4
Gas-Phase Synthesis of Nanoparticles
4.1
Fundamental Considerations
Gas-phase processes were the methods first applied to synthesize nanoparticles.
Previously, and significantly earlier, colloid chemistry had been used to obtain
nanoparticles in suspensions (¼ colloids), although after being separated from the
liquid the colloidal particles formed agglomerates. In gas-phase processes, a vapor of
the material condenses to form small particles, the concentration of which is, in
general, very low. Hence, the probability of the formation of larger agglomerates is
lesser.
The process of particle formation may be divided into four major steps:
Nucleation: a nucleus is the smallest stable unit and usually consists of two or, in
most cases, three atoms or molecules.
The nucleus acts as core for further condensation of atoms or molecules.
Therefore, the nucleus grows and forms clusters, and later a particle. Condensation is a stochastic process, which is ruled by the dynamics of gas species.
Colliding clusters or small particles may coagulate.
During coagulation by exchange of surface energy, a new particle is formed.
Having reached a certain size, the difference in surface energy will be so small
(see Chapter 3) that further coagulation of particles is impossible.
Now, the process of agglomeration starts. Agglomerates consist of two or more
individual particles. Often, one distinguishes between soft and hard agglomerates. Soft agglomerates are bound by van der Waals bonds, whereas hard
agglomerates are sintered. If at all, only soft agglomerates can be separated.
High temperatures and small particle sizes lead to the formation of hard
agglomerates; therefore, after synthesis and particle formation, the temperature
should be reduced as rapidly as possible. This quenching process reduces the
probability of the formation of hard agglomerates.
As the processes of condensation and coagulation are random by nature, the
distribution of particle sizes obtained is relatively broad. Mathematically speaking,
provided the formation of particles is a random process, the size distribution of the
particles follows a Poisson distribution, which is, in general, approximated using a
Nanomaterials: An Introduction to Synthesis, Properties and Applications, Second Edition. Dieter Vollath.
Ó 2013 Wiley-VCH Verlag GmbH & Co. KGaA. Published 2013 by Wiley-VCH Verlag GmbH & Co. KGaA.
j45
Gas-Phase Synthesis of Nanoparticles
4.1
Fundamental Considerations
Gas-phase processes were the methods first applied to synthesize nanoparticles.
Previously, and significantly earlier, colloid chemistry had been used to obtain
nanoparticles in suspensions (¼ colloids), although after being separated from the
liquid the colloidal particles formed agglomerates. In gas-phase processes, a vapor of
the material condenses to form small particles, the concentration of which is, in
general, very low. Hence, the probability of the formation of larger agglomerates is
lesser.
The process of particle formation may be divided into four major steps:
Nucleation: a nucleus is the smallest stable unit and usually consists of two or, in
most cases, three atoms or molecules.
The nucleus acts as core for further condensation of atoms or molecules.
Therefore, the nucleus grows and forms clusters, and later a particle. Condensation is a stochastic process, which is ruled by the dynamics of gas species.
Colliding clusters or small particles may coagulate.
During coagulation by exchange of surface energy, a new particle is formed.
Having reached a certain size, the difference in surface energy will be so small
(see Chapter 3) that further coagulation of particles is impossible.
Now, the process of agglomeration starts. Agglomerates consist of two or more
individual particles. Often, one distinguishes between soft and hard agglomerates. Soft agglomerates are bound by van der Waals bonds, whereas hard
agglomerates are sintered. If at all, only soft agglomerates can be separated.
High temperatures and small particle sizes lead to the formation of hard
agglomerates; therefore, after synthesis and particle formation, the temperature
should be reduced as rapidly as possible. This quenching process reduces the
probability of the formation of hard agglomerates.
As the processes of condensation and coagulation are random by nature, the
distribution of particle sizes obtained is relatively broad. Mathematically speaking,
provided the formation of particles is a random process, the size distribution of the
particles follows a Poisson distribution, which is, in general, approximated using a
Nanomaterials: An Introduction to Synthesis, Properties and Applications, Second Edition. Dieter Vollath.
Ó 2013 Wiley-VCH Verlag GmbH & Co. KGaA. Published 2013 by Wiley-VCH Verlag GmbH & Co. KGaA.
j45