Precipitation and transformation of nanostructured copper oxalate and copper/cobalt composite precursor synthesis

The aim of this thesis was to synthesise and control a nanostructured composite of copper/cobalt. Both the copper and cobalt oxalate exhibit a nanostructure. Attempts to produce nanocomposite particles in the size range of 10-70 nm were made via an oxalate co-precipitation route followed by the appropriate thermal treatment to finally obtain the metal nanocomposite. Much attention was focussed on the understanding of the copper oxalate precipitation and its further transformation into metal before the investigation of the co-precipitation system. One major challenge of this thesis work was to achieve a better understanding of the copper oxalate precipitation mechanism. This was made by following kinetic parameters in order to shed light on the various steps of the precipitation from a supersaturated solution (nucleation, growth and aggregation of nanocrystals). Following the pH as a function of time and using the thermodynamic solubility data it was possible to propose a kinetic model of copper oxalate precipitation, with the co-precipitation of slight amounts (around 0.40% wt) of malachite (CuCO3·Cu(OH)2). The copper oxalate nanostructured particle growth mechanism, from the self-assembly of nanosized buildings blocks, was confirmed for intermediate precipitation times (1-15 minutes). Evidence for such organisation of the particles was shown by a combination of XRD diffraction, SEM and AFM measurements showing the presence of steps at the particle surface with a height that corresponds to a multiple of the mean crystallite size in that particular crystallographic orientation. Further investigations were performed for the early steps of the precipitation by SAXS but either the precipitated volume fraction was too low for detection of the particles or nucleation and growth kinetics were to fast (less than one second) to be followed. The TEM cross-section analyses showed a possible core-shell assembly mechanism. The core showed a random organisation of the crystallites with a size of around 25 nm, while the crystallite in the shell with a size of around 40 nm presented certain order along the 110 axis, particularly towards the particle surface. All these details provided the opportunity to propose a new and more detailed mechanism of the copper oxalate polycrystalline particle formation. In order to master the conditions of the Cu/Cu oxalate decomposition, a good understanding of the simple copper oxalate decomposition was necessary. All along this thesis, much attention was paid to the transformation of the copper oxalate cubic particles into the metal. The objective was thus to conserve the particles cubic macrostructure morphology and their internal nanoscale spatial organisation. To this goaltwo routes were investigated: a direct transformation and another, an indirect one, that required the formation of an oxide. Both the copper oxalate and the oxide showed an anisotropic behaviour during the transformation into the metal. It is shown experime