Crystal Structure Theory and Applications, 2013, 2, 1-7 Published Online March 2013 (
Plasma-Assisted Chemical Vapor Deposition of TiO 2 Thin
Films for Highly Hydrophilic Performance
Satoshi Yamauchi1*, Yoh Imai2
1Department of Biomolecular Functional Engineering, Ibaraki University, Hitachi, Japan
2Department of Electric and Electronic Engineering, Ibaraki University, Hitachi, Japan
Email: *
Received October 31, 2012; revised December 8, 2012; accepted December 21, 2012
Titanium-oxide layer was grown on glass substrate by plasma-assisted chemical vapor deposition (PCVD) using oxygen
gas plasma excited by radio-frequency power at 13.56 MHz in the pressure as low as 3mtorr at relatively low tempera-
ture below 400˚C, and studied on the crystallographic properties with the hydrophilic behavior comparing to the layer
deposited by low-pressure chemical vapor deposition (LPCVD). Raman spectra indicated anatase-phase TiO2 layer
without amorphous-phase could be formed above 340˚C by simultaneous supply of plasma-cracked and non-cracked
titanium-tetra-iso-propoxide (TTIP) used as preliminary precursor. Surface Scanning Electron Microscope images indi-
cated the PCVD-layer consists of distinct nanometer-size plate-like columnar grains, in contrast to rugged microme-
ter-size grains in the LPCVD-layer. Extremely small water contact angle about 5˚ in dark and the quick conversion to
super-hydrophilicity by UV-irradiation with a light-power density as low as 50 W/cm2 were observed on the PCVD-
layer grown at 380˚C, while the large initial contact angle was above 40˚ and the response for the UV-irradiation was
gradual on the LPCVD-layer.
Keywords: PCVD; Titanium-Oxide Films; Anatase-TiO2; Hydrophilicity
1. Introduction
Titanium dioxide (TiO2) has been extensively investi-
gated in view of photo-induced applications using the
photo-catalytic reactions and the hydrophilicity on the
surface [1,2], in addition to electronic and optoelectronic
applications [3,4]. Commonly, anatase-phase TiO2 is pre-
ferred for the photo-induced applications because of the
efficient surface reaction by UV-irradiation compared to
another crystal phases (brookite, rutile) [5]. In addition,
the defects in the crystal should be also taken into ac-
count to control the reaction because it is recognized that
the Ti3+ sites reduced from Ti4+ at the surface by photo-
excited electrons accompanying oxygen vacancies gen-
erated by the photoexcited holes play an important role in
the photo-induced surface reaction [6]. Therefore, a low
of wet or dry process to fabricate anatase-TiO2 layer has
been advanced to control the crystallinity such as stoi-
chiometric composition, impurity concentration, surface
morphology, crystal orientation and so on. In such proc-
esses, wet process such as dip-coating, spray or sol-gel
has been widely used for the photocatalytic and hydro-
philic coating [7,8] with advantages of the low-cost and
the wide-area coatings. In contrast, dry process such as
reactive sputtering [9], electron beam evaporation [10]
and metalorganic chemical vapor deposition [11] has
been attractively studied to control the growth behavior.
Plasma-assisted chemical vapor deposition (PCVD) has
been candidate for the TiO2 synthesis process for low
temperature deposition, control of the grain structure and
the surface morphology [12-14]. In the PCVD process,
titanium tetra-iso-propoxide (TTIP) has been widely used
as the preliminary precursor in order to reduce contami-
nation in the layer [15], where O2 gas is simultaneously
used as the oxidant gas for the efficient reduction. The
metal oxide dissociated in the plasma [13] brings about
the low temperature deposition below 300˚C and the
formation of highly dense columnar grains compared to
the CVD-layers. However, the temperature to grow ana-
tase-phase TiO2 is increased above 450˚C, whereas the
anatase-TiO2 can be grown at the temperature above
300˚C by CVD [13]. The unfavorable increase of the
growth temperature is probably come from hindering the
crystal-phase formation by ion bombardment of excess
energy particles [10] and poor surface migration of the
metal-oxides on the growth surface. It is noted that the
PCVD had been demonstrated in relatively high pressure
above 0.1 torr by relatively high rf-power above 100 W
[14] aiming at enough dissociation of the precursor in the
*Corresponding author.
opyright © 2013 SciRes. CSTA
plasma. On the other, the deposition enhanced chemical
dissociation by reactive oxidants such as atomic-oxygen
results in reduction of the TTIP-dissociation energy and
the high growth rate as demonstrated by remote plasma-
enhanced CVD [16]. The useful effects brought about the
low-temperature growth including ana-tase-TiO2, how-
ever, the layer grown at 350˚C was formed by relatively
low dense grains with poor crystallinity compared to the
CVD-layer. It is considered that the results indicated the
thermal-dissociation of TTIP without enhancement by
the reactive oxygen is more effective to growth anatase-
phase TiO2.
