This is the submitted version of the journal article: Zhang, Yu; Xing, Congcong; Liu, Yu; [et al.]. «Influence of copper telluride nanodomains on the transport properties of n-type bismuth telluride». Chemical engineering journal, Vol. 418 (Aug. 2021), art. 129374. DOI 10.1016/j.cej.2021.129374 This version is available at https://ddd.uab.cat/record/271949 under the terms of the license 0DQXVFULSW &OLFNKHUHWRYLHZOLQNHG5HIHUHQFHV Influence of copper telluride nanodomains on the transport properties of n-type 1 2 bismuth telluride 3 4 5 Yu Zhang,a† Congcong Xing,a† Yu Liu,a,b* Mengyao Li,a Ke Xiao,a Seungho Lee, b Xu Han,c 6 7 Ahmad Ostovari Moghaddam,d Jordi Arbiol,c,e Maria Ibáñez,b Kai Pan,f Ying Xie,f* Andreu 8 9 Cabota,e* 10 11 a 12 Catalonia Energy Research Institute - IREC, Sant Adrià de Besòs, 08930 Barcelona, Spain. 13 14 b IST Austria, Am Campus 1, 3400 Klosterneuburg, Austria. 15 16 c 17 Catalan Institute of Nanoscience and Nanotechnology (ICN2), CSIC and BIST, Campus UAB, 18 19 Bellaterra, 08193 Barcelona, Catalonia, Spain. 20 21 22 d South Ural State University, 76, Lenin Ave., Chelyabinsk 454080, Russia 23 e 24 ICREA, Pg. Lluis Companys 23, 08010 Barcelona, Catalonia, Spain. 25 26 f Key Laboratory of Functional Inorganic Material Chemistry, Ministry of Education, 27 28 29 Heilongjiang University, 150080, Harbin, PR China 30 31 † These authors contributed equally to this work 32 33 34 *Corresponding author E-mail: yu.liu@ist.ac.at; xieying@hlju.edu.cn acabot@irec.cat. 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 60 61 62 1 63 64 65 Abstract: The high processing cost, poor mechanical properties and moderate performance of 1 2 Bi2Te3–based alloys used in thermoelectric devices limit the cost-effectiveness of this energy 3 4 conversion technology. Towards solving these current challenges, in the present work, we detail 5 6 7 a low temperature solution-based approach to produce Bi2Te3-Cu2-xTe nanocomposites with 8 9 improved thermoelectric performance. Our approach consists in combining proper ratios of 10 11 colloidal nanoparticles and to consolidate the resulting mixture into nanocomposites using a hot 12 13 14 press. The transport properties of the nanocomposites are characterized and compared with 15 16 those of pure Bi2Te3 nanomaterials obtained following the same procedure. In contrast with 17 18 19 most previous works, the presence of Cu2-xTe nanodomains does not result in a significant 20 21 reduction of the lattice thermal conductivity of the reference Bi2Te3 nanomaterial, which is 22 23 24 already very low. However, the introduction of Cu2-xTe yields a nearly threefold increase of the 25 26 power factor associated to a simultaneous increase of the Seebeck coefficient and electrical 27 28 29 conductivity at temperatures above 400 K. Taking into account the band alignment of the two 30 31 materials, we rationalize this increase by considering that Cu2-xTe nanostructures, with a 32 33 relatively low electron affinity, are able to inject electrons into Bi2Te3, enhancing in this way its 34 35 36 electrical conductivity. The simultaneous increase of the Seebeck coefficient is related to the 37 38 energy filtering of charge carriers at energy barriers within Bi2Te3 domains associated with the 39 40 41 accumulation of electrons in regions nearby a Cu2-xTe/Bi2Te3 heterojunction. Overall, with the 42 43 incorporation of a proper amount of Cu2-xTe nanoparticles, we demonstrate a 250% 44 45 46 improvement of the thermoelectric figure of merit of Bi2Te3. 47 48 49 Keywords: Bi2Te3, nanocomposites, energy filtering effect, Cu2-xTe, thermoelectricity. 50 51 52 53 54 55 56 57 58 59 60 61 62 2 63 64 65 1. Introduction 1 2 Thermoelectric (TE) devices, able to convert temperature gradients into electricity and 3 4 5 vice versa, have a plethora of potential applications, especially around ambient temperature. 6 7 However, their use is frequently limited by insufficient cost-effectiveness, related both to a 8 9 10 moderate efficiency and a relatively high cost. The efficiency of TE devices is determined by 11 12 three interdependent material parameters: electrical conductivity (σ), Seebeck coefficient (S), 13 14 and thermal conductivity (κ), which are combined in a dimensionless figure of merit 15 16 17 ZT=σS2Tκ−1[1-4]. On the other hand, the elevated cost of current TE devices, optimized toward 18 19 cooling applications, is related to the large amount of highly crystalline Bi-Sb-Te-Se alloys used 20 21 22 to manufacture them. To cut costs, devices aimed at harvesting small temperature gradients 23 24 could strongly reduce the amount of TE material used. Besides, the use of high crystallinity 25 26 27 materials is not only unnecessary but even detrimental to the device performance as crystalline 28 29 materials have associated higher thermal conductivities and poorer mechanical properties than 30 31 their polycrystalline counterparts. 