Preparation of bilayer platinum and copper hexacyanoferrate hybrid film modified electrode and its electrocatalytic properties

Article abstract of DOI:10.1149/1.2745716

Bilayer platinum and copper hexacyanoferrate hybrid films have been prepared using repetitive cyclic voltammetry (CV). The deposition process and the films' electrocatalytic properties have been investigated. The electrochemical quartz crystal microbalance and CV were used to study the deposition and growth mechanism of the copper hexacyanoferrate film and platinum/copper hexacyanoferrate hybrid films. Scanning electron microscopy was used to study the surface morphology of the film. In the platinum/copper hexacyanoferrate hybrid films, the peak potentials of the platinum component that demonstrates a proton effect in acidic aqueous solution as the redox couple attributed to copper hexacyanoferrate was found to be cation dependent but not pH dependent. The electrocatalytic reactions of NH2 OH, N2 H4, and S2 O3 2- were carried out using the platinum/copper hexacyano-ferrate hybrid films.

Full text of DOI:10.1149/1.2745716

Journal of The Electrochemical Society, 154 ͑8͒ E123-E130 ͑2007͒
E123
0013-4651/2007/154͑8͒/E123/8/$20.00 © The Electrochemical Society
Preparation of Bilayer Platinum and Copper Hexacyanoferrate
Hybrid Film Modified Electrode and Its Electrocatalytic
Properties
,z
*
Shen-Ming Chen, Sheh-Hung Li, and Soundappan Thiagarajan
Department of Chemical Engineering and Biotechnology, National Taipei University of Technology,
Taipei 106, Taiwan
Bilayer platinum and copper hexacyanoferrate hybrid films have been prepared using repetitive cyclic voltammetry ͑CV͒. The
deposition process and the films’ electrocatalytic properties have been investigated. The electrochemical quartz crystal microbal-
ance and CV were used to study the deposition and growth mechanism of the copper hexacyanoferrate film and platinum/copper
hexacyanoferrate hybrid films. Scanning electron microscopy was used to study the surface morphology of the film. In the
platinum/copper hexacyanoferrate hybrid films, the peak potentials of the platinum component that demonstrates a proton effect in
acidic aqueous solution as the redox couple attributed to copper hexacyanoferrate was found to be cation dependent but not pH
dependent. The electrocatalytic reactions of NH2OH, N2H4, and S2O23− were carried out using the platinum/copper hexacyano-
ferrate hybrid films.
© 2007 The Electrochemical Society. ͓DOI: 10.1149/1.2745716͔ All rights reserved.
Manuscript submitted October 11, 2006; revised manuscript received April 12, 2007. Available electronically June 25, 2007.
In recent years, the construction and application of modified
electrodes has received much attention with a view to the enhance-
ment of sensitivity and selectivity of electroanalytical techniques.
The use of metal hexacyanoferrate in modified electrodes has been
extensively studied.1 Metal hexacyanoferrate ͑MeHCF͒ deposited as
electrode modifiers have been extensively investigated to exploit
their electrocatalytic properties for the determination of various or-
ganic and inorganic species. For example, different MeHCFs incor-
porated into carbon paste were examined for their voltammetric re-
sponse to various cations,2,3 CoHCF film was used for the
amperometric determination of hydrogen peroxide,4 NiHCF was
employed as a biosensor component for the determination of either
ethanol5 or glucose,6 and FeHCF was used for the catalytic ampero-
metric detection of hydrazine. However, mixed metal HCF media-
tors seem not yet to have been explored for analytical purposes. The
possibility of selective detection mediated by Prussian blue was first
demonstrated by Karyakina et al.7
accepted.23 Electrocatalytic oxidation of hydroxylamine on di-
nuclear ruthenium phthalocyanine ͑RuPc͒ modified electrodes is
2
studied using cyclic voltammetry ͑CV͒ and rotating disk electrode
͑RDE͒ techniques.24 The electrochemical oxidation of hydroxy-
lamine catalyzed by 2, 2, 6, 6-tetramethyl-4-acetylpipelidine-1-oxy
͑TMAPO͒ radical at a glassy carbon ͑GC͒ electrode in phosphate
buffer solution has been studied.25
On the other hand, hydrazine is a very toxic compound that may
cause cancer and inheritable genetic damage. In an organism, it is
believed to be oxidized by peroxides into reactive intermediates ca-
pable of causing a variety of adverse side effects, including DNA
harm. However, it is widely used in large quantities by the military
and aerospace industry as a rocket propellant and as a fuel for fuel
cells, and is also employed for a diversity of applications by the
chemical, pharmaceutical, and agricultural industries.26 Recently, it
was found that its derivatives can be used as inhibitors of some
enzymes27 and HIV integrase, thus being sought as potential new
therapeutics for the treatment of AIDS.28 It is therefore obvious that
reliable and sensitive analytical methods for the determination of
hydrazine in macro as well as in microvolume samples are needed.
