Download DIRECT GROWTH OF ORIENTED MG

DIRECT GROWTH OF ORIENTED MG−FE LAYERED DOUBLE HYDROXIDE
(LDH) ON PURE MG SUBSTRATES AND IN VITRO CORROSION AND CELL
ADHESION TESTING OF LDH-COATED MG SAMPLES
Jun-Kai Lina, Jun-Yen Uan a, Chia-Ping Wub and Her-Hsiung Huang c,d
Keyword: Layered double hydroxide, Pure Mg, Coating, Transmission electron microscope, In-vitro
test
Briefs: This work elucidates a novel method for directly forming a highly-oriented Mg−Fe−CO3 LDH
coating on pure Mg substrate by treating the pure Mg sample in pH 5.6 aqueous Fe3+/HCO3−/CO32− at
50 ºC, and then immersing it in pH 9.5 aqueous HCO3−/CO32− at 50 °C.
Abstract
This work presents a novel method for directly forming highly-oriented Mg−Fe−CO3 LDH coating on
pure Mg sample by treating the sample in a pH 5.6 aqueous Fe3+/HCO3−/CO32− at 50 ºC, and then
immersing it in a pH 9.5 aqueous HCO3−/CO32− at 50 °C. The former step was performed to yield Mg2+
in aqueous solution with pH 5.6 by corroding the Mg sample. A two-layered thin film was thus formed
on the Mg substrate, of which the outer layer (~1 μm-thick) comprised fine platelet-like Mg−Fe−CO3
LDH. The latter treatment in pH 9.5 aqueous HCO3−/CO32− at 50 °C resulted in the growth of the fine
LDH platelets into a strongly-oriented Mg−Fe−CO3 LDH. Chemical analysis data suggest that the
chemical formula of the Mg−Fe−CO3 LDH is Mg5.7Fe2(OH)15.4CO3·mH2O. The method used herein
involves a metal salt-free system, which requires no addition of Mg(NO3)2 and Fe(NO3)3. Several invitro tests of the Mg−Fe−CO3 LDH coating on Mg sample were performed. Based on the measured
contact angle between the sample surface and human whole blood, the Mg−Fe−CO3 LDH coating can
improve the hydrophilicity of a pure Mg surface. According to the results of an in-vitro corrosion test
in revised simulated body fluid (R−SBF), the Mg−Fe−CO3 LDH coated sample had a much higher
1
corrosion resistance than the pure Mg substrate. This finding is attributed to the protection of the Mg
substrate from corrosion by the LDH-coating.
1. Introduction
Layered double hydroxides (LDHs) have a general formula [ M12-+x M 3x+ (OH) 2 ]x+ [ A nx/n- ]⋅mH2O,1 where
M2+ and M3+ represent divalent and trivalent metal cations, respectively, at the octahedral positions.
Notably, An− is an anion (e.g., CO32−, SO42−, OH−)1 and x can take values from 0.2 to 0.33.1 LDHs have
been extensively studied because of their potential applications as ion exchangers, catalysts,
pharmaceuticals, ultraviolet (UV) stabilizers, adsorbents, and others.2
LDHs also exhibit good
biocompatibility and low toxicity.2, 3 Various approaches have been developed to synthesize LDHs
powder.
ammonia
They include co-precipitation,4−8 hydrothermal transformation,9 the sol-gel process10 and
release
reagents
methods11
(ARRs)
(such as based
urea hydrolysis11,
12
and
hexamethylenetetramine (HMT) methods11). Of these methods, co-precipitation is the most favorable
for preparing magnesium iron hydroxycarbonate (Mg−Fe–CO3 LDH) powder.4−8 As well as its use as a
catalyst,6, 7 Mg−Fe–CO3 LDH powder can be used to prepare Mg–Fe spinel ferrite nanoparticles.8 More
importantly, Mg−Fe–CO3 LDH is an effective phosphate binder, given the potentially important use of
LDH in treating hyperphosphatemia.3,
13
Although the feasibility of preparing an LDH film on a
substrate has attracted much interest in research on Mg–Al, Zn−Al, Ni−Al and Cu−Al LDH films,14−20
almost no literature is available on coating Mg−Fe–CO3 LDH onto a substrate (such as a pure
magnesium substrate). The authors’ earlier work21 established that a porous layer composed of nanosized Mg−Al LDH can be directly formed on Mg−9 wt.% Al−1 wt.% Zn alloy by dipping the alloy in
aqueous HCO3−/CO32− at 50 ºC.
