Electrodeposition of oriented SrCO3 coatings on stainless steel substrates

Article abstract of DOI:10.1149/1.2188129

Cathodic reduction of a strontium bicarbonate bath stabilized by a variety of amine-carboxylic acids as ligands resulted in the deposition of micrometers-thick SrC O3 coatings by electrogeneration of base. The use of diethylenetriaminepentaacetic acid (DTPA) yielded SrC O3 coatings with a distinct c -axis orientation. The degree of orientation is higher in thick rather than thin coatings. The degree of orientation is also reduced when the DTPA concentration is increased or the pH is increased. These observations suggest that oriented crystal growth is due neither to the template effect of the substrate nor any additive mediated interaction due to DTPA but is solely due to the kinetics of crystallization. The Sr[DTPA] complex has the highest stability constant among the ligands studied, indicating the low degree of supersaturation in this bath compared to those stabilized by other complexing agents. SrC O3 deposition is a good model for the study of aragonite CaC O3 deposition by virtue of being isostructural with the latter and having the smallest lattice mismatch with aragonite among the heavier metal carbonates.

Full text of DOI:10.1149/1.2188129

Journal of The Electrochemical Society, 153 ͑6͒ D99-D103 ͑2006͒
D99
0013-4651/2006/153͑6͒/D99/5/$20.00 © The Electrochemical Society
Electrodeposition of Oriented SrCO3 Coatings on Stainless
Steel Substrates
Sumy Joseph and P. Vishnu Kamathz
Department of Chemistry, Central College, Bangalore University, Bangalore 560001, India
Cathodic reduction of a strontium bicarbonate bath stabilized by a variety of amine-carboxylic acids as ligands resulted in the
deposition of micrometers-thick SrCO3 coatings by electrogeneration of base. The use of diethylenetriaminepentaacetic acid
͑DTPA͒ yielded SrCO3 coatings with a distinct c-axis orientation. The degree of orientation is higher in thick rather than thin
coatings. The degree of orientation is also reduced when the DTPA concentration is increased or the pH is increased. These
observations suggest that oriented crystal growth is due neither to the template effect of the substrate nor any additive mediated
interaction due to DTPA but is solely due to the kinetics of crystallization. The Sr͓DTPA͔ complex has the highest stability
constant among the ligands studied, indicating the low degree of supersaturation in this bath compared to those stabilized by other
complexing agents. SrCO3 deposition is a good model for the study of aragonite CaCO3 deposition by virtue of being isostructural
with the latter and having the smallest lattice mismatch with aragonite among the heavier metal carbonates.
© 2006 The Electrochemical Society. ͓DOI: 10.1149/1.2188129͔ All rights reserved.
Manuscript submitted December 13, 2005; revised manuscript received January 23, 2006. Available electronically April 11, 2006.
The crystallization of inorganic materials by heterogeneous
nucleation in different living systems results in the formation of
high-performance materials. The unusual physical, chemical, and
mechanical properties of these materials is due to phase selection,
morphological control, or oriented crystallization that is engineered
by the biosystems during the heterogeneous nucleation process.1
There is considerable interest in mimicking nature’s methods by the
use of suitable additives or specially modified substrates which act
as templates by molecular recognition processes.2 CaCO3 is an ar-
chetypal example of an inorganic material, where phase selectivity
and morphological control has been exercised by the use of fatty
acids,3 block copolymers,4 and Langmuir monolayers.5
Electrochemistry is another method by which heterogeneous
nucleation can be induced on a metal/conducting substrate by the
application of a potential. Professor Switzer and co-workers have
pioneered the use of electrochemistry for the fabrication of
oriented,6 epitaxial,7 compositionally modulated,8 and nanostruc-
tured oxide films.9 Others have studied the electrochemical deposi-
tion of CaCO3 in the context of scaling problems encountered in
industrial water handling systems.10-12
Experimental
Sr͑NO3͒2, NaHCO3, and disodium salt of ethylenediaminetet-
raacetic acid ͑EDTA͒ were purchased from Aldrich Chemical Co.
͑USA͒. All other complexing agents were from Lancaster ͑U.K.͒.