The electrocatalytic properties of a CuCoHCF film modified carbon
paste electrode toward the electrocatalytic oxidation of hydrazine
using various electroanalytical procedures have been reported.29 The
mechanism and kinetics of hydrazine oxidation have been studied
under a wide range of solution conditions30 and at several electrodes
including silver,31 gold,32 nickel,33 mercury, and platinum.34-37 On
the other hand, sodium thiosulfate, also called “Hypo,” an important
compound, finds application in chemical and biological fields, espe-
cially in the photographic industry. It is becoming a prime pollutant
in the above industry. Therefore, it is very important to develop an
analytical method, which must be simple, sensitive, and reliable for
the determination of sodium thiosulfate. There are few reports in the
literature regarding the electrochemical determination of thiosulfate.
One report deals with the use of GC modified with Pt particles as the
electrode material.38
It is important to study the bilayer and multilayer hybrid films in
this new era.39 There are very few reports about the bilayer hybrid
films, and there is no previous detailed study on the preparation,
characterization, and application of copper hexacyanoferrate film
modified electrodes with platinum particles which will enhance the
electrocatalytic activity of the film.40-44 Here we report the bilayer
platinum/copper hexacyanoferrate hybrid film preparation, charac-
terization, and electrocatalytic oxidation reactions of NH2OH, N2H4,
and S2O23−, respectively. The microstructure of copper hexacyano-
ferrate film and platinum/copper hexacyanoferrate hybrid films were
characterized using scanning electron microscopy ͑SEM͒ tech-
niques. The electrochemical quartz crystal microbalance ͑EQCM͒,
Beside Prussian blue, other metal hexacyanoferrate used as me-
diators in oxidase-based biosensors are copper hexacyanoferrate
11-14
͑CuHCNFe͒,8-10 cobalt hexacyanoferrate ͑CoHCNFe͒
and
nickel hexacyanoferrates ͑NiHCNFe͒.15,16 Among the analogues of
Prussian blue, copper hexacyanoferrate shows a well-defined, re-
versible, and reproducible electrochemical response in supporting
electrolytes containing not only potassium but also other alkali
metal cations such as lithium, sodium, rubidium, or cesium, contrary
to Prussian blue which only shows good electroactivity in electro-
lytes containing K+.
On the other hand, the metal hexachloroplatinate film and metal
hexabromoplatinate film deposition process involves metal ion and
͑PtIVCl26−͒ ͑or͒ ͑PtIVBr26−͒ in an aqueous KCl ͑or͒ KBr solution.17,18
The cerium hexachloroplatinate films were electrocatalytically re-
duced to NAD+, L-cystine, SO52−, and S4O26− through the redox
couple of cerium hexachloroplatinate species.19 The iron hexachlo-
roplatinate films were electrocatalytically reduced NAD+, and O2
through the redox couple of iron hexachloroplatinate species.20 The
preparation, characterization, and electrocatalysis of tin hexachloro-
platinate films was also studied.21 On the other hand, hydroxylamine
͑HAM͒, a key intermediate in e-caprolactam production and a com-
pound with various applications in technology, is obtained by the
liquid-phase catalytic hydrogenation of nitric oxide ͑NO͒ or nitrate
at carbon-supported platinum or palladium catalysts.22 The rel-
evance of the electrochemical studies on similar and model systems
for understanding the processes practiced in industry is well
*
Electrochemical Society Active Member.
z E-mail: smchen78@ms15.hinet.net
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Journal of The Electrochemical Society, 154 ͑8͒ E123-E130 ͑2007͒
CV, and SEM were used to study the deposition and growth mecha-
nism of the copper hexacyanoferrate film and platinum/copper
hexacyanoferrate hybrid films. The electrochemical and electrocata-
lytic reaction of NH2OH was investigated using the flow injection
analysis ͑FIA͒ method.
