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SiO2-coated ZnS submicrospheres with enhanced thermal stability and photoluminescence

https://doi.org/10.1016/j.mseb.2007.07.008Get rights and content

Abstract

Uniform SiO2-coated ZnS submicrospheres were synthesized by a sol–gel method. The shell thickness was controlled from 50 to 320 nm by adjusting the concentration of tetraethyl orthosilicate (TEOS). The structure and morphology of the SiO2-coated ZnS spheres were investigated by X-ray diffraction (XRD), transmission electron microscopy (TEM), scanning electron microscopy (SEM), and energy dispersive spectroscopy (EDS). The results indicate that the thermal stability of SiO2-coated ZnS spheres has been greatly improved comparing with the uncoated ZnS. The emission intensity of SiO2-coated ZnS submicrospheres increased greatly with increasing the SiO2-shell thickness, which can be ascribed to the improved thermal stability and reduced surface defects and electron–hole recombination of ZnS spheres.

Introduction

Zinc sulfide (ZnS) is a wide band gap semiconductor with Eg = 3.66 eV at room temperature. As a traditional phosphor, ZnS has been widely used in the flat-panel display, electroluminescent devices, infrared windows, sensors, and lasers [1], [2], [3]. During the last decades, ZnS quantum dots and metal ions doped ZnS nano-particles have been extensively investigated [4], [5], [6], [7], [8]. Recently, it was confirmed that the spherically monodispersed morphology is an important factor for the low-light scattering at the surfaces, as well as the high-packing densities [9]. However, it is difficult to redisperse the nanocrystalline powders in any solvents even without agglomeration because of the high-surface energy. Therefore, in order to obtain high photoextraction efficiency, monodispersed and spherical ZnS particles are required [10], [11], [12], [13]. During the preparation process, annealing is necessary for the ZnS spheres to remove the subcrystal boundaries, which act as nonradiative centers [12]. In order to avoid the oxidation of ZnS at high temperatures, covering their surface with a transparent shell, such as Y2O3 [14], SiO2 [15], [16], [17], [18], [19], [20], TiO2 [17], and In2O3 [21], to enhance the chemical stability is a frequently used approach. Although some reports of the SiO2-coated bulk ZnS [18], [20] and ZnS nano-particles [16], [17] can be found, the SiO2-coated ZnS submicrospheres prepared by the soft chemical technique has seldom been reported, especially, the effect of the SiO2-shell thickness on the optical properties of ZnS phosphor has never been studied in detail.

In this work, the monodispersed ZnS submicrospheres were synthesized by precursor gel method. Uniform coating of SiO2 were synthesized by hydrolysis of alkoxide in iso-propanol solutions. The shell thickness of SiO2 was controlled by adjusting the experimental parameters. The morphology, structure as well as PL properties of naked and SiO2-coated ZnS submicrospheres were also investigated.

Section snippets

Synthesis of SiO2-coated ZnS submicrospheres

Firstly, ZnS submicrospheres were synthesized by precursor gel method from Zn–EDTA and thioacetamide (TAA) solutions. In a typical process, Zn(OH)2 powder was initially prepared by introducing 5.0 M NaOH to an equal volume of 2.5 M Zn(NO3)2 at room temperature followed by aging for 1 h under agitation. The resulting Zn(OH)2 suspension was washed several times by centrifugation and then freeze-dried to be used as a component of the Zn–EDTA solution described bellow. The established standard

Morphology of ZnS/SiO2 submicrospheres

Table 1 summarizes a series of experiments carried out with different weight ratios of SiO2/ZnS in solution. The weight ratio of SiO2 to ZnS is altered from 10 to 100%. It can be found that the thickness of the SiO2-shell is varied from 50 to 325 nm by altering the concentration of TEOS. The concentration of TEOS is an important issue to obtain well-dispersed coated ZnS spheres. Mixture of coated particles and silica hydrous oxide is obtained if the amount of ZnS cores is too small and the

Conclusions

In summary, uniform SiO2-coated ZnS submicrospheres were successfully prepared by using the sol–gel technology. The thickness of the SiO2-shell was controlled by varying the concentration of TEOS. EDS and TEM confirmed the formation of uniform SiO2 coatings with the thickness of 50–320 nm. The results of XRD and SEM indicate that the SiO2-shell improves both anti-oxidation ability and thermal stability of ZnS submicrospheres, which induced the higher emission intensity of coated ZnS phosphor.

Acknowledgments

This work was supported by the grant of Post-Doc Program, Chonbuk National University (2006) and Post BK 21 Program from the Ministry of Education and Human Resources Development.

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