ABSTRACT

Sr₂SiO₄:Eu phosphor for white light emitting diodes (LEDs) was synthesized by employing an as-prepared (Sr,Eu)CO₃@SiO₂ core-shell precursor as starting materials, and the effect of the core-shell precursor was also discussed on the crystal structure, particle morphology and luminescent properties of the resultant phosphor. The results showed that the hybrid β- and α′-Sr₂SiO₄: Eu phosphor with fine particle size and narrow distribution could be obtained at a lower firing temperature than that in conventional solidstate reaction method, and its formation mechanism was deduced to be (Sr,Eu)CO₃ diffusion controlled reaction process. Responded to its hybrid crystal structure, this phosphor exhibited the combined luminescence of β- and α′-Sr₂SiO₄:Eu.

1. INTRODUCTION

Since Eu²⁺-activated alkaline earth orthsilicate phosphor was firstly reported on its fluorescence by Barry in 1968 [1], Me₂SiO₄:Eu (Me = Ca, Sr, Ba) had attracted little attention until the advent of white light emitting diodes (LEDs). Due to its high light conversion efficiency for near ultraviolet (NUV) and blue light, Me₂SiO₄:Eu has been an excellent commercial phosphor for white LEDs. Compared with the most popular (Y,Gd)₃(Al,Ga)₅O₁₂:Ce (YAG:Ce) phosphor for white LEDs, Me₂SiO₄:Eu is not only suitable for blue LED but also for NUV LED; moreover, Me₂SiO₄:Eu can produce more colorful emission to satisfy the demands of the white LEDs with lower color temperature and higher color rendering index [2-6].

Currently, the commercial Me₂SiO₄:Eu phosphor is produced by high temperature solid-state reaction method [5-10]. Such a method can achieve high light conversion efficiency of phosphor; however, it usually requires high firing temperature and introduction of flux to promote crystallization, which results in big particle size (>10 μm), broad particle distribution and irregular morphology, even flux contamination for phosphor. Taking the application properties into account, the phosphors for LEDs should have suitable particle size (<10 μm) and narrow distribution besides high brightness and desirable color coordinates. So some efforts have made to improve the particle properties of Me₂SiO₄:Eu phosphor by softchemistry methods [11-14]. For instance, Chang and coauthors synthesized nanometer Sr₂SiO₄ by employing SrCO₃@SiO₂ core-shell precursor as the starting materials; however, the luminescent center Eu was not considered and doped into the Sr₂SiO₄ host in their work [14]. In the previous work, we also developed a homogenous (Sr,Eu)CO₃@SiO₂ core-shell precursor, in which (Sr,Eu)CO₃ represents the homogenous mixture of Sr²⁺ and Eu³⁺ carbonates [15]. In this study, we employed this as-prepared core-shell precursor as starting materials to synthesize Sr₂SiO₄:Eu phosphor, and investigated its effect on the crystal structure, morphology and luminescent properties of the phosphor. Based on these results, the formation mechanism and the luminescence of Sr₂SiO₄:Eu was also discussed.

2. EXPERIMENTAL

The as-prepared (Sr,Eu)CO₃@SiO₂ core-shell precursor in our previous work [15] was directly employed as the starting materials to synthesize Sr₂SiO₄:Eu phosphor in the present work, wherein the molar ratio of Sr, Eu and Si was 1.9:0.1:1. To be specific, the core-shell precursor was put into an alumina crucible and directly fired in a horizontal tube furnace at 1000˚C for 2 h under reducing atmosphere (95% N₂ + 5% H₂). And then the obtained phosphor powder was cooled to room temperature in the furnace for characterization.

FTIR measurements were performed on a Nicolet Magna-IR 760 Fourier transform infrared spectrometer using the standard KBr pellets technique, in the frequency interval 4000 - 400 cm^–1. X-ray diffraction (XRD) identification was determined by Burker D8 Advance X-ray powder diffractometer running Cu Kα radiation at 40 kV and 40 mA, and the XRD patterns were collected in the range of 15˚ ≤ 2θ ≤ 65˚. The microstructure and morphology were detected by a JEOL JSM-6335F field emission scanning electronic microscope. The emission and excitation spectra of the phosphor were acquired by using Edinburgh FLS920P fluorescence spectrometer equipped with a 450W xenon lamp as an excitation source. All the measures were carried out at room temperature.

