Synthesis of In2S3-CNT nanocomposites for selective reduction under visible light

Article abstract of DOI:10.1039/c3ta14370h

In₂S₃-carbon nanotube (In₂S ₃-CNT) nanocomposites have been prepared via a facile refluxing wet chemistry process. The as-synthesized In₂S₃-CNT nanocomposites can be used as selective and a

Full text of DOI:10.1039/c3ta14370h

Journal of
Materials Chemistry A
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Synthesis of In2S3–CNT nanocomposites for
selective reduction under visible light
Cite this: J. Mater. Chem. A, 2014, 2,
1710
ab
Min-Quan Yang,ab Bo Wengab and Yi-Jun Xu
*
In2S3–carbon nanotube (In2S3–CNT) nanocomposites have been prepared via a facile refluxing wet chemistry
process. The as-synthesized In2S3–CNT nanocomposites can be used as selective and active visible-light-
driven photocatalysts toward hydrogenation of nitroaromatics to amines in water. Photoirradiation
(l > 420 nm) of In2S3–CNT photocatalysts suspended in water containing nitroaromatics produces the
corresponding amines with high yields. The control experiments reveal that an inert atmosphere and the
addition of
a hole scavenger are both indispensable for the visible-light-driven photocatalytic
hydrogenation of nitroaromatics over In2S3–CNT. In comparison with blank In2S3, the obviously enhanced
photocatalytic performance of the In2S3–CNT photocatalyst is mainly ascribed to the unique
physicochemical properties of CNTs, which enhances the adsorptivity of the substrate and performs as an
electron reservoir to trap electrons, thereby hindering the recombination of photogenerated electron–
hole pairs. It is hoped that the current work on the facile synthesis of semiconductor In2S3–CNT
nanocomposites can broaden the applications of semiconductor-carbon based composite photocatalysts
in the field of photocatalytic selective organic transformations under mild conditions.
Received 28th October 2013
Accepted 18th November 2013
DOI: 10.1039/c3ta14370h
www.rsc.org/MaterialsA
Heterogeneous photocatalysis by semiconductor-based
materials has been attracting tremendous research interest over
Introduction
the past decades, which is now considered as one of the most
promising approaches to solve the energy and environmental
issues toward sustainable processes.9,20–28 As a green technology,
photocatalysis can be widely applied to environmental purica-
tion (non-selective process) and selective organic trans-
formations to ne chemicals in both gas and liquid phases.25,29–38
However, one of the fundamental issues limiting the applica-
bility of semiconductor photocatalysis is that the photoinduced
electron–hole pairs in the excited states are unstable and can
easily recombine at or near the surface of semiconductors,
resulting in a relatively low efficiency of semiconductor-based
photocatalysis.9,39 Thus, the optimization and manipulation of a
charge carrier transfer process on a semiconductor surface is a
key and broad theme for improving the efficiency of such
semiconductor-based photocatalysts toward target reactions.
In order to improve the photocatalytic performance of
semiconductor-based photocatalysts, a variety of strategies have
been developed. Thereinto, coupling semiconductors with
carbonaceous nanomaterials has proven to be an effective
approach for enhancing the photocatalytic activities of semi-
conductor photocatalysts.40–47 Carbon nanotubes (CNTs), as a
typical type of one-dimensional (1D) material in the carbon
family, due to their superior physicochemical, mechanical, and
electronic properties,48–50 have received a lot of attention since
their discovery in 1991.51 The large specic surface area, hollow
structure, and extraordinary mechanical and unique electronic
properties of CNTs offer them various potential applications in
In recent years, nitrophenols and some other nitroaromatics are
of great public concern because of their high pollution of water.
In particular, the nitrophenolic compounds, resulting from the
production processes of pesticides, herbicides, insecticides, and
synthetic dyes, increase the pollution.1–7 In contrast, the products
of catalytic hydrogenation of nitroaromatics, such as aromatic
amines, are potent intermediates in industrial synthesis of bio-
logically active compounds, pharmaceuticals, rubber chemicals,
photographic and agricultural chemicals.5–9 Thus, the reaction of
catalytic hydrogenation of nitroaromatics to the corresponding
amines has become academically as well as technologically
important.1 Traditionally, the hydrogenation of nitroaromatics
employs noble metal (e.g. Au, Ag, Pd, and Pt) catalysts in the
presence of excess amounts of reducing agents, such as NaBH4
and H2,8,10–18 which has proven to be an effective approach.
However, it is quite expensive to use these noble metals as cata-
lysts for catalytic applications, and another major challenge is the
aggregation of the metal nanoparticles during the catalytic reac-
tions, which oen leads to a rapid decay of their catalytic
activity.19 Therefore, developing green, simple and effective routes
for the hydrogenation of nitroaromatics is highly desirable.
aState Key Laboratory Breeding Base of Photocatalysis, College of Chemistry and
Chemical Engineering, Fuzhou University, Fuzhou, 350002, P. R. China
bCollege of Chemistry and Chemical Engineering, New Campus, Fuzhou University,
Fuzhou, 350108, P. R. China. E-mail: yjxu@fzu.edu.cn; Tel: +86 591 83779326
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energy conversion, electrocatalysts, sensors, environmental concentrated nitric acid at 140 ꢀC for 8 h. Then, the treated
remediation, and catalysts.40,52,53 In particular, the large elec- CNTs were collected by centrifugation at 8000 rpm for 15 min
tron-storage capacity and metallic conductivity of CNTs similar and washed with DI water several times until pH ¼ 7. Aer that,
ꢀ
to metals suggest that they can perform as high performance the product was dried at 60 C in an oven.
