Bull. Soc. Photogr. Imag. Japan. (2020) Vol. 30 No. 1: 1–4

LetterDevelopment of Organic and Inorganic Ternary Hybrid ThermoelectricMaterials Using Ag Nanoplates
Yukihide Shiraishi*, Satoshi Hoshino*, Keisuke Oshima*, Shinichi Hata*, Yukou Du**, and Naoki Toshima***

Abstract:

Thermoelectric technology is used to convert heat energy to electric energy and vice versa, which is an interesting technology to recover electric energy from waste heat. In this study, we focused on a ternary hybrid system consisting of carbon nanotubes, polymers, and Ag nanoplates. The Seebeck coefficient hardly changed even when poly (N-vinyl-2-pyrrolidone)(PVP)-protected Ag nanoplates were used. On theother hand, the conductivity was improved by adding the Ag nanoplates. The power factor of the hybrid film in the presence of PVP-Ag
nanoplates was 1.2 times higher than that without Ag. Further improvement was achieved by changing the protective agent for the Ag nanoplates from insulating PVP to conductive poly (3,4-ethylenedioxythiophene):poly(styrenesulfonate)(PEDOT:PSS). Based on these results, it
was found that the thermoelectric properties were improved by adding the protected Ag nanoplates, especially those protected with PEDOT:PSS.

Key words:

Silver Nanoplates, Carbon Nanotubes, Conductive Polymer, Hybrid Thermoelectric Materials

Polymer electronics has currently become an attractive and rapidly growing field.1) As an alternative to inorganic materials for use inelectronics, organic polymer materials have several advantages suchas low cost, flexibility, light weight, and printability, and much effort
has been devoted to their development for applications including
light-emitting diodes,2,3)transistors,4,5) touch sensors,6,7) and photovoltaic cells.8,9) One of the most widely used polymers in this field ispoly (3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:
PSS) due to its high electrical conductivity. Meanwhile, PEDOT:PSS has recently attracted increasing attention as a p-typeorganic thermoelectric material. 10-13)
Our group has been studying the development of polymer-based
thermoelectric materials.14-17) The thermoelectric conversion efficiency is defined as the dimensionless figure of merit, i.e. the ZT
value.
ZT =(S2σ/κ)T
where S, σ, κ and T are the Seebeck coefficient, electrical conductivity, thermal conductivity and absolute temperature, respectively.
Thermoelectric materials with a high ZT and power factor, PF(=S2σ)are desirable, yet there remains a trade-off between the σ and
S. In the area of thermoelectrics, highly conducting PEDOT:PSS
films are also becoming popular and are often used as a p-type thermoelectric material.18-20) We believe that use of organic-inorganic
composite materials is one of the effective routes to improve thethermoelectric properties and consider PEDOT:PSS to be a suitable
matrix. For example, we previously reported the thermoelectric
properties of the composite thin films of PEDOT:PSS containing

several types of metal nanoparticles as the inorganic component.14)
The effects of the addition of Au nanoparticles with different protecting ligands or different particle shape (spherical or rod) into the
PEDOT:PSS film were examined, revealing that some additives cancontribute to improvement of the thermoelectric properties, probably due to an increase in the carrier mobility or control of the carrier
concentration. Xin et al. reported the functionalization of carbon
nanotube (CNT) sheet as a flexible substrate by gold nanoplates.21)
On the other hand, Ag nanomaterials have been of interest in thephotographic science. In this study, we focused on a ternary hybrid
system consisting of PEDOT:PSS, CNTs, and Ag nanoplates.
Silver (I) nitrate (AgNO3), poly (N-vinyl-2-pyrrolidone)(PVP,
MW ≈ 40000), sodium borohydride, sodium citrate, and hydrogen
peroxide were purchased from FUJIFILM Wako Pure ChemicalCo., Ltd. The poly (3,4-ethylenedioxythiophene):poly (styrenesulfonate)(PEDOT:PSS) aqueous solution (CleviosTM PH1000; 1 : 2.5= PEDOT:PSS ratio (w/w)) was purchased from H.C. Starck withthe solid content of 1.0-1.3 wt% in water. The polyimide ﬁlm was
kindly supplied by UBE Industries, Ltd., Japan. All chemicals wereused without any further purification. Deionized water (18.2MΩ·cm) was used for all the aqueous solutions. The PVP-protected
Ag nanoplates were synthesized by a hydrogen peroxide method.22)
The nominal molar ratio, R=PVP (monomer unit of PVP) / silver (I)nitrate was 0.7. A solution of AgNO3 (0.025 mmol, 4.25 mg) in 0.1
mL PVP aqueous solution (0.0175 mmol in PVP monomer unit),
sodium citrate (96.8 mg, 0.375 mmol), 30 wt% hydrogen peroxide(0.06 mL) and H2O 24.5 mL was stirred under a N2 atmosphere at

