Electrochemical Opening of Single-Walled Carbon Nanotubes Filled with Metal Halides and with Closed Ends

Abstract

The electrochemical opening of closed-ended, pristine single-walled carbon nanotubes (SWCNTs) upon the application of either a sufficiently oxidizing or reducing electrode potential is reported. Hitherto, it has been unclear whether the side walls of SWCNTs are electrochemically active, or whether, like their multiwalled counterparts (MWCNTs), the electroactive sites on SWCNTs also reside at the edge-plane-like defects at the open ends of the tubes. Evidence is presented herein that suggests the latter case is true, i.e., that SWCNTs require edge-plane sites to be electroactive. Comparisons of the voltammetric response of end-closed SWCNTs (EC-SWCNTs), end-open (EO-SWCNTs), and SWCNTs encapsulating a metal halide filling (MX@SWCNTs, where MX represents either NaI or CuI) in aqueous electrolytes indicate that SWCNTs undergo electrochemical opening if the applied electrode potential is greater than +1.2 V vs SCE or less than -1.5 V vs SCE. This was further confirmed using ex situ X-ray photoelectron spectroscopy. The nonaqueous voltammetry of NaCl@SWCNTs, NaI@SWCNTs, CuI@SWCNTs, and ZnCl2@SWCNTs in dimethyl formamide (DMF) containing 0.1 M tetrabutyl ammonium perchlorate (TBAP) all exhibited voltammetric responses identical to that of EC-SWCNTs unless the potential was cycled beyond ca. +1.6 V vs Ag (+2.129 V vs the cobaltocene/cobaltocenium redox couple) whereupon voltammetry corresponding to the filling material was observed, again indicating that the SWCNTs had become open-ended. Evidence for quantized charging of the EC-SWCNTs is presented in terms of the unusual “bow-tie” shape of the background charging current in DMF is also presented

Quantitative nanoscale visualization of heterogeneous electron transfer rates in 2D carbon nanotube networks

“…This was particularly important because the number of defects present in the SWNTs remains at the intrinsic value. Naturally, SWNTs contain no edge-plane-like sites of the type held responsible for the EC response in MWNTs (41); also, for SWNTs grown by CVD, the ends are likely to be closed, which might be expected to lead to very slow HET kinetics (24,25). The only other type of defect site that one could reasonably consider is point defects in the sidewall, identified through electrodeposition (42).…”

“…It has been suggested (24,25,41,49,50) that sidewalls are electrochemically inactive in CNTs and that only edge-plane-like sites and open oxygenated nanotube ends are active. Our work shows this earlier hypothesis is incorrect.…”

“…A popular approach for studying electrochemistry at CNTs involves drop-casting the material onto an electroactive support (23,24), but this makes it difficult to unambiguously identify the contribution from CNTs alone. These voltammetric studies have led to an interpretation that CNTs are active only at edge-planelike defects in multiwalled nanotubes (MWNTs) and at the open oxygenated ends of MWNTs and SWNTs (23)(24)(25), with the sidewall inactive, even for simple redox couples.…”

“…These voltammetric studies have led to an interpretation that CNTs are active only at edge-planelike defects in multiwalled nanotubes (MWNTs) and at the open oxygenated ends of MWNTs and SWNTs (23)(24)(25), with the sidewall inactive, even for simple redox couples. A separate approach has been to study CNTs on an inert substrate to ensure that the electrochemical (EC) signal measured can be uniquely attributed to the CNT material used.…”

Intrinsic electrochemical activity of single walled carbon nanotube–Nafion assemblies

“…66 However, these models of reactivity are typically derived from voltammetric studies performed on CNT dispersions or films cast onto a conductive support, which makes it difficult to distinguish the role of the CNTs independently of the support material. [63][64][65][66] On the other hand, work on pristine SWNTs synthesised by catalysed chemical vapour deposition (cCVD), have found ET at the side wall to be facile. Notably, a format comprising an individual SWNT on an insulating support as the electrode, where the ends were not exposed to solution, displayed fast ET, 55 as did alternative studies on individual SWNTs.…”