Spectroscopic analyses on ROS generation catalyzed by TiO2, CeO2/TiO2 and Fe2O3/TiO2 under ultrasonic and visible-light irradiation

Article abstract of DOI:10.1016/j.saa.2012.09.067

In this work, the TiO₂, CeO₂/TiO₂ and Fe₂O₃/TiO₂ powders were irradiated, respectively, by ultrasound and visible-light, and the generation of reactive oxygen species (ROS) were estimated by the method of Oxidation-Extraction Photometry (OEP). That is, the 1,5-diphenyl carbazide (DPCI) can be oxidized by generated ROS into 1,5-diphenyl carbazone (DPCO), which can be extracted by mixed solvent of benzene and carbon tetrachloride. The DPCO extract liquor displays an obvious absorbance at 563 nm wavelength. In addition, some influencing factors, such as (ultrasonic or visible-light) irradiation time, catalyst addition amount and DPCI concentration, on the generation of ROS were also reviewed. The results indicated that the quantities of generated ROS increase with the increase of (ultrasonic or visible-light) irradiation time and catalyst addition amount. Moreover, the displayed quantities of ROS are also related with DPCI concentration. And then, several radical scavengers were used to determine the kinds of the generated ROS. At last, the researches on the sonocatalytic and photocatalytic degradation of several organic dyes have also been performed. It is wished that this paper might offer some important subjects for broadening the applications of sonocatalytic and photocatalytic technologies in future environment treatment.

Full text of DOI:10.1016/j.saa.2012.09.067

Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 101 (2013) 82–90
Contents lists available at SciVerse ScienceDirect
Spectrochimica Acta Part A: Molecular and
Biomolecular Spectroscopy
journal homepage: www.elsevier.com/locate/saa
Spectroscopic analyses on ROS generation catalyzed by TiO , CeO /TiO
2
2
2
and Fe O /TiO under ultrasonic and visible-light irradiation
2
3
2
a
b
a,
⇑
a
a
a
a,
⇑
Mingming Zou , Yumei Kong , Jun Wang , Qi Wang , Zhiqiu Wang , Baoxin Wang , Ping Fan
a
College of Chemistry, Liaoning University, Shenyang 110036, PR China
College of Pharmacy, Liaoning University, Shenyang 110036, PR China
b
h i g h l i g h t s
g r a p h i c a l a b s t r a c t
"
ROS were detected by the method of
Oxidation-Extraction Photometry
(OEP).
"
The formations of ROS were studied
during sonocatalytic and
photocatalytic process.
"
"
The kinds of ROS were determined
by adding some radical scavengers.
The sonocatalytic and photocatalytic
degradation of dyes were studied.
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 2 June 2012
2 2 2 2 3 2
In this work, the TiO , CeO /TiO and Fe O /TiO powders were irradiated, respectively, by ultrasound and
visible-light, and the generation of reactive oxygen species (ROS) were estimated by the method of Oxi-
dation-Extraction Photometry (OEP). That is, the 1,5-diphenyl carbazide (DPCI) can be oxidized by gen-
erated ROS into 1,5-diphenyl carbazone (DPCO), which can be extracted by mixed solvent of benzene
and carbon tetrachloride. The DPCO extract liquor displays an obvious absorbance at 563 nm wavelength.
In addition, some influencing factors, such as (ultrasonic or visible-light) irradiation time, catalyst addi-
tion amount and DPCI concentration, on the generation of ROS were also reviewed. The results indicated
that the quantities of generated ROS increase with the increase of (ultrasonic or visible-light) irradiation
time and catalyst addition amount. Moreover, the displayed quantities of ROS are also related with DPCI
concentration. And then, several radical scavengers were used to determine the kinds of the generated
ROS. At last, the researches on the sonocatalytic and photocatalytic degradation of several organic dyes
have also been performed. It is wished that this paper might offer some important subjects for broaden-
ing the applications of sonocatalytic and photocatalytic technologies in future environment treatment.
Crown Copyright Ó 2012 Published by Elsevier B.V. All rights reserved.
Received in revised form 11 September
2
012
Accepted 22 September 2012
Available online 29 September 2012
Keywords:
Reactive oxygen species (ROS)
TiO composite
2
Ultrasonic irradiation
Visible-light irradiation
Dye degradation
Introduction
Since the beginning of the 21st Century, the application of photo-
catalytic technology using some semiconductor metal oxides to
The discharge of organic pollutants in the environment
undoubtedly possesses serious health-risks that threatened human
survival. Now, more than ever, it is the most important and urgent
for human beings to control and treat environmental pollution.
solve the environmental problems has been received much atten-
tion [1–6]. A large number of studies show that the heterogeneous
photocatalysis through light illumination on a surface of semicon-
ductor metal oxides is an attractive advanced oxidation process
(
(
AOPs) [7–9]. It involves the generation of reactive oxygen species
Å
ꢀ
Å
ROS) like superoxide anion ( O ), hydroxyl radical ( OH), hydrogen
2
⇑
Corresponding authors. Tel.: +86 24 62207859; fax: +86 24 62202053.
E-mail address: wangjun890@126.com (J. Wang).
