Rhodamine B degradation and reactive oxygen species generation by a ZnSe-graphene/TiO2 sonocatalyst

Article abstract of DOI:10.1016/S1872-2067(14)60158-3

Nanostructured ZnSe-graphene/TiO₂ was synthesized by a hydrothermal-assisted approach. ZnSe-graphene/TiO₂ exhibited favorable adsorption of rhodamine B, a wide wavelength absorption range, and efficient charge separation. Reactive oxygen species were generated by the oxidation of 1,5-diphenyl carbazide to 1,5-diphenyl carbazone. The sonocatalytic reaction mechanism was proposed. These findings potentially broaden the applications of sonocatalytic technologies.

Full text of DOI:10.1016/S1872-2067(14)60158-3

Chinese Journal of Catalysis 35 (2014) 1825–1832
a v a i l a b l e a t w w w . s c i e n c e d i r e c t . c o m
j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c o m / l o c a t e / c h n j c
Article
Rhodamine B degradation and reactive oxygen species generation
by a ZnSe‐graphene/TiO2 sonocatalyst
Lei Zhu, Sun‐Bok Jo, Shu Ye, Kefayat Ullah, Won‐Chun Oh*
Department of Advanced Materials Science & Engineering, Hanseo University, Chungnam 356‐706, Korea
A R T I C L E I N F O
A B S T R A C T
Article history:
Nanostructured ZnSe‐graphene/TiO2 was synthesized by a hydrothermal‐assisted approach.
ZnSe‐graphene/TiO2 exhibited favorable adsorption of rhodamine B, a wide wavelength absorption
range, and efficient charge separation. Reactive oxygen species were generated by the oxidation of
1,5‐diphenyl carbazide to 1,5‐diphenyl carbazone. The sonocatalytic reaction mechanism was pro‐
posed. These findings potentially broaden the applications of sonocatalytic technologies.
© 2014, Dalian Institute of Chemical Physics, Chinese Academy of Sciences.
Received 19 April 2014
Accepted 20 May 2014
Published 20 November 2014
Keywords:
Zinc selenide
Published by Elsevier B.V. All rights reserved.
Graphene‐titanium
Hydrothermal reaction
Reactive oxygen species
Sonocatalysis
1. Introduction
ture. CdS [7], Ag2S [8], ZnSe [9], Bi2S3 [10], and CdSe [11,12] all
have been used to sensitize TiO2.
Industrial dyestuffs including textile dyes are a recognized
environmental hazard. Physical, chemical, and biological
methods have been used to treat such waste. Advanced oxida‐
tion processes include peroxone, non‐thermal plasma, pho‐
to‐Fenton, ultraviolet (UV)‐O3, UV/H2O2, and semiconductor
treatments. These organic degradation processes can achieve
the complete elimination and mineralization of various pollu‐
tants.
TiO2 is an attractive material for photoelectric conversion
and photocatalysis because of its low cost, ease of production,
high photochemical and biological stability, and low toxicity
[1,2]. Much effort has focused on extending the absorption of
TiO2 into the visible region, and thus increasing its photocata‐
lytic activity. Semiconductor quantum dots have attracted
enormous interest because of their potential in single electron
transistors [3], lasers [4], light emitting diodes [5], and infrared
photodetectors [6] operated at low current and high tempera‐
Graphene‐based composites have attracted much attention
for their potential in electronics, photocatalysis, and photovol‐
taic devices [13–15]. Graphene can enhance charge transport in
devices. This is because its delocalized electrons within its
conjugated sp2‐hybridised framework impart high conductivity.
Graphene‐based nanocomposites containing Pd, Ag, Au, TiO2,
and metal selenides have all been reported [16–20]. Metal
selenides can impart interesting electronic and optical proper‐
ties in potential applications. ZnSe has a direct band gap of ~2.7
eV, making it well suited for solar absorption. Chen et al. [21]
synthesized a N‐doped graphene/ZnSe nanocomposite (GN‐
ZnSe) by a one‐pot hydrothermal process at low temperature.
