Improved luminescence properties and thermal stability of SrSi₂O₂N₂:Eu²⁺ phosphor with single phase via the formation of Eu³⁺ on surface structure

Abstract

The green-emitting SrSi₂O₂N₂:Eu²⁺ phosphors with single phase have been successfully synthesized by a simple two-step method. During the first-step process, Sr₂SiO₄:Eu³⁺ precursor was formed and emitted red light under UV light excitation. The excessive amount of Si₃N₄ in reactants promoted the formation of pure SrSi₂O₂N₂ phase when molar ratio of Si₃N₄/Sr₂SiO₄ was located in the range of 1.1–1.5. The luminescence intensities of SrSi₂O₂N₂:Eu²⁺ phosphors were enhanced by Si₃N₄/Sr₂SiO₄ molar ratio of 1.1. Phase analysis, luminescence properties and thermal stability of the samples were intensively investigated by X-ray diffraction, SEM, XPS and TGA/DSC. The trivalent europium ions were identified by XPS results on the surface of as-synthesized SrSi₂O₂N₂:Eu²⁺ particles with approximate theoretical atomic ratio of Eu/Sr, which prevent the further oxidation of Eu²⁺ to Eu³⁺ in the bulk structure and evidently improved the thermal stability of the phosphors. For the thermal oxidation mechanism, it was suggested that the oxidation of SrSi₂O₂N₂ to form SrSiO₃ layer firstly occurred on the surface after a calcination process in air.

Introduction

In recent years, solid-state lighting based on phosphor-converted light-emitting diodes (pc-LEDs) technology has been widely used in general illumination fields due to their high efficiency, long lifetime and environmental friendly characteristics [1,2,3,4,5,6]. However, the early pc-LEDs consist of blue chip, and yellow YAG:Ce³⁺ phosphor shows a low color rendering index (Ra < 80) and cannot meet the requirement of developing interior lighting [5, 7,8,9]. For pc-LEDs, phosphors play a key role in achieving optimal white light with excellent color rendering index and high correlated color temperature. Therefore, a variety of novel green and red phosphors have been developed to improve the optical properties of pc-LEDs.

Among them, rare-earth-doped nitridosilicates and oxonitridosilicates phosphors are the most likely candidates for general illumination application, which shows high quantum efficiency, small thermal quenching and suitable excitation spectra [5, 10,11,12,13,14,15,16,17,18,19,20,21]. Nowadays, the typical nitridosilicates phosphors—Sr₂Si₅N₈:Eu²⁺ and (Ca,Sr)AlSiN₃:Eu²⁺ with long wavelength emissions, high quantum efficiency and excellent thermal stability—have been successfully acted as red components in pc-LEDs. As a member of nitridosilicates and oxonitridosilicates phosphors family, SrSi₂O₂N₂:Eu²⁺ phosphor was firstly developed by Hintzen et al. and could emit yellow–green light under UV and blue light irradiation, which was considered as a promising candidate of green phosphor to improve color rendering index and correlated color temperature for pc-LEDs [22]. Furthermore, Li et al. reported the preparation of SrSi₂O₂N₂:Eu²⁺ phosphor by using Sr₂SiO₄ as the precursors. However, an unknown phase also can be observed in XRD patterns for all the samples [23]. Until now, it is still a great challenge to prepare SrSi₂O₂N₂:Eu²⁺ phosphor with single phase by traditional solid-state reaction or other synthetic methods. The impurity phases in SrSi₂O₂N₂:Eu²⁺ phosphor will decrease the luminescence intensity and thermal stability. Therefore, it is more significant to synthesize pure phase of SrSi₂O₂N₂:Eu²⁺ with expected thermal stability.

In this paper, SrSi₂O₂N₂:Eu²⁺ phosphor with single phase has been successfully prepared by a simple two-step method. The excessive amount of Si₃N₄ in the second step promoted the formation of pure SrSi₂O₂N₂ phase when molar ratio of Si₃N₄/Sr₂SiO₄ was located in the range of 1.1–1.5. The luminescent intensity also reached to maximum with Si₃N₄/Sr₂SiO₄ molar ratio of 1.1. It is suggested that the existence of Eu³⁺ ions on the surface of as-synthesized SrSi₂O₂N₂:Eu²⁺ phosphor prevents the further oxidation of Eu²⁺ in bulk structure during the calcination process in air.

