Enhanced thermoelectric performance of n-type bi2se3 nanosheets through sn doping

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,*   Citation: Li, M.; Zhang, Y.; Zhang, T.; Zuo, Y.; Xiao, K.; Arbiol, J.; Llorca, J.; Liu, Y.; Cabot, A. Enhanced Thermoelectric Performance of n-Type Bi2Se3Nanosheets through Sn Doping. Nanomaterials 2021,11, 1827. https://doi.org/10.3390/nano11071827 Academic Editor: Jiye (James) Fang Received: 22 June 2021 Accepted: 12 July 2021 Published: 14 July 2021 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affiliations. Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). 1Catalonia Energy Research Institute—IREC, Sant Adriàde Besòs, 08930 Barcelona, Spain; [email protected] (M.L.); [email protected] (Y.Z.); [email protected] (Y.Z.); [email protected] (K.X.) 2Catalan Institute of Nanoscience and Nanotechnology (ICN2), CSIC and BIST, Campus UAB, Bellaterra, 08193 Barcelona, Spain; [email protected] (T.Z.); [email protected] (J.A.) 3Istituto Italiano di Tecnologia, Via Morego 30, 16163 Genova, Italy 4ICREA, Pg. Lluis Companys 23, 08010 Barcelona, Spain 5Institute 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.llor[email protected] 6Institute of Science and Technology Austria (IST Austria), Am Campus 1, 3400 Klosterneuburg, Austria *Correspondence: [email protected] (Y.L.); [email protected] (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. Bi 2 Se 3 is an obvious possible Tefree alternative to Bi 2 Te 3 for ambient temperature thermoelectric applications, but its performance is still low for practical applications, and additional efforts toward finding proper dopants are required. Here, we report a scalable method to produce Bi 2 Se 3 nanosheets at low synthesis temperatures. We studied the influence of different dopants on the thermoelectric properties of this material. Among the elements tested, we demonstrated that Sn doping resulted in the best performance. Sn incorporation resulted in a significant improvement to the Bi 2 Se 3 Seebeck coefficient and a reduction in the thermal conductivity in the direction of the hot-press axis, resulting in an overall 60% improvement in the thermoelectric figure of merit of Bi2Se3. Keywords: thermoelectric; Bi2Se3; Sn doping 1. Introduction Thermoelectric (TE) devices that directly and reversibly convert heat into electricity find unlimited applications [ 1 – 6 ], but their real implementation is hampered by their low 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 =S 2σ T/ κ , where Tis the absolute temperature. Efficient TE materials are characterized by high power factors, S 2σ , and low thermal conductivities, κ . However, the strong correlation between these parameters makes the optimization of the material performance extremely difficult. Several strategies have been developed to maximize ZT, including engineering the electronic band structure of the TE material through doping [ 7 ], the use of energy filtering interphases [ 8 ], and the reduction in lattice thermal conductivity through the introduction of abundant grain boundaries [9]. Commercial devices use large amounts of highly crystalline Bi 2 Te 3 -based alloys as the 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. Bi 2 Se 3 presents the same structure and similar properties as Bi 2 Te 3 , and, thus, it is an obvious possible Te-free alternative to Bi 2 Te 3 . 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 energyand charge signdependent 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 Bi 2 Se 3 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 Bi 2 Se 3 -based materials with improved performance. Thus, we further analyzed the location and chemical state of this element within Bi 2 Se 3 and its effect on the material transport properties. 