Ni/nano-TiO2 composite electrodeposits: Textural and structural modifications

Article abstract of DOI:10.1016/j.electacta.2008.06.068

Nanocomposite coatings were obtained by electrochemical codeposition of TiO₂ nano-particles (mean diameter 21 nm) with nickel, from an additive-free Watts type bath. The electrodeposition of Ni-TiO₂ composites was carried out on a ro

Full text of DOI:10.1016/j.electacta.2008.06.068

Electrochimica Acta 54 (2009) 2547–2555
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Electrochimica Acta
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Ni/nano-TiO2 composite electrodeposits: Textural and structural modifications
S. Spanou, E.A. Pavlatou∗, N. Spyrellis1
Laboratory of General Chemistry, School of Chemical Engineering, National Technical University of Athens, 9 Heroon Polytechniou Str., Zografos Campus, Athens GR-15780, Greece
a r t i c l e i n f o
a b s t r a c t
Article history:
Nanocomposite coatings were obtained by electrochemical codeposition of TiO2 nano-particles (mean
diameter 21 nm) with nickel, from an additive-free Watts type bath. The electrodeposition of Ni–TiO2
composites was carried out on a rotating disk electrode (RDE), by applying direct current. Pure Ni deposits
were also produced under the same experimental conditions for comparison. The surface morphology,
the crystallographic orientation of nickel matrix and the grain size of the deposits were investigated,
along with the distribution and the percentage, of the embedded nano-particles in nickel matrix, as
a function of pH, current density and concentration of TiO2 nano-particles in the bath. The observed
textural modifications of composite coatings are associated with specific structural modifications of Ni
crystallites provoked by the adsorption–desorption phenomena occurring on the metal surface, induced
by the presence of TiO2 nano-particles. It has been observed that the presence of TiO2 nano-particles
favours the [1 0 0] texture of nickel matrix. Moreover, the codeposition of titania nano-particles with
nickel was found to be favoured at low pH and low applied current values. As the titania incorporation
percentage is increased, a considerable grain refinement in the nanometer region was revealed followed
by an improvement of the quality of the nickel preferred orientation.
Received 31 December 2007
Received in revised form 26 June 2008
Accepted 28 June 2008
Available online 6 July 2008
Keywords:
Nickel electrodeposition
Composite coating
Titania
Preferred orientation
Nano-particles
© 2008 Elsevier Ltd. All rights reserved.
1. Introduction
distribution in the metal matrix, as well as, on the microstructural
characteristics of the metal matrix.
Metal matrix composite coatings containing TiO2 particles
exhibit interesting photoelectrochemical and photocatalytical
behaviour [1–3] accompanied by improved mechanical properties
[4–8]. Especially, Ni matrix composites reinforced by TiO2 micro-
[7,9–10], submicron- [10,11] and nano-particles [1,4–6,10,12–14]
can be successfully synthesized by electrodeposition utilizing
either sulfamate or Watts plating baths.
The codeposition of nanoscaled particles within an electro-
plating process is an attractive procedure to produce improved
materials for microtechnological applications [4]. The principal
parameters acting on the composite electrodeposition process are
the compositions of the electrolytic bath [15], the presence of
additives [15–17], the pH value [15,18], the temperature [15], the
induced hydrodynamic conditions, the imposed current conditions
[8] and of course the characteristics of the reinforcing particles (size
[10], conductivity [19], hydrophylisity/hydrophobisity [20] and sur-
face charge [15,21]). It is important to adjust the plating variables
in order to control the properties of the produced composite coat-
ings that depend on the amount of incorporated particles and their
Research related to Ni–TiO2 composite coatings has demon-
strated that the codeposition percentage of nano-titania particles
is difficult to be controlled quantitatively, because the particles are
frequently agglomerated in the metal matrix, as well as in the elec-
trolyte [10,13] due to their significant high surface energy. Given
that TiO2 particles seems to incorporate in a limited extend com-
pare to other ceramic particles, like SiC particles in example [9],
the major challenges are the enhancement of particles incorpo-
ration accompanied by their uniform distribution in the deposit.
