This is an open access article published under an ACS AuthorChoice License, which permits copying and redistribution of the article or any adaptations for non-commercial purposes. Article Cite This: ACS Nano 2019, 13, 6572−6580 www.acsnano.org Tuning Transport Properties in Thermoelectric Nanocomposites through Inorganic Ligands and Heterostructured Building Blocks Maria Ibáñez,*,†,‡,§ Aziz Genç,∥,⊥ Roger Hasler,‡,§ Yu Liu,† Oleksandr Dobrozhan,# Olga Nazarenko,‡,§ María de la Mata,∥,¶ Jordi Arbiol,∥,⊗ Andreu Cabot,#,⊗ and Maksym V. Kovalenko*,‡,§ † Institute of Science and Technology Austria, Am Campus 1, 3400 Klosterneuburg, Austria ‡ Institute of Inorganic Chemistry, Department of Chemistry and Applied Biosciences, ETH Zürich, Vladimir Prelog Weg 1, Zürich CH-8093, Switzerland § Empa-Swiss Federal Laboratories for Materials Science and Technology, Ü berlandstrasse 129, Dübendorf CH-8600, Switzerland ∥ Catalan Institute of Nanoscience and Nanotechnology (ICN2), CSIC and The Barcelona Institute of Science and Technology (BIST), Campus UAB, Bellaterra, 08193 Barcelona, Catalonia, Spain ⊥ Department of Metallurgy and Materials Engineering, Faculty of Engineering, Bartin University, 74100 Bartin, Turkey # Catalonia Energy Research Institute - IREC, Sant Adria del Besos, 08930 Barcelona, Spain ⊗ ICREA, Pg. Lluís Companys 23, 08010 Barcelona, Spain ¶ Departamento de Ciencia de los Materiales, Ing. Met. y Qca.Inorg., IMEYMAT, Universidad de Cádiz, 11510 Puerto Real, Spain * S Supporting Information ABSTRACT: Methodologies that involve the use of nano- particles as “artificial atoms” to rationally build materials in a bottom-up fashion are particularly well-suited to control the matter at the nanoscale. Colloidal synthetic routes allow for an exquisite control over such “artificial atoms” in terms of size, shape, and crystal phase as well as core and surface compositions. We present here a bottom-up approach to produce Pb−Ag−K−S−Te nanocomposites, which is a highly promising system for thermoelectric energy conversion. First, we developed a high-yield and scalable colloidal synthesis route to uniform lead sulfide (PbS) nanorods, whose tips are made of silver sulfide (Ag2S). We then took advantage of the large surface-to-volume ratio to introduce a p-type dopant (K) by replacing native organic ligands with K2Te. Upon thermal consolidation, K2Te-surface modified PbS−Ag2S nanorods yield p-type doped nanocomposites with PbTe and PbS as major phases and Ag2S and Ag2Te as embedded nanoinclusions. Thermoelectric characterization of such consolidated nanosolids showed a high thermoelectric figure-of-merit of 1 at 620 K. KEYWORDS: colloidal nanoparticles, asymmetric nanoparticles, inorganic ligands, heterostructures, catalyst assisted growth, nanocomposites, thermoelectrics I norganic nanocrystalline solids containing two or more different materials and at least one of them with grain sizes in the nanometer scale can provide significantly improved mechanical, optical, electrical, and thermal properties.1 Such mesoscale microstructure (domain size and mutual arrange- ment of grains, grain boundaries, etc.),9−11 chemical and physical processing techniques that control these parameters are paramount. Toward these goals, the bottom-up formation of such nanocomposites by the assembly and consolidation of materials are being employed in a number of applications including catalysis,2,3 energy-storage devices such as batteries nanoparticles (NPs) may be a facile, scalable, potentially low and capacitors,4 membranes for gas and ion diffusion or cost, and high yielding as well as extremely versatile, in terms molecular separation,5 and energy conversion systems such as fuel cells6 and thermoelectrics.7,8 Because the desired physical Received: January 14, 2019 properties of these materials often depend not only on the Accepted: June 11, 2019 individual compositions of constituting phases but also on the Published: June 11, 2019 © 2019 American Chemical Society 6572 DOI: 10.1021/acsnano.9b00346 ACS Nano 2019, 13, 6572−6580 ACS Nano Article