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Activity changes of glassy carbon electrodes caused by their exposure to OH radicals
Tomasz Rapecki a, Anna M. Nowicka a, Mikolaj Donten a, Fritz Scholz b, Zbigniew Stojek a,a Department of Chemistry, Warsaw University, ul. Pasteura 1, 02-093 Warsaw, Polandb Institut fr Biochemie, Universitt Greifswald, Felix-Hausdorff, Str. 4, 17487 Greifswald, Germany
a b s t r a c ta r t i c l e i n f o
Article history:
Received 11 August 2010
Received in revised form 17 August 2010
Accepted 18 August 2010Available online xxxx
Keywords:
Glassy carbon
OH radicals
Metal nucleation
Surface erosion
GC electrodes were exposed to Fenton solutions. The surface changes produced by the OH radicals of these
solution were inspected using SEM, XPS, Raman spectroscopy and electrochemistry. The OH radicals caused
erosion and roughening of the surface, selective oxidation and dissolution of sp2 carbon, and reduction of the
number of nucleation sites for silver deposition.
2010 Published by Elsevier B.V.
1. Introduction
Carbon-based electrodes are nearly ubiquitous in the laboratory
because of their availability in various forms and shapes, and their
wide potential window [1,2]. The most common method of activationof glassy carbon (GC) is to polish its surface with micro-sized
abrasives. After such treatment a fresh new surface should be
obtained, however, the polishing on some types of pads can
occasionally deactivate the surface, while the use of too big particles
of abrasive canlead to relatively large surface scratches [3]. To remove
remains of polishing material the ultrasonication is often needed. The
impurities can be also removed by electrochemical oxidative
procedures [4]. A number of electrode pretreatments to yield an
active and reproducible electrode surface have been proposed [5].
Free radicals can cause substantial changes in the morphology and
activity of the conducting surfaces. As it has already been shown for
gold, the real surface area of gold electrodes treated with OH radicals
can diminish (chemical polishing action) and the voltammetric
response of analytes can change due to changes in the electron
transfer rate and the reaction path [6,7]. The aim of this paper was to
characterize how the action of OH radicals can affect the parameters
(including theamount of bound oxygenand the surface roughness) of
GC electrodes. The corresponding changes in the chronoamperometric
electrodeposition of silver are also shown.
2. Experimental section
Cyclic voltammetry was performed with an Autolab, model 12
potentiostat (Eco-Chemie, Utrecht). A GC electrode (3 mm in
diameter, BASi, USA) was used as the working electrode; saturatedAg/AgCl and calomel electrodes and a platinum wire were used as the
reference and auxiliary electrodes, respectively.
Fenton solutions were always freshly prepared from ammonium
iron(II) sulfate hexahydrate (Merck), EDTA (Merck), 0.01 M acetate
buffer (pH 4.7) and hydrogen peroxide solution (POCh). Just before
each measurement the surface of the working electrode was polished,
immersed in the Fenton solution for a defined time interval and
washed with purified water. To avoid the change in activity of Fenton
solutions the longer exposures consisted of appropriate number of 5-
min treatments in freshly prepared solutions.
In the nucleation experiments the following supporting electro-
lytes were used: 2 mM silver nitrate (POCh, Gliwice. Poland) and 0.25
M potassium nitrate (POCh).
X-ray photoelectron spectroscopy (XPS) measurements were
performed with an ESCALAB-210 spectrometer from VG Scientific.
Raman spectra were collected in the backscattering configuration
with a Labram HR800 (Horiba Jobin Yvon) confocal microscope
system equipped with a CCD detector (1024256 pixel), using a
20 mW HeNe (632.8 nm) laser.
Scanning Electron Microscopy (SEM) images were taken with an
Ultra Plus FESEM, Zeiss, Germany, using 1-kV acceleration voltage and
a low-energy-loss back-scattered electrons detector which provided a
high contrast.
K-type GC plates (cti Chemie+Werkstoff Technik GmbH, Idstein,
Germany) used for the XPS, Raman and SEM measurements were
produced by pyrolysis of aromatic polymers at circa 1000 C.
Electrochemistry Communications xxx (2010) xxxxxx
Corresponding author.
E-mail address: [email protected] (Z. Stojek).
ELECOM-03665; No of Pages 4
1388-2481/$ see front matter 2010 Published by Elsevier B.V.
