Catalytically grown carbon nanotubes: from synthesis to toxicity

Catalytic chemical vapour deposition (CCVD) is currently the most promising technique to produce carbon nanotubes (CNTs) on a specific site as well as on a large-scale. Here, we review our recent experimental studies on CNTs grown by CCVD. The yield as well as the structural characteristics of CNTs can strongly be affected by the choice of the catalyst as well as the catalyst support. In particular, CaCO3 is found to be an excellent support material which actively contributes to the CNT growth. Our systematic study of the elastic measurements of multi-walled CNTs grown by CCVD indicates that Young's modulus is generally low, independent from their precise growth conditions. This behaviour is attributed to the high density of structural defects typically present in CCVD grown CNTs, which cannot be healed by additional heat treatment. However, Young's modulus of about 1 TPa is found for double-walled CNTs indicating that CCVD grown CNTs can also have a high strength as arc-discharge tubes as long as their diameter is small. Furthermore, CNTs are mechanically attached to scanning probe tips and tested in contact as well as in tapping mode. The scans reveal that the contact between the probe and the CNT is the major problem. Finally, we compare the cytotoxicity of CNTs with that of carbon nanoparticles as well as nanofibres. Our results indicate that toxicity strongly depends on the number of chemically active sites on the surface.

Synthesis, characterization and macroscopic manipulation of carbon nanotubes

Csilla Mikó

This thesis describes the synthesis, characterisation and macroscopic manipulation of carbon nanotubes. Two interconnected aspects were investigated: (i) growth of carbon nanotubes by catalytic chemical vapour deposition of acetylene and (ii) assembly and reinforcement of macroscopic carbon nanotube fibres. The first part describes a systematic study of the growth of carbon nanotubes by chemical vapour deposition. In particular, the effect of the growth parameters, such as catalyst composition, catalyst carrier, temperature, and retention time, on the yield and quality of CNTs was investigated. It was found that Fe2Co is the most active catalyst in the bimetallic family of Fe1-ξCoξ, whereas deviation from this ideal compound was identified as poisoning the catalytic reaction. Furthermore, the CaCO3 support was found to be an additional source of carbon. This chemical reaction was explained by the decomposition of CaCO3, which produces CO2 groups that react with acetylene to result in carbon for the growth of CNTs. Hence, it was discovered that carbonate supports are not only acting as an inert support to avoid the coarsening of catalyst particles but as an additional source of reactant during the growth. In the second part, macroscopic, oriented fibres were assembled from CNTs in order to exploit their excellent mechanical properties. Electrical as well as mechanical properties were measured. Although both semiconducting and metallic CNTs are present in the starting material, electrical measurements on the macroscopic ropes of CNTs revealed a hopping-like conductance of the fibres. Measurements of mechanical properties have pointed out that these fibres lack the strength of an individual nanotube. The main problem is the weak van der Waals interaction between the tubes which lead the tubes to slide easily along their axes. This problem was addressed by irradiation experiments: Pure covalent carbon-carbon bonds were established in CNT fibres by a low dose of electron and ultraviolet irradiation resulting in an enhancement of the shear modulus between the tubes. Moreover, electrical measurements have demonstrated that a low dose of electron irradiation can significantly increase the shear modulus via cross-linking of CNTs in the fibre, but a high electron dose disorders the graphitic structure and results in a loss of the intrinsically high Young's modulus of the individual tubes. In contrast to electrons, the ultraviolet photons do not displace carbon atoms in the nanotube, therefore the reinforcement can be achieved by keeping the graphitic structure of the tubes intact.

EPFL2005

Surface-bound nanostructures

Kyumin Lee

This thesis looks at surface-bound nanowires and nanoparticles, the mechanical and the metrological properties of which are of practical importance in the realization of nanometer-scale electronics. Atomic force microscope (AFM) was the main instrument of research. By bending suspended carbon nanotube structures with the AFM, the Young's modulus of carbon nanotubes has been measured. An efficient new technique that involves an ac-dielectrophoresis preparation of carbon nanotubes on v-groove GaAs substrates and a force-curve measurement of the stiffness has been devised. The Young's modulus of a batch of multiwall carbon nanotubes grown by a single chemical vapor deposition (CVD) process shows a strong diameter dependence, indicating that the small catalyst particles produce crystalline tubes, while the thicker particles produce low-quality tubes with an abundance of structural defects. The experimental result is a strong evidence for the metastable-catalyst growth model of carbon nanotubes in CVD — the growth kinetics of carbon nanotubes is determined by the catalyst's liquid skin, which is more stable for smaller catalysts. As the nanotube study highlighted the importance of the size of catalyst nanoparticles, the topic of accurate nanoparticle sizing by dynamic AFM was then investigated. The measured size of a surface-bound nanoparticle was found to vary with imaging parameters, and a theoretical modeling showed that the non-contact — intermittent-contact mode switching can lead to discrepancies. Experimental results confirmed that the mode switching indeed causes the largest error in size measurements. A discrepancy also exists between the all-non-contact-mode and all-intermittent-contact-mode cases, and this anomaly could be explained by the effects of particle-substrate deformation and capillary forces. Nanoparticles were prepared on surfaces by boiling colloid drops on hot surfaces, a new technique developed for the uniformly dispersed deposition of colloidal nanoparticles and nanowires. Our experiments suggest that the actual deposition occurs through the smooth dewetting of liquid microdrops at elevated temperatures. The method is applicable on a wide range of surfaces and materials. Finally, a general haptic interface for the AFM was realized. The interface can be implemented on different AFM models with little effort, and it supports both the contact- and dynamic-AFM operations. Manipulation of gold nanoparticles has been carried out by raster scanning at different dynamic AFM setpoints, a promising approach for a quantitative study of the nanoparticle-substrate friction.

EPFL2008

Striking Influence of the Catalyst Support and Its Acid–Base Properties: New Insight into the Growth Mechanism of Carbon Nanotubes

Donatello Acquaviva, Juan Carlos Andresen, Endre Horvath, Gabor Laurenczy, Arnaud Magrez, Jin Won Seo, Rita Smajda

In the accepted mechanisms of carbon nanotube (CNT) growth by catalytic chemical vapor deposition (CCVD), the catalyst support is falsely considered as a passive material whose only role is to prevent catalytic particles from coarsening. The chemical changes that occur to the carbon source molecules on the surface are mainly overlooked. Here, we demonstrate the strong influence of the support on the growth of CNTs and show that it can be tuned by controlling the acid base character of the support surface. This finding largely, clarifies the CCVD growth mechanism. The CNTs' growth stems from the support where the presence of basic sites catalyzes the aromatization and reduces the complexity of CNT precursor molecules. On basic supports, the growth is activated and CNTs are more than 1000 times longer than those produced on acidic supports. These results could be the bedrock of future development of more efficient growth of CNTs on surfaces of functional materials. Finally, the modification of the aciditiy of the catalyst support during the super growth process is also discussed.