J. Phys. Chem. C 2009, 113, 153–158
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Facile Surfactant- and Template-Free Synthesis and Luminescent Properties of One-Dimensional Lu2O3:Eu3+ Phosphors Guang Jia, Yuhua Zheng, Kai Liu, 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, People’s Republic of China ReceiVed: September 23, 2008; ReVised Manuscript ReceiVed: NoVember 6, 2008
Uniform Lu2O3:Eu3+ nanorods and nanowires have been successfully prepared through a simple solutionbased hydrothermal process followed by a subsequent calcination process without using any surfactant, catalyst, or template. On the basis of X-ray diffraction, thermogravimetric analysis and differential scanning calorimetry, and Fourier transform infrared spectroscopy results, it can be assumed that the as-obtained precursors have the structure formula of Lu4O(OH)9(NO3), which is a new phase and has not been reported. The morphology of the precursors could be modulated from nanorods to nanowires with the increase of pH value using ammonia solution. The as-formed precursors could transform to cubic Lu2O3:Eu3+ with the same morphology and a slight shrinkage in size after an annealing process. Both the Lu2O3:Eu3+ nanorods and nanowires exhibit the strong red emission corresponding to the 5D0 -7F2 transition of the Eu3+ ions under UV light excitation or low-voltage electron beam excitation. 1. Introduction Generally the properties of inorganic micro/nanostructures are fundamentally related to their chemical composition, crystal structure, surface chemistry, shape, and dimensionality. Recently, shape and dimensionality are regarded as particularly important factors that influence the chemical and physical properties of materials.1 Among the various nanostructures, onedimensional (1D) nanoscaled structures, such as nanowires, nanorods, and nanotubes, have been regarded as significant factors that may bring novel and excellent properties.2 Therefore, much attention has been paid to the fabrication of 1D nanomaterials by various synthesis techniques. The widely used method is the catalyst and template-based synthesis 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 structure therefore depends greatly on the selection of suitable catalysts or templates, which involves a complicated process and may result in impurities in the products. Thus it is necessary to develop a simple method for fabricating a 1D nanostructure without involving any catalysts or templates. Recently, much research attention has been paid to the synthesis of 1D lanthanide compounds, especially rare earth oxides, because they can be used for high-performance phosphors, catalysts, and other functional materials based on their novel electronic, optical, and chemical properties resulting from their 4f electrons.3 If rare earth compounds were fabricated in the form of a 1D nanostructure, they would be expected to be highly functionalized materials due to their shape-specific and quantum confinement effects, acting as electrically, magnetically, or optically functional host materials as well.4-7 Lutetium oxide (Lu2O3) is an excellent candidate due to its favorable physical properties, such as high melting point, phase stability, and low thermal expansion.8 Ln3+-doped Lu2O3 materials (Ln * Authors to whom correspondence should be addressed,
[email protected] or
[email protected].
) Eu, Tb, Er, Ho, Sm) are important phosphors as reported in previous studies.9-13 Various traditional synthesis methods have been used to prepare Lu2O3 and Ln3+-doped Lu2O3 materials, such as a combustion process using urea, glycine, and citric acid as fuel,9,10 a coprecipitation method,11,12 and the Pechini sol-gel procedure.13 These synthesis techniques are well-known and routinely used for fabrication of oxide phosphors. Unfortunately, these kinds of methods cannot control the morphology of the samples very well. Among the various synthesis techniques, hydrothermal treatment as a typical solution approach has proven to be an effective and convenient process in preparing various inorganic materials with a variety of controllable morphologies and architectures. Compared with other rare earth compounds,1,4-7 few studies have reported on the synthesis of lutetium oxide through hydrothermal process in previous works. Recently, the square nanosheet and flowerlike microarchitectures of lanthanide-doped Lu2O3 luminescent materials have been synthesized via the hydrothermal or solvothermal method followed by a subsequent heat-treatment process. 14,15 However, to the best of our knowledge, there have been few reports on the synthesis of 1D nanostructure Lu2O3 materials. Herein, we report the controlled synthesis of one-dimensional Lu2O3:Eu3+ materials through a simple solution-based hydrothermal process followed by a subsequent calcination process. The photoluminescent (PL) and cathodoluminescent (CL) properties of phosphors have been discussed in detail. This synthetic route is promising for the preparation of other oxidebased phosphors due to its simplicity and the low cost of the starting reagents. 2. Experimental Section Lu(NO3)3 and Eu(NO3)3 aqueous solution were obtained by dissolving Lu2O3 (99.99%) and Eu2O3 (99.99%) in dilute HNO3 solution under heating with agitation. In the preparation procedure, 1.9 mmol of Lu(NO3)3 and 0.1 mmol of Eu(NO3)3 aqueous solution were added to 35 mL of
10.1021/jp808446m CCC: $40.75 2009 American Chemical Society Published on Web 12/12/2008
154 J. Phys. Chem. C, Vol. 113, No. 1, 2009
Figure 1. XRD patterns of the hydrothermal precursor obtained at (a) pH ) 8 (S1) and (b) pH ) 10 (S2). The standard data for monoclinic phase Y4O(OH)9(NO3) (JCPDS card 79-1352) is also presented in the figure for comparison.
deionized water. Then 25 wt % of ammonia solution (A. R., Beijing Fine Chemical Company, China) was introduced rapidly to the vigorously stirred solution until pH ) 8 or 10. After additional agitation for 1 h, the as-obtained white colloidal suspension 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 washed several times with deionized water and absolute ethanol and dried at 60 °C in air. The precursors prepared at pH ) 8 and 10 were labeled as S1 and S2, respectively. The final products were obtained through a heat treatment at 800 °C in air for 2 h with a heating rate of 2 °C min-1. The calcined samples for S1 and S2 were labeled as S3 and S4, respectively. Characterization. The samples were characterized by powder X-ray diffraction (XRD) performed on a Rigaku-Dmax 2500 diffractometer. Fourier transform infrared spectroscopy (FTIR) spectra were measured with a Perking-Elmer 580B infrared spectrophotometer with the KBr pellet technique. Thermogravimetric analysis and differential scanning calorimetry (TGADSC) data were recorded with a thermal analysis instrument (SDT 2960, TA Instruments, New Castle, DE) at the 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 scanning electron microscope (SEM, 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 cathodoluminescent (CL) measurements were carried out in an ultrahigh vacuum chamber (
