Dynamic Electrochemical Impedance Spectroscopy (DEIS) as a Tool for Analyzing Surface Oxidation Processes on Boron-Doped Diamond Electrodes

Abstract

Surface oxidation processes play a key role in understanding electrochemical properties of boron-doped diamond (BDD) electrodes. The type of surface termination groups, which create the potential window of electrolytic water stability or hydrophobicity, influences such properties.
In this study the kinetics of oxidation process under anodic polarization were studied in situ by means of Dynamic Electrochemical Impedance Spectroscopy (DEIS) technique. This novel approach allows for obtaining the impedance data for non-stationary systems. It has been proven that for [B] dopant level of 10k ppm, polarization to 1.5 V vs. Ag|AgCl is sufficient to initiate transformation of the film terminating BDD electrodes. XPS analysis and wettability measurements confirmed oxidation under given conditions.

Full text

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PROOF COPY [JES-14-0569] 016406JES
Journal of The Electrochemical Society,161 (6) H1-H6 (2014) H1
0013-4651/2014/161(6)/H1/6/$31.00 ©The Electrochemical Society
Dynamic Electrochemical Impedance Spectroscopy (DEIS) as a
Tool for Analyzing Surface Oxidation Processes on Boron-Doped
Diamond Electrodes
1
2
3
Jacek Ryl,a,zRobert Bogdanowicz,bPawel Slepski,aMichal Sobaszek,b
and Kazimierz Darowickia
4
5
aDepartment of Electrochemistry, Corrosion and Material Engineering, Gdansk University of Technology,
Gdansk 80-233, Poland
6
7
bDepartment of Metrology and Optoelectronics, Gdansk University of Technology, Gdansk 80-233, Poland8
9
Surface oxidation processes play a key role in understanding electrochemical properties of boron-doped diamond (BDD) electrodes.
The type of surface termination groups, which create the potential window of electrolytic water stability or hydrophobicity, influences
such properties. In this study the kinetics of oxidation process under anodic polarization were studied in situ by means of Dynamic
Electrochemical Impedance Spectroscopy (DEIS) technique. This novel approach allows for obtaining the impedance data for non-
stationary systems. It has been proven that for [B] dopant level of 10k ppm, polarization to 1.5 V vs. Ag|AgCl is sufficient to initiate
transformation of the film terminating BDD electrodes. XPS analysis and wettability measurements confirmed oxidation under given
conditions.
10
11
12
13
14
15
16
© 2014 The Electrochemical Society. [DOI: 10.1149/2.016406jes] All rights reserved.17
18
Manuscript submitted February 17, 2014; revised manuscript received March 20, 2014. Published 00 0, 2014.19
Boron-doped diamond (BDD) electrodes have been recently stud-20
ied in-depth because of their outstanding electrochemical features21
which include a wide electrochemical potential window in aqueous22
electrolytes,1high anodic stability,2and particular electrochemical23
stability in harsh environments.3,4These properties make BDD a24
useful electrode material for applications in wastewater treatment,5–7
25
electrochemical sensing8–11 and electrocatalysis.12–14 BDD is mostly26
synthesized by hot-filament CVD (HF CVD)15,16 or microwave27
plasma-assisted CVD (MW PA CVD) and in situ doped with boron28
precursors.17 The boron dopant density influences not only the elec-29
trical properties of electrode, but also its morphology and structure30
(sp3/sp2ratio).6,18,19 Nevertheless, the electrochemical behavior of31
BDD depends on these properties.20,21
32
Boron concentration has impact on the electrochemical and elec-33
trical properties of BDD electrodes. The final density of boron dopant,34
achieved by using microwave plasma-assisted chemical vapor depo-35
sition (MW PA CVD), ranges from 1016 to 1021 atomsccm−3, for the36
dopant density of 2 ·1020 the p-type semiconducting material trans-37
form to semimetal.5
38
The processes occurring on the BDD semiconducting electrodes39
are irreversible.22 The potential window of electrolytic water stability40
rises with increasing dopant concentration. Not only the dopant level41
but also the surface termination type (hydrogen and oxygen23–25 and42
other compounds added during plasma etching, i.e. CF4,Cl
2,Arand43
CH426) of boron-doped diamond electrodes is an important factor in44
the electrode kinetics27 and its reversibility.28 Many kinds of surface45
treatment can be utilized, including dry and wet processes such as46
plasma treatment or electrochemical polarization. Recently, many in-47
