The precise engineering of thermoelectric materials using nanocrystals as their building blocks has proven to be an excellent strategy to increase energy conversion efficiency. Here we present a synthetic route to produce Sb-doped PbS colloidal nanoparticles. These nanoparticles are then consolidated into nanocrystalline PbS:Sb using spark plasma sintering. We demonstrate that the introduction of Sb significantly influences the size, geometry, crystal lattice and especially the carrier concentration of PbS. The increase of charge carrier concentration achieved with the introduction of Sb translates into an increase of the electrical and thermal conductivities and a decrease of the Seebeck coefficient. Overall, PbS:Sb nanomaterial were characterized by two-fold higher thermoelectric figures of merit than undoped PbS.

•Bio-mediated synthesis enables production of cost-effective TiO2 nanocrystals.•Tangerine peels extract mediates nanocrystals formation through hydrolysis and condensation.•The pH of the tangerine peels extract increases the nanocrystals aggregation.

Nanomaterials produced from the bottom-up assembly of nanocrystals may incorporate similar to 10(20)-10(21) cm(-3) not fully coordinated surface atoms, i.e., similar to 10(20)-10(21) cm(-3) potential donor or acceptor states that can strongly affect transport properties. Therefore, to exploit the full potential of nanocrystal building blocks to produce functional nanomaterials and thin films, a proper control of their surface chemistry is required. Here, we analyze how the ligand stripping procedure influences the charge and heat transport properties of sintered PbSe nanomaterials produced from the bottom-up assembly of colloidal PbSe nanocrystals. First, we show that the removal of the native organic ligands by thermal decomposition in an inert atmosphere leaves relatively large amounts of carbon at the crystal interfaces. This carbon blocks crystal growth during consolidation and at the same time hampers charge and heat transport through the final nanomaterial. Second, we demonstrate that, by stripping ligands from the nanocrystal surface before consolidation, nanomaterials with larger crystal domains, lower porosity, and higher charge carrier concentrations are obtained, thus resulting in nanomaterials with higher electrical and thermal conductivities. In addition, the ligand displacement leaves the nanocrystal surface unprotected, facilitating oxidation and chalcogen evaporation. The influence of the ligand displacement on the nanomaterial charge transport properties is rationalized here using a two-band model based on the standard Boltzmann transport equation with the relaxation time approximation. Finally, we present an application of the produced functional nanomaterials by modeling, fabricating, and testing a simple PbSe-based thermoelectric device with a ring geometry.

Appropriately designed Bi2Te3–Cu2−xTe nanocomposites allow an improvement of over 50% of the thermoelectric figure of merit of Bi2Te3.

In the present work, we report a solution-based strategy to produce crystallographically textured SnSe bulk nanomaterials and printed layers with optimized thermoelectric performance in the direction normal to the substrate. Our strategy is based on the formulation of a molecular precursor that can be continuously decomposed to produce a SnSe powder or printed into predefined patterns. The precursor formulation and decomposition conditions are optimized to produce pure phase 2D SnSe nanoplates. The printed layer and the bulk material obtained after hot press displays a clear preferential orientation of the crystallographic domains, resulting in an ultralow thermal conductivity of 0.55 W m-1 K-1 in the direction normal to the substrate. Such textured nanomaterials present highly anisotropic properties with the best thermoelectric performance in plane, i.e., in the directions parallel to the substrate, which coincide with the crystallographic bc plane of SnSe. This is an unfortunate characteristic because thermoelectric devices are designed to create/harvest temperature gradients in the direction normal to the substrate. We further demonstrate that this limitation can be overcome with the introduction of small amounts of tellurium in the precursor. The presence of tellurium allows one to reduce the band gap and increase both the charge carrier concentration and the mobility, especially the cross plane, with a minimal decrease of the Seebeck coefficient. These effects translate into record out of plane ZT values at 800 K.

