Photocatalytic properties of BiVO4 prepared by the co-precipitation method: Degradation of rhodamine B and possible reaction mechanisms under visible irradiation

Article abstract of DOI:10.1016/j.materresbull.2009.09.029

Bismuth vanadate (BiVO₄) was synthesized by the co-precipitation method at 200 °C. The photocatalytic activity of the oxide was tested for the photodegradation of rhodamine B under visible light irradiation. The analysis of the total organic carbon showed that the mineralization of rhodamine B over a BiVO₄ photocatalyst (～40% after 100 h of irradiation) is feasible. In the same way, a gas chromatography analysis coupled with mass spectroscopy revealed the existence of organic intermediates during the photodegradation process such as ethylbenzene, o-xylene, m-xylene, and phthalic anhydride. The modification of variables such as dispersion pH, amount of dissolved O₂, and irradiation source was studied in order to know the details about the photodegradation mechanism.

Full text of DOI:10.1016/j.materresbull.2009.09.029

Materials Research Bulletin 45 (2010) 135–141
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Photocatalytic properties of BiVO4 prepared by the co-precipitation method:
Degradation of rhodamine B and possible reaction mechanisms under visible
irradiation
*
´
´
´
A. Martınez-de la Cruz , U.M. Garcıa Perez
´
´
´
´
´
Facultad de Ingenierı´a Mecanica y Electrica, Universidad Autonoma de Nuevo Leon, Ciudad Universitaria, C.P. 66451, San Nicolas 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:
Bismuth vanadate (BiVO4) was synthesized by the co-precipitation method at 200 8C. The photocatalytic
activity of the oxide was tested for the photodegradation of rhodamine B under visible light irradiation.
The analysis of the total organic carbon showed that the mineralization of rhodamine B over a BiVO4
photocatalyst (ꢁ40% after 100 h of irradiation) is feasible. In the same way, a gas chromatography
analysis coupled with mass spectroscopy revealed the existence of organic intermediates during the
photodegradation process such as ethylbenzene, o-xylene, m-xylene, and phthalic anhydride. The
modification of variables such as dispersion pH, amount of dissolved O2, and irradiation source was
studied in order to know the details about the photodegradation mechanism.
Received 28 May 2009
Received in revised form 31 August 2009
Accepted 25 September 2009
Available online 3 October 2009
Keywords:
A. Oxides
A. Semiconductors
B. Chemical synthesis
C. X-ray diffraction
D. Catalytic properties
ß 2009 Elsevier Ltd. All rights reserved.
1. Introduction
degradation of organic materials [11,12], organic dyes [13–15] and
for the splitting of water for hydrogen and oxygen evolution
In the search of compounds with photocatalytic activity under
visible light irradiation, several efforts have been carried out in the
last decade. For example, the anatase polymorph of TiO2 has been
doped with nitrogen in order to increase its absorption in the
visible range [1]. In other direction, several authors have proposed
alternative oxides to traditional TiO2 with high photocatalytic
activity under visible light irradiation such as In1ꢀxNixTaO4 [2],
[16,17].
As it is well known BiVO4 crystallizes in three different
polymorphs, i.e. tetragonal zircon, monoclinic distorted scheelite
and tetragonal scheelite. Due to its relatively narrow band gap
energy (2.4 eV), the monoclinic form exhibits the higher photo-
catalytic activity for chemical reactions induced by visible light
irradiation [18]. For this reason, different routes have been
investigated to synthesize the BiVO4 monoclinic polymorph with
small particle size and high specific surface area. In this sense,
besides the classical solid state reaction [19], monoclinic bismuth
vanadate has been synthesized by means of: layered vanadates in
aqueous solution [5], alkoxide methods [8], sonochemistry [15],
urea combustion [18], metal organic decomposition thin films [20],
hydrothermal synthesis [21], co-precipitation [22], sol–gel [23],
and hybrid organic–inorganic precursors [24].
