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ORIGINAL PAPER
Synthesis, characterization and photocatalytic propertiesof tungsten-doped hydrothermal TiO2
J. A. Leon-Ramos • D. Kibanova • P. Santiago-Jacinto •
Y. Mar-Santiago • M. Trejo-Valdez
Received: 12 February 2010 / Accepted: 25 August 2010 / Published online: 28 September 2010
� Springer Science+Business Media, LLC 2010
Abstract Crystalline anatase phase TiO2 with photocat-
alytic properties was obtained through a sol–gel low-tem-
perature hydrothermal process. TiO2 samples doped with
tungsten oxide were also obtained by using this synthetic
approach. The photocatalytic oxidation of methylene blue
in water was monitored to study the influence of the
tungsten doping degree on the photocatalytic degradation
performance of TiO2. The degradation rate constant was
further increased by adjusting the tungsten doping degree
of hydrothermal TiO2. Also, a much faster photodegrada-
tion of methylene blue was achieved using tungsten doped
samples baked at 450�C. The results were compared with
those obtained with Degussa P25 used as photocatalyst.
The structure and optical properties of tungsten-doped
TiO2 were studied by SEM, X-ray diffraction, UV–vis and
DRIFT spectroscopy techniques.
Keywords Sol–gel � Hydrothermal synthesis � TiO2 �Photocatalysis � Tungsten-doped
1 Introduction
TiO2 is a material extensively employed in industrial and
commercial applications such as pigment in the paint
industry, as sunblocking material in cosmetics, as a binder
in medicinal fields and so on [1–3]. Interfacial electron
transfer reactions on TiO2 have centered the interest in
strategic applications for solving several problems such as
energy supply [4–6] or remediation of contaminated water
[7–9]. Titania (TiO2) is a wide bandgap semiconductor
(from 3.0 to 3.2 eV depending on the crystalline phase
[10]) which presents photoactivity upon near UV or higher
irradiation, absorbing photons and transforming them into
chemical redox energy. When a photon, with an energy
equal or higher than the bandgap of the semiconductor
(Eg), is absorbed by the material, an electron from the
valence band (VB) is promoted to the conduction band
(eCB- ) leaving a hole behind (hVB
? ). The number of photo-
generated electron-hole pairs depends on the semiconduc-
tor band structure, as well as the energy and the effective
intensity of the incident light [11]. Unfortunately, light
activation of titania occurs in the near UV region
(k B 385–400 nm), meaning that only a 5–8% fraction of
sunlight can be absorbed by the bare material [12]. Due to
the inherent relatively large band of TiO2 materials,
research has focused on lowering the threshold energy for
excitation during TiO2-assisted photocatalysis, in order to
utilize a wider fraction of solar irradiation for conversion
into chemical energy [13–15].
Coupling TiO2 with other metal oxides (e.g., WO3,
SnO2, or V2O5) is an approach that has received much
attention for improving the photocatalytic properties of
titania [16–20]. Electron-hole pairs can either initiate redox
reactions with adsorbates at the catalyst surface or can
result in recombination with the release of heat. A key
J. A. Leon-Ramos � D. Kibanova
Facultad de Quımica, Universidad Nacional Autonoma de
Mexico, C. Universitaria, Mexico, D.F C.P. 04510, Mexico
P. Santiago-Jacinto
Instituto de Fısica, Universidad Nacional Autonoma de Mexico,
C. Universitaria, Mexico, D.F C.P. 04510, Mexico
Y. Mar-Santiago � M. Trejo-Valdez (&)
ESIQIE-Instituto Politecnico Nacional, Zacatenco,
D.F C.P. 07738, Mexico
e-mail: [email protected]
123
J Sol-Gel Sci Technol (2011) 57:43–50
DOI 10.1007/s10971-010-2322-6
factor affecting the efficiency of a photocatalyst is the
electron-hole recombination rate, because it controls the
availability of photoexcited sites on the catalyst surface.
