Fabrication of ZnIn2S4-g-C3N4 sheet-on-sheet nanocomposites for efficient visible-light photocatalytic H2-evolution and degradation of organic pollutants

Article abstract of DOI:10.1039/c5ra17028a

ZnIn₂S₄-g-C₃N₄ sheet-on-sheet nanocomposites with different g-C₃N₄ contents were synthesized by a facile hydrothermal method and characterized by thermogravimetric analysis (TGA), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), Fourier transform infrared spectroscopy (FT-IR), transmission electron microscopy (TEM), high-resolution transmission electron microcopy (HRTEM), N₂ adsorption-desorption, ultraviolet-visible diffuse reflection spectroscopy (DRS), photoluminescence (PL) spectroscopy and photoelectrochemical (PEC) experiments. The photocatalytic activities of these samples were evaluated by the photocatalytic H₂-production and degradation of organic pollutants (methyl orange and phenol) under visible-light illumination (λ > 420 nm). The results showed that the ZnIn₂S₄-g-C₃N₄ composite photocatalysts displayed higher photocatalytic activity than the pristine g-C₃N₄ and ZnIn₂S₄ both for H₂-evolution and degradation of pollutants. The optimal g-C₃N₄ content was determined to be 40 wt%, and the corresponding H₂-production rate was 953.5 μmol h^-1 g^-1, which was about 1.91 times higher than that of pure ZnIn₂S₄. The enhanced photocatalytic activity of ZnIn₂S₄-g-C₃N₄ composites should be attributed to the well-matched band structure and intimate contact interfaces between ZnIn₂S₄ and g-C₃N₄, which led to the effective transfer and separation of the photogenerated charge carriers. Moreover, the ZnIn₂S₄-g-C₃N₄ composites showed excellent stability during the photocatalytic reactions under visible light. A possible mechanism of the enhanced photocatalytic activity of ZnIn₂S₄-g-C₃N₄ composites was proposed and supported by the PL and PEC results.

Full text of DOI:10.1039/c5ra17028a

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PAPER
Fabrication of ZnIn S –g-C N sheet-on-sheet
2
4
3 4
nanocomposites for efficient visible-light
Cite this: RSC Adv., 2015, 5, 97951
photocatalytic H -evolution and degradation of
2
organic pollutants
Hong Liu,* Zhitong Jin, Zhengzheng Xu, Zhe Zhang and Dan Ao
ZnIn
a facile hydrothermal method and characterized by thermogravimetric analysis (TGA), X-ray diffraction
XRD), X-ray photoelectron spectroscopy (XPS), Fourier transform infrared spectroscopy (FT-IR),
transmission electron microscopy (TEM), high-resolution transmission electron microcopy (HRTEM), N
2 4 3 4 3 4
S –g-C N sheet-on-sheet nanocomposites with different g-C N contents were synthesized by
(
2
adsorption–desorption, ultraviolet-visible diffuse reflection spectroscopy (DRS), photoluminescence (PL)
spectroscopy and photoelectrochemical (PEC) experiments. The photocatalytic activities of these samples
were evaluated by the photocatalytic H
and phenol) under visible-light illumination (l > 420 nm). The results showed that the ZnIn
2
composite photocatalysts displayed higher photocatalytic activity than the pristine g-C and ZnIn S
2
-production and degradation of organic pollutants (methyl orange
S –g-C N
2 4 3 4
N
3 4
4
both for H
0 wt%, and the corresponding H
higher than that of pure ZnIn
should be attributed to the well-matched band structure and intimate contact interfaces between ZnIn
and g-C , which led to the effective transfer and separation of the photogenerated charge carriers.
Moreover, the ZnIn –g-C composites showed excellent stability during the photocatalytic reactions
2
-evolution and degradation of pollutants. The optimal g-C
3
N
1
4
content was determined to be
ꢀ
ꢀ1
4
2
-production rate was 953.5 mmol h
g
, which was about 1.91 times
2 4 2 4 3 4
S . The enhanced photocatalytic activity of ZnIn S –g-C N composites
2 4
S
Received 23rd August 2015
Accepted 6th November 2015
3 4
N
S
2 4
3 4
N
DOI: 10.1039/c5ra17028a
under visible light. A possible mechanism of the enhanced photocatalytic activity of ZnIn
2 4 3 4
S –g-C N
www.rsc.org/advances
composites was proposed and supported by the PL and PEC results.
6,7,9–12
and photocatalytic organic synthesis have been reported.
However, the photocatalytic efficiency of pure g-C is limited
Photocatalysis, as a green technology, is attracting considerable by the high recombination rate of photogenerated electron–
1
. Introduction
3 4
N
6,9
interests due to its great potential in solving global environ- hole pairs and low surface area. To enhance its photocatalytic
1–3
mental and energy problems. TiO is the most widely studied performance, a number of methods have been exploited,
2
photocatalyst due to its unique characteristics such as high including preparing novel nanostructures (mesoporous struc-
1
3,14
7,11
photocatalytic activity, low cost, stability and nontoxicity. ture
However, TiO
only works in the ultraviolet region due to its elements,
and nanosheets ), doping with metal or nonmetal
15–18
19
deposition of noble metal, protonating by strong
2
20
21–23
wide band gap (ꢁ3.2 eV), which restricts its practical applica- acids and construction of heterojunction composite.