It should be mentioned that the highest amount of nano-titania
embedded particles does not exceed ∼3.5 vol.% [13] in the pres-
ence of organic additives by using Watts type bath. In addition,
the major published reports realised the production of Ni-nano-
TiO2 composite coatings by using additives and only few have
addressed the synthesis from an additive-free Watts bath [1,4].
It is noticeable that the presence of organic additives in the bath
introduces new parameters that influence the codeposition process
and consequently increases the complexity of the electrochemical
phenomena.
Taking into consideration all the above mentioned data, we have
conducted a comprehensive study in an extended area, regarding
the influence of the electrolytic parameters on the codeposition
process. Specifically, we investigated the effect of TiO2 loadings in
bath, applied current density and pH value, on the surface mor-
∗
Corresponding author. Tel.: +30 210 7723110, 7723130; fax: +30 210 7723088.
E-mail address: pavlatou@chemeng.ntua.gr (E.A. Pavlatou).
ISE member.
1
0013-4686/$ – see front matter © 2008 Elsevier Ltd. All rights reserved.
doi:10.1016/j.electacta.2008.06.068

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S. Spanou et al. / Electrochimica Acta 54 (2009) 2547–2555
phology, crystal size and the preferred orientation of the deposits
in correlation with the quantity of embedded nano-TiO2 particles.
intensities of a standard Ni powder sample randomly oriented. The
summation in the denominator is taken for the six “basic” lines
visible in the diffraction pattern, i.e., (1 0 0), (2 0 0), (2 2 0), (3 1 1),
(3 3 1) and (4 2 0). A preferred orientation through an axis [h k l] is
indicated by values of RTC ≥ 16.67% and the strength of the pre-
ferred orientation is maximum when RTC approaches the value of
100% [22].
Grain size of the crystallites was determined by using the (2 0 0)
and (2 2 0) X-ray diffraction peak broadening according to the Sher-
rer equation. The full-width-half-maxima (FWHM) of the peaks
were estimated after background correction and subtracting the
instrumental line broadening, which was calculated using a stan-
dard Ni random oriented powder according to the Warren equation;
2. Experimental
Pure Ni and composite Ni–TiO2 coatings were electrolytically
deposited from an additive-free nickel Watts solution under con-
ditions described in Table 1. The electrodeposition experiments
were performed on rotating disk electrode (RDE) with a constant
rotation velocity of 600 rpm under direct current conditions. The
solution temperature was maintained by water circulation at
a thermostated temperature of 50 1 ◦C. Nickel plate of 99.9%
purity, positioned on the side of the electrolytic cell, was used
as a soluble anode and a standard calomel electrode served as
a reference electrode. The initial pH of the bath was adjusted to
constant values in the range of 1–4.5. The TiO2 powder (Degussa
P25) with a mean diameter of 21 nm was used as received without
any pre-treatment. This commercial powder was prepared by the
aerosil process, and is frequently used in photocatalytic studies
and consists by a mixture of the crystalline phases of anatase
and rutile. TiO2 particles were maintained in an electrolytic bath
in suspension by continuous magnetic stirring of 320 rpm for at
least 24 h before deposition as well as, during electrolysis. The
load of particles in the electrolyte was set at 20, 50 and 100 g/L
and the applied current density varied from 0.5 to 40 A/dm2. All
the experiments were conducted by using a Wenking PGS 95
Potentio-Galvanoscan. The substrates were brass discs with a total
surface area of 0.05 dm2 that were mechanically polished in order
to eliminate the epitaxial influence on the structure of deposits and
chemically cleaned in an ultrasonic agitated bath before deposi-
tion. After electrolysis, the deposits were ultrasonically cleaned in
distilled water for 10 min in order to remove any loosely adsorbed
TiO2 particles from the surface. The thickness of the produced
coatings was at least 40 ␮m in order to obtain preferred orienta-
tions fully developed and therefore permit their determination in
such a way to attain results comparable and reliable.
