High Thermoelectric Performance in Crystallographically Textured n -type Bi 2 Te 3-x Se x Produced from Asymmetric Colloidal Nanocrystals

In the present work, we demonstrate crystallographically textured nanomaterials with exceptional thermoelectric (TE) figures of merit produced by consolidating disk-shaped Bi 2 Te 3-x Se x 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 Bi 2 Te 3-x Se x nanomaterials showed very high electrical conductivities associated to 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 Ti Bi antisites. Additionally, the presence in between grains of traces of elemental Te, a narrow band gap semiconductor with a work function well below Bi 2 Te 3-x Se x , 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 Bi 2 Te 3-x Se x 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 K 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.

The recovery of the huge amounts of energy wasted in the form of heat in transportation and in all industrial and domestic processes is an extremely appealing pursuit both from economic and environmental points of view. 1,2 This goal requires the use of a distributed network of modular and cost-effective systems that convert heat into electricity. With the steam turbines being not suitable for small scale and portable generation, solid state thermoelectric (TE) devices become a worthwhile alternative. TE devices contain n-type and p-type semiconductors connected thermally in parallel and electrically in series. They can be manufactured with virtually any size, are highly durable and can be adapted to harvest energy from any temperature gradient. 1,3 However, their widespread implementation requires improving their cost-effectiveness by increasing energy conversion efficiency and/or reducing manufacturing costs.
The energy conversion efficiency of a TE device can be analytically expressed in terms of the temperature at the hot and cold sides and a unique material-dependent figure of merit (Z), which is generally expressed in an dimensionless form: ZT = σS 2 T/κ, where T is the absolute temperature, σ the electrical conductivity, S the Seebeck coefficient and κ the thermal conductivity. 2,[4][5][6] Nanostructured materials have been proven successful in meeting the very demanding set of interrelated properties that maximize the TE figure of merit, i.e. high σ and S, and low κ. 4, 7-10 An additional advantage of nanomaterials is their superior mechanical robustness as compared to fragile single crystals. 11,12 While numerous approaches to produce nanomaterials exist, nanocrystal (NC)-based bottomup strategies allow an unparalleled combination of delicate control over material parameters with high production throughputs. 5,[13][14][15][16][17] In this regard, NC-based approaches offer clear advantages over currently more conventional nanomaterial fabrication methods, which lack sufficient control over material parameters, e.g. ball milling, 18 and/or are too costly for an eventual large volume industrial manufacturing of cost-effective TE modules, e.g. vacuum-based thin film technologies. 19 Bi2Te3-based alloys are the most widely used TE materials for operation at ambient temperatures, being commercialized for refrigeration and used in radioisotope TE generation systems since several decades ago. 8,20 In recent years, great progress has been made in improving the figure of merit of the p-type Bi2Te3-based alloy, 3,18,[21][22][23][24][25] reaching record ZT values up to 1.96 at 420 K and of 1.77 when averaged over a wide temperature range 320-500 K. 24 On the other hand, the performance of n-type Bi2Te3-based materials, Bi2Te3-xSex, lags substantially behind, with ZT values just above 1. [26][27][28][29][30][31][32][33][34] Structurally, Bi2Te3 crystallizes in a rhombohedral crystal phase ( 3̅ space group, Table S1) and can be viewed as a layered material consisting of stacked quintuple layers Te-Bi-Te-Bi-Te along the c-axis bonded together by weak van der Waals interactions ( Figure S1). 35 Such layered structure results in highly anisotropic transport properties. In particular, n-type Bi2Te2.6Se0.4 single crystals are characterized by electrical and thermal conductivities in the ab plane that are ~4.38 times and over 2 times higher than along the c axis, respectively. 36 The strong anisotropy of its transport properties requires producing Bi2Te3-xSex nanomaterials with proper crystallographic texture. Such textured Bi2Te3-xSex samples have been produced using a variety of strategies, 28, 37-41 but without significantly increasing the material figure of merit over un-textured samples, probably due to a lack of the simultaneous optimization of the microstructure, composition and point defects.
In the present work, we demonstrate that using asymmetric colloidal Bi2Te3-xSex NCs with controlled composition (x = 0, 0.2, 0.3, 0.4, 0.5 and 0.7) and a proper consolidation process, highly textured nanomaterials with high relative densities and outstanding TE figures of merit can be produced.
