Ag2Te

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Functional Nanostructured Materials (including low-D carbon) 2

High Performance and Flexible Polyvinyl Pyrrolidone/Ag/AgTe Ternary Composite Film for Thermoelectric Power Generator Qiufeng Meng, Yang Qiu, Kefeng Cai, Yufei Ding, Mengdi Wang, Hongting Pu, Qin Yao, Lidong Chen, and Jiaqing He ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.9b11217 • Publication Date (Web): 14 Aug 2019 Downloaded from pubs.acs.org on August 20, 2019

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ACS Applied Materials & Interfaces

High Performance and Flexible Polyvinyl Pyrrolidone/Ag/Ag2Te Ternary Composite Film for Thermoelectric Power Generator Qiufeng Meng

†,‡, Ϯ,

, Yang Qiu §, Kefeng Cai

*, †,

Yufei Ding †, Mengdi Wang

‡ £,

, Hongting Pu †,

Qin Yao ‡, Lidong Chen *, ‡, Jiaqing He*, § †

Key Laboratory of Advanced Civil Engineering Materials, Ministry of Education, School of Materials Science & Engineering, Tongji University, 4800 Caoan Road Shanghai 201804, China ‡

State Key Laboratory of High Performance Ceramics and Superfine Microstructure, Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai 200050, China Ϯ University

of Chinese Academy of Sciences, Beijing 100049, China

§

Physics department, Southern university of Science and Technology, 1088 XueYuan Avenue, Shenzhen 518055, China £ Shanghai

Tech University, 393 Huaxia Road Shanghai 201210, China

ABSTRACT: In this work, polyvinyl pyrrolidone (PVP) coated Ag-rich Ag2Te nanowires (NWs) were synthesized by a wet chemical method using PVP coated Te NWs as templates, and flexible PVP/Ag/Ag2Te ternary composite film on nylon membrane was prepared by vacuum assisted filtration followed by heat treatment. TEM and STEM observations of the focused ion beam (FIB) prepared sample reveal that the composite film shows a porous network-like structure and that the Ag and Ag2Te exist as nanoparticles and NWs, respectively, both bonded with PVP. The Ag nanoparticles are formed by separation of the Ag-rich Ag2Te NWs during the heat treatment. For the composite film staring from a Ag/Te initial molar ratio of 6:1, a high power factor of 216.5 μW/mK2 is achieved at 300 K, and it increases to 370.1 μW/mK2 at 393 K. Bending tests demonstrate excellent flexibility of the hybrid film. A thermoelectric (TE) prototype composed of five-leg of the hybrid film is assembled and the maximum output power of 469 nW is obtained at a temperature gradient of 39.6 K, corresponding to a maximum power density of 341 µW/cm2. This work provides an effective route to composite film with high TE performance and excellent flexibility for wearable TE generators. KEYWORDS: thermoelectric, flexible film, Ag2Te, polyvinyl pyrrolidone, generator

1

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1. INTRODUCTION In recent years, owing to the large energy consumption, there have been increasing research efforts devoted to the renewable energy 1. Among them, converting waste heat into electricity via thermoelectric (TE) generators is very promising 2. More recently, hundreds of microwatt even milliwatt level electrical energy have been produced by TE generators from small temperature difference between human body and the surrounding environment, revealing great potentials of TE generators for powering body sensors and portable electronics

3-4.

TE generators possess admirable

characteristics including simplicity, silence, reliability and environmental friendly 5. The key of a TE generator is the TE materials used. The energy conversion efficiency of a TE material is quantified by the dimensionless figure of merit ZT = S2σT/κ, where S is the Seebeck coefficient, σ presents the electrical conductivity, T is the absolute temperature, and κ is the total thermal conductivity. The numerator, S2σ, called as power factor (PF), is always used for evaluating TE performance of films, as the measurement of in-plane κ of films is a great challenge 6. Inorganic materials, such as Bi-Te alloys 7, demonstrate high ZT values around room temperature (RT), while their inflexibility hinders their practical applications in wearable TE devices. The most widely studied flexible TE materials are conducting polymers (CPs) materials

11-16.

8-10

and CP-based composite

Besides, insulating polymers recently have also been used as matrixes for preparing

inorganic/organic flexible TE composite films. For example, Chen et al.

17

printed n-type Bi2Te3-

epoxy composite and p-type Sb2Te3-epoxy composite on polyimide, and a power density of 75 μW/cm2 was obtained at a temperature difference of 20 K. Recently, Pammi et al.

18

reported the

fabrication of p-type Cu2-xSe nanowire (NW)/polyvinylidene fluoride (PVDF) flexible composite thin films using vacuum filtration followed by mechanical pressing, and the composite film exhibited good flexibility and a high PF of 105.3 W/mK2 at 298 K. Dun et al

19

prepared Cu doped

Bi2Se3/PVDF flexible composite film with PF of 103 μW/mK2 at 290 K. Very recently, Hou et al. 20

2


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prepared Bi0.5Sb1.5Te3/epoxy resin composite thick TE films through brush-printing and hot pressing curing processes and the optimized film showed a high PF value of 840 W/mK2 at 300 K, and excellent flexibility. Although insulating polymer can improve the flexibility, total electrical conductivity of the composite film is negatively affected due to its electrical insulation, hindering significantly improvement of TE properties. In order to obtain flexible TE materials with outstanding performance, various methods have been tried

21-24.

