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Fabrication of polyaniline-coated SnSeS nanosheet/ polyvinylidene difluoride composites by solution-based process and optimization for flexible thermoelectrics Hyun Ju, Dabin Park, and Jooheon Kim ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b19667 • Publication Date (Web): 19 Mar 2018 Downloaded from http://pubs.acs.org on March 20, 2018

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ACS Applied Materials & Interfaces

Fabrication of polyaniline-coated SnSeS nanosheet/polyvinylidene difluoride composites by solution-based process and optimization for flexible thermoelectrics

Hyun Ju, Dabin Park, and Jooheon Kim* School of Chemical Engineering & Materials Science, Chung-Ang University, Seoul 06974, Republic of Korea

Keywords: Thermoelectric; Nanosheet; Conductive polymer; Polyaniline; Flexible; Chalcogenide

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ABSTRACT The fabrication of dodecylbenzenesulfonic acid (DBSA)-doped polyaniline (PANI)coated SnSe0.8S0.2 (PANI-SnSeS) nanosheets and their application to flexible thermoelectric composite films are demonstrated. The thermoelectric power factor of PANI-SnSeS nanosheets was optimized by manipulating the number of PANI coating cycles, and changes in their electrical conductivity and Seebeck coefficient were investigated and analyzed by considering carrier transport properties. An optimized, solution-based procedure for introducing

inorganic

nanoparticles

comprising

PANI-SnSeS

nanosheets

into

a

polyvinylidene fluoride (PVDF) matrix maximized the power factor. A composite film with a PANI-SnSeS nanosheet-to-PVDF weight ratio of 2:1 showed outstanding durability and thermoelectric performance, exhibiting a maximum power factor of ~134 µW/m·K2 at 400 K. These results demonstrate that the incorporation of conductive PANI into SnSeS nanosheets can facilitate high-performance flexible thermoelectric applications.

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INTRODUCTION Thermoelectric devices are expected to play an important role in meeting the needs arising from the global energy crisis, and they have attracted attention owing to their potential for clean and efficient energy conversion. The thermoelectric performance of a material is estimated by its dimensionless figure of merit, ZT = (S2·σ·T)/ κ, where S, σ, κ, and T are the Seebeck coefficient, electrical conductivity, thermal conductivity, and absolute temperature, respectively. To attain a high ZT value, a material must exhibit low thermal conductivity and possess an outstanding power factor (S2·σ). Inorganic thermoelectric materials, including semiconductors (Bi2Te3, PbTe, and Sb2Te3), particular bulk materials (skutterudites and lead antimony silver telluride), and oxides (CaMnO3 and NaCo2O4) have mainly been studied thus far owing to their high ZT values.1–5 However, polymer-based flexible thermoelectric materials have recently emerged as alternatives to inorganic materials because of the low cost of their raw materials, environmentally friendly processing systems, and ease of processing. Among these materials, polyvinylidene fluoride (PVDF)-based inorganic/organic composite materials have been extensively studied because they can be easily combined with various inorganic nanoparticles to fabricate flexible composite materials. Dun et al. studied several PVDF-based systems of thermoelectric composite films including inorganic particles with varying amounts of PVDF.6–8 They developed a self-assembled Te nanorod/PVDF composite film with a high power factor of 45.8 µW/m·K2 at ~300 K.6 They also prepared Cu1.75Te nanowire/PVDF composite fabrics using a simple vacuum filtration procedure, which led to a large power factor of 23 µW/m·K2.7 Pammi et al. recently fabricated Cu2Se/PVDF composite films using vacuum filtration and subsequent mechanical pressing, thus achieving a high power factor of 3



