Mechanism study on the photocatalytic efficiency enhancement of MoS2 modified Zn-AgIn5S8 quantum dots

Article abstract of DOI:10.1039/c6ra19949f

In this work, colloidal quantum dots (QDs) of environmentally friendly multinary semiconductor Zn-AgIn₅S₈ are used for heterostructure photocatalysts with hydrothermally deposited MoS₂ focusing on the mechanism study of charge transfer processes in the photocatalytic system. The influence of MoS₂ is systematically investigated on the structure, optical properties and photocatalytic activity of the Zn-AgIn₅S₈/MoS₂ nanocomposites, where the compromise of the catalytic activity and light shielding effect and increased stability is observed with increasing MoS₂. The optimized Zn-AgIn₅S₈/MoS₂-5% composite exhibits high photocatalytic activity for nearly complete degradation of Rhodamine B within 4 min under visible light irradiation. The composite photocatalysts also show good activity on degradation of other organic pollutants including a colorless antibiotic, tetracycline. Photocatalytic mechanism study proves that the main oxidizing species is the superoxide radical formed by the reaction of photogenerated electrons and oxygen molecules, for which MoS₂ mainly acts as a cocatalyst that promotes the electron transfer from the Zn-AgIn₅S₈ QDs. Time-resolved photoluminescence spectroscopy further reveals the increased charge separation by MoS₂ from the photoluminescence quenching and lifetime decrease of the QDs, especially that of the long-lived trap states. Moreover, more efficient charge transfer from the dyes is also observed with Zn-AgIn₅S₈/MoS₂, which is distinctively different from that with the Zn-AgIn₅S₈ QDs, indicating stronger interaction between the dye and photocatalyst with the introduction of MoS₂. Based on these results, a continuous charge flow mechanism is proposed, where the electron transfer can be promoted by MoS₂ not only from the QDs but also from the dyes that are all converted to superoxide radicals and contributed to the photocatalytic activity increase.

Full text of DOI:10.1039/c6ra19949f

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Page 1 of 34
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Mechanism study on the photocatalytic efficiency
enhancement of MoS2 modified ZnꢀAgIn5S8 quantum dots
Guan Gong,# Yanhong Liu,# Baodong Mao,* Bo Wang, Lili Tan, Di Li, Yu Liu and
Weidong Shi*
School of Chemistry and Chemical Engineering, Jiangsu University, Zhenjiang
212013, P.R. China
∗Corresponding authors. Eꢀmail: maobd@ujs.edu.cn (B.M.), swd1978@ujs.edu.cn
(W.S.).
#These authors contributed equally to this work.
1

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Abstract: In this work, colloidal quantum dots (QDs) of an environment friendly
multinary semiconductor ZnꢀAgIn5S8 are used for heterostructure photocatalysts with
hydrothermally deposited MoS2 focusing on the mechanism study of charge transfer
processes in the photocatalytic system. The influence of MoS2 is systematically
investigated on the structure, optical properties and photocatalytic activity of the
ZnꢀAgIn5S8/MoS2 nanocomposites, where the compromise of the catalytic activity
and light shielding effect and increased stability is observed with increasing MoS2.
The optimized ZnꢀAgIn5S8/MoS2ꢀ5% composite exhibits high photocatalytic activity
for nearly complete degradation of Rhodamine B within 4 min under visible light
irradiation. The composite photocatalysts also show good activity on degradation of
other organic pollutants including a colorless antibiotic, tetracycline. Photocatalytic
mechanism study proves that the main oxidizing specie is the superoxide radicals
formed by the reaction of photogenerated electrons and oxygen molecules, for which
MoS2 mainly acts as a cocatalyst that promotes the electron transfer from the
ZnꢀAgIn5S8 QDs. Timeꢀresolved photoluminescence spectroscopy further reveals the
increased charge separation by MoS2 from the photoluminescence quenching and
lifetime decrease of the QDs, especially that of the longꢀlived trap states. Moreover,
more efficient charge transfer from the dyes is also observed with ZnꢀAgIn5S8/MoS2,
which is distinctively different from that with the ZnꢀAgIn5S8 QDs, indicating
stronger interaction between the dye and photocatalyst with the introduction of MoS2.
Based on these results, a continuous charge flow mechanism is proposed, where the
electron transfer can be promoted by MoS2 not only from the QDs but also from the
2

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dyes that are all converted to superoxide radicals and contributed to the photocatalytic
activity increase.
Keywords: IꢀIIIꢀVI quantum dots; MoS2; Heterostructure; Photocatalysis;
Timeꢀresolved photoluminescence.
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1. Introduction
Photocatalysis, as a widely studied sunlightꢀdriven technique, has been considered
as one of the most promising solutions for environment cleaning and renewable solar
fuel production.1ꢀ3 Generally speaking, an efficient photocatalyst requires sufficient
sun light absorption, especially in the visible range, and effective separation and
utilization of the photogenerated charge carriers. In the past decades, extensive efforts
have been devoted to the development of efficient visible light photocatalysts.4ꢀ6
Owing to the direct bandgap, high light absorption coefficient and low toxicity,
multinary I−III−VI chalcogenide nanocrystals (NCs) (I = Cu, Ag; III=Al, In, Ga;
VI=S, Se) have been comprehensively studied in the photocatalytic field,7ꢀ11. More
importantly, the IꢀIIIꢀVI NCs hold a unique advantage for energy conversion
applications that their composition can be varied in a large range while the lattice
structure being maintained, which renders them excellent tunability of the band gap
and band energy positions.12ꢀ14 Photocatalytic degradation of organic dyes has been
16
17
demonstrated for CuInS2,15 AgInS2 and AgGaS2 nanostructures. Various alloyed
and doped IꢀIIIꢀVI NCs have also been developed for photocatalysis by alloying
within the IꢀIIIꢀVI materials18 or alloying with other materials, such as ZnS19 and
CdS20, showing superior photocatalytic activity compared with the original
counterparts. For example, AgIn5S8/AgInS2,21 ZnS–AgInS2,22ꢀ23 AgInxGa1–xS2,17 and
24
AgInZnxSx+2 NC based photocatalysts have been developed with composition
dependent photocatalytic activity indicating a promising future for photocatalytic
applications by composition and structure design. However, these IꢀIIIꢀVI NCs still
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suffer from the common stability issue of sulfide materials and large effort on the
photocatalyst structure design is needed for activity and stability improvement for
practical applications.
