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Surface & Coatings Technolog

Effect of plasma treatment on hydrophilic properties of TiO2 thin films

Jun-Bo Hana, Xia Wanga, Nian Wanga, Zheng-He Weia, Guo-Ping Yua,

Zheng-Guo Zhoua, Qu-Quan Wanga,b,*

aDepartment of Physics, Wuhan University, Wuhan 430072, PR ChinabCenter of Nanoscience and Nanotechnology Research, Wuhan University, Wuhan 430072, PR China

Received 2 January 2005; accepted in revised form 27 April 2005

Available online 31 May 2005

Abstract

TiO2 films prepared by reactive sputtering technique were treated by Ar, O2 and N2 radio frequency plasma, respectively. The contact

angles of water drop on the surface of TiO2 films, which were measured by drop shape analysis, decreased remarkably with plasma treatment

for 1 min. With the increasing of plasma treatment time, the contact angles of the samples treated by O2 plasma decreased rapidly to zero

degree, while the contact angles of the samples treated by Ar and N2 plasma decreased slowly. The improvement of hydrophilic property is

due to the surface etching, ultraviolet radiation and surface oxidation of plasma treatment.

D 2005 Elsevier B.V. All rights reserved.

Keywords: Plasma treatment; TiO2 films; Hydrophilic property; Radio frequency sputtering

1. Introduction

Titanium dioxide has been extensively investigated for

its application in solar energy conversion and environmental

purification since Fujishima and Honda discovered the

photocatalytic splitting of water on TiO2 electrodes in 1972

[1,2]. As a phenomenon that is distinct from conventional

TiO2 photocatalytic oxidation reactions of adsorbed mole-

cules on surface, Wang et al. [3] reported that ultraviolet

illumination to TiO2 surface could produce a highly hydro-

philic surface, which was denoted as super-hydrophilicity.

Watanabe et al. [4] proved that the hydrophilic property

originated from the water adsorption on oxygen vacancies

created by UV light irradiation. As it can be widely used in

self-cleaning and antifogging materials, many studies have

been carried out in improving the hydrophilic property of

TiO2 thin films, such as ion doping [5–7], preparing

composite films [8,9], surface treatment [10] and surface

modification [11,12]. In this paper, we reported the effect of

0257-8972/$ - see front matter D 2005 Elsevier B.V. All rights reserved.

doi:10.1016/j.surfcoat.2005.04.036

* Corresponding author. Department of Physics, Wuhan University,

Wuhan 430072, PR China.

E-mail address: qqwang@whu.edu.cn (Q.-Q. Wang).

Ar, N2 and O2 radio frequency (r.f.) plasma treatment on the

hydrophilic properties of TiO2 films prepared by r.f.

sputtering technique.

2. Experimental

TiO2 films were deposited on glass substrates by r.f.

(13.56 MHz) sputtering method. The sputtered target was

titanium (A100�3 mm2) with a purity of 99.999%, the

distance between the target and the substrate was about 60

mm. The sputtering and reactive gas was a mixture of argon

and oxygen with a partial pressure ratio of 0.5 :0.5. The base

pressure of the sputtering chamber was 1.4�10�2 Pa, and

the sputtering pressure was about 5.2 Pa during the

deposition. The sputtering power was 240 W.

Four series of samples were heat treated and r.f. plasma

treated, respectively. One series of samples were heat treated

in air for 30 min and the treatment temperature varied from

200 to 600 -C. The other three series of samples were

treated for 1¨30 minutes at room temperature by Ar, N2 and

O2 plasma, respectively [13]. The treatment power was 115

W. Contact angles of water drop on the surface of TiO2 films

were evaluated by drop shape analysis method. The thick-

y 200 (2006) 4876 – 4878

0 100 200 300 400 500 600

0

20

40

60

80

Con

tact

ang

le (

degr

ee)

Temperature (°C)

Fig. 1. Contact angle of water drop on the surface of TiO2 films as a

function of heat treatment temperature.

0 10 20 30

0

20

40

60

80

Con

tact

ang

le (

degr

ee)

Plasma treatment time (min)

A N2

B ArC O2

Fig. 3. Contact angle of water drop on the surface of TiO2 films after (A) N2

plasma treatment, (B) Ar plasma treatment, and (C) O2 plasma treatment.

J.-B. Han et al. / Surface & Coatings Technology 200 (2006) 4876–4878 4877

ness of TiO2 films was measured to be about 300 nm by a

Form Talysurf. The structures of TiO2 films were examined

by X-ray diffraction (XRD). The composition and binding

energy of the samples were determined using X-ray photo-

electron spectroscopy (XPS).

470 465 460 455 450

Ti 2p1/2

Ar plasma treated for 10 h Ar plasma treated for 1 h

Inte

nsity

(a.

u.)

