication of size and shape controlled nano/microluminescent materials without any organic solvent, surfactant, or catalyst.

Lanthanide orthophosphates (REPO₄) have been widely investigated,^15–17 due to their high thermal and chemical stability,¹⁸ refractive index¹⁹ and quantum yield.²⁰ They have been extensively used in various fields,^21,22 such as high quality phosphors, high-performance luminescent devices, magnets, optoelectronics, catalysts, solar cells, down and up-conversion materials, and other functional materials, on account of their fascinating optical, catalytic, and magnetic characteristics. Among the different lanthanide phosphates researches, yttrium orthophosphate with unprecedented luminescent functions is chosen as a host matrix and Eu³⁺, Tb³⁺ as dopants to study the luminescence properties, based on the following characteristics:^23,24 (1) It is isostructural to metal phosphates (LnPO₄). Y³⁺ sites can be easily substituted by Ln³⁺ ions because of similar ionic size and oxidation state, (2) the host has high thermal stability, chemical stability and good biocompatibility. More importantly, (3) YPO₄ has several phases like hexagonal, monoclinic and tetragonal. These merits indicate that YPO₄ is a better host. Therein, the hexagonal phase is metastable and can be apt to transform to the thermodynamically stable phases (monoclinic phase and tetragonal phase) by high-temperature calcination, but the reverse process is very difficult.²⁵ For example, Luwang et al.²⁶ reported that the tetragonal phase YPO₄ transformed to hexagonal structure by increasing Ce³⁺ concentration, and then returned to the tetragonal structure on annealing above 900 °C. However, there are still few reports on phase transformation from thermodynamically stable phases to metastable phases and reversely to thermodynamically stable phases without any organic additives in the hydrothermal system. If phase evolution could be suitably adjusted by hydrothermal conditions, many attractive properties would be realizable. Therefore, we are working to solve those aforementioned problems.

In this work, via a facile hydrothermal conversion method, a series of Eu³⁺, Tb³⁺ and Tb³⁺/Eu³⁺ co-doped YPO₄ phosphors have been successfully synthesized by treating Y₄O(OH)₉NO₃ precursor with diammonium hydrogen phosphate for the first time. The effects of reaction conditions on structure, morphology and phase transformation process of as-synthesized samples have been studied in detail. Moreover, the photoluminescence (PL) properties of Eu³⁺, Tb³⁺ and Tb³⁺/Eu³⁺ co-doped YPO₄ phosphors have also been investigated.

2 Experimental section

2.1 Materials

Y(NO₃)₃ and Ln(NO₃)₃ (Ln = Eu and Tb) aqueous solutions were prepared by dissolving the corresponding metal oxide (99.99%) in dilute nitric acid solution under heating with agitation, respectively. Diammonium hydrogen phosphate (analytic reagent) was purchased from Tianjin Kermel Chemical Reagent Co., Ltd. Other chemicals were purchased from Beijing Beihua Fine Chemicals Co. All the other chemical reagents were of analytical grade and used directly without further purification.

2.2 Hydrothermal synthesis of Y₄O(OH)₉NO₃ precursor

The Y₄O(OH)₉NO₃ precursor were prepared via a straightfoward hydrothermal synthetic process. 20 mL Y(NO₃)₃ (0.1 M) aqueous solution was added to 15 mL distilled water. Then 25 wt% aqueous ammonia solution was added dropwise into the solution with continuous agitation until pH = 9.5. After homogenizing for 30 min, a white colloidal precipitate (total volume = 35 mL) was formed, which was transferred into a 50 mL stainless Teflon-lined autoclave and heated at 180 °C for 24 h. After the autoclave was naturally cooled to room temperature, the white precipitate were collected by centrifugation, washed with distilled water and absolute ethanol several times to remove by-products, and finally dried in air at 60 °C for 12 h.