4,5
tions. Therefore, exploring new photocatalytic materials with Among them, designing heterojunction composite is regarded
high photocatalytic activity under visible-light (ꢁ43% of the as one of the best methods to promote the separation of pho-
solar spectrum) is indispensable in this eld.
togenerated electrons and holes, which can effectively improve
As a typical metal-free polymeric semiconductor, graphitic the photocatalytic efficiency.
carbon nitride (g-C N ) is a very promising material for solar
2 4
ZnIn , as a typical member of the AB X family of semi-
3
4
2
S
4
energy utilization because of its high thermal and chemical conductors, is a good candidate for a photocatalyst under the
stability, nontoxicity, easy to prepare and desirable band gap of visible-light irradiation due to its suitable bandgap (2.34–
6
–12
24–26
2.7 eV.
The potential applications of g-C
reduction, degradation of pollutants, cubic and hexagonal lattices. Previous studies revealed that
both polymorphs of ZnIn are active for photocatalytic
hydrogen generation and pollutants degradation under visible-
3
N
4
in photocatalytic 2.48 eV).
ZnIn S exhibits two distinct polymorphs based on
2 4
hydrogen evolution, CO
2
2 4
S
27–30
Department of Chemical Engineering, School of Environmental and Chemical light irradiations and show considerable chemical stability.
Engineering, Shanghai University, 99 Shangda Road, Shanghai 200444, P. R. China.
2 4
However, the photocatalytic efficiency over pure ZnIn S is low
E-mail: liuhong@shu.edu.cn; Fax: +86-21-66137725; Tel: +86-21-66137487
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due to the short lifetimes of the photo-generated electron–hole stirring at room temperature, and the pH of the solution was
pairs. To enhance the photocatalytic performance of ZnIn S , adjusted to 2.5 by hydrochloric acid. Meanwhile, an appropriate
2
4
numerous effects have been made. For example, by controlling amount of as-prepared g-C N nanosheets was dispersed in
3
4
2
6,31–34
35,36
morphologies of ZnIn S ,
doping with metals,
and 30 mL deionized water by sonication for 30 min. Then, the
incorporation of multiwalled carbon nanotubes, reduced obtained g-C solution was added to the above solution
gradually under magnetic stirring. Aer being stirred for 3 h,
2
4
37
3 4
N
2
8,30
38
graphene oxide
or other semiconductors (such as CdS,
(ref. 40) and TiO
2
9,39
MoS
2
,
CdIn
2
S
4
2
(ref. 41)), the photocatalytic the suspension was transferred to a 100 mL Teon-lined auto-
ꢂ
performance of ZnIn
2
S
4
have been enhanced to a certain degree. clave and maintained at 80 C for 6 h. The precipitate was
However, it should be noted that the photocatalytic activity of collected by centrifugation, washed with deionized water and
ꢂ
ZnIn S still requires further improvement.
absolute ethanol for several times and dried at 60 C under
2
4
Similar to g-C N , hexagonal ZnIn S has an intrinsic layered vacuum. The as-synthesized ZnIn S –g-C N sample with
3
4
2
4
2
4
3 4
structure based on a stacking of packets of S–Zn–S–In–S–In–S 20 wt%, 30 wt%, 40 wt% and 50 wt% g-C N was labeled as
3
4
2
9
layers. It is expected that it is facile to grow layer ZnIn
g-C surface due to their analogous layered structures which comparison, pure ZnIn
can minimize the lattice mismatch. Besides this, the position of hydrothermal method without the addition of g-C
2
S
4
on ZISCN20, ZISCN30, ZISCN40 and ZISCN50, respectively. For
3
N
4
2 4
S
was prepared using the same
3 4
N
42
the conduction band (CB) of g-C
negative than that of ZnIn
provides possibility for a directional transfer of the photo-
generated electrons from g-C N to ZnIn S . Moreover, due to
3
N
4
(ꢀ1.12 eV vs. NHE ) is more nanosheets.