Among them, forming inorganic TE film on insulating polymer substrate is an effective

way to achieve both high TE performance and good flexibility

25-27.

Insulating polymers such as

poly(ethyleneterephthalate), polymide, nylon, polycarbonate, PVDF and copy paper have been used as substrates to develop flexible TE films

23, 28-29.

For instance, our group

30

most recently has

reported the preparation of n-type Ag2Se film on nylon membrane, and the hybrid film shows a high PF of 987.4 μW/mK2 at 300 K and excellent flexibility. Ag2Te is an analogue of Ag2Se. It is a narrow band-gap semiconductor (Eg ~ 0.05 eV) and possesses high carrier mobility

31-33.

Disordered Ag atoms in the lattice structure, resulting in low

lattice thermal conductivities of 0.1-0.4 W/mK near RT33. Ag2Te undergoes a phase transformation at 418 K from monoclinic to cubic phase. At RT, Ag2Te is n-type due to the higher mobility of the electrons, and the carrier concentration can be controlled by adjusting the composition, namely, the superfluous Ag is used as an electron donor, whereas the excess Te is used as an electron acceptor 34. Several groups have reported the TE performance of Ag2Te 35-37, showing Ag2Te has great potential for n-type TE materials. For instance, Ag-rich Ag2Te NWs were prepared and the corresponding heavily doped bulk sample showed a high PF of 428.8 μW/mK2 at 380 K 38. Zhang et al. 39 prepared AgxTe nanofibers through tuning Ag+ concentration, and found that the Ag-rich Ag2Te nanofibers exhibited n-type semiconductor behavior with higher PF value due to the higher electrical conductivity. Gao et al.

23

prepared n-type Ag2Te NW films on copy paper substrate by glass-fiber-

3



aided cold pressing, and the maximum PF value increased to 192 μW/mK2 at 468 K. Zeng et al.

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40

reported the preparation of Ag2Te NWs/nylon hybrid films by combination vacuum filtration and mechanical pressing, and the hybrid film showed a maximum PF value of ~100 µW/mK2 at about 300 K. Nevertheless, the flexibility and TE performance of these reported Ag2Te based hybrid films are not very satisfactory. Herein, nylon membrane supported PVP/Ag/Ag2Te ternary composite flexible films were fabricated by vacuum assisted filtration of PVP coated Ag-rich Ag2Te NWs followed by heat treatment. The PVP layer was very thin; therefore, rather than the composites prepared by other methods, the PVP phase was very homogeneously distributed in the present composite films. The PF value of the hybrid film was 216.5 W/mK2 at 300 K and 370.1 μW/mK2 at 393 K, respectively. Moreover, the electrical conductivity of the composite film decreased by only 9.4 % after 1000 bending cycles, suggesting excellent flexibility of the hybrid film. In addition, a flexible TE generator composed of five legs of the optimized hybrid film was fabricated on polyimide substrate and the output properties of the generator were studied.

2. EXPERIMENTAL SECTION 2.1 Materials. Tellurium dioxide (TeO2, ≥ 99%), hydrazine hydrate (N2H4·H2O, 85%), PVP (MW ~ 40,000), potassium hydroxide (KOH, 90%), silver nitrate (AgNO3, ≥ 99.9%), ethylene glycol (EG for short, ≥ 99%) and anhydrous ethanol (≥ 99.5%), were purchased from Sinopharm Chemical Reagent Co., Ltd. All chemicals were used as received without further purification. 2.2 Preparation of PVP Coated Ag2Te NWs. Figure S1(a) shows the preparation procedure of PVP coated Ag2Te NWs which is similar to previous work 38. Briefly, 0.1596 g TeO2, 0.2 g PVP, and 0.7481 g KOH were dissolved in 30 mL 4


