22nd International Symposium on Plasma Chemistry
July 5-10, 2015; Antwerp, Belgium
Effect of the deposition method on silver diffusion in Ag-TiO 2 thin coatings:
advantages and inconveniencies for photocatalytic applications and water
treatment
H. Fakhouri1,2, F. Arefi-Khonsari1,2, A.K. Jaiswal1,2 and J. Pulpytel1,2
1
Sorbonne Universités, UPMC Univ Paris 06, UMR 8235, Laboratoire Interfaces et Systèmes Electrochimiques, 75005
Paris, France
2
CNRS, UMR8235, LISE, 75005 Paris, France
Abstract: Titania can be modified with silver to increase the lifetime of the photo-excited
electrons and holes, however, silver can have a negative effect due to its increased mobility
and diffusion especially during the annealing process. Photocatalytic TiO 2 and Ag doped
TiO 2 coatings were deposited by injecting Ti precursor (with and without silver) in the
after-glow of an Atmospheric Pressure Plasma Jet APPJ under different conditions. The
deposited coatings presented a high porosity to give a unique advantage of the effect of
silver diffusion for photocatalytic and anti-bacterial applications. The morphology, phase
structure, chemical composition and photocatalytic properties of the coatings have been
studied.
Keywords: TiO 2 , atmospheric pressure plasma jet, spray, photocatalytic membrane
1. Introduction
The primary application of photocatalysis is the photodegradation of organic materials dissolved in water (or
any other liquid). The most prominent photocatalytic
material that has been studied is TiO 2 , however, the latter
can only reach a certain level of photocatalytic efficiency
due to its fast electron-hole pair recombination. Many
attempts have been investigated to reduce the charge
recombination within TiO 2 , including doping with silver.
But in most cases, whatever the doping process used,
silver diffuses to the surface, through the adjacent layers
to form islands, after annealing at relatively low
temperatures, in the range of 100°C to 400°C which is
used to crystallize the TiO 2 films. [1]. Hence, silver has
the disadvantage to lead to non-uniform dispersion on the
surface of titania. Another research axis is to modify the
surface properties of TiO 2 by introducing silver on the
topmost surface[2], which also forms islands on the
surface.
Atmospheric pressure plasma jets APPJ have received
significant attention due to their combination of
simplicity, low cost and wide possibilities for surface
treatment. Recently such plasmas have been used to
deposit porous thin films of TiO 2 [3] by injecting liquid
precursor of Ti in the plasma, the coating process can be
easily integrated into existing production line, to treat
complex 3D shapes, but the most interesting characteristic
of this technique is its capacity to treat photocatalytic
membranes which can efficiently combine filtration and
photocatalysis for water treatment and for other selfcleaning surfaces,
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2. Experimental details
Ag doped TiO 2 thin films were prepared by two
methods: first by RF reactive magnetron sputtering
system (SPT 120, Plasmionique). Metallic Ti and Ag
targets (50 mm diameter) were co-sputtered in a reactive
gas atmosphere containing Ar and O 2 for the deposition
of TiO 2 and Ag alone for the deposition of Ag,
fabricating 32 bilayers of Ag/TiO 2 . The deposition
pressure was fixed at 6 mTorr with corresponding Ar flow
50 sccm. Oxygen flow rate was fixed at 2.3 sccm during
the TiO 2 . The RF powers were 200 W and 50 W for the
Ti and Ag targets, respectively. The fixed experimental
conditions chosen were situated in the intermediate region
between two stable sputtering modes of Ti, reactive and
metallic [4-5].
Secondly, we introduced a liquid precursor of titanium
tetra-isoperoxide TTIP in an open-air atmospheric
pressure plasma jet APPJ (Fig.1). Different concentrations
of Ag nanoparticles (0 to 5% wt.) were added for silver
doping. As the plasma jet contains highly reactive species
(radicals, metastables, vibrationally excited molecules…)
it can promote a very rich chemistry for surface treatment.