Experimental
The electrochemistry was performed using CH Instruments
model CHI-400 and CHI-750 potentiostats. Cyclic voltammetry was
conducted using a three-electrode cell in which a BAS glassy carbon
͑GC͒ electrode, a gold electrode, a platinum electrode, and an in-
dium tin oxide ͑ITO͒ electrode ͑composed of indium tin oxide that
was sputtered on a glass substrate͒ were used as the working elec-
trodes. The GC electrode was polished with 0.05 ␮m alumina on
Buehler felt pads, and then ultrasonically cleaned for 1 min. The
auxiliary compartment contained a platinum wire, which was sepa-
rated by a medium-sized glass frit. All of the cell potentials were
taken using an Ag͉AgCl͉KCl ͑saturated solution͒ reference elec-
trode. The aqueous solutions were prepared using doubly distilled
deionized water and the solutions were deoxygenated by purging
with prepurified nitrogen gas. All of the chemicals used were of
analytical grade. The potassium hexachloroplatinate, copper hexacy-
anoferrate was purchased from Aldrich. The working electrode for
the EQCM measurements was an 8 MHz AT-cut quartz crystal with
gold electrodes. The diameter of the quartz crystal was 13.7 mm and
the gold electrode diameter was 5 mm.
Amperometry and rotating disk electrochemical experiments
͑RDE͒ were performed with a Pine Instruments electrode in con-
junction with a CH Instruments CHI-750 potentiostat connected to a
model AFMSRX analytical rotator.
The flow injection analysis ͑FIA͒ system mainly consisted of a
peristaltic pump, an injection valve, and a flow cell. The peristaltic
pump was used to deliver the buffer carrier stream. The injection
valve was equipped with an injection loop of 25 ␮L for delivery of
the solutions containing the analyte. The electrolyte buffer was de-
aereated continuously for 1 h. All FIA studies were done using
0.1 M KCl at pH 5 with a buffer aqueous solution as the carrier
stream. The SEM images and microstructure of the films were ob-
served using a Hitachi S-3000H SEM.
Figure 1. ͑A͒ Repetitive CVs of platinum particles synthesized at glassy
carbon electrode from 8 ϫ 10−4 M K2PtCl6 in a 0.1 M KNO3 at pH 2. Scan-
ning potential between 1.1 and −0.3 V. ͑B͒ Repetitive cyclic voltammo-
grams of platinum/copper͑II͒ hexacyanoferrate hybrid film growth ͑two
steps͒ in aqueous 0.001 M CuCl2 and 0.003 M K3Fe͑CN͒6, 0.1 M KCl at
pH 5 on a Pt modified GC electrode ͑Fig. 1A͒. Scanning potential range
between 0.9 and 0.4 V ͑vs Ag͉AgCl͒. Scan rate: 0.1 V/s.
Results and Discussion
The bilayer platinum and copper hexacyanoferrate hybrid films
modified electrodes.— The bilayer platinum and copper hexacyano-
ferrate hybrid films were deposited onto the GC surface to form an
electroactive film directly from a pH 2.0/pH 5.0 aqueous solution
3−
containing PtCl26− and Cu2+, Fe͑CN͒ ions. The film was prepared
6
in two steps using the CV method. Platinum particles were first
deposited onto a GC electrode from a pH 2.0 acidic aqueous solu-
tion containing PtCl26− ions ͑35 cycles͒. Figure 1A shows repetitive
CVs of a GC electrode in 8 ϫ 10−4 M PtCl62− in a 0.1 M aqueous
KNO3 solution at pH 2.0 in the scanning potential region between
1.1 and −0.3 V ͑35 cycles͒. Upon continuous cycling, the peak cur-
rents corresponding to the two redox couples ͑hydrogen adsorption/
desorption͒ at about −0.25 and −0.15 V were found to have in-
creased. This behavior indicates that deposition of platinum
occurred during cycling. In this electrochemical reaction, the
PtIICl46− was reduced to Pt on GC surface.
Electrochemical properties of the bilayer platinum/copper
hexacyanoferrate hybrid film.— The electrochemical behavior of
the platinum/copper hexacyanoferrate hybrid films were recorded
after transferring the electrode to the 0.1 M KCl aqueous solution at
pH 5.0, as shown in Fig. 2. The platinum particles showed two
redox couples at about −0.59 and −0.7 V, attributed to hydrogen
atom adsorption and desorption ͑Fig. 2͒, whereas the copper hexacy-
anoferrate showed one redox couple at about +0.7 V attributed to
͓Cu͑II͒-NC-Fe͑II͔͒/͓Cu͑II͒-NC-Fe͑III͔͒.
