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Fabrication of Nanoparticulate Porous LaOF Films through Film Growth and Thermal Decomposition of Ion-Modified Lanthanum Diacetate Hydroxide Eiji Hosono, Shinobu Fujihara,* and Toshio Kimura Department of Applied Chemistry, Faculty of Science and Technology, Keio University, 3-14-1 Hiyoshi, Kohoku-ku, Yokohama 223-8522, Japan Received December 16, 2003. In Final Form: February 19, 2004 This paper first reports fabrication of macro/nanotextured rare-earth oxyfluoride films. Usage of ionmodified lanthanum diacetate hydroxide (LDAH) as self-templates was successful in producing nanoparticulate lanthanum oxyfluoride (LaOF) films. LDAH template films were deposited on glass substrates through a chemical bath deposition in solutions composed of lanthanum acetate sesquihydrate, methanol, trifluoroacetic acid, and aqueous ammonia. The LDAH films had a unique, nestlike morphology owing to a two-dimensional hexagonal crystal growth. Modification of LDAH with trifluoroacetate ions led to formation of LaOF after pyrolyzing the template films at temperatures of 400-600 °C in air. The resultant LaOF films had a nanoparticulate porous microstructure, maintaining the morphology of the original LDAH template films. It was also successful to incorporate Eu3+ ions into LaOF through deposition of the LDAH film in a solution containing europium acetate tetrahydrate. The characteristic photoluminescence from Eu3+ was observed with an ultraviolet-light excitation at 273 nm, indicating that Eu3+ was homogeneously distributed in LaOF host crystals. Thus the ion-modification of LDAH was also demonstrated to be a useful method for preparing nanostructured rare-earth oxyfluoride materials having various cationic compositions.
Introduction In recent years, nanostructured or nanoparticulate alkaline earth and rare-earth fluoride materials (CaF2, BaF2, YF3, LaF3, etc.) have been successfully prepared using chemical solutions of well-designed compositions.1-5 Not surprisingly, however, formation of metal fluorides is achieved through a usual chemical reaction between metal ions and fluoride ions (F-) derived from HF or NH4F. Precise control of nucleation and crystal growth is therefore a key to fabricating nanoparticulate fluorides. For example, Cao et al. have employed a microemulsionmediated hydrothermal method to elaborate BaF2 whiskers using aqueous solutions of BaCl2 and HF as precursors.2 Nanostructured fluorides thus obtained are expected to find a wide variety of applications in dielectrics, optics, optoelectronics, and photonics. Metal oxyfluoride materials are also believed to have great potential for such applications because they exhibit unique electrical, optical, and electrochemical characteristics.6-8 Synthesis of nanoparticulate rare-earth oxyfluoride materials has been exclusively achieved so far by employing a mechanochemical processing based on a solid-state reaction.9,10 * To whom correspondence should be addressed. E-mail:
[email protected]. Phone: +81 (0)45-566-1581. Fax: +81 (0)45-566-1551. (1) Bender, C. M.; Burlitch, J. M.; Barber, D.; Pollock, C. Chem. Mater. 2000, 12, 1969-1976. (2) Cao, M.; Hu, C.; Wang, E. J. Am. Chem. Soc. 2003, 125, 1119611197. (3) Sun, X. M.; Li, Y. D. Chem. Commun. 2003, 1768-1769. (4) Hua, R.; Zang, C.; Shao, C.; Xie, D.; Shi, C. Nanotechnology 2003, 14, 588-591. (5) Stouwdam, J. W.; van Veggel, F. C. J. M. Nano Lett. 2002, 2, 733-737. (6) Fergus, J. W.; Chen, H. P. J. Electrochem. Soc. 2000, 147, 46964704. (7) Fujihara, S.; Kato, T.; Kimura, T. J. Mater. Sci. Lett. 2001, 20, 687-689. (8) Au, C. T.; Zhang, Y. Q.; He, H.; Lai, S. Y.; Ng, C. F. J. Catal. 1997, 167, 354-363. (9) Lee, J.; Zhang, Q. W.; Saito, F. J. Am. Ceram. Soc. 2001, 84, 863-865.