On the other hand, the technique used in this study is a
blown arc system giving rise to quite high temperature at
the nozzle exit (i.e. Tg ~ 1000-1500 K) leading to an
interesting heat transfer phenomena to tailor the
physicochemical properties of the coatings. This high
temperature then quenches quite rapidly further away
from the nozzle exit, providing the possibility to treat
temperature sensitive materials as well. More
interestingly, the control of the heat transfer gave
predominant impact on the position of the Ag doping sites
in the host material (TiO 2 ).
1
The crystal structure of thin films was characterized by
X-ray diffraction (XRD) (X’Pert Pro PW3040-Pro,
Panalytical Inc.) using a Cu Kα1 (λ = 1.5418 Å) X-ray
radiation source in Bragg-Brentano configuration. X’Pert
High Score pattern processing was used to collect and
process the data. The photocatalytic activity was
evaluated by using an aqueous solution of Rhodamine B
with an initial concentration of 5 mg/L at room
temperature. A 125 W (Philips) white lamp was used to
irradiate the samples during the photocatalytic
measurements.
porosity as compared to the sputtering technic. Moreover,
the kinetic energy of the reactive species directed to the
surface is much higher in a sputtering system operating at
a low pressure (in the order of a few mTorr), as compared
to APPJ (atmospheric pressure). In the latter, the species
that reach the substrate surface have consequently less
surface mobility which also explains the higher porosity
of the coatings.
Working gas (Air)
Holes for spiraling
the working gas
(a)
Discharge
chamber
Liquid precursor
Titanium tetraisopropoxide
Ti(OC3H7)4 - TTIP
Electrode
Rotating arc
Plasma Jet
Air
spraying system
Substrate
Fig. 1. Atmospheric pressure plasma jet (APPJ) with the
spraying system.
3. Results and discussion
a) Structure of the Ag-TiO 2 coatings
Fig. 2 shows two examples of SEM-FEG images of
annealed Ag-TiO 2 , deposited by APPJ and sputtering. It
is clear that the specific surface area and porosity of the
coatings are drastically increased with the APPJ coatings
as compared to sputtered ones. The coatings deposited by
APPJ are composed of agglomerated clusters having an
average grain size of approximately 30-40 nm, a pore size
around 0.3 to 1 µm have been observed which present a
good adhesion on different substrates.
In the case of rf reactive sputtering, the deposition rate
is typically between 1~10 nm/min for TiO2 in the oxide
mode and the films are characterized by a 1D columnar
structure. Whereas, in the case of the APPJ-spray process,
the deposition rate is estimated to be between 10-20 µm/s
and the structure observed is the consequence of a
ballistic-like growth. The difference in the porosity
between the films deposited by sputtering and APPJ can
be explained by the difference in the deposition rates
between the two techniques and the difference in the
kinetic energy of the reactive species in the plasma
reaching the surface. The deposition rate of TiO 2 by
sputtering is in the order of 0.1 nm/s, whereas for APPJ
the latter is ten orders of magnitude higher. Therefore, it
is reasonable to assume that in the APPJ process, the
species adsorbed on the surface have less time to diffuse
to the surface of the coatings which lead to a higher
2
(b)
Fig. 2. SEM-FEG images of the Ag-TiO 2 thin films
deposited by (a)rf sputtering annealed at 350 °C for 1 hr,
and (b) APPJ from TTIP liquid spray annealed at 450 °C
for 1 hr. In insert of (a) is the cross section after
annealing at 450 °C.
b) Silver diffusion in the TiO 2 layer
In the samples prepared by sputtering and annealed at
350°C, Fig. 1 (a), due to the diffusion through the TiO 2
layers, silver atoms have agglomerated on the film
surface, creating clusters. The compositional contrast
gives a clear distribution of grains on the sample surface.
Silver particles are brighter as they have the highest
density in the analyzed Ag-TiO 2 system. However, after
annealing at 450°C, most of the surface of TiO 2 was
covered with a silver layer (see the cross section).