When anatase phase TiO2 is doped with a semiconductor of
smaller bandgap, such as WO3 (Eg = 2.8 eV), improved
charge separation can result from the coupling of the two
materials [16]. Tungsten (VI) in WO3 acts as trapping site
by accepting photoexcited electrons from TiO2 valence
band generating tungsten (V). Since photogenerated holes
move in the opposite direction, they accumulate in the
valence band of TiO2 increasing the efficiency of charge
separation [21]. Electrons of tungsten (V) can then be
transferred to surface reducible species. Moreover, the
presence of WO3 can increase the acidity of titania, mod-
ifying the affinity of substrates for the catalyst surface, and
as a consequence, the adsorption equilibrium and photo-
oxidation activity of the catalyst [17].
Recent publications report interesting synthetics meth-
ods to obtain tungsten-doped TiO2 that can be used for
photooxidation of aqueous dyes [22, 23]. In these reports
an improvement of the photocatalytic activity of anatase
phase TiO2 is in effect achieved by means of doping it with
W6?. Those materials have been prepared by using simple
methods such as the classical sol–gel route or electros-
pinning-sol–gel. However, their results were not compared
with those obtained with commercial TiO2, or dye con-
versions of less than 60% were presented.
This work deals with the synthesis of TiO2 doped with
tungsten oxide by using a two step sol–gel approach cou-
pled with the hydrothermal synthetic method. Such method
is a non expensive and versatile approach that allows us to
synthesize a great variety of crystalline inorganic oxides, at
a low temperature (less than 200�C). The photocatalytic
oxidation of methylene blue in water was monitored to
study the influence of the tungsten doping degree on the
degradation performance of TiO2. Our results were com-
pared with those obtained using the commercial TiO2
Degussa P25 as photocatalyst. Also, dye conversions above
95% are observed using tungsten doped TiO2 samples,
higher than those obtained using anatase phase TiO2 as
photocatalyst.
2 Experimental section
2.1 Synthetic approach
Hydrothermal synthesis of TiO2 and tungsten-doped TiO2
was achieved with a sol–gel hydrothermal approach. TiO2
precursor was titanium i-propoxide (Ti(OC3H7)4, 97%)
solution with C = 0.4 Mol/L, pH = 1.25, and a water/
Ti(OC3H7)4 M ratio (rw) of 33. This solution (called SG1),
was directly poured into an autoclave (Parr Instruments) of
45 mL capacity, and the closed autoclave was then intro-
duced in an oven at 180�C for 8 h. Once thermal treatment
was finished, the autoclave was removed from the oven and
cooled at room temperature. The white precipitate formed
at the bottom of the autoclave container was separated from
the solution by centrifugation at 4,500 rpm, washed with a
volume of absolute ethanol and then re-dispersed in this
solvent at room temperature in an ultrasonic bath. The
dispersion was centrifuged at 4,500 rpm again, and the as-
obtained precipitate was now washed with deionized water.
The sample was re-dispersed in water, centrifuged once
more and finally dried at 100�C.
The samples of TiO2 doped with tungsten oxide were
prepared in a similar way. Sodium tungstate dehydrated
(Na2WO4�2H2O) was used as tungsten precursor. In a first
step, the tungsten salt was dissolved in some volume of de-
ionized water. In a second step, this solution was slowly
added to a flask containing the SG1 solution and some
volume of absolute ethanol to adjust the final Ti(OC3H7)4
concentration to 0.05 Mol/L. The resulting mixture was
poured in the autoclave and thermally treated at 180�C for
8 h. The as-obtained pale blue precipitates were purified in
the same way as in the non-doped TiO2 synthesis. Precursor
solutions with Na2WO4/Ti(OC3H7)4 M ratios ranging from
0.005 to 0.05 were used. The obtained tungsten-doped TiO2
samples were labeled as 005W-TiO2, 01W-TiO2, 02W-
TiO2, 03W-TiO2, 04W-TiO2, and 05W-TiO2, depending on
the Na2WO4/Ti(OC3H7)4 M ratio of the precursor solutions.
Finally, selected tungsten-doped samples were baked at
450�C.
