N-type Bi-doped SnSe Thermoelectric Nanomaterials Synthesized by

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N‑type Bi-doped SnSe Thermoelectric Nanomaterials Synthesized by a Facile Solution Method Xiaofang Li,† Chen Chen,† Wenhua Xue,‡ Shan Li,† Feng Cao,§ Yuexing Chen,∥ Jiaqing He,⊥ Jiehe Sui,# Xingjun Liu,*,†,# Yumei Wang,*,‡ and Qian Zhang*,†

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Department of Materials Science and Engineering, and Institute of Materials Genome & Big Data and §Department of Science, Harbin Institute of Technology, Shenzhen, Guangdong 518055, P. R. China ‡ Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy of Science, Beijing 100190, P. R. China ∥ Shenzhen Key Laboratory of Advanced Thin Films and Applications,College of Physics and Energy, Shenzhen University, Shenzhen 518060, P. R. China ⊥ Shenzhen Key Laboratory for Thermoelectric Materials and Department of Physics, Southern University of Science and Technology, Shenzhen, Guangdong 518055, P. R. China # State Key Laboratory of Advanced Welding and Joining, Harbin Institute of Technology, Harbin, Heilongjiang 150001, P. R. China ABSTRACT: An n-type Bi-doped SnSe was synthesized by a facile solution method followed by spark plasma sintering. We used bismuth(III) 2-ethyhexanoate as a cationic dopant precursor, which can absorb on the powder surface and then diffuse into the lattice to realize the substitution of Sn by Bi. A strip structure with low-angle boundary was constructed for effective phonon scattering. With increasing content of Bi, the carrier concentration decreased from 1.35 × 1019 cm−3 (ptype) in undoped SnSe to 4.7 × 1014 cm−3 (n-type) in Sn0.99Bi0.01Se and then increased to 1.3 × 1015 cm−3 (n-type) in Sn0.97Bi0.03Se. The Seebeck coefficient changed from positive to negative and presented n-type conducting behavior in the whole measured temperature range from 300 to 773 K, reaching a maximum absolute value of ∼900 μV K−1 at room temperature and ∼300 μV K−1 at 773 K. Considering the rich variety of metal 2-ethylhexanoates, higher thermoelectric performance is expected by different cationic doping in solutionsynthesized nanomaterials.

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INTRODUCTION Thermoelectric (TE) generator has the capability to directly convert waste heat into electricity, playing an important role in alleviating current global energy crisis.1−4 Large-scale industrial application of TE power-generating systems requires improvement of the overall thermoelectric efficiency, characterized by a dimensionless figure-of-merit ZT = S2σT/κ, where S is the Seebeck coefficient, σ is the electrical conductivity, κ is the thermal conductivity, and T is the absolute temperature in Kelvin.5,6 High performance needs high Seebeck coefficient, high electrical conductivity, and low thermal conductivity.7 Nanostructuring is extensively employed for lowering the thermal conductivity, which aroused the research on the solution synthesis of TE nanomaterials.7−17 However, further development was seriously impeded by the challenges on the doping in solution, resulting in a low electrical conductivity and a low figure-of-merit.18 In solution synthesis, “self-purification” of impurity normally causes a decreased solubility of dopant in nanocrystals.19 Oxidation-prone nanoparticle surface is easy to induce traps that may pin the Fermi level near the midgap and hinder substantial net doping.20−22 Various types of strong © XXXX American Chemical Society

surfactants often attach to the surface of particles and degrade their electrical properties.23 Anionic doping by using metal halide precursors has been proven effective to realize n-type conducting behavior,24−27 while cationic doping is scarcely investigated due to the absence of suitable dopant precursor. Metal 2-ethylhexanoates belong to the metal alkanoates with medium size (C5−C12) carbon chain length.28 With a number of unique advantages, including inexpensive, air-insensitive, nontoxic, having high solubility in organic solvents, and composited by a variety of commercially available elements, this metallorganic was widely used as an effective precursor in chemical solution deposition (CSD), sol−gel synthesis of nanofilms, and solution synthesis of semiconductor nanomaterials.29−34 SnSe is an intrinsic p-type semiconductor with an orthorhombic layer structure.35−37 Both p-type and n-type SnSe single crystals have extraordinary high ZT values.38−40 And the ZT values of most SnSe polycrystallines exceed 1.0.16,41−43 In this paper, we explored the possibility of using metal 2Received: August 16, 2018

