Preparation and Characterization of TiO<sub>2</sub> Photocatalytic Thin Film and Its Compounds by Micro-Arc Oxidation Technique

doi:10.4236/ampc.2013.38044

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Advances in Materials Physics and Chemistry, 2013, 3, 320-326

Published Online December 2013 (http://www.scirp.org/journal/ampc)

http://dx.doi.org/10.4236/ampc.2013.38044

Open Access AMPC

Preparation and Characterization of TiO2 Photocatalytic

Thin Film and Its Compounds by Micro-Arc Oxidation

Technique

Qun Ma1, Lili Ji2, Yinchang Li1, Tingting Jiang1, Junxia Wang1, Fei Li1, Hongyun Jin1,

Yongqian Wang1*

1Engineering Research Center of Nano-Geo Materials of Education China University of Geosciences, Wuhan, China

2Jiangcheng College of China University of Geosciences, Wuhan, China

Email: *[email protected]

Received October 24, 2013; revised November 25, 2013; accepted December 8, 2013

permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. In accordance of

ABSTRACT

Mesoporous TiO2 ceramic films have been prepared upon the Ti alloy substrate by the micro-arc oxidation (MAO)

technology. To enhance the photo-catalytic property of the films, Eu2O3 particles were added into the electrolyte solu-

tion of Na2CO3/Na2SiO3. Scanning electron microscope (SEM), energy dispersive (EDS), X-ray photoelectron spec-

troscopy (XPS) and X-ray diffraction (XRD) are employed to characterize the modified films. Diffuse reflectance spec-

tra (DRS) test, photo-generated current test and photo decomposition test are applied to evaluate the photo-catalytic

property of the modified films. The results show that Eu2O3 transformed into one-dimensional (1-D) nano-wires em-

bedded within the composite film, and the film has high photo-catalytic property.

Keywords: TiO2; Eu2O3; Compound; Photo-Catalytic Property

1. Introduction

Rice Titanium dioxide (TiO2) is one of the most impor-

tant semi-conductor materials, which is widely used as an

efficient photo-catalyst. In terms of environmental pro-

tection and new energy, TiO2 has obtained widespread

concern and research because of its chemical stability,

biological compatibility and photo-catalytic ability since

the report of photolysis of water on titanium dioxide

(TiO2) by Fujishima in 1972 [1]. However, the applica-

tion of TiO2 is limited. For the photo-catalytic applica-

tions, the band gap energy (Eg) of the semiconductor is

critical. Because of the wideness (3.2 eV) of the Eg of

TiO2, and the ultra-violet (UV) light absorption and weak

absorption of visible light [2], considerable efforts have

been directed to extend the absorption edge of TiO2 to-

ward the visible part of the spectrum in the last three

decades. So many researchers have been concentrating

on finding an apt process and a modification technology

to improve the photo-catalytic property of the TiO2.

TiO2 powder and TiO2 film are the most common ex-

istence forms of TiO2. However, due to the hard recycle

in polluted water and limited depth of photo-radiation,

TiO2 powder is not widely used, and thus many re-

searches have been focusing on the preparation of TiO2

film recent years [3-5]. It is well-known that there are

many different methods for preparing TiO2 film, such as

sol-gel [6-9], liquid phase deposition (LPD) [10], hy-

drolysis precipitation [11] and chemical vapor deposition

[12,13], etc. TiO2 film can also be produced via mi-

cro-arc oxidation (MAO) technology [14,15]. Generally

speaking, MAO is a useful approach for forming a ce-

ramic coating on the surface of valuable metals, such as

Al, Mg and Ti, and their alloys [16]. MAO is a process

based on the anodic oxidation and occurs at potentials

above the breakdown voltage of the oxide layer formed

on the anode surface [17]. Now, micro-arc oxidation

(MAO) is most frequently used to prepare TiO2 film. The

TiO2 film that is prepared through MAO has much high

quality, such as high wear resistance and corrosion resis-

tance [18], favorable biological activity [19] and

*Corresponding author.

Q. MA ET AL. 321

photo-catalytic activity. To enhance the efficiency of

photo-catalytic property of TiO2 film prepared by MAO,

rare earth metals with incompletely occupied 4f and

empty 5d orbit also often serve as catalysts or promote

catalysis. Besides, it has been proved that metal Eu, as

one of the rare earth metals, has the ability to improve

the photo-catalytic activity of the TiO2 films, because of

the electrons trap effect supplied by the alterable chemi-

cal valence (Eu2+ and Eu3+) sites [20].

