Core-shell Cd0.2Zn0.8S@BiOX (X = Cl, Br and I) microspheres: A family of hetero-structured catalysts with adjustable bandgaps, enhanced stability and photocatalytic performance under visible light irradiation

Article abstract of DOI:10.1039/c6dt02038k

Heterostructures consisting of two semiconductors have merited considerable attention in photocatalytic applications due to synergistic effects in complex redox processes. The incorporation of solid solutions into such architectures can further offer extra variability to control the bandgap. In this study, we report the fabrication of a series of core-shell Cd_0.2Zn_0.8S@BiOX (X = Cl, Br and I) microspheres via a solvothermal route that lead to enhanced photocatalytic performance under visible light irradiation. By optimizing the synthesis conditions, uniform and porous Cd_0.2Zn_0.8S@BiOX microspheres were achieved. The products were thoroughly characterized by X-ray diffraction studies, scanning electron microscopy, transmission electron microscopy, photoluminescence studies, absorption measurements and the photodegradation of RhB. Remarkably, the electronic structures of Cd_0.2Zn_0.8S@BiOX composites can be continuously tuned by varying the composition of BiOX to achieve the best catalytic performance under visible light irradiation. Finally, this greatly enhanced visible-light-driven photocatalytic efficiency was observed in the optimized Cd_0.2Zn_0.8S@BiOI composites when compared to their single-component counterparts, which may be attributed to increased light absorption and improved electron-hole separation. The photocatalytic mechanism has also been proposed based on the experimental evidences and the theoretical band positions of Cd_0.2Zn_0.8S@BiOI.

Full text of DOI:10.1039/c6dt02038k

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Core-Shell Cd Zn S@BiOX (X = Cl, Br, I) Microspheres: A
0.2
0.8
Family of Hetero-Structured Catalysts with Adjustable
Bandgaps, Enhanced Stability and Photocatalytic Performance
under Visible Light Irradiation
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
a,b
a,b
a,b
*a,b
*c
Yannan Zhou, Ting Wen, Binbin Chang, Baocheng Yang, Yonggang Wang
www.rsc.org/
Heterostructures consisted of two semiconductors have merited considerable attention in photocatalytic applications due to
the synergistic effect in the complex redox processes. The incorporation of solid solutions into such architectures can further
offer extra variability to control the bandgap. In this work, we report the fabrication of a series of core–shell
Cd0.2Zn0.8S@BiOX (X = Cl, Br, I) microspheres via a solvothermal route with enhanced photocatalytic performance under
visible light irradiation. By optimizing the synthesis conditions, uniform and porous Cd0.2Zn0.8S@BiOX microspheres were
achieved and the products were characterized thoroughly by Xꢀray diffraction, scanning electron microscope, transmission
electron microscope, photoluminescence, absorption measurements and the photoꢀdegradation of RhB. Remarkably, the
electric structures of Cd0.2Zn0.8S@BiOX composites can be continuously tuned by varying the composition of BiOX to
achieve the best catalytic performance under visible light irradiation. Finally, greatly enhanced visibleꢀlightꢀdriven
photocatalytic efficiency was observed in the optimized Cd0.2Zn0.8S@BiOI composites than their singleꢀcomponent
counterparts, which may attribute to the increased light absorption and the improved electronꢀhole separation. The
photocatalytic mechanism has also been proposed based on the experimental evidences and the theoretical band positions of
Cd0.2Zn0.8S@BiOI.
8
–12
applications.
All the BiOX compounds are tetragonal
Introduction
matlockite structure with a layered structure composing of a
2
+
Semiconductor photocatalysis, which shown great promise in
photocatalytic degradation of pollutants and water splitting for
hydrogen production, has attracted considerable attention due to
the growing environmental and energy issues. Presently,
many traditional photocatalysts (such as TiO
made great progress during the past three decades.
Unfortunately, the photocatalytic activity of traditional
photocatalysts is still restricted due to the recombination of
photo induced electrons and holes and the poor solar energy
utilization. Recently, a great deal of effort has been devoted to
fabricate novel photocatalysts which can enhance the efficiency
of the photocatalytic process and extend the light adsorption
layer of [Bi
2
O
2
]
and two layers of halogen ions connected by
internal static electric fields, which can facilitate the photoꢀ
1
3,14
generated electron−hole pairs separation.
