Bull. Soc. Photogr. Imag. Japan. (2020) Vol. 30 No. 2: 19–21

LetterElectrochromic Stability of Polypyrrole Films on ITO Electrode Modified withAu Nano-islands
Katsumi YAMADA, Shota TSUNEYASU, Fukuro KOSHIJI, Takayuki UCHIDA, and Toshifumi SATOH

Abstract:Island-shaped Au nano-particles were immobilized on an ITO transparent conductive film by a pulse electrolysis method capable of controlling the nucleation and the nuclear growth. Using this as an electrode, a polypyrrole thin film was immobilized by electrochemical polymerization. It was confirmed that the electrochromic stability of the polypyrrole film was greatly improved on the ITO electrode modified
with the Au nano-islands.

Key words:

ITO electrode modified with Au nano-islands, Polypyrrole, Electrochromic stability

In the study of electrochromic (EC) displays using conductive
polymers, improving the responsiveness and stability of the color
changes associated with the electrochemical redox reaction is themost important subject for their practical application. Nevertheless,there are not many direct studies for improving the stability of theEC changes. On the other hand, in the research of secondary batteries, the stability of the charge and discharge has been very actively studied. The EC device and a battery are common in that they
undergo a redox reaction at the electrode. For the redox reaction of
the conductive polymer film, ion doping and dedoping occur, andthe expansion and shrinking of the polymer chain as a result of theformation of the π-conjugated system causes a large and powerfulvolumetric change in the film. After repeated redox reactions, mechanical stress due to the volumetric changes will accumulate in thepolymer film near the electrode surface. 1, 2) This causes peeling of
the polymer film from the electrodes, resulting in a decrease in theelectrochemical activity of the entire film and the amount of the EC
color changes. In recent research on batteries, a method of strengthening the polymer stacking by forming nano-sized structures on theelectrode surface has been often utilized for improving its stability. 3, 4)This method not only enhances the stability, but also expands theeffective area of ​​the electrode and simultaneously improves the storage capacity.
Improving the EC stability of polypyrrole (PPy) was achieved using the Au nano-rod array electrodes in our previous study. 5) It wasconsidered that this method not only improved the stacking of thepolymer film, but also suppressed the peeling from the electrode by
the soft Au nano-rods following the expansion and shrinking of thepolymer film due to the redox reaction. The nano-structures with a
Au rod array were obtained by the template method, and the polycarbonate porous membrane and alumina porous plate used as thetemplate were commercial products and very expensive, and the nano-rod diameters and the number of rods were predetermined. Inorder to overcome these inconveniences, a new approach was employed to form nano-structures without templates. For the methodof forming Au nano-particles on the surface of a ITO electrode bypulse electrolysis, the size and number of particles can be freely controlled.6, 7) By using this new ITO electrode obtained by this method, the electrochemical polymerization of pyrrole would be induced
at a lower applied potential than that when a normal ITO electrodewas used. Furthermore, it was found that the deterioration of thepolymer during the polymerization was suppressed by using the newITO electrode. 8) In this study, using the island-shaped Au nano-particles immobilized ITO electrode, the stability of the change
in transmittance of the PPy film due to EC was evaluated and compared to the normal ITO electrode.
The electrolyte solution for forming Au nano-particles on theITO electrode was prepared as an aqueous solution containing 100
mM potassium sulfite and 30 mM Au (I) sodium sulfite in ultrapure
water. An electrolysis device was fabricated by sandwiching theelectrolyte solution between two glass electrodes with ITO thinfilms (manufactured by Geomatec) via a silicon O-ring (effectivearea: 2 cm2). As an example of the electrolysis conditions, a DC
voltage of -10 V was applied to the device for a short time (severalms) to deposit Au nuclei on the electrodes, then a DC voltage of-1.1 V was applied for 60 s to grow the Au nuclei. The obtainedAu-ITO electrodes were washed with ultrapure water, and thendried in air.
An aqueous solution for the polymerization containing 10 mM
pyrrole and 0.1 M lithium perchlorate was placed in a transparent
glass cell as the container, and silver chloride was immersed as areference electrode, platinum wire was immersed as a counter electrode, and Au-ITO electrodes were the working electrode. To immobilize the PPy film on the electrode, a potential scanning polymerization was carried out at the rate of 50 mV/s in the range of -500

