Iron nanoparticulate planar model systems : synthesis and applications

Abstract

It is well known that at the nanoscale, materials exhibit fascinating optical, electronic and magnetic properties that differ drastically from their bulk counterparts. The novel properties of nanostructured materials enable them to find potential applications in fields such as nanofabrication, nanodevices, nanobiology and nanocatalysis. The synthesis of diameter controlled nanoparticles is important in the field of catalysis for the formulation of a catalyst which meets the activity, stability and selectivity requirements of a particular catalytic process. A fundamental understanding of the catalyst particle behavior can be achieved by supporting these nanoparticles on model supports. Planar supports with the catalyst particles on the top, offer full access to surface spectroscopies. The synthesis of diameter controlled nanoparticles can be quite challenging with the classical approaches of precipitation and impregnation. This thesis involves the synthesis of diameter controlled iron nanoparticles and its applications in carbon nanotube (CNT) and Fischer-Tropsch (FT) studies. Our initial attempt to control iron particle size was to employ the technique of spincoating. This involved varying the concentration of the iron precursor in the spincoating solution. The direct spincoating of various concentrations of FeCl3.H2Oisopropanol solutions did not produce any particles but gave rise to an iron-hydroxychloro layer instead. A calcination treatment produced needle-like goethite structures as confirmed by spectroscopy studies and a subsequent reduction treatment produced isolated particles with a metallic iron core component and an iron oxide shell. There was no distinct change in particle diameter as a function of iron precursor concentration. The inability to synthesize monodisperse iron oxide nanoparticles via the spincoating and pretreatment (in-situ) route, prompted an investigation into an alternate route for nanoparticle synthesis. The thermal decomposition of iron carboxylate compounds like iron oleate and iron acetylacetonate lead to the synthesis of monodisperse iron oxide nanoparticles, the diameters of which could be varied over a narrow increment range. The influence of reaction temperature, iron to surfactant ratio and the technique of seed mediated growth were investigated to give rise to 4.5, 9.5, 16, 12.6, and 27 nm iron oxide nanoparticles. When supported on silica transmission electron microscope (TEM) substrates, these model systems can be imaged by TEM, then taken into a reactor, treated, and then imaged again to feature the exact set of particles measured prior to the reactor treatment. CNTs have attracted much interest recently owing to their novel properties that have led to realistic possibilities of using them in a host of commercial nanoelectronic applications. Based on their technological importance, it is essential to understand the synthesis mechanism of these materials. Thus an attempt to study the growth mode of CNTs was attempted. The monodisperse iron oxide nanoparticles were used to gain some insight into the CNT growth mode. Using microscopy techniques, it was confirmed that the particles were involved in a series of re-arrangements during the pretreatment and initial CNT growth. Thus contrary to what has been expressed by some researchers, the initial monodisperse iron oxide particles do not nucleate and grow the final CNTs. It was also shown that CNT growth occurred in two stages; an initial disordered growth, followed by a more aligned growth underneath. This was one of the reasons why we observe the lack of correlation between the initial deposited iron nanoparticle and the final CNT diameter. The availability and know how to synthesize an iron nanoparticle model system prompted a FT study. Due to the uncertainty surrounding the nature of the active phase of the working iron FT catalyst, some work was attempted to observe the chemical and morphological changes of the iron nanoparticle model catalyst, when subjected to various FT pretreatment conditions. With the H2 pretreatment a core shell morphology was observed, with the core being metallic iron and the shell, iron oxide. The CO reduction seemed to result in magnetite being the dominant species. Synthesis gas (CO + H2) exposure resulted in a mixture of metallic iron, magnetite and iron carbide.