Electrochemistry at nanoscale electrodes : individual single-walled carbon nanotubes (SWNTs) and SWNT-templated metal nanowires

Individual nanowires (NWs) and native single-walled carbon nanotubes (SWNTs) can be readily used as well-defined nanoscale electrodes (NSEs) for voltammetric analysis. Here, the simple photolithography-free fabrication of submillimeter long Au, Pt, and Pd NWs, with sub-100 nm heights, by templated electrodeposition onto ultralong flow-aligned SWNTs is demonstrated. Both individual Au NWs and SWNTs are employed as NSEs for electron-transfer (ET) kinetic quantification, using cyclic voltammetry (CV), in conjunction with a microcapillary-based electrochemical method. A small capillary with internal diameter in the range 30–70 μm, filled with solution containing a redox-active mediator (FcTMA+ ((trimethylammonium)methylferrocene), Fe(CN)64–, or hydrazine) is positioned above the NSE, so that the solution meniscus completes an electrochemical cell. A 3D finite-element model, faithfully reproducing the experimental geometry, is used to both analyze the experimental CVs and derive the rate of heterogeneous ET, using Butler–Volmer kinetics. For a 70 nm height Au NW, intrinsic rate constants, k0, up to ca. 1 cm s–1 can be resolved. Using the same experimental configuration the electrochemistry of individual SWNTs can also be accessed. For FcTMA+/2+ electrolysis the simulated ET kinetic parameters yield very fast ET kinetics (k0 > 2 ± 1 cm s–1). Some deviation between the experimental voltammetry and the idealized model is noted, suggesting that double-layer effects may influence ET at the nanoscale

Electrochemistry at pristine single-walled carbon nanotubes

This thesis aims to develop an understanding of the fundamentals and applications of electrochermistry at pristine single-walled carbon nanotubes (SWNTs), synthesised by the chemical vapour deposition (CVD) method. The SWNTs grown by CVD on the insulating SiO2 substrates were chosen for the reason being clean, free of amorphous carbon and readiness of nanotube morphology control. 2D random SWNT networks and individual ultra-long flow-aligned SWNTs were employed in the electrochemical studies throughout. SWNT networks were studied either by the microcapillary electrochemical method (MCEM) or in the format of disk-shaped ultramicroelectrodes (UMEs). By challenging the SWNT UMEs with enhanced mass-transport rates in a thin-layer cell (TLC) reversible quasi-steady state cyclic voltammogramms (CVs) were acquired, which allowed the numerical simulations of the voltammetric response and derivation limits for the standard electron transfer (ET) rate constants. Individual SWNTs also generate very high intrinsic mass-transport rates and were studied by the MCEM method, coupled with finite element modelling, highlighting that SWNT sidewalls are active towards outer-sphere redox reactions. By using a sparse surface coverage (typically less than 1%) of pristine SWNTs on an insulating substrate, it has also been demonstrated that electrodeposition of nanoparticles (NPs) is highly directional. By varying electrodeposition driving force (potential) and time one can control the NP density and size. The findings suggest that nucleation of Au on SWNTs is essentially 'instantaneous', and that the nucleation density increases with increase of the deposition potential. This knowledge has enabled the synthesis of a range of different nanostructures, from isolated Au NPs to Au nanowires (NWs), which were used as expedient platforms for analytical and electrocatalytical purposes. While some common inner-sphere redox processes do not readily undergo electrochemical reactions on the carbon nanotubes, which was established in experiments employed SWNT UMEs and individual ultra-long SWNTs, the outer-sphere redox processes were shown to be reversible on the same nanotube electrodes. Novel scanning electrochemical cell microscopy (SECCM) studies allowed individual Pt NPs, electrically connected by the sub-centimeter long SWNT, to be electrochemically assessed. Significantly, this work highlights that individual NPs have their intrinsic electrochemical characteristics

Mapping nanoscale electrochemistry of individual single-walled carbon nanotubes

We introduce a multiprobe platform for the investigation of single-walled carbon nanotubes (SWNTs) that allows the electrochemical response of an individual SWNT to be mapped at high spatial resolution and correlated directly with the intrinsic electronic and structural properties. With this approach, we develop a detailed picture of the factors controlling electrochemistry at SWNTs and propose a definitive model that has major implications for future architectures of SWNT electrode devices

Electro-oxidation of hydrazine at gold nanoparticle functionalised single walled carbon nanotube network ultramicroelectrodes