BiVO4 has been proposed as a photocatalyst to work under
visible light irradiation, using three organic dyes: methylene blue
(MB) [13], rhodamine B (rhB) [14] and methyl orange (MO) [15].
Although good photocatalytic activity concerning the bleaching of
dye solutions has been reported in these works, evidences of
mineralization to CO2 and H2O as well as details about photo-
degradation pathways have not been reported. In this work, BiVO4
was synthesized by the co-precipitation method at 200 8C and the
photocatalytic activity was evaluated in the degradation of rhB
CaIn2O4 [3], InVO4 [4], BiVO4 [5], and
g-Bi2MoO6 [6]. These
advances are very important because heterogeneous photocata-
lysis is a promising technology to solve global energy and
environmental issues.
Among the different studied oxides with activity under visible
light irradiation, BiVO4 has received special attention. This oxide
has attracted great interest due to its potential applications in
various technological fields because it exhibits interesting proper-
ties such as ferroelasticity [7], ionic conductivity [8], and
photochromism [9]. It has also been used as a non-toxic yellow
pigment for high-performance lead-free paints [10]. Recently, the
photocatalytic activity of BiVO4 has been recognized for the
* Corresponding author. Tel.: +52 81 83 29 40 20; fax: +52 81 83 32 09 04.
E-mail addresses: azael70@yahoo.com.mx, azmartin@gama.fime.uanl.mx
(A. Martı´nez-de la Cruz).
0025-5408/$ – see front matter ß 2009 Elsevier Ltd. All rights reserved.
doi:10.1016/j.materresbull.2009.09.029

136
A. Martı´nez-de la Cruz, U.M.G. Pe´rez / Materials Research Bulletin 45 (2010) 135–141
source. The emission spectrum of the Xe lamp was measured by
means of a UV–vis spectrophotometer. A negligible contribution of
UV radiation was observed (l < 390 nm), but this was filtered by the
borosilicate container. The BiVO4 photocatalytic activity was
evaluated on the degradation reaction of rhB in water. In a glass
beaker, 250 mL of rhB solution [5 mg Lꢀ1] containing 250 mg of
photocatalyst was put in ultrasonic bath for 10 min to eliminate
aggregates. In order to be sure that adsorption–desorption equili-
brium 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. During the reaction, samples of 8 mL were taken at
different time intervals and then separated through double
centrifugation (4000 rpm, 20 min). The supernatant solution was
decanted and the rhB concentration was determined through its
absorption maximum band (554 nm) using a UV–vis spectro-
photometer (Perkin Elmer Lambda 35).
Fig. 1. Representation of the molecular structure of rhodamine B.
under visible light irradiation, see Fig. 1. In order to know details of
dye photodegradation pathway, the effect of some variables such
as pH, amount of O2 dissolved in the dye solution, and irradiation
source was studied. Beyond dye bleaching, the total organic carbon
(TOC) analysis was carried out to determine the dye mineralization
degree. Additionally, the course of the reaction was followed by gas
chromatography (GC) coupled with mass spectroscopy (MS) to
detect the nature of the reaction intermediates.