2.3 Preparation of the cuboid YPO₄ samples

The 1 mmol as-prepared precursor suspension was firstly redispersed into 10 mL distilled water by ultrasonic treatment. Simultaneously, a certain quantity (NH₄)₂HPO₄ was dissolved in 30 mL distilled water to form a colorless solution. And then mixed the above two solutions, followed by additional agitation for 20 min, the resultant mixture was transferred to a 50 mL autoclave, sealed and maintained at 180 °C for 72 h. Subsequently, the resulting product was cooled down to room temperature naturally. The precipitate was separated by centrifugation, sequentially washed with deionized water and anhydrous ethanol several times, and dried in air at 70 °C for 12 h. Different reaction time (24, 36, 48, 72 h) and reactant concentrations of diammonium hydrogen phosphate (2, 4, 6, 8 mmol) were selected to investigate the conversion process from precursor to final product. A similar process was repeated under similar manner for the synthesis of YPO₄:Ln³⁺ (Ln = Eu, Tb) samples, by using the same amount of Ln(NO₃)₃ solutions instead of Y(NO₃)₃ as the starting materials.

2.4 Characterization

Phase identification was made via X-ray powder diffraction (XRD) performed on a Rigaku D/max-II B X-ray diffractometer with monochromatic Cu Kα radiation (wavelength λ = 1.5418 Å). The XRD data were collected using a scanning mode in the 2θ range from 10° to 70° with a scanning step size of 0.02° and a scanning rate of 4.0° per minute. The morphology and microstructure of the as-obtained products were analyzed by field emission scanning electron microscopy (FE-SEM, S4800, Hitachi) operated under 15 kV. The ultraviolet-visible PL excitation and emission spectra were analyzed with a Hitachi F-7000 spectrophotometer equipped with a 150 W xenon lamp as an excitation source. All the measurements were performed at room temperature.

3 Results and discussion

3.1 Phase structure, and morphology

The crystal structure and phase purity of the as-obtained samples were detected by XRD measurements. Different reaction time and reagent concentrations of diammonium hydrogen phosphate were investigated for the synthesis of single phase YPO₄ products in present system. Fig. 1a shows the X-ray diffraction pattern of the as-obtained yttrium precursor. It is self-evident that well-defined diffraction peaks can be readily indexed to the monoclinic phase of Y₄O(OH)₉NO₃ (JCPDS no. 79-1352). In order to investigate the possible formation process of hydrothermal conversion from precursor compound to target product, time-dependent XRD experiments were performed (8 : 1 molar ratio of (NH₄)₂HPO₄/precursor) and other preparation conditions kept constant. When the reaction time was carried out for 24 h, the tetragonal phase appeared and became the dominant phase in comparison with the precursor (Fig. 1b), the diffraction peaks of the product can be indexed as a mixture of the precursor and tetragonal phase (JCPDS no. 83-0658). As the reaction time increased to 36 h (Fig. 1c), it can be seen that the diffraction peaks of the Y₄O(OH)₉NO₃ precursor have almost disappeared except for the main peak at 10.08°. In addition, some strong diffraction peaks belonging to the hexagonal phase of YPO₄·0.8H₂O (JCPDS no. 42-0082) could be detected and became the prime phase. It can be indicated that most of the precursor have transformed to more stable phase during the hydrothermal process. With the increase of the reaction time to 48 h, the diffraction peaks matched well the tetragonal phase besides some minor peaks of hexagonal phase at 14.9°, 20.6°, 30.2°, 32.2°, 43.2°, respectively. Upon further increasing the time to 72 h (Fig. 1d), both the residual precursor and YPO₄·0.8H₂O disappeared completely, all diffraction peaks matched well a tetragonal phase, while no other peaks from impurities can be observed, which indicates a pure tetragonal phase. Moreover, the diffraction peaks are very intense and sharp, indicating that the powder samples with high crystallinity can be prepared by hydrothermal conversion method. It is extremely essential for phosphors, because high crystallinity generally means fewer defects and stronger luminescence.

Fig. 1 XRD patterns of as-formed Y₄O(OH)₉NO₃ (a) and the hydrothermal conversion products prepared at 180 °C for (b) 24 h, (c) 36 h, (d) 48 h, and (e) 72 h. The standard data for Y₄O(OH)₉NO₃ (JCPDS card no. 79-1352), YPO₄·0.8H₂O (JCPDS card no. 42-0082), YPO₄ (JCPDS card no. 83-0658) are also presented at the bottom as reference.

