Size-dependence of Fermi energy of gold nanoparticles loaded on titanium(iv) dioxide at photostationary state

Article abstract of DOI:10.1039/b809681c

TiO₂ particle-supported Au nanoparticles (NPs) with varying sizes and good contact (Au/TiO₂) were prepared under a constant loading amount by the deposition-precipitation method. The Fermi energy of Au NPs loaded on TiO₂ a

Full text of DOI:10.1039/b809681c

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PAPER
www.rsc.org/pccp | Physical Chemistry Chemical Physics
Size-dependence of Fermi energy of gold nanoparticles loaded
on titanium(IV) dioxide at photostationary state
a
a
b
c
Tomokazu Kiyonaga, Masashi Fujii, Tomoki Akita, Hisayoshi Kobayashi and
Hiroaki Tada*a
Received 9th June 2008, Accepted 31st July 2008
First published as an Advance Article on the web 24th September 2008
DOI: 10.1039/b809681c
TiO particle-supported Au nanoparticles (NPs) with varying sizes and good contact (Au/TiO )
2
2
were prepared under a constant loading amount by the deposition-precipitation method. The
0
Fermi energy of Au NPs loaded on TiO
2
at the photostationary state (E
F
) was determined in
0
2ꢀ
water by the use of S/S having specific interaction with Au as a redox probe. The E value
F
goes up as the mean size of Au NPs (d) increases at 3.0 r d r 13 nm. The photocatalytic
0
activities for the Au/TiO
2
-photocatalyzed reductions of 2,2 -dipyridyl disulfide and nitrobenzene
2
increase with increasing d. The photoluminescence spectra for Au/TiO and their dynamic
analysis indicated that the efficiency of the interfacial electron transfer from TiO to Au NPs
2
increases as the result of an increase in d. Density functional theoretical calculations for (Au)10
and (Au)22 model clusters showed that negatively charged Au clusters are greatly stabilized by
0
water solvation, of which energy is smaller for (Au)22. The d-dependence of E
F
could be
rationalized in terms of both the increase in the efficiency of the photoinduced electron transfer
and the strong solvation of charged Au NPs by water. The parallel correlation between the
0
0
E
F
and photocatalytic activity proves for E
F
to be a very important parameter for designing
metal NP-loaded semiconductor photocatalysts.
These are good examples of the so-called ‘‘reasonable delivery
1
. Introduction
11
photocatalytic reactions systems (RDPRS)’’, which satisfy the
following three conditions for effecting high efficiency and
selectivity: (i) separation of oxidation and reduction sites; (ii)
abundant and selective supply of oxidants and reductants to
The development of green processes for decomposing harmful
compounds and producing useful ones are crucial subjects im-
posed on chemistry. On the premise that sunlight or illuminating
light is used as a light source, semiconductor-photocatalyzed
reactions can be a promising approach to achieve them. It is well
known that the photocatalytic activity significantly increases with
reduction sites (metal surface) and oxidation sites (TiO
2
surface),
respectively; and (iii) restriction of the product readsorption. In
this case, if the Fermi energy of the metal NPs loaded on TiO
0
2
at
) can be determined, it should be
1,2
loading nanometer-sized noble metal particles. Essentially, this
results from the increase in the charge separation efficiency due to
the photostationary state (E
F
very useful to design photocatalytic organic synthesis. Since the
0
3
,4
the interfacial electron transfer from semiconductors to metals.
Among the semiconductor photocatalysts, much attention has so
far been focused on TiO because of its strong oxidation power of
EF dictates the photovoltage in dye-sensitized solar cells (DSSC),
12
it is of great importance also in DSSC. In the pioneering work
2
on n-TiO electrodes covered with discontinuous Au films, Nakato
2
the holes photogenerated in the valence band (v.b.) to oxidize
5
most toxic organic compounds. Recently, the loading of Au
2
et al. showed that the photopotential of n-TiO is not in agreement
13
with that of Au, while both the potentials shift cathodically.
This study provides important information about the action
mechanism of the metal loading on semiconductor photocatalysts.
However, most of the photocatalytic reactions were carried out in
particulate systems. Recently, the research group of Kamat has
nanoparticles (NPs) on mesoporous TiO2 has been shown to
enhance the photocatalytic activity for the decomposition of
6
phenol. On the other hand, the applications to organic synthesis
are highly expected by taking advantage of the mild reducing
7
power of the excited electrons in the conduction band (c.b.).
