Comparative study of the photocatalytic performance for the degradation of different dyes by ZnIn2S4: adsorption, active species, and pathways

Article abstract of DOI:10.1039/c7ra00199a

A comparative study of the photocatalytic performance for the degradation of rhodamine B (RhB) and methyl orange (MO) by ZnIn₂S₄ under visible light irradiation was investigated based on the adsorption of dyes, the active species generated during the photocatalytic process and the degradation pathway. The results show that 97.8% of RhB and 5.6% of MO were degraded under the same conditions, respectively. Photocatalytic degradation of RhB was obviously superior to degradation of MO. The adsorption of MO was almost negligible and 56.8% of RhB was adsorbed on ZnIn₂S₄. Superoxide radical was the key active species, and hydroxyl radical played a supplementary role during photocatalytic process. Different chemical bonds with different bond energies were destroyed when degradation of RhB and MO. Photocatalytic degradation of RhB was superior to that of MO because the excellent adsorptivity of RhB than that of MO, and the C=N bond of RhB was easier destroyed compared with azo bond of MO under the same conditions. The photocatalytic degradation of MO in a system in which superoxide radical was key active species can be improved by adding hole scavengers to inhibit recombination of holes and electrons, which would result in more electrons reducing oxygen to superoxide radical.

Full text of DOI:10.1039/c7ra00199a

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Comparative study of the photocatalytic
performance for the degradation of different dyes
by ZnIn2S4: adsorption, active species, and
pathways†
Cite this: RSC Adv., 2017, 7, 12292
*
Tingting Liu, Lei Wang, Xue Lu, Jiamin Fan, Xinxin Cai, Bo Gao, Rui Miao,
Jiaxuan Wang and Yongtao Lv
A comparative study of the photocatalytic performance for the degradation of rhodamine B (RhB) and
methyl orange (MO) by ZnIn2S4 under visible light irradiation was investigated based on the adsorption of
dyes, the active species generated during the photocatalytic process and the degradation pathway. The
results show that 97.8% of RhB and 5.6% of MO were degraded under the same conditions, respectively.
Photocatalytic degradation of RhB was obviously superior to degradation of MO. The adsorption of MO
was almost negligible and 56.8% of RhB was adsorbed on ZnIn2S4. Superoxide radical was the key active
species, and hydroxyl radical played a supplementary role during photocatalytic process. Different
chemical bonds with different bond energies were destroyed when degradation of RhB and MO.
Photocatalytic degradation of RhB was superior to that of MO because the excellent adsorptivity of RhB
than that of MO, and the C]N bond of RhB was easier destroyed compared with azo bond of MO under
the same conditions. The photocatalytic degradation of MO in a system in which superoxide radical was
key active species can be improved by adding hole scavengers to inhibit recombination of holes and
electrons, which would result in more electrons reducing oxygen to superoxide radical.
Received 6th January 2017
Accepted 16th February 2017
DOI: 10.1039/c7ra00199a
rsc.li/rsc-advances
completely mineralized into CO2 and H2O during photocatalytic
process.8
1. Introduction
A large number of researches have examined the develop-
ment of photocatalysts that can be excited under visible light
since ultraviolet (UV) light accounts for only 7% of sunlight,
while visible light accounts for more than 45%.9–12 For example,
methylene blue could be degraded by WO3 under visible light.13
Priya et al. synthesized Bi2O3/BiOCl which could effectively
mineralized antibiotics under solar light.14 Among the visible-
light-driven photocatalysts, ZnIn2S4 has received more atten-
tion because of its excellent photocatalytic activity. Fang et al.
