Synthesis of nanostructured Ni- TiO2 composite coatings by sol-enhanced electroplating

Article abstract of DOI:10.1149/1.3442366

A Ni- TiO₂ composite film was prepared by electrodeposition containing Ti precursor sol (sol-enhanced deposition). The electrochemical process, microstructures, and properties of the sol-enhanced and traditional composite coatings were studied and compared. The sol-enhanced Ni- TiO ₂ composite coating possessed a smooth surface and a compact microstructure and showed higher mechanical properties (430 HV100) compared with the traditional coatings (360 HV₁₀₀). It is believed that the strengthening effects resulted from the high dispersion of TiO₂ nanoparticles. The sol added coatings also showed slower growth of Ni grains along the [220] direction but did not change the orientation. It is suggested that the sol addition reduced the thickness of the diffusion layer and increased the limited current density. Therefore, the polarization control in the sol-enhanced process changed from the traditional concentration polarization to electrochemical polarization, avoiding the formation of loose and dendritic structures at high current density.

Full text of DOI:10.1149/1.3442366

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Journal of The Electrochemical Society, 157 ͑8͒ E122-E128 ͑2010͒
0013-4651/2010/157͑8͒/E122/7/$28.00 © The Electrochemical Society
Synthesis of Nanostructured Ni–TiO2 Composite Coatings
by Sol-Enhanced Electroplating
Weiwei Chen,a Yedong He,b and Wei Gaoa,z
aDepartment of Chemical and Materials Engineering, The University of Auckland, Auckland 1142,
New Zealand
bBeijing Key Laboratory for Corrosion, Erosion and Surface Technology, University of Science and
Technology Beijing, Beijing 100083, China
A Ni–TiO2 composite film was prepared by electrodeposition containing Ti precursor sol ͑sol-enhanced deposition͒. The electro-
chemical process, microstructures, and properties of the sol-enhanced and traditional composite coatings were studied and com-
pared. The sol-enhanced Ni–TiO2 composite coating possessed a smooth surface and a compact microstructure and showed higher
mechanical properties ͑430 HV100͒ compared with the traditional coatings ͑360 HV100͒. It is believed that the strengthening
effects resulted from the high dispersion of TiO2 nanoparticles. The sol added coatings also showed slower growth of Ni grains
along the ͓220͔ direction but did not change the orientation. It is suggested that the sol addition reduced the thickness of the
diffusion layer and increased the limited current density. Therefore, the polarization control in the sol-enhanced process changed
from the traditional concentration polarization to electrochemical polarization, avoiding the formation of loose and dendritic
structures at high current density.
© 2010 The Electrochemical Society. ͓DOI: 10.1149/1.3442366͔ All rights reserved.
Manuscript submitted March 22, 2010; revised manuscript received May 7, 2010. Published June 8, 2010.
Since the Watts solution was formulated in 1916, Ni electroplat-
The sol–gel process has been widely applied to prepare uniform
nanosized particles.19-24 Typically, the hydrolysis and condensation
reactions take place in the sol–gel process to form metal oxides or
their composite nanoparticles.25,26 The sol–gel process can be re-
garded as an in situ liquid phase synthesis of uniform nanoparticles
at room temperature, providing a precursor to introduce nanopar-
ticles into the composite electrodeposition. In the electroplating pro-
cess, the metal ions discharge on the surface of the cathode through
migration and diffusion in the electrolyte,4 which can provide an
opportunity for the in situ formed nanoparticles to integrate imme-
diately into the alloy deposit.
We have developed a novel nanostructured Ni–TiO2 composite
coating: sol-enhanced deposition by electroplating with a small
amount of transparent TiO2 sol in the electrolyte solution.27,28 This
method led to a highly dispersive distribution of TiO2 nanoparticles
in the coating, avoiding the agglomeration of TiO2 nanoparticles.
The mechanical properties were therefore significantly improved.
The present work is a systematic investigation of the electrochemi-
cal process of the sol-enhanced Ni–TiO2 composite coating. The
microstructures of the sol-enhanced and traditional composite coat-
ings were closely compared and the electrochemical process involv-
ing the composite formation were discussed.