During the course of the photodegradation reaction, the effect
of some variables such as dispersion pH, dissolved O2 and
irradiation source was analyzed. The natural pH of the dispersed
photocatalyst/dye solution was around pH 6. Two additional
experiments under acid conditions were carried out at pH 4 and 5
by adding 4 M HNO3. In the same way, experiments under basic
conditions were performed at pH 8 and 10. In these cases, a 4 M
NaOH solution was used to reach these basic conditions. In all the
experiments, the pH was adjusted before putting the dispersion in
the dark. For the experiments with O2, two levels of dissolved gas
were analyzed. The first level was under the natural conditions of
the aeration by the agitation of the dispersion with a magnetic
stirrer. In the second experiment, a continuous O2 flow of
250 mL minꢀ1 was bubbled during the course of the reaction to
saturate the solution. Finally, some experiments were performed
under UV irradiation (365) nm using a UVP lamp with an intensity
2. Experimental
2.1. Preparation of the BiVO4 sample
BiVO4 was synthesized by the co-precipitation method. Two
aqueous solutions were prepared at 70 8C. In the first one, 1.4975 g
of Bi(NO3)3ꢂ5H2O (Aldrich, 99.99%) was dissolved in 100 mL of 4 M
HNO3. The second one was prepared by dissolving 0.3610 g of
NH4VO3 in 100 mL of 2 M NH4OH. The bismuth nitrate solution
was added dropwise (ꢁ5 mL minꢀ1) to the vanadate solution with
vigorous stirring. After mixing, the solution pH was adjusted at 9
using 2 M NH4OH getting a yellow suspension that was kept at
70 8C to promote the slow evaporation of the solvent. The yellow
powder obtained after this process was used as the BiVO4
precursor by a new thermal treatment at 200 8C for 61 h. On the
other hand, for comparative purposes, a BiVO4 sample was also
synthesized by reacting stoichiometric amounts of Bi(NO3)3ꢂ5H2O
and NH4VO3 at 700 8C for 66 h.
of 1200
m .
W cmꢀ2
The dye mineralization degree was monitored by analyzing the
total organic carbon content (TOC) in the solutions with different
irradiation times. The TOC analysis was performed by adding
acidified potassium persulfate reagent to the samples for oxidation,
followedbyadigestionprocessat105 8Cduring2 h. Theevolved CO2
was analyzed by a colorimetric method in a DR/890 HACH
colorimeter according to the procedure supplied by the HACH
Company [25]. Performing a typical experiment, 250 mL of an rhB
solution [20 mg Lꢀ1] containing 750 mg of photocatalyst was
employed. The dispersion was adjusted to pH10 (the best condition)
and O2 was continuously bubbled duringtheirradiationprocess. The
formation of reaction intermediates in the rhB mineralization
process was followed by gas chromatography coupled with mass
spectroscopy. For this purpose, a gas chromatograph (Agilent
Technologies, 6890N) equipped with an HP5MS capillary column
2.2. Sample characterization
The structural characterization was carried out by powder X-
ray diffraction using a Bruker D8 Advanced diffractometer with
CuK radiation, equipped with a Vantec high speed detector. The
a
X-ray diffraction data of the samples were collected in the 2u range
of 10–708 with a step scan rate of 0.058 0.05 sꢀ1. The morphology of
the samples was analyzed by scanning electron microscopy (SEM),
using a Leica JSM 6500 field emission microscope with an
accelerating voltage of 40 kV.
(60 m ꢄ 0.25 mm ꢄ 0.18
mm) coupled with a mass spectroscopy
equipment (Agilent Technologies, 5973N) was used. The working
conditions were: sampling inlet temperature of 250 8C, temperature
programming of 60–190 8C at a rate of 10 8C/min, ion source
temperature of 230 8C with electron energy of 70 eV.
The surface area of the photocatalysts was determined by N2
adsorption–desorption measurements with a Surface Area & Pore
Size analyzer Quantachrome Instruments AUTOSORB-1. The
adsorption–desorption isotherms were evaluated at ꢀ196 8C after
the pretreatment of the sample at 200 8C for 48 h. The UV-diffuse
reflectance spectra of the oxides were measured using a UV–vis
spectrophotometer equipped with an integration sphere (Perkin
Elmer Lambda 35).
