Variation in the iron oxidation states of magnetite nanocrystals as a function of crystallite size: The impact on electrochemical capacity

We have investigated magnetite (Fe₃O₄) as an electroactive battery electrode material, where a linear relationship was observed between Fe₃O₄ crystallite size and capacity, with a negative slope. In order to better understand this novel relationship, we report here the Rietveld refinement and X-ray absorption spectroscopy (XAS) investigation of nanosized Fe₃O₄ as a function of crystallite size (7-26 nm). Rietveld refinement established that the Fe ₃O₄ samples were phase pure, while the extended X-ray absorption fine structure (EXAFS) and X-ray absorption near edge structure (XANES) provided insight into the local geometries and electronic structure of the iron centers, including oxidation state assignment. From our current and recent studies, we suggest that the surface of the Fe₃O₄ crystallites is rich in Fe³⁺, thus as the Fe₃O₄ crystallite size decreases, the electrochemical capacity increases, due to a net enrichment of Fe₃O₄ in Fe³⁺.