Materials Transactions, Vol. 47, No. 9 (2006) pp. 2335 to 2340
#2006 The Japan Institute of Metals
Synthetic Process of Environmentally-Friendly TiO2 Coating
on Magnesium by Chemical Conversion Treatment
Takayoshi Fujino and Teppei Matzuda*
Department of Applied Chemistry, Faculty of Science & Technology, Kinki University, Higashiosaka 577-8502, Japan
Titanium dioxide (TiO2 ) coatings were prepared by chemical conversion treatment of magnesium in (NH4 )2 [TiO(C2 O4 )2 ] with H2 O2 ,
then, anatase type TiO2 coatings were prepared by sintering. To identify the coating structure, coating analysis was carried out using an infrared
absorption spectrum analyzer. Based on the infrared absorption results, a component of the coating was found in the hydrolysis product of
peroxo-titanium compound. Furthermore, the coating analysis was carried out using X-ray diffractometry (XRD), and non-sintered coating was
amorphous; however, the coating sintered at more than 573 K was anatase-type titanium dioxide.
In the forming process of the conversion treatment in (NH4 )2 [TiO(C2 O4 )2 ] with H2 O2 , first, magnesium was dissolved because Hþ in the
bath reacted with the magnesium. Hydrogen ions on the magnesium surface were consumed to generated hydrogen gas. Thus, the pH of the
interface became alkali. The hydrolysis of the peroxo-titanium compound was deposited on the magnesium because pH increased on the surface.
From the XPS results and the TG-DTA results, a component of the coating is a hydrolyzation product of a peroxo-titanium compound and
Mg(OH)2 . Because Mg(OH)2 is generates in pH more than 11, it is considered that the pH on the magnesium surface is more than 11.
The coating sintered at 573 K had the highest photocatalytic activity. The photocatalytic activity of the coating sintered at 623 K was lower
than the coating heated at 573 K, which is attributed to growth of TiO2 particle. This forming process of the coating is low cost because of the
useless electrolytic decomposition process and increasing the speed of the treatment. It is possible to treat complicated form of the substrate
metal, so this method can be expected to use in various fields. Therefore this method is expected to practical use for environmental
purification. [doi:10.2320/matertrans.47.2335]
(Received March 6, 2006; Accepted July 12, 2006; Published September 15, 2006)
Keywords: magnesium, chemical conversion treatment, forming process, titanium dioxide, photocatalyst, low cost, peroxo-titanium complex
1.
Introduction
Currently, steel and aluminum, etc. are used as building
materials and part ingredients for electrodevice. However,
there are strong demands for weight-saving measures for
substrate materials, cost reductions and functionality improvements to be made in the metal surface. Amount of
export of magnesium were increasing the most, because it is
the lightest of the practical metals (its weight is 2/3 that of
aluminum, 1/2 that of titanium and 1/4 that of iron). This
means magnesium could replace aluminum in the field of
building materials and as an electrodevice.1,2) In the
automotive industry, reductions in fuel-efficiency and the
amount of CO2 being discharged are being demanded from
the viewpoint of global environment protection, so there is an
active shift to magnesium for material parts and for weightsaving. As the demand for magnesium increases in these
various fields, it is necessary to add new functionalities to the
magnesium surface. As one of surface finishing, there is a
chemical conversion treatment. A chemical conversion
coating3) can be prepared by only immersion of metal
because the coating is formed by chemical reaction. Therefore, we expect method to reduce of costs, treat the
complicated form of the substrate metal, and to increase
the speed of the treatment.
Recently, research on TiO2 has advanced because TiO2
has high oxidizability and displays super-hydrophilicity
when light is irradiated to the surface of the photocatalyst.
In addition, TiO2 with high photocatalytic activity is
harmless and has chemical stability. There are examples of
TiO2 ’s practical use for environmental purification in all
fields. At present, the immobilization of TiO2 are actively
*Graduate
Student, Kinki University
studied,4,5) because we hope to expand its use. In this
research, TiO2 was immobilized on magnesium because we
expect will support market expansion in the future. As
immobilizing TiO2 uses a chemical conversion treatment in
(NH4 )2 [TiO(C2 O4 )2 ] with H2 O2 solution, it is possible to
prepare it rapidly at low cost.6,7) In the future, we’ll also be
able to create deodorization and sterilization effect, and
prevent oil dirt and harmful gases.
