Nanocrystalline Ni-C electrodeposits prepared in electrolytes containing supercritical carbon dioxide

Article abstract of DOI:10.1149/1.3205876

Electrodeposition of Ni in a Watt's bath at different applied pressures and in the presence of supercritical CO₂ (sc-CO₂), either with or without surfactant addition, was investigated. The current efficiency was evaluated under const

Full text of DOI:10.1149/1.3205876

Journal of The Electrochemical Society, 156 ͑11͒ D457-D461 ͑2009͒
D457
0013-4651/2009/156͑11͒/D457/5/$25.00 © The Electrochemical Society
Nanocrystalline Ni–C Electrodeposits Prepared in Electrolytes
Containing Supercritical Carbon Dioxide
Sung-Ting Chung and Wen-Ta Tsaiz
Department of Materials Science and Engineering, National Cheng Kung University, Tainan 70101,
Taiwan
Electrodeposition of Ni in a Watt’s bath at different applied pressures and in the presence of supercritical CO2 ͑sc-CO2͒, either
with or without surfactant addition, was investigated. The current efficiency was evaluated under constantly applied current
density conditions. The crystal structure of the resulting Ni film was characterized by performing X-ray diffraction. The compo-
sition of the deposit was analyzed using X-ray photoelectron spectroscopy. Transmission electron microscopy was employed for
microstructure analysis. Microhardness of the deposited film was measured to distinguish the role of bath composition. The
experimental results showed that carbon-containing nanocrystalline Ni films could be obtained in the bath with the presence of
sc-CO2. A significant increase in microhardness was found for the film electrodeposited in sc-CO2 fluid, as compared with that
formed in plain aqueous electrolyte. The fine grain size and solid solution strengthening caused by carbon were responsible for the
increased hardness.
© 2009 The Electrochemical Society. ͓DOI: 10.1149/1.3205876͔ All rights reserved.
Manuscript submitted May 28, 2009; revised manuscript received July 22, 2009. Published September 2, 2009.
Electroplating of nickel incorporating supercritical carbon diox-
ide ͑sc-CO2͒ has been successfully developed.1-4 Because sc-CO2
has a low conductivity, electroplating in such a medium always
involves an aqueous electrolyte. However, highly polar substances
͑such as water͒ and high molecular weight substances are less
soluble in sc-CO2, and thus a surfactant is needed. When an effec-
tive surfactant is added to the sc-CO2/aqueous electrolyte, thermo-
dynamically stable carbon dioxide-in-water ͑CO2/W͒ microemul-
sions that contain polar microaqueous domains can be produced.
Superior properties of a Ni film electroplated in such an emulsified
fluid, compared with that electrodeposited at atmospheric pressure,
have been reported.5 However, the same study has also shown that
electroplating efficiency in this environmentally friendly sc-CO2
fluid was less than that of the conventional process.
In the absence of any surfactant, the coexistence of sc-CO2 and
the aqueous electrolyte gives rise to a system with two separate
phases. Without any surfactant, the roles of pressure and the pres-
ence of sc-CO2 on the plating efficiency are of interest and are
explored. Furthermore, CO2 can be dissolved in water to form car-
bonic acid at ambient pressure. Under high pressure conditions, such
as in a supercritical state, whether CO2 participates in the film for-
mation and modifies the composition of the deposited layer is also
of interest. The incorporation of C into the Ni deposit is thus focused
upon and highlighted in this investigation.
ited film was performed. A scanning electron microscope ͑SEM͒
was used to examine the surface morphologies, while the surface
roughness of the electrodeposited nickel films was scanned and
measured with an atomic force microscope ͑AFM͒. The chemical
states of Ni and C in the electrodeposited film were detected by
X-ray photoelectron spectroscopy ͑XPS͒ on a PHI 5000 system. The
spectra were recorded with the Al K␣ line as the excitation source.