It is worth noting that the phase transformation under hydrothermal conditions are considerably different from those upon high-temperature reactions. In this work, the unordinary mixed phase evolution from tetragonal phase to hexagonal phase occurred and then transformed to pure tetragonal phase during hydrothermal process. It is difficult to understand the phase transformation via thermodynamic process, because tetragonal phase is more stable than hexagonal phase in water system.²⁴ The metastable phases are apt to transform to the thermodynamically stable phases by high-temperature calcination. Luwang et al.²⁷ reported that the phase evolution from hexagonal to tetragonal phase when the annealing temperature reached 800 °C in the Bi³⁺ and Eu³⁺ co-doped YPO₄ samples. Zollfrank et al.²⁸ reported that the phase transformation from monoclinic nonhydrated EuPO₄ to monazite when temperature exceeding 600 °C. However, the reverse process is very difficult, and even impossible without the use of any surfactant, additive or catalyst. Zhao et al.²⁵ found that low-temperature monoclinic BiPO₄ reversely transformed to hexagonal phase when the hydrothermal temperature increased above 180 °C. For the latter case, they claimed that the transformation process might be ascribed to the reaction medium of water. However, Zhao et al. didn't study the effect of reaction time and high PO₄³⁻ ions concentration on phase transformation process. In order to understand the possible mechanism of phase evolution, a series of parallel experiments with different molar ratio (P/Y) of (NH₄)₂HPO₄/precursor (2 : 1, 4 : 1, 6 : 1, 8 : 1) were studied deeply. Fig. S1† compares XRD patterns of the samples synthesized with different molar ratios, as shown in Fig. S1a and b,† the product prepared at P/Y = 2 and 4, remained numerous precursors, which indicated the lack of PO₄³⁻ ions. With raising the P/Y value to 6 (Fig. S1c†), the hexagonal phase appeared, interestingly. Upon further extending the P/Y value to 8 (Fig. S1d†), all diffraction peaks matched well a pure tetragonal phase. This result adequately demonstrated that different reactant concentrations had a substantial impact on the evolution of crystal structure.

In order to elaborate the formation mechanism of the cuboid YPO₄ samples in detail, time-dependent experiments were carried out by keeping other reaction parameters unchanged, which is showed in Fig. 2. Fig. S2† is a panoramic SEM image observed from the as-formed Y₄O(OH)₉NO₃, clearly indicating that the precursor is totally composed of uniform nanorods with diameters of 200 nm and lengths of about 1 μm. When the precursor reacted with (NH₄)₂HPO₄ at 24 h (Fig. 2a), it can be seen that the as-obtained samples inherited its parents' morphologies, and a large number of cuboid YPO₄ samples formed on the surface of precursor, which resulted in roughness and the bigger size of precursor due to the decomposition of the precursor and crystallization of the YPO₄. When the reaction time extended to 36 h (Fig. 2b), it was obvious that abundant precursor disintegrated into cuboid nanostructures and some irregular nanocrystals. By increasing the reaction time from 36 h to 48 h (Fig. 2c), the precursor disappeared and completely decomposed to cuboid YPO₄ besides a few irregular nanocrystals. The homogeneous and uniform cuboid YPO₄ products (the diameters of about 100 nm and lengths of about 240 nm) could be acquired when the reaction time prolonged to 72 h (Fig. 2d). According to the above experimental results and analysis, a schematic illustration for the formation of the YPO₄ sample is presented in Fig. S3.† Moreover, it could be observed that the surfaces of the cuboid YPO₄ were very smooth, implying less flaws and more intense luminescence, which was in agreement with the XRD results of the sample.

Fig. 2 SEM images of the hydrothermal conversion products prepared at 180 °C for 24 h (a), 36 h (b), 48 h (c), and 72 h (d).

