Langmuir 2000, 16, 7289-7293 7289

Correlation between Surface Morphology and Hydrophilic/

Hydrophobic Conversion of MOCVD-TiO2 Films
Ha Yong Lee, Yong Hwan Park, and Kyung Hyun Ko*
Department of Materials Science & Engineering, Ajou University, Suwon 442-749, Korea

Received November 30, 1999. In Final Form: June 12, 2000

The relationship between hydrophilicity and surface morphology of TiO2 film was investigated. TiO2
films were deposited by MOCVD, and anatase films were crystallized. The rougher the surface of anatase
film, the slower the conversion from hydrophilic to hydrophobic state. Also in some films, even the sonication
could not accelerate the kinetics. However, the recovery rate to hydrophilicity under UV illumination had
a reverse dependency on the surface roughness. It was assumed that the rougher TiO2 films could have
a large concentration of Ti3+ generated by UV illumination on the surface, which plays a major role as
adsorption sites of -OH in the water as well as in the air. Therefore, the healing kinetics of Ti3+ by oxygen
in the air of a dark room could have been different among films with various surface roughnesses.
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Conclusively, the sustaining tendency of hydrophilic or hydrophobic properties could mainly depend on
the concentration of Ti3+ by UV light, and in turn on the processing conditions of film deposition.
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I. Introduction the characteristics of hydrophilic/hydrophobic conversion

of TiO2 films, would be suggested. As reported in Machi-
Recently TiO2, usually known as a typical dielectric da,13 hydrophilicity was affected by the amount of SiO2
material, has been studied actively in the area of addition, because surface morphology on TiO2 films has
environmental technologies, such as a photocatalyst for been changed with SiO2 addition. Therefore, it can be
decomposing organic compounds when TiO2 films are assumed that certain factors affecting the surface mor-
illuminated by UV light.1-9 Furthermore, the hydrophilic phology of films such as the film preparation technique
coating (0° water contact angle) has emerged as a new could play a significant role in the hydrophilic/hydrophobic
and very attractive application of TiO2. conversion cycle. Regarding surface morphology related
TiO2 films have amphiphilic properties under UV to hydrophilicity of TiO2 film, the preparation method of
illumination so that they can be utilized for many kinds many research studies has usually been sol-gel or
of applications such as anti-fogging and self-cleaning sputtering. In this work, the relationship between hy-
coatings of glasses and ceramics.10 According to R. Wang11 drophilic/hydrophobic conversions of TiO2 films deposited
and A. Fujishima,12 Ti4+ ions on TiO2 surfaces are by MOCVD has been investigated in terms of surface
converted into Ti3+ ions by UV light, and the characteristics morphology.
of charge transfer between Ti ions and adsorbents such
as hydroxyl ion and/or oxygen can be altered by UV II. Experimental Section
radiation. Consequently, the hydrophilicity of TiO2 films
