Characterization of the structure and the surface reactivity of a marcasite thin film

A thin film of marcasite, FeS₂, was synthesized under vacuum and its structure and reactivity under oxidizing conditions was investigated by means of diffraction and surface analytical techniques, respectively. Synthesis of the film was carried out by codepositing Fe and S₂ onto a Ta support. The thickness of the film could be varied from approximately 10 Å to 1 μm. High-resolution S 2p synchrotron-based photoemission showed S22−, with undetectable amounts of S²⁻ impurity that is typically present on natural sample surfaces. X-ray diffraction of the micron-thick films showed that the film crystallized in the marcasite phase of FeS₂. Atomic force microscopy indicated that the thin film had a nanometer-scale roughness suggesting the film contained defects such as steps and kinks. X-ray photoelectron spectroscopy studies found the thin marcasite film to be more reactive than natural pyrite (the most ubiquitous FeS₂ dimorph) after exposure to a gaseous O₂/H₂O environment on the basis of the amount of sulfate formation. Likely the oxidation of marcasite was dominated by its short-range order (e.g., presence of steps), because the density of nonstoichiometric defect sites (e.g., S²⁻) was low as assessed by photoelectron spectroscopy.