Facile strategy for the fabrication of noble metal/ZnS composites with enhanced photocatalytic activities

Article abstract of DOI:10.1039/c9ra07163f

The introduction of noble metal nanoparticles to photocatalysts can effectively improve the separation efficiency of the photogenerated electron-holes. Therefore, noble metal/ZnS composites were synthesized using a low-temperature solid-phase chemical method with sodium borohydride as the reducing agent. The characterization results showed that the noble metal/ZnS composites have been successfully obtained and that the noble metals were distributed on the surface of ZnS. The catalytic results suggested that the composites exhibited improved activity after introduction of noble metals, which can be attributed to the rapid migration of carriers and the enhancement of the light absorption, mainly owing to the tight combination between the ZnS and noble metals and the plasmon resonance effect of the noble metals. The catalytic mechanism was explored by using photoluminescence spectroscopy, photocurrent spectra, valence band X-ray photoelectron spectroscopy (XPS-VB) spectra and capture agent experiments, and a possible mechanism was proposed. This work provides a new strategy for the high-volume synthesis of noble metal-based composite photocatalysts, which could be helpful for sustainable development.

Full text of DOI:10.1039/c9ra07163f

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Facile strategy for the fabrication of noble metal/
ZnS composites with enhanced photocatalytic
activities†
Cite this: RSC Adv., 2020, 10, 4455
a
ab
a
a
Haiming Duanb
*
*
Yali Cao,
Xuekun Jin,‡ Fengjuan Chen,‡
Dianzeng Jia,
c
and Mengqiu Long
The introduction of noble metal nanoparticles to photocatalysts can effectively improve the separation
efficiency of the photogenerated electron–holes. Therefore, noble metal/ZnS composites were
synthesized using a low-temperature solid-phase chemical method with sodium borohydride as the
reducing agent. The characterization results showed that the noble metal/ZnS composites have been
successfully obtained and that the noble metals were distributed on the surface of ZnS. The catalytic
results suggested that the composites exhibited improved activity after introduction of noble metals,
which can be attributed to the rapid migration of carriers and the enhancement of the light absorption,
mainly owing to the tight combination between the ZnS and noble metals and the plasmon resonance
effect of the noble metals. The catalytic mechanism was explored by using photoluminescence
spectroscopy, photocurrent spectra, valence band X-ray photoelectron spectroscopy (XPS-VB) spectra
and capture agent experiments, and a possible mechanism was proposed. This work provides a new
strategy for the high-volume synthesis of noble metal-based composite photocatalysts, which could be
helpful for sustainable development.
Received 6th September 2019
Accepted 14th November 2019
DOI: 10.1039/c9ra07163f
rsc.li/rsc-advances
activity. Hence, a novel approach is urgently needed to obtain
high-activity photocatalysts with a higher carrier separation
1. Introduction
efficiency.
It has been found that the catalytic activity of ZnS can be
With the fast development of the economy, environmental
problems have become increasingly severe. Therefore, people
have adopted many ways to solve this problem, and photo-
catalytic technology has received extensive attention owing to its
unique advantages.1–3 As it is well known, photocatalytic mate-
rials are at the core of the technology. Aer decades of research,
a series of semiconductor-based photocatalysts have been
explored.4–8 Among them, ZnS has received signicant attention
owing to the high efficiency and low price. However, fast
recombination of photo-generated carriers, results in a decrease
in the separation ability, and leads to a reduced catalytic
improved by means of depositing noble metals, compounding
semiconductors, exploring new materials, and so forth.9–13
Among these techniques, depositing noble metals is an effective
approach to enhance the activity. This is because noble metal
nanoparticles have a good electrical conductivity and electron-
trapping ability, and can act as an electron medium to
generate resonance in an electric eld caused by illumination,
which is called the surface plasmon resonance effect (SPR), and
the effect can promote effective separation of the electron–
hole.14–18 For example, Ayodhya et al. synthesized ZnxAg1ꢀx
S
composites using the hydrothermal method, and the photo-
catalytic results showed that the composites exhibited an
excellent photocatalytic degradation activity to organophos-
phorus pesticides aer the introduction of Ag.19 The enhanced
performance may be due to the plasmon resonance effect which
causes the photogenerated electrons on the ZnS conduction
band to be injected into the Ag surface, thereby promoting the
efficient separation of the photogenerated carriers.20 Thus,
depositing noble metals has been considered to be an effective
strategy to enhance the photocatalytic properties.
