Cation Disorder and Bond Anharmonicity Optimize the Thermoelectric

Article pubs.acs.org/cm

Cation Disorder and Bond Anharmonicity Optimize the Thermoelectric Properties in Kinetically Stabilized Rocksalt AgBiS2 Nanocrystals Satya N. Guin and Kanishka Biswas* New Chemistry Unit, Jawaharlal Nehru Centre for Advanced Scientific Research (JNCASR), Jakkur P.O., Bangalore 560064, India S Supporting Information *

ABSTRACT: High temperature rocksalt phases of AgBiS2 and AgBiS2‑xSex (x = 0.05−0.1) have been kinetically stabilized at room temperature in nanocrytals (∼11 nm) by simple solution-based synthesis. Experimental evidence for this derives from variable temperature powder X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM), and electron diffraction analysis. The band gap of the AgBiS2 nanocrystals (∼1.0 eV) is blue-shifted by quantum confinement relative to that of the cubic bulk phase of AgBiS2. Moreover, systematic lower energy shift of the band gap in AgBiS2‑xSex nanocrystals compared to pristine nanocrystalline AgBiS2 was observed with increasing Se concentration. Existence of fascinating order−disorder type transition in these nanocrystals was evidenced by temperature dependent electrical conductivity, thermopower, and heat capacity measurements. Disordered cation sublattice and nanoscale grain boundaries coupled with strong Bi−S bond anharmonicity allow effective phonon scattering, which leads to minimal lattice thermal conductivity of the nanocrystalline AgBiS2. KEYWORDS: nanocrystals, thermoelectric, order−disorder transition, anharmonicity, phonon scattering

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INTRODUCTION

bulk materials provides another efficient route to decrease the thermal conductivity, and thus enhance the ZT. Beyond these traditional materials, cubic I−V−VI2 (where I = Cu, Ag, Au; V = As, Sb, Bi; and VI = S, Se, Te) semiconductors are renowned for their intrinsically low κlat due to the strong anharmonicity of the bonding arrangements in these compounds.13 Recent theoretical and experimental studies on a series of cubic bulk I−V−VI2 compounds have shown that the lone pair on the group V element plays an important role in deforming the lattice vibration, which results in strong anharmonicity.14 Enhanced electrical transport and ultra low thermal conductivity resulted in high thermoelectric performance in Pb or Bi doped bulk p-type AgSbSe2.15a High ZT values have been achieved in n-type Nb doped bulk AgBiSe2 synthesized by solid state reaction.15b Mention must be made that solution grown nanocrytalline AgBiSe2 exhibits promising thermoelectric performance with interesting p-n-p type conduction.15c Recently, n-type bulk BiAgSeS1‑xClx have also shown promising thermoelectric performance due to its low thermal conductivity.15d Ternary silver bismuth sulfide, AgBiS2, is a typical member of I−V−VI2 family. At room temperature, bulk AgBiS2 crystallizes

Nearly 60% of all input energy is wasted as heat. Thermoelectric materials can directly and reversibly convert waste heat into electrical energy, and will play a significant role in the future of energy management.1 The performance of thermoelectric materials is quantified by a dimensionless figure of merit, ZT = σS2T/(κel + κlat), where σ, S, T, κel and κlat are the electrical conductivity, Seebeck coefficient, temperature, electronic thermal conductivity, and lattice thermal conductivity, respectively.1 Various innovative approaches have recently been introduced to enhance the ZT of thermoelectric material, such as introduction of resonance level in the valence band,2 convergence of multiple valence bands,3 phonon glass electron crystal approach,4 nanostructuring of bulk materials,5 and alllength scale phonon scattering.6 Among the high ZT inorganic materials, lead chalcogenides2,5a,6 are the most efficient for power generation applications at high temperature, whereas bismuth chalcogenide5b,7 based materials are well-known for refrigeration near room temperature. Alternatively, Cu2‑xSe,8 In4Se3‑δ,9β-Zn4Sb3,10 and skutterudites11 have shown promising ZT values. Bottom-up wet chemical synthesis of various thermoelectric nanomaterials, such as BixSb2‑xTe3,7,12a Bi2TexSe3‑x,12b Cu2 ZnGeSe4 ,12c Cu 2 ZnSnSe 4 , 1 2 d Cu 2 CdSnSe 4 , 1 2 e Ag 2 S x Se 1 ‑ x , 1 2 f and Cu2ZnSnS412g followed by densification into nanostructured© 2013 American Chemical Society

