VOLUME 3, NUMBER 7, JULY 2003 © Copyright 2003 by the American Chemical Society
Critical Size of the Phase Transition from Cubic to Tetragonal in Pure Zirconia Nanoparticles S. Tsunekawa,* S. Ito, and Y. Kawazoe Institute for Materials Research, Tohoku UniVersity, Sendai 980-8577, Japan
J.-T. Wang Institute of Physics, Chinese Academy of Science, Beijing 100080, China Received March 4, 2003; Revised Manuscript Received April 24, 2003
ABSTRACT Pure (unstabilized) zirconia nanoparticles in the fluorite-type cubic phase are observed by transmission electron microscopy. Microscopic observations at a pressure of 5 × 10 -5 Pa suggest that the phase transition from the cubic to tetragonal phase arises for particles near 2 nm in diameter. First principles computer simulations of large zirconia clusters show that the critical size of the transition could occur for a diameter of 2.05 ± 0.15 nm, which is in good agreement with the experimental value.
There have been several reports on pure tetragonal zirconia (ZrOx; 1 e x e 2) thin films and fine particles over the past four years,1-6 where “pure” denotes “stabilized without trivalent and oversized or undersized tetravalent cation dopants” (i.e., oxygen deficiencies are introduced by thermal and/or surface effects). Although isotropic materials are generally much preferred, there have been only a few papers concerning pure cubic zirconia fine particles7,8 aside from stabilized zirconia9-12 because the pure cubic phase is formed near 2560 K13 whereas the monoclinic and tetragonal phases are formed below 1480 K14 and between 1480 and 2560 K, respectively. One report suggests that ultrafine pure zirconia particles are monoclinic for diameters above 60 nm and tetragonal between 15 and 60 nm,15 and another paper reports * Corresponding author. E-mail:
[email protected]. Fax: +81-22-2152101. 10.1021/nl034129t CCC: $25.00 Published on Web 05/23/2003
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that they are monoclinic for diameters above 30 nm.16 It has been proposed that the critical sizes are strongly dependent on any hydrostatic and/or nonhydrostatic stresses present.17 Detailed reports on the size effects of oxides on their phase transitions, called size-driven or size-induced phase transitions, have been made, particularly for perovskite-type nanoparticles18-20 in which the cubic phase appears because of an increase in ionicity with decreasing particle size. It has been shown that the interatomic distances in BaTiO3 (perovskite) increase as the particle size decreases in the cubic phase.18 This means that the covalency decreases with decreasing particle size (i.e., the ionicity increases with a decrease in the size). However, zirconia nanoparticles in the cubic phase have a fluorite structure because of an increase in oxygen vacancies with decreasing particle size. It is assumed that the difference comes from discrepancies in the
Figure 1. Electron diffraction patterns of zirconia nanoparticles in (a) sample A after several minutes of strong electron exposure and (b) sample B after several minutes of weak electron exposure, which show the tetragonal phase (P42/nmc) and the fluorite-type cubic phase (Fm3m), respectively. The Miller indices in the brackets represent those of a fluorite-type cell.
layer structures in them (i.e., layers in the perovskite structure are composed of both cations and oxygen atoms, but the fluorite structure has alternating layers of cations or oxygen atoms). In both cases, small nanoparticles prefer isotropic (cubic) to anisotropic (tetragonal) morphorogy because of the increase in surface energy with decreasing particle size. Here we report the observation of pure fluorite-type cubic zirconia nanoparticles under electron microscopy. We also discuss the size dependence of the thermodynamic stability between the cubic and tetragonal phases of large zirconia clusters from first principles computer simulations. Transparent yellowish-brown sols were produced by metallorganic decomposition in a Teflon-lined autoclave of 24 mL capacity (model 4749, Parr Instruments). The sols were formed from a complex composed of 70% zirconium(IV) propoxide 1-propanol solution and 98% triethanolamine (Aldrich fine chemicals, Sigma-Aldrich), which were mixed in a volume ratio of 1:2, respectively, in a glove box with humidity below 10%, put into the autoclave (filling up to 80% of the total volume), sealed, and maintained at 523 K for 6 h and then cooled to room temperature naturally. Two weak sols with dilution rates in 99.5% ethanol (Wako Pure Chemicals) of 1 × 10-2 and 3 × 10-4 were prepared, resulting in concentrations of 71.1 and 2.13 mol/m3 (samples A and B, respectively). They were observed after drying on a carbon mesh by electron diffractionsbright- and dark-field techniques with a transmission electron microscope (TEM: JEM-2000EX, JEOL) under a pressure of 5 × 10-5 Pa. To make a rough estimation of the composition after electron beam exposure, the peak intensities of Zr and O were measured by an energy-dispersive X-ray (EDX) analyzer. Electron diffraction patterns of samples A and B could not immediately be seen but were observed after several minutes under electron beam exposure as shown in Figure 1a and b. This phenomenon is due to the hydrolysis and crystallization of the amorphous complexes by electron irradiation. Crystallization and decomposition in and/or on amorphous films under electron microscope observations are well known.21-23 The patterns of samples A and B seem to be from the tetragonal phase (primitive cell with z ) 2) and the fluorite-type cubic phase (face-centered cell with z ) 872
Figure 2. Dark-field TEM images: (a) sample A after 3 min of strong electron exposure with the 011 reflection and (b) sample B after 3 min of strong electron exposure with the 111 reflection.