By the chemical reaction of the Mg−Al−Zn alloy in aqueous
HCO3−/CO32−, the alloy provides Mg2+ and Al3+ ions to form the Mg−Al LDH film.21 However,
ordinary commercial-grade Mg and Mg alloys have an iron content of 0.01 to 0.03 wt%.22 Iron is one
of the more detrimental impurities in Mg and Mg alloys as it greatly reduces the corrosion resistance
2
when present in Mg matrix even small amounts.22 Accordingly, this work elucidates a new approach
for providing Fe3+ to aqueous HCO3−/CO32− to form a Mg−Fe LDH film on pure Mg substrate.
Metallic magnesium is a promising biodegradable implant material for possible applications in
vascular support structures,23−25 bone fixing devices,26, 27 and other applications. The elastic modulus of
human cortical bone is 7−30 GPa.28 The elastic modulus of pure Mg is 44.1 GPa,29 which is much
lower than those of the existing metallic biomaterials (such as Ti−6Al−4V (109−112 GPa)30) and much
closer than those to human cortical bone (7−30 GPa28). The elastic moduli of the existing metallic
biomaterials differs greatly from that of natural bone tissue, inducing stress shielding effects that can
result in reduced stimulation of new bone growth and low implant stability.31 Pure Mg is, then,
mechanically compatible with nature bone. Whereas several investigations have motivated the use of
magnesium and its alloys as possibly degradable orthopedic implants for load-bearing applications,
considerable research must yet be performed to evaluate the true potential of magnesium.32 According
to clinical studies,33,
34
magnesium implants corrode too rapidly, evolving hydrogen rigorously.
Niinomi,29 Staiger et al.32 and Song et al.35 suggested that, as a first step, the corrosion rate of
magnesium-based materials in the physiological environment must be modulated. Possible methods of
controlling the corrosion rate of magnesium by adding alloying elements36 and protective coatings are
available.37 Surface treatment is regarded as a shortcut to control the corrosion rate of a biodegradable
magnesium implant.35 An ideal coating for biodegradable magnesium alloys much exhibit corrosion
resistance in the early stages of healing, and be both nontoxic and biocompatible.38 Recently, several
biocompatible materials have been coated on pure Mg to protect it from corrosion. They include
hydroxyapatite coating,39 dicalcium phosphate dehydrate coating40 and apatite coating.41 Among these
coatings, hydroxyapatite is an acidic material and is a cationic exchanger.42
In contrast to
hydroxyapatite, the LDH compound is basic and has a high anion exchange ability. Additionally, LDHs
have good biocompatibility and low toxicity.2,
3
Because of LDHs have a high anionic exchange
capacity, several pharmaceutically active compounds with negative charge at physiological pH have
3
been intercalated in the LDH interlayers to prolong drug action.43 These include anticancer drugs,44
antibiotic drugs,45 anti-inflammatory drugs,46 anticoagulant drugs for cardiovascular diseases43 and
hypertension drugs.47 Additionally, LDH can potentially be utilized to suppress the toxicity of drugs by
slowly releasing them from the interlayer in a particular amount per unit time.48 Mg−Fe−CO3 LDH
powder also has been examined as an anionic drug delivery system.49, 50 However, little is known about
the formation of an Mg−Fe−CO3 LDH coating on pure magnesium substrate. This work presents a
novel approach for directly growing highly-oriented Mg−Fe−CO3 LDH on pure Mg.
Since the
biocompatibility of Mg−Fe−CO3 LDH compound was confirmed13 and LDH is promising in drug
delivery,49, 50 this work investigates the resistance to corrosion of the Mg−Fe−CO3 LDH coating on pure
Mg in revised simulated body fluid (R−SBF).51 Moreover, the surface wettability of LDH films is
important to their application in the field of biomaterials.52 Herein, the in-vitro test concerning the
analysis of the contact angle between a liquid (human whole blood) and the Mg−Fe−CO3 LDH coating
surface was carried out. The adherence of the Mg−Fe−CO3 LDH coating was evaluated using testing
procedures based on ASTM (American Standard for Testing and Materials) standard.
2. Experimental section
A magnesium ingot with 99.9 wt. % purity was used in this work. Square coupon samples were cut
from the ingot with dimensions 20 × 20 × 1.4 mm3. Each of the samples was ground using SiC paper
(2000 grit) and then cleaned ultrasonically in acetone. The samples were dried in air. To prepare an
aqueous solution that contained Fe3+ for the formation of Mg−Fe−CO3 LDH, 10 g Fe powder was added
to 1000 ml of deionized (DI) water, and then CO2 gas was bubbled through the water. The flow rate of
CO2 gas was 1 dm3/min. The DI water became carbonated (aqueous HCO3−/CO32−) with pH 4.3 after 20
min of bubbling.21, 53, 54 The CO2 gas that did not immediately dissolve in the water was recycled. The
recycled CO2 gas was then immediately recharged into the water. The total CO2-bubbling time was 2 h.
