Photocatalytic properties of WO3 nanoparticles obtained by precipitation in presence of urea as complexing agent

Article abstract of DOI:10.1016/j.apcata.2011.03.034

WO₃ nanoparticles were synthesized by a precipitation method in presence of urea at different calcination temperatures. The characterization of the WO₃ samples was carried out by X-ray powder diffraction (XRD), transmission electron

Full text of DOI:10.1016/j.apcata.2011.03.034

Applied Catalysis A: General 398 (2011) 179–186
Contents lists available at ScienceDirect
Applied Catalysis A: General
journal homepage: www.elsevier.com/locate/apcata
Photocatalytic properties of WO nanoparticles obtained by precipitation in
3
presence of urea as complexing agent
∗
D. Sánchez Martínez, A. Martínez-de la Cruz , E. López Cuéllar
Facultad de Ingeniería Mecánica y Eléctrica, Universidad Autónoma de Nuevo León, Ciudad Universitaria, C.P. 66451, San Nicolás de los Garza, N.L., Mexico
a r t i c l e i n f o
a b s t r a c t
Article history:
WO3 nanoparticles were synthesized by a precipitation method in presence of urea at different
calcination temperatures. The characterization of the WO3 samples was carried out by X-ray pow-
der diffraction (XRD), transmission electron microscopy (TEM), scanning electron microscopy (SEM),
adsorption–desorption N2 isotherms (BET), and diffuse reflectance spectroscopy (DRS). During the pre-
cipitation process, the presence of urea led to the formation of WO3 nanoparticles with rectangular and
ovoid shapes as a function of calcination temperature. The photocatalytic activity of WO3 samples was
evaluated in the degradation of rhodamine B (rhB), indigo carmine (IC) and congo red (CR) molecules in
aqueous solution under UV and visible-light radiation. The best results were obtained with the sample
Received 18 January 2011
Received in revised form 18 March 2011
Accepted 20 March 2011
Available online 26 March 2011
Keywords:
WO3
Heterogeneous photocatalysis
Photosensitization
Organic dyes
◦
synthesized at 500 C due to a contribution of factors such as morphology, surface area and the degree
of aggregation of their particles. The photocatalytic activity for the degradation of organic dyes followed
the sequence IC > rhB > CR. When the best photocatalyst was tested, the reached mineralization degrees
after 96 h of irradiation were 29% for rhB and 86% for IC.
©
2011 Elsevier B.V. All rights reserved.
1
. Introduction
visible-light irradiation, important efforts have been carried out in
the last decade. For example, the TiO anatase polymorph has been
2
Heterogeneous photocatalysis has proved to be a useful tool for
doped with nitrogen in order to increase its absorption in the visible
range [6]. In another approach, several authors have proposed alter-
the degradation of water pollutants over the last years. As such, it is
been recognized as a green technology for the treatment of all kinds
of contaminants, especially for the removal of organic dyes with
complex molecular structure, which are usually present in wastew-
ater from the textile industry [1]. This type of industry consumes
large amounts of different dyes, which are lost in quantities of up to
native oxides than traditional TiO with high photocatalytic activity
2
under visible-light irradiation such as In1 NixTaO [7], CaIn O [8],
−x
4
2
4
InVO [9], and BiVO [10].
4
4
In view of the economical use of visible-light radiation and
because of its predominance in the solar spectrum, the devel-
opment of photocatalysts with high activity under visible-light
1
5% during the dyeing process and disposed of in the textile efflu-
ents, leading to a substantial contamination of water [2]. Organic
dyes found in textile effluents are a serious ecological problem,
since even a small amount of them in water can cause a signifi-
cant coloring and they can be toxic to aquatic as well as human
life [3]. Hence, their removal from wastewater is of fundamental
importance for the environment [4].
radiation is desirable. In this sense, tungsten trioxide (WO ) is a
visible-light responsive photocatalyst that absorbs radiation in the
region up to 480 nm [11], which makes it an attractive candidate
3
for photocatalytic applications. Additionally, WO is a material with
3
high stability in aqueous solution under acidic conditions, and it is
an inexpensive material with an Eg between 2.4 and 2.8 eV [12–14].