A main consideration in photocatalyst design is the prompt separation and
migration of the photogenerated electrons and holes, on which loading coꢀcatalysts
plays an important role.15, 25ꢀ27 Among these materials, MoS2 is a 2D layered material
with typical grapheneꢀlike structure that has attracted great attention in photocatalysis
and electrocatalysis areas due to its low cost, high abundance and unique
photoelectric properties.28ꢀ31 MoS2 has been combined with lots of semiconductors for
photocatalytic applications, such as TiO2, Bi2MoO6, BiVO4, CdS and ZnIn2S4 etc,
where it provides effective surface active sites that can prompt the surface catalytic
reaction, inhibit recombination of photoꢀgenerated electrons and holes, and prevent
32ꢀ35
the occurrence of counter reaction between active species and product.20,
Moreover, MoS2 has a unique advantage for compositing with sulfide semiconductors
that the shared S2ꢀ could reduce the lattice mismatch and defect states at the interfaces.
On the other hand, since the active sites are mainly the exposed edge sites of the 2D
S−Mo−S planes, recently more and more work has been contributed to the size
control of MoS2 nanostructures towards high quality heterostructure photocatalysts
with abundant active edge sites.20 These research works suggest a promising future of
sulfide semiconductors combined with MoS2 for photocatalytic application. However,
there has not been any report on size controlled MoS2 combined with IꢀIIIꢀVI NCs
and more profound investigation on the structure and interface property manipulation
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is still needed towards a clear understanding of the role of MoS2 in the photocatalytic
processes due to the complexity of the band energy alignment, structure, and
deposition methods.
In the present study, series of ZnꢀAgIn5S8/MoS2 nanocomposites were prepared by
combining MoS2 with monodispersed ZnꢀAgIn5S8 quantum dots (QDs) towards a
more profound understanding of the role of MoS2 in photocatalyst design. The effect
of the MoS2 content on the structure, optical properties and photocatalytic activity of
the ZnꢀAgIn5S8/MoS2 nanocomposites was systematically investigated towards better
understanding of the crucial role of MoS2 in the photocatalysis system. It was found
that the visibleꢀlightꢀdriven photocatalytic activity of ZnꢀAgIn5S8/MoS2
nanocomposites were dramatically enhanced compared to pure ZnꢀAgIn5S8 QDs. The
−
related photocatalytic mechanism study indicates that the superoxide radicals (•O2 )
are the main active specie with minor contribution from the holes. Steady state and
timeꢀresolved photoluminescence (PL) spectroscopies were used together for the first
time to investigate the role of MoS2 in the charge transfer properties of the
ZnꢀAgIn5S8/MoS2 photocatalysts in terms of PL quenching and lifetime change.
2. Experimental
2.1 Chemicals
All chemicals are analytical grade and used without further purification. Silver
nitrate (AgNO3), zinc acetate (Zn(OAc)2·2H2O), indium nitrate (In(NO3)3·4.5H2O),
thioacetamide (TAA), sodium hydroxide (NaOH), thiourea (CH4N2S),
hexaammonium heptamolybdate ((NH4)6Mo7O24·4H2O), ethylenediamine tetraacetic
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acid (EDTA) dipotassium salt (C10H14K2N2O8·2H2O), Rhodamine B (RhB),
tetracycline (TC), methyl orange (MO), methyl red (MR) and absolute ethanol were
purchased from Sinopharm Chemical Reagent Co. Ltd. Lꢀcysteine
(HSCH2CH(NH2)CO2H), 1,4ꢀbenzoquinone (BQ), isopropanol (IPA), and
5,5ꢀdimethylꢀ1ꢀpyrroline Nꢀoxide (DMPO) were purchased from Aladdin (China).
2.2 Synthesis of ZnꢀAgIn5S8 QDs and ZnꢀAgIn5S8/MoS2 composites
Pure ZnꢀAgIn5S8 QDs with Ag:In:Zn ratio of 2:10:5 were prepared by a oneꢀpot
hydrothermal method based on previous report with revisions.19 In a typical synthesis
process, 0.34 mmol of AgNO3, 1.7 mmol of In(NO3)3·4.5H2O, and 0.85 mmol of
Zn(OAc)2·2H2O were added into 5.5 mL of water and a clear solution was obtained
after stiring. Then 2 mL of 2.5 M Lꢀcysteine solution was added under vigorous
stirring. The pH value of the mixture was adjusted to 8.5 using 1.0 M NaOH solution
and then 3.25 mmol of thioacetamide was rapidly added. The solution was transferred
to a 35 ml Teflonꢀlined stainless steel autoclave, sealed and heated at 110 ˚C for 4 h.
After naturally cooling down to room temperature, the samples were collected by
ethanol precipitation and centrifugation and were further washed with water/ethanol
cycles for three times. The cleaned QDs were dispersed in water for further
characterization and photocatalytic tests.
For preparation of ZnꢀAgIn5S8/MoS2 composite materials, 300 mg of the purified
ZnꢀAgIn5S8 QDs were added to 30 mL of deionized water containing appropriate
amounts of (NH4)6Mo7O24·4H2O and thiourea, where the molar ratio of Mo:S was
kept at 7:30. Subsequently, the solution was transferred to a 50 mL Teflonꢀlined
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stainless steel autoclave and maintained at 200 ˚C for 8 h. The purification procedure
of ZnꢀAgIn5S8/MoS2 materials was the same as that of pure ZnꢀAgIn5S8 QDs. For
comparison, a series of ZnꢀAgIn5S8/MoS2 composites were prepared with different
amount of MoS2 ranging from 1 wt%, 5 wt%, 10 wt%, 15 wt% to 30 wt%.