32 33 34 Bottom-up strategies based on solution-processed building blocks offer additional 35 36 advantages in both directions. Properly designed and engineered nanocomposites allow to 37 38 39 partially decouple TE properties, adjusting the charge carrier concentration without 40 41 significantly reducing the mobility by modulation doping, increasing the Seebeck coefficient 42 43 44 through energy filtering and quantum confinement, and reducing thermal conductivity through 45 46 extensive phonon scattering[5-19]. At the same time, because bottom-up approaches do not 47 48 49 require the large scale organization of atoms into macroscopic crystal domains, the energy usage, 50 51 processing time and thus the overall production cost can be strongly reduced. As a bonus 52 53 advantage, beyond providing improved TE properties at a potentially lower cost, nanomaterials 54 55 56 allow improving mechanical performances, especially for layered materials such as Bi2Te3, 57 58 which are particularly fragile toward fracture along van der Waals planes. 59 60 61 62 3 63 64 65 In the last decade, several strategies have been developed to further improve the TE 1 2 properties of Bi2Te3-based alloys[20-30]. One successful approach to increasing the power 3 4 factor (σS2) and at the same time reduce the thermal conductivity has been the incorporation of 5 6 7 doping elements, such as Cu. Besides promoting phonon scattering, the incorporation of Cu 8 9 allows optimizing the charge carrier concentration by introducing additional carriers. These 10 11 extra carriers have been shown to significantly increase electrical conductivity while 12 13 14 moderately decreasing Seebeck coefficients, overall translating into higher power factors and 15 16 figures of merit.[31-39] Cu ions electronically dope Bi2Te3 in different ways, depending on the 17 18 19 conductivity type of the Bi2Te3-based alloy and the Cu concentration and location. When Cu+ 20 21 ions intercalate at the van der Waals gap between Bi2Te3 layers, they act as donors.[36, 39] In 22 23 24 contrast, Cu+ ions substituting Bi act as acceptors[36, 38]. Moreover, the presence of Cu may 25 26 modify the concentration of electroactive defects within Bi2Te3-based alloys, altering the 27 28 29 amount of Te vacancies and Bi and Te antisites, for instance.[37] Besides, amounts of Cu above 30 31 the solubility limit lead to the segregation of Cu2-xTe phases. This segregation reduces the 32 33 amount of Te within the alloy and consequently increases the amount of Te vacancies and/or 34 35 36 reduces the concentration of Te antisites, both acting as electron donors[36-38]. Additionally, 37 38 the presence of Cu2-xTe domains within the Bi2Te3 matrix can electronically affect the material 39 40 41 in ways that remain largely unexplored. Cu2-xTe is generally characterized by a relatively low 42 43 work function[40] and a degenerated p-type conductivity associated with its Cu deficiency.[10, 44 45 46 41-44] Thus, in Bi2Te3/Cu2-xTe heterostructures, Cu2-xTe is most probably able to inject charge 47 48 carriers into Bi2Te3[45]. 49 50 In this work, we produce Bi2Te3-Cu2-xTe nanocomposites with controlled phase ratio by 51 52 53 mixing proper amounts of Bi2Te3 and Cu2-xTe colloidal nanoparticles. We characterize their 54 55 structural and chemical properties and discuss their TE performance taking into account the 56 57 58 heterojunctions formed between the two materials. 59 60 61 62 4 63 64 65 2. Materials and methods 1 2 2.1 Chemicals and solvents 3 4 Oleylamine (OAm, technical grade 70%), oleic acid (OAc, technical grade 90%), bismuth(III) 5 6 7 chloride (BiCl3, ≥ 98%), copper(II) acetate monohydrate (Cu(AC)2, (CH3COO)2Cu·H2O, 8 9 ≥ 98%), tellurium dioxide (TeO2, ≥99%), potassium hydroxide (KOH, ≥ 98%), hydrazine 10 11 12 monohydrate (NH2NH2˜H2O, 64~65%) and polyvinylpyrrolidone (PVP, (C6H9NO)n, average 13 14 molecular weight ~55,000) were purchased from Sigma Aldrich. Tri-n-octylphosphine (TOP, 15 16 97%) was purchased from Strem. Ethylene glycol (EG, HOCH2CH2OH, 99%) was purchased 17 18 19 from Fisher. Analytical grade ethanol, isopropanol and acetone were obtained from various 20 21 sources. All chemicals were used as received without further purification. Syntheses were 22 23 24 carried out using a standard vacuum/dry argon Schlenk line. 25 26 27 2.2 Synthesis of bismuth telluride nanowires 28 29 Bi2Te3 nanowires were synthesized following a previously reported protocol.