Nucleation, growth, and passivation of cobalt and iron nanoparticles
Abstract
The properties of magnetic nanoparticles depend not only on particle size and shape, but also on their stability over time. These issues are discussed in two independent studies, the first on nucleation and growth in the ligand-mediated synthesis of cobalt nanoparticles, the second on the passivation of crystalline iron nanoparticles by zinc oxide and carbonate shells. In the first study, the generation of cobalt nanoclusters from a multinuclear (Co16) calixarene complex was examined under solvothermal conditions. The thermochemistry of the Co16-calixarene complex was characterized by attenuated total reflectance infrared spectroscopy (ATR-IR) and matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS), which revealed structural changes that could be correlated with Co nanocluster formation. Based on the available spectroscopic and transmission electron microscopy (TEM) data, we propose the intermediacy of "capped clusters" in the solvothermal synthesis of cobalt nanoparticles from Co16-calixarene and Co4-pentyne. In the second study, a novel "nano-galvanization" approach was developed in which crystalline iron nanoparticles were coated with a zinc oxide shell (Fe@ZnO), followed by its conversion into hydrozincite and zinc carbonate upon treatment with simple aqueous solutions. These nanoparticles were characterized by thermogravimetric analysis (TGA), ATR-IR, TEM, and by changes in bulk magnetic susceptibility. TEM revealed the formation of thin (2–3 nm) oxide shells upon treatment of iron nanoparticles with zinc acetate dihydrate (ZnAc) under hydrothermal conditions, strongly suggestive of Fe@ZnO nanoparticles. Carbonation of the ZnO shell was evaluated by ATR-IR, which revealed key carbonyl stretches attributable to hydrozincite and zinc carbonate (smithsonite). The TGA of unpassivated Fe and Fe@ZnO nanoparticles showed no weight loss when heated up to 400 °C, whereas zinc carbonate-coated nanoparticles exhibited loss starting at 290 °C and hydrozincite-coated nanoparticles exhibited loss starting at 220 °C. Samples of unpassivated Fe nanoparticles diluted in ZnO matrices were found to have initial χ g values of 3.0-4.5 x 10-2 cm3/g. Heating the iron nanoparticles at 100 °C under ambient conditions caused a loss in magnetic susceptibility by as much as 40% in less than 24 hours. Further loss is experienced at higher temperatures, with a loss of 70% and 85% being observed at 200 and 300 °C, respectively. The Fe@ZnO nanoparticles also experienced a similar loss over time, with some protection against oxidation demonstrated at 100 °C, but not at 200 °C. Carbonated Fe@ZnO particles (presumed to be ZnCO3) experienced lower susceptibility losses at 100 °C (10%), and also at 200 °C (15%). We find that the carbonated shells provide significant protection against oxidation-induced loss of magnetization at temperatures below the decomposition threshold of hydrozincite or smithsonite.
Degree
Ph.D.
Advisors
Wei, Purdue University.
Subject Area
Inorganic chemistry|Nanoscience
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