candidates for photocatalyst carriers or promoters.54
Fabrication of In2S3–CNT nanocomposites. The In2S3–CNT
Indium sulde (In2S3), a typical III–VI group sulde with a nanocomposites were fabricated by a facile one-step reuxing
band gap of 2.0–2.3 eV, has attracted substantial attention as a method. The preparation process is illustrated in Scheme 1. The
potential semiconductor photocatalyst.32 Due to its high weight addition ratios of CNTs were selected as 1%, 2%, 3%,
photosensitivity and photoconductivity, stable chemical and and 5%. In detail, a certain amount of acid treated CNTs was
physical characteristics and low toxicity,55,56 many attempts dispersed into 100 mL distilled water by ultrasonication to
have been made to make use of In2S3 as a visible-light-driven obtain a homogeneous CNT dispersion. Aer that, 0.25 g of
photocatalyst for degradation of organic dyes and for water indium chloride tetrahydrate (InCl3$4H2O) was added into the
splitting to hydrogen.56 However, the construction of In2S3–CNT above CNT suspension. Aer stirring for 30 min, 0.12 g of thi-
nanocomposites toward photocatalytic selective organic oacetamide (TAA) was dissolved in the above mixed solution
synthesis has remained unavailable so far.
and stirred for another 30 min. The obtained solution was
Against this background, we herein construct the In2S3–CNT reuxed at 95 ꢀC for 5 h. Aer cooling, the resulting composite
nanocomposites via a facile wet chemistry process, aiming at a was recovered by centrifugation at 8000 rpm for 15 min, washed
synergetic combination of their intrinsic outstanding properties with water, and fully dried at 60 ꢀC in an oven to obtain the nal
and, thus, enhanced photocatalytic performance to meet new In2S3–CNT nanocomposites with different weight addition
requirements imposed by specic applications in green-chem- ratios of CNTs. The blank b-In2S3 was obtained in the absence of
istry-oriented organic synthesis in water under visible light.33,34 CNTs.
The as-synthesized In2S3–CNT nanocomposites have been
successfully applied for visible-light-driven photocatalytic
hydrogenation of nitrophenols and other substituted nitro-
aromatics to the corresponding amines in water. In comparison
with blank In2S3, the In2S3–CNT nanocomposite exhibits an
obviously enhanced photocatalytic performance, which is
caused by the enhanced adsorption of the substrate and the
electron accepting and transport properties of CNTs, thereby
providing a convenient way to boost the transport and separa-
tion of photogenerated charge carriers and thus improving the
photocatalytic performance toward hydrogenation of nitro-
aromatics in water under mild conditions.
Catalyst characterization
The crystal phase properties of the samples were analyzed with
a Bruker D8 Advance X-ray diffractometer (XRD) using Ni-
ltered Cu Ka radiation at 40 kV and 40 mA in the 2q range from
5ꢀ to 80ꢀ with a scan rate of 0.02ꢀ per second. UV-vis diffuse
reectance spectra (DRS) were recorded on a Cary-500 UV-vis-
NIR spectrometer in which BaSO4 powder was used as the
internal standard to obtain the optical properties of the samples
over a wavelength range of 200–800 nm. The morphology of the
samples was determined by eld-emission scanning electron
microscopy (FE-SEM) using an FEI Nova NANOSEM 230 spec-
trophotometer. Transmission electron microscopy (TEM)
images were collected using a JEOL model JEM 2010 EX
microscope at an accelerating voltage of 200 kV. The photo-
luminescence (PL) spectra were obtained using an Edinburgh
Analytical Instrument PLS920 system with an excitation wave-
length of 420 nm. Raman spectroscopy was performed on a
Renishaw inVia Raman System 1000 with a 532 nm Nd:YAG
excitation source at room temperature.
Experimental section
Catalyst preparation
Materials. Indium chloride tetrahydrate (InCl3$4H2O), thio-
acetamide (C2H5NS, TAA), ammonium formate (HCOONH4),
potassium persulfate (K2S2O8), nitric acid (HNO3, 65%),
4-nitrophenol (C6H5NO3), 3-nitrophenol (C6H5NO3), 2-nitro-
phenol (C6H5NO3), 4-nitroaniline (C6H6N2O2), 3-nitroaniline
Photoelectrochemical measurements were performed in a
homemade three electrode quartz cell with a PAR VMP3 Multi
Potentiostat apparatus. A Pt plate was used as the counter
electrode, and Ag/AgCl electrode was used as the reference
electrode, while the working electrode was prepared on uoride
tin oxide (FTO) conductor glass. Typically, the sample powder
(C6H6N2O2),
2-nitroaniline
(C6H6N2O2),
4-nitrotoluene
(C7H7NO2), 4-nitroanisole (C7H7NO3), 1-bromo-4-nitrobenzene
(C6H4BrNO2) and 1-chloro-4-nitrobenzene (C6H4ClNO2) were all
obtained from Sinopharm Chemical Reagent Co., Ltd.
(Shanghai, China). All the materials were used as received
without further purication. The multiwalled carbon nanotubes
were purchased from Shenzhen Nanotech Port Co., Ltd., China,
with 60–100 nm in diameter, 5–15 mm in length and $95%
purity. The product catalog number of the purchased CNTs was
L.MWCNTs-60100. The deionized water for solution prepara-
tion was from local sources.
Synthesis. Acid treatment of carbon nanotubes (CNTs). The
purication and surface functionalization of CNTs were carried
out before being used for preparing nanocomposites. Typically,
Scheme 1 The schematic flowchart for preparation of In2S3–CNT
a 500 mg portion of raw CNTs was reuxed in 150 mL of nanocomposites.
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(5 mg) was ultrasonicated in 0.3 mL of N,N-dimethylformamide
(DMF, supplied by Sinopharm Chemical Reagent Co., Ltd.) to
disperse it evenly to obtain a slurry. The slurry was spread onto
FTO glass, whose side part was previously protected using
Scotch tape. The working electrode was dried overnight under
ambient conditions. A copper wire was connected to the side
part of the working electrode using conductive tape. Uncoated
parts of the electrode were isolated with epoxy resin. The
photocurrent measurement was carried out on a BAS Epsilon
workstation without bias and the electrolyte was 0.2 M aqueous
Na2SO4 solution (pH ¼ 6.8) without an additive. The electro-
chemical impedance spectroscopy (EIS) measurements were
carried out using an EIS spectrometer (CHI-660D workstation,
CH Instrument) in the three electrode cell in the presence of
0.5 M KCl solution containing 5.0 mM K3[Fe(CN)6]/K4[Fe(CN)6]
by applying an AC voltage with a 5 mV amplitude in the
frequency range from 1 Hz to 100 kHz under open circuit
potential conditions. The cyclic voltammograms were measured
in the same solution in the three electrode cell as that of the EIS
measurement. The visible light irradiation source (l > 420 nm)
was a 300 W Xe arc lamp system.