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DOI: 10.1021/acs.inorgchem.8b02324 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 1. (a) Solution-synthesized reaction apparatus and SnSe nanopowders, (b) XRD patterns of nanopowder with nominal composition Sn1−xBixSe (x = 0, 0.01, 0.02, 0.03, and 0.04), (c) low-magnification TEM image of Sn0.97Bi0.03Se nanoplates, a characteristic strip is marked, (d) [100] HRTEM of Sn0.97Bi0.03Se nanoplates and corresponding SAED patterns, and (e) antiphase domain observed on nanoplate. The surface of the nanoplate is partly oxidized. Bi-doped SnSe was also prepared by melting, ball milling, and SPS (M + BM + SPS) for comparison.24 Characterization. X-ray diffraction analysis (XRD) was conducted on a Rigaku D/max 2500 PC X-ray diffractometer with Cu Kα radiation (λ = 1.5418 Å). In situ X-ray powder diffraction was detected on a Beuker/D8 Advance X-ray diffractometer with Cu Kα radiation (λ = 1.540 60 Å). The microstructure of the fracture surface was investigated by a scanning electron microscope (SEM, Hitachi S4700) and a spherical aberration-corrected (Cs-corrected) electron microscope (JEM-ARM200F). Optical diffuse reflectance measurements were performed using a Shimadzu UV-3600 double-beam, doublemonochromator spectrophotometer operating from 200 to 2500 nm. The band gaps were calculated using the Kubelka−Munk function. Xray photoelectron spectra (XPS) were obtained by a PHI 5000 Versa Probe system (Physical Electronics) using a monochromatic Al Kα Xray source (1486.6 eV). The electrical resistivity (ρ) and Seebeck coefficient (S) were measured simultaneously on a commercial system (CTA-3) from room temperature to 773 K. The thermal conductivity κ was calculated using κ = DαCp, where D is volumetric density determined by the Archimedes method, α is the thermal diffusivity measured on a laser flash apparatus (Netzsch LFA 457), and Cp is the specific heat obtained on a differential scanning calorimetry thermal analyzer (Netzsch DSC 404 F3). The room-temperature Hall coefficient RH was measured by van der Pauw technique under a reversible magnetic field of 1.5 T. The Hall carrier concentration nH and Hall mobility μH were calculated using nH = 1/(eRH) and μH = σRH, respectively.

ethylhexanoates (explicitly Bi 2-ethylhexanoates) as a cationic dopant precursor in solution synthesis of n-type SnSe TE materials. Single-phased Bi-doped Sn1−xBixSe (x = 0.01, 0.02, and 0.03) was prepared by solution method followed by spark plasma sintering (SPS). The successful doping of Bi was synchronously confirmed by measurements of band gaps, chemical states, and Hall coefficients. Higher power factor was achieved due to the higher Seebeck coefficients compared with that of the sample prepared by melting, ball milling, and spark plasma sintering.

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EXPERIMENTAL SECTION

Sample Preparation. SnCl2 (98%, Alfa Aesar), selenium (99.999%, Alfa Aesar), bismuth(III) 2-ethyhexanoate (Alfa Aesar), NaOH (>96%, Aladdin), NaBH4 (98%, Aladdin), anhydrous ethylenediamine (>99%, Aladdin), and BiCl3 (98%, Alfa Aesar) were used without any further purification. In a typical synthesis for Sn1−xBixSe (x = 0, 0.01, 0.02, 0.03, and 0.04), 3 g of NaOH and 1 g of NaBH4 were dissolved into 120 mL of deionized water mixed with 40 mL of anhydrous ethylenediamine by stirring for 30 min at room temperature to form a transparent solution. Stoichiometric amounts of SnCl2, bismuth(III) 2-ethylhexanoate (or BiCl3), and Se powder (10 mmol) were added into this solution and heated to 423 K under vigorous stirring. After 2 h, the flask was cooled in an ice bath to precipitate the nanocrystals. The black product was collected by centrifugation, washed with deionized water and absolute ethanol several times, and dried in a vacuum oven at 50 °C for 12 h. A vacuum/dry argon gas Schlenk line was used for synthesis, and an argon-filled glovebox was used for storing and handling air- and moisture-sensitive chemicals. The obtained powder was annealed under H2/Ar at 300 °C for 30 min before densified by the SPS method at 773 K for 20 min under 50 MPa pressure to obtain a half-inch disk (relative density > 95%). A 3 mol %