In the present work, we have prepared TiO2 film upon

Ti alloy substrate via MAO technology. Meanwhile, in

order to get high efficient photo-catalytic property, rare

earth metal oxide Eu2O3 was compounded into the TiO2

film, and methyl blue dye was used to test the photo-

catalytic efficiency under UV-Vis irradiation. The grain

could be combined on the surface of the TiO2 layer di-

rectly by the MAO process of high temperature and fast

solidification, which could improve its photo-catalytic

property.

2. Experimental

All chemicals were of analytical grade and used without

further purification. In our work, Eu2O3 particles were

added into the electrolyte directly after ultrasonic disper-

sion in distilled water. We prepared uniform TiO2/Eu2O3

film upon Ti alloy substrate by using the high tempera-

ture and fast solidification of MAO process. Meanwhile,

we also prepared pour TiO2 film, so as to make a contrast

with films were produced without adding semiconductor

compound into the electrolyte. Follow these steps：

At first, we made up 5 L electrolyte. The electrolyte

was a mixed solution of sodium carbonate (20 g/L) and

sodium silicate (8 g/L). The anode was TC4 substrate

with reaction area of 20 × 20 × 5 mm. After polishing,

the substrates were cleaned by ethanol, acetone and dis-

tilled water. A stainless steel container was used as elec-

trolyte cell and cathode. MAO of titanium alloy substrate

was carried out at a constant current using a 60 KW AC

power supply. Different current densities were reached

by adjusting the voltage between two working electrodes.

The temperature was controlled below 40˚C within a

small circulating water channel. Oxidation time was ten

minutes. And then, the pure TiO2 film was prepared upon

TC4 substrate when the voltage was 400 V and stable.

Last, 10 g Eu2O3 particles were added into the same elec-

trolyte (the concentration of Eu2O3 was 2 g/L). The

TiO2/Eu2O3 composite films were prepared at different

voltage, 300 V, 350 V and 400 V. The detail experimen-

tal conditions were listed in Table 1.

The morphologies of the films and their chemical

compositions were characterized by using scanning elec-

tron microscope (SEM), energy dispersive spectrometer

(EDS) and X-ray photoelectron spectroscopy (XPS). The

crystal structure was measured by using X-ray diffract

meter (XRD) (D8 Advanced, Bruker AXS, Germany)

with Cu Ka radiation (the scanning speed is 6˚ per min-

ute). Photo-catalytic property was tested by diffuse re-

flectance spectra (DRS) test, photo-generated current test,

and photo decomposition test.

Diffuse reflectance spectra (DRS) test UV-Vis dif-

fuse reflectance spectra (DRS) of the TiO2 layers were

measured on the diffuse reflectance accessory of UV-Vis

recording spectro-photometer (UV-2550, Shimadzu, Ja-

pan).

Photo-generated current test The TiO2 layer, satura-

tion calomel electrode and the platinum electrode, which

were immersed in 1 mol/L NaOH aqueous solution, were

chosen to the working electrode, reference electrode and

counter electrode respectively. The working electrode

was irradiated by using a high pressure mercury lamp

(160 W) in 10 cm horizon distance away. The photocur-

rent was detected using an electrochemical workstation

(CHI 660C, Chenhua, Shanghai).

Photo decomposition test The methyl blue solution of

12 mg/L was pour into two beakers. The TiO2 layer

without complex and the compounded layers were im-

mersed in them, and the left was used as reference. All of

them were illuminated by a high pressure mercury lamp

(160 W) under 15cm vertical distance. The absorbance of

methyl blue solution was measured by a UV-Vis spec-

trophotometer (UV-2550, Shimadzu, Japan).

Table 1. Experimental conditions of the TiO2/Eu2O3 compounded film.

Electrical Electrolyte

Voltage

Samples

Forward Negative

Frequency Duty cycle Time Na2CO3 Na2SiO3·9H2O Eu2O3

TiO2 400 V 10 V 1 KHz 20% 10 min 20 g/L 8 g/L No

TiO2/Eu2O3 300 V 10 V 1 KHz 20% 10 min 20 g/L 8 g/L 2 g/L

TiO2/Eu2O3 350 V 10 V 1 KHz 20% 10 min 20 g/L 8 g/L 2 g/L

TiO2/Eu2O3 400 V 10 V 1 KHz 20% 10 min 20 g/L 8 g/L 2 g/L

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Q. MA ET AL.

