Available online at www.sciencedirect.com

008) 5860–5863www.elsevier.com/locate/tsf

Thin Solid Films 516 (2

High-rate deposition of photocatalytic TiO2 films by oxygen plasma assistreactive evaporation method

Tetsuya Sakai a,⁎, Yuji Kuniyoshi a, Wataru Aoki b, Sho Ezoe b, Tatsuya Endo b, Yoichi Hoshi a,b

a Tokyo Polytechnic University Graduate School, Graduate School of Engineering, Department of Electric Engineering, Japanb Tokyo Polytechnic University, Faculty of Engineering, Department of Electronics and Information Engineering, Japan

Available online 16 October 2007

Abstract

High-rate deposition of titanium dioxide (TiO2) film was attempted using oxygen plasma assisted reactive evaporation (OPARE) method.Photocatalytic properties of the film were investigated. During the deposition, the substrate temperature was fixed at 400 °C. The film depositionrate can be increased by increasing the supply of titanium atoms to the substrate, although oversupply of the titanium atoms causes oxygendeficiency in the films, which limits the deposition rate. The film structure depends strongly on the supply ratio of oxygen molecules to titaniumatoms O2/Ti and changes from anatase to rutile structure as the O2/Ti supply ratio increased. Consequently, the maximum deposition rates of77.0 nm min−1 and 145.0 nm min−1 were obtained, respectively, for the anatase and rutile film.

Both films deposited at such high rates showed excellent hydrophilicity and organic decomposition performance. Even the film with rutilestructure deposited at 145.0 nm min−1 had a contact angle of less than 2.5° by UV irradiation for 5.0 h and an organics-decompositionperformance index of 8.9 [μmol l−1 min−1] for methylene blue.© 2007 Elsevier B.V. All rights reserved.

Keywords: Titanium dioxide; Photocatalytic; Hydrophilicity; Organics-decomposition; Oxygen plasma assisted reactive evaporation (OPARE) method

1. Introduction

In order to utilize the photocatalytic properties of TiO2 [1],such as ultra-hydrophilicity [2], stain resistance, deodorization,and antimicrobial functions, TiO2 has been coated onto variousmaterials. Both ultra-hydrophilicity and stain resistance prop-erties of the film are used in the side-view mirror of a typicalautomobile. Deodorization properties of the film are nowutilized in air cleaners by coating the films onto filters, tents,tiles, and sound barriers to endow them with an antifoulingfunction, among other applications [3].

High-rate deposition of the photocatalytic TiO2 film byreactive sputtering method is known to be difficult because thesurface of Ti target is oxidized during sputtering and the amountof Ti atoms emitted from the target is decreased significantly

⁎ Corresponding author. 1583, Iiyama, Atsugi-shi, Kanagawa-ken, 243-0297,Japan. Tel.: +81 46 242 9560; fax: +81 46 242 9566.

E-mail addresses: d0685201@st.t-kougei.ac.jp (T. Sakai),hoshi@seit.t-kougei.ac.jp (Y. Hoshi).

0040-6090/$ - see front matter © 2007 Elsevier B.V. All rights reserved.doi:10.1016/j.tsf.2007.10.039

[4]. The deposition rate of the film in conventional reactivesputtering method was less than 10.0 nm min−1. In the OPAREmethod, titanium atoms and oxygen radicals are suppliedindependently to the substrate from an evaporation source and aplasma source, respectively. Therefore, OPARE is thought to bean effective deposition method to realize high-rate deposition ofthe films. We have reported that the films with highphotocatalytic activities and photo-induced hydrophilicitywere deposited using OPARE method [5–13].

In this study, we attempted deposition of the TiO2 films at ahigh rate using the OPARE method. We investigated therelationships between its structure and the photocatalyticperformances of the films. Results showed that the maximumdeposition rate of 77.0 nm min−1 was obtained for thedeposition of the films with anatase structure. The films witha rutile structure were obtained at a higher deposition rate; amaximum deposition rate of 145.0 nm min−1 was realized.