2.3 Characterizations
Structure of the samples was tested by Xꢀray diffraction (XRD, D8 ADVANCE,
Bruker, Germany), using CuꢀKα radiation source (λ= 1.54056 Å) at a scanning rate of
4.0 °/min. The transmission electron microscopy (TEM), and highꢀresolution TEM
(HRTEM) were also used to characterize the samples using a Tecnai G2 F30 SꢀTwin
(FEI) electron microscope with an accelerating voltage of 200 kV. Raman spectra
were recorded with an America ThermoFisher (DXR) Laser Raman Spectrometer
with a 532 nm laser. The photoluminescence (PL) spectra of the samples were
measured on a PerkinElmer LS 55 fluorescence spectrophotometer with an excitation
wavelength of 450 nm. Timeꢀresolved photoluminescence spectra were collected on a
QuantaMasterTM 40 spectrometer (Photon Technology International, Inc.) with
excitation wavelength of 481 nm. The electron spin resonance (ESR) spectra were
collected on a Bruker EPR A300ꢀ10/12 spectrometer.
2.4 Photocatalytic activity evaluation and mechanism study
The photocatalytic activities of the ZnꢀAgIn5S8/MoS2 composite materials were
first evaluated via the photocatalytic degradation of RhB in aqueous solution under
visible light irradiation. A 250 W Xe lamp with a 420 nm cutoff filter was used to
provide visible light irradiation. In each experiment, 10 mg of photocatalyst was
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added into a 100 mL of the RhB solution (10 mg L−1). Prior to irradiation, the
suspensions were stirred in the dark for 60 min to reach the adsorptionꢀdesorption
equilibrium. After light irradiation, 4 mL of the suspension was extracted at 2 min
intervals and centrifuged to obtain a clear liquid that will be used for concentration
measurement. In addition, to further explore its photocatalytic application, the
ZnꢀAgIn5S8/MoS2 composite photocatalysts were also investigated for degradation of
different kinds of pollutants, including tetracycline (TC), methyl orange (MO) and
methyl red (MR) with all the operations the same as that of RhB. Finally, the
concentrations of RhB, TC, MO and TR were monitored using a TUꢀ1810 UVꢀvis
spectrometer at corresponding maximum absorption wavelengths of 553, 357, 463
and 410 nm, respectively. The dyes used in this work is mainly chosen in
consideration of their hazard and the energy level alignment with the photocatalyst.
With the rapid economic development, the pollution of wastewater from dyeing
industry is the most dominant and tetracycline is an emerging antibiotic pollutant that
is potentially dangerous for producing drugꢀresistant super bacteria. And RhB is
further studied as the model dye for photodegradation mechanism because of its light
absorption range and suitable energy levels that are suitable to investigate the
interaction with our QDs based photocatalysts.
To probe the active species involved in the photocatalysis process, photocatalytic
degradation of RhB has been studied with the introduction of different kinds of
scavengers, where EDTA36 as the hole scavenger, pꢀbenzoquinone (BQ)37 as the
superoxide radical scavenger, and isopropyl alcohol (IPA)38 as the hydroxyl radical
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scavenger. All other conditions were kept the same to the above photocatalytic
experiments without scavengers. In addition, to further clarify the photocatalytic
reaction mechanism, ESR technique was employed to detect the generated radicals in
the
photocatalytic
system
under
visible
light
irradiation
using
5,5ꢀdimethylꢀ1ꢀpyrroline Nꢀoxide (DMPO) as the radical trapping agent, which can
−
−
trap the •O2 or •OH radicals and form DMPOꢀ•O2 or DMPOꢀ•OH complexes.
−
Briefly, 2.5 mg of the ZnꢀAgIn5S8/MoS2 sample was dispersed in methanol (for •O2 )
and then 30 ꢁL of DMPO was added with ultrasonic dispersion for 3 min. ESR
−
spectra of the trapped DMPOꢀ•O2 radicals were recorded before and after light
irradiation.
3. Results and discussion
3.1. Structure and property characterization
ZnꢀAgIn5S8
A1g
(a)
(b)
1%
5%
E2g
10%
30%
MoS2
MoS2
30%
15%
10%
5%
1%
ZnꢀAgIn5S8
20
30
40
50
60
70
200
300
400
500
600
700
Raman shift (cmꢀ1)
2ꢀtheta (degree)
Fig. 1. (a) XRD patterns of the pure ZnꢀAgIn5S8 QDs, MoS2 and ZnꢀAgIn5S8/MoS2
composites with different MoS2 mass ratios of 1 wt%, 5 wt%, 10 wt%, 15 wt%, 30
wt%. (b) Raman spectra of the ZnꢀAgIn5S8 quantum dots, pure MoS2 nanosheets and
ZnꢀAgIn5S8/MoS2 composites with different MoS2 mass ratios of 1 wt%, 5 wt%, 10
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wt%, 30 wt%.
