Angewandte
Chemie
DOI: 10.1002/anie.200802207
Tungsten Photocatalysts
Nanoporous-Walled Tungsten Oxide Nanotubes as Highly Active
Visible-Light-Driven Photocatalysts**
Zhi-Gang Zhao and Masahiro Miyauchi*
[
14]
[15]
Semiconductor photocatalysis is important for many current
environmental and energy issues because, in addition to
splitting water to supply clean and recyclable hydrogen
energy, it can utilize solar energy to decompose harmful
organic and inorganic pollutants present in air and aqueous
La Sn O ,
and ZnWO4
nanostructures with different
2
2
7
morphologies have been synthesized and their photocatalytic
properties explored.
The Pt-loaded nanotubular structure that results from
combining the ideas of nanostructural control with a Pt/WO3
multicomponent composite as a heterocatalyst should there-
fore be more promising for visible-light-driven photocatalysis.
The nanotubular architecture would result in a larger
effective surface area and provide more active sites in close
proximity to the Pt nanoparticles, thus enabling diffusive
transport of photogenerated electrons to these nanoparticles.
More importantly, however, the energy band at the surface
bends, and the band-bending potential may establish a
depletion region at a very short distance from the materialꢀs
surface. Such a depletion layer would effectively decrease
charge–carrier recombination rates and thereby increase the
[
1–4]
systems.
TiO is currently the best known and most widely
2
used highly efficient photocatalytic material because it is
stable and cheap. However, only a small UV fraction of solar
[5,6]
light (3–5%) can be utilized
due to its wide bandgap,
therefore an imperative and challenging issue is to develop
new and efficient visible-light-sensitive photocatalysts. Visi-
ble-light photocatalysis has recently been reported in nitro-
[
7–9]
gen-doped TiO2.
However, nitrogen doping leads to
localized states in the bandgap of TiO , and the localized
2
holes produced at the impurity level have a slower mobility
than those in the valence band. The quantum efficiency of
[
16]
nitrogen-doped TiO under visible light is therefore much
photocatalytic ability.
Another possible advantage of a
2
[
8]
lower than that under UV illumination. In contrast, tungsten
porous structure is that the incident photons are more
effectively absorbed than on a flat surface due to light
trioxide (WO ), which possess a small bandgap of between 2.4
3
[
16,17]
and 2.8 eV, has many advantages for visible-light-driven
photocatalysis, including a deeper valence band (+ 3.1 eV),
strong adsorption within the solar spectrum, stable physico-
chemical properties, and resilience to photocorrosion
scattering.
The synthesis of high-purity WO nanotubes (NTs) is
rather rare, especially using a template-free solvothermal
method. The use of anodic aluminum oxide (AAO)
biological templates may introduce impurities into the WO3
materials and will in any case complicate the separation
process. Moreover, most methods using templates or thermal
evaporation techniques are intrinsically expensive and give
low yields and purities.
template-free methods suitable for the large-scale synthesis of
3
[18,19]
or
[10]
[20]
effects.
However, pure WO3 has a lower light energy
conversion efficiency than the more widely used TiO as the
2
reduction potential of the electrons in WO is low due to its
3
low conduction band level. Abe et al. have recently proposed
that Pt loading is an attractive solution to enhance the
photocatalytic properties because loaded Pt can trap photo-
[
18–21]
The development of cheap and
[
11]
generated electrons from WO to reduce O to H O .
high-purity WO NTs is therefore the primary objective of
3
2
2
2
3
Nanoscience research can greatly impact the development
of new and more potent catalysts by designing and controlling
their photocatalytic properties, especially in terms of the
energy gap, chemical composition, and surface modification.
For example, because photocatalytic activity is closely related
this work. Choi et al. have reported a simple process for the
synthesis of tungsten oxide (W O ) nanorods in ethanol
1
8
49
solvent using tungsten hexachloride (WCl ) as a raw mate-
6
[
22]
rial. Their work has shown that the slow alcoholysis of WCl6
with ethanol results in the formation of rod-like nanoparticles
in an autoclave. This process should follow traditional self-
aggregation and Ostwald ripening growth mechanisms. It has
also been found recently that urea can be rationally used to
assemble organic crystals ranging from one-dimensional
chains to nanotubes due to its strong hydrogen-bonding
[
12]
[13]
to particle diameter and morphology,
Bi WO ,
2 6
[*] Dr. Z.-G. Zhao, Dr. M. Miyauchi
Nanotechnology Research Institute
NationalInstitute of Advanced IndustrialScience and Techno ol gy
Tsukuba Central5, 1-1-1 Higashi, Tsukuba, Ibaraki, 305-8565 (Japan)
Fax: (+81)-29-861-6299
E-mail: m-miyauchi@aist.go.jp
[23]
properties. It is therefore plausible that urea could be a
suitable ligand or directing agent in an ethanol/WCl system
6
for the synthesis of inorganic tungsten oxide nanostructures as
it can act as both a hydrogen-bond donor through its two NH
protons or a hydrogen-bond acceptor through the lone pairs
Homepage: http://staff.aist.go.jp/m-miyauchi/
[
**] This work was supported by the New Energy and Industrial
Technology Development Organization (NEDO) and was partly
conducted in the AIST Nano-Processing Facility, which is supported
by the “Nanotechnology Support Project” of the Ministry of
Education, Culture, Sports, Science and Technology of Japan.
[23]
of the C=O group. Similarly, protons resulting from the
alcoholysis of WCl may adsorb on the surface of the as-
6
prepared nanoparticles during the growth process and
interact with urea, thereby also tuning the morphology of
the final products.
Angew. Chem. Int. Ed. 2008, 47, 7051 –7055
ꢀ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
7051