Dandelion-like ZnS/carbon quantum dots hybrid materials with enhanced photocatalytic activity toward organic pollutants

Article abstract of DOI:10.1039/c6ra02840c

In this paper, dandelion-like ZnS was synthesized via a facile hydrothermal method, and then as the support to further synthesize dandelion-like ZnS/carbon quantum dot hybrid materials. Multiple techniques were applied to investigate the structures, morphologies, electronic and optical properties of the samples. It can be observed that the carbon quantum dots were distributed uniformly on the surface of the ZnS. The photocatalytic activities of the as-prepared materials were investigated by the photodegradation of methylene blue, Rhodamine B and the colorless antibiotic ciprofloxacin hydrochloride, respectively. The as-synthesized hybrid materials exhibit higher photocatalytic activity than that of the pure ZnS under simulated sunlight (λ > 380 nm), indicating a broad-spectrum of photocatalytic degradation activity. The most beneficial amount of carbon quantum dots to improve the photocatalytic activity of the ZnS is 2.0 wt%.

Full text of DOI:10.1039/c6ra02840c

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Dandelionꢀlike ZnS/carbon quantum dots hybrid materials with
enhanced photocatalytic activity toward organic pollutants
a
a
a
a
Received 00th January 20xx,
Accepted 00th January 20xx
Fangwang Ming, Jinqing Hong, Xun Xu, Zhoucheng Wang*
In this paper, dandelionꢀlike ZnS was synthesized via a facile hydrothermal method, and then as the
support to further synthesize dandelionꢀlike ZnS/carbon quantum dots hybrid materials. Multiple
techniques were applied to investigate structures, morphologies, electronic and optical properties of the
samples. It can be observed that the carbon quantum dots were distributed uniformly on the surface of
the ZnS. The photocatalytic activities of the asꢀprepared materials were investigated by the
photodegradation of methylene blue, rhodamin B and colorless antibiotic ciprofloxacin hydrochloride,
respectively. The asꢀsynthesized hybrid materials exhibit higher photocatalytic activity than that of the
pure ZnS under simulated sunlight (λ>380 nm), indicating a broadꢀspectrum of photocatalytic
degradation activity. The most beneficial amount of the carbon quantum dots to improve the
photocatalytic activity of the ZnS is 2.0 wt%.
DOI: 10.1039/x0xx00000x
www.rsc.org/
uniformly on the surface of other materials because of its small
size. What’s more, it can construct the bulkꢀtoꢀsurface channels for
the electrons owing to the excellent ability for charge
1
. Introduction
ZnS, as a vital wideꢀband gap semiconductor material, has been
widely used for manufacture of catalysis, sensors,
photoluminescence and etc.. Recently, many efforts have been
devoted to enhance the photocatalytic performance of the
ZnS, including hybridizing with noble metal and compositing with
transport. Therefore, such a semiconductorꢀconductively quantum
dots system is worthy of expectation. Recently, the CQDs have been
1
ꢀ3
15
16
introduced into the semiconductors (such as Ag
PO
3 4
,
ZnO,
17
18
TiO
2
and Fe O ) to improve the photocatalytic performances.
2 3
However, most of them evaluated the photocatalytic activity by
single organic dye and the relationships between the structure and
photocatalytic activity have not been studied.
other
carbon
materials
such
as graphene oxide
(
GO), reduced graphene oxide (RGO) and carbon nanotubes
4
ꢀ7
(
CNTs). However, the contact interfaces of these hybrid systems
are insufficient and unconsolidated, therefore can’t form perfect
interfaces on count of the largeꢀsize materials. Thus, the electronꢀ
In this work, dandelionꢀlike ZnS was synthesized and serviced as
8
the
support
to
fabricate
the
ZnS/CQDs
hybrid
hole pairs recombine preferentially, which limit the further
improving of the photocatalytic performance. Herein, carbon
quantum dots (CQDs) as the coꢀcatalyst were introduced into the
ZnS in order to eliminate the limitation.
materials. The structure, morphologies and optical properties were
investigated in detail. The unique nanostructure is assembled by the
plenty of nanowires, which makes it possesses the mesopores
structure. Additionally, the porous morphology endows it has a large
specific surface areas and good adsorption properties which are of
The CQDs has been wildly used in the applications such as lightꢀ
19
great importance for a catalyst. The photocatalytic activities of the
ZnS/CQDs were evaluated by the degradations of organic dyes
9
10,11
12
13,14
emitting devices, bioimaging,
catalysis and sensor,
due to
its low toxicity, low cost and high resistance to photoꢀbleaching.