XRD was used to characterize the phase of the asꢀprepared samples. As shown in
Fig. 1a, the pure ZnꢀAgIn5S8 QDs basically match the characteristic peaks of cubic
AgIn5S8 (JCPDS no. 25ꢀ1329).19 The peaks located at 27.3˚, 46.6˚ and 55.0˚ can be
assigned to the (311), (440) and (533) crystal planes of AgIn5S8, respectively. Owning
to the small size of the particles, all of the diffraction peaks are relatively broad and
weak. The top XRD pattern of MoS2 synthesized with similar conditions can be
indexed to hexagonal phase MoS2 (JCPDS no. 73ꢀ1508) with the peaks of 29.5˚ and
58.5˚ assigned to the (101) and (110) crystal planes of MoS2, respectively.28 For the
ZnꢀAgIn5S8/MoS2 composites, the XRD patterns are similar to that of the initial QDs
with no noticeable characteristic MoS2 peak, even with the mass ratio of MoS2
increases from 1 wt% to 30 wt%. This may be due to the small amount and the poor
crystallinity of MoS2 in the nanocomposites. Moreover, with respect to the invisibility
of the MoS2 diffraction peaks in the heterojunctions, the highly similar XRD patterns
of the ZnꢀAgIn5S8/MoS2 composites indicate that the low content of MoS2 did not
bring any influence on the crystal structure of the ZnꢀAgIn5S8 QDs.
Raman spectroscopy, as a more sensitive technique, was employed to further
investigate the structure evolution of the ZnꢀAgIn5S8/MoS2 nanocomposites in
comparison with the ZnꢀAgIn5S8 QDs and pure MoS2 nanosheets. As shown in Fig.
1b, the strong broad peaks around 318 (for ZnꢀAgIn5S8 QDs), 327 (for
ZnꢀAgIn5S8/MoS2ꢀ1%) and 343 cmꢀ1 (for ZnꢀAgIn5S8/MoS2ꢀ5%) are due to the PL
signal of the ZnꢀAgIn5S8 QDs excited by the 532 nm green laser of the Raman
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spectrometer. Different from pure QDs, a tiny peak at 372 cmꢀ1 was observed with 1
wt% and 5 wt% MoS2 that can be assigned to the E2g mode of MoS2. With the
increase of MoS2 content, the PL peak gradually disappeared and the two Raman
peaks at approximately 372 cmꢀ1 and 402 cmꢀ1 increased dramatically, which are
assigned to the E2g and A1g modes of MoS2, respectively. There are no pronounced
peak shift of the E2g and A1g modes compared to those of the pure MoS2 nanosheets.31
It is worth to mention that until MoS2 content was higher than 5%, the E2g and A1g
modes of MoS2 fully appeared in the composites. The decrease of the PL peak of the
QDs and increase of the E2g and A1g modes of MoS2 suggest the strong interaction
between the two components and the dynamic growing process with increase of MoS2
ratio, which was further verified in the optical property studies presented below.
Fig. 2. TEM images of the ZnꢀAgIn5S8/MoS2 samples with different mass ratios of
MoS2: (a) 0 wt% (ZnꢀAgIn5S8 QDs), (b) 5 wt%, (c) 30 wt% and (d, e) HRTEM
images of AgIn5S8/MoS2ꢀ5%.
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The morphology change of the ZnꢀAgIn5S8/MoS2 composites with different MoS2
content was investigated by TEM to study the hybridization process between the
ZnꢀAgIn5S8 QDs and MoS2. As shown in Fig. 2a, the quaternary ZnꢀAgIn5S8 QD
s
prepared using a low temperature hydrothermal method show sphereꢀshaped
morphology with average diameter of 4.3±0.8 nm, exhibiting relatively uniform size
with a narrow size distribution. Fig. 2b and 2c show the TEM images of the samples
with 5 wt% and 30 wt% of MoS2, respectively. It can be seen that the pure
ZnꢀAgIn5S8 QDs cover the surface of the MoS2 slices. With the increasing amount of
MoS2, the nanocomposites showed size increase and the MoS2 nanosheets became
more and more condensed as indicated by the characteristic stacked sheet structure of
2D layered materials (Fig. 2c). At the same time, the MoS2 slices also gradually
coagulate together (Fig. 2c) instead of individual quantum size particles with
increasing MoS2 content. As illustrated in Fig. 2d&2e, the MoS2 nanosheets and
ZnꢀAgIn5S8 QDs were strongly linked to each other, where the 2D sheetꢀlike
morphology of MoS2 with length over 50 nm already started to form. The enlarged
image of ZnꢀAgIn5S8/MoS2ꢀ5% demonstrated that the ZnꢀAgIn5S8 QDs were attached
to the MoS2 slices and an intimate interfaces were well formed between ZnꢀAgIn5S8
and MoS2 owning to the shared sulfide ions at the interface, which would favor the
charge carrier transfer. The HRTEM images in Fig. 2d&e clearly presented the lattice
spacing of 0.63 nm, which could be assigned as the (002) plane of MoS2,28 while the
fringes with a lattice spacing of 0.33 nm correspond to the (311) plane of the
ZnꢀAgIn5S8 QDs.19
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ZnꢀAgIn
5S
8
1%
5%
10%
15%
30%
2.1
2.2
2.3
2.4
hν
(eV)
300
400
500
600
700
800
Wavelength (nm)
Fig. 3. UV/vis absorption spectra of the ZnꢀAgIn5S8 QDs and ZnꢀAgIn5S8/MoS2
nanocomposites with different MoS2 content: 1 wt%, 5 wt%, 10 wt%, 15 wt%, and 30
wt%. The inset figures are the curves of (α
hυ)2 vs. hυ of ZnꢀAgIn5S8 QDs (upper)
for band gap estination and the photographs (lower) of the ZnꢀAgIn5S8 QDs and
ZnꢀAgIn5S8/MoS2 composites with 1 wt% and 5 wt% of MoS2 dispersed in water.