253.49 µW/m·K2 at 393 K.9 However, further improvement of the thermoelectric power factor of these composite materials remains necessary because their thermoelectric performance is poor compared to that of the pristine inorganic materials. Telluride- and selenide-based materials are potential filler materials for polymers that enhance the thermoelectric power factor owing to their high Seebeck coefficients. SnSe is one such prospective material with a high Seebeck coefficient close to room temperature,10 and its value is higher than those reported for PbTe (~370 µV/K),11 Bi2Te3 (~180 µV/K),12,13 and Sb2Te3 (~125 µV/K).14 We previously reported the fabrication of SnSe0.8S0.2 as a material with enhanced thermoelectric performance compared to pristine SnSe, where 20 atom.% of S atoms were substituted into SnSe.15 Herein, we report the fabrication of PVDF-based flexible thermoelectric composite films with a high power factor by using a solution-based process. Exfoliated SnSeS nanosheets were dispersed in solution, and dodecylbenzenesulfonic acid (DBSA) was used as a surfactant for forming the colloidal particles of SnSeS nanosheets and as a doping agent for aniline polymerization. PANI-coated SnSeS (PANI-SnSeS) nanosheet/polyvinylidene fluoride (PVDF) flexible composite films subjected to different numbers of PANI coating cycles were fabricated, and the effects of PANI coatings on the thermoelectric properties of the PANI-SnSeS nanosheet/PVDF composite films were examined at 300 K. To our knowledge, this PANI-SnSeS nanosheet/PVDF flexible composite film is novel, and the findings of this study are relevant to the development of flexible thermoelectrics and for stimulating further research.

EXPERIMENTAL SECTION 4


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Preparation of composite films. SnSeS nanosheets were fabricated via the solid-state reaction of Sn (99.999%), Se (99.999%), and S (99.999%) and the hydrothermal exfoliation of a bulk ingot, as reported in our previous research.15 SnSeS nanosheets (0.1 g) and 0.015 g of dodecylbenzenesulfonic acid (DBSA) were added to 2.5 mL of 1 M HCl solution, which was then sonicated for 30 min to prepare a colloidal solution of SnSeS nanosheets. Subsequently, 0.005 g of an aniline monomer was added to a micellar dispersion while the temperature was maintained at ~273 K in an ice bath. An HCl solution (1 mL of 1 M) containing 0.01 g of ammonium persulfate was slowly added to the mixed colloidal solution and stirred steadily for 12 h. The resulting product was filtered and washed with deionized (DI) water and ethanol three times, and dried in a vacuum oven at 333 K for 12 h. Finally, DBSA-doped PANI-coated SnSeS (PANI-SnSeS) nanosheets were fabricated. The same procedure was employed repeatedly to prepare SnSeS nanosheets subjected to different numbers of PANI coating cycles. The PANI-SnSeS nanosheet/polyvinylidene fluoride (PVDF) composite films were prepared by the following procedure: 0.05 g of PANI-SnSeS nanosheet powder and different weight ratios of PVDF (PANI-SnSeS nanosheet:PVDF = 1:1, 2:1, and 3:1) were added to 1 mL of dimethylformamide (DMF) solution and sonicated for 1 h to homogenize the mixture. The mixture was then drop-casted on a glass substrate and dried at 353 K for 24 h. Characterization. Field-emission scanning electron microscopy (FE-SEM, SIGMA) and field-emission transmission electron microscopy (FE-TEM, JEM-2100F) were used to investigate the microstructure of the materials. Elemental mapping analysis of the samples was performed by energy-dispersive X-ray spectroscopy (EDS, NORAN System 7, Thermo Scientific). The crystal structure of the materials was identified by X-ray diffraction (XRD, 5



New D8-Advance/Bruker-AXS) at 40 mA and 40 kV at a scan rate of 1°/s in the 2θ range of 5–70° with Cu-Kα radiation (0.154056 nm). Thermogravimetric analysis (TGA, TGA-2050, TA Instruments) was used to investigate the thermal degradation of the samples under an N2 atmosphere at a heating rate of 10 K/min. Disk-shaped pellets were used to measure the electrical resistivity of the samples using a four-point probe method. A custom device consisting of a pair of thermocouples and voltmeters was used to measure the Seebeck coefficient (S). Five samples of the final product were fabricated to check the reproducibility of the experiments. A bending test of the composite film was performed with a radius of ~2 mm, further details of which can be found in the experimental method of a previous report.8

