Carbon quantum dot decorated hollow In2S3 microspheres with efficient visible-light-driven photocatalytic activities

Article abstract of DOI:10.1039/c6ra01348a

Carbon quantum dot (CQDs) decorated hollow In₂S₃ microspheres were firstly synthesized by a facile hydrothermal method. CQDs with an average size of 5 nm were attached on the surfaces of hollow In₂S₃ microspheres. The photocatalytic activities of the as-prepared samples were investigated by the photocatalytic degradation of methyl orange under visible light, and the 3 wt% CQDs/In₂S₃ sample presented the most efficient photocatalytic activity which was almost 3 times the pure In₂S₃ sample. On the basis of the active species trapping experiment and ESR analysis, holes and superoxide radicals were proved to be the main active species in the photocatalytic degradation process, and a possible reaction mechanism was proposed.

Full text of DOI:10.1039/c6ra01348a

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Carbon quantum dots decorated hollow In S microspheres with
2
3
efficient visible-light-driven photocatalytic activities
a
b
c
a
a
a
Changyou Huang , Yuanzhi Hong , Xu Yan , Lisong Xiao , Kai Huang , Wei Gu ,
d
a,*
Kuili Liu , Weidong Shi
a
School of Chemistry and Chemical Engineering, Jiangsu University, Zhenjiang,
2
12013, P. R. China
b
School of Materials Science and Engineering, Jiangsu University, Zhenjiang, 212013,
P. R. China
c
School of Energy and Power Engineering, Jiangsu University, Zhenjiang, 212013, P.
R. China
d
Department of Physics and Electronic Engineering, Zhoukou Normal University,
Zhoukou, 466001, P. R. China
*
Corresponding author: Tel.: +86 511 8879 0187
Fax: +86 511 8879 1108
Eꢀmail address: swd1978@ujs.edu.cn (W. Shi)
1

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Abstract
2 3
Carbon quantum dots (CQDs) decorated hollow In S microspheres were firstly
synthesized by a facile hydrothermal method. CQDs with the average size of 5 nm
were attached on the surfaces of hollow In S microspheres. The photocatalytic
2
3
activities of the asꢀprepared samples were investigated by the photocatalytic
degradation of methyl orange under visible light, and the 3wt% CQDs/In sample
presented the most efficient photocatalytic activity which was almost 3 times of the
pure In sample. On the basic of the active species trapping experiment and ESR
2 3
S
2 3
S
analysis, holes and superoxide radicals were proved to be the main active species in
the photocatalytic degradation process, and a possible reaction mechanism was
proposed.
Keywords: CQDs, In
2
3
S , hollow microsphere, methyl orange
2

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1
. Introduction
In recent years, semiconductor photocatalytic technology has drawn increasing
attentions owing to its efficient applications in solving the environmental and energy
1
ꢀ3
issues all over the world.
Currently, the development of visibleꢀlightꢀdriven
photocatalysts has become a new trend, since the efficiently utilization of the
visibleꢀlight region which occupies the largest proportion of the incoming solar
4,5
spectrum. Meanwhile, metal sulfides have been extensively investigated and proven
6
,7
to be a group of highly efficient catalysts for photochemical reactions. Typically,
indium sulfide (In S ), an IIIꢀVI group sulfide, has attracted special interest in
2
3
8
ꢀ11
photoelectron chemical and photocatalysis.
2 3 2 3 2 3
As is known, In S (αꢀIn S , βꢀIn S ,
and γꢀIn ), especially βꢀIn , an nꢀtype semiconductor with relatively narrow band
2
S
3
2 3
S
gap of 2.0–2.3 eV and comparatively negative potential (approximate –0.9 eV vs.
NHE) of conduction band, is considered as a promising candidate for photocatalytic
12ꢀ14
applications.
2 3
Unfortunately, the pure In S possesses poor photocatalytic activity and low
quantum efficiency due to the rapid recombination of photogenerated electrons and
holes. Up to now, many efforts have made to solve this drawback and various
In
2 3
S ꢀbased photocatalytic materials with enhanced photocatalytic activities have been
1
5
reported. Juan et al. synthesized Bi S /In S core/shell microspheres with admirable
2
3
2 3
visibleꢀlight photocatalytic activities in the degradation of 2, 4ꢀdichlorophenol. Xin et
16
2 3
al. synthesized In S /carbon nanofibers/Au composite, and this ternary synergetic
system exhibited increased photocatalytic activity in the decomposition of Rhodamine
3

