J. Phys. Chem. C 2008, 112, 12777–12785
12777
Self-Assembled 3D Flowerlike Lu2O3 and Lu2O3:Ln3+ (Ln ) Eu, Tb, Dy, Pr, Sm, Er, Ho, Tm) Microarchitectures: Ethylene Glycol-Mediated Hydrothermal Synthesis and Luminescent Properties Jun Yang,† Chunxia Li,† Zewei Quan,† Cuimiao Zhang,† Piaoping Yang,‡ Yinyan Li,† Cuicui Yu,† and Jun Lin*,† State Key Laboratory of Rare Earth Resources Utilization, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, Graduate UniVersity of the Chinese Academy of Sciences, Beijing 100049, People’s Republic of China, and College of Materials Science and Chemical Engineering, Harbin Engineering UniVersity, Harbin 150001, People’s Republic of China ReceiVed: May 5, 2008; ReVised Manuscript ReceiVed: June 7, 2008
Three-dimensional flowerlike Lu2O3 and Lu2O3:Ln3+ (Ln ) Eu, Tb, Dy, Pr, Sm, Er, Ho, Tm) microarchitectures have been successfully synthesized via ethylene glycol (EG)-mediated hydrothermal method followed by a subsequent heat treatment process. X-ray diffraction, Fourier transform infrared spectroscopy, energy-dispersive X-ray spectra, thermogravimetric and differential thermal analysis, elemental analysis, inductively coupled plasma atomic absorption spectrometric analysis, ion chromatogram analysis, X-ray photoelectron spectra, scanning electron microscopy, transmission electron microscopy, photoluminescence spectra as well kinetic decays, and cathodoluminescence spectra were used to characterize the samples. Hydrothermal temperature, EG, and CH3COONa play critical roles in the formation of the lutetium oxide precursor microflowers. The reaction mechanism and the self-assembly evolution process have been proposed. The as-formed lutetium oxide precursor could transform to Lu2O3 with their original flowerlike morphology and slight shrinkage in the size after postannealing process. The as-obtained flowerlike Lu2O3:Ln3+ samples show strong light emission with different colors corresponding to different Ln3+ ions under ultraviolet-visible light excitation and lowvoltage electron beams excitation, which have potential applications in fluorescent lamps and field emission displays. 1. Introduction Generally the chemical and physical properties of inorganic micro-/nanostructures are fundamentally related to their chemical composition, size, phase, surface chemistry, shape, and also dimensionality.1 So rational control over these factors has become an important research issue in recent years,2 allowing us not only to observe unique properties of the materials but also to tune their chemical and physical properties as desired. Recently, threedimensional (3D) nanostructured architectures have been explored for a new generation of advanced devices such as supercapacitors,3a,b fuel cells,3b,c and sensors3d owing to some improved properties originating from their nanobuilding blocks and the manners in which they are organized. Therefore, the alignment of nanostructured building blocks (nanoparticles, nanorods, nanoribbons, and nanoplates) into 3D ordered superstructures has been an exciting field in recent years.4 The simplest synthetic route to 3D nanostructures is self-assembly, in which ordered aggregates are formed in a spontaneous process.5 Up to now, a wide variety of inorganic materials, including metal,6a metal oxide,6b–d hydrate,6e borate,6f molybdate,6g,h and tungstate,6i have been successfully prepared with complex 3D hierarchical shapes by the solution-phase chemical method, due to its low cost and potential advantage for large-scale production. However, exploration of reasonable synthetic methods for controlled construction of complex 3D architectures of other inorganic functional materials via a chemical self-assembly route is still an intensive and hot research * Corresponding author. E-mail:
[email protected]. † Chinese Academy of Sciences. ‡ Harbin Engineering University.