All reagents were prepared using ion-exchanged Type-I water ͑Milli
Q
Academic water purification system, specific resistance
18.3 M⍀ cm͒.
Syntheses were carried out using EG&G ͑PARC͒ Versastat
model IIA scanning potentiostat driven by model M270 ECHEM
software operated in the galvanostatic mode. A cylindrical Pt mesh
͑geometrical area 28 cm2͒ and a saturated calomel electrode ͑SCE͒
were used as anode and reference electrode, respectively. A poly-
crystalline stainless steel ͑SS 304͒ flag ͑area 4.5 cm2͒ was used as a
cathode. The deposition current, i, was varied in the range
5–30 mA cm−2, and the time of deposition, t, was 10–120 min. The
charge input, Q, was calculated as ͑i ϫ t͒ mA min cm−2. Prior to
electrodeposition all the electrodes were degreased with detergent
and electrochemically polished as described elsewhere.22
Biogenic CaCO3 crystallizes in the aragonite modification13
while mineral CaCO3 is generally found in its thermodynamically
most stable form, the calcite modification.14 There is, therefore, con-
siderable interest in the laboratory synthesis of aragonite CaCO3,
especially by the use of electrochemistry. Unfortunately, aragonite is
metastable and is formed only in the presence of Mg2+ and at high
temperatures ͑50–80°C͒,15 while under other conditions calcite is
the only product.16
Baths were prepared by mixing equal volumes ͑25 mL͒ of
Sr͑NO3͒2 ͑0.05 M͒, complexing agent ͑0.05 M͒, and NaHCO3
͑0.1 M͒, in that order. The effective concentration of Sr2+ and com-
plexing agent is 0.0166 M while that of NaHCO3 is 0.033 M. Two
such baths were prepared for each experiment. One was allowed to
stand as control while electrodeposition was carried out in the other.
The control baths were observed for up to 42 h to observe their
stability. This was done to ensure that during electrochemical depo-
sition, no simultaneous chemical reactions took place. The diethyl-
enetriaminepentaacetic acid ͑DTPA͒-stabilized bath was prepared
with Sr͑NO3͒2 and DTPA solutions of 0.04 M and a NaHCO3 solu-
tion of 0.08 M strength. All electrochemical depositions were car-
ried out at the ambient temperature ͑24–26°C͒. The Sr2+ concentra-
tion was varied in the range 0.04–0.02 M and the ͓Sr2+͔/͓DTPA͔
ratio varied from 0.75 to 1.0. SrCO3 was also chemically precipi-
tated onto bare stainless steel flags to generate “electroless coatings”
as controls. No chemical precipitation was observed until the
͓Sr2+͔/͓complexing agent͔ ratio was 20.
All coatings were characterized by powder X-ray diffraction
͑PXRD͒ by directly mounting the electrode on a Siemens D5005
diffractometer operated in a reflection geometry. Data were collected
using Cu K␣ radiation ͑␭ = 1.541 Å͒ using a continuous scan rate
of 1° 2␪ per min and then binned into 2␪ steps of 0.02°. All PXRD
profiles were fitted by the Rietveld method ͑FullProf suite͒, using
the published structure of SrCO3 ͑ICSD no: CC 202793͒ ͑Pmcn,
a = 5.1039 Å, b = 8.4022 Å, c = 6.0210 Å͒.
17
Carbonates of heavier alkaline earth metals such as SrCO3
adopt the orthorhombic structure of aragonite CaCO3 and thereby
behave as model systems for the study of the latter. SrCO3 crystal-
lization has been studied using a self-assembled monolayer,18
thermally evaporated anionic surfactant sodium bis-2-
ethylhexylsulfosuccinate ͓aerosol OT ͑AOT͔͒,19 homogeneous pre-
cipitation by urea hydrolysis,20 and by surfactant-mediated
synthesis.21 All these studies are an attempt to fine-tune SrCO3 crys-
tallization by ͑i͒ the stereochemical effect of specific functional
groups or ͑ii͒ inhibiting crystal growth along specific directions.
This has the effect of controlling the morphology of the crystallites.
In this paper, we study the electrodeposition of SrCO3 on a poly-
crystalline stainless steel ͑SS304͒ electrode and report the growth of
oriented coatings. The study of SrCO3 is especially important, as
among the orthorhombic carbonates SrCO3 has the smallest lattice
mismatch with aragonite CaCO3 and can act as an epitaxial template
for the growth of the latter.17
z E-mail: vishnukamath8@hotmail.com
Scanning electron micrographs were obtained using JEOL JSM
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Journal of The Electrochemical Society, 153 ͑6͒ D99-D103 ͑2006͒
Table I. Structure of the complexing agents used
and the stability constants of their complexes
with Sr2+
.