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14
2 3 3 4
B. Chao et al. constructed In S /gꢀC N heterojunction which exhibited favorable
photocatalytic activity in the degradation of Rhodamine B. However, some defects
will emerge in the interfaces of these heterojunctions because of the largeꢀsize of
17
materials. Thus, the photogenerated electrons and holes will be apt to recombine on
the defects and the photocatalytic activities of these heterojunctions will be decreased.
So as to overcome this limitation, carbon quantum dots materials could be used
1
8
because of uniform distribution and the formation of sufficient construction.
Carbon quantum dots (CQDs), a novel class of carbon nanomaterials, are
attracting increasing interest in wide range of applications, including chemical sensing,
19ꢀ24
bioimaging, biosensing, photodynamic therapy, and electrochemical.
Considering
the excellent electron–hole pair’s separation ability, photochemical stability, and
upꢀconversion fluorescence emission, CQDs have been applied to decorate the
25ꢀ27
28ꢀ30
photocatalysts to enhance photocatalytic performance, such as TiO
2
, C
N
3 4
,
3
1
32
33
34
Ag PO , Bi MO , NiFe , and ZnO . Nevertheless, the functions of CQDs in the
3
4
2
6
composites are still controversial, and the mechanism of improvement photoactivity
need to be further studied. To the best of our knowledge, there is no research on the
CQDs decorated hollow In
organic pollutants.
2 3
S microspheres or their application in degradation of
In this work, firstly, the CQDs decorated hollow In S microspheres were
2
3
synthesized by a facile one pot hydrothermal method. Then, multiple techniques were
used to explore the microꢀ, nanoꢀ structures and chemical composition of the
asꢀprepared materials. Clearly, CQDs with the average size of 5 nm were attached on
4

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the surfaces of hollow In
samples were explored under visible light irradiation, and methyl orange (MO) was
chosen to be the target pollutant. Consequently, CQDs/In samples exhibited
significantly enhancement of photocatalytic activity for MO degradation compared to
pure In . Moreover, the 3wt% CQDs/In sample displayed the highest
photocatalytic degradation efficiency, which was about 3 times as much as that of
pure In . Furthermore, the possible photocatalytic mechanism for the enhanced
2 3
S microspheres. The photocatalytic activities of asꢀprepared
2 3
S
2
S
3
2 3
S
2 3
S
photocatalytic activity was also investigated based on the active species trapping
experiment and electron spin resonance (ESR) analysis.
2
. Experimental
2
.1. Materials
Citric acid, ethylenediamine, indium trichloride, Lꢀcysteine, triethanolamine (TEOA),
,4ꢀbenzoquinone (BQ), isopropanol (IPA), 5,5ꢀdimethy lꢀ1ꢀpyrroline Nꢀoxide
DMPO), and methyl orange (MO) were analytical grade agents purchased from
Aladdin (China) and used as received without further purification.
.2. Fabrication of photocatalysts
1
(
2
35
CQDs were prepared by a hydrothermal method according to the previous report. In
a typical fabrication, 1.0507 g citric acid and 335 µL ethylenediamine were dilute in
1
0 mL distilled water under lowꢀspeed stirring. Then the above solution was transport
to a 30 mL Teflonꢀlined autoclave, and heated at 180 ºC for 5 h. After that, the
reactors were cooled to room temperature naturally. In order to obtain CQDs，the
product was further treated using a semipermeable membrane and dried by freeze
5