would be established. Also, it was conceivable that the II.1. Preparation of TiO2 Films. TiO2 films were deposited
existence and quantity of Ti3+/Ti4+ on the surfaces could by MOCVD on bare and rutile-presputtered glass substrate. As
be reset by the cyclic use of UV radiation and (ultrasonic starting materials, TTIP (titanium tetrakis(isopropoxide), Aldrich
Ltd.) was loaded in the bubbler. The substrate temperatures
+ dark room) treatment in which ‚OH and O2 healing of
were 400 and 500 °C, while the bubbler temperature was kept
surface occurred, resulting in restoration of hydrophobic at 60 °C and the flow rates of carried gas were 50 and 110 sccm,
state. respectively. The reactant gases were carried by nitrogen gas
Because amphiphilicity also depends on adsorption and sprayed from the nozzle to the substrate. The deposition
properties of the film surface, another factor, which affects pressure was 1 Torr. Film thickness and deposition rate were
measured by a-step.
(1) Kennedy, J. C., III; Datye, A. K. J. Catal. 1998, 179, 375. Crystalline phases of TiO2 films were measured by X-ray
(2) Fox, M. A.; Dulay, M. T. Chem. Rev. 1993, 93, 341. powder diffraction (XRD; M18XHF-SRA, McScience, Japan), and
(3) Hoffmann, M. R.; Martin, S. T.; Choi, W.; Bahmann, D. W. Chem. surface morphologies of films were investigated using scanning
Rev. 1995, 95, 69. electron microscope (SEM; S-2400, Hitachi, Japan) and atomic
(4) Shin, E. M.; Senthurchelvan, R.; Munoz, J.; Basak S.; Rajeshwar, force microscope (AFM; Ls, Park instrument). The Ti atom state
K. J. Electrochem. Soc. 1996, 143 (5), 1562.
(5) Vorontsov, A. V.; Savinov, E. N.; Baranik, G. B.; Troitsky, V. N.;
on film surface was analyzed by X-ray photoelectron spectroscopy
Parmon, V. N. Catal. Today 1997, 39, 207. (XPS; Ariesarsc 16MCD150, Vacuum Science Workshop, United
(6) Hung, C. H.; Marinas, B. I. Environ. Sci. Technol. 1997, 31, 1440. Kingdom). The stoichiometries of films were analyzed by RBS
(7) Dominguez, C.; Garcia, J.; Perdraz, M. A.; Torres, A.; Galan, M. using a 2 MeV tangem type ion accelerator.
A. Catal. Today 1998, 40, 85. II.2. Test of TiO2 Films Hydrophilic/Hydrophobic Con-
(8) Wang, T.; Wang, H.; Xu, P.; Zhao, X.; Liu, Y.; Chao, S. Thin Solid version. To make as-deposited TiO2 films into the hydrophilic
Films 1998, 334, 103.
(9) Wang, H.; Wang, T.; Xu, P. J. Mater. Sci.: Mater. Electron. 1998, state, films were illuminated by Hg lamp (10 W, Philips). Then,
9, 327. TiO2 films with hydrophilic state were sonicated for 1 h by the
(10) Wang, R.; Hashimoto, K.; Fujishima, A.; Chikuni, M.; Kitamura, ultrasonic source with a 60 Hz generator followed by storage in
A.; Shimohigoshi, M.; Watanabe, T. Adv. Mater. 1998, 10(2), 135. a dark room for up to 7 days considering the kinetics of the
(11) Wang, R.; Hashimoto, K.; Fujishima, A.; Cjikuni, M.; Kojima, conversion of hydrophilic to hydrophobic state. Thereafter, films
E.; Kitamura, A.; Shimohigoshi, M.; Watanabe, T. Nature 1997, 388,
431.
(12) Sakai, N.; Wang, R.; Fujishima, A.; Watanabe, T.; Hashimoto, (13) Machida, M.; Norimoto, K.; Watanabe, T.; Hashimoto, K.;
K. Langmuir 1998, 14, 5918. Fujishima, A. J. Mater. Sci. 1999, 34, 2569.