1
peroxide (H
O
2 2
) and singlet oxygen ( O
2
) and so on [10–12]. These
1
386-1425/$ - see front matter Crown Copyright Ó 2012 Published by Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.saa.2012.09.067

M. Zou et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 101 (2013) 82–90
83
oxidizing substances with higher reaction activity than common
oxygen molecules can completely destroy various organic pollu-
tants in wastewater [13]. By contrast, as regarding to ultrasonic
and visible-light irradiation, the efficiency of photocatalytic degra-
dation is generally higher than that of the sonocatalytic degrada-
tion. However, due to the sewage with high concentration or low
transparency, as a necessity choice the ultrasound can be applied
as an excited source replacing light irradiation. It has been reported
was evaluated during the generation of ROS. In addition, the effect
of some influencing factors, such as (ultrasonic and visible-light)
irradiation time and TiO , CeO /TiO or Fe O /TiO addition
2 2 2 2 3 2
amount on the generation of ROS were reviewed. The effect of DPCI
concentrations on the generated quantities of ROS was also consid-
ered. Meanwhile, several ROS quenchers were used to determine
the kinds of generated ROS. At last, referring to corresponding re-
ports [30–33], the sonocatalytic or photocatalytic degradation of
some organic dyes in aqueous solutions, that is, under ultrasonic
2
that titanium dioxide (TiO ) or zinc oxide (ZnO) as a sonocatalyst
could perform various sonocatalytic reactions when was irradiated
by ultrasound. And then, the accompanying sonocatalytic technol-
ogy has been successfully used in the degradation of organic com-
pounds and dye pollutants [9,14–16]. In addition, some rare earth
or visible-light irradiation combined with TiO
2 2 2
, CeO /TiO and
Fe /TiO were studied. It is wished that this paper might offer
2
O
3
2
some important references for broadening the applications of son-
ocatalytic and photocatalytic technologies and developing new
efficient catalysts in future environment treatment.
2
oxides (for example: CeO ) have broadly been used in luminous
materials, electronic ceramics, polishing powder, and also used as
efficient catalyst in photocatalytic field [17–19]. It has been re-
ported that the composite made up of TiO and CeO could bring
2 2
Experimental
a special electron transfer process which is able to facilitate the
separation of the electron–hole pairs and improve the photocata-
Materials
lytic activity of TiO
tor metal oxide has relatively narrow band gap (Eg = 2.2 eV).
Hence, the coupling of Fe with TiO causes a wide respondent
spectrum, and then improves the utilization of visible light [20–
2]. So the CeO /TiO and Fe /TiO composites were taken as
the sonocatalyst and photocatalyst for treating various organic pol-
lutants. Nevertheless, as a new AOPs, the reaction mechanism of
sonocatalytic degradation as well as the relevant ROS have not
been studied yet in detail. This paper attempts to research how
to enhance the sonocatalytic degradation efficiency and search
new sonocatalysts. Hence, it is necessary to study the sonocatalytic
degradation process and determine the kind of generated ROS.
In recent years, the electron spin resonance, chromatography,
chemiluminescent method and some other technologies have been
used in the study on the formation and identification of ROS [23–
2 2 3
[17,18]. What is more, Fe O as a semiconduc-
The nano-sized CeO
Veking Company, China) were used for preparing the CeO
and Fe /TiO composites. Malachite Green, Acid Red B, Methy-
lene Blue, Congo Red and Azo Fuchsine (AR grade, Tianjin Kaiyuan
Reagent Corporation, China) were used as model organic pollutants
2
, Fe
2
O
3
and TiO
2
(anatase phase) powders
(
2
/TiO
2
2
O
3
2
2
O
3
2
2
2
2
2
O
3
2
for evaluating the catalytic activity of TiO
TiO composites. Their molecular structures were given in Fig. 1.
,5-diphenyl carbazide (DPCI), L-Histidine (His), 2,6-Di-tert-bu-
2 2 2 2 3
, CeO /TiO and Fe O /
2
1
tyl-methylphenol (BHT), Dimethylsulfoxide (DMSO) and Vitamin
C (VC) (purity > 99.0%, Sinopharm Chemical Reagent Co., Ltd., Chi-
na) were used to determine the amount and kind of the generated
ROS. Benzene and carbon tetrachloride (AR grade, Shenyang Don-
gxing Reagent Factory, China) were used as extracting agent. The
other chemical reagents were all of analytical reagent grade, and
double distilled water was used for solution preparation.
2
6]. These detecting techniques have high precision, but factually
they are limited by some other drawbacks such as the need for
expensive laboratory equipment, the complexity of the experi-
ment, heavy detection workload and too many limiting factors.
Otherwise, due to the half-life of ROS is very short, and that a series
of experiments must dynamically be conducted, it is certainly dif-
ficult and inconvenient to directly trace and detect the ROS. The
Oxidation-Extraction Photometry (OEP) method has all through
been applied to prove the existence of various oxidizing substances
through adding 1,5-diphenyl carbazide (DPCI) as capturing reagent
Apparatus
The experimental setups are shown in Fig. 2. The Controllable
Serial-Ultrasonics apparatus (SG3200HE, Shanghai GuTel Ultra-
sonic Instrument Company, China) operating at ultrasonic fre-
quency of 40 kHz, output power of 50 W and irradiation intensity
of 1.0 W/cm through manual adjusting and visible-light irradia-
tion apparatus (EFD18D/65-E3U, Toshiba Company, Japan) operat-
ing at visible-light emitting diodes of 50 Hz power supply
2
[
27–29]. The DPCI can be oxidized by oxidizing substances into
,5-diphenyl carbazone (DPCO), which can be extracted by organic
1
solvents and show an absorbance in a certain range of wavelength.
As well known, the ROS has a very strong oxidizability. Therefore,
the DPCI should also be oxidized by ROS easily. Because of fast and
accurate detection, simple equipment requirements, low-cost re-
agent, wide detection range and simple requirements for sample,
the OEP method could be widely applied in various biological
and chemical systems. Theoretically, the quantities and kinds of
generated ROS should be evaluated by the amount of oxidized DPCI
and the adding of different ROS quencher, respectively. For exam-
1
ple, L-Histidine (His) can quench O
2
, 2,6-Di-tert-butyl-methylphe-
Å
nol (BHT) and Dimethylsulfoxide (DMSO) can do OH, and that the
Vitamin C (VC) is able to quench almost all kinds of ROS. If the
absorbance of DPCO obviously decreases after adding some kind
of ROS scavenger, it will demonstrate that there is a kind of corre-
sponding ROS existing in the system.