The CQDs can form sufficient contracted sufficiently and distributed
(
methylene blue (MB), rhodamin B (RhB)) and ciprofloxacin
hydrochloride (CIP). To the best of our knowledge, there are no
existing reports on the preparation and investigation
of photocatalytic performance of such a ZnS/CQDs system and no
reports about the degradation of CIP by ZnS system.
a
Department of Chemical and Biochemical Engineering, College of Chemistry
and Chemical Engineering, Xiamen University, Xiamen 361005, China. Tel./fax:
+86-592-2180738, E-mail: zcwang@xmu.edu.cn
2
2
. Experimental section
†
Footnotes relating to the title and/or authors should appear here.
Electronic Supplementary Information (ESI) available: [details of any
supplementary information available should be included here]. See
DOI: 10.1039/x0xx00000x
.1 Preparation of dandelionꢀlike ZnS and CQDs
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All the materials are commercially available and used as received
The photocatalytic activities were evaluated by the decomposition
without further purification. In a typical procedure, 1 mmol of of methylene blue (MB), rhodamin B (RhB) and ciprofloxacin
Zn(NO ·6H O was dissolved in 16 mL ethylenediamineꢀwater hydrochloride (CIP) under simulated sunlight irradiation. The pH
3
)
2
2
mixture solvent (1: 1 in volume ratio). After stirring for 1 h, 0.1 value was not adjusted when the reaction was conducted.
mmol of CTAB was added into them. Then the solution was stirred Experiments were carried out at ambient temperature with a
until the CTAB was fully dissolved. After that, 3 mmol of thiourea circulating water system to prevent thermal catalytic effects.
was added into the mixture solution and stirred for 2 h. Then the Simulated sunlight irradiation was provided by a 300W Xe lamp
colloid solution was heat treated in a Teflonꢀlined autoclave at 180 (NBET HSXꢀF300, China). 30 mg of the photocatalyst was totally
ꢀ
1
°
C for 24 h. After the reaction, the products were collected and dispersed in an aqueous solution of MB (50 mL, 20 mg L ), RhB
ꢀ
1
ꢀ1
washed by ethanol and deionized water several times, respectively.
(50 mL, 20 mg L ) and CIP (50 mL, 10 mg L ), respectively.
Before irradiation, the suspensions were magnetically stirred in the
dark for 30 min to obtain the absorptionꢀdesorption equilibrium. At
certain time intervals, 3 mL aliquots were sampled and centrifuged
to remove the particles. The concentrations of MB, RhB and CIP
were analyzed by recording the absorbance at the characteristic band
of 664 nm, 557 nm and 276 nm using an UVꢀvis spectrophotometer
2
0
The CQDs were synthesized according to the reported literature.
In a typical procedure, citric acid (1.0507 g) and ethylenediamine
335 µL) was dissolved in DIꢀwater (10 mL). Then the solution was
transferred to a poly (tetrafluoroethylene) (Teflon)ꢀlined autoclave
30 mL) and heated at 200 C for 5 h. After the reaction, the reactors
(
(
︒
were cooled to room temperature by water or naturally. The product,
which was brownꢀblack and transparent, was subjected to dialysis in
order to obtain the CQDs.
(
UVꢀ2550, Shimadzu), respectively.