B2 = BM2 − BS2
where BM is the measured diffraction FWHM peak and BS is the
broadening of Ni standard powder sample [22]. The standard Ni
powder was randomly oriented with a mean grain size of 0.5 ␮m.
Scanning electron microscopy (SEM) and high-resolution field
emission SEM (HRFE-SEM) was applied in order to study the
structure, morphology and composition of the surface and the
cross-sectional profile of the Ni–TiO2 coatings. Depth profiles of
the composites were obtained by using glow discharge optical
spectroscopy (GDOS) in order to extract information about the
distribution of the components versus the coating thickness. The
concentration of TiO2 particles on the surface was evaluated by
using fluorescence X-ray spectroscopy (XRF). These values were
verified also by energy dispersive X-ray spectroscopy (EDS) mea-
surements.
Experiments were carried out to determine the pH variation of
the bath at various initial pH values in the presence of two different
loads of titania in order to investigate the adsorption–desorption
phenomena of H+ taking place on the surface of titania particles.
The batches of oxide were added and kept in suspension by mag-
netic stirring at a concentration of 20 and 100 g/L in a 200-cm3 cell
containing 100 cm3 of the used Watts type solution. The initial pH of
the solution was adjusted to the required value using NH3 or HNO3
solution. The pH was modified after titania addition, and reached a
steady-state value after 2 h mixing [23] that was recorded.
X-ray diffraction analysis was carried out using a Siemens D-
5000 diffractometer, with a Cu K␣ radiation. Diffractograms were
recorded with a step size of 0.02◦ for 2ꢀ ranging from 20 to 160◦
and measuring time 1 s per step. In order to describe the structure
and estimate quantitatively the preferred orientation of the nickel
deposits, the relative texture coefficient (RTC(h k l)) was calculated,
which is defined as:
3. Results and discussion
I
h k l/Ih◦ k l
61Ih k l/Ih◦ k l
RTC(h k l)
=
× 100%
3.1. Influence of nano-TiO2 particles on nickel
electrocrystallization
ꢀ
where Ih k l are the diffraction intensities of the (h k l) lines measured
in the diffractogram of the deposit and Ih◦ k l are the corresponding
3.1.1. Pure Ni deposits
The electrocrystallization of Ni is known to be a highly inhibited
process as a consequence of hydrogen codeposition. Depending on
plating conditions, mainly pH value of the bath and current den-
sity, Amblard et al. observed a predominance of a definite inhibitor,
which selectively promotes one mode of growth and leads to a
deposit exhibiting specific structural characteristics. It has been
established that nickel deposits from an additive-free Watts type
bath under direct current conditions exhibit three inhibited tex-
tures [1 1 0], [2 1 0] and [2 1 1] attributed to the presence of atomic,
molecular forms of adsorbed hydrogen and colloidal Ni(OH)2 in
the catholyte interface, respectively. Moreover, the observed [1 0 0]
mode of growth is considered to be the most “free” from inhibiting
chemical species [24–27].
Table 1
Overview of the electrodeposition parameters for preparation of pure Ni and com-
posite Ni–TiO2 coatings
Electrolyte composition
NiSO4·6H2O
330 g/L
35 g/L
40 g/L
20, 50 and 100 g/L
NiCl2·6H2O
H3BO3
TiO2 powder Degussa P25 (dm = 21 nm)
Electrodeposition conditions
pH
1, 2, 3, 3.5, 4 and 4.4
50
Brass disc (diameter 25 mm)
600 rpm
Ni foil
0.5, 1, 2, 5, 10, 20 and 40
Direct (DC)
Temperature (◦C)
Substrate
Cathode rotation rate (␻)
Anode
The estimation of the preferred crystal orientation based on the
X-ray diffraction diagrams of pure nickel deposits from an additive-
free Watts bath resulted to the stability diagram (Fig. 1a) of textures
as a function of pH and current density. The obtained results are in
Current density (A/dm2)
Type of current
Magnetic stirring (rpm)
320

S. Spanou et al. / Electrochimica Acta 54 (2009) 2547–2555
2549
Fig. 1. Stability diagrams of crystal preferred orientations as a function of pH and applied current density of (a) pure Ni and Ni–TiO2 composites at (b) 20 g/L, (c) 50 g/L and
(d) 100 g/L TiO2 loads in the bath.