Received: May 20, 2013 Revised: July 5, 2013 Published: July 10, 2013 3225

dx.doi.org/10.1021/cm401630d | Chem. Mater. 2013, 25, 3225−3231

Chemistry of Materials

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mmol), and oleic acid (10 mL) were combined in a round-bottom flask and the mixture was heated to 100 °C for 2 h with stirring in N2 atmosphere. The resulting clear pale yellow metal oleate solution was transferred to a 25 mL Teflon lined stainless steel autoclave and sulfur powder (19.2 mg, 0.60 mmol) was added inside an N2-filled glovebox. Then the autoclave was placed in a preheated hot air oven for 2 h at 180 °C. The reaction was suddenly quenched by immerging the hot autoclave in ice-cold water. The black color nanocrystals of AgBiS2 were precipitated out from the reaction mixture by adding excess ethanol and followed by centrifugation. AgBiS2 nanocrystals were washed several times with n-hexane and ethanol and finally dried in vacuum at 60 °C for 2 h. Synthesis of Cubic AgBiS1‑xSex (x = 0.05−0.1) nanocrystals. In a typical synthesis of AgBiS0.92Se0.08, silver acetate (50 mg, 0.30 mmol), bismuth acetate (116 mg, 0.30 mmol), and oleic acid (10 mL) were taken in a round-bottom flask and the mixture was heated to 100 °C for 2 h with stirring under N2 atmosphere. In addition to that, clear Se/S precursor solution was made by the reaction of 18.46 mg (0.576 mmol) S and 1.9 mg (0.024 mmol) Se powder in oleic acid at 130 °C for 2 h under N2 atmosphere. Then the S/Se precursor solution was transferred into a 50 mL Teflon lined stainless steel autoclave and silver/bismuth oleate precursor solution was added to it inside an N2filled glovebox. Then the autoclave was placed in preheated hot air oven for 2 h at 180 °C. The black color nanocrystals were precipitated out from the reaction mixture by centrifugation and were washed several times with n-hexane and ethanol and finally dried in vacuumdried in vacuum at 60 °C for 2 h. Synthesis of Cubic Bulk AgBiS2. Ingots (∼6 g) of cubic bulk AgBiS2 were synthesized by mixing appropriate ratios of high-purity starting materials of Ag, Bi, and S in a quartz tube. The tubes were sealed under high vacuum (10−5Torr) and slowly heated to 723 K over 12 h, then heated up to 1123 K in 4 h, soaked for 10 h, and subsequently water queched to trap the high temperature cubic phase at room temperature. Capping Agent Removal and Densification. In order to remove the organic capping agent, as prepared nanocrysatals were treated with n-hexane and hydrazine solution (85% v/v) under vigorous stirring. Stirring was continued until the nanocrystals were precipitated. The supernatant solution was decanted, and the surfactant free nanocrystals were washed several times with ethanol and dried in vacuum at 60 °C for 2 h. In order to measure thermoelectric properties, surface cleaned nanocrystals were pressed at room temperature into rectangular bars (2 × 2 × 8 mm3) and disks (2 mm thickness by 8 mm diameter) shaped samples using an AIMIL die-press at 60 KN/m2 pressure. The rectangular and disk shaped samples were sealed in quartz tube under vacuum (10−5 Torr) and then sintered at 450 °C for 2 h. Powder X-ray Diffraction. Room temperature and high temperaturepowder X-ray diffraction for all of the samples were recorded using a Cu Kα (λ = 1.5406 Å) radiation on a Bruker D8 diffractometer. X-ray patterns were fitted by Rietveld profile matching using FULLPROF suit program. The lattice parameters as well as further nonstructural parameters (i.e., background, peak profile parameters including asymmetry) were refined by the profile matching method using same program. Band Gap Measurements. To probe the optical energy gap of these compounds, optical diffuse reflectance measurements were performed on finely ground powders at room temperature. The spectra were recorded at the range of 200 to 3000 nm using a PerkinElmer Lambda 900, UV/vis/NIR spectrometer. Absorption (α/Λ) data were calculated from reflectance data using Kubelka−Munk equations: α/Λ = (1 − R)2/(2R), where R is the reflectance and α and Λ are the absorption and scattering coefficients, respectively. The energy band gaps were derived from α/Λ vs E (eV) plots. X-ray Photoelectron Spectroscopy. XPS measurement has been performed with Mg−Kα (1253.6 eV) X-ray source with a relative composition detection better than 0.1% on an Omicron Nanotechnology spectrometer. Field Emission Scanning Electron Microscopy. FESEM experiment has been performed using NOVA NANO SEM 600