As a typical solid
Zn S has
1
,2
solution semiconductor of CdS and ZnS, ternary Cd1ꢀ
x
x
3
,4
5,6
2
, ZnO ) have
been intensively investigated due to the controllable band gap
energy and band positions through adjusting the composition
ratio of cadmium to zinc and the excellent visibleꢀlightꢀdriven
photocatalytic activity in water splitting and the decomposition
1
5–17
of organic compounds. Although the photocatalytic
x x
properties of BiO and Cd1ꢀ Zn S have been widely examined,
X
it is still a change to further improve the stability and suppress
the electronꢀhole pair recombination by design of novel coaxial
coreꢀshell and composite heterojunctional nanostructures.
Materials with wellꢀfined heterostructures often exhibit
superior or new functional properties compared to their singleꢀ
component counterparts. To utilize visible light more efficiently
2
,7
range, such as BiO
oxyhalide compounds BiO
oxide semiconductors, have attracted widespread attention due
to their optical properties and promising industrial
X
and Cd1ꢀ
x
Zn
x
S solid solution. Bismuth
X (X = Cl, Br, and I) as novel ternary
and improve visible light photocatalytic activity of BiO
Zn S, a lot of coupled semiconductor systems, such as
BiOCl/BiOI,
X and
Cd1ꢀ
x
x
a
18,19
,23,24
20
21,22
Institute of Nanostructured Functional Materials, Huanghe Science and Technology
BiOBrꢀgꢀC
3
N
4
13
,
BiOX/TiO
2
,
BiOX/
College, Zhengzhou, Henan 450006, China.
9
25
b
graphene,
BiOBrꢀCdS,
Cd1ꢀxZn SꢀZnS,
Cd1ꢀ
Henan Provincial Key Laboratory of Nanoꢀcomposite and Applications,
x
2
6
16
27
Zhengzhou 450006, Henan Province, China.
Zn S@ZnS, CNT/Cd1ꢀxZn S, and RGOꢀCd1ꢀxZnxS have
x
x
x
c
High Pressure Science and Engineering Center, University of Nevada Las Vegas,
been studied. The successful fabrication of heterojunctional
Nevada 89154, United States. Eꢀmail: yyggwang@gmail.com (Y. Wang).
†
BiO
Electronic Supplementary Information (ESI) available: SEM and TEM images of photocatalysts can endow the composite systems with increased
X
and CZS@BiO and
X
;
EDX results and PL spectra of CZS, BiO
X
visible light absorption and promotional photo activity.
Nevertheless, because of the different band gaps and structural
CZS@BiO
DOI: 10.1039/x0xx00000x
X;
Photocatalytic degradation of RhB on CZS and BiOX]. See
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complexity between two semiconductors, it is still a standing Synthesis of Cd0.2Zn0.8S@BiOX core@shell microspheres.
challenge to fabricate highly active and stable heteroꢀstructured In a typical synthesis, the CZS microspheres were dispersed in
catalysts with suite band gaps and structure.
In our previous studies, we have synthesized a series of and 0.5 mmol K
mesoporous Cd1ꢀ Zn S ( = 0, 0.2, 0.4, 0.6, 0.8, 1) microspheres ethylene glycol (EG) and the asꢀobtained solution was dropped
with high specific surface areas and tunable bandgap, and the slowly into the aboveꢀmentioned CZS ethanol solution. After
10 mL ethanol solution firstly. Then 0.5 mmol Bi(NO
3 3 2
) ·5H O
X (X
= Cl, Br, I) were dissolved in 10 mL
x
x
x
optimal Cd0.2Zn0.8
production performance under visible light irradiation.
S
microsphere presents the best
H
2
7
stirred at room temperature for 30 min, the resulting mixture
was transferred into a Teflonꢀlined stainless steel autoclave of
1
However, unfavourable phase stability of these sulphides 40mL capacity. The autoclave was sealed and maintained at
semiconductors is observed during the cycling test, and 160 °C for 12 h and then cooled to room temperature naturally.
additional efforts should make to overcome such problem. In The resulting samples were centrifuged and dried in an oven at
this work, we introduce a facile hydrothermal route for the 80 °C overnight and the products are denoted as CZS@BiO
fabrication of coreꢀshell Cd0.2Zn0.8S@BiO microspheres with = Cl, Br, I). The samples with various CZS/Bi(NO ·5H
improved photocatalytic activity and phase stability under weight ratios of 1:1, 1:2.5, and 1:5 were prepared in the same
visible light irradiation. Though coating with BiO on the way. The resulting composite is denoted as CZS@BiO
·5H O.
