Non-linear photocatalytic reaction induced by visible-light surface-plasmon resonance absorption of gold nanoparticles loaded on titania particles

Article abstract of DOI:10.1039/c3cc41122b

Nearly second-order (1.87th) light-intensity dependence was observed for photocatalytic oxidation of formic acid in aqueous solutions induced by visible-light surface-plasmon resonance absorption of gold nanoparticles loaded on titanium(iv) oxide prepared

Full text of DOI:10.1039/c3cc41122b

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Non-linear photocatalytic reaction induced by
visible-light surface-plasmon resonance absorption
of gold nanoparticles loaded on titania particles†
Cite this: Chem. Commun., 2013,
4
9, 3419
Received 10th February 2013,
Accepted 5th March 2013
a
a
b
a
Atsuhiro Tanaka, Keiji Hashimoto, Bunsho Ohtani and Hiroshi Kominami*
DOI: 10.1039/c3cc41122b
www.rsc.org/chemcomm
Nearly second-order (1.87th) light-intensity dependence was observed (6 nm) at their nearest corners. Tsuboi et al. reported a two-photon
for photocatalytic oxidation of formic acid in aqueous solutions photochemical reaction (photochromic ring-opening reaction of
induced by visible-light surface-plasmon resonance absorption of gold diarylethene) under irradiation with CW near-infrared light,
4c
nanoparticles loaded on titanium(IV) oxide prepared by the colloid- assisted by gap-mode SPR.
photodeposition method using a hole scavenger, indicating that the
photocatalytic reaction proceeds through a non-linear process.
2
In our previous study, we prepared gold-loaded titania (Au/TiO )
having a sharp size distribution of Au nanoparticles without
agglomeration by using the colloid photodeposition method in
Unique optical properties of gold (Au) nanoparticles have been the presence of a hole scavenger (CPH) and found that Au nano-
applied in many different fields including the fields of biochemistry, particles were successfully loaded on TiO samples having different
2
1
3g
sensing science and catalysis. Au nanoparticles show strong photo- properties without change in the original size of Au. In this study,
absorption of visible light due to surface plasmon resonance (SPR). oxidation of formic acid in aqueous suspensions of Au/TiO with
2
SPR of Au nanoparticles has been applied to a visible light- various amounts of Au loading was examined under irradiation with
À2
responding photocatalyst, and there are some reports on light from a green LED (0.12 to 2.0 mW cm ), which is much
À2
application of SPR-induced photoabsorption to chemical reac- weaker than light from a laser in the order of W cm . We found
2
,3
2d,e,3a,b,d
tions,
including oxidation of organic substrates,
that the reaction order in the present system was 1.87 with respect
to light intensity, indicating that the photocatalytic reaction
2c,3c,f
selective oxidation of aromatic alcohol to a carbonyl compound,
hydrogen formation from alcohols
organic compounds.
2f,3e,h
and selective reduction of occurred via a two-photon process. As far as we know, this is the
2h
first report on a multi-photon photocatalytic reaction induced by
A multi-photon process has recently been suggested to occur SPR of supported Au nanoparticles.
in a photochemical reaction induced by SPR absorption,
although the number of papers in the literature is limited.
No multi-photon process has been reported for a photocatalytic (HAuCl
Colloidal Au nanoparticles were prepared using the method
4
5
3
reported by Frens. To 750 cm of aqueous tetrachloroauric acid
À3
3
4
) solution (0.49 mmol dm ), 100 cm of aqueous
À3
reaction including utilization of SPR. Sundaramurthy et al. solution containing sodium citrate (39 mmol dm ) was added.
reported two-photon polymerization of a photoresist polymer The solution was heated and boiled for 1 h. After the color of the
including epoxide groups (SU-8) on gold arranged into a bowtie solution had changed from deep blue to deep red, the solution
4
a
structure under laser irradiation (wavelength: 800 nm). Ueno was boiled for further 30 min. After cooling the solution to room
3
and Misawa et al. reported two-photon polymerization of SU-8 temperature, Amberlite MB-1 (ORGANO, 60 cm ) was added to
on gold nanoblocks arranged into a checkerboard pattern on a remove excess sodium citrate. After 1 h of treatment, MB-1 was
glass substrate under irradiation with an incoherent continuous removed from the solution using a glass filter.
À2
3
wave (CW) light (intensity: 0.2 W cm , wavelength: 600–1000 nm,
2
TiO powder (Degussa P25) was suspended in 20 cm of
4b
halogen lamp). They concluded that the two-photon absorption aqueous solution of colloidal Au nanoparticles in a test tube
of incoherent light by the resist was due to enhancement of the and the test tube was sealed with a rubber septum under argon (Ar).
electromagnetic field of the incident light by SPR in the nanogaps An aqueous solution (20 cm ) of oxalic acid (50 mmol) was injected
3
into the sealed test tube. The mixture was photoirradiated at
l > 300 nm using a 400 W high-pressure mercury arc (Eiko-sha,
Osaka, Japan) under Ar with magnetic stirring in a water bath
continuously maintained at 298 K. The resultant powder was
washed repeatedly with distilled water and then dried at 310 K
overnight under air. The content of Au was fixed at 0.5 wt% and the
a
Department of Applied Chemistry, Faculty of Science and Engineering,
Kinki University, Kowakae, Higashiosaka, Osaka 577-8502, Japan.
E-mail: hiro@apch.kindai.ac.jp
b
Catalysis Research Center, Hokkaido University, Sapporo 001-0021, Japan
†
Electronic supplementary information (ESI) available: Fig. S1. See DOI: 10.1039/
c3cc41122b
This journal is c The Royal Society of Chemistry 2013
Chem. Commun., 2013, 49, 3419--3421 3419