Received 25th November, 2020; Accepted 9th December, 2020; Published 11th December, 2020
Tokyo Polytechnic University kyamada@chem.t-kougei.ac.jp

20Bull. Soc. Photogr. Imag. Japan. Vol. 30 No. 1 (2020)

mV to +1000 mV (vs.Ag-AgCl).
For evaluation of the EC stability of the PPy film, the time dependence of the transmittance of the entire electrolytic cell were
measured at a wavelength of 700 nm to the light incident from thetungsten light source using an electrolyte solution containing 0.1 M
lithium perchlorate. The resulting transmittance value of 100%,
therefore, included the entire absorption of the glass cell, the electrolyte solution, and the ITO electrode.

Transmittance at 700 nm / %Since the Au-ITO electrode obtained by pulse electrolysis had anabsorption maximum at the wavelength of 580 nm (transmittance of67%) based on plasmon scattering, the diameter of the formed island-shaped Au nano-particles was expected to be about 60 nm.7)
When constant potentials of -400 mV and +800 mV were applied
to the PPy films on the electrode in an aqueous solution containing
lithium perchlorate, color changes were observed due to their electrochromism. The PPy films (film thickness: 100 nm for ITO and
150 nm for Au-ITO) formed by potential scanning polymerizationof 20 cycles on both electrodes showed a change in the transmission
spectrum as shown in Fig.1. A change in transmittance in the visible long wavelength region based on the π-π * transition of the PPy
was observed during the switching between the oxidized and reduced states. In the case of the normal ITO electrode, the transmittance decreased to about 80% at the wavelength of 700 nm after 20
cycles of polymerization, and a 9.0% electrochromic change in thetransmittance was obtained between both redox states. In the caseof the Au-ITO electrode, plasmon scattering of the Au nano-particles immobilized on the ITO surface probably included to the absorption of the ITO electrode. Therefore, the transmittance of theAu-ITO electrode decreased to about 50% at a wavelength of 700
nm after 20 cycles of polymerization, and a 25.0% electrochromicchange in transmittance was obtained between both redox states.
Since the thickness of the PPy film on the ITO electrode wasaround 100 nm, and on the Au-ITO electrode, it was around 150
nm, it is considered that the amount of EC change in transmittanceTransmittance / %80
Au-ITO:-400 mV
Au-ITO:-800 mV
ITO:-400 mV
ITO:-800 mV

604020500550600650
Wavelength / nm(ΔT) was mainly due to the difference in the film thickness.
Using the same sample as in Fig. 1, the time dependence of thetransmittance at a wavelength of 700 nm was measured when cyclic
scanning was performed between the applied potentials of -400 mVand +800 mV at the rate of 50 mV/s. The results of the comparison
between the normal ITO electrode and the Au-ITO electrode areshown in Fig. 2. With the ITO electrode, the change in transmittance, which was initially 9.0% (ΔT1), decreased to 2.2% (ΔT20) after
20 redox cycles. On the other hand, with the Au-ITO electrode, thechange in transmittance, which was initially 25.0%, decreased to13.4% after 20 redox cycles.
The decay of ΔT was evaluated as the ratio of ΔT20 to ΔT1, [Decayof ΔT = ((ΔT20) / (ΔT1)) x 100] to be 25.0% for the ITO electrodeand 54% for the Au-ITO electrode. The decay was significantly
suppressed by using the Au-ITO electrodes, thus resulting in an improved stability.
The decay process of the ΔT was discussed for the PPy film thickness in the latter half. Figure 3 shows the results of comparing therelationship between the decay and the number of redox cycles at
each electrode by the number of polymerization cycles of PPy. On
the ITO electrode, the film thickness was about 100 nm after 20polymerization cycles, and about 150 nm at the 40th cycle. On theother hand, on the Au-ITO electrode, the PPy film thickness wasabout 150 nm after 20 polymerization cycles, and about 200 nm at
the 40th cycle. For both electrodes, the PPy film thickness tended
to increase as the polymerization cycle increased. The PPy filmthickness on the Au-ITO electrode was thicker than that on theITO even for the same polymerization cycles. This suggested thatthe Au nano-particles can promote the deposition of the PPy polymer. 8) The Au nano-particles obtained on the ITO electrode wereslightly flattened in the plane direction, so it could not be simplydetermined, but it was one of the factors affecting the increase in thefilm thickness of the PPy obtained on the Au-ITO electrode. As
shown in Fig.3, the decay of ΔT progressed as the number of redox