X
X
(
X
3
)
3
2
O
X
x:
y
X,
surface, the photo response range of Cd0.2Zn0.8S microspheres where
are successfully extended into the visible region. The intimate
contact and chemical potential difference in the coreꢀshell
microstructure could effectively facilitate the transfer efficiency
and suppress there combination rate of photoꢀexcited carriers,
resulting in extremely high activity and stability. The
x
:
y
is the weight ratio of CZS to Bi(NO
3
)
3
2
Characterization. The powder Xꢀray diffraction (XRD)
patterns of all the samples were collected at room temperature
on a Bruker D8 Advance diffractometer using a germanium
monochromatic (CuꢀKα radiation, 40 kV and 40 mA). The data
for phase identification were taken in the 2 theta range between
photocatalytic efficiency and stability of Cd0.2Zn0.8S@BiOX are
investigated in terms of the degradation of Rhodamin B (RhB),
which exhibits superior photocatalytic performance compared
1
0º and 70º and with the remaining time of 0.1 s/step. Scanning
electron microscope (SEM) images were recorded using a
Quanta 250 FEG FEI at 20 kV in gentleꢀbeam mode without
any metal coating. Transmission electron microscope (TEM)
images were recorded on
accelerating voltage of 200 kV using a carbonꢀcoated copper
grid. The textural properties were studied by N adsorptionꢀ
desorption measurements (ASAP2020, America) at liquid
nitrogen temperature. Before analysis, the samples were
degassed in vacuum at 180 ºC for 6 h. UVꢀvis diffuse
reflectance spectra (DRS) were measured by using a Uꢀ4100
with the pure BiO
X and Cd0.2Zn0.8S, indicating their potential
applications for visibleꢀlightꢀdriven catalyses. The possible
photocatalytic mechanism has also been proposed based on the
relative experiments and band positions of the two
semiconductors.
a Tecnai G2 20 TEM with an
2
Experimental
Synthesis of Cd0.2Zn0.8
S
microspheres. Cd0.2Zn0.8S
microspheres were prepared following the strategy reported in UVꢀVisꢀNIR spectrometer (Hitachi). Photoluminescence (PL)
1
7
our previous work. In a typical synthesis procedure (for the spectra were collected on a Varian Cary Eclipse fluorescence
2
+
2+
Cd : Zn = 1 : 4 sample), stoichiometric Cd(NO
Zn(NO ·6H O were dissolved in 50 vol.% diethylenetriamine laser beam or a 300 W Xeꢀlamp as the light sources.
DETA) aqueous solution, the total amount of Cd(NO ·4H
and Zn(NO ·6H O was 2 mmol. After constant stirring for 15
3 2 2
) ·4H O and spectrophotometer at room temperature with either a 550 nm
3
)
2
2
(
3
)
2
2
O
Photocatalytic activity tests. Photocatalytic activity of the asꢀ
prepared core–shell CZS@BiO microspheres were evaluated
3
)
2
2
X
min at room temperature, 2 mmol cysteine (Cys) was slowly
added with stirring for another 15 min. Cd0.2Zn0.8S microsphere
could be obtained after the autoclave was heated at 180 ºC for
4 h and then cooled down to room temperature. The asꢀ
prepared Cd0.2Zn0.8
hereafter.