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EG. Then the temperature was raised to 120 oC. Subsequently, the reactor was purged with nitrogen, and 300 L N2H4·H2O was rapidly injected. The mixture was kept at 120 oC for 1 h for the formation of Te NWs and then cooled down to RT naturally. After being washed with distilled water for three times, the products were collected by centrifugation and re-dispersed in 50 mL EG. 1.019 g AgNO3 was dissolved in another 50 mL EG solution and added dropwise into the above dispersion with vigorous stirring. The reaction proceeded at RT for 2 h, after which the product was alternately washed with distilled water and ethanol for several times and collected with centrifugation. The obtained product was dispersed in 100 mL mixed solutions of ethanol and N2H4·H2O (v/v = 10/1) under vigorous stirring for a night to remove the excessive PVP. Afterwards, PVP coated Ag2Te NWs were centrifugation and alternately washed with distilled water and ethanol for four times. The nominal molar ratio of Ag/Te was 6:1 and the obtained sample was named as 6A1T. Finally, the products were dispersed in 400 mL ethanol with sonication for 2 h, resulting in a homogeneous PVP/Ag2Te NWs dispersion. For comparison, the nominal molar ratio of Ag/Te was changed to 2:1, 3:1 and 4:1, respectively, and the preparation process was the same as described above. The obtained samples were named as 2A1T, 3A1T and 4A1T, respectively. 2.3 Fabrication of Hybrid Films. The fabrication of the nylon membrane supported hybrid films is illustrated in Figure S1(b). The PVP coated Ag2Te NWs composite films were prepared by vacuum filtration of the PVP coated Ag2Te NWs onto porous nylon membrane (with 50 mm diameter and 0.22 μm average pore size). The obtained composite films were dried at 50 oC in a vacuum for 12 h, heat treated at 200 oC and 1 MPa for 30 min in vacuum, and finally naturally cooled down to RT. 2.4 Assembly of TE Generator. The TE prototype module was composed of five strips of the optimal hybrid film. The strips (25 mm in length and 5 mm in width) were adhered to a polyimide substrate by double-sided adhesive tapes 5



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(the interval of two neighboring strips was ~5 mm). In order to reduce the contact resistance, a Au film was evaporated on two ends of each strip and then electrically connected in series using the silver paste. 2.5 Measurement and Characterizations. The in-plane electrical conductivity at RT was measured by a steady-state four-probe technique (Ecopia HMS-3000). The Seebeck coefficient at RT was confirmed by the slope of the linear relationship between temperature difference (~ 5 - 10 K) and thermal electromotive force between two ends of each sample. The temperature dependence of in-plane S and σ were simultaneously measured by the standard four-probe method in helium atmosphere (Sinkuriko, ZEM-5). The Hall coefficient was tested using the Van der Pauw method (LakeShore 8404). The phase composition of the synthesized samples was examined by X-ray diffraction (XRD) using Cu Kα radiation source (D/MAX 2550VB3+/PCII). The morphology was characterized by the field emission scanning electron microscopy (FE-SEM; FEI Nova NanoSEM 450). Transmission electron microscope (TEM) and high resolution TEM (HR-TEM) were performed using a JEM2100F electron microscope. The internal microstructure of the hybrid films was characterized by double-aberration corrected transmission electron microscope (TEM, FEI Titan @ 300kV in TEM and STEM mode) with high-angle annular dark-field (HAADF) element mappings. Focused Ion Beam (FIB, FEI Helios 600i) was used to prepare the TEM samples with the in-situ lift-out technique. The thicknesses of the composite films were determined by the thickness meter (Shanghai Liu Ling Instrument Factory) combined with the FE-SEM observation. The bending tests of the composite films were performed using a homemade equipment around cylinders with different radii. The tensile strength of nylon membrane and the hybrid film was studied using a universal mechanical testing machine (Shenzhen Suns Technology Stock Co., Ltd.).

6


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The length and width of the rectangular samples were approximately 15 mm and 5 mm, respectively. The crosshead speed was 5 mm/min. The properties of the TE generator were tested by a self-built measurement device, and the details of the principle are shown in our previous work 41. One end of the assembled generator was put on a hot plate controlled by an automatic temperature controlling module and was acted as the hot side (T+ΔT), and the other end was attached to a cold insulating block acting as the cold side (T). A voltmeter (Agilent 34970) was used to capture the output voltage (U) of the generator. The TE device was connected into a circuit with a microammeter and a variable resistance box in series. Through modulating the load resistance, U and the output current (I) were collected under a paticular temperature gradient. Temperature distribution image of the generator was obtained in real-time by an infrared (IR) camera (UTi 80, UNI-T).

3. RESULTS AND DISCUSSION For the preparation of PVP coated Ag2Te NWs, PVP coated Te NWs are first synthesized (see Figure S1). Figure S2(a) shows the XRD pattern of the PVP/Te NWs, all diffraction peaks in the pattern are in good agreement with the standard PDF card (JCPDS No. 36-1452), indicating successful preparation of hexagonal Te. The FE-SEM and TEM images show that the Te NWs are uniform and the length of the NWs is longer than 1 μm (Figure S2(b) and (c)). Figure S2(d) presents the HR-TEM image of the Te NWs. Diameter of the Te NWs is ~ 7 nm and the NWs are single crystalline 42. The surface of the Te NWs is with an amorphous layer with thickness of ~ 2 nm. Since PVP was used in the preparation process, the amorphous layer is deduced to be PVP (EDS element analysis result hereinafter confirms this deduction). Figure S3 shows the XRD patterns of the as-prepared PVP coated Ag2Te NWs starting from different Ag/Te nominal molar ratios. To ensure the complete conversion of Te to Ag2Te, the initial molar ratio of Ag/Te increasing from 2:1 to 6:1 was used. The

7



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XRD peaks can be indexed to the monoclinic Ag2Te (JCPDS No. 81-1985), indicating complete transformation from Te to Ag2Te. The morphology of the as-prepared 6A1T NWs is observed by SEM and TEM (Figure S4(a), (b) and (c)). The NWs are curved and uniform, which are similar to previous reports

37, 43.