studied the effect of reaction temperature, reaction time and
solvent on the formation of ZnIn2S4 and photocatalytic activity
for the degradation of methyl blue.15 Shen et al. synthesized
ZnIn2S4 which could effectively reduce H2O to H2 via a hydro-
thermal method based on cetyltrimethylammonium bromide.16
During the photocatalytic process, organics were degraded
through direct hole oxidation and indirect free radical oxida-
tion.17 The photocatalyst was excited when it was irradiated with
incident light whose energy was greater than or equal to the
band gap energy of the semiconductor photocatalyst. The
electrons in the valence band could transit to the conduction
band to form electron–hole pairs. The photogenerated electrons
and holes take advantage of water, hydroxides (OHꢀ) and
oxygen (O2) in the environment to form free radicals that can
Organic synthetic dyes have been used widely in many elds
such as the textile, food, cosmetic and pharmaceutical indus-
tries. In terms of their chemical structure, organic synthetic
dyes can be divided into different categories; e.g., azo, anthra-
quinone, xanthene, etc.1 Xanthene dyes are common synthetic
dyes, which are oen used as laser materials and color addi-
tives. Azo dyes are characterized by the presence of the azo bond
(–N]N–) in their molecules and account for about 70% of
synthetic dyes.2 Organic synthetic dyes have excellent stability,
which results in large amounts of wastewater containing dyes
being released into the environment.3 The color of wastewater
containing dyes can inhibit photosynthesis of aquatic plants
and cause deterioration of water quality. Carcinogenic and toxic
effects of dyes are harmful to human beings and aquatic life.4–6
As an advanced oxidation process, photocatalytic oxidation
has received increasing attention because of its high efficiency
and low energy consumption.7 Moreover, photocatalytic tech-
nology is harmless to environment because the organics can be
Key Laboratory of Membrane Separation of Shaanxi Province, Xi'an University of
Architecture and Technology, Yan Ta Road, No. 13, Xi'an 710055, China. E-mail:
wl0178@126.com; Fax: +86 02982202729; Tel: +86 02982202729
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c7ra00199a
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effectively mineralize organics into CO2. Electrons and holes dried at 60 ꢁC for photocatalytic reaction. The properties of ZnIn2S4
can also recombine during the photocatalytic process, resulting have been adequately characterized in previous literatures.21,22
in a reduction of the photocatalytic degradation efficiency.
Generally, the holes and the active species generated during
the photocatalytic process are nonselective, and the organics
can be degraded as long as the redox potential of the holes
and the active species are higher than that of the organics.18
However, a large number of studies have shown that the same
photocatalyst may perform differently for different organics.
The degradation of MO by Ag3PO4 dodecahedrons under acidic
condition was signicantly superior to that of RhB, but the
adsorption of RhB was better than that of MO.19 Chen et al.
prepared hexagonal and cubic ZnIn2S4 with different raw
materials, which had different photocatalytic performances and
2.3 Characterization
The specic surface area of the as-prepared ZnIn2S4 was deter-
mined by V-sorb 2800P BET analyzer (Gold APP Instrument
Corporation, China). The chemical composition of the as-
prepared ZnIn2S4 was characterized by K-Alpha X-ray photoelec-
tron spectroscopy (Thermo Scientic, USA). UV-Vis absorption
spectra of the as-prepared ZnIn2S4 was measured using U-4100
UV-Vis spectrophotometer (Hitachi, Japan) from 300 to 1000 nm.
2.4 Photocatalytic experiment
mechanisms for degradation of rhodamine B (RhB) and methyl The photocatalytic experiments were conducted in a CEL-WLAX
orange (MO).20 Therefore, it is necessary to explore the mecha- visible-light photochemical system (China Education Au-light,
nism of different photocatalytic performance for different China). A 500 W xenon lamp emitting 300–900 nm visible light
organics with the same photocatalyst.
was installed horizontally on top of the box, and a magnetic
In this study, the different performance for the adsorption stirrer was placed below the lamp. The distance between the
and degradation of RhB and MO by ZnIn2S4 were investigated. lamp and the magnetic stirrer was 0.6 m. In the reaction for the
The active species in the system of ZnIn2S4 photocatalytic degradation of the RhB, 0.04 g ZnIn2S4 was added into 400 mL
degradation of dyes were identied by EPR and in situ capture RhB solution (initial concentration was 15 mg Lꢀ1). In the reac-
experiments. Intermediates were identied by liquid chroma- tion for the degradation of the MO, 0.04 g or 0.12 g ZnIn2S4 was
tography mass spectrometry-ion trap-time of ight (LCMS-IT- added into 400 mL MO solution (initial concentration was 15 mg
TOF), and the pathways for photocatalytic degradation of RhB Lꢀ1 or 5 mg Lꢀ1, respectively). Before irradiation, the suspensions
and MO were proposed. The results of the adsorption perfor- were stirred magnetically for a period of time in darkness to
mance and the active species generated during the photo- establish an adsorption/desorption equilibrium. Samples were
catalytic process were combined with the pathway of ZnIn2S4 taken at certain time intervals and ltrated with 0.22 mm ltrater
photocatalytic degradation of dyes to obtain the mechanism for to remove the ZnIn2S4 for analysis.
achieving the different photocatalytic performance for degra-
The absorbance of RhB and MO was measured by UV-2600
dation of different dyes. The conclusions may provide a theo- UV-Vis spectrophotometer (Unico, China) at wavelengths of
retical basis for improved degradation of dyes.
