Ž .Thin Solid Films 392 2001 338�344

Photocatalytic TiO thin film deposited onto glass by DC2magnetron sputtering

Satoshi Takedaa,� , Susumu Suzukia, Hidefumi Odakaa, Hideo Hosonob

aResearch Center, Asahi Glass Co., Ltd, 1150 Hazawa-cho, Kanagawa-ku, Yokohama 221-8755, JapanbMaterials and Structures Laboratory, Tokyo Institute of Technology, 4259 Nagatsuda-cho, Midori-ku, Yokohama 226-8503, Japan

Abstract

A high performance photocatalytic TiO thin film was successfully obtained by reactive DC magnetron sputtering. The film was2deposited onto SiO -coated glass at a substrate temperature of 220�C using a titanium metal target in O 100% atmosphere. The2 2film showed good uniformity of thickness in a large area with the optical transmittance of �80% in the visible region. The

Ž .decomposition ability of acetaldehyde CH CHO of the film under UV irradiation was almost the same as that of the3sol�gel-derived TiO thin film but the sputtered film showed a much higher mechanical durability. The characterization of the2films was carried out using XRD, SEM, AFM, XPS and SIMS, and the electronic structures of the films were calculated using afirst-principle calculation method based on the density functional theory. It was found that the amount of incorporated 18O intothe film was larger for the films with lower photocatalytic activity when the films were annealed in 18O �N atmosphere. This2 2result indicates that the amount of oxygen vacancies, which were occupied by incorporated 18O, was larger for the films withlower photocatalytic activity. Furthermore, the introduction of structural defects associated with oxygen vacancies was found tocreate some energy levels around the mid-gap, indicating that they could work as recombination centers of photo-induced holesand electrons, causing the decrease in photocatalytic activity. Therefore, the decrease in the structural defects associated withoxygen vacancies is important for improving the photocatalytic activity of the films. � 2001 Elsevier Science B.V. All rightsreserved.

Keywords: TiO ; Photocatalytic activity; Sputtering; Oxygen vacancy2

1. Introduction

Ž .Titanium oxide TiO thin films are widely used in2various fields such as optical and protective coatingsand optical fibers because of their excellent chemicalstability, mechanical hardness and optical transmit-tance with high refractive index. Recently, TiO thin2films have become the most promising materials inenvironmental cleaning such as photocatalytic purifier� � � �1,2 and photochemical solar cells 3,4 . Many re-searchers have focused on the application of TiO2photocatalyst to purification and treatment of air and

� Corresponding author. Tel.: �81-45-374-8794; fax: �81-45-374-8892.

Ž .E-mail address: takeda@agc.co.jp S. Takeda .

water, e.g. through the photolysis of organics and toxic� �gases 2 .

TiO thin films have been prepared by a variety of2� �deposition techniques such as sol�gel processes 5 ,

� � � �chemical vapor deposition 6,7 , evaporation 8 , various� �sputtering depositions 9�11 , and ion beam-assisted

� �processes 12 . However, most of the photocatalyticTiO thin films used in the market are prepared by wet2process such as a sol�gel method. Although the filmsshow excellent photocatalytic activity, the mechanicaldurability is not enough for practical uses such asarchitectural or automotive glasses. In addition, theuniformity of the thickness in a large area is poor.These points are not desirable for architectural orautomotive applications. Furthermore, a heatingprocess at approximately 500�600�C is indispensable inthe method in order to decompose metal�organic,

0040-6090�01�$ - see front matter � 2001 Elsevier Science B.V. All rights reserved.Ž .PII: S 0 0 4 0 - 6 0 9 0 0 1 0 1 0 5 4 - 9

( )S. Takeda et al. � Thin Solid Films 392 2001 338�344 339

causing a limitation of the use of non-refractory sub-strates.