2. Materials and Methods 2.1. Chemicals and Solvents Polyvinylpyrrolidone (PVP, (C 6 H 9 NO) n , AMW ~55,000), indium(III) acetate (InC 6 H 9 O 6 , 99.99%), and bismuth(III) nitrate pentahydrate (Bi(NO 3 ) 3· 5H 2 O, ≥ 99.99%) were purchased from Sigma-Aldrich. Sodium selenite (Na 2 SeO 3 , ≥ 98%), copper (II) nitrate trihydrate (Cu(NO 3 ) 2· 3H 2 O, 99%), silver nitrate (AgNO 3 , ≥ 99.9%), ethylene glycol (EG, HOCH 2 CH 2 OH, 99%), lead (II) acetate trihydrate (PbC 4 H 6 O 4· 3H 2 O), tin(II) chloride anhydrous (SnCl 2 , 98%), and potassium hydroxide (KOH, ≥ 98%) were acquired from Fisher. Analytical grade ethanol and acetone were obtained from various sources. 2.2. Synthesis of Bi2Se3 Bi 2 Se 3 particles were produced following the approach developed by Liu et al., with slight modifications [ 1 ]. Bi 2 Se 3 (5 mmol), Bi(NO 3 ) 3· 5H 2 O (10 mmol, 4.851 g), Na 2 SeO 3 (15 mmol, 2.594 g), KOH (50 mmol, 2.806 g), and PVP (0.5 g) were first dissolved in a threeneck 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 Bi 2 Se 3 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−xMxSe3 The procedure used to produce Bi 2−x M x Se 3 was the same used to produce Bi 2 Se 3 , 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 Bi 2 Se 3 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 spectrometer (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 10−7Pa. 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 (C p ), and density of the material ( ρ ) were used to obtain the thermal conductivities ( κtotal ), where κtotal = λ C pρ . 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 (C p ), 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, Bi 2 Se 3 nanosheets grouped into flower-like particles were produced, as shown by the SEM micrographs (Figure 1a,b). Bi 2 Se 3 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 Bi 2 Se 3 phase (Figure 1d, JCPDS No. 00-033-0214). Figure 1e shows the layered rhombohedral crystal structure of Bi 2 Se 3 , which consists of five covalently 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 crystallinity and has a crystal phase consistent with the Bi 2 Se 3 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.(a,b)SEMmicrographsoftheBi 2 Se 3 particles.(c)Schemeofthescrewdislocationgrowth mechanism.(d)TheXRDpatternoftheBi 2 Se 3 particles.(e)Thelayeredrhombohedralcrystalstruc‐ tureofBi 2 Se 3 .(f)Alow‐resolutionTEMimage.(g)AnHRTEMmicrographoftheBi 2 Se 3 particles anditscorrespondingpowerspectrum.Fromthecrystallographicdomain,theBi 2 Se 3 latticefringe distancesweremeasuredtobe0.205nm,0.205nm,and0.204nm,at60.96°and120.86°,respectively, whichcouldbeilluminatedastherhombohedralBi 2 Se 3 phase,visualizedalongthe[0001]zoneaxis. Figure 1. (a,b) SEM micrographs of the Bi2Se3particles. (c) Scheme of the screw dislocation growth mechanism. ( d ) The XRD pattern of the Bi 2 Se 3 particles. ( e ) The layered rhombohedral crystal structure of Bi 2 Se 3 . ( f ) A low-resolution TEM image. ( g ) An HRTEM micrograph of the Bi 2 Se 3 particles and its corresponding power spectrum. From the crystallographic domain, the Bi 2 Se 3 lattice fringe distances were measured to be 0.205 nm, 0.205 nm, and 0.204 nm, at 60.96 ◦ and 120.86 ◦ , respectively, which could be illuminated as the rhombohedral Bi 2 Se 3 phase, visualized along the [0001] zone axis. Nanomaterials 2021,11, 1827 5 of 12 Nanomaterials2021,11,xFORPEERREVIEW5of13   TheXRDanalysisshowedthatthecrystalstructureoftheobtainednanosheets matchedtherhombohedralBi2Se3phase(Figure1d,JCPDSNo.00‐033‐0214).Figure1e showsthelayeredrhombohedralcrystalstructureofBi2Se3,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crystal‐ linityandhasacrystalphaseconsistentwiththeBi2Se3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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 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().  