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drying.
CQDs decorated hollow In
2 3
S microspheres were synthesized by a facial one pot
hydrothermal method. In detail, 1 mmol indium trichloride was dissolved to 25 mL
distilled water which containing a certain amount of CQDs. Then, 1.5 mmol
Lꢀcysteine was added, and the solution was stirring for 5 min. Afterward, the above
solution was transport to a 30 mL Teflonꢀlined autoclave, and heated at 150 ºC for 24
h. A dark red product was collected after centrifugation, and the product was washed
with deionized water and ethanol three times. Finally, the product was dried in a
vacuum drier at 60 °C for 24 h. The hollow In S microspheres and CQDs decorated
2
3
hollow In
fabricated following the similar procedure by changing the content of CQDs. To make
clarity, the CQDs decorated hollow In microspheres with expected CQDs contents
of 0, 1, 3, 5 wt% are referred to as In , 1wt% CQDs/In , 3wt% CQDs/In , and
2 3
S microspheres containing different mass ratios (1%, 3%, and 5%) were
2 3
S
2
S
3
2
S
3
2 3
S
5
wt% CQDs/In S , respectively.
2 3
2
.3. Characterization
Xꢀray diffraction (XRD) patterns were obtained on a D/MAXꢀ2500 diffractometer
Rigaku, Japan) with Cu Kα radiation (λ = 1.54178Å), the scanning rate was 7.0 º
min and the scanning range (2θ) was 5.0–80 º. Scanning electron microscopy (SEM)
(
/
and energy dispersive Xꢀray spectroscopy (EDX) patterns of asꢀprepared samples
were measured by an Sꢀ4800 field emission scanning electron microscope (FESEM,
Hitachi, Japan). The transmission electron microscopy (TEM) and highꢀresolution
transmission electron microscopy (HRTEM) were also used to characterize the
6

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morphologies of asꢀprepared samples, the TEM images were acquired on a
transmission electron microscopy (Tecnai G2 F30 SꢀTwin, FEI) operated under the
accelerating voltage of 200 kV, and HRTEM images were acquired on a transmission
electron microscopy (JEMꢀ2100F, Japan). Xꢀray photoelectron spectroscopy (XPS)
was determined by a Thermo ESCALAB 250X (America) electron spectrometer with
a 150 W Al Kα Xꢀray sources. Fourier transform infrared (FTꢀIR) spectra of the
−1
samples were acquired in the 400ꢀ4000 cm range on a Bruker Vertex 70
spectrometer using KBr as the dispersion medium. The UVꢀvis diffused reflectance
spectra (DRS) of the samples were determined on an UVꢀ2450 spectrophotometer
(Shimadzu, Japan) using BaSO
4
as the reflectance standard. The photoluminescence
TM
(PL) spectra of samples were measured on a Quanta Master
40 fluorescence
spectrofluorometer (Photon Technology International Inc.) at room temperature.
Photocurrent and electrochemical impedance spectroscopy (EIS) measurements were
performed on a CHI 660B (Chenhua, China) electrochemical workstation with a
standard threeꢀelectrode cell at room temperature. The electron spin resonance (ESR)
analysis was conducted with a Bruker EPRA 300ꢀ10/12 spectrometer under visible
light.
2
.4. Photocatalytic evaluation
Photocatalytic activities of In S and CQDs/In S samples were investigated by the
2
3
2 3
degradation of MO under visible light irradiation. A 250 W xenon lamp with a 420 nm
cutoff filter was used as the source of visible light in the photochemical reactor.
Typically, 50 mg asꢀprepared photocatalyst were added into 50 mL 10 mg/L pollutants
7