The crystal structure of the as-plated film was analyzed by X-ray
diffraction ͑XRD͒ using
a
monochromatic Cu K␣ ͑␭
= 1.540562 nm͒ radiation. The microstructure of the nickel films
was also examined with a field-emission transmission electron mi-
croscope ͑TEM͒ with an emission voltage of 200 kV. The Vickers
microhardness measurements were carried out using a microhard-
ness tester with a diamond pyramid indenter at a load of 50 g and a
duration of 15 s. An average of ten readings was taken to obtain the
hardness values of the nickel films.
Results and Discussion
Current efficiency.— The current efficiencies of electrodeposi-
tion at various conditions under an applied current density of
5 A/dm2 are depicted in Table II. The efficiencies were determined
by comparing the weight gain with the theoretical value calculated
from the applied charge using Faraday’s law. In plain aqueous elec-
trolyte and at ambient pressure, the current efficiency was 91.1%.
The applied pressure, under Ar atmosphere at 10 MPa ͑about 100
atm͒, did not cause a noticeable change in the current efficiency. At
the presence of sc-CO2 at 10 MPa, either with or without surfactant
addition, a decrease in the current efficiency ͑86.4 or 85.9%͒ was
observed, as shown in Table II. The decrease in efficiency was
mainly attributed to the lower conductivity of the electrolyte, as
pointed out by Kim et al.6 CO2 dissolved in water upon contact with
it, causing the formation of carbonic acid according to the following
reaction7,8
Experimental
For the electrodeposition, brass sheets of 1 ϫ 2.4 cm were used
as substrates. The brass substrates were successively ground with
SiC paper to a grit of #2000, followed by polishing with Al2O3
slurry to 0.3 ␮m, degreased in 0.1 M NaOH solution followed by
water rinsing, etched in 0.1 M H2SO4 solution, rinsed with deion-
ized water, and finally dried using a stream of hot air.
A high pressure cell made of stainless steel was fabricated for the
electrodeposition in a bath containing sc-CO2 fluid. The autoclave
system and the apparatus used for the Ni electrodepostion have been
described elsewhere.5 The composition of the sc-CO2-based electro-
lyte and the operating conditions are listed in Table I. The bath was
basically composed of NiCl2 and NiSO4, similar to that of a Watt’s
bath. As shown in this table, saccharin and C7H9NO2S were added
as brighteners, while octa͑ethylene oxide͒dodecyl ether ͑C12EO8͒
was added as a surfactant for the emulsified sc-CO2 bath. Elec-
trodeposition was performed on both sides of each specimen under
constant current density conditions at 5 A/dm2 for 36 min.
CO2 + H2O → H2CO3 → H+ + HCO3−
A lower pH ͑about 3͒ of the aqueous solution was obtained.9 During
electrodeposition, the amount of H+ ions generated according to the
above reaction was reduced. Consequently, the simultaneous reduc-
tion of Ni2+ and H+ caused a lower current efficiency.
The dependence of current efficiency on solution pH was con-
firmed by conducting the electrodeposition in a modified Watt’s bath
with various pH values, adjusted by the addition of ammonium wa-
ter. As demonstrated in Fig. 1, the current efficiency decreased as the
solution pH was lowered, which is consistent with previous
studies.10,11 In the bath containing sc-CO2, the lower current effi-
ciency was also attributed to the relatively higher acidity of the
electrolyte used. As shown in Table II, the current efficiency in the
After electrodeposition, material characterization of the depos-
z E-mail: wttsai@mail.ncku.edu.tw
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Journal of The Electrochemical Society, 156 ͑11͒ D457-D461 ͑2009͒
Table I. Bath composition and conditions used for Ni electrodeposition.
Deposition conditions
Electrolyte
Concentration
Parameters
Value
NiSO4·6H2O
NiCl2·6H2O
H3BO3
Saccharin
C7H9NO2S
300 g/L
50 g/L
30 g/L
0.5 g/L
0.5 g/L
0.1 vol %
Current density
Bath temperature
Deposition time
5 A/dm2
50°C
36 min
3.5
0.1 MPa
10 MPa
10 MPa
Solution pH
Aqueous electrolyte ͑exposed to air͒
Electrolyte in Ar gas
Electrolyte in sc-CO2
C
12EO8
surfactant-free sc-CO2/aqueous electrolyte mixture was almost the
same as that in the emulsified sc-CO2 bath. This was probably be-
cause the acidity did not differ much between these two baths.
