Self-Assembled Three-Dimensional Bi2Te3 NanowireâPEDOT:PSS

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Surfaces, Interfaces, and Applications

Self-Assembled Three Dimensional Bi2Te3 Nanowire-PEDOT:PSS Hybrid Nanofilm Network for Ubiquitous Thermoelectrics Warittha Thongkham, Charoenporn Lertsatitthanakorn, Kanpitcha Jiramitmongkon, Kittipong Tantisantisom, Thitikorn Boonkoom, Manit Jitpukdee, Kitiphat Sinthiptharakoon, Annop Klamchuen, Monrudee Liangruksa, and Paisan Khanchaitit ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b19767 • Publication Date (Web): 18 Jan 2019 Downloaded from http://pubs.acs.org on January 20, 2019

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ACS Applied Materials & Interfaces

Self-Assembled Three Dimensional Bi2Te3 Nanowire-PEDOT:PSS Hybrid Nanofilm Network for Ubiquitous Thermoelectrics Warittha Thongkham1, Charoenporn Lertsatitthanakorn1, Kanpitcha Jiramitmongkon2, Kittipong Tantisantisom2, Thitikorn Boonkoom2, Manit Jitpukdee3, Kitiphat Sinthiptharakoon2*, Annop Klamchuen2, Monrudee Liangruksa2, and Paisan Khanchaitit2*

1Energy

Technology Division, School of Energy, Environment and Materials, King

Mongkut’s University of Technology Thonburi, 126 Pracha-Uthit Road, Bangmod, Thungkhru, Bangkok 10140, Thailand

2National

Nanotechnology Center (NANOTEC), National Science and Technology

Development Agency (NSTDA), 111 Thailand Science Park, Phahonyothin Road, Khlong Nueng, Khlong Luang, Pathum Thani 12120, Thailand

ACS Paragon Plus Environment

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ACS Applied Materials & Interfaces 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

3Department

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of Applied Radiation and Isotopes, Faculty of Science, Kasetsart

University, 50 Ngam Wong Wan Road, Ladyaow, Chatuchak, Bangkok 10900, Thailand

KEYWORDS: nanowire-based thermoelectrics, Bi2Te3 encapsulation, topological insulator, 3D self-assembled, organic-inorganic interface

ABSTRACT: Thermoelectric generation capable of delivering reliable performance in the low-temperature range (< 150 °C) for large-scale deployment has been a challenge mainly due to limited properties of thermoelectric materials. However, realizing interdependence of topological insulators and thermoelectricity, a new research dimension on tailoring and using the topological-insulator boundary states for thermoelectric enhancement has emerged. Here, we demonstrate a promising hybrid nanowire of topological Bi2Te3 within conductive PEDOT:PSS matrix using the in-situ onepot synthesis to be incorporated into a three-dimensional network of self-assembled hybrid thermoelectric nanofilms for the scalable thermoelectric application. Significantly,


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the nanowire-incorporated film network exhibits simultaneous increase in electrical conductivity and Seebeck coefficient as opposed to reduced thermal conductivity, improving thermoelectric performance. Based on comprehensive measurements for electronic transport of individual nanowires revealing an interfacial conduction path along the Bi2Te3 core inside the encapsulating layer and that the hybrid nanowire is n-type semiconducting, the enhanced thermoelectricity is ascribed to increased hole mobility due to electron transfer from Bi2Te3 to PEDOT:PSS and importantly charge transport via the Bi2Te3-PEDOT:PSS interface. Scaling up the nanostructured material to construct a thermoelectric generator having the generic pipeline-insulator geometry, the device exhibits power factor and figure of merit of 7.45 µW m-1 K-2 and 0.048, respectively, with unprecedented output power of 130 μW and 15-day operational stability at T = 60 C. Our findings not only encourage new approach to cost-effective thermoelectric generation but it could also provide route to enhancement of other applications based on the topological nanowire.


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INTRODUCTION Current energy crises (e.g. supply shortage and atmospheric toxicity) leading to the demand for reducing consumption of fossil fuel has encouraged researchers worldwide to develop new technologies for production of sustainable power. Among them, thermoelectricity (TE) capable of converting thermal energy into electricity, numerously benefiting several applications, has been proposed.1-3 Suggested by the thermoelectric figure of merit (ZT) defined as S2T/ where S is the Seebeck coefficient, σ is the electrical conductivity, T is the absolute temperature,  is the thermal conductivity, and

S2σ is the power factor (PF), performance of thermoelectricity depends strongly on the material possessing high Seebeck coefficient and electrical conductivity while expressing low thermal conductivity. However, the conversion technology is underperforming for the low-temperature range in which wasted heat of more than half of the 60% of total energy is released.4 Although Bi2Te3-based alloys can provide the maximum ZT up to  2 at room temperature,5-6 application of the inorganic material is constrained by high cost and difficulty in processing. As such, the idea of inorganic-organic hybrids has been


