1
Bull. Soc. Photogr. Imag. Japan. (2018) Vol. 28 No. 1: 1–3

LetterPreparation of ITO Electrode Modified with Au Nano-island by Pulse Electro­deposition
Katsumi Yamada, Fukuro Koshiji, Takayuki Uchida

Abstract:

 u nanoparticles were immobilized on the ITO electrodes by electro-deposition with a pulse voltage, the optical properties of theAresulting electrode were studied. It was found that the transmittance of the resulting electrode containing the plasmon scattering
could be controlled without the change in the wavelength of the absorption maximum.

Key words:

Au nano-island, Pulse electro-deposition, Plasmon scattering

Nanoparticles (diameter: 50 -100 nm) of noble metals, such as Au
and Ag, show different colorations, such as red, violet and green,from their color in the bulk states. These colors are due to plasmon
scattering of the nano-particle, and were dependent on the metalelement, the diameter, the shape and the dielectric constant of thesurrounding medium. These nanoparticles are utilized in stained
glass as a pigment with a strong durability for long time. In recentyears, these nanoparticles were integrated to improve the power generation efficiency of organic solar cells. 1)2)3) A strong electric field(plasmon polariton) exists on the surface of the nanoparticles. Whenmaterials having an absorption band in the visible range, such asorganic molecules and semiconductors, were incorporated into theelectric field, the enhancement phenomenon of the coloration (absorption) would often be observed from the materials.
To utilize the enhancement phenomenon, it is required to arrange
these particles on the electrode or into the layer. When thin layers(less than 100 nm) of Au or Ag were immobilized on a glass substrate by DC sputtering or evaporation technique, the resulting metal layer often showed an imperfect solid layer structure with space
between the neighboring particles. A red or violet coloration was
found from the thin layer, because of the plasmon scattering fromthe metal island-structures on a nano scale. Since no electric conductivity occurred from the thin layer, these nanoparticles were electrically in an isolated state. After immobilization of the nanoparticles onto the ITO glass, the conductivity and the transmittance ofthe electrode could be improved for an unmodified ITO glass. Thisis probably due to the presence of dielectric-metal-dielectric (DMD)or multi-layered structures on a nano scale.
In recent years, the comparison of the EC characteristics of thepoly(3,4-ethylenedioxythiophene) [PEDOT] was carried out using
the ITO electrode with Au nano-particle (Au-ITO) and an unmodified ITO as a control electrode. As a result, both the contrast beReceived 4th August, 2018; Accepted 22nd August, 2018
Tokyo Polytechnic University,

E-mail: kyamada@chem.t-kougei.ac.jp

tween the colored state and the bleached state, and the switching
speed of the color change were improved by using the Au-ITO. 4)
This is expected from the plasmon absorption enhancement and theDMD structures of the Au-ITO. 5)6)In this study, the preparations of the Au-ITO electrodes were carried out by a pulse electro-deposition in order to realize the effective
plasmon enhancement of the various EC materials.
To optimize the reproducibility of the electro-deposition, a device
for the metal Au formation was fabricated by interposing the electrolyte solution between two ITO electrodes through a silicono-ring. The aqueous electrolyte solution (pH 10) was consisted ofgold (I) trisodium disulfite (10 mM) and potassium sulfite (0.1 M).
The Au layer were immobilized on the ITO surface by pulse voltage
applications with a higher voltage for a short time and a constantlower voltage for a long time. In the ITO device, one side is theworking electrode for depositing the Au layer, and the other side is
the counter electrode. The voltage applied to the working electrodeis defined as the potential difference to the counter electrode.
The transmittance spectrum in the visible band was obtained witha CCD spectrophotometer connected to fiber optics and a halogen
light source. The surface observations of the resulting electrode werecarried out using a field emission scanning electron microscope(FE-SEM).
Among the preparation methods of the Au nanoparticles, electrochemical deposition has many advantages, such as the direct immobilization of the particles onto the electrode surface, and no removalof protective surfactants, but has a disadvantage of a wide distribution of the resulting particle size. 7)8) We choose a two-electrodeelectro-deposition system with the constant voltage mode for its
simplicity.
As a result of our preliminary experiment, it was found that theAu deposit was not formed on the ITO electrode by an elec-2Bull. Soc. Photogr. Imag. Japan. Vol. 28 No. 1 (2018)

tro-deposition for a long time at the applied voltage of -1.2 V. On
the other hand, the Au deposit was formed for a short time at avoltage over -2 V. During the general electro-deposition process ofthe metal, the adsorption of the Au metal on the surface of the electrode was caused by the electrochemical reduction of the metal ions
during the initial step. The growth of the metal nucleus was thenachieved by the electro-reduction of the metal ions on the surface offormed nuxleus in the following step. Because the activation energy
for the nucleation is greater than that for the nuclear growth, bothprocesses can be independently controlled. Namely, the electro-deposition at the higher negative voltages allows the both thenucleation process and the growth process, while at the lower voltages only the growth process occurs.
As an example of the electro-deposition, the time course of theapplied voltage is shown in Figure 1. Since the degradation of theITO was induced by application at the higher negative voltages, thehigher voltage (Vp: from -7 V to -10 V) application would be momentarily performed for 2 ms as the nucleation process. The lower
voltage (Vl: -1.2 V) application was then performed for 60 s as thenuclear growth process. As a result of this electro-deposition, thetransmittance spectra of the ITO electrodes are shown in Figure 2.
Their color was a pale violet; the absorption maximum was found at
the wavelength of 580 nm. This wavelength was identified as theplasmon absorption of the Au nanoparticles having the diameter ofaround 100 nm. 9) In our results, only a decrease in the transmittancewithout the red shifting of the absorption maximum was observed
by a change in the Vp. The control of the coloration for the plasmon
scattering on the ITO electrode with immobilized Au nanoparticles
was achieved without any color change by our pulse electro-deposition. In general, the red shifting of the plasmon absorption maximum is observed with an increase in the diameter of the Au
nanoparticles. Furthermore, the decrease in the transmittance of theITO electrode was induced with an increase in the number of Au
nanoparticles due to the reduction of the effective open area for lighttransmission. Therefore, when both processes of the nucleation andthe nuclear growth occur at the same time, the decrease in transmittance for the plasmon absorption was induced along with the red
shifting of the absorption maximum. On the contrary, the shifting of
the wavelength was not observed in our system. Since the voltage
application in the nuclear growth process was performed at the sametime, the particles formed might reach the same size. It is suggested
from our results and this speculation that the number of Au nuclei
on the electrode surface can be controlled by the Vp for this electro-deposition.