GN‐ZnSe exhibited favorable electrochemical performance in
the oxygen reduction reaction, and favorable photocatalytic
activity for bleaching methyl orange under visible‐light irradia‐
tion.
3
Sonocatalytic technologies have been proposed in recent
*Corresponding author. Tel: +82‐41‐6601337; Fax: +82‐41‐6883352; E‐mail: wc_oh@hanseo.ac.kr
DOI: 10.1016/S1872‐2067(14)60158‐3 | http://www.sciencedirect.com/science/journal/18722067 | Chin. J. Catal., Vol. 35, No. 11, November 2014

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Lei Zhu et al. / Chinese Journal of Catalysis 35 (2014) 1825–1832
years. Their high efficiency without additional oxidants can
lower the cost of treatment [22]. The sonocatalytic decomposi‐
tion of organic pollutants can be enhanced in the presence of a
photocatalyst. Combining ultrasound with solid photocatalyst
particles can provide additional nuclei for cavitation bubble
formation. Ultrasound can also enhance the mass transfer of
organic pollutants between the liquid phase and catalyst sur‐
face and increase the active surface area by ultrasonic de‐ag‐
gregating. Photocatalysts can be excited by ultrasound‐induced
luminescence. Such luminescence has a wide wavelength range
and increases the production of hydroxyl radicals (•OH) in the
reaction mixture [23]. Reactive oxygen species (ROS) such as
hydrogen peroxide (30%, 250 mL). The solid product was col‐
lected by centrifugation (3000 rpm), washed with 5% HCl until
SO4− was no longer detectable with BaCl2, washed three times
with acetone, and air dried overnight in a vacuum oven. GO was
transformed into graphene oxide sheets by sonication for 30
min at 308 K.
In a typical procedure, about 300 mg of GO and 22 mg of
ZnCl2 were ultrasonically dispersed in 100 mL of ethylene gly‐
col for 1 h using a digital sonifer. This yielded a graphene oxide
nanosheet (GONS)/Zn2+ solution (denoted solution A) [25].
Na2SO3 (5 g) and Se powder in 30 mL of water were refluxed
for 1 h to form a Na2SeSO3 solution (denoted solution B). Solu‐
tion B and 6 mL of NH4OH (28 wt%) were added to solution A,
which was heated to 333 K for several minutes. A colloidal TiO2
solution in 35:15:4 ethanol:H2O:TNB (denoted solution C) was
added to the above mixture. The resulting solution was trans‐
ferred to a polytetrafluoroethylene‐lined stainless steel auto‐
clave, which was then sealed. The contents were heated to 433
K for 6 h. The reaction was allowed to cool to room tempera‐
ture. The precipitate was collected by filtration, washed thor‐
oughly with water, dried in a vacuum oven at 353 K for 12 h,
and then heated to 773 K for 1 h. ZnSe/TiO2 [26] and GR‐TiO2
[19] were similarly prepared with a little modification.
−
superoxide radical anions (•O2 ), •OH, hydrogen peroxide
(H2O2), and singlet oxygen (1O2) are also generated [24].
Understanding the sonocatalytic degradation and ROS
formed during ultrasonic irradiation may enhance the degrada‐
tion efficiency and yield new sonocatalysts. Herein, we hydro‐
thermally prepared
a
ZnSe‐graphene/TiO2 sonocatalyst
[8,19,20]. The composite was used to ultrasonically degrade
aqueous rhodamine B (RhB). The generated ROS were esti‐
mating by oxidation‐extraction photometry. The reasons for
the high sonodegradation activity of ZnSe‐graphene/TiO2 are
discussed.