39
S
2 4
(ꢀ0.68 eV vs. NHE ) and thus
2.2 Characterization
3
4
2 4
The crystal structure of the samples was identied by an X-ray
diffractometer (XRD, D-MAX-2550, l ¼ 0.15418 nm) using Cu
Ka radiation. The morphologies and microstructures of the
photocatalysts were evaluated by transmission electron
microscopy (TEM FEI TECNAI G20) and high-resolution trans-
mission electron microscopy (HRTEM FEI TECNAI G2F20). The
surface elemental component and the chemical state of the
sample were analyzed by X-ray photoelectron spectroscopy (XPS,
ESCALAB 250Xi). FT-IR spectrum was performed on a FT-IR
spectrophotometer (AVATAR370) using KBr disks at room
temperature. Ultraviolet-visible (UV-vis) diffuse reectance
spectra (DRS) of the samples were obtained on an UV-vis spec-
3
9
the valence band (VB) of ZnIn S (+1.62 eV vs. NHE ) are more
2
4
42
positive than that of g-C
of ZnIn will migrate to that of g-C
of charge carriers in composites. Therefore, it can be supposed
that combining ZnIn with g-C may result in an improved
photocatalytic efficiency compared with individual g-C and
ZnIn S . However, to the best of our knowledge, no work related
N
3 4
(+1.57 eV vs. NHE ), the holes on VB
2
S
4
3 4
N
, achieving the separation
S
2 4
3 4
N
3 4
N
2
4
to ZnIn S –g-C N photocatalyst has been reported so far.
In this study, ZnIn S –g-C N sheet-on-sheet nano-
composites with different g-C
2
4
3 4
2
4
3 4
3
N
4
contents were synthesized by
3 4
–g-C N
a facile hydrothermal method. The as-prepared ZnIn
2
S
4
composites showed a signicantly enhanced photocatalytic
performance for hydrogen evolution and degradation of organic
pollutants under visible light irradiation. A possible enhance-
ment mechanism for the improved photocatalytic activity in the
ZnIn S –g-C N composites was also proposed.
trophotometer (Hitachi U-3010) using BaSO as a reference. The
4
photoluminescence (PL) spectra were measured using a uo-
rescence spectrophotometer (Hitachi RF-5301) at room
temperature. The Brunauer–Emmett–Teller (BET) specic
surface areas of the samples were analyzed by nitrogen
adsorption using a Micromeritics ASAP 2020 nitrogen adsorp-
tion apparatus. Thermogravimetric analysis (TGA) was per-
formed on a Netzsch 409 PC thermal analyzer under nitrogen
2
4
3 4
2
2
. Experimental
.1 Materials preparation
ꢂ
ꢀ1
atmosphere with a heating rate of 10 C min
.
All the chemicals were of analytical grade from the Sinopharm
Chemical Reagent Co., Ltd.
2.3 Photocatalytic measurement
3 4 3 4
Preparation of g-C N nanosheets. The g-C N nanosheets
were prepared by a thermal oxidation etching method. Briey, The photocatalytic hydrogen-production experiment was con-
dicyandiamide was put into an alumina crucible with a cover. ducted in a closed gas circulation and evacuation system tted
The crucible was then placed in a muffle furnace and heated to with
a
top window Pyrex cell.
A 300 W xenon lamp
ꢂ
ꢂ
ꢀ1
550 C with a heating rate of 2 C min , which has been held (PLS-SXE300C, Beijing Perfectlight Co. Ltd., China) coupled
for 4 h at this temperature. When the temperature of alumina with a UV cut-off lter (l > 420 nm) was used to provide the
crucible was cooled down aer reaction, the obtained bulk visible light. In each run, 0.05 g photocatalyst was dispersed in
g-C
g-C
3
3
N
N
4
4
products were grinded into powders. Next, 400 mg bulk a 100 mL aqueous solution containing 10 mL trolamine, which
obtained as above was placed in an open ceramic served as a sacricial agent. Before the irradiation with visible
ꢂ
ꢂ
ꢀ1
container and heated to 500 C with a ramp rate of 2 C min
and maintained at this temperature for 2 h. A light yellow the O
powder of g-C nanosheets was nally obtained. temperature with cooling water. The produced H
2
,
light, the suspension was degassed with N
in the system. The reaction cell was kept at room
was detected
2
for 1 h to drive away
2
3
N
4
Preparation of ZnIn S –g-C N composites. The ZnIn S –g- using an online gas chromatography (GC7900, N carrier, 5A
2
4
3
4
2
4
2
C N composites were prepared by a hydrothermal method. In molecular sieve column, TCD detector).
3
4
detail, 0.068 g ZnCl
2
, 0.293 g InCl
3
$4H
2
O and 0.225 g thio-
Photocatalytic degradation activity of the samples was esti-
acetamide (TAA) were dissolved in 40 mL deionized water by mated by monitoring the degradation of methyl orange (MO)
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and phenol in an aqueous solution under visible light irradia-
tion. The reaction was performed in a photochemical reactor
(
BL-GHX-V, Shanghai Bilon Instruments Co., Ltd, China)
equipped with a 500 W Xe lamp combined with a 420 nm cut-off
lter as a light source. All experiments were conducted at room
temperature in air. In a typical photocatalytic experiment,
.02 g photocatalyst was added into 50 mL pollutant solutions

0
ꢀ
1
(
10 mg L ) in a reaction cell with a Pyrex jacket. Prior to irra-
diation, the suspension was magnetically stirred in the dark for
0 min to reach an adsorption–desorption equilibrium. Then,
3
the suspension was exposed to visible light irradiation under
magnetic stirring. At given time intervals, about 5 mL suspen-
sions were collected and centrifuged (12 000 rpm, 6 min) to
remove the photocatalyst particles. Then, the pollutant
concentration of the obtained solution was analyzed by a UV-vis
spectrophotometer (Agilent 8453) by checking the absorbance
at 506 nm and 270 nm for MO and phenol, respectively. The
total organic carbon (TOC) assays were carried out using a TOC
Fig. 1 TGA curves of g-C
3 4 2 4 2 4
N nanosheets, ZnIn S and ZnIn S –g-
3 4
C N
composites.
