ZnWO4/BiOI heterostructures with highly efficient visible light photocatalytic activity: The case of interface lattice and energy level match

Article abstract of DOI:10.1039/c3ta00442b

ZnWO₄/BiOI heterostructures with different constituents are synthesized via a chemical bath approach under mild conditions by tuning the Zn/Bi molar ratios. The obtained ZnWO₄/BiOI heterostructures display high photocatalytic activities in degradation of MO and photocurrent response under visible light irradiation. Combining the experimental findings, first-principles calculations are used to investigate the surface geometry structures and the work functions of the (011) and (010) surfaces of the ZnWO₄ phase and the (001) surface of the BiOI phase. The results show that the lattice and energy levels between the ZnWO₄ and BiOI phases match well with each other to be capable of forming efficient ZnWO ₄/BiOI p-n heterojunction structures. This match promotes the separation and transfer of photoinduced electron-hole pairs at the interface, resulting in the excellent photocatalytic performance of the ZnWO ₄/BiOI heterostructures. Our findings show that the formation of a heterostructure would possess the excellent photocatalytic activities only if the lattice and energy level match between the two semiconductors was satisfied, which is of great importance for designing and developing more efficient heterostructured photocatalysts.

Full text of DOI:10.1039/c3ta00442b

Journal of
Materials Chemistry A
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PAPER
ZnWO /BiOI heterostructures with highly efficient
4
visible light photocatalytic activity: the case of interface
lattice and energy level match†
Cite this: J. Mater. Chem. A, 2013, 1,
421
3
a
b
ac
b
b
b
Pan Li, Xian Zhao, Chun-jiang Jia, Honggang Sun, Liming Sun, Xiufeng Cheng,
b
a
Li Liu and Weiliu Fan*
ZnWO
4
/BiOI heterostructures with different constituents are synthesized via a chemical bath approach
/BiOI heterostructures
under mild conditions by tuning the Zn/Bi molar ratios. The obtained ZnWO
4
display high photocatalytic activities in degradation of MO and photocurrent response under visible
light irradiation. Combining the experimental findings, first-principles calculations are used to
investigate the surface geometry structures and the work functions of the (011) and (010) surfaces of
the ZnWO
levels between the ZnWO
efficient ZnWO /BiOI p–n heterojunction structures. This match promotes the separation and transfer of
photoinduced electron–hole pairs at the interface, resulting in the excellent photocatalytic performance
of the ZnWO /BiOI heterostructures. Our findings show that the formation of a heterostructure would
4
phase and the (001) surface of the BiOI phase. The results show that the lattice and energy
4
and BiOI phases match well with each other to be capable of forming
4
Received 26th September 2012
Accepted 8th January 2013
4
possess the excellent photocatalytic activities only if the lattice and energy level match between the two
semiconductors was satisfied, which is of great importance for designing and developing more efficient
heterostructured photocatalysts.
DOI: 10.1039/c3ta00442b
www.rsc.org/MaterialsA
abilities. However, heterostructured photocatalysts with excel-
lent photocatalytic performance, high stability, and low cost are
1
Introduction
Photocatalysts have drawn widespread attention due to their still rare. In order to obtain the desirable heterostructured
potential applications in environmental remediation and photocatalysts, two key factors must be considered: one is the
renewable resource production, such as water purication and interface lattice match to construct a stable heterostructure; the
1–6
hydrogen production. To date, the practical applications of other is the matching of energy levels between the two
photocatalysts are restricted by two important factors: the fast components so as to promote charge carrier separation, that is,
recombination of photogenerated electron–hole pairs and
the limited visible-light response. Thus, there have been
intense efforts to develop new highly efficient visible light
photocatalysts.
the conduction band minimum (CBM) of the narrow band gap
semiconductor should be higher than the other component
with lower CBM.
ZnWO , one of the important photocatalysts, has been
4
Heterostructured semiconductors with different phases and widely studied for water splitting and mineralization of organic
15,16
materials have attracted great interest in photocatalytic pollutants under UV irradiation.
However, the practical
research. For instance, many heterostructures such as TiO
graphene, Fe @TiO , ZnO/CdS, LaVO /TiO , SnO /a-Fe
and C N /BiPO have been synthesized. Compared to single-
phased photocatalysts, the advantages of heterostructures
include not only the extended light responsive range, but also
2
–
application of ZnWO is hampered by its large band gap energy
and fast recombination of photogenerated electron–hole pairs.