2.2 Characterization
Samples morphology was studied using a scanning electron
microscope (SEM) JEOL JSM 5600LV, equipped with
Noarn analytical system and a Cu Ka monochromator from
a Phillips (X‘Pert) diffractometer. X-ray diffraction (XRD)
analysis was performed using a Philips PW2400 wave-
length dispersive x-ray fluorescence spectrometer. Diffuse
reflectance spectra of TiO2 and tungsten-doped TiO2
samples were obtained by using a GBC UV–vis (Cintra)
spectrophotometer. The diffuse reflectance FTIR spectra
were recorded with a resolution of 4 cm-1 and accumula-
tion of 300 on a Nicolet FTIR spectrometer supplied with a
MCT detector and diffuse reflectance attachment.
2.3 Photocatalytic evaluation
Photocatalytic properties of the samples were evaluated
following the photooxidation of methylene blue. Experi-
ments were carried out in an annular cylindrical glass
reactor whose center is a sleeve which contains a UV light
source. The total volume of the reactor was 250 mL and
44 J Sol-Gel Sci Technol (2011) 57:43–50
123
the light source was a blacklight blue UVA lamp (8 W,
Hitachi). This light source provided a broad range of
wavelengths from 320 to 390 nm with kmax (emis-
sion) = 355 nm, and a light intensity of 732 lW/cm2.
Batch experiments were performed by filling the reactor
with 100 mL of an aqueous slurry composed by methylene
blue solution and the test sample. As the source of oxygen,
air was bubbled into the reactor at a flux of 23 cm3/s. The
air entered the reactor by means of four entries located at
the bottom of the glass cylinder, so that the photocatalyst
was homogeneously dispersed at all times. For all the
experiments, the photocatalyst concentration was fixed at
1 g/L. Depending on the degradation rates observed at the
reaction conditions, aliquots were sampled every 5 or
10 min and filtered through a 0.2 lm PTFE syringe filter to
remove the photocatalyst particles before analyses. Meth-
ylene blue degradation was monitored by measuring the
UV–vis absorption spectra of the resulting solution in a
wavelength range of 800–200 nm. Results obtained with
tungsten-doped TiO2 were compared with those obtained
using commercial titanium dioxide Degussa P25 (AG
Germany, 21 nm of particle diameter, specific area of
50 ± 15 m2g-1 and a crystal distribution of 80% anatase
and 20% rutile).
3 Results and discussion
3.1 Structural characterization
The X-ray diffraction patterns of hydrothermal TiO2 sam-
ples doped with tungsten oxide are presented in Fig. 1.
Miller indexes presented in Fig. 1 were computed using
Eq. 1 (Bragg’s Law applied to tetragonal lattice):
Sen2h ¼ k2
4a2h2 þ k2� �
þ l2
c2=a2
" #
ð1Þ
All the diffraction patterns presented in Fig. 1 matched
well with those reported in the JCPD 21-1272 for anatase
phase TiO2 and therefore, no rutile phase TiO2 was present
in the obtained samples. The tetragonal lattice parameters
a and c were also calculated from Eq. 1 with k =
0.154056 nm (the copper radiation wavelength) and 2hvalues of the (200) and (004) diffraction peaks. For the
non-doped hydrothermal TiO2 sample, a = 3.7829 A and
c = 9.4677 A lattice parameters were obtained. These
parameters are similar to those reported for anatase phase
TiO2 (a = 3.7842 A and c = 9.5146 A). Crystal sizes
were estimated as described elsewhere [24] by using the
Scherrer relation and the diffraction data of the samples:
Bcrystallite ¼kk
L cos hð2Þ
where k is the wavelength of the x-ray source (Cu Karadiation in this case), L is the crystal size, h is the Bragg
angle, B is the broadening of the x-ray diffraction peak, and
k is a constant. Equation 2 was derived based on the
assumptions of Gaussian line profiles and small cubic
crystals of uniform size (for which k = 0.94). However,
this equation is now frequently used to estimate the crys-
tallite sizes of both cubic and non cubic materials. The
constant k in Eq. 2 has been determined to vary between
0.89 and 1.39, but the value of 1 is commonly used [25].
