STUDIA UNIVERSITATIS BABES-BOLYAI, PHYSICA, SPECIAL ISSUE, 2003
SYNTHESIS OF METALLIC AND SEMICONDUCTING NANOSTRUCTURES
Stela Pruneanu, Liliana Olenic, G. Mihailescu, D. Lupu,
A.R. Biris, L. Radu-Tudoran1
National Institute of R&D for Isotopic and Molecular
Technologies, O.P.5, P.O. BOX 700, 3400 Cluj-Napoca,
Romania
1
Babes-Bolyai University, Faculty of Biology and
Geology, Clinicilor Street,No. 5-7, 3400 Cluj-Napoca,
Romania
ABSTRACT
Metallic and semiconducting nanostructures were obtained by template
synthesis, using nanoporous alumina membranes as matrix.
We have combined two deposition methods (chemical and
electrochemical) in order to obtain gold nanostructures within the pores of
membranes. On the other hand alumina membranes filled by cobalt
particles were used as template for nanocarbon production by CVD
method. The nanocomposites were characterized by SEM and TEM
methods.
1. INTRODUCTION
Template synthesis method has been playing an important role in
fabrication of many kinds of nanowires and nanotubes. Alumina membranes
prepared by electrochemical oxidation of aluminium, represents the appropriate
material for synthesizing the desired nanostructures. By using alumina membranes
for template synthesis, nano-fibrils, nano-wires or nano-tubules of metals [1,2],
semiconductors [3,4] and carbon [5] have been prepared.
Gold and other metals have been deposited into porous alumina
membranes by chemical or electrochemical reduction of the appropriate salt
solution. We have combined the two methods, in order to obtain gold nano-wires
of about 150nm diameter and of 3 m length.
We have used further the alumina membrane filled by cobalt for obtaining carbon
nanostructures by CVD (chemical vapor deposition) method.
EXPERIMENTAL
High purity aluminium foil was vertically mounted between the two parts
of an electrochemical cell. One compartment was filled with acidic solution (0.3M
oxalic acid) and the other with distilled water. The oxidation was performed at
constant voltage (70V) and low temperature (4…60 C) for about 3 hours. The
membrane was then filled by gold, as following described.
a) Chemical deposition of gold
After preparation, the membrane was thoroughly washed with water and
then immersed for 2 minutes in a mixture of SnCl2 and HCl. After that, the
STELA PRUNEANU, LILIANA OLENIC, G. MIHAILESCU, D.LUPU, A.R.BIRIS, L.RADU-TUDORAN
membrane was activated by treatment with a solution of PdCl2 in HCl (2 minutes)
followed by the treatment with an ammoniacal solution of AgNO3 (2 minutes).
The Ag coated membrane was then immersed in the gold deposition bath,
that contains formaldehyde as reducing agent. Au displaced the Ag particles, since
gold is a more noble metal. The Au particles were catalytic sites for the oxidation
of formaldehyde and the concurrent reduction of Au (I) to Au (0). The gold
deposition process lasted for about 5 hours.
b) Electrochemical deposition of gold
The third step was the electrochemical deposition of gold. This was
performed in a three-electrode cell, by cyclic voltammetry. The potential was
varied between 0…1.5V/SCE at a scan rate of 100 mVsec-1, for about 5 hours.
At the end of these processes, the pores of alumina membrane were filled
with gold nanowires. In order to evidence these nanowires, alumina template was
removed by etching in a solution of 2M NaOH.
c) Electrochemical deposition was used for filling the alumina membrane by cobalt
particles. Further the alumina membrane with reduced catalyst was used in CVD
experiments according to the reference material (6). The catalyst was heated from
room temperature to 550 0C under pure nitrogen flow (94 ml/min).
The cobalt oxide was reduced in additional hydrogen flow (6 ml/min) for two
hours after which the hydrogen stream was stopped and the temperature raised to
6500 C in nitrogen flow. At this temperature, acetylene was admitted in the mixture
stream at 6 ml/min for one hour and the sample was cooled under a nitrogen
stream.
RESULTS AND DISCUSSIONS
Typical TEM pictures of alumina template, prepared in oxalic acid as
described above, are presented in fig.1a. The picture evidences the porous
morphology of the membrane, having the pore diameter of around 150 nm. The
diameter is larger than one expects and this is due to the pore wall dissolution that
take place during barrier layer removal. The TEM picture evidences the
morphology of the membrane fracture, showing the channels.
The gold tubules prepared by template synthesis can be seen in the SEM image
from figure 1b. The diameter of electroplated gold nanoparticles is equivalent to
the pore diameter of alumina template membrane. The amount of gold deposited
into the pores depends on the different pore diameters and time of electrolysis. The
lengths and the aspect ratios of the gold tubules are analyzed by SEM. Gold nanowires of about 150 nm diameter and 3m length have been produced.
TEMPLATE SYNTHESIS OF METALLIC AND SEMICONDUCTING NANOSTRUCTURES
Figure 1. a) TEM picture of alumina membrane showing the pores morphology;
b) SEM picture of gold nanostructures, after dissolution of alumina template
Figure 2a presents the SEM image of the carbon deposit obtained by CVD method
after dissolution of the template membrane.
The SEM analysis points out a high quantity of carbonic product (1) on the porous
oxide (2).
The characteristics of the carbonic product have to be analyzed by TEM method.
The sample was purified by dissolving the aluminium oxide in NaOH 10% for 20
hours followed by washing and dissolution of catalyst (Co) in HCl 37% (boiling
for 2 hours). Figure 2b show the TEM image of nanocarbon structures. The
presence of bundles of aligned straight carbon nanotubes with uniform thickness
having approximately 17 nm outer diameter and approximately 8 nm inner
diameter. This fact reveals that the nanostructures are multi wall nanotubes.
Figure 2.a) SEM picture of carbonic product obtained on the surface of alumina
membranes, filled by cobalt: (1) carbonic product (2) alumina membrane
b)TEM pictures of carbonic product obtained on the surface of alumina
membrane, filled by cobalt
STELA PRUNEANU, LILIANA OLENIC, G. MIHAILESCU, D.LUPU, A.R.BIRIS, L.RADU-TUDORAN
Our results are similar to those obtained in reference material (6). Nanofibers of
about 25 nm diameters are also present in the carbon deposit, grown probably on
the initial deposit of aligned nanotubes.
4. CONCLUSIONS
By combining a chemical and an electrochemical method we have
prepared gold nanostructures, using alumina membrane as template. Aligned multiwall carbon nanotubes of rather high aspect ratio were obtained on nanoporous
alumina membranes filled by cobalt. Metallic and semiconducting nanostructures
obtained by us are similar with those obtained in reference material.
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