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Bull. Soc. Photogr. Imag. Japan. (2014) Vol. 24 No. 1: 12–17

Original Paper

Electrochromic Characteristics of NiO/Au Composite Nano-rod Array Membrane
Yasuhiro Mizukoshi* and Katsumi Yamada*

Abstract:

The Au nano-rod array membrane (Au-NRM) was prepared by using an anodic porous alumina plate (AAO) as a template. NiO
films were coaxially immobilized on the surface of the Au-NRM electrode. The electrochromic (EC) characteristics of theobtained NiO/Au composite nano-rod array were evaluated in this study. Ultrafast EC color changes within 1 second wereobtained by optimizations of the rod length and film thickness of the NiO films.

Key words:

Template method, Electrodeposition, NiO/Au composite nano-rod array membrane, Electrochromic characteristics

1. Introduction
For high-contrast and quick color change of reflective electrochromic (EC) devices, the EC absorber material in traditional
planar film EC cells must have a minority carrier diffusion lengththat is long enough to allow effective transfer of the carriers overthe entire light absorption depth. One approach to circumvent
this issue involves orthogonalization of the directions of the lightabsorption and carrier transfer in the EC absorber films. For­example, an absorber thin film immobilized on the surface of avertically oriented nano-rod array with high-aspect ratio could,
in principle, provide a sufficient optical absorption along its axial
dimension, while facilitating radial transfer of carriers over a­distance sufficiently short to compensate for the short minority
carrier transfer length (Fig. 1) 1).

In our previous study by using a modified template method 2) 3),
an Au nano-rod array membrane (Au-NRM) was prepared by removing the template in which the long rods were stacked together
in large numbers on the lower Au surface layer. The Au-NRM,
composed of solid nano-rods (1–10 mm long and 50–800 nm­diameter, aspect ratio = <100 : 1) has a characteristic bristle-like
configuration on the Au film substrate. We adapted the Au-NRM
for electrochromic applications. Conducting polymers were co­axially coated on the Au nano-rods by electropolymerization. In
this case, both improvement of the EC durability and quick response of the EC color changes were realized at the same time by
the nano-structure with a high aspect ratio covered with a thin
conducting polymer film 4) 5).
The most commonly used anodic oxide-based EC materials are
NiO and IrO2. They are able to change from a transparent state to

Fig. 1. Schematic diagrams of planar and nano-rod architectures.
Received 19th, March 2014; Accepted 8th, May 2014
*Department of Life Science and Sustainable Chemistry, Tokyo Polytechnic University, 1583, Iiyama, Atsugi, Kanagawa 243-0297, Japan

Mizukoshi and Yamada   Electrochromic Characteristics of NiO/Au Composite Nano-rod Array Membranea neutral colored one upon the extraction of protons or insertionof OH– ions. Charge-balancing electrons are simultaneously extracted from the valence band. The films are probably a mixture ofoxide and hydroxide components in the bleached state, since a
reservoir of protons seems to exist in the films. Due to the highcost and limited supply of Ir, the use of NiO is favored at least forlarge scale applications.
A variety of methods have been used to prepare the NiO films,
such as sputtering, chemical vapor deposition, electrochemical
deposition process, pulsed laser deposition, spray pyrolysis and a
sol-gel process 6)–11). Among the physical and chemical deposition
techniques, electrodeposition is of particular interest due to low
cost and feasibility of room temperature growth on a large area.
Since the electric conductivity of the NiO film is very low(10 –8 S/cm), the device with a thick NiO film would have a slower
EC response due to a significant IR drop in the film. According
to the consideration as mentioned above, an ideal vertical arrangement of the rods were required to provide a sufficient colorationfrom the NiO thin films. Therefore, we tried the electrochemical
Au deposition by using anodic porous alumina plate as the template to obtain simply and easily the Au-NRM.
The detailed conditions of the Nickel oxides (NiO)/Au coaxial
nano-rod membrane preparation, such as rod length and film
thickness, and the electrochromic characteristics of the coaxial
nano-rod membranes were evaluated in the present study.

