Systhesizing SnTe nanocrystals leading to thermoelectric performance enhancement via an ultra-fast microwave hydrothermal method

Author’s Accepted Manuscript Systhesizing SnTe nanocrystals leading to thermoelectric performance enhancement via an ultra-fast microwave hydrothermal method Zhiliang Li, Yudong Chen, Jing-Feng Li, Hong Chen, Lijun Wang, Shuqi Zheng, Guiwu Lu www.elsevier.com/locate/nanoenergy

PII: DOI: Reference:

S2211-2855(16)30295-6 http://dx.doi.org/10.1016/j.nanoen.2016.08.008 NANOEN1421

To appear in: Nano Energy Received date: 23 April 2016 Revised date: 22 July 2016 Accepted date: 3 August 2016 Cite this article as: Zhiliang Li, Yudong Chen, Jing-Feng Li, Hong Chen, Lijun Wang, Shuqi Zheng and Guiwu Lu, Systhesizing SnTe nanocrystals leading to thermoelectric performance enhancement via an ultra-fast microwave hydrothermal method, Nano Energy, http://dx.doi.org/10.1016/j.nanoen.2016.08.008 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Systhesizing

SnTe

nanocrystals

leading

to

thermoelectric

performance

enhancement via an ultra-fast microwave hydrothermal method Zhiliang Lia,b, Yudong Chena, Jing-Feng Lib, Hong Chena, Lijun Wanga, Shuqi Zhenga*, Guiwu Lua a

State Key Laboratory of Heavy Oil Processing and Department of Materials Science and

Engineering, China University of Petroleum, Beijing 102249, P. R. China b

State Key Laboratory of New Ceramics and Fine Processing, School of Materials Science and

Engineering, Tsinghua University, 100084, Beijing, P. R. China *

Corresponding author. Tel./fax: +86 010 89733200. [email protected]

Abstract SnTe is a notable member of the IV-VI semiconductors with SnSe and PbTe as two representative thermoelectric materials, which is considered to be a potentially attractive thermoelectric material due to its similar rock-salt crystal structure to PbTe. However, the current researches on SnTe are limited because of the difficult synthesis in aspect of controlling morphology and size, and its thermoelectric figure of merit is also low due to the high thermal conductivity. In this study, a simple and ultra-fast microwave hydrothermal method was designed to synthesize the SnTe particles with controlled sizes from micro-scale to nano-scale. The thermoelectric properties of the corresponding SnTe bulk materials prepared by spark plasma sintering were investigated in a wide temperature range with a focus on the size effect. Due to the enhanced phonon scattering caused by the nanometer size effect, a low thermal conductivity, 0.60 W·m-1K-1 at 803 K, was obtained in the bulk specimen using 165-nm-sized nanoparticles. The corresponding maximum ZT value at 803 K is enhanced to 0.49, which is about 2.3 times that of the SnTe bulk samples using mechanically alloyed powders. 1

Graphical abstract SnTe particles with controlled grain sizes from micro-scale to nano-scale were synthesized via a simple, ultra-fast and green microwave hydrothermal method. The ZT value ( ～ 0.49) of 165-nm-sized nanoparticle building block is about 2.3 times that of the SnTe MA powder building bulk.

Keywords: Tin Telluride; Nanoparticle; Nanometer Size Effect; Thermoelectric Property; Microwave Hydrothermal Method 1. Introduction Thermoelectric (TE) materials have attracted substantial attention over the past few decades not only due to their excellent capabilities on generating electricity from the waste heat, but also because of their broad application prospects in solid-state refrigeration [1-4]. TE properties are characterized by the dimensionless figure of merit (ZT), which is defined as ZT = S2σT/κ, where S, σ, T, κ are the Seebeck coefficient, electrical conductivity, absolute temperature, and thermal conductivity, respectively [5-8]. In medium temperature region, IV-VI semiconductors, such as SnSe and PbTe, are identified as the best commercial and most efficient TE materials at present because of their superior anisotropy on the crystal structure, and intrinsically low lattice thermal conductivity. PbTe based semiconductor compounds have been applied in the thermoelectric generators of “Curiosity”, “Galileo” Mars detectors, and “Voyager 1” spacecraft since 1980s [9-11]. The ZT value of SnSe single crystal has reached 2.6 as reported by Zhao et al., which is undoubtedly the best TE properties among all of the current materials [12, 13]. SnTe, as another member of the IV-VI semiconductors, is 2

considered to be one of the most promising thermoelectric materials not only because it is a lead-free material but also possesses the similar crystal and band structures to PbTe crystal [14-18]. However, unfortunately, the recent researches have revealed that the ZT values of SnTe crystals are still very low because of their inherently high thermal conductivities (usually higher than 2.5 W·m-1K-1 at 800 K) [19, 20]. Endotaxial CdS or ZnS nanoscale precipitates were introduced into the cubic phase SnTe by Tan et al., [21] and its thermal conductivity was reduced to 2.1 W·m-1K-1 from 3.8 W·m-1K-1 (dropped by 34%), leading to an increase in ZT values to 0.60 from 0.42 (increased by 50%). It is true that increasing Seebeck coefficients and electrical conductivity is effective in raising the ZT values in this crystal, however, in terms of the final value, more than half of the contribution on the enhanced ZT values came from the reduced thermal conductivities. Similar phenomenon was also observed on the In-doped SnTe crystal reported by Zhang et al., [22] whose thermal conductivity is reduced by 18%, resulting in 32% increase in the ZT value. Thus, the thermal conductivity probably also plays a decisive role in improving the thermoelectric properties of SnTe crystals [23, 24]. In fact, according to theoretical calculations, the total thermal conductivity (κ) is composed of electrical thermal conductivity (κe) and lattice thermal conductivity (κL), and in many semiconductors κL is usually much greater than κe [5]. Thus, enhancing phonon scattering by the size effect of nano-materials, which can significantly decrease the κL, has become one of the most effective means to increase the ZT values [25, 26]. Certainly, a vast variety of SnTe nano-structures such as quantum dots, nanoparticles, nanowires, and nanosheets have been designed to improve the ZT values, and they also can be synthesized by some traditional means such as solvothermal method [27, 28], solution phase synthesis [29-32], vapor transport method [33-38], organic matter degradation [39, 40] and so on. However, there are still some limitations that should be resolved during these strategies. First, the hydrothermal method generally requires high temperature and pressure as well as the long reaction time, which significantly hinder the applications on the large-scale production. Second, 3

