Effects of 2-buthyne-1,4-diol additive on electrodeposited Ni films from a Watts-type bath

Article abstract of DOI:10.1016/j.electacta.2010.07.031

Structures of Ni films electrodeposited from a Watts-type bath containing 2-buthyne-1,4-diol (BD) were investigated using SEM, cross-sectional SIM, XRD measurement with a pole profiling technique and electrochemical methods for controlling properties of Ni electrodeposits. Preferred orientation of Ni electrodeposits was assigned to potential domains for electrodeposition. Preferred orientation in the higher potential region was (1 1 0) or (1 0 0), that in the middle potential region were (1 1 1) and (3 1 1), and that in the lower potential region was (1 0 0). The growing axis of Ni electrodeposits seems to agree with the speculation from Pangarov's model based on the two-dimensional nuclei theory in the lower overpotential region in which the dominant growing plane is fundamentally determined by crystallization overpotential related to supersaturation of adatom, although the growth axes of Ni deposits do not always agree with the preferred orientation. For example, preferred orientation of (1 1 0) was assigned to growing (1 1 1) plane which tilts at 55° to the substrate. Adsorption of BD affects the structure and morphology of electrodeposits via an inhibitory effect related to its surface coverage depending on surface orientation, growth rate and BD concentration in the plating bath.

Full text of DOI:10.1016/j.electacta.2010.07.031

Electrochimica Acta 55 (2010) 8570–8578
Contents lists available at ScienceDirect
Electrochimica Acta
journal homepage: www.elsevier.com/locate/electacta
Effects of 2-buthyne-1,4-diol additive on electrodeposited Ni films from a
Watts-type bath
a,b,∗
a
a
a
a
Takeshi Sakamoto
, Kazuhisa Azumi , Hiroto Tachikawa , Kei Iokibe , Masahiro Seo ,
c
c
Nobuya Uchida , Yasunaga Kagaya
Division of Materials Chemistry, Graduate School of Engineering, Hokkaido University, Sapporo 060-8628, Japan
Technology Group, TDK Corporation, Saku 385-0009, Japan
Technology Group, TDK Corporation, Ichikawa 272-8558, Japan
a
b
c
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 11 May 2010
Received in revised form 11 July 2010
Accepted 11 July 2010
Available online 16 July 2010
Structures of Ni films electrodeposited from a Watts-type bath containing 2-buthyne-1,4-diol (BD) were
investigated using SEM, cross-sectional SIM, XRD measurement with a pole profiling technique and
electrochemical methods for controlling properties of Ni electrodeposits. Preferred orientation of Ni
electrodeposits was assigned to potential domains for electrodeposition. Preferred orientation in the
higher potential region was (1 1 0) or (1 0 0), that in the middle potential region were (1 1 1) and (3 1 1),
and that in the lower potential region was (1 0 0). The growing axis of Ni electrodeposits seems to agree
with the speculation from Pangarov’s model based on the two-dimensional nuclei theory in the lower
overpotential region in which the dominant growing plane is fundamentally determined by crystalliza-
tion overpotential related to supersaturation of adatom, although the growth axes of Ni deposits do not
always agree with the preferred orientation. For example, preferred orientation of (1 1 0) was assigned to
Keywords:
Ni electrodeposition
2
-Buthyne-1,4-diol
Preferred orientation
Morphology
Overpotential
◦
growing (1 1 1) plane which tilts at 55 to the substrate. Adsorption of BD affects the structure and mor-
phology of electrodeposits via an inhibitory effect related to its surface coverage depending on surface
orientation, growth rate and BD concentration in the plating bath.
©
2010 Elsevier Ltd. All rights reserved.
1
. Introduction
[3]. Thickness of air formed oxide film on Ni grain is in the order
of (1 1 0) > (1 0 0) > (1 1 1) [4], and thus low resistive electric con-
tact is obtained for Ni deposits with (1 1 1) or (1 0 0) orientation
[5,6]. Organic additives adsorb on Ni surface with their unsaturated
molecular bonds [7] to block the sites for Ni deposition and alter-
nate deposition rate depending on their crystal orientation [8]. For
example, 2-buthyne-1,4-diol (BD) has been used as one of major
organic additives for controlling structure of Ni electrodeposits due
to its triple bond.