Figure2.(a)PhotographoftheconsolidatedBi2Se3cylinder.(b,c)Schemeandimagesofthesamples usedtomeasurethematerialtransportpropertiesinthedirectionsparallel(b)andvertical(c)tothe pressureaxis. Figure3displaystheelectricalconductivity(σ),Seebeckcoefficient(S),andpower factor(PF=S2σ)oftheBi2−xMxSe3(M=Sn,Cu,Ag,Pb,In)pelletsperpendiculartothepress direction.Theelectricalconductivityoftheundopedmaterialdecreasedwithtempera‐ ture,whichpointedtoadegeneratedsemiconductorbehavior.Comparedtotheundoped material,theelectricalconductivityofallthedopedsampleswasslightlylower,except fortheAg‐dopedsamples.Ontheotherhand,thesamplesdopedwithSnandtoaminor 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 2. ( a ) Photograph of the consolidated Bi 2 Se 3 cylinder. ( b , c ) Scheme and images of the samples used to measure the material transport properties in the directions parallel (b) and vertical (c) to the pressure axis. Figure 3displays the electrical conductivity ( σ ), Seebeck coefficient (S), and power factor (PF =S 2σ ) of the Bi 2−x M x Se 3 (M = Sn, Cu, Ag, Pb, In) pellets perpendicular to the press direction. The electrical conductivity of the undoped material decreased with temperature, which pointed to a degenerated semiconductor behavior. Compared to the undoped material, the electrical conductivity of all the doped samples was slightly lower, except for the Ag-doped samples. On the other hand, the samples doped with Sn and to a minor 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. Nanomaterials2021,11,xFORPEERREVIEW6of13    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 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 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 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4.ThermoelectricpropertiesofBi2−xSnxSe3perpendiculartothepressdirection:(a)electricalconductivity,σ;(b) Seebeckcoefficient,S;(c)powerfactor,PF. Figure5showsrepresentativeSEMmicrographsoftheBi1.93Sn0.07Se3particles,which hadsimilarsizesbutdidnotdisplaytheflower‐likemorphologyobservedfromBi2Se3.As showninFigure6,theXRDcharacterizationconfirmedthattheBi1.93Sn0.07Se3particles maintainedtheirrhombohedralstructure.WiththeintroductionofSn,theXRDpeaks shiftedtolower2θangles,suggestingtheincorporationofSnwithintheBi2Se3lattice[5]. Figure 3. TE properties of Bi 2−x M x Se 3 (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 4displays the electrical conductivity, Seebeck coefficient, and power factor of Bi 2-x Sn x Se 3 materials containing different amounts of Sn. We observed that the electrical 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 Bi 1.93 Sn 0.07 Se 3 . The decrease in the electrical conductivity and increase in Nanomaterials 2021,11, 1827 6 of 12 the absolute value of the Seebeck coefficient denoted a reduction in the charge carrier concentration, which points to the presence of Sn2+ ions instead of Sn4+ at Bi3+ sites. Nanomaterials2021,11,xFORPEERREVIEW6of13    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 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 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 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4.ThermoelectricpropertiesofBi2−xSnxSe3perpendiculartothepressdirection:(a)electricalconductivity,σ;(b) Seebeckcoefficient,S;(c)powerfactor,PF. Figure5showsrepresentativeSEMmicrographsoftheBi1.93Sn0.07Se3particles,which