some toxic or malodorous reducers, such as tetrahydrofuran, phenylbenzene, sodium borohydride, and three octyl phenyl phosphine are used in the solution phase synthesis. Several organic oily solvents like octadecylene, stearic acid, and oleamide used in this method may also bring serious cleaning problems. Third, the vapor transport method is badly circumscribed because of their high cost, complex processes, and the low product yield. Finally, the controlled synthesis of the nanocrystals with different sizes or morphologies still cannot be implemented in most of these methods. Furthermore, currently, there is few literature discussing the influence of nanometer size effect on the thermoelectric properties of undoped SnTe. Thus, in this study, we designed a facile, ultra-fast, green, and high-yield microwave hydrothermal method to synthesize SnTe nanoparticles (NPs) with controlled sizes from micro-scale to nano-scale. The oriented attachment growth mechanism and morphology controlling technologies were systematically discussed. In order to prove and understand the nanometer size effect further, SnTe reference sample, as a comparison, was also prepared by ball-milling combining with spark plasma sintering (SPS). Comparing with the thermoelectric performance of the pure SnTe bulk material, ultra-low thermal conductivities (from 1.5 W·m-1K-1 to 0.60 W·m-1K-1, 323 K-800 K), relatively higher Seebeck coefficients (58～90 μVK-1, 323 K-800 K), and much higher ZT value (about 0.49 at 803 K), which can be ascribed to the enhanced phonon scattering and the intensified energy filtering effect, were discovered in the specimen that was sintered from the NPs with an average diameter of 165 nm. 2. Experimental 2.1 Materials and reactors Commercial powders of tellurium dioxide (TeO2, 99.99%), tellurium (Te, 99.99%), tin (Sn, 4

99.99%) and polyvinyl pyrrolidone (PVP, K-30, Mw = 40000) were used as starting materials. The tin dichloride dehydrate (SnCl2·2H2O), sodium hydroxide (NaOH), ethylene glycol (EG), and absolute ethyl alcohol were analytical-grade and were not purified further before using. The synthetic processes of SnTe NPs were carried out in a special microwave hydrothermal synthesis system (MHSS, XH-8000, Beijing XiangHu Science & Technology Development Co., Ltd, China). The reaction temperatures can be accurately regulated by the power programme control (from 300 W to 1500 W). A magnetic stirrer was equipped into the MHSS to obtain homogeneous solution. 2.2 Preparations Controlled synthesis of SnTe NPs: In typical synthesis, 250 mL EG, 10.0 g PVP, 1.330 g TeO2, 1.881 g SnCl2·2H2O and 1.5 g NaOH were orderly added to a 500 mL special teflon autoclave with high strength shell (polyetheretherkrtone, PEEK). Then the mixture was heat to 120 °C within 8 min in the MHSS with vigorous and continuous magnetic stirring. Subsequently, the temperature was increased to 220 °C at a heating rate of 15 °C/min, and last for 20 min with a power of 550 W. The final products were naturally cooled to room temperature and washed several times with distilled water and absolute ethanol. SnTe NPs with a mean diameter of 165 nm were obtained, and they were adequately dried to powder in a vacuum oven at 60 °C for 8 h. In the similar reaction conditions, SnTe particles with different sizes and morphologies were obtained by only adjusting the NaOH dosage from 0 g to 0. 50 g. The dense specimens were prepared in a graphite die by spark plasma sintering (SPS-211Lx, Fuji Electronic Industrial Co., Ltd, Japan) at 450 °C for 8 min under an axial pressure of 80 MPa and a vacuum of 3.0 Pa. And then, the circular samples were accurately polished to 2.0 mm in thickness, and 12.7 mm in diameter to evaluate their thermal conductivities. Finally, the dense specimens were cut into rectangular columns with the size of 12×2×3 mm to measure the

5

Seebeck coefficient and the electrical resistivity. Preparation of SnTe reference materials: As a comparison specimen, SnTe bulk samples were synthesized by the following processes. 6.380 g Te powder and 5.936 g Sn powder were put into a zirconia jar, and mixed by a high-energy planetary ball mill (BM4, Beijing Grinder Instrument Co., Ltd) at 310 rpm for 5 h. Then 3.0 g mixture was sintered in to bulk by SPS at 450 °C for 8 min under an axial pressure of 80 MPa and a vacuum of 3.0 Pa. 2.3 Characterization methods The crystal structures of the samples were investigated using a Bruker AXS XRD-D8 Focus X-ray diffractometer (XRD) equipped with graphite monochromatized CuKα radiation (λ = 0.15406 nm). A field emission scanning electron microscope (FESEM, FEI Quanta 200F) was used to characterize the morphologies of the as-prepared products. High-resolution transmission electron microscopy (HRTEM) image and selected area electron diffraction (SAED) pattern were recorded by an FEI Tecnai G2 F20 field emission transmission electron microscope with an acceleration voltage of 200 kV. The Seebeck coefficient and resistivity were measured from 323 K to 803 K using a ZEM-3 (M8) Seebeck coefficient/electric resistance measuring system (ULVAC-RIKO, Inc.) whose uncertainty is 5%. The thermal conductivity can be calculated by the equation K = λCpd, where λ, Cp, d are thermal diffusion coefficient, specific heat capacity and density, respectively. The thermal diffusion coefficient of the dense specimens was evaluated by a laser flash apparatus (LFA457, Netzsch). The specific heat capacity measurements were carried out using a NETZSCH DSC 404 C. An XS105 density meter was used to measure the densities of the SnTe samples after SPS. The particle size distributions were performed in a laser particle analyzer (Zetasizer Nano ZS). The carrier concentration and mobility were measured on the square chips with the thickness under 0.6 6