However, no work has been reported in the literature to develop methods for preparing nanostructured oxyfluoride films. Our approach to fabricating nanostructured inorganic solid films utilizes a chemical bath deposition (CBD) to grow certain precursor materials as self-templates on substrates. The CBD process consists of three fundamental chemical reaction steps: (a) formation or dissociation of solvated ionic metal-ligand complexes, (b) hydrolysis of the complexes, and (c) formation of solid phases.11 Progress of step b produces less soluble chemical species and causes a low degree of supersaturation of the solutions, which results in heterogeneous nucleation on substrates in step c. When aqueous solution systems are used in the whole process, water plays a role not only as a medium where reactions occur but also as a reactant in all the reaction steps. In cases where hydrolysis reactions proceed rapidly toward a high degree of supersaturation of the solutions, nonaqueous systems containing a very small amount of water will lead to a successful deposition of solid films. Layered hydroxide metal salts (LHMSs)12-14 containing exchangeable interlayer anions such as Cl-, Br-, I-, NO3-, or CH3COO- are materials suitable for self-template precursors. If LHMS films can be synthesized through CBD in nonaqueous solutions, they are expected to exhibit a unique morphology because of their two-dimensional crystal growth. Such films can be used as templates when they are pyrolytically transformed into pertinent metal (10) Lee, J.; Zhang, Q. W.; Saito, F. J. Alloys Compd. 2003, 348, 214-219. (11) Niesen, T. P.; De Guire, M. R. J. Electroceram. 2001, 6, 169207. (12) Aylett, B. J. In Comprehensive Inorganic Chemistry; Bailar, J. C., Emelejus, H. J., Nyholm, R., Trotman-Dickenson, A. F., Eds.; Pergamon: Oxford, 1973. (13) Oswald, H. R.; Asper, R. In Preparation and Crystal Growth of Materials with Layered Structures; Lieth, R. M. A., Ed.; D. Reidel: Dordrecht, 1977. (14) Newman, S. P.; Williams, J. J. Solid State Chem. 1999, 148, 26-40.
10.1021/la036370t CCC: $27.50 © 2004 American Chemical Society Published on Web 03/25/2004
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Hosono et al.
Table 1. Composition of the Solutions That Were Used in the CBD Process (Ac ) CH3COO) solution
La(Ac)3‚1.5H2O (mol/dm3)
Eu(Ac)3‚4H2O (mol/dm3)
methanol (mL)
TFA (mL)
HNO3aq (mL)
NH3aq (mL)
water concentrationa (mol/dm3)
A B C
0.15 0.1425 0.15
0 0.0075 0
91.534 91.534 91.534
3.466 3.466 0
0 0 3.466
5 5 5
2.02 2.04 2.78
a The water concentration was calculated taking account of all the water content in rare-earth acetate hydrates, aqueous nitric acid, and aqueous ammonia. Methanol and TFA were anhydrous.