In this paper, PCVD of TiO2 supporting the crystalli-
zation in anatase-phase by thermal-dissociation of TTIP
is demonstrated at low-temperature below 400˚C and
hydrophilic property on the anatase-phase layer is shown
with the crystallinity and the surface morphology.
2. Experimental
Figure 1 shows a bell-jar type PCVD apparatus consists
of a diffusion-pump (D.P.) and a rotary-pump (R.P.) for
TiO2 deposition. The back-pressure in the chamber was
under 1 × 105 torr. An inductively coupled electrode to
introduce radio-frequency (rf) power at 13.56 MHz was
equipped in the chamber between a substrate holder and
gas inlets. A Coil to apply DC-magnetic-field of 3000
gauss at the center of the chamber was also settled
around the bell-jar to stabilize gas plasma excited by the
rf-power, because PCVD in this study was performed in
the pressure as low as 3mtorr and the low density plasma
excited by 10 W rf-power. In the case of LPCVD, the
rf-power was not induced during the growth. The tem-
peratures of substrate holder and gas inlet for preliminary
precursor were increased by resistive-heating and con-
trolled by PID-systems.
Titanium tetra-iso-propoxide (Ti(O-i-C3H7)4 : 97%-pu-
Figure 1. An apparatus of PCVD for TiO2 deposition.
rity) was used as preliminary precursor. The liquid-phase
TTIP was charged in a quartz-cell and then purified in
vacuum at 50˚C for 3hrs to remove volatile solvent in the
liquid. The purified TTIP was vaporized at 70˚C and
introduced into the chamber without any carrier gas
through a stainless tube and a variable-valve at the tem-
peratures about 90˚C. Pure oxygen gas (99.9999%-purity)
was also introduced through a stainless tube and a vari-
able-valve. The supply ratio of TTIP/O2 was controlled
by monitoring the chamber pressure when the TTIP and
the O2 were introduced into the chamber.
Quartz plates with mirror-surface used as substrates
were rinsed in deionized water and dried by a spinner
after removal of contaminations from the surface by or-
ganic solvents and a hot H2SO4 + H2O2, and then ther-
mally cleaned at 400˚C in the chamber in low pressure
below 1 × 105 torr for 30 min before the growth. Prior to
the growth, inner-wall of the chamber was cleaned by
oxygen plasma excited by 100 W rf-power to remove
residual gas, and then titanium-oxide layers were grown
at low temperatures ranging from 150˚C to 400˚C.
Thickness of the layer was checked by a contact-type
surface profiler (DEKTAK150). The surface morphology
was observed by SEM (HITACHI S-800) and the crys-
tallinity was investigated using Raman spectrometer
(JASCO NR-1100) using the 514.5 nm line of an Ar+
laser (100 mW) as the excitation source. Hydrophilicity
on the layer was evaluated by contact angle of water
when deionized water (10 μl) was dropped on the layer in
air at 20˚C with 50%-humidity. The photo-induced hy-
drophilicity was examined as a function of UV-irradia-
tion time, in which a black-light peak at 365 nm with the
low light power density of 50 μW/cm2 was used as the
light source.
3. Results and Discussions
3.1. Growth Rate
Figure 2 shows growth rates of titanium-oxide layers by
Figure 2. Growth rates of Titanium-oxide layer by PCVD
(closed circles), LPCVD (open circles) and thermal-disso-
ciation in PCVD (closed triangles).
Copyright © 2013 SciRes. CSTA
PCVD (closed circles) and LPCVD (open circles) at va-
rious temperatures, where the O2/TTIP supply ratio was
controlled to 1. The rate by LPCVD was increased with
the growth temperature above 320˚C according to Ar-
rhenius relationship and the activation energy was ob-
tained as 164 kJ/mol. The energy value was seemed to be
larger than the other reports [13,16] using TTIP and O2
but similar to the value of 150 kJ/mol by CVD using
TTIP in N2-gas with the high flow rate [17]. It has been
recognized that oxygen reduces the dissociation energy
of TTIP, but the large activation energy indicates the
dissociation of TTIP by LPCVD in such low pressure of
3mtorr with the small supply ratio of O2/TTIP was owing
to thermal-dissociation as follow [18],
3742 3637Ti(OC H )TiO2C H2HOC H 
On the other hand, two activation energies were observed
by PCVD. The activation energy in the low temperature
below 300˚C was 4.5 kJ/mol which was in good agree-
ment with the previous report by PCVD [13] but much
lower than that by thermal-dissociation enhanced by re-
active-oxygen [16]. The significant low energy indicated
that the preliminarily precursor was dissociated to tita-
nium-oxides in the plasma as described elsewhere [13].
