Tunable luminescence evolution and energy transfer behavior of Na₃Sc₂(PO₄)₃:Ce³⁺/Tb³⁺/Eu³⁺ phosphors

Abstract

A series of color-tunable emitting Na₃Sc₂(PO₄)₃:Ce³⁺/Tb³⁺/Eu³⁺ (NSPO) phosphors were prepared by a combination of hydrothermal synthesis and low temperature calcination. The phase structure, photoluminescence and energy transfer properties of the samples were studied in detail. The tunable colors were obtained by co-doping the Tb³⁺ ions into the NSPO:Ce³⁺ or NSPO:Eu³⁺ phosphors with varying concentrations. Under UV excitation, the energy transfers from Tb³⁺ to Eu³⁺ in the NSPO host occurred mainly via a dipole–dipole mechanism, and the critical distances of the ion pairs (R_c) was calculated to be 17.94 Å by the quenching concentration method. And that, the emission colors of the NSPO:Tb³⁺,Eu³⁺ phosphors could be adjusted from green through yellow to red because of the energy transfer from Tb³⁺ to Eu³⁺. Based on its good photoluminescence properties and abundant emission colors, the NSPO:Ce³⁺/Tb³⁺/Eu³⁺ phosphors might be promising as potential candidates for solid-state lighting and display fields.

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1 Introduction

When replacing cations in certain hosts, rare earth elements are important activators for modern lighting and display applications, because they have rich emission colors for spectral conversion.^1–5 Recently, rare earth ion-doped inorganic materials have drawn a great deal of attention due to their many applications in optics, magnetics and catalysis.^6–8 In the lighting field, compared with traditional incandescent lighting, white light-emitting diodes (w-LEDs) are favored due to their higher conversion efficiency and more flexible photometric properties.^9–11 Now, the main manufacturing technology of w-LEDs relies on the combination of a blue InGaN chip and a yellow light-emitting phosphor Y₃Al₅O₁₂:Ce³⁺ (YAG). Although this method shows high efficiency, it exhibits a high correlated color and low color rendering index (R_a < 80) because of the lack of a red component.¹² Currently, a new type of w-LEDs assembly which is made by ultraviolet or near ultraviolet excitation single matrix multicolor fluorescent powders is widely used.¹³ Therefore, due to the need to increase the efficiency of white light emitting solid state devices, the development of the technique of combining a UV LED chip with tricolor (red, green and blue) phosphors has been proceeding. In this case, recent research has focused on finding converted luminescent phosphors with suitable emission colors, high efficiencies and high chemical stability.

Researchers have studied a variety of inorganic materials, including silicates, aluminates, phosphates, nitrides, molybdates and tungstates. Among them, the rare earth ion-doped phosphate materials have attracted much attention because of their abundance, good chemical stability and low sintering temperature.^14–18 Luminescence of rare earth ions in orthophosphates with the general formula M^I₃M^II(PO₄)₃ or M^I₃M^II₂(PO₄)₃ (M^I = Ca, Sr, Ba, Pb, K and Na; M^II = La, Y, Gd, Sc, Bi and In) are widely researched.^19–21 As early as the 1980s, scandium phosphate Na₃Sc₂(PO₄)₃ (NSPO) was reported and developed as an ion conductor.^22,23 During 2013–2016, Hirohumi studied the luminescent properties of Eu²⁺-doped NSPO respectively.^24,25 Recently, Huang's group studied the luminescent properties of Eu³⁺ doped NSPO phosphors and NSPO:Ce³⁺,Tb³⁺ phosphors;^26,27 they also reported Dy³⁺ and Eu³⁺ co-doped NSPO phosphors.²⁸ Among these research reports, the high temperature solid-state reaction method is the most common route to produce orthophosphates. However, it has many disadvantages such as complex experimental conditions, cumbersome procedures and high reaction temperatures (such as 1300 °C).^25–31 Therefore, from the point of view of simplicity and energy saving, there is still an urgent need to develop a simple and mild solution phase method to produce pure orthophosphate micro/nanocrystals.

Luminescent lanthanide ions can be divided into two categories: 4f–4f and 5d–4f transitions, respectively. Due to the strict ban of 4f–4f transitions, their emission peak widths are very narrow and fixed. For 5d–4f transitions, the 5d orbitals are exposed to the environment and their emission spectra are strongly influenced by the host, such as the strength of the crystal field.³² We know that the energy transfer process may be affected by the presence of carrier traps or defects in the host lattice. Therefore, it is possible for the tiny dopants with different affinities to change the trap configuration and thus significantly improve the efficiency.³³ It is clear that Tb³⁺ acts as a good sensitizer to increase the luminescence efficiency of Eu³⁺ ions in Na₃Gd(PO₄)₂,³⁴ CaYAlO₄,³⁵ SrMg₂LaW₂O₁₂ (ref. 36) and Y₂O₃ (ref. 37) phosphors. Therefore, Tb³⁺ and Eu³⁺ ions can be used as efficient luminescent centers in real devices. Ce³⁺ ions with permissive 5d–4f emission transitions from 300 to 400 nm have been widely studied as efficient activators.^38,39 In addition, Tb³⁺ may also be an important activator with only weak absorption peaks at about 300–400 nm due to the 4f–4f absorption transitions.⁴⁰ Therefore, Ce³⁺ ions co-doped in the host can not only increase the emission intensity of Tb³⁺ ions but also cause excitation in the ultraviolet region of 200–400 nm, thereby overcoming the defects of the Ce³⁺ or Tb³⁺ ions doped alone. So far, many Ce³⁺ and Tb³⁺ co-doped materials have been reported including silicates, aluminates, phosphates, fluorides and oxynitrides.^41–44 To the best of our knowledge, the energy transfer of Tb³⁺ → Eu³⁺ in NSPO materials by a simple method has little been reported in the previous literature.^24,25,27