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aqueous solution. In order to achieve the adsorptionꢀdesorption equilibrium between
photocatalysts and MO molecules, the suspension was kept stirring in the dark for 30
min. The content of pollutant in reactors was sampled (about 5 mL) every 20 minutes,
and the samples were treated by centrifugation (10000 r/min, 10 min) to remove the
photocatalysts. Finally, the concentration of residual dye in the sample was
determined at the wavelength of 464 nm over a TUꢀ1810 UVꢀvis spectrophotometer.
2
.5. Active species trapping and ESR experiment
In order to determine the key active species which are responsible for the
photocatalytic reaction, the active species trapping experiment was carried out. In
3
6,37
38,39
40,41
detail, TEOA (0.5 mmol)
, BQ (0.5 mmol)
, and IPA (0.5 mmol)
were
+
employed as the scavengers for h , •O
2
¯ and •OH in the similar photocatalytic
evaluation with 3wt% CQDs/In
confirm the result of active species trapping experiment. As we all know, •OH and
O ¯ radicals can be easily trapped by the DMPO and the intermediate of DMPOꢀ•OH
2 3
S , respectively. The ESR technique was also used to
•
2
2
and DMPOꢀ•O ¯ adducts can be detected by ESR analysis. Before the test, 10.0 mg
samples were dissolved in 1 mL deionized water or 1 mL methanol, and then 30 µL
DMPO was added, respectively. The ESR analysis was carried out under visible light,
which was similar to the photocatalytic evaluation.
3
. Results and discussion
The morphologies and microstructures of pure In
2 3 2 3
S and 3wt% CQDs/In S samples
are characterized by SEM and TEM techniques. The SEM image acquired at
lowꢀmagnification, Fig. 1a, shows that the pure In
8
2 3
S sample possesses sphericalꢀlike

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morphology with the diameter from 2 to 5 ꢁm. The highꢀmagnification SEM images
of pure In , Fig. 1b and Fig. 1c, reveal that the In microspheres are built from
2
S
3
2 3
S
small nanosheets, and there are a number of porous randomly distributed on its
surfaces. These porous could play a significant role in improving surface area,
shorting chargeꢀcarrier transport paths, and enhancing light harvesting. Fig. 1d and
Fig. 1eꢀf show the low and high magnification SEM images of 3wt% CQDs/In
sample, respectively. No significant change appears compared to pure In . That is to
say, the introduction of CQDs does not affect the morphology of In
2 3
S
2 3
S
2 3
S .
Fig. 1 SEM images of the pure In
dꢀf).
TEM image of pure In S is shown in Fig. 2a, some nanosheets can be found
2 3 2 3
S sample (aꢀc) and 3wt% CQDs/In S sample
(
2
3
from the edge of sphericalꢀlike structure. Meanwhile, the obvious contrast between
the relatively bright center and the dark edge confirms their hollow structure with the
diameter of approximate 420nm. The lattice spacing shown in Fig. 2c is 0.325 nm,
2 3
which corresponding well to the (109) crystal plane of In S . As shown in Fig. 2b, 3wt%
9

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CQDs/In
shows the HRTEM image of 3wt% CQDs/In
determined to be 0.212 nm, which is corresponding to the (100) crystal plane for
2
S
3
sample has the similar sphericalꢀlike structure with hollow nature. Fig. 2d
2 3
S
sample, the lattice spacing of CQD is
4
2ꢀ44
graphitic carbon.
At the same time, the lattice spacing around 0.325 nm can be
also found, which agree well with the crystallographic (109) spacing of In S . The
2
3
2 3
above results further demonstrate that the CQDs and In S have been coupled together
successfully.
2 3
Fig. 2 TEM and HRTEM images of the pure In S sample (a, c) and 3wt%
CQDs/In S sample (b, d).
2
3
To further certify the successful fabrication of In
2 3
S and CQDs decoration
hollow In microspheres, the asꢀprepared samples are examined by powder XRD
2 3
S
10