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introduced.7-13 One of the promising systems is Bi2Te3-incorporated PEDOT:PSS achieving higher thermoelectric efficiency for the low-temperature regime by improved phonon scattering (i.e. decreased thermal conductivity) in PEDOT:PSS due to the embedded Bi2Te3 nanostructure.10-11 However, electrical conductivity of the topological Bi2Te3 which would further enhance thermoelectric performance of the hybrid material is still suppressed by the native oxide passivation.14-16 Enabled by recent advance in bottom-up nanoprocessing, encapsulation of the Bi2Te3 nanowire by PEDOT:PSS to reduce or even remove the surficial oxide has been developed.17-18 Here, a new in-situ synthesis of the Bi2Te3-PEDOT:PSS nanowire preventing nanowire aggregation and prohibiting the oxide encapsulation is proposed. The strategy exploits the concept of dopant diffusion through a reconstructing nanolayer19-20 and takes advantage of the PSS phase as nanowire-formation stabilizer while behaving as surfactant for nanowire dispersion in the PEDOT:PSS solution. To demonstrate effect of the novel in-situ hybrid nanowire for large-scale thermoelectric deployment, a threedimensional (3D) network of hybrid nanowire-incorporated PEDOT:PSS nanofilms in a cellular foam scaffold is fabricated, for which the 3D structure would provide higher


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density of electrical conducting path and more efficiently maintain temperature gradient for thermoelectricity. Furthermore, a thermoelectric power generator using the hybrid foam is prototyped, showing unprecedented output power and device stability over several weeks under operations at < 100 °C. Considering device configuration which resembles the thermal-insulator pipeline concept, ubiquitous usage of thermoelectricity becomes promising. Our thermoelectric development scheme is depicted in Figure 1.

RESULTS AND DISCUSSION

In-situ Bi2Te3-PEDOT:PSS hybrid nanowire Shape and size of the hybrid nanowire resulting from the in-situ one-pot synthesis (Step 1 in Figure 1) were visualized by transmission electron microscopy (TEM). As seen in the left panel of Figure 2 (a), the in-situ nanowire exhibits a uniform elongated rice-shape with approximately 36.0  4.2 nm in diameter and 563.9  18.2 nm in length, including an apparent shell layer of around 18.6  2.6 nm in thickness. The nanowire also exhibits the lattice d-spacing of 0.33 nm (the upper inset of Figure 2 (a)) which is in good agreement with the (015) interplanar spacing of Bi2Te321 and corresponding to the crystalline


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structure order observed in the selected area diffraction pattern (SADP) (see the lower inset in Figure 2 (a)). Further supported by X-ray diffraction (XRD) analysis of the in-situ nanowire compared to that of the ex-situ counterpart (see Figure S1 in Supplementary information) revealing the rhombohedral crystal structure of Bi2Te3 (JCPDS No. 01-0762813), the in-situ nanowire is considered to be comprised of Bi2Te3. Mapping compositional profile of the nanowire using energy dispersive spectroscopy (EDS) as shown in Figure 2 (b), thorough distribution of Bi and Te is evident. Additionally, since the EDS spectrum of the in-situ nanowire contains the sulfur peak as seen in the lower panel of Figure 2 (c) although residual PEDOT:PSS in the solution had been removed prior to the measurement, the nanoshell layer which is also revealed by scanning electron microscopy (SEM) (the upper panel of Figure 2 (c)) suggests coherent existence of the polymeric material as the nanowire encapsulating layer. To gain more information on depth-dependent compositional profile of the in-situ nanowire, X-ray photoelectron spectroscopy (XPS) data at different etching times with argon ion bombardment was acquired. The results as shown in Figure 3 reveal several characteristics of the nanostructure. Firstly, increasing of etching time resulting in rising


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intensity of Bi4f peaks (157.9 eV and 163.2 eV) and Te3d peaks (572.7 eV and 583.2 eV) in Figure 3 (a) and 3 (b) respectively, labelled as Bi2Te3, indicates that Bi2Te3 is located inside the nanowire. Tiny signal intensities of both Bi4f and Te3d states observed at 0 min can then be ascribed to penetration depth of XPS (i.e. 5-10 nm) rendering composition beneath the polymeric nanoshell of 10-20 nm detectable. Note that since sample etching planarly removes the top volume of the horizontally-deposited nanowires, PEDOT and PSS in the shell layer become detectable regardless of the etching times. Secondly, existence of Bi-S peaks (159.6 eV and 164.9 eV) in Figure 3 (a) indicates chemical interaction between the Bi ion and the SO3- group of PSS during the thermal diffusion through the reconstructing PEDOT:PSS layer,22-23 which suggests the Bi-S composition be part of the encapsulating shell. Supported by FTIR analysis (Figure S2 in Supplementary) of the encapsulating PEDOT:PSS and the pristine PEDOT:PSS showing shift of the PSS peaks due to a tailored - interaction,24 reconstruction of the encapsulating PEDOT:PSS and interaction between Bi and SO3- during the inward Bi diffusion are more convincing. Thirdly, undetectable oxide-related peaks (i.e. Bi-O and Te-O) indicate significant reduction or even absence of oxide passivation on the internal


