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

Abstract

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 (E_1/2) 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. E_1/2 for HZ oxidation for the higher density Au NP-SWNT structure is shifted slightly more negative (by ∼25 mV) than E_1/2 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.