The high photocatalytic activity under UV radiation, low cost,
and stability to corrosion processes of the anatase polymorph of
TiO2 has positioned this material as the photocatalyst par excel-
lence for commercial applications [5]. However, its relative wide
energy band gap (Eg) of 3.2 eV limits further applications of the
material in the visible-light region (ꢀ > 390 nm). In the search
of semiconductor materials with photocatalytic activity under
The WO oxide has already been tested as a photocatalyst for the
3
degradation of the azo dyes acid orange 7 and direct blue 1 [15]. Fur-
thermore, composite nanoparticles of WO /TiO were employed as
3
2
a photocatalyst in order to degrade methylene blue [16]. Although
WO can act as a photocatalyst in the degradation processes of the
3
organic dyes cited above, this oxide has received little attention
when it comes to the purification of water polluted with organic
dyes.
Recently, we reported the photocatalytic activity of WO3
nanoparticles synthesized by simple precipitation method in the
degradation processes of rhodamine B (rhB), indigo carmine (IC)
∗ Corresponding author. Tel.: +52 81 83 29 40 20; fax: +52 81 83 32 09 04.
E-mail address: azael70@yahoo.com.mx (A. Martínez-de la Cruz).
0
926-860X/$ – see front matter © 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.apcata.2011.03.034

1
80
D. Sánchez Martínez et al. / Applied Catalysis A: General 398 (2011) 179–186
Fig. 1. Molecular structure of: (a) rhodamine B, (b) indigo carmine and (c) congo red.
and congo red (CR) [17]. Given that photocatalytic activity of the
samples was closely related with the morphology they presented,
in this work we proposed a method to modify the morphology
of WO3 nanoparticles obtained by precipitation. In this case, the
influence of urea when is used as additive during the precipitation
process was revised. In the same way, the formation of OH rad-
icals was determined when an aqueous dispersion of WO3 oxide
was exposed to UV–vis light radiation.
BaTiO3 as reference. The Eg value was determined with the fol-
lowing relationship:
n
A(hꢂ − Eg)
˛ =
(1)
hꢂ
•
where ˛ is the absorption coefficient, hꢂ is the photon energy, Eg
the energy band gap, A and n are constants, and n has a value
of n = 1/2 for materials with direct transition. To calculate the
Eg from the UV–vis spectrum, a straight line was extrapolated
following the slope of the line to the x-axis, where ˛ = 0, and, conse-
quently, Eg = hꢂ. The Brunauer–Emmett–Teller (BET) surface area of
the photocatalysts was determined by adsorption–desorption N2
isotherms by means of a Micromeritics Tristar 3000 surface area
2
. Experimental
2.1. Synthesis of samples
◦
and pore size analyzer. The isotherms were evaluated at −196 C
WO3 nanoparticles were synthesized by precipitation method
in the presence of urea. For this purpose, 0.00107 mol of ammo-
◦
after a pretreatment of the sample at 100 C during 2 h.
nium tungstate hydrate (H42N10O42W12·xH O) were dissolved by
2
2.3. Photocatalytic experiments
◦
magnetic stirring at 80 C in 100 mL of a nitric acid solution (10%
v/v, HNO ). Then0.0646 mol of urea(NH CONH )were added tothe
3
2
2
The photochemical reactor employed was a homemade device
solution under continuous stirring and maintaining a constant tem-
perature until the formation of a yellow–green solid precipitated.
This material was used as a precursor for the WO3 nanoparticles.
The precursor obtained was decomposed by thermal treatments
consisting of a borosilicate glass beaker embedded in a water jacket
◦
to maintain the reaction temperature at 25 ± 1 C. A Xe lamp of
1
0,000 K with a luminous flux of 2100 lm was used as the visible-
light source. The emission spectrum of the Xe lamp was measured
on a UV–vis spectrophotometer. After filtration by the borosili-
cate container, a contribution of UV radiation (ꢀ > 315 nm) with an
◦
◦
−1
at 450, 500 and 600 C (heating rate 10 C min ) during different
periods of time in order to obtain the monoclinic crystalline struc-
ture of the WO3 oxide. These samples will be identified hereafter
as W450, W500 and W600, respectively. For comparative purposes,
−2
intensity of 1380 ␮W cm was still observed. Additionally, some
experiments were also performed with a UV source of 365 nm. In
order to have comparative results, a UV lamp with a radiation inten-
WO nanoparticles were obtained by the same experimental pro-
3
cedure but without the addition of urea. This reference sample will
be identified as WR400.
−2
sity of 1250 ␮W cm was selected for these specific experiments.
