Microwave-assisted synthesis of porous Ag2S-Ag hybrid nanotubes with high visible-light photocatalytic activity

Article abstract of DOI:10.1002/anie.201206715

Brought to light: Ag₂S-Ag hybrid nanotubes were synthesized by rapid microwave-assisted sulfidation of Ag₂CO₃ nanorods. The relative amounts of Ag₂S and Ag in the hybrid structure can be controlled easily by var

Full text of DOI:10.1002/anie.201206715

Angewandte
Chemie
DOI: 10.1002/anie.201206715
Nanotubes
Microwave-Assisted Synthesis of Porous Ag S–Ag Hybrid Nanotubes
2
with High Visible-Light Photocatalytic Activity**
Wenlong Yang, Lei Zhang, Yong Hu,* Yijun Zhong, Hao Bin Wu, and Xiong Wen (David) Lou*
Heterostructures of semiconductors and noble metals have
recently received much attention because of their unique
optical, magnetic, electrical, and catalytic properties for
(Figure 1). Moreover, the Ag content in the porous Ag S–
2
Ag HSNTs can be easily tuned by varying synthesis param-
eters such as the TAA concentration. Different from conven-
tional templating methods, the Ag CO NRs in the present
[
1–5]
different applications.
As an example, Yang and Ying
2
3
recently designed different hybrid structures of Ag S–noble
synthesis not only serve as the scaffold for the creation of
hollow structures, but also are directly involved in the
2
metals with optimized compositions and domain sizes with
[6]
improved electrocatalytic activities. Meanwhile, hollow
micro/nanostructures with uniform morphology and good
structural stability have also attracted considerable interest
owing to their widespread applications in nanoreactors, drug
formation of the Ag S–Ag hybrids.
2
[
7,8]
delivery, gas sensors, and energy storage and conversion.
However, one-pot formation of heterostructures from distinct
materials is difficult because of the dissimilar growth pro-
cesses involved with different microscopic mechanisms and
[
9,10]
reaction rates.
Therefore, integrating different compo-
nents to form complex hybrid nanostructures through facile
procedures remains as a challenging but exciting topic in
[11,12]
materials science.
As one of the important semiconductor–noble metal
heterostructures, Ag S–Ag nanohybrids have attracted great
2
Figure 1. Illustration of the fabrication of porous Ag S–Ag HSNTs by
2
research interest owing to their excellent properties in
photoelectric and medical devices, and photocatalysis.
a one-pot microwave-assisted method.
[13–15]
Therefore, it is of great importance to develop a facile and
rational strategy for the large scale preparation of well-
The as-prepared porous Ag S–Ag HSNTs were then
2
defined Ag S–Ag heterostructures to explore their novel
evaluated as photocatalysts for both the degradation of
methyl orange (MO) and the reduction of Cr species in an
aqueous solution under visible-light irradiation. Furthermore,
2
[16]
VI
properties and applications. Microwave-assisted synthesis
is attractive and facile for rapid synthesis of nanocrystals with
[17–19]
narrow particle-size distribution and high purity.
Herein,
the COH radicals formed in different Ag S–Ag photocatalysts
2
we report a facile strategy for one-pot formation of porous
were measured by the terephthalic acid fluorescence probe
Ag S–Ag heterostructure nanotubes (HSNTs) by microwave-
method, which revealed that the Ag content in the Ag S–Ag
2
2
assisted surface sulfidation of Ag CO nanorods (NRs). This
hybrids is crucial for obtaining optimized photocatalytic
activity.
2
3
process is facile and rapid, and only involves the reaction of
Ag CO NRs with thioacetamide (TAA) in ethanol under low
The reactions involved in the formation of Ag S–Ag
2
3
2
intensity microwave irradiation (400 W) for 15 minutes
hybrids under microwave irradiation might be described as
[20,21]
follows.