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characterizations. As shown in Fig. 3a, the diffraction peaks of the pure In
2
S
3
sample
appears at 27.40°, 33.45°, 43.80° and 47.90°, which are attributed to the (109), (0012),
309), and (2212) diffraction planes of βꢀIn phase (JCPDS card no. 73ꢀ1366),
(
2 3
S
respectively. Moreover, the characteristic peak of carbon at 26° is too weak to be
observed, perhaps due to the small amount of carbon and its relatively low diffraction
26,33,45
intensity in the hybrid materials.
2 3
The EDX spectra of In S and 3wt%
CQDs/In samples were shown in Fig. S1, the presence of C, S, and In elements in
2 3
S
Fig. S1b is a direct evidence for successful synthesis of the hybrid material.
Fig. 3 XRD patterns of asꢀprepared samples.
The surface valence state and the chemical composition of asꢀprepared samples
are investigated by XPS. The survey scan XPS spectrum, Fig. 4a, shows that the 3wt%
CQDs/In S sample contain indium, sulfur, oxygen and carbon elements. Note that the
2
3
O1s signal may be from the absorbed CO
Fig. 4b, In3d peaks of pure In sample are centered at approximate 444.8 eV and
52.3 eV, which attribute to the binding energies of In3d5/2 and In3d3/2. Moreover
the phenomena of spin orbit separation between In3d5/2 and In3d3/2 (7.5 eV)
2 2
and H O on the surface of the samples. In
2 3
S
4
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3
+
15
suggests the existence of In in the In
CQDs/In sample appear a slight shift compared to that of pure In
indicates that the surface In chemical environment in the 3wt% CQDs/In
has changed and there exist interactions between CQDs and In
highꢀresolution S2p peaks of 3wt% CQDs/In S sample, shown in Fig. 4c, are located
2
S
3
sample. The In3d peaks of 3wt%
, which
sample
. The
2
S
3
2 3
S
2 3
S
2 3
S
2
3
4
6
at 161.7 eV and 162.9 eV, ascribing at the binding energy of S2p3/2 and S2p1/2.
The highꢀresolution XPS spectra of C1s analysis of In and 3wt% CQDs/In S
2
S
3
2
3
sample were shown in Fig. S2 and Fig. 4d. Fig. 4d revealed three different types of
carbon atoms: graphitic or aliphatic (C=C or CꢀC, 284.75 eV), oxygenated (C=O,
1
9
2
86.15 eV), and nitrous (CꢀNHꢀC, 287.7 eV). The result of XPS analysis reveals the
coexistence of In and CQDs in the 3wt% CQDs/In sample.
2
S
3
2 3
S
12

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Fig. 4 XPS spectra of In
In 3d (b); S 2p (c); C 1s (d).
FTꢀIR spectra are used to further ensure the presence of CQDs in CQDs/In
2 3 2 3
S and 3wt% CQDs/In S sample: Survey of the samples (a);
2 3
S
−1
−1
samples. In Fig. 5, the absorb peaks at 1373 cm and 1615 cm are attributed to InꢀS
−1
19
vibration mode. The absorb peak at 1049 cm is ascribed to epoxy group. The
−1
−1
stretching vibrations of CꢀOH at 3430 cm and CꢀH at 2923 cm can be also
4
7
observedin CQDs and CQDs/In
2 3
S samples. The result of FTꢀIR analysis indicates
that CQDs and In have been coupled together successfully.
2 3
S
Fig. 5 FTꢀIR of asꢀprepared materials with different contents of CQDs.
In order to understand the role which CQD plays in the adsorptionꢀdesorption
equilibrium, the photocatalytic reaction system was placed in the dark continuous
stirring for 50 min, and the content of pollutant in reactors was sampled every 5
minutes. As shown in the Fig. S3, CQDs play an important role in aiding dye
bandaging to the photocatalyst. Moreover, the adsorptionꢀdesorption equilibrium
between photocatalyst and MO molecules will be almost achieved after 30 min. The
2 3 2 3
photocatalytic performances of the pure In S and CQDs/In S samples with different
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contents of CQDs are explored by the degradation of MO under visible light
irradiation. As shown in Fig. 6a, the blank test without catalyst reveals that the
photolysis of MO molecules is so slowly that can be ignored. Meanwhile, the
CQDs/In S samples express a better photocatalytic behavior than that of the pure
2
3
In
photocatalytic behavior which is almost 3 times as much as that of the pure In
b reveals the UV–vis spectral absorbance changes of MO solution during the
photocatalytic degradation experimentation over 3wt% CQDs/In sample.
2
S
3
, and the 3wt% CQDs/In
2
S
3
sample expresses the most outstanding
2 3
S . Fig.
6
2 3
S
Obviously, the main absorption peak of MO molecules, decreasing rapidly over the
degradation time, are located at 464 nm. In Fig. 6c, the zeroꢀorder reaction kinetic
model is applied to fit the relationship between the measured dye concentration (C
and irradiation time (t):
t
)
C
t
= −
k
t + C
0
(1)
4
8
Where C and
k
indicate the initial dye concentration and the rate constant.
0
Clearly, the degradation of MO is very consistent with the zero order kinetic model,
and the values are shown in Fig. 6d.
k
14

