Published: February 18, 2011 r2011 American Chemical Society 3460 dx.doi.org/10.1021/ja109138p | J. Am. Chem. Soc. 2011, 133, 3460–3470 ARTICLE pubs.acs.org/JACS Nanostructures Boost the Thermoelectric Performance of PbS Simon Johnsen, † Jiaqing He, †,‡ John Androulakis, † Vinayak P. Dravid, ‡ Iliya Todorov, § Duck. Y. Chung, § and Mercouri G. Kanatzidis* ,†,§ † Department of Chemistry, Northwestern University, Evanston, Illinois 60208, United States ‡ Department of Materials Science and Engineering, Northwestern University, Evanston, Illinois 60208, United States § Materials Science Division, Argonne National Laboratory, Argonne, Illinois 60439, United States b S Supporting Information ABSTRACT: In situ nanostructuring in bulk thermoelectric materials through thermo- dynamic phase segregation has established itself as an effective paradigm for optimizing the performance of thermoelectric materials. In bulk PbTe small compositional variations create coherent and semicoherent nanometer sized precipitates embedded in a PbTe matrix, where they can impede phonon propagation at little or no expense to the electronic properties. In this paper the nanostructuring paradigm is for the first time extended to a bulk PbS based system, which despite obvious advantages of price and abundancy, so far has been largely disregarded in thermoelectric research due to inferior room temperature thermoelectric properties relative to the pristine fellow chalcogenides, PbSe and PbTe. Herein we report on the synthesis, microstructural morphology and thermoelectric properties of two phase (PbS) 1-x (PbTe) x x =0-0.16 samples. We have found that the addition of only a few percent PbTe to PbS results in a highly nanostructured material, where PbTe precipitates are coherently and semicoherently embedded in a PbS matrix. The present (PbS) 1-x (PbTe) x nanostructured samples show substantial decreases in lattice thermal conductivity relative to pristine PbS, while the electronic properties are left largely unaltered. This in turn leads to a marked increase in the thermoelectric figure of merit. This study underlines the efficiency of the nanostructuring approach and strongly supports its generality and applicability to other material systems. We demonstrate that these PbS-based materials, which are made primarily from abundant Pb and S, outperform optimally n-type doped pristine PbTe above 770 K. ’ INTRODUCTION The leading commercialized thermoelectric materials as well as those poised for commercialization in the near future are mainly telluride based 1-7 even though tellurium is extremely scarce in the Earth’s crust. 8 Hence it would be desirable to develop alternative materials which minimize Te and involve cheaper and abundant elements. Currently bulk materials such as e.g. the filled skutterudites, 9-14 Zn 3 Sb 4 , 15-17 nanostructured Si and Si 1-x Ge x , 18,19 half heuslers, 20-22 and Mg 2 Si 1-x Sn x 23,24 are being considered. Another possibility is PbS, also known as the mineral Galena, which is isostructural to PbTe adopting the rock salt structure with lattice parameters of 5.94 Å and 6.46 Å, respectively. 25 The electronic properties of the compounds are very similar. PbS has a reported room temperature band gap of 0.41 eV, whereas PbTe has a smaller gap of 0.32 eV. 26,27 Following the 10k B T rule 28 this suggests PbS will show a maximum thermoelectric figure of merit (ZT) at higher temperatures than PbTe. Coupled with the signi ficantly higher melting point of PbS (1391 K) in comparison with PbTe (1197 K) this implies that PbS-based thermoelectric materials have the potential to be used at higher temperatures. The room tempera- ture effective mass of the conduction band is slightly higher in PbS than in PbTe. 26 Consequently, the reported electron mobilities are smaller in PbS, whereas the reported room temperature Seebeck coefficients for n-type PbS are higher at similar charge carrier con- centration than in n-type PbTe. 26,29,30 Despite the promising properties, surprisingly little research has been reported on the thermoelectric properties of bulk PbS compared to the isostructural PbTe. For the most part the literature is limited to studies at room temperature or below 31-33 although high-temperature Hall measurements and Seebeck coeffi- cients have been reported. 29,34 The higher lattice thermal conduc- tivity in this material in comparison to that of PbTe 26,35,36 impedes the attainment of a high ZT. Nanostructuring has proven an efficient paradigm to lower lattice thermal conductivities and achieve higher ZT’s in bulk thermoelectric materials. 37-40 Nanostructuring through nuclea- tion and growth, spinodal decomposition, and matrix encapsula- tion is capable of not only lowering the lattice thermal con- ductivity significantly in PbTe 1,2 but also yielding improved power factors compared to those of the single-phase analogues. 4,5 Recently, the properties of several PbTe-rich compositions in the PbS-PbTe pseudobinary phase diagram were reported. 2 Due to an extensive immiscibility in this system, these samples are com- posite materials. The PbTe-rich part has shown nanometer-sized precipitates of PbS in a PbTe matrix, which gives a marked increase in the thermal resistivity of the composite at little cost to the electronic properties. Consequently a significant increase in ZT is observed at the optimized composition PbTe 0.92 S 0.08 . 2,41 In this paper we report on the synthesis, microstructural morphology, and thermoelectric properties of the PbS-rich side of the pseudobinary PbS-PbTe phase diagram, which results in Received: October 11, 2010