Photocatalytic activity of mixture of ZrO2/SnO2, ZrO2/CeO2 and SnO2/CeO2 nanoparticles

Article abstract of DOI:10.1016/j.jallcom.2011.10.049

The ZrO₂, SnO₂ and CeO₂ nanoparticles synthesized by sol-gel procedure and calcined at 550 °C. The prepared nanoparticles characterized by X-ray diffraction spectroscopy, transmission electron microscopy and IR spectrophot

Full text of DOI:10.1016/j.jallcom.2011.10.049

Journal of Alloys and Compounds 513 (2012) 359–364
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Journal of Alloys and Compounds
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Photocatalytic activity of mixture of ZrO /SnO , ZrO /CeO and SnO /CeO
2
2
2
2
2
2
nanoparticles
a,∗
b
a
H.R. Pouretedal , Z. Tofangsazi , M.H. Keshavarz
a
Faculty of Applied Chemistry, Malek-ashtar University of Technology, Shahin-Shahr, Iran
Department of Chemistry, Islamic Azad University, Shahreza Branch, Shahreza, Iran
b
a r t i c l e i n f o
a b s t r a c t
◦
2 2
The ZrO , SnO and CeO2 nanoparticles synthesized by sol–gel procedure and calcined at 550 C. The pre-
Article history:
Received 23 August 2011
Accepted 18 October 2011
Available online 25 October 2011
pared nanoparticles characterized by X-ray diffraction spectroscopy, transmission electron microscopy
and IR spectrophotometry. The structure of prepared nanoparticles were tetragonal and monoclinic as
confirmed from the XRD patterns. The photocatalytic activity of ZrO2, SnO2, CeO2 nanoparticles and the
mixture of 1:1 of ZrO2/SnO2, ZrO2/CeO2 and SnO2/CeO2 studied in 2-nitrophenol degradation reaction.
The order of photocatalytic activity is ZrO2/SnO2 > ZrO2/CeO2 > SnO2/CeO2 > ZrO2 > SnO2 > CeO2. Among
mixtures of ZrO2/SnO2, the mixture with weight ratio of 4:1 showed the highest photocatalytic activity.
The results indicated the ZrO2 nanoparticles with the more band-gap energy had an important role in
photocatalytic activity. The mixture of ZrO2/SnO2 (4:1) is also indicated the higher photocatalytic activity
in comparison to Zr0.8Sn0.2O2 nanocomposite. The complete degradation of 2-nitrophenol was obtained
at time 45 min in the presence of hydrogen peroxide (0.1 g/L) and the mixture of ZrO2/SnO2 (4:1).
Keywords:
Photocatalyst
ZrO2
SnO2
CeO2
Nanoparticles
©
2011 Elsevier B.V. All rights reserved.
1
. Introduction
range of band-gap is reported between 3.25 and 5.1 eV depending
on the preparation technique of the sample and the most frequent
and accepted value is 5.0 eV. The conductance and valence band
potentials of it is −1.0 and +4.0 V versus NHE, respectively, allowing
its use as a photocatalyst in the production of hydrogen through
The degradation of organic pollutants in water and air by photo-
catalysis, using semiconductors, such as TiO , CeO , ZrO , SnO and
2
2
2
2
so on, has attracted extensive attention during recent 20 years [1].
The use of semiconductors as photocatalyst in photodegradation
process is a technique to increase the rate of process. The semicon-
ductors can increase the degradation of most kinds of persistent
organic pollutants, such as detergents, dyes, pesticides and volatile
organic compounds, under UV-light irradiation [2–5]. However,
the fast recombination rate of photogenerated electron/hole
pairs hinders the commercialization of this technology [1]. Thus,
it is of great interest to improve the photocatalytic activity of
semiconductors for the degradation process [5,6]. The synthesis
of different photocatalysts with various compositions and the
use of photocatalysts in various conditions can be an attractive
field of researches [7,8]. Mixed oxide composite materials can
often be more efficient photocatalysts than pure substances. This
phenomenon arises through the generation of new active sites
due to interactions between two oxides as substrate and dopant,
through improved mechanical strength, thermal stability, and
surface area of doped substrate [5,6].
water decomposition [9,10]. Although ZrO presents an adsorption
2
maximum around 250 nm, some samples show a non-negligible
absorption in the near UV range (290–390 nm) and photocatalytic
reactions could be performed under irradiation in this range [10].
