Page 108 - Book Hosokawa Nanoparticle Technology Handbook

FUNDAMENTALS CH. 2 STRUCTURAL CONTROL OF NANOPARTICLES
2.4.2 Solution method thin films and the nanoparticles in recent years, but
about 30 years before, it attracted attention as the low-
Advanced materials including nanomaterials have temperature synthesis method of glasses. At present,
been attracting much attention because of the NNI the sol-gel method is well-known as a deposition
project that was proposed by the former U.S. method of the functional thin films and widely called
President Mr. Clinton. In many cases, film is a typical as chemical solution deposition (CSD) to distinguish
form of nanomaterials, while the other is the nanopar- it from chemical vapor deposition (CVD) and physi-
ticles or nanopowders. Recently, the advancement of cal vapor deposition (PVD) [4]. In this method,
the nanotechnology in the semiconductor industry is nanoparticles and thin films are deposited from the
remarkable and its convenience can be easily recog- molecular-designed precursor solutions or sols. The
nized in the daily life. For example, the thickness of most outstanding advantage over the other methods is
the dielectric layer in multi-layer ceramic capacitor the easy deposition of the thin films and the nanopar-
(MLCC), more than 100 pieces of which are used for ticles with excellent properties and well-controlled
a mobile phone, has been decreasing day by day and structures at relatively low cost. To utilize this advan-
is going to be in the order of nanometer [1]. tage efficiently, it is necessary to know as much about
Therefore, the grain-size in the dielectric layer of the science (the chemistry and the physics as well as
MLCC should be several tens of nanometers and the the engineering) in the preparation of the precursor
size effect of ferroelectric materials is going to be solution and the deposition process of thin films or
necessarily taken into account in the case of MLCC the nanoparticles. To prepare the precursor solution or
[2]. In addition, the remarkable improvement of the sol with high quality in the sol-gel method, metal
MLCC performance has been attained by controlling alkoxides are the common raw materials, because
the nanostructure of the dielectric nanoparticles in metal alkoxides are Lewis base and easy to hydrolyze
MLCC to be a core–shell structure. This type of at room temperature followed by the further polycon-
MLCC has already been commercialized [3]. In other densation to form metal oxides [5]. If the hydrolysis
words, the nanoparticles with controlled nanostruc- and condensation reactions of the metal alkoxide
ture exhibit the possibility to be the core materials of could be controlled precisely, it is possible to prepare
the nanomaterials in the 21st century. the nanohybrid particles consisting of organic com-
Here, it is useful to consider the essential demand pounds, inorganic compounds, and/or metals, leading
for the nanoparticles as the advanced materials. As for to a novel nanoparticles and thin films which are
the nanoparticles, the specific surface area is larger important not only in the chemistry but also in the
than that of the conventional fine particles, and so, the applied physics.
nanoparticles are easy to coagulate. Therefore, the However, nanocoating on the nanoparticles has
handling is often difficult, compared with the conven- hardly been achieved so far. This is because that it is
tional fine particles. To overcome this problem, it is difficult to prepare the monodispersed nanoparticles
necessary to have a narrow size distribution and a with narrow size distribution less than 100 nm, and
controlled particle size as well as the well-controlled moreover, the selective nanocoating on the surface of
surface properties. Furthermore, excellent functional- the nanoparticles is very difficult from the technical
ity is also required for the nanoparticles. The nanopar- viewpoint. In this section, two typical case studies of
ticles that meet these requests include composite hybrid nanoparticles by sol-gel method are given, in
nanoparticles or hybrid nanoparticles. which the silica (SiO ) monodispersed nanoparticles
2
There are three solution-based methods to prepare were prepared by the Stöber method [6] and the
the composite or hybrid nanoparticles with a particle functional nanofilms of several or several tens of
diameter less than 100 nm. The first method is a nanometers were nanocoated on the surface of the
nanocoating on the nanoparticle in a microemulsion monodispersed silica nanospheres. The first example
as a reaction vessel. The second one is the heteroco- is the hybrid nanoparticle of VO –SiO which can
2
2
agulation of the nanoparticles with different particle be used as the smart window by the hybridization
size, and the last one is the nanocoating on the with the plastic film. The second one is the hybrid
nanoparticle by the sol-gel method. However, there nanoparticle of TiO –SiO that exhibits photocat-
2
2
is no established method for nanocoating on a alytic property, and so on.
nanoparticle. Therefore, a novel method to prepare
the hybrid nanoparticles has increasingly been (2) Smart window by hybridization of hybrid nanoparticles
demanded. of VO -SiO and plastic film
2
2
A nanocoating technique to prepare hybrid nanoparti-
(1) Nanocoating on nanoparticles by sol-gel method cles is basically different from the coating technique
One of the excellent methods to prepare hybrid onto the surface of metals or glasses, because the reac-
nanoparticles is the alkoxide method, so-called sol- tion control of the selective deposition on the surface
gel method, in which the hydrolysis and the following of the nanoparticles is essential to prepare the hybrid
condensation reaction are well-controlled. The sol-gel nanoparticles by nanocoating through wet-chemical
method is famous as a method to deposit the functional processing. It is well-known that monodispersed silica
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