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Photocatalytic TiO2 thin film deposited onto glass by DC magnetron sputtering
Thin Solid Films 392 Ž2001. 338᎐344
Photocatalytic TiO 2 thin film deposited onto glass by DC magnetron sputtering Satoshi Takedaa,U , Susumu Suzuki a , Hidefumi Odakaa , Hideo Hosono b b
a Research Center, Asahi Glass Co., Ltd, 1150 Hazawa-cho, Kanagawa-ku, Yokohama 221-8755, Japan Materials and Structures Laboratory, Tokyo Institute of Technology, 4259 Nagatsuda-cho, Midori-ku, Yokohama 226-8503, Japan
Abstract A high performance photocatalytic TiO 2 thin film was successfully obtained by reactive DC magnetron sputtering. The film was deposited onto SiO 2-coated glass at a substrate temperature of 220⬚C using a titanium metal target in O 2 100% atmosphere. The film showed good uniformity of thickness in a large area with the optical transmittance of ; 80% in the visible region. The decomposition ability of acetaldehyde ŽCH 3 CHO. of the film under UV irradiation was almost the same as that of the sol᎐gel-derived TiO 2 thin film but the sputtered film showed a much higher mechanical durability. The characterization of the films was carried out using XRD, SEM, AFM, XPS and SIMS, and the electronic structures of the films were calculated using a first-principle calculation method based on the density functional theory. It was found that the amount of incorporated 18 O into the film was larger for the films with lower photocatalytic activity when the films were annealed in 18 O 2rN2 atmosphere. This result indicates that the amount of oxygen vacancies, which were occupied by incorporated 18 O, was larger for the films with lower photocatalytic activity. Furthermore, the introduction of structural defects associated with oxygen vacancies was found to create some energy levels around the mid-gap, indicating that they could work as recombination centers of photo-induced holes and electrons, causing the decrease in photocatalytic activity. Therefore, the decrease in the structural defects associated with oxygen vacancies is important for improving the photocatalytic activity of the films. 䊚 2001 Elsevier Science B.V. All rights reserved. Keywords: TiO 2 ; Photocatalytic activity; Sputtering; Oxygen vacancy
1. Introduction Titanium oxide ŽTiO 2 . thin films are widely used in various fields such as optical and protective coatings and optical fibers because of their excellent chemical stability, mechanical hardness and optical transmittance with high refractive index. Recently, TiO 2 thin films have become the most promising materials in environmental cleaning such as photocatalytic purifier w1,2x and photochemical solar cells w3,4x. Many researchers have focused on the application of TiO 2 photocatalyst to purification and treatment of air and U
Corresponding author. Tel.: q81-45-374-8794; fax: q81-45-3748892. E-mail address: [email protected] ŽS. Takeda..
water, e.g. through the photolysis of organics and toxic gases w2x. TiO 2 thin films have been prepared by a variety of deposition techniques such as sol᎐gel processes w5x, chemical vapor deposition w6,7x, evaporation w8x, various sputtering depositions w9᎐11x, and ion beam-assisted processes w12x. However, most of the photocatalytic TiO 2 thin films used in the market are prepared by wet process such as a sol᎐gel method. Although the films show excellent photocatalytic activity, the mechanical durability is not enough for practical uses such as architectural or automotive glasses. In addition, the uniformity of the thickness in a large area is poor. These points are not desirable for architectural or automotive applications. Furthermore, a heating process at approximately 500᎐600⬚C is indispensable in the method in order to decompose metal᎐organic,
0040-6090r01r$ - see front matter 䊚 2001 Elsevier Science B.V. All rights reserved. PII: S 0 0 4 0 - 6 0 9 0 Ž 0 1 . 0 1 0 5 4 - 9
S. Takeda et al. r Thin Solid Films 392 (2001) 338᎐344
causing a limitation of the use of non-refractory substrates. In the present study, the sputtering method was applied to obtain a photocatalytic TiO 2 thin film on glass. Part of this work has recently been reported w13x. Sputtering methods are widely used in industrial products because the high quality films; high density, high adhesion, high hardness, etc., can be obtained at low substrate temperature with good uniformity of the film thickness in a large area. Unfortunately, the photocatalytic activity of the sputtered film used for architectural glass is much lower than that of the films prepared by the wet process. Weinberger and Gerber w14x reported that a photocatalytic TiO 2 film with a thickness of ; 10 m was obtained by reactive RF magnetron sputtering, and that the film showed high photocatalytic activity because of its porous columnar morphology, resulting in a high surface area. However, the optical transmittance of the film may be low due to the large thickness. To our knowledge, a high performance photocatalytic TiO 2 thin film with both a high mechanical durability and transparency in the visible region has not been reported so far. Here, the TiO 2 thin film was deposited by DC magnetron sputtering onto glass at a substrate temperature of 220⬚C using a titanium metal target in a 100% O 2 atmosphere. From an industrial point of view, DC sputtering is desirable for the large area coating because it is more simple technology than RF sputtering and it gives a higher deposition rate. The characterization of the film was carried out using X-ray diffraction ŽXRD., scanning electron microscope ŽSEM., atomic force microscopy ŽAFM. and secondary ion mass spectrometry ŽSIMS.. The electronic structure of the film was calculating using a first-principle calculation method based on the density functional theory. The photocatalytic performance of the film was evaluated by the amount of decomposition of acetaldehyde ŽCH 3 CHO. as a function of UV irradiation time. From the results obtained, the relationship between the photocatalytic performance and the structure of the film was discussed. 2. Experimental details
339
Table 1 The sputtering conditions of TiO 2 thin films Substrate temperature ŽTs . Ž⬚C.