553 nm and 464 nm, respectively. Eight standard solutions were
prepared to generate a calibration curve in the range of 0.1–
25 mg Lꢀ1, and the concentrations of RhB and MO in the irra-
diated solution were determined from the calibration curves.
The photocatalytic degradation efficiency of the dye was evalu-
ated with C/C0, where C was the concentration of dye at each
irradiated time interval, and C0 was the initial concentration of
the dye. The UV-Vis spectra of the degradation solution at each
irradiated time interval were monitored by spectrophotometer
under full scan mode. Zeta potentials of ZnIn2S4 was measured
by zeta analyzer (Malvern, UK).
2. Materials and method
2.1 Materials and reagents
Zn(NO3)2$6H2O (AR) was purchased from Sinopharm Chemical
Reagent Co., Ltd. In(NO3)2$5H2O (AR), thioacetamide (TAA, AR),
RhB (AR), MO (AR) and ethylene diaminetetraacetic acid (EDTA,
AR) were obtained from Tianjin Kemiou Chemical Reagent
Co., Ltd. Isopropyl alcohol (AR) and benzoquinone (99%) was
purchased from the Aladdin Industrial Corporation. 5-tert-
Butoxycarbonyl 5-methyl-1-pyrroline N-oxide (BMPO) and super-
oxide dismutase (SOD) were purchased from Dojindo Molecular
Technologies, Inc and Sigma Aldrich Co., LLC, respectively. All
chemicals were used without further purication.
2.5 In situ capture experiment
In situ capture experiments were conducted to investigate the
active species generated during the ZnIn2S4 photocatalytic
process. EDTA, benzoquinone and isopropyl alcohol were used
as scavengers introduced into the photocatalytic process to
capture holes, superoxide radical (cO2ꢀ) and hydroxyl radical
(cOH), respectively. The concentrations of the EDTA and ben-
zoquinone were both 1 mmol Lꢀ1, and 0.1 mL isopropyl alcohol
was added into 400 mL reaction solution.
2.2 Synthesis of ZnIn2S4
ZnIn2S4 was prepared in accordance with previous literature as
follows:21 0.75 mmol Zn(NO3)2$6H2O, 1.5 mmol In(NO3)2$5H2O
and excessive TAA (12 mmol) were dissolved in 150 mL deionized
water. The mixture was stirred for 10 min and transferred into
a 200 mL Teon-lined autoclave. The autoclave was put into
a vacuum oven and kept at 80 ꢁC. Aer 6 h, the reaction was
2.6 Electron paramagnetic resonance spectroscopy
completed, and the autoclave was cooled to room temperature. For detection of free radicals, EPR (Bruker, Germany) was used
The sample was washed several times with water and ethanol, and to record signals of spin adducts that were produced by active
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free radicals reacting with BMPO under visible light irradiation.
Differences in the performance with respect to adsorption
BMPO was used as a scavenger to capture cO2ꢀ and cOH to form and degradation of RhB and MO by ZnIn2S4 are apparent.
relatively stable adducts, and the types of free radical were Adsorption of MO by the ZnIn2S4 was almost negligible and
distinguished according to the EPR spectra of the adducts. 56.8% of RhB was adsorbed on ZnIn2S4 in darkness. Photo-
Superoxide dismutase (SOD) is an enzyme that can catalyze catalytic degradation of RhB was obviously superior to the
cO2ꢀ, but it did not react with cOH.
degradation of MO. In 90 min of photocatalytic reaction, 97.8%
According to different experimental conditions, ZnIn2S4 of the RhB was removed whereas only 5.6% of the MO was
(0.1 g Lꢀ1 or 0.3 g Lꢀ1), BMPO (40 mM), RhB (15 mg Lꢀ1), MO degraded under the same conditions.
(5 mg Lꢀ1) and SOD (400 U mLꢀ1) were mixed together and
Generally speaking, the greater specic surface area repre-
irradiated under visible light for 4 min. Each sample was then sents the larger adsorption capacity of organic molecules, which
moved into a capillary and sealed. The capillary was put into lead to superior photocatalytic performance. Nitrogen adsorp-
a quartz tube, which was inserted in the EPR cavity. The EPR tion–desorption isotherms and the pore size distribution plot of
should be tuned before measurement. The EPR parameters for ZnIn2S4 are shown in Fig. A.1.† The specic surface area of as-
all samples were as follows: center eld ¼ 3510 G, sweep width prepared ZnIn2S4 is 79.03 m2 gꢀ1 and the type of isotherms
¼ 100 G, microwave frequency ¼ 9.85 GHz, microwave power ¼ indicates that greater uniform mesopore distribution. There-
6.325 mW, modulation frequency ¼ 100 kHz.
fore, excellent adsorption capacity of RhB was attributed to
large specic surface area of ZnIn2S4. However, specic surface
area of ZnIn2S4 is constant for adsorption of RhB and MO,
hence, it is necessary to explore the reason of different
adsorption performance for RhB and MO with ZnIn2S4.
