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Bull. Soc. Photogr. Imag. Japan. (2017) Vol. 27 No. 2: 13–18

Original Paper

Novel Preparation of Poly(3,4-ethylene dioxythiophene)-Poly(styrenesulfonate)Protected Noble Metal Nanoparticles as Organic-Inorganic Hybrid ThermoelectricMaterials
Shinichi Hata*, **, Taku Omura*, Keisuke Oshima*, Yukou Du***, Yukihide Shiraishi* and Naoki Toshima****Abstract: ere, we describe a novel synthetic method of PEDOT-PSS (poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate))-protectedH
Ag, Au and Pd nanoparticles(NPs) by a simple procedure, and thermoelectric properties of the PEDOT-PSS films doped with theseNPs. Interestingly, PEDOT-PSS films containing small amount of PEDOT-PSS-protected Ag NPs showed a higher electrical conductivity than that of pure PEDOT-PSS. As a result, the PEDOT-PSS protected Ag NPs system showed a high in-plane power
factor (35.2 µW m-1 K-2), which was relatively superior to that of conventional metal nanomaterial systems. Thus, we have succeeded
in development of high performance organic-inorganic thermoelectric conversion films.

Key words:

Conductive polymer, Electrical conductivity enhancement, Metal nanoparticles, Thermoelectric materials

1. Introduction
We depend on the non-renewable energy sources such as fossil
fuels; this is essential to life. However, fossil fuel-derived CO2 isenormous and a factor that promotes global warming. The consumption of fossil fuel is accelerated increasingly with world's population
growth in the future, and worldwide demand for renewable energy(e.g., natural heat, sunlight, wind) is expected to expand.1,2) Therefore, new energy production methods without CO2 emission are
required for the realization of a clean energy and sustainable society.3) Recently, development of thermoelectric conversion materials
that convert thermal energy into electric energy attracts considerable attention. These materials have the potential to increase energy
efficiency and to reduce CO2 emissions significantly.4) Power generation of these materials derive from the Seebeck effect, which is
caused by charge carriers (e.g., electrons, holes) in the metal or semiconductor material at the absolute temperature gradient: charge carriers diffuse from hot side to cold side, resulting in a current flow
through the closed circuit.5) Thermoelectric conversion efficiency isdefined as the dimensionless figure of merit ZT value.
ZT =(S 2σ/κ)T
where S, σ, κ and T are Seebeck coefficient, electrical conductivity,
thermal conductivity and absolute temperature, respectively. Thermoelectric materials with high ZT and Power Factor PF (=S 2σ) aredesirable, yet there remains a trade-off between σ and S. Generally,
inorganic thermoelectric materials exhibit a large Seebeck coeffi-

cient as well as high electrical conductivity, high ZT values havebeen obtained (e.g., SnSe; ZT = 2.6,6) Bi2Te3-Sb2Te3; ZT = 2.47) ).
Nevertheless, their weight, rigidity and poor processability have limited widespread application: it is very difficult to scale up large area
modules. Meanwhile, a thermoelectric conversion element composed of organic materials with electronic carriers such as conductive polymers8,9) has advantages of light weight, mechanical flexibility, low cost, and is a material with extremely high potential.10,11)
Furthermore, large area modules can be formed by the printing
method expected to have high productivity.12) Since we reported thatstretched polyphenylenevinylene derivatives with ZT = 0.1 in2007,13) many researchers have been working on the development oforganic thermoelectric materials using conductive polymers.14,15) In
particular, poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate)(PEDOT-PSS) provides a lot of convenience (e.g., high conductivity,transparency, and excellent stability) and has been actively studied inrecent years.16) According to Pipe's study, the doped level optimized
PEDOT-PSS film recorded ZT=0.42.17) Cahill and coworkers reported that the in-plane ZT value of this work was about 0.1.18) In
the future, this ZT value needs to be improved to 2.0-4.0; for theeffective use of huge unused thermal energy like natural heat andwaste heat below 150 °C.19)
Our group has been interested in carbon nanotube or conductive
polymer-metal nanoparticles (NPs) hybrid thermoelectric materials.20-22) Hybridization23) of conductive polymers such as polyaniline(PANi)24) and PEDOT-PSS,25) with inorganic NPs can improve the

Received 15th, April 2017; Accepted 22nd, August 2017

*Department of Applied Chemistry, Tokyo University of Science Yamaguchi, Daigakudori, SanyoOnoda, Yamaguchi 756-0884, Japan.