of compositions and morphologies, method to produce these to-volume ratio. On one hand, large specific surface areas are materials.12,13 A large library of materials in the form of generally found to be detrimental, as they enhance oxidation20 uniform NP dispersions exists for their combinatorial or induce high densities of defects and uncontrolled blending.8 However, when it comes to the removal of the impurities.21 On the other hand, NP surfaces may serve as a solvent and consolidation, a common hurdle is that the NPs gateway22 to introduce needed amounts of dopants into the often tend to segregate into clusters of the same size, shape, or resulting semiconducting nanocomposites23−25 or to alter the composition.14 A compelling, broadly applicable strategy to overall composition.26,27 This convenient doping strategy and overcome these limitations is to design heterostructured NPs compositional engineerability of NPs can be harnessed for comprising all desired phases and dopants in each NP,15 producing all-inorganic nanosolids with desired electronic thereby allowing predictable and uniform nanoscale distribu- properties. The proper surface treatment methodology needs tion of these constituents also in the resulting multicomponent to be conceived in each specific case to ensure that the initial nanosolids (Scheme 1).9,16−19 Herein, we present such a insulating organic surfactants are removed, the surfaces are not oxidized, and controlled impurities are introduced from the Scheme 1. Schematic of the Bottom-up Chemical chosen inorganic ligand upon consolidation. Herein, we Engineering of Inorganic Nanosolids by Assembly and outline such surface engineering methodology for colloidal Consolidation of NPsa PbS−Ag2S nanorods (NRs), whose native oleate capping molecules can be displaced with K2Te as inorganic surface ligands, eventually producing quintenary Pb−Ag−K−S−Te nanocomposites after thermal consolidation. We show tunable p-type transport of these composites by controlling the length of the original NRs, from 0.5 S cm−1 to 5 S cm−1. Additionally, the existence of multiple grain boundaries in nanocrystalline Pb−Ag−K−S−Te composites greatly reduce thermal con- ductivity, from typical bulk PbS values of ca. 2.5 W m−1 K−1,28 to 0.8 W m−1 K−1 at room temperature (rt). RESULTS AND DISCUSSION On the Synthesis and Growth Mechanism of PbS− Ag2S NRs. PbS−Ag2S NRs, each consisting of a PbS rod and a Ag2S tip, were produced in a three-step colloidal synthesis. a Heterostructured NRs with identical material A at the tips but with First, monodisperse 2−3 nm Ag NPs (see transmission long (top) and short (middle) segments of material B in between electron microscopy (TEM) images, Figure 1a) were produced allow for uniform distribution of the material A in the matrix of B. using a well-established synthesis route.29 Subsequently, Such a strategy is presented in this study using colloidal heteronanorods that combine Ag2S (material A tips) with PbS monodisperse Ag2S NPs (inset of Figure 1a) were prepared (material B, rods). In contrast, mixing two separate colloids of at rt by mixing Ag NPs with a solution of sulfur in oleylamine materials A and B commonly leads to partial phase-segregation that is (OLA:S), nearly instantly leading to the color change into hard to predict and control (bottom image). light-brown, characteristic of Ag2S NPs. In the last step, PbS− Ag2S NRs (Figure 1b,c) were obtained by injecting the crude solution of Ag2S NPs, still containing high quantity of OLA:S, strategy by synthesizing colloidal PbS rods of tunable lengths into lead oleate dissolved in octadecene (ODE). Details can be with a Ag2S tip on one side. Although centrosymmetric lead found in the Methods. chalcogenides are generally difficult to crystallize in the At one end of every NR an approximately hemispherical tip anisotropic shape, Ag2S NPs can overcome this limitation by can be discerned by high-resolution TEM (HRTEM) imaging acting as catalysts for the formation of PbS. (Figures 1c and 2). This tip is identified as an orthorhombic Another