pubs.acs.org/Langmuir © 2009 American Chemical Society
Fabrication and Luminescence Properties of One-Dimensional CaMoO4: Ln3þ (Ln = Eu, Tb, Dy) Nanofibers via Electrospinning Process Zhiyao Hou,*,†,‡ Ruitao Chai,‡ Milin Zhang,† Cuimiao Zhang,‡ Peng Chong,‡ Zhenhe Xu,‡ Guogang Li,‡ and Jun Lin*,‡ † College of Materials Science and Chemical Engineering, Harbin Engineering University, Harbin 150001, P. R. China, and ‡State Key Laboratory of Rare Earth Resource Utilization, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, P. R. China
Received May 6, 2009. Revised Manuscript Received June 15, 2009 One-dimensional CaMoO4:Ln3þ (Ln = Eu, Tb, Dy) nanofibers have been prepared by a combination method of sol-gel and electrospinning process. X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), scanning electron microscopy (SEM), transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM), photoluminescence (PL), and low voltage cathodoluminescence (CL) as well as kinetic decays were used to characterize the resulting samples. SEM and TEM analyses indicate that the obtained precursor fibers have a uniform size, and the as-formed CaMoO4:Ln3þ nanofibers consist of nanoparticles. Under ultraviolet excitation, the CaMoO4 samples exhibit a blue-green emission band with a maximum at 500 nm originating from the MoO42- groups. Due to an efficient energy transfer from molybdate groups to dopants, CaMoO4:Ln3þ phosphors show their strong characteristic emission under ultraviolet excitation and low-voltage electron beam excitation. The energy transfer process was further studied by the emission spectra and the kinetic decay curves of Ln3þ upon excitation into the MoO42- groups in the CaMoO4:x mol % Ln3þ samples (x = 0-5). Furthermore, the emission colors of CaMoO4:Ln3þ nanofibers can be tuned from blue-green to green, yellow, and orange-red easily by changing the doping concentrations (x) of Ln3þ ions, making the materials have potential applications in fluorescent lamps and field emission displays (FEDs).
1. Introduction One-dimensional (1D) nanostructures, such as nanofibers, nanotubes, nanowires, nanorods, and nanobelts, have attracted great research interest because of their potential to test fundamental quantum mechanic concepts1-5 and to play a vital role in various applications such as photonics,6 nanoelectronics,7 and data storage.8 1D nanomaterials with different compositions have been developed using various methods9-14 including the chemical or physical vapor deposition, laser ablation, solution, arc discharge, vapor-phase transport process, and a template-based method. Compared with the above processes, electrospinning is an effective and simple method for generating 1D materials with diameters ranging from tens of nanometers *Corresponding authors. E-mail:
[email protected] (Z.H.); jlin@ ciac.jl.cn (J.L.). (1) Xia, Y.; Yang, P.; Sun, Y.; Wu, Y.; Gates, B.; Yin, Y.; Kim, F.; Yan, H. Adv. Mater. 2003, 15, 353. (2) Bockrath, M.; Liang, W.; Bozovic, D.; Hafner, J. H.; Lieber, C. M.; Tinkham, M.; Park, H. Science 2001, 291, 283. (3) Huang, Y.; Quan, X. F.; Wei, Q. Q.; Lieber, C. M. Science 2001, 291, 851. (4) Li, D.; Xia, Y. Nano Lett. 2003, 3, 555. (5) Hu, J.; Odom, T. W.; Lieber, C. M. Acc. Chem. Res. 1999, 32, 435. (6) Nakamura, S. Science 1998, 281, 956. (7) Mirkin, C. A. Science 1999, 286, 2095. (8) O’Barr, R.; Yamamoto, S. Y.; Schultz, S.; Xu, W. H.; Scherer, A. J. Appl. Phys. 1997, 81, 4730. (9) Bae, S. Y.; Seo, H. W.; Park, J.; Yang, H.; Park, J. C.; Lee, S. Y. Appl. Phys. Lett. 2002, 81, 126. (10) Fu, L.; Liu, Y. Q.; Hu, P.; Xiao, K.; Yu, G.; Zhu, D. B. Chem. Mater. 2003, 15, 4287. (11) Duan, X. F.; Lieber, C. M. Adv. Mater. 2000, 12, 298. (12) Jung, J. H.; Kobayashi, H.; Van, B.; Shinkai, S.; Shimizu, T. Chem. Mater. 2002, 14, 1445. (13) Wu, Y.; Yang, P. Chem. Mater. 2000, 12, 605. (14) Huang, M. H.; Choudrey, A.; Yang, P. Chem. Commun. 2000, 12, 1063.
