Electrocatalytic nanoeffect at gold nanoparticles

Nanoelectrochemistry explores the differences in chemical behaviour at the nanoscale as compared to the macro-scale. This thesis is concerned with nanoelectrochemistry and aims to develop and apply novel experiments for the unambiguous identification of changed electrode kinetics at the nanoscale. This is challenging since electrochemical responses are controlled by diverse factors like enhanced mass transport and adsorption as well as electron transfer kinetics. A joint computational and experimental strategy is employed.
Chapter 1, 2 and 3 cover essential introductory material and basic experimental details relevant to all experiment. Fuller descriptions and details are given in the following chapters as and when needed.
Chapter 4 reports the development of an electrochemical characterization method, to achieve a fast and simple quantification of the average particle size and the number of nanoparticles deposited on a glassy carbon electrode. The method consists of surface area characterization by underpotential deposition of lead particles and the determination of the amount of gold from anodic stripping in HCl. This method is also proven to be effective by comparing the results with SEM measurements.
Next, in chapter 5, a generic strategy combining computation and experimental approach is developed in order to study the electron transfer kinetics of gold nanoparticles.The modelling part considers the kinetics of the electrochemical process on the bulk materials for different regions in the electrode, that is, the substrate (glassy carbon) and the nanoparticles (gold). Comparison of experimental and theoretical results enables the detection of changes in the electrode kinetics at the nanoscale. This approach is applied into the electro-oxidations of nitrite and L-ascorbic acid for gold nanoparticles from 20 - 90 nm. In the former, analysing the system shows that no change in electron transfer kinetics is involved in the process, even though a decrease of the over-potential and an increase in the peak current are observed. But these changes reflect mass transport effects, not electrocatalysis. A case where an authentic enhanced electron transfer kinetic change occurs is shown for the ascorbic acid system.
Finally, in chapter 6 , the above strategy is exploited further to apply a quantitative study of electron transfer kinetics for various sizes of gold nanoparticles in the oxygen reduction reaction system in sulphuric acid at 298 K. The latter is at the heart of energy transformation techniques (fuel cells, battery and so on). Compared with the electron transfer kinetics on macro gold electrodes, there is no change at gold nanoparticles from size 5 nm to 40 nm. However, in the presence of Pb(II), a strong enhancement of electron transfer kinetics is observed on 5 nm citrate capped gold nanoparticles for ORR. On the other hand, a significant decrease of electron transfer kinetics has been found for gold nanoparticles of size 2 nm for ORR. The latter observation of strong negative electrocatalysis is also observed for the hydrogen evolution reaction (HER). This represents the first report of such effects with the HER system.
Overall the thesis has established a rigurous, theoretical basis for evaluating electrocatalysis in nanoparticulate system.

Keywords: energy conversion
gold nanoparticles
electrocatalysis
electron transfer kinetics
Advisor: Compton, R