We have reported the drastic enhancement of the
2
determined the Fermi energy of irradiated Au/TiO particles in
ꢀ
14
organic solvent using a C60/C60 redox pair. Hereafter, organic
synthesis using water as a green solvent will increasingly become
important from a viewpoint of environmental conservation.
8
,9
10
TiO
2
-photocatalyzed reductions of disulfides and nitrobenzene
2
(Au/TiO and Ag/TiO ).
with loading Au and Ag NPs on TiO
2
2
0
To our knowledge, the method for determining EF in water has
a
Department of Applied Chemistry, School of Science and
Engineering, Kinki University, 3-4-1, Kowakae, Higashi-Osaka,
Osaka, 577-8502, Japan. E-mail: h-tada@apch.kindai.ac.jp;
Fax: + 81-6-6727-2024; Tel: + 81-6-6721-2332
National Institute of Advanced Industrial Science and Technology,
Midorigaoka 1-8-31, Ikeda, Osaka, 563-8577, Japan
not yet been presented.
0
In this study, the E
F
value of Au/TiO
2
in water has been
2
determined using a S/S redox pair having a specific inter-
ꢀ
b
c
15
action with Au NPs.
dependences of the E
Au/TiO
Further, the Au particle-size
and photocatalytic activity of
0
Department of Chemistry and Materials Technology, Kyoto Institute
of Technology, Matsugasaki, Sakyo-ku, Kyoto, 606-8585, Japan
F
2
were examined, and the correlation was discussed
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based on the experimental and density functional theoretical
DFT) calculation results.
temperature of the suspension was kept at 298 K by circulating
thermostatted water through an outer jacket around the cell.
The concentrations of RSSR consumed and 2-mercaptopyridine
(
(
RSH) generated were determined from the absorbances at
3
2
. Experimental
4
ꢀ1
ꢀ1
2
81 nm (emax = 1.05 ꢂ 10 mol dm cm ) and 342 nm
3 ꢀ1 3 ꢀ1
(
emax = 7.18 ꢂ 10 mol dm cm ), respectively.
TiO or Au/TiO
particles (50 mg) were added to a 1.1 ꢂ
mol dm nitrobenzene (NB) solution (50 mL, solvent =
O:acetonitrile = 99:1 v/v). The suspension was bubbled with
argon for 20 min in the dark (pH 6.8), and then irradiation
l 4 300 nm) started using a high-pressure mercury lamp
H-400P, Toshiba): the light intensity integrated from 310 to
2
.1 Materials and characterization
2
2
Au particles were loaded on TiO particles with a crystal form of
2
ꢀ3
0
ꢀ3
1
2
ꢀ1
anatase and a specific surface area of 8.1 m g (A-100, Ishihara
Sangyo) by the deposition-precipitation (DP) method using
H
2
16
4
HAuCl as a raw material. The specific surface areas of the
(
metal oxides and the mean diameters of the metal clusters (d)
were determined by the Brunauer–Emmett–Teller method and
transmission electron microscopy at an applied voltage of 300 kV
(
ꢀ2
4
00 nm was measured to be 4.0 mW cm . The argon bubbling
was continued throughout the reaction. The cell was kept at 298 K
by circulating thermostatted water through an outer jacket around
the cell. For the quantification of the reactant and product, the
irradiated solution was analyzed by high-performance liquid
chromatography (LC-6AD, SPD-6A, C-R8A (Shimadzu))
(JEM-3010, JEOL), respectively. The amounts of the metal
clusters were quantified by inductively coupled plasma spectro-
scopy (ICPS-7500, Shimadzu). The surface areas of the metal
clusters were calculated from their amounts and d values.
[
measurement conditions: column = Shim-pack CLC-ODS
2
.2 Photoinduced sulfur desorption
4
.6 ꢂ 150 mm; mobile phase H O–CH OH (3/7 v/v); flow rate =
2
3
ꢀ
1
Prior to the sulfur desorption experiments, sulfur was adsorbed on
3
Au/TiO by adding the particles (3.0 g) to an 0.97 mmol dm S8
0
.5 mL min ; l = 270 nm].
