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Bull. Soc. Photogr. Imag. Japan. (2015) Vol. 25 No. 2: 38–40

BSPIJ (Letter)

Acceleration of Electrochromic Color Change using Branched Au Nano-structured ElectrodeKatsumi Yamada*, Yuki Tanaka** and Saki Akimoto**

Abstract:

 u nano-rod array membrane was prepared using a commercially-available porous anodic aluminum oxide plate as the templateA
which consisted of branched structures. When a NiO film was immobilized on the nano-rod surface of the obtained membrane, theelectrochromic response of the NiO film was remarkably accelerated: the total process including the coloring and bleaching was accelerated 1.2 times, in particular, the bleaching process was significantly accelerated 1.5 times.

Key words:

Electrochromism, NiO, Au Nano-stump array membrane, Electrochromic response

1. Introduction
Electrochromism is the phenomenon related to changes in color
induced in several materials by a reversible electrochemical process.
One of the claimed advantages of electrochromic (EC) displays overother technologies is eye-friendly color presentation involving thehigh contrast, the vibrant, and rich looking color image of the display against the background. This is due to the fact that EC materials absorb some light spectra and reflect others, similar to dye andpigments used in printing. Therefore, the EC device has been attracting attention as the display of the electronic paper. In our studies, the EC characteristics of conducting polymer films have beenevaluated on a Au nano-rod array membrane electrode preparedusing a polycarbonate porous membrane as the template. Comparedwith the result in the polymer film on a planar electrode, an improved response and durability of the color change were successfully
achieved using the nano-structured electrode system.1,2) Although
the electrochemical and the EC reactions can be accelerated using athinner EC film, the optical density (coloration) of the EC film will
be reduced by the shortening of the optical path. When the EC
material film was immobilized on a smooth plate electrode, the direction of the light path and that of the carrier (ex. hole or electrolyteions) transfer were always parallel in the EC film. On the other
hand, the orthogonalization of these directions could be easily realized using a Au nano-rod array electrode as shown in Fig.1. 3) These
3D configurations of the EC materials produced a long light pathand short carrier transfer at the same time, thereby allowing accelerated EC color changes and deep colors.
Recently, NiO films as EC materials were coaxially immobilized on the Au nano-rod array electrode prepared using
an anodic porous alumina plate (AAO) template (pore diam-Fig.1. Schematic diagrams of planar and nano-rod architectures.3)

eter of 20 nm, thickness of 60 µm). After several experiments were carried out to determine the optimum relationship between the rod height and the NiO film thickness, theultra-fast EC response within 1 s could be realized as shownin a previous report. 3) It is still necessary, however, for oursystem to make a breakthrough to achieve much faster response sufficient for the movie rate. In this study, we evaluated EC characteristics of the NiO film on a new branched
Au nano-rod array electrode to find a remarkably accelerated EC response.

2. Experimental
All electrochemical and EC measurements were performed usinga potentiostat. A three-electrode cell consisting of an Au nano-structure membrane working electrode, Ag−AgCl reference electrode,Received: 3th, November 2015; Accepted: 18st, December 2015

*Department of Life Science and Sustainable Chemistry, Tokyo Polytechnic University, 1583, Iiyama, Atsugi, Kanagawa 243-0297, Japan**Graduate School of Engineering, Tokyo Polytechnic University, 1583, Iiyama, Atsugi, Kanagawa 243-0297, Japan

