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n-Type Ultrathin Few-layer Nanosheets of Bi Doped SnSe: Synthesis and Thermoelectric Properties Sushmita Chandra, Ananya Banik, and Kanishka Biswas ACS Energy Lett., Just Accepted Manuscript • DOI: 10.1021/acsenergylett.8b00399 • Publication Date (Web): 17 Apr 2018 Downloaded from http://pubs.acs.org on April 17, 2018

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ACS Energy Letters

n-Type Ultrathin Few-layer Nanosheets of Bi Doped SnSe: Synthesis and Thermoelectric Properties Sushmita Chandra†, Ananya Banik† and Kanishka Biswas†, ‡, * †

New Chemistry Unit (NCU) and ‡School of Advanced Materials (SAMat), Jawaharlal Nehru Centre for Advanced Scientific Research (JNCASR), Jakkur P.O., Bangalore 560064 (India) *E-mail: [email protected]

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ABSTRACT: SnSe, an environmental friendly layered chalcogenide fostered immense attention in the thermoelectric community with their high thermoelectric figure of merit in single crystals. Although the stride towards developing superior p-type SnSe as a thermoelectric material is progressing rapidly, synthesis of n-type SnSe is somewhat overlooked. Here, we report the solution phase synthesis and thermoelectric transport properties of two dimensional (2D) ultrathin (1.2-3 nm thick) few layer nanosheets (2-4 layers) of n-type SnSe. The n-type nature of the nanosheets initially originates from chlorination of the material during the synthesis. We could able to increase the carrier concentration of n-type SnSe significantly from 3.08×1017 cm-3 to 1.97×1018 cm-3 via further Bi doping which results in the increment of electrical conductivity and power factor. Furthermore, Bi-doped nanosheets exhibit ultralow lattice thermal conductivity (~ 0.3 W/mK) throughout the temperature range of 300-720 K which can be ascribed to the effective phonon scattering by interface of SnSe layers, nanoscale grain boundaries and point defects.

TOC GRAPHICS:

ClSe2Sn2+ Bi3+ -200

n-type Sn1-xBixSe nanosheets

-225

S (V/K)

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Thermoelectric materials realize direct conversion of temperature gradient to electrical voltage, thus can play an important role in effective recovery of waste heat to useful electricity. The efficiency of a thermoelectric material is determined by dimensionless figure of merit (ZT), ZT = σS2T/κ, where σ, S, κ and T are the electrical conductivity, Seebeck coefficient, thermal conductivity and the absolute temperature, respectively. 1-5 Inorganic solid materials with ultralow lattice thermal conductivity (κL) are essential for good thermoelectric performance.1 Recently, layered metal chalcogenides (LMCs) have acquired a momentous attention in the field of thermoelectrics as they exhibit record high thermoelectric figure of merit due to their intrinsically low thermal conductivity owing to their anisotropic crystal and electronic structures.6-7 Few layered metal chalcogenides are, in general, held together by weak interlayer van der Waals interactions which provide it with a novel electronic structure and high specific surface area.8-10 Two dimensional (2D) few layer nanosheets/plates of layered Bi 2X3 (X=Se, Te) have displayed promising thermoelectric performance due to their enhanced metallic surface states, high carrier mobility and low κL.11-13 Recently, ultralow κL in few layered nanosheets of natural 2D van der Waals heterostructure in homologous intergrowth series, (MX) m(Bi2X3)n, [M = Pb, Sn; X = Te, Se] have been achieved.14-16 Further, nanosheets/plates of n-type SnSe2 exhibit good thermoelectric performance at medium temperatures due to low κL.17,18 Among the layered metal chalcogenides, tin selenide (SnSe), an important p-type semiconductor, has received immense momentum in this area due to its remarkable thermoelectric performance, low toxicity and earth-abundance of the component elements. 19-22 Single crystals of SnSe have lately shown an outstanding ZT of 2.6 at 923 K, which can be attributed to its layered structure, soft chemical bonding and lattice anharmonicity. 20 However, polycrystalline SnSe is a better choice for thermoelectric applications for the ease of its 3 ACS Paragon Plus Environment