4), respectively, because the 012 reflection is absent in sample B. It is clear that the pattern of sample B is made from a cubic phase because the ring-like diffractions shown in Figure 1b indicate the absence of a preferential orientation that introduces the absence of a particular reflection. It has been shown, however, that the electron diffraction patterns are subject to an irradiation effect. (The second broad reflection in the TEM diffraction patterns is changed gradually into a distinct reflection with three peaks, 012, 020, and 112, as the electron exposure time increases.23) Irradiation effects did not occur during our TEM observations because most diffraction patterns were taken with weak electron beams and within several minutes. The lattice constants were estimated to be a ) 0.351(4) nm33 and c ) 0.515(4) nm for the tetragonal phase and a ) 0.506(6) nm for the cubic phase. These values are very close to those of the bulk crystals with a ) 0.35984(5) nm33 and c ) 0.5152(1) nm24 for the former and with a ) 0.5071 nm25 for the latter. Dark-field TEM images, which were taken with the 011 and 111 reflections from almost the same areas as those in Figure 1, are shown in Figure 2. It is suggested from Figure 2a that the tetragonal and cubic phases are mixed because there are clearly nanoparticles with sizes similar to those shown in Figure 2b. Crystallites with a plane corresponding to the 011 or 111 reflection are the most abundant from the diffraction intensities in Figure 1. A statistical calculation of each particle size, Nano Lett., Vol. 3, No. 7, 2003
which is defined as the diameter of a circle equivalent to the area of the bright-field image, was executed by computer analysis (Image-Pro Plus 4.0, Media Cybernetics). The particle sizes were estimated to be 3.3 ( 0.8 and 1.2 ( 0.6 nm for the images of the tetragonal and cubic phases, respectively, where the range indicates the standard deviation. The image intensity was little changed during electron diffraction but was enhanced during dark-field imaging because the samples were irradiated by weak electron beams for diffraction and by strong beams for dark-field imaging. As the imaging time passed, both the size and number of nanoparticles increased by more than 3 times for sample A (Figure 3a), and the size distribution was little changed but the number increased by about 2 times for sample B (Figure 3b). Typical examples are shown in the regions marked by white boxes in Figure 3a and b. It should be noted that for zirconia particles below 1.5 nm in diameter (cubic-phase crystallites) there are dynamic crystallization and evaporation (nucleation) processes. This can be seen in Figure 3b, where, for example, the clusters marked by the white boxes change both their position and size with time. In the nucleation range around a 1.5-nm diameter, the thermal hydrolysis due to electron irradiation contributes little to the distribution shift into larger particle sizes but more to the increase in the number of zirconia particles. However, in the growth range above 2 nm in diameter, it contributes to both the distribution shift and the increase in the number. Therefore, the mean value of the size distribution for sample A increases with elapsed time, whereas that for sample B changes little. A great number of zirconia particles that are 10-20 nm in diameter can be obtained by adding a quantity of water that is equivalent to or double the volume of the original sol. A key material in successfully producing zirconia particles that are less than 2 nm in diameter is 99.5% ethanol including a very small amount of water. Electron irradiation during TEM observations mainly contributes to heating the complexes in the original sol. The above results suggest that the critical size of the phase transition from cubic to tetragonal is near a 2-nm diameter. However, previous reports described the difference between cubic and tetragonal phases as being ambiguous because of impurity and/or microstrain effects,12 with the critical size varying with hydrostatic and/or nonhydrostatic stresses.17 Therefore, to reveal the stability between the cubic and tetragonal phases theoretically, the total energy for each cluster was simulated by means of first principles quantum mechanical molecular dynamics using pseudopotentials and a plane wave basis set26,27 under the generalized gradient approximation.28 All calculations were performed on a Hitachi SR8000 supercomputer. The clusters were placed in large cubic supercells. The cell size of the simulation box ranged from 1.50 to 2.50 nm for the Zr19O32 and Zr225O408 clusters, and the cutoff energy for the plane wave expansion was 396 eV. The cluster lattice parameters were frozen to the bulk values: a ) 0.3610 nm and c ) 0.5198 nm for the tetragonal phase and a ) 0.5099 nm for the cubic. The tetragonalities in the bulk crystal and the simulated cluster are 1.012 and 1.018, respectively.33 Therefore, the difference Nano Lett., Vol. 3, No. 7, 2003
Figure 3. Dark-field TEM images of the same areas for (a) sample A after (i) 3 and (ii) 6 min of medium electron exposure with the 011 reflection and for (b) sample B after (i) 6 and (ii) 12 min of medium electron exposure with the 111 reflection. The white rectangles indicate the same regions in samples A and B. 873
Figure 4. (a) Geometries of the clusters ZrmOn in the (i) cubic and (ii) tetragonal phases, where the blue and red spheres represent the zirconium and oxygen ions, respectively. (b) Size dependence of the difference in total energy between the cubic and tetragonal phases, Ec - Et. 1: Zr19O32, x ) 1.684; 2: Zr55O94, x ) 1.709; 3: Zr92O160, x ) 1.739; 4: Zr116O208, x ) 1.793; 5: Zr201O360, x ) 1.791; and 6: Zr225O408, x ) 1.813. The total energy difference in the bulk denoted by the dashed line is 64.45 meV/[ZrO2].