Target
Sputtering gas
Sputtering pressure Ž P . ŽPa.
Room temperature 140 220
Ti Ti Ti
O2 s 100% O2 s 100% O2 s 100%
0.4 0.4 2.0
Žpurity 99.9%. of 10 cm diameter. The deposition rate of the film was ; 4 nmrmin. The discharge during the deposition was stable. The TiO 2 thin film was also prepared by a sol᎐gel method. Titanium n-butoxide wTi ŽOC 4 H 9 .4 , TBTx was used as a precursor. TBT was dissolved in ethanol and acetylacetone. The SiO 2-coated glass was coated with this solution by the spin-coating process and then heated to 550⬚C for 30 min in an electric furnace. 2.2. Film properties The photocatalytic performance of the films was evaluated by measuring the amount of decomposed acetaldehyde ŽCH 3 CHO. by UV irradiation. UV light was irradiated using a black light Žcentral wavelength ; 352 nm. with ; 2 mWrcm2 . The experimental apparatus is schematically illustrated in Fig. 1. The TiO 2 film coated on a 45 = 100-mm2 substrate was set in a Pyrex vessel, and CH 3 CHO solution was injected in the vessel. The injected CH 3 CHO was vaporized immediately, and the concentration of the CH 3 CHO vapor was adjusted at 750 " 10 ppm. Thereafter, the change in CH 3 CHO concentration was measured using a gas detector ŽGASTEC. as a function of UV irradiation time. The accuracy of the measurements was "10 ppm. The mechanical durability of the films was evaluated by a Taber abrasion tester ŽTeledyne Taber 503.. The tests were carried out using CS-10F abrasion wheels loaded with 500 g. The degree of degradation was evaluated by the change in the haze value. The haze value, H, which is defined by TdrTt = 100% ŽTd , scattered light; Tt , transmitted light.. The haze value
2.1. Sample preparation SiO 2 coated soda᎐lime᎐silica glass was used as a substrate. The SiO 2 film was deposited onto the glass by reactive RF magnetron sputtering with a thickness of ; 50 nm in order to suppress alkali migration from the glass substrate. TiO 2 thin films were deposited by reactive DC magnetron sputtering with a thickness of ; 200 nm. The sputtering conditions are listed in Table 1. The base pressure in the coating chamber was less than 1.3= 10y3 Pa. The target was a titanium metal
Fig. 1. Schematic illustration of experimental apparatus for the measurement of photocatalytic activity.
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S. Takeda et al. r Thin Solid Films 392 (2001) 338᎐344
change, ⌬ H which is defined by H Ž N . y H Ž0.; N denotes the number of cycles. The optical transmission spectra of the films were measured at room temperature in air using a dual beam spectrometer ŽShimazu UV3100.. Observation of surface morphology and the quantitative analysis of the surface roughness of the films were performed using SEM ŽHitachi S-900. and AFM ŽSeiko SPI 3700., respectively. Crystalline phases of the films were identified by the glancing-angle XRD ŽRigaku Rint-2000. using Cu-K ␣ radiation operated at 50 kV᎐200 mA. The incident angle was kept at 0.5⬚. The stoichiometry of the films was firstly determined by X-ray photoelectron spectroscopy ŽXPS.. However, no difference in OrTi ratio among the films was observed. In order to obtain more accurate information about the stoichiometry, SIMS analyses with oxygen tracer Ž 18 O. gas were performed. The SIMS is widely used in various fields such as semiconductors to obtain detailed information for in-depth distribution of impurities or dopants in the materials because of its excellent sensitivity and high-depth resolution compared with XPS. In this analysis, the films were heat-treated at 500⬚C for 1 h in 18 O 2rN2 s 1r4 atmosphere, and thereafter the amount of incorporated 18 O was measured by SIMS ŽPHI Adept1010.. The Csq primary ion beam was operated at 5 keV, 200 nA and rastered on the area of 300 = 300 m2 . The angle of incidence was 60⬚ to the normal of the sample surface. The charge neutralization was accomplished using an electron flood gun. The electronic structure of the films was calculated using a first-principle calculation method based on the density functional theory w15,16x. The calculations were performed using VASP programs w17᎐20x. Two types of oxygen vacancy in anatase TiO 2 were simulated by Ž2 = 2 = 1. supercell model containing 48 atoms. One was that a single oxygen atom was removed from a Ti atom in a perfect anatase crystal Žsingle oxygen vacancy model., causing the decrease in the coordination numbers of oxygen around the titanium ŽTi. atom in the supercell Žthere are three fivefold-coordinated Ti atoms.. The other was that two oxygen atoms were removed from a certain Ti atom Ždouble oxygen vacancies model., resulting in the formation of two fourfoldcoordinated and four fivefold-coordinated Ti atoms in
Fig. 2. Change in concentration of CH 3 CHO as a function of UV irradiated time by the films obtained by sol᎐gel and sputtering method.
the supercell. The geometries of those vacancies were optimized by a conjugated gradient technique in a minimization of the Kohn᎐Sham energy functional. Ultra soft Vanderbilt-type pseudo-potentials w21x supplied by Kresse and Hafner w22x were used, where Ti 3s, 3p, 3d and 4s states and oxygen 2s and 2p states were treated as valence electrons. A cut-off of 450 eV was used for the valence electron wave functions. The total energy was calculated using a local density approximation for the exchange and correlation energy with the Ceperley and Alder form of the exchange᎐correlation potential w23x. Density of states were obtained by smearing eigenvalues of the ⌫ point.
