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Size-Tailored Synthesis and Luminescent Properties of One-Dimensional Gd2O3:Eu3+ Nanorods and Microrods Jun Yang, Chunxia Li, Ziyong Cheng, Xiaoming Zhang, Zewei Quan, Cuimiao Zhang, and Jun Lin* State Key Laboratory of Application of Rare Earth Resources, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, and Graduate UniVersity of the Chinese Academy of Sciences, Beijing 100049, People’s Republic of China ReceiVed: August 21, 2007; In Final Form: September 19, 2007
Nearly monodisperse and well-defined one-dimensional (1D) Gd2O3:Eu3+ nanorods and microrods were successfully prepared through a large-scale and facile hydrothermal method followed by a subsequent heat treatment process, without using any catalyst or template. X-ray diffraction (XRD), thermogravimetric analysis and differential scanning calorimetry (TGA-DSC), scanning electron microscopy (SEM), transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM), selected area electron diffraction (SAED), photoluminescence (PL) and cathodoluminescence (CL) spectra as well as kinetic decays were used to characterize the samples. The size of the Gd2O3:Eu3+ rods could be modulated from micro- to nanoscale with the increase of pH value using ammonia solution. The as-formed product via the hydrothermal process, Gd(OH)3:Eu3+, could transform to cubic Gd2O3:Eu3+ with the same morphology and a slight shrinking in size after a postannealing process. The formation mechanism for the Gd(OH)3 rods has been proposed. Both the Gd2O3:Eu3+ nanorods and microrods exhibit the same strong red emission corresponding to 5D0 f 7 F2 transition (610 nm) of Eu3+ under UV light excitation (257 nm) and low-voltage electron beam excitation (1-5 kV), which have potential applications in fluorescent lamps and field emission displays.
1. Introduction One-dimensional (1D) nanostructures, including nanorods, nanowires, nanotubes, and nanoprisms, have attracted extensive synthetic interest over the past several years due to their potential applications in a wide range of fields.1 More applications and new functional materials might emerge if shape-controlled nanocrystals could be achieved with high complexity.2 Of the nanomaterials, rare earth compounds have been widely used in high-performance luminescent devices, catalysts, and other functional materials based on the electronic, optical, and chemical characteristics arising from their 4f electrons.3 Most of these advanced functions depend strongly on the chemical composition and crystal structure, which are sensitive to the bonding states of rare earth ions. However, shape and dimensionality are now regarded as particularly important factors influencing the chemical and physical properties of materials. If rare earth compounds were fabricated in the form of a 1D nanostructure, they would be expected to be highly functionalized materials as a result of both shape-specific and quantum confinement effects, acting as electrically, magnetically, or optically functional host materials as well. Recently, many kinds of novel 1D structured materials have been successfully synthesized, such as III-V4 and II-VI semiconductors5 and elemental and oxide nanowires/nanorods.6,7 The widely used method to prepare 1D structures is the catalystand template-based method in which the catalysts act as the energetically favored sites for the adsorption of gas reactants while the templates are used to direct the growth of the 1D structure. The formation of 1D structures therefore depends greatly on the selection of suitable catalysts or templates, which * Corresponding author. E-mail:
[email protected].
involves a complicated process and may result in impurities in the products. One can overcome these difficulties by developing solution-phase methods for direct growth of the 1D nanostructure without involving catalysts or templates.8,9 Enlightened by the recent studies on the 1D nanostructure that can be prepared by evaporating the desired metal oxide powders at high temperatures,10 we believe that the 1D nanostructure might be prepared via a dissolution-recrystallization process in solution. As we know, with the advantages of high purity and good homogeneity, the hydrothermal synthesis method is an important technology for the preparation of low-dimension nanostructures of anisotropic nanomaterials. The advantage of the hydrothermal method resulting from the fact that neither catalyst nor template is required makes it another choice besides the template method. Up to now, there are many reports about 1D lanthanide hydroxides and oxides obtained by the hydrothermal method and postcalcining process.1c,3a,11 Among them, gadolinium hydroxide [Gd(OH)3] has been used as the catalyst and sorbent, and the precursor for the preparation of Gd2O3 by thermal dehydration, and Eu3+-doped Gd2O3 phosphor is one of the important red-emitting phosphors. Although the investigations on Gd2O3:Eu3+ phosphors are extensive, few studies have been reported on synthesis of separated, uniform, and well-defined 1D rodlike Gd2O3:Eu3+ and corresponding luminescent properties from microrods to nanorods with the hydrothermal method.12 Herein, we report the preparation of 1D rodlike Gd2O3:Eu3+ material through a large-scale and facile solution-based hydrothermal process without using any catalyst or template followed by subsequent heat treatment process, together with its photoluminescent properties for the application of a high-performance phosphor and the relations between size and luminescent property. The low cost of the starting reagents and the simplicity
10.1021/jp0767112 CCC: $37.00 © 2007 American Chemical Society Published on Web 11/13/2007
1D Gd2O3:Eu3+ Nanorods and Microrods
J. Phys. Chem. C, Vol. 111, No. 49, 2007 18149
of the synthetic route make it promising for the preparation of other oxide-based phosphors. 2. Experimental Section In a typical synthesis, 0.95 mmol Gd2O3 and 0.05 mmol Eu2O3 (both with purity of 99.99%, Shanghai Yuelong NonFerrous Metals Limited, China) were dissolved in dilute HCl solution under heating with agitation to form a solution. Here the dilute HCl solution is a mixture solution of 10 mL of deionized water and 5 mL of 36-38 wt % of HCl solution (A. R., Beijing Fine Chemical Company, China). After evaporation, 38 mL of deionized water was added to form a clear aqueous solution. Then 25 wt % of ammonia solution (A. R., Beijing Fine Chemical Company, China) was introduced dropwise to the vigorously stirred solution until pH ) 7-9. After additional agitation for 1 h, the as-obtained white colloidal precipitate was transferred to a 50 mL autoclave, sealed, and heated at 200 °C for 24 h. The autoclave was then cooled to room temperature naturally. The precursors were recovered by filtration, washed with ethanol (A. R., Beijing Fine Chemical Company, China) and distilled water several times, and dried at 100 °C in air for 6 h. The final products were retrieved through a heat treatment at 900 °C in air for 3 h. The phase purity and crystallinity of the products were examined by powder X-ray diffraction (XRD) performed on a Rigaku-Dmax 2500 diffractometer with Cu KR radiation (λ ) 0.15405 nm). TGA-DSC (thermogravimetric analysis and differential scanning calorimetry) curves 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 structure of the samples were inspected using a field emission scanning electron microscope (FESEM, XL 30, Philips) and transmission electron microscopy. Low-resolution 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. High-resolution transmission electron microscopy (HRTEM) imaging was performed using an FEI Tecnai G2 S-Twinwere with a field emission gun operating at 200 kV. Images were acquired digitally on a Gatan multiple CCD camera. 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 (
