ARTICLE IN PRESS
Journal of Crystal Growth 285 (2005) 633–641
www.elsevier.com/locate/jcrysgro
Effect of NaOH(aq) treatment on the phase transformation
and morphology of calcium phosphate deposited by an
electrolytic method
Wei-Jen Shiha, Yi-Hung Chenb, Szu-Hao Wangb, Wang-Long Lib,
Min-Hsiung Hona,c, Moo-Chin Wangd,
a
Department of Materials Science and Engineering, National Cheng Kung University, 1 Ta-Hsueh Road, Tainan 70101, Taiwan
b
Department of Mechanical Engineering, National Kaohsiung University of Applied Sciences, 415 Chien-kung Road,
Kaohsiung 80782, Taiwan
c
Dayeh University, 112 Shan-Jiau Road, Da-Tsuen, Changhua 515, Taiwan
d
Department of Materials Science and Engineering, National United University, 1 Lien-Da Road, Kung-Ching Li, Miao li 360, Taiwan
Received 20 May 2005; received in revised form 27 August 2005; accepted 28 August 2005
Available online 19 October 2005
Communicated by M. Schieber
Abstract
Calcium phosphates deposited by an electrochemical deposition process (ECD) have been prepared in the 0.04 M
Ca(H2PO4)2dH2O (MCPM) solution at 10 V, 60 1C and 80 Torr for 1 h. Subsequently, the deposits have been treated in
various concentrations of NaOH(aq) solutions at different temperatures for 1 h. When excess OH is present in the
ECD process, hydroxyapatite (HAP) referred to as pre-HAP is deposited on the Ti–6Al–4 V substrate. After NaOH(aq)
treatment, all deposits are converted to the HAP phase. The treatment in 1–2.5 M NaOH(aq) solution at over 60 1C
offers a more proper environment for the HAP conversion, referred to as post-HAP, and has an average crystallite size
of about 21 nm. For more than 5 M NaOH(aq) treatment, both pre- and post-HAP show a reduction of 5–20 nm in
particle size.
r 2005 Elsevier B.V. All rights reserved.
PACS: 81.07.Wx; 81.20.Ka; 64.70.Kb
Keywords: A1. Biomaterials; A2. Electrochemical growth; B1. Nanomaterials; B1. Phosphates
Corresponding author. Tel.: +886 37 381710; fax: +886 37 324047.
E-mail address: [email protected] (M.-C. Wang).
0022-0248/$ - see front matter r 2005 Elsevier B.V. All rights reserved.
doi:10.1016/j.jcrysgro.2005.08.042
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1. Introduction
Many efforts have been made in recent years in
the development of processing methods for depositing bioceramics on implant alloy substrates such
as Ti–6Al–4 V in order to have high strength,
suitable specific density, good processing ability,
and excellent corrosion resistance in living bodies.
Electrochemical deposition (ECD) of calcium
phosphate coatings for biomedical applications
has been investigated since the 1990s [1–3].
Calcium phosphate coatings have been deposited
on various metal substrates by an electrochemical
method, which is an attractive process of relatively
fast reaction rate at low temperatures, and ease of
control by varying electrochemical potential,
current density, electrolyte concentration and
temperature [4].
Hydroxyapatite (HAP, Ca10(PO4)6d(OH)2) is
the most interesting form of calcium phosphate
to be electrochemically deposited from several
solutions at elevated temperatures [5–7]. Silva et
al. [8] have deposited calcium phosphates (monocalcium phosphate monohydrate (MCPM, Ca(H2PO4)2dH2O), and dicalcium phosphate dihydrate
(CaHPO4d2H2O, DCPD)) on Ti–6Al–4 V alloy by
ECD, and discussed the phase transformation of
deposits in various alkali solutions, such as KOH,
NH4OH and NaOH. Calcium phosphates have a
wide range of pH stability, depending on their Ca/
P ratio. Acidic calcium phosphates, like MCPM
and DCPD, are thermodynamically unstable
under pH values greater than 6–7 and undergo
transformation into more stable calcium phosphates. Under physiological conditions (pH 7),
the most stable calcium phosphate is HAP [8].