10.1021/la9915567 CCC: $19.00 © 2000 American Chemical Society

Published on Web 08/04/2000
7290 Langmuir, Vol. 16, No. 18, 2000 Lee et al.

Figure 1. XRD patterns of TiO2 films deposited at (a) 400 °C,

(b) 500 °C on the glass (flow rate, 50 sccm; bubbler temperature,
60 °C). Figure 2. XRD patterns of TiO2 films (a) deposited by
sputtering on the glass substrate, (b) deposited at 500 °C by
were reilluminated for 120 min by the same UV light. In the MOCVD on rutile film deposited by sputtering (flow rate, 50
respective process, the contact angle of water droplet on TiO2 sccm; bubbler temperature, 60 °C).
surfaces, and thus the surface wettability, was evaluated as a
measure of hydrophilicity. 50 sccm on the glass. In the case of 400 and 500 °C, rate-
limiting steps are surface-reaction controlled and diffu-
III. Results and Discussion sion-controlled kinetics with the deposition rate of 3200
III.1. Phase of TiO2 Films. Figure 1 shows X-ray and 2000 Å/min, respectively. It is, therefore, interfered
diffraction patterns of TiO2 films deposited at 400 and that surface morphology of TiO2 film deposited at 500 °C
500 °C on glass substrates. All as-deposited films were is more rough than that at 400 °C owing to the limited
crystallized as anatase phases. Four peaks of TiO2 films time of position adjustment for incident molecules with
deposited at 400 °C are corresponding to (101), (200), (211), higher velocity.
and (220) planes, while it seemed that films deposited at By the same token, the flow rate could also affect surface
500 °C had a different set of peaks such as (101), (004), morphology via altering deposition rate. In the surface
(112), (105), (211), and (220), presumably textured.15 As reaction controlled kinetics regime such as 400 °C, the
shown in Figure 2, the phase of films deposited by deposition rate is increased with flow rate. For example,
sputtering on fresh glass was only rutile and then that of the deposition rate of 50 and 110 sccm at 400 °C are 2000
film by MOCVD on rutile-predeposited substrate was and 3300 Å/min, respectively. Meanwhile, those of the
anatase. It was found that all films have a stoichiometry diffusion-controlled regime such as 500 °C showed satu-
by measuring RBS. rated behavior (Figure 6).
III.2. Surface Morphologies of TiO2 Films. It was Rutile-predeposited substrate seemed to provide surface
observed that the surface roughnesses of anatase films features different from polished glass substrate: that is,
are considerably different depending on preparation predeposited film was deposited by sputtering and crys-
conditions such as substrate temperature, flow rate, and talline phase with (110) texture. So, it could be assumed
substrate. Figure 3 shows SEM photographs of the that the differences in surface atomic mobility and/or
surfaces of anatase films deposited with various deposition ability of lattice matching can affect the final roughness
parameters. The surface of Figure 3d was the smoothest of anatase film on this substrate resulting, in very smooth
of all, and the surface became rougher in the order of (a) surface feature.
f (c) f (b). For quantitative measurements of surface III.3. The Contact Angle Changes of Water on TiO2
morphologies of anatase films deposited at each condition, Surface. Figure 7 shows the cyclic change of contact angles
films were analyzed by AFM (Figure 4.) resulting in the of water dropped on the UV-illuminated anatase films
same trend of surface morphologies observed by SEM. that appeared in Figure 3 after 60 min of sonication and
The RMS of roughnesses of these films were (a) 129, (b) then storage in the dark room for 7 days followed by
274, (c) 217, and (d) 85 Å, respectively. reillumination of UV light. All as-deposited anatase films
The reason for different surface morphology with by MOCVD showed hydrophobic properties and im-
substrate temperature (Figure 3a,b and Figure 4a,b) is mediately changed into the hydrophilic state after UV
due to the change of deposition rate. Figure 5 shows the illumination. When hydrophilic anatase films were stored
deposition rate of TiO2 with substrate temperature with in the dark room, the degree of hydrophobicity tended to
(14) Linsebigler, A. L.; John, G. Lu.; Yates Jr. T. Chem. Rev. 1995,
increase up to certain saturation values and sonication
95, 735. seemed to accelerate this process. For example, the contact
(15) JCPDS card 21-1272. angle of anatase film showed in Figure 3a increased up
Hydrophilicity and Surface Morphology of TiO2 Film Langmuir, Vol. 16, No. 18, 2000 7291

Figure 3. Surface morphology of anatase films by scanning electron microscope deposited at (a, top left) 400 °C, (b, top right) 400
°C with 110 sccm of flow rate, (c, bottom left) 500 °C, (d, bottom right) 500 °C with 50 sccm on predeposited rutile substrate (substrate
of (a), (b), and (c): glass; flow rate of (a), (c), and (d): 50 sccm).

Figure 4. Surface morphology of anatase films by atomic force microscope deposited; (a)-(d) are as in Figure 3.
to about 75° of saturation angle in the dark room for 7 hydrophilicity could be fully recovered to the original state
days. After those films were illuminated by UV light, the and complete cycle.
7292 Langmuir, Vol. 16, No. 18, 2000 Lee et al.

