Photoactivated Properties of TiO2 Films Prepared by Magnetron Sputtering

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Photoactivated Properties of TiO2 FilmsPrepared by Magnetron Sputtering

David Herman, Jan Sıcha, Jindrich Musil*

This article reports on photoactivity of sputtered TiO2 films induced by UV irradiation. TiO2

films were prepared by dc pulsed reactive magnetron sputtering using a dual magnetronoperated in bipolar mode and equipped with Ti targets. The photoactivity of TiO2 films,characterized by the water droplet contact angle (WDCA) on the film surface after UVirradiation, was evaluated and discussed in detail. The structure of TiO2 film was measuredusing X-ray diffraction and surfacemorphology using AFM. A sharp decrease inWDCA to�108has been observed for optimal sputtering conditions. Possibility of low-temperature sputter-

sensitive substrates has been introduced.
ing (<908) of photoactive TiO2 films on heat
Introduction

TiO2 photocatalysts have attracted a great deal of atten-

tion since Fujishima and Honda published a breakthrough

experiment with a TiO2 semiconductor decomposing

H2O to hydrogen and oxygen molecules.[1] A huge effort

devoted to practical utilization of TiO2 photocatalytic

properties[2] resulted not only in a rapid increase in

published articles in this field but also in a discovery of the

phenomenon called photoinduced hydrophilicity in 1995.

The explanation of this phenomenon in 2003[3] opened

new applications based on self-cleaning, anti-fogging and

antibacterial effects. Several excellent reviews have been

already published.[4–6] In spite of the fact that a great deal

of work has already been done, there are still open

questions needed to be answered. Since the optical band

gap of crystalline TiO2 is large of about 3.1–3.2 eV, the TiO2

film exhibits properties given above only when it is

activated by UV irradiation, i.e., at wavelengths shorter

than that corresponding to the band gap. There are several

trends in the development of photoactive TiO2 films. One

of them is doping of the TiO2 film with the aim to change

its electronic structure and to enable its activation under

visible light. This is of key importance for both external

and internal applications. The second task is preparation of

D. Herman, J. Sıcha, J. MusilDepartment of Physics, University of West Bohemia, Univerzitnı22, 306 14 Plzen, The Czech RepublicFax:þ420 377 632 202;E-mail: [email protected]

Plasma Process. Polym. 2007, 4, S531–S535

� 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

TiO2 films on heat sensitive substrate [e.g., on polycarbo-

nate or poly(propylene)]. For this application, a low-

temperature method of formation of crystalline transpar-

ent TiO2 film needs to be developed. Reactive magnetron

sputtering is one of the most promising methods which

can meet this requirement. The dual pulsed magnetron

sputtering is a sophisticated method for the preparation of

thin oxide films.[7] It enables to control a series of physical

properties of created material by energy delivered to the

growing film and simultaneously to avoid the formation of

microarcs. At present, correlations between the process

parameters and properties of the resulting film are

intensively studied in many laboratories. Although the

magnetron sputtering is a convenient method to produce

high-quality TiO2 films, the low deposition rate aD of

photoactive TiO2 films is very serious problem which

needs to be overcome.[8] This article reports on low-

temperature sputtering of transparent crystalline TiO2

films. Correlations between parameters as the total

working pressure, pT, oxygen partial pressure, pO2, and

the target-to-substrate distance, ds-t, and films properties

such as the film structure, surface morphology and its

photoactivity are discussed in detail.

Experimental Part

The TiO2 films were prepared by dc pulsed reactive magnetron

sputtering using a dual magnetron equippedwith Ti (99.5) targets

of 50 mm in diameter and operated in bipolar mode at the

repetition frequency fr¼ 100 kHz and the duty cycle t/T¼ 0.5; total

sputtering gas pressure pT¼0.75 Pa and oxygen partial pressure,

DOI: 10.1002/ppap.200731303 S531

D. Herman, J. Sıcha, J. Musil

Figure 1. A map of operating points in the metallic, transition andoxide mode of reactive sputtering of TiOx films from Ti(99.5)targets at Wda¼ 56 W � cm�2, ds-t¼ 100 mm, pT¼0.75 Pa.

S532

pO2, ranging from 0 to 0.2 Pa, pulse average discharge current

Ida¼ 3 A with the target pulse power density averaged over the

whole target surface area (S¼19.6 cm2) Wda¼ 56 W � cm�2; here

t and T are the length of pulse and period, respectively. Reactive

magnetron sputtering was carried out in ArþO2 mixture. Partial

pressures of both gases, pAr and pO2, were maintained constant

during deposition. The flow of argon fAr was kept constant. The

flow of oxygen fO2was adjusted by a feedback control loop to keep

the total pressure of sputtering gas pT constant using electro-

nically controlled flow meter.

