Controllable Synthesis of Monodispersed Middle and Heavy Rare Earth Oxysulfide Nanoplates Based on the Principles of HSAB Theory

Based on the theory of hard and soft acids and bases, trivalent ions of middle and heavy rare earths belong to very hard acids, which possess weak affinity towards S2- ions but strong affinity to O2- ions. So it is difficult to synthesize middle and heavy rare earth oxysulfide nano-materials through the thermolysis method in high-boiling-point organic solvent. In this article, monodispersed oxysulfide nanoplates of Y, Eu, Gd, Er and Yb were synthesized through this thermolysis method we developed. Both sodium-doped and undoped rare earth oxysulfide nanoplates could be prepared, and the doping of sodium ions could promote the crystallization of the nanoplates. Rare earth acetylacetonates were used as metal precursors and H2S gas as the sulfurizing reagent. The reactions were conducted in oleylamine at 280 degrees C for 1 hour. The thermogravimetric analysis of the precursor showed that the initial decomposition temperature of the rare earth acetylacetonates is about 200 degrees C, which is much lower than that of rare earth oleates. The transmission electron microscopy observation and energy dispersive X-ray analyses of the intermediate products during the synthesis of the nanoplates showed that rare earth oxide nanoplates formed firstly at about 220 degrees C, and these nanoplates transformed to oxysulfide nanoplates gradually during the temperature ramping period. Density functional theory calculation was used to compare the total free energy of the oxide and oxysulfide of different rare earth elements. According to this thermodynamical comparison, we concluded that, from light rare earths to heavy rare earths, higher chemical potential of sulfur is needed to obtain the oxysulfide rather than oxide. On one hand, 1125 gas has higher sulphurizing power than sulfur. On the other hand, a comparatively low reaction temperature favors the dissolving of H2S in oleylamine. As a result, the chemical potential of sulfur in synthetic system could be effectively increased by using rare earth acetylacetonates as the precursors instead of rare earth oleates, and using H2S gas as sulphurizing agent instead of sulfur, which made it possible to prepare middle and heavy rare earth oxysulfide nanoplates in oleylamine. Fluorescent measurements showed that as-synthesized Y2O2S:Eu nanoplates could emit red light under 251 nm ultraviolet light excitation. For Y2O2S:Eu nanoplates, the fluorescence life time was shorter and quantum yield was lower in comparison with the corresponding bulk counterpart, possibly due to its much higher portion of surface atoms as well as lower crystallinity.