in the hexagonal phase (space group, P-3m1) and transforms to a cubic rocksalt structure (space group, Fm-3m) at around ∼473 K (Figure 1).16 High temperature cubic bulk AgBiS2

Figure 1. Crystal structure of the high temperature cubic phase of AgBiS2 with disordered Ag/Bi positions.

possesses disordered Ag and Bi atoms (Figure 1). The valence electronic configuration of Bi is 6s26p3 in AgBiX2 (X = S/Se/ Te), where only 6p3 electrons are involved in the bond formation with the chalcogen valence electrons while the beguiling 6s2 electrons of Bi form a lone pair. The origin of anharmonicity in Bi−X (X = S/Se/Te) bond is the electrostatic repulsion between the stereochemically active lone pair of Bi and the valence bonding charge of the chalcogen.13,14 The extent of this repulsive force will be expected to increase from tellurium to sulfur as the electronegativity increases up the chalcogen group. Thus, the anharmonicity in AgBiS2 will be higher compared to AgBiSe2/AgBiTe2. Recent theoretical calculation of Grüneisen parameters also predicts stronger anharmonicity in AgBiS2 compared to its Se/Te analogue.14 Considering the presence of strong anharmonicity of the Bi−S bond and the disorder in the Ag/Bi positions, we expect low κlat in high temperature cubic phases of AgBiS2, and find it interesting to study the thermoelectric properties. Furthermore, sulfur is among the 16 elements with the highest abundance in the earth crust,17 and this has attracted us to investigate the thermoelectric properties of AgBiS2 over its Se/Te analogue. Herein, we present a solution-based bottom-up synthesis of n-type AgBiS2 and AgBiS2‑xSex (x = 0.05−0.1) nanocrystals with rocksalt structure, which is otherwise stable only above 473 K in the bulk phase. Thermoelectrically important kinetic cubic phases have been stabilized at room temperature by solution based synthesis along with the size reduction to the nanoscale. We highlight the structural, optical, electronic charge and thermal transport properties of the nanocrystals. Existence of astonishing order−disorder type transition in these nanocrystals was evidenced for the first time by temperature dependent σ, S and heat capacity measurements. Strong phonon scattering by the disordered cation sublattice, bond anharmonicity, and the nanoscale grain boundaries minimize the κlat to as low as 0.4−0.5 Wm1−K−1 in 290−830 K range.

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EXPERIMENTAL SECTION

Reagents. Bismuth(III) acetate (Alfa Aesar, 99%), silver acetate anhydrous (Alfa Aesar, 99%), sulfur powder (Alfa Aesar, 99.999%) and selenium powder (Aldrich, 99.5+%), oleic acid (Alfa Aesar, 90%) were used for synthesis without further purification. Synthesis of Cubic AgBiS2 Nanocrystals. We have synthesized cubic AgBiS2 nanocrystals using bottom-up soft chemical synthesis. At first, silver acetate (50 mg, 0.30 mmol), bismuth acetate (116 mg, 0.30 3226



