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NaOH, and then, the suspension was heated at 708C for 1 h. The
particles were washed with distilled water to be dried under vacuum.
At the second step, after the particles were re-dispersed in EtOH
(60 mL) containing NaBH4 (11 mg), the suspension was stirred at
258C for 1 h. The resulting particles were washed with distilled water,
and dried under vacuum (BM-Au/TiO2). The second step was applied
to unmodified TiO2 to prepare small Au NP-loaded TiO2. The mean
size of the Au NPs was determined by transmission electron
microscopy at an applied voltage of 300 kV (JEM-3010, JEOL).
The Au loading amount was quantified by inductively coupled plasma
spectroscopy (ICPS-7500, Shimadzu). Diffuse reflectance UV-Vis-
NIR spectra of the samples were recorded on a Hitachi U-4000
spectrometer mounted with an integrating sphere at room temper-
ature. The reflectance (R1) was recorded with respect to a reference
of BaSO4, and the Kubelka-Munk function [F(R1)] expressing the
relative absorption coefficient was calculated by the equation
F(R1) = (1ꢀR1)2/2R1. X-ray photoelectron spectroscopic (XPS)
measurements were performed using a Kratos Axis Nova X-ray
photoelectron spectrometer with a monochromated Al Ka X-ray
source operated at 15 kVand 10 mAusing C1s as the energy reference
(284.6 eV).
Reduction of nitrobenzene derivatives: The suspension of Au/
TiO2 (10 mg) in a 2-propanol solution of nitrobenzene (10 mm,
10 mL) with KOH (10 mm) was degassed by saturation with argon for
30 minutes in the dark. Irradiation was started using a 300 W Xe lamp
(HX-500, Wacom) with a cut off filter Y-45 (AGC TECHNO
GLASS) in a double jacket type reaction cell (18 mm in diameter
and 180 mm in length). The light intensity integrated from 420 to
485 nm (I420-485) through a Y-45 optical filter was adjusted to
10.0 mWcmꢀ2. The yield and selectivity were determined by high-
performance liquid chromatography (LC-6 AD, SPD-6A, C-R8A
(Shimadzu)) [measurement conditions : column = Shim-pack CLC-
ODS (4.6 mm ꢀ 150 mm) (Shimadzu); MeOH:H2O = 7:3; flow rate =
1.0 mLminꢀ1; l = 240 nm].
Photoelectrochemical measurements: A slurry of Au/TiO2 (0.5 g)
in H2O (1 mL) with acetylacetone (50 mg) and polyethylene glycol
20000 (0.25 g) was coated on ITO-film coated glass substrates (sheet
resistance = 12 W per square), and the samples were heated in air at
773 K for 1 h to form Au/mp-TiO2/ITO electrodes [The mean size of
small Au particles increased from 2.1 to 4.3 nm (Figure S4), which was
still sufficiently smaller as compared to the mean size of large Au
particles (9.0 nm)]. The electrochemical cells comprising of the
photoanode j 5% 2-propanol + 0.1m Na2SO4 (aqueous electrolyte
solution) j photocathode were fabricated. The active area of the cell
was 3.0 ꢀ 2 cm2. Under illumination by a 300 W Xe lamp (HX-500,
Wacom) with a cut off filter Y-45 (AGC TECHNO GLASS), the
current density (nAcmꢀ2) was measured at the dark rest potential by
using a galvanostat/potentiostat (HZ-5000, Hokuto Denko).
CdS photodeposition on BM-Au/TiO2: A 2-propanol suspension
(10 mL) containing BM-Au/TiO2 particles (20 mg), S8 (13.8 mm) and
Cd(ClO4)2 (13.8 mm) was bubbled with argon for 0.5 h in the dark.
Visible-light irradiation was carried out for 24 h using a 300 W Xe
lamp (HX-500, Wacom) with a cut off filter Y-45 (AGC TECHNO
GLASS) in a double jacket type reaction cell (18 mm in diameter and
180 mm in length). The light intensity integrated from 420 to 485 nm
(I420-485) through a Y-45 optical filter was adjusted to 3 mWcmꢀ2. After
irradiation, the particles were recovered by centrifugation, and the
resulting particles were washed with ethanol three times to be dried
under vacuum.
Scheme 1. Reaction mechanism.
weak visible-light absorption of S-Au NPs can be compen-
sated by their cooperation. Nitrobenzene is strongly adsorbed
through the nitro group at the interface between the Au NP
and TiO2.[30] In those sites around L-Au NPs, nitrobenzene
undergoes eight-electron reduction to azobenzene via azox-
ybenzene by their electron pool effect. On the other hand, 2-
propanol is oxidized to acetone on the surface of S-Au NPs.
Recently, selective reduction of nitrobenzene to azobenzene
has been reported by visible-light irradiation of Au/ZrO2 at
intense light intensity (ca. 300 mWcmꢀ2) and 408C.[31] The
reaction mechanism should be different from that in this
system because the LSPR excitation-induced IET from Au
NP to the very higher lying CB(ZrO2) cannot occur.[7]
Consequently, the high visible-light activity of BM-Au/TiO2
is ascribable to the enhancement of charge separation by the
net one-directional electron transport from S- to L-Au NPs.
BM-Au/TiO2 with mean small and large particle sizes of
about 2 nm and 9 nm, respectively, has been prepared by
a two-step route consisting of the deposition-precipitation
and the chemical reduction. Visible-light irradiation (l >
430 nm) of BM-Au/rutile TiO2 induces the net electron
transport from small to large Au NPs through the conduction
band of TiO2. Because of the resulting enhancement in the
charge separation, BM-Au/rutile TiO2 exhibits a very high
level of visible-light activity for the reductions of nitro-
benzene derivatives, whereas UM-Au/rutile TiO2 only has
a low thermocatalytic activity. The corresponding azobenzene
derivatives are produced with conversion greater than 95%
and selectivity greater than 99% at 258C. Meanwhile the
design of the experimental equipment and setup for scaling-
up of the products is practically important, we anticipate that
the present strategy is widely applicable to the plasmon
photocatalysts with various metal and semiconductor compo-
nents, and further open up a new avenue for their applications
to not only oxidative but also reductive syntheses of
important chemical compounds.
Received: March 3, 2014
Revised: April 13, 2014
Published online: May 26, 2014
Experimental Section
Catalyst preparation and characterization: At the first step, L-Au/
TiO2 was prepared by the deposition-precipitation method using
HAuCl4 and urea as a starting material and a neutralizer, respectively.
The post-heating was carried out at 6008C for 4 h. L-Au/TiO2 (2 g)
was added to a 4.86 mm aqueous solution of HAuCl4 neutralized by
Keywords: gold · nanoparticles · photocatalysis ·
.
surface plasmon resonances · visible light
7308
ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2014, 53, 7305 –7309