nanomaterials Article Enhanced Thermoelectric Performance of n-Type Bi2Se3 Nanosheets through Sn Doping Mengyao Li 1 , Yu Zhang 1 , Ting Zhang 2 , Yong Zuo 1,3 , Ke Xiao 1 , Jordi Arbiol 2,4 , Jordi Llorca 5 , Yu Liu 6, * and Andreu Cabot 1,4, * 1 Catalonia Energy Research Institute—IREC, Sant Adrià de Besòs, 08930 Barcelona, Spain; limengyaorz@gmail.com (M.L.); tingchan99@gmail.com (Y.Z.); yongzuo16@gmail.com (Y.Z.); elvis.xiaoke@gmail.com (K.X.) 2 Catalan Institute of Nanoscience and Nanotechnology (ICN2), CSIC and BIST, Campus UAB, Bellaterra, 08193 Barcelona, Spain; ting.zhang@icn2.cat (T.Z.); arbiol@icrea.cat (J.A.) 3 Istituto Italiano di Tecnologia, Via Morego 30, 16163 Genova, Italy 4 ICREA, Pg. Lluis Companys 23, 08010 Barcelona, Spain 5 Institute of Energy Technologies, Department of Chemical Engineering and Barcelona Research Center in Multiscale Science and Engineering, Universitat Politècnica de Catalunya, EEBE, 08019 Barcelona, Spain; jordi.llorca@upc.edu 6 Institute of Science and Technology Austria (IST Austria), Am Campus 1, 3400 Klosterneuburg, Austria * Correspondence: yu.liu@ist.ac.at (Y.L.); acabot@irec.cat (A.C.) Abstract: The cost-effective conversion of low-grade heat into electricity using thermoelectric devices requires developing alternative materials and material processing technologies able to reduce the currently high device manufacturing costs. In this direction, thermoelectric materials that do not rely on rare or toxic elements such as tellurium or lead need to be produced using high-throughput   technologies not involving high temperatures and long processes. Bi2 Se3 is an obvious possible Te- Citation: Li, M.; Zhang, Y.; Zhang, T.; free alternative to Bi2 Te3 for ambient temperature thermoelectric applications, but its performance is Zuo, Y.; Xiao, K.; Arbiol, J.; Llorca, J.; still low for practical applications, and additional efforts toward finding proper dopants are required. Liu, Y.; Cabot, A. Enhanced Here, we report a scalable method to produce Bi2 Se3 nanosheets at low synthesis temperatures. We Thermoelectric Performance of studied the influence of different dopants on the thermoelectric properties of this material. Among the n-Type Bi2 Se3 Nanosheets through Sn elements tested, we demonstrated that Sn doping resulted in the best performance. Sn incorporation Doping. Nanomaterials 2021, 11, 1827. resulted in a significant improvement to the Bi2 Se3 Seebeck coefficient and a reduction in the thermal https://doi.org/10.3390/nano11071827 conductivity in the direction of the hot-press axis, resulting in an overall 60% improvement in the thermoelectric figure of merit of Bi2 Se3 . Academic Editor: Jiye (James) Fang Keywords: thermoelectric; Bi2 Se3 ; Sn doping Received: 22 June 2021 Accepted: 12 July 2021 Published: 14 July 2021 1. Introduction Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in Thermoelectric (TE) devices that directly and reversibly convert heat into electricity published maps and institutional affil- find unlimited applications [1–6], but their real implementation is hampered by their low iations. cost-effectiveness. The efficiency of energy conversion of a TE device is in part determined by the transport properties of the TE material: Seebeck coefficient S, electrical conductivity σ, and thermal conductivity κ. These material properties are generally grouped into a dimensionless figure of merit, ZT = S2 σT/κ, where T is the absolute temperature. Efficient Copyright: © 2021 by the authors. TE materials are characterized by high power factors, S2 σ, and low thermal conductivities, Licensee MDPI, Basel, Switzerland. κ. However, the strong correlation between these parameters makes the optimization This article is an open access article of the material performance extremely difficult. Several strategies have been developed distributed under the terms and to maximize ZT, including engineering the electronic band structure of the TE material conditions of the Creative Commons through doping [7], the use of energy filtering interphases [8], and the reduction in lattice Attribution (CC BY) license (https:// thermal conductivity through the introduction of abundant grain boundaries [9]. creativecommons.org/licenses/by/ Commercial devices use large amounts of highly crystalline Bi2 Te3 -based alloys as the 4.0/). active TE material, which accounts for a significant part of the total cost of the device. To Nanomaterials 