ꢀ
2
solution (0.5 L) and stirring for 24 h at 298 K. The particles were
then separated from the solution by centrifugation, washed with
2.4 Electronic absorption and photoluminescence (PL)
spectroscopic measurements
water three times, and dried in vacuum (S–Au/TiO
2
). After the
The electronic absorption spectra of TiO
2 2
and Au/TiO
aqueous suspension of S–Au/TiO had been purged with argon
2
particles with varying d values were recorded by the diffuse
reflectance method using an integral sphere on a Hitachi
for 15 min, irradiation was started using a high-pressure mercury
lamp (H-400P, Toshiba) in a double jacket-type reaction cell
ꢀ1
U-4000 spectrophotometer at a scan speed of 60 nm min
(31 mm in diameter and 175 mm in length, transparent to light
in the 300–800 nm range. The PL spectra of TiO
particles were measured with an excitation wavelength of
00 or 340 nm at room temperature or 77 K using a JASCO
2 2
and Au/TiO
with l 4 300 nm). Since S hardly absorbs light of l 4 300 nm,
8
Au/TiO is almost selectively excited under the conditions. The pH
2
3
value of the suspension was controlled at 13 by adding
ꢀ
FP-6500 spectrofluorometer.
3
.02 mol dm NaOH solutions. The light intensity integrated
0
ꢀ2
from 310 to 400 nm (I310–400) was measured to be 4.0 mW cm by
the use of a digital radiometer (UM-360, Konica Minolta). The
temperature of the suspension was kept at 298 K by circulating
thermostatted water through an outer jacket around the cell. After
2.5 DFT calculations
A computer simulation based on the DFT method was carried
1
out using the Gaussian 98 program. The hybrid Becke–
7
1
8,19
three-parameter–Lee–Yang–Parr functional was adopted,
2
which includes the gradient corrected exchange
the sulfurs adsorbed on Au/TiO
2
before and after irradiation
0
2ꢀ
and
(0.25 g) had been completely oxidized to SO
4
ions by adding a
ꢀ3
2
1
ꢀ3
correlation functionals together with the local functionals
and the Hartree–Fock exchange operator. The Los Alamos
effective core potential and the corresponding valence basis set
1
6% H
30 mL) and stirring for 16 h, they were quantified by ion
chromatography (PIA-1000, Shimadzu). In this analysis, a mixed
2
O
2
solution and 1.15 ꢂ 10 mol dm phosphoric acid
(
2
2,23
ꢀ3
were used for Au.
The solvation effects of water molecules
solvent consisting of 12 mmol dm p-hydroxybenzoic acid and
00
ꢀ3
0
were considered by the polarized continuum model (PCM) in
the framework of self-consistent reaction field (SCRF)
4
.0 mmol dm 2,2-bis(hydroxymethyl)-2,2 ,2 -nitrilotriethanol
were used as a mobile phase.
2
4
theory. (Au)10 and (Au)22 clusters were truncated from the
111) surface, and model the corner sites of micro particles.
(
2
.3 Photocatalytic reactions
The total energies were evaluated for the charges of +1 to ꢀ10
and +1 to ꢀ11 for (Au) and (Au) , respectively, in vacuum
ꢀ
5
ꢀ3
mol dm
The photoreaction solution of 5.4
0
ꢂ
10
,2 -dipyridyl disulfide (RSSR) was prepared by diluting an
1
0
22
2
and in water (with the PCM).
acetonitrile solution with H O (H
2
2
O:acetonitrile = 99:1 v/v).
or Au/TiO (50 mg) had
After the suspension (50 mL) of TiO
2
2
been purged with Ar for 15 min, irradiation was started using a
high-pressure mercury lamp (H-400P, Toshiba) in a double
jacket type reaction cell (31 mm in diameter and 175 mm in
length, transparent to light with l 4 300 nm). The light intensity
integrated from 310 to 400 nm (I310–400) was measured to be
3. Results and discussion
3
.1 Characterization of Au/TiO2
Generally, the photocatalytic activity of metal-loaded semi-
conductor photocatalysts depends on the metal loading
ꢀ
2
25,26
amount.