The optical property of ZnꢀAgIn5S8/MoS2 nanocomposites with increasing MoS2
ratio from 0 wt% to 30 wt% was examined using UV−vis absorption spectroscopy. As
shown in Fig. 3, the absorption edge of the pure ZnꢀAgIn5S8 QDs was estimated to be
~550 nm, while the ZnꢀAgIn5S8/MoS2 composite samples showed absorption edges of
600~650nm, which may be ascribed to the increased visible light absorption caused
by MoS2. Compared with pure ZnꢀAgIn5S8 QDs, the visible light absorption was
improved dramatically with the increase of MoS2 content with colors changing from
bright orange to dark red (lower inset of Fig. 3). The sample color change is also
partly brought by increased light scattering with particle growth, changing from the
clear orange solution (pure QDs) to the turbid orange state (1% MoS2) and further to
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dark red (5% MoS2). When the MoS2 content is higher than 5 wt%, further increase of
light absorption was observed in the whole spectral range, which can be ascribed to
the increased light scattering of the samples partly due to the size increase of
composite particles. The UVꢀvis spectra show that the introduction of MoS2 enhanced
the light absorption dramatically, though it may not necessarily contribute to the
improvement of photocatalytic activity. In addition, the band gap (Eg) of pure
ZnꢀAgIn5S8 QDs composites was estimated to be 2.28 eV from the plot of (
α
hυ)2 vs.
hυ based on the UV/Vis absorption data, as shown in the upper inset in Fig. 3
.
Scheme 1. Schematic illustration of the inꢀsitu growth process of MoS2 on the
ZnꢀAgIn5S8 QDs for the construction of heterostructure photocatalysts.
Based on the above results, we propose a possible growing mechanism for the
ZnꢀAgIn5S8/MoS2 nanocomposites (Scheme 1), where the amount of MoS2 has a
major influence on both of the morphology and the optical properties. With lower
MoS2 ratio (1 wt% and 5 wt% )，the small MoS2 particles deposit on the surface of the
QDs and the resulted ZnꢀAgIn5S8/MoS2 composite particles remain good colloidal
stability along with a color change from orange to dark red (Fig. 3). Further increase
of MoS2 results in the size increase of the composite particles due to the growth of the
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MoS2 2D slices and also brings high light absorption in a wide range and dark color.
The influence of these results was further investigated in photocatalytic applications.
3.2. Photocatalytic activity
ZnꢀAgIn5S8
1.0
(a)
4
(b)
ZAIS
1%
1%
3%
0.8
0.6
0.4
0.2
0.0
3%
5%
10%
15%
30%
MoS2
5%
3
2
1
0
10%
15%
30%
RhB
0
2
4
6
8
0
2
4
6
Time (min)
Time (min)
Fig. 4. Photocatalytic activity (a) and the pseudoꢀfirstꢀorder reaction kinetics (b) for
degradation of RhB using the ZnꢀAgIn5S8/MoS2 composites with different amount of
MoS2 under visible light irradiation.
RhB was used as the probing molecule to explore the photocatalytic degradation
activity of the ZnꢀAgIn5S8/MoS2 nanocomposites under visibleꢀlight irradiation. Fig.
4a shows the photocatalytic activities of the ZnꢀAgIn5S8/MoS2 nanocomposites with
different MoS2 contents as well as the pure ZnꢀAgIn5S8 QDs and pure MoS2
nanosheets under visibleꢀlight irradiation. It can be found that RhB is only slightly
degraded in the presence of MoS2 only or in the absence of various kinds of the
prepared samples, indicating that the photocatalytic activity by MoS2 itself can be
ignored and there is no noticeable decolorization of the dye without photocalysts.
Pure ZnꢀAgIn5S8 QDs (88%) showed relatively high photocatalytic activity for
degradation of RhB under visible light within 8 min, which may be due to the small
size that provides high surface area for the photocatalytic reaction to occur. Along
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with the introduction of different contents of MoS2, the photocatalytic performance of
ZnꢀAgIn5S8/MoS2 heterojunction could be enhanced. The ZnꢀAgIn5S8/MoS2ꢀ5%
composite shows the highest activity that RhB can be completely photodegraded in 4
min, which is much better than that of pure ZnꢀAgIn5S8 QDs. This degradation
efficiency is much higher than that of previously reported AgIn5S8 powders (89.4% in
4 h for MO),39 monodispersed AgIn5S8 microspheres (98% in 20 min for MO) or
ZnIn2S4/AgIn5S8 heteromicrospheres (99% in 50 min for RhB).40 The boosted
photocatalytic activity may be attributed to the high surface area of the small size
QDꢀbased nanocomposites and the intimate interface between ZnꢀAgIn5S8 and MoS2
due to the shared S2ꢀ anions that can facilitate the interfacial charge transfer.20, 32
Moreover, the photocatalytic degradation kinetics results indicate that the reaction
rate fit well with the pseudoꢀfirst order correlation as shown in Fig. 4b. It can be seen
that the ZnꢀAgIn5S8/MoS2ꢀ5% sample exhibits the highest degradation rate, which is
about 3.35 times higher than that of pure ZnꢀAgIn5S8 QDs. With increasing MoS2
content, the photocatalytic activity of the ZnꢀAgIn5S8/MoS2 composite materials was
enhanced at first and then weakened. When the content of MoS2 is 1wt% and 3wt%,
the photocatalytic activity has gradually enhanced comparing with the pure
ZnꢀAgIn5S8 QDs. However, when MoS2 loading exceeds 5 wt%, the photocatalytic
activity of the 10 wt%, 15 wt% and 30 wt% samples started to decrease gradually
compared to the 5 wt% sample. The poorer photocatalytic activity of the composites
than pure QDs is only observed with high ratio of MoS2 (15% and 30%). These
results indicate that the optimal loading amount of ZnꢀAgIn5S8/MoS2 sample was 5 wt%
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in terms of photocatalytic activity, which suggests that MoS2 acts as a cocatalyst
instead of light absorber and works best at relatively low loading amount. There are
two contrast impacts with the introduction of MoS2: the increased charge separation
(positive effect) and the increased light shielding (negative effect). The decrease of
photocatalytic activity at high loading amount (10ꢀ30 wt%) may be due the strong
light absorption of MoS2 that shielded the light absorption of the QDs similar to
previously reported work.41 This observation indicates that the loading amount of
MoS2 has to be compromised considering the catalytic activity enhancement and the
light shielding effect.