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(FEI, Germany) operated at 15 KV. Energy dispersive spectroscopy (EDS) analysis has been performed with an EDAX Genesis instrument attached to the SEM column. Actual concentration of Se were estimated by EDX analysis, which is nearly same as the nominal composition of the nanocrsystals. Transmission Electron Microscopy. TEM experiments were performed using a JEOL (JEM3010) transmission electron microscope (TEM) fitted with a Gatan CCD camera operating at a 300 KV accelerating voltage. In a holey carbon coated Cu grid, one drop of nanocrystals dispersed in hexane solution was taken for TEM imaging. Electrical Transport. Electrical conductivity and Seebeck coefficients were measured simultaneously under a helium atmosphere from room temperature to about 823 K on a ULVAC-RIKO ZEM-3 instrument system. The typical sample for measurement had a rectangular shape with the dimensions of ∼2 × 2 × 8 mm3. The longer direction coincides with the direction in which the thermal conductivity was measured. Heating and cooling cycles give repeatable electrical properties for a given sample (Supporting Information, SI). Thermal Transport. Thermal diffusivity, D, was directly measured in the range 300−825 K by using laser flash diffusivity method in a Netzsch LFA-457. Coins with ∼8 mm diameter and ∼2 mm thickness were used in all of the measurements. Temperature dependent heat capacity, Cp, was derived using standard sample (pyroceram) in LFA457 and also confirmed by preliminary data obtained from differential scanning calorimeter (Netzsch DSC 200F3). The total thermal conductivity, κtotal, was calculated using the formula, κtotal = DCpρ, where ρ is the density of the sample, measured using the sample dimension and mass. The density of the pellets obtained was in the range ∼92% of the theoretical density.

Figure 2. XRD patterns of as prepared AgBiS2 and AgBiS2‑xSex nanocrystals along with cubic bulk AgBiS2.

temperature XRD data can be indexed on cubic AgBiS2 structure (space group, Fm-3m). As-synthesized cubic nanocrystals and the bulk samples exhibit well-defined band gap energies in the near-IR region (Figure 3). The spectroscopically measured band gap of the

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RESULTS AND DISCUSSION Rocksalt AgBiS2 and AgBiS2‑xSex (x = 0.05−0.1) nanocrystals (∼11 nm) were synthesized by the reaction of oleate of Ag and Bi with S/Se precursor in oleic acid at 180 °C for 2 h followed by quenching of the reaction mixture in ice-cold water. Acetate salts of Ag+ and Bi3+ were used as metal precursors, and elemental S/Se dissolved in oleic acid, as the S/Se source. In the synthesis, the acetate salts converted into clear pale yellow solutions of metal oleates upon heating the reaction mixtures to 100 °C under N2 flow for 2 h. Low-temperature, solution-based synthesis was followed by sudden quenching along with the size reduction to the nanoscale stabilizes kinetic rocksalt phase of AgBiS2 at room temperature. Similar phenomena are rare and have been observed only in a few semiconductors where a metastable kinetic polytype is accessed on the nanoscale, as for example in Pb 2‑xSn xS 2 , 18 wurzite MnSe,19 Cu 2 SnSe 3 ,20 CuInS2,21and orthorhombic MnAs.22 Powder X-ray diffraction patterns (XRD) (Figure 2) of the as-synthesized nanocrytals could be indexed on the cubic AgBiS2 structure (space group, Fm-3m). Figure 2 also shows the powder XRD of cubic bulk AgBiS2 for comparison, which is synthesized by vacuum sealed tube reaction at 1123 K followed by quenching in water to stabilize the high temperature phase. Lattice parameter was obtained by Rietveld profile matching (Figure S1 of the SI). Linear expansion in the lattice parameter following the Vegards law for the solid solution was observed, with increasing Se concentration in AgBiS2‑xSex (x = 0.05−0.1) (Figure S1 of the SI). As larger Se2− anions are introduced in the place of smaller S2−, the unit cell undergoes a systematic expansion, leading to increase in lattice parameter. This gradual expansion of lattice parameter indicates an isomorphic substitution of smaller S2− position by bigger Se2−, forming a single phase solid solution rather than phase separation. We have also performed high temperature powder XRD of nanocrystalline AgBiS2 in the 300−673 K range, which clearly shows no structural change (Figure S2 of the SI). All