Structure of Ni electrodeposits has been intensively investigated
by many researchers. In 1916, Watts developed a high-speed Ni
electrodeposition solution called a Watts-type bath [9], which has
since been used worldwide. Schlöttor [10] developed bright Ni elec-
trodeposition from a sulfate solution containing Ni ions and organic
additives. Kardos et al. introduced BD as an additive to obtain a
highly smooth and sulfur-free Ni electrodeposited film [11]. In the
1960s, Ni-based double layer coating, in which a bright Ni film con-
taining sulfur was deposited on a sulfur-free Ni electrodeposited
film, was developed [12]. Fischer categorized the structures of an
electrodeposited film into six types as [13–17] FI (field-oriented
isolated crystals) type, BR (basis-oriented reproduction) type, Z
Ni electrodeposition has been widely used for corrosion protec-
tion and for attractive appearance of iron, steel, copper and many
other alloys. Ni electrodeposition has also recently been used for the
fabrication of electronic devices. For example, Ni-based corrosion
protective plating layers formed on strong magnets composed of
rare earth elements are used for actuators of data storage hard disk
drives and hybrid automobiles. Ni electrodeposition has also been
applied to connectors and printed circuit boards (PCB) for assurance
of reliable connection between electric/electronic components.
Crystallographic structure and its orientation of Ni electrode-
posits are related to various properties of plating films such as
corrosion resistance, hardness, magnetic properties, and contact
resistance [1]. For example, anodic dissolution rate of different
Ni grains is in the order of (1 1 0) > (1 0 0) > (1 1 1) [2]. Corrosion
protective duplex Ni plating layer consists of a top layer com-
posed of micro-grain crystals containing sulfur and an under layer
composed of columnar crystals, and the former acts as sacrificial
anode in corrosion reaction to protect under layer and substrate
(twinning intermediate) type, FT (field-oriented texture) type, UD
∗ Corresponding author. Tel.: +81 67661111; fax: +81 67684515.
E-mail address: faraday@cyber.email.ne.jp (T. Sakamoto).
(unoriented dispersion) type and RL (rhythmic-lamellar) type. For
example, it was reported that a sulfur-free Ni electrodeposited film
0
013-4686/$ – see front matter © 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.electacta.2010.07.031

T. Sakamoto et al. / Electrochimica Acta 55 (2010) 8570–8578
8571
Table 2
was composed of columnar crystals assigned to FT-type [3] and that
Electrodeposition conditions for different samples.
a bright Ni electrodeposited film containing sulfur was composed
of fine crystals assigned to UD-type. Winand summarized Fischer’s
classification in a Winand’s diagram [18–20]. FT-type electrode-
posits are observed in the fairly strong inhibition condition due to
adsorption of additives and are composed of elongated crystals or
columnar crystals almost perpendicular to the substrate. UD-type
electrodeposits are observed in the condition of very strong inhibi-
tion and are composed of very fine crystalline particles randomly
oriented.
2-Buthyne-1,4-diol (mmol dm−3)
Current density (A m
−2
)
Sample
A-1
A-2
A-3
B-1
B-2
B-3
C-1
C-2
C-3
0.0
0.0
0.0
0.2
0.2
0.2
1.0
1.0
1.0
30
100
300
30
100
300
30
100
300
Many researchers have reported the relationship between pre-
ferred orientation of Ni electrodeposits and inhibition effect. Reddy
et al. [21–23] proposed that the preferred orientation of Ni elec-
trodeposits tends to be (1 1 1) in free growth conditions and
changes sequentially to (1 0 0), (1 1 0) or (2 1 1) with increase in
hydrogen adsorption. Amblard et al. [24] proposed that preferred
orientation of (2 1 1) and (1 1 1) for Ni electrodeposits was provided
by the inhibition effect of colloidal Ni(OH)2 adsorption, and pre-
ferred orientation of (1 1 0) and (2 1 0) for Ni electrodeposits was
provided by the inhibition effect of hydrogen adsorption. This idea
is consistent with the finding by Motoyama et al. [25] that the
equilibrium line between (1 1 0) and (1 0 0) orientations of Ni in
the Pourbaix diagram was almost parallel to the equilibrium line
of hydrogen gas evolution. Addition of BD to a Watts-type bath
also affects the orientation of Ni electrodeposits. Macheras et al.
dition, while electrodeposition in practical production lines is done
in a stagnant or mild stirring condition. To characterize the electro-
chemical properties of the plating system, polarization curves of a
2
Cu working electrode of 0.2 cm in area was measured in the plating
bath deaerated with nitrogen gas bubbling using an electrochem-
ical measurement system (Hokuto Denko Co. model HZ-5000), an
Ag/AgCl/Sat. KCl reference electrode, and a platinum counter elec-
trode. Electrode potential was swept from the immersion potential
−
1
in a less-noble potential with a sweep rate of 2 mV s . Electrode
potential presented in this paper refers to the standard hydrogen
electrode (SHE) potential.