2021, 11, 1827. https://doi.org/10.3390/nano11071827 https://www.mdpi.com/journal/nanomaterials Nanomaterials 2021, 11, 1827 2 of 12 reduce the device cost, it is critical to develop low-cost materials not relying on scarce Te and low-cost processing strategies not based on high-temperature crystallization from high purity melts. Bi2 Se3 presents the same structure and similar properties as Bi2 Te3 , and, thus, it is an obvious possible Te-free alternative to Bi2 Te3 . Numerous attempts to improve the TE figure of merit of this material have been reported, but with moderate success [10–18]. To maximize the TE performance of a material, one first critical point is to optimize its charge carrier concentration. Three main strategies can be used in this direction: tuning of the material stoichiometry, the introduction of atomic impurities, and/or the introduction of secondary phases able to inject the proper charge carriers to the host material [19–28]. Besides optimizing the charge carrier concentration, it is fundamental to minimize the thermal conductivity using multiscale phonon scattering centers. In this direction, the introduction of atomic impurities and abundant grain boundaries are highly effective. Besides, energy barriers at grain boundaries can also introduce energy- and charge sign- dependent charge carrier scattering, which could result in significant improvements to the Seebeck coefficient [29]. Here, we present a low temperature and high yield, solution-based strategy to prepare Bi2 Se3 nanomaterials doped with different elements. The influence of different dopants on the TE properties of the material was tested. From the results obtained, we demonstrated Sn to have the highest potential to yield Bi2 Se3 -based materials with improved performance. Thus, we further analyzed the location and chemical state of this element within Bi2 Se3 and its effect on the material transport properties. 2. Materials and Methods 2.1. Chemicals and Solvents Polyvinylpyrrolidone (PVP, (C6 H9 NO)n , AMW ~55,000), indium(III) acetate (InC6 H9 O6 , 99.99%), and bismuth(III) nitrate pentahydrate (Bi(NO3 )3 ·5H2 O, ≥99.99%) were pur- chased from Sigma-Aldrich. Sodium selenite (Na2 SeO3 , ≥98%), copper (II) nitrate tri- hydrate (Cu(NO3 )2 ·3H2 O, 99%), silver nitrate (AgNO3 , ≥99.9%), ethylene glycol (EG, HOCH2 CH2 OH, 99%), lead (II) acetate trihydrate (PbC4 H6 O4 ·3H2 O), tin(II) chloride anhy- drous (SnCl2 , 98%), and potassium hydroxide (KOH, ≥98%) were acquired from Fisher. Analytical grade ethanol and acetone were obtained from various sources. 2.2. Synthesis of Bi2 Se3 Bi2 Se3 particles were produced following the approach developed by Liu et al., with slight modifications [1]. Bi2 Se3 (5 mmol), Bi(NO3 )3 ·5H2 O (10 mmol, 4.851 g), Na2 SeO3 (15 mmol, 2.594 g), KOH (50 mmol, 2.806 g), and PVP (0.5 g) were first dissolved in a three- neck flask containing EG (200 mL) under an Ar atmosphere at an ambient temperature for 0.5 h. The solution was then heated to 180 ◦ C and kept at this temperature for 3 h. Immediately after the reaction was completed, the solution was allowed to cool naturally to room temperature by removing the heating mantle. The solution was divided into several centrifuge tubes and acetone was added to collect the solid product by centrifugation. In the next step, ethanol was introduced to re-disperse the particles, and acetone was added to precipitate them again. This step was repeated twice. Next, purified Bi2 Se3 particles were dried under a vacuum overnight at room temperature and kept in the glovebox for posterior use. Around 3 g of particles were obtained per batch. 2.3. Synthesis of Bi2− x Mx Se3 The procedure used to produce Bi2−x Mx Se3 was the same used to produce Bi2 Se3 , but the proper amount of bismuth nitrate was replaced with the corresponding metal precursor, as listed in the chemical section above. 