Received 22nd October, 2019; Accepted 10th May, 2020; Published 13th May, 2020
*Department of Applied Chemistry, Sanyo-Onoda City University, Daigakudori, Sanyo-Onoda, Yamaguchi 756-0884, Japan.
**College of Chemistry, Chemical Engineering and Materials Science, Soochow University, Suzhou 215123, PR China.
***Professor Emeritus, Tokyo University of Science Yamaguchi, Japan.
Corresponding author: shiraishi@rs.socu.ac.jp (Y. Shiraishi)

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Bull. Soc. Photogr. Imag. Japan. Vol. 30 No. 1 (2020)room temperature. An aqueous solution of 0.1M sodium borohydride aq. (0.25 mL, 0.025 mmol) was then rapidly injected into thesolution, and the mixture was stirred for 30 min. The PEDOT:PSS-protected Ag nanoplates were prepared in the similarmanner. A solution of AgNO3 (0.025 mmol, 4.25 mg) in 0.1 mL
PH1000/water (v/v=1/13.6, 0.0175 mmol in PEDOT:PSS monomer unit), sodium citrate (96.8 mg 0.375 mmol), 30 wt% hydrogen
peroxide (0.06 mL) and H2O 24.5 mL was stirred under a N2 atmosphere at room temperature. An aqueous solution of 0.1M sodium
borohydride aq. (0.25 mL, 0.025 mmol) was then rapidly injected
into the solution, and the mixture was stirred for 30 min. Total liquidvolume was 24.91 mL and then characterized by UV-Vis absorbance measurement. The UV-Vis absorption spectra of the Ag
nanoplates were measured by a Shimadzu UV-2500PC recording
spectrophotometer using a quartz cell with a 10-mm optical path
length. Transmission electron microscopy (TEM) images were obtained by a JEM 1230 instrument ( JEOL, Tokyo, Japan) at the accelerated voltage of 80 kV. The TEM sample was prepared by dropping the dispersion solution onto copper grids coated with a thincarbon film.
The PEDOT:PSS / CNT (6/4) hybrid films containing various
weight ratios of PVP-protected Ag nanoplates were prepared by thedrop-cast method. PEDOT:PSS aqueous solutions (CleviosTM
PH1000) containing 0.01-5.0 wt% of Ag nanoplates were sonicated(Ultrasonic Cleaner, TAITEC) for 30 min. Homogeneous Ag nanoplates dispersions with different weight ratios of solids were obtained. One of the Ag nanoplates dispersion (0.8 mL/cm2) was
drop-cast on a polyimide sheet, which was placed on a hot plate at
40 °C for 12 h. The surface of the film was then covered with a small
amount of ethylene glycol (0.24 mL/cm2), dried at 90 °C for 12 h,then at 130 °C for 30 min,23,24) resulting in dry PEDOT:PSS / CNT(6/4) hybrid films containing PVP-protected Ag nanoplates with a
5.0±1.0 μm thickness. The thermoelectric properties of the thermoelectric conversion films were measured at least 5 times each by a