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Fig. 6 (a) The photocatalytic activities of asꢀprepared samples for MO degradation
under visible light; (b) The UV–vis spectral absorption changes of MO solution
photodegraded over 3wt% CQDs/In S sample under visible light irradiation; (c) The
2
3
zeroꢀorder reaction kinetics for MO degradation; (d) The apparent rate constants for
MO degradation.
It is expected that the introduction of CQDs would have a large effect on the
optical properties of In
explored by UV–vis absorption and Upꢀconverted PL spectroscopy. As shown in Fig.
a, the CQDs/In samples presents outstanding optical absorption and a clear red
2 3
S . Thus the optical properties of asꢀprepared samples were
7
2 3
S
shift of the optical absorption can be observed with the increasing of CQDs.
Furthermore, much broader and stronger absorption tail of CQDs/In S samples are
2
3
also clearly observed, and the absorbance is gradually increasing with the increased
dosage of CQDs. Moreover, the band gap energy (E ) of In sample, shown in Fig.
g
2 3
S
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7
b, is calculated by the following equation:
0
.5
(αhv) = A(hv – Eg)
(2)
Where α, h, v, A and Eg indicate absorption coefficient, Planck constant, light
14,49
frequency, proportionality and band gap energy, respectively.
As shown in Fig. 7c,
2 3
the band gap of the pure In S is approximate 2.0 eV, which is similar to previous
12
study. Upꢀconverted photoluminescence (PL) spectra of the asꢀprepared CQDs was
studied using different excitation wavelengths. As shown in Fig. 7c, the emission
wavelengths of pure CQDs are located at the range of 400–650 nm when excited by
long wavelength light at 600, 650, 700, 750, 800, 850, 900, 950 and 1000 nm. That is
to say, CQDs can absorb light with longer wavelength (>620 nm) and then emit
visible light (400–650 nm) which in turn excites In
pairs. To further confirm the upꢀconversion effect in CQDs decorated In
microspheres, the photocatalytic behavior of In and 3wt% CQDs/In samples
were explored for the degradation of MO dye under infrared light irradiation. As
shown in Fig. S4, we can see that CQDs/In still exhibit photocatalytic activities
under infrared light, while In do not show any obvious activities.
2 3
S to form more electron/hole
2 3
S
S
2 3
2 3
S
2 3
S
2 3
S
16

