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Ligand-Mediated Band Engineering in Bottom-Up Assembled SnTe Nanocomposites for Thermoelectric Energy Conversion Journal: Journal of the American Chemical Society Manuscript ID ja-2019-01394d.R2 Manuscript Type: Communication Date Submitted by the 17-Apr-2019 Author: Complete List of Authors: Ibáñez, Maria; Institute of Science and Technology Austria Hasler, Roger; Eidgenossische Technische Hochschule Zurich, Department of Chemistry and Applied Biosciences Genç, Aziz; ICN2, ; Bartin University, Metallurgy and Materials Engineering Liu, Yu; Institut de Recerca en Energia de Catalunya, Kuster, Beatrice; Eidgenossische Technische Hochschule Zurich, Department of Chemistry and Applied Biosciences Schuster, Maximilian; Eidgenossische Technische Hochschule Zurich, Department of Chemistry and Applied Biosciences Dobrozhan, Oleksandr; Institut de Recerca en Energia de Catalunya, Cadavid, Doris; IREC, Arbiol, Jordi; ICREA and Catalan Institute of Nanoscience and Nanotechnology (ICN2), CSIC and The Barcelona Institute of Science and Technology (BIST), Advanced Electron Nanoscopy Cabot, Andreu; Catalonia Institute for Energy Research, Dept Electrònica Kovalenko, Maksym; Eidgenossische Technische Hochschule Zurich, Department of Chemistry and Applied Biosciences; ACS Paragon Plus Environment Page 1 of 8 Journal of the American Chemical Society 1 2 3 4 5 6 7 Ligand-Mediated Band Engineering in Bottom-Up Assembled SnTe 8 Nanocomposites for Thermoelectric Energy Conversion 9 10 11 Maria Ibáñez,#,†,‡,* Roger Hasler,†,‡ Aziz Genç,∥,§ Yu Liu,# Beatrice Kuster,†,‡ Maximilian 12 Schuster,† Oleksandr Dobrozhan, Doris Cadavid, Jordi Arbiol,§,¶ Andreu Cabot,,¶ and 13 14 Maksym V. Kovalenko†,‡,* 15 16 # Institute of Science and Technology Austria, Am Campus 1, 3400 Klosterneuburg, Austria 17 18 † Institute of Inorganic Chemistry, Department of Chemistry and Applied Biosciences, ETH Zürich, Vladimir Prelog Weg 1, CH- 8093, Switzerland 19 20 ‡ Empa-Swiss Federal Laboratories for Materials Science and Technology, Dübendorf, Überlandstrasse 129, CH-8600, 21 Switzerland 22 23 ∥ Department of Metallurgy and Materials Engineering, Faculty of Engineering, Bartin University, 74100 Bartin, Turkey 24 § Catalan Institute of Nanoscience and Nanotechnology (ICN2), CSIC and BIST, Campus UAB, Bellaterra, 08193 Barcelona, 25 Catalonia, Spain 26 27  Catalonia Energy Research Institute - IREC, Sant Adria del Besos, 08930 Barcelona, Spain 28 29 ¶ ICREA, Pg. Lluís Companys 23, 08010 Barcelona, Spain 30 31 32 Supporting Information Placeholder 33 34 longstanding challenge. A material’s thermoelectric ABSTRACT: The bottom-up assembly of colloidal 35 nanocrystals is a versatile methodology to produce performance is usually evaluated through a dimensionless 36 composite nanomaterials with precisely tuned electronic figure of merit, ZT, defined as ZT =  S2 T -1 where T is the 37 properties. Beyond the synthetic control over crystal domain absolute temperature,  the electrical conductivity, S the 38 size, shape, crystal phase and composition, solution- Seebeck coefficient or thermopower, and  the thermal 39 processed nanocrystals allow exquisite surface engineering. conductivity.1 All these parameters are governed by the This provides additional means to modulate the nanomaterial electronic structure of the material and its charge and phonon 40 characteristics and particularly its electronic transport scattering properties.2 Several of current best-performing 41 properties. For instance, inorganic surface ligands can be thermoelectric materials are nanocomposites produced via 42 used to tune the type and concentration of majority carriers spinodal decomposition during solid-state synthesis, 43 or to modify the electronic band structure. Herein, we report resulting in a crystalline matrix containing nanoinclusions of 44 the thermoelectric properties of SnTe nanocomposites a second phase.3 Alternatively, the bottom-up assembly of 45 obtained from the consolidation of surface-engineered SnTe