The pH of the solution was at ~4.3 throughout this time. CO2 gas that is removed from industrial
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emissions appears to be suitable for this purpose. The Fe powder corroded in the acidic aqueous
HCO3−/CO32−, increasing the concentration of Fe3+ in the solution. In addition to the Fe ions, there were
rust and the remained Fe powders in the solution. The solution was filtered through a filter paper with
pores of ~2 μm (quantitative ashless; Advantec Toyo) to remove the Fe powder and rust. During the
filtering procedure, CO2 was bubbled into the filtered solution to keep solution acidic. An ion-specific
meter model HI 93721 (Hanna Instruments, Ltd., Villafranca Padovana, Italy) was utilized to measure
the Fe3+ content of the solution. Ion-specific analysis reveals that the Fe3+ concentration of the filtered
solution was around 200 ppm. 500 ml filtered solution was then mixed with 500 ml DI water to prepare
an aqueous solution with ~100 ppm Fe3+. The aqueous Fe3+/HCO3−/CO32− was heated to 50 °C in a
water bath. Six square coupons of pure Mg were then immersed in the aqueous Fe3+/HCO3−/CO32− at 50
°C for 45 min as CO2 gas was continuously bubbled through the solution. Herein, this treatment is
called CO2−45min treatment. A new aqueous HCO3−/CO32− solution without Fe3+ was then prepared at
room temperature by bubbling CO2 gas through 1000 ml of deionized water until the pH reached the
desired value (~pH 4.3). 1.25 M aqueous NaOH was then added dropwise into the aqueous HCO3−
/CO32− to increase its pH from 4.3 to 9.5. Some of the samples that had undergone the CO2−45min
treatment were then dipped in the pH 9.5 aqueous HCO3−/CO32− for a specified period at 50 ºC. This
treatment is denoted, for example, CO2−45min/pH9.5−20h. This notation means that the Mg sample
underwent CO2−45min treatment first, and was then dipped into pH 9.5 aqueous HCO3−/CO32− at 50 °C
for 20 h.
The crystallographic structure of the sample following the above treatments was then determined by
glancing angle X-ray diffraction (GAXRD) at a glancing angle of 1° using Cu Kα1 (1.5406 Å) radiation.
Attenuated total reflection Fourier transform infrared (ATR-FTIR) analysis was conducted on a Perkin
Elmer Spectrum RX−I spectrometer, directly yielding the IR information of the coating on the sample
surface. The spectrometer was set to perform 16 scans at a 2 cm−1 resolution from 4000 to 650 cm−1.
5
The surface microstructures of the CO2−45min sample and the CO2−45min/pH9.5−20h sample were
examined using a field-emission scanning electron microscope (SEM, JEOL JSM−6700F).
Transmission electron microscope (TEM) samples were prepared for cross-sectional observation using a
focused ion beam (FIB, FEI NOVA−600). A Pt film was deposited on the surface of the area of interest
on the sample to protect the area from damage by ions. Extraneous material was removed by the ion
beam from both sides of the region of interest until a thin specimen (~80 nm) was obtained. An FEI
Tecnai F20 TEM was utilized to determine the microstructure of the CO2−45min and
CO2−45min/pH9.5−20h samples, using an applied voltage of 200 kV.
Electron spectroscopy for
chemical analysis (ESCA, ULVAC-PHI PHI 5000) was conducted to investigate the Mg and Fe
contents and the valence of Fe ions in the Mg−Fe−CO3 LDH coating. In the ESCA experiment, Al Kα
radiation with 1.4866 eV was used as the X-ray source. The ESCA measurements were made on the
coating surface after the coating was sputtered for 15 s for cleaning. The analyzed area was 1 mm2.
During analysis, the pressure in the analysis chamber was about 10−9 Torr.
The adherence of the Mg−Fe−CO3 LDH coating was evaluated using a cross-cut tape test, consistent
with the ASTM 3359 standard test method.55 Several studies56−58 have used this method to evaluate the
adherence of their coatings to substrates. A cross-cut tester comprises a set of 11 blades with a spacing
of 1 mm between each pair of adjacent blades. Two sets of 11 cuts were made perpendicular to each
other using the cross-cut tester, forming a lattice of 100 small blocks on a coated sample. Adhesive tape
was then applied to the cross-cut area and pulled back as close to an angle of 180º as possible.55 Crosscut adhesion was evaluated according to the ASTM standard.55 The evaluations were made on a scale
of 5B to 0B.55 Adhesion was excellent at 5B, at which no coating was removed from the sample
surface.55 When the affected area exceeded 65%, the quality of adhesion was 0B.55 The wettability
characteristics of the CO2−45min/pH9.5−20h sample and pure Mg specimen were determined by
measuring the static contact angle between the surfaces and whole blood at 37 °C in a contact-angle
meter (FTA2000, First Ten Angstro).