X-ray diffraction (XRD; PANalytcal Co. model MPD) spectra were
measured using Cu K␣ radiation. Diffractgrams were recorded in
[
26] investigated the preferred orientation of Ni films electrode-
◦
◦
a range from 20 to 160 of 2ꢀ with 0.02 step per 1 s. Pole fig-
ure and elevation profile was also obtained using X-ray diffraction
equipment (XRD; Rigakudenki Co. model ATX-G). Surface morphol-
ogy of Ni electrodeposits was observed using a scanning electron
microscope (SEM; Hitachi Ltd., model S-800). For cross-sectional
observation of electrodeposits, the specimen was etched using a
focused ion beam (FIB, Hitachi Ltd., model FB-2100) with Ga ions
posited from a Watts-type bath containing several kinds of organic
additives. They showed that addition of acetylenic compounds to a
Watts-type bath greatly inhibits crystal growth.
These previous investigations have clearly indicated that the
structure of Ni electrodeposits is affected by many factors of elec-
trodeposition conditions. However, the role and effect of these
additives have not yet been elucidated. Watanabe [27] reported
that various physical properties of electrodeposits were closely
connected to their microstructures such as size and orientation
of microcrystalline, surface morphology, metallurgical structures
of substrate, and so on. In this work, therefore, structures of Ni
films electrodeposited from a Watts-type bath containing BD were
investigated as a function of electrodeposition conditions.
◦
to make a trench with angles of 90 to the surface and the etched
surface, i.e., cross-section, was imaged with an observation angle
◦
of 45 (SIM image). The resultant cross-sectional image was thus
√
compressed by 1/ 2 in the vertical axes.
3. Results and discussion
3
.1. Relationship between preferred orientation and polarization
2
. Experimental
condition
A polycrystalline copper sheet of 10 mm× 30 mm × 0.5 mm in
Crystallographic orientation of Ni films electrodeposited from
size was used for plating of the substrate. The sheet was mechan-
−
3
a Watts-type bath measured using XRD are summarized in Fig. 1
as a function of BD concentration and current density. Preferred
orientations were (2 2 0) for samples A-1 and A-2, (2 0 0) for A-2
and B-1, (1 1 1) for B-2, C-1 and C-2, and (2 0 0) and (1 1 1) for A-
, B-3 and C-3. For further analysis of crystallographic orientation,
intensity of the XRD peak of each orientation shown in Fig. 1 was
normalized using the following equation:
ically polished and dipped in solution containing 0.2 mol cm
−3
−3
NaOH, 0.15 mol dm Na CO3 and 0.2 mol dm Na SiO4 at 323 K
2
4
for 300 s for cleaning the substrate surface before electrodeposition.
−
3
A Watts-type bath containing 0.0, 0.2 or 1.0 mmol dm 2-buthyne-
,4-diol (BD) was prepared from analytical-grade chemicals as
3
1
shown in Table 1. The pH of the bath was adjusted to 4.0 by adding
sodium hydroxide, and the temperature of the bath was maintained
at 323 K during electroplating. A series of combination of BD con-
centration and current density shown in Table 2 was used to obtain
Ni films of ca. 10 ␮m in thickness in a stagnant condition. Many
previous laboratory studies have been performed in a stirring con-
I(h k l)
ICDD(h k l)
M(h k l) = I
,
(1)
where M is the normalized intensity, I is the measured intensity of
the XRD peak at a specific orientation (h k l), and IICDD is the XRD
intensity of an ICDD PDF2 database. In Fig. 2, M is the plotted against
deposition current density to compare the relative intensities for
the orientations, and the following tendencies were found: (1) pre-
ferred orientations changed with BD concentration at a low current
density, (2) M(2 0 0) drastically increased with current density, and
(3) M of some orientations tended to increase with current den-
sity. However, the relationship between orientation and deposition
condition is not so clear in these plots.