colloidally synthesized, solution-processed nanocrystals 46 nanocrystals into macroscopic pellets. A CdSe-based ligand (NCs) has recently emerged as a convenient methodology to is selected to (i) converge the light and heavy bands through produce high-ZT materials.4-5 Using this strategy, materials 47 partial Cd alloying, and (ii) generate CdSe nanoinclusions as such as Cu3Sb1−x−ySnxBiySe4,6 (Ag2Te)5(Sb2Te3)5,7 48 a secondary phase within the SnTe matrix, thereby reducing AgBi0.5Sb0.5Se2,8 PbTe-Bi0.7Sb1.3Te3,9 Bi2Te2.7Se0.3,10 and 49 the thermal conductivity. These SnTe-CdSe nanocomposites Bi0.5Sb1.5Te311 have reached ZT above 1. Despite these 50 possess thermoelectric figures of merit of up to 1.3 at 850 K, promising results, further progress requires precise surface 51 which is, to the best of our knowledge, the highest engineering of the NCs and methods to control the NC 52 thermoelectric figure of merit reported for solution-processed assembly and consolidation into bulk nanocrystalline SnTe. materials. Most colloidal syntheses deliver NCs with organic 53 molecules (ligands) coordinated at their surfaces.12 These 54 molecules are highly insulating and thus need to be removed 55 prior to NCs consolidation. Usually, such organic ligands are 56 The search for thermoelectric materials, able to efficiently thermally decomposed, chemically displaced, or exchanged 57 convert heat into electricity (and vice versa), has been a with inorganic ligands to produce fully inorganic 58 1 59 60 ACS Paragon Plus Environment Journal of the American Chemical Society Page 2 of 8 nanomaterials. However, beyond avoiding detrimental NCs, we first displaced native organic ligands from the NC 1 effects of bulky organic ligands, surface modification can be surface. For this purpose, we mixed an ammonium 2 envisioned as a powerful tool to further adjust the final thiocyanate (NH4SCN) solution in anhydrous methanol with 3 nanocomposite composition and therefore its functional SnTe NCs suspended in anhydrous tetrahydrofuran (see SI). properties. Recent work has demonstrated the possibility to NH4SCN efficiently replaced organic ligands21-22 and allowed 4 tune the type and concentration of majority carriers with the production of fully inorganic SnTe nanosolids (further 5 aliovalent electronic impurities, incorporated through surface referred to as SnTe-SCN) through hot pressing of the 6 ligands.13-14 In addition, ligands have been proposed as organic-free NCs. Details of the NC consolidation can be 7 molecular solders to facilitate charge transport between the found in the SI. Figure 2 displays the thermoelectric 8 grain boundaries.15-16 properties of SnTe-SCN nanomaterial. Hall charge carrier 9 Here, we demonstrate that ligands can be also used to concentrations of 2 x 1020 cm-3 and electrical conductivities 10 modify the electronic band structure of the final up to 720 S cm-1 were measured at room temperature. nanocomposite, while simultaneously acting as precursors However, low Seebeck coefficients (ca. 110 V K-1) and high 11 thermal conductivities (ca. 2.25 W m-1K-1) provided maximum for the formation of secondary phases during NCs 12 ZT values of just 0.35 at 850 K. consolidation. 13 In particular, we focus on the development of SnTe-based 14 nanocomposites from SnTe NCs (Figure 1). While very few 15 reports have centered on the synthesis and assembly of 16 SnTe NCs,17-19 the highest ZT value (ca. 0.5 at 800 K) 17 obtained from bottom up assembled SnTe was reached 18 using rather large (ca. 170 nm) particles consolidated via 19 spark plasma sintering. Such nanomaterials showed lower lattice thermal conductivities, 0.52 W m−1 K−1, than bulk 20 SnTe, 1.6 Wm−1K−1.20 However they featured lower electrical 21 conductivities (500 S cm-1 for NC based solid18 compared to 22 1000 S cm-1 for bulk SnTe20) and lower Seebeck coefficients 23 (90 V K-1 versus 130 V K-1). Consequently, the power 24 factor (PF=S2) was approximately 4 times smaller than that 25 of bare SnTe solids. In order to overcome such limitations, 26 we explored the use SnTe NCs as building blocks to produce 27 SnTe-based nanomaterials. 