2. Experimental
2.3. Characterization
2.1. Materials
Fourier transform infrared (FT‐IR) spectra (FTS 3000MX,
Biored Co., Korea) were recorded on a Perkin‐Elmer spectrom‐
eter from KBr pellets. Spectra were recorded over the range
4000−400 cm−1 at 4 cm‒1 resolution, with forward and reverse
mirror speeds of 10 and 6.2 kHz, respectively. Crystal struc‐
tures were observed by X‐ray diffraction (XRD, Shimatz XD‐D1,
Japan) at room temperature with Cu Kα radiation. Diffuse re‐
flectance ultraviolet‐visible (DRS UV‐vis) spectra were record‐
ed using a spectrophotometer (Neosys‐2000) equipped with an
integrating sphere assembly. Morphologies were analyzed by
scanning electron microscopy (SEM, JOEL JSM‐5200, Japan) at
an operating voltage of 3.0 keV. The SEM microscope was
equipped with an energy dispersive X‐ray (EDX) attachment.
Transmission electron microscopy (TEM, JEOL JEM‐2010, Ja‐
pan) images were collected at an accelerating voltage of 200 kV
and were used to examine particle sizes and distributions. Spe‐
cific surface areas were determined using the BET method
from N2 adsorption isotherms at 77 K using a BET analyzer
(Monosorb, USA).
Ethylene glycol and anhydrous ethanol were purchased
from Daejung Chemical Co. (Korea). ZnCl2, selenium metal
powder, and NH4OH (28%) were purchased from DaeJung
Chemicals & Metal Co., Ltd. (Korea). Anhydrous Na2SO3 (95%)
was purchased from Duksan Pharmaceutical Co., Ltd. (Korea).
Titanium(IV) n‐butoxide (TNB, C16H36O4Ti, Kanto Chemical
Company, Tokyo, Japan) was used as the Ti source in the prep‐
aration of TiO2 and graphene/TiO2 composites. Anatase TiO2
(99.7%, Sigma‐Aldrich, USA) with a particle size of <25 nm was
used as a comparative sample. RhB (99.99+%, Samchun Pure
Chemical Co., Ltd., Korea) was used as a model pollutant. All
chemicals were used without further purification, and distilled
water was used throughout experiments.
2.2. Synthesis of ZnSe‐graphene/TiO2
Graphite oxide (GO) was prepared from graphite according
to the Hummers‐Offeman method [19]. In brief, graphite pow‐
der (10 g) was dispersed in cold concentrated sulfuric acid
(230 mL, 98 wt%, dry ice bath). KMnO4 (30 g) was gradually
added under cooling and vigorous stirring to prevent the tem‐
perature from exceeding 293 K. The dry ice bath was replaced
with a water bath, and the mixture was heated to 308 K for 30
min under continuous stirring, with gas allowed to release.
Deionized water (460 mL) was slowly added, which rapidly
increased the solution temperature up to 371 K. The reaction
was allowed to proceed for 40 min to increase the degree of GO
oxidation. Reaction of the resulting bright‐yellow suspension
was terminated by adding distilled water (230 mL) followed by
2.4. Ultrasonic degradation of organic dye solutions
A controllable serial‐ultrasonic apparatus (Ultrasonic Pro‐
cessor, VCX 750, Korea) was used to irradiate the RhB solution.
The apparatus was operated at an ultrasonic frequency of 20
kHz and output power of 750 W through manual adjusting
(3.04 × 106 J). In a typical experiment, 0.2 g of control sample
and nanocomposite were added to 100 mL of RhB solution (2 ×
10−5 mol/L). The suspension was magnetically stirred for 120
min in the dark to establish adsorption equilibrium. The con‐
centration of adsorbed RhB (Cads) was then measured, and the

Lei Zhu et al. / Chinese Journal of Catalysis 35 (2014) 1825–1832
1827
suspension was subjected to ultrasonic irradiation to com‐
TiO2
ZnSe
(a)
mence degradation. The temperature was controlled at about
298 K by a water bath. A glass reactor vessel of 5 cm in diame‐
ter and 7 cm in height was used and was placed on the magnet‐
ic churn dasher. The diameter of the ultrasound tip was 1.90
cm, and the ultrasound irradiation area was 26.86 cm2. Ultra‐
sonic irradiation of the reactor vessel was carried out for 30,
60, 90, 120, and 150 min. Samples (3 mL) were withdrawn
regularly from the reactor, and the dispersed material was col‐
lected by centrifugation and analyzed by UV‐vis spectropho‐
tometry (Optizen Pop Mecasys Co., Ltd., Korea).