Wang et al. [9] have reported that HAP is found
to form at 10 V, 60 1C and 80 Torr for 1 h in the
ECD process, and since the electrolyte is exclusively of MCPM, the films obtained are from
calcium phosphates only, such as DCPD, amorphous calcium phosphate (ACP) and HAP. On the
other hand, the former research [10,11] has also
pointed out that DCPD can be converted to nanosized HA crystals by hydrolysis in NaOH(aq).
While for the conditions close to physiological pH
(pHo9), convincing evidence has been presented
that ACP–HAP conversion is a solution-mediated
process which occurs via intermediate octacalcium
phosphate (OCP), Ca4H(PO4)3d2.5H2O [13–15],
ACP–HAP transformation at alkaline media remains ambiguous. The application of the concept
proven at neutral solutions to the solutions with
higher pH values results in a discrepancy between
thermodynamic and kinetic data, indicating possible
change of the mechanism of ACP–HAP conversion
[12]. From the above considerations, it can be
inferred that a minimal rate of ACP–HAP conversion is expected at solutions which are thermodynamically stable with respect to acidic calcium
phosphate phases and contain large fractions of
calcium and phosphorus in the forms of Ca2+ and
HPO2
4 , respectively [16].
The main objective of this study is to investigate
the effect of NaOH(aq) treatment on the phase
transformation and morphology of the calcium
phosphate deposited by an electrolytic method.
The ECD process has been used at 60 1C, 10 V for
1 h in the 0.04 M MCPM aqueous solution,
followed by treatment in different concentrations
of NaOH(aq) and temperatures for 1 h. The phase
transformation and morphology have been carefully investigated by X-ray diffraction (XRD),
FT–IR, SEM and TEM.
To confine the scope of this study, the following
aspects are presented: (i) to study the phase
transformation of the deposit treated in various
concentrations of NaOH(aq) at different temperatures, (ii) to extend the exploration of the effect of
NaOH(aq) treatment on the crystallite size, (iii) to
study the effect of NaOH(aq) treatment on the
chemical bonding transition of calcium phosphate
deposit, and (iv) to compare the morphology of
the as-deposited and after NaOH(aq) treatment
samples.
2. Experimental procedure
2.1. Sample preparation
This study used a Ti–6Al–4 V alloy plate as a
cathode and a platinum plate as an anode.
The composition of the Ti–6Al–4 V alloy plate
conformed to the specification of ASTM standard
F-136. The Ti–6Al–4 V alloy plate with a size of
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635
15 15 3 mm was mechanically ground with SiC
papers from 120 to 1200 grit and polished with
0.3 mm Al2O3 powers to a mirror finish. The
Ti–6Al–4 V plate was then washed thoroughly
with running distilled water before being ultrasonically degreased with acetone and dried at
60 1C.
The saturated 0.04 M electrolyte was prepared
by adding 1 g monocalcium phosphate monohydrate (MCPM, Ca(H2PO4)2dH2O, analytical
grade, Showa Chemical Co. Ltd., Tokyo, Japan)
in 100 ml deionized water. The electrolyte was
stirred with a magnetic stirrer for 1 h to enhance
the dissolution of the calcium phosphate
(pH ¼ 3:0). Electrolysis was carried out at 10 V,
60 1C for 1 h, and the distance between the
electrodes was 3 cm with the cathode area of
1.057 cm2. The ambient pressure of 80 Torr was
selected for the electrolysis to improve the bubbles
assembling in the vicinity of the cathode surface
(Ti–6Al–4 V substrate) [9]. After deposition, the
sample was washed in distilled water and dried at
room temperature.
sample was prepared by pressing a small amount
of the as-deposited and after NaOH(aq) treatment
deposits into a standard KBr thin disk.
The surface morphology and particle size
distribution of the as-deposited and after NaOH(aq) treatment samples were examined by a field
emission scanning electron microscope (Philips
XL40 FE-SEM, Eindhoven, The Netherlands).