vestigators focus on hydrogen terminations (HT-BDD) and oxygen48
terminations (OT-BDD). BDD films deposited by microwave plasma-49
assisted chemical vapor deposition mainly present HT-BDD due to50
using hydrogen plasma.6,11
51
Transformation of HT-BDD to OT-BDD is worthy of interest. It52
can be achieved by oxygen plasma treatments, chemical oxidation or53
anodic polarization.29,30 As a result of such transformation, different54
combinations of bonds can be formed on the BDD surface such as,55
=O, –O– and –OH.31,32 On the other hand, the anodic polarization56
process is not fully reversible by the cathodic treatment, even at deep57
potentials.33 Hoffmann et al.34 anodized the BDD layers in 1M H2SO4
58
with 0.5M HNO3for a 10 s period under different polarization poten-59
tials. Based on the analysis of AFM and XPS data, it was suggested60
that for this specific period of time the oxidation level depends on61
the potential, and it is not complete even for the potential of 6 V.62
zE-mail: jacek.ryl@pg.gda.pl
The longer the oxidation process, the more complete the termination 63
layer. Chaplin et al.2used the results of XPS and CV studies to pro- 64
pose that freshly prepared BDD electrodes containing high density 65
of CHx(1 ≤x≤3) surface sites can be oxidized by a sequence 66
of direct electron transfer reactions and reactions with OH•to form 67
=C=O and -C-OH functional groups. Additionally, Salazar-Banda et 68
al.35 reported that the galvanostatic cathodic pre-treatment in steps of 69
−600 C cm−2passed using −1Acm
−2until −14000 C cm−2caused 70
an expressive physical degradation of the BDD surface. However, the 71
limited charge density value of −9Ccm
−2(at −1Acm
−2) is op- 72
timal to obtain activation of the BDD electrodes (at 800, 2000, and 73
8000 ppm of boron doping) without producing any observable physi- 74
cal degradation. 75
HT-BDD surface is hydrophobic and highly conductive, while 76
OT-BDD surface is hydrophilic and displays low conductivity. The 77
mechanism involved is not fully explained, however, one of the possi- 78
ble explanations of high conductivity of HT-BDD is related to the 79
acceptor role of hydrogen. The potential window is much wider 80
for OT than HT, and it shifts energy bands to lower values.25,36 81
Moreover, the shifts of energy bands also depend on the method 82
of surface oxidation, anodic polarization more highly shifts energy 83
bands than wet oxidation.37 Changing the termination type to OT af- 84
fects the charge transfer processes, which leads to electroanalytical 85
selectivity.38 86
Numerous electrochemical studies were performed on BDD with 87
various dopant levels, which focused on the application of cyclic 88
voltammetry (CV) or electrochemical impedance spectroscopy (EIS). 89
The impedance measurement provides a valuable information about 90
the electrochemical properties of the investigated system. The tech- 91
nique, however, has a major constraint, i.e. a need to preserve the 92
stationary conditions during the entire duration of measurement. As 93
a result of anodic polarization, polycrystalline BDD electrodes are 94
subjected to variable conditions. In order to observe online changes in 95
impedance parameters, a slight modification of the technique is neces- 96
sary. Dynamic Electrochemical Impedance Spectroscopy (DEIS) uti- 97
lizes a package of multiple sinusoids generated simultaneously over a 98
wide frequency range and acting as an excitation signal. DEIS can be 99
combined with DC measurements such as chronovoltammetry, which 100
allows the direct analysis of changes in electrical parameters result- 101
ing from excitation. Such an approach has been successfully used to 102
analyze various electrochemical processes.39–41 103
This paper focuses on the presentation of analytical capabilities 104
provided by DEIS that are suitable for studying the oxidation pro- 105
cesses on the BDD surface, in particular OT-BDD formation under 106
deep anodic polarization conditions. Despite increasing interest in 107
BDD electrochemistry, only a limited number of works describing the 108

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H2 Journal of The Electrochemical Society,161 (6) H1-H6 (2014)
Figure 1. Anodic polarization of the BDD electrode in the potential range of
up to 2.5 V. The first ( ), third ( ),fifth ( ) and seventh ( ) polarization
cycle. Reference electrode Ag|AgCl. Scan rate 0.5 mV/s. Arrow indicates
change of potential window width with number of polarization cycles.