2. Experimental
All electrochemical and electrochromic measurements were
performed by using an EG&G PAR model 263 potentiostat. A
three-electrode cell consisting of an Au-NRM working electrode,
Ag-AgCl reference electrode, and Pt wire counter electrode, wasused. All potentials are quoted vs. Ag-AgCl.
The Au thin films (thickness of 50 nm) were immobilized on
the template polycarbonate (PC) membrane (thickness of 5–10mm) and the template anodic porous alumina plate (AAO) (porediameter of 200 nm, thickness of 60 mm) by sputtering coating.
The electrochemical reduction was carried out by using the Au
coated template membrane in aqueous solution (pH 10) containing gold (I) trisodium disulfite. As the typical conditions, poten-

Fig. 2.

13

tiostatic electroreduction was carried out at the applied potentialof –1.2 V (vs. Ag-AgCl) for 3 hours.
Furthermore, the NiO/Au coaxial nano-rod membranes were
electrochemically prepared by using the resulting Au-NRM as
the working electrode (A 0.283 cm 2 area is exposed to the electrolyte solution). An aqueous solution 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 potential of –0.7 V (vs. Ag-AgCl) for
45 min 12).
The surface morphologies of the product membranes were observed by using a Hitachi S-5000 scanning electron microscope(FE-SEM). A thin film (less than 50 nm) of Pt was sputter-coatedonto the samples to minimize any charging effects. The reflection
spectra and the EC switching of the product membranes were
evaluated by using an Ocean Optics USB 2000 spectrometerequipped with a reflection probe.

3. Results and discussion
Fig. 2 shows FE-SEM images of the surface condition of the
product membranes prepared by using typical two conditions. As
shown in Fig. 2, a remarkable difference in the surface condition
was found between the Au-NRM obtained from the PC template
and that from the AAO template. The orientation of the rod directions and intervals between the rods were not very uniform for
the Au-NRM obtained from the PC template. In contrast, thedistributions of the rods obtained from the AAO template werequite perpendicular and densely packed. The obtained rod diameter is about 200 nm, and the rod density was 8 to 16 × 108 rod/
cm 2. Therefore, the rod density was 2.7 to 5.3 times the productfrom the PC template. Their rod shape reflected the pore shape of
the AAO template. It is anticipated that an ideal light absorption
by the EC material along the axis of the nano-rod will be realized
by using the Au-NRM obtained from the AAO template. During
the electrodeposition of the Au rod, the current-time characteristic showed almost constant values except for the initial process.
Although the depositions were carried out in the potentiostaticmode, the charge amount was simply proportional to the deposition time due to the constant current values. The relationship

FE-SEM images of the product membranes from PC template and AAO template.

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Bull. Soc. Photogr. Imag. Japan. Vol. 24 No. 1 (2014)

Fig. 3.

Reflection spectra of obtained Au-NRMs.

