General and Facile Method To Prepare Uniform Y2O3:Eu Hollow Microspheres Guang Jia, Mei Yang, Yanhua Song, Hongpeng You,* and Hongjie Zhang* State Key Laboratory of Rare Earth Resource Utilization, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, and Graduate UniVersity of the Chinese Academy of Sciences, Beijing 100049, P. R. China
CRYSTAL GROWTH & DESIGN 2009 VOL. 9, NO. 1 301–307
ReceiVed May 9, 2008; ReVised Manuscript ReceiVed August 21, 2008
ABSTRACT: Well-shaped Y2O3:Eu hollow microspheres have been successfully prepared on a large scale via a urea-based homogeneous precipitation technique in the presence of colloidal carbon spheres as hard templates followed by a subsequent heat treatment process. XRD results demonstrate that all the diffraction peaks of the samples can be well indexed to the pure cubic phase of Y2O3. TEM and SEM images indicate that the shell of the uniform hollow spheres, whose diameters are about 250 nm, is composed of many uniform nanoparticles with diameters of about 20 nm, basically consistent with the estimation of XRD results. Furthermore, the main process in this method was carried out in aqueous condition, without the use of organic solvents or etching agents. The as-prepared hollow Y2O3:Eu microspheres show a strong red emission corresponding to the 5D0-7F2 transition of the Eu3+ ions under ultraviolet or low voltage excitation, which might find potential applications in fields such as light phosphor powders, advanced flat panel displays, field emission display devices, and biological labeling.
1. Introduction The development of nano- or micromaterials with size- and shape-controlled morphologies may open new opportunities in exploring the chemical and physical properties of the materials.1 Remarkably, hollow nano/microspheres currently represent one of the fastest growing areas compared with other structural and geometric features.2 The fabrication of hollow microspheres or nanospheres with varying sizes and shapes has attracted fascinated interest, due to their distinct low effective densities, high specific surface areas, and potential scale-dependent applications in photonic devices, drug delivery, lightweight fillers, active material encapsulation, ionic intercalation, acoustic insulation, surface functionalization, robust catalysts/carriers, and size-selective reactions.3 The hollow spheres of rare earth doped phosphors will achieve a reduction in the amount of expensive rare earth metal. Additionally, due to the low density of hollow spherical materials, when coating screens in display applications, the phosphors can be dispersed well, enhance the uniformity, and give high packing densities of the coating. Recently, many efforts have been made in the development of different methods for the design and preparation of hollow nano- or microspheres. Template-directed synthesis, with hard templates,4-6 such as polymer latex particles, silica spheres, and metal nanoparticles, and soft templates,7-10 such as emulsion droplets, micelles, and gas bubbles, have been demonstrated to be an effective approach to prepare inorganic hollow spheres. Compared with the significant progress in the preparation of hollow nano- or microspheres of other systems, such as SnO2,11-13 ZnS,14,15 CdTe,16 SiO2,17,18 TiO2,19,20 ZnO,21 Fe3O4,22 etc., the synthesis of well-defined rare earth oxide hollow spheres has rarely been studied. Therefore, it is desirable to explore feasible, easily controllable, and easily repeatable methods for the synthesis of spherical hollow structures of rare earth metal oxides with promising novel properties. Y2O3:Eu phosphor is a well-known red phosphor that is used in fluorescent lights (FLs), field emission displays (FEDs), and * To whom correspondence should be addressed. E-mail:
[email protected] (H.Y.);
[email protected] (H.Z.).
