Synthesis and Characterization of Nickel and Iron-Nickel Nanoparticles

Abstract

Metal and metal alloy nanoparticles are actively researched due to their size and shape dependent properties. Of particular note are nickel and iron-nickel alloys which are being investigated for replacing the rare noble metal catalysts in many catalytic applications. Nickel and iron-nickel are also magnetic materials and have been researched for memory storage, bio-separation and magnetically recyclable catalysis.

This thesis describes synthetic strategies for the size and shape controlled synthesis of nickel nanoparticles and the synthesis of iron-nickel nanoparticles in solution. Solution synthesis allows a unique level of control over the syntheses through utilising surfactant molecules to favour unusual shapes. Each synthesis is studied by transmission electron microscopy, diffraction, and energy dispersive X-ray spectroscopy. In-situ small angle scattering experiments were employed to study the growth of the nickel nanoparticles.

Nickel nanoparticles were formed in Fischer-Porter bottles with between 100 kPa and 300 kPa hydrogen pressure at 140°C. It was observed that the nanoparticles size could be controlled with the use of phosphine surfactants, and was directly linked to the ratio of the phosphine surfactant used. The experiments reported in Chapter 3 showed the formation of nickel nanocubes by controlling the amount of trioctylphosphine (TOP) at 100 kPa hydrogen pressure. These cubes were proposed to form due to the stabilization of the {100} facet by a combination of the trioctylphosphine and hydrogen gas.

Chapter 4 describes the formation of branched nanostructures of nickel when 300 kPa hydrogen pressure was used. These nanostructures were observed to grow from cuboctahedral/ octahedral ƒcc nickel seeds with the arms made up of segments of hcp and ƒcc nickel. The hcp nickel was formed due to defects in the ƒcc crystal structure that formed in the kinetically controlled synthesis. The branched nanostructures were observed to mimic the growth seen with polymorphic materials. This observation is the first of such behaviour with a pure metal system. The polymorphism occurs in nickel due to its high density of defects.

In Chapter 5 the formation of iron-nickel alloys are investigated, initially in a similar methodology using Fischer-Porter bottles as reported above, and also in the hot-injection method. In hot-injection reactions the formation of iron-nickel alloys was shown to be very dependent on the precursors used, and only formed with bis(salicylaldiminato)nickel (II) and bis(ƞ⁵-1,3,5-exo-6-tetramethyl cyclohexadienyl) iron(II). Furthermore the reaction length controlled the composition of the materials as the iron precursor decomposed more slowly than the nickel precursor.

In chapter 6 in-situ synchrotron SAXS experiments were carried out studying a series of trioctylphosphine to nickel acetylacetonate ratios. These experiments give evidence for the trioctylphosphine restricting the ability of the monomers and precursor to interact with the nanoparticles surface and therefore greatly reduce the size the nanoparticles can reach.

An overall conclusion and future plans for the research are given in chapter 7 in which implications for the syntheses are presented. In particular the chapter discusses the nanoparticles uses as catalysts and expands the applicability of the syntheses to different elements and different alloy compositions.