The effect of scan rate on the electrochemical behavior of
platinum/copper hexacyanoferrate hybrid films in 0.1 M KCl pH 5.0
aqueous solution is shown in Fig. 2. The inset of Fig. 2 explains the
plots of Ipa and Ipc ͑the formal potential at about +0.7 V͒ vs the scan
rate that illustrates a close linear dependence of Ipa and Ipc on the
scan rate. This behavior was found to be consistent with a thin-layer
reversible electron transfer process for different scan rates between
10 and 200 mV/s. This behavior is also consistent with a diffusion-
less electron transfer process, and the peak current and scan rate
were related as follows45,46
Figure 1B shows repetitive CVs of copper hexacyanoferrate de-
posited onto platinum particles modified GC electrode in 1
ϫ 10−3 M CuCl2 and 3 ϫ 10−3 M Fe͑CN͒3−, 0.1 M KCl at pH 5 in
6
the scanning potential range between 0.9 and 0.4 V for 35 cycles.
Upon continuous cycling, the peak currents corresponding to the
copper hexacyanoferrate redox couple at about +0.7 V was found to
have increased. This behavior indicates that the deposition of copper
hexacyanoferrate on the platinum particle coated electrode surface
during cycling.
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Journal of The Electrochemical Society, 154 ͑8͒ E123-E130 ͑2007͒
E125
Figure 2. CVs of a platinum/copper͑II͒ hexacyanoferrate hybrid film in
0.1 M KCl pH 5 buffer aqueous solution. Scan rate ͑a͒ 0.01, ͑b͒ 0.02, ͑c͒
0.03, ͑d͒ 0.045, ͑e͒ 0.06, ͑f͒ 0.08, ͑g͒ 0.10, ͑h͒ 0.12, ͑i͒ 0.14, ͑j͒ 0.16, ͑k͒
0.18, and ͑l͒ 0.20 V/s. Inset: a plot of the peak currents Ipc and Ipa vs scan
rate at Epa = 0.8, Epc = 0.6, respectively.
I = n2F2 A⌫ /4RT
͓1͔
where, ⌫ , , A, and I represents the surface coverage concentra-
v
p
o
v
o
p
tion, the scan rate, the electrode area, and the peak current, respec-
tively.
Figure 3. ͑A͒ Repetitive CVs of Pt particles synthesized on a gold electrode
from 8 ϫ 10−4 M K2PtCl6, 0.1 M KNO3 at pH 2. Electrodeϭgold. Scan
rateϭ0.02 V/s. Inset C: ͑a͒ A plot of the cathode peak current vs scan cycle,
͑b͒ every cycle of the cathode peak current change vs scan cycle. Inset D: ͑a͒
The total frequency change ⌬f vs the scan cycle and ͑b͒ the every cycle
frequency change ⌬f vs the scan cycle. ͑B͒ The change in EQCM frequency
recorded concurrently with the consecutive CVs between 1.1 and −0.32 of
͑A͒.
The bilayer platinum/copper hexacyanoferrate hybrid film depo-
sition using the EQCM method.— The electrochemical formation
of platinum/copper hexacyanoferrate hybrid films involved platinum
particles deposition followed by copper hexacyanoferrate film depo-
sition.
Figure 3A and 3B show the change in the EQCM frequency
resulting from the deposition of the platinum film. The platinum film
was obtained from 8 ϫ 10−4 M K2PtCl6, 0.1 M KNO3 at pH 2 so-
lutions. Figure 3B indicates the change in the EQCM frequency
recorded during the first 20 segments of the repeated CV. During
scanning of the first segment, the deposition of platinum particles
takes place at around 0.19 V. The voltammetric peak current in Fig.
3A and the frequency decrease ͑or mass increase͒ in Fig. 3B re-
mained consistent with the growth of a platinum deposition on the
gold electrode surface.
a frequency change of about 50 Hz ͑70 ng͒ for a 0.196 cm2 gold
electrode was observed at −0.8 and +0.2 V. The EQCM results in
Fig. 4B also show that an decrease in frequency ͑or a increase in
mass͒ occurred during the more negative potential and an increase in
frequency ͑or a decrease in mass͒ occurred during the negative scan
of platinum/copper hexacyanoferrate hybrid film between 0.2 and
+0.8 V because of the dedoping of K+ ions. However, the reverse
segment again showed that a decrease in frequency ͑or an increase
in mass͒ occurred during the more negative potential of platinum/
copper hexacyanoferrate hybrid film between 0.75 and 0.2 V be-
cause of the doping of K+ ions at this potential due to reduction
͓Cu͑II͒-NC-Fe͑III͔͒/͓Cu͑II͒-NC-Fe͑II͔͒. Here, the K+ ions are
doped to attain the electrochemical neutrality during reduction.
