Cambio de Acitividad de GC Por Radicales OH

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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

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



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[8] G.M. Jenkins, K. Kawamura, Polymeric Carbons: Carbon Fibre, Glass and Char,Cambridge University Press, Cambridge, 1976.

[9] J.H. Knox, P. Ross, Adv. Chromatogr. 37 (1997) 73.[10] K. Ray III, R.L. McCreery, Anal. Chem. 69 (1997) 4680.[11] K.N. Kudin, B. Ozbas, H.C. Schniepp, R.K. Prud'homme, I.A. Aksay, R.Car, NanoLett.

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4 T. Rapecki et al. / Electrochemistry Communications xxx (2010) xxxxxx


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Cambio de Acitividad de GC Por Radicales OH