Communications
[7] W. P. Rothwell, W. Shen, J. H. Lunsford, J. Am. Chem. Soc. 1984,
natural force might be used to clean a surface. It would be
interesting to discover whether it is possible to design a
material that incorporates both structural color and the lotus
effect, thus mimicking the wings of a butterfly. Such a material
should be of great biological and technological importance. In
this paper, we will show one approach to fabricating such a
biomimetic decorative material by taking advantage of a
nanostructured inverse opal surface.
106, 2452 – 2453.
[8] Each unit cell of CHAcontains 36 T sites and three cages. The
HSAPO-34 used has one Si, five P, and six Al atoms per cage.
[9] J. F. Haw, P. W. Goguen, T. Xu, T. W. Skloss, W. Song, Z. Wang,
Angew. Chem. 1998, 110, 993 – 995; Angew. Chem. Int. Ed. 1998,
37, 948 – 949.
Inverse opal is a solid material that consists of a three-
dimensional network.[6,8–10] Ordered monodisperse air spheres
throughout the network contribute to an optical stop band,
the position of which can be tuned by careful control of the
periodicity of the air spheres. Colors can be observed by the
naked eye when the stop band falls in the visible region. As a
consequence of its unique optical properties, inverse opal has
been regarded as a new-generation decorative material, in
addition to its application as a photonic crystalline materi-
al.[6,11] Recently, we realized that inverse opal might also be
incorporated into the design of a hydrophobic material. The
solid material network of inverse opal contributes a rough
surface composed of well-ordered meshes. According to the
Cassie–Baxter law, the intrinsic wettability of the solid
material can be greatly reduced.[12] Such a decorative
material, which exhibits both structural color and the lotus
effect, would be environmentally friendly and energy-effi-
cient.
For practical applications, a convenient method of fab-
ricating a uniform inverse opal film over a large area is
required. In addition, the rough inverse opal surface needs to
be further optimized to imbue the surface with superhydro-
phobic character. We describe here the development of a
dipping method that can be used to meet these criteria, and
which can derive uniform inverse opal films with a nano-
structured surface. The procedure for the fabrication is as
follows: First, submicron-sized monodisperse polystyrene
spheres and nanosized particles were ultrasonically dispersed
into deionised water. Aglass substrate was then immersed
into the solution and withdrawn at a constant speed. It is
known that a mixture of spheres with different sizes cannot be
used to fabricate colloidal crystals with long-range structural
order by such a deposition method,[13–15] as phase separation
occurs, or an amorphous structure is formed. In our experi-
ment, we found that this conclusion is only partially correct. A
structure with long-range order can be derived when the ratio
of the diameters of the spheres falls into a particular regime.
Figure 1a shows an image of a structure composed of
monodisperse spheres, while Figure 1b–d displays three
images of structures composed of spheres of two sizes, with
diameter ratios of 0.94, 0.34, and 0.07, respectively. The
structure formed by the spheres of varying size depends on
the diameter ratio. Astructure with long-range order can be
observed in films composed of monodisperse spheres, how-
ever, such order is absent in films composed of spheres of two
sizes, where the diameter ratio is larger than 0.15. Usually, the
particles form a structure with discernible separation when
the ratio between the two types of sphere is larger than 0.5
(Figure 1b), while the domains formed by different types of
particles are separated when the ratio is smaller than this
value (Figure 1c). When the diameter ratio between the
Surface Effects from Nanostructure
Structural Color and the Lotus Effect**
Zhong-Ze Gu, Hiroshi Uetsuka, Kazuyuki Takahashi,
Rie Nakajima, Hiroshi Onishi, Akira Fujishima, and
Osamu Sato*
The study of biological microstructure is one of the most
important research areas in biomimicry.[1–3] Microstructure
plays many important roles in living things.[2,3] For example,
the charming blue color of the Morpho sulkowskyi butterfly
originates from light diffraction and scattering, which results
from the ordered microstructure of its scales. This form of
color is usually known as structural color, which is utilized by
animals both for protection and as a warning. Today, the study
of structural color has been extended from biology to
optics.[4–6] As well as affecting coloration, microstructure also
plays an important role in self-cleaning.[2,7] For the butterfly,
the specific nanostructure enhances the hydrophobicity of its
wings, which allows droplets of water to be dispersed more
easily. During this process, dust particles on the surface of the
wings are removed. This phenomenon is known as the “lotus
effect”, which is not only very useful for natural species, but
also for materials applications, such as for decoration where a
[*] Dr. O. Sato, Prof. Dr. Z.-Z. Gu, Dr. H. Uetsuka, K. Takahashi,
R. Nakajima, Dr. H. Onishi
Kanagawa Academy of Science and Technology
KSP Bldg. East 412, 3-2-1 Sakado, Takatsu-ku, Kawasaki-shi,
Kanagawa 213-0012 (Japan)
Fax: (+81)44-819-2070
E-mail: sato@fchem.chem.t.u-tokyo.ac.jp
Prof. Dr. Z.-Z. Gu
National Laboratory of Molecular and Biomolecular Electronics
Southeast University
Nanjing 210096 (China)
Prof. Dr. A. Fujishima
Department of Applied Chemistry
School of Engineering, The University of Tokyo
7-3-1 Hongo Bunkyo-ku, Tokyo 113-8565 (Japan)
[**] This work was partially supported by a Grant-in-Aid for Scientific
Research on Priority Areas (417) from the Ministry of Education,
Culture, Sports, Science and Technology (MEXT) of the Japanese
Government. Z.-Z.G. also thanks the Research Foundation
(60228002) of the National Nature Science Foundation of China.
894
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Angew. Chem. Int. Ed. 2003, 42, No. 8