Rapid oxygen exchange between hematite and water vapor

TL;DR

This study reveals that well-ordered hematite surfaces can rapidly exchange lattice oxygen with water vapor at low temperatures, without surface disruption, challenging previous assumptions about defect necessity.

Contribution

It demonstrates that defect-free hematite surfaces can undergo quick oxygen exchange with water vapor, supported by atomic-scale observations and density functional theory calculations.

Findings

Oxygen exchange occurs within minutes at temperatures below 70°C.

Exchange predominantly happens during water desorption, not immersion.

Lattice oxygen stability is explained by on-surface diffusion and complex formation.

Abstract

Oxygen exchange at oxide/liquid and oxide/gas interfaces is important in technology and environmental studies, as it is closely linked to both catalytic activity and material degradation. The atomic-scale details are mostly unknown, however, and are often ascribed to poorly defined defects in the crystal lattice. Here we show that even thermodynamically stable, well-ordered surfaces can be surprisingly reactive. Specifically, we show that all the 3-fold coordinated lattice oxygen atoms on a defect-free single-crystalline r-cut (1-102) surface of hematite ({\alpha}-Fe2O3) are exchanged with oxygen from surrounding water vapor within minutes at temperatures below 70 {\deg}C, while the atomic-scale surface structure is unperturbed by the process. A similar behavior is observed after liquid water exposure, but the experimental data clearly show most of the exchange happens during desorption of the final monolayer, not during immersion. Density functional theory computations show that the exchange can happen during on-surface diffusion, where the cost of the lattice oxygen extraction is compensated by the stability of an HO-HOH-OH complex. Such insights into lattice oxygen stability are highly relevant for many research fields ranging from catalysis and hydrogen production to geochemistry and paleoclimatology.