Synthesis of ultrathin films of Prussian blue by successive ion adsorption technique

Article abstract of DOI:10.1246/cl.2002.310

Ultrathin films of Prussian blue (PB) were successfully prepared on substrates by the successive ion adsorption technique that consisted of alternate immersions of a substrate into solutions of Fe²⁺ and Fe(CN)₆^3-. The PB films showed stable electrochemical redox and electrochromic properties.

Full text of DOI:10.1246/cl.2002.310

3
10
Chemistry Letters 2002
Synthesis of Ultrathin Films of Prussian Blue by Successive Ion Adsorption Technique
ꢀ
Isamu Moriguchi, Haruna Kamogawa, Kaori Hagiwara, and Yasutake Teraoka
Department of Applied Chemistry, Faculty of Engineering, Nagasaki University, 1-14 Bunkyo-machi, Nagasaki 852-8521
(Received November 16, 2001; CL-011159)
Ultrathin films of Prussian blue (PB) were successfully
prepared on substrates by the successive ion adsorption technique
that consisted of alternate immersions of a substrate into solutions of
Fe and Fe(CN)6 . The PB films showed stable electrochemical
redox and electrochromic properties.
into aqueous PD and PS alternately according to the previous
reports.1
0;11
The deposition of PB by the SIA process onto the Q-(PD/PS)3
substrate with anionic surface was monitored at pH 2.5 by means of
UV-Vis spectroscopy. The Q-(PD/PS)3/PBn films exhibited
absorption around 710nm due to the intervalence charge transfer
2þ
3À
(
IVCT) band of PB14 (Figure 1). The SIA process was optimized by
In recent years, considerable interest has been devoted to the
preparation and characterization of polynuclear transition metal
cyanides such as Prussian blue (Fe 4[Fe (CN)6]3, PB) from
viewpoints of interest in the basic aspects of their electrochemistry
and of developing devices in technologically relevant fields such as
monitoring the absorbance as a function of substrate immersion
time; 8 min and 10min of immersion times were found to be ideal
for (i) and (iii) steps, respectively. The absorbance at 710nm
increased with increasing n linearly after 3 cycles, indicating
quantitative formation and/or deposition of PB on the substrate.
Similarly, the steady increase in the PB deposition amount was
observed when the Q-(PD/PS)2/PD with cationic surface was
subjected to the SIA process which started from step (iii) followed
by steps (iv), (i) and (ii). In both cases, the increase in absorbance at
III
II
1
2
3
4
electrochromism, sensor, battery, ion-selective electrode and
5
magnetooptics. Stable films of PB analogues have been prepared
so far mainly by galvanostatic and potentiostatic electrodeposition
methods after the development by Itaya et al. There are a few
studies on chemical (non-electroplating) preparation of ultrathin
1
2
þ
710nm was observed both after the immersion into aqueous Fe
6
3À
films of PB analogues. Here we report the chemical synthesis of PB
and aqueous Fe(CN)6 except for the first immersion step. The SIA
process on a bare quartz substrate was followed by UV-Vis
spectroscopy. It was found that the quantitative and regular
deposition of PB was observed not at pH 2.0, which is close to
isoelectric point of quartz, but at pH 4.0and 0. 5. These results
clearly indicated that the either cationic or anionic surface of the
substrate is indispensable for the deposition of PB by the SIA
process not only at the adsorption of the first ionic layer but also in
the steady growth of the film. The events during the SIA process can
be postulated as follows. If the anionic substrate is first immersed
ultrathin films by the successive ion adsorption (SIA) technique.
The SIA technique consists of technologically simple processes of
alternate immersion of a substrate into a solution of one precursor
ion and then that of the other with rinsing in-between.7 The
experimental procedure of the SIA is almost the same as that of the
alternate adsorption technique, which utilizes the layer-by-layer
deposition of oppositely charged ionic species and is applied for the
{9
1
0;11
preparation of assembly films of macromolecules
2;13
and inorganic
Since the SIA technique is based on the electro-
nanoparticles.1
into aqueous Fe(NH4)2(SO4)2, Fe2 ions are electrostatically
þ
static interaction between precursor ions in the solution and on the
substrate and the successive in situ reaction, it is expected that the
film of the in situ reaction product can be grown in a well-controlled
manner under ambient conditions. The SIA technique has been so
far applied exclusively to the synthesis of metal chalcogenide thin
films, the technique for which was so-called successive ionic layer
adsorbed on the substrate. During the subsequent immersion into
3
À
aqueous K3Fe(CN)6, Fe(CN)6
ions interact and react with
3À
2þ
preadsorbed Fe to form PB. In addition, excess Fe(CN)6 ions
are deposited on the surface, which then react with Fe2 in the next
immersion. The reaction and excess adsorption repeat at each
immersion step.
þ
7
{9
adsorption and reaction.
growth of PB ultrathin film on a substrate have been achieved
In the present study, formation and
2
þ
successfully by the SIA technique using precursor ions of Fe and
Fe(CN)63 . The electrochemical properties of the PB films were
À
also investigated.
The SIA process for PB deposition was carried out by the
following steps unless stated otherwise: (i) immersion of a substrate
into 0.1 M aqueous Fe(NH4)2(SO4)2 for 8 min; (ii) rinsing with
water for 10s and drying by an N 2 blast for 30s; (iii) immersion into
0
.1 M aqueous K3Fe(CN)6 for 10min; (iv) rinsing and drying as the
same manner as (ii). The pH of aqueous solutions was adjusted at
.5–4.0 by adding a dilute aqueous H2SO4. By repeating steps (i)–
iv) n times, Prussian blue (PBn) ultrathin films were prepared at
0
(
room temperature onto quartz (Q), mica (M) and ITO substrates
with or without pre-modification of the surface by alternate
adsorption of poly(diallyldimethylammonium), PD, and poly(styr-
ene sulfate), PS. The pre-modified substrates with anionic and
cationic outermost surface layers, substrate-(PD/PS)3 and -(PD/
PS)2/PD, were prepared by immersion of well-cleaned substrates
Figure 1. UV-Vis spectra of Q-(PD/PS)3/PBn films prepared at
pH 2.5. The spectra are of the samples with odd number cycles of
the SIA up to the 19 cycles. The inset shows plots of absorbance at
710nm against n.
Copyright Ó 2002 The Chemical Society of Japan