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Bi2Te3 surface. Comparing to XPS of the ex-situ Bi2Te3 nanowire (Figure S3 (a) and S3 (b) in Supplementary) showing consistency of Bi-O (158.7 eV and 164.0 eV) and Te-O (575.6 eV and 586.0 eV) intensities with etching time, it is suggestive that the surface protection from oxide of the in-situ Bi2Te3 nanowire is associated with the nanoencapsulation by PEDOT:PSS. Moreover, inspecting the XPS spectra of the in-situ nanowires in comparison with the ex-situ nanowires in the O1s range, a metal-oxide peak at ~ 529 eV detected in the ex-situ case even at 20 min of etching time (Figure S3 (c) in Supplementary) is not visible in the in-situ case (Figure 3 (c)). The observation is in consistent with the presence and the absence of Peak Bi-O and Peak Te-O for the ex-

situ and in-situ nanowires, respectively. Based on all the structural analyses above, the in-situ nanowire is considered to consist of the PEDOT:PSS shell and the Bi2Te3 core without the oxide encapsulation as illustrated in Figure 3 (d), implying electrical influence of the Bi2Te3 topological insulator on thermoelectric properties of the hybrid nanowire-embedded material. For the nanowiregenerating one-pot chemical reactions, see Note S1 and Figure S4 in Supplementary.


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To investigate electrical transport mechanism of the in-situ topological Bi2Te3PEDOT:PSS nanowire, conductive atomic force microscopy (CAFM) was used to probe local current through the Bi2Te3-PEDOT:PSS interphase. For comparison, the ex-situ Bi2Te3 nanowire in PEDOT:PSS was also investigated. Voltage-dependent images were acquired after locational stabilization of the nanowires on HOPG to enable the contactmode CAFM scanning (See Note S2 and Figures. S5 and S6 in Supplementary). Current images (Figure 4 (a)) indicate that the in-situ Bi2Te3-PEDOT:PSS nanowire is conductive along its cross-section with non-uniform distribution of conductivity, which can be ascribed to aggregation of the separated PEDOT and PSS clusters in the encapsulating layer as depicted Figure 4 (d). Although conductivity magnitude increases with bias for both polarities, the current exponentially increases with negative voltage whereas it remains comparatively small with a slight increase for positive bias (the red curve in Figure 4 (g)). Such current profile indicates that our hybrid nanowire is an n-type semiconductor25 (See Note S3 and Figure S7 (a) – S7 (c)

in Supplementary for

corresponding electronic-band structure). Due to the ultrathin-film encapsulating PEDOT:PSS layer, it suggests that the topological surface of the Bi2Te3 core should


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influence electrical conduction through the PEDOT:PSS shell layer. As such, we propose a transport path corresponding to the CAFM data as follows; (i) the metal tip electrode, (ii) the tip-contact side of the encapsulating PEDOT:PSS layer, (iii) the conducting path of Bi2Te3, (iv) the HOPG-contact side of the PEDOT:PSS layer, (v) HOPG, and (vi) the metal plate electrode. The sequence can be vice versa depending on bias polarity and the schematic model is shown in Figure 4 (d). The interfacial conduction path is also suggested by two-point nanoprobing of the single nanowires (See Figure S8 in Supplementary), for which current can flow through the nanowire along its longitudinal direction whereas the flow is very tiny through the background residual PEDOT:PSS film. Image results of the ex-situ Bi2Te3 nanowire in PEDOT:PSS reveal two types of current contrast. Conductivity of the first nanowire (Figure 4 (b)) appears thoroughly bright similar to current contrast of the background additional PEDOT:PSS film, suggesting that the nanowire should be covered by a thin layer of the additional PEDOT:PSS. Interestingly, I-V curves of the covering PEDOT:PSS and the background PEDOT:PSS (see Figure 4 (h)) similarly expresses linear relation not appearing in the case of the in-situ nanowire. Since thickness of both PEDOT:PSS films is  10 nm (see Note S5 and Figure S9 in


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Supplementary for estimation of thickness), based on the conductive-substrate effect,2630

the gapless HOPG support is considered to influence charge conduction through the

polymeric layer. With presence of the oxide encapsulation formed during exposure of the prepared Bi2Te3 nanowires to the ambient conditions prior to ex-situ mixing with PEDOT:PSS,31-32 conduction path is proposed as shown in Figure 4 (e). The current flows through the PEDOT:PSS layer to the conductive substrate directly. Existence of the insulating oxide layer is also confirmed by the second ex-situ nanowire (Figure 4 (c)) appearing thoroughly dark since charge carriers cannot flow between the tip and the substrate through the oxide shell (see Figure 4 (f)).