accordance with textural modifications reported in literature given
that the cathode rotation speed is different. This diagram consists of
four domains: (a) at low current densities and an extended region
of pH values the [1 1 0] preferred orientation of Ni crystallites is
favoured, (b) at medium current densities and a broaden region
of pH values a [1 0 0] texture is observed, (c) at medium current
densities and less acidic baths a [2 1 1] texture is stabilised and
(d) at high current densities and strong acid media a [2 1 0] tex-
ture is favoured. It should be noticed that the usage of other baths
such as chloride, sulfamate, etc. leads to completely different crystal
structures under fixed electrodepositing conditions (temperature,
current density, pH, etc.) [28].
The structural and morphological characteristics of pure nickel
deposits that exhibit the four discrete crystal preferred orientations
according to the plating conditions described in Fig. 1a are demon-
strated in Fig. 2. In detail, Fig. 2a shows the pseudo-pentagonal
crystal symmetry typical for a nickel deposit oriented through the
[1 1 0] axis; Fig. 2b illustrates the characteristic binary symmetry
of [2 1 1] oriented crystals; Fig. 2c presents nickel crystals oriented
through [1 0 0] axis composing long fibres with twins that end up to
the shape of tetragonal pyramids and Fig. 2d exhibits the peculiar
structure of [2 1 0] oriented nickel crystallites [27]. Moreover, the
comparison of SEM images (Fig. 2) indicates that the mean grain
size of the four modes of crystal growth varies. As can be observed
deposits presenting a [1 0 0] texture exhibit the highest mean grain
size.
3.1.2. Composite Ni/TiO2 deposits
Embedding of TiO2 particles in the nickel matrix provokes
changes in the structure of pure nickel coatings with increasing
concentration of TiO2 nano-particles in the electrolyte (Fig. 1b–d).
In detail, the region of [1 0 0] oriented deposits is expanded with
increasing amounts of titania in the bath, accompanied by a
confinement of the [2 1 1] and [1 1 0] domains. Additionally, a trans-
formation of [2 1 0] preferred orientation – stable at high current
densities and low pH conditions for pure Ni deposits – to ran-
domly oriented crystallites is observed in the presence of TiO2
nano-particles.
According to literature data the codeposition of oxides with
nickel results to textural modifications compare to those observed
for pure nickel deposits and seems to be correlated to the loading
of the particles in the bath [29,30], the size [10] and surface prop-
erties of the dispersoids [18,23]. Thus, it can be concluded that the
adsorption–desorption phenomena taking place in the region of the

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S. Spanou et al. / Electrochimica Acta 54 (2009) 2547–2555
Fig. 2. SEM surface micrographs presenting the typical morphology of pure Ni crystallites oriented through (a) [1 1 0], (b) [2 1 1], (c) [1 0 0] and (d) [2 1 0] axis. The regions of
operation conditions under which these typical modes of growth are prepared are shown in Fig. 1a.
catholyte area seem to be changed when surface charged oxides
arrive at the cathode and are loosely adsorbed or even partially
submerged onto the growing nickel grains.
In contrast, the high incorporation of titania in the metallic matrix
at high current densities and low pH values exerts a much more
intense inhibition to the Ni crystal growth compared to pure nickel
deposits oriented through [2 1 0] axis, leading to the formation of
random oriented deposits (Fig. 1). It should be mentioned that the
[2 1 0] preferred orientation exhibits a “helical” peculiar structure
consisting of a compact connection by edges of tetrahedra with
conservation of a common crystallographic direction type [2 1 0]
[31]. This texture could be considered as very sensitive to its devel-
opment and therefore could be also destroyed by the presence
of titania nano-particles. Additionally, the region of random ori-
ented deposits expands with increasing content of nano-particles
in the electrolyte. Overall, all the textural modifications could be
attributed to the adsorption of inhibiting species on the surface of
TiO2 nano-particles. These specific adsorption phenomena could
result in a diminution of the activity of the inhibitors affecting the
nickel grains growth and thus imposing the free [1 0 0] texture.