On the basis of the results and analysis mentioned above, it is possible to interpret the conversion process as follows. In the early stage of the reaction, the precursor disintegrated slowly and limited Y³⁺ ions generated in the system, resulting in a slow reaction rate, which is conducive to form tetragonal phase. With the increase of reaction time, the dissolution rate of the precursor also increased, and more free Y³⁺ ions were released in the solution, which increased the reaction rate under the high PO₄³⁻ concentration. During the reaction process, the Y³⁺ ions near the nuclei were consumed quickly, which led to a considerably low concentration of Y³⁺ ions in the local micro regions. As a result, a high concentration difference formed between the micro regions near the nanocrystals and bulk solution. Consequently, the mass diffusion of Y³⁺ ions is the key to the succedent formation of crystals.²⁹ This result allows us to conjecture that the excess PO₄³⁻ ions concentration results in higher reaction rate, which may be favorable for the formation of hexagonal phase instead of tetragonal phase. Vanetsev et al.³⁰ also reported that the high PO₄³⁻ ions concentration would fasten chemical reaction rate and be in favor of forming hexagonal phosphate. As the reaction time further increased, the Y₄O(OH)₉NO₃ precursor was completely consumed, and the hexagonal phase entirely converted into pure tetragonal phase. This changing process is suitable for thermodynamic process. Based on these analyses, the above transformation might be associated with reaction time and excess phosphate ions.

3.2 Luminescence properties

It is generally acknowledged that lanthanide orthophosphate is very remarkable host matrix, which is used as luminescent material with manifold optically active lanthanide ions. Among the lanthanide ions, Eu³⁺ ion is a crucial red-emitting activator in commercial phosphors, which is ascribed to the ⁵D₀ → ⁷F_J (J = 1–6) transitions in the red spectral area. Moreover, Tb³⁺ ion emits blue light stemming from the transitions of ⁵D₃ → ⁷F_J (J = 1 and 2) and green light resulting from the transitions of ⁵D₄ → ⁷F_J (J = 3–6), which is used as an activator in green phosphors.⁵ In order to achieve the multicolour tunable luminescence, Eu³⁺, Tb³⁺and Tb³⁺/Eu³⁺ co-doped YPO₄ phosphors have been synthesized in our experiment.

Fig. 3a shows the photoluminescence excitation (PLE) and photoluminescence emission (PL) spectra of YPO₄:0.04Eu³⁺ phosphor. As seen from in Fig. 3a, the PLE spectra monitored at 592 nm presents several strong bands at 365 (⁷F₀ → ⁵D₄) nm, 385 (⁷F₀ → ⁵L₇) nm, 397 (⁷F₀ → ⁵L₆) nm, 419 (⁷F₀ → ⁵D₃) nm and 467 (⁷F₀ → ⁵D₂) nm in the 350–500 nm range, respectively. The incisive excitation peak at 397 nm is attributed to the intraconfigurational f–f transitions of Eu³⁺ ions, which is well associated with the comprehensively applied output wavelengths of UV LED chips. Upon UV excitation at 397 nm, the emission spectrum of YPO₄:0.04Eu³⁺ makes up a few characteristic transitions of Eu³⁺ within its 4f⁶ configuration, corresponded to ⁵D₀ → ⁷F₁ (592 nm), ⁵D₀ → ⁷F₂ (618 nm), ⁵D₀ → ⁷F₃ (650 nm), ⁵D₀ → ⁷F₄ (695 nm), respectively. As everyone knows, the ⁵D₀ → ⁷F₂ hypersensitive transition (ΔJ = 2) belongs to the forced electric dipole transition, and it is easily affected by the chemical environment surrounding of Eu³⁺. While the ⁵D₀ → ⁷F₁ corresponds to the magnetic dipole transition, which is difficultly influenced by the crystal field around Eu³⁺ ions.³¹ In the emission spectrum of as-prepared YPO₄:Eu³⁺ nanocrystals, the emission intensity at 592 nm is stronger than that at 617 nm. The result suggests that the Eu³⁺ ions in YPO₄ occupied sites with inversion symmetry. What's more, each emission is observed as two subpeaks by reason of Stark energy splitting, which is influenced by the crystal field around Eu³⁺ environment in the host lattice.³² The quantum yield of YPO₄:0.04Eu³⁺ phosphor is 33.02%.

Fig. 3 Excitation and emission spectra of YPO₄:0.04Eu³⁺ (a), YPO₄:0.08Tb³⁺ (b) nanocrystals.