2.5 High performance liquid chromatography
(
HPLC)
2.2 Preparation of dandelionꢀlike ZnS/CQDs hybrid
materials
The chromatograph was Agilent 1100 Infinity (USA) equipped
with
chromatographic column was
250 mm 4.6 mm i.d., 5 ꢁm). The mobile phase was water and
acetonitrile containing 0.1% formic acid (v/v), at a ratio of 78:22
a
UV detector and
a
binary gradient pump. The
To synthesize the hybrid materials, 100 mg dandelionꢀlike ZnS
was added into 30 mL deionized water which contains a certain
amount of CQDs, then the hybrid system were obtained via an oil
bath reflux at 90 °C for 3 h. The obtained samples were washed
with deionized water, and then dried in an oven at 60 °C for 12 h.
The added contents of CQDs in the ZnS/CQDs hybrid materials
were 1 wt%, 2 wt%, 4 wt%, respectively.
a
Agilent HCꢀC18 column
(
ꢀ
1
(
v/v). The mobile phase flow rate was 1.0 mL min and the column
was kept at 30 C. The injection volume was 20 ꢁL and the CIP
residual were monitored at a wavelength of 270 nm.
︒
2
.6 Electrochemical impedance spectroscopy
2.3 Characterization
measurement (EIS)
The phase and morphology of the products were characterized by
For the preparation of working electrodes, 10 mg of sample, 1 mL
of ethanol and 1 mL of ethylene glycol were mixed to produce a
suspension and then 80 ꢁL of the suspension was spread on an ITO
glass electrode (1 cm × 2 cm). Electrochemical measurements were
taken by using an electrochemical analyzer in a standard three
electrode system with a platinum plate as the counter electrode, and
saturated calomel electrode as the reference electrode. 1 M KOH
aqueous solution was utilized as the electrolyte for the measurement.
Xꢀray diffraction (XRD, Rigaku, Japan) with Cu Ka radiation (λ =
.54056 Å), scanning electron microscopy (SEM, ZEISS SIGMA,
Germany), and highꢀresolution transmission electron microscopy
HRTEM, JEOL JEMꢀ2100, Japan). The BETꢀsurface areas of the
ZnS samples were measured by N adsorption using the single point
method with TriStar II 3020 (US). Xꢀray photoelectron spectroscopy
1
(
2
(
(
XPS) measurement was performed on PHI Quantumꢀ2000 XPS
US). The UV–vis diffuse reflectance spectra were measured in the
solid state, and BaSO
4
powder was used as the substrate (Caryꢀ5000, 3. Results and discussion
Aglient, US). The photoluminescence spectroscopy of the samples 3.1 Phase structure and morphology
was measured by the fluorescence spectrophotometer (Fꢀ7000,
Hitachi, Japan). The electrochemical impedance spectroscopy (EIS)
measurements were performed on an electrochemical system (CHIꢀ
The XRD patterns of the pure ZnS and hybrid ZnS/CQDs are
presented in Fig. 1(a). The diffractogram of the pure ZnS indicates
that all diffraction peaks can be well indexed as a hexagonal wurtzite
phase structure (JCPDS No.36ꢀ1450). For the CQDs modified ZnS
6
60D, China).
2.4 Photocatalytic activity measurement
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Fig. 1 (a) XRD patterns of the pure ZnS and the CQDs/ZnS hybrid materials; (b) and (c) SEM images of the dandelionꢀlike ZnS; (d) and (e)
TEM images of CQDs/ZnS hybrid materials; (f) HRTEM image of the CQDs/ZnS hybrid materials and (g) SAED of the dandelionꢀlike ZnS.
2
Fig. 3 N adsorptionꢀdesorption isotherms of the dandelionꢀlike ZnS.
The inset shows the pore size distributions calculated by the BJH
method from the desorption branch of the isotherm.
Fig. 2 (a) TEM image of CQDs , (b) HRTEM image of CQDs and
c) the CQDs size distribution histograms.
samples, no characteristic peaks of the CQDs can be detected, which
may be attributed to its low content in the samples. Fig. 1(b) and
(
(
c) show the SEM images of the typical dandelionꢀlike
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Fig. 4 XPS spectra of the pure ZnS and 2 wt% CQDs/ZnS hybrid materials. (a) Survey of the sample; (b) Zn 2p and (c) S 2p; (d) C 1s of 2
wt% CQDs/ZnS hybrid materials.