Cerium (IV) oxide nanoparticles (as an n-type semiconductor with
a much narrower band-gap of 3.3 eV) have received much attention
because of its many interesting characteristics, such as unique
UV absorption ability, high stability at high temperature, high
hardness and reactivity [11]. Thus, CeO2 and CeO -based materials
2
have been extensively used in a wide variety of applications such
as solid oxide fuel cells (SOFCs), catalysis, luminescent materials,
gas sensors, polishing materials, and ferromagnetic oxides and so
on [12]. SnO2 is a semiconductor with band-gap energy of about
3.65 eV at bulk state and is an n-type semiconductor crystallizing
in tetragonal rutile structure. SnO2 has been reported as a suitable
gas sensing oxide and recently its composites have been studied
as promising semiconductors in the photocatalytic degradation of
wastewaters [13]. Tin (IV) oxide has been a widely studied mate-
rial because of its wide range of applications as gas sensors, heat
mirrors, and transparent electrodes for solar cells, opto-electronic
devices and in catalysis [14].
Zirconium oxide is an n-type semiconductor with band-gap
energy of 5.0 eV that used as heterogeneous catalyst. The values
∗ Corresponding author. Tel.: +98 312 5912253; fax: +98 312 5220420.
E-mail address: HR POURETEDAL@mut-es.ac.ir (H.R. Pouretedal).
Photocatalytic reaction is initiated when a photoexcited elec-
tron is promoted from the filled valence band of semiconductor
0
925-8388/$ – see front matter © 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.jallcom.2011.10.049

3
60
H.R. Pouretedal et al. / Journal of Alloys and Compounds 513 (2012) 359–364
photocatalyst (SC) to the empty conduction band as the absorbed
photon energy, hꢀ, equals or exceeds the band-gap of the semi-
conductor photocatalyst leaving behind a hole in the valence band.
Thus in concert, electron and hole pair (e –h ) is generated. The
following chain reactions have been widely postulated [1,7,8].
The degradation efficiency of 2-nitrophenol determined with measurement of
absorbance of samples by a UV–vis spectrophotometer Carry-100 using a paired
1.0 cm quartz cell. The samples centrifuged to remove the nanoparticles before
absorbance measurement. The absorbance of samples before (Ao) and after a dis-
tinct time (At) of irradiation and Beers’ law to determination of Co and Ct use for
calculate of degradation efficiency (Eq. (7)).
−
+
Photoexcitation : MO /SC + hꢀ → e− + h+ M : Zr, Ce, Sn
ꢀ
ꢁ
ꢀ
ꢁ
(1)
(2)
(3)
(4)
2
Ct
At
%
Degradation = 100 × 1 −
= 100 × 1 −
(7)
Oxygen ionosorption : (O2)ads + e− → O2•−
Co
Ao
Ionization of water : H O → OH− + H+
The pH of samples, the dosage of catalysts and the presence of hydrogen per-
2
oxide investigated on the reactivity of photocatalysts. The hydrochloric acid and
sodium hydroxide (0.01 M) and a Metrohm pH-meter with combined electrode used
to control of samples pH at range of 3–9.
Protonation of superoxides : O2 + H+ → HOO•
•
−
The hydroperoxyl radical formed (Eq. (4)) also has scavenging
property as O2 thus doubly prolonging the lifetime of photohole:
3
. Results and discussion
HOO + e− → HO2−
HOO + H+ → H O
•
(5)
(6)
−
3.1. Characterization of nanoparticles
2
2
Both the oxidation and reduction can take place at the surface
of the photoexcited semiconductor photocatalyst. Recombination
between electron and hole occurs unless oxygen is available to
The X-ray diffraction patterns of SnO , ZrO and CeO2 nanopar-
ticles calcined at temperature of 550 C are indicated in Fig. 1A–C,
2
2
◦
respectively. As seen from Fig. 1A, the diffraction peaks of (1 1 0),
•
−
scavenge the electrons to form superoxides (O2 ), its protonated
◦
(
1 0 1), (2 1 1) and (1 1 2) at 2ꢂ of 26.8, 34.0, 51.8 and 64.8 , respec-
•
form the hydroperoxyl radical (HO2 ) and subsequently H O2
2
tively, show the formation of Cassiterite type tetragonal crystals
of SnO2 which matches well with JCPDS card # 41-1445 [18]. The
average of crystalline size (DT) is calculated 3.5 nm from the (1 1 0)T
diffraction peak using Scherrer’s equation (Eq. (8)) [19].