28 29 30 31 32 33 34 35 36 37 38 39 40 41 Figure 2. (a) Electrical conductivity, ; (b) Seebeck coefficient, S; 42 (c) power factor, PF; (d) schematic of the electronic band structure; 43 (e) thermal conductivity, ; and (f) figure of merit, ZT, for SnTe 44 nanomaterial prepared with SCN (black) and CdSe (blue) surface- 45 Figure 1. TEM images of the as-prepared SnTe NCs. modified SnTe NPs. 46 47 SnTe NCs were prepared by reacting tin bromide with SnTe has a high symmetry cubic crystal structure and a 48 trioctylphosphine telluride in the presence of oleylamine and complex valence band structure with a small band gap (0.18 49 trioctylamine (experimental details can be found in eV at room temperature), which favors bipolar charge supporting information-SI). The obtained NCs were purified transport at relatively low temperatures.23 It is also 50 characterized by a large offset (0.3-0.4 eV)24-25 between the inside an argon-filled glovebox to prevent surface oxidation. 51 Figure 1 shows representative transmission electron light valence band (L) and the heavy valence band () that 52 microscopy (TEM) images of the obtained SnTe NCs, prevents the contribution of the heavier holes to the Seebeck 53 exhibiting cubic morphology and an average side length of coefficient.26 Additionally, the off-stoichiometric nature of 54 14 ± 1 nm. SnTe with a large number of Sn lattice vacancies yields very 55 In order to evaluate thermoelectric properties of SnTe- high intrinsic doping levels (p = 1021 cm-3).27 These based nanomaterials obtained from the assembly of these characteristics are far from the optimum for thermoelectric 56 applications, what translates into low Seebeck coefficients 57 58 2 59 60 ACS Paragon Plus Environment Page 3 of 8 Journal of the American Chemical Society and high thermal conductivities, and makes SnTe a poor upon mild annealing at 350 C (Figure S1). We hypothesize 1 thermoelectric material.26 Overall, the success towards that the ligand could be a variety of chalcogenidocadmates 2 producing SnTe with high ZT should rely on the carrier such as (Cd2Se3)n2n- or CdSe22- as previously reported for 3 concentration optimization, a proper band structure hydrazinum based CdSe solutions.46-47 In order to replace engineering, and the introduction of all-scale (atomic, nano the organic ligands on the NC surface by the CdSe-based 4 and meso) hierarchical scattering centers.28 ligand, a phase transfer reaction was used: a solution of 5 SnTe NCs in hexane was layered on top of a solution of the Previous reports show that the hole carrier concentration 6 CdSe ink in N-methylformamide. The change of colloidal could be reduced by introducing donors such as I,29 Bi,30 or 7 Sb,30 as well as by Sn self-compensation (Sn1+xTe).31-32 On stabilization from steric to electrostatic was evidenced by the 8 the other hand, the band structure could be modified to color change of the solutions, reaching a colorless hexane 9 introduce resonant states in the valence band near the Fermi phase and colored polar phase. The top hexane phase was 10 level via In doping20 or to promote band convergence and then discarded and the SnTe NCs were precipitated by open the band gap via Cd,20, 31, 33-35 Hg,36 Mg,37 Ca38 and centrifugation, rinsed twice with acetone and dried under 11 vacuum for future use. Mn39-40 alloying. Both strategies allowed for higher Seebeck 12 coefficients. Further strategies to reduce the lattice thermal Figure 3a shows TEM images of CdSe-functionalized 13 conductivity are based on the introduction of nanoscale SnTe NCs (SnTe@CdSe). Atomic resolution high-angle 14 secondary phases such as SrTe,30 AgBiTe2,41 AgInTe2,42 annular dark-field (HAADF) scanning TEM (STEM), 15 In2Te343, GaTe,44 CdS31 and ZnS31 within the SnTe matrix via demonstrated that the NCs were composed of a ~1 nm thick 16 solid state precipitation. amorphous shell around a perfectly crystalline ~14 nm core 17 Inspired by the enhancement of the Seebeck coefficient (Figure 