Figure 5 shows CVs of a platinum/copper hexacyanoferrate hy-
brid film in various pH aqueous solutions. Figure 5A-5C shows CVs
of the hybrid film at pH 5, 2.5, 1, respectively. The results show that
the formal potential of platinum depends on the pH, and that the
formal potential of bilayer copper hexacyanoferrate film ͑formal po-
tential at about +0.75 V͒ was independent of the pH. The redox
couples of the platinum also exhibited involvement of proton trans-
fer in the redox couple of the hybrid film, but the bilayer copper
hexacyanoferrate was almost independent of the pH. Hence,
platinum/copper hexacyanoferrate hybrid film were prepared with
two redox peak potentials that is one with pH dependent property
͑platinum͒ and another is pH independent property ͑bilayer copper
hexacyanoferrate͒.
In the EQCM experiments, the change in mass at the quartz
crystal was calculated from the change in the observed frequency
using the Sauerbrey equation
Mass change ͑⌬m͒ = ͑− 1/2͒͑f−o2͒͑⌬f͒A͑k␳͒
͓2͔
1/2
where A is the area of the gold disk coated onto the quartz crystal, ␳
is the density of the crystal, k is the shear modulus of the crystal, ⌬f
is the measured frequency change, and fo is the oscillation fre-
quency of the crystal. A frequency change of 1 Hz is equivalent to a
1.4 ng change in mass. In the electrochemical reduction reaction of
PtIICl46−, Pt was deposited on the gold electrode surface. During the
first CV scan, about 64 ng/cm2 of platinum was deposited on the
fresh gold electrode, and a total of about 607 ng/cm2 of a platinum
film was deposited on the gold electrode after the first 10 CV scans.
Here, Fig. 4A ͑a and b͒ shows the CV and EQCM results during the
deposition of copper hexacyanoferrate film.
Figure 4B shows the EQCM results of a bilayer platinum/copper
hexacyanoferrate hybrid films that was deposited on the gold elec-
trode. The EQCM results in Fig. 4B show that a decrease in fre-
quency ͑or an increase in mass͒ occurred during the formation of
Pt-O between −0.8 and +0.2 V in the forward scan. During the CV,
Figure 6 shows the CVs of a bilayer platinum/copper͑II͒ hexacy-
anoferrate film on a GC electrode, which was transferred to various
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Journal of The Electrochemical Society, 154 ͑8͒ E123-E130 ͑2007͒
Figure 4. ͑A͒ ͑a͒ Repetitive CVs of copper hexacyanoferrate hybrid film
synthesize on gold electrode from 0.001 M CuCl2 and 0.003 M
a
K3Fe͑CN͒6, 0.1 M KCl at pH 5. Electrode: Pt modified gold electrode ͑Fig.
3A͒. Scan rate: 0.02 V/s. Inset C ͑a͒ A plot of the cathode peak current vs
scan cycle, ͑b͒ every cycle of the cathode peak current change vs scan cycle.
D ͑a͒ The total frequency change ⌬f vs the scan cycle and ͑b͒ the every cycle
frequency change ⌬f vs the scan cycle. A͑b͒: The change in EQCM fre-
quency recorded concurrently with the consecutive CVs between 0.9 and 0.4
of Fig. 4A ͑a͒. ͑B͒͑a͒ CVs of platinum/copper hexacyanoferrate hybrid film
growth on a gold disk electrode in a 0.1 M KCl, pH 5.0 aqueous solution and
͑B͒͑b͒ the micro gravimetric cyclic voltammetric response of a platinum/
copper hexacyanoferrate hybrid film growth on a gold disk electrode in
0.1 M KCl, pH 5.0 aqueous solution. Scan rateϭ0.02 V/s.
͑0.1 M͒ metal chloride aqueous solutions such as LiCl, NaCl, KCl,
and RbCl solutions at pH 5.0. The voltammetric characteristics and
the formal potential ͑Eo ͒ of the copper ͑II͒ hexacyanoferrate redox
Ј
couple was dependent on the ionic radius of the Li+, Na+, K+, Rb+,
and Cs+ cations, and showed a positive shift with increasing ionic
radii of the cation, whereas the platinum peak potentials were almost
independent of the ionic radius of the cations at the above condi-
tions.