In this work, Tb³⁺–Eu³⁺ (Ce³⁺–Tb³⁺) co-doped NSPO phosphors were firstly synthesized by a combination of hydrothermal method and low temperature calcinations (800 °C). And we obtained tunable emission colors such as red (Eu³⁺), green (Tb³⁺), blue-violet (Ce³⁺), yellow (Tb³⁺–Eu³⁺) and so on. The energy transfer mechanisms, the lifetimes, luminescence properties, crystallinity and color tunability of the NSPO:Tb³⁺–Eu³⁺ phosphors were also investigated. Based on multicolor tunable luminescence, the NSPO:Ce³⁺/Tb³⁺/Eu³⁺ phosphors may have potential applications in the field of color displays, fluorescent lamps, solid-state lighting and UV-pumped LEDs.

2 Experimental section

2.1 Materials

The rare earth oxides including Sc₂O₃ (A.R.), CeO₂ (99.99%), Eu₂O₃ (99.99%) and Tb₄O₇ (99.99%) were purchased from Chuandong Chemical Reagents Company (China). The raw materials sodium citrate (Na₃C₆H₅O₇·2H₂O) (A.R.), sodium phosphate tribasic dodecahydrate (Na₃PO₄·12H₂O) (A.R.) and hydrogen peroxide (H₂O₂) were purchased from Aladdin (China). All chemicals were used directly without further purification. The RECl₃ (RE = Sc, Ce, Eu and Tb) were firstly obtained by dissolving the corresponding rare earth oxides in dilute HCl solution. In addition, it was necessary to add H₂O₂ when dissolving CeO₂ in dilute HCl solution.

2.2 Synthesis

In a typical process of preparing Na₃Sc₂(PO₄)₃ host, 2 mmol Na₃C₆H₅O₇·2H₂O was added to 30 mL deionized water. After vigorous stirring for 30 min, 2 mmol ScCl₃ was added into the above solution for 30 min. Then, it was kept stirring for another 30 min. Subsequently, 2 mmol Na₃PO₄·12H₂O was added into the above reaction solution. After additional agitation for 15 min, the obtained mixing solution was transferred into a 50 ml Teflon-lined stainless steel autoclave and maintained at 180 °C for 24 h. When autoclave cooled to room temperature naturally, the obtained samples were collected by centrifugation, washed several times with deionized water and ethanol and then dried at 60 °C for 24 h. Finally, the obtained precipitates were transferred to muffle stove with the temperature at 800 °C for 2 h to get the final products Na₃Sc₂(PO₄)₃ (marked as NSPO). A series of NSPO:Ce³⁺/Tb³⁺/Eu³⁺ and NSPO:Tb³⁺,Eu³⁺/Ce³⁺,Tb³⁺ phosphors have been synthesized using the same method above except for different stoichiometric amounts of RECl₃ (RE = Sc, Ce, Eu and Tb). Our sample synthesis has good reproducibility, which can also be seen from the XRD, SEM and PL results of the samples in the Results and discussion section.

2.3 Characterization

X-ray power diffraction (XRD) measurements were performed on a Purkinje General Instrument MSALXD3 using Cu Kα radiation (λ = 0.15406 nm). The morphology analysis was carried out on a field emission scanning electron microscopy (FESEM, Hitachi, S-4800). The PL excitation and emission spectra were detected by an F-7000 spectrophotometer (Hitachi, Japan) equipped with a 150 W xenon lamp as excitation source. The decay lifetimes were measured by FLS980 (Edinburgh Instrument).

3 Results and discussion

3.1 Phase, structure and morphology

The crystal structure and phase composition of the samples were studied by X-ray diffraction (XRD). Fig. 1A shows the XRD patterns of pure NSPO (a), NSPO:0.1Ce³⁺ (b), NSPO:0.1Tb³⁺ (c), NSPO:0.1Eu³⁺ (d), NSPO:0.1Tb³⁺,0.1Eu³⁺ (e) and NSPO:0.1Ce³⁺, 0.01Tb³ (f), respectively. All diffraction peaks can be attributed to the pure monoclinic NSPO phase, which is consistent with the standard card (JCPDS#44–0567). No other impurities are detected, indicating that the resulting products consist of a microscopic substructure with high crystallinity. The results show that the doped Ln³⁺ ions do not change the phase of the matrix significantly, and the dopant ions have been successfully dissolved in the lattice of NSPO host. It is well known that the chemical properties and ionic radius of the rare earth ions are similar, so we confirm that the Ce³⁺, Tb³⁺, Eu³⁺, Ce³⁺/Tb³⁺ or Tb³⁺/Eu³⁺ are well incorporated into the NSPO host lattice. It is worth noting that the characteristic diffraction peak (112) is shifted to a lower diffraction angle. This means that the incorporated ions cause the lattice to expand and the doped rare earth ions have replaced the ions in the body. This further shows that Ce³⁺, Tb³⁺ and Eu³⁺ are successfully constructed into NSPO host material by doping strategy. Fig. 1B presents the cell structure of the NSPO crystal and the coordination environment of cation Sc³⁺/Na⁺ with O atoms. The detailed structural parameters and crystallographic data for NSPO crystal are listed in Table S1 (ESI†). In the monoclinic NSPO structure, each Sc³⁺ ion is coordinated by its nearest six oxygen atoms to form a regular quadrangular pyramid where Sc³⁺ ion is located in the symcenter. The Na⁺ ion is coordinated by eight oxygen atoms. The NSPO compound with lattice constants a = b = 8.931 and c = 22.3267 crystallizes in the R3̄c space group. The relevant atomic parameter data are listed in Table S2 (ESI†). As shown in Fig. 1C, we found that the NSPO products are relatively uniform nanoparticles with smaller size of 300 nm comparing with the literature reports^26–28 (Fig. S1, ESI†).