Since the precision of the crystallite-size analysis by this
method is, at best, about ±10%, the estimation of values
using k = 1 is justifiable. We found that an increase in the
tungsten content of samples from 0 to 5% (mol/mol) pro-
moted a slight increase in anatase crystal size from 7.62 to
11.94 nm (see Table 1).
The diffraction patterns of tungsten-doped TiO2 samples
showed some changes in comparison with the non-doped
sample (see Fig. 1). A zoom of the (101) diffraction peaks
is showed in Fig. 1b. A careful observation of the (101)
diffraction proves that the peak position of TiO2 with dif-
ferent tungsten contents shifts slightly toward lower 2hvalues. Because the ionic radius of W6? (41 pm) is smaller
than Ti4? (53 pm) [26], a decrease of the crystal size,
20 40 60 800
2000
4000
6000
8000
10000
12000
(107
)(220
)(1
16)
(204
)(211
)(1
05)
(200
)
(004
)
(101
)
Inte
nsi
ty (
a.u
)
2 Theta (degre)
TiO2
005W-TiO2
03W-TiO2
04W-TiO2
(a)
23 24 25 26 27 28
Inte
nsi
ty (
a.u
)
2 Theta (degre)
TiO2
005-WTiO2
03W-TiO2
04W-TiO2
(b)
Fig. 1 a Diffraction patterns of the TiO2 hydrothermal sample and
the tungsten doped oxides. b A zoom of the (101) diffraction peaks
J Sol-Gel Sci Technol (2011) 57:43–50 45
123
Bcrystallite, should result in the increase of 2h value. Our
results demonstrate that the observed shift of diffraction
peak toward lower angles is not because smaller lattice
parameters expected for substitution of Ti4? by W6?.
Instead, this may be due to an increase of lattice parameters
because repulsion between W6? cations which may occur
as interstitial dopants. This information agrees with the
observed increase of Bcrystallite promoted by the increase of
tungsten doping degree of the samples.
SEM images of hydrothermal samples are shown in
Fig. 2. As shown in this figure, the hydrothermal samples
of TiO2 were made up of irregularly-shaped aggregates
with sizes between 10 and 0.1 lm. The increase of tung-
sten doping degree did not affect the particles morphology.
The as-obtained hydrothermal particles generally exist in
compact aggregates with a few loose particles. EDS anal-
ysis conducted on the surface of hydrothermal TiO2 sam-
ples revealed the presence of Ti, O, W, C and Cl (see
Table 2). According to EDS data, hydrothermal oxides
were effectively doped with tungsten and tungsten depo-
sition increased with the concentration of tungsten sodium
salt in the sol–gel precursor solutions.
3.2 Optical properties
Figure 3 shows the reflectance spectrum of hydrothermal
TiO2 samples. These data were compared with reflectance
of Degussa P25 (Fig. 3a). The reflectance spectrum of P25
showed the typical reflectance edge corresponding to the
intrinsic indirect bandgap at 390 nm (3.18 eV). The
bandgap values of anatase phase TiO2 samples doped with
tungsten were estimated from the reflectance data in a
similar way as reported in Ref. [27]. For direct bandgap
materials, the absorption coefficient, a, is directly related to
bandgap energy, Eg, by the expression:
a ¼ A hv� Eg
� �1=2with A �
q2 2m�hm�e
m�hþm�e
h i
nch2m�e
3=2
ð3Þ
where h is the Planck constant, m is the light frequency, mh*
and me* are the effective masses of the hole and the
electron respectively, q is the elementary charge, n is the real
index of refraction and c is the speed of light [28]. Also, acan be expressed as a function of the reflection data as:
2at ¼ lnRmax � Rmin
R� Rmin
� �ð4Þ
where t is the thickness of the sample, Rmax and Rmin are
the maximum and minimum values of reflectance, R is the
