Communication pubs.acs.org/cm
Low Temperature Synthesis of Complex Ba1−xSrxTi1−yZryO3 Perovskite Nanocrystals Federico A. Rabuffetti, John S. Lee, and Richard L. Brutchey* Department of Chemistry, University of Southern California, Los Angeles, California 90089-0744, United States S Supporting Information *
KEYWORDS: vapor diffusion, sol−gel, perovskite, nanocrystals
A
or a mixture of two or three bimetallic alkoxides in the appropriate stoichiometric ratios. Solutions of AB(OR)6 (R = CH2CHCH3(OCH3)) in n-butanol/2-methoxypropanol were employed as bimetallic alkoxide precursors. Hydrolysis and polycondensation occur at near-neutral pH upon diffusion of water vapor into the precursor solution, resulting in a metallorganic gel within which perovskite nanocrystals nucleate and grow.19 No postsynthesis thermal treatment is needed to induce crystallization of the gel. Three different VDSG procedures were employed to map the entire Ba1−xSrxTi1−yZryO3 composition space (see Supporting Information). Scheme 1 maps the composition space of the
lkaline earth perovskite oxides with the formula ABO3 (A = Sr, Ba; B = Ti, Zr) and their corresponding solid solutions exhibit a wide range of physical and chemical properties that make them functional materials in the areas of energy conversion and storage (dielectric spacers in capacitors,1 electrolytes in proton-conducting solid oxide fuel cells2), display technologies (phosphor hosts in field emission displays3), and heterogeneous catalysis (supports for noble metals4). The versatility of this family of materials stems from the composition dependence of their physical properties, which allows their functionality to be optimized by fine-tuning the chemical composition. The synthesis of alkaline earth perovskite oxide nanocrystals with well-defined and tunable stoichiometry is relevant from a technological standpoint. For example, the use of nanocrystals can greatly improve the density and sinterability of nanostructured ceramics employed in capacitors and solid oxide fuel cells. Recently, there has been increased interest in developing synthetic approaches for the preparation of four-cation Ba1−xSrxTi1−yZryO3 nanocrystals because of their potential as functional materials in tunable nonlinear transmission lines employed in telecommunications.5 Although several synthetic strategies for the preparation of perovskite nanocrystals of formula Ba1−xSrxTiO36,7 and BaTi1−yZryO38,9 have been developed, very few reports exist describing the synthesis of Ba1−xSrxTi1−yZryO3 nanocrystals. In such cases, four-cation perovskite phases are typically synthesized via solid state reaction,10,11 sol−gel,5,12 and combustion synthesis.13 These approaches require thermal treatment at temperatures above 700 °C to produce a crystalline and phase pure product; this results in micrometer-sized grains with a broad size distribution. Shen et al. reported the synthesis of 60−80 nm Ba1−xSrxTi1−yZryO3 nanocrystals through high-gravity reactive precipitation at 85 °C and pH > 13;14 however, nanocrystals of zirconium-rich phases (i.e., y > 0.5) could not be obtained with this approach. From this perspective, the development of a lowtemperature and near-neutral pH method for the synthesis of Ba1−xSrxTi1−yZryO3 nanocrystals mapping the entire composition space (i.e., 0 ≤ x ≤ 1, 0 ≤ y ≤ 1) is highly desirable. In this work, Ba1−xSrxTi1−yZryO3 nanocrystals with arbitrary composition were synthesized at temperatures between 22 and 115 °C via the vapor diffusion sol−gel (VDSG) method.15−18 This method relies on the kinetically controlled delivery of water vapor at the gas−liquid interface of a bimetallic alkoxide © 2012 American Chemical Society
Scheme 1. Composition Space Map of Ba1−xSrxTi1−yZryO3 Phasesa
Different vapor diffusion sol−gel procedures are depicted by ○ (25 °C), ● (60−80 °C), and ■ (115 °C) symbols. Four-cation phases are located on composition lines (1), (2), and (3).