Both films deposited at such high rates showed excellenthydrophilicity and organics-decomposition performance. Eventhe film with rutile structure deposited at 145.0 nm min−1 had acontact angle of less than 2.5° by 5.0 h UV irradiation, in

Fig. 1.

Table 1

Deposition apparatus OPAREBackground pressure [Torr] b7.5×10−6

Substrate for the sample Non-alkali glassSubstrate heating temperature [°C] 400Oxygen gas flow rate [sccm] 50.0Electron beam gun emission [mA] 200–450Electron beam gun voltage [kV] 9Microwave power [W] 600Oxygen plasma accelerating voltage [V] 30Deposition rate [nm min−1] 15.0–145.0Film thickness [nm] 1000

5861T. Sakai et al. / Thin Solid Films 516 (2008) 5860–5863

addition to a methylene blue decomposition performance indexof 8.9 [μmol l−1 min−1]. Experimental details and resultsobtained in this study will be described in the followingsections.

2. Experimental and details

Fig. 1 shows the deposition system used in this study. TheOPARE apparatus consists of a titanium electron beamevaporation source (Ti source), an electron cyclotron resonance(ECR) oxygen plasma source, and a sample holder. The Ti sourcewas divided from the deposition chamber, as shown in Fig. 1 andtitanium atoms are transported from the electron beam evapora-tion source to the substrate through stainless meshes. Thisseparation was effective to improve the stability and reproduc-ibility of the film deposition, especially at high oxygen gas supply.The distance from the substrate to the Ti source and the distancefrom the substrate to the ECR oxygen plasma source are,respectively, 370.0mm and 480.0mm. The sample holder is tiltedto the Ti source at 45.0° as shown in Fig. 1 during deposition.

The deposition chamber was evacuated to a pressure of lessthan 7.5×10−6 Torr using a rotary pump (RP) and turbomolecular pump (TMP). The oxygen gas of 99.999% purity wasintroduced into the chamber at the gas flow rates of 50.0 sccmthrough the ECR plasma source, which made the pressure in thechamber 9.2×10−4 Torr.

Ti chips of 99.99% purity were used as the evaporationmaterial. The deposition rate was changed in the range of 15.0–145.0 nm min−1. The deposition rate was controlled byadjusting the amount of injection electron current of theelectron beam gun. The substrate temperature was maintained at400 °C, where crystallized films were obtained.

Non-alkali sheet glass (No.1737; Matsunami Glass Ind. Co.,Ltd.) was used as the substrate. Typical film preparation conditionsare listed in Table 1.

The film thickness was measured using a surface profiler(3030; Decktak).The crystal structure of the TiO2 film wasevaluated using X-ray diffractometry (XRD, RINT2000/PC;Rigaku Corp.).

The surface morphology of the film was investigated usingfield-emission scanning electron microscopy (FE-SEM, S-5000;Hitachi High-Technologies Corp.). Transmittance of the film wasmeasured using a spectral photometer (V-550; Jasco Inc.).

Hydrophilicity was evaluated by measuring the changes inthe contact angles of pure water using a contact angle measuringsystem (OCA 15plus; Data Physics Corp.). The organics-decomposition characteristics were measured using the changein the methylene blue concentration. In the evaluations, blacklight of main wave length of 352.0 nm was used as the UVsource and the measurement was carried out under the UVirradiation of 1.0 mW cm−2.

3. Results and discussions

3.1. Structure and optical properties of the films

Fig. 2 shows an XRD diagram of TiO2 films deposited atvarious deposition rates. The oxygen gas flow rate was fixed at

Fig. 2. Fig. 4.