Figure 5. CVs of the platinum/copper hexacyanoferrate hybrid film in vari-
ous buffer solutions of different pH: ͑A͒ 5 ͑B͒ 2.5, and ͑C͒ ͑a͒1 ͑b͒ 2. Scan
rate: 0.1 V/s.
Surface characterization.— The microstructure of copper hexa-
cyanoferrate film and bilayer platinum/copper hexacyanoferrate hy-
brid films were characterized using SEM. Figure 7A shows the SEM
morphology of copper hexacyanoferrate film. The morphology
shows a copper hexacyanoferrate thin film with structure of scat-
tered pores in it. The SEM in Fig. 7B illustrates the bilayer
platinum/copper hexacyanoferrate film in which the copper hexacy-
anoferrate film were deposited on the spherical shapes of the pt
particles at the GC electrode. Thus, an increase in surface area was
favored by the presence of Pt particles with particles radii vary from
200 to 850 nm.
increase in the cathodic peaks between −0.2 and −0.6 V shows the
proton adsorption and reduction on platinum. This phenomenon is
due to electrochemical oxidation of NH2OH to N2O at 0.9 V fol-
lowed through the ͓Cu͑II͒-NC-Fe͑II͔͒/͓Cu͑II͒-NC-Fe͑III͔͒ redox
couple. The cathodic peak current increasing phenomenon due to
electrochemical reduction of N2O between −0.2 and −0.6 V is fol-
lowed by a chemical interaction between Pt and N2O. According to
this chemical interaction, the surface coverage of Pt particles could
have been increased and thereby increased hydrogen adsorption cur-
rent.
Electrocatalytic oxidation reaction of NH2OH at platinum/
copper hexacyanoferrate hybrid film modified electrode.— The
CVs of NH2OH oxidation obtained by using the above bilayer
platinum/copper hexacyanoferrate hybrid film modified electrode
are shown in Fig. 8A. Here, the NH2OH oxidation was carried out in
0.1 M KCl electrolyte solution ͑pH 5͒. Furthermore, the addition of
NH2OH shows the linear increase in anodic peak currents at 0.9 V,
which indicates the oxidation process of NH2OH. Meanwhile, the
͓Cu͑II͒-NC-Fe͑II͔͒ ↔ ͓Cu͑II͒-NC-Fe͑III͔͒ + e−
͓3͔
͓Cu͑II͒-NC-Fe͑III͔͒ + 2NH2OH + 4KCl
→ ͓Cu͑II͒-NC-Fe͑II͔͒ + N2O + H2O + 4HCl
The peak current corresponding to PtO reduction at around
−0.3 V increased with increase of N2O concentrations. The oxide
͓4͔
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Journal of The Electrochemical Society, 154 ͑8͒ E123-E130 ͑2007͒
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Figure 6. CVs of a glassy carbon electrode modified with a platinum/copper
hexacyanoferrate hybrid film in 0.1 M with various cation chloride solutions
͑pH 5.0͒: ͑a͒ Li+, ͑b͒ Na+, ͑c͒ K+, ͑d͒ Rb+, and ͑e͒ Cs+. Scan rate: 0.1 V/s.
layer was formed on the metal surface by N2O reduction followed
by the electrochemical reduction of the oxide layers47-50
N2O + Pt → N2 + PtO
͓5͔
PtO + 2H+ + 2e− → Pt + nH2O
͓6͔
On the reverse scan, a sharp reduction peak of nitrous oxide was
observed at −0.6 V, although the magnitude was smaller than for
Figure 8. ͑A͒ CVs of platinum/copper hexacyanoferrate hybrid film on
glassy carbon electrode in 0.1 M KCl at pH 5 with various concentrations of:
͓NH2OH͔ ϭ ͑a͒ 0.0, ͑b͒ 5 ϫ 10−4, ͑c͒ 1 ϫ 10−3, and ͑d͒ 1.5 ϫ 10−3 M. ͑a ͒
Ј
bare glassy electrode and ͓NH2OH͔ = 1.5 ϫ 10−3 M. ͑B͒ FIA responses of
platinum/copper hexacyanoferrate hybrid film additions various concentra-
tions of ͓NH2OH͔ = ͑a͒2 ϫ 10−8, ͑b͒ 2 ϫ 10−7, and ͑c͒ 2 ϫ 10−6 M at
0.9 V. Inset: a plot of the currents vs concentrations.
the negative scan. This phenomenon could be due to reduction of
nitrous oxide by adsorbed hydrogen present on the Pt surface. Here,
the synergistic effect of the platinum/copper hexacyanoferrate hy-
brid film increases the electrocatalytic reaction of hydrazine
N2O
+ H
→ N2͑g͒ + OH
͓7͔
͑ads͒
͑ads͒
͑ads͒
FIA results.— Figure 8B shows a typical FIA curve of the elec-
trocatalytic oxidation reaction of NH2OH at a bilayer platinum/
copper hexacyanoferrate hybrid film modified electrode obtained by
method FIA. Standard mixtures containing NH2OH were measured
at the flow rate of 1.0 mL min−1 using the FIA system. As expected
for the mediated catalysis, the responses were linear at the low sub-
strate such as 2 ϫ 10−8, 2 ϫ 10−7, and 2 ϫ 10−6 M of NH2OH at
0.9 V. FIA responses increased rapidly just after injection of the
sample and returned to baseline within about 5.0 s, as shown in ͑Fig.
8B͒. The sensitivity is normally defined as the slope of the calibra-
tion curve, which is estimated as the change in relative catalytic
current induced by a change in NH2OH concentrations.
Figure 7. SEM micrographs of the ITO electrode surface modified with ͑A͒
copper hexacyanoferrate film and ͑B͒ platinum/copper hexacyanoferrate hy-
brid film.
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Journal of The Electrochemical Society, 154 ͑8͒ E123-E130 ͑2007͒
brid films in pH 1.5 aqueous solutions were active, whereas the
electrocatalytic current peak potential exhibited at about +0.25 V
for N2H4 oxidation ͑see Fig. 9A͒. However, on bare GC surface no
electrochemical oxidation was observed for N2H4 molecules.
The above results indicate that the catalysis of platinum/copper
hexacyanoferrate hybrid films for N2H4 must be due to the presence
of Pt particles, which increases the active surface area of the elec-
trode surface.
A typical amperometric, i-t, experiment of N2H4 oxidation was
performed in a well-stirred solution ͑rotation speed 900 rpm͒ by
keeping the platinum/copper hexacyanoferrate hybrid film in at
0.3 V ͑vs Ag͉AgCl͒ modified disk GC electrode potential at 0.3 V
in 0.1 M KNO3 at pH 2 aqueous solution ͑Fig. 9B͒. In the ampero-
metric experiment, a good response was obtained for the nine se-
quential additions of each 1.5 ϫ 10−4 M N2H4 on the platinum/
copper hexacyanoferrate hybrid film modified GC disk electrode. It
was observed that the oxidation current of N2H4 increased with the
addition of N2H4 ͑the disk electrode͒ and reached steady state within
a short time. The disk electrode current responses showed nearly
linear relationship with the concentration of N2H4 over the range of
1.5 ϫ 10−4 M. The electrochemical oxidation of N2H4 on the bare
GC disk electrode was carried out in the above-mentioned condi-
tions and compared with the above results. The oxidation current of
N2H4 on the bare disk GC electrode showed no obvious result.
Finally, the active centers ͑platinum particles͒ with copper hexacy-
anoferrate show obvious catalytic effect and this bilayer platinum/
copper hexacyanoferrate hybrid film modified electrode could be
used for accurate measurement of N2H4.
The CVs of S2O23− oxidation at bilayer platinum/copper hexacy-
anoferrate hybrid film modified electrode are shown in Fig. 10A.