[
15,16].
In the light of the literature studies, this study was designed to
synthesis nanoparticles of ZrO , CeO2 and SnO2 by sol–gel proce-
2
dure. The different composition of prepared nanoparticles was used
in 2-nitrophenol photodegradation.
(
0.9ꢁ)
DT =
(8)
(
ˇ cos ꢂ)
2. Experimental
2
.1. Synthesis of nanoprticles
In Eq. (8), D is the average crystalline size in nm, ꢁ is the
radiation wavelength (0.154 nm), ˇ is the corrected half-width at
half-intensity and ꢂ is the diffraction peak angle.
The sol–gel method used to prepare of ZrO2, CeO2 and SnO2 nanoparticles. The
ammonium cerium (IV) nitrate ((NH4)2Ce(NO3)6, Merck), zirconium oxychloride
Zirconium dioxide is one of the most studied semiconductor
materials. Pure ZrO2 has a monoclinic crystal structure at room
temperature and transitions to tetragonal and cubic at increasing
temperatures [20]. The formation of crystalline forms of tetrag-
onal and monoclinic observes for ZrO2 nanoparticles calcined at
(
ZrOCl2·8H2O, Merck) and tin (IV) chloride (SnCl4·5H2O, Merck) used as starting
materials to prepare CeO2, ZrO2 and SnO2 nanoparticles, respectively. The 0.1 M
solutions of Ce(IV), Zr(IV) and Sn(IV) prepared and the nitric acid diluted solution
added drop by drop until these solutions became clear. Deionized and double dis-
tilled water used to prepare the solutions. The solutions aged overnight and are
called “sol”. Next, the ammonia solution (1:1) added drop by drop until the gel
samples obtained at pH of 7–8. The gel samples aged overnight. After aging, the
gel samples of Ce(OH)4, Zr(OH)4 and Sn(OH)4 dried at 100 C for 2 h and followed
by calcined at 550 C for 3 h. The light yellowish CeO2, white ZrO2 and white SnO2
nanoparticles obtained after calcinations process and then stored for further use.
◦
temperature 550 C (Fig. 1B). The tetragonal structure of ZrO2 can
◦
be considered as the distortion of the cubic structure of ZrO2 and
◦
◦
its adjacent diffraction-peak pairs ((0 0 2), (1 1 0)) at 34 , ((1 1 2),
(
◦
◦
2 0 0)) at 50 , and ((1 0 3), (2 1 1)) at 60 (2ꢂ), which originate
from the splitting of the cubic diffraction peaks of (2 0 0), (2 2 0)
and (3 1 1) [21,22]. The average crystallite size of ZrO2 crystal is
calculated from Debye–Scherrer formula [19] and found 12.3 nm.
2
.2. Characterization of nanoparticles
−
1
IR-spectra of ZrO2, CeO2 and SnO2 nanoparticles in range 4000–400 cm
recorded by using Nicolet Impact 400D FT-IR Spectrophotometer. A diffractome-
ter Bruker D8 ADVANCE Germany with anode of Cu (ꢁ = 1.5406 A˚ of Cu K␣) and
Fig. 1C show the XRD pattern of nanosized CeO powder in 2ꢂ
2
◦
◦
◦
◦
◦
◦
of 20–80 . The five peaks with 2ꢂ values of 28.6 , 33.2 , 47.5 , 56.4
and 59.2 correspond to the (1 1 1), (2 0 0), (2 2 0), (3 1 1) and (2 2 2)
filter of Ni applied to record of X-ray diffraction (XRD) patterns of nanoparticles.