mechanism of BDD oxidation processes and electron transfer across109
the oxygenated BDD electrode/electrolyte interface for various boron110
dopant levels have been published so far.111
Experimental112
Si/BDD electrodes were synthesized in an MW PA CVD system113
(Seki Technotron, Japan) on p-type Si wafers with (111) orientation.114
The substrates were seeded by sonication in nanodiamond suspension115
(crystallite size of 5–10 nm) for 1h.42,43 Next, the substrates were116
dried under a stream of nitrogen. The substrate temperature was kept117
at 1000◦C during the deposition process. During the first step of the118
procedure, substrates were etched in hydrogen plasma for 3 min.119
Excited plasma was ignited by microwave radiation (2.45 GHz). The120
plasma microwave power, optimized for diamond synthesis, was kept121
at 1300 W. The gas mixture ratio was 1% of the molar ratio of CH4-122
H2at gas volume 300 sccm of total flow rate. The base pressure was123
about 10−6Torr, and the process pressure was kept at 50 Torr. All124
samples were doped with diborane (B2H6) dopant precursor. [B]/[C]125
ratio in plasma was 10000 ppm. The time of polycrystalline layer126
growth was 6h, which resulted in the thickness of deposited films of127
approx. 2 μm.44–46
128
Electrochemical studies were carried out in a three-electrode sys-129
tem. The working electrode consisted of the BDD layer with the silver130
chloride electrode acting as a reference electrode, and platinum mesh131
as a counter electrode. All potentials reported in our paper are ex- 132
pressed on the scale of silver chloride electrode. The partial area of 133
the sample, i.e. 0.5 cm2was exposed to 1 M H2SO4solution. The 134
cell volume was 50 mL. Autolab 302N was used to apply cyclic 135
polarization in the range of potentials from the corrosion potential 136
(+0.1 V) to 2.5 V at a rate of 0.5 mV/s. After five cycles, no further 137
change in the impedance parameters had been observed. Edc signal 138
is combined with a composition of selected voltage sinusoids in the 139
frequency range from 45 kHz to 0.7 Hz. The resulting AC perturbation 140
peak-to-peak amplitude did not exceed 20 mV. AC signal generation 141
and simultaneous recording of both voltage and current signals was 142
performed using a National Instrument card PXI4461. The resulting 143
records were divided into fragments of 10-sec length, and subjected 144
to Fourier transformation. Small changes within this area allowed to 145
obtain local impedance spectra correlated with time and electrode po- 146
tential. Details of this technique have been presented elsewhere.47,48 147
The measurement was carried out using dedicated software, written 148
in LabView environment. The resulting impedance spectra were ana- 149
lyzed by means of ZSimpWin 3.21, EChem Software. 150
In order to determine changes in chemical composition on the 151
surface as a result of HT-BDD into OT-BDD transformation, XPS 152
measurements were carried out on Escalab 250Xi (ThermoFisher Sci- 153
entific) equipped with monochromatic Al X-ray source and a spot 154
diameter of 200 μm. Data analysis was performed using Avantage 155
software supplied by the manufacturer. Surface wettability was mea- 156
sured by the sessile drop method (drop volume ∼0.5 μL) using 157
the self-designed tensiometer system based on B/W CCD camera 158
(Thorlabs, DCU223M, USA). Determinations of the angle between 159
the BDD surface and the tangent of the drop were performed using 160
MATLAB 6.0 (The MathWorks, USA). 161
Results and Discussion 162
Electrochemical oxidation.— In Figure 1the anodic polarization 163
curves for the first, third, fifth and seventh polarization cycle are 164
presented; the initial presence of the complex peak in the range from 165
1.5 to 2 V followed by an increase in current, which is typical for 166
the evolution of oxygen, can be observed. The peak manifested itself 167
particularly strongly in the first cycle of anodic polarization, which 168
suggests that the relaxation process on the surface of BDD electrode 169
begins for the potential values >1.5 V. The subsequent cycles are 170
characterized by significantly lower current peaks in this region, while 171
after the fifth cycle, no further changes were observed. 172
A visualization of DEIS results in the form of changes in 173
impedance and capacitance spectra during anodic polarization are 174
presented in Figure 2a and 2b, respectively. The individual impedance 175
Figure 2. Changes in impedance spectra (a) and capacitance spectra (b) resulting from anodic polarization. Visualization in the form of Nyquist plot. Reference
electrode Ag|AgCl. Scan rate 0.5 mV/s.