Fig. 1 (A) XRD patterns of pure NSPO (a) and NSPO:Ln³⁺ (b–f); (B) the structure of the NSPO crystals and the sites of Na⁺ and Sc³⁺ polyhedrons, respectively; (C) SEM of the prepared NSPO samples.

3.2 Photoluminescence properties

A series of independently doped NSPO:Ln³⁺ (Ln = Ce/Tb/Eu) and double doped NSPO:Ln³⁺ (Ln = Tb,Eu/Ce,Tb) phosphors were synthesized and their respective luminescence spectra were studied. Eu³⁺ ions are noted activators in various host lattices. Fig. 2a displays the PLE and PL spectra of NSPO:0.1Eu³⁺ phosphors. Monitored at 619 nm, the excitation spectrum includes a weak broad band at 275 nm, which can be attributed to the charge-transfer (CT) band from O²⁻ to Eu³⁺ ions. There are some sharp peaks in the ultraviolet range of 300–400 nm, which can be assigned to the f → f characteristic transition of Eu³⁺ (⁷F₀ → ⁵H₅ at 323 nm, ⁷D₀ → ⁵D₄ at 358 nm, ⁵D₀ → ⁵L₇ at 380 nm and ⁷F₀ → ⁵L₆ at 394 nm).⁴⁵ The strongest one is located at near-UV region of 394 nm which indicates that the phosphor is suitable for excitation of UV LED chips. Upon excitation at 394 nm, the PL spectrum includes two emissions peaks at 594 and 619 nm originating from the ⁵D₀ → ⁷F₁ and ⁵D₀ → ⁷F₂ transitions of Eu³⁺ activator, respectively. We know that the relative intensity of emission is affected obviously by the replaced position of activator Eu³⁺ in host lattice. The ⁵D₀ → ⁷F₁ transition corresponding to the magnetic dipole moment is less sensitive to the crystallographic site symmetry of activator Eu³⁺ than the ⁵D₀ → ⁷F₂ transition corresponding to the electrical dipole moment transition. Therefore, if Eu³⁺ occupies a high symmetry site, the ⁵D₀ → ⁷F₁ emission will be stronger; whereas the ⁵D₀ → ⁷F₂ emission dominates the emission spectrum if Eu³⁺ ions locate in a low symmetry site (Fig. 2a).^46,47

Fig. 2 PL and PLE spectra of the NSPO:0.1Eu³⁺ phosphor (a), NSPO:0.1Tb³⁺ phosphor (b) and NSPO:0.1Tb³⁺,0.1Eu³⁺ phosphor (c), respectively.

We compared the PL and PLE of the Na₃Sc₂(PO₄)₃ doped with Eu³⁺ by both methods (ceramic and hydrothermal) in Fig. S2 (ESI†). Under excitation of 394 nm, the PL spectra of Na₃Sc₂(PO₄)₃:Eu³⁺ phosphors by both methods both exhibited four characteristic emission bands with 594(593), 619(621), 654 and 699 nm corresponding to ⁵D₀ → ⁷F₁, ⁵D₀ → ⁷F₂, ⁵D₀ → ⁷F₃ and ⁵D₀ → ⁷F₄ transitions, respectively. The emission spectrum of the sample by ceramic method appears to be more refined and has higher luminous intensity at 654 and 699 nm, which can be attributed to higher crystallinity caused by the ceramic method at higher temperature. In the PLE spectra of Na₃Sc₂(PO₄)₃:Eu³⁺ phosphors by both methods, the broad band in the 250–300 nm region was both attributed to the CTB transition from the fully filled 2p orbitals of O²⁻ ions to the partially filled 4f orbitals of Eu³⁺. Furthermore, these narrow PLE peaks located at around 323(318), 358(364), 380(384) and 394 nm were all attributed to the intra-4f transitions of Eu³⁺ ions from ⁷F₀ level to ⁵H₆, ⁵D₄, ⁵L₇ and ⁵L₆ level, respectively. Compared with Fig. S2A(a),† there was no peak at 297 nm in Fig. S2B(a)† because of the overlap of the CTB band and the ⁷F₀ → ⁵F_J transition of Eu³⁺. The excitation spectrum of the sample by ceramic method appears to be more refined, which can be also attributed to higher crystallinity caused by the ceramic method at higher temperature.

