Page 100 - Book Hosokawa Nanoparticle Technology Handbook

FUNDAMENTALS CH. 2 STRUCTURAL CONTROL OF NANOPARTICLES

T=800°C T=950°C

3 μm 3 μm

Figure 2.3.8
SEM images of Bi CaSr Cu O particle prepared by spray pyrolysis.
2
2
2
References generated by the liquid-phase method. In open sys-
tems such as a semibatch reactor, a monomer is con-
[1] Y. Azuma, M. Shimada and K. Okuyama: Chem. Vapor tinuously fed from outside the system using a
Deposition, 10(1), 11–13 (2004). constant rate pump, etc. In a closed system such as a
[2] T.T. Kodas, H. Smith: Aerosol Processing of Materials, batch reactor, the monomer is fed from inside the sys-
WILEY-VCH, p. 33 (1999). tem, using a chemical reaction such as hydrolysis of
[3] A. Seto, N. Aya, K. Okuyama and K. Hata: J. Aerosol alkoxide and urea.
Res., 13(4), 337–342 (1998). The particle formation process is classified into the
prenucleation, nucleation, and growth stages. The
[4] F. Ogino: Kagaku Kogaku Handbook, Asakura Syoten,
monomer concentration increases with time (prenu-
pp. 313–335 (2004).
cleation stage). When the precursor monomer con-
[5] T. Seto, M. Shimada and K. Okuyama: Aerosol Sci.
centration reaches a critical value, generation of
Technol., 23(2), 183–200 (1995).
nucleus begins (nucleation stage).
[6] K. Nakaso, B. Han, K.H. Ahn, M. Choi and The smallest diameter of stable nucleus is deter-
K. Okuyama: J. Aerosol Sci., 34(7), 859–881 (2003). mined by the free energy of the aggregation of nuclei
[7] N. Tohge, M. Tatsumisago, T. Minami, M. Adachi, and the surface area. Then, monomers supplied are
Y. Kousaka and K. Okuyama: J. Am. Ceram. Soc., consumed for the growth of the stable nuclei (growth
74(9), 2117–2122 (1991). stage). To synthesize a monodispersed particle, it is
desirable to separate the nucleation and the growth
2.3.2 Liquid-phase method stages. However, in general, it is difficult to separate
these stages. Consequently, the supplied monomers
Several methods have been proposed for controlling are consumed competitively by the nucleation and the
the shape and size of nanoparticles generated by the growth of the generated nuclei.
liquid-phase method: template synthesis using a sur- Therefore, to synthesize monodispersed particles,
factant [1], the hot soap method which suppresses we have to increase the nucleation rate or decrease the
growth of specific crystal faces by injection of pre- growth rate as much as possible. It is necessary to
cursor molecules into a hot surfactant [2], the solvother- know the change in the supersaturation ratio and the
mal method which uses high temperature and aggregation of nuclei with time. It is possible to obtain
pressure [3], and the microemulsion method which the situation in which the growth of seed only occurs
uses interfacial activities of reverse micelles and if the monomer concentration is kept constant
microemulsions [4]. It is important to understand the between the saturation and the supersaturation
mechanism of nanoparticle formation and its control- monomer concentrations by adding seeds to the
ling factors in order to control the shape and size of system. The crystallinity (crystalline or amorphous)
generated nanoparticles. and the type of particle growth (diffusion control or
surface reaction control) are also important.
(1) Factors controlling the shape and size of nanoparticles Furthermore, the reaction temperature, raw material
It is necessary to consider the balance between the concentration, coexistence of ion species, and solu-
supply and the consumption rates of monomer in tion pH, etc., which are directly related to the control-
order to control the shape and size of nanoparticles ling factors, are very important. In this section, the

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