An in situ electrochemical study of electrodeposited nickel and nickel-yttrium oxide composite using scanning electrochemical microscopy

Article abstract of DOI:10.1149/1.1522722

Electrodeposited nickel and nickel-yttrium oxide composite samples were studied in situ using scanning electrochemical microscopy The monitored probe currents in phosphate-citrate buffer (pH 4.2) in the presence or absence of Ru(NH₃)₆³⁺ as an oxidizing mediator near the Ni surface show that the SECM is a useful tool for studying the electrochemical activity of heterogeneous metal surfaces at micrometer scales. The SECM ultramicroelectrode probe tip provides information about the shape, activity, and location of particles, such as Y₂O₃ introduced (codeposited) in the Ni matrix of the composite. Experiments show that the Ni matrix in the composite coating is more active than the pure Ni coating. This fact is expected, because of texture changes in the Ni structure upon introduction (by codeposition) of Y₂O₃ particles. In the absence of a mediator in the solution, the electrochemical activity of heterogeneous metal surface at a microlevel is investigated by using O₂ concentration changes. The rate of reaction for O₂ reduction was found to vary locally at electrodes floating at the open-circuit potential (OCP) when compared to an electrode potentiostatically polarized at a more positive potential than the OCP. This behavior suggests that local anode and cathode regions are being observed at the OCP sample.

Full text of DOI:10.1149/1.1522722

Journal of The Electrochemical Society, 150 ͑1͒ C1-C6 ͑2003͒
C1
0
013-4651/2003/150͑1͒/C1/6/$7.00 © The Electrochemical Society, Inc.
An In Situ Electrochemical Study of Electrodeposited Nickel
and Nickel-Yttrium Oxide Composite Using Scanning
Electrochemical Microscopy
L. Veleva,*, a L. Diaz-Ballote, and David O. Wipf *
a
,z
Department of Chemistry, Mississippi State University, Mississippi State, Mississippi 39762, USA
Electrodeposited nickel and nickel-yttrium oxide composite samples were studied in situ using scanning electrochemical micros-
3
ϩ
copy ͑SECM͒. The monitored probe currents in phosphate-citrate buffer ͑pH 4.2͒ in the presence or absence of Ru(NH3)6 as an
oxidizing mediator near the Ni surface show that the SECM is a useful tool for studying the electrochemical activity of hetero-
geneous metal surfaces at micrometer scales. The SECM ultramicroelectrode probe tip provides information about the shape,
activity, and location of particles, such as Y O introduced ͑codeposited͒ in the Ni matrix of the composite. Experiments show that
2
3
the Ni matrix in the composite coating is more active than the pure Ni coating. This fact is expected, because of texture changes
in the Ni structure upon introduction ͑by codeposition͒ of Y O3 particles. In the absence of a mediator in the solution, the
2
electrochemical activity of heterogeneous metal surface at a microlevel is investigated by using O concentration changes. The rate
2
of reaction for O reduction was found to vary locally at electrodes floating at the open-circuit potential ͑OCP͒ when compared to
2
an electrode potentiostatically polarized at a more positive potential than the OCP. This behavior suggests that local anode and
cathode regions are being observed at the OCP sample.
©
2002 The Electrochemical Society. ͓DOI: 10.1149/1.1522722͔ All rights reserved.
Manuscript submitted March 4, 2002; revised manuscript received June 17, 2002. Available electronically November 15, 2002.
lasers.10-13 The probable crystal structure of Y O indicates spatial
Nickel coatings are widely used to protect iron, copper, or zinc
2
3
alloys against corrosive attack in rural or industrial atmosphere or
are used as undercoatings on brass and chromium for precious metal
composition inhomogeneity, with the presence of oxygen
9
vacancies. The crystal structure is modified fluorite (CaF ), with
2
1
coatings. Because nickel is magnetic, it is sometimes plated ͑elec-
one-fourth of the total oxygen atoms removed to maintain electrical
neutrality.