4
2
O
3
2
4
–14
2
2
2
O
3
,
7
3
4
4
In our previous work, F doping was used to improve the pho-
17
tocatalytic activity of ZnWO
4
, but its visible light absorption is
still limited. Bismuth oxyiodide (BiOI), as an important V–VI–
the enhanced photogenerated charge carrier separation VII ternary compounds with small band gap (E ¼ 1.73–1.92 eV)
g
to absorb visible light, has attracted great interest in recent
18,19
years.
Moreover, as a typical p-type semiconductor, BiOI can
a
School of Chemistry and Chemical Engineering, Shandong University, Jinan 250100,
serve as an efficient visible light photosensitizer for n-type
China. E-mail: fwl@sdu.edu.cn; Fax: +86-531-88365174; Tel: +86-531-88366330
20,21
semiconductors with a large band gap.
for BiOI coupling with ZnWO to form ZnWO
structures with high visible light photocatalytic activities.
Here, we present novel ZnWO /BiOI heterostructures with
It is a good candidate
b
State Key Laboratory of Crystal Materials, Shandong University, Jinan 250100, China
/BiOI hetero-
c
4
4
Max-Planck-Institut f u¨ r Kohlenforschung, Kaiser-Wilhelm-Platz 1, 45470 M u¨ lheim an
der Ruhr, Germany
†
Electronic supplementary information (ESI) available: TEM and HRTEM images
4
of the ZnWO4 and BiOI. See DOI: 10.1039/c3ta00442b
different constituents synthesized by a chemical bath approach
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under mild conditions. First-principles calculations are used to SEM (FESEM, Hitachi, Japan) at an accelerating voltage of 5 kV.
investigate the lattice and energy match between the ZnWO4 The transmission electron microscopy (TEM) and high resolu-
and BiOI phases by examining the surface geometry structures tion TEM (HRTEM) analyses were conducted with the use of a
and the work functions of the (011) and (010) surfaces of the JEM 2100 microscope at an accelerating voltage of 200 kV. UV-
ZnWO
that the lattice and energy levels between the ZnWO
phases match well, enabling them to form stable ZnWO
BiOI heterostructures with highly efficient photocatalytic measure the Brunauer–Emmett–Teller (BET) surface areas of
4
phase and the (001) surface of the BiOI phase. We nd Vis diffuse reectance spectra were measured with a Shimadzu
and BiOI UV-2550 spectrophotometer with an integrating sphere in
ambient conditions. A Builder 4200 instrument was used to
4
4
/
performance.
the samples at liquid nitrogen temperature.
2
Experimental details
2.4 Photocatalytic activity tests
2
.1 Synthesis of ZnWO nanorods
The photocatalytic activities of the as-prepared products were
evaluated by the degradation of methyl orange (MO) under
visible light irradiation at room temperature. A 400 W metal
halide lamp with a 420 nm cutoff lter was used as the light
source. Typically, 0.1 g of photocatalyst was added to 100 mL of
4
Zinc tungstate (ZnWO
thermal method based on our previous work. In a typical
synthesis, 1 mmol of Zn(AC) $2H O was dissolved in 10 mL of
4
) nanorods were prepared by the hydro-
22
2
2
deionized water to form a solution. Then 10 mL of 0.1 M
Na WO solution was added slowly to the above solution under
stirring. The pH value of the solution was adjusted to 9.0 using
dilute 25–28% NH $H O solution. Aer being stirred for about
0 min, the mixture was transferred into a 25 mL Teon-lined
ꢁ
1
10 mg L MO aqueous solution in a container. The solution
2
4
was stirred in the dark for 1 h to insure the adsorption–
desorption equilibrium. At 30 min intervals, about 5 mL of the
suspension was taken for the following analysis aer centrifu-
gation. The concentration of MO was monitored by a UV-visible
spectrophotometer (U-3500, Hitachi, Japan).
3
2
2
steel autoclave. Hydrothermal treatments were carried out at
ꢀ
180 C for 12 h and then the autoclave was cooled to room
temperature naturally. The white precipitates were collected
and washed with deionized water and absolute alcohol several
times. Finally the sample was dried in an oven at 60 C for 8 h.