ing has been a commercially important and versatile surface coating/
finishing process.1,2 Ni or Ni alloys have been widely deposited on
the surfaces of working parts to improve their corrosion and wear
resistance or modify magnetic and other properties.2-4 To achieve
better properties, the electrodeposited Ni or Ni alloys were modified
by codepositing second-phase particles in the matrix called compos-
ite coatings. The traditional composite coating method is a solid
particle mixing process: The second-phase particles are suspended
in the Ni electroplating solution, and then both the particles and the
Ni ions codeposit onto the specimens/parts to form composite
coatings.4 The Ni-based composite coatings were fabricated earlier
from suspensions of relatively large ͑typically micrometer level͒
particles of carbides,5 oxides,6 diamond,7 and Teflon.8,9 More re-
cently, there has been increasing emphasis on codepositing Ni ions
and superfine or nanosized particles to synthesize a new structure,
i.e., nanocomposite coatings. The superfine/nanoparticles are dis-
persed into the Ni matrix, providing significantly improved proper-
ties, such as hardness and wear resistance.10-14
The strengthening mechanisms for nanocomposite coatings can
be interpreted based on the dislocation model such as the Orowan
theory.15 In this theory, the critical condition for a dislocation to
bypass the particles in its glide plane is to bend the dislocation to a
semicircle between the particles. The dislocation with its dipoles
annihilated can move forward while dislocation loops are left be-
hind, surrounding each particle.15-17 Orowan’s criterion indicates
that mechanical properties of composite coatings increase with both
decreasing mean planar interparticle spacing and particle size.
Based on the above theory, the incorporation of second-phase
superfine/nanoparticles can be much more effective than microsized
particles in reinforcing the composite coatings. Theoretically, if the
second-phase nanoparticles are highly dispersed in composite mate-
rials, the strong interaction between dislocation lines and the nano-
particles can almost completely block the movements of disloca-
tions, leading to a huge improvement of mechanical properties.17,18
To achieve good dispersion of the nanoparticles, powder suspension
has to be physically maintained in the solution by vigorous agita-
tion, air injection, ultrasonic vibration, or adding surfactants. How-
ever, it is always difficult for nanoparticles to achieve good suspen-
sion because they have very large surface areas. The high surface
energy tends to cause agglomeration of nanoparticles in composite
coatings. Therefore, it has been a challenge to prepare highly dis-
persive nanoparticle reinforced composites or composite coatings.
Experimental
Preparation of transparent TiO2 sol.— According to the previ-
ous paper,27,29 8.68 cm3 of tetrabutylorthotitanate ͓Ti͑OBu͒ ͔ was
4
dissolved into the mixture solution of 35 cm3 ethanol and 2.82 cm3
diethanolamine ͑DEA͒. After magnetic stirring for 2 h, it was hy-
drolyzed by adding a mixture of 0.45 cm3 deionized water and
4.5 cm3 ethanol dropwise under magnetic stirring.
Electrodeposition of coatings.— Cylindrical medium carbon
steels ͑0.50 wt % C͒ with a diameter of 15 mm and thickness of 3
mm were used as the substrates. Specimens were mechanically pol-
ished using SiC paper up to a grit of #1200 and then degreased
ultrasonically in ethanol. Before electroplating, the specimens were
pretreated in 1 mol dm−3 HCl solution for 2 min at room tempera-
ture. The bath composition and plating parameters used in the
present work are listed in Table I. After 12.5 cm3 dm−3 TiO2 sol
was added into the bath, the electroplating was immediately con-
ducted. At the same time, the solution was continuously magneti-
cally stirred at a rate of 100 r min−1. The traditional Ni and
Ni–TiO2 coatings were prepared with identical bath composition
and plating parameters for comparison. The traditional Ni–TiO2
composite coatings were prepared by using the solid particle mixing
z E-mail: w.gao@auckland.ac.nz
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Journal of The Electrochemical Society, 157 ͑8͒ E122-E128 ͑2010͒
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root-mean-square microstrain of the coatings was measured by the
XRD line broadening analysis based on background subtraction and
Ni lattice planes ͑111͒, ͑200͒, and ͑220͒. Microhardness of coatings
was measured using a load of 100 g with a holding time of 15 s.
Table I. The composition of electroplating bath and processing
parameters.
Bath composition and processing parameters
Quantity
NiSO4·6H2O
NiCl2·6H2O
H3BO3
TiO2 sol
Temperature
Current
300 g dm−3
45 g dm−3
Results
40 g dm−3
Characterization of coatings.— Figure 1 shows the surface mor-
phologies of Ni coatings and traditional and sol-enhanced Ni–TiO2
nanocomposite coatings prepared at currents of 10, 50, and
100 mA cm−2. The morphologies are quite different for the tradi-
tional and sol-enhanced composite coatings. The pyramid-like Ni
nodules were formed on the surface of the traditional Ni plating at
different currents ͑Fig. 1a1-a3͒. The size of the Ni nodules became
larger when the current increased from 10 to 100 mA cm−2. A den-
dritic surface formed when the Ni coating was prepared with a high
current of 100 mA cm−2 ͑Fig. 1a3͒. The incorporation of TiO2 par-
ticles significantly changed the surface morphologies of the coat-
ings. With the low current of 10 mA cm−2, the spherical Ni nodules
similarly exist on the surfaces of both traditional and sol-enhanced
composite coatings but with different sizes ͑ϳ500 nm in Fig. 1b1
and ϳ100 nm in Fig. 1c1͒. Obviously the sol-enhanced Ni–TiO2
composite coating has a much smoother surface than the traditional
counterpart.