trodeposited͒ where magnetic properties are desired. Nickel can be
deposited with minimal internal stress and is therefore useful in
electroforming and aerospace applications. Nickel plating for engi-
neering purposes provides relatively good wear and abrasion resis-
tance ͑on molds, for example͒, and it is also used on electronic
circuit boards as a protective barrier layer against corrosive chemi-
cal environments. In several applications, nickel composites have
been formed where the nickel is codeposited with dispersed inert
A simple Ni-Cr matrix, when combined with the strengthening
effect of Y O dispersoid during mechanical alloys, provides excel-
2
3
lent creep properties and resistance to thermal fatigue. In addition,
chemical surface resistance is improved and operation in severe con-
ditions ͑industrial environments͒ without protective coatings is
14
possible. Similar effects and improvement of the corrosion resis-
tance were found for a zirconium matrix, due to the addition of
2-4
inorganic particles such as RuO , SiO , and SiC. Lately, the use
15
2
2
Y O dispersoid and for Zn-Al-Cu alloy, modified by deposition
2
3
and deposition of yttrium oxide thin films, as well as complex metal
1
6,17
of a thin layer of Y O .
2
3
oxide films containing Y O are of interest for electrochemical and
2
-7
3
18
In our previous study, solid particles of Y O were codeposited
electronic applications.5
2
3
in a matrix of plated Ni ͑from a Watts bath͒, and this composite was
compared to pure-plated Ni. Some differences in the electrochemical
behavior of both coatings were detected in polarization curve, cyclic
voltammetry, and impedance ͑EIS͒ measurements. Changes in
corrosion-current density, polarization resistance, and charge-current
density were probably due to the blocking effect of Y O particles at
Sintered yttrium oxide (Y O ) is a white polycrystalline powder,
2
3
with a high melting temperature ͑2400°C͒, high mechanical break-
down strength, and good chemical stability. It has weakly alkaline
properties and is only slightly soluble in neutral aqueous solution
forming Y(OH) . The oxide is insoluble in alkaline solutions but is
3
2
3
more soluble in acid pH, producing salts, which are hydrolyzed in
several steps, giving various positively charged cations, as shown in
Eq. 1
the composite surface. Correcting for the actual metal area was dif-
ficult because the clusters of Y O are not distributed uniformly
2
3
within the Ni composite matrix, and the particle ͑cluster͒ diameter
ranges from 0.5 to 10 ␮m. Thus, the previously measured param-
eters produce an average electrochemical response over the entire
composite area and, due to this fact, information about the local
activity of nickel near and far from the Y O particles is lost.
3
ϩ
2ϩ
ϩ
͓
Y͑H O͒ ͔ → ͓Y͑H O͒ OH͔ → ͓Y͑H O͒ ͑OH͒ ͔ ͓1͔
2
6
2
5
2
4
2
2
3
1
9
The solubility of yttrium oxide is highest at pH Ͻ 1.5 but undergoes
hydrolysis at pH Ͼ 2.
The scanning electrochemical microscope ͑SECM͒ is used here
to provide local information about the electrochemical activity of
the nickel/nickel composite surfaces. The SECM uses an ultramicro-
electrode ͑UME͒ probe, with a diameter of a few nanometers to 25
The use and deposition of yttrium oxide thin films, as well as
complex metal-oxide films containing Y O are of interest for elec-
2
3
trochemical and electronic applications.8 Because of the relatively
-10
␮m, to image topographic and chemical variations near a phase
surface. This information can be used to examine different local
large dielectric constant (⑀ of 9-18͒ of Y O and its high dielectric
r
2
3
2
0-28
electrochemical activities.
For example, images of surface reac-
strength (Ebd of 1-5 MV/cm͒, it has attracted much attention for use
as gate oxide insulators in large-scale integrated ͑LSI͒ devices, LSI
capacitors, and insulating layers of electroluminescent ͑EL͒ devices,
high-temperature ion-conducting ceramics, and rare-earth-doped
tivity are obtained by moving the UME probe parallel to a sample’s
surface at a constant distance ͑a few tip diameters͒. Since this
method does not require electrical contact with the sample, there are
few restrictions on the chemical or physical nature of the sample.
The feedback mode of the SECM uses a mediator species to provide
information about the electrochemical activity ͑with respect to the
*
Electrochemical Society Active Member.