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production and machinability.19 There are number of validating recent reports on p-type bulk polycrystalline SnSe showing good thermoelectric performance.23-29 Though, in most of the cases, reported SnSe materials are p-type while thermoelectric applications demand for both ptype and n-type materials. Recently, n-type single crystals of 6 mole% Bi-doped SnSe have been reported to show high ZT of 2.2.30 Till date, few nanostructures (eg. nanowires, nanoplates, nanoflowers and nanosheets) of SnSe have been synthesized by bottom-up wet chemical synthesis,31,32 but the materials are mostly p-type.33,34 Recently, electronic transport properties of n-type Cl-doped SnSe nanoparticles have been studied, which show moderate power factors. 35 Thus, n-type SnSe with 2D few-layered nanosheet morphology would be interesting for thermoelectric investigations as it may exhibits low κL due to excess phonon scattering from nanoscale grains and interfaces. Herein, we have demonstrated the low temperature solution phase synthesis and anisotropic thermoelectric properties of n-type few-layer (~2-4 layers) Bi-doped SnSe nanosheets (1.2-3 nm thick). Nanosheets of Sn0.94Bi0.06Se exhibit lower band gap compared to the pristine SnSe nanosheets due to the formation of Bi acceptor level near the conduction band. Bi-doped few-layer SnSe nanosheets show enhanced n-type carrier concentration compared to SnSe nanosheets, which resulted in superior electrical conductivity and power factor for spark plasma sintering (SPS) processed Sn0.94Bi0.06Se nanosheets compared to SnSe nanosheets. We have achieved higher Seebeck coefficient value in the perpendicular to the SPS pressing direction (i.e. along c-axis in SnSe) compared to parallel to SPS pressing direction (i.e. along aaxis in SnSe) due to high effective mass of the conduction band electron along the crystallographic c-axis. Furthermore, Sn0.94Bi0.06Se nanosheets exhibit ultralow lattice thermal conductivity (~ 0.3Wm-1K-1) in the temperature range of 300-720 K due to effective phonon 4 ACS Paragon Plus Environment

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scattering by interface of SnSe layers and point defects. We have achieved a ZT of 0.21 at 719 K in Sn0.94Bi0.06Se nanosheets, which is higher than that of the controlled n-type SnSe nanosheet sample. To prepare n-type SnSe nanosheets, stoichiometric amount of SnCl 4·5H2O and SeO2 were taken in oleylamine and 1,10-phenanthroline (Phen) mixture. The mixture was heated up to 200 °C under N2 atmosphere. Black colored suspension was observed immediately, which indicates the formation of SnSe. The product was precipitated out by the addition of hexane and ethanol (1:1 mixture). Here, oleylamine acts as capping reagent as well as reducing agent and Phen acts as a morphology controlling agent. The yield of the reaction was ~95% and the product was scaled up to ~7 gm for the thermoelectric measurements. The nanosheets were dispersed in cyclohexane for further characterizations. 6 mol% Bi doped SnSe (Sn 0.94Bi0.06Se) nanosheets have been synthesized by using Bi-neodecanoate as the bismuth precursor (see experimental section, supporting information, SI, Figure S1). (c)

Se

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190 195 200 205 Binding Energy (eV)

Figure 1.a) Structure of SnSe viewed down the crystallographic b-direction. b) Highly distorted SnSe7 polyhedra containing four long (dotted lines) and three short (solid lines) Sn-Se bonds. c) PXRD patterns of as-synthesized SnSe and Sn0.94Bi0.06Se nanosheets. d) Optical absorption spectra of SnSe and Sn0.94Bi0.06Se nanosheets. e) High-resolution XPS of Sn3d, Se3d, Bi4f and Cl2p in Sn0.94Bi0.06Se nanosheets. 5 ACS Paragon Plus Environment