Catalytic activity
Photocatalytic hydrogenation of nitroaromatics. The photo-
catalytic hydrogenation of nitroaromatic compounds was per-
formed at ambient temperature in an inert atmosphere. In a
typical reaction, 40 mg of the samples and 40 mg of ammonium
formate were added into 40 mL of nitroaromatic compound
solution (10 ppm) in a quartz vial. A 300 W Xe arc lamp (PLS-SXE
300C, Beijing Perfectlight Co., Ltd.) equipped with a lter to cut
off light of wavelength below 420 nm was used as the irradiation
source to trigger the photocatalytic reaction. Aer the reaction,
the mixture was centrifuged at 12 000 rpm for 10 min to remove
the catalyst completely. Aer that, the solution was analyzed
using a Varian ultraviolet-visible light (UV-vis) spectrophotom-
eter (Cary-50, Varian Co.).8 The identication of reactants and
products was also conrmed by the high performance liquid
chromatography (HPLC) analysis (Shimadzu HPLC-LC20AT
equipped with a C18 column and SPD-M20A photodiode array
detector). The whole experimental process was conducted
Fig. 1 X-ray diffraction patterns of the samples of CNTs before and
after acid treatment (a), and the samples of blank In2S3 and In2S3–CNT
nanocomposites with different weight addition ratios of CNTs (b).
surface is benecial for improving its hydrophilicity and
provides the possibility of preparing CNT-based nano-
composites via a wet chemistry process.
The crystallographic structure and phase purity of the as-
synthesized In2S3 and In2S3–CNT nanocomposites are shown in
Fig. 1b. It can be seen that the samples of In2S3–CNT with
different weight addition ratios of CNTs show similar XRD
patterns to blank In2S3. The peaks located at ca. 14.2, 23.3, 27.4,
28.7, 33.2, 43.6, 47.7, 56.6, 59.4, 66.6 and 69.8ꢀ are distinctly
indexed to the (111), (220), (311), (222), (400), (511), (440), (622),
(444), (731) and (800) crystal planes of the cubic In2S3 phase
(b-In2S3) (JCPDS no. 65-0459), respectively. Notably, no diffraction
peaks for CNTs can be observed in the In2S3–CNT nano-
composites. One possible reason might be due to the low amount
under N2 bubbling at a ow rate of 80 mL minꢁ1
.
Results and discussion
Fig. 1a displays the XRD patterns of CNTs before and aer the of CNTs in the In2S3–CNT nanocomposites, and the other is
acid treatment. The observed diffraction peaks at 26.4, 42.2, probably due to the overlapping of the main characteristic peak
54.5, and 77.2ꢀ can be attributed to the hexagonal graphite of CNTs at 26.6ꢀ with the (311) peak at 27.4ꢀ of cubic In2S3.
structures (002), (100), (004), and (110).57 Similar XRD patterns
Table 1 summarizes the average crystallite sizes of blank
of the two samples indicate that the acid treatment has not In2S3, and the samples of In2S3–CNT with different addition
destroyed the graphitic structure of the CNTs. However, aer ratios of CNTs. For all the samples, the average crystallite sizes
the acid treatment, the dispersibility of CNTs in water is of In2S3 particles are calculated from the (440) peak of the XRD
signicantly enhanced in comparison with the pristine CNTs. patterns by the Scherrer formula: D ¼ 0.89l/b cos q, where D is
This is due to the fact that the acid treatment introduces a lot of the average crystallite size, l is the Cu Ka wavelength
functional groups (e.g. carboxylic groups) on the surface of (0.15406 nm), b is the half-width of the peak in radians, and q is
CNTs, which has been well demonstrated by previous the corresponding diffraction angle.8 The average crystallite
studies.58–61 The graing of functional groups on the CNT sizes of In2S3 particles in blank In2S3 and In2S3–CNT
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Table 1 Summary of the average crystallite size of blank In2S3 and
In2S3–CNT nanocomposites with different weight addition ratios of
CNTs
It can be seen that the presence of different amounts of CNTs
has a signicant inuence on the optical properties of light
absorption for the In2S3–CNT nanocomposites. The sample of
blank In2S3 exhibits a wide range light absorption with an
absorption edge onset at 610 nm, corresponding to the band
gap (Eg) of about 2.0 eV. The addition of CNTs induces the
increased light absorption intensity in the visible light region
ranging from 610 to 800 nm as compared to blank In2S3, which
can be ascribed to the broad background absorption of CNTs in
the visible light region. This is in accordance with the color
change of the samples (Fig. 3, bottom).
Samples
Average crystallite size (nm)
Blank In2S3
18.7
19.2
19.1
18.6
18.9
In2S3–1%CNT
In2S3–2%CNT
In2S3–3%CNT
In2S3–5%CNT
In order to characterize the morphology of the samples and
analyze the effect of CNTs on the microscopic structure of the
In2S3–CNT nanocomposites, scanning electron microscopy
(SEM) analysis has been performed. Fig. 4a displays the typical
SEM images of blank In2S3, from which it can be seen that the
sample is composed of a large quantity of aggregated small
nanoparticles. When CNTs are added during the synthesis of
In2S3–CNT nanocomposites, as can be observed from Fig. 4b,
CNTs are surrounded by In2S3 nanoparticles and some of the
nanocomposites are close to each other, implying that the
introduction of CNTs into the matrix of In2S3 has no obvious
inuence on the crystallite sizes of In2S3 particles.