3. Results and discussions
3.1. Sample characterization
BiVO4 was successfully synthesized in pure form by the co-
precipitation method. The formation of the oxide was followed
by X-ray diffraction in the different steps of the synthesis as it is
shown in Fig. 2. This analysis revealed that the crystalline
framework of the monoclinic BiVO4 polymorph is formed even
from the processes of water elimination at 70 8C, according to the
JCPDS file no. 14-0688. Nevertheless, three additional diffraction
2.3. Photocatalytic reaction
The photochemical reactor employed in this work consisted of
a borosilicate glass beaker surrounded by a water-jacket to
maintain the reaction temperature at 25 ꢃ 1 8C. A Xe lamp of
10,000 K with a luminous flux of 2100 lm was used as visible light
lines were observed at 22.68, 24.68 and 33.48 in the 2u scale. The

A. Martı´nez-de la Cruz, U.M.G. Pe´rez / Materials Research Bulletin 45 (2010) 135–141
137
Fig. 2. XRD patterns of powders obtained during the BiVO4 co-precipitation
synthesis.
origin of this impurity was not determined. For the sample
calcined at 200 8C for 17 h, the diffraction lines associated with
the impurity were also observed. In order to maintain the
synthesis temperature at low values, the precursor calcination
time was extended to 61 h. As it can be observed in Fig. 2, the
precursor thermal treatment at 200 8C for 61 h led to the
formation of monoclinic BiVO4 in pure form. The X-ray
diffraction pattern of the sample synthesized at 700 8C by solid
state reaction is also shown.
Recently, Yu et al. [26] reported the formation of BiVO4 by a
similar co-precipitation method, starting from the same reactive
salts, but with different solution concentrations. They observed the
formation of monoclinic BiVO4 with relatively poor crystallinity
until 300 8C. In our case, the synthesis of BiVO4 at 200 8C could
promote its application as a photocatalyst. In general, lower
calcination temperatures are preferable because they produce
materials with higher specific surface areas.
Fig. 3. SEM analysis of the morphology of BiVO4 synthesized at 200 8C for 61 h by
the co-precipitation method.
Fig. 3 shows the morphology of particles analyzed by SEM for
the sample prepared by the co-precipitation method. The particles
with the smallest sizes were observed around 250–300 nm. These
particles are formed by the agglomeration of primary particles of
smaller dimensions. In order to estimate the crystal size of the
sample, the half-width of the strongest X-ray diffraction line of
BiVO4 was measured (Fig. 2) and a crystal size of 30 nm was
estimated by the Scherrer method [27].
The BET analysis revealed a surface area of 1.5 m2 gꢀ1 for the
sample prepared at 200 8C by the co-precipitation method and in
the case of the sample synthesized at 700 8C by the solid state
reaction, a value of 0.3 m2 gꢀ1 was found. The value observed in the
sample prepared by co-precipitation is similar to those obtained
for BiVO4 samples synthesized by other low-temperature synth-
esis methods [5,12,18,28–30]. Diffuse reflectance of both samples
was analyzed by UV–vis spectroscopy. The band gap, Eg, obtained
for the samples were 2.27 eV (co-precipitation method) and
2.31 eV (solid state reaction). These values are within the typical
interval of 2.24–2.45 eV reported for BiVO4 synthesized by
different preparation methods [12,15,31].
Fig. 4. Changes in the rhB concentration during the course of its photocatalytic
degradation (5 mg Lꢀ1) in the presence of BiVO4 (1 g Lꢀ1) at different pH values.
