5630 Chem. Mater. 2010, 22, 5630–5635 DOI:10.1021/cm102016j
Understanding Nanostructures in Thermoelectric Materials: An Electron Microscopy Study of AgPb18SbSe20 Crystals Christos B. Lioutas,† Nikolaos Frangis,*,† Iliya Todorov,‡ Duck Young Chung,§ and Mercouri G. Kanatzidis*,‡,§ †
Solid State Physics Section, Department of Physics, Aristotle University of Thessaloniki, GR-54124 Thessaloniki, Greece, ‡Department of Chemistry, Northwestern University, 2145 Sheridan Road, Evanston, Illinois 6020-3113, and §Materials Science Division, Argonne National Laboratory, Argonne, Illinois 60439 Received July 20, 2010. Revised Manuscript Received August 19, 2010
The characterization and understanding of the presence of nanostructuring in bulk thermoelectric materials requires real space atomic level information. We report electron diffraction and highresolution transmission electron microscopy studies of crystals of the system AgPb18SbSe20 (=18PbSe þ AgSbSe2) which reveal that this system is nanostructured rather than a solid solution. Nanocrystals of varying sizes are found, endotaxially grown in the matrix of PbSe (phase A), and consist of two phases, a cubic one (phase B) and a tetragonal one (phase C). Well-defined coherent interfaces between the phases in the same nanocrystals are observed. On the basis of the results of combined electron crystallography techniques, we propose reasonable structural models for the phases B and C. There are significant differences in the nanostructuring chemistry between AgPb18SbSe20 and the telluride analog AgPb18SbTe20 (LAST-18). Introduction Thermoelectric materials are becoming increasingly important in the field of energy production, conversion, and conservation. The past decade has witnessed significant enhancements in the thermoelectric performance of materials which can now achieve a figure of merit (ZT) higher than the critical value of 1.1-3 Moreover, the nanostructured multiphase systems were proved to exhibit an increased ZT value. (See the review papers.4-6) This is attributed to the reduction of the lattice thermal conductivity, probably due to phonons scattering at the interfaces of the nanocrystals with the matrix material. Lead telluride is a well-known thermoelectric material used for power generation up to 900 K with a maximum ZT of ∼0.8. The family of materials with the formula AgPbmSbTemþ2 (=mPbTe þ AgSbTe2) which achieve *Corresponding author. E-mail:
[email protected](M.G. K.);
[email protected] (N.F.).
(1) Sootsman, J. R.; Kong, H.; Uher, C.; D’Angelo, J. J.; Wu, C.-I.; Hogan, T. P.; Caillat, T.; Kanatzidis, M. G. Angew. Chem., Int. Ed. 2008, 47(45), 8618–8622. (2) Poudel, B.; Hao, Q.; Ma, Y.; Lan, Y.; Minnich, A.; Yu, B.; Yan, X.; Wang, D.; Muto, A.; Vashaee, D.; Chen, X.; Liu, J.; Dresselhaus, M. S.; Chen, G.; Ren, Z. Science 2008, 320, 634–638. (3) Snyder, G. J.; Toberer, E. S. Nat. Mater. 2008, 7, 105. (4) (a) Sootsman, J. R.; Chung, D. Y.; Kanatzidis, M. G. Angew. Chem., Int. Ed. 2009, 48, 8616–8639. (b) Kanatzidis, M. G. Chem. Mater. 2010, 22, 648–659. (5) (a) Pichanusakorn, P.; Bandaru, P. Mater. Sci. Eng. R: Rep. 2010, 67, 19–63. (b) Medlin, D. L.; Snyder, G. J. Curr. Opin. Colloid Interface Sci. 2009, 14, 226–235. (6) He, J.; Sootsman, J. R.; Girard, S. N.; Zheng, J.-C.; Wen, J.; Zhu, Y.; Kanatzidis, M. G.; Dravid, V. P. J. Am. Chem. Soc. 2010, 132, 8669–8675. (7) Hsu, K. F.; Loo; Guo, F.; Chen, W.; Dyck, J. S.; Uher, C.; Hogan, T.; Polychroniadis, E. K.; Kanatzidis, M. G. Science 2004, 303, 818.