Raman spectroscopy as a valuable tool to characterize the
nanostructures of carbon based nanomaterials has also been
carried out. Fig. 2 shows the typical Raman spectra of the blank
In2S3 and In2S3–CNT (here, taking In2S3–3%CNT as an example)
under the excitation of a 532 nm laser at room temperature. It
can be seen that there are four predominant bands located at
114, 266, 306 and 368 cmꢁ1 for blank In2S3, which correspond to
the typical vibration modes of b-In2S3.62 As for the In2S3–CNT
nanocomposite, it is notable that besides the typical peaks of
b-In2S3 at the low wavenumber, there are two additional char-
acteristic bands of CNTs at 1345 cmꢁ1 and 1580 cmꢁ1. The
structure around 1345 cmꢁ1 is called the “D band” and is
related to scattering from local defects or disorders present in
the CNT sample.58,59 The peak at 1580 cmꢁ1 is referred to as the
“G band” and it originates from the in-plane tangential
stretching of the C–C bonds in the graphitic structure.58,59 The
Raman characterization conrms the presence of In2S3 and
CNT components in the In2S3–CNT nanocomposite.
The UV-vis diffuse reectance spectra (DRS) measurement
has been performed to determine the optical properties of the
samples. Fig. 3 displays the DRS of blank In2S3 and In2S3–CNT
nanocomposites with different weight addition ratios of CNTs.
Fig. 3 Top: UV-vis diffuse reflectance spectra (DRS) of blank In2S3 and
the samples of In2S3–CNT with different weight addition ratios of
CNTs. Bottom: photographs of blank In2S3 and the samples of
In2S3–CNT.
Fig.
2 Raman spectra of the blank In2S3 and In2S3–3%CNT Fig. 4 Typical SEM images of blank In2S3 (a) and the sample of
nanocomposite.
In2S3–3%CNT (b).
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In2S3 particles are bound onto the CNT surface. Furthermore, it increased to 3%, for which the conversion of 4-NP reaches ca.
can also be observed that the morphology of the In2S3 compo- 96% under visible light irradiation for 1 h. However, further
nent in In2S3–3%CNT is not signicantly different compared to increase of the weight addition ratio of CNTs in the In2S3–CNT
that of blank In2S3.
nanocomposites leads to a rapid decrease of the photocatalytic
The microscopic morphology and structure information of activity, which is also observed in other carbon–semiconductor
the In2S3–CNT nanocomposite have been further investigated nanocomposite photocatalysts.29,31,40,63 This is understandable
by the transmission electron microscopy (TEM) analysis. It can because the relatively high weight ratio of carbon materials in
be seen from Fig. 5a–c that there is good contact between the carbon–semiconductor nanocomposites would increase the
CNTs and the In2S3 particles and the nanosized In2S3 particles opacity and shield light from samples, lowering the contact
formed along the side wall of the CNTs can be clearly observed surface of semiconductor nanoparticles with the light illumi-
(Fig. 5b). Fig. 5d shows the HR-TEM image of the In2S3–CNT nation and thus leading to
a decreased photocatalytic
nanocomposite, displaying distinct lattice fringes with a lattice activity.40,64 The result indicates that proper control of the
spacing of 0.621 nm, which can be ascribed to the (111) face of loading amount of CNTs is crucial for optimizing the synergistic
cubic In2S3. In addition, the SAED pattern in Fig. 5c indicates interaction between semiconductor In2S3 and CNTs, thus
that In2S3–CNT possesses a polycrystalline structure, which is in improving the photocatalytic activity of In2S3–CNT nano-
accordance with the result of XRD analysis. The joint SEM and composites efficiently. In addition, a control experiment over
TEM characterization indicates the good interfacial contact blank CNTs shows that it does not display visible light photo-
between CNTs and In2S3 nanoparticles. Since the separation catalytic activity and no apparent conversion of 4-NP has been
and transfer of photogenerated charge carriers in CNT–semi- observed, conrming that the primary photoactive ingredient is
conductor nanocomposites is intimately associated with the semiconductor In2S3 in the In2S3–CNT nanocomposites.
interfacial interaction between the CNT and the semiconductor,
Based on the above results, it is clear that, among the In2S3–
it should be expected that such a good interfacial contact for the CNT nanocomposites with different addition ratios of CNTs, the
In2S3–CNT nanocomposite would favor the charge carrier In2S3–3%CNT nanocomposite shows the best photocatalytic
transfer process across the interface, thereby contributing to the performance. Therefore, it is chosen as the testing candidate to
photoactivity improvement.
evaluate the photocatalytic activity for selective hydrogenation
The photocatalytic activity of the as-prepared blank In2S3 of other nitroaromatics with different substituent groups. As
and In2S3–CNT nanocomposites is initially examined by selec- shown in Table 2, in comparison with blank In2S3, the In2S3–3%
tive hydrogenation of 4-nitrophenol (4-NP) to 4-aminophenol CNT nanocomposite displays higher photocatalytic activities for
(4-AP) in water under visible light irradiation, as displayed in all testing reactions. The result demonstrates that In2S3–CNT is
Fig. 6. It can be seen that the combination of CNTs with In2S3 able to perform as an efficient visible-light-driven photocatalyst
has an important inuence on its photocatalytic activity. When for selective hydrogenation of nitroaromatics to their corre-
a small amount weight ratio of 1%CNT is added, the conversion sponding amines, and the improvement of the photocatalytic
of 4-NP is obviously enhanced, and the optimal photocatalytic activity over the In2S3–3%CNT nanocomposite should be
performance is obtained while the weight ratio of CNTs is ascribed to the introduction of the CNT material. To under-
stand the origin of CNTs in improving the photoactivity of
Fig. 6 Photocatalytic selective hydrogenation of 4-NP over blank
Fig. 5 Typical TEM images (a–c) and high-resolution TEM (HR-TEM) In2S3 and In2S3–CNT nanocomposites under visible light irradiation for
image (d) of the sample of In2S3–3%CNT; the inset of panel (c) is the 1 h in water with the addition of ammonium formate as a hole
corresponding SAED pattern.
scavenger and N2 purge at room temperature.