is very small and does not affect the critical size estimation. The oxygens on the outermost surfaces were eliminated. The simulation results are shown in Figure 4. It is found from Figure 4b that the phase transition from cubic to tetragonal could occur between two large clusters, Zr116O208 and Zr201O360 , resulting in a particle size of 2.05 ( 0.15 nm with x ) 1.792 ( 0.001 (Figure 4a). Here, the size is defined as the average diameter between the inscribed and circumscribed spheres of the cluster. It should be noted that the valence state of the Zr ions, 2x, increases gradually with increasing cluster size (i.e., the oxygen deficiency decreases as the cluster size increases). The quantitative values of the oxygen deficiency under electron irradiation have not yet 874
been obtained because the electron microscope that was employed has only a simple EDX analyzer. The above results show good agreement between the TEM observations and the computer simulation (i.e., the critical size occurs near a diameter of 2 nm). Moreover, the oxygen composition in zirconia clusters is intermediate, with the first and second models of oxygen deficiencies proposed by us,29 where the effective valence of Zr ions is near 3.6 for a diameter of 2 nm and an octant of the oxygen ions on the outermost surfaces are missing, with the remainder having an effective valence of |1|. This model comes from the fact that nanocrystalline particles with fluorite structure can remain neutral not only with the creation of oxygen deficienNano Lett., Vol. 3, No. 7, 2003
cies but also with a change in the oxygen valence from -2 to -1.30,31 The above coincidence suggests that the outermost oxygens in zirconia combine with protons present in the surroundings, as has also been seen in ceria.31,32 Acknowledgment. J.-T.W. thanks the JSPS for financial support. We are grateful to the crew of the Center for Computational Materials Science of IMR (Tohoku University) for their continuous support of the supercomputer facilities. We also thank Professor T. Sugimoto and Dr. T. Kojima of IMRAM (Tohoku University) for their advice on metallo-organic decomposition. References (1) Ji, Z.-Q.; Rigsbee, J. M. J. Am. Ceram. Soc. 2001, 84, 2841. (2) Igawa, N.; Ishii, Y. J. Am. Ceram. Soc. 2001, 84, 1169. (3) Bouvier, P.; Djurado, E.; Ritter, C.; Dianoux, A. J.; Lucazeau, G. Int. J. Inorg. Mater. 2001, 3, 647. (4) Wu, N.-L.; Wu, T.-F.; Rusakova, I. A. J. Mater. Res. 2001, 16, 666. (5) Nguyen, T.; Djurado, E. Solid State Ionics 2001, 138, 191. (6) Go´mez, R.; Lo´pez, T.; Bokhimi, X.; Mun˜oz, E.; Boldu´, J. L.; Novaro, O. J. Sol.-Gel Sci. Technol. 1998, 11, 309. (7) Roy, S.; Ghose, J. Mater. Res. Bull. 2000, 35, 1195. (8) Martin, U.; Boysen, H.; Frey, F. Acta Crystallogr., Sect. B 1993, 49, 403. (9) Li, P.; Chen, I.-W.; Penner-Hahn, J. E. J. Am. Ceram. Soc. 1994, 77, 118. (10) Li, P.; Chen, I.-W.; Penner-Hahn, J. E. J. Am. Ceram. Soc. 1994, 77, 1281. (11) Chatterjee, A.; Pradhan, S. K.; Datta, A.; De, M.; Chakravorty, D. J. Mater. Res. 1994, 9, 263. (12) Bernstein, E.; Blanchin, M. G.; Ravelle-Chapuis, R.; RodriguezCarvajal, J. J. Mater. Sci. 1992, 27, 6519.
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