LETTERS
All-solid-state Z-scheme in CdS–Au–TiO2
three-component nanojunction system
HIROAKI TADA1 *, TOMOHIRO MITSUI1 , TOMOKAZU KIYONAGA1 , TOMOKI AKITA2 AND KOJI TANAKA2
1
Department of Applied Chemistry, Faculty of Science and Engineering, Kinki University, 3-4-1, Kowakae, Higashi-Osaka, Osaka 577-8502, Japan
National Institute of Advanced Industrial Science and Technology, Midorigaoka 1-8-31, Ikeda, Osaka 563-8577, Japan
* e-mail: [email protected]
2
Published online: 10 September 2006; doi:10.1038/nmat1734
atural photosynthesis, which achieves efficient solar
energy conversion through the combined actions of
many types of molecules ingeniously arranged in a
nanospace, highlights the importance of a technique for siteselective coupling of different materials to realize artificial
high-efficiency devices1 . In view of increasingly serious energy
and environmental problems, semiconductor-based artificial
photosynthetic systems consisting of isolated photochemical
system 1 (PS1), PS2 and the electron-transfer system have
recently been developed2,3 . However, the direct coupling of the
components is crucial for retarding back reactions to increase
the reaction efficiency. Here, we report a simple technique for
forming an anisotropic CdS–Au–TiO2 nanojunction, in which
PS1(CdS), PS2(TiO2 ) and the electron-transfer system (Au) are
spatially fixed. This three-component system exhibits a high
photocatalytic activity, far exceeding those of the single- and
two-component systems, as a result of vectorial electron transfer
driven by the two-step excitation of TiO2 and CdS.
Semiconductors are a key material in modern optoelectronic
and photoelectrochemical devices, and unique electronic states
resulting from the quantum size effect and high dispersion with
downsizing make them even more interesting from the perspective
of both fundamentals and potential applications. Recently, the
coupling of semiconductors with molecules and other solids on
the nanoscale has been reported to improve the performance
of various devices, including solar cells4,5 , photoluminescence6
and electrochromic devices7 and biosensors8 . On the other
hand, in artificial photosynthetic systems consisting of isolated
semiconductor particles and redox mediators (Ox/Red), water
splitting to hydrogen and oxygen has recently been shown to
proceed via a Z-scheme2,3 . However, the Ox and Red mobile in
solutions can compete with the reduction in PS1 and the oxidation
in PS2, respectively, to reduce the reaction efficiency. Using a simple
photochemical technique, we have been able to construct a siteselective CdS–Au–TiO2 nanojunction achieving an all-solid-state
Z-scheme.
Au particles with a mean size of 3.4 nm were firmly deposited
on the anatase TiO2 {101} surface with an orientation relationship
N
of Au{111} TiO2 {101} by the deposition–precipitation method
(see Supplementary Information, Fig. S1)9 . A high-resolution
transmission electron microscopic (HRTEM) image of a sample
prepared by irradiation (lex > 320 nm) of a de-aerated S8 ethanol
solution containing Au/TiO2 particles in the presence of Cd2+
ions shows that a hemispherical core–shell-type nanoparticle is
formed on TiO2 (Fig. 1a). The lattice spacings of the core and shell,
determined to be 0.23 and 0.32 nm, respectively, are in agreement
with the values for the Au(111) plane (International Centre for
Diffraction Data, No. 04-0784) and for the hexagonal CdS(101)
plane (International Centre for Diffraction Data, No. 41-1049). The
electron energy-loss spectra (EELS) were obtained by irradiating
an electron beam focused on the support and the shell labelled
1 and 2, respectively, in Fig. 1a (Fig. 1b). The Ti and O signals
are present in spectrum 1, whereas the signals of Cd and S are
observed in spectrum 2. In addition, the molar ratio of Cd/S in
the deposits was determined to be about 1, irrespective of the
irradiation time (tp ), by inductively coupled plasma spectroscopy
and ion chromatography. Evidently, CdS deposits on the Au
surface to yield CdS-coated Au nanocrystals on the TiO2 surface
(Au@CdS/TiO2 ). We have recently specified the reduction sites in
Au/TiO2 -photocatalysed reduction of S8 to S2− ions as being the
Au surface atoms having a great affinity to sulphur10 . Thus, in the
presence of Cd2+ ions, the S2− ions produced selectively on the
Au surface are considered to bond to Cd2+ ions to form Au@CdS
on TiO2 .
