Photocatalytic Degradation of Rhodamine 6G using TiO
2
/WO
3
Bilayered Films Produced by Reactive Sputtering
L. C. Silva
1
, B. Barrocas
2
, M. E. Melo Jorge
3
and S. Sério
1
1
CEFITEC, Departamento de Física, Faculdade de Ciências e Tecnologia da Universidade Nova de Lisboa,
2829-516 Caparica, Portugal
2
Departamento de Química e Bioquímica, Faculdade de Ciências, Universidade de Lisboa,
Campo Grande C8, 1749-016 Lisboa, Portugal
3
Centro de Química e Bioquímica, Faculdade de Ciências, Universidade de Lisboa,
Campo Grande C8, 1749-016 Lisboa, Portugal
Keywords: Photocatalysis, Rhodamine 6G, Reactive Sputtering, Decolorization, Bilayered Films.
Abstract: TiO
2
/WO
3
and WO
3
/TiO
2
bilayered films were deposited onto glass substrates by DC reactive magnetron
sputtering and their photocatalytic activity was evaluated on the decolorization of Rhodamine 6G (Rh6G)
aqueous solutions. The structures, morphologies and optical properties of TiO
2
and WO
3
layers and also of
the bilayered films were studied by X-ray diffraction, field emission scanning electron microscopy and UV-
Vis spectroscopy. It was found that the bilayered films exhibit good adherence to the substrates and high
mechanic stability. The structural characterization revealed that in both nanocomposites independently of
the above layer, the main phase observed in the X-ray patterns corresponds to WO
3
and the optical
properties are similar to the WO
3
layer. The photocatalytic efficiency of the nanocrystalline bilayered films
was further compared with TiO
2
and WO
3
films also produced by sputtering and the results show that the
higher photocatalytic activity was achieved by the bilayered film with WO
3
as the upper layer.
1 INTRODUCTION
In the past years, the pollution of the wastewaters
with dyes is becoming a huge environmental
problem due to the growing use of a variety of dyes
in several industries, namely textile, paper, plastics
and cosmetic. These industries discharge large
amount of colour effluents, which are very toxic and
consequently can lead to serious ecological
problems (Roy Choudhury, 2014). Dyes normally
have a synthetic origin and complex aromatic
molecular structures, which are very stable and
difficult to biodegrade. It is also known that
wastewaters containing dyes are very difficult to
treat, since they are molecules resistant to aerobic
digestion and are stable to oxidizing agents.
Adsorption and coagulation are common
methodologies applied to treat dyes but always result
in secondary pollution (Ozmen, 2007, Crini, 2008).
Hence, it is extremely urgent to develop
ecologically clean and safe chemical technologies,
materials and processes to solve those problems. In
line with these objectives, the application of
alternative methods like the advanced oxidation
processes (AOP), such as photocatalysis has
received great attention in the last years. Advanced
oxidation processes (AOP), characterized by the
production of hydroxyl radicals (OH
.
) and
superoxide anion (O
2-
), which are generated when a
semiconductor catalyst absorbs radiation when it is
in contact with water and oxygen are a promising
technology, in which a broad range of organic dyes
can be oxidized quickly and non-selectively (Yang,
1998, Das, 1999). Among several photocatalysts,
TiO
2
was extensively studied due to its inertness,
nontoxicity, strong oxidizing activity and chemical
stability (Mills, 1997, Grätzel, 2001). Nevertheless
TiO
2
presents two important drawbacks for a wide
practical application, such as the small percentage of
the solar radiation, which has the required energy to
photogenerate electrons and holes and their high
recombination rate (Diebold, 2003, Thompson,
2006). In order to effectively use the energy of
sunlight several approaches have been used to
develop suitable photocatalysts and basically two
strategies have been proposed. One is to modify the
334
Silva, L., Barrocas, B., Jorge, M. and Sério, S.
Photocatalytic Degradation of Rhodamine 6G using TiO
2
/WO
3
Bilayered Films Produced by Reactive Sputtering.