The average

length and diameter of the NWs are ~ 700 nm and ~ 20 nm, respectively. Compared with the PVP coated Te NWs, length of the as-prepared Ag2Te NWs becomes shorter whereas the diameter becomes thicker. This is because the 98% volume expansion when hexagonal Te was converted to monoclinic Ag2Te 44. A significant mechanical stress is introduced by the large volume expansion, which is released by curving and breaking of the NWs (Figure S4(d)). Figure S4(e) shows the HRTEM image corresponding to the curving part shown in Figure S4(d). An amorphous PVP layer (~2 nm) can be clearly observed on the NW. The interplanar distances of 2.88 and 3.73 Å correspond to the lattice spaces of (-212) and (110) planes of monoclinic Ag2Te, respectively. The fast Fourier transform (FFT) pattern (Figure S4(f)) corresponding to Figure S4(e) indicates that the curving part of the NW is polycrystalline. In addition, a typical TEM image of the straight part of the NW is shown in Figure S4(g). Figure S4(h) and (i) show the HR-TEM image and the corresponding FFT pattern, respectively. The interplanar distances of 2.86, 2.88 and 3.38 Å correspond to the lattice spaces of (012), (-212) and (200) planes, respectively, of monoclinic Ag2Te, suggesting that the straight part of the NWs is well crystallized. Figure S5 shows the thicknesses of the composite films starting from different Ag/Te nominal molar ratios. Figure S6 shows the TE properties at RT of the heat treated composite films starting from different Ag/Te nominal molar ratios. The electrical conductivity of the composite film increases from 56.5 to 360.9 S/cm. The Seebeck coefficient of the composite film is negative, indicating n-type conduction. And the absolute value of the Seebeck coefficient slightly decreases from 81.6 to 77.5 μV/K. As a consequence, the PF value of the composite film increases from 37.6 to

8


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216.5 μW/mK2. These results indicate that increasing initial molar ratio of Ag/Te is beneficial to improve TE properties of the composite film, mainly because of increasing electrical conductivity, which is consistent with previous report in Ref. 39. This PF value (216.5 μW/mK2) is superior to that of previously reported Ag2Te related films (see Table 1).

Figure 1 (a) XRD pattern of the heat treated 6A1T sample. (b) Low- and (c) high-magnifications FESEM images of the heat treated 6A1T sample. In order to understand the reasons for good TE properties of the composite films, the heat treated 6A1T composite film is taken as the example and analyzed by various characterization technologies. Figure 1(a) shows the XRD pattern of the heat treated 6A1T sample. All the peaks can be indexed to the standard card of monoclinic Ag2Te (JCPDS No. 81-1985), except two weak peaks for Ag (JCPDS No. 04-0783), indicating existence of Ag phase in the heat treated 6A1T sample (Whereas in the heat treated 3A1T sample there is no Ag phase, and it is also detected in the heat treated 4A1T sample, although the peak intensity of Ag is weaker than that of the treated 6A1T sample, see Figure S7). This is because the nominal stoichiometric ratio of Ag/Te = 6:1, which is three times as much as 9



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that of the stoichiometric ratio of Ag2Te, was used in the synthesis process; the obtained NWs are Ag-rich. As the deposition of Ag is thermodynamically preferred

39,

during the heat treatment, Ag

atoms segregated from the Ag-rich NWs, resulting in the formation of the elemental Ag

45.

Since

elemental Ag exists in the sample, the sample is re-named as PVP/Ag/Ag2Te hereinafter. In addition, compared with the XRD pattern of the 6A1T sample before treatment (see Figure S3), the XRD pattern of the PVP/Ag/Ag2Te sample shows higher peak intensity, implying better crystallinity after the treatment. Figure 1(b) and (c) show the surface FE-SEM images of the PVP/Ag/Ag2Te composite film at low and high magnifications. The obtained composite film consists of one-dimensional (1D) nanostructures with an average thickness of 37 nm intertwined into a network-like structure with numerous nanopores (with sizes in the range of ~ 20 to 130 nm).

Figure 2 (a) A HAADF-STEM image of a typical region of the PVP/Ag/Ag2Te composite film, (b) overall EDS image corresponding to (a), (c) magnified image corresponding to the red square area marked in (b), and (d) EDS line scan profile along the direction of the red arrow in (c).