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Journal of The Electrochemical Society,161 (6) H1-H6 (2014) H3
Figure 3. Electric equivalent circuit used for analysis, composed of two
time constants. First one, in the range of high frequencies, defined by C1
and R1. Low frequency range time constant is defined by CPE =f(Q2,n)
and R2.
spectra were approximately semi-circular, with their size being de-176
termined by the magnitude of polarization. Particularly large changes177
occurred for the potential values >1.5 V. In the detailed picture the178
second time constant is visible for the very high frequency values, i.e.179
above 1 kHz. Another possible way of presenting DEIS results is by180
using the complex capacity system (Fig. 2b). This approach partic-181
ularly allows the observation of changes in the capacity parameters182
under anodic polarization. In this case, the local increase in capacity183
was visible in the vicinity of 1.5 V, indicating a transformation on184
the sample surface. For the potential values >2 V, capacitance spectra185
assumed the shape of straight lines as a result of oxygen evolution186
processes taking place on the surface.187
The impedance techniques are an important tool for investigat-188
ing the BDD layers, however, there is lack of uniformity in the189
equivalent circuit selection which is a key element in data analy-190
sis. Trouillon and O’Hare49 used the classic Randles circuit with the191
Warburg impedance R(C(RW)) to evaluate biofouling resistance. A192
similar circuit, including a constant phase element, CPE in the place193
of capacitance R(Q(RW)) was proposed by S.C.B. Oliveira and A.M.194
Oliveira-Brett50 in the studies of BDD termination layer formed under195
anodic and cathodic polarization. CPE is generally used to account196
for the interfacial impedance; its use is well established in modeling197
of sp2-based carbon electrodes and other non-homogenous materials.198
The explanation of such element is controversial, yet in many cases, it199
is related to the presence of different components with various capac-200
itances on the surface. It may be suitable for modeling inhomogene-201
ity at the atomic scale,51 adsorption effects52 or microscopic surface 202
roughness.53 A contribution of the Warburg diffusion impedance to 203
the analysis of oxidized BDD electrodes is sometimes represented 204
by a CPE such as RC(CQ) or RC(CQ)(CR),54 but in most cases it is 205
completely overlooked. A quite popular circuit, used by J. Hernando 206
et al.55 and Y. V. Pleskov et al.,56 consists of only one time constant 207
R(QR). In the case of oxygen terminated BDD (O-BDD) electrodes, 208
two time constants were observed in the impedance studies of a sim- 209
ple redox reaction.57 The obtained results suggest that the electron 210
transfer process is mediated by surface states, which is in agreement 211
with the model proposed in single-crystal studies.27 Denisenko et al. 212
used R(CR)(CR) circuit in the study on anodic oxidation of BDD in 213
H2SO4.37 One RC element was related to the termination layer on the 214
BDD surface, while the other described the depletion (space charge) 215
layer in BDD. Rameshan58 utilized a similar circuit in other solutions. 216
The definition of individual circuit elements varies for all of the above 217
mentioned articles. 218
In this study a circuit similar to that described by Denisenko et al.37 219
was proposed, replacing the capacitance of the second time constant 220
with CPE (Fig. 3). The Warburg impedance was not included into this 221
equivalent circuit due to the limitation of the frequency range used. 222
The two time constants, characterized by the impedance parameters 223
(R1,C
1)and(R
2,Q
2) were estimated for a high and low frequency 224
range, respectively. The interpretation of these parameters is anal- 225
ogous to that presented above. The analysis of impedance spectra 226
based on the proposed equivalent circuit provides information 227
about the processes taking place on BDD electrodes during anodic 228
polarization. It must be noted that because the first time constants are 229
smaller by a few orders of magnitude, C1and R1might carry much 230
higher estimation error. 231
The impedance parameters of the first time constant are connected 232
to the hydrogenated/oxidized termination layer, and they represent its 233
resistance and capacitance (Fig. 4a,4b). For a polarization potential 234
of 1.5 V, resistance R1exhibits weak local minimum correlated to 235
an increase in the current, which is often visible in the case of re- 236
laxation processes. A consecutive increase of R1is often due to the 237
limited amount of the reagent or diffusion control. However, further 238
polarization cycles do not show similar dependence, indicating irre- 239
versibility of the observed process. For the range of potential above 240
Figure 4. Changes in capacitance C1(a) and
resistance R1(b) of the first time constant; as
well as constant phase element impedance Q2
(c) and resistance R2(d) of the second time
constant under anodic polarization of up to
2.5V for the first ( ), third ( ), fifth ( )
and seventh ( ) polarization cycle. Arrow
marks changes of impedance parameters with
number of polarization scans.