29
a
mediator species͒ of the substrate. The feedback experiment uses
Present address: Research Center for Advanced Study, Department of Applied
Physics, Unidad Merida, C.P. 97310, Merida, Yucatan, Mexico.
E-mail: wipf@ra.msstate.edu
the probe tip electrode to generate an oxidized or reduced form of
the mediator. At close probe-substrate separation, the mediator can
z
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Journal of The Electrochemical Society, 150 ͑1͒ C1-C6 ͑2003͒
quickly diffuse to the substrate surface. Positive feedback occurs if
the mediator is returned to its original oxidation state by electron
transfer at the substrate. The probe current increases during positive
feedback due to the regeneration process. The increase in current is
a function of the probe-substrate separation and the rate of substrate-
mediator electron transfer. Negative feedback occurs at an inert sub-
strate. The probe current decreases as the probe-substrate separation
decreases due to diffusional blocking of the substrate surface. An
alternate experiment uses the probe as a scanning electrochemical
sensor. In the substrate generation/tip collection ͑SG/TC͒3 mode the
probe senses the concentration of redox-active species generated at
0
3
1,32
the substrate surface.
Here, the probe is an amperometric elec-
trode, and the signal at the tip is, in principle, proportional to the
concentration of redox active species in solution. The SG/TC is
more sensitive to concentration changes than the feedback mode.
Conversely, diffusion and convection cause the concentration of re-
dox species to extend significantly beyond the source, which makes
the spatial resolution in the SG/TC mode less than the feedback
mode.
In this paper, both feedback and SG/TC modes of the SECM are
used to examine and compare the local chemical activity of nickel
and nickel-Y O composite electrodes, first, by addition of a media-
Figure 1. Cross-sectional SEM image of electrodeposited Ni-Y2O3 compos-
ite coating. The black spots and stains represent yttrium oxide particles and
their clusters ͑aggregations͒.
2
3
tor and then by examination of local O2 concentration near the
sample surface.
Experimental
Electrochemical deposition of nickel and nickel-Y O
2
3
composite.—Deposition occurred in a classic Watts bath containing
͑
g/L͒ 250 NiSO •7H O, 60 NiCl •6H O, and 35 H BO ͑analytical
5°C and 4 A/dm current density ͑galvanostatic mode͒ on a stain-
4
2
2
2
3
3
reagent chemicals͒ at pH 3.5-4.2. The experiment was performed at
5
2
less steel cathode, which allowed a later removal of the Ni coating
for testing. The substrate was mechanically polished, etched, and
degreased before deposition of each coating. Two Ni anodes
͑
99.99%͒ were arranged on either side of the cathode to produce a
homogeneous electric field. For the composite electrodeposition,
0-70 g/L Y O powder, with particle diameter less than 0.5 ␮m,
5
2
3
͑
produced by the Institute for Pure Compounds of Bulgaria͒ was
introduced into the bath. Particles were held in suspension by air
bubble agitation from the bottom of the bath cell.
The codeposition conditions for Y O3 are such that the solid
2
oxide particles are positively charged because the solution pH is
lower than their isoelectric point ͑i.e.p. ϳ pH 7.6͒.3
3,34
In addition,
Figure 2. EDX spectra of electrodeposited Ni coating.
hydrolysis of Y O ͑Eq. 1͒ also leads to cationic particles. The Y O
2
3
2
3
particles are thus attracted and adhere to ͑adsorb͒ on the negatively
charged cathode. Under these conditions, Y O is embedded in the
2
3
growing metal layer of Ni.
Stripped coating samples were evaluated by means of a scanning
electron microscope ͑SEM͒ and energy dispersive X-ray analyzer
͑
EDX͒, to explore morphology and composition, respectively. It can
be seen ͑Fig. 1͒ that the dark particles of Y O have been incorpo-
2
3
rated in the Ni matrix as individual particles or, more often, as
clusters with a diameter greater than 2-3 ␮m. The EDX microanaly-
sis confirmed the presence of a significant amount of yttrium in the
composite, especially compared to the Ni coating ͑Fig. 2 and 3͒.
