Investigation of the behavior of the nickel catalyst in chemical vapor deposition synthesis of carbon nanopearls

Carbon nanopearls (CNPs), also known as carbon spheres and nanospheres, are of interest to the nanoscience community due to their field emission and tribology capabilities. There have been numerous reports on the properties and potential applications of CNPs; however, there have been few studies on the behavior of the catalyst during synthesis. Carbon nanopearls are limited to being used as cold cathodes and lubricants for tribology if the nickel catalyst remains. This research focused on studying the behavior of the nickel catalyst during chemical vapor deposition of CNPs. Carbon nanopearls were grown at various growth times (10 sec, 30 sec, 60 sec, 90 sec, 120 sec and 300 sec) using two different nickel catalyst sizes (20 nm nickel nanoparticles and 100 nm nickel nanoparticles). Chemical analysis was conducted using X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). This enabled observation of the chemical phases as growth time increased. Imaging of the CNPs samples was performed using transmission electron microscopy (TEM). Raman spectroscopy was performed to observe the defects and order in the graphitic structures as growth time varied. The melting temperature of the nickel nanoparticles was investigated experimentally by performing differential scanning calorimetry (DSC) on the nickel catalyst. Theoretically, the melting temperature was calculated using the Gibb-Thomson equation. The question does the Ni catalyst evaporate during synthesis of carbon nanopearls was addressed both theoretically and experimentally. Theoretically, the Kelvin effect was used to calculate the vapor pressure of the nickel nanoparticles. The vapor pressure of the nanoparticles was compared to the vapor pressure for bulk nickel, and this helped to determine if the nanoparticles were evaporating. Weight loss experiments were conducted and thermal gravimetric analysis (TGA) was performed on the nickel nanoparticles. These experiments were used to identify the temperature of evaporation. The results from this research showed that during the synthesis process, the Ni oxidized. XRD and XPS showed that the nickel oxide reduced as growth time increased, followed by the formation of a nickel carbide phase. Towards the longer growth times, the carbide decomposed leaving only nickel and graphite. TEM results revealed that the remaining nickel did not exist in the core of the carbon nanopearl, but that it was nickel that had segregated from the CNPs and agglomerated with other nickel particles. DSC identified the melting temperature of the 20 nm nickel nanoparticles to be lower than the bulk melting temperature of nickel. The Gibbs-Thomson effect was used as a guideline for determining the melting temperature of the nanoparticles. Oxidation of the nickel nanoparticles prevented determination of the evaporation temperature. Results from the Kelvin effect indicated that the Ni nanoparticles evaporate sooner than bulk nickel. However, due to XRD identifying Ni at the longer growth times, there was no evidence to conclude that the Ni had evaporated. Finally, a model for CNPs growth was presented based off the results in this research

Investigation of the Behavior of the Nickel Catalyst in Chemical Vapor Deposition Synthesis of Carbon Nanopearls

Carbon nanopearls (CNPs), also known as carbon spheres and nanospheres, are of interest to the nanoscience community due to their field emission and tribology capabilities. There have been numerous reports on the properties and potential applications of CNPs; however, there have been few studies on the behavior of the catalyst during synthesis. Carbon nanopearls are limited to being used as cold cathodes and lubricants for tribology if the nickel catalyst remains. This research focused on studying the behavior of the nickel catalyst during chemical vapor deposition of CNPs. Carbon nanopearls were grown at various growth times (10 sec, 30 sec, 60 sec, 90 sec, 120 sec and 300 sec) using two different nickel catalyst sizes (20 nm nickel nanoparticles and 100 nm nickel nanoparticles). Chemical analysis was conducted using X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). This enabled observation of the chemical phases as growth time increased. Imaging of the CNPs samples was performed using transmission electron microscopy (TEM). Raman spectroscopy was performed to observe the defects and order in the graphitic structures as growth time varied. The melting temperature of the nickel nanoparticles was investigated experimentally by performing differential scanning calorimetry (DSC) on the nickel catalyst. Theoretically, the melting temperature was calculated using the Gibb-Thomson equation. The question does the Ni catalyst evaporate during synthesis of carbon nanopearls was addressed both theoretically and experimentally. Theoretically, the Kelvin effect was used to calculate the vapor pressure of the nickel nanoparticles. The vapor pressure of the nanoparticles was compared to the vapor pressure for bulk nickel, and this helped to determine if the nanoparticles were evaporating. Weight loss experiments were conducted and thermal gravimetric analysis (TGA) was performed on the nickel nanoparticles. These experiments were used to identify the temperature of evaporation. The results from this research showed that during the synthesis process, the Ni oxidized. XRD and XPS showed that the nickel oxide reduced as growth time increased, followed by the formation of a nickel carbide phase. Towards the longer growth times, the carbide decomposed leaving only nickel and graphite. TEM results revealed that the remaining nickel did not exist in the core of the carbon nanopearl, but that it was nickel that had segregated from the CNPs and agglomerated with other nickel particles. DSC identified the melting temperature of the 20 nm nickel nanoparticles to be lower than the bulk melting temperature of nickel. The Gibbs-Thomson effect was used as a guideline for determining the melting temperature of the nanoparticles. Oxidation of the nickel nanoparticles prevented determination of the evaporation temperature. Results from the Kelvin effect indicated that the Ni nanoparticles evaporate sooner than bulk nickel. However, due to XRD identifying Ni at the longer growth times, there was no evidence to conclude that the Ni had evaporated. Finally, a model for CNPs growth was presented based off the results in this research