Figure 1. PXRD patterns of SrCO3 coatings deposited from baths stabilized
by ͑a͒ sodium nitrilotriacetate, ͑b͒ EGTA, ͑c͒ CDTA, and ͑d͒ EDTA ͑depo-
sition current 10 mA cm−2, t = 60 min.͒. Features marked by the asterisk are
due to the substrate.
(i)
Nitrate reduction
NO−3 + H2O + 2e → NO−2 + 2 OH− E° = 0.01 V
͓1͔
͓2͔
͓3͔
(ii) Hydrogen evolution
2H2O + 2e → H2 + 2 OH− Eؠ = − 0.828 V
(iii) Dissolved oxygen reduction
840A microscope directly from the coating on the substrate by
mounting small pieces of electrode on conducting carbon tape and
sputter coating gold to improve the conductivity.
Infrared spectra were recorded using Nicolet Impact model 400D
Fourier transform infrared ͑FTIR͒ spectrometer ͑KBr pellets, reso-
lution 4 cm−1͒.
O2 + 2H2O + 4e → 4 OH− Eؠ = 0.40 V
The cell potentials measured during the galvanostatic deposition
vary in the range −1.3 to − 1.6 V and point toward aggressive H2
evolution as the chief reaction responsible for electrogeneration of
OH− ions. The electrogenerated base reacts with the HCO−3 ions,
leading to the generation of carbonate ions24
Results and Discussion
HCO3− + OH− → CO32− + H2O
͓4͔
SrCO3 has a low solubility product ͑9.4 ϫ 10−10͒ due to which a
Sr2+-containing carbonate/bicarbonate bath could not be stabilized
in the absence of a complexing agent. Different complexing agents
were used to bring down the level of supersaturation of SrCO3 in the
bath. Table I ͑where NTA is nitrilotriacetic acid and CDTA is
cyclohexylene-1,2-diaminetetraacetic acid͒ gives the list of com-
plexing agents used and the value of the stability constants of their
complexes with Sr2+. Using these complexing agents, the baths
could be stabilized indefinitely. No bulk precipitation was observed
in each of the baths for up to 42 h.
Concentration of ͓OH−͔ and ͓CO23−͔ near the electrode surface has
been calculated by theory and measured by experiment by Tlili and
co-workers in a calcium bicarbonate bath.25 Although the bulk pH
Upon electrolysis, however, white crystallites are deposited on
the stainless steel cathode within 15 min under the deposition con-
ditions employed in this study. The PXRD patterns of the as-
deposited coatings obtained from baths stabilized by different com-
plexing agents are shown in Fig. 1. The Bragg peaks appear at 2␪
values of 20.3, 25.4, 29.6, 31.5 and 36.2°, respectively, and the
patterns match with that assigned to SrCO3 ͓Powder Diffraction File
͑PDF͒: 05-0418͔ in the literature. To further confirm that the deposit
is indeed of SrCO3, the experimental data in Fig. 1 were Rietveld fit
by refinement of the SrCO3 structure. Figure 2 shows an illustrative
result of one such refinement. The structural and refinement param-
eters are given in Table II. Except in the case of ethylene glycol
bis͓2–aminoethylether͔tetraacetic acid ͑EGTA͒, the coatings ob-
tained from other baths show broad peaks probably related to the
small domain size.
Figure 2. Rietveld fit of PXRD profile of the SrCO3 coating obtained from
an EDTA-stabilized bath: Observed: full line, calculated: open circles. The
lower trace is the difference profile.
SrCO3 deposition takes place at the cathode due to electrogen-
eration of base23 according to the following reactions
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Journal of The Electrochemical Society, 153 ͑6͒ D99-D103 ͑2006͒
Table II. Results of the Rietveld refinement of the structure of strontium carbonate.a
D101
Goodness of fit
parameters
Atom
x
y
z
SOF
Line shape
Sr
C
O1
O2
0.25
0.25
0.25
0.49954
0.41890
0.76033
0.91540
0.67406
0.74455
−0.10064
−0.07757
−0.04561
1
1
1
2
u = 0.8500
v = −0.0500
w = 0.2505
␩ = 0.8128
X = 0.0105
Rwp = 20.8
Rexp = 18.15
Rp = 25.3
RF = 6.107
RB = 6.511
2
␹ = 1.32
DW-stat = 1.83
a Space group: Pmcn, a = 5.022 Å, b = 8.566 Å, c = 5.997 Å
does not change, the concentration of CO23− ions rises to mM pro-
portions close to the electrode. Sr2+ ions diffuse from the bulk to the
electrode surface, leading to the precipitation of SrCO3 on the elec-
trode
residual intensity is indicative of the growth of oriented coatings.