2.7 LCMS-IT-TOF analysis
LCMS-IT-TOF (Shimadzu, Japan) was used to identify interme-
diates formed during the photocatalytic process. The separation
column was a Thermo HPLC Betasil C18 column (2.1 mm i.d. ꢂ
150 mm, 3.5 mm particles), and the column temperature was set
at 40 ꢁC. The mobile phases were methanol : water ¼ 63 : 35 (v/
v) for RhB detection and acetonitrile : 0.1% formic acid ¼
25 : 75 (v/v) for MO detection. The ow rate was maintained at
0.3 mL minꢀ1, and the injection volume was 20 mL. RhB was
detected at wavelengths of 553 nm and 500 nm under dual-
channel mode, and MO was detected at a wavelength of
464 nm. MS spectrometry was operated in positive and negative
mode for RhB and MO analyses, respectively.
Because RhB is a cationic dye and MO is an anionic dye, the
different surface charge properties of RhB and MO may have
affected the adsorptivity of the photocatalyst, which resulted in
different photocatalytic performance in the degradation of
different types of dyes.23,24 The pH of solutions of RhB and MO
were 4.81 and 5.25, respectively. The acid dissociation exponent
of the carboxyl group of RhB is 4.1 and pKa of MO is 3.4.25
Therefore, carboxyl group of RhB dissociated and presented in
system at negative charged –COOꢀ state and MO was negative
charged under investigated condition. The zeta potentials of
ZnIn2S4 was measured using a zeta analyzer (Malvern, UK) as
ꢀ35.16. The results show that the RhB, MO and ZnIn2S4 were
negatively charged under the investigated conditions and the
surface charge properties of RhB and MO were not key factors
that affected the adsorption performance of RhB and MO.
The differences in photocatalytic performance for dye
degradation based on the structure of ZnIn2S4 were examined.
The results show that ZnIn2S4 was composed of nanolamella
petals growing in the ab plane, which was favorable for
adsorption of RhB via N(Et)2 groups, whereas similar functional
groups were absent in MO.20 Therefore, adsorption of RhB on
ZnIn2S4 was better than that of MO. Above all, the differences in
adsorption performance of ZnIn2S4 for RhB and MO further
resulted in different photocatalytic performance.
3. Results and discussion
3.1 Degradation of RhB and MO by ZnIn2S4
The variation of C/C0 of RhB and MO as a function of time under
different conditions is shown in Fig. 1. The results include
adsorption of RhB and MO on ZnIn2S4 in darkness and the
photocatalytic degradation in the presence of ZnIn2S4 under
visible light irradiation.
Fig. 2 shows the variation of C/C0 of MO as a function of
irradiation time under different photocatalyst dosage and
concentration of MO. The results show that the photocatalytic
degradation efficiency of the MO increased with decreasing MO
concentration and increasing ZnIn2S4 dosage. Thus, 51.3% of
MO was removed when the initial concentration of the MO
decreased to 5 mg Lꢀ1 and the ZnIn2S4 dosage increased to 0.3 g
Lꢀ1. By increasing the ZnIn2S4 dosage and decreasing the MO
concentration, more photocatalysts could be excited and more
active species were generated, which was helpful for degrading
MO. At the same time, degradation of the MO was improved
because the amount of MO in the system declined. To investi-
gate the degradation behavior of MO, the concentration of the
Fig. 1 Degradation of RhB and MO by adsorption and photocatalysis.
The concentration of RhB and MO was 15 mg Lꢀ1 and the dosage of
ZnIn2S4 was 0.1 g Lꢀ1
.
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ꢀ
parameters of the standard spectra of BMPO/cO2 and BMPO/
cOH and the simulated spectrum are listed in Table A.1.†
Comparing the hyperne splitting parameters of the standard
spectrum and the simulated spectrum, results show that EPR
ꢀ
spectrum (b) is obtained from BMPO/cO2 and BMPO/cOH. In
other words, cO2ꢀ and cOH are simultaneously generated during
the ZnIn2S4 photocatalytic process.