reflectance at a given photon energy, hm. Metal oxides can
be either direct or indirect semiconductors depending on
whether the electronic transition is dipole allowed or
Table 1 Crystal size values of hydrothermal TiO2 samples
Sample Na2WO4/Ti(OC3H7)4 M
ratio (9100%)
Crystal size
(nm)
TiO2 0 7.62
005W-TiO2 0.5 7.66
01W-TiO2 1 8.51
02W-TiO2 2 9.61
03W-TiO2 3 9.66
04W-TiO2 4 11.2
05W-TiO2 5 11.94
Data obtained from Eq. 2 using k = 1
Fig. 2 SEM micrographs of the 01W-TiO2 sample: a hydrothermal
sample and b the sample baked at 450�C
Table 2 EDS analysis of the hydrothermal TiO2 doped with tungsten
Sample Element (Wt%)
Ti O W C Cl
005W-TiO2 81.99 16.01 1.04 – 0.97
01W-TiO2 85.62 9.5 1.54 2.87 0.48
02W-TiO2 84.46 6.93 6.82 1.42 0.37
03W-TiO2 81.33 8.08 8.4 2.0 0.2
04W-TiO2 67.69 20.58 11.24 – 0.5
05W-TiO2 55.32 31.57 12.73 – 0.38
46 J Sol-Gel Sci Technol (2011) 57:43–50
123
forbidden, being the later case phonon assisted. TiO2 is an
indirect semiconductor but nanostructured samples may
likely be direct ones. This is a general result as the con-
finement of charge carriers in a limited space causes their
wave functions to spread out in momentum space, in turn
increasing the probability of direct (Frank–Condon type)
transitions for bulk indirect semiconductors [29]. Then, the
bandgap of nanocrystalline semiconductors can be esti-
mated by plotting (hm ln [(Rmax-Rmin)/(R-Rmin)]2) versus
the incident energy, hm. As shown in the insert of Fig. 3,
the extrapolation of the straight line to hm = 0 provides the
bandgap of the sample. With this method, we estimated the
bandgap of the hydrothermal TiO2 doped with tungsten
(Table 3). The energy bandgap of commercial TiO2
Degussa P25 was also calculated and the obtained value of
3.18 eV matched well with the reported one. We found that
values obtained for our hydrothermal samples were similar
to those obtained for Reddy et al. [30]. These authors
reported Eg values between 3.3 and 3.4 eV for anatase
phase TiO2, with average particle sizes of 5–10 nm [30].
Furthermore, they showed that the direct semiconductor
approach granted more realistic bandgap values than those
obtained from the indirect one when crystal sizes were less
than 39 nm. In this work, since crystals with sizes between
7.62 and 11.94 nm were obtained, we believe the estima-
tion of bandgaps by using Eq. 4 results in a good
approximation.
3.3 Photocatalytic properties
Figure 4 shows the UV–vis spectrum of methylene blue. In
this paper, dye conversion degree induced by photocatal-
ysis was estimated by measuring the disappearance of the
strong absorption band of methylene blue located at
663 nm. As we can appreciate from Fig. 5, less than 10%
of the initial dye concentration was lost under UVA irra-
diation alone. However, the photocatalytic conversion of
the dye was successfully achieved only in the presence of
the hydrothermal TiO2 samples or Degussa P25. Disper-
sions containing tungsten-doped samples presented a much
faster methylene blue degradation compared to the non-
doped hydrothermal TiO2. A key factor that controls the
efficiency of a photocatalyst is the recombination rate of
the generated electron-hole pairs, as it controls the avail-
ability of photoexcited sites on the catalyst surface. Then,
the increase of the dye conversion determined in the
presence of tungsten-doped samples can be explained in
terms of an improvement of the charge separation which
was promoted by the coupling of tungsten oxide with the
anatase TiO2.