Figure 3. Electronic absorption spectra of as prepared AgBiS2 and AgBiS2‑xSex nanocrystals along with cubic bulk AgBiS2.

bulk cubic AgBiS2 is ∼0.8 eV, while the band gap for nanocrystalline AgBiS2 is measured to be ∼1.0 eV. An increase in the band gap in the case of nanocrystals compared to the bulk is due to the quantum confinement effect. Systematic lower energy shift of the band gap in AgBiS2‑xSex nanocrystals compared to pristine nanocrystalline AgBiS2 was observed with increasing the Se concentration (Figure 3), which supports the successful Se substitution in S sublattice of AgBiS2. In order to confirm the presence of Se in solid solution nanocrystals, we have measured the X-ray photoelectron spectra of nanocrystalline AgBiS2 and AgBiS1.92Se0.08 (Figure 4). Two strong peaks at 367.8 and 373.7 eV (Figure 4a) with a peak splitting of 5.9 eV were observed due to Ag 3d5/2 and Ag 3d3/2, which is consistent with the standard Ag(I). The peaks at 158.3 and 163.7 eV were corresponds to Bi 4f7/2 and Bi 4f5/2 (Figure 4b) . Binding energy of S 2s located at 225.3 eV (Figure 4c). Peaks at 54.1 and 57.1 eV assigned to Se 3d5/2 and Se 3d3/2 (Figure 4d), respectively, suggest the incorporation of Se in the S sublattice of AgBiS2. 3227



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Figure 4. XPS spectra of as prepared AgBiS2 and AgBiS1.92Se0.8 nanocrystals. (a) Ag 3d, (b) Bi 4f, (c) S 2s, and (d) Se 3d spectra.

Figure 5. (a) FESEM image of nanocrystalline AgBiS2; the lower inset shows the EDAX spectra of AgBiS1.92Se0.08. (b) TEM image of AgBiS2 nanocrystals; the upper inset shows the size distribution histogram of AgBiS2 nanocrytals and lower inset shows the indexed SAED pattern. (c) HRTEM image of a cubic AgBiS2 nanocrystal. (d) TEM (inset) and HRTEM images of AgBiS2 nanocrystals after surface cleaning and densification.

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Figure 6. Temperature dependent (a) electrical conductivity (σ), (b) Seebeck coefficient (S), (c) power factor (σS2), (d) specific heat (Cp), (e) total thermal conductivity (κtotal), and (f) thermoelectric figure of merit (ZT) of nanocrystalline AgBiS2 and AgBiS1.92Se0.08 sample.

structural change in the surface cleaned densified nanocrystals, as can be confirmed from the powder XRD patterns (Figure S3 of the SI) and HRTEM analysis (Figure 5d). The sample became much more compact, and the adjacent nanoscale grains are closely attached to each other with small a increase in the grain size, as can be seen from TEM (inset, Figure 5d) and SEM (Figure S3 of the SI). The temperature dependent electrical and thermal transport properties of nanocrytalline AgBiS2 and solid solution AgBiS1.92Se0.08 samples in the 300−830 K range are presented in Figure 6. Electrical conductivity (σ) and Seebeck coefficient (S) were measured simultaneously under He atmosphere in the 300−810 K range on a ULVAC-RIKO ZEM3 instrument. Typically, the nanocrytalline AgBiS2 has σ of ∼52 S cm−1 at 300 K, which rapidly falls to ∼26 S cm−1 at ∼630 K and stays almost flat in the 625−810 K range, reaches a value of ∼32 S cm−1 at ∼810 K (Figure 6a). We also observed similar sudden decrease in σ from 310 S cm−1 to 145 S cm−1 at ∼610 K in the case of AgBiS1.92Se0.08 sample. An abrupt decrease (∼50%) in σ at ∼600 K indicates an enhanced disordering in the arrangement of Ag and Bi atoms in the face centered cubic lattice of nanocrytalline AgBiS2 above 600 K. We attribute this sharp decrease in σ at this transition temperature due to an