The photocatalytic activity of WO nanoparticles was evaluated
3
in the reaction of degradation of rhodamine B (rhB, CAS 81-88-9),
indigo carmine (IC, CAS 860-22-0), and congo red (CR, CAS 573-58-
2.2. Characterization of samples
0
) in aqueous solution. The molecular structure of these organic
Structural characterization of the WO samples was carried out
by powder X-ray powder diffraction using a Bruker D8 advanced
diffractometer with CuK␣ radiation (ꢀ = 1.5418 A˚ ) coupled with a
dyes is showed in Fig. 1. In a glass beaker, 220 mL of an organic
dye solution containing 220 mg of photocatalyst were put in an
ultrasonic bath to eliminate aggregates. By considering the molar
extinction coefficient of each dye, the initial concentrations were
3
Vantec high speed detector and Ni filters. X-ray diffraction data of
◦
−1
the samples was collected in the 2ꢁ range of 10–70 with a scan
determined to be 5, 30 and 20 mg L , respectively. In order to
◦
−1
rate of 0.05 s . The morphology and particle size of the prepared
WO3 samples were investigated by scanning electron microscopy
ensure that the adsorption–desorption equilibrium of the dye on
the catalyst surface had been reached, the solution was kept in the
dark for 1 h. After this time, the light source was turned on. Dur-
ing the reaction, samples were taken from the reactor at different
time intervals and then analyzed following the procedures estab-
lished in a previous work [18]. The dye mineralization degree was
monitored by analyzing the total organic carbon content (TOC) in
(
(
FEI Nova 200 NanoSEM) and transmission electron microscopy
JEOL 2010 instrument with an accelerating voltage of 200 kV).
UV–vis diffuse reflectance absorption spectrums of the samples
were obtained by using a UV–vis spectrophotometer (PerkinElmer
Lambda 35) equipped with an integrating sphere and by using

D. Sánchez Martínez et al. / Applied Catalysis A: General 398 (2011) 179–186
181
Table 1
Physical properties of the WO3 samples synthesized under different experimental
conditions.
BET surface area (m2
g
−1
)
WO3 sample
Eg (eV)
WR400
W450
W500
W600
Commercial
2.59
2.68
2.67
2.65
2.62
16.45
10.43
6.10
3.15
3.32
W600
W500
the precipitation process seems to slow down the crystallization
process.
A study of the morphology and particle size of WO nanoparti-
3
cles was performed by SEM and TEM analyses. Fig. 3 shows selected
W450
TEM micrographs of WO synthesized at different calcination tem-
3
peratures. In the first case, the morphology of the particles of the
W450 sample was characterized by rectangular and oval shapes
with variable sizes ranging from 10 to 50 nm, as showed in Fig. 3a.
A similar morphology and particle size was observed for the W500
sample, but with a predominance of the oval particles over the rect-
angular ones (Fig. 3b). This behavior was accentuated for W600,
where only ovoid particles were present. These particles presented
a major thickness, compared with the previously observed at lower
temperatures, as shown in Fig. 3c. The ovoid morphology of the
WO3 nanoparticles can be associated with the presence of the
organic additive during the precipitation process. A TEM analysis
for the WR400 sample revealed a material with a high hetero-
geneity in the shape of its particles, which exhibited morphologies
of square and rectangular plates, see Fig. 3d. As the calcination
WR400
Commercial
JCPDS No. 01-83-0950
1
0
20
30
40
50
60
2
θ
Fig. 2. XRD patterns of WO3 powders obtained at different calcinations tempera-
◦
tures.
temperature was increased for WR400, i.e. 500 and 600 C, the mor-
phology of the nanoparticles shifted towards a prevalence of the
rectangular plate shape [17]. This situation confirms the effect of
urea in the morphology of the samples. For comparative purposes,
Fig. 3e shows a TEM micrograph of a WO3 commercial sample
the solutions with different irradiation times. Performing a typical
−
1
experiment, 200 mL of the corresponding dye solution (50 mg L
−1
for rhB and 100 mg L for IC) containing 200 mg of photocatalyst
were employed. The irradiated samples were analyzed in a Shi-
madzu VSCN8 TOC analyzer.
(99.9%, Aldrich) where bigger particles with a size of approximately
1
00 nm are observed. The morphology of the commercial sample
resembles the shape of the particles obtained by precipitation with
urea in this work.
•
The generation of OH radicals during the photocatalytic
reaction was investigated by the photoluminescence method.