3. Results
3.1. Photocatalytic performance
Fig. 2 shows the change in concentration of CH 3 CHO by TiO 2 thin films as a function of UV irradiation time. It is found that the decomposition ability of CH 3 CHO for the sputtered film deposited at room temperature ŽTs s r.t.. is the same as that of the sol᎐gel film without UV irradiation, indicating that the photocatalytic activity of Ts s r.t. is almost zero. The decomposition ability
Table 2 Mechanical durability of the sol᎐gel-derived TiO 2 thin film and the sputtered TiO 2 thin film deposited at Ts s 220⬚C, P s 2.0 Pa Sample
⌬ H Ž N s 100. Ž%.
⌬ H Ž N s 200. Ž%.
⌬ H Ž N s 300. Ž%.
Sputtered TiO2 ŽTs s 220⬚C, Ps 2.0 Pa. Sol᎐gel-derived TiO2
4.4
7.2
9.5
Delaminated
Delaminated
Delaminated
S. Takeda et al. r Thin Solid Films 392 (2001) 338᎐344
341
Fig. 3. Transmission spectra of the films obtained by sol᎐gel and sputtering method.
increases with increasing Ts . However, the ability of the film deposited at Ts s 140⬚C is still lower than that of the sol᎐gel film. The ability of the film deposited at Ts s 220⬚C significantly increases, and its ability is almost the same as that of the sol᎐gel film. 3.2. Mechanical durability Table 2 shows the results of the Taber test for the sol᎐gel film and the sputtered film of Ts s 220⬚C. It is seen that the sol᎐gel film is delaminated after the rubbing of 100 cycles. On the other hand, the sputtered film is not delaminated by the rubbing of 300 cycles. This result indicates that the mechanical durability of the sputtered films is much higher than that of the sol᎐gel film although the photocatalytic activity of the films is almost the same. 3.3. Characterization Fig. 3 shows the transmission spectra of the sol᎐gel film and the sputtered film of Ts s 220⬚C. The average transmittance of the sputtered film is ; 80% in the visible region. The uniformity of thickness for the sputtered film in a large area was also better than that of the sol᎐gel film. These results indicate that the film has a possibility to use it for architectural or automotive glasses. Fig. 4 shows the SEM photographs of the sol᎐gel film and the sputtered films deposited at different Ts . The surface morphology of the sputtered film is clearly different with and without the substrate heating. The grain size of the crystalline increases with increasing Ts . This is because Ti particles sputtered from the target effectively react with oxygen atoms on the substrate due to a thermal effect, promoting the crystalline growth. On the other hand, the surface morphology is found to be different between the sputtered film and the sol᎐gel film. No significant relationship was observed between the decomposition ability of
Fig. 4. SEM images of the films obtained by sol᎐gel and sputtering method.
CH 3 CHO and the root-mean-square roughness Žr.m.s.. estimated from the AFM measurements for the sputtered and sol᎐gel film surfaces. Fig. 5 shows the 2⌰ X-ray diffraction patterns of the sol᎐gel film and the sputtered films. There is no peak for the sputtered film of Ts s r.t., indicating that the film is amorphous. On the other hand, several peaks are clearly observed for the sputtered films of Ts s 140 and 220⬚C although the photocatalytic activity is different between them, as shown in Fig. 3. These peaks
Fig. 5. 2⌰ X-ray diffraction patterns for the films obtained by sol᎐gel methods and sputtering methods.
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could be assigned to the anatase structure of TiO 2 without other phases. The XRD pattern of the sol᎐gel film is almost the same as that of the sputtered film with substrate heating. Fig. 6 shows the SIMS depth profiles for the films heat-treated in 18 O 2rN2 atmosphere. Before the heat treatment, the secondary ion intensity of 133 Cs18 Oq was less than 10 2 counts for all the films. After the heat treatment, the intensity of 133 Cs18 Oq increases for all the films although no significant change is observed for the intensity of 133 Cs16 Oq before and after the heat treatment, indicating that 18 O is incorporated into the films. The amount of incorporated 18 O is approximately two orders of magnitude larger than that before the heat treatment. It is found that the amount of incorporated 18 O is different among the films, and that the amount of incorporated 18 O is larger for the film with lower decomposition ability of CH 3 CHO than that of the film with higher ability. The incorporation of 18 O was not observed for a single crystal of rutile TiO 2 , which is a stoichiometiric TiO 2 , indicating that 16 O᎐ 18 O isotopic exchange reaction is negligible during the heat treatment. These results suggest that the incorporated 18 O occupies an oxygen vacancy site, and that the amount of incorporated 18 O
Fig. 6. SIMS depth profiles for the films heat-treated at 500⬚C for 1 h in 18 O 2 atmosphere.
may represent the amount of oxygen vacancies in the film. Namely, the decomposition ability of CH 3 CHO, the photocatalytic activity, of the film is seen to decrease with increasing the oxygen vacancies in the film. Fig. 7 shows the density of states ŽDOS. for Ža. perfect model; Žb. single oxygen vacancy model; and Žc. double oxygen vacancies model obtained from the first-principle calculations. It is found that no energy levels ap-
Fig. 7. Density of states obtained from first-principle calculations based on the density functional theory for Ža. perfect model; Žb. single oxygen vacancy model; and Žc. double oxygen vacancies model.
