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Highly Enhanced Thermoelectric Properties of Bi/Bi2S3 Nano Composites Zhen-Hua Ge, Peng Qin, Dongsheng He, Xiaoyu Chong, Dan Feng, Yi-Hong Ji, Jing Feng, and Jiaqing He ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b14803 • Publication Date (Web): 13 Jan 2017 Downloaded from http://pubs.acs.org on January 13, 2017

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ACS Applied Materials & Interfaces

Highly Enhanced Thermoelectric Properties of Bi/Bi2S3 Nano Composites Zhen-Hua Gea, b*, Peng Qina, DongSheng Heb, Xiaoyu Chonga, Dan Fengb, Yi-Hong Jia, Jing Fenga*, and Jiaqing Heb,c* a

Faculty of Materials Science and Engineering, Kunming University of Science and Technology,

Kunming, 650093, China. b

Shenzhen Key Laboratory of Thermoelectric Materials, Department of Physics, South University of

Science and Technology of China, Shenzhen 518055, China. c

State Key Laboratory of Silicon Materials, Zhejiang University, Hangzhou 310027, China.

Corresponding authors: E-mail address: [email protected] (Z.-H. Ge), [email protected] (J. Feng), [email protected] (J.Q. He) Abstract: Bismuth sulfide (Bi2S3) has been of high interest for thermoelectric applications due to the high abundance of sulfur on Earth. However, the low electrical conductivity of pristine Bi2S3 results in a low figure of merit (ZT). In this work, Bi2S3@Bi core-shell nanowires with different Bi shell thicknesses were prepared by a hydrothermal method. The core-shell nanowires were densified to Bi/Bi2S3 nanocomposite by spark plasma sintering (SPS) technique, and the structure of nanowire was maintained in the nanocomposite due to rapid SPS processing and low sintering temperature. The thermoelectric properties of bulk samples were investigated. The electrical conductivity of a bulk sample after sintering at 673 K for 5 min using Bi2S3@Bi nanowire powders prepared by treating Bi2S3 nanowires in a hydrazine solution for 3 h is 3 orders of magnitude greater than that of a pristine Bi2S3 sample. The nano composite obtained the highest ZT value of 0.36 at 623 K. This is a new strategy for densifying core-shell powders to enhance their thermoelectric properties.

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Keywords: Bi; Bi2S3; nanowires; core-shell; thermoelectric.

Introduction Thermoelectric (TE) material is of high importance in the fields of waste heat harvest and solid-state refrigeration. TE devices are ideal for miniaturized and distributed power generation and electronic cooling.1-3 Because of these outstanding virtues, TE materials have attracted worldwide interest in many fields, including waste-heat recovery and power generation by using solar heat, car coolers, as well as temperature controller of microprocessors.

3-5

The dimensionless figure of merit (ZT)

determines the efficiency of TE devices, which is defined as ZT = α2Tσκ−1, where α, T, σ, and κ, are the Seebeck coefficient, absolute temperature, electrical conductivity, and thermal conductivity, respectively.3 The high TE performance, therefore, requires both a high power factor (α2σ) and a low value of κ.4, 6 Although it is hard to control those parameters independently due to the intercoupling of each other, the TE performance record has been broken continuously in the recent past owing to the application of new mechanisms and concepts, such as phonon-glass electron-crystal (PGEC) behavior,7 synergistic nanostructuring,2 band engineering,8

energy barrier filtering effect,9

modulation doping,10 using liquid-like ions, 11 and Critical phase transitions,12 depressing the bipolar effect.13 Semiconducting chalcogenides have received more attentions as high performance TE materials .5, 6 Among them, Bi2Te3 has demonstrated the best TE properties at room temperature, and presently holds a dominant market share for TE materials.5 However, tellurium is a rare and toxic element, alternative materials must be developed to replace it. The Bi2S3 chalcogenide has received increasing interest recently for TE application as a non-tellurium candidate.14-23 However,