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2
Fig. 7 UV–vis absorption spectra of asꢀprepared samples (a); Plots of the (αhv)
versus (hv) for In
2
S
3
sample (b); Upꢀconverted PL spectra of CQDs with different
excitation wavelengths (c).
Photocurrent and Electrochemical impedance spectroscopy (EIS) were used to
estimate the photogenerated charge carriers separation and transfer ability of the
2 3 2 3
asꢀprepared samples. Photocurrent tests of the pure In S and 3wt% CQDs/In S
samples are carried out in an onꢀandꢀoff cycle mode under visible light, and the result
is shown in Fig. 8a. Obviously, the 3wt% CQDs/In S electrode exhibits a higher
2
3
2 3
photocurrent than that of the pure In S sample. The significant improvement of
photocurrent response reveals that the photogenerated electrons and holes separate
efficiently, which can prolong the lifetime of electrons and holes, and consequently
improve the photocatalytic activity. The EIS Nyquist plots of the working electrodes
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were acquired in the dark or under illumination condition, and the result was shown in
Fig. 8b. The semicircle part at the high frequency region corresponds to the
chargeꢀtransfer process of electrolyte/electrode interface and a smaller radius implies
5
0,51
more efficient charge transfer process.
radius of the Nyquist semicircle in the high frequency region of the 3wt% CQDs/In2S3
electrode is smaller than that of the pure In electrode, which implies that the 3wt%
CQDs/In material has lower electron transfer resistance. Moreover, compared with
Clearly, when measured in the dark, the
2 3
S
2 3
S
the plot obtained in the dark, a much smaller radius was acquired under irradiation for
both electrodes, illustrating the increased electron conductivity of the electrode under
28
light. The result of EIS demonstrating that the introduction of CQDs can efficiently
reduce the electron transfer resistance of In
electron hole separation.
2 3
S , and then improve the efficiency of
Fig. 8 Photocurrent response (a) and Nyquist plots (b) of pure In S and 3wt%
2
3
CQDs/In
2 3
S samples.
In order to reveal the photocatalytic mechanism, firstly, the active species
2 3
trapping experiment is carried out over the 3wt% CQDs/In S sample. In detail, IPA,
+
−
TEOA, and BQ are used as the scavengers of •OH, h and •O
2
radicals, respectively.
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As shown in Fig. 9, when IPA is added, the photocatalystic degradation rate (80%) is
almost unchanged compared with that of no scavenger. However, when BQ is added,
the photocatalystic degradation rate (3%) significantly decreases, which indicate that
−
the •O
2
is the predominant active species. Meanwhile, after the addition of TEOA,
the photocatalystic degradation rate is also inhibited seriously (7%), which implies
+
that the h is also playing an important role in photocatalystic degradation. Therefore,
−
+
2
•O and h radicals are the major active species in the photocatalytic degradation.
Fig. 9 Effects of a series of scavengers on the photocatalytic efficiency of 3wt%
CQDs/In sample.
2 3
S
The ESR spinꢀtrap tests with DMPO is performed over 3wt% CQDs/In2S3
sample to further confirm the result of the active species trapping experiment. As
shown in Fig. 10 a, no ESR signals can be detected when the reaction is carried out in
−
the dark. However, the characteristic peaks of the DMPOꢀ•O
2
adducts can be
observed under visible light irradiation. The intensity of the ESR signal is related to
the concentration of reactive oxygen species generated in each catalyst suspension.
Thus, the weak characteristic peaks of DMPOꢀ•OH adducts, shown in Fig. 10 b, may
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−
36
2
be attributed to further reduction of •O .
2
−
Fig. 10 ESR spectra of (a) DMPOꢀ•O adducts in methanol solution and (b)
DMPOꢀ•OH adducts in aqueous solution recorded with 3wt% CQDs/In S sample
2
3
under visible light irradiation.
In order to fully understand the photocatalytic mechanism, the energy band
positions of conduction band (CB) and valence band (VB) of In
2
S
3
microspheres are
calculated with the following equation:
e
E
CB=Χ－E －0.5E
g
(3)
(4)
E
VB= ECB＋E
g
Where ECB and EVB are the CB and VB edge potential of the pure In
2
S
3
sample,
14
e
X is the absolute electronegativity of the pure In S sample (4.7 eV), E is the energy
2
3
of free electrons on the hydrogen scale (4.5 eV), and E
semiconductor which is determined by UVꢀvis absorption spectra (2.0 eV). Thus, the
sample has been calculated to −0.8 eV, which is more negative than the
g
is the bandgap energy of the
2 3
ECB of In S
−
52
potential of O