[46] In a typical 30 31 32 synthesis, a mixture of 5.6 mmol of TeO2, 56 mL of EG, 37 mmol of KOH, and 1.5 g of PVP 33 34 were added into a three-neck flask and heated to 140 °C to completely dissolve them. 35 36 37 Afterwards, 0.87 mL hydrazine monohydrate was injected to reduce Te and the solution was 38 39 maintained at 140 °C for 1 h. These initial steps resulted in the formation of Te nanowires as 40 41 described previously.[13, 45, 46] Then the temperature was raised to 160 °C and a hot solution 42 43 44 of BiCl3/EG was injected into the flask. The hot BiCl3/EG solution was prepared by adding 3.6 45 46 mmol BiCl3 into 15 mL EG in a glass vial that was heated to 100−120 °C before injection. The 47 48 49 BiCl3 solution was allowed to react with the tellurium nanowires for 1 h at 160 °C. After that, 50 51 the mixture was cooled down naturally to room temperature. The products were firstly purified 52 53 54 by adding acetone and centrifuging. Then ethanol was used to redisperse the particles and 55 56 acetone was used as antisolvent to precipitate them. In the next step, deionized water was 57 58 introduced to dissolve remaining impurities and products were precipitated by slow 59 60 61 62 5 63 64 65 centrifugation. This procedure was repeated twice. In the last step, particles were dispersed in 1 2 ethanol and precipitated with acetone. The final products were dried under vacuum and stored 3 4 in an Ar-filled glovebox until posterior use. 5 6 7 2.3 Synthesis of copper telluride nanoparticles 8 9 10 A tellurium precursor solution (TOPTe) was prepared in an argon-filled glovebox one week 11 12 before synthesis: 6.38 g tellurium powder (0.05 mol) was dissolved in 100 mL TOP to form the 13 14 0.5 M TOPTe which had a clear yellow color. In a typical synthesis of Cu2-xTe colloidal 15 16 17 nanoparticles, 0.3 mmol of Cu(AC)2 was mixed with 6 ml of OAm in a 25 mL three-neck flask. 18 19 The mixture was kept under vacuum for 20 min at room temperature, then heated to 120 ºC and 20 21 22 maintained at this temperature for 60 min still under vacuum to obtain a clear solution and 23 24 remove low boiling point impurities. Then the temperature was increased to 220 ºC under Ar. 25 26 27 After ~10 min, the initial bright yellow solution became clear light brown. At this point, 1.5 28 29 mL of 0.5 M TOPTe was injected and the light brown solution immediately changed color to 30 31 deep green. Upon injection, the temperature of the reaction mixture dropped to ~210 ºC. The 32 33 34 mixture was allowed to recover the 220 ºC and maintained at this point for 1 h. Afterwards, the 35 36 colloidal solution was rapidly cooled to room temperature with a water bath at an initial 37 38 39 approximate rate of ~80 ºC/min. During the cooling down process, when the solution reached 40 41 ~70 ºC, 2 mL of OAc were added to replace the weakly bound OAm molecules. Finally, 5 mL 42 43 44 of chloroform were added to the crude solution and the mixture was sonicated for several 45 46 minutes. The final deep green product was precipitated by centrifugation at 8000 rpm for 5 min. 47 48 49 Then it was redispersed in chloroform and precipitated one more time by centrifugation in the 50 51 presence of isopropanol. Finally, Cu2-xTe nanoparticles were re-dispersed in chloroform and 52 53 kept in an Ar-filled glovebox until their posterior use. 54 55 56 2.4 Nanomaterial consolidation 57 58 59 Bi2Te3 and Cu2-xTe nanoparticles were mixed at the proper ratio in hexane solution. The mixture 60 61 62 6 63 64 65 was sonicated for 1 h to obtain a homogeneous dispersion. Then the nanoparticles were dried 1 2 out from the solvent. The obtained nanopowder was introduced inside a tube furnace and 3 4 annealed at 450 °C for 1 h under an Ar flow. The annealed material was loaded into a graphite 5 6 7 die and uniaxially hot-pressed into cylinders (Ø 10 mm×10 mm) using a custom-made hot- 8 9 press. This process was carried out within an argon-filled glove box. The hot press temperature 10 11 was set at 290 ºC and the pressure was increased up to 50 MPa for 5 min. Relative densities of 12 13 14 the consolidated pellets were measured by the Archimedes’ method. From these cylinders, 15 16 rectangular bars of about 6 × 6 × 1 mm3 were cut in two normal