Table 1
Composition of the Watts-type bath and the electrodeposition conditions for Ni
electroplating.
−
−
−
3
3
3
Ni sulfate hexahydrate (NiSO4·6H2O)
Ni chloride hexahydrate (NiCl2·6H2O)
Boric acid (H3BO3)
1.0 mol dm
0.2 mol dm
0.7 mol dm
−
3
2
-Buthyne-1,4-diol (HOCH2CCCH2OH)
0.0, 0.2, or 1.0 mmol dm
pH
Adjusted to 4.0 by adding NaOH
323 K
30, 100 or 300 A m
Temperature
Current density
Many researchers have proposed the relationship between
preferred orientations of electrodeposits and overpotential for
−
2

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572
T. Sakamoto et al. / Electrochimica Acta 55 (2010) 8570–8578
−
3
−3
−3
Fig. 1. XRD spectra of Ni films electrodeposited from a Watts-type bath containing 0.0 mmol dm (a–c), 0.2 mmol dm (d–f) or 1.0 mmol dm (g–i) 2-buthyne-1,4-diol at
−2
current density of 30, 100 or 300 A m . Preferred orientation of Ni crystalline is assigned as follows: triangle to (2 2 0), box to (2 0 0), circle to (1 1 1), and lozenge to (3 1 1).
electrodeposition [16,17,32,28–30]. In Fig. 3, cathodic polarization
curves of the Ni electrode measured in a Watts-type bath are shown
as a function of BD concentration. This plot clearly shows that
deposition potential at the same current density shifts in a less-
noble direction with increase in BD concentration. The polarization
curves can be divided into three regions, i.e., Tafel region at lower
overpotential, diffusion control region at higher overpotential, and
transition region between them. The dashed straight lines show the
Tafel relationship of three i–E curves expressed by the following
equations:
Line 2, 0.2 M BD : ꢁ = 0.450 − 0.232 log |i|,
Line 3, 1.0 M BD : ꢁ = 0.493 − 0.238 log |i|.
(3)
(4)
Tafel slopes of lines 2 and 3 are almost identical and smaller than
that of line 1, indicating that inhibition strengths of lines 2 and 3
due to BD are almost the same in the Tafel region. Transition from
the Tafel region to the diffusion control region probably involves
competition between formation of a fresh surface of Ni deposits
and adsorption of BD onto this surface, and the rate of the latter
depend on BD concentration. In the diffusion control region, BD
does not affect Ni deposition anymore because the deposition rate
is considerably faster than the adsorption rate of BD.
Line 1, 0.0 M BD : ꢁ = 0.319 − 0.103 log |i|,
(2)
−
2
Fig. 2. Normalized XRD peak intensity, M(h k l), obtained from data shown in Fig. 1 using Eq. (1) as a function of deposition current density at 30, 100 or 300 A m and
−3
−3
−3
2
-buthyne-1,4-diol concentration at 0.0 mmol dm (a), 0.2 mmol dm (b) or 1.0 mmol dm (c). Preferred orientation of Ni crystalline is assigned as follows: triangle to
(
2 2 0), box to (2 0 0), circle to (1 1 1), and lozenge to (3 1 1).

T. Sakamoto et al. / Electrochimica Acta 55 (2010) 8570–8578
8573
Fig. 3. Cathodic polarization curves of during Ni electrodeposition in a Watts-type
−
3
bath containing 2-buthyne-1,4-diol at concentrations of (1) 0.0 mmol dm , (2)
−
3
−3
0
.2 mmol dm , and (3) 1.0 mmol dm . Symbols indicate preferred orientation of
Ni electrodeposits obtained in Fig. 1 as triangle to (2 2 0), box to (2 0 0), circle to
1 1 1), and lozenge to (3 1 1).
(
The deposition potentials, E , which are the estimated values of
d
E at each deposition current density, i, on three cathodic polariza-
tion i–E curves shown in Fig. 3 as described by the symbols. A plot
of M(h k l) against Ed shown in Fig. 4(a) clearly reveals the three
domains. Fig. 4(b) is fractional expression of relative intensities
of M(h k l) against E . Preferred orientations for each specimen is
d
determined as dominant orientation higher than 50% of M(h k l).
