The synthesis of size- and color-controlled silver nanoparticles by using microwave heating and their enhanced catalytic activity by localized surface plasmon resonance

Article abstract of DOI:10.1002/anie.201301652

Silver nanoparticles (Ag NPs) of various colors were synthesized within the mesopore structure of SBA-15 by using microwave-assisted alcohol reduction. The charge density is partially localized on the surface of these Ag NPs owing to localized surface pla

Full text of DOI:10.1002/anie.201301652

Angewandte
Chemie
Communications
Plasmonic Catalysis
Silver nanoparticles (Ag NPs) of various
colors were synthesized within the
K. Fuku, R. Hayashi, S. Takakura,
T. Kamegawa, K. Mori,
mesopore structure of SBA-15 by using
microwave-assisted alcohol reduction.
The charge density is partially localized
on the surface of these Ag NPs owing to
localized surface plasmon resonance.
This charge localization results in them
having enhanced catalytic activity under
visible light irradiation compared to Ag
NPs obtained by thermal processes.
H. Yamashita*
&&&& — &&&&
The Synthesis of Size- and Color-
Controlled Silver Nanoparticles by Using
Microwave Heating and their Enhanced
Catalytic Activity by Localized Surface
Plasmon Resonance
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DOI: 10.1002/anie.201301652
Plasmonic Catalysis
The Synthesis of Size- and Color-Controlled Silver Nanoparticles by
Using Microwave Heating and their Enhanced Catalytic Activity by
Localized Surface Plasmon Resonance**
Kojirou Fuku, Ryunosuke Hayashi, Shuhei Takakura, Takashi Kamegawa, Kohsuke Mori, and
Hiromi Yamashita*
Clean energy production, organic transformations through
energy-conserving processes, and efficient disposal of harmful
waste are desirable from the point of view of reducing
environmental problems, such as global warming and air and
water pollution. Supported metallic nanoparticle (NP) cata-
lysts are widely recognized as an important class of industrial
catalysts that resolve the above issues.[1] Significant efforts
have recently been devoted to achieving NPs with precise
architectures, in which the size, composition and morphology
are controlled; changes to these parameters enable the
catalytic activity and selectivity of such catalysts to be
tuned. The development of synthetic methods to provide
NPs suitable for particular catalytic reactions is important.[2]
Metallic NPs, such as Au, Ag, and Cu, can absorb visible
and infrared light in particular regions owing to localized
surface plasmon resonance (LSPR).[3] In simple terms, LSPR
is made up of collective oscillations of free electrons in metal
NPs driven by the electromagnetic field of incident light.[4]
This unique characteristic has given rise to a new approach to
the fabrication of visible-light-responsive photocatalysts,
known as plasmonic photocatalysts; this approach involves
the deposition of metallic NPs onto suitable semiconduc-
tors.[5] It has been proposed that for Au/TiO2 catalysts, at an
excitation wavelength corresponding to the LSPR of Au, the
Au NPs absorb photons and transfer photogenerated elec-
trons into the TiO2 conduction band.[5b,c]
As catalysis primarily occurs on the metallic surface, we
anticipated that the catalytic performance of such plasmonic
NPs could be significantly enhanced by the use of inert silica
as a support material because of the localized surface charge
density induced by the LSPR effect. More interestingly, the
color of the plasmonic NPs can be systematically tuned, owing
to changes in the LSPR absorption wavelength, by varying the
size and morphology of the particles.[6] However, the appli-
cation of plasmonic NPs with unique characteristics owing to
LSPR, such as localization of surface charge density and color
control, as catalysts have not yet been investigated.
Herein, we describe a new method for the synthesis of Ag
NPs, the color of which can be altered by changing the size
and morphology: this method involves microwave heating
and the use of SBA-15 mesoporous silica material. We also
demonstrated that the localized surface charge of these Ag
NPs results in them having enhanced catalytic activity under
visible light irradiation owing to LSPR, compared to Ag NPs
obtained by pure thermal processes. As expected, the
enhancement was dependent on the Ag NPs used. The
plasmonic metal NP catalysts could be designed with the
optimal size and color for target reactions and light sources,
thus making it possible to achieve the maximum catalytic
activity in various light environments.