This synthetic protocol, including the purification step, was characterized by material yields above 95 % and it was optimized to produce ca. 4 g of NCs per batch. Figures 1a and S2 show representative SEM micrograph of the disk-shaped Bi2Te3-xSex NCs obtained following this procedure. As measured from SEM images, NCs had average lateral dimensions of 480  100 nm and an average thickness of 35  8 nm ( Figure S3). Within the tested range, no effect of the Se concentration on the NC size was observed.
Excess amounts of Te, nominally 25 mol%, were introduced in the reaction mixture. This Te excess was found in the form of thin Te nanorods segregated from Bi2Te3-xSex nanodisks ( Figures   1 and S2). The segregation of the Te excess was very convenient for the present work, but at the same time somehow surprising. Notice that the reaction of the Te precursor in the exact same conditions as those used to produce Bi2Te3-xSex NCs, but in the absence of the Bi and Se precursors, resulted in the formation of Te nanorods ( Figure S4). Even lower reaction temperatures than those necessary to produce Bi2Te3-xSex NCs sufficed to reduce the Te precursor and form the Te nanorods.
On the other hand, the reaction of the Se precursor alone did not result in the formation of Se particles ( Figure S5). The reaction of a mixture of Se and Te precursors resulted again in Te nanorods, with no Te-Se compound ( Figure S6). We hypothesize that the formation of Bi2Te3-xSex NCs proceeded through the initial nucleation of Te NCs and the posterior incorporation of Bi and Se atoms within the structure, with the concomitant geometric and crystallographic evolution from the initial Te nanorods to the final Bi2Te3-xSex nanodisks. Because the differential energy of Se within the Bi2Te3-xSex lattice and in solution is higher than that of Te, Bi2Te3-xSex NCs with the Bi:Se ration of the precursor mixture were obtained and the excess of Te was found in the form of segregated elemental particles ( Figure S7). Bi2Te3-xSex NCs were purified by multiple precipitation and redispersion steps using several solvents to remove the different impurities, as detailed in the experimental section. Subsequently, they were dried and, to remove any remaining trace of surfactant, they were annealed at 350 °C for 60 min under argon flow inside a tube furnace ( Figure S8). The annealed Bi2Te3-xSex NCs conserved the original disk shape ( Figure S9), but during annealing their edges became smoother and the lateral size and thickness slightly increased. The size of the Te nanorods particularly increased as observed in Figures 1c and S9.
SEM-EDX analysis showed the NC composition, particularly the Bi:Se ratios, to precisely follow the nominal values (Table 1 and Figure S10), both before and after the annealing treatment. XRD analyses of the initial NCs and the annealed NCs showed no secondary phases (Figures 1b, 1d and S11), not even elemental Te due to its relatively low amount. XRD peaks linearly shifted with the Se concentration to higher diffraction angles, as it corresponds to a lattice contraction due to the substitution of Te by Se atoms ( Figure S12). The following of a Vegard's law evidenced the presence of increasing amounts of Se within the alloy crystal lattice.

Consolidation of Bi2Te3-xSex NCs
Annealed Bi2Te3-xSex NCs were hot-pressed within an inert atmosphere into cylindrical pellets (see experimental section for details). As measured by the Archimedes' method, the relative densities of the produced materials were ca. 93 %, as calculated from a linear regression with the Se content of the theoretical values for Bi2Te3 and Bi2Se3 (Table S2).  showed the grains to partially preserve the precursor disk-like geometry, but to have grown considerably in the lateral direction during the hot-pressing.  The presence of an excess of Te during the hot press step, which was in liquid state, was essential to produce highly textured nanomaterials ( Figure S13).
Under a uniaxial pressure, the minimization of both the system entropy and the energy related to the stress accommodation drives the crystals to rotate to have the c direction, the one with the lowest elastic modulus, along the pressure axis. Liquid Te aided this rotation by playing a lubricant role.
Besides NC rotation, another mechanism of crystallographic alignment is the NCs dissolution and their recrystallization with the proper crystallographic orientation. Indeed, Bi has a relatively high solubility within liquid Te at the hot pressing temperature (480 ºC), which enabled the NC dissolution and reprecipitation mechanism. 45 Dissolution takes place preferentially at the highest energy facets, i.e. those normal to the ab plane, and is aided in the pressing direction by the created 10 μm a b c stress. 46,47 Even without stress, Bi2Te3-xSex crystals naturally grow forming plates that extend along the ab crystal plane. The presence of a uniaxial pressure favors growth in the direction normal to the pressure axis. At the same time, to minimize energy under the uniaxial pressure, reprecipitating crystals align with their minimum elastic modulus, i.e. the c direction, along the direction of the stress. Thus, thin and large plates with the c direction along the pressure axis were produced.