A high-resolution CCD camera that was equipped with a
6
magnifying zoom lens was used to observe the spreading of the blood drop on sample surface. Blood
was taken from the author’s vein through the needle into the syringe. This blood was immediately
collected using a vacuum blood collection tube (Becton Dickinson Vacutainer Systems, Plymouth, UK)
with an anticoagulant (K2 EDTA) to prevent clotting. The blood sample was kept at 37 °C in a water
bath. Each measurement was made at least three times to verify the contact angle.
Electrochemical polarization tests were performed in a corrosion cell that contained 330 ml of R−SBF
at 37 ºC. The scan rate was 0.5 mVs−1. The R−SBF was prepared by dissolving NaCl (5.403 g),
NaHCO3 (0.736 g), Na2CO3 (2.036g), KCl (0.225 g), K2HPO4 (0.182 g), MgCl2·6H2O (0.310 g),
HEPES (2−(4−(2−hydroxyethyl)−1−piperazinyl), ethane sulfonic acid) (11.928 g), CaCl2 (0.293 g) and
Na2SO4 (0.072 g) in that order. It was buffered to pH 7.4 at 37 ºC by adding HEPES and NaOH. All
electrochemical measurements were made using a Princeton Applied Research model 263A
potentiostat/galvanostat with M352 software. The reference electrode was a silver/silver chloride
(Ag/AgCl) electrode and the counter electrode was a platinum flake. The working electrode was the
tested sample. The area of the surface of the working electrode that was exposed to the solution was 1
cm2. Tafel’s extrapolation method59, 60 was applied to determine the corrosion current density (Icorr).
Cathodic polarization data are preferred in this method because of they are easily measured.59, 60 At
least five of each sample were prepared to verify the electrochemical data.
Magnesium typically reacts with an aqueous solution with pH ≤ 11.5, as follows (1).61
Mg + 2H 2 O → Mg 2 + + 2OH − + H 2
(1)
In reaction (1), the evolution of one mole of hydrogen gas corresponds to the dissolution of one mole of
magnesium metal, and increases the pH of the solution.
Measurement of hydrogen evolution is
regarded as a useful and practical method that complements electrochemical experiments.62 Hence, the
corrosion rate of the sample in the R−SBF solution at 37 ºC was also determined by measuring the
volume of the evolved hydrogen from the corroding sample. A hydrogen gas collection system that was
the same as that used elsewhere,63 was utilized herein. Two samples (each with exposed area 3 cm3)
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were immersed in a beaker that contained 800 ml R−SBF. Hydrogen bubbles from the two samples
were collected in a burette. Each measurement was repeated at least four times to confirm the volume
of evolved hydrogen.
3. Results and discussion
3.1 Formation and characterization of Mg−Fe−CO3 LDH film
Figure 1 presents the GAXRD patterns of the CO2−45min sample, and of the samples after
CO2−45min/pH9.5−(3h, 6h, 9h and 20h) treatment. A weak X-ray peak at 2θ = 11.3º was observed in
the pattern of the CO2−45min sample (see the first X-ray diffractogram at the bottom of Figure 1). The
GAXRD pattern of the CO2−45min/pH9.5−3h sample included a weak peak at 11.3º. As the duration of
CO2−45min/pH9.5 treatment was increased to 6 h, two peaks at 2θ = 11.3° and 22.7° were observed.
As plotted in Figure 1, the intensity of the peaks at 2θ = 11.3º and 22.7º increased with the
CO2−45min/pH9.5 treatment time. For instance, when the duration of CO2−45min/pH9.5 treatment
increased to at least 9 h, the GAXRD patterns included intense peaks of Mg−Fe−CO3 LDH
(Mg6Fe2(OH)16CO3·mH2O) (JCPDS X−ray diffraction file No. 70−2150). The GAXRD results show
that the alkaline-based solution favored the formation of the crystalline Mg−Fe−CO3 LDH on pure Mg
A: Mg−Fe−CO3 LDH
B: Mg
A
Intensity (a.u.)