Experimental

SrSi₂O₂N₂:xEu²⁺ (x = 0.01–0.14) phosphors were synthesized using raw materials of SrCO₃ (99.9%, Sinopharm, China), SiO₂ (Sinopharm, China), α-Si₃N₄ (Aldrich) and Eu₂O₃ (99.99%, Aldrich). The samples were prepared by a two-step method according the following equation:

• 1. ${\text{SiO}}_{2} + 2{\text{SrCO}}_{3} + x{\text{Eu}}_{2} {\text{O}}_{3} \to {\text{Sr}}_{2} {\text{SiO}}_{4}:2x{\text{Eu}}^{3 + } + 2{\text{CO}}_{2} + 3x/2{\text{ O}}_{2}$

• 2. ${\text{Sr}}_{2} {\text{SiO}}_{4}:2x{\text{Eu}}^{3 + } + {\text{Si}}_{3} {\text{N}}_{4} \to 2{\text{SrSi}}_{2} {\text{O}}_{2} {\text{N}}_{2}:x{\text{Eu}}^{2 + }$

In the first step, the raw materials of SiO₂, SrCO₃ and Eu₂O₃ were weighed stoichiometrically and mixed thoroughly in an agate mortar. Then, the powder mixture was loaded into a furnace and sintered in air for 4 h at 1100 and 1180 °C, respectively. In the second step, the obtained Sr₂SiO₄:2xEu³⁺ precursor and excessive amount of Si₃N₄ were finely ground in agate mortar. The Si₃N₄/Sr₂SiO₄ molar ratio of 1.0–1.5 was employed. Subsequently, the powder mixtures were transferred to a tubular furnace and sintered at 1450 °C for 5 h under a flow condition of 3% H₂/N₂ gas (100 mL/min). After sintering, the samples were cooled with the furnace and the phosphors with green color were finally obtained.

The phase of samples was identified by X-ray powder diffraction (XRD-7000S, Shimadzu) with Cu Kα radiation (λ = 0.15374 nm) operating at 40 kV and 20 mA. The data were collected with a step of 0.02° (2θ) and a count time of 1s per step in continuous scan mode. Photoluminescence (PL) spectra were recorded by a HITACH F-4500 fluorescence spectrophotometer at room temperature. The particle morphology of the phosphors was observed by field emission electron microscopy (FESEM, JSM-7800F, JEOL). The surface elements of powders were analyzed by X-ray photoelectron spectroscopy (Thermofisher ESCALAB 250Xi). Thermogravimetric (TG) and differential scanning calorimetry (DSC) were performed using a thermometric system (Setsys 16/18, Setaram) under air flow in the temperature ranging from 25 to 1000 °C at a heating rate of 10 °C/min.

Results and discussion

Phase identification and luminescence property of precursor

A different temperature (1100 and 1180 °C) was employed in the first step to investigate the phase composition of the precursors. It is shown in Fig. 1 that raw material of SrCO₃ (PDF No.: 5-418) can be found in the precursor obtained at 1100 °C and the precursor with pure Sr₂SiO₄ phase is formed under the temperature of 1180 °C. Eu³⁺ ions were introduced into the structure of Sr₂SiO₄ during the first-step process. Therefore, no Eu₂O₃ phase can be observed in the XRD patterns and the characteristic luminescent spectra (Fig. 2) of Eu³⁺ can be obtained for the precursor of Sr₂SiO₄:Eu³⁺. The excitation spectrum of Sr₂SiO₄:Eu³⁺ precursor presents an intense band with maximum at 394 nm by monitoring the ⁵D₀–⁷F₁ transition of Eu³⁺ ion (587 nm). It is well known that the emission spectra of Eu³⁺ ion are attributed to ⁵D₀–⁷F _J (J = 0–4) transitions. In Fig. 2b, the emission spectrum of Sr₂SiO₄:Eu³⁺ precursor displays the characteristic ⁵D₀–⁷F₁ transition (587 nm) and ⁵D₀–⁷F₂ transition (613 nm) of Eu³⁺ ion.