hadsimilarsizesbutdidnotdisplaytheflower‐likemorphologyobservedfromBi2Se3.As showninFigure6,theXRDcharacterizationconfirmedthattheBi1.93Sn0.07Se3particles maintainedtheirrhombohedralstructure.WiththeintroductionofSn,theXRDpeaks shiftedtolower2θangles,suggestingtheincorporationofSnwithintheBi2Se3lattice[5]. Figure 4. Thermoelectric properties of Bi 2−x Sn x Se 3 perpendicular to the press direction: ( a ) electrical conductivity, σ ; (b) Seebeck coefficient, S; (c) power factor, PF. Figure 5shows representative SEM micrographs of the Bi 1.93 Sn 0.07 Se 3 particles, which had similar sizes but did not display the flower-like morphology observed from Bi 2 Se 3 . As shown in Figure 6, the XRD characterization confirmed that the Bi 1.93 Sn 0.07 Se 3 particles maintained their rhombohedral structure. With the introduction of Sn, the XRD peaks shifted to lower 2θangles, suggesting the incorporation of Sn within the Bi2Se3lattice [5]. The HRTEM characterization confirmed the rhombohedral phase of the Bi 1.93 Sn 0.07 Se 3 particles (Figure 7a). EELS chemical composition maps obtained from the red squared region in the HAADF STEM micrograph shown in Figure 7b displayed a homogeneous distribution of Sn, Bi and Se within the Bi1.93Sn0.07Se3nanosheet. Nanomaterials2021,11,xFORPEERREVIEW7of13    Figure5.SEMmicrographsofBi1.93Sn0.07Se3nanosheetswithdifferentmagnifications;(a)magnification5.00K×,(b)mag‐ nification10.00K×.  Figure6.(a)XRDpatternsofBi2Se3andBi1.93Sn0.07Se3;(b)expansiondiagramoftheareacorrespond‐ ingtothediffractionpeakofBi2Se3(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. 18 19 Intensity (a.u.) 2  (Degree) b a (006) 10 20 30 40 50 60 70 80 Intensity (a.u.) Bi 1.93 Sn 0.07 Se 3 Bi 2 Se 3 JCPDS No. 00-033-0214 2  (Degree) Figure 5. SEM micrographs of Bi 1.93 Sn 0.07 Se 3 nanosheets with different magnifications; ( a ) magnification 5.00 K × , ( b ) magnification 10.00 K ×. Nanomaterials 2021,11, 1827 7 of 12 Nanomaterials2021,11,xFORPEERREVIEW7of13    Figure5.SEMmicrographsofBi1.93Sn0.07Se3nanosheetswithdifferentmagnifications;(a)magnification5.00K×,(b)mag‐ nification10.00K×.  Figure6.(a)XRDpatternsofBi2Se3andBi1.93Sn0.07Se3;(b)expansiondiagramoftheareacorrespond‐ ingtothediffractionpeakofBi2Se3(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. 18 19 Intensity (a.u.) 2  (Degree) b a (006) 10 20 30 40 50 60 70 80 Intensity (a.u.) Bi 1.93 Sn 0.07 Se 3 Bi 2 Se 3 JCPDS No. 00-033-0214 2  (Degree) Figure 6. ( a ) XRD patterns of Bi 2 Se 3 and Bi 1.93 Sn 0.07 Se 3 ; ( b ) expansion diagram of the area corresponding to the diffraction peak of Bi2Se3(006). Nanomaterials2021,11,xFORPEERREVIEW8of13    Figure7.(a)AnHRTEMmicrographofaBi 1.93 Sn 0.07 Se 3 nanosheet,detailedinformationoftheorangesquaredareaandits correspondingpowerspectrum.Bi 2 Se 3 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Bi 2 Se 3 phase,along its[0001]zoneaxis.(b)EELSchemicalcompositionmapsobtainedfromtheredsquaredareaoftheSTEMmicrograph. IndividualBiN 2,3 ‐edgesat679eV(red),SnM 4,5 ‐edgesat485eV(green),SeM 2,3 ‐edgesat162eV(blue)andcompositesof Bi‐Sn,Bi‐SeaswellasBi‐Sn‐Se.(c)Bi4f,Se3dandSn3dhigh‐resolutionXPSspectraobtainedfromBi 1.93 Sn 0.07 Se 3  nanosheets. Bi4f,Se3dandSn3dhigh‐resolutionXPSspectraofBi 1.93 Sn 0.07 Se 3 particlesaredis‐ playedinFigure7c.Thehigh‐resolutionBi4fspectrumwasfittedwithtwodoublets,as‐ sociatedwithBi 3+ withinaBi 2 Se 3 chemicalenvironment(Bi4f 7/2 at158.2eV)andBiwithin amoreelectronegativeenvironment,asitcouldbeBi 2 O 3 orBi 2 SeO 2 (Bi4f 7/2 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,corre‐ spondingtoSewithintheBi 2 Se 3 (Se3d 5/2 at53.6eV),seleniuminanelementalorBi 2 SeO 2  environment(Se3d 5/2 