Networks of pristine single walled carbon nanotubes (SWNTs) grown by catalysed chemical vapour deposition (cCVD) on an insulating surface and arranged in an ultramicroelectrode (UME) format are insensitive to the electro-oxidation of hydrazine (HZ) in aqueous solution, indicating a negligible metallic nanoparticle content. Sensitisation of the network towards HZ oxidation is promoted by the deliberate and controlled electrodeposition of “naked” gold (Au) nanoparticles (NPs). By controlling the deposition conditions (potential, time) it is possible to control the size and spacing of the Au NPs on the underlying SWNT network. Two different cases are considered: Au NPs at a number density of 250 ± 13 NPs μm−2 and height 24 nm ± 5 (effective surface coverage, θ = 92%) and (ii) Au NPs of number density 22 ± 3 NPs μm−2 and height 43 nm ± 8 nm (θ = 35%). For both morphologies the HZ oxidation half-wave potential (E1/2) is shifted significantly negative by ca. 200 mV, compared to a gold disc UME of the same geometric area, indicating significantly more facile electron transfer kinetics. E1/2 for HZ oxidation for the higher density Au NP-SWNT structure is shifted slightly more negative (by 25 mV) than E1/2 for the lower density Au NP electrode. This is attributed to the lower flux of HZ at NPs in the higher number density arrangement (smaller kinetic demand). Importantly, using this approach, the calculated HZ oxidation current density sensitivities for the Au NP-SWNT electrodes reported here are higher than for many other metal NP functionalised carbon nanotube electrodes

Mapping Nanoscale Electrochemistry of Individual Single-Walled Carbon Nanotubes

We introduce a multiprobe platform for the investigation of single-walled carbon nanotubes (SWNTs) that allows the electrochemical response of an individual SWNT to be mapped at high spatial resolution and correlated directly with the intrinsic electronic and structural properties. With this approach, we develop a detailed picture of the factors controlling electrochemistry at SWNTs and propose a definitive model that has major implications for future architectures of SWNT electrode devices

Visualizing zeptomole (electro)catalysis at single nanoparticles within an ensemble

The relationship between the structural properties, such as the size and the shape, of a catalytic nanoparticle and its reactivity is a key concept in (electro) catalysis. Current understanding of this relationship is mainly derived from studies involving large ensembles of nanoparticles (NPs). However, the results necessarily reflect the average catalytic behavior of an ensemble, even though the properties of individual particles may vary widely. Here, we demonstrate a novel approach using scanning electrochemical cell microscopy (SECCM) to locate and map the reactivity of individual NPs within an electrocatalytic ensemble, consisting of platinum NPs supported on a single carbon nanotube. Significantly, our studies show that subtle variations in the morphology of NPs lead to dramatic changes in (potential-dependent) reactivity, which has important implications for the design and assessment of NP catalysts. The instrumental approach described is general and opens up new avenues of research in functional imaging, nanoscale electron transfer, and catalysis

Electrochemical nucleation and growth of gold nanoparticles on single-walled carbon nanotubes : new mechanistic insights

The electrodeposition mechanism of gold nanoparticles (NPs) on pristine single walled carbon nanotubes (SWNTs) at high driving forces has been elucidated using the microcapillary electrochemical method. Here, a small capillary (internal diameter similar to 50-100 mu m) filled with a gold plating solution, and positioned so that the capillary meniscus makes contact with a two-dimensional SWNT random network, was used to record current-time transients Nucleation and growth transients were observed in which the current increased with time to a maximum value beyond which the current decreased (planar diffusion regime) With increased driving force, the current maximum shifted dramatically to increasingly shorter times. Atomic force microscopy (AFM) analysis indicated that this was not due to significant differences in NP growth rates, but rather to increased densities of NPs formed at more cathodic potentials Detailed microscopic analysis showed that the size of the NPs initially increased with deposition time and the particle surface coverage was constant. However, at the highest driving forces the NP density decreased with deposition time and AFM revealed the presence of both larger and smaller particles at long times This was attributed to electrochemically induced Ostwald ripening, whereby larger particles grow at the expense of smaller ones. As NP nucleation and growth on SWNT two dimensional network electrodes is highly directional and enforced in particular locations, it is inappropriate to analyze electrochemical data using conventional models. There is thus a need to complement chronoamperometric measurements with high resolution microscopy to fully interpret nucleation on complex electrode surfaces