10


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For the purpose of further investigating the detailed internal microstructure of the PVP/Ag/Ag2Te composite film, TEM sample was prepared by FIB with the in-situ lift-out technique and studied by HAADF-STEM. A typical HAADF-STEM image is shown in Figure 2(a). The internal morphology of the composite film is similar to that shown in Figure 1(c). Figure 2(b) shows the overall corresponding EDS image. It is obviously by comparing Figure 2 (a) and (b) that Ag is rich at the peripheral of the Ag2Te NWs. Figure 2(d) is the EDS line scan result scanning along the arrow in Figure 2(c). It shows that the concentration of Ag at the peripheral of the Ag2Te NW is higher, also indicating the enrichment of Ag. Figure S8 shows the EDS images of elemental Ag, Te, O, N and C corresponding to the STEM image in Figure 2(a). The O, N and C originate from the PVP, corresponding to the amorphous layer on the Ag2Te NWs. Figure S9(a) shows a HAADF-HR-STEM image of a head part of a PVP coated Ag2Te NW, overall corresponding EDS image (clearly the outer surface of the NW having a higher concentration of C, implying that it is coated a layer of PVP) and EDS image of elemental Ag, Te, C, O and N. By comparing the EDS image of Ag, Te and C, it is clearly seen that two Ag nanoparticles exist at the right side of the image, indicated by the yellow dot lines. Besides, it can be seen that elements O and N are distributed on the NW, further verifying that the NWs are coated with PVP. EDS line scanning result (Figure S9(b)) along the red arrow in the overall corresponding EDS image proves that the concentration of Ag at the two Ag nanoparticle locations is much higher. The reasons for the aggregation of Ag nanoparticles in the PVP layer can be explained as follows. As the content of Ag in the as-prepared 6A1T NWs is excessive, Ag atoms are disordered in the lattice of Ag2Te NWs due to their high mobility 39. During the heat treatment, the Ag-rich Ag2Te separates into Ag and Ag2Te and the Ag nanoparticles are aggregated and “captured” by the PVP layer, resulting in a homogeneous dispersion of Ag nanoparticles in the PVP coating layer. Conductive channels can form since high conducting Ag nanoparticles are dispersed in the insulating PVP layer, leading to a preferable electrical conductivity of the heat treated sample.

11



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Figure 3 Characterization of internal microstructure of the PVP/Ag/Ag2Te composite film. (a) An overview HAADF-STEM image, (b) an enlarged image of the blue square area marked in (a), (c) HR-STEM image of the yellow square area marked in (b), (d) an enlarged HR-STEM image of the green square area marked in (c), and (e) an enlarged HR-STEM image of the red square marked in (c), the inset image is its FFT pattern Figure 3(a) shows another typical HAADF-STEM image of the composite film with low magnification, which also shows that the film is porous and consists of numerous nanowires. Figure 3(b) is an enlarged image of the blue square in (a), which more clearly shows the feature of the internal microstructure. Figure 3(c) is the HR-STEM image of the yellow square area marked in (b), which contains four crystalline domains (marked as ①, ②, ③ and ④). These crystalline domains are separated by amorphous phase (see more clearly in (d)), which must be PVP. Figure 3(e) shows the internal crystal structure of the crystalline domain. Since the as-prepared Ag2Te NWs are coated with PVP layer, Ag2Te NWs cannot directly contact with each other. The PVP layer is stable due to the low treated temperature, and most mass transfer channels between Ag2Te NWs are blocked by the coating layer. As a consequence, Ag2Te NWs only slightly become thicker. Hence, the composite film can be considered as the inorganic nanostructures (Ag nanoparticles and Ag2Te NWs) homogeneously bonded by PVP.

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According to the internal microstructure observation, there are two kinds of junctions in the composite film, namely, Ag/Ag2Te NW and PVP/Ag2Te NW (see Figure S10). The existence of the Ag nanoparticles results in numerous junctions in the composite film. Unlike the previous report

46

where Ag improves the electrical conductivity while greatly reduces the Seebeck coefficient, the existence of Ag nanoparticles in the film enhances the electrical conductivity without significantly reducing the Seebeck coefficient (see Figure S6). Previous study has shown that the energy filtering effects are more effective in one-dimensional materials 47. Since there are high dense of junctions in the composite film, energy barriers at the junctions effectively block low-energy electrons, whereas high-energy electrons can cross the junctions. Therefore, the slight decrease in the Seebeck coefficient with Ag content increasing is mainly due to the combined effects of Ag and energy filtering effects at the junctions.

Figure 4 Temperature dependence of TE properties (a) and carrier concentration and mobility (b) of the PVP/Ag/Ag2Te composite film. (c) Ln(σT1/2) as a function of 1/kT. 13



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Figure 4(a) shows the temperature dependence of TE properties of the PVP/Ag/Ag2Te composite film measured from 300 to 393 K. The electrical conductivity increases from 360.9 to 540.6 S/cm, indicating a semiconducting behavior. The electrical conductivity (σ) is related to the carrier concentration (n) and carrier mobility (µ) as follows: σ = neµ

(1),

where e is the electron charge. Figure 4(b) shows the temperature dependence of carrier concentration and mobility, which is consistent with the change trend of electrical conductivity. In particular, when the temperature increases from 300 to 393 K, n increases from 8.4 × 1018 to 1.43 × 1019 /cm3, which should be due to the thermal excitation of carriers. The carrier mobility μ is almost constant. Therefore, the increase of the electrical conductivity is mainly due to the increase of carrier concentration. The absolute value of Seebeck coefficient slightly increases from 77.5 to 82.7 μV/K. As a consequence, the PF value increases with temperature increasing, and reaches a maximum value of 370.1 μW/mK2 at 393 K. Table 1. Comparison of TE performances of the present PVP/Ag/Ag2Te composite film and previously reported n-type flexible TE films at RT a. S

PF

κ

(S/cm)

(μV/K)

(μW/mK2)

(W/mK)

86

-60

30

146

-84

Ag2Te NWs/paper film

87

Ag2Te NWs/nylon film Ag2Te nanocrystal/nylon film

Materials

σ

ZT

Ref.

