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| - X-ray Photoelectron Spectroscopy and Magnetic Studies on the Effect of Pore Size, WallThickness, and Wall Composition on Superparamagnetic Cobaltocene Mesoporous Nb, Ta,and Ti Composites
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| - Recent results in our group demonstrated that mixed oxidation state mesoporous niobium oxide cobaltocenecomposites display superparamagnetism at certain composition ratios. This was the first report of superparamagnetism in nanoscale molecular ensembles. A series of mesoporous niobium oxide materials were synthesized inorder to understand the role of pore size and thickness of the walls in the mesostructure on the magnetic properties.Mesoporous Ti oxide and Ta oxide composites were also synthesized in order to investigate the effect of changingthe wall composition on the magnetic properties of this new series of materials. All samples were characterizedby X-ray diffraction, nitrogen adsorption, ultraviolet spectroscopy, X-ray photoelectron spectroscoy, scanningelectron microscopy, and superconducting quantum interference device magnetometry. The results of this studyshowed that variation of wall thickness or pore size in the Nb system had little effect on the properties and thatsuperparamagnetism most likely arises from mixed oxidation state cobaltocene grains residing in the individualpores and not from the free electrons in the mesostructure or much larger domains. The Langevin function wasapplied to the isothermal magnetic data from the Nb composites and gave mean superparamagnetic particle sizesof ca. 14 nm in each system. The Co(II) to Co(III) ratios in these materials were approximately 1:1. The Ti andTa materials showed no sign of superparamagnetism and only very low levels of neutral cobaltocene in the pores.This suggests that a critical amount of cobaltocene is required to bring about superparamagnetic behavior.
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