Abstract C-sp2 (graphite) impurities are undesirable in synthetic diamond electrodes (C-sp3), because they can affect the electrochemical response. In this work, we demonstrate that Csp3 surfaces can be activated successfully by applying an anodic current density corresponding to sufficiently high potential where the hydroxyl radicals (●OH) are generated. The effectiveness of this activation process was verified by Raman spectroscopy, X-ray diffraction, scanning electron microscopy, and cyclic voltammetry.

1. Electrocatalysis (2013) 4:189–195
DOI 10.1007/s12678-013-0158-x
Surface Activation of C-sp3 in Boron-Doped
Diamond Electrode
Alejandro Medel & Erika Bustos & Luis M. Apátiga &
Yunny Meas
Published online: 7 September 2013
# Springer Science+Business Media New York 2013
Abstract C-sp2 (graphite) impurities are undesirable in syn-thetic
diamond electrodes (C-sp3), because they can affect the
electrochemical response. In this work, we demonstrate that C-sp3
surfaces can be activated successfully by applying an
anodic current density corresponding to sufficiently high po-tential
where the hydroxyl radicals (●OH) are generated. The
effectiveness of this activation process was verified by Raman
spectroscopy, X-ray diffraction, scanning electron microscopy,
and cyclic voltammetry.
Keywords Boron doped diamond . Anodic polarization .
Electrochemical oxidation . Hydroxyl radicals . Graphite
Introduction
Diamond is an extremely hard crystalline form of carbon that is
an excellent material for many applications (electroanalysis,
electrosynthesis, electrocatalysis, and treatment of chemical
residues in water) due to its unusual physical and chemical
properties [1]. Among the characteristics that differentiate
BDD from conventional electrodes are a wide potential win-dow,
low background current, high corrosion resistance [2],
low adsorption, and high efficiency in electrochemical oxida-tion
processes [1]. BDD electrodes can be synthesized using
different processes, such as chemical vapor deposition (CVD)
using energy-assisted plasma or hot filaments [3]. Diamonds
can be synthesized over different substrates including: Nb, Si,
Ta, Ti, W, Mo, and vitreous carbon. Although it is possible to
obtain a highly homogenous diamond surface, it has been
demonstrated that diamond (C-sp3) can only grow in a restrict-ed
region, under strict control of both the precursor stoichiom-etry
and the operating conditions [4]. Otherwise, another state
of hybridized carbon, C-sp2 (graphite) can appear. Although
graphite and diamond are allotropes of carbon, each displays
different properties. In graphite, the carbon atoms are in a
hybridized state, with an intraplanar bond length of 1.42 Å
and an intraplanar space of 3.354 Å. Diamond is wholly sp3
hybridized, with a bond length of 1.54 Å. Its tetrahedral struc-ture
results in extreme hardness and low electroconductivity, so
doping agents are needed to obtain sufficient conductivity [5].
The different characteristics of diamond and graphite lead to
completely different electrochemical properties. If both forms
of carbon are formed during synthesis, there is a possibility of
obtaining two types of electrodes, one low quality (due to the
presence of graphite impurities, C-sp2) and one high quality (C-sp3).