3D network of nanowire-embedded PEDOT:PSS nanofilms To achieve a robust 3D network of nanowire-dosed PEDOT:PSS films for large-scale thermoelectric deployment, a thermoelectric foam was fabricated by submerging the insulating melamine foam in the PEDOT:PSS solution (see Step 2 in Figure 1). After obtaining a dense and robust network of PEDOT:PSS thin films by appropriate addition of the SDS surfactant and the EG polar solvent as shown in Figure 5 (a) (see also Figures


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S10 and S11 in Supplementary), subsequent incorporation of the in-situ nanowires into the submerging solution results in well-dispersed nanowires across the films (Figure 5 (b)), in which there appear contacts among the adjacent nanowires as seen in Figure 5 (c) and 5 (d). By comparing to agglomerative morphology of the films incorporated by the

ex-situ nanowires (Figure S11 in Supplementary), excellent dispersibility of the in-situ nanowires is ascribed to PSS in the encapsulating layer. Note that incorporation of the in-

situ nanowires additionally causes an ordered linear pattern in the film structure (see Figure S12 in Supplementary), which suggests molecular rearrangement of PEDOT altering charge mobility of the film.33-35 The thermoelectric properties of the foam material as functions of the SDS, the EG, the

in-situ nanowire, and the ex-situ nanowire concentrations are shown in Figure 6. Considering Figure 6 (a) and 6 (b), beside morphological improvement of the nanofilm network, SDS and EG also enhances thermoelectricity of the material.36-38 For both SDS dosing and EG dosing, the optimal power factor of ~ 2-3 µW m-1 K-2 is acquired with electrical conductivity raised to the maximum of ~ 5  103 S m-1 while Seebeck coefficient reduced to the minimum at 3 wt.% of SDS and at 10 vol.% of EG. Increase of charge


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carrier concentration (n) due to higher density of the PEDOT:PSS films boosts electrical conductivity while decreasing Seebeck coefficient of the foam material based on the following equations39-41

  ne S  (8 2 k B2 m*T / 3eh 2 )( / 3n) 2/3

(1) (2)

where e is the carrier charge, µ is the charge carrier mobility, kB is the Bolzmann constant,

h is the Planck’s constant and m* is the effective mass of the carrier. Due to small values of Seebeck coefficient, the tracking variation of power factor with electrical conductivity is observed. Note that the turnaround of electrical conductivity and Seebeck coefficient at the higher concentrations is attributed to deteriorated morphological structure of the PEDOT:PSS nanofilms (see Figure S13 in Supplementary). Co-dosing with both SDS and EG at 3 wt.% and 10 vol.% provides electrical conductivity up to 1  104 S m-1 whereas Seebeck coefficient remains small, resulting in power factor of 5.82 µW m-1 K-2 (see the data points at 0 wt.% in Figure 6 (c)). With measured thermal conductivity (к) of 0.058 W m-1 K-1, thermoelectric efficiency of the material defined by ZT is 0.03.


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Incorporation of the in-situ nanowires into the PEDOT:PSS films dosed with 3 wt.% SDS and 10 vol.% EG further enhances thermoelectricity as shown in Figure 6 (c). Unlike the generic case where electrical conductivity and Seebeck coefficient are inversely proportional, levelling up the nanowire inclusion from 0 wt.% to 1 wt.% results in simultaneous increase of electrical conductivity and Seebeck coefficient to 1.24  104 S m-1 and 24.5 µV K-1, respectively, raising power factor up to 7.45 µW m-1 K-2. With thermal conductivity reduced to 0.047 W m-1 K-1, ZT of the thermoelectric material is amplified to the maximum at 0.048. Recalling the charge transport of the in-situ nanowires derived by CAFM, it is suggestive that free electrons transfer from n-type Bi2Te3 to PEDOT:PSS (see Note S4 and Figure S7 (d) in Supplementary). Consequently, the additional electrons de-dope the p-type PEDOT:PSS material around the nanowire, decreasing hole concentration. However, the reduction can be dominated by improvement of hole mobility due to longer mean free path in the film. Combined with the additional conduction path of hole carriers through the hybrid nanowire via the topological Bi2Te3-PEDOT:PSS interface (see Note S4 and Figure S7 (e) in Supplementary), enhanced electrical conductivity of the film is obtained. The


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embedded location of nanowires can also act as a phonon scattering center which decreases lattice thermal conductivity (l) of the material.42 Although electronic thermal conductivity (e) increases with electrical conductivity (e = LσT where L is a bridging factor such as the Lorenz factor), increase of the e is considered small because the measured thermal conductivity is detrimental ( = e

+

l). Combined with increase of

Seebeck coefficient due to the decrease of hole concentration, ZT of the foam material at 1 wt.% nanowire concentration is enhanced. If over-dosing, the hole carrier concentration can be significantly reduced such that electrical conductivity of the film appears lower regardless of the increases of hole mobility. Although phonon scattering at the nanowire location can decrease lattice thermal conductivity of the film and electronic thermal conductivity is reduced with decreasing electrical conductivity, ZT could be dramatically reduced following the power factor due to higher order of electrical conductivity (104 S m-1) compared to thermal conductivity (10-2 W m-1 K-1). Note that increase of Seebeck coefficient is considered insignificant due to its lower order (101 V K-1). The collapse of thermoelectric efficiency is evident in Figure 6 (c) for the cases of 2 wt.% and 4 wt.% of hybrid nanowire incorporation.


