(59c) Hollow Fe@SiO2 Nanorods for Oxidation-Reduction Chemistry

Core-shell materials, consisting of a reactive metal core surrounded by an inert support shell are of interest in a range of applications including catalysis, drug delivery, optics, and separation processes.  Hollow core-shell materials, in which there exists a central cavity within the metal core, are of particular interest; they present new possibilities for limiting confinement effects and hence studying the potential for an enhanced reduction-oxidation capacity for the metal cores.  We have developed a synthesis path for hollow iron core/silica shell nanoparticles (Fe@SiO₂) with ~40 x 150 nm rod shaped hollow iron cores.  The method begins with a hydrothermal synthesis of β-FeO(OH) nanorods, followed by a wrapping of these particles with a 20-30 nm porous and amorphous silica shell via a modified Stöber synthesis.  This synthesis route relies on the low density of the β-FeO(OH) nanorod intermediate.  The metal core becomes hollow only after transformation into higher density iron materials via calcination in air (to Fe₂O₃) or reduction (to elemental iron).  We have found that although a range of both hydrothermal and forced hydrolysis synthesis conditions can lead to the β-FeO(OH) intermediate, an optimization of temperature, iron precursor concentration, and pH are required for well-defined and uniform nanorods with narrow particle size distributions. 

Notably, initial evaluation of this material indicates redox capacities ~2 times higher than conventional (non-hollow) Fe@SiO₂ materials, possibly due to reduced confinement effects.  Synthesis and characterization of the materials, as well as application as oxygen carriers in chemical looping, a clean combustion technology, will be discussed in detail in the presentation.