SBA-15 mesoporous silica was prepared according to
a previously reported procedure.[7] The diffraction pattern
obtained by low-angle XRD measurement for SBA-15
exhibited two peaks at around 1.0–2.08, corresponding to
a hexagonal structure, thus indicating the formation of an
ordered mesoporous structure (see the Supporting Informa-
tion, Figure S1).[8] The N2 adsorption–desorption measure-
ments showed that SBA-15 possessed a high specific surface
area and a mesopore structure of 9.2 nm (see the Supporting
Information, Table S1).[8] Three types of Ag NPs were
prepared on SBA-15 by microwave-assisted alcohol reduction
with varying irradiation times (3 or 5 min) in the presence or
absence of sodium laurate (Lau) as a surface-modifying
ligand (Ag: 1 wt%): 1 (3 min, with sodium laurate), 2 (3 min,
without sodium laurate), and 3 (5 min, without sodium
laurate). Microwave induction heating has recently attracted
considerable attention as a potential method for the prepa-
ration of monodispersed inorganic materials because energy-
efficient microwave irradiation results in internal, rapid, and
uniform heating; this method can also be used for the simple
and energy-saving preparation of inorganic nanostructured
materials.[9] Although many studies of the preparation of
metallic colloids and clusters have been reported, there are
The charge density of such plasmonic NPs is partially
localized on the surface, and this localization is increased by
charge separation derived from the LSPR effect, in the
absence of a semiconductor to act as an electron acceptor.[4]
[*] K. Fuku, R. Hayashi, S. Takakura, Dr. T. Kamegawa, Dr. K. Mori,
Prof. Dr. H. Yamashita
Graduate School of Engineering, Osaka University
1-2 Yamadaoka, Suita, Osaka 565-0871(Japan)
E-mail: yamashita@mat.eng.osaka-u.ac.jp
Homepage: http://www.mat.eng.osaka-u.ac.jp/msp1/Englishme-
nu.htm
Dr. K. Mori, Prof. Dr. H. Yamashita
Unit of Elements Strategy Initiative for Catalysts & Batteries
Kyoto University, ESICB, Kyoto Univ. (Japan)
[**] The authors appreciate assistance from Dr. Eiji Taguchi and Prof.
Hidehiro Yasuda at the Research Center for Ultra-High Voltage
Electron Microscopy, Osaka University for TEM measurements. The
X-ray absorption experiments were performed at the Spring-8,
Harima (Japan).
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201301652.
2
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only a few examples of the microwave-assisted synthesis of
size-controlled metallic NPs on supports.[10]
As shown in the TEM images in Figure 1, SBA-15
exhibited a well-ordered mesoporous channel structure,
which was confirmed by the low-angle XRD pattern and N2
tion depended on the size, morphology, and aspect ratio of the
Ag NPs; a red shift in the LSPR absorption was observed as
the size and aspect ratio of the Ag nanoparticles increased.[6a]
For comparison, Ag deposition on SBA-15 was also per-
formed by heating by using a conventional oil bath at 413 K
with stirring. However, Ag NPs with a variety of morphol-
ogies, including spheres and nanorods, and wide size distri-
butions were formed in the presence of Lau, whereas hardly
any Ag NPs were formed in the absence of Lau (see the
Supporting Information, Figures S4 and S5).[8] It was con-
cluded that the SBA-15 mesopores and microwave heating
were required to attain rapid production and uniform growth
of the Ag nuclei.
To investigate the effect of the size and color of the Ag
NPs on their catalytic performance, H2 production from
ammonia borane (NH3BH3) was studied as the model
reaction. NH3BH3, which produces only H2 as a gas product
and possesses high H2 storage capacity (19.6 wt%), has
attracted attention as a promising next-generation hydrogen
storage material. Recently, the use of various NPs, such as Au,
Pd, Ru, Cu, Ni, Fe, and Au/Co, for the efficient production of
H2 from NH3BH3 has been widely investigated and H2 has
been stoichiometrically produced in a 3:1 (H2/NH3BH3)
molar ratio, as shown in Equation (1).[11] Noble metal
catalysts with superior catalytic activity under mild conditions
are particularly desirable as they are expected to have
widespread applications.
Figure 1. TEM images and photographs of samples (insets) of Ag/
SBA-15: a) 1, b) 2, and c) 3.
