CRYSTAL GROWTH & DESIGN
Electrochemical Sc2O3 Single Crystal Growth Nobuhito Imanaka,*,† Young Woon Kim,† Toshiyuki Masui,† and Gin-ya Adachi‡ Department of Applied Chemistry, Faculty of Engineering and Handai Frontier Research Center, Osaka University, 2-1 Yamadaoka, Suita, Osaka 565-0871, Japan, and Juri Institute for Environmental Science and Chemistry, College of Analytical Chemistry, 2-1-8 Temma, Kita-ku, Osaka 530-0043, Japan Received December 2, 2002;
2003 VOL. 3, NO. 3 289-290
Revised Manuscript Received February 24, 2003
ABSTRACT: Scandium oxide single crystals were successfully grown electrochemically by applying the Sc3+ ion-conducting Sc2(MoO4)3 solid electrolyte at 950 °C. The obtained crystals distributed in the range of 0.1-1.3 µm, and the average size was 0.56 µm. Because the melting point of Sc2O3 is as high as ca. 2500 °C, the single crystal growth of such refractory oxides as Sc2O3 is considerably difficult by the conventional thermal heating-cooling crystal growth procedure. The presently developed electrochemical single crystal growth can be simply applicable at moderate temperatures around 1000 °C for the single crystal growth of another type of rare earth refractory oxide by applying the target rare earth ion-conducting Ln2(MoO4)3 solid electrolyte series. Rare earth oxides are the series whose melting temperatures exceed 2000 °C1 and one of the high refractory oxides similar to magnesium oxide (mp 2826 °C) and alumina (mp 2054 °C), etc. Therefore, the growth of rare earth oxides in a single crystal form is a tough task and time-consuming. Some methods have been reported to grow a Sc2O3 single crystal by melting it’s powder at 2550 °C,2 growing it by a flame fusion method,3 or by flux methods.4,5 However, especially by the melt growth method, a severe fracture problem appears during cooling. In the case for the flux growth method, the flux used appears inside the single crystal and deteriorates the single crystal quality more or less. In our previous paper,6 we have demonstrated the In3+ ion conduction in the In2(MoO4)3 solid electrolyte, and recently, we have demonstrated the growth of In2O3 single crystals, which gradually vaporize approximately at 850 °C by using the In3+ ion-conducting solid electrolyte of In2(MoO4)3 by the dc electrolysis method.7 In this paper, Sc2O3 was electrochemically grown in a single crystal form at moderate temperature as low as 950 °C. Because the Sc2(MoO4)3 solid electrolyte can only conduct Sc3+ cation, the Sc2O3 single crystal with a high quality can be intentionally grown. A stoichiometric amount of Sc2O3 (purity 99.9%) and MoO3 (purity 99.9%) was mixed by a ball milling method (Pulverizette 7, FRITSCH GmbH). The mixture was calcined at 750 °C for 12 h in air. The calcined powder was ground in a mortar and reheated at 1000 °C for 12 h in air. The resulting powder was made into pellets (10 mm in diameter and 0.8 mm in thickness) and sintered at 1000 °C for 12 h in air. The sintered sample pellets were placed between two ion-blocking Pt electrodes for the crystal growth electrolysis as shown in Figure 1. The electrolysis was carried out by dc voltage at 11 V at 950 °C for 2 weeks in air. The particles obtained were characterized by means of scanning electron microscopy (SEM, Hitachi S-4300SD). Prior to SEM observation, the sample was sputter-coated with a gold layer, to minimize any possible surfacecharging effects. The particle size distribution and the average particle size were determined by measuring more than 450 particles along a fixed direction on the SEM photograph. Transmission electron microscopy (TEM) and selected area electron diffraction (SAED) measurement * To whom correspondence should be addressed. † Osaka University. ‡ College of Analytical Chemistry.
Figure 1. Schematic representation of the Sc2O3 single crystal growth method by dc electrolysis.