between the charge amount and the rod length was not directlydiscussed in this study. The Au deposition proceeded on both theinside of the AAO pores and back side surface of the sputtered
Au layer, then most reaction current was consumed by the Au
under layer formation except for the rod formation. Since thereaction of Au ions needs to pass through the small pores with thelength of 60 mm, the net current for the rod formation is about
1/5 of the observed current. Actually, the rod length can becontrolled by the deposition time.
The reflection spectra of the obtained Au-NRM are shown in
Fig. 3. The shape of the spectra was fundamentally similar to thatfrom the PC template, and the reflectance were lower than thatfrom the PC template (20–50%) 13). The spectrum involved theabsorption due to the Au inter-band transitions shorter than the
wavelength of 500 nm and plasmon scattering to the longer wavelength. It was found that the reflectance value decreased with achange in the rod length to 2–3 mm, then no drastic drop in thevalue would be observed for Au-NRM with longer rods. Since thereflection includes both the head of the rod and Au under layer,
the high reflectance value was observed from Au-NRM withshorter rods. In contrast with Au-NRM with longer rods whereincident light could not reach the reflective Au under layer, in thecase of Au-NRM with shorter rods, the reflection came from boththe head of the rod and Au under layer, thereby showing higher
reflectance. If the EC films were coaxially immobilized on thelonger nano-rods, the incident light into the EC film would not
be reflected on the under layer of the Au-NRM. Cyclic voltammetries of a redox reagent were carried out with Au-NRM of­different rod lengths. Fig. 4 shows the relationship between theobtained oxidative peak current and the scan rate. Between thecases of Au-NRMs with rod length of 1 mm and 3 mm, definitive
differences in the redox current were found with increment of theelectrode surface area. When the rod length increase from 3 mm
to 6 mm, only a slight enhancement in the redox current could beobserved. Even if the rod length of the Au-NRM changes, thesurface area of the Au under layer is always constant. Therefore,
total surface area of the Au-NRM is the addition of the surface
area for the side wall of the all rods (multiplication of the rod circumference by the rod length) and the surface area for the underFig. 4. Relationship between the obtained oxidative peak current andthe scan rate. Cyclic voltammetric analyses of K 3Fe(CN)6(2 mM) were carried out in the KNO3 aqueous solution with
Au-NRM of different rod lengths.

layer, and depends only on the rod length. In the cases of the rod
length of 1, 3 and 6 mm (Average rod density : 12 × 10 8 rod/cm 2),

the theoretical surface areas of the Au-NRM are 2.42, 6.68 and13.1 cm 2 , respectively. When the electrochemical measurement iscarried out with a flat surface electrode, the redox currents areproportional to the surface area of the electrode. As shown in Fig.
4, it was found that the redox currents from three-dimensional
electrode as the Au-NRM were not simply proportional to theelectrode surface area. Since these redox currents had a linear
relationship with the square root of the scan rate, these redox
reactions were controlled by diffusion process of the redox agentsin the solution. This suggests that it is hard to migrate redox species into deeper areas of the rod array.
Fig. 5 shows the FE-SEM image of the obtained the productfilm on the Au-NRM prepared by a typical condition (Rod from1h deposition, and product film from 40 min. deposition). The
flaky morphology was found on the Au nano-rod, which reflected
the NiO formation. Fig. 6 shows the cyclic voltammograms of the
product membrane (Rod from 3 h deposition, and product film
from 40 min. deposition) in the 1.0 M KOH aqueous solution. A
pair of well-defined redox peaks at +0.4 and +0.2 V were clearlyobserved at the scan rate of 50 mV/s. Their peaks are due to thereversible reactions of Ni 2+/Ni3+ associated with the OH– anions.
On the base of these results, the product films were identified as
NiO. As in the other nano-structured NiO films, almost no
charging current due to the electric double layer was observed inFig. 6 14).
Fig. 7 shows the changes in the reflection spectrum of the NiO/
Au composite nano-rod array membrane (Rod from 3 h deposition, and NiO film from 40 min. deposition) induced by an applied potential switching between +0.7 V and –0.3 V. Brightnesschanges were visually observed from the membrane. Especially,
the difference in the reflection values (DRef.) was maximum(around 5.5%) from the wavelengths of 650 nm to 750 nm. Upon
immersion of NiO into an alkaline solution, a spontaneous chemical conversion of NiO into a hydrous metal oxide phase Ni(OH)2
takes place. The Ni 2+/Ni3+ couple responsible for the reversible

Mizukoshi and Yamada   Electrochromic Characteristics of NiO/Au Composite Nano-rod Array Membrane

15

Fig. 5. FE-SEM image of the obtained product film on the Au-NRM prepared by a typical condition (Rod from 1h deposition, and product film from40 min. deposition).