In electrochemical oxidation processes, boron-doped
diamond-like anodes are preferred over conventional materials,
because of their capacity to destroy pollutants in CO2 and water
[6]. In these applications, a low-quality electrode can seriously
affect the electrochemical kinetics [7]. Acevedo et al. [8] eval-uated
this, using [Fe(CN)6]3−/4− as the redox pair, including
reactions which depend on electrode surface interaction, since
such kinetics are very sensitive to the presence of C-sp2. They
found that ΔEp, the most sensitive parameter of the surface
termination, was 1,100 mV for an electrode with high C-sp2
content and 80 mV for one with low content, setting the
reference value at 59/n mv for a redox system with rapid
kinetics. Thus, the kinetics is much slower in the presence of
C-sp2 [8]. Then, analyzing the electroactivity of phenol, they
found that the peak currents (Ip) were affected by the presence
of C-sp2 and were much lower in comparison with those that
contained little C-sp2 without a profile of current characteris-tics,
since the potentials are displaced to more positive values
∼1.2 V vs. Ag|AgCl. On the other hand, when the C-sp2
content is low, the profile of current characteristics is defined,
there is no potential displacement, and the oxidation peak of
A. Medel : E. Bustos : Y. Meas (*)
Centro de Investigación y Desarrollo Tecnológico en
Electroquímica, S. C., Parque Tecnológico Querétaro-Sanfandila,
C. P. 76703 Pedro Escobedo, Estado de Querétaro, México
e-mail: yunnymeas@cideteq.mx
L. M. Apátiga
Centro de Física Aplicada y Tecnología Avanzada,
C.P. 76230 Juriquilla, Estado de Querétaro, México

2. 190 Electrocatalysis (2013) 4:189–195
phenol is stable, with a value of 1.12 V vs. Ag|AgCl. Studies
performed by Guinea et al. [9], showed that the C-sp3/C-sp2
relation influences the electro-oxidation of persistent organic
compounds, indicating that a greater proportion of C-sp2 leads
to the formation of many intermediates. These experiments
indicate that the “nature” of the electrode strongly influences
its electrochemical activity. Thus, it is necessary to apply a
pretreatment in order to eliminate C-sp2, which permits surface
activation of C-sp3 with reproducible results. Among the
methods employed for the surface activation of C-sp3, through
the elimination of C-sp2, those that stand out include: anodic
polarization through the application of a fixed current density,
switching potential, and the combination of these techniques
[10]. Anodic polarization has been reported using the applica-tion
of 10 mA cm−2 for 30 min in 1 M H2SO4 [7, 11]. Under
these conditions, the surface is stabilized by the formation of
functional groups containing oxygen, which is supported by X-ray
photoelectron spectroscopy, where the ratio of O/C in-creases
from 0.08 to 0.22 after the polarization process. This
has also been demonstrated by applying a potential of +3 V vs.
SCE in 1 M HClO4 under extreme conditions of oxygen
evolution, which permits the production of an oxygenated
surface with a ratio (O/C) of 0.20 [12, 13]. This results in a
hydrophilic surface [14–17], which should be, upon modifying
the hydrogen endings, responsible for the hydrophobic nature
of the BDD surface. In turn, other works have used the same
reaction medium (1 M H2SO4), but used instead a current
density of 50 mA cm−2 [14, 18, 19] and a polarization time
of 30 min. Although these studies used a common electrolyte
(1 M H2SO4), there were variations in the current density
applied. In regards to the electrolyte, some papers have reported
anodic polarization using H3PO4 [20, 21] or 1MNa2SO4 [22,
23], by applying a current density of 50 mA cm−2 for 30 min.
Although the use of these electrolytes has been reported sepa-rately,
there has also been variation in the applied current
density [9]. Another commonly used electrolyte is 1MHClO4,
which, with an applied anodic polarization of 10 mA cm−2 and
a polarization time of 30 min, has been adopted as a standard
procedure [15–17, 24–26]. However, a variation of this proce-dure
has also been reported, which uses a current density of
30mAcm−2 for 5 min [27]. There have also been studies where
no pretreatment was done prior to evaluating the electrochem-ical
characteristics of interest [28, 29]. These previous works
have shown that the principal differences in the anodic polar-ization
process are the polarization time, the applied current
density and the supporting electrolyte used. It is important to
mention that while these procedures are effective, they do not
take into account that the quantity of C-sp2 can vary from one
electrode to another depending on the method of synthesis, so
C-sp2 can be present in high [30] or low concentrations [31] or
even be completely absent [32]. Therefore, the application of a
fixed current density may or may not be sufficient for the
complete elimination of graphite impurities, and there may be
unnecessary energy costs to performing the polarization pro-cess.
