Synthesis, Chemistry and Structure of Boron-Doped Carbon Nanotubes and Nanofibers

Abstract

The objective of this project is to synthesize novel boron-rich carbon in the form of nanotubes and nanofibers and to determine the influence of high concentrations of boron on the sp2 carbon structure. Research efforts on the influence of high concentrations of boron on the sp2 carbon structure have been undertaken through a combination of computational prediction and experimental investigation. A theoretical prediction regarding the boron influence on carbon nanotube structure is carried out using molecular and quantum mechanical simulations. The influence of substitutional boron on strain, tube diameter, tube length and tube stability is discussed. A high boron doping (10-17 at.%) introduces disorder into the host sp2 carbon structure. Boron-doped carbon nanotubes and nanofibers have been synthesized by a catalytic CVD at 750-900°C from C6H6 and BCl3. The deposition temperature and reactant ratio (BCl3/C6H6) influence the microstructure and compositions of the resulting deposits significantly. The boron concentrations of the boron-doped carbon nanotubes and nanofibers are in the range of 5-18 at.% as measured by AES. The optimum temperature to obtain a high boron content sample is around 800°C. XPS and Auger analyses suggest that the C-B bonding is sp2 hybridized and includes B-BC2 and B-C3. Small clusters of boron atoms may be present in the nanofibers deposited at high temperatures that may contribute to disorder/strain within the nanotube and nanofiber structure. The crystal structures and microstructures of boron-doped carbon nanotubes and nanofibers are inhomogeneous. There are mixed crystallite sizes of both La and Lc. Strains occur in the samples synthesized at low temperature (750°C), and lead to a small lattice parameter, d002. Submicron fibers and nanofibers coexist in each sample. The texture of filamentous carbon changes with the change in boron concentrations or deposition temperatures. The boron doping into the carbon nanotubes and nanofibers is structure-related. The platelet stacked structure is always very well-ordered with very low boron concentration (< 2% B/C). Herringbone structures are ordered with relatively low boron concentration (2-8% B/C). The tubular structure is either ordered with a B/C ratio of 3-8% B/C, or trubostratic with a B/C ratio of > 8%. A disordered transition area between herringbone and tubular structure in a nanofiber of Sample I-1 is observed where the boron concentration is 16%, i.e., consistent with a composition of C6B. Mechanisms of boron-doped nanofiber growth are proposed that involve both surface diffusion and bulk diffusion process. Temperature is the governing factor that determines the process by which either surface diffusion or bulk diffusion dominates. Boron doping also influences the ordering of the carbon layer plane stacking.