First, under microwave irradiation TAA reacts
with ethanol to form CH (NH )C(OC H )ꢀSH [Eq. (1)],
3
2
2
5
[
*] W. L. Yang, Prof. Y. Hu, Y. J. Zhong
Key Laboratory of the Ministry of Education for Advanced Catalysis
Materials, Institute of Physical Chemistry
Zhejiang Normal University, Jinhua, 321004 (P. R. China)
E-mail: yonghu@zjnu.edu.cn
which further reacts to release H S [Eq. (2)].
2
CH CSNH þ C H OH ! CH ðNH ÞCðOC H Þ-SH
ð1Þ
3
2
2
5
3
2
2
5
Dr. L. Zhang, H. B. Wu, Prof. X. W. Lou
School of Chemical and Biomedical Engineering
Nanyang Technological University
CH ðNH ÞCðOC H ÞSH þ C H OH ! CH ðNH ÞCðOC H Þ þ H S
3
2
2
5
2
5
3
2
2
5
2
2
ð2Þ
7
0 Nanyang Drive, Singapore 637457 (Singapore)
Then, H S reacts quickly with Ag CO at the surface of the
2
2
3
E-mail: xwlou@ntu.edu.sg
Homepage: http://www.ntu.edu.sg/home/xwlou
Ag CO NRs to produce a Ag S framework [Eq. (3)].
2
3
2
[
**] Y.H. greatly acknowledges the financial support from the Natural
Science Foundation of China (21171146) and Zhejiang Provincial
Natural Science Foundation of China (Y4110304).
H S þ Ag CO ! Ag S þ H O þ CO
ð3Þ
2
2
3
2
2
2
Simultaneously, metallic Ag is produced by the thermal
decomposition of Ag CO3 under microwave irradiation
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201206715.
2
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Communications
[
22]
[
Eq. (4)]).
amount of TAA in the synthesis results in the formation of
pure Ag S.
2
1
Ag
2
CO
3
! 2 Ag þ =
2
O
2
þ CO
2
ð4Þ
Scanning electron microscope (SEM) and transmission
electron microscope (TEM) characterizations provide
insights into the morphology and detailed structure of the
as-prepared Ag CO NR templates and the porous Ag S–Ag
The key to the successful synthesis of porous Ag S–Ag
HSNTs by a one-pot microwave-assisted method in the
present system is to retain the structure of the Ag CO NR
2
2
3
2
HSNTs. A panoramic view of the as-prepared Ag CO NRs
2
3
2
3
templates. As Ag CO3 can dissolve in hot water, only
templates (Figure 3a) reveals that the sample is entirely
composed of uniform nanorods about 100–150 nm in diam-
eter and 1–2 mm in length. Figure 3b shows the SEM image of
2
agglomerates of Ag S–Ag hybrids (Figure S1, see the Sup-
2
porting Information) are obtained by the microwave-assisted
in situ surface sulfidation process with water as the solvent.
Thus, a nonaqueous solvent, such as ethanol as we have
chosen for this work, should be used for the microwave-
assisted synthesis. Additionally, the concentration of TAA in
this synthesis is also an important parameter to retain the
structural integrity of the as-prepared porous Ag S–Ag
2
HSNTs and control the content of metallic Ag in the
HSNTs. Samples prepared with 0, 1.8, 3.5, 5.4, and 7.0 mm
concentrations of TAA are denoted as pure Ag, H-1, H-2, H-
3
, and pure Ag S, respectively.
2
The crystallographic structure and phase purity of the as-
prepared porous Ag S–Ag HSNTs was examined by X-ray
2
powder diffraction (XRD), and the typical result from the
sample H-2 (CTAA = 3.5 mm) is shown in Figure 2. The
diffraction peaks marked by triangles can be assigned to
Figure 3. SEM images of the as-prepared a) Ag CO NRs, b) porous
2
3
Ag S–Ag HSNTs; TEM images of c) porous Ag S–Ag HSNTs (inset:
2
2
a high-magnification image. Scale bar: 100 nm); d) a section of
a single HSNT (inset: corresponding HRTEM image. Scale bar: 5 nm).
porous Ag S–Ag HSNTs formed by the one-pot microwave-
2
assisted synthesis (sample H-2; CTAA = 3.5 mm). The well-
defined Ag CO NRs are fully converted into porous Ag S–
2
3
2
Ag HSNTs without any impurity particles or aggregates. The
Ag/S ratio determined by energy dispersive X-ray spectrom-
etry (EDS) analysis is around 3.6 (see the Supporting
Information, Figure S3 and Table S1), which suggests the
Ag S/Ag molar ratio of 0.64 in the porous Ag S–Ag HSNTs.