aKey Laboratory of Energy Materials Chemistry, Ministry of Education, Key Laboratory
of Advanced Functional Materials, Autonomous Region, Institute of Applied Chemistry,
Xinjiang University, Urumqi, Xinjiang 830046, China. E-mail: jdz0991@gmail.com
bSchool of Physics Science and Technology, Xinjiang University, Urumqi 830046,
Xinjiang, PR China. E-mail: c2008meet@163.com
cHunan Key Laboratory of Super Micro-structure and Ultrafast Process, Central South
University, Changsha 410083, China
† Electronic supplementary information (ESI) available: Fig. S1 shows the N2
adsorption–desorption isotherms for the Ag/ZnS composites. Fig. S2 exhibits
the recycling activity of the Ag/ZnS composite AGZS-2. Fig. S3 displays
a comparison of the photocatalytic activities between the Ag/ZnS composites
and Au/ZnS composites. Fig. S4 shows the recycling activity of the Au/ZnS
composite AUZS-2. See DOI: 10.1039/c9ra07163f
In general, the composition, structure and properties of
materials are closely related to their catalytic properties. It has
been found that there are many methods for fabricating noble
‡ These authors contributed equally to this work.
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metal/sulde composites, such as the microwave assisted microscopic morphology of the composites was characterized
reduction method, the photoreduction coupled water bath by eld emission transmission electron microscopy (JEOL,
method, and the in situ deposition method.21–26 However, the JEM2011F). The specic surface areas were analyzed using the
above mentioned methods have some disadvantages, such as Brunauer–Emmett–Teller (BET) method on a Micromeritics
a complicated operation, high cost, and can be difficult to ASAP 2460 apparatus. The elemental composition of the
control. Therefore, it is highly desirable to nd a simple and fast composites was analyzed using X-ray photoelectron spectros-
technique to obtain noble metal/sulde composites. At present, copy (XPS, SCIENTIFIC ESCALAB 250Xi, Thermo Scientic). The
our group has synthesized a series of nanomaterials with ultraviolet-visible absorption spectrum of the composites was
excellent photoelectric properties by using the low-temperature measured using an UV-visible spectrometer (H-3900, Hitachi).
solid-phase chemical method.27–30 However, through consulting The photoluminescence spectra of the composites were
numerous studies, it has been found that the synthesis of noble measured using a photoluminescence spectrometer (F-4500,
metals/suldes composites using this method have not been Hitachi). The photocurrent of the composites was tested on
reported so far.
an electrochemical workstation (CHI 660B, Shanghai Chenhua).
In this paper, Ag/ZnS composites and Au/ZnS composites The catalytic performance of the composites was analyzed on
were synthesized using a low-temperature solid-phase chemical a photochemical reactor (XPA-7, Nanjing Hanjiang Power
method with the aid of the reducing agent sodium borohydride Plant).
(NaBH4). The photocatalytic activity of the composites was
studied by degrading methyl orange (MO) under UV light, and
2.3 Photocatalytic tests
the photocatalytic mechanism was further discussed. It was
The catalytic performance of the composites was performed on
found that the noble metal nanoparticles in the composites
an XPS reactor with a 300 W mercury lamp source. As the energy
synthesized using the method have strong binding with ZnS,
of the light source is high, more heat is generated, and it is
which facilitates the rapid separation of the photogenerated
necessary to continuously pass condensed water through for
carriers and improves the catalytic activity.
circulated cooling. The catalytic performances of the compos-
ites were assessed using MO as a model.
