Combustion synthesis of nanophosphors

Recently, nanophosphors have become a research focus in terms of both their fundamental and practical importance. They exhibit unique chemical and physical properties compared to their bulk materials. These properties are halfway between macroscopic and microscopic substances. For example, quantum confinement effect of a nanoparticle gives rise to novel optoelectronic properties. The emission lifetime, luminescent efficiency, and concentration quenching of the phosphor strongly depends on particle size. Due to these unique properties, many potential applications in the area of optical, electrical, biological, mechanical can be developed. For example, research and development of optical switch, biomarker, new laser, etc are under the way.

Figure 1: Applications for nanophosphors.

In general, the rare-earth doped phosphors are mainly synthesized by the solid-state reaction at high temperature. However, this method requires a high processing temperature, a long processing time, repeated milling and washing with chemicals.  These processes tend to degrade the luminescence property of the particles and yield irregularly shaped particles. Sol-gel method has also been employed to synthesize nanophosphors by many researchers. The as-prepared powders obtained from the sol-gel method have low crystallinity and often require post-treatment at high temperature, which results in severe agglomerations. Combustion synthesis is a promising particle preparation method because it can employ a wide range of precursors for synthesis of a broad spectrum of functional nanoparticles. The use of combustion can avoid hollowness and provide the high temperature environment which is favorable to phosphor synthesis. The flame temperature and particle residence time, which are very important parameters determining particle characteristics, can be easily controlled by varying fuel and oxidizer flow rates. Moreover, the particle size can be controlled by varying precursor solution concentration and multi-component particles can also be obtained by adding different salts into the solution.

Figure 2: Comparison for particles synthesized between sol-gel and combustion method.

Figure 3 shows a schematic of the combustion synthesis system. The system consisted of a spray generator, a coflow burner, quartz reactor, filter and vacuum pump. The spray, which was generated by an ultrasonic spray generator, was carried into the premixed flame established by a methane/air rich mixture. The starting solution was prepared by dissolving yttrium and europium nitrate salts in water or ethanol. The particles were collected using a micron glass-fiber filter located 30 cm above the flame. The prepared particles are characterized by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The photoluminescence intensity of the particles excited by UV or infrared is measured by a spectrophotometer.

Figure 3: Experimental setup.

The as-prepared nanoparticles show strong photoluminescence under ultraviolet excitation as demonstrated in Fig. 4. Figure 5 shows SEM photographs of as-prepared nanoparticles at different overall concentrations. The average particle size decreases with the decrease of precursor concentration. The particles are generally non-aggregated and have a spherical morphology. Figure 6 shows a TEM photograph of very fine nanophosphors.

Figure 4: Photoluminescence of Y2O3:Eu nanoparticles under the excitation of UV light.

(a) 0.1 M                               (b) 0.01 M

Figure 5: SEM image of nanoparticles prepared from different precursor concentration.

Figure 6: TEM image of fine nanophosphors prepared by combustion synthesis.

Figure 7 shows the photoluminescence of the as-prepared Y2O3:Eu nanoparticles under the excitation of UV light. The particles absorb excitation energy in the range of ultra violet and emit red visible light. The emission spectra exhibit multi-peaks in the visible spectral region for all samples with the peak wavelength at 612 nm, a red emission. Figure 8 shows the influence of flame temperature on photoluminescence intensity of nanoparticles. The brightness of the as-prepared particles was strongly affected by flame temperatures. At high flame temperatures, the crystallinity of the particles becomes higher and the brightness of the as-prepared particles increases.

Figure 7: Photoluminescence intensity of as prepared Y2O3:Eu nanoparticles showing effect of Eu doping concentration.

Figure 8: Effect of flame temperature on photoluminescence intensity. Temperature in the legend represents values taken at 20 cm above the burner exit.