Figure 2. XRD pattern of the as-prepared porous Ag S–Ag HSNTs
2
2
2
(CTAA =3.5 mm).
The EDS analysis also indicates that the Ag S/Ag molar ratio
2
in the product increases when the concentration of TAA in
the synthesis is increased (see the Supporting Information,
Table S1). The geometrical structure and hollow interior of
monoclinic Ag S (JCPDS card no. 77-0072; space group: P2 /
2
1
n, a = 4.229 ꢀ, b = 6.931 ꢀ, and c = 9.091 ꢀ), and those
as-prepared porous Ag S–Ag HSNTs were further elucidated
2
marked by asterisks correspond to face-centered cubic Ag
by TEM results. Figure 3c shows the incorporation of Ag
ꢀ
(
JCPDS card no. 87-0720; space group: Fm3m, a = b = c =
nanocrystals into the porous Ag S framework, and the
2
4
.077 ꢀ). The result of XRD analysis suggests the successful
resultant porous tubes well duplicate the size and shape of
the Ag CO NRs templates. The interior void and the porous
conversion from Ag CO NRs to Ag S–Ag hybrids by this
2
3
2
2
3
one-pot microwave-irradiation method. No additional peaks
were detected, thus indicating the high purity of the sample.
shell indicate that the Ag CO NRs are fully converted into
2 3
porous Ag S–Ag HSNTs after microwave irradiation for
2
In addition, the XRD patterns of the as-prepared Ag CO3
15 minutes. Figure 3d displays a TEM image of a section of
a single HSNT, and the heterojunction region between Ag2S
and Ag is clearly shown in the HRTEM image (inset). We also
2
NRs and other samples obtained with different TAA
concentrations are shown in Figure S2 (see the Supporting
Information). These results indicate that the TAA concen-
tration in the microwave-assisted synthesis can be used to
tune the content of metallic Ag in the hybrids. Thus, only pure
Ag is obtained in the absence of TAA, whereas a sufficient
observed that the size of the primary Ag and Ag S nano-
2
crystals in the HSNTs is relatively big. As a result, the
Brunauer—Emmet–Teller (BET) specific surface area of the
2
ꢀ1
H-2 sample is only about 12.9 m g . The spatial distribution
1
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of the composition in a single HSNT was further studied by
elemental mapping. The mapping result shows uniform
distribution of Ag and S throughout the HSNT (see the
Supporting Information, Figure S4).
In the present strategy, a layer of Ag S is first generated on
2
the surface of Ag CO NRs by the microwave-assisted in situ
2
3
sulfidation process. This Ag S layer should improve the
2
structural stability and reduce the impact of released O and
2
CO gases owing to the thermal decomposition of Ag CO . As
2
2
3
a result, almost no structural deformation, collapse or obvious
shrinkage, is found in the resultant hybrid hollow structures.
For comparison, the SEM image of the pure Ag nanoparticles
obtained by directly irradiating Ag CO3 templates in the
2
absence of TAA for 15 minutes apparently reveals the
collapse of the nanorod structure (see the Supporting
Information, Figure S5a). On the other hand, with more
TAA present in the synthesis, the structure of the nanotubes
can be better retained (see the Supporting Information,
Figure S5b–d). Surface information of the porous Ag S–Ag
2
HSNTs was further acquired with the X-ray photoelectron
spectroscopy (XPS; see the Supporting Information, Fig-
ure S6) technique. The peaks at 368.2 and 374.2 eV in the Ag
3
d photoelectron spectrum (in Figure S6b) are in good
agreement with the binding energies of Ag 3d5 and Ag 3d3/
/2
0
of the metallic Ag . Meanwhile, the peaks at 367.8 and
73.8 eV could be assigned to Ag 3d and Ag 3d3/2 of Ag
5/2
Thus, the presence of both
metallic state (Ag ) and Ag ions in the Ag S–Ag HSNTs is
2
+
3
[
13]
ions in the Ag S HSTNs.
