MgO Coating Film Prepared by Electrochemical Synthesis and
Thermal Condensation
Ching-Fei Li1, and Wen-Hsien Ho2, and Shiow-Kang Yen1*
1. Department of Materials Engineering, National Chung Hsing University, 250, kuo-kuang Road,
Taichung 40227, Taiwan, ROC E-mail: [email protected]
2. Taiwan Textile Research Institute, Taipei 23674, Taiwan
Abstract
The magnesium hydroxide (Mg(OH)2) coating have been successfully
prepared via electrolytic deposition on Pt in aqueous Mg(NO3)2 solution.
The as-coated Mg(OH)2 films were condensed into MgO films after subsequent annealing .
Two major reactions which belong to various ap-
plied voltage ranges were identified: (i) O2 + 2H2O + 4e− → 4OH− (~0.5
to −0.77 V), (ii) 2H2O + 2e− → H2 + 2OH− (−0.77 to -3 V).
The efficient
deposition was carried out at the second stage, because H+ ions were depleted and many OH− ions were provided to form Mg(OH)2 on the cathodic surface.
Herein, we report the controllable fabrication of
Mg(OH)2 films by electrolytic deposition. The coated samples obtained
were further annealed and characterized by thermogravimetric differential
thermal analysis (TG-DTA), X-ray diffraction (XRD), Fourier transform
infrared (FTIR) spectroscopy, and scanning electron microscopy (SEM).
The results indicated that the coating film was highly uniform and
densely packed lamellar-like Mg(OH)2, and condensed into MgO at
350℃.
The reported method is low cost, simple and environmentally
benign.
Compared with micro-arc oxidation (MAO) which only re-
stricted on Mg alloy, this electrolytic method can be applied on other
metals.
Keywords: Electrolytic deposition, MgO film, TG-DTA, Condensed.
1
Introduction
Magnesium oxide (MgO, magnesia) is the most important product of
the magnesium compound industry.
MgO has a number of interesting
properties such as: high electrical resistivity, high transparency, good
chemical resistance, good thermal stability and high secondary electron
emission coefficient.
Therefore, MgO thin film has been widely used as
a protective layer of AC-PDP (Alternating Current Plasma Display Panels).
On other hand, the MgO films have also been applied as a protect-
ing layer for magnesium alloy to improve corrosion properties [1].
Re-
cently, there have been a number of studies on preparation of MgO film
deposition by various methods, for example, unbalanced magnetron sputtering [2,3], electrostatic spray deposition [4], ion beam sputtering [5,6],
plasma process [7], sol-gel method [8,9], and micro-arc oxidation (MAO)
[10-13].
Many ceramic films and powders were prepared by electrolytic
deposition method, such as ZnO[14], CeO2[15], ZrO2[16], Al2O3[17],
MnO2[18]. However, MgO thin films prepared by electrolytic deposition have not been reported yet. In order to evaluate such promising
candidate, the electrolytic deposition appears to be a convenient route for
elaborating oxide coating.
Electrolytic films have a high purity and the
2
thickness is easily controlled. Deposition is performed at room temperature and atmospheric pressure, with a low cost compared to conventional vacuum deposition. In this report, MgO thin film was prepared by
cathodic deposition on Pt in Mg(NO3)2 aqueous solution. Herein, we
report the controllable fabrication of highly uniform and densely packed
Mg(OH)2 by deposition potential.
The process is based on heterogene-
ous nucleation and subsequent growth of specific nanostructure on the Pt
substrate.
Cathodic reactions, surface morphology, and phase transfor-
mation of the magnesium oxide-coated films were also characterized by
polarization tests, scanning electron microscopy (SEM), thermogravimetric-differential thermal analysis (TG/DTA), Fourier transform infrared
(FTIR) spectroscopy, and X-ray diffraction (XRD).
3
Experimental
Cathodic polarization tests and deposition.