by photoꢀdegradation of RhB using a 200 mL quartz reactor
under irradiation of a 300 W Xe lamp (CELꢀHXF300) with a
4
20 nm cutꢀoff filter. A recycling water pump was used to keep
2
the reactor temperature constant at 10 ºC. The photocatalyst (1
g/L) was dispersed into 100 mL RhB solution (20 mg/L). Prior
to irradiation, a mixture with photocatalyst were stirred for 30
S microspheres were denoted as CZS
Synthesis of BiOX. In
Bi(NO ·5H O was dissolved in 10 mL ethylene glycol (EG), equilibrium. 3 mL suspension was collected and centrifuged at
then the stoichiometric amount of K = Cl, Br, I) was slowly regular intervals to measure the UVꢀvisible absorption
added with the Bi/ molar ratio of 1. The mixture was stirred spectrum by using a spectrophotometer (Hitachi, Uꢀ4100). To
a typical synthesis, 2 mmol min in darkness in order to reach the adsorption–desorption
3
)
3
2
X (X
X
for 30 min at room temperature, and then poured into a 40 mL investigate the stability of catalysts, the specimen after one trial
Teflonꢀlined stainless autoclave until 50% of the autoclave was washed and dried for the subsequent recycling test.
volume was filled. The autoclave was allowed to be heated at
Active species trapping experiments. To detecting the active
species during photocatalytic process, 1.0 mM isopropanol
160 °C for 12 h, and then cooled down to room temperature
naturally. The resulting samples were centrifuged and washed
with deionized water and ethanol thoroughly and dried at 60 °C
overnight.
(
(
IPA), 1.0 mM benzoquinone (BQ) and ammonium oxalate
−
AO) were added into the RhB solution as •OH quencher, •O
2
+
quencher and h quencher, respectively.
2
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RESULTS and DISCUSSION
present work, the 1:2.5 weight ratio of CZS to Bi(NO
was used for all the samples mentioned in this article.
3 3 2
) ·5H O
Characterizations of core-shell CZS@BiOX microspheres.
To date, a variety of strategies have been employed to synthesis
efficient visibleꢀlight photocatalytic materials. Among these
approaches, formation of core–shell structured nanocomposites/
microcomposites is considered to be a facile method to improve
the photoꢀinduced charges separation efficiency and the
photochemical activity, and has been extensively studied for the
7
,28,29
last decades.
Semiconductor core–shell composites which
often display superior physical and chemical properties over
3
0
their singleꢀcomponent counterparts can be divided into three
different cases: typeꢀI, typeꢀII and typeꢀIII band alignment
depending on the bandgaps and the electronic affinity of
semiconductors. Taking the typeꢀII band alignment for
example, the position of valance and conduction bands of the
core semiconductor is higher than that of the shell
semiconductor. The chemical potential difference causes band Fig. 1 The lowꢀmagnification SEM images of CZS (a), CZS@BiOCl (b),
CZS@BiOBr (c) and CZS@BiOI (d). Scale bar: 5 ꢁm.
bending at the interface of junction, which can induce a builtꢀin
field to accelerate the spatial separation of the electron−hole
7
,31
pairs on different sides of the coreꢀshell composites.
experiments, type II coreꢀshell CZS@BiO microspheres were
achieved via a twoꢀstep solvothermal method. The pristine
BiO microspheres present hierarchical superstructures built up
of irregular nanoplates, and the sizes of these spheres are 1~2
In our
X
X
8
,14
µm which are consistent with the previous reports (Fig. S1).
The SEM images of the CZS@BiO coreꢀshell microspheres as
well as the asꢀprepared CZS microspheres are depicted in Fig.
. The SEM images reveal that the pure CZS appears as
X
1
separate microspheres with diameters of 1~2 ꢁm and the
distinct coreꢀshell microstructure demonstrates the successful
Fig. 2 TEM images of CZS (a), CZS@BiOCl (b), CZS@BiOBr (c) and
coating of BiO
their microstructure, EDX elemental technique was applied to
the CZS@BiO composites. Fig. S2 presents that the elements
including Bi, S, Cl/Br/I and O coexist in the CZS@BiO
hybrid, which further confirms the successful formation of
CZS@BiO composites.
X
shells on the CZS cores. To further elucidate CZS@BiOI (d). Scale bar: 500 nm.