ZnSe-GR/TiO2
ZnSe-TiO2
GR-TiO2
TiO2
2.5. Evaluation of ROS
20
30
40
50
60
70
2/(o )
Six 10‐mL 1,5‐diphenyl carbazide (DPCI) stock solutions
(0.01 mol/L) were added into three 100‐mL volumetric flasks.
ZnSe‐GR/TiO2 and TiO2 (50 mg) were added to the above DPCI
solutions. All three solutions were diluted to 50 mL with water.
The DPCI and ZnSe‐GR/TiO2 concentrations of the three solu‐
tions were 2 × 10−3 mol/L and 1.00 g/L, respectively. After 180
min of ultrasonic irradiation, 10 mL of each solution was ex‐
tracted with benzene and diluted to 10 mL with benzene for
analysis by UV‐vis spectrophotometry.
40
(b)
GO
-OH
35
30
25
20
15
10
C=O
OH
C-OH
CO
ZnSe-GR/TiO2
3. Results and discussion
C=O
C-OH
C-H
1000
1500
2000
2500
3000
3500
3.1. Growth and characterization of ZnSe‐GR/TiO2
Wavenumber (cm1)
Fig. 2. (a) XRD patterns of TiO2, GR‐TiO2, ZnSe‐TiO2, and ZnSe‐GR/TiO2;
(b) FT‐IR spectra of GO and ZnSe‐GR/TiO2.
ZnSe‐GR/TiO2 was synthesized by the ethylene gly‐
col‐assisted hydrothermal dissociation of Na2SeSO3 in the
presence of GO and ZnCl2. TiO2 nanoparticles adhered to GO
functional groups on graphene oxide. Epoxy (C–O–C), hydroxyl
(–OH), carbonyl (C=O), and carboxylic acid (–COOH) groups
existed on the GO surface, so Zn2+ was strongly electrostatically
adsorbed. Generated selenium ions caused nucleation and
promoted the formation of ZnSe‐GR. TiO2 was deposited on GO
sheets or ZnSe‐GR surfaces and then crystallized into anatase
nanocrystals during heat‐treatment. Figure 1 shows the depo‐
sition of ZnSe and TiO2 on graphene.
peaks of anatase TiO2 (JCPDS 21‐1272) [27]. The (111), (220),
and (311) diffraction peaks originated from the zinc blende
structure (JCPDS 21‐1272), in agreement with previous reports
[28]. Weak new peaks appeared at 30° and 34° after reaction at
433 K for 6 h. These may have been indicated the presence of
ZnO in the composite at shorter reaction times, in agreement
with Gharibe et al. [29]. No diffractions of GO(001) or gra‐
phene(002) phase were detected in the GR‐TiO2 and
ZnSe‐GR/TiO2. GO can be reduced to graphene during sol‐
vothermal reaction, and graphene sheets can then restack to
form poorly ordered graphite. If the regular stacking of GO or
graphite is broken, for example by exfoliation, their diffraction
Figure 2(a) shows XRD patterns of TiO2, GR‐TiO2, ZnSe‐TiO2,
and ZnSe‐GR/TiO2. The pattern of TiO2 contained diffractions at
37.9°, 47.8°, 54.3°, 55.0°, and 62.7°, which were indexed to the
characteristic (004), (200), (105), (101), (211), and (204)
A
OH
B
C
O
160 oC 6 h
Colloidal
TiO2
Se2-
COO-Zn-OOC
Filtered, washed
dried, 500 oC
ZnSe
TiO2
Fig. 1. Schematic illustration of the deposition of ZnSe and TiO2 nanoparticles on a GONS.

