Synthesis and Characterisation of Shape-Controlled Metallic and Bimetallic Nanocrystals for Catalysis and Sensing

Abstract

Metallic and bimetallic nanocrystals are intensively and constantly researched due to their potential as efficient heterogeneous catalysts. The catalytic capabilities of the nanocrystals were shown to be highly dependent on their size, shape and/or elemental composition. Of many of the shapes (mainly polyhedrons) reported, nanocrystals with branched morphology have increasingly gained attention owning to their characteristics of high surface active areas and abundance of high-index facets which are both beneficial for catalytic applications. In addition to the existing systems of branched nanocrystals, the incorporation of an additional metal to the nanocrystals forming bimetallic branched nanocrystals have further enhanced the catalytic response acquired. Thus, there is a need of a reliable synthetic route in order to produce a wide variety of bimetallic branched nanocrystals.

In this thesis, the effect of the shape of platinum nanocrystals on the sensing of ethanol was first explored. Following this, the syntheses of palladium-ruthenium branched nanocrystals was conducted through seed-mediated growth and the application of the concept of polymorphism in the growth of the nanocrystals. All the syntheses were conducted in solution as solution-phase synthesis allows greater control over the nucleation and growth of nanocrystals through the utilisation of surfactant. The products of the syntheses were further studied by transmission electron microscope, electron/x-ray diffraction and energy dispersive X-ray spectroscopy.

The background of the literature on the strategies in attaining shape- and size-controlled metallic and bimetallic nanocrystals and their potential as heterogeneous catalysts was overviewed in Chapter 1. Chapter 2 elaborates the synthetic routes and characterisation techniques used in the following chapters.

In Chapter 3, platinum nanocrystals were first produced in Fischer-Porter bottles by adapting a reported work in the literature. By tuning the temperature and pressure of hydrogen gas, the control over the growth of platinum nanocrystals was achieved resulting in the formation of shape- and size-controlled platinum nanocrystals. The produced platinum nanocrystals were further grafted onto tin oxide supports and the capability of the materials in sensing ethanol was compared with tin oxide attached with spherical platinum nanocrystals. Further analysis revealed that the material consisting of shape-controlled platinum nanocrystals exhibited a higher sensitivity towards ethanol and this enhancement was attributed to the morphologies of the platinum nanocrystals which consisted of a higher concentration of crystal facet edges and steps.

Chapter 4 showcased the syntheses carried out in order to produce palladium-ruthenium branched nanocrystals through seed-mediated growth route. Monodisperse palladium seeds were first synthesised utilising a bench-top setup with adapted synthetic procedures. The palladium seeds, along with ruthenium precursor and surfactant, were employed in reactions in Fischer-Porter bottle to synthesise palladium-ruthenium branched nanocrystals. By manipulating the concentration of palladium seeds and surfactant, controlled deposition of ruthenium on palladium seeds was achieved generating homogenous branched nanocrystals. Thorough investigation on the characteristics of the branched nanocrystal and its growth mechanism was carried out and results obtained were presented in Chapter 5. The branched nanocrystals were further used in catalysing oxygen evolution reaction and showed improved catalytic activity owing to their unique branched morphology and facetted features.

Palladium overgrowth on the branched nanocrystals produced in Chapter 4 was carried out in order to generate bimetallic nanocrystals with complex branched morphologies. The syntheses and results obtained were presented in Chapter 6. Utilising both bis(acetonitrile)palladium dichloride and palladium acetylacetonate as precursor for reactions, the temperature, hydrogen pressure and concentrations of surfactant were tuned in order to achieve overgrowth with controlled morphology while eliminating monometallic byproducts produced through homogeneous nucleation. The type and concentration of precursor employed along effective surfactants was found essential in order to achieve controlled overgrowth of palladium on the seeds.

Finally, Chapter 7 presents the overall discussion and conclusions of the research in this thesis. The potential future work in relation to the progress and achievement in this work are elaborated as well.