2.5 Photoelectrochemical measurements
ꢀ
The photocurrents were measured with an electrochemical
workstation (LAN-LIKE, Tianjin, China) in a standard three-
electrode system with the sample as the working electrode, a Pt
foil as the counter electrode, and Ag/AgCl (saturated KCl) as the
reference electrode. For the fabrication of the working elec-
trodes, 40 mg of the products were mixed with a certain amount
of ethanol and Naon solution homogeneously. The as-
prepared samples were spread on a 15 mm ꢂ 20 mm indium-
tin oxide (ITO) conducting glass and allowed to dry under
ambient conditions. A 300 W Xe arc lamp (PLS-SXE300C/
300CUV, Beijing Trusttech Co., Ltd.) with a 420 nm cutoff lter
2
.2 Synthesis of ZnWO
ZnWO /BiOI heterostructures were synthesized by a facile
chemical bath method at low temperature. Bi(NO $5H O was
added slowly into a water solution containing a stoichiometric
amount of KI under stirring. Then, 1 mmol of ZnWO nanorods
4
/BiOI heterostructures
4
3
)
3
2
4
were added to the above mixed solution. The Zn/Bi molar ratios
were kept to 4 : 1, 3 : 1, 2 : 1, and 1 : 1 (denoted as ZB-4, ZB-3,
ZB-2, and ZB-1), respectively. The mixture was stirred for 30 min
at room temperature, and then heated at 80 C for 2 h in a water
bath. The precipitates were collected and washed with deion-
ized water and ethanol several times. Then the samples were
ꢀ
2 4
was used as the light source. The electrolyte was 0.5 M Na SO
aqueous solution. The photocurrents as light on and off were
measured at 0.5 V.
ꢀ
dried at 60 C for 8 h. For comparison, the single BiOI sample
was also prepared via the same procedure without the addition
of ZnWO4.
3 Computational details and models
To evaluate the effect of the heterojunction on the photo-
The calculations were performed using the plane-wave pseu-
catalytic activity of the ZnWO /BiOI heterostructures,
a
4
2
3
dopotential DFT method as implemented in the CASTEP code.
The generalized-gradient approximation with the Perdew–
mechanically physically mixed ZnWO
ZnWO
/BiOI molar ratio ¼ 2 : 1, denoted as ZnWO
prepared and evaluated.
4
and BiOI sample
(
4
4
+ BiOI) was
24
Burke–Ernzerhof (PBE) forms was used to describe the
exchange and correlation potential. Ultraso pseudopotentials
are used to deal with core electrons. The pseudopotentials for
2.3 Characterization
10
2
2
6
4
2
Zn, W, Bi, O, and I atoms represented 3d 4s , 5s 5p 5d 6s ,
2
3
2
4
2
5
Powder X-ray diffraction (XRD) patterns were obtained on a D8 6s 6p , 2s 2p and 5s 5p electron congurations, respectively.
Advance X-ray diffractometer (Bruker, Germany), using Cu Ka For ZnWO
, a Monkhorst–Pack k-point mesh of 5 ꢂ 4 ꢂ 5 yiel-
radiation at a scan rate of 0.02 2q s . X-ray photoelectron ded well converged bulk results and the calculated a, b and c
4
ꢀ
ꢁ1
˚
˚
5
˚
spectroscopy (XPS) measurements were carried out on a lattice constants were 4.75 A, 5.84 A and 4.97 A, in good
2
Thermo Fisher Scientic Escalab 250 spectrometer with mon- agreement with experimental values. For BiOI, a Monkhorst–
ochromatized Al Ka excitation, and C 1s (284.8 eV) was used to Pack k-point mesh of 6 ꢂ 6 ꢂ 3 yielded well converged bulk
˚
calibrate the peak positions of the elements. Scanning electron results and the calculated a and c lattice constants were 4.00 A
26
˚
microscopy (SEM) was performed by an S-4800 eld emission and 9.37 A, which agreed well with experimental values.
3
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For the (001)BiOI–(011)ZnWO4 interface model, a 5 ꢂ 4 six Zn/Bi molar ratios of 4 : 1, 3 : 1, 2 : 1, and 1 : 1 are denoted as
atomic layer stoichiometric BiOI (001) surface slab containing ZB-4, ZB-3, ZB-2, and ZB-1, respectively. All of the diffraction
1
20 atoms is used to match a 4 ꢂ 2 twelve atomic layer stoi- peaks shown in Fig. 1a and f can be well-indexed to monoclinic
chiometric ZnWO (011) surface slab containing 96 atoms in a ZnWO (JCPDS card no.73-0554) and tetragonal BiOI (JCPDS
4
4
supercell. For the (001)BiOI–(010)ZnWO4 interface model, a 4 ꢂ 5 card no.73-2062), respectively. The sharp diffraction peaks of
six atomic layer stoichiometric BiOI (001) surface slab con- both BiOI and ZnWO indicate their good crystallinity. No traces
taining 120 atoms is used to match a 3 ꢂ 4 eight atomic layer of other phases are examined, conrming the high purity of the
stoichiometric ZnWO (010) surface slab containing 144 atoms samples. All curves in Fig. 1b–e obviously show two sets of XRD
in a supercell. The BiOI (001) surface consists of [Bi ] layers peaks of monoclinic ZnWO and tetragonal BiOI, indicating
4
4
2
O
2
4
interleaved with double layers of halogen atoms, so it is a polar that the as-synthesized products are composite materials.
