X-ray photoelectron spectroscopy and magnetic studies on the effect of pore size, wall thickness, and wall composition on superparamagnetic cobaltocene mesoporous Nb, Ta, and Ti composites

Recent results in our group demonstrated that mixed oxidation state mesopor ous niobium oxide cobaltocene composites display superparamagnetism at cert ain composition ratios. This was the first report of superparamagnetism in nanoscale molecular ensembles. A series of mesoporous niobium oxide materia ls were synthesized in order to understand the role of pore size and thickn ess of the walls in the mesostructure on the magnetic properties. Mesoporou s Ti oxide and Ta oxide composites were also synthesized in order to invest igate the effect of changing the wall composition on the magnetic propertie s of this new series of materials. All samples were characterized by X-ray diffraction, nitrogen adsorption, ultraviolet spectroscopy, X-ray photoelec tron spectroscoy, scanning electron microscopy, and superconducting quantum interference device magnetometry. The results of this study showed that va riation of wall thickness or pore size in the Nh system had little effect o n the properties and that superparamagnetism most likely arises from mixed oxidation state cobaltocene grains residing in the individual pores and not from the free electrons in the mesostructure or much larger domains. The L angevin function was applied to the isothermal magnetic data from the Nh co mposites and gave mean superparamagnetic particle sizes of ca. 14 nm in eac h system. The Co(II) to Co(III) ratios in these materials were approximatel y 1:1. The Ti and Ta materials showed no sign of superparamagnetism and onl y very low levels of neutral cobaltocene in the pores. This suggests that a critical amount of cobaltocene is required to bring about superparamagneti c behavior.