Photocatalytic Degradation of Rhodamine 6G using TiO

/WO

Bilayered Films Produced by Reactive Sputtering

L. C. Silva

, B. Barrocas

, M. E. Melo Jorge

and S. Sério

CEFITEC, Departamento de Física, Faculdade de Ciências e Tecnologia da Universidade Nova de Lisboa,

2829-516 Caparica, Portugal

Departamento de Química e Bioquímica, Faculdade de Ciências, Universidade de Lisboa,

Campo Grande C8, 1749-016 Lisboa, Portugal

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

/WO

and WO

/TiO

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

and WO

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

and the optical

properties are similar to the WO

layer. The photocatalytic efficiency of the nanocrystalline bilayered films

was further compared with TiO

and WO

films also produced by sputtering and the results show that the

higher photocatalytic activity was achieved by the bilayered film with WO

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

), 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

was extensively studied due to its inertness,

nontoxicity, strong oxidizing activity and chemical

stability (Mills, 1997, Grätzel, 2001). Nevertheless

TiO

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

/WO

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

wide band gap of the photocatalysts (such as TiO

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

visible light is the development of heterojunctions of

different nano-semiconductors including

/TiO

, TiO

/SnO

, ZnO/TiO

and WO

/TiO

(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

/WO

and WO

/TiO

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

/WO

and WO

/TiO

(being WO

and TiO

, 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

and the partial

pressures of these gases were separately controlled

by mass flow controllers. One reference sample of

TiO

film was deposited on a glass substrate using a

metallic titanium disc (99.99% purity) as sputtering

target. The total pressure, P

, was kept constant at

0.8 Pa and the partial pressure of oxygen was set at

0.08 Pa (10% of P

). The sputtering power was 1000

W and the deposition time was 80 min. In the case

of WO

film, the total pressure, P

, was kept

constant at 1.2 Pa and the partial pressure of oxygen

was set at 0.6 Pa (50% of P

). The sputtering power

used was 350 W and the deposition time was 25

min. The TiO

/WO

and WO

/TiO

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 kV–30 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

and WO

), TiO

/WO

and WO

/TiO

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

/WO

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

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

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

-C)/C

×100 (1)

In which, C

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 UV–vis

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

and WO

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

and WO

films, are presented

in Figure 1. For the TiO

film the XRD pattern

shows relative sharp peaks indicating the

coalescence of nanocrystalline anatase-phase TiO

and exhibits a preferred orientation along the (004)

direction. In the case of the WO

film, the diffraction

peaks were assigned to the monoclinic WO

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

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

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

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

film analysed by

SEM. The image shows agglomerates of grains or

particulates, with spherical shape and average sizes

of 100–250 nm distributed over the substrate surface

with a ‘blooming flower-like’ appearance and others

with an elongated shape with an average length of

200–300 nm.

The XRD patterns of the TiO

/WO

and

/TiO

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

phase (also as the WO

reference

film, the presence of the triclinic and orthorrombic

phases cannot be discarded) and only a small

evidence of the anatase TiO

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

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

and WO

films. The inset corresponds to a SEM image of the

annealed WO

film surface.

Figure 2: XRD patterns of the TiO

/WO

and WO

/TiO

bilayered films annealed at 400ºC. - corresponds to the

anatase TiO

phase.

In the case of the nanocomposite film with the

TiO

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

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

thin films, which

are presented as inset of the corresponding films.

The thicknesses of TiO

and WO

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

, WO

and of the bilayered films. The

transmittance spectrum of the reference TiO

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

film is light yellowish and nearly transparent. The

band edge of WO

film is around 500 nm. The

transmittance spectra of the TiO

/WO

and

/TiO

bilayered systems show almost similar

nature as the WO

film. The average transmittance

of TiO

, WO

, and TiO

/WO

and WO

/TiO

are

90%, 60%, and 58%, respectively.

Figure 3: SEM images of the surface of the a) TiO

/WO

and b) WO

/TiO

nanocomposites films. The cross-section

images of the nanocomposite films are presented as inset.

The optical band-gap of the TiO

, WO

reference

films were determined from the transmission spectra

using Tauc’s relation (Tauc, 1974), given as





 



 







(2)

in which, 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

is the optical energy

band-gap and α is the absorption coefficient (in

−1

), which is obtained near the absorption edge

from the transmittance, T, using the equation

  

-







 (3)



Photocatalytic Degradation of Rhodamine 6G using TiO

/WO

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

and WO

films, the exponent takes the value

(González-Borrero, 2010), m = 2.

Figure 4: Optical transmittance spectra of the TiO

, WO

reference films and of TiO

/WO

and WO

/TiO

bilayered

films.

The optical energy band gap was determined to

be 3.33 eV for the TiO

reference film, whereas the

optical energy band-gap for the WO

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

and WO

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 UV–vis spectra of Rh6G decolorization

for the WO

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

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

100

T /%

/n m

TiO

/WO

/TiO



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

, TiO

/WO

and WO

/TiO

after 60 min

of irradiation approximately 80, 20, 65 and 45% of

Rh6G, were degraded respectively. These results

reveal that the WO

film exhibit the best

photocatalytic performance, followed by the

TiO

/WO

bilayered film. However, the WO

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

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

the

photocatalytic activity is higher than the one

observed for the other bilayered film when the upper

layer is TiO

. 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

/WO

and WO

/TiO

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

. The bilayered

films and in particular with WO

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

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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