Three-dimensional ordered assembly of thin-shell Au/TiO2 hollow nanospheres for enhanced visible-light-driven photocatalysis

Article abstract of DOI:10.1002/anie.201400966

An Au/TiO₂ nanostructure was constructed to obtain a highly efficient visible-light-driven photocatalyst. The design was based on a three-dimensional ordered assembly of thin-shell Au/TiO₂ hollow nanospheres (Au/TiO₂-3DHNSs). The designed photocatalysts exhibit not only a very high surface area but also photonic behavior and multiple light scattering, which significantly enhances visible-light absorption. Thus Au/TiO₂-3DHNSs exhibit a visible-light-driven photocatalytic activity that is several times higher than conventional Au/TiO₂ nanopowders. Order makes a difference: A three-dimensional ordered assembly of thin-shell Au/TiO₂ hollow nanospheres exhibits not only a very high surface area but also photonic behavior and multiple light scattering. The designed materials show significantly enhanced visible-light absorption and a visible-light-driven photocatalytic activity several times higher than those of conventional Au/TiO₂ nanopowders.

Full text of DOI:10.1002/anie.201400966

Angewandte
Chemie
DOI: 10.1002/anie.201400966
Nanostructures
Three-Dimensional Ordered Assembly of Thin-Shell Au/TiO2 Hollow
Nanospheres for Enhanced Visible-Light-Driven Photocatalysis**
Cao-Thang Dinh, Hoang Yen, Freddy Kleitz, and Trong-On Do*
Abstract: An Au/TiO2 nanostructure was constructed to obtain
a highly efficient visible-light-driven photocatalyst. The design
was based on a three-dimensional ordered assembly of thin-
shell Au/TiO2 hollow nanospheres (Au/TiO2-3DHNSs). The
designed photocatalysts exhibit not only a very high surface
area but also photonic behavior and multiple light scattering,
which significantly enhances visible-light absorption. Thus
Au/TiO2-3DHNSs exhibit a visible-light-driven photocatalytic
activity that is several times higher than conventional
Au/TiO2 nanopowders.
its high stability and strong visible-light absorption over
a wide range. As a consequence, several Au/TiO2 composite
systems have been developed for solar water splitting and
photocatalytic conversion of organic compounds.[5j]
Traditional methods for designing Au/TiO2 photocatalysts
mainly focus on improving the dispersion of Au NPs and the
surface area of TiO2 matrix.[5j,k] Although the photocatalytic
activity can be improved because of the higher density of
active sites, these Au/TiO2 materials usually exhibit low
visible-light absorption because of the limited contribution of
TiO2 , which absorbs only UV light. On the other hand,
incorporation of Au NPs in photonic structures was found to
enhance the surface plasmon resonance of Au NPs.[6] Materi-
als with photonic structures exhibit the slow photon effect, in
which light propagates in the material with extremely low
group velocities. When the slow photon wavelength overlaps
with the light that the material can absorb, an enhanced light
absorption can be obtained.[7] The slow photon effect has also
been demonstrated to enhance light absorption of semi-
conductors, such as TiO2,[7c–f] ZnO,[7j] WO3,[7h] or TaON,[7i] and
consequently, increase their photocatalytic activities. Com-
bining Au NPs and TiO2 that possess a photonic structure
would thus be a promising strategy to obtain efficient visible-
light-driven photocatalysts. To date, TiO2 photocatalysts with
photonic properties are primarily based on ordered macro-
porous structures.[7c–f,8] However, these ordered macroporous
TiO2 materials have a relatively low surface area compared to
their nanoparticle or mesoporous counterparts, which pre-
vents them from being efficient photocatalysts.
Herein, we report on a novel Au/TiO2 nanostructured
photocatalyst that is constructed by the three-dimensional
ordered assembly of thin-shell Au/TiO2 hollow nanospheres
(namely Au/TiO2-3DHNSs). The designed materials exhibit
not only exceedingly high surface area but also photonic
behavior originating from periodic macroscopic voids from
both the inside and the outside of hollow spheres that have
very thin shells. The multiple light scattering and slow photon
effects resulting from this unique architecture greatly enhance
the surface plasmon resonance of Au NPs, which leads to
a significant enhancement in the visible light absorption of
Au/TiO2-3DHNSs. As a result, these new photocatalysts
exhibit a photocatalytic activity that is several times higher
than conventional Au/TiO2 nanopowders, as illustrated by the
example of the photocatalytic decomposition of isopropanol
under visible-light illumination.
