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Energy, Environmental, and Catalysis Applications 7

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Preparation and Characterization of Te/PEDOT:PSS/CuTe Ternary Composite Films for Flexible Thermoelectric Power Generator Yao Lu, Yang Qiu, Qinglin Jiang, Kefeng Cai, Yong Du, Haijun Song, Mingyuan Gao, Changjun Huang, Jiaqing He, and Dehua Hu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b15252 • Publication Date (Web): 15 Nov 2018 Downloaded from http://pubs.acs.org on November 16, 2018

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ACS Applied Materials & Interfaces

Preparation and Characterization of Te/PEDOT:PSS/Cu7Te4 Ternary Composite Films for Flexible Thermoelectric Power Generator Yao Lua, Yang Qiub, Qinglin Jiangc, Kefeng Cai a*, Yong Dud, Haijun Songa, Mingyuan Gaoa, Changjun Huanga, Jiaqing Heb*, Dehua Huc* a

Key Laboratory of Advanced Civil Engineering Materials, Ministry of Education,

School of Materials Science & Engineering, Tongji University, 4800 Caoan Road, Shanghai 201804, China b

Physics Department, Southern University of Science and Technology, 1088

Xueyuan Avenue, Shenzhen 518055, China c Institute

of Polymer Optoelectronic Materials and Devices, State Key Laboratory of

Luminescent Materials and Devices, South China University of Technology, Guangzhou 510640, China d

School of Materials Science and Engineering, Shanghai Institute of Technology,

Shanghai 201418, China

ABSTRACT In this work, Te/PEDOT:PSS/Cu7Te4 ternary thermoelectric (TE) nanocomposite films were successfully fabricated by physical mixing and then drop casting. An optimum power factor of 65.3 W/mK2 was acquired from a composite film containing 95 wt% PEDOT:PSS coated Te (PC-Te) nanorods at 300 K, which was about 5 times as large as that of PC-Cu7Te4 nanorod film and about 3 times as large as 1

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that of the PC-Te nanorod film. The power factor reached 112.3 W/mK2 when the temperature was 380 K. STEM and HRSTEM were used to observe the detailed internal microstructure of the composite film, revealing that the Te nanorods were single crystalline and the Cu7Te4 rods polycrystalline. The composite film was in fact a 3D network interconnected with the PC-Te and PC-Cu7Te4 nanorods. The enhancement of the TE properties was ascribed to the synergetic effect of the two kinds of nanorods and the double-carrier filtering effect at the two heterointerfaces of Te/PEDOT:PSS and Cu7Te4/PEDOT:PSS. An eight single-leg flexible TE device consisting of the optimized composite film was fabricated, which produced a voltage of 31.2 mV and a maximum output power of 94.7 nW at a temperature gradient of 39 K, respectively. KEYWORDS: PEDOT:PSS; Copper telluride; Tellurium, Thermoelectric; Flexible.

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1. INTRODUCTION Thermoelectric (TE) materials can directly convert heat to electricity or vice versa, hence they have tremendous potential for applications in both generation power from waste heat and temperature cooling.1 The TE performance of a material is quantified by the dimensionless figure of merit, ZT (= S2σT/κ, where S, σ, and κ are the Seebeck coefficient, the electrical conductivity, the thermal conductivity, respectively, and T is the absolute temperature).2 Thus, an ideal TE material should have a high σ, a high S and a low κ at the same time. However, these three parameters always have strong interdependencies; hence, it is a big challenge to increase the ZT value. Inorganic semiconductors especially chalcogenides, such as PbTe,3 Bi2Te3,4 with narrow bandgap and high S have been extensively researched as the most efficient TE materials. However, the inorganic TE materials are rigid and brittle, which prevent them from being used as flexible and wearable TE devices. Conducting polymers (CPs), including polyaniline, polyprrole and poly(3,4ethylenedioxythiophene) (PEDOT), have been considered as promising candidates. They possess many advantages, such as nontoxic, cheap, light weight, flexible, easy synthesis, abundant, and low thermal conductivity (0.11-0.5 W/mK)5, etc. Among the CPs, poly(styrenesulfonate) doped PEDOT (PEDOT:PSS) is a rapidly developed TE material because of its water soluble, relatively high electrical conductivity (103 S/cm) and excellent stability.6 However, its low Seebeck coefficient hampers its further TE applications.7-8 Generally, constructing CP-based nanocomposites by fully taking advantage of low κ of CP and the high power factor (PF=S2σ) of inorganic TE materials

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is a useful approach to improve the TE performance, such as reduced oxide graphene (rGO)/PEDOT:PSS composite9, Bi2Te3 nanosheet/PEDOT:PSS composite10. The hybrid materials exhibit superior performance mainly because of non-linear interaction deriving from strong energy filtering effect at the nanoscale interfaces between their constituent components.11 In fact, rational ternary polymer/inorganic nanocomposite has started to attract attention due to the double-carrier filtering effect.12, 13 However, by a conventional physical mixing method, polymer and inorganic components can hardly form interactions and are not robustly tethered to one another. They easily dissociate and lead to low stability, thus severely hinder their use in TE application. 14 Hence, choosing suitable synthesis methods for effective dispersion of inorganic nanostructures in a polymer matrix to enhance wettability is a key issue. Tellurium (Te) possesses a very high Seebeck coefficient (~ 408 μV/K) 15 and Te nanocrystals can be facilely synthesized in water solution.16 Previously, See et al.

