CRYSTAL GROWTH & DESIGN
Nano Single Crystals of Yttria-Stabilized Zirconia Camille Guiot,† Ste´phane Grandjean,*,† Ste´phane Lemonnier,† Jean-Pierre Jolivet,‡ and Patrick Batail§ Laboratory of Actinide Chemistry, CEA Marcoule, BP 17171, 30200 Bagnols-sur-Ce`ze, France, LCMC, UniVersite´ Pierre et Marie Curie, UMR 7514 CNRS/UPMC, Case 174, 4 place Jussieu, F-75252 Paris Cedex 05, France and CIMA, UniVersite´ d’Angers, CNRS UMR 6200, 2 Bd LaVoisier 49045 Angers, France
2009 VOL. 9, NO. 8 3548–3550
ReceiVed March 12, 2009; ReVised Manuscript ReceiVed June 9, 2009
ABSTRACT: A simple, low-cost hydrothermal method was developed to synthesize in one step single crystals of yttria-stabilized zirconia (YSZ) nanoparticles of ca. 5 nm. Subsequent dialysis and ultrasonic dispersion afford a very stable YSZ suspension in pure water. Characterization by X-ray diffraction, transmission electron microscopy, nitrogen adsorption, and dynamic light scattering demonstrates that the particles are single crystals of the cubic Fm3m structure with a large specific surface area of 150 m2/g. Pure zirconia experiences phase transitions that induce shrinkage and then detrimental cracks during thermal cycling, but thermal stable phase structures are obtained by doping zirconia with rare earth cations such as yttria.1 Hence, yttriastabilized zirconia (YSZ) is one of the most studied high performance ceramic materials. For doping molar percentages larger than 8%, a solid solution is formed whose fluorite cubic structure is stable from room temperature up to the fusion temperature. Its good thermal and chemical stabilities, coupled with oxygen-ion conductivity, enable the use of YSZ as a solid electrolyte in solid oxide fuel cells (SOFC).2 Nanoparticles with a narrow size distribution are suitable for the production of high-quality ceramics. A large variety of chemical methods have been developed to synthesize nanocrystalline YSZ, including low temperature processes, such as sol-gel from alkoxyde precursors3,4 or metallic salts,5,6 and precipitation.7,8 These methods typically involve the formation of an amorphous precipitate followed by calcination above 400 °C in order to obtain crystalline precursors. Hydrothermal synthesis is advantageous, as nanoparticles are generated at a lower temperature, typically below 200 °C,9,10 as the solvatation properties of water are thereby greatly affected and nucleation and growth of metastable crystals are favored. Typically, current procedures11 involve a first step that consists of the precipitation of a hydrous compound followed by filtration and washing of the solid. In a second step, the amorphous solid is transferred into an autoclave and allowed to crystallize in hydrothermal conditions. Here, we report a cost-effective, highly efficient, one-step hydrothermal synthesis transforming in high yields a sol into monodisperse YSZ nanometric single crystals of ca. 5 nm. Further purification by dialysis and ultrasonic dispersion afford an YSZ suspension which is stable in water without the need for any stabilizing agent. A stable sol is prepared in a Teflon liner by dissolving, in 50 mL of pure water, zirconyl nitrate hydrate (ZrO(NO3)2 · xH2O 99% Aldrich) and yttrium nitrate hexahydrate (Y(NO3)3 · 6H2O 99.9% Aldrich) with the following concentrations: [Zr(IV]] ) 0.1 M, [Y3+] ) 0.05 M. Acetylacetone (99% Sigma-Aldrich) * Corresponding author: E-mail:
[email protected]. Tel: +33 (0) 4 66 79 16 03. Fax: +33 (0) 4 66 79 65 67. † LCA, CEA Marcoule. ‡ LCMC, Universite´ pierre et Marie Curie, CNRS. § CIMA, Universite´ d’Angers, CNRS.