**Division of Applied Chemistry, Tokyo Metropolitan University, MinamiOsawa, Hachioji, Tokyo 192-0397, Japan.

***College of Chemistry, Chemical Engineering and Materials Science, Soochow University, Suzhou 215123, PR China.

****Professor Emeritus, Tokyo University of Science Yamaguchi, Japan

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Bull. Soc. Photogr. Imag. Japan. Vol. 27 No. 2 (2017)

thermoelectric performance of conductive polymers by utilizing the
low thermal conductivity of organic constituents and high Seebeckcoefficient of inorganic constituents. However, there is no systematic knowledge about the enhancement mechanism. To enhance carrier hopping between conductive polymers, we focus on conductive
PEDOT-PSS-protected metal NPs of Ag, Au, and Pd (i.e., noble
metal elements) with excellent electrical conductivities26,27) and interesting chemical properties,28-30) because direct hybridization of
PEDOT-PSS with various metal NPs is expected to lead to acceleration of the hopping between PEDOT molecules by direct contact
of PEDOT-PSS with metal NPs. In addition, methods for systematically preparing metal NPs protected with conductive polymers
have not been reported until now.
Herein, we report on a novel synthetic method of PEDOT-PSS-protected Ag NPs, PEDOT-PSS-protected Au NPs
and PEDOT-PSS-protected Pd NPs by a simple procedure of treating noble metal ion in PEDOT-PSS aqueous solution under mild
conditions. We also describe the fabrication, thermoelectric properties of PEDOT-PSS hybrid films containing Ag, Au and Pd NPs,
and new insights into higher thermoelectric performances than thatof insulating poly(N-vinyl-2-pyrrolidone) (PVP)-protected Ag
NPs.

2. Experimental
Materials
Silver perchlorate (AgClO4) was purchased from Kojima Chemicals Co., Ltd. Hydrogen tetrachloroaurate(III), tetrahydrate (HAuCl4·4H2O), palladium(II) chloride (PdCl2), sodium borohydride,hydrochloric acid, nitric acid, sulfuric acid, hydrogen peroxide, silver
standard solution (1,000 ppm), gold standard solution (1,000 ppm),
palladium standard solution (1,000 ppm), and poly(N-vinyl-2-pyrrolidone) (PVP, MW ≈ 40000) were purchased from Wako Pure
Chemical Industries, Ltd. 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, the solid content was 1.0-1.3 wt% in water. Polyimide
ﬁlm was kindly supplied by UBE Industries, Ltd., Japan. All chemicals were used without any further purification. Deionized water(18.2 MΩ·cm) was used for all aqueous solutions.
PEDOT-PSS-protected noble metal NPs
The PEDOT-PSS-protected noble metal NPs were synthesized
by a chemical reduction method. The nominal molar ratio (R), PEDOT-PSS (monomer unit of PEDOT-PSS: 657.645 g/mol) / metal precursor, was 1.6. A solution of AgClO4 (0.132 mmol, 27.4 mg)
in 200 mL PH1000/water (v/v=1/13.6, 0.207 mmol in PEDOTPSS monomer unit) was stirred under N2 atmosphere in an ice–water bath at 0 °C for 1 h. Then, an aqueous solution of NaBH4 (10 mL,
33 mM, 0 °C) was rapidly injected into the solution, and the mixture
was stirred in an ice-water bath at 0 °C for 1 h, then characterized by
Ultraviolet-Visible (UV-Vis) absorbance measurement. Thus prepared Ag NPs dispersion was purified by filtration with an ultrafilter
membrane (Advantec, Q01000 76E, a cutoff molecular weight:

10000) and by washing with water to remove by-produced ions. The
PEDOT-PSS aggregates dispersed in the solution were removed
using a glass filter (Pyrex iwaki glass 17G4, pore size: 5~10 µm). The
PEDOT-PSS-protected Ag NPs were finally obtained as bulk materials after removal of the solvent by using a rotary evaporator followed by vacuum drying at 40 °C. The PEDOT-PSS-protected Au
NPs and PEDOT-PSS-protected Pd NPs were prepared in the similar manner.
Structural characterization of PEDOT-PSS-protected noble
metal NPs
UV-Vis absorption spectra of PEDOT-PSS-protected noble
metal colloidal dispersions were measured with a Shimadzu UV2500PC recording spectrophotometer using a quartz cell with 10
mm of optical path length. Transmission electron microscopy(TEM) images were observed with a JEM 1230 instrument ( JEOL,Tokyo, Japan) at accelerated voltage of 80 kV. A colloidal dispersion
solution 5 mg/L was obtained by dissolving the resultant bulk materials in water using Soniﬁer II model 250D ultrasonic homogenizer(Branson Ultrasonic, Emerson, Japan, Ltd.). The TEM sample wasprepared by dropping the dispersion solution onto copper grids
coated with a thin carbon film. The metal contents of PEDOT-PSS-protected noble metal NPs were determined by opticalemission spectroscopy with inductively coupled plasma (ICP-OES,
Varian 720-ES). For this purpose, the samples were solubilized inaqua regia (HCl/HNO3) or piranha solution (H2SO4/H2O2). ICP
results showed that the metal content of Ag, Au and Pd NPs of thebulk materials were 2.4, 12.9 and 6.0wt%, respectively.
Preparation of PVP-protected Ag NPs
The PVP-protected noble metal NPs were synthesized by achemical reduction method.31,32) The nominal molar ratio, R=PVP(monomer unit of PVP: 114.14 g/mol) / metal precursor was 1.6. A
solution of AgClO4 (0.528 mmol, 109.46 mg) in 800 mL PVP (0.83mmol in PVP monomer unit) was stirred under N2 atmosphere inan ice–water bath at 0 °C for 1 h. Then, an aqueous solution of
NaBH4 (10 mL, 132 mM, 0 °C) was rapidly injected into the solution, and the mixture was stirred in an ice-water bath at 0 °C for 1 h,then characterized by UV-Vis absorbance measurement. The mean
diameters of NPs were determined from the TEM images to be 3.4± 1.2 nm. These results were consistent with the previous reoprt.31)
The Ag amount was estimated by ICP measurement to be 0.58 mg/
mL.
Fabrication and evaluation of PEDOT-PSS-protected noble
metal film
The PEDOT-PSS films containing various weight ratios of Ag
NPs were prepared by the drop-casted method. PEDOT-PSS aqueous solutions (CleviosTM PH1000) containing 0.01-5.0 wt% of Ag
NPs were sonicated (Ultrasonic Cleaner, TAITEC) for 30 min. Homogeneous Ag NPs dispersions with different weight ratios of solids
were obtained. One of Ag NPs dispersion (0.8 mL/cm2) was
drop-casted on a polyimide sheet, which was placed in a hot plate at
40 °C for 12 h. Then, the surface of the film was covered with a small

Shinichi Hata, Taku Omura, Keishuk Oshima, Yukou Du, Yukihide Shiraishi and Naoki Toshima

Novel Preparation of Poly(3,4-ethylene dioxythiophene)-Poly(styrenesulfonate)-Protected Noble Metal Nanoparticles as Organic-Inorganic Hybrid Thermoelectric Materials

amount of ethyleneglycol (0.24 mL/cm2) , dried at 90 °C for 12 h
and then at 130 °C for 30 min,33,34) resulting in dry PEDOTPSS-protected Ag NPs film with a 5.0±1.0 µm thickness. The PEDOT-PSS film, PEDOT-PSS film containing Au NPs, Pd NPs
and PVP-protected Ag NPs were prepared in the similar manner.
The in-plane thermoelectric properties of the thermoelectric conversion films were measured at least 5 times each with a ULVAC
ZEM-3 M8 instrument (ULVAC-RIKO Inc., Yokohama, Kanagawa, Japan) purged with He at 330-390 K.

