Correlative pipette probe electrochemistry - electron microscopy studies of platinum nanoclusters

Mecking Ornelas, Isabel (2020). Correlative pipette probe electrochemistry - electron microscopy studies of platinum nanoclusters. University of Birmingham. Ph.D.

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Abstract

This thesis focuses on combining Scanning ElectroChemical Cell Microscopy (SECCM) and aberration-corrected High Angle Annular Dark Field (HAADF) Scanning Transmission Electron Microscopy (STEM) to obtain new insights into the relationship between the structure and electrochemical properties of nanoscale platinum. Carbon-coated Transmission Electron Microscopy (TEM) grids were utilised as substrates for electrochemical experiments, which were performed with SECCM probe tips with diameters of ca. 1 μm, forming electrochemical cells with a working electrode areas in the μm\(^2\) scale. The location of the droplet landing areas were identified using ex situ STEM, which permitted the visualisation of the whole meniscus-substrate contact area. Micrographs with dimensions of \( \approx 10^4\) nm\(^2\) were employed to obtain statistical data about the size and spatial distribution of the Pt NPs, and high magnification images, with subatomic resolution, were utilised to obtain mechanistic information about the studied systems. Two main studies are presented in this thesis. Briefly, the first focuses on the Oxygen Reduction Reaction (ORR), catalysed by mass-selected Pt nanoclusters; the second investigates the nucleation and growth of platinum during electrochemical deposition.

Mass-selected nanoclusters (NCs), with ca. 3 nm diameter, were generated in a cluster beam source and deposited onto TEM grid supports, which permitted the preparation of well-defined ensembles of Pt NCs due to the independent control of particle size, density, and impact energy during deposition. These samples were used as model catalyst systems for ORR, which is vital in energy-related applications, such as fuel cells. The SECCM setup provided a high mass-transport system with a three-phase (gas-liquid-solid) configuration, which mimics fuel cell conditions. Voltammetric studies revealed a loss of electrocatalytic activity with time which was more pronounced when Pt loading was low. Analysis of the samples utilising STEM and X-ray photoelectron spectroscopy (XPS) provided strong evidence that the degradation of the activity of the Pt catalysts was due to poisoning of the platinum surface by carbon/oxygen-containing species, generated by the reaction of reactive oxygen intermediaries of the ORR with the carbon support. Additionally, it was found that whereas Pt NCs deposited with impact energies of 1.6 eV per atom were stable on the substrate, those deposited at 0.54 eV per atom became mobile as a consequence of the ORR and formed characteristic aggregates containing up to 15 or more clusters, with edge-to-edge nearest neighbour distances of ca. 1.5 nm.

The electrochemical deposition of platinum onto carbon-coated TEM grids was investigated employing a high-throughput methodology in which the SECCM probe is sequentially brought into contact with the substrate, and where experimental parameters can be systematically varied with each landing. Here, the nucleation and growth of platinum nanostructures was studied by chronoamperometry in a series of experiments where the deposition time was held constant and the applied potential was decreased from 0V to -0.9V vs. Pt(II)/Pt(0). It was shown that particle density increases and size dispersion decreases with increasing driving force. Furthermore, it was found that single atoms, small clusters and larger particles coexist, independently of the applied potential, supporting a non-classical nucleation and aggregative growth mechanism, and a correlation was established between a structural transition of clusters at \( d \approx \) 2 nm from amorphous to monocrystalline, and the critical size at which growth by direct addition is impeded.

Type of Work:

Thesis (Doctorates > Ph.D.)

Award Type:

Doctorates > Ph.D.

Supervisor(s):