2. Experimental section
2.1 Preparation of the noble metal/ZnS composites
The procedure was performed as follows: 25 mg catalyst was
added to 50 mL MO solution (10 mg Lꢀ1). In order to reach the
adsorption equilibrium between the catalyst and MO solution,
Ag/ZnS composites were synthesized using a low-temperature
the adsorption was allowed to progress for 2 h under dark
solid-phase chemical method. First, Zn(CH3COO)2 2H2O (0.01
conditions. Subsequently, the light source was turned on for
testing. 3.5 mL of the reaction solution was taken at intervals of
20 min, centrifuged at a certain speed (rotation speed of 10 000
rpm) for 10 min, and the supernatant was taken and analyzed
mol) was added to the agate mortar. Aer grinding for 10 min,
NH2CSCH3 (0.01 mol) was added. Aer grinding for 40 min,
NaBH4 (0.05 mol) was added and the mixture was continually
milled for 40 min. Subsequently, AgCH3COO (0.0015 g) was
for the concentration of MO. The concentration of MO was
added and milled for 60 min to obtain the product. Finally, the
measured using an UV-vis spectrometer with a range of 300–
700 nm.
obtained product was washed several times with deionized
water and ethanol, and dried for 10 h at 80 ꢁC to obtain the Ag/
In order to study the active substances in the catalytic
ZnS composites AGZS-0.1. Here, when the mass percentage of
process, a capture agent experiment was carried out. The
Ag and ZnS in the nal product is 0.1, the composites are named
capture agents adopted were EDTA-Na2, ethanol and 1,4-ben-
AGZS-0.1. In a similar manner, composites AGZS-0.2, AGZS-0.5,
zoquinone. The experimental procedure was the same as that
AGZS-1, AGZS-2, and AGZS-5 were also obtained with different
described above, except that a certain amount of capture agent
was added to the system.
compositions.
In order to further study the inuence of noble metals on the
composition, structure and performance of ZnS, Au/ZnS compos-
ites were also synthesized under the same conditions as those
described above. The difference was that AgCH3COO (0.0015 g)
was replaced with AuCl$HCl$4H2O (0.021 g, the mass percentage
3. Result and discussion
3.1 Characterization of the Ag/ZnS composites
Fig. 1 exhibits the X-ray diffraction (XRD) patterns of the Ag/ZnS
of Au and ZnS was 0.1), and the Au/ZnS composite was obtained
composites prepared by the low-temperature solid-phase reac-
and designed as AUZS-0.1. By changing the mass percentage of Au
tion in the presence of NaBH4 and varying amounts of silver
and ZnS, composites AUZS-0.2, AUZS-0.5, AUZS-1, AUZS-2 and
acetate. From Fig. 1, it can be noted that the diffraction peaks of
AUZS-5 with different compositions were also prepared.
pure ZnS are consistent with the standard card of cubic ZnS
(JCPDS no. 05-0566).31 At the same time, the characteristic peaks
of the Ag/ZnS composites are in agreement with that of pure
ZnS, indicating that introduction of Ag does not change the
crystal structure of the composites. No characteristic peak for
For the convenience of comparison, pure ZnS was also
synthesized without NaBH4, AgCH3COO and AuCl$HCl$4H2O.
2.2 Characterization
The crystal structure of the composites was analyzed using an X- Ag was observed in the Ag/ZnS composites, which may be due to
ray powder diffractometer (D8 Advance, Bruker). The its low content. It is worth noting that the intensity of the
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analysis was performed. In Fig. 2c, the composite AGZS-2 has
clear lattice fringes, in which the lattice spacing of 0.30 nm
corresponds to the (111) crystal plane of cubic ZnS, and the
lattice spacing of 0.24 nm is consistent with the (111) crystal
plane of the cubic Ag, suggesting the presence of the ZnS and Ag
phases in the region.33 At the same time, there is a clear
boundary between the ZnS and Ag lattices, indicating the
formation of Ag/ZnS composites. In addition, the SAED results
(Fig. 2d) indicate that the composites have a polycrystalline
property.
Specic surface areas on the Ag/ZnS composites were also
characterized using nitrogen (N2) adsorption–desorption. As
shown in Fig. S1,† the Ag/ZnS composites all display a typical
type IV isotherm with a distinct hysteresis loop. The BET
specic surface area of the Ag/ZnS composites AGZS-0.1, AGZS-
0.2, AGZS-0.5, AGZS-1, AGZS-2 and AGZS-5 are 38.85, 48.31,
60.04, 72.36, 239.83 and 53.01 m2 gꢀ1, respectively. When the
mass percentage of Ag to ZnS is two, the composite AGZS-2
exhibited the largest specic surface area in comparison with
the others, which is helpful for the adsorption and carrier
transport, and the composite AGZS-2 displayed the highest
photocatalytic activity (as shown in Fig. 5a).