at55.2eV),probablyarisingfromthepartialoxidationofthemate‐ rialsurface,andSeinaSeO 2 chemicalenvironment(Se3d 5/2 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Sn x+ withinaSnSeandanoxidizedenviron‐ ment[31].AccordingtotheelectricalpropertiesmeasuredattheintroductionofSn,we tentativelyassignedtheSn x+ componentwithinSnSetoSn 2+ . Top‐viewandcross‐sectionSEMmicrographsofBi 2 Se 3 andBi 1.93 Sn 0.07 Se 3 (Figure8) showedthefinalpelletspresentingalaminarmicrostructurewithanevidentpreferential orientationofthemateriallayers.Bi 2 Se 3 displayedlargerandthinnerlayersthan Bi 1.93 Sn 0.07 Se 3 .WhencomparingtheXRDpatternsofthesampleheldintwonormaldirec‐ tions,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crys‐ tallographicorientationofthehot‐pressedmaterials,withthe[001]crystallographicdi‐ rectionorientedparalleltothepressureaxis. Figure 7. ( a ) An HRTEM micrograph of a Bi 1.93 Sn 0.07 Se 3 nanosheet, detailed information of the orange squared area and its corresponding power spectrum. Bi 2 Se 3 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 Bi 2 Se 3 phase, along its [0001] zone axis. ( b ) EELS chemical composition maps obtained from the red squared area of the STEM micrograph. Individual Bi N 2,3 -edges at 679 eV (red), Sn M 4,5 -edges at 485 eV (green), Se M 2,3 -edges at 162 eV (blue) and composites of BiSn, Bi-Se as well as Bi-Sn-Se. ( c ) Bi 4f, Se 3d and Sn 3d high-resolution XPS spectra obtained from Bi 1.93 Sn 0.07 Se 3 nanosheets. Nanomaterials 2021,11, 1827 8 of 12 Bi 4f, Se 3d and Sn 3d high-resolution XPS spectra of Bi 1.93 Sn 0.07 Se 3 particles are displayed in Figure 7c. The high-resolution Bi 4f spectrum was fitted with two doublets, associated with Bi 3+ within a Bi 2 Se 3 chemical environment (Bi 4f 7/2 at 158.2 eV) and Bi within a more electronegative environment, as it could be Bi 2 O 3 or Bi 2 SeO 2 (Bi 4f 7/2 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 Bi 2 Se 3 (Se 3d 5/2 at 53.6 eV), selenium in an elemental or Bi 2 SeO 2 environment (Se 3d 5/2 at 55.2 eV), probably arising from the partial oxidation of the material surface, and Se in a SeO 2 chemical environment (Se 3d 5/2 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 Sn x+ 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 Bi 2 Se 3 and Bi 1.93 Sn 0.07 Se 3 ( Figure 8 ) showed the final pellets presenting a laminar microstructure with an evident preferential orientation of the material layers. Bi 2 Se 3 displayed larger and thinner layers than Bi 1.93 Sn 0.07 Se 3 . 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 crystallographic orientation of the hot-pressed materials, with the [001] crystallographic direction oriented parallel to the pressure axis. Nanomaterials2021,11,xFORPEERREVIEW9of13    Figure8.(a,b)Cross‐sectionandtop‐viewSEMmicrographofBi2Se3.(c,d)Cross‐sectionandtop‐viewSEMmicrograph ofBi1.93Sn0.07Se3.(e,f)XRDpatternsofBi2Se3andBi1.93Sn0.07Se3measuredintwoperpendiculardirections,along(//)and perpendicular(⊥)tothepressureaxis. Figure9displaystheTEpropertiesofBi2Se3andBi1.93Sn0.07Se3measuredinthetwo directions.Weobservedσ>σ//,asexpectedfromthehigherchargecarriermobilitiesin theabcrystalplanecomparedwiththecdirection,andtheextendedsizeofthecrystal domainsinthedirectionnormaltothepressuredirectionwithinthelayeredpellets.