3. Results and Discussion
Figure 1 shows the UV-Vis spectra of (a) PEDOT-PSS solution,(b) PEDOT-PSS-protected Ag NPs, (c) PEDOT-PSS-protected
Au NPs and (d) PEDOT-PSS-protected Pd NPs, respectively. The
color of PEDOT-PSS solution as blank sample was royal blue andthe absorbance increased in the wavelength range above 500 nm. On
the other hand, the solution of PEDOT-PSS-protected Ag NPs
showed a peak at around 400 nm, which represents the surface plasmon resonance excitation of Ag NPs.35) Moreover, the PEDOT-PSS-protected Au NPs showed a dominant peak of an absorbance maximum at 540 nm, which is attributed to the plasmon
resonance of the Au NPs.36) These observations clearly demonstrate
the formation of Ag NPs in the PEDOT-PSS aqueous solution. In
the PEDOT-PSS-protected Pd NPs system, no significant absorption peak was found on the spectra of the solution treated with
NaBH4. However, the absorption in the ultraviolet-visible region
increased, suggesting that the Pd NPs were formed.37)
Size and shape of the PEDOT-PSS-protected noble metal NPs
were further confirmed by TEM observation. TEM images of the
PEDOT-PSS-protected Ag NPs, PEDOT-PSS-protected Au NPs
and PEDOT-PSS-protected Pd NPs in Figure 2 (a)-(c), respectively, showed the formation of spherical NPs with a relatively narrowsize dispersion. The mean diameters of the NPs were determined

15

from the TEM images to be (d) 3.9 ± 1.2 nm, (e) 4.5 ± 1.5 nm, and(f ) 3.9 ± 1.3 nm. These mean diameters were estimated by averaging
over 200 particles. There has been no study that tried to systematically prepare conductive polymer-protected metal NPs with a particle diameter on the order of nanometers.38,39) In this work, PEDOT-PSS-protected Ag NPs, PEDOT-PSS-protected Au NPs
and PEDOT-PSS-protected Pd NPs of single dispersion and thesmall particle size were prepared by a chemical reduction method.
We investigated the thermoelectric properties of PEDOT-PSS
film containing conductive PEDOT-PSS-protected Ag NPs andinsulating PVP-protected Ag NPs using Seebeck coefficient / electric resistance measurement system. Figure 3 summarizes (a) Seebeck coefficient S, (b) electrical conductivity σ, and (c) power factor
PF (=S 2σ) at 345 K of PEDOT-PSS films containing various weightratios of Ag NPs. All the films exhibited p-type conduction and theSeebeck coefficient of approximately 18.5 µV K-1 in each Ag NPs
concentration region. The electric conductivity value was higher in
PEDOT-PSS system than in PVP system for 0.01 to 1.0 wt% Ag
NPs concentration. The highest power factor, 32 µW m-1 K-2 wasobserved at 0.01 wt% of Ag NPs; the carrier flow within the film
accelerated by adding a small amount of conductive PEDOT-PSS
protected Ag NPs. On the other hand, thermoelectric characteristics
were not improved by the addition of insulating PVP-protected Ag
NPs. We previously reported the thermoelectric properties of PEDOT-PSS films containing surfactants or polymer-protected metal
nanomaterials (e.g., Au NPs or nanorods and Ag nanowires).25,40)

Figure 2 T
 EM image and size distribution histograms of PEDOT-PSS-protected Ag NPs (a and d), PEDOT-PSS-protected Au NPs (b ande) and PEDOT-PSS-protected Pd NPs (c and f ), respectively.

Figure 1

 V−Vis spectra of (a) PEDOT-PSS solution, (b) PEDOTU
PSS-protected Ag NPs, (c) PEDOT-PSS-protected Au NPs and(d) PEDOT-PSS-protected Pd NPs dispersion. Broken and solid
curves indicate pristine and treated with NaBH4, respectively.