2
/•O
2
(−0.33 eV vs. NHE). That is to say, the photogenerated
−
electrons in the CB of 3wt% CQDs/In S sample is enough to reduce O to •O .
2
3
2
2
However, the EVB of the pure In
2
S
3
sample, estimated to 1.2 eV, is lower than the
+
53
potential of •OH, H /H
2
O (2.72 eV vs. NHE). Thus, the EVB of 3wt% CQDs/In
2
S
3
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sample cannot yield •OH. This is consistent with the results of active species trapping
experiment and ESR analysis.
On the basic of the above experimental results, the possible mechanism of
photocatalytic degradation is put forward. As shown in scheme 1, the CQDs play an
important role in the improvement of the photocatalytic activity. The upꢀconvention
PL property of CQDs endows the CQDs decorated hollow In
broadband spectrum photocatalytic activity. When the CQDs decorated hollow In
microspheres are irradiated by shorter wavelength light (400ꢀ620nm), the electrons on
2 3
S microspheres with a
2 3
S
+
+
VB of In S are excited to the CB of In S , leaving h on VB of In S . The h can
2
3
2
3
2 3
5
4
directly oxidize MO, and the photoelectrons on the CB of In
2 3
S are apt to transform
to CQDs due to the excellent electrical conductivity and electron reservoir property of
31
−
+
CQDs. This is propitious to the separation of e and h in the process of
1
7
photocatalytic degradation. When the CQDs decorated hollow In
are irradiated by NIR light (620ꢀ1000nm), CQDs can absorb these light, and then emit
shorter wavelength light (400 to 650 nm), which in turn excites In to form more
molecular to
2 3
S microspheres
2 3
S
electron/hole pairs. The electrons accumulated on CQDs can convert O
2
−
•O
2
radicals, and the superoxide radical can break down the MO molecules into small
5
5
molecules (e.g. CO
2 2
and H O). In addition, excessive loading of blackꢀcolour CQDs
(>5wt%) will shield the active sites on the surface of catalyst and decrease the
56
intensity of light through the depth of the reaction solution. Thus, it is also crucial to
2 3
control the ratios of CQDs in the CQDs decorated hollow In S microspheres to gain
the best photocatalystic activity.
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Scheme 1 Schematic model for the important roles of CQDs for the high
photocatalytic activity in the CQDs/In sample.
2 3
S
The stability of photocatalyst is extremely important for its assessment and
application. Thus, the circulating experiment over 3wt% CQDs/In S sample is
2
3
carried out to research the photostability of the asꢀprepared photocatalysts. After each
run, the photocatalyst is collected, washed (by distilled water and absolute ethanol for
◦
three times), and dried (in vacuum at 80 C for 12 h). As illustrated in Fig 11, no
obvious change can be found after five runs of photocatalytic degradation, which
indicates that the asꢀprepared CQDs/In S samples possessing excellent photostability.
2
3
The outstanding photostability of the CQDs decorated hollow In
2
S
3
microspheres may
, which can
be attributed to the insoluble CQDs layer on the surface of In
2 3
S
3
1
effectively protect In from dissolution in aqueous solution, thus the structural
2
S
3
stability of CQDs/In S samples can be greatly enhanced during the photocatalytic
2 3
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process.
2 3
Fig. 11 The repeated photocatalytic experiments of 3wt% CQDs/In S sample for
degradation of MO under visible light irradiation.
4
. Conclusion
In summary, CQDs decorated hollow In
2
S
3
microspheres were firstly synthesized by a
microspheres
facile hydrothermal process. Obviously, CQDs decorated hollow In
2 3
S
presented enhanced photocatalytic activity and stability compared with pure In2S3
sample. The significant improvement in photocatalytic performance was attributed to
the upꢀconversion luminescence and photoinduced electron transfer properties of
+
−
CQDs. Moreover, h and •O
2
radicals were the main active species in the
photocatalytic process, and the probable photocatalystic degradation mechanism was
also proposed.
ACKNOWLEDGEMENTS
We gratefully acknowledge the National Natural Science Foundation of China
(21276116, 21477050, 21301076 and 21303074), Excellent Youth Foundation of
Jiangsu Scientific Committee (BK20140011), and ChineseꢀGerman Cooperation
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Research Project (GZ1091). Program for HighꢀLevel Innovative and Entrepreneurial
Talents in Jiangsu Province, Program for New Century Excellent Talents in University
(NCETꢀ13ꢀ0835), Henry Fok Education Foundation (141068) and Six Talents Peak
Project in Jiangsu Province (XCLꢀ025).
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