In the present study, the sputtering method wasapplied to obtain a photocatalytic TiO thin film on2

� �glass. Part of this work has recently been reported 13 .Sputtering methods are widely used in industrialproducts because the high quality films; high density,high adhesion, high hardness, etc., can be obtained atlow substrate temperature with good uniformity of thefilm thickness in a large area. Unfortunately, the pho-tocatalytic activity of the sputtered film used for archi-tectural glass is much lower than that of the filmsprepared by the wet process. Weinberger and Gerber� �14 reported that a photocatalytic TiO film with a2thickness of �10 �m was obtained by reactive RFmagnetron sputtering, and that the film showed highphotocatalytic activity because of its porous columnarmorphology, resulting in a high surface area. However,the optical transmittance of the film may be low due tothe large thickness. To our knowledge, a high perfor-mance photocatalytic TiO thin film with both a high2mechanical durability and transparency in the visibleregion has not been reported so far.

Here, the TiO thin film was deposited by DC mag-2netron sputtering onto glass at a substrate temperatureof 220�C using a titanium metal target in a 100% O2atmosphere. From an industrial point of view, DCsputtering is desirable for the large area coating be-cause it is more simple technology than RF sputteringand it gives a higher deposition rate. The characteriza-tion of the film was carried out using X-ray diffractionŽ . Ž .XRD , scanning electron microscope SEM , atomic

Ž .force microscopy AFM and secondary ion mass spec-Ž .trometry SIMS . The electronic structure of the film

was calculating using a first-principle calculationmethod based on the density functional theory. Thephotocatalytic performance of the film was evaluatedby the amount of decomposition of acetaldehydeŽ .CH CHO as a function of UV irradiation time. From3the results obtained, the relationship between the pho-tocatalytic performance and the structure of the filmwas discussed.

2. Experimental details

2.1. Sample preparation

SiO coated soda�lime�silica glass was used as a2substrate. The SiO film was deposited onto the glass2by reactive RF magnetron sputtering with a thicknessof �50 nm in order to suppress alkali migration fromthe glass substrate. TiO thin films were deposited by2reactive DC magnetron sputtering with a thickness of�200 nm. The sputtering conditions are listed in Table1. The base pressure in the coating chamber was lessthan 1.3�10�3 Pa. The target was a titanium metal

Table 1The sputtering conditions of TiO thin films2

Substrate Target Sputtering SputteringŽ . Ž .temperature T gas pressure Ps

Ž . Ž .�C Pa

Room temperature Ti O �100% 0.42140 Ti O �100% 0.42220 Ti O �100% 2.02

Ž .purity 99.9% of 10 cm diameter. The deposition rateof the film was �4 nm�min. The discharge during thedeposition was stable.

The TiO thin film was also prepared by a sol�gel2� Ž . �method. Titanium n-butoxide Ti OC H , TBT was4 9 4

used as a precursor. TBT was dissolved in ethanol andacetylacetone. The SiO -coated glass was coated with2this solution by the spin-coating process and thenheated to 550�C for 30 min in an electric furnace.

2.2. Film properties

The photocatalytic performance of the films wasevaluated by measuring the amount of decomposed

Ž .acetaldehyde CH CHO by UV irradiation. UV light3Žwas irradiated using a black light central wavelength

. 2�352 nm with �2 mW�cm . The experimental appa-ratus is schematically illustrated in Fig. 1. The TiO2film coated on a 45�100-mm2 substrate was set in aPyrex vessel, and CH CHO solution was injected in the3vessel. The injected CH CHO was vaporized immedi-3ately, and the concentration of the CH CHO vapor3was adjusted at 750�10 ppm. Thereafter, the changein CH CHO concentration was measured using a gas3

Ž .detector GASTEC as a function of UV irradiationtime. The accuracy of the measurements was �10 ppm.

The mechanical durability of the films was evaluatedŽ .by a Taber abrasion tester Teledyne Taber 503 . The

tests were carried out using CS-10F abrasion wheelsloaded with 500 g. The degree of degradation wasevaluated by the change in the haze value. The haze

Žvalue, H, which is defined by T �T �100% T , scat-d t d.tered light; T , transmitted light . The haze valuet

Fig. 1. Schematic illustration of experimental apparatus for the mea-surement of photocatalytic activity.

( )S. Takeda et al. � Thin Solid Films 392 2001 338�344340

Ž . Ž .change, � H which is defined by H N �H 0 ; N de-notes the number of cycles.

