CRYSTAL GROWTH & DESIGN
Y2O3 : Eu3+ Microspheres: Solvothermal Synthesis and Luminescence Properties Jun Yang, Zewei Quan, Deyan Kong, Xiaoming Liu, and Jun Lin* Key Laboratory of Rare Earth Chemistry and Physics, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022; and Graduate UniVersity of the Chinese Academy of Sciences, Beijing 100049, Peoples Republic of China
2007 VOL. 7, NO. 4 730-735
ReceiVed October 16, 2006; ReVised Manuscript ReceiVed December 8, 2006
ABSTRACT: Y2O3 : Eu3+ microspheres, with an average diameter of 3 µm, were successfully prepared through a large-scale and facile solvothermal method followed by a subsequent heat treatment. X-ray diffraction, Fourier transform infrared spectroscopy, energy-dispersive X-ray spectra, thermogravimetric and differential thermal analysis, inductive coupled plasma atomic absorption spectrometric analysis, scanning electron microscopy, transmission electron microscopy, photoluminescence spectra, as well kinetic decays, and cathodoluminescence spectra were used to characterize the samples. These microspheres were actually composed of randomly aggregated nanoparticles. The formation mechanisms for the Y2O3 : Eu3+ microspheres have been proposed on an isotropic growth mechanism. The Y2O3 : Eu3+ microspheres show a strong red emission corresponding to 5D0 f 7F2 transition (610 nm) of Eu3+ under ultraviolet excitation (259 nm) and low-voltage electron beams excitation (1-5 kV), which have potential applications in fluorescent lamps and field emission displays. 1. Introduction The demand for high-resolution and increased efficiency in phosphors for cathode ray tubes (CRT) and field emission displays (FEDs) has promoted the development of phosphors that perform at low voltages, typically 1-5 kV.1,2 Low voltage FEDs require the development of phosphors that are bright and efficient under low-energy excitation, are stable at high currents, and have good saturation characteristics.1 The phosphor grain size for FEDs should be in the range 1-3 µm.3,4 In particular, phosphors made up of small, ideally spherical particles are of interest because they offer the possibility of brighter cathodoluminescent performance, high definition, and much improved screen packing.2 The ideal morphology of phosphor particles includes a perfect spherical shape, narrow size distribution, and nonagglomeration. Spherical morphology of the phosphors is good for high brightness and high resolution because high packing densities and low scattering of light can be obtained by using them.5,6,7 So far, many synthetic routes have been developed to control the spherical shape, size, and distribution of phosphor particles, such as spray pyrolysis1,1 sol-gel process,8 and urea homogeneous precipitation.9,10 As the main and unsurpassed red emitting materials in fluorescent lamps and flat panel devices,1,2,7,10 Y2O3 : Eu3+ phosphors inevitably gather more attention because of its good luminescent characteristic, acceptable atmospheric stability, reduced degradation under applied voltages, and the lack of hazardous constituents as opposed to sulfide phosphors.11 Inorganic particles always show unique size- and shapedependent properties, such as shape, size, crystallinity, defects, grain boundaries, crystal structure, and preparation technique. Over the past decade, various morphologies of Y2O3 : Eu3+ have been synthesized via different methods, including nanoparticles through combustion,12,13 microemulsion,14 and chemical vapor technique;15 nanotubes fabricated by a surfactant assembly mechanism;16 nanowires induced by template-assisted growth in AAO;17 spherical particles by using spray pyrolysis method,6b urea homogeneous precipitation,9 and Pechini sol-gel process;8 * Corresponding author phone: +86-431-85262031; fax: +86-43185698041; e-mail:
[email protected].