3 4
Table 1 BET surface area and real g-C N concentrations of the as-
analyzer (Multi N/C 2100). To probe the oxidizing species in the prepared samples
photocatalytic degradation, the disodium ethylenediamine tet-
raacetate (EDTA-2Na), isopropyl alcohol (IPA), or benzoquinone
2
ꢀ1
Samples
g-C
3
N
4
content (wt%)
S
BET (m g )
(
BQ), was added in the solution. The concentrations of the
ZnIn
2
S
4
0
66.9
63.9
42.3
35.2
29.8
10.4
quenching chemicals were 2 mM.
ZISCN20
ZISCN30
ZISCN40
ZISCN50
20.5
27.4
38.6
52.9
100
2
.4 Photoelectrochemical measurements
3 4
g-C N nanosheet
Photocurrent measurements were performed on a CHI 660E
electrochemical workstation (Chenhua Instrument, Shanghai,
China) in a conventional three electrode conguration with a Pt
foil as the counter electrode and a Ag–AgCl (saturated KCl) as
the reference electrode. A 300 W Xe arc lamp severed as a light
source. A 0.5 M Na SO aqueous solution was used as the
electrolyte. The working electrodes were prepared as follows: 10
mg of the prepared photocatalyst was ground with 20 mL of
a
3 4
g-C N content can be easily determined from the correspond-
ing weight loss in the thermogram. As shown in Table 1, the real
g-C N contents are nearly consistent with the theoretical
2
4
3
4
values.
Fig. 2 shows the XRD patterns of g-C
and ZnIn S –g-C N composites with different g-C N contents.
The XRD pattern of the ZnIn S can be assigned to a hexagonal
phase of ZnIn S (JCPDS no. 03-065-2023) and the characteristic
3 4 2 4
N nanosheets, ZnIn S
poly(3,4-ethylenedioxythiophene)/poly(styrenesulfonate)
2
4
3
4
3 4
(PEDO/TPSS, 1.3–1.7%) aqueous solution and 100 mL of
2
4
distilled water to make a slurry. The slurry was then spread on
2
ꢂ
4
a 1.5 cm ꢃ 1.0 cm indium-tin oxide (ITO) glass substrate with an
ꢂ
ꢂ
peaks at 21.4 , 27.8 and 47.3 are attributed to (006), (102) and
2
active area of about 0.3 cm by the doctor-blade method, using
29
(
1
110) crystal planes of ZnIn
3.1 and 27.7 are observed in the XRD pattern of g-C N
2
S
4
.
Two distinct peaks around
adhesive tape as the space. The lm was dried in air and
annealed at 150 C for 30 min in a owing N atmosphere. The
ꢂ
ꢂ
3 4
ꢂ
2
photoresponses of the samples as light on and off were
measured at 0.0 V. Electrochemical impedance spectroscopy
(
EIS) measurements were determined at an AC voltage magni-
6
tude of 5 mV with the frequency range of 10 to 0.01 Hz with the
initial potential (0 V) in 0.01 M Na SO
2
4
.
3. Results and discussion
3.1 Structure and properties characterization
The contents of ZnIn –g-C composites were determined by
2
S
4
3 4
N
thermogravimetric analysis (TGA) under nitrogen atmosphere
ꢂ
from room temperature to 800 C with a heating rate of
ꢂ
0 C min . The decomposition of the g-C N nanosheets start
3 4
ꢀ1
1
ꢂ
ꢂ
at 560 C and is complete at ꢁ760 C which is attributed to the
burning of g-C as shown in Fig. 1. In all ZnIn –g-C
3
N
4
2
S
4
3
N
4
Fig. 2 XRD patterns of g-C
3 4
C N composites.