2.
Experimental
Magnesium (plate or mesh, AZ91, AZ80, AZ61, etc.) was
used as a basic material. Prior to use, the magnesium was
treated with a surface active agent for 5 minutes at 323 K and
then washed with distilled water. In addition, the magnesium
was etched using alkaline and acid solution to remove a thin
layer of magnesium hydroxide and to increase its surface
area. As a coating process using a chemical conversion
treatment, TiO2 coatings were prepared by dipping magnesium in (NH4 )2 [TiO(C2 O4 )2 ] with H2 O2 solution (primary
treatment). This preparation conditioning was carried out at
293–393 K for 10–120 minutes. As a secondary treatment,
the coatings were sintered at 473–773 K. The coating
thickness was calculated using the mean value of five
measurements. The optimum conditions of the coating
preparation are as follows: treatment temperature: 353 K,
treatment time: 30 min, concentration of H2 O2 : 0.04
k mol/m3 , concentration of (NH4 )2 [TiO(C2 O4 )2 ]: 0.02
k mol/m3 , pH of the solution: 2.5–3.0, sintering temperature:
550 K. All the analytical samples were prepared at the
optimum condition. In the solution of less than pH 2.5, the
coating could not be formed as magnesium only dissolves. In
the solution of more than pH 3.0, as hydrolysis of titanium
ion in the solution occurs, so the solution was approximately
2336
T. Fujino and T. Matzuda
Working conditions on measurement of photocatalytic activity.
Test piece size (cm2 )
100
Source of light
Intensity of light (mW/cm2 )
Black light
360
Distance from
source of light to test piece (cm)
10
Vessel size (cm)
7.0
Volume of
malachite green (cm3 )
Adsorption time (min)
15
Irradiation time (min)
120
40
14
12
Film thickness, th/µm
Table 1
10
8
6
4
2
0.01
3.
Results and Discussion
Figure 1 shows the relationship between the solution
concentration of (NH4 )2 [TiO(C2 O4 )2 ] and film thickness in
the primary treatment. The film thickness increased with
increasing the concentration of (NH4 )2 [TiO(C2 O4 )2 ]. Increasing the concentration of (NH4 )2 [TiO(C2 O4 )2 ] caused to
occur hydrogen gas remarkably, and the adhesion of the
prepared coating was inferior. Figure 2 shows the dependence of H2 O2 concentration for the film thickness. The
reactive rate increases with H2 O2 concentration. The film
thickness increased by additions of H2 O2 because magnesium dissolves easily. Another reason is that the excess H2 O2
not only formed a peroxo-titanium complex but also worked
as for oxidizing agent. Figure 3 shows the influence of the
treatment temperature on film thickness. A white coating was
0.02
0.03
0.04
0.05
Concentration of (NH4 )[TiO(C2O4 )2 ], c/k mol·m-3
Fig. 1 Dependence of the concentration of (NH4 )2 [TiO(C2 O4 )2 ] for the
coating thickness (Bath temperature 353 K, treatment time 30 minutes,
concentration of H2 O2 0.04 k mol/m3 ).
10.5
Film thickness, th/µm
10
9.5
9
8.5
8
7.5
7
6.5
0.02
0.03
0.04
0.05
0.06
Concentration of H2O2, c/k mol·m-3
Fig. 2 Dependence of the concentration of H2 O2 for the coating thickness
(Bath temperature 353 K, treatment time 30 minutes, concentration of
(NH4 )2 [TiO(C2 O4 )2 ] 0.02 k mol/m3 ).
