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

Abstract

Carbon nanotubes have attracted considerable interest for electrochemical, electrocatalytic, and sensing applications, yet there remains uncertainty concerning the intrinsic electrochemical (EC) activity. In this study, we use scanning electrochemical cell microscopy (SECCM) to determine local heterogeneous electron transfer (HET) kinetics in a random 2D network of single-walled carbon nanotubes (SWNTs) on an Si∕SiO 2 substrate. The high spatial resolution of SECCM, which employs a mobile nanoscale EC cell as a probe for imaging, enables us to sample the responses of individual portions of a wide range of SWNTs within this complex arrangement. Using two redox processes, the oxidation of ferrocenylmethyl trimethylammonium and the reduction of ruthenium (III) hexaamine, we have obtained conclusive evidence for the high intrinsic EC activity of the sidewalls of the large majority of SWNTs in networks. Moreover, we show that the ends of SWNTs and the points where two SWNTs cross do not show appreciably different HET kinetics relative to the sidewall. Using finite element method modeling, we deduce standard rate constants for the two redox couples and demonstrate that HET based solely on characteristic defects in the SWNT side wall is highly unlikely. This is further confirmed by the analysis of individual line profiles taken as the SECCM probe scans over an SWNT. More generally, the studies herein demonstrate SECCM to be a powerful and versatile method for activity mapping of complex electrode materials under conditions of high mass transport, where kinetic assignments can be made with confidence.W ithin the family of nanostructured materials, carbon nanotubes (CNTs) have attracted particular attention because they are readily synthesised at low cost, have exceptional electronic properties, exhibit chemical and mechanical stability, and are amenable to a wide range of simple chemical functionalization routes (1-3). These characteristics have led to CNTs being considered ideal substrates for electronics (4), sensing systems (5, 6), electrocatalytic supports (7), and batteries (8). Furthermore, the different configurations in which CNTs can be arranged broaden their versatility and allow custom design of devices for specific applications. Individual single-walled carbon nanotubes (SWNTs) (9), 2D networks (10-12), and 3D nanostructures (13) have all been employed successfully.Understanding heterogeneous electron transfer (HET) at CNTs is of considerable importance, due to the wide range of electroanalytical and electrocatalytic systems based on CNTs (14-17), and also because electrochemistry provides an attractive route to functionalize and tailor the properties of . Probing HET in 1D electrode materials is interesting fundamentally, given their inherent electronic structure and properties (21,22). However, as we highlight herein, despite many studies aimed at characterizing HET at CNTs, substantial questions remain unanswered, such as the location and rate of HET.A popular approach for studying electrochemistry at CNT...

Molecular Functionalization of Graphite Surfaces: Basal Plane versus Step Edge Electrochemical Activity

“…Note that in contrast to SECCM imaging 13,16,41,63,64 with much smaller tips, there was no modulation of the pipet position in these FSCV studies. The DC conductance current (i DC ) was adopted as the feedback signal during approach.…”

Scanning Electrochemical Cell Microscopy Platform for Ultrasensitive Photoelectrochemical Imaging

“…In fact, these have mainly focused on scanning electrochemical microscopy (SECM) based techniques, 2-11 often as a tool for catalyst screening 4,7 on a fairly large length scale and for thick deposits of photoelectrocatalytic materials. 4,7,12,13 Here, we use a scanning droplet cell platform that confines the electrochemical measurement to a small contact area 14,15 ensuring high spatial resolution and excellent signalto-noise, 16 as responses from features outside the probe area are excluded. Although scanning droplet cells have found some recent application for localized photovoltaic measurements on organic materials, 17 the droplet cell in these studies (3.5 mm in diameter) 17 operates with an aerial footprint five orders of magnitude larger than herein.…”

“…As a member of the scanning droplet technique family, 18 scanning electrochemical cell microscopy (SECCM) 14,[18][19][20][21][22][23] has emerged in recent years as a powerful technique to visualize electrochemistry at the nanoscale as well as to functionalize surfaces. 24,25 Key capabilities of SECCM are mainly due to: (i) a nanoscopic droplet size at the tip of the scanning probe (down to < 200 nm in 3 diameter in published studies 15 and < 100 nm in work in progress in our group) and (ii) an independent feedback mechanism that provides constant tip-substrate separation, unambiguously allowing surface reactivity to be separated from topography.…”

Pseudo-Single-Crystal Electrochemistry on Polycrystalline Electrodes: Visualizing Activity at Grains and Grain Boundaries on Platinum for the Fe2+/Fe3+ Redox Reaction

“…This is particularly true for the Fe 2+/3+ redox process. 47,50 Based on our recent work, 21,[24][25][26][27][28][29]33,51 SECCM provides a powerful method to investigate whether structural effects hold for the oxidation of Fe 2+ on polycrystalline platinum.…”