Electrochemistry of platinum nanoparticles

This thesis reports experimental work with two main aims: the first is the investigation of single nanoparticles with an emphasis on their porosity, and the second is to develop a more comprehensive understanding of their electrocatalytic behavior. The fundamental principles and techniques of electrochemistry are introduced in Chapter 1. Chapters 2 and 3 respectively summarize nanomaterials and their characterization methods. In particular, the technique of single particle electrochemical impacts, so-called "nanoimpacts", is used to study redox reactions at individual nanoparticles.
Chapters 4 and 5 report the porosity of platinum nanoparticles (PtNPs) via tag-redox coulometry (TRC) and surface platinum oxidation, respectively. In the former, thioltagged PtNPs are employed and current transients upon impacts with a cathode are detected. This is due to the nitro group reduction on individual PtNPs. The charge per spike is measured and the number of tag molecules can be calculated. Therefore, the "electroactive surface area" of the PtNPs is revealed. In the latter, oxidative transients upon direct impacts are observed. The charge per current spike is measured and attributed to the formation of surface platinum oxides. XPS shows PtO and PtO2 in different amounts and an average oxidation state is deduced as a function of potential. Thus the number of platinum atoms oxidized per PtNP can be estimated. Both experiments allow insight into extent to which the internal surface of the aggregate is "seen" by the solution and is electrochemically active, providing a fuller knowledge of the catalytic behavior of nanoparticles, as reflected from the reaction kinetics and the redox currents. Furthermore, results from Chapters 4 and 5 are in close agreement with each other, indicating the accuracy of the measurements in both studies.
With this appreciation of the internal nanoscale structure, Chapter 6 investigates the kinetics of the hydrogen oxidation reaction (HOR) on mesoporous particles. The steadystate current for HOR is measured on both individual platinum particles and PtNPs drop-casted substrates. The kinetic parameters are determined and contrasted between these two approaches. Nanoimpacts are required in order to correctly understand the electrochemical reaction catalyzed by nanoparticles, as the aggregation or loss of ensemble drop-casted particles leads to errors.
Chapter 7 further elaborates the role of nanomorphology of nanoparticles in catalyzing the HOR using individual mesoporous particles and low density random arrays of particles. At the single particle scale the activity of the platinum catalyst towards the hydrogen oxidation is potential dependent and exhibits two peaks, as reflected in the electrochemical current. This is due to the sensitivity of the reaction rate to the interfacial structure of PtNPs. The decrease in activity correlates directly with the potential at which the underpotential deposited hydrogen (Hupd) is removed from the catalytic interface. The contribution of the internal mesoporous nanoparticle structure towards the total particle catalytic activity is further corroborated through comparison of the current-time transients recorded for individual nanoparticles of differing morphology: solid vs mesoporous, and by evidencing the sensitivity of the single particle catalytic activity to the supporting electrolyte concentration.
In Chapter 8, the underpotential deposition and removal of hydrogen from a mesoporous PtNP surface is shown to quantify the electrochemically active surface area (ECSA) of an individual nanoparticle. This surface area of the particle is concomitantly correlated with its individual catalytic activity (current density) towards the hydrogen evolution reaction (HER). In addition, conclusions in this chapter are compared with the results in Chapters 4 and 5, showing consistency in all structural measurements of the particles.
Finally, Chapter 9 provides the main conclusions for the studies described in the thesis. The various discoveries in this thesis in respect of individual nanoparticles reflect generic methodological development, both fundamentally and applied, in studying electrochemistry at nanoscale. The overall major finding is that it is essential to recognize particle porosity to fully understand catalytic properties.

Advisor: Compton, R