A transmission electron microscope (HITACHI
FE-2000, Tokyo, Japan) was used at 200 kV to
determine the crystal structures of the samples after
NaOH(aq) treatment. In addition, the ratio of Ca/P
was determined by an energy dispersive spectrometer
(EDS). The TEM sample was prepared by dispersing
the powders of the deposits treated in various
concentrations of NaOH(aq) in an ultrasonic bath
and then collected on a copper grid.
2.2. NaOH(aq) treatment
The XRD patterns of the deposited coatings
with different applied voltages at 60 1C for 1 h are
shown in Fig. 1. All diffraction peaks conform to
the crystalline DCPD (JCPDS 72-1240), HAP
The electrolytic deposited samples were then
treated in the 0.1–10 M NaOH(aq) solutions at 30,
45, 60, and 75 1C for 1 h, respectively. After the
NaOH(aq) treatment, the specimens were gently
washed with distilled water and dried at 60 1C in
an electric oven overnight.
3. Results and discussion
3.1. Effect of NaOH(aq) treatment on the phase
transformation of calcium phosphate deposit
2.3. Sample characterization
The crystalline phases of the dried samples asdeposited and after NaOH(aq) treatment were
examined with XRD (Rigaku D-Max/IIIV, Tokyo, Japan). A monochromatic Cu Ka radiation
was selected (l ¼ 1:54052 Å). The operation tube
voltage and current were 30 kV and 20 mA,
respectively. The scanning angle (2y) of the sample
was from 201 to 551 with a scanning speed of 41/
min.
The chemical behaviour and molecular bonding
structure of the converted HA were evaluated by
an FT–IR spectrometer (PerkinElmer Spectrum
One FT–IR spectrometer, Boston, USA). The
Fig. 1. XRD pattern of deposits on Ti–6Al–4 V alloy by ECD
with different voltages at 60 1C for 1 h: (a) 5 V, (b) 10 V (D:
DCPD, H: HAP, T: Ti).
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(JCPDS 09-0432) and Ti (JCPDS 44-1294). In
Fig. 1(a), DCPD is the major phase of the film
deposited at the applied voltages of 5 V. When the
applied voltage is increased to 10 V (Fig. 1(b)),
DCPD is still the major phase, but the HAP
reflections increase dramatically and also clearly
illustrate the HAP (0 0 2) peak at 2y ¼ 25:881 and
the broadened HAP (2 1 1) peak at 2y ¼ 31:771.
Here, the HAP formed by ECD is called ‘‘preforming HAP’’ (pre-HAP).
During ECD, a lot of OH ions are formed in
the reduction of water and the concentration of
OH at high-applied voltage is sufficient to
3
convert all HPO2
4 into PO4 using the following
reaction [17]:
3
HPO2
4 þ OH ! PO4 þ H2 O:
(1)
HAP can then be deposited on the cathode
surface by the following reaction:
10 Ca2þ þ 6 PO3
4 þ 2 OH ! Ca10 ðPO4 Þ6 ðOHÞ2 .
(2)
Ban and Hasegawa [18] have found that the
crystal growth of HAP increases with electrolyte
temperature, and hence the temperature of 60 1C is
chosen. Additionally, the broadened peak of the
HAP reflection reveals a coating with poor crystallinity or composed of small crystallites, which are
similar to natural bone mineral and suitable for
tissue compatibility [17].
Fig. 2 shows the XRD patterns of the deposits
on the Ti–6Al–4 V alloy by ECD at 60 1C, 10 V for
1 h, and followed by treatment in various NaOH(aq) concentrations at 75 1C for 1 h. All the
samples deposited with DCPD and HAP show
the major HAP phase after the NaOH(aq) treatment. All diffraction peaks conform to the crystalline HAP and Ti. The broadened reflections reveal
a nano-sized distribution of the HAP crystallites.