The chemical composition of pure ZnS and the composite
AGZS-2 was studied using an XPS technique. In the survey
spectra (Fig. 3a), characteristic peaks for S 2p, C 1s, Ag 3d, O 1s,
and Zn 2p appear, in which C 1s and O 1s are derived from the
test process,34 conrming the presence of the S and Zn elements
for ZnS, and the presence of the S, Ag and Zn elements in the
compositeAGZS-2. Pure ZnS exhibits characteristic peaks of S
2p3/2 and S 2p1/2 orbitals at 161.6 and 162.6 eV (Fig. 3b),
respectively, indicating the existence of the S element.35 In
contrast, the characteristic peaks of the S element in composite
AGZS-2 were slightly red-shied to 161.7 and 162.8 eV, respec-
tively. In Fig. 3c, the pure ZnS exhibits characteristic peaks of Zn
2p3/2 and Zn 2p1/2 orbitals at 1021.9 and 1044.9 eV, respectively.
However, there is a slight red shi for the Zn 2p characteristic
peaks in the composites AGZS-2, suggesting that there are
Fig. 1 XRD patterns of pure ZnS and Ag/ZnS composites.
diffraction peak corresponding to the (111) crystal plane of ZnS
in Ag/ZnS composites gradually decreases with the increased
amount of Ag, owing to the high dispersion of Ag, indicating
that the amount of Ag had an important inuence on the
crystallinity.32
In order to clearly observe the morphology of the samples,
transmission electron microscopy (TEM) analyses were con-
ducted as shown in Fig. 2. It can be seen that the ZnS nano-
particle with a size of about 50 nm, has a slight agglomeration
(Fig. 2a). In Fig. 2b, as for the composites AGZS-2, a small
amount of Ag nanoparticles are present on the surface of ZnS.
To further study the structure of the composites, high-
resolution transmission electron microscopy (HRTEM)
Fig. 2 TEM images of ZnS (a), Ag/ZnS composite AGZS-2 (b); HRTEM Fig. 3 XPS spectra of the ZnS and Ag/ZnS composites AGZS-2 (a)
image (c), and SAED patterns of the composites AGZS-2 (d).
survey; (b) S 2p; (c) Zn 2p; and (d) Ag 3d.
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interactions between Ag and ZnS.36 The characteristic peaks at
368.0 and 373.9 eV in Fig. 3d correspond to the Ag 3d5/2 and Ag
3d3/2 orbitals of metallic Ag, respectively, indicating the pres-
ence of the Ag element in the composite AGZS-2.37 The above
described results show that there is ZnS and Ag in the
composites, that is, the Ag/ZnS composite photocatalyst is ob-
tained. XPS analysis was further performed to measure the Ag
percentage in the Ag/ZnS composites. The Ag percentage with
respect to ZnS was found to be 1.5% in the composite AGZS-2,
which further proved the expected Ag percentage in the ZnS.
The optical absorption properties of the ZnS and Ag/ZnS
composites were studied as shown in Fig. 4. It can be found
that pure ZnS has an absorption in the range of 250–400 nm,
which is consistent with the literature.38 Aer introducing Ag,
Ag/ZnS composites showed a slight blue shi, which may be due
to its small size (Fig. 4a). At the same time, with the formation of
Ag/ZnS composites, a new absorption band appeared in the
range of 450–800 nm, which is caused by the SPR effect of Ag,
further indicating the presence of Ag.39 As the amount of Ag
increases, the absorption capacity increases continuously,
indicating that the composites will exhibit improved photo-
catalytic activity.
Fig.
5
(a) Photocatalytic performance of the ZnS and Ag/ZnS
composites in the degradation of MO under UV light; and (b) the
kinetic curve.
composites obtained by NaBH4 reduction are tightly bound to
ZnS, which is benecial to the rapid separation of the photo-
generated carriers.41 On the other hand, the SPR effect of the Ag
nanoparticles enhanced the absorption of light, and improved
the catalytic performance.