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Dosing with the ex-situ nanowires at 1 wt.%, improvement of thermoelectricity by SDS and EG is nullified, in which decreased electrical conductivity down to 2.89  103 S m-1 and increased Seebeck coefficient up to 27.7 µV K-1 result in un-enhanced power factor of ~ 2.21 µW m-1 K-2 as observed in Figure 6 (d). The comparison is evident that the improved thermoelectric efficiency in Figure 6 (c) is due to inclusion of the in-situ nanowires. Note that the reduced electrical conductivity and thus power factor can be ascribed to degraded mobility of charge carriers due to aggregated-nanowire scattering center. Although it also suggests depressed electronic and lattice thermal conductivity, the dominant magnitude of electrical conductivity causes lower thermoelectric performance.

Performance of thermoelectric device A thermo-electric converter based on the nanofilm network material was prototyped as shown in Step 3 in Figure 1. The experiment setup is shown in Figure S14 in Supplementary. Output power of the device at T = 10-80 K without and with inclusion of the in-situ nanowires at 1 wt.% is shown in Figure 7 (a) and 7 (b), respectively. Although


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the power generation increases with T, it is the hybrid nanowire-based configuration which produces higher power for every T. Using the following equation

TE   (Thot  Tcold ) / Thot   ( 1  ZT  1) / ( 1  ZT  (Thot / Tcold ))   

(3)

where Thot and Tcold are the temperature of the hot and cold electrodes while T is the average temperature. Regardless of the maximum conversion efficiency of the hybrid nanowire-based device (TE) estimated to be just 3.1 × 10-3 (or 0.31%) at T ~ 80 K, the system can provide the maximum output power of 130 µW corresponding to power density of 2.76 W cm-2 with the load resistance of 10 . Conversely, the nanowire-free device produces the maximum output power of just 40 µW with a load resistance of 30 . Considering that output power of the hybrid system can also reach the maximum at T ~ 60 K, device stability was evaluated by monitoring output voltage at this low temperature difference over time. As shown in Figure 7 (c), the thermoelectric generator shows an excellent stability with small voltage fluctuation within 62.0 V and 69.5 V for 15 days. A comparison of the thermoelectric performance with other PEDOT:PSS-based devices (i.e. inorganic-polymer hybrids,43 elastic thermoelectric conducting polymer aerogels, 44 a


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fabric-based thermoelectric device,45-46 screen printed on a paper,47 and roll-to-roll on a polyethylene terephthalate (PET) foil48) is shown in Table 1. Including the finding in this work, electrical conductivity has currently been reported in the range of 0.1-770 S cm-1, Seebeck coefficient in the range of 0.7-114 V K-1, and  in the range of 0.047-0.52 W m-1 K-1. Although the ZT of 0.048 is relatively small compared to the reported maximum value of 0.39, our nanowire-based thermoelectric generator can be regarded as a promising alternative considering that it is accompanied by the unprecedented output power of 130 W and harvesting capability in the low-temperature range (i.e. the hot and cold electrodes are  80 C and  20 C, respectively).

CONCLUSION We have synthesized a hybrid nanowire of Bi2Te3 core-PEDOT:PSS shell structure using the in-situ one-pot synthesis. Combined nanoscale characterizations indicate that oxide passivation on the Bi2Te3 surface is significantly reduced by the encapsulating PEDOT:PSS layer, suggesting a tailored conduction of the hybrid nanowire by the Bi2Te3 topological insulator. A 3D network of self-assembled PEDOT:PSS nanofilms


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incorporated with the hybrid nanowires is fabricated within the insulating melamine foam using a simple dip-and-dry process. The flexible and light-weight thermoelectric foam exhibits simultaneous increase of Seebeck coefficient and electrical conductivity ascribed to additional electron transfer from Bi2Te3 into the PEDOT:PSS film. With the reduced thermal conductivity, the TE test specimen provides the figure of merit of 0.048 comparable

among

the

PEDOT:PSS-based

thermoelectricity.

Prototyping

a

thermoelectric device using the thermoelectric foam by resembling the pipe-insulation concept, a maximum output power up to 130 µW is achieved at the low temperatures (< 100 C) with unprecedented operational stability over 15 days. Due to simplicity of the hybrid-nanowire synthesis and the thermoelectric-foam fabrication, mass production of the material is promising. Moreover, considering the temperature range of thermal energy harvesting, our study emphasizes feasibility of the thermoelectric technology for costeffective and large-scale deployment in household and industry.

