Surface structure and environment-dependent hydroxylation of the nonpolar hematite (100) from density functional theory modeling

Hematite (α-Fe₂O₃) nanoparticles are typically synthesized, stored, or used in hydrous environments, and the mineral/water interfaces are important for the surface stability and reactivity of these nanoparticles. Under such conditions the exposed facets are often passivated by hydroxyl groups. The configurations of surface hydroxylation vary with environmental conditions and affect the morphology and surface chemistry. Among the low-index hematite surfaces, the {100} are the only nonpolar surfaces and are often present on nanorods or nanotubes elongated along the [001] direction. In this paper we explore the relaxation and hydroxylation of this surface using first principles thermodynamics. Our results reveal that depending on the supersaturation of water and oxygen, various extents of hydroxylation may appear. In humid or hydrous environments, undercoordinated subsurface oxygen atoms are hydrogenated. In water singly and doubly coordinated hydroxyl groups coexist with chemisorbed water molecules at the surfaces. In environments where the humidity is reduced, the surface is terminated exclusively by doubly coordinated hydroxyl groups. The clean surface occurs when the humidity is further reduced or when temperature is elevated. On the basis of these findings, we have constructed the surface phase diagrams to describe the thermodynamic stability for two different temperatures. The phase diagrams enable us to predict the density and type of hydroxylation, which is relevant to surface stability, reactivity, and catalytic properties in hydrous or humid environments.