TEM images of the samples prepared by changing the tp show
that all the Au particles are covered with CdS (Fig. 1c). The CdS
growing rate of 4.8 nm h−1 at tp < 0.5 h decreases to 0.27 nm h−1
at 0.5 < tp < 10 h (Fig. 1d). Electronic absorption spectra of TiO2 ,
Au/TiO2 and Au@CdS/TiO2 with varying CdS thickness (l ) show
that Au/TiO2 has an absorption peak (lmax ) at 530 nm due to the Au
surface plasmon resonance with the TiO2 interband transition band
at l < 385 nm. The formation of a 1.3-nm-thick CdS layer causes
significant broadening and a redshift of the Au surface plasmon
band of about 50 nm, which suggest a strong electronic interaction
between Au and CdS11 . The bandgap of CdS (Eg ) estimated from
the absorption edge is shown as a function of l in Fig. 2c: the
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LETTERS
a
b
Intensity (a.u.)
Core
Shell
2
Au{111}
= 0.232 nm
CdS{101}
= 0.318 nm
1
300
Intensity (a.u.)
2
Cd
400
500
Ti
600
O
S
TiO2
× 20
1
100
200
300
400
500
600
Energy loss (eV)
5 nm
c
6
d
5
4
t p = 0.5 h
l (nm)
tp = 0 h
3
2
1
tp = 4 h
t p = 10 h
0
0
20 nm
2
4
6
8
10
tp (h)
Figure 1 Geometrical structure and composition of an Au–CdS composite nanoparticle formed on TiO2 , and time evolution of the CdS shell layer. a, HRTEM image of
Au–CdS/TiO2 . b, EELS of the support (1, blue line) and shell layer (2, red line) in a. c, TEM images of Au@CdS/TiO2 prepared by changing tp . d, Plots of l versus t p . The l
values were determined by TEM observation carried out at an acceleration voltage of 300 kV. The error bars represent one standard deviation. HRTEM observation and EELS
analysis were carried out using a JEOL JEM 3000F electron microscope with a Gatan imaging filter (applied voltage: 300 kV) and TEM observations were carried out using a
JEOL JEM 3010 electron microscope (acceleration voltage: 300 kV).
theoretical curve is calculated using the Brus equation for spherical
CdS particles with a radius of l (ref. 12). At l < 4 nm, the Eg
increases relative to the bulk value13 (2.4 eV) which is ascribable to
the quantum size effect of the CdS shell layer because the theoretical
curve fits well with the experimental data.
To study the effects of the CdS–Au–TiO2 nanojunction on
the photocatalytic activity, methylviologen (MV2+ ) reduction was
used as a test reaction, in which sol–gel TiO2 films14 (TiO2 -TF)
were used as a support in place of TiO2 particles (Fig. 3a).
Both Au/TiO2 -TF and CdS (about 5 nm)/TiO2 -TF show higher
photocatalytic activities than TiO2 -TF; these probably arise from
the increase in the charge-separation efficiency due to the
electron transfer from TiO2 to Au with a large work function15
and from CdS to TiO2 with a conduction band (cb) edge
2
lying lower than that of CdS16 , respectively. The photocatalytic
activity of Au@CdS/TiO2 -TF far exceeds that of either the singlecomponent (1C) or 2C systems. In addition, the MV+ yield in the
photostationary state for Au@CdS/TiO2 -TF reaches 52.2%, which
is larger than that achieved in the 1C and 2C systems: the difference
in the CdS morphology between CdS/TiO2 and Au@CdS/TiO2
might make the activity comparison difficult; however, CdS (about
5 nm)/TiO2 coupled using mercaptoacetic acid (MAA) showed a
photocatalytic activity higher than those of CdS and TiO2 (ref. 17).