mm using the Hall measurement system with 2.0 Tesla magnetic field (ResiTest 8400, Toyo, Japan). 3. Results and Discussion 3.1 Characterization of the SnTe NPs Different from the traditional hydrothermal apparatuses, where stirring cannot be performed due to the sealed and ferric reactor, magnetic stirring was applied to the microwave hydrothermal synthesis system (MHSS) because the autoclaves used in this reactor were made up of polymer instead of the stainless steel. The solution temperature in the MHSS were accurately probed by a thermal detector which was deeply inserted into the solution. The overall appearance and main units of the MHSS are shown in Figure 1a. Moreover, microwave is electromagnetic wave generated from magnetrons. The strong electric and magnetic fields with an ultra-high frequency can force the polar molecules (like EG or H2O) to realign and vibrate acutely. Energy with high power is generated from the inner of the solution as a result of the violent collision and strong friction among the polar molecules. Thus, reagents in the reaction solutions are heated uniformly with an ultra-fast heating rate, and therefore, microwave-assisted synthesis can operate at a high reaction rate and short reaction time [41-44]. As the temperature recorder displayed, the temperature was accurately controlled at 220 °C as well as the program set, and the main reaction processes were finished only in 20 min (Figure 1b). The maximum vapor pressure was 0.17 MPa, which is far lower than the ultimate pressure (4.0 MPa) of PEEK autoclave. The highly polar and temperate vapor pressure also demonstrated that the EG is an ideal solvent for microwave hydrothermal method at 220 °C. The XRD pattern with diffraction angles from 15° to 90° was performed after the SnTe NPs were adequately dried. All the Bragg diffraction peaks in the pattern of the as-obtained sample (red line A in Figure 1c) are completely indexed to the crystal planes of the stand JCPDS card No. 8-487, including the (200), (220), (420) 7

planes and so on (black line B in Figure 1c). It is also proved that the as-prepared sample is cubic phase SnTe with a space group of Fm3m (No. 225), and the lattice constants are a = b = c = 0.6303 nm. The sharp peaks and the small half-peak width suggest that the sample consist of highly crystallized crystals with small diameter. This characteristic is also attested by the overall morphology (Figure 1d) and the moderate-magnification (Figure 1e) SEM images. SnTe NPs with various structures are dispersed uniformly, and their diameters ranged from 130 nm to 190 nm. Among these NPs, many regular octahedral structures with the side length approximately to 180 nm can be found, and one of their detailed morphology is clearly shown in the inset of Figure 1e. As the simulative structure shows (Figure 1f), which was established using the professional structure simulation software based on the Te and Sn atom position, the six top atoms of the octahedron are central atoms of the plane on the face-centered-cubic (FCC) cell. Thus, the eight surface planes of the octahedron belong to {111} plane family. And interestingly, due to the slow-rate and multistep of Te reductive reaction (from TeO2 to TeO32-, Te and Te2-) as well as the high ionization of SnCl2, we speculate that the most superficial atoms of the octahedron are Sn atoms (Figure 1f, the red balls), which is consistent with a simulation. In order to confirm the structure information, we took a TEM image in a single octahedral NPs, and a similar hexagon shadow was obtained (inset of Figure 1g) when the electron beams vertically pass through the (111) plane. Similar hexagon shadow was also obtained in the 3D simulation structure under the same conditions (Figure 1g). The SAED pattern was performed on the thin area of the octahedron (Figure 1h). The zone axis is [111], and the interplanar spacings from the three diffraction spots to the center transmission spot are 0.2293 nm, 0.2302 nm and 0.1308 nm, which respectively correspond well to the (-220), (-202), (-422) planes of the cubic SnTe. All these planes combined with the transmission spot can form a regular 8

parallelogram, and meet the principle of vector superposition as well. Figure 1i is a HRTEM image, the smallest interplanar spacings in different directions are 0.2295 nm and 0.2305 nm, matching well to the (-220) and (-202) planes (both of their interplanar spacings are 0.2229 nm, which can be calculated from the interplanar spacing formula or found in JCPDS Card No.8-487). All these phenomena demonstrate that the NPs are cubic SnTe once again.

Fig.1. (a) Main units of microwave hydrothermal synthesis system; (b) The temperature curve as 9

function of the reaction time; (c) The XRD pattern of SnTe NPs; (d) and (e) are SEM images under different magnifications; (f) is a simulative octahedral structure of SnTe nanoparticles; (g), (h), and (i) are overall TEM, SEAD, and HRTEM images. 3.2 Formation mechanisms and size control from micro-scale to nano-scale In order to obtain the SnTe particles with different sizes from micro-scale to nano-scale, we tried to control the reaction rates by adjusting the dosage of NaOH that act as an accelerant of reducing agent in this reaction. A relatively slow oxidation-reduction reaction rate between the TeO2 and EG occurred when there is no NaOH added into the solution. The Te4+ ions were reduced to Te atoms, then nucleated, and grew into Te nanorods along its intrinsic growth orientation [001], which has been discussed in our previous works [45-47]. However, Te atoms was not reduced to Te2- ions further because of the absence of OH- ions, and also did not react with Sn2+ ions to form SnTe crystals. Therefore, the reactions without NaOH were run incompletely, and only Te nanorods adhered lots of non-reacted reactants on their surface were generated. Their overall morphologies are displayed in Figure 2a, and their XRD pattern (red line B in Figure 2d) corresponds well to the hexagonal Te crystal (JCPDS Card No.4-555, blue line C in Figure 2d). Te nanorods were transformed into nanowires when some but insufficient NaOH granules were added to the reaction solution, and a few of SnTe NPs appeared in the final products. The reaction mechanism can be understood by the following phase chemical reaction equations: 4𝑂𝐻 − + 2𝑇𝑒𝑂2 → 2𝑇𝑒𝑂3 2− + 2𝐻2 𝑂