ULVAC ZEM-3 M8 instrument (ULVAC-RIKO Inc., Yokohama,
Kanagawa, Japan) purged with He at 330-390 K. The PEDOT:PSS/ CNT (6/4) hybrid films containing various weight ratios of PEDOT:PSS-protected Ag nanoplates were prepared by the same
method.
Colloidal dispersions of the PVP-protected Ag nanoplates wereprepared by the Zhang et al. reduction method of a mixed solutionof silver (I) nitrate and PVP in water.22) The color of the dispersionsof the PVP-protected Ag nanoplates changed from light-yellow to
deep-yellow, purple, green, and finally blue after the addition of sodium borohydride. No aggregates or sediments were observed in the
prepared PVP-protected Ag nanoplates. The time-dependant UVvis absorption spectra of the mixture of silver (I) nitrate, PVP, sodium borohydride, sodium citrate, and hydrogen peroxide are shownin Figure 1. Since the solution of silver (I) nitrate has no absorption
above 230 nm, the reduction of Ag (I) ions to form PVP-protected
Ag nanoparticles can be supported by the absorption at 423 nm dueto the plasmon oscillation characteristic of the Ag nanoparticles(Figure 1 yellow line). The intensity of the characteristic peak at
423 nm of the spherical nanoparticles quickly decreased, indicating
their decline of nanoparticles during the reaction. At the same time,
another peak at 600 nm emerged and gradually red-shifted to longer
wavelengths, implying the formation and growth of the Ag nanoplates. The formation and development of Ag nanoplates took about
20-30 min. We previously reported the preparation of PEDOT:PSS-protected noble metal nanoparticles. 25) In this study, weused PEDOT:PSS instead of PVP as a stabilizer of the Ag nanoplates as shown in Figure 2. In Figure 2, yellow line, the absorption
spectrum, has the main characteristic peak at 400 nm of the spherical nanoparticles, the shoulder peak at around 330 nm attributed tothe out-of-plane quadrupole of Ag nanoplates,26) and the broadening peak above 500 nm due to PEDOT:PSS itself. The peak at
400 nm is in roughly agreement with PVP-protected Ag nanoparticles. Red-shifts together with broadening of the surface plasmon

ー30 minー20 minー10 minFigure 1 
UV-Vis absorption spectra of colloidal dispersions of
PVP-protected Ag nanoparticles (yellow line) and Ag
nanoplates (green and blue lines).

Figure 2 UV-Vis absorption spectra of colloidal dispersions of PEDOT:PSS-protected Ag nanoparticles (yellow line) andAg nanoplates (green line).

Yukihide Shiraishi, Satoshi Hoshino, Keisuke Oshima, Shinichi Hata, Yukou Du, and Naoki Toshima

Development of Organic and Inorganic Ternary Hybrid Thermoelectric Materials Using Ag Nanoplates

3band of Ag particles are observed at 20 min of the reaction time
(Figure 2 green line). The disappearance of the shoulder peak ataround 330 nm in Figure 2 green line may be due to the diminishedthickness of the layer of PEDOT:PSS-protected Ag nanoplates.
Figure 3 shows the transmission electron micrographs and thecorresponding histograms indicating the particle size distributionsof the PVP-protected Ag nanoplates. Particles in the PVP-protected Ag nanoplates have an average diameter of 19.8 nm and mainly
distribute within the range of about 5-25 nm accompanied withsmall nanoparticles. On the other hand, the average diameter of PEDOT:PSS-protected Ag nanoplates was 28.9 nm and was lager
than that of the PVP-protected Ag nanoplates, as shown in Figure
4. The colloidal dispersion of metals can be stabilized by the additionof water-soluble natural or synthetic polymers,27) surfactants,28) andorganic ligands29) etc. These stabilizers are called protective colloids
or surrounding ligands. In this case, the surrounding ligands can beadsorbed on the surface of metallic nanomaterials, and steric repulsion between the surrounding ligands stabilizes the metallic nanomaterials. The protection ability is expressed by the gold number andthe protection value. The protection value of PVP is 50 and is highamong the synthetic polymers.30) On the other hand, the protection
ability of PEDOT:PSS for Ag nanoparticles was reported by Dai etal.31) The protection ability of PVP for metal nanoparticles may begenerally stronger than that of PEDOT:PSS. In fact, PVP has beenused by many researchers as a protective polymer. The small average
diameter of the PVP-protected Ag nanoplates may be due to thehighly protective ability.
Recently, Nakai et al. reported that purified semiconducting single-walled CNTs have a high Seebeck coefficient, which made themsuitable for use as flexible thermoelectric materials.32) We investigated the thermoelectric properties of the PEDOT:PSS / CNT hybrid
film containing Ag nanoplates using the Seebeck coefficient / electric resistance measurement system. Figure 5 summarizes (a) the
Seebeck coefficient S, (b) the electrical conductivity σ, and (c) the
power factor PF (=S2σ)at 345 K of PEDOT:PSS / CNT films containing various weight ratios of the PVP-protected Ag nanoplates
and PEDOT:PSS-protected Ag nanoplates. All of the films show ap-type conduction, and the Seebeck coefficient does not change or
only slightly increases with the PVP-protected Ag nanoplates. The
electrical conductivity increased from 441 to 519 S cm-1 according
to the increase in the PVP-protected Ag nanoplates content from
0.0 to 0.01 wt%. The highest power factor, 51.8 μW m-1 K-2, wasobserved at 0.01 wt% PVP-protected Ag nanoplates. Yoshida et al.
reported the PEDOT:PSS films containing rod-shaped Au nanorods as organic-inorganic hybrid thermoelectric materials.14)
Salah et al. recently showed that the nanocomposites of ultra-thincopper oxide nanosheets and single-wall carbon nanotubes could be
produced and studied for their thermoelectric properties.33)Incorporating a small amount of CNTs into the matrix of CuO nanosheets
enhanced the thermoelectric properties. In general, particles with alarge aspect ratio and CNTs can easily build a percolated structure.34,35) The formation of such a percolated structure gives rise to
many connections between the materials and consequently increases
the number of paths (network) for carrier transfer, leading to an enhancement of the electrical conductivity.
We next, investigated the thermoelectric properties of the PEDOT:PSS / CNT hybrid film containing PEDOT:PSS-protected
Ag nanoplates. The Seebeck coefficient in the presence of the PE-