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SnSe adopts a layered orthorhombic crystal structure (Pnma) at room temperature, in which the intralayer Sn and Se atoms are covalently bonded in the a-b plane and the layers are held together by interlayer van der Waals interactions along the c-axis (Figure 1a). Powder Xray diffraction (PXRD) patterns of as-synthesized samples have been indexed to pure orthorhombic SnSe (Figure 1c) with lattice parameters of a = 11.50 Å, b = 4.15 Å and c = 4.45 Å. The indirect optical band gap of the SnSe nanosheets were measured to be ~ 0.88 eV (Figure 1d). Further, Bi being more electronegative than Sn, is known to create an acceptor level just below the conduction band of Sn, which in-turn reduces the band gap of the Sn0.94Bi0.06Se nanosheets to ~ 0.75 eV (Figure 1d). In order to confirm Bi doping, X-ray photoelectron spectroscopy (XPS) was performed on both SnSe and Bi-doped SnSe nanosheets (Figure 1 and Figure S2, SI). Figure1e confirms the presence of Sn, Se and Bi in as-synthesized Sn0.94Bi0.06Se nanosheets. The Sn3d spin-orbit doublet peaks appeared at 488.4 eV and 496.7 eV with splitting of 8.3 eV (Figure 1e), which can be assigned to Sn3d5/2and Sn3d3/2, respectively.14 We could able to deconvolute the broad peak of selenium as Se3d5/2and Se3d3/2 at 53.1 eV and 55.45 eV respectively.17 Presence of Bi4f7/2 and Bi4f5/2 peaks at 159.6 eV and 164.95 eV can be attributed to Bi (III) states in Bi-doped SnSe sample.14 In situ chlorination of as-synthesized SnSe and Sn0.94Bi0.06Se nanosheets was further confirmed from the XPS peak at 198.7 eV (Figure 1e).17 Table 1. Compositional analysis from EDAX and ICP. Nominal Composition

EDAX Composition

SnSe

SnSe1.05

SnSe1.33

Sn0.94Bi0.06Se

Sn0.91Bi0.058Se1.47

Sn0.94Bi0.055Se1.42


ICP-AES Composition

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The actual elemental compositions of SnSe nanosheet samples were obtained further from inductively coupled plasma atomic emission spectroscopy (ICP-AES) and energy dispersive analysis of X-Rays (EDAX). Compositions determined by these two independent techniques are in good agreement with the nominal compositions (Table 1), which confirms successful control over the composition in the present synthetic technique. (a)

1.18 nm

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Sn 010

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Figure 2.a) FESEM image of SnSe nanosheets. b) AFM image of single nanosheet of SnSe having. c) TEM image of SnSe nanosheets. d) HRTEM image reveals the crystalline nature of the as-synthesized SnSe nanosheets. e) SAED pattern of a single SnSe nanosheet. f) STEM image of nanosheets and EDAX color mapping for Sn, Se and Cl. Field emission scanning electron microscopy (FESEM) images display the 2D nanosheet morphology of SnSe and Sn0.94Bi0.06Se samples (Figures 2a and 3a). SnSe and Bi-doped SnSe samples show folded nanosheet like morphology with the thickness of few nanometers (1.23nm). The ultra-thin nature of the nanosheets was confirmed by AFM, as depicted in Figure 2b 7 ACS Paragon Plus Environment


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and 3b. The height profile acquired from a typical AFM micrograph indicates the formation of 1.18 nm thick nanosheets (Figure 2b), which is nearly the thickness of two-layers SnSe (i.e. a axis of the unit cell). The lateral dimension of the as-synthesized nanosheets ranges from 0.3 to 0.5 μm. 2D nanosheet nature of the samples was further verified from transmission electron microscope (TEM) images in Figure 2c and 3c. High-resolution TEM (HRTEM) images of SnSe and Sn0.94Bi0.06Se nanosheets clearly show a lattice spacing of 3.06 Å, which corresponds to the (011) plane of SnSe (Figure 2d and 3d). (a)

(b)

Si

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Figure 3. a) FESEM image of Sn0.94Bi0.06Se nanosheets. b) AFM image of single nanosheet of Sn0.94Bi0.06Se. c) TEM image of Sn0.94Bi0.06Se nanosheets. d) HRTEM image of Sn0.94Bi0.06Se nanosheets. e) SAED pattern of a single Sn0.94Bi0.06Se nanosheet. Inset of (e) shows the STEM image of single nanosheet of Sn0.94Bi0.06Se. f) EDAX color mapping of Sn, Se, Bi and Cl from the Sn0.94Bi0.06Se nanoshhet shown in STEM image.