RESULTS AND DISCUSSION Two-dimensional SnSeS nanosheets with widths of ~200 nm and height of ~3.5 nm were successfully fabricated from a bulk ingot by Li-intercalation and subsequent exfoliation, and were characterized by various analyses as described in our previously reported paper.15 After deposition of the PANI coating on the surface of the SnSeS nanosheets, FE-TEM analysis was performed to investigate the morphology of the product. Figure 1a shows a lowmagnification FE-TEM image of the SnSeS nanosheet, exhibiting its sheet-like structure with particles of ~200 nm. The high-magnification image shown in Figure 1b indicates that the product in this study has a lattice distance of ~0.5 nm, corresponding to the (0 1 1) plane of SnSeS. Figure S1 presents the FE-SEM image and the corresponding EDS mapping images of the SnSeS nanosheet, showing the homogeneous distribution of Sn, Se, and S atoms. The elemental ratio of the SnSeS nanosheet is ~5:4:1 (Figure S2), in good agreement with the initial bulk SnSeS composition. The FE-TEM images of the PANI-SnSeS nanosheet with one 6


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PANI coating shown in Figure 1c and 1d clearly show the PANI layer coated on the surface of the SnSeS nanosheet.

Figure 1. Low- and high-magnification FE-TEM images of the (a, b) exfoliated SnSeS nanosheet and (c, d) PANI-SnSeS nanosheet with a single PANI coating. The SnSeS nanosheets exfoliated from the ingot were dispersed in solution. DBSA was used as a surfactant to form the colloidal particles of the SnSeS nanosheets, and also acted as a doping agent for aniline polymerization. Thus, the PANI-SnSeS nanosheets were successfully prepared and were characterized by various analyses as described below. XRD analysis confirmed the successful fabrication of the PANI-SnSeS nanosheets, with Figure 2a showing the XRD patterns of the SnSeS nanosheets, pristine PANI, and PANI-SnSeS nanosheets with different numbers of PANI coatings. The XRD data for the SnSeS nanosheets are in accordance with previously reported data for SnSeS.15,16 Determining the location of the PANI XRD peaks in the PANI-SnSeS sample profile was difficult because the PANI diffraction peaks of were weaker than those of SnSeS. However, TGA analysis (Figure 7



2b) bolstered the demonstration of the presence of PANI in the PANI-SnSeS nanosheets. The pristine SnSeS nanosheet sample exhibited outstanding thermal stability up to ~900 K, whereas PANI underwent extensive thermal degradation at ~500 K. Based on these results, the weight ratios of PANI on the surface of the SnSeS nanosheets with different numbers of PANI coatings (1, 2, 3, and 4) were estimated as ~8, 11, 14, and 18 wt.%, respectively.

Figure 2. (a) XRD and (b) TGA data for SnSeS nanosheets, pristine PANI, and PANI-SnSeS nanosheets with different numbers of PANI coatings. The thermoelectric properties of the prepared PANI-SnSeS samples were examined as a function of the number of PANI coatings in order to verify the potential use of PANISnSeS nanosheets in thermoelectrics. Figure 3a shows the electrical conductivity of the PANI-SnSeS nanosheet samples as a function of the number of PANI coating cycles. The electrical conductivity of the PANI-SnSeS nanosheets increased gradually with additional coating cycles, which can be attributed to the higher conductive PANI content. The deposited PANI layer formed a highly conductive network in the PANI-SnSeS composite structure. The change in the electrical conductivity was further investigated by analyzing the charge carrier transport properties of the samples. Generally, the electrical conductivity of a material can be 8


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expressed by Equation (1).