(1)

𝑇𝑒𝑂3 2− + 𝐻𝑂­𝐶𝐻2 𝐶𝐻2 ­𝑂𝐻 → 𝑂𝐻𝐶­𝐶𝐻𝑂 + 𝑇𝑒 + 2𝑂𝐻 − + 𝐻2 𝑂 𝑂𝐻𝐶­𝐶𝐻𝑂 + 𝑇𝑒𝑂3 2− → ¯𝑂𝑂𝐶­𝐶𝑂𝑂¯ + 𝑇𝑒 + 𝐻2 𝑂 3𝑇𝑒 + 6𝑂𝐻 − → 2𝑇𝑒 2− + 𝑇𝑒𝑂3 2− +3𝐻2 𝑂 𝑇𝑒 2− + 𝑆𝑛2+ → 𝑆𝑛𝑇𝑒

(4)

(5) 10

(3)

(2)

The ratio of Te nanowires to SnTe NPs was highly governed by the dosage of NaOH. Figures 2b and c are the SEM images of the products with 1.0 g or 1.25 g NaOH. The proportion of the Te nanowires decreases, and the amount of SnTe NPs increases gradually because of the increasing OHconcentrations, which made the Te crystals generated from equation (2) and (3) re-dissolve and transform to Te2- ions according to the equation (4). Their XRD patterns (black line A in Figure 2d) demonstrated that both the hexagonal Te and the cubic SnTe crystals coexisted in these samples. The reaction rate of equation (4) enhanced sharply when NaOH increased to 1.50 g. All the Te nanowires were involved into the reactions, and only pure SnTe NPs with octahedral structure or irregular shapes are found in Figures 2e and h. Their average diameters ranged from 130～190 nm, which were measured by the laser particle analyzer, and the sharp peak appears at 165 nm (inset of Figure 2h). When the dosage of NaOH increased to 2.50 g, the reaction of equation (4) further enhanced because of the increasing OH- concentration, and also result in a relatively high Te2- concentration, which made the equation (5) occur fluently. Probably, in this case, some reaction equilibrium was formed between the generated rate and consumed rate of Te2- ions, which led to the SnTe crystals grown along one of intrinsic crystal morphology, and ultimately formed the regular octahedrons shown in Figures 2f and i. Their average diameter also increased from 165 nm to 550 nm (inset of Figure 2i) due to the high yield Te2- ions. However, excess OH- ions and too violent reaction may make some lattice planes, like (00l, l≠0) planes (inset of Figure 2g), that once relatively stabilized in the low OH- concentration solution involve into the reaction dramatically. At the same time, more TeO32- ions were formed (equation (1) and (4)) and first transformed to Te crystals (equation (2) and (3)), then Te2- ions (equation (4)), and finally SnTe NPs (equation (5)), which result in a high SnTe yield. So the SnTe particles become irregular, and their average diameters increased to 5～12 μm 11

when the NaOH was added up to 5.50 g (Figures 2g and j). Thus, the grain sizes of SnTe crystals can be controlled by simply adjusting the dosage of NaOH in this microwave hydrothermal method.

Fig.2. (a), (b) and (c) are SEM images of SnTe NPs synthesized with 0, 1.0, 1.25 g NaOH; (d) are XRD patterns of sample (a) and (c); (e) and (h), (f) and (i), (g) and (j) are SEM images of SnTe particles synthesized with 1.50, 2.50, 5.50 g NaOH.

3.3 Enhanced thermoelectric properties due to the nanometer size effect In order to evaluate the thermoelectric properties of the as-prepared samples, SnTe particles with different average diameters, including 165 nm, 550 nm, and 8.2 μm were sintered to dense specimens 12

by SPS after adequately drying. Their overall morphologies of the sintered samples are nearly identical after polishing, and one of them is shown in Figure 3a. Their densities are approximately to 6.14, 6.21, and 6.33 g/cm3 measured by a density meter based on the Archimedes principle, and their relative densities based on the theoretical density (6.50 g/cm3) were around 94.5%, 95.5%, and 97.4%, respectively. All these densities have basically met the special requirements of thermoelectric properties test. Dimensions after final machining were shown in Figure 3a and its inset (round flake: ø = 12.7 mm, T = 2.0 mm; rectangular columns: 12×2×3 mm). As a reference experiment, a SnTe bulk sample was also prepared by SPS with the same parameters using the powders prepared by mechanical alloying (MA). All the diffraction peaks in the XRD pattern well corresponded to the cubic SnTe shown in Figure 3b. The detailed SEM image on the fracture surface demonstrated that the reference sample was made up of relatively large block crystals with few obvious defects and cracks (Figure 3c). The relative density is 98.2% because of this compact structure, which resulted from the high sintering pressure (80 MPa). However, as the SEM images of the fracture surfaces demonstrated the nanoparticles or microparticles are still retained as their original sizes and morphologies in their dense bulk specimens, which were respectively sintered from the particles with different average diameters of 165 nm (Figure 3d), 550 nm (Figure 3e), and 8.2 μm (Figure 3f).