Ni(OH)2 ↔ NiOOH + H+ + e –Fig. 6. Cyclic voltammograms of the product membrane (Rod from 3 hdeposition, and product film from 40 min. deposition) in the
1.0 M KOH aqueous solution.

color change upon electrochemical cycling has been identified as
the nickel hydroxide/oxy-hydroxide phases as shown in the followed equation 15) 16).

(1)

Thus, the NiO film exhibited an anodic coloring at +0.7 V andcathodic bleaching at –0.3 V. This change in the redox states
would be visually observed as the color change. The changes in
the reflectance value at the wavelength of 700 nm for the NiO/Au
composite nano-rod array membrane (Rod from 3 h deposition,
and NiO film from 40 min. deposition) during the continuous
potential switching between +0.7 V and –0.3 V were measured toevaluate the color switching speed. The response (coloring andbleaching) times are defined as the time required for an 80%
change in the total reflectance modulation at the wavelength of
700 nm. The obtained EC response was quite fast, which involved
the coloring for 0.65 s and bleaching for 1.3 s.
The dependences of the EC characteristics on the rod length
were discussed for the coaxial membrane with the same NiO film
thickness (85 nm). Fig. 8a shows the relationship between the
DRef. and rod length. According to the tendencies as shown in

Fig. 7. Changes in the reflection spectrum of the NiO/Au composite nano-rod array membrane (Rod from 3h deposition, and NiO film from 40 min.
deposition) induced by applied potential switching between +0.7 V and –0.3 V. (a) Photographs of the NiO/Au composite nano-rod arraymembrane in the colored (Ox.)/bleached (Red.) states of –0.7 and –0.3 V. (b)16Bull. Soc. Photogr. Imag. Japan. Vol. 24 No. 1 (2014)

Fig. 8. Relationship between DRef. and rod length. (a) The dependences of the DRef. on the rod length were discussed for the coaxial membrane with thesame NiO film thickness (85 nm). Relationship between DRef. and the film thickness. (b) The dependences of the DRef. on the NiO film
thickness were discussed for the coaxial membrane with the same rod length (3 mm).

Fig. 9. Relationship between the response time and the NiO film
thickness. The dependences of the response time on the NiO film
thickness were discussed for the coaxial membrane with the same
rod length (3 mm).
Table 1.

Fig. 10. Changes in the reflectance at 700 nm for the NiO/Au composite
nano-rod array membrane (Rod from 3h deposition, and NiOfilm from 40 min. deposition) during continuous potential
switching between +0.7 V and –0.3 V.

The comparison of the EC performance between our reflective system and other selected
NiO-based transparent systems. (tc) and (tb) are the response time for coloring andbleaching, respectively.

λ (nm)DT (%)

tc (s)

tb (s)

ref.

550
550
550
550
630
630

28
66.2
76
82
45
79.6

0.88
3.5
3
8
1.7
0.063

0.74
4
5
10
2
0.053

Cai et al. 14
Lin et al. 17
Yuan et al. 18
Xia et al. 19
Sonavane et al. 20
Scherer et al. 21

λ (nm)

DRef. (%)

tc (s)

tb (s)

ref.

700

5.5

0.62

0.85

Our Result

the Fig. 4, further increase in the DRef.s were not found over therod length of 3 mm. Since the response would be slower with theincreasing rod length, it was concluded that the rod length of
3 mm was the optimum condition for the EC performance.
The dependences of the EC characteristics on the NiO film
thickness were discussed for the coaxial membrane with same therod length (3 mm). Fig. 8b shows the relationship between the
DRef. and the film thickness. The DRef. increased to 5.5% withan increase in the thickness, while further increases in the DRef.
were not found over the film thickness of 50 nm. Fig. 9 shows therelationship between the response time for the color change andthe NiO film thickness. When the film thickness was 50 nm, theEC response time was 0.62 s for coloring and 0.85 s for bleaching
as shown in Fig. 10. The optimum condition for the EC response
has so far been the film thickness of 50 nm and the rod length of
3 mm. Table 1 shows the comparison of the EC performance be-Mizukoshi and Yamada   Electrochromic Characteristics of NiO/Au Composite Nano-rod Array Membrane

tween our reflective EC system and other selected NiO-basedtransparent EC systems 14) 17)–21). Our reflective EC system shows
the fastest response. By modification of wave function for the applied potential (from a simple square wave), such as over driving,
the improvement of the EC response would be achieved in thenear further. As the other important issue, the evaluation of the
durability for our system is now under way.