ZnSb/beta-Zn4Sb3 nanocomposites are produced from Zn1.1-xGexSb mixtures using a two-step process. First, proper amounts of the three elements are mixed, melted, and reacted at 800 K. During this process, the nonstoichiometric mixture is crystallized in a combination of ZnSb and beta-Zn4Sb3 phases. Then, the material is ball milled and subsequently hot pressed. Through this process, a dense ZnSb/beta-Zn4Sb3 composite, consisting of beta-Zn4Sb3 nanoinclusions embedded within a ZnSb matrix, is formed. The particular phase distribution of the final ZnSb/beta-Zn4Sb3 composites is a consequence of the harder and more brittle nature of ZnSb than Zn4Sb3, which translates into a stronger reduction of the size of the ZnSb crystal domains during ball milling. This small particle size and the high temperature generated during ball milling result in the melting of the ZnSb phase and the posterior crystallization of the two phases in a ZnSb/beta-Zn4Sb3 matrix/nanoinclusion-type phase distribution. This particular phase distribution and the presence of Ge result in excellent thermoelectric performances, with power factors up to 1.5 mW m(-1) K-2, lattice thermal conductivities down to 0.74 W m(-1) K-1, and a thermoelectric figures of merit, ZT, up to 1.2 at 650 K, which is among the highest ZT values reported for ZnSb.

The introduction of nonmetal nanoinclusions within Ag2Se results in an interphase band bending that promotes electron filtering and increase Seebeck coefficient. Similar loading of metal nanoinclusions provided an opposite effect-modulating free carrier concentration, as characterized by superior electrical conductivities and lower Seebeck coefficients.

We report the thermoelectric performance of p-type nanocrystalline SnSe obtained from the liquid phase sintering of blends of SnSe nanocrystals and Te nanorods. A cycled hot press procedure at a temperature above the Te melting point promoted the formation of crystallographically textured SnSe nanomaterials with relative densities up to 93%. After consolidation, part of this Te was found within the SnSe lattice and part remained as elemental Te between the SnSe grains. The presence of Te during the SnSe consolidation resulted in SnSe nanomaterials with higher electrical conductivities and lower Seebeck coefficients and thermal conductivities. By adjusting the amount of Te, thermoelectric figures of merit (ZT) up to 1.4 at 790 K were measured in the direction of the uniaxial pressure, coinciding with the preferential a crystallographic axis. While this value matches the highest ZT value reported at this temperature for SnSe in the [100] crystal direction, the ZT values of the consolidated SnSe along the bc plane were relatively lower due to moderately low thermal conductivities in this plane.

Low energy barriers for charge transport may improve thermoelectric performance in heterostructured inorganic materials and hybrid polymer composites. In this study, earth abundant Cu12Sb4S13 nanoparticles were synthesized and incorporated into poly (3,4-ethylenedioxythiophene) (PEDOT) matrices. The surface of nanoparticles was modified with ligands that improve carrier transport while maintaining the Seebeck coefficient. We demonstrated optimized thermoelectric figures of merit, ZT, were obtained at around 5 wt.% nanoparticle content, which we attribute to a low hole-phonon interaction, an effective phonon scattering, and mainly to the presence of a proper density of low energy barriers, 0.17 eV, between the energy states of Cu12Sb4S13 nanoparticle and PEDOT nanofiber and which selectively scatter low energy charge carriers. A 2.5-fold increase of ZT over pristine PEDOT nanofiber, ZT = 0.0098, was obtained by polymer composite at the optimal nanoparticle concentration. Remarkably, electrical conductivity and Seebeck coefficient decreased with an increase in nanoparticle content beyond 5 wt.%, which is attributed to the non-energy dependent hole-phonon interaction. These findings are expected to create an economical and new route for enhancing ZT in hybrid thermoelectric polymer composite and devices.

The bottom-up assembly of colloidal nanocrystals is a versatile methodology to produce composite nanomaterials with precisely tuned electronic properties. Beyond the synthetic control over crystal domain size, shape, crystal phase, and composition, solution-processed nanocrystals allow exquisite surface engineering. This provides additional means to modulate the nanomaterial characteristics and particularly its electronic transport properties. For instance, inorganic surface ligands can be used to tune the type and concentration of majority carriers or to modify the electronic band structure. Herein, we report the thermoelectric properties of SnTe nanocomposites obtained from the consolidation of surface-engineered SnTe nanocrystals into macroscopic pellets. A CdSe-based ligand is selected to (i) converge the light and heavy bands through partial Cd alloying and (ii) generate CdSe nanoinclusions as a secondary phase within the SnTe matrix, thereby reducing the thermal conductivity. These SnTe-CdSe nanocomposites possess thermoelectric figures of merit of up to 1.3 at 850 K, which is, to the best of our knowledge, the highest thermoelectric figure of merit reported for solution-processed SnTe.