4.0 mW cm by the use of a digital radiometer (UM-360,
In order to examine the metal particle-size
Konica Minolta). Ar bubbling and magnetic stirring of the
suspension were continued throughout the irradiation. The
dependence of the photocatalytic activity, therefore, it is
necessary to vary the metal particle size with its loading
6
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amount maintained constant. For this purpose, Au NPs were
incorporated on TiO by the deposition–precipitation method
where both heating temperature (T ) and time (t ) were
2
c
c
2
7
altered. Notice here that changing T causes the variation
c
in the crystallinity and the number of oxygen-vacancy of TiO2
to affect its photocatalytic activity. Hence, all the TiO2
particles were heated at 923 K for 4 h prior to the use for
the preparation of Au/TiO
changes of TiO with different T
TEM images (left) and Au particle size distribution (right)
of Au/TiO prepared under different heating conditions:
= 673 K, t = 1 h; (B) T = 673 K, t = 4 h;
= 873 K, t = 4 h. In every sample, Au NPs highly
2
, which allows us to neglect the
2
c
below 873 K. Fig. 1 shows
2
(
A) T
C) T
c
c
c
c
(
c
c
dispersed on the TiO surface have a fairly sharp distribution.
2
The mean size of Au NPs (d) increases from 4.2 to 13 nm as a
result of an increase in T from 673 to 873 K, while the loading
amount remains almost constant (0.43–0.46 mass%). To check
c
the state of the Au–TiO
single crystal were observed by high-resolution TEM
HRTEM). Fig. 1D is a typical HRTEM image of Au
NP (0.44 mass%). The Au particle is supported on the
TiO {101} surface with an orientation relationship of
Au{111}//TiO {101}. Most Au NPs are single crystals with
2 2
junction, Au NPs loaded on a TiO
(
2
2
the fcc structure surrounded by the low index (111) and (100)
planes, as predicted by the Wulf construction. The truncated
shape Au particle is in good contact with TiO at an atomic
2
level, having a large interfacial area because of the low
interfacial energy between Au and TiO . Large (111) facets
2
with the smallest surface energy in the fcc crystal structure are
usually observed. Thus, Au NPs with varying sizes and good
contact can be formed on the TiO
2
surface under a constant
loading amount by this DP method.
3
.2 Adsorption and photoinduced desorption of sulfur on/from
Au/TiO
2
The Au NP-loading effect on the adsorption properties of
elemental sulfur (S ) was examined. Fig. 2A shows the adsorption
isotherms of S from ethanol solutions on TiO (a) and
Au(d = 3.2 nm)/TiO (b) at 298 K. A drastic increase in the
adsorption amount of S is observed as a result of such a small
amount loading of Au. The adsorption selectivity defined as
(Au)/{n (TiO ) + n (Au)} reaches as much as 99.6%, where
(Au) and n (TiO ) are the saturated adsorption amounts on the
Au and TiO surfaces, respectively. Evidently, S is highly
8
Fig. 1 TEM images of Au/TiO2 particles prepared under various
8
2
heating conditions (left) and their size distribution of the Au particles
(right): (A) T
73 K, t = 4 h, and an HRTEM image of a Au nanoparticle loaded
on TiO (D).
c c c c c
= 673 K, t = 1 h; (B) T = 673 K, t = 4 h; (C) T =
2
8
c
8
2
n
s
s
2
s
continuously moving throughout the reaction vessel due to
stirring and convection under the present conditions, part of the
n
s
s
2
2
8
2ꢀ
sulfur desorbed as S ions would be readsorbed on the Au
selectively adsorbed on the Au(111) surfaces of Au NPs. Upon
irradiation to the Au/TiO2 in a saturated adsorption state
29
surfaces of Au/TiO , below this region. Assuming that the rates
2
2
ꢀ
S–Au/TiO ), part of the sulfur is efficiently reduced to S to
2
of sulfur desorption and readsorption are equal at the photo-
(
30
15
stationary state, one can obtain eqn (1) relating X to tp.
be desorbed into water at room temperature. The ratio of sulfur
desorbed from S–Au/TiO dispersed in water was measured for
2
X(t ) = [k /(k + k )][1 ꢀ exp{ꢀ(k + k )t }]
(1)
the samples having different d values. Fig. 2B shows the sulfur
desorption ratio (X = desorbed sulfur/initially adsorbed sulfur)
p
d
a
d
a
d p
where k and k express the rate constants of sulfur desorption
d
as a function of irradiation time (t
abruptly increases on irradiation, reaching a constant value at t
20 min. When the suspension containing 0.2 vol% TiO
p
). In every sample, the X value
a
and readsorption, respectively.