When the fit was repeated by incorporating a c-axis orientation, the
residual intensity could be accounted for and a superior fit was
achieved ͑Fig. 4c and d͒. To quantify the orientation, a modified
March’s function26 was employed wherein there are two refinable
parameters: G1, which varies from 1 ͑no orientation͒ to 0 ͑100%
orientation͒, and G2, which measures texture. The degree of orien-
tation indicated by the value of G1 increases from 0.77 in thin coat-
ings to 0.36 in thick coatings. While thin coatings have no texture
͑G2 = 0͒, thick coatings are well-textured ͑G2 = 0.62͒. The increase
in the degree of orientation and texture with thickness indicates that
the orientation is not substrate-induced. Substrate effects are more
pronounced in thin films. Thus, calcite CaCO3 grown over Si single-
crystal surfaces is oriented at small thickness, while at higher thick-
ness the film loses orientation.15
Orientation observed in thick coatings is driven by the kinetics of
crystallization. To verify this, deposition was carried out at high Q
͑600 mA min cm−2͒ at a lower ͓Sr2+͔/͓DTPA͔ ratio of 0.75 ͑see
Fig. 5͒. Despite the high Q, the degree of orientation was reduced
and the resultant PXRD pattern ͑see Fig. 5b͒ is comparable to the
one in Fig. 3b rather than Fig. 3c. The G1 value was found to be
0.42. The variation in the degree of orientation with the DTPA con-
centration shows that orientation is not due to directed crystalliza-
tion induced by the additive affect of DTPA. Further, the electroless
coatings obtained from a DTPA-containing bath were found to be
Sr2+ + CO23− → SrCO3↓
͓5͔
Deposition carried out in a wide range of conditions yielded
similar results for all complexing agents except DTPA.
The DTPA-stabilized bath gave different results. In Fig. 3 are
shown the PXRD patterns of coatings obtained from a DTPA-
stabilized bath at different deposition times ͑t = 30, 45, and 60 min͒
at a fixed deposition current ͑10 mA cm−2͒ and bath composition
͓͑Sr2+͔ = 0.04 M, ͓DTPA͔ = 0.04 M, ͓HCO−3͔ = 0.08 M͒. It is evi-
dent that with the increase in deposition time, the relative intensity
of 002 reflection increases and is higher than that observed for coat-
ings obtained from other baths. We refer to the coating obtained at
t = 30 min as “thin” and those obtained at t ജ 45 min as “thick.”
The classification of the coatings as thin and thick is done on the
basis of the charge input, Q, during the deposition. A Q value up to
300 mA min cm−2 yields a coating similar to that obtained from
other baths ͑Fig. 3a͒, whereas Q ജ 450 mA min cm−2 yields coat-
ings with a higher relative intensity of the 002 reflection ͑Fig. 3b
and c͒.
When an attempt was made to Rietveld-fit the experimental pat-
tern of a thick film by refining the structure of SrCO3, a difference
profile with a high-residual intensity at the Bragg angle correspond-
ing to the 002 reflection was obtained ͑see Fig. 4a and b͒. Such a
Figure 4. Rietveld fit of PXRD profile of a thick SrCO3 coating obtained
from a DTPA-stabilized: ͑a, b͒ the error function and calculated pattern ob-
tained without incorporating orientation, ͑c, d͒ the error function and calcu-
lated pattern obtained by incorporating 002 orientation, and ͑e͒ observed
pattern.
Figure 3. PXRD patterns of the SrCO3 deposits obtained from a DTPA-
stabilized bath ͑deposition current 10 mA cm−2͒ at different deposition
times: ͑a͒ 30 min, ͑b͒ 45 min, and ͑c͒ 60 min. Features marked by the aster-
isk are due to the substrate.
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Journal of The Electrochemical Society, 153 ͑6͒ D99-D103 ͑2006͒
Figure 5. Rietveld-fit of PXRD profile of the SrCO3 coating obtained from
a DTPA-rich bath. Observed: full line, calculated: open circles. The lower
trace is the difference profile.
Figure 7. PXRD patterns of SrCO3 coatings deposited from a dilute DTPA-
stabilized bath stabilized at ͑a͒ pH 3.5 and ͑b͒ pH 5.5. Features marked by
the asterisk are due to the substrate.
unoriented ͑see Fig. 6͒ ͑refined value of G1 = 0.87; G2 = 0͒, show-
ing that oriented growth is a result of electrodeposition.