inherent and, in the context of this study, very Ag2S phase (space group P212121) by the detailed crystallo- useful attribute of colloidal nanomaterials is their high surface- graphic analysis of the corresponding power spectrum (FFT). Figure 1. Representative TEM images of (a) Ag NPs, Ag2S NPs (inset in a) and (b) the corresponding Ag2S-PbS NRs. (c) HRTEM image of a single Ag2S-PbS NR. 6573 DOI: 10.1021/acsnano.9b00346 ACS Nano 2019, 13, 6572−6580 ACS Nano Article PbS phase. The difference in plane spacing results in a clear misfit, and there are three Ag2S planes for every two PbS planes, which is in good agreement with the calculated mismatch. The narrow size distribution of the PbS−Ag2S NPs facilitated their assembly into superlattice structures, in particular, with the perpendicular alignment to the TEM grid. A square two-dimensional (2D) projection of these NRs can thus be imaged directly (Figure 3). In order to rule out the presence of cubic PbS NPs, high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) micrographs and their corresponding surface plots (2D intensity profile) have been obtained (Figure 3c). For the same composition, HAADF intensity is determined by the probed thickness. As expected, the surface plot indicates higher intensities from rectangular profiles (vertically aligned NRs) as compared to elongated profiles (horizontally lying NRs). Ag2S is an ionic conductor with a high concentration of Ag vacancies and in which Ag cations act as a fluid.30,31 Such behavior has allowed the use of Ag2S NPs as a catalyst to growth heterostructures of different types of semiconductors, such as Ag2S−ZnS,32−34 Ag2S−CdS,32,35 and Ag2S−CoS2.36 Figure 2. HRTEM image of a single PbS−Ag2S NR. (a) Colored For the present case of PbS−Ag2S heterostructures, we assume structural map of a typical NR obtained from HRTEM, where red that Pb ions dissolve in Ag2S and occupy Ag vacancies. As the indicates the orthorhombic Ag2S phase and green indicates the concentration of Pb increases to the solubility limit, PbS face-centered cubic PbS phase. The bottom left inset corresponds to the power spectrum (FFT) used for structural map filtering, cluster nucleates and continues to grow in a fashion similar to with spots colored following the same previous color code. The well-known solution−liquid−solid-catalyzed growth.37 Clearly, top middle inset corresponds to a GPA analysis obtained on the for a given concentration of Ag2S seeds, the NR length will be heterojunction between the PbS and the Ag2S phases. Purple determined by the overall Ag/Pb ratio in the three-step arrows point to the misfit dislocations. (b) Detailed HRTEM synthesis described above. Figure 4 illustrates such length analyses of the different crystal phases and their corresponding tunability from 9 to 44 nm, with the retention of the NR indexed FFT. thickness at ca. 9−10 nm. XRD analysis, in agreement with the HRTEM study, corroborates that the NR growth direction is The FFT reveals that the tip has lattice parameters a = 0.6725 ⟨100⟩ (Figure 4b). It is important to note that the formation of nm, b = 0.4148 nm, and c = 0.7294 nm, visualized along its anisotropic rodlike morphologies is hard to accomplish at high [−103] axis, and that the NR body consists of a face-centered yield and good control over the dimensions and uniformity cubic PbS phase (space group = Fm3m) with lattice parameter because this shape is very unusual for Pb chalcogenides due to a = 0.5936 nm, visualized along its [001] axis. The inset in their highly symmetric cubic crystal structure. Few known Figure 2a shows the geometric-phase analysis (GPA) at the examples include ultranarrow (1.8 nm) PbS wires produced by interface of both crystallographic phases. Purple arrows point decomposition of lead hexadecylxanthate in trioctyl- to the misfit dislocations with a mismatch calculated from the amine;38−40 single-crystalline PbS nanowires synthesized GPA to be around 30% with respect to a perfectly relaxed NP. using a solvothermal reaction, chemical vapor transport, and The colored structural