Chemistry Reference
In-Depth Information
Fig. 5.1
Photo comparison of a silicon substrate (
on top
) a core-shell nanowires sample synthesized
without the use of surfactant (
in the mid-dle
) and a sample fully covered by nanowires with catalyst
deposi-tion carried out with the aid of an organic surfactant (
on the bottom
). The white stains in the
middle sample are areas covered by nanowires bundles
Nanoparticles
An excellent method to deposit catalyst was found to be the simple
drop casting of a nanoparticles solution. For the aim of the present work it was used
an acetone solution of magnetite (Fe
3
O
4
) nanoparticles obtained by coprecipitation
following the method proposed by Massart
1981
.
The solution was spread over the surface of silicon substrate. The acetone drying
process is fast compared to the aggregation kinetics of the nanoparticles (Velarde
et al.
2007
), so it is possible to obtain a uniform superficial distribution.
Drop Casting of Nitrates
For the growth of core-shell nanowires, a very simple
technique to obtain a metallic film on the substrate was found to be the drop casting
of an ethanol solution of nickel (II) nitrate (Ni(NO
3
)
2
) or Iron(III) nitrate Fe(NO
3
)
3
.
After HF treatment to remove native oxide, the surface of silicon is mainly com-
posed of Si atoms bonded with 2 hydrogen atoms each (Ubara et al.
1984
) and it
has a low wettability with ethanol solution because it is non-polar. To enhance the
hydrophobic surface wettability an organic surfactant (oleylamine) was added to the
catalyst solution. The hydrophobic “tail” of oleylamine molecule is in contact with
hydrophobic silicon surface, while hydrophilic “head” is facing the polar solution
(Negri
n.d
).
The drying process is performed at 40
◦
C for less than 20 min. to limit the for-
mation of silicon oxide on the surface; as a result, a uniform layer of nitrate is
obtained.
The uniformity of the nanowires bundle covering the whole substrate is visible
with bare eye (Fig.
5.1
).
5.3.3
Dewetting
A thin film deposited on a solid substrate is frequently a metastable phase and it
agglomerates when the temperature is raised to the melting point of the metal or of