surface and not stable. In order to eliminate the polarity of the Moreover, it is noted that the diffraction peaks of BiOI are
surface slab, half the amount of oxygen atoms is moved from intensied with decreasing Zn/Bi molar ratio, while the peak
one side of the slab to the other to make the crystal slab intensities of ZnWO
symmetric about its midpoint. A vacuum of approximately 15 A˚ corresponding to BiOI in the patterns of ZnWO
was placed above the slabs to separate the interaction between structures become narrower and stronger, suggesting that the
periodic images. Geometry optimization of the supercell is crystal growth of BiOI was affected by ZnWO
carried out at the G point. The cutoff energies for plane waves The X-ray photoelectron spectroscopy (XPS) measurements
are chosen to be 300 eV, and the convergence tolerance of force provide further information for the chemical composition and
4
decreased gradually. The diffraction peaks
4
/BiOI hetero-
4
.
ꢁ1
on each atom during structure relaxation is set at 0.05 eV A˚
.
surface chemical states of the ZnWO /BiOI heterostructures.
4
The interface formation energies are calculated to examine The binding energies in the XPS spectra presented are cali-
the stabilities of the model interfaces. The interface formation brated by using that of C 1s (284.8 eV). The XPS survey spectrum
energies are dened as the energy difference between the total of the ZnWO
system and the single components (ZnWO and BiOI). The strates that the heterostructure is composed of Zn, W, Bi, O and
single component energy of ZnWO is calculated by removing I elements. Fig. 2b–f shows the high-resolution XPS spectra of
BiOI from the interface; similarly, the BiOI energy is calculated Zn 2p, O 1s, and W 4f for pure ZnWO and ZnWO /BiOI heter-
4
/BiOI heterostructure (ZB-2) in Fig. 2a demon-
4
4
4
4
with ZnWO removed from the interface. To understand the ostructure, as well as the O 1s, Bi 4f, and I 3d spectra for the
4
origin of the interface charge transfer in the ZnWO /BiOI het- heterostructure and pure BiOI. The two peaks of the Zn region
4
erostructure, work functions for the ZnWO and BiOI surfaces at 1021.5 and 1044.7 eV are attributed to Zn 2p and Zn 2p1/2
4
3/2
2
+
are calculated by aligning the Fermi level relative to the vacuum (Fig. 2b), which can be ascribed to the Zn ions of the
2
7,28
energy level.
ZnWO
1/2 for ZnWO
eV, respectively, both of which have a shi of 0.5 eV to high
energy region compared with those for pure ZnWO . The O 1s
peaks for the ZnWO /BiOI heterostructure in Fig. 2c can be
4
.
However, the binding energies of Zn 2p3/2 and Zn
2
p
4
/BiOI heterostructures are 1022.0 and 1045.2
4
Results and discussion
4
4
4
.1 Structure and morphology of ZnWO /BiOI
4
heterostructures

tted into three peaks: the peak at 529.5 eV corresponds to the
2
1,29
Fig. 1 shows the XRD patterns of ZnWO
4
, BiOI, and ZnWO
4
/BiOI Bi–O bonds,
the peak at 530.6 eV is attributed to Zn–O
3
0
heterostructures. ZnWO /BiOI heterostructures with different bonds, and the peak at 532.2 eV is ascribed to the O–H bonds
4
31
of the surface-adsorbed water. Compared with pure BiOI, the
O 1s peak has a 0.7 eV shi to low binding energies. The effect
of surface charging on the shi in these binding energies
can be excluded since the photoelectron peaks for W 4f, Bi 4f
and I 3d remain at the same binding energies, as shown in
21,29,30
Fig. 2d–f.
The shis of Zn 2p peaks in the heterostructures
are attributed to the Zn bonding with O of BiOI, which is similar
32
to the shi of Zn 2p peaks in the g-C
3 4
N –ZnO composite.
These indicate the interaction between ZnWO
4
and BiOI, con-
and

rming the existence of chemical bonds between ZnWO
4
BiOI in the heterostructures.
The morphologies of ZnWO
4
4
, BiOI, and the ZnWO /BiOI
heterostructures are characterized by SEM, as shown in Fig. 3.