map also suggests the epitaxial gas-phase conversion reaction of pregrown CdS nanowires;41 relationship between the PbS and the Ag2S phases. The PbS nanorods obtained via sequential cation exchange epitaxy took place with the (020) plane (d020 = 0.207 nm) of process;42 and ultrathin PbS sheets43 or PbSe nanowires44 Ag2S phase facing the (200) plane (d200 = 0.297 nm) of the formed by oriented attachment. Figure 3. (a) TEM image of vertically aligned PbS−Ag2S NRs. (b) HRTEM micrograph showing vertically aligned NRs. Detail of the red squared region and its corresponding power spectrum (FFT). (c) STEM HAADF micrograph and its intensity profile. 6574 DOI: 10.1021/acsnano.9b00346 ACS Nano 2019, 13, 6572−6580 ACS Nano Article Surface Chemical Engineering and Consolidation into Nanosolids. PbS−Ag2S NRs were initially stabilized by long-chain carboxylate groups, which were subsequently replaced with K2Te (Figure 6a). This inorganic surface Figure 4. TEM micrographs of PbS−Ag2S NRs with different aspect ratios (a), corresponding XRD patterns (b), and depend- Figure 6. (a) Schematic of the effect of the ligand exchange ence of the NR aspect ratio with respect to the Ag/Pb ratio (c). processes at the surface of PbS used in this study. Analogous chemistry is assumed for Ag2S surfaces, not shown here. (b) XRD An experiment extending the NR growth time from 3 to 5 patterns of the as-synthesized (13 ± 1 nm) × (9 ± 1 nm) PbS− min to 1 h illustrates the reduced thermodynamic stability of Ag2S NRs, the corresponding K2Te-surface modified NRs right NRs with respect to spherical NPs, as can be seen from the after the ligand exchange (PbS−Ag2S−K 2Te) and after annealing conversion of NRs into uniform, ca. 15 nm large spherical PbS at 210 °C (PbS−Ag2S−K2Te powder); hot-pressed (HP) resulting NPs (Figure 5). This transition occurs presumably after all lead pellet (PbS−Ag2S−K 2Te, HP). functionalization can be used to introduce a p-type dopant (K) into metal chalcogenides as well as to adjust the composition by forming either metal telluride phases or by replacing some sulfur to form solid solutions.27 The ligand- exchange reaction was conducted via a phase-transfer process, in which PbS−Ag2S NRs migrated from the nonpolar phase (hexane) to the polar phase (N -methylformamide, MFA) due to the shift from steric to electrostatic mechanism of colloidal stabilization. The ligand-exchange process did not alter the XRD pattern of NRs. However, mild thermal treatments (210 °C, 10 min) induced an increase of the crystal domain size (SI) in the PbS phase as well as the emergence of a crystalline PbTe phase (Figure 6b), associated with the excess of Te2− ions in solution promoting a partial S2− to Te2− anion exchange.27,47 None of the known silver chalcogenide crystal phases could be identified by XRD, which can be attributed to the small crystal domains, low concentration, and the large quantity of peaks in the characteristic XRD pattern of orthorhombic silver chalcogenides.48−50 Figure 5. TEM images of (a) as-synthesized PbS−Ag2S NRs K2Te-functionalized PbS−Ag2S NRs were used as a (growth time of 3 min) and (b) of the same sample aged for 55 min at 180 °C, showing conversion into spherical NPs. (c) precursor for K-doped Pb−Ag−S−Te nanocomposites (Pb− HRTEM image of several spherical NPs; details of the red and Ag−K−S−Te), with the aim of constructing efficient thermo- green squared regions and their corresponding FFT. On the right, electric materials. To produce a nanocomposite with low a colored structural map is presented, wherein the red color porosity, precipitated and predried K2Te−PbS−Ag2S NRs indicates an orthorhombic Ag2S phase and the green color were first annealed at 210 °C under argon to fully remove the indicates a face-centered cubic PbS phase. physisorbed MFA solvent. Subsequently, the powder was hot- pressed into 10 mm in diameter and ca. 1 mm thick disk- oleate and sulfur precursors are consumed for the formation of shaped pellets by applying a uniaxial pressure of 40 MPa at NRs. An Ostwald-ripening-like process can be assumed.45,46 