can be seen in Fig. 4, the rhB concentration was constant all the time,
whichindicatesthatthecombinationphotocatalyst/light irradiation
was necessary to eliminate the dye from the solution. In order to
evaluate the pH effect on the photedegradation reaction, different
photocatalyst/dye solution dispersions were adjusted to pH = 4, 5, 8
and 10. Note that the natural pH of the mentioned dispersion was
around pH 6. At this pH, the rhB photodegradation in the presence of
BiVO4 obtained by co-precipitation is feasible and about 60% of the
dye was bleached after 360 min. When similar experiments were
carried out at different pH values, an increase in the photocatalytic
3.2. Photocatalytic reaction
The BiVO4 photocatalytic activity was evaluated on the
degradation of rhB in aqueous solution under visible light
irradiation. The temporal evolution of the dye concentration during
the photodegradation process by BiVO4 is shown in Fig. 4. In order to
evaluate the rhB photolysis degree under visible light irradiation, an
additional experiment without photocatalyst was carried out. As it

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A. Martı´nez-de la Cruz, U.M.G. Pe´rez / Materials Research Bulletin 45 (2010) 135–141
by attacking ꢅOH radicals on the aromatic chromophore ring,
leading to the degradation 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)
[32–34]. According to the spectral changes observed in Fig. 6, it
seems clear that the mechanism concerning the rhB photo-
degradation, which occurs by the attack of the aromatic
chromophore ring (pathway 2), predominates over the de-
ethylated processes (pathway 1), see Fig. 7. This is an important
point because
a complete rhB de-ethylation leads to the
bleaching of the solution, but not necessarily to the mineraliza-
tion of the dye. To elucidate this point, TOC analyses were
performed on dispersions with different irradiation times. In
order to have more accuracy in the measurements performed in
this experiment, an initial rhB concentration of 20 mg Lꢀ1 was
used. The dye mineralization degree was around 40% after 100 h
of visible light irradiation. This result indicates that the rhB
mineralization by the action of the BiVO4 photocatalyst is
possible. Mineralization evidences of organic dyes by BiVO4
have not been previously reported. Due to the high molar
Fig. 5. Changes in the rhB concentration during the course of its photocatalytic
degradation (5 mg Lꢀ1) in the presence of BiVO4 (1 g Lꢀ1) at pH 10 and different
amounts of dissolved O2. In the insert, variation of photocatalytic reaction rate with
pH.
reaction rate was observed as the pH was increased from acid to
basic. A significant increase in the photocatalytic reaction rate
was especially observed at pH 10. In fact, after 300 min of light
irradiation, the solution was bleached in 99%. According to our
experiments, this pH value is critical. Experiments performed at
higher pH than 10, i.e. pH = 12, revealed that photocatalytic reaction
rate does not undergo an improvement as is shown in the insert
of Fig. 5.
extinction coefficient of rhB at 554 nm (8.8 ꢄ 104 Mꢀ1 cmꢀ1
)
[35], a small quantity of the dye dissolved in water produces a
remarkable color. For this reason, rhB concentrations around
5 mg Lꢀ1 or less are typically used for dyeing process. The high
dye concentration employed in the TOC experiments
(20 mg Lꢀ1) and its relatively easy mineralization predicts an
efficient removal of rhB from industrial wastewater.
According to this result, the dispersion at pH 10 was selected
to carry out experiments with bubbled O2. Fig. 5 shows a slight
rhB oxidation by O2 without photocatalyst (ꢁ10%). The presence
of BiVO4 under these experimental conditions considerably
increases the photocatalytic reaction rate, reaching a bleaching
of dye solution of 99% after 180 min. The temporal evolution of
spectral changes during the rhB photodegradation process by
the BiVO4 photocatalyst synthesized by the co-precipitation
method is shown in Fig. 6. The main rhB absorption band located
at 554 nm is due to the presence of four ethylated groups on the
dye molecule. The visible light irradiation of the BiVO4/rhB
dispersion produces a gradual depletion of the absorbance value.
This depletion takes place without a shift of the dye absorption
band, revealing some features of the pathway followed during
the rhB photodegradation. As it is well known, the rhB
photodegradation can occur by two competitive mechanisms:
To obtain information about the intermediate products of the
mineralization process, dispersions of rhB/BiVO4 with irradiation
times of 25, 50, 75 and 100 h were analyzed by the GC/MS
technique. The main detected products were ethylbenzene, o-
xylene, m-xylene and phthalic anhydride, see Fig. 8. Under our
experimental conditions, simple organic acids such as formic acid,
acetic acid and oxalic acid were not detected. Although a detailed
analysis is necessary to establish a complete pathway of the rhB
degradation, these GC/MS results evidenced the cleavage of the rhB
aromatic structure.