The pure ZnWO sample has a rod structure (Fig. 3a), and the
4
pure BiOI sample consists of large numbers of irregular plates
4
(Fig. 3f). It can be seen that the ZnWO nanorods are well spread
on the surface of the BiOI plates in the ZnWO
4
/BiOI hetero-
structures (Fig. 3b–e).
4 4
Fig. 1 XRD patterns of (a) ZnWO ; ZnWO /BiOI heterostructures with different
Detailed structural information about the ZnWO
erostructures was further investigated by TEM and HRTEM.
4
/BiOI het-
Zn/Bi molar ratios: (b) ZB-4 (Zn/Bi ¼ 4 : 1), (c) ZB-3 (Zn/Bi ¼ 3 : 1), (d) ZB-2 (Zn/
Bi ¼ 2 : 1), (e) ZB-1 (Zn/Bi ¼ 1 : 1); and (f) BiOI.
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Fig. 2 XPS spectra for: (a) survey of ZB-2, (b) Zn 2p of pure ZnWO
4
and ZnWO
4
/BiOI heterostructure ZB-2, (c) O 1s of pure BiOI, ZnWO
4
, and ZB-2, (d) W 4f of pure
4
ZnWO and ZB-2, (e) Bi 4f of pure BiOI and ZB-2, and (f) I 3d of pure BiOI and ZB-2.
From the structural analysis of the single ZnWO
phases (see Fig. S1 in ESI†) the (011) and (010) planes make up structure ZB-2. Fig. 4b and d display an interface region in the
the side facets of the ZnWO nanorods and the basal planes of heterostructure. The border between ZnWO nanorods and BiOI
the BiOI component are identied as {001} facets. Fig. 4 shows plates is nearly invisible, suggesting that the intimate contact
4 4
and BiOI TEM images and HRTEM images of the ZnWO /BiOI hetero-
4
4
4 4
Fig. 3 SEM images of (a) ZnWO ; ZnWO /BiOI heterostructures (b) ZB-4, (c) ZB-3, (d) ZB-2, (e) ZB-1; and (f) BiOI.
3
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between them and the composition fusion may happen at the
interface. The lattice spacings of about 0.37 and 0.58 nm
correspond to the (011) and (010) planes of ZnWO , and that of
4
0
.28 nm corresponded to the (110) plane of BiOI. Thus, it
revealed that the interface is formed by the BiOI (001) plane
contacting with the ZnWO (011) and (010) planes.
UV-Vis diffuse reectance spectra of the as-prepared samples
are shown in Fig. 5. The white ZnWO only has absorbance in
4
4
the UV light range with an absorption edge about 320 nm,
whereas the deep red BiOI shows strong absorption in the
visible light range, with an absorption edge around 660 nm. All
of the ZnWO /BiOI heterostructures have remarkable absorp-
4
tion in the visible light range, and their edges range from 665 to
680 nm. The bandgaps (E
g 4
) of ZnWO and BiOI are calculated to
be 3.87 and 1.88 eV, respectively.
4 4
Fig. 5 UV-vis diffuse reflectance spectra of ZnWO , BiOI, and ZnWO /BiOI
heterostructures.
4.2 Enhancement of photocatalytic activity and
photocurrent
The photocatalytic activity of ZnWO /BiOI heterostructures was
evaluated by MO degradation under visible light irradiation (l $
4
2
, implying it is the heterojunction formed between ZnWO and
4
BiOI that enhanced photocatalytic activities signicantly. The
reason that the photocatalytic efficiency of ZnWO /BiOI heter-
ostructures changes with different BiOI contents can be
explained by the following aspects. First, higher BiOI content
means more heterojunction could be formed between ZnWO
4
20 nm). Fig. 6 shows the MO photodegradation over ZnWO
BiOI, ZnWO /BiOI heterostructures, and the physical mixture of
ZnWO and BiOI. Pure ZnWO has almost no visible light
4
,
4
4
4
4
photocatalytic activity, while pure BiOI exhibits a weak degrade
efficiency of 26% aer 4 h of visible light irradiation. All of the
4
and BiOI. Second, excess ZnWO4 on the surface of BiOI in
the ZnWO /BiOI heterostructures could hinder the visible
ZnWO /BiOI heterostructures exhibit superior photocatalytic
4
4
activity to the single-component semiconductors. Aer 4 h
irradiation with visible light, the photodegradation efficiencies
of MO are 46%, 49%, 86%, and 75% for ZB-4, ZB-3, ZB-2, and
ZB-1, respectively. The photocatalytic activity of the physical
light capture of BiOI. Thus, the ZB-2 heterostructure shows
higher photocatalytic activity than the other ZnWO
heterostructures.