S. Takeda et al. r Thin Solid Films 392 (2001) 338᎐344
pear at the mid-gap when the single oxygen atom is simply removed from a perfect anatase crystal, as shown in Fig. 7b. On the other hand, some energy levels pointed out by an arrow are clearly observed at the mid-gap in the double oxygen vacancies model, as shown in Fig. 7Žc.. 4. Discussion As mentioned before, there was no relationship between the photocatalytic performance and the surface roughness Žr.m.s.. of the films. This means that the surface roughness is not a major factor governing the photocatalytic performance of the films. As shown in Fig. 6, the amount of incorporated 18 O is larger for the film with lower photocatalytic activity than that of the film with higher photocatalytic activity. This result indicates that the amount of incorporated 18 O may represent the amount of oxygen vacancies in the film, and that the photocatalytic activity decreases with increasing oxygen vacancies in the films, as mentioned above. According to Takeuchi w24x, the generation of oxygen vacancies of the sputtered TiO 2 films was related to the reduction of TiO 2 during the sputtering process. In order to investigate how the oxygen vacancy affects the photocatalytic activity of the film, the electronic structure of the film was calculated using a first-principle calculation method. As shown in Fig. 7b, no energy level appears at the mid-gap when a single oxygen atom is simply removed from a perfect anatase crystal. On the other hand, some energy levels are clearly observed around the mid-gap in the double oxygen vacancies model, as shown in Fig. 7c. These results suggest that the increase in oxygen vacancies, which causes the locally decrease in the coordination numbers of oxygen around the Ti atom from 6 to 4᎐5, can create some energy levels around the mid-gaps. It is known that TiO 2 is a semiconductor with a band gap of ; 3.0 eV, and that UV light with wavelengths shorter than ; 400 nm can excite pairs of electrons and holes. The photo-generated electrons react with molecular oxygen ŽO 2 . to produce superoxide radical . anions Ž⭈Oy 2 , and the photo-generated holes react with water to produce hydroxyl Ž⭈OH. radicals. These reactive radicals then work together to decompose organic materials. If an energy level is present at the band-gap, the photo-generated electrons and holes are considered to be recombinated. Consequently, the probability of the formation of ⭈Oy 2 and ⭈OH radicals decreases, leading to the decrease in photocatalytic activity. Taking this into consideration, the formation of some energy levels around the mid-gap, as shown in Fig. 7c, can cause the decrease in photocatalytic activity because they can work as recombination centers of
343
photo-generated holes and electrons. The change in coordination structure of Ti is induced by the structural defects associated with oxygen vacancies. Namely, there is a possibility that the sputtered TiO 2 thin film has many more structural defects associated with oxygen vacancies than a perfect crystal, resulting in the decrease in the coordination numbers of oxygen around a Ti atom. Therefore, it is important to decrease the structural defects associated with oxygen vacancies in the film for improving the photocatalytic activity. Takeuchi w24x reported that the structural defects associated with oxygen vacancies of the sputtered TiO 2 film decreased with increasing the oxygen concentration in the sputtering gas. This result suggests that controlling the oxygen concentration during the sputtering process is a key parameter for improving the photocatalytic activity. Furthermore, the decomposition ability of CH 3 CHO is higher for the crystallized film than that of the amorphous film, as shown in Figs. 2 and 5. It is known that there are some energy levels at the band-gap for the amorphous film because of its structural disorder. This means that the photo-generated electrons and holes are recombinated, as mentioned above. Consequently, the photocatalytic activity of the amorphous film decreases compared with that of the crystallized film. In addition, the crystalline structure of the crystallized film is found to be the anatase form. The anatase form is known to exhibit excellent photocatalytic activity compared with the rutile form w25x. Namely, the formation of anatase form is also important for improving the photocatalytic activity of the film.
5. Conclusions
A photocatalytic TiO 2 thin film with high mechanical durability and transparency in the visible region has successfully been obtained by reactive DC magnetron sputtering. The film showed good uniformity of thickness over a large area with the optical transmittance of ; 80% in the visible region. The mechanical durability of the film was much higher than that of sol᎐gelderived TiO 2 thin films. The photocatalytic activity of the film was almost the same as that of the sol᎐gel films. The film was characterized by various analytical methods. The electronic structure of the film was also calculated using a first-principle calculation method. It was found that the structural defects associated with the oxygen vacancies and the crystalline structure played an important role for the photocatalytic performance of the films. Therefore, controlling these factors is important for improving the photocatalytic activity of the films.
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References w1x I. Sopyan, S. Murasawa, K. Hashimoto, A. Fujishima, in: D.E. Ollis, H. Al-Ekabi ŽEds.., Photocatalytic Purification and Treatment of Water and Air, Elsevier, Amsterdam, 1993, p. 747. w2x D.E. Ollis, H. Al-Ekabi ŽEds.., Proc. 1st Int. Conf. TiO 2 Photocatalytic Purification and Treatment of Water and Air, Elsevier, Amsterdam, 1993, p. 747. w3x A. Fujishima, K. Honda, Nature 238 Ž1972. 37. w4x M. Graetzel, Comments Inorg. Chem. 12 Ž1991. 93. w5x T. Watanabe, A. Kitamura, E. Kojima, C. Nakayama, K. Hashimoto, A. Fujishima, Chem. Lett. Ž1994. 723. w6x G.A. Battiston, R. Gerbasi, M. Porchia, A. Marigo, Thin Solid Films 239 Ž1994. 186. w7x L.M. Williams, D.W. Hess, J. Vac. Sci. Technol. A1 Ž1983. 1810. w8x T. Fujii, N. Sakata, J. Takada, Y. Miura, Y. Daitoh, J. Mater. Res. 9 Ž1994. 1468. w9x H. Tang, K. Prasad, R. Sanjines, P.E. Schmid, F. Levy, J. Appl. Phys. 75 Ž1994. 2042.