Fig. 3b displays the PLE and PL spectra of YPO₄:0.08Tb³⁺ phosphor. The excitation spectrum observed at 544 nm exhibits several strong peaks at 355 nm, 362 nm, 373 nm, 381 nm, 491 nm, corresponded to ⁷F₆ → ⁵D₂, ⁷F₆ → ⁵L₁₀, ⁷F₆ → ⁵G₅, ⁷F₆ → ⁵G₆ and ⁷F₆ → ⁵D₄, respectively. Upon excitation with 373 nm irradiation, the YPO₄:0.08Tb³⁺ sample shows four prominent lines centered at 489 nm, 544 nm, 588 nm, 621 nm, which are associated with the transitions from the ⁵D₄ excited state to the ⁷F_J (J = 6, 5, 4, 3) ground states of Tb³⁺ ions. The most highlighted peaks at 544 nm is ascribed to the ⁵D₄ → ⁷F₅ transition. Furthermore, the quantum yield of YPO₄:0.08Tb³⁺ phosphor is tested to be 14.3%.

The doping concentrations of luminescent centers is a crucial factor affecting the phosphor performance.³³ As a consequence, a series of Tb³⁺ doped YPO₄ phosphors have been synthesized with different Tb³⁺ concentration to confirm the optimal dopant amount. As shown in Fig. 4, the emission spectra measured at 373 nm doesn't observe noteworthy shift of the peak positions with adjusting the Tb³⁺ ions concentration. However, the intensities of band have changed obviously. All the samples show the intense emission peaks ranging from 400 to 700 nm, the emission intensities increase with increasing the Tb³⁺ ions concentrations and reach a maximum at y = 0.08, and then decrease with further increasing (y) (the inset in Fig. 4) owning to concentration quenching.

Fig. 4 Effect of the activator concentration on optical properties of YPO₄:yTb³⁺ (a). The inset (b) stands for the relative intensity of the 544 nm emission, as a function of the Tb³⁺ ions concentration.

Energy transfer is of great importance to improve the emission efficiency for nano/micro luminescent materials. In order to obtain the multicolour tunable luminescence, we choose the Tb³⁺ as sensitizer and Eu³⁺ as activator to investigate the energy transfer from Tb³⁺ to Eu³⁺. Fig. 5a compares the PLE spectrum of YPO₄:Eu³⁺ and PL spectrum of YPO₄:Tb³⁺. It can be seen clearly that a effective spectral overlap between the emission spectrum of Tb³⁺ and the excitation spectrum of Eu³⁺ is observed, which allows us to speculate that an efficient resonance type energy transfer may exist in the Tb³⁺/Eu³⁺ co-doped YPO₄ phosphors. In order to further demonstrate the existence of the energy transfer from Tb³⁺ to Eu³⁺, when exciting with 355 nm laser light corresponding to Tb³⁺:⁷F₆ → ⁵D₂ transition (Fig. 5b), the PL spectrum concurrently contains the weak peaks ⁵D₀ → ⁷F₁ (592 nm), ⁵D₀ → ⁷F₂ (618 nm) of Eu³⁺ ions and the strong peaks ⁵D₄ → ⁷F₆ (489 nm), ⁵D₄ → ⁷F₅ (544 nm) transition of Tb³⁺ ions in the YPO₄:0.08Tb³⁺, 0.04Eu³⁺ phosphor. Nevertheless, YPO₄:0.04Eu³⁺ phosphor has no emissions because there is no obvious absorption of Eu³⁺ at 355 nm (Fig. 3a). In addition, upon 381 nm excitation (⁵G₆ of Tb³⁺), compared with the emission spectrum of Eu³⁺ single-doped sample (Fig. 5c), some new emission peaks of Tb³⁺ can be observed in Tb³⁺/Eu³⁺ codoped sample. Moreover, in the YPO₄:0.08Tb³⁺, 0.04Eu³⁺ sample, the emission intensities of Eu³⁺:⁵D₀ → ⁷F₁, ⁵D₀ → ⁷F₂ transitions obviously increase. While the emission intensity of ⁵D₄ → ⁷F₅ (544 nm) transition of Tb³⁺ ions distinctly decreases compared with the YPO₄:0.08Tb³⁺ nanocrystal. The abovementioned spectral features give direct evidence to illustrate that the energy transfer process from Tb³⁺ to Eu³⁺ exists in YPO₄ host.^34,35

Fig. 5 PLE spectra of YPO₄:Eu³⁺ and PL spectra of YPO₄:Tb³⁺ phosphors (a), PL spectra of the YPO₄:0.04Eu³⁺, YPO₄:0.08Tb³⁺ and YPO₄:0.08Tb³⁺, 0.04Eu³⁺ samples under 355 nm and 381 nm excitation, respectively (b) and (c).