To investigate the effect of solution concentrations on cathodic reaction,
the platinum disks were electrochemically polarized in aerated 0.1, 0.01,
and 0.001 M Mg(NO3)2．6H2O aqueous solutions assigned as solutions A,
B, and C, respectively, by EG＆G Princeton Applied Research 263A Potentiostat M352 software.
To investigate the effect O2 concentration on
the cathodic reaction, the polarization test was also conducted in 0.01 M
deaerated Mg(NO3)2．6H2O solutions by N2 purging, which was assigned
as solution D.
The potential was swept from the equilibrium potential of
platinum to final potential of -3.0 V (saturated Ag/AgCl), at a scanning
rate of 0.167 mV/s.
The details of the electrochemical solutions, in-
cluding solution concentrations, pH values, O2 concentrations, and solution resistivity (ρ) are also given in Table Ι.
To further characterize the coated specimens, the cathodic deposition
of Mg(OH)2 films on platinum was carried out in solution B (aerated 0.01
M Mg(NO3)2．6H2O aqueous solutions) for 1800s, at -1.2 V(Ag/AgCl)
and at room temperature, because it revealed the better deposition efficiency.
4
Thermogravimetric analysis (TGA) and differential thermal analysis
(DTA)
In order to investigate the dehydration and condensation, the thermalgravimetric analysis (TGA) was carried out in air between room temperature and 800℃ at a heating rate 10℃/min.
Prior to tests, the pow-
ders were dried in furnace at 50℃ for 1 day and then the tests were carried out in air at a heating rate of 1℃/min, using a STA-PT1600 TG-DTA
analyzer with the simultaneous recoding of weight variations.
Fourier transformation infrared (FTIR) analysis
Chemical bonding information on metal-oxygen and hydroxyl was
conducted on a FT-IR spectrometer. The samples were collected from
cathodic deposition, and dried in furnace at 50℃ for 1 day.
The sample
was mixed with an appropriate amount of anhydrous KBr powder.
The
IR spectra were recorded on a spectrometer (Perkin Elmer Spectrum RX-I)
in the range of 400-4000 cm-1.
Surface morphology analysis and XRD
5
The surface morphology of the deposited samples was observed by
scanning electron microscopy (SEM, JEOL model JSM-6700F). The
crystal structure of Mg(OH)2 coated on Pt substrate and then annealed at
100, 300, 350, 400 and 800℃ for 1h were analyzed by grazing angle
X-ray diffraction technique in a MAC MO3X-HF diffractometer, with Cu
Kα radiation (λ=1.5418 Å), 2θ in the range 10~70°, at a scanning rate of
1° min-1, a voltage of 40 kV, and a current of 30 mA.
6
Results and Discussion
Cathodic reactions
The polarization curves were divided into two regions as shown in Fig.
1.
The diffusion-limited current density corresponding to region I for
aerated 0.1, 0.01, 0.001 M, and deaerated 0.01 M represented by A, B, C,
and D, respectively, are given in Table I.
In Mg(NO3)2 aqueous solution,
several possible cathodic reactions are suggested as follows
-
-
O2 + 2H2O + 4e → 4OH
+
-
2H + 2e → H2
-
-
-
-
NO3 + H2O + 2e → NO2 + 2OH
-
-
2H2O + 2e → H2 + 2OH
E0 = 0.204 V vs. Ag/AgCl
(1)
E0 = -0.197 V vs. Ag/AgCl
(2)
E0 = -0.187 V vs. Ag/AgCl
(3)
E0 = -1.025 V vs. Ag/AgCl
(4)
Region I (~0.5 to -0.75V).  The similar limiting current densities
were found in region Ι for solutions A, B, and C., as listed in Table I.
The possible reactions are (1), (2), and (3) due to the different concentra-
tion of NO3 and/or pH in solution A, B, and C.
However, reactions (2)
could be neglected because of the similar limiting current densities and
different pH values in this region. According to Yen investigated [], the
NO3- was affect of cathodic polarization curve by NaNO3 and pure H2O
solution.