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Lei Zhu et al. / Chinese Journal of Catalysis 35 (2014) 1825–1832
peaks can reportedly weaken or disappear [19].
(a)
(b)
The FT‐IR spectrum of graphene was rather simple, which
suggested extensive oxidation. The spectra of the crystalline
material contained well‐distinguished sharp bands, whereas
the spectra of the amorphous material were less resolved. The
hydration results established the importance of defined condi‐
tions for FT‐IR (Fig. 2(b)). The strong band at about 3400 cm−1
in the GO was assigned to the –OH stretching vibration, and
that at 1619 cm−1 to the vibration of adsorbed water. The C=O,
C–OH, and C–O stretching bands were observed at 1728, 1226,
and 1050 cm−1, respectively. In the spectrum of ZnSe‐GR/TiO2,
weak peaks at 1224 and 1367 cm−1 were assigned to the C‐OH
and C–H bands, respectively. The C=O stretching vibration was
observed at 1739 cm−1. Compared to the spectrum curves the
intensities of peaks attributed to functional groups of oxidized
graphene were weak and decreased. This was because some of
functional groups had combined with ZnSe and TiO2 particles,
resulting in a reduction of these groups content. This was in
agreement with the XRD results.
(c)
(d)
(e)
(f)
The composite was characterized by SEM, as shown in Fig.
3. GO had a flaky morphology, which reflected its layered mi‐
crostructure, as shown in Fig. 3(a,b). ZnSe/TiO2 and ZnSe‐GR/
TiO2 were also hydrothermally synthesized for comparison.
The morphology of ZnSe/TiO2 is shown in Fig. 3(c,d). A homo‐
geneous distribution with some agglomeration was observed.
The SEM images of ZnSe‐GR/TiO2 are shown in Fig. 3(e,f), and
they exhibited the well‐known properties of surface
nanostructures. This suggested that ZnSe and TiO2 particles
were well‐dispersed on the layered graphene nanosheet. Ele‐
mental compositions were analyzed by EDX, and the results are
shown in Fig. 3(g). The main elements present were Ti and O,
as evidenced by the strong peaks at 4.51, 4.92, and 0.52 keV.
The Zn and Se contents of the composite were much lower.
According to a former study [30], a small concentration of do‐
pant in the TiO2 matrix can prevent the formation of elec‐
tron‐hole recombination centers and increase negative charge
capability.
Quantitative results
(g)
40
30
20
10
0
(h)
C
O
Ti
Zn
Se
Fig. 3. SEM micrographs of as‐preparedsamples. (a,b) Graphene; (c,d)
ZnSe/TiO2; (e,f) ZnSe‐GR/TiO2; (g) EDX elemental microanalysis and (h)
element weight of ZnSe‐GR/TiO2.
0.567 nm [32]. The presence of the GR template resulted in a
lower aggregation of the TiO2 and ZnSe nanoparticles. The TiO2
and ZnSe nanoparticles also prevented the agglomeration of GR
sheets after reduction.