2.4. Nanomaterial Consolidation Dried Bi2 Se3 particles were first annealed for 1 h at 350 ◦ C under an Ar flow in a tubular furnace. Then, the annealed material was introduced into a graphite die and Nanomaterials 2021, 11, 1827 3 of 12 consolidated into cylinders (Ø 10 mm × 10 mm) at 480 ◦ C and 50 MPa of pressure for 4 min with a custom-made hot press. The relative densities of the compacted pellets were measured by Archimedes’ method. To measure transport properties in the two relevant directions, cylindrical pellets were cut into ca. 8 × 6 × 1 mm rectangular bars along the pressure axis, and 1 mm thick disks along the perpendicular to the pressure axis. 2.5. Structural and Chemical Characterization The field emission scanning electron microscope (SEM) was used to determine the nanoparticle morphology on an Auriga Zeiss. An Oxford energy dispersive X-ray spec- trometer (EDX) was used to measure the material composition at 20.0 kV. X-ray diffraction (XRD) was performed on a Bruker AXS D8 Advance diffractometer. The crystal structure of the samples was analyzed under the 200 keV Tecnai F20 field emission microscope equipped with a Gatan quantum image filter. X-ray photoelectron spectroscopy (XPS) was performed on a Specs system with the material inside the chamber at a pressure below 10−7 Pa. Data processing was carried out using the CasaXPS program. 2.6. Performance Characterization of Bulk Nanomaterial Seebeck coefficients and resistivities were measured by the static direct current method and by the standard four-probe method, respectively, in an LSR-3 Linseis system under helium. All samples were tested for at least three heating and cooling cycles. Considering the system accuracy and measurement accuracy, the measurement error of the conductivity and Seebeck coefficient was estimated to be about 4%. Thermal diffusivity (λ), constant pressure heat capacity (Cp ), and density of the material (ρ) were used to obtain the thermal conductivities (κtotal ), where κtotal = λCp ρ. The thermal diffusivities of the samples were measured by a Xenon Flash Apparatus XFA600, which has an estimated error of ca. 5%. We used a constant as the heat capacity (Cp ), which was estimated from empirical formulas by the Dulong–Petit limit (3R law). Under a magnetic field of 0.6 T, the Hall charge carrier concentrations and mobilities at room temperature were measured by the Van der Pauw method. The figures in this article do not have error bars in order to avoid cluttering the plots. 3. Results and Discussion Following the above synthesis method, Bi2 Se3 nanosheets grouped into flower-like particles were produced, as shown by the SEM micrographs (Figure 1a,b). Bi2 Se3 shows a screw dislocation growth mechanism (Figure 1c). This layer-by-layer growth mechanism is usually due to the low supersaturation in the synthesis conditions, which inhibits dendritic growth. Axial screw dislocation is the self-sustaining spiral growth of nanosheets around the axis [15,23]. The XRD analysis showed that the crystal structure of the obtained nanosheets matched the rhombohedral Bi2 Se3 phase (Figure 1d, JCPDS No. 00-033-0214). Figure 1e shows the layered rhombohedral crystal structure of Bi2 Se3 , which consists of five cova- lently bonded atomic planes of Se-Bi-Se-Bi-Se. Quintuple layers are weakly bonded by van der Waals. The HRTEM characterization showed that the material has good crys- tallinity and has a crystal phase consistent with the Bi2 Se3 rhombohedral phase (space group = R3-MH) with a = b = 4.1340 Å and c = 28.6300 Å (Figure 1f,g). Nanosheets were annealed at 350 ◦ C, and the annealed powder was hot-pressed into cylindrical pellets with a diameter of 10 mm and a height of 10 mm in a glovebox filled with argon. Samples were hot-pressed for four minutes at 480 ◦ C and 50 MPa, and then naturally cooled to an ambient temperature. The relative density of the cylinders produced by this process was about 93% of the theoretical value. From the consolidated cylinder, a rectangular bar of 8 × 6 × 1 mm was cut longitudinally, and a 1 mm thick pellet was cut transversely (Figure 2). These samples were used to measure the TE properties of materials parallel to the pressure axis (//) and perpendicular to the pressure axis (⊥). Nanomaterials 2021, 11, 1827 4 of 12 Nanomaterials 2021, 11, x FOR PEER REVIEW 4 of 13 Figure 1. Figure 1. (a,b) (a,b) SEM SEM micrographs micrographs of the Bi22Se Se33 particles. particles.