Preparation and luminescence property of Gd₂O₂S:Tb X-ray nano-phosphors using the complex precipitation method

Abstract

The Gd₂O₂S:Tb X-ray nano-phosphors were prepared using the complex precipitation method combined with low temperature sulfurized method. The mixed solution of the ammonium water and ammonium hydrogen carbonate was employed as the complex precipitant. The samples were characterized by X-ray diffraction (XRD), transmission electron microscope (TEM), photoluminescence (PL) and X-ray excited luminescence (XL) spectra. The results show that the as-prepared samples can be sulfurized completely at 900 °C with flowing N₂ containing sulfur vapor, and show the pure Gd₂O₂S phases with the hexagonal structure. The obtained X-ray phosphor particles are quasi-spherical, disperse well. And the observed particle size of the sample calcined at 900 °C is 30–50 nm, while the one calcined at 1100 °C is about 50–70 nm. Therefore, both of them have successfully realized the nano-sized, which are beneficial to enhance the resolution of imaging system efficiently. When excited by the 254 nm UV light and X-ray, the Gd₂O₂S:Tb X-phosphors exhibit the same characteristic emission of Tb³⁺, which arise from the transitions of ⁵D₃ and ⁵D₄ excited levels to ⁷F_J (J = 0–6) ground states. And the luminescence property of the nanocrystals and the process of energy transfer are studied in details.

Introduction

The CaWO₄ had been mainly used for more than eighty years since it was found that it could greatly enhance the light output from X-ray to visible light. Until the 1970s, the rare earth phosphors, especially the oxysulfide phosphors, due to its advantages such as high luminescence efficiency and innocuity, etc., have caused great attention as the host materials of X-ray phosphors. Among them, the Gd₂O₂S:Tb has been considered as one of the most efficient phosphors that convert X-ray to visible light, and can be applied to the majority of the X-ray imaging systems. With the enhancement of resolution of imaging system, the high quality of the phosphors is required. The quality of the phosphors is not only related to the purity, but also relates to the particle size, morphology and the particle size distribution closely. However, the commercial phosphor powders are produced by a solid-state reaction at present, which involves several process steps: such as raw material blending, high-temperature calcination, washing, milling, classification, filtering, drying and sieving. Traditional phosphor synthesis techniques could be improved by elimination of time-consuming multiple steps and the need for high temperatures [1]. Uniform, spherical phosphors are favored in the screening process and desired for high-quality display devices. However, it is not only difficult to achieve the nano-size but also hardly control the morphology of phosphor powders for high-temperature solid-state reactions. Therefore, grinding is required before the screening process to obtain the smaller-sized particles. But grinding generally degrades luminescent efficiencies due to the introduction of surface defects that act as non-radiative recombination sites. Therefore, the direct phosphor synthesis techniques overcoming the disadvantages of high-temperature solid-state reaction have been studied, such as precipitation method [2], [3], combustion synthesis [4], ELM method [5], and so on.

The precipitation method is more promising due to its advantages, such as the simple process, no need for complex equipment, convenient for dope and low production cost, etc. The homogeneous precipitation method, which has drawn increasing attention in the fields of nano-materials synthesis, gives rise to the formation of the uniform and small-sized particles because the precipitants sedimentate out homogeneously and slowly. The urea (CO(NH₂)₂) as the precipitant has been used most commonly in the homogeneous precipitation method to prepare phosphors [3], whereas a certain temperature is necessary and the speed of the precipitation is very slow, which results in the prolonging of the precipitation process. Moreover, low concentration and a large amount of reaction solution are needed in order to obtain the smaller-sized nano-particles, which leads to the inconvenience of the post-processes, such as filtration, etc.

In this paper, the oxysulfide X-ray nano-phosphors were prepared using the complex precipitation method with ammonium water and ammonium hydrogen carbonate as precipitant. The reaction principle of the homogeneous precipitation method is used and it can successfully overcome the disadvantages of the urea homogeneous precipitation method. Up to now, the reports of synthesis the Gd₂O₂S:Tb X-ray phosphors using this method have not been seen.