0.4

>0.02

6

103.2

0.32

0.1

19

-100

87

-

-

23

193.2

-74.6

107.5

-

-

40

9

-78

5.5

-

-

48

49

-90

40.1

0.29

0.04

49

Ni NWs/PVDF film

4701

-20.6

200

0.55

0.11

50

SWCNT/ADTAb

642

-44

124.4

8.3  1.0

4.5 × 10-3

51

Ag2Se/nylon film

497

-140

987

0.478

0.6

30

PVP/Ag/Ag2Te/nylon film

360.9

-77.5

216.5

0.2-0.4

0.15- 0.3

This work

Ag2Te nanoshuttle/PVDF film Cu0.1Bi2Se3 nanoplatelet/PVDF composite thin films (2:1)

Bi2Se3 nanoplate/PVDF composite thin films (2:1)

14


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a Some

parameters in the table are estimated from the graphs reported in the references; b SWCNT =

single walled carbon nanotube; ADTA = acridones with different tertiary amine groups at the C2 and C7 positions on the acridone ring. Carrier trapping at the junctions is assumed to dominate electrical transport properties of the composite film. The temperature dependence of the electrical conductivity may follow the equation as follows 39, 52-53:

2

σ = Lq n

(

1

1

)

2m * πkT

2

Eb

( )

exp - kT

(2),

where L is the grain size, q is the element charge, n is the carrier concentration, m* is the effective mass, k is the Boltzmann’s constant and Eb is the energy barrier at the junction. According to Eq. 2, the electrical conductivity increases with temperature increasing because more electrons gain energy and surmount the energy barrier at rising temperature, which agrees with the Hall measurement result (see Figure 4(b)). Besides, the ln(σT1/2) and 1/kT should have a linear relation (see Figure 4(c)). By fitting the plot, a barrier height of 59 meV of the sample is obtained. As for most nanoparticle based TE materials with semiconducting behavior, the energy barrier introduced by high dense junctions is often located at 55 - 70 meV 54-56, thus there are high dense junctions in the present composite film. In addition, as PVP is an insulating polymer, the energy barrier at the PVP/Ag2Te junction should be higher than the calculated value (59 meV); therefore, the Ag/Ag2Te junctions are the main transport channels for carriers. The Seebeck coefficient and electrical conductivity show a slight decoupling, i.e. both electrical conductivity and the Seebeck coefficient increase with increasing temperature. With temperature increasing, the average energy of the electrons crossing the junction increases, which gives rise to slight enhancement of the Seebeck coefficient 53. As the composite film is very difficult to separate from nylon membrane without destroying it, the κ of the composite film cannot be measured directly. It is reported that the κ of PVP is in the range of 15



0.2 - 0.5 W/mK

57,

and the κ of a bulk of Ag-rich Ag2Te NWs is ~ 0.4 W/mK

Page 16 of 29

38.

As the present

composite film contains numerous nanopores (with size ranging from ~ 20 to 130 nm), nanostructures (Ag nanoparticles and Ag2Te NWs), heterointerfaces (Ag/PVP, Ag2Te/PVP, and Ag/Ag2Te), and amorphous PVP, heat-carrying phonons in a wide spectrum of wavelengths can be scattered; hence, the κ of the composite film is estimated to be 0.2 - 0.4 W/mK, leading to a ZT value of 0.15 - 0.3. Table 1 compares TE properties of the present composite film and other related reported n-type TE films, indicating superior TE performance of the present composite film. Note that the TE property of the composite film is lower than that of the reported Ag2Se/nylon film, which is mainly because Ag2Se has better TE performance than Ag2Te near RT 58-59.

Figure 5 The ratio of electrical conductivity of the hybrid films before and after bending as a function of (a) bending radius (the inset images are the digital image of the sample in curved state (left) and the schematic diagram of bending test (right), respectively), and (b) bending times at a bending radius of ~ 5 mm (the inset image is the digital image of bending test), where σ0 and σ are