ZnS. TEM images of the CQDs/ZnS hybrid materials are shown in determined to be 0.32 nm, which corresponding to the (002) crystal
1
5
Fig. 1(d) and (e).These observations indicate that the dandelionꢀlike plane.
ZnS are assembled by plenty of nanowires and the products retain
the morphology and structure after hybridizing with CQDs. In
addition, it can be found that the diameter of ZnS nanowires is ~10
nm and some dark dots are distributed on the ZnS (Fig. 1(e) and (f)),
implying that the CQDs are deposited on the surface of the
dandelionꢀlike ZnS. The HRTEM image revealed in Fig. 1(f) shows
the lattice structure of an individual ZnS nanowire. The (002) plane
of the ZnS which illustrates an interplanar space of 0.311 nm can be
The N2 adsorptionꢀdesorption isotherm measurements were
performed to calculate the specific surface areas and pore size
distributions of the samples. As shown in Fig. 3, the hysteresis loops
(
inset) vary in the range of 0.5~1.0 P/P , indicating the formation of
0
mesopores in the dandelionꢀlike ZnS. The BET surface areas of the
2
ꢀ1
dandelionꢀlike ZnS are calculated to be 98.4 m g .
3
.2 Compositional and surface properties
observed. Moreover, the
SAED
(Fig. 1(g)) pattern
further
demonstrates the polycrystalline structure of the products. The
morphology and lattice structure of the CQDs are shown in Fig. 2(a)
and (b). It can be seen that the average particle sizes of the CQDs are
in the range of 2~4 nm (Fig. 2(c)). The lattice spacing of CQDs was
XPS measurements are carried out in order to study the chemical
bonding states of the CQDs modified ZnS samples. Fig. 4(a) shows
full scan spectrum of 2 wt% CQDs/ZnS, confirming the presence of
7
Zn 2p, S 2p and C 1s. The high resolution XPS spectrum of the Zn
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Fig. 5 (a), (c) and (e) Photocatalytic degradation of MB, RhB and CIP, respectively, with the catalysts under simulated sunlight; (b) and (d)
kinetic fit for the degradation of MB and RhB, respectively.
2
2
and S element in Fig. 4(b) and (c) show peaks at 1020 and 161.5 bond, respectively. The results of XPS analysis indicate that CQDs
eV which correspond to the Zn 2p3/2 and 2p3/2 peaks of and ZnS have been coupled together successfully.
ZnS, demonstrating that Zn and S exist in the form of ±2 valence
S
2
1
Energy Dispersive Spectrometer (EDS) measurements are
employed to determine the elements ratio and the real contents of the
CQDs in the ZnS and the hybrid materials. The results are shown in
Fig. S1 and Table S1. As expected, there is no signal of the C
element in the pure ZnS. The real contents of the CQDs in the 1
wt%, 2 wt% and 4 wt% CQDs/ZnS are 0.86 wt%, 1.72 wt% and
state. It is noted that the characteristic peak of the S 2p exhibits
obvious right shift, whereas the peaks of the Zn 2p become broaden
in the 2 wt% CQDs/ZnS materials compared with the pure
ZnS. It implies that there exist interactions between the CQDs and
ZnS. As can be observed in Fig. 4(d), the high resolution XPS
spectrum of the C 1s of the 2 wt% CQDs/ZnS can be divided into
three peaks at 284.8, 286.6 and 288.4 eV, corresponding to CꢀC
3
.39wt%, respectively. It should be noted that we use 1 wt%
CQDs/ZnS, 2 wt% CQDs/ZnS and 4 wt% CQDs/ZnS to define the
hybrid materials in the following paragraphs for convenience.
bond
with
the
sp2
orbital, CꢀOꢀC
bond and
C=O
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Fig. 6 Temporal UVꢀvis absorption spectral changes during the photocatalytic degradation of (a) MB, (b) RhB and (c) CIP, respectively, in
aqueous solution in the presence of 2 wt% CQDs/ZnS materials.
Table 1 Pseudoꢀfirstꢀorder rate constant for MB photocatalytic degradation under different photocatalysts.