3 4 2 4 2 4
N nanosheets, ZnIn S and ZnIn S –g-
composites, this weight loss region can also be observed and the
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3 4 2 4
Fig. 3 TEM images of (a) g-C N nanosheets, (b and c) ZnIn S and (d) ZISCN40, (e) HRTEM image and (f) EDS spectrum of ZISCN40.
nanosheets, corresponding to the (100) and (002) diffraction microspheres are unfolded and distributed uniformly as
6
–12
planes, respectively.
broadened diffraction peaks for ZnIn2S4 have been observed sheet-on-sheet structure (Fig. 3d). This phenomenon may be
due to the overlapping of the tow closely peaks for ZnIn ascribed to the important effect of surface groups of g-C in
the crystal growth and self-assembly process of ZnIn mate-
nano- rials. The 2D sheet-on-sheet composite structure offers the
and ZISCN40 were characterized by TEM and maximum contact area between two components (ZnIn and
HRTEM. It can be observed from Fig. 3a that pure g-C shows g-C nanosheets), which will facilitate the transfer of photo-
two-dimensional nanosheet structure with wrinkled generated carriers and improve the photocatalytic performance.
surfaces. The pristine ZnIn S sample (Fig. 3b) exhibits micro- The HRTEM image of ZISCN40 (Fig. 3e) reveals that a lattice
For all ZnIn S –g-C N composites, only nanosheets on the surface of g-C N nanosheets to form a 2D
2 4 3 4 3 4
2
S
4
3 4
N
ꢂ
ꢂ
(
27.8 ) and g-C
The morphologies and microstructures of g-C
sheets, ZnIn
3
N
4
(27.7 ).
2 4
S
3 4
N
2
S
4
2 4
S
3
N
4
3 4
N
a
a
2
4
sphere morphology with a diameter of about 1.5 mm. Further spacing of ZnIn S crystallites is ꢁ0.324 nm, corresponding to
2
4
30
observation show that individual ZnIn
2 4 2 4
S microsphere are built the (102) plane of ZnIn S . The EDS spectrum (Fig. 3f) of the
by numerous nanosheets with a thickness of about 10 nm composites clearly conrms the presence of Zn, In, S, C and N
(Fig. 3c). Aer hybridization with g-C
3
N
4
,
the ZnIn
2
S
4
elements (other peaks originate from the substrate).
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43
stretching mode of the N–H bond. And the peaks located at
ꢀ
1
about 1240, 1320, 1410, 1460, 1575 and 1640 cm are ascribed
to aromatic C–N stretching vibration modes and C]N stretch-
4
4,45
ꢀ1
ing vibration modes.
The peak at 812 cm is originated
44
from characteristic breathing vibration of triazine ring. For
ꢀ
1
ꢀ1
ZnIn
2 4
S , the broad band centered at 3430 cm and 1620 cm
are contributed to the surface absorbed water molecules and
4
6
hydroxyl groups. In the case of the ZnIn
2
S
4
–g-C
3
N
4
composite,
are also
the IR bands characteristic of g-C and ZnIn
3
N
4
2
S
4
identied, conrming that the composite photocatalyst is
composed of g-C N and ZnIn S .
3
4
2 4
The X-ray photoelectron spectra (XPS) were further employed
to elucidate the surface elemental composition of g-C
ZnIn and ZnIn –g-C and to further study the interac-
tion of ZnIn with the g-C support. The XPS survey spec-
3 4
N ,
3 4 2 4
Fig. 4 FT-IR spectra of g-C N nanosheets, ZnIn S and ZISCN40.
2
S
4
2
S
4
3 4
N
2
S
4
3 4
N
trum of ZISCN40 (Fig. 5a) conrms that the composite is mainly
composed of C, N, Zn, In and S. The C 1s spectrum of pure
Fig. 4 shows the FT-IR spectra of g-C
and the ZnIn –g-C (40%) sample. In the FT-IR spectra of
g-C , the broad peak at 3250 cm can be attributed to
3 4 2 4
N nanosheets, ZnIn S
2
S
4
3 4
N
g-C N (Fig. 5b) can be deconvoluted into three peaks at 284.7,
ꢀ
1
3 4
3
N
4
Fig. 5 XPS spectra of g-C
3
N
4
nanosheets, ZnIn
2
S
4
and ZISCN40: (a) survey spectrum of ZISCN40, (b) C 1s, (c) N 1s, (d) Zn 2p, (e) In 3d, (f) S 2p.
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Fig. 6 (a) UV-vis spectra and (b) band gap energies of as-synthesized
samples.
Fig. 7 (a) Plots of photocatalytic H
time and (b) comparison of H
ZnIn and ZnIn –g-C
irradiation.
2
evolution amount vs. irradiation
2
evolution rate for g-C
3 4
N nanosheets,
S
2 4
2
S
4
3 4
N composites under visible-light
2
86.4 and 288.3 eV. The C peak at 288.3 eV is identied as
2
sp -bonded carbon (N–C]N), the C peak at 284.7 eV is cor-
chemical bonds between g-C N and ZnIn S , rather than
responded to graphitic carbon which was usually observed on
3
4
2 4
4
4,45
a simple physical mixture.
The Brunauer–Emmett–Teller specic surface areas (SBET) of
pure g-C nanosheets, ZnIn , and the as-prepared ZnIn
g-C composites were measured by nitrogen adsorption and
summarized in Table 1. As listed in Table 1, the BET specic
the XPS characterization for carbon nitrides.
