Communications
Synthesis of Ag@SiO2: A colloidal dispersion of Ag NPs was
prepared by dropwise addition of a degassed solution of sodium
citrate (0.54 mmol) in water (16 mL) to 400 mL of boiling aqueous
solution containing AgNO3 (0.82 mmol) under vigorous stirring.
After heating to reflux for 1 h under an Ar atmosphere, the reaction
solution was cooled to room temperature. The as-prepared solution of
Ag NPs was centrifuged, and the solid washed with distilled water
several times. Next, a solution of aminopropyltrimethoxysilane
(0.042 mmol) in ethanol (6.0 mL) was added to an aqueous solution
of as-synthesized Ag NPs (1.8 L, 0.46 mm). The mixture was stirred
for 30 min at room temperature, and finally a solution of sodium
silicate (0.07 g) in water (200 mL) was added to allow sol–gel
condensation. After stirring for 24 h, the encapsulated particles
were repeatedly centrifuged and washed with copious distilled water
to give Ag@SiO2.
Again, the use of the Ag NPs without the SiO2 layer
deactivated the photoinduced oxidation ability to a significant
extent. Similar phenomena were observed in the oxidation of
1-methyl-1-cyclohexene.[14] This tendency is consistent with
3
increased phosphorescent emission from the lowest MLCT
state. The efficient quenching of the emission on addition of
O2 indicates that [Ru(bpy)3]2+ efficiently interacts with O2 in
the 3MLCT state. The Stern–Volmer equation (I0/I = 1 +
ksv[Q]) can be used to represent quenching of the emission
by O2, where I0 and I are the intensities of emission in the
absence and presence of O2, respectively, and ksv and [Q] are
the quenching rate constant and the concentration of O2,
respectively. As expected, [Ru(bpy)3]2+/Ag@SiO2 gave a ksv
value of 4.8, which is much larger than that of 1.3 observed for
[Ru(bpy)3]2+/SiO2. This result clearly indicates smooth inter-
action of the 3MLCT state with O2 in the presence of Ag NPs,
and corresponds well to the observed increase in phosphor-
escence emission intensity as well as the photoinduced
oxidation ability. Thus, the present study has unambiguously
demonstrated that interaction with the LSPR of Ag NPs can
enhance the photocatalytic activity of a dye complex anch-
ored to the surface. The importance of the thin SiO2 layer is
also confirmed. It is known that the electromagnetic field
enhanced by LSPR excitation is localized to the surface
between the metal and the dielectric with a roughly expo-
nentially decaying strength in space.[15] However, some
studies proved that the specific distance between metal and
dye play a crucial role for attaining maximum effect of
LSPR.[16] The SiO2 layer therefore not only offers chemical
inertness, transparency, and versatility for conjugation of dye
molecules, but also provides a spacer to limit quenching via
energy transfer with the core Ag NPs, and thus optimizes the
LSPR effect. We also note the importance of using Ag cores.
Some reports have proved that Ag NPs gave much higher
LSPR enhancement effects than Au NPs.[17] One possible
explanation is that the surface plasmon from Au NPs does not
couple to the excitation wavelength of the Ru complex dye,
while the plasmon resonance frequency generated on Ag NPs
may closely match the excitation frequency of the dye to offer
reasonable enhancement.
Synthesis of [Ru(bpy)3]2+/Ag@SiO2: First, Ag@SiO2 was modi-
fied by treatment with 3-(triethoxysilyl)propylsuccinic acid anhydride
(0.19 mmol) in solution in toluene (60 mL) for 24 h. The colloidal
solution was centrifuged, and the solid washed with toluene several
times and dried under vacuum overnight. Next, the obtained material
was treated under aqueous conditions to allow hydrolysis of
anhydride to carboxyl groups. After 3 h, the solution was centrifuged,
and the solid washed with distillated water several times. Finally, the
obtained material was treated with 15 mL of an aqueous solution of
[Ru(bpy)3]Cl2 (0.87 mmol) at room temperature. After 24 h, the
solution was centrifuged, and the solid washed with distilled water
several times to afford [Ru(bpy)3]2+/Ag@SiO2 (0.05 wt% Ru).
Liquid-phase photooxidation: The photocatalyst (0.02 g), a-
methylstyrene (10.0 mmol), and acetonitrile (15 mL) were added to
a quartz reaction vessel (30 mL), which was then sealed with a rubber
septum. The resulting mixture was sonicated and bubbled with
oxygen for 30 min in the dark. Subsequently the sample was
irradiated from the side with an Xe lamp (500 W; SAN-EI ELEC-
TRIC XEF-501S) through a glass filter (l > 400 nm) for 24 h with
magnetic stirring at ambient pressure and temperature. After the
reaction, the resulting solution was centrifuged and the supernatant
was analyzed by GC with an internal standard on a Shimadzu GC-14B
with a flame ionization detector equipped with TC-1 columns. The
turnover number (TON) was determined by the following equation:
TON = (acetophenone [mol])/(Ru atoms on catalyst [mol]).
Received: August 9, 2010
Published online: September 30, 2010
Keywords: nanoparticles · photooxidation · ruthenium · silver ·
.
surface plasmon resonance
In summary, we have prepared a new class of nanosized
photocatalysts composed of core/shell Ag@SiO2 NPs with an
anchored [Ru(bpy)3]2+ dye. The enhanced electromagnetic
field in the vicinity of the Ag NPs due to LSPR significantly
enhances emission intensity of the dye, which ultimately
results in the enhanced catalytic activity for photooxidation
with O2. This work provides a novel pathway to the design of
new photocatalysts enabling selective organic transformation
even under limited light exposure.
[2] a) Homogeneous Photocatalysis, Vol. 2 (Ed.: M. Chanon), Wiley,
New York, 1997; b) A. Maldotti, A. Molinari, R. Amadelli,
18, 4894; c) Y. Horiuchi, M. Shimada, T. Kamegawa, K. Mori, H.
[5] a) M. Ihara, K. Tanaka, K. Sakaki, I. Honma, K. Yamada, J.
[6] Enhancement of photocatalytic activity of TiO2-based materials
with the aid of the LSPR has already been reported, see: a) T.
Experimental Section
Materials: AgNO3 and sodium silicate were purchased from Wako
Pure Chemical Ind., Ltd. Trisodium citrate dehydrate was obtained
from Nakarai Tesque. Aminopropyltrimethoxysilane and [Ru-
(bpy)3]Cl2 were supplied by Sigma-Aldrich. 3-(Triethoxysilyl)propyl-
succinic acid anhydride was purchased from Gelest Solvents and all
commercially available organic compounds for catalytic reactions
were purified by standard procedures.
ꢀ 2010 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2010, 49, 8598 –8601
Mori, Kohsuke