Figure 6 illustrates the initial reaction rate variation
determined in slurries containing different dye concentra-
tions. These data were obtained applying the kinetic model
Table 3 Energy bandgap
values of TiO2 hydrothermal
samples and Degussa P25
Sample Bandgap (eV)
Degussa P25 3.18
TiO2 3.20
005W-TiO2 3.21
01W-TiO2 3.23
02W-TiO2 3.23
03W-TiO2 3.3
04W-TiO2 3.29
05W-TiO2 3.31
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
310 330 350 370 390 410
Ref
lect
ance
Wavelength (nm)
P25 Hydroth. TiO2 005W-TiO2 04W-TiO2 05W-TiO2
3.0 3.1 3.2 3.3 3.4 3.5 3.60
5
10
15
20
(hvv
ln(R
maax
-Rm
in))//(
R-R
minn
))2
Energy (eV)
Fig. 3 Diffuse reflectance
spectrum of P25 (opendiamond), hydrothermal TiO2
(plain line), 005W-TiO2 (opentriangle), 04W-TO2 (;), 05W-
TiO2 (plus symbol) samples.
The shown insert corresponds to
the plot of (hm ln [(Rmax-Rmin)/
(R-Rmin)]2) versus the incident
energy, hm for the 005W-TiO2
sample baked at 450�C
J Sol-Gel Sci Technol (2011) 57:43–50 47
123
developed by Langmuir and Hinshelwood [31, 32]. The
photocatalytic oxidation of several dyes (such as methylene
blue) obey the Langmuir–Hinshelwood kinetics given by
the equation:
r ¼ � kK MB½ �1þ K MB½ � ð5Þ
Where r is the rate of dye mineralization, k is the apparent
zero-order rate constant, [MB] is the dye concentration and
K is the adsorption coefficient. An equivalent expression
for the Langmuir–Hinshelwood kinetics is:
1
r0
¼ 1
kþ 1
kK MB½ �0ð6Þ
The constants k and K, can be obtained from the intercept
and slope of the line formed when 1/rate is plotted against
1/[MB]0. Once the [MB] variation as function of the
irradiation time is obtained from the experiments, a
mathematical fitting is performed. The initial rates, r0, are
finally obtained calculating the first derivate of the
[MB] = f(t) fitting expressions and substituting t = 0 min.
From the data showed in Fig. 6, a perfect linear fitting
shows that the dye degradation was achieved on TiO2
surface. Among our samples only three showed the linear
fit. These are the non-doped sample, the intermediate
doped one and the highest doped sample, whose k and
K constants are presented in Table 4. Even though the
Langmuir–Hinshelwood kinetics were not perfectly repre-
sented overall the tungsten-doped samples, from data in
Fig. 6 and Table 4, it can be appreciated that the highest
dye conversions were obtained using W/Ti ratios below 3%
(mol/mol). Our results are in accordance with recent
reports that indicate that W(VI) loaded TiO2 catalysts
0
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
200 250 300 350 400 450 500 550 600 650 700 750 800
Wavelength (nm)A
bso
rban
ce (
a.u
)
0 min.
15 min.
30 min.
45 min.
60 min
75 min.
90 min.
105 min.
120 min.
Fig. 4 UV-vis absorption
spectra of methylene blue
solution exposed to UVA
irradiation in the presence of
01W-TiO2 sample
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
0 5 10 15 20 25 30 35
Co
nver
sio
n
Time (min)
Degussa P25 Anatase TiO2 005W-TiO2
01W-TiO2 02W-TiO2
03W-TiO2 04W-TiO2 05W-TiO2
photolysis
Fig. 5 Methylene blue conversion determined in slurries containing
Degussa P25 (open diamond), hydrothermal TiO2 (open square),
005W-TiO2 (open triangle), 01W-TiO2 (), 02W-TiO2 (;), 03W-TiO2
(open circle), 04W-TiO2 (plus symbol), 05W-TiO2 (solid line) and no
photocatalyst (filled circle). The initial dye concentration was
0.06 mM
0
0.5
1
1.5
1 1.5 2 2.5 3 3.5
AnataseTiO2
005W-TiO2
01W-TiO2
02W-TiO2
03W-TiO2
04W-TiO2
05W-TiO2
-1/ r
0 *
10E
-6 (
Mo
l/L*m
in)-1
1/Co *E-5 (Mol/L)-1
Fig. 6 Correlation of MB degradation rates as a function of the dye
concentration. The showed linear fits correspond to the anatase TiO2,
the 03W-TiO2 and the 05W-TiO2 samples
48 J Sol-Gel Sci Technol (2011) 57:43–50
123
maximize their photodegradation rate when the degree of
atomic substitution lies between 1.7 and 4% depending on
the preparation procedures [22, 23, 33].