CO2-45min/pH9.5-20h
C: Fe
AB
B
A
B
A
B
C B
CO2-45min/pH9.5-9h
CO2-45min/pH9.5-6h
8
CO2-45min/pH9.5-3h
substrate. The authors’ previous works21, 54 developed a method for directly forming an Mg−Al LDH
film on a flat Mg−Al−Zn alloy sample. The alloy sample was immersed in aqueous HCO3−/CO32− with
an initial pH of around 4.3 at 50 ºC. The corrosion of the sample surface in the carbonic acid not only
increased the concentration of metal ions (including Al3+ and Mg2+) close to the surface of the sample
but also increased the pH of the solution. An alkaline-based environment is conducive to the formation
with crystalline LDH on Mg alloy substrate.21,
54
In this investigation, an aqueous solution with
trivalent ions (Fe3+) and anions (CO32−) is prepared to form LDH. The divalent ion, Mg, is the main
metallic ion to occupy the octahedral positions of the lattice of the layered hydroxide.1 The pure Mg
substrate surface was the only source of Mg2+, and is responsible for the direct growth of Mg−Fe−CO3
LDH on the Mg substrate. Notably, as presented in Figure 1, the GAXRD patterns include with weak
X-ray peaks of iron, suggesting that Fe3+ was reduced to Fe in this case. Although the mechanism of
formation of the Fe particles is beyond the scope of this work, microstructural characterization of the Fe
particles in the Mg−Fe−CO3 LDH coating will be discussed later.
Figure 2 presents the ATR-FTIR spectrum of the CO2−45min/pH9.5−20h sample. A broad absorption
band at around 3480 cm−1 (Figure 2) corresponds to the O−H stretching vibration of the hydroxyl
groups of the LDH.64 The absorption at ~3080 cm−1 has been attributed to the hydrogen-bonding of the
water molecules to carbonate ions in the interlayer.65 The band at ~1650 cm−1 (Figure 2) is ascribed to
9
the bending motion of interlayer water.13 The absorption band at ~1365 cm−1 (Figure 2) reflect the
asymmetric stretching of carbonate molecules in the interlayer.66 Based on the above analysis, the
spectrum reveals the characteristic bands of Mg−Fe−CO3 LDH. Moreover, the ATR-FTIR results
confirm that the interlayer anion of the LDH is a carbonate ion. The results of quantitative ESCA
analysis (Table 1) reveal that the Mg−Fe−CO3 LDH on the CO2−45min/pH9.5−20h sample contained
10
25.2 at% Mg and 8.7 at% Fe. The valence of the Fe ions in the LDH is trivalent. Hydrotalcite-like
compounds have a general formula [ M12−+x M 3x+ (OH) 2 ]x+[ A nx/n− ]⋅mH2O.1 The value of x is equal to M3+ /
(M2+ + M3+).1
Based on the data in Table 1, the x value of the Mg−Fe−CO3 LDH was 0.26.
Consequently, the Mg−Fe−CO3 LDH coating formed on the CO2−45min/pH9.5−20h sample was
Mg5.7Fe2(OH)15.4CO3·mH2O.
Table
1.
Mg2+
and
Fe3+
contents
of
coating
on
the
CO2−45min/pH9.5−20h sample was evaluated by ESCA analysis.
Composition in LDH (at.%)
Mg−Fe−CO3 LDH coating
Mg2+
Fe3+
Mg2+/Fe3+
25.16
8.74
2.879
Figures 3a and b present the SEM micrographs of the samples after CO2−45min treatment and
CO2−45min/pH9.5−20h treatment, respectively. As shown in Figure 3a, network-like cracks are present
on the surface of the CO2−45min sample (white arrows). Nano-sized platelet-like compounds are
observed on the surface (inset in Figure 3a).
Figure 3b shows the surface morphology of the
CO2−45min/pH9.5−20h sample. The inset in Figure 3b presents large Mg−Fe−CO3 LDH platelets on
the CO2−45min/pH9.5−20h sample. Mg−Fe−CO3 LDH platelets on the CO2−45min sample tended to
grow to a relatively large size when the sample was immersed in pH 9.5 aqueous HCO3−/CO32− at 50 °C.
Moreover, the CO2−45min/pH9.5−20h sample exhibits no surface cracks on its LDH coating, as
presented in Figure 3b. Figure 4 displays the TEM micrographs of cross-sections of the CO2−45min
sample. In Figure 4a, an outer layer and an inner layer of the coating, and the substrate, can be
identified. The Pt film on top of the coating is to protect the coating surface from ionic damage during
FIB thinning. As shown in Figure 4a, the outer layer was a thin layer that was 0.5 − 0.8 μm thick. The
11
(a)
2 μm
20 μm
(b)
2 μm
20 μm
Figure 3. SEM surface morphologies of (a) CO2−45min sample; (b)
CO2−45min/pH9.5−20h sample
thickness of the inner layer was around 1.0 − 1.2 μm. Interior cracks, as shown in Figure 4a, were
sometimes found in the inner layer. As presented in Figure 4a, the outer layer contained particles.