is then added in the molar ratio Zr:Acac ) 1:1. The initially acidic pH of this solution is increased by dropwise addition of 3 M aqueous ammonia solution until pH 7 is reached. It is important not to exceed this value to prevent rapid precipitation of the solid. This proved to be the optimal pH to prepare a stable sol, which, if left aging at room temperature, turns into a homogeneous gel in the course of a few weeks.12,13 After sealing the Teflon liner in an autoclave (Parr bomb), the fresh sol is heated to 160 °C for 3 days. Under these hydrothermal conditions, nanocrystals of YSZ are formed and self-assemble within a gel that shrinks upon expulsion of the liquid from the gel, a phenomenon known as syneresis. Cooling to room temperature affords a cylindrical macroscopic aggregate of YSZ particles in a clear supernatant. Within the macroscopic aggregate the particles appear to be loosely associated as they are readily redispersed upon mechanical stirring. The condensation reactions occurring during the hydrothermal synthesis are partly responsible for a decrease in pH, reaching 4 in the recovered supernatant. ICP-AES measurements (Perkin-Elmer Optima 3100 RL) demonstrate that almost 100% of the nominal amount of zirconium has precipitated, whereas only 40% of the yttrium is incorporated into the YSZ particles. This incorporation occurs in a reproducible way, consistently yielding a solid of global formula Zr0.83Y0.17O1.915 (or 9 mol % Y2O3-ZrO2). The supernatant is carefully removed and the particles are dialyzed 5 times with pure water using a cellulose membrane (CelluSep 3500 Da). The resulting suspension is submitted to ultrasonic stirring for 30 min in a simple cleaning bath (VWR, 45 kHz), which leads to a nearly translucent stable suspension of crystalline YSZ nanoparticles that does not sediment for weeks. This is referred to as the as-prepared suspension for the following characterization tests. The phase structure of the as-prepared suspension dried at 70 °C has been determined by X-ray diffraction (INEL CPS 120). The X-ray powder spectra displayed in Figure 1 is indexed as a fluorite cubic structure with a ) 0.5151 nm, space group Fm3m, as expected for 9% mol Y2O3-ZrO2. The crystallite size calculated from the line broadening using the Scherrer formula is 3.5-4.0 nm. Transmission electronic microscopy measurements (JEOL 2010F, 200 kV) have been performed on a sample prepared by dropping of the diluted, as-prepared suspension on a Cu grid previously coated with a holey carbon layer, and allowing it to dry at room temperature. As shown in Figure 2, the transmission
10.1021/cg900292h CCC: $40.75 2009 American Chemical Society Published on Web 06/25/2009
Crystals of Yttria-Stabilized Zirconia
Crystal Growth & Design, Vol. 9, No. 8, 2009 3549
Figure 1. X-ray powder pattern (Cu KR1 radiation) of the as-prepared suspension dried at 70 °C, together with a fit to the profile using Fullprof software. 14Au was used as an internal reference.
Figure 2. Nonaggregated YSZ nanoparticles are identified on this micrograph of an as-prepared suspension.
electron microscopy (TEM) observations provide evidence for nonaggregated, well-defined nanoparticles with a rather spherical shape and a quite uniform diameter ranging from 3 to 5 nm. High-resolution images, as shown in Figure 3, demonstrate that each particle is a single crystal. The reticular distances indexed from the electronic diffraction pattern (Figure 4) exactly match those expected from cubic YSZ. A specific surface area of 150 m2 · g-1 is determined by nitrogenadsorption(GEMINI2360)usingBrunauer-Emmett-Teller (BET) calculation method on a freeze-dried sample, yielding a mean particle size of 6.7 nm assuming that the YSZ nanopar-
Figure 3. Observation of reticular arrays on this HRTEM micrograph demonstrates that the YSZ nanoparticles are single crystals.
ticles are in fact fully dense. This is in full agreement with the size determined by electronic microscopy, taking into account the small amount of aggregation occurring prior to the measurement during the degassing treatment at 200 °C. Scattering (DLS) measurements (Zetasizer nanoZS, Malvern Instruments) of the as-prepared suspension indicate the presence of aggregates with a mean size of 45 nm (Figure 5a) which can easily be broken with further ultrasonic stirring of the suspension, as exemplified in Figure 5b. To summarize, small diameter (ca. 5 nm) single crystals of YSZ with a high specific surface area (150 m2 g-1) are readily
3550
Crystal Growth & Design, Vol. 9, No. 8, 2009
Guiot et al.