The weak
3
peak at 286.4 eV can be assigned to sp -coordinated carbon
4
4,45
N
4
2
S
4
2 4
S –
bonds from the defects on g-C
spectrum of pure g-C (Fig. 5c) can be deconvoluted into
3
N
4
surfaces.
The N 1s
3
3 4
N
3 4
N
three peaks at 398.1, 399.3 and 400.7 eV, respectively, corre-
2
ꢀ1
surface area of the pristine ZnIn
2 4
S is about 66.9 m g . For the
sponding to the nitrogen atoms in the aromatic rings (C]N–
4
7,48
ZnIn S –g-C N composites, the surface area decreases with the
C), tertiary nitrogen (N–(C)
3
) and C–N–H.
In comparison
2
4
3 4
enhancement of g-C N mass ratio because the surface area of
with pure g-C N , the negative shi of the C 1s peaks and
3
4
3
4
2
ꢀ1
g-C
The UV-vis diffuse reectance spectra of the ZnIn
composites with different weight ratios of g-C , as well as
are compiled in Fig. 6a. Pure
sample shows absorption wavelengths from the UV to
, the absorption edge
is about 600 nm, exhibiting the broad absorption region of
visible-light. The ZnIn S –g-C N composites have absorption
3 4
N nanosheets is only 10.4 m g .
positive shi of N 1s peaks are observed for the ZISCN40
sample, indicating the g-C N structure has changed aer
2 4 3 4
S –g-C N
3
4
3 4
N
interaction with ZnIn
binding energy peaks located at 1044.5 eV (Zn 2p1/2), 1021.5 eV
Zn 2p3/2), 452.4 eV (In 3d3/2), 444.9 eV (In 3d5/2), 161.5 eV (S
3/2) and 162.7 eV (S 2p1/2) can be observed from the high
resolution spectra of the pristine ZnIn in Fig. 5d–f,
respectively. The binding energy values are very close to the
2 4
S to form the heterojunction. The main
g-C
g-C
3
3
N
N
4
4
2 4
nanosheets and ZnIn S
(
2 4
the visible range up to 460 nm. For ZnIn S
2
p
2 4
S
2
4
3 4
2
9,30,38,39
edges to longer wavelengths in comparison with the pure
g-C . The observations are attributed to the interaction
between g-C The inter-
reported ones,
In and S are +2, +3 and ꢀ2, respectively. Aer modication
with g-C , slight shis toward lower binding energies can be
indicating that the valence states of Zn,
3 4
N
48,49
N
4
and ZnIn
2
S
4
in the composites.
3 4
N
3
action probably plays a signicant role in improving the sepa-
ration of the photogenerated electron–hole pairs to enhance the
observed from the high resolution spectra of ZISCN40 in
Fig. 5d–f, implying that the binding energies of the core level
electrons of those metal and sulde ions are affected due to
the possible chemical bonding actions among the composite
components. The XPS results clearly indicate the presence of
photocatalytic activity. The band gap energy of a semiconductor
n/2
can be calculated by the following equation: ahn ¼ A(hn ꢀ E )
,
g
in which a, h, n, E , and A are the absorption coefficient, Planck
g
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Fig. 9 TOC removal during (a) MO and (b) phenol degradation over
ZISCN40 under visible light illumination.
3 4 2 4
Fig. 8 Photocatalytic activities of g-C N nanosheets, ZnIn S and
2 4 3 4
ZnIn S –g-C N composites on the degradation of the target organic
pollutions under visible-light irradiation: (a) MO, (b) phenol.
photoinduced holes. As shown in Fig. 7, the photocatalytic
constant, the light frequency, the band gap, and a constant, H -production rate is negligible when g-C N alone is used as
2
3 4
respectively. The value of n depends on the type of optical photocatalyst due to fast recombination of photocatalytic elec-
transition of the semiconductor (n ¼ 1 for direct transition and tron–hole pairs, indicating that pure g-C N nanosheets is not
3
4
n ¼ 4 for indirect transition). For g-C
3 4 2 4
N and ZnIn S , the n active for photocatalytic hydrogen production. Pristine ZnIn2S4
50,51
value of 1 is used.
corresponding band gap values of ZnIn
ZISCN40, ZISCN50 and g-C
.34, 2.35, 2.38, 2.40 and 2.70 eV, respectively. The valence band enhanced. The rate of H2 evolution over ZnIn S –g-C N
Therefore, as can be seen from Fig. 6b, the without g-C N shows a low photocatalytic activity with the H
3 4
2
ꢀ1
ꢀ1
2
S
4
, ZISCN20, ZISCN30, evolution rate of 500.0 mmol h
g
. Aer introduction of
3 4
3 4 2 2 4
N nanosheets are found to be 2.32, g-C N , the H -production activity of ZnIn S is signicant
2
2
4
3
4
edge potential and the conduction band edge potential of composites increases with increasing g-C N content, achieving
3
4
ꢀ
1
ꢀ1
a semiconductor material can be determined by using the a maximum of 953.5 mmol h
g
3 4
at the g-C N content of
e
following equation: EVB ¼ X ꢀ E + 0.5E
g
, where EVB represents 40 wt%, which is about 1.91 times higher than that of pure
valence band edge potential, X is the electronegativity of the ZnIn S . A further increase in the content of g-C N leads to
2
4
3 4
semiconductor estimated by the geometric mean of the elec- a reduction in the photocatalytic activity. This decrease is
e
tronegativity of the constituent atoms. E is the energy of free probably due to the fact that excessive amount of g-C N may
3
4
45,52
electrons on the hydrogen scale (ꢁ4.5 eV), E
g
is the band gap cover the active sites on the surface of ZnIn S .