Large amounts of waste heat generated in our fossil-fuel based economy can be converted into useful electric power by using thermoelectric generators. However, the low-efficiency, scarcity, high-cost and poor production scalability of conventional thermoelectric materials are hindering their mass deployment. Nanoengineering has proven to be an excellent approach for enhancing thermoelectric properties of abundant and cheap materials such as silicon. Nevertheless, the implementation of these nanostructures is still a major challenge especially for covering the large areas required for massive waste heat recovery. Here we present a family of nano-enabled materials in the form of large-area paper-like fabrics made of nanotubes as a cost-effective and scalable solution for thermoelectric generation. A case study of a fabric of p-type silicon nanotubes was developed showing a five-fold improvement of the thermoelectric figure of merit. Outstanding power densities above 100 W/m2 at 700 °C are therefore demonstrated opening a market for waste heat recovery.

Bottom-up approaches for producing bulk nanomaterials have traditionally lacked control over the crystallographic alignment of nanograins. This limitation has prevented nanocrystal-based nanomaterials from achieving optimized performances in numerous applications. Here we demonstrate the production of nanostructured Bi xSb2- xTe3 alloys with controlled stoichiometry and crystallographic texture through proper selection of the starting building blocks and the adjustment of the nanocrystal-to-nanomaterial consolidation process. In particular, we hot pressed disk-shaped Bi xSb2- xTe3 nanocrystals and tellurium nanowires using multiple pressure and release steps at a temperature above the tellurium melting point. We explain the formation of the textured nanomaterials though a solution-reprecipitation mechanism under a uniaxial pressure. Additionally, we further demonstrate these alloys to reach unprecedented thermoelectric figures of merit, up to ZT = 1.96 at 420 K, with an average value of ZTave = 1.77 for the record material in the temperature range 320-500 K, thus potentially allowing up to 60% higher energy conversion efficiencies than commercial materials.

In the search for low-cost thermoelectric materials operating near room temperature, the potential of chalcopyrite (CuFeS2) nanocrystals is explored. Their colloidal synthesis is optimized to achieve around 40 nm sized nanocrystals with the goal to effectively reduce thermal conductivity via phonon scattering while maintaining high electrical conductivity. EDX and XPS analyses reveal that the nanocrystals are intrinsically nanostructured with a radial compositional gradient. Three strategies are explored to optimize the thermoelectric properties: i) Intrinsic doping by varying the Cu:Fe ratio. However, the effect of this variation is overcompensated by a global sulfur deficiency, making the chalcopyrite nanocrystals n-type. A high Seebeck coefficient, 5, up to -380 mu V/K is obtained, while the figure of merit remains comparably low (ZT-0.07 at 400 degrees C) because of low electrical conductivity sigma. ii) Removal of the native, insulating ligands by exchange with potassium selenide. This results in a better trade-off between S and a and hence a strongly improved ZT (0.18 at 400 degrees C). iii) Extrinsic doping via intimate mixture of chalcopyrite nanocrystals with metal nanoparticles. Sn (3 wt%) or Ag (16 wt%) nanoparticles give the best results (ZT=0.16 at 400 degrees C), inducing the concomitant reduction of thermal conductivity kappa and increase of sigma.

In the present work, we detail a fast and simple solution-based method to synthesize hexagonal SnSe2 nanoplates (NPLs) and their use to produce crystallographically textured SnSe2 nanomaterials. We also demonstrate that the same strategy can be used to produce orthorhombic SnSe nanostructures and nanomaterials. NPLs are grown through a screw dislocation-driven mechanism. This mechanism typically results in pyramidal structures, but we demonstrate here that the growth from multiple dislocations results in flower-like structures. Crystallographically textured SnSe2 bulk nanomaterials obtained from the hot pressing of these SnSe2 structures display highly anisotropic charge and heat transport properties and thermoelectric (TE) figures of merit limited by relatively low electrical conductivities. To improve this parameter, SnSe2 NPLs are blended here with metal nanoparticles. The electrical conductivities of the blends are significantly improved with respect to bare SnSe2 NPLs, what translates into a three-fold increase of the TE Figure of merit, reaching unprecedented ZT values up to 0.65.