Fig. 2b shows the PLE and PL spectra of NSPO:0.1Tb³⁺. We know that Tb³⁺ has a simple 4f configuration energy level structure with low energy state ⁷F_J (J = 6, 5, …, 0) and excited states ⁵D_3,4. The excitation spectrum (left) at room temperature (RT) is recorded by monitoring with bright green emission at 548 nm, which showed a strong spectral band at 219 nm owing to the 4f⁸ → 4f⁷5d¹ transition of the Tb³⁺ ions. Here, we can see several weak peaks in the range of 300–500 nm, which are caused by the spin and orbital forbidden intra-4f transitions (enlarged and shown in Fig. S3†). Excited at the wavelength of 219 nm, the NSPO:0.1Tb³⁺ samples have two emission groups, one is the blue emission caused by the ⁵D₃ → ⁷F_J transitions, and the other is the green emission caused by the ⁵D₄ → ⁷F_J transitions. Four typical transitions from ⁵D₄ level down to ⁷F_J levels of 4f⁸ configuration of Tb³⁺ were observed, that is, ⁵D₄ → ⁷F₆ at 488 nm, ⁵D₄ → ⁷F₄ at 587 nm, ⁵D₄ → ⁷F₃ at 621 nm and ⁵D₄ → ⁷F₅ at 548 nm (the strongest one). Furthermore, there are three weaker emission peaks at 375 nm, 417 nm and 434 nm, which can be attributed to the ⁵D₃ → ⁷F₆, ⁵D₃ → ⁷F₅ and ⁵D₃ → ⁷F₄ transitions of Tb³⁺, respectively. The emission spectra illustrate that phosphor NSPO:0.1Tb³⁺ shows an intense green emission. Comparing the excitation band of NSPO:0.1Eu³⁺ with the emission band of NSPO:0.1Tb³⁺ (Fig. 2a and b), a meaningful spectral overlap was observed, which facilitated the transfer energy from Tb³⁺ to Eu³⁺.

Fig. 2c shows the PLE and PL spectra of the typical NSPO:0.1Tb³⁺,0.1Eu³⁺ sample. The excitation spectra under the detection wavelength of 619 nm and 548 nm (Fig. 2c) are both the same as the excitation spectra of the single doped Tb³⁺ in Fig. 2b (219 nm), indicating that double doped NSPO phosphors can be used as a green and red two-color phosphors in near-UV pumped white LEDs. On the other hand, the emission peaks of the Tb³⁺–Eu³⁺ co-doped phosphors under 219 nm excitation were observed at 619 nm (⁵D₀–⁷F₂) and 548 nm (⁵D₄–⁷F₅), which are attributed to the Eu³⁺ and Tb³⁺ ions, respectively. Moreover, 595 nm (⁵D₀–⁷F₁) in the orange region of Eu³⁺ as well as 488 nm (⁵D₄–⁷F₆) in the blue region of Tb³⁺ can be also observed in the NSPO:0.1Tb³⁺,0.1Eu³⁺ phosphors. From the Fig. 2, we can see that the co-doping of the Tb³⁺ ions have greatly improved the Eu³⁺ emission under same irradiation by comparing the emission spectra of the NSPO:0.10Tb³⁺,0.1Eu³⁺ (Fig. 2c) and NSPO:0.1Eu³⁺ phosphors (Fig. 2a). Most notably, the intensity of the characteristic emitter region of Tb³⁺ is weaker than that of Eu³⁺, which provides sufficient evidence for effective energy transfer from the donor Tb³⁺ to the receptor Eu³⁺. Therefore, adjusting the concentrations of the two activators can change the relative intensities of the two emissions by the principle of energy transfer, which is a viable way to get multicolor light emission.

In order to further understand the energy transfer between the Tb³⁺ and Eu³⁺ ions, a series of composition-controlled samples were prepared. Fig. 3 displays the emission spectra of NSPO:0.1Tb³⁺,xEu³⁺ phosphors with x varying from 0 to 0.1. Under excitation at 219 nm UV light, the phosphors show the characteristic emissions of Tb³⁺ as well as sharp emission peaks of Eu³⁺. Keeping the concentration of Tb³⁺ at 0.1, the emission intensity of Eu³⁺ at 595/619 nm gradually increases with the increase of Eu³⁺ concentration, while the emission intensity of Tb³⁺ at 488/548 nm decreases. Under excitation of characteristic excitation peaks of Tb³⁺ at 219 nm, the changing of Tb³⁺ and Eu³⁺ emission relative intensity in NSPO:0.1Tb³⁺,xEu³⁺ phosphors can further prove the efficient energy transfer from Tb³⁺ to Eu³⁺ ions. In addition, the variations of the Eu³⁺ and Tb³⁺ emission intensity in NSPO:0.1Eu³⁺,yTb³⁺ phosphors is displayed in Fig. S4,† which also reflect the same result of energy transfer from Tb³⁺ to Eu³⁺.

Fig. 3 Emission spectra of the NSPO:0.1Tb³⁺,xEu³⁺ phosphors.

