Sr 2SiO 4: Eu phosphor for white light emitting diodes (LEDs) was synthesized by employing an as-prepared (Sr, Eu)CO 3@SiO 2 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′-Sr 2SiO 4: 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)CO 3 diffusion controlled reaction process. Responded to its hybrid crystal structure, this phosphor exhibited the combined luminescence of β- and α′-Sr 2SiO 4: Eu.
Since Eu2+-activated alkaline earth orthsilicate phosphor was firstly reported on its fluorescence by Barry in 1968 [
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 [
The as-prepared (Sr,Eu)CO3@SiO2 core-shell precursor in our previous work [
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.
To further investigate and clearly identify the phase structure,
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,
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 [
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
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
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
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.
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).