700750Figure1 Transmittance spectra of the PPy films (20 cycles polym.) on theITO and the Au-ITO in the redox states at the applied potentialsof -400 mV and +800 mV.

8070Au-ITO
ITO

6050403005101520Redox cycle numbers
Figure 2 Time dependences on the transmittance of PPy films (20 cyclespolym.) on the ITO electrode and the Au-ITO electrode at thewavelength of 700 nm during the applied potential cycling between -400 mV and +800 mV.

Katsumi Yamada, Shota Tsuneyasu, Fukuro Koshiji, Takayuki Uchida, and Toshifumi Satohcycles increased on both electrodes. The large difference between
the two electrodes was that a rapid drop was found on the ITO
electrode during the initial stage. During the subsequent stage,
however, the two decays seemed to have almost the same slope on
both electrodes. It is considered that the decay in the subsequent
stage is due to the reason that an aqueous solution is used as theelectrolyte solution, so that the hydrolysis of PPy occurs due to peroxidation, and the electrochemical activity is gradually deteriorated.
The rapid drop in the initial stage of the ITO electrode suggested
that the volume of the PPy film changes with the redox reaction, andthe PPy film is peeled off from the electrode. In fact, in the ITO
electrode, it was confirmed that the PPy film was peeled off whenthe electrode was removed from the electrolytic cell after the experiment. Regarding the difference in the PPy film thickness, the initial decay occurred on both electrodes, but the steeper decay was

Decay of ΔT / %100
Au-ITO:40cycles polym.
Au-ITO:20cycles polym.
ITO:40cycles polym.
ITO:20cycles polym.

80

Electrochromic Stability of Polypyrrole Films on ITO Electrode Modified with Au Nano-islands

21

found from the thicker PPy film. The volumetric change of the PPy
film associated with the redox reaction must be greater on the electrolyte solution side than on the electrode substrate side. Because
the freedom of movement is limited on the electrode substrate side,
the mechanical stress of the PPy film is enhanced by the continuous
redox reaction. It is considered that this tendency becomes stronger
as the film thickness increases.
An aqueous electrolyte solution was used in the study so that thedecay of the EC change of PPy strongly took place on both electrodes by hydrolysis due to peroxidation. However, the stacking between the electrode and PPy film was strengthened by using theAu-ITO electrode, which had nano-structures consisting of Au nano-particles. This can suppress peeling of the PPy film due to theredox reaction, and the stability of the EC changes would be finallyimproved.
In the future, the EC stability of the PPy film will be evaluated byfocusing more on the effect of the volumetric change of the film
under conditions when the hydrolysis is suppressed by using a nonaqueous solution for the polymerization and the EC experiments.

60References

401)
2)
3)

204)

05

10
15
Redox cycle numbers

20Figure 3 Electrochromic stabilities of the PPy films on the ITO electrodeand the Au-ITO electrode for 22 applied potential cycles between-400 mV and +800 mV at the scan rate of 50mV/s. The decay ofΔT was calculated from [Decay of ΔT = ((ΔT20) / (ΔT1)) x 100].

5)
6)
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8)

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