For more directly observing the changes of relative emission intensity, Fig. 4 presents the variations of Tb³⁺ at 548 nm and Eu³⁺ at 619 nm upon changing the Eu³⁺ content. It is found that the emission intensity of Tb³⁺ ions decreases monotonically with increasing the content of the Eu³⁺ ions; whereas the emission intensity of Eu³⁺ ions as well as the energy transfer efficiency gradually increases. These results further support the generation of effective energy transfer from Tb³⁺ to Eu³⁺ ions. Here, the energy transfer efficiency (η_T) of Tb³⁺ → Eu³⁺ in the NSPO host can be estimated using the following formula:⁴⁸

η_T = 1 − I_S/I_S0

where I_S and I_S0 are the luminescence intensities of Tb³⁺ in the presence and absence of Eu³⁺, respectively. It is clearly discovered that the value of η_T gradually increases with the increase of Eu³⁺, as shown in Fig. 4 (red line). When x = 0.1, the η_T is as high as 84.72% for NSPO:0.1Tb³⁺,0.1Eu³⁺ phosphors. Generally speaking, energy transfer from sensitizer (Tb³⁺) to activator (Eu³⁺) can be achieved through the exchange interaction or electric multipolar interaction.⁴⁹ If the energy transfer is caused by the exchange interaction, the critical distance between the sensitizer and the activator should be less than 3–4 Å. In contrast, electrical multipolar interaction may dominate.⁵⁰ The critical distance R_c of energy transfer was calculated by using the concentration quenching method, where the critical distance R_c can be calculated by the following formula presented by Blasse:⁵¹
Here V is the unit cell volume, x_c is the critical concentration and N refers to the number of formula units per unit cell. For the NSPO host, V = 1542.20 ų, x_c = 0.085 and N = 6. Based on the above formula, the critical distance for energy transfer is estimated to be about 17.94 Å. The value is longer than 3–4 Å, indicating that the energy transfer mechanism is controlled by electrical multipolar interaction in this system. We have further discussed the energy transfer mechanism for electrical multipolar interaction, which can be determined using the following relationship:^52,53

η₀/η ∝ C^n/3

Here η₀ and η are the luminescence quantum efficiencies of Tb³⁺ in the absence and presence of Eu³⁺, respectively; and the values of η₀/η can be approximately calculated by the ratio of related luminescence intensities (I_S0/I_S); C is the concentration of the sum of Tb³⁺ and Eu³⁺; and n = 6, 8 and 10 correspond to dipole–dipole, dipole–quadrupole and quadrupole–quadrupole interaction, respectively. The relationships between I_S0/I_S and C_{(Tb³⁺+Eu³⁺)}^n/3 are illustrated in Fig. 5 for NSPO:0.1Tb³⁺,xEu³⁺ phosphors. The best linear behavior was observed when n = 6 based on the values of the fitting parameters R², implying that the Tb³⁺ → Eu³⁺ energy transfer occurs by the dipole–dipole interaction.

Fig. 4 Relative emission intensities of Eu³⁺ at 619 nm and Tb³⁺ at 548 nm, and the ET efficiency of Tb³⁺ → Eu³⁺ in NSPO:0.1Tb³⁺,xEu³⁺ phosphors (x = 0–0.1).

Fig. 5 Dependence of I_S0/I_S of Tb³⁺ on C_{(Tb³⁺+Eu³⁺)}^6/3, C_{(Tb³⁺+Eu³⁺)}^8/3 and C_{(Tb³⁺+Eu³⁺)}^10/3, respectively.

To further validate the energy transfer phenomenon, the decay curves of Tb³⁺ emission of NSPO:0.1Tb³⁺,xEu³⁺ (x = 0, 0.01, 0.05, 0.10) phosphors are shown in Fig. 6. We can see that all decay curves can be well fitted by the first order exponential decay method using the formula:^54,55

I_(t) = I₀ exp(−t/τ)

where τ is the 1/e lifetime of Tb³⁺. According to the formula, the decay times were calculated to be 3.36, 2.84, 1.97 and 0.86 ms for NSPO:0.1Tb³⁺,xEu³⁺ with x = 0, 0.01, 0.05 and 0.10, respectively. It is easy to find that the decay lifetime of the Tb³⁺ ions decreases monotonically with the Eu³⁺ concentration increasing, which further confirms the existence of energy transfer from Tb³⁺ to Eu³⁺ ions.⁵¹ Based on the principle of energy transfer, a coordinated emission of colors can be achieved by varying the proportion of doping content. A representative CIE chromaticity diagram and coordinates of NSPO:0.1Tb³⁺,xEu³⁺ (x = 0–0.1) phosphors are calculated from the relevant PL spectra (Fig. 3), as shown in Fig. 7. With the increase of Eu³⁺ concentration, the CIE coordinates range from (0.2865, 0.4982) through (0.3657, 0.4516) to (0.5693, 0.2976); and the emission colors change from green through yellow to red, accordingly. Thus, we make sure that tunable luminescence can be achieved in the new NSPO:0.10Tb³⁺,xEu³⁺ phosphors based on energy transfer.

Fig. 6 Decay curves of Tb³⁺ in NSPO:0.1Tb³⁺,xEu³⁺ (x = 0, 0.01, 0.05, 0.10) phosphors.

Fig. 7 CIE chromaticity coordinates of NSPO:0.1Tb³⁺,xEu³⁺ (x = 0–0.1) phosphors under 219 nm excitation.