Table II. Current efficiency for Ni electrodeposited at various
conditions.
Current efficiency
Conditions of coating
͑%͒
Surface morphology.— The surface morphologies of the elec-
trodeposited Ni films, produced in plain aqueous electrolytes at dif-
ferent pressures and in sc-CO2-containing baths with and without
surfactant addition, were examined by an SEM. Figure 2a shows the
SEM image of the Ni film electrodeposited in plain aqueous elec-
trolyte at atmospheric pressure, exhibiting a rather smooth surface
appearance. Similar results were found for the Ni film deposited in a
modified Watt’s bath under an Ar atmosphere of 10 MPa. The results
indicated that the surface appearance of the Ni film electrodeposited
in an aqueous electrolyte was not affected by the pressure in which
it was exposed. With the presence of sc-CO2, but without surfactant
addition, a large number of pinholes were found on the electrode-
posited Ni film surface, as depicted in Fig. 2b. These pinholes could
be as large as 150 nm, as shown in the micrograph. During elec-
trodeposition, hydrogen reduction is commonly accompanied by
metal ion reduction, resulting in the formation of pinholes. In the
surfactant-free sc-CO2 fluid, Ni was electrodeposited at the metal/
aqueous electrolyte interface. Under such high pressure conditions,
the amount of CO2 dissolved in the aqueous solution was increased.
Once the Ni was reduced at the metal ͑substrate͒ surface, some of
the dissolved CO2 was trapped and encapsulated in the deposited Ni
film. The pressure of the trapped CO2 was so high that it erupted to
form a pinhole ͑cavity͒ at the subsurface of the Ni film once the
pressure was released when the autoclave was opened up. Whether
the pinholes revealed in Fig. 2b mainly resulted from H2 evolution
or CO2 encapsulation is worth further investigation.
Aqueous electrolyte/0.1 MPa ͑air͒
Aqueous electrolyte/10 MPa ͑Ar͒
sc-CO2/10 MPa ͑surfactant-free͒
sc-CO2/10 MPa ͑emulsion͒
91.1
90.5
85.9
86.4
100
90
80
70
60
50
Constant current density : 5 A/dm2
aqueous electrolyte/0.1 MPa (air)
0.5 1.0 1.5 2.0
3.0 3.5 4.0 4.5
pH 2v.a5lue
Figure 1. Effect of pH on current efficiency of Ni electrodeposition in aque-
ous electrolyte at ambient pressure and at a current density of 5 A/dm2.
When electrodeposition was performed in the emulsified sc-CO2
electrolyte, a flat and almost pinhole-free surface was observed, as
shown in Fig. 2c. In the emulsified bath, Ni2+ reduction occurred
when the W/CO2 microemulsion was in contact with the electrode
surface. It has been reported that H2 gas has a high solubility in
sc-CO2 fluid.12,13 When simultaneous reductions of Ni2+ and H+
occur in the W/CO2 microemulsion, the evolved H2 would be mis-
cible in the sc-CO2 phase. As a result, the pinhole formation could
be minimized and even avoided, as demonstrated in Fig. 2c.
The surface roughness of the electrodeposited films was further
examined using an AFM. Figure 3 shows surface images of nickel
films deposited at various conditions. The surface roughness of the
Ni film electrodeposited in aqueous electrolyte in air at ambient
pressure ͑0.1 MPa͒ was about 9.8 nm. At a pressure of 10 MPa
under Ar atmosphere, the surface roughness of Ni film was almost
the same as that electrodeposited in air at ambient pressure. How-
ever, the surface of the Ni film became much rougher when it was
electrodeposited in the surfactant-free sc-CO2 fluid, and the presence
of pinholes on the surface was responsible for this. For the Ni film
deposited in the emulsified sc-CO2 fluid, a significant reduction of
surface roughness ͑about 5.7 nm͒ was seen, indicating the advantage
of electrodeposition in such an environment. The average surface
roughness ͑Ra͒ values of the nickel films deposited at various con-
ditions are listed in Table III.