EXPERIMENTAL METHODS Processing of thermoelectric material and device


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Step (1): One-pot synthesis of the in-situ hybrid Bi2Te3-PEDOT:PSS nanowire Synthesis of the hybrid nanowire started with growth of Te nanowire. TeO2 (0.1 mmol) ( 99.9%, Carlo Erba) and ascorbic acid (L+) (1.867 mmol) (Carlo Erba) were added into a three-neck flask connected with a condenser. This was followed by injecting PEDOT:PSS (4 ml) (Clevios PH 1000, Hereaus) and high purity deionized (DI) water (2 ml) into the flask. The solution was stirred and heated to 90 C for 1 hour to obtain homogeneous Te nanowire. This was performed under argon atmosphere to prevent oxide formation on surface of the Te nanowire. After that, Bi(NO3)3.5H2O (0.067 mmol) (98%, Carlo Erba) was dissolved in 0.6 ml EG (> 99.5%, Carlo Erba) and injected into the solution at the temperature. Allowing the one-pot reaction to continue for 6 hours, the in-

situ Bi2Te3-PEDOT:PSS nanowire is formed. In addition, we synthesized the reported exsitu Bi2Te3 nanowire49 to realize effects of the PEDOT:PSS encapsulation formed by the in-situ process. Step (2): Preparation of hybrid thermoelectric foam PEDOT:PSS was diluted with DI water using the ratio of 1:1, followed by adding EG, SDS ( 99%, Loba Chemie Pvt. Ltd) and the in-situ Bi2Te3-PEDOT:PSS nanowires,


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respectively. The solution was then stirred for 20 minutes to obtain good nanowire dispersion. Concentration of EG was varied from 0 to 15 vol.% while SDS was varied from 0 to 5 wt.%. Content of the in-situ hybrid nanowire was varied from 0 to 4 wt.%. Melamine foams (Basotec®G, BASF Co. Ltd) - Grade G possessing low thermal conductivity (≤ 0.035 W/m.K at 10 °C), high flexibility, light weight, and durability even at high operational temperature - were cut with CO2 laser machine into 3 × 3 × 15 mm3 pieces and cleaned with DI water and ethanol to remove ashes and unwanted organic substances. To fabricate the nanowire-embedded nanofilm network, the foam was dipped into the prepared PEDOT:PSS aqueous solution under the ambient conditions for 5 minutes to allow the hybrid nanofilms to self-assemble, followed by removing and drying in an oven at 80 C for 3 h and 100 C for 2 h. Silver paste was painted on both sides of the thermoelectric foam and dried at 100 C for 1 h to form the electrodes.

Step (3): Systemization of thermoelectric device The melamine foam of 25-mm thick was cut along its cross section using the CO2 laser machine into thirteen pieces of donut shape having internal diameter (in) and external diameter (out) of 10 mm and 40 mm, respectively. The foams were cleaned with DI water


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and ethanol to remove ashes and unwanted organic substances prior to the dip coating, followed by drying in an oven at 80 C for 3 h and 100 C for 2 h. Electrodes of thermoelectric device were two 300-mm-long copper pipes of 1,in = 12 mm and 2,out = 40 mm.

Characterization of material and device nanostructures Surface morphology (i.e. shape and size) of the Bi2Te3-PEDOT:PSS nanowire and the nanowire-embedded PEDOT:PSS nanofilm network were visualized using field emission scanning electron microscope (FESEM, Hitachi SU5000), transmission electron microscope (TEM, JEOL JEM-2100), and tapping-mode AFM (Seiko SPA 400). Elemental composition of individual nanowires was characterized using energy dispersive spectroscopy (EDS). Electron diffraction pattern and X-ray diffraction (XRD) (Bruker D8 ADVANCE) were used to reveal crystallinity. Chemical states of Te, Bi, and oxide phase of the nanowire were determined using X-ray photoelectron spectroscopy (XPS) (ULVACPHI 5000) operated at 15 keV. Functional groups and bonding interactions were also identified by Fourier transform infrared spectroscopy (FTIR, Nicolet 6700). Wettability of


23


Page 24 of 49

the PEDOT:PSS solution was characterized by measuring the contact angle between a drop of the solution and the (100) silicon surface. Nanoscale local conductivity of individual nanowires on the highly oriented pyrolytic graphite (HOPG) substrate was investigated using conductive atomic force microscope (CAFM, JPK NanoScience) with Au-coated Si tips (company, version, radius of curvature  20 nm) under ambient conditions. Care was taken to ensure that the applied loading force of the tip on the sample had been minimized such that no surface damage was observed.50 Maximum current of the scanned nanowire was extracted and plotted versus the applied bias as the I-V characteristic.51 Conductivity along the longitudinal axis of the individual nanowire was examined using a multi-probe nanoprobing system (AIST).