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Table 2 Photocatalytic selective hydrogenation of different nitroaromatics to the corresponding amines over blank In2S3 and In2S3–3%CNT
under the irradiation of visible light (l > 420 nm) for 1 h with the addition of ammonium formate as a quencher for photogenerated holes and N2
purge at room temperature
Blank In2S3
In2S3–3%CNT
Entry
1
Substrate
Product
Conversion (%)
Yield (%)
44
Selectivity (%)
100
Conversion (%)
Yield (%)
74
Selectivity (%)
98
44
18
75
30
2
3
18
19
100
97
29
45
97
20
45
100
4
5
12
22
12
21
100
95
23
40
22
39
96
98
6
7
8
60
53
62
59
52
62
98
98
78
71
79
76
70
78
97
98
99
100
9
50
49
98
81
80
99
semiconductor In2S3, we have then comparatively characterized electrodes. Notably, it can be seen that the photocurrent density
the optimal sample of In2S3–3%CNT and blank In2S3. obtained on In2S3–3%CNT under visible light illumination is
Photoelectrochemical experiments have been performed to obviously enhanced as compared to that of blank In2S3. Due to
investigate the electronic interaction between In2S3 and CNTs. that the photocurrent is mainly formed by the diffusion of the
Fig. 7 displays a comparison of the photocurrent–time (I–t) photogenerated electrons to the back contact, and meanwhile
curves for blank In2S3 and In2S3–3%CNT electrodes that are the photoinduced holes are taken up by the hole acceptor in the
recorded over on–off cycles of intermittent visible light irradi- electrolyte,65 the enhanced photocurrent over In2S3–3%CNT
ation (l > 420 nm). It is clear to observe the fast photocurrent indicates the more efficient separation and longer lifetime of
response for each switch-on and switch-off event in both photogenerated charge carriers over it than that of blank In2S3,
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Fig. 9 shows the cyclic voltammograms of blank In2S3 and
In2S3–3%CNT nanocomposite, which display obvious anodic
and cathodic peaks for both samples. The peak at positive
potentials on the anodic (forward) sweep represents the oxida-
tion of ferrocyanide to ferricyanide with the loss of one elec-
tron.66 The sample of In2S3–3%CNT presents a large anodic
current density as compared to that of blank In2S3, indicating a
signicantly enhanced electron transfer rate of the In2S3–3%
CNT nanocomposite due to the incorporation of CNTs with
high electrical conductivity.
The improved efficiency in prolonging the lifetime of pho-
togenerated charge carriers of In2S3 by coupling with CNTs is
further supported by the photoluminescence (PL) spectra
measurement. The PL spectra can give information about the
photoexcited energy/electron transfer and recombination
processes, and it has been used to study the optoelectronic
properties and charge transfer efficiency of the photocatalyst
successfully.32,67 Fig. 10 shows the PL spectra of In2S3 and
In2S3–3%CNT powder samples. It is easy to observe that at an
Fig. 7 Transient photocurrent–time (I–t) curves of blank In2S3 and the
In2S3–3%CNT nanocomposite in 0.2 M Na2SO4 (pH ¼ 6.8) aqueous
solution versus Ag/AgCl electrode.
suggesting that the life span of photogenerated charge carriers
can be remarkably boosted by the strategy of combining In2S3
with CNTs in an appropriate manner.
In addition, electrochemical impedance spectra (EIS) anal-
ysis as a powerful tool in studying the charge transfer process
occurring in the three electrode system has also been per-
formed. As displayed in Fig. 8, the Nyquist plots of blank In2S3
and In2S3–3%CNT electrode materials cycled in 0.5 M KCl
solution containing 5.0 mM K3[Fe(CN)6]/K4[Fe(CN)6] all show
semicircles at high frequencies. It is known that the high-
frequency arc corresponds to the charge transfer limiting
process and can be attributed to the charge transfer resistance
at the contact interface between the electrode and the electro-
lyte solution.32,64 The smaller arc radius of the EIS Nyquist plot
corresponds to the lower electric charge transfer resistance.
Consequently, in comparison with blank In2S3, the smaller arc
radius of In2S3–3%CNT implies that it has faster interfacial
electron transfer than that of blank In2S3.
Fig. 9 Cyclic voltammograms of the as-synthesized thin films on FTO
glass with blank In2S3 and In2S3–3%CNT nanocomposite.
Fig. 8 Electrochemical impedance spectroscopy (EIS) Nyquist plots of
blank In2S3 and In2S3–3%CNT nanocomposite in 0.5 M KCl solution Fig. 10 Photoluminescence (PL) spectra of blank In2S3 and In2S3–3%
containing 5.0 mM K3[Fe(CN)6]/K4[Fe(CN)6].
CNT nanocomposite.
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excitation wavelength of 420 nm, the addition of CNTs will lead
to the PL quenching due to the preferential segregation of
energetic electrons onto CNTs, indicating that the recombina-
tion of photogenerated electron–hole pairs is efficiently
inhibited aer the combination of In2S3 with CNTs in the
sample of In2S3–3%CNT. The result is in good accordance with
the photoelectrochemical analysis and also in line with its
higher photocatalytic activities of In2S3–3%CNT than blank
In2S3.
In addition, the adsorption experiment in the dark for 4-NP
over the blank In2S3 and In2S3–CNT nanocomposite photo-
catalysts has also been performed, as displayed in Fig. 11.