As-prepared AgBiS2 nanocrystals are nearly monodisperse as can be seen from field emission scanning electron microscopy (FESEM) image (Figure 5a). Energy dispersive X-ray spectra of nanocrystalline AgBiS1.92Se0.08 sample also confirm the presence of Se in the sample (lower inset, Figure 5a). To further investigate the structural detail in nanoscale, transmission electron microscopy (TEM) and electron diffraction experiments were carried out. A typical TEM image of the assynthesized AgBiS2 nanocrystals is shown in Figure 5b. A particle size distribution histogram generated from the TEM images is plotted in the upper inset of Figure 5b. The average particle size was estimated to be ∼11 nm. The selected area electron diffraction (SAED) pattern of the 11 nm nanocrystal can be indexed based on cubic rocksalt AgBiS2 (inset, Figure 5b). The high resolution TEM (HRTEM) image shows clear lattice spacing of 3.2 Å corresponding to (111) interplanar distances of cubic AgBiS2. For the electronic and thermal transport measurements, we have carefully removed the organic capping agent by treating as synthesized nanocrystals with n-hexane and hydrazine solution (85% v/v) under vigorous stirring,23 followed by densification through pressing and sintering in a vacuum (10−5 Torr) sealed quartz tube at 450 °C for 2 h. We have not observed any 3229



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measured thermoelectric properties of the bulk sample (Figure S5 of the SI). Typically, the bulk AgBiS2 has σ of ∼3 S cm−1 at 300 K, which slowly increases to ∼28 S cm−1 at 705 K, indicating the semiconductor-like conduction. The negative sign of S indicates n-type carriers in bulk sample similar to the nanocrystals. The room temperature S for the bulk sample is ∼−337 μVK−1, which decreases to ∼−237 μVK−1 at 710 K. Observation of higher thermopower values in bulk sample than those of the nanocrystalline sample suggests the presence of lesser number of n-type carriers in bulk AgBiS2. Interestingly, we have not observed any sharp transition in temperature dependent σ as well as S data in the case of bulk AgBiS2. At room temperature, κtotal of the bulk sample shows a value of ∼0.65 Wm1−K−1 which remains flat up to 480 K and then decreases to ∼0.45 Wm1−K−1 at 725 K. Room temperature κtotal values of the bulk sample is higher than that of the nanocrystalline sample, while the high temperature κtotal values are comparable between bulk and nanocrystalline phases. The decrease in κtotal at 480 K in the case of bulk AgBiS2 may be due to the extra phonon scattering by the disordered Ag/Bi lattice at high temperature. In the case of bulk AgBiS2, maximum ZT value was achieved to be ∼0.23 at 705 K, which is comparable to that of nanocrystalline sample.