w10x S. Ben Amor, G. Baud, J.P. Besse, M. Jacqet, Thin Solid Films 293 Ž1997. 163. w11x N. Martin, C. Rousselot, D. Rondot, F. Palmino, Thin Solid Films 300 Ž1997. 113. w12x M. Gilo, N. Croitoru, Thin Solid Films 283 Ž1996. 84. w13x S. Suzuki, S. Takeda, Sputtering Plasma Process. 14 Ž3. Ž1999. 33 Žin Japanese.. w14x B.R. Weinberger, R.B. Garber, Appl. Phys. Lett. 66 Ž1995. 2409. w15x P. Hohenberg, W. Kohn, Phys. Rev. 136 Ž1964. B864. w16x W. Kohn, L.J. Sham, Phys. Rev. 140 Ž1965. A1133. w17x G. Kresse, J. Hafner, Phys. Rev. B 47 Ž1993. 58. w18x G. Kresse, Thesis, Technische Universituat ¨ at Wien, 1993. w19x G. Kresse, J. Furthmuuller, Compt. Mat. Sci. 6 Ž1996. 15. ¨ w20x G. Kresse, J. Furthmuuller, Phys. Rev. B. 54 Ž1996. 11169. ¨ w21x D. Vanderbilt, Phys. Rev. B 41 Ž1990. 7892. w22x G. Kresse, J. Hafner, J. Phys. Condens. Mater. 6 Ž1994. 8245. w23x D.M. Ceperley, B.J. Alder, Phys. Rev. Lett. 45 Ž1980. 566. w24x M. Takeuchi, Phys. Stat. Solid Ža. 55 Ž1979. 653. w25x S.N. Frank, A.J. Bard, J. Am. Chem. Soc. 99 Ž1977. 303.
Photocatalytic TiO 2 thin film deposited onto glass by DC magnetron sputtering Satoshi Takedaa,U , Susumu Suzuki a , Hidefumi Odakaa , Hideo Hosono b b
a Research Center, Asahi Glass Co., Ltd, 1150 Hazawa-cho, Kanagawa-ku, Yokohama 221-8755, Japan Materials and Structures Laboratory, Tokyo Institute of Technology, 4259 Nagatsuda-cho, Midori-ku, Yokohama 226-8503, Japan
Abstract A high performance photocatalytic TiO 2 thin film was successfully obtained by reactive DC magnetron sputtering. The film was deposited onto SiO 2-coated glass at a substrate temperature of 220⬚C using a titanium metal target in O 2 100% atmosphere. The film showed good uniformity of thickness in a large area with the optical transmittance of ; 80% in the visible region. The decomposition ability of acetaldehyde ŽCH 3 CHO. of the film under UV irradiation was almost the same as that of the sol᎐gel-derived TiO 2 thin film but the sputtered film showed a much higher mechanical durability. The characterization of the films was carried out using XRD, SEM, AFM, XPS and SIMS, and the electronic structures of the films were calculated using a first-principle calculation method based on the density functional theory. It was found that the amount of incorporated 18 O into the film was larger for the films with lower photocatalytic activity when the films were annealed in 18 O 2rN2 atmosphere. This result indicates that the amount of oxygen vacancies, which were occupied by incorporated 18 O, was larger for the films with lower photocatalytic activity. Furthermore, the introduction of structural defects associated with oxygen vacancies was found to create some energy levels around the mid-gap, indicating that they could work as recombination centers of photo-induced holes and electrons, causing the decrease in photocatalytic activity. Therefore, the decrease in the structural defects associated with oxygen vacancies is important for improving the photocatalytic activity of the films. 䊚 2001 Elsevier Science B.V. All rights reserved. Keywords: TiO 2 ; Photocatalytic activity; Sputtering; Oxygen vacancy
1. Introduction Titanium oxide ŽTiO 2 . thin films are widely used in various fields such as optical and protective coatings and optical fibers because of their excellent chemical stability, mechanical hardness and optical transmittance with high refractive index. Recently, TiO 2 thin films have become the most promising materials in environmental cleaning such as photocatalytic purifier w1,2x and photochemical solar cells w3,4x. Many researchers have focused on the application of TiO 2 photocatalyst to purification and treatment of air and U
Corresponding author. Tel.: q81-45-374-8794; fax: q81-45-3748892. E-mail address: [email protected] ŽS. Takeda..