Fig. 3 describes the XRD patterns of BiO
corresponding CZS@BiO composites ( = Cl, Br, I). The
XRD patterns of pure BiO agree well with the tetragonal
structures of BiOCl (JCPDS #73ꢀ2060), BiOBr (JCPDS #73ꢀ
061), and BiOI (JCPDS #73ꢀ2062), respectively. The core–
shell structured CZS@BiO microspheres demonstrate an
obviously coexistence of both CZS and BiO phases with no
X
, CZS and the
X
X
X
X
X
X
2
Previous investigations have demonstrated that the shellꢀ
layer thickness which directly influences their performance can
be controlled by simply changing the content of raw
X
X
other impurity phases from the diffraction patterns, indicating
that there are no appreciable chemical reactions between CZS
7
,29
materials.
composed of irregular nanoꢀsized blocks with an average
diameter of ~1ꢀ2 ꢁm (Fig. 2). BiO = Cl, Br, I) powders are
composed of nanosheets aggregated together and selfꢀ
assembled into urchinꢀlike spheres (Fig. S3). The TEM images
The TEM images reveal that pure CZS is
and BiO
X
. The diffraction patterns of CZS@BiO
X
indicate that
and the
X (X
the composites are composed of the tetragonal BiO
hexagonal CZS.
X
The optical response of samples in the visible region is
systematically evaluated by UVꢀvis DRS Fig. 4). The
of the coreꢀshell microcomposites CZS@BiO
mass ratios of CZS to Bi(NO ·5H O ranged from 1:1 to 1:5
are shown in Fig. 2 and Fig. S4. It was also found that wellꢀ
shaped coreꢀshell CZS@BiO microspheres could be obtained
in the whole examined ranges. In the case of 1:1 CZS@BiO
the formations of shell layers were incomplete due to the low
content of BiO . With regard to the 1:1.25 CZS@BiO , it
shows a coreꢀshell structure with lumpy BiOX flakes covered
on the CZS microspheres core. As the CZS/Bi(NO ·5H
mass ratio increased to 1:5, there was a dramatic density
increase of BiO shells in the achieved products, resulting in
X with different
(
3
)
3
2
transmittance cutꢀoff edge of pure CZS is determined to be 490
nm, corresponding to bandgap energy of 2.55 eV. While, for
BiOCl, BiOBr and BiOI, the bandgap values are derived to be
X
X
,
3
.12, 2.58 and 1.92 eV, respectively. The derived bandgap
values of CZS and BiO could calculate by KubelkaꢀMunk
KM) method, αhν = A(hν−Eg) , where α and Eg are the
absorption coefficient and bandgap energy, CZS ( = 1) and
BiO = 4) pertain to direct and indirect transitions,
respectively. The calculated bandgaps are provided in Table 1
X
X
X
n/2
(
n
3
)
3
2
O
X
(n
.
X
From these observed interactions between bare CZS and BiO
counterparts, we can say that the heteroꢀstructured CZS@BiO
X
X
the cottonꢀlike coatings encircled each microspheres. In the
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coreꢀshell photocatalysts are successfully fabricated. surface. More interestingly, CZS@BiO
Surprisingly, all the CZS@BiO composites are black in color a closed surface area with BiO
after the formation processes of the coreꢀshell heterojunction, should be ascribed to the complete coating of BiO
which are consistent with the high background of the microspheres. The insert displayed the pore size distribution of
absorption in the visible light range up to 800 nm. We asꢀmade samples. For BiO samples, the pore size mainly
tentatively attribute this phenomenon to the oxygen and sulfur centered at ca. 2.0 and 2.7 nm. Notably, benefiting from their
defects on the surface of the products, similar as those reported mesoporous heterostructures, CZS@BiO coreꢀshell spheres
X
composites exhibited
X
X
samples (Table 1), which
on CZS
X
X
X
3
2–34
in the previous studies.
exhibited an extra larger pore distribution centered around 12
nm besides the original pores centered around 2.7 nm, which
may greatly improve the reactants transmission efficiency and
4
0,41
the utilization of active sites on the surface.
Fig. 3 Powder XRD patterns of (a) BiOX (X = Cl, Br, I) and (b) CZS,
CZS@BiOX (X = Cl, Br, I).
Fig.
5 Nitrogen adsorptionꢀdesorption isotherms of the CZS, BiOX,
CZS@BiOX (X = Cl, Br, I). Inset shows the derived poreꢀsize distribution
curves.
Fig. 4 (a) UVꢀvis DRS of CZS, BiOX and CZS@BiOX (X = Cl, Br, I). (b)
Band gaps of CZS and BiOX established from the absorption data.
