Transition Metal Catalyst Aided Growth of Novel Carbon Nanostructures

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With their special morphologies and novel properties, the one-dimensional (1D) carbon nanostructures (CNs) have attracted a great attention over many years. Their unusual atomic architecture endows not only their extensive application capability in electronic transport, thermal conductivity, fluorescence，field emission, hydrogen storage etc., but also their ability to be used as templates for preparing other types of 1D and 2D nanostructures. As one significant member of the 1D CN’s family, carbon nanotubes (CNTs), with cylindrical and hollow characteristics, are extensively investigated with regards to their synthesis strategies, structural characteristics, growth mechanism and moreover with regards to their properties and applications. The synthesis of CNTs is usually carried out by catalytic chemical vapor deposition (CVD) techniques by employing transition metals (Fe, Co, Ni and their alloys) as catalysts. The advantage of the catalytic CVD techniques is in their large-scale production ability of the CNTs at low temperatures as well as at low costs. Besides CNTs, many other types of one dimensional carbon nanostructures namely nanocones, nanobells, nanocoils, nanohelixes etc., are also fabricated via transition metal catalyzed CVD processes by using different reaction conditions. It is interesting to note that, among these CNs, new types of polymer nanostructures (PNs), in which remarkable amount of hydrogen atoms remain, can be synthesized at a low temperature of 473 K with catalytic Cu nanoparticles. Because of different stacking modes and sizes of graphene sheets, PNs show much novel morphology compared with those of CNTs, and furthermore, they show interesting properties related to their high specific surface, such as ferromagnetic properties and potential hydrogen storage capability after carbonization. Additionally, the morphologies as well as properties of PNs strongly depend on geometries of corresponding catalysts (shape and size). These novel properties indicate that the PNs have significant potential application in future nanotechnology. However, the growth mechanism of the polymer nanofibers (PNFs), especially the size and shape effect of catalyst nanoparticles on the structure characteristic of the PNFs is still unclear. On the other hand, the fact that the PNFs were produced at much lower temperature than that of CNTs indicates a distinct growth mechanism from the vapor-liquid-solid (VLS) mechanism of CNTs synthesized at 1000 K. Due to the low reaction temperature of PNFs grown on Cu catalyst nanoparticles and the low solubility of carbon in Cu at all temperature, the growth mechanism of the PNFs is supposed to be surface diffusion mechanism, which is based on the concept that oligomers form and diffuse on catalyst particles surface instead of dissolving and transporting of carbon atoms throughout catalyst particles (VLS theory). Subsequently, Fe besides Cu nanoparticles were found to be also active to produce PNFs when similar reaction conditions were applied. Accordingly, it is reasonable to expect more transition metals, such as Co, Ni nanoparticles are also able to produce PNFs. Therefore detailed investigations of growth thermodynamic and kinetic processes of forming PNFs are urgently required. Beyond that, the aim of this proposal is not only to synthesize novel morphological PNFs employing transition metals, especially Fe, Co and Ni, nanoparticles as catalysts at low temperature, but also to systematically characterize the microstructures of PNFs and to understand the relationships between catalyst structures (size, shape, surface electronic structure) and corresponding PNFs structures. Practically, Fe, Co and Ni, nanoparticles with various size and shape will be prepared as the catalysts to synthesize different types of PNFs by using thermal CVD as the growth technique. The correlation between the microstructures of the resultant novel PNFs and the geometry (shapeand size) of the responsible catalysts will be researched in detail in order to establish a generalized understanding on the surface diffusion mechanism at low temperature. The success of this project will have not only a scientific significance but will also provide a technological boost in designing nanodevices.