Overall, both mechanisms might have taken place simultaneously. Disk-shaped NCs initially lying close to normal to the pressure axis might slightly rotate and grow in the ab directions from ions coming, through the liquid tellurium, from the dissolution of NCs lying mostly parallel to the pressure axis ( Figure 3c).
While the presence of liquid Te was essential to produce highly textured nanomaterials, close to stoichiometric cation:anion ratios are required to achieve optimum TE properties. Therefore, the removal of the relatively large excess of Te used during the consolidation process was critical.
Conveniently, during the hot-press process, the excess amount of Te was expelled through the dieplunger interface, as observed by careful analysis of the plunger insertion region of the die ( Figure   S14). Nevertheless, the complete removal of Te required cycling up and down the pressure several times during the 210 s that the material was at 480 °C (Table 1). This pressure cycling also aided texturization, i.e. an improved crystal texture was obtained in pellets produced with a cycled pressure as compared to those hot-pressed at constant pressure. We hypothesize that the compression and decompression of the pellet during each cycle, extending ca. 1 m, could aid rotation and reorientation of the NCs. Additionally, the texture improvement with cycling might in part be associated with the more efficient removal of the Te excess. When cooling down, all the liquid material randomly crystallizes, reducing texture. Thus, an efficient removal of Te during consolidation resulted in no material randomly crystallizing when cooling down at the end of the hot pressing process.
Within its experimental error, EDX analysis of the consolidated materials showed them to have close to stoichiometric cation:anion ratios (Table 1) with Te:Se ratios closely following the nominal compositions (once removed the Te excess). However, extensive STEM-EELS analysis of a thinned cross section of the pellet demonstrated the presence of abnormally large amounts of Te at a significant number of crystal interfaces (Figures 4b and S15).

Thermoelectric characterization
From the consolidated cylinders, rectangular bars were cut in two directions, along the press axis, i.e. the cylinder length, and within the plane normal to this axis, i.e. the cylinder disk plane ( Figure   5).   the ab plane, and its normal, i.e. c direction ( Figure S16).
All Bi2Te3-xSex pellets displayed a degenerated semiconductor behavior, with σ monotonically decreasing with temperature in all the temperature range measured (Figures 6a and 6b). σ decreased with the amount of Se (x, Figure 8a). Generally, Te-rich Bi2Te3 has an n-type character because σ is dominated by positively charged antisite defects, TeBi. 48 Such defects are abundant due to the similar ionic radius and electronegativity of Te and Bi, providing charge carrier concentrations in the range 10 18 -10 20 cm -3 . 48 The decrease of the σ of Bi2Te3-xSex pellets with Se was related to a reduction of the concentration of the dominant n-type point defect, TeBi. This reduction was associated with the smaller radius and higher electronegativity of Se compared with Te and Bi, which made the formation of SeBi antisites much less energetically favorable than TeBi.
Actually, the dominant point defects in n-type Bi2Se3 are not SeBi antisites, but positively charged Se vacancies, which are not favored in an ambient with an excess of chalcogen as the one used to process our nanomaterials.
As expected, all samples exhibited much higher σ in the cylinder plane (σ⊥) than in the pressing direction (σ//), e. g. σ⊥/σ// = 2.3 for Bi2Te2.7Se0.3 at 320 K, which was related to the higher charge carrier mobility in the ab crystal plane than in the c direction (Figures 6a and 6b). At first view surprisingly, σ was higher for nanomaterials than commercial samples (Figure 9a) was consistent with the low anisotropy of this parameter (Figure 9b). 36 Absolute S values of the nanomaterials were significantly lower than those of commercial ingots, consistently with their higher charge carrier concentrations. These higher charge carrier concentrations also explained the lower absolute S decrease due to the bipolar contribution observed in the high temperature region measured for the nanomaterial when compared with the commercial ingot.
For nanomaterials, PFs calculated in the pressing direction were lower than those calculated in its normal (Figures 6e, 6f and 8c). In the normal direction, PFs up to 2.86 Wm -1 K -2 at 350 K were reached. The highest PFs measured in the pressing direction were obtained for Bi2Te2.7Se0.3 and Bi2Te2.8Se0.2, and in the cylinder plane for Bi2Te3, Bi2Te2.8Se0.2 and Bi2Te2.7Se0.3. Compared with commercial ingots (Figure 9c), PFs of nanomaterials were lower than those measured in the cleavage plane of commercial ingots, due to the higher S of the later, but higher than in the direction normal to the cleavage plane, due to the low σ of the commercial sample in this direction.