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have developed an in-situ synthesis method for preparing PEDOT:PSS coated Te (PCTe) nanorods composite film. Attributing to a potential interfacial interaction in the CP/Te nanocrystal, a Seebeck coefficient of 163 V/K is obtained, but the σ of the composite film is very low (~ 19.3 S/cm). Subsequently, Song et al. 18 prepared PC-Te nanorods by the same method as in ref. [17], then integrated them with PEDOT:PSS through a vacuum-assisted filtration method. Finally, an optimized PF of 51.4 W/mK2 for a flexible composite was obtained. Choi et al. 19 also synthesized PC-Te nanorods, and mixed with rGO aqueous solution, and then vacuum filtered to prepare rGO/PEDOT:PSS/Te ternary nanocomposite film, which showed a high PF of 143 4


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W/mK2 due to a double-carrier filtering effect. In order to ensure flexibility, the rGO content in the composite film needs to be higher than 10 wt%; hence, it will increase the cost of the film since rGO is relatively expensive. Copper telluride includes many compounds with variable stoichiometric of CuxTe (1 ≤ x ≤ 2).20 The complexity of the compounds makes the preparation of their nanostructures more difficult than the other typical semiconductors. Cu7Te4 shows a metallic conductivity, with the electrical conductivity in the order of 104 S/cm 21 due to copper vacancies and the direct narrow band gap

22.

However, as a TE material, its

Seebeck coefficient is relatively low (~ 10 V/K) 23, and the reported synthesis methods for Cu7Te4 nanocrystal are costly and always produce impurity phases.24 Recently, a facile solution-based synthesis method using Te nanorods/nanowires as template to grow II-VI chalcogenides, such as Cu1.75Te 25, PbTe 26, CdTe 26, etc, has been reported. In 2016, Zaia et al. 27 synthesized PC-Te nanorods by the same method as reported in ref. [17], then controlled growth of polycrystalline Cu1.75Te within the hybrid nanocomposites by injecting copper ions into the PC-Te solution. The PEDOT:PSS/TeCu1.75Te hetero-structure nanocomposite exhibited a 22% PF enhancement compared with PEDOT:PSS/Te. However the composite film was drop-cast on a glass substrate, limiting its application for flexible TE devices. In this work, PC-Te nanorods were first in-situ chemically synthesized, then PEDOT:PSS coated polycrystal Cu7Te4 (PC-Cu7Te4) nanorods were prepared using the PC-Te nanorods as templates, finally PC-Cu7Te4/PC-Te composite films were fabricated through physical mixing and then drop-casting so as to combine the high 5



Seebeck coefficient of the PC-Te nanorods and high electrical conductivity of the PCCu7Te4 nanorods. The ternary composite film shows an enhanced PF of 65.3 W/mK2, which is ascribed to the synergistic effect and the double-carrier filtering at the two heterointerfaces between the Te and PEDOT:PSS and between the Cu7Te4 and PEDOT:PSS. Besides, an eight single-leg flexible TE device was assembled using the optimized ternary composite film.

2. EXPERIMENTAL SECTION Materials PEDOT:PSS aqueous solution (CLEVIOS PH1000) was bought from Wuhan Zhuoxin Technology Co., Ltd. Ascorbic acid, ethanol, sodium tellurite (Na2TeO3, 97%) and copper (I) chloride (CuCl) were bought from Aladdin Industrial Corporation. All the raw materials were directly used. Synthesis of the PC-Te and PC-Cu7Te4 nanorods The synthesis procedure of the PC-Te nanorods was as follows. Firstly, 56.8 mmol ascorbic acid was dissolved in 500 ml of deionized water with continuous stirring for 30 min, then adding 10 ml PEDOT:PSS with another 30 min stirring. Secondly, 3.2 mmol Na2TeO3 was added to the vigorously stirring mixture slowly. Then, the obtained suspension liquid was heated to ~90 °C and stirred for 20 h. Finally, the mixture was naturally cooled down to room temperature (~300 K), and a solution (~ 510 mL) containing PC-Te nanorods was obtained. The synthesis procedure of the PC-Cu7Te4 nanorods was described as follows. 6