(b)
(a)
2 µm
2 µm
(c)
2 µm
Figure 2. Surface morphologies of nickel films electrodeposited in ͑a͒ aque-
ous electrolyte at atmospheric pressure, ͑b͒ surfactant-free sc-CO2 fluid, and
͑c͒ sc-CO2 emulsion.
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Journal of The Electrochemical Society, 156 ͑11͒ D457-D461 ͑2009͒
D459
(a)
(c)
(b)
Table IV. 2␪„111…, d„111…, and lattice constant of Ni electrodeposited
at various conditions.
Lattice constant
Conditions of coating
2␪
d
͑Å͒
͑111͒
͑111͒
Aqueous electrolyte/0.1 MPa ͑air͒ 44.390 2.039
3.531
3.530
3.537
3.538
Aqueous electrolyte/10 MPa ͑Ar͒
sc-CO2/10 MPa ͑surfactant-free͒
sc-CO2/10 MPa ͑emulsion͒
44.410 2.038
44.320 2.042
44.310 2.043
(d)
Crystal structure.— The XRD patterns of Ni coatings prepared
at different conditions are illustrated in Fig. 4. As shown in this
figure, the nickel films electrodeposited in plain aqueous electrolyte,
at 0.1 or 10 MPa, are crystalline and exhibits a face-centered cubic
structure. The pressure applied did not cause any transformation in
the crystal structure of the Ni films, except the change in the relative
intensities between the ͑111͒ and ͑200͒ planes. The results indicate
that ͑200͒ planes are favored to form in high pressure conditions.
When electrodeposition was performed in the presence of sc-CO2 at
10 MPa, but without any surfactant addition, the diffraction peaks
broadened, revealing the nanocrystalline characteristics of the de-
posited film. An XRD pattern with broad, even peaks and much
lower intensities ͑Fig. 4͒ was obtained when electrodeposition was
performed in the sc-CO2 emulsion. The loss of crystallinity of the Ni
films electrodeposited in the electrolyte in the presence of sc-CO2
was further confirmed by TEM analysis, as described below. The
values of the d-spacing and lattice constant of the various Ni depos-
its are summarized in Table IV. The larger lattice constant for the Ni
films deposited in the electrolyte containing sc-CO2 indicates that
lattice distortion occurred in these deposits. As shown later, the dis-
solution of carbon into the Ni lattice was the main reason for the
modification of the crystal structure.
Figure 3. ͑Color online͒ AFM images showing the surface roughness of
nickel films electrodeposited in ͑a͒ aqueous electrolyte at atmospheric pres-
sure, ͑b͒ Ar atmosphere at 10 MPa, ͑c͒ surfactant-free sc-CO2 fluid, and ͑d͒
sc-CO2 emulsion.
Table III. Surface roughness of Ni electrodeposited at various
conditions.
Surface roughnessa ͑Ϯ2.4͒
Conditions of coating
͑nm͒
Aqueous electrolyte/0.1 MPa ͑air͒
Aqueous electrolyte/10 MPa ͑Ar͒
sc-CO2/10 MPa ͑surfactant-free͒
sc-CO2/10 MPa ͑emulsion͒
9.8
11.2
15.0
5.7
Chemical composition analysis.— XPS was employed for the
chemical composition analysis of the surface layer of the deposit. A
short Ar+ ion etching was performed to clean the surface of the
deposited film before XPS analysis. Figure 5 shows the survey scan
of the Ni film electrodeposited in the plain aqueous electrolyte ex-
posed in air at ambient pressure. The absence of C 1s in Fig. 5
indicates that Ar+ ion etching is adequate for the surface cleaning to
eliminate C contamination. A similar procedure was applied to other
specimens in which XPS was performed. The XPS spectra of C 1s
for the Ni films electrodeposited in the plain aqueous electrolyte at
atmospheric pressure and in the sc-CO2 emulsion are compared in
a Average of five measurements.