oxides without changing the morphology. We have already reported the film formation of layered basic zinc acetate (Zn5(OH)8(CH3COO)2‚2H2O) and its pyrolytic transformation into nanostructured zinc oxide.15 Here, we investigate the CBD process of lanthanumrelated LHMS compounds to obtain nanostructured lanthanum oxyfluoride (LaOF) films. Two kinds of LHMS compounds are known to exist in a lanthanum acetate hydroxide system:14,16-18 La(CH3COO)2(OH) and La(CH3COO)(OH)2. The former, lanthanum diacetate hydroxide (LDAH), has been prepared by a glycothermal method that was processed at 300 °C starting from mixtures of lanthanum acetate sesquihydrate (La(CH3COO)3‚1.5H2O) and 1,4-butanediol or lanthanum acetylacetonate dihydrate and toluene.16-18 In this work, the CBD process starting from La(CH3COO)3‚1.5H2O and methanol has led to formation of LDAH films even at a low temperature of 60 °C. Crystal growth of LDAH was found to follow a two-dimensional hexagonal growth mode, strongly influencing morphology of the films that could be used as self-templates. Modification of LDAH with trifluoroacetate (TFA) ions (CF3COO-) was possible by adding trifluoroacetic acid to the CBD solutions. By pyrolyzing the films of the TFA-modified LDAH, LaOF was formed instead of lanthanum oxide (La2O3). The LaOF formation was also confirmed by analyzing luminescence of Eu3+ used as a spectroscopic probe. The LaOF films thus prepared had nanoparticulate porous structures that were achieved for the first time, demonstrating that the present synthetic process would be useful for producing nanostructured oxyfluoride materials. Experimental Methods Materials. Lanthanum acetate sesquihydrate (La(CH3COO)3‚ 1.5H2O) with 99% purity and europium acetate tetrahydrate (Eu(CH3COO)3‚4H2O) with 99.9% purity were obtained from Soekawa Chemicals, Japan, and Wako Pure Chemicals, Japan, respectively. Anhydrous trifluoroacetic acid (TFA; CF3COOH) was obtained from Wako. Aqueous nitric acid (HNO3aq; 13.5 mol/ dm3), aqueous ammonia (NH3aq; 15 mol/dm3), and anhydrous methanol were used as received from Taisei Chemical, Japan. Film Fabrication Process. Three kinds of solutions with different compositions were prepared for the CBD process by dissolving La(CH3COO)3‚1.5H2O (and Eu(CH3COO)3‚4H2O for one of the solutions) in methanol with addition of TFA or nitric acid. NH3aq was added dropwise to the solutions to adjust pH values. The concentration of the rare-earth metal ion was fixed to 0.15 mol/dm3. Overall compositions of the solutions (denoted as A, B, and C) are listed in Table 1. Quartz glass plates 1 mm in thickness (Shinkyo Kogyo, Japan) were used as substrates for the deposition. The substrates were put into bottles filled with the solutions and sealed up and were kept at 60 °C for 2-24 h in a constant-temperature oven. After the deposition, the LDAH films obtained were rinsed with ethanol and were dried at room (15) Hosono, E.; Fujihara, S.; Kimura, T.; Imai, H. J. Colloid Interface Sci. 2004, 272, 391-398. (16) Inoue, M.; Kominami, H.; Otsu, H.; Inui, T. Nippon Kagaku Kaishi 1991, 1254-1260. (17) Kominami, H.; Onoue, S.; Nonaka, S.; Kera, Y. J. Ceram. Soc. Jpn. 1999, 107, 682-685. (18) Inoue, M.; Nishikawa, T.; Kominami, H.; Inui, T. J. Mater. Sci. 2000, 35, 1541-1547.
temperature. The films were then heated at 400-1000 °C for 1 h in air to be transformed into LaOF or La2O3. Characterization. The crystal structure of the films was identified by X-ray diffraction (XRD) analysis with a Rigaku RAD-C diffractometer using Cu KR radiation in the 2θ range 3-60°. The film morphology was observed by field emission scanning electron microscopy (FESEM) and transmission electron microscopy (TEM) with a Hitachi S-4700 and a Philips TECNAI F20 microscope, respectively. Organic species present in the films were examined by Fourier transform infrared (FT-IR) spectroscopy with a BIO-RAD FTS-165 spectrometer using a KBr method. The thermal decomposition behavior of LDAH was examined by thermogravimetry-differential thermal analysis (TG-DTA) with a Mac Science 2020S analyzer using a heating rate of 5 °C/min in air. The specific surface area was estimated by the BrunauerEmmett-Teller (BET) method based on the N2 adsorption. For the FT-IR, TG-DTA, and BET measurements, the films were detached from the substrate. X-ray photoelectron spectroscopy (XPS) was carried out with a JEOL JSP-9000MX spectrometer using Mg KR radiation. Photoluminescence (PL) spectra were measured at room temperature with a Shimadzu RF-5300PC spectrofluorophotometer using a xenon lamp (150 W) as a light source. Emission scans were performed with 1.5 nm band-pass emission slits. A filter, which eliminated light having wavelengths of