Fig. 5b shows the kinetic results for the ZnS and Ag/ZnS
composites. It can be seen that the photocatalytic degradation
of MO complies with a quasi-rst-order kinetic reaction.42 The
rate constants of pure ZnS and composite AGZS-2 were 0.00193
and 0.00581 minꢀ1, respectively. Compared with pure ZnS, the
degradation activity of composite AGZS-2 increased by three
times, indicating that the composites exhibit excellent photo-
catalytic activities. Furthermore, the recycling activity of the Ag/
ZnS composites has been tested, and we found that the stability
is relatively high (Fig. S2†).
According to the Tauc equation ahn ¼ A(hn ꢀ Eg)n,40 the band
gap energy of the samples can be estimated by plotting the
photon energy (hn) by (ahn)1/2, as shown in Fig. 4b. It can be seen
that the band gap energies (Eg) of ZnS, the Ag/ZnS composites
AGZS-0.1, AGZS-0.2, AGZS-0.5, AGZS-1, AGZS-2, and AGZS-5 are
3.38, 3.52, 3.49, 3.47, 3.46, 3.43 and 3.44 eV, respectively. It can
be seen that with the increase in the Ag content, the band gap of
the Ag/ZnS composites decreases rst and then increases.
Furthermore, the Eg of composite AGZS-2 is the smallest, which
may be related to its structure and properties.
In order to study the photocatalytic mechanism of the Ag/
ZnS composites, the photocurrent response, photo-
luminescence (PL) spectra, capture agents and valence band
XPS (XPS-VB) spectra measurements were conducted, as shown
in Fig. 6.
From Fig. 6a, it can be seen that the pure ZnS exhibits
a relatively low photocurrent response. Aer the introduction of
3.2 Photocatalytic activity of the Ag/ZnS composites
In order to evaluate the catalytic activity of the composites, MO
was used as the degradation target, as shown in Fig. 5a. Aer
introduction of Ag, the Ag/ZnS composites exhibited an
improved catalytic activity, which changed regularly with
different amounts of Ag. When the mass percentage of Ag to ZnS
was two, the composites AGZS-2 exhibited the highest photo-
catalytic activity, indicating that the amount of Ag plays a key
role in obtaining photocatalysts with excellent activity. In
addition, the enhanced photocatalytic activity can be attributed
to two aspects. On the one hand, the Ag nanoparticles in the
Fig. 6 (a) Photocurrent response, (b) PL spectra of pure ZnS and Ag/
ZnS composites, (c) species tests for MO degradation by composite
Fig. 4 (a) UV-vis spectra; and (b) (ahn)2 versus hn for the ZnS and Ag/ AGZS-2 under UV irradiation, and (d) XPS-VB spectrum of composites
ZnS composites.
AGZS-2.
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Ag, the Ag/ZnS composites displayed a signicantly increased
photocurrent higher than that of ZnS. In particular, the
composite AGZS-2 showed the strongest photocurrent signal,
indicating that there is an efficient electron transfer between
ZnS and Ag. Hence, the introduction of Ag can effectively
improve the separation efficiency of the photogenerated elec-
tron–holes and exhibit an enhanced activity, which can also be
veried by the photocatalytic results (Fig. 5).43
Fig. 6b shows the PL spectra of the pure ZnS and Ag/ZnS
composites. The PL intensities of the composites signicantly
decreased aer introducing Ag. Among them, the composite
AGZS-2 exhibited the lowest PL intensity, indicating that
introducing Ag can effectively inhibit the photogenerated
charge recombination and further improve the photocatalytic
activity.44
The role of active components in the catalytic process was
investigated using a capture agent experiment. Here, disodium
Fig. 7 XRD patterns of pure ZnS and Au/ZnS composites (AUZS-0.1–
edetate (EDTA-Na2), ethanol (ethanol) and p-benzoquinone (1,4- AUZS-5).
benzoquinone) were used as capture agents of holes (h+),
hydroxyl radicals ($OH) and superoxide radicals ($O2ꢀ),
respectively.45 The degradation curves of MO aer adding the
(200) crystal planes of Au, indicating that the Au/ZnS compos-
ites were obtained by solid-phase chemical reaction.46 In addi-
tion, when the amount of Au increases, the intensity of the
diffraction peaks for Au increase, while the intensity of ZnS
decreases, possibly due to the change in crystallinity.