water, e.g. through the photolysis of organics and toxic gases w2x. TiO 2 thin films have been prepared by a variety of deposition techniques such as sol᎐gel processes w5x, chemical vapor deposition w6,7x, evaporation w8x, various sputtering depositions w9᎐11x, and ion beam-assisted processes w12x. However, most of the photocatalytic TiO 2 thin films used in the market are prepared by wet process such as a sol᎐gel method. Although the films show excellent photocatalytic activity, the mechanical durability is not enough for practical uses such as architectural or automotive glasses. In addition, the uniformity of the thickness in a large area is poor. These points are not desirable for architectural or automotive applications. Furthermore, a heating process at approximately 500᎐600⬚C is indispensable in the method in order to decompose metal᎐organic,
0040-6090r01r$ - see front matter 䊚 2001 Elsevier Science B.V. All rights reserved. PII: S 0 0 4 0 - 6 0 9 0 Ž 0 1 . 0 1 0 5 4 - 9
S. Takeda et al. r Thin Solid Films 392 (2001) 338᎐344
causing a limitation of the use of non-refractory substrates. In the present study, the sputtering method was applied to obtain a photocatalytic TiO 2 thin film on glass. Part of this work has recently been reported w13x. Sputtering methods are widely used in industrial products because the high quality films; high density, high adhesion, high hardness, etc., can be obtained at low substrate temperature with good uniformity of the film thickness in a large area. Unfortunately, the photocatalytic activity of the sputtered film used for architectural glass is much lower than that of the films prepared by the wet process. Weinberger and Gerber w14x reported that a photocatalytic TiO 2 film with a thickness of ; 10 m was obtained by reactive RF magnetron sputtering, and that the film showed high photocatalytic activity because of its porous columnar morphology, resulting in a high surface area. However, the optical transmittance of the film may be low due to the large thickness. To our knowledge, a high performance photocatalytic TiO 2 thin film with both a high mechanical durability and transparency in the visible region has not been reported so far. Here, the TiO 2 thin film was deposited by DC magnetron sputtering onto glass at a substrate temperature of 220⬚C using a titanium metal target in a 100% O 2 atmosphere. From an industrial point of view, DC sputtering is desirable for the large area coating because it is more simple technology than RF sputtering and it gives a higher deposition rate. The characterization of the film was carried out using X-ray diffraction ŽXRD., scanning electron microscope ŽSEM., atomic force microscopy ŽAFM. and secondary ion mass spectrometry ŽSIMS.. The electronic structure of the film was calculating using a first-principle calculation method based on the density functional theory. The photocatalytic performance of the film was evaluated by the amount of decomposition of acetaldehyde ŽCH 3 CHO. as a function of UV irradiation time. From the results obtained, the relationship between the photocatalytic performance and the structure of the film was discussed. 2. Experimental details
339
Table 1 The sputtering conditions of TiO 2 thin films Substrate temperature ŽTs . Ž⬚C.
Target
Sputtering gas
Sputtering pressure Ž P . ŽPa.
Room temperature 140 220
Ti Ti Ti
O2 s 100% O2 s 100% O2 s 100%
0.4 0.4 2.0
Žpurity 99.9%. of 10 cm diameter. The deposition rate of the film was ; 4 nmrmin. The discharge during the deposition was stable. The TiO 2 thin film was also prepared by a sol᎐gel method. Titanium n-butoxide wTi ŽOC 4 H 9 .4 , TBTx was used as a precursor. TBT was dissolved in ethanol and acetylacetone. The SiO 2-coated glass was coated with this solution by the spin-coating process and then heated to 550⬚C for 30 min in an electric furnace. 2.2. Film properties The photocatalytic performance of the films was evaluated by measuring the amount of decomposed acetaldehyde ŽCH 3 CHO. by UV irradiation. UV light was irradiated using a black light Žcentral wavelength ; 352 nm. with ; 2 mWrcm2 . The experimental apparatus is schematically illustrated in Fig. 1. The TiO 2 film coated on a 45 = 100-mm2 substrate was set in a Pyrex vessel, and CH 3 CHO solution was injected in the vessel. The injected CH 3 CHO was vaporized immediately, and the concentration of the CH 3 CHO vapor was adjusted at 750 " 10 ppm. Thereafter, the change in CH 3 CHO concentration was measured using a gas detector ŽGASTEC. as a function of UV irradiation time. The accuracy of the measurements was "10 ppm. The mechanical durability of the films was evaluated by a Taber abrasion tester ŽTeledyne Taber 503.. The tests were carried out using CS-10F abrasion wheels loaded with 500 g. The degree of degradation was evaluated by the change in the haze value. The haze value, H, which is defined by TdrTt = 100% ŽTd , scattered light; Tt , transmitted light.. The haze value
2.1. Sample preparation SiO 2 coated soda᎐lime᎐silica glass was used as a substrate. The SiO 2 film was deposited onto the glass by reactive RF magnetron sputtering with a thickness of ; 50 nm in order to suppress alkali migration from the glass substrate. TiO 2 thin films were deposited by reactive DC magnetron sputtering with a thickness of ; 200 nm. The sputtering conditions are listed in Table 1. The base pressure in the coating chamber was less than 1.3= 10y3 Pa. The target was a titanium metal
Fig. 1. Schematic illustration of experimental apparatus for the measurement of photocatalytic activity.
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change, ⌬ H which is defined by H Ž N . y H Ž0.; N denotes the number of cycles. The optical transmission spectra of the films were measured at room temperature in air using a dual beam spectrometer ŽShimazu UV3100.. Observation of surface morphology and the quantitative analysis of the surface roughness of the films were performed using SEM ŽHitachi S-900. and AFM ŽSeiko SPI 3700., respectively. Crystalline phases of the films were identified by the glancing-angle XRD ŽRigaku Rint-2000. using Cu-K ␣ radiation operated at 50 kV᎐200 mA. The incident angle was kept at 0.5⬚. The stoichiometry of the films was firstly determined by X-ray photoelectron spectroscopy ŽXPS.. However, no difference in OrTi ratio among the films was observed. In order to obtain more accurate information about the stoichiometry, SIMS analyses with oxygen tracer Ž 18 O. gas were performed. The SIMS is widely used in various fields such as semiconductors to obtain detailed information for in-depth distribution of impurities or dopants in the materials because of its excellent sensitivity and high-depth resolution compared with XPS. In this analysis, the films were heat-treated at 500⬚C for 1 h in 18 O 2rN2 s 1r4 atmosphere, and thereafter the amount of incorporated 18 O was measured by SIMS ŽPHI Adept1010.. The Csq primary ion beam was operated at 5 keV, 200 nA and rastered on the area of 300 = 300 m2 . The angle of incidence was 60⬚ to the normal of the sample surface. The charge neutralization was accomplished using an electron flood gun. The electronic structure of the films was calculated using a first-principle calculation method based on the density functional theory w15,16x. The calculations were performed using VASP programs w17᎐20x. Two types of oxygen vacancy in anatase TiO 2 were simulated by Ž2 = 2 = 1. supercell model containing 48 atoms. One was that a single oxygen atom was removed from a Ti atom in a perfect anatase crystal Žsingle oxygen vacancy model., causing the decrease in the coordination numbers of oxygen around the titanium ŽTi. atom in the supercell Žthere are three fivefold-coordinated Ti atoms.. The other was that two oxygen atoms were removed from a certain Ti atom Ždouble oxygen vacancies model., resulting in the formation of two fourfoldcoordinated and four fivefold-coordinated Ti atoms in
Fig. 2. Change in concentration of CH 3 CHO as a function of UV irradiated time by the films obtained by sol᎐gel and sputtering method.