However, for the traditional Ni–TiO2 nanocomposite coating
prepared at the current of 50 mA cm−2, due to the fast growth of
spherical Ni nodules, a cauliflower-like surface formed with the
nodule size of ϳ5 ␮m, on which many small nodules of ϳ500 nm
in size can be seen ͑Fig. 1b2͒. In contrast, the fast growth of Ni
nodules was largely inhibited for the sol-enhanced Ni–TiO2 coat-
ings, leading to the formation of a compact structure with fewer
porosity and smoother surface ͑Fig. 1c2͒. The superfine Ni nodules
͑ϳ400 nm͒ were distributed rather homogeneously on the surface,
and the cauliflower-like structure disappeared ͑Fig. 1c2͒. The sur-
face microstructure of the sol-enhanced composite coating was not
changed much even at 100 mA cm−2 ͑Fig. 1c2 and c3͒. For the
12.5 cm3 dm−3
Room temperature
10, 50, 100 mA cm−2
method with a concentration of TiO2 nanoparticles of 10 g dm−3
The TiO2 powder ͑Sigma Co., Ltd.͒ has an anatase crystalline phase
with an average diameter of Ͻ25 nm.
.
Characterization of coatings
.— The content of TiO2 particles in
the composite coatings was calculated by the equation
weight͑TiO2͒
weight͑coating͒
C =
ϫ 100%
͓1͔
The weight of TiO2 particles in the composite coating was precisely
measured by a chemical method described below. The Ni–TiO2
composite coating was put into HNO3 solution to be fully dissolved.
TiO2 particles do not react with HNO3, so they precipitated out in
the solution. The whole solution was set aside for 2 weeks to fully
precipitate TiO2 nanoparticles. Then the TiO2 particles were sepa-
rated with a centrifuge and calcined at 90°C for 20 h to remove the
water. Finally, the dried TiO2 particles were weighed using an elec-
tronic balance with an accuracy of 0.01 mg.
The coating morphologies were analyzed using a field-emission-
scanning electron microscope. The phase structure of the coatings
was determined using X-ray diffraction ͑XRD͒ with Cu K␣ radia-
tion ͑U = 40 kV, I = 40 mA͒. Diffraction patterns were recorded
in the 2␪ range from 20 to 90° at a scanning rate of 0.02° s−1. The
Figure 1. Surface morphologies of coat-
ings: ͓͑a1͒-͑a3͔͒ Traditional Ni coatings,
͓͑b1͒-͑b3͔͒ traditional ͑solid powder mix-
ing͒ Ni–TiO2 nanocomposite coatings,
and ͓͑c1͒-͑c3͔͒ sol-enhanced Ni–TiO2
nanocomposite coatings. Each group of
coatings was prepared at 10, 50, and
100 mA cm−2
.
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Journal of The Electrochemical Society, 157 ͑8͒ E122-E128 ͑2010͒
Figure 2. Cross-sectional morphologies of coatings prepared at
100 mA cm−2: ͑a͒ The traditional Ni–TiO2 composite coating showing a
two-layer structure, ͑b͒ magnified image of layer I in ͑a͒, and ͑c͒ the sol-
enhanced Ni–TiO2 composite coating.
solid particle composite coating, with 100 mA cm−2 current den-
sity, the pillar-shaped nodules formed on the loose, dendritic surface
͑Fig. 1b3͒, a little similar to Ni coating ͑Fig. 1a3͒.
Large-scale clusters of nanosized TiO2 particles are clearly seen
on the surfaces of traditional Ni–TiO2 composite coatings, as shown
by the arrows in Fig. 1-b3. In contrast, it is difficult to identify TiO2
nanoparticles from the surface of the sol-enhanced composite coat-
ings by the scanning electron microscopy images due to their small
size and uniform distribution ͑Fig. 1c1-c3͒.
