A Forest of Sub-1.5-nm-wide Single-Walled Carbon Nanotubes over an Engineered Alumina Support

Abstract

A precise control of the dimension of carbon nanotubes (CNTs) in their vertical array could enable many promising applications in various fields. Here, we demonstrate the growth of vertically aligned, single-walled CNTs (VA-SWCNTs) with diameters in the sub-1.5-nm range (0.98 ± 0.24 nm), by engineering a catalyst support layer of alumina via thermal annealing followed by ion beam treatment. We find out that the ion beam bombardment on the alumina allows the growth of ultra-narrow nanotubes, whereas the thermal annealing promotes the vertical alignment at the expense of enlarged diameters; in an optimal combination, these two effects can cooperate to produce the ultra-narrow VA-SWCNTs. According to micro- and spectroscopic characterizations, ion beam bombardment amorphizes the alumina surface to increase the porosity, defects, and oxygen-laden functional groups on it to inhibit Ostwald ripening of catalytic Fe nanoparticles effectively, while thermal annealing can densify bulk alumina to prevent subsurface diffusion of the catalyst particles. Our findings contribute to the current efforts of precise diameter control of VA-SWCNTs, essential for applications such as membranes and energy storage devices.

Carbon Nanotubes and Related Nanomaterials: Critical Advances and Challenges for Synthesis toward Mainstream Commercial Applications

“…165 Furthermore, both ion-and electron bombardment of a sapphire surface, which is otherwise inactive for CNT growth, was shown to increase surface roughness and enable CNT forest growth. 166,167 This approach of engineering the topography of the support layer by ion bombardment promoted growth of small-diameter CNTs, 165,166 suggesting that smaller particles remained sufficiently stable for nucleation and growth. Such a pretreatment of oxide supports to improve growth has also been extended to other supports such as sputtered MgO.…”

Controlling the Tube Diameter of SWCNTs Using High Melting Point Promotors

“…[29] Pretreatment of the catalyst support by ion beam bombardment and thermal annealing was found to be an effective approach to minimize the effect of OR and obtain ultra-narrow tube diameter in SWCNT. [30] Inhibiting the Ostwald ripping was also reported by introducing H2O along with H2 during annealing to reduce the diffusion rates of catalyst atoms. [31] The nanoparticles tried to minimize their surface energy by making big clusters at high temperatures.…”

“…Thermal dewetting of the deposited metal films into finely/homogeneously nanoparticles is highly dependent on the melting point of that metal and the porosity of the under layer. [30] We hypothesis that using high melting temperature metal as a promotor, like Ru can limit the mobility of the formed Co particles and eventually decrease the effects of OR. The same observation has been reported by using platinum (Pt) with Co as a bimetallic catalyst, [32] where Pt plays a crucial role to stabilize the Co catalyst and obtain a narrower tube diameter.…”

Thermal Transport in Molecular Forests

“…Using these estimates, we can analyze different molecular forests. For example, the typical heights of CNT forests or arrays of NWs range from 0.1 to 2 mm, , while in some cases they can reach several millimeters to even a few centimeters . For bundles of PE or PEDOT, .…”