As shown in Fig. 2B, the theoretical curves calculated by the
least square method with eqn (1) fit well with the experimental
p
4
2
particles was irradiated from the upper side of the reaction vessel,
light absorption by TiO particles is limited to approximately
s d a d
data. The saturated desorption ratio (X ) given by k /(k + k )
2
increases with increasing d. A similar trend was observed also for
1
in the UV region is about 10 cm
mm of the suspension because the absorption coefficient of TiO
ꢀ1 28
2
31
4
the Pt NP-loaded TiO
2
particle system at 1.1 r d r 9.2 nm.
.
Since Au/TiO particles are
2
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Fig. 3 (A) Cyclic voltammograms of TiO
2 2 2 2
/SnO (a) and Au/TiO /SnO
ꢀ3
for starting from anodic (b) and cathodic (c) scan in a 0.5 mmol dm
ꢀ
3
Na
2
S solution containing 0.1 mol dm
NaClO
0
F
vs. d at pH 13: light intensity integrated
4
as a supporting
electrolyte. (B) Plots of E
ꢀ2
from 310 to 400 nm (I310–400) = 4.0 mW cm
.
3
2
formation of S-multilayers. On reversing the sweep direction
curve (c)), only a reduction wave corresponding to the reduction
(
2ꢀ
33
of S to S ions is observed at ꢀ1.0 V with the absence of the
reduction peak at ꢀ0.66 V due to the reduction of S-multilayers to
2ꢀ
S (see curve (b)). These results clearly indicate that S ions are
adsorbed on the Au NP surfaces as S in the dark, which was
34
Fig. 2 (A) Adsorption isotherms of S
8
2 2
on TiO (a) and Au/TiO (b) at
previously confirmed by X-ray photoelectron spectroscopy.
Thus, the present reaction at the photostationary state can be
expressed by eqn (2).
298 K. (B) Time courses for the photoinduced sulfur desorption
from S–Au/TiO : (a) d = 4.2 nm, (b) d = 6.8 nm, (c) d = 8.1 nm,
2
(d) d = 12 nm.
ꢀ
2ꢀ
S ꢃ ꢃ ꢃAu/TiO + 2e # S
(2)
ad
2
0
3
.3 Determination of E
F
The application of Nernst’s equation to eqn (2) yields eqn (3)
0
2ꢀ
0
relating EF to X . It should be noted here that EF provides
Redox properties of S ions on Au NP-loaded TiO
on F doped-SnO electrodes (Au/TiO /SnO ) were studied to
specify the adsorption state of S ions on the Au NP surfaces
of Au/TiO using cyclic voltammetry. Fig. 3A shows cyclic
voltammograms of TiO /SnO (a) and Au/TiO /SnO (b) in a
S solution containing 0.1 mol dm NaClO
Two pairs of redox waves are observed for Au/TiO /SnO (curves
b) and (c)), whereas TiO /SnO is almost inactive in the potentials
2
films formed
s
the Fermi energy of Au NPs loaded on TiO because sulfur is
2
2
2
2
2ꢀ
selectively adsorbed (and desorbed) on (and from) the Au NP
surface.
2
2
2
2
2
0
E
F
= E
0
+ (RT/2F)ln[(1 ꢀ X
s
)/X
s
]
(3)
ꢀ
3
ꢀ3
0.5 mmol dm Na
2
4
.
2ꢀ
2
2
where E is the standard redox potential of S/S (ꢀ0.447 V
0
35
(
vs. SHE).
Fig. 3B shows EF calculated from eqn (3) (left axis) and
scaled from vacuum level (right axis) as a function of d.
2
2
0
(
U/V vs. Ag/AgCl) ranging from ꢀ1.0 to + 0.3 V (curve (a)).
Apparently, the Au NPs act as a good electrocatalyst for the redox
reactions of sulfur. When the potential is scanned from the rest
potential (ꢀ0.50 V) toward the anodic direction first (curve (b)),
an oxidation wave appears at ꢀ0.11 V corresponding to the
0
Interestingly, EF rises as the d value increases at 3.0 r d r
13 nm. Note here that the EF value should also depend on the
irradiation conditions, i.e., the light wavelength and intensity.