Fig. 8 displays the PLE and PL spectra of Ce³⁺ or Tb³⁺ solely doped and Ce³⁺/Tb³⁺ co-activated NSPO phosphors. Ce³⁺ ions have been doped into the host as activator, and the luminescent properties have been explored. As shown in Fig. 8a, the excitation spectrum monitored at 367 nm for NSPO:0.1Ce³⁺ phosphors exhibit a broad band centered at 278 nm, which corresponds to the transition from the 4f state of Ce³⁺ ions to the excited Ce³⁺ ions 5d state. Under the irradiation at 278 nm, the phosphors can emit intense blue-violet light with a peak wavelength at 367 nm which can be attributed to the lowest Ce³⁺ 5d excited state to the ground state ²F_5/2 in the Ce³⁺ 4f configuration. Generally, with a low doping concentration of Tb³⁺ in the host matrix, the character emission of Tb³⁺ is rather weak. Fig. 8b displays the excitation and emission spectra of NSPO:0.01Tb³⁺. The excitation spectrum shows a strong broad absorption band (219 nm) with shoulders at 257/273 nm arising from an allowed 4f⁸–4f⁷5d transition and some weak narrow absorption peaks from 300 to 500 nm due to intra-(4f) transitions of Tb³⁺. The shoulders may originate from the forbidden component of the 4f⁸–4f⁷5d transition.⁵⁶ Based on the above PL spectrum of the Ce³⁺ single-doped sample and the PLE spectrum of the Tb³⁺ single-doped sample (Fig. 8a and b), the spectral overlap between the broad emission band of Ce³⁺ and the Tb³⁺ excitation within the spectral range of 300–400 nm. Thus, it is expected that a resonance-type ET from Ce³⁺ to Tb³⁺ may occur in the NSPO host. As shown in Fig. 8c, the PLE and PL spectra of NSPO:0.1Ce³⁺,0.01Tb³⁺ phosphors were studied. When monitored at 548 nm of Tb³⁺, the excitation spectrum consists of an excitation band (219 nm) of Tb³⁺ ions assigned to f-d transitions and a strongest excitation band (278 nm) of Ce³⁺ ions, indicating the possible energy transfer from Ce³⁺ to Tb³⁺ ions. Moreover, when excited at 278 nm, the PL spectra show not only the broad emission peak in the blue-violet region from the electric-dipole-allowed 4f–5d transitions of Ce³⁺ (367 nm), but also the strong emission of Tb³⁺at 488 nm and 548 nm (⁵D₄–⁷F_J=6,5), which also provides another evidence for the energy transfer from Ce³⁺ to Tb³⁺. To know the energy transfer process, a series of samples with different concentrations of Ce³⁺ were prepared. Fig. S5† shows the emission spectrum of the NSPO:0.01Tb³⁺,yCe³⁺(y = 0, 0.05, and 0.10) phosphors upon an excitation wavelength of 278 nm. When the Tb³⁺ doping concentration is fixed, the Tb³⁺ emission intensity increases monotonously and the Ce³⁺ emission intensity decreases as the Ce³⁺ ion concentration increases. This should be due to the enhancement of energy transfer from Ce³⁺ ions to Tb³⁺ ions.

Fig. 8 PLE and PL spectra of NSPO:0.1Ce³⁺ (a), NSPO:0.01Tb³⁺ (b) and NSPO:0.1Ce³⁺,0.01Tb³⁺ (c), respectively.

The decay curves and lifetime of Ce³⁺ in NSPO:0.1Ce³⁺,yTb³⁺ (y = 0, 0.01) are showed in Fig. 9 According to the decay behavior of Ce³⁺, their corresponding luminescence decay curve can be best fitted to the typical second-order decay method by the following equation:⁵⁷

I_(t) = A₁ exp(−t/τ₁) + A₂ exp(−t/τ₂)

Here I is the luminescence intensity at time t; A₁ and A₂ are two constants which are related with the initial intensity; τ₁ and τ₂ are rapid and slow times for the exponential components, respectively. The average lifetime (τ) can be calculated using the following equation:

τ = (A₁τ₁² + A₂τ₂²)²/(A₁τ₁ + A₂τ₂)

Fig. 9 The decay curves and lifetime of Ce³⁺ in NSPO:0.1Ce³⁺,yTb³⁺ (y = 0, 0.01).

The lifetime of Ce³⁺ is 4.67 ms when doped only with Ce³⁺ ions in NSPO phosphors. However, the lifetime of Ce³⁺ decreases to 3.25 ms with the introduction of Tb³⁺, demonstrating the energy transfer from Ce³⁺ to Tb³⁺ by non-radiative processes.⁵⁸

The proposed Ce³⁺ → Tb³⁺ ET process in the NSPO host is shown in Fig. 10. Firstly, Ce³⁺ ions can effectively absorb UV light from the ground state (²F_5/2) to the excited state (5d energy levels) by UV irradiation. Secondly, these electrons relax to the lowest vibrational level of the excited state to release excess energy into the surrounding environment and then either return to the ground states to produce the Ce³⁺ emissions; or efficiently transfers the energy to the ⁵D₃ level of Tb³⁺, followed by non-radiative relaxation to ⁵D₄ level (⁵D₃ + ⁷F₆ = ⁵D₄ + ⁷F₀) due to the same energy difference between ⁵D₃ → ⁵D₄ and ⁷F₀ → ⁷F₆ in Tb³⁺ ions.^56,59 Finally, as a result of the electrons moving from the ⁵D₄ excited state to the ⁷F_J (J = 3, 4, 5, 6) ground state, the green light is obtained.

Fig. 10 Energy level model for the ET processes of Ce³⁺ → Tb³⁺ in NSPO host.