SECM experiments.—The SECM images were obtained by scan-
ning the probe ͑a 2 ␮m diam Pt tip͒ parallel to the cross section of
the Ni coatings ͑electrodeposited Ni and Ni composite͒. The experi-
mental setup is similar to that previously reported.2
9,35-37
The probe
is mounted on a TS-75Z stage with integral encoder ͑Burleigh In-
struments, Inc.͒ for vertical movement. TSE-150 translation stages
were used for horizontal movement. Closed-loop positioning was
accomplished with a Burleigh 6000 ULN controller. An EI-400 bi-
potentiostat ͑Ensman Instrumentation͒ was used for all SECM ex-
periments. Data acquisition and position control were enabled with a
custom LabView ͑National Instruments, Austin, TX͒ program.
Samples of the Ni and Ni-Y O were embedded in epoxy and pol-
Figure 3. EDX spectra of electrodeposited Ni-Y O composite coating.
2
3
2
3
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Journal of The Electrochemical Society, 150 ͑1͒ C1-C6 ͑2003͒
C3
Figure 4. SECM image of the probe current monitored on Ni coating ͑at
OCP͒. Scanned in situ in pH 4.2 phosphate-citrate buffer with 2.0 mM
3
6
ϩ
Ru(NH3)
.
ished to expose a cross section of about 60-80 ␮m ϫ 2-3 mm of
the material for imaging. Before all experiments, samples were pol-
ished with 0.05 ␮m gamma alumina powder ͑Buehler, Inc.͒. All
potentials are referenced to a Ag/AgCl electrode.
Figure 5. Optical microscope image of Ni-Y O composite substrate. ͑The
2
3
black spots are Y O particles.͒
2
3
The electrochemical study was performed in two solutions,
a pH 4.2 phosphate-citrate buffer38 with 2 mM Ru(NH3)6
͓
3ϩ
3ϩ
the reduction of Ru(NH3)6 on the Ni coating occurs at about the
same rate over the surface. The Ni epoxy boundary is sharply de-
as (Ru(NH ) Cl )] as a mediator ͑oxidizing agent͒ and a pH 4.2
3 6 3
2
ϩ
phosphate-citrate buffer with 6 mM NaCl ͑to replace the chloride
anions that are introduced by dissociation of the ruthenium salt͒.
Neither solution was deareated during SECM experiments. This pH
buffer was chosen because it approximates an acid, polluted atmo-
spheric environment, in the absence or presence of chloride con-
tamination ͑in coastal regions͒. All SECM experiments were per-
formed at an initial probe-substrate separation of about 2-3 ␮m. This
position was set by monitoring the probe current-distance curve or
by carefully approaching the surface until electrical contact was de-
tected between the probe and sample. During image acquisition, the
probe scan rate was normally 20 ␮m/s.
fined by a three- to four-fold lower probe current ͓Ru(NH ) con-
3
6
2ϩ
centration͔. The low concentration of Ru(NH3)6 over the epoxy
substrate indicates that the epoxy is electrochemically inert.
Areas of the Ni-Y O composite substrate containing a high con-
centration of Y O particles were selected by optical microscopy
Fig. 5͒ for SECM imaging. The SECM image in Fig. 6 of the
Ni-Y O composite substrate clearly indicates the shape and loca-
tion of the larger Y O particles. Smaller particles are not resolved
but appear as slightly darker regions in the image. The concentration
of Ru(NH ) is very low over the oxide particles, indicating that
2
3
2
3
͑
2
3
2
3
2
6
ϩ
3
ϩ
2
6
Ru(NH3) is produced principally on the Ni matrix. Based on the
Results and Discussion
overall probe current in Fig. 4 and 6, the composite Ni matrix ap-
pears more active than the Ni coating. This fact could be due to
3ϩ
SECM in the presence of Ru(NH3)6 mediator.—Cyclic voltam-
metry on the Pt tip ͑UME͒ showed that the half-wave reduction
3
ϩ
2ϩ
potential for the Ru(NH3)6 /Ru(NH3)6 couple is about Ϫ0.175 V
vs. Ag/AgCl͒ in pH 4.2 phosphate-citrate buffer. The open-circuit
potentials ͑OCP͒ of the Ni and Ni-Y O composites in this solution
͑
2
3
are Ϫ0.20 and Ϫ0.21 V ͑vs. Ag/AgCl͒, respectively. Since the open
3
6
ϩ
2ϩ
circuit substrate reduces Ru(NH3)
to Ru(NH3)6 , a substrate
generation tip collection ͑SG/TC͒ SECM experiment was used, in
which the substrate was held at OCP during the SECM experiment
and the probe potential was Ϫ0.05 V to oxidize ͑collect͒ the sub-
2
ϩ
strate generated Ru(NH3)6 . The probe current is thus a concentra-
2
ϩ
tion map of the Ru(NH3)6 near the metal substrate. Depending on
3
ϩ
the substrate activity, the reduction of Ru(NH3)6 will occur at a
higher or lower rate, producing a higher or lower concentration of
3
6
ϩ
this ion. Some contribution due to feedback of the Ru(NH )
/
3
2
ϩ
Ru(NH3) is also expected given the substrate potential.