As expected from the microstructure of the nanomaterial pellets, κ in the pressing direction was much lower than in the cylinder plane for all samples, e.g. κ⊥/κ⁄⁄ = 2.5 for Bi2Te2.7Se0.3 at 320 K ( Figure 9d). In the low temperature range, κ decreased with temperature to later slightly rise with the emergence of the bipolar contribution. This bipolar contribution strongly decreased with the amount of Se introduced due to the band gap increase, as observed in Figures 7c and 7d. κ also decreased with Se due to the higher lattice disorder (Figure 8d).
After having subtracted the electronic contribution κe, κ decreased with temperature in the low temperature range and stabilized or increased at higher temperatures, when the bipolar contribution, κbi, became significant ( Figure 9e). Above the Debye temperature, ca. 145 K for Bi2Te3, 49 phonon scattering by Umklapp mechanism leads to a T −1 dependence of the lattice thermal conductivity, κL. On the other hand, κbi displays an exponential dependence with temperature and the material band gap, Eg: 50 To estimate κbi, we linearly fitted (κ-κe) vs. T −1 in the low temperature range and extrapolated data to higher temperatures ( Figure S17).
At room temperature, compared with commercial ingots, with κL ~ 1. In Bi2Te3-xSex nanomaterials the κbi contribution was substantially lower than in commercial ingots and started to be appreciated at higher temperatures, ~ 400 K (Figures 9e and S17). This   Overall, among the different compositions tested, Bi2Te2.7Se0.3 displayed the highest ZT values (Figures 7e, 7f and 9f). ZT values in the pressing direction, i.e. the direction of preferential c crystallographic orientation, were systematically higher than in its normal directions, the ab plane, due to the predominance of  suppression in c over σ gain in ab (Figures 7a and 7b). Averaging the data measured from 5 pellets with very similar composition, Bi2Te2.7Se0.3, the highest ZT value, ZT = 1.31, was reached at 438 K in the pressing direction ( Figures S18 and S19). Even more important, due to the lower bipolar contributions, very high ZT values were sustained in an extended temperature range. The average ZT for nanocrystalline Bi2Te2.7Se0.3 in the pressing direction, calculated over 180 K, from 320 K to 500 K, was ZTave = 1.15 ( Figure S19b), which represents a ca. 50 % increase over the best average value calculated for the commercial ingot, ZTave = 0.77. Additionally, nanocrystalline Bi2Te3-xSex also showed good stability when maintained at relatively high temperature, 420 K, for long periods of time (﹥60 h, Figure S20) or when cycling it between the room temperate and 500 K ( Figure S21).

CONCLUSIONS
In this work, n-type Bi2Te3-xSex NCs with disk-like asymmetric shapes and tuned composition (x = 0, 0.2, 0.3, 0.4, 0.5 and 0.7) were produced in solution. Excess amounts of Te were incorporated in the form of Te nanorods at the NC synthesis step. Such NCs were used to produce high density Bi2Te3-xSex bulk nanomaterials with marked crystallographic texture through hotpressing them at a temperature above Te melting point. The TE properties of NC-based bulk nanomaterials were measured in two perpendicular directions, along the pressure axis, i.e. the direction of preferential c crystallographic orientation, and normal to it, with preferential ab crystal plane. Nanomaterials were characterized by higher electrical conductivities but lower Seebeck coefficients than commercial materials, which was associated to the higher charge carrier concentrations of the former due to their processing in an excess of Te. Nanomaterials also showed notably lower bipolar contributions, which was in most part also related to the higher charge carrier concentrations, but which could be also influenced by a preferential scattering of minority charge carriers at Te-rich grain boundaries. Record TE figures of merit, ZT ~ 1.31 at 438 K were measured from the Bi2Te2.7Se0.3 nanomaterial in the pressing direction. In addition, the high ZT values extended over a larger temperature range, from 320 K to 500 K, resulting in ZT average values as high as ZTave = 1.15.
Analytical grade acetone and ethanol were obtained from various sources. All chemicals were used as received without further purification. Syntheses were carried out using a standard vacuum/dry argon Schlenk line.