(c) (c)Scheme Schemeofofthe thescrew screwdislocation dislocationgrowth growth mechanism. (d) mechanism. (d)The TheXRD XRDpattern of of pattern Bi2Se thethe particles. Bi32 Se 3 (e) particles.The layered (e) The rhombohedral layered crystal rhombohedral struc‐ crystal ture of Bi 2Se3. (f) A low‐resolution TEM image. (g) An HRTEM micrograph of the Bi2Se3 particles structure of Bi2 Se3 . (f) A low-resolution TEM image. (g) An HRTEM micrograph of the Bi2 Se3 and its corresponding power spectrum. From the crystallographic domain, the Bi2Se3 lattice fringe particles and its corresponding power spectrum. From the crystallographic domain, the Bi2 Se3 distances were measured to be 0.205 nm, 0.205 nm, and 0.204 nm, at 60.96° and 120.86°, respectively, lattice fringe distances were measured to be 0.205 nm, 0.205 nm, and 0.204 nm, at 60.96◦ and 120.86◦ , which could be illuminated as the rhombohedral Bi2Se3 phase, visualized along the [0001] zone axis. respectively, which could be illuminated as the rhombohedral Bi2 Se3 phase, visualized along the [0001] zone axis. with argon. Samples were hot‐pressed for four minutes at 480 °C and 50 MPa, and then naturally cooled to an ambient temperature. The relative density of the cylinders pro‐ duced by this process was about 93% of the theoretical value. From the consolidated cyl‐ inder, a rectangular bar of 8 × 6 × 1 mm was cut longitudinally, and a 1 mm thick pellet Nanomaterials cut11,transversely was2021, 1827 (Figure 2). These samples were used to measure the TE properties of5 of 12 materials parallel to the pressure axis (//) and perpendicular to the pressure axis (). Figure Figure 2.Photograph 2. (a) (a) Photograph of the consolidated of the consolidated Bi2Se(b,c) Bi2 Se3 cylinder. 3 cylinder. Scheme (b,c) Scheme and images and of the images samples oftothe used samples measure the used transport material to measure the material properties transport in the directions properties parallel in the directions (b) and vertical parallel (c) to the pressure axis.(b) and vertical (c) to the pressure axis. Figure 3 displays the electrical conductivity (σ), Seebeck coefficient (S), and power factor (PF = S2 σ) of the Bi2−x Mx Se3 (M = Sn, Cu, Ag, Pb, In) pellets perpendicular to Figure 3 displays the electrical conductivity (σ), Seebeck coefficient (S), and power the press direction. The electrical conductivity of the undoped material decreased with factor (PF = S2σ) of the Bi2−xMxSe temperature, 3 (M = which Sn, Cu, pointed Ag, to a Pb, In) pellets degenerated perpendicular semiconductor behavior.to the pressto the Compared direction. The electricalundopedconductivity of the undoped material, the electrical material conductivity of all thedecreased with doped samples wastempera‐ slightly lower, ture, which pointedexcept for the Ag-doped to a degenerated samples. On the semiconductor other hand, behavior. the samplesto Compared doped with Sn and to the undoped a minor extent, Pb displayed an increase in the absolute value of the Seebeck coefficient. Nanomaterials 2021, 11, x FOR PEER REVIEW material, the electrical conductivity of all the doped samples was slightly lower, except 6 of 13 Overall, the highest power factors were obtained with the Sn doping. Thus, we decided to for the Ag‐doped samples. On the further study theeffect otherof hand, the samples doped with Sn and to a minor this element. extent, Pb displayed an increase in the absolute value of the Seebeck coefficient. Overall, the highest power factors were obtained with the Sn doping. Thus, we decided to further study the effect of this element. Figure 3. TE Figure 3. properties of Bi TE properties BiM of2−x 2−xxSe M3x(M Se3 =(MSn, = Cu, Ag,Ag, Sn, Cu, Pb,Pb, In)In) samples samplesmeasured measuredperpendicular perpendicular to to the the press direction: (a) press direction: electrical conductivity, (a) electrical σ; (b)σ; conductivity, Seebeck coefficient, (b) Seebeck S; (c) coefficient, S; power (c) powerfactor, PF. factor, PF. Figure4 4displays Figure displaysthethe electrical electrical conductivity, conductivity,Seebeck Seebeckcoefficient, andand coefficient, power factor power of of factor Bi2-x Snx Se3 materials containing different amounts of Sn. We observed that the electrical Bi2‐xSnxSe3 materials containing different amounts of Sn. We observed that the electrical conductivity decreased and the absolute value of the Seebeck coefficient increased with conductivity decreased and the absolute value of the Seebeck coefficient increased with the amount of Sn up to a certain Sn concentration. The highest power factors were finally theobtained amountfor of Sn upSnto a Se Bi1.93 certain Sn concentration. The highest power factors were finally 0.07 3 . The decrease in the electrical conductivity and increase in obtained for Bi1.93Sn0.07Se3. The decrease in the electrical conductivity and increase in the absolute value of the Seebeck coefficient denoted a reduction in the charge carrier concen‐ tration, which points