6
An SECM image of the probe current monitored at a 2-3 ␮m
separation from the Ni coating is presented in Fig. 4. The probe
current is uniform over the Ni coating surface with an increase along
the right edge of the Ni electrode. The increase can be ascribed to a
slight tilt in the sample along both the right-left and top-bottom axes
and also to the Ni protruding from the epoxy. The tip is closest to the
surface at the top right. The smooth, featureless image indicates that
Figure 6. SECM image of the probe current monitored on Ni-Y O com-
2
3
posite substrate ͑at OCP͒. Scanned in situ in pH 4.2 phosphate-citrate buffer
3
ϩ
with 2.0 mM Ru(NH3)6
.
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Journal of The Electrochemical Society, 150 ͑1͒ C1-C6 ͑2003͒
Figure 7. SECM vertical concentration map showing the reduction current
3
6
ϩ
for Ru(NH3) . Acquired at a Ni-Y O composite substrate in phosphate-
2
3
citrate buffer at pH 4.2.
changes in the Ni structure because of the inclusion of Y O par-
2
3
ticles in the Ni matrix during the metal electrodeposition ͑such as
the preferred orientation of its crystal planes, defects, and internal
stresses͒. The greater activity is also predicted by the more negative
OCP of the composite.
Figure 8. Cyclic voltammogram ͑100 mV/s͒ at the probe ͑Pt UME͒ used in
the SECM experiment, run in phosphate-citrate buffer at pH 4.2. ͑
solution exposed to air, ͑ ͒ solution deaerated with N for 30 min.
The SECM can also provide a vertical concentration map over
the substrate. Rather than scan laterally at a fixed vertical position
across a surface, the scan proceeds laterally along one axis and
vertically along the other. An x-z vertical concentration map was
acquired at the 50 ␮m y position of Fig. 6. Data were acquired by
repetitive scans of 50 ␮m vertically from the electrode surface into
the bulk solution while incrementing the x position. The monitored
probe current is presented in Fig. 7. It shows how the cloud of
͒
2
process. However, as the activity ͑the dissolution͒ of the Ni elec-
trode changes, the concentration of the naturally present mediator
(O2) will be changed at the Ni surface.
Evidence for differences in the surface activity were found when
the probe was scanned in the vicinity of Ni-Y2O3 composite sub-
strate ͑Fig. 9͒. In this case, the O2 reduction current is very small
2
6
ϩ
Ru(NH3) ͑formed on the Ni͒ increases, when the distance ͑axis z͒
between probe and Ni composite surface decreases. ͑The dark areas
in this SECM image, those with a lower probe current at left and
right parts of axis x, represent epoxy substrate.͒
and the SECM image does not sharply define the shape of the Y O3
2
particles or the metal-epoxy boundary. The dark region along the
mid-right side indicates higher O concentration at the edge of the
3
ϩ
2
Phosphate-citrate buffer with 6 mM NaCl.—Ru͑NH3͒6 is a mild
composite and, thus, a likely region of high Ni dissolution. In addi-
tion several dark spots in the upper middle are likely to be the
location of Y O particles. Over the Ni composite substrate, O is
oxidizing agent and its presence during SECM experiments may
accelerate corrosion of the Ni substrate. Eliminating the Ru(NH3)
mediator provides a more realistic view of the activity of the Ni or
3
6
ϩ
2
3
2
consumed ͑Eq. 3͒ to support the Ni dissolution and the probe current
Ni composite surface. According to the Pourbaix diagram for the
is two times smaller than that over the epoxy substrate or Y O ,
electrochemical equilibrium of Ni in aqueous solution,39 at pH 4.2
2
3
where O reduction does not occur. A corresponding SECM image
2
and at a potential of Ϫ0.21 V ͑OCP͒, nickel will be oxidized ac-
at the Ni substrate is presented in Fig. 10A. The O reduction cur-
2
cording to the reaction
rent is also very low and in the same range as on the Ni composite.