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up to micrometers. The electrospinning technique has been developed since 1934 for the synthesis of 1D materials,15 and now it is demonstrated that a rich variety of materials can be electrospun to form uniform 1D materials,16-25 such as polymers and inorganic and hybrid (organic-inorganic) compounds. On the other hand, the sol-gel process has been proven as an efficient way to produce nanoparticles26 and nanocoatings27 of metal oxide. Sol-gel techniques can be employed to prepare precursor solutions, which have been used to deposit coatings by spinning and dipping.28 1D materials fabricated by a combination method of sol-gel process and electrospinning have become important for their exceptionally long length, uniform diameter, diverse composition and (15) Formhals, A. US Patent Specification, 1 975 504, 1934. (16) Li, X. H.; Shao, C. L.; Liu, Y. C. Langmuir 2007, 23(22), 10920. (17) Madhugiri, S.; Dalton, A.; Gutierrez, J.; Ferraris, J. P.; Kenneth, J.; Balkus, J. J. Am. Chem. Soc. 2003, 125, 14531. (18) Yao, L.; Haas, T. W.; Guiseppi-Elie, A.; Bowlin, G. L.; Simpson, D. G.; Wnek, G. E. Chem. Mater. 2003, 15, 1860. (19) Salalha, W.; Dror, Y.; Khalfin, R. L.; Cohen, Y.; Yarin, A.; Zussman, E. Langmuir 2004, 20(22), 9852. (20) Larsen, G.; Velarde-Ortiz, R.; Minchow, K.; Barrero, A.; Loscertales, I. G. J. Am. Chem. Soc. 2003, 125, 1154. (21) Hou, H. Q.; Reneker, D. H. Adv. Mater. 2004, 16, 69. (22) Bazilevsky, A. V.; Yarin, A. L.; Megaridis, C. M. Langmuir 2007, 23(5), 2311. (23) Wu, J.; Coffer, J. L. Chem. Mater. 2007, 19, 6266. (24) Ge, J. J.; Hou, H.; Li, Q.; Graham, M. J.; Greiner, A.; Reneker, D. H.; Harris, F. W.; Cheng, Z. D. J. Am. Chem. Soc. 2004, 126, 15754. (25) Li, M. J.; Zhang, J. H.; Zhang, H.; Liu, Y. F.; Wang, C. L.; Xu, X.; Tang, Y.; Yang, B. Adv. Funct. Mater. 2007, 17, 3650. (26) Yang, P.; Lu, M. K.; Song, C. F.; Zhou, G. J.; Xu, D.; Yuan, D. R. J. Phys. Chem. Solids 2002, 63, 2047. (27) Fu, X. A.; Qutubuddin, S. Colloid Surf. A 2001, 186, 245. (28) Zhai, J.; Zhang, L. Y.; Yao, X.; Hodgson, S. N. B. Surf. Coat. Technol. 2001, 138, 135.
Published on Web 07/07/2009
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Figure 1. X-ray diffraction patterns for CaMoO4:5 mol %Ln3þ nanofibers: (a) as-formed precursor fibers; (b) the CaMoO4:5 mol % Tb3þ fibers annealed at 800 °C, (c) CaMoO4:5 mol % Eu3þ annealed at 800 °C, (d) CaMoO4:5 mol % Dy3þ annealed at 800 °C; and the JCPDS card 29-0351 of CaMoO4 for comparison.