The characteristic spectra (c) and (d) that were obtained from
the mixtures of BMPO, ZnIn2S4 and RhB or MO irradiated under
visible light, are the same as EPR spectrum (b). The results
demonstrate that species of free radicals did not change when
dyes were added into the system. However, the intensities of
spectrum (c) and spectrum (d) decreased dramatically in
comparison with spectrum (b) because of free radicals that were
consumed during the process of photocatalytic degradation of
the dyes.
Fig.
2 Photocatalytic degradation of MO with different ZnIn2S4
dosage (0.1 g Lꢀ1 or 0.3 g Lꢀ1) and initial concentration of MO
(15 mg Lꢀ1 or 5 mg Lꢀ1).
Aer addition of 400 U mLꢀ1 SOD, the EPR signals were
completely suppressed, indicating the absence of O2ꢀ and cOH.
MO was 5 mg Lꢀ1 and the ZnIn2S4 dosage was 0.3 g Lꢀ1 in the
subsequent experiments.
ꢀ
The results further demonstrate the generation of cO2 during
the photocatalytic process. Moreover, cOH in the system were
generated through cO2ꢀ reacting with H+. Thus, the generation
of cOH was inhibited aer addition of SOD, which could cata-
3.2 Determination of active species
3.2.1. Identication of free radicals by EPR. Fig. 3 shows
the EPR spectra obtained using different solutions including
the following: (a) BMPO solution was irradiated under visible
light for 4 min, (b) the mixture of BMPO and ZnIn2S4 was irra-
diated under visible light for 4 min, (c) and (d) mixtures of
BMPO, ZnIn2S4 and RhB or MO respectively were irradiated
under visible light for 4 min and (e) the mixture of BMPO,
ZnIn2S4 and SOD was irradiated under visible light for 4 min.
No EPR signals were observed in the case of the BMPO solution
(a). Aer visible light irradiation of mixture (b) with ZnIn2S4
present, a four-line spectrum with relative intensities approxi-
mating 1 : 2 : 2 : 1 was recorded by EPR.
ꢀ
lyze cO2 via disproportionation reaction.
3.2.2. Identication of active species by in situ capture
experiments. Fig. 4 shows the variation of C/C0 of RhB as
a function of irradiation time before and aer scavengers were
added into the photocatalytic system. The role of the active
species was determined through the variation of C/C0 of the dyes
aer the scavengers were added into the photocatalytic system.
Only 57.4% of the RhB was degraded aer 90 min when
benzoquinone was added into the system, and the degradation of
the RhB was obviously inhibited, when compared with 97.8% of
the RhB that was degraded in the absence of scavengers.
Degradation of the RhB was slightly suppressed with the addition
of isopropyl alcohol. When EDTA was added into the system, the
concentration of the RhB decreased quickly, and 97.3% of the
RhB was removed in 60 min of photocatalytic process. Degrada-
tion of the RhB was enhanced when EDTA was added into
the system. Similar results were obtained aer scavengers were
The characteristic spectrum of adduct formed between BMPO
ꢀ
and O2 (BMPO/cO2ꢀ) was a four-line spectrum with relative
intensities of 1 : 1 : 1 : 1, and the characteristic spectrum of the
adduct formed between BMPO and cOH (BMPO/cOH) was a four-
line spectrum with relative intensities of 1 : 2 : 2 : 1.26 The EPR
spectrum (b) of the mixture of BMPO and ZnIn2S4 was simulated
using xenon (Bruker, Germany) based on g factor ¼ 2.007. The
Fig. 4 Photocatalytic degradation of RhB by ZnIn2S4 under visible light
Fig. 3 EPR spectra obtained using different solutions. (a) BMPO; (b) irradiation in the presence of scavengers: EDTA, benzoquinone, and
BMPO + ZnIn2S4; (c) BMPO + ZnIn2S4 + RhB; (d) BMPO + ZnIn2S4
MO; (e) BMPO + ZnIn2S4 + SOD.
+
isopropyl alcohol were used to capture holes, cO2ꢀ, and cOH,
respectively.
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added into the system of photocatalytic degradation of MO by X ¼ (4.24 ꢂ 3.073 ꢂ 6.234.8)1/(1 + 3 + 4.8) ¼ 4.69 eV; ECB ¼ 4.69 ꢀ
ZnIn2S4 under visible light irradiation (Fig. A.2†).