A JEOL JEM-1200EXII transmission electron microscope (TEM) operating at 120 kV
use for estimation of nanoparticles size. The supporting grids were formvar-covered,
carbon-coated, 200-mesh copper grids. BET (Brunauer–Emmett–Teller) surface area
of nanoparticles determined by using Monosorb Quantochorom.
planes indicate the formation of pure-phase CeO in a cubic fluorite
2
structure [23,24]. It is clear that all Bragg reflections for sample
agree well with those of standard CeO2 and noticeably broadened
of all reflections show the fine nature of CeO2 particles. The size of
CeO2 particles is obtained 8.2 nm by Debye–Scherrer formula.
2
.3. Photocatalytic activity of nanoprticles
The photocatalytic activity of prepared nanoprticles studied in photodegra-
The transmission electron microscopy images of SnO , ZrO2
2
dation of 2-nitrophenol. Phenol and its derivatives such as 2-nitrophenol are
industrially important chemicals and thus their presence in the environment is rela-
tively very common. Due to their high toxicity, they represent a group of dangerous
chemicals even at low concentrations [17].
and CeO2 nanoparticles are shown in Fig. 2A–C, respectively. The
spherical topography saw from the TEM images for prepared
nanoparticles. The formation of nanoparticles with size less than
20 nm confirmed by TEM images that agreement with results of
XRD analysis. The TEM analysis also shows a slightly irregular
and rounded shape for synthesized nanoparticles. The IR spec-
Photodegradation experiments perform in a photocatalytic reactor system. A
0 W mercury low pressure lamp uses as irradiation source. The lamp and the tube
7
immersed in the photoreactor cell with a light path of 3.0 cm. The photoreactor
filled with 50 ml of 10–50 mg/L of 2-nitrophenol and 0.1–0.8 g/L of photocatalysts.
The ZrO2, CeO2, SnO2 and mixture of ZrO2/CeO2, ZrO2/SnO2 and CeO2/SnO2 use as
photocatalysts. The whole reactor cooled with a water-cooled jacket on its outside
tra of prepared nanoparticles of SnO , ZrO and CeO2 showed an
2
2
1
−
absorption band at range of 500–450 cm that is related to the
vibration of M–O–M bond in MO2 with M: Sn, Zr and Ce [25,26].
BET (Brunauer–Emmett–Teller) surface area of prepared photocat-
◦
and the temperature was kept at 25 C. All reactants in the reactions stir using a
magnetic stirrer for ensure that the suspension of the catalyst was uniform during
the course of the reaction. In order to setting the adsorption/desorption equilib-
rium of 2-nitrophenol on heterogeneous catalysts surface, the reactor keep in dark
conditions within 30 min.
alysts determined by using Monosorb Quantochorom and obtained
2
2
63.4, 254.6 and 231.8 m /g for SnO , ZrO and CeO , respectively.
2
2
2

H.R. Pouretedal et al. / Journal of Alloys and Compounds 513 (2012) 359–364
361
Fig. 1. (A) XRD pattern of SnO2 nanoparticles. (B) XRD pattern of ZrO2 nanoparticles.
C) XRD pattern of CeO2 nanoparticles.
Fig. 2. (A) TEM image of SnO2 nanoparticles. (B) TEM image of ZrO2 nanoparticles.
C) TEM image of CeO2 nanoparticles.
(
(
3
.2. Photocatalytic activity of nanoparticles
each photocatalyst and thus the total amount of 0.1 g/L of mixture
of nanoparticles. As seen from Figs. 3 and 4, the highest degradation
for 2-nitrophenol obtained at pH of 5. The isoelectric points (IEP)
of metal oxide influence on the charge of their surfaces. The metal
The photodegradation of 2-nitrophenol (20 mg/L) catalyzed by
prepared nanoparticles (0.1 g/L) at different pHs and in duration of
h is shown in Figs. 3 and 4. The reactor is containing 0.05 g/L of
4
oxides of ZrO , CeO2 and SnO2 are as colloids or larger particles
2

3
62
H.R. Pouretedal et al. / Journal of Alloys and Compounds 513 (2012) 359–364
Fig. 3. Dgradation efficiency of 2-nitrophenol (20 mg/L) catalyzed by nanoparticles
0.1 g/L) in duration of 4 h.