Representative XRD patterns of nickel coatings produced at dif-
ferent TiO2 loadings in the electrolyte are presented in Fig. 3. The
diffractograms are characterised by mainly two diffraction peaks
(2 2 0) and (2 0 0), which correspond to [1 1 0] and [1 0 0] preferred
In this context, addition of titania nano-particles in the bath
followed by low current densities and pH values leads to the
replacement of [1 1 0] preferred orientation to [1 0 0] texture
(Fig. 1). This alteration implies a less intense inhibition of the Ni
crystal growth process, as the free mode growth [1 0 0] is favoured.
Moreover, the domain of [1 1 0] texture is further confined at
medium pH values and low current densities with increasing con-
centration of the nano-particles in the bath. The observed textural
modification could be correlated to the reduced concentration
of Hads inhibitors in the catholyte area under the specific elec-
trodeposition conditions induced by the presence of charged TiO2
nano-particles in catholyte area.
At high pH values and low to medium current densities the
region of deposits oriented through [2 1 1] axis is reduced and
replaced by composites exhibiting [1 0 0] texture as titania load-
ing in the bath is increased (Fig. 1). This observation indicates that
the formation of Ni(OH)2 on the cathode surface is reduced due
to the presence of TiO2 particles in the nickel/catholyte inetrface.

S. Spanou et al. / Electrochimica Acta 54 (2009) 2547–2555
2551
Fig. 3. X-ray diffraction patterns of Ni coatings with various TiO2 concentrations in the electrolyte prepared under pH 3.5 and J = 1 A/dm2: (a) pure Ni deposit and Ni–TiO2
composite coatings from bath containing (b) 20 g/L, (c) 50 g/L and (d) 100 g/L TiO2.
orientation. It should be noticed that the reflection at 2ꢀ = 25.3◦
could be assigned to the (1 0 0) plane of anatase and cannot be
clearly observed due to the relatively high intensity of nickel diffrac-
tion peaks (Fig. 3b). A change in relative intensity of the nickel
diffraction peaks occurred with increasing amounts of titania in
the electrolyte, which could be expressed through the value of the
relative texture coefficient of each crystalline orientation. In detail,
in the case of pure Ni deposits the XRD diagram is characterised
by a strong (2 2 0) diffraction line, together with a weak (2 0 0) line
(Fig. 3a), which switches to an intense (2 2 0) line for 20 g/L TiO2 in
the electrolyte (Fig. 3b). Further addition of titania in the bath leads
to an exceptional (2 0 0) diffraction line (Fig. 3c and d). The RTC(h k l)
of each (h k l) diffraction line of these deposits is presented in Fig. 4.
There is a significant improvement of the quality of the preferred
orientation by increasing the content of TiO2 nano-particles in the
electrolyte, for example in the case of (2 2 0) lines and the corre-
sponding [1 1 0] texture, the RTC is increased from 55.3% to 94.4%
by just adding 20 g/L TiO2 in the bath (Fig. 4).
Fig. 5 demonstrates the surface morphology of the four deposits
described in the X-ray diffraction data of Figs. 3 and 4. Under these
plating conditions a pure Ni polycrystalline deposit is produced ori-
ented through [1 1 0] axis with clear boarders of the grains (Fig. 5a).