4. Summary
The Au-NRMs were prepared with AAO as the template. As aresult, the distributions of the rod obtained from the AAO template were ideally perpendicular and densely packed. The NiOfilms were coaxially immobilized on the Au rod surface by anelectrodeposition by using the Au-NRM as the electrode. It was
found from this study that the fabrication conditions for best EC
performance were the rod length of 3 mm and the NiO film thickness of 50 nm. An ultrafast EC response was achieved by the
NiO/Au composite nano-rod array membrane under the optimum
conditions.

References
1) J. M. Spurgeon, H. A. Atwater, N. S. Lewis, J. Phys. Chem., C,
112, 6186 (2008).
2) C. R. Martin, Science, 266, 1961 (1994).
3) K. Yamada, R. Gasparac, C. R. Martin, J. Electrochem. Soc., 151,
14 (2004).

174) K. Yamada, K. Seya, G. Kimura, Synth. Met., 159, 188 (2009).
5) G. Kimura, K. Yamada, Synth. Met., 159, 914 (2009).
6) Y. Zhou, D. Gu, Y. Geng, F. Gan, Materials Science and Engineering B, 135, 125 (2006).
7) T. Maruyama, S. Arai, Sol. Energy Mater. Sol. Cells, 30, 257 (1993).
8) M. M. Uplane, S. H. Mujawar, A. I. Inamdar, P. S. Shinde, A. C.
Sovavane, P. S. Patil, J. Appl. Surf. Sci., 253, 9365 (2007).
9) N. Penin, A. Rougier, L. Laffont, P. Poizot, J. M. Tarascon, Sol.
Energy Mater. Sol. Cells, 90, 422 (2006).
10) B. A. Reguig, A. Khelil, L. Cattin, M. Morsli, J. C. Bernede, J.
Appl. Surf. Sci., 253, 4330 (2007).
11) E. O. Zayim, I. Turhan, F. Z. Tepehan, N. Ozer, Sol. Energy Mater.
Sol. Cells, 92, 164 (2008).
12) A. C. Sonavane, A. I. Inamdar, D. S. Dalavi, H. P. Deshmukh, P. S.
Patil, Electrochim. Acta, 55, 2344 (2010).
13) K. Yamada, J. Sone, J. Nanosci. Nanotechnol., 12, 403 (2012).
14) G. F. Cai, C. D. Gu, J. Zhang, P. C. Liu, X. L. Wang, Y. H. You, J. P.
Tu, Electrochim. Acta, 87, 341 (2013).
15) L. Bouessay, A. Rougier, P. Poizot, J. Moacovici, A. Michalowicz, J.
M. Tarascon, Electrochim. Acta, 50, 3737 (2005).
16) H. Liu, G. Yan, F. Liu, Y. Zhong, B. J. Feng, Alloys Comd., 481,
385 (2009).
17) S. Lin, F. Chen, J. Kai, J. Appl. Surf. Sci., 254, 3357 (2008).
18) Y. Yuan, X. Xia, J. Wu, Y. Chen, J. Yang, S. Guo, Electrochim.
Acta, 53, 1208 (2011).
19) X. Xia, J. Tu, J. Zhang, X. Wang, H. Huang, Sol. Energy Mater.
Sol. Cells, 92, 628 (2008).
20) A. Sonavane, A. Inamdar, P. Shinde, H. Deshmukh, R. Patil, P.
­Patil, Alloys Comd., 489, 667 (2010).
21) M. R. J. Scherer, U. Steiner, Nano Lett., 13, 3005 (2013).