Sustainable Manufacturing\nof Vertical Carbon Nanotube\n(CNT) Arrays Inside Insulating Nanoporous Membranes Using Nickel Magnetic\nNanowires (MNWs)

Carbon nanotubes (CNTs) were successfully synthesized\nusing industrial\nwaste gases by chemical vapor deposition inside vertically oriented\nnanopores of insulating membranes. Importantly, the waste products\nfrom Fischer–Tropsch synthesis were used as the carbon source\nrather than typical purified sources, and this recycling of carbon\nis important for the sustainability of our environment. Specifically\nin this work, vertical CNT arrays were achieved using nickel (Ni)\nmagnetic nanowires (MNWs) catalysts that were prepared by template\nelectrochemical deposition inside 50 μm-thick nanoporous anodized\naluminum oxide (AAO). Here, the nanopore diameter (20–200 nm)\nand Ni MNW length (45 and 25 μm) were varied to study the impact\non CNT growth characteristics. Raman spectroscopy, scanning electron\nmicroscopy, transmission electron microscopy, and field emission spectroscopy\nwere used to characterize CNTs on Ni MNWs. For long MNWs (45 μm),\nthe Ni catalyst was just below the AAO surface, so CNT diameters did\nnot change appreciably with the MNW diameter. Alternatively, for short\nMNWs (25 μm), the carbon source gases had to diffuse into the\nAAO nanopores before reacting with the Ni catalyst, and both the CNT\ndiameter and yield increased with the nanopore diameter. Highly crystalline\nCNTs were formed from particles of Ni catalyst, although for smaller\ndiameter nanopores, the Ni catalyst particle could be blocked by template\npore wall defects, resulting in subsequent amorphous nanofiber growth\nabove the blocked particle. Optimally, CNT synthesis was observed\nfor 25 μm MNWs grown in 80 nm AAO nanopores, maximizing field\nemission current at 480 μA/cm<sup>2</sup> (at electric field\n0.5 V/μm) with a turn-on field of 0.26 V/μm.