Constant current density : 5.0 A/dm2
Ni (111)
20
scCO2/10 MPa
emulsion
1400
1200
1000
Ni 2p3/2
Ni (200)
C region
18
16
14
12
10
8
scCO2/10 MPa
surfactant-free
295 290 285 280 275
aqueous electrolyte
10 MPa (Ar)
6
aqueous electrolyte
Air/0.1 MPa
4
2
Ni LMM
48
56
D4if4fraction 5A2ngle ( 2 t6h0eta 6)4
0
1000
600
400
0
800 Binding Energy [eV] 200
Figure 4. XRD spectra of nickel films electrodeposited in aqueous electro-
lyte at atmospheric pressure, in Ar atmosphere at 10 MPa, in surfactant-free
sc-CO2 fluid at 10 MPa, and in sc-CO2 emulsion at 10 MPa.
Figure 5. XPS survey spectrum of Ni film electrodeposited in aqueous elec-
trolyte at atmospheric pressure after Ar+ sputtering.
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Journal of The Electrochemical Society, 156 ͑11͒ D457-D461 ͑2009͒
(a)
(b)
(d)
C 1s
(c)
scCO2/10 MPa
emulsion
aqueous electrolyte
0.1 MPa (air)
Figure 7. Cross-sectional micrographs of nickel films electrodeposited in ͑a͒
aqueous electrolyte at atmospheric pressure, ͑b͒ Ar atmosphere at 10 MPa,
͑c͒ surfactant-free sc-CO2 fluid, and ͑d͒ sc-CO2 emulsion.
295 290 285 280 275
Binding Energy [eV]
Microstructure.— The microstructure of each electrodeposited
Ni film was examined by a TEM. Figure 8 shows the TEM micro-
graphs and the selected area diffraction ͑SAD͒ patterns of four dif-
ferent as-deposited Ni films. As demonstrated in Fig. 8a, the average
grain size of the Ni film electrodeposited in plain aqueous electro-
lyte in air at ambient pressure was about 43 nm, not too different
from that deposited in Ar atmosphere at a pressure of 10 MPa ͑Fig.
8b͒. As shown in Fig. 8c and d, a much finer microstructure was
found for the Ni film electrodeposited in the electrolyte containing
sc-CO2, either with or without surfactant. The average grain sizes
determined from the TEM micrographs were 17 and 14 nm, respec-
tively, for both cases, which are much finer than those deposited in
the electrolyte without the presence of sc-CO2 fluid. These results
Figure 6. ͑Color online͒ X-ray photoelectron spectra of the C 1s region of
nickel films electrodeposited in aqueous electrolyte at atmospheric pressure
and in sc-CO2 emulsion at 10 MPa, both etched by Ar+ ion for 1 min.
Fig. 6. In each case, Ar+ ion etching was carried out for 1 min. A
distinct C 1s peak was observed in the spectrum of the Ni film
deposited in the sc-CO2 emulsion. When Ar+ ion etching was ex-
tended to 3 min, the C peak could still be observed, confirming the
existence of C in the deposit layer. The C 1s binding energy for
graphitic carbon is 284.6 eV.14 As shown in Fig. 6, the C 1s binding
energy obtained in this investigation was 282.8 eV. The decrease in
the C 1s binding energy due to the formation of metal–carbon bonds
has been reported.15,16 Thus, the result shown in Fig. 6 indicate the
dissolution of carbon into the Ni film deposited in the sc-CO2 emul-
sion.
(b)
(a)
The formation of Ni–C deposits via sputtering or chemical syn-
thesis has been reported.17-19 However, to the best of our kowledge,
it has not been reported an electrodeposition process. In the system
containing sc-CO2, as described in this investigation, the formation
of Ni–C deposit might be associated with the following reduction
reactions
Ni2+ + 2e− → Ni
and
HCO−3 + 5H+ + 4e− → C + 3H2O
(c)
(d)
The reduction reaction of bicarbonate ion ͑HCO−3͒ might become
favorable at high pressure, as found in the system incorporating
sc-CO2.