Figure 1

XRD patterns of standard card of Sr₂SiO₄ with PDF No.: 39-1256 and precursors obtained at 1100 °C (a) and 1180 °C (b), respectively. The diffraction peaks labeled by asterisk indicated the existence of raw material of SrCO₃ (PDF No.: 5-418)

Figure 2

a Excitation and b emission spectra of the precursor obtained at 1180 °C

Effect of excessive amount of Si₃N₄ on phase composition and luminescent intensity of Sr_0.98Si₂O₂N₂:0.02Eu²⁺ phosphors

X-ray powder diffraction was performed on the prepared samples to verify the phase purity. Figure 3 shows the standard data of SrSi₂O₂N₂ crystal (ICSD No. 172877) and the representative XRD patterns of SrSi₂O₂N₂:Eu²⁺ phosphors prepared using the different molar ratio of Si₃N₄/Sr₂SiO₄. No impurity phase of Sr₂SiO₄ or SrSiO₃ is observed in Fig. 3a–e. However, there is also a diffraction peak of 2θ = 28.2° marked by filled diamond in Fig. 3a, which has been attributed to unknown phase in other literatures. When the excessive amounts (Si₃N₄/Sr₂SiO₄ molar ratio of 1.1–1.5) of Si₃N₄ were employed in the second step, all the diffraction peaks in Fig. 3b–e can be indexed to the standard data of SrSi₂O₂N₂ which own the triclinic structure with space group P1, a = 7.0802(2) nm, b = 7.2306(2) nm, c = 7.2554(2) nm, α = 88.767(3), β = 84.733(2), γ = 75.905(2) and V = 358.73(2) nm³, Z = 4. Therefore, it is indicated that the obtained phosphors in this case are pure triclinic SrSi₂O₂N₂ phase.

Figure 3

XRD patterns of standard data of SrSi₂O₂N₂ crystal (ICSD No. 172877) and SrSi₂O₂N₂:Eu²⁺ phosphors prepared using the different molar ratio of Si₃N₄/Sr₂SiO₄. a 1, b 1.1, c 1.2, d 1.3, e 1.5. The peak labeled by filled diamond means an unknown phase

The excitation and emission spectra of Sr_0.98Si₂O₂N₂:0.02Eu²⁺ phosphors prepared with different molar ratio of Si₃N₄/Sr₂SiO₄ are presented in Fig. 4. The excitation spectra monitored at 526 nm show a broad band from 300 to 470 nm. The intense green emission bands ~526 nm are observed in the emission spectra under the excitation of 370 nm, which is attributed to 4f ⁶5d–4f ⁷ transition of Eu²⁺ ions. With the Si₃N₄/Sr₂SiO₄ molar ratio increasing, the luminescence intensity increases firstly and then decreases. The absorption achieves the strongest when Si₃N₄/Sr₂SiO₄ molar ratio is 1.1.

Figure 4

Excitation and emission spectra of Sr_0.98Si₂O₂N₂:0.02Eu²⁺ phosphors prepared with different molar ratio of Si₃N₄/Sr₂SiO₄

Photoluminescence properties and morphological observation of Sr_1−x Si₂O₂N₂:xEu²⁺ phosphors

The concentration of activator plays a key role in the performance of a phosphor. Therefore, it is important to investigate the relationship between Eu²⁺ concentrations (x) of Sr_1−x Si₂O₂N₂:xEu²⁺ phosphors and photoluminescent emission intensity. A series of Sr_1−x Si₂O₂N₂:xEu²⁺ phosphors with various Eu²⁺ concentrations (x) were prepared using Si₃N₄/Sr₂SiO₄ molar ratio of 1.1. The influence of Eu²⁺ concentration (x) on luminescence properties of the prepared phosphors is shown in Fig. 5. In Fig. 5a, the emission intensity of the phosphors increases firstly with increasing Eu²⁺ concentration (x) and reaches the strongest intensity when x = 0.02; then, the emission intensity decreases due to the concentration quenching with a further increasing of x. Figure 5b exhibits the dependence of relative emission intensity and wavelength on Eu²⁺ concentration (x). The emission bands of the green phosphors ranged from 526 to 537 nm with the changes of x (0.01–0.14).

Figure 5

a Emission spectra of Sr_1−x Si₂O₂N₂:xEu²⁺ (x = 0.01–0.14) phosphors prepared with Si₃N₄/Sr₂SiO₄ molar ratio of 1.1 under 370 nm excitation. b The dependence of relative emission intensity and wavelength on Eu²⁺ concentration (x)

The particle size and morphology of precursor and phosphor were characterized by a field emission scanning electron microscope (FESEM). Figure 6a indicates that Sr₂SiO₄:Eu³⁺ precursors are pellet with particle size of ~1 μm. Polyhedral morphology with particle size of 1–10 μm can be found for SrSi₂O₂N₂:Eu²⁺ phosphors in Fig. 6b. SrSi₂O₂N₂:Eu²⁺ phosphors show clear crystal facets and growth habits of layer structure for triclinic SrSi₂O₂N₂ crystal.