Figure 3

(a) Seebeck coefﬁcient, (b) electrical conductivity and (c) thermoelectric power factor at 345 K of PEDOT-PSS film (black) and
PEDOT-PSS film containing various weight ratio Ag NPs with
PEDOT-PSS (red) or PEDOT-PSS film including Ag NPs
with PVP (blue).

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Bull. Soc. Photogr. Imag. Japan. Vol. 27 No. 2 (2017)

The Seebeck coefficient of films containing the metal nanomaterial
with the insulating layer decreased with increasing metal concentration, while the electrical conductivity increased, i.e., power factor
improved. This suggested that the insulating protective agents on themetal surface prevent contact of the metal nanomaterial with PEDOT conductive polymers. On the basis of these results, higher
thermoelectric performances are also expected to be realized by employing the conductive polymer-protected nanomaterials.
We next prepared the PEDOT-PSS film containing various
amount of Ag NPs and investigated their thermoelectric performance testing. Seebeck coefficient, electrical conductivity, and power
factor at 330-380 K of PEDOT-PSS films containing Ag NPs ofvarious weight ratios are shown in Figure 4 (a)-(c), respectively. The
Seebeck coefficients of these films gradually increased with the temperature up to around 390 K. The electrical conductivity of thesesample slightly decreased i.e., negative slope in σ vs. T. This is anindication of metal-like behavior, as evident from the previous reports.41-43) The electrical conductivity dramatically increased from
827 to 1002 S/cm with the increase of Ag NPs content from 0.0 to
0.01wt% at 380 K. As a result, the power factor of PEDOT-PSS
film containing 0.01wt% Ag NPs was around 12% higher than thatof pure PEDOT-PSS film (35.2 µW m-1 K-2). Surprisingly, the thermoelectric properties were greatly improved by adding only a small
amount of Ag NPs to PEDOT-PSS film. However, when the Ag
NPs concentration further increased to 1.0wt%, the electrical conductivity rather decreased and thermoelectric power factor thereby
decreased. We think that an increase in Ag NPs content hinder thepacking of the PEDOT molecules necessary for keeping the electrical conduction of the polymer film.
Furthermore, we investigated the temperature dependence ofthermoelectric performance for PEDOT-PSS film containing
0.01wt% of Au NPs with PEDOT-PSS and PEDOT-PSS film
containing 0.01wt% of Pd NPs with PEDOT-PSS. Figure 5 shows(a) Seebeck coefficient, (b) electrical conductivity and (c) thermoelectric power factor for the prepared films. The Seebeck coefficientdecreased only slightly with metal NPs. The electrical conductivityof PEDOT-PSS film dramatically increased with Ag NPs or Au
NPs. The electrical conductivities of PEDOT-PSS films containing
0.01wt%, 0.1 wt%, and 1.0 wt% of Au NPs at 345 K were 919 S/cm,
874 S/cm, and 853 S/cm, respectively. In the addition of Au NPs, thesame tendency was observed in the case of PEDOT-PSS film containing Ag NPs, as shown in Figure 4 (b). Meanwhile, when Pd NPs

Figure 4 T emperature dependence of (a) Seebeck coefficient, (b) electricalconductivity and (c) thermoelectric power factor for PEDOT-PSS (black) and PEDOT-PSS films with 0.01wt% (red),
0.1wt% (blue), 1.0wt% (green) Ag NPs, respectively.

were added, the increase in electrical conductivity was small. Namely, the electrical conductivity of PEDOT-PSS film increased withmetal NPs in the order of Ag > Au > Pd. This order corresponds wellwith the order of the electrical conductivity of these metals, i.e., Ag> Au > Pd. This result showed that the properties of the metal in the
PEDOT-PSS-protected metal NPs contributed to the increase inelectric conductivity of the PEDOT-PSS film. Temperature dependent conductivity measurements were the most direct way to study
experimentally the thermoelectric properties and conduction mechanism in a conjugated polymeric materials. The charge transport in
the ethyleneglycol-treated PEDOT-PSS film is identiﬁed as a
one-dimensional variable range hopping (1D VRH) model.44)
σ(T )=σ0 exp[-(T0/ T )1/2] … (1)In equation (1), σ0 is the conductivity at inﬁnite temperature, T0 is
the energy barrier between localized states. The temperature dependent electrical conductivity in the range of 330 K< T < 380 K werefitted by 1D VRH model, the Figure 6 shows the plots of ln(σ) vs.
T -1/2 for pure PEDOT-PSS film and PEDOT-PSS film containing
0.01wt% of Ag NPs film. According to the analyses of the data, theT0 value of pure PEDOT-PSS film and PEDOT-PSS film containing 0.01wt% of Ag NPs film are 63 K and 28 K, respectively. Because these results implies that the carrier hopping barrier decreases,
i.e. the electrical conductivity increases, 45) in the Ag NPs system,
PEDOT-PSS-protected Ag NPs entered between the PEDOT
molecules promoted one-dimensional carrier conduction within
PEDOT-PSS film.