The optical transmission spectra of the films weremeasured at room temperature in air using a dual

Ž .beam spectrometer Shimazu UV3100 . Observation ofsurface morphology and the quantitative analysis of thesurface roughness of the films were performed using

Ž . Ž .SEM Hitachi S-900 and AFM Seiko SPI 3700 , re-spectively. Crystalline phases of the films were identi-

Ž .fied by the glancing-angle XRD Rigaku Rint-2000using Cu-K� radiation operated at 50 kV�200 mA.The incident angle was kept at 0.5�.

The stoichiometry of the films was firstly determinedŽ .by X-ray photoelectron spectroscopy XPS . However,

no difference in O�Ti ratio among the films wasobserved. In order to obtain more accurate informationabout the stoichiometry, SIMS analyses with oxygen

Ž18 .tracer O gas were performed. The SIMS is widelyused in various fields such as semiconductors to obtaindetailed information for in-depth distribution of impu-rities or dopants in the materials because of its excel-lent sensitivity and high-depth resolution comparedwith XPS. In this analysis, the films were heat-treatedat 500�C for 1 h in 18 O �N �1�4 atmosphere, and2 2thereafter the amount of incorporated 18 O was mea-

Ž . �sured by SIMS PHI Adept1010 . The Cs primary ionbeam was operated at 5 keV, 200 nA and rastered onthe area of 300�300 �m2. The angle of incidence was60� to the normal of the sample surface. The chargeneutralization was accomplished using an electron floodgun.

The electronic structure of the films was calculatedusing a first-principle calculation method based on the

� �density functional theory 15,16 . The calculations were� �performed using VASP programs 17�20 . Two types of

oxygen vacancy in anatase TiO were simulated by2Ž .2�2�1 supercell model containing 48 atoms. Onewas that a single oxygen atom was removed from a Ti

Žatom in a perfect anatase crystal single oxygen vacancy.model , causing the decrease in the coordination num-

Ž .bers of oxygen around the titanium Ti atom in theŽsupercell there are three fivefold-coordinated Ti

.atoms . The other was that two oxygen atoms wereŽremoved from a certain Ti atom double oxygen vacan-

.cies model , resulting in the formation of two fourfold-coordinated and four fivefold-coordinated Ti atoms in

Fig. 2. Change in concentration of CH CHO as a function of UV3irradiated time by the films obtained by sol�gel and sputteringmethod.

the supercell. The geometries of those vacancies wereoptimized by a conjugated gradient technique in aminimization of the Kohn�Sham energy functional.

� �Ultra soft Vanderbilt-type pseudo-potentials 21 sup-� �plied by Kresse and Hafner 22 were used, where Ti 3s,

3p, 3d and 4s states and oxygen 2s and 2p states weretreated as valence electrons. A cut-off of 450 eV wasused for the valence electron wave functions. The totalenergy was calculated using a local density approxima-tion for the exchange and correlation energy with theCeperley and Alder form of the exchange�correlation

� �potential 23 . Density of states were obtained bysmearing eigenvalues of the � point.

3. Results

3.1. Photocatalytic performance

Fig. 2 shows the change in concentration of CH CHO3

by TiO thin films as a function of UV irradiation time.2

It is found that the decomposition ability of CH CHO3

for the sputtered film deposited at room temperatureŽ .T �r.t. is the same as that of the sol�gel film withouts

UV irradiation, indicating that the photocatalytic activ-ity of T �r.t. is almost zero. The decomposition abilitys

Table 2Mechanical durability of the sol�gel-derived TiO thin film and the sputtered TiO thin film deposited at T �220�C, P�2.0 Pa2 2 s

Ž . Ž . Ž .Sample � H N�100 � H N�200 � H N�300Ž . Ž . Ž .% % %

Sputtered TiO 4.4 7.2 9.52Ž .T �220�C, P�2.0 Pas

Sol�gel-derived TiO Delaminated Delaminated Delaminated2

( )S. Takeda et al. � Thin Solid Films 392 2001 338�344 341

Fig. 3. Transmission spectra of the films obtained by sol�gel andsputtering method.

increases with increasing T . However, the ability of thesfilm deposited at T �140�C is still lower than that ofsthe sol�gel film. The ability of the film deposited atT �220�C significantly increases, and its ability is al-smost the same as that of the sol�gel film.