Fig. 4 shows the detailed SEM micrograph of the W450, W500
and W600 samples. Here, is possible to appreciate that the mor-
phology of the particles in these samples consisted of large
agglomerates of the smaller size particles observed by the TEM
technique, as shown in Fig. 4a–c. In general, a natural tendency to
form larger agglomerates of particles was observed at lower tem-
peratures. Fig. 4d and e shows that the particles of WR400 and
commercial oxides have the tendency to form even larger agglom-
erates. This is relevant because, under our experimental conditions,
a good material dispersion in aqueous solution allows a major area
of material exposed to the radiation lamp.
Table 1 shows the corresponding energy band gap values of the
samples obtained, which are found to agree with the previously
reported for WO3 [12]. The BET surface area of WO3 samples is
reported also in Table 1. It was found that when the calcination tem-
perature of the samples increases, the BET surface area decreased,
as was expected due to the sinterization of the samples at higher
temperatures. In this case, the WR400 reference sample showed
the highest surface area value.
•
Terephthalic acid was used to readily react with OH to form
highly fluorescent product, 2-hydroxyterephthalic acid, which can
be detected by fluorescence spectrum (excitation wavelength:
3
12 nm, fluorescence peak: 426 nm) [19]. For this purpose, 220 mg
of photocatalyst were dispersed in a solution formed by tereph-
−
4
−1
−3
−1
thalic acid (5 × 10 mol L ) and NaOH (2 × 10 mol L ). The
dispersion formed was irradiated for different amounts of time.
From the fluorescence intensity, the amount of OH radical could be
estimated. The OH-trapping photoluminescence spectrums were
•
•
performed in a PerkinElmer precisely LS 55 luminescence spec-
trophotometer.
3
. Results and discussions
3.1. Characterization of samples
The powder used as a precursor to obtain the WO samples pre-
3
sented a greenish tinge. The crystalline structure of WO was found
3
◦
to form starting at 450 C, below this temperature the material was
amorphous without diffraction lines. The X-ray diffraction patterns
of the W450, W500 and W600 samples obtained by thermal treat-
ments are shown in Fig. 2. For the three samples, all diffraction
lines were correctly assigned to the monoclinic polymorph of WO3
according to the JCPDS Card No. 01-083-0950. On the other hand,
as shown in Fig. 2, in absence of urea the crystalline structure of
3.2. Photocatalytic activity
The photocatalytic activity of the WO nanoparticles was eval-
3
uated for the degradation of rhB, IC and CR molecules in aqueous
solution under Xe lamp radiation. Fig. 5 shows the data corre-
sponding to the temporal degradation of rhB when the different
WO3 is formed by a thermal treatment at temperatures as low as
synthesized WO samples were used as photocatalysts. All the WO3
samples tested were able to bleach the rhB solution, however, there
3
◦
4
00 C (WR-400). This indicates that the presence of urea during

1
82
D. Sánchez Martínez et al. / Applied Catalysis A: General 398 (2011) 179–186
◦
◦
◦
Fig. 3. TEM analysis of the morphology of the WO3 nanoparticles synthesized by precipitation in the presence of urea: (a) 450 C, (b) 500 C, (c) 600 C, (d) by simple
precipitation 400 C, and (e) commercial WO3.
◦
were differences in the rate at which this was achieved. The sample
with the best photocatalytic activity was W500, which bleached the
rhB solution after 210 min of lamp radiation. Different factors can
contribute to the high photocatalytic activity observed in W500.
At first instance, a minor aggregation of the particles of W500 and
W600 with respect to W450 was observed. As mentioned before,
this promotes a better dispersion of the photocatalyst during the
photocatalytic experiment. Regarding the surface area, W500 has a

D. Sánchez Martínez et al. / Applied Catalysis A: General 398 (2011) 179–186
183
◦
◦
◦
Fig. 4. SEM analysis of the morphology of the WO3 samples synthesized by precipitation in the presence of urea: (a) 450 C, (b) 500 C, (c) 600 C, (d) by simple precipitation
◦
4
00 C, and (e) commercial WO3.
higher exposed surface, compared to W600. On the other hand,
WR400 had the highest surface area among all the synthesized
samples but, at the same time, it tended to form the largest agglom-
erates. Additionally, the better photocatalytic activity observed for
W500 can be associated with its higher crystallinity with respect
to W450. As it is well known, the crystalline defects act as recom-
bination centers of the electron–hole pair. As a conclusion, it is
possible to think that the degradation rate of the organic dye is
the result of the contribution of all factors mentioned. The photo-
catalytic activity for the degradation of rhB followed the sequence
W500 > W600 > commercial > W450 > WR400.
