Shape and Size Control of Bimetallic PtX (X = Au, Ni, Sn) Nanoparticles

Abstract

Nanoparticles show interesting and novel properties compared to their bulk materials. These properties range from optical, magnetic, electronic to catalytic and can be influenced by shape, size and elemental composition. As the ability to control nanoparticle morphology is important in materials science these particles are actively researched. Moreover, by combining different metals multiple properties intrinsic to those elements can be accessed within a single system.

This thesis describes general synthetic approaches and underlying theory in the formation of nanoparticles. Focusing on organic solution phase synthesis, pathways to control both size and shape of nanoparticles are discussed. The concept behind the formation and possible structures of bimetallic nanoparticles are explained. Additionally, a brief overview about used characterisation techniques such as transmission electron microscopy and x-ray diffraction are given.

Metallic nanoparticles were formed using the organic solution phase synthesis within Fischer-Porter bottles. Elevated temperatures and the presence of hydrogen lead to thermal decomposition of the metallic precursor, reduction of formed metal ions and subsequent build-up of nanoparticles. For bimetallic nanoparticles the seed mediated growth technique is commonly used. By utilizing this technique bimetallic AuPt nanoparticles were formed. The impact of different surfactants, hydrogen pressure, precursors and reaction time upon the size, elemental composition and morphology of these bimetallic AuPt nanoparticles is investigated. The bimetallic structure is evaluated and experiments to control the growth of platinum onto the seed structures are conducted.

Further research deals with the formation of hexagonal close packed (hcp) nickel nanoparticles. By altering the surfactant type and concentration nickel favours to crystallise in its hcp modification rather than its most common face-centred cubic (fcc) phase. It was found that nickel packing in this hcp crystal system is forming hourglass-shaped nanoparticles. These particles are further used in seed mediated growth experiments with a platinum precursor to achieve bimetallic nanoparticles to both exploit the catalytic activity of platinum as well as the magnetic moment of nickel. It is shown that the choice of reaction conditions is crucial to achieve growth onto the nickel surface. Moreover, it was found that these nanoparticles are only selectively coated by platinum on hcp {001} facets leading to exposure of both nickel and platinum surfaces. The key results are summarised and the exploited parameters evaluated. Also, perspectives for future research are discussed and a brief outlook for the application of the investigated bimetallic systems is given.

Bimetallic tin-platinum nanoparticles were formed by coreduction of the respective tin and platinum containing metal precursors. Several metal sources for both tin and platinum were investigated upon their decomposition and the resulting nanoparticle shape and elemental composition. The formation of a bimetallic precursor containing a Pt-Sn bond is discussed. Further reaction parameters such as temperature and time are also investigated to eludicate their impact on the formed nanoparticles.

Finally, the key results are summarised and the exploited parameters evaluated. Also, perspectives for future research are discussed and a brief outlook for the application of the investigated bimetallic systems is given.

The discussion in Chapter 4 about selectively obtaining hcp Ni nanoparticles is shortened and a major focus is given on the platinum coating of these hourglass-shaped nanoparticles, as Lee et al. published a paper on "Shaped Ni nanoparticles with an unconventional hcp crystalline structure" (Chemical Communications, 2014, 50, 6353-6356) during the course of these studies, describing similar methods and findings as observed in this research.