NH3BH3 þ 2 H2O ! NH4þ þ BO2ꢀ þ 3 H2
ð1Þ
adsorption–desorption measurements. Ag NPs were success-
fully deposited on SBA-15. Spherical Ag NPs with a mean
diameter of approximately 4 nm and a narrow size distribu-
tion were formed within the mesoporous channels of SBA-15
in 1 (Figure 1a), which was prepared by microwave irradi-
ation for 3 min in the presence of Lau. In contrast, in 2
(Figure 1b) and 3 (Figure 1c), which were prepared in the
absence of surface-modifying ligands under microwave irra-
diation for 3 and 5 min, respectively, Ag nanorods were
formed parallel to the SBA-15 mesopores. The aspect ratio of
the Ag nanorods increased markedly with increasing micro-
wave irradiation time. The average diameter of these Ag
nanorods was approximately 9 nm, which was in close agree-
ment with the pore size of SBA-15. The obtained Ag NPs
were also characterized by Ag K-edge XAFS spectra (see the
Supporting Information, Figure S2).[8] The XANES spectra of
all samples were similar to that of Ag foil, suggesting the
presence of Ag in the metallic state. In the Fourier transforms
of the Ag K-edge EXAFS spectra, all samples exhibited
a peak at approximately 2.7 ꢀ, which corresponds to the
Initially, H2 production from aqueous NH3BH3 (20 mmol)
was performed under an Ar atmosphere at room temperature
(258C) in the dark to evaluate the catalytic performance of
size-controlled Ag NPs on SBA-15 (see the Supporting
Information, Figure S6).[8] No catalytic activity was observed
in the control experiment (without catalyst). In contrast, all of
the Ag/SBA-15 catalysts exhibited H2 production activity. The
catalytic activity depended on the size of the Ag NP: the
smallest Ag NP, 1 (Figure S6a), was a more efficient catalyst
than 2 (Figure S6b) and 3 (Figure S6c). The H2 production
reaction was complete (ca. 60 mmol) within a short reaction
time when the smallest Ag NPs were used. These results
suggest that catalytic H2 production from NH3BH3 in the dark
occurred on the exposed Ag atoms on the surface of the NPs.
It is apparent from the results of previously reported catalyst
systems that the TOF values achieved with Ag/SBA-15 is
0.2 hꢀ1, which is higher than those reported for other catalyst
systems, such as Cu/g-Al2O3 (0.09 minꢀ1),[11d] metallic Fe
crystallites (0.05 minꢀ1),[11e] and Ni/SiO2 (0.16 minꢀ1).[11f]
To achieve maximum efficiency of the Ag NP catalysts,
the effect of the Ag LSPR on H2 production activity was
investigated under light irradiation. Figure 2 shows the time
course of the initial H2 production on Ag/SBA-15 in the dark
or under light irradiation (wavelength l > 420 nm,
320 mWcmꢀ2). The catalytic performance of all Ag NPs was
enhanced with light irradiation. Interestingly, the rate of H2
production was significantly dependent on the color of the Ag
NPs, and increased in the following order: 1 (29%) < 2
(66%) < 3 (124%). This trend matched the corresponding
ꢀ
contiguous Ag Ag bond in the metallic nanoparticles. The
peak intensity decreased in the order of 3 > 2 > 1; this trend is
due to the smaller size of the Ag particles. The order of peak
intensity was in close agreement with the order of Ag particle
size observed by TEM (Figure 1). In addition, the colors of
these samples were yellow (1), red (2), and blue (3), which
reflected the UV/Vis spectra. All of the Ag NPs exhibited
different UV/Vis absorption characteristics, derived from the
corresponding Ag LSPR (see the Supporting Information,
Figure S3).[8] The intensity and wavelength of LSPR absorp-
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Figure 3. Comparison of initial H2 production on Ag/SBA-15 3 in the
dark (25 or 308C) or under light irradiation (l>420 nm).
This equation can be used as a function of light intensity
(I0), absorption efficiency (Kabs), the diameter of metallic NP
(r0), the thermal conductivity of the surrounding medium (k1)
and heating temperature (T1). By using the experimental
values I0 = 320 mWcmꢀ2, r0 = 4–50 nm, T1 = 298 K, and the
values calculated by Christopher et al. (Kabs = 3, k1 =
40 mWmꢀ1 Kꢀ1), the increase in temperature due to the
plasmonic heating in our reaction system was determined to
be less than 0.002 K. These results suggest that the enhanced
catalytic performance originated primarily owing to Ag LSPR
under irradiation, and the contribution from heating was
negligible.
The effect of Ag LSPR on catalytic performance was also
investigated by using a red LED lamp with a maximum
wavelength lmax = 650 nm (25 mWcmꢀ2) (see the Supporting
Information, Figure S9).[8] Although this red LED light
contains only visible light and no infrared light, a similar
enhancement of the catalytic performance was obtained for
all Ag NPs. The rate of enhancement of catalytic performance
depended significantly on the type of Ag NP and increased in
the order 1 < 2 < 3, which was consistent with the order of the
Kubelka–Munk (KM) function intensity derived from Ag
-LSPR at irradiation wavelength (lmax = 650 nm) calculated
from UV/Vis measurements (see the Supporting Information,
Figure S3).[8] There was no increase in the temperature of
these Ag/SBA-15 suspensions under this red light irradiation.