V
isible-light-driven photocatalysis for hydrogen production
and degradation of pollutants has attracted a tremendous
amount of interest over the past decades as it offers direct use
of sunlight for energy and environmental applications.[1]
Among many photocatalysts developed to date, TiO2 is
considered to be the most suitable candidate for commercial
scale-up because it is abundant, nontoxic, and stable under
photochemical conditions.[2] However, its large band gap
means that TiO2 is only active in the ultraviolet (UV) region,
which accounts for less than 5% of the total energy of the
solar spectrum. Strategies have been proposed to expand the
TiO2 optical absorption spectrum into the visible region,
which accounts for 43% of the solar spectrum, including
sensitizing TiO2 with dyes or small-band-gap quantum dots,[3]
and doping TiO2 with metal or nonmetal elements.[4]
Recently, a new approach for enhancing the visible light
photoactivity of TiO2 by the plasmonic effect of metal
nanostructures has received much attention.[5] Under visi-
ble-light illumination, plasmon-excited hot electrons in noble-
metal nanoparticles (NPs) can be transferred to the con-
duction band of an adjacent semiconductor, and then
participate in subsequent chemical reactions.[5e,f,i] Among
various plasmonic metals, Au is the most studied owing to
[*] C.-T. Dinh, Prof. T.-O. Do
Department of Chemical Engineering and Centre de recherche sur
les propriꢀtꢀs des interfaces et la catalyse (CERPIC)
Universitꢀ Laval, Quebec, G1V 0A6 (Canada)
E-mail: trong-on.do@gch.ulaval.ca
H. Yen, Prof. F. Kleitz
Department of Chemistry and
Centre de recherche sur les matꢀriaux avancꢀs (CERMA)
Universitꢀ Laval, Quebec, G1V 0A6 (Canada)
[**] This work was supported by the Natural Sciences and Engineering
Research Council of Canada (NSERC). C.T.D. thanks the FQRNT for
the Excellence Scholarship. F.K. thanks Yongbeom Seo and Prof.
Ryong Ryoo (KAIST, Korea) for access to high-resolution TEM
microscopy data.
To synthesize Au/TiO2-3DHNSs, uniform titanate nano-
disks (TNDs)[9] as titania precursors were first coated onto the
surface of SiO2 nanospheres (NSs) using a layer-by-layer
technique and poly(ethyleneimine) (PEI) as a polyelectrolyte,
to produce TND-PEI/SiO2 NSs (Figure 1). The layer-by-layer
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201400966.
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tion, followed by calcination at 5508C. It can be seen that the
NSs were packed into a face-centered-cubic structure that is
visible over a wide range. Interestingly, this 3D ordered
assembly structure was retained after removal of the SiO2
cores to obtain hollow nanospheres (HNSs; Figure 2C and
D). The high-magnification SEM image of Au/TiO2-3DHNSs
(Figure 2D) shows some broken particles which clearly
indicates the hollow structure of the NSs. Furthermore, the
presence of holes on the shell of each HNS is also observed.
These holes might be formed during the removal of the SiO2
cores. The presence of these holes, in fact, will be beneficial
for the photocatalytic application as they enhance the
diffusion of the reactants through the shell of the HNSs
(vide infra).
Figure S3 in the Supporting Information shows X-ray
diffraction (XRD) pattern of Au/TiO2-3DHNSs that corrob-
orates the formation of the anatase phase of TiO2. In addition,
the presence of diffraction peak at 44.38, corresponding to the
(200) plane of Au, is also seen, which confirms the presence of
crystalline Au. The composition of Au/TiO2-3DHNSs deter-
mined by energy dispersive X-ray spectroscopy (EDX;
Figure S4) shows the presence of 3.5 wt% of Au in the
composite.