Fig. 6 illustrates the variation of the emission spectra of the YPO₄:0.08Tb³⁺, xEu³⁺ (x = 0–0.10) phosphors. Under directly excited at 373 nm (⁷F₆ → ⁵G₅ excitation transition of Tb³⁺), the emission spectra shows the characteristic emission bands of both Tb³⁺ and Eu³⁺. With the increase of the Eu³⁺ content, the emission intensity of the sensitizer Tb³⁺ is found to decrease monotonically, due to the markedly energy transfer between Tb³⁺ and Eu³⁺. Meanwhile, the emission intensity of the activator Eu³⁺ first increases up to an optimum concentration at 0.04 and then decreases because of the concentration quenching effect. Moreover, the quantum yield of YPO₄:0.08Tb³⁺, 0.04Eu³⁺ phosphor is 11.01%.

Fig. 6 PL spectra for YPO₄:0.08Tb³⁺, xEu³⁺ phosphors as a function of the Eu³⁺ doping content (x).

The energy transfer efficiency (η_T) from the sensitizer Tb³⁺ to the activator Eu³⁺ was also investigated as a function of Eu³⁺ content, which is depicted in Fig. 7. The energy transfer efficiency can be approximately estimated according to the following equation:³⁶

where I and I₀ are the emission intensities of the sensitizer (Tb³⁺) in the presence and absence of an activator (Eu³⁺), respectively. With the increase of Eu³⁺ content, the energy transfer efficiency increases monotonically and reaches 94.30% when the concentration of Eu³⁺ is 0.10. The phenomenon may be ascribed to be the decreased the average distance between Tb³⁺ and Eu³⁺ ions. With the decrease of average distance, the excitation energy may transfer from the Tb³⁺ to close Eu³⁺ ions, which results in high energy transfer efficiency. For this reason, the critical distance (R_c) is a crucial parameter which can be calculated by the following formula:³⁷where x_c represents the critical concentration (the total concentration of sensitizer Tb³⁺ and activator Eu³⁺) at maximum luminescence intensity. N stands for the number of host cation in the YPO₄ unit cell and V is the volume of the unit cell. The crystallographic data based on the experimental and calculative values are given as follows: x_c = 0.12, N = 4, V = 287.7 ų, respectively. Accordingly, based on the eqn (2), the critical distance R_c is calculated to be 10.46 Å. As is known to all, the resonant energy transfer mechanism between a donor and an activator consists of two main aspects. One is the exchange interaction and the other one is multipole–multipole interaction. If the R_c is shorter than 5 Å, the former will occur. Otherwise, the latter will become the dominating energy transfer mechanism. The estimated critical distance is 10.46 Å, which is greater than 5 Å, indicating multipole–multipole interaction plays a important role in the process of energy transfer from Tb³⁺ to Eu³⁺ in the YPO₄ host matrix.

Fig. 7 The dependence of energy transfer efficiency η_T of YPO₄:0.08Tb³⁺, xEu³⁺ phosphors on Eu³⁺ ions concentration.

To further explain the nature of energy transfer mechanism from the Tb³⁺ to Eu³⁺ ions, Reisfeld's approximation and Dexter's energy transfer expressions of multipolar interaction are employed. The corresponding formula can be expressed by:³⁸

where I and I₀ are the fluorescence intensities of the donor with and without the acceptor, C is the total concentration of the donor and acceptor, and n represents the values of 6, 8 or 10 corresponding to dipole–dipole, dipole–quadrupole or quadrupole–quadrupole interactions, respectively. Fig. 8 shows the relationship between the I₀/I and C^n/3. The linear relevant fitting result is well at n = 10 (Fig. 8c). It turns out that energy transfer mechanism of the Tb³⁺/Eu³⁺ co-doped YPO₄ phosphors is a quadrupole–quadrupole interaction.

Fig. 8 Dependence of I₀/I of Eu³⁺ on C^6/3 (a), C^8/3 (b) and C^10/3 (c).