Preferred orientations of samples A-1, A-2 and B-1 in the poten-
tial region higher than −0.53 V (domain 1) are (2 2 0) or (2 0 0).
Preferred orientations of samples B-2, C-1 and C-2 in the poten-
tial region from −0.54 to −0.56 V (domain 2) are (1 1 1) and (3 1 1).
Preferred orientations of samples A-3, B-3 and C-3 in the potential
region lower than −0.59 V (domain 3) are (2 0 0).
Fig. 4. (a) Relationship between normalized XRD peak intensity, M(h k l), of Ni elec-
trodeposited films and deposition potential, Ed. Preferred orientation of Ni crystal
is assigned as follows: triangle to (2 2 0), box to (2 0 0), circle to (1 1 1), and lozenge
to (3 1 1). (b) Relationship between the fraction of M(h k l) and Ed.
3.2. Consideration of overpotential using the two-dimensional
nuclei theory
Pangarov evaluated Wh k l as a function of supersaturation, ꢃ,
for each lattice plane [28–30]. Supersaturation was considered as a
In order to interpret the relationship between crystallographic
driving force of crystallization and could be calculated from activity
orientation and overpotential of electrodeposition, we considered
Fischer’s model [13,16] and Pangarov’s model based on two-
dimensional nuclei theory [28–30]. Fischer considered components
of total overpotential for electrodeposition, ꢁ, as the following rela-
tionship [13,16]:
of adsorbed metal ions (M ) under an equilibrium condition, a0
z+
,
ad
and that of cathodic polarization condition, aad, using the following
equation [31,32]:
aad − a0
aad
ad
ꢃ =
=
− 1.
(7)
0
0
a
a
ꢁ
= ꢁt + ꢁd + ꢁr + ꢁ˝ + ꢁk,
(5)
ad
ad
Nernst equations under these two conditions are defined as
where ꢁt is the charge transfer overpotential, ꢁd is the diffusion
overpotential, ꢁr is the reaction overpotential, ꢁꢂ is the ohmic
potential drop and ꢁk is the crystallization overpotential. Pan-
garov derived the work, Wh k l, required to form a two-dimensional
nucleus of the (h k l) plane [28–30] as
z+
RT
zF
[M
]
∗
ϕ
= ϕ +
ln
,
(8)
(9)
0
0
ad
a
z+
RT
[M
]
ϕ = ϕ +
ln
,
0
zF
aad
Bh k l
Wh k l
=
,
(6)
where ϕ0 is the standard electrode potential, R is the gas con-
stant, and T is the temperature. The crystallization overpotential
zF
N
∗
(
ϕ − ϕ ) − A
h k l
*
where z is the valency of ions, F is the Faraday constant, N is the
Avogadro number, ϕ is an electrode potential at a given current
in Eq. (5), ꢁk, is equal to ϕ – ϕ [31] and is described using
Eqs. (8) and (9) as
*
density for electrodeposition, ϕ is an equilibrium potential for of
0
RT
zF
a
RT
ꢁk = ϕ − ϕ∗
=
ln
ad
= − zF ln(ꢃ + 1).
(10)
electrodeposition, and Ah k l and Bh k l are values related to the work
for breaking a bond of some atom with its neighbor atoms at the
aad
(
h k l) plane and its separation from the substrate. According to this
Pangarov predicted the preferred orientation for thick deposits
of fcc metals such as Ag and Cu to be [1 1 1] at ꢃ ≤ 5.2 in the low
*
equation, Wh k l depends on crystallization overpotential, ϕ – ϕ .

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574
T. Sakamoto et al. / Electrochimica Acta 55 (2010) 8570–8578
−
3
−2
Fig. 5. SEM images of Ni films electrodeposited from a Watts-type bath containing 0.0, 0.2 or 1.0 mmol dm 2-buthyne-1,4-diol at current density of 30, 100 or 300 A m .