cathode-ray tubes (CRTs). Over the past decades, various morphologies of Y2O3:Eu have been synthesized via different methods, including nanoparticles through combustion,23,24 microemulsion,25 and chemical vapor technique;26 nanotubes fabricated by a surfactant assembly mechanism;27 nanowires induced by template-assisted growth in AAO;28 spherical particles by using a spray pyrolysis method,29 urea homogeneous precipitation,30 and Pechini sol-gel process;31 and patterned thin films prepared from sol-gel soft lithography.32 However, to our knowledge, the synthesis of well-shaped Y2O3:Eu hollow spheres has rarely been studied. Recently, Li and co-workers reported a hydrothermal method to fabricate uniform carbon sphere templates.33-35 In their work, the monodisperse carbon spheres can be easily prepared from hydrothermal carbonization of glucose or sugar in aqueous solutions under mild conditions without any template. The surface of the carbon spheres is hydrophilic and has a distribution of -OH and -CdO groups similar to that of a polysaccharide, which makes surface modification unnecessary. Therefore, the carbon spheres may greatly widen their range of applications in biochemistry, diagnostics, and drug delivery, and they could be used as templates for fabricating core-shell structures or hollow materials. In the present study, we demonstrate that highly uniform Y2O3:Eu hollow spheres can be synthesized successfully on a large scale by a general, facile, and green method using colloidal carbon spheres as templates. This may open up new possibilities to synthesize hollow spheres of other oxides and extend their applications.
2. Experimental Section Analytical grade glucose and urea were purchased from Beijing Chemical Corporation and used as the starting materials without further purification. Y(NO3)3 and Eu(NO3)3 aqueous solution were obtained by dissolving Y2O3 (99.99%) and Eu2O3 (99.99%) (Wuxi Yiteng RareEarth Limited Corporation, China) in dilute HNO3 solution under heating with agitation. Preparation of Carbon Spheres. In a typical synthesis of colloidal carbon spheres, glucose (8 g) was dissolved in 35 mL of distilled water to form a clear solution. The solution was then sealed in a 50 mL Teflon-lined stainless steel autoclave and maintained at 160 °C for 9 h.
10.1021/cg8004823 CCC: $40.75 2009 American Chemical Society Published on Web 11/17/2008
302 Crystal Growth & Design, Vol. 9, No. 1, 2009 After the autoclave was cooled to room temperature naturally, the black-brown precipitates were washed with distilled water and ethanol five times and dried at 60 °C in air. Synthesis of Y2O3:Eu Hollow Microspheres. In the preparation procedure, 0.95 mmol of Y(NO3)3 and 0.05 mmol of Eu(NO3)3 aqueous solution were added to 23 mL of distilled water; Then 3.0 g of urea was dissolved in the metal solution after vigorous stirring to form a clear solution. The as-prepared carbon microspheres (90 mg) were added and well–dispersed into the above solution with the assistance of ultrasonication for 10 min. Finally, the mixture was transferred into a 100 mL three-necked round-bottom flask and heated at 90 °C for 3 h with vigorous stirring before the product was collected by centrifugation. The precursors were washed by distilled water and ethanol three times each and dried in air at 60 °C. The final products were obtained through a heat treatment at desired temperatures in air for 2 h with a heating rate of 2 °C min-1. Characterization. The samples were characterized by powder X-ray diffraction (XRD) performed on a Rigaku-Dmax 2500 diffractometer. Fourier transform infrared spectroscopy (FT-IR) spectra were measured with a Perkin-Elmer 580B infrared spectrophotometer with the KBr pellet technique. Thermogravimetric analysis and differential scanning calorimetry (TGA-DSC) data were recorded with a thermal analysis instrument (SDT 2960, TA Instruments, New Castle, DE) with a heating rate of 10 °C min-1 in an air flow of 100 mL min-1. The morphology and composition of the samples were inspected using a field emission scanning electron microscope (FESEM; XL30, Philips) equipped with an energy-dispersive X-ray spectrum (EDX; JEOL JXA-840). Transmission electron microscopy (TEM) images and selected area electron diffraction (SAED) patterns were obtained using a JEOL 2010 transmission electron microscope operating at 200 kV. Photoluminescence (PL) excitation and emission spectra were recorded with a Hitachi F-4500 spectrophotometer equipped with a 150 W xenon lamp as the excitation source. The cathodoluminescence (CL) measurements were carried out in an ultrahigh-vacuum chamber (