the supercell. The geometries of those vacancies were optimized by a conjugated gradient technique in a minimization of the Kohn᎐Sham energy functional. Ultra soft Vanderbilt-type pseudo-potentials w21x supplied by Kresse and Hafner w22x were used, where Ti 3s, 3p, 3d and 4s states and oxygen 2s and 2p states were treated as valence electrons. A cut-off of 450 eV was used for the valence electron wave functions. The total energy was calculated using a local density approximation for the exchange and correlation energy with the Ceperley and Alder form of the exchange᎐correlation potential w23x. Density of states were obtained by smearing eigenvalues of the ⌫ point.
3. Results
3.1. Photocatalytic performance
Fig. 2 shows the change in concentration of CH 3 CHO by TiO 2 thin films as a function of UV irradiation time. It is found that the decomposition ability of CH 3 CHO for the sputtered film deposited at room temperature ŽTs s r.t.. is the same as that of the sol᎐gel film without UV irradiation, indicating that the photocatalytic activity of Ts s r.t. is almost zero. The decomposition ability
Table 2 Mechanical durability of the sol᎐gel-derived TiO 2 thin film and the sputtered TiO 2 thin film deposited at Ts s 220⬚C, P s 2.0 Pa Sample
⌬ H Ž N s 100. Ž%.
⌬ H Ž N s 200. Ž%.
⌬ H Ž N s 300. Ž%.
Sputtered TiO2 ŽTs s 220⬚C, Ps 2.0 Pa. Sol᎐gel-derived TiO2
4.4
7.2
9.5
Delaminated
Delaminated
Delaminated
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Fig. 3. Transmission spectra of the films obtained by sol᎐gel and sputtering method.
increases with increasing Ts . However, the ability of the film deposited at Ts s 140⬚C is still lower than that of the sol᎐gel film. The ability of the film deposited at Ts s 220⬚C significantly increases, and its ability is almost the same as that of the sol᎐gel film. 3.2. Mechanical durability Table 2 shows the results of the Taber test for the sol᎐gel film and the sputtered film of Ts s 220⬚C. It is seen that the sol᎐gel film is delaminated after the rubbing of 100 cycles. On the other hand, the sputtered film is not delaminated by the rubbing of 300 cycles. This result indicates that the mechanical durability of the sputtered films is much higher than that of the sol᎐gel film although the photocatalytic activity of the films is almost the same. 3.3. Characterization Fig. 3 shows the transmission spectra of the sol᎐gel film and the sputtered film of Ts s 220⬚C. The average transmittance of the sputtered film is ; 80% in the visible region. The uniformity of thickness for the sputtered film in a large area was also better than that of the sol᎐gel film. These results indicate that the film has a possibility to use it for architectural or automotive glasses. Fig. 4 shows the SEM photographs of the sol᎐gel film and the sputtered films deposited at different Ts . The surface morphology of the sputtered film is clearly different with and without the substrate heating. The grain size of the crystalline increases with increasing Ts . This is because Ti particles sputtered from the target effectively react with oxygen atoms on the substrate due to a thermal effect, promoting the crystalline growth. On the other hand, the surface morphology is found to be different between the sputtered film and the sol᎐gel film. No significant relationship was observed between the decomposition ability of
Fig. 4. SEM images of the films obtained by sol᎐gel and sputtering method.
CH 3 CHO and the root-mean-square roughness Žr.m.s.. estimated from the AFM measurements for the sputtered and sol᎐gel film surfaces. Fig. 5 shows the 2⌰ X-ray diffraction patterns of the sol᎐gel film and the sputtered films. There is no peak for the sputtered film of Ts s r.t., indicating that the film is amorphous. On the other hand, several peaks are clearly observed for the sputtered films of Ts s 140 and 220⬚C although the photocatalytic activity is different between them, as shown in Fig. 3. These peaks
Fig. 5. 2⌰ X-ray diffraction patterns for the films obtained by sol᎐gel methods and sputtering methods.