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RESULTS AND DISCUSSION Facile wet chemical method is widely used to synthesize nanocrystals with controllable size and morphology for diverse applications.44,45 A high yield of SnSe nanopowder was synthesized by directly stirring all the raw materials at 423 K (see Figure 1a). The crystal structures of the as-synthesized B

DOI: 10.1021/acs.inorgchem.8b02324 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 2. Temperature dependence of (a) electrical conductivity, (b) Seebeck coefficient, (c) PF, (d) thermal conductivity, (e) ZT, and (f) average ZT measured perpendicular to the hot pressing direction compared with the previous reported data.47−51

crystal surface indicates the small proportion of oxidation to SnO2 during the reaction or post-reaction process. All the synthesized powder was subsequently condensed by SPS. The thermoelectric properties of undoped sample synthesized by this chemical method were explored and presented in Figure 2a−f compared with the previous reported data [refs 39−43]. With the lower thermal conductivity and the competitive power factor (PF), the ZT value perpendicular to the pressure direction is ∼0.6 at 773 K, which is higher than the undoped SnSe prepared by melting and close to the other p-type SnSe polycrystals, suggesting an effective method for the synthesis of SnSe.42,47 The XRD patterns of all the Sn1−xBixSe (x = 0, 0.01, 0.02, 0.03, 0.04) bulk samples perpendicular to the SPS direction are shown in Figure 3a. All the peaks can be indexed to the orthorhombic phase of SnSe (space group Pnma), except for a trace amount of Bi observed in Sn0.96Bi0.04Se. The inset shows the image of the doped pellet with high relative density ∼99% of the theoretical density (see Table 1). Figure 3b shows the in situ variabletemperature XRD of Sn0.97Bi0.03Se powder. The peak position obviously shifted to the left with increasing temperature,

powders with nominal composition Sn1−xBixSe (x = 0, 0.01, 0.02, 0.03, and 0.04) were examined by XRD, and the results are shown in Figure 1b. The major diffraction peaks of all the samples can be indexed as the orthorhombic SnSe phase (Pnma space group, JCPDS No. 48-1224) with lattice parameters of a = 11.49 Å, b = 4.13 Å, and c = 4.44 Å.46 A small amount of Bi secondary phase was observed in the doped samples. (We will trace the Bi dopant later.) The obvious difference in relative diffraction intensity in (400) and (111) planes indicates the existence of anisotropy of the samples in agreement with the microstructure shown in low-magnification transmission electron microscopy (TEM) image (Figure 1c). The product is composed of nanoplates with individual sizes of several tens nanometers and a characteristic morphology of strips having width around tens of nanometers and length around hundreds of nanometers. The high-resolution TEM (HRTEM) and the selected area electron diffraction (SAED) pattern (inserted in Figure 1d) confirm the crystallization of SnSe. The obvious antiphase domain was detected and marked by the yellow solid lines in Figure 1e, which is considered beneficial to the phonon scattering. The different (110) d-spacing of ∼0.346 nm at the C

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Figure 3. (a) XRD patterns of Sn1−xBixSe (x = 0, 0.01, 0.02, 0.03, 0.04) bulk samples perpendicular to the pressing direction of SPS. (inset) pellet. (b) In situ variable-temperature PXD patterns of Sn0.97Bi0.03Se. SEM images of the fracture morphologies in (c) perpendicular to the pressing direction (left) and parallel to the pressing direction (right) of Sn0.97Bi0.03Se. The scale bar is 2 μm. (d) Optical absorption spectra and band gaps for undoped SnSe and Bi-doped Sn1−xBixSe (x = 0.01, 0.02, 0.03, 0.04) pellets. (e) Schematic diagram to explain the formation of Bi-doped SnSe.