Apparently, the result suggests that the adsorption of the
substrate over the samples can be signicantly increased with
the introduction of CNTs into the matrix of In2S3. The increased
adsorptivity of In2S3–CNT can be attributed to the addition of
CNTs, which is also observed in other semiconductor–CNT
nanocomposites toward different photocatalytic applica-
tions.29,68 Because semiconductor-based heterogeneous photo-
Fig. 12 Control experiments for photocatalytic hydrogenation of
4-nitrophenol over the In2S3–3%CNT nanocomposite under visible
light irradiation (l > 420 nm); the reaction with the addition of
ammonium formate in a N2 atmosphere (a), reaction without a pho-
catalysis is a surface-based process, the increased adsorptivity tocatalyst (b), reaction with K2S2O8 as a scavenger for electrons (c),
reaction without ammonium formate (d) and reaction without the
purge of N2 (e).
of 4-NP on the surface of the In2S3–CNT nanocomposite is
believed to be benecial for improving the photocatalytic
performance under the visible light irradiation.
In order to explore the inuence of reaction conditions on
the photocatalytic hydrogenation of nitroaromatics over In2S3– photocatalytic conversion of 4-NP over the optimal In2S3–3%
CNT nanocomposites, additional control experiments have CNT nanocomposite is strongly inhibited in the absence of N2
been performed. As shown in Fig. 12b, the control experiment (Fig. 12d) and HCOONH4 (Fig. 12e), indicating that the inert
without the In2S3–CNT photocatalyst exhibits no conversion of atmosphere and the addition of HCOONH4 as a hole scavenger
4-NP, which ensures that the reaction is really driven by a are both indispensable for the photocatalytic hydrogenation of
photocatalytic process. When a radical scavenger for photo- 4-NP over the In2S3–CNT nanocomposites. These also suggest
generated electrons K2S2O8 is added into the reaction system that photocatalytic selective reduction of nitroaromatics over
(Fig. 12c),69,70 no obvious conversion of 4-NP can be observed, In2S3–CNT requires the appropriate control of reaction condi-
clearly demonstrating that the photogenerated electrons play a tions, ensuring that the photoreduction process driven by
decisive role in driving the photocatalytic hydrogenation of photogenerated electrons proceeds efficiently.
nitroaromatics. In addition, it can be seen that the
On the basis of the above discussion, it can be concluded
that the enhanced photocatalytic performance over In2S3–CNT
is attributed to the contribution of CNTs, which enhances the
adsorptivity of the substrate, and they act as electron acceptors
and transfer channels for enhancing the charge separation
efficiency. Accordingly, a probable reaction mechanism has
been proposed and illustrated in Fig. 13. Under the visible light
irradiation (l > 420 nm), the electrons are photoexcited from the
valence band (VB) to the conduction band (CB) of In2S3 in the
framework of In2S3–CNT, by which holes in the VB are created.
The photogenerated electrons tend to transfer to CNTs due to
the large electron-storage capacity, metallic conductivity and
lower potential of CNTs,29,54 which can efficiently facilitate the
separation of photoexcited charge carriers and lengthen the
lifetime of electron–hole pairs. Since the reduction of nitro-
aromatics is a multi-electron driven process, the efficient
separation and transfer of electrons are benecial for boosting
the reduction of nitroaromatics. On the other hand, the large
specic surface area of CNTs has also proven to improve the
adsorption of the substrate on the surface of In2S3–CNT, leading
to the fact that a high concentration of substrate nitroaromatic
compounds can be accumulated over the In2S3–CNT photo-
catalysts, thereby contributing to the photoactivity
Fig. 11 Remaining fraction of 4-NP after the adsorption–desorption
equilibrium is achieved over blank In2S3 and In2S3–CNT
nanocomposites.
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of nitroaromatics under visible light irradiation with the addi-
tion of ammonium formate as a quencher for photogenerated
holes in a N2 atmosphere. The stability of the samples has been
proven by the successive recycle test for photocatalytic hydro-
genation of 4-NP over the optimal sample of In2S3–3%CNT. As
displayed in Fig. 14, during the ve times recycle test for pho-
tocatalytic reduction of 4-NP under 1 h visible light irradiation,
the In2S3–3%CNT nanocomposite shows almost no deactiva-
tion. The conversion of 4-NP is nearly constant (ca. 96%) for
each run of the recycle photoactivity test. Therefore, In2S3–3%
CNT can perform as a stable visible-light-driven photocatalyst
toward selective photoreduction of nitroaromatics under the
current photocatalytic reaction conditions.
Conclusion
Fig. 13 Illustration of the proposed reaction mechanism for photo-
catalytic selective reduction of nitroaromatics over In2S3–CNT
nanocomposites.
In summary, we have synthesized a series of In2S3–CNT nano-
composites via an efficient and easily accessible low-tempera-
ture reuxing method. The In2S3–CNT nanocomposites have
been successfully applied to photocatalyze the hydrogenation of
nitroaromatics to the corresponding amines in water under
ambient conditions. The hydrogenation rate of In2S3 under
visible light irradiation (l > 420 nm) is obviously enhanced by
loading a suitable amount of CNTs, which improves the
adsorption of the substrate, facilitates the electron transfer, and
boosts the separation of photogenerated charge carriers. As a
result, the photocatalytic performance of In2S3–CNT is
remarkably enhanced. The optimal synergistic interaction
between In2S3 and CNTs is achieved when the content of CNTs
is ca. 3 wt%. The In2S3–CNT nanocomposites display good
photostability and recycle performance for photocatalytic
reduction of nitroaromatics under visible light. We hope this
study can enrich the applications of semiconductor–CNT
nanocomposite photocatalysts with improved photoactivity for
targeting applications in the eld of photocatalytic selective
organic synthesis.
enhancement. Besides, the photogenerated holes in the VB of
In2S3 are trapped by the hole quencher of ammonium formate
in a N2 atmosphere, which efficiently restrains the occurrence of
the oxidation process for nitroaromatics and thus guarantees
that the selective photoreduction proceeds efficiently. The
whole photocatalytic reduction of nitroaromatics can be
expressed by the following formula (Scheme 2).
Finally, the In2S3–CNT nanocomposite also has the excellent
recycle photocatalytic performance for selective hydrogenation
Scheme 2 Photocatalytic reduction of nitroaromatics to amines in
water with the addition of ammonium formate (HCOONH4) for
quenching photogenerated holes in a N2 atmosphere.