Figure 3 Transmission electron micrograph and particle size histograms of
PVP-protected Ag nanoplates. Ave = average diameter, Std = standard deviation.

Figure 4 Transmission electron micrograph and particle size histograms of
PEDOT:PSS-protected Ag nanoplates. Ave = average diameter,
Std = standard deviation.

Figure 5 (a) Seebeck coefﬁcient, (b) electrical conductivity and (c) thermoelectric power factor at 345 K of PEDOT:PSS / CNT hybrid film (white) and PEDOT:PSS / CNT hybrid film containing various weight ratios of PVP-protected Ag nanoplates (gray) and PEDOT:PSS-protected Ag nanoplates(black).

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Bull. Soc. Photogr. Imag. Japan. Vol. 30 No. 1 (2020)

DOT:PSS-protected Ag nanoplates (33.7 μV K-1) was almost unchanged. The electrical conductivity of the PEDOT:PSS / CNT
films increased from 441 to 554 S cm-1 in the presence of the PEDOT:PSS-protected Ag nanoplates. The highest power factor, 63.1
μW m-1 K-2, was observed for the 0.01 wt% PEDOT:PSS-protected
Ag nanoplates; the carrier flow within the film was accelerated byadding a small amount of the conductive PEDOT:PSS-protected
Ag nanoplates. We previously reported the thermoelectric properties of PEDOT:PSS films containing polymer-protected metal
nanoparticles.25) The power factor of PEDOT:PSS films in the presence of PEDOT:PSS-protected Ag nanoparticles(32.0 μW m-1 K-2)
was larger than that in the absence of Ag nanoparticles (27.8 μW
m-1 K-2). The improvement rate of power factor in PEDOT:PSS /CNT hybrid films containing PEDOT:PSS-protected Ag nanoplates (1.48 times) was larger than PEDOT:PSS films containing
PEDOT:PSS-protected Ag nanoparticles (1.17 times). The electrical conductivity of the films containing the metal nanomaterials
with the conductive layer increased with increasing metal concentration, i.e., the power factor improved. Because these results implies
that the carrier hopping barrier decreases, i.e. the electrical conductivity increases, in the metal nanoparticles system, PEDOT-PSS-protected metal nanoparticles entered between the PEDOT molecules promoted one-dimensional carrier conduction
within PEDOT-PSS films. In this study, we designed a suitable
metal-conductive polymer interface within the thermoelectric conversion film and realized a smooth carrier transport by using a small
amount of the PEDOT:PSS-protected Ag nanoplates. The films ofthe ternary organic-inorganic hybrid thermoelectric materials with ahigh thermoelectric performance can be easily manufactured bydrop-casting and drying the corresponding dispersions. Ag nanomaterials have been of interest in the photographic science and will
be expected to be applied to new functional materials. Thus, they are
expected to be useful for practical applications in the near future.

Acknowledgements

This study was partially supported in part by the projects KAKENHI (Nos. 18K14017 to S.H. and 19K05633 to Y.S.) from theJapan Society for the Promotion of Science Japan.

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