Synthesis of Nanoparticles and One-Dimensional Nanomaterials - Nanotechnologies for Water Environment Applications - page 30

subsequently synthesized within their pores. Because these templates have cylindrical
pores, monodisperse nanocylinders of the desired materials can be synthesized within
each pore. Then, depending on the operating parameters, these nanocylinders may be
solid (a nanorod) or hollow (a nanotubule). Despite being a suitable chemical pathway
for synthesizing one-dimensional nanostructures, there are still a number of concerns
pertaining to the use of template-based synthesis. These include (Martin, 1996):
1)

Does the precursor solution 'wet' the pore?

2) Is it possible to avoid pore blockage?

3) Is the template stable under the reaction conditions?

These concerns will be addressed below.

Electrochemical Deposition. Electrochemical deposition within the pores is
accomplished by coating one face of the membrane on an electrode and using this metal
film as a cathode for electroplating. In other words, templates such as electrically-
conductive membrane with desired channels are used for the synthesis of nanowires in
electrochemical deposition. Template is attached onto the cathode, since cations in the
deposition solution reduce at the cathode. The anode is installed in the deposition
solution parallel to the cathode. When the electric field is supplied to the system, cations
diffuse towards the cathode and these are reduced at the cathode. As a result of this
process, nanowires are grown inside the pores of the template.

In general, electrochemical deposition can be used for the synthesis of metals,
alloys, semiconductors, and electrical conductive polymers, since after the initial
deposition the electrode is separated from the precursor solution by the deposited
materials. This means that the deposited material can be used as an electrically-
conductive material to act as a current pathway. The deposition time, the length of the
deposition channel or pore, and current density are the controlling factors for
synthesizing nanostructures using this method (Whitney et al., 1993; Martin, 1996).

Gold nanowires with diameters of 8 nm were synthesized using track-etched
mica as a template. HF acid used to etch the mica also acts as an electrolyte. An Ag thin
layer was deposited on the one side of the mica, and the metal is electroplated into the
pores to form the wires. An Au electroplating solution (pH = 6) was used as a precursor
solution (Williams and Giordano, 1984). In addition, Possin (1970) synthesized several
metallic nanowires such as tin, indium, and zinc nanowires by this method (Possin,
1970).

Hollow metal tubes can also be synthesized by the electrochemical deposition
method. For growth of metal tubules, the pore walls of the template were modified to be
chemically derivatized. Due to this modification, metals preferentially deposit onto the
pore walls instead of the bottom of the electrode. For example, Au nano tubules were