high surface, which can be applied in biomedical fields, reinforced composites, catalyst supports, sensors, electronic and optical devices, as well as sacrificial templates.29-32 Metal molybdates of relatively large bivalent cations (MMoO4, ionic radius >0.099 nm, M = Ca, Ba, Pb, Sr) exist in the so-called scheelite structure form (scheelite = CaWO4), where the molybdenum atom adopts tetrahedral coordination.33 Scheelitetype molybdates of the same divalent metal are reciprocally soluble over the entire compositional range, which results in a rich family of solid-state solution compounds. CaMoO4 is important among metal molybdate families that have potential applications in various fields, such as in photoluminescence,34 microwave applications,35 white light emitting diodes,36 and laser materials.37 Among all the nanomaterials, rare earth compounds have been widely used in the fields of high-performance luminescent devices, catalysts, and other-functional materials based on their electronic, optical, and chemical characteristics arising from their 4f electrons.38 As the most frequently used activator ions in luminescent materials, Eu3þ mainly shows emission due to transitions of 5D0-7FJ (J = 1, 2, 3, 4) in the orange-red regions, Tb3þ mainly shows emission due to transitions of 5D4-7FJ (J = 6, 5, 4, 3) in the green regions, and Dy3þ mainly shows emission due to transitions of (29) Bergshoef, M. M.; Vancso, G. J. Adv. Mater. 1999, 11, 1362. (30) Wang, X. Y.; Drew, C.; Lee, S. H.; Senecal, K. J.; Kumar, J.; Samuelson, L. A. Nano Lett. 2002, 2, 1273. (31) Casper, C. L.; Stephens, J. S.; Tassi, N. G.; Chase, D. B.; Rabolt, J. F. Macromolecules 2004, 37, 573. (32) Czaplewski, D. A.; Verbridge, S. S.; Kameoka, J.; Craighead, H. G. Nano Lett. 2004, 4, 437. (33) Yu, S. H.; Liu, B.; Mo, M. S.; Huaang, J. H.; Liu, X. M.; Qian, Y. T. Adv. Funct. Mater. 2003, 13, 639. (34) (a) Graser, R.; Pitt, E.; Scharmann, A.; Zimmerer, G. Phys. Status Solidi B 1975, 69, 359. (b) Yan, S. X; Zhang, J. H.; Zhang, X.; Lu, S. Z.; Ren, X. G.; Nie, Z. G.; Wang, X. J. J. Phys. Chem. C 2007, 111, 13256. (c) Lei, F.; Yan, B. J. Solid State Chem. 2008, 181, 855. (d) Zhang, Z. J.; Chen, H. H.; Yang, X. X.; Zhao, J. T. Mater. Sci. Eng. B 2007, 145, 34. (35) Johnson, L. F.; Boyd, G. D.; Nassau, K.; Soden, R. R. Phys. Rev. 1962, 126, 1406. (36) Hu, Y. S.; Zhuang, W. D.; Ye, H. Q.; Wang, D. H.; Zhang, S. S.; Huang, X. W. J. Alloys Compd. 2005, 390, 226. (37) Barbosa, L. B.; Reyes; Ardila, D.; Cusatis, C.; Andreeta, J. P. J. Cryst. Growth 2002, 235, 327. (38) (a) Feldmann, C. Adv. Funct. Mater. 2003, 13, 101. (b) Tang, Q.; Liu, Z. P.; Li, S.; Zhang, S. Y.; Liu, X. M.; Qian, Y. T. J. Cryst. Growth 2003, 259, 208. (c) Capobianco, J. A.; Vetrone, F.; Boyer, J. C.; Speghini, A.; Bettinelli, M. Opt. Mater. 2002, 19, 259. (d) Palmer, M. S.; Neurock, M.; Olken, M. M. J. Am. Chem. Soc. 2002, 124, 8452. (e) Hasegawa, Y.; Thongchant, S.; Wada, Y.; Tanaka, H.; Kawai, T.; Sakata, T.; Mori, H.; Yanagida, S. Angew. Chem. 2002, 114, 2177. (f) Diamente, P. R.; Bruke, R. D.; Vegge, F. Langmuir 2006, 22(4), 1782. (g) Xu, Y. F.; Ma, D. K.; Chen, X. A.; Yang, D. P.; Huang, S. M. Langmuir 2009, 25, 7103–7108.
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Figure 2. FT-IR spectra of CaMoO4:5 mol % Tb3þ nanofibers: (a) as-formed precursor fibers and (b) the fibers annealed at 800 °C.
