Surface Structure/Property Relationship for (001) Surface of Magnetite
Magnetite (Fe3O4), a well-known magnetic material, is still attracting intense study because of its great application in catalyst and technology development. These useful properties are related to the coexistence and coupling of several degrees of freedom, including charge, lattice, orbital and spin. The interaction between Fe3O4 and hydrogen is one of the most important issues, which guides the development of catalytic efficiency and material practicality. In this work, natural single crystal Fe3O4 (001) surfaces are studied with a variety of techniques. It is discovered that the Fe3O4 (001) surface structure and properties are dependent on the surface preparation methods. Conventional processed surfaces in an oxygen-rich environment are found to be oxygen deficient, with a significant amount of ordered oxygen vacancies on the surface and even penetrate deep into the bulk. The more stoichiometric surface is then obtained by ozone treatment, which successfully removes most surface vacancies. Atomic hydrogen is used to probe the Fe3O4 (001) surface. On an ozone processed (OP) surface, H bonds to surface oxygen, which form hydroxyl as expected. However, on conventional processed (CP) surfaces, H is found to bond preferentially to the surface Fe atoms. This abnormal H-Fe bonding is a result of oxygen vacancies on the CP surface. One explanation is, when H is adsorbed by a CP surface, it leads to the formation and desorption of water, thus creating more oxygen vacancies and stabilizing H-Fe bonds. Our study shows that previous experimental work on CP Fe3O4 surfaces all deal with oxygen deficient surfaces, which solves the long disagreement between experimental results and theoretical predictions. The different H bonding on CP and OP surfaces can serve as a novel direction of catalysis development and hydrogen storage applications.
material processing, H adsorption, oxygen defects
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