Photocatalytic reactions under irradiation of visible light over gold nanoparticles supported on titanium(IV) oxide powder prepared by using a multi-step photodeposition method

Article abstract of DOI:10.1039/c4cy00042k

Titanium(IV) oxide (TiO₂) having both smaller and larger gold (Au) particles was successfully prepared by a multi-step (MS) photodeposition method. When 0.25 wt% Au loading per photodeposition was repeated four times, smaller and larger Au particles having average diameters of 1.4 and 13 nm, respectively, were fixed on TiO₂, and the Au/TiO₂ sample exhibited strong photoabsorption around 550 nm due to surface plasmon resonance (SPR) of the larger Au particles. Various Au/TiO₂ samples were prepared by changing the Au loading per photodeposition and the number of photodepositions. Effects of the conditions in MS photodeposition and sample calcination on Au particle distribution and photoabsorption properties were investigated. These samples were used for hydrogen (H₂) formation from 2-propanol and mineralization of acetic acid in aqueous suspensions under irradiation of visible light. In the case of H₂ formation under deaerated conditions, the reaction rate of Au/TiO₂ having both larger and smaller particles was 4 times higher than that of the Au/TiO₂ sample without smaller Au particles, indicating that smaller Au particles acted effectively as a co-catalyst, that is, as reduction sites for H₂ evolution. On the other hand, in the case of mineralization of acetic acid under aerated conditions, carbon dioxide formation rates were independent of the presence of smaller Au particles, indicating that the smaller Au particles had little effect on the mineralization of acetic acid. To extend the possibility of Au/TiO₂ for H₂ formation under irradiation of visible light, H₂ formation from ammonia (NH₃) as biomass waste was examined under deaerated conditions; NH₃ was decomposed to H ₂ and nitrogen with a stoichiometric ratio of 3:1. The Royal Society of Chemistry 2014.

Full text of DOI:10.1039/c4cy00042k

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Photocatalytic reactions under irradiation of
visible light over gold nanoparticles supported
on titanium(IV) oxide powder prepared by using
a multi-step photodeposition method†
Cite this: Catal. Sci. Technol., 2014,
4, 1931
*
Atsuhiro Tanaka, Satoshi Sakaguchi, Keiji Hashimoto and Hiroshi Kominami
Titanium(IV) oxide (TiO2) having both smaller and larger gold (Au) particles was successfully prepared by a
multi-step (MS) photodeposition method. When 0.25 wt% Au loading per photodeposition was repeated
four times, smaller and larger Au particles having average diameters of 1.4 and 13 nm, respectively, were
fixed on TiO2, and the Au/TiO2 sample exhibited strong photoabsorption around 550 nm due to surface
plasmon resonance (SPR) of the larger Au particles. Various Au/TiO2 samples were prepared by changing
the Au loading per photodeposition and the number of photodepositions. Effects of the conditions in MS
photodeposition and sample calcination on Au particle distribution and photoabsorption properties were
investigated. These samples were used for hydrogen (H2) formation from 2-propanol and mineralization
of acetic acid in aqueous suspensions under irradiation of visible light. In the case of H2 formation under
deaerated conditions, the reaction rate of Au/TiO2 having both larger and smaller particles was 4 times
higher than that of the Au/TiO2 sample without smaller Au particles, indicating that smaller Au particles
acted effectively as a co-catalyst, that is, as reduction sites for H2 evolution. On the other hand, in the
case of mineralization of acetic acid under aerated conditions, carbon dioxide formation rates were
independent of the presence of smaller Au particles, indicating that the smaller Au particles had little
effect on the mineralization of acetic acid. To extend the possibility of Au/TiO2 for H2 formation under
irradiation of visible light, H2 formation from ammonia (NH3) as biomass waste was examined under
deaerated conditions; NH3 was decomposed to H2 and nitrogen with a stoichiometric ratio of 3 : 1.