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the value of electrical resistivity for Bi2S3 is too high to be an ideal TE material, and increasing the electrical conductivity ( σ ) is therefore a priority for improving the TE properties of this material. Numerous efforts have been applied to optimizing the TE properties of Bi2S3 such as element doping,14-16 texturing,17-19 microstructure design,20 and nanostructuring.21-23 Liu et al.21 reported that nanostructured Bi2S3 bulk materials made from solution synthesized and surface-treated nano-scale networks exhibit a ZT of 0.5 at 723 K (0.35 at 623 K), which represents a 60% increase compared with non-treated samples. However, the nano-network structures were lost during bulk sintering by hot pressing. In this work, we are going to prepare the Bi/Bi2S3 composite bulk using Bi/Bi2S3 nanowire powders, the Bi was supposed to distribute along the grain boundaries, and form a highly conductive path for improving the electrical conductivity of Bi2S3. Bi2S3 nanowires and Bi2S3@Bi core-shell nanowires with different Bi shell thicknesses were prepared by a hydrothermal method. The Bi shell thickness was controlled by applying a hydrothermal treatment time of 1 h or 3 h. The nanowires were densified to nanocomposite by spark plasma sintering (SPS) technique. The nanowire structure was maintained in the Bi/Bi2S3 nanocomposite due to the rapid SPS process conducted at a low sintering temperature. The TE transport properties of bulk samples were investigated. The σ of a bulk sample subjected to SPS at 673 K for 5 min using Bi2S3@Bi nanowire powders prepared by treating Bi2S3 nanowires in a hydrazine solution for 3 h was 3 orders of magnitude greater than that of a pristine Bi2S3; moreover, the nanocomposite (Bi@Bi2S3 1 h) obtained the highest ZT value 0.36 at 623 K.

Results and discussion Figure 1 shows XRD patterns of (a) Bi2S3 nanowire powders, and the nanowire powders upon which the (b) Bi2S3@Bi 1h and (c) Bi2S3@Bi 3h samples are based. All the peaks in Figure 1(a) are



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well-matched with the standard PDF#17-0320 card, being indicative of the pure Bi2S3 phase. After 1 h hydrazine treatment, Bi peaks are detected in Fig 1(b), indicating the existence of metallic Bi. Increasing the treatment time to 3 h results in increased Bi peak intensities due to the greater concentration of metallic Bi in the powder, the formation mechanism of which has been discussed previously.25 Figs. 1(d)–(f) show XRD patterns of the pure Bi2S3, Bi2S3@Bi 1h, and Bi2S3@Bi 3h bulk samples, respectively. Compared with the XRD patterns of the initial powders, the number of XRD peaks is reduced owing to the nanowire texturing caused by the sintering process. Here, the single crystal nanowires have a strong tendency to arrange along the horizontal direction because of the pressure exerted in the vertical direction during SPS processing. Similar results have been reported for the SPS processing of Bi2S3 nanorods24 and ZnO nanorods.26 Also of note is that the Bi peaks in the Bi2S3@Bi 3h bulk samples are obviously weaker than those of the powders. The most likely reason for this result is that much of the metallic Bi was expelled during SPS processing because the SPS temperature (673 K) is greater than the melting point of Bi (544 K). Figure 2 shows FESEM images of Bi2S3 nanowires (Figure 2(a)), and the nanowires upon which the Bi2S3@Bi 1h (Figure 2b) and Bi2S3@Bi 3h (Figure 2c) samples were formed. The Bi2S3 nanowires are about 50–200 nm in diameter and 2–10 μm in length. The sizes and morphologies of the Bi2S3@Bi core-shell nanowires are very similar to those of the pure nanowires, indicating that the nanowire morphologies were maintained during the hydrazine treatment process. It is also noted that small particles are observable among the core-shell nanowires. These particles are considered to be metallic Bi. Further TEM observation was performed for the two core-shell nanowire samples. The core-shell nanowires are about 50–200 nm in diameter and 2–10 μm in length, which is in agreement with the FESEM results. TEM images for the Bi2S3@Bi 1h and Bi2S3@Bi 3h nanowire