11
Film thickness, th/µm
constant in pH 2.5–3.0. We observed the surface of the
coatings using a scanning electron microscope (SEM). As a
pretreatment for SEM observation, the surface of the
conversion coatings was coated with a 0.03 mm thick layer
of Pt-Pd using vapor deposition equipment. The analysis of
the coating was carried out using thin film X-ray diffractometry (XRD). The angles ranged from 10 to 90 because
the peak of anatase TiO2 was detected with in this range. The
absorption spectra were measured using Infrared spectroscopy (IR) analysis to estimate the coating components and the
sintered coating. A KBr tablet method was used to measure
the IR spectrum. The conversion coating (non-sintering) and
the coating sintered at 550 K were measured using X-ray
photoelectron spectroscopy (XPS) because of analysis of the
compounds in the coating. In addition, the precipitation
generated in the bath after chemical conversion treatment
was analyzed using thermal analysis equipment (TG-DTA).
We analyzed it within the range of 298–1273 K (10 K/min)
in an atmosphere of air (50 mL/min). A micro type platinum
cell was used for the sample cell. We measured the UV/vis
absorption spectra of the coatings within wavelengths of
350–800 nm. Absorbance was defined as the ratio of the
intensity of the incident and the transmitted beams. The
photocatalytic activity of the coatings was evaluated by
measuring the decease of the malachite green concentration.
After the coating was adsorbed, the solution of malachite
green solution (40 mL, 2.50 ppm) was illuminated by UV
light for 60 minutes at room temperature in the dark, Table 1
shows the conditions of photocatalytic activity measurement.
A UV light with a black light was used as a source light.
10
9
8
7
6
5
300
310
320
330
340
350
Temperature, T/K
Fig. 3 Dependence of the treatment temperature for coating thickness
(Treatment time 30 minutes, concentration of H2 O2 0.04 k mol/m3 ,
concentration of (NH4 )2 [TiO(C2 O4 )2 ] 0.02 k mol/m3 ).
Synthetic Process of Environmentally-Friendly TiO2 Coating on Magnesium by Chemical Conversion Treatment
Anatase TiO2
Intensity /a.u.
prepared by processing within a temperature range of 323 K
from 298 K. We assumed that the main ingredient of the
coating was Mg(OH)2 . A yellow coating was obtained at a
temperature of more than 333 K, and the titanium content was
increased. Figure 4 shows the results of coating structure
analysis using thin film XRD. The non-sintered coating
(Fig. 4(a)) was amorphous, however, the composite of the
coating sintered at 550 K (Fig. 4(b)) was an anatase-type
titanium dioxide to crystallize.
Scherrer formula TðnmÞ ¼ 0:9=B cos (B: half-value
width, : Wavelength of CuK ¼ 0:15414 nm).
From the formula indicated above, we established that the
grain diameter of the TiO2 crystal in the coating was 27 nm.
Figure 5 shows an SEM image of the coating (non-sintering)
and the sintered coating. The conditions of the coating
preparation for SEM observation are as follows: treatment
temperature: 353 K, treatment time: 30 min, concentration of
H2 O2 : 0.04 k mol/m3 , concentration of (NH4 )2 [TiO(C2 O4 )2 ]:
0.02 k mol/m3 , pH of the solution: 2.7, sintering temperature:
550 K. We observed a large number of adhered granular
spherical crystals on the surface. As Fig. 5(b), shows, the
TiO2 crystals (diameter of particle: approximately 30 nm)
were deposited on magnesium. Figure 6 shows the IR of the
(b)
(a)
10
20
30
40
50
60
70
90
Fig. 4 X ray diffraction pattern of coatings. (a) Non-sintered coating,
(b) Coating sintered at 550 K.
coating. From Fig. 6(a) in non-sintered coating, we detected
infrared absorption resulting from a free peroxide (O-O bond)
stretching mode at 904 cm1 . The broad peaks observed at
500 nm
(b) Sintered coating at 550 K
1 µm
80
2θ /deg
(a) Non-sintered coating
1 µm
2337
500 nm
Fig. 5 Scanning electron micrographs image of the coating surface. (a) Non-sintered coating, (b) Coating sintered at 550 K.
2338
T. Fujino and T. Matzuda
100
(b)
Peroxo titanium complex → Anatase TiO2
50
Mg(OH)2 → MgO +
8.5
8.0
H2O
DTA, /uV
T (%)
7.0
-50
6.5
(a)
MgCO3 → MgO + CO2
-100
904 cm-1
1401 cm-1
6.0
H2O
-150
273
473
Wavenumber, σ /cm
(a)
Fig. 8
1073
5.5
1273
TG-DTA curves of chemical conversion coating.