directions, along and normal 17 18 19 to the pressure axis. 20 21 22 2.5 Structural and chemical characterization 23 24 The morphology and size of the nanoparticles were characterized by transmission electron 25 26 27 microscopy (TEM) using a Zeiss Libra 120, operating at 120 kV, and field-emission scanning 28 29 electron microscopy (SEM) on an Auriga Zeiss operated at 5.0 kV. X-ray diffraction (XRD, 2θ 30 31 angle: 20° to 60°; scanning rate: 2°/min) analyses were carried out on a Bruker AXS D8 32 33 34 Advance X-ray diffractometer with Cu–Kα radiation (λ = 1.5406 Å). Chemical composition 35 36 and crystallographic structure were analyzed by high-resolution TEM (HRTEM) and electron 37 38 39 energy loss spectroscopy (EELS) using a Tecnai F20 field-emission gun microscope at 200 keV 40 41 embedded with a Gatan Quantum image filter. Additionally, the nanoparticles composition was 42 43 44 analyzed using an Oxford energy dispersive X-ray spectrometer (EDX) combined with a Zeiss 45 46 Auriga SEM at 20.0 kV. 47 48 49 2.6 Thermoelectric property measurement 50 51 52 Electrical resistivity data was obtained using a standard four-probe method. Seebeck 53 54 coefficients were determined using a static DC method. Electrical resistivity and Seebeck 55 56 coefficients were measured simultaneously in a LSR-3 Linseis system under a helium 57 58 59 atmosphere, in the temperature range between room temperature and 573 K. 3 measurements 60 61 62 7 63 64 65 were taken at each temperature in each temperature cycle. Considering the system accuracy and 1 2 the measurement precision, an error of ca. 5 % in the measurement of the electrical conductivity 3 4 and Seebeck coefficients was estimated. The thermal conductivity was calculated according to 5 6 7 κ = λCpρ, where κ is the total thermal conductivity, λ is the thermal diffusivity, Cp is heat 8 9 capacity, and ρ is the measured density of each specimen. An XFA 600 xenon flash apparatus 10 11 was applied to measure the thermal diffusivities (λ) of the samples with an estimated error of 12 13 14 ca. 5 %. Cp was calculated from empirical formulas by the Dulong–Petit limit (3R law), and 15 16 the density (ρ) values used here were measured using the Archimedes’ method. Hall charge 17 18 19 carrier concentrations (nH) and mobilities (μH) at room temperature (300 K) and high 20 21 temperature (300-600 K) were obtained from the Van der Pauw method using a magnetic field 22 23 24 of 1 T (ezHEMS, NanoMagnetics). Values provided correspond to the average of 7 25 26 measurements, from which an error of ca. 10% was estimated. 27 28 29 3. Result and discussion 30 31 32 Bi2Te3 was prepared using a previously reported two-step process based on the reaction of 33 34 Bi with Te nanowires (see details in the experimental section)[46]. The obtained particles 35 36 37 displayed a uniform nanowire geometry with a narrow distribution of diameters (30±10 nm) 38 39 and lengths (600 ± 80 nm) as observed by TEM (Figure 1a, b). XRD analysis showed the Bi2Te3 40 41 42 nanowires to be highly crystalline with the Bi2Te3 rhombohedral phase (JCPDS NO. 01-082- 43 44 0358, Figure 1d). HRTEM characterization confirmed the Bi2Te3 rhombohedral phase (space 45 46 47 group = R-3M) with a=b= 4.2999 Å and c= 30.5975 Å. EELS elemental composition maps 48 49 revealed the homogeneous composition of the nanowires (Figure 1c). 50 51 52 Cu2-xTe nanocrystals were prepared by reacting a solution of Cu(Ac)2 with TOPTe at 220 53 54 ºC in OAm (see materials and methods section). Figure 1e shows a representative TEM 55 56 micrograph of the as-synthesized Cu2-xTe nanoparticles that displayed a cubic/rectangular 57 58 59 morphology with a relatively narrow size distribution. HRTEM characterization allowed 60 61 62 8 63 64 65 assigning the crystal structure of the nanoparticles to a Cu1.5Te cubic phase (space group = Pm- 1 2 3n) with a=b=c=7.5100 Å (Figure 1f), consistently with previous reports.[47, 48]. The reference 3 4 XRD pattern simulated from the cubic Cu1.5Te crystal phase determined by HRTEM fitted well 5 6 7 the experimental results obtained from the XRD characterization of the material (Figure 1d). 8 9 EELS elemental composition maps showed a homogeneous distribution of the two elements 10 11 within the Cu2-xTe nanoparticles (Figure 1g). 