As presented in Fig. 2 (b), samples prepared by APPJ
showed similar diffusion of silver, but the latter was
limited by the morphology of the APPJ’s coatings. In fact,
silver diffused in the adjacent layer of TiO 2 to partially
cover the top part of the clusters but not the whole surface
of TiO 2 .
c) Photocatalytic activity
Fig. 3 shows the photocatalytic degradation kinetics on
glass coated samples by APPJ. No degradation of
Rhodamine B was observed after 4 hours with uncoated
substrates, which means that the photo-bleaching of the
solution was negligible in the absence of TiO 2 or AgTiO 2 . The most efficient photocatalytic activity was
found in the presence of samples prepared with 3% and
4% of Ag. Also, the half-life of Rhodamine B was
reduced 3 times with the sample having 3% wt. of silver
as compared with the pure TiO 2 . This indicates that silver
doping can increase the charge separation and the life
time of the photo-generated electron-holes in TiO 2 when
using an optimal concentration of Ag in the composite
material.
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Absorption of Rhodamine B (a.u)
0,7
0,6
TiO2
0,5 % Ag
1 % Ag
2 % Ag
3 % Ag
4 % Ag
5 % Ag
Ag-TiO2 at 200°C
0,5
0,4
0,3
[5] A. Brudnik, M. Bucko, M. Radecka, A. TrenczekZajac, K. Zakrzewska, Vacuum, 82, 936 (2008).
[6] W.K. Jung, H.C. Koo, K.W. Kim, S. Shin, S.H. Kim,
Y.H. Park, Applied and Environmental Microbiology, 74,
2171 (2008).
[7] V. Kumar, C. Jolivalt, J. Pulpytel, R. Jafari, F. ArefiKhonsari, Journal of Biomedical Materials Research Part
A, 101, 1121, (2012).
Half life
0,2
0,1
0,0
0
20
40
60
80
100
120
140
160
Time (min)
Fig. 3. Photocatalytic activity of the Ag-TiO 2 coatings
deposited by APPJ.
In addition, the degradation of a target pharmaceutical
molecule Carbamazepine, CBZ spiked at 1 mg/L in
deionized water, has been investigated in presence of the
photocatalytic coatings. The experiments showed similar
results as compared to the degradation of Rhodamine B.
These results are potentials to escort the study on
biological tests with virus and bacteria, in order to
investigate more the effect of silver ions to enhance the
oxidation of bacteria [6-7].
4. Conclusion
Injection of liquid precursor of Ag/TiO2 in APPJ can
give significant control of the porosity and crystallinity by
varying the deposition parameters such as the energy of
plasma and substrate temperature. Investigation of the
effect of post annealing showed significant improvement
in the photo activity under UV and visible irradiation for
an optimal concentration of silver in TiO 2 as compared to
thin films prepared by low pressure systems.
Overall, blown arc atmospheric pressure plasma jet
applied for TiO 2 /Ag thin coatings has shown a unique and
highly desirable control over several important physical
characteristics, which can be beneficial for many
optoelectronic and photocatalytic applications, due to the
unique structure and the good controlled charge
separation within the doped TiO 2 .
5. References
[1] J. Kulczyk-Malecka, P.J. Kelly, G. West, G.C.B.
Clarke, J.A. Ridealgh, K.P. Almtoft, A.L. Greer, Z.H.
Barber, Acta Materialia, 66, 396 (2014).
[2] L. Su Pei, A. Pandikumar, N. Ming Huang,
H.N. Lim International Journal of Hydrogen Energy, 39,
14720 (2014)
[3] H. Fakhouri, D. Ben Salem, O. Carton, J. Pulpytel, F.
Arefi-Khonsari, Journal of Physics D: Applied Physics,
46 (26) 265301 (2014).
[4] W.D. Sproul, D.J. Christie, D.C. Carter, Thin Solid
Films, 491, 1 (2005).
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