Figure 2 shows the cross-sectional morphologies of traditional
and sol-enhanced Ni–TiO2 composite coatings electroplated at
100 mA cm−2. The traditional Ni–TiO2 composite coating was di-
vided into two layers with an outer loose layer ͑layer I͒ and an inner
compact layer ͑layer II͒, as seen in Fig. 2a. Layer I indicated a
dendritic structure ͑Fig. 2b͒, identical with the surface morphology
in Fig. 1b3. In contrast, the sol-enhanced composite coating pos-
sessed a compact structure ͑Fig. 2c͒.
Figure 3 shows the content of TiO2 nanoparticles in the tradi-
tional and sol-enhanced Ni–TiO2 composite coatings. At
10 mA cm−2, the traditional composite coating contains high TiO2
͑ϳ4.3 wt %͒. The contents of TiO2 particles decreased when the
traditional composite coatings were electrodeposited at higher cur-
rents. The sol-enhanced Ni–TiO2 composite coating, however, con-
tains lower TiO2 particles ͑ϳ3 wt % at 50 mA cm−2͒ than tradi-
tional coatings. Similar contents ͑ϳ1 wt %͒ existed in both
composite coatings when they were electroplated at the high current
of 100 mA cm−2
.
Figure 4. XRD patterns of the coatings prepared with the current of ͑1͒ 10,
͑2͒ 50, and ͑3͒ 100 mA cm−2: ͑a͒ Traditional Ni coating, ͑b͒ traditional
Ni–TiO2 composite coating, and ͑c͒ sol-enhanced Ni–TiO2 composite coat-
ing.
Phase structures.— Phase structures of traditional Ni coating
and traditional and sol-enhanced Ni–TiO2 composite coatings were
analyzed by XRD, as shown in Fig. 4. At 10 mA cm−2, the three
coatings show similar phase structures with a preferential orientation
Figure 3. ͑Color online͒ The contents of TiO2 nanoparticles in composite
coatings prepared at 10, 50, and 100 mA cm−2. ͑᭿: Traditional Ni–TiO2
composite coating and red b: Sol-enhanced Ni–TiO2 composite coating.͒
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Journal of The Electrochemical Society, 157 ͑8͒ E122-E128 ͑2010͒
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Figure 6. ͑Color online͒ Microhardness of the coatings prepared at 10, 50,
and 100 mA cm−2. ͑᭿: Traditional Ni coating, red b: Traditional Ni–TiO2
composite coating, and blue ᭡: Sol-enhanced Ni–TiO2 composite coating.͒
Figure 5. ͑Color online͒ The root-mean-square microstrain of different elec-
troplating coatings prepared at 10, 50, and 100 mA cm−2. ͑᭿: Traditional Ni
coating, red b: Traditional Ni–TiO2 composite coating, and blue ᭡: Sol-
enhanced Ni–TiO2 composite coating.͒
Mechanical properties.— Figure 6 shows the microhardness of
the traditional Ni coating and the traditional and sol-enhanced
Ni–TiO2 composite coatings prepared at 10, 50, and 100 mA cm−2
.
along ͓111͔, indicating that the incorporation of TiO2 particles does
not have a significant effect on the phase structures.
The traditional and sol-enhanced Ni–TiO2 composite coatings pos-
The coatings electrodeposited at 50 and 100 mA cm−2 have
completely different phase structures. The phase structure is influ-
enced by two factors, i.e., current and incorporation of TiO2 nano-
particles. It can be seen from the XRD patterns of traditional Ni
coatings that the orientation gradually changed from ͓111͔ to ͓220͔
when the current increased from 10 to 100 mA cm−2, indicating
that the high current density induced a preferential growth of Ni
grains along ͓220͔. However, with the incorporation of TiO2 nano-
particles in the traditional composite coating, the growth of Ni
grains along ͓220͔ is significantly reduced and, finally, the orienta-
tion of the coating shifted to ͓111͔ at 50 and 100 mA cm−2. Simi-
larly, the growth of Ni grains along ͓220͔ slowed down in the sol-
enhanced Ni–TiO2 composite coating process, but the orientation of
the coating did not change, still along ͓220͔, in part reflecting the
different influence mechanism of the sol-enhanced and particle-
mixing methods. Meanwhile, a weak peak of TiO2 was detected for
the traditional Ni–TiO2 composite coating prepared at
sess nearly the same trend in the current from 10 to 100 mA cm−2
.
The highest microhardness: 430 HV100 of the sol-enhanced coating
formed at 50 mA cm−2 was observed, corresponding to 20–34%
improvement against ordinary Ni–TiO2 composite ͑360 HV100͒ and
Ni plating ͑320 HV100͒.