Figure 3
Figure 1

Change in applied voltage during Au electrodeposition.

 E-SEM image of the ITO electrode after electrodeposition ofF
Au with pulse voltage application of -8 V.
Fig.1

2490

-7 V-8 V-9 V-10 V

UnmodifiedITO80-7 V-8 V-9 V70-10 V60400Number / %Transmittance / %201612840500600

700

800-1010-191 20-292 30-393 40-494 50-595 60-696 70-797 80-898 90-999 100-10

Wavelength / nm
Figure 3 T ransmittance spectra of ITO electrode with Au nanoparticles
obtained by electro-deposition at various applied voltages.

11Diameter / nm
Figure 4 ize distributions of the Au nanoparticles obtained on the ITO
Selectrode by electro-deposition at various applied voltages.

Fig.2

Katsumi Yamada, Fukuro Koshiji, Takayuki Uchida

Preparation of ITO Electrode Modified with Au Nano-island by Pulse Electrodeposition

The FE-SEM image of the ITO electrode obtained by the electro-deposition with the Vp of -8 V is shown in Figure 3 in order toconfirm the surface condition. The particle size distributions calculated from the image analysis of the FE-SEM images is shown inFigure 4. It was confirmed from Figure 3 that the Au depositions
were particulate and immobilized on the ITO surface. We called thissurface condition a “Au nano-island”. From Figure 4, most of thediameters are around 50 nm for all the pulse voltages. This resultindicates that the wavelength of the maximum absorption at 580 nm
is attributed to the plasmon scattering of the nanoparticles having
the diameter of 50 nm. However, this relationship between the particles size and the wavelength was a little different from that of theAu spherical nanoparticles. (diameter:100 nm and wavelength: 580nm) Therefore, it is required to evaluate in detail the shape of thenanoparticle obtained from our system. From the observation of theFE-SEM images, it was confirmed that the number of the particles
obtained in the same area by the electro-deposition with pulse voltages (Vp) of -7 V, -8 V, -9 V and -10 V were 601, 626, 660 and 696,respectively. Because of a progressive increment in the particle, thetransmittance of the electrode will be smaller by enhancement of thelight shielding. The discussion about the relationship between theVp and the number of the Au nucleus in figure 2 is supported fromthese results. Namely, the Vp affected definitely the number of theAu nucleus, and did not affect the final size of the particles. Asshown in figure 4, because of the same application time for electro-deposition with the Vl (nuclear growth process), the final size ofthe particle was also close to all sample. Other electro-depositions
were carried out with the same Vp and the same application time of3

the Vp in order to evaluate the influence of the Vl application timeon the final size of the particle. Increment of the particle size, andthe red shift of the plasmon absorption maximum (accompanying
the enhancement of the absorption) were induced by the elongationof the application time of the Vl.
About the relationship between the Au nucleation and the pulse
voltage (Vp), and the shape of the resulting particles will be studied
in detail in the near future. We believe that our studies of the plasmon scattering characteristics of Au nanoparticles immobilized on
the ITO electrode can contribute to improvement of novel opto-electronic devices.

References
1)
2)
3)
4)
5)
6)
7)
8)
9)

H.A.Atwater, A.Polman, Nat.Mater., 9, 205 (2010).
S.Pillai, K.R.Catchple, T.Trupke, M.A.Green, J.Appl.Phys., 101, 093105(2007).
M.Ihara, M.Kanno, S.Inoue, Physica E, 42, 2867 (2010).
Y.Tanaka, K.Yamada, J. Soc. Photogr. Sci. Technol. Jpn., 80, 60 (2017).
Y.-C. Nah, J.Nanosci.Nanotech., 13, 3470 (2013).
T.Xu, E.C.Walter, A.Agrawal, C.Bohn, J.Velmurugan, W.Zhu,
H.J.Lezec, A.A.Talin, Nat. Commun., 7, 10479 (2016).
N.Sasai, Y. Fujiwara, M.Arai, K.Yu, T.Tatsuma, J. Electroanal. Chem.,
628, 7 (2009).
Y.-C. Nah, S.-S. Kim, J.-H. Park, D.-Y. Kim, Electrochem. Solid-State
Lett., 10, J12 (2007).
P.K.Jain, K.S.Lee, I.H.El-Sayed, M.A.El-Sayred, J.Phys.Chem. B, 110,7238 (2006).