additional scattering of charge carriers by the highly disordered Ag/Bi lattice. Such anomaly in temperature dependent σ is very rare and is found only in few bulk crystals such as Cu0.5NbS2, Ag0.5NbS2,24 and Cu2Se8 at their order−disorder transition temperatures. As the high temperature powder XRD data in the 300−673 K range for nanocrystalline AgBiS2 shows no structural change (Figure S2 of the SI), transition found in transport measurement of AgBiS2 nanocrytals is solely attributed to the enhanced disordering of the cations. The negative sign of S indicates n-type conduction in nanocrytalline AgBiS2 and AgBiS1.92Se0.08 (Figure 6b). Formation of chalcogen vacancies during the chemical synthesis may be responsible for electron carriers, which gives rise to ntype behavior. Typically, over 300−500 K range, the nanocrystalline AgBiS2 has a very low value of S up to −10 μVK−1, which sharply increases to ∼−135 μVK−1 at ∼630 K, and reaches a value of ∼−195 μVK−1 at ∼810 K. A similar rapid increase in S is also observed in the AgBiS1.92Se0.08 sample at ∼610 K, but the overall thermopower value is lower compared to pristine AgBiS2 nanocrytals, which may be due to the presence of a greater number of charge carriers in the solidsolution sample. This remarkably sharp enhancement in S at ∼600 K may be ascribed to a sudden increase in the entropy of the system,25 which also suggests a transition to highly disordered cation arrangement in AgBiS2 nanocrystals above 600 K. On the basis of the temperature dependent σ and S, maximum power factors, σS2, were estimated to be 1.2 and 1.3 μWcm−1K−2 at 810 K for AgBiS2 and AgBiS1.92Se0.08 samples, respectively (Figure 6c). The total thermal conductivity, κtotal, of the samples was estimated in the temperature range of 300−830 K using the formula, κtotal = DCpρ, where D is the thermal diffusivity, Cp is specific heat, and ρ is density of the sample. D is measured by laser flash diffusivity technique by using NETZSCH LFA 457 instrument in 300−825 K range. Interestingly, we find a λ type transition in the temperature dependent Cp in 500−620 K range (Figure 6d), which confirms an order−disorder type transition in nanocrystalline AgBiS2. Similar lambda transitions have been found in AgCrSe226 and Ag2Se,12f where cation rearrangement is responsible for the same.27 Nanocrytalline AgBiS2 has very low κtotal values (∼0.43−0.57 Wm1−K−1) in 300−825 K range (Figure 6e). Interestingly, we have observed slightly higher values of κtotal in the case of AgBiS1.92Se0.08. A strong anomaly in the temperature dependent κtotal also indicates disordering of Ag/Bi cation above 550 K. The electronic thermal conductivity (κe = LσT, where L is the Lorenz number) was estimated based on L value of 2.44 × 10−8 WΩK−2. The lattice thermal conductivity, κlat, was calculated by subtracting the κel from the κtotal and plotted in Figure S4 (SI). Typically, AgBiS2 has κlat values of ∼0.4−0.5 Wm1−K−1 in 300− 825 K range. This extremely low κlat was observed due to three possible reasons: (a) a high degree of anharmonicity in the Bi− S bonds which gives rise to strong phonon−phonon interactions; (b) effective phonon scattering by the highly disordered Ag/Bi lattice above the transition temperature; and (c) strong phonon scattering at the nanoscale grain boundaries present in AgBiS2 samples. Temperature-dependent ZT for the samples are plotted in Figure 6f. Maximum ZT was achieved to be ∼0.2 at 810 K for AgBiS2 nanocrytals, which is indeed promising and comparable or higher than that in the case of any solution grown sulfide nanocrystals.12g In order to compare the transport properties of the cubic nanocrystalline AgBiS2 with the cubic bulk AgBiS2, we have

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CONCLUSIONS In summary, high temperature rocksalt phases of AgBiS2 and AgBiS2‑xSex (x = 0.05−0.1) have been kinetically stabilized at room temperature in nanocrytals (∼11 nm) by rapid and scalable solution-based synthesis. Fascinating anomalies in temperature dependent σ, S, and Cp were observed at ∼600 K due to novel order−disorder type transition, which indeed optimizes the electronic charge transport in these nanocrystals. Disordered cation sublattice and nanoscale grain boundaries coupled with strong Bi−S bond anharmonicity allow effective phonon scattering, which lead to minimal κlat of the nanocrystalline AgBiS2. Optimization of thermoelectric performance through a combined phonon scattering approach is expected to be applicable to other nanocrystals with disordered cation/anion sublattice. Further improvement in the thermoelectric performance may be achieved by an optimization of the n-type carrier concentration through halogen doping in the S2− anion sublattice of nanocrystalline AgBiS2.

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ASSOCIATED CONTENT

S Supporting Information *

Additional figures (Figures S1−S9). 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 DST Ramanujan Fellowship, the Sheikh Saqr Laboratory, and the New Chemistry Unit. REFERENCES

(1) (a) Sootsman, J.; Chung, D. Y.; Kanatzidis, M. G. Angew. Chem., Int. Ed. 2009, 48, 8616. (b) Snyder, G. J.; Toberer, E. S. Nat. Mater. 2008, 7, 105. (c) Chen, G.; Dresselhaus, M. S.; Dresselhaus, G.;

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Cation Disorder and Bond Anharmonicity Optimize the Thermoelectric

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