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Firstly, the PC-Te nanorods contained solution was prepared by the same procedure as described in the former paragraph. Secondly, 1.6/3.2/6.4/9.6 mmol CuCl was dissolved in 10 ml of deionized water under stirring for 20 min. Finally, the solution was poured into the PC-Te nanorods contained solution (corresponding molar ratio of the precursor for Cu: Te was 0.5:1, 1:1, 2:1 and 3:1, respectively) slowly with vigorously stirring for 3 h at room temperature to allow completely reaction and PC-Cu7Te4 nanorods formation (~ 520 mL total solution). Preparation of the PC-Te/PC-Cu7Te4 composite films At first, in order to accuratly control the mass fractions, the mass concentration of the PC-Te nanorods and PC-Cu7Te4 nanorods in each solution was investigated. For example, a certain volume of the PC-Te solution was centrifuged at 11000 rpm for 15 min, and the precipitate was washed with distilled water. The centrifugation process and washing process were repeated for six times. The precipitate was finally dried in vacuum at 60 °C for 10 h. By weighing the final product, the mass concentration of the PC-Te was determined. For that of the PC-Cu7Te4, the PC-Cu7Te4 nanorods contained solution generated from 3.2 mmol CuCl (Cu:Te=1:1, the product from this ratio will be used for forming the composites) was investigated. The mass concentration of the PCTe and PC-Cu7Te4 in each solution was determined to be 0.75 mg/mL and 1.02 mg/mL, respectively. Figure 1 schematically depicted the fabrication process of the PC-Cu7Te4/PC-Te composite films. Different volume of the PC-Te solution was added into the PC-Cu7Te4 solution to form different mass fraction from 33 to 98 wt%. The resulting mixture was 7



stirred continuously for 24 h at 300 K. Purification was performed by centrifuging the precipitate at 11000 rpm for 30 min. The precipitate was re-suspended in deionized water and centrifuged again after pouring off the supernatant. This procedure was performed six times, and the precipitate was finally dispersed in 2-4 mL water (at concentration about 30 mg/mL) and sonicated for 20 min. Finally, the PC-Cu7Te4/PCTe mixed solution was drop cast onto a pre-cleaned common glass substrates (1.5 cm ×1.5 cm) and dried at 60 °C for 12 h. For comparison, the PC-Te and PC-Cu7Te4 binary composite films were also fabricated by the same procedure. Typically the substrates were sonicated in cleanser, acetone, water, and ethyl alcohol for 20 min each, followed by vacuum drying for at least 60 min.

Figure 1. Schematic plot of the fabrication of PC-Cu7Te4/PC-Te composite film

Thermoelectric device fabrication and characterization PC-Cu7Te4/95 wt% PC-Te mixture was drop-cast onto a polyimide substrate 8


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(typically 3-5 μm in thickness), followed by vacuum drying for 12 h to form film, which was used to fabricate flexible TE device. The polyimide substrate was cleaned and dried before use according to the above procedure as for glass substrates. The as-prepared composite film was cut into eight strips (25 mm × 5 mm). The strips were pasted onto a polyimide substrate and connected in series using a Ag paste. Measurements and characterizations The electrical conductivity measurement was performed using an Ecopia HMS3000 Hall effect measurement system. The Seebeck coefficient was measured by the slope of the linear relationship between the thermal electromotive force and temperature difference (~ 10 K) between two ends of each film. The electrical conductivity and Seebeck coefficient varying with temperature were measured simultaneously in a MRS3L thin-film TE test system (Wuhan Giant Instrument Technology Co., Ltd., Wuhan, China) in vacuum (≤40 Pa) from 293 to 380 K with measurement error for both the parameters being about 5%. The in-plane thermal conductivity measurement was performed using the LINSEIS TFA apparatus based on the 3-method. The thickness of samples was measured by a Bruker Dektak XT profilometer. The phase composition of the PC-Te and PC-Cu7Te4 was examined by X-ray diffraction (XRD) using Cu Kα radiation (D/MAX 2550VB3+/PCII). The composition of the films was detected by X-ray photoelectron spectroscopy (XPS) with Al Kα radiation (1486.6 eV) (ESCALAB 250Xi). The porous feature of the composite samples was analyzed by a Brunauer-EmmettTeller (BET) analyzer (Micromeritics, ASAP 2020), and the pore size distribution was 9



examined by the Barret-Joyner-Halenda (BJH) method. About 200 mg of sample was scraped from the substrate and degassed at 150 ℃ for 6 h. The measurement was carried out in an ultra-high pure N2 flow at 77.35 K. The morphology of the samples was examined by a field-emission scanning electron microscope (FE-SEM) (FEI Nova NanoSEM 450) and a high-resolution transmission electron microscope (HRTEM) (JEM-2100F). The FEI Helios 600i was used for HRSTEM measurement with high-angle annular dark-field (HAADF) elements mappings. The STEM sample was prepared by the Focused Ion Beam (FIB, FEI Helios600i) with the in-situ lift-out technique. Before the ion milling, a Pt layer was sputtered on the sample to protect its surface. The interest area was further locally capped in the FIB with ion beam deposited Pt. The major milling was done with a 30 kV Ga ion beam while the milling procedure was controlled with SEM. Final milling to minimize the damage layer on the sample was carried with 2 kV Ga ion beam. The tensile strength of the composite film on polyimide was investigated by using a universal mechanical testing machine made by Shenzhen suns technology stock Co. Ltd, and for comparison, that of the polyimide with and without undergone the same heating process for the composite film was also studied. The samples were glued to two acryl plate mounted on the tensile machine and then stretched until fracture occurred. A uniaxial extension of the samples was conducted with a constant rate of 5 mm/min. The performance of TE module was tested by a home-made measurement device. As shown in Figure S1, one end of the TE generator was put onto a heating element controlled by an automatic temperature controlling module acting as the hot side (T + 10


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ΔT), while another end was connected with a cold metal block acting as cold side (T). The temperature difference was detected by two K-type thermocouples located at the two ends of the TE generator. The TE device was connected into a circuit with a variable resistance box and a microammeter in series, with its two end points jointed with two conducting wires using sliver paste. For a particular temperature difference, by adjusting the value of load resistance, output voltage and output current were obtained by a voltmeter (Agilent 34970) and the microammeter, respectively.