Photocatalytic activity and stability. The photocatalytic
activities of coreꢀshell CZS@BiO
X
microspheres were
The photocatalytic activity can be significantly inhibited by evaluated by the degradation of RhB under visible light
the recombination of photoꢀinduced electronꢀhole pairs. PL irradiation in aqueous solution (420 nm ≤ λ ≤ 780 nm). As a
spectra can be used as a useful strategy to evaluate the model pollutant to evaluate the photocatalytic activity of a
3
5–39
recombination efficiency in a semiconductor photocatalyst.
Firstly, we focused on the visible light region around the CZS wellꢀcharacterized
emission bands (580ꢀ680 nm). The PL spectra of asꢀobtained cleavage process, redox reactions to terephthalic and benzoic
CZS, BiO and CZS@BiO
excited at 550 nm are presented in acids etc., and finally into CO and H O (Fig. S6). During the
Fig. S5. The emission intensity of the asꢀprepared CZS@BiO
process, the concentration of RhB can be easily detected by the
given catalyst, the decomposition mechanism of RhB has been
4
2ꢀ44
to go through a stepwise chromophore
X
X
2
2
X
decreases in the order of CZS@BiOI < CZS@BiOBr < intensity decrease of the emission peak around 550 nm. Fig. 6a
CZS@BiOCl. These two bands become imperceptible for the and Fig. 6b present the concentration of RhB as a function of
CZS@BiOI microspheres. Since the PL intensity is irradiation time over CZS and CZS@BiOI catalysts. The
proportional to the recombination rate of the photoꢀinduced continual fading of the coloration of RhB solutions obviously
electrons and holes, the constructing of the coreꢀshell illustrates a steady and continuous degradation of the organic
architecture do suppress the recombination of photoꢀinduced dyes. The absorption peak at 554 nm decreases gradually as the
electronꢀhole pairs effectively.
Fig. 5 depicted the N
adsorptionꢀdesorption isotherms and of the conjugated structure due to the incomplete mineralization
irradiation time increased, which demonstrates the destruction
2
pore size distributions of all asꢀobtained samples. All materials of RhB during the visible light irradiation. The curve plotted for
showed a slope of isotherm with a clear hysteresis loop at BiOCl and BiOBr shows the minute photocatalytic activity
relative pressure from 0.5 to 1.0, indicating the porosity was under visible light due to the wide band gap (Fig. S6). After
mainly composed of mesopores. Apparently, all isotherms CZS coating on the BiO
X particles, the coreꢀshell hybrid
presented a sharp capillary condensation step at the high catalyst displays enhanced photocatalytic performance, and
relative pressure, implying the existence of mesopores with a CZS@BiOI composite exhibits the highest photocatalytic
relatively large pore size or even marcopores. For the activity for the degradation of RhB. As for the individual BiOI
CZS@BiO
X samples (especially when X
= I), the curve sloop and CZS, the photocatalytic efficiency on RhB are 50% and
and the hysteresis loop increased gradually, indicating the 71%, respectively (Fig. S6). While, almost 98% of RhB is
formation of more slitꢀshaped mesopores and active sites on the degraded over CZS@BiOI after irradiation for 120 min under
4
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visible light. The kinetics of RhB degradation is revealed by asꢀobtained photocatalysts are stable enough during the
applying the pseudoꢀfirstꢀorder model and the results are listed photocatalytic process. The enhanced stability and durability of
in Table 1
.
CZS@BiOI can be attributed to the following two reasons: 1)
The accelerated separation of photoꢀgenerated electronꢀhole
pairs due to the effective heterojunction between the CZS and
BiOI, which reduce the opportunity of the undesirable
2
ꢀ
oxidization of S ions; 2) BiO
X layers in the coreꢀshell
architecture can protect the inside CZS cores from aggregation
during the photoꢀdegradation processes.
Photocatalytic mechanism of core–shell CZS@BiOX
microspheres. As discussed above, the superior photocatalytic
activity and stability of the coreꢀshell CZS@BiO
X
microspheres were attributed to the efficient separation and fast
transfer of photoꢀgenerated charges at the interface of coreꢀshell
microꢀarchitectures. To investigate the detailed photocatalytic
mechanism of CZS and CZS@BiOI core–shell microspheres,
radicals trapping experiments were conducted to estimate the
role of different reactive species by adding various scavengers
in the photocatalytic degradation of RhB. BQ, AO and IPA
−
+
Fig. 6 Absorption spectra of RhB aqueous solution in the presence of
the CZS (a) and CZS@BiOI (b). Photocatalytic degradation of RhB on
blank and CZS@BiOX (X = Cl, Br, I) under visibleꢀlight irradiation (c).
