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Effect of plasma treatment on hydrophilic properties of TiO2 thin films
Surface & Coatings Technology 200 (2006) 4876 – 4878 www.elsevier.com/locate/surfcoat
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,* b
a Department of Physics, Wuhan University, Wuhan 430072, PR China Center 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 molecules on surface, Wang et al. [3] reported that ultraviolet illumination to TiO2 surface could produce a highly hydrophilic 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
* Corresponding author. Department of Physics, Wuhan University, Wuhan 430072, PR China. E-mail address: [email protected] (Q.-Q. Wang). 0257-8972/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2005.04.036
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-
J.-B. Han et al. / Surface & Coatings Technology 200 (2006) 4876 – 4878 80
Contact angle (degree)
Contact angle (degree)
80
4877
60
40
20
0
60
A
N2
B C
Ar O2
40
20
0
0
100
200
300
400
500
600
0
10
20
30
Plasma treatment time (min)
Temperature (°C) Fig. 1. Contact angle of water drop on the surface of TiO2 films as a function of heat treatment temperature.
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.
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 photoelectron spectroscopy (XPS).
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-A was 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
3. Results and discussion Ar plasma treated for 10 h Ar plasma treated for 1 h Ti 2p3/2
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 temperature 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].
Intensity (a.u.)
3.1. Effect of heat treatment on hydrophilic property of TiO2 thin films
(a)
Ti 2p1/2
470
465
460
455
450
Binding energy (eV)
(b)
O 1s
Intensity (a.u.)
Ar treated for 10 h Ar treated for 1 h
540
535
530
525
Binding energy (eV) 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.
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.
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J.-B. Han et al. / Surface & Coatings Technology 200 (2006) 4876 – 4878
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 mechanism 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 8with 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.
References [1] K. Honda, A. Fujishima, Nature 238 (1972) 37. [2] B. O’Regan, M. Gra¨tzel, Nature 353 (1991) 737. [3] R. Wang, K. Hashimoto, A. Fujishima, M. Chikuni, E. Kojima, M. Shimohigoshi, T. Watanabe, Nature 388 (1997) 431. [4] T. Watanabe, A. Nakajima, R. Wang, M. Minabe, S. Koizumi, A. Fujishima, K. Hashimoto, Thin Solid Films 351 (1999) 260. [5] M. Nakamura, T. Aoki, Y. Hatanaka, Vacuum 59 (2000) 506. [6] Y.C. Lee, Y.P. Hong, H.Y. Lee, H. Kim, Y.J. Jung, K.H. Ko, H.S. Jung, K.S. Hong, J. Colloid Interface Sci. 267 (2003) 127. [7] Q.J. Liu, J.L. Li, Y.X. Jin, Q.H. Wang, X.H. Wu, J. Funct. Mater. Dev. 9 (2003) 391. [8] K.S. Guan, B.J. Lu, Y.S. Yin, Surf. Coat. Technol. 173 (2003) 219. [9] M. Machida, K. Norimoto, T. Watanabe, J. Mater. Sci. 34 (1999) 2569. [10] J.G. Yu, X.J. Zhao, Mater. Res. Bull. 36 (2001) 97. [11] M. Takeuchi, Y. Onozaki, Y. Matsumura, H. Uchida, T. Kuji, Nucl. Instrum. Methods Phys. Res., B Beam Interact. Mater. Atoms 206 (2003) 259. [12] C.K. Jung, S.H. Cho, S.B. Lee, T.K. Kim, M.N. Lee, J.H. Boo, Surf. Rev. Lett. 10 (2003) 635. [13] J.H. Choi, E.L. Lee, H.K. Baik, S.J. Lee, K.M. Song, M.K. Hwang, C.S. Huh, Surf. Coat. Technol. 171 (2003) 257. [14] H.Y. Yin, Z.S. Jin, S.L. Zhang, S.B. Wang, Z.J. Zhang, Sci. China, Ser B Chem. 32 (2002) 413. [15] V.E. Henrich, G. Dresselhaus, H.J. Zeiger, Solid State Commun. 24 (1977) 623. [16] L.Q. Wang, D.R. Baer, M.H. Engelhard, Surf. Sci. 320 (1994) 295. [17] K. Ishibashi, Y. Nosaka, K. Hashimoto, A. Fujishima, J. Phys. Chem., B 102 (1998) 2117. [18] M.C. Kim, D.K. Song, H.S. Shin, S.H. Baeg, G.S. Kim, J.H. Boo, J.G. Han, S.H. Yang, Surf. Coat. Technol. 171 (2003) 312. [19] N. Sakai, A. Fujishima, T. Watanabe, K. Hashimoto, J. Phys. Chem., B 105 (2001) 3023.
