Synthesis and Characterization of Mn-based Spinels Grown by Molecular Beam Epitaxy

Abstract

A developed energy infrastructure that relies predominately on renewable energy is critical to the longevity of society. Water-splitting and fuel cell devices will have a place in this infrastructure, although this technology is still in its infancy. Implementation of this technology on a global scale requires effective devices that are also economically feasible to manufacture. Modern fuel cell devices incorporate effective but expensive material components such as platinum and iridium to perform at high levels, hindering their wide-spread use. However, recent investigations into metal-oxide spinel materials containing elements such as iron, cobalt, nickel and manganese have demonstrated their potential for use in fuel cell technology as an inexpensive alternative to precious metals. Despite their importance in catalysis and fuel cell research, many of these materials are understudied on the basic material property level. Studies on these materials from a thin film perspective are particularly sparse. A thorough understanding of these materials is essential for their implementation in water splitting and fuel cell technology. In order to investigate the properties of these important materials and facilitate their future use as catalysts, samples were synthesized using molecular beam epitaxy and studied using a wide array of characterization techniques. MnFe2O4 single-orientation samples were grown with different crystal orientations in order to observe this effect on the material’s catalytic properties. While MnFe2O4’s properties are widely understood in literature, this study is the first to investigate the oxygen reduction reactivity properties of MnFe2O4 grown as an epitaxial thin film. Single-crystalline CoMn2O4 and Co-rich Cox+1Mn2-xO4 samples were grown and studied, and the material’s electronic properties were found to be constant despite variable stoichiometry. X-ray photoelectron and absorption spectroscopy revealed tetrahedral Co2+ and octahedral Mn3+ coordination, and cation-oxygen bond lengths quantifies the effect of Jahn-Teller distortion on stretched Mn3+ octahedra. X-ray Diffraction reveals the tetragonal crystal structure of the material and increased c-lattice parameter due to the Jahn-Teller effect. MnxCo3-xO4 samples from x = 0 to x = 1.28 were grown and studied, and manganese cations were found to be octahedrally coordinated. Most samples also show mixed Mn3+ and Mn4+ character, and greater c-lattice parameter and phase segregation tendencies with increasing manganese content away from Co3O4. The increased lattice parameters are likely due to Jahn-Teller-active Mn3+ octahedra, and phase segregation may occur due to structural incompatibility between cubic and tetragonal crystal structures associated with Mn4+ and Mn3+ octahedra, respectively. Mn3+ valence character is likely due to insufficient oxygen reactivity during sample growth, with Mn4+ being the proper valence state of the spinels. Cobalt valence also trends from mixed Co2+ and Co3+ towards predominately Co2+ with increasing manganese content, meaning Co2+ is the favored valence state. This study suggests that the single-crystalline phase of ideal MnCo2O4 is an inverse-type spinel comprised of Mn4+ and Co2+ valence states as opposed to the typical 2+ and 3+ states of most other spinels.