2
Series
Photocatalyst
The first order kinetic
equation
Rate constant k
R
−
1
(min )
1
2
3
4
ZnS
ꢀln(C/C
0
)=0.0183 t
0.0183
0.0278
0.0306
0.0238
0.9784
0.9765
0.9901
0.9899
1wt% CQDs/ ZnS
2wt% CQDs/ ZnS
4wt% CQDs/ ZnS
ꢀln(C/C )= 0.02779 t
0
ꢀln(C/C
0
0
)= 0.03058 t
)= 0.02381 t
ꢀln(C/C
Table 2 Pseudoꢀfirstꢀorder rate constant for RhB photocatalytic degradation under different photocatalysts.
2
Series
Photocatalyst
The first order kinetic
equation
Rate constant k
R
−
1
(min )
0.0054
0.0087
0.0114
0.0075
1
2
3
4
ZnS
0
ꢀln(C/C )=0.00539 t
0.9521
0.9674
0.9875
0.9553
1wt% CQDs/ ZnS
2wt% CQDs/ ZnS
4wt% CQDs/ ZnS
0
ꢀln(C/C )= 0.00866 t
ꢀln(C/C )= 0.01138 t
0
ꢀln(C/C )= 0.00748 t
0
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3
.3 Photocatalytic test
materials exhibit an improved photocatalytic activity for the CIP
degradation than that of the pure ZnS. The temporal UVꢀvis
absorption spectral changes during the photocatalytic degradation of
the MB, RhB and CIP with the presence of 2 wt% CQDs/ZnS are
shown in Fig. 6(aꢀc), the intensities of the characteristic peaks
become weaker with the increase of irradiation time, revealing the
degradation of the pollutants. The shifts of the characteristic
peaks indicate the presence of decomposition products of the
pollutants. The above results imply that the introduction of the
CQDs improves the photocatalytic efficiency of the ZnS effectively.
Photocatalytic activity of asꢀprepared samples was first evaluated
by the degradation of MB and RhB under simulated
sunlight. Fig. 5(a) and (c) show the photocatalytic activities of the
CQDs/ZnS hybrid materials with respect to the CQDs content under
simulated sunlight for MB and RhB. It can be seen that the
introduction of the CQDs enhances the photocatalytic performance
of the ZnS. The most beneficial amount of the CQDs to improve the
photocatalytic activity of the ZnS coating is 2 wt%. However, an
excessive addition of CQDs produces detrimental effects due to the
excess CQDs may cover the active sites of the ZnS. Meanwhile, the
The analysis of CIP decomposition has been performed using the
photocatalytic degradation kinetics of the MB and the HPLC technique. The results are shown in the Fig. 7. The peak at
RhB were investigated, the results are shown in Fig. 5(b) and (d). retention time of 4.82 min responses to the CIP, and the peaks at
The kinetic model is expressed as – ln(C/C )=kt, where k is the other retention times are the byproducts of the degradation of CIP.
0
apparent reaction constant, C is the initial concentration of the dye As shown in Fig. 7, the intensity of the peak directed to the CIP
0
and C is the concentration of the dye at different reaction times. The decreases with the increase of the irradiation time, indicating the
k value can be determined by a linear fit of the plot of –ln(C/C ) degradation of CIP. It can be found that the peaks of the degradation
0
2
versus reaction time t, and the R values are given to determine the products emerge immediately when the reaction occurs.
A
confidence intervals. The determined
k values for MB of the pure representative chromatogram after 30 min irradiation was shown in
ZnS, 1 wt% CQDs/ZnS, 2 wt% CQDs/ZnS and 4 wt% Fig. S2, and the relative kinetic evolution profile of the degradation
ꢀ
1
ꢀ1
ꢀ1
23,24
CQDs/ZnS are 0.0183 min , 0.0278 min , 0.0306 min and products was displayed in Fig. S3. As reported in other systems,
ꢀ1
0
.0238 min , respectively. Additionally, the determined
k
values the dominant product detected during the degradation of CIP was
for RhB of the pure ZnS, 1 wt% CQDs/ZnS, 2 wt% CQDs/ZnS desethylene ciprofloxacin (T1) at the retention time of 4 min, which
ꢀ
1
ꢀ1
and 4 wt% CQDs/ZnS are 0.0054 min , 0.0087 min , 0.0114 is formed by a net loss of C H at the piperazinyl substituent of
2
2
ꢀ1
ꢀ1
22
min
and
0.0075
min ,
respectively.