Received 13th January 2014,
Accepted 11th February 2014
DOI: 10.1039/c4cy00042k
www.rsc.org/catalysis
Modification of TiO2 photocatalysts has often been used
to extend their absorption wavelength from the UV region to
Introduction
Various reactions including oxidation of organic compounds,1
reduction of aromatic compounds2 and hydrogen (H2) pro-
duction3 by semiconductor photocatalysts such as titanium(IV)
oxide (TiO2) have attracted much attention. In the case of
H2 production, TiO2 represents a promising technology to use
renewable resources for clean and environmentally friendly
energy production. TiO2 is a wide band gap photocatalyst
(band gap = 3.2 eV) that can induce evolution of H2 from
various compounds under ultraviolet (UV) light irradiation.1–3
However, UV light accounts for only about 5% of the total
solar energy, while visible light accounts for about 50% of the
total solar energy. Therefore, the development of photocatalysts
using visible light is an important topic from a practical point
of view.
the visible region. To the best of our knowledge, these
photocatalysts can be roughly classified into two types. The
first type is TiO2 doped with nitrogen4 and sulfur5 as bulk
modifiers. The energy levels of nitrogen and sulfur are
inserted in the forbidden band of TiO2, resulting in the
response to visible light. The second type is semiconduc-
tors modified with metal ions such as copper ions (Cu2+)
utilizing interface charge transfer (IFCT)6 and TiO2 samples
modified with inorganic sensitizers such as platinum(IV) and
rhodium(III) chlorides.7
Unique optical properties of gold (Au) nanoparticles have
been applied in many fields including biochemistry, sensing
science, and catalysis.8 Nanoparticles of Au show strong
photoabsorption of visible light at around ca. 550 nm due to
surface plasmon resonance (SPR). SPR of Au nanoparticles
has been applied to visible light-responding photocatalysts.9–11
These photocatalysts have been widely used for SPR-induced
photoabsorption in chemical reactions, i.e., oxidation of organic
substrates,10d,e,11a–c,g selective oxidation of aromatic alcohols to
Department of Applied Chemistry, Faculty of Science and Engineering, Kinki
University, Kowakae, Higashiosaka, Osaka 577-8502, Japan.
E-mail: hiro@apch.kindai.ac.jp
† Electronic supplementary information (ESI) available: Fig. S1 and S2. See
DOI: 10.1039/c4cy00042k
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carbonyl compounds,10c,11d,f H2 formation from alcohols,10f,g,11e,h
dioxygen (O2) formation from water in the presence of hexavalent
chromium as an electron scavenger11k and selective reduction
of organic compounds.10h,11j In our previous communication,11e
we described a multi-step (MS) photodeposition method for
the preparation of Au/TiO2 samples, in which photodeposition
of Au was divided into several steps; for example, photo-
deposition of 0.25 wt% Au was repeated four times to obtain a
1.00 wt% Au/TiO2 sample. We reported that Au/TiO2 samples
prepared by the MS photodeposition method (MS-Au/TiO2)
exhibited stronger photoabsorption at around 550 nm due to
SPR and higher levels of activity for H2 production under irra-
diation of visible light. Since MS-Au/TiO2 samples exhibited a
unique bimodal distribution of Au particles, we suggested that
functions of Au/TiO2 could be shared by Au particles with dif-
ferent sizes, i.e., small Au particles as co-catalysts for H2 forma-
tion and large Au particles for photoabsorption and oxidation.
Here, we report 1) precise control of Au particle distribution by
the MS photodeposition method, 2) factors controlling proper-
ties and the photocatalytic activity of Au/TiO2 samples and 3)
the function of Au as a co-catalyst.
a sample prepared by the MS photodeposition method is
designated as Au(X × Y)/TiO2, where X and Y mean the
amount of Au in wt% per photodeposition and the number of
photodepositions, respectively. For example, Au(0.25 × 4)/TiO2
was prepared by repeating 0.25 wt% photodeposition four
times. In this study, the total amount of Au was fixed at
1.0 wt% (X × Y = 1.0).