In addition, the wavelength dependence of the enhancement
of the catalytic performance of Ag/SBA-15 having various
colors was investigated by using monochromatic light (l =
400, 440 and 460 nm, 6 mWcmꢀ2). As shown in Figure 4, the
increasing rates of catalytic performance with light intensities
were highly consistent with the LSPR absorption of these Ag
NPs. Notably, the increases in catalytic performance under
light irradiation were entirely dependent on the particle color
derived from Ag LSPR. This wavelength dependence also
suggests that the increase in catalytic performance was
derived from the Ag LSPR, and that maximum efficiency
can be attained by using Ag NPs of appropriate colors for the
light environment.
Figure 2. Time courses of initial H2 production in the presence of Ag/
&
SBA-15 in the dark (black ) or under light irradiation at wavelength
*
l>420 nm (gold ): a) 1, b) 2, and c) 3.
trend in Ag LSPR absorption intensity at wavelengths of
more than 420 nm. The off/on effect of this light irradiation
was also investigated and Ag/SBA-15 3 exhibited the highest
increase in catalytic activity (see the Supporting Information,
Figure S7).[8] After reaction in the dark for 15 min (light off),
the catalytic activity was significantly enhanced by light
irradiation (light on). It should be noted that the slope of
activity under light irradiation after reaction in the dark for
15 min was almost the same as that of activity under light
irradiation after no reaction in the dark (from 0 min). To the
best of our knowledge, this is the first demonstration for the
enhancement of catalytic activity of color-controlled Ag NPs
by the assistance of LSPR. Moreover, the catalyst could be
completely recovered without the leaching of active compo-
nents. Recovered Ag/SBA-15 1 retained its original catalytic
activity both under light irradiation and in the dark (Fig-
ure S8),[8] thus suggesting that this catalytic reaction pro-
ceeded on the Ag NPs deposited on the silica surface, but not
with detached Ag species.
Because light irradiation from a Xe lamp at l > 420 nm
contains not only visible light but also infrared light, the
above enhancement of catalytic activity under light irradi-
ation could simply be due to heating by the infrared light. In
fact, the temperature of the suspension of Ag/SBA-15 3
increased slightly (about 58C) under light irradiation, because
the Ag nanorods exhibit wide light absorption by LSPR over
the infrared region (see the Supporting Information, Fig-
ure S3).[8] In an effort to exclude the thermal effect of infrared
light, the thermal reaction at 308C was performed in the dark
with Ag/SBA-15 3. The initial H2 production in the dark at
308C was 0.74 mol%minꢀ1, which was slightly higher than the
production (0.57 mol%minꢀ1) in the dark at room temper-
ature (258C), but was significantly lower than the production
(1.35 mol%minꢀ1) under light irradiation (Figure 3). Also,
the plasmonic heating of Ag NPs derived from plasma
oscillation by LSPR, as previously reported by Christopher
et al., can be calculated as shown in Equation (2).[12]
The mechanism for catalytic H2 production from NH3BH3
has been reported for both heterogeneous and homogeneous
catalysis.[11d,13] In the case of catalysis by metallic NPs,
a plausible mechanism involves adsorption of NH3BH3 onto
the metallic NPs to form an activated complex, which is the
rate-determining step, followed by H2 production through
attack by a H2O molecule. In homogeneous catalysis using
I0Kabsr0
ð2Þ
T ¼ T1
þ
6k1
4
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performance was in close agreement with the order of
absorption intensity of the irradiated light by Ag LSPR.
From the results of experiments to investigate the thermal
effect, wavelength dependence, and the effect of a charge
scavenger, it was concluded that the H2 production reaction
was promoted by the positive charge generated by charge
separation due to Ag LSPR. This study constitutes a promis-
ing approach to the preparation of highly ordered metallic
NPs and the enhancement of catalytic performance under
mild conditions, and we expect that this strategy can be
extended to the preparation of other metallic NPs as well as
the exploitation of highly efficient catalytic processes.
Experimental Section
Materials: Tetraethoxysilane (TEOS), hydrochloric acid, 1-hexanol,
acetone, AgNO3, and sodium laurate (Lau) were purchased from
Nacalai Tesque Inc. Triblock copolymer P123 and ammonia borane
(NH3BH3) were purchased from Sigma–Aldrich Co. All chemicals
were used as received without further purification.