Chemistry Letters 2002
311
Figure 2. AFM images of surfaces of (a) M/PB1 and (b) M/PB2.
The PB deposition with the SIA cycles was also confirmed by a
quantitative mass increase in quartz crystal microbalance (QCM)
measurements using 2-amino-1-ethanethiol derivertized resona-
tors. The mass increase per each SIA cycle was nearly constant at
6
50 Æ 30 ng when the concentration of the immersing solutions (aq
2þ
3À
Fe , aq Fe(CN)6 ) was 0.1 M. X-Ray diffraction (XRD)
measurement showed that the PB produced on the substrate was
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II
15
Fe 4[Fe (CN)6]3 Á xH2O that is the insoluble form of PB.
Figure 3. (a) Cyclic voltammogram (CV) of ITO/PB10 and (b)
absorbance change at 710nm in ITO/PB 10 during the CVs, in
III
II
Assuming that a dense film of Fe 4[Fe (CN)6]3 Á 14{16H2O
À3 16
À1
(
density ¼ 1:75{1:81 gÁcm
)
is formed, the average thickness of
aqueous 0.1 M KCl (pH 6.5) with a scan rate of 10 mVÁs
.
the PB films per 1 cycle of the SIA is estimated to be 11 Æ 1 nm.
quantitative PB deposition on the substrate by the SIA process. In
situ UV-Vis spectra during the CVs were also investigated. The
absorption around 710nm due to IVCT of PB was changed with the
potential sweep as shown in Figure 3b and it was reproducibly
observed up to 20multiple scans ( þ1:3 V , À0:3 V). These results
indicate that the SIA-derived PB films show a stable electro-
chromism (coloring dark blue around þ0:5 V and bleaching around
þ1:3 V and À0:3 V).
Observations by scanning electron microscopy (SEM) on the Q-
(
PD/PS)3/PB20 film showed that a continuous PB film with ca.
.2 ꢀm thickness was formed throughout the Q-(PD/PS)3 substrate
0
and that the film consisted of densely packed nanoparticulates of PB
below 50nm (the image is not shown here). The 1 cycle thickness of
PB estimated from the SEM was ca. 10nm, which is consistent well
with evaluated by the QCM. From these results, it was concluded
that the average thickness of the PB film could be controlled of the
order of nanometer level by the number of SIA cycles considering
the proportional increases in UV-Vis absorbance and mass in QCM.
The initial stage of the formation of PB on a mica substrate at the
solution conditions of 0.1 M and pH 2.5 was investigated by atomic
force microscopy (Figure 2). A lot of nanodomains with 20–30 nm
in lateral size and 2 nm in height were produced directly on the
surface of mica substrate after 1 cycle of the SIA (Figure 2a). After
the second SIA cycle (Figure 2b), the amount and size of
nanodomains increased and almost all the substrate surface was
covered with PB nanoparticulates. These results indicated that PB
nanoparticles formed at initial stage grew with repeating the SIA
cycle to yield eventually a continuous nanoparticulate film.
The adapted SIA technique is in principle applicable to
synthesis of a variety of polynuclear transition metal cyanides. In
addition, multilayer films composed of Prussian blue and organic
polyelectrolyte layers can be also prepared by coupling with
alternate adsorption technique. Therefore, the present study can
contribute to a chemistry of Prussian blue analogues as well as a
development of functional devices hybridized between Prussian
blue analogues and organic materials.
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ꢁ
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À1
7
8
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1
agreement with the reported values of the following reactions for
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9
1
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1
III
II
À
II
II
4À
Fe 4½Fe ðCNÞ Š þ 4e $ Fe 4½Fe ðCNÞ Š3
ð1Þ
ð2Þ
6
3
6
1
3
III
II
À
III
III
3þ
Fe 4½Fe ðCNÞ Š À 3e $ Fe 4½Fe ðCNÞ Š3
6
3
6
1
1
5
6
J. Hanawalt, H. Rinn, and L. Frevel, Anal. Chem., 10, 457 (1938).
H. J. Buser, D. Schwarzenach, W. Peter, and A. Ludi, Inorg. Chem., 16, 2704
(1977).
This also agrees with the XRD results that the insoluble form of PB,
III
II
Fe 4[Fe (CN)6]3 Á xH2O, was formed. In addition, the peak current
increased proportionally with n in PBn films, confirming again the