The sample treated in the 0.1 M NaOH(aq) 75 1C
for 1 h has some residual DCPD phase at low
angles, which steadily disappear as the NaOH(aq)
concentration increases. The (2 1 1) and (1 1 2)
reflections gradually split when the NaOH(aq)
concentration increases to 2.5 M and both intensities also increase, which means the maturation of
the synthesized HAP crystallites. However, when
the concentration keeps increasing, the (2 1 1) and
Fig. 2. XRD patterns of deposits on Ti–6Al–4 V alloy by ECD
at 60 1C, 10 V for 1 h, and alkali treated with various NaOH(aq)
concentrations at 75 1C for 1 h: (a) 0.1 M, (b) 1 M, (c) 2.5 M, (d)
5 M, and (e) 10 M.
(1 1 2) reflections are overlapped. Here, the HAP
formed by the NaOH(aq) treatment is called ‘‘postforming HAP’’ (post-HAP).
Ohta et al. [10] have pointed out that DCPD is
thought to be one of the precursors of HAP and
the synthesis of HAP aggregates oriented along caxis is obtained through the hydrolysis of DCPD
based on the following reaction:
10CaHPO4 2H2 O ! Ca10 ðPO4 Þ6 ðOHÞ2
þ 18H2 O þ 12Hþ þ 4PO3
4 .
ð3Þ
The mechanism of DCPD dissolution and
conversion to HAP is a fast solid–solid phase
transition occurring at a high liquid-to-solid ratio,
generally 100:1, and the HAP product has been
identified as calcium deficient HAP (d-HAP) in the
above work.
Liu et al. [19] have pointed out that the unstable
calcium phosphate phases such as DCPD at high pH
values (pH ¼ 11213) may transform into ACP
(Ca3(PO4)2dxH2O, Ca=P ¼ 1:5) very quickly, which
then gradually converts to d-HAP (Ca10x(HPO4)x(PO4)6x(OH)2xdnH2O, where 0pxp1), and finally, the intermediate product transforms into the
stable HAP phase.
Fig. 3 shows the XRD patterns of the deposits
on the Ti–6Al–4 V alloy by ECD at 60 1C, 10 V for
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Fig. 3. XRD patterns of deposits on Ti–6Al–4 V alloy by ECD
at 60 1C, 10 V for 1 h, and alkali treated with 2.5 M NaOH(aq) at
various temperatures for 1 h: (a) 30 1C, (b) 45 1C, (c) 60 1C, and
(d) 75 1C.
1 h, and alkali treated with 2.5 M NaOH(aq) at
various temperatures for 1 h. When NaOH(aq) is
treated at low temperatures, there is still some
residual DCPD phase observed and the (2 1 1) and
(1 1 2) reflections are overlapped as one reflection.
As the electrolyte temperature is higher than 60 1C,
the DCPD phase disappears steadily and the
reflections of HAP split.
3.2. Effect of NaOH(aq) treatment on the chemical
band structure change of calcium phosphate deposit
FT–IR is a viable tool for structural investigations at the molecular level providing the knowledge of the vibration band positions for a given
helical or extended conformation [20]. Fig. 4
shows the FT–IR spectra of the deposits on the
Ti–6Al–4 V alloy by ECD at 60 1C, 10 V for 1 h,
and of alkali treated with NaOH(aq) of (a) different
concentrations at 75 1C and (b) 2.5 M at various
temperatures for 1 h. The sharp splitting of PO3
4
bands at 566 and 604 cm1 indicates the crystalline
apatite structure. Bands of 962 and 1038 cm1 are
also assigned to the P–O stretching modes. The
broad band of adsorbed water is in the range of
3200–3433 cm1, and 1599 cm1 corresponds to
the OH group in the water molecule. The
Fig. 4. FT–IR spectra of deposits on Ti–6Al–4 V alloy by ECD
at 60 1C, 10 V for 1 h, and alkali treated with NaOH(aq) of (a)
different concentrations at 75 1C and (b) 2.5 M at various
temperatures for 1 h.
stretching and vibration modes of the O–H group
show the presence of the HAP phase in support of
the XRD data. Neither Ti nor Ti–O bonding is
found in Fig. 4 indicating that the HAP deposits
are thick enough. The peak at 875 cm1 corresponding to HPO2
4 also indicates the intermediate
OCP bonding [16] mentioned in the introduction
or the residual DCPD phase observed in the XRD
patterns, but there is no noticeable absorption of
the TCP phases.
