Sol-gel-derived carbon ceramic composite electrodes bulk-modified with 1:12-silicomolybdic acid

Article abstract of DOI:10.1039/b002067m

1:12-Silicomolybdic acid (SiMo₁₂) doped carbon ceramic composite electrodes were fabricated by incorporating SiMo₁₂ and graphite powder in a methyltrimethoxysilane-based gel and characterized by cyclic and square-wave voltammetry. It was demonstrated that the chemically modified electrodes were suitable for electrocatalytic reduction of bromate. The electrodes had the remarkable advantage of surface renewal owing to bulk modification, as well as simple preparation, good mechanical and chemical stability and reproducibility.

Full text of DOI:10.1039/b002067m

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Sol–gel-derived carbon ceramic composite electrodes bulk-modiÐed
with 1 : 12-silicomolybdic acid
Peng Wang, Xiangping Wang and Guoyi Zhu*
L aboratory of Electroanalytical Chemistry, Changchun Institute of Applied Chemistry, Chinese
Academy of Sciences, Changchun 130022, P. R. China. E-mail: zhuguoyi=ns.ciac.jl.cn
L e t t e r
Received (in Montpellier, France) 10th March 2000, Accepted 4th May 2000
Published on the Web 14th June 2000
1
: 12-Silicomolybdic acid (SiMo ) doped carbon ceramic com-
Fig. 1 shows the comparative cyclic voltammograms for a
1
2
posite electrodes were fabricated by incorporating SiMo and
glassy carbon electrode, a CCE in 0.5 M H SO ] 0.1 M
1
2
2
4
graphite powder in a methyltrimethoxysilane-based gel and
characterized by cyclic and square-wave voltammetry. It was
demonstrated that the chemically modiÐed electrodes were suit-
able for electrocatalytic reduction of bromate. The electrodes
had the remarkable advantage of surface renewal owing to bulk
modiÐcation, as well as simple preparation, good mechanical
and chemical stability and reproducibility.
Na SO ] 1 mM SiMo aqueous solution and a SiMo -
2
4
12
12
doped CCE in 0.5 M H SO ] 0.1 M Na SO aqueous solu-
2
4
2
4
tion. The pairs of redox peaks I-I@, II-II@ and III-III@ corre-
spond to reduction and oxidation, respectively, each through
a two-electron process.2f,g Cyclic voltammetry reveals that the
modiÐed CCE is better for SiMo reduction than the glassy
1
2
carbon electrode and unmodiÐed CCE as regards the onset
potential. The surface coverage was measured by intergrating
the area under peak I of Fig. 1 (curve c) and was found to be
2.0 ] 10~10 mol cm~2.
In order to study the pH-dependent electrochemical behav-
ior of the SiMo -doped CCE, square-wave voltammetry with
The high capacity of heteropolyanions (HPAs) to accumulate
electrons in their framework and deliver them to an appropri-
ate substrate prompted investigations on their voltammetric
and electrocatalytic behavior in the last decades.1 Many
methods have been exploited to attach HPAs onto electrode
surfaces, such as electrodeposition, adsorption, encapsulation
in organic and inorganic polymeric matrices, layer-by-layer
deposition, self-assembly and the LB procedure.1h3 A serious
drawback in the application of these thin Ðlm modiÐed elec-
trodes is their poor long-term stability; moreover, electrode
surfaces cannot be renewed in the case of leakage, contami-
nation and passivation.1h3 It seems desirable to develop and