Figure 1. Schematic illustration of the procedure for the design of Au/
TiO2-3DHNSs: 1) Uniform coating of TNDs on the surface of colloidal
À
À
SiO2 NSs followed by the loading of AuCl4 to form TND-PEI-AuCl4
/
SiO2; 2) assembly of TND-PEI-AuCl4À/SiO2 into a 3D ordered structure
followed by calcination to form a 3D ordered assembly of Au/TiO2/
SiO2; 3) removal of SiO2 to obtain Au/TiO2-3DHNSs.
technique combined with the uniform size of TNDs allows the
formation of a homogeneous and tunable shell thickness,
which is crucial for the formation of 3D ordered assembly
À
structures. The Au precursors (AuCl4 ) were then loaded on
À
TND-PEI/SiO2 NSs to form TND-PEI-AuCl4 /SiO2 NSs. The
strong interaction between AuCl4À and the amine groups[10] in
PEI enables a homogeneous deposition of the Au precursor
on each TND-PEI/SiO2 NS. The obtained NSs were then
packed into 3D ordered assembly structure using a centrifu-
À
gation process, and then calcined at 5508C to convert AuCl4
and TNDs into Au and TiO2, respectively. Finally, the SiO2
NSs core was removed by dissolving the product in a solution
of NaOH to obtain Au/TiO2 3DHNSs.
The morphology of the Au/TiO2-3DHNSs was further
characterized by TEM and STEM (Figure 3). It is clearly seen
that the sample is an assembly of uniform HNSs (Figure 3A).
The STEM and high-magnification TEM images (Figure 3B
and C) show Au NPs that are evenly distributed on the wall of
Figure 2A and and Figure S1 in the Supporting Informa-
tion show transmission electron microscopy (TEM) images of
À
TND-PEI-AuCl4 /SiO2 NSs obtained by coating TNDs on
SiO2 NSs, followed by loading of AuCl4À. The NSs are highly
uniform in size with a diameter of 370 nm. Compared to bare
SiO2 NSs (Figure S2), the surface of TND-PEI-AuCl4À/SiO2
NSs became rougher (see insert in Figure 2A), thus indicating
the successful deposition of TNDs on the surface of SiO2.
Figure 2B shows a scanning electron microscopy (SEM)
image of 3D ordered assembly of Au/TiO2/SiO2 NSs obtained
À
by packing of the TND-PEI-AuCl4 /SiO2 using centrifuga-
Figure 3. A) TEM; B) STEM; C,D) HRTEM images of Au/TiO2-
3DHNSs.
TiO2 HNSs with an average size of around 12 nm. The
thickness of the HNS shell is approximately 10 nm. The high-
resolution TEM image (Figure 3D) shows the lattice fringes
of both TiO2 and Au, thus indicating the highly crystalline
nature of TiO2 shell and Au NPs. The lattice fringe with a d-
spacing of 0.35 nm can be assigned to the (101) lattice plane of
Figure 2. A) TEM image of TND-PEI-AuCl4À/SiO2 NSs (inset shows
a high-magnification image); B) SEM image of 3D ordered assembly
of Au-TiO2/SiO2; C,D) SEM images of Au/TiO2-3DHNSs.
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anatase TiO2, and the fringe with a d-spacing of 0.25 nm
belongs to the (111) lattice plane of Au with a face-centered-
cubic phase. Figure 3D also shows the presence of intimate
contacts between Au and TiO2. This close contact between
metal and semiconductor may enhance the charge transfer
between them, and thus the photocatalytic efficiency.[5d]
The surface chemical states of Au and Ti were charac-
terized by X-ray photoelectron spectroscopy (XPS). As
shown in Figure S5 in the Supporting Information, the peak
observed at a binding energy of 83.3 eV was ascribed to
metallic Au 4f7/2 , thus confirming that the Au species exist as
metallic Au0 in Au/TiO2-3DHNSs.[6b,11] Compared to bulk Au
(4f7/2 at 84.0 eV), the Au 4f7/2 peak in Au/TiO2-3DHNSs
exhibits a negative shift of 0.7 eV, which could be due to
electron transfer from the oxygen vacancies of TiO2 to Au
that leads to a lower binding energy of Au 4f7/2 in Au/TiO2-
3DHNSs.[11] The Ti 2p3/2 XPS spectrum of TiO2 (Figure S6)
shows a peak at a binding energy of 458.4 eV, which could be
identified as that of Ti4+ from anatase TiO2.[12] The porous
structure of the Au/TiO2-3DHNSs was characterized using N2
physical adsorption at À1968C. As shown in Figure S7, the
pore size distribution calculated by applying the nonlocal
density functional theory method on the adsorption branch
reveals maxima in the range 3–12 nm, indicating a porous
shell of Au/TiO2 HNSs.[13] The specific surface area of the
composite reaches 200 m2 gÀ1, which is several times higher
than conventional ordered macroporous TiO2 (often in the
range 20–40 m2 gÀ1),[7f,k,8a] and is comparable to most high-
surface-area TiO2 materials.[14]
at around 490 nm. The slow photon effect that occurs at the
edges of the stop band is expected to appear in the range 550–
620 nm for Au/TiO2-3DHNSs, which matches well with the
absorption spectrum of Au NPs. The wavelength matching of
surface plasmon resonance absorption and photonic band
edge would be expected to increase the surface plasmon
resonance intensity because an increase of the effective path
length of light in the photonic band edge regions would result
in a significant enhancement of the interaction between
photons and Au NPs.