4.5 ꢀ 0.5 ꢂ 2.05 ¼ ꢀ0.835 eV; EVB ¼ ꢀ0.835 + 2.05 ¼ 1.215 eV.
ꢀ
The results indicate that a large number of cO2 were
Holes in the valence band of ZnIn2S4 could not oxidize H2O
generated when ZnIn2S4 was irradiated under visible light and and OHꢀ to cOH because the potential of valence band (1.215
played an important role in the degradation of the dyes. eV) was more negative than the potential of cOH/OHꢀ (1.99 eV)
Therefore, degradation of the dyes was signicantly inhibited and cOH/H2O (2.27 eV). However, the potential of the conduc-
ꢀ
because of the addition of benzoquinone to capture cO2
.
tion band (ꢀ0.835 eV) was more negative than the potential of
ꢀ
Degradation of the dyes was slightly suppressed when isopropyl O2/cO2 (ꢀ0.28 eV), and O2 adsorbed on the surface of the
alcohol was ꢀadded because of a small amount of cOH generated photocatalyst could be reduced to cO2ꢀ as electronic acceptors.
ꢀ
through cO2 reacting with H+.
cO2 reacted with H2O and electrons to form H2O2, which was
Under visible light irradiation, electrons in the valence band further broken down into OH.
transited to the conduction band and formed electron–hole
The active species generated during the photocatalytic process
pairs. Excited electrons could also recombine with holes are described in detail as follows. Electrons in the valence band
through direct recombination and surface recombination, transited to the conduction band to form photogenerated hole–
which resulted in reduced efficiency of the photocatalytic electron pairs when the ZnIn2S4 was excited under visible light
reaction. The recombination of electrons and holes was obvi- (4). Electrons reacted with O2 adsorbed on the surface of the
ously restrained when EDTA was added into the system as ZnIn2S4 to generate cO2ꢀ (5). Peroxide hydroxyl radical (HO2) was
a scavenger to capture holes. At the same time, more electrons generated through cO2 reacting with H+ and decomposed intꢀo
ꢀ
migrated to the surface of the photocatalyst and reacted with O2 hydrogen peroxide (H2O2) (6) and (7), which combined with cO2
to form cO2ꢀ, which could enhance the degradation of the dyes to form cOH (8). Meanwhile, H2O2 also decomposed into cOH by
by ZnIn2S4 under visible light irradiation.
adsorbing energy from the visible light (9).
Generally, it is necessary for a photocatalytic reaction that
the potential of the conduction band is more negative than the
potential of the acceptor, and the potential of the valence band
is more positive than the potential of the donor. The potential of
conduction band (CB) and valence band (VB) can be calculated
according to the eqn (1) and (2) as follows:27
ZnIn2S4 + hn / ZnIn2S4(h+ + eꢀ)
(4)
(5)
(6)
(7)
(8)
(9)
ꢀ
O2 + eꢀ / cO2
cO2 + H+ / HO2c
ꢀ
2HO2c / O2 + H2O2
ECB ¼ X ꢀ Ee ꢀ 0.5Eg
EVB ¼ ECB + Eg
(1)
(2)
H2O2 + cO2 / cOH + OHꢀ + O2
ꢀ
H2O2 + hn / 2cOH
where ECB and EVB are the potential of conduction band and
valence band, respectively; X is the electronegativity of the
semiconductor, expressed as the geometric mean of the elec-
tronegativity of the constituent atoms; Ee is about 4.5 eV, Eg is
the band gap energy of the semiconductor.
UV-Vis diffuse reectance spectra and XPS spectra of ZnIn2S4
are showed in Fig. A.3 and A.4,† respectively. It can be seen that
steep adsorption edge of ZnIn2S4 is about 606 nm and the band
gap is estimated to be 2.05 eV according to the eqn (3)
ꢀ
These results indicate that cO2 is the key active species for
the degradation of RhB and MO by ZnIn2S4 under visible light
irradiation, and cOH played a supplementary role during the
photocatalytic process. Li et al. also found similar results in
photocatalytic inactivation of MS2 by metal-free g-C3N4.28
Yan et al. believe that different active species generated
during the photocatalytic process using the same photocatalyst
result in different photocatalytic performance.29 However, the
active species generated during the photocatalytic process for
the degradation of RhB were the same as those generated
during the photocatalytic degradation of MO. The results indi-
cate that the active species are not the main factors causing the
different photocatalytic performance for the degradation of RhB
and MO by ZnIn2S4. Moreover, the photocatalytic degradation
of MO could be improved when EDTA was added into the
system as a scavenger to capture holes that inhibit the recom-
bination of holes and electrons.