DOI: 10.5220/0006751203340340
In Proceedings of the 6th International Conference on Photonics, Optics and Laser Technology (PHOTOPTICS 2018), pages 334-340
ISBN: 978-989-758-286-8
Copyright © 2018 by SCITEPRESS Science and Technology Publications, Lda. All rights reserved
wide band gap of the photocatalysts (such as TiO
2
,
ZnS) by cation or anion doping and the other
approach is producing heterojunctions between them
and other semiconductors. In fact, the increase in the
photocatalytic efficiency in these heterogeneous
systems results from the suppression of the
recombination of the photogenerated charge carriers
which results from the directional transfer of the
photogenerated charges between different types of
semiconductor particles (Akhavan, 2010,
Krishnamoorthy, 1998, Nova, 2000). Therefore, a
proficient way to extend the absorbance of TiO
2
to
visible light is the development of heterojunctions of
different nano-semiconductors including
Fe
2
O
3
/TiO
2
, TiO
2
/SnO
2
, ZnO/TiO
2
and WO
3
/TiO
2
(Lin, 2008, Marci, 2001, Keller, 2003, Akhavan,
2009). These studies evidence a synergetic
photocatalytic effect for a suitable combination of
the semiconductors.
Photocatalysts can either be used as powders in a
slurry form or as supported films. The latter
configuration is advantageous since solves important
problems such as the need for separation/filtration
steps, the problematic use in continuous flow
systems and the particles aggregation (Fernandez-
Ibanez, 2003). Investigations on heterogeneous
photocatalysis have been oriented towards the
photocatalyst immobilization in film form. Many
techniques can be used to prepare catalytic material
immobilized in film form (Kavan, 1995, Shaogui,
2004, Sánchez-Mora, 2004, Celik, 2006) such as,
sol-gel method, chemical vapor deposition,
sputtering and others. Among them sputtering
method presents several advantages such as high
deposition rates, films with high purity, high
adherence, accurate control of the film thickness and
a wide industrial applicability.
This work reports a study of the preparation and
characterization of TiO
2
/WO
3
and WO
3
/TiO
2
and the
photocatalytic activity evaluation under visible light
irradiation using the Rh6G as a target contaminant.
Rh6G dye is a complex molecule and is extensively
used for coloring leather, paper, silk and wool,
which should be treated before being extruded into
the environment.
2 EXPERIMENTAL DETAILS
TiO
2
/WO
3
and WO
3
/TiO
2
(being WO
3
and TiO
2
, the
upper layer, respectively) bilayered films were
deposited by DC-reactive magnetron sputtering on
glass substrates in a custom made system. Prior to
the deposition, the substrates were cleaned
successively in acetone, isopropanol and deionized
water for 5 min each step and dried with nitrogen
gas to remove any organic contamination. A
turbomolecular pump was used to achieve a base
pressure of 10
-4
Pa (before introducing the gas
mixture). Before the sputter-deposition step of the
films, a movable shutter was interposed between the
target and the substrates, and the target was pre-
sputtered in Ar atmosphere for 5 min to clean the
target surface. The target-to-substrate distance was
kept constant at 100 mm. The gases in the system
were 99.99% pure Ar and O
2
and the partial
pressures of these gases were separately controlled
by mass flow controllers. One reference sample of
TiO
2
film was deposited on a glass substrate using a
metallic titanium disc (99.99% purity) as sputtering
target. The total pressure, P
T
, was kept constant at
0.8 Pa and the partial pressure of oxygen was set at
0.08 Pa (10% of P
T
). The sputtering power was 1000
W and the deposition time was 80 min. In the case
of WO
3
film, the total pressure, P
T
, was kept
constant at 1.2 Pa and the partial pressure of oxygen
was set at 0.6 Pa (50% of P
T
). The sputtering power
used was 350 W and the deposition time was 25
min. The TiO
2
/WO
3
and WO
3
/TiO
2
bilayered films
were prepared using the same deposition conditions
of the reference films.
The structural characterization of the films was
carried out by X-ray diffraction (XRD) on a Philips
Analytical PW 3050/60 X’Pert PRO (
2
) equipped
with X’Celerator detector and with automatic data
acquisition (X’Pert Data Collector (v2.0b) software),
using a monochromatized CuKα radiation as
incident beam, 40 kV30 mA. Diffractograms were
obtained by continuous scanning in a 2θ-range of
10º to 90º with a 2θ-step size of 0.02º and a scan
step time of 20 s.
The surface morphology and thickness of the
films were examined by field-emission scanning
electron microscope (FEG-SEM JEOL 7001F). In
order to prevent charge build up during analysis a
thin chromium film was coated on the films.
The optical properties of the films were
measured with Shimadzu UV b - 2101PC UV/VIS
spectrophotometer at room temperature within the
wavelength range 300-900 nm.
