Small and large single-walled carbon nanotubes (SWCNTs) bundles from different-sized cobalt catalyst clusters have been synthesized and prepared through chemical vapor deposition (CVD) method by using Co-acetate ethanol solution with silica nanoparticles. By controlling concentration of Co-acetate ethanol solution (0.2 wt% and 0.4 wt%), various sizes and types of bundle of SWCNTs are grown on the silica nanoparticle substrates. Synthesized SWCNT’s diameter ranged from 0.92 nm to 1.63 nm, and chirality of SWCNTs and their electronic property from high concentration solution show diverse characteristics. In high concentration solution, the large number of cobalt clusters is induced to merge on the surface of silica nanoparticles and then lots of nucleation points are provided by cobalt clusters for growth of SWNTs. These results give us a promising path to selectively synthesize various types of SWCNTs with different shapes of merged cobalt catalyst. Engineering bundle sizes of SWCNTs can be promising key for diverse applications of carbon nanotubes.
In the last few decades, carbon nanotubes (CNTs) have been attracting attention due to their unique and outstanding structural, electrical, and mechanical properties [
The goal of this experiment is to control selectively the growth size of SWCNT bundles by controlling the size of cobalt nanoparticle clusters. Figure
Schematics of single-walled carbon nanotubes synthesis by chemical vapor deposition process. (a) At first, cobalt-acetate ethanol solution (0.2 wt% and 0.4 wt%) is prepared. (b) 50 nm and 200 nm silica nanoparticles were added to Co-acetate ethanol solution. (c) Ultrasonication of the solution for 90 minutes in bath-type sonicator. Cobalt particles gather together making clusters on the surface of silica nanoparticles. (d) Previous sample is dropped on Si wafer and dried. This wafer goes through the CVD process.
Synthesis of SWCNTs was carried out by using a thermal CVD process. Sonicated solution was dropped on Si wafer (1 cm × 1 cm) by pipette. The wafer was then dried and baked in furnace at 400°C for 5 minutes in air to burn-out organic materials. Prepared substrate was placed in a quartz tube inside a furnace. After that, the quartz tube was evacuated, and high purity argon gas (99.9%) as a carrier gas was flowed into the quartz tube and simultaneously temperature was heated up to 950°C. After the desired temperature was reached, Ar gas was stopped and high purity methane (99.99%, 100 sccm, 5 min) was introduced as a carbon source for SWCNTs growth. After the reaction, methane gas was stopped and Ar gas was supplied again during the temperature cooling to room temperature.
Figure
Scanning electron microscopy images of small bundles of SWNTs. Low and high magnification SEM images of small-bundled SWNTs networks grown from 0.2 wt% Co-acetate ethanol solution. (a) Two- or three-dimensional SWNTs bundles networks are grown on 50 nm silica nanoparticles. (b) High magnification SEM image of the same sample shown at (a). Small size bundles of SWNTs are interconnected on silica nanoparticles. (c) SWNTs bundles grown on 200 nm silica nanoparticles. (d) High magnification SEM image of sample on 200 nm silica nanoparticles. The bundle size is slightly bigger than that of image (b).
Scanning electron microscopy images of large bundles of SWNTs. Low and high magnification SEM images of small-bundled SWNTs networks grown from 0.4 wt% Co-acetate ethanol solution. (a) Network of bundled SWNTs on 50 nm silica nanoparticles. (b) High magnification image of (a). SWNTs bundles are grown from silica nanoparticles and make connection with each other. (c) SEM image of SWNT bundles on 200 nm silica nanoparticles. (d) High magnification image of (c). More and bigger bundles are grown from silica particles and also make more connections with other bundles or SWNTs.
To investigate diameter distributions of as-grown SWCNTs, radial breathing mode (RBM) Raman spectra were recorded from SWCNTs synthesized with two different concentrated Co-acetate solutions on the surface of 200 nm silica particle. Figure
Raman spectra and transmission electron microscopy images from small- and large-bundled SWNTs grown from cobalt clusters on 200 nm silica particle. (a) Raman spectra of large-bundled SWNTs are colored in blue. This curve shows 6 representative peaks (152 cm−1, 208 cm−1, 219 cm−1, 228 cm−1, 236 cm−1, and 268 cm−1). Small-bundled SWNTs (red colored spectra) show 2 representative peaks (158 cm−1 and 207 cm−1). Each peak has information about corresponding nanotubes chirality and electronic property. (
We confirmed that as-synthesized CNTs have 98% purity of SWCNTs (Figures
Different sizes of SWCNT bundles grown from Co clusters are schematically displayed in Figure
Schematics bundled SWNTs grown from cobalt clusters on 200 nm silica particle. SWNTs were grown from cobalt particles (black sphere). (a) Small bundle of SWNTs is grown from small size of Co cluster. This growth occurs in sample with 0.2 wt% solution. (b) Large bundle of SWNTs attached to large size of Co cluster. This growth corresponds to the sample with high concentration (0.4 wt%).
In this study, we synthesized the small and large SWCNTs bundles from different-sized cobalt catalyst clusters, prepared through CVD method by using Co-acetate ethanol solution with silica nanoparticles. We have shown that more various sizes and types of bundle of SWCNTs are grown by using high concentration Co-acetate ethanol solution (0.4 wt%). We synthesized SWCNTs with diameter from 0.92 nm to 1.63 nm. And chirality and electronic properties of SWCNTs from high concentration solution are also various. This result is due to more and larger cobalt clusters that merged on the surface of silica nanoparticles, made from high concentration solution. And these cobalt clusters provide a lot of nucleation sites for growth of SWNTs. On the basis of our experiment, selective growth of SWCNTs can be conducted by using different shapes of merged cobalt catalyst. Further, bundle size engineering of SWCNTs can be important key for wide application of carbon nanotubes.
The authors declare that there are no conflicts of interest regarding the publication of this paper.
This work was supported by Inha University Research Grant (INHA-53349).