Eg ¼ 1240/lg.
(3)
The as-prepared photocatalyst in this study is composed of Zn,
In and S, and the molar ratio of Zn : In : S is 1 : 3 : 4.8 from
XPS analysis. The parameters used for calculating potential of
conduction band and valence band are listed in Table 1. Therefore,
Table 1 The parameters used for calculating potential of conduction
band and valence band
Element
electronegativity (eV)
Band
gap (eV)
3.3 Identication of intermediates
Element
Molar ratio
3.3.1. Identication of intermediates generated during the
degradation of RhB. The temporal UV-Vis spectras of degraded
RhB solution with an initial RhB concentration of 15 mg Lꢀ1
and a ZnIn2S4 dosage of 0.1 g Lꢀ1 are illustrated in Fig. 5.
Zn
In
S
4.24
3.07
6.23
1
3
4.8
2.05
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Fig. 5 UV-Vis spectra of degraded RhB solution under visible light
irradiation with an initial RhB concentration of 15 mg Lꢀ1 and a ZnIn2S4
dosage of 0.1 g Lꢀ1
.
It can be seen from Fig. 5 that the absorbance of RhB at
a wavelength of 553 nm obviously decreased as the reaction time
was prolonged, which indicates that RhB was gradually degraded.
Aer 30 min visible light irradiation, the maximum absorption
peak of the solution began to display slight hypsochromic shis
and then moved signicantly from 553 nm to 500 nm in 90 min.
This phenomenon resulted from the formation of a series of N-
deethylated intermediates during the initial stages of photo-
catalytic degradation of the RhB in the presence of ZnIn2S4,
under visible light. Li et al. also found similar phenomena in
the photocatalytic degradation of RhB by synthesized Bi2S3
nanoparticles/platelike Bi2WO6 heterostructures.30
HPLC chromatograms of RhB degraded samples at 0 min and
60 min are shown in Fig. A.5.† Six peaks (I–VI) were recorded at
60 min of photocatalytic process. Mass spectrograms of corre-
sponding intermediates are shown in Fig. 6. Considering the
value of m/z, one of the peaks was RhB (peak I), and the other ve
peaks were identied as N-deethylated intermediates generated
during the initial stages of photocatalytic degradation of RhB by
ZnIn2S4. The conclusions are consistent with results obtained
from hypsochromic shis of maximum absorption peaks of RhB
degraded solution. Information for the N-deethylated interme-
diates (I–VI) is listed in Table 2.
Aer N-deethylation, rhodamine was degraded into inter-
mediate VII (m/z 181.04 (M+)) with cleavage of the carbon–
nitrogen bond (C]N), which was responsible for visible
absorbance at 553 nm for RhB and formation of several
aromatic intermediates such as benzoic acid (VIII, m/z 122.55
(M+)) and phenol (IX, m/z 94.04 (M+)).31 According to the
pathway of photocatalytic degradation RhB by Bi-doped TiO2,
RhB was degraded completely via four processes: N-deethyla-
tion, cleavage of chromophores, rings opening and minerali-
zation.32 Thus, the aromatic intermediates may be degraded
into small molecule organic matters via ring opening with
increased reaction time. The degradation pathway of RhB in
this study is proposed in Scheme 1.
Fig. 6 Mass spectrograms of proposed intermediates generated during
photocatalytic degradation of RhB by ZnIn2S4 under visible light irradiation.
The results above mentioned are well agreement with the
degradation pathway of RhB under UV-LED/TiO2 process.33
Intermediates generated during degradation of RhB by TiO2/
ZnFe2O4 nanocomposite are slightly different from our study. N-
Deethylation leads to the formation of phenyl oxonium inter-
mediates and the cleavage of chromophore leads to the
formation of major phenolic intermediates.34
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Table 2 Proposed N-deethylated intermediates generated during Table 3 Proposed intermediates generated during photocatalytic
photocatalytic degradation of RhB by ZnIn2S4 under visible light degradation of MO by ZnIn2S4 under visible light irradiation
irradiation
Number
m/z (Mꢀ)
Structure
Retention
time
Peak
m/z (M+)
Proposed intermediates
A
290.06
I
II
III
IV
V
7.515
5.490
3.997
2.978
2.314
1.702
443.24
415.20
387.17
387.17
359.14
331.11
Rhodamine B
N,N-Diethyl-N0-ethylrhodamine
N,N-Diethyl-rhodamine
N-Ethyl-N0-ethylrhodamine
N-Ethylrhodamine
B
C
D
276.04
212.98
197.95
VI
Rhodamine
3.3.2. Identication of intermediates generated during the
degradation of MO. The temporal UV-Vis spectra of degraded
MO solution with an initial MO concentration of 5 mg Lꢀ1 and
ZnIn2S4 dosage of 0.3 g Lꢀ1 are illustrated in Fig. A.6.† It can be
seen from the UV-Vis spectra of the degraded MO solution that
two maximum absorption peaks exist at wavelengths of 270 nm
and 464 nm, owing to the benzene ring and azo bond (–N]N–),
which are chromophores of MO.35 The absorbance of degraded
solution at 464 nm was seldom decreased, and the maximum
absorption peak at 270 nm was not changed. The maximum
adsorption peaks did not exhibit hypsochromic shis during
the photocatalytic process.