3. RESULTS AND DISCUSSION

Figure 1 shows the FTIR spectrum of the obtained phosphor sample (curve b). For comparison, the FTIR spectrum of (Sr,Eu)CO₃@SiO₂ precursor is also presented as curve (a). Obviously, the FTIR spectrum of the precursor mainly exhibits the characteristic vibrations of SiO₂ (1093, 796 and 471 cm^–1), group (692, 856, 1456 and 1749 cm^–1), CTAB (1630 and 2823 cm^–1) and H₂O (3435 cm^–1). After the precursor was fired into phosphor, the obtained sample shows different FTIR spectrum. As displayed in curve (b), the characteristic vibrations of group, CTAB and H₂O disappear, and the characteristic vibration bands of SiO₂ are replaced by multiple bands at 1000 - 800 cm^–1 and 600 - 500 cm^–1, which could be attributed to the stretching and bending vibrations of Si-O bonds in SiO₄ tetrahedra, respectively [16]. It means that the vibration bands of the phosphor sample are shifted to lower frequency compared with that of pure SiO₂. As O/Si ratio increases from 2 (SiO₂) to 4 (orthosilicate), the Si-O bond length increases from 0.161 nm to 0.163 nm due to the presence of modifier oxides in the silica network, and the polymerization degree of SiO₄ tetrahedral gets lower, which generally corresponds to a lower vibration frequency in FTIR spectrum [17,18]. Meanwhile, it is noticed that the stretching vibrations of the phosphor sample cover a broader range (1300 - 600 cm^–1, the region between both dot lines in curve (b)) than that of pure SiO₂ (1300 - 880 cm^–1, the region between both dot lines in curve (a)) and split into three groups: 962, 908, and 839 cm^–1, which is probably arisen by the different NBO/Si ratio (NBO/Si: non-bridging oxygen per silicon) in SiO₄ tetrahedra [19].

To further investigate and clearly identify the phase structure, Figure 2 depicts the XRD pattern of the phosphor sample obtained by directly firing (Sr,Eu)CO₃@SiO₂ precursor at 1000˚C for 2 h. This pattern agrees well with the standard diffraction data of α′-Sr₂SiO₄ (JCPDS No. 39-1256) and β-Sr₂SiO₄ (JCPDS No. 38-0271), so it can be deduced that this phosphor sample is a mixture of

orthorhombic α′-Sr₂SiO₄:Eu and monoclinic β-Sr₂SiO₄:Eu. The α′ and β forms are the two modifications of Sr₂SiO₄, and the phase transition between low temperature β phase and high temperature α′ phase occurs at about 358 K [20,21], whereas α′ phase can also be stabilized at room temperature by substituting more Eu (≥0.1) or small amounts of Ba²⁺ for Sr²⁺ [22,23]. In this work, the concentration of Eu activator is 0.1, so α′-Sr₂SiO₄:Eu is stably crystallized as well as β-Sr₂SiO₄:Eu as expected. Approximately estimated from the intensity of the diffraction peaks, α′-Sr₂SiO₄:Eu has much more content percentage than β-Sr₂SiO₄:Eu in this mixture. It is noted that not only is the present synthesized temperature (1000˚C, no flux) lower than that in the conventional flux-assisted solid-state reaction method (usually 1300˚C), but also the contamination of flux can be avoided due to the absence of flux.

To investigate the effect of core-shell precursor on the morphology of the resultant Sr₂SiO₄:Eu phosphor, Figure 3 and Figure 4 demonstrate the SEM images of the precursor and phosphor sample, respectively. In this precursor (Figure 3), most of SiO₂ are induced to coat on

the surface of (Sr,Eu)CO₃ core to form (Sr,Eu)CO₃@SiO₂ core-shell structure with SiO₂ shell layer about 100 ~ 200 nm thickness; however, a few nucleate directly into SiO₂ nano-particles (100 ~ 200 nm) and locate onto the surface of core-shell structure. The formation mechanism and the composition of this precursor have been disclosed in our previous work [15]. It can be noted that this precursor has imperfect core-shell structure, and there appears an apparent gap without coated by SiO₂ on every core-shell particle. However, the existence of the gap accesses us to clearly understand the reaction mechanism between core and shell by morphology observation and comparison.

After the precursor was fired at 1000˚C for 2 h, the obtained Sr₂SiO₄:Eu phosphor sample seems to inherit the external contour of the precursor. Overall, the particles of the phosphor still appear uniform and nearspherical morphology, as shown in Figure 4(a), and the particle size is about 2 μm, slightly less than that of the precursor, which results from the condensation reaction between (Sr,Eu)CO₃ core and amorphous SiO₂ shell at the high temperature. Observed from the enlargement figure (Figure 4(b)), the condensation reaction also leads to slight aggregation and adhesion among particles, but the profile of the single particle can be clearly recognized, which indicates that every (Sr,Eu)CO₃@SiO₂ particle in the core-shell precursor would act as a separate reaction unit, and it is of the suitable stoichiometrical ratio to situ produce an isolated Sr₂SiO₄:Eu micrometer particle. More noticeably, all the particles of the Sr₂SiO₄:Eu phosphor exhibit hollow morphology with an opening, and the wall thickness is slightly thicker than that of SiO₂ shell layer of the precursor. Combined with the results of XRD analysis, it seems that the shell of the precursor has transformed into thicker Sr₂SiO₄:Eu layer, while the core has disappeared past the chemical reaction at the high temperature. That is to say, the fact that the phosphor particles still maintain near single and hollow spherical shape suggests that (Sr,Eu)CO₃ would diffuse into SiO₂ shell and react into hollow Sr₂SiO₄:Eu phosphor until the exhaust of (Sr,Eu)CO₃ core. It can be deduced intuitively that this reaction mechanism can be regarded as (Sr,Eu)CO₃ diffusion controlled process, which is consistent with the theoretical inference in Lu and Wu’ work [24].