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. Figure 8. ( a , b ) Cross-section and top-view SEM micrograph of Bi 2 Se 3 . ( c , d ) Cross-section and top-view SEM micrograph of Bi 1.93 Sn 0.07 Se 3 . ( e , f ) XRD patterns of Bi 2 Se 3 and Bi 1.93 Sn 0.07 Se 3 measured in two perpendicular directions, along (//) and perpendicular (⊥) to the pressure axis. Figure 9displays the TE properties of Bi 2 Se 3 and Bi 1.93 Sn 0.07 Se 3 measured in the two directions. We observed σ⊥ > σ// , as expected from the higher charge carrier mobilities in the ab crystal plane compared with the cdirection, and the extended size of the crystal domains in the direction normal to the pressure direction within the layered pellets. On Nanomaterials 2021,11, 1827 9 of 12 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,xFORPEERREVIEW10of13    Figure9.TEpropertiesofBi2Se3andBi1.93Sn0.07Se3parallel(//)andvertical(⊥)tothepressureaxis:(a)electricalconductiv‐ 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. WhencomparingtheTEpropertiesofBi2Se3andBi1.93Sn0.07Se3,weobservedσto slightlyincreaseandσ//toslightlydecreasewiththeintroductionofSn,whichcouldbein partrelatedtothethinnerandlargermateriallayersobservedwithinthelayeredBi2Se3 pelletscomparedwithBi1.93Sn0.07Se3.Ontheotherhand,withtheintroductionofSn,Sin‐ creasedinbothdirections,whichresultedinahigherPFinbothdirectionsforthe Bi1.93Sn0.07Se3sample. Table1displaystheHallchargecarrierconcentrationandmobilityofBi2Se3and Bi1.93Se0.07Se3pelletsatroomtemperature.Asexpectedfromtheelectricalconductivityand Seebeckcoefficientmeasurements,weobservedtheSnintroductiontoresultinadecrease inthechargecarrierconcentration,fromnH=1.8×1019cm−3forBi2Se3tonH=1.3×1019cm−3 forBi1.93Se0.07Se3.ThisresultisconsistentwiththepresenceofSn2+replacingBi3+ions,thus trappingafreeelectron.Webelievethisoptimizationofthechargecarrierconcentration tobeattheoriginofthehigherPFobtainedwiththeintroductionofSn.Wefurthercal‐ culatedtheeffectivemassoftheBi2Se3andBi1.93Sn0.07Se3materialsusingasingleparabolic band(SPB)model.Fordetailedcalculations,thecarriertransportpropertyanalysisin‐ cluded[32]: Table1.Roomtemperaturetransportpropertiesandm*oftheBi2Se3andBi1.93Se0.07Se3pellets. Materialsnн[1019cm−3] μн[cm2V−1S−1]m* / m0 Bi2Se31.77178.60.27 Bi1.93Sn0.07Se31.34239.30.29 TheSeebeckcoefficient, Figure 9. TE properties of Bi 2 Se 3 and Bi 1.93 Sn 0.07 Se 3 parallel (//) and vertical ( ⊥ ) to the pressure axis: ( a ) electrical conductivity, 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. When comparing the TE properties of Bi 2 Se 3 and Bi 1.93 Sn 0.07 Se 3 , we observed σ⊥ to slightly increase and σ// to slightly decrease with the introduction of Sn, which could be in part related to the thinner and larger material layers observed within the layered Bi 2 Se 3 pellets compared with Bi 1.93 Sn 0.07 Se 3 . On the other hand, with the introduction of Sn, Sincreased in both directions, which resulted in a higher PF in both directions for the Bi1.93Sn0.07Se3sample. Table 1displays the Hall charge carrier concentration and mobility of Bi 2 Se 3 and Bi 1.93 Se 0.07 Se 3 pellets at room temperature. As expected from the electrical conductivity and Seebeck coefficient measurements, we observed the Sn introduction to result in a decrease in the charge carrier concentration, from n H = 1.8 × 10 19 cm −3 for Bi 2 Se 3 to nH= 1.3 ×1019 cm−3 for Bi 1.93 Se 0.07 Se 3 . This result is consistent with the presence of Sn 2+ replacing Bi 3+ ions, thus trapping a free electron. We believe this optimization of the charge carrier concentration to be at the origin of the higher PF obtained with the introduction of Sn. We further calculated the effective mass of the Bi 2 Se 3 and Bi 1.93 Sn 0.07 Se 3 materials using a single parabolic band (SPB) model. For detailed calculations, the carrier transport property analysis included [32]: Table 1. Room temperature transport properties and m* of the Bi2Se3and Bi1.93Se0.07Se3pellets. Materials nH[1019 cm−3]µH[cm2V−1S−1]m*/m0 Bi2Se31.77 178.6 0.27 Bi1.93Sn0.07Se31.34 239.3 0.29