4
/BiOI
To quantitatively investigate the reaction kinetics of MO
degradation, the degradation data were analyzed with the
mixture of ZnWO
4
and BiOI is almost the same as that of pure
/BiOI heterostructure ZB-
BiOI, much lower than that of ZnWO
4
pseudo-rst-order model, ln(C/C
0
) ¼ kt, where C is the
concentration of MO at time t, C is the original concentration
0
of MO solution, and k is the reaction rate constant. The rate
constants calculated from the data plotted in Fig. 6 are
summarized in Table 1. The MO degradation rate constant of
Fig. 6 Photocatalytic activities of ZnWO
and the physical mixture of ZnWO and BiOI in the degradation of MO under
visible-light irradiation (l $ 420 nm).
4 4
, BiOI, ZnWO /BiOI heterostructures,
Fig. 4 (a and c) TEM images, and (b and d) HRTEM images of the ZnWO
4
/BiOI
4
heterostructure ZB-2.
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the ZB-2 heterostructure is much higher than those of ZnWO
and BiOI.
4
On the basis of the N adsorption–desorption isotherms, the
2
BET surface areas (ABET) of the samples were calculated and are
also summarized in Table 1. ZnWO
4
has the highest BET
, and BiOI has the lowest BET
surface area of 7.51 m g . The surface areas of the ZnWO
2
ꢁ1
surface area of 29.49 m
g
2
ꢁ1
4
/
BiOI heterostructures decrease as the content of BiOI increases.
To investigate the effects of surface area and heterojunction on
the photocatalytic activity, we normalized the reaction rate
constants k of the ZnWO /BiOI heterostructures and the phys-
4
ical mixture of ZnWO and BiOI by surface area. The normalized
4
rate constants of all ZnWO
than that of the physical mixture of ZnWO
that the heterojunction has an important effect on improving
4
/BiOI heterostructures are higher
4
and BiOI, indicating
the photocatalytic activity. Among the ZnWO
4
/BiOI hetero- Fig. 7 Recycling test on the ZnWO /BiOI heterostructure ZB-2 for the degra-
4
dation of MO under visible-light irradiation (l $ 420 nm).
structures, ZB-2 still shows the highest normalized rate
constant, suggesting that the surface area is not a major factor
inuencing the photocatalytic efficiency of our samples.
We also evaluated the photostability of the ZnWO /BiOI
heterostructure (ZB-2). As shown in Fig. 7, there is no obvious
change in the photocatalytic activity aer three cycles of pho-
todegradation, implying the good stability of the ZnWO
heterostructures.
absorption and the high separation and transfer of photo-
generated electron–hole pairs at the heterojunction interfaces.
4
The realization of such synergistic effects in the ZnWO /BiOI
4
heterostructures demands two key factors: the lattice match and
the energy level match between ZnWO and BiOI phases.
4
4
/BiOI
The interface lattice match is one important factor to form a
stable heterostructure, which can be investigated by examining
the surface geometry structure of the ZnWO and BiOI phases.
4
To better understand the effect of the heterojunction on the
high photocatalytic activity of the ZnWO /BiOI heterostructure,
4
we investigated its photoinduced charge transfer properties.
Fig. 8 shows the photocurrent responses of ZnWO , BiOI,
On the basis of the HRTEM analysis, we consider a model for
4
simulating of the interface between BiOI and ZnWO , namely
4
ZnWO
ZnWO
4
/BiOI heterostructures, and the physical mixture of
and BiOI under visible light irradiation (l $ 420 nm) at
the (001)BiOI–(011)ZnWO4 interface and the (001)BiOI–(010)ZnWO4
4
interface, as shown in Fig. 9. In the case of the (001)BiOI
011)ZnWO4 interface, it is composed of a ZnWO (011) plane and
a BiOI (001) plane (Fig. 9A). For the ZnWO (011) surface, the
lattice parameters are aZnWO4(011) ¼ 0.475 nm and bZnWO4(011)
.767 nm. The ZnWO (011) surface is made up of Zn atoms with
two dangling bonds and O atoms with one dangling bond. For
the BiOI (001) surface, the lattice parameters are aBiOI(001)
bBiOI(001) ¼ 0.400 nm. The BiOI (001) surface is terminated by O
–
0
.5 V. All of the ZnWO
current density than the single-component semiconductors and
the physical mixture of ZnWO and BiOI, demonstrating that a
4
/BiOI heterostructures exhibit higher
(
4
4
4
¼
more effective separation of photogenerated electron–hole pairs
and faster interfacial charge transfer occur in the hetero-
structures. The ZB-2 heterostructure exhibits the highest
current density, and the photocurrent changes of the samples
correspond to the photocatalytic performance variations.