A 380−400 °C for 4 min. This consolidation yielded Pb−Ag− ca. 3 nm large twinned Ag2S tip remains attached to the PbS K−S−Te nanocomposites with relative densities between NPs (Figure 5c). FFT of the HRTEM image confirmed an ∼90−92%. Their exact elemental composition was controlled orthorhombic Ag2S phase with lattice parameters a = 0.6725 by the rod lengths. Smaller NRs yield nanocomposites with nm, b = 0.4148 nm, and c = 0.7294 nm, viewed along its larger Ag/Pb ratio, since the size of the Ag2S tip remains [−120] axis; and the fcc PbS phase, viewed along its [011] unchanged for all NR sizes. On the other hand, the smaller the axis. In this case, an epitaxial relationship between the (21−1) NRs the larger is the surface-to-volume ratio and, hence, the plane of Ag2S phase and the (1−11) plane of PbS phase is also K/Pb and Te/S ratios. Nanocomposites with the following observed, in which there are 4 Ag2S planes for every 3 PbS elemental compositions as determined by ICP and Rietveld planes with an overall lattice mismatch of 4%. refinement, Pb0.81Ag0.16K0.03Te0.33S0.67 (13 × 9 nm NRs) and 6575 DOI: 10.1021/acsnano.9b00346 ACS Nano 2019, 13, 6572−6580 ACS Nano Article Pb0.89Ag0.10K0.01Te0.28S0.72 (20 × 10 nm NRs), were selected were found for Pb0.81Ag0.16K0.03Te0.33S0.67 (p = 1 × 1019 cm−3) for the subsequent thermoelectric characterization. when compared with Pb0.89Ag0.10K0.01Te0.28S0.72 (p = 8 × 1018 To investigate the nanoscale structure, cm−3). Several additional reference experiments, described Pb0.81Ag0.16K0.03Te0.33S0.67 nanocomposite was studied in detail below and summarized in Figure 9 in terms of transport by HRTEM (Figure 7), revealing a large density of nanosized Figure 7. (a) TEM image of a Pb0.81Ag0.16K0.03Te0.37S0.67 nano- composite and (b) HRTEM images along with their FFTs for typical inclusions (Ag2Te, Ag2S, PbS). crystalline domains and interfaces. Largest crystal domains correspond to PbTe and PbS crystal phases, in agreement with narrow and intense XRD reflections. A closer look at the crystal structure of some of the nanoscale inclusions reveals the Figure 9. (a) Electrical conductivity, σ; (b) Seebeck coefficient, S; presence of Ag2S, Ag2Te and PbS NPs within the Pb− (c) thermal conductivity, κ; (d) thermoelectric figure of merit, ZT, chalcogenide matrix (Figure 7b). of Pb−S, Pb−Ag−S, Pb−K−S−Te, and Pb−Ag−K−S−Te nano- Electronic and Thermal Properties. Temperature- composites. dependent measurements of the electrical conductivity and Seebeck coefficients are presented in Figure 8 for properties, had confirmed the importance of each component in the Pb0.81Ag0.16K0.03Te0.33S0.67 nanocomposite. First, the functionalization with K2Te was excluded and replaced with NH4SCN treatment, thereby reducing the number of elements in a consolidated material to three (denoted as Pb−Ag−S material). To exclude Ag, pure PbS NPs were ligand- exchanged with either K2Te (a Pb−K−S-Te material) or NH4SCN (a Pb−S material). K-free materials (Pb−Ag−S and Pb−S; open symbols in Figure 9) are characterized by lower electrical conductivities and negative Seebeck coefficients in the whole temperature range. In the case of Pb−Ag−S, as the temperature increased, a pronounced change at 450 K of the electrical conductivity as well as the Seebeck coefficient was observed, which is associated with the phase transition from the low temperature orthorhombic β-Ag2S to the high-temperature cubic α-Ag2S phase.51 The high-temperature phase contains both electrons and Ag ions that are mobile.52 The contribution of Ag ions to the transport properties increases the electrical conductivity, Figure 8. (a) 13 and 20 nm NRs used to build up but their p-type nature induces bipolar effects reducing the Pb0.81Ag0.16K0.03Te0.33S0.67 and Pb0.89Ag0.10K0.01Te0.28S0.72 nanocom- absolute value of the Seebeck coefficient to near −10 μV/K. posites, respectively, and the corresponding (b) electrical Despite the fact that Ag can partially dissolve into Pb conductivity, σ, and (c) Seebeck coefficient, S. chalcogenide and then act as a p-type dopant,53,54 