Synthesis of Nanoparticles and One-Dimensional Nanomaterials - Nanotechnologies for Water Environment Applications - page 31

synthesized into the pores of a nanoporous alumina membrane. An organocyanide and
(2-cyanoethyl)triethoxysilane were selected as the molecular anchor to bind the
electrochemically-deposited Au to the pore wall. After this process, one side of the
template was coated by Au. Then Au was deposited galvanostatically (Brumlik and
Martin, 1991). Beside these examples, by carefully choosing the electrolyte, one-
dimensional nano structures have been synthesized, such as ZnO (Zheng et al., 2002),
Cu 2 O (Liu and Zhou, 2005) and NiO (Mintz et al., 2005).
Sol-gel Deposition. The sol-gel synthesis method has been utilized to create one-
dimensional nanostructures with a porous membrane (Hulteen and Martin, 1997;
Marinakos et al., 1998). The sol-gel process is based on the use of a gel composed of
aggregated sol particles. A membrane template is immersed into the sol suspension, and
sol aggregates on the template surface. Depending on the appropriate deposition time,
sol particles fill the pores of the template and form nanostructures with a high aspect
ratio. Then, the gel is removed by a thermal treatment, thereby completing the synthesis
of desired nanostructures, such as one-dimensional Au arrays (Marinakos et al., 1998),
ZnO (Chen et al., 2005), ZrO 2 (Xu et al., 2003), and TiO 2 (Zhang et al., 2001) nanorods
and MnO 2 nanowires (Wang et al., 2005).
For example, Wang et al. (2005) demonstrated synthesis of MnO 2 nanowires, in
which manganese acetate was used as the cationic source, and citric acid and n -propyl
alcohol as the monomers for forming the polymeric matrix. Mn(CH 3 COO) 2 was added
into a mixture of citric acid and n -propyl alcohol with a pH of 8.5. This solution was
heated at 80 °C for formation of a transparent viscous sol. The anodized aluminum oxide
(AAO) templates were slowly dipped into the sol for the desired period of time. The
AAO templates containing sol into the pores then were taken out of the sol, dried at
80 °C and then heated at 400 °C. Finally, MnO 2 nanowires were separated by dissolving
the heat-treated templates in NaOH solution.
2.3.2.2 Template-Free Synthesis
This method is relatively convenient and cheap compared with the template-
based method. In the case of template-based synthesis, separation of the nanostructure
synthesized from the template is not a simple process. However, this separation step is
not required in template-free synthesis, though synthesis is difficult to achieve (Eftekhari
et al., 2005). Several methods will be briefly explained below, including the surfactant-
assisted method, sonochemical method, and hydrothermal method (Lu et al., 2006).
Surfactant-Assisted Method. In this method, the surfactant plays an important
role in synthesizing the desired one-dimensional nanostructure. When no surfactant or
suitable surfactant was used in the process, only nanospheres were obtained. The
surfactant acts as a soft template, and this template action enables the epitaxial growth of