Figure 4b enlarges the rectangular region in Figure 4a. The selected-area diffraction pattern on the
right-hand side of Figure 4b is that of the particle. The diffraction pattern of the particle was indexed
using JCPDS file No. 6−696, suggesting that the particle was an α-iron particle. The figure reveals that
the α-iron particle was not in contact with the pure Mg substrate. Nano-sized platelets covered the
CO2−45min sample, as shown previously in Figure 3a. The cross section of the platelets was as shown
in the outer layer of Figure 4a. The diffraction ring pattern on the upper-left-hand side of Figure 4b was
from the outer layer. The pattern of the outer layer was indexed (JCPDS file No. 74−1513), suggesting
the crystalline structure of Mg−Fe−CO3 LDH. The bottom left-hand side of Figure 4b presents the
12
(a)
Pt film
outer layer
Mg−Fe−CO3 LDH
inner layer
Mg substrate
Mg substrate
2 μm
(b)
Mg−Fe−CO3 LDH
Pt film
001
Fe
(1 0 1 3)
110
(1 0 1 0)
(1 0 1 5)
(1 0 1 3)
200
110
020
020
110
110
5 1/nm
200
5 1/nm
Mg substrate
5 1/nm
0.5 μm
Figure 4. (a) TEM image of the cross−section microstructure of CO2−45min sample; (b)
image of high magnification regarding the rectangular region in (a) and diffraction patterns
of different positions.
diffraction pattern of the inner layer. The poor contrast of diffraction rings suggests that the inner layer,
as shown in Figure 4, has low crystallinity. Figure 5 shows the cross-sectional TEM micrographs of the
CO2−45min/pH9.5−20h sample. Figure 5a presents a three-layered structure (excluding the Pt film) that
comprises, from top to bottom, an outer layer of oriented platelet-like compounds, a middle layer, and
an inner thick layer on the Mg substrate. According to the TEM examination, the thickness of the outer
layer was around 1.5 μm and that of the middle layer was0.5 − 1.0 μm. The range of thickness of the
inner layer was 2.0 − 3.0 μm. Figure 5b enlarges the rectangular region in Figure 5a. The diffraction
13
(a)
Pt film
outer layer
Mg−Fe−CO3 LDH
middle layer
inner layer
Mg−Fe−CO3 LDH layer
2 μm
1213
Mg−Fe−CO LDH
Pt (220)
Pt (111)
0111
Mg substrate
111
(b)
3
121
211
1010
1121
Fe
101
1121
0111
121
121
1101
110
011
2111
112
101
211
5 1/nm
5 1/nm
Mg−Fe−CO3 LDH layer
Mg−Fe−CO3 LDH
(0 0 0 14)
(2 0 2 6)
(1 1 2 2)
(1 0 1 4)
(1 0 1 3)
(1 1 2 2)
(2 0 2 0)
011
110
1010
0.5 μm
Mg−Fe−CO3 LDH layer
(1 0 1 4)
5 1/nm
5 1/nm
Figure 5. (a) TEM image of the cross−section microstructure of CO2−45min/pH9.5−20h
sample; (b) image of high magnification regarding the rectangular region in (a) and
diffraction patterns of different positions on the Mg−Fe−CO3 LDH coating. The parallel
streaks in coating and substrate were due to FIB thinning process.
pattern at the upper left-hand side of Figure 5b is that of the oriented platelet-like compound in the outer
layer. The pattern of the oriented platelet-like compound was indexed using JCPDS file No. 74−1513.
It suggests that the oriented platelet-like compound in the outer layer was crystalline Mg−Fe−CO3 LDH.
The lower-left diffraction pattern in Figure 5b is that of the middle layer. The indexing of the pattern
suggests that the fine platelets were crystalline Mg−Fe−CO3 LDH. According to Figure 5b, each unit of
platelet-like Mg−Fe−CO3 LDH in the upper layer was rooted in the middle layer. Figure 5b presents
14
two particles in the middle layer. The diffraction pattern of the particle (upper-right pattern in Figure 5b)
was indexed (JCPDS file No. 6−696), suggesting the crystal structure of α-iron. As shown in that figure,
the α-iron particles were not in contact with the pure Mg substrate. Additionally, TEM examinations
(Figure 5) at the interface between each layer and the interface right immediately above the Mg
substrate verified their satisfactory adherence to each other. According to the TEM examination results
in Figures. 4 and 5, when the CO2−45min sample (Figure 4) was immersed in pH 9.5 aqueous
HCO3−/CO32− at 50 °C, the fine platelets of Mg−Fe−CO3 LDH on the sample grew upward from the
surface into a relatively large size of LDH platelets, forming the outer layer as shown for the
CO2−45min/pH9.5 −20h sample (Figure 5).
According to Hansen and Taylor,67 Mg−Fe−CO3 LDH compounds have two polytypes, hexagonal
and rhombohedral, which differ only in the stacking of the brucite-like sheets.67
The hexagonal
polytype is usually a high-temperature form, whereas the rhombohedral polytype is a low temperature
form.67 The GAXRD patterns of Mg−Fe−CO3 LDH (Figure 1) were indexed using JCPDS file No.