2.3 Characterization
Diffuse reflectance spectra of Au/TiO2 samples were obtained
using a UV-visible spectrometer (UV-2400, Shimadzu, Kyoto)
equipped with a diffuse reflectance measurement unit (ISR-2000,
Shimadzu). The morphology of Au/TiO2 samples was observed
under a JEOL JEM-3010 transmission electron microscope
(TEM) operated at 300 kV in the Joint Research Center of
Kinki University. The spectrum and light intensity of the Xe
lamp (filtered) were determined using a spectroradiometer
USR-45D (Ushio, Tokyo).
2.4 Photocatalytic activity tests
2. Experimental
2.1 Synthesis of nanocrystalline TiO2
2.4.1 H2 formation from 2-propanol in aqueous suspen-
sions of Au/TiO2 under irradiation of visible light. Dried
photocatalyst powder (50 mg) was suspended in 50 vol%
2-propanol–water solution (5 cm3), bubbled with Ar, and
sealed with a rubber septum. The suspension was irradiated
with visible light from a 500 W xenon (Xe) lamp (Ushio, Tokyo)
filtered with a Y-48 filter (AGC Techno Glass) (450–600 nm;
83 mW cm−2) with magnetic stirring in a water bath continu-
ously kept at 298 K. The amount of H2 in the gas phase was
measured using a Shimadzu GC-8A gas chromatograph
equipped with an MS-5A column. The amount of acetone in
the liquid phase was determined using a Shimadzu GC-14A
gas chromatograph equipped with a fused silica capillary
column (HiCap-CBP20, 25 m, 0.22 mm). Toluene was used as
the internal standard sample. The reaction solution (1 cm3)
was added to a diethyl ether–water mixture (2 : 1 v/v, 3 cm3).
After the mixture had been stirred for 10 min, acetone in the
ether phase was analyzed. The amount of acetone was deter-
mined from the ratio of the peak area of acetone to the peak
area of toluene. In some experiments, another Xe lamp
(LA-410UV, Hayashi Watch Works, Tokyo) was also used to
evaluate the activity of H2 formation under irradiation of visi-
ble light coming from a light source other than the above-
mentioned Xe lamp.
2.4.2 Mineralization of acetic acid in aqueous suspensions
of Au/TiO2 under irradiation of visible light. Dried Au/TiO2
powder (50 mg) was suspended in distilled water (5 cm3) in a
test tube. The aqueous mixture was bubbled with oxygen (O2)
and the test tube was sealed with a rubber septum. Acetic
acid (50 μmol) was injected into the suspension and then
irradiated with visible light from the 500 W Xe lamp (Ushio)
with a Y-48 filter with magnetic stirring in a water bath
continuously kept at 298 K. The amount of carbon dioxide
(CO2) in the gas phase of the reaction mixture was measured
Nanocrystalline TiO2 powder was prepared using the HyCOM
(hydrothermal crystallization in organic medium) method at
573 K.12 Titanium(IV) butoxide and toluene were used as the
starting material and solvent, respectively. The product was
calcined at 723 K for 1 h in a box furnace. The crystallinity of
HyCOM-TiO2 samples was improved by calcination, and the
samples still had a large specific surface area of 97 m2 g−1 even
after calcination at 723 K. Hereafter, a calcined HyCOM-TiO2
sample is designated as TiO2(calcination temperature); for
example, a sample calcined at 723 K is shown as TiO2(723).
2.2 Preparation of Au/TiO2
Loading of Au on TiO2 was performed by a photodeposition
method. Bare TiO2 powder (198 mg) was suspended in 10 cm3
of an aqueous solution of methanol (50 vol%) in a test tube
and the test tube was sealed with a rubber septum under
argon (Ar). An aqueous solution of tetrachloroauric acid (HAuCl4)
was injected into the sealed test tube and then photo-
irradiated at λ > 300 nm by a 400 W high-pressure mercury
arc (Eiko-sha, Osaka, Japan) under Ar with magnetic stirring
in a water bath continuously kept at 298 K. The Au source was
reduced by photogenerated electrons in the conduction band
of TiO2, and the Au metal was deposited on TiO2 particles,
resulting in the formation of Au/TiO2. Analysis of the liquid
phase after each photodeposition revealed that the Au
source had been almost completely (>99.9%) deposited as
Au on the TiO2 particles. The resultant powder was washed
repeatedly with distilled water and then dried at 310 K
overnight under air. In the MS photodeposition method,
photodeposition of Au was repeated several times to obtain
an Au/TiO2 sample having a desired Au content. Hereafter,
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using a gas chromatograph (GC-8A, Shimadzu) equipped with
a Porapak QS column.