3b and SI). The power spectrum (FFT) obtained from 18 achieved in SnTe through band convergence and widening these nanoparticles revealed their core to have a crystal of the band-gap via alloying with transition metals, and structure matching an f.c.c. SnTe phase visualized along its 19 specifically with Cd;20, 31, 33-34 we designed a ligand exchange [001] zone axis. To unravel the composition of the 20 amorphous shell, STEM electron energy loss spectroscopy strategy using soluble CdSe-compounds as inorganic 21 ligands. The CdSe-based ligand was prepared by dissolving (EELS) analyses were conducted. EELS compositional maps 22 stoichiometric amounts of elemental Cd and Se in a thiol- indicated the presence of amorphous CdSe shells at the 23 amine mixture45 at room temperature and in inert- surface of SnTe cores. More examples can be found in 24 atmosphere (SI). This molecular solution (referred to as Figures S5 and S6. 25 CdSe ink) allowed the recovery of pure hexagonal CdSe 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 Figure 3. (a) General TEM micrograph of the SnTe@CdSe core-shell like nanoparticles, revealing the homogeneity in size and shape; (b) atomic resolution HAADF STEM micrograph of a core-shell nanoparticle and its corresponding power spectrum. (c) ADF STEM micrograph of 47 several SnTe@CdSe NPs and the corresponding STEM–EELS elemental composition maps: Sn (red), Te (green), Cd (blue) and Se (yellow). 48 49 Subsequently, SnTe@CdSe nanoparticles were used as increased from 110 up to 185 V K-1 at 850 K. These results 50 building blocks to produce SnTe-CdSe nanocomposites are in agreement with the predicted band structure 51 under the same conditions as SnTe-SCN nanomaterials. A modification of SnTe with the incorporation of Cd (Figure 52 6.4% content of Cd was estimated considering the ligand 2d).31 Substituting Sn atoms with isovalent Cd significantly surface coverage and verified by ICP (SI table S1). Figure 2 increased the band gap and diminished the energy 53 displays the thermoelectric properties of these materials. separation between the light-hole band and the heavy-hole 54 Compared to SnTe-SCN nanomaterials, the electrical band (Figure 2d).31 The larger band gap and the reduced 55 conductivity of SnTe-CdSe nanocomposite was reduced carrier concentration (9  1019 cm-3) of SnTe-CdSe also 56 from 720 to 380 S cm-1, while the Seebeck coefficient reduced bipolar contributions. Additionally, the convergence 57 58 3 59 60 ACS Paragon Plus Environment Journal of the American Chemical Society Page 4 of 8 of the valence bands provided a higher hole density at the nanocomposites was investigated (Figure 4, S7-S9). HRTEM 1 Fermi level. Both phenomena translated into much higher analysis showed the presence of CdSe precipitates, both 2 Seebeck coefficients in the whole temperature range, and with cubic or hexagonal phases, and with sizes between 3 3 especially at high temperatures.20, 31 Thus, much higher nm and 10 nm, within the SnTe matrix. In some cases, both power factors of ca. 1.3 mWm-1 K-2 were reached. Finally, crystal domains, SnTe and CdSe were found to be in perfect 4 thermal conductivities of 0.85 Wm-1 K-1 were systematically epitaxy due to their low lattice mismatch of 1.8 % (Figure S8). 5 acquired at 850 K which is approximately 2.7 times lower Overall, the alloy formation, the presence of nanoscale CdSe 6 than for SnTe-SCN; and corresponded to a lattice thermal secondary phases and the mesoscale SnTe domains 7 conductivities of 0.48 Wm-1 K-1 (SI); close to the theoretical provided scattering mechanisms to block propagation of 8 amorphous limit (0.5 Wm-1 K-1).36 phonons on all relevant length scales, thereby reducing the 9 To understand the observed low lattice thermal lattice thermal conductivity to the amorphous limit. 