to the presence of Sn2+ ions instead of Sn4+ at Bi3+ sites. Figure 3. TE properties of Bi2−xMxSe3 (M = Sn, Cu, Ag, Pb, In) samples measured perpendicular to the press direction: (a) electrical conductivity, σ; (b) Seebeck coefficient, S; (c) power factor, PF. Figure 4 displays the electrical conductivity, Seebeck coefficient, and power factor of Nanomaterials 2021, 11, 1827 Bi2‐xSnxSe3 materials containing different amounts of Sn. We observed that the electrical 6 of 12 conductivity decreased and the absolute value of the Seebeck coefficient increased with the amount of Sn up to a certain Sn concentration. The highest power factors were finally obtained for Bi1.93Sn0.07Se3. The decrease in the electrical conductivity and increase in the the absolute absolute value value ofSeebeck of the the Seebeck coefficient coefficient denoted denoted a reduction a reduction in the carrier in the charge charge concen‐ carrier concentration, which points to the presence of Sn2+ ions instead of Sn4+ at Bi3+ sites. tration, which points to the presence of Sn2+ ions instead of Sn4+ at Bi3+ sites. Figure 4. Figure 4. Thermoelectric properties properties of of Bi 2−Sn Bi2−x xSe x Sn 3 perpendicular x Se to to 3 perpendicular thethe press direction: press (a) (a) direction: electrical conductivity, electrical σ; (b) conductivity, σ; Seebeck coefficient, S; (c) power factor, PF. (b) Seebeck coefficient, S; (c) power factor, PF. Figure Figure 55shows showsrepresentative representativeSEM SEMmicrographs micrographsofofthe theBiBi 1.93 Sn 1.93 0.07Se Sn0.07 Se33 particles, particles, which which had had similar sizes but did not display the flower‐like morphology observed from Bi2 2Se33.. As similar sizes but did not display the flower-like morphology observed from Bi Se As shown shown in in Figure Figure6,6,the theXRD XRDcharacterization characterizationconfirmed confirmedthatthatthe theBi1.93 Sn Bi1.93 Sn 0.07Se 0.07 Se33 particles particles maintained maintained their their rhombohedral rhombohedral structure. structure. With the introduction of Sn, Sn, the the XRDXRD peaks peaks shifted shifted to to lower lower 2θ2θangles, angles,suggesting suggestingthe theincorporation incorporationof ofSn Snwithin withinthe theBi Bi22Se lattice [5]. Se33 lattice [5]. The HRTEM characterization confirmed the rhombohedral phase of the Bi1.93 Sn0.07 Se3 particles (Figure 7a). EELS chemical composition maps obtained from the red squared Nanomaterials 2021, 11, x FOR PEER REVIEW 7 of 13 region in the HAADF STEM micrograph shown in Figure 7b displayed a homogeneous distribution of Sn, Bi and Se within the Bi1.93 Sn0.07 Se3 nanosheet. Figure 5. SEM micrographs of Bi1.93Sn0.07Se3 nanosheets with different magnifications; (a) magnification 5.00 K ×, (b) mag‐ Figure 5. SEM micrographs of Bi1.93 Sn0.07 Se3 nanosheets with different magnifications; (a) magnification 5.00 K ×, (b) mag- nification 10.00 K ×. nification 10.00 K ×. a Bi1.93Sn0.07Se3 b Intensity (a.u.) Intensity (a.u.) Bi2Se3 Nanomaterials 2021, 11, 1827 Figure 5. SEM micrographs of Bi1.93Sn0.07Se3 nanosheets with different magnifications; (a) magnification 5.00 K ×, (b) mag‐ 7 of 12 nification 10.00 K ×. a Bi1.93Sn0.07Se3 b Intensity (a.u.) Intensity (a.u.) Bi2Se3 JCPDS No. 00-033-0214 (006) 10 20 30 40 50 60 70 80 18 19 2 (Degree) 2 (Degree) Nanomaterials 2021, 11, x FOR PEER REVIEW 8 of 13 Figure (a)XRD 6. (a) Figure 6. XRDpatterns patternsofof BiBi 2Se23Se 3 and and BiSn Bi1.93 1.93 Sn 3; (b) 0.07Se0.07 Seexpansion 3 ; (b) expansion diagram diagram of the of the area area corre- correspond‐ sponding ing to the to the diffraction diffraction peak ofpeak Bi2Seof3 (006). Bi2 Se3 (006). The HRTEM characterization confirmed the rhombohedral phase of the Bi1.93Sn0.07Se3 particles (Figure 7a). EELS chemical composition maps obtained from the red squared re‐ gion in the HAADF STEM micrograph shown in Figure 7b displayed a homogeneous dis‐ tribution of Sn, Bi and Se within the Bi1.93Sn0.07Se3 nanosheet. Figure 7. (a) An HRTEM micrograph of a Bi1.93 Sn0.07 Se3 nanosheet, detailed information of the orange squared area and its Figure 7. (a) An HRTEM micrograph of a Bi1.93Sn0.07Se3 nanosheet, detailed information of the orange squared area and its corresponding power spectrum. Bi2 Se3 lattice fringe distances were measured to be 0.370 nm, 0.349 nm, 0.202 nm, and corresponding power spectrum. Bi2Se3 lattice fringe distances were measured to be 0.370 nm, 0.349 nm, 0.202 nm, and 0.351 nm, at 62.89◦ , 92.61◦ and 122.99◦ , respectively, which could be illuminated as the rhombohedral Bi2 Se3 phase, along 0.351 nm, at 62.89°, 92.61° and 122.99°, respectively, which could be illuminated as the rhombohedral Bi2Se 3 phase, along itsits[0001] [0001]zone zoneaxis. axis.