Information on intermediates formed during photocatalytic
degradation of MO by ZnIn2S4 under visible light irradiation is
listed in Table 3. Mass spectrograms of corresponding inter-
mediates are shown in Fig. A.7.† MO (m/z 304.07 (Mꢀ)) was
degraded into A (m/z 290.06 (Mꢀ)) and B (m/z 276.04 (Mꢀ))
through demethylation.
Then the sulfur carbon bonds (S–C) of intermediates were
destroyed, and hydroxylation of molecules occurred to form C
(m/z 212.98 (Mꢀ)) and D (m/z 195.95 (Mꢀ)). The azo bonds may
be destroyed as reaction time is prolonged, according to other
research.36 The degradation pathway of MO is proposed in
Scheme 2.
3.4 The mechanism of different photocatalytic performance
for the degradation of RhB and MO
The mechanism of different photocatalytic performance for the
degradation of RhB and MO by means of ZnIn2S4 was discussed
in detail based on the results mentioned above.
Adsorption of RhB on ZnIn2S4 was obviously higher than that
of MO because of the specic structure of ZnIn2S4, which was
favorable for adsorption of RhB via N(Et)2 groups; similar
functional groups were absent in MO. The different adsorptivity
of ZnIn2S4 for RhB and MO further resulted in differences in
photocatalytic performance.
According to the pathway of photocatalytic degradation of
dyes, RhB was degraded through N-deethylation and cleavage of
Scheme 1 Proposed degradation pathway of photocatalytic degra- Scheme 2 Proposed degradation pathway of photocatalytic degra-
dation of RhB by ZnIn2S4 under visible light irradiation.
dation of MO by ZnIn2S4 under visible light irradiation.
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the C]N bond, which is a chromophore of RhB. Degradation of other hand, the bond energy of the azo bond of MO was higher
MO was achieved through demethylation and cleavage of the than the bond energy of the C]N bond of RhB. Thus, it is easier
azo bond.
for cO2ꢀ and cOH to destroy the C]N bond of RhB than the azo
Chemical bonds with different bond energies are destroyed bond of MO under the same conditions, and the subsequent
during degradation of dyes. It is more difficult to destroy a bond process of degradation of RhB is easier to carry out. The pho-
ꢀ
with a higher bond energy. Carbon–carbon bonds (C–C) and tocatalytic degradation of MO in a system in which cO2 is key
carbon–nitrogen bonds (C–N) were destroyed during N-deethy- active species can be improved by adding hole scavengers to
lation of RhB and demethylation of MO, respectively. The inhibit recombination of holes and electrons, which would
ꢀ
processes of N-deethylation and demethylation could be carried result in more electrons reducing O2 to cO2
out successfully because the bond energy of C–C was almost the
.
same as that of C–N.
Acknowledgements
However, the C]N bond of RhB has a bond energy of about
407 kJ molꢀ1, while the bond energy of the azo bond of MO is This work was supported by the National Natural Science
about 518 kJ molꢀ1 37 Therefore, it is easier for cO2ꢀ and cOH to Foundation of China [Grant No. 51278408]; Shanxi Province
.
destroy the C]N bond of RhB than the azo bond of MO under Science and Technology Youth Star Project [Grant No.
the same conditions, and the subsequent process of degrada- 2014KJXX-65] and the innovative research team of Xi'an
tion of RhB is easier to carry out. Thus, photocatalytic degra- University of Architecture and Technology.
dation of RhB is superior to the degradation of MO under the
same conditions. Above all, the differences in photocatalytic
performance of ZnIn2S4 for the degradation of RhB and MO
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