Cycling performance for the photocatalytic degradation of RhB over
CZS and CZS@BiOI under visible light irradiation (d).
were used to trap the •O
respectively.
2
,
h
and •OH radicals,
,
12,18,46
As shown in Fig. 7 the degradation
efficiency of RhB over CZS and CZS@BiOI is significantly
inhibited by adding AO and BQ scavengers, implying that the
+
−
h and •O
2
radicals plays major roles in the catalytic processes.
Table 1. Summarized CB, VB potentials and catalysisꢀrelated
Comparatively, the addition of IPA shows weaker influence on
the photocatalytic oxidation process of RhB over CZS and
CZS@BiOI, indicating the less participation of •OH radicals
during the photoꢀdegradation process. These evidences clearly
properties of CZS, BiO
X
and CZS@BiO
X
(
X
= Cl, Br, I).
2
−1
Material
CZS
S
BET (m /g)
98.09
30.35
43.82
70.15
44.42
45.18
64.51
g
E , eV
E
CB, eV
E
VB, eV
2.02
3.71
3.23
2.67
ꢀꢀꢀ
K (min )
2.55
3.12
2.56
1.92
ꢀꢀꢀ
ꢀ0.53
0.59
0.67
0.75
ꢀꢀꢀ
0.008
0.002
0.002
0.004
0.007
0.013
0.018
BiOCl
+
−
demonstrate that the h and •O
2
radicals play more important
BiOBr
roles than that of •OH in the photoꢀdegradation process of RhB
over CZS and CZS@BiOI catalysts.
BiOI
CZS@BiOCl
CZS@BiOBr
CZS@BiOI
ꢀꢀꢀ
ꢀꢀꢀ
ꢀꢀꢀ
ꢀꢀꢀ
ꢀꢀꢀ
ꢀꢀꢀ
It was reported that the synthesis method could affect the
morphology, size and surface area of the photocatalysts, and
thus decide the adsorption behavior and photocatalytic activity
4
5
Fig. 7 The impact of reactive species in the photoꢀdegradation process
of RhB over CZS (a) and CZS@BiOI (b). Scavengers used in the
experiments: Blue, None; Green, 1.0 mM IPA; Cyan, 1.0 mM AO;
Purple, 1.0 mM BQ.
of BiO
different synthetic routes, we provide a brief summary of our
asꢀfabricated BiO = Cl, Br, I) photocatalysts together with
X
.
To compare the features of the materials via
X (X
those reported previously in Table S1. We speculate that
different synthetic routes may lead to distinct pollutant
adsorption and light absorption behavior, and thus diverse
photocatalytic performance under visible light. The kinetic
To further evaluate the •OH radicals produced in the catalytic
process, PL signals with terephthalic acid (TA) as the probe was
collected, since TA was believed that could react with •OH radicals
readily to produce 2ꢀhydroxyterephthalic acid with highly
reaction rate constants
k values of coreꢀshell hybrid catalysts
1
8,47,48
−
1
−1
fluorescent emissions.
The PL spectra from NaOH solution of
are 0.007 min (CZS@BiOCl), 0.013 min (CZS@BiOBr)
−
1
TA (excitation at 315 nm) over CZS and CZS@BiOI are shown in
Fig. 8. Significant PL signal centered at ca. 415 nm is observed
during the photocatalytic process, which confirms the production of
and 0.018 min (CZS@BiOI), respectively. The enhanced
catalytic activity of CZS@BiOI can be attributed to the raised
light absorption capability and the improved separation and
transfer efficiency of photoꢀgenerated electronꢀhole pairs. Fig.
•
OH radical during the photocatalytic process. Nevertheless, the PL
intensities for CZS@BiOI photocatalyst are much weaker than that
of CZS photocatalyst, which indicates that the emerging hydroxyl
radical is not the main oxygen species in the degradation process of
RhB.
6d depicts the cycling runs for the photocatalytic degradation of
RhB over CZS and CZS@BiOI under visibleꢀlight irradiation.
