22nd International Symposium on Plasma Chemistry
July 5-10, 2015; Antwerp, Belgium
Synthesis of TiO 2 porous films by atmospheric dielectric barrier discharge
plasma at room temperature
Q. Chen1, J. Mertens1, J. Hubert1, J. Baneton1, Q.R. Liu1, G. Wallaert2, M.-P. Delplancke2 and F. Reniers1
1
Analytical and Interfacial Chemistry (CHANI), Faculty of Sciences, Université Libre de Bruxelles,
Avenue F.D. Roosevelt 50, 1050 Brussels, Belgium
2
4MAT, Faculty of Applied Sciences, Université Libre de Bruxelles, Avenue F.D. Roosevelt 50, 1050 Brussels, Belgium
Abstract: TiO 2 films were synthesized by atmospheric dielectric barrier discharge (DBD)
plasma at room temperature. The morphology and chemical composition of TiO 2 films
were studied by scanning electron microscopy (SEM) and X-ray photoelectron
spectroscopy (XPS). Effect of the oxygen ratio and deposition time was investigated. It is
showed that the content of O 2 is an important parameter in the formation of a porous
structure. The study of the deposition time could lead to the understanding of the
mechanisms of nucleation and growth of TiO 2 nanoparticles in the plasma and formation of
films on the substrate.
Keywords: atmospheric plasma, dielectric barrier discharge, TiO 2 porous films
1. Introduction
Titanium oxide (TiO 2 ) thin films are one of the most
important materials in optical, electronic and chemical
fields for various applications like photo-anode layers,
antireflection coatings, hydrophilic films and photocatalytic materials [1–3]. Plasma enhanced chemical
vapor deposition (PECVD) is a highly efficient method
for TiO 2 films synthesis but often requires sophisticated
discharge, high temperature and vacuum conditions [2].
In order to mitigate the drawbacks, atmospheric plasmas
have been investigated as an alternative approach and
interesting results have been achieved in synthesizing and
functionalizing specific nanomaterials [4–7]. Atmospheric
plasma such as dielectric barrier discharge (DBD) plasma
has become one of the most promising “next generation”
candidate systems for replacing high temperature vacuum
CVD or wet chemical processes [5, 6]. In this study, TiO 2
porous film was synthesized by using atmospheric DBD
plasma at room temperature.
2. Experimental
A scheme of the DBD device for TiO 2 films synthesis
used in this work is shown in Figure 1. It consists of two
flat circular copper electrodes both covered by alumina as
dielectrics. The distance between both dielectrics is kept
constant at 3 mm for all the experiments. The plasma is
generated by an AC power supply (AFS G10S-V),
connected to a high voltage transformer (AFS GT10S-V1K) operating at 2700 Hz. In order to prevent air
contamination, the reactor is pumped down to a pressure
of 0.05 Torr before and after each deposition. The reactor
is then filled up with argon until atmospheric pressure is
reached. A continuous gas flow is maintained into the
reactor during the experiments. The TTIP precursor,
heated at 373K, is evaporated by a 0.3 slm argon flow and
then mixed with argon and oxygen to obtain a total flow
P-III-6-10
of 2.5 slm. The vaporized TTIP is then carried into the
discharge through a stainless steel wire heated at 333K in
order to prevent any condensation inside the tube. The
mass flow rate of the evaporated precursor is 19.5 mg/min
which is calculated by weighing of the TTIP in the
bubbler before and after the deposition. Influence of
oxygen content is studied by changing the O 2 flow while
keeping the total flow rate at 2.5 slm. The ratio between
carrier gas and O 2 flows is marked as R Ar/O2 . The
morphology and the chemical composition of TiO 2 films
are investigated by scanning electron microscopy (SEM)
and
X-Ray
photoelectron
spectroscopy (XPS)
respectively.