2.4.3 H2 formation from ammonia in aqueous suspensions
of Au/TiO2 under irradiation of visible light. Dried photocatalyst
powder (50 mg) was suspended in an aqueous solution
(5 cm3) containing ammonia (NH3, 20 μmol) in a test tube,
bubbled with Ar, and sealed with a rubber septum. The
suspension was irradiated with visible light from the 500 W
Xe lamp (Ushio) filtered with a Y-48 filter with magnetic stir-
ring in a water bath continuously kept at 298 K. The amounts
of H2 and nitrogen (N2) in the gas phase were measured using
a Shimadzu GC-8A gas chromatograph equipped with an
MS-5A column.
Fig. 2 Effect of the number of steps (Y) on the distribution of Au
nanoparticles of Au(X × Y)/TiO2(723) samples (X × Y = 1.0) prepared by
the MS method.
3. Results and discussion
3.1 Characterization of Au/TiO2
Fig. 1 shows TEM photographs of Au(1.0 × 1)/TiO2(723) and
Au(0.25 × 4)/TiO2(723). Small Au particles were observed in
the TEM photograph of Au(1.0 × 1)/TiO2(723) and the aver-
age diameter of Au particles was determined to be 1.2 nm,
suggesting that the one-step photodeposition can be used to
highly disperse Au nanoparticles on the surface of TiO2. On
the other hand, two types of Au particles having different sizes
were observed in the TEM photographs of smaller and larger
Au nanoparticles, although the number of larger particles was
less than that of smaller particles. The average particle sizes
of smaller and larger Au nanoparticles were determined to be
1.4 nm and 13 nm, respectively.
These results indicate that the Au sources added after the
first photodeposition were deposited on Au particles previously
formed on the TiO2 surface, resulting in growth of the Au parti-
cles, and that the distribution of Au particles can be controlled
by Y. The effect of Y on the distribution of Au nanoparticles of
Au/TiO2(723) samples having 1.0 wt% Au was examined in detail
by changing Y from 1 to 10, and the results are shown in Fig. 2,
in which X was simply determined by Y (X = 1.0/Y). The distri-
bution of Au nanoparticles of Au/TiO2(723) samples changed
depending on Y, although the amount of Au loaded on TiO2
was the same (1.0 wt%). The proportion of smaller Au particles
(<5 nm) decreased with the increase in Y, while the proportion
of larger Au particles (>10 nm) increased. Finally, smaller Au
nanoparticles were not observed in Au(0.1 × 10)/TiO2(723).
Absorption spectra of Au(1.0 × 1)/TiO2(723) and Au(0.25 ×
4)/TiO2(723) are shown in Fig. S1† (ref. 11e). Photoabsorption
was observed at 550 nm, which was attributed to the SPR
of the supported Au particles.8–10 It should be noted that
Au(0.25 × 4)/TiO2(723) exhibited much stronger photoabsorption
than Au(1.0 × 1)/TiO2(723), although the amounts of Au
loaded on TiO2 were the same. Fig. 3(a) shows the intensities
of photoabsorption at 550 nm of various Au/TiO2(723) sam-
ples having 1.0 wt% Au. The intensity of photoabsorption of
the samples increased with the increase in Y up to Y = 4 and
tended to be saturated at Y > 4. This result suggests that the
intensity of photoabsorption due to the SPR of Au was affected
by the size of Au nanoparticles. Similar results have been
obtained for plasmonic Au/TiO211e and Au/CeO211f photocatalysts
and for unsupported Au nanoparticles.13 In the last case,
Shimizu et al.13 reported that such observed features coincide
with the prediction of the Mie theory.