a
resulting nanocrystals, where each composition is labeled with a four-digit number abcd that indicates its stoichiometry Baa/(a+b)Srb/(a+b)Tic/(c+d)Zrd/(c+d)O3. For example, phases 0121 and 5111 correspond to SrTi2/3Zr1/3O3 and Ba5/6Sr1/6Ti 1/2Zr1/2O3, respectively. The first procedure consists of flowing water vapor over the precursor solution at room temperature, while the other two are two- and four-step procedures, in which water vapor diffusion is followed by heating under nitrogen atmosphere. Optimization of the synthetic procedures and variables (i.e., number of vapor Received: June 6, 2012 Revised: August 4, 2012 Published: August 8, 2012 3114
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diffusion and heating stages, water vapor diffusion time, and heating temperature and time) was carried out by monitoring the crystallinity and phase purity of the resulting materials (see Supporting Information). For example, the absence of a heating step at 60 °C for 24 h following vapor diffusion resulted in the presence of a significant amount of amorphous material in the syntheses of four-cation phases 1151, 5111, 1221, and 2112. Likewise, the absence of a heating step at 115 °C for 24 h after an initial 4 h vapor diffusion step resulted in the segregation of SrZrO3 when attempting the syntheses of three-cation phases 2101 and 1201 from a mixture of BaZr(OR)6 and SrZr(OR)6. This observation suggests that this intermediate heating stage is critical to form a partially hydrolyzed trimetallic Ba−Sr−Zr alkoxide that prevents segregation of SrZrO3 during the following vapor diffusion stage. More generally, it was observed that the crystallization of a desired phase requires more complex VDSG procedures, specifically, higher temperatures, upon moving toward phases closer to the BaZrO3−SrZrO3 composition line. Indeed, the coexistence of crystalline and amorphous fractions was observed in the SrZrO3 sample. It should be recalled that the crystal structures of BaTiO3, SrTiO3, and BaZrO3 nanocrystals all exhibit cubic or pseudocubic symmetry at room temperature, while that of SrZrO3 displays orthorhombic symmetry (vide infra). From this perspective, the increasing complexity of the synthetic procedures may be reflecting the difficulty of stabilizing crystal structures exhibiting large deviations from cubic or pseudocubic symmetries under the experimental conditions explored in this work. Conventional powder X-ray diffraction (XRD) patterns of freshly prepared nanocrystals are shown in Figure 1a for compositions Ba1−xSrxTi1/3+xZr2/3−xO3 and Ba2/3−xSr1/3+xTixZr1−xO3 (0 ≤ x ≤ 2/3); these correspond to three- and four-cation phases laying along composition lines (2) and (3) of the composition space map shown in Scheme 1, respectively. With only the exception of SrZrO3, patterns shown in Figure 1a are representative of those corresponding to other compositions (see Supporting Information). All the diffraction maxima can be indexed to the centrosymmetric cubic space group Pm3m ̅ (PDF No. 79-2263, 74-1296, and 703667 for BaTiO3, SrTiO3, and BaZrO3, respectively) with lattice constant values ranging from 3.91 Å for SrTiO3 to 4.20 Å for BaZrO3. Diffraction maxima appearing in the pattern of SrZrO3 were indexed to the orthorhombic space group Pbnm (PDF No. 44-0161) with lattice constants a = 5.79 Å, b = 5.81 Å, and c = 8.18 Å. No traces of binary oxide (i.e., TiO2, ZrO2), carbonate (i.e., BaCO3, SrCO3), or pyrochlore phases were observed in any of the patterns, demonstrating the as-prepared nanocrystals are phase pure. Qualitative analysis of the patterns shows that diffraction maxima shift to higher angles upon increasing the nominal content of Sr and Ti, indicating a contraction of the cubic lattice constant a. This trend is expected on the basis of the radii of the ions sitting in the A (rSr2+ = 144 pm < rBa2+ = 161 pm) and B (rTi4+ = 61 pm < rZr4+ = 72 pm) sites of the perovskite structure. Furthermore, a decrease in the intensity of the diffraction maxima corresponding to the (100), (111), and (210) crystal planes is observed upon going from composition line (2) to (3). This can be understood by considering the analytical expression of the structure factors Fhkl involved, which are F100 = F210 = fA − f B − f O and F111 = fA − f B + 3f O, where fA, f B, and f O are the atomic scattering factors of A and B site cations and oxygen, respectively.20 Moving from composition line (2) to (3) leads to strontium and zirconium enrichment, that is, to a decrease in
Figure 1. (a) XRD patterns of Ba1−xSrxTi1−yZryO3 phases. (b) Cubic lattice constant values extracted from Rietveld analysis as a function of the substitution level. Linear fits to the data are shown as dotted lines with values of the residual R2.