13

Fig.3. (a) is the overall morphology of sample after SPS, and its insets are rectangular column and square chips for the test of thermoelectric properties and Hall measurements respectively; (b) is a XRD pattern of undoped SnTe reference sample; (c), (d), (e) and (f) are detailed SEM images on the dense specimens, which were sintered from mechanically alloyed (MA) powder (c) or particles with different diameters: 165 nm (d), 550 nm (e), and 8.2 μm (f). The thermal conductivities as function of temperatures are displayed in Figure 4a. The uncertainty of the thermal conductivity is estimated to be within 8%, considering the uncertainties for λ, Cp, and d. In the reference sample (green line D), the thermal conductivity decreased from 9.2 W·m-1K-1 (at 323 K) to 5.1 W·m-1K-1 (at 803 K), which is similar to these results reported by Kanatzidis et al., [21] Snyder et al., [48] Ren et al., [22] Zhao et al., [49] and some other reports [17-20, 24]. However, the thermal conductivities decreased dramatically when the diameters of the SnTe particles decreased from 8.2 μm to 165 nm. A lower thermal conductivity (approximately to 3.5 W·m -1K-1, blue line C, only about 69% of the reference sample) was obtained at 803 K when the dense sample was sintered 14

from the 8.2 μm particles. The thermal conductivity dropped quickly above 643 K, and reduced to 0.95 W·m-1K-1 at 803 K, which is only 18.7 % of the bulk sample when the average diameter of the NPs is 550 nm (red line B). Interestingly, when the diameter further decreased to 165 nm, the thermal conductivities in the whole temperature range from 323 K to 803 K (black line A) are far below the minimum values of reference samples. An ultra-low thermal conductivity, 0.60 W·m-1K-1, only 11.8% of the reference sample is obtained at 803 K, which is the lowest value for current undoped SnTe. The ultra-low full-ranged thermal conductivities in SnTe materials were achieved probably through the following three strategies. First, the size of SnTe NPs has a poly-disperse distribution (from 130 nm to 190 nm, although the average diameter is over 100 nm), which made some contribution to scatter the heat-carrying phonons from mid to long wavelength at the low temperature. Second, the specific surface area of the SnTe particles remarkably increased with the decreasing sizes, and plenty of nanoscale grain boundaries were formed after SPS, which obviously enhanced the scattering to the low and mid frequency phonons at the low temperature, and probably this is the main reason why the thermal conductivity decreased to 1.6～1.3 W·m-1K-1 even at the 325～575 K. Third, due to the insufficient chemical reaction, large number of point defects were also formed after SPS, which signally enhanced the phonon scattering to the high frequency phonons with short wavelengths, and finally led to a low lattice thermal conductivities in the high temperature range [50]. The enhanced phonon scattering was also demonstrated by the dramatically decreased lattice thermal conductivities (κL, Figure 4b), which were calculated by κL = κ – κe

=κ

– LσT. When the particle sizes decreased

from the reference sample to 550 nm, the κL reduced from 2.8 W·m-1K-1 to 0.52 W·m-1K-1, which almost approach to the theoretically predicted minimum lattice thermal conductivity (κmin), 0.5 W·m-1K-1, calculated using the model proposed by Cahill et al. [51, 52, 17, 23]: 15

𝜅𝑚𝑖𝑛 = 𝑘𝐵𝑉 − 𝜈

(6)

where kB, V and ν are Boltzmann constant, unit cell volume and average sound velocity (～1800 ms-1 for SnTe [19, 53]) respectively. Interestingly, the κL of the bulk specimen using 165-nm-sized nanoparticles is even slightly lower than the κmin, which could possibly result from the relatively low relative density (94.5%), instrumental error (8%) and high Lorenz number (L), 1.5×10-8 V2 K2, whose available range is (1～2.4)×10-8 V2 K2 [12]. On the other hand, the nanometer size effect may also have some influences on the electrical conductivity (Figure 4c). The electrical transport properties of the SnTe particles with micro-scale (blue line C) are similar to the reference sample (green line D). However, the electrical conductivities decreased dramatically when the particle sizes were reduced to nano-scale. The electrical conductivities of the samples made of 165 or 550 nm NPs are only 23.6% or 18.7% of the reference one. These phenomena probably stemmed from the special physical properties of SnTe. In fact, SnTe is a typical topological insulator, in the inner of SnTe crystal, energy gap (Eg) near the Fermi level (EF) is similar to the insulator, and thus, electrons cannot transport fluently in the inner of crystal. But on the surfaces of SnTe crystal (Figure 4d), generally, an electron state named Dirac steadily existed, which can overcome the Eg easily, and led to the surfaces of the crystals present metallization property with a relative high electric conductivity [33-38]. Thus, the main transportations of the holes preferentially happened on the surface of the bulk sample because of its smooth surface with few defects or crevices as shown in the Figure 4e sample D. On the contrary, although the dense block can also be formed by the micrometer particles, some big crevices stemmed from the defective matching between the angular nanoparticles still remained on its surface, which may seriously hinder the transportation of the holes, and result in a lower electrical conductivity than the reference sample 16

(Figure 4e, sample C). More structure defects and small crevices appeared when the dense samples were sintered from the 550 nm NPs (Figure 4e, sample B). The holes cannot smoothly transport by the sample surface, so the electrical conductivity is far lower than the reference sample (red line B in Figure 4c). However, the electrical conductivity of the bulk samples sintered from the 165 nm NPs did not further decreased (black line A in Figure 4c), because in this sample, the dominated carrier transport occurred on the grain boundaries rather than the specimen surface (Figure 4e, sample A). Theoretically, the smaller grain size, the more grain boundaries, and the more transport channels for holes, which led to a comparative electrical conductivities for sample A to sample B. On the other hand, the carrier mobility, which was measured at room temperature and demonstrated in Figure 4f, gives a corresponding evidence for this electrical transportation mechanism. The hole mobility reduced from 125 cm2V-1s-1 to 31 cm2V-1s-1 when the grain sizes gradually decreased to 165 nm with the increasing crystal defects. Although the hole concentrations increased with the reducing grain sizes (red bars in Figure 4f), which probably result from the increasing particle specific surface area and corresponding high density of Dirac electron state, in this case, the hole mobility made more contribution to the electrical conductivity. The nanometer size effect may also have some mild influences on the Seebeck coefficient. The maximum Seebeck coefficients increased from 84 μV/K (green line D, Figure 4g) to 90 μV/K (black line A, Figure 4g) at 803 K when the diameters of SnTe crystals decreased from the reference sample to 165 nm. In fact, as the theoretical researches demonstrated, the relationship between the relaxation time () and the energy (E) of the carrier can be defined by  = 0E-1/2, where 0 is a constant irrelevant to E,  is scattering parameter. Thus, due to the energy filtering effect [54-57], which highly relevant to the carrier energy rather than the carrier concentrations, the carriers with low 17

energy may have shorter relaxation times, also can be filtered by the NPs, defects, and grain boundaries more easily than the high-energy carriers while transmitting. Thus, more high-energy carriers can be gathered at the ends of the dense samples sintered from NPs, where plenty of NPs, defects, and grain boundaries are remained after SPS, and ultimately result in a relatively high Seebeck coefficient (black line A). On the other hand, the slightly increased Seebeck coefficient simultaneously result from the enhanced density of states (DOS) 𝑔(𝐸), which can be derived using the follow formulas [58]: 𝑆=