3.2 Effect of the number of photodepositions on photocatalytic
H2 formation
Various 1.0 wt% Au/TiO2(723) samples prepared by changing
Y from 1 to 10 were used for formation of H2 from
2-propanol in their aqueous suspensions under visible light
Fig. 3 Effect of the number of steps (Y) of Au(X × Y)/TiO2(723) samples
(X × Y = 1.0) on (a) “1-reflection” at 550 nm and (b) the rate of H2 evo-
lution from 2-propanol in aqueous suspensions under irradiation of
visible light from a Xe lamp with a Y-48 filter.
Fig. 1 TEM images of (a) Au(1.0 × 1)/TiO2(723) and (b) Au(0.25 × 4)/TiO2(723)
samples.
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irradiation from a Xe lamp with a Y-48 filter at 298 K. Just
after irradiation of visible light, H2 was evolved and the for-
mation of H2 continued almost linearly with irradiation time,
indicating that the 1.0 wt% Au/TiO2(723) samples were active
for H2 formation. The rates of H2 evolution are shown in
Fig. 3(b). The reaction rate increased until Y = 6 and the rates
of H2 evolution at Y = 1 and 6 were determined to be 0.15
and 3.1 μmol h−1, respectively, i.e., the Au(0.17 × 6)/TiO2(723)
sample exhibited a rate of H2 evolution that was ca. 21 times
higher than that of the Au(1.0 × 1)/TiO2(723) sample even
though the Au contents were the same. The intensity of SPR
absorption seems to be one of the factors controlling the
activity for H2 production. However, the rates of H2 evolution
decreased at Y = 8 and 10 even though the light intensities of
the samples were the same (Fig. 3(a)). Therefore, this ten-
dency suggests that the presence of smaller Au nanoparticles
as reduction sites was important as well as strong SPR
absorption for H2 evolution over Au/TiO2(723) samples.
Fig. 5 Effects of post-calcination temperature on (a) “1-reflection” at
550 nm of Au(0.25 × 4)/TiO2(723), (b) the rate of H2 evolution from
2-propanol, and (c) the rate of CO2 formation from acetic acid in the
presence of O2.
3.3 Effect of post-calcination
Post-calcination has been widely applied to control the parti-
cle size of Au loaded on various supports. Akita et al.14
reported that the average size of small Au particles (~2 nm)
in Au/TiO2 increased with the increase in calcination temper-
ature (>473 K) and that the small particles disappeared at
temperature >573 K. When Au(0.25 × 4)/TiO2(723) was cal-
cined at 773 K in air, smaller Au particles having an average
diameter of 1.4 nm disappeared (Fig. S2†), suggesting that
these smaller particles merged with larger Au particles.11e
Fig. 4 shows the effect of post-calcination temperature on the
distribution of Au nanoparticles. It can be clearly seen that
the proportion of smaller Au particles (<5 nm) decreased
with the increase in post-calcination temperature and that
Au particles on TiO2 gradually grew with calcination. The
effect of post-calcination temperature on photoabsorption of
Au/TiO2 samples at 550 nm is shown in Fig. 5(a).
of the sample before calcination. Overall, post-calcination of
the Au(0.25 × 4)/TiO2(723) sample had an effect on smaller
Au particles (<5 nm) but not on the SPR intensity. Therefore,
the role of smaller Au particles (<5 nm) in H2 evolution
under irradiation of visible light was investigated by using
Au(0.25 × 4)/TiO2(723) and samples obtained by calcination of
Au(0.25 × 4)/TiO2(723) at various temperatures. Fig. 5(b) shows
the effect of post-calcination temperature on the rate of H2
evolution from 2-propanol in aqueous suspensions of these
samples. The rate of H2 formation decreased with the increase
in post-calcination temperature, although the Au contents
and SPR intensities of these samples were almost the same.
These results suggest that smaller Au particles formed on
TiO2 by using the MS photodeposition method played impor-
tant roles, such as in charge separation, electron storage and
H+ reduction but not in photoabsorption, in H2 formation
under irradiation of visible light.