The preferred orientation was obtained from XRD spectra shown in Fig. 1. The deposition potential, Ed, vs. SHE shown in parenthesis was estimated from cathodic polarization
curves shown in Fig. 3.
overpotential region, [1 0 0] at 5.2 < ꢃ < 5.7 in the intermediate over-
potential region, and [1 1 0], [1 1 3] and [2 1 0] at ꢃ ≥ 5.7 in the high
overpotential region. This prediction was in good agreement with
experimental results of electrodeposition of Ag and Cu and was also
successfully applied to other metals with different crystalline types
To verify the applicability of Pangarov’s model to the present
experimental results, the relationship between preferred orienta-
tion and electrode potential shown in Fig. 4(a) was examined. In
domain 2, preferred orientation of samples B-2, C-1 and C-2 was
[1 1 1] and thus ꢁk was expected to be ꢁ ≥ −0.025 V. In domain
k
[
−
30]. The values of ꢁk were calculated to be −0.025 V at ꢃ = 5.2 and
3, preferred orientation for samples A-3, B-3 and C-3 was [1 0 0]
0.026 V at ꢃ = 5.7 using Eq. (10) with the conditions of T = 323 K
and thus ꢁk was expected to be −0.026 V < ꢁ < −0.025 V. On the
k
and z = 2. From the correspondence between ꢃ, ꢁk and preferred
other hand, measured overpotential ꢁ, i.e., the potential difference
between the deposition potential and the equilibrium potential of
−0.20 V (SHE) shown in Fig. 3, was −0.46 to −0.53 V in domain 1,
−0.54 to −0.56 V in domain 2, and ca. −0.59 V in domain 3. These
orientation described above, the preferred orientations of the fcc
lattice form are predicted to be [1 1 1] at ꢁ ≥ −0.025 V, [1 0 0] at
k
−
0.026 V < ꢁ < −0.025 V, and [1 1 0] at ꢁ ≤ −0.026 V.
k
k

T. Sakamoto et al. / Electrochimica Acta 55 (2010) 8570–8578
8575
−
3
−3
−3
Fig. 6. Cross-sectional SIM images of Ni plating films deposited from a Watts-type bath containing 0.0 mmol dm (a and b), 0.2 mmol dm (c and d) and 1.0 mmol dm (e
√
−2
and f) of 2-buthyne-1,4-diol at current density of 30 A m . (a, c, e) are original images. (b, d, f) are corrected images obtained by expanding the vertical scale by 2 (see text).
values of ꢁ were considerably larger than that of ꢁ , because ꢁ is a
the preferred orientation is [1 0 0] to −0.026 V < ꢁ < −0.025 V.
k
k
k
part of ꢁ as defined by Eq. (5). Although the contribution of ꢁ to ꢁ is
In these assignments, however, domain 1 cannot be assigned
k
considerably small, it may be possible to assign domain 2 where the
to ꢁ . The reason for this discordance is discussed in the next
k
preferred orientation is [1 1 1] to ꢁ ≥ −0.025 V and domain 3 where
subsection.
k

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T. Sakamoto et al. / Electrochimica Acta 55 (2010) 8570–8578
3.3. Morphology of Ni deposits
To investigate the reason for the inconsistency of preferred ori-
entation of Ni electrodeposits obtained in the present experiments
from that of Pangarov’s prediction in domain 1, surface and cross-
sectional morphologies of electrodeposits were observed using
SEM and SIM. As shown in SEM images in Fig. 5, surface morphology
changed depending on electrode potential and BD concentration.
In these images, two typical structures can be recognized. One
is spiral-type deposits found in the films deposited at −0.51 V as
indicated by a box in Fig. 5(d). The other is lozenge-type deposits
indicated by diamond frames in Fig. 5(e) (g) and (h), and they were
found on the films deposited at −0.54 to −0.56 V. The size of the
lozenge structures decreased with increases in BD concentration
and current density.
Fig. 6 shows cross-sectional SIM images of samples A-1, B-1
−2
and C-1 electrodeposited at 30 A m . The images on the left are
original SIM images and the images on the right were obtained by
√
multiplying the vertical scale of the left image by 2 to reproduce
Fig. 7. (1 1 0) pole figure for sample A-1 indicating the tilt angle distribution of
{1 1 1} micro-crystals to the surface plane. The symbol + corresponds to {1 1 1} of
standard (1 1 0) projection of a cubic single crystal.
the actual dimension of the cross-section because the observation
angle was 45 to the cross-section. It seems that the incipient Ni
◦
electrodeposits are influenced from substrate, but crystal size grad-
ually departs from the incipient electrodeposits, and finally the Ni
electrodeposits have a crystal size the deposition conditions with-
out substrate influence, as shown in Fig. 6. The substrate material is
not as a critical factor for structure or preferred orientation of elec-
trodeposits except for initial stage. This phenomenon was found
by Finch and Williams [33] who summarized that the effect of sub-
strate on crystal size and orientation gradually disappeared within
◦
◦
two peaks around 0 and 55 marked with +, corresponding to
[1 1 1] of standard [1 1 0] projection of a cubic single crystal. This
result indicates that the film contains micro-crystals of [1 1 1] ori-
◦
entation with tilting angle of ca. 55 to the substrate plane, and the
tilting direction is randomly distributed.