With the first addition of TiO2 nano-particles in the electrolytic
bath, the boarders of the grains become fuzzy and the mean grain
size is reduced compared to pure Ni (Fig. 5b). As mentioned above,
the preferred orientation of this composite deposit is preserved and
its quality is improved significantly (Fig. 4). By further addition of
nano-particles in the bath, the texture of the deposit is switched
from [1 1 0] to [1 0 0] and a typical cauliflower surface morphology
is observed (Fig. 5c and d). The strength of this preferred orienta-
tion is further enhanced at the highest titania loading in the bath
(Fig. 4). It is noteworthy that EDS measurements on the surface of
these three composites disclosed an increase of the codeposition
percentage of the nano-particles in the deposit by increasing the
loading in the electrolyte (i.e., 3.75 vol.% for 20 g/L to 7.3 vol.% for
100 g/L).
3.2. Quantitative analysis of the amount of TiO2 in the composite
coatings
From the evaluation of the XRF results it was found that as the
concentration of TiO2 nano-particles in the electrolyte increases,
the percentage of TiO2 nano-particles occluded in the electrode-
posit (Fig. 6) is also increased independently from the pH and
applied current value. This result is in good correlation with reports
Fig. 4. Relative texture coefficient values of (2 1 1), (2 0 0) and (2 2 0) diffraction lines
of Ni coatings with various TiO2 concentrations in the electrolyte prepared under
pH 3.5 and J = 1 A/dm2.

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S. Spanou et al. / Electrochimica Acta 54 (2009) 2547–2555
Fig. 5. SEM surface micrographs of (a) pure Ni deposit with a [1 1 0] preferred orientation and Ni–TiO2 composite coatings with (b) [1 1 0], (c) [1 0 0], and (d) [1 0 0] preferred
orientation from bath containing 20 g/L, 50 g/L and 100 g/L TiO2, respectively. All the coatings were prepared under pH 3.5 and J = 1 A/dm2.
related to the study of the TiO2 nano-particles incorporation in Ni
matrix electrodeposits under different values of applied current
density [5,13].
i.e., lower current density, or increasing the number of particles
suspended in the electrolyte.
Additionally, the experimental data of this work have revealed
that there is a correlation between the concentration of the code-
posited nano-particles and the grain size of nickel matrix (Fig. 7).
In particular, as the TiO2 loading in the electrolyte increases under
constant values of pH and current density, the TiO2 vol.% in elec-
trodeposit is also increased (see also Fig. 6), while the mean grain
size of the crystallites decreases (Fig. 7a). It should be noticed that
this observation stands as long as there is no modification in the tex-
ture of the deposits due to an increase of embedded nano-particles
into the Ni matrix. While, when the pH and the TiO2 loading in the
bath are constant, the concentration of TiO2 in deposit decreases
and the mean grain size increases by increasing the current den-
sity (Fig. 7b). A similar behaviour was reported by studies related
to the electro-codeposition of alumina in nickel matrix [30]. It
is noteworthy to mention that the maximum incorporation per-
centage of the nano-particles in the Ni matrix (9 vol.%) has been
achieved at low current densities and low pH values (Fig. 7b).
The highest amount of nano-titania embedded particles reported
However, the codeposition percentage of TiO2 nano-particles
decreases by increasing the pH value at a constant value of cur-
rent density (Fig. 6a). It seems that codeposition is favoured at
low pH values implying that TiO2 nano-particles are more posi-
tively surface charged [15] compare to high bath pH values. On the
other hand, at a constant value of pH, the incorporation percentage
increases by decreasing current density value (Fig. 6b). According
to Fransaer et al., a particle “group” will be engulfed by the grow-
ing metal when brought in contact with an electrode with enough
foothold to remain on the electrode. A situation that could give
this opportunity to the particles to be efficiently embedded in the
matrix could be achieved at low current densities [32]. Moreover,
Banovic et al. [30], as well as Chen et al. [33] proposed that increas-
ing the number of effective collisions between the oxide particles
and the cathode surface per unit volume of deposited matrix will
increase the amount incorporated into the coating. According to
them, this can be accomplished by using a slower plating rate,

S. Spanou et al. / Electrochimica Acta 54 (2009) 2547–2555
2553
Fig. 7. Variation of vol.% TiO2 content in the composite coatings Ni crystallites mean
grain size as a function of (a) concentration of the titania particles dispersed in the
electrolyte at pH 3.5 and J = 2 A/dm2 and (b) applied current density versus at pH 2
and 50 g/L TiO2 in the bath.
