Article pubs.acs.org/JACS
High Thermoelectric Performance in Electron-Doped AgBi3S5 with Ultralow Thermal Conductivity Gangjian Tan,† Shiqiang Hao,‡ Jing Zhao,†,§ Chris Wolverton,‡ and Mercouri G. Kanatzidis*,† †
Department of Chemistry and ‡Department of Materials Science and Engineering, Northwestern University, Evanston, Illinois 60208, United States § Beijing Center for Crystal Research and Development, Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing 100190, P. R. China S Supporting Information *
ABSTRACT: We report electron-doped AgBi3S5 as a new highperformance nontoxic thermoelectric material. This compound features exceptionally low lattice thermal conductivities of 0.5−0.3 W m−1 K−1 in the temperature range of 300−800 K, which is ascribed to its unusual vibrational properties: “double rattling” phonon modes associated with Ag and Bi atoms. Chlorine doping at anion sites acts as an efficient electron donor, significantly enhancing the electrical properties of AgBi3S5. In the carrier concentration range (5 × 1018−2 × 1019 cm−3) investigated in this study, the trends in Seebeck coefficient can be reasonably understood using a single parabolic band model with the electron effective mass of 0.22 me (me is the free electron mass). Samples of 0.33% Cl-doped AgBi3S5 prepared by spark plasma sintering show a thermoelectric figure of merit of ∼1.0 at 800 K.
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INTRODUCTION Thermoelectric materials are capable of directly converting a temperature difference into electric voltage, and vice versa.1−6 To make thermoelectrics practical in large-scale use, high conversion efficiency is necessary. The efficiency is evaluated by the dimensionless figure of merit ZT = S2σT/κ = S2σT/(κel + κlat), where S is the Seebeck coefficient, σ is the electrical conductivity, T is the absolute temperature, and κ is the total thermal conductivity that consists of an electrical contribution (κel) and a lattice contribution (κlat). In addition to enhancing ZT, it is also desirable that the constituent elements of thermoelectric materials are earth abundant, nontoxic, and low cost.7−12 Lead or tin chalcogenides and their alloys with other chalcogenides have been the archetypal of advanced thermoelectric materials, not only because of their low thermal conductivities but also due to their complex electronic structures that allow for excellent electrical properties.13−30 However, due to the perceived toxicity of Pb and/or the scarcity of Te and Se elements, scientists are searching for new compounds, especially those eco-friendly metal sulfides, as potential substitutes for IV−VI semiconductors.9,12,31−37 For example, Biswas et al. reported a ZT value of ∼0.6 at 773 K in the n-type Cl-doped Bi2S3.31 Liu et al. synthesized a nanostructured CoSbS compound by ball milling and obtained a maximum ZT of ∼0.5 at 873 K in the n-type Ni-doped sample.32 Xie et al. claimed a ZT of 0.26 at 630 K in n-type Zndoped CuFeS2.33 Guin et al. reported that AgBiS2 is an interesting thermoelectric material with extremely low thermal conductivities ranging between 0.4 and 0.6 Wm−1K−1 from 300 © 2017 American Chemical Society
to 823 K, though the pristine sample displays a relatively small n-type ZT < 0.2 at 823 K.34 Cu−Bi−S ternary pavonite, reported by Ahn et al.,35 is another narrow gap n-type semiconductor having an intrinsically low thermal conductivity of ∼0.5 Wm−1K−1 that is almost temperature independent. Through doping and elemental alloying, the optimized ZT value of Cu−Bi−S compound reaches ∼0.3 at 673 K. Another successful example was given by Guilmeau et al., who reported that the n-type copper-intercalated TiS2 layered compound displays a promising ZT of 0.45 at 800 K because of the concurrent optimization of electrical and thermal properties.36 Among the p-type lead-free metal sulfides, a high ZT close to unity was achieved at ∼723 K in p-type Cu12Sb4S13, an earthabundant compound based on the mineral tetrahedrite, mainly benefiting from its unusually low thermal conductivity (∼0.2 Wm−1K−1 at T > 700 K).12 The p-type Cu2−xS compound features an exceptionally low thermal conductivity (