Currently, no studies exist that show a complete outline of
all the steps, from the surface activation of C-sp3 to the elim-ination
of C-sp2 in BDD. With regards to the former, the
objective of this work is to develop a characterization method
in which the elimination of C-sp2 and the subsequent surface
activation of C-sp3 can be applied considering the nature of the
electrode of BDD utilized, due to the superficial variation of the
C-sp2 caused by the synthesis process. Considering thatmost of
the protocols for activating C-sp3 surfaces are focused on
evaluating the effect of C-sp2 impurities on the electrochemical
response, this work is focused on evaluating of the incineration
of C-sp2 by ●OH, the generation of ●OH, and the increase of the
ratio C-sp3/C-sp2 by Raman spectroscopy. The goal of this
work is to show that C-sp3 surfaces can be activated success-fully
by applying an anodic current density corresponding to a
sufficiently high potential where the ●OH radicals are
generated.
Methods and Materials
Selection of the Current Density
In this study polycrystalline boron ([B]=1,300 ppm) doped
diamond film (BDD) of 3 μm thickness was deposited on
titanium substrates (Ti/BDD) by hot filament chemical vapor
deposition (HF-CVD), which was provided by Adamant
Technologies. The process of anodic polarization was
performed under galvanostatic conditions, at 25 °C, using a
one compartment electrochemical cell, with BDD as anode
(with a geometric area of 2.185 cm2) and platinum mesh as
cathode. For the supporting electrolyte, 0.5 M H2SO4 was
used. Three different current densities (0.45, 0.91, and
1.83 mA cm−2) were evaluated for 10 min. The changes in
the surface and the removal of C-sp2 under these current
densities were monitored by Raman spectroscopy [33–35],
X-ray diffraction, and scanning electron microscopy. The
Raman spectroscopic analysis was carried out using a Bruker
Dispersive Senterra with a resolution of 9–15 cm−1, an inte-gration
time of 10 s, a laser of 785 nm, and 100 milliwatts of
potential. Crystal structure analysis using X-ray diffraction
was carried out in a Rigaku Minifles, using Cu Ka radiation,
with a 30 kv operation voltage and 15 mA of current, at a
velocity of 2°/min. The morphological changes were moni-tored
by scanning electron microscopy with a JEOL JMS-
6060LV, using an acceleration voltage of 15 kv. Analysis of
the ●OH at different current densities (0.45, 0.91, and
1.83 mA cm−2) was performed using the system described
above under constant agitation. The analysis was carried out
by UV-visible spectroscopy, using the textile colorant N,N-dimethyl-
p-nitrosoaniline (pNDA), with a concentration of
2.5×10−5 M in 0.5 M H2
SO4. The reaction volume was

3. Electrocatalysis (2013) 4:189–195 191
60 ml (pNDA+H2SO4), which was transported to a UV–vis
spectrophotometer with the help of a peristaltic pump through
a fluid cell at a velocity of 12.5 mL min−1, where the decrease
of absorbance (λ=350 nm) marked the generation potential of
●OH under the different current densities imposed. The ex-perimental
system for this analysis is shown in Fig. 1. For this
method, a high quantity of RNO (2.5×10−5 M) was used to
prevent the recombination of ●OH.
Determination of the Polarization Time
The estimate of the polarization time required to completely
eliminate the graphite impurities was obtained by polarizing
newBDDelectrodes fromthe same lot and applying a previously
selected current density for 0–15 min. Monitoring of the surface
a
b
c
250 300 350 400 450 500
changes and structures was performed using Raman spectrosco-py,
X-ray diffraction, and scanning electron microscopy every
5min, employing the conditions previouslymentioned. Finally, a
cyclic voltammetry analysis was performed, with the goal of
verifying the electroactivity of C-sp2 and the effectiveness of the
applied procedure. This analysis was done in a three electrode
cell, utilizing 0.5 M H2SO4 as the supporting electrolyte, in the
absence of oxygen, using BDD as the anode, a rod of Ti as
contra-electrode, and a mercury sulfate electrode (Hg|Hg2SO4|
K2SO4 (SAT), E°=0.640 V vs. SHE) as the reference electrode.
f
d
e
0.2
0.4
0.6
0.8
1.0
1.2
1.4
Abs
nm
Fig. 1 Experimental system for
in situ analysis ●OH. a)
Electrochemical cell, b) rectifier,
c) peristaltic pump, d) UV–vis
spectrophotometer, e) data
acquisition PC, and f) heat
exchanger
1000 1100 1200 1300 1400 1500 1600 1700 1800
35000
30000
25000
20000
15000
10000
5000
0
B
C-sp2
C-sp3
j/mA cm-2 Raman shift/cm-1
Intensity/a.u.