3.3. Possible reaction mechanisms of photodegradation of
rhB over BiVO4
The degradation of a dye by light irradiation can happen by
three mechanisms: (a) by means of a photolysis process induced by
the luminous energy from a light source, (b) by means of a
photosensitization process where the light irradiation excites an
electron from the dye and then it is injected into the conduction
band of the semiconductor with the concomitant oxidation of the
dye, and (c) by means of a true heterogeneous photocatalysis
where the promotion of an electron from the valence band to the
conduction band by light irradiation produces active sites (holes)
for the oxidation of the dye.
As it can be observed in Fig. 4, the rhB solution is stable when
it is exposed to long periods of visible light irradiation. So, the
photolysis process (a) has a negligible contribution to the
degradation process of the dye. The second possibility is the
photosensitization process (b). rhB is a xantane dye that is
widely known as a photosensitizer on TiO2 and other oxides
[36]. The maximum rhB absorption band is located at 554 nm,
so, the energy provided by the Xe lamp is enough for the
photosentization of the dye. On the basis of this feature, it is
possible that the photodegradation of rhB by BiVO4 mainly takes
place by a photosensitization process. This statement can be
supported by the experiments done using UV irradiation
(365 nm) over BiVO4/rhB dispersions. While in experiments
Fig. 6. Absorption changes in the rhB solution during the photocatalytic process
(250 mg of BiVO4 prepared at 200 8C added to 250 mL of 5 mg Lꢀ1 rhB solution at pH
10).

A. Martı´nez-de la Cruz, U.M.G. Pe´rez / Materials Research Bulletin 45 (2010) 135–141
139
Fig. 7. Possible pathways followed during the rhB photodegradation.
carried out with visible light irradiation, a decrease in the rhB
concentration of 62% was observed after 180 min. For the same
time, the UV irradiation led to a decrease of only 18% of the
initial rhB concentration. This result suggests a minor contribu-
tion of the photocatalytic process by charge separation (c) to the
degradation reaction. It was also shown that on BiVO4, the
charge separation process is inefficient as it was previously
described [37]. The photosensitization process by rhB is well
known for different systems [38–42]. In the case of BiVO4, it can
be described as follows:
rhB þ h
n
! rhBꢆ
(1)
(2)
BiVO4 þ rhB ꢆ ! rhBþꢅ þ BiVO4ðeꢀÞ
Fig. 8. Mass spectra of intermediates from the degradation of rhB for specific retention times, tR.

140
A. Martı´nez-de la Cruz, U.M.G. Pe´rez / Materials Research Bulletin 45 (2010) 135–141
ꢀꢅ
BiVO4ðeꢀÞ þ O2 ! BiVO4 þ O2
(3)
(4)
(5)
(6)
BiVO4ðeꢀÞ þ O2 þ Hþ ! H2O2
ꢀꢅ
H2O2 þ eꢀ
!
ꢅOH þ OHꢀ
rhB þ ꢅOH ! ! ! CO2 þ H2O
According to this rhB degradation pathway, the step described
by Eq. (3) is favored by the presence of O2. In this reaction,
electrons in the particle surface are scavenged by the present
molecular oxygen to yield the superoxide radical anion. This
reaction leads to subsequent reactions where in one of them the
radical ꢅOH is formed. Consequently, the photodegradation process
is improved, as it was observed in the rhB/photocatalyst dispersion
saturated with oxygen.
Nevertheless, the positive effect of alkaline pH observed on the
photodegradation process cannot be explained on the basis of a
photosensitization process. As it was previously mentioned, when
the rhB/photcatalyst dispersion was irradiated with UV light, a
small contribution to the photodegradation process was observed.