3. RESULTS AND DISCUSSION

Figure 2. (a) XRD patterns of the PC-Te film and PC-Cu7Te4 films obtained with nominal compositions of Cu/Te = 0.5:1, 1:1, 2:1, and 3:1; typical TEM images of (b) PC-Te nanorods, (c) (d) PC-Cu7Te4 nanorods, (e) a HRTEM image of the yellow square in (d).

Figure 2(a) reveals the XRD pattern of the PC-Te and PC-Cu7Te4 composite films. All the XRD peaks for the PC-Te film can be indexed to hexagonal Te (JCPDS No. 361452), indicating that the high purity Te has been synthesized.28 Slight peak broadening indicates that the product is in nanostructure.29 11



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For the PC-Cu7Te4 film, when the nominal molar ratio of [Cu]/[Te] in the starting materials is < 2:1, all the XRD peaks can be indexed to Cu7Te4 (JCPDS No. 18-0456 Cu7Te4, a = 8.32 Å, c = 7.2 Å), whereas when the molar ratio of [Cu]/[Te] in the starting materials is ≥ 2:1, a weak peak for Cu2O (JCPDS No. 34-1354) is detected, besides the XRD peaks for Cu7Te4, which is unsurprising considering that Cu is excessive and the newly formed Cu being oxidized. Note that the XRD peaks for PEDOT:PSS is not detected in all the samples due to its amorphous characteristics. Based on the XRD results, the chemical reactions involved in the process are proposed as follows: TeO32 + 2H+→TeO2 + H2O

(1),

C6H8O6 + TeO2 → C6H4O6 + Te + 2H2O

(2),

2Cu+ + C6H8O6 → 2Cu + C6H6O6 + 2H+

(3),

7Cu + 4Te → Cu7Te4

(4).

As the sample obtained with nominal composition of Cu/Te = 1:1 shows a higher crystallization and exhibits a highest PF value (see supplementary information Table S1), for preparing Te/PEDOT:PSS/Cu7Te4 ternary composites, the PC-Cu7Te4 nanorods synthesized with the nominal composition of Cu/Te = 1:1 are used. Figure 2(b) shows a typical TEM image of the PC-Te nanorods. It is seen that the nanorods are straight (average diameter ~ 30 nm) with a thin amorphous PEDOT:PSS layer. However, the PC-Cu7Te4 nanorods are winding and kinking with diameter about 30 nm (see Figure 2(c, d)). A PEDOT:PSS thin layer is also observed on the surface of the Cu7Te4 nanorods (see Figure 2(e)). The polymer layer is tightly adhered to the Cu7Te4 and Te nanorods, preventing them from oxidation.30 In addition, a hollow 12


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structure can be observed in some of the PC-Cu7Te4 nanorods in Figure 2(c, d) and clearly seen in the enlarged image (Figure 2(e)). Since the PC-Cu7Te4 nanorods and PC-Te nanorods are water-soluble, it is easy to form composite by simply physical mixing their solutions followed with drop casting without changing their morphology.

Figure 3. FESEM images of the (a) PC-Te, (b) PC-Cu7Te4/95 wt% PC-Te, (c) PC-Cu7Te4/33 wt% PC-Te and (d) PC-Cu7Te4 films. Inset in each image is the FESEM image at high magnification.

Typical surface FESEM images of the (a) PC-Te, (b) PC-Cu7Te4/95 wt% PC-Te, (c) PC-Cu7Te4/33 wt% PC-Te and (d) PC-Cu7Te4 films are shown in Figure 3. Figure 3(a) indicates that the PC-Te nanorods (with length of 900 ± 10 nm and diameter of 30 ± 10 nm) are straight and uniform, which is in good agreement with the that reported in ref. [28]. As the PC-Cu7Te4 content increases, the amount of winding nanorods increases due to the PC-Cu7Te4 nanorods being winding (Figure 3(b, d)). The size of the PC-Cu7Te4 nanorods is very similar to that of the PC-Te nanorods, which is 13



consistent with the TEM observation result (Figure 2(c)). Furthermore, EDS spectrum (Figure S2) confirms that the atomic ratio of Cu/Te is close to 7:4. Figure 3(b, c) show the uniformly distribution of the PC-Cu7Te4 and PC-Te nanorods without obvious agglomeration. Compared with 0D quantum dot and 2D nanoplate based films, 1D nanorod-based film possesses a higher surface coverage and thus higher reliability.25 The cross-sectional FESEM images show that the composite films are in fact 3D nanorod-based-network (see Figure S3). The thickness (typically 3-5 m) of the films is listed in Table S2.