and patterned thin films prepared from sol-gel soft lithography.18 However, further investigations of shape-controllable synthesis for Y2O3 : Eu3+ together with their formation mechanisms via other simple chemical synthesis routes are still of great interest. This not only will be able to enrich the synthesis science, but also make it possible to find novel and improved properties of the existing materials. With the advantages of high purity and good homogeneity, the hydrothermal or solvothermal synthesis method is an important technology for the preparation of low dimension nanostructures. Up to now, there are many reports about one-dimensional lanthanide hydroxides and oxides obtained by hydrothermal method and postcalcining process.19-35 Certainly, Y2O3 : Eu3+ 1D structures induced by hydrothermal method have been reported.28-33 However, to the best of our knowledge, few studies have focused on the synthesis or selforganization of spherical lanthanide oxides through hydrothermal or solvothermal method.36 It is well-known that under certain conditions nanoparticles can undergo self-assembly and form three-dimensional structures. The optical and electronic properties of these structures are dependent on both the initial nanoparticles and the manner in which they are organized. Herein, we report the self-assembled Y2O3 : Eu3+ microspheres synthesized by a simple solvothermal process followed further calcining treatment. These microspheres, which are composed of nanoparticles, are expected to maintain the desirable properties of Y2O3 : Eu3+ nanocrystals while being quite stable in the form of microspheres. A phenomenological growth mechanism for the microspheres has been proposed. Finally, optical properties of the resulted Y2O3 : Eu3+ microspheres were investigated in comparison with the corresponding commercial product. 2. Experimental Section 2.1. Preparation. In a typical synthesis, 0.95 mmol Y2O3 and 0.05 mmol Eu2O3 (both with purity of 99.99%, Shanghai Yuelong NonFerrous Metals Limited, China) were dissolved in dilute HCl (analytical reagent, A. R., Beijing Fine Chemical Company, China), resulting in the formation of a colorless solution of YCl3 and EuCl3. After evaporation followed by drying at 100 °C for 12 h in ambient atmosphere, a powder mixture of YCl3 and EuCl3 was obtained. Then
10.1021/cg060717j CCC: $37.00 © 2007 American Chemical Society Published on Web 03/09/2007
Synthesis and Properties of Y2O3:Eu3+ Microspheres 2.0 g CH3COONa (A. R. Beijing Fine Chemical Company, China) and 37 mL mixing solution of ethylene glycol (EG) (A. R. Beijing Fine Chemical Company, China) and water (volume ratio for EG : H2O ) 35 : 2) were added to the mixture of YCl3 and EuCl3. The solution was stirred for another 3 h. Then the transparent feedstock was charged into a 45 mL Teflon-lined stainless autoclave and heated at 180 °C for 24 h. After the autoclave was cooled to room-temperature naturally, the precursors were separated by filtration, washing with ethanol (A. R., Beijing Fine Chemical Company, China) and distilled water several times, and drying in atmosphere at 100 °C for 8 h. The final products were retrieved through a heat treatment at desired temperatures (3501400 °C) in air for 4 h. 2.2. Characterization. The samples were characterized by powder X-ray diffraction (XRD) performed on a Rigaku-Dmax 2500 diffractometer with Cu kR radiation(λ ) 0.15405 nm). Fourier transform infrared spectroscopy (FT-IR) spectra were measured with PerkingElmer 580B infrared spectrophotometer with the KBr pellet technique. Thermogravimetric and differential thermal analysis (TG-DTA) data were recorded with Thermal Analysis Instrument (SDT 2960, TA Instruments, New Castle, DE) with the heating rate of 10 °C ‚ min-1 in an air flow of 100 mL ‚ min-1. Elemental analyses of Y and Eu in the solid samples were carried out on inductive coupled plasma (ICP) atomic absorption spectrometric analysis (POEMS, TJA). The morphology of the samples was inspected using a field emission scanning electron microscopy equipped with energy-dispersive spectrometer (EDS) (FE-SEM, XL 30, Philips) and a transmission electron microscope (JEOL-2010, 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 at room temperature. The cathodoluminescent (CL) measurements were carried out in an ultrahigh-vacuum chamber (