Table 1. Room-Temperature Real Composition, Density, Hall Carrier Concentration (nH), Hall Mobility (μH), and Band Gap (Eg) of Sn1−xBixSe (x = 0, 0.01, 0.02, 0.03, 0.04) nominal comp

real comp

density (g cm−3)

nH (1 × 1014 cm−3)

μH (cm2 V−1 s−1)

Eg (eV)

SnSe Sn0.99Bi0.01Se Sn0.98Bi0.02Se Sn0.97Bi0.03Se Sn0.96Bi0.04Se

Sn0.49Se0.50 Sn0.991Bi0.009Se Sn0.991Bi0.009Se Sn0.988Bi0.012Se Sn0.984Bi0.016Se

5.24 6.04 6.15 6.14 6.08

135 000 4.7 8.5 13 17

7.57 13.91 11.62 18.78 7.22

0.95 0.87 0.84 0.83 0.85

the successful doping of Bi. A schematic diagram is presented in Figure 3e to explain the doping process by using bismuth(III) 2ethylhexanoate as a dopant precursor. Ethylenediamine was used as a stronger nucleophilic reagent and stabilizing ligand in the alkaline solution other than halide anions (such as Cl−). Because of the nucleophilicity of the ethylenediamine ions and the electrophilicity of the unsaturated metal cations,18 bismuth(III) 2-ethylhexanoate attached to the surface of SnSe nanoparticles. When annealed under H2/Ar at 300 °C, the absorbed ethylenediamine evaporated, leaving Bi attach to the Sn2+. During the SPS procedure, Bi diffused into the SnSe lattice and substituted for Sn2+ to obtain n-type Bi-doped SnSe.

indicating the diffusion of Bi atom (r = 1.63 Å) into crystal lattice to replace Sn2+ (r = 1.22 Å) at high temperature. Figure 3c shows the typical SEM images revealing in-plane (left) and out-ofplane (right) fracture morphology. The grain size increased to several micrometers after SPS. Obvious layered structures indicated the anisotropic properties in this solution synthesized samples. To confirm the doping effect of Bi, we conducted the optical diffuse reflectance measurements and calculated the band gaps of undoped SnSe and Bi-doped Sn1−xBixSe (x = 0.01, 0.02, 0.03, 0.04) samples (see Figure 3d). With increasing content of Bi, the band gap decreased from ∼0.95 eV for undoped SnSe to ∼0.83−0.87 eV for Bi-doped SnSe, suggesting D

DOI: 10.1021/acs.inorgchem.8b02324 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 4. XPS for Sn0.97Bi0.03Se powder (black line) and Sn0.97Bi0.03Se pellet (red line). Survey scans (a) and high-resolution scans of Sn 3d (b), Se 3d (c), and Bi 4f (d).

Figure 5. TEM images of Sn0.97Bi0.03Se. (a) Low-magnification TEM image showing many nanoscale strip grains indicated by the broken lines, (b) low-magnification TEM showing sharp grain boundaries, (c) HRTEM image of the boxed region in (b) with the diffractograms of adjacent grains inserted, (d) SAED pattern taken from (c), whereas two sets of diffraction spots overlapped together with an angle of 2.93° along [100] zone axis, (e) enlarged view of the boxed region in (c), along the grain boundary, edge dislocation cores indicated by circles are clearly shown, (f) filtered image of (e) clearly identifying the dislocations, (g) HRTEM image showing a nanoprecipitated.