were performed with a transmission electron microscope (Hitachi, H-800) equipped with a tilting device and operating at 200 kV. The prepared particles were rinsed out from the matrix with methanol and dispersed in it ultrasonically and then supported on an amorphous carbon film mounted on a copper grid. Images were recorded under axial illumination at an approximate Scherzer focus, with a point resolution better than 0.194 nm. The diameter of the objective aperture was 20 µm, which was large enough to include some low-indexed diffraction spots from Sc2O3. The SEM image of the prepared particles obtained by the electrolysis of Sc2(MoO4)3 at 950 °C and 11 V for 2 weeks is presented in Figure 2. Well-defined polyhedral tabular particles ranging from 0.1 to 1.3 µm in diameter were obtained by the electrolysis. From the calculation of the diameter for more than 450 particles along a fixed direction on the SEM photograph, the particle size distribution histogram of the Sc2O3 particles was obtained (Figure 3). The average particle size was 0.56 µm, and the standard deviation was estimated to be 0.18 µm. More than 85% of the particles were in the range of 0.3-0.8 µm. The dc voltage applied is as high as 11 V above the decomposition of Sc2(MoO4)3. A considerably higher dc voltage as compared with the decomposition voltage of Sc2(MoO4)3 facilitates the Sc2(MoO4)3 decomposition to Sc3+ ions and MoO3 at the anode. Because the formed MoO3 vaporizes during the electrolysis because the electrolysis was conducted at 950 °C, which is above the MoO3 sublimation temperature of around 750-800 °C, Sc3+ ions were successively produced at the anodic surface. The Sc3+
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Figure 2. SEM photograph of the cathodic surface of the Sc2(MoO4)3 pellet, which was electrolyzed at 950 °C for 2 weeks at 11 V.
Communications
Figure 5. SAED pattern for one of the prepared Sc2O3 fine particles.
of Figure 3. To obtain a clear SAED pattern, it is necessary to select a small crystal suitable for enough electron transmission. Figure 5 depicts the SAED pattern for one of the prepared fine particles. The net pattern was consistently indexed as that of [004] zone axis pattern of Sc2O3 with the c type cubic structure; that is, the electron diffraction spots can be consistently indexed as those of cubic Sc2O3, which has the c type structure of the space group Ia3 with a lattice constant of a0 ) 0.9849 nm,8 and several diffraction spots corresponding to Sc2O3 were identified to exist as a single crystal phase. The results obtained above explicitly indicate that the synthesized particle is Sc2O3 dispersed as a single crystal form on the surface of Sc2(MoO4)3 polycrystalline bulk matrixes.
Figure 3. Particle size distribution histogram of the Sc2O3 particles.
Figure 4. TEM photograph of the Sc2O3 fine particles.
cations formed at the anode conduct in the Sc2(MoO4)3 pellet by ionic conduction toward the Pt cathode during the electrolysis. At the cathode, Sc3+ is reduced to the Sc metal state due to the electrolysis and Sc metal is instantly oxidized to Sc2O3 from the surface of the Sc deposits due to electrolyzing in air. Because the Sc3+ ions are steadily supplied from inside the Sc2(MoO4)3 electrolyte by the electrolysis, Sc metal is successively supplied to the interface between the Sc2O3 deposits and the Sc2(MoO4)3 polycrystal. Therefore, Sc2O3 crystals grow during the electrolysis procedure. As a result, size similarly controlled scandium oxide single crystals are grown on the cathode surface of the Sc2(MoO4)3 solid electrolyte. For the purpose of characterization of the particle and identification of whether the particle is a single crystal or not, TEM and SAED measurements were carried out. In Figure 4, very small particles appeared in the whole region with weak agglomeration as seen in the SEM photograph. Almost all crystallites are in the range between 0.1 and 0.2 µm, which is the smallest region in the size distribution
In conclusion, single crystals of high refractory Sc2O3 were successfully grown by a simple dc electrolysis method at the temperature below 1000 °C. This growth method is applicable for various metal oxides, if the ionic species of the metal can be the migrating ion in the molybdate solid electrolyte series. Because the size of the grown single crystals is uniformly controlled by the period of the electrolysis, this method would be one of advanced single crystal methods whose melting points are considerably high. Acknowledgment. The present work was partially supported by a Grant-in-Aid for Scientific Research No. 13555241 from The Ministry of Education, Science, Sports and Culture. This work was also partially supported by the Industrial Technology Research Grant Program in ‘02 (project ID: 02A27004c) from the New Energy and Industrial Technology Development Organization (NEDO) based on funds provided by the Ministry of Economy, Trade and Industry, Japan (METI).
References (1) Adachi, G.; Imanaka, N.; Tamura, S. Chem. Rev. 2002, 102, 2405-2430. (2) Berard, M. F.; Wirkus, C. D.; Wilder, D. R. J. Am. Ceram. Soc. 1968, 51, 643-647. (3) Tippins, H. H. J. Phys. Chem. Solids 1966, 27, 1069-1071. (4) Schleid, T.; Meyer, G. J. Less-Common Met. 1989, 149, 7380. (5) Geller, S.; Romo, P.; Remeika, J. P. Z. Kristallogr. 1967, 124, 136-142. (6) Ko¨hler, J.; Imanaka, N.; Adachi, G. Z. Anorg. Allg. Chem. 1999, 625, 1890-1896. (7) Imanaka, N.; Ko¨hler, J.; Adachi, G. Electrochem. Solid-State Lett. 1999, 2, 556-558. (8) Norrestam, R. Ark. Kem. 1967, 29, 343-349.
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