0
0.45
0.91
1.83
Fig. 2 Raman spectrumof Ti/BDD polarized at current densities of 0.45,
0.91, and 1.83 mA cm−2 for 10 min, in 0.5 M H2SO4
0.0 0.5 1.0 1.5 2.0
0.6
0.5
0.4
0.3
0.2
0.1
0.0
Csp2/Csp3
-2
j/mA cm
Fig. 3 Analysis of the C-sp2/C-sp3 ratio of Ti/BDD polarized at current
densities of 0.45, 0.91, and 1.83 mA cm−2 for 10 min, in 0.5 M H2SO4

4. 192 Electrocatalysis (2013) 4:189–195
Results and Discussion
Selection of the Current Density
Figure 2 shows a Raman spectroscopic analysis of the diamond
electrode, subjected to an anodic polarization process in 0.5 M
H2SO4, with an applied current density of 0, 0.45, 0.91, and
1.83 mA cm−2 over the same area (2.185 cm2). It was observed
that at time 0, only the bands corresponding to C-sp2 and C-sp3
were observed, at a Raman displacement of 1,600 cm−1 [1, 2,
36] and 1,313.24 cm−1, respectively. Although the peak corre-sponding
to C-sp3 has been reported at a value of 1,332 cm−1,
the displacement observed at 1,313.24 cm−1 has been linked to
the presence of C-sp2 in highly doped BDD [8]. In turn, in this
first analysis, boron was not detected probably because it was
located at sites that did not contribute to the continuum of
electronic states [8]. Upon performing the polarization, it was
observed that, as the current density was increased, the
peak corresponding to C-sp2 decreased from 17,775.90 to
35000
30000
25000
20000
15000
10000
5000
663.43 u.a, after the application of a current density with the
value of 1.83 mA cm−2. In parallel, the peak corresponding to
C-sp3 diminished from 32,476.83 to 15,732.94 u.a and shifted
to 1,332 cm−1,which is characteristic of diamond [7, 32]. In this
case, the decrease in the peak corresponding to C-sp3 has been
linked to the incineration of C-sp2 at increasing current densities
and to the presence of boron [7, 32, 37, 38], whose peak,
situated in the 1,200–1,280 cm−1 range [39] significantly in-creased
after elimination of C-sp2 which results in a bigger
exposed area. Analyzing the relationship between C-sp2/C-sp3
(Fig. 3) and taking as a reference the Raman intensities, it was
possible to observemore clearly the effect of the current density
on the elimination of C-sp2. It was also observed that, as the
current density was increased to 0.45, 0.91, and 1.83 mA cm−1,
the C-sp2/C-sp3 ratio decreased almost to zero. The importance
of this analysis has been reported in the literature, indicating
that the C-sp2/C-sp3 proportion can directly influence the elec-trochemical
response of interest [9]. Although the anodic po-larization
was performed over the same area, the results
obtained through Raman spectroscopy showed that the appli-cation
of 1.83mA cm−2 was sufficient for eliminating almost all
the C-sp2. In order to analyze the accumulated effect that the
current densities 0.45 and 0.91 mA cm−2 could have on the
Table 1 Analysis of the effect of the current density applied on the
removal of C-sp2 during the anodic polarization process of BDD
j/mAcm−2 Ei Raman
intensity
(a.u.)
C-sp2
inc
(%)
C-sp2
res
(%)
ta.p. C-sp2/
C-sp3
C-sp3/
C-sp2
0 – 17,775.90 0 100 0 0.54 1.82
0.45 1.86 2,306.23 87.02 12.98 10 0.14 6.82
0.91 1.90 1,621.73 29.68 70.32 20 0.1 9.70
1.83 1.96 663.43 59.09 40.91 30 0.042 23.71
Ei interfacial potential, C-sp2
inc graphite incinerated, C-sp2
res residual
graphite, ta.p. accumulated polarization time
0.45 mA cm-2
0.91 mA cm-2
1.83 mA cm-2
0 2 4 6 8 10
1.0
0.8
0.6
0.4
0.2
0.0
A/Ao (absorbance)
time (min)
Fig. 4 Absorbance normalized values of 2.5×10−5 M pNDA+0.5 M
H2SO4 solution, obtained at 1 min, intervals during 10 min, of
galvanostatic electrolysis, at 0.45, 0.91, and 1.83 mA cm−2
1000 1100 1200 1300 1400 1500 1600 1700 1800
0
time (min)
C-sp3
C-sp2
Raman shift/cm-1
Intensity/a.u.