2 2 2 2 3 2
In this work, TiO , CeO /TiO and Fe O /TiO , taken as the son-
ocatalyst and photocatalyst, were irradiated by ultrasound and vis-
ible-light, and the generated ROS was estimated by the OEP
method. In this process, the synergistic effect of TiO
2 2 2
, CeO /TiO
Fig. 1. Molecular structures of Malachite Green, Acid Red B, Methylene Blue, Congo
or Fe /TiO and ultrasonic irradiation or visible-light irradiation
2
O
3
2
Red and Azo Fuchsine.

8
4
M. Zou et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 101 (2013) 82–90
Fig. 2. Experimental setups. ((a) Ultrasonic irradiation and (b) visible-light irradiation.)
Fig. 3. (a) XRD and (b) SEM of CeO
2
2
/TiO , Fe
2
O
3
/TiO
2
and TiO
2
powders.
frequency, 28 W output power and irradiation intensity of 0.2 W/
cm were used as irradiation sources, respectively. Muffle furnace
Preparation of CeO
2
/TiO
2
and Fe
2
O
3
/TiO
2
composites
2
(
(
SX2-4-10, Great Wall Furnace Company, China) and electric oven
101-1, Shanghai Experiment Apparatus Company, China) were
The CeO
rect mechanical mixing at molar ratio of CeO
TiO = 4:1 and ground thoroughly in an agate mortar. In order to
2
/TiO
2
and Fe
2
O
3
/TiO
2
composites were prepared by di-
2
:TiO = Fe :-
2
2 3
O
used to prepare CeO
2
/TiO
2
and Fe
2
O
3
/TiO
2
composites. X-ray dif-
2
fractometer (XRD) (Rigaku Rint-2500, Rigaku Corporation, Japan)
and scanning electron microscopy (SEM) (JEOL JSM-5610LV, Hitachi
Corporation, Japan) were used to determine the XRD and SEM of
further improve the dispersion of two oxides, the mixed oxides
were added into 20 mL of distilled water and sonicated in an ultra-
sonic apparatus for 6.0 min. After that, the mixed suspension was
filtered and the mixture precipitation was dried at 100 °C for
10 h and then calcined at 500 °C for 40 min. After abrading ade-
CeO
2
/TiO
2
and Fe
2
O
3
/TiO
2
composites. For comparison, the XRD
powder were also determined. The cor-
and SEM of nano-sized TiO
2
responding results are shown in Fig. 3. The UV–vis spectrophotom-
eter (Cary 50, Varian Company, USA) was used to estimate the
generation of ROS and the degradation process of organic dyes.
quately, the CeO
and a series of studies were carried out. In addition, the nano-sized
TiO (anatase phase) powders were also treated with the same
2 2 2 3 2
/TiO and Fe O /TiO composites were obtained
2

M. Zou et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 101 (2013) 82–90
85
method as above. The surface areas of the prepared CeO
Fe /TiO composites were determined by using nitrogen adsorp-
tion method and calculated by BET equation. The values are 30–
2
/TiO
2
and
Evaluation of reactive oxygen species (ROS)
2
O
3
2
Experiments were performed according to the method of Oxida-
tion-Extraction Photometry (OEP) reported in the references [27–
2
2
4
5 m /g and 35–50 m /g, respectively, for CeO
TiO composites. And the surface areas of nano-sized CeO
and TiO (anatase phase) powders are 10–15 m /g,
5–20 m /g and 35–65 m /g, respectively.
2
/TiO
2
and Fe
2 3
O /
ꢀ
2
2
2
, Fe
2
O
3
29]. Firstly, six 10.00 mL DPCI stock solutions (1.00 ꢁ 10 mol/L)
2
2
were added into six 100 mL volumetric flasks, respectively. And
2
2
1
2 2 2 3 2 2
then two 100 mg CeO /TiO , Fe O /TiO and TiO powders were
Fig. 4. UV–vis spectra of DPCO solution in the presence of CeO
2
/TiO
2 2 3 2 2
, Fe O /TiO and TiO under (a) ultrasonic irradiation and (b) visible-light irradiation.
[DPCI] = 1.00 ꢁ 10ꢀ mol/L, [CeO
3
/TiO ] = [Fe O /TiO ] = [TiO ] = 1.00 g/L, T = 298 K and t = 60 min.).
2 2 2 3 2 2
(
ꢀ
3
Fig. 5. Absorbance of DPCO solution with (a) irradiation time and (b) catalyst amount. ([DPCI] = 1.00 ꢁ 10 mol/L, [CeO
and t = 60. 1: ultrasonic irradiation; 2: visible-light irradiation.).
2 2 2 3 2 2
/TiO ] = [Fe O /TiO ] = [TiO ] = 1.00 g/L, T = 298 K

8
6
M. Zou et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 101 (2013) 82–90
added to above six solutions, respectively. All of the six solutions
were diluted to 100 mL with double distilled water. For all six solu-
tions, the final DPCI concentration and catalyst amount were
irradiation time from 0 min to 75 min at 15 min intervals and the
CeO /TiO , Fe /TiO or TiO amount from 0.00 g/L to 1.25 g/L at
2
2
2
O
3
2
2
0.25 g/L intervals were changed and the influences on the genera-
tion of ROS were investigated. The corresponding results were
shown in Fig. 5. In addition, the DPCI concentration was changed
ꢀ
3
1
.00 ꢁ 10 mol/L and 1.00 g/L, respectively. Among them, three
solutions, containing CeO /TiO , Fe /TiO and TiO powers,
2
2
2
O
3
2
2
ꢀ3
ꢀ3
ꢀ3
respectively, were irradiated by ultrasonic irradiation away from
light. And the other three corresponding solutions were irradiated
by visible-light. For all samples the solution temperature was con-
trolled at 25.00 ± 0.02 °C. After 60 min (ultrasonic or visible-light)
irradiation, from each sample 10.00 mL solution was taken exactly
and extracted with mixed solvent of benzene and carbon tetrachlo-
ride (1:1 volume ratio). And then, all extracted solutions were di-
luted to 10.00 mL with the same mixed benzene–carbon
tetrachloride solution and their UV–vis spectra were determined.