Two curves can be divided into the same three stages, but no
apparent evidence shows the nitrate reduction. It is mean that the NO3was not participated in the reaction of magnesium nitrate solutions.
Thus, nitrate reduction (reaction 3) in this study can be excluded.
7
On
other hand, the similar concentration is only for O2.
(1) is suggested in this region.
Therefore, reaction
This argument is further confirmed by
the much lower current density for solution D, which was deaerated by
N2 purging.
Region ΙΙ (-0.77 to -3 V). 
separated in region II.
Three polarization curves were clearly
Three different current densities in region II were
found for solution A, B, and C.
Much more hydrogen bubbles were
found in this region. This drastic one is reaction (4), and current density
is inversely proportional to solution conductivity (κ), as listed in Table I.
The current density should depend on the concentration of dissolved
magnesium nitrate due to the different electrical resistance values in solution A, B, and C.
A>B>C.
The order of solution conductivity is concentration
The cathodic polarization curve in MgCl2 solution of different
concentration is shown in Fig. 2.
Therefore, the electrical resistance
was major effect the current density depend on the concentration of dissolved magnesium nitrate.
The most efficient deposition was also found
in this region, because much more OH- ions were supplied and more
Mg2+ ions migrated to the cathode to form Mg(OH)2, by the following
reaction
Mg2+ + 2OH- → Mg(OH)2
(5)
8
Figure 2 shows the variation of current density with time at constant
controlled potential experiments, applied voltage -1.2 V.
The current
density is dependent of time for constant controlled potential experiments,
as shown in Fig. 2. The current density was decreased with increased
deposition times due to the passivation films (Mg(OH)2) was formed on
the electrode surface.
The electrode active areas were reduced as depo-
sition time increased.
Crystal structures and phase transformation
The decomposition path of the magnesium hydroxide (Mg(OH)2)
coating, was studied by DTA/TGA analysis.
The TGA/DTA profiles of
the deposited Mg(OH)2 are shown in Fig. 2.
The TG analysis diagram
can be divided into two regions as two endothermic peaks were formed in
the DTA diagrams.
The first endothermic peak, which was formed be-
tween 25 and 125℃, resulted from the evaporation of water contained in
Mg(OH)2.
The weight loss of absorbed water was determined as 7.61%.
The second endothermic peak appears in the temperature range of
250-400℃, which can be ascribed to the condensation of Mg(OH)2 into
MgO.
The observed value of Mg(OH)2 weight loss is 29.1%, which is
nearly equal to theoretical value 28.5%.
9
At this stage, the weight loss
was resulted from the condensation of hydroxide.
Mg(OH)2 → MgO + H2O↑
(6)
The XRD diagrams of as-deposited magnesium hydroxide powders,
and after annealed at 300, 350, 400, and 800℃, are respectively shown in
Fig. 3 (a), (b), (c), (d), and (e).
All the peaks in Fig. 3(a) and (b) can be
indexed to hexagonal Mg(OH)2 crystallites with lattice constants comparable to the values of JCPDS 84-2164. There are no parasite peaks in
the XRD pattern of Fig. 3(a), which indicates that Mg(OH)2 powders obtained by the electrochemical deposition method in this study are relatively pure.
This result supports the proposed reaction (5).
At higher
temperature, crystals of MgO were initially detected at 350℃, as shown
in Fig. 3(c).
There are several stronger diffraction peaks of hexagonal
structured magnesium hydroxide and two relative weak peaks at 2θ=
42.76° and 62.1° which can be indexed to (200) and (220) planes of cubic
magnesium oxide, respectively. This means that the condensation of
reaction (6) occurred and finished at 400℃, as shown in Fig 3(d), where
only MgO was found.
These broad diffraction peaks became higher and
sharper at 800℃, revealing the higher crystallinity, as shown in Fig. 3(e).