0
10 5
15
Fig. 5 Raman spectrum of Ti/BDD pretreated by anodic polarization at
1.83 mA cm−2, in 0.5MH2SO4 during 0, 5, 10, and 15 min
40 45 50 55 60 65 70 75 80
6000
5000
4000
3000
2000
1000
0
Ti
Ti
TiH
(111)
C-sp3
(220)
C-sp3
time (min)
2 ( ° )
Intensity/a.u
0
5
10
15
Ti
Fig. 6 XRD analysis of the crystalline structure of BDD, pretreated by
anodic polarization at 1.83 mA cm−2 for 0, 5, 10, and 15 min

5. Electrocatalysis (2013) 4:189–195 193
a1 a2 a3 a4
b1 b2 b3 b4
c1 c2 c3 c4
Fig. 7 SEM analysis of the BDD surface, pretreated by anodic polarization at 1.83 mA cm−2. a1–a4 ×1,000, b1–b4 ×5,000, and c1–c4 ×10,000 at 0, 5,
10, and 15 min, respectively
elimination of C-sp2, an analysis was performed, taking as a
reference the Raman intensities of the peaks corresponding to
C-sp3 and C-sp2. In this analysis, the value of the peak intensity
after the application of the different current densities was taken
to be 100 %. Table 1 shows that at polarization time 0 (tp=0),
with a current density of 0.45 mA cm−2, the peak intensity
corresponding to C-sp2 decreases from an initial value of
17,775.90 to a value of 2,306.23, representing 87.02%graphite
incinerated (C-sp2
inc%) and 12.98 % graphite residue (C-sp2
res%). On the other hand, after the application of a current
density of 0.91 mA cm−2, the Raman intensity of C-sp2 dimin-ished
from 2,306.23 to 1,621.73, for a C-sp2
inc% of 29.68 %
and a C-sp2
res% of 70.32 %. Upon application of a current
density of 1.83mAcm−2, the intensity decreased from1,621.73
to 663.43, representing a C-sp2
inc%of 59.09%and a C-sp2
res%
of 40.91 %. From this analysis, we can conclude that, although
the major portion of C-sp2 is incinerated by the application of
an initial current density of 0.45 mA cm−2, when a current of
0.91 mA cm−2 is applied, the removal percentage is lower and
increases with the application of 1.83 mA cm−2, as this last
value was used to estimate the polarization time needed to
completely eliminate C-sp2. Another interesting result was that
when a current density of 1.83mAcm−2,was applied the C-sp3/
C-sp2 ratio increased from 1.82 to 23.71, illustrating the effect
of current density on the activation process of C-sp3 surfaces.
Considering these results and taking into account that
1.83 mA cm−2 was the “ideal” current density for the removal
of C-sp2, an analysis of interfacial potential corresponding to
the densities of 0.45, 0.91, and 1.83 mA cm−2 was performed,
indicating values of 1.86, 1.90, and 1.96 V vs. Hg|Hg2SO4,
respectively. That is to say, potentials located within the zone of
decomposition medium where the ●OH are generated. In order
to determine whether ●OH participated in the removal of C-sp2,
a qualitative analysis of these species was performed. The
results (Fig. 4) show that at a current density of
1.83 mA cm−2, the decrease in the absorbance of the RNO is
due to the generation of more ●OH [40, 41]. This result was
expected, since increasing current density favors the elimina-tion
of C-sp2, permitting the surface activation of C-sp3. These
results are consistent with the results published in the literature,
which indicate that graphite is strongly oxidized in an acidic
2.0
1.5
1.0
0.5
0.0
b
without treatment
with treatment
0.0 0.5 1.0 1.5 2.0
j / mAcm-2
E / V vs Hg/Hg2SO4
a
C-sp2
Fig. 8 Analysis by cyclic voltammetry of the BDD, without treatment
(a) and with treatment (b) applying 1.83 mA cm−2 in 0.5 M H2SO4

6. 194 Electrocatalysis (2013) 4:189–195
medium under anodic polarization conditions of oxygen evo-lution,
where active intermediates such as ●OH contribute to
this corrosion [42]. In this sense, the incineration of C-sp2, due
to the imposed current can be explained through the reaction
mechanisms of water oxidation [43] whereC-sp2 is oxidized to
CO2 and water, a reaction that occurs in parallel to oxygen
evolution.