As for composite AUZS-2, the Au nanoparticles at about
20 nm are distributed on the surface of ZnS (Fig. 8a). While, in
Fig. 8b, the crystal plane distances of 0.31 and 0.26 nm corre-
spond to the (111) plane of ZnS and the (111) plane of Au,
respectively.47 This is in agreement with the XRD results,
demonstrating the successful synthesis of the Au/ZnS compos-
ites. The SAED pattern in the inset further indicates the poly-
crystalline properties of the composites.
above capture agents are shown in Fig. 6c. When EDTA-Na2 was
added, the degradation rate to MO by the composite AGZS-2 is
obviously improved, indicating that h+ is not an active compo-
nent that affects the degradation process of MO. The degrada-
tion efficiency was inhibited to some extent aer ethanol was
added, indicating that $OH is one of the active components, but
is not the main component. When 1,4-benzoquinone was added
to the reaction system, the degradation rate was signicantly
ꢀ
reduced, indicating that $O2 is the main activꢀe component
during MO degradation. It can be seen that $O2 is the main
active species that affects MO degradation, and $OH is the
second.
The position of the band determines the redox capacity of
the photogenerated carriers during the catalytic degradation
process. Therefore, XPS-VB spectra measurements of ZnS and
composites AGZS-2 were performed, and the results are shown
in Fig. 6d. The valence band edge (EVB) of ZnS and composite
AGZS-2 are 0.49 and 0.96 eV, respectively, wherein the potential
of the valence band of composite AGZS-2 is higher than that of
ZnS (0.49 eV). This indicates that composite AGZS-2 has
a stronger oxidizing power and catalytic activity than that of
pure ZnS, probably due to the presence of Ag. Combined with
the UV-vis results, the Eg values of the ZnS and composite AGZS-
2 are 3.38 and 3.43 eV, respectively. According to the formula Eg
¼ EVB ꢀ ECB, the determined conduction band edge (ECB) values
of the ZnS and composite AGZS-2 are ꢀ2.89 and ꢀ2.47 eV,
respectively.
The survey spectrum (Fig. 9a) conrmed that there are S, Zn
and Au elements in the composite AUZS-2. In Fig. 9b, the
characteristic peaks of the S 2p3/2 and S 2p1/2 orbitals of the
composite AUZS-2 are slightly red-shied to 161.9 and 162.8 eV
compared to the pure ZnS, respectively, suggesting there are
interactions between Au and ZnS. As for the composite AUZS-2,
characteristic peaks for the Zn 2p3/2 and Zn 2p1/2 orbitals appear
at 1021.8 and 1044.8 eV, respectively (Fig. 9c). The two peaks for
Au 4f5/2 at 88.7 and 91.6 eV are assigned to Au0 and Au1+,
3.3 Characterization and activity of the Au/ZnS composites
In Fig. 7, the XRD patterns of the Au/ZnS composites are basi-
cally the same as those of ZnS (JCPDS no. 05-0566). When the
mass percentage of Au to ZnS is 0.5, the composite AUZS-0.5
exhibits two diffraction peaks at 38.3ꢁ and 44.3ꢁ. In compar-
ison with the standard card (JCPDS no. 65-8601), the corre-
Fig. 8 TEM images of the Au/ZnS composite AUZS-2 (a); HRTEM
sponding diffraction peaks can be designated to the (111) and image (b), and SAED patterns (inset).