A strange result is that the traditional Ni–TiO2 composite coating
possesses a lower microhardness ͑ϳ330 HV100͒ than the Ni coating
͑ϳ360 HV100͒ when the coatings were prepared at the high current
of 100 mA cm−2. In contrast, the sol-enhanced Ni–TiO2 composite
coating has a higher microhardness of ϳ380 HV100. Figure 6 shows
that the microhardness of coatings is determined by both the incor-
poration of TiO2 particles and processing parameters ͑current den-
sity͒, which affect the microstructure of coatings. As mentioned be-
fore, at 100 mA cm−2, although the contents of TiO2 particles are
similar in both composite coatings ͑Fig. 3͒, the traditional Ni–TiO2
composite coating showed a porous, dendritic surface ͑Fig. 1b3 and
2a and b͒, probably leading to the decrease in microhardness. In
contrast, the compact structure in the sol-enhanced composite coat-
ing keeps a higher microhardness of ϳ380 HV100 ͑Fig. 1c3 and 2c͒.
50 mA cm−2
.
Figure 5 shows the root-mean-square microstrain of the tradi-
tional Ni coating and the traditional and sol-enhanced Ni–TiO2 com-
posite coatings. It is widely accepted that the microstrain of com-
posite coatings is mainly resulted from the lattice distortion due to
the incorporation of second-phase particles. The same trend exists in
both traditional and sol-enhanced Ni–TiO2 composite coatings, i.e.,
the microstrain increases when the current increases, peaking at
50 mA cm−2, and then declines. Both composite coatings have al-
most the same microstrain of ϳ0.22% when the coatings were elec-
trodeposited at 50 mA cm−2. In this case, as we have described
previously, the TiO2 nanoparticles ͑ϳ4 wt %͒ agglomerated to
form large clusters in the traditional Ni–TiO2 composite coatings,
compared to the highly dispersive distribution of TiO2 nanoparticles
in the sol-enhanced composite coating.28 We propose that the strong
lattice distortion occurred when the TiO2 nanoparticles were highly
dispersed in Ni matrix, leading to a large microstrain, although there
Discussion
The above analysis indicates that the electrochemical processes
were quite different for the traditional and sol-enhanced Ni–TiO2
composite coatings. The orientation of coatings is mainly deter-
mined by grain growth. In electrochemical deposition, the micro-
structure of coatings is mainly controlled by overpotential. The de-
tailed mechanisms are discussed below.
Grain growth during sol-enhanced electroplating process.— The
effect of sol addition on the Ni electroplating can be analyzed from
the XRD patterns in Fig. 7. The grain orientation of the traditional
Ni coating was along ͓220͔ at the beginning of deposition ͑Fig. 7a͒.
However, after the sol was added into the electrolyte, the relative
intensity of the peak ͓220͔ decreased, indicating that the growth of
Ni grains along ͓220͔ was slowed down. With the deposition pro-
cess, the effect of TiO2 sol on the Ni plating became larger, evi-
denced by the decreasing intensity of the ͓220͔ peak, as shown in
Fig. 7b-d. Meanwhile, the peaks of ͓200͔ were almost unchanged.
Obviously, the sol addition significantly affects the Ni grains grow-
was
a
smaller amount of nanoparticles ͑ϳ3 wt %͒. At
100 mA cm−2, the traditional composite coating possessed a higher
microstrain than the sol-enhanced composite coating, although both
coatings have a similar content of TiO2 particles ͑Fig. 3͒. The higher
microstrain is probably related to the dendritic structure in the tra-
ditional composite coating ͑Fig. 2a and b͒.
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Journal of The Electrochemical Society, 157 ͑8͒ E122-E128 ͑2010͒
Figure 7. ͑Color online͒ XRD patterns of
the traditional Ni coating ͑red lines͒ and
the sol-enhanced Ni–TiO2 composite coat-
ing ͑black lines͒ prepared at 50 mA cm−2
for ͑a͒ 30 s, ͑b͒ 2 min, ͑c͒ 5 min, and ͑d͒
10 min.
ing along ͓220͔. As the surface energy determines the preferential
growth direction, the addition of sol probably changes the surface
energy of different planes.30
TiO2 nanoparticles. In contrast, for the solid powder mixing
Ni–TiO2 composite coatings, the nanoparticles were agglomerated
up to several hundred nanometers when they were added into the
electroplating solution, so the actual situation is that the Ni ions
surrounded the large clusters of TiO2 particles ͑Fig. 8b͒.