According to the real composition detected by EDS (shown in Table 1), ∼1−1.5 atm % Bi was doped into SnSe in spite of the high concentration of nominal Bi. X-ray photoelectron spectra (XPS) were also examined to detect the composition and the chemical states of all the

elements in Sn0.97Bi0.03Se powder and Sn0.97Bi0.03Se pellet. Figure 4a depicts the XPS survey scan of the two samples, confirming the presence of Sn, Se, Bi, C, N, and O in both samples. High-resolution XPS scans of Sn (b), Se (c), and Bi (d) are also presented in Figure 3. Sn (3d5/2) and Sn (3d3/2) peaks E

DOI: 10.1021/acs.inorgchem.8b02324 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 6. Temperature dependence of (a) electrical conductivity, (b) Seebeck coefficient, and (c) PF for Sn1−xBixSe (x = 0, 0.01, 0.02, 0.03, 0.04) in comparison with Sn0.97Bi0.03Se synthesized using BiCl3 as dopant precursor and Sn0.97Bi0.03Se prepared by melting, ball milling, and SPS (M + BM + SPS). (d) Relationship between room-temperature Hall carrier concentration (nH) and Bi concentration x (○) and relationship between roomtemperature Hall mobility (μH) and Bi concentration x (●) in Sn1−xBixSe (x = 0, 0.01, 0.02, 0.03, 0.04).

in Figure 5e). To distinctly show the structure of the dislocation core, the filtering processing for Figure 5e was performed (see Figure 5f). The inserted half plane indicated by the symbol ⊥ is parallel to (400) plane. In addition, we also found some nanoprecipitates at the grain boundary marked by dashed circle showing in Figure 5g. Actually all the complex microstructures are also observed in the undoped SnSe, which are not shown here. All these microstructures are believed beneficial to the effective phonon scattering.53,54 The incorporation of Bi in SnSe was further confirmed by results of temperature-dependent electrical conductivity (a), Seebeck coefficient (b), PF (c), and room-temperature Hall carrier concentration and mobility (d) for Sn1−xBixSe (x = 0, 0.01, 0.02, 0.03, 0.04) shown in Figure 6. We also included TE properties of Sn0.97Bi0.03Se synthesized using BiCl3 as dopant precursor and Sn0.97Bi0.03Se prepared by melting, ball milling, and SPS (M + BM + SPS) for comparison. The highest electrical conductivity of undoped SnSe is ∼5500 S m−1 at 473 K, higher than those of all the reported undoped SnSe polycrystals prepared by melting and other methods, like microwave-assisted solvothermal method, etc.42 The electrical conductivity of all the doped samples increased with increasing temperature, showing characteristic of semiconductor. Bi doping decreased the electrical conductivity, which is due to the decreased Hall carrier concentration as shown in Figure 6d. The Hall carrier concentration decreased from 1.35 × 1019 cm−3 (p-type) in undoped SnSe to 4.7 × 1014 cm−3 (n-type) in Sn0.99Bi0.01Se, and it increased again to 1.7 × 1015 cm−3 (n-type) in Sn0.96Bi0.04Se. The Hall mobility, however, increased by doping with Bi, possibly because of the increased density when Bi was doped (see Table 1). With increasing Bi concentration, the electrical conductivity increased and kept unchanged when x = 0.04, indicating that the doping limit is around x = 0.03, consistent with the XRD results (see Figure 3a). The electrical conductivity

were observed at binding energies of 485.48 and 493.94 eV (Figure 4b), respectively, which can be assigned to Sn2+. The high-binding-energy shoulder peaks at 486.7 and 495.4 eV correspond to the chemical shifts for SnO2, consistent with the microstructure analysis in Figure 1f. Similarly, Se (3d5/2) and Se (3d3/2) peaks were observed at binding energies of 486.6 and 495 eV (Figure 4c), respectively, which are the characteristic values for SnSe.52 Two peaks corresponding to Bi (4f7/2) at 158.7 eV and Bi (4f5/2) at 163.6 eV were detected and shown in the pellet samples (see Figure 4d), but they are absent in the powder samples, further confirming the doping of Bi in SnSe after spark plasma sintering in agreement with the previous analysis. The peaks observed at 159.8 and 165.8 eV in both samples possibly can be assigned to Bi2O3. The C 1s, N 1s, and O 1s peaks are because of ethylenediamine or 2-ethylhexanoate molecules bound to the particle surface. We also studied the microstructures of the SPS bulk samples using TEM and presented in Figure 5. The grains crystallize into many nanoscale strips with spacing of ∼40−500 nm and distribute in different region (plotted by yellow broken line in Figure 5a). Straight and sharp grain boundaries between the strips are observed in the enlarged image (Figure 5b). HRTEM image of the boxed region in Figure 5b is shown in Figure 4c with the diffractograms of adjacent grains inserted, respectively. The upper and lower diffractograms can be indexed as [011] and [021] according to Pnma orthorhombic structure of Sn0.97Bi0.03Se. The SAED pattern taken from Figure 5c is shown in Figure 5d with the red and yellow dots representing the diffraction spots from neighbor grains. From the enlarged view of boxed region in Figure 5d, the angle between the adjacent [011] and [021] grains is ∼2.93°, which demonstrates that it is a low-angle grain boundary. When the low-angle grain boundary was enlarged, we found many dislocations with extra atomic planes around each dislocation core (marked by dashed circles F