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scavenger (PH) in which Au particles (an average size: 7.2 nm)
were directly formed by reduction of HAuCl with photogenerated
4
electrons and deposited on the surface of TiO and the spectrum of
2
PH-Au(0.5)/TiO (P25) is also shown in Fig. 2. Fig. 2 clearly shows
2
that the CPH-Au(0.5)/TiO (P25) exhibited stronger photoabsorption
2
2
than PH-Au(0.5)/TiO (P25). Based on spectral simulation using
6
the finite-difference time-domain method, Kazuma et al. reported
that the extinction (= absorption + scattering + reflection) peak
due to SPR of metal nanoparticles was strongly affected by the size
2
Fig. 1 TEM images of (a) colloidal Au nanoparticles and (b) Au(0.5)/TiO (P25)
2
of metals and contact area with the TiO substrate. Difference in
prepared by CPH.
photoabsorption of CPH- and PH-Au(0.5)/TiO (P25) would be
2
explained mainly by the difference in the particle size of Au
nanoparticles.
sample is designated here as Au(0.5)/TiO
loaded on TiO were determined using atomic absorption spectro-
metry after dissolving the Au fixed on TiO with aqua regia. Colloidal
2
(P25). The amounts of Au
2
Photocatalytic oxidation of formic acid in aqueous suspensions
2
of Au(0.5)/TiO (P25) prepared by CPH was examined under irradia-
2
Au nanoparticles were almost quantitatively loaded on TiO when
CPH was used. The morphology of Au particles and Au(0.5)/
2
tion with light from a green LED with various light intensities (0.12
to 2.0 mW cm , Fig. S1, ESI†). In this experiment, a green LED was
À2
2
TiO (P25) was observed under a JEOL JEM-3010 transmission
electron microscope (TEM) operated at 300 kV in the Joint Research
Center of Kinki University.
used as the light source since the light from a green LED matches
SPR-absorption of Au and minimizes various effects other than the
3b,c,f
effect of light intensity.
Fig. 3(a) shows the time course of evolution of CO from formic
Dried Au/TiO
2
powder (50 mg) was suspended in distilled water
2
3
(
5 cm ) in a test tube. The aqueous mixture was bubbled with
acid in aqueous suspensions of Au(0.5)/TiO
with green light of various intensities (0.12–2.0 mW cm ).
Just after the commencement of irradiation, CO was
evolved and formation of CO continued almost linearly with
2
(P25) under irradiation
À2
oxygen (O ) and the test tube was sealed with a rubber septum. An
2
aqueous formic acid solution was injected into the suspension and
then the suspension was irradiated with visible light from a green
LED at 298 K. Spectra and light intensity of the green LED were
determined using a USR-45D spectroradiometer (Ushio, Tokyo).
2
2
irradiation time, indicating pseudo zero-order kinetics of this
reaction with respect to concentration of formic acid. Fig. 3(b)
shows the effect of light intensity on the rate of evolution of CO2
from formic acid. Generally, first-order kinetics, i.e., linear
correlations between the reaction rate and light intensity,
are observed in photochemical and photocatalytic reactions.
The amount of carbon dioxide (CO
the reaction mixture was measured using a gas chromatograph
GC-8A, Shimadzu) equipped with Porapak QS columns.
2
) in the gas phase of
(
Fig. 1(a) shows a TEM image of colloidal Au nanoparticles,
revealing that Au nanoparticles have an average particle size