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samples are shown in Figs. 3. Figs. 3(b) and (e) confirm that the nanowires are of a core-shell structure. As shown in Figure 3(c), the shell thickness is about 5 nm for the Bi2S3@Bi 1h nanowire sample, and, as shown in Figure 3(f), the shell thickness for the Bi2S3@Bi 3h nanowire sample is about 10 nm. Fractographs of the pure Bi2S3, Bi2S3@Bi 1h, and Bi2S3@Bi 3h bulk samples fractured in two different directions are presented in Figure 4. The morphologies for any given sample differ markedly in the two directions. This is in agreement with the XRD results representative of texturing within the bulk samples. Figure 4(a) presents a fractograph of a pure Bi2S3 sample fractured in the horizontal direction, showing the strip-like grains derived from the nanowires. After SPS processing, the original nanowires were maintained in the bulk sample due to the low sintering temperature and relatively short holding time 5 min. Figs. 4(c) and (d) present the morphologies of the Bi2S3@Bi 1h and Bi2S3@Bi 3h samples in the horizontal direction, which are very similar to that of the pure Bi2S3 bulk sample. However, pores are observable in the Bi2S3@Bi 3h sample, and the grain boundaries have a molten morphology. This is because of the low melting point of the metallic Bi that forms the shells, which was partially expelled in its molten state during SPS processing. In fact, molten Bi was observed being extruded from the cracks of the graphite die during sintering. Figs. 4(b), (d), and (f) present fractographs of pure Bi2S3, Bi2S3@Bi 1h, and Bi2S3@Bi 3h, respectively, fractured in the vertical direction, i.e., parallel to the direction of the applied pressure, which are indicative of the cross sections of the pure and core-shell nanowires. In order to further investigate the structure of the bulk sample, the TEM observation was performed for the sample Bi2S3@Bi 1h. Figure 5(a) shows the low resolution dark field image of the sample Bi2S3@Bi 1h, the morphology of the sample is wire-like, which is agreement with the results



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of FESEM observation. However, in TEM image, the white nanowires with dark edges indicating that the compositions of the nanowires and edges are different. Figure 5(b), 5(b1) and 5(b2) illustrate that the core of nanowire is Bi2S3 and the edge is Bi. The low resolution image shows the strip-like grain and the the grain boundary phases were observed as the red arrows marked in Figure 5c. The high resolution image of the grain boundary was shown in Figure 5d, indicating that the Bi layer with the thickness of 10 nm existed in grain boundary. The Bi layer thickness in the grain boundary is agreement with the powder TEM observation results that the core-shell nanowoires have the 5 nm Bi shells. The TEM observation of the bulk sample Bi2S3@Bi 1h suggested that the core-shell structure of Bi@Bi2S3 nanowires was maintained in the bulk after SPS sintering. The TE properties of bulk samples were measured in equivalent directions, as illustrated by the strip-like horizontal sample presented at the top of the inset of Figure 6a. Figure 6(a) presents the measured temperature dependence of σ for the three bulk samples. For the pure Bi2S3 sample, σ is initially 5 S/m, and increases to 174 S/m with rising temperature, showing a semiconductor behavior. For the Bi2S3@Bi 1h and Bi2S3@Bi 3h samples, the values of σ are initially 172 S/m and 3,750 S/m, respectively, which are 2 and 3 orders of magnitude greater than that of pristine Bi2S3. The values of n and µ were extrapolated from the measured results for each sample (shown in Table 1), and the results indicate that both n and µ increased with increasing hydrazine treatment time. The reason why the Bi2S3@Bi samples demonstrated substantially greater values for σ relative to that of pure Bi2S3 will be discussed later. The Bi/Bi2S3 3 h composite have a high electrical conductivity which is 3 orders of magnitude higher than that of pristine Bi2S3 sample. The reason is the introducing of metal Bi with a high electrical conductivity. 27 The Bi/Bi2S3 core-shell nanorods were sintered to bulk composites, the


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wire-like structure was maintained in bulk. The metal Bi distributed in grain boundary significantly improved the electrical conductivity of composites. This is very easy to understand that the Bi will increase the electrical conductivity, but the regular method for adding Bi to Bi2S3 have a big problem of Bi distributing in matrix. The electrical conductivity of 3 h sample decreases with increasing temperature, the reason is that the conducting behavior of sample was changed from semiconductor to metal due to too much metal Bi in grain boundary. Figure 6(b) shows the temperature dependence of α for each of the samples. The negative