12 13 14 15 a) b) c) 16 17 18 19 20 21 22 23 Bi2Te3 R3-MH 24 25 d) e) g) 26 Bi2Te3-Cu2-xTe 27 28 Intensity (a.u.) 29 Cu2-xTe 30 31 200 nm 32 33 Bi2Te3 f) 34 35 Bi2Te3 36 Cu1.5Te 37 38 20 40 60 39 40 2T (Degree) [100] Cu1.5Te Pm-3n 41 42 Figure 1. (a) Representative high-angle annular dark-field scanning transmission electron microscopy 43 44 (HAADF-STEM) and bright-field TEM micrographs of Bi2Te3 nanowires. (b) HRTEM micrograph, 45 46 47 detail of the orange squared region and its corresponding power spectrum. (c) STEM micrograph and 48 49 EELS chemical composition maps obtained from the red squared area in the STEM micrograph. Bi N6,7- 50 51 edge at 157 eV (red), Te M4,5-edge at 572 eV (green). (d) Powder XRD pattern of as-synthesized Bi2Te3 52 53 and Cu2-xTe nanoparticles and of a consolidated Bi2Te3-Cu2-xTe nanocomposite. The reference pattern 54 55 56 for Bi2Te3 (JCPDS 01-082-0358) and the simulated XRD pattern of Cu1.5Te are also displayed. (e) 57 58 Representative TEM image of Cu2-xTe nanocubes. (f) HRTEM micrograph, detail of the yellow squared 59 60 region and its corresponding power spectrum. The Cu1.5Te lattice fringe distances were measured to be 61 62 9 63 64 65 0.708 nm, 0.528 nm and 0.731 nm, at 45.68º and 86.43º, which was interpreted as the Cu1.5Te cubic 1 phase, visualized along its [100] zone axis. (g) STEM micrograph and EELS chemical composition 2 3 4 maps obtained from the red squared area in the STEM micrograph. Cu L2,3-edges at 931 eV (red), Te 5 6 M4,5-edge at 572 eV (green). 7 8 9 10 11 Colloidal Bi2Te3 nanowires and Cu2-xTe nanocrystals were mixed at proper mass ratios (0 12 13 ≤ Bi:Cu ≤ 26) in hexane (Figure 2a). SEM characterization of the dried mixture revealed a 14 15 16 homogeneous dispersion of the two types of particles (Figure 2b). After solvent evaporation, 17 18 the obtained Bi2Te3-Cu2-xTe nanopowder was annealed at 450 °C to remove organic residues. 19 20 21 After this process, particles maintained their initial geometry and crystal phase, and both phases 22 23 remained homogeneously dispersed (Figures S1, 2c). 24 25 Afterwards, the annealed powder was loaded inside a graphite die and hot-pressed into 26 27 28 cylindrical pellets that achieved relative densities in the range 85-89 % (Table S1). XRD 29 30 patterns of the Bi2Te3-Cu2-xTe pellets showed the main peaks of the Bi2Te3 phase and a smaller 31 32 33 XRD peak at 2T = 26.7º that was assigned to the Cu1.5Te phase (Figure 3a). SEM 34 35 characterization showed the consolidated pellet to be formed of elongated structures resembling 36 37 38 the initial Bi2Te3 nanowires, with no obvious grain growth after hot pressing (Figure 2d). EDX 39 40 analysis of the Bi:Cu=21 composite showed a Cu/Bi/Te molar ratio around 1/7.6/11.4, i.e. a 41 42 43 Te/(Bi+Cu) molar ratio close to the stoichiometric 3/2. Similar Te/(Bi+Cu) ratios were obtained 44 45 for Bi:Cu=13 and Bi:Cu=26 samples (Figure S2). Additionally, EDX compositional maps 46 47 48 showed a relatively homogeneous distribution of Cu at a 5 Pm scale (Figure 3b). 49 50 51 52 53 54 55 56 57 58 59 60 61 62 10 63 64 65 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 Figure 2. (a) Schematic illustration of the preparation of Bi2Te3-Cu2-xTe nanocomposites. (b) SEM 27 28 image of the dried mixture of Cu2-xTe and Bi2Te3 nanoparticles. (c) SEM images of annealed Bi2Te3 and 29 30 31 Bi2Te3-Cu2-xTe nanopowders. Scale bars = 200 nm. (d) Schematic illustration of sample processing, 32 33 including optical photographs of the initial colloidal dispersion of the two types of particles, the dried 34 35 powder and the final pellet, and a cross-section SEM image of the hot-pressed material. 36 37 38 a) Bi:Cu=26 b) Bi 39 40 Bi:Cu=21 Intensity (a.u.) 41 42 Bi:Cu=13 43 44 Cu1.5Te Bi:Cu=8 5 μm 5 μm 45 46 Te Cu Bi2Te3 47 48 Bi2Te3 JCPDS NO. 01-082-0358 49 50 51 20 30 40 50 60 5 μm 52 2T (degree) 5 μm 53 54 55 Figure 3. (a) XRD pattern of consolidated Bi2Te3-Cu2-xTe pellets with different compositions. (b) SEM 56 57 micrograph of a Bi2Te3-Cu2-xTe (Bi:Cu=21) pellet and corresponding EDX composition maps for Bi 58 59 (red), Te (orange) and Cu (green). 60 61 62 11 63 64 65 The TE properties of Bi2Te3-Cu2-xTe nanocomposites were characterized along two 1 2 directions: parallel and normal to the pressure axis (Figure S3). Despite the asymmetric 3 4 geometry of the Bi2Te3 building blocks used to produce the composites and the 5 6 7 anisotropic crystal structure of Bi2Te3, similar transport properties were obtained in both 8 9 directions. We associate this experimental result to the random orientation of the crystallites 10 11 within the nanocomposite, i.e. to the lack of crystallographic texture. SEM characterization 12 13 14 confirmed the long-range random distribution of crystal domains (Figure 4c, d). Additional 15 16 XRD characterization of the samples being hold in the two normal directions displayed similar 17 18 19 relative peak intensities (I(0015)/I(110)), further confirming the homogeneous orientation of the 20 21 Bi2Te3 crystal domains (Figure 4a,b). On the grounds of lacking anisotropic transport properties, 22 23 24 in the following, we focus on discussing the results obtained from the direction normal to the 25 26 pressure axis. 