The oxidation process of S2O23− at a bilayer platinum/copper
hexacyanoferrate hybrid film modified electrode was carried out in
0.1 M KCl electrolyte solution ͑pH 5͒. Upon the addition of S2O32−
,
an anodic peak current appeared at around 0.9 V, which corresponds
to the oxidation peak of S2O2− and the cathodic peak located at
−0.25 V shows the proton ads3orption/desorption on platinum. This
phenomenon may be due to electrochemical oxidation of S2O32−
to S4O26− at 0.9 V followed by mediated oxidation of
͓Cu͑II͒-NC-Fe͑II͔͒/͓Cu͑II͒-NC-Fe͑III͔͒ redox couple. Further-
more, the increasing cathodic peak current at −0.25 V phenomenon
may be due to electrochemical reduction of the oxidized product of
S2O23− ͑0.9 V͒ through proton adsorption/desorption peak on plati-
num. The scheme represents the oxidation of S2O23− at bilayer
platinum/copper hexacyanoferrate hybrid film
Figure 9. ͑A͒ CVs of a platinum/copper hexacyanoferrate hybrid film on
glassy carbon electrode in 0.1 M KNO3 at pH 1.5 with various concentra-
tions of: ͑A͒ ͓N2H4͔ = ͑a͒ 0.0, ͑b͒ 1.5 ϫ 10−2, ͑c͒ 3 ϫ 10−2, ͑d͒ 4.5
ϫ 10−2, and ͑e͒ 6 ϫ 10−2 M. ͑a ͒ bare glassy electrode and ͓N H ͔ = 6
Ј
2
4
ϫ 10−4 M. ͑B͒ Amperometric responses of 9 sequential additions of N2H4
͑each 1.5 ϫ 10−4 M͒ at ͑a͒ bare GC ͑b͒ platinum/copper hexacyanoferrate
hybrid film in 0.1 M KNO3 at pH 2 at 0.3 V ͑vs Ag͉AgCl͒. Rotating rate:
900 rpm. Inset: Relationship between oxidation current vs ͓N2H4͔.
Here, the platinum particles function as active centers on the
hexacyanoferrate film, which further increases the catalytic activity
of the film. From these results, it was concluded that the synergistic
effect of bilayer platinum/copper hexacyanoferrate shows a very
good response compared with copper hexacyanoferrate film.51 The
sensitivity of the proposed FIA in the linear range was determined to
be 0.8 ␮A/log nM.
.
The rotating disk electrode technique was employed to study the
“D” of the S2O23− oxidation in above mentioned experimental con-
ditions. Figure 10B shows the RDE voltammograms of the S2O32−
oxidation at the platinum/copper hexacyanoferrate hybrid film modi-
fied electrode at various rotation rates in the 0.1 M KCl at pH 5. In
the case of the rotating disk electrode experiment, the analysis was
based on the dependence of the limiting current to the rotation rate.
The diffusion coefficient “D” was calculated using the Levich plot
obtained through the RDE experiment in the above condition. The
Levich equation is given below
Electrocatalytic oxidation of N2H4 and S2O2Ϫ by platinum/
copper hexacyanoferrate hybrid film.— The electr3ocatalytic oxida-
tion of N2H4 by the bilayer platinum/copper hexacyanoferrate hy-
brid films in acidic aqueous solutions was also performed. The re-
sults of the electrocatalytic CVs demonstrate that the electrocatalytic
oxidation of N2H4 by bilayer platinum/copper hexacyanoferrate hy-
Downloaded on 2014-12-09 to IP 169.230.243.252 address. Redistribution subject to ECS terms of use (see ecsdl.org/site/terms_use) unless CC License in place (see abstract).

Journal of The Electrochemical Society, 154 ͑8͒ E123-E130 ͑2007͒
E129
of platinum/copper hexacyanoferrate hybrid films. The platinum/
copper hexacyanoferrate hybrid film was electrocatalytic oxidation
active for NH2OH, N2H4, and S2O2− in acidic aqueous solution. The
electrocatalytic oxidation current3 develops through the oxidized
form of the copper hexacyanoferrate redox couple. Here, the depos-
ited platinum particle increased the active surface area of the elec-
trode. Moreover platinum particles act as active centers onto the
copper hexacyanoferrate film. Therefore, the synergistic effect of
platinum/copper hexacyanoferrate bilayer film shows a more obvi-
ous catalytic effect than do monolayer films. These films can be
used for the electrocatalytic reduction of the oxidized products of
NH2OH and S2O23− that are produced by the electrocatalytic oxida-
tion of the same electrode by the presence of platinum particles in
the hybrid films. The electrocatalytic reaction for NH2OH was in-
vestigated using the FIA and amperometric methods. The rotating
disk electrode technique was employed to study the “D” of the
S2O2− oxidation in the above mentioned experimental conditions.
The3 diffusion coefficient “D” was found to be 1.871
ϫ 10−7 cm2 s−1
.
Acknowledgment
This work was supported by the National Science Council of the
Taiwan ͑R.O.C͒.
National Taipei University of Technology assisted in meeting the publi-
cation costs of this article.
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