In contrast, the activation energy between 300˚C and
380˚C was determined as 163 kJ/mol (closed-triangles in
Figure 2) after removal the growth rate extrapolated
from the rate in the low temperature region performed by
the plasma-cracked precursors. The activation energy
above 300˚C was coin-cident to that by LPCVD. It was
reported that activation energy of TTIP-dissociation was
reduced by plasma-excited oxygen [16], however, the
enhanced dissociation was not observed in this work be-
cause the collision probability between TTIP and oxygen
was relatively small in such low pressure with the small
O2/TTIP sup-ply ratio. The decreased growth rate above
400˚C was believed to be caused by the depletion of the
precursor at the growth surface due to the high desorp-
tion coefficient or the volumetric dissociation in the
gas-phase. It should be concluded from these results that
the PCVD above 300˚C was performed by supply of the
metal-oxides dissociated in the plasma, which was ex-
pected to form highly dense grain growth by the high-
sticking coefficient, and thermal dissociation of TTIP,
which was expected to enhance the crystallization into
the anatasephase.
3.2. Raman Spectra
Figure 3 shows Raman spectra of 600 nm-thick tita-
nium-oxide layers grown by PCVD at various tempera-
tures. Typical peaks corresponding to anatase-TiO2 were
appeared at 144, 399, 514 and 639 cm1 in the layers
grown above 340˚C, where the peaks were attributed to
Eg, B1g, B2g and Eg vibration modes respectively [19].
However, any peaks except a weak broadband around
450 cm1, which was recognized due to amorphous phase
of TiOx [20], could not be observed for the layer grown
at 250˚C.
The broadband was decreased with the deposition
temperature and disappeared in the layer grown above
340˚C. The broadband was appeared for the layer grown
at the temperature ranging from 300˚C to 330˚C but the
weak Eg-band could be also observed. It is noted here
that the low-temperature about 300˚C to form anatase-
TiO2 was much lower than 450˚C by PCVD reported
previously [13], but consistent with the temperature for
anatase-TiO2 growth by LPCVD using TTIP as single
precursor [21]. The significantly low temperature to form
anatase-TiO2 suggested that the crystallization could be
supported by the non-cracked TTIP which was simul-
taneously supplied with the plasma-cracked precursors
and thermally dissociated on the growth surface. Figures
4(a) and (b) show growth rate ratio by the CVD-mode
(GRthermal) and the PCVD-mode (GRplasma) thermal-disso-
ciation as a function of the growth temperature and inte-
grated intensity of Eg-mode peak at 144 cm1 as a func-
tion of the GRthermal/GRplasma ratio, respectively. The ratio
of GRthermal/GRplasma except the open circle (grown at
400˚C) was rapidly increased with the growth tempera
Figure 3. Raman spectra of PCVD-layers with 600 nm-
thickness grown at various temperatures.
(a) (b)
Figure 4. (a) Rel ative growth rate by thermal dissociation
(GRtherma l) compared to growth by plasma cracked pre-
cursors (GRplasma) as a function of the growth tempera-
ture; and (b) Eg-intensity as a function of GR thermal/GRplasma,
where the open circles shows for the layer grown at
Copyright © 2013 SciRes. CSTA
ture above 300˚C according to rapid increase of the
thermal dissociation of TTIP. In contrast, the intensity of
Eg-mode was rapidly increased and gradually saturated
with the ratio of GRthermal/GRplasma, which indicated crys-
tallization in anatase-phase was enhanced by relatively
small amount of the thermally dissociated TTIP com-
pared to the supply of the plasma-cracked precursors. In
the case of the growth at 400˚C, decrease of the Eg-band
intensity (open circle in Figure 4(b)) and the weak
broadband due to amorphous-phase (Figure 3) suggested
the dissociated precursor in the gas-phase was simulta-
neously supplied on the growth surface.