Obviously, CIP. Therefore, the possible reaction pathway of the CIP in this
the photocatalytic degradation rate is improved by the introduction work is that parts of the CIP transformed to mineral with the release
of CQDs. The 2 wt% CQDs content hybrid sample exhibits the of C H2 and CO molecule, forming CO2 and H O eventually. In
2
2
highest degradation rate which is about 1.67 and 2.11 times higher addition, the remaining CIP transformed to the organic
than that of the pure ZnS sample for MB and RhB, respectively. The intermediates. The degradation of the CIP requires a meticulous
specific data are listed in Table 1 and Table 2.
investigation in future work.
Furthermore, the maximum absorption wavelength shifts from
25ꢀ27
6
64 to 637 nm owing to the demethylation process of MB.
The
shifts from 557 to 530 nm caused by the deethylation of the RhB
was attributed to the attack of the active oxygen species on the Nꢀ
28,29
ethyl group.
The intermediates of MB and RhB transformed into
CO , H O and mineral acids eventually.
2
2
3
.4 Optical and electrochemical properties
In order to explore the mechanisms behind the enhanced
photocatalytic activity, the optical and electrochemical properties
have been investigated by UVꢀvis diffuse reflectance spectra, PL
(
photoluminescence) spectra and EIS (electrochemical impedance
spectroscopy), respectively. The UVꢀvis diffuse reflectance
spectra are shown in Fig. 8. The pure ZnS exhibits the absorption in
the region ranging from 200 nm to 400 nm. This absorption
originates from the charge transfer response of ZnS from the valence
band to the conduction band upon excitation by light. The absorption
regions get broaden and the absorption intensity increase with the
introduction of CQDs, corresponding to the optical absorption of
CQDs. Red shifting for CQDs/ZnS hybrid materials samples
probably promote the formation of electronꢀhole pairs, resulting in
the improved photocatalytic activity. In addition, the PL spectra of
the CQDs/ZnS were studied to reveal the charge transfer, migration
Fig. 7 HPLC chromatograms of the CIP degradation of 2 wt%
CQDs/ZnS under simulated sunlight irradiation.
Furthermore, nowadays, the widespread use of antibiotics may
cause various adverse effects on the ecosystem proliferation of
bacterial drug resistance. Herein, CIP, the broadꢀspectrum antibiotic
agent, was degradation by the asꢀprepared samples. As shown in
Fig. 5(e), the results reveal that the 2 wt% CQDs/ZnS hybrid
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and recombination processes in photocatalytic. It is well known that
the weaker intensity of the PL represents lower recombination
3
0
probability of photoexcited charge carriers. Fig. 9 provides the PL
spectra of the pure ZnS and the hybrid materials. It is obvious that
the intensity of the emission band decreases significantly when the
CQDs is anchored on the ZnS surface compared with the pure
ZnS, and the 2 wt% CQDs/ZnS possesses the lowest intensity,
suggesting an efficient transfer of photoexcited electrons from ZnS
to CQDs.
Based on the above results, one can draw a conclusion that the
introduction of the CQDs reduces the electrons and holes
recombination rate, which promotes the effective charge separation
of ZnS and thus improves the photocatalytic activities. For further
illustrating this assumption, EIS measurements for pure ZnS and
CQDs/ZnS hybrid material samples were carried out to study the
process of electron transfer, and the results are shown in Fig. 10. The
semicircle in the high frequency region can be ascribed to chargeꢀ
transfer resistance, showing the charge transfer through the
Fig. 8 UV–vis diffuse reflectance spectra of pure ZnS and
CQDs/ZnS hybrid materials.
31
electron/electrolyte interface. Obviously, the diameters of the
Nyquist circle at the high frequency region decrease with the
increase of the CQDs content due to the good conductivity of the
CQDs, indicating the lower electron transfer resistance. It means the
occurrence of a faster interfacial charge transfer to the electron
acceptor (CQDs), which contributes to the separation of electronꢀ
32,33
hole pairs.
