Synthesis of Nanoparticles and One-Dimensional Nanomaterials - Nanotechnologies for Water Environment Applications - page 29

2) An impurity is always necessary.

3) A small globule is always found on the tip of the nanostructure during the growth

period.

In summary, growth species are first evaporated at a high temperature, and then
these species are diffused and dissolved into a liquid droplet. Because of the large
accommodation coefficient at the surface of a liquid, a liquid droplet acts as a preferred
deposition site. The supersaturation condition in the droplet leads to precipitation. The
precipitation stage will undergo a nucleation and crystal growth step.

Amorphous silica nanowires were prepared using a mixture of silicon powder
and silica powder, with Fe powder being as a catalyst. This mixture was placed inside
the quartz tube which was pumped to 20 mTorr and heated at 850 °C. After further
heating at 1200 °C, the treated mixture was ablated by using excimer laser of 246 nm in
wavelength under the argon condition. The final product was amorphous silica
nanowires at a diameter of ~15 nm and a length up to hundreds micrometers (Yu et al.,
1998).

Beside this material, semiconductor nanowires of the III-IV group materials (i.e.,
GaAs, GaP, GaAsP, InAs, InP, and InAsP), II-IV group materials (i.e., ZnS, ZnSe, CdS,
and CdSe) and IV-IV group alloy (SiGe) have been synthesized via the VLS method
(Duan and Lieber, 2000). The nanowires have diameters ranging from three to tens of
nanometers, and lengths extending to tens of micrometers. In addition, various oxide
nanowires have been synthesized using this method, such as ZnO (Bae et al., 2004),
MgO (Chen et al., 2002), TiO 2 (Wu et al., 2005), and SnO 2 (Dai et al., 2002).

2.3.2 Solution Phase Growth

Growth of one-dimensional nanostructures in a solution has been developed
based on several methods. Solution phase reactions require a relatively low temperature,
which means that these methods can reduce the complexity and cost of fabrication (Lu et
al., 2006). The template-based and template-free methods are representative of solution
phase growth methods.

2.3.2.1 Template-Based Synthesis

Template-based synthesis can be utilized for the preparation of a variety of
micro- and nanomaterials for conductive polymers (Parthasarathy and Martin, 1994;
Penner and Martin, 1986), metals (Nishizawa et al., 1995), semiconductors (Lakshmi et
al., 1997), carbon (Parthasarathy et al., 1995), and oxides (Limmer et al., 2002). To this
end, porous templates such as anodized alumina membranes, mesoporous materials, and
radiation track-etched polymer membranes have been employed, with the nanostructures