The ultrasonic and visible-light irradiation intensities are 1.0 W/
from 0.00 ꢁ 10 mol/L to 2.50 ꢁ 10 mol/L at 0.50 ꢁ 10 mol/L
intervals and the influence on the quality of determined ROS was
also reviewed. The result was offered in Fig. 6.
The determination of the kind of reactive oxygen species (ROS)
Several special radical scavengers were used to detect the kinds
of ROS generated during sonocatalytic and photocatalytic reac-
tions. According to the quenching degree, which kind of ROS can
be judged. As well known, His can quench the singlet state molec-
2
2
cm and 0.2 W/cm , respectively. The obtained results were given
in Fig. 4.
1
ular oxygen ( O
2
), while BHT and DMSO can do the hydroxyl rad-
Å
icals ( OH). And that, the VC can quench almost all kinds of ROS
[
34,35]. Firstly, five 2.50 mL DPCI stock solutions and five 25 mg
CeO /TiO , Fe /TiO or TiO were added into five 25.00 mL
volumetric flasks, respectively. 2.50 mL His, VC, BHT and DMSO
The influence factors on the generation of reactive oxygen species
2
2
2
O
3
2
2
(
ROS)
ꢀ3
stock solutions (5.00 ꢁ 10 mol/L) were added into above four
volumetric flasks, respectively. All five solutions were diluted to
25.00 mL with double distilled water. For all samples the final DPCI
Currently, in this work, the CeO
2
/TiO
2
, Fe
2
O
3
/TiO
2
or TiO
2
ꢀ
3
amount and DPCI concentration were 1.00 g/L and 1.00 ꢁ 10
-
mol/L, respectively. The other experimental conditions such as
2 2 2 3 2 2
concentration and catalyst (CeO /TiO , Fe O /TiO or TiO ) amount
ꢀ
3
2
6
5.00 ± 0.02 °C solution temperature, 100 mL total volume and
0 min ultrasonic or visible-light irradiation time were adopted ex-
were 1.00 ꢁ 10 mol/L and 1.00 g/L, respectively. And the final
ꢀ3
His, VC, BHT and DMSO concentrations are all 5.00 ꢁ 10 mol/L.
cept the special statement. Besides, the ultrasonic and visible-light
All of the solution was averagely transferred into two conical
Fig. 6. Absorbance of DPCO solution with different DPCI concentration in the presence of CeO
irradiation. ([CeO /TiO ] = [Fe /TiO ] = [TiO ] = 1.00 g/L, T = 298 K and t = 60 min.).
2 2 2 3 2 2
/TiO , Fe O /TiO and TiO under (a) ultrasonic irradiation and (b) visible-light
2
2
2
O
3
2
2
2
Fig. 7. Absorbance of DPCO solution in the presence of various ROS quenching reagents under (a) ultrasonic irradiation and (b) visible-light irradiation with combined CeO /
ꢀ
3
ꢀ3
TiO
t = 60 min.).
2
,
Fe
2
O
3
/TiO
2
and TiO
2
.
([DPCI] = 1.00 ꢁ 10 mol/L, [CeO
2
/TiO
2
] = [Fe
2
O
3
/TiO
2
] = [TiO
2
] = 1.00 g/L, [His] = [VC] = [BHT] = [DMSO] = 5.00 ꢁ 10 mol/L, T = 298 K and

M. Zou et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 101 (2013) 82–90
87
2 2 2 3 2 2
Fig. 8. Effect of (a) ultrasonic irradiation and (b) visible-light irradiation time on the degradation of some organic dyes in the presence of CeO /TiO , Fe O /TiO and TiO .
(
2 2 2 3 2 2
[CeO /TiO ] = [Fe O /TiO ] = [TiO ] = 1.00 g/L, [dye] = 10 mg/L, T = 298 K and t = 120 min.).
flasks. Then one group of the conical flasks was placed in an ultra-
sonic apparatus away from light. After 60 min ultrasonic irradia-
tion, from each sample 10.00 mL solution was taken exactly and
extracted by the mixed solvent of benzene and carbon tetrachlo-
ride (1:1 volume ratio). And then, they were diluted to 10.00 mL
with the same mixed benzene–carbon tetrachloride solution. The
UV–vis spectra of all the solutions were determined. The other
group of the conical flask was directly irradiated by visible-light
and the same experiments were carried out as above method. All
results were provided in Fig. 7.
wavelength equal to 0.154 nm, b is the full width at half maximum
and h is the half diffraction angle (18.14°)).
Scanning electron microscopy (SEM) was also used to descry
the size and shape of TiO
Moreover, from Fig. 3b, it was found that the treated CeO
and Fe /TiO powders agglomerated up to some extent and the
sizes of them were around 80–100 nm, which is larger than pure
nano-sized TiO particle whose sizes are 60–80 nm. Hence, the
2
, CeO
2
/TiO
2
and Fe
2
O
3
/TiO
2
particles.
2
/TiO
2
2
O
3
2
2
crystallite size (55–75 nm) calculated by Scherrer’s equation were
in good agreement with the ones of the particles observed by SEM.
Sonocatalytic and photocatalytic degradation of some organic dyes
UV–vis spectra of DPCO solutions in the presences of CeO
2 2 2 3
/TiO , Fe O /
TiO and TiO under ultrasonic and visible-light irradiation
2
2
In order to evaluate the feasibility of sonocatalytic and photo-
catalytic degradation using two prepared composites (CeO
2 2
/TiO
The ROS could be detected by UV–vis absorption spectrum
according to the method of Oxidation-Extraction Photometry
OEP) [27–29]. Under ultrasonic or visible-light irradiation, the
semiconductor metal oxides are transferred into the excited state.
