Mechanical interlocking of SWNTs with N-rich macrocycles for efficient ORR electrocatalysis.

Substitutional N-doping of single-walled carbon nanotubes is a common strategy to enhance their electrocatalytic properties in the oxygen-reduction reaction (ORR). Here, we explore the encapsulation of SWNTs within N-rich macrocycles as an alternative strategy to display electroactive sites on the surface of SWNTs. We design and synthesize four types of mechanically interlocked derivatives of SWNTs (MINTs) by combining two types of macrocycles and two types of SWNT samples. Comprehensive electrochemical characterization of these MINTs and their reference SWNTs allows us to establish structure-activity relationships. First, we show that all MINT samples are superior electrocatalysts compared to pristine SWNTs, which serves as general validation of our strategy. Secondly, we show that macrocycles displaying both N atoms and carbonyl groups perform better than those with N atoms only. Finally, we demonstrate that a tighter fit between macrocycles and SWNTs results in enhanced catalytic activity and stability, most likely due to a more effective charge-transfer between the SWNTs and the macrocycles. These results, focusing on the ORR as a testbed, show the possibility of understanding electrocatalytic performance of SWNTs at the molecular level and thus enable the design of more active and more stable catalysts in the future.

Positive and negative regulation of carbon nanotube catalysts through encapsulation within macrocycles.

One of the most attractive applications of carbon nanomaterials is as catalysts, due to their extreme surface-to-volume ratio. The substitution of C with heteroatoms (typically B and N as p- and n-dopants) has been explored to enhance their catalytic activity. Here we show that encapsulation within weakly doping macrocycles can be used to modify the catalytic properties of the nanotubes towards the reduction of nitroarenes, either enhancing it (n-doping) or slowing it down (p-doping). This artificial regulation strategy presents a unique combination of features found in the natural regulation of enzymes: binding of the effectors (the macrocycles) is noncovalent, yet stable thanks to the mechanical link, and their effect is remote, but not allosteric, since it does not affect the structure of the active site. By careful design of the macrocycles' structure, we expect that this strategy will contribute to overcome the major hurdles in SWNT-based catalysts: activity, aggregation, and specificity.