Katsumi Yamada, Yuki Tanaka and Saki Akimoto

Acceleration of Electrochromic Color Change using Branched Au Nano-structured Electrodeand Pt wire counter electrode, was used. All potentials are quoted vs.
Ag−AgCl. The Au thin films (thickness of 50 nm) were immobilizedon the AAO plate template (pore diameter of 20 nm, thickness of 60µm) by sputter coating. The electrochemical reduction was carriedout using the Au coated template in an aqueous solution (pH 10)
containing gold (I) trisodium disulfite. In a typical conditions, thepotentiostatic electroreduction was carried out at the applied potential of –1.0 V (vs. Ag−AgCl) for 90 minutes. Using an ultraviolet
curing resin, the product template was immobilized on the glassplate. The template/glass was then immersed in 1.0 M NaOH aqueous solution for 2 hours to remove the AAO. This procedure wascarried out to expose the Au nano-structure. Furthermore, the NiO/
Au composite membranes were electrochemically prepared using
the resulting Au nano-structure membrane as the working electrode(a 0.283 cm2 area is exposed to the electrolyte solution). An aqueoussolution containing nickel (II) chloride 6-hydrate (25 mM) was employed as the electrolyte solution. As the typical conditions, the potentiostatic electroreduction was carried out at the applied potentialof –0.6 V (vs. Ag−AgCl) for 15 minutes. The surface morphologiesof the product membranes were observed by a scanning electron microscope (FE-SEM). A thin film (less than 50 nm thick) of Pt wassputter-coated onto the samples in order to minimize any charging
effects. In order to evaluate the EC characteristics, three electrodes
device with quartz cell was composed of a 1.0 M KOH aqueoussolution, the NiO/Au nano-structure membrane, Ag−AgCl, and Pt
wire as the electrolyte, working electrode, reference electrode andcounter electrode, respectively. The reflection spectra and the EC
switching of the product membranes were evaluated using a spectrophotometer equipped with a reflection probe.

3. Results and Discussion
Surface observations of the obtained Au membrane were carriedout by FE-SEM, and the resulting image is shown in Fig.2a. Complex structures were found in the image which contained somebranched nano-rod structures. The average height (length) of therod structure was about 2 µm by the electrolysis for 90 min. The diameters of the nano-rods changed from 20 nm near the Au baseplate (the length is about 50 nm) to 200 nm at the top of the rod. In
this study, we labelled the straight rod membrane NRM and thebranched rod membrane NSM. After electroreduction of the nickel(II) chloride, NSM was covered with flaky deposit of NiO, thethickness of which was about 50 nm. In the FE-SEM observation,
relatively thicker film of 50 nm was immobilized to confirm the formation of NiO easily.
The electrochemical characteristics of the thin NiO film wereevaluated in a 1.0 M KOH aqueous solution using the NRM andNSM electrodes. The NRM was composed of the smooth andstraight rods with the height of 2 µm and the diameter of 200 nm.
The cyclic voltammograms (CV) are shown in Fig.3 which wereobtained at the scan rate of 20 mV/s. The NiO film on the NRM
was slightly thicker than that on the NSM; the film thickness wasaround 15 nm. For both electrodes, the redox couple (Ni II/Ni III)
was approximately +0.35 V (vs. Ag−AgCl) based on the CV. Their39Fig.2. FE-SEM images of (a) the product membranes from AAO templatewith the branched pores and (b) the product film on the Au nanostructured membrane after electroreduction of NiCl2.

Fig.3. Cyclic voltammograms of the NiO(15 nm)/NSM and the NiO(15nm)/NRM in the 1.0 M KOH aqueous solution.

currents were also about the same values. In contrast, the different
feature was the shape of the reductive wave from the NiO/NRM.
The reductive wave appeared to contain two peaks at +0.26 V (vs.
Ag−AgCl) and +0.31 V (vs. Ag−AgCl). A Ni(OH)2 thin layer wasformed on the NiO film in the KOH aqueous solution. The Ni(OH)2
thin layer would be NiOOH by electrochemical oxidation. The color
change in the metal oxide from colorless to brown occurred during
this reaction. These peaks were probably attributed to the electrochemical reduction of β-NiOOH at the positive potential andγ-NiOOH at the negative potential. γ-NiOOH was formed byover-oxidation of β-NiOOH, and was hard to reduce compared tothe β-NiOOH.4) Since the small shoulder of the oxidation current
was found at +0.55 V (vs. Ag−AgCl) in the CV with the NRM, theformation of γ-NiOOH was probably facilitated on this electrode.
Concerning the redox reaction between Ni(OH)2 and NiOOH, thereduction of γ-NiOOH on the NRM required the more negative
applied potential compared to the reduction of β-NiOOH on theNSM.
According to the CV, the NiO films on the NSM and NRM exhibited an anodic coloring at +0.7 V (vs. Ag−AgCl) and cathodic
bleaching at –0.3 V (vs. Ag−AgCl). These changes in the redox states
could be visually observed as brightness changes. The changes in thereflectance value at the wavelength of 600 nm for the NiO films on
the NSM and NRM during the continuous potential switching between +0.7 V (vs. Ag−AgCl) and –0.3 V (vs. Ag−AgCl) were mea-40Bull. Soc. Photogr. Imag. Japan. Vol. 25 No. 2 (2015)