Process of sol-enhanced electrodeposition.— Experimental re-
sults indicated that the mechanical properties of the sol-enhanced
Ni–TiO2 composite coatings were mainly determined by the highly
dispersive TiO2 nanoparticles.28 The organic components in the
TiO2 sol mainly influenced the physical and chemical behaviors of
the sol. Therefore, the electrochemical process of the sol-enhanced
electrodeposition is discussed based on the effect of the TiO2 nano-
particles.
The mechanism of the composite electrodeposition has been in-
vestigated since the 1960s,31-35 and three main mechanisms were
proposed4,35: ͑i͒ electrophoretic movement of the positively charged
particles to the cathode, ͑ii͒ adsorption of the particles at the elec-
trode surface by van der Waals forces, and ͑iii͒ mechanical inclusion
of the particles into the layer. According to the above mechanisms, it
is proposed that two key factors in our present experiment, i.e.,
formation and movement of TiO2 nanoparticles, should be empha-
sized to explain the sol-enhanced electrochemical process.
Movement of TiO nanoparticles
.— As discussed above, the TiO2
2
nanoparticles existed in two forms during electroplating, i.e., physi-
cal adsorption onto the freshly deposited surface and hydrate Ni
ion-adsorbed TiO2 nanoparticles. We have previously described the
formation of the sol-enhanced composite coatings partially under the
control of physical adsorption of TiO2 nanoparticles.28 Here, we
mainly analyze the influence of hydrate Ni ion-adsorbed TiO2 nano-
particles.
The overall composite deposition process can be shown in five
steps ͑Fig. 9a͒.4 These steps describe the process of particles from
the solution to their incorporation in the metal matrix. The first stage
postulates the formation of an electroactive ionic cloud surrounding
the particles as soon as the particles are introduced into the electro-
lyte. Under the action of convection, these ionically enveloped par-
ticles are transported to the hydrodynamic boundary layer, migrate
across the layer, and then are conveyed by diffusion to the cathode.
Formation of TiO nanoparticles
.— The formation of TiO2 nano-
2
particles has been discussed in a previous paper.28 As a summary,
when a small amount of transparent sol solution was added into the
conventional electroplating solution, the concentrated hydrate Ni
ions in the solution destabilized the sol, leading to polymerization of
TiO2 sol. It was reported that the condensation process of Ti mac-
romolecule ions started before the completion of hydrolysis in neu-
tral and basic conditions, and the formation of an ordered structure
was hindered.36,37 Thus, the amorphous TiO2 nanoparticles formed
in the electrolyte.
Once the nanoparticles formed in the electrolyte, some of them
were immediately physically adsorbed onto the freshly deposited
surface based on the Martin–Williams model.32 Some of them were
immediately adsorbed by hydrate Ni ions due to their large surface
areas based on the Whithers model.31 They were highly dispersed in
the electrolyte, as schematically shown in Fig. 8a. The ethanol and
DEA probably also contributed to the dispersion of the ion-adsorbed
Figure 8. A schematic drawing to show the hydrate Ni ions and adsorbed
TiO2 particles: ͑a͒ The sol-enhanced and ͑b͒ the traditional particle-mixing
electrodeposition process.
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Journal of The Electrochemical Society, 157 ͑8͒ E122-E128 ͑2010͒
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the limited current density iL did not change significantly. Therefore,
at 100 mA cm−2, the concentration polarization controlled the elec-
trochemical process, leading to the formation of a loose and den-
dritic structure ͑Fig. 1b3 and 2a and b͒. The process of sol-enhanced
electroplating was controlled by electrochemical polarization, result-
ing in a compact structure, as seen in Fig. 1c3 and 2c.
The experimental confirmation of the above-described mecha-
nism is needed and now in progress. We are now designing an
electrochemical microsetup to investigate the effect of sol addition
on the diffusion layer, activation energy, nucleation, and grain
growth. The process seems complex based on some preliminary ex-
perimental results. In the present paper, the comparison of structures
and properties between the traditional and sol-enhanced Ni–TiO2
composite films was emphasized; therefore, the discussions were
mainly based on the above differences. Soon, the detailed mecha-
nism for the sol-enhanced composite plating will be presented based
on electrochemical polarization theory and TEM observation.
Figure 9. ͑a͒ Process steps in codeposition and incorporation of a solid
particle into the deposit: ͑1͒ Formation of an ion cloud around the particle,
͑2͒ transport by means of convection, ͑3͒ transport by diffusion, ͑4͒ reduction
reaction, and ͑5͒ adsorption. ͑b͒ The schematic diagram of the concentration-
depth profile during the sol-enhanced electroplating.