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very high ZT values at 800 K was presented in several publications.7-10 The member with m = 18, i.e., the material AgPb18SbTe20 exhibits the highest ZT (∼1.6). The electron microscopic study of these samples revealed endotaxially dispersed nanocrystals (i.e., regions of a few nanometers in size that are rich in Ag-Sb and are surrounded by a PbTe matrix). By the term “endotaxy”, we mean the disposition (or growth) of a nanocrystalline particle inside a solid crystalline matrix with coherent defect free interfaces on its entire surface.8,11 For comparison with AgPb18SbTe20, we have also prepared the selenium analogue AgPb18SbSe20. The latter is a PbSe based derivative which is now beginning to attract attention as a promising thermoelectric material because of its favorable electronic band structure and low thermal conductivity.12 Therefore, the AgPbmSbSe2þm system (though not yet optimized) is also of high interest because of its relationship to the binary homologue. In order to understand and improve the thermoelectric (8) Quarez, E.; Hsu, K. F.; Pcionek, R.; Frangis, N.; Polychroniadis, E. K.; Kanatzidis, M. G. J. Am. Chem. Soc. 2005, 127, 9177–9190. (9) Androulakis, J.; Lin, C.-H.; Kong, H.-J.; Uher, C.; Wu, C.-I.; Hogan, T.; Cook, B. A.; Caillat, T.; Paraskevopoulos, K. M.; Kanatzidis, M. G. J. Am. Chem. Soc. 2007, 129(31), 9780–9788. (10) Han, M.-K.; Hoang, K.; Kong, H.; Pcionek, R.; Uher, C.; Paraskevopoulos, K. M.; Mahanti, S. D.; Kanatzidis, M. G. Chem. Mater. 2008, 20(10), 3512–3520. (11) Bonev, I. Acta Crystallogr. 1972, A28, 508–512. (12) (a) Parker, D.; Singh, D. J. Phys. Rev. B 2010, 82, 035204. (b) Liang, W. J.; Rabin, O; Hochbaum, A. I.; Fardy, M.; Zhang, M.; Yang, P. D. Nano Res. 2009, 2, 394–399. (c) Qiu, X. F.; Zhao, Y. X.; Steward, I. M.; Dyck, J. S.; Burda, C. Dalton Trans. 2010, 39, 1095– 1100. (d) Wang, R. Y.; Feser, J. P.; Lee, J. S.; Talapin, D. V.; Segalman, R.; Majumdrar, A. Nano Lett. 2008, 8, 2283–2288. (e) Cai, K. F.; He, X. R.; Avdeev, M.; Cui, J. L.; Li, H. J. Solid State Chem. 2008, 181, 1434–1438.
Published on Web 09/08/2010
r 2010 American Chemical Society
Article
Chem. Mater., Vol. 22, No. 19, 2010
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properties of these materials, it is important to understand their microstructure and atomic level inhomogeneities that are critical in affecting their electronic properties. The present work deals with the nanoscale characterization of this material using high resolution transmission electron microscopy and reveals that the selenides of AgPb18SbSe20 are in fact inhomogeneous on the nanoscale, exhibiting both similarities but also significant differences with the tellurides. We report for the first time the identification of new phases in AgPb18SbSe20 observed in the coherently embedded endotaxial nanocrystals, and we discuss their implications on the thermoelectric properties and future material optimization. Experimental Section Ingots with a nominal composition AgPb18SbSe20 (=18PbSe þ AgSbSe2) were prepared by annealing, in quartz tubes under vacuum, mixtures of appropriate stoichiometric quantities of Ag, Pb, Sb, and Se at 950 C for 4 h and cooling to 450 C in 40 h (type I). Type II material was grown in a similar fashion, but the cooling was to room temperature over 48 h. The rationale for the two different conditions was to investigate any dependence on processing conditions. Specimens suitable for electron microscopy observations were prepared either by crashing the material and collecting flakes on a copper grid or by drilling discs of 3 mm diameter and thickness