That is, some electrons are transited from valence band (VB) to
conduction band (CB). In the meantime, the electron–hole pairs
are formed. The electrons and holes react with the molecular oxy-
and Fe /TiO ) under ultrasonic and visible-light irradiation,
2
O
3
2
some organic dyes (Malachite Green, Acid Red B, Methylene Blue,
Congo Red and Azo Fuchsine) with different chemical compositions
and molecular structures are selected as the model organic pollu-
tants. The manipulative experiments were performed with the cat-
(
2 2 2 3 2
alyst of CeO /TiO or Fe O /TiO (1.00 g/L) for the degradation of
different organic dyes in aqueous solutions (10.00 mg/L). Under
ultrasonic or visible-light irradiation within 120 min at 20 min
intervals, a small quantity of treated dye solutions were taken at
every turn and the UV–vis spectra were determined. For the TiO ,
2
the same process was also carried out by the above method. The
obtained results were given in Fig. 8.
2 2
gen (O ) dissolved in aqueous solution and water molecules (H O)
absorbed on the surface of catalyst particles, respectively, produc-
Å
ꢀ
Å
ing the superoxide anion ( O ) and hydroxyl radicals ( OH). Owing
2
to the strong oxidation ability, these ROS can oxidize 1,5-diphenyl
carbazide (DPCI) into 1,5-diphenyl carbazone (DPCO) which can be
extracted by the mixed solvent of benzene and carbon tetrachlo-
ride and show a strong absorbance at 563 nm wavelength. Sequen-
tially, the produce and output of ROS can be detected easily.
Fig. 4 displays that, whether under ultrasonic or visible-light
Results and discussion
2 2
XRD and SEM of CeO /TiO , Fe
2 3
O /TiO
2
and TiO
2
2 2 2 3 2 2
irradiation, in the presences of CeO /TiO , Fe O /TiO and TiO
the absorption peaks of DPCO in the DPCI solutions at 563 nm all
show an obvious increase compared with the corresponding ones
without any irradiation. And that, for any catalyst the absorbance
of DPCO under visible-light irradiation is much higher than the cor-
responding one under ultrasonic irradiation. It indicates that the
visible-light can directly excite the semiconductor metal oxides
Fig. 3a shows the X-ray diffraction (XRD) patterns of pure TiO
powder, CeO /TiO and Fe /TiO composites with Ti/(Ce and Fe)
molar proportion of 4:1. From corresponding characteristic 2h val-
ues (2h = 25.00°) of the diffraction peaks in XRD pattern, it can be
2
2
2
2
O
3
2
confirmed that the pure TiO
CeO /TiO and Fe /TiO composites are all identified as anatase
phase. In addition, the 2h = 28.00° and 2h = 36.00° of diffraction
peaks should be taken for the characteristic peaks of CeO and
Fe [36,37], respectively. The average crystallite size of the
nano-sized TiO as well as in CeO /TiO and Fe /TiO composites
2 2
powder and TiO part in prepared
2
2
2
O
3
2
(CeO
tion with relatively high efficiency. In addition, the absorption
peaks of DPCI solutions arranges as the order of CeO /TiO > Fe
TiO > TiO . The results can be explained that owing to a narrow
band gap (Eg = 2.2 eV) the Fe has a wide light respondent spec-
trum. It makes the Fe /TiO
2 2 2 3 2 2
/TiO , Fe O /TiO and TiO ) to perform photocatalytic reac-
2
2
2
2 3
O /
2
O
3
2
2
2
2
2
2
O
3
2
2 3
O
are 55–75 nm estimated according to the Scherrer’s equation:
D = k(k/bcosh) (where k is a constant equal to 0.89, k is the X-ray
2
O
3
2
composite effectively absorb visible-
ꢀ
lights, which products more photogenerated electrons (e ) and

8
8
M. Zou et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 101 (2013) 82–90
+
+
ꢀ
holes (h ). These holes (h ) and electrons (e ) could react with the
ROS [34,35]. Based on the quenching resulted from different radi-
cal scavengers, the kinds of generated ROS can be judged.
It is observed from Fig. 7 that, for both ultrasonic and visible-
light irradiation, in the absence of any radical scavenger the high
absorbance of DPCO in the DPCI solution at 563 nm can be found
molecular oxygen (O
molecules (H O) absorbed on the surface of TiO
various ROS. Moreover, the CeO has more width band gap than
that of Fe . So the combination of CeO
2
) dissolved in aqueous solution and water
2
2
particles forming
2
2
O
3
2
with TiO
2
facilitates
ꢀ
+
the separation of photogenerated electrons (e ) and holes (h ).
2 2 2 3 2 2
for all three catalysts (CeO /TiO , Fe O /TiO and TiO ). It indicates
Thus, the CeO
among three catalysts. That is, the CeO
duce the ROS to oxidize the DPCI.
2
/TiO
2
composite displays highest catalytic activity
that a great amount of ROS are generated and then the most of
DPCI are oxidized to DPCO. Of course, for any catalyst the absor-
bance under visible-light irradiation is much higher than corre-
sponding that under ultrasonic irradiation. Moreover, the order is
2 2
/TiO can effectively pro-
also CeO
2 2 2 3 2 2
/TiO > Fe O /TiO > TiO . Apparently, as a sensitive com-
Influence of (ultrasonic and visible-light) irradiation time and catalyst
CeO /TiO , Fe /TiO and TiO ) amount on the generation of ROS
pound to ROS, the DPCI can be oxidized into DPCO more easily in
(
2
2
O
2 3
2
2
the presence of CeO
TiO
CeO
TiO
2
/TiO
2
than in the presences of Fe
2
O
3
/TiO
2
or
or
2
. That is, under ultrasonic or visible-light irradiation the
From Fig. 5a it can be seen that for two courses the absorbance
Å
2
/TiO
2 2 3 2
can generate relatively more OH than Fe O /TiO
of DPCO in the DPCI solution at 563 nm both increase with the in-
crease of ultrasonic and visible-light irradiation time. However, the
absorbance under visible-light irradiation in the presence of any
catalyst is much higher than corresponding that under ultrasonic
irradiation at any irradiation time. This proves that the visible-light
can effectively excite the semiconductor oxides to generate the
2
does.