3.2. Mechanical durability

Table 2 shows the results of the Taber test for thesol�gel film and the sputtered film of T �220�C. It issseen that the sol�gel film is delaminated after therubbing of 100 cycles. On the other hand, the sputteredfilm is not delaminated by the rubbing of 300 cycles.This result indicates that the mechanical durability ofthe sputtered films is much higher than that of thesol�gel film although the photocatalytic activity of thefilms is almost the same.

3.3. Characterization

Fig. 3 shows the transmission spectra of the sol�gelfilm and the sputtered film of T �220�C. The averagestransmittance of the sputtered film is �80% in thevisible region. The uniformity of thickness for the sput-tered film in a large area was also better than that ofthe sol�gel film. These results indicate that the film hasa possibility to use it for architectural or automotiveglasses.

Fig. 4 shows the SEM photographs of the sol�gelfilm and the sputtered films deposited at different T .sThe surface morphology of the sputtered film is clearlydifferent with and without the substrate heating. Thegrain size of the crystalline increases with increasing T .sThis is because Ti particles sputtered from the targeteffectively react with oxygen atoms on the substratedue to a thermal effect, promoting the crystallinegrowth. On the other hand, the surface morphology isfound to be different between the sputtered film andthe sol�gel film. No significant relationship wasobserved between the decomposition ability of

Fig. 4. SEM images of the films obtained by sol�gel and sputteringmethod.

Ž .CH CHO and the root-mean-square roughness r.m.s.3estimated from the AFM measurements for the sput-tered and sol�gel film surfaces.

Fig. 5 shows the 2� X-ray diffraction patterns of thesol�gel film and the sputtered films. There is no peakfor the sputtered film of T �r.t., indicating that thesfilm is amorphous. On the other hand, several peaksare clearly observed for the sputtered films of T �140sand 220�C although the photocatalytic activity is dif-ferent between them, as shown in Fig. 3. These peaks

Fig. 5. 2� X-ray diffraction patterns for the films obtained bysol�gel methods and sputtering methods.

( )S. Takeda et al. � Thin Solid Films 392 2001 338�344342

could be assigned to the anatase structure of TiO2without other phases. The XRD pattern of the sol�gelfilm is almost the same as that of the sputtered filmwith substrate heating.

Fig. 6 shows the SIMS depth profiles for the filmsheat-treated in 18 O �N atmosphere. Before the heat2 2treatment, the secondary ion intensity of 133Cs18 O�

was less than 102 counts for all the films. After theheat treatment, the intensity of 133Cs18 O� increasesfor all the films although no significant change isobserved for the intensity of 133Cs16 O� before andafter the heat treatment, indicating that 18 O is incor-porated into the films. The amount of incorporated 18 Ois approximately two orders of magnitude larger thanthat before the heat treatment. It is found that theamount of incorporated 18 O is different among thefilms, and that the amount of incorporated 18 O is largerfor the film with lower decomposition ability ofCH CHO than that of the film with higher ability. The3incorporation of 18 O was not observed for a singlecrystal of rutile TiO , which is a stoichiometiric TiO ,2 2indicating that 16 O� 18 O isotopic exchange reaction isnegligible during the heat treatment. These resultssuggest that the incorporated 18 O occupies an oxygenvacancy site, and that the amount of incorporated 18 O

Fig. 6. SIMS depth profiles for the films heat-treated at 500�C for 1 hin 18 O atmosphere.2

may represent the amount of oxygen vacancies in thefilm. Namely, the decomposition ability of CH CHO,3the photocatalytic activity, of the film is seen to de-crease with increasing the oxygen vacancies in the film.