DOI: 10.1021/acs.inorgchem.8b02324 Inorg. Chem. XXXX, XXX, XXX−XXX

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of all the doped samples synthesized by using bismuth(III) 2ethyhexanoate as a dopant precursor is higher than that of Sn0.97Bi0.03Se synthesized by using BiCl3 as a dopant precursor, but lower than that of Sn0.97Bi0.03Se prepared by M + BM + SPS at temperature lower than 673 K. At higher temperature, all the doped samples have almost the same electrical conductivity. The Seebeck coefficient of undoped SnSe is positive (see Figure 6b). When Bi was doped into Sn site, all Bi-doped SnSe samples have negative Seebeck coefficient at lower temperature, consistent with the room-temperature Hall measurements. With increasing temperature, all the Seebeck coefficients decreased, even changed to positive at T > 700 K in Sn0.97Bi0.03Se synthesized by using BiCl3 as a dopant precursor. This phenomena also happened in solution synthesized Cl-doped SnSe by using SnCl2 as a dopant precursor with n-type to p-type transition at ∼500 K.27 However, all the doped samples synthesized by using bismuth(III) 2-ethyhexanoate as a dopant precursor presented stable n-type conducting behavior in the whole measured temperature range from 300 to 773 K. The highest roomtemperature absolute Seebeck is ∼900 μV K−1 for Sn0.99Bi0.01Se. The absolute Seebeck is still as high as 300 μV K−1 at 773 K for Sn0.97Bi0.03Se. Contributed from the higher Seebeck coefficient, the power factors of all the doped samples synthesized by using bismuth(III) 2-ethyhexanoate as a dopant precursor are higher than those of the two compared samples, especially when temperature is higher than 573 K (see Figure 6c).

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CONCLUSIONS

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AUTHOR INFORMATION

REFERENCES

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Using bismuth(III) 2-ethyhexanoate as a dopant precursor, we successfully synthesized n-type Bi-doped SnSe nanomaterials. All doped samples presented stable n-type conducting behavior in the whole measured temperature range from 300 to 773 K. A maximum room-temperature Seebeck coefficient ca. −900 μV K−1 was achieved in Sn0.99Bi0.01Se, which is beneficial to the higher electrical properties. A strip structure with low-angle boundary was constructed for effective phonon scattering. Further studies on the other metal 2-ethylhexanoate are expected to increase TE properties in both p- and n-type nanomaterials. This method can be extensively applied to cationic doping in solution synthesis for different applications.

Corresponding Authors

*E-mail: [email protected]. (X.-J.L.) *E-mail: [email protected]. (Y.-M.W.) *E-mail: [email protected]. (Q.Z.) ORCID

Qian Zhang: 0000-0001-5975-9781 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS

This work was funded by the National Nature Science Foundation of China (11674078, 11474329, and 51632005), Shenzhen Fundamental Research Projects (JCYJ20160427184825558 and KQTD2016022619565991), Startup Foundation for Advanced Talents from Shenzhen, and Startup Foundation from Harbin Institute of Technology (Shenzhen). G

DOI: 10.1021/acs.inorgchem.8b02324 Inorg. Chem. XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.inorgchem.8b02324 Inorg. Chem. XXXX, XXX, XXX−XXX