Figure 6

FESEM images of a Sr₂SiO₄:Eu³⁺ precursor and b SrSi₂O₂N₂:Eu²⁺ phosphors

Thermal stability analysis

The thermal quenching performance of SrSi₂O₂N₂:Eu²⁺ phosphor is usually studied as an intrinsic property of phosphor. In contrast to the recoverability of thermal quenching, thermal stability is regarded as plastic performance and cannot be recovered when thermal attack is removed [21, 24]. High thermal stability of a phosphor becomes more significant for its practical application in white LEDs. Thermal degradation usually relates to the valence band of activator and the chemical composition of the host lattice. In this work, the thermal stability of SrSi₂O₂N₂:Eu²⁺ has been investigated by a calcined method in air for 4 h at temperature of 400 °C, which will be in favor of the understanding of thermal degradation or thermal oxidation of phosphor. As shown in Fig. 7a, the luminescent intensity of SrSi₂O₂N₂:Eu²⁺ phosphor declines by ~30% after calcination at 400 °C. It has been found that the phase composition (Fig. 7b) and morphologies (Fig. 7c, d) of the samples after calcination at 400 °C are consistent with the ones of as-synthesized samples. Figure 8a shows TG–DSC curves of the as-synthesized SrSi₂O₂N₂:Eu²⁺ phosphor. It can be seen from TG curves that an appreciable weight gain begins at 800 °C and a massive weight gain appears after 900 °C owing to the oxidation of phosphor. The thermal oxidation temperature is confirmed as 915 °C by extension lines in Fig. 8a. The exothermic variation tendencies in the DSC curves also demonstrate the oxidation process of the phosphor in accordance with the results of TG. Based on the results of TG–DSC, Sr_0.95Si₂O₂N₂:0.05Eu²⁺ phosphor was sintered at 1200 °C in air (denoted as SSON-1200) to investigate the oxidation of phosphor. In Fig. 8b, all diffraction peaks of SSON-1200 sample are attributed to crystalline SrSiO₃ phase (PDF card No. 34-0099). Apparently, SrSi₂O₂N₂ was oxidized as SrSiO₃ phase during the thermal oxidation process.

Figure 7

a Photoluminescence intensity under 370 nm excitation and b XRD patterns for SSON and SSON-400 samples, respectively. SEM images of c SSON and d SSON-400

Figure 8

a TG–DSC curves of the as-synthesized Sr_0.95Si₂O₂N₂:0.05Eu²⁺ phosphor. b XRD patterns of standard data of SrSiO₃ phase (PDF Card No. 34-0099) and Sr_0.95Si₂O₂N₂:0.05Eu²⁺ phosphor sintered at 1200 °C (SSON-1200) for 2 h in air

XPS was also employed to determine the surface states of as-synthesized Sr_0.95Si₂O₂N₂:0.05Eu²⁺ phosphor (denoted as SSON) and the samples calcined at 400 °C (denoted as SSON-400), as shown in Fig. 9. From Fig. 9a, peaks assigned to core levels of Eu 3d, O 1s, N 1s, Sr 3p, Sr 3d, Si 2p are identified. Two intense peaks at 1164.4 and 1134.8 eV in Fig. 9b are attributed to the core levels of Eu 3d _3/2 and Eu 3d _5/2, respectively, which indicates the oxidation states of europium ions are mainly trivalent both for SSON and SSON-400 samples. Considering that XPS is a surface detection technology and the characteristic 4f ⁶5d–4f ⁷ transition of Eu²⁺ ions for the samples, Eu³⁺ ions are only located on the surface of SSON and SSON-400 particles. As shown in Fig. 9c, the O 1s region for both samples all can be fitted into two peaks centered at 532.1 and 530.3 eV, respectively, which means at least two kinds of oxygen species are present. The peak at 532.1 eV is due to Si–O bands, while the peak at 530.3 eV is associated with Sr–O bands. Then, the relative peak intensity of Sr–O band (P_Sr–O) and Si–O band (P_Si–O) can be calculated by the corresponding fit peak area. The relative peak intensity (0.426) of SSON-400 is a little more than that (0.415) of SSON, meaning larger amount of Sr–O bands in SSON-400. In Fig. 9d, double peaks at 134.8 and 133.1 eV are ascribed to the core levels of Sr 3d _3/2 and Sr 3d _5/2, respectively. The theoretical atomic ratio of Eu to Sr was quantitatively determined as 0.053 for Sr_0.95Si₂O₂N₂:0.05Eu²⁺ phosphor. The Eu/Sr ratios determined by XPS analysis for SSON and SSON-400 samples are 0.052 and 0.051, respectively, closing to the theoretical atomic ratio of 0.053. The results suggest that Eu concentration on the surface of phosphors is consistent with the ones in the bulk.