70−2150 (rhombohedral), while TEM diffraction patterns of Mg−Fe−CO3 LDH (Figure 4 and Figure 5)
were indexed using JCPDS file No. 74−1513 (hexagonal). Since the GAXRD analysis was performed
at room temperature, the GAXRDs patterns of Mg−Fe−CO3 LDH could be indexed for the
rhombohedral structure using JCPDS file No. 70−2150 but could not be indexed using the JCPDS file
No. 74−1513. Previous studies68−70 have indicated that the actual sample temperature may rise under
electron beam heating when the sample is observed using TEM. Therefore, the electron beam may
have heated Mg−Fe−CO3 LDH during TEM analysis. Accordingly, the TEM diffraction patterns of
Mg−Fe−CO3 LDH were indexed as a hexagonal structure using JCPDS file No. 74−1513.
Figure 6a displays the surface of the CO2−45min/pH9.5−20h sample after a cross-cut tape test based
on ASTM 3359.55 First, the sample surface was cut, and then an adhesive tape was stuck on the crosscut area. The tape was pulled back at an angle close to 180º. As shown in Figure 6a, none of the
15
squares of the lattice was detached. According to ASTM D3359,55 the adhesion quality consistent with
the test result in Figure 6a is 5B. The 5B ranking suggests that the Mg−Fe−CO3 LDH coating adhered
excellently to the pure Mg substrate.
Figure 6b displays an SEM micrograph of the
CO2−45min/pH9.5−20h sample after the cross-cut tape test. As shown in Figure 6b, the platelet-like
Mg−Fe−CO3 LDHs remained after the cross-cut tape test. Furthermore, residual adhesive was present
on the sample surface following the cross-cut test.
(a)
3 mm
(b)
residual
adhesive
1 μm
Figure 6. (a) Optical surface observation of Mg−Fe−CO3 LDH coating on the
CO2−45min/pH9.5−20h sample after cross−cut tape test; (b) SEM micrograph of
the Mg−Fe−CO3 LDH coated sample after the cross−cut tape test.
16
3.2 In-vitro tests
The wettability characteristics of the test sample surface were examined by making contact angle
measurements. A whole blood droplet was dropped on sample surface. The contact angles between the
surface of the test sample and the whole blood film were then measured. Figure 7a plots the contact
angle of whole blood on the surface of the CO2−45min/pH9.5−20h sample against contact time. For
comparison, the change in the contact angle of the blood on the pure Mg substrate as a function of
contact time is also shown. The blood contact angle on the CO2−45min/pH9.5−20h sample declined
sharply to 18.4º, and then more slowly to 11.0º in 0.5 s. Thereafter, as the contact time increased to 2 s,
the contact angle was slightly reduced to 8.4º. The figure also reveals that the blood contact angle on
the pure Mg substrate declined to ~83.9° at 0.25 s and then decreased slightly further to ~72.4° as the
contact time increased from 0.25 to 2 s. Figures 7b and c present the optical micrographs of the whole
blood droplets on different sample surfaces after a contact time of 2 s. Figure 7b illustrates the blood
droplet flattened out on the surface of the CO2−45min/pH9.5−20h sample. Figure 7c presents the shape
of the blood droplet on the pure Mg substrate surface. The results show that the Mg−Fe−CO3 LDH
coating exhibited a high hydrophilicity with the human whole blood. According to Yang et al.,52 the
water contact angle of as-prepared Zn−Al LDH film was measured to be 122º, indicating a hydrophobic
surface, but after calcination at high temperature (600 ºC for 2 h), the water contact angle became 65º,
suggesting a hydrophilic surface. Herein, the as-prepared Mg−Fe−CO3 LDH film can exhibit excellent
hydrophilicity. Based on earlier studies,71, 72 a hydrophilic surface has a greater affinity for cells than
does a hydrophobic surface. Electrochemical polarization tests were performed in R−SBF at 37 ºC to
determine the electrochemical properties of the CO2−45min sample, the CO2−45min/pH9.5−20h sample
and pure Mg substrate. Figure 8 plots the polarization curves of the three samples. For each case in
Figure 8, at least five experiments were conducted to confirm the data. The corrosion potential (Ecorr) of
the
CO2−45min/pH9.5−20h
sample
was
around
−1.521
VAg/AgCl;
that
of
17
(a)
: Pure Mg sample
: CO2−45min/pH9.5−20h sample
Contact angle (degree)
90
80
70
20
10
0
0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0 2.2
Contact time (s)
(b)
whole blood
Mg−Fe−CO3 LDH on pure Mg sample
1mm
(c)
whole blood
Pure Mg sample
1mm
Figure 7. (a) Blood contact angle on the pure Mg sample and on the
CO2−45min/pH9.5−20h sample as a function of contact time; optical micrographs of the
shape of whole blood droplet on the surface of (b) Mg−Fe−CO3 LDH coated sample and
(c) pure Mg sample after contact time of 2 s.