Conclusions
A Ni–TiO2 composite film has been prepared by sol-enhanced
electroplating. In this process, a small amount of transparent TiO2
sol was added into the traditional electroplating Ni solution, forming
highly dispersive TiO2 nanoparticles, which codeposited with Ni to
form a nanocomposite coating. The in situ formation of fine TiO2
nanoparticles originating from sol changed the electrochemical be-
havior and polarization mechanism, avoiding the loose and dendritic
surface structure at higher current deposition. The highly dispersive
nanoparticles and the dense structure led to the significant improve-
ment in mechanical properties of Ni–TiO2 composite coatings pre-
pared at a high current density.
After the ionic cloud is entirely or partly reduced, the particles are
deposited and incorporated in the metal matrix as the metal ions are
discharged, so “burying” the inert particles.
There are three main mechanisms involved in the delivery of
ions to the cathode surface, i.e., migration ͑under a potential gradi-
ent͒, diffusion ͑under a concentration gradient͒, and convection
͑movement of the electrolyte solution itself͒. It is believed that the
overall contribution to the supply of ions from the migration process
is very small and can be neglected.4 The convection resulted from
the movement of bulk solution is determined by stirring. Such
movement of solution ceases to be significant in the diffusion layer,
and movement of ions across the diffusion layer takes place by
diffusion. The driving force for diffusion is the concentration gradi-
ent, more suitably expressed as the concentration polarization. Ac-
cording to the electrochemical theory, there are two important po-
larizations in the electroplating process: electrochemical ͑activation͒
polarization and concentration polarization. Concentration polariza-
tion can be increased by reducing the thickness of the diffusion
layer. There exists a limited current density ͑iL͒, which is deter-
mined by concentration polarization, expressed as
Acknowledgments
The project is partially funded by New Zealand FRST IIOF grant
UoA 0601 and an Auckland UniServices grant. We thank the tech-
nical staff in the Department of Chemical and Materials Engineering
and Research Centre for Surface and Materials Science for assis-
tance.
The University of Auckland assisted in meeting the publication costs of
this article.
References
1. O. P. Watts, Trans. Am. Electrochem. Soc., 29, 395 ͑1916͒.
2. G. A. Di Bari, in Morden Electroplating, 4th ed., M. Schlesinger and M. Paunovic,
Editors, p. 139, John Wiley & Sons, New York ͑2000͒.
cϱ
iL = zFD
͓2͔
␦
N
3. S. Field and A. D. Weill, Electro-Plating, 2nd ed., p. 136, Sir Isaac Pitman & Sons,
London ͑1935͒.
4. N. Kanani, Electroplating: Basic Principles, Processes and Practice, Elsevier Ad-
vanced Technology, Oxford ͑2004͒.
5. J. Sadowski-Mazur, M. E. Warwick, and R. Walker, Trans. Inst. Met. Finish., 64,
142 ͑1976͒.
6. V. Sova and H. Bollhalder, Plat. Surf. Finish., 75, 53 ͑1988͒.
7. J. Zahavi and J. Hazan, Plat. Surf. Finish., 70, 57 ͑1983͒.
8. S. S. Tulsi, Trans. Inst. Met. Finish., 61, 147 ͑1983͒.
9. P. R. Ebdon, Plat. Surf. Finish., 75, 65 ͑1988͒.
where z is the number of electrons per ion being transferred, F is
Faraday’s constant, cϱ is the ion concentration in bulk solution, and
␦
is the thickness of the diffusion layer. When the current is below
N
iL, the electroplating process is controlled by electrochemical polar-
ization; otherwise, the process is controlled by concentration polar-
ization. In the latter case, metal ions tend to discharge preferentially
at the tips of the protrusions, and the nucleus is difficult to grow,
which tends to result in the formation of a loose and dendritic sur-
face.
10. R. R. Oberle, M. R. Scanlon, R. C. Cammarata, and P. C. Searson, Appl. Phys.
Lett., 66, 19 ͑1995͒.
Figure 9b shows a schematic diagram of the concentration-depth
profile during the sol-enhanced electroplating. During the deposition
process, a large amount of hydrate Ni ions was adsorbed on the
surface of the newly formed TiO2 nanoparticles due to the large
surface areas of the nanosized particles. The ion-adsorbed nanopar-
ticles were conveyed to the diffusion layer under the effect of stir-
ring. An ion-rich area formed at the edge of the diffusion layer ͑Fig.
9b͒, where the ion concentration was so large that it reached the
concentration ͑cϱ͒ of the bulk electrolyte. In this case, the thickness
of the diffusion layer decreased from ␦N to ␦N-sol, leading to the
increase in the limited current density ͑iL͒. Therefore, the critical
current density to form the dendritic surface increased during the
sol-enhanced deposition process.