(b)
Sample:coating
(c)
Sample:coating
O 1s
458.3
Before
sintering
873
peroxo titanium complex
decomposition
-1
Fig. 6 FT-IR spectra of the coating on aluminum. (a) Non-sintered coating,
(b) Coating sintered at 550 K.
Ti 2p
673
Temperature, T/K
4000 3500 3000 2500 2000 1500 1000 500
Intensity / a.u.
TGA, /mg
7.5
0
O 1s
Sample:Powder
532.3
531.7
529.8
Before
sintering
530.1
Before
sintering
464.1
533.2
532.1
530.3
458.6
Sintering
464.5
530.7
Sintering
Sintering
468
464
460
456
452 536 534 532 530 528 526 540
535
530
525
Binding energy, E/eV
Fig. 7 XPS of Ti 2p and O 1s electron binding energy of the coating. (a) 2p spectrum of the coating, (b) O 1s spectrum of the coating,
(c) O 1s spectrum of the powder.
2700–3667 cm1 and the clear peak at 1628 cm1 are
attributed to water molecules. The peak at 1401 cm1 is
due to the stretching vibration of N-H bonds in NH4 þ . The
peak at 2361 cm1 is attributed to KBr. Those peaks were
disappeared by sintering as shows Fig. 6(b). Figure 7 shows
the XPS data of the coating and powder as well as the
spectrums of Ti 2p and O 1s. The spectra must be calibrated
with the standard sample, such as C 1s. Normally, the simple
calibration is done by C 1s (approximately 284.6 eV)
spectrum of adventitious carbon that exists on all samples.
Figure 7(a) shows that Ti 2p was detected in two peaks,
458.6 and 464.5 eV in the sintered coating. From the two
peaks, the coating composite sintered at 550 K was an
anatase-type TiO2 to crystallize. The O 1s spectrum of the
coating is indicated in Fig. 7(b). The chemical conversion
coating (non-sintering) contained Mg (OH)2 from a peak of
531.7 eV. And, the peak at 529.8 eV is characteristic of
titanium oxides. And then, in sintering coating, O 1s
spectrum at 530.3 eV is attributed to anatase TiO2 . After
the coating on magnesium had been removed, the coating
was made a fine particle. And, O 1s spectrum of the sample
was measured by XPS. The spectrum shows in Fig. 7(c). The
powder contained MgO was confirmed that from a peak of
530.7 eV after sintering. In addition, the existence of MgCO3
was also confirmed from a peak of 533.2 eV. These peaks
were not detected with XRD because it was an amorphous
substance. MgCO3 brings in coating because Mg(OH)2 reacts
with CO2 in the air or water. The deposit in the solution after
chemical conversion treatment was analyzed using a TGDTA measurement. The measurement results are indicated in
Fig. 8. We confirmed the peak of the wide endothermic
reaction around 376 K in a DTA spectrum, This is considered
according to the dehydration reaction. In a TG spectrum,
decrease of nineteen percent was detected to temperatures of
about 523 K, because this is regarded as the cutting of the
dehydration reaction and the resolution of the peroxo-
Synthetic Process of Environmentally-Friendly TiO2 Coating on Magnesium by Chemical Conversion Treatment
Absorbance, /a.u.
0.5
0.45
by the addition of excessive amounts of H2 O2 .8–10) We can
also estimate that a peroxo-titanium bond brings similarity to
this solution.11–15) After magnesium was immersed in this
solution, hydrogen gas was immediately generated from the
magnesium surface. The forming process was as follows:
(a)
0.4
0.35
0.3
(b)
0.25
0.2
0.15
0.1
350 400
2339
500
600
700
800
Wavelength, I/nm
Fig. 9 UV-vis spectra of the conversion coatings at wavelength 300–
800 nm. (a) Non-sintered coating, (b) Coating sintered at 550 K.