Figure 5 shows the Eg-band spectra of PCVD- and
LPCVD-TiO2 layers grown at 380˚C with 600 nm-
thickness, where the background was numerically re-
moved in the spectra. The spectrum with the FWHM of
10.7 cm1 of the PCVD-layer was slightly sharper than
that of the LPCVD-sample with the FWHM of 12.0 cm1.
Additionally, Raman-spectrum shift of the PCVD-TiO2
was small below 1 cm1, while the shift of the LPCVD-
layer was about 2 cm1. It is recognized that the Ra-
man-spectrum shift is originated from residual stress in
the layer [22] on the substrate with different thermal-
expansion coefficient. Previously, Alhomoudi et al. re-
ported Eg-band of anatase-TiO2 layer was shifted toward
higher wavenumber above 3 cm-1 broadening about 23
cm1 in the thick layer about 600 nm, where the layer
was grown by reactive-sputtering around 300˚C, and
concluded the spectrum shift and broadening were
caused by residual compressive stress and large distribu-
tion of the orientation of grains in the layer, respectively
[23]. In contrast, both of the shift and the broadening
were much smaller in the PCVD-TiO2 layer as shown
above, which clearly indicated the residual stress in the
PCVD-layer was relaxed and speculated the layer con-
sisted of uniform grains.
3.3. Surface Morphology
Figures 6(a) an d (b) show surface SEM images of 600
nm-thick PCVD-layer and LPCVD-layer grown at 380˚C,
Figure 5. Eg-band spectra of PCVD-(solid-line) and LP-
CVD-layer (dotted-line), where the both layers were grown
at 380˚C.
respectively. Relatively large and rugged grains with
sub-micrometer size were found in the LPCVD-layer,
which indicated the growth was performed by the low
density nucleation at the initial stage and/or the secon-
dary nucleation in the deep grain boundaries. In contrast,
the PCVD-layer was formed by highly dense plate-like
nano-grains with the width around 45 nm. It can be easily
recognized that the significant difference of the grain
feature compared to the LPCVD-layer was caused by
highly dense nucleation at the initial stage and prefe-
rential growth along the thickness. The nucleation with
high density could be performed by the plasma cracked
metal-oxides with the high sticking coefficients, then the
grains were grown by simultaneous supply of the plas-
ma-cracked precursors and non-cracked TTIP, where the
grain growth was governed by supply of the plasma-
cracked precursors in PCVD-mode as shown in Figure
4(b). In addition, it is considered that the distinct nano-
grains in the PCVD-layer resulted in relaxation of the
bi-axial stress observed in the Raman spectrum.
3.4. Hydrophilicity
Figure 7 shows contact angle of water on the PCVD-
layers grown at various temperatures ranging from 250˚C
to 380˚C as a function of irradiation time of UV-light
Figure 6. Surface SEM images of 600 nm-thick TiO2 layers
grown by (a) PCVD and (b) LPCVD at 380˚C.
Figure 7. Contact angle of water on TiO2 layers grown at
various temperatures by PCVD as a function of UV-irra-
diation time. The inset shows variation of the angle on
CVD-TiO2 layer grown at 380˚C for UV-irradiation time.
Copyright © 2013 SciRes. CSTA
peak at 365 nm-wavelength with the low power density
of 50 μW/cm2. The inset shows the dependence of
LPCVD-TiO2 layer grown at 380˚C. Here, the grown
layer was initially treated in atmosphere by the UV-irra-
diation for a few hours required to achieve super-hy-
drophilicity and exposed in air in dark for 1 month, and
then the contact angle on the layers irradiated by the
UV-light for each time was evaluated after the initial
contact angle was checked before the UV-irradiation.
The hydrophilicity on the PCVD-layer including the
initial angle and the hydrophilization feature by the
UV-irradiation was significantly dependent on the gro-
wth temperature. The contact angle was around 40˚ be-
fore the UV-irradiation and not responsible to the irradia-
tion on the layer grown at low temperature below 250˚C.
Nakamura et al. previously showed the amorphous tita-
nium-oxide deposited at low temperature below 200˚C
by PCVD included hydroxyl which plays an important
role of the hydrophilicity and the contact angle on the
layer was responsible for UV-irradiation [24]. However,
absorption due to -OH bond could not be observed in
FTIR spectrum of the amorphous-phase PCVD-layer
grown at 250˚C, and the contact angle was not responsi-
ble for the UV-irradiation. On the other, the contact angle
on the layer consists of anatase- and amorphous-phase
mixture grown at 300˚C was reduced by the UV-irradia-
tion but the initial contact angle was similar to the amor-
phous-sample. In contrast, the initial contact angle on the
layer grown above 340˚C, which consists of anatase-
phase grains without amorphous-phase, was obviously
decreased with the growth temperature and the prompt
hydrophilization was observed by the UV-irradiation.