After adding radical scavengers, the absorbances of DPCO in the
DPCI solution are obviously weakened. Especially, for the addition
of VC the absorbance nearly disappear, which proves that the VC
can indeed quench almost all kinds of ROS. For ultrasonic irradia-
tion, after adding His, BHT and DMSO the absorbances all decrease.
However, for adding His, the absorbance is obviously higher than
that for adding BHT and DMSO. It illustrates that, under ultrasonic
irradiation, the catalysts (TiO
erate more OH than
the absorbances of DPCO in the DPCI solution all decrease equally
for adding His, BHT and DMSO. It demonstrates that, under visible-
ROS again. Moreover, it estimates that the CeO
by whether ultrasound or visible-light can generate more ROS than
the Fe /TiO and TiO . Of course, for any catalyst the quantities
2 2
/TiO irradiated
2
O
3
2
2
2
, CeO
2 2 2 3 2
/TiO and Fe O /TiO ) can gen-
of generated ROS both increase with the increase of ultrasonic or
visible-light irradiation time.
Å
1
O . Whereas, under visible-light irradiation,
2
From Fig. 5b it can be seen that, under whether ultrasonic or
visible-light irradiation, the absorbance of DPCO in the DPCI solu-
tion increases at beginning with the increase of catalyst amounts,
and then gradually decreases. It indicates that, in absence of any
catalyst, whether ultrasonic or visible-light irradiation only can
light irradiation, the TiO
2
, CeO
2
/TiO
2
2
and Fe O
3
/TiO
2
generate al-
most equal amounts of 1O
and OH.
Å
2
Degradation of organic dyes in the presence of CeO
and TiO
2 2 2 3 2
/TiO , Fe O /TiO
generate limited ROS. Once the catalyst (TiO
CeO /TiO ) is added, under ultrasonic or visible-light irradiation
much more ROS would be generated. Moreover, along with the in-
crease of catalyst (TiO , Fe /TiO and CeO /TiO ) amount, the
2 2 3 2
, Fe O /TiO or
2
under ultrasonic and visible-light irradiation
2
2
Under ultrasonic and visible-light irradiation, the degradations
of several organic dyes with different chemical compositions and
molecular structures within 120 min at intervals of 20 min were
reviewed [38]. It is observed in Fig. 8 that, whether under ultra-
sonic or visible-light irradiation, the degradation ratios of five dyes
all increase gradually with the increase of irradiation time. How-
ever, for any organic dye, the degradation ratios under visible-light
irradiation are slightly higher than corresponding ones under
ultrasonic irradiation. It indicates that, under the same conditions,
2
2
O
3
2
2
2
absorbance of DPCO rapidly increases before 0.75 g/L. This indi-
cates that more and more ROS are generated with the catalyst
amount increases. Nevertheless, when the catalyst (CeO
2 2
/TiO ,
Fe /TiO and TiO ) amount adds more than 0.75 g/L, the absor-
2
O
3
2
2
bance of DPCO begins to decrease. It reveals that the quantity of
generated ROS decrease along with the further increase of catalyst
amount. Maybe, the reason is that the overmuch catalyst particles
adsorb a certain amount of DPCI molecules or hinder the transmis-
sion of ultrasound and visible-light.
the direct visible-light irradiation can effectively excite the TiO
2
or
TiO composites (CeO /TiO and Fe /TiO ) as photocatalysts to
2
2
2
2
O
3
2
degradate any organic dye. In addition, for both ultrasonic and
visible-light irradiation, the degradation ratios increase as the or-
Influence of DPCI concentration on the determination of ROS
2 2 2 3 2 2
der of CeO /TiO > Fe O /TiO > TiO . It is similar to the process of
It is observed from Fig. 6 that, without DPCI, there is not
any absorbance at any wavelength even under ultrasonic and
visible-light irradiation. It demonstrates that the spectral absorp-
tion only comes from the oxidation of DPCI. Under ultrasonic or
visible-light irradiation, the absorbance of DPCO in the DPCI solu-
tion obviously increases along with the increase of DPCI concentra-
tion. It is because more and more DPCI has been oxidized into
DPCO with the increase of DPCI concentration. This indicates that
the DPCO concentration relates to the quantity of generated ROS.
ROS generation as mentioned above. It means that the degradation
of organic dyes is mainly through ROS oxidation mechanism.
Furthermore, because of different chemical compositions and
molecular structures, these dyes give a series of different degrada-
tion ratios.
Possible mechanism on the generation of ROS in the presence of
2 2 2 3 2 2
CeO /TiO , Fe O /TiO and TiO under ultrasonic and visible-light
irradiation
Kinds of generated ROS under ultrasonic and visible-light irradiation
The method of ultrasonic and visible-light irradiation combin-
ing with semiconductor metal oxides (such as TiO ) can generate
2
To understand the mechanism of the sonocatalytic and photo-
catalytic degradation reactions, it is necessary to determine the
kinds of generated ROS. Here, several radical scavengers are used
various reactive oxygen species (ROS). In general, they are called
as sonocatalytic and photocatalytic reaction process, respectively.