fA and an increase in f B, respectively. This, in turn, diminishes the intensity of the corresponding diffraction maxima. The stoichiometry of the as-prepared nanocrystals was confirmed by Rietveld analysis of their XRD patterns, and the composition of four arbitrarily selected four-cation nanocrystals were corroborated by ICP−AES elemental analysis (see Supporting Information). Details of the structural refinement procedures and results are given in the Supporting Information; for clarity, it should be mentioned here that a single perovskite phase with fixed stoichiometry identical to the nominal composition of the reaction mixture was refined. Values of the lattice constant a are plotted in Figure 1b as a function of the nominal substitution level. Linear fits to the lattice constant values extracted from Rietveld analysis show that substitution of Sr for Ba in the A site and of Zr for Ti in the B site leads to a monotonic linear decrease or increase in the lattice constant, respectively. The validity of Vegard’s law demonstrates the formation of true solid solutions with homogeneous distribution of the cations in the A and B sites at or very near to the nominal stoichiometry. More importantly, it demonstrates the ability of the VDSG method to produce four-cation phases with well-defined and tunable composition by simply changing the ratios of the starting bimetallic alkoxide precursors. Noteworthy, however, is the fact that the quality of the linear fit degrades slightly upon moving toward phases laying closer to the BaZrO3−SrZrO3 composition line; indeed, the residual R2 3115
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nanocomposite capacitors and as supports in heterogeneous catalysis.
decreases according to composition line (2) > composition line (1) > composition line (3) > Ba1−xSrxZrO3. This result confirms our earlier observation regarding the increasing difficulty of crystallizing the desired perovskite phase upon moving toward SrZrO3. The morphology of as-prepared nanocrystals was investigated by means of transmission electron microscopy (TEM); TEM images of the four-cation phases 1151, 2112, and 1511 are shown in Figure 2. Images and particle size distribution
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ASSOCIATED CONTENT
* Supporting Information S
Synthetic procedures and optimization of synthetic conditions; Rietveld analysis of XRD patterns; elemental analysis of selected samples; TEM images and particle size distribution histograms; FT-IR spectrum. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work is supported by the Department of Energy Office of Basic Energy Sciences under Grant No. DE-FG02-11ER46826. R.L.B. acknowledges the Research Corporation for Science Advancement for a Cottrell Scholar Award.
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REFERENCES
(1) Tagantsev, A. K.; Sherman, V. O.; Astafiev, K. F.; Venkatesh, J.; Setter, N. J. Electroceram. 2003, 11, 5. (2) Fabbri, E.; Bi, L.; Tanaka, H.; Pergolesi, D.; Traversa, E. Adv. Funct. Mater. 2011, 21, 158. (3) Vecht, A.; Smith, D.; Chadha, S. S.; Gibbons, C. S.; Koh, J.; Morton, D. J. Vac. Sci. Technol. 1994, 12, 781. (4) Enterkin, J. A.; Setthapun, W.; Elam, J. W.; Christensen, S. T.; Rabuffetti, F. A.; Marks, L. D.; Stair, P. C.; Poeppelmeier, K. R.; Marshall, C. L. ACS Catal. 2011, 1, 629. (5) Chen, C.