1.0
(a)1.0
0.8
(b)
0.8
0.6
0.4
0.2
0.0
0.6
0.4
0.2
0.0
0
4
8
12
16
20
24
0
4
8
12
16
20
24
Time (min)
Time (min)
Fig. 5 Cycle runs of photocatalytic degradation of RhB under visible light irradiation
.
over the ZnꢀAgIn5S8/MoS2 samples with 5 wt% (a) and 30 wt% (b) of MoS2.
In order to evaluate the stability and reusability of the ZnꢀAgIn5S8/MoS2
composite photocatalysts, cycle runs of photocatalytic degradation of RhB were
carried out under visibleꢀlight irradiation. As shown in Fig. 5a, the efficiency of
ZnꢀAgIn5S8/MoS2ꢀ5% decreases from 100% in the 1st run to 65% in the 3rd run,
which may be due to the incomplete recycle of the small sized photocatalyst. On the
other hand, the photocatalytic activity of ZnꢀAgIn5S8/MoS2ꢀ30% decreases from 100%
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in the 1st run to 90% in the 3rd run (Fig. 5b), e.g. the stability of the
ZnꢀAgIn5S8/MoS2 photocatalysts was enhanced with the increase of MoS2. A
plausible explanation is that the presence of extra MoS2 contributed to the efficient
separation of the photoꢀgenerated electrons and holes. The photocatalytic activity of
the AgIn5S8/MoS2ꢀ30% sample has a little deactivation (<10%) after three successive
cycles for the degradation of RhB under visibleꢀlight irradiation, indicating that our
photocatalyst possesses relatively high stability for practical applications. 32
MO
MR
TC
1.0
0.8
0.6
0.4
0.2
0.0
0
2
4
6
8
10
12
Time (min)
Fig. 6. Photocatalytic activity of the ZnꢀAgIn5S8/MoS2ꢀ5% nanocomposite for the
degradation of TC, MO and MR under visible light irradiation.
To further investigate their photocatalytic applications, the ZnꢀAgIn5S8/MoS2ꢀ5%
sample was also evaluated for degradation of other dyes (MO and MR) and an
antibiotic (TC). Fig. 6 shows the photocatalytic performance on the degradation of TC,
MO and MR under visible light. It was found that the ZnꢀAgIn5S8/MoS2ꢀ5%
nanocomposite could also efficiently decompose these organic pollutants, where TC
could be degraded (74%) in 4 min and MO and MR could be completely degraded
within 12 and 8 min. Hence, it is implied that our ZnꢀAgIn5S8/MoS2 photocatalysts
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display excellent photocatalytic activity in the visibleꢀlight range and has great
potential in environment purification.
3.3. Photocatalytic mechanism study
(a)
♦
dark
light
♦
1.0
0.8
0.6
0.4
0.2
0.0
(b) superoxide radical
♦
BQ
EDTA
IPA
♦
♦
♦
♦
♦
♦
♦
No quencher
♦
♦
0
2
4
6
8
10
12
3460
3480
3500
3520
3540
Time (min)
Magnetic Field (G)
Fig. 7. (a) Trapping experiment of the active species during photocatalytic
degradation of RhB over ZnꢀAgIn5S8/MoS2ꢀ5% nanocomposite alone and with the
−
addition of EDTA (quencher of h+), BQ (quencher of •O2 ), and IPA (quencher of •OH)
under visible light irradiation. (b) ESR spectra of the superoxide radical adducts
trapped by DMPO in ZnꢀAgIn5S8/MoS2ꢀ5% methanol dispersion under visible light
irradiation.
Active species. To better understand the photocatalysis mechanism, trapping
experiments of the active species were carried out during photocatalytic degradation
of RhB. As shown in Fig. 7a, for ZnꢀAgIn5S8/MoS2ꢀ5%, the photocatalytic
degradation of RhB shows no obvious change (red curve) with IPA (quencher of •OH)
added compared with no quencher, which indicates that •OH is not the main reactive
specie. When EDTA (quencher of h+) was added into the solution, the degradation of
RhB slowed down slightly, but was still completed within 12 min, which indicates
that the photogenerated holes play a minor role in the photocatalytic system. In
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−
contrast, addition of BQ (•O2 scavenger) caused complete quenching of
−
photodegradation, implying that •O2 is the major reactive specie and plays a
predominant role in the photocatalytic process. Therefore, it is clearly illustrated that
−
•O2 from the reaction of photogenerated electrons and O2 are the main oxidative
species, while the photogenerated h+ also contributed in certain extent for the
degradation of RhB in the ZnꢀAgIn5S8/MoS2 composite photocatalyst system.
In order to further reveal the reactive oxidation species formed over the
ZnꢀAgIn5S8/MoS2 materials, ESR spectroscopy was performed after light irradiation.
It is known that DMPO is generally used as a radical trapping agent forming
−
DMPOꢀ•O2 or DMPOꢀ•OH.37ꢀ38 As shown in Fig. 7b, the twelve characteristic peaks
−
of DMPO−•O2 can be clearly observed in the methanol dispersion of
ZnꢀAgIn5S8/MoS2ꢀ5% nanocomposite after visible light irradiation, indicating that the
−
•O2 radicals were produced from the reaction of photogenerated electrons and O2
molecules in the photocatalytic systems. Combined with the trapping experiment (Fig.
−
7a), it can be inferred that •O2 played a major role in the photocatalytic system.
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Scheme 2. The proposed photocatalytic mechanism of the ZnꢀAgIn5S8/MoS2
nanocomposites for degradation of organic pollutants under visible light irradiation:
the MoS2 enhanced electron injection from the QDs, the subsequently increased
interaction between the dyes and photocatalysts, and the formation of superoxide
radicals.