The photocatalytic activity of the reference films
(TiO
2
and WO
3
), TiO
2
/WO
3
and WO
3
/TiO
2
bilayered films was evaluated by the photocatalytic
decolorization of Rh6G aqueous solution at ambient
temperature under visible light irradiation. The
photodegradation experiments were carried out in a
photoreactor refrigerated by water circulation. The
reaction vessel with a capacity of 250 mL,
Photocatalytic Degradation of Rhodamine 6G using TiO
2
/WO
3
Bilayered Films Produced by Reactive Sputtering
335
manufactured from borosilicate glass accommodates
a cooled central quartz tube, in which a 450 W
Hanovia medium-pressure mercury-vapour lamp
with 0.37 W/cm
2
of watt density is placed to
irradiate the dye solution. Of the total irradiated
energy, 40-48% is in the ultraviolet range and 40-
43% in the visible region of the electromagnetic
spectrum. In order to ensure illumination by only
visible light, a cut off filter was placed around the
cold trap to completely eliminate any radiation with
wavelength below 400 nm. A magnetic stirrer was
used to guarantee the solution homogenization.
The catalytic photodegradation experiments
were performed carried out using one film with 9.72
cm
2
geometric area immersed on 175 mL of 5 ppm
Rh6G aqueous solution prepared with ultrapure
water. Prior to irradiation, the solution was
magnetically stirred in absence of irradiation (in
dark) for 1 h to establish the adsorption/degradation
equilibrium between Rh6G and the photocatalyst.
For convenience of the reader this period was
represented by -10 min on the graphic
representations. After this 1 h period the lamp was
turned on. During irradiation, the solutions were
sampled at regular intervals and analysed by UV-vis
spectroscopy. The absorption of the Rh6G solutions
and the rate of decolorization were observed in
terms of change in intensity at λmax of the dye (526
nm). The decolorization efficiency percentage (DE)
% of the catalyst was determined using the equation:
DE(%)=(C
0
-C)/C
0
×100 (1)
In which, C
0
is the dye initial concentration (t = 0
min) and C is the dye concentration after
photoirradiation, which can be estimated from the
characteristic absorption maximum in UVvis
spectroscopy at 526 nm. The blank experiment
(Rh6G photolysis) was conducted in the presence of
irradiation without any photocatalyst.
The structural stability and surface morphology
of the photocatalysts were analyzed by XRD and
diffuse reflectance infrared fourier transform
spectroscopy (DRIFTS) before and after the
photodegradation experiments.
3 RESULTS AND DISCUSSION
The structural characterization revealed that the as-
prepared TiO
2
and WO
3
films are amorphous or
poorly crystallized. In order to allow the crystalline
growth, the films were thermal annealed at 400ºC in
tubular furnace in air during 4h. The XRD patterns,
for the annealed TiO
2
and WO
3
films, are presented
in Figure 1. For the TiO
2
film the XRD pattern
shows relative sharp peaks indicating the
coalescence of nanocrystalline anatase-phase TiO
2
and exhibits a preferred orientation along the (004)
direction. In the case of the WO
3
film, the diffraction
peaks were assigned to the monoclinic WO
3
phase,
which gave the best fit. Nevertheless, triclinic,
orthorhombic and monoclinic diffraction peaks
almost overlap for many 2θ values, namely the
(002), (020) and (200) reflections at 2θ around 23-
24º and it is difficult to discriminate between these
three phases. According to the phase diagram, the
monoclinic and triclinic structures are the most
common structures and coexist in WO
3
at
temperatures lower than 500 °C, the orthorhombic
phase between 330 and 740 °C and, finally, a
tetragonal structure up to 1230 °C (Naidu, 1991).
Moreover, previous studies show that the diffraction
pattern features at 2θ around 23-24º can suggest a
mixture of the orthorhombic and the monoclinic
WO
3
phases (Marsen, 2007), but can also arise from
preferred orientation or from the presence of
crystallographic shear planes (Jimenez, 2003).
Therefore, in this work for the developed WO
3
films
it cannot be discarded the presence also of the
triclinic and orthorrombic phases due to the
aforementioned reasons.
The inset in Figure 1 shows the surface
morphology of the annealed WO
3
film analysed by
SEM. The image shows agglomerates of grains or
particulates, with spherical shape and average sizes
of 100250 nm distributed over the substrate surface
with a ‘blooming flower-like’ appearance and others
with an elongated shape with an average length of
200300 nm.
The XRD patterns of the TiO
2
/WO
3
and
WO
3
/TiO
2
bilayers films are depicted in Figure 2.