In Fig. 4(a), the concentration of the NaOH
solution seems to have no effect on the structure of
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the converted HAP, namely, all the IR peaks have
nearly no shift but differences in morphology are
observed in the SEM images. The effect of treating
temperature on the HAP bonding is not obvious as
in Fig. 4(b), but the HPO2
bonding decreases
4
with treating temperature. In addition, a small
disturbance from 632 to 667 cm1 means that the
O–H bonding in HAP is weakly affected by
treating temperatures.
3.3. Effect of NaOH(aq) treatment on the
crystallite size of calcium phosphate deposit
As shown in Figs. 2 and 3, the sampledependent broadening of the XRD patterns
consists of the contributions from crystallite size
and strain. Size broadening occurs when individual
crystallites become so small that the complete
destructive interference of X-rays does not occur
at angles close to Bragg angle because of the
limited number of crystal planes. The crystallite
size of the synthesized HA is expressed by the
Scherrer’s formula as follows [21]:
x ¼ 0:9l=bs cos ys ,
(4)
where x is the crystallite size of the synthesized
HAP, l denotes the wavelength of CuKa
(l ¼ 1:5418 Å), bs is the full-width at half-maximum intensity (FWHM), and y is the Bragg’s
angle. By examining the peak width obtained from
the distinct families of crystal planes, the apparent
particle size in a particular direction can be
determined.
Fig. 5 shows the crystallite size of the deposits
on the Ti–6Al–4 V alloy by ECD at 60 1C, 10 V for
1 h and alkali treated with different NaOH(aq)
concentrations at various temperatures for 1 h.
The (0 0 2) peak is the most distinct reflection in
the XRD pattern and hence taken in calculation.
The solid marks are for the crystallite sizes when
treated at 75 1C, which show an increasing trend
with NaOH(aq) concentration from 20 to 24 nm.
However, as compared to the XRD result in Fig.
2, better crystallization is observed at 2.5 M but
nothing is noted in Fig. 5. The crystallite size of the
pre-HAP is calculated as about 17 nm, which is
smaller than the post-HAP and the difference is
clear in the SEM result.
Fig. 5. Crystallite sizes of deposits on Ti–6Al–4 V alloy by
ECD at 60 1C, 10 V for 1 h and alkali treated with different
NaOH(aq) concentrations at various temperatures for 1 h.
The open marks in Fig. 5 reveal the crystallite
sizes of the treated species where the crystallite size
is about 23 nm for 60 1C and decreases to about
21 nm for over 60 1C. Therefore, the effect of the
treatment temperature on the crystallite size
matches the XRD results in Fig. 3.
3.4. Effect of NaOH(aq) treatment on the
morphology of calcium phosphate deposit
Fig. 6(a) shows the SEM micrograph of the
deposits on the Ti–6Al–4 V substrate under 10 V in
the electrolyte at 60 1C without NaOH treatment.
Plate-like precipitates identified as DCPD are
observed in the specimen with some nano-sized
needle-like precipitates identified as pre-HAP in Fig.
6(b) [22]. Figs. 6(c)–(e) show the SEM micrographs
of the plate-like precipitate surface after treated with
NaOH(aq) concentrations 0.1, 2.5, and 10 M, respectively at 75 1C for 1 h. In Fig. 6(c), the post-HAP
aggregates are in random orientations and in various
sizes, indicating low recrystallization from original
DCPD base. According to Lazić’s kinetic curves at
different pH values [16], we see higher the concentration of the NaOH(aq) solution, shorter the time
taken for conversion.