No obvious decay of the photoꢀdegradation efficiency was
observed after five circulatory experiments, suggesting that the
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−
−
51
composite to produce •O
2
(O
2
/•O
2
= ꢀ0.046 eV) and then
decompose RhB. Nevertheless, the VB of CZS (2.02 eV) is less
ꢀ
positive than the standard reduction potential of •OH/OH (1.99
5
2
+
eV), suggesting that the h on the surface of CZS cannot
ꢀ
oxidize OH into •OH radicals, the main source of the •OH
radicals in the detection experiment may produce by the •O
reducing H
−
2
2
O into •OH radicals. The photoꢀdegradation activity
of RhB declined marginally when IPA was added, therefore the
RhB photoꢀdegradation of CZS@BiOI composites may be
initiated by the direct surface hole and electron transfer rather
than the oxidation by •OH. Furthermore, one should be aware
that the overall photocatalytic performance of a given material
relied on the cooperation of the pollutant adsorption, light
harvesting, and efficient separation of photoꢀinduced charge
carrier. Therefore, the enhanced catalytic performance of the
optimized CZS@BiOI heteroꢀstructures can be attributed to the
improved visible light absorption, and the accelerated
separation of photoꢀgenerated electronꢀhole pairs.
Fig. 8 The PL spectra of TA over the CZS (a) and CZS@BiOI
microspheres (b) excited at 315 nm in NaOH solution.
Conclusions
In conclusion, we report a facile method to fabricate a family of
core–shell Cd0.2Zn0.8S@BiO
Cl, Br, I) onto Cd0.2Zn0.8
microspheres and photocatalytic processes of CZS@BiOI under visibleꢀ Cd0.2Zn0.8S@BiO microspheres
X
microspheres by coating BiO
microspheres. The
exhibit enhanced
X
(
X
=
S
Fig.
9 Schematic illustration of the formation of CZS@BiOX
X
light irradiation.
photocatalytic activities and stability toward visibleꢀlightꢀ
driven degradation of RhB compared to their singleꢀcomponent
counterparts. The excellent photocatalytic performance of the
products can be attributed to the synergistic effect derived from
the wellꢀfined heteroꢀstructures between Cd0.2Zn0.8S and BiOX,
which can improve the light absorption and accelerate the
separation of the photoꢀgenerated electrons and holes. Our
In order to understand the photocatalytic mechanism, CB and
VB positions of CZS and BiO
experimental data and empirical equations. The results are
shown in Table 1. Taking BiOI as example, band edge
X were calculated based on the
4
9,50
positions are given by the following equations:
1
/(3)
E
E
c
v
= (
χ
(Bi)
ꢂ
E
χ
(O)
ꢂ
χ
(I)
)
ꢀ1/2
E
g
+ E
0
(1)
(2)
work develops
a
simple preparation route to obtain
=
E
c
+
g
semiconductor composites for various energy and environmentꢀ
related applications.
The absolute electroꢀnegativities of the constituent atoms Bi, O,
and I are χ(Bi) = 4.69, χ(O) = 7.54, and χ(I) = 6.76, respectively.50
Acknowledgements
E
C
, E
bandgap energy of the semiconductor respectively. E
determined by the UVꢀvis DRS, and E = –4.5 eVVAS at pH = 0.
Based on these results, we provided the schematic illustration
of fabrication CZS@BiO microspheres and photocatalytic
V
, and E
g
are the energy levels of the CBM, VBM and the
g
were
This work is supported by the National Natural Science
Foundation of China (21301063), Natural Science Foundation
of Education Department of Henan Province (12B150015) and
Sciꢀtech Development Program of Zhengzhou in 2012
0
X
mechanism scheme of CZS@BiOI under visibleꢀlight
irradiation in Fig. 9. When the CZS@BiOI microsphere is
exposed to visible light, both semiconductors are activated. As
for CZS@BiOI, the redox potential of both CB and VB (ECB = ꢀ
(
20120325).
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Graphical Abstract
A family of heterostructured core-shell Cd Zn S@BiOX (X = Cl, Br, I) microspheres with
0.2
0.8
adjustable bandgaps, enhanced stability and visible-light-driven photocatalytic performance were
fabricated via a facile solvothermal route.