Sustainable manufacturing of vertical carbon nanotube arrays inside insulating nanoporous membranes using nickel nanowires
Carbon nanotubes (CNTs) were successfully synthesized using industrial waste gases by chemical vapor deposition inside the vertically-oriented nanopores of insulating membranes. The gaseous waste products from Fischer-Tropsch synthesis were used as the carbon source . In this study, vertical CNT arrays were achieved using nickel (Ni) nanowires catalysts that were prepared by template electrochemical deposition inside 50 um-thick nanoporous anodized aluminum oxide (AAO). Here, the nanopore diameter (20 and 200 nm) and Ni MNW length (45 and 25 um) were varied to study the impact on CNT growth characteristics. Raman spectroscopy, scanning electron microscopy, transmission electron microscopy, and field emission were used to characterize CNTs on Ni MNWs. For long MNWs (45 um), the Ni catalyst was just below the AAO surface, so CNT diameters did not change appreciably with MNW diameter. Alternatively, for short MNWs (25 um), 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 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 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 150 µA (at electric field 0.5 V/µm) with a turn-on field of 0.26 V/µm. These aligned CNTs can further be used to produce matrix composite heating elements for welding of plastics.