Fig. 5. The effect of weight fraction of ZrO2 in mixture of SnO2/ZrO2 on the dgrada-
tion efficiency of 2-nitrophenol.
(
in aqueous solution. The surface of them is generally assumed to
be covered with surface hydroxyl species of M–OH. At pH values
effect of excess heterogeneous particles in the solution and scat-
tering of light [28,29].
−
above the IEP, the predominate surface species is M–O , while at
+
pH values below the IEP, M–OH2 species predominate [8]. The IEP
3.3. The kinetic of photodegradation
of ZrO , CeO and SnO2 is at pH of 4–7 [27]. Thus, the charge of
2
2
catalysts is positive in pH < 7 and negative in pH > 7. In pH of 5, the
-nitrophenol molecules are neutral and maximum absorption of
their occur on the surface of catalysts. In pH values over 7, the repul-
sive of phenolate ions with negative charge of surface of catalysts
due to a considerable reduction of degradation efficiency.
The degradation experiments for UV–vis irradiation of 2-
2
nitrophenol aqueous solutions containing mixture of SnO /ZrO2
2
(
1:4) follow first-order kinetics with respect to the concentration
of the pollutant in the bulk solution (C) [30,31]:
−
dC
The order of photocatalytic activity of catalysts is
r =
dt
= kappC
(9)
SnO /ZrO > CeO /ZrO > SnO /CeO > ZrO > SnO > CeO .
Zir-
2
2
2
2
2
2
2
2
2
conium oxide is an n-type semiconductor with band-gap energy of
in which kapp is the apparent first-order rate constant (with the
same restriction of C = Co at t = 0, with Co being the initial content
in the bulk solution after dark adsorption and t the reaction time),
and is affected by 2-nitrophenol concentration. Integration of this
equation will lead to the expected relation:
5
.0 eV. Apparently, the more band-gap energy of ZrO2 versus CeO2
and SnO2 (3.3–3.7 eV) is due to an increasing of lifetime of photo-
generated electrons and holes and decrease of recombination rate
of them [16,8]. Thus, the mixtures contain ZrO2 include SnO /ZrO2
2
and CeO /ZrO show the more activity versus SnO /CeO2 mixture.
ꢂ
ꢃ
2
2
2
Co
Fig. 5 shows the effect of weight fraction of ZrO2 in mixture of
SnO /ZrO (total amount of 0.1 g/L) on the photocatalytic activity
ln
= kapp
t
(10)
C
2
2
of it in 2-nitrophenol degradation. As seen, the highest reactiv-
The values of kapp can be obtained directly from the regression
ity is obtained for mixture of SnO /ZrO2 with xZrO2 = 0.8. As the
analysis the linear curve of plot of ln(Co/C) versus t. The results are
indicated in Table 1.
2
other words, a mixture of SnO /ZrO with weight ratio of 1:4 show
2
2
the most degradation for 2-nitrophenol. Also, increasing of total
amount of mixture of photocatalyst up to 0.5 g/L is due to increas-
3.4. Synthesis of Sn0.2Zr0.8O2 composite
ing of rate of degradation. However, the weight ratio of SnO /ZrO is
2
2
1
:4. But, the loading of 0.5 g/L show a reduction in degradation effi-
ciency. The degradation efficiencies in duration 3 h obtained 56.4,
2.7, 69.8 and 66.2 for the dosage of 0.1, 0.3, 0.5 and 0.8 g/L, respec-
In order to study of circumstance of catalyst on the photoac-
4+
tivity of it, Sn0.2Zr0.8O2 composite prepared. The solution of Sn
6
and Zr4+ with mole ratio of 1:4 used as starting materials. After
◦
tively. The decrease in photodegradation efficiency beyond 0.5 g/L
of mixture of photocatalysts may be attributed to the screening
aging, the composite gel of Sn0.2Zr0.8O2 dried at 100 C for 2 h and
◦
then calcined at 550 C for 3 h. The XRD pattern and TEM image
of Sn0.2Zr0.8O2 composite are shown in Fig. 6A and B, respectively.