5862 T. Sakai et al. / Thin Solid Films 516 (2008) 5860–5863

50.0 sccm. The figure clarifies that the film structure changesfrom an anatase to a rutile structure as the deposition rateincreases. It is noteworthy that the film with a single-phaseanatase structure was obtained at the deposition rate of less than77.0 nm min−1. Further increase in the deposition rateengenders formation of films with a rutile structure; the filmdeposited at 145.0 nm min−1 has a rutile structure. These valuesof deposition rate are more than 10.0 times higher than thoseobtained in a conventional reactive sputtering method. Theseresults suggest that the OPARE method is a promisingdeposition technique to obtain the TiO2 films at a high rate. Itis also noteworthy that both anatase and rutile films areobtainable reproducibly by controlling the OPARE depositionconditions.

Fig. 3 shows the FE-SEM images of the films deposited atvarious deposition rates. The oxygen gas flow rate was fixed at50.0 sccm in the deposition. Grain sizes of about 30.0–150.0 nm are observed in the anatase films obtained atdeposition rates of 18.0–77.0 nm min−1. The rutile filmsobtained at deposition rates of 129.0–145.0 nm min−1 havegrain sizes of 100.0–150.0 nm. The crystallite size of the filmsestimated from the width of X-ray diffraction peaks is less than30.0 nm, which suggests that each grain comprises smallcrystallites. Surface morphology of the film changed little withthe deposition rate, but the surface configuration of the grains inthe film changes from a spherical shape to a notched one, asshown in Fig. 3.

Fig. 3.

Fig. 4 shows the transmittance spectra of films deposited atvarious deposition rates. The oxygen gas flow rate was fixed at50.0 sccm. Even the film deposited at the deposition rate of145.0 nm min−1 has an average transmittance value of greaterthan 70.0% in the visible region, although the absorption edgeshifts to a longer wavelength. This is thought to be caused bythe change in crystal structure from anatase to rutile. That is, theabsorption edge around 380.0 nm originates in the film with theanatase structure, and the absorption edge around 400.0 nm iscaused by the film with the rutile structure.

3.2. Photocatalytic characteristics

Fig. 5 shows the changes in the contact angles of waterdroplets on the film surface deposited at various depositionrates. The contact angles of the films and glass substrate beforeUV irradiation take values in the range from 6.7 to 89.3°. Thecontact angle of the film with anatase structure deposited at77.0 nm min−1 was decreased to less than 2.0° after UVirradiation for 1.0 h, whereas the contact angle of the film withrutile structure deposited at 145.0 nm min−1 was decreased toless than 2.5° after the UV irradiation for 5.0 h. All films

Fig. 5.

Fig. 6.

5863T. Sakai et al. / Thin Solid Films 516 (2008) 5860–5863

deposited below 145.0 nm min−1 showed ultra-hydrophilicity,irrespective of their crystal structure. While on the other hand,the contact angle of glass substrate was not decreased to lessthan 85.0° after the UV irradiation for 24.0 h. The glasssubstrate has no hydrophilicity characteristic.

Fig. 6 shows the changes in the concentrations of methyleneblue (MB) under UV irradiation of the films deposited at variousdeposition rates. The figures clarify that the MB concentrationdecreases continuously, concomitant with the UV irradiationtime up to 3.0 h.

The decomposition performance index is generally used toevaluate the organics-decomposition performance [14]. Theindex is defined as the slope of the MB concentration changesfor the UV irradiation time. It indicates the amount of organicsubstances decomposed in unit time.

The indexes for films deposited at 77.0 nm min−1 and145.0 nmmin−1 were, respectively, 9.7 [μmol l−1 min−1] and 8.9[μmol l−1 min−1]. The film deposited at 54.0 nm min−1 had theindex of 11.4 [μmol l−1 min−1]. While on the other hand, theindex for glass substrate was 1.6 [μmol l−1 min−1]. These resultsindicate that our films, deposited at such a high deposition rate,have similar MB decomposition performance to that of filmsobtained using reactive sputtering method [15–17].