8

𝜅𝐵

𝑚∗ 𝑇(3𝑛)2⁄3

3𝑒ℎ

∗ 𝑚𝑥

𝑚0∗

𝑆

𝑛

= 𝑆𝑥 (𝑛𝑥 )2⁄3

𝑔(𝐸) =

0

0

(𝑚∗ ) ⁄ √2𝐸 ℎ

(7) (8) (9)

where h, e and E are electronic charge, Planck constant and energy level, 𝑆𝑥 , 𝑛𝑥 and 𝑚𝑥∗ are Seebeck coefficients, carrier concentrations and effective mass of carriers from different samples respectively. According to the equation (8), 𝑚∗ increased with the enhanced n (show in Figure 4f ) when the grain sizes gradually decreased to nano scale, indicating an enhancement in 𝑔(𝐸) according to the equation (9), which was generally perceived as a key factor to improve the Seebeck coefficient. Although the Seebeck coefficient of the 165 nm NPs sample increased slightly, its power factor (S2σ, 3.64 μW·m-1K-2 at 803 K, black line A of Figure 4h) is still lower than the reference sample (13.5 μW·m-1K-2 at 803 K, green line D of Figure 4h) because of its lower electrical conductivity. However, considering the effect of the ultra-low thermal conductivity, the ZT value of the sample A sintered from the 165 nm NPs finally increased to 0.49 at 803 K (black line A, in Figure 4i), which is about 230% of the reference sample, and also higher than the state-of-the-art works [37-40, 48, 49]. 18

Thus, these phenomena give a glimpse of this viewpoint that the thermal conductivity can be decreased by adjusting the phonon scattering due to the nanometer size effect, and the energy filtering effect caused by the nanometer size effect also has some contributions to the Seebeck coefficient. If the low power factor of the SnTe NPs can be enhanced without increasing the thermal conductivity by adjusting the carrier concentration with the help of element doping, the thermoelectric properties will be increased further.

Fig.4. The performance curves as function of temperatures from 203 K to 803 K and electrical transporting mechanism of the dense samples sintered from the 165 nm, 550 nm nanoparticles (NPs), 8.2 μm microparticles (MPs), and mechanically alloyed (MA) powder: (a) total thermal 19

conductivities, (b) lattice thermal conductivities, (c) electrical conductivities, (d) and (e) electrical transporting mechanism, (f) hole mobility, carrier concentration and the ratio of 𝑚𝑥∗ ⁄𝑚0∗ as function of the decreased grain sizes, (g) Seebeck coefficients, (h) power factors, and (i) ZT values.

4. Conclusion In summary, SnTe particles with different sizes and shapes were successfully synthesized using a simple, ultra-fast, and green microwave hydrothermal method. Their diameters can be controlled from 165 nm to 8.2 μm by simply adjusting the dosage of NaOH from 1.50 g to 5.50 g. Their enhanced thermoelectric properties were systematically studied using the phonon scattering theory and the energy filtering effect caused by the nanometer size effect. The thermal conductivity is dramatically reduced when their diameters decreased from 8.2 μm to 165 nm. An ultra-low thermal conductivity, 0.60 W·m-1K-1 at 803 K was obtained due to the enhanced phonon scattering effect intruded by refined grains, grain boundaries and point defects in the dense sample sintered using the 165 nm nanoparticles. This value is only 11.8% of the reference sample, which is the lowest thermal conductivity for the undoped SnTe so far. Comparing with the reference sample, a much higher ZT value, 0.49 at 803 K was obtained in the 165 nm NPs sample, which is also higher than the vast majority of other works. The microwave hydrothermal method, the technologies on the controlling grain sizes or morphologies, and the nanometer size effect may also offer new insights into the synthesis-structure-property relationships of a broad class of semiconductor thermoelectric materials. Acknowledgements This work was financially supported by the National Natural Science Foundation of China. (Nos. 51171208 and 51271201) 20

References [1] L. E. Bell, Science 321 (2008) 1457. [2] G. Q. Zhang, B. Kirk, L. A. Jauregui, H. R. Yang, X. F. Xu, Y. P. Chen, Y. Wu, Nano Lett. 12 (2012) 56. [3] H. Y. Lv, H. J. Liu, J. Shi, X. F. Tang, C. Uher, J. Mater. Chem. A. 1 (2013) 6831. [4] L. P. Hu, T. J. Zhu, Y. G. Wang, H. H. Xie, Z. J. Xu, X. B. Zhao, NPG Asia Materials. 6 (2014) e88. [5] F. J. Disalvo, Science 285 (1999) 703. [6] R. Venkatasubramanian, E. Siivola, T. Colpitts, B. O'Quinn, Nature 413 (2001) 597. [7] Y. Y. Qi, Z. Wang, M. L. Zhang, F. H. Yang, X. D. Wang, J. Mater. Chem. A. 1 (2013) 6110. [8] S. A. Yamini, T. Ikeda, A. Lalonde, Y. Z. Pei, S. X. Dou, G. J. Snyder, J. Mater. Chem. A. 1 (2013) 8725. [9] Z. H. Dughaish, Physica B. 322 (2002) 205. [10] Y. Gelbstein, Z. Dashevsky, M. P. Dariel, J. Appl. Phys. 104 (2008) 033702. [11] K. Biswas, J. Q. He, I. D. Blum, C. I. Wu, T. P. Hogan, D. N. Seidman, V. P Dravid, M. G. Kanatzidis, Nature 489 (2012) 414. [12] L. D. Zhao, S. H. Lo, Y. S. Zhang, H. Sun, G. J. Tan, C. Uher, C. Wolverton, V. P. Dravid, M. G. Kanatzidis, Nature 508 (2014) 373. [13] L. D. Zhao, G. J. Tan, S. Q. Hao, J. Q. He, Y. L. Pei, H. Chi, H. Wang, S. K. Gong. H. B. Xu, V. P. Dravid, C. Uher, G. J. Snyder, C. Wolverton, M. G. Kanatzidis, Science 351 (2016) 141. [14] R. F. Brebrick, A. J. Strauss, Phys. Rev. 131 (1963) 104. [15] L. M. Rogers, J. Phys. D: Appl. Phys. 1 (1968) 845. 21