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H4 Journal of The Electrochemical Society,161 (6) H1-H6 (2014)
Figure 5. Relationship between the 1/Q2values and potential E for the first
polarization cycle. The linear correlation at low polarization potentials indi-
cates p-type semiconductivity of BDD electrode. For E >1.5 V, double-layer
capacitance has influence on the CPE.
1.5 V, an increase in R1is stronger during the next polarization cycles.241
The transformation of hydrogenated termination on the BDD elec-242
trode surface to other functional groups (i.e. =O, –OH) shows similar243
behavior.50 The boundary phase is also an area where capacitance is244
generated. Any change in the composition of the termination layer245
involves a capacitance change which is only within one order of mag-246
nitude. The capacitance parameter C1drops as a result of consecutive247
polarization cycles, as it would be in the case of formation of the248
adsorption layer on the BDD surface.249
Changes in the resistance (R2) and capacitance values in the form250
of CPE (Q2) are presented in Figure 4c,4d. Initially, R2only slightly251
depends on the potential of anodization; it equals 40–50 kand252
drops by two orders of magnitude with the appearance of the current253
peak at 1.5 V. After reaching this point, R2displays a local plateau.254
The interpretation of such behavior should be linked to the oxidation255
processes on BDD electrode. A further decrease in the value of R2
256
for the potential >2 V results from oxygen evolution at high anodic257
polarization potentials. The consecutive polarization scans show an258
increase in R2values by up to one order of magnitude in the entire259
potential range.260
Impedance of CPE is equal to Q2(ωi)−n. Within the experiment261
the value of n parameter was fluctuating around 0.9, suggesting that262
CPE behavior is similar to that of a capacitor. A physical explana-263
tion of Q2is complex; it is based on the experimental data. In the264
case of interpreting this parameter as a double layer capacitance,265
one should observe its changes in the range of potential characteris- 266
tic for relaxation processes (adsorption or electrochemical reaction). 267
Such relation can be seen in the proximity of polarization potential of 268
1.5 V (Fig. 4c). However, below this potential, double layer capaci- 269
tance should not be a subject to change. In the analyzed case, the initial 270
increase in CPE can be explained by the dual nature of this parameter. 271
Fig. 5shows the Mott-Schottky dependence 1/Q2during the first po- 272
larization cycle as a function of potential. A linear correlation, typical 273
for p-type semiconductors, is evident until the potential reaches 1.5 V. 274
As soon as the oxidation processes are initiated, the linear correlation 275
is no longer present. Therefore, the parameter Q must be regarded as 276
a parallel combination of two elements, i.e. depletion-layer capaci- 277
tance and double-layer capacitance. The peak in CPE at 1.5 V is most 278
visible for the first cycle; it becomes negligible for the consecutive 279
polarization scans, where the layer is already formed, proving a direct 280
connection between the CPE and oxidation process of the termination 281
layer. 282
XPS and wettability analysis.— XPS was performed for hydro- 283
genated and oxidized BDD electrodes to get qualitative and quantita- 284
tive information on the oxidation level and termination groups on the 285
BDD surface. Figure 6show the C1s spectral region before and after 286
anodic polarization to 2.5 V. 287
Hydrogen plasma-treated BDD sample (Fig. 6a) contains two main 288
components, i.e. one at 284.6 eV (noted as C1), and another shifted at 289
+0.6 eV (noted as C2). For the anodic polarization of BDD electrodes 290
in 0.5M H2SO4, Wang et al. assigned those peaks to hydrogenated and 291
non-hydrogenated diamond surface, respectively.31 The very location 292
of peak C1 can shift with the dopant level in the sample. The energy 293
value of 284.6 eV is in good agreement with similar results reported 294
for polycrystalline electrode with the boron dopant level at 1e19 cm−3.295
However, for different dopant levels, this peak can shift even up to 296
+1eV.