Interestingly, calcination of the Au(0.25 × 4)/TiO2(723)
sample had almost no effect on the SPR, probably because
further increase in the particle size had little effect on the SPR
Photocatalytic reaction in the presence of O2, i.e., photo-
catalytic mineralization, does not require a co-catalyst because
O2 is a good electron acceptor and one-electron reduction of O2
by electrons in the conduction band of TiO2 is possible without
the aid of a co-catalyst. Therefore, Au(0.25 × 4)/TiO2(723) and
samples obtained by calcination of Au(0.25 × 4)/TiO2(723) at
various temperatures were used for mineralization of acetic
acid in aqueous suspensions in the presence of O2 under irra-
diation of visible light, and the effect of post-calcination on the
rate of CO2 formation is shown in Fig. 5(c). The rates of CO2
formation were almost the same regardless of the temperature
of post-calcination in contrast to the rates of H2 formation
(Fig. 5(b)). The different temperature dependency of CO2 for-
mation indicates that the smaller Au nanoparticles had little
effect on the mineralization of acetic acid in the presence
Fig. 4 Change in the distribution of Au particles by post-calcination of
Au(0.25 × 4)/TiO2(723).
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of O2. Two different dependencies support the idea that the
smaller Au particles formed on TiO2 work as a co-catalyst
rather than as the main photocatalyst in H2 evolution, i.e.,
smaller Au particles have key roles in charge separation,
electron storage and H+ reduction rather than in photo-
absorption, electron injection and substrate oxidation.
Au particles in contrast to the Au(1.0 × 1)/TiO2(Z) samples.
The behavior of the Au(0.25 × 4)/TiO2(Z) samples indicated
that the photoabsorption of Au(0.25 × 4)/TiO2(Z) samples was
determined by the larger Au particles in the range of 13–15 nm.
H2 and acetone were formed in all anatase-type and
anatase–rutile-mixed TiO2 samples, whereas H2 evolution
from rutile-type TiO2 samples was negligible. Since the con-
duction band of rutile-type TiO2 is lower than that of anatase-
type TiO2, the position of the conduction band of the TiO2
support seems to be important for Au/TiO2 in H2 evolution
under the present reaction conditions. In the case of the
Au(1.0 × 1)/TiO2(Z) samples, H2 yield for 10 h reached a
maximum (8.6 μmol) at Z = 1073 and further elevation in the
calcination temperature led to a gradual decrease in the pho-
tocatalytic activity. The yield of H2 seems to be determined
complexly depending on the SBET, crystal structure, particle
size and photoabsorption due to SPR. On the other hand, the
temperature dependency of the rate of Au(0.25 × 4)/TiO2(Z)
samples was simple. The Au(0.25 × 4)/TiO2(723) sample
exhibited the largest H2 yield (28 μmol) probably because the
largest SBET and the smallest average size of Au particles con-
tributed to the adsorption of a large amount of 2-propanol
and efficient H2 evolution on the smaller Au particles, respec-
tively. The ratios of the amount of H2 formation to the amount
of acetone over various Au(X × Y)/TiO2(Z) samples are also
shown in Table 1. Values around unity were obtained in all
samples except rutile-type TiO2 (Z = 1273 K). Good agreement
with yields of H2 and acetone in these samples shows that
the stoichiometric dehydrogenation of 2-propanol occurred
as expressed in eqn (1):
3.4 Effect of pre-calcination temperature
To examine the effects of physical properties and crystal
structure, HyCOM-TiO2 samples calcined at various temper-
atures (Z) [Z = 723–1273 K] were used as supports for Au
particles. With the elevation in calcination temperature, SBET
of TiO2 gradually decreased. Using the MS photodeposition
method, 1.0 wt% Au particles were loaded on TiO2(Z), and
Au(1.0 × 1)/TiO2(Z) and Au(0.25 × 4)/TiO2(Z) samples were pre-
pared. These samples were used for H2 evolution from
2-propanol under irradiation of visible light. Table 1 shows
the yields of H2 in the gas phase and acetone in the liquid
phase after 10 h of irradiation as well as the specific surface
areas of TiO2, crystalline phases of TiO2, intensities of photo-
absorption at 550 nm due to SPR, and average sizes of smaller
and larger Au particles. In the series of Au(1.0 × 1)/TiO2(Z)
samples, only smaller Au particles were observed and the
average size of the smaller Au particles increased with the ele-
vation in calcination temperature. The increase in particle
size can be explained by the decrease in SBET of TiO2 with cal-
cination, i.e., dispersion of Au on TiO2 became worse with the
decrease in SBET of TiO2. The intensity of photoabsorption
due to SPR increased with the elevation in calcination temper-
ature, although the Au contents were the same (1.0 wt%).