F9/2-6H13/2 in the blue region and 4F9/2-6H15/2 in the yellow region, respectively. As far as we know, there are no reports concerning the preparation of Ln3þ ion doped CaMoO4 1D phosphors via an electrospinning process. Accordingly, here we employed electrospinning method to prepare 1D CaMoO4 and CaMoO4:Ln3þ (Ln = Eu, Tb, Dy) materials and investigated their photoluminescent and cathodoluminescent properties in detail. 4
2. Experimental Section Chemicals and Materials. Polyvinylpyrrolidone (PVP, Mw=1300000) was purchased from Aldrich, ammounium molybdate (NH4)6Mo7O24 3 4H2O (g99.0%, analytical reagent, A. R.) was purchased from Beijing Beihua Chemicals Co., Ltd., and calcium nitrate Ca(NO3)2 3 4H2O (g99.0%, analytical reagent, A. R.), citric acid monohydrate C6H8O7 3 H2O (g99.5%, A. R.), nitric acid HNO3 (A. R.), hydrogen peroxide (g30%, A. R.), and ethanol were all purchased from Beijing Fine Chemical Co. Tb4O7, Eu2O3, and Dy2O3 (99.99%) were purchased from Science and Technology Parent Company of Changchun Institute of Applied Chemistry. Eu(NO3)3 and Dy(NO3)3 were prepared by dissolving Eu2O3 and Dy2O3 in dilute nitric acid HNO3, and Tb(NO3)3 was prepared by dissolving Tb4O7 in dilute nitric acid HNO3 containing H2O2 and then evaporating the water in the solutions by heating. All of the initial chemicals in this work were used without further purification. Preparation. 1D CaMoO4:x mol %Ln3þ (x = 0-5) samples were prepared by an electrospinning process followed by annealing at high temperature. In a typical procedure for the preparation of CaMoO4 (x = 0), 1 mmol of Ca(NO3)2 3 4H2O and 0.1766 g of (NH4)6Mo7O24 3 4H2O were dissolved in 20 mL of deionized water (1:20 molar ratio of metal ions to H2O), and the pH value of the solution was kept between 2 and 3 by addition of HNO3. Then the above solution was mixed with 80 mL of ethanol containing citric acid as a chelating agent for the metal ions. The molar ratio of metal ions to citric acid was 1:2. A certain amount of polyvinylpyrrolidone was added (with a 7 wt % in the water-ethanol solution to adjust the viscosity). The solution was stirred for 4 h to form a homogeneous hybrid sol for further electrospinning. It should be mentioned that the key strategy of sol-gel/electrospinning method to obtain 1D inorganic nanomaterials was to form a solution with viscoelastic behavior similar to that of a conventional polymer solution. In our experiments, the purposes of controlling the volume ratio of water to alcohol and the weight percentage of PVP were to adjust the viscoelastic behavior, making the hybrid solution suitable for further electrospinning. The distance between the spinneret (a metallic needle) and collector (a grounded conductor) was fixed at 16.5 cm and the high-voltage supply DOI: 10.1021/la9016189
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Figure 3. SEM images of the as-formed precursor for CaMoO4:5 mol % Tb3þ nanofibers: images with low magnification (a) and high magnification (b). Those annealed at 800 °C: images with low magnification (c), large magnification (d). was maintained at 15 kV. The spinning rate was controlled at 0.5 mL 3 h-1 by a syringe pump (TJ-3A/W0109-1B, Boading Longer Precision Pump Co., Ltd., China). In this way, 1D CaMoO4:xmol % Ln3þ hybrid precursor samples were obtained. The CaMoO4:Ln3þ hybrid samples were prepared in a similar process as above, with the doping concentration of Tb3þ, Eu3þ, and Dy3þ of x mol % in CaMoO4 (x = 0.5, 1, 3, 5). Finally, the as-prepared hybrid precursor samples were annealed to the desired temperature 800 °C with the heating rate of 2 °C 3 min-1 and held there for 4 h in air. The actual doping concentration analyses of the as prepared CaMoO4:x mol % Ln3þ (x = 0.5, 1, 3, 5) samples were performed with ICP technique, as summarized in Table S1 (Supporting Information). Characterization. The X-ray diffraction (XRD) patterns of the samples were carried out on a Rigaku-Dmax 2500 diffractometer using Cu KR radiation (λ = 0.15405 nm). FT-IR spectra were performed on a Perkin-Elmer 580B infrared spectrophotometer using the KBr pellet technique. The morphology of the samples was inspected using a field emission scanning electron microscope (Philips XL 30). Transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM) micrographs were obtained from a FEI Tecnai G2 S-Twin transmission electron microscope with a field emission gun operating at 200 kV. Inductively coupled plasma (ICP) optical emission spectrometer (ICP-OES, ICAP 6300, Thermal Scientific) was used to determine the actual doping molar concentrations of Ln3þ in CaMoO4:Ln3þ samples. The photoluminescence (PL) measurements were performed with a Hitachi F-4500 spectrophotometer equipped with a 150 W xenon lamp as the excitation source. The cathodoluminescence (CL) measurements were conducted in an ultrahigh-vacuum chamber (