Fig. 9 shows the schematic illustration of energy transfer from Tb³⁺ to Eu³⁺. Upon 373 nm excitation, it can be seen that the energy level of Eu³⁺ (⁵D_0,1) is located below energy level of Tb³⁺ (⁵D₄). In addition, there is a effective overlap between ⁵D₄ → ⁷F_J (J = 1–6) emission of Tb³⁺ and the ⁷F₁ → ⁵D_1,2 absorption of Eu³⁺, which indicates that energy transfer is valid. Therefore, the ground state electrons of Eu³⁺ ions may be excited to the excited state ⁵D₁ by energy transfer from Tb³⁺ ions. Subsequently, the ⁵D₁ energy level of Eu³⁺ relaxes to the ⁵D₀ energy level, and finally transfers to the ⁷F_J (J = 1–4) ground states, which improves the characteristic emission of Eu³⁺.

Fig. 9 Schematic energy level diagram standing for the energy transfer and energy transfer mechanism in the Tb³⁺/Eu³⁺ co-doped YPO₄ phosphors.

Fig. 10 shows the calculated CIE chromaticity coordinates of YPO₄:0.08Tb³⁺, xEu³⁺ samples under excitation at 373 nm, and the corresponding data in detail are listed in Table 1. The color tone of phosphors can be modulated from green to primrose yellow and then to white by adjusting the doped concentration of Eu³⁺ ions from 0 to 0.08. Furthermore, the chromaticity coordinate of YPO₄:0.08Tb³⁺, 0.06Eu³⁺ phosphor is (0.335, 0.346), which is close to that of the standard white light (0.33, 0.33). The correlated color temperature (CCT) of the samples has also been calculated and the corresponding data are given in Table 1. Based on the above results, the yttrium phosphate phosphors with tunable white light emission may have potential application as white-emitting phosphors in white-LEDs.

Fig. 10 The CIE chromaticity diagram of the YPO₄:0.08Tb³⁺, xEu³⁺ (x = 0, 0.02, 0.04, 0.06, 0.08) phosphors.

Table 1 The CIE chromaticity coordinates (x, y) and CCT parameters of YPO₄:0.08Tb³⁺, xEu³⁺ (x = 0, 0.02, 0.04, 0.06, 0.08) phosphors upon excitation at 373 nm

Label	Sample compositions	CIE (x, y)	CCT (K)
a	YPO4:0.08Tb^3+	(0.260, 0.505)	7250
b	YPO4:0.08Tb^3+, 0.02Eu^3+	(0.292, 0.439)	6690
c	YPO4:0.08Tb^3+, 0.04Eu^3+	(0.325, 0.405)	5737
d	YPO4:0.08Tb^3+, 0.06Eu^3+	(0.335, 0.346)	5387
e	YPO4:0.08Tb^3+, 0.08Eu^3+	(0.357, 0.338)	4483

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4 Conclusion

In summary, for the first time, we developed a facile hydrothermal conversion method for the synthesis of uniform and monodispersed cuboid YPO₄ samples using Y₄O(OH)₉NO₃ as a sacrificial precursor. The reaction conditions (hydrothermal treatment time and concentration of (NH₄)₂HPO₄) play crucial roles in obtaining pure YPO₄ products with uniform cuboid morphology and high crystallinity. Interestingly, it can be observed that the abnormal mixed phase evolution from tetragonal to hexagonal occurred, and then transformed to pure tetragonal phase by changing the reaction conditions. The energy transfer process from Tb³⁺ to Eu³⁺ has also been systematically investigated. The energy transfer mechanism between Tb³⁺ and Eu³⁺ has been demonstrated to be a quadrupole–quadrupole interaction. Moreover, the energy transfer efficiency (η_T) and average donor–acceptor distance (R_c) were calculated to be 94.30% and 10.46 Å, respectively. The emitting colors of Tb³⁺/Eu³⁺ co-doped YPO₄ phosphors could be adjusted from green to the primrose yellow and then to white by tuning the Eu³⁺ doped concentration. These results reveal that this kind of material with variable and tunable colors could be potentially used in white-LEDs.

Acknowledgements

This present work was financially supported by China Geological Survey (Grant No. 12120113088300); and the Key Technology and Equipment of Efficient Utilization of Oil Shale Resources, No. OSR-5.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra22913a

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