After treated in 2.5 M NaOH(aq), the post-HAP
aggregates in Fig. 6(d) are in a well-aligned situation
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Fig. 6. SEM images of deposits on Ti–6Al–4 V alloy by ECD at
60 1C, 10 V for 1 h: (a) as-deposited and (b) zoomed image,
alkali treated with NaOH(aq) of (c) 0.1 M, (d) 2.5 M, (e) 10 M at
75 1C, and (f) 2.5 M at 30 1C for 1 h.
and composed of uniform sheath-like precipitates, as
reported in the previous research with commercial
DCPD conversion [11]. Hence, the difference in the
morphology of the pre- and post-HAP can be
observed in Figs. 6(b) and (d). However, the
treatment with higher concentration of 10 M inhibits
the crystallite alignment but leads to differences from
the under-treated one in Fig. 6(c). For the concen-
639
tration of NaOH solution more than 5 M, the
powders were pulverized and agglomerated owing to
the decreasing electrostatic repulsion of the HAP
particles in the solution with increasing ionic
strength [10].
Fig. 6(f) is the SEM image of sample treated
with 2.5 M NaOH(aq) at 30 1C, which is more close
to the under-treated situation in Fig. 6(c), which
matches the XRD results and indicates that low
treating temperature also reduces the conversion.
Fig. 7 is the TEM image of the deposits on the
Ti–6Al–4 V alloy by ECD at 60 1C, 10 V for 1 h, and
alkali treated with various concentrations of NaOH(aq) at 75 1C for 1 h. Fig. 7(a) shows that in the
under-treated situation the pre- and post-HAP are
stacked on a thin-film-like substance and look like
the original DCPD plates. Fig. 7(b) is the zoom
image of part of the aggregates, which are composed
of needle-like and rod-like crystallites. In Fig. 7(c),
the needle-like crystallite is the pre-HAP from ECD
identified as a single crystal oriented along (0 0 2).
Most crystallites with a rod-like shape originate from
DCPD as the post-HAP with lower symmetry by the
NaOH(aq) treatment. With higher treatment concentrations of NaOH(aq), the image for 10 M in Fig. 7(d)
shows the appearance of tiny aggregates of 5–20 nm
size and the decreasing number of needle-like and
rod-like crystallite. This situation occurs when both
pre- and post-HAP are broken into pieces by overhigh concentration environment. Compared to the
result in Fig. 4, the aggregate size does not match the
crystallite size calculated, which may be due to the
agglomeration or unbroken HAP crystallite. Since
no other calcium phosphates are found in the XRD
patterns and the FT–IR peaks, the pulverized
aggregates are still deemed to be the HAP phase.
4. Conclusion
The effects of NaOH(aq) treatment on the phase
transformation and morphology of calcium phosphate deposited by electrolytic method have been
studied. The results are summarized as follows:
(1) The calcium phosphate coating deposited on
the Ti–6Al–4 V alloy substrate by electrolytic
deposition in the 0.04 M Ca(H2PO4)2dH2O
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Fig. 7. TEM image of deposits on Ti–6Al–4 V alloy by ECD at 60 1C, 10 V for 1 h, and alkali treated with various concentrations of
NaOH(aq) at 75 1C for 1 h: (a), (b) 0.1 M, (c) 2.5 M, and (d) 10 M.
(MCPM) solution at 10 V, 60 1C, 1 h and
80 Torr shows the presence of both DCPD
and nano-sized pre-forming HAP. The DCPD
converts to the post-forming HAP phase after
NaOH(aq) treatment.
(2) All the coatings deposited by ECD and alkali
treated in various concentration of NaOH(aq)
at different temperatures for 1 h comprise the
only HAP phase as determined by XRD and
FT–IR.
(3) Both low treating temperature and low NaOH(aq) concentration favour the formation of
the post-HAP in an under-treated situation as
random precipitate sizes. Treatment in the
1–2.5 M NaOH(aq) solution at a temperature
higher than 60 1C offers a more proper
environment for the HAP formation with a
crystallite size of about 21 nm.
(4) For NaOH solutions with a concentration of
more than 5 M, both pre- and post-HAP are
noted to undergo a size reduction and form
aggregates of 5–20 nm size and agglomerate
owing to the electrostatic attraction of the
HAP particles.
Acknowledgements
The authors gratefully acknowledge the financial support by the National Science Council
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(NSC93-2216-E-151-005), Mr. H.Y. Yao for
TEM/EDS experiments, Mr. F.C. Wu for SEM
photography, and Prof. M. P. Hung and H.S. Liu
for discussion in manuscript preparation.
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