Sustainable Manufacturing of Vertical Carbon Nanotube (CNT) Arrays Inside Insulating Nanoporous Membranes Using Nickel Magnetic Nanowires (MNWs)

Carbon nanotubes (CNTs) were successfully synthesized using industrial waste gases by chemical vapor deposition inside vertically oriented nanopores of insulating membranes. Importantly, the waste products from Fischer–Tropsch synthesis were used as the carbon source rather than typical purified sources, and this recycling of carbon is important for the sustainability of our environment. Specifically in this work, vertical CNT arrays were achieved using nickel (Ni) magnetic nanowires (MNWs) catalysts that were prepared by template electrochemical deposition inside 50 μm-thick nanoporous anodized aluminum oxide (AAO). Here, the nanopore diameter (20–200 nm) and Ni MNW length (45 and 25 μm) were varied to study the impact on CNT growth characteristics. Raman spectroscopy, scanning electron microscopy, transmission electron microscopy, and field emission spectroscopy were used to characterize CNTs on Ni MNWs. For long MNWs (45 μm), the Ni catalyst was just below the AAO surface, so CNT diameters did not change appreciably with the MNW diameter. Alternatively, for short MNWs (25 μm), the carbon source gases had to diffuse into the AAO nanopores before reacting with the Ni catalyst, and both the CNT diameter and yield increased with the nanopore diameter. Highly crystalline CNTs were formed from particles of Ni catalyst, although for smaller diameter nanopores, the Ni catalyst particle could be blocked by template pore wall defects, resulting in subsequent amorphous nanofiber growth above the blocked particle. Optimally, CNT synthesis was observed for 25 μm MNWs grown in 80 nm AAO nanopores, maximizing field emission current at 480 μA/cm2 (at electric field 0.5 V/μm) with a turn-on field of 0.26 V/μm.