The FTIR spectrum of deposition Mg(OH)2 powders in KBr pellet,
10
annealed at 300, 350, 400, and 800℃ are shown in Fig. 4 (a), (b), (c), (d),
and (e), respectively. The sharp and strong peak at 3698 cm-1 is due to
the O-H stretching vibrations in the Mg(OH)2 crystal structure [19-20].
The band of absorption at 1390 cm-1 was corresponded to the O-H in
plane bending [21].
A broad absorption band at 3400 cm-1 implies the
transformation from free protons into a proton-conductive state in brucite.
A 1645 cm-1 peak indicates the bent vibration of H-O-H.
There is pos-
sibility that these bands at 1645 and 3400 cm-1 can also be attributed to
O-H vibration in absorbed water on the sample surface [21,22].
The
presence of Mg-O bonds is observed at around 440 cm-1.[23] In the
spectrum of the as-coated sample one can see strong absorption around
1645and 3400 cm-1 corresponding to surface absorbed water.
Absorbed
water comes out at temperature less than 300℃, while O-H band in crystal structure remain until 400℃. The FTIR results further support the
arguments discussed in TGA and XRD.
Surface morphology of Mg(OH)2 and MgO coated films
Deposition potentials will greatly affect the structural and morphological features of resulting Mg(OH)2 films.
11
To understand the formation
mechanism of needle- or lamellar-like Mg(OH)2 on the substrate, the kinetic process of crystal growth was studied carefully in 0.01 M Mg(NO3)2
solution at room temperature.
The analysis by SEM, showed the mor-
phology of Mg(OH)2 films originating from different deposition potentials.
At the same times, the brucite phase was observed, which shows
microstructural differences among four deposition potentials (Fig. 5).
The experimental result revealed no precipitation of Mg(OH)2 in the
aqueous solution of 0.01 M Mg(NO3)2.
For 0.01 M Mg(NO3)2 (pH=5.88)
aqueous solution, the concentration of OH- is 7.56×10-9 M in bulk solution. According to reaction (5) and the following equation [24],
Ksp=[Mg2+][OH-], (where Ksp=7.1×10-12, at 25℃)
(7)
the concentration of OH- should be greater than 2.42×10-4 M, to the form
Mg(OH)2 by direct precipitation in the bulk solution.
Therefore, in or-
der to form Mg(OH)2 coating, cathodic reactions (1) and/or (4) should
proceed to increase the concentration of OH- in equation (7) nearby the
electrode.
In the Fig. 5(a), we can see the needle-like nuclei grew on the substrate
sparsely.
Since the production rate of OH- ions in reaction (1) at -0.7 V
(5.01×10-5 A/cm2 as shown in Fig. 1 solution B region II) controlled by
12
the diffusion rate of O2 was very slow, only few heterogeneous nucleations were found on the metal substrate.
At more negative voltage such
as -0.8 V (8.71×10-5 A/cm2), the resultant Mg(OH)2 film came out with
lamellar-shaped porous continuous network form as shown in Fig. 5(b).
Due to the more production of OH- enhanced by reaction (4), as shown in
Fig. 1 solution B region III, the lamellar-like Mg(OH)2 sprouted from the
stem and eventually expanded into fully developed continuous lamellar
porous network.
When the deposition voltage was raised to -1 V
(1.148×10-3 A/cm2), the ratio of lamellar structure was decreased and
more particles among lamellae were found, as shown in figure 5 (c).
Much more OH- ions were produced in reaction (4), as shown in Fig. 1
solution B region III and the migration of Mg2+ was further enhanced,
leading to higher nucleation rate on the specimen and resulting in the
(001) preferred orientation, as shown in Fig. 6 (c). Figure 6 shown the
On the other hand, the crystal orientation was random at -0.8 V, as shown
in Fig. 6(b).
For the same reason, the highly uniform and densely
packed particles structure can be obtained at -1.2 V (2.39×10-3 A/cm2), as
shown in Fig. 5(d). Similarly, the (001) preferred orientation was found,
as shown in Fig. 6(d). It is possible that the nucleation rate increases
13
faster than growth rate, and the deposition of each layer proceeds with the
formation of a large number of nuclei. The particle size is decreased
with increasing applied voltage due to the nuclei overlap and growth centers impinge on each other, where growth stops at the point of contact, as
shown in Fig. 5(d), which revealing the particle size about 70 nm.