In principle, for sonocatalytic reaction process the explanation of
possible mechanism should be based on the ultrasonic cavitation
effect. As well known, under ultrasonic irradiation, the adiabatic
expansion of the gas or vapor in the micro-bubbles happens. And
then, the micro-bubbles implode after reaching the limit. This
for this purpose. In general, l-Histidine (His) can quench singlet
1
molecular oxygen ( O
2
), and 2,6-Di-tert-butyl-methylphenol
(
(
BHT) and Dimethylsulfoxide (DMSO) can do the hydroxyl radicals
OH). And that, the Vitamin C (VC) can quench almost all kinds of
Å

M. Zou et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 101 (2013) 82–90
89
process will generate high temperature and high pressure in their
surroundings and inners, which usually results in the short and lo-
cal sonoluminescence and hot spots [39]. These energies are suffi-
cient to activate the semiconductor metal oxides which selectively
accumulate in aqueous phase. The sonoluminescence caused by
ultrasonic cavitation generates the lights with wide wavelength
range. And the hot spots also produced by ultrasonic cavitation ef-
fect in water medium can generally reach high temperature of
2 2 3 2 2
TiO , Fe O /TiO or TiO addition amount. Meanwhile, the deter-
mined quantity of generated ROS is also related to the used 1,5-di-
phenyl carbazide (DPCI) concentration. In addition, the results
show that several conventional dyes as the model organic pollu-
tants can effectively be degraded by using CeO
2 2 2 3 2
/TiO , Fe O /TiO
and TiO powders under ultrasonic or visible-light irradiation. Par-
2
ticularly, the research results offered some important proofs about
sonocatalytic and photocatalytic degradation mechanisms of or-
ganic pollutants in wastewater. Because of the effective separation
5
000 °C–10,000 °C. Under the excitation of such light or hot spot,
ꢀ
+
some electrons are transited from valence band (VB) of semicon-
of photogenerated electrons (e ) and holes (h ) and prolongation
of light response range, the combination of CeO and Fe con-
tributes to the enhancement of photocatalytic and sonocatalytic
activities of TiO . It is expected that this work could offer some
ductor metal oxides (for example: TiO
At the same time, the electron (h )-hole (e ) pairs are formed.
2
) to conduction band (CB).
2
2 3
O
+
ꢀ
+
Being similar to the photocatalytic process, these electrons (h )
2
ꢀ
and holes (e ) react with the molecular oxygen (O
2
) dissolved in
O) absorbed on the sur-
particles, respectively. This process will produce the
valuable references for promoting the application of sonocatalytic
and photocatalytic degradation in waste water treatment.
aqueous solution and water molecules (H
face of TiO
superoxide radical anions ( O ) and hydroxyl radicals ( OH) with
2
2
Å
ꢀ
Å
2
Acknowledgments
Å
ꢀ
strong oxidation ability. Of which, the
O
could also becomes
2
Å
the OH through a series of chemical reactions. Moreover, the ex-
The authors greatly acknowledge the National Natural Science
Foundation of China, Liaoning Province Natural Science Foundation
of Education Department, Liaoning Province Natural Science Foun-
dation of Science and Technology Department and liaoning univer-
sity ‘‘211’’ project for financial support. The authors also thank our
colleagues and other students for their participating in this work.
cited TiO
2
particles can transfer the appropriate energy to active
3
the ground (triplet) state molecular oxygen ( O
excited (singlet) state molecular oxygen ( O
2
) to give birth to
1
2
) [20,21,40]. Hence,
in view of the comparability of sonocatalytic and photocatalytic
generation of ROS, the semiconductor metal oxides excited by
ultrasonic cavitation could also lead to the generation of the ROS.
Å
1
These ROS including OH and
2
O will oxidize a wide range of or-
ganic compounds, like dye, pesticide and surfactant and so on.
Apparently, the effective generation of ROS is based on the combi-
nation of simultaneous ultrasonic irradiation and semiconductor
metal oxides. The possible process is thought as following.
References
[
1] S. Rodrigues, K.T. Ranjit, S. Uma, I.N. Martyanov, K. Klabunde, J. Adv. Mater. 17
2005) 2467–2471.
(
[
[
[
2] H. Kisch, W. Macyk, Chem. Phys. Chem. 3 (2002) 399–400.
3] A.P. Davis, D.L. Green, Environ. Sci. Technol. 33 (1999) 609–617.
4] H. Choi, A.C. Sofranko, D.D. Dionysiou, Adv. Funct. Mater. 16 (2006) 1067–
Ultrasonic irradiationðcaviation effectÞ ! visible-light þ heat
1074.
[
5] Z.M. El-Bahy, A.A. Ismail, R.M. Mohamed, J. Hazard. Mater. 166 (2009) 138–
143.
½
½
TiO
TiO
2
ꢂ þ visible-light or heat ! ½TiO
2
ꢂꢃ
[
[
[
6] M.J. Zhan, X. Yang, Q.M. Xian, L.R. Kong, Chemosphere 63 (2006) 378–386.
7] M. Saquib, M. Muneer, Dyes Pigm. 56 (2003) 37–49.
8] M. Muruganandham, M. Swaminathan, Sol. Energy Mater. Sol. Cells 81 (2004)
ꢂꢃ ! ½TiO
ꢂ þ h þ e
þ
ꢀ
2
2
439–457.
þ
Å
þ
[9] S. Kaur, V. Singh, Ultrason. Sonochem. 14 (2007) 531–537.
10] N.H. Ince, G. Tezcanli, R.K. Belen, I.G. Apikyan, Appl. Catal. B: Environ. 29
H
2
O þ h ! OH þ H
[
(
2001) 167–176.
ꢀ
Å
ꢀ
O
2
þ e ! O
[11] N.H. Ince, G. Tezcanli, Dyes Pigm. 49 (2001) 145–153.
2
[
[
12] J.K. Kim, I.S. Metcalfe, Chemosphere 69 (2007) 689–696.
13] J. Wang, Y.Y. Zhang, Y. Guo, L. Zhang, R. Xu, Z.Q. Xing, S.X. Wang, X.D. Zhang,
Dyes Pigm. 80 (2009) 271–278.