= · ·

(1)

where, σ, n, e, and µ are the electrical conductivity, charge carrier concentration, elemental charge per carrier, and charge carrier mobility, respectively. Table S1 presents the carrier mobility and concentration values of the PANI-SnSeS samples with different numbers of PANI coating cycles. The carrier concentration increased with an increase in the number of coating cycles, whereas the carrier mobility decreased. In this study, PANI has a high carrier concentration originating from the DBSA doping; hence, increasing the number of coating cycles resulted in a high PANI content in the PANI-SnSeS nanosheets, thereby increasing the carrier concentration and electrical conductivity. The PANI-SnSeS samples had positive Seebeck coefficients (Figure 3b), representing their intrinsic p-type nature. The plot shows that a thicker PANI layer reduces the Seebeck coefficient of the PANI-SnSeS nanosheets. According to the electron transport model, the Seebeck coefficient is inversely proportional to the carrier concentration as1

=

· · ··

· ∗ · · (·)

(2)

where, S, kB, m*, and h are the Seebeck coefficient, Boltzmann constant, effective mass of the carriers, and Planck's constant, respectively. The reduction of the Seebeck coefficient as the number of coating cycles increased may be caused by an increase in the carrier concentration, in agreement with the trend of the carrier concentration values in Table S1. The thermoelectric power factor (S2·σ) of the PANI-SnSeS sample exhibits a maximum value of 252 µW/m·K2 for the sample subjected to two coating cycles because the enhancement of the electrical conductivity outweighed the decline in the Seebeck coefficient as a result of the 9



optimum PANI coating.

Figure 3. (a) Electrical conductivity, (b) Seebeck coefficient, and (c) power factor values of PANI-SnSeS samples as a function of the number of PANI coating cycles, at 300 K. Flexible thermoelectric composite films were fabricated by incorporating PANISnSeS nanosheets into PVDF. Figure 4a shows a digital photograph of the PANI-SnSeS nanosheet/PVDF composite film with a PANI-SnSeS nanosheet-to-PVDF weight ratio of 2:1. The film was black and flexible as illustrated in the image. The inset in Figure 4a shows the top surface of the prepared film, revealing its ~15 mm size. The microscopic morphology of that sample was further investigated by FE-SEM. The surface of the PANI-SnSeS nanosheet/PVDF film was clearly curved, as seen in the FE-SEM image (Figure 4b). Figures 4c and 4d show surface and cross-sectional FE-SEM images of the composite film, which reveal the even distribution of the PANI-SnSeS nanosheets in the PVDF matrix. To demonstrate the possibility of applying PANI-SnSeS nanosheet/PVDF composite films for flexible thermoelectrics, the thermoelectric properties of the composite films were examined under different conditions, and the results are discussed in the next section.

10


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Figure 4. (a) Digital photograph of the PANI-SnSeS nanosheet/PVDF composite film. The inset shows the top surface of the prepared film. (b) Low-magnification FE-SEM images of the PANI-SnSeS nanosheet/PVDF composite film. Also shown are high-magnification views of (c) surface and (d) cross-sectional FE-SEM images of the composite film. In Figure (d), white arrows indicate the PANI-SnSeS nanosheets distributed in the matrix. The weight ratio of the PANI-SnSeS nanosheets to PVDF was 2:1. Figure 5a shows the variation of the electrical conductivity of the PANI-SnSeS nanosheet/PVDF composite films with different amounts of PVDF as a function of the number of bending cycles. The electrical conductivity decreased as the number of bending cycles increased because the durability of the composite samples declined with repeated bending. The composite sample with the ratio of PANI-SnSeS nanosheet to PVDF ratio exhibited the highest electrical conductivity owing to the high concentration of PANI-SnSeS nanosheets in the film. However, the electrical conductivity of this sample decreased more sharply than that of the other two samples because the large number of inorganic particles in 11



its polymer matrix interfered with its ability to remain flexible, reducing its durability and lowering its electrical conductivity. The Seebeck coefficient and power factor of the composite films (Figure 5b and 5c) trend similarly to the electrical conductivity, showing that the high content of PANI-SnSeS nanosheets in the composite film lowered the durability and thermoelectric properties of the materials. Consequently, the PANI-SnSeS nanosheet/PVDF composite film with the PANI-SnSeS nanosheet-to-PVDF ratio of 2:1 was optimal, with outstanding durability and thermoelectric performance.