The micrograph of sample annealed at 400℃, as shown in Fig. 7, can
be compared with that of the corresponding as-deposited films (Fig. 5).
Although the morphologies are similar, the particle size became finer, especially for the lamellar structure, estimated about 30-50 nm, since the
condensation in reaction (7) occurred with annealing process.
the nano porosity became more obvious.
14
Also,
Conclusion
A novel method of coating MgO on Pt was successfully conducted in
Mg(NO3)2 aqueous solution by cathodic synthesis. It is found that the
cathodic polarization curves in 0.01M Mg(NO3)2 should be divided into
three stages: (i) O2 + 4H + 4e− → 2H2O (~0.5 to 0.185 V), (ii) O2 + 2H2O
+ 4e− → 4OH− (0.185 to −0.77 V), (iii) 2H2O + 2e− → H2 + 2OH− (−0.77
to -3 V).
The efficient deposition condition was found at the region III.
The nucleation and deposition rate of Mg(OH)2 was increased with increasing applied voltage.
The crystal orientation of Mg(OH)2 was ran-
dom at -0.7~-0.8 V but (001) preferred at -1.0~-1.2 V. To get the highly
uniform and densely packed Mg(OH)2 film, the applied voltage should be
near -1.2 V.
At this voltage (-1.2 V), the grain size was about 70nm.
The Mg(OH)2 as-coated film was condensed into MgO around 350℃,
and fully crystallized at 400℃.
The Mg(NO3)2 aqueous solution used in
this method is cheep, simple, and environmentally benign. Compared
with micro-arc oxidation (MAO) which was only restricted to Mg alloys,
this electrolytic method can be applied on the other metals.
15
Acknowledgments
The authors are grateful for the support of this research by the National Science Council, Republic of China under contract No. NSC
95-2221-E-005-073.
16
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19
Table I. Molar concentration, pH, O2 (mg/L), resistivity of solution (ρ),
and the first and second limiting current densities of polarization
curves in Mg(NO3)2 solution. All solutions were aerated with
air except for solution D, which was deaerated with N2 gas.
Solution
Molar concentration of Mg(NO3)2
Purging gas
pH
O2% (mg/L)
ρ(Ω-cm)
Limiting current density at region I (µA/cm2)
Limiting current density at region Ⅱ (mA/cm2)
20
A
B
C
D
0.1
4.88
7.78
11.83
4.26
63.09
0.01
5.88
7.36
56.85
1.41
63.09
0.001
6.44
7.20
500.00
0.45
55.95
0.01
N2
5.94
0.25
61.34
…
13.80
Table II. Molar concentration, pH, O2 (mg/L), conductivity of solution (κ),
and the first and second limiting current densities of polarization curves in MgCl2 solution. All solutions were aerated with
air at room temperature.
Solution
A
Molar concentration of MgCl2
pH
O2% (mg/L)
κ(mS/cm)
Limiting current density at region I (µA/cm2)
Limiting current density at region II (mA/cm2)
21
0.1
6.01
7.81
18.44
74.64
164.66
B
0.01
6.33
7.42
2.29
71.44
24.77
C
0.001
6.45
7.28
0.553
67.6
6.11
Table captions
Table I. Molar concentration, pH, O2 (mg/L), resistivity of solution (ρ),
and the first and second limiting current densities of polarization
curves in Mg(NO3)2 solution. All solutions were aerated with
air except for solution D, which was deaerated with N2 gas.
Table II. Molar concentration, pH, O2 (mg/L), conductivity of solution (κ),
and the first and second limiting current densities of polarization curves in MgCl2 solution. All solutions were aerated with
air at room temperature.