Figure 7. Changes of contact angle after 1 h of sonication and

subsequent storage in the dark room for 7 days followed by UV
illumination; (a)-(d) are as in Figure 3. Region 1, storage time
Figure 5. Substrate temperature dependence of deposition in dark room; region 2, UV illumination time.
rate of TiO2 film; (bubbler temperature, 60 °C, flow rate, 50
sccm). From the results mentioned in sections III.2 and III.3,
it was found that changes in hydrophilicity of TiO2 films
have been very closely related to surface morphology. TiO2
films with high roughness, such as anatase films deposited
at 400 °C with 110 sccm of flow rate (film b), had the
slowest conversion to hydrophobic state, resulting in the
smallest contact angle after 7 days of dark room storage.
This film also showed the fastest recovery of hydrophilicity
under UV illumination. The recovery rate to hydrophilicity
increased in the order of (d) f (a) f (c) f (b), which
coincided with the order of roughness.
According to R. Wang,10 from the investigation of water-
wetted surface of hydrophilic film by Fourier transform
infrared spectroscopy (FT-IR), it was claimed that IR bands
could be assigned to the stretching of OH groups sup-
posedly chemisorbed on Ti3+ on the surfaces. From XPS
data, Ti 2p spectra of two samples (parts a and c of Figure
3, respectively) are shown in Figure 8. While the peak of
sample with rougher surface shows a shoulder around
457 eV, there is no such trace in the peak corresponding
to the smoother one. This overlap is consistent with the
creation of Ti3+ on the surface by UV illumination,16 and
Figure 6. Effects of precursor flow rate on the deposition rate
of MOCVD TiO2.
the contribution of Ti3+ /Ti4+on the surface is about 15%.
It was also found that for both samples in Figure 8,
As depicted in Figure 7, the characteristics of cyclic adsorbed OH groups on TiO2 film with rough surface were
change of contact angle, and so the hydrophilic T detected12,16 but OH groups on smooth surface film were
hydrophobic state, were different from specimen to not detected from XPS measurement. (Figure 9) Therefore,
specimen. For the film denoted by (b) in Figures 3 and 7, it is conceivable that anatase films with high roughness
the saturation contact angle of the specimen was as low such as Figure 3c would have more Ti3+ by generated UV
as 15° and the conversion kinetics from hydrophilic to light, and thus more adsorption sites of OH groups on the
hydrophobic state was the slowest of all. Furthermore, TiO2 surface than those on smooth films (Figure 3a).
there seemed to be minimal effect of sonication for this Because both smooth and rough surface showed super-
film. On the contrary, other films showed fast conversion hydrophilicity after UV illumination (Figure 7), it can be
kinetics after sonication so that the contact angle increased assumed that Ti3+ and OH groups concentration of two
up to 60-100% of the saturation values, demonstrating samples were similar and that the differences in concen-
large acceleration efficiency of sonication for surface state tration of both films were generated during dark room
conversion. In the recovering stage to the original hy- storage.
drophilic state, the kinetics was reversed. The contact According to N. Skai,12 the oxygen in the air was
angle of film b which has the lowest hydrophobic property responsible for oxidizing of hydrophilic surface via Ti3+ f
of all, returned to 0° right after 10 min of UV illumination. Ti4+ state during storage in the dark room. Therefore, it
However, films c and a required 50-100 min to be fully was seemed that during the dark storage period, so-called
recovered. Most significantly, in the case of film d, the “oxygen healing”, could be a candidate for hydrophilic/
hydrophilic surface state did not return at all, showing
virtually no change from the saturation value even after (16) Li-Qiong, W.; Baer, D. R.; Engelhard, M. H. Surf. Sci. 1994, 302,
2 h of UV illumination. 295.
Hydrophilicity and Surface Morphology of TiO2 Film Langmuir, Vol. 16, No. 18, 2000 7293

Figure 8. Ti 2p XPS spectra of TiO2 thin films at (a, top) 400

°C and (b, bottom) 500 °C on glass (flow rate: 50 sccm). The Figure 9. O 1s XPS spectra of TiO2 thin films at (a, top) 400
samples are illuminated by UV light for 2 h and then stored °C and (b, bottom) 500 °C on glass (flow rate: 50 sccm). The
in the dark room for 1 day. samples are illuminated by UV light for 2 h and then stored
in the dark room for 1 day.
hydrophobic conversion mechanism. In light of this
hypothesis, it is likely that the smoother surface can be hydrophilicity within as short as 10 min of UV illumina-
more susceptible to oxidation of Ti3+ during dark room tion, the smoother films such as anatase film deposited
storage and, therefore, that the regeneration of hydrophilic at 500 °C on rutile predeposited substrate had 70°of contact
state by UV illumination would take more time than the after the same treatment but sustained the hydrophobic
rough surface. state even after 2 h of illumination. Assuming the kinetics
of surface conversion depends on the concentration of so-
IV. Conclusions
called unhealed Ti3+ generated by UV illumination, the
It was suggested that the characteristics of hydrophilic/ rougher the surface, the slower conversion to hydropho-
hydrophobic conversion of MOCVD-TiO2 films are very bicity and the faster recovery to hydrophilic state because
closely related to their surface morphologies. While the more Ti3+ on the surfaces could exist. Therefore, deposition
water contact angle of anatase film with the largest conditions such as substrate temperature and flow rate
roughness, which was deposited at 400 °C with 110 sccm of precursor can alter the surface conversion of TiO2 films.
of flow rate, was about 15° after 1 h of sonication followed
by subsequent dark room treatment for 7 days and restored LA9915567