UV‐vis absorption spectra of the different samples were
measured to investigate their visible light response and are
shown in Fig. 5. TiO2, ZnSe/TiO2, and ZnSe‐GR/TiO2 strongly
absorbed in the UV region. The spectrum of TiO2 exhibited the
characteristic sharp absorption edge at 400 nm (3.2 eV). The
absorption bands of ZnSe‐GR/TiO2 differed from those of na‐
noscale TiO2 and ZnSe/TiO2. TiO2 had a white appearance,
whereas ZnSe/TiO2 was pale yellow and ZnSe‐GR/TiO2 was
TEM images of ZnSe/TiO2 and ZnSe‐GR/TiO2 are shown in
Fig. 4. The composite exhibited uniform particle sizes. The TiO2
particles were cubic with an average size of 15–20 nm. The
average size of the well‐dispersed ZnSe nanoparticles was
10–15 nm. The lattice spacing of 0.35 nm was assigned to the
(101) plane of anatase [31]. The interplanar spacings of (110)
lattice plane families of ZnSe, with a lattice parameter of α =
(a)
(b)
(c)
d = 0.35 nm
d = 0.567 nm
5 nm
d = 0.35 nm
Fig. 4. TEM images of ZnSe/TiO2 (a) and ZnSe‐GR/TiO2 (b,c).

Lei Zhu et al. / Chinese Journal of Catalysis 35 (2014) 1825–1832
1829
1.5
1.0
0.5
0.0
1.8
1.6
(a)
ZnSe-GR/TiO2
1.4
1.2
1.0
0.8
0.6
0.4
0.2
0.0
Without sample
Nanoscale TiO2
ZnSe/TiO2
ZnSe-GR/TiO2
GR-TiO2
ZnSe/TiO2
TiO2
200
400
600
Wavelength (nm)
800
1000
90
80
70
60
50
40
30
20
10
0
400
500
600
700
(b)
Wavelength (nm)
TiO2
ZnSe/TiO2
Fig. 6. UV‐vis absorption spectra of samples at sorption equilibrium,
showing RhB absorption features.
surface area of ZnSe‐TiO2 was 25.34 m2/g, which was higher
than that of TiO2. GR/TiO2 had the largest surface area and
therefore adsorbed the most RhB. After establishing adsorption
equilibrium, GR‐TiO2 and ZnSe‐GR/TiO2 removed 68% and
55% of the RhB from solution, respectively. TiO2 removed only
11% of RhB.
3.50 3.25 3.00 2.75 2.50 2.25 2.00 1.75 1.50 1.25 1.00
Eg (eV)
Fig. 5. (a) UV‐vis absorption spectra of TiO2, ZnSe/TiO2, and
ZnSe‐GR/TiO2; (b) Variation of (αhv)2 with photon energy (hv) of TiO2
and ZnSe/TiO2.
3.2.2. Sonocatalytic activity
Sonocatalysis is an alternative to photocatalysis for degrad‐
ing pollutants in waste water [32]. The effects of ultrasonic
irradiation on RhB degradation in the presence of TiO2,
GR‐TiO2, ZnSe/TiO2, and ZnSe‐GR/TiO2 were investigated. After
ultrasonic irradiation 150 min, ZnSe‐GR/TiO2 exhibited the
highest degradation of RhB (82.9%), as shown in Fig. 7(a).
GR‐TiO2 and ZnSe/TiO2 degraded 47.3% and 34.1% of RhB,
respectively. Calculated –ln(Ct/Cads) values were approximately
linear with increasing irradiation time, as shown in Fig. 7(b)
and Table 1. The RhB degradation rate constant for
ZnSe‐GR/TiO2 was 10.32 × 10–3 min–1 under ultrasonic irradia‐
tion, which was much higher than those for TiO2, GR‐TiO2, and
ZnSe/TiO2.
Cycling experiments of the sonocatalytic degradation of RhB
in the presence of ZnSe‐GR/TiO2 were conducted to investigate
the stability of the sonocatalytic activity. Figure 8 shows that
ZnSe‐GR/TiO2 exhibited no significant loss in photocatalytic
activity after four successive RhB degradations. This indicates
that ZnSe‐GR/TiO2 was stable and could not be photocorroded
during the photocatalytic oxidation of RhB. Thus, ZnSe‐GR/TiO2
is a promising sonocatalyst for environmental purification. The
modification of graphene improved the photocatalytic perfor‐
mance and increased the stability of ZnSe and TiO2 nanocrys‐
gray‐black. Therefore, the quantitative consideration is void
between TiO2, ZnSe/TiO2, and ZnSe‐GR/TiO2. The enhanced
light‐harvesting efficiency of GR‐TiO2 could have arisen from
chemical bonding between TiO2 and GR. Ti–O–C bonding could
have facilitated charge transfer upon excitation of ZnSe and
TiO2 [33,34].