titanium complex in the coating. An endothermic peak at
493 K is a dehydration reaction when Mg(OH)2 changes into
MgO. The exothermic peak was confirmed at 550 K. Therefore, an amorphous peroxo-titanium hydrate crystallizes to an
anatase type TiO2 is at 550 K. The exothermic peak at 690 K
was a change point from MgCO3 to MgO. The exothermic
peak at 1073 K was a crystal change to a rutile-type titanium
dioxide from an anatase type TiO2 . The UV/vis spectra of the
non-sintered coating and the coating sintered at 550 K are
shown in Fig. 9. The wavelength range was 350–800 nm. The
absorption spectrum of the non-sintered coating was characterized by a strong band at 520 nm. The position of the
absorption was strongly influenced by the peroxo titanium
compound. The position of the coating absorption sintered at
550 K was below approximately 400 nm, which is the same
absorbance peak as the peak of the existing anatase-type
TiO2 . Based on the results obtained, we can presume
that there was a forming process, as shown in Fig. 10.
(NH4 )2 [TiO(C2 O4 )2 ] is a colorless, transparent liquid, but
the color of the (NH4 )2 [TiO(C2 O4 )2 ] solution changed to
yellow when H2 O2 was added. Peroxo-titanic acid is formed
Magnesium
Mg ! Mg2þ þ 2e
2Hþ þ 2e ! H2 "
ð1Þ
ð2Þ
Mg2þ þ 2OH ! Mg(OH)2
ð3Þ
(NH4 )2 [TiO(C2 O4 )2 ] þ H2 O2 þ 2H2 O
! (NH4 )2 TiO(O2 )(OH)2 þ 4Hþ þ 2(C2 O4 )2
ð4Þ
First, magnesium was dissolved in a treatment solution and
the hydrogen ions on the magnesium surface were consumed.
Thus, the pH of the interface was increased because H2 gas
brings (Formulas (1) and (2)) and then, Mg(OH)2 was
brought to the magnesium surface (Formulas (3)). At the
same time the hydrolysis product of the titanium peroxo
compound was deposited with the separation of Mg(OH)2
because the pH increased on the surface (Formula (4)).
Because Mg(OH)2 is generates in pH more than 11, it is
considered that the pH on the magnesium surface is more
than 11. An amorphous titanium-peroxo compound in the
coating was transformed to a crystalline phase by sintering.
The TiO2 coating obtained at 473–673 K was white. From
XRD and XPS result, the component of the coating was
anatase type TiO2 . In addition, from XPS (Fig. 8(b)), we
detected Mg(OH)2 in the coating (non-sintered coating), then
the Mg(OH)2 changed into MgO and MgCO3 by sintering
more than 493 K.
Figure 11 shows the results of the photocatalitic activity of
the sintered coatings. TiO2 coatings were confirmed by
sintering at 673 K. The activity of the coating sintered at
673 K was lower than the coating sintered at 573 K. This is
probably due to the decrease of the surface area of the coating
because TiO2 crystals were grown with increasing sintering
temperatures.
4.
Conclusion
(1) The coating was prepared by treating a magnesium
plate in a solution of (NH4 )2 [TiO(C2 O4 )2 ] and H2 O2 .
Etching
Peroxo titanium
compound
Anatase TiO2,
Mg(OH)2
MgCO3,
MgO
Sintering
Chemical conversion treatment
in (NH 4 )2 [TiO(C2O4 )2 ] H 2 O2
Fig. 10 Forming process of chemical conversion coating.
2340
T. Fujino and T. Matzuda
Adsorption time
2.5
Concentration, /ppm
Acknowledgements
Ultraviolet irradiation time
: Blank
: 573 K
: 673 K
2
1.5
This work was ‘‘University-Industry Joint Research’’
project for Private Universities and supported by funding
from the Ministry of Education, Culture, Sports, Science and
Technology private school grant, 2002–2006. The authors
thank them for their immense assistance in the development
of functional chemical conversion coatings and its applicability.
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1
0
20
40
60
80
100
120 150
Time, t/min
Fig. 11 Photocatalytic activity of conversion coatings.
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(3) We expect this method to be industrialized because it is
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