Precipitation and Transformation of Nanostructured Copper Oxalate and Copper/Cobalt Composite Precursor Synthesis

PRECIPITATION AND TRANSFORMATION OF NANOSTRUCTURED COPPER OXALATE AND COPPER/COBALT COMPOSITE PRECURSOR SYNTHESIS THÈSE NO 3083 (2004) PRÉSENTÉE À LA FACULTÉ SCIENCES ET TECHNIQUES DE L'INGÉNIEUR Institut des matériaux SECTION DES MATÉRIAUX ÉCOLE POLYTECHNIQUE FÉDÉRALE DE LAUSANNE POUR L'OBTENTION DU GRADE DE DOCTEUR ÈS SCIENCES PAR Lucica Cristina SOARE DEA de physique de la matière, Université Paul Sabatier, Toulouse, France et de nationalité roumaine acceptée sur proposition du jury: Prof. H. Hofmann, directeur de thèse Dr P. Bowen, rapporteur Dr H. Cölfen, rapporteur Prof. M. Pijolat, rapporteur Prof. A. Renken, rapporteur Lausanne, EPFL 2004 To my mother and my father with gratitude… … … Remerciements Je tiens à remercier toutes les personnes qui ont contribué à la réalisation de ce travail, de prés ou de loin en particuliers: Le Prof. Heinrich Hofmann, mon directeur de thèse, le directeur du Laboratoire de Technologie des Poudres et initiateur de ce sujet, pour ses nombreux conseils et son intérêt tout au long de travail. Le Dr. Paul Bowen, pour son aide dans les moments clés de la thèse, les nombreuses discussions partagées, ses encouragements, sa patience, son aide pour la correction du manuscrit de thèse; il fut un chef et restera un ami; "if you can't be good be careful". Le Dr. Jacques Lemaître, pour son aide importante au début de ma thèse, son excellent enseignement scientifique et ses conseils. Le Dr. Nathalie Jongen, collègue de bureau et amie, qui a suivi de prés le début de ce travail de thèse. Le Dr. Sandrine Rousseau pour son soutien et ses encouragements au long de la thèse et les analyses ICP. Le Prof. Frank Kubel (Wien, University of Technology) - pour sa gentillesse et les nombreuses discussions sur la caractérisation des poudres par diffraction de rayons X, la détermination de la structure de l'oxalate de cuivre, et la visite d'une ville magnifique Wien. Le Prof. A. Philippe Buffat (Centre Interdépartemental de Microscopie Electronique - CIME Lausanne), pour son aide dans un moment important de la thèse, pour les nombreuses heures qu'il m'a consacré pour les diffractogrammes TEM sur l'oxalate de cuivre, malgré les conditions difficiles d'observation. Je tiens à le remercier également pour l'opportunité d'accès au HRSEM. Le Dr. Ollivier Pujol (Centre Interdépartemental de Microscopie Electronique - CIME Lausanne), collègue de travail, je teins à le remercier pour toutes les discussions tout au long de ce travail et l'enseignement de la microscopie. Mme. Daniel Laub (Centre Interdépartemental de Microscopie Electronique - CIME Lausanne), pour les superbes coupes qu'elle a réalisées ainsi que pour les diffractogrammes TEM sur les oxydes de cuivre et le cuivre métallique. Les Prof. Michèle Pijolat et Dr. Francoise Valdivieso (Sciences des Processus Industriels et Naturels - SPIN St-Etienne), de m'avoir accueillie dans leur laboratoire, leurs excellentes explications scientifiques et la mise en œuvre du troisième chapitre de thèse. Le Dr. Heinz Aminitsch (Institute of Biophysics and X-ray Structure Research, Graz), or "Planet Beam Time" pour la découverte du monde SAXS, son esprit et manière de travailler originale, pour la validation des mesures effectuées. L'Office Fédéral de l'Education et de la Science pour son soutien financier projet COST 523 n° C90- 0020 OFES: 20100-056643.99/1. Mes étudiants de projet de semestre: Michael Siegfried, Aude Hauert, Alban Dubach, pour leurs contributions à ce projet. Je voudrais également remercier tous mes collègues du LTP pour la bonne ambiance et leur humeur spécifique. Enfin je n'ai pas oublié mes amis, Sébastien Jiguet et Séverine Lamberet de m'avoir soutenue et encouragée dans les moments difficiles de la thèse, ma sœur Gabriela, Frédéric Juillerat et Wolfgang Kerbe d'avoir eu la patience du corriger l'anglais du manuscrit, Ingrid Schneider, Bernadette Müller et Petra Passuello avec qui j'ai partagé les vacances. Precipitation and Transformation of Nanostructured Copper Oxalate and Copper/Cobalt Composite Precursor Synthesis Summary 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 goal Precipitation and Transformation of Nanostructured Copper Oxalate and Copper/Cobalt Composite Precursor Synthesis two 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 experimentally that the anisotropy, nanostructure and inhomogeneity of the initial nanocrystallites of both the oxalate and the oxide have an important influence on the mechanism and evolution of transformation into the metal. The particle morphology was shown to be lost for a transformation yield of α>0.80 in the case of the direct transformation from the oxalate whereas the morphology was kept up to the metallic state when passing via the oxide. A kinetic model was proposed for both systems using the method of the sudden change in temperature and pressure. The kinetics analysis did not permit a total understanding of the transformations studied, as several stages were shown to have complex and concurrently competing mechanisms. However, a geometrical model was proposed using the ex-situ analysis of the samples as a function of the reaction yield for both routes. For the initial stages of the copper oxide reduction under He/H2 atmospheres, the kinetic analysis showed hydrogen dissociation as a rate-limiting step. With a view to producing a cobalt/copper composite, preliminary experiments were carried out for the co-precipitation of the Co/Cu oxalate with different cationic ratios. Thermodynamic calculations showed the formation of the two solids was possible independent of the ratio Co/Cu. Experimentally, however a co-precipitation was obtained only for a ratio Co/Cu of 1, whereas for other ration only one single phase (either copper oxalate or cobalt oxalate) was formed. A second route was investigated using cobalt oxalate or cobalt oxide seeds. The amount of the cobalt detected by TEM in the precipitate using either the oxalate or from the oxide seeds was around 3%wt, which is lower than the value of 10%wt necessary to provide desired magnetic properties in the resulting precipitate. The exact nature and spatial distribution of the cobalt was not ascertained and further analysis of the nanostructure by TEM needs to be carried out to confirm the premise of the seed route. This thesis has shown that it is possible to get to a deeper understanding of the kinetics and the mechanism of the copper oxalate precipitation using the appropriate techniques. The cubic macrostructure can be conserved from the initial CuO nanoparticles (13 nm) to metallic Cu (42 nm) by a controlled transformation in a reducing atmosphere. Preliminary experiments made on the possible formation of a Co/Cu composite via the use of the cobalt oxide seeds as heterogeneous nuclei for the copper oxalate precipitation seems most promising. Precipitation and Transformation of Nanostructured Copper Oxalate and Copper/Cobalt Composite Precursor Synthesis Version Abrégée Le but de la thèse était de synthétiser et de contrôler la nanostructuration d'un composite métallique Co/Cu. Les deux oxalates de cobalt et cuivre montrant une nanostructure, avec crystallite de 10-70 nm. La production d'un nanocomposite de type "mosaïque" a été envisagée par la voie de la co- précipitation de ces deux oxalates, suivie par un traitement thermique approprié afin d'obtenir le nanocomposite métallique.

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