The DRS spectra of TiO2 and ZnSe/TiO2 were transformed
using the Kubelka‐Munk function of the measured reflectance:
K = (1 – R)2/2R = F(R)
where K is the transformed reflectance, R is the reflectance (%),
and F(R) is the remission or Kubelka‐Munk function. The band
gap (Eg) and absorption coefficient (α) were related by:
αhv = A (hv – Eg)1/2
where v and A are the frequency and a constant, respectively. If
the compound scattered in a perfectly diffuse manner, then K =
2α. The following expression then applies:
[F(R)hv]2 = A(hv – Eg)
The Eg of ZnSe/TiO2 was estimated to be 2.5 eV, which was
red‐shifted from the typical Eg of TiO2 (3.25 eV) in Fig. 5(b).
3.2. Degradation of RhB
Table 1
3.2.1. Adsorption ability
Sonodegradation rate (kapp) constants and BET surface areas of TiO2,
To evaluate the adsorption ability of the composite, the re‐
actor was placed on a magnetic churn dasher and stirred for 30
min in the dark to establish sorption equilibrium, as shown in
Fig. 6. The amount of RhB adsorbed by ZnSe‐GR/TiO2 was
higher than that adsorbed by TiO2. This was attributed to the
larger surface area of ZnSe‐GR/TiO2, as shown in Table 1. The
GR‐TiO2, ZnSe/TiO2, and ZnSe‐GR/TiO2.
Sample
BET surface area (m2/g)
kapp (min–1)
6.95 × 10–4
2.82 × 10–3
4.22 × 10–3
10.32 × 10–3
TiO2
11.59
25.34
51.95
49.63
ZnSe/TiO2
GR‐TiO2
ZnSe‐GR/TiO2

1830
Lei Zhu et al. / Chinese Journal of Catalysis 35 (2014) 1825–1832
2.0
(a)
(b)
1.0
0.8
0.6
0.4
0.2
0.0
Ultrasonic
Ultrasonic
Nanoscale TiO2
ZnSe/TiO2
ZnSe-GR/TiO2
GR-TiO2
Nanoscale TiO2
1.5
1.0
0.5
0.0
ZnSe/TiO2
GR-TiO2
ZnSe-GR/TiO2
-120
0
30
60
90
120
150
0
20
40
60
80
100 120 140 160
Ultrasonic irradiation time (min)
Ultrasonic irradiation time (min)
Fig. 7. Removal of RhB (a) and –ln(Ct/Cads) plots (b) with increasing ultrasonic irradiation time in the presence of TiO2, ZnSe/TiO2, GR‐TiO2, and
ZnSe‐GR/TiO2.
tals. This is beneficial for practical application, as the enhanced
photocatalytic activity and suppressed catalyst deactivation
will lead to more cost‐effective operation.
spots pyrolyze H2O to form •OH , as shown in Reaction (1) [37].
Sonoluminescence involves intense UV light. TiO2 particles are
excited and act as a photocatalyst during sonication. Usually
sonochemical reaction pathways for the degradation of organic
compounds by the sonolysis of water as the solvent inside the
collapsing cavitation bubbles under extremely high tempera‐
ture. In the presence of a sonocatalyst, ultrasound results in the
sonolysis of water coupled with catalysts producing elec‐
tron‐hole pairs, as shown in Reaction (2). Electron‐hole pairs
3.2.3. Generation of ROS
Figure 9(a) shows the UV‐vis absorption spectra of 1,5‐di‐
phenyl carbazone (DPCO)‐containing extracts from ZnSe‐GR/
TiO2 sonocatalysis experiments. DPCI was oxidized to DPCO.