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could be assigned to the anatase structure of TiO 2 without other phases. The XRD pattern of the sol᎐gel film is almost the same as that of the sputtered film with substrate heating. Fig. 6 shows the SIMS depth profiles for the films heat-treated in 18 O 2rN2 atmosphere. Before the heat treatment, the secondary ion intensity of 133 Cs18 Oq was less than 10 2 counts for all the films. After the heat treatment, the intensity of 133 Cs18 Oq increases for all the films although no significant change is observed for the intensity of 133 Cs16 Oq before and after the heat treatment, indicating that 18 O is incorporated into the films. The amount of incorporated 18 O is approximately two orders of magnitude larger than that before the heat treatment. It is found that the amount of incorporated 18 O is different among the films, and that the amount of incorporated 18 O is larger for the film with lower decomposition ability of CH 3 CHO than that of the film with higher ability. The incorporation of 18 O was not observed for a single crystal of rutile TiO 2 , which is a stoichiometiric TiO 2 , indicating that 16 O᎐ 18 O isotopic exchange reaction is negligible during the heat treatment. These results suggest that the incorporated 18 O occupies an oxygen vacancy site, and that the amount of incorporated 18 O
Fig. 6. SIMS depth profiles for the films heat-treated at 500⬚C for 1 h in 18 O 2 atmosphere.
may represent the amount of oxygen vacancies in the film. Namely, the decomposition ability of CH 3 CHO, the photocatalytic activity, of the film is seen to decrease with increasing the oxygen vacancies in the film. Fig. 7 shows the density of states ŽDOS. for Ža. perfect model; Žb. single oxygen vacancy model; and Žc. double oxygen vacancies model obtained from the first-principle calculations. It is found that no energy levels ap-
Fig. 7. Density of states obtained from first-principle calculations based on the density functional theory for Ža. perfect model; Žb. single oxygen vacancy model; and Žc. double oxygen vacancies model.
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pear at the mid-gap when the single oxygen atom is simply removed from a perfect anatase crystal, as shown in Fig. 7b. On the other hand, some energy levels pointed out by an arrow are clearly observed at the mid-gap in the double oxygen vacancies model, as shown in Fig. 7Žc.. 4. Discussion As mentioned before, there was no relationship between the photocatalytic performance and the surface roughness Žr.m.s.. of the films. This means that the surface roughness is not a major factor governing the photocatalytic performance of the films. As shown in Fig. 6, the amount of incorporated 18 O is larger for the film with lower photocatalytic activity than that of the film with higher photocatalytic activity. This result indicates that the amount of incorporated 18 O may represent the amount of oxygen vacancies in the film, and that the photocatalytic activity decreases with increasing oxygen vacancies in the films, as mentioned above. According to Takeuchi w24x, the generation of oxygen vacancies of the sputtered TiO 2 films was related to the reduction of TiO 2 during the sputtering process. In order to investigate how the oxygen vacancy affects the photocatalytic activity of the film, the electronic structure of the film was calculated using a first-principle calculation method. As shown in Fig. 7b, no energy level appears at the mid-gap when a single oxygen atom is simply removed from a perfect anatase crystal. On the other hand, some energy levels are clearly observed around the mid-gap in the double oxygen vacancies model, as shown in Fig. 7c. These results suggest that the increase in oxygen vacancies, which causes the locally decrease in the coordination numbers of oxygen around the Ti atom from 6 to 4᎐5, can create some energy levels around the mid-gaps. It is known that TiO 2 is a semiconductor with a band gap of ; 3.0 eV, and that UV light with wavelengths shorter than ; 400 nm can excite pairs of electrons and holes. The photo-generated electrons react with molecular oxygen ŽO 2 . to produce superoxide radical . anions Ž⭈Oy 2 , and the photo-generated holes react with water to produce hydroxyl Ž⭈OH. radicals. These reactive radicals then work together to decompose organic materials. If an energy level is present at the band-gap, the photo-generated electrons and holes are considered to be recombinated. Consequently, the probability of the formation of ⭈Oy 2 and ⭈OH radicals decreases, leading to the decrease in photocatalytic activity. Taking this into consideration, the formation of some energy levels around the mid-gap, as shown in Fig. 7c, can cause the decrease in photocatalytic activity because they can work as recombination centers of
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photo-generated holes and electrons. The change in coordination structure of Ti is induced by the structural defects associated with oxygen vacancies. Namely, there is a possibility that the sputtered TiO 2 thin film has many more structural defects associated with oxygen vacancies than a perfect crystal, resulting in the decrease in the coordination numbers of oxygen around a Ti atom. Therefore, it is important to decrease the structural defects associated with oxygen vacancies in the film for improving the photocatalytic activity. Takeuchi w24x reported that the structural defects associated with oxygen vacancies of the sputtered TiO 2 film decreased with increasing the oxygen concentration in the sputtering gas. This result suggests that controlling the oxygen concentration during the sputtering process is a key parameter for improving the photocatalytic activity. Furthermore, the decomposition ability of CH 3 CHO is higher for the crystallized film than that of the amorphous film, as shown in Figs. 2 and 5. It is known that there are some energy levels at the band-gap for the amorphous film because of its structural disorder. This means that the photo-generated electrons and holes are recombinated, as mentioned above. Consequently, the photocatalytic activity of the amorphous film decreases compared with that of the crystallized film. In addition, the crystalline structure of the crystallized film is found to be the anatase form. The anatase form is known to exhibit excellent photocatalytic activity compared with the rutile form w25x. Namely, the formation of anatase form is also important for improving the photocatalytic activity of the film.
5. Conclusions
A photocatalytic TiO 2 thin film with high mechanical durability and transparency in the visible region has successfully been obtained by reactive DC magnetron sputtering. The film showed good uniformity of thickness over a large area with the optical transmittance of ; 80% in the visible region. The mechanical durability of the film was much higher than that of sol᎐gelderived TiO 2 thin films. The photocatalytic activity of the film was almost the same as that of the sol᎐gel films. The film was characterized by various analytical methods. The electronic structure of the film was also calculated using a first-principle calculation method. It was found that the structural defects associated with the oxygen vacancies and the crystalline structure played an important role for the photocatalytic performance of the films. Therefore, controlling these factors is important for improving the photocatalytic activity of the films.
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