Low Temperature Synthesis of Nanophosphors for White Light Emission

Abstract

Nanophosphors are considered to be very critical for the development of lighting and display due to some new features from nanosized materials compared to bulk phosphors, for example, surface-site luminescence. The current methods to synthesize nanophosphors are based on high temperature methods, such as hydrothermal method, co-precipitation method, and so-gel method. Usually, temperature dictates the nanocrystals growth speed, which may affect the size and crystallinity of the nanocrystals. pH value is another very important parameter to control the nanocrystal growth, which is believed to be associated with nanomaterial morphology and structure. In our work, we developed a low temperature (2oC) method to synthesize some tungstate nanophosphors, for example CaWO4:Eu, in aqueous solution under different pH values. Nanowires, nanorods, nanospheres are obtained by the developed low temperature synthesis method. Self-assembled microstructures and isolated nanoparticles can be obtained by controlling the pH values. Nanowires are produced as the Eu doping concentration is 10%. For comparison, high temperature synthesis method (150oC) are also used in the synthesis of tungstate nanophosphors. The influence of temperature, pH vales and Eu doping concentrations on the luminescence properties of tungstate nanophosphors are systemically studied, and the mechanism of the luminescence are demonstrated. Small sized nanophosphors are produced as the synthesis temperature is 2oC compared to high temperature. The photoluminescence of CaWO4:Eu includes the green-blue emission band due to the tungstate group in the host and the red emission peaks from Eu3+. The relative intensity of the green-blue emission to the red emission can be considerably tuned by the pH values, which affects the energy transfer efficiency from tungstate group to Eu3+. Eu doping concentration also can be used to adjust the color of the luminescence of tungstate nanophosphors. We optimize the parameters including pH vales, and Eu doping to obtain the white light emission. Figure 1 shows the color coordinates of the nanophosphors prepared by us. It can be seen that the range of color coordinates can be tuned by our method covers blue, green, white, red and yellow regions, which shows the feasibility of our method for the synthesis of nanophosphors toward white light. Low reaction temperature synthesis method provides a platform to synthesize the nanophosphors toward color tunable and while light emission for lighting and display. Figure 1: color coordinates of the luminescence of CaWO4:Eu nanophosphors Figure 1