4. Conclusions

In this study, high-rate deposition of TiO2 films wasattempted using OPARE method. In the OPARE method, thefilm structure depends strongly on the supply ratio of O2/Ti tothe substrate: a film with a single-phase anatase structure isobtained at an O2/Ti supply ratio above 40.0; a film with rutilestructure was obtained at an O2/Ti supply ratio below 40.0.Therefore, the maximum deposition rate to obtain a film withanatase structure was much lower than the deposition rate of the

film with the rutile structure because the maximum oxygen gassupply for the deposition chamber was limited to less than50.0 sccm in our deposition system. Consequently, thedeposition rates of 77.0 nm min−1 and 145.0 nm min−1 wereobtained, respectively, for deposition of the anatase film and therutile film.

These films deposited at a high rate showed excellenthydrophilicity and organics-decomposition performance, i.e.,the film had contact angles of water of 2.5° or less by UVirradiation for 5.0 h and a decomposition performance index of11.4 [μmol l−1 min−1].

The results described above indicate that the OPARE methodis effective to obtain TiO2 films with excellent photocatalyticproperties at a high deposition rate.

Acknowledgments

The authors are very grateful to Osamu Kamiya (Guestinvestigator of Tokyo Polytechnic University Graduate School ofEngineering), Yusuke Onai, and Keisuke Ichikawa (Student ofTokyo Polytechnic University Graduate School of Engineering)for encouragement and helpful discussions.

References

[1] A. Fujishima, K. Hond, Nature 238 (1972) 37.[2] T. Kawai, T. Sakuta, Nature 286 (1980) 474.[3] K. Hashimoto, H. Irie, A. Fujishima, Jpn. J. Appl. Phys. 44 (2005) 8269.[4] S. Ichimura, H. Ebisu, T. Nonami, K. Kato, Jpn. J. Appl. Phys. 44 (2005)

5164.[5] P.K. Song, Y. Irie, Y. Shigesato, Thin Solid Films 496 (2006) 121.[6] D. Noguchi, T. Sakai, Y. Nagatomo, Jpn. J. Appl. Phys. 45 (2006) 1775.[7] R.P. Netterfield, W.G. Sainty, P.J. Martin, S.H. Sie, Appl. Opt. 24 (1985)

2267.[8] I. Shimizu, Y. Setsuhara, S. Miyake, H. Saitou, M. Kumagai, Jpn. J. Appl.

Phys. 39 (2000) 4138.[9] I. Shimizu, M. Kumagai, H. Saitou, Y. Setsuhara, Y. Makino, S. Miyake,

Proc. 12th Int. Conf. on Implantation Technology, Kyoto, June 1998.[10] S. Miyake, K. Honda, T. Kohno, Y. Setsuhara, A. Chayahara, M. Satou,

J. Vac. Sci. Technol. A 10 (1992) 3253.[11] I. Shimizu, Y. Setsuhara, S. Miyake, J. Musil, H. Saitou, M. Kumagai, Jpn.

J. Appl. Phys. 42 (2003) 634.[12] S. Ohno, D. Sato, M. Kon, Y. Sato, M. Yoshikawa, P. Frach, Y. Shigesato,

Jpn. J. Appl. Phys. 43 (2004) 8234.[13] T. Sakai, Y. Kuniyoshi, W. Aoki, S. Ezoe, T. Endo, Y. Hoshi, (13th

Winter Meet. 2006) Photocatalytic Symposium P-8.[14] Y. Katsumata, S. Abe, (8th Winter Meet 2001) Photocatalytic Symposium

D-01.[15] D. Noguchi, Y. Kawamata, Y. Nagatomo, Jpn. J. Appl. Phys. 42 (2003)

5255.[16] D. Noguchi, Y. Kawamata, Y. Nagatomo, Jpn. J. Appl. Phys. 43 (2004)

1581.[17] D. Noguchi, Y. Kawamata, Y. Nagatomo, Jpn. J. Appl. Phys. 43 (2004)

4351.