Microwave assisted co-precipitation synthesis and photoluminescence characterization of spherical Sr₂P₂O₇:Ce³⁺, Tb³⁺ phosphors

Highlights

• A novel spherical phosphor was synthesized using microwave-assisted co-precipitation method.

• The energy transfer mechanism of Ce³⁺–Tb³⁺ was discussed in detail.

• The phosphors have excellent luminescence properties and thermal stability.

Abstract

A novel spherical green phosphor, Sr₂P₂O₇:Ce³⁺, Tb³⁺ was prepared by co-precipitation method using a microwave heating system. The synthesis was performed at 1000 °C for 15 min. The phase structure, morphology, photoluminescence properties and thermal stability were investigated. The photoluminescence intensity of green emission from Tb³⁺ was enhanced remarkably by co-doping Ce³⁺ in the Tb³⁺ solely doped Sr₂P₂O₇ phosphor because of energy transfer from Ce³⁺ to Tb³⁺ ions. The novel phosphors have excellent thermal stability and remarkable CIE chromaticity coordinates of (0.258, 0.605), indicating its potential application of green emitting phosphor in lighting field. The microwave-assisted co-precipitation method was found to be simple, rapid and advantageous in achieving the required stoichiometry and homogeneity of precursor materials in comparison with the conventional solid state reaction route.

Introduction

Inorganic luminescent materials have been extensively studied for applications in field emission displays, photo-electric devices, high definition television, security devices, etc. [1], [2], [3]. Up to now, many phosphors have been reported, including silicates, borates, aluminates, sulfides, oxy-nitrides/nitrides, and so on [4], [5], [6], [7], [8]. In the past years, pyrophosphate phosphor materials have attracted considerable attention due to their low power consumption, high efficiency, chemical stability, and environmental friendliness etc. [9], [10], [11], [12], [13]. At present, research on multicolor emitting phosphor for ultraviolet excitation was usually designed according to the energy transfer between different activators (such as Ce³⁺–Mn²⁺, Eu²⁺–Mn²⁺ and Ce³⁺–Tb³⁺) in some hot topic. To the best of our knowledge, both Ce³⁺ and Tb³⁺ ions are important activators, which are widely used in phosphor for fluorescent lamps. Ce³⁺ and Tb³⁺ co-doped materials have been applied in green light emitting phosphors, because the energy transfer from Ce³⁺ to Tb³⁺ is usually very effective in obtaining bright luminescence of Tb³⁺. In many luminescence materials containing Tb³⁺, such as La₂Sn₂O₇:Ce³⁺, Tb³⁺, Ca₈Mg(SiO₄)₄Cl₂:Ce³⁺, Tb³⁺ and LaPO₄:Ce³⁺, Tb³⁺, [14], [15], [16], [17], [18], [19], [20], Ce³⁺ has been successful in sensitizing the luminescence of Tb³⁺.

The phosphor materials are usually synthesized by the solid state reaction method, in which relatively high temperature and prolonged heating time are required. However, microwave heating techniques has the advantage of short reaction time, low sintering temperatures, low activation energy and rapid heating rate. Hence, it has become a rapidly developing research field [21], [22], [23], [24], [25]. Moreover, the solid state reaction method has some unavoidable disadvantages, including uncontrollable morphology and non-homogeneous mixtures on a microscopic scale. The ideal morphology of phosphors materials is a perfect sphere. Spherical particles can reduce light scattering, at the same time improve the packing density of phosphor, so as to effectively increase the light intensity. To extend phosphors to high resolution applications, fine phosphor particles with a spherical morphology, homogeneous composition, and good properties are highly desirable. Recently, wet chemistry methods such as hydrothermal synthesis, sol–gel method and spray thermal decomposition, have been developed [26], [27], [28]. Using these methods, homogeneous products can be achieved by atomically mixing on the molecular level at lower crystallization temperatures. Unfortunately, most of these chemical methods suffer from complex and time consuming procedures. Compared with these methods, the co-precipitation method is obviously advantageous in industrial applications, such as safe reaction processes, inexpensive facilities, low production cost, and large amounts of products. However, to the best of our knowledge, few reports have been made regarding the microstructural properties and luminescent characteristics of Sr₂P₂O₇ doped and co-doped with Ce³⁺, Tb³⁺ ions by using microwave-assisted co-precipitation method.

In this study, uniform microspheres of Sr₂P₂O₇:Ce³⁺, Tb³⁺ were synthesized via co-precipitation method assisted by microwave heating. The homogeneous products can be obtained at a relatively low reaction temperature within short processing time. The phase characterizations, morphologies, luminescence properties, and optimal doping concentrations of Ce³⁺ and Tb³⁺ were investigated in order to search for new phosphor materials for potential applications. The energy transfer mechanism of Ce³⁺–Tb³⁺ is also discussed in detail.