322

3. Results and Discussions

The SEM morphologies of the pure TiO2 film at 400V

are shown in Figure 1. From Figure 1, we can see

clearly that the pure TiO2 film is porous with diameter of

pores (the diameter of pores is about 1 - 5 µm). Figure 2

shows the SEM morphologies of the TiO2/Eu2O3 com-

posite films at 300 V, 350 V and 400 V. Meanwhile, Fig-

ure 2 gives its two different magnifications photos at 400

V. It can be seen that the MAO TiO2 surface exhibits a

typical crater and porous microstructure with a diameter

around 1 - 2 µm. At 300 V and 350 V, the surface of the

TiO2/Eu2O3 composite film present micro pore structure,

as shown in (a) and (b), which is just ordinary film pre-

pared by MAO technology. However, at the high voltage

400 V, there are many one-dimensional (1-D) structures

homogeneously embedded within the film, as shown in

(c-1) and (c-2). Due to the small size effect, these 1-D

structures can enhance the photo-catalytic property of

TiO2 film. So, our researches were concentrated on the

TiO2/Eu2O3 composite film prepared at 400V. In order to

get more detail information about the 1-D nano-wires

materials on the surface of TiO2/Eu2O3 film, we made

size statistics about it. It can be noticed that these 1-D

nano-wires have the average length of 1.26 μm and the

average width of 87.5 nm from Figure 3.

In order to prove that the 1-D nano-wires were Eu2O3,

Figure 1. SEM morphologies of TiO2 film without composites. (a) Low magnification; (b) High magnification.

(a) (b)

2 µm 2

µm

500 nm

(c-1) (c-2)

2 µm

Figure 2. SEM morphologies of the TiO2/Eu2O3 film by different voltage. (a) 300 V, (b) 350 V, (c-1) 400 V, (c-2) 400 V.

Open Access AMPC

Q. MA ET AL. 323

(a) (b)

Figure 3. Statistical size distribution of the nano-wires on TiO2/Eu2O3 composite film: (a) Length; (b) Width.

which were added into the electrolyte and entered into

the micro-porous of the TiO2 films, we made EDS test.

EDS measurement revealed that these 1-D nano-wires

were composed of Eu and O elements, as shown in Fig-

ure 4, which implies that original Eu2O3 particles have

transformed into 1-D nano-wires during MAO process.

At the same time, we know that Ti and some O elements

come from the TiO2 film, and Au elements come from

SEM observation of injection of the sample, and Na ele-

ments come from the electrolyte.

XPS technique was applied to confirm Eu2O3 exis-

tence in the TiO2/Eu2O3 composite film and investigate

the chemical states of the elements, whose result is pre-

sented in Figure 5. It can be seen that the peaks mainly

contain the Ti 2p, O 1s, C 1s and Eu 3d5, which also

demonstrates the existence of Eu2O3 inside the film.

Figure 6(a) illustrates XRD patterns of the pure TiO2

and the TiO2/Eu2O3 composite film (400V). It can be

noticed clearly that both of them mainly contain anatase

phase with few rutile phase. At the same time, we know

that 1-D nano-wires have little affection to the crystal

structure of the films. Figure 6(b) displays the enlarged

XRD peaks of anatase TiO2 plans A (101) in the 2θ re-

gion of 24.0˚ - 27.0˚. We measured the full width half

maximum (FWHM) of the main peak of anatase TiO2

plans (101) in the 25.3˚. The result is that the FWHM of

the pure TiO2 film is 0.24 and the TiO2/Eu2O3 composite

film is 0.28. Using the Scherrer Formula, we calculated

that in pure TiO2 film, the grain size of anatase TiO2 is

about 33 - 34 nm but in the TiO2/Eu2O3 composite film,

the size is around 28 - 29 nm. From calculation results,

we found that the grain size of anatase in the TiO2/Eu2O3

composite film decreased obviously, compared with pure

TiO2 film. The reason is that rare earth metals has in-

completely occupied 4f and empty 5d orbit and the

growth of TiO2 grains was inhibited after Eu2O3 was

compounded in TiO2 films.

For the sake of studying the photo-catalytic properties

of the TiO2/Eu2O3 composite film (400 V), diffuse re-

flectance spectra (DRS) test was employed and its result

is presented in Figure 7. Obviously, the composite film

exhibits a higher absorption of UV and visible light with

an Einstein shift about 10 nm. Before Eu2O3 was com-

pounded, the pure TiO2 film only has little absorption of

visible light. The results reveal that improved absorption

in UV and visible light ranges is contributed from the

addictive phase Eu2O3. Eu3+ possesses abundant energy

levels, so Eu2O3 can increase the absorption of UV and

visible light and improve the photo-catalytic properties of

the composite film.

To study the photo-catalytic efficiency of the TiO2/

Eu2O3 composite film (400 V), we also measured the

intensity of photo-generated current of it, as shown in

Figure 8. The reason why the TiO2/Eu2O3 composite

film has a higher intensity of photon generated current is

that in Eu2O3, iron Eu3+ has an incomplete 4f orbital track

and an empty 5d orbital track, which tends to produce

multi-electron configuration and therefore can effectively

inhibit the recombination of photo-electrons and holes.

