Shape-Controlled Synthesis of Gold-Palladium Bimetallic Nanoparticles for Catalysis, and their Phase Properties

Abstract

Bimetallic nanoparticles have been actively researched in recent years due to their potential use as heterogeneous and electrochemical catalysts. This is primarily due to their enhancement of activity and/or selectivity over conventional monometallic nanoparticles. In addition, shape control of metal nanoparticles has also achieved much interest, as both surface plasmon resonances and catalytic properties can be dependent on particle shape or the surface facets present. However, there is still limited information on how shape or the composition of a bimetallic nanoparticle affects the resulting properties. Furthermore, in order to investigate their properties, direct control over the synthesis is required. In this thesis, the shape control of Au-Pd nanoparticles was investigated using seed-mediated growth in the solution-phase. Seed-mediation allows for control over the end product by use of a seed with a desired shape and composition, and allows for greater ability to design nanocrystals for a desired property. Chapters 1 and 2 of this thesis overview the background of the literature on the shape-controlled synthesis of mono- and bimetallic nanoparticles, their potential catalytic properties, and the methods and characterisation used in the following results chapters.

In Chapter 3, gold nanocrystals were synthesised using a reliable and reproducible synthesis which gave a monodisperse product. The gold nanocrystals were subsequently used as seeds for further growth of gold-palladium branched particles using bis(acetonitrile) palladium dichloride and a mixture of oleic acid and oleylamine surface-capping agents. Chapter 4 expands on the preliminary work completed in Chapter 3, whereby the palladium precursor was exchanged with palladium acetylacetonate. This precursor is more stable and as a result allowed for the tuning of various reaction conditions, including temperature and surfactant concentration. As a result, monodisperse icosahedral Au-Pd core-shell nanocrystals were synthesised. These particles were then used towards the oxidation of benzyl alcohol to benzaldehyde, and showed that the shell thickness of the Au-Pd core-shell can be critical in determining the activity and/or selectivity of the catalytic reaction. The importance of the gold seed shape was investigated in Chapter 5 by replacing the icosahedral gold nanoparticles with larger gold nanocubes and quasi-spherical particles.

This resulted in a variety of shapes, which were dependent on the kinetics of the reaction. From the results outlined here-in, it was determined that gold-palladium nanoparticles could be synthesised with control over shape determined primarily from the rate of surface palladium growth. For instance, by changing in surfactant moiety from amine to acid, the particles could be tuned from core-shell polyhedra to branched particles. In addition, the gold nanoparticles were concluded to slow the growth of the palladium shell, as gold was not expected to interact with the palladium precursor or hydrogen gas to result in autocatalytic growth.

Chapter 6, the final results chapter, studied the melting properties of Au-Pd nanoparticles by heating in-situ using Aberration-Corrected TEM. The particles were observed to undergo structural changes, including Au-Pd segregation, melting behaviour, and then alloying. Of primary interest was the observation of linear ordered alloy superstructures which appeared over 550 ᴼC and which remained after cooling to room temperature. Finally, Chapter 7 provides the overall discussion and conclusions of this research, in addition to outlining potential future work.