Acknowledgements
The support by the National Natural Science Foundation of
China (NSFC) (20903023, 21173045), the Award Program for
Minjiang Scholar Professorship, the Natural Science Founda-
tion (NSF) of Fujian Province for Distinguished Young Investi-
gator Grant (2012J06003), Program for Changjiang Scholars and
Innovative Research Team in Universities (PCSIRT0818),
Program for Returned High-Level Overseas Chinese Scholars of
Fujian Province, and the Project Sponsored by the Scientic
Research Foundation for the Returned Overseas Chinese
Scholars, State Education Ministry, is gratefully acknowledged.
Notes and references
1 S. Panigrahi, S. Basu, S. Praharaj, S. Pande, S. Jana, A. Pal,
S. K. Ghosh and T. Pal, J. Phys. Chem. C, 2007, 111, 4596–
4605.
2 J. Du and Z. Xia, Anal. Methods, 2013, 5, 1991–1995.
3 T. Vincent and E. Guibal, Langmuir, 2003, 19, 8475–8483.
Fig. 14 The recycle experiment of photocatalytic reduction of 4-NP
over the In2S3–3%CNT nanocomposite under visible light irradiation
(l > 420 nm) with the addition of ammonium formate as a quencher for
photogenerated holes in a N2 atmosphere.
1718 | J. Mater. Chem. A, 2014, 2, 1710–1720
This journal is © The Royal Society of Chemistry 2014

View Article Online
Paper
Journal of Materials Chemistry A
4 M. Liang, L. Wang, X. Liu, W. Qi, R. Su, R. Huang, Y. Yu and 33 G. Palmisano, V. Augugliaro, M. Pagliaro and L. Palmisano,
Z. He, Nanotechnology, 2013, 24, 245601. Chem. Commun., 2007, 3425–3437.
5 N. Sahiner, H. Ozay, O. Ozay and N. Aktas, Appl. Catal., B, 34 G. Palmisano, E. Garcia-Lopez, G. Marci, V. Loddo,
2010, 101, 137–143.
6 S. U. Sonavane, M. B. Gawande, S. S. Deshpande,
S. Yurdakal, V. Augugliaro and L. Palmisano, Chem.
Commun., 2010, 46, 7074–7089.
A. Venkataraman and R. V. Jayaram, Catal. Commun., 2007, 35 X. Lang, H. Ji, C. Chen, W. Ma and J. Zhao, Angew. Chem., Int.
8, 1803–1806. Ed., 2011, 50, 3934–3937.
7 J. S. D. Kumar, M. M. Ho and T. Toyokuni, Tetrahedron Lett., 36 C. Chen, W. Ma and J. Zhao, Chem. Soc. Rev., 2010, 39, 4206–
2001, 42, 5601–5603. 4219.
8 M.-Q. Yang, X. Pan, N. Zhang and Y.-J. Xu, CrystEngComm, 37 Z. Xiong and X. S. Zhao, J. Mater. Chem. A, 2013, 1, 7738–
2013, 15, 6819–6828. 7744.
9 M.-Q. Yang and Y.-J. Xu, Phys. Chem. Chem. Phys., 2013, 15, 38 H. Cheng, B. Huang, Y. Liu, Z. Wang, X. Qin, X. Zhang and
19102–19118. Y. Dai, Chem. Commun., 2012, 48, 9729–9731.
10 Y. Zhang, S. Liu, W. Lu, L. Wang, J. Tian and X. Sun, Catal. 39 Q. Xiang, J. Yu and M. Jaroniec, Chem. Soc. Rev., 2012, 41,
Sci. Technol., 2011, 1, 1142–1144. 782–796.
11 N. Zhang and Y.-J. Xu, Chem. Mater., 2013, 25, 1979– 40 J. Yu, B. Yang and B. Cheng, Nanoscale, 2012, 4, 2670–2677.
1988.
41 Z.-R. Tang, F. Li, Y. Zhang, X. Fu and Y.-J. Xu, J. Phys. Chem.
C, 2011, 115, 7880–7886.
12 X. Li, X. Wang, S. Song, D. Liu and H. Zhang, Chem.–Eur. J.,
2012, 18, 7601–7607.
13 J. Li, C.-y. Liu and Y. Liu, J. Mater. Chem., 2012, 22, 8426–
42 G. Li, B. Jiang, X. Li, Z. Lian, S. Xiao, J. Zhu, D. Zhang and
H. Li, ACS Appl. Mater. Interfaces, 2013, 5, 7190–7197.
43 L. Han, P. Wang and S. Dong, Nanoscale, 2012, 4, 5814–
5825.
8430.
14 J. Zeng, Q. Zhang, J. Chen and Y. Xia, Nano Lett., 2009, 10,
30–35.
44 W. Tu, Y. Zhou, Q. Liu, Z. Tian, J. Gao, X. Chen, H. Zhang,
J. Liu and Z. Zou, Adv. Funct. Mater., 2012, 22, 1215–1221.
45 W. Tu, Y. Zhou, Q. Liu, S. Yan, S. Bao, X. Wang, M. Xiao and
Z. Zou, Adv. Funct. Mater., 2013, 23, 1743–1749.
15 K. Esumi, R. Isono and T. Yoshimura, Langmuir, 2003, 20,
237–243.
16 A. Corma and P. Serna, Science, 2006, 313, 332–334.
17 Y. Shiraishi, Y. Togawa, D. Tsukamoto, S. Tanaka and 46 Y. Fu, H. Chen, X. Sun and X. Wang, Appl. Catal., B, 2012,
T. Hirai, ACS Catal., 2012, 2, 2475–2481. 111–112, 280–287.
18 A. Corma, P. Concepcion and P. Serna, Angew. Chem., Int. 47 Y. Hu, X. Gao, L. Yu, Y. Wang, J. Ning, S. Xu and X. W. Lou,
Ed., 2007, 46, 7266–7269. Angew. Chem., Int. Ed., 2013, 52, 5636–5639.