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 60 61 62 12 63 64 65 a) Bi2Te3 b) Bi:Cu=21 (0 1 5) (0 1 5) (1 0 10) (1 0 10) 1 (0 1 11) (0 1 11) 2 (1 0 1) (2 0 5) (0 2 10) (2 0 5) (1 0 1) (0 2 10) (2 0 8) 3 4 Intensity (a.u.) Intensity (a.u.) 5 6 (1 1 0) (1 1 0) 7 (0 0 15) (0 0 15) 8 9 10 11 Bi2Te3 JCPDS NO. 01-082-0358 Bi2Te3 JCPDS NO. 01-082-0358 12 13 14 20 30 40 50 60 20 30 40 50 60 15 2T (Degree) 2T (Degree) 16 17 a c) Top view Cross section view 18 19 20 21 22 23 24 5 μm 5 μm 25 26 d) b Top view Cross section view 27 28 29 30 31 32 33 5 μm 5 μm 34 35 36 37 Figure 4. (a,b) XRD patterns of consolidated (a) Bi2Te3 and (b) Bi2Te3-Cu1.5Te (Bi:Cu=21) pellets in 38 39 two normal directions, perpendicular and parallel to the pressure axis, as marked within the graphs. (c,d) 40 41 42 Top-view and cross-section SEM micrographs of consolidated (c) Bi2Te3 and (b) Bi2Te3-Cu1.5Te 43 44 (Bi:Cu=21) pellets. 45 46 47 48 Figure 5 shows the TE properties of a series of Bi2Te3-Cu2-xTe nanocomposites obtained 49 50 51 by combining different amounts of Bi2Te3 and Cu2-xTe particles (Bi:Cu=13, 21 and 26). The 52 53 data obtained from a nanocrystalline Bi2Te3 sample produced by annealing and hot pressing 54 55 56 Bi2Te3 nanowires is also plotted as a reference. The data obtained from a Cu2-xTe pellet 57 58 produced by hot-pressing Cu2-xTe nanoparticles is provided in Figure S4. 59 60 61 62 13 63 64 65 AT room temperature, Bi2Te3 displayed moderate electrical conductivities (a 104 S m-1) 1 2 and Seebeck coefficients (a 130 PV K-1), which slowly increased with temperature (Figure 3 4 5a,b). The incorporation of Cu2-xTe resulted in a slight decrease of the room temperature 5 6 7 electrical conductivity and Seebeck coefficient (Figure 5a,b), which translated into lower power 8 9 factors (Figure 5c). On the other hand, the Cu2-xTe pellet showed much larger electrical 10 11 12 conductivities (0.8 x 106 S m-1) but much lower Seebeck coefficients (0.5 PV K-1, Figure S4). 13 14 Hall measurements showed the room temperature charge carrier concentration to increase 15 16 17 with the presence of Cu2-xTe, from n = 1.4x1019 cm-3 for Bi2Te3 to n = 1.8 x 1019 cm-3 for Bi2Te3- 18 19 Cu1.5Te (Bi:Cu=18, Figure 6). On the other hand, the Hall charge carrier concentration of the 20 21 22 Cu2-xTe pellet was much higher, p = 3.7 x 1021 cm-3. We associate the larger charge carrier 23 24 concentration obtained in Bi2Te3-Cu1.5Te composites with the spillover of charge carriers from 25 26 27 the Cu2-xTe phase to the Bi2Te3 matrix, in line with the much lower electronic affinity of the 28 29 former and its highly degenerated semiconductor character[22, 26]. On the other hand, carrier 30 31 mobilities decreased with the presence of Cu2-xTe (Figure 6b), consistently with a promoted 32 33 34 charge carrier scattering. 35 36 Taking into account the Hall charge carrier concentration and mobilities, the lower 37 38 39 electrical conductivity measured at ambient temperature in Bi2Te3-Cu2-xTe nanocomposites 40 41 compared with bare Bi2Te3 was associated with the lower electron mobilities within the 42 43 44 composites. On the other hand, the lower Seebeck coefficients measured at ambient temperature 45 46 were consistent with the larger charge carrier concentrations measured in the nanocomposites, 47 48 as observed with a Pisarenko plot using a single parabolic band model and an effective mass 49 50 51 m*=0.40 m0 (Figure 6d). 52 53 54 55 56 57 58 59 60 61 62 14 63 64 65 1 2 3 4 5 6 7 8 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 Figure 5. Temperature dependence of TE properties of Bi2Te3-Cu2-xTe pellets: (a) electrical conductivity, 40 41 σ; (b) Seebeck coefficient, S; (c) power factor or S2σ; (d) total thermal conductivity, κtotal; (e) lattice 42 43 thermal conductivity, κL; (f) TE figure of merit, ZT. 44 45 46 47 48 All nanocomposites were characterized by strong dependences of the electrical 49 50 conductivity and Seebeck coefficient with temperature. This strong temperature dependence 51 52 53 was ascribed to the important role played by energy barriers on the charge transport properties, 54 55 i.e. charge carriers strongly relied on an activation energy to move through nanocomposites. 