Controlled CVD Growth of Carbon Nanotubes on Monolayer Graphene Substrates

Abstract

Graphene and carbon nanotubes exhibit exceptional electrical and material properties. Full utilization of these properties in three dimensional nano-architectures towards envisioned applications necessitates that the integrity of the graphene layer during the CNT growth step is maintained. Recent efforts in fabricating 3D composite nanostructures consisting of 2D graphene and 1D nanomaterials of carbon and conducting polymer are of interest for a number of applications, including next-generation, high capacity, fast-discharge supercapacitors. For these types of energy storage applications, the advantages of graphene arising from the nature of its dimensionality, such as large surface area-to-volume ratio and excellent conductivity, may be compromised due to self-aggregation and charge transfer between the graphene flakes, 1D nanomaterials, and current collector during fabrication. The direct growth of 1D nanostructures such as carbon nanotubes directly on graphene to yield hybrid 3D nano-architectures would, by design, circumvent this self-aggregation, while maintaining low contact resistance to enable effective electron transfer. In our previous work, CNTs were directly grown by chemical vapor deposition (CVD) on graphene using CH4 gas as a carbon source, and the performance of the resultant 3D nanoarchitecture as an advanced electrical double layer capacitor was characterized. However, during this earlier CNT growth, the graphene layer was often found to be etched away at so-called “etched tracks”. We attribute the formation of these etched tracks to hydrogenation by the catalyst nanoparticles (Nickel (Ni)nanoparticle + Cgraphene + 2H2 → Ni + CH4) and their subsequent expansion to carbothermal reduction (SiO2 + Cgraphene → SiO(g) +CO(g)) and hydrogen etching at the tracks’ defect points. This graphene etching process during CNT growth has thus far not been extensively studied in the literature. Therefore, in this work, we grow CNTs atop graphene substrates using C2H4 to demonstrate that the high hydrocarbon conversion rate of this gas, at relatively lower temperature than the more stable CH4 used in our previous study, allows optimization of CNT growth through fine tuning the process parameters including growth temperature, gas flow rate and gas ratio. Further, we optimize the supersaturation state of the catalyst nanoparticles by adjusting the seed density, temperature and carbon source concentration. We also confirm that the controlled use of C2H4is essential for balancing the competing processes of carbon deposition and carbon removal, which ultimately blocks undesired etching of the graphene substrate during the CNT growth process.