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The single crystalline nature of the nanosheets is confirmed from selected-area electron diffraction (SAED) pattern recorded from single nanosheet region (Figure 2e and 3e), which are indexed based on the orthorhombic SnSe structure. EDAX elemental color mapping has been done during scanning transmission electron microscopy (STEM) imaging which shows homogeneous distribution of all the respective elements in SnSe and Sn 0.94Bi0.06Se nanosheets (Figure 2f and 3f), which also confirms the successful Bi doping in SnSe. In order to investigate thermoelectric properties of SnSe and Bi-doped SnSe few-layer nanosheets, capping reagent has been removed by heat-treatment of as-synthesized powder at 500 °C under N2 flow. The conventional hydrazine treatment route for removal of capping agent was avoided because SnSe reacts with hydrazine to form a SnSe-hydrazine hydrate adduct. 36 During this heat treatment, no change in structure and composition was found, as indicated by PXRD patterns (Figure S3, SI). The FTIR spectra confirms the removal of organic capping agent in the heating process (Figure S4, SI). To measure the thermoelectric properties, the surface cleaned nanosheets were pressed into pellets using SPS process at a pressure of 40 MPa and 450 °C under vacuum. Obtained density of the pellets was found to be ~95 % of the theoretical density. The sample became more compact and the adjacent nanosheets are arranged on top of each other with negligible change in the nanoscale-grain size as seen from the FESEM images of the SPS-processed samples (Figure S5, SI). As SnSe shows anisotropic transport properties originating from its layered structure (as depicted from PXRD in Figure S6, SI), thermoelectric transport properties were measured both in parallel (||) and perpendicular (⊥) to the pressing direction of SPS. Figure 4 presents the temperature dependent thermoelectric properties (both in || & ⊥ to SPS pressing directions) of SnSe and Sn0.94Bi0.06Se nanosheets in the temperature range of 3009 ACS Paragon Plus Environment


720 K. Electrical conductivity (σ) and Seebeck coefficient (S) were measured simultaneously under He atmosphere by a ULVAC-RIKO ZEM-3 instrument. Figure 4a represents temperature dependence of σ for both SnSe and Sn0.94Bi0.06Se samples. Being a semiconductor, electrical conductivity increases linearly with temperature for both the samples. The electrical conductivity measured along perpendicular to the SPS pressing direction is more than those from parallel to the pressing direction, which can be attributed to favorable electronic transport in perpendicular to pressing direction (i.e. the c-axis, along the layers in SnSe, Figure 1a), as also observed elsewhere.27,37,38 Room temperature σ for SnSe nanosheets measured perpendicular to pressing direction is to be 0.28 Scm−1, which increases to 9 Scm−1 at 719 K. Bi-doped nanosheets show superior electrical conductivity as compared to that of pristine SnSe. At room temperature, the σ values for the Sn0.94Bi0.06Se nanosheets (for the ⊥ to pressing direction) was 1.20 Scm-1, which increases to 12.5 Scm-1 with increase in temperature. The Hall coefficient of both the SnSe and Sn0.94Bi0.06Se nanosheets is negative at room temperature, which confirms the n-type conduction. The n-type nature of SnSe can be attributed to the in-situ chlorination during synthesis as confirmed from ICP and EDAX analysis (Table 1). The room temperature n-type carrier concentration (n) for Sn0.94Bi0.06Se nanosheets was estimated based on Hall measurement to be 1.97 × 10 18 cm−3, which is significantly higher compared to that of the pure SnSe nanosheet (3.08 × 1017 cm−3). Substitution of Bi3+ at Sn2+ site contributes excess electron to conduction band, which results in enhanced n-type carrier concentration and electrical conductivity. Negative Seebeck coefficient (S) indicates the presence of n-type carriers in both the samples (Figure 4b). Typically, Sn0.94Bi0.06Se nanosheets (⊥ to pressing direction) has S value of


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(a)

(b)

SnSe ( || ) SnSe ( ) Sn0.94Bi0.06Se ( || )

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S (V/K)

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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 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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0.1