The XRD analysis shows that a higher lattice constant was ob-
served when the Ni film was deposited in the electrolyte with the
presence of sc-CO2. This expansion of the lattice constant can be
attributed to the solid solution of C in the Ni lattice.
Cross-section micrograph.— The cross-sectional micrographs of
the various deposits are shown in Fig. 7. The results show that with
a deposition time of 36 min, the thickness of the deposit varies with
deposition conditions. In sc-CO2-containing electrolytes, with or
without surfactant addition, the film was thinner than those elec-
trodeposited in plain aqueous electrolytes. These results are consis-
tent with those of the weight change measurements.
Figure 8. TEM micrographs and SAD patterns of as-plated Ni coating pro-
duced at i = 5 A/dm2 and in ͑a͒ aqueous electrolyte at atmospheric pressure,
͑b͒ Ar atmosphere at 10 MPa, ͑c͒ surfactant-free sc-CO2 fluid, and ͑d͒
sc-CO2 emulsion.
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Journal of The Electrochemical Society, 156 ͑11͒ D457-D461 ͑2009͒
D461
sc-CO2 due to the increased participation of proton reduction. Pin-
holes were found in the Ni film electrodeposited in the surfactant-
free sc-CO2 bath, but these were eliminated when the deposition
took place in an emulsified sc-CO2 bath. Consequently, the surface
roughness of the latter film was much lower than that of the former.
A nanocrystalline Ni film was obtained, and the grain size became
much finer when the sc-CO2 bath was used. XPS analysis showed
that the solution of carbon occurred to form Ni–C electrodeposits in
the sc-CO2 bath. The fine grain size and the solid solution of C in Ni
gave rise to a substantial increase in the microhardness of the elec-
trodeposited Ni–C film.
Table V. Grain size and microhardness of Ni electrodeposited at
various conditions.
Grain size
Microhardness
Conditions of coating
͑nm͒
͑Hv͒
Aqueous electrolyte/0.1 MPa ͑air͒
Aqueous electrolyte/10 MPa ͑Ar͒
sc-CO2/10 MPa ͑surfactant-free͒
sc-CO2/10 MPa ͑emulsion͒
43
45
17
14
446 Ϯ 21
412 Ϯ 16
701 Ϯ 14
736 Ϯ 23
Acknowledgments
agree with the broadened XRD patterns shown in Fig. 4. da Rocha et
al.20 have reported that the emulsion of a saline solution with
sc-CO2 contains numerous micelles with radii in the range of several
micrometers. Yoshida et al.2 have also indicated that plating in the
emulsion is similar to pulse plating. With the characteristic nature of
the electrolyte and the unique electrodeposition mechanism, a much
finer microstructure could thus be obtained, especially with the pres-
ence of a sc-CO2 emulsion, as revealed in the TEM micrographs of
Fig. 8d. The four diffraction rings shown in Fig. 8 are identified as
the ͑111͒, ͑200͒, ͑220͒, and ͑311͒ reflections of the face-centered
cubic Ni. The well-defined electron diffraction patterns indicated
that the Ni films were crystalline in nature but with a nanoscale
grain size.
The authors thank the National Science Council of the Republic
of China for partially supporting the use of the instruments for sur-
face characterization under contract no. NSC 96-2221-E-006-109.
The authors also thank the High Valued Instrument Center of I-Shou
University for assisting in the TEM analysis.
National Cheng Kung University assisted in meeting the publication
costs of this article.
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␴ = ␴ + kyd−1/2
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o
where ␴ is the tensile yield strength, ␴ is the intrinsic stress re-
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Conclusion
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Electrodeposition of Ni was performed in a modified Watt’s bath
in the presence of sc-CO2, either with or without surfactant addition.
However, the current efficiency was lower in the bath containing
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