To confirm that the 3DHNS structure has an enhance-
ment effect on the light harvesting, the absorption spectrum
of Au/TiO2-3DHNSs sample and that of the same material,
but crushed in an agate mortar to completely destroy the
3DHNS structure (Figure S8 in the Supporting Information),
were analyzed (Figure 4B). It is clearly seen that, the
uncrushed sample exhibits an enhanced light absorption,
especially in the range 500–800 nm, compared to the crushed
sample. In fact, the UV/Vis absorption spectrum of the
crushed Au/TiO2-3DHNSs appeared to be similar to that of
conventional Au NPs dispersed on commercial titanium
dioxide, P25 Degussa, 3.5 wt% of Au (Au/TiO2-P25; Fig-
ure S9). The result confirms that the macroporous structure
plays a crucial role in visible light absorption enhancement.
Furthermore, it is also noted that the light absorption
enhancement of the uncrushed sample compared to the
crushed one occurs not only in the photonic-band-edge region
(550–620 nm), but in the whole range of the UV/Vis region.
This result indicates that, beside the slow photon effect, the
multiple light scattering is also important for the light
absorption enhancement. The inset in Figure 4B shows
a digital photo of the Au/TiO2-3DHNS that shows several
colors. These colors result from the light reflectance on
different planes of the Au/TiO2-3DHNS particles that orig-
inates from the photonic behavior of the materials.[15]
The optical properties of Au/TiO2-3DHNSs were inves-
tigated using diffuse reflectance UV/Vis spectroscopy with
TiO2-3DHNSs as reference sample. As shown in Figure 4A,
the reflection spectra for both samples begin to attenuate
sharply as the wavelength becomes shorter than 380 nm. This
The photocatalytic activity of Au/TiO2-3DHNSs was
investigated in the photocatalytic decomposition of isopro-
panol to CO2 under visible-light illumination (l ꢀ 420 nm).
Figure 5A shows the amount of CO2 generated during the
course of 10 h of visible light illumination using Au/TiO2-
3DHNSs and Au/TiO2-P25 as a reference sample. It can be
Figure 4. A) UV/Vis diffuse reflectance spectra for Au/TiO2 3DHNSs
and the TiO2-3DHNSs reference sample. The shaded region in (A)
shows the presence of photonic stop band. B) UV/Vis absorption
spectra of Au/TiO2-3DHNSs before and after being crushed. Inset in
(B) shows a digital photo of Au/TiO2-3DHNSs.
behavior is caused by strong intrinsic absorption of light in the
anatase TiO2 semiconductor. In the case of Au/TiO2-
3DHNSs, another decrease in reflection spectrum centered
at 580 nm is due to the surface plasmon resonance of Au NPs.
Interestingly, both samples exhibit photonic behavior as
evidenced by the presence of Bragg reflection peaks centered
Figure 5. Amount of CO2 generated A) under visible-light illumination
(lꢀ420 nm) for 10 h and B) under UV-light illumination (l=365 nm)
for 4 h using 1) Au/TiO2-P25, 2) Au/TiO2-3DHNSs, 3) crushed Au/
TiO2-3DHNSs, 4) Au/TiO2-HNSs (without the 3D ordered structure) as
photocatalysts.
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seen that Au/TiO2-3DHNSs can produce 7.4 mmol of CO2 ,
which is 6.1 times higher than that produced by Au/TiO2-P25
(1.2 mmol) under the same reaction conditions. The excellent
photocatalytic activity of Au/TiO2-3DHNSs under visible
light is attributed to the novel nanostructure.
nanoporous structure, which consists of ordered voids both
inside and outside the ultrathin shell HNSs.