From the above discussion, Ni electrodeposits seem to grow
in a manner predicted from the two-dimensional nuclei theory in
which the growth plane is basically determined by crystallization
overpotential related to supersaturation, but the growth axis is not
always consistent with the preferred orientation axis. Models of
electrodeposition of Ni films on Cu substrates are summarized in
Fig. 8.
3
␮m from the interface, and the crystal size and orientation could
be characterized by the electrodeposition condition.
In these images, grain size of Ni electrodeposits becomes small
with increase in BD concentration. Sample A-1 is composed of thick
columns inclined to the growth direction and the maximum incline
◦
angle is ca. 58 against the substrate plane. On the other hand,
samples B-1 and C-1 are composed of narrow columns oriented
perpendicularly to the substrate plane. In all samples, grain size is
small at the interface between the film and substrate and increases
with film growth.
Fig. 8(a) shows a model for sample A-1. At the initial stage of
electrodeposition, high density of nucleation causes growth of fine
and dense columns in a direction normal to the substrate plane.
Most of these columns are annihilated in competitive growth, and
only a small number of thick columns survive and grow continu-
ously at the (1 1 1) plane in the growing direction with tilting angle
Sample B-1 shows a spiral structures in the SEM image shown
in Fig. 5(d) and strong (2 0 0) and weak (1 1 1) peaks in XRD spec-
tra shown in Fig. 1(d). A cross-sectional SIM image of sample
B-1 (Fig. 6(d)) indicates that the electrodeposits are composed of
small crystals and large spiral crystals among them. The formation
mechanism of such spiral structures in electrodeposits has been
investigated by many researchers [13–20,32,34–36]. Generally, the
crystal growth process involves surface diffusion of adatoms from
terraces to steps and settlement at kinks [37] (TSK model). If a
screw dislocation exists on the surface of the (1 0 0) plane, the spi-
ral crystal grows in a direction normal to the surface, while the
actual growth plane is (1 1 1) which orientation declines to the sur-
face of the (1 0 0) plane as schematically shown in the top image of
Fig. 8(b).
◦
of ca. 55 to the substrate plane, resulting in apparent orientation
of [1 1 0] observed in XRD measurement.
Fig. 8(b) is a model for sample B-1, in which columnar crys-
tals grow on fine crystals. At the bottom of the film, a considerable
amount of BD adsorbs on the surface, resulting in reduction of grain
size due to an inhibitory effect on crystal growth. This effect is
weakened but continues during the deposition process to keep the
column thin. The top of each column continues to grow in a spi-
ral manner shown in the top drawing in Fig. 8(b), i.e., orientation of
the growing surface, [1 1 1], inclines to the mean growing direction,
[1 0 0].
Fig. 8(c) is a model for sample C-1, and probably B-2 and C-2, in
which the Ni film shows [1 1 1] orientation in XRD results as shown
in Fig. 1, and the film is composed of lozenge-type crystals. The top
drawing in Fig. 8(c) shows growth of lozenge-type crystals in which
fast-growing plane tend to disappear and slow-growing plane tend
to survive [39–41]. Such a manner of growth was observed for a KCl
crystal with a high concentration of Pb impurity [41].