This indicates that the true heterogeneous photocatalysis mechan-
ism (c) takes place simultaneously with the photosensitization
process (b), although in a minor scale. Under this scheme, the
alkaline pH of the dispersion promotes a process where the charge
separation in the semiconductor bands plays an important role in
the photocatalytic activity of the material. The pathway degrada-
tion by heterogeneous photocatalysis can be described throughout
the following equations:
Fig. 9. Schematic representation of the effect of hydroxide ions on the rhB
photocatalytic degradation.
BiVO4. The presence of dissolved O2 in the dispersion exerts a
positive effect on both mechanisms and the last is considerably
improved by the presence of hydroxide ions at alkaline pH.
4. Conclusions
2BiVO4 þ h
rhB þ BiVO4ðhþÞ ! rhBþꢅ þ BiVO4
H2O þ BiVO4ðhþÞ ! ꢅOH þ Hþ þ BiVO4
n
! BiVO4ðhþÞ þ BiVO4ðeꢀÞ
(7)
(8)
(9)
BiVO4 oxide was synthesized at low temperatures by the co-
precipitation method and characterized by conventional techni-
ques. The photocatalytic activity of the material was evaluated on
the photodegradation of rhodamine B dissolved in aqueous
solution. The material exhibited good capacity to bleach the dye
solution. Moreover, the total organic carbon analysis showed that
the mineralization of rhodamine B over BiVO4 photocatalysts is
feasible. In the same way, the analysis of samples of rhodamine B/
BiVO4 dispersions by gas chromatography revealed the presence of
intermediates that were formed as products of the aromatic ring
fragmentation of the dye. The role of variables such as dissolved O2,
irradiation source and pH was evaluated on the photodegradation
process. It was shown that O2 acts as an efficient electron
scavenger in the photosensitization process. In the same way, the
hydroxide ion acts as trap of holes and promotes the photode-
gradation of rhodamine B by heterogeneous photocatalysis.
OHꢀ þ BiVO4ðhþÞ ! ꢅOH þ BiVO4
(10)
ꢀꢅ
BiVO4ðeꢀÞ þ O2 ! BiVO4 þ O2
BiVO4ðeꢀÞ þ O2 þ Hþ ! HO2 þ BiVO4
(11)
(12)
(13)
(14)
(15)
ꢀꢅ
ꢅ
ꢅ
HO2 þ Hþ ! H2O2
H2O2 þ eꢀ
!
ꢅOH þ OHꢀ
rhB þ ꢅOH ! ! ! CO2 þ H2O
Due to the relatively low band gap of BiVO4, ꢁ2.3 eV, the
necessary energy to activate the process described by Eq. (7) can be
supplied by both vis and UV irradiation under our experimental
conditions. Note that by following this mechanism, the step
described by Eq. (10) is favored by the presence of hydroxide ions
in the aqueous solution. As the dispersion pH is increased, a higher
concentration of hydroxide ions is incorporated to the surface of
the photocatalyst. This is comprehensible by taking into account
that rhB is a cationic dye, see Fig. 1. The adsorption of hydroxide
ions on the BiVO4 surface allows the oxidation of the ions by direct
reaction with the holes (h+) generated on the valence band
(Eq. (10)). As a product of this reaction, the hydroxyl radical (ꢅOH)
is formed and it subsequently acts as an oxidant of organic
molecules adsorbed on the surface of the photocatalyst, see Fig. 9.
In this sense, hydroxide ions act as an efficient trap of the holes
photogenerated as products of the charge separation induced by
light irradiation in the semiconductor. In the same way, this
mechanism is also favored by the presence of the dissolved O2 as it
was shown by Eq. (11).
Acknowledgements
´
´
We wish to thank Universidad Autonoma de Nuevo Leon
(UANL) for its invaluable support through the PAICYT 2007-2008
projects and CONACYT for the 81546 project support. We also want
´
to thank Departamento de Ecomateriales y Energıa, Facultad de
´
Ingenierıa Civil (UANL) for its assistance on the characterization of
materials.
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