In conclusion, we have demonstrated the construction of
three-dimensional ordered assembly of thin-shell Au/TiO2
HNSs from SiO2 NSs and TNDs. The designed Au/TiO2-
3DHNS materials exhibit a high surface area, photonic
behavior, and enhanced light scattering effects. These unique
properties permit Au/TiO2-3DHNSs to absorb visible light
with great efficiency. As a result, the designed photocatalysts
exhibit significantly enhanced visible-light-driven photocata-
lytic activity compared to conventional Au/TiO2 nanopow-
ders. Given that the photonic behavior of the 3DHNS
structure can be tuned by varying the size of SiO2 core
while maintaining a high surface area, we believe that the
3DHNS structure not only offers a powerful tool for the
construction of efficient photocatalysts, but will also have
a wide impact in all applications where high surface area and
effective light absorption are essential.
To confirm the effects of the structure on the photo-
catalytic properties, we evaluated the photocatalytic activities
of the crushed Au/TiO2-3DHNSs and Au/TiO2-HNSs without
the 3D ordered structure (Figure S10 in the Supporting
Information). As shown in Figure 5A, the photocatalytic
activity of the crushed Au/TiO2-3DHNSs sample was signifi-
cantly lower than that of the uncrushed sample. The crushed
Au/TiO2-3DHNSs sample produced 1.7 mmol of CO2 after
10 h of reaction, which is 4.3 times smaller than the uncrushed
sample, even though this crushed sample has a slightly higher
surface area (210 m2 gÀ1) than that of the uncrushed sample
(200 m2 gÀ1). Although the photocatalytic activity of the
crushed Au/TiO2-3DHNSs is significantly reduced, it is still
1.4 times higher than that of Au/TiO2-P25. The higher activity
of the crushed sample might be due to its higher surface area
(210 m2 gÀ1) as compared to that of Au/TiO2-P25 (around
50 m2 gÀ1). As also seen in Figure 5A, Au/TiO2-HNSs (e.g.,
without the 3D ordered structure) can generate 2.5 mmol of
CO2 which is 1.5 times higher than that produced by crushed
Au/TiO2-3DHNSs under the same conditions. However,
compared to that of Au/TiO2-HNSs with the 3D ordered
assembly structure, the photocatalytic activity of Au/TiO2-
HNSs without 3DNSs is 3 times lower. These results reveal
that not only the hollow structure but also the 3D ordered
assembly structure play an important role for the enhance-
ment of the photocatalytic activity. The 3D ordered assembly
of HNSs with periodic voids exhibits the slow photon effect
which enhances the visible light absorption, and as a conse-
quence, increases the photocatalytic activity of Au/TiO2-
3DHNSs.
Received: January 28, 2014
Published online: && &&, &&&&
Keywords: gold · nanostructures · ordered assembly ·
.
photocatalysis · titanium dioxide
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To study the effect of light scattering on the photocatalytic
activity of Au/TiO2-3DHNSs, the photocatalytic experiments
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3DHNSs can produce 94.1 mmol of CO2 which is 2.8, 2.1, and
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TiO2-P25
(33.1 mmol),
crushed
Au/TiO2-3DHNSs
(43.2 mmol), and Au/TiO2-HNSs (67.3 mmol), respectively.
The crushed and uncrushed Au/TiO2-3DHNSs samples and
Au/TiO2-HNSs possess similar intrinsic properties and com-
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the multiple light scattering resulting from its unique 3DHNS
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of TiO2 with ordered macroporous structure.[7c,f] The
enhanced light scattering in 3DHNSs compared to TiO2
ordered macroporous materials may be due to its highly
4
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Angew. Chem. Int. Ed. 2014, 53, 1 – 6
ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org
5
These are not the final page numbers!

.
Angewandte
Communications
Communications
Nanostructures
C.-T. Dinh, H. Yen, F. Kleitz,
T.-O. Do*
&&&& — &&&&
Three-Dimensional Ordered Assembly of
Thin-Shell Au/TiO2 Hollow Nanospheres
for Enhanced Visible-Light-Driven
Photocatalysis
Order makes a difference: A three-
significantly enhanced visible-light
absorption and a visible-light-driven
photocatalytic activity several times
higher than those of conventional Au/
TiO2 nanopowders.
dimensional ordered assembly of thin-
shell Au/TiO2 hollow nanospheres exhib-
its not only a very high surface area but
also photonic behavior and multiple light
scattering. The designed materials show
6
www.angewandte.org
ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2014, 53, 1 – 6
These are not the final page numbers!