Figure 4. a) Photocatalytic degradation of MO, and b) photocatalytic
2
0
+
VI
reduction of Cr in the presence of different photocatalysts. C is the
2
concentration of MO after light irradiation for a certain period, and Co
is the concentration of the MO after reaching adsorption/desorption
equilibrium in dark.
again confirmed.
Recent studies have suggested that Ag S–Ag heterostruc-
tures might possess advantages such as favorable charge
2
transfer from Ag to Ag S and a wide absorbance range of
2
[16]
light. Thus, this new type of porous HSNTs was investigated
as a promising photocatalyst in the present work. Figure 4
shows the photocatalytic activities of the as-prepared porous
The superior photocatalytic performance of the porous
Ag S–Ag HSNTs may be ascribed to the enhanced charge-
2
transfer process in the hybrid nanostructures. The work
function of Ag and the bottom of the conduction band of
Ag S–Ag HSNTs, which were evaluated for the degradation
2
VI
of organic dye MO and the reduction of aqueous Cr under
Ag S are 4.26 and 4.42 eV, respectively, relative to the vacuum
2
[
23]
visible-light irradiation. Figure 4a presents the visible-light
energy level. Therefore, the photoexcited electrons in the
photodegradation behaviors of MO using pure Ag S and
conduction band of Ag S can transfer to Ag in the hybrid
2
2
porous Ag S–Ag HSNTs that were obtained with different
nanostucture easily. The induced charge separation could
suppress the recombination of excited electrons and holes,
and hence increasing the photocatalytic activity. However,
excess Ag content may reduce the catalytic efficiency of the
2
concentrations of TAA as photocatalysts, and commercial
Degussa P25 nanoparticles for comparison. C is the concen-
tration of MO after light irradiation for a certain period, and
[13]
C is the concentration of the MO after reaching adsorption/
o
Ag S–Ag HSNTs owing to the reduced availability of the
2
desorption equilibrium in dark. After irradiation of 30 min,
nearly 92.1% of the MO is degraded by the sample H-2,
semiconductor surface for light absorption and pollutant
adsorption. As a result, the sample H-2 photocatalyst with
[24]
whereas other samples including pure Ag S, P25, H-1 and H-3
a moderate Ag S/Ag molar ratio exhibits superior activity
2
2
exhibit lower activities with degradation rates of about 7.8%,
over other samples.
2
5.4%, 13.9%, and 82.5%, respectively. Figure 4b displays
Furthermore, the COH radicals formed in different porous
VI
the photocatalytic reduction of Cr catalyzed by pure Ag S,
P25, and different porous Ag S–Ag HSNTs under visible-light
illumination. Similarly, the sample H-2 exhibits the highest
photocatalytic activity among the five samples. We have
further studied the stability and reusability of the photo-
catalyst by collecting and reusing the same photocatalysts for
HSNTs and pure Ag S photocatalysts could be probed using
2
2
[25]
a method described previously. It is well known that COH
reacts with terephthalic acid (TA) in basic solution to
generate 2-hydroxy-terephthalic acid (TAOH), which emits
a unique fluorescence signal with the peak centered at
2
[
26]
426 nm.
Significant fluorescent signals associated with
5
cycles (see the Supporting Information, Figure S7). The
TAOH are generated upon visible-light irradiation of the
different photocatalysts suspended in a TA solution for
10 minutes (Figure 5). It is clearly demonstrated that the
photoexcited holes are powerful enough to oxidize surface-
results show that there is only neglible loss of the photo-
catalytic activity; this loss might be partly caused by the loss of
the photocatalysts during each collection and rinsing step.
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Received: August 20, 2012
Published online: October 11, 2012
1
Keywords: microwave chemistry · nanotubes · photochemistry ·
.
semiconductors · silver
1
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Angew. Chem. Int. Ed. 2012, 51, 11501 –11504