The tetragonal structure of composite with particles size of less
than 20 nm confirmed by XRD pattern and TEM image. The tetrag-
onal phase of composite proved with JCPDS card # 37-1413 and
Table 1
−
1
2
The apparent rate constants (kapp, h ) and regression coefficients (R ) of 2-
nitrophenol degradation catalyzed by nanoparticles of mixture of SnO2/ZrO2 (1:4)
and composite of Sn0.2Zr0.8O2.
Co, mg/L
mixture of SnO2/ZrO2
1:4)
composite of
Sn0.2Zr0.8O2
(
kapp
R2
kapp
R2
10
20
30
40
50
1.283
1.040
0.685
0.518
0.390
0.924
0.955
0.973
0.963
0.959
0.345
0.314
0.272
0.186
0.160
0.950
0.981
0.983
0.989
0.997
Fig. 4. Dgradation efficiency of 2-nitrophenol (20 mg/L) catalyzed by mixture of
nanoparticles (0.1 g/L) in duration of 4 h.

H.R. Pouretedal et al. / Journal of Alloys and Compounds 513 (2012) 359–364
363
Fig. 7. The reusability of mixture of SnO2/ZrO2 (1:4) as photocatalyst in degradation
of 2-nitrophenol.
3
.6. The effect of H O
2 2
As seen from results, the complete degradation of 2-nitrophenol
obtained at time 4 h. In order to reduction of time of degradation,
the hydrogen peroxide as an oxidant added to the reactor. A degra-
dation of more than 99% is seen at presence of 0.1 g/L of H O2
2
and 0.5 g/L of mixture of SnO /ZrO2 (1:4) duration of 45 min. Also,
2
the kapp value for 2-nitrophenol (20 mg/L) degradation calculated
−
1
•
0
.178 min . The concentration of OH radical increase in the pres-
ence of H O2 because it inhibit the electron–hole recombination
2
according to the Eq. (11) [33].
MO (e ) + H O → MO + OH + OH•
−
−
(11)
2
2
2
2
Also, the electrons in conduction band can be adsorbed by H O2
2
and thus the charge separation promoted in semiconductor. On the
other hand, H O forms hydroxyl radical according the Eqs. (12) and
2
2
(13) [34].
H O + e → OH− + OH•
−
(12)
(13)
2
2
−
•
→ OH− + OH• + O2
H O + O
2
2
2
Thus, the rate of degradation of a pollutant can considerably
increase with presence hydrogen peroxide beside of a photocata-
lyst.
Fig. 6. (A) XRD pattern of Sn0.2Zr0.8O2 composite. (B) TEM image of Sn0.2Zr0.8O2
composite.
4. Conclusion
The sol–gel procedure used to prepare of ZrO , CeO2 and SnO2
2
2
9-1484 for ZrO2 and SnO , respectively [21,32]. The composition
2
nanoparticles. The use of mixture of photocatalysts improved the
activity of catalysts and therefore the rate of photodegradation of 2-
nitrophenol. The composition of mixture as well as pH of samples
influenced on the reactivity of catalysts. So that, the mixture of
of composite obtained 78.8% of ZrO2 and 21.2% of SnO2 by XRD
analysis that show the structure of composite is Sn0.21Zr0.79O . BET
2
(
Brunauer–Emmett–Teller) surface area of Sn0.2Zr0.8O2 nanocom-
2
posite obtained 241.3 m /g.
ZrO /SnO2 with weight ratio of 4:1 and total amount of 0.5 g/L in
2
The apparent first-order rate constant of 2-nitrophenol degra-
pH 5 showed the highest activity. The performance of nanoparticles
dation catalyzed by Sn0.2Zr0.8O composite are collected in Table 1.
The obtained results show the more activity of mixture of
SnO /ZrO (1:4) versus composite of Sn0.2Zr0.8O2 as photocatalyst
2
mixture of ZrO /SnO2 as catalyst was more than nanocomposite of
2
Sn0.2Zr0.8O2.
2
2
in photodegradation reaction of 2-nitrophenol.
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