The topological nature of two-dimensional (2D) chalcogenide platelets, can present novel opportunities in thin, flexible thermoelectrics. In this work, metal dopants are added to the reactive edges of 2D Bi2Te3 platelets. We show that along this active edge, an atomically well-ordered heterojunction is formed and facile charge exchange is created, onto the platelet and proximal to its known topological states. Temperature dependent conductivity suggests that local band bending across the interface may act as an injection energy filter for dopant-originated carriers. Moreover, as carrier density increases with increasing edge-dopant, carrier scattering does not appear to increase dramatically. As a result, an apparent decoupling between electrical conductivity and Seebeck coefficient occurs, leading to a surprisingly high power factors (PF): For example, the PF increases in Bi2Te3 platelets by eight times when doped with Cu. First principles calculations show that the electronics of the semiconductor-metal interfaces are quite different for edge and facial configurations, thus the site of metal dopant is believed to play an important role in the expected thermoelectric performance. Finally, this work suggests that the topological sensitivity of dopant placement should be considered in the rational design of high performance thermoelectric composites.

In the present work, we demonstrate crystallographically textured n-type Bi2Te3-xSex nanomaterials with exceptional thermoelectric figures of merit produced by consolidating disk-shaped Bi2Te3-xSex colloidal nanocrystals (NCs). Crystallographic texture was achieved by hot pressing the asymmetric NCs in the presence of an excess of tellurium. During the hot press, tellurium acted both as lubricant to facilitate the rotation of NCs lying close to normal to the pressure axis and as solvent to dissolve the NCs approximately aligned with the pressing direction, which afterward recrystallize with a preferential orientation. NC-based Bi2Te3-xSex nanomaterials showed very high electrical conductivities associated with large charge carrier concentrations, n. We hypothesize that such large n resulted from the presence of an excess of tellurium during processing, which introduced a high density of donor Te-Bi, antisites. Additionally, the presence in between grains of traces of elemental Te, a narrow band gap semiconductor with a work function well below Bi2Te3-xSex might further contribute to increase n through spillover of electrons, while at the same time blocking phonon propagation and hole transport through the nanomaterial. NC-based Bi2Te3-xSex nanomaterials were characterized by very low thermal conductivities in the pressing direction, which resulted in ZT values up to 1.31 at 438 K in this direction. This corresponds to a ca. 40% ZT enhancement from commercial ingots. Additionally, high ZT values were extended over wider temperature ranges due to reduced bipolar contribution to the Seebeck coefficient and the thermal conductivity. Average ZT values up to 1.15 over a wide temperature range, 320 to 500 K, were measured, which corresponds to a ca. 50% increase over commercial materials in the same temperature range. Contrary to most previous works, highest ZT values were obtained in the pressing direction, corresponding to the c crystallographic axis, due to the predominance of the thermal conductivity reduction over the electrical conductivity difference when comparing the two crystal directions.