Synthesis of Bi2Te3-xSex NCs with Te excess:
Bi2Te3-xSex NCs with Te excess were synthesized following a modified polyol process using EG as a high boiling point solvent and reducing agent. 23,[31][32][33][42][43][44] In a typical synthesis of 5 mmol of Bi2Te3-xSex (x = 0, 0. mmol of Na2TeO3, 50 mmol of KOH and 0.5 g of PVP were dissolved in 200 ml of EG within a 500 ml three-neck flask by stirring under argon atmosphere at room temperature for 20 min. Then the solution was heated to 185 ºC. During this process, above 150 ºC, the color of the solution gradually changed from slightly brown to black. Finally, the reaction mixture was kept at 185 °C for 3 h. After this time, the solution was naturally cooled to room temperature by removing the heating mantle. NCs were purified in a multi-step process using various solvents to remove the different impurities. Notice that a proper NC purification was essential to produce nanomaterials with optimized TE properties. In a first step, the solid product was collected by adding acetone to the solution and centrifuging it. In a second step, ethanol was used to redisperse the NCs and acetone (Vethanol/Vacetone = 1:2) to precipitate them again. In a third step, deionized water was added to solubilize remaining impurities and NCs were precipitated by slow centrifugation. This 3 rd step was repeated twice. In a last step, NCs were redispersed with ethanol and precipitated with acetone, like in step two. Finally, NCs were dried under vacuum at room temperature. This synthesis protocol was optimized to produce almost 4 g of NCs per batch, which was the amount required for a complete characterization of their TE properties at the laboratory scale. using a custom-made hot press. This process was carried out within a glove box having an argon atmosphere. The hot press temperature was set at 480 ºC and pressure was increased up to 70 MPa and released for 5 times during the 210 s that the material was kept at 480 ºC. This pressure cycling was found fundamental to remove the excess of tellurium from the sample and to reach proper crystal texture. When materials with an excess of tellurium were hot pressed in these conditions, the relative densities of the compacted pellets were measured by the Archimedes' method and found to be around 93 % of the theoretical value. From these cylinders, rectangular bars of about 8 × 6 × 1 mm 3 were cut in two normal directions, along the pressing direction and within the cylinder plane. As a reference material, we characterized a commercial n-type ingot obtained from Huabei Cooling Device Co., Ltd, China. Within the EDX experimental error (ca. 3 %), the composition of the commercial ingot was Bi2Te2.68Se0.27. Using the Archimedes' method, we measured a density of this ingot to be ~7.7 g/cm 3 .
Structural and chemical characterization: X-ray diffraction (XRD, 2θ angle: 20° to 80°; scanning rate: 5 °/min) analyses were carried out on a Bruker AXS D8 ADVANCE X-ray diffractometer with Cu-Kα radiation (λ = 1.5406 Å). Size and morphology of initial NCs were examined by transmission electron microscopy (TEM) using a ZEISS LIBRA 120, operating at 120 kV, and field-emission scanning electron microscopy (SEM) on an Auriga Zeiss operated at 5.0 kV. SEM micrographs were used to obtain an estimate of the NC average size. 100 NCs were measured for this purpose in each sample. Crystallographic structure and chemical composition were analyzed by HRTEM and Electron Energy Loss Spectroscopy (EELS), respectively, using a Tecnai F20 field-emission gun microscope at 200 keV with an embedded Gatan QUANTUM image filter. The material composition was analyzed using an Oxford energy dispersive X-ray spectrometer (EDX) attached to a Zeiss Auriga SEM at 20.0 kV. Thermoelectric property measurement: Seebeck coefficients were measured by using a static DC method. Electrical resistivity data was obtained by a standard four-probe method. Both the Seebeck coefficient and the electrical resistivity were measured simultaneously in a LSR-3 LINSEIS system in the temperature range between room temperature and 498 K, under helium atmosphere. At each temperature, 3 measurements were taken. Taking into account the system accuracy and the measurement precision, an error of ca. 4 % in the measurement of the electrical conductivity and Seebeck coefficient was estimated. The thermal conductivity was calculated by κ = λCpρ, where λ is the thermal diffusivity, Cp is the heat capacity, and ρ is the mass density of the specimen. A XFA 600 Xenon Flash Apparatus was used to determine the thermal diffusivities (λ) of the samples with an estimated error of ca. 5 %. The constant pressure heat capacity (Cp) was estimated from empirical formulas by the Dulong-Petit limit (3R law), and the density (ρ) values used here were calculated using the Archimedes' method. To avoid cluttering the plots, error bars were not