Environmental Engineering Reference
In-Depth Information
8.2.3.2
Vapour Deposition Methods
Chemical Vapour Deposition (CVD) methods are based on established methods
for the manufacture of semiconductors (Van Zant, 2000). These systems have con-
ventionally been used to deposit thin fi lms of silicon and other semi-conductors on
to semi-conductor wafers. Vapour is formed in a reaction chamber by pyrolysis,
reduction, oxidation and nitridation. Deposited fi lm growth, in several stages begin-
ning with nucleation as the fi rst few atoms, deposit on the surface. These fi rst atoms
form islands that spread and coalesce into a continuous fi lm. After this transition
fi lm is formed, growth continues until thicker fi lm develops. Areas of growth on the
wafer are controlled using various patterning processes (also known as photolithog-
raphy or photomasking) in which deposition patterns are etched on to the surface
layers of the wafers.
CVD methods have been used to produce nanoparticles from many different
materials including TiO
2
and ZnO (Nakaso
et al.
, 2002). However, the most impor-
tant application is the synthesis of carbon nanotubes where CVD is considered to
offer one of the most effective routes for scaling up to industrial production (Singh
et al.
, 2003 ).
8.2.3.3
Colloidal Methods
The third major group is the colloidal methods. These are well established ' conven-
tional' wet chemistry precipitation processes in which solutions of the different ions
are mixed under controlled conditions of temperature and pressure to form insol-
uble precipitates. Colloidal methods provide a simple route to the synthesis of
nanoparticles. This approach enables the relatively straightforward production of
signifi cant quantities of nanoparticle material at modest capital cost. As with other
approaches, much of the recent emphasis has been on the development of more
monodisperse particles with better defi ned shape.
The earliest reported colloids were metals. Preparation of metallic colloids dates
back several centuries, but scientifi c investigation of their preparation or properties
was fi rst reported by Faraday (1857) in his experiments with gold. Development of
colloidal theory, processes and methods have been ongoing since that time. Many
comprehensive reviews are available, including Hiemenz and Rajagopalan (1997)
and Holmberg (2002), which describe this science in great detail. Nanomaterials
produced by colloidal process include metals, metal oxides, organics, and
pharmaceuticals.
8.2.3.4
Attrition Methods
The fi nal group of methods is that containing the mechanical attrition methods. In
contrast to the previous three groups where nanoparticles were built ' bottom - up '
from individual molecules, in attrition methods nanoparticles are produced top
down from larger particles.
Size reduction by grinding and milling is a very well established industrial process
used to produce progressively fi ner forms of materials, including minerals such as
clay, coal and metals. Production rates of materials can be of the order of tonnes
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