Therefore, an
2
4
energy of the semiconductor. The CB edge potential ECB can be appropriate g-C N loading amount is crucial to achieve the
3
4
determined by ECB ¼ E ꢀ E . The X value of pure ZnIn S and optimized photocatalytic activity of ZnIn S photocatalyst.
VB
g
2
4
2 4
g-C N is about 4.86 and 4.63 eV. The E of bare ZnIn S and
The photocatalytic degradation performances of as-prepared
3
4
VB
2 4
g-C
3
N
4
nanosheets can be assigned to be +1.52 and +1.48 eV, samples were also evaluated by using MO and phenol as the
respectively, and the corresponding ECB of ZnIn
can be estimated to be ꢀ0.80 and ꢀ1.22 eV, respectively.
2
S
4
and g-C
3
N
4
model pollutant under visible light irradiation (Fig. 8). The
blank experiments (without any photocatalyst) show that direct
photolysis of MO and phenol under visible light can be
3 4
neglected. Fig. 8a shows that the pure g-C N nanosheets has
a poor activity, in which only 5.7% of MO is decomposed aer
3.2 Photocatalytic activity
irradiation for 120 min. Moreover, only 72.6% of MO can be
Photocatalytic hydrogen evolution activities of as-prepared
samples were evaluated under visible light (l > 420 nm) irradi-
ation using trolamine as the sacricial reagent to quench
removed by ZnIn
coupling with g-C
2
S
4
for the same irradiation time. Aer
2 4
, the photodegradation activity of ZnIn S
3
N
4
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Fig. 10 Photocatalytic degradation of MO (a) and phenol (b) with the
addition of hole and radical scavenger.
Fig. 11 Cycling runs for (a) the photocatalytic hydrogen evolution and
(b) MO degradation in the presence of ZISCN40 under visible light
illumination.
is remarkably enhanced. It can be observed that the optimum
g-C N mass ratio is 40 wt%. The degradation ratio of MO is up
3 4
to 95.3% in the presence of ZISCN40 aer 120 min irradiation.
Fig. 8b displays the photodegradation activity of phenol over the
pollutants. It indicates that both the holes and hydroxyl radicals
are the active species in the photocatalytic degradation of MO
and phenol, while cO2 can be negligible in the reaction.
ꢀ
To reveal the stability of as-synthesized photocatalysts, the
cycling hydrogen evolution and MO degradation experiments
were performed over ZISCN40 under the same conditions. As
shown in Fig. 11, there is no signicant decrease in
3 4 2 4
as-prepared samples. Both pure g-C N and ZnIn S samples
show low activities, in which only 21.4% and 52.5% of phenol
are degraded aer reacting for 240 min, respectively. In
contrast, all the ZnIn S –g-C N samples display much
2
4
3 4
H
2
-production rate and photodegradation rate aer four
consecutive cycles, indicating the good stability of the ZnIn
g-C N composite during photocatalytic reactions.
improved activities. The trend of activity change of these
ZnIn –g-C samples for the photodegradation of phenol is
2 4
S –
2
S
4
3 4
N
the same as that of MO degradation. The degradation ratio of
phenol on the optimal photocatalyst (ZISCN40) reaches to
3 4
7
2
2.3% aer 240 min irradiation. In addition, aer 120 min or
40 min irradiation the mineralization rate of MO and phenol
3
.3 Mechanism for the enhanced photocatalytic activity of
ZnIn S –g-C N composites
over ZISCN40 reaches to 70.0% and 58.0%, respectively (Fig. 9).