Electrons were excited from the valence to conduction bands
upon ultrasonic irradiation. Electron‐hole pairs formed on the
ZnSe‐GR/TiO2 surface. Electrons and holes reacted with dis‐
produce •OH and •O2 , which degrade the dyes to CO2, H2O, and
−
•
−
•
solved O2 and adsorbed H2O to produce O2 and OH, respec‐
1.0
•
(a)
tively. OH then oxidized DPCI to DPCO. DPCO was extracted
(4)
(1) Sonocatalyst + ultrasonic (0 min)
0.8
with benzene, and its absorbance at 560 nm was detected. The
(2) ZnSe-GR/TiO2 + ultrasonic (60 min)
(3) ZnSe-GR/TiO2 + ultrasonic (90 min)
(4) ZnSe-GR/TiO2 + ultrasonic (120 min)
•
production of OH was also detected. The DPCO absorbance
increased with increasing time [35]. Figure 9(b) shows the
UV‐vis absorption spectra of DPCO‐containing extracts in the
presence of TiO2 under ultrasonic irradiation. The results show
that ZnSe‐GR/TiO2 was a more efficient sonocatalyst.
The sonocatalytic degradation of dyes in the presence of
TiO2 has been widely reported. Dye oxidation depends on •OH
production [36] according to a mechanism involving hot spots
and sonoluminescence. Cavitation first increases from the nu‐
cleation of bubbles, resulting in hot spots in solution. These hot
0.6
(3)
0.4
0.2
(2)
(1)
0.0
1.0
(b)
(1) Sonocatalyst + ultrasonic (0 min)
(2) Pure TiO2 + ultrasonic (60 min)
(3) Pure TiO2 + ultrasonic (90 min)
(4) Pure TiO2 + ultrasonic (120 min)
0.8
0.6
0.4
0.2
0.0
1.0
(4)
1st run
2nd run
3th run
4th run
0.8
0.6
0.4
0.2
0.0
(3)
(2)
(1)
300
400
500
600
700
0
90
180
Time (min)
270
360
Wavelength (nm)
Fig. 9. UV‐vis absorption spectra of DPCO‐containing extracts from
ultrasonic irradiation in the presence of ZnSe‐GR/TiO2 (a) and TiO2 (b)
with increasing time.
Fig. 8. Successive sonocatalytic degradations of RhB by ultrasonic irra‐
diation in the presence of ZnSe‐GR/TiO2.

Lei Zhu et al. / Chinese Journal of Catalysis 35 (2014) 1825–1832
1831
ZnSe-graphene/TiO2
-
O2
·
O2
H2O/OH-
Ultrasonic
irradiation
OH
·
RhB
ROS
Fig. 10. Proposed mechanism for the sonocatalytic degradation of RhB and generation of reactive oxygen species (ROS) by ZnSe‐GR/TiO2.
inorganic compounds, as shown in Reactions 3–5 [38].
H2O + ultrasound → •OH + •H
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Graphical Abstract
Chin. J. Catal., 2014, 35: 1825–1832 doi: 10.1016/S1872‐2067(14)60158‐3
Rhodamine B degradation and reactive oxygen species
generation by a ZnSe‐graphene/TiO2 sonocatalyst
ZnSe-graphene/TiO2
Lei Zhu, Sun‐Bok Jo, Shu Ye, Kefayat Ullah, Won‐Chun Oh*
Hanseo University, Korea
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O2
·
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Ultrasonic
irradiation
H2O/OH-
OH
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ZnSe‐graphene/TiO2 was hydrothermally prepared, and its degra‐
dation of rhodamine B and generation of reactive oxygen species
(ROS) under ultrasonic irradiation were investigated.
ROS

1832
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