α values imply a n-type conducting behavior, where the major carrier is the electron. The value of α is inversely proportional to σ. Owing to this relationship between α and σ, the highest absolute values of α in the range of −480 to −550 μV/K were obtained for the pure Bi2S3 sample, and the lowest absolute values were obtained for the Bi2S3@Bi 3h sample in the range of −150 to −220 μ V/K. The power factor (PF) is defined as PF = α2σ. As shown in Figure 6(c), the PF of all samples increased with increasing temperature due to the increased values of α and σ. The highest PF value attained by the pure Bi2S3 sample was about 50 μW/mK2 at 623 K, whereas the Bi2S3@Bi 3h sample attained a highest PF value of 225 μW/mK2, which is greater than that of the pure Bi2S3 sample by a factor of nearly 5. The temperature dependence of the thermal transport properties along the same direction as that of the electrical transport properties was measured, as illustrated by the square vertical sample presented at the side of the inset of Figure 7(a). The temperature dependence of the specific heat is shown in Figure 7(a), where the specific heat is observed to increase from 0.22 to 0.27 J/gK with increasing temperature for all samples. The values for all the samples are very similar because the



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specific heat is an inherent parameter of a material that is difficult to change. Figure 7(b) shows the temperature dependence of κ for each of the samples. The values of κ for all samples decreased with increasing temperature, indicating that κ is dominated by the lattice thermal conductivity. The pure Bi2S3 sample exhibits the lowest value with κ = 0.4 W/mK at 323 K. This may be the result of the special interface between nanowires, which significantly scatters phonons. A low value of κ has been reported for a solution phase synthesized sample relative to samples fabricated by other synthesis methods. 2, 21 The lattice thermal conductivities of all the bulk sample were calculated by κl=κ-κe, κe=LσT, where κl, κ, κe, L,σ,T are lattice thermal conductivity, total thermal conductivity, carrier thermal conductivity, Lorenz number, electrical conductivity and absolute temperature, respectively. The results were shown in Figure 7(c), indicating the lattice thermal conductivity is predominating. The calculated ZT values for each of the samples are shown in Figure 7(c). The pure Bi2S3 sample attained a ZT value of 0.15 at 623 K, which is in agreement with previously reported values. The Bi2S3@Bi 3h sample attained the highest ZT value of 0.36 at a temperature of 623 K, which is twice that of the pure sample, and represents a relatively large value for a solution phase synthesized Bi2S3 system. Actually, mixed samples comprised of two or more different components typically exhibit poor TE properties compared with those of pure-phase samples. However, because of the unique core-shell interface with the shell materials and core materials growing together, the as-prepared Bi2S3/Bi nano composite samples exhibit better TE properties than either pure Bi2S3 or metallic Bi. This is a new strategy for enhancing the TE properties of bulk materials by densifying core-shell structured powders. The core-shell powder also can be used as a doping agent for introducing additional interfaces in bulk materials. For the synthesis method employed in the present work, the


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initial hydrothermal process can be changed to mechanic alloying, colloidal synthesis, or some other method, prior to treating the prepared powders by hydrazine in the second hydrothermal process to obtain core-shell powders. Because the shell thickness was controlled by the hydrothermal time, we can more readily obtain core-shell powders with easily controllable sizes, morphologies, and shell thicknesses as precursor powders for further enhancing TE properties. Metal sulfides are expected to be promising TE materials in the near future. 29

Conclusion Bi2S3@Bi core-shell nanowires with different Bi shell thicknesses were prepared by a two-step hydrothermal method. The core-shell nanowires were densified by SPS at 673 K for 5 min, where the nanowire and core-shell structure were maintained in the bulk materials owing to rapid SPS processing and low sintering temperature. Measurements of the TE properties indicated that the electrical conductivity of the bulk sample sintered by SPS using Bi2S3@Bi nanowire powders prepared by treatment in a hydrazine solution for 3 h was 3 orders of magnitude greater than that of the pristine Bi2S3; moreover, this sample obtained the peak ZT of 0.36 at 623 K.

Experimental section Chemicals used in this work are of analytical grade, and were purchased from Alfa Aesar. The core-shell nanowires were synthesized by a two-step hydrothermal method as described in our previous work. 24, 25 The hydrothermal reaction temperature is 180 oC for either 1 h or 3 h to obtain nanowires with different shell thicknesses. The details of synthesis process were shown in supporting information. The core-shell powders were checked by X-ray diffraction (XRD; Bruker, Germany). Transmission electron microscopy (TEM) observations were performed by using a JEOL