-F.; Reagor, D. W.; Russell, S. J.; Marksteiner, Q. R.; Earley, L. M.; Dalmas, D. A.; Volz, H. M.; Guidry, D. R.; Papin, P. A.; Yang, P. J. Am. Ceram. Soc. 2011, 94, 3727. (6) Niederberger, M.; Garnweitner, G.; Pinna, N.; Antonietti, M. J. Am. Chem. Soc. 2004, 126, 9120. (7) Wei, X.; Xu, G.; Ren, Z.; Wang, Y.; Shen, G.; Han, G. J. Cryst. Growth 2008, 310, 4132. (8) Qi, J. Q.; Wang, Y.; Chen, W. P.; Li, L. T.; Chan, H. L. W. J. Nanopart. Res. 2006, 8, 959. (9) Veith, M.; Mathur, S.; Lecerf, N.; Huch, V.; Decker, T.; Beck, H. P.; Eiser, W.; Haberkorn, R. J. Sol-Gel Sci. Technol. 1999, 15, 145. (10) Joseph, J.; Vimala, T. M.; Raju, J.; Murthy, V. R. K. J. Phys. D: Appl. Phys. 1999, 32, 1049. (11) Thongtha, A.; Angsukased, K.; Riyamongkol, N.; Bongkarn, T. Ferroelectrics 2010, 403, 68. (12) Kumar, M.; Garg, A.; Bhatnagar, M. C. Physica B 2008, 403, 1819. (13) Thongtha, A.; Angsukased, K.; Bongkarn, T. Smart Mater. Struct. 2010, 19, 124001. (14) Shen, Z.; Shao, L.; Chen, J.; Yun, J. Mater. Lett. 2005, 59, 2232. (15) Brutchey, R. L.; Morse, D. E. Angew. Chem., Int. Ed. 2006, 45, 6564. (16) Beier, C. W.; Cuevas, M. A.; Brutchey, R. L. J. Mater. Chem. 2010, 20, 5074. (17) Rabuffetti, F. A.; Brutchey, R. L. Chem. Commun. 2012, 48, 1437. (18) Rabuffetti, F. A.; Lee, J. S.; Brutchey, R. L. Adv. Mater. 2012, 24, 1434. (19) Rabuffetti, F. A.; Brutchey, R. L. J. Am. Chem. Soc. 2012, 134, 9475. (20) Bouwma, J.; De Vries, K. J.; Burggraaf, A. J. Phys. Status Solidi A 1976, 35, 281.
Figure 2. TEM images of four-cation phases 1151, 2112, and 1511. An HRTEM image of an individual nanocrystal corresponding to the 1151 phase is shown; the spacing of the lattice fringes corresponding to the {110} crystal planes is indicated.
histograms for other compositions explored in this work are given in the Supporting Information. The nanocrystal average size and size distribution for each sample were estimated by assuming spherical shape and measuring the diameter of 100 particles. Mean diameters and standard deviations of 11.6 ± 1.5, 19.8 ± 2.4, and 8.44 ± 1.23 nm were obtained for phases 1151, 2112, and 1511, respectively. More generally, the average nanocrystal size varied between 8.44 ± 1.23 and 26.0 ± 2.4 nm for phases 1511 and 0121, respectively. These results demonstrate that the VDSG method is suitable for the preparation of perovskite nanocrystals of well-defined composition in the sub-30 nm size range. High-resolution TEM (HRTEM) imaging of individual nanocrystals corresponding to the 1151 phase shows the presence of well-defined lattice fringes corresponding to the {110} crystal planes, demonstrating these are apparently single crystalline particles. The spacing of the lattice fringes (d = 0.28 nm) is in good agreement with XRD data for this composition. This feature is representative of individual nanocrystals belonging to all the compositions explored in this work. In summary, single-crystalline, sub-30 nm Ba1−xSrxTi1−yZryO3 (0 ≤ x ≤ 1, 0 ≤ y ≤ 1) nanocrystals were synthesized via the vapor diffusion sol−gel method at temperatures ranging from 22 to 115 °C and near-neutral pH. The compositional flexibility of the method affords the synthesis of nanocrystals of complex four-cation perovskites of arbitrary and well-defined stoichiometry. This particular feature makes the VDSG method an attractive alternative for the preparation of precursors for nanostructured, nonlinear dielectric ceramics requiring high density and excellent sinterability. This strategy is also applicable to the synthesis of perovskite oxide nanocrystals that may be employed “as-prepared” in polymer−oxide 3116
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