16


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the electrical conductivity before and after bending tests, respectively. (c) The stress-strain curves of nylon membrane and the PVP/Ag/Ag2Te composite film. (d) Typical HR-STEM image of the heterointerface between PVP/Ag/Ag2Te composite film and the nylon membrane. To test the reliability of the flexible nylon membrane supported PVP/Ag/Ag2Te hybrid film, it is bent to different radii and the electrical conductivity is tested as a function of bending radius (Figure 5(a)). σ0 and σ are the electrical conductivity before and after bending tests, respectively. The σ is almost constant with the radius decreasing, and it decreases by only 1.7 % when the bending radius reduces to 2.5 mm. Figure 5(b) shows the change in σ with bending times at a bending radius of ~ 5 mm. The σ is almost constant after bending for 500 cycles, and decreases by only ~ 9.4 % after bending for 1000 cycles. In addition, there are no macroscopic cracks on the composite film after bending for 1000 times. These results show that the mechanical flexibility of the hybrid films is excellent and is superior to those of the previous reports

23, 60-62,

and even better than that of

conducting polymer, poly(3,4-ethylenedioxythiophene):poly(styrensulfonate) (PEDOT:PSS)

63

(see

Table S1). Nevertheless, the flexibility is also not as good as that of the Ag2Se/nylon hybrid film

30

probably because of the present film being not sintered. Figure 5(c) compares the stress-strain curves of nylon and the hybrid film. The tensile strength and strain of nylon membrane are 23 MPa and ~ 8.5%, respectively, whereas those of the hybrid film are 95 MPa and ~ 24%, respectively, indicating strength enhancement of the hybrid film. The conspicuous mechanical property guarantees that the hybrid film can withstand a harsh stretching, which is important for wearable devices. In order to make clear the reasons for the superior flexibility of the heat treated hybrid film, the detailed microstructure near the heterointerface between PVP/Ag/Ag2Te composite and nylon membrane was investigated. Figure S11 shows the HAADF-STEM image of the heterointerface between PVP/Ag/Ag2Te film (upper part) and nylon membrane (lower part), overall corresponding 17



Page 18 of 29

EDS image and EDS images of element C, Ag and Te. The element C is ascribed to both the PVP coating layer and the CONH group of nylon. It can be seen from the EDS elemental images of Ag and Te that a small amount of Ag and Te are detected in nylon membrane. This is because the nylon filter membrane is porous (pore size ~ 200 nm) so that some tips of the PVP coated Ag2Te NWs may penetrate into the pores during the vacuum filtration and they compacted together during the heat treatment, which is good to the combination between the composite film and the nylon membrane. Figure 5(d) shows a typical HR-STEM image of the heterointerface between PVP/Ag/Ag2Te composite film and nylon membrane. The composite film is well bonded with the amorphous nylon membrane, which is beneficial to improve flexibility and tensile properties of the hybrid films. Therefore, the reasons for the favorable flexibility of the hybrid films are explained as follows. (1) Nylon membrane possesses intrinsic excellent flexibility. (2) The PVP/Ag/Ag2Te composite film and nylon membrane are well combined. (3) The composite film, consisting of PVP coated Ag2Te NWs intertwined into a porous network, itself has a certain flexibility. The amorphous PVP definitely enhances the flexibility of the hybrid film. A flexible TE generator composed of five strips of the optimal composite film serially connected is fabricated on a polyimide substrate. Figure S12(a) presents the schematic illustration of the asprepared TE device. Figure S12(b) is the digital image of the TE generator. Figure S12(c) and (d) show the thermal infrared image taken during the measurement to display temperature profile. When one end of the generator is heated, temperature gradient is built between two ends of each leg.

18


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Figure 6 (a) The output voltage and output power as a function of the output current at different temperature gradients. (b) The open-circuit voltage and maximum power density as a function of temperature gradient. Figure 6(a) and Figure S13 show the output voltage (U) and output power (P) as a function of the output current (I) at different temperature gradients. The U increases with increasing temperature gradient. An inverse relationship is observed between the I and U, and the output power (P) is parabolic with I. Besides, P can be expressed as follows: P = E2R / (R + Rin)2

(3),

where E is the open circuit voltage, R is the external load resistance, and Rin is the internal resistance of the TE generator. When R matches Rin, the output power achieves a maximum value. As a consequence, when ΔT = 39.6 K, a maximum output power of 469 nW is obtained. Figure 6(b) shows the open-circuit voltage and the maximum power density as a function of temperature gradients. When the temperature difference increases from 7.1 to 39.6 K, the corresponding E increases from 3.1 to 16.5 mV. The power density is achieved by dividing the output power by the number (N) and the cross-sectional area of TE legs

48, 61.

As a result, the maximum power densities

of the TE generator are 26.1, 77.7, 240.9 and 341 μW/cm2, respectively, at the temperature gradients of 12, 20, 32.1 and 39.6 K, respectively, which are higher than those of the previous reported flexible

19



Page 20 of 29

TE generators based on organic-inorganic composites at the similar temperature gradients (Table 2) 30, 41, 64-70,

suggesting high TE performance of the composite film.