Figure 4. Wavelength dependence of the enhancement of catalytic
performance of Ag/SBA-15: a) 1, b) 2, and c) 3 under irradiation by
monochromatic light (l=400, 440 and 460 nm) and red LED light
(lmax =650 nm).
Preparation of Ag nanoparticles on SBA-15 mesoporous silica:
SBA-15 was prepared according to a previous report, involving the
sol–gel method using TEOS as a silica source and triblock copolymer
P123 as a template.[7] Typically, a 1-hexanol suspension of SBA-15
powder (0.396 g), an aqueous solution of AgNO3 (0.037 mmol), and
Lau (10 mg), as surface-modifying ligand for Ag, were irradiated by
microwave (500 W, 2450 ꢁ 30 MHz) under an Ar atmosphere. After
filtration and washing with acetone and distilled water, the resulting
powder was dried at 343 K overnight under air. Size control of the Ag
nanoparticles was achieved by varying the irradiation time (3 or
5 min) and in the presence or absence of Lau, resulting in the
formation of 1 wt% Ag nanoparticles supported on SBA-15 (Ag/
SBA-15) of three types (1, 2, and 3).
Catalytic activity test: H2 production from NH3BH3: H2 produc-
tion from NH3BH3 was carried out in an aqueous suspension of
various Ag/SBA-15 powders. The Ag/SBA-15 powder (20 mg) was
suspended in distilled water (5 mL) in a test tube. After bubbling Ar
through the mixture, NH3BH3 (20 mmol) was injected into the mixture
through a rubber septum. The test tube was photoirradiated at
a wavelength l > 420 nm (320 mWcmꢀ2), by monochromatic light
(l = 400, 440 and 460 nm, 6 mWcmꢀ2) from a 500 W Xe lamp through
metal halides, high H2 production activity is obtained by the
addition of a Lewis acid such as Co2+ ions to the reaction
mixture; this increased activity can be simply explained by
cooperative activation of Lewis basic NH3BH3 with the
assistance of the electron-deficient Lewis acid sites. Thus, we
predict that the enhancement of the catalytic activity for H2
production from NH3BH3 by Ag LSPR is due to the
increasing surface charge density of the Ag nanoparticles
derived from the charge separation effect under light
irradiation.
To confirm that the enhancement in the H2 production is
due to Ag LSPR, NaHCO3 (100 mmol) was added as a positive
charge scavenger to a suspension of Ag/SBA-15 3 under light
irradiation by a red LED lamp (lmax = 650 nm) and a Xe lamp
ꢀ
through a glass filter (l > 420 nm). HCO3 adsorbs highly
onto the Ag NP surface, where it can interact with the
generated H+ (positive charge) on the Ag NPs.[5d] The
enhancement in the H2 production was decreased signifi-
cantly by the presence of NaHCO3 (see the Supporting
Information, Figure S10).[8] The inhibition ratio of catalytic
a glass filter or a band-pass filter, or at a maximal wavelength lmax
=
650 nm (25 mWcmꢀ2) by a red LED lamp, with magnetic stirring at
room temperature (258C). The amount of H2 in the gas phase was
measured by using a Shimadzu GC-8A gas chromatograph equipped
with a MS-5A column.
ꢀ
activity in the presence of HCO3 under light irradiation is
significantly higher than that observed for the reaction in the
dark (see the Supporting Information, Figure S11).[8] Ag NPs
produce cyclical charge separation by wavelength-dependent
collective oscillations of free electrons, resulting in the
formation of a transitory deficient site of electron population
Received: February 26, 2013
Revised: April 10, 2013
Published online: && &&, &&&&
Keywords: mesoporous silica · microwave chemistry ·
(plasmon).[4] It was concluded that HCO3 adsorbed on the
ꢀ
.
nanoparticles · silver · surface plasmon resonance
Ag NPs reacted with the positive charge resulting from charge
separation derived from the Ag LSPR plasma oscillation, and
activation of the NH3BH3 was markedly suppressed.
In summary, size- and color-controlled Ag NPs were
successfully synthesized within the mesopore structure of
SBA-15 by microwave-assisted alcohol reduction. The Ag
NPs exhibited different catalytic activities when used for H2
production from NH3BH3 in the dark; higher catalytic
performance was observed for smaller Ag NPs. In addition,
the catalytic performance was enhanced by light irradiation.
It should be noted that the order of increasing catalytic
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