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Figure 4. STEM images of the PC-Cu7Te4/95 wt% PC-Te composite film, (a) cross-sectional image, (b) enlarged image of the square marked in (a), HRSTEM images of (c) a PC-Te nanorod and (d) a PC-Cu7Te4 nanorod, insets in (c) and (d) are enlarged images of the square marked in c and d, respectively, and HAADF elemental mapping, (e) a cross-sectional image of a PC-Cu7Te4 nanorod containing a few grains with different orientations coated with a thick layer of PEDOT:PSS, (f) a side view of a PC-Cu7Te4 nanorod containing 3 different-orientation grains coated with a thin layer of PEDOT:PSS, (g) a side view of a PC-Cu7Te4 nanorod containing three grains with different orientations and a clear interface, the rod coated with a thin layer of PEDOT:PSS, (h) schematic illustration of the formation process of the PC-Cu7Te4 nanorod based on the STEM observation

Figure 4(a) shows a typical cross-sectional STEM image of the PC-Cu7Te4/95 wt% PC-Te composite film. The film consists of uniformly distributed nanorods interconnected into a network. Several hexagonal cross-section of nanorods are observed and marked by yellow dotted lines in Figure 4(a), which is more clearly shown in Figure 4(b). The nanoporous network is helpful to decrease the thermal conductivity of the film. Figure 4(c) shows a HRSTEM image of a PC-Te nanorod, a distinct crystal lattice fringe with lattice spacing of 3.234 Å, in accordance with the (101) plane of the hexagonal Te. Figure 4(d) shows a HRSTEM image of a PC-Cu7Te4 nanorod, with a lattice spacing of 7.21 Å, corresponding to the d-spacing of (100) plane. The HAADFSTEM EDS mapping of the Cu7Te4 nanorod is shown in Figure 4(e), which clearly demonstrates a well-defined and uniformly distribution of Cu and Te elements.31 Notes that the hollow structures can be observed in the central of some PC-Cu7Te4 nanorods (Figure 2(c-e)). Such special structure has been often observed in chalcogenide nanocrystals synthesized in a solution process according to the nanoscale 15



Kirkendall effect.32 In order to well understand the growth mechanism of the Cu7Te4 nanorods, we specifically observed the sample with STEM. Figure 4(e-g) show three typical STEM images of the PC-Cu7Te4 nanorods. Figure 4(e) is the cross-section image of a PC-Cu7Te4 nanorod containing six different orientation nanograins (The lattice spacing is 0.42 nm, 0.36 nm, 0.32 nm, 0.24 nm, 0.25 nm and 0.27 nm, corresponding to the (110), (002), (102), (003), (202) and (210) plane of the hexagonal Cu7Te4, respectively). Figure 4(f) shows a side view of a part of a PCCu7Te4 nanorod, which consists of a well crystalline with (100) lattice plane and two grains with (202) and (102) planes. Figure 4(g) shows a side view of another PC-Cu7Te4 nanorod, which also consists of three grains with different orientations and with an obvious interface. We deduce that the three typical images present the three growth stages of the PC-Cu7Te4 nanorods. On the basis of the STEM observation, the growing process of the Cu7Te4 nanorods is proposed and illustrated in Figure 4(h). And the details are described as follows. (I) At first, the uniformly distributed Cu+ ions are reduced to Cu atoms by ascorbic acid. The Cu atoms diffuse into the interface between Te core and PEDOT:PSS shell due to the concentration difference and react with Te to generate Cu7Te4 crystalline seeds. (II) As the diffusion of Cu atoms continues, the Cu7Te4 seeds grow up into grains. (III) Since the diffusion rate of Te is faster than that of Cu 33, vacancies are generated at the core, then merge into voids by the continuous outward diffusion of Te atoms, accompanying with the persistent growth of the Cu7Te4 grains. (IV) The distribution of Cu atoms in solution is not homogeneous inevitably. For some nanorods, Te atoms consume earlier, voids grow to hollow structure in the 16


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central region of the Cu7Te4 nanorod. For others, Te and Cu atoms consume simultaneously, no hollow structure forms. Small Cu7Te4 grains coherently grow into bigger grains, with clear interfaces due to misaligned crystal orientations, finally leading to the curving rods. Such special nanostructure can reduce the thermal conductivity in principle,34 but has little effect on the electrical conductivity since the rods are with homojunctions.