2
4
3 4
To understand the photocatalytic mechanism, the main It has been widely accepted that the separation efficiency of
active oxidants in the photocatalytic degradation process were electrons and holes played a crucial role in the photocatalytic
identied. It is well-known that the oxidants generated in the reaction. Photoluminescence (PL) analysis was applied to reveal
photocatalytic process can be measured through trapping by the efficiency of charge carrier trapping and separation of the
IPA (a scavenger of hydroxyl radicals), EDTA-2Na (a scavenger of photoinduced electrons and holes in semiconductors. Fig. 12a
ꢀ
holes) or BQ (a scavenger of cO
2
). It can be clearly seen from shows the PL spectra of pure g-C N nanosheets, ZnIn S and
3
4
2 4
Fig. 10 that the addition of EDTA-2Na greatly reduces the pho- ZISCN40 sample with an excitation wavelength of 385 nm. It can
todegradation rate of MO and phenol in the ZISCN40 suspen- be seen that the pure g-C N has a strong emission peak at
3
4
sion, whereas the addition of BQ has little effect on the around 460 nm, which corresponds to band gap recombination
photodegradation rate of MO and phenol. In addition, the of electron–hole pairs. The PL intensity of bare ZnIn S is weak
2
4
addition of IPA as hydroxyl radicals' scavenger causes a minor probably because the amount of photogenerated electron–hole
change in the photocatalytic degradation of the above pairs is less under the same irradiation conditions.53,54 Similar
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Scheme 1 Mechanism for the enhanced photocatalytic activity of
ZnIn S –g-C N composites.
2 4 3 4
visible light irradiation, which is greatly larger than that for bare
g-C and ZnIn . It suggests the remarkably enhanced
3
N
4
2 4
S
carrier separation ratio over ZISCN40. To further conrm the
above results, electrochemical impedance spectroscopy (EIS),
a useful measurement to characterize charge carrier trans-
portation, was also performed. As can be seen from Fig. 12c, the
diameter of the Nyquist semicircle for the ZISCN40 sample is
smaller than that of ZnIn
the hybrid composite has a lower resistance than that of ZnIn
and g-C The PL and photoelectrochemical results
conrmed the superior charge transfer and recombination
inhibition in the ZnIn –g-C composite photocatalyst,
2 4 3 4
S and g-C N , which indicates that
2 4
S
3 4
N .
2
S
4
3 4
N
which is responsible for the enhanced photocatalytic activity.
The high separation efficiency of photo-generated carriers
should be ascribed to the suitable band potentials of g-C N4
3
nanosheets and ZnIn
and transportation of electron–hole pairs at the interface of
ZnIn –g-C composite under visible light is proposed and
illustrated in Scheme 1. Under the visible light illumination,
both g-C and ZnIn can be excited and produce photo-
2 4
S . The possible mechanism of separation
2
S
4
3 4
N
3
N
4
2 4
S
generated electron–hole pairs. Since the CB edge potential of
g-C N nanosheets (ꢁꢀ1.22 eV vs. NHE) is more negative than
3
4
that of ZnIn S (ꢁꢀ0.80 eV vs. NHE), the photoinduced elec-
2
4
trons on the CB of g-C
Fig. 12 (a) PL spectra, (b) transient photocurrent responses and (c) ZnIn . While the corresponding VB edge potential of ZnIn
electrochemical impedance spectroscopy of g-C nanosheets,
3 4
N can directly transfer to the CB of
2
S
4
2
S
3
N
4
4
N
3 4
(
ꢁ+1.52 eV vs. NHE) is more positive than that of g-C
ZnIn and ZISCN40 under visible light irradiation.
2 4
S
nanosheets (ꢁ+1.48 eV vs. NHE), the photogenerated holes on
the VB of ZnIn can immigrate to the VB of g-C . Thus, the
2
S
4
3 4
N
photogenerated electrons and holes move in opposite direc-
tions, which can effectively reduce the recombination proba-
bility and enhance the charge separation efficiency, leading to
a remarkable enhancement of the photocatalytic activity.
5
4
results were also observed on the BiOBr/g-C
3
N
4
,
Ag
ZnS/g-C
(ref. 59) hetero-
junction systems by other groups. The ZnIn –g-C
composite has a lower peak than that of g-C N , which indicates
3 4
PO /
N ,
3 4
55
56
57
51
g-C
BiVO
3
N
4
,
Fe
2
O
3
/g-C
3
N
4
,
ZnFe
2
O
4
/g-C
/g-C
3
N
N
3 4
4
,
4
/g-C
3
N
4
(ref. 58) and MnFe
2
O
4
2
S
4
3 4
N
3
4
4. Conclusions
that the composite has a lower electrons and holes recombi-
nation rate. Fig. 12b shows the photocurrent transient response
for the electrodes of bare g-C N nanosheets, ZnIn S and
3 4 2 4
ZISCN40 under visible light irradiation. As it can be seen, with
the light switched-on and -off cycles, the ZISCN40 sample
exhibits the highest photocurrent transient response under
2 4 3 4
Novel ZnIn S –g-C N sheet-on-sheet nanocomposites were
prepared by a facile hydrothermal method. Under visible light
irradiation, the obtained ZnIn S –g-C N composites exhibited
2
4
3 4
a signicantly enhanced photocatalytic activity for hydrogen
evolution and degradation of pollutants than pure ZnIn and
2 4
S
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g-C
3
N
4
. The optimal content of g-C
3
N
4
was found to be 40 wt%. 18 G. Liu, P. Niu, C. H. Sun, S. C. Smith, Z. G. Chen, G. Q. Lu and
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