Fig. 6. Volume percentage of nanosized TiO2 particles in the composite coating at
different TiO2 loadings as a function of (a) pH and constant value of current density
J = 5 A/dm2 and (b) current density and at constant pH value of 4.4.
the uniform distribution of titania nano-particles in the depth of
the deposit (Fig. 10).
in literature does not exceed the ∼3.5 vol.% by using Watts type
bath, though in presence of organic additives [10]. Moreover, at the
conditions under which the highest incorporation of titania nano-
particles is achieved the mean grain size of composite coatings
crystallites is approximately 20 nm. Pure nickel deposits exhibit for
the same [1 0 0] preferred orientation with the composite a min-
imum grain size of ∼45 nm. Therefore, the embedding of titania
nano-particles yields to a considerable nickel grain refinement in
the nanoscale region under specific electrolytic conditions (titania
loading, pH and current density) compared to pure nickel deposits.
This grain downsize of electrodeposited composites is defined by
the competition between nucleation and crystal growth [34]. How-
ever, the ascription of the observed grain refinement is not yet
well defined. Studies concerning the incorporation of TiO2 nano-
particles in nickel have shown that the dispersed nano-particles
either increase the nucleation [5] or restrain the crystal size rather
by inhibiting crystal growth than by providing new nucleation sur-
faces [10].
3.3. Particle reactivity with H+
In order to interpret the observed textural modifications of Ni
matrix induced by the presence of TiO2 particles in the deposits,
the nano-particles reactivity with charged ions in the electrolytic
bath and especially with H+ has been examined. It is known that
in electroplating metal-oxide composite coatings, it is necessary to
investigate the effects of adding the metal oxide to the electrolyte
on resulting electrolyte pH [29]. It has been shown [23,35] that
the pH change which occurs, in aqueous solution, after oxide par-
ticle addition, depends on the acid–base properties of the powders
and indicates the magnitude of the H+ exchange on their surface.
Fig. 11 shows the change in pH (noted ꢁpH) caused by TiO2 addi-
tion into nickel Watts type baths at different pH values and various
titania loadings in the bath. After introducing the oxide powders
The experimental findings by applying SEM and HRFE-SEM
techniques have demonstrated that the distribution of the TiO2
nano-particles on the surface is uniform without serious problems
of agglomeration (Fig. 8a). It seems that the incorporation of the
titania nano-particles takes place between the grains as denoted in
Fig. 8b. Finally, Fig. 9 presents the cross-sectional SEM micrograph
of Ni–TiO2 revealing a compact composite with a good adhesion
with the substrate, while a representative GDOS profile discloses
an increase in the bath pH occurred (zone I), and at specific pH
∼
value no change is observed after powder addition (ꢁpH3.5 0).
=
This pH is dependent on the load of the powder in the solution and
is characteristic for each oxide usually called as point of zero charge
(PZC). At pH values larger than the PZC point, a negative change in
pH occurred (zones II and III). According to literature data [23,36],
a positive change in pH can be interpreted as an adsorption of pro-
tons on the powder surface which becomes partially hydroxylated

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S. Spanou et al. / Electrochimica Acta 54 (2009) 2547–2555
Fig. 10. GDOS depth profile for a Ni–TiO2 composite prepared on brass discs under
pH 3.5, J = 0.5 A/dm2 and 100 g/L TiO2 in the bath. The codeposition percentage of
titania remains almost constant across the depth of the deposit at the value of
7.7 vol.%.
according to the following reaction:
–MOH + H+ → –MOH2+ → –M+ + H2O
(1)
In contrast, a decrease in bath pH occurs when particles tend to
release protons into the solution, owing to the following reaction:
–MOH + H2O → –MO− + H3O+
(2)
In the present case, the results obtained show the relationship
between the degree of acceptance or release of a proton by the
oxide group and the load of the powder in the solution. Specifically,
higher concentration of TiO2 in the electrolyte leads to a higher H+
exchange with the solution.