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Fig. 11 (a) Photocatalytic performance of ZnS and Au/ZnS composites
for the degradation of MO under UV light, and (b) the kinetic curve.
the highest photocatalytic performance. The enhancement of
the photocatalytic ability can be ascribed to the close combi-
nation of Au and ZnS and the SPR effect of Au, which facilitates
the rapid separation of photogenerated carriers and thus
improves the catalytic performance.50
Fig. 9 XPS spectra of the Au/ZnS composite AUZS-2: (a) survey; (b) S
In order to compare the photocatalytic activities of the Au/
ZnS composites and Ag/ZnS composites, Fig. S3† was exam-
ined. It can be found that 44% of MO was degraded by the Au/
ZnS composite AUZS-2 aer irradiation for 120 min, lower than
that of the Ag/ZnS composite AGZS-2 (49%), indicating that the
Ag/ZnS composites possess a higher photocatalytic activity than
that of the Au/ZnS composites.
Fig. 11b indicates that the Au/ZnS composite photocatalytic
degradation of MO conforms to a quasi-rst-order kinetic
process. The rate constant of the composite AUZS-2 was
0.00471 minꢀ1, three times higher than that of ZnS, indicating
that the Au/ZnS composite exhibits an excellent photocatalytic
performance aer introducing Au. In addition, we can see that
the Au/ZnS composites also exhibited a stable photocatalytic
activity (Fig. S4†).
2p; (c) Zn 2p; and (d) Au 4f.
respectively (Fig. 9d), indicating the presence of the Au element
in composite AUZS-2.48 The above described results show that
there are two phases of ZnS and Au in the composites, that is,
the Au/ZnS composites are obtained. From the XPS analysis, it
can be found that the Au percentage with respect to ZnS was
1.9% in the composite AUZS-2, in agreement with the expected
results.
Aer the introduction of Au, the Au/ZnS composites have
a signicant blue shi compared with that of ZnS (Fig. 10a). At
the same time, with the formation of the Au/ZnS composites,
a new absorption band appears in the range of 450–800 nm,
which is caused by the plasmon resonance effect of Au, indi-
cating the presence of Au in the composites.49 According to the
Tauc equation ahn ¼ A(hn ꢀ Eg)n, the Eg of the Au/ZnS
composites AUZS-0.1, AUZS-0.2, AUZS-0.5, AUZS-1, AUZS-2,
and AUZS-5 are 3.38, 3.57, 3.53, 3.54, 3.57, 3.55 and 3.59 eV,
respectively (Fig. 10b). It can be seen that the Eg of composite
AUZS-2 is relatively small, which may be related to the structure
and properties.
Subsequently, the photocatalytic mechanism of the Au/ZnS
composites was further studied as shown in Fig. 12. In
Fig. 12a, the photocurrent of the Au/ZnS composites increases
The photocatalytic activity of the Au/ZnS composites were
also explored as shown in Fig. 11. As the amount of Au
increases, the photocatalytic activity of the Au/ZnS composites
rst increases and then decreases (Fig. 11a). When the mass
percentage of Au to ZnS is two, the composite AUZS-2 exhibited
Fig. 12 (a) Photocurrent response, (b) PL spectra of the Au/ZnS
composites, (c) species tests for degrading MO using composite
Fig. 10 (a) UV-vis spectra; and (b) (ahn)2 versus hn of the ZnS and Au/ AUZS-2 under UV irradiation, and (d) XPS-VB spectrum of composite
ZnS composites.
AUZS-2.
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signicantly aer introduction of Au, and composite AUZS-2
exhibits the strongest photocurrent signal, indicating that
introducing Au can effectively improve the separation efficiency
of the photogenerated charges.51 In Fig. 12b, the PL intensity of
the composites signicantly decreased aer introducing Au.
Wherein the composite AUZS-2 displayed the lowest PL inten-
sity, implying an excellent separation ability of photogenerated
charges, which was consistent with the results of the photo-
current analysis (Fig. 12a).52 The resultsꢀof the capture agent
experiment (Fig. 12c) indicate that $O2 is the main active
substance, followed by $OH. In addition, a XPS-VB spectrum
study was performed on composite AUZS-2, as shown in
Fig. 12d. The determined EVB of composite AUZS-2 is 1.31 eV,
which is higher than that of ZnS (0.56 eV), indicating that
composite AUZS-2 has a strong oxidizing ability.53 Combined
with UV-vis results, the Eg of the composite AUZS-2 was found to
be 3.55 eV. According to the formula Eg ¼ EVB ꢀ ECB, it can be
determined that the ECB of composite AUZS-2 is ꢀ2.24 eV.