The TiO2 films were deposited on unheated Na-containing

microscope glass slides (26�26� 1 mm3). TiOx films were

reactively sputtered at different values of pO2, see Figure 1. In

this figure, the values of pO2used in sputtering of TiOx films are

denoted by letters A, B, C, and D. Selection of the different values of

pO2makes it possible to control not only transparency of a TiOx

film but also its stoichiometry x¼O/Ti and its phase composition

as well. The sputtering at Wda¼56 W � cm�2 and ds-t¼ 100 mm

ensured that the substrate surface temperature Tsurf during TiOx

film deposition was lower than 160 8C. An increase in Tsurf during

film deposition is caused by the film and substrate heating from

sputtered targets, plasma, condensing and bombarding particles.

The temperature of substrate surface Tsurf was measured by a

thermostrip pasted to glass surface. More details are given in

ref.[7,9–11]

Hydrophilicity of TiO2 films was characterized by the water

droplet contact angle (WDCA) air on its surface after UV irradiation

at l¼365 nm andWir¼ 0.9mW � cm�2 (TLD Blacklight Blue Lamps

made by Philips, the Netherlands) for 20, 60, and 300 min. The

following procedure was used. After deposition, TiO2 films were

kept in a dark box for 1 week. Prior to the UV irradiation, TiO2

samples were cleaned by isopropylalcohol and dried in flowing air



at room temperature for 30 min. Distilled deionized water was

used for measurements. Droplets (4 ml) were put on the surface of

a TiO2 film by a micropipette from zero height, i.e., with no falling

of water droplet. Each droplet was put into a new original position

andwas removed after measurement. TheWDCA awas evaluated

by Surface Energy Evaluation system made at the Masaryk

University Brno, the Czech Republic. We experienced the

measurement error �18 and rapidly increasing for a<108.The structure of TiO2 films was determined by X-ray diffraction

(XRD) using anXRD spectrometer Dron 4.07 in the Bragg–Brentano

configuration with CoKa (l¼0.179021 nm) radiation. The surface

roughness Ra of TiO2 films was computed from atomic force

microscopy (AFM) images using a scanning probe microscopy

system MetrisTM 2001 NC produced by Burleigh Instruments Inc.,

USA.[12] Optical properties, i.e., transmittance T and optical band

gap Eg have been determined from UV-vis transmittance spectra

using the spectrometer Specord M400 (Carl Zeiss Inc., Germany).

The optical band gap Eg was evaluated from the transmittance

spectra using Tauc plots.[13]

Results and Discussion

Hydrophilicity of three series of �1 mm thick TiO2 films

prepared by reactive magnetron sputtering at different

deposition conditions was investigated in detail. Based on

this investigation, the following correlations between

photoinduced hydrophilicity and phase composition, sur-

face roughness, crystallinity, and microstructure of TiO2

films were found.

Hydrophilicity Versus Phase Composition

Many experiments carried out so far clearly show that

there is a general trend in evolution of the phase

composition of reactively sputtered TiOx films with

increasing partial pressure of oxygen pO2, for instance,

see Figure 2. From this figure, it is seen that the phase

composition of sputtered TiOx films gradually varies from

X-ray amorphous through (i) rutile and (ii) mixture of

rutileþ anatase to anatase with increasing pO2. Typical

XRD patterns of TiOx films with different phase composi-

tion are given in Figure 3. The growth of anatase phase

with increasing pO2is in good agreementwith experiments

of other researchers.[14–16] From this experiment, two

important issues can be drawn.

1. In
spite of the fact that the rutile is a high-T TiO2 phase,
it is formed on unheated substrate at lower values of pO2

compared with those needed to form the low-T TiO2

anatase phase.

2. T
iO2 films with pure anatase phase are formed in the
oxide mode of sputtering only.

No reason for the formation of high-T rutile phase in

sputtered TiOx film has been discovered till now. We

believe that the oxygen added to high energy (1 to several

DOI: 10.1002/ppap.200731303

Photoactivated Properties of TiO2 Films Prepared . . .

Figure 2. Evolution of deposition rate aD, phase composition, intensity of A(101)anatase X-ray reflection IA(101), roughness and WDCA air after UV irradiation for 1 hon the surface of �1 mm thick sputtered TiOx films with increasing pO2

. Depositionconditions: Wda¼ 56 W � cm�2, ds-t¼ 100 mm, and Tsurf� 160 8C.

tens of eV) Ti in reactive sputtering can act as ‘‘doping’’

element sputtered Ti in TiOx compound and high energy of

atoms are sufficient to form high-T rutile phase. In this

context the formation of high-T rutile phase at low

substrate temperature Tsurf� Thtp is a result of the rapid

cooling that accompanies the highly nonequilibrium

sputter deposition process operating at an atomic

level;[17]Thtp is here the formation temperature of high-T

phase at equilibrium. This hypothesis is based on our

previous experiments which already confirmed the for-

mation of high-T phases in Ti-based alloys; high-T c-bTi(Fe)

and c-bTi(Cr).[17] Our experiments indicate that formation

of high-T rutile phase is created at the conditions far form

the equilibrium (low pO2, low pT, low Tsurf, high aD) while

low-T anatase is preferred at the conditions closer to

equilibrium (high pO2, high pT, high Tsurf, low aD).