0
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3
.4 Au particle-size dependence of the photocatalytic activity
absorption of NB centered at 270 nm. Since isosbestic points
are situated at 210 nm, 220 nm and 245 nm, the 6-electron
reduction of NB to AN proceeds almost selectively (eqn (5)).
of Au/TiO
2
2
As a first test reaction, the Au/TiO -photocatalyzed reduction of
9
RSSR to RSH, a typical RDPRS, was carried out. Fig. 4A
shows the electronic absorption spectral change of the reaction
Au=TiO ;hv
f-NO2 þ 6H þ 6eꢀ ƒƒƒƒ ƒƒ ! f-NH2 þ 2H2O
þ
2
ð5Þ
2
solutions under UV-light irradiation in the presence of Au/TiO .
Fig. 4D shows the turnover frequency (TOF) for the Au/TiO -
2
With irradiation, a new absorption centered at 342 nm of RSH
appears, while the absorption band with a peak at 233 nm of
RSSR weakens. Also, the presence of isosbestic points at 218,
photocatalyzed reduction of NB as a function of d (3.0 o d o
ꢀ3
1
3 nm): [CH OH] = 100 mmol dm . On the assumption that
3
the surface Au atom is the reduction site, the TOF was
calculated from the loading amount and d value. Au
NP-loading gives rise to a drastic increase in the photocatalytic
activity, whereas no reduction occurs without Au NPs (d = 0).
Also, the TOF increases as the d value increases. The profiles of
the d-dependences of the photocatalytic activities (Fig. 4B and
247, and 295 nm indicates that this reaction almost selectively
proceeds (eqn (4)).
Au=TiO2;hv
RSSR þ 2H þ 2eꢀ ƒƒƒƒ ƒƒ ! 2RSH
þ
ð4Þ
Further, Au particle-size effect on the photocatalytic activity for
the RSSR reduction was studied. Fig. 4B shows plots of the
0
D) well resemble that of the EF vs. d plots (Fig. 3B). Thus, the
increase in the activity for the Au/TiO -photocatalyzed reduc-
2
initial rate of reaction (v
expresses that for the TiO
significantly increases as a result of the Au NP loading. The v
0
) vs. d, where the v
0
value for d = 0
0
tion with increasing d can be attributed to the increase in E ,
F
2
system. The photocatalytic activity
i.e., the increase in the electronic potential of Au NPs coupled
0
2
with TiO .
value increases with increasing d in spite of the decrease in the
Au surface area.
3
.5 Au particle-size dependence of the photoluminescence
2
Au/TiO -photocatalyzed reduction of nitrobenzene (NB)
intensity of Au/TiO
2
also belonging to RDPRS was performed as a second test
1
reaction. The addition of CH OH as a hole scavenger causes
0
Photoluminescence (PL) measurements provide valuable informa-
tion on the interfacial electron transfer in the metal-semiconductor
3
the NB reduction to proceed, whereas no reduction occurred in
3
6
water for both the TiO
electronic absorption spectral change of the reaction solution
with UV-light irradiation in the presence of Au/TiO : [CH OH]
100 mmol dm . Irradiation yields a new absorption around
30 nm due to aniline (AN), accompanied by a decrease in the
2
and Au/TiO
2
systems. Fig. 4C shows
coupled systems. Fig. 5A shows electronic absorption spectra
(left) and PL spectra (right) with an excitation wavelength of
0
0
2
3
300 nm at room temperature for TiO
0
2
(a, a ), Au(d = 4.2 nm)/
(c, c ). In the absorption
ꢀ
3
=
TiO
2
(b, b ), and Au(d = 13 nm)/TiO
2
2
spectra, Au/TiO
2
has a broad absorption around 550 nm due to
ꢀ
5
ꢀ3
ꢀ2
Fig. 4 (A) Electronic absorption spectral change of a 5.4 ꢂ 10 mol dm RSSR solution with irradiation (l 4 300 nm, I310–400 = 4.0 mW cm ).
ꢀ3
ꢀ3
(
B) Plots of v
0
vs. d for the Au/TiO
2
-photocatalyzed RSSR reduction. (C) Electronic absorption spectral change of a 1.1 ꢂ 10 mol dm NB
ꢀ2
solution with irradiation (l 4 300 nm, I310–400 = 4.0 mW cm ). (D) Plots of TOF vs. d for the Au/TiO
2
-photocatalyzed NB reduction.