0
4
¼
4.3 Mechanism of enhancement of photocatalytic activity
According to the above results, the excellent visible light pho-
tocatalytic activity of ZnWO /BiOI heterostructures can be
4
attributed to the synergetic effects of strong visible light
Table 1 Textural and photocatalytic properties of ZnWO
4
/BiOI heterostructures
ꢁ3
ꢁ2
0
a
A
BET
k (ꢂ 10
,
k
(ꢂ 10
ꢁ2
,
2
ꢁ1
ꢁ1
ꢁ1
Samples
(m g
)
h
)
g h
m )
ZnWO
ZB-4
ZB-3
ZB-2
ZB-1
4
29.49
25.44
21.26
20.28
17.68
18.20
7.51
0.29
7.39
7.80
24.20
16.87
3.07
—
2.90
3.67
11.93
9.54
1.69
—
ZnWO
BiOI
4
+ BiOI
Fig. 8 Photocurrent responses of ZnWO
4
, BiOI, ZnWO
4
/BiOI heterostructures,
3.42
and the physical mixture of ZnWO
4
and BiOI under visible light irradiation (l $
a
0
k values are k values normalized by the surface areas.
420 nm) at 0.5 V vs. Ag/AgCl.
3
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Journal of Materials Chemistry A
atoms with two dangling bonds. In the (001)BiOI–(011)ZnWO4 direction of (001)BiOI–(011)ZnWO4, that is, 5.3% with ve bBiOI(001)
interface model, a 5 ꢂ 4 BiOI (001) surface slab is used to match matching to four bZnWO4(010), while along the a direction, the
a 4 ꢂ 2 ZnWO (011) surface slab (Fig. 9A(c)). As shown in mismatch is about 7.4% with four aBiOI(001) matching to three
4
Fig. 9A(d), the optimized lattice compatibility along the a aZnWO4(010), as is shown in Fig. 9B(d). The interface formation
direction is ve aBiOI(001) matching to four aZnWO4(011), with the energies for the (001)BiOI–(011)ZnWO4 interface and the (001)BiOI
–
ꢁ
2
˚
lattice mismatch of (5aBiOI(001) ꢁ 4aZnWO4(011))/aZnWO4(011)
.3%. Similarly, four bBiOI(001) match two bZnWO4(011) along the b ꢁ66.16 meV A , respectively, indicating the stability of the
direction, with the lattice mismatch of 4.4%. As for the systems.
The energy level match is the other important factor to affect
¼
(010)ZnWO4 interface are calculated to be ꢁ69.66 meV A and
ꢁ2
˚
5
(
001)BiOI–(010)ZnWO4 interface, a 4 ꢂ 5 BiOI (001) surface slab is
used to match a 3 ꢂ 4 ZnWO (010) surface slab (Fig. 9B(c)). The the properties of heterostructures, which can be investigated by
4
lattice mismatch along the direction b is the same as that the a examining the work functions of the ZnWO and BiOI phases.
4
Work function F is calculated by aligning the Fermi level rela-
tive to the vacuum energy level. It may change with the surface
conditions (composition, orientation and adsorption, etc.),
which is results from the changing surface electric eld caused
by surface electron distribution. Due to the different surface
4
compositions, the calculated work functions for ZnWO (011)-
(
4 ꢂ 2) and (010)-(3 ꢂ 4) surfaces are 5.02 eV and 4.76 eV,
respectively, as shown in Fig. 10a and c. As for the BiOI (001)
surface, the different orientation results in a very slight differ-
ence in the work function, 6.74 eV for the BiOI (001)-(5 ꢂ
4
) surface and 6.70 eV for the BiOI (001)-(4 ꢂ 5) surface
(Fig. 10b and d).