1
0
0
0
0
.8
.6
.4
.2
0
Without photocatalyst
Commercial
W450
W500
W600
WR400
The temporal evolution of spectrum changes during the rhB
photodegradation process by W500 is shown as Supplementary
data in S1. The main rhB absorption band located at 554 nm is
due to the presence of four ethylated groups on the dye molecule.
The irradiation of the W500/rhB dispersion with Xe lamp produced
a gradual depletion of the absorbance value. This depletion takes
place almost without a shift of the dye absorption band, revealing
some features of the pathway followed during the rhB photodegra-
dation. As was previously established, the rhB photodegradation
0
50
100
150
200
250
300
350
Time (minutes)
Fig. 5. Evolution of the rhB concentration during its photocatalytic degradation
−1
(
5 mg L ) by WO3 nanoparticles using a Xe lamp as source of radiation, pHo 6.0.
The concentration of rhB was determined through its absorption band maximum
554 nm).
•
can occur by two competitive mechanisms: by attacking OH radi-
cals on the aromatic chromophore ring, leading to the degradation
(

1
84
D. Sánchez Martínez et al. / Applied Catalysis A: General 398 (2011) 179–186
1
.8
.6
1
Without photocatalyst
0.9
0
0
0
0
Commercial
W450
W500
W600
0.8
Without photocatalyst
Commercial
W450
W500
W600
WR400
0
0
0
.7
.6
.5
.4
.2
WR400
0
0
50
100
150
200
250
300
350
0
50
100
150
200
250
300
350
Time (minutes)
Time (minutes)
Fig. 7. Evolution of the CR concentration during its photocatalytic degradation
Fig. 6. Evolution of the IC concentration during its photocatalytic degradation
−1
(
20 mg L ) by WO3 nanoparticles using a Xe lamp as source of radiation, pHo 6.0.
−
1
(
30 mg L ) by WO3 nanoparticles using a Xe lamp as source of radiation, pHo 6.0.
The concentration of IC was determined through its absorption band maximum
610 nm).
The concentration of CR was determined through its absorption band maximum
498 nm).
(
(
situation was observed in the degradation of IC where after 96 h
of radiation a mineralization degree of 86% was reached for a solu-
tion of IC (100 mg L ). This data confirms the high photocatalytic
of the rhB structure and reduction of the absorption band without
a wavelength shift (pathway 1) or by a successive de-ethylation
from the aromatic rings, causing significant blue wavelength shifts
according to the formation of the different de-ethylated rhodamine
intermediates (pathway 2) [20–22]. According to the spectrum
changes observed in S1, it seems clear that the mechanism concern-
ing the rhB photodegradation occurs by the attack of the aromatic
chromophore ring, and predominates over the de-ethylated pro-
cesses.
Fig. 6 shows the high photocatalytic activity of the WO3 sam-
ples for the degradation process of IC, which was even higher than
that observed with rhB. Note that the initial IC concentration used
in these experiments was 6 times higher than the one employed
for rhB due to their respective molar extinction coefficients. Once
again, in all cases the samples were able to bleach the IC solution.
In fact, this process was completed after only 150 min of radiation
for the samples calcined at higher temperatures (W500 and W600).
The sequence of the photocatalytic activities of the WO3 samples
was W500 > W600 > W450 > WR400 > commercial.
−1
activity of WO3 for the degradation process of IC. The observed
difference in the mineralization degree of rhB and IC can be associ-
ated with the recalcitrant behavior of the products formed during
the photodegradation of rhB, as was previously reported [23,24].
In the same way, the origin of this important difference can be
related to the mechanism that operates in each dye’s photodegra-
dation process. In order to elucidate this point, some experiments
were performed varying the source of radiation. The degradation
of rhB and IC by the action of a photocatalyst can take place by
two ways. The first of which is by a true photocatalytic process,
where radiation on the photocatalyst promotes an electron from its
valence band to the conduction band, and then the electron–hole
pair is formed. The second possibility is through a photosensitiza-
tion process, where the radiation excites an electron from the dye
and then, it is injected to the conduction band of the semiconduc-
tor oxide. In both processes, a series of consecutive reactions leads
to the eventual mineralization of organic dyes to CO2 and H O. In
2
The temporal evolution of spectrum changes during the IC pho-
todegradation process by W500 followed a similar behavior that
observed for rhB, see data Supplementary S2. The depletion in
the absorbance value without a shift of the dye absorption band
revealed a possible direct attack of the aromatic chromophore ring
of IC by radicals formed.