11. I. Shao, P. M. Vereecken, R. C. Cammarata, and P. C. Searson, J. Electrochem.
Soc., 149, C610 ͑2002͒.
12. K. C. Chan, C. L. Wang, K. F. Zhang, and G. Pang, Scr. Mater., 51, 605 ͑2004͒.
13. W. Wang, F. Y. Hou, H. Wang, and H. T. Guo, Scr. Mater., 53, 613 ͑2005͒.
14. B. S. Xu, H. D. Wang, S. Y. Dong, B. Jiang, and W. Y. Tu, Electrochem. Commun.,
7, 572 ͑2005͒.
15. E. Orowan, in Symposium on Internal Stresses in Metals and Alloys, Institute of
Metals, p. 451 ͑1948͒.
16. M. F. Ashby, in Physics of Strength and Plasticity, A. S. Argon, Editor, p. 113, MIT
Press, Cambridge, MA ͑1969͒.
17. J. R. Vinson and T. W. Chou, Composite Materials and Their Use in Structures, p.
124, Applied Science, London ͑1975͒.
18. S. Dobatkin and X. Sauvage, in Bulk Nanostructured Materials, M. J. Zehetbauer
and Y. T. Zhu, Editors, p. 595, Wiley-VCH Verlag GmbH, Weinheim ͑2009͒.
19. W. Stöber, A. Fink, and E. Bohn, J. Colloid Interface Sci., 26, 62 ͑1968͒.
20. J. Livage, Bull. Mater. Sci., 22, 201 ͑1999͒.
21. M. J. Iqbal and M. N. Ashiq, Scr. Mater., 56, 145 ͑2007͒.
22. Q. L. Zhang, F. Wu, H. Yang, and D. Zou, J. Mater. Chem., 18, 5339 ͑2008͒.
23. A. E. Gash, T. M. Tillotson, J. H. Satcher, J. F. Poco, L. W. Hrubesh, and R. L.
Simpson, Chem. Mater., 13, 999 ͑2001͒.
In contrast, during the traditional composite coating process, be-
cause of poor adsorption of hydrous Ni2+ onto the surface of the
agglomerated TiO2 particles, the thickness of the diffusion layer and
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E128
Journal of The Electrochemical Society, 157 ͑8͒ E122-E128 ͑2010͒
24. C. N. Chervin, B. J. Clapsaddle, H. W. Chiu, A. E. Gash, J. H. Satcher, Jr., and S.
M. Kauzlarich, Chem. Mater., 18, 4865 ͑2006͒.
25. M. Kallala, C. Sanchez, and B. Cabane, Phys. Rev. E, 48, 3692 ͑1993͒.
26. T. Ogihara, M. Ikeda, M. Kato, and N. Mizutani, J. Am. Ceram. Soc., 72, 1598
͑1989͒.
27. W. W. Chen and W. Gao, Provisional Pat. NZ578038, New Zealand ͑2009͒.
28. W. W. Chen, Y. D. He, and W. Gao, Surf. Coat. Technol., 204, 2487 ͑2010͒.
29. J. J. Qiu, Z. G. Jin, Z. F. Liu, X. X. Liu, G. Q. Liu, W. B. Wu, X. Zhang, and X. D.
Gao, Thin Solid Films, 515, 2897 ͑2007͒.
31. J. C. Withers, Prod. Finish., 26, 62 ͑1962͒.
32. R. V. Williams and P. W. Martin, Trans. Inst. Met. Finish., 42, 182 ͑1964͒.
33. E. A. Brandes and D. Goldthorpe, Metalurgia (Bucharest), 76, 195 ͑1967͒.
34. J. P. Celis, J. R. Roos, C. Buelens, and J. Fransaer, Trans. Inst. Met. Finish., 69,
133 ͑1991͒.
35. A. Hovestad and L. J. J. Janssen, J. Appl. Electrochem., 25, 519 ͑1995͒.
36. M. Gopal, W. J. Mobery Chan, and L. C. De Jonghe, J. Mater. Sci., 32, 6001
͑1997͒.
37. Z. L. Tang, J. Y. Zhang, Z. Cheng, and Z. T. Zhang, Mater. Chem. Phys., 77, 314
͑2003͒.
30. T. Watanabe, Nano-plating: Microstructure Control Theory of Plated Film and
Database of Plated Film Microstructure, Elsevier, Amsterdam ͑2004͒.
Downloaded on 2015-02-06 to IP 128.120.194.195 address. Redistribution subject to ECS terms of use (see ecsdl.org/site/terms_use) unless CC License in place (see abstract).