in our composite Ag doping was not efficient enough to change Pb0.81Ag0.16K0.03Te0.33S0.67 and Pb0.89Ag0.10K0.01Te0.28S0.72 mate- majority of carriers in the Pb−Ag−S nanocomposite. On the rials. Both nanosolids exhibit positive Seebeck coefficients contrary, the K2Te treatment induces a change of the type of indicating a p-type electronic transport. Smaller NRs yielded the majority carriers in the nanocomposites (Pb−S−K−Te nanocomposites with higher electrical conductivities in the and Pb−Ag−K−S−Te), seen as a change of the sign of whole temperature range, in agreement with the higher Seebeck coefficient from negative to positive and larger contents of a dopant (K) in the material produced with electrical conductivities in the whole temperature range smaller NRs. Correspondingly, larger carrier concentrations indicating an efficient p-type doping (solid symbols). 6576 DOI: 10.1021/acsnano.9b00346 ACS Nano 2019, 13, 6572−6580 ACS Nano Article Ag-free nanocomposites (blue squares) exhibited larger ethanol as nonsolvent. Isolated NPs were dried under vacuum and thermal conductivities than Pb−Ag−S or Pb−Ag−K−S−Te, then redispersed in hexane for further use. indicating a significant role of Ag for enhanced phonon Synthesis of Ag/Ag2S NPs/Seeds. Ag NPs with an average scattering. The presence of Ag2S and Ag2Te NPs in the Pb− diameter of 2−3 nm were produced using a modified approach of that reported by Wang et al.29 In a typical reaction, AgNO3 (0.17 g, 1 chalcogenide matrix with the mutual lattice mismatch might be mmol), Fe(NO3)3·9H2O (0.04 g, 0.01 mmol), OA (10 mL, 31.6 the reason for a more efficient phonon scattering. Additionally, mmol), and OLA (10 mL, 30.5 mmol) were mixed and degassed the ionic nature of silver chalcogenides compounds, with high under Ar at rt for 0.5 h. Afterward, the reaction mixture was heated to mobility of Ag ions, can further reduce the thermal 120 °C at a rate of 5 °C min−1 and kept at this temperature for an conductivity by scattering short-wavelength phonons.55 additional 60 min. To form Ag2S seeds, Ag NPs were mixed with a On the basis of the measured electrical conductivities, OLA/S solution (5 mL/0.16 g, 5 mmol). Immediately after OLA/S Seebeck coefficients, and thermal conductivities, one can was mixed with the Ag NPs, the solution changed color from yellow estimate a thermoelectric figure-of-merit (ZT = σS2Tκ−1) of to light brown, indicating the formation of Ag2S seeds. Synthesis of PbS−Ag2S NRs. PbO (1.115 g, 5 mmol), OA (12.5 the obtained materials. The highest ZT value of ca. 1 at 620 K mL, 0.04 mol), and ODE (25 mL) were combined in a three-neck was obtained for Pb−Ag−K−S−Te nanocomposites, which is flask. This mixture was degassed under vacuum at rt and 100 °C for at 3-fold higher with respect to Pb−S−Te nanocomposites. least 0.5 h each to form a lead oleate complex and to remove low- boiling-point impurities. Then the solution was flushed with Ar, and CONCLUSIONS the temperature was raised to 180 °C. At this temperature the OLA/ S−Ag solution was rapidly injected at 180 °C into the lead oleate In summary, a possibility of the fully rational control of complex solution. After 3 min, the reaction mixture was quickly electrical and thermal characteristics of a multicomponent cooled to rt using a water bath. The obtained NRs were washed inside thermoelectric material, Pb−Ag−K−S−Te in this study, by the the glovebox by three precipitation/redispersion steps using hexane as multistep bottom-up engineering is presented. In particular, a solvent and 2-propanol as a nonsolvent. The washed NRs were dried shown is the preassembly of Pb, Ag, and S atoms into a Ag2S− using vacuum/Ar and then redispersed in hexane for further use. PbS NR morphology with tunable PbS rod lengths by means of K2Te Synthesis. K2Te was synthesized in liquid ammonia from the elemental potassium and tellurium in stoichiometric quantities. colloidal synthesis from monodisperse Ag NPs that were Typically, 29.2 mmol of K and 14.6 mmol of Te were placed into a consequently converted into Ag2S NP seeds, which are 