Measurement of thermoelectric properties and performance. The thermoelectric power factor and electrical conductivity were measured with Seebeck coefficient/Electrical resistance measuring system (Model ZEM-3, ULVAC). Actual electrical conductivity of the 3D foam network of the PEDOT:PSS films was recalculated from the measured value due to high porosity (~ 99%) of the foam structure,


24

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resulting in two orders of magnitude higher than the measured value. All the measurements were performed at room temperature; otherwise, stated in the main text. The thermal property was determined using thermal conductivity analysis (Hot Disk TCA, TPS 2500S) under the ambient conditions as shown in Figure S15, in which two foam specimens sandwiching the kapton sensor in a stainless-steel hood to avoid temperature disturbance due to the environment were heated up by passing constant current through the sensor at power of 0.006 W, allowing thermal conductivity of the foam to be analyzed. Measuring time and specimen size were 5 s and 5 × 5 × 2.5 cm3, respectively. Thermoelectric performance including current, voltage, and output power was examined using an in-house electrical measurement setup. At the time of measurement, cold water ( 15-20 C) was passed through the core of the inner copper pipe electrode while the surface of the outer copper pipe electrode was heated to a temperature ranging from 30 C to 100 C, creating a temperature difference (T). For stability test of device operation, the hot-side temperature was set to about 80 C while the cold-side was about 20 C.


25


Page 26 of 49

ASSOCIATED CONTENT Supporting Information. Supporting Information is available free of charge on the ACS Publications website. XRD patterns of the in-situ and the ex-situ nanowires, FTIR spectra of the pristine PEDOT:PSS and the in-situ nanowire, XPS spectra of Bi4f, Te3d and O1s states of the ex-situ nanowire, CAFM images and tapping-mode AFM images, Electronic-band model of the

in-situ nanowire derived from the CAFM analysis, Two-point nanoprobing measurement, Height profiles of the ex-situ Bi2Te3 nanowire covered by PEDOT:PSS, SEM images showing the 3D foam scaffold and PEDOT:PSS nanofilm inside the foam, Film thickness using cross-sectional FIB-SEM image, SEM images of film at overdose SDS, Measurement setup for TE output power.

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] and [email protected]


26

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Author Contributions

P.K. conceived the project and designed the overall experiments. W.T. helped design and mainly performed all the experiments. K.J. contributed synthesis of the hybrid nanowires. M.J. provided the measurement setup for the thermoelectric device performance. K.S. designed and performed the CAFM characterization bridged toward mechanism of the enhanced thermoelectric properties. K.T. helped analyze and interpret the CAFM data. T.B. contributed the XPS data analysis. A.K., M.L. and C.L. provided useful suggestion and discussion on data interpretation. W.T. and K.S. wrote the manuscript.

Notes The authors declare no competing financial interest.

ACKNOWLEDGMENT The authors gratefully acknowledge financial support from Thailand Graduate Institute of Science and Technology (TGIST 01-57-052) and National Nanotechnology Center (P1750180). Synchrotron Light Research Institute (SLRI), NSTDA Characterizations and


27


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Testing Service Center (NCTC), National Advanced Nano-Characterization Center (NANC), Thermoelectric and Nanotechnology Research Center (TNRC), and National Institute of Advanced Industrial Science and Technology (AIST, Japan) are cordially appreciated for accesses to the equipment and facilities. We specially thank Prof. Supapan Seraphin for a fruitful discussion and suggestion.

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39


1

In-situ synthesized Bi2Te3 NW

Formation of Te nanowire

2

Page 40 of 49

Diffusion of Bi ions to form Bi2Te3 nanowire

In-situ Bi2Te3-PEDOT:PSS nanowire

TE test specimen

Self-assembled PEDOT:PSS film

SDS & EG Dip coating

In-situ Bi2Te3-PEDOT:PSSmixture

3

Foam ligament

1 mm

1 mm

Melamine foam 3 x 3 x 15 mm

Silver paste electrode

TE device prototype

300 mm

SDS & EG Dip coating

5 mm

Cu pipes

5 mm

In-situ Bi2Te3-PEDOT:PSS mixture

Melamine foam  in =10 mm,  out = 40 mm

Nanofilm network

Figure 1. Schematic illustration for processing steps of the hybrid TE material and device: Step (1) One-pot synthesis of the in-situ Bi2Te3-PEDOT:PSS hybrid nanowire, Step (2) Fabrication of the self-assembled 3D network of nanowire-embedded PEDOT:PSS


40

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nanofilms as TE test specimen, and Step (3) Construction of the insulator-pipeline TE device for large-scale deployment of low-temperature energetic harvesting.

Figure 2. (a) TEM image of the single in-situ Bi2Te3 nanowire with the high magnification and the corresponding SADP shown in the upper and the lower insets, respectively. The left panel illustrates the single nanowire. (b) EDS mapping of Bi and Te along the hybrid nanowires shown in the top panel. (c) SEM image of the in-situ Bi2Te3 nanowires and their EDS spectrum.