(b) (b)EELS EELSchemical chemicalcomposition composition maps maps obtained obtained from from the the red red squared squared area area ofofthe theSTEM STEMmicrograph. micrograph. Individual Bi N2,3‐edges at 679 eV (red), Sn M4,5‐edges at 485 eV (green), Se M2,3‐edges at 162 eV (blue) and compositesofofBi- Individual Bi N 2,3 -edges at 679 eV (red), Sn M4,5 -edges at 485 eV (green), Se M 2,3 -edges at 162 eV (blue) and composites Sn, Bi-Se Bi‐Sn, Bi‐Seas as well as Bi-Sn-Se. well (c) Bi(c) as Bi‐Sn‐Se. 4f, Bi Se 4f, 3d and Se 3dSn and 3d high-resolution XPS spectra Sn 3d high‐resolution obtained XPS spectrafrom Bi1.93 Sn obtained 0.07 SeBi from nanosheets. 3 1.93 Sn0.07Se3 nanosheets. Bi 4f, Se 3d and Sn 3d high‐resolution XPS spectra of Bi1.93Sn0.07Se3 particles are dis‐ played in Figure 7c. The high‐resolution Bi 4f spectrum was fitted with two doublets, as‐ sociated with Bi3+ within a Bi2Se3 chemical environment (Bi 4f7/2 at 158.2 eV) and Bi within Nanomaterials 2021, 11, 1827 8 of 12 Bi 4f, Se 3d and Sn 3d high-resolution XPS spectra of Bi1.93 Sn0.07 Se3 particles are displayed in Figure 7c. The high-resolution Bi 4f spectrum was fitted with two doublets, associated with Bi3+ within a Bi2 Se3 chemical environment (Bi 4f7/2 at 158.2 eV) and Bi within a more electronegative environment, as it could be Bi2 O3 or Bi2 SeO2 (Bi 4f7/2 at 159.4 eV) [30]. This oxidation of the particles surface is related to their transport and handling in the air [4]. The high-resolution Se 3d XPS spectrum was fitted with three doublets, corresponding to Se within the Bi2 Se3 (Se 3d5/2 at 53.6 eV), selenium in an elemental or Bi2 SeO2 environment (Se 3d5/2 at 55.2 eV), probably arising from the partial oxidation of the material surface, and Se in a SeO2 chemical environment (Se 3d5/2 at 58.6 eV) [30,31]. Finally, the high-resolution Sn 3d XPS spectrum was difficult to fit owing to the small amount of this element, but the broadness of the peaks pointed to the presence of at least two Sn chemical states that should be tentatively assigned to Snx+ within a SnSe and an oxidized environment [31]. According to the electrical properties measured at the introduction of Sn, we tentatively assigned the Snx+ component within SnSe to Sn2+ . Top-view and cross-section SEM micrographs of Bi2 Se3 and Bi1.93 Sn0.07 Se3 (Figure 8) showed the final pellets presenting a laminar microstructure with an evident preferen- tial orientation of the material layers. Bi2 Se3 displayed larger and thinner layers than Bi1.93 Sn0.07 Se3 . When comparing the XRD patterns of the sample held in two normal directions, parallel and perpendicular to the pressure axis (Figure 8e,f), we observed that the relative XRD peak intensity clearly differed. This result confirmed the preferential Nanomaterials 2021, 11, x FOR PEER REVIEW 9 of 13 crystallographic orientation of the hot-pressed materials, with the [001] crystallographic direction oriented parallel to the pressure axis. Figure Figure 8. 8. (a,b) (a,b)Cross-section Cross‐sectionand andtop-view top‐viewSEM SEMmicrograph micrograph ofof Bi2BiSe2Se 3 . 3(c,d) . (c,d)Cross-section and Cross‐section top-view and top‐viewSEM SEM micrograph micrograph of of1.93 Bi Bi1.93 SnSn SeSe 0.070.07 3 .3.(e,f) (e,f)XRD XRDpatterns patternsofof Bi2BiSe 2Se 3 and 3 and BiBi Sn 1.93 1.93 Sn Se33 measured 0.07Se 0.07 measured in in two twoperpendicular perpendiculardirections, directions,along along(//) (//) and and perpendicular((⊥) perpendicular ⊥) toto the the pressure pressure axis. axis. Figure Figure 99 displays displaysthe theTE TEproperties propertiesofofBiBi 2 Se3 3and 2Se andBiBi 1.93Sn 1.93 Sn0.07 measured in the two 0.07Se33 measured two directions. We observed σ⊥ >>σσ//// , as , as expected expected from from thethe higher higher charge charge carrier carrier mobilities mobilities in in ab ab thethe crystal crystal planecompared plane comparedwith thec cdirection, withthe direction,and andthethe extended extended size of the crystal crystal domains in the direction direction normal normal to to the the pressure pressure direction direction within within the the layered layered