Table 1 The porous feature of PC-Cu7Te4/95 wt% PC-Te composite films cast at different temperatures. Casting

Average pore size BET surface area Pore volume

temperature (℃)

(nm)

(m2/g)

(cm3/g)

40

47.85

24.85

0.30

60

52.49

25.77

0.34

80

44.78

28.43

0.32

100

45.66

30.22

0.34

To reveal the effect of the drop casting condition on the composite samples, the PC-CuTe/95 wt% PC-Te films cast at different temperatures were analyzed by the BET and BJH method. Table 1 lists the average pore size, surface area and pore volume from the BET measurement. As shown in Table 1, the casting temperature has an effect on porous structure. When the film casting temperature rises from 40 to 100℃，the BET surface area increases from 24.85 to 30.22 g/cm3, suggesting an increasing number of pores. Hence, the electrical conductivity of the PC-Cu7Te4/95 wt% PC-Te film decreases with the increase of casting temperature (Figure 4(a)). In addition, most of 17



the pores are ~110 - 130 nm in size (Figure 4(b)), indicating an existence of macroporous microstructure (diameter ＞50 nm) in the 3D nanorod-based-network. Such porous microstructure is beneficial for lowering the thermal conductivity of the films. XPS was used to investigate the surface chemical compositions of the samples. Figure S5(a) shows the XPS survey spectra. For the PEDOT:PSS film, there exhibit two distinct S 2p characteristic peaks of S atom (Figure S5(b)). The peak with a binding energy of 162-166 eV corresponds to the S atoms in the PEDOT units, and that with a binding energy of 166-170 eV originates from the S atoms in the PSS units.18 The ratio of the S 2p peak integral area of PSS to that of PEDOT is 3.3, which is used to estimate the relative composition of PSS to PEDOT at the sample surface.28 When PEDOT:PSS is integrated with Te and Cu7Te4, the low intensity S 2p peaks prove a small coating amount of PEDOT:PSS. In Figure S5(c), two strong peaks at 573.2 and 583.6 eV are detected, corresponding to the Te 3d5/2 and Te 3d3/2 peaks.35 Note that the characteristic Te 3d peaks shift to a lower energy with the increase of Cu+ concentration. In the chemical reaction, Te atoms are apt to obtain electrons from Cu atoms, thus leading to a decrease of the binding energy of Te 3d. Figure S5(d) shows the additional Cu 2p3/2 (932.58 eV) and Cu 2p1/2 (952.4 eV) peaks.36 XPS results verify the good combination of PEDOT:PSS with Te 18 and PEDOT:PSS with Cu7Te4.37

18


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Figure 5. Electrical conductivity, Seebeck coefficient, and power factor (a), carrier concentration, mobility (b) of the composite samples with different PC-Te contents at 300 K, (c) Band structure of the interfacial band diagram of Te/PEDOT:PSS and Cu7Te4/PEDOT:PSS,

(d) in-plane

thermal conductivity and ZT of the PC-Cu7Te4/PC-Te composite films as a function of PC-Te content at 300 K.

The TE properties of PC-Cu7Te4, PC-Te, and PC-Cu7Te4/PC-Te composite films with different PC-Te contents at 300 K are present in Figure 5. The PC-Cu7Te4 film possesses a low S of 11.5 V/K and σ of 925.9 S/cm. The electrical conductivity is higher than that of other Cu7Te4 films consists of 0D nanoparticles 38 or 2D nanosheets 20,

indicating that the Cu7Te4 nanorod-based 3D network constructs quick transport

channels for electrons. On the contrary, PC-Te film shows a relatively high S of 151 V/K, and low σ of 11.5 S/cm. With the PC-Te content increasing, the σ of the composite films decreases, while the S shows an opposite tendency. The decrease of σ is mainly caused by a poor 19



Page 20 of 32

electrical conductivity of PC-Te. Whereas the enhancement of S is mainly ascribed to the intrinsic high S of the PC-Te nanorods. Finally, an optimized PF value of 65.3 W/mK2 is obtained from the sample with 95 wt% PC-Te, which is about 5 times larger than that of PC-Cu7Te4 film and about 3 times larger than that of the PC-Te film. The PF value is superior to that of the PANi/SWCNT/Te

39,

PEDOT/TiO2/ZnO

40

and

PEDOT/graphene/CNT13 ternary composite films; whereas it is still inferior to that of the Te/Cu1.75Te/PEDOT:PSS 27 and rGO/Te/PEDOT:PSS 19 hybrid films. Hall-effect measurement results are shown in Figure 5(b), which can be used for further understanding the changes of TE performance. By comparing Figure 5(a) and (b), it is seen that the change tendency of the Seebeck coefficient with the PC-Te content is quite similar to that of the carrier mobility with the PC-Te content, whereas it is just opposite to that of the carrier concentration with the PC-Te content. There are two reasons for the increase of the S with the PC-Te content. One is the PC-Te with an intrinsic high Seebeck coefficient, as mentioned above; the other is the film with higher carrier mobility and lower carrier concentration due to the interfacial energy-filtering effect. It has been previously reported that the energy filtering effect exists at the polymer/ inorganic nanocrystal interface, in which low-energy carriers are selectively scattered, while high-energy carriers cross the energy barrier, leading to an increase in the Seebeck efficient.41 The effect is more effective when the nanocomposite possesses 1D morphology. 42 In the ternary composite, the induced interface can serve as an energy filter to 20