doi:10.1016/j.elecom.2010.08.026
Contents lists available at ScienceDirect
Electrochemistry Communications
j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c o m / l o c a t e / e l e c o m
Please cite this article as: T. Rapecki, et al., Activity changes of glassy carbon electrodes caused by their exposure to OH radicals, Electrochem.Commun. (2010), doi:10.1016/j.elecom.2010.08.026
http://dx.doi.org/10.1016/j.elecom.2010.08.026http://dx.doi.org/10.1016/j.elecom.2010.08.026http://dx.doi.org/10.1016/j.elecom.2010.08.026mailto:[email protected]://dx.doi.org/10.1016/j.elecom.2010.08.026http://www.sciencedirect.com/science/journal/13882481http://dx.doi.org/10.1016/j.elecom.2010.08.026http://dx.doi.org/10.1016/j.elecom.2010.08.026http://www.sciencedirect.com/science/journal/13882481http://dx.doi.org/10.1016/j.elecom.2010.08.026mailto:[email protected]://dx.doi.org/10.1016/j.elecom.2010.08.026 -
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3. Result and discussions
3.1. Surface structure changes
Fig. 1 depicts a SEM image of the GC surface after mechanical
polishing and before exposing it to OH. Typical SEM images recorded
after immersion of the electrode in Fenton's solution for 30 and
60 min are presented in Fig. 1. A progress in the corrosion of the GC
surface with time of the OH
action is clearly visible in themicrographs. After 30 min of the radical treatment the surface is
still marked with a net of very fine scratches formed during its
mechanical polishing before the radical treatment; however, the first
effects of the erosion of the material can be already noticed. The net of
scratches is almost invisible for the samples treated for 45 min and
completely vanishes after 60 min. The disappearing of the scratches
on the GC surface indicates the removal of a layer of carbon from the
GC surface treated. It can be a combined process of mechanical
crumbling and chemical oxidation.
According to the model proposed by Jenkins and Kawamura [8],
glassy carbon consists of long sheets (microfibrils) of hexagonally
oriented, sp2-hybridized carbon atoms. However, unlike graphite,
there is no precise orientation of the carbon atoms from layer to layer
[9], also, some sp3 carbon atoms can be expected. XPS experiments
showed that the amount of oxygen at the surface layer increased
(from 8 to 35%) with time of electrode treatment by OH
radicals up to45 min. and then surprisingly dropped below 30%. Apparently, OH
radicals first oxidized the sp2 carbon atoms to alcohol, carbonyl or
carboxyl groups and finally detached those carbon atoms as CO2. At
the same time the amount of the sp3 bonds (diamond-like)
significantly increased (from 11 to 16%) while the percentage of
carbon sp2 (graphite type) decreased (from 60 to 31%), which is a
significant change in sp3/sp2 molar ratio.
The above XPS results are in good agreement with the Raman data.
In the range 600 1800 cm1 the bare GC exhibits two characteristic
intense Raman bands at ~1360 cm1 (diamond band) and ~1600 cm1
(graphite band) [10]. Both the graphite and the diamond bands
undergo significant changes upon interactions with OH radicals.
Intensities of both bands increase (up to 30 min), but only the
graphite-type band changes its position: it shifts towards higher
frequencies. This shift towards higher energies is characteristic for the
oxidation state of carbon [11].
3.2. Electroactivity changes
We have examined the electrodeposition and nucleation of silver
on pure GC and GC treated with OH. Typical experimental current
transients for 0.12 V vs. SCE (overpotential of 0.44 V) obtained with a
pure GC electrode and the GC electrodes treated with OH are
presented in Fig. 2A. The first parts of the chronoamperometric curves
reflect an increase in the current related to the nucleation (increase in
number of active sites) and growth of silver particles on the surface of
GC electrode. The current reaches a maximum and then starts to
decrease due to the overlap of diffusion fields around the nuclei. The
current maximum of silver deposition decreases and occurs at longertimes as the time of dipping of GC electrode in Fenton solution is
prolonged. These results suggest that the number of active sites and
silver nuclei decrease with an extension of time of OH interaction
with GC substrate.
A comparison between the experimental curves and the theoret-
ical data allows determining the nucleation type. According to the
model of diffusion-controlled growth of hemispherical particles
proposed by Scharifker and Hills [12], two limiting nucleation
mechanisms can be considered: the progressive and the so-called
instantaneous. The progressive nucleation is related to the growth of
number of nucleation sites activated during the course of the
electrodeposition process. The so-called instantaneous nucleation
corresponds to the growth of the nuclei on a smaller number of active
sites, all activated at the same time during the initial phase of theelectroreduction.
The theoretical currenttime transients for the two considered
cases: instantaneous and progressive nucleation can be described by
the equations:
i2
i2m
!=
1:9542
t= tm1exp 1:2564
t
tm
2
1
i2
i2m
!=
1:2254
t= tm1exp 2:3367
t
tm
2
22
where i is current density and tis time; im and tm are the coordinates
of the peak.
0 min
30 min
200 nm
200 nm
60 min
200 nm
Fig. 1. Micrographs of GC surface obtained after: 0, 30 and 60 min of OH radicals'
treatment. Concentrations of Fe2+
, EDTA and H2O2 are 1, 1 and 10 mM, respectively.