4 Conclusions

In summary, we have successfully synthesized a series of color tunable NSPO:Ce³⁺/Eu³⁺/Tb³⁺ phosphors by a combination of hydrothermal method and low temperature calcinations (800 °C). The obtained NSPO:Ce³⁺/Eu³⁺/Tb³⁺ phosphors were relatively uniform nanoparticles with size of 300 nm. The energy transfer of Tb³⁺/Eu³⁺ co-doping as well as their tunable multicolor luminescence in Na₃Sc₂(PO₄)₃ host lattices were realized. Due to energy transfer of Tb³⁺ → Eu³⁺, the emission colors of the resulting phosphor can be changed from green (0.2865, 0.4982) to red (0.5693, 0.2976) through yellow (0.3657, 0.4516) by adjusting the concentration of Eu³⁺. The obtained NSPO:Ce³⁺/Eu³⁺/Tb³⁺ phosphors can be promising as a potential candidate for the applications for solid-state lighting and display fields.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This project is financially supported by the Fundamental Research Funds for the Central Universities (XDJK2016C147 and XDJK2018C050) and the National Natural Science Foundation of China (51302229).

Notes and references

1. P. Pust, V. Weiler, C. Hecht, A. Tucks, S. A. Wochnik, K. A. Henss, D. Wiechert, C. Scheu, J. P. Schmidt and W. Schnick, Nat. Mater., 2014, 13, 891–896.
2. A. A. Setlur, E. V. Radkov, C. S. Henderson, J. H. Her, A. M. Srivastava, N. Karkada, M. S. Kishore, N. P. Kumar, D. Aesram, A. Deshpande, B. L. Kolodin, S. Grigorov and U. Happek, Chem. Mater., 2010, 22, 4076–4082.
3. K. A. Denault, J. Brgoch, M. W. Gaultois, A. Mikhailovsky, R. Petry, H. Winkler, S. P. DenBaars and R. Seshadri, Chem. Mater., 2014, 26, 2275–2282.
4. Z. Ci, Q. Sun, S. Qin, M. Sun, X. Jiang, X. Zhang and Y. Wang, Phys. Chem. Chem. Phys., 2014, 16, 11597–11602.
5. X. Li, F. Liu, J. Y. Howe, J. Zhang, X. J. Wang, Z. Gu, C. Sun, R. S. Meltzer and Z. Pan, Light: Sci. Appl., 2013, 2, 3250–3257.
6. Y. Q. Li, A. C. A. Delsing, G. D. With and H. T. Hintzen, Chem. Mater., 2005, 17, 3242–3248.
7. K. Hanaoka, K. Kikuchi, T. Terai, T. Komatsu and T. Nagano, Chemistry, 2008, 14, 987–995.
8. C. H. Liang, F. B. Li, C. S. Liu, J. Lu and X. G. Wang, Dyes Pigm., 2008, 76, 477–488.
9. H. A. Höppe, Angew. Chem., Int. Ed., 2009, 48, 3572–3582.
10. E. F. Schubert and J. K. Kim, Science, 2005, 308, 1274–1278.
11. T. Justel, H. Nikol and C. Ronda, Angew. Chem., 1998, 37, 3084–3103.
12. W. Y. Tian, K. X. Song, F. F. Zhang, P. Zheng, J. X. Deng and J. Jiang, et al., J. Alloys Compd., 2015, 638, 249–253.
13. Z. G. Xia, H. Y. Du, J. Y. Sun, D. M. Chen and X. F. Wang, Mater. Chem. Phys., 2010, 119, 7–10.
14. J. K. Sheu, S. J. Chang, C. Kuo, Y. K. Su, L. Wu, Y. Lin, W. Lai, J. Tsai, G.-C. Chi and R. Wu, IEEE Photonics Technol. Lett., 2003, 15, 18–20.
15. T. S. Chan, R. S. Liu and I. Baginskiy, Chem. Mater., 2008, 20, 1215–1217.
16. N. Guo, W. Lü, Y. Jia, W. Lv, Q. Zhao and H. You, ChemPhysChem, 2013, 14, 192–197.
17. D. Geng, M. Shang, Y. Zhang, H. Lian and J. Lin, Inorg. Chem., 2013, 52, 13708–13718.
18. W. Wu and Z. Xia, RSC Adv., 2013, 3, 6051–6057.
19. C. C. Lin, C. C. Shen and R. S. Liu, Chem.–Eur. J., 2013, 19, 15358–15365.
20. H. B. Liang, Y. Tao, J. H. Xu, H. He, H. Wu and W. X. Chen, J. Solid State Chem., 2004, 117, 901–908.
21. Z. J. Wang, S. Q. Lou and P. L. Li, J. Alloys Compd., 2014, 586, 536–541.
22. V. V. Tkachev, V. I. Ponomarev and L. O. Atovmyan, Zhurnal Strukturnoj Khimii, 1984, 25, 128–134.
23. B. I. Lazoryak, V. B. Kalinin, S. Y. Stefanovich and V. A. Efremov, Dokl. Akad. Nauk SSSR, 1980, 250, 861–864.
24. N. Guo, Y. H. Zheng, Y. C. Jia, H. Qiao and H. P. You, J. Phys. Chem. C, 2012, 116, 1329–1334.