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scatter the low-energy carriers, and then enhanced TE properties are obtained. 43 In the present work, the Te and Cu7Te4 nanorods (core) are coated with PEDOT:PSS (shell) in favor of emerging a large quantity of Te/PEDOT:PSS and Cu7Te4/PEDOT:PSS interfaces in the ternary nanocomposite. By taking advantage of the energy filtering effect at the two different interfaces, the Seebeck coefficient of the nanocomposite is enhanced. The Te/PEDOT:PSS and Cu7Te4/PEDOT:PSS qualitatively interfacial band diagrams are illustrated in Figure 5(c). The work function of the Te nanorods is supposed to be the same as that of bulk Te (～4.95 eV).44 The work function of Cu7Te4 and PEDOT:PSS is 4.8 eV and 4.71 eV, respectively, according to ref. [23] (Figure S6). The energy barrier at the Te/PEDOT:PSS and Cu7Te4/PEDOT:PSS interfaces is 0.24 and 0.09 eV, respectively. The low-energy carriers will be blocked while high-energy carriers cross the interface. High-energy carriers can transfer more “heat” than lowenergy ones, thus resulting in an increase in S and enhancing the TE performance.45 The electrical conductivity (σ) is related to carrier concentration (n) and the carrier mobility (: σ = neμ

(5).

As shown in Figure 5(b), the carrier mobility increases with the increase of PC-Te content, especially when the PC-Te content is greater than 75 wt%, while the carrier concentration decreases quickly with the PC-Te content. Finally, the electrical conductivity exhibits a downward trend with the PC-Te content (Figure 5(a)). Figure 5(d) shows that the in-plane thermal conductivity of four selected samples at 300 K. In the four samples, the PC-Cu7Te4 film exhibits a higher thermal conductivity 21



because of it having higher electronic contribution (it has higher electrical conductivity, see Fig. 5(a)). The thermal conductivity of the PC-Te film is about 0.2 W/mK, which is close to that for PEDOT:PSS/Te composite film reported in ref.[18]. With the PC-Te content increases, the thermal conductivity of the composite films decreases. Finally, a value of 0.198 W/mK (which is very close to that of the PC-Te film) is obtained from the sample containing 95 wt% PC-Te. The reasons for the composite film showing such a low thermal conductivity are as follows: one is that it has porous microstructure (pores with sizes ranging from a few nanometers to hundreds of nanometers can scatter phonons of a wide range of frequencies 3); one is that it has numerous nanointerfaces which can also scatter phonons; and another one is that the Te nanorods and Cu7Te4 nanorods coated with a PEDOT:PSS nanolayer, which is with intrinsic low thermal conductivity. Consequently, the room temperature ZT value of the composite films can be evaluated, and that of the PC-Cu7Te4/95 wt% PC-Te composite film achieves a maximum ZT ∼ 0.1.

22


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Figure 6. (a) The electrical conductivity, (b) the relation of ln (σ) vs. T-1/3, (c) the Seebeck coefficient, (d) the PF of the PC-Te film, PC-Cu7Te4 film and PC-Cu7Te4/95 wt% PC-Te film from 292 to 380 K.

Temperature dependence of TE properties of the PC-Te, PC-Cu7Te4 and PCCu7Te4/95 wt% PC-Te nanocomposite films are measured from 292 to 380 K, for the aim of understanding the conduction mechanism. Figure 6(a) shows that the temperature-dependent conductivity of the PC-Cu7Te4, exhibiting a metallic behavior, which is exemplified by the near linear negative slope with respect to temperature. 25,46 The plot of ln (σ) vs. T-1/3 (Figure 6(b)) shows straight curves for the three composite films in the range of 299 K < T < 380 K, indicating that the conduction of the samples follow the variable range hopping mechanism.47 It illustrates that the little amount of PEDOT:PSS layer also contributes to a high-speed transport of the carriers in the 3D nanorod-based-network. Figure 6(c) shows that the S of these three nanocomposites 23



increases with temperature increasing, with the positive slope indicating a majority of hole carriers.25 For the PC-Cu7Te4/95 wt% PC-Te composite film, the increase in the electrical conductivity (from 25.3 to 42 S/cm) upon test temperature is accompanied by substantial increase in S (121.7 to 163.5 V/K). Hence, a high power factor of 112.3 W/mK2 at 380 K is obtained (Figure 6(d)).

Figure 7. (a) Flexibility of the film. The change of the Seebeck coefficient and electrical resistance of the PC-Cu7Te4/95 wt% PC-Te film drop-cast onto the polyimide substrate over different bending times with the insets exhibiting a bending test, where the bending diameter is 5 mm, (b) a digital photo of the TE device fabricated using the as-prepared film, (c) the relationship between output voltage and temperature difference, (d) output voltage and power as a function of load resistance at various temperature differences.