41


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Figure 3. XPS spectra of (a) Bi4f state and (b) Te3d state of the in-situ nanowire at different etching times which is equivalent to different depths from the nanowire surface.


42

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(c) XPS spectra of the O1s state of the in-situ nanowire (d) A three-dimensional illustration of the Bi2Te3 nanowire in-situ encapsulated within the modified PEDOT:PSS nanolayer.

Topography

Current

Topography

Current

Topography

Current

- 0.2 V - 0.1 V 0V + 0.1 V + 0.2 V + 0.35 V

(a) AFM Tip

Bi2Te3

(b)

(c)

AFM Tip

AFM Tip

Bi2Te3

Bi2Te3

HOPG

HOPG

HOPG

(d)

PEDOT:PSS

PEDOT:PSS

PEDOT:PSS

(e)

(f) 60 nm

10 nA

0 nm

0 nA

PEDOT-rich Domain PSS-rich Domain Bi2Te3 HOPG

(g)

(h)

conducting path

Figure 4. Voltage-dependent CAFM images of (a) the representative in-situ Bi2Te3PEDOT:PSS nanowire, (b) the ex-situ Bi2Te3 nanowire covered by PEDOT:PSS, and (c) the bare ex-situ Bi2Te3 nanowire. Schematic models of conducting paths through (d) the


43


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in-situ Bi2Te3-PEDOT:PSS nanowire and (e) the ex-situ Bi2Te3 covered by PEDOT:PSS, and (f) the bare ex-situ nanowire. Maximum current versus applied voltage on (g) the in-

situ nanowire (red-circle) comparing to the background residual PEDOT:PSS (blacksquare) and (h) the ex-situ nanowire covered by PEDOT:PSS (red-circle) comparing to the background external PEDOT:PSS (black-square). Height and current scale bars of all the images are shown in the bottom right corner of the figure.


44

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(a)

(b)

20 m

200 m

(c)

1 m

5 m

(d)

500 nm

Figure 5. (a) The SEM images of the 3D foam scaffold consisting of PEDOT:PSS films at 3wt.% SDS and 10 vol.% EG doped with the in-situ Bi2Te3 nanowires. (b) The zoomed image of the single PEDOT:PSS film. (c) and (d) The high-resolution FIB-SEM images of thin film morphology.


45


(a)

Page 46 of 49

(b)

In-situ Bi2Te3 NWs

(c)

Ex-situ Bi2Te3 NWs

(d)

Figure 6. Electrical conductivity ( ), Seebeck coefficient ( ) and power factor ( ) of the thermoelectric foam (a) with 0-5 wt.% of SDS, (b) with 0-15 vol.% of EG, (c) with the in-

situ Bi2Te3 nanowires for 3 wt.% SDS and 10 vol.% EG, and (d) with 0-4 wt.% of the exsitu nanowires for 3 wt.% SDS and 10 vol.% EG.


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(a)

(b)

(c)

Figure 7. Output power versus load resistance of the TE prototype device (a) without Bi2Te3 nanowires and (b) with 1 wt.% Bi2Te3 nanowires at different temperature gradients. (c) Stability testing of output voltage and temperature difference between the inner and the outer electrodes as a function of operational time.


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Table 1. Comparisons of the thermoelectric properties and performance with other PEDOT:PSS-based materials.



S (µV K-1)

(W m-1 K-1)

dip coating

123.72

24.5

0.047

printing

214.86

114.97

0.22

3% EG

dip coating

13.59

18

PEDOT:PSS aerogel, Khan et al.44

CNF GOPS

freeze drying

0.1

PEDOT:PSS ( polyester fabric), Du et al.46

5% DMSO

dip coating

PEDOT:PSS (paper), Wei et al.47

5 % EG

PEDOT:PSS (PET foil), Søndergaard et al.48

propanol

In-situ Bi2Te3-PEDOT:PSS hybrid (3D melamine foam), this study Te-PEDOT:PSS composite (PET), Bae et al.43 PEDOT:PSS (cotton cellulose fibers: non-woven fabric), Kirihara et al.45

Additive s

 (S cm-1)

Materials (substrates)

3% SDS 10% EG 80% H2SO4

Process

ZT

OP (W)

0.048 (at RT) 0.39 (at 300 K)

130 (80 K)* 10.59 × 10-3 (10 K)*

0.10

0.0013 (at 295 K)

34 (55 K)*

37

-

-

-

0.5-3

15.316.3

0.12

screenprinting

770

18

0.42-0.52

roll-to-roll

-

0.7-3.5

-

0.0000950.5 (at 300 K) 0.01 (at RT) -

12.29 × 10-3 (75.2 K)* 50 (100 K)* 60 × 10-6 (70 K)*

- indicates no report; CNF = cellulose nanofibrills; GOPS = silane glycidoxypropyl trimethoxysilane; RT = room temperature; OP = output power; * indicates value of T


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For Table of Contents Only


49

Self-Assembled Three-Dimensional Bi2Te3 NanowireâPEDOT:PSS

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