Charge transfer and reaction processes. On the basis of the above results, a
probable mechanism is proposed for the enhanced photocatalytic activity of
ZnꢀAgIn5S8/MoS2 nanocomposites. It was reported that AgIn5S8 has direct bandgaps
47
of 1.7ꢀ1.8 eV, as one of the most potential candidates for the visibleꢀlightꢀdriven
photocatalytic applications. Here the effect of Zn doping of AgIn5S8 QDs in the whole
system can be divided into three parts: changing absorption threshold, shifting the
band energy levels and reducing the defect states. And the crucial and positive
influence is increasing the conduction band and reducing the defect states according
to the literatures. As illustrated in Scheme 2, the ZnꢀAgIn5S8 QDs can be excited
under visibleꢀlight illumination, leading to the generation of electronꢀhole pairs. The
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photoexcited electrons can be transfered from CB of the ZnꢀAgIn5S8 QDs to MoS2
due to the proper band energy alignment, and the separated electrons accumulated in
−
the CB of MoS2 can reduce the adsorbed O2 to form •O2 radicals, which is a powerful
oxidative specie that can react with organic pollutants and convert them to small
molecules. Meanwhile, the photogenerated holes left in the VB of ZnꢀAgIn5S8 can
directly oxidize organic pollutants adsorbed on the surface of the photocatalyst,
though it only accounts for a small portion of the photodegradation. Here MoS2 is
more like a cocatalyst that can enhance the charge separation and accept the excited
−
electrons in the photocatalyst system, which results in the efficient production of •O2 ,
the main oxidative specie. To better explain the charge separation process in the
ZnꢀAgIn5S8/MoS2 composites, the band edge potentials of the ZnꢀAgIn5S8 QDs were
estimated by the following equations:
ECB=XꢀEe ꢀ0.5Eg
EVB=ECB +Eg
(1)
(2)
where ECB and EVB are the conduction and valence band edge potential, X is the
electronegativity of the semiconductor from the geometric mean of the
electronegativity of the constituent atoms, Ee is the energy of free electrons on the
hydrogen scale (~4.5 eV), and Eg is the band gap of the semiconductor (2.28 eV here
for the ZnꢀAgIn5S8 QDs as estimated from Fig. 3). The Zn effect on QD Ecb and Evb
energy levels is mainly determined by the electronegativity of the ZnꢀAgIn5S8
semiconductor. It is the geometric mean value of the absolution electronegativity of
each atom of the compound. According to the initial ratio of each element, Zn doping
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of AgIn5S8 QDs forms an alloy, such as AgIn5S8ꢀ(ZnS)5 。 After calculating, X
approximately equals to 4.9 eV. Combined with the above equation, the CB and VB
potentials are about ꢀ0.74 and 1.54 V for the ZnꢀAgIn5S8 QDs. It was reported that the
CB and VB potentials of AgIn5S8 are ꢀ0.66 and 1.10 eV, while the electronegativity of
AgIn5S8 is 4.72 eV.48 Our results indicate that Zn doping of AgIn5S8 QDs could
increase the conduction band from ꢀ0.66 eV to ꢀ0.74 eV, which increases the reducing
ability of the photogenerated electrons. When the ZnꢀAgIn5S8 QDs is excited,
electrons can be efficiently transferred to the MoS2 cocatalyst and reduce the
−
surfaceꢀadsorbed O2 into •O2 (ꢀ0.046 V vs NHE), inducing the formation of the
active oxidizing species.32, 36
On the other hand, it was reported that the strong interaction between the dyes
and photocatalsts plays an important role in lots of photocatalytic processes.4 In this
ZnꢀAgIn5S8/MoS2 system, we propose that the dye/photocatalst interaction can be
effectively enhanced with the existence of MoS2 since the photogenerated electrons in
the QDs can be promptly transferred. In the photocatalytic reaction, electrons from
RhBꢀLUMO (ꢀ1.42V vs NHE)4 can be injected into the ZnꢀAgIn5S8 QDs conduction
band (ꢀ0.74 V vs NHE). The matching band energy levels (RhB, ZnꢀAgIn5S8 and
MoS2) are shown in Scheme 2 and this synergistic effect are favorable for
photogenerated electron−hole separation and transfer at both interfaces. To better
demonstrate the effect of MoS2 in the photocatalytic system, the charge separation
processes were further investigated by PL quenching and lifetime measurements for
both of the QDs and RhB as discussed below.
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(a)
(b)
ZnꢀAgIn
1%
5%
5S8
ZnꢀAgIn
1%
5%
5S8
450
300
150
0
500
600
700
800
0
200
400
600
800
Time (ns)
Wavelength (nm)
Fig. 8. (a) Evolution of the PL spectra and (b) corresponding PL decay curves, and
biꢀexponential fitting curves of the ZnꢀAgIn5S8 QDs and ZnꢀAgIn5S8/MoS2 samples
with MoS2 mass ratios of 1 wt% and 5 wt%
.
Table 1. PL decay parameters of the ZnꢀAgIn5S8 QDs and ZnꢀAgIn5S8/MoS2
composites.
A1 / %
36.1
A2 / %
63.9
61.5
9.8
τ
1 / ns
τ
2 / ns
τave / ns
9.95
27.09
6.08
238.75
216.45
95.54
156.05
143.6
14.85
ZnꢀAgIn5S8 QDs
ZnꢀAgIn5S8/MoS2ꢀ1%
ZnꢀAgIn5S8/MoS2ꢀ5%
38.5
90.2
Enhanced charge transfer between the two components by MoS2. In order to
explore the charge separation and recombination processes in the prepared
photocatalysts, PL spectroscopy was used to analyze the photocatalytic mechanism.