Figure 5 Th ermoelectric parameters as a function of temperature for PEDOT-PSS film including 0.01wt% Ag NPs (red), PEDOT-PSS
film containing 0.01wt% Au NPs (blue) and PEDOT-PSS film
including 0.01wt% Pd NPs (green): (a) Seebeck coefficient, (b)electrical conductivity and (c) thermoelectric power factor, respectively.

Figure 6 ependence of natural logarithm of conductivity versus T for
D
PEDOT-PSS (black) and PEDOT-PSS with 0.01wt% Ag NPs(red).

Shinichi Hata, Taku Omura, Keishuk Oshima, Yukou Du, Yukihide Shiraishi and Naoki Toshima

Novel Preparation of Poly(3,4-ethylene dioxythiophene)-Poly(styrenesulfonate)-Protected Noble Metal Nanoparticles as Organic-Inorganic Hybrid Thermoelectric Materials

Additionally, the thermoelectric performances in this works werecompared with that of PANi system. The electrical conductivities at
325 K for PEDOT-PSS or PANi-metal NPs system in this andprevious studies are summarized in Figure 7. As in the case of PEDOT-PSS system, addition of conductive polymer-protected metalNPs (i.e., PANi-protected Au NPs) promoted current-flow in the
conductive polymer film also in the PANi system. This indicates thatthe utilization of the hybridization of conductive polymer films andconductive polymer-protected metal NPs probably achieve higher
electrical conductivity. From these results, the conductive polymer-protected metal NPs could be expected to function as chargetransport promoter between the conductive polymer chains withinorganic thermoelectric conversion film. In this study, we designed asuitable metal-conductive polymer interface within thermoelectric
conversion film and realized smooth carrier transport by using asmall amount of PEDOT-PSS-protected Ag NPs.

4. Summary
In conclusion, we have succeeded in the fabricating an organic–
inorganic hybrid thermoelectric materials consisting of PEDOTPSS molecules and PEDOT-PSS protected noble metal NPs, i.e.
Ag, Au and Pd, prepared by simple and convenient method. The
electrical conductivity of PEDOT-PSS film significantly increased
by adding PEDOT-PSS-protected Ag NPs, while thermoelectric
power factor was enhanced. Surprisingly, we have demonstrated the
optimized fabrication and interfacial carrier transport of metal
nanomaterial-PEDOT-PSS hybrid films with high performance.
That is, fabrication of conductive polymer–metal NPs hybrid material is a promising method for improving the thermoelectric performance of conductive polymers. These results present an important
example for the control and understanding of carrier transport within organic thermoelectric conversion materials, and provides a guiding principle of the improved thermoelectric devices for the realization of sustainable society.
Acknowledgements
This work was partially supported by Konica Minolta Science andFigure 7

 lectrical conductivity at 325 K of PANi or PEDOT-PSS (black)Efilms with 0.01 wt% Ag NPs (red), PEDOT-PSS film with 0.01
wt% Au NPs (blue) and PEDOT-PSS film with 0.01 wt% Pd
NPs (green), respectively.

17

Technology Foundation of The Society of Photography and Imaging of Japan to Y.S. and a Grant-in-Aid for Scientific Research (C)(No. 15K04613 to Y.S.) from the Ministry of Education, Culture,
Sports, Science, and Technology (MEXT), Japan.

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