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2100F microscope operated at 200 kV. The as-prepared Bi2S3 nanowire powders and Bi2S3@Bi core-shell nanowires with different shell thicknesses were densified by sintering at 673 K for 5 min using an SPS system (Sumitomo SPS1050, Japan). Three bulk samples were obtained. The sintered bulk sample comprised of pure Bi2S3 nanowires is denoted herein as pure Bi2S3. The sintered samples comprised of Bi2S3@Bi core-shell nanowires with different hydrazine treatment times of 1 h and 3 h are denoted herein as Bi2S3@Bi 1h and Bi2S3@Bi 3h, respectively. The phase structure of the sintered samples was analyzed by XRD. The morphologies of fractographs were observed by field emission scanning electron microscopy (FESEM; Zeiss Merlin, Germany). The TE properties were evaluated along the sample section perpendicular to the pressing direction of SPS. The Hall coefficients (RH) of the samples were measured at 323 K using a physical properties measurement system (PPMS-9T, Quantum Design Inc., USA). The details of TE properties measurements are available in supporting information.

Associated Content Supporting information The calculation detail of lattice thermal conductivity and stability of the nanocomposites were supplied as Supporting Information Notes The authors declare no competing ﬁnancial interest.

Acknowledgements This work was supported by the National Natural Science Foundation of China (Grant No. 51501086)


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and also supported by the Science, Technology, and Innovation Commission of Shenzhen Municipality (Grant Nos. JCYJ20140612140151884 and JCYJ20150831142508365), Natural Science Foundation of Guangdong Province (Grant No. 2015A030308001) and the leading talents of Guangdong province Program (Grant No. 00201517).

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Figure

Figure 1 XRD patterns of nanowire powders: (a) pure Bi2S3 nanowire powder; (b) Bi2S3@Bi nanowire powder prepared by treating Bi2S3 nanowires in a hydrazine solution for 1 h (Bi2S3@Bi 1h); (c) Bi2S3@Bi nanowire powder prepared by treating Bi2S3 nanowires in a hydrazine solution for 3 h (Bi2S3@Bi 3h) and XRD patterns of bulk samples (d) pure Bi2S3 bulk sample; (e) bulk Bi2S3@Bi 1h sample; (f) bulk Bi2S3@Bi 3h sample. The red triangles represent the diffraction peak of metal Bi.



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Figure 2 FESEM images of (a) pure Bi2S3 nanowires, (b) Bi2S3@Bi nanowires prepared by treating Bi2S3 nanowires in a hydrazine solution for 1 h, and (c) Bi2S3@Bi nanowires prepared by treating Bi2S3 nanowires in a hydrazine solution for 3 h.

Figure 3 TEM images (a)-(c): Bi2S3@Bi nanowires prepared by treating Bi2S3 nanowires in a hydrazine


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solution for 1 h, and (d)-(f): Bi2S3@Bi nanowires prepared by treating Bi2S3 nanowires in a hydrazine solution for 3 h. The insets of Figure 3b are the SAED for both core area and shell area of Bi2S3@Bi nanowires prepared by treating Bi2S3 nanowires in a hydrazine solution for 1 h.

Figure 4 FESEM images of bulk samples fractured in the different directions indicated by the insets: (a), (b) pure Bi2S3 sample; (c), (d) Bi2S3@Bi 1h sample; and (e), (f) Bi2S3@Bi 3h sample.



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Figure 5 TEM image (dark field) of Bi2S3@Bi 1h bulk sample (a, b) and EDS results (b1) line scanning, (b2) face element scanning result acquired from figure 5b. Low resolution image of Bi2S3@Bi 1h bulk sample (c), the red arrows shows the metal Bi in the grain boundary, (d) the high resolution TEM image of Bi2S3@Bi 1h bulk sample acquired from Figure 5(c).


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Figure 6 Temperature dependence of electrical conductivity (a), Seebeck coefficient (b), and power factor (c) for pure Bi2S3, Bi2S3@Bi 1h, and Bi2S3@Bi 3h samples.



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Figure 7 Temperature dependence of (a) specific heat, (b) thermal conductivity, (c) lattice thermal conductivity and (d) ZT value with error bar for each bulk sample.

Table 1 Carrier concentration and mobility of bulk sample Sample name

n (× ×1019cm-3)

Pure Bi2S3 Bi2S3@Bi 1 h Bi2S3@Bi 1 h

0.25 1.9 20


μ (cm2V-1S-1) 1.2 5.3 11.5

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TOC


Bi2S3

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