2
ϩ
Ϫ
Ni ꢀ Ni ϩ 2e
͓2͔
The pH and OCP correspond to an area of the diagram in which
water is stable and hydrogen evolution is not favored and, therefore,
the corresponding reduction reaction is likely dissolved oxygen. The
relevant half reaction in this buffered solution is
ϩ
Ϫ
O ϩ 4H ϩ 4e ꢀ 2H O
͓3͔
2
2
Imaging of the O concentration ͑consumed during the cathodic
2
reaction on Ni͒ with the SECM gives an estimation by proxy of the
2
ϩ
Ni formation at the substrate ͑the anodic reaction͒. A voltammo-
gram at the probe electrode in this buffer solution shows a wave for
oxygen reduction and its disappearance upon N sparging ͑Fig. 8͒.
2
Images were acquired by holding the probe electrode at a potential
of Ϫ0.3 V in order to reduce O ͑Fig. 8͒, while the Ni or Ni com-
2
posite substrate was unbiased and was floating at the OCP. The
experiment in this case is a mixture of a negative feedback and a
SG/TC experiment. Neither the tip nor substrate potential is suffi-
cient to effect a mass-transfer-limited reduction of O ͑the substrate
2
OCP varied slightly between experiments but was always about
Ϫ0.21 V vs. Ag/AgCl͒. This allows the tip to sample the O con-
Figure 9. SECM image of the probe current monitored on Ni-Y O com-
2
2
3
centration without significantly perturbing it through the electrolysis
posite substrate ͑at OCP͒ in phosphate-citrate buffer at pH 4.2
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Journal of The Electrochemical Society, 150 ͑1͒ C1-C6 ͑2003͒
C5
artifacts in the image. These are most apparent on the epoxy side of
the image and should be ignored.͒ For example, Fig. 11 presents the
SECM image for the probe reduction current of oxygen found in a
vicinity of the Ni composite. This figure is interesting in that it
shows the evolution of the chemical environment as the substrate
potential is moved from OCP to ϩ100 mV vs. OCP. Acquiring a
raster image requires a fixed amount of time and thus the y axis in
Fig. 11 can be considered to be the equivalent of a time axis. Ini-
tially (y ϭ 0), the image is equivalent to that observed at OCP ͑Fig.
9͒. As time increases, the image becomes less resolved and the tip
current decreases. At short times, the image has good contrast be-
tween the Ni and epoxy matrix and is very similar in appearance to
the identical region of Fig. 9 ͑at OCP͒. This contrast is due to the
presence of an O2 gradient between the epoxy ͑high O ) and Ni
2
͑
less O ). At longer times (y Ͼ 100) the image shows less overall
2
current as the O is depleted near the substrate surface by anodic
2
2
ϩ
dissolution of the Ni ͑thus generating a significant Ni concentra-
tion in the vicinity of the substrate͒. The region of depleted O2
extends beyond the Ni composite surface due to the effect of diffu-
sion. The implication of this result is that O reduction occurs more
2
uniformly across the surface when the Ni substrate is polarized at
100 mV more positive potential ͑vs. OCP͒ than when the substrate is
floating at the OCP. Polarization apparently overcomes local anodic
and cathodic activity of the open circuit electrode, eliminating varia-
tions in O concentration. The time to acquire the image in Fig. 11 is
2
about 20 min. The fact that the change in image does not occur
instantly upon polarization, but evolves slowly is an indication that
the net oxygen consumption is small and roughly similar at the
polarized and open circuit electrode ͑Fig. 9͒. These images also
2
ϩ
indicate that Ni reduction at the tip is not a primary source of the
2
ϩ
image contrast. Ni reduction would produce an increase in ca-
thodic current at the probe electrode. An alternate explanation is that
ϩ
the probe electrode is reducing H ion and thus the decrease in
ϩ
26
cathodic current is attributed to a local decrease in H ion. This
explanation is unlikely since the Pourbaix diagram indicates that the
Figure 10. SECM image of the probe current monitored on Ni coating sub-
strate. Scanned in situ in pH 4.2 phosphate-citrate buffer. ͑A͒ At OCP and
ϩ
H
reduction at Ni is not favored at these potentials. In addition, the
͑
B͒ at anodic polarization of 100 mV ͑vs. OCP͒.