By the comparison of Figure 1 with Figure 2 it can
be detected that the XRD patterns of the bilayered
films exhibit a main phase attributed to the
monoclinic WO
3
phase (also as the WO
3
reference
film, the presence of the triclinic and orthorrombic
phases cannot be discarded) and only a small
evidence of the anatase TiO
2
is observed in both
heterojunctions independently of the above layer.
The surface morphology of the bilayered films was
investigated by SEM and the images are presented in
Figure 3. From the analysis of the images it is
observed that when the above layer is WO
3
the film
morphology consist of agglomerates of nano-sized
grains or particulates, distributed over the substrate
surface with a ‘blooming flower-like’ appearance
(see Figure 3a) and an average size of 300-500 nm.
AOMatSens 2018 - Special Session in Advanced Optical Materials, Sensors and Devices
336
Figure 1: XRD patterns of the annealed TiO
2
and WO
3
films. The inset corresponds to a SEM image of the
annealed WO
3
film surface.
Figure 2: XRD patterns of the TiO
2
/WO
3
and WO
3
/TiO
2
bilayered films annealed at 400ºC. - corresponds to the
anatase TiO
2
phase.
In the case of the nanocomposite film with the
TiO
2
layer as the upper layer (see Figure 3b), the
surface morphology is completely different although
are visible agglomerates considerably smaller in
comparison with the other nanocomposite with the
WO
3
layer above. It is also noticeable a less dense
surface with voids between the agglomerates.
The thicknesses were evaluated from FE-SEM
cross-section images of the TiO
2
thin films, which
are presented as inset of the corresponding films.
The thicknesses of TiO
2
and WO
3
layers were
approximately 990 nm and 2100 nm, respectively, in
both bilayered films independently of the order of
the layers.
Figure 4 shows the optical transmittance spectra
of the TiO
2
, WO
3
and of the bilayered films. The
transmittance spectrum of the reference TiO
2
film
shows the usual interference pattern in the range of
low absorption with a sharp fall of transmittance at
the band edge around 350 nm. The annealed WO
3
film is light yellowish and nearly transparent. The
band edge of WO
3
film is around 500 nm. The
transmittance spectra of the TiO
2
/WO
3
and
WO
3
/TiO
2
bilayered systems show almost similar
nature as the WO
3
film. The average transmittance
of TiO
2
, WO
3
, and TiO
2
/WO
3
and WO
3
/TiO
2
are
90%, 60%, and 58%, respectively.
Figure 3: SEM images of the surface of the a) TiO
2
/WO
3
and b) WO
3
/TiO
2
nanocomposites films. The cross-section
images of the nanocomposite films are presented as inset.
The optical band-gap of the TiO
2
, WO
3
reference
films were determined from the transmission spectra
using Tauc’s relation (Tauc, 1974), given as



(2)
in which, E
phot
(E
phot
= 1239/λ) is the excitation
energy (in eV), with the wavelength λ in nanometers
and m is a parameter accounting for the different
band-gap transition modes, E
g
is the optical energy
band-gap and α is the absorption coefficient (in
cm
1
), which is obtained near the absorption edge
from the transmittance, T, using the equation
-

(3)
Photocatalytic Degradation of Rhodamine 6G using TiO
2
/WO
3
Bilayered Films Produced by Reactive Sputtering
337
in which, d is the thickness of the film (Sério, 2011,
Sério, 2012). In Eq. (2), the exponent m depends
upon the type of optical transitions in the material.
For indirect transitions, which is the case of TiO
2
and WO
3
films, the exponent takes the value
(González-Borrero, 2010), m = 2.
Figure 4: Optical transmittance spectra of the TiO
2
, WO
3
reference films and of TiO
2
/WO
3
and WO
3
/TiO
2
bilayered
films.
The optical energy band gap was determined to
be 3.33 eV for the TiO
2
reference film, whereas the
optical energy band-gap for the WO
3
reference film
comes out to be 1.97 eV. These values are in
agreement with the published results for these type
of oxides (Sério, 2011, Barrocas, 2013) and also
evidence that the TiO
2
and WO
3
will be activated in
the UV and visible range, respectively.
The photocatalytic activity of the bilayered and
reference films was investigated under visible light
irradiation on the decolorization of 5 ppm Rh6G
solution for a period of 120 min. The Rh6G
degradation was monitored by measuring the
absorbance of the samples taken at regular intervals
during a period of 120 min. The blank experiment
without catalyst (photolysis) was also investigated.