pellets. pellets. On On the other hand, similar Seebeck coefficients were obtained in both directions, pointing out that this parameter has little relationship with grain boundary scattering in this material. Nanomaterials 2021, 11, 1827 9 of 12 Nanomaterials 2021, 11, x FOR PEER REVIEW 10 of 13 the other hand, similar Seebeck coefficients were obtained in both directions, pointing out that this parameter has little relationship with grain boundary scattering in this material. Figure 9. TE properties of Bi2 Se3 and Bi1.93 Sn0.07 Se3 parallel (//) and vertical (⊥) to the pressure axis: (a) electrical conduc- Figure 9. (b) tivity, σ; TE Seebeck properties of Bi2Se3 S; coefficient, and(c)Bipower 1.93Sn0.07Se3 parallel (//) and vertical (⊥) to the pressure axis: (a) electrical conductiv‐ factor, PF; (d) total thermal conductivity, κ; (e) lattice thermal conductivity, κ L ; ity, σ; (b) Seebeck coefficient, S; (c) power factor, PF; (d) total thermal conductivity, κ; (e) lattice thermal conductivity, κL; and (f) the TE figure of merit, ZT. and (f) the TE figure of merit, ZT. When comparing the TE properties of Bi2 Se3 and Bi1.93 Sn0.07 Se3 , we observed σ⊥ to slightly When increase and σthe comparing // toTEslightly decrease properties of Bi2with the Bi Se3 and introduction of Sn, 1.93Sn0.07Se3, we which could observed σ to be in part related to the thinner and larger material layers observed slightly increase and σ// to slightly decrease with the introduction of Sn, which could within the layered be in Bi2 Se part 3 pellets related to compared the thinnerwithandBilarger 1.93 Sn0.07 Se3 . On material the other layers hand,within observed with the theintroduction layered Bi2Seof 3 Sn, S increased pellets comparedinwithbothBi directions, which resulted in a higher PF in both directions for the 1.93Sn0.07Se3. On the other hand, with the introduction of Sn, S in‐ Bi1.93 Sn0.07 creased in Se 3 sample. both directions, which resulted in a higher PF in both directions for the Bi1.93Sn0.07Se3 sample. the Hall charge carrier concentration and mobility of Bi2 Se3 and Table 1 displays Bi1.93Table Se0.071Sedisplays 3 pellets at theroom Halltemperature. charge carrier Asconcentration expected from themobility and electricalofconductivity Bi2Se3 and and Seebeck coefficient measurements, we observed the Bi1.93Se0.07Se3 pellets at room temperature. As expected from the electrical Sn introduction to result and conductivity in a decrease in the charge carrier concentration, from n = 1.8 × 1019 cm−3 for Bi Se to Seebeck coefficient measurements, we observed the Sn H 19 cm−3 for Bi 2 3 introduction to result in a decrease 2+ nH = 1.3 × in the charge3+carrier concentration, from nH = 1.8 × 10 cm for Bi2Se3 to nH = 1.3 × 10of 10 1.93 Se 0.07 Se 3 . This result is 19 consistent −3 with the presence Sn−3 19 cm replacing Bi ions, thus trapping a free electron. We believe this optimization of the charge for Bi1.93Se0.07Se3. This result is consistent with the presence of Sn2+ replacing Bi3+ ions, thus carrier concentration to be at the origin of the higher PF obtained with the introduction trapping a free electron. We believe this optimization of the charge carrier concentration of Sn. We further calculated the effective mass of the Bi2 Se3 and Bi1.93 Sn0.07 Se3 materials to be at the origin of the higher PF obtained with the introduction of Sn. We further cal‐ using a single parabolic band (SPB) model. For detailed calculations, the carrier transport culated the effective mass of the Bi2Se3 and Bi1.93Sn0.07Se3 materials using a single parabolic property analysis included [32]: band (SPB) model. For detailed calculations, the carrier transport property analysis in‐ cluded [32]: Table 1. Room temperature transport properties and m* of the Bi2 Se3 and Bi1.93 Se0.07 Se3 pellets. Table 1.Materials Room temperature transport n [1019properties cm−3 ] and µ m* of [cmthe 2VBi−21SeS3−and 1 ] Bi1.93Se0.07Se3 pellets. m*/m H H 0 Bi2 Se3 Materials 1.77 nн [10 19 cm−3] 178.6 μн [cm 2 V−1 S−1] 0.27 0 m*/m Bi1.93 BiSn 2Se0.07 3 Se3 1.34 1.77 239.3 178.6 0.29 0.27 Bi1.93Sn0.07Se3 1.34 239.3 0.29 The Seebeck coefficient, Nanomaterials 2021, 11, 1827 10 of 12 The Seebeck coefficient,   κ B (r + 5/2)· Fr+3/2 (η ) S(η ) = −η (1) e (r + 3/2)· Fr+1/2 (η ) The Hall carrier concentration, 2 2 1 (2m∗ ·κ B T )3/2 (r + 3/2) · Fr+1/2 (η ) nH = = · (2) e· R H 3π 2 }3 (2r + 3/2)· F2r+1/2 (η ) The Hall mobility, " # eπ }4 Cl (2r + 3/2)· F2r+1/2 (η ) µH = √ (3) 2(κ B T ) 3/2 Ede f 2 ( m∗ )5/2 (r + 3/2)2 · Fr+1/2 (η ) R ∞ εx where Fx (η ) = 0 dε is the Fermi integral. 