TEMPLATE SYNTHESIS OF METALLIC AND SEMICONDUCTING NANOSTRUCTURES
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
1Babes-Bolyai University, Faculty of Biology and Geology, Clinicilor Street,No. 5-7, 3400 Cluj-Napoca, Romania
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.
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  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.
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 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 nano-wires of about 150 nm diameter and 3m length have been produced.
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
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.
By combining a chemical and an electrochemical method we have prepared gold nanostructures, using alumina membrane as template. Aligned multi-wall 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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