500 mL Schlenk vessel. Dry ammonia was then condensed into this immediately used for formation of PbS rods. Ag2S−PbS NRs reaction vessel, which was cooled with a dried ice/acetone bath. A were then surface-functionalized with K2Te, followed by beige powder was obtained after complete evaporation of ammonia. consolidation into all-inorganic five-component Pb−Ag−K− K2Te powder was stored and handled in an Ar-filled glovebox. S−Te nanomaterials, whose chemical composition is adjust- Ligand Exchange with K2Te. A 7 mM K2Te solution in MFA was able by the size of the initial rods. Efficient suppression of prepared in an Ar-filled glovebox. Anhydrous hydrazine was added to thermal conductivity was attained due to nanoscale homoge- generate a reducing environment (4 μL per milliliter of MFA). Equal neity of the mixing of the grains of several binary and ternary volumes of K2Te/MFA solution and hexane solution of NPs (5 mg/ mL) were combined in a vial and vigorously stirred at rt in an Ar-filled crystal phases. Efficient p-type transport was imparted by glovebox for 16 h. After its complete discoloration, the hexane phase efficient substitutional doping with K ions. Overall, the was decanted and the remaining polar phase was rinsed with pure combined effect of such engineering is a high ZT value of hexane. Acetone was added to the remaining polar phase, and the ca. 1 at 620 K. mixture was centrifuged to precipitate the NPs. NPs were then washed one more time with acetone, centrifuged, dried under EXPERIMENTAL METHODS vacuum, and stored in the glovebox for further use. Ligand Exchange with NH4SCN. The native ligands were removed Chemicals and Materials. Lead(II) oxide (PbO, 99.9%), oleic by mixing 6 mL of a 130 mM NH4SCN solution in methanol with 1 g acid (OA, 90%, technical grade), 1-octadecene (ODE, 90%, technical of NPs suspended in anhydrous chloroform. NPs were then purified grade), sulfur (S, 99.998%, trace metals basis), oleylamine (OLA, min. using chloroform and methanol to remove free carboxylic acid and 95%), silver nitrate (AgNO3, ≥ 99.8%), iron(III) nitrate nonahydrate excess NH4SCN, respectively. (Fe(NO3)3·9H2O, 99.99%), K (cubes in mineral oil 99.5%, trace Consolidation of NPs into Pellets. In all cases, surface-modified metals basis), Te powder (99.999%), N-methylformamide (MFA, NPs were dried from solution under vacuum. Afterward, NPs were 99%), and hydrazine (N2H4) were obtained from Sigma-Aldrich. annealed at 210 °C on a heating plate in an Ar-filled glovebox for Anhydrous hexane, ethanol, 2-propanol, and acetone were obtained approximately 20 h to remove remaining volatile organics before the from various sources. All chemicals were used as received without pellet fabrication. NPs powders were pressed using a custom-made further purification. MFA was dried over 4 Å molecular sieves at room hot press. In this system, the heat was provided by an induction coil temperature (rt) under an Ar flow for 20 h and then filtered using operated in the RF range applied directly to a graphite die acting as a hydrophobic syringe filters. Standard airless techniques were used: a susceptor. This setup configuration allows increasing temperature at a vacuum/dry argon Schlenk line for synthesis and an argon glovebox similar rate as spark plasma sintering. Inside the glovebox, powders for storage and handling of air- and moisture-sensitive chemicals. were ground into fine powder and loaded into a 10 mm diameter PbS NPs with a mean edge size of 11 nm were prepared similarly to graphite die lined with 0.13 mm thick graphite paper. The filled die previously reported procedures.23 In a typical synthesis, PbO (4.46 g, was placed in the hot press system. The densification profile applied 20 mmol) and OA (50 mL, 0.158 mol) were mixed with 100 mL of an axial pressure of ∼40 MPa before the die was heated to 300 °C. ODE. This mixture was degassed at rt and 100 °C for 0.5 h each to The temperature was held between 400 and 420 °C for 5 min. The form the lead oleate complex and remove low boiling point impurities. pressure was then removed