The difference in the behavior of DTPA from other complexing
agents probably arises due to the higher stability constant of the
Sr–DTPA complex compared to the others. The consequent slow
release of free Sr2+ ions from the Sr–DTPA complex results in a
slower rate of deposition of SrCO3 from the DTPA-stabilized bath.
Low rates of deposition permit the preferential growth of the ther-
modynamically most stable crystal planes leading to oriented
growth. In the orthorhombic structure of SrCO3, the plane of CO2−
ions is perpendicular to the c-crystallographic axis. Oriented grow3th
along this axis takes place by the stacking of the planar CO23− ions
one on top of another. Such a stacking maximizes the bonding in-
teraction making this crystal plane thermodynamically stable.
The kinetics of deposition of SrCO3 are also affected by the pH
of the bath. The pH affects the extent of supersaturation of SrCO3 in
the solution as well as the rate of nucleation of SrCO3. Since the
͓HCO−3͔ concentration is kept constant in all the experiments, the
extent of supersaturation of SrCO3 depends on the free ͓Sr2+͔ con-
centration. Free ͓Sr2+͔ is generated by the decomposition of the
complex
SrHY2− + 2H+ Û Sr2+ + H3Y2−
͓6͔
H3Y2− is the predominant species of DTPA at the experimental
pH.27 The complex stability is higher and the free Sr2+ concentration
lower at higher pH. However, at high pH, SrCO3 has a lower solu-
bility and thereby a faster rate of nucleation compared to low pH.
Thus, the overall kinetics of SrCO3 deposition is a fine interplay
of these opposing factors. We did the following experiments to ex-
plore these various factors.
1. The ͓Sr–DTPA͔ complex concentration was reduced from
0.04 to 0.02 M to reduce the degree of supersaturation.
2. The pH of the bath was raised from 3.5 to 5.5 at 0.02 M
concentration to enhance the nucleation rate.
While the first experiment yielded a lower degree of orientation ͑
G1 = 0.71; G2 = 0͒, the second experiment yielded a nearly unori-
ented coating ͑G1 = 0.84; G2 = 0͒ ͑Fig. 7͒. At a lower concentra-
tion, the coating obtained is similar to that of the thin variety. The
effect is similar to that of decreasing the ͓Sr2+͔/͓EDTA͔ ratio. De-
creasing the supersaturation by either means results in thin deposits.
The loss of orientation on increasing the pH is probably due to the
rapid deposition of SrCO3 brought about by the enhanced rate of
nucleation. The orientation appears to be controlled by the kinetics
of nucleation.
In Fig. 8, the SEMs of coatings obtained from the DTPA stabi-
lized bath are compared with those of the coatings obtained from
EDTA-stabilized bath. It is evident that in both cases the stainless
steel surface has been completely covered over. However, the mor-
phology of the particles are different. The EDTA-stabilized bath
yields spherical particles, which are evident in the micrographs ob-
tained at both low and high magnification. The spherical morphol-
ogy arises due to loss of faceting as a result of redissolution, which
is aggressive at the crystallite edge. The presence of strong ligating
agents enhances the dissolution. The coatings obtained from other
baths, except for DTPA, also comprise spherical particles ͑data not
shown͒. The coating obtained from the DTPA-stabilized bath has a
platelike morphology. There is no evidence of redissolution, despite
the high stability constant of the Sr͓DTPA͔ complex. This is prob-
ably because the coatings are oriented and comprised of thermody-
namically stable crystal faces, which resist dissolution. The coating
obtained by chemical precipitation from a DTPA-containing bath
͑electroless coating͒ is very similar to that obtained by electrochemi-
cal deposition from an EDTA-stabilized bath. This clearly shows
Figure 6. Rietveld fit of PXRD profile of a SrCO3 coating obtained by
electroless deposition from a DTPA bath. Observed: full line, calculated:
open circles. The lower trace is the difference profile.
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Journal of The Electrochemical Society, 153 ͑6͒ D99-D103 ͑2006͒
D103
namely, the fabrication of oriented coatings of inorganic materials.
Electrochemical deposition is also a soft chemical alternative to
techniques such as pulsed laser deposition, sputtering, and chemical
vapor deposition, which are currently employed for the synthesis of
thin/thick films and coatings.
Acknowledgments
The authors thank the University Grants Commission, Govern-
ment of India, for financial support under the Major Research
Project program. The authors thank the Solid State and Structural
Chemistry Unit and the Department of Metallurgy, Indian Institute
of Science, for powder X-ray diffraction and electron microscopy
facilities, respectively.
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coatings obtained by electrodeposition from DTPA stabilized baths
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