The durability of continual bending on the Seebeck coefficient and resistance (R) of the as-prepared PC-Cu7Te4/95 wt% PC-Te composite film is shown in Figure 7(a). In 24


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particular, the R of the film increases by about 55% after 350 times bending and it almost keeps stable even undergone more bending times. This should be due to the degradation of the nanorod to nanorod junctions, and it reaches a maximum degradation when it is being bent for about 350 times. Similarly, the S decreases slightly by about 21% after 500 times bending, where the decrease is also due to the degradation of the nanorod to nanorod junctions. The stress-strain curve and the Young's modulus of the composite film on the polyimide substrate were also investigated (Figure S7). The Young's modulus (E) of is obtained according to the stress-strain curve and determined by the equation, E=’/

(6),

where ’ is the stress, and  is the strain. The tensile strength of the PC-Cu7Te4/95 wt% PC-Te/polyimide film, polyimide with and without undergone the same heating process for the composite film is 41, 43, and 65 MPa, respectively (Figure S7(a)). This suggests that the heat-treatment has a great effect on the strength of the polyimide. The heattreated polyimide and the PC-Cu7Te4/95 wt% PC-Te composite film on polyimide show similar strength and Young's modulus (Figure S7). Hence, we speculate that the mechanical property of the composite film on polyimide mainly depends on the polyimide substrate. The superior mechanical property ensures that the composite films can withstand in a harsh mechanical environment. A ﬂexible TE prototype device consisting of 8 legs (25 mm × 5 mm) with Ag paste as electrode is fabricated (Figure 7(b)). Figure 7(c) exhibits the relationship between open circuit voltage (VO) and temperature gradient (T). When T is 18.2, 27.1, 31.4 25



Page 26 of 32

and 39.1 K, VO of the device is 14.2, 21.4, 26.3 and 31.2 mV, respectively. The VO can be defined as: 48 𝑉𝑂 = 𝑁𝑆∆𝑇

(7),

where N is the number of TE legs. For a 39.1 K temperature gradient, the calculated output voltage of a p-leg per 1 K T is about 103 V, which is much greater than the value, ~ 80.7 V, measured at 300 K. This is mainly because of the enhanced Seebeck coefficient value of every leg with increased temperature during the test, as proved by the Seebeck coefficient measurement at different temperature gradients in Figure 7(c). In addition, the VO value of the TE device at a 39.1 K temperature gradient decreases by only about 10% after 70 times twisting, showing a good flexibility of our TE device (Figure S8). The output voltage and power of the device as a function of load resistors (Rload) are shown in Figure 7(d). When the Rload (~ 1200 Ω) matches with internal resistance of the module, a maximum output power of ~ 94.7 nW is generated at a 39.1 K temperature gradient. By dividing the cross-sectional area for heat flow and the number of legs, the power density ~ 39.5 W/cm2 is obtained.49 The output power density is relatively higher than that of other PEDOT:PSS-based TE generators.19 The output voltage can be further increased by increasing the number of legs or combining with n-type materials to assemble p-n couples.

4. CONCLUSIONS In summary, we have fabricated PC-Cu7Te4/PC-Te ternary TE nanocomposite 26


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films to combine the high Seebeck coefficient of the PC-Te and high electrical conductivity of the PC-Cu7Te4 for TE applications by a facile method. An optimized power factor of 65.3 W/mK2 has been obtained from a composite film containing 95 wt% PC-Te nanorods at 300 K, which is much higher than that of the individual components. It is ascribed to the synergetic effect of the nanorods and the double-carrier filtering effect at the two heterointerfaces of Te/PEDOT:PSS and Cu7Te4/PEDOT:PSS. Moreover, the power factor increases with increasing temperature and reaches 112.3 W/mK2 at 380 K. A prototype device composed of 8 legs of the film connected with silver paste is designed, which exhibits a maximum power density of 39.5 W/cm2 at a 39 K temperature gradient. This work reveals a successful approach to improve the properties of flexible TE materials by introducing more heterointerfaces and demonstrates an effective method for fabrication of PEDOT:PSS/chalcogenide TE nanocomposites. ■ ASSOCIATED CONTENT Supporting Information. Schematic of the performance measurement of the TE generator. EDS spetrum of the PC-Cu7Te4 nanorods. Cross-sectional SEM images of the PC-Cu7Te4/PC-Te composite films. TE parameters and pore size distribution for the PC-Cu7Te4/95 wt% PC-Te composite samples with different film casting temperatures. XPS spectra of the pristine PEDOT:PSS, PC-Te, PC-Cu7Te4 and their nanocomposite films. Schematic illustration of the band structure. The stress-strain curve and the Young's modulus of the polyimide substrate and the PC-Cu7Te4/95 wt% PC-Te composite film. Fatigue property of the TE 27



device. ■ AUTHOR INFORMATION Corresponding Author * E-mail: [email protected]；[email protected]; [email protected] ORCID Yao Lu: 0000-0002-8734-1056 Kefeng Cai: 0000-0002-7543-1628 Notes The authors declare no competing financial interest. ■ ACKNOWLEDGEMENTS This work was supported by the Key Program of National Natural Science Foundation of China (5163210), National Basic Research Program of China (973 Program) under Grant No.2013CB632500, and the foundation of the State Key Lab of Advanced Technology for Material Synthesis and Processing (Wuhan University of Technology). Innovation Commission of Shenzhen Municipality (Grant Nos. KQTD2016022619565991and KQCX2015033110182370). National Natural Science Foundation of China (61611530550). National Natural Science Foundation of China (51473052).

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