This result shows good agreement with the PL
analysis, demonstrating that the introduction of the CQDs is a
resultful way to improve the photocatalytic activity of the ZnS.
To sum up, a schematic illustration of the photocatalytic process
of the CQDs/ZnS hybrid materials is shown in Fig. 11. When the
ZnS is irradiated by the simulated sunlight, the electrons can be
excited from the valence band (VB) to the conduction band (CB) of
the ZnS (eq (1)), producing the holes on the VB. Generally, these
photoꢀgenerated electrons and holes recombine rapidly and only few
charges participate in the photocatalytic process. Nevertheless, when
the CQDs are introduced to form hybrid materials, the electrons on
the CB of the ZnS tend to transfer to CQDs owing to their excellent
electronic conductivity (eq (2)), resulting in an effective separation
of the photoꢀgenerated electronꢀhole pairs. Thus, the photocatalytic
activity of the hybrid materials is enhanced. Eventually, the electrons
react with absorbed molecular oxygen forming the superoxide
Fig. 9 PL spectra of pure ZnS and CQDs/ZnS hybrid materials.
ꢀ
8,34
radical anion ·O
2
(eq (3)).
In aqueous medium, the
photogenerated holes in the VB of ZnS can be easily trapped by the
hydroxyl ions in the solution, producing extremely strong oxidant
Fig. 10 Nyquist plots of pure ZnS and CQDs/ZnS hybrid materials.
8
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2
X. Fang, Y. Bando, M. Liao, U. K. Gautam, C. Zhi, B. Dierre, B.
Liu, T. Zhai, T. Sekiguchi, Y. Koide, D. Golberg, Adv. Mater.,
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Pandey, J. Mater. Chem. B, 2014, 2, 522.
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015, 5, 18342.
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T. Zhu, C. Zhang, G. W. Ho, J. Phys. Chem. C, 2015, 119, 1667.
C. J. Chang, K. W. Chu, M. H. Hsu, C. Y. Chen, Int. J. Hydrogen
Energ, 2015, 40, 14498.
Fig. 11 Schematic of the separation and transfer of photogenerated
charges in the CQDs/ZnS hybrid material combined with the
possible reaction mechanism of photocatalytic procedure.
7
8
9
1
J. Cao, Q. Liu, D. Han, S. Yang, J. Yang, T. Wang, H. Niu, RSC
Adv., 2014, 4, 30798.
J. Di, J. Xia, Y. Ge, H. Li, H. Ji, H. Xu, Q. Zhang, H. Li, M. Li,
Appl. Catal., B, 2015, 168ꢀ169, 51.
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L. Lee, W. K. Choi, Nat. nanotechnol., 2012, 7 , 465.
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Lin, B. A. Harruff, L. M. Veca, D. Murray, S. Y. Xie, Y. P. Sun, J.
Am. Chem. Soc., 2007, 129, 11318.
radicals ·OH (eq (4)). The hydroxyl radicals will recombine to form
H O (eq (5)), and the H O may react with the superoxide radical
2 2 2 2
anions to regenerate hydroxyl radicals (eq (6)). These photoꢀ
generated active factors can react with the organic pollutants (MB,
RhB, CIP), degradating into carbon dioxide and water eventually (eq
4
(7)).
11 S. T. Yang, L. Cao, P.G. Luo, F. Lu, X. Wang, H. Wang, M. J.
Meziani, Y. Liu, G. Qi, Y. P. Sun, J. Am. Chem. Soc., 2009, 131,
11308.
2 R. Liu, H. Huang, H. Li, Y. Liu, J. Zhong, Y. Li, S. Zhang, Z.
Kang, ACS Catal., 2014, 4, 328.
−
+
ZnS + hυ → e + h
(1)
(2)
(3)
ZnS
ZnS
−
−
1
e
e
→ e
ZnS
CQDs
−
−
+ O → ⋅O
CQDs
2
2
13 Z. Qian, X. Shan, L. Chai, J. Ma, J. Chen, H. Feng, ACS Appl.
+
Mater. Inter., 2014, 6, 6797.
