Electrochemistry of single entities and ensembles of metal-based nanoparticles
Keywords: electroanalysis
sizing
electrocatalysis
single particle
optical
electrochemistry
dark field microscopy
metal nanoparticles
nano-impacts

This thesis aims at both electrochemically investigating mainly at the single entity level the physicochemical properties of metal-based nanoparticles and seeking to study unusual electrochemical activity of metal nanoparticle ensembles by using mainly an optical approach. Three experimental stages are presented accordingly:
(1) Electrochemical detection and sizing of single metallic nanoparticles with a critical focus on the study of the quantitative methodology;
(2) Further investigations of the surfaces (composition, structure and catalytic activity) of metal-based core-shell nanoparticles both at the particle ensemble and single particle level;
(3) Opto-electrochemical investigation of a substrate-mediated electrochemical dissolution of metal nanoparticle ensembles.
To begin with, the first chapter briefly reviews the key basics of fundamental electrochemistry, and the second chapter introduces the metal-based nanoparticles, the major electrochemical technique of ‘nano-impacts’ and the alternative technique of dark field microscopy used in this thesis.
Chapter 3 reports a generic quantitative methodology for electrochemically detecting and sizing individual nanoparticles using the complete oxidation of silver nanoparticles in halide solutions as a model system. In particular, this established methodology of nano-impacts emphasises the use of lower filtering frequencies in the data acquisition (prior to any analysis) for the accurate description of the charge passed during a transient nano-event. For the critical comparison between electrochemical and electron microscopic measurements, the effect of the nanoparticle diffusion coefficient on the probability of it being observed in the electrochemical responses and the effects of non-spherical shapes of nanoparticles visible in the microscopic images are also discussed. On this basis, Chapter 4 further shows the advantages of applying the nano-impact technique for sizing small (d < 30 nm), multimodal-sized nanoparticles that are inherently beyond the measuring capacity of the conventional light scattering methods of dynamic light scattering and nanoparticle tracking analysis.
Chapter 5 next demonstrates the electrochemical characterisation of the surface compositions of Co@Co(OH)2 nanoparticles at the single particle level. By contrast, the very thin surface layer of Co(OH)2 is not characterised by either X-ray diffraction or transmission electron microscopy. Moreover, the aggregation of the nanoparticles in aqueous solutions is implied via the impact electrochemistry. Chapter 6 focuses on the mechanistic study of the mediated hydroxide ion oxidation by Co3O4 nanoparticles in alkaline conditions. Hydrogen peroxide, rather than dioxygen, is found to be the predominant initial oxidation product of the electron transfer. The nano-impact results further point out the rate determining step and the limiting kinetics of the catalytic reaction. Furthermore, as the work next compares the electrochemical behaviour of bare Co3O4 with that of core-shell Co3O4@SiO2 nanoparticles, the surface silica of the synthesised core-shell particles is inferred to be highly porous or broken. This is not otherwise easily distinguished, for example, by electron microscopy.
Chapter 7 reports a substrate-mediated electrochemical dissolution into aqueous solution of silver nanoparticles driven via electron transfer to trace dissolved oxygen. Investigation of a range of solid materials (glass, indium titanium oxide, glassy carbon and platinum) used to support the nanoparticles shows that conductive supports greatly catalyse the dissolution and by means of complementary and synergistic optical and electrochemical measurements it is inferred that the electron transfer can occur at locations on the support surface remote from the silver nanoparticles.