Determination of Polarization Time
To standardize the polarization time, Raman spectroscopy, X-ray
diffraction (XRD), and scanning electron microscopy
(SEM) analysis were performed. Figure 5 shows the Raman
spectroscopy analysis of BDD, subjected to an anodic polar-ization
process, applying the previously selected current den-sity
(1.83mAcm−2). The analysiswas performed every 5min.
It was observed that, after a polarization time of 15 min, the
peak corresponding to the presence of C-sp2 disappeared
completely. In order to verify changes in the crystalline struc-ture
of the diamond, XRD analysis was performed. Figure 6
shows the diffractograms after the polarization of BDD taken
every 5 min. In this analysis the preferential crystallographic
planes corresponding to C-sp3 were (111) and (220) [39], at
43.93° and 75.36° of 2θ, respectively. The diffraction peak
corresponding to C-sp2 was not localized, since the graphite
was probably in an amorphous form, which was not a limita-tion
in the Raman spectroscopy analysis [44]. Furthermore, it
was observed that the 220 diffraction peak increased from its
initial value of 1,033.87 to 2,843.89, while the 111 peak
increased from an initial value of 1,564.25 to 4,889.25, with
a difference of 1,810.02 and 3,325 u.a, respectively, attributed
to the removal of C-sp2 from the surface of the BDD. This
result is in agreement with the results obtained using Raman
spectroscopy, indicating the complete disappearance of C-sp2
on the BDD surface.
Figure 7 shows the morphological analysis of BDD, taken
every 5 min. In this analysis the morphology of BDD
consisted of randomly ordered crystals and well-defined
phases [7, 11]. Before pretreatment (a1), no well-defined
BDD crystals were observed. However, after treatment, these
crystals acquired their characteristic forms (Fig. a2, a3, and a4),
reaching their maximum definition at 15 min. The analysis
was performed over the same area, and itwas assumed that the
definition of the crystals corresponded to the removal of C-sp2
over the BDD surface, results that are consistent with the
analysis performed by Raman spectroscopy and X-ray diffrac-tion.
In order to verify the effectiveness of the applied proce-dure,
a cyclic voltammetry analysis was performed. In Fig. 8,
it can be observed than C-sp2 shows an oxidation peak around
1.4 V vs. Hg|Hg2SO4 [8, 45, 46], which disappears complete-ly
after the application of 1.83 mA cm−2 for 15 min, showing
that the electrode stays frees of impurities and the surface of
C-sp3 is activated.
Conclusions
Surface activation of C-sp3 through the elimination of C-sp2 was
achieved by applying 1.83 mA cm−2 in 0.5 H2SO4 for 15min. In
this work we have shown that C-sp3 surfaces can be activated
successfully by applying an anodic current density corresponding
to sufficiently high potential where ●OH radicals are generated.
Acknowledgments The authors thank the Mexican Council of Science
and Technology (CONACyT) for the financing granted for the implementation
of this study through the Sector Research Fund for Education—Basic Science-
4955 and the Joint Fund of the State Government ofVeracruz Key-96313. The
authors also thank Dr. LuisMiguel Apátiga Castro, as well as Dr. Eric Rivera
Muñoz, Dra. Genoveva Hernández Padrón, and Alicia del Real López, from
Center for Applied Physics and Advanced Technology (CFATA), for their
support in the characterization of BDD during the different pretreatments.
Alejandro Medel thanks CONACyT for the fellowship that it granted.
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