ADVERTISEMENT RETURN TO ISSUEPREVCommunicationNEXTTuning p-Type Transport in Bottom-Up-Engineered Nanocrystalline Pb Chalcogenides Using Alkali Metal Chalcogenides as Capping LigandsMaria IbáñezMaria IbáñezInstitute of Inorganic Chemistry, Department of Chemistry and Applied Biosciences, ETH Zürich, Vladimir Prelog Weg 1, Zürich CH-8093, SwitzerlandEmpa-Swiss Federal Laboratories for Materials Science and Technology, Überlandstrasse 129, Dübendorf CH-8600, SwitzerlandMore by Maria Ibáñezhttp://orcid.org/0000-0001-5013-2843, Roger HaslerRoger HaslerInstitute of Inorganic Chemistry, Department of Chemistry and Applied Biosciences, ETH Zürich, Vladimir Prelog Weg 1, Zürich CH-8093, SwitzerlandEmpa-Swiss Federal Laboratories for Materials Science and Technology, Überlandstrasse 129, Dübendorf CH-8600, SwitzerlandMore by Roger Hasler, Yu LiuYu LiuCatalonia Energy Research Institute - IREC, Sant Adria del Besos, 08930 Barcelona, SpainMore by Yu Liu, Oleksandr DobrozhanOleksandr DobrozhanCatalonia Energy Research Institute - IREC, Sant Adria del Besos, 08930 Barcelona, SpainMore by Oleksandr Dobrozhan, Olga NazarenkoOlga NazarenkoInstitute of Inorganic Chemistry, Department of Chemistry and Applied Biosciences, ETH Zürich, Vladimir Prelog Weg 1, Zürich CH-8093, SwitzerlandEmpa-Swiss Federal Laboratories for Materials Science and Technology, Überlandstrasse 129, Dübendorf CH-8600, SwitzerlandMore by Olga Nazarenko, Doris CadavidDoris CadavidCatalonia Energy Research Institute - IREC, Sant Adria del Besos, 08930 Barcelona, SpainMore by Doris Cadavid, Andreu CabotAndreu CabotCatalonia Energy Research Institute - IREC, Sant Adria del Besos, 08930 Barcelona, SpainICREA, Pg. Lluís Company 23, 08010 Barcelona, SpainMore by Andreu Cabothttp://orcid.org/0000-0002-7533-3251, and Maksym V. Kovalenko*Maksym V. KovalenkoInstitute of Inorganic Chemistry, Department of Chemistry and Applied Biosciences, ETH Zürich, Vladimir Prelog Weg 1, Zürich CH-8093, SwitzerlandEmpa-Swiss Federal Laboratories for Materials Science and Technology, Überlandstrasse 129, Dübendorf CH-8600, Switzerland*M. V. Kovalenko. E-mail: [email protected]More by Maksym V. Kovalenkohttp://orcid.org/0000-0002-6396-8938Cite this: Chem. Mater. 2017, 29, 17, 7093–7097Publication Date (Web):August 23, 2017Publication History Received17 July 2017Revised20 August 2017Published online23 August 2017Published inissue 12 September 2017https://doi.org/10.1021/acs.chemmater.7b02967Copyright © 2017 American Chemical SocietyRIGHTS & PERMISSIONSACS AuthorChoiceArticle Views2051Altmetric-Citations23LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InReddit PDF (2 MB) Get e-AlertsSupporting Info (1)»Supporting Information Supporting Information SUBJECTS:Alkali metals,Chalcogenides,Doping,Electrical conductivity,Ligands Get e-Alerts

The conversion of thermal energy to electricity and vice versa by means of solid state thermoelectric devices is extremely appealing. However, its cost-effectiveness is seriously hampered by the relatively high production cost and low efficiency of current thermoelectric materials and devices. To overcome present challenges and enable a successful deployment of thermoelectric systems in their wide application range, materials with significantly improved performance need to be developed. Nanostructuration can help in several ways to reach the very particular group of properties required to achieve high thermoelectric performances. Nanodomains inserted within a crystalline matrix can provide large charge carrier concentrations without strongly influencing their mobility, thus allowing to reach very high electrical conductivities. Nanostructured materials contain numerous grain boundaries that efficiently scatter mid- and long-wavelength phonons thus reducing the thermal conductivity. Furthermore, nanocrystalline domains can enhance the Seebeck coefficient by modifying the density of states and/or providing type- and energy-dependent charge carrier scattering. All these advantages can only be reached when engineering a complex type of material, nanocomposites, with exquisite control over structural and chemical parameters at multiple length scales. Since current conventional nanomaterial production technologies lack such level of control, alternative strategies need to be developed and adjusted to the specifics of the field. A particularly suitable approach to produce nanocomposites with unique level of control over their structural and compositional parameters is their bottom-up engineering from solution-processed nanoparticles. In this work, we review the state-of-the-art of this technology applied to the thermoelectric field, including the synthesis of nanoparticles of suitable materials with precisely engineered composition and surface chemistry, their combination and consolidation into nanostructured materials, the strategies to electronically dope such materials and the attempts to fabricate thermoelectric devices using nanoparticle-based nanopowders and inks.

Sn- and Bi-doped Cu3SbSe4-based thermoelectric devices.