Fig.4. Changes in the reflectance at 600 nm for the NiO(15 nm)/NSN andthe NiO(15 nm)/NRM induced by the switching of potential applications between +0.7 V and –0.3 V (vs.Ag-AgCl).

sured in order to evaluate the color switching speed. As a result, thetime dependences of the reflectance after a potential application are
shown in Fig.4. The response (coloring or bleaching) times are defined as the time required for a 70% change in the total reflectancemodulation at the wavelength of 600 nm. The EC response for theNSM was quite fast: 1.01 s for coloring and 0.83 s for bleaching. For
the NRM, the response time was 0.98 s for coloring and 1.26 s forbleaching. This result indicates that the total response time (coloringand bleaching) for the NSM will be about 82% of that for the NRM.
Thus, a considerable acceleration of the NiO EC change wasachieved using the NSM.
Concerning the coloring process, this acceleration was attributed
to the change from Ni(OH)2 to NiOOH. Since the electric conductivity of NiOOH was higher than that of Ni(OH)2, the conductive
layer would be smoothly extended from the electrode side to thesolution side. In the bleaching process, an insulating layer (Ni(OH)2)
was formed by the reduction of NiOOH in the area nearest to theelectrode. The reduction of NiOOH in an area slightly distant fromthe electrode was then inhibited by the formed insulating layer. In
most cases, the EC bleaching process of NiO required a longer timecompared to the coloring process. We are now proposing a mechanism for the acceleration of the bleaching process on the NSM. The
only difference in the experimental conditions was the shape of thenano-rod; the other conditions, such as the NiO immobilization,
were common to both of the nano-electrodes. According to the peakseparation on the CV (as shown in Fig.3), the conductivities of theNiO film were probably different on the NRM versus on the NSM.
It is expected that a difference in the electric environment on theelectrode surface is present between the NRM and the NSM. The
dielectric-metal-dielectric (DMD) structure must be discussed as

the potential candidate mechanism. Many studies about the DMD
structure were carried out to improve the electric conductivity andthe transmittance of the transparent electrode.5-7) At first, metals
such as Ag and Au were used as the thin layer in these studies, but
metal nano-particles with island structures were actually immobilized on the substrate. The localized surface plasmon resonance onthe metal layer of the DMD structure has been discussed in recentyears.8) Although the comments about the reflectance spectrum ofboth electrodes (NRM and NSM) were omitted in the letter, thedrop in the reflectance for the NSM was enhanced at a wavelength
longer than 500 nm compared with the NRM. This drop is due toplasmon scattering, which is often observed from a metal nano-structure. As mentioned above, the nano-rods of the NSM werebranched and ranging in the diameter from 20 nm to 50 nm near theAu substrate. Since the NiO films were coaxially immobilized on thebranched Au nano-rod, the DMD (NiO/Au/NiO) structure might
be formed in this system. There is a possibility that the conductivityof the NiO film on the NSM is enhanced by the DMD structure.
Therefore, we plan to study the influence of the plasmon electricfield of the NSM on the conductivity of the prepared NiO film.

4. Summary
Array membranes of the Au nano-rod with a branched structure
were prepared in this study. NiO films were electrochemically immobilized using the obtained nano-rod membrane as the electrode.
The EC response of the NiO film which included a coloring andbleaching process was accelerated 1.2 times by a branched nano-rod
electrode compared to a straight nano-rod one. The difference in theelectrochemical transformation from NiOOH to Ni(OH)2 is responsible for the acceleration of the EC response. The experimental
results obtained in this study seem to be stimulating for such studies
as rechargeable batteries and sensor devices.

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