order to elucidate which mechanism is involved, some experiments
were performed varying the source of radiation. The photocatalytic
degradation of rhB over the WO photocatalyst was performed, but
3
now the photocatalyst/dye dispersion was irradiated with UV light
(365 nm). As it can be seen in Fig. 9a, the photocatalytic activity of
W500 decreased considerably, and after 150 min of UV-radiation
Fig. 7 shows the data of the temporal degradation of CR when
WO samples were used as photocatalysts. In general, after 300 min
of light radiation the CR solutions remained colored. The diffi-
culty to degrade CR is associated with the presence of the two azo
3
100
(
–N N–) groups in its molecular structure, see Fig. 1. The recalci-
8
6
4
0
0
0
trant nature of the azo dyes is due to the high energy necessary to
break the azo bond. Due to this result, additional experiments with
CR were not carried out.
To determine if the mineralization of organic dyes is feasible
by WO3 under the Xe lamp radiation, experiments to determine
the variation of total organic carbon were performed in the course
of the photocatalytic reaction for the degradation of rhB and IC,
as is shown in Fig. 8. These experiments were carried out with
RhB
IC
20
0
◦
the photocatalyst synthesized at 500 C, i.e. W500. To achieve a
greater accuracy in the quantification concerning the experiment
−
1
with rhB, the initial concentration was modified to 50 mg L . This
data showed that the complete mineralization of rhB by the WO3
photocatalyst is not feasible (29% after 96 h). The bleaching of the
dye solution is just associated with the inactivation of the chro-
mophore groups in the molecular structure of rhB. A different
0
25
50
75
100
Time (hours)
Fig. 8. Changes in the TOC during the mineralization of rhB and IC using a Xe lamp
as source of radiation, pHo 6.0.

D. Sánchez Martínez et al. / Applied Catalysis A: General 398 (2011) 179–186
185
only 12% of the dye solution was bleached. In the same period, the
solution was bleached in a 95% when it was irradiated with the
Xe lamp. This experiment revealed that the degradation of rhB by
1
.8
.6
.4
a
0
0
0
WO seems to occur predominantly through the photosensitization
3
of the organic dye by the action of visible-light radiation. Fig. 9b
shows a similar experiment but in this case an IC solution was irra-
diated with the UV source in the presence of W500. Although the
photocatalytic activity decreased when the dispersion was irradi-
ated with UV-radiation a considerable activity was still observed
in this experiment. This situation suggests that the two possible
mechanisms of dye degradation take place simultaneously, i.e. true
photocatalysis and the photosensitization process. As was previ-
ously mentioned, the mineralization degrees achieved after 96 h
of Xe lamp radiation were 29% for rhB and 86% for IC. This behav-
ior can be explained on the basis of the proposed photocatalytic
degradation mechanism of rhB. By taking into account that the
photosensitization process is the predominant degradation mecha-
nism, the presence of rhB promotes degradation by this mechanism.
When the solution is bleached, the photosensitization process is
avoided and the mineralization of rhB is considerably inhibited. On
the other hand, for IC the temporal TOC value decreases consider-
ably during the experiment. Due to the fact that the IC degradation
occurs by simultaneous photosensitization and true photocatalysis
processes, the bleaching of the solution does not affect considerably
the mineralization process.
Without photocatalyst
W500 UV365
W500 LV
0.2
0
0
50
100
150
200
250
300
350
Time (minutes)
1
b
0
0
0
.8
.6
.4
Whitout photocatalyst
W500-UV365
W500-LV
0.2
0
•
In order to investigate the participation of OH radicals in the
courseof thephotocatalytic degradation of organic dyes inpresence
of WO , a test with terephthalic acid was performed. This test is rec-
3
0
50
100
150
200
250
300
350
•
ognized as an efficient method to determine the formation of OH
Time (minutes)
radicals when a semiconductor is exposed to a radiation source. Due
to the formation of a unique product during the reaction of tereph-
Fig. 9. Evolution of the (a) rhB and (b) IC concentration during its photocatalytic
degradation by WO3 nanoparticles under UV and vis irradiation.