56 57 58 This activation energy was estimated at 130 meV from a linear fit of Ln(VT1/2) with (kT)-1. As 59 60 the temperature increased, the electrical conductivity of nanocomposites increased up to values 61 62 15 63 64 65 two to threefold higher than those of bare Bi2Te3. The absolute value of the Seebeck coefficient 1 2 showed a parallel evolution, strongly increasing up to 460 K, but decreasing at higher 3 4 temperatures when bipolar effects became dominant. Surprisingly, despite the larger charge 5 6 7 carrier concentrations measured in nanocomposites and associated to the electron spillover from 8 9 Cu2-xTe to Bi2Te3, nanocomposites were also characterized by much higher Seebeck 10 11 coefficients at temperatures above 400 K (Figures 5b). These abnormally high Seebeck 12 13 14 coefficients are more clearly perceived when plotting data within a Pisarenko plot, as displayed 15 16 in Figure 6d. Overall, in all Bi2Te3-Cu2-xTe composites, the concomitant increase of electrical 17 18 19 conductivity and Seebeck coefficient resulted in a significant increase of the power factor, up 20 21 to 1.32 at 554 K. 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 Figure 6. (a) Temperature dependence of the Hall charger carrier concentration and (b) mobility for 54 55 56 Bi2Te3 and Bi2Te3-Cu2-xTe (Bi:Cu = 13, 21, 26). (c) Dependence of the charge carrier concentration and 57 58 mobility on the Bi:Cu ratio at two temperatures, 300 K and 460 K. d) |S| vs. n for the different samples 59 60 at 300 K (solid symbols) and 460 K (open symbols). Dot lines show the theoretical |S| vs. n dependence 61 62 16 63 64 65 considering a SPB model and the effective mass (m*) that matches the Seebeck coefficient of bare Bi2Te3 1 at 300 K (black line) and 460 K (purple line). 2 3 4 5 6 We associate the large S values measured from Bi2Te3-Cu2-xTe composites to an energy- 7 8 dependent differential scattering of charge carriers at energy barriers, i.e. to an energy filtering 9 10 11 effect. Figure S5 shows the band alignment of the two materials as obtained from density 12 13 functional theory (DFT) calculations. Owing to the large difference in electron affinity between 14 15 16 Cu2-xTe and Bi2Te3, when nanodomains of the two materials are combined, the heavily doped 17 18 Cu2-xTe injects electrons to Bi2Te3, modifying the electron energy band structure of both 19 20 21 materials at their interphase (Figure 7a). Within the nanocomposites, the Bi2Te3 region nearby 22 23 the heterojunction becomes flooded with electrons and its energy bands suffer an important 24 25 downward bending. Taking into account the small diameter of the Bi2Te3 nanowires, all the 26 27 28 nanowire region in contact with a Cu2-xTe domain may become flooded with electrons. These 29 30 electrons need to overcome an energy barrier to exit the created potential well and freely move 31 32 33 through the nanowire which in part explains the stronger temperature dependence of the 34 35 transport properties of the composites (Figure 7b,c). Besides, the presence of these potential 36 37 38 wells can generate a differential charge carrier mobility as a function of the carrier energy. High 39 40 energy electrons should be able to freely move through Bi2Te3 domains within Bi2Te3-Cu2-xTe 41 42 43 composites without a major influence of the potential wells. On the other hand, low energy 44 45 electrons within Bi2Te3 might become trapped or scattered in these potential wells. We 46 47 hypothesize this energy filtering mechanism to be responsible of the higher Seebeck 48 49 50 coefficients displayed by Bi2Te3-Cu2-xTe composites. 51 52 The enhancement of the Seebeck coefficient associated with the energy filtering effect 53 54 55 competes with the decrease of the Seebeck coefficient associated with the larger charge carrier 56 57 concentrations. These two competing effects result in a non-monotonous dependence of the 58 59 60 Seebeck coefficient with the amount of Cu2-xTe introduced. The highest Seebeck coefficients 61 62 17 63 64 65 were reached with the Bi2Te3-Cu2-xTe nanocomposite with Bi:Cu=21, which provided a S = - 1 2 170 μV/K at 458 K. 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 Figure 7. (a) Electron energy band diagram at a heterojunction between Cu2-xTe and Bi2Te3. (c) Charge 28 29 distribution at the heterojunction between a Bi2Te3 nanowire and a Cu2-xTe cubic particle. (d) Energy 30 31 band diagram of a Bi2Te3 nanowire in contact with a Cu2-xTe nanoparticle. 