Electrochemistry of single nanoparticles

This thesis presents experimental work with two main aims: one is the detection and characterization of single electrocatalytic nanoparticles with an emphasis on metal nanoparticle decorated carbon nanotubes, and the other is to develop a more comprehensive understanding of nanoscale electrocatalysis by exploring the differences in the electrocatalytic behaviour of single nanotubes as compared to their ensembles. The first chapter introduces the basic principles and techniques of electrochemistry and the second chapter gives a brief introduction to the nanomaterials and electrochemical methods used in this thesis.
Chapter 3 first reports the detection of individual carbon nanotubes with the 'nano-impact' method using the production of the under-potential deposited hydrogen (HUPD) from proton reduction on palladium nanoparticle decorated carbon nanotubes (CNT-Pds). The current spikes corresponding to the reduction of proton forming HUPD on the individual impacting CNT-Pds were detected via chronoamperometry. From the knowledge of the palladium nanoparticle sizes and their coverage on the CNTs it was further demonstrated how the magnitude of impact charge allows an assessment of the nanotube lengths. The correspondence between electrochemical sizing and microscopy results directly shows that a CNT-Pd is electroactive along its entire length, evidencing the reactivity and conductivity of the CNTs and further indicating the complete HUPD saturation of the palladium nanoparticles decorated on the carbon nanotubes.
Chapter 4 then extends electrocatalytic nano-impacts of single CNT-Pd to investigate the hydrogen oxidation reaction (HOR). A facile procedure which enables rapidly and easily gathering single nanotube voltammetry was established and validated. Single CNT-Pds impact a micro wire electrode from dilute suspension and adhere to it for a sufficiently long time for cyclic voltammetry to be conducted. The single adsorbed decorated CNT is electro-catalytic towards HOR which thus occurs exclusively on it and cyclic voltammetry on single decorated carbon nanotube is realized. Single nanotube voltammetry of oxygen reduction reaction (ORR) was further studied and reported in Chapter 5. It demonstrates that the reduction of oxygen on Pd involves the initial formation of absorbed superoxide with fast electron transfer kinetics but with a low apparent transfer coefficient due to double layer effects arising from the Coulombic interactions of the adsorbed superoxide species inside the double layer. The mechanism of ORR on Pd was re-evaluated.
Chapter 6 compares the oxidation of formate and of methanol at the single CNT-Pds and ensemble levels. Pd oxide formation as a competitive reaction with formate or methanol oxidation is significantly inhibited at high overpotentials at single nanotubes in comparison with what is seen with ensembles. The discovery of the superior electro-catalytic performance under high mass transport conditions created by single nanotubes suggests requirements for the design of better fuel cell development and offers the scope for new catalytic approaches to be developed.
Chapter 7 is focused on the study of the origins of the current fluctuation observed in the single nanotube voltammetry by comparing the responses of HOR at a single CNT-Pds immobilised on the gold surface to analogous data on a carbon substrate. No significant distinction between the gold and carbon was found, demonstrating that the physical motion of the nanotube at the electrode is likely responsible for the modulation of current. A simple and feasible methodology based on the 'nano-impact' method is then developed to enable the measurement of the resistance across individual carbon nanotube-electrode contacts. It reveals that the major component of the measured resistance is associated with the single CNT-gold contact, which has major implications for the widespread use of CNTs as electro-catalysts and as catalyst supports.

Advisor: Compton, R