Ž . Ž .Fig. 7 shows the density of states DOS for a perfectŽ . Ž .model; b single oxygen vacancy model; and c double

oxygen vacancies model obtained from the first-princi-ple calculations. It is found that no energy levels ap-

Ž . Ž .Fig. 7. Density of states obtained from first-principle calculations based on the density functional theory for a perfect model; b single oxygenŽ .vacancy model; and c double oxygen vacancies model.

( )S. Takeda et al. � Thin Solid Films 392 2001 338�344 343

pear at the mid-gap when the single oxygen atom issimply removed from a perfect anatase crystal, as shownin Fig. 7b. On the other hand, some energy levelspointed out by an arrow are clearly observed at themid-gap in the double oxygen vacancies model, as

Ž .shown in Fig. 7 c .

4. Discussion

As mentioned before, there was no relationshipbetween the photocatalytic performance and the sur-

Ž .face roughness r.m.s. of the films. This means that thesurface roughness is not a major factor governing thephotocatalytic performance of the films. As shown inFig. 6, the amount of incorporated 18 O is larger for thefilm with lower photocatalytic activity than that of thefilm with higher photocatalytic activity. This result indi-cates that the amount of incorporated 18 O may repre-sent the amount of oxygen vacancies in the film, andthat the photocatalytic activity decreases with increas-ing oxygen vacancies in the films, as mentioned above.

� �According to Takeuchi 24 , the generation of oxygenvacancies of the sputtered TiO films was related to2the reduction of TiO during the sputtering process.2

In order to investigate how the oxygen vacancy af-fects the photocatalytic activity of the film, the elec-tronic structure of the film was calculated using afirst-principle calculation method. As shown in Fig. 7b,no energy level appears at the mid-gap when a singleoxygen atom is simply removed from a perfect anatasecrystal. On the other hand, some energy levels areclearly observed around the mid-gap in the doubleoxygen vacancies model, as shown in Fig. 7c. Theseresults suggest that the increase in oxygen vacancies,which causes the locally decrease in the coordinationnumbers 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 is a semiconductor with a band2gap of �3.0 eV, and that UV light with wavelengthsshorter than �400 nm can excite pairs of electronsand holes. The photo-generated electrons react with

Ž .molecular oxygen O to produce superoxide radical2Ž �.anions �O , and the photo-generated holes react with2

Ž .water to produce hydroxyl �OH radicals. These reac-tive radicals then work together to decompose organicmaterials. If an energy level is present at the band-gap,the photo-generated electrons and holes are con-sidered to be recombinated. Consequently, theprobability of the formation of �O� and �OH radicals2decreases, leading to the decrease in photocatalyticactivity.

Taking this into consideration, the formation of someenergy levels around the mid-gap, as shown in Fig. 7c,can cause the decrease in photocatalytic activity be-cause they can work as recombination centers of

photo-generated holes and electrons. The change incoordination structure of Ti is induced by the structuraldefects associated with oxygen vacancies. Namely, thereis a possibility that the sputtered TiO thin film has2many more structural defects associated with oxygenvacancies than a perfect crystal, resulting in the de-crease in the coordination numbers of oxygen around aTi atom. Therefore, it is important to decrease thestructural defects associated with oxygen vacancies inthe film for improving the photocatalytic activity.

� �Takeuchi 24 reported that the structural defects asso-ciated with oxygen vacancies of the sputtered TiO film2decreased with increasing the oxygen concentration inthe sputtering gas. This result suggests that controllingthe oxygen concentration during the sputtering processis a key parameter for improving the photocatalyticactivity.

Furthermore, the decomposition ability of CH CHO3is higher for the crystallized film than that of theamorphous film, as shown in Figs. 2 and 5. It is knownthat there are some energy levels at the band-gap forthe amorphous film because of its structural disorder.This means that the photo-generated electrons andholes are recombinated, as mentioned above. Conse-quently, the photocatalytic activity of the amorphousfilm decreases compared with that of the crystallizedfilm. In addition, the crystalline structure of the crystal-lized film is found to be the anatase form. The anataseform is known to exhibit excellent photocatalytic activ-

� �ity compared with the rutile form 25 . Namely, theformation of anatase form is also important for improv-ing the photocatalytic activity of the film.