2 T. Rapecki et al. / Electrochemistry Communications xxx (2010) xxxxxx
Please cite this article as: T. Rapecki, et al., Activity changes of glassy carbon electrodes caused by their exposure to OH radicals, Electrochem.Commun. (2010), doi:10.1016/j.elecom.2010.08.026
http://dx.doi.org/10.1016/j.elecom.2010.08.026http://dx.doi.org/10.1016/j.elecom.2010.08.026 -
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The dimensionless plots, (j(t)/jm)2 vs. t/tm, for selected experi-
mental chronoamperometric curves are presented in Fig. 2B. They are
matched with the theoretical plots for the cases of pure progressive
and instantaneous nucleation. In thecase of deposition of silveron the
surface of pure, untreated GC substrate, the progressive nucleation of
silver was dominating. After the treatment with radicals the situation
changed and the instantaneous mechanism of silver nucleation on the
glassy carbon surface was observed. These results indicate that the
treatment of GC surface with OH radicals affects the nucleation
processes on this substrate in such a way that the number of active
sites on the surface of GC electrode, at the selected overpotential of
deposition, decrease.
The decrease of nuclei population density with the extension of
time of the treatment of GC electrode with OH radicals was also
examined by determination (after recording appropriate chronoam-
perograms) of thenumber of nucleifrom the in-situ SEMphotographs
of the surface of GC substrate (area examined: 0.007 cm2). The results
are presented in Fig. 2C and compared with the theoretical number of
nuclei calculated according to the reference [12] for both limitingcases of the nucleation mechanism. The nuclei population density, N0,
of instantaneous nucleation mechanism was calculated using the
equation:
N0 = 0 :06521
8C0Vmol 0:5
zFC0 2
i2mt2m
3
where n is number of electron involved, Fis the Faraday constant, Vmolis the molar volume and C0 is the concentration of species in solution.
For the progressive nucleation mechanism the density of nuclei at
saturation, Ns, is given by
Ns =AN
2K0
D 0:5
4
AN is the growth rate of the nuclei:
AN
=4:6733
t2mK0
D5
and K is defined as
K0
=4
3
8C0M
0:56
where D is the diffusion coefficient of the metal ion, M is the atomic
mass and is the density of the silver deposit.
A comparison of the experimental results (obtained from the SEM
images) and the theoretical data obtained for the two consideredmodels proves that the results obtained for the untreated GC surface
are closer to the progressive model while those obtained for the
etched surface fits better the instantaneous model. An increase in
the exposition time of GC to OH radicals leads also to a decrease in
the nucleation rate. Most importantly, the results indicate that the
number of nucleation sites is decreasing as the OH attack lasts. This
is very surprising as the OH attack is obviously accompanied by a
surface roughening (see Fig. 1).
4. Conclusions
The attack of OH on glassy carbon material leads to a surface
erosion through selective oxidation of the sp2 carbon atoms. In
consequence a surface roughening and decrease of number of activenucleation sites for metal deposition occurs. The latter clearly
demonstrates that surface roughness and the number of active
nucleation sites are inversely-proportional related. This is challenging
more detailed studies on the true nature of active sites. Thesp2 carbon
atoms are regarded as more active sites compared to sp3 [13,14]. The
electrodeposition of silver is transformed from a progressive to an
instantaneous nucleation during the OH treatment. The change in
percentage of sp2 carbon and the possibility of its oxidation and
selective removal stands behind the change in activity of GC surface.
Acknowledgement
The support for this work by the Polish Ministry of Science and
Higher Education Grant N N204 244534 is gratefully acknowledged.
t / s
0 5 10 15 20 25 30
i/m
Acm-2
-0.4
-0.3
-0.2
-0.1
0.0
a
b
cd
e
t/tm
0 1 2 3 4
(i/im
)2
0.0
0.2
0.4
0.6
0.8
1.0
0 min15 min30 min
Instantaneous
Progressive
t / min
0 15 30 45 60
N*106/cm-2
0.0
0.5
1.0
1.5
2.0
2.5 SEMinst.
prog.
A
B
C
Fig. 2. A: Current transients for electrocrystallization of silver at GC electrode obtained
after: (a) 0, (b) 15, (c) 30, (d) 45, and (e) 60 min of OH radicals treatment.Overpotential: 0.44 V. B: Non-dimensional (i/im)2 vs. (t/tm) plots of current transients
of silver electrodeposition process. C: Changes of nuclei population density vs. time of
interactions of GC with OH radicals. Data determined from SEM experiments and
theoretical calculations for progressive and instantaneous mechanisms.
3T. Rapecki et al. / Electrochemistry Communications xxx (2010) xxxxxx
Please cite this article as: T. Rapecki, et al., Activity changes of glassy carbon electrodes caused by their exposure to OH radicals, Electrochem.Commun. (2010), doi:10.1016/j.elecom.2010.08.026
http://dx.doi.org/10.1016/j.elecom.2010.08.026http://dx.doi.org/10.1016/j.elecom.2010.08.026 -
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4 T. Rapecki et al. / Electrochemistry Communications xxx (2010) xxxxxx
Please cite this article as: T. Rapecki, et al., Activity changes of glassy carbon electrodes caused by their exposure to OH radicals, Electrochem.Commun. (2010), doi:10.1016/j.elecom.2010.08.026
http://dx.doi.org/10.1016/j.elecom.2010.08.026http://dx.doi.org/10.1016/j.elecom.2010.08.026