These results indicate that photoabsorption due to SPR can
be easily controlled by the calcination temperature of TiO2
before Au photodeposition. In all of the Au(0.25 × 4)/TiO2(Z)
samples, bimodal distributions of Au particles were observed.
The average size of smaller Au particles increased with the
decrease in SBET of TiO2, as in the case of Au(1.0 × 1)/TiO2(Z)
samples, while the average sizes of larger Au particles were
almost the same. Photoabsorption intensities at 550 nm of the
Au(0.25 × 4)/TiO2(Z) samples were almost the same regardless
of the calcination temperature and average size of the smaller
(CH3)2CHOH → (CH3)2CO + H2
(1)
As also clearly shown in Table 1, H2 yields of the Au(0.25 ×
4)/TiO2(Z) samples were 2–19 times higher than those of the
Au(1.0 × 1)/TiO2(Z) samples. Since various factors intricately
affect properties and functions of Au particles and final
properties of Au/TiO2, it is generally difficult to prepare an
Au/TiO2 sample satisfying all properties as a plasmonic
photocatalyst by using a usual photodeposition method
(Y = 1). The results of this study indicate that an Au/TiO2
Table 1 Specific surface areas and crystal structures of various of TiO2 samples and H2 evolution from 2-propanol in aqueous suspensions of
Au/TiO2 under irradiation of visible lighta
Au(1.0 × 1)/TiO2(Z)
Au(0.25 × 4)/TiO2(Z)
b
d
d
SBET
/
Crystal
DAu
Yf h (H2)/ Ygh (Ac)/
DAu
Yf h (H2)/ Yg h (Ac)/
Z/K m2 g−1 structurec (smaller/larger) 1-Re μmol
μmol
Y(H2)/Y(Ac) (smaller/larger) 1-Re μmol
μmol
Y(H2)/Y(Ac)
723 97
823 91
973 54
1073 31
1173 13
1273 2.3
A
A
A
A
A, R
R
1.2/—
1.4/—
2.5/—
3.6/—
4.2/—
5.5/—
0.39 1.5
0.40 2.3
0.42 4.1
0.45 8.6
0.48 2.4
0.55 trace
1.5
2.4
4.2
7.8
2.3
trace
1.0
0.96
0.98
1.1
1.0
—
1.4/13
1.5/13
2.8/14
3.8/14
4.5/14
5.6/15
0.70 28
0.70 27
0.71 24
0.72 20
0.73 5.6
0.72 trace
29
26
23
22
5.7
trace
0.97
1.0
1.0
0.91
0.98
—
a
b
c
d
TiO2 was synthesized by the HyCOM method and calcined at Z (K) shown in parenthesis. BET surface area. A: anatase, R: rutile. DAu
:
e
f
g
h
average size of Au nanoparticles in Au/TiO2. Photoabsorption at 550 nm due to SPR. H2 yield for 10 h. Acetone yield for 10 h. 50 mg of
photocatalyst and visible light in the range of 450–600 nm (83 mW cm−2).
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sample with excellent properties as a plasmonic photocatalyst
can be easily prepared by using the MS photodeposition method.
3.5 Effect of cut-off filters
Fig. 6 shows the effect of cut-off filters set in the Xe lamp on
H2 evolution from 2-propanol in aqueous suspensions of
Au(0.25 × 4)/TiO2(723). Fig. 6 is different from an ordinary action
spectrum, and the results indicate both the effects of the
wavelength of light and the amount of photons. The rate of
H2 formation decreased with the increase in the filter
number, corresponding to the decrease in photoabsorption.
Specifically, a large decrease in the rate was observed when
the O58 cut-off filter was used. In the case of Au/cerium(IV)
oxide, photoabsorption in the range of 600–700 nm mainly
due to light scattering scarcely contributed to the photocata-
lytic reaction.11c Similarly, the large decrease in the rate
observed in the Au(0.25 × 4)/TiO2(723) sample is explained by
the same reason.