5. Conclusions

A photocatalytic TiO thin film with high mechanical2durability and transparency in the visible region hassuccessfully been obtained by reactive DC magnetronsputtering. The film showed good uniformity of thick-ness over a large area with the optical transmittance of�80% in the visible region. The mechanical durabilityof the film was much higher than that of sol�gel-derived TiO thin films. The photocatalytic activity of2the film was almost the same as that of the sol�gelfilms. The film was characterized by various analyticalmethods. The electronic structure of the film was alsocalculated using a first-principle calculation method. Itwas found that the structural defects associated withthe oxygen vacancies and the crystalline structureplayed an important role for the photocatalytic perfor-mance of the films. Therefore, controlling these factorsis important for improving the photocatalytic activity ofthe films.

( )S. Takeda et al. � Thin Solid Films 392 2001 338�344344

References

� �1 I. Sopyan, S. Murasawa, K. Hashimoto, A. Fujishima, in: D.E.Ž .Ollis, H. Al-Ekabi Eds. , Photocatalytic Purification and Treat-

ment of Water and Air, Elsevier, Amsterdam, 1993, p. 747.� � Ž . st2 D.E. Ollis, H. Al-Ekabi Eds. , Proc. 1 Int. Conf. TiO Photo-2

catalytic Purification and Treatment of Water and Air, Else-vier, Amsterdam, 1993, p. 747.

� � Ž .3 A. Fujishima, K. Honda, Nature 238 1972 37.� � Ž .4 M. Graetzel, Comments Inorg. Chem. 12 1991 93.� �5 T. Watanabe, A. Kitamura, E. Kojima, C. Nakayama, K.

Ž .Hashimoto, A. Fujishima, Chem. Lett. 1994 723.� �6 G.A. Battiston, R. Gerbasi, M. Porchia, A. Marigo, Thin Solid

Ž .Films 239 1994 186.� � Ž .7 L.M. Williams, D.W. Hess, J. Vac. Sci. Technol. A1 1983

1810.� �8 T. Fujii, N. Sakata, J. Takada, Y. Miura, Y. Daitoh, J. Mater.

Ž .Res. 9 1994 1468.� �9 H. Tang, K. Prasad, R. Sanjines, P.E. Schmid, F. Levy, J. Appl.

Ž .Phys. 75 1994 2042.

� �10 S. Ben Amor, G. Baud, J.P. Besse, M. Jacqet, Thin Solid FilmsŽ .293 1997 163.

� �11 N. Martin, C. Rousselot, D. Rondot, F. Palmino, Thin SolidŽ .Films 300 1997 113.

� � Ž .12 M. Gilo, N. Croitoru, Thin Solid Films 283 1996 84.� � Ž . Ž .13 S. Suzuki, S. Takeda, Sputtering Plasma Process. 14 3 1999

Ž .33 in Japanese .� � Ž .14 B.R. Weinberger, R.B. Garber, Appl. Phys. Lett. 66 1995

2409.� � Ž .15 P. Hohenberg, W. Kohn, Phys. Rev. 136 1964 B864.� � Ž .16 W. Kohn, L.J. Sham, Phys. Rev. 140 1965 A1133.� � Ž .17 G. Kresse, J. Hafner, Phys. Rev. B 47 1993 58.� �18 G. Kresse, Thesis, Technische Universituat at Wien, 1993.¨� � Ž .19 G. Kresse, J. Furthmuuller, Compt. Mat. Sci. 6 1996 15.¨� � Ž .20 G. Kresse, J. Furthmuuller, Phys. Rev. B. 54 1996 11169.¨� � Ž .21 D. Vanderbilt, Phys. Rev. B 41 1990 7892.� � Ž .22 G. Kresse, J. Hafner, J. Phys. Condens. Mater. 6 1994 8245.� � Ž .23 D.M. Ceperley, B.J. Alder, Phys. Rev. Lett. 45 1980 566.� � Ž . Ž .24 M. Takeuchi, Phys. Stat. Solid a 55 1979 653.� � Ž .25 S.N. Frank, A.J. Bard, J. Am. Chem. Soc. 99 1977 303.