# Atomic-Scale Insights into Surface Reconstruction and Dissolution of Hematite: The Formation of Water Cages and Protonation Effects

## TL;DR

This study uses simulations to explore how hematite dissolves in water at the atomic level, revealing why it resists full dissolution but forms surface defects.

## Contribution

The paper provides atomic-scale insights into the dissolution of hematite surfaces, including the role of water cages and protonation effects.

## Key findings

- The first major transition in dissolution requires breaking hydrogen bonds and overcoming steric hindrance.
- The final release of Fe has a high free-energy barrier due to a rigid water cage and loss of proton access.
- Hematite's resistance to full dissolution is linked to its tendency to form surface defects and adatom structures.

## Abstract

Dissolution of iron oxides in water plays a critical role in corrosion, mineral cycling, and surface reactivity; yet, the atomic-scale mechanisms governing Fe release remain poorly understood. Here, we employ ab initio molecular dynamics and well-tempered metadynamics simulations to investigate the stepwise dissolution of surface Fe atoms from the -Fe2O3(0001) surface in aqueous solution. The dissolution process initiates from a stable surface configuration in which Fe is coordinated to three lattice oxygen atoms and one water molecule. It proceeds through a series of metastable states involving additional water coordination, proton-assisted Fe-O bond weakening, and eventual detachment from the substrate. The first major transition, requiring 46.5 kJ/mol, involves breaking the hydrogen-bonding net and overcoming steric hindrance to allow adsorption of a second water molecule. Intermediate barriers (10.9–30.3 kJ/mol) are associated with further coordination and bond cleavage steps. In contrast, the final release of Fe into the solution, corresponding to a state coordinated with four water molecules and no lattice oxygen, exhibits a much higher free-energy barrier of ~93.0 kJ/mol. This barrier arises from the formation of a rigid hydrogen-bonded water cage and the loss of proton access to the remaining surface oxygen site, as confirmed by radial distribution function analysis. Our findings reveal why -Fe2O3(0001) is highly resistant to complete dissolution yet prone to surface roughening, defect formation, and adatom structures under aqueous conditions.