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the surface plasmon resonance of Au NPs along with a strong
absorption band below 385 nm due to the interband transition of
TiO . Also, the intensity of the Au surface plasmon band for
2
Au(d = 13 nm)/TiO is greater than that for Au(d = 4.2 nm)/
2
37
TiO2. On the other hand, in the PL spectrum of TiO , a broad
2
emission band is present in the range from 380 to 700 nm. The
main band at 424 nm (B
Also, a sharp band at 470 nm (B
from (oxygen) vacancy levels. Further, a band at 575 nm (B
result from impurity levels because pure TiO does not have this
PL band. There are two kinds of electron-trap sites in the TiO
particles used in this study. When Au NPs are loaded on TiO , the
1
) can be assigned to the band-band PL.
) is attributed to the emission
) may
2
3
2
38
2
2
relative PL intensity decreases, although the spectral profile is
invariant. A similar trend has recently been reported also for the
39
Ag/TiO particles. Interestingly, the decreasing degree of the
2
PL intensity for Au(d = 13 nm)/TiO is greater than that for
2
Au(d = 4.2 nm)/TiO . The PL intensity is thought to decrease
2
with the Au-NP loading because of the reduction in the recombi-
2
nation due to the interfacial electron transfer from TiO to Au.
Further, the efficiency of the interfacial electron transfer is
suggested to increase as the d value increases in the range below
1
3 nm.
Fig. 5B shows PL intensity at 530 nm (I) vs. time curves for
TiO (a), Au(d = 4.2 nm)/TiO (b), and Au(d = 13 nm)/TiO
c) with an excitation wavelength of 340 nm at 77 K. Each data
2
2
2
(
can be well fitted by a double-exponential function (eqn (6)),
and Table 1 summarizes the fitting parameters.
I(t) = c
1
exp(ꢀt/t
1
) + c
2
exp(ꢀt/t
2
)
(6)
where c
1
(c
2
) and t (t ) are the weight and life-time of the
1
2
longer-life (the shorter-life) component, respectively.
As a result of Au-NP loading on TiO , two significant
changes are apparent. One is the increase in the ratio of c
to c , and another is the decreases in t and t . Also, these
2
2
1
1
2
tendencies are more pronounced in Au(d = 13 nm)/TiO2.
Fig. 5C shows delayed PL spectra of TiO2 (a), Au(d =
4.2 nm)/TiO2 (b), and Au(d = 13 nm)/TiO2 (c) with an
excitation wavelength of 340 nm: delayed time = 25 ms. Each
spectrum has a broad PL band at l 4 450 nm, of which peak
position is close to that of B
3
in Fig. 5A. The direct electron-
hole recombination is a ultrafast event occurring at times
4
0
1
2 3
ps. Accordingly, B and B are tentatively assignable to
the shorter-life PL from shallow trap sites at ca. 0.6 eV below
the c.b. and the longer-life PL from deep trap sites at ca. 1.0 eV
below the c.b., respectively. A new path of the excited electron
Fig. 5 (A) Electronic absorption spectra (left) and PL spectra (right) of
TiO
2
2 2
(a), Au(d = 4.2 nm)/TiO (b) and Au(d = 13 nm)/TiO
(
c): measurement temperature = 298 K. (B) Decay curves of PL at
transfer from TiO to Au explains the decrease in the PL life
2
2 2 2
530 nm TiO (a), Au(d = 4.2 nm)/TiO (b) and Au(d = 13 nm)/TiO
time with the Au-NP loading. In this manner, the conclusion
can be drawn also from the kinetic analysis of the PL spectra
that the efficiency of the interfacial electron transfer from TiO2
(
(
c): measurement temperature = 77 K. (C) Delayed PL spectra of TiO
2
2 2
a), Au(d = 4.2 nm)/TiO (b) and Au(d = 13 nm)/TiO (c) measured at
77 K: delay time = 25 ms, measurement temperature = 77 K.
to Au increases as the d value increases.. This is further
0
consistent with the experimental results that the E
F
value
increases to improve the photocatalytic activity for the
Au/TiO -photocatalzyed reduction with increasing d.