The luminescent properties of α′-Sr₂SiO₄:Eu or β- Sr₂SiO₄:Eu have been researched widely [25-28]; however, the hybrid luminescence of the co-existent phase of α′- and β-Sr₂SiO₄:Eu has seldom been separately distinguished. In order to systematically understand the dependence of luminescent properties on the crystal structure of Sr₂SiO₄:Eu phosphor, we further investigated and analyzed the photoluminescence of this hybrid phosphor sample.

Figures 5(a) and (b) show the emission and excitation spectra of the phosphor sample, respectively. As illustrated in Figure 5(a), the peak of the dominant emission band moves from 555 to 568 nm with the increase of the excitation wavelength from 320 to 450 nm. Whichever the excitation wavelength is, a turn at 532 nm is observed in the emission spectra as clearly shown in the inset of Figure 5(a), which implies that the peak at 532 should be classified as a different emission band from the dominant one in the emission spectra. Both emission bands are the response to the co-existence of α′-Sr₂SiO₄:Eu and β-Sr₂SiO₄:Eu. The orientations of β and α′ lattices are related by a simple rotation of x axis, and β-Sr₂SiO₄:Eu with stronger Si-O bond valence produces stronger crystal field and then yields shorter wavelength emission compared with α′-Sr₂SiO₄:Eu, so it can be deduced that the emission band centered at 532 nm originates from β-Sr₂SiO₄:Eu, and its emission peak position is independent on the excitation wavelength; while the dominant emission band at longer wavelength should be ascribed to α′-Sr₂SiO₄:Eu, and this emission can occur blueor red-shift with the change of the excitation wavelength. It can be noted that α′-Sr₂SiO₄:Eu shows stronger emission intensity than β-Sr₂SiO₄:Eu in our sample, which is contrary to the results confirmed by many work [10,22,26]. The reason is that α′-Sr₂SiO₄:Eu has much more mass percentage than β-Sr₂SiO₄:Eu in this phosphor sample. Both of the emission band are produced by the 5d-4f transition of the activator Eu²⁺ occupied the ten oxygen coordinated Sr²⁺(I). As well known, there is another Sr²⁺ site, Sr²⁺(II), surrounded by nine oxygen in β- and α′-Sr₂SiO₄:Eu. When Eu²⁺ replaces Sr²⁺(II), a shorter blue light emission band will be produced with peak at about 470 nm. As shown in Figure 5(a), the 470 nm blue emission is obviously resolved as gibbous shoulder when the excitation wavelength is as short as 320 nm, while it disappears when the excitation wavelength is set at 400 or 450 nm. On the other hand, the phosphor sample has the strongest emission peak intensity when excited by 400 nm (NUV, near ultraviolet), and the intensity still maintains about 80% under 450nm blue light excitation. That is to say, the Sr₂SiO₄:Eu phosphor can applied to combine with NUV-LED or blue

LED.

The applicability of this Sr₂SiO₄:Eu phosphor for white LED can also be certificated by the excitation spectrum. Three excitation spectra were illustrated in Figure 5(b), with monitoring emission wavelength at 568, 532 and 470 nm, respectively. Obviously, there is a very broad excitation band in 300 - 500 nm spectral regions when monitoring wavelength at 532 and 568 nm, which assures that this phosphor has high efficient light conversion for NUV and blue light. As for 470 nm blue emission, it has much lower efficiency, and its suitable excitation region is narrower (250 - 400 nm). That is the reason that 470 nm emission is nearly unresolved when this phosphor is excited by long-wavelength light.

4. CONCLUSIONS

The hybrid α′- and β-Sr₂SiO₄:Eu phosphor was successfully developed by firing (Sr,Eu)CO₃@SiO₂ core-shell precursor directly at 1000˚C, and it appeared uniformly hollow near-spherical morphology with particle size about 2 μm. The morphology was resulted from the coreshell structure of the precursor and the reaction mechanism between (Sr,Eu)CO₃ core and SiO₂ shell. The mechanism was acquired visually to be (Sr,Eu)CO₃ diffusion controlled reaction process. Responded to its hybrid crystal structure, the phosphor exhibited the combined luminescence of α′- and β-Sr₂SiO₄:Eu. α′-Sr₂SiO₄:Eu has the longer emission wavelength, and its emission peak can blueor red-shift with the change of the excitation wavelength; while the 532 nm emission of β-Sr₂SiO₄:Eu is independent on the excitation wavelength. Compared with the conventional high temperature solid-state reaction method, this method requires lower firing temperature and no flux contamination, and produces finer particles with narrow size distribution. More importantly, the reaction mechanism will provide some ideas to improve the particle performance of silicate phosphors, for instance, the spherical and solid Sr₂SiO₄:Eu phosphor could be obtained by employing a spherical SiO₂@ (Sr,Eu)CO₃ core-shell precursor as starting materials.

5. ACKNOWLEDGEMENTS

The authors would like to acknowledge the support from the National Hi-Tech. R&D Program of China (863 Program, 2010AA03A404, 2011AA03A101) and the Hong Kong Polytechnic University Grant (Grant No. J-BB9R).

REFERENCES

NOTES

^*Corresponding authors.