Synthesis of Nanoparticles and One-Dimensional Nanomaterials - Nanotechnologies for Water Environment Applications - page 28

4) The equipment and processing are simple, and it is relatively easy to control the

reaction conditions. Therefore, hydrothermal synthesis can be considered a good
method for nanoparticle preparation.

2.3 One-Dimensional Nanomaterials

Owing to the specific physical and chemical properties of nanomaterials, as well
as their potential application as nanoparticles, nanowires, nanotubes, nanofibers and
nanorods, etc., nanomaterials have attracted great attention. Generally, these structures
are referred to as one-dimensional structures, as the diameter range of nanofibers is from
several nanometers to several hundred microns, whereas that of nanowires and nanorods
does not exceed a few hundred nanometers.

Various methods have been developed to achieve efficient synthesis and
formation of one-dimensional nanostructures. According to the synthesis condition,
these methods can be mainly classified as vapor phase growth (Huang et al., 2001),
solution phase growth (Trentler et al., 1995), and template-based methods (Zhang and
Zhu, 2002), among others. This section will provide a brief introduction of the synthesis
methods required for one-dimensional nanostructures.

2.3.1 Vapor Phase Growth

In general, a high temperature and a vacuum are required for the growth of one-
dimensional nanostructures by the vapor phase method. In order to control the diameter,
aspect ratio, and crystallinity, several methods such as thermal chemical vapor
deposition (CVD), direct thermal evaporation (Dai et al., 2003) , and metal-organic
chemical vapor deposition (Park et al., 2002) have been utilized. Basically, these
synthesis methods are based on the vapor-liquid-solid (VLS) mechanism.

In VLS growth, impurities, commonly referred to as second phase materials, act
as catalysts. A catalyst forms a liquid droplet by itself or, more often than not, after
mixing with the growth material(s) during the growth. The VLS process is initiated by
the formation of a liquid droplet that contains both a catalyst and a source material. The
liquid droplet acts as a preferred site for the deposition of the source material from the
vapor phase. When the liquid droplet reaches the supersaturation condition with the
source material, a one-dimensional nanostructure is induced to grow due to the
precipitation of the source material.

Wagner and Ellis (1964) first proposed the VLS mechanism while observing the
growth of a Si whisker. Three important facts emerged from their research:
1) There is no axial screw dislocation.