Table 1 Kinetic parameters for the decay of excited TiO
Au/TiO
2
2
and
2
TiO
737
4.43
199
26.5
2
Au(4.2 nm)/TiO
2
Au(13.1 nm)/TiO
2
3
.6 Theoretical considerations to the Au particle-size
0
c1
680
2.27
267
17.6
664
dependence of E
F 2
for Au/TiO
t
1
c
2
t
2
/ms
/ms
0.93
282
10.0
Downsizing the metal particle at a given loading amount
necessarily increases its number density. It was previously
6
558 | Phys. Chem. Chem. Phys., 2008, 10, 6553–6561
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shown by dissolving 2D-Poisson’s equation for metal particle-
loaded TiO that the metal NPs produce the electric filed at the
metal/TiO2 interface to enhance the charge separation.
than the flat terrace sites, a popular Au10 cluster is used upset to
represent a defect site (Fig. 6a). To examine the size depen-
dency, a larger Au22 cluster is made by adding twelve Au atoms
as a third layer to the upset Au10 cluster (Fig. 6b). Fig. 7A
shows the n-dependence of m in vacuum for (Au)10 (a) and
2
4
1,42
However, when too many metal particles are loaded on a
single TiO2 particle, the charge separation effect is rather
vitiated due to the overlap of the interfacial electric field. This
reasonably explains the conclusion drawn from the PL
measurements that the efficiency of the interfacial electron
2
transfer from TiO to Au NPs increases with increasing d at
3
.0 r d r 13 nm.
In order to obtain general understanding of the
0
d-dependence of E , the chemical potential (m) for model
Au clusters was calculated from eqn (7) by the DFT
F
calculations as a function of excess electron number n.
m = ꢀ(I
p
+ E
A
)/2
(7)
where I
p
and E
A
are ionization potential and electron affinity,
respectively.
0
The value of m can be compared to the EF value experimen-
tally determined. Fig. 6 shows the structures of the model
clusters ((Au)10 (a) and (Au)22 (b)) used for the calculations,
which were truncated from the Au(111) plane. The Au10 cluster
is frequently used for theoretical studies of adsorption of simple
molecules to fcc(111) surface, such as Ni, Cu, Pd, Ag, Pt, and
7
Au. In those cases, the Au hexagon represents the (111)
surface, and the three Au atoms for the second layer. In this
study, since the reaction on the Au NP surface possibly occurs
at some defect sites (kink or corner sites) on the surface rather
Fig. 7 (A) Plots of the m value for the Au clusters calculated in
vacuum as a function of the number of excess electrons (n): (a) (Au)10
b) (Au)22. (A) Plots of the m value calculated in water by the PCM
model as a function of n: (a) (Au)10, (b) (Au)22. (C) Difference in the
m value between in vacuum and in water as a function of n: (a) (Au)10
(b) (Au)22
,
(
Fig. 6 Structures of (Au)10 (a) and (Au)22 (b) model clusters used for
,
the DFT calculations.
.
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(
Au)22 (b). In each system, the m value steeply increases with an
further accounted for in terms of both the increase in the
increase in n, and the increment of m for (Au)10 is larger than
that for (Au)22 at the same n. Recently, the research group
2
efficiency of the photoinduced electron transfer from TiO to
Au and the decrease in the stabilization of charged Au NPs by
water solvation. This method is applicable to various metal-
semiconductor coupling systems, providing important infor-
mation on the design of metal NP-loaded semiconductor
photocatalytic reactions.
of Kamat has prepared Au(3 o d o 8 nm)/TiO particles using
2
3-mercaptopropionic acid (3-MPA) as a bifunctional coupling
agent, measuring the Fermi energy of Au/TiO in a toluene-
2
ꢀ
43
ethanol mixed solvent using C60/C60 as a redox pair. As a
0
result, they observed a trend opposite to ours that the E
F
increases with decreasing d, attributing this to the quantum size
effect of Au NPs. Clearly, the change in m in Fig. 7A is
overestimated; however, the trend is in agreement with that
reported by Kamat et al. Also, the research group of Goodman
has revealed using scanning tunneling microscopy/spectroscopy
Acknowledgements
This work was partially supported by a Grant-in-Aid for
Scientific Research (C) No. 16550169 and (B) No. 20350097
from the Ministry of Education, Science, Sport, and Culture,
Japan.
that an Au particle formed on the surface of a TiO single
2
crystal undergoes metal-to-nonmetal transformation when its
dimension is smaller than 3.5 nm in diameter ꢂ 1.0 nm
4
4
in height. Therefore, in low polar solvents such as a
toluene–ethanol mixed solvent (relative dielectric constant,
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