22
Figure captions
Figure 1. Cathodic polarization curves of platinum (a) in aerated 0.1 M
(solution A), 0.01 M (solution B), 0.001 M (solution C), and in
deaerated 0.01 M (solution D) Mg(NO3)2 · 6H2O aqueous
solutions assigned to curves A, B, C, and D, respectively, (b) in
aerated 0.1 M (solution A), 0.01 M (solution B), 0.001 M
(solution C) MgCl2 · 6H2O aqueous solutions assigned to curves
A, B, and C, respectively.
Figure 2. The variation of current density with time at applied voltage
-1.2 V
Figure 3. TG/DTA diagrams of the as-deposited powders tested in air at
temperature raising rate of 10℃/min.
Figure 4. X-ray Diffraction of (a) the as-deposited powders, and after annealed at (b) 300℃, (c) 350℃, (d) 400℃, and (e) 800℃, respectively.
Figure 5. The FTIR spectra of coating film (a) the as-deposited, annealed
at (b) 300℃, (c) 350℃, (d) 400℃, and (e) 800℃, respectively.
Figure 6. SEM observations of Mg(OH)2 film deposited for 1800 sec at (a)
-0.7 V, (b) -0.8 V, (c) -1.0 V, and (d) -1.2 V.
Figure 7. GAXRD diffraction patterns of (a) standard diffraction pattern
23
of Mg(OH)2, and the as-deposited samples at (b) -0.8 V for
3600s, (c) -1 V for 1800s, and (d) -1.2 V for 1800s.
Figure 8. SEM observations of the MgO films deposited at (a) -0.7 V, (b)
-0.8 V, (c) -1.0 V, and (d) -1.2 V annealed at 400℃ for 1hr.
24
1.0
E(V vs. Ag/AgCl)
0.5
0.0
region I
−
O2 + 2H2O + 4e →
D
−
4OH
-0.5
-1.0
region II
−
2H2O + 2e →
-1.5
−
H2 + 2OH
A
A
B
C
D
-2.0
C
B
-2.5
-3.0
-9
-8
-7
-6
-5
-4
-3
-2
-1
log(I(A/cm2))
(a)
1.0
E(V vs. Ag/AgCl)
0.5
0.0
region I
O2 + 2H2O + 4e →
-0.5
-1.0
region III
2H2O + 2e →
-1.5
-2.5
-3.0
-9
-
H2 + 2OH
A
A
B
C
-2.0
-8
C
-7
-6
-
4OH
-5
-4
log(I(A/cm2))
(b)
Fig. 1
25
-3
B
-2
-1
Current density (mA/cm2)
-3.0
-3.5
-4.0
-4.5
-5.0
-5.5
-6.0
0
200
400
600
800 1000 1200 1400 1600 1800 2000
time (sec)
Fig. 2
26
2
TG
DTA
0
105
100
95
85
80
-4
75
70
-6
65
60
-8
0
100
200
300
400
500
Temperature (℃ )
Fig. 3
27
600
700
55
800
TG(%)
DTA(μ V/mg)
90
-2
Mg(OH)2
MgO
Intensity[a.u.]
(e)
(d)
(c)
(b)
(a)
20
30
40
50
2θ [Degrees]
Fig. 4
28
60
70
(e)
Transmittance
(d)
(c)
(b)
(a)
H2O bending
H2O bending
OH in plane bending
O-H stretching
4000
3500
Mg-O stretching
3000
2500
2000
1500
Wavenumbers(cm-1)
Fig. 5
29
1000
500
(a)
(b)
(c)
(d)
Fig. 6
30
2000
1500
(d)
Pt
Mg(OH)2
1000
500
2000
(c)
1000
500
2000
(b)
1500
1000
500
(013)
(100)
1000
(111)
(a)
(110)
1500
(012)
2000
(011)
500
(001)
Intensity [a.u.]
1500
0
20.0
30.0
40.0
50.0
2θ[Degrees]
Fig. 7
31
60.0
70.0
(a)
(b)
(c)
(d)
Fig. 8
32