[16] D. J. Singh, Funct. Mater. Lett. 3 (2010) 223. [17] G. J. Tan, F. Y. Shi, H. Sun, L. D. Zhao, C. Uher, V. P. Dravid, M. G. Kanatzidis, J. Mater. Chem. A. 2 (2014) 20849. [18] H. J. Wu, C. Chang, D. Feng, Y. Xiao, X. Zhang, Y. L. Pei, L. Zheng, D. Wu, S. K. Gong, Y. Chen, J. Q. He, M. G. Kanatzidis, L. D. Zhao, Energy Environ. Sci. 8 (2015) 3298. [19] G. J. Tan, F. Y. Shi, S. Q. Hao, H. Chi, T. P. Bailey, L. D. Zhao, C. Uher, C. Wolverton, V. P. Dravid, M. G. Kanatzidis, J. Am. Chem. Soc. 137 (2015) 11507. [20] L. D. Zhao, X. Zhang, H. J. Wu, G. J. Tan, Y. L. Pei, Y. Xiao, C. Chang, D. Wu, H. Chi, L. Zheng, S. K. Gong, C. Uher, J. Q. He, M. G. Kanatzidis, J. Am. Chem. Soc. 138 (2016) 2366. [21] G. J. Tan, L. D. Zhao, F. Y. Shi, W. D. Jeff, S. H. Lo, H. Sun, C. Wolverton, V. P. Dravid, C. Uher, M. G. Kanatzidis, J. Am. Chem. Soc. 136 (2014), 7006. [22] Q. Zhang, B. L. Liao, Y. C. Lan, K. Lukas, W. S. Liu, K. Esfarjani, C. Opeil, D. Broido, G. Chen, Z. F. Ren, PNAS. 110 (2013) 13261. [23] G. J. Tan, F. Y. Shi, J. W. Doak, H. Sun, L. D. Zhao, P. L. Wang, C. Uher, C. Wolverton, V. P. Dravid, M. G. Kanatzidis, Energy Environ. Sci. 8 (2015) 267. [24] G. J. Tan, F. Y. Shi, S. Q. Hao, H. Chi, L. D. Zhao, C. Uher, C. Wolverton, V. P. Dravid, M. G. Kanatzidis, J. Am. Chem. Soc. 137 (2015) 5100. [25] B. Poudel, Q. Hao, Y. Ma, Y. C. Lan, A. Minnich, B. Yu, X. Yan, D. Z. Wang, A. Muto, D. Vashaee, X. Y. Chen, J. M. Liu, M. S. Dresselhaus, G. Chen, Z. F. Ren, Science 320 (2008) 634. [26] H. Z. Zhao, J. H. Sui, Z. J. Tang, Y. C. Lan, Q. Jie, D. Kraemer, K. McEnaney, A. Guloy, G. Chen, Z. F. Ren, Nano Energy 7 (2014) 97. [27] W. Jiang, Z. L. Yang, D. Weng, J. W. Wang, Y. F. Lu, M. J. Zhang, Z. Z. Yang, Chin. Chem. Lett. 22

25 (2014) 849. [28] J. J. Ning, K. K. Men, G. J. Xiao, B. Zou, L. Wang, Q. Q. Dai, B. B. Liu, Z. G. Zou, CrystEngComm. 12 (2010) 4275. [29] S. Schlecht, M. Budde, L. Kienle, Inorg. Chem. 41 (2002) 6001. [30] M. V. Kovalenko, W. Heiss, E. V. Shevchenko, J. S. Lee, H. Schwinghammer, A. P. Alivisatos, D. V. Talapin, J. Am. Chem. Soc. 129 (2007) 11354. [31] Y. Xu, N. A. Salim, J. M. Hodgkiss, R. D. Tilley, Cryst. Growth Des. 11 (2011) 2721. [32] I. T. Sines, D. D. Vaughn II, A. J. Biacchi, C. E. Kingsley, E. J. Popczun, R. E. Schaak, Chen. Mater. 24 (2012) 3088. [33] M. Safdar, Q. S. Wang, M. Mirza, Z. X. Wang, K. Xu, J. He, Nano Lett. 13 (2013) 5344. [34] Z. Li, S. Shao, N. Li, K. McCall, J. Wang, S. X. Zhang, Nano Lett. 13 (2013) 5443. [35] M. Saghir, M. R. Lees, S. J. York, G. Balakrishnan, Cryst. Growth Des. 14 (2014) 2009. [36] M. Safdar, Q. S. Wang, M. Mirza, Z. X. Wang, J. He, Cryst. Growth Des. 14 (2014) 2502. [37] J. Shen, Y. Jung, A. S. Disa, F. J. Walker, C. H. Ahn, J. J. Cha, Nano Lett. 14 (2014) 4183. [38] E. Z. Xu, Z. Li, J. A. Martinez, N. Sinitsyn, H. Htoon, N. Li, B. Swartzentruber, J. A. Hollingsworth, J. Wang, S. X. Zhang, Nanoscale. 7 (2015) 2869. [39] P. Boudjouk, M. P. Remington, D. G. Grier, W. Triebold, B. R. Jarabek, Organometallics 18 (1999) 4534. [40] V. G. Leontyev, L. D. Ivanova, K. Bente, V. F. Gremenok, Cryst. Res. Technol. 47 (2012) 561. [41] Y. Jiang, Y. J. Zhu, G. F. Cheng, Cryst. Growth Des. 6 (2006) 2175. [42] L. K. Pan, X. J. Liu, Z. Sun, C. Q. Sun, J. Mater. Chem. A. 1 (2013) 8299. [43] J. B. Zhu, Q. Y. Li, W. T. Bi, L. F. Bai, X. D. Zhang, J. F. Zhou, Y. Xie, J. Mater. Chem. A. 1 23