It is well known that a TiO2 film with the best

photoinduced hydrophilicity exhibits the lowest contact

Figure 3. Evolution of XRD patterns from sputtered TiOx films with increasing pO2

used in their deposition.



angle. The correlation between measured

hydrophilicity and the phase composition

of sputtered TiOx films is displayed in

Figure 2. From this figure, it is seen that

the best photoinduced hydrophilicity is

exhibited by TiO2 films containing either a

mixture of rutileþ anatase phases or

anatase phase only. These films are

produced at the boundary of the transition

mode and oxidemode of sputtering and in

the oxidemode of sputtering, respectively,

i.e., at low deposition rates aD� 25 nm �min�1. The surface roughness Ra of these

films at first increases up to �18 nm

(pO2� 0.09 Pa) and then decreases to about

10 nm with increasing pO2. It was found

that transparent TiO2 film with anatase

structure sputtered at pO2¼ 0.2 Pa (film D)

exhibits the very low value of air¼ 13 8Calready after 20 min of UV irradiation,

though the films is smooth with relatively

low roughness Ra� 10 nm.

Here, it is also worthwhile to note that an improvement

of the anatase crystallinity has a certain limit. When

anatase crystallinity surpasses a critical value (not

determined yet), the UV induced hydrophilicity

decreases.[12]

Hydrophilicity Versus Total Working Pressure

Generally, it is known that the crystallinity of sputtered

films can be improved when the substrate surface

temperature Tsurf increases or the total working pressure

pT decreases. In this section, the effect of pT on the TiO2 film

crystallization is investigated solely. Tsurf is kept as low as

possible because we try to develop a technological process

suitable for production of transparent crystalline TiO2

films on heat sensitive polymer substrates, for instance, on

polycarbonate (Tsurf� 90 8C).The total pressure pT influences collisions between

particles and so the energy of particles

incident on the surface of growing film.

The mean free path between particles

increases with decreasing pT. This results

in an increase in the energy delivered to the

growing film by condensing and bombard-

ing particles and an improvement of the

crystallization of TiO2 film. However, simul-

taneously also the phase composition of

TiO2 film varies, see Table 1. The content of

rutile phase in the TiO2 film decreases with

increasing pT. We believe that this trend

should be expected because the production

www.plasma-polymers.org S533

D. Herman, J. Sıcha, J. Musil

Table 1. Photoactivity, surface roughness and other properties of �1 mm thick TiO2 films prepared at fr¼ 100 kHz, Ida¼ 3 A, ds-t¼ 100 mm,pO2

¼0.15 Pa and different values of pT.

pT Structure air20 min air1 h air5 h Ra aD Tl¼ 550 nm Eg

Pa deg deg deg nm nm �minS1 % eV

0.60 rR a 45 22 17 16 6 66 3.10

0.75 rR a 30 13 12 12 7 68 3.11

0.80 aR r 12 9 9 8.5 7 66 3.10

0.90 aR r 20 8 7 7.5 7 72 3.15

1.00 Anatase 10 8 7 8 8 74 3.19

1.50 X-ray amorph. 11 10 10 8 15 72 3.23

3.00 X-ray amorph. 11 11 10 5.5 16 75 3.25

S534

of atomic oxygen increases with increasing pT due to

(i) increase in number of collisions and (ii) shift of

sputtering deeper to the oxide mode where the production

of high-T rutile phase at low values of Tsurf is stopped.

From Table 1 it is seen that amorphous films system-

atically exhibit values>3.2 eV. Eg for the filmwith anatase

phase only corresponds almost precisely to the anatase

bulk value 3.2 eV and film comprising both anatase and

rutile exhibit slightly lower Eg¼ 3.10–3.15 eV which is in

agreement with literature.[18] Values of transmittance

correspond to the film roughness due to light scattering on

film surface. The highest transmittance (75%) has been

observed for the smoothest film of Ra¼ 5.5 nm.

The following issues can be drawn from experimental

data given in the Table 1.

1. The good UV induced hydrophilicity characterized by

low air� 108 is exhibited by transparent (i) TiO2 films

composed of mixture anataseþ rutile, (ii) TiO2 films

with anatase structure only, and (iii) X-ray amorphous

Figure 4. Evolution of air on the surface of �1 mm thick TiO2 film sputtered at ds-t¼ 100 mmand pT¼0.75 Pa after UV irradiation for 20, 60, and 300 min with increasing average targetpower loading Wda.