Figure 9 shows the result of the degradation experi-

ments of methyl blue. It revealed that the photo-catalytic

performance of the TiO2/Eu2O3 composite film (400 V)

exhibited two times higher than that of the pure TiO2

film. We can see that after four hours, the degradation of

methyl blue reaches to 90% when using the TiO2/Eu2O3

composite film (400 V), while only 45% when using the

film without Eu2O3.

4. Conclusion

In conclusion, the TiO2 films were prepared upon Ti al-

loy substrate by micro-arc oxidation (MAO) process. The

Open Access AMPC

Q. MA ET AL.

324

films had a porous structure with a rough surface which

is suitable for modification. When rare earth metal oxide

Eu2O3 was added into the electrolyte, a composite film

was produced during MAO and Eu2O3 transformed into

one-dimensional (1-D) nano-wires embedded within the

composite film. Comparing with the pure films, the

composite films exhibited high efficient photo-catalytic

properties, such as high absorption in UV and visible

light ranges, high photo-generated current intensity and

high photo degradation rate.

(a) (b)

Figure 4. EDS analysis of the TiO2/Eu2O3 composite film.

0.00E+00

1.00E+05

2.00E+05

3.00E+05

4.00E+05

5.00E+05

6.00E+05

02004006008001000

Counts / s

Binding En ergy (eV )

Surv ey A l 30 0W PE 1 0 0e V

0.00E+00

1.00E+04

2.00E+04

3.00E+04

4.00E+04

5.00E+04

6.00E+04

450452454456458460462464466468

Counts / s

Binding Energy (eV)

Ti 2p Al 300W P E 25eV

Ti2p

0.00E+00

1.00E+05

2.00E+05

3.00E+05

4.00E+05

5.00E+05

6.00E+05

02004006008001000

Counts / s

Binding En ergy (eV)

Survey Al 300W PE 100eV

0.00E+00

1.00E+04

2.00E+04

3.00E+04

4.00E+04

5.00E+04

6.00E+04

450452454456458460462464466468

Counts / s

Binding Energy (eV)

Ti 2p Al 300W P E 25eV

Ti2p

450

Counts/s

(a) (b)

7.40E+04

7.60E+04

7.80E+04

8.00E+04

8.20E+04

8.40E+04

1130114011501160

Counts / s

Binding Energy (eV)

Eu 3d Al 300W P E 25eV

Eu3d5

7.40E+04

7.60E+04

7.80E+04

8.00E+04

8.20E+04

8.40E+04

1130114011501160

Counts / s

Binding Energy (eV)

Eu 3d Al 300W P E 25eV

Eu3d5

Counts/s

(c)

Figure 5. XPS spectra of the TiO2/Eu2O3 composite film.

Open Access AMPC

Q. MA ET AL. 325

(a) (b)

Figure 6. XRD patterns of the TiO2 and the TiO2/Eu2O3 composite films prepared by MAO (a)survey, (b) anatase (101) peak.

200 300 400 500600 700800

0. 3

0. 4

0. 5

0. 6

0. 7

0. 8

0. 9

1. 0

1. 1

1. 2

1. 3

Abs./a.u.

wavelength/nm

Ti O2/Eu2O3

TiO2

Einstein shift

1.3

1.2

1.1

1.0

0.9

0.8

0.7

0.6

0.5

0.4

0.3

200 300 400 500 600 700 800

Figure 7. UV-Vis spectra of the TiO2 film and the TiO2/

Eu2O3 composite film.

-20020 40 60 80100120140160

-0.2

0. 0

0. 2

0. 4

0. 6

0. 8

1. 0

OFF OFF

Cur rent/uA

Time/se

TiO2/Eu2O3

TiO2

OFF ON

lam p-hou se control

Figure 8. Photo-generated current of pure TiO2 and TiO2/

Eu2O3 composite film.

TiO

/Eu

(400 V)

TiO

100

Abs/a.u.

0 50 100 150 200 250 300

Time/min

Figure 9. The concentration of methylene blue photo-de-

graded by pure TiO2 and TiO2/Eu2O3 film.

5. Acknowledgement

The work was supported by the National Natural Science

Foundation of China (21203170, 21201156), the Hall of

Hubei Province Science and Technology Project

(2011CDB347) and the Fundamental Research Funds for

the Central Universities, China University of Geosci-

ences (Wuhan, CUG130401), National Training Pro-

grams of Innovation and Entrepreneurship for Under-

graduates (201310491015). The financial support was

gratefully appreciated.

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