10 conductivities, the mesoscale structure of SnTe-CdSe 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 Figure 4. (a) Schematic of the bottom-up process. (b) HRTEM micrograph obtained from the cross-section of the SnTe-CdSe pellet sample 40 with the corresponding power spectrum (FFT) from the marked yellow squared area. In the bottom line phase filtered images corresponding to 41 a CdSe precipitate (red); a SnTe crystal domain visualized along the [001] zone axis (green) and another SnTe crystal domain also visualized along the [001] zone axis (blue). The inset below corresponds to a phase filtered RGB composition map showing the different superposed 42 phases found. 43 44 In conclusion, a scalable high-yield colloidal synthetic route ASSOCIATED CONTENT 45 to prepare SnTe NCs with narrow size distribution was 46 presented. In order to optimize the thermoelectric Supporting information containing details of the chemicals 47 performance of bottom-up SnTe-based nanosolids, a new used, NCs synthesis; ligand exchange reactions; pellet NC ligand-exchange process using CdSe-based compounds fabrication, materials characterization (X-ray diffraction 48 was presented. Upon thermal consolidation, the surface- patterns, HRTEM, electrical and thermal characterization); 49 deposited CdSe units partially alloyed with SnTe to modify its stability measurement and additional notes is available free 50 band structure and partially crystallized in CdSe of charge via the Internet at http://pubs.acs.org. 51 nanodomains within the SnTe matrix. Alloying resulted in an 52 increase of the Seebeck coefficient due to the influence of Cd AUTHOR INFORMATION 53 in the electronic band structure of SnTe. Besides, the mesoscale structuration allowed reducing the thermal * mibanez@ist.ac.at; mvkovalenko@ethz.ch 54 conductivity to the amorphous limit. Overall, SnTe-CdSe 55 nanocomposites with ZT of ca. 1.3 at 850 K were produced. ACKNOWLEDGMENT 56 57 58 4 59 60 ACS Paragon Plus Environment Page 5 of 8 Journal of the American Chemical Society This work was financially supported by the European Union (10) Liu, Y.; Zhang, Y.; Lim, K. H.; Ibáñez, M.; Ortega, S.; Li, M.; 1 (EU) via FP7 ERC Starting Grant 2012 (Project David, J.; Martí-Sánchez, S.; Ng, K. M.; Arbiol, J.; Kovalenko, M. V.; 2 NANOSOLID, GA No. 306733). M.I. was supported by IST Austria, and by ETH Zurich via ETH career seed grant Cadavid, D.; Cabot, A., High Thermoelectric Performance in 3 (SEED-18 16-2). Y.L acknowledges funding from the Crystallographically Textured n-Type Bi2Te3–xSex Produced from 4 European Union’s Horizon 2020 research and innovation 5 Asymmetric Colloidal Nanocrystals. ACS Nano 2018, 12 (7), 7174- programme under the Marie Sklodowska-Curie grant 6 agreement No 754411. IREC acknowledges funding from 7184. 7 Generalitat de Catalunya (2014SGR1638). JA acknowledge (11) Liu, Y.; Zhang, Y.; Ortega, S.; Ibáñez, M.; Lim, K. H.; Grau- 8 funding from Generalitat de Catalunya 2014 SGR 1638 and the Spanish MINECO coordinated projects between IREC Carbonell, A.; Martí-Sánchez, S.; Ng, K. M.; Arbiol, J.; Kovalenko, M. 9 and ICN2 TNT-FUELS and e-TNT (MAT2014-59961-C2-2- V.; Cadavid, D.; Cabot, A., Crystallographically Textured 10 R). ICN2 acknowledges support from the Severo Ochoa Nanomaterials Produced from the Liquid Phase Sintering of BixSb2– 11 Program (MINECO, Grant SEV-2013-0295) and is funded by 12 the CERCA Programme / Generalitat de Catalunya. xTe3 Nanocrystal Building Blocks. 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M.; 3 4 (45) McCarthy, C. L.; Brutchey, R. L., Solution Processing of Jang, J.; Lee, B.; Sun, C.; Talapin, D. V., New Forms of CdSe: 5 Chalcogenide Materials Using Thiol-Amine "Alkahest" Solvent Molecular Wires, Gels, and Ordered Mesoporous Assemblies. J. Am. 6 Systems. Chem. Comm. 2017, 53 (36), 4888-4902. Chem. Soc. 2017, 139 (9), 3368-3377. 7 (46) Dolzhnikov, D. S.; Zhang, H.; Jang, J.; Son, J. S.; Panthani, M. 8 G.; Shibata, T.; Chattopadhyay, S.; Talapin, D. V., Composition- 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 7 59 60 ACS Paragon Plus Environment Journal of the American Chemical Society Page 8 of 8 1 2 3 4 5 6 7 8 9 10 11 12 13 14 “For Table of Contents Only” 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 8 60 ACS Paragon Plus Environment