Fig. 7 (a) Intensity of visible light irradiated to reaction systems from
a Xe lamp with a Y-44 cut-off filter and effect of light intensity on
(b) the rate of H2 formation from 2-propanol in aqueous suspensions
of Au(1.0 × 1)/TiO2(723) (circles) and Au(0.25 × 4)/TiO2(723) (squares)
and (c) the ratio of the rates.
3.6 Effect of light intensity
Au(1.0 × 1)/TiO2(723) and Au(0.25 × 4)/TiO2(723) samples
were used for H2 formation from 2-propanol in aqueous
suspensions under irradiation by light from a Xe lamp with a
Y-44 filter with various light intensities (3.2–46 mW cm−2) as
shown in Fig. 7(a). Fig. 7(b) shows the effect of light intensity
on the rate of H2 formation. The rates increased with the
increase in light intensity in both samples, and the rates
of Au(0.25 × 4)/TiO2(723) were always higher than those of
Au(1.0 × 1)/TiO2(723). We noted that the light intensity had
different effects on the rate of H2 formation over the two
samples. The ratio of the rates of H2 formation over the two
samples (R4/R1) is shown in Fig. 7(c). The values of R4/R1
decreased with the increase in light intensity, which was
due to the decrease in efficiency of Au(0.25 × 4)/TiO2(723)
(Fig. 7(b)). The decrease in efficiency of photocatalytic reac-
tions under irradiation of intense light is often observed in
semiconductor photocatalysts and is generally explained by
increased electron–hole recombination probability. Similarly,
the decrease in the rates of Au(0.25 × 4)/TiO2(723) under
irradiation of intense light is explained by the same reason.
In other words, the efficiency of Au(0.25 × 4)/TiO2(723) is
more pronounced under irradiation of weaker light.
3.7 H2 formation from NH3
To extend the possibility of Au/TiO2 for H2 formation under
visible light, NH3 as biomass waste in water was used as
a substrate. Fig. 8 shows time courses of the evolution
of H2 and N2 from NH3 in an aqueous suspension of
Au(0.25 × 4)/TiO2(723) under irradiation of visible light. Since
H2 and N2 increased linearly with photoirradiation time, the
rates of H2 and N2 evolution were determined to be 0.84 and
Fig. 8 Time courses of evolution of H2 and N2 from NH3 in an
aqueous suspension of Au(0.25 × 4)/TiO2(723) under irradiation of
visible light from a Xe lamp with a Y-48 filter. After 40 h of irradiation
and evacuation, additional NH3 (20 μmol) was injected and the suspen-
sion was irradiated again.
Fig. 6 Effect of cut-off filter sets in the Xe lamp on H2 evolution from
2-propanol in aqueous suspensions of Au(0.25 × 4)/TiO2(723).
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0.29 μmol h−1, respectively. After photoirradiation for 35 h,
H2 and N2 evolution was almost saturated at around 30 and
10 μmol (H2/N2 ratio of 3.0), which corresponded to the
initial amount of NH3 (20 μmol), indicating that NH3 was
almost completely decomposed to H2 and N2 (eqn (2)) over
Au(0.25 × 4)/TiO2(723) under irradiation of visible light.
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2NH3 → N2 + 3H2
(2)
To evaluate the stability of Au/TiO2 in H2 and N2 pro-
duction from NH3, Au(0.25 × 4)/TiO2(723) was used again.
Irradiation of the reaction mixtures under visible light again
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from 40 h to 80 h without deactivation.
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Acknowledgements
This work was partly supported by a Grant-in-Aid for Scientific
Research (no. 23560935) from the Ministry of Education, Culture,
Sports, Science, and Technology (MEXT) of Japan. The authors
(H. K. and A. T.) are grateful for financial support from the
Iketani Science and Technology Foundation. One of the authors
(A. T.) appreciates the Japan Society for the Promotion of
Science (JSPS) for the Research Fellowship for Young Scientists.
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