Fig. 13 Photocatalytic mechanism for the degradation of MO over the
Ag/ZnS composites.
addition, as the photogenerated electrons aggregate on the Ag
nanoparticles, the Fermi level increases and is higher than the
O2/$O2ꢀ potential (ꢀ0.046 V vs. NHE), and then the electron can
3.4 Photocatalytic mechanism of the noble metal/ZnS
composites
ꢀ
reduce O2 to $O2 (eqn (3)). The same conclusion can also be
obtained in the degradation of MO by the Au/ZnS composites.
Based on the experimental results of composition, structure,
photocatalytic activity, photocurrent, PL spectra, XPS-VB and
active species, the reasons for the improvement in the photo-
catalytic activity of ZnS aer introducing noble metals can be
explained as follows.
+
OH + hVB /$OH
(1)
(2)
(3)
O2 + 2H+ + 3eCB / OHꢀ +$OH
ꢀ
Here, we take the Ag/ZnS composite as an example. First, the
improved catalytic activity of the Ag/ZnS composites is mainly
due to the rapid generation and migration of photogenerated
charges. According to the results of the above mentioned UV-vis
and XPS-VB tests, the ECB and the EVB position of ZnS are ꢀ2.89
and 0.49 eV, respectively. According to the literature, the Fermi
level of Ag is located at 0.4 eV (vs. a normal hydrogen electrode
(NHE)).54 Under UV irradiation, ZnS is excited and generates
electron–hole pairs. Driven by the difference in the ZnS
conduction band potential and the Ag Fermi level potential,
photogenerated electrons of ZnS will rapidly migrate to the Ag
surface, effectively reducing the recombination of the photo-
generated electrons and holes on the surface of ZnS, which can
also be conrmed by the results of the photocurrent and PL
spectra (Fig. 6). However, when the mass percentage of Ag to
ZnS increases to ve, a new carrier recombination center is
O2 + eCB /$O2
In summary, the mechanism of the catalytic degradation of
MO by noble metal/ZnS composites can be briey described as
follows. Under UV irradiation, photogenerated electrons and
holes are generated on the surface of ZnS, and electrons rapidly
migrate to noble metal nanoparticles, thereby achieving the
effective separation of electrons and holes on the surface of ZnS.
Next, electrons on the noble metal nanoparticles react with O2
to obtain the active species $O2ꢀ and $OH. Eventually, the active
species react with MO and then converts it to H2O and CO2,
thereby achieving the degradation of MO.
4. Conclusions
generated, thereby reducing the catalytic activity of the Ag/ZnS Noble metal/ZnS composites were obtained using a low-
composites. temperature solid-phase chemical reaction in the presence of
Secondly, the active species involved in the reaction is the NaBH4. UV-vis analysis showed that the noble metal/ZnS
direct cause of the enhanced photocatalytic activity of the Ag/ composites showed a new absorption band in the range of
ZnS composites. Aer photogenerated electron–hole separa- 450–800 nm, indicating the presence of noble metal in the
tion, it is further converted into an active species ($OH, h+, composites, which is consistent with the XPS results. The noble
$O2ꢀ) to complete the degradation of MO. As can be seen from metal/ZnS composites exhibited enhanced photocatalytic
Fig. 13, the valence band position of ZnS (0.99 eV) is higher than activities compared with pure ZnS. When the mass percentage
the OHꢀ/$OH potential (1.99 V vs. NHE). Therefore, $OH (eqn of the noble metal to ZnS is two, the composites displayed the
(1)) cannot be obtained by oxidizing OHꢀ. However, $OH highest photocatalytic value, suggesting that the content of
appears in the Ag/ZnS composites as shown in Fig. 6c. This may noble metals has a great inuence on the performance. The
be due to the fact that aer introduction of Ag, the reaction time excellent activities of the noble metal/ZnS composites are
of the electron is increased, and then a multi-electron reaction mainly due to the rapid migration of carriers and the
with oxygen occurs, indirectly generating $OH (eqn (2)).55 In enhancement of the light absorption capacity, ascribed to the
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