18
the CO2−45min sample was about −1.580 VAg/AgCl, and that of the pure Mg substrate was approximately
−1.920 VAg/AgCl.
The Mg substrate had a corrosion current density (Icorr) ~423 μA/cm2 and the
CO2−45min sample had Icorr ~400 μA/cm2. The Icorr of the CO2−45min/pH9.5−20h sample was only
~14 μA/cm2, indicating that the CO2−45min/pH9.5−20h sample had a relatively low corrosion rate in
R−SBF. The electrochemical polarization test is a short-term test (30 min per test cycle). To verify the
corrosion performance of the CO2−45min/pH9.5−20h sample in R−SBF at 37 ºC, long-term tests were
also conducted. Figure 9 plots the hydrogen evolution volume against immersion time in R−SBF at 37
ºC. For the CO2−45min/pH9.5−20h sample, the accumulated volume of hydrogen at 300 min was ~0.04
Potential (V) vs. AgCl
-1.0
-1.2
CO2-45min/pH9.5-20h
CO2-45min
Pure Mg sample
-1.4
-1.6
-1.8
-2.0
-2.2
-2.4
1E-7
1E-6
1E-5
1E-4
1E-3
0.01
−2
0.1
Current density (A·cm )
Figure 8. Polarization curves of the pure Mg sample, the CO2−45min sample and
CO2−45min/pH9.5−20h sample.
The electrochemical tests were performed in
R−SBF at 37 ºC
ml/cm2. Thereafter, the hydrogen volume increased to ~0.17 ml/cm2 at an immersion time of 1440 min,
and then to ~0.30 ml/cm2 at 2880 min. Contrarily, the pure Mg sample vigorously reacted with the
R−SBF solution at 37 °C. Figure 9 reveals that the pure Mg sample produced 0.2 ml/cm2 hydrogen in
10 min, rapidly increasing to 1.3 ml/cm2 in 300 min. The results of the immersion corrosion test were
19
consistent with the electrochemical results (short-term test, Figure 8), verifying that an Mg−Fe−CO3
LDH coating improves in-vitro corrosion performance than Mg metal in R−SBF at 37 ºC.
The CO2-45min/pH 9.5 treatment caused the growth of the nano-sized Mg-Fe-CO3 LDH (Figure 4) to
the micro-sized LDH platelets (Figure 5). That is, this method could control the formation of the LDH
coating on pure Mg sample (Figure 1). Corrosion test results (Figures 8 and 9) suggest that the LDH
coating on pure Mg can possibly modulate the corrosion rate (i.e., degrading rate) of the base metal to a
Volume of evolved hydrogen (ml·cm−2)
relatively low level in bio-environment.
: Pure Mg sample
: CO2−45min/pH9.5−20h sample
1.8
1.6
1.4
1.2
1.0
0.8
0.4
0.2
0.0
0
500
1000
1500
2000
2500
3000
Immersion time (min)
Figure 9. Hydrogen evolution volumes of Mg sample and the
CO2−45min/pH9.5−20h sample as a function of the immersion time in
R−SBF at 37 ºC
4. Conclusions
This work presents a novel approach for directly growing a highly-oriented Mg−Fe−CO3 LDH
coating on a pure Mg metal. In the method, pure Mg metal is firstly be treated in acidic aqueous
Fe3+/HCO3−/CO32− solution at 50 °C; it is then dipped in an alkaline HCO3−/ CO32− bath at 50 °C.
Chemical analysis verified that the formula of the Mg−Fe−CO3 LDH on pure Mg substrate was
20
Mg5.7Fe2(OH)15.4CO3·mH2O. In-vitro test results for the corrosion resistance of the Mg−Fe−CO3 LDH
coating in revised simulated body fluid reveal that the LDH coating markedly protects the pure Mg
substrate against corrosion. On the other hand, the proposed approach may be extended to a wide range
of Mg−M−CO3 LDH ( Mg12-+x M 3x+ (OH) 2 (CO 32- ) x/2 ⋅ mH 2 O ) that will be formed on a pure Mg surface by
dipping the pure Mg sample in acidic aqueous M3+/HCO3−/CO32−, and then in an alkaline HCO3−/ CO32−
bath.
Acknowledgements
Founding for this study came in part from the Ministry of Education grant (Republic of China,
Taiwan) under the ATU plan. This work was also financially supported by the National Science
Council of Taiwan (Contract No. NSC 98−2221−E−005−028). The authors are grateful for their
support.
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