These findings are indicative of the progress of MV2+ reduction
by the electrons excited to the cb(CdS) with a high potential in
Au@CdS/TiO2 , whereas the cb(TiO2 ) electrons reduce MV2+ in the
2C system16 . The MV+ yield for Au@CdS/TiO2 -TF as a function of
l indicates that the photocatalytic activity of Au@CdS/TiO2 reaches
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LETTERS
a
b
0.1
0.05
λ ex > 300 nm
l = 5.2 nm
l = 4.1 nm
l = 3.8 nm
Absorbance
Absorbance
l = 2.6 nm
l = 2.4 nm
λ ex > 400 nm
Before
irradiation
Au@CdS (l = 1.3 nm)
/TiO2
Au/TiO2
TiO2
350
400
450
500
λλ(nm)
550
600
650
400
500
600
700
800
λλ(nm)
3.0
c
300
0.05
d
2.9
0.03
0.01
2.7
dA/ dλ
λ
Eg (eV)
2.8
2.6
550
650
–0.01
2.5
–0.03
2.4
2.3
2
3
4
l (nm)
5
6
–0.05
λλ(nm)
Figure 2 Optical properties of Au@CdS/TiO2 . a, Electronic absorption spectra of Au@CdS/TiO2 with varying l. b, Change in electronic absorption spectra of Au@CdS/TiO2
with irradiation. c, Size dependence of the Eg (CdS): the curve is calculated by the Brus equation of Eg = Eg (bulk) + (h̄ 2 π2 /2l 2 )(1/m e∗ 2 + 1/m h∗ 2 ) − 1.8e 2 /εl using the
values of m e∗ = 0.21m0 , m h∗ = 0.8m0 and ε(CdS) = 8.45. The error bars represent one standard deviation. d, Derivative spectra of b.
a maximum at l ∼ 3 nm (Fig. 3b). This probably results from the
balance of the increase in the cb-electrons(CdS) potential due to the
quantum size effect (Fig. 2c) and the decrease in light absorption.
The lmax of metal nanoparticles is related to electron density
(n) by lmax = 4πc(2ε0 m/e2 n)1/2 (ref. 18), where ε0 is the vacuum
permittivity, e and m are the charge and mass of the electron,
respectively, and c is the speed of light. Thus, the Au nanoparticles
can be regarded as a sensor for electron transfer in the 3C system.
The electronic absorption spectra of Au@CdS/TiO2 before and
after ultraviolet (lex > 300 nm) and visible light (lex > 400 nm)
irradiation in de-aerated ethanol show that the lmax redshifts
by 24 nm after visible light irradiation, whereas it blueshifts by
20 nm after ultraviolet light irradiation (Fig. 2b,d). A similar
trend was also observed in water, although the amounts of the
shift decreased (see Supplementary Information, Table S1). The
influence of semiconductor charging during the reaction on the
Au surface plasmon resonance might be excluded in these ex situ
optical measurements.
The essential reaction scheme can be discussed on the basis
of the energy band diagram of the 3C system (Fig. 4). For
lex > 400 nm irradiation, the cb-electrons(CdS) would be used
for MV2+ reduction rather than transferred to Au because
of the increase in their surface population due to the CdS
size quantization19 ; the injection of the holes left in the CdS
valence band (vb) into Au reduces n, explaining the redshift
of lmax (electron transfer I , Au → CdS). For lex > 300 nm, the
vb-holes(TiO2 ) with a strong oxidation power oxidize the solvent,
and the electrons left in the cb(TiO2 ) flow into Au, which increases
n to cause the blueshift of lmax (electron transfer II, TiO2 → Au).
Thus, simultaneous electron transfer I and II (that is, vectorial
electron transfer of TiO2 → Au → CdS) should occur as a result
of excitation of both TiO2 and CdS under the conditions in
MV2+ reduction. Furthermore, to specify the reduction sites of
Au@CdS/TiO2 , Pt photodeposition was carried out by irradiating
visible and ultraviolet light in the 3C system. If the electron
transfer from CdS to TiO2 rapidly takes place through their
contact with Au@CdS/TiO2 , Pt should be photodeposited on TiO2 .