Figure 5. (a) Electrical conductivity, (b) Seebeck coefficient, and (c) power factor values of the PANI-SnSeS nanosheet/PVDF composite films with different amounts of PVDF (PANISnSeS nanosheets:PVDF = 1:1, 2:1, and 3:1), as a function of bending cycles. Figure 6 shows the temperature dependence of the thermoelectric properties of the PANI-SnSeS nanosheet/PVDF composite film with a PANI-SnSeS nanosheet-to-PVDF ratio of 2:1. The electrical conductivity of the composite film increased as the temperature increased to ~350 K, and then declined with further increase (Figure 6a), similar to the trends for SnSe-based semiconductor materials.17–19 Figure 6b shows the Seebeck coefficient of the film, which gradually increased with the temperature from ~343 µV/K at 300 K to a maximum value of ~395 µV/K at 500 K. The power factor value of the composite film also 12


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increases with increasing temperature attributed to the simultaneously increased electrical conductivity and Seebeck coefficient up to a temperature of 400 K, and then decreases with increasing temperature. The maximum power factor of ~134 µW/m·K2 for the composite film (Figure 6c), was obtained at 400 K, which is higher than that of the BiSe3 nanoplate/PVDF,8 Te nanorod/PVDF,6 Te nanowire/PEDOT:PSS,20 Cu0.1Bi2Se3 nanoplate/PVDF,8 and Te nanorod/PANI21 composite films shown in Figure 6d.

Figure 6. (a) Electrical conductivity, (b) Seebeck coefficient, and (c) power factor values of the PANI-SnSeS nanosheet/PVDF composite film with a PANI-SnSeS nanosheet-to-PVDF ratio of 2:1 as a function of temperature. (d) Power factor value of the sample at 300 K compared to the previously reported materials.

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CONCLUSION SnSeS nanosheets were coated with conductive DBSA-doped PANI, and nanosheets were prepared from a colloidal solution. The PANI coating layer was clearly observed by FE-TEM imaging, and the amount of PANI deposited, based on the number of coating cycles, was estimated by TGA. The maximum thermoelectric power factor of the PANI-SnSeS nanosheets was attained with two coating cycles, where the enhancement of the electrical conductivity effectively outweighed the reduction of the Seebeck coefficient, which was attributed to the optimum PANI coating. The electrical conductivity and Seebeck coefficient of the PANI-SnSeS nanosheets were affected by charge transport. Therefore, the charge carrier properties of the PANI-SnSeS nanosheets were investigated and discussed, and showing that carrier concentration was an important factor for manipulating thermoelectric properties. PANI-SnSeS nanosheet/PVDF flexible films were prepared from a solution-based mixture of PANI-SnSeS nanosheets and PVDF, providing flexible black films. Evaluation of the bending cycle-dependent thermoelectric properties of the PANI-SnSeS nanosheet/PVDF composite films with different amounts of PVDF demonstrated that the sample with a PANISnSeS nanosheet-to-PVDF weight ratio of 2:1 was optimal with excellent durability and thermoelectric performance, producing a power factor of ~134 µW/m·K2 at 400 K. This study suggests that the incorporation of conducting polymer-coated SnSeS nanosheets into a PVDF flexible composite film is a promising strategy for obtaining high-performance flexible thermoelectric devices.

ASSOCIATED CONTENT Supporting Information. Additional figures and a table are included in the Supporting 14


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Information, as follows: FE-SEM image and corresponding EDS mapping images of SnSeS nanosheets (Figure S1); EDS spectrum of SnSeS nanosheets (Figure S2); Carrier concentration and mobility values of PANI-SnSeS nanosheet samples with different PANI coating cycles (Table S1).

AUTHOR INFORMATION Corresponding Author * Tel: +82-2-820-5763, Fax: +82-2-812-3495, E-mail address: [email protected] (J. Kim) Notes The authors declare no competing financial interest.

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Table of contents graphic 411x171mm (96 x 96 DPI)


polyvinylidene

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