Fig. 8. HRFE-SEM surface micrographs of Ni–TiO2 composite coatings: (a) a com-
posite coating produced at pH 4.0, J = 10 A/dm2 and 100 g/L TiO2 loading indicating a
good distribution of the nano-particles with Ni crystal oriented through [1 0 0] axis
and (b) characteristic shape of Ni crystals oriented through [1 1 0] axis produced at
pH 3.5, J = 0.5 A/dm2 and 100 g/L TiO2 loading in the bath.
Our experimental data revealed a replacement of [1 1 0] pre-
ferred orientation to [1 0 0] under low current densities and low
to medium pH values in presence of titania nano-particles in the
composites. This textural modification is provoked by the proton
adsorption on the titania particle surfaces according to Eq. (1),
which inhibits the formation of Hads at the cathode/catholyte inter-
face that impose the [1 1 0] mode of nickel growth. It should be
noticed that this inhibition takes place to such an extent that the
metallic surface is less hindered by any adsorbed chemical species
related to hydrogen codeposition and thus, a [1 0 0] free mode of
growth is favoured. Additionally, in the case of very acidic baths and
high current densities, the adsorption of H+ on the titania particles
provokes a local alkalization of the cathode/electrolyte interface
Fig. 9. SEM micrograph of Ni–TiO2 coating in cross-section. The deposit was pro-
duced at pH 3.5, J = 1 A/dm2 and 50 g/L TiO2 in the bath. The codeposition percentage
of titania reaches the value of 5.5 vol.%.
Fig. 11. Variation of initial pH value for an additive-free Watts type bath as a function
of concentration of TiO2 nano-particles in the bath.

S. Spanou et al. / Electrochimica Acta 54 (2009) 2547–2555
2555
produced by the partially hydroxylated layer around the particles
as denoted in Eq. (1). Such situation indicates that hydrogen evo-
lution is restrained to some extent and consequently the [2 1 0]
preferred crystalline orientation is diminished. In addition, the sen-
sibility of [2 1 0] texture should be taken into account, where the
presence of titania nano-particles on the {1 1 1} twins could lead
to the destruction of [2 1 0] structure [31].
On the other hand, when the pH is higher than the PZC, titania
particles tend to release protons in the bath (Eq. (2)) resulting to a
local decrease of pH in the catholyte area, which in turns frustrates
the formation/precipitation of Ni(OH)2 that inhibits every mode
of growth except [2 1 1]. Therefore, the region of deposits oriented
through [2 1 1] axis, obtained at high pH values and low to medium
current densities, is reduced and replaced by composites exhibiting
[1 0 0] texture during progressive addition of TiO2 nano-particles in
the bath. Overall, the experimental data revealed that codeposition
of TiO2 nano-particles is favoured at low pH values and current
densities, implying that there is a plentiful adsorption of H+ on
the titania surface and as the particle surfaces become positively
charged they will be strongly adsorbed on the cathode leading to an
enhanced electrolytic codeposition. Similar results were reported
also for the case of Ni/yttria [29] and Zn/titania modified silica [18]
composite coatings.
entation. In detail, as the TiO2 nano-particles vol.% in the deposit
increases the mean grain size of the nickel matrix decreases and
the relative texture coefficient of each texture is improved, result-
ing in the production of a nanostructured nickel matrix in the range
of ∼20 nm. TiO2 nano-particles in the composite coatings are dis-
persed uniformly in the nickel matrix on the surface as well as,
through the cross-section of the deposits and the incorporation of
the nano-particles take place between the grains. Concluding, the
experimental data proved that is possible to enhance the incorpo-
ration of titania nano-particles in a nanostructured nickel matrix
by applying low pH and current density values.
Acknowledgement
The authors would like to acknowledge greatly Dr. Manfred
Baumgärtner for providing us the HRFE-SEM and GDOS measure-
ments.
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