and the die cooled to room temperature. Then the solution was flushed with Ar, and the temperature was The resulting pellets were ca. 90% dense, ca. 1 mm thick, 10 mm in raised to 210 °C. At this temperature, a sulfur precursor, prepared by diameter, and air stable. The density of the pressed pellets was dissolving elemental sulfur (0.64 g, 20 mmol) in OLA (20 mL, 0.061 measured by the Archimedes method. mol), was rapidly injected. The reaction mixture was maintained X-ray Diffraction. XRD analysis was performed directly on the as- between 195 and 210 °C for 5 min and then quickly cooled to rt using synthesized NPs before and after the ligand exchange as well as after a water bath. The obtained NPs were washed inside the glovebox by the annealing and on the final pellets. The measurements were done three precipitation/redispersion steps using hexane as solvent and in a Bruker D8 Advance powder diffractometer equipped with an M. 6577 DOI: 10.1021/acsnano.9b00346 ACS Nano 2019, 13, 6572−6580 ACS Nano Article Braun 50 m position sensitive detector, Bragg−Brentano geometry, Yu Liu: 0000-0001-7313-6740 Cu Kα1 radiation (1.54059 Å), focusing Ge monochromator. Oleksandr Dobrozhan: 0000-0001-9238-7596 (S)TEM Characterization. High-resolution transmission electron microscopy (HRTEM) and high angle annular dark field scanning Jordi Arbiol: 0000-0002-0695-1726 TEM (HAADF STEM) images have been obtained by means of a FEI Andreu Cabot: 0000-0002-7533-3251 Tecnai field emission gun microscope with a 0.19 nm point-to-point Maksym V. Kovalenko: 0000-0002-6396-8938 resolution at 200 keV equipped with an embedded Quantum Gatan Notes image filter (Quantum GIF) for spectrum imaging (SI) EELS analyses. Images have been analyzed by means of Gatan Digital The authors declare no competing financial interest. Micrograph software. Structure phase color maps have been generated also with the latest software in order to differentiate the different ACKNOWLEDGMENTS heterostructures.56 Thermoelectric Characterization. Electrical Properties. The This work was financially supported by the European Union pressed samples were polished, maintaining the disk-shape morphol- (EU) via FP7 ERC Starting Grant 2012 (Project NANO- ogy. Final pellets had a 10 mm diameter and were approximately 1 SOLID, GA No. 306733). M.I. was supported by IST Austria mm thick. The Seebeck coefficient was measured using a static DC and by ETH Zurich via an ETH career seed grant (SEED-18 method. Electrical resistivity data were obtained by a standard four- 16-2). Y.L. acknowledges funding from the European Union’s probe method. Both the Seebeck coefficient and the electrical Horizon 2020 research and innovation programme under the resistivity were simultaneously measured with accuracies better than Marie Sklodowska-Curie Grant Agreement No. 754411. IREC 1% in a LSR-3 LINSEIS system from rt to 650 K, under helium receives funding from Generalitat de Catalunya atmosphere. Samples were held between two alumel electrodes and two probe thermocouples with spring-loaded pressure contacts. A (2014SGR1638). ICN2 acknowledge funding from Generalitat resistive heater on the lower electrode created temperature differ- de Catalunya 2017 SGR 327 and the Spanish MINECO entials in the sample to determine the Seebeck coefficient. Note: The project ENE2017-85087-C3. ICN2 is supported by the Severo results presented in the manuscript are an average of the results Ochoa program from Spanish MINECO (Grant No. SEV- obtained after measuring two pellets produced under identical 2017-0706) and is funded by the CERCA Programme/ conditions. The measurements between different samples had Generalitat de Catalunya. standard deviations below 10%. Additionally, each pellet was measured three times, providing very little hysteresis between the heating and cooling cycles. The major difference was found in the first REFERENCES measurement. Therefore, all of the measurements presented in the (1) Kao, J.; Thorkelsson, K.; Bai, P.; Rancatore, B. J.; Xu, T. 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