As shown in Fig. 8a, the ZnꢀAgIn5S8 QDs show a strong PL peak centered at 646 nm
with excitation of 450 nm. With the increase of MoS2 content, PL intensity of the
ZnꢀAgIn5S8/MoS2 composites decreased significantly from 468 to 106 (for 1 wt%)
and 14 (for 5 wt%) and reached almost zero when the loading amount exceeded 5 wt%
(data not presented). This phenomenon was also observed in the Raman spectra as
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shown in Fig. 1b. The PL intensity decrease implies the enhanced charge separation
and eliminated radiative recombination of the photoꢀgenerated electrons and holes in
the composites. This result clearly indicates that MoS2 has a major influence on the
charge separation and subsequently photocatalytic activities of ZnꢀAgIn5S8/MoS2
samples. The ZnꢀAgIn5S8/MoS2ꢀ5% composite shows the highest photocatalytic
performance compared with the other samples, which reveals efficient transfer of
photoꢀexcited electrons from ZnꢀAgIn5S8 QDs to MoS2 and the suppressed radiative
recombination of the photoꢀgenerated charge carriers
.
To further probe the mechanism of electron−hole recombination in the
ZnꢀAgIn5S8/MoS2 samples, timeꢀresolved PL spectroscopy was used to assess the PL
lifetimes (Fig. 8b). The PL decay curve of the prepared sample can be well fitted with
the biexponential function:
I(t) =A1exp(ꢀt/τ1)+A2exp(ꢀt/τ2)
τave = (A1τ1+A2τ2)/(A1+A2)
(3)
(4)
where I(t) is the PL intensity at delay time of t, A1 and A2 are the relative weights of
the decay components, τ1 and τ2 represent the lifetimes of two decay pathways. In
comparison with ZnꢀAgIn5S8 QDs (τave=156.05 ns), the PL lifetime of
ZnꢀAgIn5S8/MoS2 composites with 1 wt% and 5 wt% MoS2 (τave=143.6 and 14.85 ns,
respectively) decrease rapidly with increasing MoS2. The addition of MoS2 results in
an efficient PL quenching and lifetime decrease. It was reported that the fast decay
component (τ1) can be attributed to the surface trap state recombination, while the
slowly decay component (τ2) can be ascribed to deep donorꢀacceptor recombination
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process.19, 42ꢀ43 When loaded with MoS2, lifetime of both the surface states and deep
trap states were dramatically decreased (Table 1) indicating the effective quenching of
the photogenerated carriers, which can be attributed to the efficient electron transfer
from surface states or deep trap states near CB of the ZnꢀAgIn5S8 QDs to MoS2 that is
competing with the radiative recombination process. Moreover, a more efficient
quenching of the long lived deep trap states (A2 changing from 63.9% to 61.5% and
9.8% with increasing MoS2) than the short lived surface trap states (A1 in Table 1.)
was observed, which suggests that introduction of MoS2 makes the electron transfer
−
and •O2 formation processes more favorable than the electron transfer to the deep
trap states within the QDs. The electrons transferred to MoS2 will further react with
−
O2 to form •O2 radicals for photocatalyic reactions.33 These timeꢀresolved PL results
are in good agreement with the decrease of PL intensity and increase of photocatalytic
activity, which further demonstrates that MoS2 acts as a cocatalyst, rather than a light
absorber, and mainly contributes to the increased charge separation during the
photocatalytic process.
Enhanced interaction between the dye and photocatalysts by MoS2. It has been
reported that the interaction between dyes and semiconductor QDs, such as CdSe and
CdS plays an important role in the charge transfer and photoelectric conversion
processes.44ꢀ46 To better understand the role of MoS2 in our photocatalytic system,
more profound study was performed in terms of PL quenching and lifetime change of
RhB upon the addition of the ZnꢀAgIn5S8 QDs and the ZnꢀAgIn5S8/MoS2ꢀ5%
composite, which showed distinctively different behaviors. As shown in Fig. 9a, when
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quantitative (20 ꢁL) ZnꢀAgIn5S8 QDs solution (3 mg/mL) was added to RhB solution
continually, the PL peak intensity at 586 nm of RhB gradually decreased, while PL of
the ZnꢀAgIn5S8 QDs (around 600ꢀ750 nm) increased slowly. It was found that the PL
intensity was reduced from 730 for RhB to 385 for 1:10 mixed solution (the condition
of the photocatalytic reaction), but the quenching effect was slowed down with further
addition of the QDs. In sharp contrast, nearly 100 times PL quenching of RhB by
addition of ZnꢀAgIn5S8/MoS2ꢀ5% was observed, from 730 to below 10 for
RhB:ZnꢀAgIn5S8/MoS2=1:10 (Fig. 9b), e.g. nearly complete PL quenching in the
actual photocatalytic reaction condition. Fig. 9c shows the relationship of the PL
intensity vs. ratio of RhB/photocatalysts, which clearly indicates the much more
efficient PL quenching by the ZnꢀAgIn5S8/MoS2 composite than the QDs. We propose
that this enhanced interaction between RhB and ZnꢀAgIn5S8/MoS2 photocatalysts can
be attributed to the MoS2 assisted electron transfer processes illustrated in Scheme 2.
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Fig. 9. PL spectra evolution of the RhB solution with continuous addition of the
ZnꢀAgIn5S8 QDs (a) and ZnꢀAgIn5S8/MoS2ꢀ5% nanocomposites (b); (c) The
corresponding PL intensity change vs. mass ratio of the dye/photocatalysts (the arrow
position corresponds to the actual ratio for the photocatalytic reaction); (d) PL decay
curves, corresponding singleꢀexponential fitting curves and lifetime data of RhB, RhB
with ZnꢀAgIn5S8 QDs and RhB with ZnꢀAgIn5S8/MoS2ꢀ5%, respectively.
Further exploration by the timeꢀresolved PL spectra of RhB with and without
MoS2 gives more profound understanding of the charge separation and recombination
processes. As shown in Fig. 9d, for RhB/ ZnꢀAgIn5S8 (1:4) mixture, the PL intensity
at 586 nm was decreased, but the lifetime remains almost unchanged (2.32±0.04 ns)
compared to that of the original RhB solution (2.31±0.04 ns), which indicates that this
29