carried out at a temperature as low as 160 °C is cost-effective. As-prepared stable suspensions offer great potential as precursors for advanced materials used in various applications such as catalysts, nanofiltration membranes, etc. Thin films deposited by dip-coating from this suspension are currently being characterized and their transport properties15 evaluated. Likewise, rare-earth doped nanosingle crystals16,17 and their electronic structure and luminescent properties are currently under investigation in our laboratory. Acknowledgment. We acknowledge support from ICSM (Institut de Chimie Se´parative de Marcoule, UMR 5257, France). Wahib Saikaly and Martiane Cabie´ from the CP2M (Centre Pluridisciplinaire de Microscopie e´lectronique et de Microanalyse), University of Science and Technology Saint Je´roˆme, Marseilles, France, are kindly acknowledged for their expert TEM experiments.
References
Figure 4. Indexation of the electron diffraction pattern of YSZ nanoparticles matches the reticular distances expected from cubic YSZ.
synthesized in a one-step procedure, that is, without the need for a dried-state intermediate. The product has a rather uniform particle size and is of high purity. The hydrothermal procedure
Figure 5. DLS size distribution by volume of (a) the as-prepared YSZ suspension and (b) that of the deaggregated suspension following 2 h of ultrasonic stirring.
(1) Yashima, M.; Kakihana, M.; Yoshimura, M. Solid State Ionics; Proceedings of the 10th International Conference on Solid State Ionics; Singapore, December 3-8, 1995; Elsevier: Amsterdam, 1996; Vol. 86-88, pp 1131-1149. (2) Badwal, S. P. S. Solid State Ionics 2001, 143, 39–46. (3) Changrong, X.; Huaqiang, C.; Hong, W.; Pinghua, Y.; Guangyao, M.; Dingkun, P. J. Membr. Sci. 1999, 162, 181–188. (4) Van der Donk, G. J. W.; Serra, J. M.; Meulenberg, W. A. J. NonCryst. Solids 2008, 354, 3723–3731. (5) Kuo, C.-W.; Lee, Y.-H.; Fung, K.-Z.; Wang, M.-C. Non-Cryst. Solids 2005, 351, 304–311. (6) Kuo, C.-W.; Lee, Y.-H.; Hung, I.-M.; Wang, M.-C.; Wen, S.-B.; Fung, K.-Z.; Shih, C.-J. J. Alloys Compd. 2008, 453, 470–475. (7) Kumar, C.; Manohar, P. Ionics 2007, 13, 333–335. (8) Zhang, Y.-W.; Yan, Z.-G.; Liao, F.-H.; Liao, C.-S.; Yan, C.-H. Mater. Res. Bull. 2004, 39, 1763–1777. (9) Dell’Agli, G.; Esposito, S.; Mascolo, G.; Mascolo, M. C.; Pagliuca, C. J. Eur. Ceram. Soc. 2005, 25, 2017–2021. (10) Xu, G.; Jiang, W.; Qian, M.; Chen, X.; Li, Z.; Han, G. Cryst. Growth Des. 2009, 9 (1), 13–16. (11) Zhu, Q.; Fan, B. Solid State Ionics 2005, 176, 889–894. (12) Grandjean, S.; Chapelet-Arab, B.; Lemonnier, S.; Robisson, A.-C.; Vigier, N. MRS. Symp. Proc. 2006, 893, 0893-JJ08-03.1. (13) Lemonnier, S. Synthe`se par voie douce d’oxydes polymetalliques incluant des actinides: re´activite´ et structure de la solution au solide, Ph.D. Thesis, University of Paris VI Pierre et Marie Curie, 2006. (14) Rodriguez-Carvajal, J. Abstracts of the Satellite Meeting on Powder Diffraction of the XVth Congress of the International Union of Crystallography; Toulouse, France, 1990; p 127. (15) Li, Q.; Xia, T.; Liu, X. D.; Ma, X. F.; Meng, J.; Cao, X. Q. Mater. Sci. Eng., B 2007, 138, 78–83. (16) Ramos-Brito, F.; Alejo-Armenta, C.; Garcı´a-Hipo´lito, M.; Camarillo, E.; Herna´ndez A, J.; Murrieta, S. H.; Falcony, C. Opt. Mater. 2008, 30, 1840–1847. (17) Reisfeld, R.; Zelner, M.; Patra, A. J. Alloys Compd. 2000, 300-301, 147–151.
CG900292H