In Figure 5 is depicted a representative time
dependent UVvis spectra of Rh6G decolorization
for the WO
3
catalyst. Similar time evolution decays
were obtained for other catalysts.
From Figure 5, it is observed that the
characteristic absorption peaks of Rh6G decrease
with the increase of irradiation time. It is further
observed that the Rh6G presents three characteristic
absorption peaks: the peak at 247 nm is attributed to
the benzene ring structure, the peak around 275 nm
is related to the naphthalene ring structure and the
third peak at 526 nm is the characteristic peak of
Rh6G (Barrocas, 2013, Barrocas, 2014). The
intensity of the peak at 526 nm dropped with the
Figure 5: UV-vis absorption data for photocatalytic
degradation of Rh6G solution upon irradiation with visible
light, using WO
3
film as catalyst.
increase of irradiation time, which means that the
chromophoric unsaturated conjugated bond in the
dye molecule was gradually destroyed. The peak of
benzene ring and naphthalene ring also decreased
progressively with exposure time. These results
indicate that the photocatalytic degradation not only
destroys the conjugate system, but also partly
decomposes the benzene and naphthalene rings in
Rh6G molecule.
The rate of Rh6G decolorization was recorded
considering the change in intensity of the absorption
peak at 526 nm, during the 120 min of photo-
irradiation. Also the decay with time of the other
Rh6G absorption bands is in accordance with
complete degradation of the dye. The obtained
results are present in Figure 6 for all the materials
studied.
Figure 6: Rh6G degradation percentage evolution during
the photocatalytic degradations of a 5 ppm aqueous
solution using the reference films and the bilayered films
as catalysts. The solid lines are guidelines for easy
viewing.
300 400 500 600 700 800 900
0
10
20
30
40
50
60
70
80
90
100
T /%
l
/n m
TiO
2
WO
3
TiO
2
/WO
3
WO
3
/TiO
2
AOMatSens 2018 - Special Session in Advanced Optical Materials, Sensors and Devices
338
As can be seen by Figure 6 analysis all the
materials tested demonstrated to be catalytic in this
photodegradation process. Without any catalyst,
after 60 min of irradiation, less than 10% of Rh6G,
were degraded. On the other hand, in the presence of
WO
3
, TiO
2
, TiO
2
/WO
3
and WO
3
/TiO
2
after 60 min
of irradiation approximately 80, 20, 65 and 45% of
Rh6G, were degraded respectively. These results
reveal that the WO
3
film exhibit the best
photocatalytic performance, followed by the
TiO
2
/WO
3
bilayered film. However, the WO
3
film
suffers of low mechanical stability, which isn’t
observed for the bilayered films independently of the
upper layer. It is well known that the materials
catalytic performance results from the balance of
several factors such as the surface area, crystal
structure, adsorption activity, band gap and so on. In
this work and analyzing the band gap values
obtained for these oxides the catalytic results point
out that the lower band gap obtained for the WO
3
films and also for the bilayered films seems to be a
preponderant factor on the films photocatalytic
activity. It is further observed that in the case of the
bilayered films, when the upper layer is WO
3
the
photocatalytic activity is higher than the one
observed for the other bilayered film when the upper
layer is TiO
2
. This difference in the catalytic activity
may be attributed to the different surface
morphology observed for these two types of
bilayered films.
4 CONCLUSIONS
TiO
2
/WO
3
and WO
3
/TiO
2
bilayered films were
deposited on glass substrates by DC-reactive
magnetron sputtering. The bilayered films exhibit
good adherence to the substrates and high mechanic
stability. In both composites independently of the
above layer, the main phase observed in the X-ray
patterns corresponds to the WO
3
. The bilayered
films and in particular with WO
3
as the upper layer
exhibit high catalytic performance for Rh6G
degradation under visible irradiation which can be
mainly attributed to the lower band gap in
comparison with TiO
2
and also to the surface
morphology. These findings are encouraging for
future studies on the improvement of bilayered films
photocatalytic efficiency and promote its practical
application in environmental remediation using solar
irradiation.
ACKNOWLEDGEMENTS
The authors acknowledge the financial support from
FEDER, through Programa Operacional Factores de
Competitividade COMPETE and Fundação para a
Ciência e a Tecnologia FCT, for the projects
UID/MULTI/00612/2013 and UID/FIS/00068/2013.
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