exploit a new, simple and reliable procedure for preparing
HPA-modiÐed electrodes.
1
2
an excellent sensitivity was adopted to accurately measure the
half-wave potentials (E ). Fig. 2 shows the Osteryoung
1
@2
square-wave voltammograms for the SiMo -doped CCE in
1
2
acidic aqueous solutions with di†erent pH. It can be clearly
seen that with increasing pH, the three redox potentials all
gradually shift in the more negative potential direction and
the peak currents also decrease. Reduction of SiMo immo-
bilized in the CCE matrix is accompanied by the uptake of
protons from the solution to the wetting section of the elec-
trode to maintain charge neutrality. The shift of E and the
1
2
1
@2
SolÈgel-derived hybrid organic-inorganic materials are
receiving much attention since they can exhibit the properties
associated with both the organic moieties and the inorganic
framework.4 Since the carbon ceramic composite electrode
change of peak current can be explained by the Nernst equa-
tion and FickÏs Ðrst law, respectively. As shown in Fig. 3, plots
of E
of the three successive redox waves vs. pH for the
1
@2
SiMo -doped CCE present good linearity in the pH range
1
2
(
CCE) was Ðrst prepared by LevÏs group,5 a great amount of
from 0.00 to 3.53. Slopes in this pH range are [59.6, [66.7
and [64.5 mV pH~1 for the I-I@, II-II@ and III-III@ couples,
respectively, which are close to the theoretical value of [59
mV pH~1 for the 2e~/2H` redox process. As a matter of fact,
analogous behavior in SiMo -modiÐed thin Ðlm electrodes
work has been devoted to the fabrication of chemically modi-
Ðed CCEs and to their use as sensors for metal ions, glucose,
amino acids and other important chemical and biological sub-
stances.6 An interesting feature of CCEs is that the active
section of the electrodes is not clogged upon repeated pol-
ishing, due to the brittleness of the solÈgel silicate backbone,
and thus the electrodes can be renewed by a mechanical
polish after every use or contamination. The present work
reports on the fabrication, voltammetric and electrocatalytic
1
2
behavior of the Ðrst HPA-modiÐed CCE,
silicomolybdic acid (SiMo ) doped CCE.
a
1 : 12-
1
2
UnmodiÐed CCEs were prepared according to the pro-
cedure described by Lev and co-workers5,6ahe, which was also
used to fabricate SiMo -doped CCEs with some modiÐ-
1
2
cations. Surfaces of unmodiÐed and SiMo -doped CCEs were
1
2
both mirror-like and homogeneous. A drop of water deposited
on the surfaces did not spread, indicating their apparent
hydrophobic nature.
As is known, SiMo
is unstable in neutral and basic
12
aqueous solutions and undergoes a series of hydrolysis pro-
cesses, but is fairly stable in acidic aqueous solution.1 So the
electrochemical experiments were all performed in acidic
aqueous solutions.
Fig. 1 Comparative cyclic voltammograms of (a) a glassy carbon
electrode, (b) a CCE in 0.5 M H SO ] 0.1 M Na SO ] 1 mM
2
4
2
4
SiMo and (c) a SiMo -doped CCE in 0.5 M H SO ] 0.1 M
12
12
2
4
Na SO aqueous solution. Scan rate is 250 mV s~1.
2
4
DOI: 10.1039/b002067m
New J. Chem., 2000, 24, 481È483
481
This journal is ( The Royal Society of Chemistry and the Centre National de la Recherche ScientiÐque 2000

