(Invited) Missing Link Between Solid State and Solution Reactions for the Nanophosphors

Since the conventional solid state reactions are very slow process around room temperature, the most of inorganic phosphor materials have been synthesized by the high temperature solid state reaction method. The high temperature reaction leads to irregular particle morphology of phosphors. Therefore, the solution synthesis methods such as the co-precupitation, hydrothermal and sol-gel method will become even more important. However, some liquid phase reactions required to dry the solvent and special equipment for the separations. Recently, we reported novel soft chemical synthesis method, water assisted low temperature solid state reaction (WASSR) method[1,2]. This process is very simple and can synthesize the ceramic materials just by mixing of raw materials added a small amount of water. For example, RbVO₃ and YVO₄phosphors can be synthesized at room temperature just by mixing the raw materials with a small amount of water.In this study, we presents the luminescence properties of nano-phosphors prepared by our original WASSR processing and also discussed the reaction mechanism of WASSR method.

For the synthesis of YVO₄ using the WASSR method, a stoichiometric ratio mixture of Y₂O₃ (0.2769 g) (C. I. Kasei Co., Ltd. 3N) and V₂O₅ (0.2231 g) (Kojundo Chemical Laboratory Co., Ltd. 4N) was mixed using a mortar. A small amount of de-ionized water was added to the mixture in a ratio of 11.4 mol% (approximately 0.05 g), and mixed using mortar for 3 h to synthesize to a single phase. Standard YVO₄was also synthesized by the conventional solid-state reaction method for comparison. This reference sample was calcined at 1473 K for 10 h in air. Powder X-ray diffraction (XRD) data were obtained using an X-ray diffractometer (MX-Labo, Mac Science Ltd.). The particle morphologies were observed by a scanning electron microscope (SEM, JSM-5310MVB, JEOL Ltd.). The emission and excitation spectra were measured at room temperature with a spectrofluorometer (JASCO Corp. FP-6500/6600).

The particle size of the YVO₄ sample prepared by the WASSR method was clearly smaller than that of the sample prepared by the conventional solid-state reaction method. The average particle sizes were approximately under 100 nm for the sample synthesized by the WASSR method and few micrometers size for the sample synthesized by the conventional solid-state reaction method, respectively. Internal quantum efficiency of (Y_0.90Eu_0.10)VO₄ phosphor sample synthesized by WASSR method was 32 % under 300 nm excitation. This value is relatively high value for the conventional nano YVO₄:Eu³⁺phosphors. The samples mixed or heated in the absence of the water are the mixture of raw materials Y₂O₃ and V₂O₅. No diffraction peaks of YVO₄ phase are present in the XRD patterns. The solubility of the Y₂O₃ and V₂O₅ raw materials in water are 0.018 g/Ɩ and 0.700 g/Ɩ, respectively. Total amount of dissolved raw materials in water is negligible low in the reaction of the WASSR method. This result suggests that the reaction is also different from a solution reaction, which occurs by the dissolution of the raw materials. In addition, the WASSR method proceeds without strong mechanical load. Therefore, the reaction mechanism of WASSR method is not a mechano-chemical reaction. In addition, we can be successfully synthesized numerous ceramic materials, such as SrMoO₄, BaTiO₃, LaPO₄:Ce³⁺,Tb³⁺, SrAl₂O₄, Ba₂SiO₄ and NaEuMo₂O₈ at low temperature by the WASSR method. Our proposed low cost and low temperature synthesis technique is promising for an industrial application for ceramic materials synthesis processing.

[1] KANEKO, T., KIM, S. W., TODA, A., UEMATSU, K., ISHIGAKI, T., TODA, K., SATO, M., KOIDE, J., TODA, M., KUDO, Y., MASAKI. T. & YOON, D. H., 2015. Synthesis of YVO4 nano particles by novel room temperature synthesis method. Science of Advanced Materials, 7, 1502-1505.

[2] TODA, K., KIM, S. W., HASEGAWA, T., WATANABE, M., KANEKO, T., TODA, A., ITADANI, A., SATO, M., UEMATSU, K., ISHIGAKI, T., KOIDE, J., TODA, M., KUDO, Y., MASAKI. T. & YOON, D. H., 2016. Novel Soft Chemical Synthesis Methods of Ceramic Materials. Key Engineering Materials, 690, 268-271.