of 13 nm. Fig. 1(b) shows TEM images of Au(0.5)/TiO (P25)
2
prepared by CPH using Au colloids. Au particles were observed
in the images, indicating that Au nanoparticles were deposited
on the TiO surface by CPH. The average diameter of Au
2
particles of the samples was determined to be 13 nm, which
was in good agreement with the average diameter of original
colloidal Au nanoparticles before Au loading. Fig. 2 shows an
absorption spectrum of Au(0.5)/TiO (P25) prepared by CPH. A
2
strong photoabsorption attributed to SPR of the supported Au
nanoparticles was observed at around 550 nm. For comparison,
we also prepared Au(0.5)/TiO
2
(P25) by using a traditional
photodeposition method in the presence of methanol as a hole
Fig. 3 (a) Time course of evolution of CO
2
from formic acid in aqueous
suspensions of Au(0.5)/TiO
2
(P25) prepared by CPH under irradiation with visible
À2
light (0.44–2.0 mW cm ), (b) effect of light intensity on the rate of evolution of
Fig. 2 Absorption spectra of Au(0.5)/TiO
2
(P25) prepared by CPH (upper) and a
2
CO , and (c) double logarithm plots of the light intensity of visible light and the
reference sample prepared by a traditional photodeposition method (lower).
reaction rate.
3
420 Chem. Commun., 2013, 49, 3419--3421
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distribution, particle density and photoabsorption between
2
Au(0.5)/TiO (P25) prepared by CPH and that prepared by
3
g
PH. At least one of those differences is attributable to the
drastic change in the reaction order with respect to light
intensity. We think that the use of colloidal Au particles and
deposition on the TiO
are important for the non-linear photocatalytic reaction
over CPH-Au(0.5)/TiO (P25) even under weak photoirradiation.
Two-photon absorption induced by SPR of Au nanoparticles in
CPH-Au(0.5)/TiO (P25) might account for the non-linearity,
although we have no clear mechanism for the two-photon
absorption. We are now conducting various control experiments
to clarify the key factor in the non-linear catalytic reaction and to
clarify the possibility of two-photon or step-wise two-photon
2
surface without change in particle size
2
2
Fig. 4 Double logarithm plots of the light intensity of visible light and the
reaction rate of evolution of CO
prepared by PH.
2 2
from formic acid over Au(0.5)/TiO (P25)
Under the conditions of intense light irradiation, the reaction induced photocatalytic reaction.
rate tends to be saturated. Interestingly, the rate in the present
This work was partly supported by a Grant-in-Aid for Scientific
reaction system increased with increase in light intensity, Research (no. 23560935) from the Ministry of Education, Culture,
indicating that the reaction order in the present system Sports, Science, and Technology (MEXT) of Japan and by the
was larger than unity with respect to light intensity. Double Cooperative Research Program of the Catalysis Research Center,
logarithm plots of the light intensity of visible light and the Hokkaido University (grant # 10A0006 and 12B1006).
2
reaction rate of Au(0.5)/TiO (P25) are shown in Fig. 3(c). Linear
correlations between them were observed, and the slope of the
plots that corresponded to the reaction order with respect to
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Chem. Commun., 2013, 49, 3419--3421 3421