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Figure 4.Temperature dependent a) Electrical conductivity, (σ) b)Seebeck coefficient, (S) c) Power factor,(σS2) d) Total thermal conductivity, (κtotal) e) Lattice thermal conductivity, (κL) and f) Thermoelectric figure of merit, (ZT) of SnSe and Sn0.94Bi0.06Se nanosheets measured along the SPS direction (indicated by squares) and perpendicular to the SPS direction (indicated by spheres). -219 μV/K at 300 K, which increases to -285 μV/K at 719 K. Interestingly, we observe the S value is higher in the perpendicular to the pressing direction (along the c-axis in SnSe structure) than that of the parallel to the pressing direction, which is earlier observed in n-type Cl/Br-doped bulk SnSe.37,38 Previous electronic structure calculation demonstrated the presence of a heavy conduction band (Γ-Z direction i.e. the c-axis in SnSe) along with the principal conduction band 11 ACS Paragon Plus Environment


in SnSe.39 Thus, high effective mass (m*) of electron along the c-axis of SnSe (⊥ to the pressing direction) resulted high S value compared to that of the parallel to the pressing direction. Sn0.94Bi0.06Se nanosheets sample exhibit higher power factor of 100 µW/mK 2 at 719 K which significantly higher than that of the SnSe (Figure 4c). The total thermal conductivity, κtotal (Figure 4d) was estimated in the temperature range of 300−720 K using the formula, κtotal = DCpρ, where D is the thermal diffusivity, Cp is specific heat, and ρ is density of the sample. Temperature dependent (300-720 K) D has been measured by laser flash diffusivity technique in NETZSCH LFA 457 instrument. The κL was acquired by subtracting the electronic thermal conductivity, κele, from the κtotal. The electronic thermal conductivities, κele were evaluated (Figure S7, SI) using Wiedemann-Franz Law, κele = LσT, where L is the Lorenz number which is calculated by fitting reduced chemical potential derived from temperature-dependent Seebeck coefficient using single parabolic band conduction and dominant acoustic phonon scattering of carriers. Sn0.94Bi0.06Se nanosheets show lower κL in the parallel to SPS pressing direction (i.e. a-axis in SnSe) compared to that of the perpendicular to the SPS pressing direction (i.e. c-axis of SnSe), which is attributed to phonon scattering by the interfaces between the layers. Typically, Sn0.94Bi0.06Se nanosheets exhibit ultralow κL of ~0.3 Wm-1K-1 over 300–720 K measured perpendicular to SPS pressing direction which is in close proximity to the theoretical minimum, κmin of SnSe (0.26 Wm-1K-1).27 n-type Sn0.94Bi0.06Se fewlayer nanosheets (⊥ to the SPS pressing direction) demonstrate ZT of 0.21 at 719 K, which is higher than that of n-type SnSe nanosheet sample (Figure 4f). In summary, n-type few-layer (2-4 layers) Bi-doped SnSe 2D nanosheets (1.2-3 nm thick) have been synthesized via facile low temperature solution based route. The n-type nature of Sn0.94Bi0.06Se can be attributed to the in-situ chlorination and donor dopant nature of Bi. The 12 ACS Paragon Plus Environment

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presence of nanoscale grain boundaries and layered anisotropic structure enables the heat carrying phonons to get scattered significantly, thereby decreasing κL to as low as ~ 0.3 Wm-1K-1 over 300–720 K range. A combination of low thermal conductivity coupled with high power factor originated due to the enhancement of carrier concentration to 1.97 x 1018 cm-3, gives rise to a ZT of 0.21 at 719 K for 6% Bi-doped SnSe nanosheets, which is higher than that of n-type SnSe nanosheets.

ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website. Experimental section, photographs of reaction, XPS spectrum, PXRD, FTIR spectra, FESEM images of SPS-processed samples, temperature dependent κele, reversibility and reproducibility of ZT and table for carrier concentration. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]. ORCID Kanishka Biswas: 0000-0001-9119-2455 Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This work was supported by the SERB, DST (EMR/2016/000651) project and Sheik Saqr Laboratory, JNCASR. S.C. sincerely thanks JNCASR for the fellowship.



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