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,* b
a Department of Physics, Wuhan University, Wuhan 430072, PR China Center 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 molecules on surface, Wang et al. [3] reported that ultraviolet illumination to TiO2 surface could produce a highly hydrophilic 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
* Corresponding author. Department of Physics, Wuhan University, Wuhan 430072, PR China. E-mail address: [email protected] (Q.-Q. Wang). 0257-8972/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2005.04.036
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-
J.-B. Han et al. / Surface & Coatings Technology 200 (2006) 4876 – 4878 80
Contact angle (degree)
Contact angle (degree)
80
4877
60
40
20
0
60
A
N2
B C
Ar O2
40
20
0
0
100
200
300
400
500
600
0
10
20
30
Plasma treatment time (min)
Temperature (°C) Fig. 1. Contact angle of water drop on the surface of TiO2 films as a function of heat treatment temperature.
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.
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 photoelectron spectroscopy (XPS).
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-A was 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
3. Results and discussion Ar plasma treated for 10 h Ar plasma treated for 1 h Ti 2p3/2
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 temperature 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].
Intensity (a.u.)
3.1. Effect of heat treatment on hydrophilic property of TiO2 thin films
(a)
Ti 2p1/2
470
465
460
455
450
Binding energy (eV)
(b)
O 1s
Intensity (a.u.)
Ar treated for 10 h Ar treated for 1 h
540
535
530
525
Binding energy (eV) 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.
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.
4878
J.-B. Han et al. / Surface & Coatings Technology 200 (2006) 4876 – 4878
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 mechanism 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 8with 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.
References [1] K. Honda, A. Fujishima, Nature 238 (1972) 37. [2] B. O’Regan, M. Gra¨tzel, Nature 353 (1991) 737. [3] R. Wang, K. Hashimoto, A. Fujishima, M. Chikuni, E. Kojima, M. Shimohigoshi, T. Watanabe, Nature 388 (1997) 431. [4] T. Watanabe, A. Nakajima, R. Wang, M. Minabe, S. Koizumi, A. Fujishima, K. Hashimoto, Thin Solid Films 351 (1999) 260. [5] M. Nakamura, T. Aoki, Y. Hatanaka, Vacuum 59 (2000) 506. [6] Y.C. Lee, Y.P. Hong, H.Y. Lee, H. Kim, Y.J. Jung, K.H. Ko, H.S. Jung, K.S. Hong, J. Colloid Interface Sci. 267 (2003) 127. [7] Q.J. Liu, J.L. Li, Y.X. Jin, Q.H. Wang, X.H. Wu, J. Funct. Mater. Dev. 9 (2003) 391. [8] K.S. Guan, B.J. Lu, Y.S. Yin, Surf. Coat. Technol. 173 (2003) 219. [9] M. Machida, K. Norimoto, T. Watanabe, J. Mater. Sci. 34 (1999) 2569. [10] J.G. Yu, X.J. Zhao, Mater. Res. Bull. 36 (2001) 97. [11] M. Takeuchi, Y. Onozaki, Y. Matsumura, H. Uchida, T. Kuji, Nucl. Instrum. Methods Phys. Res., B Beam Interact. Mater. Atoms 206 (2003) 259. [12] C.K. Jung, S.H. Cho, S.B. Lee, T.K. Kim, M.N. Lee, J.H. Boo, Surf. Rev. Lett. 10 (2003) 635. [13] J.H. Choi, E.L. Lee, H.K. Baik, S.J. Lee, K.M. Song, M.K. Hwang, C.S. Huh, Surf. Coat. Technol. 171 (2003) 257. [14] H.Y. Yin, Z.S. Jin, S.L. Zhang, S.B. Wang, Z.J. Zhang, Sci. China, Ser B Chem. 32 (2002) 413. [15] V.E. Henrich, G. Dresselhaus, H.J. Zeiger, Solid State Commun. 24 (1977) 623. [16] L.Q. Wang, D.R. Baer, M.H. Engelhard, Surf. Sci. 320 (1994) 295. [17] K. Ishibashi, Y. Nosaka, K. Hashimoto, A. Fujishima, J. Phys. Chem., B 102 (1998) 2117. [18] M.C. Kim, D.K. Song, H.S. Shin, S.H. Baeg, G.S. Kim, J.H. Boo, J.G. Han, S.H. Yang, Surf. Coat. Technol. 171 (2003) 312. [19] N. Sakai, A. Fujishima, T. Watanabe, K. Hashimoto, J. Phys. Chem., B 105 (2001) 3023.