Especially, the contact angle on the PCVD- layer grown
at 380˚C was extremely small about 5˚ before the UV-
irradiation and quickly converted to super-hydrophilicity
within 5 min by the UV-irradiation. It is recognized that
Ti3+ sites reduced from Ti4+ in TiO2 crystal cause the
hydrophilic conversion. The influence of surface mor-
phology should be also considered in the hydrophilicity,
however, the grain structure as shown in Figure 3 was
scarcely dependent on the growth temperature. Therefore,
it can be concluded that the hydrophilic property de-
pendent on the growth temperature was originated from
the density of Ti3+ sites at the surface. On the other, the
large initial contact angle about 40˚ and the gradual re-
sponse for the UV-irradiation were observed on LPCVD-
layer grown at 380˚C (inset of Figure 7), while the layer
consists of anatase-TiO2 without the amorphous-phase.
Capillary effect in addition to chemical property on the
surface should be taken into account to recognize the
significantly different feature of hydrophilic properties.
Previously, Katsumata et al. showed the hydrophilic
conversion during UV-irradiation was enhanced on the
sol-gel derived TiO2 layer consists of nano-grains with
the size around 50 nm by comparison of the property on
the layer consists of relatively large 150 - 200 nm grains
[25]. They found out that the difference of hydrophili-
cizing rate by UV-irradiation was originated from ho-
mogeneity of the surface and suggested the hydrphiliciz-
ing was enhanced by two-dimensional capillary effect on
the homogeneous microstructure. Although the grain-
structure in the PCVD layer as shown in Figure 6(a) was
different from the sol-gel derived layer, the surface con-
sists of nano-grains was homogeneous compared to re-
markably heterogeneous surface of the LPCVD as shown
in Figure 6(b). It has been also recognized that the hy-
drophilicity after UV-irradiation is gradually degraded in
dark since the life-time of -OH bond at the surface is not
so long, however, long-time storage behavior for the su-
per-hydrophilicity above 5 hrs was observed on the
PCVD-TiO2 layer. This useful behavior was speculated
by percolation of water into the nano-scale slits between
the grains as shown in Figure 6(a), where the percolated
water was probably kept in the pores for long-time
avoiding the evaporation compared on the surface. Of
course, the storage behavior could not be observed on the
LPCVD-layer, in which the contact angle was gradually
increased to 40˚ within 1 hr after exposure in dark.
4. Conclusion
Titanium-oxide layers were grown on quartz substrates
by PCVD using TTIP and oxygen mixed gas plasma with
O2/TTIP supply ratio of 1 in the pressure as low as 3
mtorr at low temperatures below 400˚C, and then char-
acterized by the growth behavior and the hydrophilicity
comparing to the layer by LPCVD. The PCVD was per-
formed by plasma-cracked precursors and thermally dis-
sociated TTIP, while the LPCVD was owing to thermal
dissociation of TTIP. Raman spectra investigated ana-
tase-TiO2 could be grown above 300˚C by the PCVD.
Further, it was revealed that the crystallinity of ana-
tase-TiO2 was significantly dependent on the ratio of the
PCVD-mode and the CVD-mode during the growth,
where non-cracked TTIP played a role to enhance the
crystallization of the plasma-cracked precursors on the
growth surface. SEM observations showed that the
PCVD anatase-phase TiO2 layer was formed by homo-
geneous nano-size columnar grains, whereas the LPCVD
layer consists of heterogeneous micro-size grains. Initial
contact angle of water before UV-light irradiation was
dependent on the growth temperature and drastically de-
creased on the layer grown above 340˚C. The initial con-
tact angle about 5˚ on the PCVD layer grown at 380˚C
showed the excellent hydrophilicity without UV-irradia-
tion was performed by the PCVD. The contact angle on
the PCVD-layer was quickly reduced and showed su-
per-hydrophilicity within 5 min by UV-light irradiation
Copyright © 2013 SciRes. CSTA
with the low light power density of 50 μW/cm2, while the
long time about 3 hrs was required for the LPCVD layer.
The PCVD-layer also showed interesting storage behav-
ior of the super-hydrophilicity as long as 5 hrs in dark
after the UV-irradiation, whereas the contact angle on the
LPCVD-layer was increased up to 40˚ within 1 hr in
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