14] T.P. Pandiyan, O.M. Rivas, J.O. Martinez, G.B. Amezcua, M.A. Martinez-Carrillo,
J. Photochem. Photobiol. A: Chem. 146 (2002) 149–155.
15] G.A. Epling, C. Lin, Chemosphere 46 (2002) 561–570.
16] W.Z. Tang, Z. Zhang, H. An, M.O. Quintana, D.F. Torres, Environ. Technol. 18
(1997) 1–12.
Å
ꢀ
þ
Å
O þ H ! HO
2
2
[
ÅHO
Å
2
þ H
2
O ! H
2
O
2
þ OH
[
[
Å
H
2
O
2
þ visible-light or heat ! 2 OH
[
17] B.T. Jiang, S.Y. Zhang, X.Z. Guo, B.K. Jin, Y.P. Tian, Appl. Surf. Sci. 255 (2009)
5
975–5978.
ꢃ
3
1
½
TiO
2
ꢂ þ O
2
! ½TiO
2
ꢂ þ O
2
[
[
18] F. Galindo, R. Gómez, M. Aguilar, J. Mol. Catal. A: Chem. 281 (2008) 119–125.
19] K. Nakagawa, Y. Murata, M. Kishida, M. Adachi, M. Hiro, K. Susa, Macromol.
Chem. Phys. 104 (2007) 30–39.
In face, it includes the photocatalytic process. Of course, be-
cause of the direct excitation, under visible-light irradiation the
TiO displays slightly higher activity than that under ultrasonic
irradiation. In addition, in order to enhance the sonocatalytic and
photocatalytic activities of TiO , the combination with other some
semiconductor metal oxides (for example: CeO and Fe ) is nec-
[20] X. Zhang, L. Lei, Appl. Surf. Sci. 254 (2008) 2406–2412.
[
21] E.S. Mora, E.G. Barojas, E.R. Rojas, R.S. González, Sol. Energy Mater. Sol. Cells 91
2007) 1412–1415.
2
(
[
22] L. Diamandescu, M. Feder, D. Tarabasanu-Mihaila, F. Vasiliu, Appl. Catal. A:
Gen. 325 (2007) 270–275.
23] N. Miyoshi, T. Igarashi, P. Riesz, Ultrason. Sonochem. 7 (2000) 121–124.
24] M.M. Rahman, K. Ninomiya, C. Ogino, N. Shimizu, Ultrason. Sonochem. 17
2
[
[
2
2 3
O
essary. In this work, in the organic dye degradation the advantages
are obviously displayed.
(
2010) 738–743.
[25] J. Wang, B.D. Guo, X.D. Zhang, Z.H. Zhang, J.T. Han, J. Wu, Ultrason. Sonochem.
2 (2005) 331–337.
26] T.H. Yu, J. Bai, K. Hu, Z.B. Wang, Ultrason. Sonochem. 10 (2003) 33–35.
[27] X.J. Chen, H.H. Xu, Y.J. Wang, S. Hu, Z.X. Zhang, Y.M. Zhang, Agr. Sci. China 6
2007) 458–465.
1
[
Conclusions
(
[
28] J. Wang, Y.W. Guo, B. Liu, X.D. Jin, L.J. Liu, R. Xu, Y.M. Kong, B.X. Wang, Ultrason.
Sonochem. 18 (2011) 177–183.
29] J. Wang, L.J. Liu, B. Liu, Y. Guo, Y.Y. Zhang, R. Xu, S.X. Wang, X.D. Zhang,
Spectrochim. Acta A 75 (2010) 366–374.
This study reveals that, when the CeO
TiO powders were irradiated by ultrasound and visible-light, var-
ious reactive oxygen species (ROS), including not less than singlet
2 2 2 3 2
/TiO , Fe O /TiO and
2
[
1
Å
[30] Y. Segura, R. Molina, F. Martínez, J.A. Melero, Ultrason. Sonochem. 16 (2009)
17–424.
[
state molecular oxygen ( O
generated. The quantities of generated ROS increase with the in-
crease of ultrasonic or visible-light irradiation time and CeO
2
) and hydroxyl radicals ( OH), can be
4
31] N. Shimizu, C. Ogino, M.F. Dadjour, T. Murata, Ultrason. Sonochem. 14 (2007)
184–190.
2
/

9
0
M. Zou et al. / Spectrochimica Acta Part A: Molecular and Biomolecular Spectroscopy 101 (2013) 82–90
[
[
[
[
32] N. Shimizu, C. Ogino, M.F. Dadjour, K. Ninomiya, Ultrason. Sonochem. 15
2008) 988–994.
33] M. Kubo, K. Matsuoka, A. Takahashi, N. Shibasaki-Kitakawa, T. Yonemoto,
Ultrason. Sonochem. 12 (2005) 263–269.
[36] W.B. Yue, W.Z. Zhou, J. Mater. Chem. 17 (2007) 4947–4952.
[37] S. Luther, N. Borgfeld, J. Kim, J.G. Parsons, Microchem. J. 101 (2012) 30–36.
[38] J. Wang, Y.H. Lv, L.Q. Zhang, B. Liu, R.Z. Jiang, G.X. Han, R. Xu, X.D. Zhang,
Ultrason. Sonochem. 17 (2010) 642–648.
[39] A.S. Diaz-Barriga, A. Hernandez-Ceruelos, E. Madrigal-Bujaidar, G. Chamorro,
Phytother. Res. 16 (2002) 754–757.
(
34] S. Umemura, N. Yumita, K. Umemura, R. Nishigaki, Cancer Chemother. Pharm.
4
3 (1999) 389–393.
35] D. Renke, S. Richard, F. David, M.J. Han, S.K. Choong, S. Pill-Soon, J. Nat. Prod. 55
1992) 1241–1251.
[40] O. Carter, G.S. Bailey, R.H. Dashwood, J. Nutr. 134 (2004) 3441–3444.
(