Studies on preparation and properties of nanophase metal oxides

Abstract: This thesis is about science on an atomic scale - aninterdisciplinary approach to nanoparticles. Studies of thepreparation of nanoparticles using both physical and chemicalmethods are presented. Understanding and controlling factorsfor particle growth during synthesis are the keys to successfulfabrication of materials with tailored properties. The sizeeffects on electronic, magnetic, optical, as well as oxygenstorage and catalytic properties have been investigated. Therelationship between the structure and physical properties hasbeen examined. The erudition of the fundamentals and thecontrol of the synthesis enables the tailoring of new materialson the atomic scale.The preparation of nanoparticles using a new technique basedon laser evaporation under reactive atmosphere in an upwardthermal diffusion cloud chamber was attempted. Unlike otherlaser evaporation - gas condensation techniques, high vacuumwas not needed and atmospheric pressure sufficed. Particlesbelow 10 nm in diameter were successfully prepared without anynecking, despite the high pressure. No further treatments likepost-oxidation or carburization were needed. It was possible toprepare many different materials: metals, oxides, carbides andnitrides. "New" materials like molybdenum carbide (MoC4)nn=1-4 series have been made on a macroscopicscale. Molybdenum was found to catalyze the formation offullerenes, especially the higher ones, as well as thepolymerization of ethylene.Silicon and iron oxides were synthesized using laservaporization. Silicon oxide nanoparticles evincephotoluminescence. The oxygen-deficient structure of silicagives rise to this phenomenon. The magnetic behavior of ironoxides can be explained in terms of superparamagnetism. Theoxidation state of iron oxides could be controlled by thepartial pressure of oxygen in the preparation chamber.Chemical methods were applied for synthesis of nanophasemetal oxides. The particle growth and agglomeration of dopedcerium oxalates precipitated using the coprecipitation methodwas studied. Agglomeration was found to be diffusion controlledand proceeded through the cluster-cluster collision mechanism.Optimization of particle morphology by controlling thesurfactant selection, pH, dilution and counter ions wasperformed. DLVO theory was used to explain the particlegrowth.The structure, oxygen storage capacity and surface chemistryof cerium-zirconium oxides prepared by laser vaporization andcoprecipitation methods were compared. The reducibility ofcerium oxide in laser evaporated samples was significantlyenhanced and activated at much lower temperatures and this wasattributed to the reduction of surface hydroxyl groups.Coprecipitated samples had the highest carbon contamination,which was possibly due to undecomposed carbonates or surfacecharges that favored carbon dioxide absorption.The synthesis of doped ceria applying differentprecipitation sequences was studied and the catalytic activitywas compared to a commercial catalyst. Calcium doping increasesthe oxygen storage capacity remarkably in rhodium catalysts.The lambda window was equal to that obtained for the referencesample despite the lower surface area.Studies of the catalytic activity of doped cerium oxideswith and without precious metals were carried out. Highcatalytic activity was obtained for manganese andneodymium-zirconium doped samples. Good thermal stability wasobserved for neodymium-zirconium doped ceria. The light-offtemperature of doped cerium oxide catalysts towards COoxidation was a function of particle size. The surface area onthe other hand was found to be related to the electronegativityof the dopant. The heat of formation of the oxidesseemed to bedirectly related to the heat of adsorption of propene and aminimum light-off temperature was found. The catalytic activityof palladium-impregnated ceria samples was a function of theacidity of the cerium oxide support.Keywords:nanoparticles, laser vaporization,coprecipitation, SiO2, iron oxide, CeO2, doping, photoluminescence, superparamagnetism,surface chemistry, oxygen storage capacity, catalytic activity,modeling