On the other hand, tungsten-doped samples that were
baked at 450�C presented degradation efficiencies higher
than that of the non-doped TiO2 and just slightly lower than
that of Degussa P25 (see Fig. 7; Table 5). From the com-
parison of the SEM micrographs shown in Fig. 2a and b, it
can be appreciated that no important changes of the texture
occurred when the hydrothermal sample was baked at
450�C. However, DRIFT determinations of hydrothermal
TiO2 baked at 450�C showed the loss of remaining
organics as evidenced by the disappearance of the IR bands
located at 2,987, 2,935 and 2,860 cm-1, corresponding to
symmetric and asymmetric vibrations modes of –CH2– and
–CH3 groups (see Fig. 8). The loss of this organic material
trapped after the sol–gel hydrothermal synthesis might be
involved in the increase of the degradation rate.
4 Conclusions
We have obtained tungsten-doped TiO2 oxides by using a
sol–gel hydrothermal approach. The tungsten doping pro-
moted the increase of crystal size and bandgap values of
the anatase TiO2. The TiO2 samples doped with tungsten
presented faster dye conversion rates than the non-doped
anatase TiO2 sample. The highest degradation rates of
methylene blue degradation were obtained with samples
whose W/Ti content was below 3% mol/mol. The degra-
dation rate of the dye was improved by using hydrothermal
samples baked at 450�C. A thermal treatment at this
Table 4 Apparent zero order constant, k, and adsorption coefficient,
K, of hydrothermal TiO2 samples
Sample k (Mol/L*min) K (Mol/L) Correlation
coefficient
TiO2 4.4565E-6 1.5E 6 0.98616
03W-TiO2 1.498E-5 0.743081 0.99313
05W-TiO2 5.025E-5 0.36498 0.9459
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
0 10 20 30 40 50 60
Cx/
Co
Time (min)
Degussa P25 01WTiO2 03W-TiO2 01WTiO2 450 C 03W-TiO2 450 C
Fig. 7 Normalized dye
concentration plotted as a
function of irradiation time in
the presence of Degussa P25
(open diamond), 01W-TiO2
(filled triangle), 01W-TiO2
baked at 450�C (open square),
03W-TiO2 (;), 03W-TiO2
baked at 450�C (filled square).
The initial methylene blue
concentration was 0.09 mM
Table 5 Initial reaction rates in the presence of TiO2 samples baked at 450�C
Sample Degussa P25 005W-TiO2 01W-TiO2 02W-TiO2 03W-TiO2
-r0 (Mol/Lmin) 15.67 9 10-6 9.14 9 10-6 7.77 9 10-6 7.7 9 10-6 9.62 9 10-6
Data is compared with the initial rate determined with Degussa P25. Initial methylene blue concentration was 0.098 mM
1.62
1.72
1.82
1.92
2.02
2.12
2400290034003900
Ab
sorb
ance
Wavenumber (cm-1)
2987
2935
2860
(a)
(b)
Fig. 8 DRIFT spectrum of a 01W-TiO2 sample, b 01W-TiO2 sample
baked at 450�C
J Sol-Gel Sci Technol (2011) 57:43–50 49
123
temperature promoted the loss of the remaining synthesis
organics in these samples.
Acknowledgments The authors express gratitude to R. Hernandez
Reyes, M. Aguilar Franco (IF-UNAM) and Ivan Puente Lee (FQ-
UNAM, SEM) for technical support. This project was supported in
part by the CONACYT-Mexico through grants No. 80024 and 102919
and by the IPN through grant No. 20100836. The authors are also
thankful to Sergio O. Flores Valle facilities at the Laboratorio de
Catalisis y Materiales, ESIQIE-IPN.
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