However, annular dark-field scanning TEM images and EELS
of the Pt-photodeposited Au@CdS/TiO2 (Pt/Au@CdS/TiO2 ) have
demonstrated that Pt is deposited almost selectively on CdS under
both conditions (see Supplementary Information, Fig. S2). This
finding that CdS acts as the reduction sites of Au@CdS/TiO2
indicates that the back electron transfer from CdS to TiO2 is a
minor path, strongly supporting the vectorial electron transfer
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LETTERS
2.5
a
Vacuum level (eV)
Au@CdS(l = 2.96 nm)/TiO2
CdS/TiO2
Au/TiO2
MV+ (10–4 M)
2.0
CdS
–3
TiO2
–4
e–
TiO2
cb
Reduction
e–
cb
DOx
2
λ 0
E (R/O)
λ
DRed2
Au
1.5
–5
λ ex > 400 nm
–6
1.0
vb
λ ex > 300 nm
vb
h+
Red1
–7
Oxidation
0.5
vb
h+
Ox1
0
0
20
40
60
t p (min)
80
100
Figure 4 Energy band diagram scheme of the CdS–Au–TiO2 system. E 0 (R/O) is
the standard electrode potential of MV+ /MV2+ . DRed2 and DOx2 represent the
distribution function for occupied and unoccupied states, respectively, and l is the
reorganization energy.
70
b
Yield of MV + (%)
60
50
of an all-solid-state Z-scheme for visible-light-induced efficient
artificial photosynthetic systems.
40
METHODS
30
20
0
1
2
3
l (nm)
4
5
6
Figure 3 Photocatalytic activity of Au@CdS/TiO2 . a, Time courses for
photocatalytic reduction of MV2+ . b, Dependence of the MV+ yield after 100-min
irradiation on l. The error bars represent one standard deviation.
in the 3C system: although the TiO2 –CdS contact area increases
with the growth of CdS, the ratio of the contact area to the CdS
surface area never exceeds 50% for hemispherical CdS and Au
particles. This Z-scheme simultaneously generates vb-holes(TiO2 )
with a strong oxidation power and cb-electrons(CdS) with a strong
reduction power, which explains the high photocatalytic activity
and high yield for MV2+ reduction. The photocatalytic activity of
Pt/Au@CdS/TiO2 for H2 generation from H2 O was studied further
(see Supplementary Information, Fig. S3). Although no H2 was
detected in the Pt/CdS system, Pt/Au@CdS/TiO2 yielded H2 with
an almost constant rate. These results can also be interpreted within
the framework of the Z-scheme20 (that is, the electron supply from
TiO2 to CdS via Au restricts the self-decomposition of CdS due to
the oxidation of surface S2− ions by the vb-holes(CdS)).
The photoinduced reductive desorption of sulphur can occur
for semiconductors with a cb edge greater than −4.6 eV versus
vacuum21 . In addition, anion-doped TiO2 , of which the vb edge is
raised with the cb edge almost maintained, was shown to exhibit
photocatalytic activities under visible light irradiation22 . Owing
to the versatility, rational coupling of the components and their
dimensional control will enable the development of our prototype
4
SAMPLE PREPARATION
TiO2 particles (anatase, surface area = 8.1 m2 g−1 ) were used as a support. The
pH of a 4.86 × 10−3 M aqueous solution (100 ml) of HAuCl4 ·4H2 O was
adjusted to 6.0 with a 1 mol dm−3 NaOH aqueous solution. The solution
turned from yellow to lighter yellow, accompanied by ligand exchange from
[AuCl4 ]− to [Au(OH)4−x Clx ]− . To this solution, 10 g of TiO2 particles were
added and magnetically stirred at 343 K for 1 h. The particles were washed with
distilled water three times, and then heated at 673 K for 4 h in air
(Au(0.33 mass%)/TiO2 ). After an Au/TiO2 (1 g) ethanol suspension (250 ml)
containing S8 (0.344 mmol) and Cd(ClO4 )2 · 6H2 O (3.46 mmol) had been
bubbled with argon for 30 min in the dark, irradiation was carried out for a
given period with a high-pressure mercury lamp at 298 K; the light intensity
integrated from 320 to 400 nm ( I320−400 ) was 3.7 mW cm−2 . CdS/TiO2 -TF was
prepared using MAA as a bifunctional coupling agent. A Cd(ClO4 )2 · 6H2 O
aqueous solution (1.0 mmol dm−3 , 50 ml) and a Na2 S · 9H2 O aqueous solution
(1.0 mmol dm−3 , 50 ml) containing MAA (1.0 mmol dm−3 ) were mixed, and
stirred for 10 min. After TiO2 film-coated glass substrates had been immersed
in the suspension of MAA-capped CdS nanoparticles (about 5 nm) with
stirring for 18 h, the sample was washed with distilled water and dried. The
loading with Pt on Au@CdS/TiO2 and CdS was carried out by
photoplatinization. After Au@CdS/TiO2 (or CdS) particles (0.2 g) had been
dispersed into a 1.93 mmol dm−3 aqueous H2 PtCl6 solution and de-aerated
with Ar for 60 min, the dispersion was illuminated by a 500 W Xe lamp through
a 430 nm cutoff filter (lex > 400 nm, I420−485 = 3.7 mW cm−2 ) or a 300–400 nm
bandpath filter (300 < lex < 400 nm, I320−400 = 4.0 mW cm−2 ) for 6 h at 298 K.