59 The third peak (noted as C3) observed in the case of hy- 297
drogenated sample is shifted by +1.4 eV, which corresponds to the 298
oxidized carbon atoms C-OH and C-OC due to the unavoidable ex- 299
posure of the sample to atmospheric conditions. Fig. 6b shows the 300
results of the anodic polarization of BDD electrode for the poten- 301
tial range up to 2.5 V. Peak C1s includes two additional components 302
related to the oxidized forms of carbon; C4 is shifted by +2.2 V, 303
while C5, by +3.9 eV. These peaks correspond to C=O and COOH 304
bonds. Such interpretation is in agreement with previously published 305
results.31,37,59,60 306
In BDD electrodes sp2carbon may also influence the properties of 307
anodized layer, serving as a mediator for the charge transfer.61 On the 308
basis of Raman bands intensity, measured elsewhere6the sp3/sp2ratio 309
of BDD electrode was equal to 37. Electrochemical etching, which 310
occurs as a result of anodic polarization, additionally removes sp2-C; 311
Figure 6. High resolution XPS spectra and the surface wettability measurement for hydrogen plasma-treated BDD electrode (a), and for the same electrode after
one (b) and five (c) anodic polarization cycles in the range up to +2.5 V.

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Journal of The Electrochemical Society,161 (6) H1-H6 (2014) H5
Table I. Percent contribution of each C1s obtained by peak fitting for HT-BDD and electrochemically formed OT-BDD electrodes (after first and
fifth polarization cycle).
Surface termination C1 [%.at] C2 [%.at] C3 [%.at] C4 [%.at] C5 [%.at]
HT-BDD: plasma treatment 75 21 4 ∼0∼0
OT-BDD: after 1st cycle 45 36 10 6 3
OT-BDD: after 5th cycle 24 45 17 8 6
thus resulting in reduced activity of the BDD surface. During anodic312
polarization in H2SO4,sp2-hybridized carbon corrodes and is removed313
from the BDD surface in the oxidized CO/CO2form.61 As a result,314
sp2component (peak at 283.3 eV 62) was not taken into consideration315
due to its negligible size.316
As a result of the anodic polarization treatment, the amount of hy-317
drogenated carbon (C1) drops thrice (Table I). The contribution of the318
identified carbon-oxygen functional groups (C3, C4 and C5) increases319
from approximately 4% (resulting from atmospheric oxidation only)320
to over 31%, giving a proof of reconfiguration to OT-BDD. Impor-321
tantly, for the polarization potential of 2.5 V, at least three different322
types of oxidized carbon were observed. Their formation may be a323
result of the competition between various mechanisms of HT-BDD324
oxidation 60 or be initiated at different electrode potentials.325
The surface wettability was measured after hydrogen plasma treat-326
ment and anodic oxidation. The contact angle of hydrogen plasma-327
treated sample was 71◦, proving that the surface of BDD sample is328
hydrogen terminated due to its hydrophobicity. After the full anodic329
polarization, the measured contact angle was 26◦. A change in the330
contact angle of over 40◦serves as a proof of overlapping anodic331
oxidation taking place on the sample surface.332
Conclusions333
DEIS technique had proven its usefulness in an in situ study of334
changes in electric parameters that result from the oxidation processes335
on the BDD surface. The impedance parameters showed a behavior336
typical of electrochemical processes taking place on the semiconduc-337
tive electrode. Anodic polarization of 1.5 V is sufficient to start the338
transformation of initially H-terminated BDD. A complete oxidation339
of BDD surface was reached in under five polarization cycles, whereas340
no further change in the impedance parameters was observed.341
Oxidation of the terminating layer was proved on the basis of high342
resolution XPS analysis and wettability measurements. XPS study343
demonstrated that the transformation can be characterized by differ-344
ent mechanisms, leading to different states of carbon oxidation on345
the surface. The variation of DEIS parameters might indicate that the346
different oxidation processes take place after reaching certain polar-347
ization level, however further research is needed in order to investigate348
this hypothesis.349
Acknowledgments350
The authors gratefully acknowledge financial support from351
the Polish National Science Center (NCN) under grant no.352
2011/03/D/ST7/03541.353
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Queries
Q1: AU: Please verify volume and page number in Ref. 8.
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