19 H. Chong, P. Li, J. Xiang, F. Fu, D. Zhang, X. Ran and M. Zhu, 48 A. Kongkanand and P. V. Kamat, ACS Nano, 2007, 1, 13–21.
Nanoscale, 2013, 5, 7622–7628. 49 Y. K. Kim and H. Park, Energy Environ. Sci., 2011, 4, 685–694.
20 N. Zhang, Y. Zhang and Y.-J. Xu, Nanoscale, 2012, 4, 5792– 50 L. Liu, W. Ma and Z. Zhang, Small, 2011, 7, 1504–1520.
´
5813.
51 M.-Q. Yang, N. Zhang and Y.-J. Xu, ACS Appl. Mater.
Interfaces, 2013, 5, 1156–1164.
21 Q. Xiang and J. Yu, J. Phys. Chem. Lett., 2013, 4, 753–759.
´
22 J. Zhang, J. Yu, M. Jaroniec and J. R. Gong, Nano Lett., 2012, 52 G. Ovejero, J. L. Sotelo, M. D. Romero, A. Rodrıguez,
˜
´
´
12, 4584–4589.
M. A. Ocana, G. Rodrıguez and J. Garcıa, Ind. Eng. Chem.
23 Q. Xiang, J. Yu and M. Jaroniec, J. Am. Chem. Soc., 2012, 134,
6575–6578.
24 W. Fan, Q. Zhang and Y. Wang, Phys. Chem. Chem. Phys.,
2013, 15, 2632–2649.
25 W. Fan, Q. Lai, Q. Zhang and Y. Wang, J. Phys. Chem. C, 2011,
115, 10694–10701.
26 X. Chen, C. Li, M. Gratzel, R. Kostecki and S. S. Mao, Chem.
Soc. Rev., 2012, 41, 7909–7937.
Res., 2006, 45, 2206–2212.
53 P.-X. Hou, C. Liu and H.-M. Cheng, Carbon, 2008, 46, 2003–
2025.
54 D. M. Guldi, G. M. A. Rahman, F. Zerbetto and M. Prato, Acc.
Chem. Res., 2005, 38, 871–878.
55 Y. H. Kim, J. H. Lee, D.-W. Shin, S. M. Park, J. S. Moon,
J. G. Nam and J.-B. Yoo, Chem. Commun., 2010, 46, 2292–
2294.
´
´
´
27 A. Kubacka, M. Fernandez-Garcıa and G. Colon, Chem. Rev., 56 W. Qiu, M. Xu, X. Yang, F. Chen, Y. Nan, J. Zhang, H. Iwai
2011, 112, 1555–1614. and H. Chen, J. Mater. Chem., 2011, 21, 13327–13333.
28 W. Zhou, Z. Yin, Y. Du, X. Huang, Z. Zeng, Z. Fan, H. Liu, 57 W. Li, C. Liang, W. Zhou, J. Qiu, Z. Zhou, G. Sun and Q. Xin,
J. Wang and H. Zhang, Small, 2013, 9, 140–147. J. Phys. Chem. B, 2003, 107, 6292–6299.
29 Y.-J. Xu, Y. Zhuang and X. Fu, J. Phys. Chem. C, 2010, 114, 58 V. Datsyuk, M. Kalyva, K. Papagelis, J. Parthenios, D. Tasis,
2669–2676.
A. Siokou, I. Kallitsis and C. Galiotis, Carbon, 2008, 46,
30 Y. Zhang, N. Zhang, Z.-R. Tang and Y.-J. Xu, ACS Nano, 2012,
833–840.
6, 9777–9789.
59 I. D. Rosca, F. Watari, M. Uo and T. Akasaka, Carbon, 2005,
43, 3124–3131.
31 Y. Zhang, Z.-R. Tang, X. Fu and Y.-J. Xu, ACS Nano, 2011, 5,
¨ ´
´
7426–7435.
60 L. Stobinski, B. Lesiak, L. Kover, J. Toth, S. Biniak,
G. Trykowski and J. Judek, J. Alloys Compd., 2010, 501,
77–84.
32 M.-Q. Yang, B. Weng and Y.-J. Xu, Langmuir, 2013, 29, 10549–
10558.
This journal is © The Royal Society of Chemistry 2014
J. Mater. Chem. A, 2014, 2, 1710–1720 | 1719

View Article Online
Journal of Materials Chemistry A
Paper
´
´
61 F. Aviles, J. V. Cauich-Rodrıguez, L. Moo-Tah, A. May-Pat and 66 X. Pan, Y. Zhao, S. Liu, C. L. Korzeniewski, S. Wang and
R. Vargas-Coronado, Carbon, 2009, 47, 2970–2975. Z. Fan, ACS Appl. Mater. Interfaces, 2012, 4, 3944–3950.
62 Y. Xiong, Y. Xie, G. Du, X. Tian and Y. Qian, J. Solid State 67 J. Yang, H. Yan, X. Wang, F. Wen, Z. Wang, D. Fan, J. Shi and
Chem., 2002, 166, 336–340. C. Li, J. Catal., 2012, 290, 151–157.
63 M.-Q. Yang and Y.-J. Xu, J. Phys. Chem. C, 2013, 117, 21724– 68 Y. Zhang, Z.-R. Tang, X. Fu and Y.-J. Xu, ACS Nano, 2010, 4,
21734. 7303–7314.
64 N. Zhang, M.-Q. Yang, Z.-R. Tang and Y.-J. Xu, J. Catal., 2013, 69 A. Syouan and K. Nakashima, J. Colloid Interface Sci., 2007,
303, 60–69. 313, 213–218.
65 Z. Chen, S. Liu, M.-Q. Yang and Y.-J. Xu, ACS Appl. Mater. 70 Y. Zhang, Z. Chen, S. Liu and Y.-J. Xu, Appl. Catal., B, 2013,
Interfaces, 2013, 5, 4309–4319.
140–141, 598–607.
1720 | J. Mater. Chem. A, 2014, 2, 1710–1720
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