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the modiÐed electrodes also show good electrocatalytic activ-
ity for reduction of iodate, nitrite and nitride oxide.
The hydrophobic methyltrimethoxysilane monomer results
in a controlled wetting of the composite electrode in aqueous
solutions. Hence, a bulk-modiÐed electrode can be polished
using emery paper and a fresh surface exposed whenever
needed. This is especially useful for electrocatalytic studies
since the catalytic activity is known to decrease when the elec-
-
trode is fouled. Indeed, ten successive polishings of a SiMo1
2
doped CCE resulted in a relative standard deviation (RSD) of
3.7% for the peak currents measured by square-wave voltam-
metry. In addition, little leakage was found when the electrode
was immersed in 0.5 M H SO aqueous solution for 10 days.
2
4
We think that the high stability of the SiMo -doped CCEs is
12
related to the stability of the silicate matrix, the limited wetted
section controlled by the methyl group, the strong adsorption
Fig. 2 Osteryoung square-wave voltammograms of a SiMo -doped
1
2
CCE in aqueous solutions (H SO ] 0.1 M Na SO ) with pH of 0.00,
2
4
2
4
0
.54, 1.00, 1.51, 1.99, 2.50, 3.00 and 3.53. Scan rate is 100 mV s~1.
of SiMo
on graphite (due to the large neutral cage
12
(
Mo O ),7 the aromatic properties of SiMo and the conju-
n 3n
12
gated Ti-bonds of graphite), and the possible interactions
has been described and discussed previously.2g It is evident
that the modiÐed CCE can provide a very favorable environ-
to perform electron and proton transfers.
between the SiMo and silanol groups.
In conclusion, we have demonstrated a practical and
straightforward method for the construction of HPA bulk-
modiÐed electrodes, which are exempliÐed here by the
12
ment for SiMo1
2
The reduction of BrO ~ is totally irreversible at a glassy
3
carbon electrode or an unmodiÐed CCE in acidic aqueous
SiMo -doped CCEs and which promise to compete well with
solution. In the present work, we found that the SiMo
doped CCE showed catalytic activity toward the reduction
-
12
traditional HPA-modiÐed thin Ðlm electrodes. The SiMo
-
1
2
12
doped CCEs exhibit reversible voltammetric behavior, high
activity for electrochemical reduction, good chemical and
mechanical stability, good reproducibility and the distinct
advantage of surface renewal by polishing in the event of
surface fouling. In addition, the concept described can be
extended to fabricate HPA-modiÐed large-scale electrodes for
electrolysis and microelectrodes for electroanalysis.
of BrO ~. The bromate is mainly reduced by the four-
3
electron- (H SiMoVIMoVO
)
and six-electron-reduced
4
8
4 40
(
H SiMoVIMoVO ) species in the presence of protons to yield
4
6
6 40
the reduced species containing bromine, and the two-electron-
reduced species is regenerated. It can also be seen from Fig. 4
that the six-electron-reduced species has a higher catalytic
activity toward bromate than the four-electron-reduced
species. So the catalytic activity of SiMo toward bromate
1
2
increases with the extent that SiMo is reduced. In addition,
Experimental
1
2
Reagents
Methyltrimethoxysilane (MTMOS, 97%) was purchased from
ACROS Chemical Company and used without further puri-
Ðcation. High purity graphite powder was a product of
Shanghai Carbon Plant. Pure SiMo was obtained from
1
2
Beijing Xinhua Chemical Plant, and water content in
H SiMo
O
É 28H O was determined by thermogravimetric
4
12 40
2
analysis. Ultrapure water obtained from a Millipore Milli-Q
water puriÐcation system was used throughout the experi-
ments.
Physical measurements
Electrochemical studies were carried out with a computer-
controlled CHI 660 Electrochemical Workstation interfaced
to a conventional three-electrode cell. The working electrode
was a glassy carbon electrode (d \ 3 mm), an unmodiÐed
CCE, or a modiÐed CCE. An Ag/AgCl (saturated KCl) elec-
trode was used as a reference electrode and a Pt gauze as a
counter electrode. Solutions were deaerated by argon bub-
bling prior to the experiments and the electrochemical cell was
kept under argon atmosphere throught the experiments. A
Cole-Parmer 3TM pH meter was used for pH measurements.
Fig. 3 Plots of E vs. pH for the SiMo
@2
12-doped CCE.
1
Fabrication of SiMo -doped CCEs
1
2
The SiMo -doped CCEs were fabricated as follows. A solu-
1
2
tion of 0.75 ml methanol containing 1.88 mg SiMo , 0.25 ml
12
MTMOS and 0.025 ml hydrochloric acid (11 M) was ultra-
sonically mixed for 2 min, then 1.875 g graphite powder was
added and the mixture shaken on a vortex agitator for an
additional 3 min. The mixture was added to glass tubes with 3
mm inner diameter and dried for 24 h at 30 ¡C. After drying,
the electrodes were polished with no. 3 emery paper and the
Fig. 4 Cyclic voltammograms of a SiMo -doped CCE in 0.5 M
1
2
H SO ] 0.1 M Na SO aqueous solutions containing (b) 0, (c) 0.5,
2
4
2
4
(
d) 2, (e) 4 mM NaBrO , and (a) an unmodiÐed CCE in the same
3
solution as (e). Scan rate is 40 mV s~1.
482
New J. Chem., 2000, 24, 481È483

View Article Online
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made by silver paint through the back of the electrode.
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4
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Acknowledgements
The Ðnancial support of the Ministry of Science & Tech-
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