magnitudes of the current changes are consistent with the change in
probe electrode current at Ϫ0.3 V in the absence and presence of O2
as seen in the cyclic voltammograms ͑CVs͒ of Fig. 8. A final pos-
ϩ
Applying an anodic polarization ͑100 mV vs. OCP͒ at Ni ͑Fig.
0B͒ or Ni composite ͑Fig. 11͒ accelerates the nickel dissolution in
sible explanation is that, due to the consumption of H on the ca-
Ϫ
1
thodic sites ͑Eq. 3͒, there is a locally higher concentration of OH
2
ϩ
this buffer solution ͑in the presence of free chloride ions͒. ͑Note that
Fig. 10B was acquired with a relatively coarse resolution of the 5
␮
ions, which react with Ni ions to form a nickel hydroxide, cover-
ing the Ni surface and causing a local loss of SECM image resolu-
tion. This behavior would mask variations in activity at the electrode
surface.
m/scan line. This, and the presence of noise, produces aliasing
A vertical concentration map ͑Fig. 12͒ provides more evidence of
O consumption at the anodic polarized Ni substrate ͑100 mV vs.
2
OCP͒. The map is acquired at 50 ␮m along the axis y in Fig. 10. The
monitored tip current shows a decrease in oxygen concentration
when the tip approaches the Ni substrate and the lighter cloud de-
fines the thickness of sample and its border with the epoxy substrate.
It can be seen that in the bulk of the solution ͑100 ␮m over the Ni
substrate͒, the oxygen concentration is ϳ30% higher. This is consis-
tent with the SECM results of Gilbert and co-workers at a titanium
2
8
surface.
Conclusions
Electrodeposited nickel and Ni-Y O composite samples were
2
3
compared using in situ SECM in mixed feedback and substrate
generation-tip collection ͑SG/TC͒ modes. The experiments were
done in phosphate-citrate buffer ͑at pH 4.2͒ in the absence or pres-
3
6
ϩ
ence of Ru(NH3) as an oxidizing agent ͑mediator͒. SECM images
with the addition of a mediator clearly indicate regions of higher and
lower electrochemical activity on Ni and Ni composite surface. The
SECM images with the presence of a mediator define the shape,
activity, and location of particles, such as Y O introduced ͑code-
posited͒ in the Ni matrix of the composite. The Ni matrix in the
Figure 11. SECM image of the probe current monitored on Ni-Y O com-
posite substrate at an anodic polarization of 100 mV ͑vs. OCP͒ in phosphate-
2
3
2
3
citrate buffer at pH 4.2.
composite coating appears more active than the pure Ni coating.
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C6
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Figure 12. SECM vertical concentration map of reduction of oxygen on Ni
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vestigated by using O concentration changes in the absence of an
2
additional mediator in the solution. The rate of reaction for O re-
2
duction was found to locally vary at electrodes floating at the OCP
when compared to an electrode potentiostatically polarized at 100
mV more positive than the OCP. This behavior suggests that local
anode and cathode regions are being observed at the OCP sample.
This is intriguing and should be of interest when comparing metallic
corrosion data acquired at OCP or potentiostatically.
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The authors acknowledge the National Science Foundation for
support by grant DBI-9987028. L.D. thanks Mexican CONACYT
for his sabbatical scholarship during this period. The authors are
grateful to T. Muleshkov and N. Muleshkov for their assistance with
the deposition of coatings samples and also to Professor M. Kai-
sheva for providing Y O sample.
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Mississippi State University assisted in meeting the publication costs of
this article.
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