Figure 9

a Wide scan spectra of SSON (uncalcined sample) and SSON-400 (calcined sample at 400 °C); and XPS spectra of Eu 3d (b), O 1s (c), and Sr 3d (d), respectively

Thermal oxidation mechanism

As evidenced by the core-level Eu 3d XPS spectra, the trivalent europium ions are identified on the surface of SrSi₂O₂N₂:Eu phosphors. Therefore, europium ions of the phosphors are consisted of Eu³⁺ ions on the surface and Eu²⁺ ions in the bulk because SrSi₂O₂N₂:Eu phosphors show the characteristic 4f ⁶5d–4f ⁷ transition of Eu²⁺ ions. On the basis of the above-mentioned discussions, the thermal oxidation mechanism of SrSi₂O₂N₂:Eu phosphor is schematically illustrated in Fig. 10. SrSi₂O₂N₂ crystal shows a similar layer structure with BaMgAl₁₀O₁₇:Eu²⁺ phosphor in which Sr²⁺ (or Eu²⁺) are sandwiched in between the SiON₃ layers. It has been reported that thermal degradation of BaMgAl₁₀O₁₇:Eu²⁺ phosphor is due to the oxidation of the divalent europium ions according to the following oxidation process: (i) adsorption of oxygen atoms on the surface of phosphors; (ii) diffusion of the divalent europium ions into the conduction layer; (iii) transfer of an electron from a dopant ion to a adsorbed oxygen ion [25]. Considering the similar lattice structure, SSON phosphor will go through a same thermal oxidation process with BaMgAl₁₀O₁₇:Eu²⁺ phosphor. Eu²⁺ ions in the host crystal of SrSi₂O₂N₂ were well protected from oxidation by the existence of Eu³⁺ ions on the surface of phosphor during a heat treatment process because Eu³⁺ ions layer has an adverse effect on (i) adsorption of oxygen atoms on the surface of phosphors and (ii) diffusion of the divalent europium ions. Thus, the thermal oxidation of SrSi₂O₂N₂:Eu phosphors in this work is different from the ones reported in other literatures, in which the oxidation of activators appears firstly. In our cases, O₂ mainly attacks on the host crystal of SrSi₂O₂N₂ to form the corresponding oxide compound layer (SrSiO₃) demonstrated by larger amount of Sr–O bands in SSON-400.

Figure 10

Schematic illustration of thermal oxidation mechanism of SrSi₂O₂N₂:Eu phosphor

Figure 11 shows the Commission International de L’Eclairage (CIE) chromaticity coordinates and the emitting images for as-synthesized SrSi₂O₂N₂:Eu²⁺ phosphors with different europium concentration. The intense green emission can be observed for Sr_0.98Si₂O₂N₂:0.02Eu²⁺ phosphor with color coordinates of (x = 0.245, y = 0.659). With increasing Eu²⁺ contents, the chromaticity coordinates of SrSi₂O₂N₂:Eu²⁺ phosphors change from green (x = 0.241, y = 0.658) to yellow green region(x = 0.299, y = 0.648) in accordance with the inset images of SrSi₂O₂N₂:Eu²⁺ phosphors under 370-nm light excitation.

Figure 11

Commission Internationale de L’Eclairage (CIE) chromaticity diagram of SrSi₂O₂N₂:Eu²⁺ phosphors. Inset is image of SrSi₂O₂N₂:Eu²⁺ phosphors under 370-nm light excitation

Conclusions

In summary, SrSi₂O₂N₂:Eu²⁺ phosphors with single phase were conveniently prepared by a two-step method using molar Si₃N₄/Sr₂SiO₄ ratio of 1.1–1.5. The luminescent intensity of SrSi₂O₂N₂:Eu²⁺ phosphors reached to maximum when the molar ratio of Si₃N₄/Sr₂SiO₄ was 1.1. The emission bands of the green phosphors ranged from 526 to 537 nm with the increases in Eu concentration. Trivalent europium ions were identified on the surface of the as-synthesized samples with approximate theoretical atomic ratio of Eu/Sr and play a key role to avoid oxidation of Eu²⁺ ions in the bulk during a heat treatment process. Furthermore, the thermal oxidation of SrSi₂O₂N₂:Eu²⁺ phosphors was attributed to the formation of the SrSiO₃ layer on the surface of SrSi₂O₂N₂ particles.