(2013) 8154. [44] Z. L. Li, R. Y. Teng, S. Q. Zheng, Y. Z. Zhang, T. Huang, G. W. Lu, J. Cryst. Growth. 406 (2014) 104. [45] Z. L. Li, S. Q. Zheng, Y. Z. Zhang, R. Y. Teng, T. Huang, C. F. Chen, G. W. Lu, J. Mater. Chem. A. 1 (2013) 15046. [46] Z. L. Li, S. Q. Zheng, T. Huang, Y. Z. Zhang, R. Y. Teng, G. W. Lu, J. Alloys Comp. 617 (2014) 247. [47] Z. L. Li, S. Q. Zheng, Y. Z. Zhang, H. Chen, T. Huang, G. W. Lu, J. Electron. Mater. 44 (2015) 2061. [48] M. Zhou, Z. M. Gibbs, H. Wang, Y. M. Han, C. N. Xin, L. F. Li, G. J. Snyder, Phys. Chem. Chem. Phys. 16 (2014) 20741. [49] Y. Chen, M. D. Nielsen, Y. B. Gao, T. J. Zhu, X. B. Zhao, J. P. Heremans, Adv. Energy Mater. 2 (2012) 58. [50] C. J. Vineis, A. Shakouri, Arun. Majumdar, M. G. Kanatzidis, Adv. Mater. 22 (2010) 3970. [51] D. G. Cahill, S. K. Watson, R. O. Pohl, Phys. Rev. B: Condens. Matter Mater. Phys. 46 (1992) 6131. [52] A. Banik, B. Vishal, S. Perumal, R. Datta, K. Biswas, Energy Environ. Sci. DOI: 10.1039/C6EE00728G. [53] P. Bauer Pereira, I. Sergueev, S. Gorsse, J. Dadda, E. Muller, R. P. Hermann, Phys. Status Solidi B 250 (2013) 1300. [54] X. Chen, D. Parker, D. J. Singh, Sci. Rep. 3 (2013) 3168. [55] P. K. Rawat, B. Paul, P. Banerji, ACS Appl. Mater. Interfaces 6 (2014) 3995. 24

[56] A. M. Dehkordi, S. Bhattacharya, T. Darroudi, J. W. Grff, U. Schwingenschlögl, H. N. Alshareef, T. M. Tritt, Chem. Mater. 26 (2014) 2478. [57] S. R. S. Kumar, A. Z. Barasheed, H. N. Alshareef, ACS Appl. Mater. Interfaces 5 (2013) 7268. [58] J. H. Li, Q. Tan, J-F. Li, D. W. Liu, F. Li, Z. Y. Li, M. M. Zou, K. Wang, Adv. Funct. Mater. 23 (2013) 4317.

Dr. Zhiliang Li is currently a postdoctoral fellow in School of Materials Science and Engineering at Tsinghua University, China. He obtained his Ph.D. in Department of Materials Science and Engineering, China University of Petroleum, Beijing in 2015. He specializes in solution phase chemistry, nano-technology and thermoelectric materials.

Yudong Chen is currently a master’s degree candidate at China University of Petroleum, Beijing. He received his bachelor’s degree from Department of Materials Science and Engineering, China University of Petroleum, Beijing in 2015. His current research focuses on the nanostructured thermoelectric materials. 25

Dr. Jing-Feng Li is a “Cheung-Kong Scholar” distinguished professor and a vice dean of School of Materials Science and Engineering, Tsinghua University. He obtained his Ph.D. from Tohoku University in 1991. After working in Tohoku University as assistant professor from 1992 to 1997 and then associate professor until 2002, he joined Tsinghua University as a full professor in 2002. His current research interests include thermoelectric and piezoelectric materials. Prof. Li is now an Editor-in-Chief of Journal of Materiomics and editorial committee members for several international journals including Journal of Asian Ceramic Societies and NPG Asia Materials.

Hong Chen is currently a candidate for his master’s degree in Department of Materials Science and Engineering, China University of Petroleum, Beijing. He received his bachelor’s degree from China University of Petroleum, Beijing in 2014. His current research focuses on nanostructures and thermoelectric materials.

Lijun Wang is currently a Ph.D. candidate in the Science College of China University of Petroleum, Beijing. She received her bachelor’s degree from Liaochen University, Shandong, China, and master degree in Dalian Polytechnic University, Dalian, China. Her current research interests include the synthesis, assembly and thermoelectric properties of size-, shape-, and structure-controlled metal-chalcogenide nanocrystals.

Dr. Shuqi Zheng is currently a professor in the Department of Materials Science 26

and Engineering at China University of Petroleum, Beijing. He received his Ph.D. in Department of Materials Engineering at Shandong University in 2002 and master degree in Department of Materials Engineering at Shandong University of Technology in 1999. His current research is mainly on the synthesis and characterization of nanostructured thermoelectric materials.

Dr. Guiwu Lu is currently a professor in the Department of Physics at China University of Petroleum, Beijing. He obtained his Ph. D. from the School of Physics and Microelectronics, Shandong University, China in 2002. His research interests include the synthesis, characterization and theory simulation on the multifunctional crystal materials.

Highlights 

SnTe nanoparticles were prepared via a simple, ultra-fast microwave hydrothermal method.



Diameters of SnTe nanoparticles can be controlled from micro-scale to nano-scale.



Thermoelectric properties were remarkably enhanced by the nanometer size effect.

27