The valence-band (VB) edge positions of BiOI and ZnWO
4
can be estimated by Mulliken electronegativity theory
EVB ¼ X + 0.5Eg
where X is the absolute electronegativity of the semiconductor,
which is dened as the geometric mean of the electronegativity
of the constituent atoms. EVB is the VB edge potential and E is
g
the band gap of the semiconductor. The conduction band (CB)
position can be obtained by ECB ¼ E ꢁ E . The X values for
VB
g
4
BiOI and ZnWO were determined to be 5.94 and 6.23 eV,
respectively. According to the above equations, the EVB and ECB
values of BiOI are calculated to be 6.88 and 5.00 eV, respectively;
Fig. 9 Model for simulating the interface between BiOI and ZnWO
4
. (A)
(011)-(4 ꢂ 2) surface, (b) BiOI (001)-
5 ꢂ 4) surface, (c) side view of the (001)BiOI–(011)ZnWO4 interface, and (d) sche-
(
(
001)BiOI–(011)ZnWO4 interface: (a) ZnWO
4
matic of the interfacial lattice mismatch. The black lines represent the ZnWO
lattice and the red lines represent the BiOI lattice. (B) (001)BiOI–(010)ZnWO4
interface: (a) ZnWO
4
Fig. 10 The electrostatic potentials for (a) ZnWO
(5 ꢂ 4), (c) ZnWO (010)-(3 ꢂ 4), and (d) BiOI (001)-(4 ꢂ 5) surfaces. The red
dashed lines represent the Fermi level E and the blue dotted lines represent the
vacuum level E . Insets: the surface geometry structure models.
4
(011)-(4 ꢂ 2), (b) BiOI (001)-
4
(010)-(3 ꢂ 4) surface, (b) BiOI (001)-(4 ꢂ 5) surface, (c) side
4
view of the (001)BiOI–(010)ZnWO4 interface, and (d) schematic of the interfacial
F
lattice mismatch.
0
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the EVB and ECB values of ZnWO
.29 eV, respectively.
On the basis of the obtained work function and band edge the valence band of BiOI could be excited up to the conduction
positions of BiOI and ZnWO , a schematic diagram of the band band. Because the CBM potential of BiOI is more active than
4
are calculated to be 8.16 and heterostructure, the CBM of BiOI is higher than ZnWO
4
, as
4
shown in Fig. 11A. Under visible light irradiation, electrons in
4
conguration and the charge separation at the interface of that of ZnWO
ZnWO /BiOI heterostructures is shown in Fig. 11. As shown in migrate from BiOI to the CB of ZnWO
Fig. 11A, BiOI is a typical p-type semiconductor whose Fermi BiOI valence band. Moreover, the built-in electric eld could
level is close to the valence band, while ZnWO is an n-type further facilitate the separation of the electron–hole pairs,
4
, the photogenerated electrons would easily
4
4
, leaving the holes on the
4
semiconductor with the Fermi level close to the conduction causing the electrons generated within BiOI to move across the
band. Because of the larger work function of BiOI, the Fermi interface to the region of ZnWO . It is shown that the hetero-
4
level of BiOI is lower than that of ZnWO . When BiOI contacts junctions at the interface between ZnWO and BiOI can greatly
4
4
ZnWO4 to form a p–n junction, the electrons ow from n- promote the separation of electron–hole pairs, resulting in the
ZnWO
4
to p-BiOI, resulting in a positive section in the n-ZnWO
4
enhanced photocatalytic activities of the heterostructures. The
region next to the junction; holes ow from p-BiOI to n-ZnWO
4
,
formation of ZnWO4(010)/BiOI(001) heterostructure also benets
causing a negative section in the p-BiOI region near the junc- the interface charge transfer for their matched energy levels, as
tion. When the two phases acquire an equalized Fermi level, a shown in Fig. 11B.
built-in electric eld directed from n-ZnWO
4
to p-BiOI is
established at the same time, which can stop the charge diffu-
5
Conclusions
sion between ZnWO and BiOI. Meanwhile, the energy bands of
4
ZnWO shi downward 1.72 eV along with its Fermi level in this
process. Consequently, in the formed ZnWO4(011)/BiOI(001)
4
4
In summary, novel ZnWO /BiOI heterostructures have been
synthesized via a chemical bath approach under mild condi-
tions. The as-prepared ZnWO /BiOI heterostructures show high
4
photocatalytic activities in degradation of MO and photocurrent
response under visible light irradiation. First-principles calcu-
lations were used to investigate the lattice and energy level
match in the ZnWO
surface geometry structures and the work functions of the
ZnWO and BiOI phases. Our work shows that the excellent
4
/BiOI heterostructures by examining the
4
photocatalytic activities of the heterostructure are attributed to
the lattice and energy level match between the two semi-
conductors, which is important for the design and development
of more efficient heterostructured photocatalysts.
Acknowledgements
This work is supported by the National Natural Science Foun-
dation of China (Grant no. 21173131, 51172127 and 91022034),
Excellent Youth Foundation of Shandong Scientic Committee
(Grant no. JQ201015), Youth Scientist (Doctoral) Foundation of
Shandong Province of China (Grant no. BS2009CL038), and the
Independent Innovation Foundation of Shandong University
(Grant no. 2012TS212).
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Journal of Materials Chemistry A
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