Effect of preparation condition on the luminescent properties of Zn₂SiO₄:Mn²⁺ phosphors

Owing to its properties of high-luminescent efficiency, chemical stability and wet-resistence, Zn₂SiO₄ exhibits more excellent performance than the sulfides as luminescent host material. Zn₂SiO₄:Mn²⁺ (willemite) is an efficient phosphor for plasma display, cathodoluminescent and electroluminecent displays. Over the past few decades, researchers have studied intensely about factors influencing on the photoluminescent properties of Zn₂SiO₄:Mn²⁺, such as calcining temperatures, concentration of Mn²⁺, atmosphere of heat treatment, various preparation methods. However, the pH value of the Zn₂SiO₄:Mn²⁺ precursor sol also plays an important role in its luminescent properties. Herein, we investigate the effects of pH value of the precursor sol, heating atmosphere (oxidizing, neutral protecting and reducing) and calcining temperature on the photoluminescent intensity of Zn₂SiO₄:Mn²⁺ phosphor powder. Zn₂SiO₄:Mn²⁺ phosphors have been prepared via a sol-gel route. Different pH values of the sol resulted in different geling time and the sol with pH value lower than 3 could not be gelatinized. After pre-heating at 600 °C, Zn₂SiO₄:Mn²⁺ precursors were calcined in different atmosphere (air, N₂ and mixture of 10% H₂ + 90% N₂) at heating temperature varing from 800 °C to 1 100 °C. The structure of Zn₂SiO₄:Mn²⁺ has been characterized by X-ray diffraction (XRD), which shows that all the phase of phosphors is with α-Zn₂SiO₄ phase. The phosphor prepared from the sol with pH of 4.34 has the best crystallinity. Scanning electron microscope (SEM) is used to measure the particle size of the Zn₂SiO₄: Mn²⁺ phosphors and the mean size of the phosphor obtained from the sol with pH value of 4.34 is about 100 nm, larger than the size of the phosphors obtained from sols with pH values of 5.32 and 6.53. The results of photoluminescent (PL) spectra indicate that the PL intensity of the phosphor increases with increasing the pH value of precursor sol, and the maximum was obtained when the pH value of precursor sol is 4.34. Because the phosphor obtained from the sol with pH value of 4.34 has best crystallinity, and its particle size is larger, leading to smaller specific surface area. When the specific surface area is smaller, there are lower defects in the surface of particles and fewer nonradiative transitions appear. Additionally, the hydrolysis and polycondensation reactions of the precursor sol with pH value of 4.34 have been carried throughly and properly, resulting in the well dispersion of the Mn ions. The emission intensity of Zn₂SiO₄:Mn²⁺ phosphors calcined in the mixture of nitrogen and hydrogen is stronger than those of the phosphors calcined in the other heating atmosphere. Reducing atmosphere could easily split Mn-Mn pair and produce large number of isolated Mn ions. With H₂ existing in the heating atmosphere, more Zn ions incline to be volatilized and more Mn ions would substitute for Zn ions. Therefore, the PL intensity of Zn_SiO₄:Mn²⁺ phosphors is the strongest when the pH value of the precursor sol is 4.34 and the phosphors are calcined in the mixture of N₂ and H₂ at 900 °C.