[
1 1 0] orientation in sample A-1 can be explained as follows. The
cross-sectional SIM image in Fig. 6(b) shows that the crystalline col-
◦
◦
umn inclines between 58 and 90 against the surface plane. Since
the apparent incline angle of columns in cross-section changes
depending on the direction of inclination, it is speculated that many
◦
columns incline around 58 . If these columns grew in [1 1 1] orien-
tation, the apparent orientation of this crystal is detected as [1 1 0]
in XRD measurement, because the (1 1 1) plane inclines at 54.8 to
the (1 1 0) plane in an fcc crystal. To confirm this, pole figure ele-
vation profile of sample A-1 was measured to obtain the angular
distribution of crystalline orientation of micro-crystals in a single
sample to the surface plane by the stereographic projection method
Structures and morphology of electrodeposits are affected by
inhibition strength of BD related to its coverage on the surface and
thus to its concentration in the bath. It is reasonable to speculate
that the columnar crystal of sample B-1 was formed by continuous
growth under the condition of weak inhibition, while fine crystals
of sample C-1 were formed by intermittent growth under the condi-
tion of strong inhibition. The coverage of BD depends on orientation
◦
[
38]. The pole profile of sample A-1 shown in Fig. 7 reveals strong

T. Sakamoto et al. / Electrochimica Acta 55 (2010) 8570–8578
8577
Fig. 8. Schematic representation of electrodeposited Ni films on Cu substrates. (a) Sample A-1. BR-type crystals incline to fine columnar Ni crystals. (b) Sample B-1. FT-type
columnar Ni crystals grow spirally on UD-type crystals. (c) Sample C-1. Fine lozenge UD-type crystals grow on the substrate.
of the surface. For example, coverage of the (1 1 1) plane seems to
be smaller than that, for example, of the (1 0 0) plane because the
deposition rate of Ni on the former is faster than that on the latter.
Eventually, the (1 1 1) plane with faster deposition rate disappears
as shown in the top drawing in Fig. 8(c), and, for example, the (1 1 0)
plane with slower deposition rate becomes predominant on the
deposited surface [39,42].
As discussed in this paper, the inhibitory effect of BD on
the electroplating process is influenced by many factors such as
overpotential and crystal orientation and provides variation of
morphology and structures of plating films. It seem reasonable that
BD molecule adsorbs on Ni surface mainly with its triple bond as
on Pt [43] and Au [44]. Bron and Holtze proposed the adsorbed
orientation model in which BD adsorbs on Au with strong inter-
action at the multiple bond and partial interaction at OH groups
profiling technique and electrochemical methods. The relationship
between structures and crystallographic orientation of electrode-
posits and deposition condition was evaluated. The results obtained
and conclusions based in the results are follows:
(
1) Preferred orientation of Ni electrodeposited films can be
assigned to potential domains for electrodeposition. Preferred
orientation in the potential region higher than −0.53 V (SHE)
(
domain 1) was (1 1 0) or (1 0 0), that in the region from −0.54
to −0.56 V (domain 2) were (1 1 1) and (3 1 1), and that in the
region lower than −0.59 V (domain 3) was (1 0 0).
(
2) Preferred orientation observed by XRD measurement does not
always agree with orientation of the actual growing plane
because the growing plane may tilt to the geometric surface
of the specimen. For example, preferred orientation of (1 1 0)
observed in domain 1 can be assigned to growing (1 1 1) plane
[
44]. In the present case it is supposed that BD molecule adsorbs on
Ni with co-deposition of Ni atoms and provides inhibitory effect.
BD was co-deposited into the Ni deposits resulting in 0.220 wt% of
carbon content in sample C-3 and 0.005 wt% in sample A-3 mea-
sured by total organic carbon analysis. Other adsorbents such as
◦
which tilts 55 to the substrate, and preferred orientation of
(
(
1 0 0) observed in domain 2 can be assigned to spiral growing
1 1 1) plane.
hydrogen [21–23] and Ni(OH)2 [24] and anions such like SO42 or
−
(3) The growing axis of Ni electrodeposits in the lower overpoten-
tial region seems to agree with the speculation from Pangarov’s
model based on the two-dimensional nuclei theory in which the
growth plane is fundamentally determined by crystallization
overpotential related to supersaturation of adatoms, although
the growth axes of Ni deposits do not always agree with the
preferred orientation.
−
Cl also have similar effects as shown in many previous studies.
The cause and condition of spiral or slanting growth of crystals and
their control remain as issues for future work.
4
. Conclusions
(
4) Adsorption of BD affects the structures and morphology of Ni
electrodeposits via an inhibitiory effect related to its surface
coverage depending on deposition rate, surface orientation and
BD concentration in the plating bath.
In this work, structure of Ni films electrodeposited from a Watts-
type bath containing 2-buthyne-1,4-diol (BD) was investigated
using SEM, cross-sectional SIM and XRD measurement with a pole

8
578
T. Sakamoto et al. / Electrochimica Acta 55 (2010) 8570–8578
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