•
thalic acid with OH radicals, the 2-hydroxyterephthalic acid, and
•
its photoluminescence properties, an estimated of the OH radi-
cal concentration can be calculated from the photoluminescence
•
spectrum of the product formed. Fig. 10 shows the OH-trapping
4
3
2
1
00
00
00
00
0
800
a)
b)
600
3
2
2
00 min
70 min
10 min
300 min
270 min
210 min
150 min
120 min
400
1
50 min
120 min
90 min
60 min
30 min
0 min
9
6
3
0 min
0 min
0 min
200
0
0
min
3
50
400
450
500
550
350
400
450
500
550
Wavelength(nm)
Wavelength(nm)
1
000
400
c)
d)
8
6
4
2
00
00
00
00
0
3
00
2
40 min
210 min
240 min
180 min
150 min
120 min
90 min
60 min
30 min
0 min
1
1
1
80 min
50 min
20 min
90 min
200
6
3
0 min
0 min
1
00
0
0
min
3
50
400
450
500
550
350
400
450
500
550
Wavelength(nm)
Wavelength(nm)
•
Fig. 10. OH-trapping photoluminescence spectra of WO3 (W500) and TiO2 (P25) in a solution of terephthalic acid at room temperature (excitation at 312 nm, emission at
26 nm): (a) WO3, UV-light radiation (365 nm), (b) WO3, Xe lamp radiation, (c) TiO2, UV-light radiation (365 nm) and (d) TiO2, Xe lamp radiation.
4

1
86
D. Sánchez Martínez et al. / Applied Catalysis A: General 398 (2011) 179–186
photoluminescence spectrums of sample WO (W500) in a tereph-
thalic acid solution at room temperature under: (a) UV-light
radiation (365 nm), and (b) Xe lamp radiation. In the same way,
solutions. Moreover, the mineralization of indigo carmine was also
completed after 96 h of light radiation. In general, the best photo-
catalytic activity was obtained with the sample calcined at 500 C.
3
◦
experiments with both radiation sources but now using TiO P25 as
photocatalyst were also performed, see Fig. 10c UV-light radiation
The reason behind its higher activity was associated with factors
such as morphology, aggregation degree of its particles, and, to a
lesser extent, surface area. Throughout the experiments with vis-
and UV-radiation, it was possible to conclude that the degrada-
tion of rhB by WO3 seems to occur predominantly through the
photosensitization of the organic dye by the action of visible-light
irradiation, whereas the degradation of IC simultaneously happens
by photosensitization and true photocatalytic processes. Finally,
2
and Fig. 10d Xe lamp radiation. In first instance, it is clear that the
•
formation of OH radicals is feasible when WO is used as photocat-
3
•
alyst. The concentration of OH radicals is notably increased when
the dispersion was exposed to the Xe lamp radiation. Although
both lamps have similar intensity of UV radiation, the emission
spectrums of the lamps are necessary to characterize completely
the radiation source. Nevertheless, an effective comparison can be
•
the formation and participation of OH radicals in the degradation
realized if TiO P25 is used as reference.
of organic dyes when WO is used as photocatalyst was confirmed.
2
3
•
The formation of OH radicals when the dispersion of TiO was
2
irradiated with UV was more than double in relation with the
results obtained with the visible-light source. The chemical compo-
Acknowledgements
sition of TiO P25 included the polymorphs anatase (80%) and rutile
We wish to thank Universidad Autónoma de Nuevo León (UANL)
for the support provided through the PAICYT-2009 project and
CONACYT for supporting the 81546 project. We also want to thank
the Departamento de Ecomateriales y Energía of the Facultad de
Ingeniería Civil (UANL) for the assistance with the materials char-
acterization.
2
(
20%), both polymorph oxides with band gaps > 3.1 eV and suscep-
tible to be excited only by UV radiation under 390 nm [25]. The
•
activity observed for the formation of OH radicals under Xe lamp
irradiation is due to the presence of UV radiation not filtered by
the borosilicate container. On the other hand, the higher activity of
•
WO in the formation of OH radicals when it is exposed to visible-
3
light source is indicative that this type of radiation has the main
contribution to the photocatalytic process. This result is in agree-
ment with the recent work of Kim et al. [26], where they relate
Appendix A. Supplementary data
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.apcata.2011.03.034.
•
the formation of OH radicals when a dispersion of platinized WO3
(
Pt/WO ) photocatalyst was exposed to visible-light radiation. The
3
•
generation of OH radicals in the dispersion of WO (W500) would
3
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