4 Y. Li, Y. Zhong, Y. Zhang, W. Weng, S. Li, Sens. Actuators, B,
2015, 206, 735.
5 H. Zhang, H. Huang, H. Ming, H. Li, L. Zhang, Y. Liu, Z. Kang,
J. Mater. Chem., 2012, 22, 10501.
6 H. Yu, H. Zhang, H. Huang, Y. Liu, H. Li, H. Ming, Z. Kang,
New J. Chem., 2012, 36, 1031.
h
+ H O →
⋅
OH
(4)
(5)
ZnS
2
1
1
1
1
1
1
2
2
⋅OH → H O
2
2
−
−
H O +
⋅
O2 → OH +
⋅
OH + O2
(6)
(7)
2
2
⋅
OH + dye → H O + CO + intermediates
2
2
4
.
Conclusions
7 P. S. Saud, B. Pant, A. M. Alam, Z. K. Ghouri, M. Park, H. Y.
Kim, Ceram. Int., 2015, 41, 11953.
In summary, the CQDs/ZnS hybrid materials were fabricated via a
simple
strategy. Importantly, the
enhanced photocatalytic decomposition activities of both organic
dyes (MB and RhB) and colorless antibiotic (CIP) under simulated
sunlight irradiation. The 2 wt% CQDs/ZnS hybrid materials possess
8 H. Zhang, H. Ming, S. Lian, H. Huang, H. Li, L. Zhang, Y. Liu,
Z. Kang, S.T. Lee, Dalton Trans., 2011, 40, 10822.
9 Z. Guo , C. Shao, M. Zhang, J. Mu, Z. Zhang, P. Zhang, B.
Chen and Y. Liu, J. Mater. Chem., 2011, 21, 12083.
0 S. Zhu, Q. Meng, L. Wang, J. Zhang, Y. Song, H. Jin, K. Zhang,
H. Sun, H. Wang, B. Yang, Angew. Chem., 2013, 52, 3953.
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2 B. Y. Yu, S. Y. Kwak, J. Mater. Chem., 2012, 22, 8345.
3 X. Zhang, R. Li, M. Jia, S. Wang, Y. Huang, C. Chen, Chem. Eng.
J., 2015, 274, 290.
and
efficient
hydrothermal
asꢀprepared
and
bath
products
reflux
exhibit
the
optimal photocatalytic performance. The
optical
and
2
2
2
electrochemical properties of the products are investigated and prove
that the enhancement of photocatalytic activities are ascribed to the
improved charge separation efficiency which caused by CQDs. The
reaction mechanism has been discussed as well. This work may
provide inspirations in improving the photocatalytic activity of
semiconductor materials.
2
4 M. Sturini, A. Speltini, F. Maraschi, A. Profumo, L. Pretali, E. A.
Irastorza, E. Fasani, A. Albini, Appl. Catal. B: Environ., 2012,
119–120, 32.
2
2
5 M. A. Rauf , M. A. Meetani, A. Khaleel, A. Ahmed, Chem. Eng.
J., 2010, 157, 373.
Acknowledgements
6
C. Yogi, K. Kojima, N. Wada, H. Tokumoto, T. Takai, T.
The authors thank the National Natural Science Foundation of
China (51372212).
Mizoguchi, H. Tamiaki, Thin Solid Films, 2008, 516, 5881.
27 A. Orendorz, C. Ziegler, H. Gnaser, Appl. Surf. Sci., 2008, 255,
011.
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0 | J. Name., 2012, 00, 1-3
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Graphical Abstract
Dandelion-like ZnS/carbon quantum dots hybrid materials with
enhanced photocatalytic activity toward organic pollutants
a
a
a
a
Fangwang Ming, Jinqing Hong, Xun Xu, Zhoucheng Wang*
a
Department of Chemical and Biochemical Engineering, College of Chemistry and Chemical
Engineering, Xiamen University, Xiamen 361005, China
This work may provide important inspirations in improving the photocatalytic activity.