The solids (Pt/Au@CdS/TiO2 or Pt/CdS) were repeatedly washed with ethanol
and dried under vacuum.
OPTICAL MEASUREMENTS
After suspensions of Au@CdS/TiO2 particles (0.2 g) in ethanol or water (50 ml)
were irradiated (lex > 400 nm or lex > 300 nm) for 4 h, the solvent was
removed by evaporation, and the solids were further dried in a vacuum
desiccator for 0.5 h. The diffuse reflectance spectra of the Au@CdS/TiO2
particles were recorded on a Hitachi U-4000 spectrophotometer equipped with
an integrating sphere to be transformed into the absorption spectra.
PHOTOCATALYTIC ACTIVITY TEST
MV+ is easily oxidized by oxygen in air, and thus the reactions were carried out
in a closed reaction vessel with an optical cell using a sol–gel TiO2 film as a
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LETTERS
support in place of TiO2 particles, which enabled optical measurements to
determine the MV+ concentrations (εat 605 nm = 1.1 × 104 mol−1 dm3 cm−1 )
without opening the system. As the l value for the thin-film system could not be
determined by electron microscopy, the corresponding value for the particulate
system under the same conditions is shown. After Au@CdS/TiO2 -TF samples
(8 mm × 40 mm) had been immersed in a 0.4 mM MV2+ ethanol solution
followed by de-aeration with argon bubbling for 1 h, irradiation was carried out
using a 500 W Xe lamp under I320−400 = 8.0 mW cm−2 at 305 K. As another test
reaction, photocatalytic H2 generation from water was examined for
Pt/Au@CdS/TiO2 or Pt/CdS. After the photocatalyst (0.02 g) had been
dispersed in water (20 ml) and de-aerated with Ar for 15 min, the suspension
was irradiated by a 500 W Xe lamp through a 300–400 nm bandpath filter
(300 < lex < 400 nm, I320−400 = 4.0 mW cm−2 ). The reaction temperature was
kept at 298 or 323 K by circulating thermostatted water through an outer jacket
around the reaction cell. The H2 that evolved under illumination was analysed
by gas chromatography (Shimadzu GC-8A); tcd column SHINCARBON ST,
carrier gas was Ar, both the injection and column temperatures were 323 K.
Received 26 April 2006; accepted 2 August 2006; published 10 September 2006.
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Acknowledgements
H.T. thanks T. Kawahara for discussions throughout this work. This work was partially supported by a
Grant-in-Aid for Scientific Research (C) No. 16550169 from the Ministry of Education, Science, Sport,
and Culture, Japan.
Correspondence and requests for materials should be addressed to H.T.
Supplementary Information accompanies this paper on www.nature.com/naturematerials.
Author contributions
H.T.: project planning, data analysis. T.M. and T.K.: experimental work (electronic absorption spectra,
TEM and photocatalytic reactions). T.A. and K.T.: experimental work (HRTEM and EELS).
Competing financial interests
The authors declare that they have no competing financial interests.
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