(a)Ti 2p3/2

3. Results and discussion

3.1. Effect of heat treatment on hydrophilic property of TiO2

thin films

Fig. 1 shows the relation between the contact angle of

water drop on the surface of TiO2 films and the different

heat treatment temperature. The contact angle decreases

from about 66- to about 21- as the heat treatment temper-

ature increases from room temperature to 400 -C, and it

reaches about 7- when the heat treatment temperature is 600

-C. The improvement of hydrophilicity is assumed to be

caused by the release of organic contaminants on TiO2

surface [14].

Fig. 2. Images of water drop on the surface of TiO2 films (A) not treated,

(B) heat treated at 400 -C for 30 min, and (C) Ar plasma treated for 1 min.

3.2. Effect of plasma treatment on hydrophilic property of

TiO2 thin films

Fig. 2 is the images of the water drop on the surface of

TiO2 films. Sample 2-Awas untreated, sample 2-B was heat

treated at 400 -C for 30 min, and sample 2-C was treated by

Ar plasma for 1 min. The contact angles of the three

Binding energy (eV)

540 535 530 525

Ar treated for 10 h Ar treated for 1 h

Binding energy (eV)

O 1s (b)

Inte

nsity

(a.

u.)

Fig. 4. XPS spectra of (a) Ti 2p and (b) O 1s for TiO2 films treated by Ar

plasma for 1 h and 10 h, respectively.

J.-B. Han et al. / Surface & Coatings Technology 200 (2006) 4876–48784878

samples are 66-, 21- and 8-, respectively. By comparing

samples 2-B with 2-C, the improvement of hydrophilicity of

TiO2 films is believed mainly due to the plasma treatment,

which also can be proved by the results of XRD described

below.

Fig. 3 shows the contact angles of the water drop on the

surface of TiO2 films as a function of plasma treatment time.

Samples 3-A, 3-B and 3-C were treated by N2, Ar and O2

plasma, respectively. For all the samples, the contact angles

decrease sharply from about 66- to nearly 8- with the

plasma treatment of 1 min. With the increasing of plasma

treatment time, the contact angles of O2 plasma treated

samples decrease rapidly to zero degree, and the contact

angles of Ar and N2 plasma treated samples decrease slowly.

3.3. Structure and binding state of TiO2 films

XRD result reveals that both the as-deposited and plasma

treated TiO2 films are amorphous, and plasma treatment

does not change the crystal phase of the films. When TiO2

films were heat treated at above 500 -C, the peaks of rutile

(101) and rutile (211) were observed.

Fig. 4(a)(b) are the XPS spectra of Ti 2p and O 1s of

TiO2 films treated by Ar plasma for 1 h and 10 h,

respectively. Peaks of Ti 2p3 / 2 are located at 458.5 eV

and 458.3 eV for the two corresponding films, showing that

the peak position shifts 0.2 eV to lower binding energy side.

Ti 2p3 / 2 peak of the sample treated for 10 h is broader than

the one treated for 1 h, indicating the presence of defect sites

[15,16]. As shown in Fig. 4(b), O 1s peaks are located at

529.7 eV for both samples. A shoulder is observed on

higher binding energy side for TiO2 films treated for 10 h,

indicating that the oxygen vacancies are introduced by Ar

plasma treatment [17].

The reaction of different plasma treatment involves three

processes: surface etching, ultraviolet radiation and surface

oxidation [13]. Surface etching caused by the bombardment

of the atoms and activated species cleans and etches the

surface of the films [18]. Ultraviolet radiation and surface

oxidation lead to the generation of oxygen vacancies, which

are favoring the adsorption of dissociative water, therefore

the hydrophilicity of the films is improved. As shown in

Fig. 4, Ar plasma treatment generates oxygen vacancies that

can improve the hydrophilicity of TiO2 films. The mecha-

nism of the improvement of N2 plasma treatment is similar

to that of Ar plasma treatment.

As for the O2 plasma treated samples, the improvement

of hydrophilicity is also due to the oxidation of the oxygen

atoms and activated species which are dissociated in the

plasma. During the treatment, the lattice oxygen is oxidized

to be a neutral O radical. By coupling two neutral radicals,

an O2 molecule is produced, and an oxygen defect is formed

on the surface [19]. Therefore, the hydrophilicity of O2

plasma treated samples is enhanced.

4. Conclusions

TiO2 films prepared by r.f. sputtering technique were

treated by N2, Ar and O2 plasma for 1¨30 min, respectively.

The contact angles of the water drop on the surface of the

samples decrease remarkably from about 66- to nearly 8-with plasma treatment for 1 min. The improvement of

hydrophilicity is mainly due to the function of surface

etching, ultraviolet radiation, and surface oxidation of

plasma treatment. With the increasing of plasma treatment

time, the contact angle of O2 plasma treated samples

decreases rapidly to zero degrees, while the contact angles

of Ar and N2 treated samples decrease slowly, which shows

that the surface oxidation of oxygen atoms and activated

species generated in O2 plasma can improve the hydrophilic

property of samples more efficiently.

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