Table 2. Comparison of TE properties of flexible TE generators Materials a

E, mV

Pmaxb, nW

Ag2Se/nylon

18

460

TiS2HA0.01NMF0.003 foil

1.4

Te/PEDOT:PSS Te/PEDOT:PSS/Cu7Te4 PEDOT:PSS (p)/ TiS2[(HA)(NMF)] (n) PEDOT:PSS (p)/ (PEDOT)xV2O5 (n) Te/PEDOT:PSS/rGO (p)/ PEI-doped SWCNTs (n) Au-doped CNT/PANI webs (p) / PEI-doped CNT webs (n) PEDOT:PSS/Cu3Se1/nylon

PVP/Ag/Ag2Te/nylon

a

Maximum power

ΔT, K

N

Ref.

230

30

4

30

24

32

20

1

64

13.4

47.7

57.2

40

9

41

31.2

94.7

39.5

39.1

8

65

-

-

80

40

26.1

0.34

0.266

20

4

67

58

-

0.65

50

8

68

-

1700

0.4

20

16

328

91

30

9

8.2

106.8

77.7

20

5

13.5

331.4

240.9

32.1

5

16.5

469

341

39.6

5

density, µW/cm2

5

66

pairs

7

69

pairs

NMF = N-methylformamide; rGO = reduced graphene oxide; PEI = polyethylenimine;

70

This work

b

Pmax=

maximum output power.

4. CONCLUSIONS In summary, flexible n-type nylon membrane supported PVP/Ag/Ag2Te hybrid films have been fabricated by vacuum filtration followed by heat treatment at a relatively low temperature and pressure. A maximum power factor of 370.1 μW/mK2 is obtained at 393 K for the composite film 20


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starting from a Ag/Te initial molar ratio of 6:1. The electron transport is governed by the interfacial barrier mechanism. After 1000 bending cycles around a rod with radius of ~5 mm, electrical conductivity of the composite film decreases by only 9.4 %, indicating excellent flexibility. A flexible TE generator consisting of five legs of the optimal hybrid film has been assembled on the polyimide substrate. A maximum output power of 469 nW is generated at ΔT = 39.6 K, corresponding to a power density of 341 μW/cm2. This work provides an efficient route to TE films with high performance for flexible TE devices. ASSOCIATED CONTENT Supporting Information Schematic demonstrating fabrication of PVP/Ag2Te NWs dispersion and nylon membrane supported PVP/Ag/Ag2Te hybrid films; XRD pattern and morphologies of the as-synthesized PVP coated Te NWs; XRD patterns of the as-synthesized PVP/Ag2Te NWs prepared by different Ag/Te nominal molar ratios; FE-SEM and TEM characterizations of the as-prepared 6A1T NWs; cross-sectional FESEM images of the heat treated composite films starting from different Ag/Te nominal molar ratios; room temperature TE properties of the heat treated composite films starting from different Ag/Te nominal molar ratios; XRD patterns of the heat treated 3A1T and 4A1T samples; EDS images of Ag, Te, O, N and C corresponding to the STEM image of the PVP/Ag/Ag2Te sample; a HAADF-HRSTEM image of a rod headpart in the PVP/Ag/Ag2Te composite film and overall corresponding EDS image; EDS images of elemental Ag, Te, C, O and N; EDS line scanning plot along the arrow in the overall EDS image; schematic diagrams of two kinds of junctions in the PVP/Ag/Ag2Te composite film; comparison of bending properties between the present hybrid film and those of reported flexible TE materials; a HAADF-STEM image of a heterointerface between PVP/Ag/Ag2Te film and nylon membrane and overall corresponding EDS image; EDS images of elemental C, Ag and Te; schematic illustration of the as-assembled thermoelectric device consisting of five legs of the optimal hybrid

21



Page 22 of 29

film; the digital image of the fabrication device stuck on a polyimide film; schematic illustration for the power output measurement of the TE device; the thermal infrared image taken during the measurement; the output voltage and output power as a function of the output current at different temperature gradients AUTHOR INFORMATION Corresponding Authors *E-mail: [email protected] (K.F. Cai). *E-mail: [email protected] (L.D. Chen). *E-mail: [email protected] (J.Q. He). ORCID 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 (51632010, 51632005), and Innovation Commission of Shenzhen Municipality (Grant Nos. KQTD2016022619565991 and KQCX2015033110182370). REFERENCES (1) Jao, Y. T.; Li, Y. C.; Xie, Y.; Lin, Z. H. A Self-Powered Temperature Sensor Based on Silver Telluride Nanowires. ECS J. Solid State Sc. 2017, 6, N3055-N3057. (2) Maria Theresa de, L.; Harold, C.; Michael, K. Solar Thermoelectric Generators Fabricated on a Silicon-on-Insulator Substrate. J. Micromech. Microeng. 2014, 24, 085011. (3) Bahk, J. H.; Fang, H.; Yazawa, K.; Shakouri, A. Flexible Thermoelectric Materials and Device Optimization for Wearable Energy Harvesting. J. Mater. Chem. C 2015, 3, 10362-10374. (4) Kim, S. J.; We, J. H.; Cho, B. J. A Wearable Thermoelectric Generator Fabricated on a Glass Fabric. Energ. Environ. Sci. 2014, 7, 1959-1965.

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Graphical abstract 67x38mm (300 x 300 DPI)


Ag2Te

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