TiO2 films.

2. X-ray amorphous TiO2 films with

the good UV induced hydrophili-

city (air� 108) have (i) the smooth-

est surface (Ra� 6 nm), (ii) high

values of optical band gap Eg(�3.24 eV), and are (iii) produced

at relatively high deposition rates

aD (�15 nm �min�1), i.e., twice as

rapidly as good hydrophilic TiO2

filmswith either anatase structure

or composed of mixture of anata-

seþ rutile.

3. Good hydrophilicity of X-ray

amorphous TiO2 films indicates

that their nanostructure can

enhance UV induced functions.

This investigation is now under

way in our laboratories.



Hydrophilicity Versus Target Power Density

Hydrophilicity of TiO2 films can be controlled also by the

value of average target power density Wda, see Figure 4.

Similar to pT Wda also influences the phase composition of

TiO2 films. In this experiment, changes in the phase

composition are caused mainly by variation of Tsurf which

decreases with decreasing Wda. A small WDCA (air� 158)on the surface of TiO2 film produced at Wda� 12 W � cm�2,

i.e., at Tsurf� 908, clearly shows that TiO2 films with good

hydrophilicity can be sputtered also on heat sensitive

substrates such as polycarbonate.

Hydrophilicity Versus Microstructure

The experiments described above indicate that not only

phase composition but also the microstructure decides on

the hydrophilicity of sputtered TiO2 films. Better hydro-

philicity is observed on TiO2 films sputtered at lower

values of Tsurf and exhibiting finer mictrostructure,

Figure 5 shows a dramatic effect of Tsurf on hydrophilicity

DOI: 10.1002/ppap.200731303

Photoactivated Properties of TiO2 Films Prepared . . .

Figure 5. Water droplet contact angle a of �1 mm thick TiO2 film with almostthe same surface roughness Ra� 7 nm sputtered at (a) Tsurf¼ 160 8C and(b) Tsurf> 250 8C with different microstructures as a function of UV irradiationtime tir.

of sputtered TiO2 films. Smaller grains are expected in the

films sputtered at a lower value of Tsurf. To determine the

size of grains HRTEM investigations of these films are now

under way. These results will be published elsewhere.

Conclusion

The film photoactivity characterized by a decrease in the

water contact angle a after UV irradiation has been studied

on �1 mm thick TiO2 films prepared on unheated glass

substrates by reactive magnetron sputtering. It has been

found that crystalline TiO2 thin films can be prepared at

low values of Tsurf� 160 8C when sputtered from pure

Ti(99.5) targets at relatively high averaged target power

loading Wda> 30 W � cm�2, ds-t¼ 100 mm, pO2> 0.3 Pa and

pT ranging from 0.75 to 1.5 Pa. The main results can be

summarized as follows.

1. Experiments carried out at different values of partial

pressure of oxygen pO2show that the nanocrystalline

anatase TiO2 films with no rutile phase can be prepared

only in a deep oxide mode, i.e., at pO2far away from the

transition mode of sputtering, and at low deposition

rates aD� 7 nm �min�1. These films exhibit the best

photoactivity. The water contact angle a on the surface

of these films decreases to 108 after 60 min of UV

irradiation.

2. The TiO2 films with mixed anataseþ rutile structure

prepared in the transition mode at higher values of

aD� 25 nm �min�1 also exhibit an excellent photo-

activity almost as good as that of the anatase films

mainly due to their higher roughness (Ra¼ 14–18 nm).

In contrast, TiO2 films with rutile structure exhibit

lower photoactivity compared to those with anatase

structure.3. Microstructure of TiO2 films seems to be of key

importance for photoactivity. Our experiments indicate



that TiO2 films exhibit traces of anatase

XRD reflections only and being close to an

amorphous structure, although their sur-

face is relatively smooth (Ra¼ 5.5 nm),

exhibit better photoactivity than films with

a good crystalline anatase structure. At

present, the optimum size of the grains and

their separation in amorphous matrix is,

however, not known.

4. TiO2 films with good hydrophilicity

(air< 158) can be sputtered at low values

of Tsurf� 908 in the oxide mode of sputter-

ing. This finding is of key importance

because it opens up a way to deposition

of photoactive TiO2 on heat sensitive

substrate, e.g., on polycarbonate.

Acknowledgements: This work was supported in part by theMinistry of Education of the Czech Republic under project no.MSM 4977751302 and in part by the European Community underproject PHOTOCOAT no. GRD1-2001-40701.

Received: September 1, 2006; Revised: November 10, 2006;Accepted: November 30, 2006; DOI: 10.1002/ppap.200731303

Keywords: films; low temperature sputtering; nanocrystallinefilms; pulsed discharges; structure; TiO2 thin films; wettability

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Photoactivated Properties of TiO2 Films Prepared by Magnetron Sputtering

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