25. K. Hirohumi, Master thesis, Niigata University, 2013.
26. H. Guo, X. Y. Huang and Y. J. Zeng, J. Alloys Compd., 2018, 741, 300–306.
27. H. Guo, B. Devakumar, B. Li and X. Y. Huang, Dyes Pigm., 2018, 151, 81–88.
28. R. Vijayakumar, H. Guo and X. Y. Huang, Dyes Pigm., 2018, 156, 8–16.
29. X. C. Wang, Z. Y. Zhao, Q. S. Wu, C. Wang, Q. Wang, Y. Y. Li and Y. H. Wamg, J. Mater. Chem. C, 2016, 4, 8795–8801.
30. Y. S. Liu, D. T. Tu, H. M. Zhu and X. Y. Chen, Chem. Soc. Rev., 2013, 42, 6924–6958.
31. X. Teng, Y. H. Zhu, W. Wei, S. C. Wang, J. F. Huang, R. Naccache, W. B. Hu, A. L. Y. Tok, Y. Han, Q. C. Zhang, Q. Y. Fan, W. Huang, J. A. J. Capobianco and L. Huang, J. Am. Chem. Soc., 2012, 134, 8340–8343.
32. Y. J. Ding, X. Teng, H. Zhu, L. L. Wang, W. B. Pei, J. J. Zhu, L. Huang and W. Huang, Nanoscale, 2013, 5, 11928–11932.
33. H. Y. Jiao and Y. H. Wang, J. Electrochem. Soc., 2009, 156, 117–120.
34. H. Yamamoto and K. Urade, J. Electrochem. Soc., 1982, 129, 2069–2074.
35. D. Wang, Y. Wang and J. W. He, Mater. Res. Bull., 2012, 47, 142–145.
36. D. Q. Geng, G. G. Li, M. M. Shang, C. Peng, Y. Zhang, Z. Y. Cheng and J. Lin, Dalton Trans., 2012, 41, 3078–3086.
37. K. Pavani, J. Suresh Kumar and L. Rama Moorthy, J. Alloys Compd., 2014, 586, 722–729.
38. M. Back, M. Boffelli, A. Massari, R. Marin, F. Enrichi and P. Riello, J. Nanopart. Res., 2013, 15, 1753–1758.
39. N. Guo, Y. H. Song, H. P. You, G. Jia, M. Yang and K. Liu, et al., Eur. J. Inorg. Chem., 2010, 29, 4636–4642.
40. W. P. Chen, H. B. Liang, B. Han, J. P. Zhong and Q. Su, J. Phys. Chem. C, 2009, 113, 17194–17199.
41. J. Ding, Q. Zhang, J. Cheng and X. Liu, J. Alloys Compd., 2010, 495, 205–208.
42. Z. G. Xia and R. S. Liu, J. Phys. Chem. C, 2012, 116, 15604–15609.
43. L. E. Muresan, Y. Karabulut, A. I. Cadis, I. Perhaita, A. Canimoglu and J. Garcia Guinea, J. Alloys Compd., 2016, 658, 356–366.
44. Q. H. Zhang, H. Y. Ni, L. L. Wang and F. M. Xiao, Ceram. Int., 2016, 42, 6115–6120.
45. Z. F. Yang, Y. H. Hu, L. Chen, X. J. Wang and G. F. Ju, Mater. Sci. Eng,. B: Solid State. Mater. Adv. Technol., 2015, 193, 27–31.
46. L. Macalik, P. E. Tomaszewski, R. Lisiecki and J. Hanuza, J. Solid State Chem., 2008, 181, 2591–2600.
47. Z. X. Tao, T. Tsuboi, Y. L. Huang, W. Huang, P. Q. Cai and H. J. Seo, Inorg. Chem., 2014, 53, 4161–4168.
48. S. Y. Xin, Y. H. Wang, G. Zhu, X. Ding, W. Y. Geng and Q. Wang, Dalton Trans., 2015, 44, 16099–16106.
49. P. I. Paulose, G. Jose, V. Thomas, N. V. Unnikrishnan and M. K. R. Warrier, J. Phys. Chem. Solids, 2003, 64, 841–846.
50. R. Reisfeld, E. Greenberg, R. Velapoldi and B. Barnett, J. Chem. Phys., 1972, 56, 1698–1705.
51. B. M. Antipeuko, I. M. Bataev, V. L. Ermolaev, E. I. Lyubimov and T. A. Privalova, Opt. Spectrosc., 1970, 29, 177–184.
52. G. Blasse, J. Solid State Chem., 1986, 62, 207–211.
53. H. Jiao, F. Liao, S. Tian and X. J. Jing, J. Electrochem. Soc., 2003, 150, 220.
54. D. L. Dexter and J. A. Schulman, J. Chem. Phys., 1954, 22, 1063–1070.
55. Z. G. Xia, J. Q. Zhuang, A. Meijerink and X. P. Jing, Dalton Trans., 2013, 42, 6327–6336.
56. J. Hao, S. A. Studenikin and M. Cocivera, J. Appl. Phys., 2001, 90, 5064–5069.
57. H. L. Li, Z. L. Wang, S. J. Xu and J. H. Hao, J. Electrochem. Soc., 2009, 156, 112–116.
58. J. Y. Sun, X. Y. Zhang, Z. G. Xia and H. Y. Du, Mater. Res. Bull., 2011, 46, 2179–2182.
59. D. W. Wen and X. J. Shi, Dalton Trans., 2013, 42, 16621–16629.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c8ra07827k

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