Flow controllers
O2
Ar
TTIP
bubbler
Power supply &
transformer
Heated wire
Oil bath
(373 K)
Grounded
electrode
Figure 1. Scheme of the DBD deposition setup.
3. Results
Figure 2 shows SEM images of the product collected on
the silicon wafer for different R Ar/O2 . When the R Ar/O2 is 3,
a smooth film of TiO 2 homogeneous nanoparticles with
an average size of 20 nm is obtained. As the R Ar/O2
decreases from 3 to 1, some porosity appears without any
change in the morphology of the nanoparticles. These
observations suggest that the addition of oxygen in the
plasma has an impact on the organization of the film.
1
Same results have been noticed by A. Shelemin [8] with
N 2 /O 2 system where oxygen may act as a scavenger to
remove a part of the organic phase. Besides, the presence
of O 2 induces the formation of a filamentary discharge,
which will be enhanced as O 2 increased. The strong effect
among the filamentary and nanoparticles may be an
explanation for the formation of porous structural TiO 2
film.
time is increasing could be explained by electrostatic
repulsions between nanoparticles [5].
Figure 2. SEM images of TiO 2 films prepared by
atmospheric pressure DBD with different ratios of Ar/O 2 :
(a) R Ar/O2 = 3, (b) R Ar/O2 = 2 and (c) R Ar/O2 = 1. Discharge
power 40 W, t = 5 min.
Figure 4. XPS spectra of TiO 2 film prepared by
atmospheric pressure DBD. Discharge power 40 W,
R Ar/O2 = 2, t = 5 min.
Figure 3. SEM images of TiO 2 films prepared by
atmospheric pressure DBD with different deposition times:
(a) t = 10 min and (b) t = 15 min. Discharge power 40 W,
R Ar/O2 = 2.
The effect of deposition time on the morphology of the
film is shown on the SEM images in Figure 3. In contrast
with Figure 2(b), we can see that when increasing
deposition duration, the film growth from several layers
of particles to a porous structure. This finding suggests
that the nanoparticles of TiO 2 rather appear in the
discharge volume and then deposit onto the substrates as
ready-made granules. This process could be dependent of
the ratio between TTIP/O 2 /electrons in the plasma [6, 9].
Once in the plasma, the TTIP precursor is excited by the
highly energetic species like electrons and metastables
species. This could lead to the formation of radicals or
ions. When the active species collide and react with each
other, some homogeneous reactions such as nucleaction
of Ti(O) 4 happen in the bulk of the plasma [8], then they
will transform to the TiO 2 nanoparticles and accumulate
on the coating. The formation of a porous structure as the
2
TiO 2 synthesized films have also been characterized by
XPS. Results are presented in Figure 4. The peaks
positions are referenced with respect to carbon (C 1s) at
285.0 eV. As it can be seen on the XPS survey scan
(Figure 4a), it is confirmed that the deposited films are
only composed of Ti, O, and residual C. The high
resolution XPS spectra of Ti 2p, O 1s, and C 1s are shown
in Figures 4b–d. The Ti 2p 3/2 peak at 458.3 eV and Ti
2p 1/2 peak at 464.0 eV present a energy gap of 5.7 eV
which can be attributed to Ti4+ from TiO 2 [4, 10]. The O
1s spectrum (Figure 4c) is deconvoluted into three peaks.
The peak at 529.8 eV is assigned to the lattice oxygen
(O2-) of TiO 2 . The two other O 1s peaks correspond to
OH surface groups (531.5 eV) and C-O bonded species
(532.6 eV) [10]. The C 1s signal (Figure 4d) is
deconvoluted into four components, which corresponds to
C-C/C-H, C-O, C=O and O-C=O, respectively [8].
4. Acknowledgements
The authors would like to thank the China Scholarship
P-III-6-10
Council (CSC, Grant no. 201407040053) for a PhD
scholarship and the Belgian Federal Government IAP PSI (physical chemistry of plasma surface interactions)
P6-08 network for their financial support.
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