32 33 34 35 36 The thermal conductivities of all the nanomaterials were very low when compared with 37 38 that of highly crystalline Bi2Te3. Surprisingly, the total thermal conductivities of Bi2Te3-Cu2- 39 40 41 xTe nanocomposites were not significantly lower than those of the Bi2Te3 nanomaterial (Figure 42 43 5e). A slight decrease of the lattice thermal conductivity obtained in some samples and 44 45 46 associated with an enhanced phonon scattering in the composite was compensated by an 47 48 increase of the electronic contribution. 49 50 Figure 5f displays the TE figure of merit obtained from Bi2Te3-Cu2-xTe nanocomposites 51 52 53 and the bare Bi2Te3 nanomaterial. For Bi2Te3, a maximum ZT = 0.38 was achieved at 560 K, 54 55 which is a notable value for pure Bi2Te3.[45] As for Bi2Te3-Cu2-xTe nanocomposites, the largest 56 57 58 ZT value was obtained from the Bi:Cu=21 sample, reaching a ZT = 0.95 at 576 K. The 59 60 extraordinary improvement of figure of merit obtained with Bi2Te3-Cu2-xTe nanocomposites 61 62 18 63 64 65 was associated to two factors: (1) An effective electron spillover from Cu2-xTe nanoinclusions 1 2 to the Bi2Te3 matrix that promoted electrical conductivity; and (2) the energy filtering of charge 3 4 carriers at energy wells introduced within the Bi2Te3 matrix by the nearby presence of Cu2-xTe 5 6 7 domains, and which significantly enhanced the Seebeck coefficient. 8 9 10 11 4. Conclusion 12 13 14 In summary, we detailed a simple bottom-up procedure to produce Bi2Te3-Cu2-xTe 15 16 nanocomposites with an adjusted composition based on combining proper ratios of solution- 17 18 19 processed Bi2Te3 and Cu2-xTe nanoparticle building blocks. With the introduction of Cu2-xTe, 20 21 the electrical conductivity of Bi2Te3 was greatly improved at temperatures above 400 K by 22 23 24 charge carrier spillover from the lower work function Cu2-xTe domains to the Bi2Te3 matrix. 25 26 This electron spillover from the Cu2-xTe valence band to the Bi2Te3 conduction band resulted in 27 28 29 regions of accumulated electrons within the Bi2Te3 domains. In these composites, the Seebeck 30 31 coefficient was significantly increased by the filtering of low energy carriers at the potential 32 33 wells created in Bi2Te3 domains next to a Bi2Te3/Cu2-xTe heterojunction. The enhanced Seebeck 34 35 36 coefficients and improved electrical conductivities resulted in a threefold increase in power 37 38 factor. Nanocomposites did not provide lower thermal conductivities than bare Bi2Te3 since the 39 40 41 thermal conductivity of all the tested nanomaterials was already very low. Overall, a significant 42 43 improvement of the figure of merit was observed, from 0.35 obtained for nanocrystalline Bi2Te3 44 45 46 to 0.95 for a Bi2Te3-Cu2-xTe containing around 4 wt% of Cu1.5Te. 47 48 49 50 Declaration of Competing Interest 51 52 53 The authors declare that they have no known competing financial interests or personal 54 55 relationships that could have appeared to influence the work reported in this paper. 56 57 58 59 60 Acknowledgements 61 62 19 63 64 65 This work was supported by the European Regional Development Funds and by the Generalitat 1 2 de Catalunya through the project 2017SGR1246. Y.Z, C.X, M.L, K.X and X.H thank the China 3 4 Scholarship Council for the scholarship support. MI acknowledges financial support from IST 5 6 7 Austria. YL acknowledges funding from the European Union’s Horizon 2020 research and 8 9 innovation program under the Marie Sklodowska-Curie grant agreement No. 754411. ICN2 10 11 acknowledges funding from Generalitat de Catalunya 2017 SGR 327 and the Spanish MINECO 12 13 14 project ENE2017-85087-C3. ICN2 is supported by the Severo Ochoa program from the Spanish 15 16 MINECO (grant no. SEV-2017-0706) and is funded by the CERCA Program/Generalitat de 17 18 19 Catalunya. Part of the present work has been performed in the framework of Universitat 20 21 Autònoma de Barcelona Materials Science PhD program. 22 23 24 25 26 Reference 27 28 29 [1] C. Wood, Materials for thermoelectric energy conversion, Rep. Prog. Phys. 51 (1988) 459. 30 31 [2] G.J. Snyder, E.S. Toberer, Complex thermoelectric materials, Nat. Mater. 7 (2008), 105- 32 33 114. 34 35 36 [3] J.R. Sootsman, D.Y. Chung, M.G. Kanatzidis, New and old concepts in thermoelectric 37 38 materials, Angew. Chem. Int. Ed. 48 (2009) 8616-8639. 39 40 41 [4] S. Ortega, M. Ibáñez, Y. Liu, Y. Zhang, M.V. Kovalenko, D. Cadavid, A. 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