topic. In the controlled construction of self-assembly of 1D or 2D nanobuilding blocks into 3D novel nanoarchitectures, copolymers and surfactants always play important roles due to their directing functions during the aggregation process as well as their stabilizing effects in equilibrium systems.6h,7 Relative to the isotropic self-assembly of spherical or near-spherical nanoparticles, the self-assembly of anisotropic nanostructures, such as nanoplates, nanosheets, nanorods, and nanotubes, requires more effort.6i Rare earth oxides have been extensively used in highperformance luminescent devices, magnets, catalysts, and other functional materials because of their electronic, optical, and chemical characteristics resulting from the 4f electronic shells.8 These properties depend strongly on the material chemical composition, crystal structure, shape, and dimensionality, which are sensitive to the bonding states of rare earth ions. To date, many rare earth hydroxides with various morphologies have been synthesized via a hydrothermal route due to the advantages of high purity and good homogeneity, and the corresponding structured rare earth oxides were made by calcining the precursors.9 The morphologies of the rare earth oxides were only focused on the nanotubes, nanorods, nanowires, and nanoflakes in previous reports. However, the studies on the synthesis of complex 3D architectures of rare earth oxides were relatively rare.6d So, how to achieve the controlled construction of 3D microarchitectures from these 1D or 2D nanostructured rare earth oxides via a chemical self-assembly route is a great challenge for material chemists. In addition, few studies have reported on the synthesis of lutetium oxide through hydrothermal
10.1021/jp803945w CCC: $40.75 2008 American Chemical Society Published on Web 07/29/2008
12778 J. Phys. Chem. C, Vol. 112, No. 33, 2008 process in previous works.9c Lutetium oxide (Lu2O3) is an excellent candidate for lanthanide ion substitution due to its favorable physical properties, such as high melting point, phase stability, and low thermal expansion.10a Lutetium (Lu) may be a more favorable cation than yttrium (Y) for trivalent lanthanide dopant emission due to the intensity-borrowing mechanism mixing the 4f and 5d orbitals of the Ln3+ ion via the lattice valence band levels.10b–d Ln3+-doped Lu2O3 materials (Ln ) Eu, Tb, Er, Ho, Sm) are important phosphors as reported in previous studies.11 The traditional synthesis procedures are the combustion (propellent) process using urea (glycine/ citric acid) as fuel,11b–e the reverse-strike coprecipitation method,11a and the Pechini procedure.12a These synthesis techniques are very wellknown and routinely used for fabrication of oxide phosphors. Unfortunately, these kinds of methods cannot control the morphology of the phosphor samples very well. Herein, we report a controllable route for the production of novel self-assembled 3D flowerlike lutetium oxide microarchitectures by an ethylene glycol (EG)-mediated hydrothermal process under nearly neutral conditions for the first time. Similar organic compound-assisted process has been adopted previously by others for the synthesis of rare earth oxides.12b–d By calcination at an elevated temperature, the as-synthesized lutetium oxide precursor was transformed into lutetium oxide, which maintained its original flowerlike morphology. The reaction mechanism leading to the lutetium oxide precursor and the self-assembly process were discussed. As an example of potential applications, various lanthanide ions (Ln3+ ) Eu3+, Tb3+, Dy3+, Pr3+, Sm3+, Er3+, Ho3+, Tm3+) have been doped into the 3D flowerlike lutetium oxide microarchitectures and the optical properties were investigated in detail. 2. Experimental Section 2.1. Materials. The initial chemicals, including Lu2O3, Eu2O3, Tb4O7, Dy2O3, Pr6O11, Sm2O3, Er2O3, Ho2O3, and Tm2O3 (all with purity g99.99%, Changchun Applied Chemistry Science and Technology Limited, China), HCl, ethylene glycol (EG), CH3COONa, and ethanol (all with purity of A. R., Beijing Fine Chemical Company, China), were used without further purification. 2.2. Preparation. 2.2.1. Lu2O3. In a typical synthesis, 1 mmol Lu2O3 was dissolved in dilute HCl, resulting in the formation of a colorless solution of LuCl3. After evaporation followed by drying at 100 °C for 12 h in ambient atmosphere, a powder of LuCl3 was obtained. Then 2.0 g of CH3COONa, 3 mL of distilled water, and 34 mL of ethylene glycol were added to the powder of LuCl3. The solution was stirred for another 3.5 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 and distilled water several times, and drying in atmosphere at 100 °C for 6 h. The final products (Lu2O3) were retrieved through a heat treatment at 800 °C in air for 4 h. 2.2.2. Lu2O3:Ln3+. First, Eu2O3, Tb4O7, Dy2O3, Pr6O11, Sm2O3, Er2O3, Ho2O3, and Tm2O3 were dissolved in dilute HCl, respectively, resulting in the formation of a colorless stock solution of LnCl3 (Ln ) Eu, Tb, Dy, Pr, Sm, Er, Ho, Tm) with 0.02 mol/L. In a typical synthesis, stoichiometric of Lu2O3 and LnCl3 (solution) were dissolved into dilute HCl, resulting in the formation of a colorless solution of LuCl3 and LnCl3 (Ln ) Eu, Tb, Dy, Pr, Sm, Er, Ho, Tm; Ln3+/(Lu3+ + Ln3+) ) 0.1-8 mol %). Then Lu2O3:Ln3+ products can be obtained through the same processes like Lu2O3 products as stated above.
Yang et al. 2.3. Characterization. Powder X-ray diffraction (XRD) measurements were 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 Perking-Elmer 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 analyze of H in the solid samples was carried out on VarioEL (Elementar Analysensysteme GmbH). Elemental analyses of Na and Cl in the solid samples were carried out on inductive coupled plasma (ICP) atomic absorption spectrometric analysis (POEMS, TJA) and ion chromatogram (IC) system (Dionex ICS-1000), respectively. The X-ray photoelectron spectra (XPS) were taken on a VG ESCALAB MK II electron energy spectrometer using Mg KR (1253.6 eV) as the X-ray excitation source. The morphology and structure of the samples were inspected using a field emission scanning electron microscopy (FE-SEM, XL 30, Philips) equipped with energy-dispersive X-ray (EDX) spectrometer and a transmission electron microscope. Low-resolution transmission electron microscopy (TEM) images and selective area electron diffraction (SAED) patterns were obtained using a JEOL 2010 transmission electron microscope operating at 150 kV. High-resolution transmission electron microscopy (HRTEM) images were performed using 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 (