1+ e ( ε − η ) In the above equations, S, µH , η, κ B , e, r, RH , h̄, Cl , Edef , and m* are the Seebeck coefficient, the carrier mobility, the reduced Fermi level, the Boltzmann constant, the electron charge, the carrier scattering factor (r = −1/2 for acoustic phonon scattering), the Hall coefficient, the reduced plank constant, the elastic constant for longitudinal vibrations, the deformation potential coefficient, and the density of state effective mass, respectively. Using the experimental S, the reduced Fermi level η can be obtained by Equation (1). Substituting the estimated η and the measured “nH ” and “µH ” into Equations (2) and (3), the effective mass “m*” can be calculated. The results show that m* increased slightly with the Sn doping (Table 1). Consistent with the layered structure of the pellets and their crystallographic texture, the thermal conductivity of the materials measured in the direction normal to the pressure axis was much higher than in the pressure axis, κ ⊥ > κ // . The introduction of Sn resulted in lower κ ⊥ but higher κ // values, which is related to the different microstructure of the layers, with thicker and smaller Bi1.93 Se0.07 Se3 plates compared to those within the Bi2 Se3 pellet. Overall, the Bi1.93 Sn0.07 Se3 samples measured in the pellet plane displayed the highest PF and ZT values, 0.65 m Wm−1 K−2 and 0.41, respectively. These values are among the highest published for Bi2 Se3 -based materials, as displayed in Table 2. Table 2. A comprehensive summary of the thermoelectric performance of the previously reported Bi2 Se3 . ZT at Room Temperature ZTmax Materials σ (104 S/m) S (µV/K) κ (W/mK) ZT σ (104 S/m) S (µV/K) κ (W/mK) ZT K2.5 Bi8.5 Se14 [33] 0.4 −100 0.6 0.05 1.8 −160 0.38 1 (873 K) Bi0.7 Sb0.3 Se [34] 4.5 −138 0.5 0.45 4.2 −160 0.7 0.8 (425 k) Bi5.6 Sb2.4 Se7 [35] 4.8 −168.5 0.73 0.55 (300 K) Bi2 Se3 [28] 2 −120 0.5 0.17 1.95 −155 0.4 0.48 (427 K) Bi2 Se3 [21] 1.78 −90 0.42 0.1 3 −120.7 0.49 0.35 (400 K) Bi2 Se3 HA0.11 DMSO0.06 14.8 −80 1.52 0.187 [36] Bi2 Se3 [12] 2.5 −60 0.55 0.05 2 −100 0.6 0.18 (480 K) Cu0.1 Bi2 Se3 [37] 1.46 −84 0.32 0.1 (290 K) Bi2 Se3 [20] 0.212 −115 0.75 0.011 0.6755 −150 0.83 0.096 (523 K) This work 5.87 −101 1 0.195 4.72 −116 0.91 0.41 (577 K) 4. Conclusions We report here a large-scale method to synthesize n-type Bi2 Se3 nanosheets with different dopants. Samples doped with Sn provided the highest TE performances. The composition was optimized at Bi1.93 Se0.07 Se3 . Upon Sn doping, we observed an increase in the Seebeck coefficient and the power factor compared to pure Bi2 Se3 . As a result, a Nanomaterials 2021, 11, 1827 11 of 12 power factor up to 0.65 m Wm−1 K−2 and a ZT of 0.41 were obtained for the Bi1.93 Sn0.07 Se3 pellet measured in the direction perpendicular to the pressure axis, which represents a 60% increase over pure Bi2 Se3 . Author Contributions: Conceptualization, M.L.; methodology, M.L. and Y.L.; software, M.L. and Y.Z. (Yong Zuo); formal analysis, M.L., J.A. and J.L.; data curation, T.Z., Y.Z. (Yu Zhang) and K.X.; writing—original draft preparation, M.L.; writing—review and editing, A.C.; supervision, Y.L., A.C. All authors have read and agreed to the published version of the manuscript. Funding: All sources of funding are acknowledged in the acknowledgements section. Data Availability Statement: Data are contained within the article. Acknowledgments: M.L., Y.Z., T.Z. and K.X. thank the China Scholarship Council for their scholar- ship support. Y.L. acknowledges funding from the European Union’s Horizon 2020 research and innovation program under the Marie Sklodowska-Curie grant agreement No. 754411. J.L. thanks the ICREA Academia program and projects MICINN/FEDER RTI2018-093996-B-C31 and G.C. 2017 SGR 128. ICN2 acknowledges funding from the Generalitat de Catalunya 2017 SGR 327 and the Spanish MINECO ENE2017-85087-C3. Conflicts of Interest: The authors declare no conflict of interest. References 1. Liu, Y.; Zhang, Y.; Lim, K.H.; Ibáñez, M.; Ortega, S.; Li, M.; David, J.; Martí-Sànchez, S.; Ng, K.M.; Arbiol, J.; et al. High Thermo- electric Performance in Crystallographically Textured n type Bi2 Te3-x Sex Produced from Asymmetric Colloidal Nanocrystals. ACS Nano 2018, 12, 7174–7184. [CrossRef] 2. 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